The allosteric site of class

The allosteric site of class C receptors is generally located on the extracellular side of the TM domain (Brauner-Osborne et al., 2007, Christopher et al., 2015, Dore et al., 2014, Gregory et al., 2011, Harpsoe et al., 2017, Kniazeff et al., 2011, Pin and Prezeau, 2007, Urwyler, 2011, Wu et al., 2014). Recently, crystal structures of mGlu1 and mGlu5 TM domains have been solved, each in the presence of a NAM (Christopher et al., 2015, Dore et al., 2014, Wu et al., 2014). These structures reveal that the NAMs for mGlu1 (FITM) and mGlu5 (mavoglurant; HTL14242 and its analogue) have overlapping but distinct binding pockets, with the mGlu5 modulators bound deeper into the TM core (Christopher et al., 2015, Dore et al., 2014, Wu et al., 2014). In addition, the FITM-binding site is analogous to the orthosteric ligand-binding site of class A GPCRs (Rosenbaum et al., 2009, Wu et al., 2014). Previous mutagenesis studies indicate that different allosteric modifiers of a particular class C GPCR often occupy similar spaces and bind to an overlapping set of residues, as mutations that decrease the binding of one modifier can also hinder others (Conn et al., 2009, Gregory et al., 2011, Jensen and Brauner-Osborne, 2007, Urwyler, 2011). Furthermore, many of the known PAMs and NAMs of class C receptors share a common site within the overlapping FITM/mavoglurant binding pocket that is enclosed by TM helices 2, 3, 5, 6 and 7 (TM2, TM3, TM5, TM6 and TM7) (Christopher et al., 2015, Conn et al., 2009, Dore et al., 2014, Gregory et al., 2011, Harpsoe et al., 2017, Jensen and Brauner-Osborne, 2007, Kniazeff et al., 2011, Pin and Prezeau, 2007, Urwyler, 2011, Wu et al., 2014). At the same time, class C TM domains are known to possess regions that serve as distinct prostaglandin receptors for specific allosteric modulators (Conn et al., 2009, Gregory et al., 2011, Jensen and Brauner-Osborne, 2007, Urwyler, 2011). It has been shown that CPPHA, a PAM of mGlu5, binds within the TM domain at an unrelated site from most known mGlu5 allosteric modulators (Chen et al., 2008). In another example, CaS receptor contains separate allosteric sites within its TM domain for phenylalkylamines like the PAM Calindol, and structurally different molecules like the NAM ‘BMS compound 1’ (Arey et al., 2005, Hu et al., 2006, Jensen and Brauner-Osborne, 2007). Allosteric modulators are attractive drug candidates because of their potential for adjusting receptor action without risking side effects from full activation or complete inhibition (Brauner-Osborne et al., 2007, Christopoulos, 2002, Kniazeff et al., 2011, Pin and Prezeau, 2007, Urwyler, 2011). The various PAMs of GABAB receptor have shown therapeutic effects in pre-clinical studies on anxiety (Cryan et al., 2004), depression (Cryan et al., 2004), and drug addiction (Filip and Frankowska, 2007, Lhuillier et al., 2007, Mombereau et al., 2004, Slattery et al., 2005, Smith et al., 2004, Sturchler et al., 2017). Furthermore, these allosteric modulators lack the muscle-relaxing and sedative effects associated with the orthosteric drug baclofen (Kniazeff et al., 2011, Pin and Prezeau, 2007, Urwyler, 2011). These side effects also limit the use of baclofen in behavioral pharmacological studies and help to further research interests in PAMs and NAMs (Dalvi and Rodgers, 1996). However, detailed receptor-allosteric modulator interactions still await structural elucidation of GABAB receptor TM domain.
Trafficking control The intracellular domain of GABAB receptor controls the surface expression of the intact receptor (Margeta-Mitrovic et al., 2000). The GABAB1 subunit is retained inside the cell when expressed alone (Couve et al., 1998), and is only transported to the cell surface when it is chaperoned by GABAB2 in a heterodimeric pair (Margeta-Mitrovic et al., 2000, Pagano et al., 2001). Recently it has been discovered that the endoplasmic reticulum (ER) membrane protein, PRAF2, directly binds to GABAB1, and is responsible for trapping the subunit within the ER (Doly et al., 2016). GABAB1 exits the ER only after the bound PRAF2 is displaced by GABAB2. The assembled heterodimer then progresses to the Golgi apparatus, and in turn the cell surface where the receptor can function (Doly et al., 2016).