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.
INTRODUCTIONThe cytoskeleton of vertebrate cells consists of three major types of protein networks: intermediate filaments (IF), microfilaments (MF), and microtubules (MT). Intermediate filaments are the most diverse of the three because they are encoded by Ͼ65 genes, making the IF superfamily one of the 100 largest in the human genome (Hesse et al., 2001). These genes are developmentally regulated, resulting in the cell type-specific expression of IF. This is clearly evident in the nervous system, where at least seven different IF proteins are expressed, ranging from the complex neurofilament (NF) heteropolymers composed of the type IV triplet proteins NF-L, NF-M, and NF-H, to the simpler homopolymers of the type III IF protein peripherin (Leung et al., 1998). Peripherin forms the major IF system of peripheral and enteric neurons, and it is abundantly expressed in PC12 cells, a widely used model for studies of peripheral neurons (Parysek and Goldman, 1987;Leonard et al., 1988; Portier et al., 1983a,b).One of the hallmarks of mature or terminally differentiated neurons is their remarkable shape, highlighted by extremely long cytoplasmic processes such as axons. The initiation, extension, and maintenance of axons involve the coordinated interactions of different cytoskeletal proteins (Mueller, 1999;Dickson, 2002). To date, only MT and MF have been considered essential for growth cone activity and axon outgrowth (Letourneau, 1996). In contrast, the contribution of neural IF to these processes has not been defined.The expression patterns of neural IF are highly correlated with different phases of axonal development. Type III IF, such as peripherin and vimentin, are present throughout early stages of outgrowth, and later type IV NF triplet proteins are expressed as axons reach maturity (Cochard and Paulin, 1984;Troy et al., 1990a). In mature neurons, NF appear to be major determinants of axon caliber, and thus conduction velocity (Lasek et al., 1983;Hoffman et al., 1987). However, little is known about ...
