Sphingomyelinase: A novel therapeutic target for retinal ischemia.
Retinal ischemia is a common cause of visual impairment and blindness and linked to multiple retinal degenerative diseases including glaucoma, anterior ischemic neuropathy, retinal and choroidal vessel occlusion, diabetic retinopathy and traumatic optic neuropathy. However, due to the absence of effective therapies for retinal ischemia these diseases remain the most common causes of and blindness in the industrialized world. Therefore, it is important to understand the underlying events that lead to ischemic retinal degeneration and to develop new therapeutic agents that target these processes. Sphingolipid-signaling regulates various biological processes, including vascular angiogenesis, cell differentiation, apoptosis, chemotaxis, inflammation and neuronal degeneration. In the eye, sphingolipid-signaling plays important roles in the homeostasis of photoreceptors and retinal pigment epithelium, and modulation of conventional outflow resistance. However, the roles of sphingolipids in retinal degeneration associated with ischemic insult are unknown. The long-term goals of this project are to determine if sphingolipid-signaling contributes to the detrimental effects of retinal ischemia, and to identify strategies that prevent or attenuate the injurious effects of retinal ischemia.
Sphingomyelinase (SMase) is one of the important enzymes for regulation the endogenous levels and turnover of the sphingolipid molecules. It catalyzes the hydrolysis of sphingomyelin to the proinflammatory/proapoptotic second messenger ceramide. The activation of SMase is an early response to various cellular stresses and precedes ceramide production, which accelerates the signaling pathways involved in cell death. The hypothesis for this project is: Increasing SMase activity and expression plays a central role in the events responsible for retinal ischemia injury.
To investigate this hypothesis we propose two specific aims. 1) Investigate the potential of modulating sphingomylin-ceramide hydrolysis as strategy to prevent ischemia induced retinal degeneration. This aim will employ the acid SMase, neutral SMase knockout mice and pharmaceutical approaches to investigate if suppressing of SMase activity can protect the retina from ischemic injury. Retinal function and morphology will be assessed by electroretinograms and morphometric analyses. Time-dependent changes in expression, activity of SMase will be identified by Western and activity assay. Changes in individual sphingolipid species that mediate these stress-signaling events will be identified by liquid chromatography-mass spectrometry at the Lipidomics Shared Resource at MUSC. 2) Determine the cellular and molecular consequences of the SMase activation under ischemia-related stress in vitro. This aim will utilize primary human optic nerve head astrocytes culture to elucidate the role of SMase activation in regulating inflammatory cytokines and their downstream signaling responses under the ischemia-related stress (serum, oxygen and glucose deprivation). SMase inhibitors and RNA interference will be administrated to the cells to evaluate the involvement of specific SMase subtypes in the pathophysiology of ischemic retinal injury.
Collectively, studies in this application will begin to elucidate the roles for SMase and their metabolic products in retinal ischemia and provide a rational basis for the development of SMase inhibitors as neuroprotective agents for the treatment of ischemic retinal disorders. A central feature of this project is the collaboration with Lipidomics Shared Resource at MUSC to determine the roles of sphingolipids following ischemic retinal injury. We expect that the collection of new data from this application will lead to two original publications and submission of a new R01 application by February of 2015.