Basement membrane is a dense sheet-like extracellular matrix that encapsulates and separates tissue compartments. The ability of cells to invade through basement membrane barriers is important during normal and disease processes. For example, basement membrane breaching is required for embryo implantation, neural crest migration, heart development, leukocyte trafficking and cancer cell metastasis-a critical step in tumor progression and the leading cause of patient death. Studying how cells breach basement membrane in vertebrates has been hindered by the difficulty of imaging and experimentally examining cell-basement membrane interactions in vivo. As a result, how cells cross basement membrane remains poorly understood. Our laboratory has developed methods that combine high-resolution live-cell imaging with the highly- stereotyped and genetically tractable model of anchor cell invasion in Caenorhabditis elegans to uncover the molecular mechanisms regulating basement membrane breaching in vivo. We have determined that the C. elegans c-fos oncogene homologue, fos-1a functions in the anchor cell to specifically mediate basement membrane penetration. In fos-1a mutant animals, the anchor cell extends cellular processes that flatten at an intact basement membrane. The complete repertoire of FOS-1A protein transcriptional targets mediating basement membrane breaching is not known. Recently, we have found that FOS-1A regulates the expression of three MMPs in the anchor cell during the time of invasion. Matrix metalloproteinases are overexpressed in cells responsible for tissue remodeling, wound healing, and cancer and are hypothesized to enzymatically facilitate BM removal. Matrix metalloproteinases localize to invasive machinery and are required for extracellular matrix degradation in metastatic cancer cell lines in vitro. Due to the high number of matrix metalloproteinases expressed in vertebrates and the difficulty of directly examining cell invasion in vivo, the relevance, and potential function of matrix metalloproteinases in cell invasion through basement membrane is unclear. The goal of the proposed research is to use the strengths of the model of anchor cell invasion in C. elegans - genetic analysis, live cell-imaging, molecular perturbation - to determine the function of matrix metalloproteinases during basement membrane breaching. In addition, I will perform a sensitized genetic screen to identify novel genes and pathways that function downstream of FOS-1A with matrix metalloproteinases to promote invasion. Completion of the aims in this proposal will increase our knowledge of the genetic pathways regulating cell invasion and the functional significance of FOS-1A-directed matrix metalloproteinase-driven basement membrane breaching. This work outlined in this proposal will directly impact human health by identifying specific pathways that could be targeted to limit invasive behavior.