Gene targeting of BPAG1: abnormalities in mechanical strength and cell migration in stratified epithelia and neurologic degeneration

Plakins: a family of versatile cytolinker proteins

By connecting cytoskeletal elements to each other and to junctional complexes, the plakin family of cytolinkers plays a crucial role in orchestrating cellular development and maintaining tissue integrity. Plakins are built from combinations of interacting domains that bind to microfilaments, microtubules, intermediate filaments, cell-adhesion molecules and members of the armadillo family. Plakins are involved in both inherited and autoimmune diseases that affect the skin, neuronal tissue, and cardiac and skeletal muscle. Here, we describe the members of the plakin family and their interaction partners, and give examples of the cellular defects that result from their dysfunction.

Deletion of the cytoplasmatic domain of BP180/collagen XVII causes a phenotype with predominant features of epidermolysis bullosa simplex

BP180/collagen XVII is a hemidesmosomal transmembrane molecule serving as cell-surface receptor. Mutations in its gene cause junctional epidermolysis bullosa. Here, we report a patient with mutations in the gene for BP180/collagen XVII, COL17A1, but predominant phenotypic features of epidermolysis bullosa simplex. At birth, the proband presented with bullous lesions on the trunk, face, and hands. Ultrastructurally, hemidesmosomes were fairly normal, but the attachment of intermediate filaments with the hemidesmosomal plaques appeared to be impaired. Blister formation demonstrated both intraepidermal and junctional cleavage. Immunofluorescence staining with antibodies to keratins, several hemidesmosomal proteins, and the extracellular domain of BP180/collagen XVII showed normal staining patterns, whereas an antibody against the intracellular domain of BP180/collagen XVII yielded a negative immunofluorescence signal. Analysis of BP180/collagen XVII cDNA revealed a 1172 bp deletion corresponding to an in-frame deletion from Ile-18 to Asn-407 from the intracellular domain of the polypeptide. Mutation analysis of the COL17A1 gene disclosed a paternal nonsense mutation, R1226X, and a large maternal genomic deletion extending from intron 2 to intron 15, but no mutations in basal keratin genes. These findings underline the functional importance of the intracellular BP180/collagen XVII domain for the interaction of hemidesmosomes with keratin intermediate filaments and for the spatial stability of basal keratinocytes, and provide a functional explanation for the epidermolysis-bullosa- simplex-like phenotype. Further, the data demonstrate that defects in a given gene can cause unexpected phenotypes of epidermolysis bullosa categories, depending on the function of the affected protein domain.

Transient translocation of hemidesmosomal bullous pemphigoid antigen 1 from cytosol to membrane fractions by 12-O-tetradecanoylphorbol-13-acetate treatment and Ca2+-switch in a human carcinoma cell line

We previously showed that 12-O-tetradecanoylphorbol-13-acetate (TPA) and Ca2+-switch from low (0.07 mM) to normal (1.87 mM) concentration in culture medium, which were also linked to activation of protein kinase C (PKC), lead to phosphorylation of 180 kDa-bullous pemphigoid antigen (BPAG) 2, but not of 230 kDa-BPAG1, and possibly to its disassembly from hemidesmosomes in a human squamous cell carcinoma cell line (DJM-1). In this study, we examined the effects of TPA and Ca2+-switch on intracellular localization of BPAG1 by immuno-blotting and immuno-fluorescence microscopy with monoclonal antibodies to the antigen after sub-cellular fractionation. In DJM-1 cells cultured in low Ca2+ medium, BPAG1 was detected as phosphate buffered saline-soluble (cytosolic), Triton X-100 soluble (roughly membrane-associated) and Triton X-100 insoluble (cytoskeleton-bound) forms, whereas in normal Ca2+-grown cells only as cytosolic and cytoskeleton-bound forms. In normal Ca2+-cultured cells, TPA (50 nM) caused a complete translocation of BPAG1 from cytosol to membrane fractions within 10 min, that was inhibited by pretreatment with H7 (a selective PKC inhibitor) at 40 microM. After 30 min and 4 h of TPA-treatment, BPAG1 was exclusively detected in cytoskeleton fractions. Morphologically, immuno-fluorescence microscopy showed that treatment caused a marked reduction of BPAG1 from the cytoplasm and generated a linear pattern at cell-cell contacts, suggesting translocation of BPAG1 from the cytosol to the plasma membrane. In contrast, the Ca2+-switch from low to normal caused a prominent increase of BPAG1, both in cytosolic and membrane-associated forms after 4 h, that was inhibited both with H7 and cycloheximide (an inhibitor of protein synthesis) at 70 microM, suggesting a role for PKC and BPAG1 synthesis in these Ca2+-induced effects. These results suggest that TPA and Ca2+-switch induced BPAG1 translocation to membrane fractions possibly mediated by PKC-activation. Furthermore, whereas TPA affects the redistribution of BPAG1 among their pools without inducing their synthesis, Ca2+-switch induces both membrane translocation and synthesis of BPAG1, suggesting involvement of signaling other than PKC pathways in control of BPAG1 synthesis.

EGF-R signaling through Fyn kinase disrupts the function of integrin alpha6beta4 at hemidesmosomes: role in epithelial cell migration and carcinoma invasion

We have examined the mechanism and functional significance of hemidesmosome disassembly during normal epithelial cell migration and squamous carcinoma invasion. Our findings indicate that a fraction of EGF receptor (EGF-R) combines with the hemidesmosomal integrin alpha6beta4 in both normal and neoplastic keratinocytes. Activation of the EGF-R causes tyrosine phosphorylation of the beta4 cytoplasmic domain and disruption of hemidesmosomes. The Src family kinase inhibitors PP1 and PP2 prevent tyrosine phosphorylation of beta4 and disassembly of hemidesmosomes without interfering with the activation of EGF-R. Coimmunoprecipitation experiments indicate that Fyn and, to a lesser extent, Yes combine with alpha6beta4. By contrast, Src and Lck do not associate with alpha6beta4 to a significant extent. A dominant negative form of Fyn, but not Src, prevents tyrosine phosphorylation of beta4 and disassembly of hemidesmosomes. These observations suggest that the EGF-R causes disassembly of hemidesmosomes by activating Fyn, which in turn phosphorylates the beta4 cytoplasmic domain. Neoplastic cells expressing dominant negative Fyn display increased hemidesmosomes and migrate poorly in vitro in response to EGF. Furthermore, dominant negative Fyn decreases the ability of squamous carcinoma cells to invade through Matrigel in vitro and to form lung metastases following intravenous injection in nude mice. These results suggest that disruption of hemidesmosomes mediated by Fyn is a prerequisite for normal keratinocyte migration and squamous carcinoma invasion.

Gene targeting of envoplakin, a cytoskeletal linker protein and precursor of the epidermal cornified envelope

Envoplakin, a member of the plakin family of cytoskeletal linker proteins, is localized in desmosomes of stratified epithelial cells and is a component of the epidermal cornified envelope. Gene targeting in mouse embryonic stem cells was used to generate a null allele of envoplakin. No envoplakin transcripts from the targeted allele could be detected in the skin of newborn mice. Mice homozygous for the targeted allele were born in the normal Mendelian ratio and were fertile. They did not develop any discernible pathological phenotype up to the age of 1 year. The ultrastructural appearance of cornified envelopes from adult epidermis was indistinguishable between wild-type and knockout mice, and there was no evidence that the absence of envoplakin affected the subcellular distribution of periplakin or desmoplakin, two other plakins found in desmosomes. The proportion of immature cornified envelopes in the epidermis of newborn mice was greater in envoplakin-null animals than in heterozygous littermates or wild-type mice, and the envelopes had a larger surface area. This correlated with a slight delay in barrier acquisition during embryonic development. We conclude that although envoplakin is part of the scaffolding on which the cornified envelope is assembled, it is not essential for envelope formation or epidermal barrier function.

