Structural and biochemical features of fractionated spermatid manchettes and sperm axonemes of the azh/azh mutant mouse

Expression Dynamics Indicate Potential Roles of KIF17 for Nuclear Reshaping and Tail Formation during Spermiogenesis in Phascolosoma esculenta

Kinesin family member17 (KIF17), a homologous dimer of the kinesin-2 protein family, has important microtubule-dependent and -independent roles in spermiogenesis. Little is known about KIF17 in the mollusk, Phascolosoma esculenta, a newly developed mariculture species in China. Here, we cloned the open reading frame of Pe-kif17 and its related gene, Pe-act, and performed bioinformatics analysis on both. Pe-KIF17 and Pe-ACT are structurally conserved, indicating that they may be functionally conserved. The expression pattern of kif17/act mRNA performed during spermiogenesis revealed their expression in diverse tissues, with the highest expression level in the coelomic fluid of P. esculenta. The expressions of Pe-kif17 and Pe-act mRNA were relatively high during the breeding season (July-September), suggesting that Pe-KIF17/ACT may be involved in spermatogenesis, particularly during spermiogenesis. Further analysis of Pe-kif17 mRNA via fluorescence in situ hybridization revealed the continuous expression of this mRNA during spermiogenesis, suggesting potential functions in this process. Immunofluorescence showed that Pe-KIF17 co-localized with α-tubulin and migrated from the perinuclear cytoplasm to one side of the spermatid, forming the sperm tail. Pe-KIF17 and Pe-ACT also colocalized. KIF17 may participate in spermiogenesis of P. esculenta, particularly in nuclear reshaping and tail formation by interacting with microtubule structures similar to the manchette. Moreover, Pe-KIF17 with Pe-ACT is also involved in nuclear reshaping and tail formation in the absence of microtubules. This study provides evidence for the role of KIF17 during spermiogenesis and provides theoretical data for studies of the reproductive biology of P. esculenta. These findings are important for spermatogenesis in mollusks.

Cylicins are a structural component of the sperm calyx being indispensable for male fertility in mice and human

Cylicins are testis-specific proteins, which are exclusively expressed during spermiogenesis. In mice and humans, two Cylicins, the gonosomal X-linked Cylicin 1 (Cylc1/CYLC1) and the autosomal Cylicin 2 (Cylc2/CYLC2) genes, have been identified. Cylicins are cytoskeletal proteins with an overall positive charge due to lysine-rich repeats. While Cylicins have been localized in the acrosomal region of round spermatids, they resemble a major component of the calyx within the perinuclear theca at the posterior part of mature sperm nuclei. However, the role of Cylicins during spermiogenesis has not yet been investigated. Here, we applied CRISPR/Cas9-mediated gene editing in zygotes to establish Cylc1- and Cylc2-deficient mouse lines as a model to study the function of these proteins. Cylc1 deficiency resulted in male subfertility, whereas Cylc2-/-, Cylc1-/yCylc2+/-, and Cylc1-/yCylc2-/- males were infertile. Phenotypical characterization revealed that loss of Cylicins prevents proper calyx assembly during spermiogenesis. This results in decreased epididymal sperm counts, impaired shedding of excess cytoplasm, and severe structural malformations, ultimately resulting in impaired sperm motility. Furthermore, exome sequencing identified an infertile man with a hemizygous variant in CYLC1 and a heterozygous variant in CYLC2, displaying morphological abnormalities of the sperm including the absence of the acrosome. Thus, our study highlights the relevance and importance of Cylicins for spermiogenic remodeling and male fertility in human and mouse, and provides the basis for further studies on unraveling the complex molecular interactions between perinuclear theca proteins required during spermiogenesis.

CAMSAP1 role in orchestrating structure and dynamics of manchette microtubule minus-ends impacts male fertility during spermiogenesis

The manchette is a crucial transient structure involved in sperm development, with its composition and regulation still not fully understood. This study focused on investigating the roles of CAMSAP1 and CAMSAP2, microtubule (MT) minus-end binding proteins, in regulating manchette MTs, spermiogenesis, and male fertility. The loss of CAMSAP1, but not CAMSAP2, disrupts the well-orchestrated process of spermiogenesis, leading to abnormal manchette elongation and delayed removal, resulting in deformed sperm nuclei and tails resembling oligoasthenozoospermia symptoms. We investigated the underlying molecular mechanisms by purifying manchette assemblies and comparing them through proteomic analysis, and results showed that the absence of CAMSAP1 disrupted the proper localization of key proteins (CEP170 and KIF2A) at the manchette minus end, compromising its structural integrity and hindering MT depolymerization. These findings highlight the significance of maintaining homeostasis in manchette MT minus-ends for shaping manchette morphology during late spermiogenesis, offering insights into the molecular mechanisms underlying infertility and sperm abnormalities.

The tubulin code in mammalian sperm development and function

Microtubules are cytoskeletal elements that play key roles throughout the different steps of sperm development. As an integral part of the sperm flagellum, the molecular machine that generates sperm motility, microtubules are also essential for the progressive swimming of sperm to the oocyte, which is a prerequisite for fertilisation. Given the central role of microtubules in all steps of spermatogenesis, their functions need to be tightly controlled. Recent work has showcased tubulin posttranslational modifications as key players in sperm development and function, with aberrations often leading to male infertility with a broad spectrum of sperm defects. Posttranslational modifications are part of the tubulin code, a mechanism that can control microtubule functions by modulating the properties of their molecular building blocks, the tubulin proteins. Here we review the current knowledge on the implications of the tubulin code in sperm development and functions and its importance for male fertility.

Sperm bauplan and function and underlying processes of sperm formation and selection

The spermatozoon is a highly differentiated and polarized cell, with two main structures: the head, containing a haploid nucleus and the acrosomal exocytotic granule, and the flagellum, which generates energy and propels the cell; both structures are connected by the neck. The sperm's main aim is to participate in fertilization, thus activating development. Despite this common bauplan and function, there is an enormous diversity in structure and performance of sperm cells. For example, mammalian spermatozoa may exhibit several head patterns and overall sperm lengths ranging from ∼30 to 350 µm. Mechanisms of transport in the female tract, preparation for fertilization, and recognition of and interaction with the oocyte also show considerable variation. There has been much interest in understanding the origin of this diversity, both in evolutionary terms and in relation to mechanisms underlying sperm differentiation in the testis. Here, relationships between sperm bauplan and function are examined at two levels: first, by analyzing the selective forces that drive changes in sperm structure and physiology to understand the adaptive values of this variation and impact on male reproductive success and second, by examining cellular and molecular mechanisms of sperm formation in the testis that may explain how differentiation can give rise to such a wide array of sperm forms and functions.

Molecular characterization, dynamic transcription, and potential function of KIF3A/KIF3B during spermiogenesis in Opsariichthys bidens

Spermiogenesis is the final phase of spermatogenesis, wherein the spermatids differentiate into mature spermatozoa via complex morphological transformation. In this process, kinesin plays an important role. Here, we observed the morphological transformation of spermatids and analyzed the characterization, dynamic transcription, and potential function of kinesin KIF3A/KIF3B during spermiogenesis in Chinese hook snout carp (Opsariichthys bidens). We found that the full-length cDNAs of O. bidens kif3a and kif3b were 2544 and 2806 bp in length comprising 119 bp and 259 bp 5' untranslated region (UTR), 313 bp and 222 bp 3' UTR, and 2112 bp and 2325 bp open reading frame encoding 703 and 774 amino acids, respectively. Ob-KIF3A/KIF3B proteins have three domains, namely N-terminal head, coiled-coil stalk, and C-terminal tail, and exhibit high similarity with homologous proteins in vertebrates and invertebrates. Ob-kif3a/kif3b mRNAs were ubiquitously expressed in all tissues examined, with the highest expression in the brain and stage-IV testis. Immunofluorescence results showed that Ob-KIF3A was co-localized with tubulin and the mitochondria. Particularly, in early spermatids, Ob-KIF3A, tubulin, and the mitochondrial signals were evenly distributed in the cytoplasm, whereas in middle spermatids, they were distributed around the nucleus. In the late stage, the signals were concentrated on one side of the nucleus, where the tail is formed, whereas in mature sperms, they were detected in the midpiece and flagellum. These results indicate that Ob-KIF3A/KIF3B may participate in nuclear reshaping, flagellum formation, and mitochondrial aggregation in the midpiece during spermiogenesis.

