Synapse Remodeling, Compliments of the Complement System

Article (PDF Available)inCell 131(6):1034-6 · January 2008with14 Reads
DOI: 10.1016/j.cell.2007.11.031 · Source: PubMed
A growing body of evidence indicates that some proteins known for their immune functions also have distinct nonimmune functions in the normal uninjured central nervous system. In this issue, Stevens et al. (2007) demonstrate an unexpected requirement for molecules of the complement cascade in the remodeling of synaptic connections in the developing visual system.


1034 Cell 131, December 14, 2007 ©2007 Elsevier Inc.
Baehrecke, E.H. (2002). Nat. Rev. Mol. Cell
Biol. 3, 779–787.
Berry, D.L., and Baehrecke, E.H. (2007). Cell,
this issue.
Blommaart, E.F., Krause, U., Schellens, J.P.,
Vreeling-Sindelarova, H., and Meijer, A.J.
(1997). Eur. J. Biochem. 243, 240–246.
Gorski, S.M., Chittaranjan, S., Pleasance,
E.D., Freeman, J.D., Anderson, C.L., Varhol,
R.J., Coughlin, S.M., Zuyderduyn, S.D., Jones,
S.J., and Marra, M.A. (2003). Curr. Biol. 13,
Kuranaga, E., and Miura, M. (2007). Trends
Cell Biol. 17, 135–144.
Levine, B. (2007). Nature 446, 745–747.
Maiuri, M.C., Zalckvar, E., Kimchi, A., and
Kroemer, G. (2007). Nat. Rev. Mol. Cell Biol.
8, 741–752.
Schweichel, J.U., and Merker, H.J. (1973).
Teratology 7, 253–266.
Scott, R.C., Juhasz, G., and Neufeld, T.P.
(2007). Curr. Biol. 17, 1–11.
Xie, Z., and Klionsky, D.J. (2007). Nat. Cell
Biol. 9, 1102–1109.
Traditionally, immune molecules have
been associated with neurons only in
the context of pathological conditions
such as brain injury, neuroinamma-
tion, and autoimmune disorders. How-
ever, our denition of neuroimmunol-
ogy is expanding, based on mounting
evidence that certain proteins that were
originally identied in the immune sys-
tem also have nonimmune functions in
the central nervous system (e.g., Bou-
langer et al., 2001; Goddard et al., 2007;
Huh et al., 2000; Loconto et al., 2003;
Oliveira et al., 2004; Syken et al., 2006).
The work now presented by Stevens
et al. (2007) reinforces this emerging
concept and introduces a new set of
playersmembers of the complement
The complement cascade is an arm
of the innate immune system and is
composed of over 30 small proteins
and protein fragments that are nor-
mally found in inactive forms in the
blood. The complement cascade can
be triggered via three basic mecha-
nisms: the classical, lectin, and alterna-
tive pathways. The classical pathway is
initiated by binding of the complement
protein C1q. All three pathways con-
verge on complement C3, which trig-
gers a sequence of proteolytic events
that amplify the signal and can lead to
formation of the cell-killing membrane
attack complex. In these cascades,
both C1q and C3 selectively bind to
pathogens and potentially toxic cellular
debris and mark them for destruction
and clearance by phagocytosis.
In the current study, Stevens et al.
found using a gene chip screen that
mRNA encoding C1q is upregulated by
puried neurons from the developing
mouse eye (retinal ganglion cells) in vitro
in response to astrocytes. Punctate
C1q immunoreactivity was detected
at postnatal day 5 (P5) in the devel-
oping retina and in the dorsal lateral
geniculate nucleus (dLGN), where reti-
nal ganglion cell axons from both eyes
initially send exuberant, overlapping
projections. These retinal projections
undergo activity-dependent remodel-
ing during therst two postnatal weeks,
such that selective strengthening of
some connections and weakening of
others results in the establishment of
the adult pattern of distinct, nonover-
lapping eye-specic layers. Imaging
of the dLGN during this remodeling (at
P5) showed that some C1q protein was
colocalized with either the postsynap-
tic marker PSD95 or the presynaptic
marker SV2, whereas less C1q was
detected at sites of close apposition
between the two markers. Because
such close apposition is a hallmark of
mature stable synapses, this pattern
is consistent with the presence of C1q
at nascent or retracting synaptic con-
nections. Importantly, the timing of C1q
expression coincided closely with the
Synapse Remodeling, Compliments of
the Complement System
Lawrence Fourgeaud
and Lisa M. Boulanger
Division of Biological Sciences, Section of Neurobiology, University of California, San Diego, 9500 Gilman Drive, La Jolla, CA
92093, USA
Silvio Varon Chair in Neuroregeneration
DOI 10.1016/j.cell.2007.11.031
A growing body of evidence indicates that some proteins known for their immune functions
also have distinct nonimmune functions in the normal uninjured central nervous system.
