Regional differences in composition of the perforatorium and outer periacrosomal layer of the rat spermatozoon as revealed by immunocytochemistry

Sperm-borne glutathione-S-transferase omega 2 accelerates the nuclear decondensation of spermatozoa during fertilization in mice†

The postacrosomal sheath (PAS) of the perinuclear theca (PT) is the first compartment of the sperm head to solubilize into the ooplasm upon sperm-oocyte fusion, implicating its constituents in zygotic development. This study investigates the role of one such constituent, glutathione-S-transferase omega 2 (GSTO2), an oxidative-reductive enzyme found in the PAS and perforatorial regions of the PT. GSTO2 uses the conjugation of reduced glutathione, an electron donor shown to be compulsory in sperm disassembly within the ooplasm. The proximity of GSTO2 to the condensed sperm nucleus led us to hypothesize that this enzyme may facilitate nuclear decondensation by reducing disulfide bonds before the recruitment of GSTO enzymes from within the ooplasm. To test this hypothesis, we utilized a cell permeable isozyme-specific inhibitor, which fluoresces when bound to the active site of GSTO2, to functionally inhibit spermatozoa before performing intracytoplasmic sperm injections (ICSI) in mice. The technique allowed for targeted inhibition of solely PT-residing GSTO2, as all that is required for complete zygotic development is the injection of the mouse spermatozoon head. ICSI showed that inhibition of PT-anchored GSTO2 caused a delay in sperm nuclear decondensation, and further resulted in untimely embryo cleavage, and an increase in fragmentation beginning at the morula stage. The confounding effects of these developmental delays ultimately resulted in decreased blastocyst formation. This study implicates PT-anchored GSTO2 as an important facilitator of nuclear decondensation and reinforces the notion that the PAS-PT is a critical sperm compartment harboring molecules that facilitate zygotic development.

The perforatorium and postacrosomal sheath of rat spermatozoa share common developmental origins and protein constituents†

The perinuclear theca (PT) is a cytosolic protein capsule that surrounds the nucleus of eutherian spermatozoa. Compositionally, it is divided into two regions: the subacrosomal layer (SAL) and the postacrosomal sheath (PAS). In falciform spermatozoa, a third region of the PT emerges that extends beyond the nuclear apex called the perforatorium. The formation of the SAL and PAS differs, with the former assembling early in spermiogenesis concomitant with acrosome formation, and the latter dependent on manchette descent during spermatid elongation. The perforatorium also forms during the elongation phase of spermiogenesis, suggesting that like the PAS, its assembly is facilitated by the manchette. The temporal similarity in biogenesis between the PAS and perforatorium led us to compare their molecular composition using cell fractionation and immunodetection techniques. Although the perforatorium is predominantly composed of its endemic protein FABP9/PERF15, immunolocalization indicates that it also shares proteins with the PAS. These include WBP2NL/PAWP, WBP2, GSTO2, and core histones, which have been implicated in early fertilization and zygotic events. The compositional homogeny between the PAS and perforatorium supports our observation that their development is linked. Immunocytochemistry indicates that both PAS and perforatorial biogenesis depend on the transport and deposition of cytosolic proteins by the microtubular manchette. Proteins translocated from the manchette pass ventrally along the spermatid head into the apical perforatorial space prior to PAS deposition in the wake of manchette descent. Our findings demonstrate that the perforatorium and PAS share a mechanism of developmental assembly and thereby contain common proteins that facilitate fertilization.

HIPK4 is essential for murine spermiogenesis

Mammalian spermiogenesis is a remarkable cellular transformation, during which round spermatids elongate into chromatin-condensed spermatozoa. The signaling pathways that coordinate this process are not well understood, and we demonstrate here that homeodomain-interacting protein kinase 4 (HIPK4) is essential for spermiogenesis and male fertility in mice. HIPK4 is predominantly expressed in round and early elongating spermatids, and Hipk4 knockout males are sterile, exhibiting phenotypes consistent with oligoasthenoteratozoospermia. Hipk4 mutant sperm have reduced oocyte binding and are incompetent for in vitro fertilization, but they can still produce viable offspring via intracytoplasmic sperm injection. Optical and electron microscopy of HIPK4-null male germ cells reveals defects in the filamentous actin (F-actin)-scaffolded acroplaxome during spermatid elongation and abnormal head morphologies in mature spermatozoa. We further observe that HIPK4 overexpression induces branched F-actin structures in cultured fibroblasts and that HIPK4 deficiency alters the subcellular distribution of an F-actin capping protein in the testis, supporting a role for this kinase in cytoskeleton remodeling. Our findings establish HIPK4 as an essential regulator of sperm head shaping and potential target for male contraception.

Leydig cells express the FABP9 in human testis

BACKGROUND

Previous studies have shown that the FABP9/PERF15 gene is expressed in mice and in some other mammals in the testicles and in the spermatozoa, and its possible effect on the ability of the sperm to form and maintain the nucleus until fertilization.

