Proteolytic processing of the laminin alpha3 G domain mediates assembly of hemidesmosomes but has no role on keratinocyte migration

Junctional epithelium and hemidesmosomes: Tape and rivets for solving the "percutaneous device dilemma" in dental and other permanent implants

The percutaneous device dilemma describes etiological factors, centered around the disrupted epithelial tissue surrounding non-remodelable devices, that contribute to rampant percutaneous device infection. Natural percutaneous organs, in particular their extracellular matrix mediating the "device"/epithelium interface, serve as exquisite examples to inspire longer lasting long-term percutaneous device design. For example, the tooth's imperviousness to infection is mediated by the epithelium directly surrounding it, the junctional epithelium (JE). The hallmark feature of JE is formation of hemidesmosomes, cell/matrix adhesive structures that attach surrounding oral gingiva to the tooth's enamel through a basement membrane. Here, the authors survey the multifaceted functions of the JE, emphasizing the role of the matrix, with a particular focus on hemidesmosomes and their five main components. The authors highlight the known (and unknown) effects dental implant - as a model percutaneous device - placement has on JE regeneration and synthesize this information for application to other percutaneous devices. The authors conclude with a summary of bioengineering strategies aimed at solving the percutaneous device dilemma and invigorating greater collaboration between clinicians, bioengineers, and matrix biologists.

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.

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.

Laminin 332 in cancer: When the extracellular matrix turns signals from cell anchorage to cell movement

Laminin 332 is crucial in the biology of epithelia. This large extracellular matrix protein consists of the heterotrimeric assembly of three subunits - α3, β3, and γ2 - and its multifunctionality relies on a number of extracellular proteolytic processing events. Laminin 332 is central to normal epithelium homeostasis by sustaining cell adhesion, polarity, proliferation, and differentiation. It also supports a major function in epithelial tissue formation, repair, and regeneration by buttressing cell migration and survival and basement membrane assembly. Interest in this protein increased after the discovery that its expression is perturbed in tumor cells, cancer-associated fibroblasts, and the tumor microenvironment. This review summarizes current knowledge regarding the established involvement of the laminin 332 γ2 chain in tumor invasiveness and discusses the role of its α3 and β3 subunits.

Epidermolysis bullosa: Molecular pathology of connective tissue components in the cutaneous basement membrane zone

Epidermolysis bullosa (EB), a group of heritable skin fragility disorders, is characterized by blistering, erosions and chronic ulcers in the skin and mucous membranes. In some forms, the blistering phenotype is associated with extensive mutilating scarring and development of aggressive squamous cell carcinomas. The skin findings can be associated with extracutaneous manifestations in the ocular as well as gastrointestinal and vesico-urinary tracts. The phenotypic heterogeneity reflects the presence of mutations in as many as 20 different genes expressed in the cutaneous basement membrane zone, and the types and combinations of the mutations and their consequences at the mRNA and protein levels contribute to the spectrum of severity encountered in different subtypes of EB. This overview highlights the molecular genetics of EB based on mutations in the genes encoding type VII and XVII collagens as well as laminin-332. The mutations identified in these protein components of the extracellular matrix attest to their critical importance in providing stability to the cutaneous basement membrane zone, with implications for heritable and acquired diseases.

Epidermal Basement Membrane in Health and Disease

Skin, as the organ protecting the individual from environmental aggressions, constantly meets external insults and is dependent on mechanical toughness for its preserved function. Accordingly, the epidermal basement membrane (BM) zone has adapted to enforce tissue integrity. It harbors anchoring structures created through unique organization of common BM components and expression of proteins exclusive to the epidermal BM zone. Evidence for the importance of its correct assembly and the nonredundancy of its components for skin integrity is apparent from the multiple skin blistering disorders caused by mutations in genes coding for proteins associated with the epidermal BM and from autoimmune disorders in which autoantibodies target these molecules. However, it has become clear that these proteins not only provide mechanical support but are also critically involved in tissue homeostasis, repair, and regeneration. In this chapter, we provide an overview of the unique organization and components of the epidermal BM. A special focus will be given to its function during regeneration, and in inherited and acquired diseases.

