Connexin 43 in astrocytes contributes to motor neuron toxicity in amyotrophic lateral sclerosis

Perspective and Therapeutic Potential of the Noncoding RNA-Connexin Axis

Noncoding RNAs (ncRNAs) are a class of nucleotide sequences that cannot be translated into peptides. ncRNAs can function post-transcriptionally by splicing complementary sequences of mRNAs or other ncRNAs or by directly engaging in protein interactions. Over the past few decades, the pervasiveness of ncRNAs in cell physiology and their pivotal roles in various diseases have been identified. One target regulated by ncRNAs is connexin (Cx), a protein that forms gap junctions and hemichannels and facilitates intercellular molecule exchange. The aberrant expression and misdistribution of connexins have been implicated in central nervous system diseases, cardiovascular diseases, bone diseases, and cancer. Current databases and technologies have enabled researchers to identify the direct or indirect relationships between ncRNAs and connexins, thereby elucidating their correlation with diseases. In this review, we selected the literature published in the past five years concerning disorders regulated by ncRNAs via corresponding connexins. Among it, microRNAs that regulate the expression of Cx43 play a crucial role in disease development and are predominantly reviewed. The distinctive perspective of the ncRNA-Cx axis interprets pathology in an epigenetic manner and is expected to motivate research for the development of biomarkers and therapeutics.

Astroglial connexin 43 is a novel therapeutic target for chronic multiple sclerosis model

In chronic stages of multiple sclerosis (MS) and its animal model, experimental autoimmune encephalitis (EAE), connexin (Cx)43 gap junction channel proteins are overexpressed because of astrogliosis. To elucidate the role of increased Cx43, the central nervous system (CNS)-permeable Cx blocker INI-0602 was therapeutically administered. C57BL6 mice with chronic EAE initiated by MOG35-55 received INI-0602 (40 mg/kg) or saline intraperitoneally every other day from days post-immunization (dpi) 17-50. Primary astroglia were employed to observe calcein efflux responses. In INI-0602-treated mice, EAE clinical signs improved significantly in the chronic phase, with reduced demyelination and decreased CD3+ T cells, Iba-1+ and F4/80+ microglia/macrophages, and C3+GFAP+ reactive astroglia infiltration in spinal cord lesions. Flow cytometry analysis of CD4+ T cells from CNS tissues revealed significantly reduced Th17 and Th17/Th1 cells (dpi 24) and Th1 cells (dpi 50). Multiplex array of cerebrospinal fluid showed significantly suppressed IL-6 and significantly increased IL-10 on dpi 24 in INI-0602-treated mice, and significantly suppressed IFN-γ and MCP-1 on dpi 50 in the same group. In vitro INI-0602 treatment inhibited ATP-induced calcium propagations of Cx43+/+ astroglial cells to similar levels of those of Cx43-/- cells. Astroglial Cx43 hemichannels represent a novel therapeutic target for chronic EAE and MS.

Proteomic profiling of interferon-responsive reactive astrocytes in rodent and human

Astrocytes are a heterogeneous population of central nervous system glial cells that respond to pathological insults and injury by undergoing a transformation called "reactivity." Reactive astrocytes exhibit distinct and context-dependent cellular, molecular, and functional state changes that can either support or disturb tissue homeostasis. We recently identified a reactive astrocyte sub-state defined by interferon-responsive genes like Igtp, Ifit3, Mx1, and others, called interferon-responsive reactive astrocytes (IRRAs). To further this transcriptomic definition of IRRAs, we wanted to define the proteomic changes that occur in this reactive sub-state. We induced IRRAs in immunopanned rodent astrocytes and human iPSC-differentiated astrocytes using TNF, IL1α, C1Q, and IFNβ and characterized their proteomic profile (both cellular and secreted) using unbiased quantitative proteomics. We identified 2335 unique cellular proteins, including IFIT2/3, IFITM3, OASL1/2, MX1/2/3, and STAT1. We also report that rodent and human IRRAs secrete PAI1, a serine protease inhibitor which may influence reactive states and functions of nearby cells. Finally, we evaluated how IRRAs are distinct from neurotoxic reactive astrocytes (NRAs). While NRAs are described by expression of the complement protein C3, it was not upregulated in IRRAs. Instead, we found ~90 proteins unique to IRRAs not identified in NRAs, including OAS1A, IFIT3, and MX1. Interferon signaling in astrocytes is critical for the antiviral immune response and for regulating synaptic plasticity and glutamate transport mechanisms. How IRRAs contribute to these functions is unknown. This study provides the basis for future experiments to define the functional roles of IRRAs in the context of neurodegenerative disorders.

Astrocytes Drive Divergent Metabolic Gene Expression in Humans and Chimpanzees

The human brain utilizes ∼20% of all of the body's metabolic resources, while chimpanzee brains use <10%. Although previous work shows significant differences in metabolic gene expression between the brains of primates, we have yet to fully resolve the contribution of distinct brain cell types. To investigate cell type-specific interspecies differences in brain gene expression, we conducted RNA-seq on neural progenitor cells, neurons, and astrocytes generated from induced pluripotent stem cells from humans and chimpanzees. Interspecies differential expression analyses revealed that twice as many genes exhibit differential expression in astrocytes (12.2% of all genes expressed) than neurons (5.8%). Pathway enrichment analyses determined that astrocytes, rather than neurons, diverged in expression of glucose and lactate transmembrane transport, as well as pyruvate processing and oxidative phosphorylation. These findings suggest that astrocytes may have contributed significantly to the evolution of greater brain glucose metabolism with proximity to humans.

Astrocyte-Neuron Interactions Contributing to Amyotrophic Lateral Sclerosis Progression

Amyotrophic lateral sclerosis (ALS) is a complex disease impacting motor neurons of the brain, brainstem, and spinal cord. Disease etiology is quite heterogeneous with over 40 genes causing the disease and a vast ~90% of patients having no prior family history. Astrocytes are major contributors to ALS, particularly through involvement in accelerating disease progression. Through study of genetic forms of disease including SOD1, TDP43, FUS, C9orf72, VCP, TBK1, and more recently patient-derived cells from sporadic individuals, many biological mechanisms have been identified to cause intrinsic or glial-mediated neurotoxicity to motor neurons. Overall, many of the normally supportive and beneficial roles that astrocytes contribute to neuronal health and survival instead switch to become deleterious and neurotoxic. While the exact pathways may differ based on disease-origin, altered astrocyte-neuron communication is a common feature of ALS. Within this chapter, distinct genetic forms are examined in detail, along with what is known from sporadic patient-derived cells. Overall, this chapter highlights the interplay between astrocytes and neurons in this complex disease and describes the key features underlying: astrocyte-mediated motor neuron toxicity, excitotoxicity, oxidative/nitrosative stress, protein dyshomeostasis, metabolic imbalance, inflammation, trophic factor withdrawal, blood-brain/blood-spinal cord barrier involvement, disease spreading, and the extracellular matrix/cell adhesion/TGF-β signaling pathways.

Genetically encoded barcodes for correlative volume electron microscopy

While genetically encoded reporters are common for fluorescence microscopy, equivalent multiplexable gene reporters for electron microscopy (EM) are still scarce. Here, by installing a variable number of fixation-stable metal-interacting moieties in the lumen of encapsulin nanocompartments of different sizes, we developed a suite of spherically symmetric and concentric barcodes (EMcapsulins) that are readable by standard EM techniques. Six classes of EMcapsulins could be automatically segmented and differentiated. The coding capacity was further increased by arranging several EMcapsulins into distinct patterns via a set of rigid spacers of variable length. Fluorescent EMcapsulins were expressed to monitor subcellular structures in light and EM. Neuronal expression in Drosophila and mouse brains enabled the automatic identification of genetically defined cells in EM. EMcapsulins are compatible with transmission EM, scanning EM and focused ion beam scanning EM. The expandable palette of genetically controlled EM-readable barcodes can augment anatomical EM images with multiplexed gene expression maps.

