Hippo pathway proteins could synergistically regulate germ c

Hippo pathway proteins could synergistically regulate germ cell development with some specific receptors. Kassab et al. [27] found that epidermal growth factor (EGF) receptor was localized in testicular germ TMPyP4 tosylate and played important roles in bovine spermatogenesis. Likewise, insulin-like growth factor 1 (IGF-I) receptor was expressed in spermatogonia [28], and IGF-I signaling cascade was important for self-renewal and differentiation of germ cell [29]. Moreover, the activation of Hippo pathway effector YAP1 could promote cell proliferation and differentiation in response to EGF and IGF-I signaling [30,31]. In addition, Hippo signaling pathway proteins functioned as essential components of the mitotic exit network, which played crucial roles in spermatogenesis [32]. Combined with our results that the high expressions of Hippo pathway components existed in the germ cells, these results indicated the potential role of Hippo signaling in spermatogenesis in response to cytokines. After spermatozoa left the seminiferous tubules, it would reside in the epididymis, which was required for the acquisition of normal motility and fertilization, and these processes in the epididymis were androgen dependent [33]. Much evidence suggested that there were cross-talks between Hippo pathways and androgen signaling in cell survival and homeostasis [34,35]. Functionally, the growth factors [eg. vascular endothelial growth factor (VEGF) and hepatocyte growth factor (HGF)] were widely distributed in the epididymis [36]. Moreover, the roles of HGF in epithelial-to- mesenchymal transition and VEGF signaling during developmental angiogenesis were regulated by the Hippo pathway [37,38]. All these results indicate that Hippo pathway may regulate spermatozoa development in the epididymis by mediating the secretion of growth factors and cytokines. Furthermore, calcium crystal was easy to form in the lumen of rete testis, efferent ducts and epididymis, which could negatively regulate semen quality [39], while Hippo pathways may suppress calcium crystal formation by maintaining calcium oscillation and disrupting intracellular calcium overload [40]. Additionally, Hippo pathway compounds in epithelial cells of epididymis could contribute to bacterial infection prevention and maintain the semen quality [41], as well as sustain the redox balance and provide suitable microenvironment for the spermatozoa [9,42]. In this study, the main localization of Hippo pathway proteins in the epididymis and ductus deferens indicated Hippo pathway may be involved in the regulation of spermatozoa normal motility and fertilization in male reproductive tract. The increased expressions of Hippo pathway proteins in cauda epididymis of postpubertal Hu sheep indicated their important roles in male sexual maturation. Functional spermatozoa were stored in the cauda epididymis and remained immotile until ejaculation. During ejaculation, epididymal spermatozoa were mixed with secretions of male accessory sex glands (ASG) and the cytoplasmic droplet was shed from spermatozoa, which allowed the spermatozoa to motility, capacitation and further acrosome reaction [43]. The prominent staining of LATS1 protein in the acrosomal head of the ejaculated spermatozoa in our study indicated the potential role of LATS1 in spermatozoa acrosome reaction or fertilization, which could explain the decreased fertility of male Lats1−/− mice [11]. In addition, the strong expression of LATS2 and YAP1 proteins in the midpiece (a lot of mitochondria existed) of the ejaculated spermatozoa indicated the important roles of LATS2 and YAP1 in energy metabolism, which provided ATP to keep spermatozoa motility. This observation was consistent with previous research that there was a cross-talk between mitochondrial function and Hippo pathway in controlling cell proliferation and tissue homeostasis [44]. Furthermore, substantial evidence suggests that ASG fluid interactions can increase ATP requirement, intracellular calcium and cyclic AMP (cAMP) levels of the spermatozoa, which was essential for spermatozoa motility and subsequent capacitation [45,46]. Additionally, the Hippo pathway was a key signaling branch of cAMP in regulating cellular functions [47]. Altogether, in this context, the significantly increase of tested Hippo pathway components except for MST1 in ejaculation spermatozoa, emphasized the viewpoint that Hippo pathway components may play important roles in spermatozoa motility and capacitation.