Ligand-independent role of the beta 4 integrin subunit in the formation of hemidesmosomes

Junctional epidermolysis bullosa: genotype-phenotype correlations

Junctional epidermolysis bullosa most commonly results from mutations in theLAMA3, LAMB3, LAMC2, COL17A1, ITGA6 and ITGB4genes. Junctional epidermolysis bullosa is characterized by clinical heterogeneity. To date, scientific findings allow to evaluate correlations between the severity of clinical manifestations and genetic defects underlying in the development of the disease. A systematic literature search was performed using PubMed and RSCI, and keywords including junctional epidermolysis bullosa, laminin 332, collagen XVII, 64 integrin. The review includes description of clinical findings of junctional epidermolysis bullosa, mutation location and types, its impact on protein production and functions. To evaluate the impact of gene mutation on protein functions, this review explores the structure and functions of lamina lucida components, including laminin 332, collagen XVII and 64 integrin, which are frequently associated with the development of junctional epidermolysis bullosa. The correlation between severe types of junctional epidermolysis bullosa and mutations resulting in premature stop codon generation and complete absence of protein expression has been described. Although, genotype-phenotype correlations should be analyzed carefully due to mechanisms which enable to improve protein expression.

Loss of α6β4 Integrin-Mediated Hemidesmosomes Promotes Prostate Epithelial Cell Migration by Stimulating Focal Adhesion Dynamics

Epithelial cell adhesion is mediated by actin cytoskeleton-linked focal adhesions (FAs) and intermediate filament-associated hemidesmosomes (HDs). HDs are formed by α6β4-integrins and mediate stable anchoring to the extracellular matrix (ECM) while FAs containing β1-integrins regulate cell migration. Loss of HDs has been reported in various cancers such as prostate cancer where it correlates with increased invasive migration. Here we have studied cell migration properties and FA dynamics in genetically engineered prostate epithelial cell lines with intact or disrupted HDs. Disruption of HDs by depleting α6- or β4-integrin expression promoted collective cell migration and modulated migratory activity. Dynamic analysis of fluorescent protein-tagged FA marker proteins revealed faster FA assembly and disassembly kinetics in HD-depleted cells. FRAP analysis showed that loss of HDs correlated with faster diffusion rates of focal adhesion kinase (FAK) and vinculin in and out of FAs. These data suggest that loss of α6β4-mediated HDs promote cell migration and FA assembly dynamics by influencing the molecular diffusion rates of FAK.

Regulation of hemidesmosome dynamics and cell signaling by integrin α6β4

Hemidesmosomes (HDs) are specialized multiprotein complexes that connect the keratin cytoskeleton of epithelial cells to the extracellular matrix (ECM). In the skin, these complexes provide stable adhesion of basal keratinocytes to the underlying basement membrane. Integrin α6β4 is a receptor for laminins and plays a vital role in mediating cell adhesion by initiating the assembly of HDs. In addition, α6β4 has been implicated in signal transduction events that regulate diverse cellular processes, including proliferation and survival. In this Review, we detail the role of α6β4 in HD assembly and beyond, and we discuss the molecular mechanisms that regulate its function.

A novel miR-365-3p/EHF/keratin 16 axis promotes oral squamous cell carcinoma metastasis, cancer stemness and drug resistance via enhancing β5-integrin/c-met signaling pathway

Background

Targeting the c-Met signaling pathway has become a therapeutic strategy in multiple types of cancer. We unveiled a novel c-Met regulating mechanism that could be applied as a modality for oral squamous cell carcinoma (OSCC) therapy.

Methods

Upregulation of keratin 16 (KRT16) was found by comparing isogenic pairs of low and high invasive human OSCC lines via microarray analysis. OSCC cells with ectopic expression or silencing of KRT16 were used to scrutinize functional roles and associated molecular mechanisms.

Results

We observed that high KRT16 expression significantly correlated with poorer pathological differentiation, advanced stages, increased lymph nodes metastasis, and decreased survival rate from several Taiwanese OSCC patient cohorts. We further revealed that miR-365-3p could target ETS homologous factor (EHF), a KRT16 transcription factor, to decrease migration, invasion, metastasis and chemoresistance in OSCC cells via inhibition of KRT16. Under confocal microscopic examination, c-Met was found possibly partially associates with KRT16 through β5-integrin. Colocalization of these three proteins may facilitate c-Met and β5-integrin–mediated signaling in OSCC cells. Depletion of KRT16 led to increased protein degradation of β5-integrin and c-Met through a lysosomal pathway leading to inhibition of their downstream Src/STAT3/FAK/ERK signaling in OSCC cells. Knockdown of KRT16 enhanced chemosensitivity of OSCC towards 5-fluorouracil (5-FU). Various combination of c-Met inhibitor (foretinib), protein tyrosine kinase inhibitor (genistein), β5-integrin antibody, and 5-FU markedly augmented cytotoxic effects in OSCC cells as well as tumor killing effects in vitro and in vivo.

Conclusions

Our data indicate that targeting a novel miR-365-3p/EHF/KRT16/β5-integrin/c-Met signaling pathway could improve treatment efficacy in OSCC.

Electronic supplementary material

The online version of this article (10.1186/s13046-019-1091-5) contains supplementary material, which is available to authorized users.

Keratins Stabilize Hemidesmosomes through Regulation of β4-Integrin Turnover

Epidermal integrity and wound healing depend on remodeling of cell-matrix contacts including hemidesmosomes. Mutations in β4-integrin and plectin lead to severe epidermolysis bullosa (EB). Whether mutations in keratins K5 or K14, which cause EB simplex, also compromise cell-matrix adhesion through altering hemidesmosomal components is not well investigated. In particular, the dependence of β4-integrin endocytosis and turnover on keratins remains incompletely understood. Here, we show that the absence of keratins causes loss of plectin-β4-integrin interaction and elevated β4-integrin phosphorylation at Ser1354 and Ser1362. This triggered a caveolin-dependent endocytosis of β4-integrin but not of other integrins through Rab5 and Rab11 compartments in keratinocytes. Expressing a phospho-deficient β4-integrin mutant reduces β4-integrin endocytosis and rescues plectin localization in keratin-free cells. β4-integrin phosphorylation in the absence of keratins resulted from elevated Erk1/2 activity downstream of increased EGFR and PKCα signaling. Further, increased Erk1/2 phosphorylation and altered plectin localization occur in keratin-deficient mouse epidermis in vivo. Strikingly, expression of the K14-R125P EBS mutant also resulted in plectin mislocalization and elevated β4-integrin turnover, suggesting disease relevance. Our data underscore a major role of keratins in controlling β4-integrin endocytosis involving a plectin-Erk1/2-dependent mechanism relevant for epidermal differentiation and pathogenesis.