OBJECTIVE

Since the FABP9 homologue gene exists in humans, and so far no research has been done to indicate the exact location of this gene in the organism, it is necessary to find a better interpretation of its possible performance by its localization in the testis.

MATERIAL AND METHODS

Biopsied testicular tissue samples after sectioning and embedding on class slide were subjected to IHC with specific monoclonal antibody and underwent final staining with hematoxylin and eventually evaluated by light microscope.

RESULTS

The antibody could successfully bind and detect its related protein, FABP9, in Leydig cells rather than spermatogenic cells.

CONCLUSION

The expression of FABP9 in a different cell type rather than spermatogenic cells in other mammals, reports of a plausible different function for the gene product like its involvement in fertility potential in homo sapiens.

Phospholipase C zeta and calcium oscillations at fertilisation: The evidence, applications, and further questions

Oocyte activation is a fundamental event at mammalian fertilisation, initiated by a series of characteristic calcium (Ca²⁺) oscillations in mammals. This characteristic pattern of Ca²⁺ release is induced in a species-specific manner by a sperm-specific enzyme termed phospholipase C zeta (PLCζ). Reduction or absence of functional PLCζ within sperm underlies male factor infertility in humans, due to mutational inactivation or abrogation of PLCζ protein expression. Underlying such clinical implications, a significant body of evidence has now been accumulated that has characterised the unique biochemical and biophysical properties of this enzyme, further aiding the unique clinical opportunities presented. Herein, we present and discuss evidence accrued over the past decade and a half that serves to support the identity of PLCζ as the mammalian sperm factor. Furthermore, we also discuss the potential novel avenues that have yet to be examined regarding PLCζ mechanism of action in both the oocyte, and the sperm. Finally, we discuss the advances that have been made regarding the clinical therapeutic and diagnostic applications of PLCζ in potentially treating male infertility as a result of oocyte activation deficiency (OAD), and also possibly more general cases of male subfertility.

Thiol changes during epididymal maturation: a link to flagellar angulation in mouse spermatozoa?

Caput epididymal wild-type spermatozoa and cauda epididymal spermatozoa from mice null for the adenylyl cyclase Adcy10 gene are immotile unless stimulated by a membrane-permeant cyclic AMP analogue. Both types of spermatozoa exhibit flagellar angulation where the head folds back under these conditions. As sperm proteins undergo oxidation of sulfhydryl groups and the flagellum becomes more stable to external forces during epididymal transit, we hypothesized that ADCY10 is involved in a mechanism regulating flagellar stabilization. Although no differences were observed in global sulfhydryl status between caput and cauda epididymal spermatozoa from wild-type or Adcy10-null mice, two-dimensional fluorescence difference gel electrophoresis was performed to identify specific mouse sperm proteins containing sulfhydryl groups that became oxidized during epididymal maturation. A-kinase anchor protein 4, fatty acid-binding protein 9 (FABP9), glutathione S-transferase mu 5 and voltage-dependent anion channel 2 exhibited changes in thiol status between caput and cauda epididymal spermatozoa. The level and thiol status of each of these proteins were quantified in wild-type and Adcy10-null cauda epididymal spermatozoa. No differences in the abundance of any protein were observed; however, FABP9 in Adcy10-null cauda epididymal spermatozoa contained fewer disulfide bonds than wild-type sperm cells. In caput epididymal spermatozoa, FABP9 was detected in the cytoplasmic droplet, principal piece, midpiece, and non-acrosomal area of the head. However, in cauda epididymal spermatozoa, this protein localized to the perforatorium, post-acrosomal region and principal piece. Together, these results suggest that thiol changes during epididymal maturation have a role in the stabilization of the sperm flagellum.

Spermatozoa of the old endemic rodents of Australia - the possible functional significance of their ventral processes

Spermatozoa of the plains mouse (Pseudomys australis), like those of most Australian old endemic rodents, contain, in addition to an apical hook, two further processes that extend from the upper concave surface of the head, the ventral processes. This study shows that these processes contain thiol-rich cytoskeletal proteins, which presumably help to maintain their rigidity during sperm transport, together with the overlying cell membrane having abundant intramembranous proteins. To determine the possible functional significance of these processes, an in vitro study of spermatozoon-zona binding was undertaken. The findings suggest that initial sperm binding occurs by way of the cell membrane over the acrosome of the apical hook and that, subsequently, the lateral surfaces of the ventral processes also become tightly bound to the zona matrix. These ventral processes may therefore have evolved to increase sperm adhesion to the outer zona surface and/or to enhance stabilisation of the spermatozoon at the time of zona binding and initial penetration of the egg coat.

