A requirement of serotonergic p38α mitogen-activated protein kinase for peripheral immune system activation of CNS serotonin uptake and serotonin-linked behaviors.

Alterations in central serotonin (5-hydroxytryptamine, 5-HT) neurotransmission and peripheral immune activation have been linked to multiple neuropsychiatric disorders, including depression, schizophrenia and autism. The antidepressant-sensitive 5-HT transporter (SERT, SLC6A4), a critical determinant of synaptic 5-HT inactivation, can be regulated by pro-inflammatory cytokine signaling. Systemic innate immune system activation via intraperitoneal lipopolysaccharide (LPS) injection rapidly elevates brain SERT activity and 5-HT clearance. Moreover, the pro-inflammatory cytokine interleukin (IL)-1ß rapidly stimulates SERT activity in raphe nerve terminal preparations ex vivo, effects that are attenuated by pharmacological p38 MAPK inhibition. To establish a role of serotonergic p38α MAPK signaling in LPS/IL-1ß-induced SERT regulation and attendant behavioral responses, we pursued studies in mice that afford conditional elimination of p38α MAPK in 5-HT neurons (p38α(5HT-)). We found p38α(5HT-) and control (p38α(5HT+)) littermates to be indistinguishable in viability and growth and to express equivalent levels of SERT protein and synaptosomal 5-HT transport activity. Consistent with pharmacological studies, however, IL-1ß fails to increase SERT activity in midbrain synaptosomes prepared from p38α(5HT-) animals. Moreover, although LPS elevated plasma corticosterone and central/peripheral pro-inflammatory cytokines in p38α(5HT-) animals, elevations in midbrain SERT activity were absent nor were changes in depressive and anxiety-like behaviors observed. Our studies support an obligate role of p38α MAPK signaling in 5-HT neurons for the translation of immune activation to SERT regulation and 5-HT-modulated behaviors.

Molecular genetics of mouse serotonin neurons across the lifespan.

New molecular genetics approaches have been developed over the past several years to study brain serotonin (5-HT) neuron development and the roles of 5-HT neurons in behavior and physiology. These approaches were enabled by manipulation of the gene encoding the Pet-1 ETS transcription factor whose expression in the brain is restricted to developing and adult 5-HT neurons. Targeting of the Pet-1 gene led to the development of a mouse line with a severe and stable deficiency of embryonic 5-HT-synthesizing neurons. The Pet-1 transcription regulatory region has been used to create several new 5-HT neuron-type transgenic tools that have greatly increased the experimental accessibility of the small number of brain 5-HT neurons. Permanent and specific marking of 5-HT neurons with Pet-1-based transgenic tools have now been used for flow cytometry, whole cell electrophysiological recordings, progenitor fate mapping, and live time lapse imaging of these neurons. Additional tools provide multiple strategies for conditional temporal targeting of gene expression in 5-HT neurons at different stages of life. Pet-1-based approaches have led to advances in understanding the role of 5-HT neurons in respiration, thermoregulation, emotional behaviors, maternal behavior, and the mechanism of antipsychotic drug actions. In addition, these approaches have begun to reveal the molecular basis of 5-HT neuron heterogeneity and the transcriptional mechanisms that direct 5-HT neuron-type identity, maturation, and maintenance.

Contribution of 5-HT to locomotion - the paradox of Pet-1(-/-) mice.

Serotonin (5-HT) plays a critical role in locomotor pattern generation by modulating the rhythm and the coordinations. Pet-1, a transcription factor selectively expressed in the raphe nuclei, controls the differentiation of 5-HT neurons. Surprisingly, inactivation of Pet-1 (Pet-1(-/-) mice) that causes a 70% reduction in the number of 5-HT-positive neurons in the raphe does not impair locomotion in adult mice. The goal of the present study was to investigate the operation of the locomotor central pattern generator (CPG) in neonatal Pet-1(-/-) mice. We first confirmed, by means of immunohistochemistry, that there is a marked reduction of 5-HT innervation in the lumbar spinal cord of Pet-1(-/-) mice. Fictive locomotion was induced in the in vitro neonatal mouse spinal cord preparation by bath application of N-methyl-d,l-Aspartate (NMA) alone or together with dopamine and 5-HT. A locomotor pattern characterized by left-right and flexor-extensor alternations was observed in both conditions. Increasing the concentration of 5-HT from 0.5 to 5 µm impaired the pattern in Pet-1(-/-) mice. We tested the role of endogenous 5-HT in the NMA-induced fictive locomotion. Application of 5-HT(2) or 5-HT(7) receptor antagonists affected the NMA-induced fictive locomotion in both heterozygous and homozygous mice although the effects were weaker in the latter strain. This may be, at least partly, explained by the reduced expression of 5-HT(2A) R as observed by means of immunohistochemistry. These results suggest that compensatory mechanisms take place in Pet-1(-/-) mice that make locomotion less dependent upon 5-HT.

