Sphingomyelinase Activity and Expression in Optic Nerve and Retina: Effects of Ocular Hypertensive and Ischemia

Abstract

Purpose :
Sphingomyelinases (SMase) are important sensors for inflammatory cytokine and apoptotic signaling, catalyzing hydrolysis of sphingomyelin to ceramide. Studies have shown that increased SMase activity can lead to retinal injury. In most tissues, two SMase are responsible for stress-induced increases in ceramide: acid sphingomyelinase (ASMase) and Mg²⁺-dependent neutral sphingomyelinase (NSMase). The purpose of this study was to investigate how ocular hypertensive and ischemia alter ASMase and NSMase activity and expression.

Methods : Brown Norway rats were utilized in these studies. To assess if ischemic and ocular-hypertensive stress alter ASMase and NSMase activity, retinas and optic nerves were isolated 90 minutes following retinal ischemia, or 7 days following the elevation in IOP. Ocular ischemia was induced by elevating IOP to 160 mm Hg for 45 minutes. Chronic ocular hypertension (4 - 8 mm Hg) was induced injecting 1×10⁵ of 15 µm microbeads into the anterior chamber. SMase activity was determined using Amplex Red SMase assay kit. Tissue expression of SMase enzymes were determined by immunohistochemistry.

Results : The expression of ASMase was observed in the optic nerve and ganglion cell layer. The expression of NSMase was observed in the optic nerve and photoreceptor and ganglion cell layers. Basal activities for ASMase in the retina and optic nerve were 56.7±1.9 and 108±17 mU/mg protein, respectively. Basal activities for NSMase in the retina and optic nerve were 12.3±2.1 and 37.9±8.7 mU/mg protein, respectively. Ocular hypertension significantly increased ASMase activity in the retina (117%) and optic nerve (149%). Ocular ischemia significantly increased ASMase activity in the retina (117%), but not in the optic nerve. No significant changes in NSMase activity were measured following ocular ischemia or hypertension.

Conclusions : Our results provide evidence that ASMase and NSMase are expressed in the retina and optic nerve, basal ASMase activity is significantly higher than NSMase activity, and ASMase activity is significantly increased by ocular ischemia or hypertension. These results support the idea that ASMase is the primary enzyme responsible for sphingomyelin metabolism in the optic nerve and retina under normal conditions, and following ocular ischemic and hypertensive stress.

This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.