Phosphatidylinositol 3-kinase, phosphoinositide-specific phospholipase-Cγ and protein kinase-C signal myelin phagocytosis mediated by complement receptor-3 alone and combined with scavenger receptor-AI/II in macrophages

Galectin-3 (MAC-2) controls phagocytosis and macropinocytosis through intracellular and extracellular mechanisms

Galectin-3 (Gal-3; formally named MAC-2) is a β-galactoside-binding lectin. Various cell types produce Gal-3 under either normal conditions and/or pathological conditions. Gal-3 can be present in cells' nuclei and cytoplasm, secreted from producing cells, and associated with cells' plasma membranes. This review focuses on how Gal-3 controls phagocytosis and macropinocytosis. Intracellular and extracellular Gal-3 promotes the phagocytosis of phagocytic targets/cargo (e.g., tissue debris and apoptotic cells) in “professional phagocytes” (e.g., microglia and macrophages) and “non-professional phagocytes” (e.g., Schwann cells and astrocytes). Intracellularly, Gal-3 promotes phagocytosis by controlling the “eat me” signaling pathways that phagocytic receptors generate, directing the cytoskeleton to produce the mechanical forces that drive the structural changes on which phagocytosis depends, protrusion and then retraction of filopodia and lamellipodia as they, respectively, engulf and then internalize phagocytic targets. Extracellularly, Gal-3 promotes phagocytosis by functioning as an opsonin, linking phagocytic targets to phagocytic receptors, activating them to generate the “eat me” signaling pathways. Macropinocytosis is a non-selective endocytic mechanism that various cells use to internalize the bulk of extracellular fluid and included materials/cargo (e.g., dissolved nutrients, proteins, and pathogens). Extracellular and intracellular Gal-3 control macropinocytosis in some types of cancer. Phagocytosed and macropinocytosed targets/cargo that reach lysosomes for degradation may rupture lysosomal membranes. Damaged lysosomal membranes undergo either repair or removal by selective autophagy (i.e., lysophagy) that intracellular Gal-3 controls.

Exploring the Pro-Phagocytic and Anti-Inflammatory Functions of PACAP and VIP in Microglia: Implications for Multiple Sclerosis

Multiple sclerosis (MS) is a chronic neuroinflammatory and demyelinating disease of the central nervous system (CNS), characterised by the infiltration of peripheral immune cells, multifocal white-matter lesions, and neurodegeneration. In recent years, microglia have emerged as key contributors to MS pathology, acting as scavengers of toxic myelin/cell debris and modulating the inflammatory microenvironment to promote myelin repair. In this review, we explore the role of two neuropeptides, pituitary adenylate cyclase-activating polypeptide (PACAP) and vasoactive intestinal peptide (VIP), as important regulators of microglial functioning during demyelination, myelin phagocytosis, and remyelination, emphasising the potential of these neuropeptides as therapeutic targets for the treatment of MS.

Role of Fractalkine in promoting inflammation in sepsis-induced multiple organ dysfunction

OBJECTIVE

Fractalkine, CX3CL1, is involved in the directional movement of chemokine cells, immune response, inflammatory response, tissue repair, and other processes. However, its role in sepsis is not well known.

METHODS

We measured circulating Fractalkine in adult patients with sepsis. Effects of Fractalkine on the survival, inflammation, tissue injury, and bacterial clearance were assessed using the WT or CX3CL^-/- murine model of cecal ligation and puncture (CLP)-induced sepsis.

RESULTS

Serum Fractalkine concentrations were significantly elevated in adult patients with sepsis compared to healthy adults. Increased Fractalkine correlated positively with the number of blood leukocytes and the level of inflammatory cytokines, including IL-6, IL-1β, IL-17A, IFN-γ, and TNF-α, and correlated negatively with IL-10 in clinical sepsis. Recombinant Fractalkine impaired survival whereas Fractalkine gene knockout or anti-Fractalkine antibody improved survival in the murine model of CLP-induced sepsis. Fractalkine administration increased inflammatory response, evident by higher levels of cytokines (TNF-α, IL-1β, IL-17A, IFN-γ, and IL-6 but not IL-10), and tissue damage (lung, liver, and kidney) in CLP-induced sepsis. Fractalkine reduced bacterial clearance in CLP-induced polymicrobial sepsis by reducing macrophage or neutrophil phagocytosis and intracellular elimination of E. coli.

CONCLUSIONS

Fractalkine aggravates sepsis by increasing inflammation and decreasing bacterial clearance, and is a potential tool for anti-sepsis therapy.

PLCG2 protective variant p.P522R modulates tau pathology and disease progression in patients with mild cognitive impairment

A rare coding variant (rs72824905, p.P522R) conferring protection against Alzheimer's disease (AD) was identified in the gene encoding the enzyme phospholipase-C-γ2 (PLCG2) that is highly expressed in microglia. To explore the protective nature of this variant, we employed latent process linear mixed models to examine the association of p.P522R with longitudinal cognitive decline in 3595 MCI patients, and in 10,097 individuals from population-based studies. Furthermore, association with CSF levels of pTau181, total tau, and Aβ1-42 was assessed in 1261 MCI patients. We found that MCI patients who carried the p.P522R variant showed a slower rate of cognitive decline compared to non-carriers and that this effect was mediated by lower pTau181 levels in CSF. The effect size of the association of p.P522R with the cognitive decline and pTau181 was similar to that of APOE-ε4, the strongest genetic risk factor for AD. Interestingly, the protective effect of p.P522R was more pronounced in MCI patients with low Aβ1-42 levels suggesting a role of PLCG2 in the response to amyloid pathology. In line with this hypothesis, we observed no protective effect of the PLCG2 variant on the cognitive decline in population-based studies probably due to the lower prevalence of amyloid positivity in these samples compared to MCI patients. Concerning the potential biological underpinnings, we identified a network of co-expressed proteins connecting PLCG2 to APOE and TREM2 using unsupervised co-regulatory network analysis. The network was highly enriched for the complement cascade and genes differentially expressed in disease-associated microglia. Our data show that p.P522R in PLCG2 reduces AD disease progression by mitigating tau pathology in the presence of amyloid pathology and, as a consequence, maintains cognitive function. Targeting the enzyme PLCG2 might provide a new therapeutic approach for treating AD.