Variational modularity at the cell level: insights from the sperm head of the house mouse

Background

Modularity is an important feature in the evolvability of organisms, since it allows the occurrence of complex adaptations at every single level of biological systems. While at the cellular level the modular organization of molecular interactions has been analyzed in detail, the phenotypic modularity (or variational modularity) of cell shape remains unexplored. The mammalian spermatozoon constitutes one of the most complex and specialized cell types found in organisms. The structural heterogeneity found in the sperm head suggests an association between its inner composition, shape and specificity of function. However, little is known about the extent of the connections between these features. Taking advantage of the house mouse sperm morphology, we analyzed the variational modularity of the sperm head by testing several hypotheses related to its structural and functional organization. Because chromosomal rearrangements can affect the genotype-phenotype map of individuals and thus modify the patterns of covariation between traits, we also evaluate the effect of Robertsonian translocations on the modularity pattern of the sperm head.

Results

The results indicated that the house mouse sperm head is divided into three variational modules (the acrosomal, post-acrosomal and ventral spur module), which correspond to the main regions of the cytoskeletal mesh beneath the plasma membrane, i.e., the perinuclear theca. Most of the covariation is concentrated between the ventral spur and the acrosomal and post-acrosomal modules. Although the Rb fusions did not alter the main modularity pattern, they did affect the percentages of covariation between pairs of modules.

Conclusions

The structural heterogeneity of the cytoskeleton is responsible for the modular organization of the sperm head shape, corroborating the role that this structure has in maintaining the cell shape. The reduction in percentages of shape covariation between pairs of modules in Rb sperms suggests that chromosomal rearrangements could induce changes in the genotype-phenotype map. Nevertheless, how these variations affect sperm fertilization success is yet to be elucidated.

Mice lacking FABP9/PERF15 develop sperm head abnormalities but are fertile

The male germ cell-specific fatty acid-binding protein 9 (FABP9/PERF15) is the major component of the murine sperm perforatorium and perinuclear theca. Based on its cytoskeletal association and sequence homology to myelin P2 (FABP8), it has been suggested that FABP9 tethers sperm membranes to the underlying cytoskeleton. Furthermore, its upregulation in apoptotic testicular germ cells and its increased phosphorylation status during capacitation suggested multiple important functions for FABP9. Therefore, we investigated specific functions for FABP9 by means of targeted gene disruption in mice. FABP9(-/-) mice were viable and fertile. Phenotypic analysis showed that FABP9(-/-) mice had significant increases in sperm head abnormalities (~8% greater than their WT cohorts); in particular, we observed the reduction or absence of the characteristic structural element known as the "ventral spur" in ~10% of FABP9(-/-) sperm. However, deficiency of FABP9 affected neither membrane tethering to the perinuclear theca nor the fatty acid composition of sperm. Moreover, epididymal sperm numbers were not affected in FABP9(-/-) mice. Therefore, we conclude that FABP9 plays only a minor role in providing the murine sperm head its characteristic shape and is not absolutely required for spermatogenesis or sperm function.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 2: changes in spermatid organelles associated with development of spermatozoa

Spermiogenesis is a long process whereby haploid spermatids derived from the meiotic divisions of spermatocytes undergo metamorphosis into spermatozoa. It is subdivided into distinct steps with 19 being identified in rats, 16 in mouse and 8 in humans. Spermiogenesis extends over 22.7 days in rats and 21.6 days in humans. In this part, we review several key events that take place during the development of spermatids from a structural and functional point of view. During early spermiogenesis, the Golgi apparatus forms the acrosome, a lysosome-like membrane bound organelle involved in fertilization. The endoplasmic reticulum undergoes several topographical and structural modifications including the formation of the radial body and annulate lamellae. The chromatoid body is fully developed and undergoes structural and functional modifications at this time. It is suspected to be involved in RNA storing and processing. The shape of the spermatid head undergoes extensive structural changes that are species-specific, and the nuclear chromatin becomes compacted to accommodate the stream-lined appearance of the sperm head. Microtubules become organized to form a curtain or manchette that associates with spermatids at specific steps of their development. It is involved in maintenance of the sperm head shape and trafficking of proteins in the spermatid cytoplasm. During spermiogenesis, many genes/proteins have been implicated in the diverse dynamic events occurring at this time of development of germ cells and the absence of some of these have been shown to result in subfertility or infertility.

Male infertility caused by spermiogenic defects: lessons from gene knockouts

Spermiogenesis refers to the process by which postmeiotic spermatids differentiate into elongated spermatids and eventually spermatozoa. During spermiogenesis, round spermatids undergo dynamic morphologic changes, which include nuclear condensation and elongation, formation of flagella and acrosome, reorganization of organelles and elimination of cytoplasm upon spermiation. This cellular differentiation process is unique to male haploid germ cells, which may explain why approximately half of the testis-specific genes are exclusively expressed in spermiogenesis. The spermiogenesis-specific expression implies that these genes contribute to either structural or functional aspects of future sperm. Many such genes have been inactivated in mice and some of these gene knockout mice display male infertility due to nonfunctional sperm which display no or various degrees of structural abnormalities. Since the majority of these spermiogenesis-specific genes are highly conserved between mice and humans, findings from knockout mouse studies may be applicable to human infertility. Here, I briefly review some of these spermatid-specific gene knockouts. The mouse studies strongly suggest that sperm quality rather than quantity is a better indicator of male fertility and novel assays should be developed to determine sperm functionality.

