Connexin43 in Germ Cells Seems to Be Dispensable for Murine Spermatogenesis

Kinesins in Mammalian Spermatogenesis and Germ Cell Transport

In mammalian testes, the apical cytoplasm of each Sertoli cell holds up to several dozens of germ cells, especially spermatids that are transported up and down the seminiferous epithelium. The blood-testis barrier (BTB) established by neighboring Sertoli cells in the basal compartment restructures on a regular basis to allow preleptotene/leptotene spermatocytes to pass through. The timely transfer of germ cells and other cellular organelles such as residual bodies, phagosomes, and lysosomes across the epithelium to facilitate spermatogenesis is important and requires the microtubule-based cytoskeleton in Sertoli cells. Kinesins, a superfamily of the microtubule-dependent motor proteins, are abundantly and preferentially expressed in the testis, but their functions are poorly understood. This review summarizes recent findings on kinesins in mammalian spermatogenesis, highlighting their potential role in germ cell traversing through the BTB and the remodeling of Sertoli cell-spermatid junctions to advance spermatid transport. The possibility of kinesins acting as a mediator and/or synchronizer for cell cycle progression, germ cell transit, and junctional rearrangement and turnover is also discussed. We mostly cover findings in rodents, but we also make special remarks regarding humans. We anticipate that this information will provide a framework for future research in the field.

Members of the AP-1 Family of Transcription Factors Regulate the Expression of Gja1 in Mouse GC-1 Spermatogonial Cells

Gap junctions, mainly formed by Gja1 (Connexin43), play an essential role in the regulation of proliferation and differentiation of spermatogonia in the testis. Regulation of the abundance of Gja1 in spermatogonia involves various processes, including gene transcription, mRNA maturation, protein synthesis, post-translational modifications, plasma membrane integration and protein degradation. However, gene expression of Gja1 is abnormally decreased in most testicular germ cell tumors. Hence, a better understanding of the mechanisms of transcriptional regulation of Gja1 in spermatogonia is essential to understand how the loss of its expression occurs during the development of testicular cancer. As in other cell types, activator protein-1 (AP-1) transcription factors may be involved in such regulatory process. Thus, AP-1 members were overexpressed in GC-1 cells to assess their impact on Gja1 expression. We showed that Jun and Fosl2 cooperate to activate the Gja1 promoter in GC-1 cells. Furthermore, the recruitment of Jun to the proximal region (−153 to +46 bp) of the Gja1 promoter has been confirmed via chromatin immunoprecipitation. Protein kinase A and calcium-calmodulin protein kinase I also contribute to the activation of Gja1 expression by improving the cooperation between AP-1 factors. Therefore, the reduction in Gja1 expression in testicular germ cell tumors may involve a loss of cooperation between AP-1 factors.

Action and Interaction between Retinoic Acid Signaling and Blood–Testis Barrier Function in the Spermatogenesis Cycle

Spermatogenesis is a complex process occurring in mammalian testes, and constant sperm production depends on the exact regulation of the microenvironment in the testes. Many studies have indicated the crucial role of blood–testis barrier (BTB) junctions and retinoic acid (RA) signaling in the spermatogenesis process. The BTB consists of junctions between adjacent Sertoli cells, comprised mainly of tight junctions and gap junctions. In vitamin A-deficient mice, halted spermatogenesis could be rebooted by RA or vitamin A administration, indicating that RA is absolutely required for spermatogenesis. Accordingly, this manuscript will review and discuss how RA and the BTB regulate spermatogenesis and the interaction between RA signaling and BTB function.