Comparison of Sirtuin 3 Levels in ALS and Huntington's Disease-Differential Effects in Human Tissue Samples vs. Transgenic Mouse Models

Neurodegenerative diseases are characterized by distinct patterns of neuronal loss. In amyotrophic lateral sclerosis (ALS) upper and lower motoneurons degenerate whereas in Huntington’s disease (HD) medium spiny neurons in the striatum are preferentially affected. Despite these differences the pathophysiological mechanisms and risk factors are remarkably similar. In addition, non-neuronal features, such as weight loss implicate a dysregulation in energy metabolism. Mammalian sirtuins, especially the mitochondrial NAD+ dependent sirtuin 3 (SIRT3), regulate mitochondrial function and aging processes. SIRT3 expression depends on the activity of the metabolic master regulator peroxisome proliferator-activated receptor gamma coactivator 1-alpha (PGC-1α), a modifier of ALS and HD in patients and model organisms. This prompted us to systematically probe Sirt3 mRNA and protein levels in mouse models of ALS and HD and to correlate these with patient tissue levels. We found a selective reduction of Sirt3 mRNA levels and function in the cervical spinal cord of end-stage ALS mice (superoxide dismutase 1, SOD1^G93A). In sharp contrast, a tendency to increased Sirt3 mRNA levels was found in the striatum in HD mice (R6/2). Cultured primary neurons express the highest levels of Sirt3 mRNA. In primary cells from PGC-1α knock-out (KO) mice the Sirt3 mRNA levels were highest in astrocytes. In human post mortem tissue increased mRNA and protein levels of Sirt3 were found in the spinal cord in ALS, while Sirt3 levels were unchanged in the human HD striatum. Based on these findings we conclude that SIRT3 mediates the different effects of PGC-1α during the course of transgenic (tg) ALS and HD and in the human conditions only partial aspects Sirt3 dysregulation manifest.

High-resolution respirometry of fine-needle muscle biopsies in pre-manifest Huntington's disease expansion mutation carriers shows normal mitochondrial respiratory function

Alterations in mitochondrial respiration are an important hallmark of Huntington's disease (HD), one of the most common monogenetic causes of neurodegeneration. The ubiquitous expression of the disease causing mutant huntingtin gene raises the prospect that mitochondrial respiratory deficits can be detected in skeletal muscle. While this tissue is readily accessible in humans, transgenic animal models offer the opportunity to cross-validate findings and allow for comparisons across organs, including the brain. The integrated respiratory chain function of the human vastus lateralis muscle was measured by high-resolution respirometry (HRR) in freshly taken fine-needle biopsies from seven pre-manifest HD expansion mutation carriers and nine controls. The respiratory parameters were unaffected. For comparison skeletal muscle isolated from HD knock-in mice (HdhQ111) as well as a broader spectrum of tissues including cortex, liver and heart muscle were examined by HRR. Significant changes of mitochondrial respiration in the HdhQ knock-in mouse model were restricted to the liver and the cortex. Mitochondrial mass as quantified by mitochondrial DNA copy number and citrate synthase activity was stable in murine HD-model tissue compared to control. mRNA levels of key enzymes were determined to characterize mitochondrial metabolic pathways in HdhQ mice. We demonstrated the feasibility to perform high-resolution respirometry measurements from small human HD muscle biopsies. Furthermore, we conclude that alterations in respiratory parameters of pre-manifest human muscle biopsies are rather limited and mirrored by a similar absence of marked alterations in HdhQ skeletal muscle. In contrast, the HdhQ111 murine cortex and liver did show respiratory alterations highlighting the tissue specific nature of mutant huntingtin effects on respiration.

