Microglial migration mediated by ATP-induced ATP release from lysosomes

Adenosine 5'-triphosphate (ATP) inhibits schwann cell demyelination during Wallerian degeneration

Adenosine 5'-triphosphate (ATP) is implicated in intercellular communication as a neurotransmitter in the peripheral nervous system. In addition, ATP is known as lysosomal exocytosis activator. In this study, we investigated the role of extracellular ATP on demyelination during Wallerian degeneration (WD) using ex vivo and in vivo nerve degeneration models. We found that extracellular ATP inhibited myelin fragmentation and axonal degradation during WD. Furthermore, metformin and chlorpromazine, lysosomal exocytosis antagonists blocked the effect of ATP on the inhibition of demyelination. Thus, these findings indicate that ATP-induced-lysosomal exocytosis may be involved in demyelination during WD.

P2X4 receptors control the fate and survival of activated microglia

Microglia, the resident immune cells of the central nervous system, responds to brain disarrangements by becoming activated to contend with brain damage. Here we show that the expression of P2X4 receptors is upregulated in inflammatory foci and in activated microglia in the spinal cord of rats with experimental autoimmune encephalomyelitis (EAE) as well as in the optic nerve of multiple sclerosis patients. To study the role of P2X4 receptors in microgliosis, we activated microglia with LPS in vitro and in vivo. We observed that P2X4 receptor activity in vitro was increased in LPS-activated microglia as assessed by patch-clamp recordings. In addition, P2X4 receptor blockade significantly reduced microglial membrane ruffling, TNFα secretion and morphological changes, as well as LPS-induced microglial cell death. Accordingly, neuroinflammation provoked by LPS injection in vivo induced a rapid microglial loss in the spinal cord that was totally prevented or potentiated by P2X4 receptor blockade or facilitation, respectively. Within the brain, microglia in the hippocampal dentate gyrus showed particular vulnerability to LPS-induced neuroinflammation. Thus, microglia processes in this region retracted as early as 2 h after injection of LPS and died around 24 h later, two features which were prevented by blocking P2X4 receptors. Together, these data suggest that P2X4 receptors contribute to controlling the fate of activated microglia and its survival.

P2X7 purinoceptors contribute to the death of Schwann cells transplanted into the spinal cord

The potential to use Schwann cells (SCs) in neural repair for patients suffering from neurotrauma and neurodegenerative diseases is well recognized. However, significant cell death after transplantation hinders the clinical translation of SC-based therapies. Various factors may contribute to the death of transplanted cells. It is known that prolonged activation of P2X7 purinoceptors (P2X7R) can lead to death of certain types of cells. In this study, we show that rat SCs express P2X7R and exposure of cultured SCs to high concentrations of ATP (3-5 mM) or a P2X7R agonist, 2'(3')-O-(4-benzoylbenzoyl)ATP (BzATP) induced significant cell death rapidly. High concentrations of ATP and BzATP increased ethidium uptake by SCs, indicating increased membrane permeability to large molecules, a typical feature of prolonged P2X7R activation. SC death, as well as ethidium uptake, induced by ATP was blocked by an irreversible P2X7R antagonist oxidized ATP (oxATP) or a reversible P2X7R antagonist A438079. oxATP also significantly inhibits the increase of intracellular free calcium induced by minimolar ATP concentrations. Furthermore, ATP did not cause death of SCs isolated from P2X7R-knockout mice. All these results suggest that P2X7R is responsible for ATP-induced SC death in vitro. When rat SCs were treated with oxATP before transplantation into uninjured rat spinal cord, 35% more SCs survived than untreated SCs 1 week after transplantation. Moreover, 58% more SCs isolated from P2X7R-knockout mice survived after being transplanted into rat spinal cord than SCs from wild-type mice. This further confirms that P2X7R is involved in the death of transplanted SCs. These results indicate that targeting P2X7R on SCs could be a potential strategy to improve the survival of transplanted cells. As many other types of cells, including neural stem cells, also express P2X7R, deactivating P2X7R may improve the survival of other types of transplanted cells.

