Protein Gq Modulates Termination of Phototransduction and Prevents Retinal Degeneration

Abstract

Background: Appropriate termination of photoresponse is critical for photoreceptors to achieve high temporal resolution and to prevent excessive Ca 2ϩ -induced cell toxicity. Results: We isolated a novel G␣ q mutant allele and revealed that metarhodopsin/G q interaction affects Arr2-Rh1 binding. Conclusion: G q modulates the termination of phototransduction and prevents retinal degeneration. Significance: Our study revealed the novel role of G q in phototransduction deactivation and in retinal degeneration.Appropriate termination of the phototransduction cascade is critical for photoreceptors to achieve high temporal resolution and to prevent excessive Ca 2؉ -induced cell toxicity. Using a genetic screen to identify defective photoresponse mutants in Drosophila, we isolated and identified a novel G␣ q mutant allele, which has defects in both activation and deactivation. We revealed that G q modulates the termination of the light response and that metarhodopsin/G q interaction affects subsequent arrestin-rhodopsin (Arr2-Rh1) binding, which mediates the deactivation of metarhodopsin. We further showed that the G␣ q mutant undergoes light-dependent retinal degeneration, which is due to the slow accumulation of stable Arr2-Rh1 complexes. Our study revealed the roles of G q in mediating photoresponse termination and in preventing retinal degeneration. This pathway may represent a general rapid feedback regulation of G protein-coupled receptor signaling.Heterotrimeric G proteins play pivotal roles in mediating extracellular signals from hormones, neurotransmitters, peptides, as well as sensory stimuli to intracellular signaling pathways (1, 2). In Drosophila photoreceptors, G proteins are essential for the activation of the phototransduction cascade (3, 4). Photon absorption leads to the photoisomerization of chromophores, resulting in the formation of activated metarhodopsin. In turn, metarhodopsin activates heterotrimeric G proteins and PLC.2 The activation of PLC leads to transient receptor potential and transient receptor potential-like channels opening and extracellular Ca 2ϩ influx (5-8). It is also critical for each step of the phototransduction cascade to be terminated appropriately, which is essential for the high temporal resolution of fly vision (9, 10). The most important step in phototransduction termination is the deactivation of metarhodopsin. During this step, arrestin (Arr2) plays an important role by displacing the G q ␣ subunit and allowing it to bind with rhodopsin (Rh1) (11,12). Unlike other G proteincoupled receptors (GPCRs), the phosphorylation of fly rhodopsin is not required for its deactivation (13) but is essential for its endocytosis (11). In contrast, the dephosphorylation of rhodopsin by retinal degeneration C (RDGC) is essential for receptor deactivation (14). Ca 2ϩ also plays critical roles in regulating the termination of the photoresponse in Drosophila (8,15,16). Several proteins that mediate this Ca 2ϩ -regulated termination have been identified, such as eye-specific protein kinase C...

Gαq splice variants mediate phototransduction, rhodopsin synthesis, and retinal integrity in Drosophila

“…Among the genes covered by the deficient chromosome Df(2R)Gαq1.3, the Gαq gene plays an essential role in Drosophila phototransduction (12,20). To test whether Gαq gene mutations contribute to the significantly reduced light responses in nlr mutants, we obtained a Gαq 221c mutant allele that removes a 359 bp fragment around the translation start site of all Gαq splice variants (21).…”

“…The Gαq gene encodes several Gαq splice variants, among which the Gαq-RD variant generates Gαq1 isoform protein, and other splice variants generate Gαq3 isoform protein (9). Although both strong alleles of norpA and trpl;trp double mutants show completely abolished photoresponses (4,10,11), the Gαq1 null mutant allele (Gαq 961 ) still displays a residual light response (12). These data indicate that other Gαq splice variants, or the Gq α subunits encoded by additional genes, contribute to the residual light responses in Gαq1 null mutants.…”

“…Rh1/Arr2 complex formation is thought to attribute to impaired Ca 2+ influx-activated CaM kinase Ⅱ, which usually phosphorylates Arr2 to release Arr2 from Rh1 (17,18). However, neither Gαq 1 nor Gαq 961 mutants undergo rapid retinal degeneration (12,19), exhibiting only slight retinal degeneration after keeping them in 12-h light/12-h dark cycles for 21 days (12). The disagreeing retinal degeneration phenotype between Gαq and norpA mutant is therefore unclear.…”

<b><i>Drosophila</i></b> as a Potential Model for Ocular Tumors

“…Activated oncogenes can be readily expressed ectopically; however, modeling the effects of the loss of human tumor suppressor genes is limited by the requirement for the presence and expression of the relevant homologs in the fly. Importantly, almost all of the genes implicated in uveal melanoma are conserved in Drosophila (table 1), including ubiquitous oncogenes or tumor suppressors, such as IGFR1, B-RAF and PTEN, which play conserved roles in growth and proliferation control in all metazoans, as well as highly conserved tissue-specific factors, such as G protein α q subunits GNAQ/GNA11 [27], which in Drosophila is required for the termination of phototransduction and prevents retinal degeneration [28]. The insult most closely associated with metastasis of uveal melanoma cells is somatic loss-of-function mutations in the BRCA1-associated protein 1 ( BAP1 ) tumor suppressor, which have been reported in nearly 50% of primary uveal melanomas and 84% of metastasizing tumors [29,30].…”

Sub-functionalization of dorsal and ventral eyes in a whirligig beetle (Coleoptera: Gyrinidae)

“…Inside the photoreceptor cells, there are special molecular pigments, better known as opsins, which allow for color vision in these animals. Opsins are G-protein coupled receptors used and adapted for light perception that under certain conditions have shown a large plasticity of responses to environmental change [14][15][16]. In some nocturnal insects adapted for dark vision, green photopigments (Rh1, Rh2, or LW-opsin) together with some coexpression of UV sensitive opsins (Rh4, Rh3, or UV-opsin) are uniformly expressed in photoreceptor cells along the length of the omatidia.…”