Selenium (Se) is an essential micronutrient element, and the biological significance of Se is predominantly dependent on its incorporation as selenocysteine (Sec), the genetically encoded 21st amino acid in protein synthesis, into the active site of selenoproteins, which have broad functions, ranging from redox regulation and anti-inflammation to the production of active thyroid hormones. Compared to its counterpart Cys, there are only limited probes for selective recognition of Sec, and such selectivity is strictly restricted at low pH conditions. We reported herein the design, synthesis, and biological evaluations of a series of potential Sec probes based on the mechanism of nucleophilic aromatic substitution. After the initial screening, the structural determinants for selective recognition of Sec were recapitulated. The follow-up studies identified that probe 19 (Sel-green) responds to Sec and other selenols with more than 100-fold increase of emission in neutral aqueous solution (pH 7.4), while there is no significant interference from the biological thiols, amines, or alcohols. Sel-green was successfully applied to quantify the Sec content in the selenoenzyme thioredoxin reductase and image endogenous Sec in live HepG2 cells. With the aid of Sel-green, we further demonstrated that the cytotoxicity of different selenocompounds is correlated to their ability metabolizing to selenols in cells. To the best of our knowledge, Sel-green is the first selenol probe that works under physiological conditions. The elucidation of the structure-activity relationship for selective recognition of selenols paves the way for further design of novel probes to better understand the pivotal role of Sec as well as selenoproteins in vivo.

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Copper promotion of angiogenesis has been known for more than two decades, but the mechanism of action of copper has not been explored until recently. Copper stimulation of factors involved in vessel formation and maturation, such as vascular endothelial growth factor (VEGF), is mainly responsible for its angiogenesis effect. Copper is required for the activation of hypoxia-inducible factor-1 (HIF-1), a major transcription factor regulating the expression of VEGF. Copper would be transported into nucleus by a copper chaperon for superoxide dismutase-1. Copper is required for HIF-1 interaction with the hypoxia-responsive element of the target genes and ensures the formation of HIF-1 transcriptional complex, thus activating the expression of target genes including VEGF. On the other hand, excess copper can stabilize HIF-1alpha, the rate-limiting component of HIF-1, leading to its accumulation in cytoplasm and thus HIF-1 activation. The essential role of copper in production of VEGF makes it implicated in anti-angiogenesis therapy, such as the application of copper chelators in cancer therapy. However, suppression of angiogenesis is involved in the progression of heart hypertrophy and its transition to heart failure, therefore copper supplementation improves hypertrophic heart disease conditions. This dilemma of copper implications in cancer therapy and heart hypertrophy dictates a comprehensive understanding of a patient's condition before an implementation of copper manipulation therapy for different diseases. In this context, a development of diagnosis for copper metabolic changes as well as a tissue-specific copper manipulation would greatly benefit patients with an implication of copper manipulation therapy.

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Healing skin wounds with anatomic and functional integrity, especially under chronic pathological conditions, remain an enormous challenge. Due to their outstanding regenerative potential, mesenchymal stem cells (MSCs) have been explored in many studies to determine the healing ability for difficult-to-treat diseases. In this article, we review current animal studies and clinical trials of MSC-based therapy for chronic wounds, and discuss major challenges that confront future clinical applications. We found that a wealth of animal studies have revealed the versatile roles and the benefits of MSCs for chronic wound healing. MSC treatment results in enhanced angiogenesis, facilitated reepithelialization, improved granulation, and accelerated wound closure. There are some evidences of the transdifferentiation of MSCs into skin cells. However, the healing effect of MSCs depends primarily on their paracrine actions, which alleviate the harsh microenvironment of chronic wounds and regulate local cellular responses. Consistent with the findings of preclinical studies, some clinical trials have shown improved wound healing after transplantation of MSCs in chronic wounds, mainly lower extremity ulcers, pressure sores, and radiation burns. However, there are some limitations in these clinical trials, especially a small number of patients and imperfect methodology. Therefore, to better define the safety and efficiency of MSC-based wound therapy, large-scale controlled multicenter trials are needed in the future. In addition, to build a robust pool of clinical evidence, standardized protocols, especially the cultivation and quality control of MSCs, are recommended. Altogether, based on current data, MSC-based therapy represents a promising treatment option for chronic wounds.

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