Role of Homocysteine in Retinal Neurodegeneration

Abstract

Purpose: Epidemiological studies have linked type 2 diabetes mellitus (T2DM) with an increased risk of Alzheimer's disease (AD). Histopathological, molecular, and biochemical abnormalities have commonalities in boh these major disease, which has lead to AD recently termed as "Type 3 Diabetes". Several neurodegenerative conditions that affect the brain have manifestations in the eye, and ocular symptoms often precede conventional diagnosis of such CNS disorders. High levels of Homocysteine (Hcy) in the plasma has been associated with both AD and T2DM. The aim of this study was to investigate whether retinal Hcy is associated with retinal neurodegeneration in animal models of AD (TASTPM transgenic mice) and T2DM (Goto-Kakizaki (GK) rats).

Methods: Eyes of 3, 12 and 18 months GK-rats and 2,4,8, and 12 months TASTPM mice and age-matched controls were fixed in 4% paraformaldehyde following their termination. The GK-rats eyes were dissected and the cornea, lens and ­vitreous were removed, whereas the eyes of TASTPM were directly embedded in paraffin. 4um sections were stained for homocysteine, amyloid beta (Aβ), Amyloid Precursor Protein (APP), and caspase-3, using immunofluorescence. Confocal microscopy images were analysed in a masked fashion for the mean intensity staining in the Retinal Ganglion Cells Layer.

Results: When compared with aged-matched control, increased hcy fluorescence was found in 12-month and 18-month GK-rats retinas (p>0.05 and p>0.005 respectively). Hcy was significantly increased in TASTPM mice at 4 months (p<0.05), preceding the cognitive impairment onset at 6 months. In both models, this was found to occur at smilar time points for increased levels of Aβ, APP, and cleaved caspase-3 expression, with TASTPM mice showing significant increases at 4 months in Aβ, APP (p<0.05), and cleaved caspase-3 ( p<0.005).

Conclusions: As far as we are aware, this is the first study that shows a role of homocysteine in retinal neurodegeneration in GK-rats and TASTPM mice. Hcy is increased with aging and the worsening of the diabetes in GK-rats, and in both models appears to be linked with RGC apoptosis and Aβ expression. This highlights the association of T2DM with AD, and provides further evidence that the GK-rat may be a good model of AD.