Chromatin reorganization in rat spermatids during the disappearance of testis-specific histone, H1t, and the appearance of transition proteins TP1 and TP2

The Ultrastructure and Composition of Bovine Spermatozoa

This chapter provides a cytological and compositional evaluation of the various compartments and sub-compartments making up the bull spermatozoon. The intention is to define the sperm head and tail compartments from an ultrastructural perspective and attribute to them their protein constituents gathered from both traditional and modern proteomic approaches. Common to both approaches, the compositional analysis is dependent on the fractionation and isolation of the sperm compartments combined with polyacrylamide gel electrophoresis (PAGE) and Western blotting to detect the identities of the proteins, and immunocytochemistry to confirm their residency. As will be appreciated, the identity of a particular sperm protein together with its residency provide valuable insights not only into its role, but also to the role of the specific sperm compartment it occupies, in development and/or fertilization. Attention is also given in this chapter to the consequences (on sperm structure and fertility) of inactivating genes that play key roles in sperm formation, especially if their phenotypes appear to match common bull sperm abnormalities. The keywords below cover the sperm head and tail compartments addressed in this chapter.

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.

Core Histones Are Constituents of the Perinuclear Theca of Murid Spermatozoa: An Assessment of Their Synthesis and Assembly during Spermiogenesis and Function after Gametic Fusion

The perinuclear theca (PT) of the eutherian sperm head is a cytoskeletal-like structure that houses proteins involved in important cellular processes during spermiogenesis and fertilization. Building upon our novel discovery of non-nuclear histones in the bovine PT, we sought to investigate whether this PT localization was a conserved feature of eutherian sperm. Employing cell fractionation, immunodetection, mass spectrometry, qPCR, and intracytoplasmic sperm injections (ICSI), we examined the localization, developmental origin, and functional potential of histones from the murid PT. Immunodetection localized histones to the post-acrosomal sheath (PAS) and the perforatorium (PERF) of the PT but showed an absence in the sperm nucleus. MS/MS analysis of selectively extracted PT histones indicated that predominately core histones (i.e., H3, H3.3, H2B, H2A, H2AX, and H4) populate the murid PT. These core histones appear to be de novo-synthesized in round spermatids and assembled via the manchette during spermatid elongation. Mouse ICSI results suggest that early embryonic development is delayed in the absence of PT-derived core histones. Here, we provide evidence that core histones are de novo-synthesized prior to PT assembly and deposited in PT sub-compartments for subsequent involvement in chromatin remodeling of the male pronucleus post-fertilization.

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

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

A rat H1t-GFP transgene recapitulates endogenous H1t expression pattern in mouse

H1 histones bind to linker DNA. H1t (H1f6), a testis-specific linker histone variant, is present in pachytene spermatocytes and spermatids. The expression of H1t histone coincides with the acquisition of metaphase I competence in pachytene spermatocytes. Here we report the generation of H1t-GFP transgenic mice. The H1t-GFP (H1 histone testis-green fluorescence protein) fusion protein expression recapitulates the endogenous H1t expression pattern. This protein appears first in mid pachytene spermatocytes in stage V seminiferous tubules, persists in round spermatids and elongating spermatids, but is absent in elongated spermatids. The strong green fluorescence signal, due to the high abundance of H1t-GFP, is maintained in spermatocytes after induction towards metaphase I through treatment with okadaic acid. Therefore, H1t-GFP can be used as a visual marker for monitoring the progression of meiosis in vitro and in vivo, as well as fluorescence-activated cell sorting (FACS) sorting of germ cells.

WBP2 shares a common location in mouse spermatozoa with WBP2NL/PAWP and like its descendent is a candidate mouse oocyte-activating factor

The sperm-borne oocyte-activating factor (SOAF) resides in the sperm perinuclear theca (PT). A consensus has been reached that SOAF most likely resides in the postacrosomal sheath (PAS), which is the first region of the PT to solubilize upon sperm–oocyte fusion. There are two SOAF candidates under consideration: PLCZ1 and WBP2NL. A mouse gene germline ablation of the latter showed that mice remain fertile with no observable phenotype despite the fact that a competitive inhibitor of WBP2NL, derived from its PPXY motif, blocks oocyte activation when coinjected with WBP2NL or spermatozoa. This suggested that the ortholog of WBP2NL, WBP2, containing the same domain and motifs associated with WBP2NL function, might compensate for its deficiency in oocyte activation. Our objectives were to examine whether WBP2 meets the developmental criteria established for SOAF and whether it has oocyte-activating potential. Immunoblotting detected WBP2 in mice testis and sperm and immunofluorescence localized WBP2 to the PAS and perforatorium of the PT. Immunohistochemistry of the testes revealed that WBP2 reactivity was highest in round spermatids and immunofluorescence detected WBP2 in the cytoplasmic lobe of elongating spermatids and colocalized it with the microtubular manchette during PT assembly. Microinjection of the recombinant forms of WBP2 and WBP2NL into metaphase II mouse oocytes resulted in comparable rates of oocyte activation. This study shows that WBP2 shares a similar testicular developmental pattern and location with WBP2NL and a shared ability to activate the oocyte, supporting its consideration as a mouse SOAF component that can compensate for a WBP2NL.

The developmental origin and compartmentalization of glutathione-s-transferase omega 2 isoforms in the perinuclear theca of eutherian spermatozoa

The perinuclear theca (PT) is a condensed, nonionic detergent resistant cytosolic protein layer encapsulating the sperm head nucleus. It can be divided into two regions: the subacrosomal layer, whose proteins are involved in acrosomal assembly during spermiogenesis, and the postacrosomal sheath (PAS), whose proteins are implicated in sperm–oocyte interactions during fertilization. In continuation of our proteomic analysis of the PT, we have isolated two prominent PT-derived proteins of 28 and 31 kDa from demembranated bovine sperm head fractions. These proteins were identified by mass spectrometry as isoforms of glutathione-s-transferase omega 2 (GSTO2). Immunoblots probed with anti-GSTO2 antibodies confirmed the presence of the GSTO2 isoforms in these fractions while fluorescent immunocytochemistry localized the isoforms to the PAS region of the bull, boar, and murid PT. In addition to the PAS labeling of GSTO2, the performatorium of murid spermatozoa was also labeled. Immunohistochemistry of rat testes revealed that GSTO2 was expressed in the third phase of spermatogenesis (i.e., spermiogenesis) and assembled in the PAS and perforatorial regions of late elongating spermatids. Fluorescent immunocytochemistry performed on murine testis cells co-localized GSTO2 and tubulin on the transient microtubular-manchette of elongating spermatids. These findings imply that GSTO2 is transported and deposited in the PAS region by the manchette, conforming to the pattern of assembly found with other PAS proteins. The late assembly of GSTO2 and its localization in the PAS suggests a role in regulating the oxidative and reductive state of covalently linked spermatid/sperm proteins, especially during the disassembly of the sperm accessory structures after fertilization.