Towards Post-Meiotic Sperm Production: Genetic Insight into Human Infertility from Mouse Models

Declined quality and quantity of sperm is currently the major cause of patients suffering from infertility. Male germ cell development is spatiotemporally regulated throughout the whole developmental process. While it has been known that exogenous factors, such as environmental exposure, diet and lifestyle, et al, play causative roles in male infertility, recent progress has revealed abundant genetic mutations tightly associated with defective male germline development. In mammals, male germ cells undergo dramatic morphological change (i.e., nuclear condensation) and chromatin remodeling during post-meiotic haploid germline development, a process termed spermiogenesis; However, the molecular machinery players and functional mechanisms have yet to be identified. To date, accumulated evidence suggests that disruption in any step of haploid germline development is likely manifested as fertility issues with low sperm count, poor sperm motility, aberrant sperm morphology or combined. With the continually declined cost of next-generation sequencing and recent progress of CRISPR/Cas9 technology, growing studies have revealed a vast number of disease-causing genetic variants associated with spermiogenic defects in both mice and humans, along with mechanistic insights partially attained and validated through genetically engineered mouse models (GEMMs). In this review, we mainly summarize genes that are functional at post-meiotic stage. Identification and characterization of deleterious genetic variants should aid in our understanding of germline development, and thereby further improve the diagnosis and treatment of male infertility.

Observations on Dag-like defect of spermatozoa induced by treatment of the phytotherapeutic Quassia amara/quassin in the mouse model

Gross alterations in the morphology of spermatozoa, teratozoospermia, invariably render them incapable of fertilisation. One of the contributory factors to teratozoospermia is failure of spermatozoon to shed the cytoplasmic droplet even after their arrival at epididymis. Quassia amara and quassin are of medicinal value with special reference to malaria. Nevertheless, there are also reports implicating Quassia/quassin in male reproductive toxicity. We were interested in finding if its therapeutic application would jeopardise male fertility. So, we tested it for male reproductive toxicity by analysing, among other aspects, abnormal sperm morphologies, and made a systematic analysis of the spermatozoa of treated mice before they are spermiated and until they arrive at the cauda epididymis. The spermatozoa not only failed to shed the cytoplasmic droplet during epididymal transit but swell to a very large size and were angulated, resulting in Dag-like defect or lasso shape. A link between cytoplasmic droplet that was retained and lasso shape of tail was indicated. This article traces the structural changes in spermatozoa that lead to angulation, flexion and coiling of the tail, caused due to retention of cytoplasmic droplet, and explains one of the mechanisms of toxicant-induced teratozoospermia.

Comprehensive Analysis of Mouse Cancer/Testis Antigen Functions in Cancer Cells and Roles of TEKT5 in Cancer Cells and Testicular Germ Cells

The cancer/testis antigen (CTA) genes were identified as human genes preferentially expressed in cancer cells and testis, but the contribution of CTAs to cancer and male germ cell development is unclear. In this study, we comprehensively examined mouse CTA functions and found that the majority of CTAs are involved in growth and/or survival of cancer cells. We focused on one mouse CTA gene, Tekt5, for its detailed functional analysis. Tekt5 knockdown (KD) in ovarian cancer cells caused G₁ arrest and apoptosis, and p27^kip1 was concomitantly upregulated. Tekt5 KD also resulted in decreased levels of acetylated α-tubulin and subsequent fragmentation of β-III-tubulin, upregulation of HDAC6 that deacetylates α-tubulin, and nuclear accumulation of SMAD3 that induces p27^kip1 expression. Because depolymerization of tubulin is known to cause translocation of SMAD3 to the nucleus, these results together suggested that TEKT5 negatively regulates Hdac6 expression and consequently maintains cell cycle via stabilization of tubulin. We also found that the number of spermatids was significantly decreased and acetylated α-tubulin levels were decreased in vivo by KD of Tekt5 in testis. Because acetylated α-tubulin is required for sperm morphogenesis, these results suggest that TEKT5 is necessary for spermiogenesis via maintenance of acetylated α-tubulin levels.

Descriptive Analysis of LAP1 Distribution and That of Associated Proteins throughout Spermatogenesis

Spermatogenesis comprises highly complex differentiation processes. Nuclear envelope (NE) proteins have been associated with these processes, including lamins, lamina-associated polypeptide (LAP) 2 and the lamin B-receptor. LAP1 is an important NE protein whose function has not been fully elucidated, but several binding partners allow predicting putative LAP1 functions. To date, LAP1 had not been associated with spermatogenesis. In this study, LAP1 expression and cellular/subcellular localization during spermatogenesis in human and mouse testes is established for the first time. The fact that LAP1 is expressed during nuclear elongation in spermiogenesis and is located at the spermatids' centriolar pole is singularly important. LAP1 binds to members of the protein phosphatase 1 (PP1) family. Similar localization of LAP1 and PP1γ2, a testis-specific PP1 isoform, suggests a shared function for both proteins during spermiogenesis. Furthermore, this study suggests an involvement of LAP1 in manchette development and chromatin regulation possibly via interaction with acetylated α-tubulin and lamins, respectively. Taken together, the present results indicate that, by moving to the posterior pole in spermatids, LAP1 can contribute to the achievement of non-random, sperm-specific chromatin distribution, as well as modulate cellular remodeling during spermiogenesis. In addition, LAP1 seems to be associated with dynamic microtubule changes related to manchette formation and flagella development.

Ccdc181 is a microtubule-binding protein that interacts with Hook1 in haploid male germ cells and localizes to the sperm tail and motile cilia

Disruption of murine Hook1 results in a disturbed spermatogenesis and consequently leads to male infertility in mice. Within these mice abnormal sperm development starts with a disorganization of the microtubular manchette in elongating spermatids that leads to an abnormal head shape as well as to distinctive structural changes in the flagella of the sperm. To elucidate Hook1 function in male germ cell differentiation a yeast two-hybrid screen was performed using a murine testicular library, which leads to the identification of several putative Hook1 interacting proteins. One of the isolated cDNA fragments encodes for the coiled-coil domain containing protein 181 (Ccdc181). The putative interaction of Ccdc181 with Hook1 was verified by FRET analysis and interacting regions were identified using yeast two-hybrid assays. Furthermore, Ccdc181 seems to interact directly with microtubules and localizes to the microtubular manchette of elongating spermatids, resembling the previously reported localization of Hook1. According to the observed immunostaining pattern the RNA expression of Ccdc181 is less prominent in pre-meiotic stages of sperm development but increases in the haploid phase of spermatogenesis and seems to be restricted to male germ cells. However, Ccdc181 expression is also observed to a lower extent in somatic tissues, particularly, in tissues containing ciliated epithelia. Additionally, Ccdc181 protein is found to localize to the sperm flagella and to the basal half of motile cilia, whereas Ccdc181 was not detected in primary non-motile cilia. Furthermore, we showed that Ccdc181 is a putative interacting partner of the different catalytic subunits of Pp1, raising the hypothesis that Ccdc181 plays a role in mediating ciliary motility.

