Regional and temporal patterns of retinal α-crystallins expression during secondary retinal ganglion cell degeneration.

Abstract

Purpose : Understanding how retinal ganglion cells (RGCs) respond to noxious stress in glaucomatous RGC degeneration would help guide to develop new treatments for glaucoma. We evaluated stress protein regulation in the retina with an experimental model that separates primary and secondary RGC degeneration. Methods : Unilateral partial optic nerve transection (pONT) was performed on the temporal side of optic nerve in adult Wistar rats; the contralateral eye was used as control. To quantify primary and secondary RGC degeneration, the topographical density of RGCs at 1 and 8 weeks after pONT was determined by immunohistochemistry with Rbpms antibody. Protein expression in 4 retinal quadrants (superior, temporal, inferior and nasal) collected at 2 weeks after pONT were compared to their contralateral controls with 8-plex iTRAQ quantitative proteomic analysis coupled with a QTOF mass spectrometer (n=4). Western blotting and immunohistochemistry followed (n=4). Results : The density of RGCs in the temporal quadrants was significantly decreased to 83.4% (n=8; P=0.026) at 1 week and 27.7% (n=12; P<0.0001) at 8 weeks after pONT. For the nasal quadrants, there was no RGC loss at 1 week but 56.4% (n=12; P=0.0001) RGCs were remained at 8 weeks, which was considered secondary degeneration. At 2 weeks after pONT, a total of 2364 proteins were identified and 285 proteins were found to be regulated for 4 pairs of samples. In nasal quadrants, 36 proteins were upregulated and 20 proteins were down-regulated (30% changes; P<0.05) while 9 out of those upregulated proteins were the members of the crystallin family. Increased expression of αA and αB crystallin in the nasal quadrant was confirmed with Western blotting compared to the other quadrants in the same eye and to controls. Consistently, increased immunoreactivity was detected in the nasal retina compared to the temporal retina by immunohistochemistry. There was a mild increased αA crystallin expression in all retinal layers whereas αB crystallin expression was predominantly increased in the RGC layer cells 2 weeks after pONT. Both αA and αB crystallin expression were remarkably diminished at 8 weeks after pONT. Conclusions : There were temporal and regional differences of αA and αB crystallin expression in the retina after pONT suggesting a self-defense mechanism involved in the retina during the progression of RGC degeneration.