ATP-binding cassette transporter A7 enhances phagocytosis of apoptotic cells and associated ERK signaling in macrophages

Current understanding of the mechanisms for clearance of apoptotic cells-a fine balance

Apoptosis is an important cell death mechanism by which multicellular organisms remove unwanted cells. It culminates in a rapid, controlled removal of cell corpses by neighboring or recruited viable cells. Whilst many of the molecular mechanisms that mediate corpse clearance are components of the innate immune system, clearance of apoptotic cells is an anti-inflammatory process. Control of cell death is dependent on competing pro-apoptotic and anti-apoptotic signals. Evidence now suggests a similar balance of competing signals is central to the effective removal of cells, through so called 'eat me' and 'don't eat me' signals. Competing signals are also important for the controlled recruitment of phagocytes to sites of cell death. Consequently recruitment of phagocytes to and from sites of cell death can underlie the resolution or inappropriate propagation of cell death and inflammation. This article highlights our understanding of mechanisms mediating clearance of dying cells and discusses those mechanisms controlling phagocyte migration and how inappropriate control may promote important pathologies.

ABCA7 expression is associated with Alzheimer's disease polymorphism and disease status

Genome-wide association studies (GWAS) have implicated a series of single nucleotide polymorphisms (SNPs) in Alzheimer's disease (AD) risk. Elucidating the function of these SNPs is critical to identify the underlying pathways and, potentially, novel therapeutic agents. SNPs within the gene ATP binding cassette A7 (ABCA7) reached significance in these studies, warranting investigation into their actions. Here, we analyzed ABCA7 expression in a set of human brain samples as a function of AD-associated SNPs and AD status. We report that the rs3764650T allele that decreases AD risk is associated with increased ABCA7 expression. However, ABCA7 expression is increased in AD individuals. We interpret our findings as suggesting a model wherein increased ABCA7 expression reduces AD risk and that the increased ABCA7 observed in AD reflects an inadequate compensatory change.

Association of ATP-binding cassette transporter variants with the risk of Alzheimer's disease

AIM

A number of studies have demonstrated that ABCB1 and BCRP (ABCG2) actively transport Aβ. We aimed to investigate the association of genetic variants of selected multidrug transporters with Alzheimer's disease (AD) in histopathologically confirmed AD cases and controls.

MATERIALS & METHODS

DNA from brain tissue of 71 AD cases with Consortium to Establish a Registry for Alzheimer's Disease (CERAD) neuropathological stages B/C and 81 controls was genotyped for selected variants in ABCA1, ABCA7, ABCB1, ABCC2 and ABCG2. In addition, the APOE4 status was analyzed.

RESULTS

The novel ABCA7 SNP, rs3752246, tended to be associated with AD in our study. Variants in ABCB1 were significantly less frequent in AD cases older than 65 years of age and among females. This association of ABCB1 2677G>T (rs2032582) was more pronounced in APOE4-negative cases (p = 0.005). However, only ABCC2 3972C>T (rs3740066) was significantly associated with AD risk after logistic regression analysis including all variants. Other transporters showed a lack of association.

CONCLUSION

Our results support the hypothesis that ABCB1 and possibly other ABC-transporters are involved in the process of Aβ accumulation in the aging brain and may modulate the risk for AD in an allele-specific manner, and thus might represent a new target for prevention and treatment of AD.

Pathways to neurodegeneration: mechanistic insights from GWAS in Alzheimer's disease, Parkinson's disease, and related disorders

The discovery of causative genetic mutations in affected family members has historically dominated our understanding of neurodegenerative diseases such as Alzheimer's disease (AD), Parkinson's disease (PD), frontotemporal dementia (FTD), and amyotrophic lateral sclerosis (ALS). Nevertheless, most cases of neurodegenerative disease are not explained by Mendelian inheritance of known genetic variants, but instead are thought to have a complex etiology with numerous genetic and environmental factors contributing to susceptibility. Although unbiased genome-wide association studies (GWAS) have identified novel associations to neurodegenerative diseases, most of these hits explain only modest fractions of disease heritability. In addition, despite the substantial overlap of clinical and pathologic features among major neurodegenerative diseases, surprisingly few GWAS-implicated variants appear to exhibit cross-disease association. These realities suggest limitations of the focus on individual genetic variants and create challenges for the development of diagnostic and therapeutic strategies, which traditionally target an isolated molecule or mechanistic step. Recently, GWAS of complex diseases and traits have focused less on individual susceptibility variants and instead have emphasized the biological pathways and networks revealed by genetic associations. This new paradigm draws on the hypothesis that fundamental disease processes may be influenced on a personalized basis by a combination of variants - some common and others rare, some protective and others deleterious - in key genes and pathways. Here, we review and synthesize the major pathways implicated in neurodegeneration, focusing on GWAS from the most prevalent neurodegenerative disorders, AD and PD. Using literature mining, we also discover a novel regulatory network that is enriched with AD- and PD-associated genes and centered on the SP1 and AP-1 (Jun/Fos) transcription factors. Overall, this pathway- and network-driven model highlights several potential shared mechanisms in AD and PD that will inform future studies of these and other neurodegenerative disorders. These insights also suggest that biomarker and treatment strategies may require simultaneous targeting of multiple components, including some specific to disease stage, in order to assess and modulate neurodegeneration. Pathways and networks will provide ideal vehicles for integrating relevant findings from GWAS and other modalities to enhance clinical translation.

Proprotein convertase subtilisin/kexin type 9 (PCSK9) can mediate degradation of the low density lipoprotein receptor-related protein 1 (LRP-1)

Elevated LDL-cholesterol (LDLc) levels are a major risk factor for cardiovascular disease and atherosclerosis. LDLc is cleared from circulation by the LDL receptor (LDLR). Proprotein convertase subtilisin/kexin 9 (PCSK9) enhances the degradation of the LDLR in endosomes/lysosomes, resulting in increased circulating LDLc. PCSK9 can also mediate the degradation of LDLR lacking its cytosolic tail, suggesting the presence of as yet undefined lysosomal-targeting factor(s). Herein, we confirm this, and also eliminate a role for the transmembrane-domain of the LDLR in mediating its PCSK9-induced internalization and degradation. Recent findings from our laboratory also suggest a role for PCSK9 in enhancing tumor metastasis. We show herein that while the LDLR is insensitive to PCSK9 in murine B16F1 melanoma cells, PCSK9 is able to induce degradation of the low density lipoprotein receptor-related protein 1 (LRP-1), suggesting distinct targeting mechanisms for these receptors. Furthermore, PCSK9 is still capable of acting upon the LDLR in CHO 13-5-1 cells lacking LRP-1. Conversely, PCSK9 also acts on LRP-1 in the absence of the LDLR in CHO-A7 cells, where re-introduction of the LDLR leads to reduced PCSK9-mediated degradation of LRP-1. Thus, while PCSK9 is capable of inducing degradation of LRP-1, the latter is not an essential factor for LDLR regulation, but the LDLR effectively competes with LRP-1 for PCSK9 activity. Identification of PCSK9 targets should allow a better understanding of the consequences of PCSK9 inhibition for lowering LDLc and tumor metastasis.

