Collagen XVII is destabilized by a glycine substitution mutation in the cell adhesion domain Col15

Mutational analysis of epidermolysis bullosa in Taiwan by whole-exome sequencing complemented by RNA sequencing: a series of 77 patients

BACKGROUND

Epidermolysis bullosa (EB) is a heterogeneous group of hereditary skin diseases characterized by skin fragility. Primary data on Taiwanese population remain scarce.

METHODS

We gathered clinical information from EB patients at National Cheng Kung University Hospital from January, 2012, to June, 2021. Diagnostic tests including transmission electron microscopy, immunofluorescence studies, and whole-exome sequencing (WES) were performed. The pathogenicity of novel splice-site mutations was determined through reverse transcriptase-PCR of skin mRNA followed by Sanger and/or RNA sequencing.

RESULTS

Seventy-seven EB patients from 45 families were included: 19 EB simplex, six junctional EB, and 52 dystrophic EB. Pathogenic variants were identified in 37 of 38 families (97.4%), in which WES was used as a first-line tool for mutational analysis; RNA sequencing determined pathogenic variants in the remaining one family. A total of 60 mutations in EB-related genes were identified, including 22 novel mutations. The mutations involved KRT5, KRT14, PLEC, COL17A1, LAMB3, LAMA3, ITGB4, and COL7A1. Over one-quarter of DEB patients had EB pruriginosa.

CONCLUSIONS

The distinct clinical presentation and molecular pathology of EB in Taiwan expand our understanding of this disorder. WES was an effective first-line diagnostic tool for identifying EB-associated variants. RNA sequencing complemented WES when multiple potentially pathogenic splice-site mutations were found.

Design of Recombinant Spider Silk Proteins for Cell Type Specific Binding

Cytophilic (cell-adhesive) materials are very important for tissue engineering and regenerative medicine. However, for engineering hierarchically organized tissue structures comprising different cell types, cell-specific attachment and guidance are decisive. In this context, materials made of recombinant spider silk proteins are promising scaffolds, since they exhibit high biocompatibility, biodegradability, and the underlying proteins can be genetically functionalized. Here, previously established spider silk variants based on the engineered Araneus diadematus fibroin 4 (eADF4(C16)) are genetically modified with cell adhesive peptide sequences from extracellular matrix proteins, including IKVAV, YIGSR, QHREDGS, and KGD. Interestingly, eADF4(C16)-KGD as one of 18 tested variants is cell-selective for C2C12 mouse myoblasts, one out of 11 tested cell lines. Co-culturing with B50 rat neuronal cells confirms the cell-specificity of eADF4(C16)-KGD material surfaces for C2C12 mouse myoblast adhesion.

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.

BP180/Collagen XVII: A Molecular View

BP180 is a type II collagenous transmembrane protein and is best known as the major autoantigen in the blistering skin disease bullous pemphigoid (BP). The BP180 trimer is a central component in type I hemidesmosomes (HD), which cause the adhesion between epidermal keratinocytes and the basal lamina, but BP180 is also expressed in several non-HD locations, where its functions are poorly characterized. The immunological roles of intact and proteolytically processed BP180, relevant in BP, have been subject to intensive research, but novel functions in cell proliferation, differentiation, and aging have also recently been described. To better understand the multiple physiological functions of BP180, the focus should return to the protein itself. Here, we comprehensively review the properties of the BP180 molecule, present new data on the biochemical features of its intracellular domain, and discuss their significance with regard to BP180 folding and protein-protein interactions.

Transmembrane collagens-Unexplored mediators of epidermal-dermal communication and tissue homeostasis

Tissue homeostasis is maintained through constant, dynamic and heterogeneous communication between cells and their microenvironment. Proteins that are at the same time active at the intracellular, cell periphery and deeper extracellular levels possess the ability to, on the individual molecular level, influence the cells and their microenvironment in a bidirectional manner. The transmembrane collagens are a family of such proteins, which are of notable interest for tissue development and homeostasis. In skin, expression of all transmembrane collagens has been reported and deficiency of transmembrane collagen XVII manifests with distinct skin phenotypes. Nevertheless, transmembrane collagens in skin remain understudied despite the association of them with epidermal wound healing and dermal fibrotic processes. Here, we present an overview of transmembrane collagens and put a spotlight on them as regulators of epidermal-dermal communication and as potential players in fibrinogenesis.

The intercellular expression of type-XVII collagen, laminin-332, and integrin-β1 promote contact following during the collective invasion of a cancer cell population

Cancer cells can invade as a population in various cancer tissues. This phenomenon is called collective invasion, which is associated with the metastatic potential and prognosis of cancer patients. The collectiveness of cancer cells is necessary for collective invasion. However, the mechanism underlying the generation of collectiveness by cancer cells is not well known. In this study, the phenomenon of contact following, where neighboring cells move in the same direction via intercellular adhesion, was investigated. An experimental system was created to observe the two-dimensional invasion using a collagen gel overlay to study contact following in collective invasion. The role of integrin-β1, one of the major extracellular matrix (ECM) receptors, in contact following was examined through the experimental system. Integrin-β1 was localized to the intercellular site in squamous carcinoma cells. Moreover, the intercellular adhesion and contact following were suppressed by treatment of an integrin-β1 inhibitory antibody. ECM proteins such as laminin-332 and type-XVII collagen were also localized to the intercellular site and critical for contact following. Collectively, it was demonstrated that the activity of integrin-β1 and expression of ECM proteins in the intercellular site promote contact following in the collective invasion of a cancer cell population.

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.

The Changing Landscape in the Genetic Etiology of Human Tooth Agenesis

Despite much progress in understanding the genetics of syndromic tooth agenesis (TA), the causes of the most common, isolated TA remain elusive. Recent studies have identified novel genes and variants contributing to the etiology of TA, and revealed new pathways in which tooth development genes belong. Further, the use of new research approaches including next-generation sequencing has provided increased evidence supporting an oligogenic inheritance model for TA, and may explain the phenotypic variability of the condition. In this review, we present current knowledge about the genetic mechanisms underlying syndromic and isolated TA in humans, and highlight the value of incorporating next-generation sequencing approaches to identify causative and/or modifier genes that contribute to the etiology of TA.

