Resting microglia directly monitor the functional state of synapses in vivo and determine the fate of ischemic terminals. J Neurosci. 2009;29:3974-3980. [PMID: 19339593 DOI: 10.1523/jneurosci.4363-08.2009]

The role of microglia in the healthy brain

Microglia were recently shown to play unexpected roles in normal brain development and adult physiology. This has begun to dramatically change our view of these resident "immune" cells. Here, we briefly review topics covered in our 2011 Society for Neuroscience minisymposium "The Role of Microglia in the Healthy Brain." This summary is not meant to be a comprehensive review of microglia physiology, but rather to share new results and stimulate further research into the cellular and molecular mechanisms by which microglia influence postnatal development, adult neuronal plasticity, and circuit function.

In vivo dynamics of innate immune sentinels in the CNS

The innate immune system is comprised of cellular sentinels that often serve as the first responders to injury and invading pathogens. Our basic understanding of innate immunity is derived from research conducted in peripheral lymphoid tissues. However, it is now recognized that most non-lymphoid tissues throughout the body are equipped with specialized innate immune cells that are uniquely adapted to the niches in which they reside. The central nervous system (CNS) is a particularly interesting compartment because it contains a population of post-mitotic cells (neurons) that are intolerant of robust, cytopathic inflammatory responses observed in many peripheral tissues. Thus, evolutionary adaptations have fitted the CNS with a unique array of innate immune sentinels that facilitate the development of local inflammatory responses but attempt to do so in a manner that preserves the integrity of its post-mitotic residents. Interestingly, studies have even suggested that CNS resident innate immune cells contribute to the homeostasis of this compartment and promote neural activity. In this review we discuss recent advances in our understanding of CNS innate immune sentinels and how novel imaging approaches such as intravital two-photon laser scanning microscopy (TPLSM) have shed light on these cells during states of health and disease.

Microglia and memory: modulation by early-life infection

The proinflammatory cytokine interleukin-1β (IL-1β) is critical for normal hippocampus (HP)-dependent cognition, whereas high levels can disrupt memory and are implicated in neurodegeneration. However, the cellular source of IL-1β during learning has not been shown, and little is known about the risk factors leading to cytokine dysregulation within the HP. We have reported that neonatal bacterial infection in rats leads to marked HP-dependent memory deficits in adulthood. However, deficits are only observed if unmasked by a subsequent immune challenge [lipopolysaccharide (LPS)] around the time of learning. These data implicate a long-term change within the immune system that, upon activation with the "second hit," LPS, acutely impacts the neural processes underlying memory. Indeed, inhibiting brain IL-1β before the LPS challenge prevents memory impairment in neonatally infected (NI) rats. We aimed to determine the cellular source of IL-1β during normal learning and thereby lend insight into the mechanism by which this cytokine is enduringly altered by early-life infection. We show for the first time that CD11b(+) enriched cells are the source of IL-1β during normal HP-dependent learning. CD11b(+) cells from NI rats are functionally sensitized within the adult HP and produce exaggerated IL-1β ex vivo compared with controls. However, an exaggerated IL-1β response in vivo requires LPS before learning. Moreover, preventing microglial activation during learning prevents memory impairment in NI rats, even following an LPS challenge. Thus, early-life events can significantly modulate normal learning-dependent cytokine activity within the HP, via a specific, enduring impact on brain microglial function.

Regulation of microglia by ionotropic glutamatergic and GABAergic neurotransmission

Recent studies have indicated that constitutive functions of microglia in the healthy adult central nervous system (CNS) involve immune surveillance, synapse maintenance and trophic support. These functions have been related to the ramified structure of 'resting' microglia and the prominent motility in their processes that provide extensive coverage of the entire extracellular milleu. In this review, we examine how external signals, and in particular, ionotropic neurotransmission, regulate features of microglial morphology and process motility. Current findings indicate that microglial physiology in the healthy CNS is constitutively and reciprocally regulated by endogenous ionotropic glutamatergic and GABAergic neurotransmission. These influences do not act directly on microglial cells but indirectly via the activity-dependent release of ATP, likely through a mechanism involving pannexin channels. Microglia in the 'resting' state are not only dynamically active, but also constantly engaged in ongoing communication with neuronal and macroglial components of the CNS in a functionally relevant way.

Microglia activation triggers astrocyte-mediated modulation of excitatory neurotransmission

Fine control of neuronal activity is crucial to rapidly adjust to subtle changes of the environment. This fine tuning was thought to be purely neuronal until the discovery that astrocytes are active players of synaptic transmission. In the adult hippocampus, microglia are the other major glial cell type. Microglia are highly dynamic and closely associated with neurons and astrocytes. They react rapidly to modifications of their environment and are able to release molecules known to control neuronal function and synaptic transmission. Therefore, microglia display functional features of synaptic partners, but their involvement in the regulation of synaptic transmission has not yet been addressed. We have used a combination of pharmacological approaches with electrophysiological analysis on acute hippocampal slices and ATP assays in purified cell cultures to show that activation of microglia induces a rapid increase of spontaneous excitatory postsynaptic currents. We found that this modulation is mediated by binding of ATP to P2Y1R located on astrocytes and is independent of TNFα or NOS2. Our data indicate that, on activation, microglia cells rapidly release small amounts of ATP, and astrocytes, in turn, amplified this release. Finally, P2Y1 stimulation of astrocytes increased excitatory postsynaptic current frequency through a metabotropic glutamate receptor 5-dependent mechanism. These results indicate that microglia are genuine regulators of neurotransmission and place microglia as upstream partners of astrocytes. Because pathological activation of microglia and alteration of neurotransmission are two early symptoms of most brain diseases, our work also provides a basis for understanding synaptic dysfunction in neuronal diseases.

Genetic approaches to study glial cells in the rodent brain

The development, function, and pathology of the brain depend on interactions of neurons and different types of glial cells, namely astrocytes, oligodendrocytes, microglia, and ependymal cells. Understanding neuron-glia interactions in vivo requires dedicated experimental approaches to manipulate each cell type independently. In this review, we first summarize techniques that allow for cell-specific gene modification including targeted mutagenesis and viral transduction. In the second part, we describe the genetic models that allow to target the main glial cell types in the central nervous system. The existing arsenal of approaches to study glial cells in vivo and its expansion in the future are key to understand neuron-glia interactions under normal and pathologic conditions.