Temporal deposition and spatial distribution of cytoskeletal proteins in the sperm head of an Australian rodent

The Australian murine rodent, the plains mouse (Pseudomys australis), possesses a highly complex sperm head, in which there are, in addition to an apical hook, two ventral processes that extend from its upper concave surface. The present study set out to determine the temporal deposition and distribution of the proteins within these structures during late spermiogenesis by light and electron microscopy using various antibodies to bull and laboratory rat sperm-head cytoskeletal proteins. The findings show that there are two phases of protein deposition. In the first phase, perinuclear theca proteins are deposited at the base of the ventral processes around the acrosomal extensions of the developing spermatids. In the second phase, as the ventral processes expand, actin and then perforatorial proteins are laid down during which time the processes become progressively more bilaterally flattened. These various proteins are moulded together to give rise to the two very large cytoskeletal structures that extend from the upper concave surface of the sperm head. They may be involved in binding the spermatozoon to the outer surface of the zona pellucida and/or in aiding the spermatozoon in zona penetration at the time of fertilisation.

Quantification of PERF 15 mRNA in tissue sections from rat testes

We previously conducted basic research to quantify in situ hybridization (ISH) signals in rat testes. In this experimental model, we selected ribosomal RNA (rRNA) as the hybridizable RNA in paraffin sections, since it allowed us to easily analyze ISH signals expressed with digoxygenin (DIG)-labeled probes quantitatively through “posterization” of the images. We applied this method to analyze the quantification of transcript, PERF 15 mRNA. PERF 15 is expressed specifically in the testes and localized in the rigid cytoskeletal structure of the sperm head, and has been considered to be involved in the apoptotic process of spermatogenic cells. Quantification of the signals may help to clarify the detailed function of PERF 15. We further analyzed the signals concomitant with a confocal laser scanning microscope. The peak of PERF 15 mRNA expression was found in diplotene spermatocytes, and the amount of PERF 15 mRNA was greatest in late pachytene and diplotene spermatocytes and early spermatids, followed by early pachytene spermatocytes, and then late spermatids. PERF 15 may be involved in the events leading to meiotic division, in which apoptosis is also involved. The present study may help to determine the concentration of mRNA in tissue sections.

Segregation of micron-scale membrane sub-domains in live murine sperm

Lipid rafts, membrane sub-domains enriched in sterols and sphingolipids, are controversial because demonstrations of rafts have often utilized fixed cells. We showed in living sperm that the ganglioside G(M1) localized to a micron-scale membrane sub-domain in the plasma membrane overlying the acrosome. We investigated four models proposed for membrane sub-domain maintenance. G(M1) segregation was maintained in live sperm incubated under non-capacitating conditions, and after sterol efflux, a membrane alteration necessary for capacitation. The complete lack of G(M1) diffusion to the post-acrosomal plasma membrane (PAPM) in live cells argued against the transient confinement zone model. However, within seconds after cessation of sperm motility, G(M1) dramatically redistributed several microns from the acrosomal sub-domain to the post-acrosomal, non-raft sub-domain. This redistribution was not accompanied by movement of sterols, and was induced by the pentameric cholera toxin subunit B (CTB). These data argued against a lipid-lipid interaction model for sub-domain maintenance. Although impossible to rule out a lipid shell model definitively, mice lacking caveolin-1 maintained segregation of both sterols and G(M1), arguing against a role for lipid shells surrounding caveolin-1 in sub-domain maintenance. Scanning electron microscopy of sperm freeze-dried without fixation identified cytoskeletal structures at the sub-domain boundary. Although drugs used to disrupt actin and intermediate filaments had no effect on the segregation of G(M1), we found that disulfide-bonded proteins played a significant role in sub-domain segregation. Together, these data provide an example of membrane sub-domains extreme in terms of size and stability of lipid segregation, and implicate a protein-based membrane compartmentation mechanism.

Patterns of expression of sperm flagellar genes: early expression of genes encoding axonemal proteins during the spermatogenic cycle and shared features of promoters of genes encoding central apparatus proteins*

Sperm are motile cells. Thus, a significant component of the spermatogenic cycle is devoted to the formation of flagellum, a process that must be coordinated to insure proper construction. To document the temporal pattern of flagellar gene expression, we employed real-time PCR to assess changes in accumulation of a cohort of genes encoding axoneme, outer dense fibre (ODF) and fibrous sheath (FS) proteins during the first wave of spermatogenesis in the mouse. Axoneme genes were expressed first at the pachytene spermatocyte stage, followed by expression of transcripts encoding ODF and FS components. However, there were differences among these families with respect to the time of initial expression and the rate of mRNA accumulation. To gain understanding of factors that determine these patterns of expression, we cloned the promoters of three axoneme central apparatus genes (Pf6, Spag6 and Pf20). These promoters shared common features including the absence of a TATA box, and putative binding sites for several factors implicated in spermatogenesis (CREB/CREM, SOX17 and SPZ1) as well as ciliogenesis (FOXJ1). Collectively, our findings demonstrate a sequential pattern of expression of flagellar component genes, differential times of expression or rates of transcript accumulation within each class and shared promoter features within a class.