Connexins Control Glial Inflammation in Various Neurological Diseases

Connexins (Cxs) form gap junctions through homotypic/heterotypic oligomerization. Cxs are initially synthesized in the endoplasmic reticulum, then assembled as hexamers in the Golgi apparatus before being integrated into the cell membrane as hemichannels. These hemichannels remain closed until they combine to create gap junctions, directly connecting neighboring cells. Changes in the intracellular or extracellular environment are believed to trigger the opening of hemichannels, creating a passage between the inside and outside of the cell. The size of the channel pore depends on the Cx isoform and cellular context-specific effects such as posttranslational modifications. Hemichannels allow various bioactive molecules, under ~1 kDa, to move in and out of the host cell in the direction of the electrochemical gradient. In this review, we explore the fundamental roles of Cxs and their clinical implications in various neurological dysfunctions, including hereditary diseases, ischemic brain disorders, degenerative conditions, demyelinating disorders, and psychiatric illnesses. The influence of Cxs on the pathomechanisms of different neurological disorders varies depending on the circumstances. Hemichannels are hypothesized to contribute to proinflammatory effects by releasing ATP, adenosine, glutamate, and other bioactive molecules, leading to neuroglial inflammation. Modulating Cxs' hemichannels has emerged as a promising therapeutic approach.

The Key Role of Astrocytes in Amyotrophic Lateral Sclerosis and Their Commitment to Glutamate Excitotoxicity

In the last two decades, there has been increasing evidence supporting non-neuronal cells as active contributors to neurodegenerative disorders. Among glial cells, astrocytes play a pivotal role in driving amyotrophic lateral sclerosis (ALS) progression, leading the scientific community to focus on the "astrocytic signature" in ALS. Here, we summarized the main pathological mechanisms characterizing astrocyte contribution to MN damage and ALS progression, such as neuroinflammation, mitochondrial dysfunction, oxidative stress, energy metabolism impairment, miRNAs and extracellular vesicles contribution, autophagy dysfunction, protein misfolding, and altered neurotrophic factor release. Since glutamate excitotoxicity is one of the most relevant ALS features, we focused on the specific contribution of ALS astrocytes in this aspect, highlighting the known or potential molecular mechanisms by which astrocytes participate in increasing the extracellular glutamate level in ALS and, conversely, undergo the toxic effect of the excessive glutamate. In this scenario, astrocytes can behave as "producers" and "targets" of the high extracellular glutamate levels, going through changes that can affect themselves and, in turn, the neuronal and non-neuronal surrounding cells, thus actively impacting the ALS course. Moreover, this review aims to point out knowledge gaps that deserve further investigation.

Spatiotemporal molecular dynamics of the developing human thalamus

The thalamus plays a central coordinating role in the brain. Thalamic neurons are organized into spatially distinct nuclei, but the molecular architecture of thalamic development is poorly understood, especially in humans. To begin to delineate the molecular trajectories of cell fate specification and organization in the developing human thalamus, we used single-cell and multiplexed spatial transcriptomics. We show that molecularly defined thalamic neurons differentiate in the second trimester of human development and that these neurons organize into spatially and molecularly distinct nuclei. We identified major subtypes of glutamatergic neuron subtypes that are differentially enriched in anatomically distinct nuclei and six subtypes of γ-aminobutyric acid-mediated (GABAergic) neurons that are shared and distinct across thalamic nuclei.

Spatiotemporal molecular dynamics of the developing human thalamus

The thalamus plays a central coordinating role in the brain. Thalamic neurons are organized into spatially-distinct nuclei, but the molecular architecture of thalamic development is poorly understood, especially in humans. To begin to delineate the molecular trajectories of cell fate specification and organization in the developing human thalamus, we used single cell and multiplexed spatial transcriptomics. Here we show that molecularly-defined thalamic neurons differentiate in the second trimester of human development, and that these neurons organize into spatially and molecularly distinct nuclei. We identify major subtypes of glutamatergic neuron subtypes that are differentially enriched in anatomically distinct nuclei. In addition, we identify six subtypes of GABAergic neurons that are shared and distinct across thalamic nuclei.

One-Sentence Summary

Single cell and spatial profiling of the developing thalamus in the first and second trimester yields molecular mechanisms of thalamic nuclei development.

Gap junctions regulate the development of neural circuits in the neocortex

Gap junctions between cells are ubiquitously expressed in the developing brain. They are involved in major steps of neocortical development, including neurogenesis, cell migration, synaptogenesis, and neural circuit formation, and have been implicated in cortical column formation. Dysfunctional gap junctions can contribute to or even cause a variety of brain diseases. Although the role of gap junctions in neocortical development is better known, a comprehensive understanding of their functions is far from complete. Here we explore several critical open questions surrounding gap junctions and their involvement in neural circuit development. Addressing them will greatly impact our understanding of the fundamental mechanisms of neocortical structure and function as well as the etiology of brain disease.

RNA-binding proteins as a common ground for neurodegeneration and inflammation in amyotrophic lateral sclerosis and multiple sclerosis

The neurodegenerative and inflammatory illnesses of amyotrophic lateral sclerosis and multiple sclerosis were once thought to be completely distinct entities that did not share any remarkable features, but new research is beginning to reveal more information about their similarities and differences. Here, we review some of the pathophysiological features of both diseases and their experimental models: RNA-binding proteins, energy balance, protein transportation, and protein degradation at the molecular level. We make a thorough analysis on TDP-43 and hnRNP A1 dysfunction, as a possible common ground in both pathologies, establishing a potential link between neurodegeneration and pathological immunity. Furthermore, we highlight the putative variations that diverge from a common ground in an atemporal course that proposes three phases for all relevant molecular events.

Redistribution of the astrocyte phenotypes in the medial vestibular nuclei after unilateral labyrinthectomy

Astrocytes are highly heterogeneous and involved in different aspects of fundamental functions in the central nervous system (CNS). However, whether and how this heterogeneous population of cells reacts to the pathophysiological challenge is not well understood. To investigate the response status of astrocytes in the medial vestibular nucleus (MVN) after vestibular loss, we examined the subtypes of astrocytes in MVN using single-cell sequencing technology in a unilateral labyrinthectomy mouse model. We discovered four subtypes of astrocytes in the MVN with each displaying unique gene expression profiles. After unilateral labyrinthectomy, the proportion of the astrocytic subtypes and their transcriptional features on the ipsilateral side of the MVN differ significantly from those on the contralateral side. With new markers to detect and classify the subtypes of astrocytes in the MVN, our findings implicate potential roles of the adaptive changes of astrocyte subtypes in the early vestibular compensation following peripheral vestibular damage to reverse behavioral deficits.

Astrocytes: Dissecting Their Diverse Roles in Amyotrophic Lateral Sclerosis and Frontotemporal Dementia

Amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD) are fatal neurodegenerative disorders often co-occurring in the same patient, a feature that suggests a common origin of the two diseases. Consistently, pathological inclusions of the same proteins as well as mutations in the same genes can be identified in both ALS/FTD. Although many studies have described several disrupted pathways within neurons, glial cells are also regarded as crucial pathogenetic contributors in ALS/FTD. Here, we focus our attention on astrocytes, a heterogenous population of glial cells that perform several functions for optimal central nervous system homeostasis. Firstly, we discuss how post-mortem material from ALS/FTD patients supports astrocyte dysfunction around three pillars: neuroinflammation, abnormal protein aggregation, and atrophy/degeneration. Furthermore, we summarize current attempts at monitoring astrocyte functions in living patients using either novel imaging strategies or soluble biomarkers. We then address how astrocyte pathology is recapitulated in animal and cellular models of ALS/FTD and how we used these models both to understand the molecular mechanisms driving glial dysfunction and as platforms for pre-clinical testing of therapeutics. Finally, we present the current clinical trials for ALS/FTD, restricting our discussion to treatments that modulate astrocyte functions, directly or indirectly.

Targeting connexin 43 expression via scaffold mediated delivery of antisense oligodeoxynucleotide preserves neurons, enhances axonal extension, reduces astrocyte and microglial activation after spinal cord injury

Injury to the central nervous system (CNS) provokes an inflammatory reaction and secondary damage that result in further tissue damage and destruction of neurons away from the injury site. Upon injury, expression of connexin 43 (Cx43), a gap junction protein, upregulates and is responsible for the spread and amplification of cell death signals through these gap junctions. In this study, we hypothesise that the downregulation of Cx43 by scaffold-mediated controlled delivery of antisense oligodeoxynucleotide (asODN), would minimise secondary injuries and cell death, and thereby support tissue regeneration after nerve injuries. Specifically, using spinal cord injury (SCI) as a proof-of-principle, we utilised a fibre-hydrogel scaffold for sustained delivery of Cx43asODN, while providing synergistic topographical cues to guide axonal ingrowth. Correspondingly, scaffolds loaded with Cx43asODN, in the presence of NT-3, suppressed Cx43 up-regulation after complete transection SCI in rats. These scaffolds facilitated the sustained release of Cx43asODN for up to 25 days. Importantly, asODN treatment preserved neurons around the injury site, promoted axonal extension, decreased glial scarring, and reduced microglial activation after SCI. Our results suggest that implantation of such scaffold-mediated asODN delivery platform could serve as an effective alternative SCI therapeutic approach.

