‘Poking’ microtubules bring about nuclear wriggling to position nuclei

Abstract

Nuclei wriggle in the cells of the follicle epithelium of the Drosophila pre-vitellogenic egg primordia. Although similar phenomena have been reported for a number of cultured cell types and some neurons in the zebra fish embryo, the mechanism and importance of the process remained unexplained. Wriggling involves the succession of sudden and random minor turns of the nuclei, about three twists in a minute with roughly 12° per twist, one of which lasts typically for 14 seconds. Wriggling is brought about by the growing microtubules seeded throughout the cell cortex, which, while poking the nuclei, buckle and exert 5–40 piconewtons over about 16 seconds - as it appears - through the nuclear pore complexes. While wriggling, the nuclei drift about 5 µm in a day in the immensely growing follicle cells along the apical-basal axis from the apical to the basal cell region. An over twofold excess of the microtubules nucleated in the apical cell region, as compared to those seeded in the basal cell cortex, makes the nuclei drift along the apical-basal axis. Nuclear wriggling and positioning appear to be tightly related processes: they cease simultaneously - when the nuclei become anchored by the actin cytoskeleton -, moreover, colchicine or taxol treatments eliminate both nuclear wriggling and positioning. We propose that the wriggling nuclei reveal a thus far not described nuclear positioning mechanism.