Available via license: CC BY-NC-ND 4.0
Content may be subject to copyright.
Contributions of Microtubule Dynamics and Transport to
Presynaptic and Postsynaptic Functions
Chandra S. J. Miryala1, Elizabeth D. Holland2, Erik W. Dent1,*
1Department of Neuroscience, University of Wisconsin, School of Medicine and Public Health,
Madison, WI 53705
2Neuroscience Training Program, University of Wisconsin-Madison, Madison, WI 53705
Abstract
Microtubules (MT) are elongated, tubular, cytoskeletal structures formed from polymerization
of tubulin dimers. They undergo continuous cycles of polymerization and depolymerization,
primarily at their plus ends, termed dynamic instability. Although this is an intrinsic property
of MTs, there are a myriad of MT-associated proteins that function in regulating MT dynamic
instability and other dynamic processes that shape the MT array. Additionally, MTs assemble into
long, semi-rigid structures which act as substrates for long-range, motor-driven transport of many
different types of cargoes throughout the cell. Both MT dynamics and motor-based transport play
important roles in the function of every known type of cell. Within the last fifteen years many
groups have shown that MT dynamics and transport play ever-increasing roles in the neuronal
function of mature neurons. Not only are neurons highly polarized cells, but they also connect
with one another through synapses to form complex networks. Here we will focus on exciting
studies that have illuminated how MTs function both pre-synaptically in axonal boutons and
post-synaptically in dendritic spines. It is becoming clear that MT dynamics and transport both
serve important functions in synaptic plasticity. Thus, it is not surprising that disruption of MTs,
either through hyperstabilization or destabilization, has profound consequences for learning and
memory. Together, the studies described here suggest that MT dynamics and transport play key
roles in synaptic function and when disrupted result in compromised learning and memory.
Introduction
The synapse has long been known to be a primary point of rapid communication between
presynaptic axons and postsynaptic dendrites. Both presynaptic and postsynaptic sites are
specializations in the axon and dendrite, respectively, composed of distinct membrane
receptors, adhesion proteins, scaffolding proteins, signaling molecules and cytoskeletal
components (Krueger-Burg et al., 2017; Sheng and Hoogenraad, 2007). Broadly, synapses
are divided into inhibitory and excitatory types. Within the central nervous system (CNS),
excitatory synapses are generally associated with dendritic spines, small mushroom-shaped
*Corresponding Author: Department of Neuroscience, School of Medicine and Public Health, University of Wisconsin-Madison,
Madison, WI 53705, ewdent@wisc.edu.
Declaration of Competing Interests
Declarations of interest: none
HHS Public Access
Author manuscript
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Published in final edited form as:
Mol Cell Neurosci
. 2022 December ; 123: 103787. doi:10.1016/j.mcn.2022.103787.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
protuberances arrayed along dendrites, while inhibitory synapses occur on dendrite shafts
and spine necks (Bucher et al., 2020). In this review we will concentrate on the cytoskeletal
contributions to both presynaptic and postsynaptic compartments of central nervous system
excitatory neurons, such as the pyramidal cells in the cortex and the principal neurons in
the hippocampus. Specifically, this review will focus on the dynamic microtubule (MT)
cytoskeleton and the emerging role it plays in synaptic function.
Both presynaptic and postsynaptic specializations require cytoskeletal components to
function properly, including actin filaments (f-actin), MTs, intermediate filaments and septin
filaments. Although we will focus on MTs in this review it is worthwhile introducing
actin filaments as well. Indeed, the most heavily concentrated cytoskeletal polymer in
postsynaptic dendritic spines and presynaptic axonal boutons are actin filaments. Actin
filaments are comprised of globular actin subunits (g-actin) that polymerize into a helical
structure approximately 7 nm in diameter, which can elongate several microns in length.
Neurons contain a myriad of actin-associated proteins that affect actin filament geometry,
stability and motility (Dent et al., 2011). Actin filaments are generally bundled together in
either parallel or antiparallel arrays or are part of a branched actin network. These forms of
actin networks play distinct roles in dendritic spines and axonal boutons. We will touch on
some of these actin-associated proteins and how they contribute to f-actin/MT associations
at synapses.
Actin forms filaments from actin monomers, while MTs polymerize from tubulin dimers,
composed of an alpha and a beta subunit. These alpha-beta tubulin dimers associate end-to-
end forming protofilaments, which themselves associate with one another to form a tube
composed of 13–16 protofilaments that is approximately 25nm in diameter (Roll-Mecak,
2020). Within most cells, including neurons, MT can elongate to over one hundred microns
in length (Brown et al., 1993). Like actin filaments, MTs can also bundle into parallel
and antiparallel arrays and branch in coordination with MT-associated proteins. Both actin
filaments and MTs are polar polymers, with each end having different polymerization and
depolymerization kinetics. However, unlike actin filaments, which generally polymerize
from one end (the plus or barbed end) and depolymerize from the other end (minus or
pointed end), MTs undergo a process termed dynamic instability (Mitchison and Kirschner,
1984). Dynamic instability is a stochastic process by which the plus end of MTs undergoes
cycles of growth and shrinkage. The transition of growth to shrinkage is termed catastrophe,
while the transition from shrinkage to growth is termed rescue. Dynamic instability allows
MTs to quickly reorganize and efficiently explore all regions of a cell, which is particularly
important for neurons, because of their extremely polarized morphology (Figure 1).
The polarized structure of neurons also suggests they may require more decentralized
nucleation and growth of MTs. MTs nucleate from tubulin dimers, forming a ringlike
structure that continues to polymerize to form a tube, from the centrosome through the
actions of the gamma tubulin ring complex (γTURC) and remain attached to this organelle
(Akhmanova and Kapitein, 2022). The centrosome is positioned in the cell body, but the
length of dendritic and axonal processes can be hundreds or thousands of times the diameter
of a cell body. Electron microscopy studies of neurons have shown that most MTs are not
attached to the centrosome (Baas and Black, 1990) and individual MTs do not extend the
Miryala et al. Page 2
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
length of dendrites or axons (Baas et al., 1989). In neurons, although MTs polymerize from
the centrosome, they are severed and can be transported throughout the neuron (Rao and
Baas, 2018). Because the minus ends of MTs in neurons are generally not associated with
the centrosome they can stabilized by other proteins. These minus-end binding proteins
(-TIPs) include microtubule associated proteins (MAPs) such as the CAMSAP/Patronin
family, end binding proteins (EBs), severing proteins, such as katanin, the minus-end motor
cytoplasmic dynein and γ-TURCs not associated with the centrosome (Akhmanova and
Steinmetz, 2019; Kuo and Howard, 2021). These proteins regulate the dynamics of the
minus ends of MTs and thus play important roles in the overall MT dynamics in neurons.
These -TIPs have not been studied as extensively as plus end binding proteins (+TIPs),
especially in neurons, and deserve further study.
Several studies have demonstrated that MTs can nucleate and polymerize from other cellular
structures in axons and dendrites. In dendrites, MTs can nucleate at Golgi outposts (Horton
and Ehlers, 2003; Horton et al., 2005; Ori-McKenney et al., 2012) and a recent study in
C. elegans
demonstrated local MT nucleation in dendritic growth cones in association with
Rab11-containing endosomes (Liang et al., 2020). Nucleation and polymerization of MTs
from these endosomes were shown to be a source of minus-end-out MTs in dendrites. The
recent discovery of MT nucleation from existing MTs in neurons, through the activity the
HAUS/augmin complex and γ-tubulin, provides yet another structure onto which new MT
nucleation and growth can occur in both axons and dendrites (Cunha-Ferreira et al., 2018;
Sanchez-Huertas et al., 2016). Such growth of new MTs from existing MTs results in MT
branching that is synonymous with the well-known actin nucleation and growth by the
Arp2/3 complex, which results in a branched actin network (Pollard, 2007). Thus, MTs can
nucleate throughout the neuronal cytoplasm and undergo cycles of growth and shrinkage,
making for a very dynamic MT network.
Perhaps, more than any other advance, the discovery and imaging of MT plus-end binding
proteins allowed for the appreciation of MT dynamics in neurons (Stepanova et al., 2003).
Although many different types of proteins can associate with the growing ends of MTs
(Akhmanova and Steinmetz, 2008) a canonical member of this +TIP group of proteins
is the family of end-binding proteins (EB1–3) (Akhmanova and Steinmetz, 2015). When
EB3-EGFP was first visualized in neurons it was surprising to see how dynamic MTs were
in the thin processes of axons and dendrites (Stepanova et al., 2003). EB3 cycles on and off
the growing ends of MTs, creating a comet-like structure that moves through the cytoplasm
at approximately 5–15um/min (Dixit et al., 2009; Dragestein et al., 2008). Previous studies
of MTs in neurons required injection of labeled tubulin (Dent et al., 1999; Dent and Kalil,
2001) or expression of GFP-tubulin (Kimura et al., 2005), which is incorporated throughout
the MT polymer. Since MTs are packed together in dendrites and axons, MTs labeled with
tubulin resulted in a continuously fluorescent axon and dendrite. Only where processes
flattened, such as at the neuronal growth cone or at axon branch points, could individual MT
dynamics be discerned (Dent et al., 1999; Dent and Kalil, 2001). However, upon imaging
EB3-EGFP it was obvious that MTs were dynamic throughout axons and dendrites. Further
work showed this was indeed the case in cultured neurons, in hippocampal slices and
in vivo
(Yau et al., 2016). Surprisingly, even in mature neurons (63DIV) MTs remained
dynamic, polymerizing and depolymerizing throughout dendrites (Hu et al., 2008). Thus,
Miryala et al. Page 3
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
imaging EB3 comets has shown that MTs continue to polymerize in well established, stably
polarized neurons throughout their lifetimes. Interestingly, EML2-S was recently discovered
to concentrate near the ends of depolymerizing MTs (Hotta et al., 2022). Imaging this
protein, in conjunction with EB3, may allow the construction of a more complete picture of
MT dynamics in neurons.
