4.2 The male sexual dimorphism-related GnRH signaling pathway
GnRH is secreted from the hypothalamus and plays a key role in the control of vertebrate reproductive functions by binding to its specific membrane receptor, GnRHR, triggering the synthesis and release of pituitary gonadotropins, including follicle-stimulating hormone (FSH), LH, and CG, to induce gonads that produce sex steroids for gametogenesis (Fernald and White, 1999; Okubo and Nagahama, 2008). In the hypothalamus-pituitary-gonadal axis, the GnRH signaling cascade control process is essential for maintaining gonadotropin synthesis and release. It plays an important role in developing secondary sex characteristics (Bosch et al., 2021).
Almost all gene families in the GnRH signaling pathway have been identified in the male O. bidens genome, and multiple expanded genes are present in the node steps of this pathway. First, GnRHR transmits its signal to activate phospholipase C by coupling channel proteins, such as guanine nucleotide-binding protein subunits Gs and Gq/11. Subsequently, the subtypes of G proteins and their receptors represent the components of a highly versatile signal transduction system, centrally involved in hormone release and actions (Wettschureck and Offermanns, 2005). Receptor-G protein coupling requires diacylglycerol (DAG) and inositol 1,4,5-trisphosphate (IP3) after ligand-dependent receptor activation, and the G protein occasionally occurs in a cell-specific manner (Krsmanovic et al., 2003; Ruf et al., 2003). The intracellular PKC pathway supports the primary mechanism for hormone release and action activated via DAG by releasing intracellular calcium and activating the calcium signaling pathway stimulated via IP3 (Du et al., 2015).
In the present study, out of 138 genes involved in the GnRH signaling pathway in male O. bidens , 78 showed at least a 3-fold increase in essential nodes regulating gonadotropin hormone secretion by pituitary gonadotrope cells. The top three gene families were CaMKII, p38MAPK, and cytosolic phospholipase A2 (PLA2). It has been reported that the Ca2+ increase is fast and transient upon binding of most hormone receptors and that CaMKII, which is a ubiquitous serine/threonine protein kinase, is enriched in the IP3 pathway and activated by Ca2+ signals and calmodulin (CaM) to phosphorylate diverse substrates involved in metabolism and hormone release (Illario et al., 2003). LH synthesis and secretion are influenced by cytokines transported to the pituitary gland through the blood and act on endocrine pituitary cells. The p38MAPK and PLA2 genes are enriched in the PKC pathway, and when p38 MAPK is inhibited by a small-molecule inhibitor, the LH expression decrease at mRNA levels (Jian et al., 2021). PLA2 is a large family of calcium-dependent phospholipases with the ability to directly interact with G-proteins and kinases (Song et al., 1999), resulting in short sequestration in activating regulated LH secretion (Melamed et al., 2012). LH production induces growth hormone (GH) and regulates GH secretion and synthesis (Chen et al., 2018a; Fontaine et al., 2020; Fung et al., 2017). Our results indicate that the expanded genes in the GnRH signaling pathway play an important role in sexual dimorphism in male O. bidens .