The hemidesmosomal protein bullous pemphigoid antigen 1 and the integrin beta 4 subunit bind to ERBIN. Molecular cloning of multiple alternative splice variants of ERBIN and analysis of their tissue expression

The bullous pemphigoid antigen 1 (eBPAG1) is a constituent of hemidesmosomes (HDs), cell-substrate adhesion complexes in stratified epithelia. Although its COOH terminus interacts with intermediate filaments, its NH(2) terminus is important for its recruitment into HDs. To identify proteins that interact with the NH(2) terminus of human eBPAG1, we performed a yeast two-hybrid screen, which uncovered a protein belonging to the LAP/LERP (for LRR and PDZ domain) protein family with 16 NH(2)-terminal leucine-rich repeats and a COOH-terminal PDZ domain. The gene for this LAP/LERP protein comprises at least 26 exons located on the long arm of chromosome 5. In most human tissues, several transcripts were detected differing in the coding region situated upstream of or within the PDZ domain. One of the encoded variants was found to correspond to the recently described protein ERBIN. In yeast and in vitro binding experiments, ERBIN was shown to interact not only with eBPAG1 but also with the COOH-terminal region of the cytoplasmic domain of the integrin beta4 subunit, another component of HDs. Antibodies raised against the COOH terminus showed that ERBIN is expressed in keratinocytes. In transfected epithelial cells the protein, however, was not localized in HDs but was either diffusely distributed over the cytoplasm or concentrated at the basolateral plasma membrane. Because ERBIN had been shown previously to interact with the transmembrane tyrosine kinase receptor Erb-B2, which in turn associates with the integrin beta4 subunit, we suggest that ERBIN provides a link between HD assembly and Erb-B2 receptor signaling.

The BPAG1 locus: Alternative splicing produces multiple isoforms with distinct cytoskeletal linker domains, including predominant isoforms in neurons and muscles

Bullous pemphigoid antigen 1 (BPAG1) is a member of the plakin family with cytoskeletal linker properties. Mutations in BPAG1 cause sensory neuron degeneration and skin fragility in mice. We have analyzed the BPAG1 locus in detail and found that it encodes different interaction domains that are combined in tissue-specific manners. These domains include an actin-binding domain (ABD), a plakin domain, a coiled coil (CC) rod domain, two different potential intermediate filament-binding domains (IFBDs), a spectrin repeat (SR)-containing rod domain, and a microtubule-binding domain (MTBD). There are at least three major forms of BPAG1: BPAG1-e (302 kD), BPAG1-a (615 kD), and BPAG1-b (834 kD). BPAG1-e has been described previously and consists of the plakin domain, the CC rod domain, and the first IFBD. It is the primary epidermal BPAG1 isoform, and its absence that is the likely cause of skin fragility in mutant mice. BPAG1-a is the major isoform in the nervous system and a homologue of the microtubule actin cross-linking factor, MACF. BPAG1-a is composed of the ABD, the plakin domain, the SR-containing rod domain, and the MTBD. The absence of BPAG1-a is the likely cause of sensory neurodegeneration in mutant mice. BPAG1-b is highly expressed in muscles, and has extra exons encoding a second IFBD between the plakin and SR-containing rod domains of BPAG1-a.

Formation of a normal epidermis supported by increased stability of keratins 5 and 14 in keratin 10 null mice

The expression of distinct keratin pairs during epidermal differentiation is assumed to fulfill specific and essential cytoskeletal functions. This is supported by a great variety of genodermatoses exhibiting tissue fragility because of keratin mutations. Here, we show that the loss of K10, the most prominent epidermal protein, allowed the formation of a normal epidermis in neonatal mice without signs of fragility or wound-healing response. However, there were profound changes in the composition of suprabasal keratin filaments. K5/14 persisted suprabasally at elevated protein levels, whereas their mRNAs remained restricted to the basal keratinocytes. This indicated a novel mechanism regulating keratin turnover. Moreover, the amount of K1 was reduced. In the absence of its natural partner we observed the formation of a minor amount of novel K1/14/15 filaments as revealed by immunogold electron microscopy. We suggest that these changes maintained epidermal integrity. Furthermore, suprabasal keratinocytes contained larger keratohyalin granules similar to our previous K10T mice. A comparison of profilaggrin processing in K10T and K10(-/-) mice revealed an accumulation of filaggrin precursors in the former but not in the latter, suggesting a requirement of intact keratin filaments for the processing. The mild phenotype of K10(-/-) mice suggests that there is a considerable redundancy in the keratin gene family.

Epithelial development and differentiation in the mammary gland is not dependent on alpha 3 or alpha 6 integrin subunits

In the mammary gland, both laminin and integrins have been shown to be required for normal ductal morphogenesis during development in vivo, and for functional differentiation in culture models. Major integrin receptors for laminins in the mammary gland are alpha 3 beta 1, alpha 6 beta 1, and alpha 6 beta 4. However, the specific subunits that contribute to laminin-mediated mammary cell function and development have not been identified. In this study, we use a genetic approach to test the hypothesis that laminin-binding integrins are required for the function of the mammary gland in vivo. Rudiments of embryonic mammary gland were shown to develop in the absence of these integrin subunits. Postnatal development of the mammary gland was studied in integrin null tissue that had been transplanted into the mammary fat pads of syngeneic hosts. In mammary epithelium lacking alpha 6 integrin, the beta 4 subunit was not apparent and hemidesmosome formation was only rudimentary. However, despite this deficiency, normal ductal morphogenesis and branching of the mammary gland occurred and myoepithelial cells were distributed normally with respect to luminal cells. Mammary alveoli devoid of alpha 3 or alpha 6 integrin formed in pregnancy and were histologically and functionally identical to those in wild-type mammary gland. The tissue underwent full morphological differentiation, and the epithelial cells retained the ability to synthesize beta-casein. This work demonstrates that mammary tissue genetically lacking major laminin-binding integrin receptors is still able to develop and function.

Glial cell line-derived neurotrophic factor-responsive and neurotrophin-3-responsive neurons require the cytoskeletal linker protein dystonin for postnatal survival

We have investigated the fate of different neurotrophin-responsive subpopulations of dorsal root ganglion neurons in dystonia musculorum (dt) mice. These mice have a null mutation in the cytoskeletal linker protein, dystonin. Dystonin is expressed by all sensory neurons and cross links actin filaments, intermediate filaments, and microtubules. The dt mice undergo massive sensory neurodegeneration postnatally and die at around 4 weeks of age. We assessed the surviving and degenerating neuronal populations by comparing the dorsal root ganglion (DRG) neurons and central and peripheral projections in dt mice and wildtype mice. Large, neurofilament-H-positive neurons, many of which are muscle afferents and are neurotrophin-3 (NT-3)-responsive, were severely decreased in number in dt DRGs. The loss of muscle afferents was correlated with a degeneration of muscle spindles in skeletal muscle. Nerve growth factor (NGF)-responsive populations, which were visualized using calcitonin gene-related peptide and p75, appeared qualitatively normal in the lumbar spinal cord, DRG, and hindlimb skin. In contrast, glial cell line-derived neurotrophic factor (GDNF)-responsive populations, which were visualized using the isolectin B-4 and thiamine monophosphatase, were severely diminished in the lumbar spinal cord, DRG, and hindlimb skin. Analysis of NT-3, NGF, and GDNF mRNA levels using semiquantitative reverse transcriptase-polymerase chain reaction revealed normal trophin synthesis in the peripheral targets of dt mice, arguing against decreased trophic synthesis as a possible cause of neuronal degeneration. Thus, the absence of dystonin results in the selective survival of NGF-responsive neurons and the postnatal degeneration of many NT-3- and GDNF-responsive neurons. Our results reveal that the loss of this ubiquitously expressed cytoskeletal linker has diverse effects on sensory subpopulations. Moreover, we show that dystonin is critical for the maintenance of certain DRG neurons, and its function may be related to neurotrophic support.

