Reactive astrocytes promote proteostasis in Huntington's disease through the JAK2-STAT3 pathway.

Huntington's disease is a fatal neurodegenerative disease characterized by striatal neurodegeneration, aggregation of mutant Huntingtin and the presence of reactive astrocytes. Astrocytes are important partners for neurons and engage in a specific reactive response in Huntington's disease that involves morphological, molecular and functional changes. How reactive astrocytes contribute to Huntington's disease is still an open question, especially because their reactive state is poorly reproduced in experimental mouse models. Here, we show that the JAK2-STAT3 pathway, a central cascade controlling astrocyte reactive response, is activated in the putamen of Huntington's disease patients. Selective activation of this cascade in astrocytes through viral gene transfer reduces the number and size of mutant Huntingtin aggregates in neurons and improves neuronal defects in two complementary mouse models of Huntington's disease. It also reduces striatal atrophy and increases glutamate levels, two central clinical outcomes measured by non-invasive magnetic resonance imaging. Moreover, astrocyte-specific transcriptomic analysis shows that activation of the JAK2-STAT3 pathway in astrocytes coordinates a transcriptional program that increases their intrinsic proteolytic capacity, through the lysosomal and ubiquitin-proteasome degradation systems. This pathway also enhances their production and exosomal release of the co-chaperone DNAJB1, which contributes to mutant Huntingtin clearance in neurons. Together, our results show that the JAK2-STAT3 pathway controls a beneficial proteostasis response in reactive astrocytes in Huntington's disease, which involves bi-directional signalling with neurons to reduce mutant Huntingtin aggregation, eventually improving disease outcomes.

Functional diversity of astrocytes in neural circuit regulation.

Although it is well established that all brain regions contain various neuronal subtypes with different functions, astrocytes have traditionally been thought to be homogenous. However, recent evidence has shown that astrocytes in the mammalian CNS display distinct inter- and intra-regional features, as well as functional diversity. In the CNS, astrocyte processes fill the local environment in non-overlapping domains. Therefore, a potential advantage of region-specified astrocytes might be their capacity to regulate local development or optimize local neural circuit function. An overview of the regional heterogeneity of neuron-astrocyte interactions indicates novel ways in which they could regulate normal neurological function and shows how they might become dysregulated in disease.

Evidence for glutamine synthetase function in mouse spinal cord oligodendrocytes.

Glutamine synthetase (GS) is a key enzyme that metabolizes glutamate into glutamine. While GS is highly enriched in astrocytes, expression in other glial lineages has been noted. Using a combination of reporter mice and cell type-specific markers, we show that GS is expressed in myelinating oligodendrocytes (OL) but not oligodendrocyte progenitor cells of the mouse and human ventral spinal cord. To investigate the role of GS in mature OL, we used a conditional knockout (cKO) approach to selectively delete GS-encoding gene (Glul) in OL, which caused a significant decrease in glutamine levels on mouse spinal cord extracts. GS cKO mice (CNP-cre+ :Glulfl/fl ) showed no differences in motor neuron numbers, size or axon density; OL differentiation and myelination in the ventral spinal cord was normal up to 6 months of age. Interestingly, GS cKO mice showed a transient and specific decrease in peak force while locomotion and motor coordination remained unaffected. Last, GS expression in OL was increased in chronic pathological conditions in both mouse and humans. We found a disease-stage dependent increase of OL expressing GS in the ventral spinal cord of SOD1(G93A) mouse model of amyotrophic lateral sclerosis. Moreover, we showed that GLUL transcripts levels were increased in OL in leukocortical tissue from multiple sclerosis but not control patients. These findings provide evidence towards OL-encoded GS function in spinal cord sensorimotor axis, which is dysregulated in chronic neurological diseases.

The JAK/STAT3 pathway is a common inducer of astrocyte reactivity in Alzheimer's and Huntington's diseases.

Astrocyte reactivity is a hallmark of neurodegenerative diseases (ND), but its effects on disease outcomes remain highly debated. Elucidation of the signaling cascades inducing reactivity in astrocytes during ND would help characterize the function of these cells and identify novel molecular targets to modulate disease progression. The Janus kinase/signal transducer and activator of transcription 3 (JAK/STAT3) pathway is associated with reactive astrocytes in models of acute injury, but it is unknown whether this pathway is directly responsible for astrocyte reactivity in progressive pathological conditions such as ND. In this study, we examined whether the JAK/STAT3 pathway promotes astrocyte reactivity in several animal models of ND. The JAK/STAT3 pathway was activated in reactive astrocytes in two transgenic mouse models of Alzheimer's disease and in a mouse and a nonhuman primate lentiviral vector-based model of Huntington's disease (HD). To determine whether this cascade was instrumental for astrocyte reactivity, we used a lentiviral vector that specifically targets astrocytes in vivo to overexpress the endogenous inhibitor of the JAK/STAT3 pathway [suppressor of cytokine signaling 3 (SOCS3)]. SOCS3 significantly inhibited this pathway in astrocytes, prevented astrocyte reactivity, and decreased microglial activation in models of both diseases. Inhibition of the JAK/STAT3 pathway within reactive astrocytes also increased the number of huntingtin aggregates, a neuropathological hallmark of HD, but did not influence neuronal death. Our data demonstrate that the JAK/STAT3 pathway is a common mediator of astrocyte reactivity that is highly conserved between disease states, species, and brain regions. This universal signaling cascade represents a potent target to study the role of reactive astrocytes in ND.

