Immunoelectron microscopic distribution of actin in hamster spermatids and epididymal, capacitated and acrosome-reacted spermatozoa

Ouabain-induced activation of phospholipase C zeta and its contributions to bovine sperm capacitation

The sperm-derived oocyte activating factor, phospholipase C zeta (PLC ζ), is the only PLC isoform reported in cattle. The objectives were to (1) localize PLC ζ in fresh and capacitated bovine sperm and (2) investigate the activation of PLC ζ during bull sperm capacitation and contributions of PLC activity to this process. We confirmed interaction of testis-specific isoform of Na/K-ATPase (ATP1A4) with PLC ζ (immunolocalization and immunoprecipitation) and tyrosine phosphorylation (immunoprecipitation) of PLC ζ (a post-translational protein modification commonly involved in activation of PLC in somatic cells) during capacitation. Furthermore, incubation of sperm under capacitating conditions upregulated PLC-mediated hyperactivated motility, tyrosine phosphoprotein content, acrosome reaction, and F-actin formation (flow cytometry), implying that PLC activity is enhanced during capacitation and contributing to these capacitation processes. In conclusion, we inferred that PLC ζ is activated during capacitation by tyrosine phosphorylation through a mechanism involving ATP1A4, contributing to capacitation-associated biochemical events.

Identification and cellular location of glutamine synthetase in human sperm

Glutamine synthetase (GS) catalyzes the de novo synthesis of glutamine, an amino acid that has been shown to influence sperm motility in mammals. To date, no information is available about GS content in human sperm. In this study, we have characterized the presence and cellular location of GS in fresh human normozoospermic samples. We have detected a single band corresponding to GS by Western blot. Confocal analysis has revealed GS immunoreactivity in the post-acrosomal head region. Moreover, double-labeling experiments with either F-actin or calicin have demonstrated GS confinement in the post-acrosomal region of the perinuclear theca. These data have been validated by a post-embedding ultra-structural study. The presence of GS in the post-acrosomal region of the perinuclear theca suggests that human sperm can carry out in glutamine synthesis.

Glucose-regulated protein precursor (GRP78) and tumor rejection antigen (GP96) are unique to hamster caput epididymal spermatozoa

The immotile testicular mammalian spermatozoon gets transformed into a motile spermatozoon during 'epididymal maturation'. During this process, the spermatozoa transit from the caput to the cauda epididymis and undergo a number of distinct morphological, biophysical and biochemical changes, including changes in protein composition and protein modifications, which may be relevant to the acquisition of motility potential. The present proteome-based study of the hamster epididymal spermatozoa of caput and cauda led to the identification of 113 proteins spots using Matrix-assisted laser desorption/ionization tandem mass spectrometry (MALDI-MS/MS) analysis. Comparison of these 113 protein spots indicated that 30 protein spots (corresponding to 20 proteins) were significantly changed in intensity. Five proteins were increased and eleven were decreased in intensity in the cauda epididymal spermatozoa. In addition, two proteins, glucose-regulated protein precursor (GRP78) and tumor rejection antigen (GP96), were unique to the caput epididymal spermatozoa, while one protein, fibrinogen-like protein 1, was unique to cauda epididymal spermatozoa. A few of the five proteins, which increased in intensity, were related to sperm metabolism and ATP production during epididymal maturation. The changes in intensity of a few proteins such as ERp57, GRP78, GP96, Hsp60, Hsp70, and dihydrolipoamide S-acetyltransferase were validated by immunoblotting. The present study provides a global picture of the changes in protein composition occurring during hamster sperm epididymal maturation, besides being the first ever report on the proteome of hamster spermatozoa.

