Neural stem cell (NSC) transplantation induces recovery in animal models of central
nervous system (CNS) diseases. Although the replacement of lost endogenous cells was
originally proposed as the primary healing mechanism of NSC grafts, it is now clear
that transplanted NSCs operate via multiple mechanisms, including the horizontal exchange
of therapeutic cargoes to host cells via extracellular vesicles (EVs). EVs are membrane
particles trafficking nucleic acids, proteins, metabolites and metabolic enzymes,
lipids, and entire organelles. However, the function and the contribution of these
cargoes to the broad therapeutic effects of NSCs are yet to be fully understood. Mitochondrial
dysfunction is an established feature of several inflammatory and degenerative CNS
disorders, most of which are potentially treatable with exogenous stem cell therapeutics.
Herein, we investigated the hypothesis that NSCs release and traffic functional mitochondria
via EVs to restore mitochondrial function in target cells. Untargeted proteomics revealed
a significant enrichment of mitochondrial proteins spontaneously released by NSCs
in EVs. Morphological and functional analyses confirmed the presence of ultrastructurally
intact mitochondria within EVs with conserved membrane potential and respiration.
We found that the transfer of these mitochondria from EVs to mtDNA-deficient L929
Rho(0) cells rescued mitochondrial function and increased Rho(0) cell survival. Furthermore,
the incorporation of mitochondria from EVs into inflammatory mononuclear phagocytes
restored normal mitochondrial dynamics and cellular metabolism and reduced the expression
of pro-inflammatory markers in target cells. When transplanted in an animal model
of multiple sclerosis, exogenous NSCs actively transferred mitochondria to mononuclear
phagocytes and induced a significant amelioration of clinical deficits. Our data provide
the first evidence that NSCs deliver functional mitochondria to target cells via EVs,
paving the way for the development of novel (a)cellular approaches aimed at restoring
mitochondrial dysfunction not only in multiple sclerosis, but also in degenerative
neurological diseases.