RAB10 Interacts with the Male Germ Cell-Specific GTPase-Activating Protein during Mammalian Spermiogenesis

According to recent estimates, 2%–15% of couples are sterile, and approximately half of the infertility cases are attributed to male reproductive factors. However, the reasons remain undefined in approximately 25% of male infertility cases, and most infertility cases exhibit spermatogenic defects. Numerous genes involved in spermatogenesis still remain unknown. We previously identified Male Germ Cells Rab GTPase-Activating Proteins (MGCRABGAPs) through cDNA microarray analysis of human testicular tissues with spermatogenic defects. MGCRABGAP contains a conserved RABGAP catalytic domain, TBC (Tre2/Bub2/Cdc16). RABGAP family proteins regulate cellular function (e.g., cytoskeletal remodeling, vesicular trafficking, and cell migration) by inactivating RAB proteins. MGCRABGAP is a male germ cell-specific protein expressed in elongating and elongated spermatids during mammalian spermiogenesis. The purpose of this study was to identify proteins that interact with MGCRABGAP during mammalian spermiogenesis using a proteomic approach. We found that MGCRABGAP exhibited GTPase-activating bioability, and several MGCRABGAP interactors, possible substrates (e.g., RAB10, RAB5C, and RAP1), were identified using co-immunoprecipitation (co-IP) and nano liquid chromatography-mass spectrometry/mass spectrometry (nano LC-MS/MS). We confirmed the binding ability between RAB10 and MGCRABGAP via co-IP. Additionally, MGCRABGAP–RAB10 complexes were specifically colocalized in the manchette structure, a critical structure for the formation of spermatid heads, and were slightly expressed at the midpiece of mature spermatozoa. Based on these results, we propose that MGCRABGAP is involved in mammalian spermiogenesis by modulating RAB10.

LRGUK1 is part of a multiprotein complex required for manchette function and male fertility

Infertility occurs in 1 in 20 young men and is idiopathic in origin in most. We have reported that the leucine-rich repeat (LRR) and guanylate kinase-like domain containing, isoform (LRGUK)-1 is essential for sperm head shaping, via the manchette, and the initiation of sperm tail growth from the centriole/basal body, and thus, male fertility. Within this study we have used a yeast 2-hybrid screen of an adult testis library to identify LRGUK1-binding partners, which were then validated with a range of techniques. The data indicate that LRGUK1 likely achieves its function in partnership with members of the HOOK family of proteins (HOOK-1-3), Rab3-interacting molecule binding protein (RIMBP)-3 and kinesin light chain (KLC)-3, all of which are associated with intracellular protein transport as cargo adaptor proteins and are localized to the manchette. LRGUK1 consists of 3 domains; an LRR, a guanylate kinase (GUK)-like and an unnamed domain. In the present study, we showed that the GUK-like domain is essential for binding to HOOK2 and RIMBP3, and the LRR domain is essential for binding to KLC3. These findings establish LRGUK1 as a key component of a multiprotein complex with an essential role in microtubule dynamics within haploid male germ cells.-Okuda, H., DeBoer, K., O'Connor, A. E., Merriner, D. J., Jamsai, D., O'Bryan, M. K. LRGUK1 is part of a multiprotein complex required for manchette function and male fertility.

Formation and function of the manchette and flagellum during spermatogenesis

The last phase of spermatogenesis involves spermatid elongation (spermiogenesis), where the nucleus is remodeled by chromatin condensation, the excess cytoplasm is removed and the acrosome and sperm tail are formed. Protein transport during spermatid elongation is required for correct formation of the sperm tail and acrosome and shaping of the head. Two microtubular-based protein delivery platforms transport proteins to the developing head and tail: the manchette and the sperm tail axoneme. The manchette is a transient skirt-like structure surrounding the elongating spermatid head and is only present during spermatid elongation. In this review, we consider current understanding of the assembly, disassembly and function of the manchette and the roles of these processes in spermatid head shaping and sperm tail formation. Recent studies have shown that at least some of the structural proteins of the sperm tail are transported through the intra-manchette transport to the basal body at the base of the developing sperm tail and through the intra-flagellar transport to the construction site in the flagellum. This review focuses on the microtubule-based mechanisms involved and the consequences of their disruption in spermatid elongation.

Coiled-coil domain containing 42 (Ccdc42) is necessary for proper sperm development and male fertility in the mouse

Spermiogenesis is the differentiation of spermatids into motile sperm consisting of a head and a tail. The head harbors a condensed elongated nucleus partially covered by the acrosome-acroplaxome complex. Defects in the acrosome-acroplaxome complex are associated with abnormalities in sperm head shaping. The head-tail coupling apparatus (HTCA), a complex structure consisting of two cylindrical microtubule-based centrioles and associated components, connects the tail or flagellum to the sperm head. Defects in the development of the HTCA cause sperm decapitation and disrupt sperm motility, two major contributors to male infertility. Here, we provide data indicating that mutations in the gene Coiled-coil domain containing 42 (Ccdc42) is associated with malformation of the mouse sperm flagella. In contrast to many other flagella and motile cilia genes, Ccdc42 expression is only observed in the brain and developing sperm. Male mice homozygous for a loss-of-function Ccdc42 allele (Ccdc42(KO)) display defects in the number and location of the HTCA, lack flagellated sperm, and are sterile. The testes enriched expression of Ccdc42 and lack of other phenotypes in mutant mice make it an ideal candidate for screening cases of azoospermia in humans.

A comparative overview of the sperm centriolar complex in mammals and birds: Variations on a theme

This paper presents an overview of the structure, function and anomalies of the sperm centriolar complex (CC) on a comparative basis between mammals and birds. The information is based on selected references from the literature supplemented by original observations on spermiogenesis and sperm structure in disparate mammalian (cheetah and cane rat) and avian (ostrich, rhea and emu) species. Whereas the basic structure of the CC (a diplosome surrounded by pericentriolar material) is similar in Aves and Mammalia, certain differences are apparent. Centriole reduction does not generally occur in birds, but when present as in oscines, involves the loss of the proximal centriole. In ratites, the distal centriole forms the core of the entire midpiece and incorporates the outer dense fibres in addition to initiating axoneme formation. The elements of the connecting piece are not segmented in birds and less complex in basic design than in mammals. The functions of the various components of the CC appear to be similar in birds and mammals. Despite obvious differences in sperm head shape, the centrosomal anomalies afflicting both vertebrate groups demonstrate structural uniformity across species and display a similar range of defects. Most abnormalities result from defective migration and alignment of the CC relative to the nucleus. The most severe manifestation is that of acephalic sperm, while angled tail attachment, abaxial and multiflagellate sperm reflect additional defective forms. The stump-tail defect is not observed in birds. A comparison of defective sperm formation and centrosomal dysfunction at the molecular level is currently difficult owing to the paucity of relevant information on avian sperm.

[The testicular microtubule-associated protein Tau: Where, when during spermatogenesis?]

The Tau protein (Tubulin Associated Unit) is a phosphoprotein of the microtubule-associated protein family (MAPs). Its role is the regulation of the microtubule polymerization. The Tau protein is naturally present in brain, heart, muscle, lung, kidney, pancreas and liver. An expression of Tau protein and RNA messengers was also highlighted in the testis that is an organ rich in microtubules. The role of microtubules is essential in the stabilization of the cellular shape and in cell divisions. In the testis, Tau protein could be involved in the division process of the spermatogenesis by acting on the microtubular dynamics in the arrangement of the spermatozoon polarity. This review synthesizes the current knowledge, the localization and the main functions of the Tau protein focused on the testis. The localization and the potential roles of the Tau protein during the spermatogenesis are discussed by emphasizing the link with the microtubular structures of seminiferous tubules.

The LINC complex component Sun4 plays a crucial role in sperm head formation and fertility

LINC complexes are evolutionarily conserved nuclear envelope bridges, physically connecting the nucleus to the peripheral cytoskeleton. They are pivotal for dynamic cellular and developmental processes, like nuclear migration, anchoring and positioning, meiotic chromosome movements and maintenance of cell polarity and nuclear shape. Active nuclear reshaping is a hallmark of mammalian sperm development and, by transducing cytoskeletal forces to the nuclear envelope, LINC complexes could be vital for sperm head formation as well. We here analyzed in detail the behavior and function of Sun4, a bona fide testis-specific LINC component. We demonstrate that Sun4 is solely expressed in spermatids and there localizes to the posterior nuclear envelope, likely interacting with Sun3/Nesprin1 LINC components. Our study revealed that Sun4 deficiency severely impacts the nucleocytoplasmic junction, leads to mislocalization of other LINC components and interferes with the formation of the microtubule manchette, which finally culminates in a globozoospermia-like phenotype. Together, our study provides direct evidence for a critical role of LINC complexes in mammalian sperm head formation and male fertility.

