From cell surface to signalling and back: the life of the mammalian FSH receptor

Follicle-stimulating hormone receptor (FSHR) is a class A G protein-coupled receptor that belongs to the subfamily of glycoprotein hormone receptors (GPHRs). The interaction of FSH with FSHR triggers downstream signaling pathways that play a central role in mammalian reproduction, such as folliculogenesis in females and the maintenance of spermatogenesis in males. This warrants a detailed investigation into FSHR, from its genesis, to the post-translational modifications that enable it to become functionally competent, followed by its trafficking to the cell membrane. Subsequently, FSH-stimulated G(s) uncoupling and transduction of G protein-mediated signaling pathways takes place, after which the receptor undergoes beta-arrestin-mediated internalization and may trigger other noncanonical signaling pathways. The majority of the FSH-FSHR complexes are recycled back to the cell surface and only a small proportion are routed to lysosomal degradation pathways, thus completing the lifecycle of the FSH receptor. Information about important epitopes and aspects of FSH receptor function has been gleaned from a number of sources, including structure-function studies on both naturally occurring and induced mutations, single nucleotide polymorphisms, peptides and antipeptide antibodies corresponding to predicted functional residues, X-ray crystallography analysis and high resolution imaging studies, in addition to the information available for the other GPHRs. In this review, we have traversed through the life cycle of the FSH receptor and discuss the reproductive pathophysiologies that could result from an impairment in receptor function, as may arise from defects during its journey from its birth to its degradation. Moreover, the unresolved questions and challenges that require exploration have been highlighted.