SYK interaction with ITGβ4 suppressed by Epstein-Barr virus LMP2A modulates migration and invasion of nasopharyngeal carcinoma cells

Epstein-Barr virus (EBV)-encoded Latent Membrane Protein 2A (LMP2A) is an EBV latency-associated protein regularly expressed in nasopharyngeal carcinoma (NPC). In B cells, LMP2A activity resembles that of a constitutively activated antigen receptor, which recruits the Syk tyrosine kinase to activate a set of downstream signaling pathways. LMP2A also downregulates cellular Syk levels. In the present study, we demonstrate that Syk interacts with the integrin β4 subunit (ITGβ4) of integrin α6β4 in epithelial cells and that concurrent LMP2A expression interferes with this interaction by competitive binding to Syk. We find that both Syk and LMP2A have an effect on ITGβ4 cell surface expression. However, in LMP2A expressing cells, ITGβ4 remains concentrated at the cellular protrusions, an expression pattern characteristic of motile cells, including NPC-derived epithelial cells. This effect of LMP2A on ITGβ4 localization is associated with a greater propensity for migration and invasion in-vitro, and may contribute to the invasive property of LMP2A-expressing NPC.

Two distinct integrin-mediated mechanisms contribute to apical lumen formation in epithelial cells

BACKGROUND

Formation of apical compartments underlies the morphogenesis of most epithelial organs during development. The extracellular matrix (ECM), particularly the basement membrane (BM), plays an important role in orienting the apico-basal polarity and thereby the positioning of apical lumens. Integrins have been recognized as essential mediators of matrix-derived polarity signals. The importance of β1-integrins in epithelial polarization is well established but the significance of the accompanying α-subunits have not been analyzed in detail.

PRINCIPAL FINDINGS

Here we demonstrate that two distinct integrin-dependent pathways regulate formation of apical lumens to ensure robust apical membrane biogenesis under different microenvironmental conditions; 1) α2β1- and α6β4-integrins were required to establish a basal cue that depends on Rac1-activity and guides apico-basal cell polarization. 2) α3β1-integrins were implicated in positioning of mitotic spindles in cysts, a process that is essential for Cdc42-driven epithelial hollowing.

SIGNIFICANCE

Identification of the separate processes driven by particular integrin receptors clarifies the functional hierarchies between the different integrins co-expressed in epithelial cells and provides valuable insight into the complexity of cell-ECM interactions thereby guiding future studies addressing the molecular basis of epithelial morphogenesis during development and disease.

Serine phosphorylation of the integrin beta4 subunit is necessary for epidermal growth factor receptor induced hemidesmosome disruption

Hemidesmosomes (HDs) are multiprotein adhesion complexes that promote attachment of epithelial cells to the basement membrane. The binding of alpha6beta4 to plectin plays a central role in their assembly. We have defined three regions on beta4 that together harbor all the serine and threonine phosphorylation sites and show that three serines (S1356, S1360, and S1364), previously implicated in HD regulation, prevent the interaction of beta4 with the plectin actin-binding domain when phosphorylated. We have also established that epidermal growth factor receptor activation, which is known to function upstream of HD disassembly, results in the phosphorylation of only one or more of these three residues and the partial disassembly of HDs in keratinocytes. Additionally, we show that S1360 and S1364 of beta4 are the only residues phosphorylated by PKC and PKA in cells, respectively. Taken together, our studies indicate that multiple kinases act in concert to breakdown the structural integrity of HDs in keratinocytes, which is primarily achieved through the phosphorylation of S1356, S1360, and S1364 on the beta4 subunit.

Current insights into the formation and breakdown of hemidesmosomes

Hemidesmosomes are multiprotein adhesion complexes that promote epithelial stromal attachment in stratified and complex epithelia. Modulation of their function is of crucial importance in a variety of biological processes, such as differentiation and migration of keratinocytes during wound healing and carcinoma invasion, in which cells become detached from the substrate and acquire a motile phenotype. Although much is known about the signaling potential of the alpha6beta4 integrin in carcinoma cells, the events that coordinate the disassembly of hemidesmosomes during differentiation and wound healing remain unclear. The binding of alpha6beta4 to plectin has a central role in hemidesmosome assembly and it is becoming clear that disrupting this interaction is a crucial event in hemidesmosome disassembly. In addition, further insight into the functional interplay between alpha3beta1 and alpha6beta4 has contributed to our understanding of hemidesmosome disassembly and cell migration.

Modeling and Experimental Validation of the Binary Complex of the Plectin Actin-binding Domain and the First Pair of Fibronectin Type III (FNIII) Domains of the β4 Integrin

The binding of plectin to the beta4 subunit of the alpha6beta4 integrin is a critical step in the formation of hemidesmosomes. An important interaction between these two proteins occurs between the actin-binding domain (ABD) of plectin and the first pair of fibronectin type III (FNIII) domains and a small part of the connecting segment of beta4. Previously, a few amino acids, critical for this interaction, were identified in both plectin and beta4 and mapped on the crystal structures of the ABD of plectin and the first pair of FNIII domains of beta4. In the present study, we used this biochemical information and protein-protein docking calculations to construct a model of the binary complex between these two protein domains. The top scoring computational model predicts that the calponin-homology 1 (CH1) domain of the ABD associates with the first and the second FNIII domains of beta4. Our mutational analysis of the residues at the proposed interface of both the FNIII and the CH1 domains is in agreement with the suggested interaction model. Computational simulations to predict protein motions suggest that the exact model of FNIII and plectin CH1 interaction might well differ in detail from the suggested model due to the conformational plasticity of the FNIII domains, which might lead to a closely related but different mode of interaction with the plectin-ABD. Furthermore, we show that Ser-1325 in the connecting segment of beta4 appears to be essential for the recruitment of plectin into hemidesmosomes in vivo. This is consistent with the proposed model and previously published mutational data. In conclusion, our data support a model in which the CH1 domain of the plectin-ABD associates with the groove between the two FNIII domains of beta4.

Integrin clipping: a novel adhesion switch?

During human prostate cancer progression, the majority of normally expressed integrins are suppressed with the exception of the alpha6, alpha3, and beta1 integrins. We have shown that in prostate cancer, the alpha6 integrin is found paired with the beta1 integrin and that a novel form of the alpha6 integrin that lacks a large portion of the extracellular domain (alpha6p) exists. The alpha6pbeta1 integrin is found in human prostate cancer tissue specimens as well as tissue culture cell lines and is formed on the cell surface. This review discusses the mechanism of alpha6pbeta1 production and the potential functions of this integrin variant. Our current working model predicts that the alpha6pbeta1 integrin maintains the intracellular cytoskeletal connections associated with the heterodimer while allowing for an alteration in cell adhesion. The mechanism provides a selective advantage for cancer cell metastasis.