TRPV1-dependent and -independent alterations in the limbic cortex of neuropathic mice: impact on glial caspases and pain perception

During neuropathic pain, caspases are activated in the limbic cortex. We investigated the role of TRPV1 channels and glial caspases in the mouse prelimbic and infralimbic (PL-IL) cortex after spared nerve injury (SNI). Reverse transcriptase-polymerase chain reaction, western blots, and immunfluorescence showed overexpression of several caspases in the PL-IL cortex 7 days postinjury. Caspase-3 release and upregulation of AMPA receptors in microglia, caspase-1 and IL-1β release in astrocytes, and upregulation of Il-1 receptor-1, TRPV1, and VGluT1 in glutamatergic neurons, were also observed. Of these alterations, only those in astrocytes persisted in SNI Trpv1(-/-) mice. A pan-caspase inhibitor, injected into the PL-IL cortex, reduced mechanical allodynia, this effect being reduced but not abolished in Trpv1(-/-) mice. Single-unit extracellular recordings in vivo following electrical stimulation of basolateral amygdala or application of pressure on the hind paw, showed increased excitatory pyramidal neuron activity in the SNI PL-IL cortex, which also contained higher levels of the endocannabinoid 2-arachidonoylglycerol. Intra-PL-IL cortex injection of mGluR5 and NMDA receptor antagonists and AMPA exacerbated, whereas TRPV1 and AMPA receptor antagonists and a CB(1) agonist inhibited, allodynia. We suggest that SNI triggers both TRPV1-dependent and independent glutamate- and caspase-mediated cross-talk among IL-PL cortex neurons and glia, which either participates or counteracts pain.

The role of glial cells in synapse elimination

Excessive synapses generated during early development are eliminated extensively to form functionally mature neural circuits. Synapses in juvenile and mature brains are highly dynamic, and undergo remodeling processes through constant formation and elimination of dendritic spines. Although neural activity has been implicated in initiating the synapse elimination process cell-autonomously, the cellular and molecular mechanisms that transduce changes in correlated neural activity into structural changes in synapses are largely unknown. Recently, however, new findings provide evidence that in different species, glial cells, non-neuronal cell types in the nervous system are crucial in eliminating neural debris and unwanted synapses through phagocytosis. Glial cells not only clear fragmented axons and synaptic debris produced during synapse elimination, but also engulf unwanted synapses thereby actively promoting synapse elimination non-cell autonomously. These new findings support the important role of glial cells in the formation and maintenance of functional neural circuits in development as well as in adult stages and neurodegenerative diseases.

Physiological roles of microglia during development

In all the species examined thus far, the behavior of microglia during development appears to be highly stereotyped. This reproducibility supports the notion that these cells have a physiological role in development. Microglia are macrophages that migrate from the yolk sac and colonize the central nervous system early during development. The first invading yolk-sac macrophages are highly proliferative and their role has not yet been addressed. At later developmental stages, microglia can be found throughout the brain and tend to preferentially reside at specific locations that are often associated with known developmental processes. Thus, it appears that microglia concentrate in areas of cell death, in proximity of developing blood vessels, in the marginal layer, which contains developing axon fascicles, and in close association with radial glial cells. This review describes the main features of brain colonization by microglia and discusses the possible physiological roles of these cells during development.

Cortical microhemorrhages cause local inflammation but do not trigger widespread dendrite degeneration

Microhemorrhages are common in the aging brain, and their incidence is correlated with increased risk of neurodegenerative disease. Past work has shown that occlusion of individual cortical microvessels as well as large-scale hemorrhages can lead to degeneration of neurons and increased inflammation. Using two-photon excited fluorescence microscopy in anesthetized mice, we characterized the acute and chronic dynamics of vessel bleeding, tissue compression, blood flow change, neural degeneration, and inflammation following a microhemorrhage caused by rupturing a single penetrating arteriole with tightly-focused femtosecond laser pulses. We quantified the extravasation of red blood cells (RBCs) and blood plasma into the brain and determined that the bleeding was limited by clotting. The vascular bleeding formed a RBC-filled core that compressed the surrounding parenchymal tissue, but this compression was not sufficient to crush nearby brain capillaries, although blood flow speeds in these vessels was reduced by 20%. Imaging of cortical dendrites revealed no degeneration of the large-scale structure of the dendritic arbor up to 14 days after the microhemorrhage. Dendrites close to the RBC core were displaced by extravasating RBCs but began to relax back one day after the lesion. Finally, we observed a rapid inflammatory response characterized by morphology changes in microglia/macrophages up to 200 µm from the microhemorrhage as well as extension of cellular processes into the RBC core. This inflammation persisted over seven days. Taken together, our data suggest that a cortical microhemorrhage does not directly cause significant neural pathology but does trigger a sustained, local inflammatory response.

Sensory input regulates spatial and subtype-specific patterns of neuronal turnover in the adult olfactory bulb

Throughout life, new neurons are added and old ones eliminated in the adult mouse olfactory bulb. Previous studies suggested that olfactory experience controls the process by which new neurons are integrated into mature circuits. Here we report novel olfactory-experience-dependent mechanisms of neuronal turnover. Using two-photon laser-scanning microscopy and sensory manipulations in adult live mice, we found that the neuronal turnover was dynamically controlled by olfactory input in a neuronal subtype-specific manner. Olfactory input enhanced this turnover, which was characterized by the reiterated use of the same positions in the glomeruli by new neurons. Our results suggest that olfactory-experience-dependent modification of neuronal turnover confers structural plasticity and stability on the olfactory bulb.

Transcranial two-photon imaging of the living mouse brain

This protocol describes imaging of the living mouse brain through a thinned skull using two-photon microscopy. This transcranial two-photon laser-scanning microscope (TPLSM) imaging method allows high-resolution imaging of fluorescently labeled neurons, microglia, astrocytes, and blood vessels, as well as subcellular structures such as dendritic spines and axonal varicosities. The surgical procedure that is required to allow imaging thins the cranium so that it becomes optically transparent. Once learned, the surgery can be performed in ∼30 min, and imaging can follow immediately. The procedure can be repeated multiple times, allowing brain cells and tissues to be studied in the same animals over short or long time intervals, depending on the design of the experiment. Two-photon imaging through a thinned and intact skull avoids side effects caused by skull removal and is a minimally invasive method for studying the living mouse brain under physiological and pathological conditions.