The spermatozoon of Eurasian murine rodents: Its morphological diversity and evolution

The murine rodents are the most speciose subfamily of mammals. Here the morphology of the spermatozoon, as determined by scanning and transmission electron microscopy of representative species from four Eurasian clades, is described. Much interspecific variability in all components of the spermatozoon was found to occur, although most species have a bilaterally flattened sperm head with a single apical hook of variable length and orientation. Ultrastructural observations indicate that this apical hook invariably contains a nuclear projection as well as a large extension of the subacrosomal cytoskeleton, as a perforatorium rostrally, and a complex asymmetrical acrosomal extension. These spermatozoa also have relatively long tails that are attached to the lower concave surface of the sperm head. Uniquely, in species in the Apodemus clade, the apical hook is orientated caudally. In a few species a highly derived sperm head morphotype that does not contain an apical hook is present. These sperm heads vary in morphology from being globular in two species of Bandicota, to bilaterally flattened and paddle-shaped in Tokudaia and Micromys. In spermatozoa of the latter two genera the subacrosomal cytoskeleton, which is less extensive than in species with a hooked sperm head, forms an apical extension, but that is not the case in Bandicota. In all species where the sperm head lacks an apical hook the acrosome is more symmetrical. The sperm tail is much shorter in these species, with attachment to the head occurring on the ventral surface in Tokudaia and basal in Micromys and the two species of Bandicota. As the sperm head morphotype with a complex apical hook is present in all the major clades of murine rodents, it is likely to be a plesiomorphic character within each of these clades, with the nonhooked sperm heads, which vary greatly in structure between species of the different lineages, probably being independently derived. The ultrastructural organization of the sperm head of Bandicota, but not those of Micromys or Tokudaia, suggest divergence in some of the morphological events associated with sperm-egg interaction at the time of fertilization.

Characterization of boar sperm cytoskeletal cylicin II as an actin-binding protein

The presence of actin-binding proteins in the perinuclear theca of boar spermatozoa has been investigated, using stepwise extractions of proteins from sperm heads. Proteins extracted with the alkaline buffer 1M Na(2)CO(3), pH 11, were found to contain a 66kDa protein that binds F-actin in actin pelleting assays. Sequence studies and immunological characterization with antibodies specific for human cylicin II identified the 66kDa protein as the homologue of bovine and human cylicin II. Immunocytochemical studies showed the presence of porcine cylicin II in the acrosomal region of round spermatids and in the postacrosomal region of late spermatids and spermatozoa, in agreement with the previously described localization of cylicins. Taken together, the results suggest that cylicin II, a protein of the sperm perinuclear cytoskeleton, is a novel actin-binding protein, which probably plays a role in the actin-related events that occur during spermiogenesis and the early events of fertilization.

Formation and organization of the mammalian sperm head

The formation and organization of a mammalian sperm head occurs through diverse cellular and molecular processes during spermiogenesis. Such cellular events include sequential changes in the nucleus and the acrosome-which is derived from the Golgi apparatus-in concert with prominent bundles of microtubules, the manchette. However, these complex processes are readily impaired by a variety of intrinsic and extrinsic factors, eventually causing various types of male infertility--such as teratozoospermia--which include the deformation of the acrosome and nucleus. In order to comprehend such idiopathic male infertility syndromes, it is important to clarify the mechanism involved in sperm head formation and organization. In addition to the manchette, two key structures in these events are the acroplaxome and the perinuclear theca. The acroplaxome forms the acrosome plate with periodic intermediate filament bundles of the marginal ring at the leading edge of the acrosome, and its nature has recently been characterized. The perinuclear theca, which is located in the perinuclear region in the sperm head, contains not only a cytoskeletal element to maintain the shape of the sperm head but also functional molecules leading to oocyte activation during fertilization. This review discusses recent developments regarding the formation and organization of the mammalian sperm head in relation to its relevant functions.

