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Astrocytes tightly control neuronal connectivity and function in the brain through direct contact with synapses1–5. These glial cells become reactive during disease pathogenesis6–8including Parkinson’s disease (PD)9–14. However, it remains unknown if astrocyte dysfunction is an initiating factor of PD pathogenesis and whether astrocytes can be targeted to stop or reverse the synaptic dysfunction seen in PD. Usingin vitroandin vivomethods, we found that the PD-linked gene Lrrk2 controls astrocyte morphology via regulating the phosphorylation of ERM proteins (Ezrin, Radixin, and Moesin), a structural component of the perisynaptic astrocyte processes. ERM phosphorylation is robustly elevated both in mice and humans carrying the LRRK2 G2019S Parkinsonism mutation. Importantly, the reduction of the ERM phosphorylation, specifically in the LRRK2 G2019S in adult astrocytes, is sufficient to restore excitatory synapse number and function deficits in the LRRK2 G2019S knock-in mouse cortex. These results show a role for Lrrk2 in controlling astrocyte morphogenesis and synaptogenic function and reveal that early astrocyte dysfunction in PD could be causal to disruptions in cortical excitatory synaptic connectivity. The astrocytic dysfunction can be corrected by dampening ERM phosphorylation, pinpointing astrocytes as critical cellular targets for PD therapeutics.

SUMMARYAstrocytes strongly promote the formation and maturation of synapses by secreted proteins. To date, several astrocyte-secreted synaptogenic proteins controlling different stages of excitatory synapse development have been identified. However, the identities of astrocytic signals that induce inhibitory synapse formation remain elusive. Here, through a combination ofin vitroandin vivoexperiments, we identified Neurocan as an astrocyte-secreted inhibitory synaptogenic protein. Neurocan is a chondroitin sulfate proteoglycan that is best known as a protein localized to the perineuronal nets. However, Neurocan is cleaved into two after secretion from astrocytes. We found that the resulting N- and C-terminal fragments have distinct localizations in the extracellular matrix. While the N-terminal fragment remains associated with perineuronal nets, the Neurocan C-terminal fragment localizes to synapses and specifically controls cortical inhibitory synapse formation and function. Neurocan knockout mice lacking the whole protein or only its C-terminal synaptogenic region have reduced inhibitory synapse numbers and function. Through super-resolution microscopy andin vivoproximity labeling by secreted TurboID, we discovered that the synaptogenic domain of Neurocan localizes to somatostatin-positive inhibitory synapses and strongly regulates their formation. Together, our results unveil a mechanism through which astrocytes control circuit-specific inhibitory synapse development in the mammalian brain.

Within eukaryotic cells, translation is regulated independent of transcription, enabling nuanced, localized, and rapid responses to stimuli. Neurons respond transcriptionally and translationally to synaptic activity. Although transcriptional responses are documented in astrocytes, here we test whether astrocytes have programmed translational responses. We show that seizure activity rapidly changes the transcripts on astrocyte ribosomes, some predicted to be downstream of BDNF signaling. In acute slices, we quantify the extent to which cues of neuronal activity activate translation in astrocytes and show that this translational response requires the presence of neurons, indicating that the response is non-cell autonomous. We also show that this induction of new translation extends into the periphery of astrocytes. Finally, synaptic proteomics show that new translation is required for changes that occur in perisynaptic astrocyte protein composition after fear conditioning. Regulation of translation in astrocytes by neuronal activity suggests an additional mechanism by which astrocytes may dynamically modulate nervous system functioning.

SummarySynaptogenesis is essential for circuit development; however, it is unknown whether it is critical for the establishment and performance of goal-directed voluntary behaviors. Here, we show that operant-conditioning via lever-press for food reward-training in mice induces excitatory synapse formation onto a subset of Anterior Cingulate Cortex neurons projecting to the dorsomedial striatum (ACC->DMS). Training-induced synaptogenesis is controlled by the Gabapentin/Thrombospondin receptor α2δ-1, which is an essential neuronal protein for proper intracortical excitatory synaptogenesis. Using germline and conditional knockout mice, we found that deletion of α2δ-1 in the adult ACC->DMScircuit diminishes training-induced excitatory synaptogenesis. Surprisingly, this manipulation did not impact learning but instead resulted in a profound increase in effort exertion without affecting sensitivity to reward value or changing contingencies. Bidirectional optogenetic manipulation of ACC->DMSneurons rescued or phenocopied the behaviors of the α2δ-1 cKO mice highlighting the importance of synaptogenesis within this cortico-striatal circuit in regulating effort exertion.

