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Flii Control: Balancing Migration and Adhesion

2009; Elsevier BV; Volume: 129; Issue: 8 Linguagem: Inglês

10.1038/jid.2009.171

1523-1747

Kristina Kligys, Jonathan Jones,

Skin and Cellular Biology Research

Wound healing in the skin requires a compromise between adhesion and migration. Both processes include modulation of the cytoskeleton, cell-surface receptors, and receptor ligands., In this issue, Kopecki et al. demonstrate that overexpression of Flii, an actin-remodeling protein, impedes wound healing but inhibits hemidesmosome formation. In contrast, Flii deficiency results in enhanced wound healing while promoting hemidesmosome assembly. We discuss potential mechanisms that could explain how this unique gelsolin family member might regulate both stable keratinocyte adhesion and motility. Wound healing in the skin requires a compromise between adhesion and migration. Both processes include modulation of the cytoskeleton, cell-surface receptors, and receptor ligands., In this issue, Kopecki et al. demonstrate that overexpression of Flii, an actin-remodeling protein, impedes wound healing but inhibits hemidesmosome formation. In contrast, Flii deficiency results in enhanced wound healing while promoting hemidesmosome assembly. We discuss potential mechanisms that could explain how this unique gelsolin family member might regulate both stable keratinocyte adhesion and motility. Ten years ago, it was widely accepted that the laminin-332 receptor α6β4 integrin mediated formation of stable anchoring complexes, or hemidesmosomes, in skin. Its loss was demonstrated to be the molecular mechanism underlying skin blistering in junctional epidermolysis bullosa, validating its role in mediating stable cell adhesion (Jones et al., 1998Jones J.C. Hopkinson S.B. Goldfinger L.E. Structure and assembly of hemidesmosomes.Bioessays. 1998; 20: 488-494Crossref PubMed Scopus (175) Google Scholar). Furthermore, α6β4 integrin was not thought to have an important role in the migration of cells, given that hemidesmosomes disassemble during wound healing. Rather, α6β4 integrin was believed to play a role once epithelialization of a wound was complete by mediating the stable adhesion of cells to dermal structures in the wound bed (Jones et al., 1998Jones J.C. Hopkinson S.B. Goldfinger L.E. Structure and assembly of hemidesmosomes.Bioessays. 1998; 20: 488-494Crossref PubMed Scopus (175) Google Scholar). In addition, antibody inhibitor studies suggested that another laminin-332 receptor, α3β1 integrin, mediated migration of epidermal cells on laminin matrix and thus was an important regulator of epithelialization of wounds (Jones et al., 1998Jones J.C. Hopkinson S.B. Goldfinger L.E. Structure and assembly of hemidesmosomes.Bioessays. 1998; 20: 488-494Crossref PubMed Scopus (175) Google Scholar; Nguyen et al., 2000Nguyen B.P. Ryan M.C. Gil S.G. Carter W.G. Deposition of laminin 5 in epidermal wounds regulates integrin signaling and adhesion.Curr Opin Cell Biol. 2000; 12: 554-562Crossref PubMed Scopus (216) Google Scholar). Recent data have challenged these concepts, however. There is now evidence that α3β1 integrin retards epidermal motility, and results from our own laboratory have implicated α6β4 integrin in the modulation of signals that regulate keratinocyte migration (Margadant et al., 2009Margadant C. Raymond K. Kreft M. Sachs N. Janssen H. Sonnenberg A. Integrin alpha3beta1 inhibits directional migration and wound re-epithelialization in the skin.J Cell Sci. 2009; 122: 278-288Crossref PubMed Scopus (110) Google Scholar; Sehgal et al., 2006Sehgal B.U. DeBiase P.J. Matzno S. Chew T.L. Claiborne J.N. Hopkinson S.B. et al.Integrin beta4 regulates migratory behavior of keratinocytes by determining laminin-332 organization.J Biol Chem. 2006; 281: 35487-35498Crossref PubMed Scopus (94) Google Scholar). It is becoming increasingly clear that the function of integrins depends on tissue and cell context and that integrins cannot be pigeonholed as either adhesive or motility receptors. Likewise, data presented by Kopecki et al., 2009Kopecki Z. Arkell R. Powell B.C. Cowin A.J. Flightless I regulates hemidesmosome formation and integrin-mediated cellular adhesion and migration during wound repair.J Invest Dermatol. 