The major subacrosomal occupant of bull spermatozoa is a novel histone H2B variant associated with the forming acrosome during spermiogenesis

Recent studies on the structural composition of mammalian sperm heads have shown a congregate of unidentified proteins occupying the periphery of the mammalian sperm nucleus, forming a layer of condensed cytosol. These proteins are the perinuclear theca (PT) and can be categorized into SDS-soluble and SDS-insoluble components. The present study focused on identifying the major SDS-insoluble PT protein, which we localized to the subacrosomal layer of bovine spermatozoa and cloned by immunoscreening a bull testicular cDNA library. The isolated clones encode a protein of 122 amino acids that bears 67% similarity with histone H2B and contains a predicted histone fold motif. The novel amino terminus of the protein contains a potential bipartite nuclear targeting sequence. Hence, we identified this prominent subacrosomal component as a novel H2B variant, SubH2Bv. Northern blot analyses of SubH2Bv mRNA expression showed that it is testis-specific and is also present in murid testes. Immunocytochemical analysis showed SubH2Bv intimately associates, temporally and spatially, with acrosome formation. While the molecular features of SubH2Bv are common to nuclear proteins, it is never seen developmentally within the nucleus of the spermatid. Considering its developmental and molecular characteristics, we have postulated roles of SubH2Bv in acrosome assembly and acrosome-nuclear docking.

Tpx-1 is a component of the outer dense fibers and acrosome of rat spermatozoa

Previously we reported the cloning of a member of the cysteine-rich secretory protein family, tpx-1, from a testis expression library using an outer dense fiber (ODF)-specific antiserum. Using immunohistochemical and immunoelectron microscopic techniques and Western blotting of purified sperm tail components, we have determined that tpx-1 exists as 25 and 27 kDa proteins in two components of rat spermatid: the ODFs and the acrosome. Tpx-1 mRNA is first expressed in the late pachytene spermatocytes, but the production of these tpx-1 proteins is translationally delayed for 4-5 days before being incorporated into the developing sperm acrosome, surrounding the elongating and condensing spermatid nucleus. Concurrent with sperm head formation, tpx-1 protein was incorporated into the developing sperm tail, and specifically the ODFs. The tpx-1 protein was seen within structures resembling granulated bodies in the cytoplasmic lobe of elongating spermatids and was incorporated subsequently into the growing tail in a manner consistent with ODF development. In addition, tpx-1 protein was localized at the ultrastructural level of the connecting piece of the neck and longitudinal columns of the fibrous sheath, suggesting common protein components in these cytoskeletal structures. As such, tpx-1 may have functional significance in the processes of sperm head development and tail function.

Actin-binding properties and colocalization with actin during spermiogenesis of mammalian sperm calicin

The nucleus of mammalian spermatozoa is surrounded by a rigid layer, the perinuclear theca, which is divided into a subacrosomal layer and a postacrosomal calyx. Among the proteins characterized in the perinuclear theca, calicin is one of the main components of the calyx. Its sequence contains three kelch repeats and a BTB/POZ domain. We have studied the association of boar calicin with F-actin and the distribution of boar and human calicin during spermiogenesis compared with the distribution of actin. Calicin was purified from boar sperm heads under nondenaturating conditions. The molecule bound actin with high affinity (K(d) = approximately 5 nM), and a stoichiometry of approximately one calicin per 12 actin monomers was observed. Gel filtration studies showed that calicin forms homomultimers (tetramers and higher polymers). According to immunocytochemical results, calicin is present (together with actin) in the acrosomal region of round spermatids and is mainly localized in the postacrosomal region of late spermatids and spermatozoa. Taken together, the results suggest that the affinity of calicin to F-actin allows targeting of calicin at the subacrosomal space of round spermatids, and that its ability to form homomultimers contributes to the formation of a rigid calyx.

Protein composition of the ventral processes on the sperm head of Australian hydromyine rodents

The sperm head of the plains rat, an Australian hydromyine rodent, is highly complex in structure and contains, in addition to an apical hook, two large ventral processes (VPs) that extend from its upper concave surface and that are largely composed of a huge extension of the sperm head cytoskeleton surrounded by postacrosomal dense lamina. In this study we have attempted to determine their protein composition. For this, the VPs were isolated, the proteins within them separated by SDS-PAGE, and the resultant polypeptide bands Western blotted and probed with antibodies against laboratory rat perforatorial and bull perinuclear theca sperm proteins. Antibodies were also used to determine the perforatorial and perinuclear theca proteins by immunogold labeling of transmission electron microscopic sections. The results indicate that the material within the VPs is largely composed of perforatorial cross-reacting proteins together with F-actin with the dominant protein being PERF 15. The perinuclear theca proteins are, by contrast, restricted to a narrow region adjacent to the acrosomal and nuclear membranes. In conclusion, this study has shown that the VPs of the spermatozoa of Australian rodents are perforatorial-like appendages that contain similar proteins to the perforatorium of the apical hook together with F-actin; their functional significance remains unknown.

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.

