Function of Rescued GPCRs with Missense Mutations

As just discussed, it is important to consider the relationship of the in vitro and in vivo responses of a mutant GPCR to a pharmacological chaperone with respect to clinical outcome. It is expected that a mutated GPCR that shows no response in vitro would not respond in vivo. However, predicting the in vivo response of mutated GPCR that does respond in vitro is more difficult, as cultured cells differ significantly from a living organism. For example, given that mutated receptors tend to be less stable than their wild-type counterparts, increased turnover rates could limit the time a rescued receptor is available to interact with endogenous ligand at the cell surface. In addition, the phar-macokinetic profile of the pharmacological chaperone, including its tissue distribution and clearance rate, could influence the net response, especially if the elimination half-life is long or tissue penetration is poor (see below). Furthermore, physiological factors need to be considered. For instance, in the case of IHH, pulsatile release of GnRH and subsequent binding to GnRHR in pituitary gonadotropes is critical for downstream effects, with LH release favored by a more rapid GnRH pulse frequency and FSH release favored by a slower pulse frequency. Hence, both temporal and quantitative rescue of GnRHR may be necessary to restore full physiological function. Similarly, in the case of adRP, interaction of opsin with 11-cis-retinal is not only critical for receptor stabilization, but also for achieving the optimal spectral properties characterized by a Vax of 500 nm. It has been shown that rescue of P23H rhodopsin with 11-cis-7-ring-retinal leads to transient chromophore binding and a Vax of 490 nm, properties that may provide only limited function of the receptor in the retina [135].

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