Function of RAD6B and RNF8 in spermatogenesis

Histone ubiquitination regulates sperm formation and is important for nucleosome removal during spermatogenesis. RNF8 is an E3 ubiquitin ligase, and RAD6B is an E2 ubiquitin-conjugating enzyme. Both proteins participate in DNA damage repair processes via histone ubiquitination. Loss of RNF8 or RAD6B can lead to sterility in male mice. However, the specific mechanisms regulating these ubiquitin-mediated processes are unclear. In this study, we found that RNF8 knockout mice were either subfertile or sterile based on the numbers of offspring they produced. We explored the mechanism by which RAD6B and RNF8 knockouts cause infertility in male mice and compared the effects of their loss on spermatogenesis. Our results demonstrate that RAD6B can polyubiquitinate histones H2 A and H2B. In addition, RNF8 was shown to monoubiquitinate histones H2 A and H2B. Furthermore, we observed that absence of histone ubiquitination was not the only reason for infertility. Senescence played a role in intensifying male sterility by affecting the number of germ cells during spermatogenesis. In summary, both histone ubiquitination and senescence play important roles in spermatogenesis.

What is the role of histone H1 heterogeneity? A functional model emerges from a 50 year mystery

For the past 50 years, understanding the function of histone H1 heterogeneity has been mired in confusion and contradiction. Part of the reason for this is the lack of a working model that tries to explain the large body of data that has been collected about the H1 subtypes so far. In this review, a global model is described largely based on published data from the author and other researchers over the past 20 years. The intrinsic disorder built into H1 protein structure is discussed to help the reader understand that these histones are multi-conformational and adaptable to interactions with different targets. We discuss the role of each structural section of H1 (as we currently understand it), but we focus on the H1's C-terminal domain and its effect on each subtype's affinity, mobility and compaction of chromatin. We review the multiple ways these characteristics have been measured from circular dichroism to FRAP analysis, which has added to the sometimes contradictory assumptions made about each subtype. Based on a tabulation of these measurements, we then organize the H1 variants according to their ability to condense chromatin and produce nucleosome repeat lengths amenable to that compaction. This subtype variation generates a continuum of different chromatin states allowing for fine regulatory control and some overlap in the event one or two subtypes are lost to mutation. We also review the myriad of disparate observations made about each subtype, both somatic and germline specific ones, that lend support to the proposed model. Finally, to demonstrate its adaptability as new data further refines our understanding of H1 subtypes, we show how the model can be applied to experimental observations of telomeric heterochromatin in aging cells.

RFX2 Is a Major Transcriptional Regulator of Spermiogenesis

Spermatogenesis consists broadly of three phases: proliferation of diploid germ cells, meiosis, and finally extensive differentiation of the haploid cells into effective delivery vehicles for the paternal genome. Despite detailed characterization of many haploid developmental steps leading to sperm, only fragmentary information exists on the control of gene expression underlying these processes. Here we report that the RFX2 transcription factor is a master regulator of genes required for the haploid phase. A targeted mutation of Rfx2 was created in mice. Rfx2^-/- mice are perfectly viable but show complete male sterility. Spermatogenesis appears to progress unperturbed through meiosis. However, haploid cells undergo a complete arrest in spermatid development just prior to spermatid elongation. Arrested cells show altered Golgi apparatus organization, leading to a deficit in the generation of a spreading acrosomal cap from proacrosomal vesicles. Arrested cells ultimately merge to form giant multinucleated cells released to the epididymis. Spermatids also completely fail to form the flagellar axoneme. RNA-Seq analysis and ChIP-Seq analysis identified 139 genes directly controlled by RFX2 during spermiogenesis. Gene ontology analysis revealed that genes required for cilium function are specifically enriched in down- and upregulated genes showing that RFX2 allows precise temporal expression of ciliary genes. Several genes required for cell adhesion and cytoskeleton remodeling are also downregulated. Comparison of RFX2-regulated genes with those controlled by other major transcriptional regulators of spermiogenesis showed that each controls independent gene sets. Altogether, these observations show that RFX2 plays a major and specific function in spermiogenesis.

Failure of spermatogenesis, which is presumed to often result from genetic defects, is a common cause of male sterility. Although numerous genes associated with defects in male spermatogenesis have been identified, numerous cases of genetic male infertility remain unelucidated. We report here that the transcription factor RFX2 is a master regulator of gene expression programs required for progression through the haploid phase of spermatogenesis. Male RFX2-deficient mice are completely sterile. Spermatogenesis progresses through meiosis, but haploid cells undergo a complete block in development just prior to spermatid elongation. Gene expression profiling and ChIP-Seq analysis revealed that RFX2 controls key pathways implicated in cilium/flagellum formation, as well as genes implicated in microtubule and vesicle associated transport. The set of genes activated by RFX2 in spermatids exhibits virtually no overlap with those controlled by other known transcriptional regulators of spermiogenesis, establishing RFX2 as an essential new player in this developmental process. RFX2-deficient mice should therefore represent a valuable new model for deciphering the regulatory networks that direct sperm formation, and thereby contribute to the identification of causes of human male infertility.

Triclosan exhibits a tendency to accumulate in the epididymis and shows sperm toxicity in male Sprague-Dawley rats

Triclosan (TCS) is considered a potent endocrine disruptor that causes reproductive toxicity in non-mammals, but it is still unclear exactly whether TCS has adverse effects on the sperm or reproductive organs in mammals. In this study, we aimed to evaluate the distribution status of TCS in male reproductive organs of rats, and seek the correlation with the TCS-induced sperm toxicity or reproductive organ damage. Male rats were intragastrically administered with TCS at a dose of 50 mg/kg, the kinetics of TCS in the plasma and reproductive organs were investigated. TCS in testes and prostates both showed a lower-level distribution compared to that in the plasma, which indicates it has no tendency to accumulate in those organs. However, TCS in the epididymides showed a longer elimination half-life (t1/2 z), a longer the mean retention time (MRT), and a lower clearance (CLZ /F) compared with those in the plasma. Besides, the ratios of mean area under the concentration-time curve (AUC)(0-96 h(epididymides/plasma)) and AUC(0-∞(epididymides/plasma)) were 1.13 and 1.51, respectively. These kinetic parameters suggest TCS has an accumulation tendency in the epididymides. Based on this, we investigated the TCS-induced sperm toxicity and histopathological changes of reproductive organs in rats. TCS was given intragastrically at doses of 10, 50, and 200 mg/kg for 8 weeks. Rats treated with the high dose (200 mg/kg) of TCS showed a significant decrease in daily sperm production (DSP), changes in sperm morphology and epididymal histopathology. Considering the histopathological change in the epididymides, TCS may induce the epididymal damage due to the epididymal accumulation of that.

