Eaten alive! Cell death by primary phagocytosis: 'phagoptosis'

Microglial phagocytosis of live neurons

Microglia, the brain's professional phagocytes, can remove dead and dying neurons as well as synapses and the processes of live neurons. However, we and others have recently shown that microglia can also execute neuronal death by phagocytosing stressed-but-viable neurons - a process that we have termed phagoptosis. In this Progress article, we discuss evidence suggesting that phagoptosis may contribute to neuronal loss during brain development, inflammation, ischaemia and neurodegeneration.

Microglial dynamics and role in the healthy and diseased brain: a paradigm of functional plasticity

The study of the dynamics and functions of microglia in the healthy and diseased brain is a matter of intense scientific activity. The application of new techniques and new experimental approaches has allowed the identification of novel microglial functions and the redefinition of classic ones. In this review, we propose the study of microglial functions, rather than their molecular profiles, to better understand and define the roles of these cells in the brain. We review current knowledge on the role of surveillant microglia, proliferating microglia, pruning/neuromodulatory microglia, phagocytic microglia, and inflammatory microglia and the molecular profiles that are associated with these functions. In the remodeling scenario of microglial biology, the analysis of microglial functional states will inform about the roles in health and disease and will guide us to a more precise understanding of the multifaceted roles of this never-resting cells.

Progress in translational research on intracerebral hemorrhage: is there an end in sight?

Intracerebral hemorrhage (ICH) is a common and often fatal stroke subtype for which specific therapies and treatments remain elusive. To address this, many recent experimental and translational studies of ICH have been conducted, and these have led to several ongoing clinical trials. This review focuses on the progress of translational studies of ICH including those of the underlying causes and natural history of ICH, animal models of the condition, and effects of ICH on the immune and cardiac systems, among others. Current and potential clinical trials also are discussed for both ICH alone and with intraventricular extension.

Upregulation of EHD2 after intracerebral hemorrhage in adult rats

EHD2, a member of the Eps15 homology domain (EH domain) family, is important for protein interactions during vesicular trafficking. Previous studies have proved that EHD2 can regulate trafficking from the plasma membrane in the process of endocytosis. However, its function in central nervous system diseases is still with limited understanding. In this frame, we found that EHD2 expression was upregulated in the perihematomal caudate in adult rats after intracerebral hemorrhage (ICH). Double immunofluorescence staining revealed that EHD2 was colocalized with neurons and activated microglias after ICH. Besides, we detected that neuronal apoptosis markers (TUNEL and caspase-3), and microglial activation marker (CD68), also known as a marker of macrophage, were colocated with EHD2. The vitro study also indicated that EHD2 was linked with neuronal apoptosis and microglial phagocytosis. All our findings suggested that EHD2 might be involved in the pathophysiology of ICH.

Comparison of the innate immune responses of porcine monocyte-derived dendritic cells and splenic dendritic cells stimulated with LPS

Dendritic cell (DC) subsets form a remarkable cellular network that regulate innate and adaptive immune responses. Although pigs are the most approximate model to humans, little is known about the regulation of monocyte-derived DCs (moDCs) and splenic DCs (SDCs) in the initiation of immune responses under inflammatory conditions. We investigated the activation and maturation of porcine moDC and SDC subpopulations following LPS stimulation. Porcine monocytes that would differentiate into moDCs were isolated. SDCs were isolated directly from the porcine spleen. Following LPS stimulation, phagocytosis activity, TLR4/MyD88-dependent gene expression, co-stimulatory molecule, and pro-inflammatory cytokine (TNF-α, IL-1β) and chemokine (IL-8) expressions were increased in both cell subsets. Furthermore, moDCs showed higher levels of gene and protein expression compared with SDCs. Interestingly, moDCs were found to be more responsive via the TLR4/TRAF-dependent signalling pathway of activation. Only SDCs expressed higher level of IL-12p40 gene and protein, whereas, IFN-γ gene and protein expression were likely to be unchanged after LPS stimulation in both cell subtypes. These data demonstrate that porcine moDCs display a greater ability to initiate innate immune responses, and could be used as a model to investigate immune responses against Ags.

