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bound to its endogenous ligand, neurokinin A (NKA), NK2 can adopt distinct and
sequential conformations, stabilized by two high-affinity binding sites (
Palanche
et al., 2001
). The first conformation, A1L, is thought to facilitate rapid dissociation
of NKA from the receptor followed by G
a
q-induced calcium mobilization, whereas
the second conformation, A2L, results in slower ligand dissociation kinetics and
leads to G
a
s-induced cAMP production. An allosteric ligand for NK2R, LPI805,
preferentially stabilized the A1L conformation, diminishing the intensity of G
a
s-
mediated cAMP accumulation (
Maillet et al., 2007
). In a follow-up study, the authors
were able to generate derivatives of the original allosteric ligand, which generated dis-
tinct selectivity profiles, acting as a PAM for calcium signaling and an NAM for cAMP
production (
Valant,Maillet,etal.,2009
). Another example of an allosteric modulator
directly regulating coupling between the receptor and the G protein in a biased manner
was shown for the prostaglandin F2
a
(PGF2
a
) receptor (FP). We developed a small
molecule peptidemimic, PDC113.824, derived originally from the sequence of the sec-
ond extracellular loop of FP (
Goupil et al., 2010
). The peptide itself was initially char-
acterized as a tocolytic in a mouse model of preterm labor (
Peri et al., 2002
).
PDC113.824 was demonstrated to stabilize a specific conformation of FP receptor,
where G
a
q-induced PKC-ERK1/ERK2 activation was potentiated and G
a
12-induced,
Rho-mediated cytoskeletal rearrangement was inhibited following PGF2
a
stimulation
(
Goupil et al., 2010
). Interestingly, neither effector pathway was modulated by
PDC113.824 alone. However, basal levels of GTP
g
[
35
S] incorporation were altered
by PDC113.824 for both G
a
qandG
a
12, suggesting that the allosteric ligand recog-
nized two distinct preformed, receptor/G protein complexes. This observation has im-
plications regarding the actual molecular targets of biased and/or allosteric ligands.
Moreover, the repercussions of such biased signaling were manifested
ex vivo
and
in vivo
by inhibition of myometrial contraction and lipopolysaccharide- or PGF2
a
-in-
duced preterm labor in mice, respectively, in the presence of PDC113.824. Interest-
ingly, as for LPI805, the dissociation kinetics of [
3
H]-PGF2
a
from FP were more
rapid in the presence of PDC113.824, and G
a
q coupling was enhanced. These studies
suggest that allosteric modulators, which enhance GTP binding to G proteins in the ab-
sence of orthosteric ligand, may lead to alterations in orthosteric ligand affinity for the
receptor, leading to specific G protein-dependent signaling patterns (see
Fig. 9.2
).
GPCRs were long considered to be monomeric entities; however, it has become
clear in recent years that most if not all GPCRs can form dimers and possibly higher-
order structures (see
Bulenger, Marullo, & Bouvier, 2005; H
ยด
bert & Bouvier, 1998;
Kniazeff, Prezeau, Rondard, Pin, & Goudet, 2011; Milligan, 2009; Prinster, Hague,
& Hall, 2005
for review). Although some GPCRs can function in a monomeric state
(
Whorton et al., 2007, 2008
), dimerization seems critical in regulating all aspects of
GPCR function (receptor trafficking to and from the cell surface, ligand binding,
G protein coupling, and downstream signaling, reviewed in
Terrillon & Bouvier,
2004
). While allosteric and biased ligands are useful ways to control GPCR signaling
more selectively (
Fig. 9.1
A and B), the potential relevance of these ligands to indi-
vidual GPCRs is both increased and complicated by the existence of receptor homo-
and heterooligomers (
Fig. 9.1
C and D). The notion that GPCRs can modulate each
another allosterically in the context of a heterodimer, as the G protein does with
