Microglial TNFα controls daily changes in synaptic GABAARs and sleep slow waves

Microglia sense the changes in their environment. How microglia actively translate these changes into suitable cues to adapt brain physiology is unknown. We reveal an activity-dependent regulation of cortical inhibitory synapses by microglia, driven by purinergic signaling acting on P2RX7 and mediated by microglia-derived TNFα. We demonstrate that sleep induces microglia-dependent synaptic enrichment of GABAARs in a manner dependent on microglial TNFα and P2RX7. We further show that microglia-specific depletion of TNFα alters slow waves during NREM sleep and blunt memory consolidation in sleep-dependent learning tasks. Together, our results reveal that microglia orchestrate sleep-intrinsic plasticity of synaptic GABAARs, sculpt sleep slow waves, and support memory consolidation.

Microglial TNFα orchestrates protein phosphorylation in the cortex during the sleep period and controls homeostatic sleep

Sleep intensity is adjusted by the length of previous awake time, and under tight homeostatic control by protein phosphorylation. Here, we establish microglia as a new cellular component of the sleep homeostasis circuit. Using quantitative phosphoproteomics of the mouse frontal cortex, we demonstrate that microglia-specific deletion of TNFα perturbs thousands of phosphorylation sites during the sleep period. Substrates of microglial TNFα comprise sleep-related kinases such as MAPKs and MARKs, and numerous synaptic proteins, including a subset whose phosphorylation status encodes sleep need and determines sleep duration. As a result, microglial TNFα loss attenuates the build-up of sleep need, as measured by electroencephalogram slow-wave activity and prevents immediate compensation for loss of sleep. Our data suggest that microglia control sleep homeostasis by releasing TNFα which acts on neuronal circuitry through dynamic control of phosphorylation.

Microglia states and nomenclature: A field at its crossroads

Microglial research has advanced considerably in recent decades yet has been constrained by a rolling series of dichotomies such as "resting versus activated" and "M1 versus M2." This dualistic classification of good or bad microglia is inconsistent with the wide repertoire of microglial states and functions in development, plasticity, aging, and diseases that were elucidated in recent years. New designations continuously arising in an attempt to describe the different microglial states, notably defined using transcriptomics and proteomics, may easily lead to a misleading, although unintentional, coupling of categories and functions. To address these issues, we assembled a group of multidisciplinary experts to discuss our current understanding of microglial states as a dynamic concept and the importance of addressing microglial function. Here, we provide a conceptual framework and recommendations on the use of microglial nomenclature for researchers, reviewers, and editors, which will serve as the foundations for a future white paper.

The Heterogeneity of Ly6Chi Monocytes Controls Their Differentiation into iNOS+ Macrophages or Monocyte-Derived Dendritic Cells

Inflammation triggers the differentiation of Ly6C^hi monocytes into microbicidal macrophages or monocyte-derived dendritic cells (moDCs). Yet, it is unclear whether environmental inflammatory cues control the polarization of monocytes toward each of these fates or whether specialized monocyte progenitor subsets exist before inflammation. Here, we have shown that naive monocytes are phenotypically heterogeneous and contain an NR4A1- and Flt3L-independent, CCR2-dependent, Flt3⁺CD11c⁻MHCII⁺PU.1^hi subset. This subset acted as a precursor for FcγRIII⁺PD-L2⁺CD209a⁺, GM-CSF-dependent moDCs but was distal from the DC lineage, as shown by fate-mapping experiments using Zbtb46. By contrast, Flt3⁻CD11c⁻MHCII⁻PU.1^lo monocytes differentiated into FcγRIII⁺PD-L2⁻CD209a⁻iNOS⁺ macrophages upon microbial stimulation. Importantly, Sfpi1 haploinsufficiency genetically distinguished the precursor activities of monocytes toward moDCs or microbicidal macrophages. Indeed, Sfpi1^+/− mice had reduced Flt3⁺CD11c⁻MHCII⁺ monocytes and GM-CSF-dependent FcγRIII⁺PD-L2⁺CD209a⁺ moDCs but generated iNOS⁺ macrophages more efficiently. Therefore, intercellular disparities of PU.1 expression within naive monocytes segregate progenitor activity for inflammatory iNOS⁺ macrophages or moDCs.