Abnormal sperm in mice lacking the Taf7l gene

TFIID is a general transcription factor required for transcription of most protein-coding genes by RNA polymerase II. TAF7L is an X-linked germ cell-specific paralogue of TAF7, which is a generally expressed component of TFIID. Here, we report the generation of Taf7l mutant mice by homologous recombination in embryonic stem cells by using the Cre-loxP strategy. While spermatogenesis was completed in Taf7l(-/Y) mice, the weight of Taf7l(-/Y) testis decreased and the amount of sperm in the epididymides was sharply reduced. Mutant epididymal sperm exhibited abnormal morphology, including folded tails. Sperm motility was significantly reduced, and Taf7l(-/Y) males were fertile with reduced litter size. Microarray profiling revealed that the abundance of six gene transcripts (including Fscn1) in Taf7l(-/Y) testes decreased more than twofold. In particular, FSCN1 is an F-action-bundling protein and thus may be critical for normal sperm morphology and sperm motility. Although deficiency of Taf7l may be compensated in part by Taf7, Taf7l has apparently evolved new specialized functions in the gene-selective transcription in male germ cell differentiation. Our mouse studies suggest that mutations in the human TAF7L gene might be implicated in X-linked oligozoospermia in men.

Cytoskeleton localization in the sperm head prior to fertilization

Three major cytoskeletal proteins, actin, tubulin and spectrin, are present in the head of mammalian spermatozoa. Although cytoskeletal proteins are implicated in the regulation of capacitation and the acrosome reaction (AR), their exact role remains poorly understood. The aim of this study was to compare the distribution of the sperm head cytoskeleton before and after the AR in spermatozoa representing a range of acrosome size and shape. Spermatozoa from the human and three rodents (rat, hamster and grey squirrel) were fixed before and after the AR in appropriate medium in vitro. Indirect immunofluorescent localization of cytoskeletal proteins was undertaken with antibodies recognizing actin, spectrin and alpha-tubulin. Preparations were counterstained with propidium iodide and examined by epifluorescent and confocal microscopy. Our results clearly demonstrated changes in localization of cytoskeleton during the AR, mainly in the apical acrosome with further changes to the equatorial segment and post-acrosomal regions. The pattern of cytoskeletal proteins in the sperm head of all the species was similar in respect to various sub-compartments. These observations indicated that the sperm head cortical cytoskeleton exhibits significant changes during the AR and, therefore, support the image of cytoskeletal proteins as highly dynamic structures participating actively in processes prior to fertilization.

A perinuclear theca substructure is formed during epididymal guinea pig sperm maturation and disappears in acrosome reacted cells

The perinuclear theca (PT) is a unique cytoskeletal mammalian sperm structure that surrounds the nucleus. Using negatively stained whole-mount preparations, we detected a PT substructure on the apical region of the postacrosomal theca layer of guinea pig spermatozoa. The PT substructure consists of projections resembling eyelashes, circling the sperm head. The PT substructure was absent in caput but appeared in corpus epidydimal spermatozoa. The same finding was observed in sheep and rabbit spermatozoa. The PT substructure persisted in capacitating spermatozoa, but was absent in acrosome reacted gametes. No labeling of the PT substructure was observed by the immunogold technique using antibodies against calmodulin, spectrin, myosin, and vimentin. A 34-kDa band appeared as a possible PT substructure protein. The PT was positive to the antibodies and the presence of the above-mentioned proteins was confirmed by Western blot. F-actin gold label was observed in mature spermatozoa on the PT substructure base zone. Results using cytochalasin D and phalloidin point to a role of F-actin in the PT substructure formation/disassembly processes. Ca(2+), bicarbonate, and proteases might be involved in the mechanism of the substructure disassembly. Novel PT morphological changes occurring during sperm epidydimal maturation and at acrosome reaction, respectively, are discussed in relation to the PT stability and function.

Calcium-dependent actin filament-severing protein scinderin levels and localization in bovine testis, epididymis, and spermatozoa