Reactive astrocytes overexpress TSPO and are detected by TSPO positron emission tomography imaging.

Astrocytes and microglia become reactive under most brain pathological conditions, making this neuroinflammation process a surrogate marker of neuronal dysfunction. Neuroinflammation is associated with increased levels of translocator protein 18 kDa (TSPO) and binding sites for TSPO ligands. Positron emission tomography (PET) imaging of TSPO is thus commonly used to monitor neuroinflammation in preclinical and clinical studies. It is widely considered that TSPO PET signal reveals reactive microglia, although a few studies suggested a potential contribution of reactive astrocytes. Because astrocytes and microglia play very different roles, it is crucial to determine whether reactive astrocytes can also overexpress TSPO and yield to a detectable TSPO PET signal in vivo. We used a model of selective astrocyte activation through lentiviral gene transfer of the cytokine ciliary neurotrophic factor (CNTF) into the rat striatum, in the absence of neurodegeneration. CNTF induced an extensive activation of astrocytes, which overexpressed GFAP and become hypertrophic, whereas microglia displayed minimal increase in reactive markers. Two TSPO radioligands, [(18)F]DPA-714 [N,N-diethyl-2-(2-(4-(2-[(18)F]fluoroethoxy)phenyl)-5,7-dimethylpyrazolo[1,5-a]pyrimidin-3-yl)acetamide] and [(11)C]SSR180575 (7-chloro-N,N-dimethyl-5-[(11)C]methyl-4-oxo-3-phenyl-3,5-dihydro-4H-pyridazino[4,5-b]indole-1-acetamide), showed a significant binding in the lenti-CNTF-injected striatum that was saturated and displaced by PK11195 [N-methyl-N-(1-methylpropyl)-1-(2-chlorophenyl)-isoquinoline-3-carboxamide]. The volume of radioligand binding matched the GFAP immunopositive volume. TSPO mRNA levels were significantly increased, and TSPO protein was overexpressed by CNTF-activated astrocytes. We show that reactive astrocytes overexpress TSPO, yielding to a significant and selective binding of TSPO radioligands. Therefore, caution must be used when interpreting TSPO PET imaging in animals or patients because reactive astrocytes can contribute to the signal in addition to reactive microglia.

Astrocytes and neuropsychiatric symptoms in neurodegenerative diseases: Exploring the missing links.

Neurodegenerative diseases (NDs) are characterized by primary symptoms, such as cognitive or motor deficits. In addition, the presence of neuropsychiatric symptoms (NPS) in patients with ND is being increasingly acknowledged as an important disease feature. Yet, their neurobiological basis remains unclear and mostly centered on neurons while overlooking astrocytes, which are crucial regulators of neuronal function underlying complex behaviors. In this opinion article, we briefly review evidence for NPS in ND and discuss their experimental assessment in preclinical models. We then present recent studies showing that astrocyte-specific dysfunctions can lead to NPS. Because many astrocyte alterations are also observed in ND, we suggest that they might underlie ND-associated NPS. We argue that there is a need for dedicated preclinical studies assessing astrocyte-based therapeutic strategies targeting NPS in the context of ND.

[New technologies to unveil the role of brain glial cells]. / De nouvelles techniques pour dévoiler le rôle des cellules gliales du cerveau.

Brain function relies on complex interactions between neurons and different types of glial cells, such as astrocytes, microglia and oligodendrocytes. The relatively young field of "gliobiology" is thriving. Thanks to various technical innovations, it is now possible to address challenging biological questions on glial cells and unravel their multiple roles in brain function and dysfunction.

