Crystallins in retinal ganglion cell survival and regeneration

Abstract

Crystallins are heterogeneous proteins classified into alpha, beta, and gamma families. Although crystallins were first identified as the major structural components of the ocular lens with a principal function to maintain lens transparency, further studies have demonstrated the expression of these proteins in a wide variety of tissues and cell types. Alpha crystallins (alpha A and alpha B) share significant homology with small heat shock proteins and have chaperone-like properties, including the ability to bind and prevent the precipitation of denatured proteins and to increase cellular resistance to stress-induced apoptosis. Stress-induced upregulation of crystallin expression is a commonly observed phenomenon and viewed as a cellular response mechanism against environmental and metabolic insults. However, several studies reported downregulation of crystallin gene expression in various models of glaucomatous nerodegeneration suggesting that that the decreased levels of crystallins may affect the survival properties of retinal ganglion cells (RGCs) and thus, be associated with their degeneration. This hypothesis was corroborated by increased survival of axotomized RGCs in retinas overexpressing alpha A or alpha B crystallins. In addition to RGC protective functions of alpha crystallins, beta and gamma crystallins were implicated in RGC axonal regeneration. These findings demonstrate the importance of crystallin genes in RGC survival and regeneration and further in-depth studies are necessary to better understand the mechanisms underlying the functions of these proteins in healthy RGCs as well as during glaucomatous neurodegeneration, which in turn could help in designing new therapeutic strategies to preserve or regenerate these cells.

Figure 1

In situ analysis of the crystallin expression in the retina. The expression of crystallins was primarily observed in the ganglion cell layer (GCL). Relatively weak staining can also be seen in the inner nuclear layer (INL) and, to a much lesser degree, in the outer nuclear layer (ONL). Crystallin-positive cells in the GCL were colocalized with RGCs retrogradely labeled with Fluorogold (FG). IPL, inner plexiform layer. Scale bars =20 μm.

Figure 2

Regulation of crystallin mRNA and protein expression in ocular hypertension rat model. mRNA levels of alpha and beta crystallin genes 2 (blue solid bars) and 5 (red solid bars) weeks after IOP elevation. mRNA levels of the six cristallin genes, expression of which was affected by IOP elevation as detected by gene profiling, was analyzed. Consistent with the gene profiling data, transcription of alpha A, alpha B, beta A1/A3, beta A2, beta A4 and beta B2 was transiently downregulated at 2 weeks but was similar to that of control at 5 weeks after IOP increase. Protein levels of alpha A, alpha B, beta A1/A3 and beta B2 were lower at 2 weeks (blue hatched bars) and at 5 weeks (red hatched bars) after IOP elevation compared to controls. Beta A2 and beta A4 crystallin proteins were undetected in the retinal extract, most likely due to their low expression level.