We assessed the levels and localization of the actin filament-severing protein scinderin, in fetal and adult bovine testes, and in spermatozoa during and following the epididymal transit. We performed immunoblots on seminiferous tubules and interstitial cells isolated by enzymatic digestion, and on bovine chromaffin cells, spermatozoa, aorta, and vena cava. Immunoperoxidase labeling was done on Bouin's perfusion-fixed testes and epididymis tissue sections, and on spermatozoa. In addition, immunofluorescence labeling was done on spermatozoa. Immunoblots showed one 80-kDa band in chromaffin cells, fetal and adult tubules, interstitial cells, spermatozoa, aorta, and vena cava. Scinderin levels were higher in fetal than in adult seminiferous tubules but showed no difference between fetal and adult interstitial cells. Scinderin levels were higher in epididymal than in ejaculated spermatozoa. Scinderin was detected in a region corresponding with the subacrosomal space in the round spermatids and with the acrosome in the elongated spermatids. In epididymal spermatozoa, scinderin was localized to the anterior acrosome and the equatorial segment, but in ejaculated spermatozoa, the protein appeared in the acrosome and the post-equatorial segment of the head. In Sertoli cells, scinderin was detected near the cell surface and within the cytoplasm, where it accumulated near the base in a stage-specific manner. In the epididymis, scinderin was localized next to the surface of the cells; in the tail, it collected near the base of the principal cells. In Sertoli cells and epididymal cells, scinderin may contribute to the regulation of tight junctional permeability and to the release of the elongated spermatids by controlling the state of perijunctional actin. In germ cells, scinderin may assist in the shaping of the developing acrosome and influence the fertility of the spermatozoa.

Actin localization in ram spermatozoa: effect of freezing/thawing, capacitation and calcium ionophore-induced acrosomal exocytosis

We have analyzed, by immunofluorescence, the localization of actin in ram spermatozoa, its colocalization with the actin-binding protein, gelsolin, and the effect of freeze/thawing, in vitro capacitation, and induced acrosomal exocytosis on its distribution. The monoclonal anti-actin and anti-gelsolin antibodies used recognized single bands at 43,000 and 90,000 kDa, respectively. In all spermatozoa, intense actin staining was observed in the whole length of the flagellum and, depending on the protocol used, in the neck and postacrosomal region of the head. Comparison of three staining methods, together with the use of NBD-phallacidin, allowed us to characterize ram sperm actin as a monomeric, intracellular, membrane-associated protein. Gelsolin was also present in ram spermatozoa and precisely colocalized with actin. Processes involving alterations in membrane structure such as freezing/thawing, in vitro capacitation, and calcium ionophore-induced acrosomal exocytosis provoked changes in the exposure of actin to the antibody. This strongly suggests a physical association of this protein to the plasma membrane, most likely by its intracellular side. The possible role of actin in sperm function is discussed.

Immunocytochemical detection of actin and 53 kDa polypeptide in the epididymal spermatozoa of rat and mouse

BACKGROUND

Presence of immunocytochemically detectable actin in the rat and mouse sperm head has been enigmatic for years. In this study, we demonstrate actin in the perinuclear theca and show that the detection of actin epitopes in the rat and mouse epididymal spermatozoa can effectively be enhanced by pre-extraction of sperm cells with SDS.

METHODS

The study with one monoclonal and one polyclonal anti-actin antibody was carried out at conventional and confocal fluorescence and electron microscope level, and by immunoblotting of proteins isolated from the head and tail fractions.

RESULTS

In the head of the control methanol-acetone fixed rat spermatozoa, the polyclonal antibody gave a stronger immunostaining in the postacrosomal area and in the perforatorium than the monoclonal antibody. In the mouse sperm head, the monoclonal antibody labeled the ventral edge of the postacrosomal area and slightly the perforatorium, whereas the polyclonal antibody stained the entire perinuclear space. In the SDS-extracted spermatozoa, an intense postacrosomal and perforatorial labeling was obtained with both antibodies but, in particular in the rat spermatozoa, the middle lateral portion of the postacrosomal segment remained unlabeled. Sonication seemed to cause structural modifications which specifically impeded staining with the monoclonal antibody. Both antibodies detected actin in the basal plate and the monoclonal antibody in the neck. Amorphous matrix of the connecting piece showed immunogold labeling. In the tail, the monoclonal antibody recognized actin and a relatively basic 53 kDa polypeptide, whereas the polyclonal antibody reacted with several protein bands. SDS-soluble actin of the tail was addressed to the midpiece and the SDS-insoluble 53 kDa protein profoundly to the outer dense fibers of the principal piece.

