Astrocyte power fuels neurons during stroke.

In a paper published in Nature July 2016, a group of investigators coordinated by the Harvard neuroscientist Eng H. Lo set out to investigated the role of the intercellular transfer of mitochondria from astrocytes to neurons following ischemic stroke [1]. Mitochondria are intracellular organelles involved in several cellular processes, including oxidative phosphorylation, metabolism of amino acids, lipids, and steroids, and regulating multiple intracellular signalling cascades, such as cell-cycle, antiviral responses and cell death [2]. Mitochondria are also emerging players in intercellular communications. For instance, the generation of mitochondria-derived vesicles (MDVs) – a subset of small vesicular carriers that transport mitochondrial proteins and lipids to other intracellular organelles – drives the presentation of mitochondrial antigens on MHC class I molecules in immune cells, thus eliciting a local inflammatory response [3]. Similarly, the release of mitochondrial DNA or proteins in the extracellular space is highly immunogenic and represents a danger signal that activates leucocytes [4]. Intact mitochondria can also be exchanged between cells, with important biological and functional implications. Donor cells transfer mitochondria to target cells via gap junctions [5], or via a Rho GTPase-dependent transport through nanometer wide tubular extensions called tunnelling nanotubes that connect adjacent cells [6]. Another described mechanism of intercellular mitochondrial transfer is via the release of free organelles or their inclusion in extracellular membrane vesicles (EVs) [7]. The horizontal transfer of mitochondria is able to rescue impaired aerobic respiration, inhibit apoptosis, increase cell survival and proliferation, induce chemoresistance, and enhance phagocytic activity. Intercellular mitochondria exchange is multidirectional and associated with target cell-specific outcomes. For example, mitochondria derived from mesenchymal stem cells (MSC) promote partial de-differentiation and protection against ischaemia/reperfusion injury in target cardiomyocytes, and cardiomyocyte-derived mitochondria induce the expression of cardio-specific proteins in target MSCs. Despite all this previously published evidence, the mechanisms regulating the intercellular transfer of mitochondria, its environmental triggers and its relevance in re-establishing tissue homeostasis after injury are yet to be fully understood. Using a combination of electron microscopy, fluorescenceactivated cell sorting (FACS) and tunable resistive pulse sensing (TRPS) analysis, Hayakawa and colleagues showed that the conditioned medium from astrocytes contains structurally intact mitochondrial particles capable of producing ATP and consuming O2, while concurrently conserving a functional membrane potential [1]. To investigate whether astrocyte-derived mitochondria play a role in re-establishing tissue homeostasis after an ischaemic injury, the authors used the in vitro oxygen/ glucose deprivation (OGD) model of hypoxia, which impairs mitochondrial function in neurons. Under these experimental conditions, astrocyte-derived mitochondria were taken up by neurons subjected to OGD, increasing their viability and aerobic respiration, and dendrite length (fig. 1). This rescue effect was abolished when the mitochondrial particles were filtered from the conditioned medium, made dysfunctional via inhibition of the mitochondrial form of aconitase (which plays a key role in carbohydrate metabolism), or when mitochondria-free ATPliposomes were delivered to OGD neurons. To assess the mechanisms of mitochondria secretion by astrocytes the authors focused on the expression of CD38. CD38 catalyses the synthesis of cyclic ADP-ribose (cADPR) in mitochondrial membranes, increases in astrocytes in response to glutamate release from neurons [1] and has been implicated in neuroglial crosstalk. Of note, overexpression of CD38 in astrocytes led to a significant increase in cyclase activity, extracellular ATP and O2 consumption in secreted mitochondrial particles [1]. Importantly, the rescue effect of astrocyte-derived mitochondria on OGD neurons was also impaired when astrocytes were pre-treated with a small interfering RNA (siRNA) to silence Cd38 [1]. Some of the most provocative data from this paper arise from the in vivo analysis of mitochondrial transfer. Injecting FACS-sorted astrocyte-derived mitochondrial particles