Deletion of Abca7 increases cerebral amyloid-β accumulation in the J20 mouse model of Alzheimer's disease

ATP-binding cassette transporter A7 (ABCA7) is expressed in the brain and has been detected in macrophages, microglia, and neurons. ABCA7 promotes efflux of lipids from cells to apolipoproteins and can also regulate phagocytosis and modulate processing of amyloid precursor protein (APP) to generate the Alzheimer's disease (AD) amyloid-β (Aβ) peptide. Genome-wide association studies have indicated that ABCA7 single nucleotide polymorphisms confer increased risk for late-onset AD; however, the role that ABCA7 plays in the brain in the AD context is unknown. In the present study, we crossed ABCA7-deficient (A7(-/-)) mice with J20 amyloidogenic mice to address this issue. We show that ABCA7 loss doubled insoluble Aβ levels and thioflavine-S-positive plaques in the brain. This was not related to changes in APP processing (assessed by analysis of full-length APP and the APP β C-terminal fragment). Apolipoprotein E regulates cerebral Aβ homeostasis and plaque load; however, the apolipoprotein E concentration was not altered by ABCA7 loss. Spatial reference memory was significantly impaired in both J20 and J20/A7(-/-) mice compared with wild-type mice; however, there were no cognitive differences between J20 and J20/A7(-/-) mice. There were also no major differences detected in hippocampal or plaque-associated microglial/macrophage markers between J20 and J20/A7(-/-) mice, whereas the capacity for bone marrow-derived macrophages derived from A7(-/-) mice to take up oligomeric Aβ was reduced by 51% compared with wild-type mice. Our results suggest that ABCA7 plays a role in the regulation of Aβ homeostasis in the brain and that this may be related to altered phagocyte function.

Genetic insights in Alzheimer's disease

In the search for new genes in Alzheimer's disease, classic linkage-based and candidate-gene-based association studies have been supplanted by exome sequencing, genome-wide sequencing (for mendelian forms of Alzheimer's disease), and genome-wide association studies (for non-mendelian forms). The identification of new susceptibility genes has opened new avenues for exploration of the underlying disease mechanisms. In addition to detecting novel risk factors in large samples, next-generation sequencing approaches can deliver novel insights with even small numbers of patients. The shift in focus towards translational studies and sequencing of individual patients places each patient's biomaterials as the central unit of genetic studies. The notional shift needed to make the patient central to genetic studies will necessitate strong collaboration and input from clinical neurologists.

Pathway analysis of the human brain transcriptome in disease

Pathway analysis is a powerful method for discerning differentially regulated genes and elucidating their biological importance. It allows for the identification of perturbed or aberrantly expressed genes within a biological context from extensive data sets and offers a simplistic approach for interrogating such data sets. With the growing use of microarrays and RNA-Seq, data for genome-wide studies are growing at an alarming rate, and the use of deep sequencing is revealing elements of the genome previously uncharacterised. Through the employment of pathway analysis, mechanisms in complex diseases may be explored and novel causatives found primarily through differentially regulated genes. Further, with the implementation of next generation sequencing, a deeper resolution may be attained, particularly in identification of isoform diversity and SNPs. Here, we look at a broad overview of pathway analysis in the human brain transcriptome and its relevance in teasing out underlying causes of complex diseases. We will outline processes in data gathering and analysis of particular diseases in which these approaches have been successful.

Expression of novel Alzheimer's disease risk genes in control and Alzheimer's disease brains

Late onset Alzheimer’s disease (LOAD) etiology is influenced by complex interactions between genetic and environmental risk factors. Large-scale genome wide association studies (GWAS) for LOAD have identified 10 novel risk genes: ABCA7, BIN1, CD2AP, CD33, CLU, CR1, EPHA1, MS4A6A, MS4A6E, and PICALM. We sought to measure the influence of GWAS single nucleotide polymorphisms (SNPs) and gene expression levels on clinical and pathological measures of AD in brain tissue from the parietal lobe of AD cases and age-matched, cognitively normal controls. We found that ABCA7, CD33, and CR1 expression levels were associated with clinical dementia rating (CDR), with higher expression being associated with more advanced cognitive decline. BIN1 expression levels were associated with disease progression, where higher expression was associated with a delayed age at onset. CD33, CLU, and CR1 expression levels were associated with disease status, where elevated expression levels were associated with AD. Additionally, MS4A6A expression levels were associated with Braak tangle and Braak plaque scores, with elevated expression levels being associated with more advanced brain pathology. We failed to detect an association between GWAS SNPs and gene expression levels in our brain series. The minor allele of rs3764650 in ABCA7 is associated with age at onset and disease duration, and the minor allele of rs670139 in MS4A6E was associated with Braak tangle and Braak plaque score. These findings suggest that expression of some GWAS genes, namely ABCA7, BIN1, CD33, CLU, CR1 and the MS4A family, are altered in AD brains.

Mammalian P4-ATPases and ABC transporters and their role in phospholipid transport

Transport of phospholipids across cell membranes plays a key role in a wide variety of biological processes. These include membrane biosynthesis, generation and maintenance of membrane asymmetry, cell and organelle shape determination, phagocytosis, vesicle trafficking, blood coagulation, lipid homeostasis, regulation of membrane protein function, apoptosis, etc. P(4)-ATPases and ATP binding cassette (ABC) transporters are the two principal classes of membrane proteins that actively transport phospholipids across cellular membranes. P(4)-ATPases utilize the energy from ATP hydrolysis to flip aminophospholipids from the exocytoplasmic (extracellular/lumen) to the cytoplasmic leaflet of cell membranes generating membrane lipid asymmetry and lipid imbalance which can induce membrane curvature. Many ABC transporters play crucial roles in lipid homeostasis by actively transporting phospholipids from the cytoplasmic to the exocytoplasmic leaflet of cell membranes or exporting phospholipids to protein acceptors or micelles. Recent studies indicate that some ABC proteins can also transport phospholipids in the opposite direction. The importance of P(4)-ATPases and ABC transporters is evident from the findings that mutations in many of these transporters are responsible for severe human genetic diseases linked to defective phospholipid transport. This article is part of a Special Issue entitled Phospholipids and Phospholipid Metabolism.

Role of Abca7 in mouse behaviours relevant to neurodegenerative diseases

ATP-binding cassette transporters of the subfamily A (ABCA) are responsible for the translocation of lipids including cholesterol, which is crucial for neurological function. Recent studies suggest that the ABC transporter ABCA7 may play a role in the development of brain disorders such as schizophrenia and Alzheimer’s disease. However, Abca7’s role in cognition and other behaviours has not been investigated. Therefore, we characterised homozygous Abca7 knockout mice in a battery of tests for baseline behaviours (i.e. physical exam, baseline locomotion and anxiety) and behaviours relevant to schizophrenia (i.e. prepulse inhibition and locomotor response to psychotropic drugs) and Alzheimer’s disease (i.e. cognitive domains). Knockout mice had normal motor functions and sensory abilities and performed the same as wild type-like animals in anxiety tasks. Short-term spatial memory and fear-associated learning was also intact in Abca7 knockout mice. However, male knockout mice exhibited significantly impaired novel object recognition memory. Task acquisition was unaffected in the cheeseboard task. Female mice exhibited impaired spatial reference memory. This phenomenon was more pronounced in female Abca7 null mice. Acoustic startle response, sensorimotor gating and baseline locomotion was unaltered in Abca7 knockout mice. Female knockouts showed a moderately increased motor response to MK-801 than control mice. In conclusion, Abca7 appears to play only a minor role in behavioural domains with a subtle sex-specific impact on particular cognitive domains.

