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Abnormal activation of microtubule affinity-regulating kinase 4 (MARK4) and its phosphorylation of the microtubule-associated protein tau are believed to play a role in the pathogenesis of Alzheimer's disease, and MARK4 inhibition can be a strategy to develop disease-modifying therapy. Here we report the development of a membrane-permeable peptide that inhibits MARK4 activity in an allosteric manner. The SARS-CoV-2-derived protein Orf9b inhibited MARK4-mediated tau phosphorylation in primary neurons and Drosophila. Orf9b inhibited MARK4 activity in an allosteric manner and did not inhibit the activity of MARK2, which is another MARK family member and is closely related to MARK4. Co-expression of Orf9b in the fly retina expressing human tau and MARK4 suppressed phosphorylation of tau at the microtubule-binding repeats and tau-induced neurodegeneration. We identified the minimal sequence of Orf9b required to suppress MARK4 activity and fused it to a cell-permeable sequence (TAT-Orf9b10-18_78–95). Extracellular supplementation of TAT-Orf9b10-18_78–95 inhibited MARK4 activity in primary neurons, and feeding TAT-Orf9b10-18_78–95 to a fly model of tauopathy lowered phospho-tau levels and suppressed neurodegeneration. These results suggest that TAT-Orf9b10-18_78–95 is a unique class of MARK4 inhibitor and can be used to modify tau toxicity.

Declines in mitochondrial functions are associated with aging. The combination of 5‐aminolevulinic acid (5‐ALA) and sodium ferrous citrate (SFC) improves mitochondrial functions in cultured cells. In this study, we investigated the effects of dietary supplementation with 5‐ALA and SFC (5‐ALA/SFC) on the healthspan and life span of Drosophila melanogaster. Adult Drosophila fruit flies were fed cornmeal food containing various concentrations of 5‐ALA/SFC. Locomotor functions, life span, muscle architecture, and age‐associated changes in mitochondrial function were analyzed. We found that feeding 5‐ALA/SFC mitigated age‐associated declines in locomotor functions and extended organismal life span. Moreover, 5‐ALA/SFC preserved muscle architecture and maintained the mitochondrial membrane potential in aged animals. Since 5‐ALA phosphate/SFC is used as a human dietary supplement, our results suggest that it could be used to slow the age‐related declines in muscle functions, prevent age‐associated clinical conditions such as frailty, and extend healthspan and life span.