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The increased risk of disease and decreased capacity to respond to tissue insult in the setting of aging results from complex changes in homeostatic mechanisms, including the regulation of oxidative stress and cellular heterogeneity. In aged skin, the healing capacity is markedly diminished resulting in a high risk for chronic wounds. Stem cell-based therapies have the potential to enhance cutaneous regeneration, largely through trophic and paracrine activity. Candidate cell populations for therapeutic application include adult mesenchymal stem cells, embryonic stem cells and induced pluripotent stem cells. Autologous cell-based approaches are ideal to minimize immune rejection but may be limited by the declining cellular function associated with aging. One strategy to overcome age-related impairments in various stem cell populations is to identify and enrich with functionally superior stem cell subsets via single cell transcriptomics. Another approach is to optimize cell delivery to the harsh environment of aged wounds via scaffold-based cell applications to enhance engraftment and paracrine activity of therapeutic stem cells. In this review, we shed light on challenges and recent advances surrounding stem cell therapies for wound healing and discuss limitations for their clinical adoption.

BackgroundHuman adipose stem cells (ASCs) have emerged as a promising treatment paradigm for skin wounds. Recent works demonstrate that the therapeutic effect of stem cells is partially mediated by extracellular vesicles, which comprise exosomes and microvesicles. In this study, we investigate the regenerative effects of isolated microvesicles from ASCs and evaluate the mechanisms how ASC microvesicles promote wound healing.MethodsAdipose stem cell-derived microvesicles (ASC-MVs) were isolated by differential ultracentrifugation, stained by PKH26, and characterized by electron microscopy and dynamic light scattering (DLS). We examined ASC-MV effects on proliferation, migration, and angiogenesis of keratinocytes, fibroblasts, and endothelial cells both in vitro and in vivo. Next, we explored the underlying mechanisms by gene expression analysis and the activation levels of AKT and ERK signaling pathways in all three kinds of cells after ASC-MV stimulation. We then assessed the effect of ASC-MVs on collagen deposition, neovascularization, and re-epithelialization in an in vivo skin injury model.ResultsASC-MVs could be readily internalized by human umbilical vein endothelial cells (HUVECs), HaCAT, and fibroblasts and significantly promoted the proliferation, migration, and angiogenesis of these cells both in vitro and in vivo. The gene expression of proliferative markers (cyclin D1, cyclin D2, cyclin A1, cyclin A2) and growth factors (VEGFA, PDGFA, EGF, FGF2) was significantly upregulated after ASC-MV treatment. Importantly, ASC-MVs stimulated the activation of AKT and ERK signaling pathways in those cells. The local injection of ASC-MVs at wound sites significantly increased the re-epithelialization, collagen deposition, and neovascularization and led to accelerated wound closure.ConclusionsOur data suggest that ASC-MVs can stimulate HUVEC, HaCAT, and fibroblast functions. ASC-MV therapy significantly accelerates wound healing, and the benefits of ASC-MVs may due to the involvement of AKT and ERK signaling pathways. This illustrates the therapeutic potential of ASC-MVs which may become a novel treatment paradigm for cutaneous wound healing.Electronic supplementary materialThe online version of this article (10.1186/s13287-019-1152-x) contains supplementary material, which is available to authorized users.

There is a high mortality in patients with diabetes and severe pressure ulcers. For example, chronic pressure sores of the heels often lead to limb loss in diabetic patients. A major factor underlying this is reduced neovascularization caused by impaired activity of the transcription factor hypoxia inducible factor-1 alpha (HIF-1α). In diabetes, HIF-1α function is compromised by a high glucose-induced and reactive oxygen species-mediated modification of its coactivator p300, leading to impaired HIF-1α transactivation. We examined whether local enhancement of HIF-1α activity would improve diabetic wound healing and minimize the severity of diabetic ulcers. To improve HIF-1α activity we designed a transdermal drug delivery system (TDDS) containing the FDA-approved small molecule deferoxamine (DFO), an iron chelator that increases HIF-1α transactivation in diabetes by preventing iron-catalyzed reactive oxygen stress. Applying this TDDS to a pressure-induced ulcer model in diabetic mice, we found that transdermal delivery of DFO significantly improved wound healing. Unexpectedly, prophylactic application of this transdermal delivery system also prevented diabetic ulcer formation. DFO-treated wounds demonstrated increased collagen density, improved neovascularization, and reduction of free radical formation, leading to decreased cell death. These findings suggest that transdermal delivery of DFO provides a targeted means to both prevent ulcer formation and accelerate diabetic wound healing with the potential for rapid clinical translation.wound healing | diabetes | drug delivery | small molecule | angiogenesis D iabetes mellitus affects over 25 million people in the United States (1, 2) and costs nearly $250 billion per year (3). Chronic diabetic wounds and decubiti are important long-term sequalae of both diabetes mellitus types 1 and 2 (4). There is a high mortality in diabetic patients who develop decubiti (5-7), and owing to prolonged disability and the high rates of recurrence these wounds represent an especially severe complication of diabetes (8). This is further underscored by the fact that diabetic nonhealing wounds are the leading cause of nontraumatic amputations in the United States (3, 9-11). As such, there is a clear need for new approaches to effectively manage and treat diabetic ulcers.The propensity for wound development in diabetes is associated with a reduced capacity for ischemia-driven neovascularization (12, 13). Hypoxia inducible factor-1 (HIF-1), which consists of a highly regulated α-subunit and a constitutively expressed β-subunit, is a critical transcriptional regulator of the normal cellular response to hypoxia, promoting progenitor cell recruitment, proliferation, survival, and neovascularization (14, 15). In nondiabetics, hypoxia causes stabilization of HIF-1α protein by preventing the normal rapid proteasomal degradation of HIF-1α. It does this by inhibiting the prolyl hydroxylases (PHDs), which hydroxylate specific prolyl residues on HIF-1α. Without proline hydroxylation HIF-1α is not ...

