Astrocytes send out long processes that are terminated by endfeet at the vascular surface and regulate vascular functions as well as homeostasis at the vascular interface. To date, the astroglial mechanisms underlying these functions have been poorly addressed. Here we demonstrate that a subset of messenger RNAs is distributed in astrocyte endfeet. We identified, among this transcriptome, a pool of messenger RNAs bound to ribosomes, the endfeetome, that primarily encodes for secreted and membrane proteins. We detected nascent protein synthesis in astrocyte endfeet. Finally, we determined the presence of smooth and rough endoplasmic reticulum and the Golgi apparatus in astrocyte perivascular processes and endfeet, suggesting for local maturation of membrane and secreted proteins. These results demonstrate for the first time that protein synthesis occurs in astrocyte perivascular distal processes that may sustain their structural and functional polarization at the vascular interface.

Summary Local translation is a conserved mechanism conferring cells the ability to quickly respond to local stimuli. In the brain, it has been recently reported in astrocytes, whose fine processes contact blood vessels and synapses. Yet the specificity and regulation of astrocyte local translation remain unknown. We study hippocampal perisynaptic astrocytic processes (PAPs) and show that they contain the machinery for translation. Using a refined immunoprecipitation technique, we characterize the entire pool of ribosome-bound mRNAs in PAPs and compare it with the one expressed in the whole astrocyte. We find that a specific pool of mRNAs is highly polarized at the synaptic interface. These transcripts encode an unexpected molecular repertoire, composed of proteins involved in iron homeostasis, translation, cell cycle, and cytoskeleton. Remarkably, we observe alterations in global RNA distribution and ribosome-bound status of some PAP-enriched transcripts after fear conditioning, indicating the role of astrocytic local translation in memory and learning.

Astrocytes are the most numerous type of neuroglia in the brain and have a predominant influence on the cerebrovascular system; they control perivascular homeostasis, the integrity of the blood–brain barrier, the dialogue with the peripheral immune system, the transfer of metabolites from the blood, and blood vessel contractility in response to neuronal activity. These regulatory processes occur in a specialized interface composed of perivascular astrocyte extensions that almost completely cover the cerebral blood vessels. Scientists have only recently started to study how this interface is formed and how it influences cerebrovascular functions. Here, we review the literature on the astrocytes' role in the regulation of the cerebrovascular system. We cover the anatomy and development of the gliovascular interface, the known gliovascular functions, and molecular factors, the latter's implication in certain pathophysiological situations, and recent cutting‐edge experimental tools developed to examine the astrocytes' role at the vascular interface. Finally, we highlight some open questions in this field of research.

Astrocytes are morphologically complex and use local translation to regulate distal functions. To study the distribution of mRNA in astrocytes, we combined mRNA detection via in situ hybridization with immunostaining of the astrocyte-specific intermediate filament glial fibrillary acidic protein (GFAP). mRNAs at the level of GFAPimmunolabelled astrocyte somata, and large and fine processes were analysed using AstroDot, an ImageJ plug-in and the R package AstroStat. Taking the characterization of mRNAs encoding GFAP-α and GFAP-δ isoforms as a proof of concept, we showed that they mainly localized on GFAP processes. In the APPswe/PS1dE9 mouse model of Alzheimer's disease, the density and distribution of both α and δ forms of Gfap mRNA changed as a function of the region of the hippocampus and the astrocyte's proximity to amyloid plaques. To validate our method, we confirmed that the ubiquitous Rpl4 (large subunit ribosomal protein 4) mRNA was present in astrocyte processes as well as in microglia processes immunolabelled for ionized calcium binding adaptor molecule 1 (Iba1; also known as IAF1). In summary, this novel set of tools allows the characterization of mRNA distribution in astrocytes and microglia in physiological or pathological settings.