Evaluation of monoacylglycerol lipase as a therapeutic target in a transgenic mouse model of ALS

Amyotrophic lateral sclerosis (ALS) is a fatal neurodegenerative disease of the motor neuron system with limited therapeutic options. While an increasing number of ALS patients can be linked to a small number of autosomal-dominantly inherited cases, most cases are termed sporadic. Both forms are clinically and histopathologically indistinguishable, raising the prospect that they share key pathogenic steps, including potential therapeutic intervention points. The endocannabinoid system is emerging as a versatile, druggable therapeutic target in the CNS and its dysregulation is an early hallmark of neurodegeneration. Whether this is a defense mechanism or part of the pathogenesis remains to be determined. The neuroprotective and anti-inflammatory endocannabinoid 2-arachidonoylglycerol (2-AG), which is degraded by monoacylglycerol lipase (MAGL), accumulates in the spinal cords of transgenic models of ALS. We tested the hypothesis that this 2-AG increase is a protective response in the low-copy SOD1^G93A mouse model of ALS. We show that oral application of the MAGL inhibitor KML29 delays disease onset, progression and survival. Furthermore, we could demonstrate that KML29 reduced proinflammatory cytokines and increased brain-derived neurotrophic factor (BDNF) expression levels in the spinal cord, the major site of neurodegeneration in ALS. Moreover, treatment of primary mouse neurons and primary mousecroglia with 2-AG confirmed the neuroprotective and anti-inflammatory action by increasing BDNF and arginase-1 and decreasing proinflammatory cytokines in vitro. In summary, we show that elevating 2-AG levels by MAGL inhibition is a therapeutic target in ALS and demonstrate that the endocannabinoid defense mechanisms can be exploited therapeutically in neurodegenerative diseases. This article is part of the Special Issue entitled "A New Dawn in Cannabinoid Neurobiology".

Peripheral monocytes are functionally altered and invade the CNS in ALS patients

Amyotrophic lateral sclerosis (ALS) is a devastating progressive neurodegenerative disease affecting primarily the upper and lower motor neurons. A common feature of all ALS cases is a well-characterized neuroinflammatory reaction within the central nervous system (CNS). However, much less is known about the role of the peripheral immune system and its interplay with CNS resident immune cells in motor neuron degeneration. Here, we characterized peripheral monocytes in both temporal and spatial dimensions of ALS pathogenesis. We found the circulating monocytes to be deregulated in ALS regarding subtype constitution, function and gene expression. Moreover, we show that CNS infiltration of peripheral monocytes correlates with improved motor neuron survival in a genetic ALS mouse model. Furthermore, application of human immunoglobulins or fusion proteins containing only the human Fc, but not the Fab antibody fragment, increased CNS invasion of peripheral monocytes and delayed the disease onset. Our results underline the importance of peripheral monocytes in ALS pathogenesis and are in agreement with a protective role of monocytes in the early phase of the disease. The possibility to boost this beneficial function of peripheral monocytes by application of human immunoglobulins should be evaluated in clinical trials.

Toxic gain of function from mutant FUS protein is crucial to trigger cell autonomous motor neuron loss

FUS is an RNA-binding protein involved in amyotrophic lateral sclerosis (ALS) and frontotemporal dementia (FTD). Cytoplasmic FUS-containing aggregates are often associated with concomitant loss of nuclear FUS Whether loss of nuclear FUS function, gain of a cytoplasmic function, or a combination of both lead to neurodegeneration remains elusive. To address this question, we generated knockin mice expressing mislocalized cytoplasmic FUS and complete FUS knockout mice. Both mouse models display similar perinatal lethality with respiratory insufficiency, reduced body weight and length, and largely similar alterations in gene expression and mRNA splicing patterns, indicating that mislocalized FUS results in loss of its normal function. However, FUS knockin mice, but not FUS knockout mice, display reduced motor neuron numbers at birth, associated with enhanced motor neuron apoptosis, which can be rescued by cell-specific CRE-mediated expression of wild-type FUS within motor neurons. Together, our findings indicate that cytoplasmic FUS mislocalization not only leads to nuclear loss of function, but also triggers motor neuron death through a toxic gain of function within motor neurons.

