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The intermediate filament proteins (desmin, vimentin, nestin, synemins and paranemin) synthesized by muscle cells depends on the type of muscle and its stage of development. Desmin is present in all muscles at all stages of development. The others appear transiently or in only certain muscles. The muscles of mice lacking desmin and those of human having a mutated desmin gene that encodes a nonfunctional desmin are abnormal. The severity of the human disease depends on the location of the mutation; it may cause skeletal myopathies, cardiomyopathies or altered vascular elasticity. This report summarizes the function of the desmin gene in the skeletal and smooth muscles, the gene regulation and desmin-related myopathies.

Astroglial cells (Fig. 1) are the most abundant cells in the mammalian central nervous system (CNS), and we only now start to fully realize their importance both in health and disease. Upregulation of intermediate filaments (IFs) has been a well-known hallmark of astrocytes activated by a disease process. This chapter focuses on the function of IF proteins and IFs in astroglial cells both in health and in pathological situations, such as severe mechanical or osmotic stress, hypoxia, brain and spinal cord injury as well as in CNS regeneration.

Neuronal intermediate filaments (IFs) are major components of the cytoskeleton in neurons and are composed of a family of neuronal IF proteins that include peripherin, α-internexin, and neurofilament triplet proteins (NFTPs), designated NF-L, NF-M, and NF-H for low, medium, and high molecular weight subunits, respectively. NFTPs are determinants of axonal caliber, while α-internexin may play a role in neuronal regeneration and peripherin in proper development of a defined population of sensory neurons. The composition and proper stoichiometry of these proteins are important for IF assembly and transport. Abnormal IF accumulations in neuronal perikarya and proximal axons are pathological hallmarks of many neurodegenerative diseases. Studies of transgenic mouse models as well as identification of NF-H mutations in sporadic amyotrophic lateral sclerosis (ALS) and NF-L mutations in Charcot-Marie-Tooth disease indicate that neuronal IFs have a direct role in pathogenesis of these diseases. Overexpression of individual NFTP subunit or peripherin in transgenic mice causes abnormal accumulation of IFs in motor neurons and motor dysfunction, reminiscent of ALS, whereas overexpression of α-internexin causes Purkinje cell death and motor coordination deficits. Deletion of NF-L or overexpression of NF-H extends the lifespan of transgenic mice expressing mutated Cu/Zn superoxide dismutase (SOD1), suggesting that NFTPs may also contribute to SOD1 mutation-linked ALS. The misaccumulated IFs in neuronal perikarya and proximal axons are disorganized and aberrantly phosphorylated. The kinases and phosphatases that are known to control phosphorylation of NFTPs are deregulated in the disease state. Studies of transgenic mouse models show that axonal transport of neuronal IFs and other cellular components is impaired in the disease state, ultimately leading to neurodegeneration. Formation of disorganized and aberrantly phospohorylated IFs, changes in stoichiometric levels of neuronal IF proteins due to dysregulation of their gene expression, and reduced levels of functional motor proteins may contribute in part to this defective axonal transport. Recent studies raise the possibility that cis-acting and trans-acting determinants of NF-L mRNA stability may also be involved in degeneration of motor neurons. Taken together, these data suggest that abnormal IF accumulations in neurons may not simply be the by-products of neurodegenerative diseases but may instead play a contributory role in pathogenesis of these diseases.

Neurofilaments belong to the Class IV family of Intermediate filaments and are neuron-specific. They are classed into three distinct groups according to their molecular masses; NF-H (heavy chain), NF-M (middle-chain) and NF-L (light chain). Together with microtubules and their associated proteins, neurofilaments make up the dynamic axonal cytoskeleton. Neurofilaments comprise a central alpha helical coil-coiled domain flanked by an amino terminal head domain and in the case of NF-H and NF-M, a hypervariable carboxy-terminal tail domain. Neurofilaments participate in dynamic properties of the axonal cytoskeleton such as axon outgrowth, axonal transport and the control of axonal caliber. They contain multiple phosphorylation sites in their amino-head and carboxy-tail domains that are phosphorylated topographically by a number of kinases and the phosphorylation of neurofilaments is related to their functions. Expression and phosphorylation of neurofilaments is developmentally regulated and most of the phosphorylation occurs in the tail domains in the mature nervous system. Kinases that have been found to phosphorylate neurofilaments include PKA, CKI, CKII CaMK and the proline directed kinases. The proline directed kinases such as Cdk5, members of the MAP kinase family (p38, SAPK and ERK1/2) and GSK3 phosphorylate the KSP repeats found in NF-M and NF-H tail domains. Aberrant hyperphosphorylation of neurofilaments in dendrites and cell bodies are seen in neurodegenerative diseases such as Amyotrophic Lateral Sclerosis, Alzheimer’s disease, Parkinson’s disease, Pick’s disease and Dementia with Lewy bodies. Thus, defects in compartmentalization of cytoskeletal protein phosphorylation may contribute to pathology seen in these diseases. Neurofilament phosphorylation is affected by signal transduction pathways. Calcium influx into neurons causes the phosphorylation of NF-M through the activation of the ERK1/2. Integrin mediated signaling also causes the phosphorylation of NF-H through the activation of Cdk5 activity. Recent studies have also shown that kinase cascades can be affected by myelin associated glycoprotein (MAG), a major glial protein found in periaxonal membranes of glial cells. MAG appears to be involved in bi-directional signaling affecting axonal properties such as axonal caliber, phosphorylation of neurofilaments and mediating the activity of ERK1/2 and Cdk5. Thus, signal transduction pathways are involved in the phosphorylation of the KSP repeats found in NF-M and NF-H.

