OBJECTIVEImpairments in mitochondrial function have been proposed to play a role in the etiology of diabetic sensory neuropathy. We tested the hypothesis that mitochondrial dysfunction in axons of sensory neurons in type 1 diabetes is due to abnormal activity of the respiratory chain and an altered mitochondrial proteome.RESEARCH DESIGN AND METHODSProteomic analysis using stable isotope labeling with amino acids in cell culture (SILAC) determined expression of proteins in mitochondria from dorsal root ganglia (DRG) of control, 22-week-old streptozotocin (STZ)-diabetic rats, and diabetic rats treated with insulin. Rates of oxygen consumption and complex activities in mitochondria from DRG were measured. Fluorescence imaging of axons of cultured sensory neurons determined the effect of diabetes on mitochondrial polarization status, oxidative stress, and mitochondrial matrix-specific reactive oxygen species (ROS).RESULTSProteins associated with mitochondrial dysfunction, oxidative phosphorylation, ubiquinone biosynthesis, and the citric acid cycle were downregulated in diabetic samples. For example, cytochrome c oxidase subunit IV (COX IV; a complex IV protein) and NADH dehydrogenase Fe-S protein 3 (NDUFS3; a complex I protein) were reduced by 29 and 36% (P < 0.05), respectively, in diabetes and confirmed previous Western blot studies. Respiration and mitochondrial complex activity was significantly decreased by 15 to 32% compared with control. The axons of diabetic neurons exhibited oxidative stress and depolarized mitochondria, an aberrant adaption to oligomycin-induced mitochondrial membrane hyperpolarization, but reduced levels of intramitochondrial superoxide compared with control.CONCLUSIONSAbnormal mitochondrial function correlated with a downregulation of mitochondrial proteins, with components of the respiratory chain targeted in lumbar DRG in diabetes. The reduced activity of the respiratory chain was associated with diminished superoxide generation within the mitochondrial matrix and did not contribute to oxidative stress in axons of diabetic neurons. Alternative pathways involving polyol pathway activity appear to contribute to raised ROS in axons of diabetic neurons under high glucose concentration.

OBJECTIVE-Reactive oxygen species (ROS) are pro-oxidant factors in distal neurodegeneration in diabetes. We tested the hypothesis that sensory neurons exposed to type 1 diabetes would exhibit enhanced ROS and oxidative stress and determined whether this stress was associated with abnormal axon outgrowth.RESEARCH DESIGN AND METHODS-Lumbar dorsal root ganglia sensory neurons from normal or 3-to 5-month streptozotocin (STZ)-diabetic rats were cultured with 10 or 25-50 mmol/l glucose. Cell survival and axon outgrowth were assessed. ROS were analyzed using confocal microscopy. Immunofluorescent staining detected expression of manganese superoxide dismutase (MnSOD) and adducts of 4-hydroxy-2-nonenal (4-HNE), and MitoFluor Green dye detected mitochondria.RESULTS-Dorsal root ganglion neurons from normal rats exposed to 25-50 mmol/l glucose did not exhibit oxidative stress or cell death. Cultures from diabetic rats exhibited a twofold (P Ͻ 0.001) elevation of ROS in axons after 24 h in 25 mmol/l glucose compared with 10 mmol/l glucose or mannitol. Perikarya exhibited no change in ROS levels. Axonal outgrowth was reduced by approximately twofold (P Ͻ 0.001) in diabetic cultures compared with control, as was expression of MnSOD. The antioxidant N-acetyl-cysteine (1 mmol/l) lowered axonal ROS levels, normalized aberrant axonal structure, and prevented deficits in axonal outgrowth in diabetic neurons (P Ͻ 0.05).CONCLUSIONS-Dorsal root ganglia neurons with a history of diabetes expressed low MnSOD and high ROS in axons. Oxidative stress was initiated by high glucose concentration in neurons with an STZ-induced diabetic phenotype. Induction of ROS was associated with impaired axonal outgrowth and aberrant dystrophic structures that may precede or predispose the axon to degeneration and dissolution in human diabetic neuropathy.

OBJECTIVEImpairments in mitochondrial physiology may play a role in diabetic sensory neuropathy. We tested the hypothesis that mitochondrial dysfunction in sensory neurons is due to abnormal mitochondrial respiratory function.RESEARCH DESIGN AND METHODSRates of oxygen consumption were measured in mitochondria from dorsal root ganglia (DRG) of 12- to- 22-week streptozotocin (STZ)-induced diabetic rats, diabetic rats treated with insulin, and age-matched controls. Activities and expression of components of mitochondrial complexes and reactive oxygen species (ROS) were analyzed.RESULTSRates of coupled respiration with pyruvate + malate (P + M) and with ascorbate + TMPD (Asc + TMPD) in DRG were unchanged after 12 weeks of diabetes. By 22 weeks of diabetes, respiration with P + M was significantly decreased by 31–44% and with Asc + TMPD by 29–39% compared with control. Attenuated mitochondrial respiratory activity of STZ-diabetic rats was significantly improved by insulin that did not correct other indices of diabetes. Activities of mitochondrial complexes I and IV and the Krebs cycle enzyme, citrate synthase, were decreased in mitochondria from DRG of 22-week STZ-diabetic rats compared with control. ROS levels in perikarya of DRG neurons were not altered by diabetes, but ROS generation from mitochondria treated with antimycin A was diminished compared with control. Reduced mitochondrial respiratory function was associated with downregulation of expression of mitochondrial proteins.CONCLUSIONSMitochondrial dysfunction in sensory neurons from type 1 diabetic rats is associated with impaired rates of respiratory activity and occurs without a significant rise in perikaryal ROS.