Evidence that microglia mediate the neurobiological effects of chronic psychological stress on the medial prefrontal cortex

Psychological stress contributes to the development of clinical depression. This has prompted many preclinical studies to investigate the neurobiology of this relationship, however, the effects of stress on glia remain unclear. In this study, we wished to determine, first, how exposure to chronic psychological stress affects microglial activity within the prefrontal cortex (PFC) and, second, whether the observed changes were meaningfully related to corresponding changes in local neuronal activity and PFC-regulated behavior. Therefore, we examined markers of microglial activation, antigen presentation, apoptosis, and persistent neuronal activation within the PFC after exposure to repeated restraint stress. We also examined the effect of stress on spatial working memory, a PFC-dependent function. Finally, we tested the ability of a microglial activation inhibitor (minocycline) to alter the impact of chronic stress on all of these endpoints. Stressor exposure produced positively correlated increases in microglial and long-term neuronal activation in the PFC but not antigen presentation or apoptosis. As expected, it also impaired spatial working memory. Importantly, minocycline reduced the impact of stress on neuronal activation and working memory, as well as microglial activation. These results suggest a role for microglia in mediating the effects of stress on PFC neuronal function and PFC-regulated behavior.

Selective transfer of exosomes from oligodendrocytes to microglia by macropinocytosis

The transfer of antigens from oligodendrocytes to immune cells has been implicated in the pathogenesis of autoimmune diseases. Here, we show that oligodendrocytes secrete small membrane vesicles called exosomes, which are specifically and efficiently taken up by microglia both in vitro and in vivo. Internalisation of exosomes occurs by a macropinocytotic mechanism without inducing a concomitant inflammatory response. After stimulation of microglia with interferon-γ, we observe an upregulation of MHC class II in a subpopulation of microglia. However, exosomes are preferentially internalised in microglia that do not seem to have antigen-presenting capacity. We propose that the constitutive macropinocytotic clearance of exosomes by a subset of microglia represents an important mechanism through which microglia participate in the degradation of oligodendroglial membrane in an immunologically 'silent' manner. By designating the capacity for macropinocytosis and antigen presentation to distinct cells, degradation and immune function might be assigned to different subtypes of microglia.

Neuronal and microglial mechanisms of neuropathic pain

Neuropathic pain is generally defined as a chronic pain state resulting from peripheral and/or central nerve injury. Effective treatment for neuropathic pain is still lacking, due in part to poor understanding of pathological mechanisms at the molecular level. Neuronal mechanisms of neuropathic pain, especially synaptic plasticity, are the major focus of many investigators. N-methyl-D-aspartate (NMDA) receptor dependent synaptic plasticity at the spinal and cortical levels is believed to contribute to enhanced sensory responses after injury. Glial cells, including astrocytes and microglia, have recently been implicated in neuropathic pain. These glial cells form close interactions with neurons and thus may modulate nociceptive transmission under pathological conditions. In this review, we present recent progress in the study of neuronal and microglial mechanisms underlying neuropathic pain. We propose that activity-dependent neuronal plasticity is a key target for treatment in neuropathic pain.

Capillary blood flow around microglial somata determines dynamics of microglial processes in ischemic conditions

Microglia are the resident immune cells in the brain. Under normal conditions, resting ramified microglia constantly extend and retract fine processes while performing immunological surveillance. In ischemia, microglia become activated as demonstrated by morphological changes during deramification leading to transformation from ramified to amoeboid form. In vivo two-photon microscopy of enhanced green fluorescent protein (EGFP)-expressing microglia in mouse neocortex was used to examine microglial dynamics during the early periods of focal and global ischemia. A penumbra-like "area-at-risk" surrounded by a square-shaped area of severely hypoperfused tissue was created by laser-induced photothrombosis. The dynamics of microglial processes in the area-at-risk was strongly correlated with capillary blood flow (BF) measured within 10 μm of microglial somata. Changes in BF around distal microglial processes (>30 μm from somata) had no effect on microglial dynamics. A severe reduction of capillary BF near somata by 84% ± 6% resulted in initiation of microglial deramification, suggesting activation. A moderate decrease in BF near somata by 22% ± 5% or increase by 87% ± 10%, reflecting a redistribution of capillary BF, had no effect on microglial morphology. Complete BF loss during cardiac arrest (CA) or transient bilateral common carotid artery occlusion (BCCAO) entirely stalled all microglial processes without structural changes. Reperfusion after BCCAO induced recovery of microglial dynamics to preocclusion values. These findings suggest that during ischemia, the severe drop in BF around microglial somata coincides with morphological activation. However, this activation requires some residual BF, because complete perfusion loss (as during BCCAO and CA) did not support microglial deramification.

Synaptic pruning by microglia is necessary for normal brain development

Microglia are highly motile phagocytic cells that infiltrate and take up residence in the developing brain, where they are thought to provide a surveillance and scavenging function. However, although microglia have been shown to engulf and clear damaged cellular debris after brain insult, it remains less clear what role microglia play in the uninjured brain. Here, we show that microglia actively engulf synaptic material and play a major role in synaptic pruning during postnatal development in mice. These findings link microglia surveillance to synaptic maturation and suggest that deficits in microglia function may contribute to synaptic abnormalities seen in some neurodevelopmental disorders.