Protein kinase C-alpha phosphorylation of specific serines in the connecting segment of the beta 4 integrin regulates the dynamics of type II hemidesmosomes

Although the regulation of hemidesmosome dynamics during processes such as epithelial migration, wound healing, and carcinoma invasion is important, the mechanisms involved are poorly understood. The integrin alpha 6 beta 4 is an essential component of the hemidesmosome and a target of such regulation. Epidermal growth factor (EGF) can induce hemidesmosome disassembly by a mechanism that involves serine phosphorylation of the beta 4 integrin subunit. Using a combination of biochemical and mutational analyses, we demonstrate that EGF induces the phosphorylation of three specific serine residues (S(1356), S(1360), and S(1364)) located within the connecting segment of the beta 4 subunit and that phosphorylation on these residues accounts for the bulk of beta 4 phosphorylation stimulated by EGF. Importantly, phosphorylation of these serines is critical for the ability of EGF to disrupt hemidesmosomes. Using COS-7 cells, which assemble hemidesmosomes type II upon exogenous expression of the alpha 6 beta 4 integrin, we observed that expression of a beta 4 construct containing Ser-->Ala mutations of S(1356), S(1360), and S(1364) reduced the ability of EGF to disrupt hemidesmosomes and that this effect appears to involve cooperation among these phosphorylation sites. Moreover, expression of Ser-->Asp mutants that mimic constitutive phosphorylation reduced hemidesmosome formation. Protein kinase C-alpha (PKC-alpha) is the kinase responsible for phosphorylating at least two of these serines, based on in vitro kinase assays, peptide mapping, and mutational analysis. Together, these results highlight the importance of serine phosphorylation in regulating type II hemidesmosome disassembly, implicate a cluster of serine residues within the connecting segment of beta 4, and argue for a key role for PKC-alpha in regulating these structures.

A dynamic podosome-like structure of epithelial cells

Focal contacts and hemidesmosomes are cell-matrix adhesion structures of cultured epithelial cells. While focal contacts link the extracellular matrix to microfilaments, hemidesmosomes make connections with intermediate filaments. We have analyzed hemidesmosome assembly in 804G carcinoma cells. Our data show that hemidesmosomes are organized around a core of actin filaments that appears early during cell adhesion. These actin structures look similar to podosomes described in cells of mesenchymal origin. These podosome-like structures are distinct from focal contacts and specifically contain Arp3 (Arp2/3 complex), cortactin, dynamin, gelsolin, N-WASP, VASP, Grb2 and src-like kinase(s). The integrin alpha3beta1 is localized circularly around F-actin cores and co-distributes with paxillin, vinculin, and zyxin. We also show that the maintenance of the actin core and hemidesmosomes is dependent on actin polymerization, src-family kinases, and Grb2, but not on microtubules. Video microscopy analysis reveals that assembly of hemidesmosomes is preceded by recruitment of beta4 integrin subunit to the actin core before its positioning at hemidesmosomes. When 804G cells are induced to migrate, actin cores as well as hemidesmosomes disappear and beta4 integrin subunit becomes co-localized with dynamic actin at leading edges. We show that podosome-like structures are not unique to cells of mesenchymal origin, but also appear in epithelial cells, where they seem to be related to basement membrane adhesion.

Hemidesmosomes: molecular organization and their importance for cell adhesion and disease

In the skin, basal epithelial cells constantly divide to renew the epidermis. The newly formed epithelial cells then differentiate in a process called keratinization, ultimately leading to the death of these cells and a pile-up of cell material containing vast amounts of keratins. The basal keratinocytes in skin are attached to their underlying basement membrane via specialized adhesion complexes termed hemidesmosomes (HDs). These complexes ascertain stable adhesion of the epidermis to the dermis, and mutations in components of these complexes often result in tissue fragility and blistering of the skin. In this review, we will describe the various hemidesmosomal proteins in detail as well as, briefly, the protein families to which they belong. Specifically, we will report the protein-protein interactions involved in the assembly of hemidesmosomes and their molecular organization. Some signaling pathways involving primarily the alpha6beta4 integrin will be discussed, since they appear to profoundly modulate the assembly and function of hemidesmosomes. Furthermore, the importance of these hemidesmosomal components for the maintenance of tissue homeostasis and their involvement in various clinical disorders will be emphasized. Finally, we will present a model for the assembly of HDs, based on our present knowledge.

Role of binding of plectin to the integrin beta4 subunit in the assembly of hemidesmosomes

We have previously shown that plectin is recruited into hemidesmosomes through association of its actin-binding domain (ABD) with the first pair of fibronectin type III (FNIII) repeats and a small part of the connecting segment (residues 1328-1355) of the integrin beta4 subunit. Here, we show that two proline residues (P1330 and P1333) in this region of the connecting segment are critical for supporting beta4-mediated recruitment of plectin. Additional binding sites for the plakin domain of plectin on beta4 were identified in biochemical and yeast two-hybrid assays. These sites are located at the end of the connecting segment (residues 1383-1436) and in the region containing the fourth FNIII repeat and the C-tail (residues 1570-1752). However, in cells, these additional binding sites cannot induce the assembly of hemidesmosomes without the interaction of the plectin-ABD with beta4. Because the additional plectin binding sites overlap with sequences that mediate an intramolecular association of the beta4 cytoplasmic domain, we propose that they are not accessible for binding and need to become exposed as the result of the binding of the plectin-ABD to beta4. Furthermore, these additional binding sites might be necessary to position the beta4 cytoplasmic domain for an optimal interaction with other hemidesmosomal components, thereby increasing the efficiency of hemidesmosome assembly.

The association of the tetraspanin D6.1A with the alpha6beta4 integrin supports cell motility and liver metastasis formation

The metastatic subline of a rat pancreatic adenocarcinoma differs from the non-metastasizing subline by overexpression of 5 membrane molecules: CD44 variant isoforms, EpCAM, the tetraspanin D6.1A, an uPAR-related molecule and, as described here, the alpha6beta4 integrin. An antibody-defined molecule was identified by mass spectrometry and cloning as alpha6beta4 integrin. Transfection-induced expression of alpha6beta4 in the non-metastasizing subline did not support migration on laminin 5 or tumor progression. However, when the non-metastasizing subline was doubly transfected to express alpha6beta4 and the D6.1A tetraspanin, intraperitoneally injected tumor cells frequently formed liver metastasis. For the following reasons we assume that metastasis formation is supported by an interaction between alpha6beta4 and D6.1A. (i) The 2 molecules can associate and co-localize. (ii) Co-localization is strengthened by PKC stimulation. (iii) PKC stimulation, which induces a migratory phenotype, leads to a redistribution of alpha6beta4/D6.1A complexes. In resting cells, the molecules co-localize at the trail of the cell; during PKC stimulation they become transiently internalized and are (re-)expressed in the leading lamella. Thus, in the appropriate milieu, i.e. intraperitoneally, alpha6beta4 changes from an adhesion-supporting towards a migration-supporting molecule by its association with a tetraspanin. The findings provide a convincing experimental explanation for the repeatedly described involvement of alpha6beta4 in tumor progression.