Microglia exacerbate white matter injury via complement C3/C3aR pathway after hypoperfusion

Microglial activation participates in white matter injury after cerebral hypoperfusion. However, the underlying mechanism is unclear. Here, we explore whether activated microglia aggravate white matter injury via complement C3-C3aR pathway after chronic cerebral hypoperfusion. Methods: Adult male Sprague-Dawley rats (n = 80) underwent bilateral common carotid artery occlusion for 7, 14, and 28 days. Cerebral vessel density and blood flow were examined by synchrotron radiation angiography and three-dimensional arterial spin labeling. Neurobehavioral assessments, CLARITY imaging, and immunohistochemistry were performed to evaluate activation of microglia and C3-C3aR pathway. Furthermore, C3aR knockout mice were used to establish the causal relationship of C3-C3aR signaling on microglia activation and white matter injury after hypoperfusion. Results: Cerebral vessel density and blood flow were reduced after hypoperfusion (p<0.05). Spatial learning and memory deficits and white matter injury were shown (p<0.05). These impairments were correlated with aberrant microglia activation and an increase in the number of reactive microglia adhering to and phagocytosed myelin in the hypoperfusion group (p<0.05), which were accompanied by the up-regulation of complement C3 and its receptors C3aR (p<0.05). Genetic deletion of C3ar1 significantly inhibited aberrant microglial activation and reversed white matter injury after hypoperfusion (p<0.05). Furthermore, the C3aR antagonist SB290157 decreased the number of microglia adhering to myelin (p<0.05), attenuated white matter injury and cognitive deficits in chronic hypoperfusion rats (p<0.05). Conclusions: Our results demonstrated that aberrant activated microglia aggravate white matter injury via C3-C3aR pathway during chronic hypoperfusion. These findings indicate C3aR plays a critical role in mediating neuroinflammation and white matter injury through aberrant microglia activation, which provides a novel therapeutic target for the small vessel disease and vascular dementia.

Microglia: Brain cells on the move

In the last decade, tremendous progress has been made in understanding the biology of microglia - i.e. the fascinating immigrated resident immune cell population of the central nervous system (CNS). Recent literature reviews have largely dealt with the plentiful functions of microglia in CNS homeostasis, development and pathology, and the influences of sex and the microbiome. In this review, the intriguing aspect of their physical plasticity during CNS development will get specific attention. Microglia move around (mobility) and reshape their processes (motility). Microglial migration into and inside the CNS is most prominent throughout development and consequently most of the data described in this review concern mobility and motility in the changing environment of the developing brain. Here, we first define microglia based on their highly specialized age- and region-dependent gene expression signature and associated functional heterogeneity. Next, we describe their origin, the migration route of immature microglial cells towards the CNS, the mechanisms underlying their invasion of the CNS, and their spatiotemporal localization and surveying behaviour inside the developing CNS. These processes are dependent on microglial mobility and motility which are determined by the microenvironment of the CNS. Therefore, we further zoom in on the changing environment during CNS development. We elaborate on the extracellular matrix and the respective integrin receptors on microglia and we discuss the purinergic and molecular signalling in microglial mobility. In the last section, we discuss the physiological and pathological functions of microglia in which mobility and motility are involved to stress the importance of microglial 'movement'.

Galectin-3 (MAC-2) Controls Microglia Phenotype Whether Amoeboid and Phagocytic or Branched and Non-phagocytic by Regulating the Cytoskeleton

Myelin surrounding central nervous system (CNS) axons breaks down in multiple sclerosis (MS) and following traumatic axonal injury. Myelin-debris so produced is harmful to repair since it impedes remyelination in MS and the regeneration of traumatized axons. These devastating outcomes are largely due to inefficient removal by phagocytosis of myelin-debris by microglia. Therefore, revealing mechanisms that control phagocytosis is vital. We previously showed that in phagocytosis, filopodia and lamellipodia extend/engulf and then retract/internalize myelin-debris. Moreover, cofilin activates phagocytosis by advancing the remodeling of actin filaments (i.e., existing filaments disassemble and new filaments assemble in a new configuration), causing filopodia/lamellipodia to protrude, and furthermore, Galectin-3 (formally named MAC-2) activates phagocytosis by enhancing K-Ras.GTP/PI3K signaling that leads to actin/myosin-based contraction, causing filopodia/lamellipodia to retract. To understand further how Galectin-3 controls phagocytosis we knocked-down (KD) Galectin-3 expression in cultured primary microglia using Galectin-3 small-hairpin RNA (Gal-3-shRNA). KD Galectin-3 protein levels reduced phagocytosis extensively. Further, inhibiting nucleolin (NCL) and nucleophosmin (NPM), which advance K-Ras signaling as does Galectin-3, also reduced phagocytosis. Strikingly and unexpectedly, knocking down Galectin-3 resulted in a dramatic transformation of microglia morphology from “amoeboid-like” to “branched-like,” rearrangement of actin filaments and inactivation of cofilin. Thus, Galectin-3 may control microglia morphology and phagocytosis by regulating the activation state of cofilin, which, in turn, affects how actin filaments organize and how stable they are. Furthermore, our current and previous findings together suggest that Galectin-3 activates phagocytosis by targeting the cytoskeleton twice: first, by advancing cofilin activation, causing filopodia/lamellipodia to extend/engulf myelin-debris. Second, by advancing actin/myosin-based contraction through K-Ras.GTP/PI3K signaling, causing filopodia/lamellipodia to retract/internalize myelin-debris.