SEPT12-microtubule complexes are required for sperm head and tail formation

The septin gene belongs to a highly conserved family of polymerizing GTP-binding cytoskeletal proteins. SEPTs perform cytoskeletal remodeling, cell polarity, mitosis, and vesicle trafficking by interacting with various cytoskeletons. Our previous studies have indicated that SEPTIN12+/+/+/- chimeras with a SEPTIN12 mutant allele were infertile. Spermatozoa from the vas deferens of chimeric mice indicated an abnormal sperm morphology, decreased sperm count, and immotile sperm. Mutations and genetic variants of SEPTIN12 in infertility cases also caused oligozoospermia and teratozoospermia. We suggest that a loss of SEPT12 affects the biological function of microtublin functions and causes spermiogenesis defects. In the cell model, SEPT12 interacts with α- and β-tubulins by co-immunoprecipitation (co-IP). To determine the precise localization and interactions between SEPT12 and α- and β-tubulins in vivo, we created SEPTIN12-transgene mice. We demonstrate how SEPT12 interacts and co-localizes with α- and β-tubulins during spermiogenesis in these mice. By using shRNA, the loss of SEPT12 transcripts disrupts α- and β-tubulin organization. In addition, losing or decreasing SEPT12 disturbs the morphogenesis of sperm heads and the elongation of sperm tails, the steps of which are coordinated and constructed by α- and β-tubulins, in SEPTIN12+/+/+/- chimeras. In this study, we discovered that the SEPTIN12-microtubule complexes are critical for sperm formation during spermiogenesis.

The dynamic cytoskeleton of the developing male germ cell

Mammalian spermatogenesis is characterised by dramatic cellular change to transform the non-polar spermatogonium into a highly polarised and functional spermatozoon. The acquisition of cell polarity is a requisite step for formation of viable sperm. The polarity of the spermatozoon is clearly demonstrated by the acrosome at the apical pole of the cell and the flagellum at the opposite end. Spermatogenesis consists of three basic phases: mitosis, meiosis and spermiogenesis. The final phase represents the period of greatest cellular change where cell-type specific organelles such as the acrosome and the flagellum form, the nucleus migrates to the plasma membrane and elongates, chromatin condenses and residual cytoplasm is removed. An important feature of spermatogenesis is the change in the cytoskeleton that occurs throughout this pathway. In this review, the author will provide an overview of these transformations and provide insight into possible modes of regulation of these rearrangements during spermatogenesis. Although primary focus will be given to the microtubule cytoskeleton, the importance of actin filaments to the cellular transformation of the male germ cell will also be discussed.

Molecular control of rodent spermatogenesis

Spermatogenesis is a complex developmental process that ultimately generates mature spermatozoa. This process involves a phase of proliferative expansion, meiosis, and cytodifferentiation. Mouse models have been widely used to study spermatogenesis and have revealed many genes and molecular mechanisms that are crucial in this process. Although meiosis is generally considered as the most crucial phase of spermatogenesis, mouse models have shown that pre-meiotic and post-meiotic phases are equally important. Using knowledge generated from mouse models and in vitro studies, the current review provides an overview of the molecular control of rodent spermatogenesis. Finally, we briefly relate this knowledge to fertility problems in humans and discuss implications for future research. This article is part of a Special Issue entitled: Molecular Genetics of Human Reproductive Failure.

Cytoskeletal track selection during cargo transport in spermatids is relevant to male fertility

Spermatids generate diverse and unusual actin and microtubule populations during spermiogenesis to fulfill mechanical and cargo transport functions assisted by motor and non-motor proteins. Disruption of cargo transport may lead to teratozoospermia and consequent male infertility. How motor and non-motor proteins utilize the cytoskeleton to transport cargos during sperm development is not clear. Filamentous actin (F-actin) and the associated motor protein myosin Va participate in the transport of Golgi-derived proacrosomal vesicles to the acrosome and along the manchette. The acrosome is stabilized by the acroplaxome, a cytoskeletal plate anchored to the nuclear envelope. The acroplaxome plate harbors F-actin and actin-like proteins as well as several other proteins, including keratin 5/Sak57, Ran GTPase, Hook1, dynactin p150Glued, cenexin-derived ODF2, testis-expressed profilin-3 and profilin-4, testis-expressed Fer tyrosine kinase (FerT), members of the ubiquitin-proteasome system and cortactin. Spermatids express transcripts encoding the non-spliced form of cortactin, a F-actin-regulatory protein. Tyrosine phosphorylated cortactin and FerT coexist in the acrosome-acroplaxome complex. Hook1 and p150Glued, known to participate in vesicle cargo transport, are sequentially seen from the acroplaxome to the manchette to the head-tail coupling apparatus (HTCA). The golgin Golgi-microtubule associated protein GMAP210 resides in the cis-Golgi whereas the intraflagellar protein IFT88 localizes in the trans-Golgi network. Like Hook1 and p150Glued, GMAP210 and IFT88 colocalize at the cytosolic side of proacrosomal vesicles and, following vesicle fusion, become part of the outer and inner acrosomal membranes before relocating to the acroplaxome, manchette and HTCA. A hallmark of the manchette and axoneme is microtubule heterogeneity, determined by the abundance of acetylated, tysosinated and glutamylated tubulin isoforms produced by post-translational modifications. We postulate that the construction of the male gamete requires microtubule and F-actin tracks and specific molecular motors and associated non-motor proteins for the directional positioning of vesicular and non-vesicular cargos at specific intracellular sites.

GMAP210 and IFT88 are present in the spermatid golgi apparatus and participate in the development of the acrosome-acroplaxome complex, head-tail coupling apparatus and tail

We describe the localization of the golgin GMAP210 and the intraflagellar protein IFT88 in the Golgi of spermatids and the participation of these two proteins in the development of the acrosome-acroplaxome complex, the head-tail coupling apparatus (HTCA) and the spermatid tail. Immunocytochemical experiments show that GMAP210 predominates in the cis-Golgi, whereas IFT88 prevails in the trans-Golgi network. Both proteins colocalize in proacrosomal vesicles, along acrosome membranes, the HTCA and the developing tail. IFT88 persists in the acrosome-acroplaxome region of the sperm head, whereas GMAP210 is no longer seen there. Spermatids of the Ift88 mouse mutant display abnormal head shaping and are tail-less. GMAP210 is visualized in the Ift88 mutant during acrosome-acroplaxome biogenesis. However, GMAP210-stained vesicles, mitochondria and outer dense fiber material build up in the manchette region and fail to reach the abortive tail stump in the mutant. In vitro disruption of the spermatid Golgi and microtubules with Brefeldin-A and nocodazole blocks the progression of GMAP210- and IFT88-stained proacrosomal vesicles to the acrosome-acroplaxome complex but F-actin distribution in the acroplaxome is not affected. We provide the first evidence that IFT88 is present in the Golgi of spermatids, that the microtubule-associated golgin GMAP210 and IFT88 participate in acrosome, HTCA, and tail biogenesis, and that defective intramanchette transport of cargos disrupts spermatid tail development.

UBQLN1 interacts with SPEM1 and participates in spermiogenesis

Spermiogenesis represents the process through which haploid male germ cells differentiate from round spermatids into elongated spermatids and eventually the male gametes called spermatozoa. Haploid cell differentiation is unique to male germ cell development and many unique genes/proteins essential for this process have been discovered. SPEM1 is one of these spermiogenesis-essential proteins encoded by a testis-specific gene exclusively expressed in the developing spermatids. Inactivation of Spem1 in mice results in deformed spermatozoa characterized by "head-bent-back" abnormalities with 100% penetrance. Using yeast two-hybrid screening assays, we identified UBQLN1 as one of the SPEM1-interacting partners. UBQLN1 and SPEM1 were colocalized to the manchette of elongating spermatids. Since UBQLN1 functions through binding and directing poly-ubiquitinated proteins to the proteasome for degradation, interactions between UBQLN1 and SPEM1 suggest a role in the regulation of protein ubiquitination during spermiogenesis.

