Tyrosine kinase inhibitors block sperm-induced egg activation in Xenopus laevis

Fertilization of Xenopus laevis eggs triggers a wave of increased [Ca²⁺](i). The exact signal transduction pathway culminating in this Ca²⁺ wave remains unknown. To determine whether increases in tyrosine kinase activity are part of this pathway, we microinjected tyrosine kinase inhibitors into unfertilized eggs. Upon fertilization, signs of activation were monitored, such as fertilization envelope liftoff and the Ca²⁺ wave (for eggs microinjected with lavendustin A). Various concentrations of lavendustin A and tyrphostin B46 were microinjected, as well as inactive forms of these compounds (lavendustin B and tyrphostin A1) to provide negative controls. Peptide A, a 20-amino-acid peptide derived from the SH2 region of pp60(v-src) tyrosine kinase, was also microinjected. Peptide A inhibits tyrosine kinase activity but not PKA or PKG activity. Dose-response curves for lavendustin A, tyrphostin B46, and peptide A show clear inhibition of vitelline envelope liftoff by these three compounds. Confocal imaging of eggs coinjected with lavendustin A and Oregon Green-dextran showed that the Ca²⁺ wave was inhibited under normal insemination conditions but that the block of the Ca²⁺ wave could be overcome with very high sperm densities. A phenomenon of small local Ca²⁺ increases termed 'hot spots' seen in lavendustin A containing eggs is also described. Since this inhibition of egg activation by tyrosine kinase inhibitors can be overcome by Ca²⁺ microinjection, the inhibitors must act on a step in the signal transduction cascade that is upstream of the Ca²⁺ wave.