Astrocytic response to traumatic brain injury to rescue neuronal mitochondrial dysfunction through mitochondrial transfer

As highly dynamic organelles, mitochondria play an essential role in neuronal survival and synaptic function. Excitotoxicity is as a critical factor that promotes mitochondrial dysfunction after traumatic brain injury (TBI). Intercellular mitochondrial transfer and exogenous mitochondrial transplantation are emerging concepts to understand mitochondrial trafficking in response to mitochondrial dysfunction; however, robust in vivo evidence remains limited on the extent of these processes in the central nervous system (CNS). There is a significant knowledge gap in our understanding of mitochondrial transfer mechanisms under both normal physiological conditions and after experimental TBI. Mouse lines expressing mitochondrial green-fluorescent dendra-2 (mtD2) and GFP (mtGFP) targeted to inner and outer mitochondrial membranes, respectively, were used to study astrocyte-specific (Aldh1l1-CreER; mtD2f/f - AmtD2 and Aldh1l1-CreER; mtGFPf/f - AmtGFP) and neuron-specific (CamK2aCre; mtD2f/f - NmtD2 and CamK2aCre; mtGFPf/f - NmtGFP) mitochondrial dynamics and bioenergetics in acute TBI and excitotoxicity. At 24 hrs following TBI, neurons in the NmtD2 mouse brain exhibited rapid and significant alterations in mitochondrial morphology, including changes in total mitochondrial volume, volume distribution, and sphericity. Synaptic neuronal (SN) mitochondria display robust deficits in mitochondrial bioenergetics and complex protein levels while non-synaptic neuronal (NSN) mitochondria show State III bioenergetics and complex proteins at control levels. These findings are accompanied by a marked increase in astrocyte-derived mitochondria (AmtGFP) transfer to neurons at 24 hrs post-injury, compared to control animals, but no increase in transfer to neuronal synapses. While TBI also altered astrocytic mitochondrial morphology in the cortex, astrocytic mitochondrial bioenergetics remained preserved. Single-cell RNA-seq analysis of astrocytes revealed significant transcriptional reprogramming following TBI, characterized by the upregulation of genes associated with mitochondrial homeostasis and the machinery for organelle trafficking. In vitro co-cultures of primary cortical astrocytes and neurons demonstrated that astrocytes can transfer mitochondria to neurons via direct contact and that NMDA-mediated excitotoxicity further enhanced this astrocyte-to-neuron mitochondrial transfer. Furthermore, astrocytic-derived extracellular vesicles containing mitochondria (EV-mito) deliver mitochondria to neurons and EV-mediated mitochondrial transfer significantly ameliorated NMDA-induced mitochondrial dysfunction in primary cortical neurons. Together, these findings show that astrocytes take on a TBI-related phenotype that facilitates dynamic changes in mitochondrial networks and mitochondrial trafficking to neurons. Astrocytic transfer of respiratory-competent mitochondria support is an intrinsic neuroprotective response to injury that supports mitochondrial function in neuronal soma, dendrites, and axons but not at the neuronal synapse. Finally, we show therapeutic potential of exogenous mitochondrial transfer, particularly via EV-mito, for treating neurological disorders associated with excitotoxicity, such as TBI.