The triple helix of collagens - an ancient protein structure that enabled animal multicellularity and tissue evolution

The cellular microenvironment, characterized by an extracellular matrix (ECM), played an essential role in the transition from unicellularity to multicellularity in animals (metazoans), and in the subsequent evolution of diverse animal tissues and organs. A major ECM component are members of the collagen superfamily -comprising 28 types in vertebrates - that exist in diverse supramolecular assemblies ranging from networks to fibrils. Each assembly is characterized by a hallmark feature, a protein structure called a triple helix. A current gap in knowledge is understanding the mechanisms of how the triple helix encodes and utilizes information in building scaffolds on the outside of cells. Type IV collagen, recently revealed as the evolutionarily most ancient member of the collagen superfamily, serves as an archetype for a fresh view of fundamental structural features of a triple helix that underlie the diversity of biological activities of collagens. In this Opinion, we argue that the triple helix is a protein structure of fundamental importance in building the extracellular matrix, which enabled animal multicellularity and tissue evolution.

Whole-Exome Sequencing Identifies Novel Variants for Tooth Agenesis

Tooth agenesis is a common craniofacial abnormality in humans and represents failure to develop 1 or more permanent teeth. Tooth agenesis is complex, and variations in about a dozen genes have been reported as contributing to the etiology. Here, we combined whole-exome sequencing, array-based genotyping, and linkage analysis to identify putative pathogenic variants in candidate disease genes for tooth agenesis in 10 multiplex Turkish families. Novel homozygous and heterozygous variants in LRP6, DKK1, LAMA3, and COL17A1 genes, as well as known variants in WNT10A, were identified as likely pathogenic in isolated tooth agenesis. Novel variants in KREMEN1 were identified as likely pathogenic in 2 families with suspected syndromic tooth agenesis. Variants in more than 1 gene were identified segregating with tooth agenesis in 2 families, suggesting oligogenic inheritance. Structural modeling of missense variants suggests deleterious effects to the encoded proteins. Functional analysis of an indel variant (c.3607+3_6del) in LRP6 suggested that the predicted resulting mRNA is subject to nonsense-mediated decay. Our results support a major role for WNT pathways genes in the etiology of tooth agenesis while revealing new candidate genes. Moreover, oligogenic cosegregation was suggestive for complex inheritance and potentially complex gene product interactions during development, contributing to improved understanding of the genetic etiology of familial tooth agenesis.

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.

Recessive bullous dermolysis of the newborn in preterm siblings with a missense mutation in type VII collagen

Bullous dermolysis of the newborn is a dominant or recessive inherited subtype of dystrophic epidermolysis bullosa characterized by the tendency to spontaneously stop blistering within the first months of life. Here we report two siblings with bullous dermolysis of the newborn who were born prematurely and have a novel recessive mutation, p.Pro2259Leu, in the triple helix domain of type VII collagen. We discuss the possible relationship between genotype and prematurity and clinical manifestations in these patients.

LAMB3 mutations causing autosomal-dominant amelogenesis imperfecta

Amelogenesis imperfecta (AI) can be either isolated or part of a larger syndrome. Junctional epidermolysis bullosa (JEB) is a collection of autosomal-recessive disorders featuring AI associated with skin fragility and other symptoms. JEB is a recessive syndrome usually caused by mutations in both alleles of COL17A1, LAMA3, LAMB3, or LAMC2. In rare cases, heterozygous carriers in JEB kindreds display enamel malformations in the absence of skin fragility (isolated AI). We recruited two kindreds with autosomal-dominant amelogenesis imperfecta (ADAI) characterized by generalized severe enamel hypoplasia with deep linear grooves and pits. Whole-exome sequencing of both probands identified novel heterozygous mutations in the last exon of LAMB3 that likely truncated the protein. The mutations perfectly segregated with the enamel defects in both families. In Family 1, an 8-bp deletion (c.3446_3453del GACTGGAG) shifted the reading frame (p.Gly 1149Glufs*8). In Family 2, a single nucleotide substitution (c.C3431A) generated an in-frame translation termination codon (p.Ser1144*). We conclude that enamel formation is particularly sensitive to defects in hemidesmosome/basement-membrane complexes and that syndromic and non-syndromic forms of AI can be etiologically related.

Ectodomain shedding generates Neoepitopes on collagen XVII, the major autoantigen for bullous pemphigoid

As a type II transmembrane protein in basal keratinocytes, collagen XVII provides stable adhesion between epidermis and dermis in the skin. Its ectodomain can be shed from the cell surface, and autoantibodies in certain blistering diseases preferentially recognize the shed form. Major epitopes of collagen XVII are clustered within the juxtamembranous noncollagenous 16th A domain, and ectodomain shedding occurs within this region, suggesting that cleavage generates neoepitopes. However, the candidate cleavage sites have been controversial, and the mechanism of neoepitope generation is unclear. In this study, we investigated cleavage sites in the noncollagenous 16th A domain to understand the generation of neoepitopes and their pathological role. Polyclonal Abs recognizing the stretch Leu(524)-Gly(532) preferentially reacted with the shed ectodomain, but not with the full-length form, indicating that a neoepitope was localized at this site. The neoepitope-specific Ab fixed complement and induced granulocyte-dependent dermal-epidermal separation in cryosections of normal human skin. The physiological cleavage sites were identified using mass spectrometry. N termini were found at Asp(514), Leu(524), Glu(525), and Gly(526), among which Asp(514) and Glu(525) were blocked by acetylation and pyroglutaminate. In silico prediction of B cell epitopes indicated that the antigenicity of the Leu(524)-Gly(532) region increased substantially after shedding, regardless of the cleavage sites. Correspondingly, neoepitopes were found in the skin and blister fluids of patients with bullous pemphigoid, and bullous pemphigoid sera reacted with the peptide Leu(524)-Gly(532). Taken together, these data demonstrate that physiological shedding of collagen XVII generates neoepitopes, which may serve as a target of blister-inducing autoantibodies.

