Mitochondrial Transfer Into the Cerebrospinal Fluid in the Setting of Subarachnoid Hemorrhage

Mitochondria are dynamic organelles that actively redistribute within tissues to meet changing bioenergetic requirements.1 Recent studies suggest that in the setting of neurological disease, transfer of mitochondria within the central nervous system may represent a key driver of neurological recovery. For example, following transient cerebral ischemia in mice, astrocytic cells release functional mitochondria that subsequently enter neurons. This transfer of mitochondria amplifies cell survival signaling in the neurons and suppression of mitochondrial transfer is associated with worsened neurological outcomes.2 Much remains to be learned about the role of mitochondrial transfer in the setting of human neurological diseases, and this mechanism had not been explored in the setting of subarachnoid hemorrhage (SAH). To begin answering this question, Chou et al3 studied both a rat model and human patients with and without nontraumatic SAH. In the rats they used a well-characterized model of internal carotid artery puncture to induce SAH. They then collected posthemorrhage cerebrospinal fluid (CSF) samples and assessed neurological deficits. They also studied patients with spontaneous nontraumatic SAH who had ventriculostomies placed for clinical reasons. They collected posthemorrhage CSF and measured neurological deficits at the time of presentation and at 3-mo follow-up. They compared rodents with SAH (n = 24) to a control group of rodents without SAH (n = 6), and they compared SAH patients (n = 41) to a control group of humans undergoing lumbar drain trials for normal pressure hydrocephalus (n = 27). Next, they analyzed CSF samples using a variety of techniques: (1) electron microscopy, to visually inspect the extracellular contents; (2) western blot analysis, to study the profile of extracellular proteins; (3) fluorescence-activated cell sorter analysis, to label and identify mitochondria; (4) a unique dye-based analysis (JC-1 dye), to measure the membrane potential of mitochondria; and (5) flow cytometry, to identify specific markers that could reveal the cell-type origin of extracellular mitochondria. In the rodent model they found that extracellular mitochondrial membrane potentials were decreased with SAH and that decreased mitochondrial membrane potentials (ie, membrane depolarization) correlated with worsened neurological deficits (Figure 1).Figure 1.: In the rodent SAH model, extracellular mitochondrial function was associated with neurological outcomes. A, JC1 red indicates intact mitochondria and JC1 green indicates collapsed mitochondria. The JC1 ratio (red/green) is a surrogate marker of extracellular CSF mitochondrial membrane potential, where elevated JC1 ratio corresponds to elevated membrane potential and decreased JC1 ratio corresponds to decreased membrane potential; B, Neuroscore (ie, worsened neurological deficit) was significantly higher in the rodents with SAH than in the sham control group; and C, Regression analysis showed that worsened neurological deficit was significantly correlated with decreased extracellular CSF mitochondrial membrane potential. Reprinted with permission from Chou SH, Lan J, Esposito E, et al, Extracellular Mitochondria in Cerebrospinal Fluid and Neurological Recovery After Subarachnoid Hemorrhage, Stroke, 48, 2231-2237, http://stroke.ahajournals.org/content/48/8/2231.long.In the human patients, in addition to demonstrating the presence of functional extracellular mitochondria in the CSF, they found that membrane potentials were decreased in the SAH cohort and that decreased membrane potentials correlated with worsened neurological deficits (Figure 2). Specifically, worse Hunt and Hess grade at the time of presentation was associated with decreased membrane potentials on post-bleed-day 1, and better modified Rankin Scale outcomes at 3-mo follow-up was associated with recovery of membrane potentials by post-bleed-day 3.Figure 2.: In the human SAH patients extracellular mitochondrial function was associated with clinical outcomes. A and B are beyond the scope of this editorial. Our focus is on C and D. C, Hunt and Hess grade at clinical presentation was associated with decreased membrane potential, and D good clinical outcome (modified Rankin Scale score 0-2 at 3-mo follow-up) was associated with increased extracellular CSF mitochondrial membrane potential. Reprinted with permission from Chou SH, Lan J, Esposito E, et al, Extracellular Mitochondria in Cerebrospinal Fluid and Neurological Recovery After Subarachnoid Hemorrhage, Stroke, 48, 2231-2237, http://stroke.ahajournals.org/content/48/8/2231.long.Finally, flow cytometry analysis suggested that the abundant extracellular mitochondria in the CSF of patients with SAH may be of astrocytic origin as opposed to endothelial or microglial lineage. The findings of this important research study argue for further dedicated investigation into mitochondrial transfer within the central nervous system as a previously unrecognized physiological mechanism of neurological recovery in the setting of SAH. How and why mitochondrial transfer into the extracellular CSF occurs in the setting of SAH remains to be elucidated. Additionally, any causal link between extracellular mitochondrial viability and clinical outcomes remains to be defined. Nevertheless, the authors are to be congratulated for executing this thought-provoking and novel study, which calls attention to new, exciting avenues of investigation.