Effect of glial cells on remyelination after spinal cord injury

Remyelination plays a key role in functional recovery of axons after spinal cord injury. Glial cells are the most abundant cells in the central nervous system. When spinal cord injury occurs, many glial cells at the lesion site are immediately activated, and different cells differentially affect inflammatory reactions after injury. In this review, we aim to discuss the core role of oligodendrocyte precursor cells and crosstalk with the rest of glia and their subcategories in the remyelination process. Activated astrocytes influence proliferation, differentiation, and maturation of oligodendrocyte precursor cells, while activated microglia alter remyelination by regulating the inflammatory reaction after spinal cord injury. Understanding the interaction between oligodendrocyte precursor cells and the rest of glia is necessary when designing a therapeutic plan of remyelination after spinal cord injury.

IFNβ secreted by microglia mediates clearance of myelin debris in CNS autoimmunity

Introduction

Multiple sclerosis (MS) is a chronic demyelinating disorder of the central nervous system (CNS) leading to progressive neurological disability. Interferon β (IFNβ) represents a standard treatment for relapsing-remitting MS and exogenous administration of IFNβ exhibits protective effects in experimentally induced CNS autoimmunity. Also, genetic deletion of IFNβ in mice leads to an aggravation of disease symptoms in the MS model of experimental autoimmune encephalomyelitis (EAE). However, neither the underlying mechanisms mediating the beneficial effects nor the cellular source of IFNβ have been fully elucidated.

Results

In this report, a subpopulation of activated microglia was identified as the major producers of IFNβ in the CNS at the peak of EAE using an IFNβ-fluorescence reporter mouse model. These IFNβ expressing microglia specifically localized to active CNS lesions and were associated with myelin debris in demyelinated cerebellar organotypic slice cultures (OSCs). In response to IFNβ microglia showed an enhanced capacity to phagocytose myelin in vitro and up-regulated the expression of phagocytosis-associated genes. IFNβ treatment was further sufficient to stimulate association of microglia with myelin debris in OSCs. Moreover, IFNβ-producing microglia mediated an enhanced removal of myelin debris when co-transplanted onto demyelinated OSCs as compared to IFNβ non-producing microglia.

Conclusions

These data identify activated microglia as the major producers of protective IFNβ at the peak of EAE and as orchestrators of IFNβ-induced clearance of myelin debris.

Electronic supplementary material

The online version of this article (doi:10.1186/s40478-015-0192-4) contains supplementary material, which is available to authorized users.

PI3K signaling and Stat92E converge to modulate glial responsiveness to axonal injury

Activation of glial cells following axon injury is mediated by a positive feedback loop downstream of the glial phagocytic receptor Draper, allowing the strength of the response to match the severity of injury.

Glial cells are exquisitely sensitive to neuronal injury but mechanisms by which glia establish competence to respond to injury, continuously gauge neuronal health, and rapidly activate reactive responses remain poorly defined. Here, we show glial PI3K signaling in the uninjured brain regulates baseline levels of Draper, a receptor essential for Drosophila glia to sense and respond to axonal injury. After injury, Draper levels are up-regulated through a Stat92E-modulated, injury-responsive enhancer element within the draper gene. Surprisingly, canonical JAK/STAT signaling does not regulate draper expression. Rather, we find injury-induced draper activation is downstream of the Draper/Src42a/Shark/Rac1 engulfment signaling pathway. Thus, PI3K signaling and Stat92E are critical in vivo regulators of glial responsiveness to axonal injury. We provide evidence for a positive auto-regulatory mechanism whereby signaling through the injury-responsive Draper receptor leads to Stat92E-dependent, transcriptional activation of the draper gene. We propose that Drosophila glia use this auto-regulatory loop as a mechanism to adjust their reactive state following injury.

Acute injuries of the central nervous system (CNS) trigger a robust reaction from glial cells—a non-neuronal population of cells that regulate and support neural development and physiology. Although this process occurs after all types of CNS trauma in mammals, how it is activated and its precise role in recovery remain poorly understood. Using the fruit fly Drosophila melanogaster as a model, we previously identified a cell surface receptor called Draper, which is required for the activation of glia after local axon injury (“axotomy”) and for the removal of degenerating axonal debris by phagocytosis. Here, we show that regulation of Draper protein levels and glial activation through the Draper signaling pathway are mediated by the well-conserved PI3K and signal transducer and activator of transcription (STAT) signaling cascades. We find that STAT transcriptional activity is activated in glia in response to axotomy, and identify an injury-responsive regulatory element within the draper gene that appears to be directly modulated by STAT. Interestingly, the intensity of STAT activity in glial cells after axotomy correlates tightly with the number of local severed axons, indicating that Drosophila glia are able to fine-tune their response to neuronal injury according to its severity. In summary, we propose that the initial phagocytic competence of glia is regulated by setting Draper baseline levels (via PI3K), whereas injury-activated glial phagocytic activity is modulated through a positive feedback loop that requires STAT-dependent activation of draper. We speculate that the level of activation of this cascade is determined by glial cell recognition of Draper ligands present on degenerating axon material, thereby matching the levels of glial reactivity to the amount of injured axonal material.

