Müller cells, the major type of glial cells in the retina, are responsible for the homeostatic and metabolic support of retinal neurons. By mediating transcellular ion, water, and bicarbonate transport, Müller cells control the composition of the extracellular space fluid. Müller cells provide trophic and anti‐oxidative support of photoreceptors and neurons and regulate the tightness of the blood‐retinal barrier. By the uptake of glutamate, Müller cells are more directly involved in the regulation of the synaptic activity in the inner retina. This review gives a survey of recently discoved new functions of Müller cells. Müller cells are living optical fibers that guide light through the inner retinal tissue. Thereby they enhance the signal/noise ratio by minimizing intraretinal light scattering and conserve the spatial distribution of light patterns in the propagating image. Müller cells act as soft, compliant embedding for neurons, protecting them in case of mechanical trauma, and also as soft substrate required for neurite growth and neuronal plasticity. Müller cells release neuroactive signaling molecules which modulate neuronal activity, are implicated in the mediation of neurovascular coupling, and mediate the homeostasis of the extracellular space volume under hypoosmotic conditions which are a characteristic of intense neuronal activity. Under pathological conditions, a subset of Müller cells may differentiate to neural progenitor/stem cells which regenerate lost photoreceptors and neurons. Increasing knowledge of Müller cell function and responses in the normal and diseased retina will have great impact for the development of new therapeutic approaches for retinal diseases.
The unravelling of the polarized distribution of AQP4 in perivascular astrocytic endfeet has revitalized the interest in the role of astrocytes in controlling water and ion exchange at the brain-blood interface. The importance of the endfeet is based on the premise that they constitute a complete coverage of the vessel wall. Despite a number of studies based on different microscopic techniques this question has yet to be resolved. We have made an electron microscopic 3D reconstruction of perivascular endfeet in CA1 (stratum moleculare) of rat hippocampus. The endfeet inter-digitate and overlap, leaving no slits between them. Only in a few sites do processes-tentatively classified as processes of microglia-extend through the perivascular glial sheath to establish direct contact with the endothelial basal lamina. In contrast to the endfoot covering of the endothelial tube, the endfoot covering of the pericyte is incomplete, allowing neuropil elements to touch the basal lamina that enwraps this type of cell. The 3D reconstruction also revealed large bundles of mitochondria in the endfoot processes that came in close apposition to the perivascular endfoot membrane. Our data support the idea that in pathophysiological conditions, the perivascular astrocytic covering may control the exchange of water and solutes between blood and brain and that free diffusion is limited to narrow clefts between overlapping endfeet. (C) 2010 Wiley-Liss, Inc.
Inflammation is implicated in the progressive nature of neurodegenerative diseases, such as Parkinson's disease, but the mechanisirts are poorly understood. A single systemic lipopolysaccharide (LPS, 5 mg/kg, i.p.) or tumor necrosis factor alpha (TNF alpha, 0.25 mg/kg, i.p.) injection was administered in adult wild-type mice and in mice lacking TNF alpha receptors (TNF R1/R2(-/-)) to discern the mechanisms of inflammation transfer from the periphery to the brain and the neurodegenerative consequences. Systemic LPS administration resulted in rapid brain TNFa increase that remained elevated for 10 months, while peripheral TNF alpha (serum and liver) had subsided by 9 h (serum) and 1 week (liver). Systemic TNF alpha and LPS administration activated microglia and increased expression of brain pro-inflammatory factors (i.e., TNF alpha, MCP-1, IL-1 beta and NF-kappa B p65) in wild-type mice, but not in TNF R1/R2(-/-) mice. Further, LPS reduced the number of tyrosine hydroxylase-immunoreactive neurons in the substantia nigra (SN) by 23% at 7-months post-treatment, which progressed to 47% at 10 months. Together, these data demonstrate that through TNF alpha, peripheral inflammation in adult animals can: (1) activate brain microglia to produce chronically elevate roinflammatory factors; (2) induce delayed and progressive loss of DA neurons in the SN. These findings provide valuable insight into the potential pathogenesis and self-propelling nature of Parkinson's disease. (c) 2007 Wiley-Liss, Inc.
