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Published on: November 4, 2022
Effect of long-term chronic hyperhomocysteinemia on retinal structure and function in the cystathionine-β-synthase
Haiyan Xiao1, Jing Wang1, Shannon R Barwick1
1Department of Cellular Biology and Anatomy, Medical College of Georgia, Augusta University, Augusta, GA, USA; James and Jean Culver Vision Discovery Institute, Augusta University, Augusta, GA, USA.
Insights
Chronic, moderate hyperhomocysteinemia (Hhcy) in Cbs+/- mice did not cause significant retinal damage or vision loss compared to wild-type mice. Moderate Hhcy appears tolerated by the retina, suggesting protective cellular mechanisms are active.
Area of Science:
- Ophthalmology
- Neuroscience
- Biochemistry
Background:
- Elevated homocysteine (Hcy) is linked to human retinal diseases like glaucoma.
- The pathogenic role of moderate Hcy elevation remains unclear.
- Cystathionine β-synthase (CBS) deficient mice (Cbs+/-) model moderate hyperhomocysteinemia (Hhcy).
Purpose of the Study:
- To investigate the long-term effects of moderate Hhcy on retinal structure and function.
- To determine if chronic Hhcy leads to visual impairment or retinal pathology.
Main Methods:
- Comprehensive analysis of retinal function (IOP, visual acuity, ERG) and structure (SD-OCT, histology) in 20-month-old Cbs+/- and wild-type (WT) mice.
- Immunohistochemical analysis for ganglion cells (Brn3a), oxidative stress (dihydroethidium), and gliosis (GFAP).
Main Results:
- No significant differences in intraocular pressure (IOP), visual acuity, contrast sensitivity, or electroretinography (ERG) between Cbs+/- and WT mice.
- Histology and morphometric analysis showed no significant differences in retinal structure or ganglion cell count.
- Oxidative stress and gliosis were similar between 20-month-old Cbs+/- and WT mice, though increased in older mice generally.
Conclusions:
- Chronic, moderate Hhcy, at least from CBS deficiency, does not cause significant retinal structural or functional deficits in mice.
- Moderate Hhcy appears to be tolerated by the retina, possibly due to compensatory cellular survival mechanisms.
- Findings suggest severe Hhcy is toxic, but moderate levels may not be pathogenic in the retina.
Abstract:
Elevated levels of the excitatory amino acid homocysteine (Hcy) have been implicated in retinal diseases in humans including glaucoma and macular degeneration. It is not clear whether elevated Hcy levels are pathogenic. Models of hyperhomocysteinemia (Hhcy) have proven useful in addressing this including mice with deficiency in the enzyme cystathionine β-synthase (CBS). Cbs+/- mice have a ∼two-fold increase in plasma and retinal Hcy levels. Previous studies of visual function and structure in Cbs+/- mice during the first 10 months of life revealed mild ganglion cell loss, but minimal electrophysiological alterations. It is not clear whether extended, chronic exposure to moderate Hhcy elevation will lead to visual function loss and retinal pathology. The present study addressed this by performing comprehensive analyses of retinal function/structure in 20 month Cbs+/- and Cbs+/+ (WT) mice including IOP, SD-OCT, scotopic and photopic ERG, pattern ERG (pERG), and visual acuity. Eyes were harvested for histology and immunohistochemical analysis of Brn3a (ganglion cells), dihydroethidium (oxidative stress) and GFAP (gliosis). The analyses revealed no difference in IOP between groups for age/strain. Visual acuity measured ∼0.36c/d for mice at 20 months in Cbs+/- and WT mice; contrast sensitivity did not differ between groups at either age. Similarly SD-OCT, scotopic/photopic ERG and pERG revealed no differences between 20 month Cbs+/- and WT mice. There was minimal disruption in retinal structure when eyes were examined histologically. Morphometric analysis revealed no significant differences in retinal layers. Immunohistochemistry revealed ∼5 RGCs/100 μm retinal length in both Cbs+/- and WT mice at 20 months. While there was greater oxidative stress and gliosis in older (20 month) mice versus young (4 month) mice, there was no difference in these parameters between the 20 month Cbs+/- and WT mice. We conclude that chronic, moderate Hhcy (at least due to deficiency of Cbs) is not accompanied by retinal structural/functional changes that differ significantly from age-matched WT littermates. Despite considerable evidence that severe Hhcy is toxic to retina, moderate Hhcy appears tolerated by retina suggesting compensatory cellular survival mechanisms.