Postnatal regulation of limb proprioception by muscle-derived neurotrophin-3

To investigate the effects of neurotrophin-3 (NT-3) on postnatal proprioceptive neurons and their targets, transgenic mice were generated that use the myosin light chain 1 (mlc) promoter to overexpress NT-3 in skeletal muscle. Ribonuclease protection assays revealed that NT-3 overexpression in hindlimb skeletal muscle began at embryonic day 14 (E14) and continued throughout adulthood. Overexpression of NT-3 during late embryogenesis resulted in increased numbers of large sensory and small fusimotor axons. Within a week of birth, mlc/NT-3 mice retract their limbs to the torso when lifted by the tail. Footprint analysis revealed that mlc/NT-3 mice had significant abnormalities in their gait compared with wild-types. Beam walking and rotorod analysis confirmed the poor limb control by mlc/NT-3 mice. These locomotive deficits progressively worsened with age and were likely related to the formation of morphologically abnormal muscle spindles. The most common spindle anomaly was the presence of excessive intrafusal bag fibers within individual muscle spindles. To assess the role of NT-3 in recovery from nerve injury, sciatic nerve crushes were performed in young adult mice. Two days after injury, mlc/NT-3 mice displayed significantly improved sciatic functional indexes and a significant increase in muscle spindles that remained associated with axons. The latter finding suggests that excess NT-3 in muscle may retard the degeneration of proprioceptive axons after nerve crush. Long-term survival after nerve injury in mlc/NT-3 mice did not induce further changes in spindle number or morphology. These findings demonstrate that, in addition to promoting embryonic proprioceptive neuron survival, postnatal overexpression of NT-3 in muscle leads to abnormal spindle formation and deficits in locomotive control. However, our results also show that NT-3 may be therapeutic for proprioceptive axons immediately after nerve injury by delaying axon degeneration.

Towards a mechanistic understanding of tumor invasion--lessons from the alpha6beta 4 integrin

This review explores the mechanistic basis of carcinoma migration and invasion by focusing on the contribution of integrins. Integrins are essential for invasion not only for their ability to mediate physical interactions with extracellular matrices, but also for their ability to regulate signaling pathways that control actin dynamics and cell movement, as well as for growth and survival. Our comments center on a unique member of the integrin family, the alpha 6 beta 4 integrin, which is a receptor for the laminin family of basement membrane components. Numerous studies have implicated this integrin in the invasion of solid tumors and have provided a rationale for studying the mechanistic basis of its contribution to the invasive process. Such studies have revealed novel insights into the mechanism of carcinoma invasion that involve both the dynamics of cell migration and signaling pathways that regulate this migration.

Downregulation of hemidesmosomal proteins in nasopharyngeal carcinoma cells

Hemidesmosome (HD) is a transmembrane complex that mediates attachment of epithelial cells to the basement membrane. Abnormal expression of HD components has been reported in several types of human cancers and is believed to play a role in tumor invasion and metastasis. Using differential gene display, we have identified downregulation of BPAG1 expression in nasopharyngeal carcinoma cells. BPAG1 is a major component of hemidesmosome. In the present study, we have extended our work to investigate the expression pattern of other components in the HD complex, namely, BPAG2, ITGalpha6 and ITGbeta4 in three distinct biological groups of nasopharyngeal epithelial cells: (a) non-malignant nasopharyngeal epithelial cells established from primary culture of nasopharyngeal explants, (b) non-malignant nasopharyngeal epithelial cells immortalized by viral oncogenes, SV40 or HPV16E6E7, and (c) nasopharyngeal carcinoma (NPC) cells. Both non-malignant primary cultured nasopharyngeal epithelial cells and immortalized nasopharyngeal epithelial cell lines expressed all the HD components examined, although the immortalized cells expressed a lower level of HD components compared with the non-malignant nasopharyngeal cells established from primary culture. In contrast, downregulation of HD components is commonly observed in nasopharyngeal carcinoma cells. Loss of HD expression in NPC may be associated with the undifferentiated properties of NPC cells and may have prognostic significance.

Structure and function of a vimentin-associated matrix adhesion in endothelial cells

The alpha4 laminin subunit is a component of endothelial cell basement membranes. An antibody (2A3) against the alpha4 laminin G domain stains focal contact-like structures in transformed and primary microvascular endothelial cells (TrHBMECs and HMVECs, respectively), provided the latter cells are activated with growth factors. The 2A3 antibody staining colocalizes with that generated by alphav and beta3 integrin antibodies and, consistent with this localization, TrHBMECs and HMVECs adhere to the alpha4 laminin subunit G domain in an alphavbeta3-integrin-dependent manner. The alphavbeta3 integrin/2A3 antibody positively stained focal contacts are recognized by vinculin antibodies as well as by antibodies against plectin. Unusually, vimentin intermediate filaments, in addition to microfilament bundles, interact with many of the alphavbeta3 integrin-positive focal contacts. We have investigated the function of alpha4-laminin and alphavbeta3-integrin, which are at the core of these focal contacts, in cultured endothelial cells. Antibodies against these proteins inhibit branching morphogenesis of TrHBMECs and HMVECs in vitro, as well as their ability to repopulate in vitro wounds. Thus, we have characterized an endothelial cell matrix adhesion, which shows complex cytoskeletal interactions and whose assembly is regulated by growth factors. Our data indicate that this adhesion structure may play a role in angiogenesis.

The 'ins' and 'outs' of intermediate filament organization

A major function shared by several types of cytoplasmic intermediate filaments (IFs) is to stabilize cellular architecture against the mechanical forces it is subjected to. As for other fibrous cytoskeletal arrays, a crucial determinant of this function is the spatial organization of IFs in the cytoplasm. However, very few crossbridging proteins are specific for IFs - most IF-associated proteins known to exert a structural role act to tether IFs to other major cytoskeletal elements, such as F-actin, microtubules or adhesion complexes. In addition, IFs are endowed with the ability to participate in their own organization. This intriguing property is probably connected to the unusual degree of sequence diversity and sequence-specific regulation that characterize IF genes and their proteins. This dependence upon a combination of extrinsic and intrinsic determinants contributes to distinguish IFs from other fibrous cytoskeletal polymers and is key to their function.

Forced expression of keratin 16 alters the adhesion, differentiation, and migration of mouse skin keratinocytes

Injury to the skin results in an induction of keratins K6, K16, and K17 concomitant with activation of keratinocytes for reepithelialization. Forced expression of human K16 in skin epithelia of transgenic mice causes a phenotype that mimics several aspects of keratinocyte activation. Two types of transgenic keratinocytes, with forced expression of either human K16 or a K16-C14 chimeric cDNA, were analyzed in primary culture to assess the impact of K16 expression at a cellular level. High K16-C14-expressing and low K16-expressing transgenic keratinocytes behave similar to wild type in all aspects tested. In contrast, high K16-expressing transgenic keratinocytes show alterations in plating efficiency and calcium-induced differentiation, but proliferate normally. Migration of keratinocytes is reduced in K16 transgenic skin explants compared with controls. Finally, a subset of high K16-expressing transgenic keratinocytes develops major changes in the organization of keratin filaments in a time- and calcium concentration-dependent manner. These changes coincide with alterations in keratin content while the steady-state levels of K16 protein remain stable. We conclude that forced expression of K16 in progenitor skin keratinocytes directly impacts properties such as adhesion, differentiation, and migration, and that these effects depend upon determinants contained within its carboxy terminus.