Collagen XVII

Collagen XVII has been identified as having a role in inherited junctional epidermolysis bullosa non-Herlitz (JEB-other, MIM #226650). The role of collagen XVII in both autoimmune and genetic blistering disorders demonstrates its relevance to dermal-epidermal adhesion. Collagen XVII is a major structural component of the hemidesmosome (HD), a highly specialized multiprotein complex that mediates the anchorage of basal epithelial cells to the underlying basement membrane in stratified, pseudostratified, and transitional epithelia. This article examines the genetic and pathological features of collagen XVII.

Maternal serum adiponectin multimers in gestational diabetes

OBJECTIVE

Adiponectin, an adipokine with profound insulin-sensitizing effect, consists of heterogeneous species of multimers. These oligomeric complexes circulate as low-molecular-weight (LMW) trimers, medium-molecular-weight (MMW) hexamers and high-molecular-weight (HMW) isoforms and can exert differential biological effects. The aims of this study were to determine whether there is a change in circulating adiponectin multimers in the presence of gestational diabetes mellitus (GDM), overweight/obesity or with a treatment with sulfonylurea or insulin in patients with GDM.

STUDY DESIGN

This cross-sectional study included women with: 1) normal pregnancy (n=149); and 2) patients with GDM (n=72). Thirty-three patients with GDM were managed with diet alone. Among the others 39 diabetic patients, 17 were treated with Glyburide and 22 with insulin. The study population was further stratified by first trimester body mass index (BMI) (normal weight <25 kg/m(2) vs. overweight/obese > or =25 kg/m(2)). Serum adiponectin multimers (total, HMW, MMW and LMW) concentrations were determined by ELISA.

RESULTS

1) The median maternal serum of total, HMW, MMW and LMW were lower in patients with GDM than in those with normal pregnancies (P<0.001 for all comparisons); 2) patients with GDM had a lower HMW/total adiponectin ratio and a higher MMW/total and LMW/total adiponectin ratio than those with a normal pregnancy (P<0.001 for all comparisons); and 3) among GDM patients, there were no differences in the concentrations and relative distribution of adiponectin multimers between those who were managed with diet, and those who were treated with pharmacological agents.

CONCLUSION

1) GDM is characterized by a distinctive pattern of concentrations and relative distribution of adiponectin multimers akin to Type 2 diabetes mellitus; 2) dysregulation of adiponectin multimeres can provide a mechanistic basis for the association between adiposity and GDM.

Autosomal dominant junctional epidermolysis bullosa

BACKGROUND

Epidermolysis bullosa (EB) encompasses a heterogeneous group of inherited skin disorders associated with trauma-induced blistering. The junctional forms of EB (JEB), Herlitz JEB, non-Herlitz JEB and JEB associated with pyloric atresia have all been attributed to autosomal recessive inheritance. We describe a 7-year-old girl with defective dental enamel, trauma-induced blistering and subsequent scarring. Her mother, a carrier of the mutation p.G627V in the collagen XVII gene (COL17A1) had evidence of hypoplastic dental enamel without skin blistering. Her grandmother had non-Herlitz JEB as a result of a compound heterozygous mutation in COL17A1 (p.G627V and c.3514ins25).

OBJECTIVES

To explore the molecular, ultrastructural and immunofluorescence findings of the first case of dominant JEB.

METHODS

Mutational analysis of COL17A1 was performed on the proband's genomic DNA. In addition, transmission electron microscopy and immunofluorescence microscopy were performed on a nonlesional skin biopsy from the proband and an unrelated healthy control.

RESULTS

Direct sequencing revealed a heterozygous glycine substitution mutation, p.G627V, in COL17A1. No discernible morphological abnormalities were found on transmission electron microscopy; however, immunofluorescence microscopy revealed findings of an altered distribution pattern for type XVII collagen epitopes close to the dermal-epidermal junction.

CONCLUSION

This report describes the first case of dominant JEB. Although some heterozygous mutations in COL17A1 are known to cause dental abnormalities none were associated with skin fragility. The dominant-negative interference between the proband's mutated type XVII collagen and the wild-type allele appears to render the skin prone to trauma-induced blister formation. Alternatively, other undisclosed modifying genetic or epigenetic factors might explain why the patient gets blistering whereas her mother, who has the same COL17A1 mutation, has no skin fragility.

Adiponectin multimers in maternal plasma

OBJECTIVE

Adiponectin is an anti-diabetic, anti-atherogenic, anti-inflammatory, and angiogenic adipokine that circulates in oligomeric complexes including: low molecular weight (LMW) trimers, medium molecular weight (MMW) hexamers, and high molecular weight (HMW) isoforms. The aim of this study was to determine whether there are changes in adiponectin multimers in pregnancy and as a function of maternal weight.

STUDY DESIGN

In this cross-sectional study, plasma concentrations of total, HMW, MMW, and LMW adiponectin were determined in women included in three groups: (1) normal pregnant women of normal body mass index (BMI) (n = 466), (2) overweight pregnant women (BMI >or=25; n = 257), and (3) non-pregnant women of normal weight (n = 40). Blood samples were collected once from each woman between 11 and 42 weeks of gestation. Plasma adiponectin multimer concentrations were determined by enzyme-linked immunosorbent assay (ELISA). Non-parametric statistics were used for analysis.

RESULTS

(1) The median HMW adiponectin concentration and the median HMW/total adiponectin ratio were significantly higher, and the median LMW adiponectin concentration was significantly lower in pregnant women than in non-pregnant women. (2) Among pregnant women, the median plasma concentration of total, HMW, and MMW adiponectin was significantly higher in normal weight women than in overweight patients. (3) Maternal HMW was the most prevalent adiponectin multimer regardless of gestational age or BMI status. (4) There were no significant differences in the median concentration of total, MMW, and LMW adiponectin and their relative distribution with advancing gestation.

CONCLUSION

Human pregnancy is characterized by quantitative and qualitative changes in adiponectin multimers, especially the most active isoform, HMW adiponectin.

Maternal serum adiponectin multimers in gestational diabetes

OBJECTIVE

Adiponectin, an adipokine with profound insulin-sensitizing effect, consists of heterogeneous species of multimers. These oligomeric complexes circulate as low-molecular-weight (LMW) trimers, medium-molecular-weight (MMW) hexamers and high-molecular-weight (HMW) isoforms and can exert differential biological effects. The aims of this study were to determine whether there is a change in circulating adiponectin multimers in the presence of gestational diabetes mellitus (GDM), overweight/obesity or with a treatment with sulfonylurea or insulin in patients with GDM.