Protamines and spermatogenesis in Drosophila and Homo sapiens : A comparative analysis

The production of mature and motile sperm is a detailed process that utilizes many molecular players to ensure the faithful execution of spermatogenesis. In most species that have been examined, spermatogenesis begins with a single cell that undergoes dramatic transformation, culminating with the hypercompaction of DNA into the sperm head by replacing histones with protamines. Precise execution of the stages of spermatogenesis results in the production of motile sperm. While comparative analyses have been used to identify similarities and differences in spermatogenesis between species, the focus has primarily been on vertebrate spermatogenesis, particularly mammals. To understand the evolutionary basis of spermatogenetic variation, however, a more comprehensive comparison is needed. In this review, we examine spermatogenesis and the final packaging of DNA into the sperm head in the insect Drosophila melanogaster and compare it to spermatogenesis in Homo sapiens.

Localization of cathepsin D in mouse reproductive tissues and its acquisition onto sperm surface during epididymal sperm maturation

Sperm maturation in the epididymis involves multiple complex events, that include the adsorption of epididymal secretory proteins, re-organization and removal of sperm surface ligands. In this study, we investigated the existence and distribution of cathepsin D (CAT-D) transcripts and proteins in mouse reproductive tissues and proposed a transfer mechanism of CAT-D to the sperm surface. CAT-D transcripts were highly expressed in cultured Sertoli cells, but not in germ cells. The transcriptional level was relatively higher in the caput epididymis (CP) than in the cauda epididymis (CD). At the translational level, CAT-D was detected in testicular somatic cells and in the principal and basal cells in the CP. The expression of CAT-D was fairly specific to the clear cells in the CD. All forms of CAT-D were detected in ultracentrifuged epididymosomes. In conjunction with the expression levels in epididymal epithelium and epididymosomes, CAT-D expression level on the sperm surface was relatively high in CP sperm, but gradually declined toward the CD. Overall, our results indicated that CAT-D was not inherent to sperm themselves, but rather of epididymal origin and was presumably transported to the sperm surface via epididymosomes.

Mili and Miwi target RNA repertoire reveals piRNA biogenesis and function of Miwi in spermiogenesis

Germ cells implement elaborate mechanisms to protect their genetic material and to regulate gene expression during differentiation. Piwi proteins bind piRNAs, a class of small germline RNAs whose biogenesis and functions are still largely elusive. We employed high throughput sequencing after crosslinking and immunoprecipitation (HITS-CLIP) coupled with RNA-Seq to characterize the genome-wide target RNA repertoire of Mili (Piwil2) and Miwi (Piwil1), two Piwi proteins expressed in mouse postnatal testis. We report the in vivo pathway of primary piRNA biogenesis and implicate distinct nucleolytic activities that process Piwi-bound precursor transcripts. Our studies indicate that pachytene piRNAs are the end products of RNA processing. HITS-CLIP demonstrates that Miwi binds spermiogenic mRNAs directly, without utilizing piRNAs as guides, and independent biochemical analyses of testis mRNA-ribonucleoproteins (mRNPs) establishes that Miwi functions in the formation of mRNP complexes that stabilize mRNAs essential for spermiogenesis.

MMP2 and acrosin are major proteinases associated with the inner acrosomal membrane and may cooperate in sperm penetration of the zona pellucida during fertilization

Sperm-zona pellucida (ZP) penetration during fertilization is a process that most likely involves enzymatic digestion of this extracellular coat by spermatozoa. Since the inner acrosomal membrane (IAM) is the leading edge of spermatozoa during penetration and proteins required for secondary binding of sperm to the zona are present on it, the IAM is the likely location of these enzymes. The objectives of this study were to identify and characterize proteinases present on the IAM, confirm their localization and provide evidence for their role in fertilization. Gelatin zymography of detergent extracts of the IAM revealed bands of enzymatic activity identified as serine and matrix metallo-proteinases (MMPs). Specific inhibitors to MMPs revealed that MMP activity was due to MMP2. Immunoblotting determined that the serine protease activity on the zymogram was due to acrosin and also confirmed the MMP2 activity. Immunogold labeling of spermatozoa at the electron microscope level showed that acrosin and MMP2 were confined to the apical and principal segments of the acrosome in association with the IAM, confirming our IAM isolation technique. Immunohistochemical examination of acrosin and MMP2 during spermiogenesis showed that both proteins originate in the acrosomic granule during the Golgi phase and later redistribute to the acrosomal membrane. Anti-MMP2 antibodies and inhibitors incorporated into in vitro fertilization media significantly decreased fertilization rates. This is the first study to demonstrate that MMP2 and acrosin are associated with the IAM and introduces the possibility of their cooperation in enzymatic digestion of the ZP during penetration.

Fusion failure of dense-cored proacrosomal vesicles in an inducible mouse model of male infertility

The acrosome is a specialized secretory vesicle located in the head of spermatozoa and has an essential role during fertilization. This organelle and the sperm nucleus have aberrant morphologies in forms of male infertility in humans (teratozoospermia), often associated with poor motility (asthenoteratozoospermia). To further our understanding of the aetiology of these conditions, we have performed a pathological investigation of a model of asthenoteratozoospermia that can be induced in mice by N-butyldeoxynojirimycin (NB-DNJ). We have found that, in mice treated with NB-DNJ, instead of an acrosome forming over the round spermatid nucleus, multivesicular bodies (MVB) accumulate in the vicinity of this nucleus. Electron microscopy has revealed that proacrosomic vesicles or granules (PAG) secreted during the Golgi phase of spermiogenesis do not fuse together to form an acrosomic vesicle, but rather attach transiently to the spermatid nucleus. Immunocytochemistry has shown that acrosomal membrane proteins and cytosolic acrosome-associated proteins are redirected to MVB in affected testes, whereas glycoproteins originating in the dense core of the PAG are degraded. Thus, the major effect of NB-DNJ is to inhibit membrane fusion of Golgi-derived secretory vesicles destined for acrosome formation, raising the possibility that these vesicles are critically affected in forms of (astheno)teratozoospermia.

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

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

RFX2 is a candidate downstream amplifier of A-MYB regulation in mouse spermatogenesis

Background

Mammalian spermatogenesis involves formation of haploid cells from the male germline and then a complex morphological transformation to generate motile sperm. Focusing on meiotic prophase, some tissue-specific transcription factors are known (A-MYB) or suspected (RFX2) to play important roles in modulating gene expression in pachytene spermatocytes. The current work was initiated to identify both downstream and upstream regulatory connections for Rfx2.

Results

Searches of pachytene up-regulated genes identified high affinity RFX binding sites (X boxes) in promoter regions of several new genes: Adam5, Pdcl2, and Spag6. We confirmed a strong promoter-region X-box for Alf, a germ cell-specific variant of general transcription factor TFIIA. Using Alf as an example of a target gene, we showed that its promoter is stimulated by RFX2 in transfected cells and used ChIP analysis to show that the promoter is occupied by RFX2 in vivo. Turning to upstream regulation of the Rfx2 promoter, we identified a cluster of three binding sites (MBS) for the MYB family of transcription factors. Because testis is one of the few sites of A-myb expression, and because spermatogenesis arrests in pachytene in A-myb knockout mice, the MBS cluster implicates Rfx2 as an A-myb target. Electrophoretic gel-shift, ChIP, and co-transfection assays all support a role for these MYB sites in Rfx2 expression. Further, Rfx2 expression was virtually eliminated in A-myb knockout testes. Immunohistology on testis sections showed that A-MYB expression is up-regulated only after pachytene spermatocytes have clearly moved away from the tubule wall, which correlates with onset of RFX2 expression, whereas B-MYB expression, by contrast, is prevalent only in earlier spermatocytes and spermatogonia.