In addition to MT dynamics, MTs serve the important function of acting as a substrate for
long-distance transport of material throughout neurons. The polarity of MTs is detected by
motor proteins that either move toward the plus end (kinesins) or the minus end (cytoplasmic
dynein) of MTs (Cason and Holzbaur, 2022). There are many different kinesin proteins
in neurons, but one major form of cytoplasmic dynein. Each type of kinesin binds a
specific set of adapter proteins, which results in the transport of specific cargo, including
vesicles, proteins and mRNA, throughout the neuron (Cason and Holzbaur, 2022). Kinesin
and dynein motors are also capable of transporting MTs themselves within neurons (Del
Castillo et al., 2019; Rao and Baas, 2018). If motor proteins are bound, through their
cargo-binding domain, to stable structures, such as the plasma membrane, submembranous
actin cytoskeleton or to other less compliant MTs, they can continue to motor along MTs.
This results in kinesins translocating MTs minus-end-leading, while cytoplasmic dynein
translocates MTs with their plus ends leading. Therefore, the motor-based transport of
MT polymers in neurons provides a further mechanism by which MTs remain dynamic in
neurons. Together, MT polymerization/depolymerization and motor-based transport function
in coordination to translocate material throughout axonal and dendritic processes.
Microtubule dynamics and transport in presynaptic axons and boutons
Axons are long, thin processes extending from the cell body of a neuron that branch
extensively in the neuropil. CNS axons contact many other neurons through axonal
swellings termed axonal boutons (Figure 1). Axons are composed of bundles of MTs
extending along the length of the axon. However, each MT is shorter than the axon but
overlaps proximally and distally with other MTs, creating a continuous array of MTs
throughout the axon and all branches (Baas et al., 1988). Due to their polarity, MTs are
not uniform structures, rather they contain both dynamic and stable regions. To mark these
stable and dynamic regions researchers have exploited the fact that the α-tubulin subunit
in the tubulin dimer is tyrosinated at the C-terminal end of the protein. Newly polymerized
MTs are therefore described at tyrosinated and can be identified as such by antibodies that
specifically recognize this posttranslational modification of tubulin. Over time, MTs become
both acetylated, at separate residues (K40 being the most studied), and detyrosinated at
the C-terminus. Thus, older, presumably more stable MTs can be labeled with antibodies
that recognize acetylated or detyrosinated tubulin (Figure 1C). Interestingly, when labeled
immunocytochemically with these antibodies there is a fairly sharp demarcation between
acetylated and tyrosinated regions of MTs in neurons (Brown et al., 1993). Almost all MTs
in axons are oriented in a plus-end-out fashion (Yau et al., 2016). This parallel array of
MTs has implications for motor-driven transport of cargo. In axons, anterograde transport
of cargo is exclusively the domain of plus-end-directed kinesins, while retrograde transport
is carried out by cytoplasmic dynein. Two important cargos that are transported throughout
Miryala et al. Page 4
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
the axon and are related to synapses are synaptic vesicle precursors (SVPs) and dense core
vesicles (DCVs).
Early studies employing electron microscopy suggested that synaptic vesicles were
associated with MTs at presynaptic sites (Bird, 1976; Gordon-Weeks et al., 1982; Gray,
1975). Moreover, several studies have identified tubulin in presynaptic fractions (see
references in (Waites et al., 2021)), while others have shown MTs play an important
role at the
Drosophila
neuromuscular junction (see references in (Dent, 2020)). However,
relatively little research has focused on the connection of MTs and vesicles at the synapse
in mammalian neurons, until recently. Several contemporary studies have shown that MTs
are likely critical for the delivery of both SVPs and DCVs to presynaptic compartments
at axonal boutons (Figure 2). One study showed that DCVs containing synaptotagmin-IV
(Syt-4) are trafficked by the Kinesin-3 motor KIF1A in axons (Bharat et al., 2017). As
these vesicles transit down the axon, increases in the activity of the protein kinase JNK near
synapses phosphorylates Syt-4, releasing the DCVs from MTs. After releasing from MTs,
the DCVs are captured by presynaptic actin filaments. Although MT dynamics were not
analyzed directly in this study, it is likely that DCV release occurs at MT plus ends, as has
been shown in the postsynaptic spine (see below) (McVicker et al., 2016).
Two studies discovered complementary mechanisms by which MTs contribute to the
transport and delivery of SVPs to synaptic boutons. One study, by the Holzbaur group,
showed that in mature axons MT polymerization oftentimes originates and terminates in
presynaptic terminals at
en passant
synapses of cultured hippocampal neurons (Guedes-Dias
et al., 2019). Disruption of MT dynamics by pharmacological means resulted in decreased
delivery of SVPs to synapses. Interestingly, like DCVs mentioned above, the SVPs used
KIF1A for transport in the axon. The authors discovered that KIF1A had a decreased
affinity for the “GTP cap” that exists at the plus ends of dynamic MTs. The decreased
affinity of KIF1A as it reached the plus ends of MTs and data showing MT plus ends are
often associated with synapses suggest that this is a mechanism for concentrating SVPs
specifically at synapses. A second study showed that MT nucleation preferentially occurs
at presynaptic sites in mature hippocampal neurons in culture and in slice preparations (Qu
et al., 2019). This presynaptic nucleation was activity-dependent and required γ-tubulin
and the HAUS/augmin complex, indicating that new MT nucleation and potentially MT
branching (Cunha-Ferreira et al., 2018; Sanchez-Huertas et al., 2016) is important for
delivery of SVPs to
en passant
synapses. These newly nucleated MTs served as conduits
for transport of SVPs between boutons. Most significantly, loss of these nucleated MTs
resulted in impaired high-frequency neurotransmitter release (Qu et al., 2019). Together,
these results suggest that MT nucleation, dynamic instability and motor driven transport play
fundamental roles in concentrating DCVs and SVPs to axonal
en passant
synapses.
Both studies mentioned above suggest that SVP anterograde movement along plus-end-out
MTs and their unloading at
en passant
synapses are one mechanism to concentrate SVPs
at synapses. A very recent study has shown that retrograde transport of SVPs via dynein
can occur at the minus end of MTs in
C. elegans
(Balseiro-Gomez et al., 2022). In
this study, the authors showed that VAB-8/KIF26, an immotile kinesin, localizes to MT
minus ends and promotes dynein pausing to facilitate SVP delivery to synapses (Figure 2).
Miryala et al. Page 5
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
Interestingly, the levels of VAB-8/KIF26 at MT minus ends are determined by neurexin
and frizzled signaling. In combination with the studies mentioned above, it appears that
SVP accumulation at presynaptic sites can occur through both anterograde and retrograde
transport, with offloading occurring at either the plus or minus ends of MTs, respectively.
Together, these studies suggest that both plus and minus end positioning of MTs within
the axon is critical for SVP targeting to synapses. However, these studies do not preclude
the likelihood that presynaptic actin and myosin motor proteins also play important roles
in trafficking recycling synaptic vesicles within the axon (Chenouard et al., 2020; Gramlich
and Klyachko, 2017). Nevertheless, MTs appear as important players in both the long-
distance transport and specific targeting of SVPs and DCVs to presynaptic specializations.
Microtubule dynamics in postsynaptic dendrites and spines
Dendrites are generally shorter and thicker processes, compared to axons, and in excitatory
neurons their dendritic array is studded by small mushroom-shaped growths termed spines.
Importantly, the MT array of dendrites is constructed in a different way than axons (Figure
1). Unlike axons, where the MTs are entirely plus-end-out, in mature dendrites MTs are
arranged in an antiparallel fashion, with approximately equal numbers of plus-end-out and
minus-end-out MTs (Baas et al., 1988; Stepanova et al., 2003; Yau et al., 2016). However,
in
Drosophila
and
C. elegans
dendrites contain mostly minus-end-out MTs (Goodwin et
al., 2012; Stone et al., 2008). It is unclear why this difference exists, but it clearly has
implications for both MT dynamics and transport. Because of the different arrangement of
MTs in axons and dendrites plus-end-directed kinesins only move cargo anterogradely in
axons, while they can move both anterogradely and retrogradely in dendrites. Similarly,
dynein moves only retrogradely in axons but can move in both directions in dendrites.