CONCLUSIONS

Intense labeling of actin in the SDS-extracted rat and mouse spermatozoa was presumably due to the generated demasking of actin epitopes embedded in the perinuclear cytoplasm. The results are important in confirming that actin in the rat and mouse sperm head is not lost during spermiogenesis but apparently contributes to the three-dimensional packing of the mature perinuclear cytoplasm. This study further demonstrates the importance of the methods used in sample preparation and advantages of confocal microscopy when attempting to detect cytoskeletal proteins which, as in spermatozoa, may occur in small quantities.

Insertional mutation that causes acrosomal hypo-development: its relationship to sperm head shaping

A family of transgenic mice (OVE 219) was generated by microinjection of a tyrosinase minigene (Ty811C). The transgenic mice demonstrate an atypical and variable coat color pattern and the homozygous males show abnormalities of spermatogenesis that are variably expressed from animal to animal. Heterozygous mice proved to have normal spermatogenesis and along with non-transgenic mice were used as controls to study the abnormalities in spermatogenesis in OVE 219 homozygous males. These abnormalities shed light on the features controlling normal spermatogenesis. In some homozygous males early spermiogenesis was disrupted as the flagellar microtubules became disorganized within the flagellar process. What appeared to be crystalline tubulin was noted within some of the rounded flagellar processes. Sperm with this defect did not develop a flagellum. In other homozygous males defects were apparent by step 6 or 7 of spermiogenesis when the acrosome did not grow and spread over the nucleus as noted in control animals. The modified nuclear envelope underlying the acrosome continued to develop and spread well beyond one margin of the acrosome. Since the modified nuclear envelope grew independently of the acrosome, the acrosome was not the controlling factor in determining the spread of the modified nuclear envelope. Micrographs revealed that Sertoli ectoplasmic specialization failed to form over most regions of the spermatid head lacking a normal acrosome. In homozygous males, the manchette took origin (proximally) in close relation to the modified nuclear envelope and never in relation to the edge of the spreading acrosome, a feature indicating that manchette placement was influenced by the position of the modified nuclear envelope and not the edge of the acrosome. Thus the modification in the nuclear envelope may be the primary event to signal acrosomal spread and manchette development. In spermatids where the manchette developed from an ectopic site, the result was abnormal caudal head shaping. In some spermatids a portion of the manchette was lacking. When this occurred the caudal head was rounded in the region of the missing manchette. In a minority of spermatids there was no evidence for a manchette. The entire caudal head was gently rounded. These data support the growing body of evidence that the caudal sperm head is shaped, in part, by the manchette. The OVE 219 family of mice provides a useful model to understand the processes involved in periods of spermiogenesis that are critical to development of a normally shaped sperm head.

Differential distribution of glutamylated tubulin during spermatogenesis in mammalian testis

The distribution of glutamylated tubulin has been analyzed in mammalian testis using the specific mAb GT335 by immunoelectron microscopy and immunoblotting. In spermatozoa of various species, immunogold labeling showed the presence of glutamylated tubulin in all of the microtubules of axoneme and centrioles, whereas the microtubule network of the spermatid manchette was unlabeled. In earlier germ cells, centriole was the only microtubule structure to be labeled. A similar distribution was observed using the anti-acetylated tubulin antibody (6-11B-1), confirming previous results of Hermo et al. [Anat. Rec. 229:31-50, 1991]. However, among testicular somatic cells, microtubules of some Sertoli cell branches were not acetylated but glutamylated. 2-D PAGE of mouse and hamster sperm extracts showed a high level of alpha and beta-tubulin heterogeneity, comparable to that found in brain. Immunoblotting with GT335 revealed a large amount of glutamylated tubulin resolved into numerous alpha as well as beta-tubulin isoforms. This suggests that the major testis-specific tubulin isotypes (m alpha 3/7 and m beta 3) are also glutamylatable. These results show a subcellular sorting of posttranslationally modified tubulin isoforms in spermatids, glutamylation being associated with the most stable microtubule structures.

