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Ken Lindsay, Jonathon Linton, James Hurley; Unique expression and regulation of glycolytic enzyme PKM2 in Photoreceptor cells and the role of enzymatic activity modulating metabolism of the retina. Invest. Ophthalmol. Vis. Sci. 2013;54(15):692.
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The photoreceptor cell is one of the most energy demanding cells in the body. While being a post-mitotic cell, the daily turnover of Rod Outer Segment Discs imposes a need for protein and lipid synthesis that rivals that of a rapidly dividing cell. This intense metabolic demand suggests that the expression of glycolytic enzymes associated with increases in anabolism, typically associated with tumors and other rapidly dividing cells may be expressed in the retina and be regulated to fuel these processes. Initial observations demonstrated that the retina expresses a form of pyruvate kinase, PKM2, previously thought to be primarily associated with cancer cells. The purpose of this study was to analyze the localization and possible regulation of PKM2 in retinas.
We analyzed the distribution of PKM2, by enzymatic activity assays and immunoblotting of serial sections of light and dark adapted rat retinas. Western Blot analyses of serial sections used Rhodopsin, Recoverin and Synaptotagmin as landmarks for orientation of enzymes throughout the retina. An antibody specific to PKM2 was used to localize this isoform in the retina.
Serial Sectioning revealed that PKM2 is present only in the photoreceptor layer of the retina. While total expression of PKM2 is insensitive to light, the fraction of enzymatic activity in the photoreceptor layer increased significantly in Dark Adapted retinas (n = 7). Immunoblot analyses showed that phosphorylation of Y105 on PKM2, typically associated with decreased PKM2 enzyme activity, occurs independently of light.
Our findings show that the PKM2 isoform of pyruvate kinase is present specifically in the photoreceptor layer of rat retinas. We also found that the fraction of pyruvate kinase activity that is in the outer retina is diminished by light adaptation. Additional studies will address how PKM2 helps photoreceptors to meet their unusual metabolic demands.
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