Visual arrestin interaction with clathrin adaptor AP-2 regulates photoreceptor survival in the vertebrate retina

Abstract

Arrestins bind ligand-activated, phosphorylated G protein-coupled receptors (GPCRs) and terminate the activation of G proteins. Additionally, nonvisual arrestin/GPCR complex can initiate G proteinindependent intracellular signals through their ability to act as scaffolds that bring other signaling molecules to the internalized GPCR. Like nonvisual arrestins, vertebrate visual arrestin (ARR1) terminates G protein signaling from light-activated, phosphorylated GPCR, rhodopsin. Unlike nonvisual arrestins, its role as a transducer of signaling from internalized rhodopsin has not been reported in the vertebrate retina. Formation of signaling complexes with arrestins often requires recruitment of the endocytic adaptor protein, AP-2. We have previously shown that Lys296 → Glu (K296E), which is a naturally occurring rhodopsin mutation in certain humans diagnosed with autosomal dominant retinitis pigmentosa, causes toxicity through forming a stable complex with ARR1. Here we investigated whether recruitment of AP-2 by the K296E/ARR1 complex plays a role in generating the cell death signal in a transgenic mouse model of retinal degeneration. We measured the binding affinity of ARR1 for AP-2 and found that, although the affinity is much lower than that of the other arrestins, the unusually high concentration of ARR1 in rods would favor this interaction. We further demonstrate that p44, a splice variant of ARR1 that binds light-activated, phosphorylated rhodopsin but lacks the AP-2 binding motif, prevents retinal degeneration and rescues visual function in K296E mice. These results reveal a unique role of ARR1 in a G protein-independent signaling cascade in the vertebrate retina.

Identification of Receptor Binding-induced Conformational Changes in Non-visual Arrestins

“…Fixed position(s) of the C-tail in both nonvisual subtypes can promote interactions with clathrin and other partners that arrestin-1 does not bind. Interestingly, even the C-tail of arrestin-1 can interact with AP2 upon its release by receptor binding (65).…”

Induction of the Unfolded Protein Response by Constitutive G-protein Signaling in Rod Photoreceptor Cells

“…Therefore G-protein signaling is prolonged in the ARR1 or GRK1 knock-out mice, and the retina degenerates even under low ambient light (15,16). Light damage in this pathway is prevented in the transducin G t ␣ (encoded by the GNAT1 gene) knock-out background, indicating a dependence on phototransduction (8,17,18). How constitutive G-protein signaling leads to photoreceptor cell death is not well understood but is an important question to address inasmuch as defects in ARR1 and GRK1 genes occur in the human population (19,20), and certain mutations in other phototransduction genes generate "equivalent light" by driving the phototransduction cascade (21).…”

The Physiological Roles of Arrestin-1 in Rod Photoreceptor Cells

“…Indeed, recent evidence showed that K296E recruits AP2 and other key endocytic proteins, such as endophilin and clathrin, to the outer segment (Moaven et al 2013). Interestingly, p44 rescued retinal degeneration and restored visual function to K296E mice (Moaven et al 2013). These results implicate recruitment of endocytic proteins by the K296E/arrestin-1 complex in generating the cell death signal.…”

“…Given that AP2 binding alone can recruit clathrin (Laporte et al 2000), it is possible that arrestin-1 is able to recruit endocytic proteins via AP2. Indeed, recent evidence showed that K296E recruits AP2 and other key endocytic proteins, such as endophilin and clathrin, to the outer segment (Moaven et al 2013). Interestingly, p44 rescued retinal degeneration and restored visual function to K296E mice (Moaven et al 2013).…”