Microtubule configurations and post-translational alpha-tubulin modifications during mammalian spermatogenesis

Kinesin-14 KIFC1 promotes acrosome formation and chromatin maturation during mouse spermiogenesis

KIFC1, a member of kinesin-14 subfamily motors, is essential for meiotic cell division and acrosome formation during spermatogenesis. However, the functions of KIFC1 in the formation and maintenance of the acrosome in male germ cells remain to be elucidated. In this study, we report the structural deformities of acrosomes in the in vivo KIFC1 inhibition mouse models. The proacrosomal vesicles diffuse into the cytoplasm and form atypical acrosomal granules. This phenotype is consistent with globozoospermia patients and probably results from the failure of the Golgi-derived vesicle trafficking and actin filament organization. Moreover, the multinucleated and undifferentiated spermatogenic cells in the epidydimal lumen after KIFC1 inhibition reveal the specific roles of KIFC1 in regulating post-meiotic maturation. Overall, our results uncover KIFC1 as an essential regulator in the trafficking, fusion and maturation of acrosomal vesicles during spermiogenesis.

Differentiation Disorders of Chara vulgaris Spermatids following Treatment with Propyzamide

Microtubules are cytoskeletal cell elements that also build flagella and cilia. Moreover, these structures participate in spermatogenesis and form a microtubular manchette during spermiogenesis. The present study aims to assess the influence of propyzamide, a microtubule-disrupting agent, on alga Chara vulgaris spermatids during their differentiation by means of immunofluorescent and electron microscopy methods. Propyzamide blocks the functioning of the β-tubulin microtubule subunit, which results in the creation of a distorted shape of a sperm nucleus at some stages. Present ultrastructural studies confirm these changes. In nuclei, an altered chromatin arrangement and nuclear envelope fragmentation were observed in the research as a result of incorrect nucleus-cytoplasm transport behavior that disturbed the action of proteolytic enzymes and the chromatin remodeling process. In the cytoplasm, large autolytic vacuoles and the dilated endoplasmic reticulum (ER) system, as well as mitochondria, were revealed in the studies. In some spermatids, the arrangement of microtubules present in the manchette was disturbed and the structure was also fragmented. The observations made in the research at present show that, despite some differences in the manchette between Chara and mammals, and probably also in the alga under study, microtubules participate in the intramanchette transport (IMT) process, which is essential during spermatid differentiation. In the present study, the effect of propyzamide on Chara spermiogenesis is also presented for the first time; however, the role of microtubule-associated proteins in this process still needs to be elucidated in the literature.

A Non-Synonymous Point Mutation in a WD-40 Domain Repeat of EML5 Leads to Decreased Bovine Sperm Quality and Fertility

This study is part of a concerted effort to identify and phenotype rare, deleterious mutations that adversely affect sperm quality, or convey high developmental and fertility potential to embryos and ensuing progeny. A rare, homozygous mutation in EML5 (EML5^R1654W), which encodes a microtubule-associated protein with high expression in testis and brain was identified in an Angus bull used extensively in artificial insemination (AI) for its outstanding progeny production traits. The bull’s fertility was low in cross-breeding timed AI (TAI) (Pregnancy/TAI = 25.2%; n = 222) and, in general, AI breeding to Nellore cows (41%; n = 822). A search of the 1,000 Bull Genomes Run9 database revealed an additional 74 heterozygous animals and 8 homozygous animals harboring this exact mutation across several different breeds (0.7% frequency within the 6,191 sequenced animals). Phenotypically, spermatozoa from the homozygous Angus bull displayed prominent piriform and tapered heads, and outwardly protruding knobbed acrosomes. Additionally, an increased retention of EML5 was also observed in the sperm head of both homozygous and heterozygous Angus bulls compared to wild-type animals. This non-synonymous point mutation is located within a WD40 signaling domain repeat of EML5 and is predicted to be detrimental to overall protein function by genomic single nucleotide polymorphism (SNP) analysis and protein modeling. Future work will examine how this rare mutation affects field AI fertility and will characterize the role of EML5 in spermatogenesis.

MEIG1 determines the manchette localization of IFT20 and IFT88, two intraflagellar transport components in male germ cells

Sperm flagella formation is a complex process that requires cargo transport systems to deliver structural proteins for sperm flagella assembly. Two cargo transport systems, the intramanchette transport (IMT) and intraflagellar transport (IFT), have been shown to play critical roles in spermatogenesis and sperm flagella formation. IMT exists only in elongating spermatids, while IFT is responsible for delivering cargo proteins in the developing cilia/flagella. Our laboratory discovered that mouse meiosis expressed gene 1 (MEIG1), a gene essential for sperm flagella formation, is present in the manchette of elongating spermatids. IFT complex components, IFT20 and IFT88, are also present in the manchette of the elongating spermatids. Given that the three proteins have the same localization in elongating spermatids and are essential for normal spermatogenesis and sperm flagella formation, we hypothesize that they are in the same complex, which is supported by co-immunoprecipitation assay using mouse testis extracts. In the Meig1 knockout mice, neither IFT20 nor IFT88 was present in the manchette in the elongating spermatids even though their localizations were normal in spermatocytes and round spermatids. However, MEIG1 was still present in the manchette in elongating spermatids of the conditional Ift20 knockout mice. In the sucrose gradient assay, both IFT20 and IFT88 proteins drifted from higher density fractions to lighter ones in the Meig1 knockout mice. MEIG1 distribution was not changed in the conditional Ift20 knockout mice. Finally, testicular IFT20 and IFT88 protein and mRNA levels were significantly reduced in Meig1 knockout mice. Our data suggests that MEIG1 is a key protein in determining the manchette localization of certain IFT components, including IFT20 and IFT88, in male germ cells.