Crucial role of the specificity-determining loop of the integrin beta4 subunit in the binding of cells to laminin-5 and outside-in signal transduction

Within each hemidesmosome, alpha6beta4 integrin plays a crucial role in hemidesmosome assembly by binding to laminin-5 in the basement membrane zone of epithelial tissue. Recent analyses have implicated "specificity-determining loops" (SDLs) in the I-like domain of beta integrin in regulating ligand binding. Here, we investigated the function of an SDL-like motif within the extracellular I-like domain of beta4 integrin. We generated point mutations within the SDL of beta4 integrin tagged with green fluorescent protein (GFP-beta4K150A and GFP-beta4Q155L). We also generated a mutation within the I-like domain of the beta4 integrin, lying outside the SDL region (GFP-beta4V284E). We transfected constructs encoding the mutated beta4 integrins and a GFP-conjugated wild type beta4 integrin (GFP-beta4WT) into 804G cells, which assemble hemidesmosomes, and human endothelial cells, which express little endogenous beta4 integrin. In transfected 804G cells, GFP-beta4WT and GFP-beta4V284E colocalize with hemidesmosome proteins, whereas hemidesmosomal components in cells expressing GFP-beta4K150A and GFP-beta4Q155L are aberrantly localized. In endothelial cells, GFP-beta4WT and mutant proteins are co-expressed at the cell surface with alpha6 integrin. When transfected endothelial cells are plated onto laminin-5 matrix, GFP-beta4WT and GFP-beta4V284E localize with laminin-5, whereas GFP-beta4K150A and GFP-beta4Q155L do not. GFP-beta4WT and GFP-beta4V284E expressed in endothelial cells associate with the adaptor protein Shc when the cells are stimulated with laminin-5. However, GFP-beta4K150A and GFP-beta4Q155L fail to associate with Shc even when laminin-5 is present, thus impacting downstream signaling. These results provide evidence that the SDL segment of the beta4 integrin subunit is required for ligand binding and is involved in outside-in signaling.

Specificity of binding of the plectin actin-binding domain to beta4 integrin

Plectin is a major component of the cytoskeleton and links the intermediate filament system to hemidesmosomes by binding to the integrin beta4 subunit. Previously, a binding site for beta4 was mapped on the actin-binding domain (ABD) of plectin and binding of beta4 and F-actin to plectin was shown to be mutually exclusive. Here we show that only the ABDs of plectin and dystonin bind to beta4, whereas those of other actin-binding proteins do not. Mutations of the ABD of plectin-1C show that Q131, R138, and N149 are critical for tight binding of the ABD to beta4. These residues form a small cavity, occupied by a well-ordered water molecule in the crystal structure. The beta4 binding pocket partly overlaps with the actin-binding sequence 2 (ABS2), previously shown to be essential for actin binding. Therefore, steric interference may render binding of beta4 and F-actin to plectin mutually exclusive. Finally, we provide evidence indicating that the residues preceding the ABD in plectin-1A and -1C, although unable to mediate binding to beta4 themselves, modulate the binding activity of the ABD for beta4. These studies demonstrate the unique property of the plectin-ABD to bind to both F-actin and beta4, and explain why several other ABD-containing proteins that are expressed in basal keratinocytes are not recruited into hemidesmosomes.

Alpha 6 beta 4 integrin regulates keratinocyte chemotaxis through differential GTPase activation and antagonism of alpha 3 beta 1 integrin

Growth factor-induced cell migration and proliferation are essential for epithelial wound repair. Cell migration during wound repair also depends upon expression of laminin-5, a ligand for alpha 6 beta 4 integrin. We investigated the role of alpha 6 beta 4 integrin in laminin-5-dependent keratinocyte migration by re-expressing normal or attachment-defective beta 4 integrin in beta 4 integrin null keratinocytes. We found that expression of beta 4 integrin in either a ligand bound or ligand unbound state was necessary and sufficient for EGF-induced cell migration. In a ligand bound state, beta 4 integrin supported EGF-induced cell migration though sustained activation of Rac1. In the absence of alpha 6 beta 4 integrin ligation, Rac1 activation became tempered and EGF chemotaxis proceeded through an alternate mechanism that depended upon alpha 3 beta 1 integrin and was characterized by cell scattering. alpha 3 beta 1 integrin also relocalated from cell-cell contacts to sites of basal clustering where it displayed increased conformational activation. The aberrant distribution and activation of alpha 3 beta 1 integrin in attachment-defective beta 4 cells could be reversed by the activation of Rac1. Conversely, in WT beta 4 cells the normal cell-cell localization of alpha 3 beta 1 integrin became aberrant after the inhibition of Rac1. These studies indicate that the extracellular domain of beta 4 integrin, through its ability to bind ligand, functions to integrate the divergent effects of growth factors on the cytoskeleton and adhesion receptors so that coordinated keratinocyte migration can be achieved.

Immunofluorescence analysis of the basement membrane zone components in human anagen hair follicles

The ultrastructure and constituent components of the basement membrane zone (BMZ) of the interfollicular epidermis have been well characterized. However, little is known about the junctions between dermal papilla and the surrounding epithelial cells of the hair bulb, as well as those junctions between connective tissues and epithelial cells outside the hair follicle. In the present study, immunofluorescence was used to determine the expression of various BMZ components, particularly plectin, BP230, BP180, alpha6beta4 integrin, laminin 5 and type VII collagen, in anagen hair follicles from human scalp. All the BMZ components examined showed essentially the same immunofluorescence staining pattern. Specifically, staining of the upper portion of the hair follicle demonstrated expression of all BMZ components with a labeling intensity similar to that found in the interfollicular epidermis. Staining in the lower portion of the hair follicle, however, was markedly different: all the BMZ components showed a gradual decrease in staining intensity. Particularly, outside the hair bulb, the linear staining was diminished and even discontinuous in some areas. Finally, between dermal papilla and epithelial cells inside the hair bulb, there was a strong immunoreactivity for all the BMZ components except for BP230, which was completely negative. The present study also confirmed a previous reported ultrastructural finding that hemidesmosomes are not apparent in the hair bulb's exterior BMZ nor in the dermal papilla junctions. Instead, peculiar cloudy materials were seen in both the lamina densa and the adjacent epithelium outside the hair bulb. Taken together, the diminished expression of all the BMZ components outside the hair bulb, as well as the complete absence of BP230 at the dermal papilla junction, seem to be responsible for the incomplete ultrastructure of hemidesmosomes in these regions. Furthermore, the results in the present study led us to speculate that the expression of BMZ components inside and outside the hair bulb are markedly decreased in the transient regions of the hair follicle as compared with their expression in the permanent region, signified by the upper portion of the hair follicle. When the hair follicle moves upward in catagen or downward in anagen, the complete structure of the hemidesmosome may stabilize the upper portion to the surrounding connective tissues, while the incomplete hemidesmosome may facilitate the movement of the transient region.

