The cytoskeleton and disease: genetic disorders of intermediate filaments

Hyper-adhesion in desmosomes: its regulation in wound healing and possible relationship to cadherin crystal structure

The resistance of tissues to physical stress is dependent upon strong cell-cell adhesion in which desmosomes play a crucial role. We propose that desmosomes fulfil this function by adopting a more strongly adhesive state, hyper-adhesion, than other junctions. We show that the hyper-adhesive desmosomes in epidermis resist disruption by ethylene glycol bis(2-aminoethyl ether)-N,N,N′N′-tetraacetic acid (EGTA) and are thus independent of Ca2+. We propose that Ca2+ independence is the normal condition for tissue desmosomes. Ca2+ independence is associated with an organised arrangement of the intercellular adhesive material exemplified by a dense midline. When epidermis is wounded, desmosomes in the wound-edge epithelium lose hyper-adhesiveness and become Ca2+ dependent, i.e. readily dissociated by EGTA. Ca2+-dependent desmosomes lack a midline and show narrowing of the intercellular space. We suggest that this indicates a less-organised, weakly adhesive arrangement of the desmosomal cadherins, resembling classical cadherins in adherens junctions. Transition to Ca2+ dependence on wounding is accompanied by relocalisation of protein kinase C α to desmosomal plaques suggesting that an `inside-out' transmembrane signal is responsible for changing desmosomal adhesiveness. We model hyper-adhesive desmosomes using the crystal packing observed for the ectodomain of C-cadherin and show how the regularity of this 3D array provides a possible explanation for Ca2+ independence.

Sperm tail abnormalities in mutant mice with neo(r) gene insertion into an intron of the keratin 9 gene

Keratin 9 (K9) is one of the components of the perinuclear ring of the manchette found in developing spermatids but is predominantly expressed in the epidermis of the footpad (palm and sole in human epidermis). As an initial step to determine the function of K9 protein in sperm development, we have generated a mutant mouse by homologous recombination of the targeting vector containing the disrupted K9 gene in which the neo(r) gene was inserted into the intron 6. This insertion resulted in the expression of two K9 mRNAs: a wild-type K9 mRNA, in which intron 6 with the neo(r) gene was completely spliced out, and a mutated form in which only a portion of the intron 6 between neo(r) gene and exon 7 was spliced out. While both heterozygous (K9(+/neo)) and homozygous (K9(neo/neo)) mutant mice expressed the wild-type form of K9 protein, the expression profile of the wild-type K9 in K9(neo/neo) mutants was modified. In addition, the open reading frame of the aberrant mRNA terminated at the exon 6/intron 6 splice site, resulting in a truncated K9 protein. Both K9(+neo) and K9(neo/neo) male mice displayed spermatids with ectopic manchette. Coiled tails were seen in maturing spermatids and epididymal sperm of mutant mice and sperm with deformed tails displayed forward motility. A predominant sperm anomaly was residual cytoplasm at the end of the mitochondria-containing middle piece tail segment. The residual cytoplasm displayed vesicles with random in situ motion, suggesting a transport impediment toward the distal end of the sperm tail. All mutant mice were fertile. Surprisingly, in oocyte nuclear injection experiments using K9(neo/neo) sperm donor, 76% of the resulting animals displayed a deletion of the neo(r) gene from the intron 6 of the mutated K9 allele. Results of this study support the view that intron 6 influences the transcriptional efficiency of the K9 gene by decreasing production of wild-type K9 and changing the expression of K9 proteins.

ROCK-I regulates closure of the eyelids and ventral body wall by inducing assembly of actomyosin bundles

Rho-associated kinase (ROCK) I mediates signaling from Rho to the actin cytoskeleton. To investigate the in vivo functions of ROCK-I, we generated ROCK-I–deficient mice. Loss of ROCK-I resulted in failure of eyelid closure and closure of the ventral body wall, which gave rise to the eyes open at birth and omphalocele phenotypes in neonates. Most ROCK-I^−/− mice died soon after birth as a result of cannibalization of the omphalocele by the mother. Actin cables that encircle the eye in the epithelial cells of the eyelid were disorganized and accumulation of filamentous actin at the umbilical ring was impaired, with loss of phosphorylation of the myosin regulatory light chain (MLC) at both sites, in ROCK-I^−/− embryos. Stress fiber formation and MLC phosphorylation induced by EGF were also attenuated in primary keratinocytes from ROCK-I^−/− mice. These results suggest that ROCK-I regulates closure of the eyelids and ventral body wall through organization of actomyosin bundles.

Glial fibrillary acidic protein mutations in infantile, juvenile, and adult forms of Alexander disease

Alexander disease is a progressive, usually fatal neurological disorder defined by the widespread and abundant presence in astrocytes of protein aggregates called Rosenthal fibers. The disease most often occurs in infants younger than 2 years and has been labeled a leukodystrophy because of an accompanying severe myelin deficit in the frontal lobes. Later onset forms have also been recognized based on the presence of abundant Rosenthal fibers. In these cases, clinical signs and pathology can be quite different from the infantile form, raising the question whether they share the same underlying cause. Recently, we and others have found pathogenic, de novo missense mutations in the glial fibrillary acidic protein gene in most infantile patients examined and in a few later onset patients. To obtain further information about the role of glial fibrillary acidic protein mutations in Alexander disease, we analyzed 41 new patients and another 3 previously described clinically, including 18 later onset patients. Our results show that dominant missense glial fibrillary acidic protein mutations account for nearly all forms of this disorder. They also significantly expand the catalog of responsible mutations, verify the value of magnetic resonance imaging diagnosis, indicate an unexpected male predominance for the juvenile form, and provide insights into phenotype-genotype relations.

The novel protein PTPIP51 exhibits tissue- and cell-specific expression

The expression patterns of both mRNA and protein of the novel protein tyrosine phosphatase interacting protein 51 (PTPIP51) were studied in various organs by in situ hybridization, immunoblotting, and immunocytochemistry. The protein was found in all mammalian species investigated: guinea pig, rat, mouse, pig, and human. The presence of the protein was, however, restricted to specific organs. High levels of PTPIP51 were found in epidermis and seminiferous epithelium. The expression appears to be associated with distinct stages of differentiation. While basal cells in the epidermis and spermatogonia showed no perceptible amount of PTPIP51, keratinocytes of suprabasal layers and differentiating first-order spermatocytes up to spermatids exhibited high expression. In skeletal muscle, the presence of PTPIP51 was restricted to fibers of the fast twitch type. In surface epithelia containing ciliated cells, the protein was associated with the microtubular structures responsible for ciliary movement. Furthermore, specific structures of the central nervous system, for example, neurons of the hippocampal region, ganglion cells of the autonomic nervous system, and axons of the peripheral nervous system showed a distinct staining pattern with the antibody to PTPIP51. Our data suggest that PTPIP51 might be involved in the regulation of cellular processes associated with differentiation, movement, or cytoskeletal organization.