Release of lysophospholipid 'find-me' signals during apoptosis requires the ATP-binding cassette transporter A1

Efficient engulfment of apoptotic cells is essential in multi-cellular organisms in order to prevent inflammatory responses. Apoptotic cells secure this process by releasing 'find-me' signals for the attraction of phagocytes. A major 'find-me' signal liberated from apoptotic cells is lysophosphatidylcholine (LPC). So far, however, the mechanisms underlying LPC release are poorly understood. In this study, we demonstrate that pharmacological inhibition and RNAi-mediated knock-down of the lipid transporter ABCA1 in apoptotic cells completely abolished phagocyte attraction. Moreover, ectopic expression of ABCA1 significantly enhanced monocyte migration to supernatants of apoptotic cells. Hence, ABCA1 represents a novel regulator of LPC release during apoptosis.

Novel late-onset Alzheimer disease loci variants associate with brain gene expression

OBJECTIVE

Recent genome-wide association studies (GWAS) of late-onset Alzheimer disease (LOAD) identified 9 novel risk loci. Discovery of functional variants within genes at these loci is required to confirm their role in Alzheimer disease (AD). Single nucleotide polymorphisms that influence gene expression (eSNPs) constitute an important class of functional variants. We therefore investigated the influence of the novel LOAD risk loci on human brain gene expression.

METHODS

We measured gene expression levels in the cerebellum and temporal cortex of autopsied AD subjects and those with other brain pathologies (∼400 total subjects). To determine whether any of the novel LOAD risk variants are eSNPs, we tested their cis-association with expression of 6 nearby LOAD candidate genes detectable in human brain (ABCA7, BIN1, CLU, MS4A4A, MS4A6A, PICALM) and an additional 13 genes ±100 kb of these SNPs. To identify additional eSNPs that influence brain gene expression levels of the novel candidate LOAD genes, we identified SNPs ±100 kb of their location and tested for cis-associations.

RESULTS

CLU rs11136000 (p = 7.81 × 10(-4)) and MS4A4A rs2304933/rs2304935 (p = 1.48 × 10(-4)-1.86 × 10(-4)) significantly influence temporal cortex expression levels of these genes. The LOAD-protective CLU and risky MS4A4A locus alleles associate with higher brain levels of these genes. There are other cis-variants that significantly influence brain expression of CLU and ABCA7 (p = 4.01 × 10(-5)-9.09 × 10(-9)), some of which also associate with AD risk (p = 2.64 × 10(-2)-6.25 × 10(-5)).

CONCLUSIONS

CLU and MS4A4A eSNPs may at least partly explain the LOAD risk association at these loci. CLU and ABCA7 may harbor additional strong eSNPs. These results have implications in the search for functional variants at the novel LOAD risk loci.

Effects of deletion of macrophage ABCA7 on lipid metabolism and the development of atherosclerosis in the presence and absence of ABCA1

ABCA7, a close relative of ABCA1 which facilitates cholesterol efflux to lipid-poor apoproteins, has been implicated in macrophage lipid efflux and clearance of apoptotic cells in in vitro studies. In the current study, we investigated the in vivo effects of macrophage ABCA7 deficiency on lipid metabolism and atherosclerosis. Chimeras with dysfunctional ABCA7 in macrophages and other blood cells were generated by transplantation of bone marrow from ABCA7 knockout (KO) mice into irradiated low-density lipoprotein receptor (LDLr) KO mice. Unexpectedly, macrophage ABCA7 deficiency did not significantly affect atherosclerosis susceptibility of LDLr KO mice after 10 weeks Western-type diet feeding. However, ABCA7 deficiency was associated with 2-fold (p<0.05) higher macrophage ABCA1 mRNA expression levels. Combined disruption of ABCA1 and ABCA7 in bone-marrow-derived cells increased atherosclerotic lesion development (1.5-fold (p>0.05) as compared to wild type transplanted mice. However, single deletion of ABCA1 had a similar effect (1.8-fold, p<0.05). Macrophage foam cell accumulation in the peritoneal cavity was reduced in ABCA1/ABCA7 dKO transplanted animals as compared to single ABCA1 KO transplanted mice, which was associated with increased ABCG1 expression. Interestingly, spleens of ABCA1/ABCA7 double KO transplanted mice were significantly larger as compared to the other 3 groups and showed massive macrophage lipid accumulation, a reduction in CD3+ T-cells, and increased expression of key regulators of erythropoiesis. In conclusion, deletion of ABCA7 in bone marrow-derived cells does not affect atherogenesis in the arterial wall neither in the absence or presence of ABCA1. Interestingly, combined deletion of bone marrow ABCA1 and ABCA7 causes severe splenomegaly associated with cellular lipid accumulation, a reduction in splenic CD3+ T cells, and induced markers of erythropoeisis. Our data indicate that ABCA7 may play a role in T cell proliferation and erythropoeisis in spleen.

Primary phagocytosis of neurons by inflamed microglia: potential roles in neurodegeneration

Microglial phagocytosis of dead or dying neurons can be beneficial by preventing the release of damaging and/or pro-inflammatory intracellular components. However, there is now evidence that under certain conditions, such as inflammation, microglia can also phagocytose viable neurons, thus executing their death. Such phagocytic cell death may result from exposure of phosphatidylserine (PS) or other eat-me signals on otherwise viable neurons as a result of physiological activation or sub-toxic insult, and neuronal phagocytosis by activated microglia. In this review, we discuss the mechanisms of phagocytic cell death and its potential roles in Alzheimer’s Disease, Parkinson’s Disease, and Frontotemporal Dementia.

Genetics of dementia

Genetic factors are now recognized to play an important role in most age-related dementias. Although other factors, including aging itself, contribute to dementia, in this review the authors focus on the role of specific disease-causing genes and genetic factors in the most common age-related dementias. They review each dementia within the context of a genes/environment continuum, with varying levels of genetic versus environmental influence. All major classes of dementia will be discussed but greatest attention will be given to the most common dementia, Alzheimer's disease, for which several new genetic factors were recently identified.

A-Subclass ATP-Binding Cassette Proteins in Brain Lipid Homeostasis and Neurodegeneration

The A-subclass of ATP-binding cassette (ABC) transporters comprises 12 structurally related members of the evolutionarily highly conserved superfamily of ABC transporters. ABCA transporters represent a subgroup of "full-size" multispan transporters of which several members have been shown to mediate the transport of a variety of physiologic lipid compounds across membrane barriers. The importance of ABCA transporters in human disease is documented by the observations that so far four members of this protein family (ABCA1, ABCA3, ABCA4, ABCA12) have been causatively linked to monogenetic disorders including familial high-density lipoprotein deficiency, neonatal surfactant deficiency, degenerative retinopathies, and congenital keratinization disorders. Recent research also point to a significant contribution of several A-subfamily ABC transporters to neurodegenerative diseases, in particular Alzheimer's disease (AD). This review will give a summary of our current knowledge of the A-subclass of ABC transporters with a special focus on brain lipid homeostasis and their involvement in AD.

