High Glucose Induces Mitochondrial Morphology and Metabolic Changes in Retinal Pericytes

Hyperglycemia-induced apoptosis has been implicated as the underlying cause of vascular and neuronal cell death in the diabetic retina.1–3 The death of endothelial cells and pericytes in the early stages of diabetic retinopathy leads to acellular capillaries and pericytes ghosts, respectively, which in turn accelerate the development of structural lesions characteristic of diabetic retinopathy.4–6 In addition, capillary degeneration as a result of vascular cell loss can lead to increased permeability and play a role in triggering the neovascularization associated with later stages of retinopathy. Thus, understanding the mechanism by which hyperglycemia or HG accelerates apoptosis of endothelial cells and pericytes in the diabetic retina is vital before a therapeutic intervention can be developed. Pericytes are smooth-muscle–like cells with contractile properties, and they provide structural integrity to the retinal microvasculature. Importantly, loss of retinal pericytes has been hypothesized to be the initial lesion to form during diabetes,7,8 which can influence vessel stability, endothelial proliferation, and angiogenesis contributing to the progression of diabetic retinopathy.9 Several mechanisms have been proposed to explain the accelerated apoptosis of retinal pericytes under HG or diabetic condition. Increased advanced glycation end products,10,11 polyol pathway activation,8,12 upregulation of protein kinase C and TNF-α,13–16 and oxidative stress17 have all been identified as mechanisms underlying the apoptosis of retinal pericytes associated with diabetic retinopathy. However, it remains unclear how HG triggers such detrimental changes in retinal pericytes, leading to apoptosis. Oxidative stress is known to increase in diabetic retinas and trigger proapoptotic actions of mitochondria, including the release of cytochrome c.18,19 In particular, oxidative stress is known to be a key regulator of retinal vascular cell injury during HG insult, and mitochondrial dysfunction has been shown to be an agonizing factor in reactive oxygen species (ROS) overproduction in an HG or diabetic condition.19 Although mitochondrial DNA mutations,20,21 Bax accumulation,19 and changes in the ROS scavenging enzymes22,23 are documented characteristics of mitochondrial dysfunction in retinal vascular cells, it is unclear whether mitochondrial morphology changes play a role in promoting mitochondrial dysfunction in diabetic retinopathy. In various cell types, including rat hepatocytes, myoblasts, ventricular myocytes, bovine aortic endothelial cells, and mouse smooth muscle cells, exposure to HG has been shown to induce mitochondrial fragmentation.24,25 Recently, we have shown that HG induces mitochondrial fragmentation and metabolic changes, leading to apoptosis in rat retinal endothelial cells.26 However, it is unknown whether mitochondrial morphology is affected by HG in retinal pericytes and whether HG affects mitochondrial oxygen consumption, an index for mitochondrial metabolic activity.24,25,27 The purpose of this study was to establish whether apoptosis in retinal pericytes involves changes in mitochondrial shape, membrane potential, oxygen consumption, and extracellular acidification and whether these changes are attributable to the effect of HG.