Desmin: molecular interactions and putative functions of the muscle intermediate filament protein

Desmin is the intermediate filament (IF) protein occurring exclusively in muscle and endothelial cells. There are other IF proteins in muscle such as nestin, peripherin, and vimentin, besides the ubiquitous lamins, but they are not unique to muscle. Desmin was purified in 1977, the desmin gene was characterized in 1989, and knock-out animals were generated in 1996. Several isoforms have been described. Desmin IFs are present throughout smooth, cardiac and skeletal muscle cells, but can be more concentrated in some particular structures, such as dense bodies, around the nuclei, around the Z-line or in costameres. Desmin is up-regulated in muscle-derived cellular adaptations, including conductive fibers in the heart, electric organs, some myopathies, and experimental treatments with drugs that induce muscle degeneration, like phorbol esters. Many molecules have been reported to associate with desmin, such as other IF proteins (including members of the membrane dystroglycan complex), nebulin, the actin and tubulin binding protein plectin, the molecular motor dynein, the gene regulatory protein MyoD, DNA, the chaperone alphaB-crystallin, and proteases such as calpain and caspase. Desmin has an important medical role, since it is used as a marker of tumors' origin. More recently, several myopathies have been described, with accumulation of desmin deposits. Yet, after almost 30 years since its identification, the function of desmin is still unclear. Suggested functions include myofibrillogenesis, mechanical support for the muscle, mitochondrial localization, gene expression regulation, and intracellular signaling. This review focuses on the biochemical interactions of desmin, with a discussion of its putative functions.

Intermediate filaments: molecular structure, assembly mechanism, and integration into functionally distinct intracellular Scaffolds

The superfamily of intermediate filament (IF) proteins contains at least 65 distinct proteins in man, which all assemble into approximately 10 nm wide filaments and are principal structural elements both in the nucleus and the cytoplasm with essential scaffolding functions in metazoan cells. At present, we have only circumstantial evidence of how the highly divergent primary sequences of IF proteins lead to the formation of seemingly similar polymers and how this correlates with their function in individual cells and tissues. Point mutations in IF proteins, particularly in lamins, have been demonstrated to lead to severe, inheritable multi-systemic diseases, thus underlining their importance at several functional levels. Recent structural work has now begun to shed some light onto the complex fine tuning of structure and function in these fibrous, coiled coil forming multidomain proteins and their contribution to cellular physiology and gene regulation.

Self-assembly of the cytoskeletal glial fibrillary acidic protein is inhibited by an isoform-specific C terminus

The predominant isoform of glial fibrillary acidic protein (GFAP), GFAPalpha, is the characteristic building block of the cytoskeletal intermediate filaments in astrocytes. Isoform GFAPepsilon, produced by alternative splicing of the GFAP gene, includes a new tail domain that confers a presenilin binding capacity. We here show that the GFAPepsilon tail prevents GFAPepsilon homodimerization and homomeric filament formation, whereas the ability to form heterodimers and filaments with GFAPalpha is retained. Furthermore, GFAPepsilon shows decreased affinity for several GFAPalpha-interacting proteins. A GFAPepsilon tail mutation that results in gain of GFAPepsilon dimerization and filament formation abolishes presenilin binding. This mutation also abolishes interaction between the tail and the coiled-coil domain of GFAPepsilon. Together, this indicates that direct interaction between the coiled-coil and tail domains may serve as an inhibitory mechanism for homomeric dimerization and filament formation. We propose that the GFAPepsilon isoform represents a new functionally distinct component of GFAP intermediate filaments.

Intermediate filaments are required for C. elegans epidermal elongation

Cytoplasmic intermediate filaments (cIFs) are thought to provide mechanical strength to vertebrate cells; however, their function in invertebrates has been largely unexplored. The Caenorhabditis elegans genome encodes multiple cIFs. The C. elegans ifb-1 locus encodes two cIF isoforms, IFB-1A and IFB-1B, that differ in their head domains. We show that both IFB-1 isoforms are expressed in epidermal cells, within which they are localized to muscle-epidermal attachment structures. Reduction in IFB-1A function by mutation or RNA interference (RNAi) causes epidermal fragility, abnormal epidermal morphogenesis, and muscle detachment, consistent with IFB-1A providing mechanical strength to epidermal attachment structures. Reduction in IFB-1B function causes morphogenetic defects and defective outgrowth of the excretory cell. Reduction in function of both IFB-1 isoforms results in embryonic arrest due to muscle detachment and failure in epidermal cell elongation at the 2-fold stage. Two other cIFs, IFA-2 and IFA-3, are expressed in epidermal cells. We show that loss of function in IFA-3 results in defects in morphogenesis indistinguishable from those of embryos lacking ifb-1. In contrast, IFA-2 is not required for embryonic morphogenesis. Our data indicate that IFB-1 and IFA-3 are likely the major cIF isoforms in embryonic epidermal attachment structures.

An autocrine/paracrine loop linking keratin 14 aggregates to tumor necrosis factor alpha-mediated cytotoxicity in a keratinocyte model of epidermolysis bullosa simplex

Epidermolysis bullosa simplex (EBS) is a blistering cutaneous disease featuring protein aggregates. Here we investigate the molecular mechanisms linking protein aggregates to cell death in a cellular model of EBS in which HaCaT keratinocytes are transfected with plasmids expressing various mutant forms of keratin 14 (K14). In HaCaT cells, mutant K14 was found to form ubiquitinated protein aggregates that suppressed 20 S proteasome function instead of being degraded by 20 S proteasome. Keratinocytes with mutant K14-induced phosphorylation of the stress-activated kinase c-Jun, as well as up-regulation of unfolding protein Bip, indicates induction of endoplasmic reticulum stress. HaCaT cells were susceptible to apoptosis by activation of caspases-3, and -8, but not caspase-9 or -12. Tumor necrosis factor-alpha (TNFalpha) in the culture medium was increased in keratinocytes with mutant K14 compared with wild K14, and the addition of neutralizing anti-TNFalpha antibody to the culture medium rescued keratinocytes from cell death. Thus, TNFalpha release and the subsequent activation of the TNFalpha receptor by an autocrine/paracrine pathway links protein aggregates to cell death in this keratinocyte EBS cellular model. Furthermore, mutation in K14 reduced its affinity to TNFalpha receptor-associated death domain (TRADD), suggesting that the susceptibility of keratinocytes to caspase-8-mediated apoptosis is increased in mutated K14 because of impairment of the cytoprotective mechanism mediated by K14-TRADD interaction.

Plasma Membrane Disruption: Repair, Prevention, Adaptation

Many metazoan cells inhabit mechanically stressful environments and, consequently, their plasma membranes are frequently disrupted. Survival requires that the cell rapidly repair or reseal the disruption. Rapid resealing is an active and complex structural modification that employs endomembrane as its primary building block, and cytoskeletal and membrane fusion proteins as its catalysts. Endomembrane is delivered to the damaged plasma membrane through exocytosis, a ubiquitous Ca2+-triggered response to disruption. Tissue and cell level architecture prevent disruptions from occurring, either by shielding cells from damaging levels of force, or, when this is not possible, by promoting safe force transmission through the plasma membrane via protein-based cables and linkages. Prevention of disruption also can be a dynamic cell or tissue level adaptation triggered when a damaging level of mechanical stress is imposed. Disease results from failure of either the preventive or resealing mechanisms.

Functional complexity of intermediate filament cytoskeletons: from structure to assembly to gene ablation

The cell biology of intermediate filament (IF) proteins and their filaments is complicated by the fact that the members of the gene family, which in humans amount to at least 65, are differentially expressed in very complex patterns during embryonic development. Thus, different tissues and cells express entirely different sets and amounts of IF proteins, the only exception being the nuclear B-type lamins, which are found in every cell. Moreover, in the course of evolution the individual members of this family have, within one species, diverged so much from each other with regard to sequence and thus molecular properties that it is hard to envision a unifying kind of function for them. The known epidermolytic diseases, caused by single point mutations in keratins, have been used as an argument for a role of IFs in mechanical "stress resistance," something one would not have easily ascribed to the beaded chain filaments, a special type of IF in the eye lens, or to nuclear lamins. Therefore, the power of plastic dish cell biology may be limited in revealing functional clues for these structural elements, and it may therefore be of interest to go to the extreme ends of the life sciences, i.e., from the molecular properties of individual molecules including their structure at the atomic level to targeted inactivation of their genes in living animals, mouse, and worm to define their role more precisely in metazoan cell physiology.