STUDY DESIGN

This cross-sectional study included women with: 1) normal pregnancy (n=149); and 2) patients with GDM (n=72). Thirty-three patients with GDM were managed with diet alone. Among the others 39 diabetic patients, 17 were treated with Glyburide and 22 with insulin. The study population was further stratified by first trimester body mass index (BMI) (normal weight <25 kg/m(2) vs. overweight/obese > or =25 kg/m(2)). Serum adiponectin multimers (total, HMW, MMW and LMW) concentrations were determined by ELISA.

RESULTS

1) The median maternal serum of total, HMW, MMW and LMW were lower in patients with GDM than in those with normal pregnancies (P<0.001 for all comparisons); 2) patients with GDM had a lower HMW/total adiponectin ratio and a higher MMW/total and LMW/total adiponectin ratio than those with a normal pregnancy (P<0.001 for all comparisons); and 3) among GDM patients, there were no differences in the concentrations and relative distribution of adiponectin multimers between those who were managed with diet, and those who were treated with pharmacological agents.

CONCLUSION

1) GDM is characterized by a distinctive pattern of concentrations and relative distribution of adiponectin multimers akin to Type 2 diabetes mellitus; 2) dysregulation of adiponectin multimeres can provide a mechanistic basis for the association between adiposity and GDM.

C-terminal truncation impairs glycosylation of transmembrane collagen XVII and leads to intracellular accumulation

Collagen XVII, a type II transmembrane protein in hemidesmosomes, is involved in the anchorage of stratified epithelia to the underlying mesenchyme. Its functions are regulated by ectodomain shedding, and its genetic defects lead to epidermal detachment in junctional epidermolysis bullosa (JEB), a heritable skin fragility syndrome, but the molecular disease mechanisms remain elusive. Here we used a spontaneously occurring homozygous COL17A1 deletion mutant in JEB to discern glycosylation of collagen XVII. The mutation truncated the distal ectodomain and positioned the only N-glycosylation site 34 amino acids from the newly formed C terminus, which impaired efficient N-glycosylation. Immunofluorescence staining of authentic JEB keratinocytes and of COS-7 cells transfected with the mutant indicated intracellular accumulation of collagen XVII precursor molecules. Cell surface biotinylation and quantification of ectodomain shedding demonstrated that only about 15% of the truncated collagen XVII reached the cell surface. The cell surface-associated molecules were N-glycosylated in a normal manner, in contrast to the molecules retained within the cells, indicating that N-glycosylation of the ectodomain is required for targeting of collagen XVII to the plasma membrane and that reduced accessibility of the N-glycosylation site negatively regulates this process. Functional consequences of the strong reduction of collagen XVII on the cell surface included scattered deposition of cell adhesion molecule laminin 5 into the extracellular environment and, as a consequence of faulty collagen XVII-laminin ligand interactions, aberrant motility of the mutant cells.

Kindlin-1 is a phosphoprotein involved in regulation of polarity, proliferation, and motility of epidermal keratinocytes

A novel family of focal adhesion proteins, the kindlins, is involved in attachment of the actin cytoskeleton to the plasma membrane and in integrin-mediated cellular processes. Deficiency of kindlin-1, as a result of loss-of-function mutations in the KIND1 gene, causes Kindler syndrome, an autosomal recessive genodermatosis characterized by skin blistering, progressive skin atrophy, photosensitivity and, occasionally, carcinogenesis. Here we characterized authentic and recombinantly expressed kindlin-1 and show that it is localized in basal epidermal keratinocytes in a polar fashion, close to the cell surface facing the basement membrane, in the areas between the hemidesmosomes. We identified two forms of kindlin-1 in keratinocytes, with apparent molecular masses of 78 and 74 kDa, corresponding to phosphorylated and desphosphorylated forms of the protein. In kindlin-1-deficient skin, basal keratinocytes show multiple abnormalities: cell polarity is lost, proliferation is strongly reduced, and several cells undergo apoptosis. In vitro, deficiency of kindlin-1 in keratinocytes leads to strongly reduced cell proliferation, decreased adhesion, undirected motility, and intense protrusion activity of the plasma membrane. Taken together, these results show that kindlin-1 plays a role in keratinocyte adhesion, polarization, proliferation, and migration. It is involved in organization and anchorage of the actin cytoskeleton to integrin-associated signaling platforms.

Adiponectin and adiponectin receptors in insulin resistance, diabetes, and the metabolic syndrome

Adiponectin is an adipokine that is specifically and abundantly expressed in adipose tissue and directly sensitizes the body to insulin. Hypoadiponectinemia, caused by interactions of genetic factors such as SNPs in the Adiponectin gene and environmental factors causing obesity, appears to play an important causal role in insulin resistance, type 2 diabetes, and the metabolic syndrome, which are linked to obesity. The adiponectin receptors, AdipoR1 and AdipoR2, which mediate the antidiabetic metabolic actions of adiponectin, have been cloned and are downregulated in obesity-linked insulin resistance. Upregulation of adiponectin is a partial cause of the insulin-sensitizing and antidiabetic actions of thiazolidinediones. Therefore, adiponectin and adiponectin receptors represent potential versatile therapeutic targets to combat obesity-linked diseases characterized by insulin resistance. This Review describes the pathophysiology of adiponectin and adiponectin receptors in insulin resistance, diabetes, and the metabolic syndrome.

