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inhibited G protein-mediated signaling by KISS1R. Conversely, overexpression of
a catalytically inactive GRK2 mutant (K220R-GRK2) led to increased G protein
signaling by the KISS1R [
40
]. GRKs are enzymes that selectively phosphorylate
activated GPCRs, leading to recruitment and binding of arrestin to the phosphory-
lated receptor, preventing further G protein coupling or signaling and triggering
arrestin-dependent GPCR internalization [
56
,
57
]. Accordingly, KISS1R has been
shown to colocalize with
-arrestin-2, which provides further support for arrestin-
mediated internalization of KISS1R [
58
]. Additionally, Saereszewsky et al. showed
that overexpression of
β
-arrestin-2 potentiates the phosphorylation of ERK in fi bro-
blasts lacking arrestin or G
q/11
and transfected with KISS1R. Conversely, overex-
pression of
β
-arrestin-1 in these fi broblasts inhibits KISS1R-dependent ERK
phosphorylation, which suggests that this KISS1R effect may specifi cally require
β
β
-arrestin-2 [
58
].
Following internalization, GPCRs are eventually sorted for recycling or destruc-
tion. Destruction of GPCRs is classically mediated by lysosomal degradation,
which is responsible for long-term GPCR desensitization [
55
]. Intriguingly, confo-
cal microscopy of KISS1R failed to detect lysosomal targeting of KISS1R under
any experimental conditions studied, including stimulation with a supraphysiologi-
cal concentration of kisspeptin (10
−7
M) for up to 5 h [
41
]. This surprising fi nding
was confi rmed by western blot analysis, which showed that levels of KISS1R pro-
tein are not affected by treatment with a lysosome inhibitor, which suggests that,
unlike most GPCRs, lysosomal degradation of KISS1R is low or absent [
41
].
Conversely, treatment of the same cells with an inhibitor of proteasomal degrada-
tion resulted in massive increases in KISS1R protein when compared to untreated
cells, which suggests that KISS1R may undergo proteasomal (rather than lyso-
somal) degradation [
41
]. Although unusual, proteasomal degradation has been
reported for some GPCRs [
59
,
60
].
Further investigation using confocal microscopy revealed persistent membrane
localization of KISS1R for up to 5 h of stimulation with supraphysiological concen-
trations of kisspeptin [
41
]. This suggests a dynamic pattern of recycling of internal-
ized KISS1R back to the cell surface, which was confi rmed in studies measuring the
rate of membrane traffi cking of KISS1R using
125
I-kisspeptin binding [
41
]. This
behavior indicates that a substantial amount of kisspeptin-dependent KISS1R sig-
naling is, in fact, due to the binding of kisspeptin to newly recycled KISS1R, which
demonstrates that KISS1R recycling is relevant for its function [
41
].
Inactivating Mutations in KISS1R and Normosmic IHH
Central hypogonadotropic hypogonadism refers to delayed or absent sexual matura-
tion due to a central nervous system defect that results in low circulating gonadotro-
pins and sex steroids [
2
]. These cases are further classifi ed as idiopathic when the
source of the hypogonadotropic hypogonadism cannot be identifi ed. IHH affects
1-10 individuals per 100,000 births and its incidence is sexually dimorphic, with
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