Alterations in the hypothalamic melanocortin pathway in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis, the most common adult-onset motor neuron disease, leads to death within 3 to 5 years after onset. Beyond progressive motor impairment, patients with amyotrophic lateral sclerosis suffer from major defects in energy metabolism, such as weight loss, which are well correlated with survival. Indeed, nutritional intervention targeting weight loss might improve survival of patients. However, the neural mechanisms underlying metabolic impairment in patients with amyotrophic lateral sclerosis remain elusive, in particular due to the lack of longitudinal studies. Here we took advantage of samples collected during the clinical trial of pioglitazone (GERP-ALS), and characterized longitudinally energy metabolism of patients with amyotrophic lateral sclerosis in response to pioglitazone, a drug with well-characterized metabolic effects. As expected, pioglitazone decreased glycaemia, decreased liver enzymes and increased circulating adiponectin in patients with amyotrophic lateral sclerosis, showing its efficacy in the periphery. However, pioglitazone did not increase body weight of patients with amyotrophic lateral sclerosis independently of bulbar involvement. As pioglitazone increases body weight through a direct inhibition of the hypothalamic melanocortin system, we studied hypothalamic neurons producing proopiomelanocortin (POMC) and the endogenous melanocortin inhibitor agouti-related peptide (AGRP), in mice expressing amyotrophic lateral sclerosis-linked mutant SOD1(G86R). We observed lower Pomc but higher Agrp mRNA levels in the hypothalamus of presymptomatic SOD1(G86R) mice. Consistently, numbers of POMC-positive neurons were decreased, whereas AGRP fibre density was elevated in the hypothalamic arcuate nucleus of SOD1(G86R) mice. Consistent with a defect in the hypothalamic melanocortin system, food intake after short term fasting was increased in SOD1(G86R) mice. Importantly, these findings were replicated in two other amyotrophic lateral sclerosis mouse models based on TDP-43 (Tardbp) and FUS mutations. Finally, we demonstrate that the melanocortin defect is primarily caused by serotonin loss in mutant SOD1(G86R) mice. Altogether, the current study combined clinical evidence and experimental studies in rodents to provide a mechanistic explanation for abnormalities in food intake and weight control observed in patients with amyotrophic lateral sclerosis. Importantly, these results also show that amyotrophic lateral sclerosis progression impairs responsiveness to classical drugs leading to weight gain. This has important implications for pharmacological management of weight loss in amyotrophic lateral sclerosis.

Serotonin 2B receptor slows disease progression and prevents degeneration of spinal cord mononuclear phagocytes in amyotrophic lateral sclerosis

Microglia are the resident mononuclear phagocytes of the central nervous system and have been implicated in the pathogenesis of neurodegenerative diseases such as amyotrophic lateral sclerosis (ALS). During neurodegeneration, microglial activation is accompanied by infiltration of circulating monocytes, leading to production of multiple inflammatory mediators in the spinal cord. Degenerative alterations in mononuclear phagocytes are commonly observed during neurodegenerative diseases, yet little is known concerning the mechanisms leading to their degeneration, or the consequences on disease progression. Here we observed that the serotonin 2B receptor (5-HT2B), a serotonin receptor expressed in microglia, is upregulated in the spinal cord of three different transgenic mouse models of ALS. In mutant SOD1 mice, this upregulation was restricted to cells positive for CD11b, a marker of mononuclear phagocytes. Ablation of 5-HT2B receptor in transgenic ALS mice expressing mutant SOD1 resulted in increased degeneration of mononuclear phagocytes, as evidenced by fragmentation of Iba1-positive cellular processes. This was accompanied by decreased expression of key neuroinflammatory genes but also loss of expression of homeostatic microglial genes. Importantly, the dramatic effect of 5-HT2B receptor ablation on mononuclear phagocytes was associated with acceleration of disease progression. To determine the translational relevance of these results, we studied polymorphisms in the human HTR2B gene, which encodes the 5-HT2B receptor, in a large cohort of ALS patients. In this cohort, the C allele of SNP rs10199752 in HTR2B was associated with longer survival. Moreover, patients carrying one copy of the C allele of SNP rs10199752 showed increased 5-HT2B mRNA in spinal cord and displayed less pronounced degeneration of Iba1 positive cells than patients carrying two copies of the more common A allele. Thus, the 5-HT2B receptor limits degeneration of spinal cord mononuclear phagocytes, most likely microglia, and slows disease progression in ALS. Targeting this receptor might be therapeutically useful.