An intact sperm nuclear matrix may be necessary for the mouse paternal genome to participate in embryonic development

We have been interested in determining the minimally required elements in the sperm head that are necessary in order for the paternal genome to participate in embryogenesis. We used an ionic detergent, mixed alkyltrimethylammonium bromide (ATAB), plus dithiothreitol (DTT) to remove the acrosome and almost all of the perinuclear theca, leaving only the sperm nucleus morphologically intact. We also tested the stability of the sperm nuclear matrix by the ability to form nuclear halos. Sperm nuclei washed in freshly prepared 0.5% ATAB + 2 mM DTT completely decondensed when extracted with salt, but nuclei washed in the same buffer that was 1 wk old, and then extracted with salt, produced nuclear halos, indicating stable nuclear matrices. When we treated sperm heads with freshly prepared ATAB+DTT and injected them into oocytes, none of the oocytes developed into live offspring. In contrast, sperm heads treated in the same way but with 1-wk-old ATAB+DTT solution could support development of about 30% of the oocytes to live offspring. Electron microscopy demonstrated that most of the perinuclear theca had been removed in both cases. These data suggest that at least in the mouse, the only component of the spermatozoa that is crucial for participation in embryologic development is the sperm nucleus with a stable nuclear matrix.

Distribution and possible role of perinuclear theca proteins during bovine spermiogenesis

The perinuclear theca (PT) is a unique cytoskeletal element that encapsulates the mammalian sperm nucleus. It is divided into subacrosomal and postacrosomal regions. The objective of this study was to analyze and stage the intracellular distribution of several prominent bull PT proteins during spermatogenesis. For this purpose, polyclonal antibodies raised and affinity-purified against the 15.5-, 25-, 28-, 32-, 36-, and 60-kDa bull PT polypeptides were used as probes on sections of aldehyde-fixed testes. Immunoperoxidase staining revealed that the PT polypeptides first appeared early in spermiogenesis, concomitant with early steps of development of the acrosomic system. Immunogold labeling further showed that these polypeptides were peripherally associated with the entire acrosomal membrane, before and during the attachment of the acrosomic vesicle onto the spermatid's nucleus. Once the acrosome had capped the nucleus the labeling resided mainly in the subacrosomal region of the spermatid, between the inner acrosomal membrane and nuclear envelope. Later, during the elongation of the spermatid's nucleus, the labeling with all antibodies except the anti-15.5-kDa antibody extended caudally over the assembling postacrosomal sheath. This study suggests that the perinuclear theca proteins play an instrumental role in the attachment, spreading, and binding of the acrosome onto the nucleus of spermatids.

The fluorescent dye 3, 3' dihexyloxacarbocyanine iodide selectively stains the midpiece and apical region of the heads of murid rodent spermatozoa

Fluorescence microscopy of caudal epididymal spermatozoa stained with 3, 3' dihexyloxacarbocyanine iodide (DiOC6(3)) showed intense fluorescence along the concave surface of the apical hook of spermatozoa of Rattus species and along the upper concave margin of the sperm head in Mus musculus. In the spermatozoa of Hydromys chrysogaster, Melomys cervinipes, and Pseudomys australis, the two ventral processes also fluoresced brightly. In P. australis, fluorescence in the apical hook of sperm heads was largely localized to its upper and lower surfaces. The sperm of N. alexis did not show consistent positive fluorescence. The localization of fluorescence in these spermatozoa after staining with DiOC6(3) was mainly restricted to regions where a large accumulation of perinuclear theca material lies beneath the plasmalemma. The reason for this remains to be determined, but DiOC6(3) may be useful for quickly demonstrating areas of abundant perinuclear thecal material in sperm heads of eutherian mammals by light microscopy.

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.

Extractions reveal specific argentophilic proteins in rat and bull sperm heads

BACKGROUND

Silver-stainability (argentophilia) of cytoplasmic structures occurring in spermatids have been localized into the organizing perinuclear theca, but the biochemical nature and structural associations of these proteins with the cytoskeletal and membranous elements are unresolved and, therefore, were the aim of the present study.

METHODS

Light and electron microscopic analysis of the silver-stainability in the rat spermatids and spermatozoa was carried out in the intact testis tissue and epididymal spermatozoa and after their chemical and mechanical extraction. Correlation of argentophilia with specific proteins of rat and bovine spermatids and spermatozoa was investigated using a recently developed technique for silver nitrate staining of proteins on nitrocellulose.

RESULTS

Sequential formation of the silver-stainable domains seemed to proceed from the argentophilic acrosomal ring. Various extractions indicated that argentophilia in the spermatids and spermatozoa was mainly associated with the perinuclear theca and to some extent to the plasma membrane. Hyamine-soluble extract from spermatozoa of rat and bull revealed only a single argentophilic protein of 130 kDa. Hyamine and SDS-soluble extracts of rat testis tissue contained an additional group of argentophilic polypeptides of lower molecular weight (115, 94, 36, 23, and 21 kDa).

CONCLUSIONS

Reduction in the number of argentophilic proteins appears to be involved in a series of changes in the cyto-architecture of developing spermatids. Tentative cytoskeletal nature of argentophilic proteins remains to be identified. Nevertheless, they may have important physical relations with the higher-order organization of the sperm head cytoskeleton and overlying membranes.

