Membranous extracellular matrix (ECM)-based scaffolds are one of the most promising biomaterials for skin wound healing, some of which, such as acellular dermal matrix, small intestinal submucosa, and amniotic membrane, have been clinically applied to treat chronic wounds with acceptable outcomes. Nevertheless, the wide clinical applications are always hindered by the poor mechanical properties, the uncontrollable degradation, and other factors after implantation. To highlight the feasible strategies to overcome the limitations, in this review, we first outline the current clinical use of traditional membranous ECM scaffolds for skin wound healing and briefly introduce the possible repair mechanisms; then, we discuss their potential limitations and further summarize recent advances in the scaffold modification and fabrication technologies that have been applied to engineer new ECM-based membranes. With the development of scaffold modification approaches, nanotechnology and material manufacturing techniques, various types of advanced ECM-based membranes have been reported in the literature. Importantly, they possess much better properties for skin wound healing, and would become promising candidates for future clinical translation.

Regenerative medicine based on stem cell research has the potential to provide advanced health care for human beings. Recent studies demonstrate that stem cells in human urine can serve as an excellent source of graft cells for regenerative therapy, mainly due to simple, low-cost, and noninvasive cell isolation. These cells, termed human urine-derived stem cells (USCs), are highly expandable and can differentiate into various cell lineages. They share many biological properties with mesenchymal stem cells, such as potent paracrine effects and immunomodulation ability. The advantage of USCs has motivated researchers to explore their applications in regenerative medicine, including genitourinary regeneration, musculoskeletal repair, skin wound healing, and disease treatment. Although USCs have showed many positive outcomes in preclinical studies, and although the possible applications of USCs for animal therapy have been reported, many issues need to be addressed before clinical translation. This article provides a comprehensive review of USC biology and recent advances in their application for tissue regeneration. Challenges in the clinical translation of USC-based therapy are also discussed. Impact statement: Recently, stem cells isolated from urine, referred to as urine-derived stem cells (USCs), have gained much interest in the field of regenerative medicine. Many advantages of human USCs have been found for cell-based therapy: (i) the cell isolation procedure is simple and low cost; (ii) they have remarkable proliferation ability, multidifferentiation potential, and paracrine effects; and (iii) they facilitate tissue regeneration in many animal models. With the hope to facilitate the development of USC-based therapy, we describe the current understanding of USC biology, summarize recent advances in their applications, and discuss future challenges in clinical translation.

Bioactive wound dressings that are capable of regulating the local wound microenvironment have attracted a very large interest in the field of regenerative medicine. Macrophages have many critical roles in normal wound healing, and the dysfunction of macrophages significantly contributes to impaired or non-healing skin wounds. Regulation of macrophage polarization towards an M2 phenotype provides a feasible strategy to enhance chronic wound healing, mainly by promoting the transition of chronic inflammation to the proliferation phase of wound healing, upregulating the level of anti-inflammatory cytokines around the wound area, and stimulating wound angiogenesis and re-epithelialization. Based on this, modulation of macrophage functions by the rational design of bioactive scaffolds has emerged as a promising way to accelerate delayed wound healing. This review outlines current strategies to regulate the response of macrophages using bioactive materials, with an emphasis on extracellular matrix-based scaffolds and nanofibrous composites.

