Axonal Transport and the Cytoskeleton of Nerve cells

The long length of axons makes them critically dependent on intracellular transport for their growth and survival. This movement is called axonal transport.

The cargoes of axonal transport are very diverse, including membranous organelles and transport vesicles, as well as non-membranous cargoes such as cytoskeletal polymers, cytosolic protein complexes, ribosomes and messenger RNAs. These cargoes move along cytoskeletal polymer tracks, powered by molecular motor proteins.

We are particularly interested in neurofilaments, which are the intermediate filaments of nerve cells, and one of three classes of cytoskeletal polymers that comprise the nerve cell cytoskeleton. Neurofilaments play a critical role in the growth and maintenance of axon diameter, which is an important determinant of axonal conduction velocity.

The axonal transport of neurofilaments is impaired in many neurodegenerative diseases such as amyotrophic lateral sclerosis (Lou Gehrig's disease), and mutations in neurofilament proteins can also give rise to peripheral neuropathies such as Charcot-Marie-Tooth disease.

Neurofilaments move rapidly in axons but their overall rate of movement is slow because they spend most of their time pausing. The balance of movements and pauses determines the number of neurofilaments in the axon, and thus we believe that the transport properties of neurofilaments are an important determinant of axonal morphology.

We use live-cell fluorescence imaging in combination with a molecular, genetic and biochemical approaches to investigate the axonal transport and assembly dynamics of neurofilaments in cultured nerve cells.

Our goal is to understand the molecular mechanism of neurofilament assembly and transport and how these processes are regulated in health and disease.

Assembly Dynamics of Intermediate Filaments

Actin filaments and microtubules lengthen and shorten by addition and loss of subunits at their ends, but it is not known whether this is also true for intermediate filaments. In fact, several studies suggest that intermediate filaments may lengthen by end-toend annealing and that addition and loss of subunits is not confined to the filament ends.

To test these hypotheses, we are investigating the assembly dynamics of neurofilament proteins and other intermediate filament proteins in cultured cells using cell fusion, photobleaching, and photoactivation strategies in combination with conventional and photoactivatable fluorescent fusion proteins.

We have obtained the first direct evidence that neurofilaments and vimentin filaments in vivo lengthen by end-to-end annealing of assembled filaments and that they can exchange subunits along their length with no preferential addition or loss of subunits to the filament ends, a process which we term intercalary subunit exchange.

We are currently working to define the molecular basis of these unique dynamic properties and to understand their significance for intermediate filament function in cells.