This project's long term aim is to clarify the roles that insulin and the insulin-like growth factors (IGFs) may play in the development and function of the nervous system. This will be accomplished through an investigation of the interactions of these peptides with the homologous and heterologous receptor systems at different stages of ontogeny. The project will compare the receptors for insulin and IFG-I and IFG-II in the brain and liver of developing chick embryo using a variety of techniques. The ontogenic development of each population of receptors will be examined by radioreceptor assays which will characterize the binding kinetics, receptor concentrations, and the affinity of each population of receptors for the homologous and heterologous hormones. The structures of the receptors, including differences in the structure of the insulin receptors of brain versus liver, will be examined during ontogeny by polyacrylamide gel electrophoresis, adsorption to wheat germ agglutinin, and susceptibility to digestion by neuraminidase. The cross-reactivity of receptor populations with anti-receptor antibodies will be studied by immunoprecipitation assays and an attempt will be made to isolate antisera capable of reacting with only one receptor population. Coupling of receptors to intracellular events during development will be examined by studies of hormone-stimulated phosphorylation of both receptors and exogenous substrates. In addition, the ability of each peptide to stimulate such phosphorylation through binding to the heterologous receptors will be evaluated. Since nervous tissue does not require insulin for metabolic regulation, the presence of both insulin and insulin receptors in the central nervous system raises the question of a unique role for insulin in brain; recent evidence suggests effects on both neurotransmission and brain growth. Both neurotransmitter and hormonal effects of peptides are mediated through binding to cell surface receptors, causing activation of a program of events intrinsic to the receptor; in the case of insulin, this is made more complex by the fact that insulin binds not only to its specific high-affinity receptor, but also to receptors for the IGFs. In addition, insulin receptors in adult rat brain differ structurally from those in other tissues, suggesting the possibility that a different program of events may be mediated. Thus, this ontogenic study of the development, structure, cross reactivity, and coupling of these receptors to intracellular events in the brain, and comparison to the findings in liver, where insulin's effects are relatively well understood, should shed light on whether insulin may have a unique effect on developing brain and how such an effect might be mediated. These findings may have clinical implications for states of hormone excess or deficiency during periods of brain development, for possible malfunctions of these hormone-receptor systems in disorders of brain development and function, and for effects of these peptides on differentiating and transformed tissues.