Multimolecular signaling complexes enable Syk-mediated signaling of CD36 internalization

CD36 is a versatile receptor known to play a central role in the development of atherosclerosis, the pathogenesis of malaria, and the removal of apoptotic cells. Remarkably, the short cytosolically exposed regions of CD36 lack identifiable motifs, which has hampered elucidation of its mode of signaling. Using a combination of phosphoprotein isolation, mass spectrometry, superresolution imaging, and gene silencing, we have determined that the receptor induces ligand internalization through a heteromeric complex consisting of CD36, β1 and/or β2 integrins, and the tetraspanins CD9 and/or CD81. This receptor complex serves to link CD36 to the adaptor FcRγ, which bears an immunoreceptor tyrosine activation motif. By coupling to FcRγ, CD36 is able to engage Src-family kinases and Syk, which in turn drives the internalization of CD36 and its bound ligands.

CD11b/CD18 (Mac-1) is a novel surface receptor for extracellular double-stranded RNA to mediate cellular inflammatory responses

During viral infection, extracellular dsRNA is a potent signaling molecule that activates many innate immune cells, including macrophages. TLR3 is a well-known receptor for extracellular dsRNA, and internalization of extracellular dsRNA is required for endosomal TLR3 activation. Preserved inflammatory responses of TLR3-deficient macrophages to extracellular dsRNA strongly support a TLR3-independent mechanism in dsRNA-mediated immune responses. The present study demonstrated that CD11b/CD18 (Mac-1 [macrophage-1 Ag]), a surface integrin receptor, recognized extracellular dsRNA and induced macrophage immune responses. CD11b deficiency reduced inflammatory cytokine induction elicited by polyinosinic:polycytidylic acid (poly I:C; a synthetic dsRNA) in mouse sera and livers, as well as in cultured peritoneal macrophages. dsRNA-binding assay and confocal immunofluorescence showed that Mac-1, especially the CD11b subunit, interacted and colocalized with poly I:C on the surface of macrophages. Further mechanistic studies revealed two distinct signaling events following dsRNA recognition by Mac-1. First, Mac-1 facilitated poly I:C internalization through the activation of PI3K signaling and enhanced TLR3-dependent activation of IRF3 in macrophages. Second, poly I:C induced activation of phagocyte NADPH oxidase in a TLR3-independent, but Mac-1-dependent, manner. Subsequently, phagocyte NADPH oxidase-derived intracellular reactive oxygen species activated MAPK and NF-κB pathways. Our results indicate that extracellular dsRNA activates Mac-1 to enhance TLR3-dependent signaling and to trigger TLR3-independent, but Mac-1-dependent, inflammatory oxidative signaling, identifying a novel mechanistic basis for macrophages to recognize extracellular dsRNA to regulate innate immune responses. This study identifies Mac-1 as a novel surface receptor for extracellular dsRNA and implicates it as a potential therapeutic target for virus-related inflammatory diseases.

Complement receptor-3 negatively regulates the phagocytosis of degenerated myelin through tyrosine kinase Syk and cofilin

BACKGROUND

Intact myelin, which normally surrounds axons, breaks down in Wallerian degeneration following axonal injury and during neurodegenerative diseases such as multiple sclerosis. Clearance of degenerated myelin by phagocytosis is essential since myelin impedes repair and exacerbates damage. CR3 (complement receptor-3) is a principal phagocytic receptor in myelin phagocytosis. We studied how tyrosine kinase Syk (spleen tyrosine kinase) and cofilin control phagocytosis of degenerated myelin by CR3 in microglia and macrophages. Syk is a non-receptor tyrosine kinase that CR3 recruits to convey cellular functions. Cofilin is an actin-depolymerizing protein that controls F-actin (filamentous actin) remodeling (i.e., disassembly and reassembly) by shifting between active unphosphorylated and inactive phosphorylated states.

RESULTS

Syk was continuously activated during prolonged phagocytosis. Phagocytosis increased when Syk activity and expression were reduced, suggesting that normally Syk down regulates CR3-mediated myelin phagocytosis. Levels of inactive p-cofilin (phosphorylated cofilin) decreased transiently during prolonged phagocytosis. In contrast, p-cofilin levels decreased continuously when Syk activity and expression were continuously reduced, suggesting that normally Syk advances the inactive state of cofilin. Observations also revealed inverse relationships between levels of phagocytosis and levels of inactive p-cofilin, suggesting that active unphosphorylated cofilin advances phagocytosis. Active cofilin could advance phagocytosis by promoting F-actin remodeling, which supports the production of membrane protrusions (e.g., filopodia), which, as we also revealed, are instrumental in myelin phagocytosis.

CONCLUSIONS

CR3 both activates and downregulates myelin phagocytosis at the same time. Activation was previously documented. We presently demonstrate that downregulation is mediated through Syk, which advances the inactive phosphorylated state of cofilin. Self-negative control of phagocytosis by the phagocytic receptor can be useful in protecting phagocytes from excessive phagocytosis (i.e., "overeating") during extended exposure to particles that are destined for ingestion.

