Diabetes causes mitochondrial dysfunction in sensory neurons that may contribute to peripheral neuropathy. Ciliary neurotrophic factor (CNTF) promotes sensory neuron survival and axon regeneration and prevents axonal dwindling, nerve conduction deficits and thermal hypoalgesia in diabetic rats. In this study, we tested the hypothesis that CNTF protects sensory neuron function during diabetes through normalization of impaired mitochondrial bioenergetics. In addition, we investigated whether the NF-κB signal transduction pathway was mobilized by CNTF. Neurite outgrowth of sensory neurons derived from streptozotocin (STZ)-induced diabetic rats was reduced compared to neurons from control rats and exposure to CNTF for 24 h enhanced neurite outgrowth. CNTF also activated NF-κB, as assessed by Western blotting for the NF-κB p50 subunit and reporter assays for NF-κB promoter activity. Conversely, blockade of NF-κB signaling using SN50 peptide inhibited CNTF-mediated neurite outgrowth. Studies in mice with STZ-induced diabetes demonstrated that systemic therapy with CNTF prevented functional and structural indices of peripheral neuropathy along with deficiencies in dorsal root ganglion (DRG) NF-κB p50 expression and DNA binding activity. DRG neurons derived from STZ-diabetic mice also exhibited deficiencies in maximal oxygen consumption rate and associated spare respiratory capacity that were corrected by exposure to CNTF for 24 h in an NF-κB-dependent manner. We propose that the ability of CNTF to enhance axon regeneration and protect peripheral nerve from structural and functional indices of diabetic peripheral neuropathy is associated with targeting of mitochondrial function, in part via NF-κB activation, and improvement of cellular bioenergetics.

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BackgroundA luminex-based screen of cytokine expression in dorsal root ganglia (DRG) and nerve of type 1 diabetic rodents revealed interleukin-1 (IL-1α) and IL-1β to be significantly depressed. We, therefore, tested the hypothesis that impaired IL-1α and IL-1β expression in DRG may contribute to aberrant axon regeneration and plasticity seen in diabetic sensory neuropathy. In addition, we determined if these cytokines could optimize mitochondrial bioenergetics since mitochondrial dysfunction is a key etiological factor in diabetic neuropathy.ResultsCytokines IL-1α and IL-1β were reduced 2-fold (p<0.05) in DRG and/or nerve of 2 and 5 month streptozotocin (STZ)-diabetic rats. IL-2 and IL-10 were unchanged. IL-1α and IL-1β induced similar 2 to 3-fold increases in neurite outgrowth in cultures derived from control or diabetic rats (p<0.05). STAT3 phosphorylation on Tyr705 or Ser727 was depressed in DRG from STZ-diabetic mice and treatment of cultures derived from STZ-diabetic rats with IL-1β for 30 min raised phosphorylation of STAT3 on Tyr705 and Ser727 by 1.5 to 2-fold (p<0.05). shRNA-based or AG490 inhibition of STAT3 activity or shRNA blockade of endogenous IL-1β expression completely blocked neurite outgrowth. Cultured neurons derived from STZ-diabetic mice were treated for 24 hr with IL-1β and maximal oxygen consumption rate and spare respiratory capacity, both key measures of bioenergetic fidelity that were depressed in diabetic compared with control neurons, were enhanced 2-fold. This effect was blocked by AG490.ConclusionsEndogenous synthesis of IL-1β is diminished in nerve tissue in type 1 diabetes and we propose this defect triggers reduced STAT3 signaling and mitochondrial function leading to sup-optimal axonal regeneration and plasticity.