In addition to their orientation, MT composition has been a focus of study. Many classic
studies, primarily from the Baas and Black groups, have shown that MTs in both axons and
dendrites have a composite structure, with a stable region toward the minus end of the MT
and a dynamic region toward the plus end (Baas and Black, 1990; Baas et al., 1991; Brown
et al., 1993; Li and Black, 1996; Rochlin et al., 1996; Wang et al., 1996). However, it should
be noted that these studies used cultured sympathetic neurons from the superior cervical
ganglia and analyzed MTs via immunocytochemistry (ICC) and electron microscopy (EM).
Over the last 30 years this model of composite MTs has generally dominated the literature.
Moreover, it was generally assumed that this same composite model of MTs occurred in
other types of neurons, such as hippocampal and cortical neurons.
However, recent work by the Kapitein group, studying MTs in dendrites of cultured
hippocampal neurons, has suggested that this model may need some revision (Katrukha
et al., 2021; Tas et al., 2017). In two recent studies this group used super resolution
microscopy, coupled with a motor-PAINT technique, as well as ICC and expansion
microscopy to interrogate the structure of MTs in dendrites. Implementing these techniques,
they showed that minus-end-out MTs are concentrated in the central region of the dendrite,
are acetylated, and are more stable than the dynamic, tyrosinated MTs that are localized
around this central MT core, near the plasma membrane (Katrukha et al., 2021; Tas et al.,
2017). The model based on the above studies from the 1990’s posits a random distribution
Miryala et al. Page 6
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
of composite MTs, each containing acetylated and tyrosinated regions. These new data
suggest a new model whereby MTs in hippocampal dendrites are primarily either acetylated
or tyrosinated and organized in a center-acetylated/peripheral-tyrosinated fashion (Figure
1). However, the Kapitein group was not able to follow individual MTs in their entirety,
even with expansion microscopy and super-resolution imaging. Thus, they could not exclude
the possibility some or most of the MTs in mature hippocampal neurons are composite
in nature. Moreover, such a model where minus-end-out MTs are entirely stable would
predict few, if any, retrograde EB3 comets in dendrites. However, it has been demonstrated
clearly in several studies, using EB3 imaging of growing MTs in dendrites, that retrograde
EB3 comets originate throughout the dendritic tree of hippocampal neurons (Hu et al.,
2008; Stepanova et al., 2003; Yau et al., 2016). Moreover, if all the stable MTs are
minus-end-out the kinesins that favor stable MTs would only be able to transport material
in the retrograde direction. Thus, a combination of these models is likely to be a more
accurate representation of MT organization in mature neurons. In such a model acetylated,
stable MTs are concentrated in the central region of the dendrite but are also capable of
polymerizing in the retrograde direction and thus contain a composite structure, tipped by
a region of tyrosinated tubulin (Figure 1). Additionally, the plus-end-out MTs are likely to
be mostly tyrosinated and more dynamic than the centrally located MTs, but may contain
a shorter, stable, acetylated region toward their minus end. Moreover, there are likely small
populations of dynamic minus-end-out MTs and stable plus-end-out MTs in dendrites as
well. This model of MT composition and organization in dendrites has clear implications for
both MT dynamics and movement of motors along MTs in dendrites and spines.
MT dynamics in dendrites and spines will be covered first. We will then touch upon motor-
driven transport and try to integrate these two processes. Although dendrites and axons must
maintain their integrity, oftentimes for the life of the organism, MT turnover is constantly
occurring through dynamic instability and severing. Since MTs can explore the entire cell
through cycles of polymerization and depolymerization, it suggests they would be able to
explore all regions of a neuron including dendrites and dendritic spines. Curiously, standard
preparatory and EM imaging techniques result in micrographs where MTs are confined to
the dendritic shaft, with no MTs entering dendritic spines (Harris et al., 2022). However, by
using unusual preparatory techniques (see Dent, 2017 for more detail) George Gray showed
that MTs enter dendritic spines in the cerebral cortex and can extend into the post synaptic
density (Gray et al., 1982; Westrum et al., 1983; Westrum et al., 1980). Were these studies
just artifacts of the preparation? To answer this question several labs used fluorescently
labeled tubulin and EB proteins to determine the dynamics of MTs in dendrites of mature
hippocampal and cortical neurons in culture. Consistently, four independent groups showed
that MTs continued to polymerize in dendrites over the lifetime of the cultures and explored
the entire neuronal cytoplasm, including dendritic spines (Gu et al., 2008; Hu et al., 2008;
Jaworski et al., 2009; Mitsuyama et al., 2008). Further studies by these and other labs have
confirmed that MTs polymerize into and out of dendritic spines.
MT entry into dendritic spines does not appear to be a random process. Although
dynamic instability is a stochastic process (Mitchison and Kirschner, 1984), suggesting MTs
polymerize and depolymerize in a random manner, MT entry into dendritic spines appears to
have some specificity. One interesting association of MT entry of spines is that it is activity
Miryala et al. Page 7
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
dependent. MTs appear to target dendritic spines more often in synaptically active neurons,
compared to less active neurons (Hu et al., 2008). Moreover, both BDNF application and
chemical long-term potentiation (cLTP) enhance MT invasions of dendritic spines (Hu et al.,
2011; Merriam et al., 2011; Merriam et al., 2013), while chemically induced LTD results in
fewer invasions (Kapitein et al., 2011). These studies also showed that MT invasions were
dependent on NMDA receptor activity.
How does synaptic activity regulate MT invasions of spines? It is well known that synaptic
activity, in the form of long-term potentiation (LTP), increases calcium influx in dendritic
spines through AMPA and NMDA receptors (Fortin et al., 2010; Lu et al., 2001). LTP-
induced calcium influx activates several downstream pathways resulting in increased actin
polymerization in individual spines that are potentiated (Bosch et al., 2014; Fukazawa et al.,
2003; Honkura et al., 2008). A parsimonious explanation of how actin filaments (f-actin)
could influence MT invasion of spines is that f-actin could serve as a “gate” or “ramp” in
the axon shaft and could physically deflect MTs into specific spines that are potentiated
(Figure 3). A more complex model would include both MT plus-end-associated proteins
and actin-associated proteins interacting with one another to position MTs to polymerize
into dendritic spines. The current data suggest that both processes could be functioning
at the intersection of the dendrite shaft and the neck of dendritic spines (Merriam et al.,
2013; Schatzle et al., 2018). The earlier study by Merriam and colleagues showed that
actin polymerization after chemical LTP played a central role in MT polymerization into
spines (Merriam et al., 2013). Additionally, this group showed that the actin-binding protein
drebrin, which has been shown to interact with EB3 (Geraldo et al., 2008), decreased MT
invasions when knocked-out and increased invasions when overexpressed. Consistently, the
subsequent work by Schatzle and colleagues confirmed that f-actin in the dendrite shaft
was instrumental for MT invasion (Schatzle et al., 2018). However, this group tested several
actin-associated proteins, including drebrin, and found only cortactin knockdown decreased
MT entries into spines, suggesting that cortactin, rather than drebrin, was necessary for MT
entries. Moreover, this group suggested, based on using a mutant EB3 construct without a
C-terminal acidic domain, that EB3 binding to other proteins is not required for MT entry.
Together these results suggest that f-actin is required for MT entry of dendritic spines and
activity-dependent actin remodeling in the dendrite shaft, below potentiated spines, plays an
important role in deflecting actively polymerizing MTs into dendritic spines. Furthermore,
actin-associated proteins are likely to be indirectly involved in MT entry of spines by
sculpting the f-actin network at the dendrite shaft/spine neck junction.
The studies mentioned above would suggest that regulating MT polymerization kinetics
would affect MT entries into dendritic spines. Indeed, a previous study incubated
hippocampal neurons in low concentrations of nocodazole, which does not depolymerize
MTs but decreases their dynamics (growth and shrinkage) and showed a decrease in MT
entries of spines (Jaworski et al., 2009). However, one must consider that bath application of
this compound will affect all MTs in the neuron, both presynaptically and postsynaptically,
as well as in any glial cells in the culture. Thus, it is quite a non-specific treatment. However,
a recent study used a conditional knockout (cKO) mouse to specifically affect postsynaptic
MT dynamics (Zheng et al., 2022). They discovered that conditionally depleting the
Kinesin-13 protein KIF2C/MCAK, a known MT depolymerizing protein (Ogawa et al.,
Miryala et al. Page 8
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
2004), resulted in increased MT entries into dendritic spines under basal conditions (Zheng
et al., 2022). Moreover, they showed that the presence of tyrosinated MTs in dendritic spines
increased after BDNF treatment, consistent with a previous study (Hu et al., 2011), and this
increase was dependent on KIF2C (Zheng et al., 2022). However, they also demonstrated
that MT dynamics in the dendrite shaft were affected in KIF2C cKO neurons. Thus, it is not
clear where KIF2C is acting, in the dendrite shaft, spine or both. Future work may be able to
sort out this enigma.
Transport of cargo along MTs in dendrites and spines
As mentioned above, a fundamental function of MTs is to provide a substrate to transport
cargoes long distances within cells. Thus, an additional question that arises is what might
MTs be transporting into and out of dendritic spines and is there any specificity to this
transport? MT polymerization rates are on the order of 5–15μm/min and MTs generally
remain in spines that they target for tens of seconds to several minutes (Hu et al., 2008;
Jaworski et al., 2009). Although this is a relatively short period of time, the rate of transport
of motor proteins are generally on the order of 0.3–1μm/sec (18–60μm/min). Thus, the time
MTs are present in spines would allow substantial transport of cargo into or out of spines.