Acroplaxome, an F-actin-keratin-containing plate, anchors the acrosome to the nucleus during shaping of the spermatid head

Nuclear shaping is a critical event during sperm development as demonstrated by the incidence of male infertility associated with abnormal sperm ad shaping. Herein, we demonstrate that mouse and rat spermatids assemble in the subacrosomal space a cytoskeletal scaffold containing F-actin and Sak57, a keratin ortholog. The cytoskeletal plate, designated acroplaxome, anchors the developing acrosome to the nuclear envelope. The acroplaxome consists of a marginal ring containing keratin 5 10-nm-thick filaments and F-actin. The ring is closely associated with the leading edge of the acrosome and to the nuclear envelope during the elongation of the spermatid head. Anchorage of the acroplaxome to the gradually shaping nucleus is not disrupted by hypotonic treatment and brief Triton X-100 extraction. By examining spermiogenesis in the azh mutant mouse, characterized by abnormal spermatid/sperm head shaping, we have determined that a deformity of the spermatid nucleus is restricted to the acroplaxome region. These findings lead to the suggestion that the acroplaxome nucleates an F-actin-keratin-containing assembly with the purpose of stabilizing and anchoring the developing acrosome during spermatid nuclear elongation. The acroplaxome may also provide a mechanical planar scaffold modulating external clutching forces generated by a stack of Sertoli cell F-actin-containing hoops encircling the elongating spermatid nucleus.

Mallory body formation in primary biliary cirrhosis is associated with increased amounts and abnormal phosphorylation and ubiquitination of cytokeratins

BACKGROUND/AIMS

Animal studies revealed a key role of toxic bile acids in the regulation of hepatocytic cytokeratin (CK) expression and Mallory body (MB) formation. In this study, we compared CK expression, phosphorylation, and ubiquitination in primary biliary cirrhosis (PBC), chronic hepatitis C (CHC) and control livers to determine whether bile acid-induced CK alterations are associated with cytoskeletal alterations and MB formation in a prototypic chronic cholestatic liver disease.

METHODS

CK 8 and CK 18 mRNA and protein levels were investigated by reverse transcriptase-polymerase chain reaction and Western blotting. Intermediate filament (IF) cytoskeletal alterations were assessed by immunofluorescence microscopy using antibodies against CKs, CK phosphoepitopes, MBs, and ubiquitin.

RESULTS

Despite unchanged mRNA levels, CK 8 and CK 18 protein levels were significantly elevated in PBC suggesting stabilization of CKs, possibly due to decreased degradation. CK-IF alterations in PBC comprised increased density with abnormal phosphorylation of the IF network of hepatocytes in acinar zone 1 and in the periphery of cirrhotic nodules. In addition, in these areas hepatocytes with diminished IF network containing MBs consisting of abnormally phosphorylated and ubiquitinated CK were observed.

CONCLUSIONS

These findings support our concept that IF cytoskeletal alterations and MB formation in cholestatic liver diseases are related to bile acid-induced cell stress.

Genetics of dark skin in mice

Chemical mutagenesis in the mouse is a powerful approach for phenotype-driven genetics, but questions remain about the efficiency with which new mutations ascertained by their phenotype can be localized and identified, and that knowledge applied to a specific biological problem. During a global screen for dominant phenotypes in about 30,000 animals, a novel class of pigmentation mutants were identified by dark skin (Dsk). We determined the genetic map location, homozygous phenotype, and histology of 10 new Dsk and 2 new dark coat (Dcc) mutations, and identified mutations in Agouti (Met1Leu, Dcc4), Sox18 (Leu220ter, Dcc1), Keratin 2e (Thr500Pro, Dsk2), and Egfr (Leu863Gln, Dsk5). Cutaneous effects of most Dsk mutations are limited to melanocytes, except for the Keratin 2e and Egfr mutations, in which hyperkeratosis and epidermal thickening precede epidermal melanocytosis by 3-6 wk. The Dsk2 mutation is likely to impair intermediate filament assembly, leading to cytolysis of suprabasal keratinocytes and secondary hyperkeratosis and melanocytosis. The Dsk5 mutation causes increased tyrosine kinase activity and a decrease in steady-state receptor levels in vivo. The Dsk mutations represent genes or map locations not implicated previously in pigmentation, and delineate a developmental pathway in which mutations can be classified on the basis of body region, microscopic site, and timing of pigment accumulation.

Deletion Mutagenesis of the Amino-Terminal Head Domain of Vimentin Reveals Dispensability of Large Internal Regions for Intermediate Filament Assembly and Stability

Previous studies have shown that the non-alpha-helical head domain of vimentin is required for polymerization of intermediate filaments (IFs) and, furthermore, a nonapeptide highly conserved among type III IF subunit proteins at their extreme amino-terminus is essential for this process. Recombinant DNA technology was employed to produce specific vimentin deletion mutant proteins (for in vitro studies) or vimentin protein expression plasmids (for in vivo studies), which were used to identify other regions of the vimentin head domain important for polymerization. Various vimentin proteins lacking either residues 25-38, 44-95, or 40-95 polymerized into wild-type or largely normal IFs, both in vitro and in vivo. Vimentin proteins lacking residues 44-69 or 25-63 failed to form IFs in vitro, but assembled into IFs in vivo. Vimentin proteins lacking residues 25-68, 44-103, or 88-103 failed to form IFs in vitro or in vivo. Taken together with previous results, these data demonstrate that the middle of the vimentin non-alpha-helical head domain, which is known to be the site of nucleic acid binding, is completely dispensable for IF formation, whereas both ends of the vimentin non-alpha-helical head domain are required for IF formation. The simplest explanation for these results is that the middle of the vimentin non-alpha-helical head domain loops out, thereby permitting the juxtaposition of the ends of the head domain and their productive interaction with other protein domains (probably the C-terminus of the rod domain) during IF polymerization. The ability of some of the mutant proteins to form IFs in vivo, but not in vitro, suggests that as-yet-unknown cellular proteins may interact with and, in some cases, enable polymerization of IFs, even though they are not absolutely required for IF formation by wild-type vimentin.

Beyond structure: do intermediate filaments modulate cell signalling?

Intermediate filament (IF) proteins form the largest family of cytoskeletal proteins in mammalian cells. The function of these proteins has long been thought to be only structural. However, this single function does not explain their diverse tissue- and differentiation-specific expression patterns. Evidence is now emerging that IF also act as an important framework for the modulation and control of essential cell processes, in particular, signal transduction events. Here, we review the most recent developments in this growing and exciting new field.

A new splice variant of glial fibrillary acidic protein, GFAP epsilon, interacts with the presenilin proteins

We describe a new human isoform, GFAP epsilon, of the intermediary filament protein GFAP (glial fibrillary acidic protein). GFAP epsilon mRNA is the result of alternative splicing and a new polyadenylation signal, and thus GFAP epsilon has a new C-terminal protein sequence. This provides GFAP epsilon with the capacity for specific binding of presenilin proteins in yeast and in vitro. Our observations suggest a direct link between the presenilins and the cytoskeleton where GFAP epsilon is incorporated. Mutations in GFAP and presenilins are associated with Alexander disease and Alzheimer's disease, respectively. Accordingly, GFAP epsilon should be taken into consideration when studying neurodegenerative diseases.