Cells with a complex shape often use mRNA distribution and local translation to regulate distal functions. These mechanisms have recently been described in astrocytes, the processes of which contact and functionally modulate neighbouring synapses and blood vessels. In order to study the distribution of mRNA in astrocytes, we developed a three-dimensional histological method that combines mRNA detection via in situ hybridization with immunostaining of the astrocyte-specific intermediate filament glial fibrillary acidic protein (GFAP). Three-dimensional confocal images were analyzed using AstroDot, a custom Image J plug-in developed in-house for the identification and quantification of mRNAs in GFAP-immunolabelled astrocyte somata, large processes and fine processes. The custom R package AstroStat was used to analyze the AstroDot results. Taking the characterization of mRNAs encoding the astrocyte-specific GFAP a and d isoforms in the hippocampus as a proof of concept, we showed that Gfap a and Gfap d mRNAs mainly colocalized with GFAP in astrocyte processes. Gfap a mRNA was more abundant than Gfap d mRNA, and was predominantly found in fine processes. Upon glial activation in the APPswe/PS1dE9 mouse model of Alzheimer's disease, the same overall patterns were found but we noted strong variations in Gfap a and Gfap d mRNA density and distribution as a function of the part of the hippocampus and the astrocyte's proximity to beta-amyloid (Ab) plaques. In astrocytes not associated with Ab, Gfap a mRNA levels were only slightly elevated, and Gfap d mRNA was distributed within the fine processes; these effects were more prominent in CA3 than in CA1. In contrast, levels of both mRNAs were markedly elevated in the fine processes of Ab-associated astrocytes in both CA1 and CA3. In order to validate our new method, we confirmed that Rpl4 mRNA (a ubiquitously expressed mRNA encoding the large subunit ribosomal protein 4) was present in large and fine processes in both astrocytes and microglia. In summary, we have developed a novel, reliable set of tools for characterizing mRNA densities and distributions in the somata and processes of astrocytes and microglia in physiological or pathological settings. Furthermore, our results suggest that intermediate filaments are crucial for distributing mRNA within astrocytes and for modulating specific Gfap mRNA profiles in Alzheimer's disease.

Astrocytes send out long processes that are terminated by endfeet at the vascular surface and regulate vascular functions in particular through the expression of a specific molecular repertoire in perivascular endfeet. We recently proposed that local translation might sustain this structural and functional polarization. More specifically we showed that a subset of mRNAs is distributed in astrocyte endfeet and characterized this transcriptome. We also identified among these endfeet RNAs, the ones bound to ribosomes, the polysomal astrocyte endfeet mRNAs, which we called the endfeetome. Here, we describe experimental strategies to identify mRNAs and polysomes in astrocyte perivascular endfeet, which are based on the combination of gliovascular unit purification and astrocyte specific translating ribosome affinity purification.

Summary Translation of distally localized mRNAs is an evolutionary mechanism occurring in polarized cells. It has been observed in astrocytes, whose processes contact blood vessels and synapses. Here, we describe a protocol for the purification of the entire pool of ribosome-bound mRNAs in perisynaptic astrocytic processes (PAPs). Our procedure combines the preparation of synaptogliosomes with a refined translating ribosome affinity purification technique. This approach can be used in any brain region to probe the physiological relevance of local translation in PAPs. For complete details on the use and execution of this protocol, please refer to Mazaré et al. (2020) .

Together with the compartmentalization of mRNAs in distal regions of the cytoplasm, local translation constitutes a prominent and evolutionarily conserved mechanism mediating cellular polarization and the regulation of protein delivery in space and time. The translational regulation of gene expression enables a rapid response to stimuli or to a change in the environment, since the use of pre-existing mRNAs can bypass time-consuming nuclear control mechanisms. In the brain, the translation of distally localized mRNAs has been mainly studied in neurons, whose cytoplasmic protrusions may be more than 1000 times longer than the diameter of the cell body. Importantly, alterations in local translation in neurons have been implicated in several neurological diseases. Astrocytes, the most abundant glial cells in the brain, are voluminous, highly ramified cells that project long processes to neurons and brain vessels, and dynamically regulate distal synaptic and vascular functions. Recent research has demonstrated the presence of local translation at these astrocytic interfaces that might regulate the functional compartmentalization of astrocytes. In this Review, we summarize our current knowledge about the localization and local translation of mRNAs in the distal perisynaptic and perivascular processes of astrocytes, and discuss their possible contribution to the molecular and functional polarity of astrocytes.