P2Y4 receptor-mediated pinocytosis contributes to amyloid beta-induced self-uptake by microglia

Brain disturbances, like injuries or aberrant protein deposits, evoke nucleotide release or leakage from cells, leading to microglial chemotaxis and ingestion. Recent studies have identified P2Y12 purinergic receptors as triggers for microglial chemotaxis and P2Y6 receptors as mediators for phagocytosis. However, pinocytosis, known as the internalization of fluid-phase materials, has received much less attention. We found that ATP efficiently triggered pinocytosis in microglia. Pharmacological analysis and knockdown experiments demonstrated the involvement of P2Y4 receptors and the phosphatidylinositol 3-kinase/Akt cascade in the nucleotide-induced pinocytosis. Further evidence indicated that soluble amyloid beta peptide 1-42 induced self-uptake in microglia through pinocytosis, a process involving activation of P2Y4 receptors by autocrine ATP signaling. Our results demonstrate a previously unknown function of ATP as a "drink me" signal for microglia and P2Y4 receptors as a potential therapeutic target for the treatment of Alzheimer's disease.

Myeloid cells expressing VEGF and arginase-1 following uptake of damaged retinal pigment epithelium suggests potential mechanism that drives the onset of choroidal angiogenesis in mice

Whilst data recognise both myeloid cell accumulation during choroidal neovascularisation (CNV) as well as complement activation, none of the data has presented a clear explanation for the angiogenic drive that promotes pathological angiogenesis. One possibility that is a pre-eminent drive is a specific and early conditioning and activation of the myeloid cell infiltrate. Using a laser-induced CNV murine model, we have identified that disruption of retinal pigment epithelium (RPE) and Bruch's membrane resulted in an early recruitment of macrophages derived from monocytes and microglia, prior to angiogenesis and contemporaneous with lesional complement activation. Early recruited CD11b(+) cells expressed a definitive gene signature of selective inflammatory mediators particularly a pronounced Arg-1 expression. Accumulating macrophages from retina and peripheral blood were activated at the site of injury, displaying enhanced VEGF expression, and notably prior to exaggerated VEGF expression from RPE, or earliest stages of angiogenesis. All of these initial events, including distinct VEGF (+) Arg-1(+) myeloid cells, subsided when CNV was established and at the time RPE-VEGF expression was maximal. Depletion of inflammatory CCR2-positive monocytes confirmed origin of infiltrating monocyte Arg-1 expression, as following depletion Arg-1 signal was lost and CNV suppressed. Furthermore, our in vitro data supported a myeloid cell uptake of damaged RPE or its derivatives as a mechanism generating VEGF (+) Arg-1(+) phenotype in vivo. Our results reveal a potential early driver initiating angiogenesis via myeloid-derived VEGF drive following uptake of damaged RPE and deliver an explanation of why CNV develops during any of the stages of macular degeneration and can be explored further for therapeutic gain.

Regulation of membrane trafficking by signalling on endosomal and lysosomal membranes

  Endosomal and lysosomal membrane trafficking requires the coordination of multiple signalling events to control cargo sorting and processing, and endosome maturation. The initiation and termination of signalling events in endosomes and lysosomes is not well understood, but several key regulators have been identified, which include small GTPases, phosphoinositides, and Ca2+. Small GTPases act as master regulators and molecular switches in a GTP-dependent manner, initiating signalling cascades to regulate the direction and specificity of endosomal trafficking. Phosphoinositides are membrane-bound lipids that indicate vesicular identities for recruiting specific cytoplasmic proteins to endosomal membranes, thus allowing specificity of membrane fusion, fission, and cargo sorting to occur within and between specific vesicle compartments. In addition, phosphoinositides regulate the function of membrane proteins such as ion channels and transporters in a compartment-specific manner to mediate transport and signalling. Finally, Ca2+, a locally acting second messenger released from intracellular ion channels, may provide precise spatiotemporal regulation of endosomal signalling and trafficking events. Small GTPase signalling can regulate phosphoinositide conversion during endosome maturation, and electrophysiological studies on isolated endosomes have shown that endosomal and lysosomal Ca2+ channels are directly modulated by endosomal lipids. Thus trafficking and maturation of endosomes and lysosomes can be precisely regulated by dynamic changes in GTPases and membrane lipids, as well as Ca2+ signalling. Importantly, impaired phosphoinositide and Ca2+ signalling can cause endosomal and lysosomal trafficking defects at the cellular level, and a spectrum of lysosome storage diseases.

