Amyloid precursor protein protects against a rodent model of optic neuropathy and is abundant in the human vitreous

Abstract

Purpose: Age is a major risk factor for both Alzheimer’s disease (AD) and glaucoma. There is evidence of increased rates of glaucoma in AD patients. AD pathology includes extracellular amyloid beta (Aβ) plaques and Aβ has been found in glaucomatous retina. Aβ is produced as part of beta cleavage of the amyloid precursor protein (APP). However, the majority of APP cleavage is via an alpha pathway producing the neuroprotective molecule soluble APPα (sAPPα). RGCs express APP and sAPPα has been detected in bovine vitreous; presumably released by RGCs. Soluble APPα has demonstrated protection against a wide range of insults including the mitochondrial complex I inhibitor rotenone in vitro. Mitochondrial defects have recently been described in glaucoma; we hypothesized that sAPPα may protect RGCs against mitochondrial toxic injury in vivo.

Methods: An established mitochondrial toxin model of optic neuropathy was used in C57BL/6J mice (N=5-6 per group) by intravitreal delivery of rotenone (4µg). Recombinant human sAPPα (6ng) was delivered 30 minutes post-rotenone, also intravitreally. Retinal sections were obtained from whole eyes harvested 24 hours later. Analysis was by TUNEL assay and immunohistochemistry. To examine APP in human eyes an aged series of donor retina (N=83) and vitreous humour (N=41) was analysed for APP and sAPPα using western blotting. Soluble APPα was also measured in vitreous (N=47) collected after vitrectomy surgery in diabetic patients.

Results: We found that sAPPα treatment after rotenone injury reduced apoptosis in both the ganglion cell (p<0.05) and inner nuclear (p<0.001) layers. In the ageing human retina we found decreasing amounts of APP (p<0.05) with unchanged levels of sAPPα in the retina or vitreous. Vitreous from patients affected by diabetes show increased sAPPα compared to non-diabetic controls (p<0.05).

Conclusions: We have established that exogenous sAPPα is a potent neuroprotectant against rotenone toxicity in the rodent retina. Decreasing APP in ageing human retina along with unchanged sAPPα levels in the vitreous may suggest an upregulation of alpha cleavage in an attempt at neuroprotection. Similarly increased sAPPα in diabetic vitreous may indicate a protective action perhaps in response to increased hyperglycaemia-induced oxidative stress. This initial protective attempt may eventually become overwhelmed at which time RGC loss ensues.

Keywords: 413 aging • 615 neuroprotection • 688 retina