Myosin heavy-chain isoform composition and distribution in developing and adult human aortic smooth muscle.

The myosin heavy-chain (MHC) composition of developing and adult human aortic smooth muscle (SM) was studied by SDS-polyacrylamide gel electrophoresis, Western blotting and indirect immunofluorescence using a panel of anti-MHC antibodies. On 5% SDS gels, three bands of 204, 200 and 196 kDa apparent molecular mass were identified in fetal, infant and adult stages of development. In the extracts from thoracic aorta (upper level), the 204, and 200-kDa bands (designated as SM-1 and SM-2, respectively) were recognized by SM-G4 and SMMS-1 antibodies, raised against a SM antigen, whereas the 196-kDa band was reactive with nonmuscle (NM)-F6 and NM-G2 antiplatelet MHC antibodies. Western blotting and immunofluorescence tests performed on bovine brain and other human NM tissues using NM-F6 and NM-G2 indicated that antigenic targets of the two antibodies resembled that of so-called IIB and IIA NM myosin found in the bovine system, respectively. In the aortic media, SM-1 was expressed throughout development, while SM-2 was upregulated during late fetal and postnatal development. Similarly, the 196-kDa band showed two distinct patterns of immunoreactivity with the anti-NM-MHC antibodies: with NM-G2, antigenicity was equal at all the developmental stages examined, whereas with NM-F6, it diminished during postnatal development. In the upper level, the cellular distribution of NM-G2 and NM-F6 immunoreactivities was similar in the early fetus but quite distinct at later stages of development. In infant and adult subjects, SM cells (SMC) reactive with NM-F6 accumulated predominantly within the intimal layer as well as in some areas of the underlying media as cell foci, whereas NM-G2 homogeneously stained the two layers. In the aorta near the diaphragm (lower level), both antibodies stained the thickened intima but not the underlying media. These data are consistent with the existence of developmental, stage-specific molecular and cellular transitions during vascular SMC maturation in human aortic media. In addition, these data suggest that IIB-like myosin may be expressed in SMC involved specifically in intimal thickening.