Abstract
Purpose: :
Astrocyte reactivity is a common feature of both acute and chronic neurodegenerative conditions. In glaucoma, astrocyte reactivity and hypertrophy is associated generally with degeneration of the retinal ganglion cell (RGC) projection along the optic nerve to central targets in the brain. Here we compared the progression of astrocyte hypertrophy with that of RGC axon loss in the optic nerve and synapse loss in the superior colliculus (SC), the primary target for RGCs in the rodent brain.
Methods: :
We used the DBA/2J mouse model of hereditary glaucoma, selecting mice at various stages of progression between 1 and 13 months of age. Following aldehyde perfusion, astrocyte hypertrophy and axon survival were quantified in cross-sections of optic nerve using light and electron microscopy coupled with immune-labeling against GFAP (astrocytes) and phosphorylated neurofilaments (axons). Astrocyte hypertrophy and RGC synapse survival in the SC were compared using immuno-labeling for GFAP and estrogen-related receptor beta (ERRβ) or vesicular glutamate transporter 2 (VGluT2) to assess synapses. Both were measured against active axonal transport of cholera toxin beta (CTB) subunit from the retina.
Results: :
Prior to overt degeneration of the DBA/2J optic nerve, the mean cross-sectional area of RGC axons enlarged 50-70% with increased axoplasmic disorder and accumulation of phosphorylated neurofilaments. Concurrently, astrocyte ramification between axon fascicles diminished substantially so that overall axon density was conserved. In the SC, astrocyte hypertrophy in the superficial RGC recipient zone increased with diminished CTB transport from the retina and accumulation of phosphorylated neurofilaments but prior to loss of ERRβ- and VGluT2-labeled RGC synapses. While expression of GFAP mRNA increased with transport loss in the SC, even young DBA/2J had 5-fold greater levels than C57 counterparts.
Conclusions: :
Axonopathy is an early feature of pathogenesis in the DBA/2J and other animal models of glaucoma. In the optic nerve, diminished astrocyte ramification with axonal enlargement prior to degeneration could serve to maintain local metabolic balance as axon demand increases. In the SC, as axonal transport fails, increased astrocyte hypertrophy prior to loss of RGC synapses similarly may reflect a local mechanism to compensate for loss of retina-derived signaling.
Keywords: astrocyte • optic nerve • synapse