TITLE: De nouvelles techniques pour dévoiler le rôle des cellules gliales du cerveau. ABSTRACT: L'exécution des fonctions cérébrales requiert des interactions optimales entre les neurones et les différents types de cellules gliales (astrocytes, microglies et oligodendrocytes). Le domaine de la gliobiologie, qui s'intéresse aux cellules gliales, est en pleine expansion. Les innovations techniques permettent désormais d'aborder des questions biologiques complexes quant aux rôles de ces cellules dans le fonctionnement physiologique et pathologique du cerveau. Dans cette synthèse, nous décrivons comment certaines de ces avancées techniques nous ont permis d'en apprendre davantage sur les origines et les rôles fonctionnels des cellules gliales. Nous illustrons également comment ces techniques et les découvertes qui en ont découlé, peuvent être transposées en clinique et pourraient, dans un futur proche, offrir des nouvelles perspectives thérapeutiques.

Complex roles for reactive astrocytes in the triple transgenic mouse model of Alzheimer disease.

In Alzheimer disease (AD), astrocytes undergo complex changes and become reactive. The consequences of this reaction are still unclear. To evaluate the net impact of reactive astrocytes in AD, we developed viral vectors targeting astrocytes that either activate or inhibit the Janus kinase-signal transducer and activator of transcription 3 (JAK2-STAT3) pathway, a central cascade controlling astrocyte reaction. We aimed to evaluate whether reactive astrocytes contribute to tau as well as amyloid pathologies in the hippocampus of 3xTg-AD mice, an AD model that develops tau hyper-phosphorylation and amyloid deposition. JAK2-STAT3 pathway-mediated modulation of reactive astrocytes in 25% of the hippocampus of 3xTg-AD mice did not significantly influence tau phosphorylation or amyloid processing and deposition at early, advanced, and terminal disease stage. Interestingly, inhibition of the JAK2-STAT3 pathway in hippocampal astrocytes did not improve spatial memory in the Y maze but it did reduce anxiety in the elevated plus maze. Our unique approach to specifically manipulate reactive astrocytes in situ show they may impact behavioral outcomes without influencing tau or amyloid pathology.

Astrocyte layers in the mammalian cerebral cortex revealed by a single-cell in situ transcriptomic map.

Although the cerebral cortex is organized into six excitatory neuronal layers, it is unclear whether glial cells show distinct layering. In the present study, we developed a high-content pipeline, the large-area spatial transcriptomic (LaST) map, which can quantify single-cell gene expression in situ. Screening 46 candidate genes for astrocyte diversity across the mouse cortex, we identified superficial, mid and deep astrocyte identities in gradient layer patterns that were distinct from those of neurons. Astrocyte layer features, established in the early postnatal cortex, mostly persisted in adult mouse and human cortex. Single-cell RNA sequencing and spatial reconstruction analysis further confirmed the presence of astrocyte layers in the adult cortex. Satb2 and Reeler mutations that shifted neuronal post-mitotic development were sufficient to alter glial layering, indicating an instructive role for neuronal cues. Finally, astrocyte layer patterns diverged between mouse cortical regions. These findings indicate that excitatory neurons and astrocytes are organized into distinct lineage-associated laminae.

Kir4.1-Dependent Astrocyte-Fast Motor Neuron Interactions Are Required for Peak Strength.

Diversified neurons are essential for sensorimotor function, but whether astrocytes become specialized to optimize circuit performance remains unclear. Large fast α-motor neurons (FαMNs) of spinal cord innervate fast-twitch muscles that generate peak strength. We report that ventral horn astrocytes express the inward-rectifying K+ channel Kir4.1 (a.k.a. Kcnj10) around MNs in a VGLUT1-dependent manner. Loss of astrocyte-encoded Kir4.1 selectively altered FαMN size and function and led to reduced peak strength. Overexpression of Kir4.1 in astrocytes was sufficient to increase MN size through activation of the PI3K/mTOR/pS6 pathway. Kir4.1 was downregulated cell autonomously in astrocytes derived from amyotrophic lateral sclerosis (ALS) patients with SOD1 mutation. However, astrocyte Kir4.1 was dispensable for FαMN survival even in the mutant SOD1 background. These findings show that astrocyte Kir4.1 is essential for maintenance of peak strength and suggest that Kir4.1 downregulation might uncouple symptoms of muscle weakness from MN cell death in diseases like ALS.

Modulation of astrocyte reactivity improves functional deficits in mouse models of Alzheimer's disease.

Astrocyte reactivity and neuroinflammation are hallmarks of CNS pathological conditions such as Alzheimer's disease. However, the specific role of reactive astrocytes is still debated. This controversy may stem from the fact that most strategies used to modulate astrocyte reactivity and explore its contribution to disease outcomes have only limited specificity. Moreover, reactive astrocytes are now emerging as heterogeneous cells and all types of astrocyte reactivity may not be controlled efficiently by such strategies.Here, we used cell type-specific approaches in vivo and identified the JAK2-STAT3 pathway, as necessary and sufficient for the induction and maintenance of astrocyte reactivity. Modulation of this cascade by viral gene transfer in mouse astrocytes efficiently controlled several morphological and molecular features of reactivity. Inhibition of this pathway in mouse models of Alzheimer's disease improved three key pathological hallmarks by reducing amyloid deposition, improving spatial learning and restoring synaptic deficits.In conclusion, the JAK2-STAT3 cascade operates as a master regulator of astrocyte reactivity in vivo. Its inhibition offers new therapeutic opportunities for Alzheimer's disease.