Delayed wound healing in CXCR2 knockout mice

Previous studies demonstrated that the CXC chemokine, MGSA/GRO-alpha and its receptor, CXCR2, are expressed during wound healing by keratinocytes and endothelial cells at areas where epithelialization and neovascularization occur. The process of wound healing is dependent on leukocyte recruitment, keratinocyte proliferation and migration, and angiogenesis. These processes may be mediated in part by CXC chemokines, such as interleukin-8 and MGSA/GRO-alpha. To examine further the significance of CXC chemokines in wound healing, full excisional wounds were created on CXCR2 wild-type (+/+), heterozygous (+/-), or knockout (-/-) mice. Wounds were histologically analyzed for neutrophil and monocyte infiltration, neovascularization and epithelialization at days 3, 5, 7, and 10 postwounding. The CXCR2 -/- mice exhibited defective neutrophil recruitment, an altered temporal pattern of monocyte recruitment, and altered secretion of interleukin-1beta. Significant delays in wound healing parameters, including epithelialization and decreased neovascularization, were also observed in CXCR2 -/- mice. In vitro wounding experiments with cultures of keratinocytes established from -/- and +/+ mice revealed a retardation in wound closure in CXCR2 -/- keratinocytes, suggesting a role for this receptor on keratinocytes in epithelial resurfacing that is independent of neutrophil recruitment. These in vitro and in vivo studies further establish a pathophysiologic role for CXCR2 during cutaneous wound repair.

Characterization of AMC-HN-9, a cell line established from an undifferentiated carcinoma of the parotid gland: expression of alpha6beta4 with the absence of BP180 and 230

We recently reported the development of a cell line, AMC-HN-9, established from an undifferentiated carcinoma (UDC) of the parotid gland. AMC-HN-9 consists mostly of spindle-shaped cells, has poor in vitro adhesiveness and an in vitro appearance that is different from that of other epithelial cell lines. To test the hypothesis that structural or functional abnormalities of the hemidesmosomes might contribute to the morphological appearance and biology of UDCs, we studied the expression of hemidesmosomal proteins in AMC-HN-9. Flow cytometry, indirect immunofluorescence, immunoprecipitation, reverse transcriptase-polymerase chain reaction, and cytogenetic analysis were used. AMC-HN-9 cells express the alpha6 and beta4 integrin subunits at nearly the same intensity as head and neck squamous cell carcinoma cell lines. However, AMC-HN-9 does not express BP180 and BP230, although there is no gross deletion of the loci of the BP180 and BP230 genes, suggesting that a more subtle mechanism has silenced these genes. In conclusion, the failure to express certain hemidesmosomal proteins is a likely explanation for the functional and morphologic characteristics of UDC cells both in vivo and in vitro.

An induction gene trap for identifying a homeoprotein-regulated locus

An important issue in developmental biology is the identification of homeoprotein target genes. We have developed a strategy based on the internalization and nuclear addressing of exogenous homeodomains, using an engrailed homeodomain (EnHD) to screen an embryonic stem (ES) cell gene trap library. Eight integrated gene trap loci responded to EnHD. One is within the bullous pemphigoid antigen 1 (BPAG1) locus, in a region that interrupts two neural isoforms. By combining in vivo electroporation with organotypic cultures, we show that an already identified BPAG1 enhancer/promoter is differentially regulated by homeoproteins Hoxc-8 and Engrailed in the embryonic spinal cord and mesencephalon. This strategy can therefore be used for identifying and mutating homeoprotein targets. Because homeodomain third helices can internalize proteins, peptides, phosphopeptides, and antisense oligonucleotides, this strategy should be applicable to other intracellular targets for characterizing genetic networks involved in a large number of physiopathological states.

Structure and regulation of the envoplakin gene

Envoplakin, a member of the plakin family of proteins, is a component of desmosomes and the epidermal cornified envelope. To understand how envoplakin expression is regulated, we have analyzed the structure of the mouse envoplakin gene and characterized the promoters of both the human and mouse genes. The mouse gene consists of 22 exons and maps to chromosome 11E1, syntenic to the location of the human gene on 17q25. The exon-intron structure of the mouse envoplakin gene is common to all members of the plakin family: the N-terminal protein domain is encoded by 21 small exons, and the central rod domain and the C-terminal globular domain are coded by a single large exon. The C terminus shows the highest sequence conservation between mouse and human envoplakins and between envoplakin and the other family members. The N terminus is also conserved, with sequence homology extending to Drosophila Kakapo. A region between nucleotides -101 and 288 was necessary for promoter activity in transiently transfected primary keratinocytes. This region is highly conserved between the human and mouse genes and contains at least two different positively acting elements identified by site-directed mutagenesis and electrophoretic mobility shift assays. Mutation of a GC box binding Sp1 and Sp3 proteins or a combined E box and Krüppel-like element interacting with unidentified nuclear proteins virtually abolished promoter activity. 600 base pairs of the mouse upstream sequence was sufficient to drive expression of a beta-galactosidase reporter gene in the suprabasal layers of epidermis, esophagus, and forestomach of transgenic mice. Thus, we have identified a regulatory region in the envoplakin gene that can account for the expression pattern of the endogenous protein in stratified squamous epithelia.

Prenatal onset of axonopathy in Dystonia musculorum mice

Dystonia musculorum (dt) is a recessive hereditary neuropathy of the mouse. Affected animals display loss of limb coordination and twisting of the trunk. Sensory nerve fibers of these mice are severely reduced in number, and the remaining fibers present numerous axonal swellings. The gene defective in dt, dystonin (Dst), encodes a cytoskeletal linker protein that forms the bridge between F-actin and intermediate filaments. Dst is expressed during embryogenesis, whereas overt phenotype in dt mice only appears during the second week after birth. Here we show that axonal swellings are present in sensory nerve fibers of dt embryos as early as E15.5, before myelination and radial axonal growth have begun. Thus disease progression is gradual in dt mice, having begun during embryogenesis. In dt embryos, microtubule network disorganization and cytoplasmic organelle accumulation within axonal swellings were consistently observed. In addition, a few of the axonal swellings presented intermediate filament accumulation. These results demonstrate that dystonin is required for cytoskeleton organization during axonogenesis. They also suggest that axonal transport defects, through microtubule network perturbation, may be the primary mechanism of neurodegeneration in dt mice.

IgG autoantibodies from bullous pemphigoid patients recognize multiple antigenic reactive sites located predominantly within the B and C subdomains of the COOH-terminus of BP230

Bullous pemphigoid is a subepidermal bullous disorder characterized by an autoantibody response against the bullous pemphigoid antigen 230 (BP230) and the bullous pemphigoid antigen 180 (BP180), a cytoplasmic component and a transmembrane component, respectively, of hemidesmosomes. Although immunodominant sequences within the extracellular domain of BP180 have been identified, characterization of the antigenic sites on BP230 is still incomplete. To identify autoantibody-reactive sites on BP230 and to examine whether the targeted regions are contained within functionally important domains, recombinant fragments encompassing almost the entire BP230 were used to assess the reactivity of 25 bullous pemphigoid sera by immunoblotting. Our results demonstrate that (i) the region bearing the B and C subdomains of the COOH-terminus of BP230 contains immunodominant sequences recognized by the majority of bullous pemphigoid sera; (ii) additional autoantibody- reactive sites are present over extended regions of the NH2-terminal half of BP230 without evidence for antigenic cross-reactivity between the NH2- and COOH-termini of BP230; and, finally, (iii) autoantibodies reacting with the BP230 tail predominantly belong to the IgG4 and IgG1 subclasses, suggesting that both autoreactive TH2 and autoreactive TH1 cells regulate the autoantibody response to immunodominant sequences of BP230. As the COOH- terminus of BP230 mediates the attachment of keratin intermediate filaments to the hemidesmosomal plaque, whereas its NH2-terminus contains sequences important for its interaction with other constituents of hemidesmosomes, autoantibodies to BP230 might precipitate subepidermal blister formation and perpetuate the disease not only by eliciting an inflammatory reaction but also by interfering with the function of BP230 and thus the stability of hemidesmosomes.

