Trafficking and quality control of the gonadotropin releasing hormone receptor in health and disease

Abstract

SUMMARYIn order to serve as enzymes, receptors and ion channels, proteins require structural precision. This is monitored by a cellular quality control system (QCS) that rejects misfolded proteins and thereby protects the cell against aberrant activity. Misfolding can result in protein molecules that retain intrinsic function, yet become misrouted within the cell; these cease to perform normally and result in disease. A therapeutic opportunity exists to correct misrouting and rescue mutants using "pharmacoperones," (small molecular folding templates, often peptidomimetics, which promote correct folding and rescue) thereby restoring function and potentially curing the underlying disease. Because of its small size, the GnRH (gonadotropin-releasing hormone) receptor (GnRHR) is an excellent model for GPCRs (G protein-coupled receptor) and has allowed elucidation of the precise biochemical mechanism of pharmacoperone action necessary for rational design of new therapeutic agents. This review summarizes what has been learned about the structural requirements of the GnRHR that govern its interaction with the QCS and now presents the potential for the rational design of pharmacoperones. Because of the role of protein processing, this approach is likely to be applicable to other GCPCs and other proteins in general.

Pharmacological Chaperones Restore Function to MC4R Mutants Responsible for Severe Early-Onset Obesity

“…In many of these conformational diseases, the mutation occurs in receptor domains that do not directly affect ligand binding or G protein coupling, opening the possibility for interventions that could restore receptor function by rescuing folding and cell surface expression (Bernier et al, 2004;Conn et al, 2007). Such functional rescue has been achieved for several GPCRs, indicating that pharmacologically selective compounds, termed pharmacological chaperones (PCs), can stabilize the misfolded receptors to facilitate their export from the ER to the plasma membrane where they are active (Morello et al, 2000;Petäjä -Repo et al, 2002;Noorwez et al, 2004;Bernier et al, 2006;Robben et al, 2007;Conn and Janovick, 2009). The clinical effectiveness of a PC approach has been tested for one such disease, nephrogenic diabetes insipidus, where a vasopressin antagonist, acting as PC, rescued the function of ER-retained V2-vasopressin receptor mutants and significantly improved the kidney function of patients with nephrogenic diabetes insipidus (Bernier et al, 2006).…”

Salt Bridges Overlapping the Gonadotropin-Releasing Hormone Receptor Agonist Binding Site Reveal a Coincidence Detector for G Protein-Coupled Receptor Activation

“…Primate GnRHRs, unlike their rat and mouse orthologs, are poorly trafficked from the endoplasmic reticulum (ER) to the plasma membrane (PM). Approximately half of the newly synthesized receptor molecules are misfolded and retained in the ER (Leañ os-Miranda et al, 2002;Conn et al, 2006a;Conn and Janovick, 2009b). This distribution of human GnRHR (hGnRHR) between the ER and PM is highly sensitive to point mutations (Knollman et al, 2005) whose effects on receptor trafficking are the underlying cause of certain forms of disease (Leañ os-Miranda et al, 2002Janovick et al, 2006).…”

Salt bridge integrates GPCR activation with protein trafficking

“…4 Middle, all cells were rescued by In3, then In3 was washed out and each of the five pharmacoperones was added to assess constitutive activation; this observation shows that the pharmacoperones also behave as inverse agonists, agents that inhibit constitutive activity (compare with dashed line, showing IP production by mutant E 90 K that was not rescued). Because the pharmacoperones used in this study all form a bridge between receptor residues D 98 and K 121 (4,11,19), other interactions notwithstanding, they stabilize the TM2-TM3 relation, an event that allows correct trafficking but precludes constitutive activity, because of occupancy. When the pharmacoperones are washed out and the metabolically stable GnRH agonist, Buserelin, is present after rescue of E 90 K by each of the pharmacoperones, effector coupling is observed.…”

“…Because WT receptors are frequently only partially expressed at the plasma membrane (11,19), these findings may explain why inverse agonists (in this case, pharmacoperones), which stabilize receptors in the configuration that is acceptable to the QCS, have been shown to produce greater receptor up-regulation in many systems than do neutral antagonists (22). This effect may occur by increasing the percentage of protein that passes the QCS.…”

“…The drugs are, respectively, from the following chemical classes: indoles, quinolones (both Q89 and Q103), erythromycin macrolides and (N-1-(2,6-difluorobenzyl)-2,4-dioxo-3-phenyl-1,2,3,4-tetrahydrothieno[2,3-d]pyrimidin-6-yl]phenyl}-N'-methoxyurea). Chemical structures (11,19) and the mechanism of action on the hGnRHR (4) have been reported. Transient Transfection.…”