Astrocytic mitochondrial transfer to brain endothelial cells and pericytes <i>in vivo</i> increases with aging

Intercellular mitochondrial transfer (IMT) is an intriguing biological phenomenon where mitochondria are transferred between different cells and notably, cell types. IMT is physiological, occurring in normal conditions, but also is utilized to deliver healthy mitochondria to cells in distress. Transferred mitochondria can be integrated to improve cellular metabolism, and mitochondrial function. Research on the mitochondrial transfer axis between astrocytes and brain capillaries <i>in vivo</i> is limited by the cellular heterogeneity of the neurovascular unit. To this end, we developed an inducible mouse model that expresses mitochondrial Dendra2 only in astrocytes and then isolated brain capillaries to remove all intact astrocytes. This method allows the visualization of <i>in vivo</i> astrocyte- endothelial cell (EC) and astrocyte-pericyte IMT. We demonstrate evidence of astrocyte-EC and astrocyte-pericyte mitochondrial transfer within brain capillaries. We also show that healthy aging enhances mitochondrial transfer from astrocytes to brain capillaries, revealing a potential link between brain aging and cellular mitochondrial dynamics. Finally, we observe that astrocyte-derived extracellular vesicles transfer mitochondria to brain microvascular endothelial cells, showing the potential route of <i>in vivo</i> IMT. These results represent a breakthrough in our understanding of IMT in the brain and a new target in brain aging and neurovascular metabolism.