Laminin 332 processing impacts cellular behavior

Laminin 332, composed of the α3, β3 and γ2 chains, is an epithelial-basement membrane specific laminin variant. Its main role in normal tissues is the maintenance of epithelial-mesenchymal cohesion in tissues exposed to external forces, including skin and stratified squamous mucosa. After being secreted and deposited in the extracellular matrix, laminin 332 undergoes physiological maturation processes consisting in the proteolytic processing of domains located within the α3 and the γ2 chains. These maturation events are essential for laminin 332 integration into the basement membrane where it plays an important function in the nucleation and maintenance of anchoring structures. Studies in normal and pathological situations have revealed that laminin 332 can trigger distinct cellular events depending on the level of its proteolytic cleavages. In this review, the biological and structural characteristics of laminin 332 domains are presented and we discuss whether they trigger specific functions.

Agrin and synaptic laminin are required to maintain adult neuromuscular junctions

As synapses form and mature the synaptic partners produce organizing molecules that regulate each other’s differentiation and ensure precise apposition of pre- and post-synaptic specializations. At the skeletal neuromuscular junction (NMJ), these molecules include agrin, a nerve-derived organizer of postsynaptic differentiation, and synaptic laminins, muscle-derived organizers of presynaptic differentiation. Both become concentrated in the synaptic cleft as the NMJ develops and are retained in adulthood. Here, we used mutant mice to ask whether these organizers are also required for synaptic maintenance. Deletion of agrin from a subset of adult motor neurons resulted in the loss of acetylcholine receptors and other components of the postsynaptic apparatus and synaptic cleft. Nerve terminals also atrophied and eventually withdrew from muscle fibers. On the other hand, mice lacking the presynaptic organizer laminin-α4 retained most of the synaptic cleft components but exhibited synaptic alterations reminiscent of those observed in aged animals. Although we detected no marked decrease in laminin or agrin levels at aged NMJs, we observed alterations in the distribution and organization of these synaptic cleft components suggesting that such changes could contribute to age-related synaptic disassembly. Together, these results demonstrate that pre- and post-synaptic organizers actively function to maintain the structure and function of adult NMJs.

The Asian-American E6 variant protein of human papillomavirus 16 alone is sufficient to promote immortalization, transformation, and migration of primary human foreskin keratinocytes

We examined how well the human papillomavirus (HPV) E6 oncogene can function in the absence of the E7 oncogene during the carcinogenic process in human keratinocytes using a common HPV variant strongly associated with cervical cancer: the Asian-American E6 variant (AAE6). This E6 variant is 20 times more frequently detected in cervical cancer than the prototype European E6 variant, as evidenced by independent epidemiological data. Using cell culture and cell-based functional assays, we assessed how this variant can perform crucial carcinogenesis steps compared to the prototype E6 variant. The ability to immortalize and transform primary human foreskin keratinocytes (PHFKs) to acquire resilient phenotypes and the ability to promote cell migration were evaluated. The immortalization capability was assayed based on population doublings, number of passages, surpassing mortality stages 1 and 2, human telomerase reverse transcriptase (hTERT) expression, and the ability to overcome G(1) arrest via p53 degradation. Transformation and migration efficiency were analyzed using a combination of functional cell-based assays. We observed that either AAE6 or prototype E6 proteins alone were sufficient to immortalize PHFKs, although AAE6 was more potent in doing so. The AAE6 variant protein alone pushed PHFKs through transformation and significantly increased their migration ability over that of the E6 prototype. Our findings are in line with epidemiological data that the AA variant of HPV16 confers an increased risk over the European prototype for cervical cancer, as evidenced by a superior immortalization, transformation, and metastatic potential.

