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homozygous LuRKO/LHCGR
B
/LHCGR
S
animals with WT females and compar-
ing litter sizes and numbers of litters to those of WT males crossed with WT females.
Our studies showed that not only were the homozygous LuRKO/LHCGR
B
/
LHCGR
S
males fertile but also their fertility was comparable to WT males.
23.3
DISCUSSION
Our data outline the methods by which we have explored GPCR transactivation using
the LHCGR as a model receptor. We have shown that coexpression of LHCGR
B
and LHCGR
S
in vitro
results in receptor transactivation, and the degree of func-
tional rescue observed is dependent on the cell surface coexpression levels of
LHCGR
B
and LHCGR
S
. In a LuRKO background, using BAC transgenics to
coexpress LHCGR
B
and LHCGR
S
, we have shown functional rescue of the hypo-
gonadism and infertility of LuRKO animals. Our work presents the first model that
demonstrates class A GPCR homodimerization, via transactivation, is a physiolog-
ically relevant mechanism of LHCGR functionality
in vivo
.
The structural determinates between LHCGR
B
and LHCGR
S
resulting in
transactivation are yet to be conclusively demonstrated. While one study suggests
that the minimal functional unit to observe transactivation is the extracellular domain
of LHCGR anchored by CD8 to activate a LHCGR
B
(
Ji et al., 2002
), others have
observed that the first TM domain of LHCGR is required for transactivation (
Osuga,
Hayashi, et al., 1997; Osuga, Kudo, et al., 1997
). Whether these structural require-
ments reflect the dimer interface is also unclear; while there is no information on
potential LHCGR dimer interfaces, BRET studies of the closely related follicle stim-
ulating hormone receptor (FSHR) demonstrated that either the N-terminal extracel-
lular domain or the 7TM domain was sufficient for receptor-receptor interactions
(
Guan et al., 2010
), indicating that either transactivation or the dimer interface most
likely involves multiple regions of the receptor. Transactivation appears to be suf-
ficient for mediating LHCGR functions in male mice; however, the role of cisactiva-
tion in LHCGR signaling remains unknown.
There are certain caveats to consider when discussing receptor transactivation,
and we acknowledge that LHCGR transactivation will not be observed with all
LHCGR
B
and LHCGR
S
that are coexpressed. It has been previously demonstrated
that the degree of functional rescue through transactivation is dependent on the po-
sition of the mutation in the LHCGR
B
, with mutations at the very N-terminal region
of the extracellular domain favoring transactivation and mutations at the C-terminal
of the extracellular domain approaching the hinge region, detrimental for transacti-
vation (
Lee, Ji, & Ji, 2002; Lee, Ji, Ryu, et al., 2002
).
With the increasing sensitivity of techniques for determining G protein-
dependent and G protein-independent signaling events, it is becoming increasingly
evident that GPCRs can couple to a diverse array of signaling pathways.
Dimerization presents a functional mechanism by which the preference for coupling
to a specific pathway can be modulated. It is conceivable that regulation of the com-
position of a di/oligomer may change the preference of pathway activation, acting to
fine tune the signal diversity of a specific receptor subtype.
