The essential role of male germline-specific protein MAPS in pachynema progression and fertility

Abstract

Meiosis enables the generation of haploid gametes from corresponding diploid progenitor cells with high fidelity and ensures genetic diversity through recombination. Meiotic prophase I, as a critical element of gametogenesis, is classified into five stages based on the chromosomal configurations, namely leptotene, zygotene, pachytene, diplotene, and diakinesis stages. Following DNA replication, chromatin undergoes compaction into condensed chromosomes, accompanied by the generation of programmed double-strand breaks (DSBs) during leptonema. Chromosome pairing is initiated in the subsequent zygotene stage, wherein DSB end processing and strand invasion take place. Full synapsis of autosomal chromosome pairs is achieved during pachynema, involving a series of events, including DSB repair resolution, crossover formation, and meiotic sex chromosome inactivation (MSCI). Following this, chromosome segregation occurs during the diplotene stage, and the nuclear envelope collapses in the diakinesis stage. Various proteins are recruited and degraded in the complicated process in meiotic prophase I; however, the identified proteins are still limited. In this study, we identified a male pachynema-specific protein (Maps) gene, expressed specifically in the mouse testis and pachytene spermatocytes. In the nucleus, MAPS was diffusely expressed and also accumulated on the sex body area in pachytene and diplotene spermatocytes. The knockout of Maps results in the elimination of spermatids and male infertility in mice, indicating that Maps is essential for spermatogenesis. MAPS functions differently in spermatocytes in pubertal and adult mice. In pubertal mice, the depletion of Maps in spermatocytes leads to pachytene arrest with aberrant DNA repair, and defective formation of crossover and XY body. The lack of Maps causes early pachytene arrest with significant apoptosis in male mice. However, adult Maps−/− spermatocytes exhibit seemingly normal crossover formation, XY chromosome morphology, and recombination protein recruitment but still show pachytene arrest. Adult Maps−/− spermatocytes undergo apoptosis eventually and exhibit delayed progression from early to mid- and late pachynema. We further carried out the mechanistic studies of MAPS in pachytene spermatocytes. Maps−/− spermatocytes exhibit transcriptional dysregulation of sex chromosomes and autosomes in pubertal and adult mice. The dysregulated transcription is correlated with altered chromatin accessibility. MAPS also regulates cellular ubiquitination both in vitro and in vivo. Altered cellular protein and histone ubiquitination may contribute to the significantly changed chromatin accessibility and dysregulated transcriptional profiles in Maps−/− spermatocytes. Moreover, N-terminal amino acids 2–9 are essential for MAPS foci formation. MAPS foci are not co-localized with heterochromatin, nucleoli, or nuclear speckles, but MAPS forms condensates that incorporate nucleosome arrays in vitro. Hence, MAPS may directly regulate chromatin organization. In conclusion, this thesis focuses on the function and mechanisms of MAPS in meiotic prophase I. We found that MAPS is requisite for pachynema progression and male fertility. Adult mice lacking Maps exhibit retarded progression from early to mid- and late pachynema. We proposed that variations of cellular ubiquitination in Maps−/− spermatocytes might be implicated in the altered chromatin accessibility. Subsequently, the altered chromatin accessibility could lead to global transcription dysregulation, eventually causing the pachytene arrest phenotype.