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.

The testis-specific LINC component SUN3 is essential for sperm head shaping during mouse spermiogenesis

Sperm head shaping is a key event in spermiogenesis and is tightly controlled via the acrosome-manchette network. Linker of nucleoskeleton and cytoskeleton (LINC) complexes consist of Sad1 and UNC84 domain-containing (SUN) and Klarsicht/ANC-1/Syne-1 homology (KASH) domain proteins and form conserved nuclear envelope bridges implicated in transducing mechanical forces from the manchette to sculpt sperm nuclei into a hook-like shape. However, the role of LINC complexes in sperm head shaping is still poorly understood. Here we assessed the role of SUN3, a testis-specific LINC component harboring a conserved SUN domain, in spermiogenesis. We show that CRISPR/Cas9-generated Sun3 knockout male mice are infertile, displaying drastically reduced sperm counts and a globozoospermia-like phenotype, including a missing, mislocalized, or fragmented acrosome, as well as multiple defects in sperm flagella. Further examination revealed that the sperm head abnormalities are apparent at step 9 and that the sperm nuclei fail to elongate because of the absence of manchette microtubules and perinuclear rings. These observations indicate that Sun3 deletion likely impairs the ability of the LINC complex to transduce the cytoskeletal force to the nuclear envelope, required for sperm head elongation. We also found that SUN3 interacts with SUN4 in mouse testes and that the level of SUN4 proteins is drastically reduced in Sun3-null mice. Altogether, our results indicate that SUN3 is essential for sperm head shaping and male fertility, providing molecular clues regarding the underlying pathology of the globozoospermia-like phenotype.

Mechanism of Acrosome Biogenesis in Mammals

During sexual reproduction, two haploid gametes fuse to form the zygote, and the acrosome is essential to this fusion process (fertilization) in animals. The acrosome is a special kind of organelle with a cap-like structure that covers the anterior portion of the head of the spermatozoon. The acrosome is derived from the Golgi apparatus and contains digestive enzymes. With the progress of our understanding of acrosome biogenesis, a number of models have been proposed to address the origin of the acrosome. The acrosome has been regarded as a lysosome-related organelle, and it has been proposed to have originated from the lysosome or the autolysosome. Our review will provide a brief historical overview and highlight recent findings on acrosome biogenesis in mammals.

The sperm-associated antigen 6 interactome and its role in spermatogenesis

Mammalian SPAG6, the orthologue of Chlamydomonas reinhardtii PF16, is a component of the central apparatus of the '9 + 2' axoneme that controls ciliary/flagellar motility, including sperm motility. Recent studies revealed that SPAG6 has functions beyond its role in the central apparatus. Hence, we reexamined the role of SPAG6 in male fertility. In wild-type mice, SPAG6 was present in cytoplasmic vesicles in spermatocytes, the acrosome of round and elongating spermatids and the manchette of elongating spermatids. Spag6-deficient testes showed abnormal spermatogenesis, with abnormalities in male germ cell morphology consistent with the multi-compartment pattern of SPAG6 localization. The armadillo repeat domain of mouse SPAG6 was used as a bait in a yeast two-hybrid screen, and several proteins with diverse functions appeared multiple times, including Snapin, SPINK2 and COPS5. Snapin has a similar localization to SPAG6 in male germ cells, and SPINK2, a key protein in acrosome biogenesis, was dramatically reduced in Spag6-deficient mice which have defective acrosomes. SPAG16L, another SPAG6-binding partner, lost its localization to the manchette in Spag6-deficient mice. Our findings demonstrate that SPAG6 is a multi-functional protein that not only regulates sperm motility, but also plays roles in spermatogenesis in multiple cellular compartments involving multiple protein partners.

Developmental Testicular Expression, Cloning, and Characterization of Rat HDAC6 In Silico

We had previously reported presence of histone deacetylase 6 (HDAC6) in sperm and demonstrated its tubulin deacetylase activity and role in sperm motility in rat. In the present study we report its abundant expression in testis, epididymis, accessory sex organs, brain, and adrenal. In the testis, HDAC6 transcript and protein were observed throughout development. We therefore cloned the gene from rat testis using primers for hdac6 (accession number XM_228753.8) in order to determine the role of acetylation/deacetylation in spermatogenesis. The cloned rat hdac6 gene is ~3.5 kb with 28 exons and 1152 amino acids. We noted 4 single nucleotide polymorphisms (SNPs) on exons 2 (G/A), 5 (A/G), 7 (T/C), and 26 (G/T), respectively, in this sequence when compared to XM_228753.8. These were further validated at both cDNA and gene level. These SNPs resulted in 2 amino acids changes, namely, glycine → arginine and valine → phenylalanine at protein level. Cloned hdac6 overexpressed in HEK293T cells demonstrated significant overexpression by IIF. Alpha-tubulin acetylation analysis of the overexpressed cell lysate demonstrated that the protein was bioactive. This is the first study showing the ontogenic expression in the testis and reporting experimentally validated sequence of rat HDAC6 and its structural and functional annotation in silico. This sequence has been submitted to GenBank (Accession number Rattus KY009929.1).

