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R. M. Yetemian, B. M. Brown, C. M. Craft; Deciphering the Role of Phosphorylation in Cone Photoreceptor Survival and Death. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4655.
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Phosphorylation by G-protein-coupled receptor kinase 1 (GRK1) is vital for the deactivation of photoactivated opsins and may contribute to the phosphorylation of enzymes imperative for cone photoreceptor homeostasis. We hypothesize that GRK1 phosphorylates and regulates the activity of the multifunctional metabolic enzyme, Phosphoglucose Isomerase (PGI), which also exhibits both cytokine and neuroleukin activity, contributing to tumor progression and neuronal differentiation, respectively. To identify the importance of phosphorylation in the mechanisms of cellular proliferation and apoptosis regulation in cones, the relationship between GRK1 and PGI will be deciphered.
Comparative protein expression via 2D gel electrophoresis and EttanTM DIGE overlay technology was performed using protein homogenates from pure cone retinas of neural retina leucine zipper (Nrl-/-) and Nrl-/-/Grk1-/- knockout mice. Mass spectrometry (MS) confirmed the identity of a differentially expressed protein. Phosphorylation assays and enzyme activity assays were conducted using retinal homogenates and recombinant proteins.
The 2D gel analyses revealed differential expression of 2 forms of PGI, both with a molecular weight of 63 kDA but with distinct isoelectric points (PI). An acidic form of PGI was expressed in the Nrl-/- sample, in contrast to the basic form in the Nrl-/-/Grk1-/- homogenate. Following identity confirmation of PGI by MS, the variance in PI was hypothesized to be different phosphorylation states of PGI. The role of GRK1 as a putative kinase involved in the regulation of PGI activity in cone photoreceptor homeostasis is the subject of our ongoing investigation.
We propose the Nrl-/-/Grk1-/- retina is an excellent model to study cone cell dystrophy due to an observed light independent, age-related degeneration of cones. Based on the posttranslational protein expression pattern observed, a lack of PGI phosphorylation in the absence of GRK1 in the mouse cone photoreceptor may be critical for the daily metabolic state of cones. The regulation of PGI or its cytokine-related activity may be essential for maintaining retinal homeostasis.
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