Alzheimer's Disease: APP, Gamma Secretase, APOE, CLU, CR1, PICALM, ABCA7, BIN1, CD2AP, CD33, EPHA1, and MS4A2, and Their Relationships with Herpes Simplex, C. Pneumoniae, Other Suspect Pathogens, and the Immune System

Alzheimer's disease susceptibility genes, APP and gamma-secretase, are involved in the herpes simplex life cycle, and that of other suspect pathogens (C. pneumoniae, H. pylori, C. neoformans, B. burgdorferri, P. gingivalis) or immune defence. Such pathogens promote beta-amyloid deposition and tau phosphorylation and may thus be causative agents, whose effects are conditioned by genes. The antimicrobial effects of beta-amyloid, the localisation of APP/gamma-secretase in immunocompetent dendritic cells, and gamma secretase cleavage of numerous pathogen receptors suggest that this network is concerned with pathogen disposal, effects which may be abrogated by the presence of beta-amyloid autoantibodies in the elderly. These autoantibodies, as well as those to nerve growth factor and tau, also observed in Alzheimer's disease, may well be antibodies to pathogens, due to homology between human autoantigens and pathogen proteins. NGF or tau antibodies promote beta-amyloid deposition, neurofibrillary tangles, or cholinergic neuronal loss, and, with other autoantibodies, such as anti-ATPase, are potential agents of destruction, whose formation is dictated by sequence homology between pathogen and human proteins, and thus by pathogen strain and human genes. Pathogen elimination in the ageing population and removal of culpable autoantibodies might reduce the incidence and offer hope for a cure in this affliction.

The dysregulation of the monocyte/macrophage effector function induced by isopropanol is mediated by the defective activation of distinct members of the AP-1 family of transcription factors

Isopropanol is the second most common cause of short-chain alcohol acute intoxication. Nonethanolic short-chain alcohols mediate their immunomodulatory effect by interfering with nuclear factor of activated T cells (NFAT) activation with or without additional activator protein-1 (AP-1) involvement. In the present study, we examined the immunomodulation induced by isopropanol in conditions that are not reliant on NFAT: the inflammatory cytokine response of lipopolysaccharide (LPS)-stimulated monocytes. Our hypothesis was that isopropanol acute exposure would have an attenuated effect or no consequence in this setting. To our surprise, the impairment of AP-1 activation was sufficient to mediate a severe and dose-dependent phenotype in human monocytes in vitro at alcohol concentrations as low as 0.16% (or 26 mM). There were three outcomes: interleukin (IL)-1β/IL-8 were unaltered; IL-6 was upregulated; and tumor necrosis factor alpha (TNF-α)/CCL2 were downregulated. The effector function of human monocyte-derived macrophages was also compromised. Our results showed that Toll-like receptor 4 early signaling was preserved, as isopropanol did not change the kinase activity of the IL-1 receptor-associated kinase 1 in LPS-stimulated cells. The nuclear factor-κB signaling cascade and the p38/c-Jun N-terminal kinase modules of the mitogen-activated protein kinase pathway were alcohol insensitive. Conversely, the activation of extracellular signal-regulated protein kinase and, ultimately, of c-Fos and JunB were impaired. The alcohol-induced cytokine dysregulation was confirmed in a mouse model of isopropanol intoxication in which the production of TNF-α in response to LPS challenge was virtually abolished. The magnitude of this alcohol effect was sufficiently high to rescue animals from LPS-induced toxic shock. Our data contribute to the dismal body of information on the immunotoxicology of isopropanol, one of the most ubiquitous chemicals to which the general population is significantly exposed.

Complement activation as a biomarker for Alzheimer's disease

There is increasing evidence from genetic, immunohistochemical, proteomic and epidemiological studies as well as in model systems that complement activation has an important role in the pathogenesis of Alzheimer's disease (AD). The complement cascade is an essential element of the innate immune response. In the brain complement proteins are integral components of amyloid plaques and complement activation occurs at the earliest stage of the disease. The complement cascade has been implicated as a protective mechanism in the clearance of amyloid, and in a causal role through chronic activation of the inflammatory response. In this review we discuss the potential for complement activation to act as a biomarker for AD at several stages in the disease process. An accurate biomarker that has sufficient predictive, diagnostic and prognostic value would provide a significant opportunity to develop and test for effective novel therapies in the treatment of AD.

Alzheimer's disease genetics: lessons to improve disease modelling

In the present review, we look back at the recent history of GWAS (genome-wide association studies) in AD (Alzheimer's disease) and integrate the major findings with current knowledge of biological processes and pathways. These topics are essential for the development of animal models, which will be fundamental to our complete understanding of AD.

Interleukin-6 protects human macrophages from cellular cholesterol accumulation and attenuates the proinflammatory response

Cholesterol-laden monocyte-derived macrophages are phagocytic cells characteristic of early and advanced atherosclerotic lesions. Interleukin-6 (IL-6) is a macrophage secretory product that is abundantly expressed in atherosclerotic plaques but whose precise role in atherogenesis is unclear. The capacity of macrophages to clear apoptotic cells, through the efferocytosis mechanism, as well as to reduce cellular cholesterol accumulation contributes to prevent plaque progression and instability. By virtue of its capacity to promote cellular cholesterol efflux from phagocyte-macrophages, ABCA1 was reported to reduce atherosclerosis. We demonstrated that lipid loading in human macrophages was accompanied by a strong increase of IL-6 secretion. Interestingly, IL-6 markedly induced ABCA1 expression and enhanced ABCA1-mediated cholesterol efflux from human macrophages to apoAI. Stimulation of ABCA1-mediated cholesterol efflux by IL-6 was, however, abolished by selective inhibition of the Jak-2/Stat3 signaling pathway. In addition, we observed that the expression of molecules described to promote efferocytosis, i.e. c-mer proto-oncogene-tyrosine kinase, thrombospondin-1, and transglutaminase 2, was significantly induced in human macrophages upon treatment with IL-6. Consistent with these findings, IL-6 enhanced the capacity of human macrophages to phagocytose apoptotic cells; moreover, we observed that IL-6 stimulates the ABCA1-mediated efflux of cholesterol derived from the ingestion of free cholesterol-loaded apoptotic macrophages. Finally, the treatment of human macrophages with IL-6 led to the establishment of an anti-inflammatory cytokine profile, characterized by an increased secretion of IL-4 and IL-10 together with a decrease of that of IL-1β. Taken together, our results indicate that IL-6 favors the elimination of excess cholesterol in human macrophages and phagocytes by stimulation of ABCA1-mediated cellular free cholesterol efflux and attenuates the macrophage proinflammatory phenotype. Thus, high amounts of IL-6 secreted by lipid laden human macrophages may constitute a protective response from macrophages to prevent accumulation of cytotoxic-free cholesterol. Such a cellular recycling of free cholesterol may contribute to reduce both foam cell formation and the accumulation of apoptotic bodies as well as intraplaque inflammation in atherosclerotic lesions.

HMG-CoA reductase inhibitors enhance phagocytosis by upregulating ATP-binding cassette transporter A7

We recently reported that the endogenous ATP-binding cassette transporter (ABC) A7 strongly associates with phagocytosis, being regulated by sterol regulatory element binding protein 2. We therefore examined the effect of statins on phagocytosis in vitro and in vivo through the SREBP-ABCA7. Phagocytosis was found to be enhanced by pravastatin, rosuvastatin and simvastatin and cyclodextrin in J774 macrophages, as cellular cholesterol was reduced and expressions of the cholesterol-related genes were modulated, including an increase of ABCA7 mRNA and decrease of ABCA1 mRNA. Conversely, knock-down of ABCA7 expression by siRNA ablated enhancement of phagocytosis by statins. In vivo, pravastatin enhanced phagocytosis in wild-type mice, but not in ABCA7-knockout mice. We thus concluded that statins enhance phagocytosis through the SREBP-ABCA7 pathway. These findings provide a molecular basis for enhancement of the host-defense system by statins showing that one of their "pleiotropic" effects is in fact achieved through their reaction to a primary target.

