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Biology of Reproduction, 2020, 1–13
doi:10.1093/biolre/ioaa071
Research Article
Research Article
Loss of myosin VI expression affects
acrosome/acroplaxome complex morphology
during mouse spermiogenesis†
Przemysław Zakrzewski1, Maria Jolanta Re˛dowicz2, Folma Buss3and
Marta Lenartowska1,4
1Department of Cellular and Molecular Biology, Faculty of Biological and Veterinary Sciences, Nicolaus Copernicus
University in Toru ´
n, Torun, Poland, 2Laboratory of Molecular Basis of Cell Motility, Nencki Institute of Experimental
Biology, Polish Academy of Sciences, Warsaw, Poland, 3Cambridge Institute for Medical Research, The Keith Peters
Building, University of Cambridge, Cambridge, UK and 4Centre for Modern Interdisciplinary Technologies, Nicolaus
Copernicus University in Toru ´
n, Torun, Poland
*Correspondence: Department of Cellular and Molecular Biology, Faculty of Biological and Veterinary Sciences, Nicolaus
Copernicus University in Toru´
n, Lwowska 1, 87-100 Torun, Poland. Tel: +48 56 611 31 86; E-mail: przezak@doktorant.umk.pl
†Grant Support: This project was supported by PRELUDIUM grant from the National Science Centre (Poland) to P.Z.
(2017/25/N/NZ3/00487), an ETIUDA doctoral scholarship from the National Science Centre (Poland) to P.Z.
(2018/28/T/NZ3/00002), a traveling fellowship for P.Z. funded by The Company of Biologists (JCSTF-171105) and a Medical
Research Council program grant to F.B. (MR/S007776/1).
Conference Presentation: Presented in part at the 43rd FEBS Congress, 7–12 July 2018, Prague, Czech Republic, and the
44th FEBS Congress, 6–11 July 2019, Krakow, Poland.
F.B. and M.L. contributed equally to this work and are joint senior authors.
Received 2 February 2020; Revised 24 March 2020; Accepted 14 May 2020
Abstract
During spermiogenesis in mammals, actin filaments and a variety of actin-binding proteins are
involved in the formation and function of highly specialized testis-specific structures. Actin-based
motor proteins, such as myosin Va and VIIa, play a key role in this complex process of spermatid
transformation into mature sperm. We have previously demonstrated that myosin VI (MYO6)
is also expressed in mouse testes. It is present in actin-rich structures important for spermatid
development, including one of the earliest events in spermiogenesis—acrosome formation. Here,
we demonstrate using immunofluorescence, cytochemical, and ultrastructural approaches that
MYO6 is involved in maintaining the structural integrity of these specialized actin-rich structures
during acrosome biogenesis in mouse. We show that MYO6 together with its binding partner
TOM1/L2 is present at/around the spermatid Golgi complex and the nascent acrosome. Depletion
of MYO6 in Snell’s waltzer mice causes structural disruptions of the Golgi complex and affects the
acrosomal granule positioning within the developing acrosome. In summary, our results suggest
that MYO6 plays an anchoring role during the acrosome biogenesis mainly by tethering of different
cargo/membranes to highly specialized actin-related structures.
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2P. Zakrzewski et al., 2020
Summary Sentence
Myosin VI is required for the maintenance of correct morphology of testis-specific actin-containing
structures important for acrosome development in mouse such as the Golgi complex and the
acrosome–acroplaxome complex.
Key words: acrosome/cap, acroplaxome, actin cytoskeleton, Golgi complex, myosin VI, Snell’s waltzer mice.
Introduction
Actin filaments are ubiquitously present in eukaryotic cells and play
a fundamental role in cell differentiation and function. The actin
cytoskeleton together with actin-binding proteins (ABPs) provides
both structural support and functional f lexibility, performing key
roles in many cellular processes including cell motility, intracellular
trafficking, and cell division. In addition, microfilaments form cell
type-specific cytoskeletal arrays that are crucial for the subcellular
organization, asymmetric positioning of specialized compartments,
maintenance of cell shape, and formation of highly specialized cell
structures. The actin cytoskeleton is also crucial for a number of
cellular processes during spermatogenesis and is thus essential for
male fertility [1].
Spermiogenesis, the last step of sperm development, is a complex
transformation process of unpolarized spermatids into morpholog-
ically mature spermatozoa. In mammals this process is typically
divided into three successive steps: the Golgi phase, acrosome/cap
phase, and maturation phase. One of the earliest events during
spermiogenesis is acrosome formation, an apical nuclear cap rich
in hydrolytic enzymes, which is required for sperm penetration of
the oocyte at fertilization. Biogenesis of the acrosome begins with
trafficking of proacrosomal vesicles toward the spermatid nucleus
from both the trans-Golgi network and the endocytic pathway [2].
These vesicles adhere to and fuse to form the acrosomal vesicle along
the acroplaxome, a cytoskeletal actin-rich plate that anchors the
developing acrosome to the nuclear envelope [3]. The large acroso-
mal vesicle then progressively spreads over the spermatid nucleus
to form a distinct cap-like structure. As spermiogenesis proceeds,
a transient cytoskeletal structure, the manchette, develops to exert
mechanical force for nuclear compaction/elongation and to serve as
a scaffold for transport of molecules to the growing flagellum. The
acroplaxome also expands during maturation phase, thus potentially
supporting the manchette complex in nuclear shaping [4]. Two addi-
tional testis-specific structures are formed to support spermiogenesis
efficiently: (i) apical ectoplasmic specialization that anchors the
developing spermatid to the Sertoli cell during its movement in the
seminiferous tubules and (ii) tubulobulbar complexes at the Sertoli–
spermatid interface [5,6]. Finally, excess cytoplasm (the residual
body) is removed from the maturing spermatid and phagocytozed
by the Sertoli “nurse” cell.