Expanding the COL7A1 Mutation Database: Novel and Recurrent Mutations and Unusual Genotype – Phenotype Constellations in 41 Patients with Dystrophic Epidermolysis Bullosa

Dystrophic epidermolysis bullosa (DEB), a heterogeneous hereditary skin disorder characterized by trauma-induced blistering and scarring, affects thousands of families worldwide. The clinical manifestations extend from minor nail dystrophy to severe life-threatening blistering, making early molecular diagnosis and prognostication of utmost importance for the affected families. DEB is caused by mutations in the COL7A1 gene encoding collagen VII in the skin. Molecular diagnostics and genotype-phenotype correlations in DEB remain complex owing to the gene structure, large variety of mutations, high rate of novel mutations, complex protein structure and assembly, and the heterogeneity of phenotypes. Here, we report an efficient strategy for COL7A1 mutation detection using direct automated DNA sequencing and implementation of software tools. With this approach, COL7A1 mutations of 41 DEB families were disclosed. Twenty-four mutations were novel and two recurrent. Elucidation of biological consequences of the mutations helped define disease mechanisms, but also revealed several unusual genotypic and/or phenotypic constellations, which impeded the diagnostics and prognostication. In addition, the studies disclosed a de novo mutation in recessive DEB and two new polymorphisms in the COL7A1 gene.

Molecular mechanisms of junctional epidermolysis bullosa: Col 15 domain mutations decrease the thermal stability of collagen XVII

Mutations in the collagen XVII gene, COL17A1, are associated with junctional epidermolysis bullosa. Most COL17A1 mutations lead to a premature termination codon (PTC), whereas only a few mutations result in amino acid substitutions or deletions. We describe here two novel glycine substitutions, G609D and G612R, and a splice site mutation resulting in a deletion of three Gly-X-Y amino acid triplets. In order to investigate the molecular pathomechanisms of non-PTC mutations, G609D and G612R and two previously known substitutions, G627V and G633, and deletion of the amino acids 779-787 were introduced into recombinant collagen XVII. The thermal stability of the mutated collagens was assessed using trypsin digestions at incremental temperatures. All the four glycine substitutions significantly destabilized the ectodomain of collagen XVII, which manifested as 16 degrees C-20 degrees C lower T(m) (midpoint of the helix-to-coil transition). These results were supported by secondary structure predictions, which suggested interruptions of the collagenous triple helix within the largest collagenous domain, Col15. In contrast, deletion of the three full Gly-X-Y triplets, amino acids 779-787, had no overall effect on the stability of the ectodomain, as the deletion was in register with the triplet structure and also generated compensatory changes in the NC15 domain.

The collagen superfamily: from the extracellular matrix to the cell membrane

The collagen superfamily is highly complex and shows a remarkable diversity in molecular and supramolecular organization, tissue distribution and function. However, all its members share a common structural feature, the presence of at least one triple-helical domain, which corresponds to a number of (Gly-X-Y)n repeats (X being frequently proline and Y hydroxyproline) in the amino acid sequence. Several sub-families have been determined according to sequence homologies and to similarities in the structural organization and supramolecular assembly. In the present review, we focus on the newly described fibrillar collagens, fibrillar-associated collagens with interrupted triple helix, membrane collagens and multiplexins. Recent advances in the characterization of proteins containing triple-helical domains but not referred to as collagens are also discussed.

Dystroglycan in skin and cutaneous cells: beta-subunit is shed from the cell surface

In skin, hemidesmosomal protein complexes attach the epidermis to the dermis and are critical for stable connection of the basal epithelial cell cytoskeleton with the basement membrane (BM). In muscle, a similar supramolecular aggregate, the dystrophin glycoprotein complex links the inside of muscle cells with the BM. A component of the muscle complex, dystroglycan (DG), also occurs in epithelia. In this study, we characterized the expression and biochemical properties of authentic and recombinant DG in human skin and cutaneous cells in vitro. We show that DG is present at the epidermal BM zone, and it is produced by both keratinocytes and fibroblasts in vitro. The biosynthetic precursor is efficiently processed to the alpha- and beta-DG subunits; and, in addition, a distinct extracellular segment of the transmembranous beta-subunit is shed from the cell surface by metalloproteinases. Shedding of the beta-subunit releases the alpha-subunit from the DG complex on the cell surface into the extracellular space. The shedding is enhanced by IL-1beta and phorbol esters, and inhibited by metalloproteinase inhibitors. Deficiency of perlecan, a major ligand of alpha-DG, enhanced shedding suggesting that lack of a binding partner destabilizes the epithelial DG complex and makes it accessible to proteolytic processing.

Sorting of furin in polarized epithelial and endothelial cells: expression beyond the Golgi apparatus

The conversion of proteins into their mature forms underlies the functionality of many fundamental cellular pathways. One posttranslational modification leading to maturation of precursor proteins consists of the cleavage of their prodomain at pairs of basic amino acids by enzymes of the subtilisin-like mammalian proprotein convertase family. One of these enzymes, furin, acts in the constitutive secretory pathway of almost every cell type. However, in spite of furin's major roles in many pathophysiological processes, the exact subcellular sites of processing and activation of its substrates remain elusive. In this study, furin antigenic sites were tracked in subcellular compartments of various tissues and corresponding cell lines by high-resolution immunogold electron microscopy, Western blotting, cell transfection, and in vivo gene delivery of the furin cDNA. In addition to the Golgi apparatus, furin was assigned to endosomes and plasma membranes of polarized intestinal and renal epithelial cells and endothelial cells of the continuous, fenestrated, and discontinuous capillaries. Roles of furin in endothelial permeability, basement membrane turnover, and shedding of transmembrane proteins are supported by our data.

Shedding of collagen XVII/BP180: structural motifs influence cleavage from cell surface

Collagen XVII/BP180, an epithelial adhesion molecule, belongs to the group of collagenous transmembrane proteins, which are characterized by ectodomain shedding. We recently showed that ADAMs can cleave collagen XVII, but also that furin participates in this process (Franzke, C. W., Tasanen, K., Schäcke, H., Zhou, Z., Tryggvason, K., Mauch, C., Zigrino, P., Sunnarborg, S., Lee, D. C., Fahrenholz, F., and Bruckner-Tuderman, L. (2002) EMBO J. 21, 5026-5035). To define the cleavage region in the juxtamembranous NC16A linker domain and assess its structure and requirements for shedding, we constructed deletion mutants of the NC16A domain, expressed them in COS-7 cells, and analyzed their structural integrity and shedding behavior. A mutant lacking the furin consensus sequence was shed in a normal manner, demonstrating that furin does not cleave collagen XVII but rather activates ADAMs (a disintegrin and metalloproteinase). Large deletions of the NC16A domain prevented shedding, and analysis of defined smaller deletions pointed to the stretch of amino acid residues 528-547 as important for sheddase recognition and cleavage. Secondary protein structure predictions showed that deletion of this stretch resulted in an NC16A domain with a positive net charge and an amphipathic alpha-helix, which can cause conformational changes in the collagen XVII homotrimer. Assessment of triple-helix folding of the mutants revealed a lower thermal stability of all non-shed variants than of wild-type collagen XVII or the shed mutants. In contrast, deletion of the putative nucleation site for triple-helix folding of collagenous transmembrane proteins did not affect folding of collagen XVII. The data indicate that the conformation of the NC16A domain and steric availability of the cleavage site influence shedding and is important for folding of collagen XVII.