Confluence switch signaling regulates ECM composition and the plasmin proteolytic cascade in keratinocytes

In culture, cell confluence generates signals that commit actively growing keratinocytes to exit the cell cycle and differentiate to form a stratified epithelium. Using a comparative proteomic approach, we studied this 'confluence switch' and identified a new pathway triggered by cell confluence that regulates basement membrane (BM) protein composition by suppressing the uPA-uPAR-plasmin pathway. Indeed, confluence triggers adherens junction maturation and enhances TGF-β and activin A activity, resulting in increased deposition of PAI-1 and perlecan in the BM. Extracellular matrix (ECM)-accumulated PAI-1 suppresses the uPA-uPAR-plasmin pathway and further enhances perlecan deposition by inhibiting its plasmin-dependent proteolysis. We show that perlecan deposition in the ECM strengthens cell adhesion, inhibits keratinocyte motility and promotes additional accumulation of PAI-1 in the ECM at confluence. In agreement, during wound-healing, perlecan concentrates at the wound-margin, where BM matures to stabilize keratinocyte adhesion. Our results demonstrate that confluence-dependent signaling orchestrates not only growth inhibition and differentiation, but also controls ECM proteolysis and BM formation. These data suggest that uncontrolled integration of confluence-dependent signaling, might favor skin disorders, including tumorigenesis, not only by promoting cell hyperproliferation, but also by altering protease activity and deposition of ECM components.

Epitope mapping of function-blocking monoclonal antibody CM6 suggests a "weak" integrin binding site on the laminin-332 LG2 domain

Laminin-332 (Ln-332) is an extracellular matrix molecule that regulates cell adhesion, spreading, and migration by interaction with cell surface receptors such as alpha3beta1 and alpha6beta4. Previously, we developed a function-blocking monoclonal antibody against rat Ln-332, CM6, which blocks hemidesmosome assembly induced by Ln-332-alpha6beta4 interactions. However, the location of its epitope on Ln-332 has remained unclear. In this study, we show that the CM6 epitope is located on the laminin G-like (LG)2 module of the Ln-332 alpha3 chain. To specify the residues involved in this epitope, we produced a series of GST-fused alpha3 LG2 mutant proteins in which rat-specific acids were replaced with human acids by a site-directed mutagenesis strategy. CM6 reactivity against these proteins showed that CM6 binds to the (1089)NERSVR(1094) sequence of rat Ln-332 LG2 module. In a structural model, this sequence maps to an LG2 loop sequence that is exposed to solvent according to predictions, consistent with its accessibility to antibody. CM6 inhibits integrin-dependent cell adhesion on Ln-332 and inhibits cell spreading on both Ln-332 and recombinant LG2 (rLG2; but not rLG3), suggesting the presence of an alpha3beta1 binding site on LG2. However, we were unable to show that rLG2 supports adhesion in standard assays, suggesting that LG2 may contain a "weak" integrin binding site, only detectable in spreading assays that do not require washes. These results, together with our previous findings, indicate that binding sites for alpha3beta1 and alpha6beta4 are closely spaced in the Ln-332 LG domains where they regulate alternative cell functions, namely adhesion/migration or hemidesmosome anchoring.

Emerging concepts in engineering extracellular matrix variants for directing cell phenotype

Directing specific, complex cell behaviors, such as differentiation, in response to biomaterials for regenerative medicine applications is, at present, a mostly unrealized goal. To date, current technological advances have been inspired by the reductionist point of view, focused on developing simple and merely adequate environments that facilitate simple cellular adhesion. However, even if extracellular matrix (ECM)-derived peptides, such as Arg-Gly-Asp (RGD), have largely demonstrated their utility in supporting cell adhesion, their lack of biological specificity is simply not optimal for controlling more integrated processes, such as cell differentiation. These more complex cellular processes require specific integrin-signaling scaffolds and presumably synergistic integrin and growth factor-receptor signaling. This article will introduce some current efforts to engineer ECM variants that incorporate additional levels of complexity for directing greater integrin specificity and synergistic ECM growth factor signaling toward directing cell phenotype.