Regulation of transcription in the neuronal nicotinic receptor subunit gene cluster by a neuron-selective enhancer and ETS domain factors.

Expression of neurotransmitter receptors encoded by the nicotinic acetylcholine receptor (nAchR) subunit gene cluster depends on coexpression of the beta4, alpha3, and alpha5 subunits in certain kinds of neurons. One way in which coexpression might be achieved is through the regulation of promoters in the cluster by neuron-selective enhancers. The beta43' enhancer is located between the beta4 and alpha3 promoters and it directs cell type-specific expression in cell lines. It is not known, however, whether beta43' is active in neurons. Therefore, we assayed beta43' in dissociated rat sympathetic ganglia cultures, which contain nAchR-positive neurons as well as nAchR-negative non-neuronal cells. Reporters controlled by the alpha3 promoter and beta43' were expressed in a neuron-selective manner; greater than 90% and up to 100% of luciferase expression was detected in neurons. Neuron selectivity was maintained when beta43' was placed next to ubiquitously active viral promoters. In contrast, replacing beta43' with the SV40 enhancer eliminated neuron selectivity. The enhancer is composed of at least two separate but functionally interdependent elements, each of which interacts with a different type of ETS domain factor. These findings support a model in which beta43' controls neuronal expression of one or more genes in the cluster through interactions with a combination of ETS factors.

Transcriptional control of the neuronal nicotinic acetylcholine receptor gene cluster by the beta43' enhancer, Sp1, SCIP and ETS transcription factors.

Receptors assembled from the products of a neuronal beta4alpha3alpha5 NAChR gene cluster depend on these genes being coordinately regulated in particular populations of neurons. Little is known, however, about the transcriptional mechanisms that are likely to underlie their co-expression in correct neuronal cell types. We have identified several regulatory elements and transcription factors that influence transcription of the alpha3 and beta4 genes. The promoters of these genes appear to contain a common cis element that binds Sp1 transcription factors. They can be activated by the POU-domain factor SCIP and activation does not require SCIP binding sites. Between these two promoters is a cell type specific enhancer called beta43'. This enhancer has little activity in non-neuronal cells and is preferentially active in particular populations of central neurons. The clustered genes are potential targets of ETS factors as the ETS domain factor, Pet-1 can activate beta43'-dependent transcription. The neuron-selective properties of beta43' and its location suggest that it is a component of the cis regulatory information required to control expression of the beta4 and alpha3 genes in specific populations of neurons.

The ETS domain factor Pet-1 is an early and precise marker of central serotonin neurons and interacts with a conserved element in serotonergic genes.

Serotonin (5-HT) plays a crucial neuromodulatory role in numerous physiological and behavioral functions, and dysfunction of the serotonergic system has been implicated in several psychiatric disorders. Despite the widespread importance of the central serotonergic neurotransmitter system, little is known about the molecular mechanisms controlling the development of 5-HT neurons. We previously identified an ETS domain transcription factor, Pet-1, that is expressed in a small number of tissues, including the brain. Here, we show that expression of Pet-1 RNA in the brain is restricted to, and marks, the entire rostrocaudal extent of rat serotonergic hindbrain raphe nuclei. Remarkably, Pet-1 RNA colocalizes with tryptophan hydroxylase-positive neurons in raphe nuclei but not with their nonserotonergic neuron or non-neuronal neighbors. Pet-1 RNA is limited to two domains in the developing hindbrain, which precedes the appearance of 5-HT in each domain by approximately a half day. Conserved Pet-1 binding sites are present in or near the promoter regions of the human and mouse 5-HT1a receptor, serotonin transporter, tryptophan hydroxylase, and aromatic L-amino acid decarboxylase genes whose expression is characteristic of the serotonergic neuron phenotype. These sites are capable of supporting transcriptional activation through interactions with the Pet-1 ETS domain and can function as enhancers. Together, our findings establish Pet-1 as an early and precise marker of 5-HT neurons and suggest that it functions specifically in the differentiation and maintenance of these neurons.