Microglia release ATP by exocytosis

Microglia survey the brain environment by sensing several types of diffusible molecules, among which extracellular nucleotides released/leaked from damaged cells have central roles. Microglia sense ATP or other nucleotides by multiple P2 receptors, after which they change into several different phenotypes. However, so far, it is largely unknown whether microglia themselves release ATP and, if so, by what mechanism. Here we show that exocytosis is the mechanism by which microglia release ATP. When we stimulated microglia with ionomycin, they released ATP and the release was dependent on Ca²⁺, vesicular H⁺-ATPase, or SNAREs but independent of connexin/pannexin hemichannels. VNUT was found to be expressed in microglia and exhibited no colocalization with lysosome. We also visualized the exocytosis of ATP by a quinacrine-based fluorescent time-lapse imaging. Moreover, we found that lipopolysaccharide increased the ionomycin-induced release of ATP, which was dependent on the increase in VNUT. Taken together, our data suggested that exocytosis is the mechanism of ATP release from microglia. When activated, they would release ATP by increasing VNUT-dependent exocytotic mechanisms.

Constitutive and agonist stimulated ATP secretion in leukocytes

Release and reception of extracellular ATP by leukocytes plays a critical role in immune responses to infection, injury and cardiovascular disease. Leukocytes of both the innate, adaptive immune and central nervous system express a repertoire of cell surface receptors for ATP (P2X and P2Y receptors) and its metabolites. ATP acts as a damage-associated molecule pattern (DAMP) released by injured or dying cells. Detection of released ATP by neighboring leukocytes initiates inflammation and wound healing. However, recent evidence from our group and others suggests ATP release by leukocytes themselves serves to regulate homeostatic mechanisms and coordinate responses to external pro-inflammatory cues. Examples include the homeostatic control of intracellular calcium and regulation of migratory guidance during chemotactic response to external cues. Though there has been some progress in elucidating ATP release mechanisms of some mammalian cells types, release conduits and coupling signal transduction machinery remain larger elusive for leukocytes. Our recent studies suggest a role for secretory lysosomes in releasing ATP in monocytes. Though poorly defined, targeting ATP release mechanisms in leukocytes have great anti-inflammatory potential.

Classical and unconventional pathways of vesicular release in microglia

Emerging evidence indicates that activation of microglia, the immune cells of the brain, is strictly associated to both secretion of soluble molecules and release of extracellular membrane vesicles (EMVs) into the pericellular space. Through these processes, microglia heavily influence brain cell functions, either propagating inflammation and causing damage to neurons or playing a supportive, neuroprotective role. In this review, we highlight the emerging concepts related to vesicular mechanisms of secretion operating in microglial cells, with the aim of dissecting how microglia communicate with other cell types within the brain microenvironment in health and disease.

Neurotransmitter signaling in the pathophysiology of microglia

Microglial cells are the resident immune cells of the central nervous system. In the resting state, microglia are highly dynamic and control the environment by rapidly extending and retracting motile processes. Microglia are closely associated with astrocytes and neurons, particularly at the synapses, and more recent data indicate that neurotransmission plays a role in regulating the morphology and function of surveying/resting microglia, as they are endowed with receptors for most known neurotransmitters. In particular, microglia express receptors for ATP and glutamate, which regulate microglial motility. After local damage, the release of ATP induces microgliosis and activated microglial cells migrate to the site of injury, proliferate, and phagocytose cells, and cellular compartments. However, excessive activation of microglia could contribute to the progression of chronic neurodegenerative diseases, though the underlying mechanisms are still unclear. Microglia have the capacity to release a large number of substances that can be detrimental to the surrounding neurons, including glutamate, ATP, and reactive oxygen species. However, how altered neurotransmission following acute insults or chronic neurodegenerative conditions modulates microglial functions is still poorly understood. This review summarizes the relevant data regarding the role of neurotransmitter receptors in microglial physiology and pathology.

Rare contacts between synapses and microglial processes containing high levels of Iba1 and actin--a postembedding immunogold study in the healthy rat brain

Although microglia is recognised as the cell-mediating innate immunity in the brain, emerging evidence suggests a role of microglia in synaptic communication and modulation. The ability of microglia to move in the neuropil and contact synapses is crucial for such a function. However, the frequency of microglial contact with synapses is not known. Microglia motility is regulated by actin polymerisation and its interaction with ionising calcium-binding adaptor protein 1 (Iba1). In order to move and make contact with synapses, delicate microglial processes should contain high levels of actin and Iba1. To study this we refined an electron microscopic postembedding immunogold method enabling us to identify and quantitatively study different microglial constituents in intact brain tissue. We show that Iba1 and actin were colocalised at high densities in delicate processes in the rat frontal cortex, and that these delicate processes of microglia contact synaptic elements. About 3.5% of the synapses received direct contact from microglia. There was a marked inverse correlation between the densities of Iba1/actin gold particles and the area of the microglial processes, suggesting that the most delicate processes possess the machinery to provide movement in the neuropil. The low frequency of microglia interaction with synaptic elements suggests that microglia have a limited role in overall regulation of synaptic activity.