Features of microglia and neuroinflammation relevant to environmental exposure and neurotoxicity

Microglia are resident cells of the brain involved in regulatory processes critical for development, maintenance of the neural environment, injury and repair. They belong to the monocytic-macrophage lineage and serve as brain immune cells to orchestrate innate immune responses; however, they are distinct from other tissue macrophages due to their relatively quiescent phenotype and tight regulation by the CNS microenvironment. Microglia actively survey the surrounding parenchyma and respond rapidly to changes such that any disruption to neural architecture or function can contribute to the loss in regulation of the microglia phenotype. In many models of neurodegeneration and neurotoxicity, early events of synaptic degeneration and neuronal loss are accompanied by an inflammatory response including activation of microglia, perivascular monocytes, and recruitment of leukocytes. In culture, microglia have been shown to be capable of releasing several potentially cytotoxic substances, such as reactive oxygen intermediates, nitric oxide, proteases, arachidonic acid derivatives, excitatory amino acids, and cytokines; however, they also produce various neurotrophic factors and quench damage from free radicals and excitotoxins. As the primary source for pro-inflammatory cytokines, microglia are implicated as pivotal mediators of neuroinflammation and can induce or modulate a broad spectrum of cellular responses. Neuroinflammation should be considered as a balanced network of processes whereby subtle modifications can shift the cells toward disparate outcomes. For any evaluation of neuroinflammation and microglial responses, within the framework of neurotoxicity or degeneration, one key question in determining the consequence of neuroinflammation is whether the response is an initiating event or the consequence of tissue damage. As examples of environmental exposure-related neuroinflammation in the literature, we provide an evaluation of data on manganese and diesel exhaust particles.

A role for microglia in synaptic plasticity?

Remodeling of brain circuits, including the formation, modification and elimination of synaptic structures, occurs throughout life as animals adapt to their environment. Until very recently, known mechanisms for experience-dependent synaptic plasticity had placed neurons and their structural interactions with astrocytes in the spotlight. However microglia, the immune cells of the brain, are very active even in the absence of pathological insults and their processes periodically contact dendritic spines and axon terminals in vivo.1-3 This intriguing behavior prompted us to explore, using electron microscopy and two-photon in vivo imaging in the primary visual cortex of juvenile mice, a possible role for quiescent microglia in the modification of synaptic structures.4 Our work uncovered subtle changes in the behavior of microglia during manipulations of visual experience including regulation of perisynaptic extracellular spaces, contact with subsets of structurally dynamic and transient dendritic spines, and phagocytic engulfment of intact synapses. Based on these results, here we further discuss three means of synapse modification or elimination that could be mediated by microglia in the context of normal experience-dependent plasticity.

Exploring the neuroimmunopharmacology of opioids: an integrative review of mechanisms of central immune signaling and their implications for opioid analgesia

Vastly stimulated by the discovery of opioid receptors in the early 1970s, preclinical and clinical research was directed at the study of stereoselective neuronal actions of opioids, especially those played in their crucial analgesic role. However, during the past decade, a new appreciation of the non-neuronal actions of opioids has emerged from preclinical research, with specific appreciation for the nonclassic and nonstereoselective sites of action. Opioid activity at Toll-like receptors, newly recognized innate immune pattern recognition receptors, adds substantially to this unfolding story. It is now apparent from molecular and rodent data that these newly identified signaling events significantly modify the pharmacodynamics of opioids by eliciting proinflammatory reactivity from glia, the immunocompetent cells of the central nervous system. These central immune signaling events, including the release of cytokines and chemokines and the associated disruption of glutamate homeostasis, cause elevated neuronal excitability, which subsequently decreases opioid analgesic efficacy and leads to heightened pain states. This review will examine the current preclinical literature of opioid-induced central immune signaling mediated by classic and nonclassic opioid receptors. A unification of the preclinical pharmacology, neuroscience, and immunology of opioids now provides new insights into common mechanisms of chronic pain, naive tolerance, analgesic tolerance, opioid-induced hyperalgesia, and allodynia. Novel pharmacological targets for future drug development are discussed in the hope that disease-modifying chronic pain treatments arising from the appreciation of opioid-induced central immune signaling may become practical.

CCL2/MCP-1 modulation of microglial activation and proliferation

Background

Monocyte chemoattractant protein (CCL2/MCP-1) is a chemokine that attracts cells involved in the immune/inflammatory response. As microglia are one of the main cell types sustaining inflammation in brain, we proposed here to analyze the direct effects of MCP-1 on cultured primary microglia.

Methods

Primary microglia and neuronal cultures were obtained from neonatal and embryonic Wistar rats, respectively. Microglia were incubated with different concentrations of recombinant MCP-1 and LPS. Cell proliferation was quantified by measuring incorporation of bromodeoxyuridine (BrdU). Nitrite accumulation was measured using the Griess assay. The expression and synthesis of different proteins was measured by RT-PCR and ELISA. Cell death was quantified by measuring release of LDH into the culture medium.

Results

MCP-1 treatment (50 ng/ml, 24 h) did not induce morphological changes in microglial cultures. Protein and mRNA levels of different cytokines were measured, showing that MCP-1 was not able to induce proinflammatory cytokines (IL-1β, IL6, MIP-1α), either by itself or in combination with LPS. A similar lack of effect was observed when measuring inducible nitric oxide synthase (NOS2) expression or accumulation of nitrites in the culture media as a different indicator of microglial activation. MCP-1 was also unable to alter the expression of different trophic factors that were reduced by LPS treatment. In order to explore the possible release of other products by microglia and their potential neurotoxicity, neurons were co-cultured with microglia: no death of neurons could be detected when treated with MCP-1. However, the presence of MCP-1 induced proliferation of microglia, an effect opposite to that observed with LPS.

Conclusion

These data indicate that, while causing migration and proliferation of microglia, MCP-1 does not appear to directly activate an inflammatory response in this cell type, and therefore, other factors may be necessary to cause the changes that result in the neuronal damage commonly observed in situations where MCP-1 levels are elevated.

CD14 and TRIF govern distinct responsiveness and responses in mouse microglial TLR4 challenges by structural variants of LPS

Toll-like receptor (TLR) 4 responds to a range of agonists in infection and injury, but is best known for the recognition of bacterial lipopolysaccharides (LPS). Assembly in heterologous receptor complexes as well as signaling through both MyD88 and TRIF adaptor proteins, as unmatched by other TLRs, could underlie its versatile response options, probably also in a cell type-dependent manner. We show that microglia, the CNS macrophages, react to diverse LPS variants, including smooth (S) and rough (R) LPS chemotypes, with cytokine/chemokine induction, MHC I expression and suppression of myelin phagocytosis. The TLR4 co-receptor CD14 was shown in peritoneal macrophages to be essential for S-LPS effects and the link of both S- and R-LPS to TRIF signaling. In contrast, cd14(-/-) microglia readily respond to S- and R-LPS, suggesting an a priori high(er) sensitivity to both chemotypes, while CD14 confers increased S- and R-LPS potencies and compensates for their differences. Importantly, CD14 controls the magnitude and shapes the profile of cyto/chemokine production, this influence being itself regulated by critical LPS concentrations. Comparing reactive phenotypes of microglia with deficiencies in CD14, MyD88 and TRIF (cd14(-/-), myd88(-/-), and trif(lps2)), we found that distinct signaling routes organize for individual functions in either concerted or non-redundant fashion and that CD14 has contributions beyond the link to TRIF. Modulation of response profiles by key cytokines finally reveals that the microglial TLR4 can differentiate between the class of LPS structures and a self-derived agonist, fibronectin. It thus proves as a sophisticated decision maker in infectious and non-infectious CNS challenges.

