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Endocrinology Vol. 140, No. 1 233-243
Copyright © 1999 by The Endocrine Society

ARTICLES

Insulin/Insulin-Like Growth Factor-I Hybrid Receptors with High Affinity for Insulin Are Developmentally Regulated during Neurogenesis1

Mario García-de Lacoba, Cristina Alarcón, Enrique J. de la Rosa and Flora de Pablo

Department of Cell and Developmental Biology, Centro de Investigaciones Biológicas, Consejo Superior de Investigaciones Científicas, E-28006 Madrid, Spain

Address all correspondence and requests for reprints to: Flora de Pablo, Department of Cell and Developmental Biology, Centro de Investigaciones Biológicas, Consejo Superior de Investigaciones Científicas, Velázquez 144, E-Madrid, Spain. E-mail: cibfp1f{at}fresno.csic.es

The extensive colocalization of insulin receptor (IR) and insulin-like growth factor-I receptor (IGFR) messenger RNAs during central nervous system development, together with the effects of insulin and IGF-I in neurogenesis, raises the question of how stage- and factor-specific signaling occurs. Thus, it is necessary to characterize the receptor proteins present in vivo to start addressing this issue. Here we have studied the chick embryonic neuroretina at day 6 (E6), when it is predominantly proliferative, and at E12, when neuronal differentiation is advanced. Developmentally regulated high-affinity binding sites for both insulin and IGF-I were detected at E6 and E12. In proliferative neuroretina, typical IGFR with the highest affinity for IGF-I coexisted with separate atypical insulin binding sites, which had similar high affinity for insulin and IGF-I. Immunoprecipitation of ligand-cross-linked receptors with specific antibodies for the IR {alpha}-subunit, the IR ß-subunit, or the IGFR ß-subunit demonstrated the presence of IR/IGFR hybrids. They were more abundant in E6 than in E12 retina. These hybrid receptors bound most of radiolabeled insulin, but little radiolabeled IGF-I, at tracer concentrations. At E12, the specificity of the insulin binding sites changed, and it was closer to that found with IR in liver, where hybrids were undetectable. The basal autophosphorylation level of these atypical hybrid receptors was high, although insulin and, even more so, IGF-I modestly increased the phosphorylation of two IR ß-subunits of 95 and 105 kDa. The high-affinity/low-discriminative IR/IGFR hybrids predominantly found in a proliferative stage of neurogenesis can mediate the effects of proinsulin and insulin, previously demonstrated in organoculture at this stage. More importantly, this hybrid receptor may be physiologically relevant for the action of the locally produced proinsulin found in early neurogenesis.




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