Epididymal hypo-osmolality induces abnormal sperm morphology and function in the estrogen receptor alpha knockout mouse

Estrogen receptor-alpha (ESR1) is highly expressed in the efferent ductules of all species studied as well as in the epididymal epithelium in mice and other select species. Male mice lacking ESR1 (Esr1KO) are infertile, but transplantation studies demonstrated that Esr1KO germ cells are capable of fertilization when placed in a wild-type reproductive tract. These results suggest that extratesticular regions, such as the efferent ductules and epididymis, are the major source of pathological changes in Esr1KO males. Previous studies have shown alterations in ion and fluid transporters in the efferent duct and epididymal epithelia of Esr1KO males, leading to misregulation of luminal fluid pH. To determine the effect of an altered epididymal milieu on Esr1KO sperm, we assayed sperm morphology in the different regions of the epididymis. Sperm recovered from the epididymis exhibited abnormal flagellar coiling and increased incidence of spontaneous acrosome reactions, both of which are consistent with exposure to abnormal epididymal fluid. Analysis of the epididymal fluid revealed that the osmolality of the Esr1KO fluid was reduced relative to wild type, consistent with prior reports of inappropriate fluid absorption from the efferent ductules. This, along with the finding that morphological defects increased with transit through the epididymal duct, suggests that the anomalies in sperm are a consequence of the abnormal luminal environment. Consistent with this, incubating Esr1KO sperm in a more wild-type-like osmotic environment significantly rescued the abnormal flagellar coiling. This work demonstrates that Esr1KO mice exhibit an abnormal fluid environment in the lumen of the efferent ducts and epididymis, precluding normal sperm maturation and instead resulting in progressive deterioration of sperm that contributes to infertility.

Rnf19a, a ubiquitin protein ligase, and Psmc3, a component of the 26S proteasome, tether to the acrosome membranes and the head-tail coupling apparatus during rat spermatid development

We report the cDNA cloning of rat testis Rnf19a, a ubiquitin protein ligase, and show 98% and 93% protein sequence identity of testicular mouse and human Rnf19a, respectively. Rnf19a interacts with Psmc3, a protein component of the 19S regulatory cap of the 26S proteasome. During spermatid development, Rnf19a and Psmc3 are initially found in Golgi-derived proacrosomal vesicles. Later on, Rnf19a, Psmc3, and ubiquitin are seen along the cytosolic side of the acrosomal membranes and the acroplaxome, a cytoskeletal plate linking the acrosome to the spermatid nuclear envelope. Rnf19a and Psmc3 accumulate at the acroplaxome marginal ring-manchette perinuclear ring region during spermatid head shaping and in the developing sperm head-tail coupling apparatus and tail. Rnf19a and Psmc3 may interact directly or indirectly with each other, presumably pointing to the participation of the ubiquitin-proteasome system in acrosome biogenesis, spermatid head shaping, and development of the head-tail coupling apparatus and tail.

Rat hd mutation reveals an essential role of centrobin in spermatid head shaping and assembly of the head-tail coupling apparatus

The hypodactylous (hd) locus impairs limb development and spermatogenesis, leading to male infertility in rats. We show that the hd mutation is caused by an insertion of an endogenous retrovirus into intron 10 of the Cntrob gene. The retroviral insertion in hd mutant rats disrupts the normal splicing of Cntrob transcripts and results in the expression of a truncated protein. During the final phase of spermiogenesis, centrobin localizes to the manchette, centrosome, and the marginal ring of the spermatid acroplaxome, where it interacts with keratin 5-containing intermediate filaments. Mutant spermatids show a defective acroplaxome marginal ring and separation of the centrosome from its normal attachment site of the nucleus. This separation correlates with a disruption of head-tail coupling apparatus, leading to spermatid decapitation during the final step of spermiogenesis and the absence of sperm in the epididymis. Cntrob may represent a novel candidate gene for presently unexplained hereditary forms of teratozoospermia and the "easily decapitated sperm syndrome" in humans.

Expression of Fer testis (FerT) tyrosine kinase transcript variants and distribution sites of FerT during the development of the acrosome-acroplaxome-manchette complex in rat spermatids

We report the association of testicular Fer, a non-receptor tyrosine kinase, with acrosome development and remodeling of the acrosome-associated acroplaxome plate during spermatid head shaping. A single gene expresses two forms of Fer tyrosine kinases in testis: a somatic form (FerS) and a truncated testis-type form (FerT). FerT transcript variants are seen in spermatocytes and spermatids. FerS transcripts are not detected in round spermatids but are moderately transcribed in spermatocytes. FerT protein is associated with the spermatid medial/trans-Golgi region, proacrosomal vesicles, the cytosolic side of the outer acrosome membrane and adjacent to the inner acrosome membrane facing the acroplaxome. FerT coexist in the acroplaxome with phosphorylated cortactin, a regulator of F-actin dynamics. We propose that FerT participates in acrosome development and that phosphorylated cortactin may contribute to structural changes in F-actin in the acroplaxome during spermatid head shaping.

RIM-BP3 is a manchette-associated protein essential for spermiogenesis

During spermiogenesis, round spermatids are converted into motile sperm in mammals. The mechanisms responsible for sperm morphogenesis are poorly understood. We have characterized a novel protein, RIM-BP3, with a specialized function in spermatid development in mice. The RIM-BP3 protein is associated with the manchette, a transient microtubular structure believed to be important for morphogenesis during spermiogenesis. Targeted deletion of the RIM-BP3 gene resulted in male infertility owing to abnormal sperm heads, which are characterized by a deformed nucleus and a detached acrosome. Consistent with its role in morphogenesis, the RIM-BP3 protein physically associates with Hook1, a known manchette-bound protein required for sperm head morphogenesis. Interestingly, RIM-BP3 does not interact with the truncated Hook1 protein characterized in azh (abnormal spermatozoon head) mutant mice. Moreover, RIM-BP3 and Hook1 mutant mice display several common abnormalities, in particular with regard to the ectopic positioning of the manchette within the spermatid, a presumed cause of sperm head deformities. These observations suggest an essential role for RIM-BP3 in manchette development and function through its interaction with Hook1. As the occurrence of deformed spermatids is one of the common abnormalities leading to malfunctional sperm, identification of RIM-BP3 might provide insight into the molecular cue underlying causes of male infertility in humans.

Genomic origin, processing and developmental expression of testicular outer dense fiber 2 (ODF2) transcripts and a novel nucleolar localization of ODF2 protein

Outer dense fibers are a major constituent of the sperm tail and outer dense fiber 2 (ODF2) protein is one of their major components. ODF2 shares partial homology with cenexin 1 and cenexin 2, regarded as centriolar proteins. We show that ODF2 and cenexin 2 transcripts are the product of differential splicing of a single gene, designated Cenexin/ODF2 and that cenexin 1 is an incomplete clone of ODF2. ODF2 terminates in exon 20b whereas in cenexin 2 this exon is spliced out and translation terminates in exon 24. We demonstrate a transcriptional switch during rat testicular development, from somatic-type to testis-type ODF2 and cenexin transcripts during the onset of meiosis. The switch is completed when spermiogenesis is established. ODF2 immunoreactive sites were visualized in the acroplaxome, along the sperm tail and the centrosome-derived sperm head-to-tail coupling apparatus. An unexpected finding was the presence of ODF2 antigenic sites, but not cenexin antigenic sites, in the dense fibrillar component of the nucleolus of Sertoli cells, spermatogonia and primary spermatocytes. The characterization of the genomic origin, processing and developmental expression of ODF2 transcript isoforms and their protein products can help reconcile differences in the literature on the role of ODF2 and cenexin in the centrosome. Furthermore, the finding of ODF2 in the dense fibrillar component of the nucleolus suggests that this protein, in addition to its presence in sperm outer dense fibers and centrosome, highlights and adds to the nucleolar function during spermatogenesis and early embryogenesis.

Intramanchette transport during primate spermiogenesis: expression of dynein, myosin Va, motor recruiter myosin Va, VIIa-Rab27a/b interacting protein, and Rab27b in the manchette during human and monkey spermiogenesis

AIM

To show whether molecular motor dynein on a microtubule track, molecular motor myosin Va, motor recruiter myosin Va, VIIa-Rab27a/b interacting protein (MyRIP), and vesicle receptor Rab27b on an F-actin track were present during human and monkey spermiogenesis involving intramanchette transport (IMT).