GFAP mutations in Alexander disease

Alexander disease is a rare but often fatal disease of the central nervous system. Infantile, juvenile and adult forms have been described that present with different clinical signs, but are unified by the characteristic presence in astrocytes of Rosenthal fibers-protein aggregates that contain glial fibrillary acidic protein (GFAP) and small stress proteins. The chance discovery that mice expressing a human GFAP transgene formed abundant Rosenthal fibers suggested that mutations in the GFAP gene are a cause of Alexander disease. Sequencing results from several laboratories have indeed now identified GFAP coding mutations in most cases of the disease, including both the infantile and juvenile forms. These mutations have been found in the 1A, 2A and 2B segments of the conserved central rod domain of GFAP, and also in the variable tail region. All changes detected are heterozygous missense mutations, and none has been found in any parent of a patient that has been tested. This indicates that most cases of Alexander disease arise through de novo, dominant, GFAP mutations. Many of these mutations are homologous to ones described in other intermediate filament diseases. These other diseases have been attributed to a dominant loss of function, as the intermediate filament network is usually disrupted and a similar phenotype is observed in mice in which the corresponding intermediate filament gene has been inactivated. However, astrocytes of Alexander disease patients have normal appearing intermediate filaments, and GFAP null mice do not display the symptoms or pathology of Alexander disease. Thus, Alexander disease likely results from a dominant gain of function. Drawing upon the homology of many of the Alexander disease mutations to those found in other intermediate filament diseases, it is suggested that the gain of function is due to a partial block of filament assembly that leads to accumulation of an intermediate that participates in toxic interactions.

Keratin-mediated resistance to stress and apoptosis in simple epithelial cells in relation to health and disease

Epithelial cells such as hepatocytes exhibit highly polarized properties as a result of the asymmetric distribution of subsets of receptors at unique portions of the surface membrane. While the proper targeting of these surface receptors and maintenance of the resulting polarity depend on microtubules (MTs), the Golgi sorting compartment, and different actin-filament networks, the contribution of keratin intermediate filaments (IFs) has been unclear. Recent data show that the latter cytoskeletal network plays a predominant role in providing resistance to various forms of stress and to apoptosis targeted to the surface membrane. In this context, we first summarize our knowledge of the domain- or assembly-related features of IF proteins and the dynamic properties of IF networks that may explain how the same keratin pair K8/K18 can exert multiple resistance-related functions in simple epithelial cells. We then examine the contribution of linker protein(s) that integrate interactions of keratin IFs with MTs and the actin-cytoskeleton network, polarity-dependent surface receptors and cytoplasmic organelles. We next address likely molecular mechanisms by which K8/K18 can selectively provide resistance to a mechanical or toxic stress, or to Fas-mediated apoptosis. Finally, these issues on keratin structure-function are examined within a context of pathological anomalies emerging in tissue architecture as a result of natural or targeted mutations, or posttranslational modifications at specific amino acid residues. Clearly. the data accumulated in recent years provide new and significant insights on the role of K8/K18, particularly under conditions where polarized cells resist to stressful or apoptotic insults.

Keratin expression in human tissues and neoplasms

Keratin expression in human tissues and neoplasms Keratin filaments constitute type I and type II intermediate filaments (IFs), with at least 20 subtypes named keratin 1-20. Since certain keratin subtypes are only expressed in some normal human tissues but not others, and vice versa, various tissues have been subclassified according to the pattern of keratin staining. Simple epithelia generally express the simple epithelial keratins 7, 18, 19, and 20, while complex epithelia express complex epithelial keratins 5/6, 10, 14, and 15. When an epithelium undergoes malignant transformation, its keratin profile usually remains constant. The constitution and expression patterns of keratin filaments in human epithelial neoplasms are complex and often distinctive. In this article, we first briefly review the molecular and cell biology of keratin filaments. We then focus on the expression patterns of keratin filaments in various human neoplasms.

Keratins: unraveling the coordinated construction of scaffolds in spermatogenic cells

Recent work shows that two groups of keratins are expressed during mammalian spermatogenesis. One group, belonging to the classic epidermis-type keratins, is present in spermatogonia, spermatocytes, and spermatids. A member of this group, Sak57, a keratin 5 homologue, has been shown to co-align with microtubules and provide a scaffolding shell while also strengthening intercellular cytoplasmic bridges conjoining members of spermatogonial and spermatocyte cohorts. The other, keratin 9, is a component of the perinuclear ring of the manchette, a microtubular structure developed during the elongation and condensation of the spermatid nucleus. The second group, the outer dense fiber (Odf) proteins, is expressed preferentially during mammalian spermiogenesis. The family of Odf proteins-Odf1, Odf2, and Odf3-includes an expanding group of proteins co-assembled along the axoneme during the development of the sperm tail. Investigations on the assembly of epidermis-type and Odf sperm tail-targeted keratins are now focused on a group of chaperone-like Odf-binding molecules, designated Spags. Spags appear to drive Odfs to a precise destination. A daunting task is to determine how members of the family of keratins get the signal to produce linear scaffolds in specific spermatogenic cell populations and transport keratins to microtubule-containing structures such as the manchette and axoneme.

Heightened intrathecal release of axonal cytoskeletal proteins in multiple sclerosis is associated with progressive disease and clinical disability

The pathologic basis of disease progression in multiple sclerosis (MS) is thought to involve axonal degeneration, which contributes to the accumulation of neurological disability. Recent reports suggest that intrathecal concentrations of the neurofilament protein in relapsing remitting MS correlate with disease activity and the degree of disability. We sought to investigate the intrathecal levels of other cytoskeletal components of axons, primarily actin, tubulin and the light subunit of neurofilament (NFL) in patients with progressive MS and relevant controls and correlate results with clinical parameters of disease severity. Cerebrospinal fluid (CSF) concentrations of actin, tubulin and NFL were significantly increased in MS patients when compared to corresponding levels in patients with other inflammatory or non-inflammatory neurological diseases. Moreover, the intrathecal release of actin and tubulin, and to a lesser extent NFL, was significantly more marked in patients with primary and secondary progressive MS when compared to patients with relapsing remitting disease and was correlated with clinical disability. Our findings suggest that progressive MS is associated with the heightened intrathecal release of axonal cytoskeletal proteins, and that CSF actin, tubulin and NFL are reliable markers of axonal damage.

Regulated expression of human filaggrin in keratinocytes results in cytoskeletal disruption, loss of cell-cell adhesion, and cell cycle arrest

Filaggrin is an intermediate filament (IF)-associated protein that aggregates keratin IFs in vitro and is thought to perform a similar function during the terminal differentiation of epidermal keratinocytes. To further explore the role of filaggrin in the cytoskeletal rearrangement that accompanies epidermal differentiation, we generated keratinocyte cell lines that express human filaggrin using a tetracycline-inducible promoter system. Filaggrin expression resulted in reduced keratinocyte proliferation and caused an alteration in cell cycle distribution consistent with a post-G1 phase arrest. Keratin filament distribution was disrupted in filaggrin-expressing lines, while the organization of actin microfilaments and microtubules was more mildly affected. Evidence for direct interaction of filaggrin and keratin IFs was seen by overlay assays of GFP-filaggrin with keratin proteins in vitro and by filamentous filaggrin distribution in cells with low levels of expression. Cells expressing moderate to high levels of filaggrin showed a rounded cell morphology, loss of cell-cell adhesion, and compacted cytoplasm. There was also partial or complete loss of the desmosomal proteins desmoplakin, plakoglobin, and desmogleins from cell-cell borders, while the distribution of the adherens junction protein E-cadherin was not affected. No alterations in keratin cytoskeleton, desmosomal protein distribution, or cell shape were observed in control cell lines expressing beta-galactosidase. Filaggrin altered the cell shape and disrupted the actin filament distribution in IF-deficient SW13 cells, demonstrating that filaggrin can affect cell morphology independent of the presence of a cytoplasmic IF network. These studies demonstrate that filaggrin, in addition to its known effects on IF organization, can affect the distribution of other cytoskeletal elements including actin microfilaments, which can occur in the absence of a cytoplasmic IF network. Further, filaggrin can disrupt the distribution of desmosome proteins, suggesting an additional role(s) for this protein in the cytoskeletal and desmosomal reorganization that occurs at the granular to cornified cell transition during terminal differentiation of epidermal keratinocytes.

