Regulation of G protein-coupled receptor function by Na+/H+ exchange regulatory factors

Abstract

Many G protein-coupled receptors (GPCR) exert patterns of cell-specific signaling and function. Mounting evidence now supports the view that cytoplasmic adapter proteins contribute critically to this behavior. Adapter proteins recognize highly conserved motifs such as those for Src homology 3 (SH3), phosphotyrosine-binding (PTB), and postsynaptic density 95/discs-large/zona occludens (PDZ) docking sequences in candidate GPCRs. Here we review the behavior of the Na+/H+ exchange regulatory factor (NHERF) family of PDZ adapter proteins on GPCR signalling, trafficking, and function. Structural determinants of NHERF proteins that allow them to recognize targeted GPCRs are considered. NHERF1 and NHERF2 are capable also of modifying the assembled complex of accessory proteins such as β-arrestins, which have been implicated in regulating GPCR signaling. In addition, NHERF1 and NHERF2 modulate GPCR signaling by altering the G protein to which the receptor binds or affect other regulatory proteins that affect GTPase activity, protein kinase A, phospholipase C, or modify downstream signaling events. Small molecules targeting the site of NHERF1-GPCR interaction are being developed and may become important and selective drug candidates.

Fig. 1.

Schematic representation of human NHERF proteins discussed in this review. PDZ domains are indicated by the red rectangular boxes and EBDs by purple ellipses. The relative scale is shown on the bottom.

Fig. 2.

Position of the carboxyl-terminal PTHR (PTHR-CT ligand) in the binding groove of the PDZ2 domain (only the peptide backbone is shown). The α2-helix is displayed as a ribbon. The carboxylate binding loop (Gly-163, Tyr-164, Gly-165, and Phe-166) and β2 strand (Asn-167 and Leu-168) are shown as the polypeptide backbone chain (stick representation). The carbon atoms of the PTHR-CT ligand are yellow, the oxygen atoms are red, and the nitrogen atoms are blue. H-bond interactions are indicated by black dotted lines. The proximity of Ser-162 to this site is evident. Predicted electrostatic interactions between the Glu -3 side chain and Arg-180 (β3 strand) are depicted. (Graphic kindly prepared by Dr. T. Mamonova.)

Fig. 3.

Models of NHERF effects on GPCR signaling. Activation (+) or inhibition (−) of signaling pathways is shown with blue or red lines, respectively. A, E-T-V-M-binding domain at the carboxyl terminus of the PTHR binds to PDZ1 of NHERF1 or PDZ2 of NHERF2. In turn, through the EBD, NHERF1 associates with merlin, ezrin, radixin, or moesin (MERM) with the actin cytoskeleton. NHERF1 interactions with the PTHR increase the binding for Gα_q [NHERF1 and -2] or Gα_i [NHERF2] proteins (Wang et al., 2010). Upon PTH stimulation, NHERF1 increases PLCβ activity, internalizes the Npt2a Na-P_i cotransporter, and decreases β-arrestin recruitment to the receptor. B, upon agonist stimulation, the D-S-S-L-binding domain at the carboxyl terminus of the β2-AR) binds to PDZ1 of NHERF1 or PDZ1 or PDZ2 of NHERF2 (Lau and Hall, 2001). Interactions of the β₂AR with NHERFs increase cAMP accumulation and also increase (NHERF1) or decrease (NHERF2) CFTR of anion extrusion (Naren et al., 2002; Taouil et al., 2003; Singh et al., 2009). Inhibitory effects of NHERF1 on NHE3 transporter activity decrease after NHERF1 releases the transporter when recruited to β2-AR. C, the D-T-S-L-binding domain at the carboxyl terminus of the P2Y1R binds PDZ2 of NHERF2 (Hall et al., 1998a; Fam et al., 2005), tethering the receptor indirectly to PLCβ and increasing calcium (Ca²⁺) signaling (Fam et al., 2005). NHERF2 associates with ezrin through the EBD and in turn ezrin bind PKA, which increases CFTR Cl⁻ efflux (Guerra et al., 2004). D, the DSTL-binding sequence at the carboxyl terminus of the LPA2R binds to PDZ2 of NHERF2, thereby tethering the receptor indirectly to PLCβ and increasing Akt and ERK1/2 phosphorylation (P) and activity upon LPA stimulation (Oh et al., 2004; Yun et al., 2005). NHERF2 interactions with LPA₂R also increase the binding for Gα_q or Gα_i (Li et al., 2005; Lee and Zheng, 2010). Recruitment of Gα_i decreases adenylyl cyclase activity, reducing cAMP accumulation, and decreasing Cl⁻ transport by the CFTR (Li et al., 2005; Singh et al., 2009).

Fig. 4.

Alignment of NHERF1 by species. The consensus sequence and percentage conservation are shown at the bottom.