Re-epithelialization rate and protein expression in the suction-induced wound model: comparison between intact blisters, open wounds and calcipotriol-pretreated open wounds

We have investigated re-epithelialization following induction of suction blisters in humans in intact blisters, open wounds, i.e. blister roofs removed immediately after blister induction, and calcipotriol-pretreated open wounds. Intact blisters simulate blister healing in bullous disease, while open wounds simulate re-epithelialization during wound healing. Re-epithelialization was clearly faster in open wounds than in intact blisters, and was not affected by calcipotriol pretreatment. Bullous pemphigoid antigen 2 (BP180), bullous pemphigoid antigen 1 (BP230), plectin/hemidesmosomal 1 protein (HD1), laminin 5, laminin alpha5, laminin beta1, type VII collagen, tenascin-C, beta4, alphavbeta5, alpha5 and alpha9 integrins were studied in intact blisters and open wounds by immunohistochemistry. Hemidesmosomal plaque proteins BP230 and plectin/HD1, which connect the keratin cytoskeleton to the hemidesmosome, appeared earlier at the leading edge in intact blisters than in open wounds. Band-like immunostaining in the basement membrane for laminin 5, alpha5 and beta1 chains was continuous in blister bases, but partially discontinuous in open wound bases. The other antigens studied showed similar expression in intact blisters and open wounds. BP180, BP230, plectin/HD1, beta4 integrin, laminin 5 and tenascin-C expression were further studied in calcipotriol-pretreated open wounds. Calcipotriol did not affect the expression of these antigens. The immunohistochemical results suggest that the keratin cytoskeleton is linked to the basal plasma membrane of migrating basal cells via BP230 and plectin/HD1 earlier in the more slowly re-epithelializing blisters than in open wounds. An intact laminin sheath may inhibit keratinocyte migration in intact blisters.

An epidermal plakin that integrates actin and microtubule networks at cellular junctions

Plakins are cytoskeletal linker proteins initially thought to interact exclusively with intermediate filaments (IFs), but recently were found to associate additionally with actin and microtubule networks. Here, we report on ACF7, a mammalian orthologue of the Drosophila kakapo plakin genetically involved in epidermal-muscle adhesion and neuromuscular junctions. While ACF7/kakapo is divergent from other plakins in its IF-binding domain, it has at least one actin (K(d) = 0.35 microM) and one microtubule (K(d) approximately 6 microM) binding domain. Similar to its fly counterpart, ACF7 is expressed in the epidermis. In well spread epidermal keratinocytes, ACF7 discontinuously decorates the cytoskeleton at the cell periphery, including microtubules (MTs) and actin filaments (AFs) that are aligned in parallel converging at focal contacts. Upon calcium induction of intercellular adhesion, ACF7 and the cytoskeleton reorganize at cell-cell borders but with different kinetics from adherens junctions and desmosomes. Treatments with cytoskeletal depolymerizing drugs reveal that ACF7's cytoskeletal association is dependent upon the microtubule network, but ACF7 also appears to stabilize actin at sites where microtubules and microfilaments meet. We posit that ACF7 may function in microtubule dynamics to facilitate actin-microtubule interactions at the cell periphery and to couple the microtubule network to cellular junctions. These attributes provide a clear explanation for the kakapo mutant phenotype in flies.

The alpha isoform of protein kinase C is involved in signaling the response of desmosomes to wounding in cultured epithelial cells

Initiation of reepithelialization upon wounding is still poorly understood. To enhance this understanding, we focus here on changes in the adhesive state of desmosomes of cultured Madin-Darby canine kidney cells in response to wounding of confluent cell sheets. Previous results show that desmosomal adhesion in Madin-Darby canine kidney cells changes from a calcium-dependent state to calcium independence in confluent cell sheets. We show that this change, which requires culture confluence to develop, is rapidly reversed upon wounding of confluent cell sheets. Moreover, the change to calcium dependence in wound edge cells is propagated to cells hundreds of micrometers away from the wound edge. Rapid transition from calcium independence to calcium dependence also occurs when cells are treated with phorbol esters that activate PKC. PKC inhibitors, including the conventional isoform inhibitor Gö6976, cause rapid transition from calcium dependence to calcium independence, even in subconfluent cells. The cellular location of the alpha isoform of PKC correlates with the calcium dependence of desmosomes. Upon monolayer wounding, PKCalpha translocates rapidly to the cell periphery, becomes Triton X-100 insoluble, and also becomes concentrated in lamellipodia. The PKCalpha translocation upon wounding precedes both the increase in PKC activity in the membrane fraction and the reversion of desmosomes to calcium dependence. Specific depletion of PKCalpha with an antisense oligonucleotide increases the number of cells with calcium-independent desmosomes. These results show that PKCalpha participates in a novel signaling pathway that modulates desmosomal adhesion in response to wounding.

short stop is allelic to kakapo, and encodes rod-like cytoskeletal-associated proteins required for axon extension

short stop (shot) is required for sensory and motor axons to reach their targets in the Drosophila embryo. Growth cones in shot mutants initiate at the normal times, and they appear normal with respect to overall morphology and their abilities to orient and fasciculate. However, sensory axons are unable to extend beyond a short distance from the cell body, and motor axons are unable to reach target muscles. The shot gene encodes novel actin binding proteins that are related to plakins and dystrophin and expressed in axons during development. The longer isoforms identified are predicted to contain an N-terminal actin binding domain, a long central triple helical coiled-coil domain, and a C-terminal domain that contains two EF-hand Ca(2+) binding motifs and a short stretch of homology to the growth arrest-specific 2 protein. Other isoforms lack all or part of the actin binding domains or are truncated and contain a different C-terminal domain. Only the isoforms containing full-length actin binding domains are detectably expressed in the nervous system. shot is allelic to kakapo, a gene that may function in integrin-mediated adhesion in the wing and embryo. We propose that Shot's interactions with the actin cytoskeleton allow sensory and motor axons to extend.

The N terminus of the transmembrane protein BP180 interacts with the N-terminal domain of BP230, thereby mediating keratin cytoskeleton anchorage to the cell surface at the site of the hemidesmosome

In epidermal cells, the keratin cytoskeleton interacts with the elements in the basement membrane via a multimolecular junction called the hemidesmosome. A major component of the hemidesmosome plaque is the 230-kDa bullous pemphigoid autoantigen (BP230/BPAG1), which connects directly to the keratin-containing intermediate filaments of the cytoskeleton via its C terminus. A second bullous pemphigoid antigen of 180 kDa (BP180/BPAG2) is a type II transmembrane component of the hemidesmosome. Using yeast two-hybrid technology and recombinant proteins, we show that an N-terminal fragment of BP230 can bind directly to an N-terminal fragment of BP180. We have also explored the consequences of expression of the BP230 N terminus in 804G cells that assemble hemidesmosomes in vitro. Unexpectedly, this fragment disrupts the distribution of BP180 in transfected cells but has no apparent impact on the organization of endogenous BP230 and alpha6beta4 integrin. We propose that the BP230 N terminus competes with endogenous BP230 protein for BP180 binding and inhibits incorporation of BP180 into the cell surface at the site of the hemidesmosome. These data provide new insight into those interactions of the molecules of the hemidesmosome that are necessary for its function in integrating epithelial and connective tissue types.