Actin in stratified squamous keratinized epithelium

Actin filament distribution patterns were revealed in a stratified squamous keratinized epithelium using phalloidin-fluorescent and immunogold labeling techniques applied on bovine ruminal pilar as a model tissue. In non-keratinized cell types, actin concentrates on the microfilament-rich cellular cortex as well as on cytoplasmic processes and protrusions. In cornified cells labeling is distributed diffusely over the amorphous cytoplasm. A constant feature in all cell types is plasmalemmal labeling. Desmosomes exhibit deposition on their plasmalemmal leaflets, the dense central stratum and plaques. Desmosomal as well as cytoplasmic keratin filament bundles also label for actin, the latter often in a cross-banded manner. These cellular distribution patterns of actin filaments are discussed with respect to the significance of the microfilaments in the process of cell shape determination, stratification, and cell adhesion.

Actin polymerization in boar spermatozoa: fertilization is reduced with use of cytochalasin D

The aggregational state of actin in boar spermatozoa after capacitation and the acrosome reaction has been examined by several methods. In vitro fertilization (IVF) experiments were conducted in the presence and absence of cytochalasin D (CD) to evaluate the role of actin polymerization in the events of fertilization. The fertilizing capacity was very high in controls, but, when CD (an inhibitor of the polymerization of actin) was added to the capacitation medium, there was a marked decrease in the fertilizing capacity of the boar spermatozoa. There was a further decrease when CD was present during both capacitation and fertilization processes. In addition to the IVF tests, biochemical and immunoelectron microscopic methods were used to analyze the state of aggregation of actin in boar spermatozoa after capacitation, and the acrosome reaction. By immunoelectron microscopy with a phalloidin probe, there were no gold particles, indicating the presence of F-actin on boar sperm heads capacitated and acrosome-reacted in media containing CD. By sodium dodecyl sulfate-polyacrylamide gel electrophoresis there were differences in NP-40 solubility, reflecting actin polymerization, between CD-treated and untreated sperm. These results suggest that actin polymerizes during capacitation and the acrosome reaction and that this polymerization is essential to the fertilization process.

Actin filaments in the ruminal epithelium of cattle

Phalloidin and immunogold labeling were used to localize actin filaments in the ruminal pilar epithelium of adult cattle on the light and electron microscopic level. Incubation with rhodaminyl-phalloidin permitted an overview of general actin distribution pattern. Distinct subcellular localization of actin was revealed with the biotin-streptavidin bridge technique in a postembedding procedure. Superior fixation of membranes and filaments was achieved with tannic acid-glutaraldehyde. Surface-etching prior to immunoincubation ensured restoration of actin immunoreactivity. Patterns of distribution in non-keratinized and keratinized epithelial cells as well as in pericytes and endothelial cells point to cell-specific cytoskeletal and motile functions of actin filaments.

F-actin in acrosome-reacted boar spermatozoa

Biochemical and immunoelectron microscopic methods have been used to analyze the distribution of actin in boar spermatozoa and its state of aggregation before and after acrosome reaction. F-actin was detected on sperm head and tail by electron microscopy using an improved phalloidin probe: incubation with a fluorescein-phalloidin complex and an anti-fluorescein antibody, followed by labeling with protein A-gold complex. Gold particles, indicating the presence of F-actin, were localized on the sperm surface of the acrosome-reacted spermatozoa. Specific labeling was localized (1) between the outer acrosomal membrane and the plasma membrane in the equatorial region, (2) between the outer surface of the fibrous sheath and the plasma membrane in the postacrosomal region, (3) around the connecting piece and the neck region, and (4) on the external surface of the fibrous sheath in the principal piece of the tail. Furthermore, after NP-40 extraction, the SDS-PAGE revealed a difference in solubility between reacted and unreacted boar spermatozoa, reflecting actin polymerization. We conclude that most actin in the acrosome reacted boar spermatozoa is polymeric.

Localization of actin, alpha-actinin, and tropomyosin in bovine spermatozoa and epididymal epithelium