In this issue, Stevens et al. (2007) demonstrate an unexpected requirement for molecules
of the complement cascade in the remodeling of synaptic connections in the developing
visual system.
Cell 131, December 14, 2007 ©2007 Elsevier Inc. 1035
period of active remodeling in the visual
system, with the highest levels at early
postnatal ages (P5) when remodeling is
ongoing and declining expression after
the second postnatal week (P15) when
remodeling is largely complete.
To characterize the function of C1q
in the development of the mouse visual
system, Stevens et al. rst looked at
the structure of retinogeniculate projec-
tions in C1q-decient mice and found
that retinal ganglion cell axons from
the two eyes retained a higher degree
of anatomical overlap than in wild-type
mice. This was apparent both late in
the remodeling process (P10) and
after it is normally complete (P30). The
authors then performed elegant elec-
trophysiological recordings on relay
neurons in the P30 dLGN. At this age,
the majority of relay cells in wild-type
animals receive active inputs from only
one or two retinal axons. In contrast,
the majority of these relay cells in mice
lacking C1q were innervated by four or
more functional retinal inputs, indicating
that the persistent anatomical overlap
of inputs from the two eyes correlates
with aberrant retention of functional
connections. Furthermore, a remark-
ably similar phenotype was observed
in mice decient for C3, supporting the
possibility that C1q may affect synapse
remodeling through activation of either
all or part of the classical complement
cascade. The authors further showed
that although many more retinal gan-
glion cell inputs were retained in mice
decient in C1q, the majority of the syn-
apses were weak, with each LGN neu-
ron receiving only one or two strong
inputs. This polarization of synaptic
strength is thought to be an early step
in remodeling and may normally lead to
anatomical retraction of the functionally
weaker inputs. This key result suggests
that the normal remodeling process
has begun in neurons lacking C1q but
then is either interrupted or delayed.
Stevens et al. also examined C1q
expression in the DBA/2J mouse
model of glaucoma, a disorder charac-
terized by degeneration of retinal gan-
glion cells. They found that C1q immu-
noreactivity was strikingly elevated
in the retinas of adult mice with either
early or moderate glaucoma, mimick-
ing the labeling seen at earlier ages in
wild-type mice. This raises the tantaliz-
ing possibility that the pathogenesis of
glaucoma, and possibly other neuro-
degenerative disorders, involves reac-
tivation of a developmental remodeling
pathway involving complement.
How might complement proteins
contribute to synapse remodeling? The
authors hypothesize that C1q and C3
act as tags to mark synapses for elimi-
nation (Figure 1), perhaps analogous
to the way they mark pathogens and
debris for clearance in their immune
capacity. In this model, astrocytes
stimulate retinal ganglion cells that are
electrically active in the appropriate
levels and patterns to release soluble
C1q and C3. These molecules then act
as spreading “punishment signals” that
bind to neighboring weaker synapses
resulting in their physical removal, pos-
sibly through phagocytosis by acti-
vated microglia (Figure 1).
The current study demonstrates
that complement cascade proteins are
required for remodeling of the devel-
oping retinogeniculate projection, but
details of the propose d model remain to
be tested. For example, are the levels or
function of C1q regulated by electrical
activity in the developing visual system,
and are neurons the primary source of
complement proteins in the dLGN? If
so, do strong synapses also selectively
upregulate protective factors, such as
known membrane-bound complement
regulatory proteins, in order to escape
“punishment” by their own secreted
complement proteins (Figure 1)? It will
also be important to determine when
these molecules act. For example, is it
early in the remodeling process, when
they might label imprecise connections
and initiate their removal, or later, when
they might recruit cellular effectors to
clean up debris in the wake of selective
axon weakening? Importantly, the cur-
rent studies clearly demonstrate that
loss of C1q causes retention of inap-
propriate connections; it is unknown if
the converse, an increase in C1q, is suf-
cient to cause removal of synapses, or
if C1q is simply permissive for normal
remodeling processes. Also, as C1q
and C3 are both secreted soluble pro-
teins, their presence in neurons implies
the existence of as yet unknown neu-
ronal complement receptors.