Why should we care about astrocytes in a motor neuron disease?

Amyotrophic lateral sclerosis (ALS) is the most common motor neuron disease in adults, causing progressive degeneration of motor neurons, which results in muscle atrophy, respiratory failure and ultimately death of the patients. The pathogenesis of ALS is complex, and extensive efforts have focused on unravelling the underlying molecular mechanisms with a large emphasis on the dying motor neurons. However, a recent shift in focus towards the supporting glial population has revealed a large contribution and influence in ALS, which stresses the need to explore this area in more detail. Especially studies into astrocytes, the residential homeostatic supporter cells of neurons, have revealed a remarkable astrocytic dysfunction in ALS, and therefore could present a target for new and promising therapeutic entry points. In this review, we provide an overview of general astrocyte function and summarize the current literature on the role of astrocytes in ALS by categorizing the potentially underlying molecular mechanisms. We discuss the current efforts in astrocyte-targeted therapy, and highlight the potential and shortcomings of available models.

FUS-ALS hiPSC-derived astrocytes impair human motor units through both gain-of-toxicity and loss-of-support mechanisms

BACKGROUND

Astrocytes play a crucial, yet not fully elucidated role in the selective motor neuron pathology in amyotrophic lateral sclerosis (ALS). Among other responsibilities, astrocytes provide important neuronal homeostatic support, however this function is highly compromised in ALS. The establishment of fully human coculture systems can be used to further study the underlying mechanisms of the dysfunctional intercellular interplay, and has the potential to provide a platform for revealing novel therapeutic entry points.

METHODS

In this study, we characterised human induced pluripotent stem cell (hiPSC)-derived astrocytes from FUS-ALS patients, and incorporated these cells into a human motor unit microfluidics model to investigate the astrocytic effect on hiPSC-derived motor neuron network and functional neuromuscular junctions (NMJs) using immunocytochemistry and live-cell recordings. FUS-ALS cocultures were systematically compared to their CRISPR-Cas9 gene-edited isogenic control systems.

RESULTS

We observed a dysregulation of astrocyte homeostasis, which resulted in a FUS-ALS-mediated increase in reactivity and secretion of inflammatory cytokines. Upon coculture with motor neurons and myotubes, we detected a cytotoxic effect on motor neuron-neurite outgrowth, NMJ formation and functionality, which was improved or fully rescued by isogenic control astrocytes. We demonstrate that ALS astrocytes have both a gain-of-toxicity and loss-of-support function involving the WNT/β-catenin pathway, ultimately contributing to the disruption of motor neuron homeostasis, intercellular networks and NMJs.

CONCLUSIONS

Our findings shine light on a complex, yet highly important role of astrocytes in ALS, and provides further insight in to their pathological mechanisms.

Protein kinase B (AKT) upregulation and Thy-1-αvβ3 integrin-induced phosphorylation of Connexin43 by activated AKT in astrogliosis

BACKGROUND

In response to brain injury or inflammation, astrocytes undergo hypertrophy, proliferate, and migrate to the damaged zone. These changes, collectively known as "astrogliosis", initially protect the brain; however, astrogliosis can also cause neuronal dysfunction. Additionally, these astrocytes undergo intracellular changes involving alterations in the expression and localization of many proteins, including α_vβ₃ integrin. Our previous reports indicate that Thy-1, a neuronal glycoprotein, binds to this integrin inducing Connexin43 (Cx43) hemichannel (HC) opening, ATP release, and astrocyte migration. Despite such insight, important links and molecular events leading to astrogliosis remain to be defined.

METHODS

Using bioinformatics approaches, we analyzed different Gene Expression Omnibus datasets to identify changes occurring in reactive astrocytes as compared to astrocytes from the normal mouse brain. In silico analysis was validated by both qRT-PCR and immunoblotting using reactive astrocyte cultures from the normal rat brain treated with TNF and from the brain of a hSOD1^G93A transgenic mouse model. We evaluated the phosphorylation of Cx43 serine residue 373 (S373) by AKT and ATP release as a functional assay for HC opening. In vivo experiments were also performed with an AKT inhibitor (AKTi).

RESULTS

The bioinformatics analysis revealed that genes of the PI3K/AKT signaling pathway were among the most significantly altered in reactive astrocytes. mRNA and protein levels of PI3K, AKT, as well as Cx43, were elevated in reactive astrocytes from normal rats and from hSOD1^G93A transgenic mice, as compared to controls. In vitro, reactive astrocytes stimulated with Thy-1 responded by activating AKT, which phosphorylated S373Cx43. Increased pS373Cx43 augmented the release of ATP to the extracellular medium and AKTi inhibited these Thy-1-induced responses. Furthermore, in an in vivo model of inflammation (brain damage), AKTi decreased the levels of astrocyte reactivity markers and S373Cx43 phosphorylation.

CONCLUSIONS

Here, we identify changes in the PI3K/AKT molecular signaling network and show how they participate in astrogliosis by regulating the HC protein Cx43. Moreover, because HC opening and ATP release are important in astrocyte reactivity, the phosphorylation of Cx43 by AKT and the associated increase in ATP release identify a potential therapeutic window of opportunity to limit the adverse effects of astrogliosis.

SOD1G93A Astrocyte-Derived Extracellular Vesicles Induce Motor Neuron Death by a miRNA-155-5p-Mediated Mechanism

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease characterized by upper and lower motor neuron (MN) degeneration. Astrocytes surrounding MNs are known to modulate ALS progression. When cocultured with astrocytes overexpressing the ALS-linked mutant Cu/Zn superoxide dismutase (SOD1G93A) or when cultured with conditioned medium from SOD1G93A astrocytes, MN survival is reduced. The exact mechanism of this neurotoxic effect is unknown. Astrocytes secrete extracellular vesicles (EVs) that transport protein, mRNA, and microRNA species from one cell to another. The size and protein markers characteristic of exosomes were observed in the EVs obtained from cultured astrocytes, indicating their abundance in exosomes. Here, we analyzed the microRNA content of the exosomes derived from SOD1G93A astrocytes and evaluated their role in MN survival. Purified MNs exposed to SOD1G93A astrocyte-derived exosomes showed reduced survival and neurite length compared to those exposed to exosomes derived from non-transgenic (non-Tg) astrocytes. Analysis of the miRNA content of the exosomes revealed that miR-155-5p and miR-582-3p are differentially expressed in SOD1G93A exosomes compared with exosomes from non-Tg astrocytes. Kyoto Encyclopedia of Genes and Genomes (KEGG) analysis indicates that miR-155-5p and miR-582-3p predicted targets are enriched in the neurotrophin signaling pathway. Importantly, when levels of miR-155-5p were reduced by incubation with a specific antagomir, SOD1G93A exosomes did not affect MN survival or neurite length. These results demonstrate that SOD1G93A-derived exosomes are sufficient to induce MN death, and miRNA-155-5p contributes to this effect. miRNA-155-5p may offer a new therapeutic target to modulate disease progression in ALS.

Connexin 30 Deficiency Ameliorates Disease Progression at the Early Phase in a Mouse Model of Amyotrophic Lateral Sclerosis by Suppressing Glial Inflammation

Connexin 30 (Cx30), which forms gap junctions between astrocytes, regulates cell adhesion and migration, and modulates glutamate transport. Cx30 is upregulated on activated astroglia in central nervous system inflammatory lesions, including spinal cord lesions in mutant superoxide dismutase 1 (mSOD1) transgenic amyotrophic lateral sclerosis (ALS) model mice. Here, we investigated the role of Cx30 in mSOD1 mice. Cx30 was highly expressed in the pre-onset stage in mSOD1 mice. mSOD1 mice with knockout (KO) of the Cx30 gene (Cx30KO-mSOD1 mice) showed delayed disease onset and tended to have an extended survival period (log-rank, p = 0.09). At the progressive and end stages of the disease, anterior horn cells were significantly preserved in Cx30KO-mSOD1 mice. In lesions of these mice, glial fibrillary acidic protein/C3-positive inflammatory astroglia were decreased. Additionally, the activation of astrocytes in Cx30KO-mSOD1 mice was reduced compared with mSOD1 mice by gene expression microarray. Furthermore, expression of connexin 43 at the pre-onset stage was downregulated in Cx30KO-mSOD1 mice. These findings suggest that reduced expression of astroglial Cx30 at the early disease stage in ALS model mice protects neurons by attenuating astroglial inflammation.

