Carboxyl domain of glutamate receptor directs its coupling to metabolic pathways

The cyclic AMP phenotype of fragile X and autism

Cyclic AMP (cAMP) is a second messenger involved in many processes including mnemonic processing and anxiety. Memory deficits and anxiety are noted in the phenotype of fragile X (FX), the most common heritable cause of mental retardation and autism. Here we review reported observations of altered cAMP cascade function in FX and autism. Cyclic AMP is a potentially useful biochemical marker to distinguish autism comorbid with FX from autism per se and the cAMP cascade may be a viable therapeutic target for both FX and autism.

Mitochondrial Ca2+ uptake requires sustained Ca2+ release from the endoplasmic reticulum

We analyzed the role of inositol 1,4,5-trisphosphate-induced Ca(2+) release from the endoplasmic reticulum (ER) (i) in powering mitochondrial Ca(2+) uptake and (ii) in maintaining a sustained elevation of cytosolic Ca(2+) concentration ([Ca(2+)](c)). For this purpose, we expressed in HeLa cells aequorin-based Ca(2+)-sensitive probes targeted to different intracellular compartments and studied the effect of two agonists: histamine, acting on endogenous H(1) receptors, and glutamate, acting on co-transfected metabotropic glutamate receptor (mGluR1a), which rapidly inactivates through protein kinase C-dependent phosphorylation and thus causes transient inositol 1,4,5-trisphosphate production. Glutamate induced a transient [Ca(2+)](c) rise and drop in ER luminal [Ca(2+)] ([Ca(2+)](er)), and then the ER refilled with [Ca(2+)](c) at resting values. With histamine, [Ca(2+)](c) after the initial peak stabilized at a sustained plateau, and [Ca(2+)](er) decreased to a low steady-state value. In mitochondria, histamine evoked a much larger mitochondrial Ca(2+) response than glutamate ( approximately 15 versus approximately 65 microm). Protein kinase C inhibition, partly relieving mGluR1a desensitization, reestablished both the [Ca(2+)](c) plateau and the sustained ER Ca(2+) release and markedly increased the mitochondrial Ca(2+) response. Conversely, mitochondrial Ca(2+) uptake evoked by histamine was drastically reduced by very transient ( approximately 2-s) agonist applications. These data indicate that efficient mitochondrial Ca(2+) uptake depends on the preservation of high Ca(2+) microdomains at the mouth of ER Ca(2+) release sites close to mitochondria. This in turn depends on continuous Ca(2+) release balanced by Ca(2+) reuptake into the ER and maintained by Ca(2+) influx from the extracellular space.

Distinct physiological roles of the Gq-coupled metabotropic glutamate receptors Co-expressed in the same neuronal populations

The group I metabotropic glutamate receptors, mGluR1 and mGluR5, exhibit a high degree of sequence homology, and are often found co-expressed in the same neuronal populations. These receptors couple to a broad array of effector systems, and are implicated in diverse physiological and pathophysiological functions. Due to the high degree of sequence homology, and the findings that these receptors couple identically in recombinant systems, it has been generally assumed that these two group I mGluR subtypes would exhibit redundant function when coexpressed in the same neurons. With the advent of subtype-selective pharmacological tools, it has become possible to tease apart the functions of mGluR1 and mGluR5 in the same neuron. The emerging picture is one of diverse function, which implies differential regulation. Interestingly, the group I mGluRs are modulated by a rich variety of regulatory systems, which may explain how these receptors can mediate divergent actions when present in the same cell.

Structural, signalling and regulatory properties of the group I metabotropic glutamate receptors: prototypic family C G-protein-coupled receptors

In 1991 a new type of G-protein-coupled receptor (GPCR) was cloned, the type 1a metabotropic glutamate (mGlu) receptor, which, despite possessing the defining seven-transmembrane topology of the GPCR superfamily, bore little resemblance to the growing number of other cloned GPCRs. Subsequent studies have shown that there are eight mammalian mGlu receptors that, together with the calcium-sensing receptor, the GABA(B) receptor (where GABA is gamma-aminobutyric acid) and a subset of pheromone, olfactory and taste receptors, make up GPCR family C. Currently available data suggest that family C GPCRs share a number of structural, biochemical and regulatory characteristics, which differ markedly from those of the other GPCR families, most notably the rhodopsin/family A GPCRs that have been most widely studied to date. This review will focus on the group I mGlu receptors (mGlu1 and mGlu5). This subgroup of receptors is widely and differentially expressed in neuronal and glial cells within the brain, and receptor activation has been implicated in the control of an array of key signalling events, including roles in the adaptative changes needed for long-term depression or potentiation of neuronal synaptic connectivity. In addition to playing critical physiological roles within the brain, the mGlu receptors are also currently the focus of considerable attention because of their potential as drug targets for the treatment of a variety of neurological and psychiatric disorders.

