Wound healing is a complicated biological process that consist of partially overlapping inflammatory, proliferation and tissue remodelling phases. A successful wound healing depends on a proper activation and subsequent termination of the inflammatory phase. The failure to terminate the inflammation halts the completion of wound healing and is a known reason for formation of chronic wounds. Previous studies have shown that wound closure is delayed in plasminogen-deficient mice, and a role for plasminogen in dissection of extracellular matrix was suggested. However, our finding that plasminogen is transported to the wound by inflammatory cells early during the healing process, where it potentiates inflammation, indicates that plasminogen may also have other roles in the wound healing process. Here we report that plasminogen-deficient mice have extensive fibrin and neutrophil depositions in the wounded area long after re-epithelialisation, indicating inefficient debridement and chronic inflammation. Delayed formation of granulation tissue suggests that fibroblast function is impaired in the absence of plasminogen. Therefore, in addition to its role in the activation of inflammation, plasminogen is also crucial for subsequent steps, including resolution of inflammation and activation of the proliferation phase. Importantly, supplementation of plasminogen-deficient mice with human plasminogen leads to a restored healing process that is comparable to that in wild-type mice. Besides of being an activator of the inflammatory phase during wound healing, plasminogen is also required for the subsequent termination of inflammation. Based on these results, we propose that plasminogen may be an important future therapeutic agent for wound treatment.

Wound healing is a complex physiologic process that proceeds in overlapping, sequential steps. Plasminogen promotes fibrinolysis and potentiates the inflammatory response during wound healing. We have tested the hypothesis that the novel plasminogen receptor, Plg-RKT, regulates key steps in wound healing. Standardized burn wounds were induced in mice and time dependence of wound closure was quantified. Healing in Plg-RKT−/− mice was significantly delayed during the proliferation phase. Expression of inflammatory cytokines was dysregulated in Plg-RKT−/− wound tissue. Consistent with dysregulated cytokine expression, a significant delay in wound healing during the proliferation phase was observed in mice in which Plg-RKT was specifically deleted in myeloid cells. Following wound closure, the epidermal thickness was less in Plg-RKT−/− wound tissue. Paradoxically, deletion of Plg-RKT, specifically in keratinocytes, significantly accelerated the rate of healing during the proliferation phase. Mechanistically, only two genes were upregulated in Plg-RKT−/− compared with Plg-RKT+/+ wound tissue, filaggrin, and caspase 14. Both filaggrin and caspase 14 promote epidermal differentiation and decrease proliferation, consistent with more rapid wound closure and decreased epidermal thickness during the remodeling phase. Fibrin clearance was significantly impaired in Plg-RKT−/− wound tissue. Genetic reduction of fibrinogen levels to 50% completely abrogated the effect of Plg-RKT deletion on the healing of burn wounds. Remarkably, the effects of Plg-RKT deletion on cytokine expression were modulated by reducing fibrinogen levels. In summary, Plg-RKT is a new regulator participating in different phases of cutaneous burn wound healing, which coordinately plays a role in the interrelated responses of inflammation, keratinocyte migration, and fibrinolysis.

The ability of cells to promote plasminogen activation on their surfaces is now well recognized, and several distinct cell surface proteins have been demonstrated to function as plasminogen receptors. Here, we review studies demonstrating that plasminogen bound to cells, in addition to plasminogen directly bound to fibrin, plays a major role in regulating fibrin surveillance. We focus on the ability of specific plasminogen receptors on eukaryotic cells to promote fibrinolysis in the in vivo setting by reviewing data obtained predominantly in murine models. Roles for distinct plasminogen receptors in fibrin surveillance in intravascular fibrinolysis, immune cell recruitment in the inflammatory response, wound healing, and lactational development are discussed.

Plasminogen is a key regulatory protein that functions to both promote fibrinolysis and to potentiate the inflammatory response during wound healing. Here we have tested the hypothesis that the novel plasminogen receptor, Plg‐RKT, regulates distinct steps in the wound healing process. Method Full‐thickness standardized burn wounds (1 cm in diameter) were induced in mice and the wound area was quantified at different time points. Wound tissue was subjected to qPCR and ELISA for cytokine expression and immunohistochemistry and Western blotting for fibrin deposition and leukocyte recruitment. Results From day 9 after injury, healing in Plg‐RKT−/− mice was significantly delayed compared with healing in Plg‐RKT+/+ littermates. Expression of cytokines, IL‐1β, CCL‐20, TNFα, and IL‐10 was dysregulated in Plg‐RKT−/− tissue although no genotype‐dependent differences in recruitment of macrophages or neutrophils to the wound area were detected. Consistent with dysregulated cytokine expression, a significant delay in wound healing during the proliferation phase (days 9 and 10) was observed in wound tissue from mPlg‐RKT−/− mice in which Plg‐RKT was specifically deleted in myeloid cells. Regarding fibrinolysis, fibrin clearance was significantly impaired in Plg‐RKT−/− wound tissue. Genetic reduction of fibrinogen levels of Plg‐RKT−/− mice to 50% completely abrogated the effect of Plg‐RKT deletion on healing of burn wounds and, remarkably, effects of Plg‐RKT deletion on cytokine expression were modulated by the reduction in fibrinogen levels. In conclusion, Plg‐RKT plays a key role in wound healing by regulating both fibrinolysis and the inflammatory response and fibrin plays a role in regulating Plg‐RKT function on myeloid cells. Support or Funding Information United States National Institutes of Health grants HL 081046 to LAM and HL 149511 to LAM and RJP and Merit Review Award #5I01BX002026 from the U.S. Department of Veterans Affairs to RJP.

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