In recent years the Hippo signaling pathway has emerged as

In recent years, the Hippo signaling pathway has emerged as an important regulator balancing cell proliferation and differentiation [[5], [6], [7]]. The pathway components, first described in the fruit fly Drosophila melanogaster, are highly conserved across different phyla [6]. The signaling pathway consists of a group of Ser/ Thr kinases that regulate the activity of transcriptional co-activator YAP and a closely related paralog TAZ [6,8]. The Hippo pathway, when active, negatively regulates YAP/TAZ activity by phosphorylation and subsequent degradation of YAP/TAZ in the cytoplasm [8]. In contrast, when the pathway is inactive, YAP/TAZ are transcriptionally active and translocate to the nucleus for regulating gene expression [8]. Inactivating mutations in the Hippo kinase homologs- STK3 and STK4 is associated with cellular over proliferation and development of cancer, primarily due to activation of YAP [5]. The Hippo pathway does not have a dedicated receptor and the Stk3/Stk4 kinases integrate inputs from multiple sources to phosphorylate and activate the Large tumor suppressor Kinases- Lats1 and Lats2 [8]. The Lats kinases get phosphorylated upon activation and inactivate YAP. Phosphorylated YAP is either sequestered in the cytoplasm by binding to 14-3-3 proteins or is degraded by the ubiquitin mediated proteosomal degradation pathway [8]. Deregulated expression of YAP/TAZ is a hallmark of most cancer types and the Hippo pathway components are attractive therapeutic targets for the treatment of cancer [5,9]. The Hippo pathway has been shown to play a crucial role in the development and maintenance of many organ and cell types [6,10,11]. However, regulatory role of this pathway in the mammalian testes is not well known, although this organ is actively engaged in cell division and differentiation of germ cells. A recent study has demonstrated a role of Hippo signaling in the Sertoli e1 ubiquitin of Atlantic salmon [12]. YAP and TAZ have also been shown to regulate the expression of sex differentiation markers in mouse testes [13]. In a previous study, we have reported that YAP is an important regulator of cyclic AMP signaling in pubertal (19-day old) functionally mature Sc [14]. However, the post natal expression pattern of Hippo signaling pathway genes in infantile and pubertal Sc is not known. In the present study, we have assessed the expression of the Hippo pathway components in infant (5-day old) and pubertal (19-day old) rat Sc, in-vitro. Additionally, we have looked into the role of Follicle Stimulating Hormone (FSH) in regulating YAP expression and phosphorylation in Sc. Our results suggested a change in the expression pattern of the Hippo pathway components during the transition of Sc from a highly proliferative infantile state to a differentiated functionally mature state at the onset of puberty.
Materials and methods
Results
Discussion In the present study, the expression pattern of Hippo signaling pathway components was analyzed in infant (immature) and pubertal (functionally mature) rat Sertoli cells in vitro. The timely transition of Sc from a proliferative state during infancy to a differentiated, non proliferative state at the onset of puberty is a crucial determinant of fertility. Disruption in Sc number due to impaired proliferation adversely affects spermatogenic output during adulthood and impaired functional maturation due to non responsiveness to hormones (viz. FSH) and/or deregulated signaling pathways adversely affect germ cell division and differentiation subsequently leading to infertility/ sub-fertility. Infant Sc are immature and cannot support spermatogenesis [2] ; cessation of Sertoli cell proliferation and establishment of a differentiated functionally mature state occurs at the onset of puberty [2]. A number of factors, both hormonal and non hormonal have been reported to regulate the timely cessation of proliferation and induction of Sc differentiation [2,20,21]. For instance, experimentally induced hypothyroidism in rats is known to prolong the proliferative phase leading to delayed puberty, however it results in an increase in sperm production, primarily due to increased numbers of Sertoli cells in the testes [4,20]. Similarly, differential FSH action in infant and pubertal Sc regulates Sc proliferation and functional maturation, respectively. FSH predominantly activates the PI3 kinase pathway during infancy to induce Sc proliferation [21]. In contrast, FSH augments cyclic-AMP production in pubertal Sc to up regulate expression of genes important for germ cell division and differentiation [16,21].