Editorial: Never enough--on the multiplicity and uniqueness of transcriptional regulators in postmeiotic male germ cells

The Cif proteins from Wolbachia prophage WO modify sperm genome integrity to establish cytoplasmic incompatibility

Inherited microorganisms can selfishly manipulate host reproduction to drive through populations. In Drosophila melanogaster, germline expression of the native Wolbachia prophage WO proteins CifA and CifB cause cytoplasmic incompatibility (CI) in which embryos from infected males and uninfected females suffer catastrophic mitotic defects and lethality; however, in infected females, CifA expression rescues the embryonic lethality and thus imparts a fitness advantage to the maternally transmitted Wolbachia. Despite widespread relevance to sex determination, evolution, and vector control, the mechanisms underlying when and how CI impairs male reproduction remain unknown and a topic of debate. Here, we use cytochemical, microscopic, and transgenic assays in D. melanogaster to demonstrate that CifA and CifB proteins of wMel localize to nuclear DNA throughout the process of spermatogenesis. Cif proteins cause abnormal histone retention in elongating spermatids and protamine deficiency in mature sperms that travel to the female reproductive tract with Cif proteins. Notably, protamine gene knockouts enhance wild-type CI. In ovaries, CifA localizes to germ cell nuclei and cytoplasm of early-stage egg chambers; however, Cifs are absent in late-stage oocytes and subsequently in fertilized embryos. Finally, CI and rescue are contingent upon a newly annotated CifA bipartite nuclear localization sequence. Together, our results strongly support the Host modification model of CI in which Cifs initially modify the paternal and maternal gametes to bestow CI-defining embryonic lethality and rescue.

Gcse, a novel germ-cell-specific gene, is differentially expressed during meiosis and gametogenesis

Gametogenesis is a complex process wherein germ cells develop from primordial diploid cells into haploid gametes. To understand the mechanisms controlling gametogenesis, we identified a novel germ-cell-specific gene, Gcse. Gcse produces two major transcripts that are 1589 bp (Gcse-l) and 906 bp (Gcse-s) in length. Northern blotting and reverse transcription-polymerase chain reaction (RT-PCR) analyses of multiple tissues reveal that Gcse-l is expressed in both adult testes and ovaries, but Gcse-s is expressed only in adult testes. During female gonad development, Gcse-l is expressed from embryonic day 13.5 to adulthood, specifically in oocytes, and maintained in ovulated and fertilized eggs. However, Gcse-s signals were detected only in ovulated oocytes and fertilized eggs but not in adult ovary. During male gonad development, strong Gcse-l signals were detected in late pachytene spermatocytes and round spermatids. However, Gcse-s transcripts exist only in round spermatids. Furthermore, the expression of GCSE-L proteins and their subcellular localizations within cells are stage specific. GCSE-L is detected in the nucleus of late pachytene spermatocytes. During meiosis, GCSE-L is translocated to acrosome regions in spermatids and maintained in the acrosome of spermatozoa. GCSE-L colocalizes with acrosin and lectin peanut agglutinin in the Golgi apparatus. However, GCSE-S proteins are expressed only in the nucleus of spermatids. From these results, we suggest that GCSE proteins play roles in meiosis and may be involved in acrosome biogenesis during spermiogenesis.

Male germ cell apoptosis: regulation and biology

Cellular apoptosis appears to be a constant feature in the adult testis and during early development. This is essential because mammalian spermatogenesis is a complex process that requires precise homeostasis of different cell types. This review discusses the latest information available on male germ cell apoptosis induced by hormones, toxins and temperature in the context of the type of apoptotic pathway either the intrinsic or the extrinsic that may be used under a variety of stimuli. The review also discusses the importance of mechanisms pertaining to cellular apoptosis during testicular development, which is independent of exogenous stimuli. Since instances of germ cell carcinoma have increased over the past few decades, the current status of research on apoptotic pathways in teratocarcinoma cells is included. One other important aspect that is covered in this review is microRNA-mediated control of germ cell apoptosis, a field of research that is going to see intense activity in near future. Since knockout models of various kinds have been used to study many aspects of germ cell development, a comprehensive summary of literature on knockout mice used in reproduction studies is also provided.

Surfing the wave, cycle, life history, and genes/proteins expressed by testicular germ cells. Part 4: intercellular bridges, mitochondria, nuclear envelope, apoptosis, ubiquitination, membrane/voltage-gated channels, methylation/acetylation, and transcription factors

As germ cells divide and differentiate from spermatogonia to spermatozoa, they share a number of structural and functional features that are common to all generations of germ cells and these features are discussed herein. Germ cells are linked to one another by large intercellular bridges which serve to move molecules and even large organelles from the cytoplasm of one cell to another. Mitochondria take on different shapes and features and topographical arrangements to accommodate their specific needs during spermatogenesis. The nuclear envelope and pore complex also undergo extensive modifications concomitant with the development of germ cell generations. Apoptosis is an event that is normally triggered by germ cells and involves many proteins. It occurs to limit the germ cell pool and acts as a quality control mechanism. The ubiquitin pathway comprises enzymes that ubiquitinate as well as deubiquitinate target proteins and this pathway is present and functional in germ cells. Germ cells express many proteins involved in water balance and pH control as well as voltage-gated ion channel movement. In the nucleus, proteins undergo epigenetic modifications which include methylation, acetylation, and phosphorylation, with each of these modifications signaling changes in chromatin structure. Germ cells contain specialized transcription complexes that coordinate the differentiation program of spermatogenesis, and there are many male germ cell-specific differences in the components of this machinery. All of the above features of germ cells will be discussed along with the specific proteins/genes and abnormalities to fertility related to each topic.