However, the motor proteins that enter spines along MTs would have to favor dynamic
MTs. Markers for new/dynamic MTs include tyrosinated tubulin, whereas acetylated or
detyrosinated tubulin is a marker for old/stable MTs. Thus, motors entering spines along
MTs would likely prefer tyrosinated MTs. Generally, Kinesin-1 motor proteins prefer to
track along acetylated MTs (Cai et al., 2009; Konishi and Setou, 2009), while Kinesin-3
motors are less selective, but prefer tyrosinated MTs (Guardia et al., 2016; Lipka et al.,
2016). These data are consistent with the finding that KIF1A, a Kinesin-3 family member,
was discovered to transport synaptotagmin-IV-containing vesicles (likely DCVs) directly
into dendritic spines of cultured hippocampal neurons (McVicker et al., 2016). This group
referred to MTs delivering cargo directly into spines as the “direct-deposit” model (Figure
3).
However, there are other mechanisms for trafficking cargo in and out of dendritic spines.
Cytoplasmic material can move in and out of spines via regulated diffusion. Translocation
of cytoplasmic proteins is a complex process, both spatially and temporally, depending on
the type of protein (Bosch et al., 2014) and beyond the scope of this review. Two other
mechanisms for trafficking membrane-associated cargo and organelles include membrane
diffusion and MT motor to actomyosin hand-off (Dent, 2017). If MTs directly enter spines,
why would it be necessary for a handoff of cargo to myosin motor proteins that would
move the cargo into the spine along f-actin? One possibility is that MT entry into dendritic
spines is transient. MTs are present in spines for only a few minutes. Thus, the MTs
that enter spines are inherently dynamic, composed of tyrosinated tubulin. Any motor that
does not use tyrosinated MTs to transport material could therefore not enter spines directly
on these dynamic MTs (Figure 3). For example, cargo that is transported by Kinesin-1
(i.e., KIF5 motors) is likely to use stable, acetylated/detyrosinated MTs in dendrites, as
mentioned above (Cai et al., 2009; Konishi and Setou, 2009). Moreover, recent data indicate
many stable MTs in dendrites are located in the center of the dendrite shaft, in regions
of dendrites near the cell body (Katrukha et al., 2021; Tas et al., 2017). For both reasons,
Miryala et al. Page 9
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
cargo transported by kinesins that prefer stable MTs would require interaction with a myosin
motor to be transported into dendritic spines along actin filaments. Interestingly, several
organelles, including endoplasmic reticulum (ER) and mitochondria, which are known to use
KIF5 (Cai et al., 2007; Tanaka et al., 1998), do not enter spines directly on MTs (Figure
3). Rather, these organelles require Myosin-Va to transport into dendritic spines (McVicker
et al., 2016; Wagner et al., 2011). Similarly, recycling endosomes, which contain AMPA
receptors, require Myosin-Va/b to enter spines (Correia et al., 2008; Esteves da Silva et al.,
2015; Wang et al., 2008). Nevertheless, recent evidence indicates that KIF13A, a Kinesin-3
motor, is required for delivery of endosomally-localized AMPA receptors to synapses during
LTP (Gutierrez et al., 2021). Although this study did not specifically test if these endosomes
entered spines along MTs, they favor a Myosin-V-based handoff mechanism, based on the
previously mentioned studies.
These aforementioned studies have focused on cargo delivery into dendritic spines. What
about cargo removal from spines? Less is known about removal of material along MTs
that enter dendritic spines. Based on their polarity, cytoplasmic dynein would be required
for transport of material out of spines along MTs. Although such movement has not been
shown directly, one study demonstrated that puncta of dynein intermediate chain (DIC) and
neuroligin moved out of dendritic spines in tandem (Schapitz et al., 2010). However, MTs
were not imaged in this study, so it is unclear if a MT was present in the spine during
this event. Thus, these puncta may represent vesicles containing dynein, but they could
move along actin filaments via myosin-based transport. Indeed, AMPA receptor endocytosis
during LTD requires Myosin VI in cerebellar Purkinje neurons (Wagner et al., 2019).
A recent study also demonstrated the MT motor protein KIF5A/C and adapter protein
protrudin work together to remove endosomes containing Rab11 and AMPA receptors
during LTD (Brachet et al., 2021). It is likely the interaction of endosomes, KIF5A/C and
MTs occurs in the dendritic shaft due to the affinity of KIF5 motors for stable MTs (Cai et
al., 2009; Konishi and Setou, 2009) and the location of stable MTs in the center of dendrites
(Katrukha et al., 2021; Tas et al., 2017), as mentioned above.
However, motors can also influence MT dynamics. A recent study by the Shen group
showed that dynein heavy chain (DHC-1), has an additional function to its well-documented
involvement in cargo transport
in C.elegans
(Yogev et al., 2017). In their study, the authors
showed that in DHC-1 mutants MTs have increased dynamicity and MT looping, resulting
in cargo accumulations at the tip of the dendrite. This outcome observed in DHC-1
mutants is suggestive of an important role in stabilizing the MT structure and preventing
aberrations in cargo transport. The study also indicates that dynein motors are not just
cargo transporters, but help maintain the stability of the MTs in
C.elegans
dendrites. It is
intriguing that MTs, which serve as railroads for transport of cargo via motor proteins, are
also regulated by the same motor proteins.
Together, these studies indicate a complex and dynamic network of events governing
entry and exit of MTs, material and organelles into and out of dendritic spines (Figure
3). Furthermore, there is clearly much more work to be done to determine both how
the cytoskeleton functions in delivery of material to and removal from dendritic spines.
Miryala et al. Page 10
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
Although these mechanisms are inherently interesting and important to study, how might
they affect the role of spines in complex processes such as learning and memory?
Microtubule dynamics in learning and memory
It is becoming increasingly evident that MT dynamicity plays a critical role in typical brain
function, particularly learning and memory formation. Long term potentiation (LTP), an
electrophysiological means of assessing the formation of memories
ex vivo
(Nabavi et al.,
2014), has been shown to be affected by alterations in MT dynamics. Hyper-stabilization
of MTs with the chemotherapy drug paclitaxel results in a deficit of LTP in murine
brain slices (Shumyatsky et al., 2005). Additionally, destabilization of MTs with the MT-
depolymerizing agent nocodazole has been reported to reduce LTP within hippocampal
mossy fibers (Barnes et al., 2010) and Shaffer collaterals (Jaworski et al., 2009) in brain
slices. Therefore, it appears that disrupting the dynamic nature of MTs either by stabilization
or destabilization disturbs brain function. However, it should be noted that studies that rely
on pharmacological disruption of MTs result in both presynaptic (axonal) and postsynaptic
(dendritic) disruption of MT dynamics. Thus, it is unclear whether presynaptic MT
dynamics, postsynaptic MT dynamics and/or the subset of postsynaptic MTs that enter
dendritic spines are the cause of these deficits.
Assessment of learning and memory with behavioral testing is another means by which
the effects of MT dynamicity have been examined. MT destabilization with nocodazole is
associated with both a general reduction of dendritic spines and decreased memory, assessed
by contextual fear conditioning (Fanara et al., 2010). This study suggests that the structural
plasticity of dendritic spines that is required for learning may be controlled through MT
dynamics, specifically MT turnover. Another group detected a reduction in contextual fear
conditioning if the MT stabilizing drug paclitaxel was injected into the dentate gyrus 30
minutes after training, whereas paclitaxel administration 8 hours after training enhanced
contextual fear conditioning (Uchida et al., 2014). Moreover, injection of nocodazole 8
hours after training disrupted fear conditioning. These results led these researchers to posit
that there is an early phase of hyper-dynamic MTs and a later phase of hyper-stable MTs
after contextual fear conditioning. The idea that MT stabilization may contribute to learning
was also reported in a study where the MT-stabilizing drug Epothilone D rescued spatial
learning in an Alzheimer’s mouse model, as assessed by Y maze (Zhang et al., 2012). This
result is consistent with the belief that Alzheimer’s results from the destabilization of MTs.
Additionally, it is important to note that these general pharmacological treatments may be
affecting MT-dependent transport in addition to MT dynamicity. Therefore, the disruption
of LTP, memory formation and/or learning may be from altered MT dynamicity, disrupted
MT-dependent transport, or the compounded nature of altering both processes.
When discussing the role of MT dynamics in learning and memory formation, it is important
to consider the effects of MT-associated proteins (MAPs) on MTs. The MAP stathmin alters
MT dynamicity through several mechanisms. Dephosphorylated or hypo-phosphorylated
stathmin can weakly bind to tubulin heterodimers, sequestering them and preventing MT
assembly (Di Paolo et al., 1997; Gavet et al., 2002). However, triple phosphorylation of
stathmin at three key serine residues will inhibit its binding affinity for tubulin, allowing
Miryala et al. Page 11
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
for MT assembly to commence (Di Paolo et al., 1997; Gavet et al., 2002). Additionally,
dephosphorylated/hypo-phosphorylated stathmin appears to promote MT catastrophe by
direct binding to protofilaments at the minus ends of dynamic MTs (Gupta et al., 2013).