Activation of the peripheral immune system elicits a coordinated response from the central nervous system. Key to this immune to brain communication is that glia, microglia, and astrocytes, interpret and propagate inflammatory signals in the brain that influence physiological and behavioral responses. One issue in glial biology is that morphological analysis alone is used to report on glial activation state. Therefore, our objective was to compare behavioral responses after in vivo immune (lipopolysaccharide, LPS) challenge to glial specific mRNA and morphological profiles. Here, LPS challenge induced an immediate but transient sickness response with decreased locomotion and social interaction. Corresponding with active sickness behavior (2–12 h), inflammatory cytokine mRNA expression was elevated in enriched microglia and astrocytes. Although proinflammatory cytokine expression in microglia peaked 2‐4 h after LPS, astrocyte cytokine, and chemokine induction was delayed and peaked at 12 h. Morphological alterations in microglia (Iba‐1 + ) and astrocytes (GFAP + ), however, were undetected during this 2–12 h timeframe. Increased Iba‐1 immunoreactivity and de‐ramified microglia were evident 24 and 48 h after LPS but corresponded to the resolution phase of activation. Morphological alterations in astrocytes were undetected after LPS. Additionally, glial cytokine expression did not correlate with morphology after four repeated LPS injections. In fact, repeated LPS challenge was associated with immune and behavioral tolerance and a less inflammatory microglial profile compared with acute LPS challenge. Overall, induction of glial cytokine expression was sequential, aligned with active sickness behavior, and preceded increased Iba‐1 or GFAP immunoreactivity after LPS challenge. GLIA 2016;64:300–316 Glial induction of cytokines was sequential, aligned with active sickness behavior, and preceded increased Iba1 or GFAP immunoreactivity after LPS challenge. Microglial Iba1 or astrocytic GFAP immunoreactivity are unreliable indicators of activation.
Cell-based therapies are attractive approaches to promote myelin repair. Recent studies demonstrated a reduction in disease burden in mice with experimental allergic encephalomyelitis (EAE) treated with mouse mesenchymal stem cells (MSCs). Here, we demonstrated human bone marrow-derived MSCs (BM-hMSCs) promote functional recovery in both chronic and relapsing-remitting models of mouse EAE, traced their migration into the injured CNS and assayed their ability to modulate disease progression and the host immune response. Injected BM-hMSCs accumulated in the CNS, reduced the extent of damage and increased oligodendrocyte lineage cells in lesion areas. The increase in oligodendrocytes in lesions may reflect BM-hMSC-induced changes in neural fate determination, since neurospheres from treated animals gave rise to more oligodendrocytes and less astrocytes than nontreated neurospheres. Host immune responses were also influenced by BM-hMSCs. Inflammatory T-cells including interferon gamma producing Th1 cells and IL-17 producing Th17 inflammatory cells and their associated cytokines were reduced along with concomitant increases in IL-4 producing Th2 cells and anti-inflammatory cytokines. Together, these data suggest that the BM-hMSCs represent a viable option for therapeutic approaches. (C) 2009 Wiley-Liss, Inc.
There is increasing evidence that astrocytes play important roles in immune regulation in the brain. Astrocytes express toll-like receptors (TLR) and build up responses to innate immune triggers by releasing proinflammatory molecules. We investigate signaling pathways and released molecules after astrocyte TLR4 activation. Purified rodent brain astrocyte cultures were treated with the TLR4 activator bacterial lipopolysaccharide (LPS). Tools used to interfere with this system include small interference RNA, inhibitory drugs, and MyD88 or Stat1 deficient mice. LPS induced early activation of the transcription factor NF kappa B, through the MyD88 adaptor, and expression of TNF-alpha, VCAM-1, IL-15, and IL-27. LPS also induced delayed Jak1/Stat1 activation, which was MyD88-independent but was not mediated by IFN-beta. Jak1/Stat1 activation induced the expression of negative cytokine regulator SOCS-1 and CXCL10 chemokine (IP-10). Mitogen-activated protein kinases (MAPK) were also involved in TLR4 signaling in a MyD88-independent fashion. p38 exerted a strong influence on LPS-induced gene expression by regulating the phosphorylation of Stat1 and the transcriptional activity of NF kappa B, while JNK regulated the Jak1/Stat1 pathway, and ERK1/2 controlled the expression of Egr-1 and influenced MyD88-dependent MMP-9 expression. Interplay between these signals was evidenced by the increased induction of MMP-9 in Stat1-deficient cells challenged with LPS, suggesting that Stat1 negatively regulates the expression of MMP-9 induced by LPS. Therefore, astrocytes are responsive to TLR4 activation by inducing a complex set of cell-dependent molecular reactions mediated by NF kappa B, MAPK and Jak1/Stat1 signaling pathways. Here we identified cross-talking signals generating a proinflammatory environment that will modulate the response of surrounding cells. (C) 2010 Wiley-Liss, Inc.