Bidirectional tuning of microglia in the developing brain: from neurogenesis to neural circuit formation

The developing brain employs multi-step processes to construct neural circuitry. Recent studies have highlighted that microglia, traditionally known to be the resident immune cells in the brain, have essential roles in these processes, which range from neurogenesis to establishing synaptic connections. Microglia play bidirectional roles for maintaining proper circuitry: eliminating unnecessary cells, axons, and synapses, while supporting the neighboring ones. Although these processes are performed in different parts of the neuron, similar molecular mechanisms are possibly involved. This paper reviews recent progress on the knowledge of the roles of microglia in brain development, and further discusses the application of this knowledge in therapies for brain disorders and injuries.

Microglial cells in organotypic cultures of developing and adult mouse retina and their relationship with cell death

Organotypic cultures of retinal explants allow the detailed analysis of microglial cells in a cellular microenvironment similar to that in the in situ retina, with the advantage of easy experimental manipulation. However, the in vitro culture causes changes in the retinal cytoarchitecture and induces a microglial response that may influence the results of these manipulations. The purpose of this study was to analyze the influence of the retinal age on changes in retinal cytoarchitecture, cell viability and death, and microglial phenotype and distribution throughout the in vitro culture of developing and adult retina explants. Explants from developing (3 and 10 postnatal days, P3 and P10) and adult (P60) mouse retinas were cultured for up to 10 days in vitro (div). Dead or dying cells were recognized by TUNEL staining, cell viability was determined by flow cytometry, and the numbers and distribution patterns of microglial cells were studied by flow cytometry and immunocytochemistry, respectively. The retinal cytoarchitecture was better preserved at prolonged culture times (10 div) in P10 retina explants than in P3 or adult explants. Particular patterns of cell viability and death were observed at each age: in general, explants from developing retinas showed higher cell viability and lower density of TUNEL-positive profiles versus adult retinas. The proportion of microglial cells relative to the whole population of retinal cells was higher in explants fixed immediately after their dissection (i.e., non-cultured) from adult retinas than in those from developing retinas. This proportion was always higher in non-cultured explants than in explants at 10 div, suggesting the death of some microglial cells during the culture. Activation of microglial cells, as revealed by their phenotypical appearance, was observed in both developing and adult retina explants from the beginning of the culture. Immunofluorescence with the anti-CD68 antibody showed that some activated microglial cells were CD68-positive but others were CD68-negative. Flow cytometry using CD68-labeling revealed that the percentage of CD68-positive microglial cells was much higher in developing than in adult retina explants, despite the activation of microglia in both types of explants, indicating that CD68-labeling was more closely related to the maturity degree of microglia than to their activation. Some swollen activated microglial cells entered the outer nuclear layer in developing and adult cultured retinal explants, whereas this layer was devoid of microglia in non-cultured explants. There was no apparent correlation between the distribution of microglia and that of TUNEL-labeled profiles. However, some swollen activated microglial cells in the outer and inner nuclear layers engulfed clusters of cell nuclei that were negative or weakly positive for TUNEL. This engulfment activity of microglia mimicked that observed in degenerative pathologies of the retina. We conclude that organotypic cultures from developing retinas show a higher rate of cell viability and better preservation of the normal cytoarchitecture in comparison to those obtained from adult retinas. In addition, the features of microglial response in cultured retinal explants show them to be a useful model for studying interactions between microglial cells and degenerating neurons in retinal diseases.

Microglial CD33-related Siglec-E inhibits neurotoxicity by preventing the phagocytosis-associated oxidative burst

Sialic acid-binding Ig-like lectins (Siglecs) are members of the Ig superfamily that recognize sialic acid residues of glycoproteins. Siglec-E is a mouse CD33-related Siglec that preferentially binds to sialic acid residues of the cellular glycocalyx. Here, we demonstrate gene transcription and protein expression of Siglec-E by cultured mouse microglia. Siglec-E on microglia inhibited phagocytosis of neural debris and prevented the production of superoxide radicals induced by challenge with neural debris. Soluble extracellular Siglec-E receptor protein bound to the neural glycocalyx. Coculture of mouse microglia and neurons demonstrated a neuroprotective effect of microglial Siglec-E that was dependent on neuronal sialic acid residues. Increased neurotoxicity of microglia after knockdown of Siglece mRNA was neutralized by the reactive oxygen species scavenger Trolox. Data suggest that Siglec-E recognizes the intact neuronal glycocalyx and has neuroprotective function by preventing phagocytosis and the associated oxidative burst.