Physiology of microglia

Microglial cells are the resident macrophages in the central nervous system. These cells of mesodermal/mesenchymal origin migrate into all regions of the central nervous system, disseminate through the brain parenchyma, and acquire a specific ramified morphological phenotype termed "resting microglia." Recent studies indicate that even in the normal brain, microglia have highly motile processes by which they scan their territorial domains. By a large number of signaling pathways they can communicate with macroglial cells and neurons and with cells of the immune system. Likewise, microglial cells express receptors classically described for brain-specific communication such as neurotransmitter receptors and those first discovered as immune cell-specific such as for cytokines. Microglial cells are considered the most susceptible sensors of brain pathology. Upon any detection of signs for brain lesions or nervous system dysfunction, microglial cells undergo a complex, multistage activation process that converts them into the "activated microglial cell." This cell form has the capacity to release a large number of substances that can act detrimental or beneficial for the surrounding cells. Activated microglial cells can migrate to the site of injury, proliferate, and phagocytose cells and cellular compartments.

Long-term changes of spine dynamics and microglia after transient peripheral immune response triggered by LPS in vivo

Background

An episode of peripheral immune response may create long-lasting alterations in the neural network. Recent studies indicate a glial involvement in synaptic remodeling. Therefore it is postulated that both synaptic and glial changes could occur under the peripheral inflammation.

Results

We tested this possibility by in vivo two-photon microscopy of dendritic spines after induction of a peripheral immune response by lipopolysaccharide (LPS) treatment of mice.

We observed that the spines were less stable in LPS-treated mice. The accumulation of spine changes gradually progressed and remained low over a week after LPS treatment but became significantly larger at four weeks. Over eight weeks after LPS treatment, the fraction of eliminated spines amounted to 20% of the initial population and this persistent destabilization resulted in a reduction of the total spine density.

We next evaluated glial activation by LPS administration. Activation of microglia was confirmed by a persistent increase of Iba1 immunoreactivity. Morphological changes in microglia were observed two days after LPS administration and were partially recovered within one week but sustained over a long time period.

Conclusions

These results indicate long-lasting aggravating effects of a single transient peripheral immune response on both spines and microglia. The parallel persistent alterations of both spine turnover and the state of microglia in vivo suggest the presence of a pathological mechanism that sustains the enhanced remodeling of neural networks weeks after peripheral immune responses. This pathological mechanism may also underlie long-lasting cognitive dysfunctions after septic encephalopathy in human patients.

Rapid synaptic remodeling in the adult somatosensory cortex following peripheral nerve injury and its association with neuropathic pain

Structural and functional plastic changes in the primary somatosensory cortex (S1) have been observed following peripheral nerve injury that often leads to neuropathic pain, which is characterized by tactile allodynia. However, remodeling of cortical connections following injury has been believed to take months or years; this is not temporally correlated with the rapid development of allodynia and S1 hyperexcitability. Here we first report, by using long-term two-photon imaging of postsynaptic dendritic spines in living adult mice, that synaptic connections in the S1 are rewired within days following sciatic nerve ligation through phase-specific and size-dependent spine survival/growth. Spine turnover in the S1 area corresponding to the injured paw markedly increased during an early phase of neuropathic pain and was restored in a late phase of neuropathic pain, which was prevented by immediate local blockade of the injured nerve throughout the early phase. New spines that generated before nerve injury showed volume decrease after injury, whereas more new spines that formed in the early phase of neuropathic pain became persistent and substantially increased their volume during the late phase. Further, preexisting stable spines survived less following injury than controls, and such lost persistent spines were smaller in size than the surviving ones, which displayed long-term potentiation-like enlargement over weeks. These results suggest that peripheral nerve injury induces rapid and selective remodeling of cortical synapses, which is associated with neuropathic pain development, probably underlying, at least partially, long-lasting sensory changes in neuropathic subjects.

Glia: initiators and progressors of pathology in Parkinson's disease

BACKGROUND

Glia are traditionally known as support cells for neurons, and their role in neurodegeneration has been largely considered secondary to neuronal dysfunction. We review newer concepts on glial function and assess glial changes in Parkinson's disease (PD) at the time of disease initiation when α-synuclein is accumulating in brain tissue but there is limited neuronal loss, and also as the disease progresses and neuronal loss is evident.

RESULTS

Of the two main types of astrocytes, only protoplasmic astrocytes are involved in PD, where they become nonreactive and accumulate α-synuclein. Experimental evidence has shown that astrocytic α-synuclein deposition initiates the noncell autonomous killing of neurons through microglial signaling. As the disease progresses, more protoplasmic astrocytes are affected by the disease with an increasing microglial response. Although there is still controversy on the role microglia play in neurodegeneration, there is evidence that microglia are activated early in PD and possibly assist with the clearance of extracellular α-synuclein at this time. Microglia transform to phagocytes and target neurons as the disease progresses but appear to become dysfunctional with increasing amounts of ingested debris. Only nonmyelinating oligodendroglial cells are affected in PD, and only late in the disease process.

CONCLUSIONS

Glial cells are responsible for the progression of PD and play an important role in initiating the early tissue response. In particular, early dysfunction and α-synuclein accumulation in astrocytes causes recruitment of phagocytic microglia that attack selected neurons in restricted brain regions causing the clinical symptoms of PD.

