Matrix metalloproteinases (MMPs) were discovered because of their role in amphibian metamorphosis, yet they have attracted more attention because of their roles in disease. Despite intensive scrutiny in vitro, in cell culture and in animal models, the normal physiological roles of these extracellular proteases have been elusive. Recent studies in mice and flies point to essential roles of MMPs as mediators of change and physical adaptation in tissues, whether developmentally regulated, environmentally induced or disease associated.The founding member of the matrix metalloproteinase (MMP) family, collagenase, was identified in 1962 by Gross and Lapiere, who found that tadpole tails during metamorphosis contained an enzyme that could degrade fibrillar collagen 1,2 . Subsequently, an interstitial collagenase, collagenase-1 or MMP1, was found in diseased skin and synovium 3 . In vitro, MMP1 initiates degradation of native fibrillar collagens, crucial components of vertebrate extracellular matrix (ECM), by cleaving the peptide bond between Gly775-Ile776 or Gly775-Lys776 in native type I, II or III collagen molecules 3,4 . Further research led to the discovery of a family of structurally related proteinases (23 in human, 24 in mice), now referred to as the MMP family.Interest in MMPs increased in the late 1960s and early 1970s following observations that MMPs are upregulated in diverse human diseases including rheumatoid arthritis and cancer. Importantly, high levels of MMPs often correlated with poor prognosis in human patients (reviewed in REF. 5 ). However, recent clinical data indicate that the relationship between § These authors contributed equally to this paper. Competing interests statementThe authors declare no competing financial interests. DATABASESThe following terms in this article are linked online to: Entrez Genome: http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=gene DmMmp1 | DmMmp2 NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author ManuscriptMMPs and disease is not simple; for example, increased MMP activity can enhance tumour progression or can inhibit it (reviewed in REF. 6 ). This complex relationship between MMP expression and cancer has increased the basic and clinical interest in understanding MMP function in vivo, but it has also focused attention on MMPs and pathology, and relatively less attention has been focused on the normal roles of these enzymes. MMP proteolysisMMPs are members of the metzincin group of proteases, which are named after the zinc ion and the conserved Met residue at the active site 11,12 . Recent work has generated a unified peptidase nomenclature 13 Functions of MMP proteolysisHistorically, MMPs were thought to function mainly as enzymes that degrade structural components of the ECM. However, MMP proteolysis can create space for cells to migrate, can produce specific substrate-cleavage fragments with independent biological activity, can MMPs in bone modelling and remodellingBone is an important site of ongoing tissue remodelling during development...

The matrix metalloproteinase (MMP) family is heavily implicated in many diseases, including cancer. The developmental functions of these genes are not clear, however, because the >20 mammalian MMPs can be functionally redundant. Drosophila melanogaster has only two MMPs, which are expressed in embryos in distinct patterns. We created mutations in both genes: Mmp1 mutants have defects in larval tracheal growth and pupal head eversion, and Mmp2 mutants have defects in larval tissue histolysis and epithelial fusion during metamorphosis; neither is required for embryonic development. Double mutants also complete embryogenesis, and these represent the first time, to our knowledge, that all MMPs have been disrupted in any organism. Thus, MMPs are not required for Drosophila embryonic development, but, rather, for tissue remodeling.

Summary Bromine is ubiquitously present in animals as ionic bromide (Br−) yet has no known essential function. Herein, we demonstrate that Br− is a required cofactor for peroxidasin-catalyzed formation of sulfilimine crosslinks, a post-translational modification essential for tissue development and architecture found within the collagen IV scaffold of basement membranes (BMs). Bromide, converted to hypobromous acid, forms a bromosulfonium-ion intermediate that energetically selects for sulfilimine formation. Dietary Br-deficiency is lethal in Drosophila while Br-replenishment restores viability, demonstrating its physiologic requirement. Importantly, Br-deficient flies phenocopy the developmental and BM defects observed in peroxidasin mutants and indicate a functional connection between Br−, collagen IV, and peroxidasin. We establish that Br− is required for sulfilimine formation within collagen IV, an event critical for BM assembly and tissue development. Thus, bromine is an essential trace element for all animals and its deficiency may be relevant to BM alterations observed in nutritional and smoking related disease.