Microglia: key elements in neural development, plasticity, and pathology

A century after Cajal identified a "third element" of the nervous system, many issues have been clarified about the identity and function of one of its major components, the microglia. Here, we review recent findings by microgliologists, highlighting results from imaging studies that are helping provide new views of microglial behavior and function. In vivo imaging in the intact adult rodent CNS has revolutionized our understanding of microglial behaviors in situ and has raised speculation about their function in the uninjured adult brain. Imaging studies in ex vivo mammalian tissue preparations and in intact model organisms including zebrafish are providing insights into microglial behaviors during brain development. These data suggest that microglia play important developmental roles in synapse remodeling, developmental apoptosis, phagocytic clearance, and angiogenesis. Because microglia also contribute to pathology, including neurodevelopmental and neurobehavioral disorders, ischemic injury, and neuropathic pain, promising new results raise the possibility of leveraging microglia for therapeutic roles. Finally, exciting recent work is addressing unanswered questions regarding the nature of microglial-neuronal communication. While it is now apparent that microglia play diverse roles in neural development, behavior, and pathology, future research using neuroimaging techniques will be essential to more fully exploit these intriguing cellular targets for effective therapeutic intervention applied to a variety of conditions.

Extracellular ATP inhibits Schwann cell dedifferentiation and proliferation in an ex vivo model of Wallerian degeneration

After nerve injury, Schwann cells proliferate and revert to a phenotype that supports nerve regeneration. This phenotype-changing process can be viewed as Schwann cell dedifferentiation. Here, we investigated the role of extracellular ATP in Schwann cell dedifferentiation and proliferation during Wallerian degeneration. Using several markers of Schwann cell dedifferentiation and proliferation in sciatic explants, we found that extracellular ATP inhibits Schwann cell dedifferentiation and proliferation during Wallerian degeneration. Furthermore, the blockage of lysosomal exocytosis in ATP-treated sciatic explants is sufficient to induce Schwann cell dedifferentiation. Together, these findings suggest that ATP-induced lysosomal exocytosis may be involved in Schwann cell dedifferentiation.

A novel method using an acedan-based Zn(DPA) probe to monitor ATP localization in an in vivo system

In in vitro and in vivo systems, understanding localization and the functional role of ATP is essential, but effective methods to monitor ATP in cells and tissues are limited. Although quinacrine dihydrochloride is a well-known fluorescent dye used to detect ATP, it is limited in its use because it shows non-specific nuclear staining both in vitro and in vivo. A commercial luciferin-luciferase bioluminescence assay has also been used to detect ATP, but it can not be easily used in vivo. Thus, to effectively monitor ATP in vivo, we employed a novel two-photon ATP fluorescent probe, acedan-based Zn(DPA). Using the acedan-based Zn(DPA) probe, we show that this probe produces high quality images of ATP in lung, spleen, liver and spinal cord tissues.

Possible ATP release through lysosomal exocytosis from primary sensory neurons

The adenosine triphosphate (ATP) plays important roles under physiological and pathological conditions such as traumatic brain injury, neuroinflammation and neuropathic pain. In the present study, we set out to study the role of lysosomal vesicles on ATP release from the dorsal root ganglion neurons. We found that the lysosomal vesicles, which contain the quinacrine-positive fluorescence and express the vesicular nucleotide transporter (VNUT), were localized within the soma and growth cone of the cultured dorsal root ganglion neurons. In addition, the number of the quinacrine staining was decreased by application of lysosomal exocytosis activators, and this decrease was suppressed by the metformin and vacuolin-1, which suppressed lysosomal exocytosis. Thus, these findings suggest that ATP release via the lysosomal exocytosis may be one of the pathways for ATP release in response to stimulation.