Gene expression requires two steps – transcription and translation – which can be regulated independently to allow nuanced, localized, and rapid responses to cellular stimuli. Neurons are known to respond transcriptionally and translationally to bursts of brain activity, and a transcriptional response to this activation has also been recently characterized in astrocytes. However, the extent to which astrocytes respond translationally is unknown. We tested the hypothesis that astrocytes also have a programmed translational response by characterizing the change in transcript ribosome occupancy in astrocytes using Translating Ribosome Affinity Purification(TRAP) subsequent to a robust induction of neuronal activity in vivo via acute seizure. We identified a change in transcripts on astrocyte ribosomes, highlighted by a rapid decrease in transcripts coding for ribosomal and mitochondrial components, and a rapid increase in transcripts related to cytoskeletal dynamics, motor activity, ion transport, and cell communication. This indicates a set of dynamic responses, some of which might be secondary to activation of Receptor Tyrosine Kinase(TRK) signaling. Using acute slices, we quantified the extent to which individual cues and sequela of neuronal activity can activate translation acutely in astrocytes. We identified both BDNF and ion concentration changes as contributors to translation induction, with potassium using both action-potential sensitive and insensitive components. We showed this translational response requires the presence of neurons, indicating the response is non-cell autonomous. We also show that this induction of new translation extends into peripheral astrocyte processes (PAPs). Accordingly, proteomics following fear conditioning in mice, showed that new translation influences peri-synaptic astrocyte protein composition in vivo under physiological conditions. Regulation of translation in astrocytes by neuronal activity suggests an additional mechanism by which astrocytes may dynamically modulate nervous system functioning.Main PointsAstrocytes have a programmed, transcript-specific translational response to neuronal activity.Both BDNF and K+, cues of neuronal activity, trigger this response.This response requires the presence of neurons.This response alters the astrocytic protein composition at the synapse.

Central nervous system injury often leads to functional impairment due, in part, to the formation of an inhibitory glial scar following injury that contributes to poor regeneration. Astrocytes are the major cellular components of the glial scar, which has led to the belief that they are primarily inhibitory following injury. Recent work has challenged this by demonstrating that some astrocytes are required for spinal cord regeneration and astrocytic roles in recovery depend on their phenotype. In this work, two mixed populations containing primarily either fibrous or protoplasmic astrocytes were derived from mouse embryonic stem cells (mESCs). Motoneuron and V2a interneuron growth on live cultures, freeze-lysed cultures, or decellularized extracellular matrix (ECM) from astrocytes were assessed. Both neuronal populations were found to extend significantly longer neurites on protoplasmic-derived substrates than fibrous-derived substrates. Interestingly, neurons extended longer neurites on protoplasmic-derived ECM than fibrous-derived ECM. ECM proteins were compared with in vivo astrocyte expression profiles, and it was found that the ESC-derived ECMs were enriched for astrocyte-specific proteins. Further characterization revealed that protoplasmic ECM had significantly higher levels of axon growth promoting proteins, while fibrous ECM had significantly higher levels of proteins that inhibit axon growth. Supporting this observation, knockdown of spondin-1 improved neurite growth on fibrous ECM, while laminin α5 and γ1 knockdown decreased neurite growth on protoplasmic ECM. These methods allow for scalable production of specific astrocyte subtype-containing populations with different neuronal growth support capacities, and can be used for further studies of the functional importance of astrocyte heterogeneity.

Quaking RNA binding protein(QKI) is essential for oligodendrocyte development as myelination requires MBP mRNA regulation and localization by the cytoplasmic isoforms(e.g. QKI-6). QKI-6 is also highly expressed in astrocytes, which were recently demonstrated to have regulated mRNA localization. Here, we show via CLIPseq that QKI-6 binds 3’ UTRs of a subset of astrocytic mRNAs, including many enriched in peripheral processes. Binding is enriched near stop codons, which is mediated partially by QKI binding motifs(QBMs) yet spreads to adjacent sequences. We developed CRISPR TRAPseq: a viral approach for mosaic, cell-type specific gene mutation with simultaneous translational profiling. This enabled study of QKI-deleted astrocytes in an otherwise normal brain. Astrocyte-targeted QKI deletion altered translation and maturation, while also increasing synaptic density within the astrocyte’s territory. Overall, our data indicate QKI is required for astrocyte maturation and demonstrate an approach for a highly targeted translational assessment of gene knockout in specific cell-types in vivo.

Nociceptin and its receptor are widely distributed throughout the brain in regions associated with reward behavior, yet how and when they act is unknown. Here we dissected the role of a nociceptin peptide circuit in reward seeking. We generated a prepronociceptin (Pnoc)-Cre mouse line that revealed a unique subpopulation of paranigral ventral tegmental area (pnVTA) neurons enriched in prepronociceptin. Fiber photometry recordings during progressive ratio operant behavior revealed pnVTA(Pnoc) neurons become most active when mice stop seeking natural rewards. Selective pnVTA(Pnoc) neuron ablation, inhibition and conditional VTA nociceptin receptor (NOPR) deletion increased operant responding, revealing that the pnVTA(Pnoc) nucleus and VTA NOPR signaling are necessary for regulating reward motivation. Additionally, optogenetic and chemogenetic activation of this pnVTA(Pnoc) nucleus caused avoidance and decreased motivation for rewards. These findings provide insight into neuromodulatory circuits that regulate motivated behaviors through identification of a previously unknown neuropeptide-containing pnVTA nucleus that limits motivation for rewards.

