Forward single‐cell sequencing into clinical application: Understanding of ageing and rejuvenation from clinical observation to single‐cell solution

Ageing is characterized by physiological characteristics as the part of natural lifespan and by pathological healthspan(definition) processes in response to stress, environmental changes, and diseases. Ageing is a pattern of morphological and functional decline of cells in tissues and circulation, that is correlated with the genetic backgrounds of individuals and with molecular regulatory pathways that may resist ageing influences. The discovery and development of interventions for molecular and cellular rejuvenation is a new and revolutionary field of clinical and translational medicine. Understanding the genetic and epigenetic mechanisms by which health span and lifespan can be regulated, manipulated, and extended is important for identifying and validating "rejuvenating interventions". These mechanisms include signal regulation, metabolic interference, reprogramming, heterochronic parabiosis, target-based drugability, and senescent editing.1-3 One of Clinical and Translational Medicine's missions is to advance spatiotemporal molecular medicine by advancing single-cell sequencing into clinical application, translating single-cell measurements into routine practice, and identifying disease-specific biomarkers and therapies.4-6 Advanced technologies such as single-cell RNA sequencing (scRNA-seq) and spatial transcriptomics have broad applications in clinical tissue or fluid samples to understand intra- and intercellular communications and network interactions within the tissue or within the circulation.7-10 The current Editorial extends our understanding of ageing and rejuvenation from clinical observation in clinical and translational medicine to molecular mechanisms of heterochronic parabiosis in the single-cell solution (Figure 1). The Editorial calls special attention to the identification and development of ageing-associated and specific biomarkers and targets that may impact ageing metabolism, molecular regulation, and genetic alteration. Potential targets for influencing ageing are systemic and local metabolic disorders that occur gradually during the ageing process. For example, Melatonin is a functionally pleiotropic indoleamine molecule and a key regulator of energy metabolism that interferes with cell proliferation and growth and induces apoptosis by regulating carboxylesterase 1 expression and epigenetic modification, lipid droplet accumulation, endoplasmic reticulum stress, de novo androgen synthesis.11 Xie et al. examined the role of melatonin in ageing-associated osteoporosis, a common phenome in the elderly.12 Using bone marrow stromal cells RNA-seq, chromatin immunoprecipitation sequencing, and transposase-accessible chromatin with high-throughput sequencing, it was shown that downregulation of the histone methyltransferase nuclear receptor binding SET domain protein 2 expression was accompanied by low levels of melatonin in the human bone marrow and was associated with the severity of senile osteoporosis. The sequencing data indicate that the manipulations of molecular metabolisms and epigenetic regulations of melatonin may be a complementary therapy for ageing-associated diseases. The tripeptide glutathione is a metabolic element that plays important role in balancing intracellular mitochondria-dominated oxidation, energy production, glucose metabolism, and cell biological behaviours. Preclinical evidence indicates that the deficiency of tripeptide glutathione may accelerate the ageing process from youth to the elderly. Recent 36-week open-label clinical trials of supplemental combination of glycine and N-acetylcysteine in young and older adults demonstrated that the supplementation for 24 weeks could improve the levels of red-blood-cell tripeptide glutathione, oxidative stress, mitochondrial function, inflammatory responses, endothelial barrier function, insulin resistance, genomic-damage, cognition, strength, gait-speed, exercise capacity, and body-fat and waist-circumference.13 This is a critical step to translating the concept of metabolic manipulation in anti-ageing and rejuvenation into the recovery from pathological processes or drug toxicity, although this particular study was still preliminary and has the limitation of a small sample size. It is a challenge to discover and develop ageing specific biomarkers in the circulation on basis of the metabolites from ageing cells as well as to validate and translate those measurable molecules for clinical practice implementation. Contents of ω-3 polyunsaturated fatty acids in the human retina play decisive roles in the maintenance of retinal structure and function, and metabolism, and potentially prevent age-related macular degeneration. Because it is not possible to directly measure retinal ω-3 content, Acar et al. in the Biomarkers of Lipid Status and Metabolism in Retinal ageing project (2021) assessed lipidomic profiles in the circulation and developed a prediction model to evaluate the predictive strength for determining retinal ω-3 content.14 Of polyunsaturated fatty acids, cholesteryl esters species with ω-3, rather than those with ω-6, were correlated with retinal contents. This observation requires further validation in a large population of patients to examine the association between blindness and response to therapy. Zhu et al. investigated trans-omic profiles of clinical phenomes and lipidomes of patients with lung cancer among lung cancer subtypes, genders, ages, stages, metastatic status, nutritional status, and clinical phenome severity, and found that patterns of circulating lipidomic profiles varied by age.15 For example, plasma levels of lysophosphatidylcholine were elevated in patients with lung cancer at ages < 60, lysophosphatidylinositol at ages between 60 and 70, and lysophosphatidylethanolamine at ages > 70. The isomers of the same lipid element can be different across ages of patients with lung cancer, for example, isomers of lysophosphatidylserine 15:1 and 20:3 elevated in ages < 60, while 15:0 and 20:1-2