Secretion of ATP from Schwann cells through lysosomal exocytosis during Wallerian degeneration

The present study demonstrates that adenosine triphosphate (ATP) is released from Schwann cells through lysosomal exocytosis during Wallerian degeneration and in response to stimulation. In primary Schwann cell cultures, ATP was stored in lysosomal vesicles. ATP could then induce Ca(2+)-dependent lysosomal exocytosis. Among three stimulants of lysosomal exocytosis (glutamate, NH(4)Cl and zymosan), only NH(4)Cl was sufficient to induce ATP release from ex vivo sciatic nerve explants at 3 days in vitro. Lysosomal exocytosis inhibitors (metformin, chlorpromazine and vacuolin-1) reversed the effect of NH(4)Cl-enhanced ATP release, replicating the state of explants treated with NH(4)Cl in the absence of lysosomal exocytosis inhibitors. Furthermore, we observed ATP release through lysosomal exocytosis during Wallerian degeneration in sciatic explant cultures using the recently identified vesicular nucleotide transporter (VNUT). From these experiments, we conclude that the exocytosis of lysosomes in Schwann cells during Wallerian degeneration is Ca(2+)-dependent, and that it induces ATP release from Schwann cells.

Effects of oxygen-glucose deprivation on microglial mobility and viability in developing mouse hippocampal tissues

As brain-resident immune cells, microglia (MG) survey the brain parenchyma to maintain homeostasis during development and following injury. Research in perinatal stroke, a leading cause of lifelong disability, has implicated MG as targets for therapeutic intervention during stroke. Although MG responses are complex, work in developing rodents suggests that MG limit brain damage after stroke. However, little is known about how energy-limiting conditions affect MG survival and mobility (motility and migration) in developing brain tissues. Here, we used confocal time-lapse imaging to monitor MG viability and mobility during hypoxia or oxygen-glucose deprivation (OGD) in hippocampal tissue slices derived from neonatal GFP-reporter mice (CX3CR1(GFP/+) ). We found that MG remain viable for at least 6 h of hypoxia but begin to die after 2 h of OGD, while both hypoxia and OGD reduce MG motility. Unexpectedly, some MG retain or recover motility during OGD and can engulf dead cells. Additionally, MG from younger neonates (P2-P3) are more resistant to OGD than those from older ones (P6-P7), indicating increasing vulnerability with developmental age. Finally, transient (2 h) OGD also increases MG death, and although motility is rapidly restored after transient OGD, it remains below control levels for many hours. Together, these results show that MG in neonatal mouse brain tissues are vulnerable to both transient and sustained OGD, and many MG die within hours after onset of OGD. Preventing MG death may, therefore, provide a strategy for promoting tissue restoration after stroke.

P2X7 signaling promotes microsphere embolism-triggered microglia activation by maintaining elevation of Fas ligand

Background

The cerebral microvascular occlusion elicits microvascular injury which mimics the different degrees of stroke severity observed in patients, but the mechanisms underlying these embolic injuries are far from understood. The Fas ligand (FasL)-Fas system has been implicated in a number of pathogenic states. Here, we examined the contribution of microglia-derived FasL to brain inflammatory injury, with a focus on the potential to suppress the FasL increase by inhibition of the P2X₇-FasL signaling with pharmacological or genetic approaches during ischemia.

Methods

The cerebral microvascular occlusion was induced by microsphere injection in experimental animals. Morphological changes in microglial cells were studied immunohistochemically. The biochemical analyses were used to examine the intracellular changes of P2X₇/FasL signaling. The BV-2 cells and primary microglia from mice genetically deficient in P2X₇ were used to further establish a linkage between microglia activation and FasL overproduction.

Results

The FasL expression was continuously elevated and was spatiotemporally related to microglia activation following microsphere embolism. Notably, P2X₇ expression concomitantly increased in microglia and presented a distribution pattern that was similar to that of FasL in ED1-positive cells at pathological process of microsphere embolism. Interestingly, FasL generation in cultured microglia cells subjected to oxygen-glucose deprivation-treated neuron-conditioned medium was prevented by the silencing of P2X₇. Furthermore, FasL induced the migration of BV-2 microglia, whereas the neutralization of FasL with a blocking antibody was highly effective in inhibiting ischemia-induced microglial mobility. Similar results were observed in primary microglia from wild-type mice or mice genetically deficient in P2X₇. Finally, the degrees of FasL overproduction and neuronal death were consistently reduced in P2X₇^−/− mice compared with wild-type littermates following microsphere embolism insult.

Conclusion

FasL functions as a key component of an immunoreactive response loop by recruiting microglia to the lesion sites through a P2X₇-dependent mechanism. The specific modulation of P2X₇/FasL signaling and aberrant microglial activation could provide therapeutic benefits in acute and subacute phase of cerebral microembolic injury.