Glial Gap Junction Pathology in the Spinal Cord of the 5xFAD Mouse Model of Early-Onset Alzheimer's Disease

Gap junctions (GJs) are specialized transmembrane channels assembled by two hemi-channels of six connexin (Cx) proteins that facilitate neuroglial crosstalk in the central nervous system (CNS). Previous studies confirmed the crucial role of glial GJs in neurodegenerative disorders with dementia or motor dysfunction including Alzheimer's disease (AD). The aim of this study was to examine the alterations in astrocyte and related oligodendrocyte GJs in association with Aβ plaques in the spinal cord of the 5xFAD mouse model of AD. Our analysis revealed abundant Aβ plaque deposition, activated microglia, and astrogliosis in 12-month-old (12M) 5xFAD mice, with significant impairment of motor performance starting from 3-months (3M) of age. Additionally, 12M 5xFAD mice displayed increased immunoreactivity of astroglial Cx43 and Cx30 surrounding Aβ plaques and higher protein levels, indicating upregulated astrocyte-to-astrocyte GJ connectivity. In addition, they demonstrated increased numbers of mature CC1-positive and precursor oligodendrocytes (OPCs) with higher immunoreactivity of Cx47-positive GJs in individual cells. Moreover, total Cx47 protein levels were significantly elevated in 12M 5xFAD, reflecting increased oligodendrocyte-to-oligodendrocyte Cx47-Cx47 GJ connectivity. In contrast, we observed a marked reduction in Cx32 protein levels in 12M 5xFAD spinal cords compared with controls, while qRT-PCR analysis revealed a significant upregulation in Cx32 mRNA levels. Finally, myelin deficits were found focally in the areas occupied by Aβ plaques, whereas axons themselves remained preserved. Overall, our data provide novel insights into the altered glial GJ expression in the spinal cord of the 5xFAD model of AD and the implicated role of GJ pathology in neurodegeneration. Further investigation to understand the functional consequences of these extensive alterations in oligodendrocyte-astrocyte (O/A) GJ connectivity is warranted.

Early and extensive alterations of glial connexins, distal oligodendrogliopathy type demyelination, and nodal/paranodal pathology are characteristic of multiple system atrophy

The pathological hallmark of multiple system atrophy (MSA) is aberrant accumulation of phosphorylated α-synuclein in oligodendrocytes, forming glial cytoplasmic inclusions (GCIs). Extensive demyelination occurs particularly in the olivopontocerebellar and striatonigral pathways, but its precise mechanism remains elusive. Glial connexins (Cxs), which form gap junction channels between astrocytes and oligodendrocytes, play critical roles in myelin maintenance, and have not been studied in MSA. Therefore, we immunohistochemically investigated glial Cx changes in the cerebellar afferent fibers in 15 autopsied patients with MSA. We classified demyelinating lesions into three stages based on Klüver-Barrera staining: early (Stage I), intermediate (Stage II), and late (Stage III) stages showing subtle, moderate, and severe myelin reduction, respectively. Myelin-associated glycoprotein, but not myelin oligodendrocyte glycoprotein, was preferentially decreased in Stage I, suggesting distal oligodendrogliopathy type demyelination. Accumulation of phosphorylated α-synuclein in oligodendrocytes was frequently seen in Stage I but less frequently observed in Stages II and III. Tubulin polymerization-promoting protein (TPPP/p25α)-positive oligodendrocytes were preserved in Stage I but successively decreased in Stages II and III. Even at Stage I, Cx32 was nearly absent from myelin, despite the relative preservation of other nodal proteins, such as neurofascin, claudin-11/oligodendrocyte-specific protein, and contactin-associated protein 1, which successively decreased in the later stages. Cx32 was re-distributed in the oligodendrocyte cytoplasm and co-localized with GCIs. Cx47 gradually decreased at the oligodendrocyte surface in a stage-dependent manner but was not co-localized with GCIs. Astrocytic Cx43 was down-regulated in Stage I but up-regulated in Stages II and III, reflecting astrogliosis. Cx43/Cx47 gap junctions significantly decreased from Stage I to III. Activated microglia/macrophages and T cells infiltrated in Stage I rather than Stages II and III. Therefore, early and extensive alterations of glial Cxs, particularly Cx32 loss, occur in MSA and may accelerate distal oligodendrogliopathy type demyelination and nodal/paranodal dysfunction through disruption of inter-glial communication.

Connexins Signatures of the Neurovascular Unit and Their Physio-Pathological Functions

Central nervous system (CNS) homeostasis is closely linked to the delicate balance of the microenvironment in which different cellular components of the neurovascular unit (NVU) coexist. Intercellular communication plays a pivotal role in exchanges of signaling molecules and mediators essential for survival functions, as well as in the removal of disturbing elements that can lead to related pathologies. The specific signatures of connexins (Cxs), proteins which form either gap junctions (GJs) or hemichannels (HCs), represent the biological substrate of the pathophysiological balance. Connexin 43 (Cx43) is undoubtedly one of the most important factors in glia–neuro–vascular crosstalk. Herein, Cxs signatures of every NVU component are highlighted and their critical influence on functional processes in healthy and pathological conditions of nervous microenvironment is reviewed.

Potential proteomic alteration in the brain of Tg(SOD1*G93A)1Gur mice: A new pathogenesis insight of amyotrophic lateral sclerosis

The pathogenesis of amyotrophic lateral sclerosis (ALS) remains unclear. The recent studies have suggested that the protein abnormalities could play some important roles in ALS because several protein mutations were found in individuals with this disease. However, proteins that are currently known to be associated with ALS only explain the pathogenesis of this disease in a minority of cases, thus, further screening is needed to identify other ALS-related proteins. In this study, we systematically analyzed and compared the brain proteomic alterations between a mouse model of ALS, the Tg(SOD1*G93A)1Gur model, and wild-type mice using isobaric tags for relative and absolute quantitation (iTRAQ) as well as bioinformatics methods. The results revealed some significant up- and downregulated proteins at the different developmental stages in the ALS-like mice as well as the possibly related cellular components, molecular functions, biological processes, and pathways in the development of ALS. Our results identified some possible proteins that participate in the pathogenesis of ALS as well as the cellular components that are damaged by these proteins, we additionally identified the molecular functions, the biological processes, and the pathways of these proteins as well as the molecules that are associated with these pathways. This study represents an important preliminary investigation of the role of proteomic abnormalities in the pathogenesis of ALS, both in human patients and other animal models. We present some novel findings that may serve as a basis for further investigation of abnormal proteins that are involved in the pathogenesis of ALS.

Neurotoxic Astrocytes Directly Converted from Sporadic and Familial ALS Patient Fibroblasts Reveal Signature Diversities and miR-146a Theragnostic Potential in Specific Subtypes

A lack of stratification methods in patients with amyotrophic lateral sclerosis (ALS) is likely implicated in therapeutic failures. Regional diversities and pathophysiological abnormalities in astrocytes from mice with SOD1 mutations (mSOD1-ALS) can now be explored in human patients using somatic cell reprogramming. Here, fibroblasts from four sporadic (sALS) and three mSOD1-ALS patients were transdifferentiated into induced astrocytes (iAstrocytes). ALS iAstrocytes were neurotoxic toward HB9-GFP mouse motor neurons (MNs) and exhibited subtype stratification through GFAP, CX43, Ki-67, miR-155 and miR-146a expression levels. Up- (two cases) and down-regulated (three cases) miR-146a values in iAstrocytes were recapitulated in their secretome, either free or as cargo in small extracellular vesicles (sEVs). We previously showed that the neuroprotective phenotype of depleted miR-146 mSOD1 cortical astrocytes was reverted by its mimic. Thus, we tested such modulation in the most miR-146a-depleted patient-iAstrocytes (one sALS and one mSOD1-ALS). The miR-146a mimic in ALS iAstrocytes counteracted their reactive/inflammatory profile and restored miR-146a levels in sEVs. A reduction in lysosomal activity and enhanced synaptic/axonal transport-related genes in NSC-34 MNs occurred after co-culture with miR-146a-modulated iAstrocytes. In summary, the regulation of miR-146a in depleted ALS astrocytes may be key in reestablishing their normal function and in restoring MN lysosomal/synaptic dynamic plasticity in disease sub-groups.