Conclusion

With an expanding list of likely target genes, RFX2 is potentially an important transcriptional regulator in pachytene spermatocytes. Rfx2 itself is a good candidate to be regulated by A-MYB, which is essential for meiotic progression. If Alf is a genuine RFX2 target, then A-myb, Rfx2, and Alf may form part of a transcriptional network that is vital for completion of meiosis and preparation for post-meiotic differentiation.

Identification and characterization of a novel mouse and human MOPT gene containing MORN-motif protein in testis

A novel testis-derived membrane occupation and recognition nexus (MORN)-motif protein was identified in mouse testis (MOPT) by subtraction screening methods and found to be localized on chromosome 17E3, spanning approximately 7kb. Sequence analysis showed that MOPT contains 669 base pair nucleotides of open reading frame and the corresponding 79 amino acids. The protein is predicted to have theoretical molecular mass of 9000 Da and an expected isoelectric point of 5.8 and seems to have unique sequences except for MORN-motif domain. The transcript of MOPT is highly and specifically expressed in adult testis as well as skeletal muscle. Moreover, MOPT transcript and protein are confined mainly to round and elongated spermatids, except for a few individual dispersed spermatocytes, and increase in abundance at subsequent stages. MOPT first appeared in the proacrosomic vesicles of the early Golgi phase spermatids and was translocated from the head cap of elongated spermatid to the nucleus of mature spermatozoa at the final stage of spermiogenesis. Our study suggests that MOPT may play an important role in dynamic regulation of acrosome biogenesis during late spermiogenesis.

Spatiotemporal organization of AT- and GC-rich DNA and their association with transition proteins TP1 and TP2 in rat condensing spermatids

Transition protein 1 (TP1) and TP2 replace histones during midspermiogenesis (stages 12-15) and are finally replaced by protamines. TPs play a predominant role in DNA condensation and chromatin remodeling during mammalian spermiogenesis. TP2 is a zinc metalloprotein with two novel zinc finger modules that condenses DNA in vitro in a GC-preference manner. TP2 also localizes to the nucleolus in transfected HeLa and Cos-7 cells, suggesting a GC-rich preference, even in vivo. We have now studied the localization pattern of TP2 in the rat spermatid nucleus. Colocalization studies using GC-selective DNA-binding dyes chromomycin A3 and 7-amino actinomycin D and an AT-selective dye, 4',6-diamidino-2-phenylindole, indicate that TP2 is preferentially localized to GC-rich sequences. Interestingly, as spermatids mature, TP2 and GC-rich DNA moves toward the nuclear periphery, and in the late stages of spermatid maturation, TP2 is predominantly localized at the nuclear periphery. Another interesting observation is the mutually exclusive localization of GC- and AT-rich DNA in the elongating and elongated spermatids. A combined immunofluorescence experiment with anti-TP2 and anti-TP1 antibodies revealed several foci of overlapping localization, indicating that TP1 and TP2 may have concerted functional roles during chromatin remodeling in mammalian spermiogenesis.

Identification and characterization of a novel testis-specific gene CKT2, which encodes a substrate for protein kinase CK2

Protein kinase CK2 is a serine/threonine kinase known to phosphorylate numerous substrates. CK2 is implicated in several physiologic and pathologic processes, particularly in cancer biology. CK2 is comprised of several subunits, including CK2α, CK2α′ and CK2β. Inactivation of CK2α′ leads to chromatin degeneration of germ cells, resulting in male sterility. To identify additional targets of CK2α′ in testes and to determine the role of CK2α′ in germ cell nuclear integrity, GST pull-down and protein–protein interaction assays were conducted. A novel testis-specific gene, CKT2 (CK2 Target protein 2), was found whose product interacts with and is phosphorylated by CK2 in vitro and in vivo. CKT2 is a 30.2 kDa protein with one coiled-coil domain and six putative phosphorylation sites. High expression of CKT2 correlated with chromatin condensation of spermatids in murine testes. Findings reported herein demonstrate that CKT2 is a target protein of native CK2α′ in testes and suggest that CKT2 plays a role in chromatin regulation of male germ cells.

Spermiogenesis and chromatin condensation in the common tree shrew, Tupaia glis

We have investigated the cellular characteristics, especially chromatin condensation and the basic nuclear protein profile, during spermiogenesis in the common tree shrew, Tupaia glis. Spermatids could be classified into Golgi phase, cap phase, acrosome phase, and maturation phase. During the Golgi phase, chromatin was composed of 10-nm and 30-nm fibers with few 50-nm to 60-nm knobby fibers. The latter were then transformed into 70-nm knobby fibers during the cap phase. In the acrosome phase, all fibers were packed into the highest-order knobby fibers, each about 80-100 nm in width. These chromatin fibers became tightly packed in the maturation phase. In a mature spermatozoon, the discoid-shaped head was occupied by the acrosome and completely condensed chromatin. H3, the core histone, was detected by immunostaining in all nuclei of germ cell stages, except in spermatid steps 15-16 and spermatozoa. Protamine, the basic nuclear protein causing the tight packing of sperm chromatin, was detected by immunofluorescence in the nuclei of spermatids at steps 12-16 and spermatozoa. Cross-immunoreactivity of T. glis H3 and protamine to those of primates suggests the evolutionary resemblance of these nuclear basic proteins in primate germ cells.

Spermatozoal sensitive biomarkers to defective protaminosis and fragmented DNA

Human sperm DNA damage may have adverse effects on reproductive outcome. Infertile men possess substantially more spermatozoa with damaged DNA compared to fertile donors. Although the extent of this abnormality is closely related to sperm function, the underlying etiology of ensuing male infertility is still largely controversial. Both intra-testicular and post-testicular events have been postulated and different mechanisms have been proposed to explain the presence of damaged DNA in human spermatozoa. Three among them, i.e. abnormal chromatin packaging, oxidative stress and apoptosis, are the most studied and discussed in the present review. Furthermore, results from numerous investigations are presented, including our own findings on these pathological conditions, as well as the techniques applied for their evaluation. The crucial points of each methodology on the successful detection of DNA damage and their validity on the appraisal of infertile patients are also discussed. Along with the conventional parameters examined in the standard semen analysis, evaluation of damaged sperm DNA seems to complement the investigation of factors affecting male fertility and may prove an efficient diagnostic tool in the prediction of pregnancy outcome.

