Cytoplasmic Intermediate Filament Proteins Are Expressed in a Tissue-Specific Manner

Sequence analysis of IF proteins reveals that they fall into five distinct homology classes, four of which are localized to the cytoplasm. These IF classes show a strong correspondence to the developmental origin of the cell type in which the IF protein is expressed (Table 18-1). We discuss the fifth class—the nuclear lamins—separately, as they perform functions distinct from the cytoplasmic intermediate filaments.

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The keratins that make up IF protein classes I and II are found in epithelia; class III IF proteins are generally found in cells of mesodermal origin; and class IV IF proteins compose the neurofilaments found in neurons. The lamins, which make up class V, are found lining the nuclei of all animal tissues. Here we briefly summarize the four homology classes found in the cytoplasm and discuss their roles in specific tissues.

Keratins Keratins provide strength to epithelial cells. The first two IF protein homology classes are the so-called acidic and basic keratins. There are about 50 genes in the human genome encoding keratins, about evenly split between the acidic and basic classes. These keratin subunits assemble into an obligate dimer, so that each dimer consists of one basic chain and one acidic chain; these dimers are then assembled into a filament as described in the previous section.

The keratins are by far the most diverse of the IF protein families, with basic and acidic keratin pairs showing different expression patterns between distinct epithelia as well as differentiation-specific regulation. Among these are the so-called hard keratins that make up hair and nails. These keratins are rich in cysteine residues that become oxidized to form disulfide bridges, thereby strengthening the proteins. This property is exploited by hair stylists: if you do not like the shape of your hair, the disulfide bonds in your hair keratin can be reduced, the hair reshaped, and the disulfide bonds re-formed by oxidation—the result is “permanent” hair curling or hair straightening.

The so-called soft keratins, or cytokeratins, are found in epithelial cells. The epidermal-cell layers that make up the skin provide a good example of the function of these keratins. The lowest layer of cells, the basal layer, which is in contact with the basal lamina, proliferates constantly, giving rise to cells called keratinocytes. After they leave the basal layer, the keratinocytes differentiate and express abundant cytokeratins. The cytokeratins associate with specialized attachment sites between cells, thereby providing sheets of cells that can withstand abrasion. The keratinocytes eventually die, leaving dead cells from which all cell organelles have disappeared. This dead cell layer provides an essential barrier to water evaporation, without which we could not survive. The life of a skin cell, from birth to its loss from the animal as a skin flake, is about one month.

In all epithelia, keratin filaments associate with desmosomes, which link adjacent cells together, and hemidesmosomes, which link cells to the extracellular matrix, thereby giving cells and tissues their strength. These structures are described in more detail in Chapter 20.

In addition to simply providing structural support, there is increasing evidence that keratin filaments provide some organization to organelles and participate in signal transduction pathways. For example, in response to tissue injury, rapid cell growth is induced. It has been shown that in epithelial cells, the growth signal requires an interaction between a cell-growth-signaling molecule and a specific keratin.

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Desmin The class III IF proteins include vimentin, found in mesenchymal cells; GFAP (glial fibrillary acidic protein), found in glial cells; and desmin, found in muscle cells. Desmin provides strength and organization to muscle cells (see cartoons in Table 18-1).

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In smooth muscle, desmin filaments link cytoplasmic dense bodies, to which the contractile myofibrils are also attached, to the plasma membrane to ensure that cells resist overstretching. In skeletal muscle, a lattice composed of a band of desmin filaments surrounds the sarcomere. The desmin filaments encircle the Z disk and are cross-linked to the plasma membrane. Longitudinal desmin filaments cross to neighboring Z disks within the myofibril, and connections between desmin filaments around Z disks in adjacent myofibrils serve to cross-link myofibrils into bundles within a muscle cell. The lattice is also attached to the sarcomere through interactions with myosin thick filaments. Because the desmin filaments lie outside the sarcomere, they do not actively participate in generating contractile forces. Rather, desmin plays an essential structural role in maintaining muscle integrity. In transgenic mice lacking desmin, for example, this supporting architecture is disrupted and Z disks are misaligned. The locations and morphology of mitochondria in these mice are also abnormal, suggesting that these intermediate filaments may also contribute to the organization of organelles.

Neurofilaments Type IV intermediate filaments consist of the three related subunits—NF-L, NF-M, and NF-H (for NF light, medium, and heavy)—that make up the neurofilaments found in the axons of neurons (see Figure 18-2). The three subunits differ mainly in the size of their C-terminal domains, and all form obligate heterodimers. Experiments with transgenic mice reveal that neurofilaments are necessary to establish the correct diameter of axons, which determines the rate at which nerve impulses are propagated down them.

The structural integrity of the skin is essential in order to withstand abrasion. In humans and mice, the K4 and K14 keratin isoforms form heterodimers that assemble into protofilaments. A mutant K14 with deletions in either the N- or the C-terminal domain can form heterodimers in vitro, but does not assemble into protofilaments. The expression of such mutant keratin proteins in cells causes IF networks to break down into aggregates. Transgenic mice that express a mutant K14 protein in the basal stem cells of the epidermis display gross skin abnormalities, primarily blistering of the epidermis, that resemble the human skin disease epidermolysis bullosa simplex (EBS). Histological examination of the blistered area reveals a high incidence of dead basal cells. The death of these cells appears to be caused by mechanical trauma from rubbing of the skin during movement of the limbs. Without their normal bundles of keratin filaments, the mutant basal cells become fragile and easily damaged, causing the overlying epidermal layers to delaminate and blister (Figure 18-50). Like the role of desmin filaments in supporting muscle tissue, the general role of keratin filaments appears to be to maintain the structural integrity of epithelial tissues by mechanically reinforcing the connections between cells.

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EXPERIMENTAL FIGURE 18-50 Transgenic mice carrying a mutant keratin gene exhibit blistering similar to that in the human disease epidermolysis bullosa simplex. Histological sections through the skin of a normal mouse and a transgenic mouse carrying a mutant K14 keratin gene are shown. In the normal mouse, the skin consists of a hard outer epidermal layer covering and in contact with the soft inner dermal layer. In the skin from the transgenic mouse, the two layers are separated (arrow) due to weakening of the cells at the base of the epidermis.
[Republished with permission from Elsevier: from Coulombe et al., “Point mutations in human keratin 14 genes of epidermolysis bullosa simplex patients,” Cell, 1991, 66:6, pp.1301-1311; permission conveyed through Copyright Clearance Center, Inc.]