Cx43 hemichannels contribute to astrocyte-mediated toxicity in sporadic and familial ALS

Our results demonstrate that connexin 43 hemichannels are the conduits for amyotrophic lateral sclerosis (ALS) astrocyte-mediated motor neuron toxicity and disease spread, acting as a common mechanism that can target both familial ALS and sporadic ALS populations. Furthermore, our present work provides proof of principle that tonabersat, as a drug already studied in clinical trials for other indications, could serve as a potential ALS therapeutic.

Connexin 43 (Cx43) gap junctions and hemichannels mediate astrocyte intercellular communication in the central nervous system under normal conditions and contribute to astrocyte-mediated neurotoxicity in amyotrophic lateral sclerosis (ALS). Here, we show that astrocyte-specific knockout of Cx43 in a mouse model of ALS slows disease progression both spatially and temporally, provides motor neuron (MN) protection, and improves survival. In addition, Cx43 expression is up-regulated in human postmortem tissue and cerebrospinal fluid from ALS patients. Using human induced pluripotent stem cell–derived astrocytes (hiPSC-A) from both familial and sporadic ALS, we establish that Cx43 is up-regulated and that Cx43-hemichannels are enriched at the astrocyte membrane. We also demonstrate that the pharmacological blockade of Cx43-hemichannels in ALS astrocytes using GAP 19, a mimetic peptide blocker, and tonabersat, a clinically tested small molecule, provides neuroprotection of hiPSC-MN and reduces ALS astrocyte-mediated neuronal hyperexcitability. Extending the in vitro application of tonabersat with chronic administration to SOD1^G93A mice results in MN protection with a reduction in reactive astrocytosis and microgliosis. Taking these data together, our studies identify Cx43 hemichannels as conduits of astrocyte-mediated disease progression and a pharmacological target for disease-modifying ALS therapies.

Astrocytic Glutamatergic Transmission and Its Implications in Neurodegenerative Disorders

Several neurodegenerative disorders involve impaired neurotransmission, and glutamatergic neurotransmission sets a prototypical example. Glutamate is a predominant excitatory neurotransmitter where the astrocytes play a pivotal role in maintaining the extracellular levels through release and uptake mechanisms. Astrocytes modulate calcium-mediated excitability and release several neurotransmitters and neuromodulators, including glutamate, and significantly modulate neurotransmission. Accumulating evidence supports the concept of excitotoxicity caused by astrocytic glutamatergic release in pathological conditions. Thus, the current review highlights different vesicular and non-vesicular mechanisms of astrocytic glutamate release and their implication in neurodegenerative diseases. As in presynaptic neurons, the vesicular release of astrocytic glutamate is also primarily meditated by calcium-mediated exocytosis. V-ATPase is crucial in the acidification and maintenance of the gradient that facilitates the vesicular storage of glutamate. Along with these, several other components, such as cystine/glutamate antiporter, hemichannels, BEST-1, TREK-1, purinergic receptors and so forth, also contribute to glutamate release under physiological and pathological conditions. Events of hampered glutamate uptake could promote inflamed astrocytes to trigger repetitive release of glutamate. This could be favorable towards the development and worsening of neurodegenerative diseases. Therefore, across neurodegenerative diseases, we review the relations between defective glutamatergic signaling and astrocytic vesicular and non-vesicular events in glutamate homeostasis. The optimum regulation of astrocytic glutamatergic transmission could pave the way for the management of these diseases and add to their therapeutic value.

The role of inflammation in neurodegeneration: novel insights into the role of the immune system in C9orf72 HRE-mediated ALS/FTD

Neuroinflammation is an important hallmark of amyotrophic lateral sclerosis (ALS) and frontotemporal lobar degeneration (FTLD). An inflammatory reaction to neuronal injury is deemed vital for neuronal health and homeostasis. However, a continued activation of the inflammatory response can be detrimental to remaining neurons and aggravate the disease process. Apart from a disease modifying role, some evidence suggests that neuroinflammation may also contribute to the upstream cause of the disease. In this review, we will first focus on the role of neuroinflammation in the pathogenesis of chromosome 9 open reading frame 72 gene (C9orf72) hexanucleotide repeat expansions (HRE)-mediated ALS/FTD (C9-ALS/FTD). Additionally, we will discuss evidence from ex vivo and in vivo studies and finally, we briefly summarize the trials and progress of anti-inflammatory therapies.

The Multifaceted Neurotoxicity of Astrocytes in Ageing and Age-Related Neurodegenerative Diseases: A Translational Perspective

In a healthy physiological context, astrocytes are multitasking cells contributing to central nervous system (CNS) homeostasis, defense, and immunity. In cell culture or rodent models of age-related neurodegenerative diseases (NDDs), such as Alzheimer’s disease (AD) and Parkinson’s disease (PD), numerous studies have shown that astrocytes can adopt neurotoxic phenotypes that could enhance disease progression. Chronic inflammatory responses, oxidative stress, unbalanced phagocytosis, or alteration of their core physiological roles are the main manifestations of their detrimental states. However, if astrocytes are directly involved in brain deterioration by exerting neurotoxic functions in patients with NDDs is still controversial. The large spectrum of NDDs, with often overlapping pathologies, and the technical challenges associated with the study of human brain samples complexify the analysis of astrocyte involvement in specific neurodegenerative cascades. With this review, we aim to provide a translational overview about the multi-facets of astrocyte neurotoxicity ranging from in vitro findings over mouse and human cell-based studies to rodent NDDs research and finally evidence from patient-related research. We also discuss the role of ageing in astrocytes encompassing changes in physiology and response to pathologic stimuli and how this may prime detrimental responses in NDDs. To conclude, we discuss how potentially therapeutic strategies could be adopted to alleviate or reverse astrocytic toxicity and their potential to impact neurodegeneration and dementia progression in patients.

Multi-omic analysis of selectively vulnerable motor neuron subtypes implicates altered lipid metabolism in ALS

Amyotrophic lateral sclerosis (ALS) is a devastating disorder in which motor neurons degenerate, the causes of which remain unclear. In particular, the basis for selective vulnerability of spinal motor neurons (sMNs) and resistance of ocular motor neurons to degeneration in ALS has yet to be elucidated. Here, we applied comparative multi-omics analysis of human induced pluripotent stem cell-derived sMNs and ocular motor neurons to identify shared metabolic perturbations in inherited and sporadic ALS sMNs, revealing dysregulation in lipid metabolism and its related genes. Targeted metabolomics studies confirmed such findings in sMNs of 17 ALS (SOD1, C9ORF72, TDP43 (TARDBP) and sporadic) human induced pluripotent stem cell lines, identifying elevated levels of arachidonic acid. Pharmacological reduction of arachidonic acid levels was sufficient to reverse ALS-related phenotypes in both human sMNs and in vivo in Drosophila and SOD1G93A mouse models. Collectively, these findings pinpoint a catalytic step of lipid metabolism as a potential therapeutic target for ALS.

GAP junctions: multifaceted regulators of neuronal differentiation

Gap junctions are intercellular membrane channels consisting of connexin proteins, which contribute to direct cytoplasmic exchange of small molecules, substrates and metabolites between adjacent cells. These channels play important roles in neuronal differentiation, maintenance, survival and function. Gap junctions regulate differentiation of neurons from embryonic, neural and induced pluripotent stem cells. In addition, they control transdifferentiation of neurons from mesenchymal stem cells. The expression and levels of several connexins correlate with cell cycle changes and different stages of neurogenesis. Connexins such as Cx36, Cx45, and Cx26, play a crucial role in neuronal function. Several connexin knockout mice display lethal or severely impaired phenotypes. Aberrations in connexin expression is frequently associated with various neurodegenerative disorders. Gap junctions also act as promising therapeutic targets for neuronal regenerative medicine, because of their role in neural stem cell integration, injury and remyelination.