Possible Function of Caudal Nuclear Pocket

Many temporarily functioning proteins are generated during the replacement of nucleoproteins in the nuclei of late spermatids and seem to be degraded in the nucleus. This study was designed to clarify the involvement of the ubiquitin-proteasome degradation system in the nucleus of rat developing spermatids. Thus, we studied the nuclear distribution of polyubiquitinated proteins (pUP) and proteasome in spermiogenic cells and sperm using postembedding immunoelectron microscopy. We divided the nuclear area of late spermatids into two regions: (1) a dense area composed of condensed chromatin and (2) a nuclear pocket in the neck region. The latter was located in the caudal nuclear region and was surrounded by redundant nuclear envelope. We demonstrated the presence of pUP in the dense area and nuclear pocket, proteasome in the nuclear pocket, and clear spots in the dense area of rat spermatids. Using quantitative analysis of immunogold labeling, we found that fluctuation of pUP and proteasome levels in late spermatogenesis was mostly synchronized with disappearance of histones and transitional proteins reported previously. In the nuclei of human sperm, pUP was detected in the dense area, whereas proteasome was in the nuclear vacuoles and clear spots. These results strongly suggest that pUP occur in the dense nuclear area of developing spermatids and that the ubiquitin-proteasome system is more actively operational in the nuclear pocket than dense area. Thus, the nuclear pocket might be the degradation site for temporarily functioning proteins generating during condensation of chromatin in late spermatids.

Rat Spag5 associates in somatic cells with endoplasmic reticulum and microtubules but in spermatozoa with outer dense fibers

The leucine zipper motif has been identified as an important and specific interaction motif used by various sperm tail proteins that localize to the outer dense fibers. We had found that rat Odf1, a major integral ODF protein, utilizes its leucine zipper to associate with Odf2, another major ODF protein, Spag4 which localizes to the interface between ODF and axonemal microtubule doublets, and Spag5. The rat Spag5 sequence indicated a close relationship with human Astrin, a microtubule-binding spindle protein suggesting that Spag5, like Spag4, may associate with the sperm tail axoneme. RT PCR assays indicated expression of Spag5 in various tissues and in somatic cells Spag5 localizes to endoplasmic reticulum and microtubules, as expected for an Astrin orthologue. MT binding was confirmed both in vivo and in in vitro MT-binding assays: somatic cells contain a 58 kDa MT-associated Spag5 protein. Western blotting assays of rat somatic cells and male germ cells at different stages of development using anti-Spag5 antibodies demonstrated that the protein expression pattern changes during spermatogenesis and that sperm tails contain a 58 kDa Spag5 protein. Use of affinity-purified anti-Spag5 antibodies in immuno electron microscopy shows that in rat elongated spermatids and epididymal sperm the Spag5 protein associates with ODF, but not with the axonemal MTs. This observation is in contrast to that for the other Odf1-binding, MT-binding protein Spag4, which is present between ODF and axoneme. Our data demonstrate that Spag5 has different localization in somatic versus male germ cells suggesting the possibility of different function.

The extracellular protein coat of the inner acrosomal membrane is involved in zona pellucida binding and penetration during fertilization: characterization of its most prominent polypeptide (IAM38)

A consequence of the acrosome reaction is to expose the inner acrosomal membrane (IAM), which is a requirement for the sperm's ability to secondarily bind to and then penetrate the zona pellucida (ZP) of the mammalian oocyte. However, the proteins on the IAM responsible for binding and presumably penetrating the zona have not been identified. This issue can be resolved if direct information is made available on the composition of the IAM. For this purpose, we devised a methodology in order to obtain a sperm head fraction consisting solely of the IAM bound to the detergent-resistant perinuclear theca. On the exposed IAM surface of this fraction, we defined an electron dense protein layer that we termed the IAM extracellular coat (IAMC), which was visible on sonicated and acrosome-reacted sperm of several mammalian species. High salt extraction removed the IAMC coincident with the removal of a prominent 38 kDa polypeptide, which we termed IAM38. Antibodies raised against this polypeptide confirmed its presence in the IAMC of intact, sonicated and acrosome-reacted sperm. By immunoscreening of a bovine testicular cDNA library and sequencing the resulting clones, we identified IAM38 as the equivalent of porcine Sp38 [Mori, E., Kashiwabara, S., Baba, T., Inagaki, Y., Mori, T., 1995. Amino acid sequences of porcine Sp38 and proacrosin required for binding to the zona pellucida. Dev. Biol., 168, 575-583], an intra-acrosomal protein with ZP-binding ability, whose precise localization in sperm was unknown. The blockage of IVF at the level of the zona with anti-IAM38 antibodies and the retention of IAM38 after sperm passage through the zona support its involvement in secondary sperm-zona binding. This study provides a novel approach to obtain direct information on the peripheral and integral protein composition of the IAM for identifying other candidates for sperm-zona interactions.

Triple knockouts reveal gene interactions affecting fertility of male mice

Triple knockout mice were used to investigate the interactions of five genes that were expressed in meiotic and haploid spermatogenic cells in mice, transition protein 2 (Tnp2), proacrosin (Acr), histone H1.1 (H1.1), histone H1t (H1t), and sperm mitochondria-associated cysteine-rich protein (Smcp). TNP2 functions in the replacement of histones and the initial condensation of the spermatid nucleus. The linker histone subtypes H1.1 and H1t are expressed at high levels in meiotic and early haploid cells. ACR, a protease that is stored as a proenzyme in the acrosome, is activated during the acrosome reaction and functions in binding of sperm to the zona pellucida. SMCP is a structural protein in the outer membranes of sperm mitochondria that functions in motility. Previous work demonstrates that homozygous knockout mice lacking each of these proteins individually exhibit no defect in fertility on mixed genetic backgrounds. In contrast, the present study demonstrates that five triple knockout lines, Acr/H1.1/Smcp, Acr/Tnp2/Smcp, Tnp2/H1.1/Smcp, Acr/H1t/Smcp, Tnp2/H1t/Smcp, exhibit drastic reductions in fertility on mixed genetic backgrounds. Analysis of fertility parameters reveal that the decreased fertility is due to line-dependent defects in sperm motility in vitro correlated with reduced migration in the female reproductive tract, and decreased fertilization due to defects in adhesion of sperm to the zona pellucida, the membrane surrounding the egg. It was also found that triple knockout males, that are hemizygous for one locus and homozygous for two other loci, are as subfertile as homozygous triple knockout males, a phenomenon known as haploinsufficiency. These findings demonstrate that male fertility involves synergistic interactions of genes that function in sperm motility and sperm-egg adhesion during fertilization.