Mutual exacerbation of peroxisome proliferator-activated receptor γ coactivator 1α deregulation and α-synuclein oligomerization

OBJECTIVE

Aggregation of α-synuclein (α-syn) and α-syn cytotoxicity are hallmarks of sporadic and familial Parkinson disease (PD), with accumulating evidence that prefibrillar oligomers and protofibrils are the pathogenic species in PD and related synucleinopathies. Peroxisome proliferator-activated receptor γ coactivator 1α (PGC-1α), a key regulator of mitochondrial biogenesis and cellular energy metabolism, has recently been associated with the pathophysiology of PD. Despite extensive effort on studying the function of PGC-1α in mitochondria, no studies have addressed whether PGC-1α directly influences oligomerization of α-syn or whether α-syn oligomers impact PGC-1α expression.

MATERIALS AND METHODS

We tested whether pharmacological or genetic activation of PGC-1α or PGC-11α knockdown could modulate the oligomerization of α-syn in vitro by using an α-syn -fragment complementation assay.

RESULTS

In this study, we found that both PGC-1α reference gene (RG-PGC-1α) and the central nervous system (CNS)-specific PGC-1α (CNS-PGC-1α) are downregulated in human PD brain, in A30P α-syn transgenic animals, and in a cell culture model for α-syn oligomerization. Importantly, downregulation of both RG-PGC-1α and CNS-PGC-1α in cell culture or neurons from RG-PGC-1α-deficient mice leads to a strong induction of α-syn oligomerization and toxicity. In contrast, pharmacological activation or genetic overexpression of RG-PGC-1α reduced α-syn oligomerization and rescued α-syn-mediated toxicity.

INTERPRETATION

Based on our results, we propose that PGC-1α downregulation and α-syn oligomerization form a vicious circle, thereby influencing and/or potentiating each other. Our data indicate that restoration of PGC-1α is a promising approach for development of effective drugs for the treatment of PD and related synucleinopathies.

Fumaric acid esters stimulate astrocytic VEGF expression through HIF-1α and Nrf2

Fumaric acid esters (FAE) are oral analogs of fumarate that have recently been shown to decrease relapse rate and disease progression in multiple sclerosis (MS), prompting to investigate their protective potential in other neurological diseases such as amyotrophic lateral sclerosis (ALS). Despite efficacy in MS, mechanisms of action of FAEs are still largely unknown. FAEs are known to activate the transcription factor Nrf2 and downstream anti-oxidant responses through the succination of Nrf2 inhibitor KEAP1. However, fumarate is also a known inhibitor of prolyl-hydroxylases domain enzymes (PhD), and PhD inhibition might lead to stabilization of the HIF-1α transcription factor under normoxic conditions and subsequent activation of a pseudo hypoxic response. Whether Nrf2 activation is associated with HIF-1α stabilization in response to FAEs in cell types relevant to MS or ALS remains unknown. Here, we show that FAEs elicit HIF-1α accumulation, and VEGF release as its expected consequence, in astrocytes but not in other cell types of the central nervous system. Reporter assays demonstrated that increased astrocytic VEGF release in response to FAEs was dependent upon both HIF-1α and Nrf2 activation. Last, astrocytes of transgenic mice expressing SOD1(G93A), an animal model of ALS, displayed reduced VEGF release in response to FAEs. These studies show that FAEs elicit different signaling pathways in cell types from the central nervous system, in particular a pseudo-hypoxic response in astrocytes. Disease relevant mutations might affect this response.