Glial dysfunction in the pathogenesis of α-synucleinopathies: emerging concepts

Parkinson's disease (PD), dementia with Lewy bodies (DLB) and multiple system atrophy (MSA) are adult onset neurodegenerative disorders characterised by prominent intracellular α-synuclein aggregates (α-synucleinopathies). The glial contribution to neurodegeneration in α-synucleinopathies was largely underestimated until recently. However, brains of PD and DLB patients exhibit not only neuronal inclusions such as Lewy bodies or Lewy neurites but also glial α-synuclein aggregates. Accumulating experimental evidence in PD models suggests that astrogliosis and microgliosis act as important mediators of neurodegeneration playing a pivotal role in both disease initiation and progression. In MSA, oligodendrocytes are intriguingly affected by aberrant cytoplasmic accumulation of α-synuclein (glial cytoplasmic inclusions, Papp-Lantos bodies). Converging evidence from human postmortem studies and transgenic MSA models suggests that oligodendroglial dysfunction both triggers and exacerbates neuronal degeneration. This review summarises the wide range of responsibilities of astroglia, microglia and oligodendroglia in the healthy brain and the changes in glial function associated with ageing. We then provide a critical analysis of the role of glia in α-synucleinopathies including putative mechanisms promoting a chronically diseased glial microenvironment which can lead to detrimental neuronal changes, including cell loss. Finally, major therapeutic strategies targeting glial pathology in α-synucleinopathies as well as current pitfalls for disease-modification in clinical trials are discussed.

Transmitter- and hormone-activated Ca(2+) responses in adult microglia/brain macrophages in situ recorded after viral transduction of a recombinant Ca(2+) sensor

In vitro studies show that microglia, the resident immune cells of the brain, express neurotransmitter and neuropeptide receptors which are linked to Ca(2+) signaling. Here we describe an approach to obtain Ca(2+) recordings from microglia in situ. We injected a retrovirus encoding a calcium sensor into the cortex of mice 2 days after stimulation of microglial proliferation by a stab wound injury. Microglial cells were identified with tomato lectin in acute slices prepared 3, 6, 21 and 42 days after the injury. The membrane current profile and the ameboid morphology indicated that microglial cells were activated at day 6 while at day 42 they resembled resting microglia. We recorded transient Ca(2+) responses to application of ATP, endothelin-1, substance P, histamine and serotonin. The fluorescence amplitude of ATP was increased only at day 6 compared to other time points, while responses to all other ligands did not vary. Only half of the microglial cells that responded to ATP also responded to endothelin-1, serotonin and histamine. Substance P, in contrast, showed a complete overlap with the ATP responding microglial population at day 6, at day 42 this population was reduced to 55%. Cultured cells were less responsive to these ligands. This study shows that in situ microglia consist of heterogeneous populations with respect to their sensitivity to neuropeptides and -transmitters.

Spreading depression sends microglia on Lévy flights

Spreading depression (SD) is thought to cause migraine aura, and perhaps migraine, and includes a transient loss of synaptic activity preceded and followed by increased neuronal excitability. Activated microglia influence neuronal activity and play an important role in homeostatic synaptic scaling via release of cytokines. Furthermore, enhanced neuronal function activates microglia to not only secrete cytokines but also to increase the motility of their branches, with somata remaining stationary. While SD also increases the release of cytokines from microglia, the effects on microglial movement from its synaptic activity fluctuations are unknown. Accordingly, we used time-lapse imaging of rat hippocampal slice cultures to probe for microglial movement associated with SD. We observed that in uninjured brain whole microglial cells moved. The movements were well described by the type of Lévy flight known to be associated with an optimal search pattern. Hours after SD, when synaptic activity rose, microglial cell movement was significantly increased. To test how synaptic activity influenced microglial movement, we enhanced neuronal activity with chemical long-term potentiation or LPS and abolished it with TTX. We found that microglial movement was significantly decreased by enhanced neuronal activity and significantly increased by activity blockade. Finally, application of glutamate and ATP to mimic restoration of synaptic activity in the presence of TTX stopped microglial movement that was otherwise seen with TTX. Thus, synaptic activity retains microglial cells in place and an absence of synaptic activity sends them off to influence wider expanses of brain. Perhaps increased microglial movements after SD are a long-lasting, and thus maladaptive, response in which these cells increase neuronal activity via contact or paracrine signaling, which results in increased susceptibility of larger brain areas to SD. If true, then targeting mechanisms that retard activity-dependent microglial Lévy flights may be a novel means to reduce susceptibility to migraine.

Multiparametric assessment of acute and subacute ischemic neuronal damage: a small animal positron emission tomography study with rat photochemically induced thrombosis model

We evaluated sequential changes in rat brain function up to 14 days after focal ischemic insult with a small animal positron emission tomography (PET). Unilateral focal ischemic cerebral damage was induced by left middle cerebral artery occlusion with a photochemically induced thrombosis (PIT) method. PET scans were conducted with [(11)C](R)-PK11195 ([(11)C](R)-PK) for peripheral benzodiazepine receptor (PBR), [(11)C]flumazenil ([(11)C]FMZ) for central benzodiazepine receptor (CBR), and [(18)F]fluoro-2-deoxy-D-glucose ([(18)F]FDG) for glucose metabolism at before (as "Normal") and after PIT. At 1 and 3 days after PIT, [(18)F]FDG indicated lower uptake in the infarct area. Interestingly, unexpectedly high-[(18)F]FDG uptake was observed in the peri-infarct area surrounding the infarct area at day 7. The high-[(18)F]FDG uptake region completely overlapped with the high-[(11)C](R)-PK uptake region at day 7, which resulted in the underestimation of neuronal damage. Immunohistochemical data also suggested that the high-[(18)F]FDG uptake peak at day 7 was caused by inflammation including microglial cell activation. In contrast, imaging with [(11)C]FMZ indicated cortical neuronal damage on days 7 and 14 without any disturbance by microglial formation. These results indicated that [(18)F]FDG might not be a suitable ligand for ischemic neuronal damage detection from acute to subacute phases.

