Biased Signaling and Allosteric Modulation at the FSHR

Abstract

Knowledge on G protein-coupled receptor (GPCRs) structure and mechanism of activation has profoundly evolved over the past years. The way drugs targeting this family of receptors are discovered and used has also changed. Ligands appear to bind a growing number of GPCRs in a competitive or allosteric manner to elicit balanced signaling or biased signaling (i.e., differential efficacy in activating or inhibiting selective signaling pathway(s) compared to the reference ligand). These novel concepts and developments transform our understanding of the follicle-stimulating hormone (FSH) receptor (FSHR) biology and the way it could be pharmacologically modulated in the future. The FSHR is expressed in somatic cells of the gonads and plays a major role in reproduction. When compared to classical GPCRs, the FSHR exhibits intrinsic peculiarities, such as a very large NH2-terminal extracellular domain that binds a naturally heterogeneous, large heterodimeric glycoprotein, namely FSH. Once activated, the FSHR couples to Gαs and, in some instances, to other Gα subunits. G protein-coupled receptor kinases and β-arrestins are also recruited to this receptor and account for its desensitization, trafficking, and intracellular signaling. Different classes of pharmacological tools capable of biasing FSHR signaling have been reported and open promising prospects both in basic research and for therapeutic applications. Here we provide an updated review of the most salient peculiarities of FSHR signaling and its selective modulation.

Keywords: G protein; GPCR; bias; follicle-stimulating hormone; reproduction; signaling; trafficking; β-arrestin.

Figure 1

Orthosteric and allosteric sites in the FSHR. (A) Cartoon and surface view of the transmembrane regions of the FSHR showing P1 and P2 allosteric sites. (B) Complex between the ectodomain of the FSHR (gray) and FSH (violet: alpha chain, pink: beta chain). The colored spheres represent sulphated Tyr355. (C) Residues involved in FSH binding are shown in red. (D) Close-up on the interaction between sulphated Tyr335 (colored spheres) and FSH.

Figure 2

FSHR signaling and trafficking. Upon FSH binding, the FSHR mainly activates Gαs protein, leading to conversion of ATP to cAMP by adenylyl cyclases and activation of intracellular effector kinases, including PKA. After stimulation, GRK phosphorylates and desensitizes the FSHR. Phosphorylated FSHR recruits β-arrestin, which in turn induces its own signaling, including ERK activation, as well as receptor internalization in the endosomes. FSHR potentially activates G protein-dependent and -independent signaling from the endosomal compartment, before quickly recycling back to the plasma membrane. Effector proteins drive the cellular responses, including gene transcription, cell proliferation and differentiation. APPL1, Adaptor protein, phosphotyrosine interacting with PH domain and leucine zipper 1; CREB, cAMP response element binding protein; ERK, extracellular signal-regulated kinase; FSH, Follicle-stimulating hormone; GRK, G protein-coupled receptor kinase; PKA, protein Kinase A.

Figure 3

Ligand bias at the FSHR. Balanced agonists or PAM at the FSHR induce both G protein and β-arrestin recruitment. FSH binding to the FSHR can be prevented using small competitive ligands, antibodies directed against the binding pocket of the FSHR or directly against FSH. Biased signaling toward Gαi, Gαs, or β-arrestin recruitment can result from glycosylation forms of FSH (fully glycosylated FSH24 vs partially glycosylated FSH21-18), antibody or small chemical compounds.

Figure 4

Mutation-induced receptor bias at the FSHR. Mutations can lead to biased signal transduction at the FSHR upon exposure to fully glycosylated FSH (FSH24). Green, Gs-biased mutants; purple, β-arrestin-biased mutant.