In addition to altering MT dynamics, stathmin has been identified as a contributor to
neurogenesis, spinogenesis, and memory formation within the dentate gyrus of adult mice
(Martel et al., 2016). Thus, stathmin may be a critical component for regulating MT
dynamics in typical brain function.
Considering the regulatory function of stathmin on MT dynamicity, it is not surprising that
several studies suggest synaptic plasticity and learning may be associated with stathmin.
Knockout animals devoid of stathmin display deficits in LTP when assessed by cued and
contextual fear conditioning, but not spatial learning tasks, such as the Morris water maze
(Shumyatsky et al., 2005). This is likely a reflection of the expression profile of stathmin
within the adult murine brain, where it is highly expressed within the lateral nucleus (LA)
of the amygdala, as opposed to the hippocampus. Another report aligned with this proposed
expression, in which stathmin knockout animals presented with enhanced extinction of cued
fear conditioned memories, which was associated with decreased amygdala activity (Martel
et al., 2012). Additionally, transgenic animals expressing an unphosphorylatable form of
stathmin display a reduction in contextual fear memory, suggesting that phosphorylation of
stathmin, which regulates MT dynamicity, may be the underlying mechanism driving its role
in memory formation (Uchida et al., 2014).
Stathmin appears to be bi-phasically regulated by learning. This regulation appears to be
driven by changes in phosphorylation and transient binding affinity for tubulin (Uchida et
al., 2014; Uchida and Shumyatsky, 2015). In the early stage of fear conditioning, 15–60
minutes after behavioral training, stathmin is de-phosphorylated at its three key serine
residues: Ser16, Ser25, and Ser38. This de-phosphorylation results in dephosphorylated
or hypo-phosphorylated stathmin with increased MT binding affinity and MT-destabilizing
activity. Throughout the second, late phase of fear conditioning, approximately 8 hours after
training, there is a resurgence of phosphorylation at the Ser16 and Ser38 residues, which
decreases stathmin’s MT-destabilizing activity and results in hyper-stabilized MTs. Local
injection of paclitaxel during the early phase disrupts fear memory, however injection during
the late phase enhances fear memory. These results confirm that it is the stathmin-mediated
alterations to MT stability, rather than other effects of stathmin, that contributes to its
function in this learning paradigm.
As discussed above, MTs play a large role in both presynaptic and postsynaptic trafficking
of cargo within mature neurons. Therefore, understandably, stathmin-dependent regulation
of MT dynamicity may have downstream effects on other processes like intracellular
transport. The late phase of stathmin activity, associated with hyper-stabilized MTs, results
in an increase of GluA2 AMPA-type glutamate receptor subunits at the synapse (Uchida et
al., 2014). The hyper-stabilized MTs may be enhancing dendritic transport as a means of
promoting memory formation, as it is well understood that AMPA-type glutamate receptors
of critical for both contextual memory (Mitsushima et al., 2011) and synaptic plasticity
(Correia et al., 2008). Interestingly, it has recently been suggested that the molecular
motor KIF5C contributes to both spatial memory and contextual fear memory through
Miryala et al. Page 12
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
transportation of RNA protein complexes near dendritic spines to allow for local translation
(Swarnkar et al., 2021). The motor protein KIF2C also contributes to synaptic plasticity
with conditional knockout animals displaying impaired working memory and memory
maintenance, as well as conditioned fear learning, but through a different mechanism
(Zheng et al., 2022). The MT depolymerizing properties of KIF2C are critical for its role in
synaptic plasticity, suggesting that it may employ a mechanism similar to stathmin following
learning. However, follow up studies are still necessarily to elucidate if KIF2C is affecting
MT-dependent transport or affecting synaptic plasticity in another manner.
Conclusions and Future Directions
The studies described above clearly indicate that MTs play a heretofore underappreciated
role in both pre- and postsynaptic structure and function. MTs have long been known to
provide ‘railways’ for transport of cargos throughout dendritic and axonal arbors. However,
until relatively recently MTs have not been implicated in synaptic function, either in the
presynaptic bouton or the postsynaptic dendritic spine. The studies described here tell
a different story. MTs appear to play seminal roles in neuronal synaptic function and
plasticity. MT dynamics and MT-based transport of material must be coordinated for proper
communication between neurons and disruption of these processes results in compromised
learning and memory. However, there have been relatively few studies that have specifically
focused on how MT dynamics and motor-based transport are coordinated. Several studies
mentioned in this review describe how motor proteins prefer either more stable or more
dynamic MTs, but relatively little is known about how changing MT dynamics, either at the
plus or minus end, might specifically affect transport of cargoes in the neuron. Future work
will undoubtedly shed new light on how these two processes are intertwined.
Although we did not discuss developmental and neurodegenerative diseases, such as
Alzheimer’s Disease, research is beginning to show disruption of MT dynamics and
transport in the underlying etiology of such conditions. Further understanding of the cell
biology underlying the role MTs and their associated proteins play at the synapse will
undoubtedly provide important foundational information that can be applied to the treatment
of such devastating diseases.
Grant Support
This work was supported by NIH grant R01-NS098372 and R01-NS115400 to ewd and NIH/NINDS T32
NS105602 to edh.
REFERENCES
Akhmanova A, Kapitein LC, 2022. Mechanisms of microtubule organization in differentiated animal
cells. Nat Rev Mol Cell Biol 23, 541–558. [PubMed: 35383336]
Akhmanova A, Steinmetz MO, 2008. Tracking the ends: a dynamic protein network controls the fate
of microtubule tips. Nat Rev Mol Cell Biol 9, 309–322. [PubMed: 18322465]
Akhmanova A, Steinmetz MO, 2015. Control of microtubule organization and dynamics: two ends in
the limelight. Nat Rev Mol Cell Biol 16, 711–726. [PubMed: 26562752]
Akhmanova A, Steinmetz MO, 2019. Microtubule minus-end regulation at a glance. Journal of cell
science 132.
Miryala et al. Page 13
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
Baas PW, Black MM, 1990. Individual microtubules in the axon consist of domains that differ in both
composition and stability. The Journal of cell biology 111, 495–509. [PubMed: 2199458]
Baas PW, Black MM, Banker GA, 1989. Changes in microtubule polarity orientation during the
development of hippocampal neurons in culture. The Journal of cell biology 109, 3085–3094.
[PubMed: 2592416]
Baas PW, Deitch JS, Black MM, Banker GA, 1988. Polarity orientation of microtubules in
hippocampal neurons: uniformity in the axon and nonuniformity in the dendrite. Proceedings of
the National Academy of Sciences of the United States of America 85, 8335–8339. [PubMed:
3054884]
Baas PW, Slaughter T, Brown A, Black MM, 1991. Microtubule dynamics in axons and dendrites.
Journal of neuroscience research 30, 134–153. [PubMed: 1795398]
Balseiro-Gomez S, Park J, Yue Y, Ding C, Shao L, etinkaya S, Kuzoian C, Hammarlund M, Verhey
KJ, Yogev S, 2022. Neurexin and frizzled intercept axonal transport at microtubule minus ends to
control synapse formation. Dev Cell 57, 1802–1816 e1804. [PubMed: 35809561]
Barnes SJ, Opitz T, Merkens M, Kelly T, von der Brelie C, Krueppel R, Beck H, 2010. Stable mossy
fiber long-term potentiation requires calcium influx at the granule cell soma, protein synthesis, and
microtubule-dependent axonal transport. J Neurosci 30, 12996–13004. [PubMed: 20881117]
Bharat V, Siebrecht M, Burk K, Ahmed S, Reissner C, Kohansal-Nodehi M, Steubler V, Zweckstetter
M, Ting JT, Dean C, 2017. Capture of Dense Core Vesicles at Synapses by JNK-Dependent
Phosphorylation of Synaptotagmin-4. Cell reports 21, 2118–2133. [PubMed: 29166604]
Bird MM, 1976. Microtubule--synaptic vesicle associations in cultured rat spinal cord neurons. Cell
and tissue research 168, 101–115. [PubMed: 1268927]
Bosch M, Castro J, Saneyoshi T, Matsuno H, Sur M, Hayashi Y, 2014. Structural and molecular
remodeling of dendritic spine substructures during long-term potentiation. Neuron 82, 444–459.
[PubMed: 24742465]
Brachet A, Lario A, Fernandez-Rodrigo A, Heisler FF, Gutierrez Y, Lobo C, Kneussel M, Esteban JA,
2021. A kinesin 1-protrudin complex mediates AMPA receptor synaptic removal during long-term
depression. Cell reports 36, 109499. [PubMed: 34348158]
Brown A, Li Y, Slaughter T, Black MM, 1993. Composite microtubules of the axon: quantitative
analysis of tyrosinated and acetylated tubulin along individual axonal microtubules. Journal of cell
science 104 (Pt 2), 339–352. [PubMed: 8505364]
Bucher M, Fanutza T, Mikhaylova M, 2020. Cytoskeletal makeup of the synapse: Shaft versus spine.
Cytoskeleton (Hoboken) 77, 55–64. [PubMed: 31762205]
Cai D, McEwen DP, Martens JR, Meyhofer E, Verhey KJ, 2009. Single molecule imaging reveals
differences in microtubule track selection between Kinesin motors. PLoS Biol 7, e1000216.