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Murine Ly6C^hiCD115⁺ monocytes are heterogeneous

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DC-related genes (Cd209a and MHCII) are expressed in a subset of FcγRIII⁺ monocytes

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GM-CSF-dependent CD209a⁺ moDCs are generated by FcγRIII⁺CD209a⁺MHCII⁺ monocytes

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iNOS⁺ macrophages are generated by FcγRIII⁺CD209a⁻MHCII⁻ monocytes

Microglia control the glycinergic but not the GABAergic synapses via prostaglandin E2 in the spinal cord

Microglia can influence the excitatory responses of neurons, but less is known about how these immune cells in the brain may influence inhibitory neurotransmitters. Cantaut-Belarif et al. report that prostaglandin production by Toll-like receptor–stimulated microglia can influence the glycinergic but not GABAergic responses of neurons by altering the lateral diffusion of glycine receptors specifically within the synaptic membrane.

Microglia control excitatory synapses, but their role in inhibitory neurotransmission has been less well characterized. Herein, we show that microglia control the strength of glycinergic but not GABAergic synapses via modulation of the diffusion dynamics and synaptic trapping of glycine (GlyR) but not GABA_A receptors. We further demonstrate that microglia regulate the activity-dependent plasticity of glycinergic synapses by tuning the GlyR diffusion trap. This microglia–synapse cross talk requires production of prostaglandin E2 by microglia, leading to the activation of neuronal EP2 receptors and cyclic adenosine monophosphate–dependent protein kinase. Thus, we now provide a link between microglial activation and synaptic dysfunctions, which are common early features of many brain diseases.

Tumor Necrosis Factor-α and Neuronal Development

Tumor necrosis factor-α (TNFα) is a prototypic inflammatory cytokine up-regulated in most if not all neurodegenerative diseases. Many studies have reported variable roles in the adult or pathological brain. In contrast, the implication of TNFα in developmental neuronal cell death has been well documented in few studies. In sympathetic and trigeminal neurons, TNFα acts in an autocrine manner to induce immediate cell death on neurotrophic factor deprivation. In the spinal cord, TNFα is transiently produced by macrophages and commits motoneurons to become competent to die 2 days later. TNFα is also likely to induce immediate and delayed prodeath effects in adult and pathological tissues. Data obtained in embryonic systems will thus help to develop new therapeutic approaches to pathological neuronal death in adults.

Microbial priming of plant and animal immunity: symbionts as developmental signals

The functional similarity between root and gut microbiota, both contributing to the nutrition and protection of the host, is often overlooked. A central mechanism for efficient protection against pathogens is defense priming, the preconditioning of immunity induced by microbial colonization after germination or birth. Microbiota have been recruited several times in evolution as developmental signals for immunity maturation. Because there is no evidence that microbial signals are more relevant than endogenous ones, we propose a neutral scenario for the evolution of this dependency: any hypothetic endogenous signal can be lost because microbial colonization, reliably occurring at germination or birth, can substitute for it, and without either positive selection or the acquisition of new functions. Dependency of development on symbiotic signals can thus evolve by contingent irreversibility.

DAP12 and CD11b contribute to the microglial-induced death of dopaminergic neurons in vitro but not in vivo in the MPTP mouse model of Parkinson's disease

Background

Parkinson’s disease (PD) is a neurodegenerative disorder characterized by a loss of dopaminergic neurons (DN) in the substantia nigra (SN). Several lines of evidence suggest that apoptotic cell death of DN is driven in part by non-cell autonomous mechanisms orchestrated by microglial cell-mediated inflammatory processes. Although the mechanisms and molecular network underlying this deleterious cross-talk between DN and microglial cells remain largely unknown, previous work indicates that, upon DN injury, activation of the β2 integrin subunit CD11b is required for microglia-mediated DN cell death. Interestingly, during brain development, the CD11b integrin is also involved in microglial induction of neuronal apoptosis and has been shown to act in concert with the DAP12 immunoreceptor. Whether such a developmental CD11b/DAP12 pathway could be reactivated in a pathological context such as PD and play a role in microglia-induced DN cell death is a tantalizing hypothesis that we wished to test in this study.