Common variants at ABCA7, MS4A6A/MS4A4E, EPHA1, CD33 and CD2AP are associated with Alzheimer's disease

We sought to identify new susceptibility loci for Alzheimer's disease through a staged association study (GERAD+) and by testing suggestive loci reported by the Alzheimer's Disease Genetic Consortium (ADGC) in a companion paper. We undertook a combined analysis of four genome-wide association datasets (stage 1) and identified ten newly associated variants with P ≤ 1 × 10(-5). We tested these variants for association in an independent sample (stage 2). Three SNPs at two loci replicated and showed evidence for association in a further sample (stage 3). Meta-analyses of all data provided compelling evidence that ABCA7 (rs3764650, meta P = 4.5 × 10(-17); including ADGC data, meta P = 5.0 × 10(-21)) and the MS4A gene cluster (rs610932, meta P = 1.8 × 10(-14); including ADGC data, meta P = 1.2 × 10(-16)) are new Alzheimer's disease susceptibility loci. We also found independent evidence for association for three loci reported by the ADGC, which, when combined, showed genome-wide significance: CD2AP (GERAD+, P = 8.0 × 10(-4); including ADGC data, meta P = 8.6 × 10(-9)), CD33 (GERAD+, P = 2.2 × 10(-4); including ADGC data, meta P = 1.6 × 10(-9)) and EPHA1 (GERAD+, P = 3.4 × 10(-4); including ADGC data, meta P = 6.0 × 10(-10)).

C1q regulation of dendritic cell development from monocytes with distinct cytokine production and T cell stimulation

The causative association of complement C1q deficiency with systemic lupus erythematosus (SLE), which inevitably involves the breakdown of tolerance, remains poorly explained. Its non-hepatic, macrophage and dendritic cell (DC) origin may be highly relevant. In tissues, C1q is produced by DCs and macrophages which deposits around these cells and we ask whether this pericellular form of C1q regulates DC development from monocytes. DCs cultured on immobilized C1q (C1q-DCs) show similar MHC, CD40, CD80, CD86, CD83 and CCR7 expression as normal DCs, but these cells exhibit increased phagocytosis of apoptotic cells and elevated IL-10 but reduced IL-12 and IL-23 production. Intracellularly, C1q-DCs exhibit increased ERK, p38 and p70S6 kinase activity. By mixed leukocyte reaction, C1q-DCs show reduced Th1 and Th17 induction from allogeneic CD4(+) T cells. LPS and IFNγ, which cause normal DCs to induce increased CD25 expression on CD4(+) T cells, attenuate C1q-DC induction of CD25. These imply that the DC pericellular C1q may induce tolerogenic properties in developing DCs.

Roles of ATP-binding cassette transporter A7 in cholesterol homeostasis and host defense system

ATP-binding cassette transporter (ABC) A7 is an ABC family protein that is a so-called full-size ABC transporter, highly homologous to ABCA1, which mediates the biogenesis of high-density lipoprotein (HDL) with cellular lipid and helical apolipoproteins. ABCA7 mediates the formation of HDL when exogenously transfected and expressed; however, endogenous ABCA7 was shown to have no significant impact on the generation of HDL and was found to be associated with phagocytosis regulated by sterol regulatory element binding protein 2. Since phagocytosis is one of the fundamental functions of animal cells as an important responsive reaction to infection, injury and apoptosis, ABCA7 seems to be one of the key molecules linking sterol homeostasis and the host defense system. In this context, HDL apolipoproteins were shown to enhance phagocytosis by stabilizing ABCA7 against calpain-mediated degradation and increasing its activity, shedding light on a new aspect of the regulation of the host-defense system.

Development of a cell-based, high-throughput screening assay for cholesterol efflux using a fluorescent mimic of cholesterol

Reverse cholesterol transport is the process by which extrahepatic cells, including macrophage-derived foam cells in arterial atherosclerotic plaque, transport excessive cholesterol back to the liver for bile acid synthesis and excretion, thus lowering the peripheral lipid burden. Cholesterol efflux from peripheral cells is the first step in this process, and finding drugs and interventions that promote this event is an important endeavor. Radioisotope-labeled cholesterol traditionally has been employed in measuring efflux efficiency, but this reagent has limitations for high-throughput screening. We developed an alternative method to measure cholesterol efflux in macrophage-derived foam cells using a novel fluorescent cholesterol mimic comprising the Pennsylvania Green fluorophore, attached by a linker containing a glutamic acid residue, to a derivative of N-alkyl-3β-cholesterylamine. Compared with the traditional radioisotope-based assay, this fluorescence-based assay gave similar results in the presence of known modulators of cholesterol efflux, such as cyclic AMP, and different cholesterol acceptors. When the fluorescent probe was employed in a high-throughput screening format, a variety of chemicals and bioactive compounds with known and unknown effects on cholesterol efflux could be tested simultaneously by plate-reader in a short period of time. Treatment of THP-1-derived macrophages with inhibitors of the membrane transporter ATP-binding cassette A1, such as glyburide or a specific antibody, significantly reduced the export of this fluorescent compound, indicating that ATP-binding cassette A1 represents the primary mediator of its cellular efflux. This fluorescent mimic of cholesterol provides a safe, sensitive, and reproducible alternative to radioactive assays in efflux experiments and has great potential as a valuable tool when incorporated into a drug discovery program.

Phagocytosis of dying cells: influence of smoking and static magnetic fields

It is becoming evident that failure in the removal of dying cells causes and/or promotes the onset of chronic diseases. Impairment of phagocytosis of apoptotic cells can be due not only to genetic or molecular malfunctioning but also to external/environmental factors. Two of these environmental factors have been recently reported to down regulate the clearance of apoptotic cells: cigarette smoke and static magnetic fields. Cigarette smoke contains highly reactive carbonyls that modify proteins which directly/indirectly affects cellular function. Human macrophages interacting with carbonyl or cigarette smoke modified extracellular matrix (ECM) proteins dramatically down regulated their ability to phagocytose apoptotic neutrophils. It was postulated that changes in the ECM environment as a result of cigarette smoke affect the ability of macrophages to remove apoptotic cells. This decreased phagocytic activity was as a result of sequestration of receptors involved in the uptake of apoptotic cells towards that of recognition of carbonyl adducts on the modified ECM proteins leading to increased macrophage adhesion. Downregulation of the phagocytosis of apoptotic cells was also described when performed in presence of static magnetic fields (SMFs) of moderate intensity. SMFs have been reported to perturb distribution of membrane proteins and glycoproteins, receptors, cytoskeleton and trans-membrane fluxes of different ions, especially calcium [Ca(2+)]i, that in turn, interfere with many different physiological activities, including phagocytosis. The effects of cigarette smoke and SMF on the phagocytosis of dying cells will be here discussed.

PPARγ activation normalizes resolution of acute sterile inflammation in murine chronic granulomatous disease

Absence of a functional nicotinamide adenine dinucleotide phosphate (NADPH) oxidase predisposes chronic granulomatous disease (CGD) patients to infection, and also to unexplained, exaggerated inflammation. The impaired recognition and removal (efferocytosis) of apoptotic neutrophils by CGD macrophages may contribute to this effect. We hypothesized that peroxisome proliferator-activated receptor γ (PPARγ) activation during CGD inflammation is deficient, leading to altered macrophage programming and decreased efferocytosis, and that PPARγ agonism would enhance resolution. using the gp91(phox-/-) murine model of X-linked CGD in a well-characterized model of sterile, zymosan-induced peritonitis, it was demonstrated that PPARγ expression and activation in CGD macrophages were significantly deficient at baseline, and acquisition was delayed over the course of inflammation relative to that of wild-type. Efferocytosis by macrophages reflected PPARγ activation during peritonitis and was impaired in CGD mice (versus wild-type), leading to accumulation of apoptotic neutrophils. Importantly, provision of the PPARγ agonist, pioglitazone, either prophylactically or during inflammation, significantly enhanced macrophage PPARγ-mediated programming and efferocytosis, reduced accumulation of apoptotic neutrophils, and normalized the course of peritonitis in CGD mice. As such, PPARγ may be a therapeutic target for CGD, and possibly other inflammatory conditions where aberrant macrophage programming and impaired efferocytosis delay resolution of inflammation.