When and how do seizures kill neurons, and is cell death relevant to epileptogenesis?

The effect of seizures on neuronal death and the role of seizure-induced neuronal death in acquired epileptogenesis have been debated for decades. Isolated brief seizures probably do not kill neurons; however, severe and repetitive seizures (i.e., status epilepticus) certainly do. Because status epilepticus both kills neurons and also leads to chronic epilepsy, neuronal death has been proposed to be an integral part of acquired epileptogenesis. Several studies, particularly in the immature brain, have suggested that neuronal death is not necessary for acquired epileptogenesis; however, the lack of neuronal death is difficult if not impossible to prove, and more recent studies have challenged this concept. Novel mechanisms of cell death, beyond the traditional concepts of necrosis and apoptosis, include autophagy, phagoptosis, necroptosis, and pyroptosis. The traditional proposal for why neuronal death may be necessary for epileptogenesis is based on the recapitulation of development hypothesis, where a loss of synaptic input from the dying neurons is considered a critical signal to induce axonal sprouting and synaptic-circuit reorganization. We propose a second hypothesis - the neuronal death pathway hypothesis, which states that the biochemical pathways causing programmed neurodegeneration, rather than neuronal death per se, are responsible for or contribute to epileptogenesis. The reprogramming of neuronal death pathways - if true - is proposed to derive from necroptosis or pyroptosis. The proposed new hypothesis may inform on why neuronal death seems closely linked to epileptogenesis, but may not always be.

Caspase-1 activity in microglia stimulated by pro-inflammagen nanocrystals

Although caspase-1 is a key participant in inflammation, there is no sensitive assay to measure its enzymatic activity in real time in cells or animals. Here we describe a nanosensor for caspase-1 ratiometric measurements, consisting of a rhodamine-labeled, caspase-1 cleavable peptide linked to quantum dots (QDs). Microglia cells were stimulated by lipopolysaccharide (LPS) and by hybrid nanoparticles LPS-QDs. These stimuli activated caspase-1 in microglia monolayers and in the mouse brain, while a selected caspase inhibitor markedly reduced it. LPS-QDs entered into the lysosomal compartment and led to an enlargement of these cellular organelles in the exposed microglia. Both lysosomal swelling and mitochondrial impairment contributed to caspase-1 activation and to the consequent interleukin-1β release. The results from these studies highlight how the unique properties of QDs can be used to create versatile biotools in the study of inflammation in real time in vivo.

Apoptosis in Living Animals Is Assisted by Scavenger Cells and Thus May Not Mainly Go through the Cytochrome C-Caspase Pathway

Because billions of cells die every day in their bodies, animals have evolutionarily developed apoptosis to preserve the tissue environment from adverse effects of dead cells, a process achieved via phagocytosis of the cell corpses by professional or amateur phagocytes that are collectively referred to as scavengers. Hence, apoptosis is a merger of two procedures separately occurring inside the dying and the scavenger cells, respectively. The task of apoptosis research is to study how these death procedures occur without hurting the host tissues, and recruitment of in vitro system into the study must be justified for this purpose. Cells in culture have no motivation to preserve the environment, and their death does not involve corpse clearance by scavengers. Therefore, programmed cell death in culture should be redefined, for example as stress-induced cell death, to avoid many sources of confusions, since the word “apoptosis” had already been defined, prior to the era of cell culture, as a silent and beneficial cell suicide with corpse clearance as a distinctive hallmark. We should start over again on apoptosis research by determining whether different physiological apoptotic procedures in animals involve the cytochrome c-caspase pathway, since it has been established from cultured cells as a central mechanism of “apoptosis” but whether it overarches any physiological apoptotic procedure in animals is still unclear. Probably, cells in living animals are programmed to use scavengers to assist their apoptosis but cells in culture have no scavengers to help and thus need to go mainly through the cytochrome c-caspase pathway.