In multiple sclerosis, endogenous oligodendrocyte precursor cells (OPCs) attempt to remyelinate areas of myelin damage. During disease progression, however, these attempts fail. It has been suggested that modulating the inflammatory environment of the lesion might provide a promising therapeutic approach to promote endogenous remyelination. Microglia are known to play a central role in neuroinflammatory processes. To investigate the microglia phenotype that supports remyelination, we performed genome-wide gene expression analysis of microglia from the corpus callosum during demyelination and remyelination in the mouse cuprizone model, in which remyelination spontaneously occurs after an episode of toxin-induced primary demyelination. We provide evidence for the existence of a microglia phenotype that supports remyelination already at the onset of demyelination and persists throughout the remyelination process. Our data show that microglia are involved in the phagocytosis of myelin debris and apoptotic cells during demyelination. Furthermore, they express a cytokine and chemokine repertoire enabling them to activate and recruit endogenous OPCs to the lesion site and deliver trophic support during remyelination. This study not only provides a detailed transcriptomic analysis of the remyelination-supportive microglia phenotype but also reinforces the notion that the primary function of microglia is the maintenance of tissue homeostasis and the support of regeneration already at the earliest stages in the development of demyelinating lesions. (c) 2011 Wiley Periodicals, Inc.
Inflammation is a common component of acute injuries of the central nervous system (CNS) such as ischemia, and degenerative disorders such as Alzheimer's disease. Glial cells play important roles in local CNS inflammation, and an understanding of the roles for microRNAs in glial reactivity in injury and disease settings may therefore lead to the development of novel therapeutic interventions. Here, we show that the miR‐181 family is developmentally regulated and present in high amounts in astrocytes compared to neurons. Overexpression of miR‐181c in cultured astrocytes results in increased cell death when exposed to lipopolysaccharide (LPS). We show that miR‐181 expression is altered by exposure to LPS, a model of inflammation, in both wild‐type and transgenic mice lacking both receptors for the inflammatory cytokine TNF‐α. Knockdown of miR‐181 enhanced LPS‐induced production of pro‐inflammatory cytokines (TNF‐α, IL‐6, IL‐1β, IL‐8) and HMGB1, while overexpression of miR‐181 resulted in a significant increase in the expression of the anti‐inflammatory cytokine IL‐10. To assess the effects of miR‐181 on the astrocyte transcriptome, we performed gene array and pathway analysis on astrocytes with reduced levels of miR‐181b/c. To examine the pool of potential miR‐181 targets, we employed a biotin pull‐down of miR‐181c and gene array analysis. We validated the mRNAs encoding MeCP2 and X‐linked inhibitor of apoptosis as targets of miR‐181. These findings suggest that miR‐181 plays important roles in the molecular responses of astrocytes in inflammatory settings. Further understanding of the role of miR‐181 in inflammatory events and CNS injury could lead to novel approaches for the treatment of CNS disorders with an inflammatory component.
The past decade has witnessed a revolution in our understanding of microglia. These immune cells were shown to actively remodel neuronal circuits, leading to propose new pathogenic mechanisms. To study microglial implication in the loss of synapses, the best pathological correlate of cognitive decline across chronic stress, aging, and diseases, we recently conducted ultrastructural analyses. Our work uncovered the existence of a new microglial phenotype that is rarely present under steady state conditions, in hippocampus, cerebral cortex, amygdala, and hypothalamus, but becomes abundant during chronic stress, aging, fractalkine signaling deficiency (CX 3 CR1 knockout mice), and Alzheimer's disease pathology (APP‐PS1 mice). Even though these cells display ultrastructural features of microglia, they are strikingly distinct from the other phenotypes described so far at the ultrastructural level. They exhibit several signs of oxidative stress, including a condensed, electron‐dense cytoplasm and nucleoplasm making them as “dark” as mitochondria, accompanied by a pronounced remodeling of their nuclear chromatin. Dark microglia appear to be much more active than the normal microglia, reaching for synaptic clefts, while extensively encircling axon terminals and dendritic spines with their highly ramified and thin processes. They stain for the myeloid cell markers IBA1 and GFP (in CX 3 CR1‐GFP mice), and strongly express CD11b and microglia‐specific 4D4 in their processes encircling synaptic elements, and TREM2 when they associate with amyloid plaques. Overall, these findings suggest that dark microglia, a new phenotype that we identified based on their unique properties, could play a significant role in the pathological remodeling of neuronal circuits, especially at synapses. GLIA 2016;64:826–839 We describe a new microglial phenotype. These cells appear extremely active at the synapse and show signs of oxidative stress. They are abundant during chronic stress, aging, fractalkine signaling deficiency, and Alzheimer's disease pathology.
