Mitochondria are organelles produced by archaea that are required for the production of ATP (1). The organism exhibits semi-autonomous genetic systems, independent genomes, and unique genetic codes that are similar to those found in bacteria (2). Recently, a short open reading frame (sORF) encoded in the mitochondrial genome has been discovered (3, 4). These sORF produce bioactive peptides, collectively known as mitochondrial-derived peptides (MDP), which have a wide range of physiological functions and can explain how mitochondria communicate within and between cells in a specific disease environment (5). Mitochondrial-derived peptides may answer the key biological problems that have plagued the field for decades (such as mitochondrial-nuclear communication, metabolic dysfunction, etc.) (6). Whether in the form of mitochondrial-derived peptide itself or in terms of sORF, mitochondrial-derived peptide is suitable for research as a therapeutic agent (1).

Studies discovered mitochondrial-derived peptide called MOTS-c has been shown to significantly reduce the level of pro-inflammatory factors in mice and increase anti-inflammatory factors and insulin-stimulated glucose treatment rates, as well as glucose homeostasis (7–9). Furthermore, human studies showed that exercise increased MOTS-c levels in skeletal muscle and blood circulation, indicating that MOTS-c is a mitochondrial-derived peptide induced by skeletal muscle exercise (9, 10). Additionally, more and more studies have revealed the importance of MOTS-c in regulating obesity and diabetes (11, 12), longevity (13), and cardiovascular disease (14). Specifically, this paper discusses the application of mitochondrial-derived peptides, including MOTS-c, in the treatment of diseases and anticipates the future development direction of MOTS-c combining synthetic biology to provide new ideas on how it can be developed and applied.

MOTS-c, one of the newly discovered sORF-encoded peptides, is a 16-amino acid polypeptide encoded by the mitochondrial 12S rRNA gene and localized to mitochondria under resting conditions ( Figure 1 ) (7). Translation of MOTS-c peptide occurs exclusively in the cytoplasm, as mitochondrial translation, using the mitochondria-specific genetic code, results in tandem codons. As a result, the polyadenylated transcript would be exported from the mitochondria. The sequence of MOTS-c peptides, especially the first 11 residues, is highly conserved among 14 species, including humans and mice (7).

MOTS-c, as a mitochondrial coding regulator, has endocrine-like and nuclear transcriptional regulation on muscle metabolism (10, 15), insulin sensitivity (16, 17) and body weight (18). Approximately 11.9-fold increase in endogenous MOTS-c levels have been found in skeletal muscle following exercise (compared to pre-exercise values), and this increase can remain for 4 hours following exercise (19). In addition, circulating endogenous MOTS-c levels increase 1.6-fold during exercise, 1.5-fold after exercise, and then return to baseline levels after 4 hours. These results imply that exercise stimulates the expression of MOTS-c, which is encoded by mitochondria, in humans (19). Exercise is accomplished through a signal network that crosses physiological functions in real time. Those with limited movement can benefit tremendously from kinesimatology, which also accelerates metabolism (10). MOTS-c, as a new type of mitochondrial signal molecule, may stimulate exercise-mediated physiological responses to increase endurance (13). Therefore, MOTS-c can be used as a motion simulator to mediate the function of the motion signal system (20).

Originally, MOTS-c was identified in the process of genetic and pharmacological screening of metabolic regulation in human cells (21). In fact, its metabolic effects under various pathophysiological conditions have been confirmed by several studies. It has been reported that MOTS-c can promote the entry of glucose into cells through 5’-monophosphate-activated protein kinase (AMPK) pathway to participate in glycolysis (22). Moreover, MOTS-c can improve insulin sensitivity of skeletal muscle and inhibit weight gain and insulin resistance caused by high-fat diet (6). However, human experiments confirmed that only obese Chinese male children/adolescents (5-14 years old) had significantly lower (20.3%) intra-cycle MOTS-C levels (22). In a cohort study, plasma MOTS-c levels in men were negatively correlated with fasting insulin levels, glycosylated hemoglobin and body mass index (23). However, another cohort study of 31- to 38-year-old adults found no correlation between body mass index and plasma MOTS-c levels (10). This contradiction may be related to the individual differences of the subjects.

MOTS-c has also been found to be involved in the regulation of nuclear gene expression by binding to transcription factors (5, 19, 24). Under resting conditions, MOTS-c is mainly distributed in the mitochondria outside the nucleus (5, 7, 19). When metabolic stress occurs, MOTS-c in cells can be transferred to the nucleus in an AMPK-dependent manner and bind to transcription factors regulated by ARE, thus improving the stress resistance of cells (16, 24). In addition, the entry of MOTS-c into the nucleus requires hydrophobic groups, which means that MOTS-c may need the help of other proteins that need further verification (5, 25). Furthermore, MOTS-c plays the role of endocrine-like factors by regulating nuclear transcription to restore homeostasis. The function may have important implications for age-related diseases in response to metabolic stress by promoting intracellular homeostasis, and MOTS-c gene polymorphisms have been found to be associated with human lifespan (18, 20). Moreover, MOTS-c may improve diabetes by inhibiting insulin resistance and diet-induced obesity (26). And MOTS-c can promote glucose utilization, inhibit oxidative stress, activate NF-κB to inhibit inflammation, and effectively protect coronary artery endothelial cell dysfunction (9).

