**6. MOTS-c**

MOTS-c (Mitochondrial ORF (Open Reading Frame) within the Twelve S rRNA c) is encoded in the 12S rRNA region of the mtDNA, and is a 16-amino acid peptide with the sequence *H-MRWQEMG YIFYPRKLR-OH* and a molecular weight 2174.7 Da (Figure 4). It is associated with insulin resistance and is found in plasma, brain, liver, and muscle tissues. MOTS-c enhances insulin sensitivity and regulates plasma metabolites in three metabolic pathways, namely sphingolipid metabolism, monoacylglycerol metabolism, and dicarboxylate metabolism. MOTS-c indirectly decreases cellular oxidative stress by reducing plasma oxidized glucose levels. MOTS-c also activates AKT phosphorylation and AMPK pathways. Moreover, MOTS-c reduces skeletal muscle fatigue and improves performance in vivo [57]. This is the first report by Cohen et al. that delineates the specific roles of MOTS-c and opens new avenues for further research with this MDP. MOTS-c enhances mitochondrial respiration in senescent cells by regulating fatty acid oxidation and increasing the senescence-related effectors [58]. Since MOTS-c levels also decline with age, it has been implicated in the regulation of lifespan and healthspan in organisms. This lifespan/healthspan prolonging ability of MOTS-c may be attributed to (1) MOTS-c-mediated increase in intracellular NAD<sup>+</sup>, a key redox metabolic coenzyme that activates sirtuins, which in turn regulate aging; and (2) MOTS-c-mediated reduction in methionine metabolism via inhibition of the folate/methionine cycle [59]. Although the function of MOTS-c as a crucial player in cell longevity, mitochondrial function, and metabolic homeostasis has been well-established, its specific protective function in the eye is ye<sup>t</sup> to be reported. Since the AMD etiology involves a metabolic component as

well as mitochondrial dysregulation, it would be interesting to investigate the cytoprotective effects of MOTS-c in AMD pathology (Figure 5).

**Figure 4.** MOTS-c ORF in the human mitochondrial DNA.

**Figure 5.** Effects of MOTS-c.

CohBar, a clinical-stage biotechnology company focused on the research and development of mitochondria-based therapeutics, has successfully completed a Phase 1a clinical study and initiated the Phase 1b stage of a double-blind, placebo-controlled clinical trial of CB4211 as a potential treatment for nonalcoholic steatohepatitis (NASH) and obesity. CB4211 is the first therapeutic candidate based on a mitochondrial-derived peptide to enter clinical testing in humans. The completed Phase 1a stage of the CB4211 clinical study evaluated safety and tolerability, and the drug was safe and well tolerated after seven days of dosing. The Phase 1b stage of the study will be an assessment of safety, tolerability, and activity in obese subjects with nonalcoholic fatty liver diseases (NAFLD). Assessments will include changes in liver fat assessed by MRI-PDFF, body weight, and biomarkers relevant to NASH and obesity [60].

### **7. Conclusions and Future Directions**

The dry and wet forms of AMD have some common denominators in terms of AMD disease pathology, since the characteristic features of dry AMD, i.e., RPE cell apoptosis and accumulation of drusen deposits (in which amyloid-β is a key component), may subsequently lead to choroidal neovascularization observed in wet AMD pathology. To our knowledge, di fferential e ffects of MDPs in dry AMD versus wet AMD have not been delineated and reported yet.

Since this manuscript presents the literature that establishes the role of MDPs in mitigating RPE cell apoptosis and reducing amyloid-β-induced toxicity, it suggests that MDPs are potential therapeutic candidates for treatment of dry AMD, and may delay its progression to the late form, i.e., wet AMD. However, the specific therapeutic e ffects of MDPs in reducing or preventing choroidal neovascularization in wet AMD need to be characterized. Experimentally, the e ffects of MDPs on the angiogenesis-promoting factors should be tested to evaluate if MDPs are able to downregulate the pro-angiogenic factors and related pathways, and thereby prevent or suppress neovascularization in wet AMD.

In conclusion, MDPs, especially Humanin (and its analogs) and SHLPs, provide cytoprotection in ocular diseases, including AMD, and may be considered as potential therapeutic targets for AMD. However, the use of MDPs as therapeutic agents for AMD will require development of appropriate delivery techniques and formulations. Currently, nanoparticles encapsulating Humanin are being tested in some labs to subsequently facilitate its e fficient delivery and sustained action. In a recent study by Li et al., it was demonstrated that the Humanin peptide mediates elastin-like polypeptide nano-assembly and protects human RPE cells against oxidative stress-induced apoptotic cell death. This technology may facilitate cellular delivery of biodegradable nanoparticles with potential protection against AMD [61]. Furthermore, with the development of precision medicine, the use of MDPs can be customized for AMD therapy to match patient needs.

**Author Contributions:** S.N.: Wrote and edited the manuscript; M.C.K.: Corresponding author. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is supported by Arnold and Mabel Beckman foundation, NEI R01 EY0127363, Discovery Eye Foundation, Polly and Michael Smith, Edith and Roy Carver, Iris and B. Gerald Cantor Foundation, Unrestricted Departmental Grant from Research to Prevent Blindness, UCI School of Medicine, and support of the Institute for Clinical and Translational Science (ICTS) at University of California Irvine. S.N. is a recipient of the 2017 Genentech/ ARVO AMD Translational Research Fellowship and the 2016 RPB pilot research grant.

**Conflicts of Interest:** The authors declare no conflict of interest.
