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...

Mitochondrial morphology is determined by a dynamic equilibrium between organelle fusion and fission, but the significance of these processes in vertebrates is unknown. The mitofusins, Mfn1 and Mfn2, have been shown to affect mitochondrial morphology when overexpressed. We find that mice deficient in either Mfn1 or Mfn2 die in midgestation. However, whereas Mfn2 mutant embryos have a specific and severe disruption of the placental trophoblast giant cell layer, Mfn1-deficient giant cells are normal. Embryonic fibroblasts lacking Mfn1 or Mfn2 display distinct types of fragmented mitochondria, a phenotype we determine to be due to a severe reduction in mitochondrial fusion. Moreover, we find that Mfn1 and Mfn2 form homotypic and heterotypic complexes and show, by rescue of mutant cells, that the homotypic complexes are functional for fusion. We conclude that Mfn1 and Mfn2 have both redundant and distinct functions and act in three separate molecular complexes to promote mitochondrial fusion. Strikingly, a subset of mitochondria in mutant cells lose membrane potential. Therefore, mitochondrial fusion is essential for embryonic development, and by enabling cooperation between mitochondria, has protective effects on the mitochondrial population.

SUMMARY Carcinomas typically invade as a cohesive multicellular unit, a process termed collective invasion. It remains unclear how different subpopulations of cancer cells contribute to this process. We developed three-dimensional (3D) organoid assays to identify the most invasive cancer cells in primary breast tumors. Collective invasion was led by specialized cancer cells that were defined by their expression of basal epithelial genes, such as cytokeratin-14 (K14) and p63. Furthermore, K14+ cells led collective invasion in the major human breast cancer subtypes. Importantly, luminal cancer cells were observed to convert phenotypically to invasive leaders following induction of basal epithelial genes. Although only a minority of cells within luminal tumors expressed basal epithelial genes, knockdown of either K14 or p63 was sufficient to block collective invasion. Our data reveal that heterotypic interactions between epithelial subpopulations are critical to collective invasion. We suggest that targeting the basal invasive program could limit metastatic progression.