Transcriptional analysis of acetylcholine receptor alpha 3 gene promoter motifs that bind Sp1 and AP2.

In this study, we performed an analysis of the neuronal nicotinic acetylcholine receptor alpha 3 subunit gene promoter region, -238/+47, to identify cis and trans elements that are important for basal activity in PC12 cells. Sequence analyses of the alpha 3 promoter and footprint assays revealed an Sp1 binding site between -79 and -57 (termed the alpha 3 GA motif) and an AP2 binding site between -30 and -7. Using mobility shift analysis, we found that PC12 cell extracts contain proteins that specifically bind to the alpha 3 GA motif and are immunologically related to Sp1. Mutation of the alpha 3 GA motif, which prevented binding of Sp1, resulted in a 75% decrease in promoter activity. Mutation of the AP2 site resulted in only a minor loss of promoter activity, which is consistent with the lack of AP2 binding activity in PC12 extracts. In Drosophila Schneider line 2 (S2) cell cotransfection assays, Sp1 activated the alpha 3 promoter in a GA motif-dependent manner. Furthermore, multimerization of the GA motif upstream of the beta-globin TATA box conferred Sp1 responsiveness. Our results indicate that Sp1 can activate transcription through direct interaction with the alpha 3 GA motif and that this motif plays a major role in alpha 3 promoter basal activity in PC12 cells.

The developmental increase in ACh current densities on rat sympathetic neurons correlates with changes in nicotinic ACh receptor alpha-subunit gene expression and occurs independent of innervation.

Determining factors that control the expression of neurotransmitter receptors and the mechanisms by which these factors operate is essential to understand how synapses form during development and how receptor numbers change in the adult. In this study, we have investigated one such factor, the influence of innervation, on the developmental expression of nicotinic ACh receptors (nAChRs) on neonatal rat sympathetic neurons, both in terms of ACh current densities, and in terms of mRNA levels for the transcripts that encode these receptors. To date, nine genes have been cloned that encode neuronal nAChRs subunits in mammals. We demonstrate that mRNA encoding five nAChR subunits, alpha 3, alpha 5, alpha 7, beta 2, and beta 4, are present in neonatal rat sympathetic neurons. We show that mRNA levels for alpha 3 and alpha 7 subunits increase by more than threefold over the first 2 postnatal weeks, a period when most synapses are forming on the neurons; however, we observed no significant change in mRNA levels for alpha 5, beta 2, or beta 4. Using whole-cell voltage-clamp techniques, we show that the increase in alpha-subunit mRNA correlates with increases in ACh current densities, which double over the same period. To investigate the role of innervation, we cut the preganglionic nerve at birth and measured subunit mRNA levels and ACh current densities in denervated neurons 1-2 weeks later. Our results indicate that the preganglionic nerve differentially affects the mRNA level for the five nAChR transcripts, yet it has little influence on the developmental increase in ACh current densities.

Characterization of an acetylcholine receptor alpha 3 gene promoter and its activation by the POU domain factor SCIP/Tst-1.

