Biology of GDNF in Diabetic Neuropathy

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Diabetic neuropathy (DN) is a serious neural complication that develops in many diabetic patients. Small unmyelinated sensory fibers are commonly affected, leading to abnormal cutaneous sensation and pain. It is believed that interrelated mechanisms contribute to DN and insufficient neurotrophic support has recently been added to the list of possible deficits. Nociceptive neurons affected in small-fiber DN respond either to nerve growth factor (NGF) or glial cell line-derived neurotrophic factor (GDNF). Whereas studies have noted deficits in NGF support to sensory neurons in DN, little is known about the role of GDNF. The long- term goal of this project is in an animal model of diabetes, examine the biology of GDNF-related ligands and receptors and correlate deficits with physiological and behavioral deficits that are caused by diabetes. Our purpose is to uncover mechanisms underlying the development of DN, thus providing information that will aid in developing novel treatments for DN. Our previous studies demonstrate that the central processes of GDNF-responsive neurons are sensitive to diabetes and GDNF administration can reverse deficits in GDNF-responsive spinal terminals. This proposal will test the hypotheses that GDNF support to primary sensory neurons is impaired in diabetes and anatomical/physiological deficits in GDNF-responsive neurons can lead to impaired responses to cutaneous stimuli. The first specific aim will characterize deficits in GDNF/GDNF-receptor synthesis and transport in STZ-induced diabetic mice, and test whether GDNF treatment can improve ligand/receptor abnormalities. The second aim will characterize deficits in GDNF-responsive fibers in the spinal cord and skin, and then test the ability of GDNF to stimulate sensory axon growth and reinnervation. The final aim will characterize deficits in GDNF-responsive neurons by performing electrophysiological recordings of single, identified neurons in an in vitro skin-nerve preparation. Physiological deficits will be correlated with abnormalities in the response of diabetic mice to noxious mechanical, chemical and thermal stimuli. The capacity of GDNF to modify neuronal physiology and behavioral responses to cutaneous stimuli will also be tested. In sum, this grant proposes to use molecular, anatomical, physiological and behavioral approaches to understand the biology of GDNF in DN and to test whether GDNF has therapeutic actions on cutaneous neurons affected in diabetic animals. Results from this study will 1) provide evidence that impaired GDNF support contributes to the development of DN and 2) establish GDNF as a candidate to be used in treatments to improve cutaneous function in DN.