Background: Rotator cuff tears are common, affecting more than 60% of individuals older than 80 years, and they have been implicated in 70% of patients with shoulder pain. M1 polarization-related inflammation has been reported to be associated with poor healing outcomes of rotator cuff injury, and leptin, an adipokine, has been reported to be a potential activator of inflammation. However, whether leptin affects rotator cuff repair remains unknown. Questions/purposes: Using in vitro cell experiments and an in vivo rat rotator cuff tear model, we therefore asked: (1) Does leptin promote the M1 polarization of macrophages in vitro and in vivo? (2) Does leptin impair biomechanical strength, the histologic structure of the tendon-bone interface, bone mineral density (BMD), or gait in the rotator cuff tear scenario? (3) Does leptin promote M1 polarization by upregulating the tumor necrosis factor (TNF) pathway? Methods: The impact of leptin on M1 macrophage polarization in vitro was determined by reverse transcription–polymerase chain reaction (RT-PCR), the Western blot test, and immunofluorescence staining. The effect of leptin on tendon-bone healing was assessed in an in vivo rat rotator cuff tear model by comparing a leptin group with a suture group in terms of gait, biomechanical tensile strength, the histologic structure of the tendon-bone interface, and BMD. In the in vivo experiments, 8-week-old male Sprague Dawley rats were used, adapting a previously developed rat rotator cuff tear model. The supraspinatus tendon was resected from the greater tuberosity bilaterally, and then the tendon was secured to its anatomical footprint using the transosseous single-row technique. In total, 30 rats were randomized into two groups (suture, leptin) by drawing lots (15 rats in each group). They were assessed at 2, 4, and 8 weeks after the surgery. In the suture group, 100 µL of normal saline was injected into the subacromial space after the deltoid muscle was restitched to the original position. In the leptin group, 100 µL of leptin solution (200 ng/mL) was injected into the subacromial space after the deltoid muscle was restitched to the original position. Biomechanical properties including maximal failure load, stiffness, and tensile failure stress were determined to assess the biomechanical strength at 4 and 8 weeks after the surgery. Histologic staining was conducted to compare the structure of the tendon-bone interface between treatment groups. Micro-MRI and micro-CT assessments were conducted to compare the overall healing outcome and BMD between treatment groups. Gait analysis was conducted to compare the stride length and strength between treatment groups. M1 macrophage polarization in vivo at the tendon-bone interface was assessed by immunofluorescence staining. Finally, to explore the underlying mechanism of the effects of leptin, Necrostatin-1 (Nec-1) was used to block the TNF signaling pathway in the in vitro macrophage study, and RT-PCR and Western blot were used to explore the underlying mechanism. Results: Leptin enhanced LPS-induced M1 polarization of macrophages in vitro, showing increased gene expression of CD86, Nos2, and TNF-α as well as increased protein expression of CD86, TNF-α, interleukin-6 (IL-6), and inducible NO synthase (iNOS). The in vivo polarization showed that the M1 polarization of macrophages at the tendon-bone interface was promoted. At 2 weeks postoperatively, there were more M1 cells in the leptin group (53 ± 5 versus 77 ± 8, mean difference 24 [95% confidence interval (CI) 11 to 37]; p = 0.002), although the proportion of M1 cells (ratio of the number of M1 cells to the total number of macrophages) was not higher (18.6% ± 2.9% versus 21.5% ± 1.7%, mean difference 2.9% [95% CI -2.8% to 8.7%]; p = 0.36). At 4 weeks postoperatively, the leptin group exhibited more M1 cells (31 ± 4 versus 50 ± 6, mean difference 19 [95% CI 6 to 32]; p = 0.008) and at a higher proportion (16.4% ± 2.6% versus 23.0% ± 3.0%, mean difference 6.6% [95% CI 0.8% to 12.4%]; p = 0.03). The in vivo experiments showed that leptin impaired tendon-bone healing. At 4 weeks postoperatively, the biomechanical properties of both groups were not different in terms of maximal failure load (12.7 ± 1.6 N versus 12.4 ± 1.8 N, mean difference -0.3 N [95% CI -2.6 to 1.8]; p = 0.91), stiffness (5.1 ± 0.7 N/mm versus 4.6 ± 0.8 N/mm, mean difference -0.5 N/mm [95% CI -1.3 to 0.5]; p = 0.44), and tensile failure stress (2.0 ± 0.3 N/mm² versus 2.0 ± 0.3 N/mm², mean difference 0.0 N/mm² [95% CI -0.4 to 0.4]; p = 0.99). At 8 weeks postoperatively, the leptin group showed worse maximal failure load (17.6 ± 1.4 N versus 14.1 ± 1.4 N, mean difference -3.5 N [95% CI -5.7 to -1.3]; p = 0.002), stiffness (7.0 ± 0.6 N/mm versus 5.2 ± 0.6 N/mm, mean difference -1.8 N/mm [95% CI -2.7 to -0.9]; p < 0.001), and tensile failure stress (3.4 ± 0.3 N/mm² versus 2.8 ± 0.4 N/mm², mean difference -0.6 N/mm² [95% CI -1.0 to -0.2]; p = 0.007). Results of histologic staining, image assessments, and gait analysis also demonstrated that leptin impaired the healing process. In vitro experiments showed that leptin upregulated the gene expression of molecules in the TNF pathway, including CCL2 and receptor-interacting protein kinase 1 (RIPK1), and M1 markers, such as TNF-α, CD86, and Nos2; the addition of Nec-1 neutralized the effect of leptin on macrophage polarization, reducing the expression of M1 markers, including TNF-α, CD86, and Nos2, and blocking the TNF signaling pathway, including CCL2 and RIPK. The protein expression exhibited similar trends. Conclusion: Based on the results of this study, leptin appears to impair tendon-bone healing in a rat model of rotator cuff tear, promote M1 macrophage polarization at the tendon-bone interface, and upregulate the TNF signaling pathway in macrophages to promote M1 polarization. Clinical Relevance: Obesity and fatty infiltration of the rotator cuff muscle are associated with poor healing of rotator cuff tears. In this study, the effect of leptin, an adipose factor, on tendon-bone healing and the underlying mechanism were explored. Future studies might focus on developing novel approaches to improve the tendon-bone healing in patients with obesity by targeting leptin or the TNF signaling pathway with the aid of biomaterials.

Accumulating studies reveal that mesenchymal stem cells (MSCs) promote skin wound healing mainly through the paracrine effects. Exosomes, one of the crucial paracrine mediators in wound healing, are cell-derived nanosized membranous vesicles containing diverse bioactive cargoes. With the potent ability of modulating skin cell behaviors, MSC-derived exosomes (MSC-Exos) are regarded as a promising nanomaterial for regenerative wound therapy. Under hostile conditions, MSC-Exos are efficient in protecting skin cells from severe damage and restoring their function. According to recent studies, MSC-Exos possess remarkable pro-healing effects in a variety of skin wounds, typically resulting in increased wound closure, inhibited scar tissue formation, and better restoration of skin function. To further enhance the therapeutic potential of MSC-Exos, the development of applicable pretreatment strategies and the optimization of exosome delivery are under intensive investigation. Herein, we summarize current research progress of MSC-Exos for skin wound treatment, with an emphasis on the biological effects of these nanovesicles, the repair mechanisms, and future challenges in clinical translation.