Microtubule actin cross-linking factor (MACF): a hybrid of dystonin and dystrophin that can interact with the actin and microtubule cytoskeletons

We cloned and characterized a full-length cDNA of mouse actin cross-linking family 7 (mACF7) by sequential rapid amplification of cDNA ends-PCR. The completed mACF7 cDNA is 17 kb and codes for a 608-kD protein. The closest relative of mACF7 is the Drosophila protein Kakapo, which shares similar architecture with mACF7. mACF7 contains a putative actin-binding domain and a plakin-like domain that are highly homologous to dystonin (BPAG1-n) at its NH(2) terminus. However, unlike dystonin, mACF7 does not contain a coiled-coil rod domain; instead, the rod domain of mACF7 is made up of 23 dystrophin-like spectrin repeats. At its COOH terminus, mACF7 contains two putative EF-hand calcium-binding motifs and a segment homologous to the growth arrest-specific protein, Gas2. In this paper, we demonstrate that the NH(2)-terminal actin-binding domain of mACF7 is functional both in vivo and in vitro. More importantly, we found that the COOH-terminal domain of mACF7 interacts with and stabilizes microtubules. In transfected cells full-length mACF7 can associate not only with actin but also with microtubules. Hence, we suggest a modified name: MACF (microtubule actin cross-linking factor). The properties of MACF are consistent with the observation that mutations in kakapo cause disorganization of microtubules in epidermal muscle attachment cells and some sensory neurons.

Cytoskeletal linkers: new MAPs for old destinations

A new isoform of the actin-neurofilament linker protein BPAG has been found that binds to and stabilizes axonal microtubules. This and other newly identified microtubule-associated proteins are likely to be just the tip of an iceberg of multifunctional proteins that stabilize and crosslink cytoskeletal filament networks.

Differential display and cloning of the hippocampal gene mRNas in senescence accelerated mouse

Identification of genes that are specifically expressed in the hippocampus of senescence accelerated mouse (SAM) is important for understanding the molecular basis of the pathological changes in the brain and of the deterioration of learning and memory in SAM-prone/8 (SAMP8), a substrain of SAM. The differential display technique was applied to compare mRNAs expression between SAMP8 and SAM-resistance 1 (SAMR1), another substrain of SAM. Complementary DNA fragments corresponding to several apparently differentially expressed mRNAs were recovered and sequenced. Six differentially expressed cDNA bands were identified. Sequence analyses demonstrated that W4 and W5 cDNA fragments corresponded to unknown genes. W1 and W6 showed 66.1 and 62.3% homology to rat GTP-exchange protein (eIF-2B) and rat phospholipase D, respectively, while W2 and W3 showed 89.2 and 90.8% homology to human bullous pemphigoid antigen and human glucogen debranching enzyme isoform 1/2/3/4/6, respectively. The results suggested that these genes are closely related to the malfunction of the brain in SAM.

Binding of integrin alpha6beta4 to plectin prevents plectin association with F-actin but does not interfere with intermediate filament binding

Hemidesmosomes are stable adhesion complexes in basal epithelial cells that provide a link between the intermediate filament network and the extracellular matrix. We have investigated the recruitment of plectin into hemidesmosomes by the alpha6beta4 integrin and have shown that the cytoplasmic domain of the beta4 subunit associates with an NH(2)-terminal fragment of plectin that contains the actin-binding domain (ABD). When expressed in immortalized plectin-deficient keratinocytes from human patients with epidermol- ysis bullosa (EB) simplex with muscular dystrophy (MD-EBS), this fragment is colocalized with alpha6beta4 in basal hemidesmosome-like clusters or associated with F-actin in stress fibers or focal contacts. We used a yeast two-hybrid binding assay in combination with an in vitro dot blot overlay assay to demonstrate that beta4 interacts directly with plectin, and identified a major plectin-binding site on the second fibronectin type III repeat of the beta4 cytoplasmic domain. Mapping of the beta4 and actin-binding sites on plectin showed that the binding sites overlap and are both located in the plectin ABD. Using an in vitro competition assay, we could show that beta4 can compete out the plectin ABD fragment from its association with F-actin. The ability of beta4 to prevent binding of F-actin to plectin explains why F-actin has never been found in association with hemidesmosomes, and provides a molecular mechanism for a switch in plectin localization from actin filaments to basal intermediate filament-anchoring hemidesmosomes when beta4 is expressed. Finally, by mapping of the COOH-terminally located binding site for several different intermediate filament proteins on plectin using yeast two-hybrid assays and cell transfection experiments with MD-EBS keratinocytes, we confirm that plectin interacts with different cytoskeletal networks.

Regulation of epidermal bullous pemphigoid antigen 1 (BPAG1) synthesis by homeoprotein transcription factors

In a recent gene-trap screen, we identified the gene coding for Epidermal Bullous Pemphigoid Antigen 1 (BPAG1) as a putative transcriptional target of Engrailed and of other homeoproteins with a glutamine in position 50 of their homeodomain. We now show that the nuclear addressing of the homeodomains of Engrailed (EnHD) and Antennapedia (AntpHD) upregulates BPAG1e transcription in immortalized human keratinocytes (GMA24FIA) expressing En1. This upregulation is not observed with AntpHD-Q50A, a variant of AntpHD in which a single mutation abolishes its high-affinity binding to target DNA, thus strongly suggesting that BPAG1e upregulation homeodomains reflects their specific recognition of homeoprotein-binding sites in the BPAG1e locus. This is further confirmed by DNase I footprinting and electrophoretic mobility shift assays that reveal, within the cloned BPAG1e promoter, several sites of direct interaction with EnHD and Engrailed. Co-transfection experiments in GMA24FIA human keratinocytes, COS-7 simian fibroblasts, and CHP-100 human neuroepithelial cells show that Engrailed, Hoxa-5, and Hoxc-8 regulate BPAG1e promoter activity and that this regulation is context-dependent. Finally, using a mouse line with LacZ inserted within the En1 locus, we identify the keratinocytes of the ventral paws, including the epithelial cells of the eccrine tubules, as a strong site of En1 expression throughout adulthood. We therefore propose that BPAG1e, a 230 kDa keratin-binding protein expressed in keratinocytes and participating in the maintenance of hemidesmosomes at the dermis-epidermis border, is directly regulated by homeoprotein transcription factors.

Protein kinase C-dependent mobilization of the alpha6beta4 integrin from hemidesmosomes and its association with actin-rich cell protrusions drive the chemotactic migration of carcinoma cells

We explored the hypothesis that the chemotactic migration of carcinoma cells that assemble hemidesmosomes involves the activation of a signaling pathway that releases the alpha6beta4 integrin from these stable adhesion complexes and promotes its association with F-actin in cell protrusions enabling it to function in migration. Squamous carcinoma-derived A431 cells were used because they express alpha6beta4 and migrate in response to EGF stimulation. Using function-blocking antibodies, we show that the alpha6beta4 integrin participates in EGF-stimulated chemotaxis and is required for lamellae formation on laminin-1. At concentrations of EGF that stimulate A431 chemotaxis ( approximately 1 ng/ml), the alpha6beta4 integrin is mobilized from hemidesmosomes as evidenced by indirect immunofluorescence microscopy using mAbs specific for this integrin and hemidesmosomal components and its loss from a cytokeratin fraction obtained by detergent extraction. EGF stimulation also increased the formation of lamellipodia and membrane ruffles that contained alpha6beta4 in association with F-actin. Importantly, we demonstrate that this mobilization of alpha6beta4 from hemidesmosomes and its redistribution to cell protrusions occurs by a mechanism that involves activation of protein kinase C-alpha and that it is associated with the phosphorylation of the beta4 integrin subunit on serine residues. Thus, the chemotactic migration of A431 cells on laminin-1 requires not only the formation of F-actin-rich cell protrusions that mediate alpha6beta4-dependent cell movement but also the disruption of alpha6beta4-containing hemidesmosomes by protein kinase C.