PGC-1α is a male-specific disease modifier of human and experimental amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis (ALS) is a devastating, adult-onset neurodegenerative disorder of the upper and lower motor systems. It leads to paresis, muscle wasting and inevitably to death, typically within 3-5 years. However, disease onset and survival vary considerably ranging in extreme cases from a few months to several decades. The genetic and environmental factors underlying this variability are of great interest as potential therapeutic targets. In ALS, men are affected more often and have an earlier age of onset than women. This gender difference is recapitulated in transgenic rodent models, but no underlying mechanism has been elucidated. Here we report that SNPs in the brain-specific promoter region of the transcriptional co-activator PGC-1α, a master regulator of metabolism, modulate age of onset and survival in two large and independent ALS populations and this occurs in a strictly male-specific manner. In complementary animal studies, we show that deficiency of full-length (FL) Pgc-1α leads to a significantly earlier age of onset and a borderline shortened survival in male, but not in female ALS-transgenic mice. In the animal model, FL Pgc-1α-loss is associated with reduced mRNA levels of the trophic factor Vegf-A in males, but not in females. In summary, we indentify PGC-1α as a novel and clinically relevant disease modifier of human and experimental ALS and report a sex-dependent effect of PGC-1α in this neurodegenerative disorder.

Adenovirus vector-based in vitro neuronal cell model for Huntington's disease with human disease-like differential aggregation and degeneration

BACKGROUND

Neuronal degeneration, in particular in the striatum, and the formation of nuclear and cytoplasmic inclusions are characteristics of Huntington's disease (HD) as a result of the expansion of a polyglutamine tract located close to the N-terminus of huntingtin (htt). Because of the large (10-kb) size of the htt cDNA, expression of full-length htt in primary neurons has proved difficult in the past.

METHODS

We generated a new chronic in vitro model that is based on high-capacity adenovirus vector-mediated transduction of primary murine striatal and cortical neurons. Because the vector has a large capacity for transport of foreign DNA, it was possible to quantitatively express in these primary cells normal and mutant full-length htt (designed as fusion proteins with enhanced green fluorescent protein) in addition to its truncated versions. Pathological changes caused by mutant htt were characterized.

RESULTS

The model mimicked several features observed in HD patients: prominent nuclear inclusions in cortical but not in striatal neurons, preferential neuronal degeneration of striatal neurons and neurofilament fragmentation in this cell type. Compared with expressed truncated mutant htt, the expression of full-length mutant htt in neurons resulted in a much slower appearance of pathological changes. Different from cortical neurons, the vast majority of nuclei in striatal cells contained only diffusely distributed N-terminal htt fragments. Cytoplasmic inclusions in both cell types contained full-length mutant htt.

CONCLUSIONS

This model and the adenovirus vectors used will be valuable for studying the function of htt and the pathogenesis of HD at molecular and cellular levels in different neuronal cell types.

A greatly extended PPARGC1A genomic locus encodes several new brain-specific isoforms and influences Huntington disease age of onset

PGC-1α has been implicated in the pathogenesis of neurodegenerative disorders. Several single-nucleotide polymorphisms (SNPs) located in two separate haplotype blocks of PPARGC1A have shown associations with Huntington's disease (HD) and Parkinson's disease, but causative SNPs have not been identified. One SNP (rs7665116) was located in a highly conserved 233 bp region of intron 2. To determine whether rs7665116 is located in an alternative exon, we performed 5'-RLM-RACE from exon 3 and discovered multiple new transcripts that initiated from a common novel promoter located 587 kb upstream of exon 2, but did not contain the conserved region harboring rs7665116. Using real-time polymerase chain reaction, RNase protection assays and northern blotting, we show that the majority of these transcripts are brain specific and are at least equally or perhaps more abundant than the reference sequence PPARGC1A transcripts in whole brain. Two main transcripts containing independent methionine start codons encode full-length brain-specific PGC-1α proteins that differ only at their N-termini (NTs) from PGC-1α, encoded by the reference sequence. Additional truncated isoforms containing these NTs that are similar to NT-PGC-1α exist. Other transcripts may encode potential dominant negative forms, as they are predicted to lack the second LXXLL motif that serves as an interaction site for several nuclear receptors. Furthermore, we show that the new promoter is active in neuronal cell lines and describe haplotypes encompassing this region that are associated with HD age of onset. The discovery of such a large PPARGC1A genomic locus and multiple isoforms in brain warrants further functional studies and may provide new tissue-specific targets for treating neurodegenerative diseases.