It has been long recognized that the actin cytoskeleton together
with various ABPs participates in assembly and remodeling of
these unique, testis-specific structures including the acrosome–
acroplaxome complex [1]. The experimental disruption of actin
filaments results in detachment of the acrosome from the spermatid
nucleus and deformation of the expanding edge of the acrosomal
sac, suggesting that the correct actin assembly is crucial for
acrosomogenesis [7]. A number of actin regulators, such as
cortactin and profilin, are present in the acrosome–acroplaxome-
manchette complex and thus may regulate actin dynamics during
the acrosome biogenesis and the head shaping [8,9]. Furthermore,
actin-based motors such as myosin Va and VIIa are components
of the acroplaxome and are associated with vesicles in the
manchette during acrosome biogenesis and sperm flagellum
development [10,11].
In addition to myosin Va and VIIa, myosin VI (MYO6) is also
expressed in mouse testes, where it is present in actin-rich struc-
tures during acrosome biogenesis such as the Golgi complex and
the acrosome-acroplaxome complex [[12]; summarized in Figure 1].
MYO6 is expressed in most cell types and tissues, and since it is
the only pointed-end-directed actin-based motor, it participates in
several cellular processes including endocytosis, Golgi organization
and function, basolateral targeting and sorting in the secretory
pathway, epithelial integrity, and cell adhesion and migration [13–
15]. Similar to vertebrates, loss of MYO6 in Drosophila is not lethal,
but causes a variety of well-documented phenotypes during embryo-
genesis and development [16–18], including severe abnormalities
during Drosophila spermiogenesis (called spermatid individualiza-
tion) leading to male sterility [19,20]. Mutation in the Myo6 gene in
Snell’s waltzer mice (sv/sv) causes deafness as a result of neurosensory
epithelia degeneration in the inner ear [21]. Moreover, the sv/sv mice
display several other defects such as changes in Golgi morphology,
reduced secretion, and defective endocytosis and autophagy [22–
25]. Moreover, data from our lab shows that male sv/sv mice have
significantly reduced fertility and exhibit disruption of the spatial
organization of the apical tubulobulbar complexes during the late
stage of spermiogenesis [26]. Here, we show, by comparing the mor-
phology of developing spermatids of control and MYO6-deficient
male mice, that this myosin is present in actin-rich structures during
acrosome biogenesis and supports the integrity of these testis-specific
structures at this stage of mouse sperm development.
Materials and methods
Ethics statement
All animal work was performed at the Nencki Institute of Experi-
mental Biology (Warsaw, Poland) or at the University of Cambridge,
Cambridge Institute for Medical Research in accordance with the
UK Animals (Scientific Procedures) Act of 1986 (PPL70/8460). The
mice were bred and housed under pathogen-free conditions. Animal
were housed and euthanized in compliance with the European Com-
munities Council directives adopted by the Polish Parliament (Act of
15 January 2015 on the use of animals in scientific investigations)
and with the UK Animals (Scientific Procedures) Act 1986 and
Laboratory Animal Science Association (LASA) guidelines.
Animals
Three-month-old male Snell’s waltzer mice (sv/sv, C57BL/6 back-
ground) were used in the study. A spontaneously mutated sv allele
encodes Myo6 gene with a 130-bp deletion resulting in the intro-
duction of a stop codon in the neck region of MYO6 [21]. No
MYO6 is detected in any tissue of homozygous sv/sv mice, including
testis (Figure 2A). Each experiment was performed at least three
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Myosin VI deficiency affects acrosomogenesis in mouse, 2020 3
Figure 1. Schematic representation of the acrosome biogenesis and the localization of MYO6 during this process in mouse. Black dots, MYO6; SC, Sertoli cell;
SpT, spermatid; red line, apical ectoplasmic specialization and yellow line, acroplaxome.
times using a pair of control (heterozygous, sv/+) and mutant
(sv/sv) males from one litter. Importantly, heterozygous mice express
approximately equal amount of MYO6 in comparison with wild-
type mice [22].
Antibodies and reagents
For the primary antibodies used in this study, please see supplemen-
tary Tab l e S1. The secondary antibodies used for immunofluores-
cence were goat anti-rabbit/mouse Alexa Fluor 488/568 (Invitrogen).
For immunogold labeling, Protein A gold conjugates (Department
of Cell Biology, University of Utrecht, the Netherlands) were used
to detect primary antibodies. The secondary anti-mouse/rabbit anti-
bodies used for immunoblotting were horseradish peroxidase (HRP)
conjugated (Sigma). F-actin was visualized using Alexa Fluor 488
Phalloidin (Invitrogen). Nuclei were counterstained with Hoechst
33342 (Thermo Scientific). Acrosomes were stained either with
Coomassie Brilliant Blue (Sigma-Aldrich) or lectin PNA from peanut
Alexa Fluor 488 conjugate (Invitrogen). Normal rabbit IgG (Sigma-
Aldrich) was used as a negative control during immunofluorescence
and immunogold experiments [26].
Immunoblotting
Testes dissected from sv/+and sv/sv males (n= 3 for both mutant
and control males) were homogenized with a Dounce tissue grinder
in protein extraction buffer (50 mM Tris-HCl pH 7.5, 0.5% Triton
X-100, 150 mM NaCl, 5% glycerol) supplemented with 1 ×cOm-
plete Protease Inhibitor Cocktail (Roche). The homogenates were
centrifuged twice at 15 000 ×gfor 10 min at 4 ◦C, and protein
concentration of the supernatants was determined using the Bio-
Rad DC Protein Assay according to the manufacturer’s instructions.
Equal amounts of protein extract were separated by electrophore-
sis on a 7% SDS-PAGE gels, and then transferred to Amersham
PVDF Hybond P membrane (GE Healthcare). Blocked blots were
probed with the primary antibodies overnight at 4 ◦C, washed, and
incubated for 1 h with the corresponding anti-rabbit secondary IgG
antibody conjugated with HRP. Signals were detected with the Amer-
sham ECL Advance Western Blotting Detection Kit according to
the manufacturer’s guidelines (GE Healthcare). All immunoblotting
experiments were repeated three times.
