Lipid droplet motion in early Drosophila embryos

M. A. Welte, S. P. Gross, M. Postner, S. M. Block, E. F. Wieschaus (1998). "Developmental regulation of vesicle transport in Drosophila embryos: forces and kinetics" Cell 92:547-557. Abstract


The complexities of organelle transport have posed a formidable challenge for traditional biochemical and cell-biological studies. Complex regulatory mechanisms can in principle be dissected by genetic analysis. In many genetically tractable systems, however, direct observation of motion is a challenge. This drawback makes it difficult to interpret how a particular mutation alters motor properties. We have developed a new model system that overcomes these problems, lipid-droplet motion in Drosophila embryos.

In Drosophila embryos, microtubules oriented along apical-basal directions support saltatory organelle movement. Organelle traffic includes lipid droplets, ubiquitous storage organelles for neutral lipids. The overall distribution of droplets shifts twice during early embryogenesis: inward (basal) transport just before cellularization is followed by outward (apical) transport during gastrulation.

Using video-enhanced microscopy, optical tweezers, and a novel squashed-mount embryo preparation, we tracked single droplets and measured the forces they generated. Droplet stalling forces change developmentally, in a roughly quantized fashion, consistent with variation in the number of active motors. Net transport direction appears to result from the balance of plus- and minus-end excursions: in all cases analyzed, the microscopic parameters of droplet motion predict the correct direction of net transport, and the predicted rates of bulk displacement are sufficient to account for the rates observed.

We characterized a mutation, klarsicht , that affects droplet transport. In the mutants, net droplet transport fails to switch from basal to apical during gastrulation. Mutations in klar similarly disrupt a switch in the direction of nuclear migration during eye development, suggesting the existence of a general, Klar-based transport system. At the level of individual droplets, Klar facilitates changes in force and is necessary for high force levels, possibly by coordinating the activity of multiple motors.



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