Intermediate Filaments

The major filamentous components of the cytoskeleton are MT, actin filaments, intermediate filaments (IF), and a class of delicate interconnecting fibrils called the microtrabecular lattice (MTL) which will be described later in this chapter. Here the "unknown" members of the cytoskeleton, intermediate filaments are reviewed (Lazarides, 1980). Intermediate filaments represent the most nebulous and chemically variable subgroup among the cytoskeleton. Five classes of IF have been distinguished which are built from polypeptides containing mostly alpha helix rod domains (Chapter 6). Under the electron microscope, IF appear as relatively featureless, 8-12 nanometer wide unbranched filaments. Treatment with certain fixative agents can cause IF to unravel into several 2-3 nanometer protofilaments, or 4-5 nanometer protofibrils whose number may vary from filament to filament, and even along the same filament. IF may form from parallel coiling of the alpha helix domains and under some circumstances, can form a polygonal meshwork with a 52 nanometer repeat of 810 nanometer wide filaments. These can also aggregate into crystal-like arrays with 24 nanometer spaced transverse bands. As such, IF may be involved in structures otherwise described as microtrabecular lattice, or cytomatrix.

Lazarides (1980) has shown that different types of IF associate with specific types of cells. For example, subunit structure defines five major classes of IF: 1) keratin ("tonofilaments"), which are found in epithelial cells, 2) desmin filaments predominantly found in smooth, skeletal and cardiac muscle cells, 3) vimentin filaments, found in mesenchymal cells, 4) neurofilaments, found in neurons (Figure 5.18), and 5) glial filaments, found in glial cells. Often two or more of these classes co-exist in the same cell.

Neurofilaments appear to function as a three dimensional structural lattice providing tensile strength to axons. Extruded axoplasm is a highly structured gel rich in neurofilaments; exposure of the gel to calcium ion results in degradation of neurofilaments and conversion of the gel to a more watery sol state, a phenomenon generally attributed to dissolution of actin filaments.

All IF including neurofilaments appear to be phosphorylated although the function that this could serve is not understood. IF are a distinct class separate from MT and actin filaments, and Lazarides argues their biochemical and morphological properties indicate they are involved in mechanically integrating the various components of the cytoplasmic space. IF remain poorly understood; their dense presence in neurons is mysterious, perhaps they participate in some way in the cognitive functions of the nervous system. In Chapter 8, Barnett's theory of neurofilaments as "string transform" memory banks coupled to MT will be described.

Figure 5.18: Cross section of small nerve axon. A. Microtubules with radial MAPs. B. Neurofilaments, outnumbering microtubules about 10:1. C. Vesicle being transported by microtubule axoplasmic transport. D. Crosslinked

actin/neurofilament gel (microtrabecular lattice). E. Myelin layers. F. Filamin bracketing actin. By Paul Jablonka.

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