2009; 129: 2031-2045Abstract Full Text Full Text PDF PubMed Scopus (52) Google Scholar demonstrate that the protein Flii, a member of the gelsolin family of actin-remodeling proteins, appears to have paradoxical roles in intact skin versus healing wounds. Flii, also a member of the gelsolin family of actin-severing proteins, was previously described as a negative regulator of wound healing, such that overexpression impedes keratinocyte migration and delays the closure of wounds (Kopecki and Cowin, 2008Kopecki Z. Cowin A.J. Flightless I: an actin-remodelling protein and an important negative regulator of wound repair.Int J Biochem Cell Biol. 2008; 40: 1415-1419Crossref PubMed Scopus (55) Google Scholar). Kopecki et al., 2009Kopecki Z. Arkell R. Powell B.C. Cowin A.J. Flightless I regulates hemidesmosome formation and integrin-mediated cellular adhesion and migration during wound repair.J Invest Dermatol. 2009; 129: 2031-2045Abstract Full Text Full Text PDF PubMed Scopus (52) Google Scholar now demonstrate that Flii not only regulates wound healing but also affects hemidesmosome assembly. Specifically, Flii overexpression inhibits hemidesmosome formation. This result is quite surprising and seems, at first glance, to be counterintuitive. One might have expected that under conditions in which wound healing is inhibited, hemidesmosome assembly would be favored, and vice versa. Yet Kopecki et al. demonstrate that, whereas epidermal basal cells of mice deficient in Flii assemble more hemidesmosomes than their wild-type counterparts, their wounds heal significantly faster than those of wild-type mice. Conversely, the basal epidermal cells in the intact skin of mice in which Flii is overexpressed contain fewer hemidesmosomes compared to those found in wild-type skin, but the wounds in these mice heal significantly more slowly than those of control animals (Kopecki et al., 2009Kopecki Z. Arkell R. Powell B.C. Cowin A.J. Flightless I regulates hemidesmosome formation and integrin-mediated cellular adhesion and migration during wound repair.J Invest Dermatol. 2009; 129: 2031-2045Abstract Full Text Full Text PDF PubMed Scopus (52) Google Scholar). Several mechanisms could explain how overexpression of Flii protein inhibits hemidesmosome assembly but also impedes wound healing. Flii expression may regulate integrin ligand deposition, integrin heterotrimer expression, expression of proteins that regulate the activity of integrins, or all of the above. In fact, Kopecki et al., 2009Kopecki Z. Arkell R. Powell B.C. Cowin A.J. Flightless I regulates hemidesmosome formation and integrin-mediated cellular adhesion and migration during wound repair.J Invest Dermatol. 2009; 129: 2031-2045Abstract Full Text Full Text PDF PubMed Scopus (52) Google Scholar provide data that support all of these possibilities. They demonstrate that laminin protein expression is regulated in response to Flii expression levels, such that Flii-deficient mouse skin displays increased laminin deposition, whereas the opposite is true in skin in which Flii is overexpressed. The authors do not state the exact isoform of laminin analyzed. However, increased laminin deposition may enhance hemidesmosome assembly because the laminin-332 heterotrimer is an integral protein component of the hemidesmosome. Moreover, laminin-332 provides a surface over which cells can migrate, potentially promoting faster wound healing. Alternatively, a loss in laminin expression is likely to inhibit both hemidesmosome formation and wound epithelialization. In addition, changes in the expression and/or balance of integrins in keratinocytes would be expected to contribute to changes in adhesion and migration of skin cells. Kopecki et al. provide some data to support this possibility by demonstrating changes in integrin expression using a quantification of wounded transgenic skin tissue stained with integrin antibodies. Finally, the authors quantified expression of the tetraspanin molecule CD151 in their transgenic mice during wound healing. CD151 interacts with laminin-binding integrins, including α3β1 and α6β4 integrins, and its loss results in impaired wound healing due to modulation in integrin-mediated signal transduction (Sterk et al., 2000Sterk L.M. Geuijen C.A. Oomen L.C. Calafat J. Janssen H. Sonnenberg A. The tetraspan molecule CD151, a novel constituent of hemidesmosomes, associates with the integrin alpha6beta4 and may regulate the spatial organization of hemidesmosomes.J Cell Biol. 2000; 149: 969-982Crossref PubMed Scopus (185) Google Scholar; Wright et al., 2004Wright M.D. Geary S.M. Fitter S. Moseley G.W. Lau L.M. Sheng K.C. et al.Characterization of mice lacking the tetraspanin superfamily member CD151.Mol Cell Biol. 2004; 24: 5978-5988Crossref PubMed Scopus (146) Google Scholar). Intriguingly, CD151 expression is elevated in Flii-deficient wounds. This increase may enhance wound healing by upregulating the activity of integrins and might enhance hemidesmosome assembly in intact skin, although it should also be noted that CD151-null mice assemble hemidesmosomes that are normal in appearance and number. Flii contains six