Cholesterol homeostasis and retinal stress in anterior ischemic optic neuropathy

Purpose: Anterior ischemic optic neuropathy (AION) is characterized by acute stress of retinal ganglion cells (RGC). Cholesterol-24S-hydroxylase (CYP46A1) is specifically expressed in RGC where it may exert neuroprotective properties by preventing from cholesterol accumulation via catalyzing the formation of 24S-hydroxycholesterol. Previous findings hypothesize the housekeeping importance of CYP46A1 in the maintenance of cholesterol in the retina. In order to better understand the role of CYP46A1 in the retina, we aimed to define whether CYP46A1 expression and 24S-hydroxycholesterol levels were modified during RGC stress in AION. Methods: Ninety nine male Sprague Dawley rats were randomized in 2 experimental groups: AION or Optic Nerve Ischemia (ONI) (n=45 in each group), and 1 control group of animals (n=9). AION was induced in one eye of anesthetized animals by illumination of the optic nerve head using 532nm laser light (500 µm, 50mW, 12x 1-second pulses) after intravenous injection of Rose Bengal (2.5mM, 1mL/kg). The same procedure, without Rose Bengal injection, was used to induce ONI. The rats of the experimental groups were evaluated at day 3, 14, 30, 60 and 90 post-laser. Scotopic electroretinography was recorded. Glial activation was assessed by means of GFAP expression using western blot analysis, as well as CYP46A1 expression. Cytokines and chemokines were analyzed by flow cytometry in plasma and vitreous. Cholesterol and 24S-hydroxycholesterol were quantified in plasma and retina using gas chromatography. Plasma 24S-hydroxycholesterol was analyzed in 41 AION patients collected during acute phases of AION, until >60 days after ischemia. Results: A trend towards a decrease of the electroretinographic response (a- and b-wave amplitude) was observed in AION rats at day 30. MCP-1 concentrations were significantly increased at day 3 in the vitreous of the eye submitted to AION (132±31 vs 44±13pg/mg protein, p=0.046) and plasma of AION rats (145±16 vs 93±10pg/mg protein, p=0.05), whereas no difference was found in INO groups. Despite stress induced by AION and expected loss of RGC content, CYP46A1 and 24S-hydroxycholesterol levels were maintained at a constant level. Plasma 24S-hydroxycholesterol tended to be increased in AION patients between days 2 and 7 of AION (p>0.05). Conclusions: Despite RGC stress triggered by AION, CYP46A1 and 24S-hydroxycholesterol were maintained at constant levels in the retina, highlighting the resilience of CYP46A1 in the retina and its crucial importance in the maintenance of cholesterol homeostasis therein.