[PubMed: 19823565]
Cai Q, Pan PY, Sheng ZH, 2007. Syntabulin-kinesin-1 family member 5B-mediated axonal transport
contributes to activity-dependent presynaptic assembly. The Journal of neuroscience 27, 7284–
7296. [PubMed: 17611281]
Cason SE, Holzbaur ELF, 2022. Selective motor activation in organelle transport along axons. Nat Rev
Mol Cell Biol.
Chenouard N, Xuan F, Tsien RW, 2020. Synaptic vesicle traffic is supported by transient actin
filaments and regulated by PKA and NO. Nature communications 11, 5318.
Correia SS, Bassani S, Brown TC, Lise MF, Backos DS, El-Husseini A, Passafaro M, Esteban
JA, 2008. Motor protein-dependent transport of AMPA receptors into spines during long-term
potentiation. Nature neuroscience 11, 457–466. [PubMed: 18311135]
Cunha-Ferreira I, Chazeau A, Buijs RR, Stucchi R, Will L, Pan X, Adolfs Y, van der Meer C,
Wolthuis JC, Kahn OI, Schatzle P, Altelaar M, Pasterkamp RJ, Kapitein LC, Hoogenraad CC,
2018. The HAUS Complex Is a Key Regulator of Non-centrosomal Microtubule Organization
during Neuronal Development. Cell reports 24, 791–800. [PubMed: 30044976]
Del Castillo U, Norkett R, Gelfand VI, 2019. Unconventional Roles of Cytoskeletal Mitotic Machinery
in Neurodevelopment. Trends in cell biology 29, 901–911. [PubMed: 31597609]
Dent EW, 2017. Of microtubules and memory: implications for microtubule dynamics in dendrites and
spines. Mol Biol Cell 28, 1–8. [PubMed: 28035040]
Miryala et al. Page 14
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
Dent EW, 2020. Dynamic microtubules at the synapse. Current opinion in neurobiology 63, 9–14.
[PubMed: 32062144]
Dent EW, Callaway JL, Szebenyi G, Baas PW, Kalil K, 1999. Reorganization and movement of
microtubules in axonal growth cones and developing interstitial branches. J Neurosci 19, 8894–
8908. [PubMed: 10516309]
Dent EW, Gupton SL, Gertler FB, 2011. The growth cone cytoskeleton in axon outgrowth and
guidance. Cold Spring Harb Perspect Biol 3.
Dent EW, Kalil K, 2001. Axon branching requires interactions between dynamic microtubules and
actin filaments. Journal of Neuroscience 21, 9757–9769. [PubMed: 11739584]
Di Paolo G, Lutjens R, Osen-Sand A, Sobel A, Catsicas S, Grenningloh G, 1997. Differential
distribution of stathmin and SCG10 in developing neurons in culture. Journal of neuroscience
research 50, 1000–1009. [PubMed: 9452014]
Dixit R, Barnett B, Lazarus JE, Tokito M, Goldman YE, Holzbaur EL, 2009. Microtubule plus-end
tracking by CLIP-170 requires EB1. Proceedings of the National Academy of Sciences of the
United States of America 106, 492–497. [PubMed: 19126680]
Dragestein KA, van Cappellen WA, van Haren J, Tsibidis GD, Akhmanova A, Knoch TA, Grosveld F,
Galjart N, 2008. Dynamic behavior of GFP-CLIP-170 reveals fast protein turnover on microtubule
plus ends. The Journal of cell biology 180, 729–737. [PubMed: 18283108]
Esteves da Silva M, Adrian M, Schatzle P, Lipka J, Watanabe T, Cho S, Futai K, Wierenga CJ,
Kapitein LC, Hoogenraad CC, 2015. Positioning of AMPA Receptor-Containing Endosomes
Regulates Synapse Architecture. Cell reports 13, 933–943. [PubMed: 26565907]
Fanara P, Husted KH, Selle K, Wong PY, Banerjee J, Brandt R, Hellerstein MK, 2010. Changes
in microtubule turnover accompany synaptic plasticity and memory formation in response to
contextual fear conditioning in mice. Neuroscience 168, 167–178. [PubMed: 20332016]
Fortin DA, Davare MA, Srivastava T, Brady JD, Nygaard S, Derkach VA, Soderling TR, 2010.
Long-term potentiation-dependent spine enlargement requires synaptic Ca2+-permeable AMPA
receptors recruited by CaM-kinase I. J Neurosci 30, 11565–11575. [PubMed: 20810878]
Fukazawa Y, Saitoh Y, Ozawa F, Ohta Y, Mizuno K, Inokuchi K, 2003. Hippocampal LTP is
accompanied by enhanced F-actin content within the dendritic spine that is essential for late LTP
maintenance in vivo. Neuron 38, 447–460. [PubMed: 12741991]
Gavet O, El Messari S, Ozon S, Sobel A, 2002. Regulation and subcellular localization of
the microtubule-destabilizing stathmin family phosphoproteins in cortical neurons. Journal of
neuroscience research 68, 535–550. [PubMed: 12111843]
Geraldo S, Khanzada UK, Parsons M, Chilton JK, Gordon-Weeks PR, 2008. Targeting of the F-actin-
binding protein drebrin by the microtubule plus-tip protein EB3 is required for neuritogenesis.
Nature cell biology 10, 1181–1189. [PubMed: 18806788]
Goodwin PR, Sasaki JM, Juo P, 2012. Cyclin-dependent kinase 5 regulates the polarized trafficking of
neuropeptide-containing dense-core vesicles in Caenorhabditis elegans motor neurons. J Neurosci
32, 8158–8172. [PubMed: 22699897]
Gordon-Weeks PR, Burgoyne RD, Gray EG, 1982. Presynaptic microtubules: organisation and
assembly/disassembly. Neuroscience 7, 739–749. [PubMed: 7041009]
Gramlich MW, Klyachko VA, 2017. Actin/Myosin-V- and Activity-Dependent Inter-synaptic Vesicle
Exchange in Central Neurons. Cell reports 18, 2096–2104. [PubMed: 28249156]
Gray EG, 1975. Presynaptic microtubules and their association with synaptic vesicles. Proc R Soc
Lond B Biol Sci 190, 367–372. [PubMed: 240166]
Gray EG, Westrum LE, Burgoyne RD, Barron J, 1982. Synaptic organisation and neuron microtubule
distribution. Cell and tissue research 226, 579–588. [PubMed: 7139692]
Gu J, Firestein BL, Zheng JQ, 2008. Microtubules in dendritic spine development. The Journal of
neuroscience 28, 12120–12124. [PubMed: 19005076]
Guardia CM, Farias GG, Jia R, Pu J, Bonifacino JS, 2016. BORC Functions Upstream of Kinesins 1
and 3 to Coordinate Regional Movement of Lysosomes along Different Microtubule Tracks. Cell
reports 17, 1950–1961. [PubMed: 27851960]
Miryala et al. Page 15
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
Guedes-Dias P, Nirschl JJ, Abreu N, Tokito MK, Janke C, Magiera MM, Holzbaur ELF, 2019.
Kinesin-3 Responds to Local Microtubule Dynamics to Target Synaptic Cargo Delivery to the
Presynapse. Curr Biol 29, 268–282 e268. [PubMed: 30612907]
Gupta KK, Li C, Duan A, Alberico EO, Kim OV, Alber MS, Goodson HV, 2013. Mechanism for
the catastrophe-promoting activity of the microtubule destabilizer Op18/stathmin. Proceedings of
the National Academy of Sciences of the United States of America 110, 20449–20454. [PubMed:
24284166]
Gutierrez Y, Lopez-Garcia S, Lario A, Gutierrez-Eisman S, Delevoye C, Esteban JA, 2021. KIF13A
drives AMPA receptor synaptic delivery for long-term potentiation via endosomal remodeling. The
Journal of cell biology 220.
Harris KM, Hubbard DD, Kuwajima M, Abraham WC, Bourne JN, Bowden JB, Haessly A,
Mendenhall JM, Parker PH, Shi B, Spacek J, 2022. Dendritic Spine Density Scales with
Microtubule Number in Rat Hippocampal Dendrites. Neuroscience 489, 84–97. [PubMed:
35218884]
Honkura N, Matsuzaki M, Noguchi J, Ellis-Davies GC, Kasai H, 2008. The subspine organization
of actin fibers regulates the structure and plasticity of dendritic spines. Neuron 57, 719–729.
[PubMed: 18341992]
Horton AC, Ehlers MD, 2003. Dual modes of endoplasmic reticulum-to-Golgi transport in dendrites
revealed by live-cell imaging. J Neurosci 23, 6188–6199. [PubMed: 12867502]
Horton AC, Racz B, Monson EE, Lin AL, Weinberg RJ, Ehlers MD, 2005. Polarized secretory
trafficking directs cargo for asymmetric dendrite growth and morphogenesis. Neuron 48, 757–771.
[PubMed: 16337914]
Hotta T, McAlear TS, Yue Y, Higaki T, Haynes SE, Nesvizhskii AI, Sept D, Verhey KJ, Bechstedt
S, Ohi R, 2022. EML2-S constitutes a new class of proteins that recognizes and regulates the
dynamics of tyrosinated microtubules. Curr Biol.