gelsolin-related domains toward its carboxy terminal tail and a protein–protein interacting domain, called the leucine-rich repeat domain (LRR), at its amino terminus (Kopecki and Cowin, 2008Kopecki Z. Cowin A.J. Flightless I: an actin-remodelling protein and an important negative regulator of wound repair.Int J Biochem Cell Biol. 2008; 40: 1415-1419Crossref PubMed Scopus (55) Google Scholar). Interactions of Flii with other proteins through the LRR domain appear to be important for Flii function because wounds treated with neutralizing antibodies generatedFlii is a new target to develop stratagems that enhance wound healing. against the LRR of Flii heal faster than those of untreated controls (Cowin et al., 2007Cowin A.J. Adams D.H. Strudwick X.L. Chan H. Hooper J.A. Sander G.R. et al.Flightless I deficiency enhances wound repair by increasing cell migration and proliferation.J Pathol. 2007; 211: 572-581Crossref PubMed Scopus (72) Google Scholar). To assess potential Flii binding partners in skin cells, Kopecki et al., 2009Kopecki Z. Arkell R. Powell B.C. Cowin A.J. Flightless I regulates hemidesmosome formation and integrin-mediated cellular adhesion and migration during wound repair.J Invest Dermatol. 2009; 129: 2031-2045Abstract Full Text Full Text PDF PubMed Scopus (52) Google Scholar performed a series of coimmunoprecipitation studies. Although Flii does not appear to complex with either β1- or β4-containing integrin heterodimers, Flii robustly binds talin. Talin is a cytoskeleton linker protein that interacts with the cytoplasmic tails of all β-integrin subunits, with the exception of the β4 subunit (Harburger and Calderwood, 2009Harburger D.S. Calderwood D.A. Integrin signalling at a glance.J Cell Sci. 2009; 122: 159-163Crossref PubMed Scopus (569) Google Scholar). Integrin–talin interaction induces an activation of receptor functions. Thus, Flii may regulate the pool of talin available for binding to and activating integrins. In such a model, overexpression of Flii would result in sequestration of talin, leading to decreased integrin activation and impaired wound healing. Conversely, loss of Flii would increase the pool of talin available to activate integrins, a possible explanation for the enhanced wound healing observed in Flii-deficient skin, because all but β4 subunit-containing integrins would be “hyper”-activated under such conditions. Of course, the mechanism we are proposing does not explain the enhanced numbers of hemidesmosomes observed in Flii-depleted skin, because talin does not regulate the activity of α6β4 integrin. However, one could envisage that an increase in CD151 expression, as a consequence of Flii deficiency, leads to enhanced nucleation or stabilization of hemidesmosomes in intact skin, independent of integrin activation induced by talin during wound healing. Finally, it is important to remember that Flii is a gelsolin family member. Whereas Flii overexpression retards motility and its deficiency enhances wound healing, gelsolin is known to regulate actin stress fiber functions, and its deficiency results in defective chemotaxis (Arora et al., 1999Arora P.D. Janmey P.A. McCulloch C.A. A role for gelsolin in stress fiber-dependent cell contraction.Exp Cell Res. 1999; 250: 155-167Crossref PubMed Scopus (25) Google Scholar). In fact, it is unclear at this point whether the actin-severing property of Flii plays an important role in its ability to regulate wound healing in the skin. During wound healing, the interaction of Flii with talin and its ability to regulate integrin-mediated adhesion may predominate. In this regard, it is interesting to note that lack of Flii expression induces embryonic lethality in mice (Kopecki and Cowin, 2008Kopecki Z. Cowin A.J. Flightless I: an actin-remodelling protein and an important negative regulator of wound repair.Int J Biochem Cell Biol. 2008; 40: 1415-1419Crossref PubMed Scopus (55) Google Scholar) and that the actin-severing activity of Flii may therefore be more important to processes necessary for proper development. In summary, Kopecki et al., 2009Kopecki Z. Arkell R. Powell B.C. Cowin A.J. Flightless I regulates hemidesmosome formation and integrin-mediated cellular adhesion and migration during wound repair.J Invest Dermatol. 2009; 129: 2031-2045Abstract Full Text Full Text PDF PubMed Scopus (52) Google Scholar have uncovered unforeseen functions for Flii in both hemidesmosome assembly and wound healing. Further work is needed to define precisely how Flii functions at the molecular level. However, as the authors point out, Flii is an intriguing new target for skin biologists who focus on developing new stratagems to enhance wound healing. KK and JCRJ are supported by NIAMS AR054184.