Scarring and tissue fibrosis represent a significant source of morbidity in the United States. Despite considerable research focused on elucidating the mechanisms underlying cutaneous scar formation, effective clinical therapies are still in the early stages of development. A thorough understanding of the various signaling pathways involved is essential to formulate strategies to combat fibrosis and scarring. While initial efforts focused primarily on the biochemical mechanisms involved in scar formation, more recent research has revealed a central role for mechanical forces in modulating these pathways. Mechanotransduction, which refers to the mechanisms by which mechanical forces are converted to biochemical stimuli, has been closely linked to inflammation and fibrosis and is believed to play a critical role in scarring. This review provides an overview of our current understanding of the mechanisms underlying scar formation, with an emphasis on the relationship between mechanotransduction pathways and their therapeutic implications.

Wnt signaling plays a central regulatory role across a remarkably diverse range of functions during embryonic development, including those involved in the formation of bone and cartilage. Wnt signaling continues to play a critical role in adult osteogenic differentiation of mesenchymal stem cells. Disruptions in this highly-conserved and complex system leads to various pathological conditions, including impaired bone healing, autoimmune diseases and malignant degeneration. For reconstructive surgeons, critically sized skeletal defects represent a major challenge. These are frequently associated with significant morbidity in both the recipient and donor sites. The Wnt pathway is an attractive therapeutic target with the potential to directly modulate stem cells responsible for skeletal tissue regeneration and promote bone growth, suggesting that Wnt factors could be used to promote bone healing after trauma. This review summarizes our current understanding of the essential role of the Wnt pathway in bone regeneration and repair.

Effective skin regeneration therapies require a successful interface between progenitor cells and biocompatible delivery systems. We previously demonstrated the efficiency of a biomimetic pullulan-collagen hydrogel scaffold for improving bone marrow-derived mesenchymal stem cell survival within ischemic skin wounds by creating a "stem cell niche" that enhances regenerative cytokine secretion. Adipose-derived mesenchymal stem cells (ASCs) represent an even more appealing source of stem cells because of their abundance and accessibility, and in this study we explored the utility of ASCs for hydrogel-based therapies. To optimize hydrogel cell seeding, a rapid, capillary force-based approach was developed and compared with previously established cell seeding methods. ASC viability and functionality following capillary hydrogel seeding were then analyzed in vitro and in vivo. In these experiments, ASCs were seeded more efficiently by capillary force than by traditional methods and remained viable and functional in this niche for up to 14 days. Additionally, hydrogel seeding of ASCs resulted in the enhanced expression of multiple stemness and angiogenesis-related genes, including Oct4, Vegf, Mcp-1, and Sdf-1. Moving in vivo, hydrogel delivery improved ASC survival, and application of both murine and human ASC-seeded hydrogels to splinted murine wounds resulted in accelerated wound closure and increased vascularity when compared with control wounds treated with unseeded hydrogels. In conclusion, capillary seeding of ASCs within a pullulan-collagen hydrogel bioscaffold provides a convenient and simple way to deliver therapeutic cells to wound environments. Moreover, ASC-seeded constructs display a significant potential to accelerate wound healing that can be easily translated to a clinical setting.

IntroductionPathophysiologic changes associated with diabetes impair new blood vessel formation and wound healing. Mesenchymal stem cells derived from adipose tissue (ASCs) have been used clinically to promote healing, although it remains unclear whether diabetes impairs their functional and therapeutic capacity.MethodsIn this study, we examined the impact of diabetes on the murine ASC niche as well as on the potential of isolated cells to promote neovascularization in vitro and in vivo. A novel single-cell analytical approach was used to interrogate ASC heterogeneity and subpopulation dynamics in this pathologic setting.ResultsOur results demonstrate that diabetes alters the ASC niche in situ and that diabetic ASCs are compromised in their ability to establish a vascular network both in vitro and in vivo. Moreover, these diabetic cells were ineffective in promoting soft tissue neovascularization and wound healing. Single-cell transcriptional analysis identified a subpopulation of cells which was diminished in both type 1 and type 2 models of diabetes. These cells were characterized by the high expression of genes known to be important for new blood vessel growth.ConclusionsPerturbations in specific cellular subpopulations, visible only on a single-cell level, represent a previously unreported mechanism for the dysfunction of diabetic ASCs. These data suggest that the utility of autologous ASCs for cell-based therapies in patients with diabetes may be limited and that interventions to improve cell function before application are warranted.