Intégrité de l'ADN des spermatozoïdes comme élément diagnostique et pronostique de la fertilité masculine

Recent progress in reproductive biology has improved comprehension physiology of the spermatozoa and on the fertilization mechanisms. This new knowledge has carried out the elaboration of tests on male fertility based on sperm genomic integrity. This review presents some of these techniques and brings a reflexion element on the application and use of sperm DNA integrity in the investigation of male fertility. The single cell gel electrophoresis (COMET assay), Sperm Chromatin Structure Assay (SCSA), In Situ Nick Translation (NT: Nick Translation) and Terminal Uridine Nick-End Labelling (TUNEL assay) are actually the most currently used techniques for the measure of sperm DNA integrity in research clinic. From a certain point of view, TUNEL assay, SCSA, COMET assay and NT assay are complementary. The TUNEL and COMET can measure single and double strand breaks of DNA, the SCSA can detect the abnormalities in the chromatin compaction and the NT assay can detect the single strand breaks of DNA. The exact origin of sperm DNA fragmentation is not established yet. However, several mechanisms have been proposed: defect in the chromatin compaction during spermiogenesis; reactive oxygen species production by immature spermatozoa; apoptosis during spermatogenesis. It becomes important to consider the possible consequences of the oocyte fertilization by a spermatozoon having a high degree of DNA fragmentation. The use in routine of some of these tests must however pass by a standardization of the inter laboratory protocols and obviously, by the establishment of both in vivo and in vitro discriminating threshold values in order for these tests to present a good predictive value for pregnancy outcome.

How to pack the genome for a safe trip

The transformation of the somatic chromatin into a unique and highly compact structure occurring during the post-meiotic phase of spermatogenesis is one of the most dramatic known processes of chromatin remodeling. Paradoxically, no information is available on the mechanisms controlling this specific reorganization of the haploid cell genome. The only existing hints suggest a role for histone variants, as well as for stage-specific post-translational histone modifications,before and during the incorporation of testis-specific basic nuclear proteins. Moreover, the exact functions of the latter remain obscure. This chapter summarizes the major chromatin-associated events taking place during the post-meiotic differentiation of male haploid cells in mammals and discusses some of the basic issues that remain to be solved to finally understand chromatin remodeling during spermatogenesis.

Rat kinesin light chain 3 associates with spermatid mitochondria

We recently discovered that in rat spermatids, kinesin light chain KLC3 can associate with outer dense fibers, major sperm tail components, and accumulates in the sperm midpiece. Here, we show that mitochondria isolated from rat-elongating spermatids have bound KLC3. Immunoelectron microscopy indicates that the association of KLC3 with mitochondria coincides with the stage in spermatogenesis when mitochondria move from the plasma membrane to the developing midpiece. KLC3 is able to bind in vitro to mitochondria from spermatids as well as somatic cells employing a conserved kinesin light chain motif, the tetratrico-peptide repeats. Expression of KLC3 in fibroblasts results in formation of large KLC3 clusters close to the nucleus, which also contain mitochondria: no other organelles were present in these clusters. Mitochondria are not present in KLC3 clusters after deletion of KLC3's tetratrico-peptide repeats. Our results indicate that the rat spermatid kinesin light chain KLC3 can associate with mitochondria.

Rat sperm AS-A: subcellular localization in testis and epididymis and surface distribution in epididymal sperm

In this study, we investigated the subcellular compartmentalization of arylsulfatase-A (AS-A) in the testis and epididymis as well as the surface distribution in rat epididymal sperm. Testicular AS-A was compartmentalized specifically to the area underneath the outer acrosomal membrane of the acrosomal granule and to the dorsal aspect of the sperm acrosome. Epididymal AS-A was synthesized in the endoplasmic reticular (ER) network of principal cells and secreted into epididymal lumen as evident by its reactivity in the apical cytoplasm and vesicles therein underneath stereocilia. In clear cells, AS-A reactivity was found throughout the cytoplasmic machineries involved in endocytosis. Surface distribution of AS-A was initially detectable at the concave ridge as early as in sperm of the initial segment (IS). AS-A was additionally localized to the post-acrosomal region in caput (CP), corpus (CO) and cauda (CD) epididymal sperm. The expression levels of surface AS-A gradually increased during sperm transit from IS to CD epididymidis. These results favored the adsorption of AS-A from epididymal fluid onto the sperm surface, rather than shunting from the acrosome as a consequence of capacitation-associated membrane priming.

TIF1delta, a novel HP1-interacting member of the transcriptional intermediary factor 1 (TIF1) family expressed by elongating spermatids

TIF1 (transcriptional intermediary factor 1) proteins are encoded by an expanding family of developmental and physiological control genes that are conserved from flies to man. These proteins are characterized by an N-terminal RING-B box-coiled-coil (RBCC) motif and a C-terminal PHD finger/bromodomain unit, and have been implicated in epigenetic mechanisms of transcriptional repression involving histone modifiers and heterochromatin-binding proteins. We describe here the isolation and functional characterization of a fourth murine TIF1 gene, TIF1delta. The predicted TIF1delta protein displays all the structural hallmarks of a bona fide TIF1 family member and resembles the other TIF1s in that it can exert a deacetylase-dependent silencing effect when tethered to a promoter region. Moreover, like TIF1alpha and TIF1beta, TIF1delta can homodimerize and contains a PXVXL motif necessary and sufficient for HP1 (heterochromatin protein 1) binding. Although TIF1alpha and TIF1beta also bind nuclear receptors and Kruppel-associated boxes specifically and respectively, TIF1delta appears to lack nuclear receptor- and Kruppel-associated box binding activity. Furthermore, TIF1delta is unique among the TIF1 family proteins in that its expression is largely restricted to the testis and confined to haploid elongating spermatids, where it associates preferentially with HP1 isotype gamma (HP1gamma) and forms discrete foci dispersed within the centromeric chromocenter and the surrounding nucleoplasm. Collectively, these data are consistent with specific, nonredundant functions for the TIF1 family members in vivo and suggest a role for TIF1delta in heterochromatin-mediated gene silencing during postmeiotic phases of spermatogenesis.

RFX2 is a potential transcriptional regulatory factor for histone H1t and other genes expressed during the meiotic phase of spermatogenesis

H1t is a novel linker histone variant synthesized in mid- to late pachytene spermatocytes. Its regulatory region is of interest because developmentally specific expression has been impressed on an otherwise ubiquitously expressed promoter. Using competitive band-shift assays and specific antisera, we have now shown that the H1t-60 CCTAGG palindrome motif region binds members of the RFX family of transcriptional regulators. The testis-specific binding complex contains RFX2, probably as a homodimer. Other DNA-protein complexes obtained from testis as well as somatic organs contain RFX1, primarily as a heterodimer. Western blots confirmed that RFX2 expression is greatly enhanced in adult testis and that RFX2 is equally prominent in highly enriched populations of late pachytene spermatocytes and round spermatids. Immunohistochemistry carried out on mouse testis showed that RFX2 is strongly expressed in pachytene spermatocytes, remains high in early round spermatids, and declines only in advance of nuclear condensation. Maximum expression correlates well with the appearance of H1t. In contrast, RFX1 immunoreactivity in germ cells was only detected in late round spermatids. RFX-specific band complexes were also identified for both the mouse lamin C2 and Sgy promoters, using either testis nuclear extracts or in vitro-synthesized RFX2. These results call attention to RFX2 as a transcription factor with obvious potential for the regulation of gene expression during meiosis and the early development of spermatids.