The role of alpha3beta1 integrin in determining the supramolecular organization of laminin-5 in the extracellular matrix of keratinocytes

Analyses of mice with targeted deletions in the genes for alpha3 and beta1 integrin suggest that the alpha3beta1 integrin heterodimer likely determines the organization of the extracellular matrix within the basement membrane of skin. Here we tested this hypothesis using keratinocytes derived from alpha3 integrin-null mice. We have compared the organizational state of laminin-5, a ligand of alpha3beta1 integrin, in the matrix of wild-type keratinocytes with that of laminin-5 in the matrix of alpha3 integrin-null cells. Laminin-5 distributes diffusely in arc structures in the matrix of wild-type mouse keratinocytes, whereas laminin-5 is organized into linear, spike-like arrays by the alpha3 integrin-null cells. The fact that alpha3 integrin-null cells are deficient in their ability to assemble a proper laminin-5 matrix is also shown by their failure to remodel laminin-5 when plated onto surfaces coated with purified laminin-5 protein. In sharp contrast, wild-type keratinocytes organize exogenously added laminin-5 into discrete ring-like organizations. These findings led us next to assess whether differences in laminin-5 organization in the matrix of the wild-type and alpha3 integrin-null cells impact cell behavior. Our results indicate that alpha3 integrin-null cells are more motile than their wild-type counterparts and leave extensive trails of laminin-5 over the surface on which they move. Moreover, HEK 293 cells migrate significantly more on the laminin-5-rich matrix derived from the alpha3 integrin-null cells than on the wild-type keratinocyte laminin-5 matrix. In addition, alpha3 integrin-null cells show low strength of adhesion to surfaces coated with purified laminin-5 compared to wild-type cells although both the wild type and the alpha3 integrin-null keratinocytes adhere equally strongly to laminin-5 that has been organized into arrays by other epithelial cells. These data suggest: (1) that alpha3beta1 integrin plays an important role in determining the incorporation of laminin-5 into its proper higher-order structure within the extracellular matrix of keratinocytes and (2) that the organizational state of laminin-5 has an influence on laminin-5 matrix function.

Analysis of the interactions between BP180, BP230, plectin and the integrin alpha6beta4 important for hemidesmosome assembly

Hemidesmosomes (HDs) are multi-protein complexes that promote stable adhesion of epithelial cells to the underlying extracellular matrix. We assessed the interactions between different hemidesmosomal components with each other, mapped the binding sites and studied the importance of these interactions for HD assembly in yeast two-hybrid and cell-transfection assays. The results show that: (1) bullous pemphigoid antigen (BP) 180 binds not only to BP230, but also to plectin. The interactions between these proteins are facilitated by the Y subdomain in the N-terminal plakin domain of BP230 and plectin, and residues 145-230 of the cytoplasmic domain of BP180; (2) different, but overlapping, sequences on BP180 mediate binding to beta4, which, in turn associates with BP180 via its third fibronectin type III repeat; (3) sequences in the N-terminal extremity of BP230 mediate its binding to beta4, which requires the C-terminal end of the connecting segment up to the fourth FNIII repeat of the beta4 subunit. (4) Finally, cell-transfection studies showed that the localization of BP230 into hemidesmosome-like structures depends on its Z-Y subdomains as well as on the availability of BP180. By having further uncovered interactions between various hemidesmosomal components, mapped the involved binding sites and dissected a hierarchy of interactions relevant for their topogenic fate, our findings give novel insights into the molecular organization of hemidesmosomes.

Deletion of a cytoplasmic domain of integrin beta4 causes epidermolysis bullosa simplex

Integrin alpha6beta4 is a hemidesmosomal transmembrane molecule involved in maintaining basal cell-matrix adhesion through interaction of the large intracytoplasmic tail of the beta4 subunit with the keratin intermediate filament network, at least in part through its binding with plectin and BP180/type XVII collagen. Here we report a patient with predominant features of epidermolysis bullosa simplex due to a mutation in the integrin beta4 gene. The patient, a 49-y-old female, had mild blistering of hands and feet from birth on, dystrophy of the nails with onychogryposis, and enamel hypoplasia. She had no alopecia and no history of pyloric atresia. Electron microscopy and antigen mapping of a skin blister revealed that the level of separation was intraepidermal, low in the basal keratinocytes through the attachment plaque of the hemidesmosome. Immuno-fluorescence microscopy revealed absent binding of monoclonal antibody 450-11 A against the third fibronectin III repeat on the intracellular domain of integrin beta4, whereas binding was reduced with monoclonal antibodies recognizing epitopes on amino-terminal and carboxy-terminal ends of the polypeptide. At the molecular level the phenotype was caused by a novel 2 bp deletion 4733delCT in ITGB4, resulting in in-frame skipping of exon 36 and a deduced 50 amino acid deletion (1450-1499) within the third fibronectin type III repeat in the cytoplasmic domain of the integrin beta4 polypeptide. Immunoblot analysis demonstrated a 5 kDa shorter beta4 polypeptide. The 4733delCT mutation was heterozygously present in the DNA. The patient is also expected to be heterozygous for a null allele, as no full-size protein was detected in vitro and the epitope 450-11 A was absent in vivo. These data show that deletion of the third fibronectin type III repeat in the cytoplasmic domain of integrin beta4, which is thought to interact with BP180/type XVII collagen, is clinically pathogenic and results in a mild phenotype with predominant features of epidermolysis bullosa simplex.

Dynamics of the alpha6beta4 integrin in keratinocytes

The integrin alpha6beta4 has been implicated in two apparently contrasting processes, i.e., the formation of stable adhesions, and cell migration and invasion. To study the dynamic properties of alpha6beta4 in live cells two different beta4-chimeras were stably expressed in beta4-deficient PA-JEB keratinocytes. One chimera consisted of full-length beta4 fused to EGFP at its carboxy terminus (beta4-EGFP). In a second chimera the extracellular part of beta4 was replaced by EGFP (EGFP-beta4), thereby rendering it incapable of associating with alpha6 and thus of binding to laminin-5. Both chimeras induce the formation of hemidesmosome-like structures, which contain plectin and often also BP180 and BP230. During cell migration and division, the beta4-EGFP and EGFP-beta4 hemidesmosomes disappear, and a proportion of the beta4-EGFP, but not of the EGFP-beta4 molecules, become part of retraction fibers, which are occasionally ripped from the cell membrane, thereby leaving "footprints" of the migrating cell. PA-JEB cells expressing beta4-EGFP migrate considerably more slowly than those that express EGFP-beta4. Studies with a beta4-EGFP mutant that is unable to interact with plectin and thus with the cytoskeleton (beta4(R1281W)-EGFP) suggest that the stabilization of the interaction between alpha6beta4 and LN-5, rather than the increased adhesion to LN-5, is responsible for the inhibition of migration. Consistent with this, photobleaching and recovery experiments revealed that the interaction of beta4 with plectin renders the bond between alpha6beta4 and laminin-5 more stable, i.e., beta4-EGFP is less dynamic than beta4(R1281W)-EGFP. On the other hand, when alpha6beta4 is bound to laminin-5, the binding dynamics of beta4 to plectin are increased, i.e., beta4-EGFP is more dynamic than EGFP-beta4. We suggest that the stability of the interaction between alpha6beta4 and laminin-5 is influenced by the clustering of alpha6beta4 through the deposition of laminin-5 underneath the cells. This clustering ultimately determines whether alpha6beta4 will inhibit cell migration or not.

