Metabolic Dysfunction Is Restricted to the Sciatic Nerve in Experimental Diabetic Neuropathy

Abstract

High glucose levels in the peripheral nervous system (PNS) have been implicated in the pathogenesis of diabetic neuropathy (DN). However, our understanding of the molecular mechanisms that cause the marked distal pathology is incomplete. We performed a comprehensive, system-wide analysis of the PNS of a rodent model of DN. We integrated proteomics and metabolomics from the sciatic nerve (SN), the lumbar 4/5 dorsal root ganglia (DRG), and the trigeminal ganglia (TG) of streptozotocin-diabetic and healthy control rats. Even though all tissues showed a dramatic increase in glucose and polyol pathway intermediates in diabetes, a striking upregulation of mitochondrial oxidative phosphorylation and perturbation of lipid metabolism was found in the distal SN that was not present in the corresponding cell bodies of the DRG or the cranial TG. This finding suggests that the most severe molecular consequences of diabetes in the nervous system present in the SN, the region most affected by neuropathy. Such spatial metabolic dysfunction suggests a failure of energy homeostasis and/or oxidative stress, specifically in the distal axon/Schwann cell-rich SN. These data provide a detailed molecular description of the distinct compartmental effects of diabetes on the PNS that could underlie the distal-proximal distribution of pathology.Diabetic neuropathy (DN) will develop in ;30-50% of patients with diabetes. DN typically presents with sensory symptoms in a distal glove-and-stocking distribution (1,2). It is a poorly understood complication of diabetes, and currently, few treatments are available (3). Raised glucose has long been believed to instigate pathology in DN either through direct neurotoxicity or from the activation of secondary pathways (4,5). However, exactly how these pathways cause nerve conduction velocity deficits, neuropathic pain, distal axonopathy, and numbness in the extremities continues to elude us, and many clinical trials aimed at specific targets have failed due to lack of efficacy (3).Crossover exists between proposed pathogenic pathways in DN, but how these interact is unclear. This question can be approached by implementing extensive -omic technologies to measure many transcripts, proteins, or metabolites in parallel. Gene microarrays were among the first of these technologies to be used, and transcriptomic analyses have been performed on tissues such as the dorsal root ganglia (DRG) from streptozotocin (STZ)-diabetic rats compared with healthy controls (6), the sciatic nerve (SN) of db/db mice compared with those from db/+ mice (7), and sural nerve biopsy specimens from human patients whose neuropathy progressed over 1 year compared with those whose neuropathy did not (based on myelinated fiber density loss) (8). Common changes across these gene array studies highlight altered carbohydrate and lipid metabolism.Because gene transcript levels do not always correlate with protein expression due to varying transcriptional/translational

Insulin prevents aberrant mitochondrial phenotype in sensory neurons of type 1 diabetic rats

“…It can be inferred from all these studies that mitochondrial OXPHOS protein expression, especially within Complex IV and its activity, is depressed under diabetic conditions. Finally, to illustrate the complexity of interactions in diabetes in the nervous system proteomic studies of distal sciatic nerve of type 1 diabetic rats reveal elevations in specific components of the mitochondrial electron transport chain (Freeman, et al, 2016). …”

“…Unmyelinated axons are more energetically demanding than myelinated axons, consuming 2.5–10-fold more energy per action potential (Wang et al, 2008). There is mounting evidence that diabetes suppresses mitochondrial function in dorsal root ganglia (DRG) (Chowdhury et al, 2010; Freeman et al, 2016; Ma et al, 2014; Roy Chowdhury et al, 2012; Sas et al, 2016; Urban et al, 2012). We have previously proposed that hyperglycemia-induced down-regulation of the AMP-activated protein kinase (AMPK)/peroxisome proliferator-activated receptor γ co-activator 1-α (PGC-1α) signaling axis can result in axon degeneration and failure to regenerate (Calcutt et al, 2017; Chowdhury, et al, 2013; Fernyhough, 2015; Roy Chowdhury, et al, 2012).…”

Schwann cell interactions with axons and microvessels in diabetic neuropathy

“…Hyperglycaemia has also been shown to drive remodelling of the Schwann cell mitochondrial proteome that leads to increased expression of the α and β subunits of ATP synthase, and to cause suboptimal respiratory capacity by increasing the overall rate of oxygen consumption, suggesting that high glucose levels contribute to mitochondrial dysfunction and decrease the efficiency of oxidative phosphorylation in Schwann cells 84 . In a study published in 2016, mitochondrial damage, with upregulation of multiple subunits of complexes I, III, IV, V and of mitochondrial Rho GTPase 1, was described in a rodent model of type 1 diabetes 85 . These changes were observed only in peripheral nerves and not in the sensory or trigeminal ganglia, suggesting that they reflect disrupted Schwann cell metabolism.…”

“…Furthermore, evidence from a mouse model of peripheral neuropathy secondary to Schwann cell mitochondrial dysfunction suggests that disruption of Schwann cell mitochondria causes lipid metabolism to shift away from fatty acid synthesis toward lipid oxidation, resulting in early depletion of myelin lipid components and accumulation of acylcarnitine lipid intermediates, leading to axonal degeneration and neuro pathy 83 . Perturbation of lipid metabolism has also been found in the peripheral nerves of mice with streptozotocin (STZ)-induced diabetes; these perturbations include a reduction in short-chain triacylglycerols, changes in major structural and/or membrane lipids, and diminished levels of palmitic, stearic and eicosanoic fatty acids 85,90 . Exposure of human Schwann cells to high extracellular glucose levels reduced their synthesis of phospholipids, and this effect was counteracted by an ARI, implicating high rates of glucose metabolism via the polyol pathway in dysregulation of Schwann cell lipid metabolism 91 .…”

Metabolomics in diabetic complications

“…Aconitase activity in the dorsal root ganglia of diabetic mice was downregulated to a lesser extent. These findings were corroborated by Freeman, et al , who described a neuronal proximal-to-distal increase in oxidative phosphorylation, polyol pathway upregulation, and mitochondrial dysfunction in three types of nervous tissue in diabetic rats using GC- and LC-MS. 91 However, a recent investigation of a triple antioxidant therapy regimen on cardiovascular autonomic neuropathy showed no benefit on disease progression and other therapeutic avenues should be explored. 11 …”