Intrinsic axonal degeneration pathways are critical for glaucomatous damage

Abstract

Glaucoma is a neurodegenerative disease affecting 70million people worldwide. For some time, analysis of human glaucoma and animal models suggested that RGC axonal injury in the optic nerve head (where RGC axons exit the eye) is an important early event in glaucomatous neurodegeneration. During the last decade advances in molecular biology and genome manipulation have allowed this hypothesis to be tested more critically, at least in animal models. Data indicate that RGC axon degeneration precedes soma death. Preventing soma death using mouse models that are mutant for BAX, a proapoptotic gene, is not sufficient to prevent the degeneration of RGC axons. This indicates that different degeneration processes occur in different compartments of the RGC during glaucoma. Furthermore, the Wallerian degeneration slow allele (Wld(s)) slows or prevents RGC axon degeneration in rodent models of glaucoma. These experiments and many others, now strongly support the hypothesis that axon degeneration is a critical pathological event in glaucomatous neurodegeneration. However, the events that lead from a glaucomatous insult (e.g. elevated intraocular pressure) to axon damage in glaucoma are not well defined. For developing new therapies, it will be necessary to clearly define and order the molecular events that lead from glaucomatous insults to axon degeneration.

Keywords: Astrocyte; Axon; BAX; Glaucoma; Microglia; Optic nerve; Optic nerve head; Retinal ganglion cell; Wld(s).

Figure 1. Wallerian degeneration slow (Wld^s) allele protects or delays axon degeneration in two models of glaucoma

(A–F) In DBA/2J mice, the Wld^s allele protected both RGC axons (A–C) and soma (D–F) at two key time points (Libby et al 2005a). A and B show representative images of optic nerve cross sections stained with paraphenylene diamine from a wild type DBA/2J eye with severe glaucoma (A) and a DBA/2J.Wld^s eye with no glaucoma (B). (C) Assessment of over 100 eyes showed that Wld^s more than doubled the number of eyes with no glaucoma (green, hashed bars) compared to wild type DBA/2J mice. (D–F) Wld^s also protected RGC somata. In eyes with no optic nerve damage, RGC somata were also preserved. (G–O) In a model where IOP elevation was induced experimentally in rats, Wld^s delayed axon degeneration (G–H) but had no effect on somal survival (J–O). (G–H) At 2 weeks following IOP elevation, proximal axons form Wld^s rats (H) had significantly less axon damage compared to wild type rats (G). Interestingly, in the wild type rats, greater axon damage was observed in proximal axons (I, squares) compared to distal axons (I, triangles) two weeks after IOP elevation. This data suggests dying back is not the major mechanism of axon degeneration in this model. (J–O) Wld^s does not protect the soma. Higher magnification confocal stacks showing loss of dendritic arborization and cell body shrinkage in glaucomatous RGCs from both wild-type (K, M) and Wld^s rats (L, N) as compared to control RGCs from untreated retinas (J). The majority of control RGCs showed more than one immunopositive process (arrow in J depicts example RGC with six processes) while, in particular, 4 weeks of glaucoma triggered massive loss of arborization that resulted in RGCs with only one (K and L, arrows) or no processes (M and N, arrows). A–F reproduced from Howell et. al., 2007 (J. Cell Biology). G–O reproduced from Beirowski et. al., 2008 (Eur. J. Neuro.). Permission requested.

Figure 2. Dying back as a mechanism for axon degeneration in glaucoma

(A–C) Schlamp et al used the diI labeling technique in DBA/2J mice to show that axon degeneration followed a die-back pattern from the distal end of the optic nerve to the proximal end. The images shown are dorsal views of a healthy young mouse (A), and two old mice with degenerating nerves that typically exhibit reduced label distally (in relation to cell body) (B, C). Ch = Chiasm, RON = right optic nerve, LON = left optic nerve. (D–F) Crish et al stained optic nerve cross sections stained with Toluidine blue to identify and quantify the number of degenerating axons (or profiles, arrowed). Sections were assessed from the proximal (D) and the distal (E) portions of the optic nerve. (F) A greater number of degenerating profiles were observed in distal optic nerve sections compared to proximal optic nerve sections. This supports dying back as the major mechanism of axon degeneration in DBA/2J glaucoma. A–C reproduced from Schlamp et. al., 2006 (BMC Neuro). D–F reproduced from Crish et. al., 2010 (PNAS).