The complement cascade can recruit
the adaptive immune response, in part
by regulating expression of members
of the major histocompatibility complex
(MHC) class I gene family. Notably, the
increased overlap in retinal projections
Figure 1. Complement Protein Function in Developmental Synapse Remodeling
As proposed by Stevens et al. (2007), the complement proteins C1q and C3 may be released
by strong synapses made by retinal ganglion cells (RGCs) in the developing dorsal lateral
geniculate nucleus (dLGN). These molecules may bind to neighboring weaker synapses,
marking them for destruction perhaps through phagocytosis by activated microglia. Strong
synapses also may express complement regulatory proteins that could protect them from
complement-mediated destruction.
1036 Cell 131, December 14, 2007 ©2007 Elsevier Inc.
in the LGN of mice lacking C1q or C3 is
remarkably similar to that seen in mice
decient for cell surface MHC class I (A.
Datwani and C.J. Shatz, personal com-
munication). Thus, it is possible that
C1q and C3 act by inducing expression
of MHC class I in the developing visual
system. Future work might resolve
this question by determining whether
MHC class I levels are reduced in mice
lacking C1q or C3 (or vice versa) and
assessing whether the effects of MHC
class I deciency and C1q deciency
on visual system development occlude
one another, which would suggest that
they act in functionally convergent
Activated microglia are the only
cells of the brain known to express
C3 receptors (Gasque et al., 1998),
indicating that they may participate
in synaptic remodeling mediated by
C3 (Figure 1). If microglia contribute
to this process, one prediction is that
higher numbers of microglia should
be present in the developing brain
specically during the period of active
remodeling. This is in fact the case in
some brain regions (e.g., Maslinska et
al., 1998). High numbers of activated
microglia have also recently been
reported in postmortem brain sam-
ples from patients with autism (Vargas
et al., 2005), raising the possibility that
microglia might contribute to patho-
logical changes in connectivity in this
neurodevelopmental disorder.
The results reported by Stevens et
al. add to growing evidence that the
immune system and nervous sys-
tem make different use of some of
the same molecular machinery. This
molecular overlap could act as a point
of either benecial or harmful cross-
talk between the two systems in injury
and disease states and hints at new
therapeutic directions for a wide vari-
ety of neurological disorders. Much
additional work is needed to elucidate
the precise molecular mechanisms by
which proteins of the innate and adap-
tive immune system also participate in
normal brain development and plastic-
ity. Fortunately, this work will be greatly
facilitated by the knowledge and exper-
imental tools established when these
proteins were rst characterized in the
immune system.
Boulanger, L.M., Huh, G.S., and Shatz, C.J.
(2001). Curr. Opin. Neurobiol. 11, 568–578.
Gasque, P., Singhrao, S.K., Neal, J.W., Wang, P.,
Sayah, S., Fontaine, M., and Morgan, B.P. (1998).
J. Immunol. 160, 3543–3554.
Goddard, C.A., Butts, D.A., and Shatz, C.J.
(2007). Proc. Natl. Acad. Sci. USA 104, 6828–
Huh, G.S., Boulanger, L.M., Du, H., Riquelme,
P.A., Brotz, T.M., and Shatz, C.J. (2000). Science
290, 2155–2159.
Loconto, J., Papes, F., Chang, E., Stowers, L.,
Jones, E.P., Takada, T., Kumanovics, A., Fis-
cher Lindahl, K., and Dulac, C. (2003). Cell 112,
Maslinska, D., Laure-Kamionowska, M., and Kal-
iszek, A. (1998). Folia Neuropathol. 36, 145–151.
Oliveira, A.L., Thams, S., Lidman, O., Piehl, F.,
Hokfelt, T., Karre, K., Linda, H., and Cullheim, S.
