Astrocytes have emerged for playing key roles in tissue remodeling during brain repair, however the underlying mechanisms remain poorly understood. We show that acute injury and blood-brain barrier (BBB) disruption trigger the formation of a prominent mitochondrial-enriched compartment in astrocytic end-feet which regulates local metabolic signalling domains. Disruption of this mitochondrial clustering impaired vascular recovery, indicating an important role of astrocytes in tissue repair.

Introduction

By virtue of their intimate specialized connection with neurons and the brain vasculature, astrocytes regulate essential aspects of brain energy metabolism but are also invariably involved in most neurodegenerative and inflammatory disorders.In particular,astrocytes have emerged for playing key roles in tissue remodeling during brain repair, and recent workindicates that they have the capability to strongly influence the extent of axon regeneration following injury. However, to which extent astrocytes can modulate the recovery of the vascular compartment is unclear.

Region-specific remodelling of astrocyte mitochondrial network following brain injury

In this project we addressed the key question whether astrocytes may play an active role in the local structural remodeling of the microvasculature in a model of stab-wound injury. Using a combination of state-of-the-art imaging, genetic and proteomic approaches we investigated with unprecedented sub-cellular resolution the changes in organelle networks experienced by reactive astrocytes in an injury setting in vivo, whether these may have any role in regulating tissue repair and if so, what are the underlying mechanisms. We found that the remodelling of the mitochondrial network in astrocytes reacting to injury and BBB disruption is part of a metabolic adaptive response that culminates in the generation of a spatially-defined mitochondrial-enriched domain in perivascular end-feet, and which is ultimately required for vascular repair. In particular, we identified this metabolic program to be regulated by mitochondrial fusion and mitochondrial tethering to ER membranes, the latter being particularly abundant in astrocytic end-feet (Fig. 1A-D).

Mitochondrial fusion dynamics regulates astrocyte mitochondrial remodelling and vascular repair after injury 

Astrocyte-specific deletion of the outer mitochondrial membrane GTPase mitofusin 2 (Mfn2), which regulates mitochondrial fusion dynamics, prevented injury-induced perivascular accumulation of mitochondria, altered the extent of mitochondria-ER tethering and led to disrupted mitochondrial Ca²⁺ uptake in astrocyte end-feet, ultimately impairing the recovery of the vascular network in the injured area (Fig. 1E-H). Importantly, by delivering astrocyte-specific adeno-associated viruses (AAVs) encoding for synthetic linkers that specifically anchor mitochondria to ER membranes, thereby enhancing perivascular accumulation of astrocytic mitochondria, we could demonstrate that vascular repair can be restored in absence of mitochondrial fusion. These results establish a new mechanism for astrocytic mitochondrial fusion in orchestrating the metabolic adaptations of brain tissue in vivo and unravel a key role for astrocytes in sustaining microvasculature remodelling during repair.

Perspectives

While this study reveals the essential role of a “metabolic” reactive state in astrocytes in regulating the functional recovery of damaged brain vasculature, with obvious key implications for a number of neurological diseases characterized by vascular conditions, it also raises a number of important questions. In particular, as complimen-tary experiments indicate that the effects observed here do not primarily depend upon mitochondrial oxidative phosphorylation (OXPHOS), the specific mechanisms by which a local accumulation of astrocytic mitochondria around vessels may facilitate vascular repair remain to be identified. For example, is there a local gradient of signalling molecules or metabolites which fuel the formation of new vessels, and what exactly this signalling may be that depends from a local bulk accumulation of mitochondria? How do changes in the reactive state of perivascular astrocytes translate into metabolic changes at the level of the whole vascular unit? Future work will address these specific questions.

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2. Göbel, J., et al.(2018). Spatiotemporal control of mitochondrial network dynamics in astroglial cells. Biochem Biophys Res Commun 500, 17-25.

3. Motori, E., et al.(2013). Inflammation-induced alteration of astrocyte mitochondrial dynamics requires autophagy for mitochondrial network maintenance. Cell Metab 18, 844-859.