Fibulins are a newly recognized family of extracellular matrix proteins. The five known members of the family share an elongated structure and many calcium-binding sites, owing to the presence of tandem arrays of epidermal growth factor-like domains. They have overlapping binding sites for several basement-membrane proteins, tropoelastin, fibrillin, fibronectin and proteoglycans, and they participate in diverse supramolecular structures. New insights into their biological roles are now emerging from studies of transgenic mice and of some inherited human diseases.

beta 1 integrins are ubiquitously expressed receptors that mediate cell-cell and cell-extracellular matrix interactions. To analyze the function of beta1 integrin in skin we generated mice with a keratinocyte-restricted deletion of the beta 1 integrin gene using the cre-loxP system. Mutant mice developed severe hair loss due to a reduced proliferation of hair matrix cells and severe hair follicle abnormalities. Eventually, the malformed hair follicles were removed by infiltrating macrophages. The epidermis of the back skin became hyperthickened, the basal keratinocytes showed reduced expression of alpha 6 beta 4 integrin, and the number of hemidesmosomes decreased. Basement membrane components were atypically deposited and, at least in the case of laminin-5, improperly processed, leading to disruption of the basement membrane and blister formation at the dermal-epidermal junction. In contrast, the integrity of the basement membrane surrounding the beta 1-deficient hair follicle was not affected. Finally, the dermis became fibrotic. These results demonstrate an important role of beta 1 integrins in hair follicle morphogenesis, in the processing of basement membrane components, in the maintenance of some, but not all basement membranes, in keratinocyte differentiation and proliferation, and in the formation and/or maintenance of hemidesmosomes.

The C-terminal domain NC1 of mouse collagen XVIII (38 kDa) and the shorter mouse and human endostatins (22 kDa) were prepared in recombinant form from transfected mammalian cells. The NC1 domain aggregated non-covalently into a globular trimer which was partially cleaved by endogenous proteolysis into several monomers (25-32 kDa) related to endostatin. Endostatins were obtained in a highly soluble, monomeric form and showed a single N-terminal sequence which, together with other data, indicated a compact folding. Endostatins and NC1 showed a comparable binding activity for the microfibrillar fibulin-1 and fibulin-2, and for heparin. Domain NC1, however, was a distinctly stronger ligand than endostatin for sulfatides and the basement membrane proteins laminin-1 and perlecan. Immunological assays demonstrated endostatin epitopes on several tissue components (22-38 kDa) and in serum (120-300 ng/ml), the latter representing the smaller variants. The data indicated that the NC1 domain consists of an N-terminal association region (~50 residues), a central protease-sensitive hinge region (~70 residues) and a C-terminal stable endostatin domain (~180 residues). They also demonstrated that proteolytic release of endostatin can occur through several pathways, which may lead to a switch from a matrix-associated to a more soluble endocrine form.

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