Activation of phospholipase-Cγ and protein kinase C signal pathways helps the survival of spinal motoneurons injured by root avulsion

The signaling transduction processes involved in avulsion-induced motoneuron (MN) death have not been elucidated. Using the brachial plexus root avulsion rat model, we showed that avulsion-activated phosphorylation of phospholipase-Cγ (PLCγ) and protein kinase C (PKC) occurred in injured spinal MNs within 72 h of injury. Moreover, some MNs positive for PLCγ and PKC are also positive for avulsion-induced neuronal nitric oxide synthase (nNOS). Inhibition of PLCγ/PKC signal pathway, either with PLCγ inhibitor, 1-[6-((17β-3-methoxyestra-1,3,5(10)-trien-17-yl) amino)hexyl]-1H-pyrrole-2,5-dione, or with PLCγ siRNA augmented avulsion-induced MN death. 1-[6-((17β-3-methoxyestra-1,3,5(10)-trien-17-yl) amino)hexyl]-1H-pyrrole-2,5-dione also inhibited PKC phosphorylation and exacerbated avulsion-induced reductions in the nNOS protein level in injured spinal segments. Moreover, activation of PLCγ/PKC signal pathway with PKC activator, phorbol-12-myristate-13-acetate, decreased avulsion-induced MN death. The temporal profile of PLCγ/PKC signaling appears to be crucial for the survival of spinal MNs after root avulsion. Our data suggest that PLCγ mediates, while PKC and nNOS are associated with, the avulsion-induced MN death in brachial plexus root avulsion.

Cellular uptake mechanisms and toxicity of quantum dots in dendritic cells

Quantum dots (QDs) are nanoparticles with strong fluorescent emission and are novel tools used in biomedical applications, but the toxicity and mechanism of cellular uptake are poorly understood. QD655-COOH (negative charge, 18 nm) consist of a cadmium/selenide core and a zinc sulfide shell with a carboxylic acid coating with an emission wavelength of 655 nm.

MATERIALS & METHODS

Peripheral blood mononuclear cells were isolated from porcine blood by gradient centrifugation, and monocytes, which are CD14 positive, were purified. Monocytes were differentiated into dendritic cells (DCs) with GM-CSF and IL-4.

RESULTS

Monocytes showed cellular uptake of QD655-COOH, while lymphocytes did not. Monocyte differentiation into DCs increased the cellular uptake by sixfold when dosed with 2 nM of QD655-COOH. Transmission electron microscopy depicted QD655-COOH in the cytoplasmic vacuoles of DCs. Twelve endocytic inhibitors demonstrated QD655-COOH endocytosis in DCs, which was recognized by clathrin and scavenger receptors and regulated by F-actin and phospholipase C. In addition, DC maturation with lipopolysaccharide (LPS) caused an increase in QD655-COOH uptake compared with DCs without LPS stimulation. Viability assays, including 96AQ, CCK-8, alamar blue and ApoTox, exhibited minimal toxicity in DCs dosed with QD655-COOH at 24 h. However, glutathione levels showed a significant decrease with 10 nM of QD655-COOH. Finally, QD655-COOH exposure was associated with a decrease in CD80/CD86 expression after LPS stimulation, suggesting suppression with DC maturation.

CONCLUSION

These findings shed light on the mechanism of QD655-COOH uptake in DCs and that cellular uptake pathways are dependent on cell type and cell differentiation.

Wallerian degeneration: the innate-immune response to traumatic nerve injury

Traumatic injury to peripheral nerves results in the loss of neural functions. Recovery by regeneration depends on the cellular and molecular events of Wallerian degeneration that injury induces distal to the lesion site, the domain through which severed axons regenerate back to their target tissues. Innate-immunity is central to Wallerian degeneration since innate-immune cells, functions and molecules that are produced by immune and non-immune cells are involved. The innate-immune response helps to turn the peripheral nerve tissue into an environment that supports regeneration by removing inhibitory myelin and by upregulating neurotrophic properties. The characteristics of an efficient innate-immune response are rapid onset and conclusion, and the orchestrated interplay between Schwann cells, fibroblasts, macrophages, endothelial cells, and molecules they produce. Wallerian degeneration serves as a prelude for successful repair when these requirements are met. In contrast, functional recovery is poor when injury fails to produce the efficient innate-immune response of Wallerian degeneration.

Translocator protein (18 kDa)/peripheral benzodiazepine receptor specific ligands induce microglia functions consistent with an activated state

In the brain, translocator protein (18 kDa) (TSPO), previously called peripheral benzodiazepine receptor (PBR), is a glial protein that has been extensively used as a biomarker of brain injury and inflammation. However, the functional role of TSPO in glial cells is not well characterized. In this study, we show that the TSPO-specific ligands R-PK11195 (PK) and Ro5-4864 (Ro) increased microglia proliferation and phagocytosis with no effect on migration. Both ligands increased reactive oxygen species (ROS) production, and this effect may be mediated by NADPH-oxidase. PK and Ro also produced a small but detectable increase in IL-1β release. We also examined the effect of PK and Ro on the expression of proinflammatory genes and cytokine release in lipopolysaccharide (LPS) and adenosine triphosphate (ATP) activated microglia. PK or Ro had no effect on LPS-induced increase of pro-inflammatory genes, but they both decreased the ATP-induced increase of COX-2 gene expression. Ro, but not PK, enhanced the LPS-induced release of IL-1β. However, Ro decreased the ATP-induced release of IL-1β and TNF-α, and PK decreased the ATP-induced release of TNF-α. Exposure to Ro in the presence of LPS increased the number of apoptotic microglia, an effect that could be blocked by PK. These findings show that TSPO ligands modulate cellular functions consistent with microglia activation. Further, when microglia are activated, these ligands may have therapeutic potential by reducing the expression of pro-inflammatory genes and cytokine release. Finally, Ro-like ligands may be involved in the elimination of activated microglia via apoptosis.