Methods

To test the possibility that DAP12 could be involved in microglia-associated DN injury, we used both in vitro and in vivo toxin-based experimental models of PD recapitulating microglial-mediated non-cell autonomous mechanisms of DN cell death. In vitro, enriched mesencephalic neuronal/microglial co-cultures were exposed to the dopaminergic neurotoxin 1-methyl-4-phenylpyridinium (MPP+) whereas in vivo, mice were administrated with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) according to acute or subchronic mode. Mice deficient for DAP12 or CD11b were used to determine the pathological function of the CD11b/DAP12 pathway in our disease models.

Results

Our results show that DAP12 and CD11b partially contribute to microglia-induced DN cell death in vitro. Yet, in vivo, mice deficient for either of these factors develop similar neuropathological alterations as their wild-type counterparts in two different MPTP mouse models of PD.

Conclusion

Overall, our data suggest that DAP12 and CD11b contribute to microglial-induced DN cell death in vitro but not in vivo in the MPTP mouse model of PD. Therefore, the CD11b/DAP12 pathway may not be considered as a promising therapeutic target for PD.

Microglial control of neuronal activity

Fine-tuning of neuronal activity was thought to be a neuron-autonomous mechanism until the discovery that astrocytes are active players of synaptic transmission. The involvement of astrocytes has changed our understanding of the roles of non-neuronal cells and shed new light on the regulation of neuronal activity. Microglial cells are the macrophages of the brain and they have been mostly investigated as immune cells. However, recent data discussed in this review support the notion that, similarly to astrocytes, microglia are involved in the regulation of neuronal activity. For instance, in most, if not all, brain pathologies a strong temporal correlation has long been known to exist between the pathological activation of microglia and dysfunction of neuronal activity. Recent studies have convincingly shown that alteration of microglial function is responsible for pathological neuronal activity. This causal relationship has also been demonstrated in mice bearing loss-of-function mutations in genes specifically expressed by microglia. In addition to these long-term regulations of neuronal activity, recent data show that microglia can also rapidly regulate neuronal activity, thereby acting as partners of neurotransmission.

Depletion of microglia from primary cellular cultures

Primary cultures are an important in vitro tool to study cellular processes and interactions. These cultures are complex systems, composed of many cell types, including neurons, astrocytes, oligodendrocytes, microglia, NG2 cells, and endothelial cells. For some studies it is necessary to be able to study a pure culture of one cell type, or eliminate a particular cell type, to better understand its function. There exist cell culture protocols for making pure astrocyte or microglia cultures. Here we present two protocols to produce cultures depleted for microglia: in the first case, from a mixed astrocyte-microglia culture and, in the second, for eliminating microglia from neuronal cultures.

Physiological roles of microglia during development

In all the species examined thus far, the behavior of microglia during development appears to be highly stereotyped. This reproducibility supports the notion that these cells have a physiological role in development. Microglia are macrophages that migrate from the yolk sac and colonize the central nervous system early during development. The first invading yolk-sac macrophages are highly proliferative and their role has not yet been addressed. At later developmental stages, microglia can be found throughout the brain and tend to preferentially reside at specific locations that are often associated with known developmental processes. Thus, it appears that microglia concentrate in areas of cell death, in proximity of developing blood vessels, in the marginal layer, which contains developing axon fascicles, and in close association with radial glial cells. This review describes the main features of brain colonization by microglia and discusses the possible physiological roles of these cells during development.