Keratin 14 point mutations at codon 119 of helix 1A resulting in different epidermolysis bullosa simplex phenotypes

Epidermolysis bullosa simplex is a heterogeneous group of inherited bullous disorders due to mutations in keratins 5 and 14. We report two different mutations in keratin 14 at codon 119 of the helix initiation peptide, each with different phenotypic expression. One, a sporadic case that clinically resembles Dowling-Meara epidermolysis bullosa simplex, resulted from conversion of methionine to threonine (M119T). The other, a multigeneration family with the Koebner phenotype, resulted from a previously unreported methionine to valine substitution (M119V). We suggest that loss of hydrophobicity during conversion of methionine to threonine is responsible for the more severe presentation of the first family, whereas maintenance of the hydrophobic nature of the amino acid with conversion to valine resulted in a less severe variant of epidermolysis bullosa simplex. Although most prior mutations in the highly conserved boundary motif of the alpha-helix have resulted in the Dowling-Meara subtype, our findings confirm that it is not always possible to predict the epidermolysis bullosa simplex severity on the basis of the location of the mutation along the keratin polypeptide. The specific amino acid substitution may be more critical in some cases.

Molecular motors and their functions in plants

Molecular motors that hydrolyze ATP and use the derived energy to generate force are involved in a variety of diverse cellular functions. Genetic, biochemical, and cellular localization data have implicated motors in a variety of functions such as vesicle and organelle transport, cytoskeleton dynamics, morphogenesis, polarized growth, cell movements, spindle formation, chromosome movement, nuclear fusion, and signal transduction. In non-plant systems three families of molecular motors (kinesins, dyneins, and myosins) have been well characterized. These motors use microtubules (in the case of kinesines and dyneins) or actin filaments (in the case of myosins) as tracks to transport cargo materials intracellularly. During the last decade tremendous progress has been made in understanding the structure and function of various motors in animals. These studies are yielding interesting insights into the functions of molecular motors and the origin of different families of motors. Furthermore, the paradigm that motors bind cargo and move along cytoskeletal tracks does not explain the functions of some of the motors. Relatively little is known about the molecular motors and their roles in plants. In recent years, by using biochemical, cell biological, molecular, and genetic approaches a few molecular motors have been isolated and characterized from plants. These studies indicate that some of the motors in plants have novel features and regulatory mechanisms. The role of molecular motors in plant cell division, cell expansion, cytoplasmic streaming, cell-to-cell communication, membrane trafficking, and morphogenesis is beginning to be understood. Analyses of the Arabidopsis genome sequence database (51% of genome) with conserved motor domains of kinesin and myosin families indicates the presence of a large number (about 40) of molecular motors and the functions of many of these motors remain to be discovered. It is likely that many more motors with novel regulatory mechanisms that perform plant-specific functions are yet to be discovered. Although the identification of motors in plants, especially in Arabidopsis, is progressing at a rapid pace because of the ongoing plant genome sequencing projects, only a few plant motors have been characterized in any detail. Elucidation of function and regulation of this multitude of motors in a given species is going to be a challenging and exciting area of research in plant cell biology. Structural features of some plant motors suggest calcium, through calmodulin, is likely to play a key role in regulating the function of both microtubule- and actin-based motors in plants.

The role of chaperone-assisted folding and quality control in inborn errors of metabolism: protein folding disorders

Molecular chaperones are present in the various compartments of the cell and assist the folding of newly synthesized proteins. Compared to wild-type proteins, missense mutant proteins are generally synthesized in a normal fashion, but may be impaired in their folding. A broad array of diseases that are due to misfolding of mutant proteins may be labelled conformational diseases: aggregation diseases, such as Alzheimer disease; diseases caused by negative dominance from misfolded structural proteins, such as hypertrophic cardiomyopathy; and disorders where the misfolded protein is degraded by intracellular proteases. Many metabolic disorders belong to this last category, where the so-called protein quality control systems, comprising chaperones and proteases, attempt to eliminate folding intermediates or misfolded proteins. On the basis of in vitro experiments with a limited number of missense mutations identified in patients with phenylalanine hydroxylase and fatty acid oxidation deficiencies, we discuss the cellular fate of missense mutant proteins. We find that the balance between folding to functional conformers, retention (holding) and degradation of folding intermediates or misfolded proteins is dependent on the nature of the mutation and on the efficiency of the quality control. For example, low temperature may promote formation of functional conformers, while elevated temperature usually promotes retention and degradation. We conclude that disorders caused by many missense mutations are complex diseases in which the mutation itself is a necessary major primary component, but that its effect may be modified by cellular conditions and possibly by genetic variations in the quality control systems. We suggest that this new knowledge about cell handling may open new avenues of understanding of the cell pathology and treatment of patients with metabolic disorders.

Epithelial structural proteins of the skin and oral cavity: function in health and disease

Epithelial tissues function to protect the organism from physical, chemical, and microbial damage and are essential for survival. To perform this role, epithelial keratinocytes undergo a well-defined differentiation program that results in the expression of structural proteins which maintain the integrity of epithelial tissues and function as a protective barrier. This review focuses on structural proteins of the epidermis and oral mucosa. Keratin proteins comprise the predominant cytoskeletal component of these epithelia. Keratin filaments are attached to the plasma membrane via desmosomes, and together these structural components form a three-dimensional array within the cytoplasm of epithelial cells and tissues. Desmosomes contain two types of transmembrane proteins, the desmogleins and desmocollins, that are members of the cadherin family. The desmosomal cadherins are linked to the keratin cytoskeleton via several cytoplasmic plaque proteins, including desmoplakin and plakoglobin (gamma-catenin). Epidermal and oral keratinocytes express additional differentiation markers, including filaggrin and trichohyalin, that associate with the keratin cytoskeleton during terminal differentiation, and proteins such as loricrin, small proline-rich proteins, and involucrin, that are cross-linked into the cornified envelope by transglutaminase enzymes. The importance of these cellular structures is highlighted by the large numbers of genetic and acquired (autoimmune) human disorders that involve mutations in, or autoantibodies to, keratins and desmosomal and cornified envelope proteins. While much progress has been made in the identification of the structural proteins and enzymes involved in epithelial differentiation, regulation of this process is less clear. Both calcium and retinoids influence epithelial differentiation by altering the transcription of target genes and by regulating activity of enzymes critical in epithelial differentiation, such as transglutaminases, proteinases, and protein kinases. These studies have furthered our understanding of how epithelial tissue and cell integrity is maintained and provide a basis for the future treatment of skin and oral disorders by gene therapy and other novel therapeutics.

Anomalous apical plasma membrane phenotype in CK8-deficient mice indicates a novel role for intermediate filaments in the polarization of simple epithelia

Previous results from our laboratory have indicated a requirement for CK intermediate filaments (IF) for the organization of the apical domain in polarized epithelial cells in culture. The results seemed to be challenged by the phenotype of cytokeratin (CK) 8-deficient mice, which comprises only colorectal hyperplasia, female sterility and a weaker hepatocyte integrity. In this work localization with anti-CK antibodies indicated that many Ck8-/- epithelia still form IF in CK8-deficient mice, perhaps because of the expression of the promiscuous CK7. In the small intestine, only villus enterocytes lacked IFs. These cells appeared to lose syntaxin 3, and three apical membrane proteins (alkaline phosphatase, sucrase isomaltase and cystic fibrosis transmembrane conductance regulator) as they progressed along the villus. At the distal third of the villi, gamma-tubulin was found scattered within the cytoplasm of enterocytes, in contrast to its normal sub-apical localization, and the microtubules were disorganized. These results could not be attributed to increased numbers of apoptotic or necrotic cells. The only other cell type we found without IFs in CK8 null mice, the hepatocyte, displayed increased basolateral levels of one apical marker (HA4), indicating a correlation between the lack of intermediate filaments and an apical domain phenotype. These data suggest a novel function for intermediate filaments organizing the apical pole of simple polarized epithelial cells.

