DIFFERENTIAL EXPRESSION AND FUNCTIONS OF CORTICAL MYOSIN IIA AND IIB ISOTYPES DURING MEIOTIC MATURATION, FERTILIZATION, AND MITOSIS IN MOUSE OOCYTES AND EMBRYOS

To explore the role of nonmuscle myosin II isoforms during mouse gamet ogenesis, fertilization, and early development, localization and micro injection studies were performed using monospecific antibodies to myos in IIA and IIB isotypes. Each myosin II antibody recognizes a 205-kDa protein in oocytes, but not mature sperm. Myosin IIA and IIB demonstra te differential expression during meiotic maturation and following fer tilization: only the IIA isoform detects metaphase spindles or accumul ates in the mitotic cleavage furrow. In the unfertilized oocyte, both myosin isoforms are polarized in the cortex directly overlying the met aphase-arrested second meiotic spindle. Cortical polarization is alter ed after spindle disassembly with Colcemid: the scattered meiotic chro mosomes initiate myosin IIA and microfilament assemble in the vicinity of each chromosome mass. During sperm incorporation, both myosin II i sotypes concentrate in the second polar body cleavage furrow and the s perm incorporation cone. In functional experiments, the microinjection of myosin IIA antibody disrupts meiotic maturation to metaphase II ar rest, probably through depletion of spindle-associated myosin IIA prot ein and antibody binding to chromosome surfaces. Conversely, the micro injection of myosin IIB antibody blocks microfilament-directed chromos ome scattering in Colcemid-treated mature oocytes, suggesting a role i n mediating chromosome-cortical actomyosin interactions. Neither myosi n II antibody, alone or coinjected, blocks second polar body formation , in vitro fertilization, or cytokinesis. Finally, microinjection of a nonphosphorylatable 20-kDa regulatory myosin Light chain specifically blocks sperm incorporation cone disassembly and Impedes cell cycle pr ogression, suggesting that interference with myosin II phosphorylation influences fertilization. Thus, conventional myosins break cortical s ymmetry in oocytes by participating in eccentric meiotic spindle posit ioning, sperm incorporation cone dynamics, and cytokinesis. Although m urine sperm do not express myosin II, different myosin II isotypes may have distinct roles during early embryonic development.