Keratinocytes from patients lacking collagen XVII display a migratory phenotype

Acquired or inherited junctional epidermolysis bullosa are skin diseases characterized by a separation between the epidermis and the dermis. In inherited nonlethal junctional epidermolysis bullosa, genetic analysis has identified mutations in the COL17A1 gene coding for the transmembrane collagen XVII whereas patients with acquired diseases have autoantibodies against this protein. This suggests that collagen XVII participates in the adhesion of basal keratinocytes to the extracellular matrix. To test this hypothesis, we studied the behavior of keratinocytes with null mutations in the COL17A1 gene. Initial adhesion of mutant cells to laminin 5 was comparable to controls and similarly dependent on alpha3beta1 integrins. The spreading of mutant cells was, however, enhanced, suggesting a propensity to migrate, which was confirmed by migration assays. In addition, laminin 5 deposited by collagen XVII-deficient keratinocytes was scattered and poorly organized, suggesting that correct integration of laminin 5 within the matrix requires collagen XVII. This assumption was supported by the co-distribution of the two proteins in the matrix of normal human keratinocytes and by protein-protein-binding assays showing that the C-terminus of collagen XVII binds to laminin 5. Together, the results unravel an unexpected role of collagen XVII in the regulation of keratinocyte migration.

Effect of glycine substitutions on alpha5(IV) chain structure and structure-phenotype correlations in Alport syndrome

The phenotype variety caused by glycine substitutions in alpha5(IV) chain in X-linked Alport syndrome (XLAS) prompted the complexity of structure changes of alpha5(IV) chain that was little to know now. In this study, we expressed a domain of alpha5(IV) chain containing different glycine substitutions (G1015V and G1030S, respectively) which were revealed in two XLAS pedigrees with different phenotype severities and the corresponding domain of a control in Escherichia coli. The recombinant proteins were characterized by immunoblot and mass spectrometry and analyzed the secondary structure by using circular dichroism (CD) spectroscopy. CD analysis showed that the recombinant protein containing G1015V mutation identified in the pedigree of juvenile-onset XLAS exhibited 12.9% alpha-helix that was not found in the control recombinant protein. The spectrum of the recombinant protein containing G1030S mutation identified in the pedigree of adult-onset XLAS was slightly different from that of the control, that is, mostly with the random coil and the beta-sheet, while without alpha-helix. These results demonstrated that two kinds of glycine substitutions, although in the same domain of alpha5(IV) chain, displayed the distinctly different secondary structures. The changes of the secondary structure could explain the phenotypic diversities of XLAS, which would be hardly understood solely by analyzing genomic DNA or mRNA of alpha5(IV) chain.

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.

Impaired multimerization of human adiponectin mutants associated with diabetes. Molecular structure and multimer formation of adiponectin

Adiponectin is an adipocyte-derived hormone, which has been shown to play important roles in the regulation of glucose and lipid metabolism. Eight mutations in human adiponectin have been reported, some of which were significantly related to diabetes and hypoadiponectinemia, but the molecular mechanisms of decreased plasma levels and impaired action of adiponectin mutants were not clarified. Adiponectin structurally belongs to the complement 1q family and is known to form a characteristic homomultimer. Herein, we demonstrated that simple SDS-PAGE under non-reducing and non-heat-denaturing conditions clearly separates multimer species of adiponectin. Adiponectin in human or mouse serum and adiponectin expressed in NIH-3T3 or Escherichia coli formed a wide range of multimers from trimers to high molecular weight (HMW) multimers. A disulfide bond through an amino-terminal cysteine was required for the formation of multimers larger than a trimer. An amino-terminal Cys-Ser mutation, which could not form multimers larger than a trimer, abrogated the effect of adiponectin on the AMP-activated protein kinase pathway in hepatocytes. Among human adiponectin mutations, G84R and G90S mutants, which are associated with diabetes and hypoadiponectinemia, did not form HMW multimers. R112C and I164T mutants, which are associated with hypoadiponectinemia, did not assemble into trimers, resulting in impaired secretion from the cell. These data suggested impaired multimerization and/or the consequent impaired secretion to be among the causes of a diabetic phenotype or hypoadiponectinemia in subjects having these mutations. In conclusion, not only total concentrations, but also multimer distribution should always be considered in the interpretation of plasma adiponectin levels in health as well as various disease states.

Alterations of collagen XVII expression during transformation of oral epithelium to dysplasia and carcinoma

Collagen XVII (BP180) is a hemidesmosomal transmembrane component that has been hypothesized to participate in keratinocyte adhesion and motility. Using immunohistochemical (IHC) and in situ hybridization (ISH) methods, we showed downregulation of collagen XVII in basal cells in mild dysplasias and upregulation in suprabasal keratinocytes in moderate and severe dysplasias as well as in the central cells of grade II and III squamous cell carcinomas (SCCs). Overexpression of collagen XVII was found at the invasive front of the tumors. Collagen XVII and its cleaved ectodomain were characterized from culture extracts and precipitates of oral keratinocytes, tongue carcinoma cells, and tumor tissue extract. Malignant cell lines exhibited increased collagen XVII expression in immunoblotting analysis. In oral keratinocytes, collagen XVII gene expression was significantly induced by PMA but not by the inflammatory cytokines TGF-beta1, TNF-alpha, EGF, IL-1beta, and IL-6. These results indicate altered expression of collagen XVII at different stages of carcinogenesis and suggest a correlation between overexpression of collagen XVII and tumor progression. The reduced collagen XVII expression at the early step of carcinogenesis may reflect disturbed keratinocyte adhesion to the basement membrane.

