Rapid transport of neural intermediate filament protein

Arrival, reversal, and departure of neurofilaments at the tips of growing axons

We have investigated the movement of green fluorescent protein-tagged neurofilaments at the distal ends of growing axons by using time-lapse fluorescence imaging. The filaments moved in a rapid, infrequent, and asynchronous manner in either an anterograde or retrograde direction (60% anterograde, 40% retrograde). Most of the anterograde filaments entered the growth cone and most of the retrograde filaments originated in the growth cone. In a small number of cases we were able to observe neurofilaments reverse direction, and all of these reversals occurred in or close to the growth cone. We conclude that neurofilament polymers are delivered rapidly and infrequently to the tips of growing axons and that some of these polymers reverse direction in the growth cone and move back into the axon. We propose that 1) growth cones are a preferential site of neurofilament reversal in distal axons, 2) most retrograde neurofilaments in distal axons originate by reversal of anterograde filaments in the growth cone, 3) those anterograde filaments that do not reverse direction are recruited to form the neurofilament cytoskeleton of the newly forming axon, and 4) the net delivery of neurofilament polymers to growth cones may be controlled by regulating the reversal frequency.

Intermediate filaments are dynamic and motile elements of cellular architecture

Recent evidence showing that intermediate filaments (IFs) are dynamic, motile elements of the cytoskeletal repertoire of vertebrate cells has overturned the long-standing view that they simply form static 'space filling' cytoplasmic networks. In fact, many types of IF are now known to engage in a remarkable array of movements that are closely associated with their assembly, disassembly and subcellular organization. Some of these motile properties are intrinsic to IFs and others are attributable to molecular crosstalk with either microtubules or actin-containing microfilaments. This crosstalk is, to a large extent, mediated by molecular motors, including conventional kinesin and cytoplasmic dynein. These motors are responsible for the high-speed delivery of nonfilamentous IF precursors and short filaments to specific regions of the cytoplasm, where they assemble into long IFs. Interestingly, the patterns and speeds of IF movements vary in different cell types and even within different regions of the same cell. These differences in motility may be related to their interactions with different types of molecular motor and/or other factors, such as IF-associated proteins.

Identification of novel principles of keratin filament network turnover in living cells

It is generally assumed that turnover of the keratin filament system occurs by exchange of subunits along its entire length throughout the cytoplasm. We now present evidence that a circumscribed submembranous compartment is actually the main site for network replenishment. This conclusion is based on the following observations in living cells synthesizing fluorescent keratin polypeptides: 1) Small keratin granules originate in close proximity to the plasma membrane and move toward the cell center in a continuous motion while elongating into flexible rod-like fragments that fuse with each other and integrate into the peripheral KF network. 2) Recurrence of fluorescence after photobleaching is first seen in the cell periphery where keratin filaments are born that translocate subsequently as part of the network toward the cell center. 3) Partial keratin network reformation after orthovanadate-induced disruption is restricted to a distinct peripheral zone in which either keratin granules or keratin filaments are transiently formed. These findings extend earlier investigations of mitotic cells in which de novo keratin network formation was shown to originate from the cell cortex. Taken together, our results demonstrate that the keratin filament system is not homogeneous but is organized into temporally and spatially distinct subdomains. Furthermore, the cortical localization of the regulatory cues for keratin filament turnover provides an ideal way to adjust the epithelial cytoskeleton to dynamic cellular processes.

A role for intermediate filaments in determining and maintaining the shape of nerve cells

To date, the functions of most neural intermediate filament (IF) proteins have remained elusive. Peripherin is a type III intermediate filament (IF) protein that is expressed in developing and in differentiated neurons of the peripheral and enteric nervous systems. It is also the major IF protein expressed in PC12 cells, a widely used model for studies of peripheral neurons. Dramatic increases in peripherin expression have been shown to coincide with the initiation and outgrowth of axons during development and regeneration, suggesting that peripherin plays an important role in axon formation. Recently, small interfering RNAs (siRNA) have provided efficient ways to deplete specific proteins within mammalian cells. In this study, it has been found that peripherin-siRNA depletes peripherin and inhibits the initiation, extension, and maintenance of neurites in PC12 cells. Furthermore, the results of these experiments demonstrate that peripherin IF are critical determinants of the overall shape and architecture of neurons.