Aquestion that still challenges cell and tissue biologists is that of the driving forces that have selected for and conserved the numerous intermediate filament proteins in vertebrates. We are only beginning to understand the functions of this family of cyto-skeleton structures and we have very few experimental tools for testing comparative functions of these proteins in a satisfactory way. A major turning point came with the discoveries that mutations in keratin intermediate filament genes were responsible for a large number of inherited skin fragility disorders. These disease links showed unequivocally that intermediate filament proteins, at least in barrier epithelia like skin, provide essential stress resilience for cells in tissues. The keratin genes account for three quarters of all the intermediate filament genes identified in the human genome, and it seems highly likely that any function attributable these structural proteins will also be important for the nonkeratin intermediate filaments. Once the stress resistance function of intermediate filaments is taken as a fact, rather than a persistent speculation, this knowledge can guide further experimental analysis and design to allow us to at last move closer to understanding the biology of these enigmatic cytoskeleton filaments.

Keratin K6 constitutes a special case among the keratin intermediate filaments. It is constitutively expressed in several stratified epithelia, but is also induced by several stimuli, many of which are related to hyperproliferation. In addition, this keratin is, unlike others, encoded by several genes, which give rise to similar but not identical forms. In recent years, considerable advances have been made in the identification of new K6 genes and the understanding of the function of K6. Here I review the present knowledge about the human, murine and bovine keratin K6 genes, in particular with regard to the differences in sequence among the different isoforms and their different regulation. Hints about the possible K6 biological function that are suggested by the study of murine models of overexpression and gene inactivation, as well as by the study of human diseases due to mutations in K6, are also discussed.

Keratin synthesis is regulated at the level of transcription. Each keratin gene appears to be regulated by a characteristic constellation of transcription factors and DNA binding sites. Often these occur in clusters and complexes, providing a mechanism for fine-tuning the expression levels. Most commonly, the important regulatory sites are found in the promoter regions, infrequently coding and downstream sequences also play a role. Transcription factors Sp1, AP1 and AP2 are important components in regulation of many keratin genes, and the nuclear receptors for retinoic acid and thyroid hormone also regulate majority of keratins. In addition, the expression of most keratin genes can be modulated by extracellular signals, such as growth factors. Universal or general regulators for all keratin genes have not been found; apparently each keratin protein has its own, characteristic circuits and machinery for regulation of expression.

Keratins K8 and K18 are the major components of the intermediate-filament cytoskeleton of simple epithelia. They are mostly expressed in internal one-layered epithelia. K8 and K18 are considered the ancestral precursor genes for the multiple specialized type II and type I keratin classes, respectively, and constitute the first keratin genes to be expressed in the embryo. Herein we focused on the current knowledge of the functions ascribed to simple epithelial keratins and the diseases associated with them. Because K8/K18 expression is naturally associated with cells with a great proliferation potential, such as cells of embryonic structures and cells in the later stages of cancer, we centered on the relationship between altered expression of K8/K18 and development and progression of tumors.

The keratin cytoskeleton of hepatocytes is affected in a variety of chronic liver diseases, such as alcoholic and nonalcoholic steatohepatitis (ASH, NASH), copper toxicosis, cholestasis and hepatocellular carcinoma. In these diseases hepatocytes reveal a derangement or even loss of the cytoplasmic keratin intermediate filament cytoskeleton and formation of cytoplasmic inclusions (Mallory bodies) consisting of misfolded and aggregated keratin as well as a variety of stress proteins. Keratin gene knock-out mice demonstrated that keratins fulfil besides a structural role providing mechanical stability to hepatocytes a role as target and modulator of toxic stress responses in that keratins interact with a variety of stress-related signaling pathways, are preferred targets of stress-induced protein misfolding and are substrates for caspases. Furthermore, the identification of mutations in keratin genes in patients with liver cirrhosis suggests that keratins act as genetic modifiers in liver diseases.

The main function of the epidermis is to provide an essential barrier between the individual and the environment. This primarily stems from a finely regulated process of differentiation occurring in this stratified epithelium. Keratins are the most abundant proteins in this tissue and provide resistance against mechanical stress to the epidermal cells. Moreover, characteristic changes in the pattern of expression of this family of proteins takes place during the process of differentiation in epidermis. The K5/K14 pair, characteristic of basal proliferative cells, is switched off at the differentiation onset, and the expression of keratin pair K1/K10 concomitantly starts. In addition, under several pathological or physiological hyperproliferative situations K1/K10 pair expression is down-regulated. These facts have led to the assumption that specific functions can be exerted by each specific keratin polypeptide. Here we will summarize the recent and past effort to elucidate these particular roles. Collectively, the data have indicated particular roles for keratin K10, as a possible modulator of epidermal homeostasis contributing to the modulation of several signaling pathways in keratinocytes. The possibility that these functions are probably not unique among the large family of keratins will open new exciting and interesting scientific fields.