Preliminary data suggest the presence of a delta-to-beta cell endocrine axis within the islet in which intraislet somatostatin inhibits insulin secretion. The somatostatin receptor subtypes responsible for the inhibition of insulin are unknown, however, preliminary data suggest that SSTR1 and SSTR5 are important regulators of insulin secretion and that there are species differences. We have also demonstrated the importance of the axis in that gene ablation of SSTR5 and/or SSTR1 in mice results in significant age- and sex-dependent alterations in insulin secretion, glucose regulation and islet morphology, and ultimately in diabetes. The purpose of this competitive renewal proposal is to prove the hypotheses that a) intraislet somatostatin is an important inhibitory regulator of insulin secretion via SSTR5, b) systemic somatostatin is an important inhibitory regulator of insulin secretion via SSTR1 and c) disruption of the inhibitory effect of intraislet and/or systemic somatostatin by gene ablation of SSTR5 and/or SSTR1 will result in diabetes in mice. We hope to translate the observations in mice to human islets and determine the role of SSTR5 and SSTR1 in the human endocrine pancreas in three specific aims:
Specific Aim 1. to determine whether a) intraislet somatostatin inhibits insulin secretion via SSTR5 in mice, b) disruption of the inhibitory effect of intraislet somatostatin by gene ablation of SSTR5 results in altered insulin secretion, glucose homeostasis and islet morphology and ultimately diabetes in older mice and c) the alterations are age- and sex-dependent.
Specific Aim 2. to determine whether a) intraislet somatostatin inhibits insulin secretion via SSTR1 in mice, b) disruption of the inhibitory effect of intraislet somatostatin by gene ablation of SSTR1 results in altered insulin secretion, glucose homeostasis and islet morphology and ultimately diabetes in younger mice, and c) the alterations are age- and sex-dependent, and d) double gene ablation of SSTRI/SSTR5 results in altered insulin secretion, glucose homeostasis and islet morphology and ultimately diabetes in mice.
Specific Aim 3. to determine whether a) intraislet somatostatin inhibits insulin secretion via SSTR1 and SSTR5 in human islets and b) the regulatory effects of SSTR1 and SSTR5 are age- and sex-dependent. For this proposal, four mouse colonies, already established in our laboratory, will be studied over time: 1) SSTR5 1/- mice, 2) beta cell-specific SSTR5 -/- mice, 3) SSTR1 -/-mice and 4) SSTRI&5 -/- mice. One additional mouse colony will be developed: 1) beta cell-specific SSTR-/- mice. In vivo and in vitro physiology studies will be performed using the following techniques: 1) intraperitoneal glucose tolerance tests in mice 2) isolated perfused mouse pancreas, 3) isolated mouse islet cultures, and 4) isolated perfused human pancreas. The role of SSTR1 and SSTR5 on human and mouse insulin secretion will be determined by examining insulin responses to varying levels of glucose, selective SSTR1 and SSTR5 agonists and to a potent somatostatin monoclonal antibody. Immunohistochemistry, Western blot, Northern blot, Southern blot and RT-PCR will be used to determine whether SSTR1 and SSTR5 gene ablation alters expression of selected regulatory proteins, such as other SSTRs, PDX-1, PCNA, c-Myc, and TGF-beta and Smads in mouse islets. Light microscopy will be used to study alterations in structure of the endocrine pancreas following SSTR1 and SSTR5 gene ablation in mice. The proposed studies will determine whether SSTR1 and SSTR5 are the predominant inhibitory regulators of insulin secretion in human and mouse islets and whether there are pathophysiologic consequences to ablating these receptors in mouse beta cells. Translation of observation in these molecularly-engineered mouse islets to human islets will hopefully provide greater insights into physiologic pathways regulating human insulin secretion, which potentially could benefit in the diagnosis and treatment of patients with diabetes.
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