Release of biological activities from quiescent fibronectin by a conformational change and limited proteolysis by matrix metalloproteinases

Neuronal laminins and their cellular receptors

The laminins are a family of extracellular matrix glycoproteins expressed throughout developing neural tissues. The laminins are potent stimulators of neurite outgrowth in vitro for a variety of cell types, presumably reflecting an in vivo role in stimulating axon outgrowth. In recent years, the laminins have been shown to occur in several distinct isoforms; currently, the precise functional differences between the laminin variants are not well understood. A variety of neuronal surface receptors have been identified for one laminin isoform, laminin-1. These receptors include several members of the integrin family, as well as non-integrin laminin-binding proteins such as LBP-110, the 67 kDa laminin-receptor, alpha-dystroglycan, and beta 1,4 galactosyltransferase. Little is currently known about receptors for other laminin isoforms.

Substratum-dependent stimulation of fibroblast migration by the gelatin-binding domain of fibronectin

Nanomolar concentrations of native fibronectin and its RGDS-containing cell-binding domain have previously been reported to stimulate fibroblast migration in the transmembrane (or ‘Boyden chamber’) assay; in contrast, the gelatin-binding domain (GBD) of fibronectin has consistently been reported to be devoid of migration-stimulating activity in this assay. We have examined the effects of fibronectin and several of its purified functional domains on the migration of human skin fibroblasts in what is presumably a more physiologically relevant assay involving the movement of cells into a 3-D matrix of native type I collagen fibrils. We report that: (a) femtomolar concentrations of GBD stimulate fibroblast migration into such collagen matrices; and (b) fibronectin, as well as peptides containing all other of its functional domains, do not exhibit migration-stimulating activity when tested in the femtomolar to nanomolar concentration range (i.e. 0.1 pg/ml to 1 microgram/ml). The correct assignment of migration-stimulating activity to GBD, rather than to a contaminant, was confirmed by: (a) the use of several fibronectin and GBD purification protocols; (b) the neutralization of GBD migration-stimulating activity by monoclonal antibodies directed against epitopes present in this domain; (c) the time-dependent generation of migration-stimulating activity by the proteolytic degradation of native fibronectin; and (d) obtaining an identical dose-response curve with a genetically engineered GBD peptide. The cryptic migration-stimulating activity of GBD was not affected by the presence of serum or native fibronectin, but was inhibited by TGF-beta 1. Parallel experiments using the transmembrane assay confirmed that GBD was devoid of migration-stimulating activity in this assay when membranes coated with gelatin were used, but revealed that significant stimulation of migration was achieved with membranes coated with native type I collagen. Cells preincubated with GBD for 24 hours whilst growing on plastic tissue culture dishes and then plated onto native collagen matrices in the absence of further GBD also displayed an elevated migration compared to controls. Taken together, these observations suggest that: (a) the interaction of GBD with a putative cell surface receptor (and not the collagen substratum) initiates a persistent alteration in cell phenotype which is manifest by an increase in migratory activity when these cells are cultured on a native collagen substratum; and (b) GBD may play a hitherto unrecognised role in the control of cell migration in response to the local release of proteases during pathological processes, such as tumour invasion and wound repair.

Immunopurification and characterization of a collagenase/gelatinase domain issued from basement membrane fibronectin

The proteolytic potential of cellular fibronectin fragments issued from a basement membrane hydrolysate was investigated. Three different gelatinase activities (47, 43 and 37 kDa), located by gelatin zymography, were isolated using successively heparin-agarose, gelatin-agarose and immunopurification with polyclonal antibodies directed against bovine plasma fibronectin. These fragments were also characterized using a monoclonal antibody directed against the extra-domain EDA of cellular fibronectin as a probe. A collagenase activity, reliably indicated by the gelatin zymography pattern, was also found using MCA-Pro-Leu-Gly-Leu-DPA-Ala-Arg-NH2, the intramolecularly quenched fluorogenic substrate of collagenases. From these results, cellular fibronectin was found to be able to exhibit a proteolytic function after limited proteolysis. This MMP-like function could be associated with tissue remodeling in both normal and pathological states, such as metastasis, angiogenesis and tissue repair.

The C-terminal (haemopexin-like) domain structure of human gelatinase A (MMP2): structural implications for its function

In common with most other matrix metalloproteinases, gelatinase A has a non-catalytic C-terminal domain that displays sequence homology to haemopexin. Crystals of this domain were used by molecular replacement to solve its molecular structure at 2.6 A resolution, which was refined to an R value of 17.9%. This structure has a disc-like shape, with the chain folded into a beta-propeller structure that has pseudo four-fold symmetry. Although the topology and the side-chain arrangement are very similar to the equivalent domain of fibroblast collagenase, significant differences in surface charge and contouring are observable on 1 side of the gelatinase A disc. This difference might be a factor in allowing the gelatinase A C-terminal domain to bind to natural inhibitor TIMP-2.