Hu X, Ballo L, Pietila L, Viesselmann C, Ballweg J, Lumbard D, Stevenson M, Merriam E, Dent
EW, 2011. BDNF-induced increase of PSD-95 in dendritic spines requires dynamic microtubule
invasions. The Journal of neuroscience 31, 15597–15603. [PubMed: 22031905]
Hu X, Viesselmann C, Nam S, Merriam E, Dent EW, 2008. Activity-dependent dynamic microtubule
invasion of dendritic spines. The Journal of neuroscience 28, 13094–13105. [PubMed: 19052200]
Jaworski J, Kapitein LC, Gouveia SM, Dortland BR, Wulf PS, Grigoriev I, Camera P, Spangler SA,
Di Stefano P, Demmers J, Krugers H, Defilippi P, Akhmanova A, Hoogenraad CC, 2009. Dynamic
microtubules regulate dendritic spine morphology and synaptic plasticity. Neuron 61, 85–100.
[PubMed: 19146815]
Kapitein LC, Yau KW, Gouveia SM, van der Zwan WA, Wulf PS, Keijzer N, Demmers J, Jaworski J,
Akhmanova A, Hoogenraad CC, 2011. NMDA receptor activation suppresses microtubule growth
and spine entry. J Neurosci 31, 8194–8209. [PubMed: 21632941]
Katrukha EA, Jurriens D, Salas Pastene DM, Kapitein LC, 2021. Quantitative mapping of dense
microtubule arrays in mammalian neurons. Elife 10.
Kimura T, Watanabe H, Iwamatsu A, Kaibuchi K, 2005. Tubulin and CRMP-2 complex is transported
via Kinesin-1. J Neurochem 93, 1371–1382. [PubMed: 15935053]
Konishi Y, Setou M, 2009. Tubulin tyrosination navigates the kinesin-1 motor domain to axons. Nature
neuroscience 12, 559–567. [PubMed: 19377471]
Krueger-Burg D, Papadopoulos T, Brose N, 2017. Organizers of inhibitory synapses come of age.
Current opinion in neurobiology 45, 66–77. [PubMed: 28460365]
Kuo YW, Howard J, 2021. Cutting, Amplifying, and Aligning Microtubules with Severing Enzymes.
Trends in cell biology 31, 50–61. [PubMed: 33183955]
Li Y, Black MM, 1996. Microtubule assembly and turnover in growing axons. J Neurosci 16, 531–544.
[PubMed: 8551337]
Liang X, Kokes M, Fetter RD, Sallee MD, Moore AW, Feldman JL, Shen K, 2020. Growth cone-
localized microtubule organizing center establishes microtubule orientation in dendrites. Elife 9.
Lipka J, Kapitein LC, Jaworski J, Hoogenraad CC, 2016. Microtubule-binding protein doublecortin-
like kinase 1 (DCLK1) guides kinesin-3-mediated cargo transport to dendrites. The EMBO journal
35, 302–318. [PubMed: 26758546]
Miryala et al. Page 16
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
Lu W, Man H, Ju W, Trimble WS, MacDonald JF, Wang YT, 2001. Activation of synaptic NMDA
receptors induces membrane insertion of new AMPA receptors and LTP in cultured hippocampal
neurons. Neuron 29, 243–254. [PubMed: 11182095]
Martel G, Hevi C, Wong A, Zushida K, Uchida S, Shumyatsky GP, 2012. Murine GRPR and stathmin
control in opposite directions both cued fear extinction and neural activities of the amygdala and
prefrontal cortex. PLoS ONE 7, e30942. [PubMed: 22312434]
Martel G, Uchida S, Hevi C, Chevere-Torres I, Fuentes I, Park YJ, Hafeez H, Yamagata H, Watanabe
Y, Shumyatsky GP, 2016. Genetic Demonstration of a Role for Stathmin in Adult Hippocampal
Neurogenesis, Spinogenesis, and NMDA Receptor-Dependent Memory. J Neurosci 36, 1185–
1202. [PubMed: 26818507]
McVicker DP, Awe AM, Richters KE, Wilson RL, Cowdrey DA, Hu X, Chapman ER, Dent EW, 2016.
Transport of a kinesin-cargo pair along microtubules into dendritic spines undergoing synaptic
plasticity. Nature communications 7, 12741.
Merriam EB, Lumbard DC, Viesselmann C, Ballweg J, Stevenson M, Pietila L, Hu X, Dent EW, 2011.
Dynamic microtubules promote synaptic NMDA receptor-dependent spine enlargement. PLoS
ONE 6, e27688. [PubMed: 22096612]
Merriam EB, Millette M, Lumbard DC, Saengsawang W, Fothergill T, Hu X, Ferhat L, Dent EW,
2013. Synaptic regulation of microtubule dynamics in dendritic spines by calcium, f-actin, and
drebrin. The Journal of neuroscience 33, 16471–16482. [PubMed: 24133252]
Mitchison T, Kirschner M, 1984. Dynamic instability of microtubule growth. Nature 312, 237–242.
[PubMed: 6504138]
Mitsushima D, Ishihara K, Sano A, Kessels HW, Takahashi T, 2011. Contextual learning requires
synaptic AMPA receptor delivery in the hippocampus. Proceedings of the National Academy of
Sciences of the United States of America 108, 12503–12508. [PubMed: 21746893]
Mitsuyama F, Niimi G, Kato K, Hirosawa K, Mikoshiba K, Okuya M, Karagiozov K, Kato Y, Kanno
T, Sanoe H, Koide T, 2008. Redistribution of microtubules in dendrites of hippocampal CA1
neurons after tetanic stimulation during long-term potentiation. Italian journal of anatomy and
embryology = Archivio italiano di anatomia ed embriologia 113, 17–27. [PubMed: 18491451]
Nabavi S, Fox R, Proulx CD, Lin JY, Tsien RY, Malinow R, 2014. Engineering a memory with LTD
and LTP. Nature 511, 348–352. [PubMed: 24896183]
Ogawa T, Nitta R, Okada Y, Hirokawa N, 2004. A common mechanism for microtubule destabilizers-
M type kinesins stabilize curling of the protofilament using the class-specific neck and loops. Cell
116, 591–602. [PubMed: 14980225]
Ori-McKenney KM, Jan LY, Jan YN, 2012. Golgi outposts shape dendrite morphology by functioning
as sites of acentrosomal microtubule nucleation in neurons. Neuron 76, 921–930. [PubMed:
23217741]
Pollard TD, 2007. Regulation of actin filament assembly by Arp2/3 complex and formins. Annu Rev
Biophys Biomol Struct 36, 451–477. [PubMed: 17477841]
Qu X, Kumar A, Blockus H, Waites C, Bartolini F, 2019. Activity-Dependent Nucleation of Dynamic
Microtubules at Presynaptic Boutons Controls Neurotransmission. Curr Biol.
Rao AN, Baas PW, 2018. Polarity Sorting of Microtubules in the Axon. Trends in neurosciences 41,
77–88. [PubMed: 29198454]
Rochlin MW, Wickline KM, Bridgman PC, 1996. Microtubule stability decreases axon elongation but
not axoplasm production. J Neurosci 16, 3236–3246. [PubMed: 8627361]
Roll-Mecak A, 2020. The Tubulin Code in Microtubule Dynamics and Information Encoding. Dev
Cell 54, 7–20. [PubMed: 32634400]
Sanchez-Huertas C, Freixo F, Viais R, Lacasa C, Soriano E, Luders J, 2016. Non-centrosomal
nucleation mediated by augmin organizes microtubules in post-mitotic neurons and controls
axonal microtubule polarity. Nature communications 7, 12187.
Schapitz IU, Behrend B, Pechmann Y, Lappe-Siefke C, Kneussel SJ, Wallace KE, Stempel AV, Buck F,
Grant SG, Schweizer M, Schmitz D, Schwarz JR, Holzbaur EL, Kneussel M, 2010. Neuroligin 1 is
dynamically exchanged at postsynaptic sites. J Neurosci 30, 12733–12744. [PubMed: 20861378]
Miryala et al. Page 17
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
Schatzle P, Esteves da Silva M, Tas RP, Katrukha EA, Hu HY, Wierenga CJ, Kapitein LC, Hoogenraad
CC, 2018. Activity-Dependent Actin Remodeling at the Base of Dendritic Spines Promotes
Microtubule Entry. Curr Biol 28, 2081–2093 e2086. [PubMed: 29910073]
Sheng M, Hoogenraad CC, 2007. The postsynaptic architecture of excitatory synapses: a more
quantitative view. Annual review of biochemistry 76, 823–847.