Novel aspect of perinuclear theca assembly revealed by immunolocalization of non-nuclear somatic histones during bovine spermiogenesis

The perinuclear theca (PT) is an important accessory structure of the sperm head, yet its biogenesis is not well defined. To understand the developmental origins of PT-derived somatic histones during spermiogenesis, we used affinity-purified antibodies against somatic-type histones H3, H2B, H2A, and H4 to probe bovine testicular tissue using three different immunolocalization techniques. While undetectable in elongating spermatid nuclei, immunoperoxidase light microscopy showed all four somatic histones remained associated to the caudal head region of spermatids from steps 11 to 14 of the 14 steps in bovine spermiogenesis. Immunogold electron microscopy confirmed the localization of somatic histones on two nonnuclear structures, namely transient manchette microtubules of step-9 to step-11 spermatids and the developing postacrosomal sheath of step-13 and -14 spermatids. Immunofluorescence demonstrated somatic histone immunoreactivity in the developing postacrosomal sheath, and on anti-beta-tubulin decorated manchette microtubules of step-12 spermatids. Focal antinuclear pore complex labeling on the base of round spermatid nuclei was detected by electron microscopy and immunofluorescence, occurring before the nucleoprotein transition period during spermatid elongation. This indicated that, if nuclear histone export precedes their degradation, this process could only occur in this region, thereby questioning the proposed role of the manchette in nucleocytoplasmic trafficking. Somatic histone immunodetection on the manchette during postacrosomal sheath formation supports a role for the manchette in PT assembly, signifying that some PT components have origins in the distal spermatid cytoplasm. Furthermore, these findings suggest that somatic histones are de novo synthesized in late spermiogenesis for PT assembly.

Transition nuclear proteins are required for normal chromatin condensation and functional sperm development

The histone-to-protamine transition is important in the formation of spermatozoa. In mammals this involves two steps: replacement of histones by transition nuclear proteins (TPs) and replacement of TPs by protamines. To determine the functions of the TPs and their importance for sperm development, we generated mice lacking both TPs, since mice lacking only TP1 or TP2 were fertile. Our results indicated that TP1 and TP2 had partially complemented each other. In mice lacking both TPs, nuclear shaping, transcriptional repression, histone displacement, and protamine deposition proceeded relatively normally, but chromatin condensation was irregular in all spermatids, many late spermatids showed DNA breaks, and protamine 2 was not posttranslationally processed. Nevertheless, genomic integrity was maintained in mature spermatids, since efficient fertilization and production of offspring were achieved by intracytoplasmic sperm injection. However, many mature spermatids were retained in the testis, epididymal spermatozoa were drastically reduced in number and were highly abnormal, and the mice were sterile. Most epididymal spermatozoa were incapable of fertilization even using intracytoplasmic sperm injection. Thus, in mammals TPs are required for normal chromatin condensation, for reducing the number of DNA breaks, and for preventing the formation of secondary defects in spermatozoa, eventual loss of genomic integrity, and sterility.

Transcription in haploid male germ cells

Major modifications in chromatin organization occur in spermatid nuclei, resulting in a high degree of DNA packaging within the spermatozoon head. However, before arrest of transcription during midspermiogenesis, high levels of mRNA are found in round spermatids. Some transcripts are the product of genes expressed ubiquitously, whereas some are generated from male germ cell-specific gene homologs of somatic cell genes. Others are transcript variants derived from genes with expression regulated in a testis-specific fashion. The haploid genome of spermatids also initiates the transcription of testis-specific genes. Various general transcription factors, distinct promoter elements, and specific transcription factors are involved in transcriptional regulation. After meiosis, spermatids are genetically but not phenotypically different, because of transcript and protein sharing through cytoplasmic bridges connecting spermatids of the same generation. Interestingly, different types of mRNAs accumulate in the sperm cell nucleus, raising the question of their origin and of a possible role after fertilization.

Chromatin organization and basic nuclear proteins in the male germ cells ofRana tigerina

The process of chromatin condensation during spermiogenesis in Rana tigerina is similar to the heterochromatization in somatic cells, where 30 nm fibers are coalesced together into a dense mass in spermatozoa without changing their initial size and nucleosomal organization. This conclusion was supported by the finding that the full set of core histones (H2A, H2B, H3, H4) are still present in sperm chromatin, but histone H1 is replaced by its variant, H1V. Rabbit anti-sera were raised against histone H3, H1, H1V, and H5 (H1 variant in chick erythrocyte). Anti-histone H1 antiserum cross-reacted with histone H1V, which implied the presence of a common epitope. Anti-histone H1V and H5 also showed cross-reaction with each other but not with histone H1, which implied the presence of a common epitope not shared by histone H1. Immunocytochemical studies, using the above antibodies as probes, showed that histones H3 is present in all steps of spermatogenic and spermiogenic cells, and somatic cells including red blood cells, Sertoli cells, and Leydig cells, while histone H1 is present in all of the cells mentioned except in spermatozoa where it is replaced by histone H1V. Histone H1V appears in the early spermatids starting from spermatid 1 (St1), and it persists throughout the course of spermatid differentiation into spermatozoa. Histone H1V is also found in chromosomes of metaphase spermatocyte and red blood cells. Thus histone H1V may cause the final and complete condensation of chromatin in Rana spermatozoa, a process which is similar to the heterochromatization occurring in somatic cells such as metaphase chromosome and chick erythrocyte nucleus.

Male mice lacking three germ cell expressed genes are fertile

In recent years, much knowledge about the functions of defined genes in spermatogenesis has been gained by making use of mouse transgenic and gene knockout models. Single null mutations in mouse genes encoding four male germ cell proteins, transition protein 2 (Tnp-2), proacrosin (Acr), histone H1.1 (H1.1), and histone H1t (H1t), have been generated and analyzed. Tnp-2 is believed to participate in the removal of the nuclear histones and initial condensation of the spermatid nucleus. Proacrosin is an acrosomal protease synthesized as a proenzyme and activated into acrosin during the acrosome reaction. The linker histone subtype H1.1 belongs to the group of main-type histones and is synthesized in somatic tissues and germ cells during the S-phase of the cell cycle. The histone gene H1t is expressed exclusively in spermatocytes and may have a function in establishing an open chromatin structure for the replacement of histones by transition proteins and protamines. Male mutant mice lacking any of these proteins show no apparent defects in spermatogenesis or fertility. To examine the synergistic effects of these proteins in spermatogenesis and during fertilization, two lines of triple null mice (Tnp-2-/-/Acr-/-/H1.1-/- and Tnp-2-/-/Acr-/-/H1t-/-) were established. Both lines are fertile and show normal sperm parameters, which clearly demonstrate the functional redundancy of these proteins in male mouse fertility. However, sperm only deficient for Acr (Acr-/-) are able to compete significantly with sperm from triple knockout mice Tnp-2-/-/Acr-/-/H1.1-/- (70.7% vs. 29.3%) but not with sperm from triple knockout mice Tnp-2-/-/Acr-/-/H1t-/- (53.6% vs. 46.4%). These results are consistent with a model that suggests that some sperm proteins play a role during sperm competition.