Conventional electron microscopy
For detailed ultrastructural analysis, dissected testes were immersion
fixed in 2% (v/v) glutaraldehyde in 0.1 M cacodylate buffer (pH 7.4)
for 2 h at room temperature. Tunicae albugineae of the testes were
punctured with a syringe needle to facilitate fixative penetration.
Pre-fixed samples were then cut into small pieces and further fixed
overnight at 4 ◦C. Next, the samples were post-fixed with 1% (v/v)
osmium tetroxide in cacodylate buffer for 1 h at 4 ◦C, dehydrated
in ethanol, and embedded in Spurr resin (Sigma-Aldrich) according
to the standard protocol. Ultrathin sections (cross-sections of sem-
iniferous tubules) were collected on copper grids, post-stained with
2.5% uranyl acetate and 0.4% lead citrate solutions, and examined
and imaged on a JEM 1400 transmission electron microscope (JEOL
Co.) equipped with 11 Megapixel TEM Morada G2 digital camera
(EMSIS GmbH) or an FEI Tecnai G2 Spirit BioTwin transmission
electron microscope equipped with a Gatan CCD camera. Acquired
images were processed using Adobe Photoshop CS6. Ultrastructural
images are representative from experiments repeated at least three
times.
Sample preparation for immunocytochemical studies
Epithelial fragments for immunofluorescence were prepared as fol-
lows: the dissected testes from sv/+and sv/sv males were decapsu-
lated and minced in 4% formaldehyde in 1 ×PBS (pH 7.4) and fixed
overnight at 4 ◦C. The fixed seminiferous tubules segments were
aspirated gently through 18-gauge and 21-gauge syringe needles
[27]. Larger fragments of tissue were allowed to settle to the bottom
of the tube, before the supernatant was removed and centrifuged
(1 min at 4000 ×g). The pellet was resuspended in PBS, and the cell
suspension was added onto poly-L-lysine-coated coverslips. After
10 min, the coverslips were plunged into ice-cold acetone and air
dried.
Immunolocalization studies
For immunofluorescence studies, samples were blocked with 1%
BSA/0.1% Triton X-100 before incubating with primary antibodies
overnight at 4 ◦C, which were detected with secondary antibodies
conjugated with different fluorochromes. DNA was stained with
Hoechst. Epi-fluorescence images were captured on Zeiss Axio
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4P. Zakrzewski et al., 2020
Figure 2. The acrosomogenesis in MYO6-deficient spermatids is altered. (A) Immunoblotting of crude protein extracts from sv/+and sv/sv testes with anti-MYO6
and anti-actin antibodies. No MYO6 is detectable in sv/sv tissue. (B) The graphs depicting the mean percentage of sv/+and sv/sv cells that displayed the Golgi
defects visualized with a use of immunofluorescence (IF) (a) and ultrastructural (EM) (b) analysis. For immunofluorescence, counts were performed on >90
cells from n= 3 independent experiments. For ultrastructure analysis, counts were performed on at least four ultrathin sections of seminiferous tubules from
randomly chosen fragments of fixed testes from n= 3 independent experiments.. Error bars indicate SD. ∗P≤0.05; ∗∗∗P<0.001. (C) Immunofluorescence
localization of TGN38 (red) and GM130 (green) in sv/+and sv/sv round spermatids. Bars 2 μm. (D) Ultrastructural analysis of sv/+and sv/sv spermatids during
the consecutive phases of acrosomogenesis. ag, acrosomal granule; av, acrosomal vesicle; er, endoplasmic reticulum; g, Golgi complex; m, mitochondria; n,
nucleus; pav, proacrosomal vesicle; pg, proximal Golgi; and white star, acroplaxome. All other indications are explained in the text. Bars 1 μm.
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Myosin VI deficiency affects acrosomogenesis in mouse, 2020 5
Imager.Z2 upright microscope and the acquired images were pro-
cessed with Zeiss ZEN 2.6 (blue edition) and Adobe Photoshop CS6
software.
For post-embedding immunogold labeling for electron micro-
scopy, samples were fixed with 4% formaldehyde and 0.25% glu-
taraldehyde in 1 ×PBS (pH 7.4), dehydrated in ethanol, before
embedding in LR Gold resin (Sigma-Aldrich) according to the stan-
dard protocol. Samples in resin were cut with a diamond knife into
ultrathin sections, collected onto nickel grids, and blocked 5 min at
room temperature with 1% BSA in PBS (pH 7.4). The sections were
then incubated overnight with primary antibody in 0.1% BSA/PBS
at 4 ◦C and the next day with 10 nm gold-conjugated protein A (at
dilution 1:50 in 0.1% BSA/PBS) for 60 min at room temperature.
Finally, the sections were post-fixed with 1% glutaraldehyde in
PBS, post-stained with 2.5% uranyl acetate and 0.4% lead citrate
solutions, and examined and imaged on an FEI Tecnai G2 Spirit
BioTwin transmission electron microscope equipped with a Gatan
CCD Camera. Acquired images were processed with Adobe Pho-
toshop CS6. Representative images for immunolocalization studies
were collected from experiments repeated at least three times.