(2004). Proc. Natl. Acad. Sci. USA 101, 17843–
Stevens, B., Allen, N.J., Vazquez, L.E., Howell,
G.R., Christopherson, K.S., Nouri, N., Micheva,
K.D., Mehalow, A.K., Huberman, A.D., Stafford,
B., et al. (2007). Cell, this issue.
Syken, J., Grandpre, T., Kanold, P.O., and Shatz,
C.J. (2006). Science 313, 1795–1800.
Vargas, D.L., Nascimbene, C., Krishnan, C.,
Zimmerman, A.W., and Pardo, C.A. (2005). Ann.
Neurol. 57, 67–81.
The malaria parasite (Plasmodium)
is a unicellular, obligate intracellular
protozoan that must invade, colonize,
replicate within, and emerge from
various cells types of the mamma-
lian host or mosquito vector in order
to complete its life cycle. Although
the basic structure of Plasmodium is
comparable to a standard eukaryotic
cell, it is capable of producing dis-
tinct invasive forms and specialized
organelles that are evolved to recog-
nize and invade the correct cell type.
For growth and multiplication in the
bloodstream of its vertebrate host,
The Exoneme Helps Malaria Parasites to
Break out of Blood Cells
Chris J. Janse
and Andrew P. Waters
Department of Parasitology, Leiden University Medical Centre, Albinusdreef 2, 2333ZA Leiden, The Netherlands
DOI 10.1016/j.cell.2007.11.026
Malaria parasites must invade the erythrocytes of its host, to be able to grow and multiply.
Having depleted the host cell of its nutrients, the parasites break out to invade new eryth-
rocytes. In this issue of Cell, Yeoh et al. (2007) discover a new organelle, the exoneme, that
contains a protease SUB1, which helps the parasite to escape from old erythrocytes and
invade new ones.
    • "They are distributed throughout the brain and retina, represent approximately 12% of the adult brain cells, and play a pivotal role in the innate immune response [1]. In normal conditions, microglia support synaptogenesis through the local synthesis of neurotrophic factors [2], [3] and the regulation of synaptic transmission and remodeling [4],[5]. In response to acute neurodegenerative disease, they transform from a ramified basal homeostatic phenotype to an activated phagocytic phenotype and release pro-inflammatory mediators, such as IL1β and TNFα. "
    [Show abstract] [Hide abstract] ABSTRACT: Background and Objective Tetrandrine (TET) is a bisbenzylisoquinoline alkaloid extracted from Stephania tetrandra Moore. Recent studies have suggested that TET can reduce the inflammatory response in microglia, but the mechanisms remain unclear. The aim of this study is to investigate whether TET can inhibit lipopolysaccharide (LPS)-induced microglial activation and clarify its possible mechanisms. Study Design/Materials and Methods Cell viability assays and cell apoptosis assays were used to determine the working concentrations of TET. Then, BV2 cells were seeded and pretreated with TET for 2 h. LPS was then added and incubated for an additional 24 hours. qRT-PCR and ELISA were used to measure the mRNA or protein levels of IL1β and TNFα. Western blotting was utilized to quantify the expression of CD11b and cell signaling proteins. Results TET at optimal concentrations (0.1 µM, 0.5 µM or 1 µM) did not affect the cell viability. After TET pretreatment, the levels of IL1β and TNFα (both in transcription and translation) were significantly inhibited in a dose-dependent manner. Further studies indicated that phospho-p65, phospho-IKK, and phospho-ERK 1/2 expression were also suppressed by TET. Conclusions Our results indicate that TET can effectively suppress microglial activation and inhibit the production of IL1β and TNFα by regulating the NF-kB and ERK signaling pathways. Together with our previous studies, we suggest that TET would be a promising candidate to effectively suppress overactivated microglia and alleviate neurodegeneration in glaucoma.