Two different mutations in the cytoplasmic domain of the integrin beta 4 subunit in nonlethal forms of epidermolysis bullosa prevent interaction of beta 4 with plectin

The integrin alpha 6 beta 4 plays a crucial role in the assembly and maintenance of hemidesmosomes. Previous work has shown that the recruitment of plectin into hemidesmosomes is dependent on beta 4 and involves a region of the beta 4 cytoplasmic domain, which contains the first two fibronectin (FNIII) repeats and a short region of the connecting segment. Two missense mutations (R1225H and R1281W) in beta 4 that are responsible for nonlethal forms of epidermolysis bullosa are located in the second FNIII repeat. One of them is confined to a loop region that connects two beta strands (EC') whereas the other is located at the N-terminal end of the second FNIII repeat. We here report that these mutations render beta 4 unable to interact with plectin and prevent the localization of plectin in hemidesmosomes. Substitution of a lysine residue (K1279W) that forms part of the same loop as R1281 had no effect on the ability of beta 4 to recruit plectin. Furthermore, we show that an extended loop structure in beta 4, composed of the amino acids DDN (1262--1264), which resembles the RGD integrin-binding loop in fibronectin, is not involved in the binding to plectin. These results further demonstrate that binding of beta 4 to plectin is essential for the proper formation and function of hemidesmosomes and that loss of the interaction between beta 4 and plectin is associated with a mild form of epidermolysis bullosa.

A signaling adapter function for alpha6beta4 integrin in the control of HGF-dependent invasive growth

alpha6beta4 integrin and the Met receptor for HGF have been shown independently to promote invasive growth. We demonstrate here that Met selectively associates with alpha6beta4. In carcinoma cells expressing Met alone, HGF does not exert significant biological effects. Ectopic expression of alpha6beta4 restores HGF-regulated processes. Following Met activation, alpha6beta4 is tyrosine phosphorylated and combines with Shc and PI3K, generating an additional signaling platform that potentiates HGF-triggered activation of Ras- and PI3K-dependent pathways. In the presence of an alpha6beta4 mutant defective for Shc recruitment, Met cannot sustain HGF-mediated responses. Surprisingly, a truncated beta4 unable to bind laminins retains the activity of wild-type alpha6beta4. Such findings invoke an unexpected role for alpha6beta4 in cancer invasion as a functional amplifier of biochemical outputs rather than a mechanical adhesive device.

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.

Functional comparison of the alpha3A and alpha3B cytoplasmic domain variants of the chicken alpha3 integrin subunit

Integrin alpha3beta1 can be alternatively spliced to generate alpha3A and alpha3B cytoplasmic domain variants that are conserved among vertebrates. To identify distinct functions of these variants, we transfected cells with intact alpha3 integrins or chimeric receptors. alpha3Abeta1 and alpha3Bbeta1 each localized to focal contacts in keratinocytes on an extracellular matrix rich in laminin-5, to which both are known to bind with high affinity. However, alpha3B accumulated intracellularly in keratinocytes on collagen, suggesting that laminin binding may stabilize alpha3Bbeta1 surface expression. Neither alpha3 cytoplasmic domain affected recruitment of chimeric alpha5 integrins to fibronectin-induced focal contacts, and either substituted for the alpha5 cytoplasmic domain in alpha5beta1-mediated cell migration. However, the alpha5/alpha3B chimera localized to cell-cell borders in MDCK or CHO cells to a lesser extent than did the alpha5/alpha3A chimera. To determine whether the alpha3 cytoplasmic domains conferred distinct localization to a nonintegrin protein, we transfected cells with interleukin-2 receptor (IL-2R) chimeras containing the alpha3 cytoplasmic domains. The IL-2R/alpha3A chimera was expressed efficiently on the cell surface, while the IL-2R/alpha3B chimera accumulated intracellularly. Our findings suggest that the alpha3B cytoplasmic domain harbors a retention signal that is regulated in an intact integrin and can alter cell surface expression and distribution of alpha3beta1.

The interaction of plectin with actin: evidence for cross-linking of actin filaments by dimerization of the actin-binding domain of plectin

Plectin is a major component of the cytoskeleton and is expressed in a wide variety of cell types. It plays an important role in the integrity of the cytoskeleton by cross-linking the three filamentous networks and stabilizing cell-matrix and cell-cell contacts. Sequence analysis showed that plectin contains a highly conserved actin-binding domain, consisting of a pair of calponin-like subdomains. Using yeast two-hybrid assays in combination with in vitro binding experiments, we demonstrate that the actin-binding domain of plectin is fully functional and preferentially binds to polymeric actin. The sequences required for actin binding were identified at the C-terminal end of the first calponin homology domain within the actin-binding domain of plectin. We found that the actin-binding domain of plectin is able to bundle actin filaments and we present evidence that this is mediated by the dimerization of this domain. In addition we also show that plectin and another member of the plakin family, dystonin, can heterodimerize by their actin-binding domains. We propose a new mechanism by which plectin and possibly also other actin-binding proteins can regulate the organization of the F-actin network in the cell.

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.

Tyrosine phosphorylation of the beta 4 integrin cytoplasmic domain mediates Shc signaling to extracellular signal-regulated kinase and antagonizes formation of hemidesmosomes

Ligation of the alpha(6)beta(4) integrin induces tyrosine phosphorylation of the beta(4) cytoplasmic domain, followed by recruitment of the adaptor protein Shc and activation of mitogen-activated protein kinase cascades. We have used Far Western analysis and phosphopeptide competition assays to map the sites in the cytoplasmic domain of beta(4) that are required for interaction with Shc. Our results indicate that, upon phosphorylation, Tyr(1440), or secondarily Tyr(1422), interacts with the SH2 domain of Shc, whereas Tyr(1526), or secondarily Tyr(1642), interacts with its phosphotyrosine binding (PTB) domain. An inactivating mutation in the PTB domain of Shc, but not one in its SH2 domain, suppresses the activation of Shc by alpha(6)beta(4). In addition, mutation of beta(4) Tyr(1526), which binds to the PTB domain of Shc, but not of Tyr(1422) and Tyr(1440), which interact with its SH2 domain, abolishes the activation of ERK by alpha(6)beta(4). Phenylalanine substitution of the beta(4) tyrosines able to interact with the SH2 or PTB domain of Shc does not affect incorporation of alpha(6)beta(4) in the hemidesmosomes of 804G cells. Exposure to the tyrosine phosphatase inhibitor orthovanadate increases tyrosine phosphorylation of beta4 and disrupts the hemidesmosomes of 804G cells expressing recombinant wild type beta(4). This treatment, however, exerts a decreasing degree of inhibition on the hemidesmosomes of cells expressing versions of beta(4) containing phenylalanine substitutions at Tyr(1422) and Tyr(1440), at Tyr(1526) and Tyr(1642), or at all four tyrosine phosphorylation sites. These results suggest that beta(4) Tyr(1526) interacts in a phosphorylation-dependent manner with the PTB domain of Shc. This event is required for subsequent tyrosine phosphorylation of Shc and signaling to ERK but not formation of hemidesmosomes.