Genes encoding neuronal nicotinic acetylcholine receptors exhibit restricted patterns of expression in the nervous system. We are interested in elucidating the molecular mechanisms responsible for establishing these patterns of expression. This paper presents the characterization of regulatory elements upstream of the neuronal nicotinic acetylcholine receptor alpha 3 gene. We have identified a GC-rich multistart site promoter adjacent to the alpha 3 coding region. Similar alpha 3 start sites were identified in PC12 cells and sympathetic ganglion neurons, suggesting similar control mechanisms in the clonal line and peripheral neurons. The start site region lacks TATA-like sequences but does contain initiator-like sequences. We show, in transient transfection assays, that the POU domain transcription factor, SCIP/Tst-1, specifically activates alpha 3 in a neural context. Other POU domain factors tested only weakly activated or repressed alpha 3. Unexpectedly, we found that alpha 3 basal activity and SCIP/Tst-1 activation of alpha 3 is not dependent on the SCIP/Tst-1 binding sites found upstream of the gene. In addition, mutations in the SCIP/Tst-1 coding region that prevent the factor from binding to DNA with high affinity do not obliterate alpha 3 activation. These results lead us to propose that alpha 3 activation by SCIP/Tst-1 is achieved via protein-protein interactions between SCIP/Tst-1 and a specific complement of transcription factors that act directly on the promoter.

The expression of nicotinic acetylcholine receptors by PC12 cells treated with NGF.

The expression of neuronal nicotinic ACh receptors (nAChRs) and the subunits that compose these receptors by PC12 cells exposed to NGF has been studied. The analysis of total RNA reveals that the neuronal nAChR subunits alpha 3, alpha S, beta 2, beta 3, and beta 4, but not alpha 2 and alpha 4, are expressed in our PC12 cells. Within 48 hr of adding NGF to cultures, the RNA corresponding to alpha 3, alpha 5, beta 3, and beta 4 is decreased, but the beta 2 RNA increases for up to 6 d after NGF treatment. To determine the influence of NGF treatment on subunit protein expression, subunit-specific antisera were prepared. Immunocytochemistry detected antigen for alpha 3, alpha 5, beta 2, beta 3, and beta 4 (but not alpha 2 and alpha 4) in both NGF-treated and nontreated PC12 cells. The expression of nAChR subunit proteins, as measured by direct binding of antibodies to PC12 cells, does not change subsequent to 6 d of treatment with NGF. Whole-cell recording of PC12 cells shows that both the individual cell current density and response to the agonist cytisine were not altered after 5-7 d in NGF. However, the number of cells exhibiting detectable ACh-induced currents doubled. These results indicate that NGF increases the number of PC12 cells expressing ACh-sensitive nAChR currents but the activation is not the result of altering the amounts of individual nAChR subunit proteins. These data, taken together with the decrease in most nAChR subunit RNAs (except beta 2), suggest that NGF regulation of nAChRs may be through a posttranscriptional mechanism.

Nicotine binding and nicotinic receptor subunit RNA after chronic nicotine treatment.

DBA mice were chronically treated with nicotine by continuous intravenous infusion of 4.0 mg/kg/hr for 10 d. Drug-treated mice were tolerant to the acute effects of nicotine on locomotor activity and body temperature. The effect of chronic treatment on the amount of L-3H-nicotine binding and RNA encoding for alpha 4, the most widely expressed nicotinic alpha-subunit, was measured in three brain regions. Chronic treatment increased L-3H-nicotine binding in cortex and midbrain but had no effect in cerebellum. In contrast, chronic treatment had no effect on the levels of mRNA encoding for alpha 4 in any of the three brain regions. Subsequently brains were sectioned and L-3H-nicotine binding was measured using quantitative autoradiographic methods. In addition, the relative amounts of mRNA for the major nicotinic receptor subunits (alpha 4 and beta 2), as well as for three additional minor subunits (alpha 2, alpha 3, and alpha 5), were determined by in situ hybridization histochemistry followed by quantitation of image intensity. Chronic nicotine treatment resulted in increases in the amount of L-3H-nicotine binding in many but not all brain areas measured. In contrast, chronic treatment had little effect on the intensity of the hybridization signal for the nicotinic subunit mRNA. The results suggest that chronic treatment with nicotine under conditions resulting in maximal steady-state increases in L-3H-nicotine binding has little effect on RNA levels encoding any of four nicotinic alpha-subunits and the beta 2-subunit.

The characterization and localization of the glutamate receptor subunit GluR1 in the rat brain.