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.

Proteome of the rete testis fluid from tropically-adapted Morada Nova rams

The rete testis has a close relationship with sperm development and may have other functions besides serving as an intercalated channel. The aim of this study was to identify and characterize the proteins of rete testis fluid (RTF) from tropically-adapted Morada Nova rams. Testicles obtained from six Morada Nova rams were dissected and the head of the epididymis was separated to access the efferent ducts. Rete testis fluid was obtained by gentle massage of the testis. The fluid was centrifuged to remove cell debris and sperm. RTF samples (containing 400μg protein) were separated by 2-D SDS-PAGE and gels, analyzed using PDQuest software (Bio Rad, USA). Proteins were identified using tandem mass spectrometry. Gene ontology and protein network were analyzed using the software tool for searching annotations of proteins (STRAP) and STRING database. Gels had, on average, 227±13.5 spots and 51% of the proteins were found above 40kDa, corresponding to 65% of the intensity of all spots detected. Based on gene ontology analysis, the most common biological processes associated with RTF proteins were regulation (24.3%) and cellular process (23.3%). Binding (27.3%) and catalytic activity (19.3%) corresponded to the most frequent molecular functions. Albumin, clusterin, serotransferrin, immunoglobulin gamma-1 chain and alpha-2-HS-glycoprotein were the most abundant proteins in the ram rete testis fluid. In conclusion, proteins identified in the ram rete testis fluid are linked to several physiological processes associated with sperm protection and spermatogenesis.

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.

TMF/ARA160 Governs the Dynamic Spatial Orientation of the Golgi Apparatus during Sperm Development

TMF/ARA160 is known to be a TATA element Modulatory Factor (TMF). It was initially identified as a DNA-binding factor and a coactivator of the Androgen receptor. It was also characterized as a Golgi-associated protein, which is essential for acrosome formation during functional sperm development. However, the molecular roles of TMF in this intricate process have not been revealed. Here, we show that during spermiogenesis, TMF undergoes a dynamic change of localization throughout the Golgi apparatus. Specifically, TMF translocates from the cis-Golgi to the trans-Golgi network and to the emerging vesicles surface, as the round spermatids develop. Notably, lack of TMF led to an abnormal spatial orientation of the Golgi and to the deviation of the trans-Golgi surface away from the nucleus of the developing round spermatids. Concomitantly, pro-acrosomal vesicles derived from the TMF^-/- Golgi lacked targeting properties and did not tether to the spermatid nuclear membrane thereby failing to form the acrosome anchoring scaffold, the acroplaxome, around the cell-nucleus. Absence of TMF also perturbed the positioning of microtubules, which normally lie in proximity to the Golgi and are important for maintaining Golgi spatial orientation and dynamics and for chromatoid body formation, which is impaired in TMF^-/- spermatids. In-silico evaluation combined with molecular and electron microscopic analyses revealed the presence of a microtubule interacting domain (MIT) in TMF, and confirmed the association of TMF with microtubules in spermatogenic cells. Furthermore, the MIT domain in TMF, along with microtubules integrity, are required for stable association of TMF with the Golgi apparatus. Collectively, we show here for the first time that a Golgi and microtubules associated protein is crucial for maintaining proper Golgi orientation during a cell developmental process.

SKAP, an outer kinetochore protein, is required for mouse germ cell development

In sexually reproducing organisms, accurate gametogenesis is crucial for the transmission of genetic material from one generation to the next. This requires the faithful segregation of chromosomes during mitotic and meiotic divisions. One of the main players in this process is the kinetochore, a large multi-protein complex that forms at the interface of centromeres and microtubules. Here, we analyzed the expression profile and function of small kinetochore-associated protein (SKAP) in the mouse. We found that two distinct SKAP isoforms are specifically expressed in the germline: a smaller isoform, which is detected in spermatogonia and spermatocytes and localized in the outer mitotic and meiotic kinetochores from metaphase to telophase, and a larger isoform, which is expressed in the cytoplasm of elongating spermatids. We generated SKAP-deficient mice and found that testis size and sperm production were severely reduced in mutant males. This phenotype was partially caused by defects during spermatogonia proliferation before entry into meiosis. We conclude that mouse SKAP, while being dispensable for somatic cell divisions, has an important role in the successful outcome of male gametogenesis. In germ cells, analogous to what has been suggested in studies using immortalized cells, SKAP most likely stabilizes the interaction between kinetochores and microtubules, where it might be needed as an extra safeguard to ensure the correct segregation of mitotic and meiotic chromosomes.