Actin, alpha-actinin, and tropomyosin were localized in the testicular, epididymal, and ejaculated spermatozoa and in the epithelium of the bovine epididymis by means of specific antibodies using an indirect immunofluorescence technique. Immunocytochemical results were confirmed by the western blot analysis. Independent of the method of fixation, washing, or sonication, actin, alpha-actinin, and tropomyosin were all consistently localized in the neck of the spermatozoa. Actin and tropomyosin present in the postacrosomal area could be removed by sonication, whereas alpha-actinin in the basal plate appeared to be resistant to the treatment. In the unwashed spermatozoa alpha-actinin-specific immunofluorescence was seen over the acrosomal area, whereas in the washed sperm it appeared as a narrow cap at the margin of the head. In the latter location, its distribution was similar to that of tropomyosin. In the majority of preparations, tropomyosin could be localized in the principal piece of the tail. Even though some actin-specific immunofluorescence could be identified in the principal piece of the tail of the testicular and epididymal spermatozoa, a strong immunoreaction appeared only in the ejaculated spermatozoa. In the principal cells of the epididymal epithelium, specific fluorescence for actin, alpha-actinin, and tropomyosin occurred in the apical junctional complex. Basal bodies of the solitary cilia of the epididymal epithelium were labelled with antitropomyosin and anti-alpha-actinin antibodies. Besides offering new information about the cytoskeletal composition of the mammalian sperm, the present results support the hypothesized homology between the connecting piece of the sperm neck and the basal body of the cilia.

Species-specific localization of actin in mammalian spermatozoa: fact or artifact?

Actin has been characterized and localized in sperm cells of many mammals. Nevertheless, the reported localizations obtained by different methods and/or antibodies varied from species to species and even for the same species. To clarify the question, sperm actin distribution was reinvestigated under uniform technical conditions. Immunogold post-embedding procedures were performed using a polyclonal and two monoclonal antibodies of known specificity to localize actin in spermatids and spermatozoa of rabbit, mouse, rat, monkey, and human. In these species, actin (F-actin) was detected with the three antibodies between the nucleus and the acrosome of round and elongating spermatids. Species-specific changes occurred in maturing spermatids. In the rabbit, actin labeling decreased and disappeared from the tip to the base of the subacrosomal layer. In testicular and epididymal spermatozoa actin was detected only with a monoclonal antibody (Amersham) successively in the neck, postacrosomal area, and subacrosomal bulges. In mouse late spermatids a transitory labeling of the neck was detected only with the polyclonal antiactin. In testicular and epididymal spermatozoa an actin labeling was observed in the principal piece of the tail. In rat, monkey, and human sperm cells actin remained undetected. These results suggest that there is a redistribution of actin in late spermatids and spermatozoa which is a species-specific process but not an artifact of methodological origin. Thus, a function for actin in sperm, if any, remains to be demonstrated.

Perinuclear cytoskeleton of acrosome-less spermatids in the blind sterile mutant mouse

The perinuclear cytoskeleton of mammalian spermatids is thought to play a major role in nucleus-acrosome association and in shape changes of the head during spermiogenesis. To test these hypotheses acrosome-less spermatids in blind-sterile mutant mice were investigated for the development of the subacrosomal layer. Immunogold procedures were used for the detection of actin and calmodulin. In addition to various other abnormalities many acrosome-less round and elongating spermatids developed a subacrosomal layer with an actin and calmodulin distribution similar to that observed in normal spermatids. However, in mutant elongating spermatids the apical part of the nucleus was truncated and/or folded. The expected elongation and shaping of the nucleus only occurred in its caudal part associated with an hypertrophied and somewhat ectopic manchette. These abnormalities and those previously observed in mutant and experimental models indicated that the subacrosomal layer may form independently of the acrosome. It is suggested that the subacrosomal filamentous actin is a transitory scaffolding which might be involved in the assemblage of other proteins of the perinuclear cytoskeleton. However, by itself, this layer is not sufficient to ensure a normal shaping of the nucleus. Acrosome-nucleus interactions mediated by the subacrosomal layer seem necessary to shape the cranial spermatid head. The manchette appears to be involved only in the caudal nuclear shaping.