Plasmin induces degradation and dysfunction of laminin 332 (laminin 5) and impaired assembly of basement membrane at the dermal-epidermal junction

BACKGROUND

The epidermal basement membrane (BM), located at the dermal-epidermal junction (DEJ), plays important roles not only in adhesion between epidermis and dermis, but also in controlling skin functions. In sun-exposed skin, the BM becomes disrupted and multilayered. In order to explore the impairment of BM assembly, we have used a skin-equivalent (SE) as a model of BM damage and previously clarified the involvement of matrix metalloproteinases (MMPs) in impairment of BM assembly.

OBJECTIVES

In this work, we examined the role of urokinase-type plasminogen activator (uPA) and plasmin in impairment of BM assembly at the DEJ by using the SE, as ultraviolet irradiation to the skin increases uPA as well as MMPs.

METHODS

SEs were used as a model of formation and damage of BM. Human uPA was detected by enzyme-linked immunosorbent assay and zymography, and gelatinases such as MMP-2 and MMP-9 were detected by zymography. Human plasminogen was added at 0.06 micromol L(-1) (about 3% of plasma level) to increase plasmin to a pathological level. N-terminal peptide sequence analysis of plasmin-treated laminin 332 was carried out to identify alpha3, beta3 and gamma2 chains of laminin 332 and their cleavage sites of each chain. Plasmin-treated laminin 332 was analysed in keratinocyte adhesion activity and binding to type VII collagen.

RESULTS

Human uPA was detected in addition to MMP-2 and MMP-9, in conditioned medium of SE. Although the BM was well organized in the presence of an MMP inhibitor alone, the activated plasmin disorganized the BM even in the presence of the inhibitor. The impairment of BM assembly made the epidermis thinner as compared with that of a control cultured in the presence of MMP inhibitor, indicating that the BM affects the polarity and differentiation of the epidermis. The addition of aprotinin, a serine proteinase inhibitor, and tranexamic acid, a uPA-plasmin inhibitor, inhibited the plasmin-induced impairment of BM assembly and facilitated BM reorganization, thereby improving the epidermal structure. N-terminal peptide sequence analysis of plasmin-treated laminin 332 revealed the removal of a 5- or 10-kDa fragment, including the cell adhesion region, from the G3 domain of the alpha3 chain, and the LN domain, which binds to the noncollagenous 1 domain in type VII collagen, from the beta3 chain. Plasmin-treated laminin 332 showed lower keratinocyte adhesion activity and reduced binding to type VII collagen.

CONCLUSIONS

These results suggest that uPA and plasmin are involved in the impairment of BM assembly and epidermal differentiation, and that these effects arise at least partly through direct degradation of laminin 332.

Syndecan-1 interaction with the LG4/5 domain in laminin-332 is essential for keratinocyte migration

Laminin 5/laminin 332 (LN332) is an adhesion substrate for epithelial cells. After secretion of LN332, a regulated cleavage occurs at the carboxy-terminus of its alpha3 subunit, which releases a tandem of two globular modules named LG4/5. We show that the presence of the LG4/5 domain in precursor LN332 decreases its integrin-mediated cell adhesion properties in comparison with mature LN332. Whereas cell adhesion to the recombinant LG4/5 fragment relies solely on the heparan sulfate proteoglycan (HSPG) receptor syndecan-1, we reveal that both syndecan-1 and the alpha3beta1 integrin bind to precursor LN332. We further demonstrate that syndecan-1 mediated cell adhesion to the LG4/5 fragment and pre-LN332 allows the formation of fascin-containing protrusions, depending on the GTPases Rac and Cdc42 activation. Reducing syndecan-1 expression in normal keratinocytes prevents cell protrusions on pre-LN332 with subsequent failure of the peripheral localization of the alpha3beta1 integrin. We finally show that cell migration on pre-LN332 requires syndecan-1. Therefore, the LG4/5 domain in precursor LN332 appears to trigger intracellular signaling events, which participate in keratinocyte motility.