Diversity of cell death pathways: insight from the fly ovary

Multiple types of cell death exist including necrosis, apoptosis, and autophagic cell death. The Drosophila ovary provides a valuable model to study the diversity of cell death modalities, and we review recent progress to elucidate these pathways. At least five distinct types of cell death occur in the ovary, and we focus on two that have been studied extensively. Cell death of mid-stage egg chambers occurs through a novel caspase-dependent pathway that involves autophagy and triggers phagocytosis by surrounding somatic epithelial cells. For every egg, 15 germline nurse cells undergo developmental programmed cell death, which occurs independently of most known cell death genes. These forms of cell death are strikingly similar to cell death observed in the germlines of other organisms.

Inflammation induces multinucleation of Microglia via PKC inhibition of cytokinesis, generating highly phagocytic multinucleated giant cells

Microglia are brain macrophages, which can undergo multinucleation to give rise to multinucleated giant cells that accumulate with ageing and some brain pathologies. However, the origin, regulation and function of multinucleate microglia remain unclear. We found that inflammatory stimuli, including lipopolysaccharide, amyloid β, α-synuclein, tumour necrosis factor-α and interferon γ, but not interleukin-4, induced multinucleation of cultured microglia: primary rat cortical microglia and the murine microglial cell line BV-2. Inflammation-induced multinucleation was prevented by a protein kinase C (PKC) inhibitor Gö6976 (100 nM) and replicated by a PKC activator phorbol myristate acetate (160 nM). Multinucleation was reversible and not because of cell fusion or phagocytosis, but rather failure of cytokinesis. Time-lapse imaging revealed that some dividing cells failed to abscise, even after formation of long cytoplasmic bridges, followed by retraction of bridge and reversal of cleavage furrow to form multinucleate cells. Multinucleate microglia were larger and 2-4 fold more likely to phagocytose large beads and both dead and live PC12 cells. We conclude that multinucleate microglia are reversibly generated by inflammation via PKC inhibition of cytokinesis, and may have specialized functions/dysfunctions including the phagocytosis of other cells. Inflammation resulted in the accumulation of multiple nuclei per cell in cultured microglia. This multinucleation was reversible and due to a PKC-dependent block of the last step of cell division. Multinucleate microglia were larger and had a greater capacity to phagocytose other cells, suggesting they might remove neurons in the brain.

Phagocytosis executes delayed neuronal death after focal brain ischemia

Delayed neuronal loss and brain atrophy after cerebral ischemia contribute to stroke and dementia pathology, but the mechanisms are poorly understood. Phagocytic removal of neurons is generally assumed to be beneficial and to occur only after neuronal death. However, we report herein that inhibition of phagocytosis can prevent delayed loss and death of functional neurons after transient brain ischemia. Two phagocytic proteins, Mer receptor tyrosine kinase (MerTK) and Milk fat globule EGF-like factor 8 (MFG-E8), were transiently up-regulated by macrophages/microglia after focal brain ischemia in vivo. Strikingly, deficiency in either protein completely prevented long-term functional motor deficits after cerebral ischemia and strongly reduced brain atrophy as a result of inhibiting phagocytosis of neurons. Correspondingly, in vitro glutamate-stressed neurons reversibly exposed the "eat-me" signal phosphatidylserine, leading to their phagocytosis by microglia; this neuronal loss was prevented in the absence of microglia and reduced if microglia were genetically deficient in MerTK or MFG-E8, both of which mediate phosphatidylserine-recognition. Thus, phagocytosis of viable neurons contributes to brain pathology and, surprisingly, blocking this process is strongly beneficial. Therefore, inhibition of specific phagocytic pathways may present therapeutic targets for preventing delayed neuronal loss after transient cerebral ischemia.