Collagenous transmembrane proteins: collagen XVII as a prototype

Collagenous transmembrane proteins are an emerging group of biologically versatile molecules which function as both cell surface receptors and matrix molecules. The seven group members have interesting structural similarities: they are integral membrane proteins in type II orientation and have one or more collagenous domains in the extracellular C-terminus; interspersed by non-collagenous stretches which confer structural flexibility to the ectodomain. A conserved coiled-coil sequence (linker domain) immediately adjacent to the extracellular face of the cell membrane presumably serves as a nucleus for trimerization and triple-helix folding of each collagen. Intriguingly, the ectodomains of at least some of these molecules are proteolytically shed from the cell surface, releasing a shorter form of the collagen into the extracellular matrix. Collagenous transmembrane proteins are expressed in many different tissues and cells, and are involved in a broad spectrum of biological functions, reaching from epithelial and neural cell adhesion, and epithelial-mesenchymal interactions during morphogenesis to host defense against microbial agents. Several group members are involved in the molecular pathology of genetic and acquired human diseases including epidermolysis bullosa, ectodermal dysplasia, bullous pemphigoid or Alzheimer disease. An extensively investigated member is collagen XVII, a keratinocyte surface protein, which attaches the epidermis to the basement membrane in the skin. In this review, the structure and functions of the currently known collagenous transmembrane proteins are summarized and, as a 'prototype' of the group, collagen XVII and its biology and pathophysiology are delineated.

Genotype-phenotype correlation in italian patients with dystrophic epidermolysis bullosa

Dystrophic epidermolysis bullosa (DEB) is a rare skin disorder that is clinically heterogeneous and is transmitted either in dominant (DDEB) or recessive (RDEB) mode. Nevertheless, all variants of DEB are caused by mutations in type VII collagen gene (COL7A1). We report an analysis of COL7A1 mutations in 51 Italian DEB patients, 27 affected with Hallopeau-Siemens RDEB, 19 with non Hallopeau-Siemens RDEB, two with DDEB, two with pretibial RDEB, and one with inversa RDEB. Forty-one mutations were identified, 18 of which are novel. Mutation consequences were analyzed at the mRNA and protein level and genotype-phenotype correlation was determined. Recessive inheritance of a new case of pretibial RDEB was also established. In RDEB patients, six recurrent mutations were identified: 7344G-->A, 425A-->G, 8441-14del21, 4783-1G-->A, 497insA, and G1664A, the last three being found only in Italian patients. Indeed, haplotype analysis supported propagation of ancestral mutated alleles within the Italian population for these particular mutations. Altogether recurrent mutations account for approximately 43% of RDEB alleles in Italian patients and therefore new DEB patients should first be screened for the presence of these mutations.

Transmembrane collagen XVII, an epithelial adhesion protein, is shed from the cell surface by ADAMs

Collagen XVII, a type II transmembrane protein and epithelial adhesion molecule, can be proteolytically shed from the cell surface to generate a soluble collagen. Here we investigated the release of the ectodomain and identified the enzymes involved. After surface biotinylation of keratinocytes, the ectodomain was detectable in the medium within minutes and remained stable for >48 h. Shedding was enhanced by phorbol esters and inhibited by metalloprotease inhibitors, including hydroxamates and TIMP-3, but not by inhibitors of other protease classes or by TIMP-2. This profile implicated MMPs or ADAMs as candidate sheddases. MMP-2, MMP-9 and MT1-MMP were excluded, but TACE, ADAM-10 and ADAM-9 were shown to be expressed in keratinocytes and to be actively involved. Transfection with cDNAs for the three ADAMs resulted in increased shedding and, vice versa, in TACE-deficient cells shedding was significantly reduced, indicating that transmembrane collagen XVII represents a novel class of substrates for ADAMs. Functionally, release of the ectodomain of collagen XVII from the cell surface was associated with altered keratinocyte motility in vitro.

Characterization of recombinant soluble macrophage scavenger receptor MARCO

MARCO is a type II transmembrane protein of the class A scavenger receptor family. It has a short N-terminal cytoplasmic domain, a transmembrane domain, and a large extracellular part composed of a 75-residue long spacer domain, a 270-residue collagenous domain, and a 99-residue long scavenger receptor cysteine-rich (SRCR) domain. Previous studies have indicated a role for this receptor in anti-microbial host defense functions. In this work we have produced the extracellular part of MARCO as a recombinant protein, and analyzed its binding properties. The production of this protein, soluble MARCO (sMARCO), has made it possible for the first time to study MARCO and its binding properties in a cell-free system. Using circular dichroism analyses, a protease-sensitive assay, and rotary shadowing electron microscopy, sMARCO was shown to have a triple-helical collagenous structure. Rotary shadowing also demonstrated that the molecules often associate with each other via the globes. sMARCO was found to bind avidly both heat-killed and living bacteria. Lipopolysaccharide, an important component of the outer membrane of Gram-negative bacteria, was shown to be a ligand of MARCO. Studies with different bacterial strains indicated that the O-side chain of lipopolysaccharide is not needed for the bacterial recognition. Finally, the C-terminal SRCR domain was also produced as a recombinant protein, and its bacteria-binding capability was studied. Although the transfection experiments with transmembrane MARCO variants have indicated a crucial role for this domain in bacterial binding, the monomeric domain exhibited low, barely detectable bacteria-binding activity. Thus, it is possible that cooperation between the SRCR domain and the collagenous domain is needed for high-affinity bacterial binding, or that the SRCR domain has to be in a trimeric form to effectively bind to bacteria.

