The role of conexin 43 in murine Sertoli cell development

Sertoli cells are the only somatic cell type present within the seminiferous tubules of the testis. The support of these somatic cells is essential for testis formation and spermatogenesis. They are called mother or nurse cells, implicating their intimate association with germ cells and also their central role in germ cell development. In mice, at embryonic day 11 (E11), Sertoli cells start to proliferate and continue to proliferate until two weeks postnatal. Puberty involves cessation of mitosis and progression of maturation/differentiation in Sertoli cells. Although Sertoli cell development has been extensively studied using various animal models, it is not yet fully understood. Thyroid hormone (T3) is a critical regulator of Sertoli cell development. Later studies identified that thyroid hormone effects on Sertoli cell development were mediated by some of the cell cycle proteins namely, p27Kip1, p21Cip1 and SKP2 (also known as CDKN1B, CDKN1A and p45, respectively). Roles of these cell cycle regulators on Sertoli cell development have been studied and Sertoli cell numbers were significantly altered in the absence of one or more of those cell cycle regulators. In addition, some studies suggested a mediatory role of the predominant testicular gap junction protein, connexin 43 (CX43), in the thyroid hormone effect on Sertoli cell development. To understand the specific role of CX43 in Sertoli cell development, I generated an animal model in which Cx43 is specifically deleted in Sertoli cells to circumvent the neonatal lethality of the Cx43 global knockout. Results indicated that in the absence of CX43, Sertoli cells failed to mature fully and also a significant population of Sertoli cells proliferated indefinitely into adulthood. The Sertoli cell phenotype of this animal model is unique and distinctly different from other animal models. Further exploration of Sertoli cell development was pursued using the Sertoli cell-specific CX43 knockout (SC-Cx43 KO) animal model. The first aim was to understand the proliferative pattern of SC-Cx43 KO Sertoli cells in vitro and also to determine whether these Sertoli cells respond differently to mitogenic and/or maturation induction stimuli. Results showed that Sertoli cells in this animal model switch between the resting and active phases of the cell cycle in vitro. Although Sertoli cells are proliferating even beyond the normal proliferative period in SC- Cx43 KO mice, their proliferation was unaltered by either mitogenic and/or maturation induction stimuli in vitro. The rest of the aims focused on understanding whether some major cell cycle regulators (SKP2, p27 Kip1 and p21Cip1), which were already shown to affect Sertoli cell development, were involved in the indefinite proliferation in Sertoli cells lacking CX43. In the second aim, I tested whether concomitant deletion of SKP2 in SC-Cx43 KO mice will stop the proliferation of Sertoli cells observed in SC-Cx43 KO mice. The results showed that Sertoli cell proliferation is stopped in SC-Cx43 KO Sertoli cells when they also lack SKP2. In the third aim, I tested whether the concomitant deletion of either p27Kip1 or p21Cip1 in the SC-Cx43 KO mice will potentiate the proliferation of Sertoli cells noticed in SC-Cx43 KO mice. The results showed that the proliferation of Sertoli cells was not potentiated by knocking out either p27Kip1 or p21Cip1 in SC- Cx43 KO mice. However, Sertoli cell proliferation is persistent but significantly lower than SC-Cx43 KO mice when either p27 Kip1 or p21Cip1 is deleted in SC-Cx43 KO mice. The collective findings of the present study provide important insights into the role of CX43 in Sertoli cell maturation and also the factors involved in the mechanism by which CX43 regulates Sertoli cell proliferation.