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.

Distribution of filamentous actin in and around spermatids and in spermatozoa of Australian conilurine rodents

The distribution of filamentous actin around the maturing sperm head and in spermatozoa of four species of Australian conilurine rodents was investigated at the light and electron microscopic levels. Similar results were obtained for all the species studied. Mechanically isolated spermatids had NBD-phallacidin-positive longitudinal bands of fluorescence over the dorsolateral surface and, in late spermatids, bands of bright fluorescence passed perpendicularly from the dorsal convex to ventral concave surface. TEM observations indicated that these regions corresponded to filaments of ectoplasmic specializations and granular filamentous material around the tubulobulbar complexes, respectively. In testicular and cauda spermatozoa NBD-phallacidin fluorescent material was present in the two ventral processes that extended from the upper concave surface of the sperm head; also fainter material occurred along the concave border and as a dorsocaudal spur. Its distribution was identical for testicular and cauda spermatozoa. TEM of late spermatids showed that in the ventral process closest to the apical hook there were between 170 and 245 filaments, which attached to the inner surface of the postacrosomal dense lamina; in the more caudal ventral process about 70 filaments occurred. No filaments were, however, visible in the mature spermatozoon but, after immunocytochemical labelling for actin, deposition of gold particles was evident over ventral processes of both late spermatids and cauda spermatozoa. Within the female tract these ventral processes made contact with the zona matrix and were taken into the egg cytoplasm unchanged in morphology. The possible functional significance of the filamentous actin in these structures is discussed.

Biogenesis of specialized cytoskeletal elements of rat spermatozoa

Our results on the formation of the ODF and perforatorium are diagrammatically summarized in Figures 30 and 31. The developmental expression of proteins making up these two cytoskeletal elements differs in timing, duration and intracellular localization. The ODF proteins are synthesized exclusively during the latter part of spermiogenesis, well after transcriptional activity in the haploid germ cell nucleus has ended. This implies that these major integral proteins of the tail are translationally regulated and that mechanisms must exist for the storage and eventual release of the mRNAs encoding these proteins. The perforatorial proteins, on the other hand, begin to be synthesized during the meiotic prophase reaching a peak of production in early spermiogenesis just before the initiation of the condensation of the spermatid's nucleus, at which time RNA synthesis stops. Another major difference between ODF and perforatorial protein production is that there seems to be a coordinated activity between the synthesis and the assembly of the ODF proteins, whereas there appears to be an almost 25 day delay between the initial meiotic synthesis and final condensation of perforatorial proteins in the subacrosomal space at the end of spermiogenesis. As for the intracellular localization of ODF and perforatorial proteins both have unprecedented distributions. The ODF proteins appear to be concentrated in a particular type of granular body which is especially abundant in the elongated spermatid at the time of peak ODF assembly. The perforatorial proteins, on the other hand, appear to be concentrated in the nuclei of pachytene spermatocytes and round spermatids until their displacement into the cytoplasm during nuclear condensation. Both forms of localization suggest a storage role for these proteins uniquely adapted by the spermatid to regulate the assemblies of the respective cytoskeletal elements.

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.

Origin and distribution of perforatorial proteins during spermatogenesis of the rat: an immunocytochemical study

The perforatorium is the subacrosomal portion of the perinuclear theca that encapsulates the nucleus of spermatozoa. In the rat, the perforatorium is a curved pointed structure, which in cross section is triangular in outline over the apical half and beyond the tip of the nucleus. The perforatorium, composed of several proteins, appears as a distinct structural entity only at the very end of spermiogenesis. In this study, polyclonal antibodies prepared against the entire isolated perforatorial fraction and against the major 16 and 34 kDa perforatorial polypeptides were used to determine the distribution of perforatorial proteins in germinal cells at various steps of differentiation. Immunoperoxidase staining at the LM level and quantitative immunogold labeling at the EM level were used. The labeling patterns with all three antibody preparations were identical. The immunolabeling first appeared in early pachytene spermatocytes and increased progressively, with a statistically significant upward trend, in both the nuclei and cytoplasm of spermatocytes and spermatids until step 9 of spermiogenesis. Up to this step the labeling concentration was significantly higher over the nucleus than over the cytoplasm. During nuclear condensation in steps 9 and 12 spermatids, there was a progressive loss of all the labeling over the nucleus and a corresponding increase of labeling over the cytoplasm. During steps 16-18, the early signs of condensation of perforatorial proteins occurred next to the inner acrosomal membrane. Then during step 19 there was a sudden condensation of perforatorial proteins into a definitive perforatorium. Thus proteins destined to form this cytoskeletal structure reside in both the nucleus and cytoplasm of spermatocytes and spermatids until nuclear condensation of the latter. Thereafter, they are restricted to the spermatid's cytoplasm and finally condense around the elongated nucleus at the end of spermiogenesis.