Expression, localization, and functional model of cholesterol transporters in lactating and nonlactating mammary tissues of murine, bovine, and human origin

Members of the ATP-binding cassette (ABC) transporters play a pivotal role in cellular lipid efflux. To identify candidate cholesterol transporters implicated in lipid homeostasis and mammary gland (MG) physiology, we compared expression and localization of ABCA1, ABCG1, and ABCA7 and their regulatory genes in mammary tissues of different species during the pregnancy-lactation cycle. Murine and bovine mammary glands (MGs) were investigated during different functional stages. The abundance of mRNAs was determined by quantitative RT-PCR. Furthermore, transporter proteins were localized in murine, bovine, and human MGs by immunohistochemistry. In the murine MG, ABCA1 mRNA abundance was elevated during nonlactating compared with lactating stages, whereas ABCA7 and ABCA1 mRNA profiles were not altered. In the bovine MG, ABCA1, ABCG1, and ABCA7 mRNAs abundances were increased during nonlactating stages compared with lactation. Furthermore, associations between mRNA levels of transporters and their regulatory genes LXRα, PPARγ, and SREBPs were found. ABCA1, ABCG1, and ABCA7 proteins were localized in glandular MG epithelial cells (MEC) during lactation, whereas during nonlactating stages, depending on species, the proteins showed distinct localization patterns in MEC and adipocytes. Our results demonstrate that ABCA1, ABCG1, and ABCA7 are differentially expressed between lactation and nonlactating stages and in association with regulatory genes. Combined expression and localization data suggest that the selected cholesterol transporters are universal MG transporters involved in transport and storage of cholesterol and in lipid homeostasis of MEC. Because of the species-specific expression patterns of transporters in mammary tissue, mechanisms of cholesterol homeostasis seem to be differentially regulated between species.

Phenotypic high-throughput screening in atherosclerosis research: focus on macrophages

Atherosclerosis is a complex disease characterized by arterial lesions consisting of macrophage foam cells, smooth muscle cells, lymphocytes and other cell types. As atherosclerotic lesions mature, they can rupture and thereby trigger thrombosis that can result in tissue infarction. Macrophage foam cells develop in the subendothelial space when cells take up cholesterol from modified forms of low-density lipoprotein (LDL) and other apolipoprotein B-containing lipoproteins. Current therapies to limit atherosclerosis focus on altering the plasma lipid composition, most commonly by reducing circulating LDL levels. No current therapy is specifically designed to alter the cellular composition of atherosclerotic lesions. To address this deficit, phenotypic high-throughput drug screens have been developed to identify compounds that reduce the uptake of oxidized LDL by macrophages or to identify compounds that increase the efflux of cholesterol from macrophages. Additional phenotypic screens can be envisaged that address cellular processes in active atherosclerotic lesions including macrophage apoptosis and efferocytosis.

Helical apolipoproteins of high-density lipoprotein enhance phagocytosis by stabilizing ATP-binding cassette transporter A7

We previously reported that the endogenous ATP-binding cassette transporter (ABC)A7 strongly associates with phagocytic function rather than biogenesis of high-density lipoprotein (HDL), being regulated by sterol-regulatory element binding protein (SREBP)2. Phagocytic activity was found enhanced by apolipoprotein (apo)A-I and apoA-II more than twice the maximum in J774 and mouse peritoneal macrophages. Therefore we investigated the molecular basis of this reaction in association with the function of ABCA7. Similar to ABCA1, ABCA7 was degraded, likely by calpain, and apoA-I and apoA-II stabilize ABCA7 against degradation. Cell surface biotinylation experiments demonstrated that endogenous ABCA7 predominantly resides on the cell surface and that the apolipoproteins increase the surface ABCA7. The increase of phagocytosis by apolipoproteins was retained in the J774 cells treated with ABCA1 siRNA and in the peritoneal macrophages from ABCA1-knockout mice, but it was abolished in the J774 cells treated with ABCA7 siRNA and in the peritoneal macrophages from ABCA7-knockout mice. Phagocytosis was decreased in the cells in the peritoneal cavity of the ABCA7-knockout mouse compared with the wild-type control. We thus concluded that extracellular helical apolipoproteins augment ABCA7-associated phagocytosis by stabilizing ABCA7. The results demonstrated direct enhancement of the host defense system by HDL components.

Binding of PDZ-RhoGEF to ATP-binding cassette transporter A1 (ABCA1) induces cholesterol efflux through RhoA activation and prevention of transporter degradation

ATP-binding cassette transporter A1 (ABCA1)-mediated lipid efflux to apolipoprotein A1 (apoA-I) initiates the biogenesis of high density lipoprotein. Here we show that the Rho guanine nucleotide exchange factors PDZ-RhoGEF and LARG bind to the C terminus of ABCA1 by a PDZ-PDZ interaction and prevent ABCA1 protein degradation by activating RhoA. ABCA1 is a protein with a short half-life, and apoA-I stabilizes ABCA1 protein; however, depletion of PDZ-RhoGEF/LARG by RNA interference suppressed the apoA-I stabilization of ABCA1 protein in human primary fibroblasts. Exogenous PDZ-RhoGEF expression activated RhoA and increased ABCA1 protein levels and cholesterol efflux activity. Likewise, forced expression of a constitutively active RhoA mutant significantly increased ABCA1 protein levels, whereas a dominant negative RhoA mutant decreased them. The constitutively active RhoA retarded ABCA1 degradation, thus accounting for its ability to increase ABCA1 protein. Moreover, stimulation with apoA-I transiently activated RhoA, and the pharmacological inhibition of RhoA or the dominant negative RhoA blocked the ability of apoA-I to stabilize ABCA1. Finally, depletion of RhoA or RhoGEFs/RhoA reduces the cholesterol efflux when transcriptional regulation via PPARgamma is eliminated. Taken together, our results have identified a novel physical and functional interaction between ABCA1 and PDZ-RhoGEF/LARG, which activates RhoA, resulting in ABCA1 stabilization and cholesterol efflux activity.

Macrophage LRP-1 controls plaque cellularity by regulating efferocytosis and Akt activation

OBJECTIVE

The balance between apoptosis susceptibility and efferocytosis of macrophages is central to plaque remodeling and inflammation. LRP-1 and its ligand, apolipoprotein E, have been implicated in efferocytosis and apoptosis in some cell types. We investigated the involvement of the macrophage LRP-1/apolipoprotein E axis in controlling plaque apoptosis and efferocytosis. Method and Results- LRP-1(-/-) macrophages displayed nearly 2-fold more TUNEL positivity compared to wild-type cells in the presence of DMEM alone or with either lipopolysaccharide or oxidized low-density lipoprotein. The survival kinase, phosphorylated Akt, was barely detectable in LRP-1(-/-) cells, causing decreased phosphorylated Bad and increased cleaved caspase-3. Regardless of the apoptotic stimulation and degree of cell death, LRP-1(-/-) macrophages displayed enhanced inflammation with increased IL-1 beta, IL-6, and tumor necrosis factor-alpha expression. Efferocytosis of apoptotic macrophages was reduced by 60% in LRP-1(-/-) vs wild-type macrophages despite increased apolipoprotein E expression by both LRP-1(-/-) phagocytes and wild-type apoptotic cells. Compared to wild-type macrophage lesions, LRP-1(-/-) lesions had 5.7-fold more necrotic core with more dead cells not associated with macrophages.