Combined inhibition of autophagy and caspases fails to prevent developmental nurse cell death in the Drosophila melanogaster ovary

During the final stages of Drosophila melanogaster oogenesis fifteen nurse cells, sister cells to the oocyte, degenerate as part of normal development. This process involves at least two cell death mechanisms, caspase-dependent cell death and autophagy, as indicated by apoptosis and autophagy markers. In addition, mutations affecting either caspases or autophagy partially reduce nurse cell removal, leaving behind end-stage egg chambers with persisting nurse cell nuclei. To determine whether apoptosis and autophagy work in parallel to degrade and remove these cells as is the case with salivary glands during pupariation, we generated mutants doubly affecting caspases and autophagy. We found no significant increase in either the number of late stage egg chambers containing persisting nuclei or in the number of persisting nuclei per egg chamber in the double mutants compared to single mutants. These findings suggest that there is another cell death mechanism functioning in the ovary to remove all nurse cell remnants from late stage egg chambers.

ROCK mediates the inflammatory response in thrombin induced microglia

To investigate whether the ROCK pathway is involved in thrombin-induced microglial inflammatory response, thrombin-induced microglia were pretreated with the thrombin inhibitor argatroban or a ROCK inhibitor Y-27632. Microglial inflammatory response was evaluated by phagocytosis of fluorescein labeled latex beads analyses and inflammatory mediators' expression such as nitric oxide (NO) and tumor necrosis factor-alpha (TNF-а). Compared to non-induced microglia, thrombin-induced microglia show significantly enhanced phagocytotic capacity and increased ROCK, NO and TNF-а expression. Pretreatment of thrombin-induced microglia with argatroban or Y-27632 significantly decreased phagocytotic capacity and reduced ROCK, NO and TNF-α expression. Therefore, the ROCK pathway may play a vital role in the mechanisms by which thrombin induces microglia in the inflammatory response.

Evidence of neurodegeneration in autism spectrum disorder

Autism spectrum disorder (ASD) is a neurological disorder in which a significant number of children experience a developmental regression characterized by a loss of previously-acquired skills and abilities. Loss of neurological function in ASD, as observed in affected children who have regressed, can be explained as neurodegeneration. Although there is research evidence of neurodegeneration or progressive encephalopathy in ASD, the issue of neurodegeneration in ASD is still under debate. Evidence of neurodegeneration in the brain in ASD includes: (1) neuronal cell loss, (2) activated microglia and astrocytes, (3) proinflammatory cytokines, (4) oxidative stress, and (5) elevated 8-oxo-guanosine levels. The evidence from this review suggests that neurodegeneration underlies the loss of neurological function in children with ASD who have experienced regression and loss of previously acquired skills and abilities, and that research into treatments to address the issue of neurodegeneration in ASD are warranted.

Rotenone induces neuronal death by microglial phagocytosis of neurons

Rotenone, a common pesticide and inhibitor of mitochondrial complex I, induces microglial activation and loss of dopaminergic neurons in models of Parkinson's disease. However, the mechanisms of rotenone neurotoxicity are still poorly defined. Here, we used primary neuronal/glial cultures prepared from rat cerebella to investigate the contribution of microglia to neuronal cell death induced by low concentrations of rotenone. Rotenone at 2.5 nm induced neuronal loss over several days without increasing the numbers of necrotic or apoptotic neurons, and neuronal loss/death could be prevented by selective removal of microglia. Rotenone increased microglial proliferation and phagocytic activity, without increasing tumour necrosis factor-α release. Rotenone-induced neuronal loss/death could be prevented by inhibition of phagocytic signalling between neurons and microglia with: cyclo(Arg-Gly-Asp-d-Phe-Val) (to block the microglial vitronectin receptor); MRS2578 (to block the microglial P2Y6 receptor); or either annexin V or an antibody against phosphatidylserine (to block exposed phosphatidylserine, a well-characterized neuronal 'eat-me' signal). As inhibition of phagocytosis by five different means prevented neuronal loss without increasing neuronal death, these data indicate that rotenone neurotoxicity is at least partially mediated by microglial phagocytosis of otherwise viable neurons (phagoptosis). Thus, neuronal loss in Parkinson's disease and other neurological diseases might be prevented by blocking phagocytic signalling.

Autophagy proteins regulate cell engulfment mechanisms that participate in cancer

Recent evidence has uncovered cross-regulation of mechanisms of cell engulfment by proteins of the autophagy pathway, in what is called LC3-Associated Phagocytosis, or LAP. By LAP, lysosome fusion to phagosomes and the degradation of engulfed extracellular cargo are facilitated by autophagy proteins that lipidate LC3 onto phagosome membranes. Here we discuss the contexts where LAP is known to occur by focusing on potential roles in tumorigenesis, including predicted consequences of LAP inhibition.

