No abstract

Mitochondria are localized to regions of the cell where ATP consumption is high and are dispersed according to changes in local energy needs. In addition to motion directed by molecular motors, mitochondrial distribution in neuronal cells appears to depend on the docking of mitochondria to microtubules and neurofilaments. We examined interactions between mitochondria and neurofilaments using fluorescence microscopy, dynamic light scattering, atomic force microscopy, and sedimentation assays. Mitochondria-neurofilament interactions depend on mitochondrial membrane potential, as revealed by staining with a membrane potential sensitive dye (JC-1) in the presence of substrates/ADP or uncouplers (valinomycin/carbonyl cyanide p-(trifluoromethoxy)phenylhydrazone) and are affected by the phosphorylation status of neurofilaments and neurofilament sidearms. Antibodies against the neurofilament heavy subunit disrupt binding between mitochondria and neurofilaments, and isolated neurofilament sidearms alone interact with mitochondria, suggesting that they mediate the interactions between the two structures. These data suggest that specific and regulated mitochondrial-neurofilament interactions occur in situ and may contribute to the dynamic distribution of these organelles within the cytoplasm of neurons.

Neuronal cytoskeletal elements such as neurofilaments, F-actin, and microtubules are actively translocated by an as yet unidentified mechanism. This report describes a novel interaction between neurofilaments and microtubule motor proteins that mediates the translocation of neurofilaments along microtubules in vitro. Native neurofilaments purified from spinal cord are transported along microtubules at rates of 100-1000 nm/s to both plus and minus ends. This motion requires ATP and is partially inhibited by vanadate, consistent with the activity of neurofilament-bound molecular motors. Motility is in part mediated by the dynein/dynactin motor complex and several kinesin-like proteins. This reconstituted motile system suggests how slow net movement of cytoskeletal polymers may be achieved by alternating activities of fast microtubule motors.

One cellular function of intermediate filaments is to provide cells with compliance to small deformations while strengthening them when large stresses are applied. How IFs accomplish this mechanical role is revealed by recent studies of the elastic properties of single IF protein polymers and by viscoelastic characterization of the networks they form. IFs are unique among cytoskeletal filaments in withstanding large deformations. Single filaments can stretch to more than 3 times their initial length before breaking, and gels of IF withstand strains greater than 100% without damage. Even after mechanical disruption of gels formed by crossbridged neurofilaments, the elastic modulus of these gels rapidly recovers under conditions where gels formed by actin filaments are irreversibly ruptured. The polyelectrolyte properties of IFs may enable crossbridging by multivalent counterions, but identifying the mechanisms by which IFs link into bundles and networks in vivo remains a challenge.