Expression of metabotropic glutamate receptor 1 isoforms in the substantia nigra pars compacta of the rat

Metabotropic glutamate receptors, which are linked via G-proteins to second messenger systems, have been implicated in the physiological regulation of dopaminergic neurons of the substantia nigra pars compacta as well as in neurodegeneration. Of the eight known metabotropic glutamate receptors, metabotropic glutamate receptor 1 is the most abundant subtype in the substantia nigra pars compacta. Metabotropic glutamate receptor 1 is alternatively spliced at the carboxy terminal region to yield five variants: 1a, 1b, 1c, 1d and a form recently identified in human brain, 1g. We used an antibody recognizing metabotropic glutamate receptor 1, and another recognizing the splice form la only, to study the localization of these receptors in dopaminergic neurons identified by the presence of tyrosine hydroxylase. Metabotropic glutamate receptor immunoreactivity was present within the somata, axons, and dendrites of substantia nigra pars compacta neurons. The 1a splice form specific antibody, however, did not label these cells, suggesting that they express a metabotropic glutamate receptor 1 splice form different from 1a. In situ hybridization with splice form-specific oligonucleotide probes was used to determine which of the other known metabotropic glutamate receptor 1 splice forms might be present in the substantia nigra pars compacta. Each probe produced a very distinct labelling pattern in the rat brain with the exception of the 1g specific probe which produced only background signal. Substantia nigra pars compacta neurons were most intensely labelled by the metabotropic glutamate receptor 1d splice form specific probe. Metabotropic glutamate receptor 1a was expressed weakly whereas there was no detectable 1b, c, or g signal in the substantia nigra pars compacta. These data demonstrate that metabotropic glutamate receptor 1 protein is present within the perikarya and processes of dopaminergic neurons in the substantia nigra pars compacta. The majority of this protein is not the 1a splice form, which is abundant in other brain regions, and may be the 1d isoform. Since splicing alters the carboxy terminus of the receptor, it is likely to affect the interaction of the receptor with intracellular signalling systems.

Functional coupling of human metabotropic glutamate receptor hmGlu1d: comparison to splice variants hmGlu1a and -1b

Functional coupling of the human mGlu1 splice variants was examined by heterologous expression. In cells stably (CHO) or transiently (A9) expressing the hmGlu1d receptor. agonists elevated intracellular calcium with a rank order of potency typical of a group I mGlu receptor (quisqualate > L-glutamate > (S)-dihydroxyphenylglycine > (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD)). These responses were reduced by the antagonist (+)-alpha-methyl-4-carboxyphenylglycine (MCPG), by pretreatment with pertussis toxin and phorbol ester, and by removal of extracellular calcium. In transiently transfected HEK293 cells, the hmGlu1b and -1d receptors increased inositol monophosphate (IP) production only in the presence of glutamate, whereas hmGlu1a coupled even in the absence of agonist. This was not due to differences in receptor expression levels as assessed by immunoblotting. Adenylate cyclase activity in HEK293 cells expressing the hmGlu1 variants was neither stimulated nor inhibited by glutamate. In A9 cells hmGlu1a-mediated calcium/fluo-3 fluorescence was sensitive to depletion of intracellular calcium stores by thapsigargin, but the hmGlu1d response was resistant. Thus, hmGlu1d receptors can be distinguished from hmGlu1a by their lack of agonist-independent coupling and their dependence on extracellular calcium.

A cluster of basic residues in the carboxyl-terminal tail of the short metabotropic glutamate receptor 1 variants impairs their coupling to phospholipase C

Among phospholipase C-coupled metabotropic glutamate receptors (mGluRs), some have a surprisingly long carboxyl-terminal intracellular domain (mGluR1a, -5a, and -5b), and others have a short one (mGluR1b, -1c, and -1d). All mGluR1 sequences are identical up to 46 residues following the 7th transmembrane domain, followed by 313, 20, 11, and 26 specific residues in mGluR1a, mGluR1b, mGluR1c, and mGluR1d, respectively. Several functional differences have been described between the long isoforms (mGluR1a, -5a, and -5b) and the short ones (mGluR1b, -1c, and -1d). Compared with the long receptors, the short ones induce slower increases in intracellular Ca2+, are activated by higher concentration of agonists, and do not exhibit constitutive, agonist-independent activity. To identify the residues responsible for these functional properties, a series of truncated, chimeric, and mutated receptors were constructed. We found that the deletion of the last 19 carboxyl-terminal residues in mGluR1c changed its properties into those of mGluR1a. Moreover, the exchange of the long carboxyl-terminal domain of mGluR5a with that of mGluR1c generated a chimeric receptor that possessed functional properties similar to those of mGluR1c. Mutagenesis of specific residues within the 19 carboxyl-terminal residues of mGluR1c revealed the importance of a cluster of 4 basic residues in defining the specific properties of this receptor. Since this cluster is part of the sequence common to all mGluR1 variants, we conclude that the long carboxyl-terminal domain of mGluR1a suppresses the inhibitory action of this sequence element.

