Melatonin is a hormone, synthesized in the pineal gland, which primarily controls the circadian rhythm of the body. In recent years, melatonin has also been shown to regulate metabolism, provide neuroprotection, and act as an anti-inflammatory, free radical scavenger. There has also been a recent research interest in the role of melatonin in regulating mesenchymal stromal cells (MSCs). MSCs are pivotal for their ability to differentiate into a variety of different tissues. There is also increasing evidence for the therapeutic prospects of MSCs via paracrine signaling. In addition to secreting cytokines and chemokines, MSCs can secrete extracellular vesicles (EVs), allowing them to respond to injury and promote tissue regeneration. While there has been a major research interest in the use of MSCs for regenerative medicine, the clinical application is limited by many risks, including tumorigenicity, senescence, and sensitivity to toxic environments. The use of MSC-derived EVs for cell-free therapy can potentially avoid the disadvantages of MSCs, which makes this an exciting prospect for regenerative medicine. Prior research has shown that MSCs, via paracrine mechanisms, can identify receptor-independent responses to melatonin and then activate a series of downstream pathways, which exert a variety of effects, including anti-tumor and anti-inflammatory effects. Here we review the synthesis of melatonin, its mechanisms of action, and the effect of melatonin on MSCs via paracrine signaling. Furthermore, we summarize the current clinical applications of melatonin and discuss future prospects.

In this review, we discuss recent publications on asthma and review the studies that have reported on the different aspects of the prevalence, risk factors and prevention, mechanisms, diagnosis, and treatment of asthma. Many risk and protective factors and molecular mechanisms are involved in the development of asthma. Emerging concepts and challenges in implementing the exposome paradigm and its application in allergic diseases and asthma are reviewed, including genetic and epigenetic factors, microbial dysbiosis, and environmental exposure, particularly to indoor and outdoor substances. The most relevant experimental studies further advancing the understanding of molecular and immune mechanisms with potential new targets for the development of therapeutics are discussed. A reliable diagnosis of asthma, disease endotyping, and monitoring its severity are of great importance in the management of asthma. Correct evaluation and management of asthma comorbidity/multimorbidity, including interaction with asthma phenotypes and its value for the precision medicine approach and validation of predictive biomarkers, are further detailed. Novel approaches and strategies in asthma treatment linked to mechanisms and endotypes of asthma, particularly biologicals, are critically appraised. Finally, due to the recent pandemics and its impact on patient management, we discuss the challenges, relationships, and molecular mechanisms between asthma, allergies, SARS-CoV-2, and COVID-19.