Amongst neurological diseases, multiple sclerosis (MS) presents an attractive target for regenerative medicine. This is because the primary pathology, the loss of myelin‐forming oligodendrocytes, can be followed by a spontaneous and efficient regenerative process called remyelination. While cell transplantation approaches have been explored as a means of replacing lost oligodendrocytes, more recently therapeutic approaches that target the endogenous regenerative process have been favored. This is in large part due to our increasing understanding of (1) the cell types within the adult brain that are able to generate new oligodendrocytes, (2) the mechanisms and pathways by which this achieved, and (3) an emerging awareness of the reasons why remyelination efficiency eventually fails. Here we review some of these advances and also highlight areas where questions remain to be answered in both the biology and translational potential of this important regenerative process. GLIA 2014;62:1905–1915 Remyelination is a spontaneously occurring regenerative process in the CNS that follows primary demyelination. It involves the generation of new oligodendrocytes from a widespread population of adult progenitor cells. Increasing knowledge about the biology of remyelination have led to the identification of several therapeutic targets by which remyelination might be pharmacologically enhanced in chronic demyelinating disease such as multiple sclerosis.
CB1 and CB2 receptors are activated by a plethora of cannabinoid compounds, be they endogenously-produced, plant-derived or synthetic. These receptors are expressed by microglia, astrocytes and astrocytomas, and their activation regulates these cells' differentiation, functions and viability. Recent studies show that glial cells also express cannabinoid-like receptors, and that their activation regulates different cell functions, but also control cell viability. This review summarizes this evidence, and discusses how selective compounds targeting cannabinoid-like receptors constitute promising therapeutics to manage neuroinflammation and eradicate malignant astrocytomas. Importantly, the selective targeting of cannabinoid-like receptors should provide therapeutic relieve without inducing the typical psychotropic effects and possible addictive properties associated with the use of Delta 9-tetrahydrocannabinol, the main psychotropic ingredient produced by the plant Cannabis sativa (C) 2010 Wiley-Liss, Inc.
The origin of α‐synuclein (α‐syn)‐positive glial cytoplasmic inclusions found in oligodendrocytes in multiple system atrophy (MSA) is enigmatic, given the fact that oligodendrocytes do not express α‐syn mRNA. Recently, neuron‐to‐neuron transfer of α‐syn was suggested to contribute to the pathogenesis of Parkinson's disease. In this study, we explored whether a similar transfer of α‐syn might occur from neurons to oligodendrocytes, which conceivably could explain how glial cytoplasmic inclusions are formed. We studied oligodendrocytes in vitro and in vivo and examined their ability to take up different α‐syn assemblies. First, we treated oligodendrocytes with monomeric, oligomeric, and fibrillar forms of α‐syn proteins and investigated whether α‐syn uptake is dynamin‐dependent. Second, we injected the same α‐syn species into the mouse cortex to assess their uptake in vivo . Finally, we monitored the presence of human α‐syn within rat oligodendroglial cells grafted in the striatum of hosts displaying Adeno‐Associated Virus‐mediated overexpression of human α‐syn in the nigro‐striatal pathway. Here , we show that oligodendrocytes take up recombinant α‐syn monomers, oligomers and, to a lesser extent, fibrils in vitro in a concentration and time‐dependent manner, and that this process is inhibited by dynasore. Further, we demonstrate in our injection model that oligodendrocytes also internalize α‐syn in vivo . Finally, we provide the first direct evidence that α‐syn can transfer to grafted oligodendroglial cells from host rat brain neurons overexpressing human α‐syn. Our findings support the hypothesis of a neuron‐to‐oligodendrocyte transfer of α‐syn, a mechanism that may play a crucial role in the progression and pathogenesis of MSA. GLIA 2014;62:387–398 We demonstrate that oligodendrocytes take up recombinant α‐synuclein monomers, oligomers and, to a lesser extent, fibrils in vitro and in vivo . This uptake is dynamin‐dependent. We also demonstrate that α‐synuclein can transfer into grafted oligodendroglial cells from neurons in the host brain.