A MEIG1/PACRG complex in the manchette is essential for building the sperm flagella

A key event in the process of spermiogenesis is the formation of the flagella, which enables sperm to reach eggs for fertilization. Yeast two-hybrid studies revealed that meiosis-expressed gene 1 (MEIG1) and Parkin co-regulated gene (PACRG) interact, and that sperm-associated antigen 16, which encodes an axoneme central apparatus protein, is also a binding partner of MEIG1. In spermatocytes of wild-type mice, MEIG1 is expressed in the whole germ cell bodies, but the protein migrates to the manchette, a unique structure at the base of elongating spermatid that directs formation of the flagella. In the elongating spermatids of wild-type mice, PACRG colocalizes with α-tubulin, a marker for the manchette, whereas this localization was not changed in the few remaining elongating spermatids of Meig1-deficient mice. In addition, MEIG1 no longer localizes to the manchette in the remaining elongating spermatids of Pacrg-deficient mice, indicating that PACRG recruits MEIG1 to the manchette. PACRG is not stable in mammalian cells, but can be stabilized by MEIG1 or by inhibition of proteasome function. SPAG16L is present in the spermatocyte cytoplasm of wild-type mice, and in the manchette of elongating spermatids, but in the Meig1 or Pacrg-deficient mice, SPAG16L no longer localizes to the manchette. By contrast, MEIG1 and PACRG are still present in the manchette of Spag16L-deficient mice, indicating that SPAG16L is a downstream partner of these two proteins. Together, our studies demonstrate that MEIG1/PACRG forms a complex in the manchette and that this complex is necessary to transport cargos, such as SPAG16L, to build the sperm flagella.

Summary: In the manchette, a structure at the base of the elongating spermatid, the proteins MEIG1 and PACRG act in a complex to control cargo transport and direct formation of the flagellum.

Mechanisms of spermiogenesis and spermiation and how they are disturbed

Haploid round spermatids undergo a remarkable transformation during spermiogenesis. The nucleus polarizes to one side of the cell as the nucleus condenses and elongates, and the microtubule-based manchette sculpts the nucleus into its species-specific head shape. The assembly of the central component of the sperm flagellum, known as the axoneme, begins early in spermiogenesis, and is followed by the assembly of secondary structures needed for normal flagella. The final remodelling of the mature elongated spermatid occurs during spermiation, when the spermatids line up along the luminal edge, shed their residual cytoplasm and are ultimately released into the lumen. Defects in spermiogenesis and spermiation are manifested as low sperm number, abnormal sperm morphology and poor motility and are commonly observed during reproductive toxicant administration, as well as in genetically modified mouse models of male infertility. This chapter summarizes the major physiological processes and the most commonly observed defects in spermiogenesis and spermiation, to aid in the diagnosis of the potential mechanisms that could be perturbed by experimental manipulation such as reproductive toxicant administration.

Microtubules and spermatogenesis

Microtubules are dynamic polymers of tubulin subunits that underpin many essential cellular processes, such as cell division and migration. Spermatogenesis is the process by which spermatogenic stem cells undergo mitotic and meiotic division and differentiation to produce streamlined spermatozoa capable of motility and fertilization. This review summarizes the current knowledge of microtubule-based processes in spermatogenesis. We describe the involvement of microtubule dynamics in Sertoli cell shape and function, as well as in the mitotic and meiotic division of germ cells. The roles of microtubules in sperm head shaping, via the development and function of the manchette, and in sperm flagella development are also discussed. The review brings together data from microscopy studies and genetically modified mouse models, and reveals that the regulation of microtubule dynamics is essential for 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.

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.

PP1 forms an active complex with TLRR (lrrc67), a putative PP1 regulatory subunit, during the early stages of spermiogenesis in mice

Mammalian spermatogenesis is a highly regulated developmental pathway that demands dramatic rearrangement of the cytoskeleton of the male germ cell. We have described previously a leucine rich repeat protein, TLRR (also known as lrrc67), which is associated with the spermatid cytoskeleton in mouse testis and is a binding partner of protein phosphatase-1 (PP1), an extremely well conserved signaling molecule. The activity of PP1 is modulated by numerous specific regulators of which TLRR is a candidate. In this study we measured the phosphatase activity of the TLRR-PP1 complex in the adult and the developing mouse testis, which contains varying populations of developing germ cell types, in order to determine whether TLRR acts as an activator or an inhibitor of PP1 and whether the phosphatase activity of this complex is developmentally regulated during spermatogenesis. Additionally, we assayed the ability of bacterially expressed TLRR to affect the enzymatic activity of PP1. Furthermore, we examined phosphorylation of TLRR, and elements of the spermatid cytoskeleton during the first wave of spermatogenesis in the developing testis. We demonstrate here that the TLRR complex is associated with a phosphatase activity in adult mouse testis. The relative phosphatase activity of this complex appears to reach a peak at about 21 days after birth, when pachytene spermatocytes and round spermatids are abundant in the seminiferous epithelium of the mouse testis. TLRR, in addition to tubulin and kinesin-1B, is phosphorylated during the first wave of spermatogenesis. These findings indicate that the TLRR-PP1 complex is active prior to translocation of TLRR toward the sperm flagella and that TLRR, and constituents of the spermatid cytoskeleton, may be subject to regulation by reversible phosphorylation during spermatogenesis in murine testis.