Epididymal sperm count and acrosomal reaction
Caudae epididymides from sv/+and sv/sv males were dissected,
cut in several places with a sharp scalpel blade, and incubated for
15 min in Whittens-HEPES medium (100 mM NaCl, 4.4 mM KCl,
1.2 mM KH2PO4, 1.2 mM MgSO4, 5.4 mM glucose, 0.8 mM
pyruvic acid, 4.8 mM lactic acid, and 20 mM HEPES) at 37 ◦Cto
allow the sperm to swim out from the incisions. A proportion of the
collected sperm was diluted 1:10 with distilled water and transferred
to a hemocytometer for sperm counting. Another fraction of the
sperm was diluted 1:1 with a 2 ×capacitation media (Whittens-
HEPES medium supplemented with 30 mM NaHCO3and10mg/ml
BSA) and incubated for 60 min at 37 ◦C. After the capacitation,
the acrosomal reaction was induced by incubation for 30 min in
10 μM A23187 ionophore (Sigma-Aldrich). Both capacitated and
acrosome-reacted sperm were fixed with 5% formaldehyde in PBS
(pH 7.4) for 10 min in room temperature, washed, smeared on glass
slides, and air dried. Acrosomes were visualized by immersion of
slides in Coomassie Brilliant Blue staining solution (Coomassie dye in
50% methanol and 10% acetic acid). For sperm head and acrosome
structure analysis, epididymal sperm were stained with Hoechst and
lectin PNA Alexa Fluor 488 conjugate (1:100 dilution in 1 ×PBS) for
10 min at room temperature. Representative images were collected
from experiments repeated at least three times.
Statistical analysis
The obtained results were presented as the mean ±SD. The statistical
significance in each experiment was analyzed using an unpaired two-
tailed Student t-tests. The data were considered significant when
P<0.05. All data analysis was performed using GraphPad Prism
6 for Windows.
Results
In this study, we focus on the comparative analysis of acrosome
biogenesis during the Golgi and the acrosome/cap phases (Figure 1)
in MYO6-deficient and control mice using cyto/immunocytochem-
istry and epi-fluorescence microscopy as well as electron microscopy
for ultrastructural studies. Our results show that MYO6 deficiency
in mouse testes leads to structural defects in developing spermatids
affecting correct acrosome formation.
MYO6 depletion causes structural disruptions during
early acrosome biogenesis
Our previous results have shown that in wild-type mice, MYO6
is present at actin-rich structures involved in acrosome formation,
such as the Golgi complex and the acrosome–acroplaxome complex
(Figure 1)[12]. We therefore analyzed here whether this motor
protein has a role in acrosomogenesis by testing whether MYO6
is important for maintaining the morphology of the testis-specific
structures during acrosome biogenesis. We used testes dissected
from sv/sv mice, which do not express any MYO6 (Figure 2A), and
compared organization of the Golgi complex in sv/sv and control
spermatids (Figure 2C). Developing spermatids at the Golgi stage
were labeled with antibodies to GM130 (a marker for the cis-
Golgi) and to TGN38 (specific for the trans-Golgi). As shown in
Figure 2C.a, the functional sub-domains of the Golgi complex are
correctly oriented and stacked relative to spermatid nuclei in sv/+
spermatids; the staining for cis-Golgi corresponded to the semi-
circular shape of the organelle observed at the ultrastructural level
(compare Figure 2C.a and D.a). In contrast, in sv/sv spermatids, the
mislocalization of these Golgi markers suggests a partial disorga-
nization of the Golgi complex that in some cases was asymmetri-
cally oriented (Figure 2C.b) or completely inverted (Figure 2C.c). In
most extreme examples, we could observe a severe loss of Golgi
morphology and no segregation between the cis-Golgi and trans-
Golgi sub-domains (Figure 2C.d). Finally, quantification and sta-
tistical analysis of these phenotypes revealed that in control mice
7.00 ±3.15% of spermatids exhibited Golgi structural defects
compared to 14.86 ±2.37% observed in sv/sv mice, indicating a
two-fold increase of Golgi disorganization in mutant spermatids
(Figure 2B.a;P= 0.026; n= 3 individual experiments on a pair of
sv/+and sv/sv littermates each).
Next, we examined in detail the ultrastructure of spermatids
during acrosomogenesis in sv/sv and control testes. At the beginning
of acrosome biogenesis, we observed in control spermatids a Golgi
complex that is located near the spermatid nucleus and forms a
semi-circular structure with its concave trans-side rich in vesicles,
facing the nucleus (Figure 2D.a). In control testes, the Golgi stacks
have tightly aligned flattened cisternae and no separation between
the individual cisternae. In sv/sv spermatids, however, the Golgi
cisternae did not form regular stacks but were separated from each
other by numerous vesicles resulting in the loss of the typical Golgi
structure (Figure 2D.b–c, arrows). Some of these vesicles present in
the sv/sv spermatids were highly enlarged (Figure 2D.b–c, arrow-
heads) and we observed multiple proacrosomal vesicles (Figure 2D.d,
white arrows), some of which were located at a distance from the
upper pole of nucleus (Figure 2D.d, black arrow). Another structural
disruption was identified in MYO6-deficient spermatids at the Golgi
stage, where the acrosomal granules were not docked symmetrically
within the acrosome sacs. In control spermatid, the acrosomal gran-
ule was attached symmetrically to the inner acrosomal membrane
at the middle of the acroplaxome (Figure 2D.e,star),whereasin
sv/sv spermatids these granules were often mislocated (Figure 2D.f).
This phenomenon was also observed during the next stage, the early
acrosome/cap stage (Figure 2D.g, compare with Figure 2D.h, stars).
Interestingly, at this later phase of acrosomogenesis the disturbed
Golgi morphology was still visible in sv/sv spermatids (Figure 2D.g,
compare with Figure 2D.h, arrows). Quantification of the percentage
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6P. Zakrzewski et al., 2020
of spermatids with ultrastructural disruptions of the Golgi complex
revealed that in control mice 9.49 ±3.61% of spermatids exhibited
Golgi structural defects compared to 40.83 ±2.91% in sv/sv mice,
indicating a four-fold increase of these defects in mutant spermatids
(Figure 2B.b;P= 0.0003; n= 3 individual experiments on a pair of
sv/+and sv/sv littermates each).
