Quantitative cellular characterization of extracellular mitochondria uptake and delivery

Mitochondria are essential intracellular organelles responsible for energy production. Over the past two decades, unconventional intercellular mitochondrial transfer has been reported, but the nature of the transport intermediates, the efficiency of the process, and the cellular mechanisms involved in their uptake and putative integration by acceptor cells remain poorly understood. This gap in knowledge is especially significant given the potential therapeutic applications of mitochondrial transplantation. In this study, we use quantifiable cell biology and biochemical approaches to assess intercellular mitochondria exchange. Our findings suggest that low amount of free mitochondria can be released into conditioned media and subsequently internalized by recipient cells, primarily via fluid-phase uptake, although alternative or concurrent endocytic pathways may also contribute. Notably, we show that a subset of internalized mitochondria escapes the endosomal compartment, reaches the cytosol, and may integrate into the host cell's pre-existing mitochondrial network.