Regulatory Processes Governing the Cell Surface Expression of LH and FSH Receptors - GPCR Signalling Complexes: Synthesis, Assembly, Trafficking and Specificity - page 110

et al. 2002 ; Fanelli et al. 2004 ; Piersma et al. 2007 ; Segaloff 2009 ; Vasseur et al.
2003 ; Smits et al. 2003 ; Montanelli et al. 2004a, 2004b ; De Leener et al. 2006 ) .
Interestingly, in spite of their high degree of homology, the hLHR and hFSHR
exhibit markedly different degrees of susceptibility to mutation-induced constitu-
tive activity, with hFSHR constitutively active mutants being far less active than
hLHR constitutively active mutants (Zhang et al. 2007 ) . Furthermore, constitutively
active mutants of the hFSHR exhibit a greater sensitivity to activation by supra-
maximal concentrations of the other glycoprotein hormones as compared to consti-
tutively active mutants of the LHR and TSHR (Vasseur et al. 2003 ; Smits et al.
2003 ; Montanelli et al. 2004a, b ; De Leener et al. 2006 ) . Although the activated
LHR and FSHR signal primarily through Gs (at least in gonadal cells), they have
been shown to stimulate other G proteins as well (Hirakawa and Ascoli 2003 ;
Srisuparp et al. 2003 ; Lai et al. 2008 ; Donadeu and Ascoli 2005 ; Escamilla-
Hernandez et al. 2008 ) .
Prior to the cloning of the cDNAs for the LHR and FSHR, there was a great deal
of controversy regarding whether each of the receptors was composed of a single
protein or was a multi-protein complex (Segaloff and Ascoli 1993 ) . This was
resolved when it could be shown that cells transfected with cDNA encoding the
single receptor polypeptide could bind and respond to hormone (McFarland et al.
1989 ; Sprengel et al. 1990 ). Now, more than 20 years later, it is appreciated that the
LHR and FSHR associate with themselves and to form dimeric and oligomeric
complexes. The role of the dimerization/oligomerization of the LHR and FSHR on
their biogenesis and trafficking to the plasma membrane, as well as their signaling
properties on the cell surface, are discussed herein.

7.2 Synthesis of the Gonadotropin Receptors

The gonadotropin receptors are first synthesized as immature receptors with
high-mannose containing N-linked carbohydrates. As such, the immature recep-
tors are sensitive to endoglycosidase H, but are insensitive to neuraminidase,
consistent with their localization to the ER (Davis et al. 1995, 1997 ; Mizrachi
and Segaloff 2004 ). In contrast, the higher molecular weight mature receptor
proteins are insensitive to endoglycosidase H, but are sensitive to neuraminidase
(Davis et al. 1995, 1997 ; Mizrachi and Segaloff 2004 ) . Pulse-chase experiments
have confirmed the precursor nature of the immature form of the receptor (Hipkin
et al. 1992 ; Quintana et al. 1993 ) and protease experiments have confirmed
the localization of the mature receptor to the cell surface (Fabritz et al. 1998 ) .
As would be expected, the sites of attachment of the N-linked carbohydrates are
within the extracellular domains of the LHR and FSHR. The rat (r) LHR was
shown to contain N-glycans on each of the six predicted sites of glycosylation
(Davis et al. 1997 ). Analyses of the rLHR in tunicamycin-treated cells showed
that it exhibited normal high affinity binding and hormone-stimulated cAMP
production, con fi rming that the N-glycans of the LHR are not directly involved in