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

Abstract

The alternatively spliced isoform of the nonmuscle myosin heavy chain MB (MHC-IIB) with an insert of 21 amino acids near the actin-binding region, MHC-IIB(B2), is expressed specifically in the brain and spinal cord in Mammalia 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 existed as the B2 inserted isoform (MHC-IIB(B2)) in the cerebrum of the guinea-pig, rabbit and pig, while the non-B2 inserted isoform (MHC-IIB(ΔB2)) was the dominant form in the cerebrum of the rat, mouse and hamster. In the guinea-pig, the amount of MHC-IIB(B2) expressed in the cerebrum was low compared to MHC-IIB(ΔB2) during the first postnatal week, but it increased to comparable levels during postnatal development. In the rat, the amount of MHC-IIB(B2) protein in the cerebrum remains low compared to its expression elsewhere in the brain throughout life. Our results regarding the distribution of MHC-IIB(B2) in the adult brain lead us to classify species into two types; one type expresses significantly less MHC-IIB(B2) in the cerebrum than in other portions of the brain, and the second type expresses it at comparable levels throughout the brain. Based on these results, we hypothesize that MHC-IIB(B2) modulates the role of MHC-IIB(ΔB2) in the regulation of synaptic structure and function in the mature brain, and that the requirements for such function by MHC-IIB(B2) have shifted gradually from the cerebellum to the cerebrum with evolutionary increases in brain size.