Lactadherin/MFG-E8 is essential for microglia-mediated neuronal loss and phagoptosis induced by amyloid β

Nanomolar β-amyloid peptide (Aβ) can induce neuronal loss in culture by activating microglia to phagocytose neurons. We report here that this neuronal loss is mediated by the bridging protein lactadherin/milk-fat globule epidermal growth factor-like factor 8 (MFG-E8), which is released by Aβ-activated microglia, binds to co-cultured neurons and opsonizes neurons for phagocytosis by microglia. Aβ stimulated microglial phagocytosis, but did not opsonize neurons for phagocytosis. Aβ (250 nM) induced delayed neuronal loss in mixed glial-neuronal mouse cultures that required microglia and occurred without increasing neuronal apoptosis or necrosis. This neuronal death/loss was prevented by antibodies to MFG-E8 and was absent in cultures from Mfge8 knockout mice (leaving viable neurons), but was reconstituted by addition of recombinant MFG-E8. Thus, nanomolar Aβ caused neuronal death by inducing microglia to phagocytose otherwise viable neurons via MFG-E8. The direct neurotoxicity of micromolar Aβ was not affected by MFG-E8. The essential role of MFG-E8 in Aβ-induced phagoptosis, suggests this bridging protein as a potential therapeutic target to prevent neuronal loss in Alzheimer's disease.

Janus-faced microglia: beneficial and detrimental consequences of microglial phagocytosis

Microglia are the resident brain macrophages and they have been traditionally studied as orchestrators of the brain inflammatory response during infections and disease. In addition, microglia has a more benign, less explored role as the brain professional phagocytes. Phagocytosis is a term coined from the Greek to describe the receptor-mediated engulfment and degradation of dead cells and microbes. In addition, microglia phagocytoses brain-specific cargo, such as axonal and myelin debris in spinal cord injury or multiple sclerosis, amyloid-β deposits in Alzheimer's disease, and supernumerary synapses in postnatal development. Common mechanisms of recognition, engulfment, and degradation of the different types of cargo are assumed, but very little is known about the shared and specific molecules involved in the phagocytosis of each target by microglia. More importantly, the functional consequences of microglial phagocytosis remain largely unexplored. Overall, phagocytosis is considered a beneficial phenomenon, since it eliminates dead cells and induces an anti-inflammatory response. However, phagocytosis can also activate the respiratory burst, which produces toxic reactive oxygen species (ROS). Phagocytosis has been traditionally studied in pathological conditions, leading to the assumption that microglia have to be activated in order to become efficient phagocytes. Recent data, however, has shown that unchallenged microglia phagocytose apoptotic cells during development and in adult neurogenic niches, suggesting an overlooked role in brain remodeling throughout the normal lifespan. The present review will summarize the current state of the literature regarding the role of microglial phagocytosis in maintaining tissue homeostasis in health as in disease.

Janus-faced microglia: beneficial and detrimental consequences of microglial phagocytosis

Microglia are the resident brain macrophages and they have been traditionally studied as orchestrators of the brain inflammatory response during infections and disease. In addition, microglia has a more benign, less explored role as the brain professional phagocytes. Phagocytosis is a term coined from the Greek to describe the receptor-mediated engulfment and degradation of dead cells and microbes. In addition, microglia phagocytoses brain-specific cargo, such as axonal and myelin debris in spinal cord injury or multiple sclerosis, amyloid-β deposits in Alzheimer's disease, and supernumerary synapses in postnatal development. Common mechanisms of recognition, engulfment, and degradation of the different types of cargo are assumed, but very little is known about the shared and specific molecules involved in the phagocytosis of each target by microglia. More importantly, the functional consequences of microglial phagocytosis remain largely unexplored. Overall, phagocytosis is considered a beneficial phenomenon, since it eliminates dead cells and induces an anti-inflammatory response. However, phagocytosis can also activate the respiratory burst, which produces toxic reactive oxygen species (ROS). Phagocytosis has been traditionally studied in pathological conditions, leading to the assumption that microglia have to be activated in order to become efficient phagocytes. Recent data, however, has shown that unchallenged microglia phagocytose apoptotic cells during development and in adult neurogenic niches, suggesting an overlooked role in brain remodeling throughout the normal lifespan. The present review will summarize the current state of the literature regarding the role of microglial phagocytosis in maintaining tissue homeostasis in health as in disease.