Loss of MYO6 affects the acrosomal granule
positioning
We next analyzed the ultrastructure of sv/sv spermatids during the
later cap phase, when the acrosome starts to flatten and forms a
cap-like structure containing the electron-dense acrosomal granule
(Figure 3A.a). The most striking and prominent structural disruption
at this stage was the altered position of the acrosomal granule within
sv/sv developing cap. While in control spermatid the acrosomal
granule was anchored at the middle of the acroplaxome (Figure 3A.a,
star), in the absence of MYO6 this central localization was lost
(Figure 3A.b). In addition, in some sv/sv spermatids the acrosomal
granules were detached from the acroplaxomes (Figure 3A.c)and
its electron-dense content was reduced (Figure 3A.d). We have also
observed a few “empty” acrosome sacs without any acrosomal
granules in sv/sv spermatids; this phenotype corresponded usually
with significantly enlarged vesicles adjacent to the deformed acro-
somes (Figure 3A.e). In the next developmental step, the f lattened
cap further spread over the spermatid nucleus and became attached
anteriorly to the Sertoli cell through the newly formed apical ecto-
plasmic specialization (Figure 3A.f, asterisks). In this phase, the
asymmetry of the acrosome was still clearly visible in sv/sv sper-
matids (Figure 3A.g–h; compare with Figure 3A.f and A.g,black
arrows with a bar). Our quantification revealed that in control
mice only 11.69 ±2.23% of spermatids exhibited defects in the
acrosomal structure compared to 56.26 ±4.30% of MYO6-deficient
spermatids, indicating almost five-fold increase of atypical acrosome
phenotype in mutant spermatids (Figure 3B;P<0.0001; n=3
individual experiments on a pair of sv/+and sv/sv littermates each).
In summary,the loss of MYO6 in developing mouse spermatids leads
to structural disruptions of the Golgi complex and affects formation
of the acrosome during spermiogenesis.
MYO6 together with its binding partner TOM1/L2 are
localized at the Golgi complex and developing
acrosome
Acrosome biogenesis involves the delivery of proacrosomal vesicles
from either the Golgi complex or from late endosomes/lysosomes to
the nascent acrosome [28]. We previously demonstrated that MYO6
localizes to the Golgi stacks during the acrosomogenesis in wild-
type mouse spermatids [12]. Indeed, our present immunof luorescent
analysis shows that MYO6 is associated with the Golgi complex
including the cis-Golgi domain (Figure 4A.a–a’) and the region
corresponding to the trans-Golgi network (Figure 4A.a–a”)insv/+
spermatids, whereas no signal was observed in sv/sv cells confirming
the specificity of our MYO6 antibodies (Figure 4A.b–b”). This con-
firms our previous results (using immunogold technique) showing
that MYO6 localized to both Golgi sub-domains in wild-type mice
males [12]. Targeting of MYO6 to different cellular compartments
in mammalian cells involves a range of binding partners [15]andso
we next analyzed which adaptor protein may function with MYO6
during acrosome biogenesis. We tested several MYO6 interacting
proteins including TOM1/L2, GIPC1, optineurin, DOCK7, LRCH3,
and LARG for their localization, and only observed the presence
of TOM1/L2 in close vicinity to the Golgi complex. As demon-
strated in Figure 4, TOM1/L2 that interacts with MYO6 in the
endocytic pathway is present on vesicular-like structures localized
between the spermatid nucleus and the Golgi complex labeled with
anti-GM130 antibody, both in the control and sv/sv spermatids
(Figure 4B.a–d).These results indicate that both MYO6 and its part-
ner TOM1/L2 are present at/around the Golgi complex, although in
different compartments. While MYO6 is associated with both cis-
Golgi and trans-Golgi sub-domains, TOM1/L2 is present in a post-
Golgi region that can either correspond to the trans-Golgi network
or an endocytic compartment.
We next determined the localization of MYO6, TOM1/L2,
and F-actin during the later acrosome phase, when the absence
of MYO6 in sv/sv spermatids caused abnormal positioning of
the acrosomal granule within the acrosome sac (Figures 2 and 3).
First, we assessed the localization of MYO6 in control spermatids,
where we observed a strong signal for MYO6 in the middle of
the acroplaxome plate labeled for F-actin with the fluorochrome-
conjugated phalloidin (Figure 5A.a, arrow) and a diffuse signal in
the cytoplasm (Figure 5A). The area of MYO6 localization increased
when the acrosome flattened and spread over the spermatid
nucleus with a strong signal along the acroplaxome (Figure 5A.b–c,
arrows). Visualizing the acrosome en face toward the microscope
camera showed a strong concentration of MYO6 in the center
of the actin-rich acroplaxome (Figure 5A.d). We next determined
the localization of TOM1/L2 and found that it was present in
the center of the acroplaxome (Figure 5B, arrows) in the same
region of the acroplaxome plate that was also positive for MYO6
(compare Figure 5B.a–d and A.a–d). Moreover, although TOM1/L2
was detectable in spermatids dissected from the sv/sv male mice
(Figure 5C.a–d), it was clearly mislocalized in the acroplaxome
plate of the MYO6-deficient spermatids (compare Figure 5C.a–c and
B.a–c, arrows).
TOM1/L2 contains a central GAT domain that binds ubiquitin
that enables interaction of TOM1/L2 with ubiquitinylated proteins
[29]. Indeed, we identified a strong signal for ubiquitin in the area
corresponding to the acrosomal sac both in the control and mutant
spermatids (Figure 5D). Interestingly, as observed for TOM1/L2,
the ubiquitin-positive acrosomal granule was asymmetrically docked
in the acroplaxome plate in sv/sv spermatids (Figure 5D.b). These
results suggest that TOM1/L2 binds to ubiquitinylated proteins
present in the membrane of the acrosomal granule to tether the
granule via MYO6 to actin filaments in the acroplaxome plate,
thereby anchoring the acrosomal granule in a central position within
the sac (see Figure 8).
Next, we investigated the actin organization at the acroplax-
ome by staining actin filaments using fluorescent phalloidin. No
obvious changes were observed in the F-actin network organization
overlying the upper part of the spermatid nucleus beneath the
acrosome in both control and mutant spermatids (Figure 5D.c–d).
