MOTS-c is a new member of the mitochondrial polypeptide family, encoded by the 12S rRNA short ORF of mtDNA. MOTS-c was first discovered by Lee et al. in 2015 ^[1][5]. They looked for potential ORF in human 12S rRNA and found one that could be translated into a 16-amino-acid peptide, which they termed MOTS-c. The primary structure of MOTS-c is Met-Arg-Trp-Gln-Glu-Met-Gly-Tyr-Ile-Phe-Tyr-Pro-Arg-Lys-Leu-Arg, and the first 11 amino acid residues are highly conserved. Lee et al. also found that MOTS-c was widely expressed in numerous tissues, including the brain, heart, liver, muscles, testes, kidney, spleen, large intestine and small intestine. MOTS-c treatment in mice prevented age-dependent and high-fat-diet-induced insulin resistance, as well as diet-induced obesity ^[1][5].

MOTS-c is an adaptive signaling protein that can translocate to the nucleus under metabolic stress ^[2][7]. MOTS-c engages in intracellular nuclear-mitochondrial signal transmission, regulates nuclear gene expression, and maintains mitochondrial function and cell resilience ^[2][7]. MOTS-c, on the other hand, has a hormonal effect. By controlling the synthesis of antioxidant response kinases within a range and activating related metabolic pathways, it affects glucose, lipid and bone metabolism ^[3][4][8,9]. Since MOTS-c plays an important role in the pathological metabolic processes of several metabolic disorders, MOTS-c is expected to become a diagnostic marker and an effective treatment approach for these disorders ^[5][10].

2. MOTS-c Inhibits Pathological Metabolic Processes

2.1. MOTS-c Reduces Insulin Resistance

Insulin resistance is defined as impaired signal transduction and biological actions in response to insulin stimulation, resulting in a decrease in the ability of insulin to increase glucose uptake and utilization ^[6][16]. Insulin resistance is a risk factor for non-alcoholic steatohepatitis, metabolic syndrome, and type 2 diabetes (T2D) ^[7][8][9][17,18,19]. MOTS-c has been demonstrated to increase insulin sensitivity in mice by preventing fat accumulation through reducing the pathways of sphingolipid metabolism, monoacylglycerol metabolism, and dicarboxylate metabolism ^[10][20]. MOTS-c treatment reversed age-dependent insulin resistance in mouse skeletal muscle, demonstrating that the occurrence of insulin resistance is related to MOTS-c ^[1][11][5,21]. MOTS-c can correct the diabetes-induced abnormal cardiac structures and functions. MOTS-c mimics exercise-induced cardio-protection in diabetic rats by activating NRG1-ErbB4 signaling pathway ^[12][22]. By increasing the expression of mitochondrial biogenesis genes TFAM, COX4, and NRF1, which are markers for mitochondrial biogenesis, MOTS-c induces GLUT4 translocation and promotes glucose uptake ^[13][23]. MOTS-c also improve the glucose metabolism of insulin-resistant rats fed with high-fat diets. The mechanism is that MOTS-c increases the AMPK activity and enhances the insulin sensitivity of skeletal muscle cells, resulting in improved glucose metabolism ^[1][5]. This result indicates that MOTS-c can be employed as a new insulin sensitizer, which is helpful in treating T2D.

2.2. MOTS-c Prevents Obesity

Obesity is a risk factor for insulin resistance and hyperinsulinemia ^[14][25]. MOTS-c in the maternal blood serum of obese women significantly increases compared to healthy women ^[15][6]. It has been found in previous studies that MOTS-c injections in the mouse could reduce the level of IL-6 and TNF-α, two obesity-related inflammatory factors ^[16][17][26,27]. Similarly, the AMPK pathway is activated after MOTS-c is given to high-fat diet-induced mice. The expression of the downstream GLUT4 rises, which reduce the incidence of obesity ^[1][13][5,23]. 

