During cutaneous wound healing, keratinocyte proliferation and migration are critical for re-epithelialization. In addition, the epidermis secretes growth factors, cytokines, proteases, and matricellular proteins into the wound microenvironment that modify the extracellular matrix and stimulate other wound cells that control the inflammatory response, promote angiogenesis, and facilitate tissue contraction and remodeling. Wound keratinocytes express at least seven different integrins - the major cell adhesion receptors for the extracellular matrix - that collectively control essential cell-autonomous functions to ensure proper re-epithelialization, including migration, proliferation, survival, and basement membrane assembly (Fig. 1, arrow 1). Moreover, it has become evident in recent years that some integrins can regulate paracrine signals from wound epidermis that stimulate other wound cells involved in angiogenesis, contraction and inflammation (Fig. 1, arrows 2 and 3). Importantly, it is likely that abnormal integrin expression or function in the epidermis contributes to wound pathologies such as over-exuberant healing (e.g., hypertrophic scar formation) or diminished healing (e.g., chronic wounds). In this review, we discuss current knowledge of integrin function in the epidermis, which implicates them as attractive therapeutic targets to promote wound healing or treat wound pathologies. We also discuss challenges that arise from the complex roles that multiple integrins play in wound epidermis, which may be regulated through extracellular matrix remodeling that determines ligand availability. Indeed, understanding how different integrin functions are temporally coordinated in wound epidermis, and which integrin functions go awry in pathological wounds, will be important to determine how best to target them clinically to achieve maximum therapeutic benefit.

How mechanical cues from the extracellular environment are translated biochemically to modulate the effects of TGF-β on myofibroblast differentiation remains a crucial area of investigation. We report here that the focal adhesion protein, Hic-5 (also known as TGFB1I1), is required for the mechanically dependent generation of stress fibers in response to TGF-β. Successful generation of stress fibers promotes the nuclear localization of the transcriptional co-factor MRTF-A (also known as MKL1), and this correlates with the mechanically dependent induction of α smooth muscle actin (α-SMA) and Hic-5 in response to TGF-β. As a consequence of regulating stress fiber assembly, Hic-5 is required for the nuclear accumulation of MRTF-A and the induction of α-SMA as well as cellular contractility, suggesting a crucial role for Hic-5 in myofibroblast differentiation. Indeed, the expression of Hic-5 was transient in acute wounds and persistent in pathogenic scars, and Hic-5 colocalized with α-SMA expression in vivo. Taken together, these data suggest that a mechanically dependent feed-forward loop, elaborated by the reciprocal regulation of MRTF-A localization by Hic-5 and Hic-5 expression by MRTF-A, plays a crucial role in myofibroblast differentiation in response to TGF-β.

After cutaneous injury, keratinocytes secrete paracrine factors that regulate wound cell functions; dysregulation of this signaling can lead to wound pathologies. Previously, we established that keratinocyte integrin a3b1 promotes wound angiogenesis through paracrine stimulation of endothelial cells. We hypothesize here that a3b1-dependent paracrine signaling from keratinocytes regulates the differentiation state of myofibroblasts. We report that epidermal a3-knockout mice exhibit more wound myofibroblasts and fewer cyclooxygenase 2 (Cox-2)-positive dermal cells than controls. We also found that conditioned medium from a3-expressing mouse keratinocytes (MKa3 þ), but not from a3-null MK cells (MKa3 e), induces expression of Cox-2 in fibroblasts in a time-and dose-dependent manner and that this induction is mediated by IL-1a. Compared with MKa3 e cells, MKa3 þ cells secrete more IL-1a and less IL-1RA, a natural IL-1 receptor antagonist. Treatment with an IL-1a neutralizing antibody, recombinant IL-1RA, or IL-1 receptoretargeting small interfering RNA suppresses MKa3 þ conditioned medium-dependent induction of Cox-2 expression in fibroblasts. Finally, active recombinant IL-1a is sufficient to induce Cox-2 in fibroblasts and to inhibit transforming growth factor-beinduced a-SMA expression. Our findings support a role for keratinocyte integrin a3b1 in controlling the secretion of IL-1a, a paracrine factor that regulates the wound myofibroblast phenotype.

Growth and repair processes, both normal and pathological, require reciprocal interactions between cells and their microenvironment. Integrins are bidirectional, cell surface receptors that transduce mechanical and chemical signals to and from the extracellular matrix. We recently reported that keratinocyte α3β1 is required for interleukin (IL)-1α secretion. Importantly, IL-1α regulates fibroblast Cox-2 expression and prostaglandin E2 (PGE 2 ) secretion, thereby linking keratinocyte integrin function to a paracrine signal that suppresses the myofibroblast phenotype. We now report that fibroblast integrin α4β1 is required for this IL-1α-induced, Cox-2 expression. Moreover, Cox-2 induction by IL-1α requires Nuclear factor erythroid 2-related factor 2 (Nrf2), the master regulator of redox homeostasis; and integrin α4β1 is necessary to maintain IL-1α-dependent, Nrf2 levels. Treating fibroblasts with a Nrf-2 activating compound inhibits TGFβ-dependent, alpha smooth muscle actin (α-SMA) expression and stress fibre formation. Our data suggest that fibroblast integrin α4β1 regulates-depending on microenvironmental cues-the differentiated state of fibroblasts through a signalling network in which IL-1α, Cox-2 and Nrf2 participate.

Enhancing diabetic wound healing with naturally derived tissue-engineered biological dressings M Safoine, C Paquette, A Cô té and J Fradette Centre de recherche en organogenèse expérimentale de lUniversité Laval/LOEX, Quebec, PQ, Canada Long term diabetes often leads to chronic wounds refractory to usual treatments, thus representing a major challenge for the healthcare system. Cell-based therapies are actively investigated to enhance wound repair. Various cell types can be used to produce biological dressings that can be applied on wounds. Adipose-derived stem cells (ASC) are an attractive cell source considering their abundancy and therapeutic properties. In this study, ASC-based dressings improved global skin healing in a diabetic murine model. The entire dressing production was realized under a clinically-compatible serum-free system using the self-assembly approach of tissue engineering, leading to natural tissues rich in human cells and matrix components. These dressings were applied to full thickness 8-mm splinted skin wounds created on the back of polygenic diabetic NONcNZO10/LtJ mice. Global wound closure kinetics evaluated by macroscopic imaging showed that ASC-based dressings accelerated wound closure by 83% at day 8, 57% at day 12 and 35% at day 16 (p<0.001, n¼10) compared to untreated wounds which consistently had a 1-week delay during the healing process. On histological sections, treated wounds exhibited regenerated skin of better quality with a more organized, homogeneous and 1.6 fold thicker granulation tissue (p<0.001, n¼7). Neovascularization, assessed by CD31+ labeling, was 1.3 fold higher in the treated wounds (p<0.05, n¼8). In this study, we described the first entirely fetal bovine serum-free system allowing the production of naturally derived scaffold-free ASC-based biological dressings facilitating clinical translation. These new tissue-engineered dressings represent promising candidates for promoting diabetic cutaneous healing in vivo, by stimulating, among others, granulation tissue formation and neovascularization.