Adhesion complexes implicated in intestinal epithelial cell-matrix interactions

This article review summarizes data on cell-substratum adhesion complexes involved in the regulation of cellular functions in the intestine. We first focus on the molecular composition of the two main adhesion structures-the beta1 integrin-adhesion complex and the hemidesmosome-found in vivo and in two human intestinal cell lines. We also report the key findings on the cellular behavior and response to the extracellular matrix that involve integrins, the main transmembrane anchors of these complexes. How the dynamics of cell/extracellular matrix interactions contribute to cell migration, proliferation, differentiation, and tumorigenicity is discussed in the light of the data provided by the human intestinal cells.

The tetraspan molecule CD151, a novel constituent of hemidesmosomes, associates with the integrin alpha6beta4 and may regulate the spatial organization of hemidesmosomes

CD151 is a cell surface protein that belongs to the tetraspan superfamily. It associates with other tetraspan molecules and certain integrins to form large complexes at the cell surface. CD151 is expressed by a variety of epithelia and mesenchymal cells. We demonstrate here that in human skin CD151 is codistributed with alpha3beta1 and alpha6beta4 at the basolateral surface of basal keratinocytes. Immunoelectron microscopy showed that CD151 is concentrated in hemidesmosomes. By immunoprecipitation from transfected K562 cells, we established that CD151 associates with alpha3beta1 and alpha6beta4. In beta4-deficient pyloric atresia associated with junctional epidermolysis bullosa (PA-JEB) keratinocytes, CD151 and alpha3beta1 are clustered together at the basal cell surface in association with patches of laminin-5. Focal adhesions are present at the periphery of these clusters, connected with actin filaments, and they contain both CD151 and alpha3beta1. Transient transfection studies of PA-JEB cells with beta4 revealed that the integrin alpha6beta4 becomes incorporated into the alpha3beta1-CD151 clusters where it induces the formation of hemidesmosomes. As a result, the amount of alpha3beta1 in the clusters diminishes and the protein becomes restricted to the peripheral focal adhesions. Furthermore, CD151 becomes predominantly associated with alpha6beta4 in hemidesmosomes, whereas its codistribution with alpha3beta1 in focal adhesions becomes partial. The localization of alpha6beta4 in the pre-hemidesmosomal clusters is accompanied by a strong upregulation of CD151, which is at least partly due to increased cell surface expression. Using beta4 chimeras containing the extracellular and transmembrane domain of the IL-2 receptor and the cytoplasmic domain of beta4, we found that for recruitment of CD151 into hemidesmosomes, the beta4 subunit must be associated with alpha6, confirming that integrins associate with tetraspans via their alpha subunits. CD151 is the only tetraspan identified in hemidesmosomal structures. Others, such as CD9 and CD81, remain diffusely distributed at the cell surface. In conclusion, we show that CD151 is a major component of (pre)-hemidesmosomal structures and that its recruitment into hemidesmosomes is regulated by the integrin alpha6beta4. We suggest that CD151 plays a role in the formation and stability of hemidesmosomes by providing a framework for the spatial organization of the different hemidesmosomal components.

Formation of hemidesmosome-like structures in the absence of ligand binding by the (alpha)6(beta)4 integrin requires binding of HD1/plectin to the cytoplasmic domain of the (beta)4 integrin subunit

Hemidesmosomes are adhesion structures that mediate anchorage of epithelial cells to the underlying basement membrane. We have previously shown that the (alpha)6(beta)4 integrin can induce the assembly of these multi-protein structures independent of binding to its ligand laminin-5 (ligand-independent formation of hemidesmosomes). Our results suggested a role for HD1/plectin, which binds to the cytoplasmic domain of the (beta)4 integrin subunit, in controlling the clustering of hemidesmosomal components at the basal side of the cell. Using keratinocytes derived from patients lacking HD1/plectin, we now show that ligand-independent formation of hemidesmosomal clusters indeed requires HD1/plectin, in contrast to the ligand-dependent assembly of hemidesmosomes. No clustering of the (alpha)6(beta)4 integrin, or of the bullous pemphigoid antigens BP180 and BP230, was seen when HD1/plectin-deficient keratinocytes were plated on fibronectin or type IV collagen. In (β)4-deficient keratinocytes, expression of an interleukin 2 receptor (IL2R) transmembrane chimera containing the (beta)4 cytoplasmic tail with the mutation R1281W, which abrogates HD1/plectin binding, resulted in a diffuse distribution of the chimeric receptor. In contrast, a (beta)4(R1281W) mutant that can associate with (alpha)6 and bind ligand, was found to be directed to the basal surface of the cells, at sites where laminin-5 was deposited. In addition, this mutant induced clustering of BP180 and BP230 at these sites. Together, these results show that the formation of hemidesmosomes requires binding of either ligand or HD1/plectin to the (beta)4 integrin subunit. Intriguingly, we found that IL2R/(beta)4 chimeras become localized in pre-existing hemidesmosomes of HD1/plectin-deficient keratinocytes, and that this localization requires a domain in the (beta)4 cytoplasmic tail that is also required for HD1/plectin binding (residues 1115–1356). Because this part of (beta)4 lacks the BP180 binding site, and since we show in this study that it is unable to interact with the same part on another (beta)4 molecule, we suggest that the chimera becomes incorporated into hemidesmosomes of HD1/plectin-deficient keratinocytes by interacting with an as yet unidentified hemidesmosomal component.

Induction of cell scattering by expression of beta1 integrins in beta1-deficient epithelial cells requires activation of members of the rho family of GTPases and downregulation of cadherin and catenin function

Adhesion receptors, which connect cells to each other and to the surrounding extracellular matrix (ECM), play a crucial role in the control of tissue structure and of morphogenesis. In this work, we have studied how intercellular adhesion molecules and beta1 integrins influence each other using two different beta1-null cell lines, epithelial GE11 and fibroblast-like GD25 cells. Expression of beta1A or the cytoplasmic splice variant beta1D, induced the disruption of intercellular adherens junctions and cell scattering in both GE11 and GD25 cells. In GE11 cells, the morphological change correlated with the redistribution of zonula occluden (ZO)-1 from tight junctions to adherens junctions at high cell confluency. In addition, the expression of beta1 integrins caused a dramatic reorganization of the actin cytoskeleton and of focal contacts. Interaction of beta1 integrins with their respective ligands was required for a complete morphological transition towards the spindle-shaped fibroblast-like phenotype. The expression of an interleukin-2 receptor (IL2R)-beta1A chimera and its incorporation into focal adhesions also induced the disruption of cadherin-based adhesions and the reorganization of ECM-cell contacts, but failed to promote cell migration on fibronectin, in contrast to full-length beta1A. This indicates that the disruption of cell-cell adhesion is not simply the consequence of the stimulated cell migration. Expression of beta1 integrins in GE11 cells resulted in a decrease in cadherin and alpha-catenin protein levels accompanied by their redistribution from the cytoskeleton-associated fraction to the detergent-soluble fraction. Regulation of alpha-catenin protein levels by beta1 integrins is likely to play a role in the morphological transition, since overexpression of alpha-catenin in GE11 cells before beta1 prevented the disruption of intercellular adhesions and cell scattering. In addition, using biochemical activity assays for Rho-like GTPases, we show that the expression of beta1A, beta1D, or IL2R-beta1A in GE11 or GD25 cells triggers activation of both RhoA and Rac1, but not of Cdc42. Moreover, dominant negative Rac1 (N17Rac1) inhibited the disruption of cell-cell adhesions when expressed before beta1. However, all three GTPases might be involved in the morphological transition, since expression of either N19RhoA, N17Rac1, or N17Cdc42 reversed cell scattering and partially restored cadherin-based adhesions in GE11-beta1A cells. Our results indicate that beta1 integrins regulate the polarity and motility of epithelial cells by the induction of intracellular molecular events involving a downregulation of alpha-catenin function and the activation of the Rho-like G proteins Rac1 and RhoA.