METHODS

Spermiogenic cells were obtained from three men with obstructive azoospermia and normal adult cynomolgus monkey (Macaca fascicularis). Immunocytochemical detection and reverse transcription-polymerase chain reaction (RT-PCR) analysis of the proteins were carried out. Samples were analyzed by light microscope.

RESULTS

Using RT-PCR, we found that dynein, myosin Va, MyRIP and Rab27b were expressed in monkey testis. These proteins were localized to the manchette, as shown by immunofluorescence, particularly during human and monkey spermiogenesis.

CONCLUSION

We speculate that during primate spermiogenesis, those proteins that compose microtubule-based and actin-based vesicle transport systems are actually present in the manchette and might possibly be involved in intramanchette transport.

[The acrosome: comparative morphology and development, contribution of a human familial globozoospermia case report]

Sperm acrosome is known to play a role in the fertilization of the majority of animal species studied. As a general rule, the acrosome appeared as soon as the fertilization occurred out of aquaeous phase. The biochemical content of acrosome as well as its release mode could suggest it is a simple lysosome. But this would by pass its important morphogenic role in spermiogenesis. Its development is strongly linked to the development of the microtubules manchette system. Molecular data of animal mutagenesis contribute to the understanding of acrosome biogenesis mechanisms. Globozoospermia is a rare but severe human teratozoospermia, characterized by ejaculates entirely consisting of round-headed spermatozoa that lack an acrosome. It originates from a disturbed acrosome biogenesis. Recently, the genetic study of a familial globozoospermia led to highlight a homozygote mutation of the gene SPATA16, linked to the globozoospermic phenotype. This study contributes to the understanding of the mechanisms implied in human acrosome formation.

Factors maintaining normal sperm tail structure during epididymal maturation studied in Gopc-/- mice

Gopc (Golgi-associated PDZ- and coiled-coil motif-containing protein)(-/-) mice are infertile, showing globozoospermia, coiled tails, and a stratified mitochondrial sheath. Transmission electron microscope (TEM) images of the spermatozoa were studied quantitatively to analyze disorganization processes during epididymal passage. Factors maintaining straight tail and normal mitochondrial sheath were also studied by TEM and immunofluorescent microscopy. Sperm tails retained a normal appearance in the proximal caput epididymidis. Tail disorganization started between the proximal and the middle caput epididymidis, and the latter is the major site for it. The tail moved up through the defective posterior ring and coiled around the nucleus to various degrees. Tail coiling occurred in the caput epididymidis suggesting it was triggered by cytoplasmic droplet migration. SPATA19/spergen-1, a candidate mitochondrial adhesion protein, remained on the stratified mitochondria, while GPX4/PHGPx, a major element of the mitochondrial capsule, was unevenly distributed on them. From these findings, we speculate GPX4 is necessary to maintain normal sheath structure, and SPATA19 prevents dispersal of mitochondria, resulting in a stratified mitochondrial sheath formation in Gopc(-/-) spermatozoa. The epididymal epithelium was normal in structure and LRP8/apoER2 expression suggesting that tail abnormality is due to intrinsic sperm factors. Three cell structures are discussed as requisite factors for maintaining a straight tail during epididymal maturation: 1) a complete posterior ring to prevent invasion of the tail into the head compartment, 2) stable attachment of the connecting piece to the implantation fossa, and 3) a normal mitochondrial sheath supported by SPATA19 and supplied with sufficient and normally distributed GPX4.

Most expressed transcripts in sexual organs and other tissues

This study characterizes the most highly expressed transcripts of 15 intact tissues in mice by using the serial analysis of gene expression (SAGE) strategy which indicates the relative level of expression for each transcript matched to the tag. We show that the most abundant transcripts in the prostate, testis, and skeletal muscle characterize the main functions of these organs as an exocrine gland of male reproduction, spermatogenesis, and contraction, respectively. In addition, the top nine most abundant transcripts in the testis are tissue-specific genes while the most abundant transcripts in the prostate are also abundantly expressed in the liver. Furthermore, the most abundant transcripts in the ovary, mammary gland, and vagina are related to steroidogenesis, adipocytes, and keratinization, respectively, whereas genes involved in the cell defence are abundantly expressed in the liver, lung, bone, mammary gland, and adipose tissue. These findings suggest that the top 10 transcripts are sufficient to characterize each tissue of the body.

Sperm tail abnormalities in mutant mice with neo(r) gene insertion into an intron of the keratin 9 gene

Keratin 9 (K9) is one of the components of the perinuclear ring of the manchette found in developing spermatids but is predominantly expressed in the epidermis of the footpad (palm and sole in human epidermis). As an initial step to determine the function of K9 protein in sperm development, we have generated a mutant mouse by homologous recombination of the targeting vector containing the disrupted K9 gene in which the neo(r) gene was inserted into the intron 6. This insertion resulted in the expression of two K9 mRNAs: a wild-type K9 mRNA, in which intron 6 with the neo(r) gene was completely spliced out, and a mutated form in which only a portion of the intron 6 between neo(r) gene and exon 7 was spliced out. While both heterozygous (K9(+/neo)) and homozygous (K9(neo/neo)) mutant mice expressed the wild-type form of K9 protein, the expression profile of the wild-type K9 in K9(neo/neo) mutants was modified. In addition, the open reading frame of the aberrant mRNA terminated at the exon 6/intron 6 splice site, resulting in a truncated K9 protein. Both K9(+neo) and K9(neo/neo) male mice displayed spermatids with ectopic manchette. Coiled tails were seen in maturing spermatids and epididymal sperm of mutant mice and sperm with deformed tails displayed forward motility. A predominant sperm anomaly was residual cytoplasm at the end of the mitochondria-containing middle piece tail segment. The residual cytoplasm displayed vesicles with random in situ motion, suggesting a transport impediment toward the distal end of the sperm tail. All mutant mice were fertile. Surprisingly, in oocyte nuclear injection experiments using K9(neo/neo) sperm donor, 76% of the resulting animals displayed a deletion of the neo(r) gene from the intron 6 of the mutated K9 allele. Results of this study support the view that intron 6 influences the transcriptional efficiency of the K9 gene by decreasing production of wild-type K9 and changing the expression of K9 proteins.

The acrosome-acroplaxome-manchette complex and the shaping of the spermatid head

A combination of exogenous contractile forces generated by a stack of F-actin-containing hoops embracing the apical region of the elongating spermatid nucleus and an endogenous modulating mechanism dependent on the spermatid-containing acrosome-acroplaxome-manchette complex may play a cooperative role in the shaping of the spermatid head. In addition, the manchette is a key element in the transport of vesicles and macromolecules to the centrosome and developing spermatid tails as well as in nucleocytoplasmic transport. The proposed model of spermatid head shaping is based on: 1) currently known structural and molecular components of the F-actin hoops, the main cytoskeletal element of the Sertoli cell ectoplasmic specializations; 2) the molecular features of acrosome biogenesis; 3) the assembly of a subacrosomal cytoskeletal plate called the acroplaxome; and 4) the spatial relationship of the acrosome-acroplaxome complex with the manchette, a transient microtubular/actin-containing structure. During acrosome biogenesis, the acroplaxome becomes the nucleation site to which Golgi-derived proacrosomal vescicles tether and fuse. The acroplaxome has at least two functions: it anchors the developing acrosome to the elongating spermatid head. It may also provide a mechanical scaffolding plate during the shaping of the spermatid nucleus. The plate is stabilized by a marginal ring with junctional complex characteristics, adjusting to exogenous clutching forces generated by the stack of Sertoli cell F-actin-containing hoops applied to the elongating spermatid head. A tubulobulbar complex, formed by cytoplasmic processes protruding from the elongating spermatid head extending into the adjacent Sertoli cell, is located at the concave side of the spermatid head. The tubulobulbar complex might provide stabilizing conditions, together with the actin-afadin-nectin-2/nectin-3 adhesion unit, to enable sustained and balanced clutching exogenous forces applied during the elongation of the spermatid head.

