Species differences in the distribution of the nonmuscle myosin heavy chain IIB inserted isoform in the brain

The alternatively spliced isoform of the nonmuscle myosin heavy chain IIB ( MHC-IIB) with an insert of 21 amino acids near the actin-binding region, MH C-IIB(B2), is expressed specifically in the brain and spinal cord in Mammal ia and Aves. We performed immunoblot analyses to elucidate the distribution of MHC-IIB(B2) in the brains of various animals. Nearly half of MHC-IIB ex isted as the B2 inserted isoform (MHC-IIB(B2)) in the cerebrum of the guine a-pig, rabbit and pig, while the non-B2 inserted isoform (MHC-IIB(Delta B2) ) was the dominant form in the cerebrum of the rat, mouse and hamster. In t he guinea-pig, the amount of MHC-IIB(B2) expressed in the cerebrum was low compared to MHC-IIB(Delta B2) during the first postnatal week, but it incre ased to comparable levels during postnatal development. In the rat, the amo unt of MHC-IIB(B2) protein in the cerebrum remains low compared to its expr ession elsewhere in the brain th roughout life. Our results regarding the d istribution of MHC-IIB(B2) in the adult brain lead us to classify species i nto two types; one type expresses significantly less MHC-IIB(B2) in the cer ebrum than in other portions of the brain, and the second type expresses it at comparable levels throughout the brain. Based on these results, we hypo thesize that MHC-IIB(B2) modulates the role of MHC- IIB(Delta B2) in the re gulation of synaptic structure and function in the mature brain, and that t he requirements for such function by MHC-IIB(B2) have shifted gradually fro m the cerebellum to the cerebrum with evolutionary increases in brain size.