In vitro characteristics of early epidermal progenitors isolated from keratin 14 (K14)-deficient mice: insights into the role of keratin 17 in mouse keratinocytes

Keratin 14 (K14) is believed to play a pivotal role in the maintenance of epidermal cell shape and contributing to their resistance to mechanical trauma, thereby protecting the cells from lysing. Mice harboring a K14 null mutation produce phenotypic characteristics of epidermolysis bullosa simplex, a skin blistering disease (Lloyd et al., 1995, J Cell Biol 129:1329-1344). K14 null animals die several days after birth, making the detailed study of the consequences of K14 deletion in epidermal cell physiology in vivo particularly difficult. To define the consequences of K14 loss more precisely, we used an in vitro approach by isolating K14-/- cell lines and studying epidermal differentiation in the K14 null background. Several keratinocyte cell lines were generated from 6-day-old mice homozygous for a targeted disruption of the K14 gene (lines designated MKC-5, MKC-23, and MKC-33) and from their wild-type littermates (lines designated MKC-1 and MKC-6). Under low Ca2+ (0.066 mM) and low serum (2%) conditions, both wild-type and mutant cells were able to adhere to collagen type I-coated dishes and form epithelial sheets. They maintained basal epidermal cell characteristics and continued to proliferate without obvious signs of terminal differentiation; however, K14-/- cells proliferated two- to threefold slower than did their wild-type counterparts. The distribution of K5, the natural partner of K14, at the immunofluorescence level was also normal looking in the K14-/- MKC-5 cells, but with fewer filaments detectable, consistent with the approximately 20% reduction in K5 detectable on immunoblots. K17 expression was increased approximately 40% in the K14-/- cells. The levels of K15 and K16 were not different in the MKC-5 and MKC-6 cell lines, suggesting that they are not contributing factors to the stabilization of K5 in the mutant cells. K8, K19, and vimentin were undetectable in both lines. Both MKC-5 and MKC-6 cells underwent morphological and biochemical differentiation in response to a switch to high Ca2+ medium. These findings indicate that K14-/- MKC-5 cells preserve the morphological, biochemical, and physiological characteristics of epidermal cells for an extensive period of time in vitro, likely due to the compensatory expression of K17. The culturing capacity of these cells also permits the analysis of keratinocyte growth and differentiation in the absence of K14. In addition, the culturing methods we describe will be useful for the generation of epithelial cell lines from a wealth of increasingly available knockout mouse strains with early lethality.

Regulation of the type II hemidesmosomal plaque assembly in intestinal epithelial cells

Hemidesmosomes (HDs) are cellular junctions that anchor epithelial cells to the extracellular matrix (ECM) and are associated morphologically with the cytoskeleton. Hemidesmosomal molecular components include two proteins involved in linking intermediate filaments, HD1/plectin and BP230, and two transmembrane proteins, BP180 and the alpha6beta4 integrin, a laminin receptor. In cells lacking BP230 and BP180, HD1/plectin still associates with alpha6beta4 integrin, forming HD-like structures, called type II HDs. In the present study, we used an intestinal epithelial cell line that expresses HD1/plectin and the alpha6beta4 integrin to investigate the regulation of assembly of these proteins in type II HDs. These compounds were found to be clustered at sites of cell-ECM contact and their polarized localization was influenced by either cell confluency or extracellular matrix deposition. Conventional and immunoelectron microscopy showed that HD1/plectin and the beta4 integrin subunit are colocalized in an adhesion structure. Using cytoskeleton-disrupting drugs and confocal microscopy, we demonstrated that type II HDs are made up of numerous individual plaques whose assembly into a cluster requires actin filaments, but not microtubules.

Integrators of the cytoskeleton that stabilize microtubules

Sensory neurodegeneration occurs in mice defective in BPAG1, a gene encoding cytoskeletal linker proteins capable of anchoring neuronal intermediate filaments to actin cytoskeleton. While BPAG1 null mice fail to anchor neurofilaments (NFs), BPAG1/NF null mice still degenerate in the absence of NFs. We report a novel neural splice form that lacks the actin-binding domain and instead binds and stabilizes microtubules. This interaction is functionally important; in mice and in vitro, neurons lacking BPAG1 display short, disorganized, and unstable microtubules defective in axonal transport. Ironically, BPAG1 neural isoforms represent microtubule-associated proteins that when absent lead to devastating consequences. Moreover, BPAG1 can functionally account for the extraordinary stability of axonal microtubules necessary for transport over long distances. Its isoforms interconnect all three cytoskeletal networks, a feature apparently central to neuronal survival.

Dystonin-deficient mice exhibit an intrinsic muscle weakness and an instability of skeletal muscle cytoarchitecture

Dystonia musculorum (dt) was originally described as a hereditary sensory neurodegeneration syndrome of the mouse. The gene defective in dt encodes a cytoskeletal linker protein, dystonin, that is essential for maintaining neuronal cytoskeletal integrity. In addition to the nervous system, dystonin is expressed in a variety of other tissues, including muscle. We now show that dystonin cross-links actin and desmin filaments and that its levels are increased during myogenesis, coinciding with the progressive reorganization of the intermediate filament network. A disorganization of cytoarchitecture in skeletal muscle from dt/dt mice was observed in ultrastructural studies. Myoblasts from dt/dt mice fused to form myotubes in culture; however, terminally differentiated myotubes contained incompletely assembled myofibrils. Another feature observed in dt/dt myotubes in culture and in skeletal muscle in situ was an accumulation and abnormal distribution of mitochondria. The diaphragm muscle from dt/dt mice was weak in isometric contractility measurements in vitro and was susceptible to contraction-induced sarcolemmal damage. Altogether, our data indicate that dystonin is a cross-linker of actin and desmin filaments in muscle and that it is essential for establishing and maintaining proper cytoarchitecture in mature muscle.

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.

Defects of basement membrane and hemidesmosome structure correlate with malignant phenotype and stromal interactions in HaCaT-Ras xenografts

Benign and malignant HaCaT-ras clones, derived from immortalized HaCaT cells were grown as nude mouse surface transplants rendering a human tumor progression model. Searching for malignancy-related alterations, the deposition, localization and mRNA of basement membrane and hemidesmosome components were analysed by immunofluorescence, in situ hybridization and electron microscopy. Initially, at 1 week epithelia of benign and malignant cells revealed a similarly low polarity and an enlarged 'activated basal' compartment, reflected by partial dislocation and extended pericellular staining of the hemidesmosome constituent integrin alpha 6 beta 4 seen by immunofluorescence. Whereas benign grafts eventually normalized, closely resembling grafts of HaCaT cells, malignant growth was correlated with a persisting epithelial activation state and continuing higher expression of alpha 6 (by immunofluorescence and in situ hybridization). The basement membrane components bullous pemphigoid antigen 1, laminin-5 and collagen IV exhibited a largely linear distribution at 1 week. However, in the malignant cell transplants initially minor basement membrane discontinuities became more severe at around 2 weeks, associated with close stromal cell contacts, angiogenesis and invasion. Most striking were basement membrane alterations seen by electron microscopy. At 1 week stretches of basement membrane had developed in malignant transplants, though to a much lesser extent than in benign specimens. With invasion these basement membrane structures mostly disappeared despite persistent although variable immunofluorescence, suggesting high turnover without ultrastructural assembly. The hemidesmosome structures were defective throughout, completely lacking anchoring plaques with keratin filaments, whereas they were still associated with basement membrane deposits. Thus, malignant HaCaT-ras transplants, while initially resembling regenerating wounds, revealed an increasing loss of tissue polarity and basement membrane structures, which seemed to be accelerated upon stromal cell contacts.