Activated microglia proliferate at neurites of mutant huntingtin-expressing neurons

In Huntington's disease (HD), mutated huntingtin (mhtt) causes striatal neurodegeneration which is paralleled by elevated microglia cell numbers. In vitro corticostriatal slice and primary neuronal culture models, in which neuronal expression of mhtt fragments drives HD-like neurotoxicity, were employed to examine wild type microglia during both the initiation and progression of neuronal pathology. As neuronal pathology progressed, microglia initially localized in the vicinity of neurons expressing mhtt fragments increased in number, demonstrated morphological evidence of activation, and expressed the proliferation marker, Ki67. These microglia were positioned along irregular neurites, but did not localize with mhtt inclusions nor exacerbate mhtt fragment-induced neurotoxicity. Prior to neuronal pathology, microglia upregulated ionized calcium binding adaptor molecule 1 (Iba1), signaling a functional shift. With neurodegeneration, interleukin-6 and complement component 1q were increased. The results suggest a stimulatory, proliferative signal for microglia present at the onset of mhtt fragment-induced neurodegeneration. Thus, microglia effect a localized inflammatory response to neuronal mhtt expression that may serve to direct microglial removal of dysfunctional neurites or aberrant synapses, as is required for reparative actions in vivo.

Age-related alterations in the dynamic behavior of microglia

Microglia, the primary resident immune cells of the central nervous system (CNS), exhibit dynamic behavior involving rapid process motility and cellular migration that is thought to underlie key functions of immune surveillance and tissue repair. Although age-related changes in microglial activation have been implicated in the pathogenesis of neurodegenerative diseases of aging, how dynamic behavior in microglia is influenced by aging is not fully understood. In this study, we employed live imaging of retinal microglia in situ to compare microglial morphology and behavioral dynamics in young and aged animals. We found that aged microglia in the resting state have significantly smaller and less branched dendritic arbors, and also slower process motilities, which probably compromise their ability to survey and interact with their environment continuously. We also found that dynamic microglial responses to injury were age-dependent. While young microglia responded to extracellular ATP, an injury-associated signal, by increasing their motility and becoming more ramified, aged microglia exhibited a contrary response, becoming less dynamic and ramified. In response to laser-induced focal tissue injury, aged microglia demonstrated slower acute responses with lower rates of process motility and cellular migration compared with young microglia. Interestingly, the longer term response of disaggregation from the injury site was retarded in aged microglia, indicating that senescent microglial responses, while slower to initiate, are more sustained. Together, these altered features of microglial behavior at rest and following injury reveal an age-dependent dysregulation of immune response in the CNS that may illuminate microglial contributions to age-related neuroinflammatory degeneration.

Annual Research Review: Transgenic mouse models of childhood-onset psychiatric disorders

Childhood-onset psychiatric disorders, such as attention deficit hyperactivity disorder (ADHD), autism spectrum disorder (ASD), mood disorders, obsessive compulsive spectrum disorders (OCSD), and schizophrenia (SZ), affect many school-age children, leading to a lower quality of life, including difficulties in school and personal relationships that persist into adulthood. Currently, the causes of these psychiatric disorders are poorly understood, resulting in difficulty diagnosing affected children, and insufficient treatment options. Family and twin studies implicate a genetic contribution for ADHD, ASD, mood disorders, OCSD, and SZ. Identification of candidate genes and chromosomal regions associated with a particular disorder provide targets for directed research, and understanding how these genes influence the disease state will provide valuable insights for improving the diagnosis and treatment of children with psychiatric disorders. Transgenic mouse models are one important approach in the study of human diseases, allowing for the use of a variety of experimental approaches to dissect the contribution of a specific chromosomal or genetic abnormality in human disorders. While it is impossible to model an entire psychiatric disorder in a single mouse model, these models can be extremely valuable in dissecting out the specific role of a gene, pathway, neuron subtype, or brain region in a particular abnormal behavior. In this review we discuss existing transgenic mouse models for childhood-onset psychiatric disorders. We compare the strength and weakness of various transgenic mouse models proposed for each of the common childhood-onset psychiatric disorders, and discuss future directions for the study of these disorders using cutting-edge genetic tools.

Extracellular ATP differentially modulates Toll-like receptor 4-mediated cell survival and death of microglia

The survival and death rates of inflammatory cells directly control their number and are substantially associated with the degree of inflammation. Microglia, key players in neuroinflammation, often cause excessive reactions implicated in neurological diseases. However, the mechanisms that determine microglial fate under pathological conditions remain to be elucidated. Here, we report that activation by lipopolysaccharide (LPS, a Toll-like receptor 4 ligand), an inflammation inducer, primarily promotes survival of microglia, but as its concentration is increased it induces cell death, resulting in decreased cell number. Moreover, extracellular ATP, which is released upon tissue damage, further enhanced the survival induced by a low LPS concentration and the death induced by a high LPS concentration. The survival-promoting effect of ATP was mimicked by non-hydrolyzable ATP analog, adenosine 5'-O-(3-thiotriphosphate), and also by the P2X(7) receptor agonist, 2'(3')-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate, and was suppressed by the P2X(7) antagonists, Brilliant Blue G and A 438079. On the contrary, the death of LPS-activated microglia was not affected by adenosine 5'-O-(3-thiotriphosphate), but enhanced by adenosine, ATP breakdown product. Thus, extracellular ATP modulates microglial survival and death in different ways involving P2X(7) receptor activation and ATP degradation to adenosine, respectively. Such Toll-like receptor 4/purinergic signaling may provide a fine regulatory system of neuroinflammation through modulating the microglial cell number.

Microglial morphology and dynamic behavior is regulated by ionotropic glutamatergic and GABAergic neurotransmission

PURPOSE

Microglia represent the primary resident immune cells in the CNS, and have been implicated in the pathology of neurodegenerative diseases. Under basal or "resting" conditions, microglia possess ramified morphologies and exhibit dynamic surveying movements in their processes. Despite the prominence of this phenomenon, the function and regulation of microglial morphology and dynamic behavior are incompletely understood. We investigate here whether and how neurotransmission regulates "resting" microglial morphology and behavior.

METHODS

We employed an ex vivo mouse retinal explant system in which endogenous neurotransmission and dynamic microglial behavior are present. We utilized live-cell time-lapse confocal imaging to study the morphology and behavior of GFP-labeled retinal microglia in response to neurotransmitter agonists and antagonists. Patch clamp electrophysiology and immunohistochemical localization of glutamate receptors were also used to investigate direct-versus-indirect effects of neurotransmission by microglia.