Cloning and developmental analysis of murid spermatid-specific thioredoxin-2 (SPTRX-2), a novel sperm fibrous sheath protein and autoantigen

Thioredoxins compose a growing family of proteins that participate in different cellular processes via redox-mediated reactions. We report here the cloning, developmental expression, and location of murid Sptrx-2. Mouse and rat SPTRX-2 proteins display a high homology to their human ortholog in the thioredoxin and NDP kinase domains, and the coding genes are located at syntenic positions. Northern blotting and in situ hybridization confirmed the testis-specific expression of murine Sptrx-2 mRNA, mostly in round spermatids. Immunohistochemical analysis of the 19 steps of rat spermiogenesis showed that SPTRX-2 expression becomes prominent in the cytoplasmic lobe of step 15-18 spermatids and diminishes in step 19 just before spermiation. However, in the spermatid tail, SPTRX-2 immunoreactivity increased from step 15 to 19 and was confined to the principal piece. By immunogold electron microscopy, SPTRX-2 was first detected scattered throughout the cytoplasm of the axoneme in step 14-15 spermatids, but began to be incorporated by step 16 into the fibrous sheath (FS). During steps 17-18, the labeling increased over the ribs and columns of the assembled FS. It peaked in step 19 and remained in the FS of epididymal spermatozoa. Immunoblots of isolated FS obtained from spermatozoa confirmed that SPTRX-2 is an integral component of the FS and a post-obstruction autoantigen in vasectomized rats. Our data indicate that SPTRX-2 incorporation into the FS lags well behind FS assembly, suggesting it is required during the final stages of sperm tail maturation in the testis and/or epididymis, where extensive disulfide bonding of FS proteins occurs.

Somatic histones are components of the perinuclear theca in bovine spermatozoa

The perinuclear theca is a non-ionic detergent-resistant, electron-dense layer surrounding the condensed nucleus of mammalian sperm. The known proteins originating from the perinuclear theca have implicated the structure in a variety of important cellular processes during spermiogenesis and fertilization. Nonetheless, the composition of the perinuclear theca remains largely unexplored. We have isolated a group of low molecular mass (14-19 kDa) perinuclear theca-derived proteins from acrosome-depleted bovine sperm heads by salt (1 M KCl) extraction and have identified them as core somatic histones. N-terminal sequencing and immunoblotting with anti-histone antibodies confirmed the presence of both intact and proteolytically cleaved somatic histones H3, H2B, H2A, and H4. Identical proteins were isolated using 2% SDS or 1 N HCl extractions. Subsequent acid and SDS extractions of intact bovine sperm revealed the presence of all four intact histone subtypes, with minimal proteolysis. Two-dimensional acid/urea/Triton-SDS-PAGE, coupled with immunoblotting analysis, confirmed the somatic nature of these perinuclear theca-derived histones. Estimates of the abundance of perinuclear theca-derived histones showed that up to 0.2 pg per sperm of each histone subtype was present. Immunogold labeling at the ultrastructural level localized all four core somatic histones to the post-acrosomal sheath region of bovine epididymal sperm, when probed with affinity-purified anti-histone antibodies. Little immunoreactivity was detected in residual perinuclear theca structures following the extractions. Taken together, these findings indicate the unprecedented and stable localization of non-nuclear somatic histones in bovine sperm perinuclear theca.

Association of kinesin light chain with outer dense fibers in a microtubule-independent fashion

Conventional kinesin I motor molecules are heterotetramers consisting of two kinesin light chains (KLCs) and two kinesin heavy chains. The interaction between the heavy and light chains is mediated by the KLC heptad repeat (HR), a leucine zipper-like motif. Kinesins bind to microtubules and are involved in various cellular functions, including transport and cell division. We recently isolated a novel KLC gene, klc3. klc3 is the only known KLC expressed in post-meiotic male germ cells. A monoclonal anti-KLC3 antibody was developed that, in immunoelectron microscopy, detects KLC3 protein associated with outer dense fibers (ODFs), unique structural components of sperm tails. No significant binding of KLC3 with microtubules was observed with this monoclonal antibody. In vitro experiments showed that KLC3-ODF binding occurred in the absence of kinesin heavy chains or microtubules and required the KLC3 HR. ODF1, a major ODF protein, was identified as the KLC3 binding partner. The ODF1 leucine zipper and the KLC3 HR mediated the interaction. These results identify and characterize a novel interaction between a KLC and a non-microtubule macromolecular structure and suggest that KLC3 could play a microtubule-independent role during formation of sperm tails.

Characterization of human thioredoxin-like 2. A novel microtubule-binding thioredoxin expressed predominantly in the cilia of lung airway epithelium and spermatid manchette and axoneme

We describe here the cloning and characterization of a novel member of the thioredoxin family, thioredoxin-like protein 2 (Txl-2). The Txl-2 open reading frame codes for a protein of 330 amino acids consisting of two distinct domains: an N-terminal domain typical of thioredoxins and a C-terminal domain belonging to the nucleoside-diphosphate kinase family, separated by a small interface domain. The Txl-2 gene spans approximately 28 kb, is organized into 11 exons, and maps at locus 3q22.3-q23. A splicing variant lacking exon 5 (Delta 5Txl-2) has also been isolated. By quantitative real time PCR we demonstrate that Txl-2 mRNA is ubiquitously expressed, with testis and lung having the highest levels of expression. Unexpectedly, light and electron microscopy analyses show that the protein is associated with microtubular structures such as lung airway epithelium cilia and the manchette and axoneme of spermatids. Using in vitro translated proteins, we demonstrate that full-length Txl-2 weakly associates with microtubules. In contrast, Delta 5Txl-2 specifically binds with very high affinity brain microtubule preparations containing microtubule-binding proteins. Importantly, Delta 5Txl-2 also binds to pure microtubules, proving that it possesses intrinsic microtubule binding capability. Taken together, Delta 5Txl-2 is the first thioredoxin reported to bind microtubules and might therefore be a novel regulator of microtubule physiology.

Telomeres in mammalian male germline cells

Telomeres are terminal chromosomal domains that protect chromosome ends from degradation and fusion and promote complete replication of DNA. Telomeres are involved in the regulation of cellular replicative lifespan and tumorigenesis. These important functions of the telomeres have evoked high interest: numerous studies have resulted in a detailed description of telomere composition and structure in somatic cells. Much less is known about telomeres in germline cells. Emerging novel features and unique behavior of telomeres in the process of gamete differentiation suggest that they may have additional germline-specific function(s). This review describes recent studies revealing changes in the telomere organization in the course of differentiation from the germline stem cells to mature sperm in mammals. Similarities and differences between somatic and spermatogenic cells in telomere nuclear localization, protein composition, DNA length, telomerase activity, and chromatin structure are discussed. The exceptional features of the germline telomeres may be important for regulation of telomerase activity during spermatogenesis, homologous chromosome pairing during recombination, as well as for male pronucleus development and ordered chromosome withdrawal post-fertilization.