Microglia cell culture: a primer for the novice

Microglial cells are the resident immune cells of the central nervous system. Progress in the recent decade has clearly established that microglial cells participate or even actively drive neurological disease. Much of our current knowledge has been generated by investigating microglial cells in cell culture. The aim of this chapter is to give the uninitiated a basic and adaptable protocol for the culturing of microglial cells. We discuss the challenges of microglial cell culture and provide a collection of tips which reflect our 25+ years of collective experience.

Cannabinol delays symptom onset in SOD1 (G93A) transgenic mice without affecting survival

Therapeutic options for amyotrophic lateral sclerosis (ALS), the most common adult-onset motor neuron disorder, remain limited. Emerging evidence from clinical studies and transgenic mouse models of ALS suggests that cannabinoids, the bioactive ingredients of marijuana (Cannabis sativa) might have some therapeutic benefit in this disease. However, Delta(9)-tetrahydrocannabinol (Delta(9)-THC), the predominant cannabinoid in marijuana, induces mind-altering effects and is partially addictive, compromising its clinical usefulness. We therefore tested whether cannabinol (CBN), a non-psychotropic cannabinoid, influences disease progression and survival in the SOD1 (G93A) mouse model of ALS. CBN was delivered via subcutaneously implanted osmotic mini-pumps (5 mg/kg/day) over a period of up to 12 weeks. We found that this treatment significantly delays disease onset by more than two weeks while survival was not affected. Further research is necessary to determine whether non-psychotropic cannabinoids might be useful in ameliorating symptoms in ALS.

Experimental autoimmune encephalomyelitis disrupts endocannabinoid-mediated neuroprotection

Focal cerebral ischemia and traumatic brain injury induce an escalating amount of cell death because of harmful mediators diffusing from the original lesion site. Evidence suggests that healthy cells surrounding these lesions attempt to protect themselves by producing endocannabinoids (eCBs) and activating cannabinoid receptors, the molecular target for marijuana-derived compounds. Indeed, activation of cannabinoid receptors reduces the production and diffusion of harmful mediators. Here, we provide evidence that an exception to this pattern is found in experimental autoimmune encephalomyelitis (EAE), a mouse model of multiple sclerosis. We show that cell damage induced by EAE does not lead to increase in eCBs, even though cannabinoid receptors are functional because synthetic cannabinoid agonists are known to confine EAE-induced lesions. This lack of eCB increase is likely due to IFN-gamma, which is released by primed T cells invading the CNS. We show that IFN-gamma disrupts the functionality of purinergic P2X7 receptors, a key step controlling eCB production by microglia, the main source of eCBs in brain. Accordingly, induction of EAE in P2X7-/- mice results in even lower eCB levels and more pronounced cell damage than in wild-type mice. Our data suggest that the high level of CNS IFN-gamma associated with EAE disrupts eCB-mediated neuroprotection while maintaining functional cannabinoid receptors, thus providing additional support for the use of cannabinoid-based medicine to treat multiple sclerosis.

The endocannabinoid anandamide protects neurons during CNS inflammation by induction of MKP-1 in microglial cells

Endocannabinoids are released after brain injury and believed to attenuate neuronal damage by binding to CB(1) receptors and protecting against excitotoxicity. Such excitotoxic brain lesions initially result in primary destruction of brain parenchyma, which attracts macrophages and microglia. These inflammatory cells release toxic cytokines and free radicals, resulting in secondary neuronal damage. In this study, we show that the endocannabinoid system is highly activated during CNS inflammation and that the endocannabinoid anandamide (AEA) protects neurons from inflammatory damage by CB(1/2) receptor-mediated rapid induction of mitogen-activated protein kinase phosphatase-1 (MKP-1) in microglial cells associated with histone H3 phoshorylation of the mkp-1 gene sequence. As a result, AEA-induced rapid MKP-1 expression switches off MAPK signal transduction in microglial cells activated by stimulation of pattern recognition receptors. The release of AEA in injured CNS tissue might therefore represent a new mechanism of neuro-immune communication during CNS injury, which controls and limits immune response after primary CNS damage.

