Role of Acid Sphingomyelinase in Ocular Hypertension Retinal Ganglion Cell Degeneration

Abstract

Purpose : Acid sphingomyelinase (ASMase) is an important sensor for inflammatory cytokine and apoptotic signaling. It catalyzes the hydrolysis of sphingomyelin to ceramide. Previous studies have shown that inhibiting ASMase expression protects the retina from ischemic injury. The purpose of this study is to investigate if ASMase contributes to the sequela of events leading to retinal ganglion cell degeneration (RGC) in ocular hypertensive eyes.

Methods : Wild-type (WT) and ASMase^+/-mice (10-12 weeks) were utilized in these studies. Localization of ASMase and sphingomyelin were determined by immunohistochemistry and MLADI mass spectrometry imaging, respectively. Intraocular pressure (IOP) was elevated unilaterally for 28 days by injecting 1×10⁵ of 15 µm microbeads. IOPs were monitored by means of a TonoLab rebound tonometer. On day 28, RGC numbers were measured by counting Brn3a labeled retinal cells.

Results : In WT mice, ASMase is highly expressed in the optic nerve head and inner retinal layers. Little or no ASMase expression was observed in the outer retina. Sphingomyelin was concentrated in optic nerve head. Beads-injected eyes in WT and ASMase^+/- mice exhibited significant elevation in IOP (days 10 to 28) when compared to contralateral control eyes. No significant difference in IOPs between corresponding eyes from WT and ASMase mice were measured. In WT mice, RGC density in ocular hypertensive eyes (3198.0 ± 246.9 cells/mm²) was significantly lower than contralateral control eyes (4804.5 ± 165.4 cells/mm²). In ASMase^+/- mice, RGC density in hypertensive eyes (4076.2 ± 131.6 cells/mm²) was not significant different from contralateral eyes (4474.1 ± 86.2 cells/mm²). Comparing ipsilateral retinas from ocular hypertensive WT and ASMase^+/- mice demonstrated that RGC density was significantly higher in ASMase^+/- mice. No significant difference in RGC density was measured between ASMase^+/- and WT contralateral retinas.

Conclusions : Our results demonstrate that ASMase and its substrate, sphingomyelin, are collocated in the optic nerve head and inner retinal layers. The reduction in ASMase expression prevented the degeneration of RGC somas in ocular hypertension mice. These data support the idea that ASMase-mediated sphingolipid metabolism plays a role in the development of glaucomatous optic neuropathy.

This is an abstract that was submitted for the 2017 ARVO Annual Meeting, held in Baltimore, MD, May 7-11, 2017.