DISASSEMBLY OF ACTIN-FILAMENTS LEADS TO INCREASED RATE AND FREQUENCY OF MITOCHONDRIAL MOVEMENT ALONG MICROTUBULES

In activated sea urchin coelomocytes, cytoplasmic organelles move alon g distinct actin and microtubule dependent pathways, actin-based motil ity is driven by an unconventional myosin, and microtubule disassembly does not effect actin-dependent organelle motility [D'Andrea et al., 1994: J. Cell Sci. 107:2081-2094]. Given the growing evidence for pote ntial interactions between components of the actin and microtubule cyt oskeletons, we examined the effect of actin filament disassembly on th e movement of mitochondria along microtubules in activated coelomocyte s. Coelomocytes treated with cytochalasin B (CB), to disrupt actin fil aments, exhibited a thinning of the cytoplasm, enhanced lateral undula tion of microtubules, and ceased centripetal cortical flow of actin. I nterestingly, the loss of actin filaments resulted in a similar to 1.5 -fold increase in the average velocity of outward and inward moving mi tochondria and increased the frequency of centripetal movement. To tes t if enhanced motility along microtubules was a consequence of decreas ed actin-myosin interaction, coelomocytes were treated with 2,3-butane dione monoxime (BDM), a potent inhibitor of myosin activity [Cramer an d Mitchison, 1995: J. Cell Biol. 131:179-189]. BDM inhibited all types of actin-based motility observed in these cells including retrograde cortical flow, protrusion and retraction of the cell edge, and movemen t of intracellular organelles. Surprisingly, BDM treatment stopped the movement of mitochondria in CB-exposed cells, suggesting that BDM can also act as an inhibitor of organelle movement along microtubules. Co llectively, these data demonstrated that microtubule-dependent mitocho ndrial motility and microtubule movement were sensitive to changes in the assembly state of the actin cytoskeleton. Cell Motil. Cytoskeleton 40:368-378, 1998. (C) 1998 Wiley-Liss, Inc.