Shumyatsky GP, Malleret G, Shin RM, Takizawa S, Tully K, Tsvetkov E, Zakharenko SS, Joseph J,
Vronskaya S, Yin D, Schubart UK, Kandel ER, Bolshakov VY, 2005. stathmin, a gene enriched in
the amygdala, controls both learned and innate fear. Cell 123, 697–709. [PubMed: 16286011]
Stepanova T, Slemmer J, Hoogenraad CC, Lansbergen G, Dortland B, De Zeeuw CI, Grosveld F, van
Cappellen G, Akhmanova A, Galjart N, 2003. Visualization of microtubule growth in cultured
neurons via the use of EB3-GFP (end-binding protein 3-green fluorescent protein). J Neurosci 23,
2655–2664. [PubMed: 12684451]
Stone MC, Roegiers F, Rolls MM, 2008. Microtubules have opposite orientation in axons and
dendrites of Drosophila neurons. Mol Biol Cell 19, 4122–4129. [PubMed: 18667536]
Swarnkar S, Avchalumov Y, Espadas I, Grinman E, Liu XA, Raveendra BL, Zucca A, Mediouni S,
Sadhu A, Valente S, Page D, Miller K, Puthanveettil SV, 2021. Molecular motor protein KIF5C
mediates structural plasticity and long-term memory by constraining local translation. Cell reports
36, 109369. [PubMed: 34260917]
Tanaka Y, Kanai Y, Okada Y, Nonaka S, Takeda S, Harada A, Hirokawa N, 1998. Targeted disruption
of mouse conventional kinesin heavy chain, kif5B, results in abnormal perinuclear clustering of
mitochondria. Cell 93, 1147–1158. [PubMed: 9657148]
Tas RP, Chazeau A, Cloin BMC, Lambers MLA, Hoogenraad CC, Kapitein LC, 2017. Differentiation
between Oppositely Oriented Microtubules Controls Polarized Neuronal Transport. Neuron 96,
1264–1271 e1265. [PubMed: 29198755]
Uchida S, Martel G, Pavlowsky A, Takizawa S, Hevi C, Watanabe Y, Kandel ER, Alarcon JM,
Shumyatsky GP, 2014. Learning-induced and stathmin-dependent changes in microtubule stability
are critical for memory and disrupted in ageing. Nature communications 5, 4389.
Uchida S, Shumyatsky GP, 2015. Deceivingly dynamic: Learning-dependent changes in stathmin and
microtubules. Neurobiology of learning and memory 124, 52–61. [PubMed: 26211874]
Wagner W, Brenowitz SD, Hammer JA 3rd, 2011. Myosin-Va transports the endoplasmic reticulum
into the dendritic spines of Purkinje neurons. Nature cell biology 13, 40–48. [PubMed: 21151132]
Wagner W, Lippmann K, Heisler FF, Gromova KV, Lombino FL, Roesler MK, Pechmann Y, Hornig
S, Schweizer M, Polo S, Schwarz JR, Eilers J, Kneussel M, 2019. Myosin VI Drives Clathrin-
Mediated AMPA Receptor Endocytosis to Facilitate Cerebellar Long-Term Depression. Cell
reports 28, 11–20 e19. [PubMed: 31269433]
Waites C, Qu X, Bartolini F, 2021. The synaptic life of microtubules. Current opinion in neurobiology
69, 113–123. [PubMed: 33873059]
Wang J, Yu W, Baas PW, Black MM, 1996. Microtubule assembly in growing dendrites. J Neurosci 16,
6065–6078. [PubMed: 8815889]
Wang Z, Edwards JG, Riley N, Provance DW Jr., Karcher R, Li XD, Davison IG, Ikebe M, Mercer JA,
Kauer JA, Ehlers MD, 2008. Myosin Vb mobilizes recycling endosomes and AMPA receptors for
postsynaptic plasticity. Cell 135, 535–548. [PubMed: 18984164]
Westrum LE, Gray EG, Burgoyne RD, Barron J, 1983. Synaptic development and microtubule
organization. Cell and tissue research 231, 93–102. [PubMed: 6850800]
Westrum LE, Jones DH, Gray EG, Barron J, 1980. Microtubules, dendritic spines and spine
appratuses. Cell and tissue research 208, 171–181. [PubMed: 6996822]
Yau KW, Schatzle P, Tortosa E, Pages S, Holtmaat A, Kapitein LC, Hoogenraad CC, 2016. Dendrites
In Vitro and In Vivo Contain Microtubules of Opposite Polarity and Axon Formation Correlates
with Uniform Plus-End-Out Microtubule Orientation. J Neurosci 36, 1071–1085. [PubMed:
26818498]
Yogev S, Maeder CI, Cooper R, Horowitz M, Hendricks AG, Shen K, 2017. Local inhibition
of microtubule dynamics by dynein is required for neuronal cargo distribution. Nature
communications 8, 15063.
Miryala et al. Page 18
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
Zhang B, Carroll J, Trojanowski JQ, Yao Y, Iba M, Potuzak JS, Hogan AM, Xie SX, Ballatore C,
Smith AB 3rd, Lee VM, Brunden KR, 2012. The microtubule-stabilizing agent, epothilone D,
reduces axonal dysfunction, neurotoxicity, cognitive deficits, and Alzheimer-like pathology in
an interventional study with aged tau transgenic mice. J Neurosci 32, 3601–3611. [PubMed:
22423084]
Zheng R, Du Y, Wang X, Liao T, Zhang Z, Wang N, Li X, Shen Y, Shi L, Luo J, Xia J, Wang Z, Xu
J, 2022. KIF2C regulates synaptic plasticity and cognition in mice through dynamic microtubule
depolymerization. Elife 11.
Miryala et al. Page 19
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
Highlights
•Microtubule dynamics and transport play important roles at the synapse
•Microtubule dynamics play a role in learning and memory
•Microtubules are associated with synaptic and dense core vesicles
presynaptically
•Microtubules polymerize into and transport material into dendritic spines
Miryala et al. Page 20
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
Figure 1. Orientation and composition of microtubules in dendrites and axons.
A. Image of a pyramidal neuron (orange), showing apical and basal dendrites, which are
studded with spines. The axon is shown below the cell body (orange blending into green)
with axonal boutons, which form
en passant
synapses with other neurons. B. A magnified
region of the dendrite showing the distribution and composition of MTs. In the dendrite,
the bulk of MTs form two populations. One population is the dynamic, plus-end-out MTs
(yellow with a small purple stable region at the minus end) that are concentrated close to the
plasma membrane. A second population are the stable, minus-end-out MTs (purple, with a
small yellow dynamic region at the plus end) that are positioned centrally in the dendrite. In
the box below, the dendrite (orange) is shown as a slice with the two populations of MTs
shown “end on” as purple (stable) and yellow (dynamic) dots However, it is likely that there
Miryala et al. Page 21
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
is a smaller population of MTs that maintain the opposite polarity (dynamic plus-end-in
and stable plus-end-out). C. A magnified region of the axon and expanded synaptic bouton.
In the axon MTs contain both an acetylated/detyrosinated stable region and a tyrosinated
dynamic region. At present it is unclear if there are distinct populations, like those in the
dendrite. Note that at the axonal bouton there is a concentration of both plus and minus ends.
Miryala et al. Page 22
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
Figure 2. Microtubules assist in concentrating synaptic vesicle precursors and dense core vesicles
at synaptic sites.
An axon and synaptic bouton are shown above, with the outline of a dendritic spine shown
below. There are three different mechanisms for concentrating vesicles at the synapse. A.
Dense core vesicles (DCVs) are transported anterogradely along MTs by the motor protein
KIF1A. When a DCV nears a synapse JNK phosphorylates synaptotagmin-IV (Syt-4) cargo,
which releases the DCV from the KIF1A motor. B. Synaptic vesicle precursors (SVPs)
are transported anterogradely along MTs by KIF1A, which has a decreased affinity for
the GTP-tubulin cap at the plus ends of MTs. The decreased affinity releases it from the
MT and it is captured at the actin-rich synapse. These GTP-tubulin-rich MT plus ends are
concentrated in the bouton. C. Dynein transports SVPs retrogradely toward the minus end
of MTs, which are also concentrated at the synapse. When dynein reaches the minus end,
it pauses due to the presence of the immobile kinesin VAB-8/KIF26 and releases the SVP
at the synapse. Thus, both anterograde and retrograde transport of SVPs can contribute to
accumulation of SVPs at synapses. P – proximal axon, D – distal axon.
Miryala et al. Page 23
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
Figure 3. Two potential mechanisms for MT-based entry and exit of material in dendritic spines.
A. The dynamic MT direct deposit model posits that the dynamic, peripherally localized
MTs in dendrites are the primary source of polymerizing MTs into dendritic spines. Thus,
motor proteins that prefer movement along dynamic MTs, such as KIF1A, use these
dynamic MTs to transport cargo into dendritic spines. An example of such cargo are vesicles
containing Syt-4 protein (likely DCVs). Dynein could then use the transiently present MTs
to transport material out of spines along these MTs. B. The stable MT/kinesin to actin/
myosin hand-off model suggests that material that is transported along the stable MTs in the
central region of the dendrite by motors such as KIF5 cannot directly enter dendritic spines,
since stable MTs do not enter dendritic spines. Rather, such cargo would need to engage
myosin motor proteins, such as Myosin 5, which translocates cargo, such as the endoplasmic
reticulum (ER) shown here into the spine (mitochondria also use this mechanism). In the
example shown, KIF5 is transporting the ER on a stable plus-end-out MT, even though most
of the stable, centrally located MTs are minus-end-out. With the bulk of the stable MTs
being minus-end-out, dynein could hand-off material to myosin from distal regions of the
dendrite.
Miryala et al. Page 24
Mol Cell Neurosci
. Author manuscript; available in PMC 2023 January 13.
Author Manuscript Author Manuscript Author Manuscript Author Manuscript