Transcriptional and proteomic profiling of flatfish (Solea senegalensis) spermatogenesis

The Senegalese sole (Solea senegalensis) is a marine flatfish of high economic value and a target species for aquaculture. The efforts to reproduce this species in captivity have been hampered by the fact that farmed males (F1) often show lower sperm production and fertilization capacity than wild-type males (F0). Our knowledge on spermatogenesis is however limited to a few studies. In a previous work, we identified by 2-D DIGE several potential protein markers in testis for the poor reproductive performance of F1 males. Therefore, the objectives of the present study were, first, to investigate changes in genes and proteins expressed in the testis throughout spermatogenesis in F0 males by using a combination of transcriptomic and proteomic approaches and, second, to further compare the testis proteome between late spermatogenic stages of F0 and F1 fish to identify potential indicators of hampered reproductive performance in F1 fish. We identified approximately 400 genes and 49 proteins that are differentially expressed during the progression of spermatogenesis and that participate in processes such as transcriptional activation, the ubiquitin-proteasome system, sperm maturation and motility or cytoskeletal remodeling. Interestingly, a number of these proteins differed in abundance between F0 and F1 fish, pointing toward alterations in cytoskeleton, sperm motility, the ubiquitin-proteasome system and the redox state during spermiogenesis as possible causes for the decreased fertility of F1 fish.

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.

Expression pattern of acetylated alpha-tubulin in porcine spermatogonia

Mammalian spermatogonial stem cells reside on the basement membrane of the seminiferous tubules. The mechanisms responsible for maintenance of spermatogonia at the basement membrane are unclear. Since acetylated alpha-tubulin (Ac-alpha-Tu) is a component of long-lived, stable microtubules and deacetylation of alpha-tubulin enhances cell motility, we hypothesized that acetylation of alpha-tubulin might be associated with positioning of spermatogonia at the basement membrane. The expression pattern of Ac-alpha-Tu at different stages of testis development was characterized by immunohistochemistry for Ac-alpha-Tu and spermatogonia-specific proteins (PGP 9.5, DAZL). In immature pig testes, Ac-alpha-Tu was present exclusively in gonocytes at 1 week of age, and in a subset of spermatogonia at 10 weeks of age. At this age, spermatogonia are migrating toward the tubule periphery and Ac-alpha-Tu appeared polarized toward the basement membrane. In adult pig testes, Ac-alpha-Tu was detected in few single or paired spermatogonia at the basement membrane as well as in spermatids and spermatozoa. Only undifferentiated (DAZL-), proliferating (determined by BrdU incorporation) spermatogonia expressed high levels of Ac-alpha-Tu. Comparison with the expression pattern of beta-tubulin and tyrosinated alpha-tubulin confirmed that only Ac-alpha-Tu is specific to germ cells. The unique pattern of Ac-alpha-Tu in undifferentiated germ cells during postnatal development suggests that posttranslational modifications of microtubules may play an important role in recruiting and anchoring spermatogonia at the basement membrane. Mol. Reprod. Dev. 77: 348-352, 2010. (c) 2009 Wiley-Liss, Inc.

Gene organization, evolution and expression of the microtubule-associated protein ASAP (MAP9)

Background

ASAP is a newly characterized microtubule-associated protein (MAP) essential for proper cell-cycling. We have previously shown that expression deregulation of human ASAP results in profound defects in mitotic spindle formation and mitotic progression leading to aneuploidy, cytokinesis defects and/or cell death. In the present work we analyze the structure and evolution of the ASAP gene, as well as the domain composition of the encoded protein. Mouse and Xenopus cDNAs were cloned, the tissue expression characterized and the overexpression profile analyzed.

Results

Bona fide ASAP orthologs are found in vertebrates with more distantly related potential orthologs in invertebrates. This single-copy gene is conserved in mammals where it maps to syntenic chromosomal regions, but is also clearly identified in bird, fish and frog. The human gene is strongly expressed in brain and testis as a 2.6 Kb transcript encoding a ~110 KDa protein. The protein contains MAP, MIT-like and THY domains in the C-terminal part indicative of microtubule interaction, while the N-terminal part is more divergent. ASAP is composed of ~42% alpha helical structures, and two main coiled-coil regions have been identified. Different sequence features may suggest a role in DNA damage response. As with human ASAP, the mouse and Xenopus proteins localize to the microtubule network in interphase and to the mitotic spindle during mitosis. Overexpression of the mouse protein induces mitotic defects similar to those observed in human. In situ hybridization in testis localized ASAP to the germ cells, whereas in culture neurons ASAP localized to the cell body and growing neurites.