Expression of functionally distinct variants of the beta(4)A integrin subunit in relation to the differentiation state in human intestinal cells

Integrins are important mediators of cell-laminin interactions. In the small intestinal epithelium, which consists of spatially separated proliferative and differentiated cell populations located, respectively, in the crypt and on the villus, laminins and laminin-binding integrins are differentially expressed along the crypt-villus axis. One exception to this is the integrin alpha(6)beta(4), which is thought to be ubiquitously expressed by intestinal cells. However, in this study, a re-evaluation of the beta(4) subunit expression with different antibodies revealed that two forms of beta(4) exist in the human intestinal epithelium. Furthermore, we show that differentiated enterocytes express a full-length 205-kDa beta(4)A subunit, whereas undifferentiated crypt cells express a novel beta(4)A subunit that does not contain the COOH-terminal segment of the cytoplasmic domain (beta(4)A(ctd-)). This new form was not found to arise from alternative beta(4) mRNA splicing. Moreover, we found that these two beta(4)A forms can associate into alpha(6)beta(4)A complexes; however, the beta(4)A(ctd-) integrin expressed by the undifferentiated crypt cells is not functional for adhesion to laminin-5. Hence, these studies identify a novel alpha(6)beta(4)A(ctd-) integrin expressed in undifferentiated intestinal crypt cells that is functionally distinct.

Integrins: a role as cell signalling molecules

Integrins form the major family of proteins that mediates cell-matrix interactions. As well as an adhesive function, it is increasingly apparent that integrins can transduce messages via classic signalling pathways and impact upon such fundamental cellular processes as proliferation, apoptosis, differentiation, and motility. Dysregulation of these processes are a feature of many malignancies. Altered integrin expression has been observed in many human tumours, and perturbation of integrin function or expression in experimental systems has demonstrated that altered integrin signalling may directly contribute to the development of the malignant phenotype.

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.

Brains and brawn: plectin as regulator and reinforcer of the cytoskeleton

Plectin is a 580 kDa intracellular protein, previously shown to link intermediate filaments with microtubules, actin filaments, and membrane components. Disruption of the plectin gene in humans and in mice results in severe skin blistering and muscular degeneration, consistent with plectin's structural role in stabilizing cells against mechanical force. However, recent work by Andra et al. characterizing cells from plectin-deficient mice demonstrates that in addition to this structural role, plectin also modulates the dynamics of the actin cytoskeleton. This makes plectin unusual in that it serves both to reinforce and crosslink intermediate filament attachments to membranes and other cytoskeletal polymers and to regulate actin dynamics in cells.

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.

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.

Interaction of BP180 (type XVII collagen) and alpha6 integrin is necessary for stabilization of hemidesmosome structure

The hemidesmosome is a multimolecular complex that integrates the extracellular matrix with the keratin cytoskeleton and that stabilizes epithelial attachment to connective tissue. A 180 kDa protein (BP180, type XVII collagen), first identified by its reactivity with autoantibodies in the serum of patients with a blistering skin disease called bullous pemphigoid (BP), is a transmembrane component of the hemidesmosome with a collagen-like extracellular domain. Here, using recombinantly expressed molecules and the yeast two-hybrid assay, we have identified alpha6 integrin as a BP180-binding partner. The association between specific domains of the BP180 and alpha6 integrin molecules is inhibited by a 14 mer peptide, whose sequence is identical to amino acid residues 506-519 in the noncollagenous region of the ectodomain of the BP180 molecule, as well as by antibodies raised against this peptide. The 14 mer peptide sequence is part of an epitope recognized by autoantibodies that are pathogenic in BP. In vivo, when 804G cells are plated into medium containing the same peptide, they fail to assemble hemidesmosomes. Furthermore, although BP180 and certain cytoplasmic components of the hemidesmosome colocalize in the peptide-treated cells, they are aberrantly distributed and fail to show extensive association with (alpha6beta4 integrin. Taken together, our results indicate that BP180 is a novel transmembrane ligand of the alpha6beta4 integrin heterodimer. In addition, our data provide support for the possibility that BP180 and alpha6 integrin interaction is not only mediated by the BP epitope but is necessary for hemidesmosome formation.

Hemidesmosome formation is initiated by the beta4 integrin subunit, requires complex formation of beta4 and HD1/plectin, and involves a direct interaction between beta4 and the bullous pemphigoid antigen 180

Hemidesmosomes (HDs) are stable anchoring structures that mediate the link between the intermediate filament cytoskeleton and the cell substratum. We investigated the contribution of various segments of the beta4 integrin cytoplasmic domain in the formation of HDs in transient transfection studies using immortalized keratinocytes derived from an epidermolysis bullosa patient deficient in beta4 expression. We found that the expression of wild-type beta4 restored the ability of the beta4-deficient cells to form HDs and that distinct domains in the NH2- and COOH-terminal regions of the beta4 cytoplasmic domain are required for the localization of HD1/plectin and the bullous pemphigoid antigens 180 (BP180) and 230 (BP230) in these HDs. The tyrosine activation motif located in the connecting segment (CS) of the beta4 cytoplasmic domain was dispensable for HD formation, although it may be involved in the efficient localization of BP180. Using the yeast two-hybrid system, we could demonstrate a direct interaction between beta4 and BP180 which involves sequences within the COOH-terminal part of the CS and the third fibronectin type III (FNIII) repeat. Immunoprecipitation studies using COS-7 cells transfected with cDNAs for alpha6 and beta4 and a mutant BP180 which lacks the collagenous extracellular domain confirmed the interaction of beta4 with BP180. Nevertheless, beta4 mutants which contained the BP180-binding region, but lacked sequences required for the localization of HD1/plectin, failed to localize BP180 in HDs. Additional yeast two- hybrid assays indicated that the 85 COOH-terminal residues of beta4 can interact with the first NH2-terminal pair of FNIII repeats and the CS, suggesting that the cytoplasmic domain of beta4 is folded back upon itself. Unfolding of the cytoplasmic domain may be part of a mechanism by which the interaction of beta4 with other hemidesmosomal components, e.g., BP180, is regulated.