Background Studies in psychiatric genetics have identified >100 loci associated with disease risk, yet many of these loci are distant from protein coding genes. Recent characterization of the transcriptional landscape of cell lines and whole tissues has suggested widespread transcription in both coding and noncoding regions of the genome, including differential expression from loci that produce regulatory noncoding RNAs that function within the nucleus; however, the nuclear transcriptome of specific cell types in the brain has not been previously investigated. Methods We defined the nuclear transcriptional landscape of the three major cellular divisions of the nervous system using flow sorting of genetically labeled nuclei from bacTRAP mouse lines. Next, we characterized the unique expression of coding, noncoding, and intergenic RNAs in the mature mouse brain with RNA-Seq and validation with independent methods. Results We found diverse expression across the cell types of all classes of RNAs, including long noncoding RNAs, several of which were confirmed as highly enriched in the nuclei of specific cell types using anatomic methods. We also discovered several examples of cell type–specific expression of tandem gene fusions, and we report the first cell type–specific expression of circular RNAs—a neuron-specific and nuclear-enriched RNA arising from the gene Hnrnpu . Conclusions These data provide an important resource for studies evaluating the function of various noncoding RNAs in the brain, including noncoding RNAs that may play a role in psychiatric disease.

Neuroligins are a family of cell adhesion molecules, which are best known for their functions as postsynaptic components of the trans-synaptic neurexin-neuroligin complexes. Neuroligins are highly conserved across evolution with important roles in the formation, maturation and function of synaptic structures. Mutations in the genes that encode for neuroligins have been linked to a number of neurodevelopmental disorders such as autism and schizophrenia, which stem from synaptic pathologies. Owing to their essential functions in regulating synaptic connectivity and their link to synaptic dysfunction in disease, previous studies on neuroligins have focused on neurons. Yet a recent work reveals that neuroligins are also expressed in the central nervous system by glial cells, such as astrocytes and oligodendrocytes, and perform important roles in controlling synaptic connectivity in a non-cell autonomous manner. In this review, we will highlight these recent findings demonstrating the important roles of glial neuroligins in regulating the development and connectivity of healthy and diseased brains.

Local translation in neuronal processes is key to the alteration of synaptic strength that contributes to long term potentiation and learning and memory. Here, we present evidence that astrocytes have ribosomes in their peripheral and perisynaptic processes, and that de novo protein synthesis occurs in the astrocyte periphery. We also developed a new biochemical approach to profile and define a set of candidate transcripts that are locally translated in astrocyte processes, several of which were validated in vivo using in situ hybridization of sparsely labeled cells. Computational analyses indicate that localized translation is both sequence dependent and enriched for particular biological functions. This includes novel pathways such as fatty acid synthesis as well as pathways consistent with known roles for astrocyte processes, such as GABA and glutamate metabolism. Finally, enriched transcripts also include key glial regulators of synaptic refinement, suggesting that local production of astrocyte proteins may support microscale alterations of adjacent synapses.Significance StatementCellular compartments are specialized for particular functions. In astrocytes, the peripheral processes, particularly near synapses, contain proteins specialized for reuptake of neurotransmitters and ions, and have been shown to alter their morphology in response to activity. Regulated transport of a specific subset of nuclear-derived mRNAs to particular compartments is thought to support the specialization of these compartments and allow for local regulation of translation. In neurons, local translation near activated synapses is thought to generate the proteins needed for the synaptic alterations that constitute memory. We demonstrate that astrocytes also have sequence-dependent local translation in their peripheral processes, including transcripts with roles in regulating synapses. This suggests local translation in astrocyte processes may also play a role in modulating synapses.

Precise spatiotemporal control of mRNA translation machinery is essential to the development of highly complex systems like the neocortex. However, spatiotemporal regulation of translation machinery in the developing neocortex remains poorly understood. Here, we show that an RNA- binding protein, Hu antigen R (HuR), regulates both neocorticogenesis and specificity of neocortical translation machinery in a developmental stage-dependent manner in mice. Neocortical absence of HuR alters the phosphorylation states of initiation and elongation factors in the core translation machinery. In addition, HuR regulates the temporally specific positioning of functionally related mRNAs into the active translation sites, the polysomes. HuR also determines the specificity of neocortical polysomes by defining their combinatorial composition of ribosomal proteins and initiation and elongation factors. For some HuR-dependent proteins, the association with polysomes likewise depends on the eukaryotic initiation factor 2 alpha kinase 4, which associates with HuR in prenatal developing neocortices. Finally, we found that deletion of HuR before embryonic day 10 disrupts both neocortical lamination and formation of the main neocortical commissure, the corpus callosum. Our study identifies a crucial role for HuR in neocortical development as a translational gatekeeper for functionally related mRNA subgroups and polysomal protein specificity.