Conclusion

The conservation of ASAP indicated in our results reflects an essential function in vertebrates. We have cloned the ASAP orthologs in mouse and Xenopus, two valuable models to study the function of ASAP. Tissue expression of ASAP revealed a high expression in brain and testis, two tissues rich in microtubules. ASAP associates to the mitotic spindle and cytoplasmic microtubules, and represents a key factor of mitosis with possible involvement in other cell cycle processes. It may have a role in spermatogenesis and also represents a potential new target for antitumoral drugs. Possible involvement in neuron dynamics also highlights ASAP as a candidate target in neurodegenerative diseases.

Lymphocyte chemotaxis is regulated by histone deacetylase 6, independently of its deacetylase activity

In this work, the role of HDAC6, a type II histone deacetylase with tubulin deacetylase activity, in lymphocyte polarity, motility, and transmigration was explored. HDAC6 was localized at dynamic subcellular structures as leading lamellipodia and the uropod in migrating T-cells. However, HDAC6 activity did not appear to be involved in the polarity of migrating lymphocytes. Overexpression of HDAC6 in freshly isolated lymphocytes and T-cell lines increased the lymphocyte migration mediated by chemokines and their transendothelial migration under shear flow. Accordingly, the knockdown of HDAC6 expression in T-cells diminished their chemotactic capability. Additional experiments with HDAC6 inhibitors (trichostatin, tubacin), other structural related molecules (niltubacin, MAZ-1391), and HDAC6 dead mutants showed that the deacetylase activity of HDAC6 was not involved in the modulatory effect of this molecule on cell migration. Our results indicate that HDAC6 has an important role in the chemotaxis of T-lymphocytes, which is independent of its tubulin deacetylase activity.

Assembly of spermatid acrosome depends on microtubule organization during mammalian spermiogenesis

The acrosome is a secretory vesicle attached to the nucleus of the sperm. Our hypothesis is that microtubules participate in the membrane traffic between the Golgi apparatus and acrosome during the first steps of spermatid differentiation. In this work, we show that nocodazole-induced microtubule depolarization triggers the formation of vesicles of the acrosomal membrane, without detaching the acrosome from the nuclear envelope. Nocodazole also induced fragmentation of the Golgi apparatus as determined by antibodies against giantin, golgin-97 and GM130, and electron microscopy. Conversely, neither the acrosome nor the Golgi apparatus underwent fragmentation in elongating spermatids (acrosome- and maturation-phase). The microtubule network of round spermatids of azh/azh mice also became disorganized. Disorganization correlated with fragmentation of the acrosome and the Golgi apparatus, as evaluated by domain-specific markers. Elongating spermatids (acrosome and maturation-phase) of azh/azh mice also had alterations in microtubule organization, acrosome, and Golgi apparatus. Finally, the spermatozoa of azh/azh mice displayed aberrant localization of the acrosomal protein sp56 in both the post-acrosomal and flagellum domains. Our results suggest that microtubules participate in the formation and/or maintenance of the structure of the acrosome and the Golgi apparatus and that the organization of the microtubules in round spermatids is key to sorting acrosomal proteins to the proper organelle.

Cytoskeleton localization in the sperm head prior to fertilization

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

Role of microtubule-dependent membrane trafficking in acrosomal biogenesis

The role of microtubule-based trafficking in acrosomal biogenesis was examined by studying the effects of colchicine on spermiogenesis. In electron micrographs of untreated cap-phase mouse spermatids, coated vesicles were always seen on the apex and caudal margins of the developing acrosomal cap. The increase in volume and the accumulation of materials in the acrosome during the Golgi and cap phases were observed to occur via fusion of vesicles at various sites on the growing acrosome. By studying the acid phosphatase localization pattern and colchicine-treated spermatids, the role of clathrin-coated vesicles became clear. Coated vesicle formation at the caudal margin of the acrosome appeared to be responsible for the spreading and shaping of the acrosome over the surface of the nucleus and also established distinct regional differences in the acrosome. In colchicine-treated spermatids, the Golgi apparatus lost its typical membranous stack conformation and disintegrated into many small vesicles. Acrosome formation was retarded, and there was discordance of the spread of the acrosomal cap with that of the modified nuclear envelope. Many symplasts were also found because of the breakdown of intercellular bridges. Colchicine treatment thus indicated that microtubule-dependent trafficking of transport vesicles between the Golgi apparatus and the acrosome plays a vital role in acrosomal biogenesis. In addition, both anterograde and retrograde vesicle trafficking are extensively involved and seem to be equally important in acrosome formation.

Dynamic changes in the cytoskeleton during human spermiogenesis

OBJECTIVE

To investigate the structural changes in the cytoskeleton (microtubules, microfilaments) and examine the expression of centrosomal functional proteins during human spermiogenesis.

DESIGN

Immunofluorescent staining of human spermiogenic cells.

SETTING

University hospital and IVF clinic.

PATIENT(S)

Human testicular tissues were obtained by testicular sperm aspiration (TESA) under informed consent. Three cases of obstructive azoospermia, with confirmed normal spermatogenesis, were examined.

INTERVENTION(S)

Spermatogenic cells were fixed with microtubule-stabilizing buffer. Immunocytochemical detection of microtubules, microfilaments, and centrosome was performed using monoclonal antibodies against alpha- and beta-tubulin, phalloidin, and functional centrosomal proteins.

