Vimentins

The leucine zipper DNA-binding proteins, described in Chapter 10, are examples of globular proteins that use coiled coils to form both homo- and heterodimers. A variety of fibrous proteins also have heptad repeats in their sequences and use coiled coils to form oligomers, mainly dimers and trimers. Among these are myosin, fibrinogen, actin cross-linking proteins such as spectrin and dystrophin as well as the intermediate filament proteins keratin, vimentin, desmin, and neurofilament proteins. 

Inside the typical smooth muscle cell, the cytoplasmic filaments course around the nuclei filling most of the cytoplasm between the nuclei and the plasma membrane. There are two filamentous systems in the smooth muscle cell which run lengthwise through the cell. The first is the more intensively studied actin-myosin sliding filament system. This is the system to which a consensus of investigators attribute most of the active mechanical properties of smooth muscle. It will be discussed in detail below. The second system is the intermediate filament system which to an unknown degree runs in parallel to the actin-myosin system and whose functional role has not yet been completely agreed upon. The intermediate filaments are so named because their diameters are intermediate between those of myosin and actin. These very stable filaments are functionally associated with various protein cytoarchitectural structures, microtubular systems, and desmosomes. Various proteins may participate in the formation of intermediate filaments, e.g., vimentin. 

The analytic validity of an abstract parallel elastic component rests on an assumption. On the basis of its presumed separate physical basis, it is ordinarily taken that the resistance to stretch present at rest is still there during activation. In short, it is in parallel with the filaments which generate active force. This assumption is especially attractive since the actin-myosin system has no demonstrable resistance to stretch in skeletal muscle. However, one should keep in mind, for example, that in smooth muscle cells there is an intracellular filament system which runs in parallel with the actin-myosin system, the intermediate filament system composed of an entirely different set of proteins, (vimentin, desmin, etc.), whose mechanical properties are essentially unknown. Moreover, as already mentioned, different smooth muscles have different extracellular volumes and different kinds of filaments between the cells. 

Two major types of muscle fibers are found in humans white (anaerobic) and red (aerobic). The former are particularly used in sprints and the latter in prolonged aerobic exercise. During a sprint, muscle uses creatine phosphate and glycolysis as energy sources in the marathon, oxidation of fatty acids is of major importance during the later phases. Nonmuscle cells perform various types of mechanical work carried out by the structures constituting the cytoskeleton. These strucmres include actin filaments (microfilaments), micrombules (composed primarily of a- mbulin and p-mbulin), and intermediate filaments. The latter include keratins, vimentin-like proteins, neurofilaments, and lamins. 

Figure 7.3 The ER 1D5 peptide only binds to the ER 1D5 mAb (upper left and lower right). Other mAbs do not bind. The antibody abbreviations in the lower panel are Her2 11G5, and 9C2, human epidermal growth factor receptor type 2 clones 11G5 and 9C2 Mela-HMB45, melanocyte-specific antibody clone HMB45 Vimen V9, vimentin clone V9 Anti-LCA, anti-leukocyte common antigen clones PD7/26 and 2B11, combined as a cocktail Mouse poly IgG, mouse polyclonal IgG. Reproduced with permission from Sompuram et al.7...

Suurmeijer AJH, Boon ME. Optimizing keratin and vimentin retrieval in formalin-fixed, paraffin-embedded tissue with the use of heat and metal salts. Appl. Immunohistochem. 1993 31 144-146. 

Not all neurons have NFs. Indeed, one entire phylum in the animal kingdom, arthropods, expresses only type V nuclear lamins so arthropod cells have no IF cytoskeletal structures at all. In addition, mature oligodendrocytes lack IFs although their embryonic precursors contain vimentin. Clearly, the IFs are not essential for cell survival. Yet, in large myelinated fibers, NFs make up the bulk of axonal volume and represent a substantial fraction of the total protein in brain. In most organisms, IFs in both glia... 

Actin, spectrin, tau, vimentin, P-catenin, gelsolin, kinectin... 

Although intermediate filaments are not universally associated with the cytoskeleton, neutrophils possess intermediate filaments of the vimentin type. Vimentin is a rod-shaped molecule of relative molecular mass 57 kDa that readily polymerises under physiological conditions to produce stable filaments 10-12 nm in diameter. Intermediate filaments are more robust than microfilaments and microtubules, and in neutrophils they form an open network of single filaments in the perinuclear space. 

Schwartz, B., P. Vicart, C. Delouis, and D. Paulin. 1991. Mammalian cell lines can be efficiently established in vitro upon expression of the SV40 large T antigen driven by a promoter sequence derived from the human vimentin gene. Biol Cell 73(1) 7-14. 

Dent, J. A., Poison, A. G., and Klymkowsky, M. W. (1989) A whole-mount immnnocytochemical analysis of the expression of the intermediate filament protein vimentin in Xenopns. Develop. 105, 61-74. 

Battifora, H. (1991) Assessment of antigen damage in immunohistochemistry the vimentin internal control. Anat. Pathol. 96, 669-671. 

The five intermediate filaments and their respective tissues are listed in Table 3. Only the intermediate filament, cytokeratin, is selected as useful in the initial classification of tumors. The other intermediate filaments can cause diagnostic confusion because (1) they are usually not expressed in their poorly differentiated counterparts (especially GFAP, NFP, and Desmin) and (2) they are often coexpressed on many types of tumors. Poorly differentiated neuroectodermal tumors may often express more than two intermediate filaments. Vimentin demonstrates the most lineage infidelity. 

In fact, because of the ubiquitous expression of vimentin in tissues and its partial susceptibility to formalin fixation, some authors advocate including vimentin routinely as an internal positive control on all cases (/7). Since many useful molecules also show sensitivity to fixation or processing for histology, vimentin positivity may be used as a gauge of the general preserved antigenicity of the tissue being examined. 

Some sarcomas besides epithelioid sarcoma or synovial sarcoma show coexpression of vimentin and keratin (18). Examining these tumors for the expression of other epithelial markers will help clarify their true nature. 

The components of the intermediate filaments belong to five related protein families. They are specific for particular cell types. Typical representatives include the cytokeratins, desmin, vimentin, glial fibrillary acidic protein (GFAP), and neurofilament. These proteins all have a rod-shaped basic structure in the center, which is known as a superhelix ( coiled coil see keratin, p. 70). The dimers are arranged in an antiparallel fashion to form tet-ramers. A staggered head-to-head arrangement produces protofilaments. Eight protofilaments ultimately form an intermediary filament. 

In addition to the above proteins, a number of other proteins are also typical of muscle—including titin (the largest known protein), a- and j3-actinin, desmin, and vimentin. 

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