The selective regulation of alpha Vbeta 1 integrin expression is based on the hierarchical formation of alpha V-containing heterodimers

The integrin beta1 subunit can form a heterodimer with 12 different alpha subunits. According to the present model, the expression level of any alphabeta complex is regulated by the availability of the specific alpha subunit, whereas beta1 subunit is constantly present in a large excess. The expression of several heterodimers containing the alphaV subunit seems to be regulated by an identical mechanism. The fact that many cells express alphaVbeta1 heterodimer, and that this fibronectin/vitronectin receptor may be selectively regulated, compromises the present model of the regulation of beta1 and alphaV integrins. We have tried to solve this problem by assuming that distinct alphabeta heterodimers are formed with different tendency. To test the hypothesis, we analyzed WM-266-4 melanoma cells transfected with a cDNA construct coding for an intracellular single-chain anti-alphaV integrin antibody. We could see 70-80% reduction in the cell surface expression of alphaV subunit. However, the only one of the alphaV integrins reduced on the cell surface was alphaVbeta1. This suggests that the cell surface expression level of alphaVbeta1 is dependent on the number of alphaV subunits available after the formation of other alphaV-containing heterodimers. Thus, there seems to be a hierarchy in the complex formation between alphaV and its different beta-partners. These observations explain how alphaVbeta1 can be specifically regulated without concomitant changes in the expression of other alphaV or beta1 integrins.

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

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

The cell adhesion domain of type XVII collagen promotes integrin-mediated cell spreading by a novel mechanism

Type XVII collagen (BP180) is a keratinocyte transmembrane protein that exists as the full-length protein in hemidesmosomes and as a 120-kDa shed ectodomain in the extracellular matrix. The largest collagenous domain of type XVII collagen, COL15, has been described previously as a cell adhesion domain (Tasanen, K., Eble, J. A., Aumailley, M., Schumann, H., Baetge, J, Tu, H., Bruckner, P., and Bruckner-Tuderman, L. (2000) J. Biol. Chem. 275, 3093-3099). In the present work, the integrin binding of triple helical, human recombinant COL15 was tested. Solid phase binding assays using recombinant integrin alpha(1)I, alpha(2)I, and alpha(10)I domains and cell spreading assays with alpha(1)beta(1)- and alpha(2)beta(1)-expressing Chinese hamster ovary cells showed that, unlike other collagens, COL15 was not recognized by the collagen receptors. Denaturation of the COL15 domain increased the spreading of human HaCaT keratinocytes, which could migrate on the denatured COL15 domain as effectively as on fibronectin. Spreading of HaCaT cells on the COL15 domain was mediated by alpha(5)beta(1) and alpha(V)beta(1) integrins, and it could be blocked by RGD peptides. The collagen alpha-chains in the COL15 domain do not contain RGD motifs but, instead, contain 12 closely related KGD motifs, four in each of the three alpha-chains. Twenty-two overlapping, synthetic peptides corresponding to the entire COL15 domain were tested; three peptides, all containing the KGD motif, inhibited the spreading of HaCaT cells on denatured COL15 domain. Furthermore, this effect was lost by mutation from D to E (KGE instead of KGD). We suggest that the COL15 domain of type XVII collagen represents a specific collagenous structure, unable to interact with the cellular receptors for other collagens. After being shed from the cell surface, it may support keratinocyte spreading and migration.

Properties of the collagen type XVII ectodomain. Evidence for n- to c-terminal triple helix folding

Collagen XVII is a transmembrane component of hemidesmosomal cells with important functions in epithelial-basement membrane interactions. Here we report on properties of the extracellular ectodomain of collagen XVII, which harbors multiple collagenous stretches. We have recombinantly produced subdomains of the collagen XVII ectodomain in a mammalian expression system. rColXVII-A spans the entire ectodomain from deltaNC16a to NC1, rColXVII-B is similar but lacks the NC1 domain, a small N-terminal polypeptide rColXVII-C encompasses domains deltaNC16a to C15, and a small C-terminal polypeptide rColXVII-D comprises domains NC6 to NC1. Amino acid analysis of rColXVII-A and -C demonstrated prolyl and lysyl hydroxylation with ratios for hydroxyproline/proline of 0.4 and for hydroxylysine/lysine of 0.5. A small proportion of the hydroxylysyl residues in rColXVII-C ( approximately 3.3%) was glycosylated. Limited pepsin and trypsin degradation assays and analyses of circular dichroism spectra clearly demonstrated a triple-helical conformation for rColXVII-A, -B, and -C, whereas the C-terminal rColXVII-D did not adopt a triple-helical fold. These results were further substantiated by electron microscope analyses, which revealed extended molecules for rColXVII-A and -C, whereas rColXVII-D appeared globular. Thermal denaturation experiments revealed melting temperatures of 41 degrees C (rColXVII-A), 39 degrees C (rColXVII-B), and 35 degrees C (rColXVII-C). In summary, our data suggest that triple helix formation in the ectodomain of ColXVII occurs with an N- to C-terminal directionality.

Hemizygosity for a glycine substitution in collagen XVII: unfolding and degradation of the ectodomain

Defects of collagen XVII, a keratinocyte adhesion protein, are associated with epidermal detachment in junctional epidermolysis bullosa. Although some missense mutations in the collagen XVII gene COL17A1 have been described, the molecular mechanisms leading to disease have remained elusive in these cases. Here we assessed the biologic consequences of a missense mutation by studying the folding and stability of wild-type and mutated recombinant collagen XVII domains. The mutation occurred in a junctional epidermolysis bullosa patient who was compound heterozygous for the novel glycine substitution mutation G633D and the novel nonsense mutation R145X. Collagen XVII mRNA was significantly reduced, indicating nonsense-mediated mRNA degradation and hemizygosity of the patient for the G633D substitution. As glycine residues within the collagen triple helices are important for stable conformation, the thermal stability of the wild-type and mutated eukaryotic recombinant Col15 domain of collagen XVII was assessed. The stability of the mutated fragment was clearly reduced. The midpoint of the helix-to-coil transition, Tm, was 5 degrees C lower than that of wild-type rCol15, indicating abnormal triple-helix folding and susceptibility to proteolysis. Consistently, immunoassays demonstrated reduced amounts of the full-length collagen XVII and absence of the soluble ectodomain in keratinocyte cultures, and lack of the ectodomain from the junctional epidermolysis bullosa skin. These observations show that the glycine substitution G633D in collagen XVII causes abnormal folding and susceptibility to degradation, and thus perturbs the physiologic adhesive functions of collagen XVII in the skin.