2.3. MOTS-c Improve Muscle Function

Previous research has shown that MOTS-c was naturally muscle-targeting, which could speed up skeletal muscle metabolism while generating energy ^[18][31]. MOTS-c increases the level of glycolysis and ATP in dystrophic muscle cells, enhancing the absorption and activity of phosphorodiamidate morpholino oligomer ^[18][31]. Additionally, it was found that by giving a long-term combined administration of MOTS-c and phosphorodiamidate morpholino oligomer to Duchenne muscular dystrophy (DMD)-affected mice, their muscular functions significantly improved ^[18][31]. Therefore, MOTS-c can be utilized as an adjuvant to enhance the therapeutic effect of oligonucleotide-mediated exon-skipping therapy for insulin-resistance-induced skeletal muscle atrophy ^[18][19][31,32]. Myostatin is a negative regulator of skeletal muscle mass and mediates insulin-resistance-induced skeletal muscle wasting ^[20][21][34,35]. In human circulation, the level of myostatin is adversely linked with plasma MOTS-c ^[19][22][32,36]. Through MOTS-c treatment, the myostatin level decreases in plasma and skeletal muscles. MOTS-c prevents muscle atrophy in diet-induced obese mice by inhibiting the activity of an upstream transcription factor for myostatin and muscle wasting gene, FOXO1 ^[19][32]. Serum myostatin has a close relationship with endothelial function.

2.4. MOTS-c Promotes Bone Metabolism

Osteoblasts and osteoclasts are the two most important cells in bone metabolism. Osteoblasts are cells found on bone surfaces that are responsible for bone formation by synthesizing the organic components of the bone matrix and coordinating the mineralization of the skeleton ^[23][40]. Osteoclasts are multinucleated cells derived from hematopoietic stem cells or monocyte/macrophage precursor cells that absorb the organic and inorganic compounds released from the impaired bone, during which the degraded compound matrix goes into the bloodstream in the form of Ca²⁺ and so on for recycling ^[24][41]. Severe bone formation defects and increased bone resorption can easily lead to metabolic bone diseases such as osteoporosis, rickets, and bone fragility ^[25][26][27][42,43,44]. Studies have demonstrated that MOTS-c improved bone metabolism abnormalities and acted as an anti-osteoporosis agent by suppressing osteoclast differentiation ^[4][28][9,45]. MOTS-c activate the TGF-β/Smad signaling pathway by up-regulating the expression level of TGF-β1/β2 and Smad7, promoting the synthesis of type I collagen in osteoblasts, and thereby improving osteoporosis ^[28][45]. MOTS-c also encourage bone marrow mesenchymal stem cells to develop into osteoblasts and advance the process of bone formation via activating the TGF-β/Smad signaling pathway ^[28][45]. 

2.5. MOTS-c Enhances Immune Regulation

Type 1 diabetes (T1D) is an autoimmune disease characterized by the destruction of pancreatic β-cells, resulting in insufficient insulin secretion ^[29][47]. MOTS-c prevents autoimmune β cell destruction by targeting T cells in non-obese diabetic mice ^[30][48]. Treatment with MOTS-c for non-obese diabetic mice shows that MOTS-c regulates the T cell phenotype and function through inhibiting TCR/mTORC1 signaling ^[30][48]. MOTS-c ameliorated the development of hyperglycemia and reduced islet-infiltrating immune cells ^[30][48]. Meanwhile, MOTS-c suppresses autoimmune diabetes, suggesting that MOTS-c could delay the onset of T1D and has the effect of early prevention. MOTS-c was found to be significantly reduced in patients with multiple sclerosis, an autoimmune disease of the central nervous system ^[31][49]. These studies indicate that MOTS-c has the physiological function of enhancing immune regulation. MOTS-c may also be a potential molecule for the treatment of inflammation-related disorders. MOTS-c protect rat cardiomyocytes from H₂O₂-induced inflammation and oxidative stress by suppressing NF-κB and activating the Nrf2-ARE pathway ^[32][50]. In a mouse formalin test, MOTS-c could activate the AMPK pathway, which suppressed pro-inflammatory cytokine (TNF-α, IL-1β, and IL-6) production and promoted anti-inflammatory cytokine (IL-10) secretion ^[16][26]. In addition, MOTS-c suppressed formalin-induced ERK, JNK, and P38 ^[16][26].

2.6. MOTS-c Postpone Aging

The level of circulating MOTS-c decreases with age, suggesting that it may participate in regulating the process of aging-related diseases ^[33][52]. According to the studies, MOTS-c can improve the physical performance of young mice, enhance the physical capacity of old mice and improve mouse healthspan ^[11][21]. Maintaining the cell metabolic balance is regarded as the main way to prevent aging-related disease. It hypothesizes that MOTS-c delay aging by regulating the expression of nuclear genes, maintaining metabolic homeostasis, and modulating cell adaptability to metabolic stress as the mechanisms mentioned above. In animal models, MOTS-c considerably delays the onset of age-related physical impairments ^[11][21]. 