RESULTS

Retinal microglial morphology and dynamic behavior were not cell-autonomously regulated but are instead modulated by endogenous neurotransmission. Morphological parameters and process motility were differentially regulated by different modes of neurotransmission and were increased by ionotropic glutamatergic neurotransmission and decreased by ionotropic GABAergic neurotransmission. These neurotransmitter influences on retinal microglia were however unlikely to be directly mediated; local applications of neurotransmitters were unable to elicit electrical responses on microglia patch-clamp recordings and ionotropic glutamatergic receptors were not located on microglial cell bodies or processes by immunofluorescent labeling. Instead, these influences were mediated indirectly via extracellular ATP, released in response to glutamatergic neurotransmission through probenecid-sensitive pannexin hemichannels.

CONCLUSIONS

Our results demonstrate that neurotransmission plays an endogenous role in regulating the morphology and behavior of "resting" microglia in the retina. These findings illustrate a mode of constitutive signaling between the neural and immune compartments of the CNS through which immune cells may be regulated in concert with levels of neural activity.

Neuropathogenesis of Noonan syndrome is mediated by inflammatory microglia

Microglia are resident hematopoietic cells that play important roles in the damaged or degenerating adult nervous system. Microglia are involved in neuropathogenesis of various diseases. Microglia are also essential for neuroprotection and comprise an essential component of the neural stem cell niche. The activation of microglia is an important phenomenon associated with several neurological disorders that arise from infections to developmental abnormalities and behavioral pathologies. Noonan syndrome (NS) is associated with mutations in the PTPN11 gene and also accounts for mental retardation in children. Interestingly, in mouse models of NS, mutations in the PTPN11 gene resulted in dysregulation of neural progenitors. The present study describes the activation of microglia in the NS mouse model, which results in an inflammatory phenotype with expression of IL-1b and defective phagocytosis. To test whether microglia from NS mice are important for neural precursor maintenance or self-renewal, embryonic neural precursors from the cortex of WT mice were cultured. Microglia from NS and WT mice were then added to cortical precursor cells which showed that microglia from NS mice inhibited astrogenesis. Together, these results demonstrate that microglia can dysregulate neural precursor development in NS, and suggest that alterations in microglial number as a consequence of genetic or pathological events may perturb neural development by directly affecting embryonic neural precursors.

Ct3d: tracking microglia motility in 3D using a novel cosegmentation approach

MOTIVATION

Cell tracking is an important method to quantitatively analyze time-lapse microscopy data. While numerous methods and tools exist for tracking cells in 2D time-lapse images, only few and very application-specific tracking tools are available for 3D time-lapse images, which is of high relevance in immunoimaging, in particular for studying the motility of microglia in vivo.

RESULTS

We introduce a novel algorithm for tracking cells in 3D time-lapse microscopy data, based on computing cosegmentations between component trees representing individual time frames using the so-called tree-assignments. For the first time, our method allows to track microglia in three dimensional confocal time-lapse microscopy images. We also evaluate our method on synthetically generated data, demonstrating that our algorithm is robust even in the presence of different types of inhomogeneous background noise.

AVAILABILITY

Our algorithm is implemented in the ct3d package, which is available under http://www.picb.ac.cn/patterns/Software/ct3d; supplementary videos are available from http://www.picb.ac.cn/patterns/Supplements/ct3d.

Changing face of microglia

Microglia are resident brain cells that sense pathological tissue alterations. They can develop into brain macrophages and perform immunological functions. However, expression of immune proteins by microglia is not synonymous with inflammation, because these molecules can have central nervous system (CNS)-specific roles. Through their involvement in pain mechanisms, microglia also respond to external threats. Experimental studies support the idea that microglia have a role in the maintenance of synaptic integrity. Analogous to electricians, they are capable of removing defunct axon terminals, thereby helping neuronal connections to stay intact. Microglia in healthy CNS tissue do not qualify as macrophages, and their specific functions are beginning to be explored.

The impact of glial activation in the aging brain

The past decade or so has witnessed a rekindling of interest in glia requiring a re-evaluation of the early descriptions of astrocytes as merely support cells, and microglia as adopting either a resting state or an activated state in a binary fashion. We now know that both cell types contribute to the optimal functioning of neurons in the healthy brain, and that altered function of either cell impacts on neuronal function and consequently cognitive function. The evidence indicates that both astrocytic and microglial phenotype change with age and that the shift from the resting state is associated with deterioration in synaptic function. In this review, we consider the rapidly-expanding array of functions attributed to these cells and focus on evaluating the changes in cell activation that accompany ageing.

Dysregulated neuronal-microglial cross-talk during aging, stress and inflammation

Communication between neurons and microglia is essential for maintaining homeostasis in the central nervous system (CNS) during both physiological and inflammatory conditions. While microglial activation is necessary and beneficial in response to injury or disease, excessive or prolonged activation can have deleterious effects on brain function and behavior. To prevent inflammation-associated damage, microglia reactivity is actively modulated by neurons in the healthy brain. Age or stress-induced disruption of normal neuronal-microglial communication could lead to an aberrant central immune response when additional stressors are applied. Recent work suggests that both aging and stress shift the CNS microenvironment to a pro-inflammatory state characterized by increased microglial reactivity and a reduction in anti-inflammatory and immunoregulatory factors. This review will discuss how heightened neuroinflammation associated with aging and stress may be compounded by the concomitant loss of neuronally derived factors that control microglial activation, leaving the brain vulnerable to excessive inflammation and neurobehavioral complications upon subsequent immune challenge.

Microglia: key innate immune cells of the brain

Microglia are the histiocytes of the central nervous system. These long-lived cells undergo very little turnover in normal physiological states; however, in pathological conditions, increased egress from the bone marrow and chemoattractive signals in the brain can substantially modulate the indigenous population. Although they were initially conceived of as "resting" cells, recent data suggest that they would be more aptly described as "surveillance" cells. Microglia are specifically adapted to sense various types of danger and differentially react with a classical or alternative reparative response. Our understanding of macrophage function has shifted away from focusing on cell lineage to a more systems-based biology of gene networks accomplishing the detoxification and immune functions. With our greater appreciation of microglial involvement in the innate immune response, we have entered a new era in which the modulation of microglia can be proposed as a means of modulating neurological disease.