Mutations in GFAP, encoding glial fibrillary acidic protein, are associated with Alexander disease

Alexander disease is a rare disorder of the central nervous system of unknown etiology. Infants with Alexander disease develop a leukoencephalopathy with macrocephaly, seizures and psychomotor retardation, leading to death usually within the first decade; patients with juvenile or adult forms typically experience ataxia, bulbar signs and spasticity, and a more slowly progressive course. The pathological hallmark of all forms of Alexander disease is the presence of Rosenthal fibers, cytoplasmic inclusions in astrocytes that contain the intermediate filament protein GFAP in association with small heat-shock proteins. We previously found that overexpression of human GFAP in astrocytes of transgenic mice is fatal and accompanied by the presence of inclusion bodies indistinguishable from human Rosenthal fibers. These results suggested that a primary alteration in GFAP may be responsible for Alexander disease. Sequence analysis of DNA samples from patients representing different Alexander disease phenotypes revealed that most cases are associated with non-conservative mutations in the coding region of GFAP. Alexander disease therefore represents the first example of a primary genetic disorder of astrocytes, one of the major cell types in the vertebrate CNS.

Effects of sulfur mustard on the basal cell adhesion complex

Among the most intriguing questions about sulfur mustard (di(2-chloroethyl) sulfide) is why basal cells are the primary targets of its vesicating lesions. To investigate this problem, replicate cultures of human epidermal keratinocytes (HEK) were grown from normal skin and exposed to 400 microM sulfur mustard (HD) for 5 min. Using fluorescein isothiocyanate (FITC)-conjugated antibodies, confocal laser microscopy and image analyses, we found that in early passages, sham-treated HEK maintained in a 0.15 mM Ca2+ medium continued to express keratins K5 and K14 as well as alpha6beta4-integrin. Both K5 and K14 are intermediate filaments characteristic of basal cells and linked with attachment mechanisms effecting epidermolysis bullosa simplex, a family of blistering skin diseases. Acute exposure to HD caused a statistically significant (P < 0.01) 30.74% decrease in K14 fluorescence within 1 h of exposure. Within 2 h of exposure, K14 fluorescence decreased to near-zero values. The loss in expression of K14 was progressive and occurred well before the expected appearance of in vivo blisters, which have a dose-dependent, clinical latent phase of 8-24 h. Acute exposure to HD also caused a statistically significant (P < 0.002) decrease in expression of beta4, an integrin which has been associated with junctional epidermolysis bullosa (JEB). Disruption of K14 and alpha6beta4-integrin may be early events in the HD injury pathway; however, they had no immediate or obvious effect on cell to substrate attachment.

Defective folding and rapid degradation of mutant proteins is a common disease mechanism in genetic disorders

Many disease-causing point mutations do not seriously compromise synthesis of the affected polypeptide but rather exert their effects by impairing subsequent protein folding or stability of the folded protein. This often results in rapid degradation of the affected protein. The concepts of such 'conformational disease' are illustrated by reference to cystic fibrosis, phenylketonuria and short-chain acyl-CoA dehydrogenase deficiency. Other cellular components such as chaperones and proteases, as well as environmental factors, may combine to modulate the phenotype of such disorders and this may open up new therapeutic approaches.

Identification with a recombinant antibody of an inner-ear cytokeratin, a marker for hair-cell differentiation

Extensive biochemical characterization of cells in the inner ear has been hampered by a lack of tools with which to identify inner-ear proteins. By using a single-chain antibody fragment isolated from a bacteriophage-displayed library, we have identified a cytokeratin that is abundant in nonsensory cells of the frog inner ear. Although the progenitors of hair cells exhibit strong immunoreactivity to this cytokeratin, the signal declines in immature hair cells and vanishes as the cells mature. The correlation between diminished immunoreactivity and hair-cell differentiation indicates that the cytokeratin is down-regulated during the transition from a nonsensory to a sensory cell and suggests that the marker is an early index of hair-cell differentiation.

Cytokeratin 8 protects from hepatotoxicity, and its ratio to cytokeratin 18 determines the ability of hepatocytes to form Mallory bodies

In alcoholic hepatitis, a severe form of alcohol-induced toxic liver injury, as well as in experimental intoxication of mice with the porphyrinogenic drugs griseofulvin and 3,5-diethoxycarbonyl-1, 4-dihydrocollidine, hepatocytes form cytoplasmic protein aggregates (Mallory bodies; MBs) containing cytokeratins (CKs) and non-CK components. Here we report that mice lacking the CK8 gene and hence CK intermediate filaments in hepatocytes, but still expressing the type I partner, ie, the CK18 gene, do not form MBs but suffer from extensive porphyria and progressive toxic liver damage, leading to the death of a considerable number of animals (7 of 12 during 12 weeks of intoxication). Our observations show that 1) in the absence of CK8 as well as in the situation of a relative excess of CK18 over CK8 no MBs are formed; 2) the loss of CK8 is not compensated by other type II CKs; and 3) porphyria and toxic liver damage are drastically enhanced in the absence of CK8. Our results point to a protective role of CKs in certain types of toxic liver injury and suggest that MBs by themselves are not harmful to hepatocytes but may be considered as a product of a novel defense mechanism in hepatocytes.

Arsenite induces DNA-protein crosslinks and cytokeratin expression in the WRL-68 human hepatic cell line

The induction of DNA-protein crosslinks (DPC) has been proposed as an indicator of early biological effects due to the fact that known or suspected carcinogens induce an increased proportion of proteins tightly bound to DNA. Arsenic, a human carcinogen, is reduced and methylated mainly in liver cells generating a number of intermediate reactive forms which could lead to the formation of DNA-protein crosslinks. The induction of DPC by arsenite [As(III)] was investigated in the WRL-68 human hepatic cell line, testing the possibility that cytokeratins or cytokeratin-like proteins, due to their high content of SH groups, could participate in DPC. The formation and decay of DPC was dose-related. Arsenite was the only intracellular species present since no methylated As forms could be detected. Thus, DPC can be attributed to the presence of arsenite, an important species present in liver during As exposure, whose permanence in the tissue would depend on the methylation rate of the organism. Several cytokeratins were identified by immunoblotting among the proteins crosslinked with DNA, including cytokeratin 18 (CK18), a specific liver intermediate filament. An augmented presence of CK18 was detected in treated cultures by immunoblotting of total protein PAGE. In liver cells cytokeratin synthesis is tightly correlated with differentiation programs, thus arsenite could not only be damaging DNA but also modifying differentiation patterns in this tissue.

