B2 exon splicing of nonmuscle myosin heavy chain IIB is differently regulated in developing and adult rat brain.

Two isoforms of nonmuscle myosin heavy chain IIB (MHC-IIB) are generated by alternative splicing; MHC-IIB(B2) differs from MHC-IIB(DeltaB2) by the insertion of B2 exon cassette near the actin binding region. Here we examined expressions of the two splice variants in developing and adult rat brains by in situ hybridization with isoform-specific oligonucleotide probes. In adult, MHC-IIB(DeltaB2) mRNA was highly expressed in neurons of the cerebral cortex, hippocampus, and cerebellum, whereas MHC-IIB(B2) mRNA was mainly distributed in the brainstem and cerebellum, with the highest level in Purkinje cells. During development, MHC-IIB(DeltaB2) mRNA was predominantly expressed in various regions of embryonic and neonatal brains, whereas MHC-IIB(B2) mRNA was low during embryonic stages. Up-regulation of MHC-IIB(B2) started in the cerebellum during early postnatal stages when dendritogenesis and synaptogenesis occur actively in Purkinje cells. We further employed immunofluorescence using two antibodies (one recognizing both splicing variants and another specific to MHC-IIB(B2)), and found similar and dense localization in cell bodies and dendrites of Purkinje cells. Therefore, splicing of the B2 exon cassette undergoes distinct temporal and spatial regulations in the brain in vivo, and the different exon usage seems unlikely to affect the somato-dendritic localization of MHC-IIB.