In most eukaryotic cells, microtubules and filamentous actin (F-actin) provide tracks on which intracellular
organelles move using molecular motors. Here we report that cytoplasmic movement of both
mitochondria and lysosomes is slowed by F-actin meshwork formation in pancreatic duct epithelial
cells (PDEC). Mitochondria and lysosomes were labeled with fluorescent Mitotracker Red CMXRos and
Lysotracker Red DND-99, respectively, and their movements were monitored using epi-fluorescence and
confocal microscopy. Mitochondria and lysosomes moving actively at rest stopped rapidly within several
seconds after an intracellular Ca2+ rise induced by activation of P2Y2 purinergic receptors. The ‘freezing’ of
the organelles was inhibited by blocking the Ca2+ rise or by pretreatment with latrunculin B, an inhibitor
of F-actin formation. Indeed, this freezing effect on the organelles was accompanied by the formation of
F-actin in the whole cytoplasm as stained with Alexa 488-phalloidin in fixed PDEC. For real-time monitoring
of F-actin formation in live cells, we expressed sGFP-fimbrin actin binding domain2 (fABD2) in PDEC.
Rapid recruitment of the fluorescent probe near the nucleus and lysosomes suggested dense F-actin formation
around intracellular structures. The development of F-actin paralleled that of organelle freezing.
We conclude that rapid Ca2+-dependent F-actin formation physically restrains intracellular organelles
and reduces their mobility non-selectively in PDEC.