CONCLUSIONS

Macrophage LRP-1 deficiency increases cell death and inflammation by impairing phosphorylated Akt activation and efferocytosis. Increased apolipoprotein E expression in LRP-1(-/-) macrophages suggests that the LRP-1/apolipoprotein E axis regulates the balance between apoptosis and efferocytosis, thereby preventing necrotic core formation.

Defective phagocytosis of apoptotic cells by macrophages in atherosclerotic lesions of ob/ob mice and reversal by a fish oil diet

RATIONALE

The complications of atherosclerosis are a major cause of death and disability in type 2 diabetes. Defective clearance of apoptotic cells by macrophages (efferocytosis) is thought to lead to increased necrotic core formation and inflammation in atherosclerotic lesions.

OBJECTIVE

To determine whether there is defective efferocytosis in a mouse model of obesity and atherosclerosis.

METHODS AND RESULTS

We quantified efferocytosis in peritoneal macrophages and in atherosclerotic lesions of obese ob/ob or ob/ob;Ldlr(-/-) mice and littermate controls. Peritoneal macrophages from ob/ob and ob/ob;Ldlr(-/-) mice showed impaired efferocytosis, reflecting defective phosphatidylinositol 3-kinase activation during uptake of apoptotic cells. Membrane lipid composition of ob/ob and ob/ob;Ldlr(-/-) macrophages showed an increased content of saturated fatty acids (FAs) and decreased omega-3 FAs (eicosapentaenoic acid and docosahexaenoic acid) compared to controls. A similar defect in efferocytosis was induced by treating control macrophages with saturated free FA/BSA complexes, whereas the defect in ob/ob macrophages was reversed by treatment with eicosapentaenoic acid/BSA or by feeding ob/ob mice a fish oil diet rich in omega-3 FAs. There was also defective macrophage efferocytosis in atherosclerotic lesions of ob/ob;Ldlr(-/-) mice and this was reversed by a fish oil-rich diet.

CONCLUSIONS

The findings suggest that in obesity and type 2 diabetes elevated levels of saturated FAs and/or decreased levels of omega-3 FAs contribute to decreased macrophage efferocytosis. Beneficial effects of fish oil diets in atherosclerotic cardiovascular disease may involve improvements in macrophage function related to reversal of defective efferocytosis and could be particularly important in type 2 diabetes and obesity.

Apoptotic engulfment pathway and schizophrenia

BACKGROUND

Apoptosis has been speculated to be involved in schizophrenia. In a previously study, we reported the association of the MEGF10 gene with the disease. In this study, we followed the apoptotic engulfment pathway involving the MEGF10, GULP1, ABCA1 and ABCA7 genes and tested their association with the disease.

METHODOLOGY/PRINCIPAL FINDINGS

Ten, eleven and five SNPs were genotyped in the GULP1, ABCA1 and ABCA7 genes respectively for the ISHDSF and ICCSS samples. In all 3 genes, we observed nominally significant associations. Rs2004888 at GULP1 was significant in both ISHDSF and ICCSS samples (p = 0.0083 and 0.0437 respectively). We sought replication in independent samples for this marker and found highly significant association (p = 0.0003) in 3 Caucasian replication samples. But it was not significant in the 2 Chinese replication samples. In addition, we found a significant 2-marker (rs2242436 * rs3858075) interaction between the ABCA1 and ABCA7 genes in the ISHDSF sample (p = 0.0022) and a 3-marker interaction (rs246896 * rs4522565 * rs3858075) amongst the MEGF10, GULP1 and ABCA1 genes in the ICCSS sample (p = 0.0120). Rs3858075 in the ABCA1 gene was involved in both 2- and 3-marker interactions in the two samples.

CONCLUSIONS/SIGNIFICANCE

From these data, we concluded that the GULP1 gene and the apoptotic engulfment pathway are involved in schizophrenia in subjects of European ancestry and multiple genes in the pathway may interactively increase the risks to the disease.

Dysfunctional cystic fibrosis transmembrane conductance regulator inhibits phagocytosis of apoptotic cells with proinflammatory consequences

Cystic fibrosis (CF) is caused by mutated CF transmembrane conductance regulator (CFTR) and is characterized by robust airway inflammation and accumulation of apoptotic cells. Phagocytosis of apoptotic cells (efferocytosis) is a pivotal regulator of inflammation, because it prevents postapoptotic necrosis and actively suppresses release of a variety of proinflammatory mediators, including IL-8. Because CF is associated with accumulation of apoptotic cells, inappropriate levels of IL-8, and robust inflammation, we sought to determine whether CFTR deficiency specifically impairs efferocytosis and its regulation of inflammatory mediator release. Here we show that CFTR deficiency directly interferes with efferocytosis by airway epithelium, an effect that is not due to altered binding of apoptotic cells to epithelial cells or altered expression of efferocytosis receptors. In contrast, expression of RhoA, a known negative regulator of efferocytosis, is substantially increased in CFTR-deficient cells, and inhibitors of RhoA or its downstream effector Rho kinase normalize efferocytosis in these cells. Impaired efferocytosis appears to be mediated through an amiloride-sensitive ion channel, because amiloride restores phagocytic competency in CFTR-deficient cells. Finally, ineffective efferocytosis in CFTR-deficient cells appears to have proinflammatory consequences, because apoptotic cells enhance IL-8 release by these cells, but not by wild-type controls. Therefore, in CF, dysregulated efferocytosis may lead to accumulation of apoptotic cells and impaired regulation of the inflammatory response and, ultimately, may suggest a new therapeutic target.

Transport of lipids by ABC proteins: interactions and implications for cellular toxicity, viability and function

Members of the ATP-binding cassette (ABC) family of membrane-bound transporters are involved in multiple aspects of transport and redistribution of various lipids and their conjugates. Most ABC transporters localize to the plasma membrane; some are associated with liquid-ordered cholesterol-/sphingolipid-rich microdomains, and to a lesser extent the membranes of the Golgi and endoplasmic reticulum. Hence, ABC transporters are well placed to regulate plasma membrane lipid composition and the efflux and redistribution of structural phospholipids and sphingolipids during periods of cellular stress and recovery. ABC transporters can also modulate cellular sensitivity to extrinsic pro-apoptotic signals through regulation of sphingomyelin-ceramide biosynthesis and metabolism. The functionality of ABC transporters is, in turn, modulated by the lipid content of the microdomains in which they reside. Cholesterol, a major membrane microdomain component, is not only a substrate of several ABC transporters, but also regulates ABC activity through its effects on microdomain structure. Several important bioactive lipid mediators and toxic lipid metabolites are also effluxed by ABC transporters. In this review, the complex interactions between ABC transporters and their lipid/sterol substrates will be discussed and analyzed in the context of their relevance to cellular function, toxicity and apoptosis.

