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 a-motor neurons (FaMNs) 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 FaMN 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 FaMN 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

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.The study was supported by the Paul G. Allen Foundation Distinguished Investigator Program (E.M.U. and D.H.R.), the Dr. Miriam and Sheldon G. Adelson Medical Research Foundation (D.H.R., D.G. and G. C.), BRAIN initiative (1U01 MH105991 to D.G.) and National Institute of Health (1R01 MH109912 to D.G.; P01NS08351 to D.H.R.), National Institute of Health Research and the European Union Seventh Framework (to P.H.), NINDS Informatics Center for Neurogenetics and Neurogenomics (P30 NS062691 to G.C.), Wellcome Trust core support (M.H., O.A.B.), European Research Council (281961 to M.G.H.), Fonds Wetenschappelijk Onderzoek (G066715N and 1523014N to M.G.H.), Stichting Alzheimer Onderzoek (S#16025 to M.G.H.) and VIB Institutional Support and Tech Watch funding (to M.G.H.), Howard Hughes Medical Institute and the Wellcome Trust (to D.H.R.)

Modulation de la voie JAK2/STAT3 in vivo : comprendre les caractéristiques fonctionnelles des astrocytes réactifs et leur contribution dans les maladies neurodégénératives.

Astrocyte reactivity is a hallmark of pathological conditions in the CNS including neurodegenerative diseases (ND) such as Alzheimer’s (AD) and Huntington’s (HD) diseases. Reactive astrocytes (RA) are identified by morphological changes but their functional features and influence on neurons are poorly understood, especially in ND. Therefore, we aimed at 1) identifying the signaling cascades involved in astrocyte reactivity in ND, 2) evaluating RA contribution to disease phenotype in ND models and 3) deciphering RA functional features. The JAK2/STAT3 pathway is a known trigger of astrocyte reactivity in CNS injuries. Here, we show that this pathway is a common inducer of astrocyte reactivity in AD and HD models. We developed new viral vectors to target this cascade in astrocytes and manipulate astrocyte reactivity in vivo. We used these vectors to determine the contribution of RA to neuronal dysfunction in HD mouse models. We found that RA do not primarily influence disease phenotype in HD. Last, we targeted the JAK2/STAT3 pathway in WT mice to characterize RA functional features in vivo. We show RA undergo transcriptional changes of numerous genes involved in metabolism, protein degradation pathways and immune response. Moreover, we show that astrocyte reactivity alters synaptic plasticity in the mouse hippocampus. Our results identify the JAK2/STAT3 pathway as a central cascade for astrocyte reactivity. The viral vectors developed in this project represent powerful tools to decipher the roles of RA in various ND models and to characterize RA functional features in vivo. Better understanding RA functions may lead to the identification of new therapeutic targets for ND.Les astrocytes deviennent réactifs dans les maladies neurodégénératives (MND) comme la maladie d’Alzheimer (MA) et de Huntington (MH) mais les conséquences fonctionnelles de cette réactivité sont peu connues. Dans cette étude, nous avons évalué 1) les voies de signalisation impliquées dans la réactivité astrocytaire, 2) la contribution des astrocyte réactifs (AR) à la dysfonction neuronale dans des modèles de MND et 3) les caractéristiques fonctionnelles des AR.Nous avons montré que la voie JAK2/STAT3 est responsable de la réactivité astrocytaire dans des modèles murins de la MA et la MH. Nous avons développé de nouveaux vecteurs viraux ciblant cette voie dans les astrocytes, in vivo. Grâce à ces outils, nous avons étudié la contribution des AR à la dysfonction neuronale dans deux modèles murins de la MH. Nos résultats suggèrent que les AR ne jouent pas un rôle central dans ces modèles de pathologie. En ciblant la voie JAK2/STAT3, nous avons induit la réactivité astrocytaire chez la souris sauvage et avons montré que cette voie régule la transcription de gènes impliqués dans des fonctions cellulaires importantes. De plus, nous avons observé que l’activation des astrocytes conduit à une diminution de la plasticité synaptique dans le cerveau de souris.En conclusion, nous avons montré que la voie JAK2/STAT3 est une voie centrale dans les AR. Nous avons développé des vecteurs viraux innovants pour évaluer 1) la contribution des AR à la dysfonction neuronale dans des modèles de MND et 2) les propriétés fonctionnelles des AR in vivo. L’étude des AR permettra d’identifier de nouvelles cibles moléculaires pour manipuler ces cellules pléiotropes à des fins thérapeutiques

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

International audienceNeurodegenerative diseases (ND) are characterized by primary symptoms such as cognitive or motor deficits. In addition, the presence of neuropsychiatric symptoms (NPS) in ND patients 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 NDassociated NPS. We argue that there is a need for dedicated preclinical studies assessing astrocytebased therapeutic strategies targeting NPS in the context of ND

De nouvelles techniques pour dévoiler le rôle des cellules gliales du cerveau

International audienceBrain 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.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

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

Elusive roles for reactive astrocytes in neurodegenerative diseases

International audienceAstrocytes 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 complex STATes of astrocyte reactivity: How are they controlled by the JAK–STAT3 pathway?

International audienceAstrocytes play multiple important roles in brain physiology. In pathological conditions, they become reac-tive, 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 astro-cyte reactivity. Only then, we will be able to precisely delin-eate the therapeutic potential of reactive astrocytes in each neurological disease context.

Reactive astrocytes promote proteostasis in Huntington's disease

International audienceAstrocytes are essential partners for neurons and their role in Huntington’s disease (HD) is emerging. In HD, astrocytes change and become reactive. Astrocyte reactivity is characterized by morphological changes but its functional impact is still unclear. To understand the roles of reactive astrocytes in HD, we have developed viral vectors that infect selectively astrocytes in vivo and either block or induce reactivity, through manipulation of the JAK2-STAT3 pathway. We used these vectors to modulate astrocyte reactivity in two complementary mouse models of HD [knock-in Hdh140 mice and lentivirus-mediated expression of a fragment of mutated Huntingtin (mHtt) in striatal neurons]. In these two models, we found that reactive astrocytes decrease the number and size of mHtt aggregates. How can reactive astrocytes reduce the aggregation of mHtt within neurons? We performed whole-genome transcriptomic analysis of acutely sorted reactive astrocytes to identify genes regulated by the JAK2-STAT3 pathway in astrocytes. We found an enrichment in genes linked to autophagy-lysosome and ubiquitin-proteasome systems, suggesting that reactive astrocytes have an enhanced capacity for protein degradation and could siphon mHtt away from neurons. Moreover, we identified several chaperones upregulated in reactive astrocytes. Chaperones prevent protein aggregation and can be released extracellularly. They could reduce mHtt aggregation within neurons themselves. Our data show that astrocytes develop a protective response in HD that involves bidirectional signaling with neurons to reduce mHtt aggregation. Reactive astrocytes are not only defective cells as usually reported, but also acquire enhanced capacities to promote mHtt clearance, which has strong therapeutic relevance for HD