Galectin-3/MAC-2, Ras and PI3K activate complement receptor-3 and scavenger receptor-AI/II mediated myelin phagocytosis in microglia

The removal of degenerated myelin is essential for repair in Wallerian degeneration that follows traumatic injury to axons and in autoimmune demyelinating diseases (e.g., multiple sclerosis). Microglia can remove degenerated myelin through phosphatidylinositol-3-kinase (PI3K)-dependent phagocytosis mediated by complement receptor-3 (CR3/MAC-1) and scavenger receptor-AI/II (SRAI/II). Paradoxically, these receptors are expressed in microglia after injury but myelin is not phagocytosed. Additionally, Galectin-3/MAC-2 is expressed in microglia that phagocytose but not in microglia that do not phagocytose, suggesting that Galectin-3/MAC-2 is instrumental in activating phagocytosis. S-trans, trans-farnesylthiosalicylic (FTS), which inhibits Galectin-3/MAC-2 dependent activation of PI3K through Ras, inhibited phagocytosis. K-Ras-GTP levels and PI3K activity increased during normal phagocytosis and decreased during FTS-inhibited phagocytosis. Galectin-3/MAC-2, which binds and stabilizes active Ras, coimmunoprecipitated with Ras and levels of the coimmunoprecipitate increased during normal phagocytosis. A role for Galectin-3/MAC-2 dependent activation of PI3K through Ras, mostly K-Ras, is thus suggested. An explanation may thus be offered for deficient phagocytosis by microglia that express CR3/MAC-1 and SRAI/II without Galectin-3/MAC-2 and efficient phagocytosis when CR3/MAC-1 and SRAI/II are co-expressed with Galectin-3/MAC-2.

The role of Galectin-3/MAC-2 in the activation of the innate-immune function of phagocytosis in microglia in injury and disease

Microglia are a self-sustained population of immune/myeloid cells present throughout the central nervous system (CNS). Microglia are in a "resting" state in the normal adult CNS. They turn "active" in injury and disease (e.g., trauma, neurodegeneration, and infection). Activated microglia can be beneficial as well as detrimental/neurotoxic. The innate-immune function of phagocytosis of tissue debris, neurotoxic factor, and pathogens is a beneficial function of microglia. The current manuscript reviews the role of Galectin-3 (known also as MAC-2; Galectin-3/MAC-2) in the activation of the phagocytosis of degenerated myelin that is mediated by complement receptor-3 (known also as MAC-1; CD11b/CD18; alphaMbeta2 integrin) and SRA (scavenger receptor-AI/II). Observations suggest that Galectin-3/MAC-2 may act as a molecular switch that activates phagocytosis by up-regulating and prolonging KRas-GTP-dependent PI3K (phosphatidylinositol 3-kinase) activity. A similar mechanism may regulate the phagocytosis of other tissue debris, neurotoxic factors and pathogens in neurodegenerative and infectious diseases.

Myelin-phagocytosing macrophages in isolated sciatic and optic nerves reveal a unique reactive phenotype

Macrophages are key effectors in demyelinating diseases of the central and peripheral nervous system by phagocytosing myelin and releasing immunoregulatory mediators. Here, we report on a distinct, a priori anti-inflammatory reaction of macrophages phagocytosing myelin upon contact with damaged nerve tissue. Macrophages rapidly invaded peripheral (sciatic) and central (optic) nerve tissues in vitro, readily incorporated myelin and expressed high levels of phagocytosis-associated molecules (e.g., Fc and scavenger receptors). In contrast, factors involved in antigen presentation (MHC class-II, CD80, CD86) revealed only a restricted expression. In parallel, a highly ordered appearance of cytokines and chemokines was detected. IL-10, IL-6, CCL22, and CXCL1 were immediately but transiently induced, whereas CCL2, CCL11, and TGFbeta revealed more persisting levels. Such a profile would attract neutrophils, monocytes/macrophages, and Th2 cells as well as bias for a Th2-supporting environment. Importantly, proinflammatory/Th1-supporting factors, such as TNFalpha, IL-12p70, CCL3, and CCL5, were not induced. Still the simultaneous presence of TGFbeta and IL-6 could assist Th17 development, further depending on yet not present IL-23. The release pattern was clearly distinct from reactive phenotypes induced in isolated macrophages and microglia upon treatment with IL-4, IL-13, bacterial lipopolysaccharide, IFNgamma, or purified myelin. Nerve-exposed macrophages thus commit to a unique functional orientation.

Microglia: active sensor and versatile effector cells in the normal and pathologic brain

Microglial cells constitute the resident macrophage population of the CNS. Recent in vivo studies have shown that microglia carry out active tissue scanning, which challenges the traditional notion of 'resting' microglia in the normal brain. Transformation of microglia to reactive states in response to pathology has been known for decades as microglial activation, but seems to be more diverse and dynamic than ever anticipated--in both transcriptional and nontranscriptional features and functional consequences. This may help to explain why engagement of microglia can be either neuroprotective or neurotoxic, resulting in containment or aggravation of disease progression. Moreover, little is known about the heterogeneity of microglial responses in different pathologic contexts that results from regional adaptations or from the progression of a disease. In this review, we focus on several key observations that illustrate the multi-faceted activities of microglia in the normal and pathologic brain.