Tyro3 receptor tyrosine kinases in the heterogeneity of apoptotic cell uptake

Mononuclear phagocytes comprise a mobile, broadly dispersed and highly adaptable system that lies at the very epicenter of host defense against pathogens and the interplay of the innate and adaptive arms of immunity. Understanding the molecular mechanisms that control the response of mononuclear phagocytes to apoptotic cells and the anti-inflammatory consequences of that response is an important goal with implications for multiple areas of biomedical sciences. This review details current understanding of the heterogeneity of apoptotic cell uptake by different members of the mononuclear phagocyte family in humans and mice. It also recounts the unique role of the Tyro3 family of receptor tyrosine kinases, best characterized for Mertk, in the signal transduction leading both to apoptotic cell ingestion and the anti-inflammatory effects that result.

A pathway for phagosome maturation during engulfment of apoptotic cells

Removal of apoptotic cells is critical for the physiological well-being of the organism and defects in corpse removal have been linked to disease states. Genes regulating corpse recognition and internalization have been identified, but few molecules involved in the processing of internalized corpses are known. Through a combination of targeted and unbiased reverse genetic screens in Caenorhabditis elegans, and studies in mammalian cells, we have identified genes required for maturation of apoptotic-cell-containing phagosomes. We have further ordered these candidates, which include the GTPases RAB-5 and RAB-7 and the HOPS complex, into a coherent linear pathway for the maturation of apoptotic cells within phagosomes. In depth analysis of two additional candidate genes, the phosphatidylinositol 3 kinase (PI(3)K) vps-34 (A001762) and dyn-1/dynamin, showed an accumulation of internalized, but undegraded, corpses within abnormal Rab5-negative phagosomes. We ordered these candidates in our pathway, with DYN-1 functioning upstream of VPS-34 in the recruitment and/or retention of RAB-5 to the phagosome. Finally, we have also identified a previously undescribed biochemical complex containing Vps34, dynamin and Rab5(GDP), thus providing a mechanism for Rab5 recruitment to the nascent phagosome.

ATP-binding cassette transporter A7 regulates processing of amyloid precursor protein in vitro

ATP-binding cassette transporter A7 (ABCA7) is expressed in the brain and, like its closest homolog ABCA1, belongs to the ABCA subfamily of full-length ABC transporters. ABCA1 promotes cellular cholesterol efflux to lipid-free apolipoprotein acceptors and also inhibits the production of neurotoxic beta-amyloid (Abeta) peptides in vitro. The potential functions of ABCA7 in the brain are unknown. This study investigated the ability of ABCA7 to regulate cholesterol efflux to extracellular apolipoprotein acceptors and to modulate Abeta production. The transient expression of ABCA7 in human embryonic kidney cells significantly stimulated cholesterol efflux (fourfold) to apolipoprotein E (apoE) discoidal lipid complexes but not to lipid-free apoE or apoA-I. ABCA7 also significantly inhibited Abeta secretion from Chinese hamster ovary cells stably expressing human amyloid precursor protein (APP) or APP containing the Swedish K670M671-->N670L671 mutations when compared with mock-transfected cells. Studies with fluorogenic substrates indicated that ABCA7 had no impact on alpha-, beta-, or gamma-secretase activities. Live cell imaging of Chinese hamster ovary cells expressing APP-GFP indicated an apparent retention of APP in a perinuclear location in ABCA7 co-transfected cells. These studies indicate that ABCA7 has the capacity to stimulate cellular cholesterol efflux to apoE discs and regulate APP processing resulting in an inhibition of Abeta production.

The autophagic machinery is necessary for removal of cell corpses from the developing retinal neuroepithelium

Autophagy is a homoeostatic process necessary for the clearance of damaged or superfluous proteins and organelles. The recycling of intracellular constituents also provides energy during periods of metabolic stress, thereby contributing to cell viability. In addition, disruption of autophagic machinery interferes with embryonic development in several species, although the underlying cellular processes affected remain unclear. Here, we investigate the role of autophagy during the early stages of chick retina development, when the retinal neuroepithelium proliferates and starts to generate the first neurons, the retinal ganglion cells. These two developmental processes are accompanied by programmed cell death. Upon treatment with the autophagic inhibitor 3-methyladenine, retinas accumulated numerous TdT-mediated dUTP nick-end labelling-positive cells that correlated with a lack of the 'eat-me' signal phosphatidylserine (PS). In consequence, neighbouring cells did not engulf apoptotic bodies and they persisted as individual cell corpses, a phenotype that was also observed after blockade of phagocytosis with phospho-L-Serine. Supplying the retinas with methylpyruvate, a cell-permeable substrate for ATP production, restored ATP levels and the presentation of PS at the cell surface. Hence, engulfment and lysosomal degradation of apoptotic bodies were also re-established. Together, these data point to a novel role for the autophagic machinery during the development of the central nervous system.

Journey to the grave: signaling events regulating removal of apoptotic cells

Programmed cell death is critical both for organ formation during development and during adult life, when billions of cells must be removed every day. The culmination of the apoptotic process is the specific recognition and engulfment of the apoptotic cell by a phagocyte. A number of recent studies have revealed a series of evolutionarily conserved proteins that link corpse recognition to membrane movement, facilitating the internalization of the target and its subsequent degradation. Two potential signaling modules have been identified: one involving the CED-12/ELMO and CED-5/Dock180 proteins, which function as a bipartite guanine nucleotide exchange factor (GEF) for Rac1, and a second involving CED-1/LRP1 (a potential engulfment receptor) and the adaptor protein CED-6/GULP. Recognition of the apoptotic cell modulates cytokine secretion by the phagocyte, resulting in an anti-inflammatory state distinct from that induced by necrotic cells. The recent molecular delineation of the phagocytic process and the identification of novel signaling proteins involved in engulfment have provided an exciting new platform for future studies into this biologically important process.

Mapping ATP-binding cassette transporter gene expression profiles in melanocytes and melanoma cells

ATP-binding cassette (ABC) transporters regulate the transport of a variety of physiologic substrates. Moreover, several human ABC proteins are responsible for drug exclusion in compound-treated tumor cells, providing cellular mechanisms for the development of multidrug resistance and, therefore, playing an important role in malignant transformation. As only limited information exists on the role of ABC transporters in melanoma, the aim of the study was to generate a complete expression profile of ABC transporters in this tumor entity. Using a TaqMan low-density array for 47 human ABC transporters, mRNA expression analysis was performed from normal human epidermal melanocytes (NHEM P2 and NHEM P3), nine different cell lines originating from primary melanoma (Mel Ei, Mel Juso, Mel Ho and Mel Wei), and metastases of malignant melanoma (Mel Im, Mel Ju, SK Mel 28, HTZ 19 and HMB2). Cell line-specific expression levels were compared with gene expression in pooled RNA from a variety of other human tissues. High expression levels were detected in pooled tissue RNA as well as in cells of melanocytic origin for ABCA5, ABCB2, ABCB6, ABCD3, ABCD4, ABCF1, ABCF2 and ABCF3, whereas ABCB5 revealed a melanocyte-specific high transcript level. In relation to normal melanocytes, ABCB3, ABCB6, ABCC2, ABCC4, ABCE1 and ABCF2 were significantly increased in melanoma cell lines, whereas ABCA7, ABCA12, ABCB2, ABCB4, ABCB5 and ABCD1 showed lower expression levels. In summary, we present here for the first time an ABC-transporter mRNA expression profile in melanoma in comparison to normal melanocytes. The differentially regulated ABC transporters detected by our approach may be candidate genes involved in melanoma tumorigenesis, progression and therapy resistance and could therefore be of great importance to identify novel options for melanoma therapy.