The cloning of cDNAs that encode functional glutamate receptors makes it possible to produce antibodies that can be used as high-affinity probes for the localization and characterization of these receptors in the mammalian brain. We have made antibodies to different regions of the first cloned member of this family, GluR1, using bacterially overproduced antigen. On Western blots, these antisera detect glycoprotein(s) of 105 kDa present in crude membranes of the hippocampus and cerebellum. The 105-kDa band is associated with postsynaptic densities, and it is observed in cultured cells upon transfection with the GluR1 cDNA. Although glutamate receptors are thought to be the most prevalent excitatory ligand-gated ion channel in the mammalian brain, immunohistochemistry reveals that the receptors recognized by these antisera are localized predominantly in neurons of the cerebellum and some structures of the limbic system, including the hippocampus, the central nucleus of the amygdala, and portions of the septum. This pattern of expression is, in general, consistent with the distribution of GluR1 mRNA as determined by in situ hybridization histochemistry. Our results suggest that glutamate excitatory circuits recognized by these antisera are predominantly found in regions of the limbic system that are reciprocally interconnected.

Pharmacological and functional diversity of neuronal nicotinic acetylcholine receptors.

Recent molecular cloning studies have identified several genes encoding alpha and beta subunits of the nicotinic acetylcholine receptor. These genes have distinct, although overlapping, patterns of expression in the brain and peripheral ganglia. Multiple nicotinic receptors with distinct pharmacological and functional properties can be made in oocytes by pairwise combination of different alpha-type subunits with different beta-type subunits. Both alpha and beta subunits contribute to the pharmacological and functional diversity. Evan Deneris and colleagues explain how oocyte expression studies, in concert with immunological and electrophysiological analysis in vivo, are beginning to reveal the subunit compositions of different neuronal nicotinic receptor subtypes.

Cloning of a novel glutamate receptor subunit, GluR5: expression in the nervous system during development.

We have isolated cDNAs encoding a glutamate receptor subunit, designated GluR5, displaying 40%-41% amino acid identity with the kainate/AMPA receptor subunits GluR1, GluR2, GluR3, and GluR4. This level of sequence similarity is significantly below the approximately 70% intersubunit identity characteristic of kainate/AMPA receptors. The GluR5 protein forms homomeric ion channels in Xenopus oocytes that are weakly responsive to L-glutamate. The GluR5 gene is expressed in subsets of neurons throughout the developing and adult central and peripheral nervous systems. During embryogenesis, GluR5 transcripts are detected in areas of neuronal differentiation and synapse formation.

Alpha 3, alpha 5, and beta 4: three members of the rat neuronal nicotinic acetylcholine receptor-related gene family form a gene cluster.

We have identified two additional members of the neuronal nicotinic acetylcholine receptor (nAChR)-related gene family. cDNA clones for one new gene, designated alpha 5, were isolated from rat hippocampus and rat PC12 cell line cDNA libraries. The alpha 5 gene encodes a protein of 48,800 daltons (424 amino acids) which exhibits significant overall amino acid sequence identity with the previously cloned rat nAChR subunits alpha 1 (49%), alpha 2 (55%), alpha 3 (52%), and alpha 4 (49%). Features characteristic of other nAChR alpha-subunits are present such as conserved cysteine residues at positions 127, 141, 191, and 192, and four strongly hydrophobic domains. A second addition to the nAChR-related gene family, designated beta 4, is encoded in overlapping rat genomic clones lambda DD15 and lambda RG518A. The beta 4 gene, encoding a mature protein of 53,300 daltons (475 amino acids), consists of 6 exons and has a transcription unit length of approximately 18 kilobase pairs. The beta 4 gene encoded protein shows considerable amino acid sequence identity with nAChR beta 1 (43%), beta 2 (64%), and beta 3 (44%) subunits. Northern blots showed that, along with alpha 3 and beta 2, transcripts for both the alpha 5 and beta 4 genes are present in the PC12 cell line, while in situ hybridization experiments demonstrated expression of the alpha 5 and beta 4 genes in a small number of nuclei in the central nervous system. Finally, the genes that encode the beta 4, alpha 3, and alpha 5 proteins are transcribed with convergent polarities and form a tightly linked gene cluster spanning approximately 60 kilobase pairs.