Endocannabinoids accumulate in spinal cord of SOD1 G93A transgenic mice

Approximately 2% of amyotrophic lateral sclerosis (ALS) cases are caused by mutations in the super oxide dismutase 1 (SOD1) gene and transgenic mice for these mutations recapitulate many features of this devastating neurodegenerative disease. Here we show that the amount of anandamide (AEA) and 2-arachidonoylglycerol (2-AG), two endocannabinoids that have neuroprotective properties, increase in spinal cord of SOD1(G93A) transgenic mice. This increase occurs in the lumbar section of spinal cords, the first section to undergo neurodegeneration, and is significant before overt motor impairment. Our results show that chronic neurodegeneration induced by a genetic mutation increases endocannabinoid production possibly as part of an endogenous defense mechanism.

P2X7 receptors control 2-arachidonoylglycerol production by microglial cells

Endogenous cannabinoid ligands (endocannabinoids) produced by neurons, astrocytes, and microglial cells activate cannabinoid receptors, the molecular target for marijuana's bioactive ingredient Delta(9)-tetrahydrocannabinol. The molecular mechanism underlying the production of the most abundant endocannabinoid, 2-arachidonoylglycerol (2-AG), is unclear. A prevalent hypothesis proposes that activation of metabotropic receptors coupled to the phosphatidylinositol-specific phospholipase C and diacylglycerol (DG) lipase pathway will systematically lead to increases in 2-AG production. Here, we show that ATP increases 2-AG production by cultured microglial cells in a phosphatidylinositol-specific phospholipase C and DG lipase-dependent manner. However, efficacious activation of metabotropic P2Y purinergic receptors coupled to phosphatidylinositol-specific phospholipase C does not increase 2-AG production. This suggests that ionotropic, and not metabotropic, purinergic receptors control 2-AG production at an unexpected enzymatic step of its metabolic pathway. We show that activation of P2X(7) ionotropic receptors, which are highly permeable to calcium, is necessary and sufficient to increase 2-AG production in microglial cells. We also show that the sustained rise in intracellular calcium induced by activation of P2X(7) receptors directly increases DG lipase activity while inhibiting the activity of monoacylglycerol lipase, the enzyme that degrades 2-AG. This inverse sensitivity of DG lipase and monoacylglycerol lipase to calcium constitutes an original and efficient modality for sustained accumulation of 2-AG. Because prolonged increases in 2-AG amounts in brain parenchyma are thought to orchestrate neuroinflammation, the enzymatic steps involved in 2-AG synthesis and degradation by microglial cells constitute appealing targets for therapy aimed at controlling exacerbated neuroinflammation.

Nonpsychotropic cannabinoid receptors regulate microglial cell migration

During neuroinflammation, activated microglial cells migrate toward dying neurons, where they exacerbate local cell damage. The signaling molecules that trigger microglial cell migration are poorly understood. In this paper, we show that pathological overstimulation of neurons by glutamate plus carbachol dramatically increases the production of the endocannabinoid 2-arachidonylglycerol (2-AG) but only slightly increases the production of anandamide and does not affect the production of two putative endocannabinoids, homo-gamma-linolenylethanolamide and docosatetraenylethanolamide. We further show that pathological stimulation of microglial cells with ATP also increases the production of 2-AG without affecting the amount of other endocannabinoids. Using a Boyden chamber assay, we provide evidence that 2-AG triggers microglial cell migration. This effect of 2-AG occurs through CB2 and abnormal-cannabidiol-sensitive receptors, with subsequent activation of the extracellular signal-regulated kinase 1/2 signal transduction pathway. It is important to note that cannabinol and cannabidiol, two nonpsychotropic ingredients present in the marijuana plant, prevent the 2-AG-induced cell migration by antagonizing the CB2 and abnormal-cannabidiol-sensitive receptors, respectively. Finally, we show that microglial cells express CB2 receptors at the leading edge of lamellipodia, which is consistent with the involvement of microglial cells in cell migration. Our study identifies a cannabinoid signaling system regulating microglial cell migration. Because this signaling system is likely to be involved in recruiting microglial cells toward dying neurons, we propose that cannabinol and cannabidiol are promising nonpsychotropic therapeutics to prevent the recruitment of these cells at neuroinflammatory lesion sites.

