Background: Mannose receptor family members are candidate mediators of intracellular collagen degradation. Results: Despite common candidate collagen-binding domains and endocytic capacity throughout the family, only uPARAP/Endo180 and MR internalize collagens. Conclusion: A multi-domain interplay in the active receptors governs collagen endocytosis. Significance: Identification of the principal collagen receptors allows elucidation of the biological importance of intracellular collagen degradation.

Background: Therapeutic strategies for MMP targeting have limited selectivity. Results: A novel, specific mAb against MT1-MMP selectively blocks proMMP-2 activation. This antibody inhibited lymphatic vessel sprouting. Conclusion:A single function of a multifunctional MMP was blocked selectively and completely. MT1-MMP mediated proMMP-2 activation is involved in lymphangiogenesis. Significance: This supports development of therapeutic MMP targeting and the understanding of lymphangiogenesis.

Matrix metalloproteinases (MMPs) and, especially membrane type 1 (MT1)-MMP/MMP-14, are promising drug targets in malignancies. In contrast with multiple small-molecule and protein pan-inhibitors of MT1–MMP cleavage activity, the murine 9E8 monoclonal antibody targets the MMP-2-activating function of cellular MT1–MMP alone, rather than the general proteolytic activity and the pro-migratory function of MT1–MMP. Furthermore, the antibody does not interact in any detectable manner with other members of the membrane type (MT)-MMP family. The mechanism of this selectivity remained unknown. Using mutagenesis, binding and activity assays, and modeling in silico, we have demonstrated that the 9E8 antibody recognizes the MT-loop structure, an eight residue insertion that is specific for MT–MMPs and that is distant from the MT1–MMP active site. The binding of the 9E8 antibody to the MT-loop, however, prevents tissue inhibitor of metalloproteinases-2 (TIMP-2) association with MT1–MMP. As a result, the 9E8 antibody incapacitates the TIMP-2-dependent MMP-2-activating function alone rather than the general enzymatic activity of human MT1–MMP. The specific function of the 9E8 antibody we determined directly supports an essential, albeit paradoxical, role of the protein inhibitor (TIMP-2) in MMP-2 activation via a unique membrane-tethered mechanism. In this mechanism, the formation of a tri-molecular MT1–MMPTIMP-2MMP-2 complex is required for both the capture of the soluble MMP-2 proenzyme by cells and then its well-controlled conversion into the mature MMP-2 enzyme. In sum, understanding of the structural requirements for the 9E8 antibody specificity may pave the way for the focused design of the inhibitory antibodies against other individual MMPs.

Restoration of correct neural activity following central nervous system (CNS) damage requires the replacement of degenerated axons with newly outgrowing, functional axons. Unfortunately, spontaneous regeneration is largely lacking in the adult mammalian CNS. In order to establish successful regenerative therapies, an improved understanding of axonal outgrowth and the various molecules influencing it, is highly needed. Matrix metalloproteinases (MMPs) constitute a family of zincdependent proteases that were sporadically reported to influence axon outgrowth. Using an ex vivo retinal explant model, we were able to show that broad-spectrum MMP inhibition reduces axon outgrowth of mouse retinal ganglion cells (RGCs), implicating MMPs as beneficial factors in axonal regeneration. Additional studies, using more specific MMP inhibitors and MMP-deficient mice, disclosed that both MMP-2 and MT1-MMP, but not MMP-9, are involved in this process. Furthermore, administration of a novel antibody to MT1-MMP that selectively blocks pro-MMP-2 activation revealed a functional co-involvement of these proteinases in determining RGC axon outgrowth. Subsequent immunostainings showed expression of both MMP-2 and MT1-MMP in RGC axons and glial cells. Finally, results from combined inhibition of MMP-2 and b1-integrin were suggestive for a functional interaction between these molecules. Overall, our data indicate MMP-2 and MT1-MMP as promising axonal outgrowth-promoting molecules.