Continuous phosphatidylinositol metabolism is required for cleavage of crane fly spermatocytes

Successful cleavage of animal cells requires co-ordinated regulation of the actomyosin contractile ring and cleavage furrow ingression. Data from a variety of systems implicate phosphoinositol lipids and calcium release as potential regulators of this fundamental process. Here we examine the requirement for various steps of the phosphatidylinositol (PtdIns) cycle in dividing crane fly (Nephrotoma suturalis) spermatocytes. PtdIns cycle inhibitors were added to living cells after cleavage furrows formed and began to ingress. Inhibitors known to block PtdIns recycling (lithium), PtdIns phosphorylation (wortmannin, LY294002) or phosphatidylinositol 4,5-bisphosphate [PtdIns(4,5)P₂] hydrolysis [U73122 (U7)] all stopped or slowed furrowing. The effect of these drugs on cytokinesis was quite rapid (within 0-4 minutes), so continuous metabolism of PtdIns appears to be required for continued cleavage furrow ingression. U7 caused cleavage furrow regression concomitant with depletion of F-actin from the contractile ring, whereas the other inhibitors caused neither regression nor depletion of F-actin. That U7 depletes furrow-associated actin seems counterintuitive, as inhibition of phospholipase C would be expected to increase cellular levels of PtdIns(4,5)P₂ and hence increase actin polymerization. Our confocal images suggest, however, that F-actin might accumulate at the poles of U7-treated cells, consistent with the idea that PtdIns(4,5)P₂ hydrolysis may be required for actin filaments formed at the poles to participate in contractile ring assembly at the furrow.