Glucagon secretion and islet neuropathy

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DESCRIPTION (provided by applicant): The glucagon response to insulin-induced hypoglycemia (IIH) is markedly impaired early in type 1 diabetes. We have previously demonstrated that this glucagon response is largely autonomically mediated in nondiabetic animals and humans. More recently we discovered an early sympathetic islet neuropathy (eSIN) in human subjects with type 1 diabetes and animal models thereof and therefore hypothesized that eSIN contributes to this glucagon impairment. The purpose of this grant is to determine the mechanism by which 1) only sympathetic nerves are lost, 2) only from the islets and 3) only in autoimmune diabetes. The mechanism that accounts for all three is based on emerging concepts in developmental neurobiology. We hypothesize that brain derived neurotrophic factor (BDNF), secreted by B-lymphocytes invading the islet during the development of autoimmune diabetes, shifts the balance of islet neurotrophins away from "axonal maintenance" via Trk receptors towards "axonal pruning" via the p75 pan-neurotrophin receptor (NTR), causing a selective loss of islet sympathetic nerves and thereby dysfunction of the sympathetic alpha-cell pathway. Therefore our first Specific Aim is to demonstrate the necessity of BDNF for the loss of islet sympathetic nerves. We will localize BDNF in the diabetic islet by a combination of transgenically tagging BDNF and immunohistochemically staining the tag. We will quantify BDNF mRNA in these islets by laser capturing them and using RT-PCR. We will demonstrate the sympathetic neurotoxicity of BDNF by knocking out the BDNF gene and thereby sparing these nerves and preventing the impairment of the glucagon response to sympathetic nerve activation. In our second Specific Aim we will demonstrate the necessity of B-lymphocytes by knocking them out, thus preventing both the loss of islet sympathetic nerves and the impairment of the glucagon response to sympathetic activation. In our third Specific Aim we will prevent the loss of islet sympathetic nerves by knocking out p75NTR, again preserving the glucagon response to sympathetic nerve activation. Significance: Determining the mechanism for the loss of islet sympathetic nerves in type 1 diabetes will allow us to prevent or reverse that loss. Since the sympathetic-alpha cell pathway is activated during IIH and contributes to the glucagon response to IIH, we expect such prevention to improve this glucagon response in type 1 diabetes. Normalizing this glucagon response will reduce the incidence and severity of insulin induced hypoglycemia, allowing more intensive treatment of patients with type 1 diabetes, thereby reducing the chronic hyperglycemia that causes the long-term complications of this disease.