Imaging immunological processes from blood to brain in amyotrophic lateral sclerosis

Neuropathology studies of amyotrophic lateral sclerosis (ALS) and animal models of ALS reveal a strong association between aberrant protein accumulation and motor neurone damage, as well as activated microglia and astrocytes. While the role of neuroinflammation in the pathology of ALS is unclear, imaging studies of the central nervous system (CNS) support the idea that innate immune activation occurs early in disease in both humans and rodent models of ALS. In addition, emerging studies also reveal changes in monocytes, macrophages and lymphocytes in peripheral blood as well as at the neuromuscular junction. To more clearly understand the association of neuroinflammation (innate and adaptive) with disease progression, the use of biomarkers and imaging modalities allow monitoring of immune parameters in the disease process. Such approaches are important for patient stratification, selection and inclusion in clinical trials, as well as to provide readouts of response to therapy. Here, we discuss the different imaging modalities, e.g. magnetic resonance imaging, magnetic resonance spectroscopy and positron emission tomography as well as other approaches, including biomarkers of inflammation in ALS, that aid the understanding of the underlying immune mechanisms associated with motor neurone degeneration in ALS.

Decreased connexin43 expression in the hippocampus is related to the antidepressant effect of amitriptyline in neuropathic pain mice

Downregulation of astrocytic connexin43 (Cx43) has been observed in several brain regions in rodents and patients with depression. However, its specific role in this effect remains unknown. Moreover, chronic pain can induce depressive disorders. Therefore, the current study examined the relationship between Cx43 expression and depressive-like behavior in a neuropathic pain model. Neuropathic pain was induced by spared nerve injury (SNI) in mice. Depressive-like behavior was evaluated using the forced swim test. Expression of Cx43 in the hippocampus was evaluated using Western blotting and real-time PCR. SNI downregulated Cx43 protein in the contralateral hippocampus of mice, whereas expression of hippocampal Cx43 mRNA was not altered following SNI. Although SNI mice showed longer immobility time compared with sham mice during the forced swim test, duration of depressive-like behavior was not correlated with the expression of Cx43 in the hippocampus of SNI mice. Repeated intraperitoneal administration of amitriptyline ameliorated SNI-induced depressive-like behavior. Furthermore, the antidepressant effect of amitriptyline was correlated with decreased hippocampal Cx43 expression in SNI mice. The current findings suggest that the alteration of Cx43 expression in the hippocampus may not be involved in the induction of depressive disorder but may influence the efficacy of antidepressants. Therefore, the level of Cx43 expression in the hippocampus could be a key parameter to evaluate individual differences in antidepressant effects in patients with depressive disorder.

Patient-Specific Cells for Modeling and Decoding Amyotrophic Lateral Sclerosis: Advances and Challenges

Motor neuron loss or degeneration is the typical characteristic of amyotrophic lateral sclerosis (ALS), which often leads to weakness, paralysis, or even death. The underlying mechanisms of motor neuron degeneration and ALS progression remain elusive, and there is no effective treatment for ALS. The advances of stem cells and reprogramming techniques has made it possible to generate patient-specific motor neurons as cell models for studying disease mechanisms and drug discovery. This review comprehensively discusses recent approaches to generate motor neurons from stem cells and somatic cells and highlights the application of induced motor neurons to modeling ALS diseases, dissecting the pathogenesis, and screening new drugs. New perspectives are also discussed on generating patient-specific motor neuron subtypes that are affected by ALS or creating 3D spinal cord organoid models for better recapitulating and understanding ALS.

Astroglial Connexins in Neurodegenerative Diseases

Astrocytes play a crucial role in the maintenance of the normal functions of the Central Nervous System (CNS). During the pathogenesis of neurodegenerative diseases, astrocytes undergo morphological and functional remodeling, a process called reactive astrogliosis, in response to the insults to the CNS. One of the key aspects of the reactive astrocytes is the change in the expression and function of connexins. Connexins are channel proteins that highly expressed in astrocytes, forming gap junction channels and hemichannels, allowing diffusional trafficking of small molecules. Alterations of astrocytic connexin expression and function found in neurodegenerative diseases have been shown to affect the disease progression by changing neuronal function and survival. In this review, we will summarize the role of astroglial connexins in neurodegenerative diseases including Alzheimer’s disease, Huntington’s disease, Parkinson’s disease, and amyotrophic lateral sclerosis. Also, we will discuss why targeting connexins can be a plausible therapeutic strategy to manage these neurodegenerative diseases.

Astroglial Connexins in Neurological and Neuropsychological Disorders and Radiation Exposure

Radiotherapy is a common treatment for brain and spinal cord tumors and also a risk factor for neuropathological changes in the brain leading to different neurological and neuropsychological disorders. Astroglial connexins are involved in brain inflammation, development of Alzheimer's Disease (AD), depressive, epilepsy, and amyotrophic lateral sclerosis, and are affected by radiation exposure. Therefore, it is speculated that radiation-induced changes of astroglial connexins may be related to the brain neuropathology and development of neurological and neuropsychological disorders. In this paper, we review the functional expression and regulation of astroglial connexins expressed between astrocytes and different types of brain cells (including oligodendrocytes, microglia, neurons and endothelial cells). The roles of these connexins in the development of AD, depressive, epilepsy, amyotrophic lateral sclerosis and brain inflammation have also been summarized. The radiation-induced astroglial connexins changes and development of different neurological and neuropsychological disorders are then discussed. Based on currently available data, we propose that radiation-induced astroglial connexins changes may be involved in the genesis of different neurological and neuropsychological disorders which depends on the age, brain regions, and radiation doses/dose rates. The abnormal astroglial connexins may be novel therapeutic targets for the prevention of radiation-induced cognitive impairment, neurological and neuropsychological disorders.

Recovery of Depleted miR-146a in ALS Cortical Astrocytes Reverts Cell Aberrancies and Prevents Paracrine Pathogenicity on Microglia and Motor Neurons

Reactive astrocytes in Amyotrophic Lateral Sclerosis (ALS) change their molecular expression pattern and release toxic factors that contribute to neurodegeneration and microglial activation. We and others identified a dysregulated inflammatory miRNA profile in ALS patients and in mice models suggesting that they represent potential targets for therapeutic intervention. Such cellular miRNAs are known to be released into the secretome and to be carried by small extracellular vesicles (sEVs), which may be harmful to recipient cells. Thus, ALS astrocyte secretome may disrupt cell homeostasis and impact on ALS pathogenesis. Previously, we identified a specific aberrant signature in the cortical brain of symptomatic SOD1-G93A (mSOD1) mice, as well as in astrocytes isolated from the same region of 7-day-old mSOD1 mice, with upregulated S100B/HMGB1/Cx43/vimentin and downregulated GFAP. The presence of downregulated miR-146a on both cases suggests that it can be a promising target for modulation in ALS. Here, we upregulated miR-146a with pre-miR-146a, and tested glycoursodeoxycholic acid (GUDCA) and dipeptidyl vinyl sulfone (VS) for their immunoregulatory properties. VS was more effective in restoring astrocytic miR-146a, GFAP, S100B, HMGB1, Cx43, and vimentin levels than GUDCA, which only recovered Cx43 and vimentin mRNA. The miR-146a inhibitor generated typical ALS aberrancies in wild type astrocytes that were abolished by VS. Similarly, pre-miR-146a transfection into the mSOD1 astrocytes abrogated aberrant markers and intracellular Ca²⁺ overload. Such treatment counteracted miR-146a depletion in sEVs and led to secretome-mediated miR-146a enhancement in NSC-34-motor neurons (MNs) and N9-microglia. Secretome from mSOD1 astrocytes increased early/late apoptosis and FGFR3 mRNA in MNs and microglia, but not when derived from pre-miR-146a or VS-treated cells. These last strategies prevented the impairment of axonal transport and synaptic dynamics by the pathological secretome, while also averted microglia activation through either secretome, or their isolated sEVs. Proteomic analysis of the target cells indicated that pre-miR-146a regulates mitochondria and inflammation via paracrine signaling. We demonstrate that replenishment of miR-146a in mSOD1 cortical astrocytes with pre-miR-146a or by VS abrogates their phenotypic aberrancies and paracrine deleterious consequences to MNs and microglia. These results propose miR-146a as a new causal and emerging therapeutic target for astrocyte pathogenic processes in ALS.