Intracytoplasmic sperm injection effects in infertile azh mutant mice

Several reports in the literature describe men with infertility resulting from abnormal sperm head shape or decapitation defects of their spermatozoa. These defects are similar to those shown for the spermatozoa from azh (abnormal spermatozoon head shape) mice. The present study examines the efficiency and effects of intracytoplasmic sperm injection (ICSI) in successive generations of azh mice generated with this method. Three successive generations of azh mice were produced with ICSI. In all three ICSI series, more than 80% of 2-cell embryos were obtained, and more than 35% of embryos transferred gave rise to normal live offspring. In addition, ICSI was used to cross homozygous azh/azh males with homozygous azh/azh females, and live offspring were obtained. The ICSI-derived males were tested for their fecundity and abnormalities of sperm morphology. Spermatozoa from ICSI-derived azh/+ males did not show any impairment of fecundity in in vitro fertilization. These spermatozoa successfully fertilized oocytes from both C57BL/6 and B6D2F1 females, with fertilization rates ranging from 70%- 92%. The proportion of morphologically normal spermatozoa was similar in azh/+ males from three successive generations of ICSI (57.8%, 54.8%, and 49.0%, respectively), and no differences were noted when comparing ICSI-derived males with males derived by mating (57.6%) and with wild-type controls (61.6%). Detailed analysis differentiating between specific types of anomalies of sperm morphology did not reveal significant differences among the examined groups. The results of the present study demonstrate that ICSI does not enhance the azh mutation phenotype in the offspring and brings no risks when applied continuously. Moreover, serial (successive generations) ICSI is highly efficient in maintaining valuable mice with fertility problems.

The Acroplaxome Is the Docking Site of Golgi-Derived Myosin Va/Rab27a/b-Containing Proacrosomal Vesicles in Wild-Type and Hrb Mutant Mouse Spermatids1

Acrosome biogenesis involves the transport and fusion of Golgi-derived proacrosomal vesicles along the acroplaxome, an F-actin/keratin 5-containing cytoskeletal plate anchored to the spermatid nucleus. A significant issue is whether the acroplaxome develops in acrosomeless mutant mice. Male mice with a Hrb null mutation are infertile and both spermatids and sperm are round-headed and lack an acrosome. Hrb, a protein that contains several NPF motifs (Asn-Pro-Phe) and interacts with proteins with Eps15 homology domains, is regarded as critical for the docking and/or fusion of Golgi-derived proacrosomal vesicles. Here we report that the lack of an acrosome in Hrb mutant spermatids does not prevent the development of the acroplaxome. Yet the acroplaxome in the mutant contains F-actin but is deficient in keratin 5. We also show that the actin-based motor protein myosin Va and its receptor, Rab27a/b, known to be involved in vesicle transport, are present in the Golgi and Golgi-derived proacrosomal vesicles in wild-type and Hrb mutant mouse spermatids. In the Hrb mutant, myosin-Va-bound proacrosome vesicles tether to the acroplaxome, where they flatten and form a flat sac, designated pseudoacrosome. As spermiogenesis advances, round-shaped spermatid nuclei of the mutant display several nuclear protrusions, designated nucleopodes. Nucleopodes are consistently found at the acroplaxome- pseudoacrosome site. Our findings support the interpretation that the acroplaxome provides a focal point for myosin-Va/ Rab27a/b-driven proacrosomal vesicles to accumulate, coalesce, and form an acrosome in wild-type spermatids and a pseudoacrosome in Hrb mutant spermatids. We suggest that nucleopodes develop at a site where a keratin 5-deficient acroplaxome may not withstand tension forces operating during spermatid nuclear shaping.

The actin-based motor myosin Va is a component of the acroplaxome, an acrosome-nuclear envelope junctional plate, and of manchette-associated vesicles

Protein and vesicle cargos can be mobilized during spermiogenesis by intramanchette transport utilizing microtubule-based protein motors (kinesins and dyneins). However, actin-based unconventional myosin motors may also play a significant role in targeting vesicle cargos to subcellular compartments during sperm development. Here we report that myosin Va, an actin-based motor protein, is a component of the acroplaxome of rodent spermatids. The acroplaxome is an F-actin/keratin-containing scaffold plate with a marginal ring fastening the caudal recess of the developing acrosome to the nuclear envelope during spermatid nuclear shaping. In contrast to the acroplaxome, fluorescently labeled phalloidin does not produce an obvious F-actin signal in the manchette. However, immunogold electron microscopy detects moderate but specific beta-actin immunoreactivity along interconnected tube-like bundles of manchette microtubules. We also show that the membrane of vesicles co-fractionated with intact manchettes by sucrose gradient ultracentrifugation display immunogold-labeled myosin Va. Myosin Va vesicle localization is known to correlate with Rab proteins, monomeric GTPases of the Ras superfamily which recruit myosin Va/VIIa motor proteins through intermediate proteins. RT-PCR analysis demonstrates that transcripts for Rab27a and Rab27b and Slac2-c (a protein that links Rab27a/b to myosin Va/VIIa) are expressed in testis. These results indicate that two independent cytoskeletal tracks, F-actin in the acroplaxome and presumably in the manchette, and manchette microtubules, may facilitate short-range (from the Golgi to the acrosome) and long-range (from the manchette to the centrosome and axoneme) mobilization of appropriate cargos during spermiogenesis.

Intratesticular signals for progression of germ cell stages in vertebrates

The mechanisms underlying the complexity of spermatogenesis and spermiogenesis have deeply been studied in recent years. Transgenic animals, gene-targeting techniques, and lower vertebrate animal models have led to the discovery of some of the intratesticular signals involved in germ cell progression. This review wish to give the state of the art about it with particular emphasis on the comparative approach.

Acroplaxome, an F-actin-keratin-containing plate, anchors the acrosome to the nucleus during shaping of the spermatid head

Nuclear shaping is a critical event during sperm development as demonstrated by the incidence of male infertility associated with abnormal sperm ad shaping. Herein, we demonstrate that mouse and rat spermatids assemble in the subacrosomal space a cytoskeletal scaffold containing F-actin and Sak57, a keratin ortholog. The cytoskeletal plate, designated acroplaxome, anchors the developing acrosome to the nuclear envelope. The acroplaxome consists of a marginal ring containing keratin 5 10-nm-thick filaments and F-actin. The ring is closely associated with the leading edge of the acrosome and to the nuclear envelope during the elongation of the spermatid head. Anchorage of the acroplaxome to the gradually shaping nucleus is not disrupted by hypotonic treatment and brief Triton X-100 extraction. By examining spermiogenesis in the azh mutant mouse, characterized by abnormal spermatid/sperm head shaping, we have determined that a deformity of the spermatid nucleus is restricted to the acroplaxome region. These findings lead to the suggestion that the acroplaxome nucleates an F-actin-keratin-containing assembly with the purpose of stabilizing and anchoring the developing acrosome during spermatid nuclear elongation. The acroplaxome may also provide a mechanical planar scaffold modulating external clutching forces generated by a stack of Sertoli cell F-actin-containing hoops encircling the elongating spermatid nucleus.

Meiosis in male PL/J mice: a genetic model for gametic aneuploidy

Sperm from mice of the PL/J strain have a high frequency of sperm-head morphology abnormalities. Fluorescence in situ hybridization (FISH) methods revealed that PL/J sperm are also characterized by a high frequency of aneuploidy. The traits of abnormal sperm head morphology and aneuploidy are associated with numerous meiotic abnormalities. Spermatocytes of PL/J mice exhibit chromosome asynapsis during meiotic prophase as well as reduced crossing over, revealed by analysis of both MLH1 foci in pachytene spermatocytes and chiasmata seen at the first meiotic metaphase. During the first meiotic division, roughly one-third of the PL/J spermatocytes exhibit aberrant spindle morphology, with abnormalities including monopolar spindles, split spindle poles, and incomplete spindle formation and centrosomal abnormalities. F1 progeny of a cross between PL/J and C57BL/6J did not exhibit a high frequency of either sperm aneuploidy or sperm head morphology aberrations, as would be expected if the PL/J traits were dominant. Among progeny of a backcross of F1 mice to PL/J, none of 16 males assessed exhibited elevated frequencies of sperm head morphology abnormalities. Four of the individuals exhibited elevated sperm aneuploidy, but not at the levels of the PL/J parents. Thus, it is likely that the aberrant PL/J traits are due to several genes and/or modifiers affecting the generation of both sperm aneuploidy and abnormal sperm head morphology.

