|The epidermal basement membrane is a composite of separate laminin- or collagen IV-containing networks connected by aggregated perlecan, but not by nidogens.|
Behrens, DT; Villone, D; Koch, M; Brunner, G; Sorokin, L; Robenek, H; Bruckner-Tuderman, L; Bruckner, P; Hansen, U
The Journal of biological chemistry
The basement membrane between the epidermis and the dermis is indispensable for normal skin functions. It connects, and functionally separates, the epidermis and the dermis. To understand the suprastructural and functional basis of these connections, heterotypic supramolecular aggregates were isolated from the dermal-epidermal junction zone of human skin. Individual suprastructures were separated and purified by immunomagnetic beads, each recognizing a specific, molecular component of the aggregates. The molecular compositions of the suprastructures were determined by immunogold electron microscopy and immunoblotting. A composite of two networks was obtained from fibril-free suspensions by immunobeads recognizing either laminin 332 or collagen IV. After removal of perlecan-containing suprastructures or after enzyme digestion of heparan sulfate chains, a distinct network with a diffuse electron-optical appearance was isolated with magnetic beads coated with antibodies to collagen IV. The second network was more finely grained and comprised laminin 332 and laminins with α5-chains. The core protein of perlecan was an exclusive component of this network whereas its heparan sulfate chains were integrated into the collagen IV-containing network. Nidogens 1 and 2 occurred in both networks but did not form strong molecular cross-bridges. Their incorporation into one network appeared to be masked after their incorporation into the other one. We conclude that the epidermal basement membrane is a composite of two structurally independent networks that are tightly connected in a spot-welding-like manner by perlecan-containing aggregates.
|Rescue of mammary epithelial cell apoptosis and entactin degradation by a tissue inhibitor of metalloproteinases-1 transgene.|
Alexander, C M, et al.
J. Cell Biol., 135: 1669-77 (1996)
We have used transgenic mice overexpressing the human tissue inhibitor of metalloproteinases (TIMP)-1 gene under the control of the ubiquitous beta-actin promoter/enhancer to evaluate matrix metalloproteinase (MMP) function in vivo in mammary gland growth and development. By crossing the TIMP-1 transgenic animals with mice expressing an autoactivating stromelysin-1 transgene targeted to mammary epithelial cells, we obtained a range of mice with genetically engineered proteolytic levels. The alveolar epithelial cells of mice expressing autoactivating stromelysin-1 underwent unscheduled apoptosis during late pregnancy. When stromelysin-1 transgenic mice were crossed with mice overexpressing TIMP-1, apoptosis was extinguished. Entactin (nidogen) was a specific target for stromelysin-1 in the extracellular matrix. The enhanced cleavage of basement membrane entactin to above-normal levels was directly related to the apoptosis of overlying mammary epithelial cells and paralleled the extracellular MMP activity. These results provide direct evidence for cleavage of an extracellular matrix molecule by an MMP in vivo.
|Mapping of nidogen binding sites for collagen type IV, heparan sulfate proteoglycan, and zinc.|
Reinhardt, D, et al.
J. Biol. Chem., 268: 10881-7 (1993)
Recombinant nidogen fragments comprising the globular domains G1 plus G2, the rod-like domain, and the rod connected to the globe G3 were prepared from the culture media of transfected human cell clones. In addition, domains G1 and G2 were separated from each other after cleavage with chymotrypsin. The purified fragments were characterized by N-terminal sequences, electrophoresis, electron microscopy, and radioimmunoassays and the cell clones by Northern hybridization. Transfection with a construct comprising a large part of domain G3 showed high mRNA levels but no secreted protein, indicating a protein folding problem. All these fragments were used as soluble and/or immobilized ligands in binding assays. This demonstrated major binding sites on domain G2 for collagen IV and heparan sulfate proteoglycan. Affinity chromatography on zinc- and cobalt-loaded columns showed binding of domains G2 and G3 and the rod. Protein binding, but not metal binding, was abolished by reduction and alkylation of nidogen. This allowed for the isolation of several zinc-binding tryptic peptides, four from G2, two from the rod, and one from the G3 domain. Most of these short peptides contained several histidines that are likely to mediate binding. Zinc inhibited efficiently G3-mediated nidogen binding to laminin at 4 degrees C (IC50 approximately 5 microM) but less at higher temperatures. Similarly, zinc inhibited binding to collagen IV and proteoglycan at low temperatures but not at high (37 degrees C) temperatures. This indicates a complex modulation of nidogen binding to other basement membrane proteins by some, but not all, transition metals. Whether the particularly striking effects shown for zinc are of biological relevance remains to be established.