Lead roles for supporting actors: critical functions of inner ear supporting cells

Many studies that aim to investigate the underlying mechanisms of hearing loss or balance disorders focus on the hair cells and spiral ganglion neurons of the inner ear. Fewer studies have examined the supporting cells that contact both of these cell types in the cochlea and vestibular end organs. While the roles of supporting cells are still being elucidated, emerging evidence indicates that they serve many functions vital to maintaining healthy populations of hair cells and spiral ganglion neurons. Here we review recent studies that highlight the critical roles supporting cells play in the development, function, survival, death, phagocytosis, and regeneration of other cell types within the inner ear. Many of these roles have also been described for glial cells in other parts of the nervous system, and lessons from these other systems continue to inform our understanding of supporting cell functions. This article is part of a Special Issue entitled "Annual Reviews 2013".

Neutrophils regulate tissue Neutrophilia in inflammation via the oxidant-modified lipid lysophosphatidylserine

Resolution of neutrophilia characteristic of acute inflammation requires cessation of neutrophil recruitment and removal of tissue neutrophils. Based on in vitro studies, a role in these events was hypothesized for oxidant-generated lysophosphatidylserine (lyso-PS) on recruited neutrophils signaling via the G2A receptor on macrophages. Peritoneal exudate neutrophils harvested from wild type (WT) mice had 5-fold more lyso-PS (lyso-PS(high)) than those of gp91(phox)(-/-) (lyso-PS(low)) mice. Ex vivo engulfment of lyso-PS(high) neutrophils (95% viable) by WT peritoneal macrophages was quantitatively similar to UV-irradiated apoptotic blood neutrophils, although the signaling pathway for the former was uniquely dependent on macrophage G2A. In contrast, lyso-PS(low) neutrophils were poorly engulfed unless presented with exogenous lyso-PS. Enhanced clearance of lyso-PS(high) neutrophils was also seen in vivo following their adoptive transfer into inflamed peritonea of WT but not G2A(-/-) mice, further supporting a requirement for signaling via G2A. To investigate downstream effects of lyso-PS/G2A signaling, antibody blockade of G2A in WT mice reduced macrophage CD206 expression and efferocytosis during peritonitis. Conversely, adoptive transfer of lyso-PS(high) neutrophils early in inflammation in gp91(phox)(-/-) mice led to accelerated development of efferocytic(high) and CD206(high) macrophages. This macrophage reprogramming was associated with suppressed production of pro-inflammatory mediators and reduced neutrophilia. These effects were not seen if G2A was blocked or lyso-PS(low) neutrophils were transferred. Taken together, the results demonstrate that oxidant-generated lyso-PS made by viable tissue neutrophils is an endogenous anti-inflammatory mediator working in vivo to orchestrate the "early" and rapid clearance of recruited neutrophils as well as the reprogramming of "resolving" macrophages.

LRRK2 kinase inhibition prevents pathological microglial phagocytosis in response to HIV-1 Tat protein

BACKGROUND

Human Immunodeficiency Virus-1 (HIV-1) associated neurocognitive disorders (HANDs) are accompanied by significant morbidity, which persists despite the use of combined antiretroviral therapy (cART). While activated microglia play a role in pathogenesis, changes in their immune effector functions, including phagocytosis and proinflammatory signaling pathways, are not well understood. We have identified leucine-rich repeat kinase 2 (LRRK2) as a novel regulator of microglial phagocytosis and activation in an in vitro model of HANDs, and hypothesize that LRRK2 kinase inhibition will attenuate microglial activation during HANDs.

