JCB: ARTICLE Y G O L O I B L L E C F O L A N R U O J E H T c-Met is essential for wound healing in the skin Jolanta Chmielowiec,1 Malgorzata Borowiak,2 Markus Morkel,1 Theresia Stradal,3 Barbara Munz,4 Sabine Werner,5 Jürgen Wehland,3 Carmen Birchmeier,2 and Walter Birchmeier1 1Department of Cancer Biology and 2Department of Neuroscience, Max-Delbrück-Center for Molecular Medicine, 13125 Berlin, Germany 3Department of Cell Biology, Helmholtz Centre for Infection Research, D-38124 Braunschweig, Germany 4Institute of Physiology, Charité-Medical University Berlin, 14195 Berlin, Germany 5Institute of Cell Biology, ETH Zürich, Hönggerberg, CH-8093 Zürich, Switzerland W ative process in adult mammals. We exam- ound healing of the skin is a crucial regener- ined wound healing in conditional mutant mice, in which the c-Met gene that encodes the receptor of hepatocyte growth factor/scatter factor was mutated in the epidermis by cre recombinase. c-Met–deﬁcient keratinocytes were unable to contribute to the reepitheli- alization of skin wounds. In conditional c-Met mutant mice, wound closure was slightly attenuated, but oc- curred exclusively by a few (5%) keratinocytes that had escaped recombination. This demonstrates that the wound process selected and ampliﬁed residual cells that express a functional c-Met receptor. We also cultured primary keratinocytes from the skin of conditional c-Met mutant mice and examined them in scratch wound as- says. Again, closure of scratch wounds occurred by the few remaining c-Met–positive cells. Our data show that c-Met signaling not only controls cell growth and migra- tion during embryogenesis but is also essential for the generation of the hyperproliferative epithelium in skin wounds, and thus for a fundamental regenerative pro- cess in the adult. Introduction The mammalian skin functions as a barrier to many forms of environmental stress, therefore wounds of the skin need to be repaired efficiently (Martin, 1997; Werner and Grose, 2003). Wounding of skin can damage both epidermis and dermis; thus, wound healing requires reepithelialization of the epidermis and the formation of new dermal structures, called granulation tis- sue. During reestablishment of the epithelial barrier, keratino- cytes from outside the wound migrate over the injured dermis and the granulation tissue. At the wound edges, these keratino- cytes form the so-called hyperproliferative epithelium (HE), which strongly proliferates and migrates to replenish the wounded area with new cells. Cells from the HE displace the fibrin clot over time (Martin, 1997; Singer and Clark, 1999; Ito et al., 2005; Santoro and Gaudino, 2005). The HE (Fig. 1 A, HE) is characterized by the expression of keratins 6 and 16, which are also present in the hair follicle, but not in the un- injured epidermis (Fuchs, 1990; Takahashi et al., 1998; Wojcik et al., 2001). Impairment of wound healing, e.g., in diabetes, Correspondence to Walter Birchmeier: wbirch@mdc-berlin.de M. Morkel’s present address is Max-Planck-Institute for Molecular Genetics, 14195 Berlin, Germany. Abbreviations used in this paper: E, embryonic day; HE, hyperproliferative epi- thelium; HGF/SF, hepatocyte growth factor/scatter factor; KGF, keratinocyte growth factor; P, postnatal day. The online version of this article contains supplemental material. © The Rockefeller University Press $15.00 The Journal of Cell Biology, Vol. 177, No. 1, April 9, 2007 151–162 http://www.jcb.org/cgi/doi/10.1083/jcb.200701086 can result in the development of chronic wounds (Singer and Clark, 1999; Falanga, 2005; Morasso and Tomic-Canic, 2005). Various signaling systems coordinate the wound healing process, as demonstrated by the analysis of growth factors, their receptors, and downstream signaling components (Scheid et al., 2000; Werner and Grose, 2003). For instance, genetic evidence obtained in mice indicates that signaling of the EGF receptor and the keratinocyte growth factor (KGF/FGF7) receptor are important for reepithelialization of wounds (Werner et al., 1994; Repertinger et al., 2004; Shirakata et al., 2005). Furthermore, down-regulation of the TGFβ receptor in keratinocytes reduces the rate of reepithelialization (Amendt et al., 2002). Smad3 is a downstream component of TGFβ signaling; in contrast, Smad3 mutant mice show an increased rate of reepithelialization and reduced monocyte infiltration during wound healing (Ashcroft et al., 1999). c-Jun and STAT3 participate in the signaling of growth factors, interleukins, and integrins; conditional mutation of c-Jun and STAT3 in the epidermis delays wound closure (Sano et al., 1999; Li et al., 2003). Cell culture models have been used to simulate wound closure. In such experiments, monolayers of epithelial cells are scratch wounded, the migration of the cells is traced, and the molecular mechanisms that control migration are studied (Raghavan et al., 2003; Kodama et al., 2004). Movement of cells into scratch wounds requires modulation of cell adhesion and JCB 151