Metabolism and Growth in a Teleost Fish Model of Diabetes Mellitus: Insulin Regulation in an Ectothermic Vertebrate

点击翻译按钮获取中文摘要

9600783 Kelley A unique model of diabetes mellitus has been developed in a marine fish, the goby Gillichthys mirabilis. In contrast to the anatomical arrangement in other animals in which pancreatic endocrine cells (i.e., those producing insulin and glucagon) are located in the same tissue as exocrine cells (i.e., digestive enzyme-producing cells), this goby possesses a single pancreatic endocrine organ, called the "islet", which contains purely hormone-producing cells. The goby thereby allows for a simple surgical "isletectomy", which removes completely the endocrine cells and produces the outright symptomatology of insulin-dependent diabetes mellitus (IDDM). In contrast to mammalian models, however, in which IDDM results from the singular loss of the insulin-producing cells ((-cells), isletectomy in the goby removes simultaneously (-cells as well as (-cells (the source of glucagon). In mammals, "counterregulatory" glucagon, acting unchallenged by insulin, is believed to be a primary factor in driving the hyperglycemia during the onset phase of IDDM. There have been no experimental models available, however, in which both (- and (-cells can be removed in order to test this hypothesis directly. The PI aims to characterize the diabetogenic role of glucagon during IDDM onset in the goby model, with special attention given to the metabolic mechanisms occurring in the liver and blood. The influence of adrenal glucocorticoid (cortisol), a diabetogenic hormone important post-onset in diabetic mammals, will also be investigated. In addition to metabolic perturbation, the Ix goby exhibits a strong diabetic growth inhibition, including reductions in insulin-like growth factor (IGF) bioactivity (an indicator of positive somatic) growth) and a significant increase in a low molecular weight IGF-binding problem that may be related to the metabolically-sensitive, IGF-inhibitory "IGFBP-1" of mammals. Taking advantage of this apparent metabolic regulation of the IGFBP-1-like protein in the Ix g oby, the PI aims to characterize this protein's endocrine/metabolic regulation and its growth-regulatory role, and to attempt the cloning of its gene using a PCR-based strategy. The PI is particularly interested in whether measurement of this protein may serve as a convenient indicator of health, growth status, and stress in natural and aquacultured fish populations. Owing to the prior lack of a model of insulin deficiency in lower vertebrates, studies on the biological role of insulin in ectotherms ("cold-blooded" animals) have been limited to hormone injection studies on intact animals. The Ix goby model will allow the PI to gain previously-unattainable insight on metabolic regulation "before" the evolution of endothermy ("warm-bloodedness"). It has been hypothesized that the development of the physiology of endothermy, as has occurred in mammalian and avian evolution, necessitated the development of a more efficient metabolic regulatory system with an increased dependence on the regulatory influence of insulin. In the Ix goby, the PI has found that insulin-regulatable glucose transport ion peripheral muscle tissues becomes insulin resistant, and that insulin treatment of Ix gobies renders the glucose transport system once again responsive to insulin action. These data suggest that insulin may exert a dual regulation on fish glucose transporters comparable to that in mammals, i.e., acute effects on transporter dynamics/activity, long-term effects on transporter gene expression. Importantly, the data also establish a regulatable experimental system amenable to another specific aim: to characterize the insulin regulation and cellular mechanisms of the glucose transport system in the goby, especially as it relates to that of the endothermic mammal; the PI then plans to use a PCR-based strategy to clone the goby counterpart of glut-4 (the primary insulin-regulated glucose transporter of mammals). In the proposed studies, interrelationships between metabolic and growth mechanisms will be e mphasized, in order to gain an integrated picture of this ectotherm's physiological system.