MAIN OUTCOME MEASURE(S)

Samples were examined using epifluorescence and laser scanning confocal microscopes.

RESULT(S)

During the Sb2 period, microtubules formed the manchette structure, which extended from the equator of the nucleus through the cytoplasm. Microfilaments were organized in the periacrosamal region during spermiogenesis (Sa to Sd). Although centrin was observed throughout the spermiogenic period, gamma-tubulin was detected only in the Sb2 period.

CONCLUSION(S)

Dynamic cytoskeletal movement was observed during human spermiogenesis. Cytoskeletal rearrangements in the Sb2 period appear to play important roles in the morphologic changes that occur during human spermiogenesis. Studies of the cytoskeletal system during spermiogenesis may help identify some causes of male infertility (e.g., teratozoospermia, maturation arrest).

Cytoskeletal dynamics during mammalian gametegenesis and fertilization: Implications for human reproduction

From gamete to neonate, human fertilization is a series of cell motilities (motion and morphological changes). Cytoskeletons play a role in cell motility as they work as a field worker in the cell. The present study is a review of dynamic motility of cytoskeletons (microfilaments and microtubules) during mammalian gamategenesis and fertilization. Dynamic and proper organization of cytoskeletons is crucial for the completion of oocyte maturation and spermatogenesis. By intracytoplasmic sperm injection, some difficulties in fertilization by sperm entry into the egg cytoplasm are overcome. However, the goal of fertilization is the union of the male and female genome, and sperm incorporation into an oocyte is nothing but the beginning of fertilization. Sperm centrosomal function, which introduces microtubule organization and promotes pronuclear apposition and first mitotic spindle formation, plays the leading role in the 'motility' of post-intracytoplasmic sperm injection events in fertilization. The present review introduces novel challenges in functional assessment of the human sperm centrosome. Furthermore, microtubule organization during development without the sperm centrosome (e.g. parthenogenesis) is mentioned. (Reprod Med Biol 2005; 4: 179-187).

Rho family GTPase Rnd2 interacts and co-localizes with MgcRacGAP in male germ cells

The male-germ-cell Rac GTPase-activating protein gene (MgcRacGAP) was initially described as a human RhoGAP gene highly expressed in male germ cells at spermatocyte stage, but exhibits significant levels of expression in most cell types. In somatic cells, MgcRacGAP protein was found to both concentrate in the midzone/midbody and be required for cytokinesis. As a RhoGAP, MgcRacGAP has been proposed to down-regulate RhoA, which is localized to the cleavage furrow and midbody during cytokinesis. Due to embryonic lethality in MgcRacGAP -null mutant mice and to the lack of an in vitro model of spermatogenesis, nothing is known regarding the role and mode of action of MgcRacGAP in male germ cells. We have analysed the expression, subcellular localization and molecular interactions of MgcRacGAP in male germ cells. Whereas MgcRacGAP was found only in spermatocytes and early spermatids, the widespread RhoGTPases RhoA, Rac1 and Cdc42 (which are, to various extents, in vitro substrates for MgcRacGAP activity) were, surprisingly, not detected at these stages. In contrast, Rnd2, a Rho family GTPase-deficient G-protein was found to be co-expressed with MgcRacGAP in spermatocytes and spermatids. MgcRacGAP was detected in the midzone of meiotic cells, but also, unexpectedly, in the Golgi-derived pro-acrosomal vesicle, co-localizing with Rnd2. In addition, a stable Rnd2-MgcRacGAP molecular complex could be evidenced by glutathione S-transferase pull-down and co-immunoprecipitation experiments. We conclude that Rnd2 is a probable physiological partner of MgcRacGAP in male germ cells and we propose that MgcRacGAP, and, quite possibly, other RhoGAPs, may participate in signalling pathways involving Rnd family proteins.

Control of membrane fusion during spermiogenesis and the acrosome reaction

Membrane fusion is important to reproduction because it occurs in several steps during the process of fertilization. Many events of intracellular trafficking occur during both spermiogenesis and oogenesis. The acrosome reaction, a key feature during mammalian fertilization, is a secretory event involving the specific fusion of the outer acrosomal membrane and the sperm plasma membrane overlaying the principal piece of the acrosome. Once the sperm has crossed the zona pellucida, the gametes fuse, but in the case of the sperm this process takes place through a specific membrane domain in the head, the equatorial segment. The cortical reaction, a process that prevents polyspermy, involves the exocytosis of the cortical granules to the extracellular milieu. In lower vertebrates, the formation of the zygotic nucleus involves the fusion (syngamia) of the male pronucleus with the female pronucleus. Other undiscovered membrane trafficking processes may also be relevant for the formation of the zygotic centrosome or other zygotic structures. In this review, we focus on the recent discovery of molecular machinery components involved in intracellular trafficking during mammalian spermiogenesis, notably related to acrosome biogenesis. We also extend our discussion to the molecular mechanism of membrane fusion during the acrosome reaction. The data available so far suggest that proteins participating in the intracellular trafficking events leading to the formation of the acrosome during mammalian spermiogenesis are also involved in controlling the acrosome reaction during fertilization.