While histological changes in microglia have long been recognized as a pathological feature of Alzheimer's disease (AD), recent genetic association studies have also strongly implicated microglia in the etiology of the disease. Coding and noncoding polymorphisms in several genes expressed in microglia—including APOE , TREM2 , CD33 , GRN , and IL1RAP —alter AD risk, and therefore could be considered as entry points for therapeutic intervention. Furthermore, microglia may have a substantial effect on current amyloid β (Aβ) and tau immunotherapy approaches, since they are the primary cell type in the brain to mediate Fc receptor‐facilitated antibody effector function. In this review, we discuss the considerations in selecting microglial therapeutic targets from the perspective of drug discovery feasibility, and consider the role of microglia in ongoing immunotherapy clinical strategies. GLIA 2016;64:1710–1732 Microglial phenotypes are altered in Alzheimer's disease (AD). Microglial‐expressed genes that confer risk for AD are potential entry points for therapeutic intervention. Microglia are important components of the efficacy of Aβ and tau immunotherapy clinical approaches.
Central nervous system (CNS) trauma involves extensive cellular damage that is due, in part, to an innate inflammatory response induced by extracellular ATP. The innate immune response is regulated by pattern recognition receptors (PRRs), which include NOD‐like receptors (NLRs). The PRRs and signaling cascades that regulate innate glial responses to CNS injury remain largely undefined. In this report, we show that human astrocytes express the NLR protein 2 (NLRP2) inflammasome that is activated by the danger associated molecular pattern (DAMP) ATP. The NLRP2 inflammasome is a multiprotein complex that consists of NLRP2, the adaptor protein apoptosis‐speck‐like protein containing a caspase recruitment domain (ASC) and caspase‐1. NLRP2 also interacts with the P2X7 receptor and the pannexin 1 channel. Stimulation of human astrocytes with ATP resulted in activation of the NLRP2 inflammasome leading to the processing of inflammatory caspase‐1 and interleukin‐1β (IL‐1β). ATP‐induced activation of the NLRP2 inflammasome was inhibited by the pannexin 1 inhibitor probenecid and by the P2X7 receptor antagonist Brilliant Blue G (BBG). siRNA knockdown of NLRP2 significantly decreased NLRP2 levels and caspase‐1 processing in human astrocytes in response to ATP. Our findings suggest that the astrocytic NLRP2 inflammasome is an important component of the CNS inflammatory response and that the NLRP2 inflammasome may be a therapeutic target to inhibit inflammation induced by CNS injury.
Network activity in the brain is associated with a transient increase in extracellular K + concentration. The excess K + is removed from the extracellular space by mechanisms proposed to involve Kir4.1‐mediated spatial buffering, the Na + /K + /2Cl − cotransporter 1 (NKCC1), and/or Na + /K + ‐ATPase activity. Their individual contribution to [K + ] o management has been of extended controversy. This study aimed, by several complementary approaches, to delineate the transport characteristics of Kir4.1, NKCC1, and Na + /K + ‐ATPase and to resolve their involvement in clearance of extracellular K + transients. Primary cultures of rat astrocytes displayed robust NKCC1 activity with [K + ] o increases above basal levels. Increased [K + ] o produced NKCC1‐mediated swelling of cultured astrocytes and NKCC1 could thereby potentially act as a mechanism of K + clearance while concomitantly mediate the associated shrinkage of the extracellular space. In rat hippocampal slices, inhibition of NKCC1 failed to affect the rate of K + removal from the extracellular space while Kir4.1 enacted its spatial buffering only during a local [K + ] o increase. In contrast, inhibition of the different isoforms of Na + /K + ‐ATPase reduced post‐stimulus clearance of K + transients. The astrocyte‐characteristic α2β2 subunit composition of Na + /K + ‐ATPase, when expressed in Xenopus oocytes, displayed a K + affinity and voltage‐sensitivity that would render this subunit composition specifically geared for controlling [K + ] o during neuronal activity. In rat hippocampal slices, simultaneous measurements of the extracellular space volume revealed that neither Kir4.1, NKCC1, nor Na + /K + ‐ATPase accounted for the stimulus‐induced shrinkage of the extracellular space. Thus, NKCC1 plays no role in activity‐induced extracellular K + recovery in native hippocampal tissue while Kir4.1 and Na + /K + ‐ATPase serve temporally distinct roles. GLIA 2014;62:608–622 Kir4.1 and Na + /K + −ATPase serve distinct roles in recovery of activity‐induced [K + ] o in hippocampus. The α2β2 Na + /K + −ATPase is specifically geared for controlling [K + ] o . NKCC1 is not involved in stimulus‐induced shrinkage of the extracellular space.