The functional diversity of the neuronal nicotinic acetylcholine receptors is increased by a novel subunit: beta 4.

A new nicotinic acetylcholine receptor (nAChR) subunit, beta 4, was identified by screening a rat genomic library. In situ hybridization histochemistry revealed expression of the beta 4 gene in the medial habenula of adult rat brains. The primary structure of this subunit was deduced from a cDNA clone isolated from a PC12 cDNA library. Functional nAChRs were detected in Xenopus oocytes injected in pairwise combinations with in vitro synthesized RNAs encoding beta 4 and either the alpha 2, alpha 3, or alpha 4 subunit. Unlike the alpha 3 beta 2 receptor, the alpha 3 beta 4 receptor is not blocked by bungarotoxin 3.1, indicating that the beta subunit can affect the sensitivity of neuronal nAChRs to this toxin. These results extend the functional diversity of nicotinic receptors in the nervous system.

Genes encoding neuronal nicotinic acetylcholine receptors.

Four genes (alpha 2, alpha 3, alpha 4, and beta 2), which encode proteins homologous to the Torpedo electric organ and vertebrate muscle nicotinic acetylcholine receptors, have been identified by cloning rat brain cDNAs. Injection of transcripts derived from these cDNAs into Xenopus laevis oocytes results in the formation of three nicotinic acetylcholine receptors. Two of these receptors, alpha 3/beta 2 and alpha 4/beta 2, have the characteristics of ganglionic nicotinic receptors. The third (alpha 2/beta 2) exhibits a previously undescribed pharmacology and thus represents a novel subtype that may be expressed in the brain. The wide distribution of alpha 2, alpha 3, alpha 4, and beta 2 transcripts in the brain indicates that neuronal nicotinic acetylcholine receptors are a major neurotransmitter receptor system.

Beta 3: a new member of nicotinic acetylcholine receptor gene family is expressed in brain.

Screening of a rat brain cDNA library with a radiolabeled probe made from an alpha 3 cDNA (Boulter, J., Evans, K., Goldman, D., Martin, G., Treco, D., Heinemanns, S., and Patrick, J. (1986) Nature 319, 368-374) resulted in the isolation of a clone whose sequence encodes a protein, beta 3, which is homologous (40-55% amino acid sequence identity) to previously described neuronal nicotinic acetylcholine receptor subunits. The encoded protein has structural features found in other nicotinic acetylcholine receptor (nAChR) subunits. Two cysteine residues that correspond to cysteins 128 and 142 of the Torpedo nAChR alpha subunit are present in beta 3. Absent from beta 3 are 2 adjacent cysteine residues that correspond to cysteines 192 and 193 of the Torpedo subunit. In situ hybridization histochemistry, performed using probes derived from beta 3 cDNAs, demonstrated that the beta 3 gene is expressed in the brain. Thus, beta 3 is the fifth member of the nAChR gene family that is expressed in the brain. The pattern of beta 3 gene expression partially overlaps with that of the neuronal nAChR subunit genes alpha 3, alpha 4, or beta 2. These results lead us to propose that the beta 3 gene encodes a neuronal nAChR subunit.

Functional expression of a new pharmacological subtype of brain nicotinic acetylcholine receptor.

A new type of agonist-binding subunit of rat neuronal nicotinic acetylcholine receptors (nAChRs) was identified. Rat genomic DNA and complementary DNA encoding this subunit (alpha 2) were cloned and analyzed. Complementary DNA expression studies in Xenopus oocytes revealed that the injection of messenger RNAs (mRNAs) for alpha 2 and beta 2 (a neuronal nAChR subunit) led to the generation of a functional nAChR. In contrast to the other known neuronal nAChRs, the receptor produced by the injection of alpha 2 and beta 2 mRNAs was resistant to the alpha-neurotoxin Bgt3.1. In situ hybridization histochemistry showed that alpha 2 mRNA was expressed in a small number of regions, in contrast to the wide distribution of the other known agonist-binding subunits (alpha 3 and alpha 4) mRNAs. These results demonstrate that the alpha 2 subunit differs from other known agonist-binding alpha-subunits of nAChRs in its distribution in the brain and in its pharmacology.