Developmental expression of brain β-spectrin isoform messenger RNAs

Abstract

We have investigated the expression of brain βSpIIa and βSpIb (previously referred to as the β-subunits of brain spectrin (240/235) and brain spectrin (240/235E), respectively) during mouse brain development. The 9 kb transcript which encodes βSpIIa is present in fetal mouse brain tissue and increases to a maximal level in a 30-day-old mouse. There is a coordinate accumulation of the 7.8 kb αSpIIa mRNA (with βSpIIa) during mouse brain development. The coordinate expression of αSpIIa and βSpIIa at the mRNA and protein level allows formation of (αSpIIa/βSpIIa) 2 tetramers (brain spectrin(240/235)) early in premitotic neuronal development; and avoids turnover of unassembled α and β-subunits. An 11 kb transcript which encodes βSpIb is not produced in embryonic tissue, and is first seen in a 6-day-old mouse. The protein translation products βSpIIa and βSpIb have previously been demonstrated by our laboratory to first appear in fetal mouse brain tissue and at postnatal day 6–8, respectively [ J. Neurosci., 7 (1987) 864–874]. The expression of βSpIb mRNA on postnatal day 6–8, and the appearance of brain spectrin(240/235E) in postmitotic and postmigratory neurons of the cerebellum at this same time; suggests that brain spectrin(240/235E) is involved in differentiated functions of the neuron (formation of cell-cell contacts, formation of dendritic processes and postsynaptic contacts). Thus, the data from the present study demonstrates that the expression of these two neuronal β-spectrin isoforms is regulated at the level of mRNA expression.