Role of soluble myosin in cortical contractions of Xenopus eggs

Abstract

The layer of cytoplasm underlying the plasmalemma of Xenopus eggs has contractile activity which is of vital importance in fertilization and early development, being involved in such processes as sperm engulfment, cortical granule exocytosis, development of the axes of embryonic symmetry and cleavage. In amphibian eggs this layer is also involved in wound healing and changes of cellular shape at gastrulation. Two kinds of contractile structures can be distinguished near the surface of Xenopus eggs. To characterize the mechanism and regulation of this contractile activity, we have experimentally induced cortical contractions in bisected living Xenopus eggs. We have shown previously that cortical contractions are induced by calcium ions in the bisected egg. Here we show that extraction of soluble cytoplasmic components prevents the calcium-induced contractions, but that addition of exogenous soluble myosin restores them. In oocytes, both soluble and insoluble components of the cortical cytoplasm are unable to support contraction. Thus, during meiotic maturation of oocytes into eggs, both of the components of the cortical cytoplasm must change so as to become competent for contraction.

Protein kinase C acts downstream of calcium at entry into the first mitotic interphase of Xenopus laevis.

“…2) Experimental treatments that increase egg [Ca2+] -such as application of calcium ionophore, pricking with a glass needle, microinjection of calcium, or microinjection of inositol 1,4,5 trisphosphate (IP3)-trigger egg activation (Gingell, 1970;Schroeder and Strickland, 1974;Wolf, 1974a;Hollinger and Schuetz, 1976;Picard et al, 1985;Kubota et al, 1987). 3) Treatments that prevent an increase in [Ca2+]i abolish events associated with interphase entry, including cortical granule exocytosis, cortical contraction, chromosome decondensation, and nuclear envelope reformation in response to activating stimuli (Gingell, 1970;Schroeder and Strickland, 1974;Christensen et al, 1984;Lohka and Masui, 1984;Kline, 1988).…”

3 Actin and Actin-Associated Proteins in Xenopus Eggs and Early Embryos: Contribution to Cytoarchitecture and Gastrulation

“…Cortices from oocytes prior to maturation (prior to attaining metaphase II arrest) are refractile to such contractions. A Xenopus myosin II-like molecule was found to be soluble under these conditions but necessary for the contractions (Christensen et al, 1984).…”

“…The contractile cortex of the zygote is 0.5-3 µm thick, excludes yolk platelets, and contains cortical granules, pigment granules, and actin filaments (Merriam et al, 1983;Christensen et al, 1984). Within 20-30 min the center of gravity is shifted to the vegetal pole, so that all fertilized eggs are oriented with the animal side up, while unfertilized eggs orient randomly, often on their sides.…”

“…Only in the absence of exogenous ATP is a 200-kDa putative myosin found associated with the contractile gel. Cortices of eggs and embryos can be dissected off the underlying cytoplasm (Franke et al, 1976), or eggs can be cut in half (Christensen et al, 1984). In either of these preparations, the contraction can be elicited and the necessary components extracted and reconstituted.…”

“…These include gelsolin (Ankenbauer et al, 1988), spectrin (Giebelhaus et al, 1987), and myosin (Christensen et al, 1984). Gelsolin is an actin filament-severing and barbed end-binding protein that nucleates filaments from the pointed, or slow-growing, end.…”