Biochemical and immunohistochemical characterization of the isoforms of myosin and actin in human placenta.

Human placenta has long been known to contain large quantities of smooth muscle-type myosin and actin, while precise isoform compositions of its contractile proteins are not known. To determine the isoform compositions, myosin and actin were extracted from human term placentas and subjected to sodium dodecyl sulfate-polyacrylamide gel electrophoresis and Western blotting by using isoform-specific monoclonal anti-myosin and anti-actin antibodies. The placental myosin was found to be composed of about 65% of a nonmuscle-type heavy chain isoform (MIIA), each about 15% of two smooth muscle-type heavy chain isoforms (SM1 and SM2) and about 5% of a brain/fetus-type heavy chain isoform (MIIB2). Whereas the MIIA isoform was present in both vascular and extravascular tissues, the SM1 isoform was localized almost only in the vascular tissue. Similarly, human term placenta was found to contain approximately 60, 30, and 10% of β-nonmuscle, α-smooth muscle, γ-smooth muscle actin isoforms, respectively. The β-nonmuscle actin was located primarily in the extravascular tissue, while the α-smooth muscle actin was located mostly in the vascular tissue. The extravascular tissue of the human term placenta thus appears to be composed of almost only nonmuscle-type isoforms of contractile proteins. The vascular tissue appears to be composed of both smooth muscle-type and nonmuscle-type isoforms of contractile proteins.