Structure and function of hemidesmosomes: more than simple adhesion complexes

The attachment of cells to the extracellular matrix is of crucial importance in the maintenance of tissue structure and integrity. In stratified epithelia such as in skin as well as in other complex epithelia multiprotein complexes called hemidesmosomes are involved in promoting the adhesion of epithelial cells to the underlying basement membrane. In the past few years our understanding of the role of hemidesmosomes has improved considerably. Their importance has become apparent in clinical conditions, in which absence or defects of hemidesmosomal proteins result in devastating blistering diseases of the skin. Molecular genetic studies have increased our knowledge of the function of the various components of hemidesmosomes and enabled the characterization of protein-protein interactions involved in their assembly. It has become clear that the alpha6beta4 integrin, a major component of hemidesmosomes, is able to transduce signals from the extracellular matrix to the interior of the cell, that critically modulate the organization of the cytoskeleton, proliferation, apoptosis, and differentiation. Nevertheless, our knowledge of the mechanisms regulating the functional state of hemidesmosomes and, hence, the dynamics of cell adhesion, a process of crucial importance in development, wound healing or tumor invasion, remains limited. The aims of this review are to highlight the recent progresses of our knowledge on the organization and assembly of hemidesmosomes, their involvement in signaling pathways as well as their participation in clinical pathologic conditions.

The intermediate filament protein peripherin is the specific interaction partner of mouse BPAG1-n (dystonin) in neurons

The dystonia musculorum (dt) mouse suffers from severe degeneration of primary sensory neurons. The mutated gene product is named dystonin and is identical to the neuronal isoform of bullous pemphigoid antigen 1 (BPAG1-n). BPAG1-n contains an actin-binding domain at its NH2 terminus and a putative intermediate filament-binding domain at its COOH terminus. Because the degenerating sensory neurons of dt mice display abnormal accumulations of intermediate filaments in the axons, BPAG1-n has been postulated to organize the neuronal cytoskeleton by interacting with both the neurofilament triplet proteins (NFTPs) and microfilaments. In this paper we show by a variety of methods that the COOH-terminal tail domain of mouse BPAG1 interacts specifically with peripherin, but in contrast to a previous study (Yang, Y., J. Dowling, Q.C. Yu, P. Kouklis, D.W. Cleveland, and E. Fuchs. 1996. Cell. 86:655-665), mouse BPAG1 fails to associate with full-length NFTPs. The tail domains interfered with the association of the NFTPs with BPAG1. In dt mice, peripherin is present in axonal swellings of degenerating sensory neurons in the dorsal root ganglia and is downregulated even in other neural regions, which have no obvious signs of pathology. Since peripherin and BPAG1-n also display similar expression patterns in the nervous system, we suggest that peripherin is the specific interaction partner of BPAG1-n in vivo.

Biology and function of hemidesmosomes

Hemidesmosomes are cell-substratum adhesion sites that connect the extracellular matrix to the keratin cytoskeleton. Our knowledge of the function of these structures has greatly increased as a result of studies on patients with aberrant expression of hemidesmosome components and studies using targeted inactivation of mouse genes encoding these components. Insight into the formation of hemidesmosomes, as well as into protein-protein interactions that occur in these junctional complexes, has recently been gained by in vitro cell transfections, blot overlay and yeast two-hybrid assays. In addition, recent results indicate that the alpha6 beta4 integrin is involved in the transduction of signals that are induced by the extracellular matrix and which modulate processes as diverse as cell proliferation, differentiation, apoptosis, migration and tissue morphogenesis. Thus it seems that hemidesmosomes do not merely maintain dermo-epidermal adhesion and tissue integrity, but that they are also implicated in intracellular signaling. Here we discuss recently published data on the biology and function of hemidesmosomes.

Mutation analysis and molecular genetics of epidermolysis bullosa

Cutaneous basement membrane zone (BMZ) consists of a number of attachment structures that are critical for stable association of the epidermis to the underlying dermis. These include hemidesmosomes, anchoring filaments and anchoring fibrils which form an interconnecting network extending from the intracellular milieu of basal keratinocytes across the dermal-epidermal basement membrane to the underlying dermis. Aberrations in this network structure, e.g. due to genetic lesions in the corresponding genes, can result in fragility of the skin at the level of the cutaneous BMZ. The prototype of such diseases is epidermolysis bullosa (EB), a heterogeneous group of genodermatoses characterized by fragility and blistering of the skin, often associated with extracutaneous manifestations, and inherited either in an autosomal dominant or autosomal recessive manner. Based on constellations of the phenotypic manifestations, severity of the disease, and the level of tissue separation within the cutaneous BMZ, EB has been divided into clinically distinct subcategories, including the simplex, hemidesmosomal, junctional and dystrophic variants. Elucidation of BMZ gene/protein systems and development of mutation detection strategies have allowed identification of mutations in 10 different BMZ genes which can explain the clinical heterogeneity of EB. These include mutations in the type VII collagen gene (COL7A1) in the dystrophic (severely scarring) forms of EB; mutations in the laminin 5 genes (LAMA3, LAMB3 and LAMC2) in a lethal (Herlitz) variant of junctional EB; aberrations in the type XVII collagen gene (COL17A1) in non-lethal forms of junctional EB; mutations in the alpha6 and beta4 integrin genes in a distinct hemidesmosomal variant of EB with congenital pyloric atresia; and mutations in the plectin gene (PLEC1) in a form of EB associated with late-onset muscular dystrophy. Identification of mutations in these gene/protein systems attests to their critical importance in the overall stability of the cutaneous BMZ. Furthermore, elucidation of mutations in different variants of EB has direct clinical applications in terms of refined classification, improved genetic counseling, and development of DNA-based prenatal testing in families with EB.

Skin fragility and hypohidrotic ectodermal dysplasia resulting from ablation of plakophilin 1

We report a 2-year-old boy with an unusual autosomal recessively inherited skin disease comprising trauma-induced skin fragility and congenital ectodermal dysplasia affecting hair, nails and sweat glands. Skin biopsy showed widening of intercellular spaces between keratinocytes and ultrastructural findings of small, poorly formed desmosomes with reduced connections to the keratin filament cytoskeleton. Immunohistochemical analysis revealed a complete absence of staining for the accessory desmosomal plaque protein plakophilin 1 (PKP1; band 6 protein). The affected individual was a compound heterozygote for null mutations on both alleles of the PKP1 gene. Both mutations occurred within the amino terminus of PKP1, the domain which normally binds the cytoskeletal keratin filament network to the cell membrane. Apart from its localization within desmosomal plaques, PKP1 may also be present within the cytoplasm and nucleus and has putative roles in signal transduction and regulation of gene activity. The clinicopathological observations in this patient demonstrate the relevance of PKP1 to desmosome formation, cutaneous cell-cell adhesion and epidermal development and demonstrate the specific manifestations of human functional knockout mutations in this gene.

Hemidesmosomal molecular changes in dermatitis herpetiformis; decreased expression of BP230 and plectin/HD1 in uninvolved skin

Recent BP230-knockout experiments with subsequent blistering and recently identified plectin/HD1 mutations in epidermolysis bullosa simplex patients suggest that defective expression of BP230 and plectin/HD1 may predispose to blister formation in human skin. We have studied the expression of the epithelial adhesion complex as well as the basement membrane and anchoring fibril antigens in uninvolved dermatitis herpetiformis skin to find out if alterations can be detected in these structures predisposing to the blister formation typical of the disease. Ten uninvolved dermatitis herpetiformis skin specimens, which all showed clear granular deposits of IgA under the basement membrane in direct immunofluorescence and five normal skin specimens, were studied by indirect immunofluorescence technique. Six uninvolved dermatitis herpetiformis skin specimens showed distinctly decreased immunoreaction for BP230 and four uninvolved dermatitis herpetiformis skin specimens showed distinctly decreased immunoreaction for plectin/HD1. All five skin controls showed strong immunoreactions for BP230 and plectin/HD1. Other hemidesmosomal proteins including BP180 and integrin alpha6beta4, as well as basement membrane proteins laminin-5, laminin-1, nidogen and type IV collagen, and the anchoring fibril protein type VII collagen showed a normal strong expression. Our results suggest that alterations in BP230 and plectin/HD1 may contribute or predispose to blister formation in dermatitis herpetiformis skin.