Development of Brain Insulin Receptors: Structural and Functional Studies of Insulin Receptors from Whole Brain and Primary Cell Cultures*

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Abstract
We studied the structural and functional characteristics of insulin receptors from rat brain and liver from late gestation through adulthood as well as from cultured neuronal and glial cells from neonatal rats. Specific insulin binding was present on membrane preparations from brain and liver at all stages of development studied, with maximal binding in neonates > 19-day-old fetuses > adults for both brain and liver. Maximal specific binding to cultured neuronal and glial cell membranes was simlar (6.2% vs. 7.1%, respectively). [125I]Iodinsulin crosslinking to the insulin receptor demonstrated that the mol wt (Mr) of the brain .alpha.-subunit was less than that of the liver .alpha.-subunit at all stages. [125I]Iodoinsulin cross-linking also demonstrated that the glial cell .alpha.-subunit (Mr, 130,000) migrated on sodium dodecyl sulfate-polyacrylamide gel electrophoresis to a position intermediate between the liver (Mr, 135,000) and brain (Mr, 119,000), whereas the neuronal cell .alpha.-subunit (Mr, 118,000) comigrated with the brain .alpha.-subunit. In solubilized lectin-purified preparations from brain and liver during development as well as from neuronal and glial cells, insulin stimulated phosphorylation of the .beta.-subunit. The Mr of the brain .beta.-subunit, as determined by migration on sodium dodecyl sulfate-polyacryalmide gel electrophoresis, was less than that of the liver .beta.-subunit. The neuronal cell .beta.-subunit comigrated with the brain .beta.-subunit while the glial cell .beta.-subunit migrated to a position intermediate between the brain and liver .beta.-subunit. Solubilized lectin-purified preparations from all tissues demonstrated insulin-stimulable phosphorylation of exogenous substrates. From these studies we conclude that 1) functional insulin receptors are present in the brain during development in the rat; and 2) the structural differences demonstrated between neuronal and glial cell and between brain and nonneuronal insulin receptors taken together with previously demonstrated functional differences of the insulin receptor on these tissues suggest a unique function for insulin receptors on neuronal tissues.