Oligodendrocyte-encoded Kir4.1 function is required for axonal integrity.

Glial support is critical for normal axon function and can become dysregulated in white matter (WM) disease. In humans, loss-of-function mutations of KCNJ10, which encodes the inward-rectifying potassium channel KIR4.1, causes seizures and progressive neurological decline. We investigated Kir4.1 functions in oligodendrocytes (OLs) during development, adulthood and after WM injury. We observed that Kir4.1 channels localized to perinodal areas and the inner myelin tongue, suggesting roles in juxta-axonal K+ removal. Conditional knockout (cKO) of OL-Kcnj10 resulted in late onset mitochondrial damage and axonal degeneration. This was accompanied by neuronal loss and neuro-axonal dysfunction in adult OL-Kcnj10 cKO mice as shown by delayed visual evoked potentials, inner retinal thinning and progressive motor deficits. Axon pathologies in OL-Kcnj10 cKO were exacerbated after WM injury in the spinal cord. Our findings point towards a critical role of OL-Kir4.1 for long-term maintenance of axonal function and integrity during adulthood and after WM injury.

The complex STATes of astrocyte reactivity: How are they controlled by the JAK-STAT3 pathway?

Astrocytes play multiple important roles in brain physiology. In pathological conditions, they become reactive, which is characterized by morphological changes and upregulation of intermediate filament proteins. Besides these descriptive hallmarks, astrocyte reactivity involves significant transcriptional and functional changes that are far from being fully understood. Most importantly, astrocyte reactivity seems to encompass multiple states, each having a specific influence on surrounding cells and disease progression. These diverse functional states of reactivity must be regulated by subtle signaling networks. Many signaling cascades have been associated with astrocyte reactivity, but among them, the JAK-STAT3 pathway is emerging as a central regulator. In this review, we aim (i) to show that the JAK-STAT3 pathway plays a key role in the control of astrocyte reactivity, (ii) to illustrate that STAT3 is a pleiotropic molecule operating multiple functions in reactive astrocytes, and (iii) to suggest that each specific functional state of reactivity is governed by complex molecular interactions within astrocytes, which converge on STAT3. More research is needed to precisely identify the signaling networks controlling the diverse states of astrocyte reactivity. Only then, we will be able to precisely delineate the therapeutic potential of reactive astrocytes in each neurological disease context.

Elusive roles for reactive astrocytes in neurodegenerative diseases.

Astrocytes play crucial roles in the brain and are involved in the neuroinflammatory response. They become reactive in response to virtually all pathological situations in the brain such as axotomy, ischemia, infection, and neurodegenerative diseases (ND). Astrocyte reactivity was originally characterized by morphological changes (hypertrophy, remodeling of processes) and the overexpression of the intermediate filament glial fibrillary acidic protein (GFAP). However, it is unclear how the normal supportive functions of astrocytes are altered by their reactive state. In ND, in which neuronal dysfunction and astrocyte reactivity take place over several years or decades, the issue is even more complex and highly debated, with several conflicting reports published recently. In this review, we discuss studies addressing the contribution of reactive astrocytes to ND. We describe the molecular triggers leading to astrocyte reactivity during ND, examine how some key astrocyte functions may be enhanced or altered during the disease process, and discuss how astrocyte reactivity may globally affect ND progression. Finally we will consider the anticipated developments in this important field. With this review, we aim to show that the detailed study of reactive astrocytes may open new perspectives for ND.

The neuroprotective agent CNTF decreases neuronal metabolites in the rat striatum: an in vivo multimodal magnetic resonance imaging study.

Ciliary neurotrophic factor (CNTF) is neuroprotective against multiple pathologic conditions including metabolic impairment, but the mechanisms are still unclear. To delineate CNTF effects on brain energy homeostasis, we performed a multimodal imaging study, combining in vivo proton magnetic resonance spectroscopy, high-performance liquid chromatography analysis, and in situ glutamate imaging by chemical exchange saturation transfer. Unexpectedly, we found that CNTF expression through lentiviral gene transfer in the rat striatum significantly decreased the levels of neuronal metabolites (N-acetyl-aspartate, N-acetyl-aspartyl-glutamate, and glutamate). This preclinical study shows that CNTF remodels brain metabolism, and suggests that decreased levels of neuronal metabolites may occur in the absence of neuronal dysfunction.