Cell-cell channel formation and lectins

The oocyte cell-cell channel assay was used to investigate determinants of the rate of channel formation. After injection of connexin-specific mRNA, oocytes accumulate a pool of precursors from which cell-cell channels can form after oocytes are paired. Channel formation was found to be increased if oocytes are pretreated with lectins before pairing. Several lectins differing in their carbohydrate binding affinities can exert this effect. Lectin-specific sugars suppress the effect on cell-cell channel formation only if the sugar is mixed with the lectin before application to the oocyte. If the lectin is first bound to the oocyte and then the sugar is added, no significant inhibition is seen. The promotion of channel formation by lectins is enhanced by adding an incubation period in regular medium after lectin treatment, before pairing of the oocytes. Electron microscopic studies with gold-conjugated lectins show that the lectin receptors are clustered on the free membrane surface and are taken up in endocytotic vesicles. These data suggest that the observed acceleration of cell-cell channel formation by lectins can be attributed to the removal of steric hindrance, which is a consequence of clustering of the bulky glycoprotein lectin receptors as well as of the removal from the surface by endocytosis.