Abstract
Purpose:
Although retinal ganglion cells (RGCs) are slowly lost as part of normal aging, accelerated loss occurs in both anterior ischemic optic neuropathy (AION) and glaucoma, diseases that primarily affect adults over age 50. While the events leading to the age-selective vulnerability and degeneration of RGCs after ischemic injury are not well understood, defects in axonal transport may play a role. This study investigates the effects of aging on the murine optic nerve and the impact of ischemia on axonal transport.
Methods:
C57BL/6 and Thy1-YFP mice (expressing yellow fluorescent protein in about 20 RGCs) at 3-, 7- and 12-months old were studied. Optic nerve head ischemia was induced by laser-assisted photochemical thrombosis. The posterior pole was imaged in vivo by confocal scanning laser ophthalmoscopy (cSLO, Spectralis) and in vitro by microscopy of retinal whole-mounts. To assess axonal transport, bilateral intravitreal injections of AlexaFluor488-conjugated cholera toxin B (A488-CTB) were given 2 hrs after inducing AION in one eye. Animals were dissected at 24 hrs for frozen serial coronal sections through the superior colliculus.
Results:
Within 30 min after AION, cSLO revealed more severe disruption of YFP protein expression in the RGCs of older Thy1-YFP mice compared to younger adult mice. This disruption in YFP expression worsened over several days, faster than the expected loss of signal due to RGC axonal degeneration, and was confirmed in vitro by microscopy. Using A488-CTB to study axonal transport, we found severe impairment of anterograde transport after AION, similar to the effect of colchicine, a known inhibitor of microtubule function and axonal transport. Following AION, there was near-complete loss of A488-CTB signal in the contralateral superior colliculus (P < 0.0001). In vivo fundus imaging 24 hrs after ischemia confirmed A488-CTB uptake by RGCs in both AION and control eyes by cSLO and the expected changes from acute AION by spectral-domain optical coherence tomography.
Conclusions:
Rapid disruption of YFP protein expression in retinal ganglion cell axons following optic nerve head ischemia revealed a selective vulnerability in older adult mice, possibly related to impaired axonal transport. Using intravitreally injected A488-labeled cholera toxin B, we demonstrated for the first time that anterograde axonal transport is severely impaired within 2 hours after experimental AION.
Keywords: 613 neuro-ophthalmology: optic nerve •
413 aging •
551 imaging/image analysis: non-clinical