Mitochondrial calcium (mtCa²⁺) uptake via the mitochondrial calcium uniporter (MCU) couples calcium homeostasis and energy metabolism. mtCa²⁺ uptake via MCU is rate-limiting for mitochondrial activation during muscle contraction, but its pathophysiological role and therapeutic application remain largely uncharacterized. By profiling human muscle biopsies, patient-derived myotubes, and preclinical models, we discovered a conserved downregulation of mitochondrial calcium uniporter regulator 1 (MCUR1) during skeletal muscle aging that associates with human sarcopenia and impairs mtCa²⁺ uptake and mitochondrial respiration. Through a screen of 5,000 bioactive molecules, we identify the natural polyphenol oleuropein as a specific MCU activator that stimulates mitochondrial respiration via mitochondrial calcium uptake 1 (MICU1) binding. Oleuropein activates mtCa²⁺ uptake and energy metabolism to enhance endurance and reduce fatigue in young and aged mice but not in muscle-specific MCU knockout (KO) mice. Our work demonstrates that impaired mtCa²⁺ uptake contributes to mitochondrial dysfunction during aging and establishes oleuropein as a novel food-derived molecule that specifically targets MCU to stimulate mitochondrial bioenergetics and muscle performance.

Wnt signaling affects fundamental development pathways and, if aberrantly activated, promotes the development of cancers. Wnt signaling is modulated by different factors, but whether the mitochondrial energetic state affects Wnt signaling is unknown. Here, we show that sublethal concentrations of different compounds that decrease mitochondrial ATP production specifically downregulate Wnt/β-catenin signaling in vitro in colon cancer cells and in vivo in zebrafish reporter lines. Accordingly, fibroblasts from a GRACILE syndrome patient and a generated zebrafish model lead to reduced Wnt signaling. We identify a mitochondria-Wnt signaling axis whereby a decrease in mitochondrial ATP reduces calcium uptake into the endoplasmic reticulum (ER), leading to endoplasmic reticulum stress and to impaired Wnt signaling. In turn, the recovery of the ATP level or the inhibition of endoplasmic reticulum stress restores Wnt activity. These findings reveal a mechanism that links mitochondrial energetic metabolism to the control of the Wnt pathway that may be beneficial against several pathologies.

Ca²⁺ transport across the inner membrane of mitochondria (IMM) is of major importance for their functions in bioenergetics, cell death and signaling. It is therefore tightly regulated. It has been recently proposed that LETM1—an IMM protein with a crucial role in mitochondrial K⁺/H⁺ exchange and volume homeostasis—also acts as a Ca²⁺/H⁺ exchanger. Here we show for the first time that lowering LETM1 gene expression by shRNA hampers mitochondrial K⁺/H⁺ and Na⁺/H⁺ exchange. Decreased exchange activity resulted in matrix K⁺ accumulation in these mitochondria. Furthermore, LETM1 depletion selectively decreased Na⁺/Ca²⁺ exchange mediated by NCLX, as observed in the presence of ruthenium red, a blocker of the Mitochondrial Ca²⁺ Uniporter (MCU). These data confirm a key role of LETM1 in monovalent cation homeostasis, and suggest that the effects of its modulation on mitochondrial transmembrane Ca²⁺ fluxes may reflect those on Na⁺/H⁺ exchange activity.