Horizontal mitochondrial transfer and mitochondrial transplantation in skin: protection against UVR-induced ROS damage and enhancement of cell proliferation and wound healing.

BACKGROUND: Skin homeostasis, protection against ultraviolet radiation (UVR), and wound repair depend on coordinated interactions among melanocytes, keratinocytes, and fibroblasts. Horizontal mitochondrial transfer (HMT) is a naturally occurring form of intercellular communication in which mitochondria move between cells and may contribute to stress adaptation, cellular recovery, and tissue resilience. Artificial mitochondrial transfer (AMT), performed in vitro or ex vivo, and mitochondrial transplantation (MT), involving the direct administration of isolated mitochondria in vivo, seek to therapeutically harness these biological mechanisms. However, HMT among resident skin cells and its relationship to mitochondria-based regenerative strategies remain poorly understood.

METHODS: HMT among human melanocytes, keratinocytes, and fibroblasts was evaluated under basal conditions and following UVR exposure. Direct 2D coculture and transwell systems were used to assess predominantly contact-dependent and contact-independent HMT, respectively, using fluorescence microscopy. AMT was performed by delivering isolated mitochondria from human fibroblast, human Wharton's jellymesenchymal stem/stromal cells (WJ-MSCs), or mouse bone marrow MSCs (BM-MSCs) to recipient fibroblasts, followed by assessment of mitochondrial uptake, reactive oxygen species (ROS) production, and cell proliferation. The regenerative effects of locally administered MSC-derived mitochondria were subsequently evaluated in murine and porcine primary-intention wound models: mouse BM-MSC-derived mitochondria were used in murine wounds, whereas human WJ-MSC-derived mitochondria were used in porcine wounds. Outcomes were assessed using histological analysis, the wound healing index (WHI), and, in pigs, spatial quantification of Ki67-positive cells.

RESULTS: HMT from melanocytes to keratinocytes increased significantly after UVR exposure and occurred predominantly under direct coculture conditions, reaching approximately 39%, compared with less than 9% in transwell assays. HMT in the other donor-recipientcombinationsremainedbelow4%.AMTusing human WJ-MSC-derived mitochondria reduced UVR-induced ROS production, while mitochondria derived from both human WJ-MSCs and mouse BM-MSCs significantly increased fibroblast proliferation, although the magnitude of these effects depended on the mitochondrial dose. In murine wounds, locally administered BM-MSC-derived mitochondria enhanced early histological repair and produced effects comparable to those observed after intact BM-MSC administration. In porcine wounds, WJ-MSC-derived mitochondria increased the WHI, improved collagen-containing tissue organization, and enhanced Ki67 positivity within epidermal and dermal regions directly involved in wound repair.

CONCLUSIONS: These findings identify HMT as a cell-type-specific response in the skin, with preferential HMT from melanocytes to keratinocytes that is enhanced by UVR exposure. They also demonstrate that MSC-derived mitochondria can reduce oxidative stress, stimulate fibroblast proliferation, and promote early cutaneous repair after local administration. Together, the results establish a translational link between endogenous mitochondrial exchange and the therapeutic use of AMT and MT, supporting further development of mitochondria-based, cell-free strategies for skin injury and impaired wound healing.