Nitric oxide regulates astrocyte maturation in the hippocampus: involvement of NOS2

Neurons and astrocytes are generated sequentially from radial glia. Once neurogenesis is completed, radial glia starts to differentiate into immature astrocytes. Astrocytes then maturate and change their morphology and electrophysiological properties. Neurotrophic cytokines or bone morphogenetic proteins have been identified as inducers of the developmental switch from neurogenesis to astrogenesis. However, the factors and mechanisms regulating the late differentiation of radial glia and the subsequent astrocyte maturation are poorly described. We used two independent approaches to examine the role of nitric oxide (NO) in the process of astrogenesis and maturation of astrocytes. First using a pharmacological approach, we depleted NO from developing hippocampus by intraventricular injection of a specific scavenger. Then by a genetic approach, we analyzed a nitric oxide synthase-2 (NOS2) knockout mouse. In both models, we found that differentiation of RC2-positive radial glia into late GFAP-positive radial glia was impaired. The cell-fate analysis after incorporation of BrdU demonstrated that astrogenesis was not altered upon NOS2 deficiency. Maturation of astrocytes was assessed by electrophysiological recordings at P7 and functional analysis. In wild type, 20% of astrocytes were immature as shown by their non-linear I/V relationship and high membrane resistance, whereas in NOS2-/- hippocampus, 51% of the astrocytes displayed an immature profile. The reduced branching of astrocytes upon NOS2 deficiency and their low content in connexin-43 further confirmed their immature profile. Our results highlight a novel developmental role of NO and NOS2 in the differentiation of radial glia and the maturation of astrocytes.

A long nuclear-retained non-coding RNA regulates synaptogenesis by modulating gene expression

A growing number of long nuclear-retained non-coding RNAs (ncRNAs) have recently been described. However, few functions have been elucidated for these ncRNAs. Here, we have characterized the function of one such ncRNA, identified as metastasis-associated lung adenocarcinoma transcript 1 (Malat1). Malat1 RNA is expressed in numerous tissues and is highly abundant in neurons. It is enriched in nuclear speckles only when RNA polymerase II-dependent transcription is active. Knock-down studies revealed that Malat1 modulates the recruitment of SR family pre-mRNA-splicing factors to the transcription site of a transgene array. DNA microarray analysis in Malat1-depleted neuroblastoma cells indicates that Malat1 controls the expression of genes involved not only in nuclear processes, but also in synapse function. In cultured hippocampal neurons, knock-down of Malat1 decreases synaptic density, whereas its over-expression results in a cell-autonomous increase in synaptic density. Our results suggest that Malat1 regulates synapse formation by modulating the expression of genes involved in synapse formation and/or maintenance.

Prenatal activation of microglia induces delayed impairment of glutamatergic synaptic function

Background

Epidemiological studies have linked maternal infection during pregnancy to later development of neuropsychiatric disorders in the offspring. In mice, experimental inflammation during embryonic development impairs behavioral and cognitive performances in adulthood. Synaptic dysfunctions may be at the origin of cognitive impairments, however the link between prenatal inflammation and synaptic defects remains to be established.

Methodology/Principal Findings

In this study, we show that prenatal alteration of microglial function, including inflammation, induces delayed synaptic dysfunction in the adult. DAP12 is a microglial signaling protein expressed around birth, mutations of which in the human induces the Nasu-Hakola disease, characterized by early dementia. We presently report that synaptic excitatory currents in mice bearing a loss-of-function mutation in the DAP12 gene (DAP12^KI mice) display enhanced relative contribution of AMPA. Furthermore, neurons from DAP12^KI P0 pups cultured without microglia develop similar synaptic alterations, suggesting that a prenatal dysfunction of microglia may impact synaptic function in the adult. As we observed that DAP12^KI microglia overexpress genes for IL1β, IL6 and NOS2, which are inflammatory proteins, we analyzed the impact of a pharmacologically-induced prenatal inflammation on synaptic function. Maternal injection of lipopolysaccharides induced activation of microglia at birth and alteration of glutamatergic synapses in the adult offspring. Finally, neurons cultured from neonates born to inflamed mothers and cultured without microglia also displayed altered neuronal activity.

Conclusion/Significance

Our results demonstrate that prenatal inflammation is sufficient to induce synaptic alterations with delay. We propose that these alterations triggered by prenatal activation of microglia provide a cellular basis for the neuropsychiatric defects induced by prenatal inflammation.