In vivo destabilization of dynamic microtubules by HDAC6-mediated deacetylation

Trichostatin A (TSA) inhibits all histone deacetylases (HDACs) of both class I and II, whereas trapoxin (TPX) cannot inhibit HDAC6, a cytoplasmic member of class II HDACs. We took advantage of this differential sensitivity of HDAC6 to TSA and TPX to identify its substrates. Using this approach, alpha-tubulin was identified as an HDAC6 substrate. HDAC6 deacetylated alpha-tubulin both in vivo and in vitro. Our investigations suggest that HDAC6 controls the stability of a dynamic pool of microtubules. Indeed, we found that highly acetylated microtubules observed after TSA treatment exhibited delayed drug-induced depolymerization and that HDAC6 overexpression prompted their induced depolymerization. Depolymerized tubulin was rapidly deacetylated in vivo, whereas tubulin acetylation occurred only after polymerization. We therefore suggest that acetylation and deacetylation are coupled to the microtubule turnover and that HDAC6 plays a key regulatory role in the stability of the dynamic microtubules.

Alteration of the C-terminal amino acid of tubulin specifically inhibits myogenic differentiation

Detyrosination is an evolutionarily conserved post-translational modification of microtubule polymers that is known to be enhanced during early morphological differentiation of cultured myogenic cells (Gundersen, G. G., Khawaja, S., and Bulinski, J. C. (1989) J. Cell Biol. 109, 2275-2288). We proposed that altering the C terminus of alpha-tubulin by detyrosination plays a role in morphological differentiation. To test our hypothesis, we treated L6 myoblasts with 3-nitrotyrosine (Eiserich, J. P., Estevez, A. G., Bamberg, T. V., Ye, Y. Z., Chumley, P. H., Beckman, J. S., and Freeman, B. A. (1999) Proc. Natl. Acad. Sci. U. S. A. 96, 6365-6375), a nontoxic inhibitor that resulted in high level inhibition of microtubule detyrosination and low level incorporation of nitrotyrosine into microtubules. Even though microtubule stabilization or modification by acetylation still occurred normally, morphological differentiation was blocked; myoblasts neither elongated significantly nor fused. Nitrotyrosine treatment prevented synthesis or activation of markers of myogenic differentiation, including muscle-specific myosin, alpha-actin, integrin alpha(7), and myogenin. Consistent with this, myoblast integrin beta(1A) remained highly expressed. In contrast, the increase in beta-catenin level characteristic of early myogenesis was unaffected by treatment. These results show that the identity of the C-terminal residue of alpha-tubulin modulates microtubule activity, possibly because binding to or signaling from modified microtubules is required for the myogenic program.

Proteome analysis of rat spermatogonia: reinvestigation of stathmin spatio-temporal expression within the testis

Stathmin is a protein known to be involved in various cell processes including cell proliferation and differentiation. It has already been described in the testis but its recent identification using a proteomic approach in mitotic spermatogenetic stem cells named spermatogonia (Guillaume et al., 2000) has lead us to reinvestigate its expression within the testis. Stathmin and its mRNAs were studied in isolated cells by Western and Northern blots and in situ using immunohistochemistry. We demonstrated that stathmin is indeed expressed in spermatogonia, and that it is also intensively expressed in the meiotic spermatocytes and in the first generations of spermatids. Furthermore, we showed aggregations of the protein in the cytoplasm of the later generations of spermatids preceding its elimination at the time of spermiation. Our Northern blots reveal the presence of two stathmin transcripts of 1.1 and 3.2 kb within the testis from the fetal stage onwards, in spermatogonia, spermatocytes, and spermatids. However, the 3.2 kb RNA transcript was barely detectable in the spermatids. Stathmin expression is known to be associated with microtubule dynamics. Therefore, its expression in the germ line is most probably related to the extremely complex structural cellular rearrangements occurring in germ cells during spermatogenesis. However, the exact role of stathmin and the reason of the existence of two transcripts in the male germ lineage awaits further investigation.

Calmin, a protein with calponin homology and transmembrane domains expressed in maturing spermatogenic cells

A cDNA named calmin of approximately 3.2 kb was isolated by RNA differential display applied to developing mouse skin. Calmin cDNA encodes 1021 amino acids with two calponin homology (CH) domains in tandem on the N-terminal side and a transmembrane domain on the C-terminal side. The region covering the CH domains showed a high level of homology with beta-spectrin, alpha-actinin, and dystrophin. Among the proteins with the tandem CH domains, calmin is unique in having a transmembrane domain. Three alternative splicing sites were identified at the 3'-side of calmin, giving rise to polymorphic protein products with or without the transmembrane domain. The calmin transcript was detected in adult testis, liver, kidney, and large intestine; the expression in testis was far stronger than that in the other tissues. In situ hybridization and immunostaining revealed that calmin was expressed in maturing spermatogenic cells at later stages. Human calmin cDNA was also isolated, and its exon/intron organization was determined.