Staining of actin filaments, however, allowed us to detect posi-
tion of the acrosomal granule attachment within the sac as a less
actin-dense area (Figure 5D.c–d, arrows) and to visualize the acro-
some asymmetry in sv/sv spermatids. Quantification of this abnor-
mal phenotype indicated an almost four-fold increase of acrosome
asymmetry in sv/sv spermatids in comparison to the control sper-
matids (6.47 ±1.94% of asymmetric sv/+acrosomes in comparison
to 24.62 ±2.01% of sv/sv acrosomes; P= 0.0004; n= 3 indi-
vidual experiments on a pair of sv/+and sv/sv littermates each)
(Figure 5E). In summary, the loss of MYO6 in developing mouse
spermatids leads to structural disruptions of the Golgi complex
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Myosin VI deficiency affects acrosomogenesis in mouse, 2020 7
Figure 3. The acrosomogenesis in MYO6-deficient spermatids is altered (continued). (A) Ultrastructural analysis of sv/+and sv/sv spermatids during
the consecutive phases of acrosomogenesis. ag, acrosomal granule; asterisks, apical ectoplasmic specialization; av, acrosomal vesicle; cy, cytoplasm; m,
mitochondria; n, nucleus; and white star, acroplaxome. All other indications are explained in the text. Bars 1 μm. (B) The graphs depicting the mean percentage
of sv/+and sv/sv cells that displayed the acrosome defects visualized with ultrastructural (EM) analysis. Counts were performed on at least four ultrathin
sections of seminiferous tubules from randomly chosen fragments of fixed testes from n= 3 independent experiments. Error bars indicate SD.∗∗∗∗P≤0.0001.
and affects formation and position of the acrosomal granule during
spermiogenesis.
Finally, we examined the exact localization of TOM1/L2
at the ultrastructural level in the developing acrosome using
immunogold labeling (Figure 6). Consistent with our previous
immunofluorescence results, we observed gold particles representing
TOM1/L2 localization below the acrosomal granule at the
position of the acroplaxome in control spermatids (Figure 6.a–a’,
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8P. Zakrzewski et al., 2020
Figure 4. MYO6 and its binding partner TOM1/L2 are present at the Golgi complex. (A) Immunofluorescence localization of MYO6 (red) and GM130 (green) in
sv/+and sv/sv round spermatids. Bars 2 μm. (B) Immunofluorescence localization of TOM1/L2 (TOM1 on images) (red) and GM130 (green) in sv/+and sv/sv
round spermatids. Bars 2 μm.
arrowheads). In MYO6-depleted spermatids, we saw a similar
pattern for TOM1/L2 localization at the acroplaxome, below the
asymmetrically docked acrosomal granule (Figure 6.b–b’), indicating
that in absence of MYO6, TOM1/L2 is still localized to this cellular
compartment. These results suggest that MYO6 is one of the
factors determining the correct positioning of this specific acrosomal
sub-compartment.
The loss of MYO6 does not affect sperm function
To determine whether loss of MYO6 expression in mouse testes
affects reproductive functions, we first assessed the overall morphol-
ogy of 3-month-old mouse testes, which showed that testes dissected
from sv/sv males were slightly smaller compared to the control
organs (Figure 7A). Indeed, the average weight of heterozygous
gonads was 102.50 ±2.67 mg [sample size (n) = 8] while for
homozygous mice this value was 91.13 ±5.36 mg [sample size
(n) = 8]. Thus, the weight of sv/sv testes was reduced by 11.09%
(P<0.0001) compared to the control organs (Figure 7B). However,
testis to body weight ratio of sv/sv males was significantly larger than
in control mice (sv/+=0.38±0.01, sv/sv =0.47±0.05; P= 0.0175;
n=4pairsofsv/+and sv/sv littermates), reflecting the difference
in body weight between control and mutant males (Figure 7C). It
should be noted that sv/sv mice are smaller and they have less body
fat compared to control mice.
Next, we determined whether structural defects observed
in the mutant developing spermatids impact on the sperm
production and evaluated the number of sperm collected from
the caudae epididymides of 3-month-old sv/+and sv/sv littermates
(Figure 7D). The production of total sperm and the sperm count
was reduced by 14.19% in the sv/sv males compared to the
control mice [sv/+= (1.55 ±0.19) ×107,samplesize(n)=14;
sv/sv = (1.33 ±0.26) ×107,samplesize(n) = 12; P= 0.0173].
We then analyzed the sperm head and acrosome structure of sv/sv
mice using Hoechst and lectin PNA conjugate staining. Although we
observed several spermatozoa with evident morphological defects in
the head/acrosome shapes (Figure 7E), our analysis did not reveal
any significant differences between mutant and control epididymal
sperm [Figure 7F;sv/+=3.00±0.50% and sv/sv =2.83±0.76%
of sperm with head/acrosome malformations; P= 0.7676; n=3
individual experiments on a pair of sv/+and sv/sv littermates each].
To test the acrosome function of the sv/sv spermatozoa in vitro,
we quantified the spontaneous acrosome reaction. We then treated
the sperm with the calcium ionophore A23187 which triggers
the acrosome reaction by increasing Ca2+concentration in the
sperm head (Figure 7G and H) and provides information about the
potential sperm fertilizing ability. Incubation in capacitation medium
caused a similar spontaneous acrosome reaction in epididymal sperm
from control and mutant mice (Figure 7H;sv/+= 36.80 ±4.91%,
sv/sv = 38.33 ±5.45%; P= 0.6922; n= 4 individual experiments on a
pair of sv/+and sv/sv littermates each) and also after exposure to the
calcium ionophore A23187 no significant changes between control
and mutant mice were observed (Figure 7H;sv/+= 87.18 ±5.46%,
sv/sv = 85.85 ±7.66%; P= 0.7876; n= 4 individual experiments on
apairofsv/+and sv/sv littermates each). Therefore, although the
number of epididymal spermatozoa in MYO6-deficient mice was
significantly reduced, their ability to perform the acrosomal reaction
was not impaired.
