Five chemically distinct classes of intermediate filaments can be identified within higher eukaryotic cells. Each class is characteristic of a particular cell type. These filaments may function to integrate mechanically the various structures of the cytoplasmic space in a way that is tailored to the differentiated state of the cell.

Intermediate filaments as mechanical integrators of cellular space

The occurrence of vimentin, a specific intermediate filament protein, has been studied by immunoflourescence microscopy in tissue of adult and embryonic brain as well as in cell cultures from nervous tissue By double imminofluorescence labeling, the distribution of vimentin has been compared with that of subunit proteins of other types of intermediate filaments (glial fibrillary acidic [GFA] protein, neurofilament protein, prekeratin) and other cell-type specific markers (fibronectin, tetanus toxin receptor, 04 antigen) In adult brain tissue, vimentin is found not only in fibroblasts and cells of larger blood vessels but also in ependymal cells and astrocytes In embryonic brain tissue, vimentin is detectable as early as embryonic day 11, the earliest stage tested, and is located in radial fibers spanning the neural tube, in ventricular cells, and in blood vessels At all stages tested, oligodendrocytes and neurons do not express detectable amounts of vimentin In primary cultures of early postnatal mouse cerebellum, a coincident location of vimentin and GFA protein is seen in astrocytes, and both types of filament proteins are included in the perinuclear aggregates formed upon exposure of the cells to colcemid In cerebellar cell cultures of embryonic-day-13 mice, vimentin is seen in various cell types of epithelioid or fibroblastlike morphology but is absent from cells expressing tetanus toxin receptors Among these embryonic, vimentin-positive cells, a certain cell type reacting neither with tetanus toxin nor with antibodies to fibronectin or GFA protein has been tentatively identified as precursor to more mature astrocytes The results show that, in the neuroectoderm, vimentin is a specific marker for astrocytes and ependymal cells It is expressed in the mouse in astrocytes and glial precursors well before the onset of GFA protein expression and might therefore serve as an early marker of glial differentiation Our results show that vimentin and GFA protein coexist in one cell type not only in primary cultures in vitro but also in the intact tissue in situ

https://rupress.org/jcb/article-pdf/90/2/435/1266911/435.pdf

Immunocytochemical demonstration of vimentin in astrocytes and ependymal cells of developing and adult mouse nervous system.

All classes of intermediate filaments share a common antigenic determinant defined by a monoclonal antibody

Granulation tissue fibroblasts (myofibroblasts) develop several ultrastructural and biochemical features of smooth muscle (SM) cells, including the presence of microfilament bundles and the expression of alpha-SM actin, the actin isoform present in SM cells and myoepithelial cells and particularly abundant in vascular SM cells. Myofibroblasts have been suggested to play a role in wound contraction and in retractile phenomena observed during fibrotic diseases. When contraction stops and the wound is fully epithelialized, myofibroblasts containing alpha-SM actin disappear, probably as a result of apoptosis, and the scar classically becomes less cellular and composed of typical fibroblasts with well-developed rough endoplasmic reticulum but with no more microfilaments. In contrast, alpha-SM actin expressing myofibroblasts persist in hypertrophic scars and in fibrotic lesions of many organs, including stroma reaction to epithelial tumours, where they are allegedly involved in retractile phenomena as well as in extracellular matrix accumulation. The mechanisms leading to the development of myofibroblastic features remain to be investigated. In vivo and in vitro investigations have shown that gamma-interferon exerts an antifibrotic activity at least in part by decreasing alpha-SM actin expression whereas heparin increases the proportion of alpha-SM actin positive cells. Recently, we have observed that the subcutaneous administration of transforming growth factor-beta 1 to rats results in the formation of a granulation tissue in which alpha-SM actin expressing myofibroblasts are particularly abundant. Other cytokines and growth factors, such as platelet-derived growth factor, basic fibroblast growth factor and tumour necrosis factor-alpha, despite their profibrotic activity, do not induce alpha-SM actin in myofibroblasts. In conclusion, fibroblastic cells are relatively undifferentiated and can assume a particular phenotype according to the physiological needs and/or the microenvironmental stimuli. Further studies on fibroblast adaptation phenomena appear to be useful for the understanding of the mechanisms of development and regression of pathological processes such as wound healing and fibrocontractive diseases.

Heterogeneity of myofibroblast phenotypic features: an example of fibroblastic cell plasticity.

It took more than 100 years before it was established that the proteins that form intermediate filaments (IFs) comprise a unified protein family, the members of which are ubiquitous in virtually all differentiated cells and present both in the cytoplasm and in the nucleus. However, during the past 2 decades, knowledge regarding the functions of these structures has been expanding rapidly. Many disease-related roles of IFs have been revealed. In some cases, the molecular mechanisms underlying these diseases reflect disturbances in the functions traditionally assigned to IFs, i.e., maintenance of structural and mechanical integrity of cells and tissues. However, many disease conditions seem to link to the nonmechanical functions of IFs, many of which have been defined only in the past few years.

/pdf/introducing-intermediate-filaments-from-discovery-to-disease-4jkc88w10v.pdf

Introducing intermediate filaments: from discovery to disease.

Extraction of chicken embryo fibroblasts (CEF) or baby hamster kidney (BHK) cells with 1% Triton X-100 and 0.6 M KCl leaves an insoluble cytoskeletal residue composed primarily of the 52,000 Mr subunit of intermediate filaments (F-IFP). In addition, CEF cytoskeletons exhibit a minor component with Mr of 50,000, identified as alpha-desmin, one of the two major isoelectric variants of the intermediate filament subunit from smooth muscle. BHK cytoskeletons contain the 50,000 Mr mammalian desmin variant. Cytoskeletons prepared from chicken embryonic myotubes contain F-IFP and both alpha- and beta-desmin. These data suggest that two distinct 10-nm filament subunits coexist in a single cell. One-dimensional peptide analysis of F-IFP and desmin from avian and mammalian cells reveals significant interspecies homology, as well as homology between F-IFP and desmin from the same species. Peptide analyses of 32P-labeled intermediate filament subunits suggest that there is considerable similarity in the phosphorylation sites of these proteins. These results indicate that F-IFP and desmin might be evolutionally related.

Coexistence of desmin and the fibroblastic intermediate filament subunit in muscle and nonmuscle cells: identification and comparative peptide analysis.

Paul B. Bell

Papers

Coexistence of desmin and the fibroblastic intermediate filament subunit in muscle and nonmuscle cells: identification and comparative peptide analysis.