Modification of proteins by 4-hydroxy-2-nonenal (4-HNE) has been proposed to cause neurotoxicity in a number of neurodegenerative diseases, including distal axonopathy in diabetic sensory neuropathy. We tested the hypothesis that exposure of cultured adult rat sensory neurons to 4-HNE would result in the formation of amino acid adducts on mitochondrial proteins and that this process would be associated with impaired mitochondrial function and axonal regeneration. In addition, we compared 4-HNE-induced axon pathology with that exhibited by neurons isolated from diabetic rats. Cultured adult rat dorsal root ganglion (DRG) sensory neurons were incubated with varying concentrations of 4-HNE. Cell survival, axonal morphology, and level of axon outgrowth were assessed. In addition, video microscopy of live cells, western blot, and immunofluorescent staining were utilized to detect protein adduct formation by 4-HNE and to localize actively respiring mitochondria. 4-HNE induced formation of protein adducts on cytoskeletal and mitochondrial proteins, and impaired axon regeneration by approximately 50% at 3 microM while having no effect on neuronal survival. 4-HNE initiated formation of aberrant axonal structures and caused the accumulation of mitochondria in these dystrophic structures. Neurons treated with 4-HNE exhibited a distal loss of active mitochondria. Finally, the distal axonopathy and the associated aberrant axonal structures generated by 4-HNE treatment mimicked axon pathology observed in DRG sensory neurons isolated from diabetic rats and replicated aspects of neurodegeneration observed in human diabetic sensory neuropathy.

Rabies virus infection of dorsal root ganglia (DRG) was studied in vitro with cultured adult mouse DRG neurons. Recent in vivo studies of transgenic mice that express the yellow fluorescent protein indicate that neuronal process degeneration, involving both dendrites and axons, occurs in mice infected with the challenge virus standard (CVS) strain of rabies virus by footpad inoculation. Because of the similarities of the morphological changes in experimental rabies and in diabetic neuropathy and other diseases, we hypothesize that neuronal process degeneration occurs as a result of oxidative stress. DRG neurons were cultured from adult ICR mice. Two days after plating, they were infected with CVS. Immunostaining was evaluated with CVS-and mock-infected cultures for neuron specific ␤-tubulin, rabies virus antigen, and amino acid adducts of 4-hydroxy-2-nonenal (4-HNE) (marker of lipid peroxidation and hence oxidative stress). Neuronal viability (by trypan blue exclusion), terminal deoxynucleotidyl transferase-mediated dUTP nick end-labeling (TUNEL) staining, and axonal growth were also assessed with the cultures. CVS infected 33 to 54% of cultured DRG neurons. Levels of neuronal viability and TUNEL staining were similar in CVS-and mock-infected DRG neurons. There were significantly more 4-HNE-labeled puncta at 2 and 3 days postinfection in CVS-infected cultures than in mock-infected cultures, and axonal outgrowth was reduced at these time points in CVS infection. Axonal swellings with 4-HNE-labeled puncta were also associated with aggregations of actively respiring mitochondria. We have found evidence that rabies virus infection in vitro causes axonal injury of DRG neurons through oxidative stress. Oxidative stress may be important in vivo in rabies and may explain previous observations of the degeneration of neuronal processes.Rabies is an acute viral infection of the central nervous system (CNS) that is usually fatal in humans and animals (10). Despite its lethality, under natural conditions only relatively mild histopathological lesions are typically found in association with a paucity of degenerative neuronal changes (9, 21). Li et al. (13) showed severe destruction and disorganization of axons and disruption of synaptic structures in silver-stained hippocampal sections from mice infected intracerebrally with the pathogenic N2C strain of rabies virus. Infection with the challenge virus standard (CVS) strain of rabies virus has recently been evaluated using hindlimb footpad inoculation in adult transgenic mice expressing yellow fluorescent protein (26). In this model, dendritic beading and axonal swellings were prominent, and neuronal process degeneration appeared to account for the severe clinical disease and fatal outcome. The axonal swellings exhibited striking morphological similarities to the neurodegenerative changes that occur in the axons of neurons in diabetic sensory and autonomic neuropathy and in human immunodeficiency virus (HIV) infection (4,12,24,35). Studies of cultured adult dorsal root ganglio...