Abstract
Purpose::
Reduced levels of folate are associated with hyperhomocysteinemia. Recent clinical studies implicate elevated homocysteine (HCY), a sulfur-containing excitotoxic amino acid, in age-related maculopathy, retinal vein occlusion, and glaucoma. The present study addresses whether HCY induced retinopathy can be ameliorated by folate supplementaion.
Methods::
Mice heterozygous for a mutation of the cystathionine beta synthase gene (cbs +/-) develop a 4 fold increase in normal plasma HCY levels which can be increased to 7 fold by feeding a high methionine diet (HMD). Three groups of mice were studied: Hi-HCY (cbs +/- mice maintained on 0.5% methionine in drinking water), FS-HCY (cbs +/- mice maintained on HMD plus 0.01% folate in drinking water), and controls (cbs +/+ maintained on regular water). Treatment was started at 3 weeks of age. At 15 and 30 weeks post-onset of treatment, retinal cryosections were subjected to systematic mophometric analysis using the Zeiss Axio Vision program. The thickness of the retinal ganglion cell (RGC) layer, inner nuclear layer (INL), outer nuclear layer (ONL), and plexiform layers was determined as were the number of RGCs in retinal cross sections.
Results::
By 15 weeks Hi-HCY mice had significantly fewer ganglion cells compared to controls (9.7 cells/100 µm ± 1.23 versus 15.5 cells/100 µm ± 1.02, respectively). Supplementation with folate decreased the RGC loss to 13.9 cells/100 µm ± 0.57 in FS-HCY retinas. The thickness of the INL was similarly affected. By 30 weeks Hi-HCY mice showed a decrease in INL thickness of 17% while FS-HCY mice had only an 8% reduction compared to controls. Interestingly, both the Hi-HCY and FS-HCY mice showed similar decreases in the thickness of the inner and outer plexiform layers.
Conclusions::
Folate supplementation attenuated some but not all of the effects of elevated HCY on the retina. The mechanism of HCY induced retinopathy is probably multifactoral, not due to decreased folate levels alone.
Keywords: neuroprotection • retinal degenerations: cell biology • nutritional factors