Discussion
We have previously shown [12] that MYO6 is present in sev-
eral actin-containing structures involved in mouse spermiogenesis,
including the Golgi complex and the acrosome–acroplaxome com-
plex (summarized in Figure 1). Here, we now demonstrate that
this unique motor protein is involved in maintaining the structural
integrity of these testis-specific structures that are crucial during the
acrosome biogenesis in mouse.
Loss of MYO6 disrupts organization of the Golgi
complex during acrosomogenesis
The actin cytoskeleton is required for the organization and function
of the Golgi complex and depolymerization of actin filaments
alters Golgi morphology and leads to dilation or fragmentation
of its cisternae and to changes in the number of Golgi-derived
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Myosin VI deficiency affects acrosomogenesis in mouse, 2020 9
Figure 5. MYO6 with its binding partner TOM1/L2 maintains the proper localization the acrosomal granule. (A) Immunofluorescence localization of MYO6 (red)
and actin (green) in sv/+spermatids during the consecutive phases of spermiogenesis. Bars 2 μm. (B) Immunofluorescence localization of TOM1/L2 (TOM1 on
images) (red) and actin (green) in sv/+spermatids during consecutive phases of spermiogenesis. Bars 2 μm. (C) Immunofluorescence localization of TOM1/L2
(TOM1 on images) (red) and actin (green) in sv/sv spermatids during consecutive phases of spermiogenesis. Bars 2 μm. (D) Immunofluorescence localization
of ubiquitin (red) and actin (green) in sv/+and sv/sv spermatids (a–b); and fluorescence visualization of the actin-rich acroplaxome (ax, green) in sv/+and sv/sv
spermatids (c–d, white arrows showing the acrosomal granule docking site). Bars 2 μm. (E) The graph depicting the mean percentage of cells that displayed
eccentric docking of the acrosomal granule. Counts were performed on >50 cells from n= 3 independent experiments. Error bars indicate SD. ∗∗∗P<0.001.
vesicles [30–32]. Several myosin motor proteins including MYO6
have been linked the Golgi complex and depletion of MYO6
causes changes in size of the Golgi complex and reductions in
post-Golgi membrane trafficking in several cell types [22,33–35].
Our recent ultrastructural study revealed that in wild-type mouse
spermatids MYO6 is present throughout the Golgi stack from
the cis- to the trans-side [12]. Here, we now show significant
defects in Golgi organization in MYO6-depleted spermatids,
including disorganized and incorrectly oriented Golgi complexes
with separated stacks, swollen cisternae, and atypical accumulation
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10 P. Zakrzewski et al., 2020
Figure 6. TOM1/L2 localizes to acroplaxome underneath the acrosomal granule in maturing spermatids. ag, acrosomal granule; cy, cytoplasm; and n, nucleus.
Bars 1 μm (a–b), 250 nm (a’–b’).
Figure 7. The loss of MYO6 results in the testis-associated phenotypic changes in Snell’s waltzer males. (A) Image showing freshly isolated testis from sv/+and
sv/sv males. Bar 0.5 cm. (B) Graph depicting the mean weight of sv/+(n=8)andsv/sv (n= 8) testis. Error bars indicate SD. ∗∗∗∗P≤0.0001. (C) Graph depicting
the mean testis to body weight ratio of sv/+(n=4)andsv/sv (n= 4) males. Error bars indicate SD. ∗P≤0.05. (D) Graph depicting the mean total number of
sperm isolated from sv/+(n= 14) and sv/sv (n= 12) caudae epididymides. Error bars indicate SD. ∗P≤0.05. (E) Fluorescence images showing normally (a)
and abnormally (b) shaped sperm heads stained with Hoechst and lectin PNA conjugate. Bar 2 μm. (F) Graph depicting the mean percentage of sperm with
head/acrosome malformation isolated from caudae epididymides of sv/+(n=3)andsv/sv (n= 3) males. Counts were performed on 200 sperm. Error bars
indicate SD. ns, non-significant difference. (G) Light microscope images showing the head of unreacted (a) and acrosome-reacted (b) sv/+sperm. Black arrow
shows the lack of acrosome staining after the acrosome reaction. Bar 2 μm. (H) The graph depicting the mean number of sv/+and sv/sv acrosome-reacted
sperm after a spontaneous and ionophore-induced acrosomal reaction. Counts were performed on >150 sperm from n= 4 independent experiments. Error bars
indicate SD. ns, non-significant difference.
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Myosin VI deficiency affects acrosomogenesis in mouse, 2020 11
Figure 8. The model of MYO6-TOM1/L2 function in the acrosome formation. In the acroplaxome, below the acrosomal membrane, MYO6 binds with its motor
domain to the actin filaments and with its tail to TOM1/L2. TOM1/L2interacts via GAT domain with ubiquitinated transmembrane receptors docking the acrosomal
granule in the center of the acrosome. In the absence of MYO6, the acrosomal granule is docked asymmetrical due to the displacement of TOM1/L2.
of vesicles between the separated cisternae. We therefore propose
a structural role for MYO6 in linking the Golgi membranes to the
surrounding actin filaments important for maintenance of Golgi
stack orientation and morphology. These changes in Golgi integrity
and morphology may also impair post-Golgi trafficking events
linked to acrosome formation during acrosomogenesis that involves
two distinct trafficking pathways—the anterograde pathway from
the trans-Golgi network and the retrograde route via the endocytic
pathway [28].
