Mutations in the CD18 gene encoding the common β-chain of β2 integrins result in impaired wound healing in humans and mice suffering from leukocyte adhesion deficiency syndrome type 1 (LAD1). Transplantation of adipose tissue-derived mesenchymal stem cells (MSCs) restores normal healing of CD18-/- wounds by restoring the decreased TGF-β1 concentrations. TGF-β1 released from MSCs leads to enhanced myofibroblast differentiation, wound contraction, and vessel formation. We uncover that MSCs are equipped with a sensing mechanism for TGF-β1 concentrations at wound sites. Low TGF-β1 concentrations as occurring in CD18-/- wounds induce TGF-β1 release from MSCs, whereas high TGF-β1 concentrations suppress TGF-β1 production. This regulation depends on TGF-β receptor sensing and is relayed to microRNA-21 (miR-21), which subsequently suppresses the translation of Smad7, the negative regulator of TGF-β1 signaling. Inactivation of TGF-β receptor, or overexpression or silencing of miR-21 or Smad7, abrogates TGF-β1 sensing, and thus prevents the adaptive MSC responses required for tissue repair.

Nonhealing chronic wounds in the constantly growing elderly population represent a major public health problem with high socioeconomic burden. Yet, the underlying mechanism of age-related impairment of wound healing remains elusive. Here, we show that the number of dermal cells expressing cyclin-dependent kinase inhibitor p21 was elevated upon skin injury, particularly in aged population, in both man and mouse. The nuclear expression of p21 in activated wound fibroblasts delayed the onset of the proliferation phase of wound healing in a p53-independent manner. Further, the local and transient inhibition of p21 expression by in vivo delivered p21-targeting siRNA ameliorated the delayed wound healing in aged mice. Our results suggest that the increased number of p21+ wound fibroblasts enforces the age-related compromised healing, and targeting p21 creates potential clinical avenues to promote wound healing in aged population.

We here studied whether reactive oxygen species (ROS) released upon activation of the phagocyte NADPH oxidase NOX2 in wound macrophages (Mϕ) are required for proper wound healing and, if so, whether the leukocyte adhesion molecules β2 integrins control ROS release. β2 integrin-deficient (CD18-/-) mice revealed a severely impaired wound healing with impaired granulation tissue formation and angiogenesis due to insufficient transforming growth factor-β1 (TGF-β1) release from CD18-/- Mϕ. Targeting the NOX2 activator rotenone to wound Mϕ fully restored reduced ROS levels measured by in vivo imaging with redox sensitive dye, reconstituted impaired TGF-β1 activation and granulation tissue formation as assessed by ELISA and Western blot analyses of wound tissue lysates, and eventually rescued wound healing of CD18-/- mice. This effect was completely abolished by the NOX2 inhibitor ebselen, indicating that NOX2 activation with ROS release are impaired and causal for delayed wound healing in CD18-/- conditions. NADPH-based fluorescence lifetime imaging demonstrated that NOX2 assembly and activation were severely impaired at CD18-/- macrophages membrane upon adhesion to apoptotic cells. Moreover, mice lacking the p40phox NADPH subunit (p40phox-/-) with impaired oxidase function presented significantly reduced ROS levels and defective wound healing, very much resembling wound closure of CD18-/- mice. Impaired TGF-β1 activation by p40phox-/- wound Mϕ was causal for reduced angiogenesis, myofibroblasts differentiation and wound contraction of p40phox-/- mice. Injection of p40phox-/- Mϕ around wound margins could not compensate β2 integrin deficiency and restore the impaired wound healing of CD18-/- mice, supporting a central role of the β2 integrin-NADPH oxidase pathway in macrophages for tissue repair and inflammation.

We have described a novel population of dermal ABCB5+ multipotent stromal cells (MSCs) with a surface marker expression profile similar to conventional MSCs and immunomodulatory properties in macrophage activation distinct from dermal fibroblasts. ABCB5+ MSCs displayed self-renewal potency as assessed by means of a self-renewal assay in vitro. In addition, a tripotent differentiation capacity into adipogenic, osteogenic and cartilage lineages was observed on a single cell-derived clonal level. ABCB5+ MSCs co-expressed the stem cell marker SSEA-4 in vivo and in vitro as well as the stem cell-associated transcription factor Sox2 in vitro. Furthermore, we uncovered a functional role of ABCB5 in cell cycle regulation of ABCB5+ MSCs. Using ABCB5 as a single specific marker, we characterized dermal MSCs and their niche in situ of human skin in youth and age. ABCB5+ MSCs were shown to reside in perivascular and interfollicular niches in human and murine skin. We detected a significant decrease in their number concomitant with a change from predominantly perivascular to interfollicular localisation of MSCs with organismal ageing in humans. Interestingly, this was correlated with a decrease of osteopontin provided by perivascular NG2+ niche pericytes. Most interestingly, in a mouse model of osteopontin deletion, we could show a significant decrease of ABCB5+ MSC numbers in the dermis. Further experiments will provide mechanistic insight into niche dependent paracrine regulation of stem cell biology by different protease cleaved osteopontin isoforms and their corresponding receptors on MSCs. In a murine ABCB5 lineage tracing model the function of endogenous MSCs and osteopontin will be addressed using in vivo wound healing assays.

Diabetes is a debilitating, life-threatening disease accounting in 2015 for the death of 5 million people worldwide. According to new estimations, 415 million adults currently suffer from the disease, and this number is expected to rise to 642 million by 2040. High glucose blood levels also affect the skin among systemic organs, and skin disorders can often predict the onset of this metabolic disorder. In this review, we address the pathomechanistic effects of diabetes on the skin and give an overview on the most common skin diseases associated with diabetes.