Microglial control of neuronal death and synaptic properties

Microglia have long been characterized by their immune function in the nervous system and are still mainly considered in a beneficial versus detrimental dialectic. However a review of literature enables to shed novel lights on microglial function under physiological conditions. It is now relevant to position these cells as full time partners of neuronal function and more specifically of synaptogenesis and developmental apoptosis. Indeed, microglia can actively control neuronal death. It has actually been shown in retina that microglial nerve growth factor (NGF) is necessary for the developmental apoptosis to occur. Similarly, in cerebellum, microglia induces developmental Purkinje cells death through respiratory burst. Furthermore, in spinal cord, microglial TNFalpha commits motoneurons to a neurotrophic dependent developmental apoptosis. Microglia can also control synaptogenesis. This is suggested by the fact that a mutation in KARAP/DAP12, a key protein of microglial activation impacts synaptic functions in hippocampus, and synapses protein content. In addition it has been now demonstrated that microglial brain-derived neurotrophin factor (BDNF) directly regulates synaptic properties in spinal cord. In conclusion, microglia can control neuronal function under physiological conditions and it is known that neuronal activity reciprocally controls microglial activation. We will discuss the importance of this cross-talk which allows microglia to orchestrate the balance between synaptogenesis and neuronal death occurring during development or injuries.

Impaired synaptic function in the microglial KARAP/DAP12-deficient mouse

Several proteins are expressed in both immune and nervous systems. However, their putative nonimmune functions in the brain remain poorly understood. KARAP/DAP12 is a transmembrane polypeptide associated with cell-surface receptors in hematopoeitic cells. Its mutation in humans induces Nasu-Hakola disease, characterized by presenile dementia and demyelinization. However, alteration of white matter occurs months after the onset of neuropsychiatric symptoms, suggesting that other neuronal alterations occur in the early phases of the disease. We hypothesized that KARAP/DAP12 may impact synaptic function. In mice deficient for KARAP/DAP12 function, long-term potentiation was enhanced and was partly NMDA receptor (NMDAR) independent. This effect was accompanied by changes in synaptic glutamate receptor content, as detected by the increased rectification of AMPA receptor EPSCs and increased sensitivity of NMDAR EPSCs to ifenprodil. Biochemical analysis of synaptic proteins confirmed these electrophysiological data. In mutants, the AMPA receptor GluR2 subunit expression was decreased only in the postsynaptic densities but not in the whole membrane fraction, demonstrating specific impairment of synaptic receptor accumulation. Alteration of the BNDF-tyrosine kinase receptor B (TrkB) signaling in the mutant was demonstrated by the dramatic decrease of synaptic TrkB with no change in other regulatory or scaffolding proteins. Finally, KARAP/DAP12 was detected only in microglia but not in neurons, astrocytes, or oligodendrocytes. KARAP/DAP12 may thus alter microglial physiology and subsequently synaptic function and plasticity through a novel microglia-neuron interaction.

Morphologically identified glycinergic synapses in the hippocampus

Inhibitory transmission in the hippocampus is predominantly GABAergic, but electrophysiological data evidenced strychnine-sensitive glycine-induced currents. However, synaptic currents have not been reported. Here, we describe, for the first time, the presence of GlyR clusters in several areas of the hippocampus as well as in cultured hippocampal neurons. In contrast with spinal cord, hippocampal GlyRs contain alpha2 but no alpha1 subunit. Optical and electron microscopy indicates that GlyRs can be synaptic as well as extrasynaptic. Synaptic GlyRs were apposed to glycinergic boutons characterized by the expression of the vesicular and the plasma membrane transporters of glycine (VIAAT and GlyT2, respectively). Double labeling with calcium-binding proteins showed that GlyT2 could be detected in boutons innervating both excitatory cells (soma and dendrites) and interneurons. Finally, GlyR clusters could be detected at synaptic sites with the GABAA receptor gamma2 subunit and gephyrin, suggesting that mixed GABA/glycine synapses might exist in the hippocampus.