Mini-Review: Induced pluripotent stem cells and the search for new cell-specific ALS therapeutic targets

Amongst the most important discoveries in ALS pathobiology are the works demonstrating that multiple cell types contribute to disease onset and progression. However, a significant limitation in ALS research is the inability to obtain tissues from ALS patient brain and spinal cord during the course of the disease. In vivo modeling has provided insights into the role of these cell subtypes in disease onset and progression. However, in vivo models also have shortcomings, including the reliance on a limited number of models based upon hereditary forms of the disease. Therefore, using human induced pluripotent stem cells (iPSC) reprogrammed from somatic cells of ALS patients, with both hereditary and sporadic forms of the disease, and differentiated into cell subtypes of both the central nervous system (CNS) and peripheral nervous system (PNS), have become powerful complementary tools for investigating basic mechanisms of disease as well as a platform for drug discovery. Motor neuron and other neuron subtypes, as well as non-neuronal cells have been differentiated from human iPSC and studied for their potential contributions to ALS pathobiology. As iPSC technologies have advanced, 3D modeling with multicellular systems organised in microfluidic chambers or organoids are the next step in validating the pathways and therapeutic targets already identified. Precision medicine approaches with iPSC using either traditional strategies of screening drugs that target a known pathogenic mechanism as well as "blind-to-target" drug screenings that allow for patient stratification based on drug response rather than clinical characteristics are now being employed.

Defective Oligodendroglial Lineage and Demyelination in Amyotrophic Lateral Sclerosis

Motor neurons and their axons reaching the skeletal muscle have long been considered as the best characterized targets of the degenerative process observed in amyotrophic lateral sclerosis (ALS). However, the involvement of glial cells was also more recently reported. Although oligodendrocytes have been underestimated for a longer time than other cells, they are presently considered as critically involved in axonal injury and also conversely constitute a target for the toxic effects of the degenerative neurons. In the present review, we highlight the recent advances regarding oligodendroglial cell involvement in the pathogenesis of ALS. First, we present the oligodendroglial cells, the process of myelination, and the tight relationship between axons and myelin. The histological abnormalities observed in ALS and animal models of the disease are described, including myelin defects and oligodendroglial accumulation of pathological protein aggregates. Then, we present data that establish the existence of dysfunctional and degenerating oligodendroglial cells, the chain of events resulting in oligodendrocyte degeneration, and the most recent molecular mechanisms supporting oligodendrocyte death and dysfunction. Finally, we review the arguments in support of the primary versus secondary involvement of oligodendrocytes in the disease and discuss the therapeutic perspectives related to oligodendrocyte implication in ALS pathogenesis.

Mast Cell and Astrocyte Hemichannels and Their Role in Alzheimer's Disease, ALS, and Harmful Stress Conditions

Considered relevant during allergy responses, numerous observations have also identified mast cells (MCs) as critical effectors during the progression and modulation of several neuroinflammatory conditions, including Alzheimer’s disease (AD) and amyotrophic lateral sclerosis (ALS). MC granules contain a plethora of constituents, including growth factors, cytokines, chemokines, and mitogen factors. The release of these bioactive substances from MCs occurs through distinct pathways that are initiated by the activation of specific plasma membrane receptors/channels. Here, we focus on hemichannels (HCs) formed by connexins (Cxs) and pannexins (Panxs) proteins, and we described their contribution to MC degranulation in AD, ALS, and harmful stress conditions. Cx/Panx HCs are also expressed by astrocytes and are likely involved in the release of critical toxic amounts of soluble factors—such as glutamate, adenosine triphosphate (ATP), complement component 3 derivate C3a, tumor necrosis factor (TNFα), apoliprotein E (ApoE), and certain miRNAs—known to play a role in the pathogenesis of AD, ALS, and other neurodegenerative disorders. We propose that blocking HCs on MCs and glial cells offers a promising novel strategy for ameliorating the progression of neurodegenerative diseases by reducing the release of cytokines and other pro-inflammatory compounds.

Connexins as therapeutic targets in neurological and neuropsychiatric disorders

Astrocytes represent the reticular part of the central nervous system; gap junctions formed by connexins Cx43, Cx30- and Cx26 provide for homocellular astrocyte-astrocyte coupling, whereas connexins Cx30, Cx32, Cx43, and Cx47 connect astrocytes and oligodendrocytes. Astroglial networks are anatomically and functionally segregated being homologous to neuronal ensembles. Connexons, gap junctions and hemichannels (unpaired connexons) are affected in various neuropathologies from neuropsychiatric to neurodegenerative diseases. Manipulation of astrocytic connexins modulates the size and outreach of astroglial syncytia thus affecting astroglial homeostatic support. Modulation of astrocytic connexin significantly modifies pharmacological profile of many CNS drugs, which represents an innovative therapeutic approach for CNS disorders; this approach is now actively tested in pre-clinical and clinical studies. Wide combination of connexin modulators with CNS drugs open new promising perspectives for fundamental studies and therapeutic strategies.

Contribution of astrocytes to neuropathology of neurodegenerative diseases

Classically, the loss of vulnerable neuronal populations in neurodegenerative diseases was considered to be the consequence of cell autonomous degeneration of neurons. However, progress in the understanding of glial function, the availability of improved animal models recapitulating the features of the human diseases, and the development of new approaches to derive glia and neurons from induced pluripotent stem cells obtained from patients, provided novel information that altered this view. Current evidence strongly supports the notion that non-cell autonomous mechanisms contribute to the demise of neurons in neurodegenerative disorders, and glia causally participate in the pathogenesis and progression of these diseases. In addition to microglia, astrocytes have emerged as key players in neurodegenerative diseases and will be the focus of the present review. Under the influence of pathological stimuli present in the microenvironment of the diseased CNS, astrocytes undergo morphological, transcriptional, and functional changes and become reactive. Reactive astrocytes are heterogeneous and exhibit neurotoxic (A1) or neuroprotective (A2) phenotypes. In recent years, single-cell or single-nucleus transcriptome analyses unraveled new, disease-specific phenotypes beyond A1/A2. These investigations highlighted the complexity of the astrocytic responses to CNS pathology. The present review will discuss the contribution of astrocytes to neurodegenerative diseases with particular emphasis on Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis and frontotemporal dementia. Some of the commonalties and differences in astrocyte-mediated mechanisms that possibly drive the pathogenesis or progression of the diseases will be summarized. The emerging view is that astrocytes are potential new targets for therapeutic interventions. A comprehensive understanding of astrocyte heterogeneity and disease-specific phenotypic complexity could facilitate the design of novel strategies to treat neurodegenerative disorders.

Integration of single-cell and bulk RNA sequencing data reveals key cell types and regulators in traumatic brain injury

Traumatic brain injury (TBI) is a leading cause of disability and mortality worldwide, whose symptoms ranging from mild to severe, even life-threatening. However, specific cell types and key regulators involved in traumatic brain injury have not been well elucidated. In this study, utilizing single-cell RNA-seq (scRNA-seq) data from mice with TBI, we have successfully identified and characterized 13 cell populations including astrocytes, oligodendrocyte, newly formed oligodendrocytes, microglia, two types of endothelial cells, five types of excitatory and two types of inhibitory neurons. Differential expression analysis and gene set enrichment analysis (GSEA) revealed the upregulation of microglia and endothelial markers, along with the downregulation of markers of excitatory neurons in TBI. The cell-cell communication analysis revealed that microglia and endothelial cell might interact through the interaction of Icam1-Il2rg and C1qa-Cd93, and microglia might also communicate with each other via Icam1-Itagm. The autocrine ligand-receptor in microglia might result in activation of TYROBP causal network via Icam1-Itgam. The cell-cell contact between microglia and endothelial cell might activate integrin signaling pathways. Moreover, we also found that genes involved in microglia activation were highly downregulated in Tyrobp/Dap12-deficient microglia, indicating that the upregulation of Tyrobp and TYROBP causal network in microglia might be a candidate therapeutic target in TBI. In contrast, the excitatory neurons were involved in maintaining normal brain function, and their inactivation might cause dysfunction of nervous system in TBI patients. In conclusion, the present study has discerned major cell types such as microglia, endothelial cells and excitatory neurons, and revealed key regulator such as TYROBP, C1QA, and CD93 in TBI, which shall improve our understanding of the pathogenesis of TBI.

Astroglial connexins in epileptogenesis

The astroglial network connected through gap junctions assembling from connexins physiologically balances the concentrations of ions and neurotransmitters around neurons. Astrocytic dysfunction has been associated with many neurological disorders including epilepsy. Dissociated gap junctions result in the increased activity of connexin hemichannels which triggers brain pathophysiological changes. Previous studies in patients and animal models of epilepsy indicate that the reduced gap junction coupling from assembled connexin hemichannels in the astrocytes may play an important role in epileptogenesis. This abnormal cell-to-cell communication is now emerging as an important feature of brain pathologies and being considered as a novel therapeutic target for controlling epileptogenesis. In particular, candidate drugs with ability of inhibition of connexin hemichannel activity and enhancement of gap junction formation in astrocytes should be explored to prevent epileptogenesis and control epilepsy.

