Abstract
Mitochondria are essential for survival of eukaryotic cells since they are responsible for ATP synthesis, calcium buffering and apoptosis. Thus, specialized transport and anchoring mechanisms are necessary to distribute and position mitochondria in response to local needs for ATP and calcium buffering. Mitochondrial transport and positioning are particularly challenging in neurons because of the unique geometry and metabolic needs of neurons. In neurons, mitochondria are transported from the soma, where mitochondria biogenesis takes place, to ATP-demanding sites as synaptic terminals and growth cones. Mitochondria remain at these sites until they are transported back to the soma for recycling or degradation. This long-range bidirectional transport mainly depends on microtubules and microtubule-based motor proteins as kinesins and dynein, while short-range mitochondrial transport and docking mainly rely on actin and actin-based motors myosins. In contrast to actin and microtubules, intermediate filaments (IFs) do not possess polarity or associated motor proteins. Therefore, how IFs take part in mitochondrial positioning is relatively unclear. Previous studies suggest that IFs may serve as docking sites for mitochondria, or affect mitochondrial transport through microtubule-based motors. In this study, we investigate the functions of IFB-1, a C. elegans IF protein, in the mitochondrial transport system of C. elegans amphid neurons. Amphids are sensory organs consisting of a pair of sensilla running along the lateral sides of the worm head. We established wild-type and ifb-1 mutant worms carrying fluorescently-labeled mitochondria in the amphid neurons, and recorded time-lapse images from these worms and their isolated neuronal cells. We found that mutations of IFB-1 lead to larger pools of stationary mitochondria, suggesting a role of IFB-1 in the balance of stationary phases and motile phases of mitochondrial transport. In addition, loss of one isoform of IFB-1 reduces mitochondrial transport velocities and frequencies of changes of directions, but increases pausing frequencies as well as moving persistencies. To explain these complex effects of IFB-1 on mitochondrial motility, we propose a model in which IFB-1 influences mitochondrial transport through affecting kinesin or kinesin-associated proteins.