Planarians as a Model to Assess <em>In Vivo</em> the Role of Matrix Metalloproteinase Genes during Homeostasis and Regeneration

<div><p>Matrix metalloproteinases (MMPs) are major executors of extracellular matrix remodeling and, consequently, play key roles in the response of cells to their microenvironment. The experimentally accessible stem cell population and the robust regenerative capabilities of planarians offer an ideal model to study how modulation of the proteolytic system in the extracellular environment affects cell behavior <em>in vivo</em>. Genome-wide identification of <em>Schmidtea mediterranea</em> MMPs reveals that planarians possess four <em>mmp</em>-like genes. Two of them (<em>mmp1</em> and <em>mmp2</em>) are strongly expressed in a subset of secretory cells and encode putative matrilysins. The other genes (<em>mt-mmpA</em> and <em>mt-mmpB</em>) are widely expressed in postmitotic cells and appear structurally related to membrane-type MMPs. These genes are conserved in the planarian <em>Dugesia japonica</em>. Here we explore the role of the planarian <em>mmp</em> genes by RNA interference (RNAi) during tissue homeostasis and regeneration. Our analyses identify essential functions for two of them. Following inhibition of <em>mmp1</em> planarians display dramatic disruption of tissues architecture and significant decrease in cell death. These results suggest that <em>mmp1</em> controls tissue turnover, modulating survival of postmitotic cells. Unexpectedly, the ability to regenerate is unaffected by <em>mmp1(RNAi).</em> Silencing of <em>mt-mmpA</em> alters tissue integrity and delays blastema growth, without affecting proliferation of stem cells. Our data support the possibility that the activity of this protease modulates cell migration and regulates anoikis, with a consequent pivotal role in tissue homeostasis and regeneration. Our data provide evidence of the involvement of specific MMPs in tissue homeostasis and regeneration and demonstrate that the behavior of planarian stem cells is critically dependent on the microenvironment surrounding these cells. Studying MMPs function in the planarian model provides evidence on how individual proteases work <em>in vivo</em> in adult tissues. These results have high potential to generate significant information for development of regenerative and anti cancer therapies.</p>