The Role Of Acid Sphingomyelinase In Shifting The Balance Between Pro-inflammatory And Reparative Bone Marrow Cells In Diabetic Retinopathy

Abstract

Purpose : Diabetes-induced low-grade chronic inflammation, retinal endothelial cell damage and inadequate vascular repair are partly due to the increased activation of bone marrow (BM)-derived inflammatory monocytes infiltrating the retina, and compromised function of BM-derived reparative circulating angiogenic cells (CACs). We now propose that a metabolic link leading to activated monocytes and dysfunctional CACs in diabetes involves upregulation of a central enzyme of sphingolipid signaling, acid sphingomyelinase (ASM).

Methods : C57BL/6.ASM^-/-Xgfp⁺ bone marrow chimeric mice were made diabetic with using streptozotocin injections. Inflammatory and cell-specific markers were measured by RT-PCR and immunohistochemistry. Retinal vascular permeability was measured by FITC-albumin. Acellular capillaries were measured by elastase digest protocol. ASM was inhibited in gfp⁺ CACs by siRNA, intravireally injected into WT mice and their co-localization with retinal vasculature was observed by confocal microscopy. Gfp⁺ CACs in BM were counted by flow cytometry to measure CAC homing efficiency.

Results : Selective inhibition of ASM in the BM prevented diabetes-induced activation of BM-derived microglia-like cells and normalized pro-inflammatory cytokine levels in the retina. Selective inhibition of ASM in BM also improved retinal vascular pathology as measured by vascular permeability and formation of acellular capillary in the diabetic retina. Inhibition of ASM in diabetic CACs improved homing of these vascular reparative cells to damaged retinal vessels. Moreover, diabetic CACs injected into the vitreous lose the ability to migrate back to their BM niche, whereas inhibition of ASM improves their in vivo homing efficiency.

Conclusions : Collectively, these findings indicate that BM pathology in diabetes could play a role in both increased pro-inflammatory state and inadequate vascular repair contributing to diabetic retinopathy. Further, selective modulation of sphingolipid metabolism in BM-derived cell populations in diabetes normalizes this reparative/pro-inflammatory cell balance, and can be explored as a novel therapeutic strategy for treating diabetic retinopathy.

This is an abstract that was submitted for the 2016 ARVO Annual Meeting, held in Seattle, Wash., May 1-5, 2016.