A type VII collagen subdomain mutant is thermolabile and shows enhanced proteolytic degradability - Implications for the pathogenesis of recessive dystrophic epidermolysis bullosa?

Type VII collagen is the major constituent of anchoring fibrils. It has a central collagenous domain that is surrounded by a small C-terminal non-collagenous domain (NC2) and a large N-terminal non-collagenous (NC1) domain. Mutations in type VII collagen can lead to hereditary skin blistering disease dystrophic epidermolysis bullosa (DEB). Most of the pathogenic missense mutations are within the collagenous domain. NC1 domain mediates interactions with other extracellular matrix molecules and only very few missense mutations within NC1 causing DEB have been reported. Interestingly, fibronectin III like (FNIII) domain 8 in the human protein can harbour different mutations at position 886 with one (R886P) leading to recessive DEB, whereas the others do not. We characterized subdomains of murine NC1, the FNIII domains 7-8, and the individual domains FNIII7 and FNIII8 by NMR- and CD-spectroscopy. We analysed the influence on stability for a mutation causing DEB and a non-pathogenic mutation. Whereas the silent mutation behaves as the wild type, the pathogenic mutation leads to a dramatic decrease in thermal stability of the FNIII8 domain. The melting temperature lowered from 77°C to 40°C compared to the wild type protein. This renders the domain susceptible to protease cleavage which could be shown by degradation tests with cathepsin G, cathepsin K, and MMP9. Our data show partial unfolding of type VII collagen due to the mutation causes an increased degradation. This could lead to skin blistering and opens new concomitant treatment options in some types of type VII collagen related skin blistering diseases.

Structural and biophysical characterization of the type VII collagen vWFA2 subdomain leads to identification of two binding sites.

Type VII collagen is an extracellular matrix protein, which is important for skin stability; however, detailed information at the molecular level is scarce. The second vWFA (von Willebrand factor type A) domain of type VII collagen mediates important interactions, and immunization of mice induces skin blistering in certain strains. To understand vWFA2 function and the pathophysiological mechanisms leading to skin blistering, we structurally characterized this domain by X-ray crystallography and NMR spectroscopy. Cell adhesion assays identified two new interactions: one with ß1 integrin via its RGD motif and one with laminin-332. The latter interaction was confirmed by surface plasmon resonance with a KD of about 1 mm. These data show that vWFA2 has additional functions in the extracellular matrix besides interacting with type I collagen.

Skin melanocytes and fibroblasts show different changes in choline metabolism during cellular senescence.

Unmodified cells undergo only a limited number of cell divisions until they enter a state termed cellular senescence. Other triggers like cytotoxic compounds can also induce cell senescence. Since cell senescence represents a major mechanism of tumor suppression this cellular state has attracted increasing attention. Different markers like senescence-associated ß-galactosidase (SAßGal), senescence-associated heterochromatic foci (SAHF) or certain metabolic changes have been identified to be characteristic for senescent cells; however, data is often limited to fibroblasts - the cardinal model system for cellular senescence. In order to investigate whether metabolic changes during senescence are cell type independent, skin fibroblasts and skin melanocytes have been examined. Expression of the senescence marker p16 could be detected in skin fibroblasts but not in melanocytes of this specific donor, rendering the senescent phenotype not fully ascertained for the melanocytes. Metabolic profiles of senescent cells and controls have been determined using NMR spectroscopy. Changes in metabolism are different for fibroblasts and melanocytes. Senescent melanocytes showed lower levels of phosphocholine whereas for fibroblasts in accordance with literature, levels of glycerophosphocholine were increased during senescence. Although no general metabolic marker for cellular senescence exists, the same metabolic pathway seems to be affected for both cell types.