Intracellular Follicle-Stimulating Hormone Receptor Trafficking and Signaling

Abstract

Models of G protein-coupled receptor (GPCR) signaling have dramatically altered over the past two decades. Indeed, GPCRs such as the follicle-stimulating hormone receptor (FSHR) have contributed to these new emerging models. We now understand that receptor signaling is highly organized at a spatial level, whereby signaling not only occurs from the plasma membrane but distinct intracellular compartments. Recent studies in the role of membrane trafficking and spatial organization of GPCR signaling in regulating gonadotropin hormone receptor activity has identified novel intracellular compartments, which are tightly linked with receptor signaling and reciprocally regulated by the cellular trafficking machinery. Understanding the impact of these cell biological mechanisms to physiology and pathophysiology is emerging for certain GPCRs. However, for FSHR, the potential impact in both health and disease and the therapeutic possibilities of these newly identified systems is currently unknown, but offers the potential to reassess prior strategies, or unveil novel opportunities, in targeting this receptor.

Keywords: FSH receptor; GPCR; cAMP; endocytosis; endosome; signaling; trafficking.

Figure 1

Summary of G protein and arrestin-mediated signal pathways activated by FSHR. Upon ligand binding FSHR has been reported to couple to Gαs, Gαq/11, and Gαi/o heterotrimeric G proteins to mediate its downstream effects, both those in the gonads such as spermatogenesis, steroidogenesis, and follicular development, and more recently at non-gonadal sites, see text for further details. The archetypal view of GPCR signaling occurs in a G protein-dependent manner, however, agonist-activated, phosphorylated receptor recruits arrestin, for G protein signal desensitization and internalization via clathrin-coated pits. Furthermore, arrestin mediates signaling pathways independent of G proteins. This can occur through the scaffolding protein binding signaling proteins such as components of MAPK pathways after G protein activation, or independent of FSHR by complexing and activating ribosomal protein, p70S6K.

Figure 2

Model summarizing current understanding of gonadotropin hormone receptor post-endocytic pathways and receptor signaling from VEEs. The trafficking of FSHR and LHR to the VEE is inextricably linked to the receptor's signal output, whereby the manipulation of which at distinct steps results in different trafficking and signal profiles. (A) Following ligand-activation FSHR internalizes to the very early endosome (VEE). The VEE differs from the early endosome (EE) in its smaller size and neither contain EEA1 nor Rab5, classical markers for the EE. During receptor-mediated endocytosis of FSHR into a clathrin-coated pit, the PDZ domain protein, GIPC, is recruited at the cytosolic interface of the GPCR. Receptor then enters the complex endosomal network where it is primarily localized to the VEE. There are two types of VEE depicted, one contains the adaptor protein, APPL1 (see text), and one is without. GIPC dissociates before FSHR enters the VEE. From the VEE receptor is able to elicit downstream signaling cascades, including cAMP generation and ERK1/2 activation. This cAMP/PKA signal phosphorylates APPL1 on Serine 410. Receptor is trafficked to APPL1 positive VEEs, where the unphosphorylated APPL1 negatively regulates endosomal cAMP signaling. The phosphorylated APPL1 is required for receptor recycling back to the plasma membrane. (B) The receptor can be rerouted from the VEE to the EE by the loss of GIPC or disruptions in the receptors ability to interact with this PDZ protein. Loss of GIPC results in the trafficking of the receptor from the VEE pathway to the EE and loss of plasma membrane recycling. While endosomal cAMP signaling is not affected, ERK signaling profile is more transient as the receptor only rapidly passes through the VEE to the EE. (C) Loss of APPL1 does not alter the endosomal organization of the receptor but inhibits recycling. The “trapped” receptor in the VEE compartment results in increases in endosomal cAMP signaling due to APPL1's role in negative regulation of G protein signaling, but without impacting ERK signal profile. (D) Manipulating the ability of APPL1 to be phosphorylated on Serine 410, either by inhibition of PKA activity or mutation of serine 410 to alanine, specifically inhibits recycling but not cAMP endosomal signaling.