Sperm actin and calmodulin during fertilization in the hamster: an immune electron microscopic study

The distribution of actin and CaM in hamster spermatozoa was examined during the early events of fertilization using postembedding immunogold procedures. Actin was immunolocalized with a polyclonal antibody and two monoclonal antibodies. CaM was immunodetected with a polyclonal antibody. In epididymal sperm, actin labeling was found solely in the principal piece of the flagellum. CaM labeling was observed in the postacrosomal lamina, subacrosomal ring, and tip of the perforatorium. These distributions were not modified after capacitation and acrosome reaction. During the successive steps of sperm-egg fusion actin remained undetected in the sperm head whereas its location did not change in the flagellum. CaM distribution remained unmodified until the sperm head begins to decondense. At later stages of sperm head decondensation the postacrosomal lamina and its CaM labeling disappeared, whereas gold particles were still detected in the subacrosomal layer. The predominant location of actin into the egg cortex, particularly the microvillus-free area was confirmed. Except for the CaM labeling of the meiotic spindle, no special CaM location could be found throughout the egg. Thus, in hamster, a role for sperm actin in sperm-egg fusion appears unlikely. In contrast the CaM present in the Ca(2+)-rich postacrosomal lamina could be involved in the regulation of egg activation.

Distribution of actin isoforms within cells of the seminiferous epithelium of the rat testis: evidence for a muscle form of actin in spermatids

Recently, a cDNA that coded for an enteric smooth muscle gamma-actin (SMGA) that was expressed in post-meiotic mouse testicular cells was identified. To determine the cellular location(s) of the protein encoded by this cDNA, this SMGA was probed for by immunocytochemistry in the cells of the seminiferous epithelium with two different monoclonal antibodies (Mabs), B4 and HUC 1-1, known to be muscle actin selective. As a control, we also examined the immunoreactivity of a third Mab, C4, that reacts with all non-muscle and muscle vertebrate isoactins. Using light and electron microscopy, a progressive increase in immunolabeling was observed with the muscle selective HUC 1-1 Mab over a loose actin filamentous network distributed throughout the cytoplasm of steps 4-16 spermatids. Thereafter, the labeling decreased such that at step 17 spermatids, only cytoplasmic labeling in the tail of the spermatids was observed. No labeling of this network was noted with the C4 or B4 Mabs. However, myoid cells enveloping seminiferous tubules and smooth muscle cells of interstitial blood vessels demonstrated comparable intense labeling with each of the three Mabs. The C4 Mab intensely labeled actin filaments of the Sertoli-Sertoli and Sertoli-spermatid ectoplasmic specializations. Also well labeled were numerous actin filaments found in the apical Sertoli cell processes encapsulating the heads of late step 19 spermatids at stage VII of the cycle of the seminiferous epithelium. In addition, actin filamentous bundles enveloping tubulobulbar complexes of the late spermatids within the Sertoli cell apical processes were intensely labeled. The actin filaments in the Sertoli apical processes and surrounding the tubulobulbar complexes were also strongly immunolabeled with the HUC 1-1 Mab. The C4 Mab but not the B4 or HUC 1-1 Mabs, recognized actin in the subacrosomal space of steps 4-18 spermatids. This study suggests that there are muscle isoforms of actin within the cytoplasm of developing spermatids and within apical processes of Sertoli cells.

Localization of calmodulin in perinuclear structures of spermatids and spermatozoa: a comparison of six mammalian species

The distribution of Calmodulin was examined during spermiogenesis and sperm epididymal maturation in rabbit, hamster, mouse, rat, monkey, and human. An affinity-purified antibody to Calmodulin was used to characterize this protein in sperm extracts by immunoblot analysis. Post-embedding immunogold procedures were used to localize Calmodulin at the ultrastructural level. The pattern of Calmodulin distribution was similar in the six species studied. A diffuse labeling was observed in round spermatids. Gold particles accumulated first in the subacrosomal layer of elongating spermatids. The perinuclear ring was also labeled. During the maturation phase of spermatids, Calmodulin labeling extended to the postacrosomal sheath. Dramatic changes occurred at spermiation so that in testicular sperm Calmodulin immunostaining was predominant in the postacrosomal sheath. Some labeling was still detected in restricted areas of the subacrosomal layer. This feature varied from species to species. Calmodulin location did not change during sperm epididymal maturation. A role for Calmodulin in the control of manchette development and regulation of subacrosomal actin aggregation state during spermiogenesis is proposed. The unique location of Calmodulin in the postacrosomal sheath of all species that have been studied in this work, together with the known presence of calcium in this area suggest a pivotal role for Calmodulin in sperm-egg fusion process.