Kinesin-1 and Dynein Are the Primary Motors for Fast Transport of Mitochondria inDrosophilaMotor Axons

Abstract

To address questions about mechanisms of filament-based organelle transport, a system was developed to image and track mitochondria in an intact Drosophila nervous system. Mutant analyses suggest that the primary motors for mitochondrial movement in larval motor axons are kinesin-1 (anterograde) and cytoplasmic dynein (retrograde), and interestingly that kinesin-1 is critical for retrograde transport by dynein. During transport, there was little evidence that force production by the two opposing motors was competitive, suggesting a mechanism for alternate coordination. Tests of the possible coordination factor P150 Glued suggested that it indeed influenced both motors on axonal mitochondria, but there was no evidence that its function was critical for the motor coordination mechanism. Observation of organelle-filled axonal swellings ("organelle jams" or "clogs") caused by kinesin and dynein mutations showed that mitochondria could move vigorously within and pass through them, indicating that they were not the simple steric transport blockades suggested previously. We speculate that axonal swellings may instead reflect sites of autophagocytosis of senescent mitochondria that are stranded in axons by retrograde transport failure; a protective process aimed at suppressing cell death signals and neurodegeneration. INTRODUCTIONOrganelle transport is central to the organization, developmental fate, and functions of asymmetric cells. A bipolar neuron is an extreme case, with the somato-dendritic region and the axon containing different sets of proteins and organelles. In general, much of the machinery for synthesizing and recycling neuronal components is clustered near the nucleus. Because an axon, often with an axial ratio of many thousands, usually contains Ͼ99% of the neuronal cytoplasm, active transport of new components away from the cell body and of spent components and trophic materials back toward the cell body is critical. Disruptions of axonal transport are thought to contribute to the pathologies of Alzheimer's, amyotrophic lateral sclerosis, and other neurodegenerative diseases (reviewed by Mandelkow and Mandelkow, 2002;Hirokawa and Takemura, 2005).Long-distance organelle transport in axons is driven by motor proteins that move along parallel microtubules whose plus ends are mostly oriented toward the axon terminal (reviewed by Hollenbeck and Saxton, 2005). There are multiple types of microtubule motors in postmitotic vertebrate neurons, including kinesins that can move toward either plus-or minus-ends and at least one cytoplasmic dynein that moves toward minus-ends (Martin et al., 1999b;Hirokawa and Takemura, 2005;Hollenbeck and Saxton, 2005). Which motors bind and move which axonal components? When multiple types of motors bind a single cargo, do they collaborate or compete, how are they regulated, and how do their combined activities generate an optimal distribution of that type of cargo?Axonal mitochondria are critical for the physiology of neurons and are particularly well suited for studying active tr...

Subpixel colocalization reveals amyloid precursor protein-dependent kinesin-1 and dynein association with axonal vesicles

“…To further understand APP vesicle-motor subunit association, we investigated the role that KLC1 plays in determining the amount of DHC1 on APP vesicles. Previous work found impairment of retrograde transport following reduction or inhibition of kinesin-1, suggesting coordination between opposite-polarity motors (17,31,(38)(39)(40)(41). We tested whether retrograde transport defects caused by reduced kinesin-1 function might be a result of decreased dynein association with APP vesicles.…”

Microtubule-mediated transport of the tumor-suppressor protein Merlin and its mutants

“…We selected mitochondria movement for this analysis because mitochondria, like Merlin particles (see below), are moved by conventional kinesin (30). Time-lapse movies of mitochondria labeled with MitoTracker Red show that expression of Mer(K70E) GFP did not affect movement of mitochondria (Movie S6).…”

Intracellular transport driven by cytoskeletal motors: General mechanisms and defects

“…Mitochondria are present in almost every eucaryotic cells and can be transported along microtubules. Remarkably mitochondria are also to be transported along the axon [288,162,280] although their size is much larger than the normal interdistance of MTs in the axon (∼ 20 − 30nm [72]). …”