The effects of intracellular signalling pathway inhibitors on phagocytosis by haemocytes of Manduca sexta

The intracellular signalling pathways activated during phagocytosis by larval haemocytes of a lepidopteran, Manduca sexta, were investigated. Using fluorescein-labelled Escherichia coli as bioparticles, a fluorescence-based assay was used to quantify phagocytosis by haemocytes in monolayers in vitro, and the intracellular signalling pathways involved in phagocytosis were examined using inhibitors. Pathways known to be involved in phagocytosis by mammalian cells were selected for the study in haemocytes, and the amino acid sequences of human isoforms of the selected protein targets were used to conduct searches of two completed databases of insect proteins, those of Drosophila melanogaster and Anopheles gambiae and EST databases of moths Bombyx mori and M. sexta. Decreases in phagocytosis produced by pathway inhibitors indicated that tyrosine phosphorlylation phosphatidylinositol 3-kinase (PI3-kinase) and mitogen-activated protein kinase/extracellular-regulated kinase (ERK/MAPK) were required for internalisation of bacteria. Inhibition of Syk, a mammalian kinase, also decreased phagocytosis. JNK/SAPK did not seem to be involved in phagocytosis. The presence of protein phosphatases probably regulates phagocytosis at the intracellular level by dephosphorlyation of serine/threonine residues.

cAMP cascade (PKA, Epac, adenylyl cyclase, Gi, and phosphodiesterases) regulates myelin phagocytosis mediated by complement receptor-3 and scavenger receptor-AI/II in microglia and macrophages

The removal by phagocytosis of degenerated myelin is central for repair in Wallerian degeneration that follows traumatic injury to axons and in autoimmune demyelinating diseases (e.g., multiple sclerosis). We tested for roles played by the cAMP cascade in the regulation of myelin phagocytosis mediated by complement receptor-3 (CR3/MAC-1) and scavenger receptor-AI/II (SRAI/II) separately and combined in mouse microglia and macrophages. Components of the cAMP cascade tested are cAMP, adenylyl cyclase (AC), Gi, protein kinase A (PKA), exchange protein directly activated by cAMP (Epac), and phosphodiesterases (PDE). PKA inhibitors H-89 and PKI(14-22) amide inhibited phagocytosis at normal operating cAMP levels (i.e., those occurring in the absence of reagents that alter cAMP levels), suggesting activation of phagocytosis through PKA at normal cAMP levels. Phagocytosis was inhibited by reagents that elevate endogenous cAMP levels to above normal: Gi-inhibitor Pertussis toxin (PTX), AC activator Forskolin, and PDE inhibitors IBMX and Rolipram. Phagocytosis was inhibited also by cAMP analogues whose addition mimics abnormal elevations in endogenous cAMP levels: nonselective 8-bromo-cAMP, PKA-specific 6-Benz-cAMP, and Epac-specific 8-CPT-2'-O-Me-cAMP, suggesting that abnormal high cAMP levels inhibit phagocytosis through PKA and Epac. Altogether, observations suggest a dual role for cAMP and PKA in phagocytosis: activation at normal cAMP levels and inhibition at higher. Furthermore, a balance between Gi-controlled cAMP production by AC and cAMP degradation by PDE maintains normal operating cAMP levels that enable efficient phagocytosis.

Non-PKC DAG/phorbol-ester receptor(s) inhibit complement receptor-3 and nPKC inhibit scavenger receptor-AI/II-mediated myelin phagocytosis but cPKC, PI3k, and PLCgamma activate myelin phagocytosis by both

Complement-receptor-3 (CR3/MAC-1), scavenger-receptor-AI/II (SRAI/II), and Fcgamma-receptor (FcgammaR) can mediate myelin phagocytosis in macrophages and microglia. Paradoxically, after injury to CNS axons these receptors are expressed but myelin is not phagocytosed, suggesting that phagocytosis is subject to regulation between efficient and inefficient states. In the present work, we focus on CR3/MAC-1 and SRAI/II-mediated myelin phagocytosis. Phagocytosis by CR3/MAC-1 and SRAI/II was inhibited by cPKC inhibitor Go-6976, general-PKC inhibitors Ro-318220 and calphostin-C, and BAPTA/AM, which chelates intracellular Ca2+ required for cPKC activation. Signaling/activation by cPKC are thus suggested. PMA, which mimics diacylglycerol (DAG) as an activator of cPKC, novel-PKC (nPKC), and non-PKC DAG-driven molecule(s), produced a dose-dependent dual effect on phagocytosis by CR3/MAC-1 and SRAI/II, i.e., augmentation at low concentrations and inhibition at high concentrations. Inhibition of phagocytosis by CR3/MAC-1 was enhanced by combining inhibiting concentrations of PMA with PKC inhibitors Go-6976 or Ro-318220, suggesting inhibition by PMA/DAG-driven non-PKC molecule(s). In contrast, inhibition of phagocytosis by SRAI/II was enhanced by combining inhibiting concentrations of PMA with cPKC inhibitor Go-6976 but not with general-PKC inhibitor Ro-318220, suggesting inhibition by nPKC. Phagocytosis by CR3/MAC-1 and SRAI/II was further inhibited by PI3K inhibitors wortmannin and LY-294002 and PLCgamma inhibitor U-73122. Altogether, our observations suggest that CR3/MAC-1 and SRAI/II-mediated myelin phagocytosis share activation by PI3K, PLCgamma and cPKC. The two differ, however, in that non-PKC DAG-driven molecule(s) inhibit CR3/MAC-1-mediated phagocytosis, whereas nPKC inhibit SRAI/II-mediated phagocytosis. Each of these signaling steps may be targeted for regulating CR3/MAC-1 and/or SRAI/II-mediated phagocytosis between efficient and inefficient states.

