Calnexin Is Essential for Rhodopsin Maturation, Ca2+ Regulation, and Photoreceptor Cell Survival

Abstract

In sensory neurons, successful maturation of signaling molecules and regulation of Ca2+ are essential for cell function and survival. Here, we demonstrate a multifunctional role for calnexin as both a molecular chaperone uniquely required for rhodopsin maturation and a regulator of Ca2+ that enters photoreceptor cells during light stimulation. Mutations in Drosophila calnexin lead to severe defects in rhodopsin (Rh1) expression, whereas other photoreceptor cell proteins are expressed normally. Mutations in calnexin also impair the ability of photoreceptor cells to control cytosolic Ca2+ levels following activation of the light-sensitive TRP channels. Finally, mutations in calnexin lead to retinal degeneration that is enhanced by light, suggesting that calnexin's function as a Ca2+ buffer is important for photoreceptor cell survival. Our results illustrate a critical role for calnexin in Rh1 maturation and Ca2+ regulation and provide genetic evidence that defects in calnexin lead to retinal degeneration.

Phototransduction and retinal degeneration in Drosophila

“…Another ER transmembrane molecular chaperon, calnexin, is required in photoreceptor cells specifically for rhodopsin maturation [70]. Given that other calnexins interact with monoglycosylated glycans present in folding intermediates of glycoproteins, it is plausible that the Drosophila calnexin facilitates the transport of Rh1 through the secretory pathway by binding to the N-linked oligosaccharide attached to asparagine 20 [70].…”

“…In the absence of NINAA, Rh1 accumulates in the ER and retinal degeneration ensues. Mutations in the gene encoding another rhodopsin chaperone, calnexin (cnx), result in a phenotype bearing some similarities to ninaA mutant flies [70]. Rh1 accumulates in the ER and there is retinal degeneration; however, in contrast to ninaA, the retinal degeneration in the cnx mutant is enhanced in the presence of light.…”

“…Rh1 accumulates in the ER and there is retinal degeneration; however, in contrast to ninaA, the retinal degeneration in the cnx mutant is enhanced in the presence of light. This latter aspect of the cnx phenotype most likely reflects an additional role of calnexin as a Ca 2+ buffer [70]. Rab6 also appears to be required for rhodopsin maturation, since expression of a dominant negative form of Rab6 causes defective rhodopsin maturation, and trafficking and triggers retinal degeneration [64].…”

“…The severity of the Trp P365 phenotype is reduced by overexpressing CalX [157], consistent with the conclusion that the retinal degeneration in Trp P365 is a consequence of Ca 2+ overload. Loss of calnexin (cnx) also results in retinal degeneration and this phenotype is due to increased Ca 2+ levels in addition to the defect in Rh1 maturation as described above, since Cnx serves to buffer Ca 2+ in the photoreceptor cell body [70]. The lightdependent retinal degeneration that results from elimination of the NINAC p174 isoform (myosin III) [194] may be due to higher free Ca 2+ levels, since elimination of p174 dramatically decreases the concentration of rhabdomeral CaM [168,169], which is normally ∼0.5 mM [168] and serves as the major Ca 2+ buffer in the photoreceptor cells.…”

An escort for GPCRs: implications for regulation of receptor density at the cell surface

“…Cycles of glucosylation and deglucosylation continue until the glycoprotein has either reached its native conformation or is targeted for degradation 3 . As expected from the GPCR glycosylation profile, multiple reports illustrate the interaction between GPCRs and carbohydrate-binding chaperones [6][7][8][9][10][11][12] .…”