MOTS-c can raise the intracellular level of NAD⁺, whose concentration gradually declines with age ^[1][34][5,54]. NAD⁺ is an evolutionarily highly conserved coenzyme with multi-faceted cell functions, including energy metabolism, molecular signaling processes, epigenetic regulation, and DNA repair ^[35][36][55,56]. NAD⁺ levels increase under conditions that increase lifespan and healthspan, such as dietary restriction and exercise, and decrease during ageing or under conditions that decrease lifespan and healthspan, such as a high-fat diet, which supports the working model that decreased NAD⁺ levels might contribute to the ageing process ^[37][57]. 

3. Clinical Application of MOTS-c

Currently, known physiological functions of MOTS-c include reducing insulin resistance, preventing obesity, improving muscle function, promoting bone metabolism, enhancing immune regulation, and postponing aging through the gene expression and signaling pathways described above. Table 12 summarizes the diseases associated with MOTS-c dysfunction. Table 23 summarizes the genes and pathways involved in the physiological function of MOTS-c.

The Functions of MOTS-c	The Disease Associated to MOTS-c Dysfunction	References
Reduces insulin resistance	type 2 diabetes, diabetes-induced abnormal cardiac structures and functions	[1][13]	[5,23]
Prevents obesity	obesity, vascular calcification	[1][3][38]	[5,8,29]
Improves muscle function	duchenne muscular dystrophy, energy-deficient muscle illnesses	[18][19][39]	[31,32,33]
Promotes bone metabolism	osteoporosis	[28][40]	[45,46]
Enhances immune regulation	type 1 diabetes, multiple sclerosis, inflammation-related disorders, methicillin-resistant staphylococcus aureus infection	[30][31]	[48,49]
Postpones aging	aging-related diseases	[33][34][41]	[52,53,54]

The Functions of MOTS-c	The Genes and Pathways Involved in the Functions of MOTS-c	References
Reduces insulin resistance	AMPK, NRG1, ErbB4, TFAM, COX4, NRF1, GLUT4	[1][12][13]	[5	[	,22	42	,23	]	,24]
Prevents obesity	AMPK, GLUT4, IL-6, TNF-α, AT-1, ET-B, PGC1α, UCP1, Dio2, ERK	[1][13][16][43][44]	[5,23,26,28,30]
MOTS-c improves muscle function	AMPK, STAT3, FOXO1, APPL1, SIRT1, PGC1α	[1]]	[5	[19]	,32	[39][45	,33,38]
Promotes bone metabolism	AMPK, TGF-β, SMAD7, OPG, RANKL, NFκB, STAT1	[28][40]	[45,46]
Enhances immune regulation	TCR, mTORC1, NF-κB, Nrf2, TNF-α, IL-1β, IL-6, IL-10, ERK, JNK, P38, AhR, Stat3	[16][30][32][46]	[26,48,50,51]
Postpones aging	JAK	[1][47]	[5,59]

The development of peptide-based drugs as an effective therapeutic strategy is currently one of the hottest topics in drug development. MOTS-c is not the first active peptide discovered with a wide range of effects. Previously, the development of peptide drugs has been reported many times. The disadvantages, including low efficacy, toxicity, and drug resistance, limit the clinical application of peptide drugs ^[48][49][50][65,66,67]. Previous animal experiments have proved that MOTS-c alone can reduce obesity and insulin resistance.

MOTS-c could be used to treat obesity and age-related diseases as a peptide drug. MOTS-c levels in obese people fell significantly. Interestingly, MOTS-c expression is significantly higher in brown adipose tissue (BAT) ^[44][30]. BAT is a thermogenic organ that is involved in energy expenditure and represents an attractive target to treat obesity ^[51][70]. White adipose tissue (WAT), on the other hand, functions as an endocrine organ and serves as a reservoir of energy in the form of triglycerides ^[52][71]. Obesity resulting from a chronic imbalance between energy intake and energy output with an increased WAT mass entails an increased risk of several chronic illnesses, including T2D, insulin resistance, and cardiovascular diseases (CVD) ^[53][54][55][72,73,74].