Expression of scavenger receptors in glial cells. Comparing the adhesion of astrocytes and microglia from neonatal rats to surface-bound beta-amyloid

Astrocytes and microglia associate to amyloid plaques, a pathological hallmark of Alzheimer disease. Microglia are activated by and can phagocytose beta-amyloid (Abeta). Scavenger receptors (SRs) are among the receptors mediating the uptake of fibrillar Abeta in vitro. However, little is known about the function of the astrocytes surrounding the plaques or the nature of their interaction with Abeta. It is unknown whether glial cells bind to nonfibrillar Abeta and if binding of astrocytes to Abeta depends on the same Scavenger receptors described for microglia. We determined the binding of glia to Abeta by an adhesion assay and evaluated the presence of scavenger receptors in glial cells by immunocytochemistry, immunohistochemistry of brain sections, and immunoblot. We found that astrocytes and microglia from neonatal rats adhered in a concentration-dependent manner to surfaces coated with fibrillar Abeta or nonfibrillar Abeta. Fucoidan and poly(I), known ligands for SR-type A, inhibited adhesion of microglia and astrocytes to Abeta and also inhibited Abeta phagocytosis. In contrast, a ligand for SR-type B like low density lipoprotein, did not compete glial adhesion to Abeta. Microglia presented immunodetectable SR-BI, SR-AI/AII, RAGE, and SR-MARCO (macrophage receptor with collagenous structure, a member of the SR-A family). Astrocytes presented SR-BI and SR-MARCO. To our knowledge, this is the first description of the presence of SR-MARCO in astrocytes. Our results indicate that both microglia and astrocytes adhere to fibrillar and nonfibrillar Abeta. Adhesion was mediated by a fucoidan-sensitive receptor. We propose that SR-MARCO could be the Scavenger receptor responsible for the adhesion of astrocytes and microglia to Abeta.

Various protein kinases regulate human sperm acrosome reaction and the associated phosphorylation of Tyr residues and of the Thr-Glu-Tyr motif

Acrosome reaction (AR) is an exocytotic process of fundamental importance for the spermatozoon to fertilize the oocyte. The mechanisms mediating this process are only partially defined. The aim of the present study was to investigate the role of various kinases and the extracellular signal-regulated kinase (ERK) pathway in the induction of the AR and associated phosphorylation of tyrosine (Tyr) residues and of the threonine-glutamic acid-tyrosine (Thr-Glu-Tyr) motif that occurs in 80 and 105 kDa proteins (p80/p105). Human spermatozoa were capacitated and AR was induced with lysophosphatidylcholine in the presence of inhibitors of various kinases and of the ERK pathway. Phosphorylation of Tyr and of Thr-Glu-Tyr peaked 15 min after the induction of the AR. Both phosphorylations were prevented by inhibitors of protein kinase C, MEK, phosphoinositide 3-kinase and Akt but not by protein kinase A inhibitors. Phosphorylation of Thr-Glu-Tyr, but not Tyr, was decreased by inhibitors of protein tyrosine kinase and Grb2-SH2. All the inhibitors prevented lysophosphatidylcholine-induced AR, indicating the involvement of PKC, PKA, PTK, PI3K, Akt and the ERK pathway. These results show that phosphorylation of Tyr and Thr-Glu-Tyr are associated with the AR and are differently regulated by the various kinases emphasing the complexity of this process.

Expression of the Ets transcription factor Erm is regulated through a conventional PKC signaling pathway in the Molt4 lymphoblastic cell line

Erm, a member of the PEA3 group within the Ets family of transcription factors, is expressed in murine and human lymphocytes. Here, we show that in the human Molt4 lymphoblastic cell line, the erm gene expression is regulated by the conventional PKC (cPKC) pathway. To better characterize the molecular mechanism by which cPKC regulates Erm transcription in Molt4 cells, we tested proximal promoter deletions of the human gene, and identified a specific cPKC-regulated region between positions -420 and -115 upstream of the first exon.

Ciclopirox olamine lotion 1%: bioequivalence to ciclopirox olamine cream 1% and clinical efficacy in tinea pedis

Studies were conducted to assess the bioequivalence of a new antimycotic formulation, ciclopirox olamine lotion 1%, to an established compound, ciclopirox olamine cream 1%. Results of in vitro studies, using skin samples from human cadavers and domestic pigs, demonstrated that the two formulations equally penetrate all layers of the stratum corneum and inhibit the growth of Trichophyton mentagrophytes and Candida albicans. In vivo studies in guinea pigs and in human volunteers demonstrated the comparable therapeutic efficacy of the lotion and the cream in experimental trichophytosis. In addition, a multicenter, double-blind clinical trial was undertaken to compare ciclopirox olamine lotion 1% with the vehicle alone in the treatment of patients with tinea pedis. Patients with plantar, interdigital, or vesicular tinea pedis were enrolled in the studies. Patients were treated for 28 days. Clinical and mycological responses were determined during treatment and two weeks posttreatment. Ciclopirox olamine lotion 1% was found to be significantly more effective than its vehicle in the treatment of patients with common tinea pedis. Minor localized side effects (pruritus, burning sensation) were reported in 2% of 89 patients treated with ciclopirox olamine lotion 1%. The results demonstrate the bioequivalence of ciclopirox olamine lotion 1% and ciclopirox olamine cream 1% and confirm the clinical effectiveness and safety of the lotion in the treatment of tinea pedis, a generally recalcitrant fungal infection. It is concluded that ciclopirox olamine lotion 1% can be used as an alternative to ciclopirox olamine cream 1% for treatment of tinea pedis, tinea versicolor, tinea cruris, tinea corporis, and cutaneous candidiasis when the convenience and/or cosmetic elegance of a lotion is desired.