Forkhead/winged-helix transcription factor Whn regulates hair keratin gene expression: molecular analysis of the nude skin phenotype

The molecular characteristics of the nude phenotype (alopecia and thymic aplasia) in humans and rodents are unknown. The nude locus encodes Whn, a transcription factor of the forkhead/winged-helix class. Expression of Whn in HeLa cells induced expression of human hair keratin genes Ha3-II and Hb5. Correspondingly, in nude mice, which are homozygous for a loss-of-function mutation of Whn, expression of mouse mHa3 and mHb5 hair keratin genes is severely reduced. Characterization of a previously identified nude allele, nu(Y), revealed a mis-sense mutation (R320C) in the DNA binding domain of Whn. This mutant protein is unable to activate hair keratin gene expression in HeLa cells. When the Whn transcription factor was expressed in two parts, one containing the N-terminal DNA binding domain and the other the C-terminal activation domain, no activation of hair keratin genes in HeLa cells was observed. However, when these two proteins were noncovalently linked by means of synthetic dimerizers, hair keratin gene expression was induced. This finding suggests that target gene activation by Whn depends on the structural integrity and physical proximity of DNA binding and activation domains, providing a molecular framework to explain the loss-of-function phenotypes of all previously characterized nude mutations. Our results implicate Whn as a transcriptional regulator of hair keratin genes and reveal the nude phenotype as the first example of an inherited skin disorder that is caused by loss of expression rather than mutation of keratin genes.

Exempting homologous pseudogene sequences from polymerase chain reaction amplification allows genomic keratin 14 hotspot mutation analysis

In patients with the major forms of epidermolysis bullosa simplex, either of the keratin genes KRT5 or KRT14 is mutated. This causes a disturbance of the filament network resulting in skin fragility and blistering. For KRT5, a genomic mutation detection system has been described previously. Mutation detection of KRT14 on a DNA level is, however, hampered by the presence of a highly homologous but nontranscribed KRT14 pseudogene. Consequently, mutation detection in epidermolysis bullosa simplex has mostly been carried out on cDNA synthesized from KRT5 and KRT14 transcripts in mRNA isolated from skin biopsies. Here we present a genomic mutation detection system for exons 1, 4, and 6 of KRT14 that encode the 1A, L1-2, and 2B domains of the keratin 14 protein containing the mutation hotspots. After cutting the KRT14 pseudogene genomic sequences with restriction enzymes while leaving the homologous genomic sequences of the functional gene intact, only the mutation hotspot-containing exons of the functional KRT14 gene are amplified. This is followed by direct sequencing of the polymerase chain reaction products. In this way, three novel mutations could be identified, Y415H, L419Q, and E422K, all located in the helix termination motif of the keratin 14 rod domain 2B, resulting in moderate, severe, and mild epidermolysis bullosa simplex phenotype, respectively. By obviating the need of KRT14 cDNA synthesis from RNA isolated from skin biopsies, this approach substantially facilitates the detection of KRT14 hotspot mutations.

Hepatocyte cytokeratins are hyperphosphorylated at multiple sites in human alcoholic hepatitis and in a mallory body mouse model

Alcoholic hepatitis (AH) is associated with cytokeratin 8 and 18 (CK8/18) accumulation as cytoplasmic inclusion bodies, termed Mallory bodies (MBs). Studies with MB mouse models and cultured hepatocytes suggested that CK8/18 hyperphosphorylation might be involved in MB formation. However, no data exist on phosphorylation of CK8/18 in human AH. In this study, antibodies that selectively recognize phosphorylated epitopes of CK8 or CK18 were used to analyze CK8/18 phosphorylation states in normal human and murine livers, human AH biopsies, and livers of 3,5-diethoxycarbonyl-1, 4-dihydrocollidine (DDC)-intoxicated mice, the last serving as model for MB induction. Hepatocyte cytokeratins become hyperphosphorylated at multiple sites in AH and in DDC-intoxicated mice. Hyperphosphorylation of CK8/18 occurred rapidly, after 1 day of DDC intoxication and preceded architectural changes of the cytoskeleton. In long-term DDC-intoxicated mice as well as in human AH, MBs preferentially contain hyperphosphorylated CK8/18 as compared with the cytoplasmic cytokeratin intermediate filament network suggesting that CK8/18 hyperphosphorylation may play a contributing role in MB pathogenesis. Furthermore, the site-specific phosphorylation of cytokeratin in different stages of MB induction provides indirect evidence for the involvement of a variety of protein kinases known to be activated in stress responses, mitosis, and apoptosis.

Vimentin intermediate filament protein as differentiation marker of optic vesicle epithelium in the chick embryo

For the study of the differentiation process of optic vesicle epithelium into neural retina, pigment epithelium and pars caeca retinae, vimentin intermediate filament protein in retinal epithelial cells was detected immunohistochemically in chick embryo at stages 11-21. In the late stage of optic vesicle development (stage 14), optic vesicle epithelium was classified into the following 3 different portions on the basis of vimentin staining intensity: latero-central epithelium under the lens placode, medio-central epithelium facing the latero-central epithelium, and peripheral epithelium connecting the latero-central and medio-central epithelia. Latero-central epithelium, the future neural retina, exhibited strongest staining of vimentin of the 3 portions. In contrast, medio-central epithelium, the future pigment epithelium, showed weakest staining. Moderate staining was observed in peripheral epithelium, the future pars caeca retinae. These differences in levels of vimentin expression were observed during optic cup formation. The present results clearly demonstrate that differentiation of retinal epithelium into neural retina, pigment epithelium and pars caeca retinae occurs in the late stage of the optic vesicle, and that retinal differentiation is reflected by the amount of vimentin in epithelial cells.

A missense mutation in the desmin rod domain is associated with autosomal dominant distal myopathy, and exerts a dominant negative effect on filament formation

In some myopathies of distal onset, the intermediate filament desmin is abnormally accumulated in skeletal and cardiac muscle. We report the first point mutation in desmin cosegregating with an autosomal dominant form of desmin-related myopathy. The L345P desmin missense mutation occurs in a large, six generation Ashkenazi Jewish family. The mutation is located in an evolutionarily highly conserved position of the desmin coiled-coil rod domain important for dimer formation. L345P desmin is incapable of forming filamentous networks in transfected HeLa and SW13 cells. We conclude that the L345P desmin missense mutation causes myopathy by interfering in a dominant-negative manner with the dimerization-polymerization process of intermediate filament assembly.

Using transgenic models to study the pathogenesis of keratin-based inherited skin diseases

In the past decade, the production of transgenic animals whose genome is modified to contain DNA transgenes of interest has significantly contributed to expand our understanding of the molecular etiology and pathobiology of several inherited skin diseases. This technology has led to the discovery that mutations affecting specific keratin genes are responsible for a wide spectrum of inherited bullous diseases, which are collectively characterized by blistering after minor trauma. Type I and type II keratin proteins are restricted to, and very abundant in, epithelial cells, where they occur as a pancytoplasmic network of cytoskeletal filaments. Although it had long been suspected that a primary function of keratin filaments may be to contribute to the physical strength of epithelial sheets, a formal demonstration came from studies of transgenic mouse models and patients suffering from keratin-based blistering diseases. Here we review the basic characteristics of keratin gene and their proteins and relate them to the molecular pathogenesis of relevant inherited skin blistering diseases. A particular emphasis is placed on the role of transgenic mouse models in the past, current, and future studies of these genodermatoses.