Association of gephyrin with synaptic and extrasynaptic GABAA receptors varies during development in cultured hippocampal neurons

Several studies have reported extrasynaptic clusters of GABAA receptors in hippocampal neurons. Yet their functional relevance as well as their evolution in relation with gephyrin during synaptogenesis remain unknown. We have analyzed the expression pattern of the main proteins of the GABAergic synapses during synaptogenesis in cultured hippocampal neurons. We found that GABAergic terminals, characterized by VIAAT and GAD-65 expression, differentiated 3 to 7 days after the glutamatergic endings. At the postsynaptic side, the GABAAR- beta3 subunit was first diffuse and then clustered when GABAergic terminals differentiated and gephyrin formed large clusters. Colocalization of these proteins was high and increased with development. At later stages, GABAAR beta3 clusters colocalized with gephyrin at synaptic but also at extrasynaptic sites. GABAAR gamma2 subunits were directly expressed as clusters which were first extrasynaptic and not associated with gephyrin. Subsequently, the GABAAR gamma2 subunits associated with gephyrin at synaptic and/or extrasynaptic sites. Our data indicate that formation of GABAAR gamma2 subunit clusters is gephyrin independent.

Distribution and pharmacology of alpha 6-containing nicotinic acetylcholine receptors analyzed with mutant mice

The alpha6 subunit of the nicotinic acetylcholine receptor (nAChR) is expressed at very high levels in dopaminergic (DA) neurons. However, because of the lack of pharmacological tools selective for alpha6-containing nAChRs, the role of this subunit in the etiology of nicotine addiction remains unknown. To provide new tools to investigate this issue, we generated an alpha6 nAChR knock-out mouse. Homozygous null mutants (alpha6-/-) did not exhibit any gross neurological or behavioral deficits. A careful anatomic and molecular examination of alpha6-/- mouse brains demonstrated the absence of developmental alterations in these animals, especially in the visual and dopaminergic pathways, where the alpha6 subunit is normally expressed at the highest levels. On the other hand, receptor autoradiography revealed a decrease in [3H]nicotine, [3H]epibatidine, and [3H]cytisine high-affinity binding in the terminal fields of retinal ganglion cells of alpha6-/- animals, whereas high-affinity [125I]alpha-conotoxinMII (alphaCtxMII) binding completely disappeared in the brain. Moreover, inhibition of [3H]epibatidine binding on striatal membranes, using unlabeled alphaCtxMII or cytisine, revealed the absence of alphaCtxMII-sensitive and cytisine-resistant [3H]epibatidine binding sites in alpha6-/- mice, although the total amount of binding was unchanged. Because alphaCtxMII, a toxin formerly thought to be specific for alpha3beta2-containing nAChRs, is known to partially inhibit nicotine-induced dopamine release, these results support the conclusion that alpha6 rather than alpha3 is the partner of beta2 in the nicotinic modulation of DA neurons. They further show that alpha6-/- mice might be useful tools to understand the mechanisms of nicotine addiction, although some developmental compensation might occur in these mice.

The neuron-restrictive silencer element: a dual enhancer/silencer crucial for patterned expression of a nicotinic receptor gene in the brain

The neuron-restrictive silencer element (NRSE) has been identified in several neuronal genes and confers neuron specificity by silencing transcription in nonneuronal cells. NRSE is present in the promoter of the neuronal nicotinic acetylcholine receptor beta2-subunit gene that determines its neuron-specific expression in the nervous system. Using transgenic mice, we show that NRSE may either silence or enhance transcription depending on the cellular context within the nervous system. In vitro in neuronal cells, NRSE activates transcription of synthetic promoters when located downstream in the 5' untranslated region, or at less than 50 bp upstream from the TATA box, but switches to a silencer when located further upstream. In contrast, in nonneuronal cells NRSE always functions as a silencer. Antisense RNA inhibition shows that the NRSE-binding protein REST contributes to the activation of transcription in neuronal cells.