Intramanchette transport (IMT): managing the making of the spermatid head, centrosome, and tail

Intramanchette transport (IMT) and intraflagellar transport (IFT) share similar molecular components: a raft protein complex transporting cargo proteins mobilized along microtubules by molecular motors. IFT, initially discovered in flagella of Chlamydomonas, has been also observed in cilia of the worm Caenorhabditis elegans and in mouse ciliated and flagellated cells. IFT has been defined as the mechanism by which protein raft components (also called IFT particles) are displaced between the flagellum and the plasma membrane in the anterograde direction by kinesin-II and in the retrograde direction by cytoplasmic dynein 1b. Mutation of the gene Tg737, encoding one of the components of the raft protein complex, designated Polaris in the mouse and IFT88 in both Chlamydomonas and mouse, results in defective ciliogenesis and flagellar development as well as asymmetry in left-right axis determination. Polaris/IFT88 is detected in the manchette of mouse and rat spermatids. Indications of an IMT mechanism originated from the finding that two proteins associated with the manchette (Sak57/K5 and TBP-1, the latter a component of the 26S proteasome) repositioned to the centrosome and sperm tail once the manchette disassembled. IMT has the features of the IFT machinery but, in addition, facilitates nucleocytoplasmic exchange activities during spermiogenesis. An example is Ran, a small GTPase present in the nucleus and cytoplasm of round spermatids and in the manchette of elongating spermatids. Upon disassembly of the manchette, Ran GTPase is found in the centrosome region of elongating spermatids. Because defective molecular motors and raft proteins result in defective flagella, cilia, and cilia-containing photoreceptor cells in the retina, IMT and IFT are emerging as essential mechanisms for managing critical aspects of sperm development. Details of specific role of Ran GTPase in nucleocytoplasmic transport and its relocation from the manchette to the centrosome to the sperm tail await elucidation.

Ran, a GTP-binding protein involved in nucleocytoplasmic transport and microtubule nucleation, relocates from the manchette to the centrosome region during rat spermiogenesis

Ran, a Ras-related GTPase, is required for transporting proteins in and out of the nucleus during interphase and for regulating the assembly of microtubules. cDNA cloning shows that rat testis, like mouse testis, expresses both somatic and testis-specific forms of Ran-GTPase. The presence of a homologous testis-specific form of Ran-GTPase in rodents implies that the Ran-GTPase pathway plays a significant role during sperm development. This suggestions is supported by distinct Ran-GTPase immunolocalization sites identified in developing spermatids. Confocal microscopy demonstrates that Ran-GTPase localizes in the nucleus of round spermatids and along the microtubules of the manchette in elongating spermatids. When the manchette disassembles, Ran-GTPase immunoreactivity is visualized in the centrosome region of maturing spermatids. The circumstantial observation that fractionated manchettes, containing copurified centrin-immunoreactive centrosomes, can organize a three-dimensional lattice in the presence of taxol and GTP, points to the role of Ran-GTPase and associated factors in microtubule nucleation as well as the potential nucleating function of spermatid centrosomes undergoing a reduction process. Electron microscopy demonstrates the presence in manchette preparations of spermatid centrosomes, recognized as such by their association with remnants of the implantation fossa, a dense plate observed only at the basal surface of developing spermatid and sperm nuclei. In addition, we have found importin beta1 immunoreactivity in the nucleus of elongating spermatids, a finding that, together with the presence of Ran-GTPase in the nucleus of round spermatids and the manchette, suggest a potential role of Ran-GTPase machinery in nucleocytoplasmic transport. Our expression and localization analysis, correlated with functional observations in other cell systems, suggest that Ran-GTPase may be involved in both nucleocytoplasmic transport and microtubules assembly, two critical events during the development of functional sperm. In addition, the manchette-to-centrosome Ran-GTPase relocation, together with the similar redistribution of various proteins associated to the manchette, suggest the existence of an intramanchette molecular transport mechanism, which may share molecular analogies with intraflagellar transport.

Sperm axoneme: a tale of tubulin posttranslation diversity

Two microtubule-containing structures are assembled during spermiogenesis: a transient manchette and a stable axoneme. Both structures contain microtubules enriched in posttranslationally modified tubulins. Despite the existence of a spectrum of tubulin isotypes postulated by the multi-tubulin hypothesis, further extended by an elaborated array of posttranslational modifications, it is unknown how this diversity influences microtubule function. There is increasing evidence that different alpha beta-tubulin isotypes can affect the structure and function of microtubules. It is also becoming increasingly clear that eukaryotic cells encode other tubulin proteins expressed by the tubulin superfamily: gamma, delta epsilon, zeta eta, and FtsZ have been identified so far. Although the role of gamma-tubulin in the nucleation of microtubule assembly is well established, the function of delta-, epsilon-, zeta-, eta-, and FtsZ-tubulins is less understood. The members of the tubulin superfamilies found in spermatids include the alpha beta-tubulin dimer, in addition to gamma-tubulin in the centrosome, and delta-tubulin in the perinuclear ring region of the mouse spermatid manchette, the centrosome region, and flagellum. Posttranslational modifications in tubulin isotypes are predominant in the C-terminus exposed on the outside surface of the microtubule. This target site may influence the interaction of microtubule-associated proteins, including motor proteins, and therefore determine the functional specificity of tubulin isotypes. It remains to be determined whether other newcomers to the superfamily of tubulins contain sites prone to posttranslational modification.

Bypassing natural sperm selection during fertilization: the azh mutant offspring experience and the alternative of spermiogenesis in vitro

Molecular aspects of spermiogenesis can be studied using mouse mutants and spermatids developed in vitro. The azh/azh mutant is an attractive model system because structural abnormalities in the sperm head and the ectopic position of the manchette are associated with tail bending and looping. Spermatids, developing an axoneme in vitro and capable of cell motility, offer the possibility of the dynamic analysis of tail development. Offspring generated by intracytoplasmic injection of azh/azh sperm heads into normal mouse oocytes complement the mouse mutant approach. A central question of sperm tail development is the role of the manchette, a transient microtubular structure assembled soon after the organization of the axoneme. The fractionation of intact manchettes by gradient centrifugation has enabled a biochemical analysis of constitutive tubulin isotypes and transiently associated proteins. For example, keratins Sak57, Odf1, and Odf2 are initially stored in the manchette before being sorted to the outer dense fibers and fibrous sheath of the developing spermatid tail. Additional proteins associated with the manchette include two proteases, the 26S proteasome and N-arginine convertase (both sorted to the developing spermatid tail), a spermatid perinuclear RNA binding protein, Spag4, an Odf1-binding protein, and type 4 cAMP-specific phosphodiesterase D. Keratin 9 and delta-tubulin are two proteins found in the perinuclear ring of the manchette, the insertion site of the microtubular mantle. Available data indicate that the manchette is a highly dynamic structure providing microtubular tracks to structural proteins participating in the sperm tail development.

Offspring from normal mouse oocytes injected with sperm heads from the azh/azh mouse display more severe sperm tail abnormalities than the original mutant

Sperm with abnormalities in the position and shape of the head were obtained from the azh/azh mutant and injected into the cytoplasm of mature mouse oocytes to determine whether sperm from the offspring display both head (club shape) and tail (looping, folding, and fusion) abnormalities observed in the mutant donor. Although quantitative differences were observed among the three examined offspring, we found that abnormalities in sperm head shape were less frequent than in the donor mutant, but that tail malformations predominated. In addition, we found that the frequency of tail abnormalities increased during sperm epididymal transit. A typical defect was the multiple folding of the sperm tail and eventual fusion of closely apposed plasma membranes. As a consequence, sperm forward motility and natural fertility were compromised. Results of this study indicate that the azh/azh mutant and offspring generated by intracytoplasmic sperm injection provide a valuable model for determining the role of the manchette and keratin-containing outer dense fibers and fibrous sheath during spermiogenesis. Furthermore, our findings stress the risk of enhancing a phenotypic abnormality caused by mutant male genotypes introduced through bypassing the biologic mechanisms of natural sperm selection during fertilization.