With the rapid development of synthetic biology technology, the role of genetic engineering bacteria in the treatment of various diseases has become more and more prominent (36, 37). Genetically engineered bacteria are bacteria that use DNA recombination technology to transfer the target gene into bacteria (such as E. coli) to express and produce the desired protein (38, 39). Engineered bacteria can achieve targeted gene reprogramming, selective functional recombination and precise space-time control, so they are widely used in medical and pharmaceutical industries (40). Genetically engineered bacteria have been developed as diagnostic and therapeutic tools for the treatment of many diseases, including cancer (41), diabetes (38), inflammatory bowel disease (42, 43) and viral infection (44). Chen Zhiyi of Nanhua University and others designed a kind of ultrasound-responsive bacteria, which can induce the expression of foreign gene IFN-γ in an ultrasound-controlled way, and improve the anti-tumor efficacy of the engineering bacteria in vitro and in vivo (42). AmirZarrinpar of the University of California genetically engineered natural E. coli isolated from the intestines of mice to express specific genes and improve diabetes (38). In addition, the use of bacteria as carriers to deliver drugs to tumors and other lesions, or to modify bacteria to express or deliver targeted drugs will greatly improve the therapeutic effect (39, 45, 46). In addition to using genetically engineered bacteria as drug delivery carriers, extracellular vesicles (EVs), especially exocrine bodies, originate from cells through exocytosis and are absorbed by target cells, which can transmit biological signals or even deliver drugs between local or distant cells (47). However, accurate, efficient and selective identification, separation and quantification of exocrine remains a challenge (48). Compared with exosomes, genetically engineered bacteria have stronger biological activity, higher effective drug concentration and more stable drug structure in drug delivery.

MOTS-c, as a small molecular active peptide, has been reported to have potential applications in aging, insulin resistance, cardiovascular disease and inflammation ( Figure 1 ). In the future, the use of synthetic biology technology to introduce MOTS-c into probiotics to achieve its accurate and controllable expression is of great significance to human health and the prevention of various diseases ( Figure 2 ). However, the virulence and uncontrollable immune response of bacteria greatly limit the clinical trial and application of bacterial therapy. It is an urgent problem for researchers to improve the safety and therapeutic effect of bacteria (37). At present, the most commonly used methods are gene-modified bacteria and surface-modified bacteria, and some natural bacteria such as Lactobacillus (LAB) and Escherichia coli Nissle1917 (EcN) had been proved to be safe clinically (49, 50). Using these bacteria as chassis bacteria will greatly improve the clinical effectiveness.

Recently, the positive effects of MOTS-c, one of the mitochondria-derived peptides, on various diseases have been gradually discovered and reported. MOTS-c targets skeletal muscle and can enhance glucose metabolism. Therefore, MOTS-c plays an important role in the regulation of cardiovascular, diabetes, exercise and longevity. It is a new mitochondrial signaling mechanism and plays a role in regulating intracellular and intercellular metabolism. It has been reported that MOTS-c is a kind of exercise-induced mitochondrial coding regulator, and the level of MOTS-c in skeletal muscle and blood of mice decreases with age. Systemic injection of MOTS-c can restore the level of MOTS-c in aged mice and successfully reverse age-related skeletal muscle insulin resistance. At the same time, muscle cells overexpressing MOTS-c can improve glucose uptake, which is mainly related to the activation of AMPK pathway. Additionally, as an endocrine factor, MOTS-c is likely to exert its effects through cellular entry, which is an ongoing investigation at multiple ends, including its cellular uptake. Understanding the detailed molecular details of MOTS-c is an ongoing endeavor. It was reported that after 30 minutes of treatment, MOTS-c can exert its effects by entering the cells and exerting its effects. There is still a great deal of unclarity as to how MOTS-c enters cells without being degraded and retains its biological activity. A future study of the mechanism by which MOTS-c enters cells will be of great importance in order to determine how it exerts its clinical therapeutic effect. As a potential target for treatment development, MOTS-c is expected to be used in the treatment development of a variety of diseases. Synthetic biology techniques, such as gene editing and genetic engineering, can greatly improve biological activity and deliver MOTS-c directly to the acting site, thus further expanding the therapeutic application of MOTS-c. However, the subsequent biosafety problem has always been the focus of attention. How to reduce or eliminate the toxicity of genetically engineered bacteria to the body is a key scientific problem to be solved urgently.

YZ: writing—original draft. ZW: review and editing, supervision. TW: review and editing, supervision. All authors have read and agreed to the published version of the manuscript. All authors contributed to the article and approved the submitted version.