Biological function of laminin-5 and pathogenic impact of its deficiency

The basement membrane glycoprotein laminin-5 is a key component of the anchoring complex connecting keratinocytes to the underlying dermis. It is secreted by keratinocytes as a cross-shaped heterotrimer of alpha3, beta3 and gamma2 chains and serves as a ligand of various transmembrane receptors, thereby regulating keratinocyte adhesion, motility and proliferation. In intact skin, laminin-5 provides essential links to both the hemidesmosomal alpha6beta4 integrin and the collagen type VII molecules which form the anchoring fibrils inserting into the dermis. If the basement membrane is injured, laminin-5 production increases rapidly. It then serves as a scaffold for cell migration, initiates the formation of hemidesmosomes and accelerates basement membrane restoration at the dermal-epidermal junction. Mutations of the laminin-5 genes or auto-antibodies against one of the subunits of laminin-5 may lead to a significant lack of this molecule in the epidermal basement membrane zone. The major contributions of laminin-5 to the resistance of the epidermis against frictional stress but also for basement membrane regeneration and repair of damaged skin are reflected by the phenotype of Herlitz junctional epidermolysis bullosa, which is caused by an inherited absence of functional laminin-5. This lethal disease becomes manifest in widespread blistering of skin and mucous membranes, impaired wound healing and chronic erosions containing exuberant granulation tissue. Here, we discuss current understanding of the biological functions of laminin-5, the pathogenic impact of its deficiency and implications on molecular approaches towards a therapy of junctional epidermolysis bullosa.

Cdc42 expression in keratinocytes is required for the maintenance of the basement membrane in skin

Cdc42 is a small GTPase, which acts as a molecular switch to regulate a wide variety of cellular functions, such as actin cytoskeleton organization, cell proliferation, apoptosis, cell migration and in particular, cell polarity. Formation and maintenance of the basement membrane is a polarized process, which requires directed secretion, deposition and organization of basement membrane components at the basal side of epithelial cells. In the current study, we analyzed the maintenance of skin basement membrane in mice with a keratinocyte-restricted deletion of the Cdc42 gene. In the absence of Cdc42, basement membrane components became aberrantly deposited and the processing of laminin 5 was impaired in parts of the dermal-epidermal junction. These impairments became more severe with age and corresponded to local defects of the basement membrane in 4.5-month-old mutant mice. However, both, structure and number of hemidesomosomes were not significantly changed in the Cdc42 mutant skin compared with the control mice and no blister formation was observed in mutant skin. These data indicate that Cdc42 in keratinocytes is important for maintenance of the basement membrane of skin.

Integrin beta4 regulates migratory behavior of keratinocytes by determining laminin-332 organization

Whether alpha6beta4 integrin regulates migration remains controversial. beta4 integrin-deficient (JEB) keratinocytes display aberrant migration in that they move in circles, a behavior that mirrors the circular arrays of laminin (LM)-332 in their matrix. In contrast, wild-type keratinocytes and JEB keratinocytes, induced to express beta4 integrin, assemble laminin-332 in linear tracks over which they migrate. Moreover, laminin-332-dependent migration of JEB keratinocytes along linear tracks is restored when cells are plated on wild-type keratinocyte matrix, whereas wild-type keratinocytes show rotation over circular arrays of laminn-332 in JEB keratinocyte matrix. The activities of Rac1 and the actin cytoskeleton-severing protein cofilin are low in JEB keratinocytes compared with wild-type cells but are rescued following expression of wild-type beta4 integrin in JEB cells. Additionally, in wild-type keratinocytes Rac1 is complexed with alpha6beta4 integrin. Moreover, Rac1 or cofilin inactivation induces wild-type keratinocytes to move in circles over rings of laminin-332 in their matrix. Together these data indicate that laminin-332 matrix organization is determined by the alpha6beta4 integrin/actin cytoskeleton via Rac1/cofilin signaling. Furthermore, our results imply that the organizational state of laminin-332 is a key determinant of the motility behavior of keratinocytes, an essential element of skin wound healing and the successful invasion of epidermal-derived tumor cells.

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.