The transcription factor CREMtau and cAMP regulate promoter activity of the Na,K-ATPase alpha4 isoform

The Na,K-ATPase is an essential enzyme of the plasma membrane that plays a key role in numerous cell processes that depend on the transcellular gradients of Na(+) and K(+). Among the various isoforms of the catalytic subunit of the Na,K-ATPase, alpha4 exhibits the most limited pattern of expression, being restricted to male germ cells. Activity of alpha4 is essential for sperm function, and alpha4 is upregulated during spermatogenesis. The present study addressed the transcriptional control of the human Na,K-ATPase alpha4 gene, ATP1A4. We describe that a 5' untranslated region of the ATP1A4 gene (designated -339/+480 based on the ATP1A4 transcription initiation site) has promoter activity in luciferase reporter assays. Computer analysis of this promoter region revealed consensus sites (CRE) for the cyclic AMP (cAMP) response element modulator (CREM). Accordingly, dibutyryl cAMP (db-cAMP) and ectopic expression of CREMtau, a testis specific splice variant of CREM were able to activate the ATP1A4 promoter driven expression of luciferase in HEK 293 T, JEG-3 and GC-1 cells. Further characterization of the effect of db-cAMP and CREMtau on deleted constructs of the ATP1A4 promoter (-339/+80, and +25/+480), and on the -339/+480 region carrying mutations in the CRE sites showed that db-cAMP and CREMtau effect required the CRE motif located 263 bp upstream the transcription initiation site. EMSA experiments confirmed the CRE sequence as a bonafide CREMtau binding site. These results constitute the first demonstration of the transcriptional control of ATP1A4 gene expression by cAMP and by CREMtau, a transcription factor essential for male germ cell gene expression.

Tisp40, a spermatid specific bZip transcription factor, functions by binding to the unfolded protein response element via the Rip pathway

TISP40, a mouse spermatid-specific gene, encodes a CREB/CREM family transcription factor that is predominantly expressed during spermiogenesis. We report here that TISP40 generates two types of proteins, Tisp40alpha and Tisp40beta, both of which contain a transmembrane domain and localize to the endoplasmic reticulum (ER). In contrast, mutant proteins lacking the transmembrane domain (Tisp40alpha/betaDeltaTM) primarily localize to the nucleus. Endoglycosidase H treatment shows that the C-terminus of Tisp40alpha/beta is glycosylated. Protease experiments demonstrate that Tisp40alpha/beta are Type II transmembrane proteins that are released into the nucleus by a two-step cleavage mechanism called 'regulated intramembrane proteolysis' (Rip). Unlike previously published observations, Tisp40alpha does not bind to the NF-kappaB site; instead, it specifically binds to the unfolded protein response element (UPRE). Luciferase assays reveal that Tisp40betaDeltaTM activates transcription through UPRE. Northern blot analysis shows that Tisp40alpha/betaDeltaTM proteins up-regulate EDEM (ER degradation of enhancing alpha-manosidase-like protein) mRNA. These observations unveil a novel event in mouse spermiogenesis and show that the final stage of transcriptional regulation is controlled by the Rip pathway.

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.

Testis-specific expression of rat mitochondrial glycerol-3-phosphate dehydrogenase in haploid male germ cells

Mitochondrial glycerol-3-phosphate dehydrogenase (mGPDH) is regulated by multiple promoters in a tissue-specific manner. We characterized the testis-specific promoter C of the mGPDH gene and investigated the cellular localization of mGPDH within the testis. Electrophoretic mobility shift experiments identified a cAMP-response element (CRE) site at -57 that was active in the testis. An in vitro-translated CRE modulator (CREM) protein was able to bind this CRE site, and an anti-CREM antibody interfered with this complex. Ectopic expression of the testis-specific transcriptional activator CREMtau and protein kinase A in human hepatocarcinoma HepG2 cells activated a promoter C-driven luciferase construct in transient transfection experiments. Furthermore, mGPDH expression was undetectable in testis of CREM-deficient mice. The cellular localization of mGPDH expression and translation in adult rat testis was determined by in situ hybridization and immunohistochemistry techniques. The mGPDH transcripts were detected solely in postmeiotic germ cells. Expression of mGPDH was restricted from round spermatids to early elongating spermatids. The mGPDH protein was delayed in postmeiotic germ cells, restricted from late elongating spermatids to mature spermatids. Our results indicate that rat mGPDH is expressed by a testis-specific promoter from haploid male germ cells in a stage-specific manner.