METHODS

We treated BV-2 immortalized mouse microglia cells with the HIV-1 trans activator of transcription (Tat) protein in the absence or presence of LRRK2 kinase inhibitor (LRRK2i). We used Western blot, qRT-PCR, immunocytochemistry and latex bead engulfment assays to analyze LRRK2 protein levels, proinflammatory cytokine and phagocytosis receptor expression, LRRK2 cellular distribution and phagocytosis, respectively. Finally, we utilized ex vivo microfluidic chambers containing primary hippocampal neurons and BV-2 microglia cells to investigate microglial phagocytosis of neuronal axons.

RESULTS

We found that Tat-treatment of BV-2 cells induced kinase activity associated phosphorylation of serine 935 on LRRK2 and caused the formation of cytoplasmic LRRK2 inclusions. LRRK2i decreased Tat-induced phosphorylation of serine 935 on LRRK2 and inhibited the formation of Tat-induced cytoplasmic LRRK2 inclusions. LRRK2i also decreased Tat-induced process extension in BV-2 cells. Furthermore, LRRK2i attenuated Tat-induced cytokine expression and latex bead engulfment. We examined relevant cellular targets in microfluidic chambers and found that Tat-treated BV-2 microglia cells cleared axonal arbor and engulfed neuronal elements, whereas saline treated controls did not. LRRK2i was found to protect axons in the presence of Tat-activated microglia, as well as AnnexinV, a phosphatidylserine-binding protein. In addition, LRRK2i decreased brain-specific angiogenesis inhibitor 1 (BAI1) receptor expression on BV-2 cells after Tat-treatment, a key receptor in phosphatidylserine-mediated phagocytosis.

CONCLUSION

Taken together, these results implicate LRRK2 as a key player in microglial inflammation and, in particular, in the phagocytosis of neuronal elements. These studies show that LRRK2 kinase inhibition may prove an effective therapeutic strategy for HANDs, as well as other neuroinflammatory conditions.

Primary phagocytosis of viable neurons by microglia activated with LPS or Aβ is dependent on calreticulin/LRP phagocytic signalling

Background

Microglia are resident brain macrophages that can phagocytose dead, dying or viable neurons, which may be beneficial or detrimental in inflammatory, ischaemic and neurodegenerative brain pathologies. Cell death caused by phagocytosis of an otherwise viable cell is called ‘primary phagocytosis’ or ‘phagoptosis’. Calreticulin (CRT) exposure on the surface of cancer cells can promote their phagocytosis via LRP (low-density lipoprotein receptor-related protein) on macrophages, but it is not known whether this occurs with neurons and microglia.

Methods

We used primary cultures of cerebellar neurons, astrocytes and microglia to investigate the potential role of CRT/LRP phagocytic signalling in the phagocytosis of viable neurons by microglia stimulated with lipopolysaccharide (LPS) or nanomolar concentrations of amyloid-β peptide_1-42 (Aβ). Exposure of CRT on the neuronal surface was investigated using surface biotinylation and western blotting. A phagocytosis assay was also developed using BV2 and PC12 cell lines to investigate CRT/LRP signalling in microglial phagocytosis of apoptotic cells.

Results

We found that BV2 microglia readily phagocytosed apoptotic PC12 cells, but this was inhibited by a CRT-blocking antibody or LRP-blocking protein (receptor-associated protein: RAP). Activation of primary rat microglia with LPS or Aβ resulted in loss of co-cultured cerebellar granule neurons, and this was blocked by RAP or antibodies against CRT or against LRP, preventing all neuronal loss and death. CRT was present on the surface of viable neurons, and this exposure did not change in inflammatory conditions. CRT antibodies prevented microglia-induced neuronal loss when added to neurons, while LRP antibodies prevented neuronal loss when added to the microglia. Pre-binding of CRT to neurons promoted neuronal loss if activated microglia were added, but pre-binding of CRT to microglia or both cell types prevented microglia-induced neuronal loss.

Conclusions

CRT exposure on the surface of viable or apoptotic neurons appears to be required for their phagocytosis via LRP receptors on activated microglia, but free CRT can block microglial phagocytosis of neurons by acting on microglia. Phagocytosis of CRT-exposing neurons by microglia can be a direct cause of neuronal death during inflammation, and might therefore contribute to neurodegeneration and be prevented by blocking the CRT/LRP pathway.