Differential regulation of neuronal nicotinic acetylcholine receptor subunit gene promoters by Brn-3 POU family transcription factors

The regulatory region of the neuronal nicotinic acetylcholine (nACh) receptor alpha 2 subunit gene is activated by the Brn-3b POU family transcription factor but not by the closely related factors Brn-3a and Brn-3c. This pattern of regulation has not previously been observed for other neuronally expressed genes, several of which, such as those encoding alpha-internexin or SNAP-25, are activated by Brn-3a and Brn-3c but repressed by Brn-3b. The alpha 3 nACh receptor subunit gene is also shown to be activated by Brn-3a but is repressed by Brn-3b and Brn-3c. In contrast, the Brn-3 POU family transcription factors have no effects on either the alpha 7 or beta 4 nACh receptor subunit genes. The actions of Brn-3b on the alpha 2 subunit are thus in contrast to the inhibitory actions of Brn-3b on several promoters that are activated by Brn-3 alpha. The different actions of the Brn-3 POU factors on the range of nACh receptor genes tested suggests that the novel stimulation of the alpha 2 subunit by Brn-3b is specific to this subunit and not a general feature of nACh receptor genes.

Promoter elements conferring neuron-specific expression of the beta 2-subunit of the neuronal nicotinic acetylcholine receptor studied in vitro and in transgenic mice

Several genes encoding subunits of the neuronal nicotinic acetylcholine receptors have been cloned and regulatory elements involved in the transcription of the alpha 2 and alpha 7-subunit genes have been described. Yet, the detailed mechanisms governing the neuron-specific transcription and the spatio-temporal expression pattern of these genes remain largely uninvestigated. The beta 2-subunit is the most widely expressed neuronal nicotinic receptor subunit in the nervous system. We have studied the structural and regulatory properties of the 5' sequence of this gene. A fragment of 1163 bp of upstream sequence is sufficient to drive the cell-specific transcription of a reporter gene in both transient transfection assays and in transgenic mice. Deletion analysis and site-directed mutagenesis of this promoter reveal two negative elements and one positive element. The positively-acting sequence includes one functional E-box. One of the repressor elements is located in the transcribed region and is the NRSE/RE1 sequence already described in promoters of neuronal genes. In this paper, we describe the neuron-specific promoter of the gene encoding the neuronal nicotinic acetylcholine receptor beta 2-subunit.

The neuronal nicotinic acetylcholine receptor alpha 2 subunit gene promoter is activated by the Brn-3b POU family transcription factor and not by Brn-3a or Brn-3c

The regulatory region of the neuronal nicotinic acetylcholine receptor alpha 2 subunit gene, which contains six copies of the octamer-related sequence CCCCATGCAAT, is activated by the Brn-3b POU family transcription factor but not by the closely related factors Brn-3a and Brn-3c. This effect is in contrast to the previously documented inhibitory effect of Brn-3b on octamer-containing promoters that are activated by Brn-3a and Brn-3c. Activation of the alpha 2 gene by Brn-3b requires that both the POU domain and other N-terminal sequences are derived from Brn-3b and is dependent on the intactness of the alpha 2 gene regulatory region, being lost in truncated derivatives containing one, two, or four copies of the octamer-related sequence. Surprisingly, however, these truncated derivatives are activated by Brn-3c. These effects are discussed in terms of both the influence of the target sequence and its context in the promoter on activation by the various forms of Brn-3 as well as of the processes that restrict expression of the alpha 2 subunit gene to a few cells in the nervous system.

Abnormal avoidance learning in mice lacking functional high-affinity nicotine receptor in the brain

Nicotine affects many aspects of behaviour including learning and memory through its interaction with neuronal nicotinic acetylcholine receptors (nAChR). Functional nAChRs are pentameric proteins containing at least one type of alpha-subunit and one type of beta-subunit. The involvement of a particular neuronal nicotinic subunit in pharmacology and behaviour was examined using gene targeting to mutate beta 2, the most widely expressed nAChR subunit in the central nervous system. We report here that high-affinity binding sites for nicotine are absent from the brains of mice homozygous for the beta 2-subunit mutation. Further, electrophysiological recording from brain slices reveals that thalamic neurons from these mice do not respond to nicotine application. Finally, behavioural tests demonstrate that nicotine no longer augments the performance of beta 2-1- mice on passive avoidance, a test of associative memory. Paradoxically, mutant mice are able to perform better than their non-mutant siblings on this task.