In mouse testes, the MYO6-small- and no-insert splice variants
are expressed [12],whichhavebeenshowntoplayaroleincargo
sorting and vesicle tethering and/or trafficking through the actin-
rich regions in eukaryotic cells [36–39]. However, there is little
evidence that MYO6 similar to myosin Va is involved in transport-
ing proacrosomal granules along the actin filaments [10,11]. We,
therefore, propose that loss of Golgi morphology, the separation of
the Golgi saccules, and the swelling of the Golgi cisternae observed
in MYO6-deficient mouse spermatids support an anchoring role for
MYO6 in the structural organization of the Golgi complex during
acrosome biogenesis. In this model, actin assembly regulated by
MYO6 and probably other ABPs provides the structural support that
maintains Golgi morphology.Loss of MYO6 disturbs Golgi integrity,
which may impact on the formation and transport of cargoes in the
secretory pathway to the nascent acrosome–acroplaxome complex.
MYO6 together with TOM1/L2 maintains the symmetry
of developing acrosome
Despite its importance in the fertilization process, very little is
known about the molecular basis of acrosome development, how the
acrosome is attached to the nuclear envelope, and how the acrosomal
granule is kept in its correct position. There are only a few mouse
mutants that show changes in the ectopic localization of the acroso-
mal granule similarly to sv/sv mice: these are Zpbp1−/−(zona pel-
lucida binding protein 1), Dpy19l2−/−(testis-specific member of an
uncharacterized gene family), Acrbp−/−(acrosin-binding protein),
and Pcsk4−/−(protein convertase subtilisin/kexin type 4) [40–43].
In these mutants, the asymmetric localization of the acrosomal gran-
ule may be caused by destabilization of the multi-layered structure
of the descending acrosome or the defective assembly/compaction of
the acrosomal matrix proteins. However, in contrast to these mutants
that show a more severe acrosome malformation, the observed
dysfunction in the sv/sv developing acrosome is restricted to the
asymmetric localization of the granule. The acrosome–acroplaxome
complex contains actin filaments and actin-based molecular motors
[3,10], including MYO6 as previously shown [12]. Our present
results suggest that MYO6 maintains the central position of the
acrosome/acrosomal granule by anchoring this organelle to the sper-
matid nucleus. This hypothesis is consistent with the role of MYO6
in tethering of membranes to cortical actin filaments during the
development of the intestinal brush border cells and the cochlear hair
cells in the mouse inner ear [44,45]. We also observed an elevated
number of proacrosmal vesicles in sv/sv spermatids. A similar defect
affecting the fusion of proacrosomal vesicles with the acrosome was
observed in TNAP-Atg7−/−mice, in which ATG7 was selectively
inactivated in germ cells [46]. The enzyme ATG7 lipidates the protein
LC3 and the membrane-associated LC3 is required for autophago-
some–lysosome fusion in the autophagy pathway. The acrosome is
a lysosome-related organelle and so the authors postulated that in
a similar way loss of ATG7 activity may cause defects in fusion of
proacrosomal vesicles with the acrosome. Thus, since loss of MYO6
has also been shown to lead to an accumulation of autophagosomes
due to a defect in autophagosome–lysosome fusion, the similar
phenotypes in the MYO6 and ATG7-KO may both be linked to a
function in the autophagy pathway [47].
Our results further show that not only MYO6 but also TOM1/L2
is present in the developing acrosome–acroplaxome complex in
mouse. The exact cellular function/s of this endocytic MYO6 adaptor
protein in mammals are less well understood; however, the MYO6-
TOM1/L2 complex is likely to play a role in endocytic cargo sorting
and has been shown to facilitate the maturation of autophagosomes
enabling their fusion with the lysosome [47]. In addition, MYO6
facilitates the tethering of early endosomes to cortical microfilaments
important for maturation of nascent endosomes and downstream
signaling events, which precedes the cargo processing in the early
endosomes [48]. Taken together these findings support an anchoring
role of MYO6 during the acrosome formation in mouse.A hypothet-
ical model of MYO6 action at the acrosome–acroplaxome complex
is summarized in Figure 8 .
Conclusions
The ultrastructural disruptions observed in MYO6-deficient sper-
matids in Snell’s waltzer mice suggests that in mammals, similar to
invertebrates, MYO6 plays an anchoring role during the key events
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12 P. Zakrzewski et al., 2020
of spermiogenesis by either organizing the actin cytoskeleton or by
tethering of different cargo/membranes to testis-specific actin struc-
tures. Other myosin motors, such as Va and VIIa, are also required
in mammalian spermiogenesis for membrane trafficking events dur-
ing acrosomogenesis and spermatid adhesion to the Sertoli cell
[10,11,49–51]. In contrast to Snell’s waltzer males, however, myosin
VIIa-deficient rat males show premature release of spermatids and
numerous defects in spermatozoa [51]. Although no morphological
defects were observed in sv/sv sperm, it is possible that spermatids
with abnormally formed acrosomes, as we observed in sv/sv mice,
are phagocytozed by Sertoli cells. This may explain the slightly
reduced number of sperm (this work) and lower fertility of sv/sv
males (our previous work [26]). Taken together our findings suggest
that the actin cytoskeleton, a number of different ABPs, and several
myosin motor proteins including MYO6 play highly specialized
sequential roles during the complex process of spermiogenesis in
mammals.
Supplementary data
Supplementary data are available at BIOLRE online.
Acknowledgments
We want to thank Dr Vira Chumak, Ms Małgorzata Topolewska,and Dr Liliia
Lehka from the Laboratory of Molecular Basis of Cell Motility (Nencki Insti-
tute of Experimental Biology, Polish Academy of Sciences, Warsaw, Poland) for
their invaluable help with animals and tissue harvesting and Dr Christopher
Batters from the Cambridge Institute for Medical Research (University of
Cambridge, Cambridge, UK) for critical reading of the manuscript.
Conflicts of Interest
The authors declare that no competing interests exist.
Author Contributions
P.Z., M.L., and F.B. conceived and designed the experiments. P.Z.
performed the experiments. P.Z., M.L., F.B., and M.J.R. analyzed the
data. P.Z., M.L., and F.B. wrote the paper.
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