Transplantation of Neural Precursors Derived from Induced Pluripotent Cells Preserve Perineuronal Nets and Stimulate Neural Plasticity in ALS Rats

A promising therapeutic strategy for amyotrophic lateral sclerosis (ALS) treatment is stem cell therapy. Neural progenitors derived from induced pluripotent cells (NP-iPS) might rescue or replace dying motoneurons (MNs). However, the mechanisms responsible for the beneficial effect are not fully understood. The aim here was to investigate the mechanism by studying the effect of intraspinally injected NP-iPS into asymptomatic and early symptomatic superoxide dismutase (SOD)1^G93A transgenic rats. Prior to transplantation, NP-iPS were characterized in vitro for their ability to differentiate into a neuronal phenotype. Motor functions were tested in all animals, and the tissue was analyzed by immunohistochemistry, qPCR, and Western blot. NP-iPS transplantation significantly preserved MNs, slowed disease progression, and extended the survival of all treated animals. The dysregulation of spinal chondroitin sulfate proteoglycans was observed in SOD1^G93A rats at the terminal stage. NP-iPS application led to normalized host genes expression (versican, has-1, tenascin-R, ngf, igf-1, bdnf, bax, bcl-2, and casp-3) and the protection of perineuronal nets around the preserved MNs. In the host spinal cord, transplanted cells remained as progenitors, many in contact with MNs, but they did not differentiate. The findings suggest that NP-iPS demonstrate neuroprotective properties by regulating local gene expression and regulate plasticity by modulating the central nervous system (CNS) extracellular matrix such as perineuronal nets (PNNs).

Metabolic and immune dysfunction of glia in neurodegenerative disorders: Focus on iPSC models

The research on neurodegenerative disorders has long focused on neuronal pathology and used transgenic mice as disease models. However, our understanding of the chronic neurodegenerative process in the human brain is still very limited. It is increasingly recognized that neuronal loss is not caused solely by intrinsic degenerative processes but rather via impaired interactions with surrounding glia and other brain cells. Dysfunctional astrocytes do not provide sufficient nutrients and antioxidants to the neurons, while dysfunctional microglia cannot efficiently clear pathogens and cell debris from extracellular space, thus resulting in chronic inflammatory processes in the brain. Importantly, human glia, especially the astrocytes, differ significantly in morphology and function from their mouse counterparts, and therefore more human-based disease models are needed. Recent advances in stem cell technology make it possible to reprogram human patients' somatic cells to induced pluripotent stem cells (iPSC) and differentiate them further into patient-specific glia and neurons, thus providing a virtually unlimited source of human brain cells. This review summarizes the recent studies using iPSC-derived glial models of Alzheimer's disease, Parkinson's disease, and amyotrophic lateral sclerosis and discusses the applicability of these models to drug testing. This line of research has shown that targeting glial metabolism can improve the survival and function of cocultured neurons and thus provide a basis for future neuroprotective treatments.

Human Astrocytes Model Derived from Induced Pluripotent Stem Cells

Induced pluripotent stem cell (iPSC)-based disease modeling has a great potential for uncovering the mechanisms of pathogenesis, especially in the case of neurodegenerative diseases where disease-susceptible cells can usually not be obtained from patients. So far, the iPSC-based modeling of neurodegenerative diseases has mainly focused on neurons because the protocols for generating astrocytes from iPSCs have not been fully established. The growing evidence of astrocytes' contribution to neurodegenerative diseases has underscored the lack of iPSC-derived astrocyte models. In the present study, we established a protocol to efficiently generate iPSC-derived astrocytes (iPasts), which were further characterized by RNA and protein expression profiles as well as functional assays. iPasts exhibited calcium dynamics and glutamate uptake activity comparable to human primary astrocytes. Moreover, when co-cultured with neurons, iPasts enhanced neuronal synaptic maturation. Our protocol can be used for modeling astrocyte-related disease phenotypes in vitro and further exploring the contribution of astrocytes to neurodegenerative diseases.

Glia in Neurodegeneration: The Housekeeper, the Defender and the Perpetrator

Over the past decade, research has unveiled the intimate relationship between neuroinflammation and neurodegeneration. Microglia and astrocytes react to brain insult by setting up a multimodal inflammatory state and act as the primary defenders and executioners of neuroinflammatory structural and functional changes. Microglia and astrocytes also play critical roles in the maintenance of normal brain function. This intricate balance of homeostatic and neuroinflammatory functions can influence the onset and the course of neurodegenerative diseases. The emergent role of the microglial-astrocytic axis in neurodegenerative disease presents many druggable targets that may have broad therapeutic benefits across neurodegenerative disease. Here, we provide a brief review of the basal function of both microglia and astrocytes, how they are changed in disease states, the significant differences between mouse and human glia, and use of human induced pluripotent stem cells derived from patients to study cell autonomous changes in human astrocytes and microglia.

Targeting connexin 43 provides anti-inflammatory effects after intracerebral hemorrhage injury by regulating YAP signaling

Background

In the central nervous system (CNS), connexin 43 (Cx43) is mainly expressed in astrocytes and regulates astrocytic network homeostasis. Similar to Cx43 overexpression, abnormal excessive opening of Cx43 hemichannels (Cx43Hcs) on reactive astrocytes aggravates the inflammatory response and cell death in CNS pathologies. However, the role of excessive Cx43Hc opening in intracerebral hemorrhage (ICH) injury is not clear.

Methods

Hemin stimulation in primary cells and collagenase IV injection in C57BL/6J (B6) mice were used as ICH models in vitro and in vivo. After ICH injury, the Cx43 mimetic peptide Gap19 was used for treatment. Ethidium bromide (EtBr) uptake assays were used to measure the opening of Cx43Hcs. Western blotting and immunofluorescence were used to measure protein expression. qRT-PCR and ELISA were used to determine the levels of cytokines. Coimmunoprecipitation (Co-IP) and the Duolink in situ proximity ligation assay (PLA) were applied to measure the association between proteins.

Results

In this study, Cx43 expression upregulation and excessive Cx43Hc opening was observed in mice after ICH injury. Delayed treatment with Gap19 significantly alleviated hematoma volume and neurological deficits after ICH injury. In addition, Gap19 decreased inflammatory cytokine levels in the tissue surrounding the hematoma and decreased reactive astrogliosis after ICH injury in vitro and in vivo. Intriguingly, Cx43 transcriptional activity and expression in astrocytes were significantly increased after hemin stimulation in culture. However, Gap19 treatment downregulated astrocytic Cx43 expression through the ubiquitin-proteasome pathway without affecting Cx43 transcription. Additionally, our data showed that Gap19 increased Yes-associated protein (YAP) nuclear translocation. This subsequently upregulated SOCS1 and SOCS3 expression and then inhibited the TLR4-NFκB and JAK2-STAT3 pathways in hemin-stimulated astrocytes. Finally, the YAP inhibitor, verteporfin (VP), reversed the anti-inflammatory effect of Gap19 in vitro and almost completely blocked its protective effects in vivo after ICH injury.

Conclusions

This study provides new insight into potential treatment strategies for ICH injury involving astroglial Cx43 and Cx43Hcs. Suppression of abnormal astroglial Cx43 expression and Cx43Hc opening by Gap19 has anti-inflammatory and neuroprotective effects after ICH injury.

Astrocytes: From the Physiology to the Disease

Astrocytes are key cells for adequate brain formation and regulation of cerebral blood flow as well as for the maintenance of neuronal metabolism, neurotransmitter synthesis and exocytosis, and synaptic transmission. Many of these functions are intrinsically related to neurodegeneration, allowing refocusing on the role of astrocytes in physiological and neurodegenerative states. Indeed, emerging evidence in the field indicates that abnormalities in the astrocytic function are involved in the pathogenesis of multiple neurodegenerative diseases, including Alzheimer's Disease (AD), Parkinson's Disease (PD), Huntington's Disease (HD) and Amyotrophic Lateral Sclerosis (ALS). In the present review, we highlight the physiological role of astrocytes in the CNS, including their communication with other cells in the brain. Furthermore, we discuss exciting findings and novel experimental approaches that elucidate the role of astrocytes in multiple neurological disorders.