Negative regulatory elements upstream of a novel exon of the neuronal nicotinic acetylcholine receptor alpha 2 subunit gene

The expression of the nicotinic acetylcholine receptor alpha 2 subunit gene is highly restricted to the Spiriform lateralis nucleus of the Chick diencephalon. As a first step toward understanding the molecular mechanism underlying this regulation, we have investigated the structural and regulatory properties of the 5' sequence of this gene. A strategy based on the ligation of an oligonucleotide to the first strand of the cDNA (SLIC) followed by PCR amplification was used. A new exon was found approximately 3kb upstream from the first coding exon, and multiple transcription start sites of the gene were mapped. Analysis of the flanking region shows many consensus sequences for the binding of nuclear proteins, suggesting that the 1 kb flanking region contains at least a portion of the promoter of the gene. We have analysed the negative regulatory elements present within this region and found that a silencer region located between nucleotide -144 and +76 is active in fibroblasts as well as in neurons. This silencer is composed of six tandem repeat Oct-like motifs (CCCCATGCAAT), but does not bind any member of the Oct family. Moreover these motifs were found to act as a silencer only when they were tandemly repeated. When two, four or five motifs were deleted, the silencer activity of the motifs unexpectedly became an enhancer activity in all cells we have tested.

Chicken neuronal acetylcholine receptor alpha 2-subunit gene exhibits neuron-specific expression in the brain and spinal cord of transgenic mice

Transgenic mice carrying the complete structural gene of the alpha 2 subunit of the chicken neuronal nicotinic acetylcholine receptor (nAChR) and 7 kilobase pairs (kbp) of 5' upstream and 3 kbp of 3' downstream sequences have been generated. The transgene was stably integrated in transgenic lines and transmitted to their progeny. Avian transgene expression was predominant in the central nervous system as detected by specific alpha 2-subunit cDNA amplification. Moreover, in at least two independent mouse lines, its expression appeared to be neuron-specific and reproducibly restricted to subregions in the brain and spinal cord, as revealed by in situ hybridization histochemistry. Most cranial motor nuclei were positive, and several of the alpha 2-subunit transgene-expressing structures corresponded to cholinergic areas in rodents. This study reveals that regulatory mechanisms giving rise to neuronal-specific gene expression have been conserved at least in part between birds and mammals.

Differential expression of the neuronal acetylcholine receptor alpha 2 subunit gene during chick brain development

In situ hybridization histochemistry reveals localized expression of the nicotinic acetylcholine receptor (nAChR) alpha 2 subunit mRNA restricted to the lateral spiriform nucleus (SpL) of the chick diencephalon. The alpha 2 nAChR transcripts are not detected in immature SpL neurons at 4.5-5 days of embryonic development. They begin to accumulate in the SpL at embryonic day 11 and increase until the newborn stage. Specific alpha 2 cDNA amplification by the polymerase chain reaction shows that during this period, the absolute content of alpha 2 mRNA increases about 20-fold. The expression of the alpha 2 nAChR gene is thus developmentally regulated and appears concomitant with the entry of cholinergic fibers into the SpL, as demonstrated by choline acetyltransferase immunohistochemistry.

Chromosomal localization of the mouse genes coding for alpha 2, alpha 3, alpha 4 and beta 2 subunits of neuronal nicotinic acetylcholine receptor

The chromosomal localization of four neuronal nicotinic acetylcholine receptor subunit genes was performed by following the mendelian segregation of their corresponding alleles in backcrosses involving the mouse species Mus spretus and the laboratory strains C57BL/6 or BALB/c. A similar analysis previously performed with muscle nicotinic acetylcholine receptor subunits revealed that the genes coding for the alpha and beta subunits are respectively located on chromosome 2 and 11, whereas the gamma and delta subunit coding genes are linked and located on mouse chromosome 1. In this study, we show that the genes coding for the neuronal nicotinic acetylcholine receptor alpha 2, alpha 3 and beta 2 subunits are dispersed on three different mouse chromosomes, viz. 14, 9 and 3 respectively. Moreover, the alpha 4 subunit gene is located on chromosome 2 but is not genetically linked to the alpha 1 subunit gene.