Molecular Biology of the Kisspeptin Receptor: Signaling, Function, and Mutations - Kisspeptin Signaling in Reproductive Biology - page 135

Intracellular Traffi cking of KISS1R
KISS1R signaling is essential for the onset and duration of puberty; and abnormal
KISS1R signaling may lead to pubertal disorders. As an example, functional assays
performed to confi rm the association of a gain-of-function mutation in KISS1R
with the CPP phenotype of the patient indicated that the mutation affects the dura-
tion of KISS1R signaling, suggesting an effect of the mutation on KISS1R desensi-
tization. Thus, investigation of precise mechanisms regulating activation and
duration of KISS1R signaling may uncover new proteins or new mechanisms under-
lying pubertal disorders in patients carrying no mutations in genes currently known
to be involved in reproductive disorders.
Recent advances in this area include studies of ligand-induced KISS1R desensi-
tization and internalization as well as KISS1R fate after internalization. As described
for other GPCRs, KISS1R was shown to undergo time-dependent ligand-induced
receptor desensitization, which occurs in spite of the continuous presence of ligand
[ 41 ]. This confi rms and further explains previous observations of desensitization of
biological effects in response to continuous stimulation of KISS1R in vivo [ 47 - 49 ].
These observations include selective desensitization of the LH response to kiss-
peptin stimulation in female rats [ 47 ], as well as in agonadal Rhesus monkeys [ 48 ],
both of which did not prevent subsequent gonadotropin secretion in response to
other secretagogues. Moreover, desensitization of the gonadotropin response to
continuous Kiss1r stimulation in male rats is reported to result in testicular degen-
eration [ 49 ]. The testicular degeneration is a consequence of the desensitization of
KISS1R signaling, which is typically observed upon continuous stimulation in vitro
[ 6 , 41 ]. Desensitization of biological responses to kisspeptin stimulation such as LH
secretion has been described after continuous infusion of kisspeptin in Rhesus mon-
keys [ 48 , 50 ] or humans [ 51 ]. Surprisingly, continuous infusion of submaximal
doses of kisspeptin resulted in a sustained increase in LH pulse frequency in men,
suggesting that some responses to kisspeptin elude desensitization [ 51 ]. In women
treated with a bolus of kisspeptin, serum LH responses varied according to the
phase of the menstrual cycle, suggesting variations in the sensitivity to kisspeptin
across the menstrual cycle [ 52 ].
Acute desensitization of GPCRs is a consequence of uncoupling of the receptor
from its signaling pathway, which is typically followed by receptor internalization.
This appears to be the case for KISS1R, which has been shown to undergo time-
dependent desensitization, despite continuous presence of ligand [ 41 ]. Internalization
of GPCR is often mediated by arrestin, a membrane-resident protein that binds phos-
phorylated receptors, prevents further G protein coupling and signaling, and triggers
receptor internalization [ 53 - 56 ]. Accordingly, a study using fl uorescence microscopy
showed that a Flag-tagged KISS1R undergoes rapid ligand-induced internalization in
HEK cells [ 40 ]. This was confi rmed in time-course experiments in which internaliza-
tion and recycling of KISS1R was measured using 125 I-kisspeptin binding [ 41 ].
KISS1R internalization may be triggered, at least in part, by G protein-coupled
receptor kinase-2 (GRK2)-mediated phosphorylation, as overexpression of GRK2