Myokine Cathepsin B as a Key Muscle–Brain Axis Regulator Mediates Treadmill-Running-Induced Hippocampal Neurogenesis and Cognitive Improvement in Mice

This study aimed to explore the impact of treadmill running at different intensities and durations on hippocampal neurogenesis and cognitive function in mice, with a focus on the interorgan communication mechanism mediated by the extracellular vesicle (EV) cargo cathepsin B (CTSB) via the muscle–brain axis. We define the intensity of treadmill running mice based on measurements of maximum oxygen uptake. The findings from treadmill running studies at varying intensities and durations in C57BL/6J mice revealed that treadmill running improved hippocampal neurogenesis and memory in wild-type (WT) mice in an intensity-dependent manner. Omics and UK Biobank cohort analyses identified muscle-derived CTSB as a key exercise-responsive factor, whose expression may be regulated by O-linked N -acetylglucosaminylation. Overexpression of O-linked N -acetylglucosaminyltransferase (OGT) prolonged the half-life of CTSB and inhibited its ubiquitination-mediated degradation, whereas inhibition of OGT accelerated its degradation. Mechanistically, treadmill running may promote the secretion of muscle-derived CTSB into the bloodstream via EVs and its subsequent delivery to the hippocampus through activation of the OGT/CTSB signaling. In WT mice, knockdown of muscular CTSB partially reversed the treadmill-running-induced improvements in hippocampal neurogenesis and memory, while overexpression of muscular OGT further enhanced the release of muscle-derived CTSB. Moreover, in amyloid precursor protein/presenilin 1 mice, treadmill running potentially improved cognitive function, reduced amyloid-β deposition, neurofibrillary degeneration, and neuroinflammation by up-regulating muscular CTSB. Knockdown of muscular CTSB attenuated the benefits of treadmill running, while overexpression of CTSB further enhanced the exercise-induced effects. Overall, this study demonstrates that treadmill running may activate the muscular OGT/CTSB signaling axis, promoting the secretion of the myokine CTSB protein into the circulatory system via EVs and its transport to the brain, thereby improving hippocampal neurogenesis and cognitive function in both WT and amyloid precursor protein/presenilin 1 mice. These findings highlight the role of myokine CTSB as a pivotal modulator in muscle–brain axis communication mechanism, with its stability regulated by O-linked N -acetylglucosaminylation.