The catalog of human hair keratins. I. Expression of the nine type I members in the hair follicle

The human type I hair keratin subfamily comprises nine individual members, which can be subdivided into three groups. Group A (hHa1, hHa3-I, hHa3-II, hHa4) and B (hHa7, hHa8) each contains structurally related hair keratins, whereas group C members hHa2, hHa5, and hHa6 represent structurally rather unrelated hair keratins. Antibodies produced against these individual hair keratins, first analyzed for specificity by one- dimensional Western blots of total hair keratins, were used to establish the two-dimensional catalog of the human type I hair keratin subfamily. The catalog comprises two different series of type I hair keratins: a strongly expressed, Coomassie-stainable series containing hair keratins hHa1, hHa3-I/II, hHa4, and hHa5, and a weakly expressed, immunodetectable series harboring hHa2, hHa6 hHa7, and hHa8. In situ hybridization and immunohistochemical expression studies on scalp follicles show that two hair keratins, hHa2 and hHa5, define the early stage of hair differentiation, i.e. hHa5 expression in hair matrix and hHa5/hHa2 coexpression in the early hair cuticle cells. Whereas cuticular differentiation proceeds without the expression of further type I hair keratins, matrix cells embark on the cortical pathway by sequentially expressing hHa1, hHa3-I/II, and hHa4, which are supplemented by hHa6 at an advanced stage of cortical differentiation, and hHa8, which is expressed heterogeneously in cortex cells. Thus, six type I hair keratins are involved in the terminal differentiation of anagen hairs. The expression of hHa7 is conspicuously different from that of the other hair keratins in that it does not occur in the large anagen follicles of terminal scalp hairs but only in central cortex cells of the rare and small follicle type that gives rise to vellus hairs.

Hereditary skin diseases of hemidesmosomes

Studies of hereditary blistering skin diseases (epidermolysis bullosa) and targeted gene mutation experiments in knockout mice have greatly improved our understanding of hemidesmosomes and their associated structures in the cytoskeleton and basement membrane of the skin and mucous membranes. At least 10 molecules are recruited in hemidesmosome complexes, where they interact in a complex way. Hemidesmosomes are not simple adhesion devices, but also transduce signals for cell spreading, cell proliferation and basement membrane organisation. The dynamics of a hemidesmosome raises the metaphor of a self-assembling suspension bridge which evokes activities on both sides of the river. This review summarises our current knowledge of the molecular pathology of hemidesmosomes caused by hereditary skin disease or gene targeting experiment.

A premature stop codon mutation in the 2B helix termination peptide of keratin 5 in a German epidermolysis bullosa simplex Dowling-Meara case

Epidermolysis bullosa simplex (EBS) is caused by defective assembly of keratin intermediate filaments in basal keratinocytes and recent studies indicated causal mutations in the keratin KRT5 and KRT14 genes. In this study, we describe a novel KRT5 mutation in a German sporadic case of EBS Dowling-Meara. Transition of G to T (nucleotide position 2334) leads to a premature stop codon (E477stop, residue 93 of the 2B helix) in the last residue of the highly conserved helix-termination peptide K/LLEGE of the 2B rod domain of keratin K5. This represents the first premature stop codon mutation identified within the K/LLEGE motif of any disorder reported so far that is caused by keratin mutations.

Exocrine pancreatic disorders in transsgenic mice expressing human keratin 8

Keratins K8 and K18 are the major components of the intermediate-filament cytoskeleton of simple epithelia. Increased levels of these keratins have been correlated with various tumor cell characteristics, including progression to malignancy, invasive behavior, and drug sensitivity, although a role for K8/K18 in tumorigenesis has not yet been demonstrated. To examine the function of these keratins, we generated mice expressing the human K8 (hk8) gene, which leads to a moderate keratin-content increase in their simple epithelia. These mice displayed progressive exocrine pancreas alterations, including dysplasia and loss of acinar architecture, redifferentiation of acinar to ductal cells, inflammation, fibrosis, and substitution of exocrine by adipose tissue, as well as increased cell proliferation and apoptosis. Histological changes were not observed in other simple epithelia, such as the liver. Electron microscopy showed that transgenic acinar cells have keratins organized in abundant filament bundles dispersed throughout the cytoplasm, in contrast to control acinar cells, which have scarce and apically concentrated filaments. The phenotype found was very similar to that reported for transgenic mice expressing a dominant-negative mutant TGF-beta type II receptor (TGFbetaRII mice). We show that these TGFbetaRII mutant mice also have elevated K8/K18 levels. These results indicate that simple epithelial keratins play a relevant role in the regulation of exocrine pancreas homeostasis and support the idea that disruption of mechanisms that normally regulate keratin expression in vivo could be related to inflammatory and neoplastic pancreatic disorders.

Modulation of cell proliferation by cytokeratins K10 and K16

The members of the large keratin family of cytoskeletal proteins are expressed in a carefully regulated tissue- and differentiation-specific manner. Although these proteins are thought to be involved in imparting mechanical integrity to epithelial cells, the functional significance of their complex differential expression is still unclear. Here we provide new data suggesting that the expression of particular keratins may influence cell proliferation. Specifically, we demonstrate that the ectopic expression of K10 inhibits the proliferation of human keratinocytes in culture, while K16 expression appears to promote the proliferation of these cells. Other keratins, such as K13 or K14, do not significantly alter this parameter. K10-induced inhibition is reversed by the coexpression of K16 but not that of K14. These results are coherent with the observed expression pattern of these proteins in the epidermis: basal, proliferative keratinocytes express K14; when they terminally differentiate, keratinocytes switch off K14 and start K10 expression, whereas in response to hyperproliferative stimuli, K16 replaces K10. The characteristics of this process indicate that K10 and K16 act on the retinoblastoma (Rb) pathway, as (i) K10-induced inhibition is hampered by cotransfection with viral oncoproteins which interfere with pRb but not with p53; (ii) K10-mediated cell growth arrest is rescued by the coexpression of specific cyclins, cyclin-dependent kinases (CDKs), or cyclin-CDK complexes; (iii) K10-induced inhibition does not take place in Rb-deficient cells but is restored in these cells by cotransfection with pRb or p107 but not p130; (iv) K16 efficiently rescues the cell growth arrest induced by pRb in HaCaT cells but not that induced by p107 or p130; and (v) pRb phosphorylation and cyclin D1 expression are reduced in K10-transfected cells and increased in K16-transfected cells. Finally, using K10 deletion mutants, we map this inhibitory function to the nonhelical terminal domains of K10, hypervariable regions in which keratin-specific functions are thought to reside, and demonstrate that the presence of one of these domains is sufficient to promote cell growth arrest.

Ontogeny and regional variability of keratin 2e (K2e) in developing human fetal skin: a unique spatial and temporal pattern of keratin expression in development

Keratin 2e (K2e) is expressed in the upper spinous and granular cells of adult epidermis. A highly specific polyclonal antibody was made against a C-terminal peptide of K2e and used to observe K2e expression at different developmental stages. At 12.5 weeks estimated gestational age (EGA) K2e was detected in trunk skin in scattered cells in the intermediate layer. At 13.5 weeks EGA, greater numbers of intermediate cells were stained with variable intensity, and staining in this pattern increased with age. Epidermal sheets from 14 weeks EGA showed that K2e + cells were excluded from developing hair follicles. At 135 days EGA, the following regional patterns were observed: in cheek, trunk, dorsal and ventral knee, elbow and dorsal hand there was moderate to intense staining of upper intermediate keratinocytes excluding cells of the hair canals and sweat ducts. The periumbilical region distinctly lacked K2e staining, while more distal areas showed increasing numbers of K2e + cells. The earliest expression of K2e was at 10 weeks EGA in the presumptive nail bed of developing digits. By 13.5 weeks EGA this pattern had shifted to the proximal nail fold, and K2e was absent in the nail bed. K2e was excluded from developing sweat glands and ducts and from developing hair follicles at the hair germ and early peg stages. By 15 weeks EGA in the fetal hair follicle small groups of cells were K2e + and by 19 weeks K2e + cells were seen at the level of the matrix. Some overlap in staining was detected for K2e with K10, and in palmar skin with K9; however, mostly the filamentous staining patterns for these keratins were distinctive. This study shows that the complex patterns of temporal and regional expression of K2e differ from known patterns for other epidermal keratins and suggest different regulation and function for this epidermal keratin.