Astrocytes in culture produce anandamide and other acylethanolamides

Anandamide (arachidonylethanolamide) is an endocannabinoid that belongs to the acylethanolamide lipid family. It is produced by neurons in a calcium-dependent manner and acts through cannabinoid CB1 receptors. Other members of the acylethanolamide lipid family are also produced by neurons and act through G-protein-coupled receptors: homo-gamma-linolenylethanolamide (HEA) and docosatetraenylethanolamide (DEA) act through CB1 receptors, palmitylethanolamide (PEA) acts through CB2-like receptors, and oleylethanolamide (OEA) acts through receptors that have not yet been cloned. Although it is clear that anandamide and other acylethanolamides play a major role in neuronal signaling, whether astrocytes also produce these lipids is unknown. We developed a chemical ionization gas chromatography/mass spectrometry method that allows femtomole detection and quantification of anandamide and other acylethanolamides. Using this method, we unambiguously detected and quantified anandamide, HEA, DEA, PEA, and OEA in mouse astrocytes in culture. Stimulation of mouse astrocytes with ionomycin, a calcium ionophore, enhanced the production of anandamide, HEA, and DEA, whereas PEA and OEA levels were unchanged. Endothelin-1, a peptide known to act on astrocytes, enhanced the production of anandamide, without affecting the levels of other acylethanolamides. These results show that astrocytes produce anandamide, HEA, and DEA in a calcium-dependent manner and that anandamide biosynthesis can be selectively stimulated under physiologically relevant conditions. The relative levels of acylethanolamides in astrocytes from rat and human were different from the relative levels of acylethanolamides in mouse astrocytes, indicating that the production of these lipids differs between species. Because astrocytes are known to express CB1 receptors and inactivate endocannabinoids, our finding strongly suggests the existence of a functional endocannabinoid signaling system in these cells.

Phagocytic clearance of apoptotic neurons by Microglia/Brain macrophages in vitro: involvement of lectin-, integrin-, and phosphatidylserine-mediated recognition

Microglia, the tissue macrophages of the brain, play a crucial role in recognition and phagocytic removal of apoptotic neurons. The microglial receptors for recognition of apoptotic neurons are not yet characterized. Here we established a co-culture model of primary microglia and cerebellar granule neurons to examine the receptor systems involved in recognition/uptake of apoptotic neurons. Treatment with 100 microM S-nitrosocysteine induced apoptosis of cerebellar neurons as indicated by nuclear condensation and phosphatidylserine exposure to the exoplasmic leaflet of the plasma membrane. Microglial cells were added to neurons 2 h after apoptosis induction and co-cultured for 6 h in the presence of ligands that inhibit recognition by binding to respective receptors. Binding/phagocytosis was determined after combined 4', 6-diamidino-2-phenylindole/propidium iodide (for apoptotic/necrotic neurons) and lectin staining (for microglia). Uptake of apoptotic neurons was reduced by N-acetylglucosamine or galactose, suggesting that recognition involves asialoglycoprotein-like lectins. Furthermore, the inhibition of microglial binding/uptake of apoptotic neurons by RGDS peptide suggests a role of microglial vitronectin receptor. As microglia selectively bind lipid vesicles enriched in phosphatidylserine and O-phospho-L-serine interfered with the uptake of apoptotic neurons, an involvement of phosphatidylserine receptor is rather likely. Apoptotic neurons do not release soluble signals that serve to attract or activate microglia. Collectively, these results suggest that apoptotic neurons generate a complex surface signal recognized by different receptor systems on microglia.