    Full-text · Article · Aug 2014
    • "Notably, C1qa, C1qb, C1qc, H2-Aa (HLA-DQA1), and H2-Ab1 (HLA-DQB1) were upregulated in the brains of both Shn-2 KO mice and patients with schizophrenia. C1q is proposed to act as a spreading ‘punishment signals' that bind to weaker synapses resulting in their physical removal (Fourgeaud and Boulanger, 2007). In this regard, it is of interest to note that the expression of the genes related to ‘synaptic transmission,' the dysregulation of which is also thought to be involved in schizophrenia (Mirnics et al, 2001; Stephan et al, 2006), tended to be downregulated in the brains of both Shn-2 KO mice and schizophrenic patients (Supplementary Table 3). "
    [Show abstract] [Hide abstract] ABSTRACT: Schnurri-2 (Shn-2), an NF-kappa B site-binding protein, tightly binds to the enhancers of major histocompatibility complex (MHC) class I genes and inflammatory cytokines, which have been shown to harbor common variant single nucleotide polymorphisms associated with schizophrenia. Although genes related to immunity are implicated in schizophrenia, there has been no study showing that their mutation or knockout results in schizophrenia. Here, we show that Shn-2 knockout mice have behavioral abnormalities that resemble those of schizophrenics. The mutant brain demonstrated multiple schizophrenia-related phenotypes, including transcriptome/proteome changes similar to those of postmortem schizophrenia patients, decreased parvalbumin and GAD67 levels, increased theta power on electroencephalograms, and a thinner cortex. Dentate gyrus granule cells failed to mature in mutants, a previously proposed endophenotype of schizophrenia. Shn-2 knockout mice also exhibited mild chronic inflammation of the brain, as evidenced by increased inflammation markers (including GFAP and NADH/NADPH oxidase p22 phox), and genome-wide gene expression patterns similar to various inflammatory conditions. Chronic administration of anti-inflammatory drugs reduced hippocampal GFAP expression, and reversed deficits in working memory and nest building behaviors in Shn-2 KO mice. These results suggest that genetically-induced changes in immune system can be a predisposing factor in schizophrenia.Neuropsychopharmacology accepted article preview online, 6 February 2013; doi:10.1038/npp.2013.38.
    Full-text · Article · Feb 2013
    • "Expression of major histocompatibility complex class I (MHC I) by neurons and glial cells has been implicated in the synaptic elimination process during development and after lesions in adulthood [1-3]. More recently, molecules from the classic complement pathway have also been implicated in the process of refinement of neural circuits and as important players in the response to peripheral nerve injury [4]. This classic model for studying the retrograde reaction to axon transection has been widely used, and has improved our understanding of the mechanisms underlying synapse elimination and of the interactions between neurons and glial cells after injury [2,5,6]. "
    [Show abstract] [Hide abstract] ABSTRACT: Background Glial cells are involved in the synaptic elimination process that follows neuronal lesions, and are also responsible for mediating the interaction between the nervous and immune systems. Neurons and glial cells express Toll-like receptors (TLRs), which may affect the plasticity of the central nervous system (CNS). Because TLRs might also have non-immune functions in spinal-cord injury (SCI), we aimed to investigate the influence of TLR2 and TLR4 on synaptic plasticity and glial reactivity after peripheral nerve axotomy. Methods The lumbar spinal cords of C3H/HePas wild-type (WT) mice, C3H/HeJ TLR4-mutant mice, C57BL/6J WT mice, and C57BL/6J TLR2 knockout (KO) mice were studied after unilateral sciatic nerve transection. The mice were killed via intracardiac perfusion, and the spinal cord was processed for immunohistochemistry, transmission electron microscopy (TEM), western blotting, cell culture, and reverse transcriptase PCR. Primary cultures of astrocytes from newborn mice were established to study the astrocyte response in the absence of TLR2 and the deficiency of TLR4 expression. Results The results showed that TLR4 and TLR2 expression in the CNS may have opposite effects on the stability of presynaptic terminals in the spinal cord. First, TLR4 contributed to synaptic preservation of terminals in apposition to lesioned motor neurons after peripheral injury, regardless of major histocompatibility complex class I (MHC I) expression. In addition, in the presence of TLR4, there was upregulation of glial cell-derived neurotrophic factor and downregulation of interleukin-6, but no morphological differences in glial reactivity were seen. By contrast, TLR2 expression led to greater synaptic loss, correlating with increased astrogliosis and upregulation of pro-inflammatory interleukins. Moreover, the absence of TLR2 resulted in the upregulation of neurotrophic factors and MHC I expression. Conclusion TLR4 and TLR2 in the CNS may have opposite effects on the stability of presynaptic terminals in the spinal cord and in astroglial reactions, indicating possible roles for these proteins in neuronal and glial responses to injury.
    Full-text · Article · Oct 2012
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