Mammalian sex chromosomes. III. Activity of pseudoautosomal steroid sulfatase enzyme during spermatogenesis in Mus musculus

Chromosome Synapsis and Recombination in Male-Sterile and Female-Fertile Interspecies Hybrids of the Dwarf Hamsters (Phodopus, Cricetidae)

Hybrid sterility is an important step in the speciation process. Hybrids between dwarf hamsters Phodopus sungorus and P.campbelli provide a good model for studies in cytological and genetic mechanisms of hybrid sterility. Previous studies in hybrids detected multiple abnormalities of spermatogenesis and a high frequency of dissociation between the X and Y chromosomes at the meiotic prophase. In this study, we found that the autosomes of the hybrid males and females underwent paring and recombination as normally as their parental forms did. The male hybrids showed a significantly higher frequency of asynapsis and recombination failure between the heterochromatic arms of the X and Y chromosomes than the males of the parental species. Female hybrids as well as the females of the parental species demonstrated a high incidence of centromere misalignment at the XX bivalent and partial asynapsis of the ends of its heterochromatic arms. In all three karyotypes, recombination was completely suppressed in the heterochromatic arm of the X chromosome, where the pseudoautosomal region is located. We propose that this recombination pattern speeds up divergence of the X- and Y-linked pseudoautosomal regions between the parental species and results in their incompatibility in the male hybrids.

Cloning and functional characterization of the mouse sodium-dependent organic anion transporter Soat (Slc10a6)

The sodium-dependent organic anion transporter SOAT is a member of the Solute Carrier Family SLC10. In man, this carrier is predominantly expressed in the testis and has transport activity for sulfoconjugated steroid hormones. Here, we report on cloning, expression analysis and functional characterization of the mouse Soat (mSoat) and compare its characteristics with the human SOAT carrier. Quantitative mRNA expression analysis for mSoat in male mice revealed very high expression in lung and further high expression in testis and skin. Immunohistochemical studies showed expression of the mSoat protein in bronchial epithelial cells of the lung, in primary and secondary spermatocytes as well as round spermatids within the seminiferous tubules of the testis, in the epidermis of the skin, and in the urinary epithelium of the bladder. Stably transfected mSoat-HEK293 cells revealed sodium-dependent transport for dehydroepiandrosterone sulfate (DHEAS), estrone-3-sulfate, and pregnenolone sulfate (PREGS) with apparent Km values of 60.3μM, 2.1μM, and 2.5μM, respectively. In contrast to human SOAT, which has a preference for DHEAS as a substrate, mSoat exhibits the highest transport rate for PREGS, likely reflecting differences in the steroid pattern between both species. In conclusion, although certain differences between human SOAT and mSoat exist regarding quantitative gene expression in endocrine and non-endocrine tissues, as well as in the transport kinetics for steroid sulfates, in general, both can be regarded as homologous carriers.

UBR2 of the N-end rule pathway is required for chromosome stability via histone ubiquitylation in spermatocytes and somatic cells

The N-end rule pathway is a proteolytic system in which its recognition components (N-recognins) recognize destabilizing N-terminal residues of short-lived proteins as an essential element of specific degrons, called N-degrons. The RING E3 ligases UBR2 and UBR1 are major N-recognins that share size (200 kDa), conserved domains and substrate specificities to N-degrons. Despite the known function of the N-end rule pathway in degradation of cytosolic proteins, the major phenotype of UBR2-deficient male mice is infertility caused by arrest of spermatocytes at meiotic prophase I. UBR2-deficient spermatocytes are impaired in transcriptional silencing of sex chromosome-linked genes and ubiquitylation of histone H2A. In this study we show that the recruitment of UBR2 to meiotic chromosomes spatiotemporally correlates to the induction of chromatin-associated ubiquitylation, which is significantly impaired in UBR2-deficient spermatocytes. UBR2 functions as a scaffold E3 that promotes HR6B/UbcH2-dependent ubiquitylation of H2A and H2B but not H3 and H4, through a mechanism distinct from typical polyubiquitylation. The E3 activity of UBR2 in histone ubiquitylation is allosterically activated by dipeptides bearing destabilizing N-terminal residues. Insufficient monoubiquitylation and polyubiquitylation on UBR2-deficient meiotic chromosomes correlate to defects in double strand break (DSB) repair and other meiotic processes, resulting in pachytene arrest at stage IV and apoptosis. Some of these functions of UBR2 are observed in somatic cells, in which UBR2 is a chromatin-binding protein involved in chromatin-associated ubiquitylation upon DNA damage. UBR2-deficient somatic cells show an array of chromosomal abnormalities, including hyperproliferation, chromosome instability, and hypersensitivity to DNA damage-inducing reagents. UBR2-deficient mice enriched in C57 background die upon birth with defects in lung expansion and neural development. Thus, UBR2, known as the recognition component of a major cellular proteolytic system, is associated with chromatin and controls chromatin dynamics and gene expression in both germ cells and somatic cells.

H2AX is required for chromatin remodeling and inactivation of sex chromosomes in male mouse meiosis

During meiotic prophase in male mammals, the X and Y chromosomes condense to form a macrochromatin body, termed the sex, or XY, body, within which X- and Y-linked genes are transcriptionally repressed. The molecular basis and biological function of both sex body formation and meiotic sex chromosome inactivation (MSCI) are unknown. A phosphorylated form of H2AX, a histone H2A variant implicated in DNA repair, accumulates in the sex body in a manner independent of meiotic recombination-associated double-strand breaks. Here we show that the X and Y chromosomes of histone H2AX-deficient spermatocytes fail to condense to form a sex body, do not initiate MSCI, and exhibit severe defects in meiotic pairing. Moreover, other sex body proteins, including macroH2A1.2 and XMR, do not preferentially localize with the sex chromosomes in the absence of H2AX. Thus, H2AX is required for the chromatin remodeling and associated silencing in male meiosis.

Common methylation characteristics of sex chromosomes in somatic and germ cells from mouse, lemur and human

DNA methylation of sex chromosomes was analysed using anti-5-methylcytosine antibodies on metaphase chromosomes of somatic cells from three species: human, lemur and mouse. Germ cells were also studied in male mouse. In female cells (human and mouse), the late replicating X was always the less methylated chromosome. Compared with autosomes, the methylation of both X chromosomes was always lower in fibroblasts than in lymphocytes and the difference was always greater in mouse than in human. In human, mouse and lemur male cells, the labelling of the unique X chromosome was quite similar to that of the early replicating X from female cells. Except for the heterochromatic region of the human Y chromosome, strongly methylated, the overall methylation of the Y chromosome was low. In mouse testicular cells, a variety of DNA methylation patterns was observed according to the cell type and the state of differentiation. Finally, the only structures of sex chromosomes which remain methylated in all conditions correspond to their pseudoautosomal regions.

Sex chromosomes, recombination, and chromatin conformation

We review what is known about the transcriptional inactivation and condensation of heteromorphic sex chromosomes in contrast to the activation of homomorphic sex chromosomes during meiotic prephase in animals. We relate these cytological and transcriptional features to the recombination status of the sex chromosomes. We propose that sex chromosome condensation is a meiotic adaptation to prevent the initiation of potentially damaging recombination events in nonhomologous regions of the X and Y chromosome.