The rat mGlu1d receptor splice variant shares functional properties with the other short isoforms of mGlu1 receptor

Three splice variants of the rat metabotropic glutamate receptor 1 (mGlu1a, 1b and 1c receptors) have been characterized so far. All have the same sequence up to the 46th residue following the 7th transmembrane domain, followed by different carboxyl-terminal tails. Whereas mGlu1b and mGlu1c receptors possess a short intracellular carboxyl-terminal tail, the mGlu1a receptor has a very long one. Compared to cells expressing mGlu1b or mGlu1c receptors, a higher agonist potency and basal phospholipase C activity were detected in cells expressing mGlu1a receptors. Another variant with a short carboxyl-terminal tail, the HmGlu1d receptor, has been recently isolated from human brain. Here we show that the mGlu1d receptor variant also exists in the rat. Like all rat mGlu1 receptor variants, the mGlu1d receptor activates phospholipase C upon stimulation with mGlu1 receptor agonists. Although the rank order of agonist potency is the same on mGlu1a and mGlu1d receptors, agonists are less potent in stimulating phospholipase C in mGlu1d receptor-expressing cells than in cells expressing mGlu1a receptors. Moreover, like the other short variants it has no significant constitutive activity. These results indicate that the mGlu1d receptor shares similar functional properties with the other short mGlu1 receptor splice variants, and further suggests that the long carboxyl-terminal tail of the mGlu1a receptor increases phospholipase C coupling efficacy.

Pharmacology and functions of metabotropic glutamate receptors

In the mid to late 1980s, studies were published that provided the first evidence for the existence of glutamate receptors that are not ligand-gated cation channels but are coupled to effector systems through GTP-binding proteins. Since those initial reports, tremendous progress has been made in characterizing these metabotropic glutamate receptors (mGluRs), including cloning and characterization of cDNA that encodes a family of eight mGluR subtypes, several of which have multiple splice variants. Also, tremendous progress has been made in developing new highly selective mGluR agonists and antagonists and toward determining the physiologic roles of the mGluRs in mammalian brain. These findings have exciting implications for drug development and suggest that the mGluRs provide a novel target for development of therepeutic agents that could have a significant impact on neuropharmacology.

Tachykinins alter inositol phosphate formation, but not cyclic AMP levels, in primary cultures of neonatal rat spinal neurons through activation of neurokinin receptors

The naturally occurring tachykinins, substance P, neurokinin A and neurokinin B, induce the formation of inositol phosphates or cAMP in a variety of tissues but their effects on neurons have not been resolved. We used primary cultures of neonatal rat spinal cord to determine whether neurokinin receptors mediate changes in these second messengers in spinal neurons. We found that substance P, neurokinin A and neurokinin B induced the formation of inositol phosphates in a concentration-dependent manner with similar potencies (EC50S: 3.6, 5.7 and 21.3 nM, respectively), but at concentrations tested (0.1-1.0 microM) these peptides had no effect on cAMP levels. All three tachykinins induced the formation of inositol phosphates predominately by activation of neurokinin1 receptors. CP-96,345 and WIN 51,708, neurokinin1 receptor antagonists, attenuated the response to substance P, neurokinin A and neurokinin B. GR 103,537, a neurokinin2 receptor antagonist, had no effect on the responses induced by any of the tachykinins. Furthermore, the selective neurokinin1 receptor agonist, GR-73632, induced the formation of inositol phosphates in a concentration-dependent manner, whereas the selective neurokinin2 receptor agonist, GR-64349, generated inositol phosphates only at the highest concentration tested (10 microM). Senktide, a neurokinin3 receptor agonist, did not induce the formation of inositol phosphates at any of the concentrations tested (0.01-10 microM). Inositol phosphate formation appeared to be due to a direct effect of the tachykinins on neuronal neurokinin1 receptors. These results suggest that biological responses in spinal neurons following activation of neurokinin1 receptors are mediated mainly by the hydrolysis of phosphoinositol 4,5-bisphosphate to form inositol 1,4,5-trisphosphate and diacylglycerol. It remains to be determined which of these second messengers mediates the increased neuronal excitability and depolarization that occurs in response to substance P.

The messenger RNAs encoding metabotropic glutamate receptor subtypes are expressed in different neuronal subpopulations of the rat suprachiasmatic nucleus

Glutamate is the principal transmitter of retinal projections to the rodent suprachiasmatic nucleus, a circadian clock synchronized with the light-dark cycle through the activation of glutamate receptors of the ionotropic type. In vitro, an intracellular mobilization of calcium can be induced by glutamate within cells of the suprachiasmatic nucleus maintained in a calcium-free medium, suggesting a participation of metabotropic glutamate receptors coupled to phospholipase C. Using in situ hybridization histochemistry, we examined the expression of messenger RNAs encoding the mGluR1 and mGluR5 subtypes of metabotropic glutamate receptors in the suprachiasmatic nucleus of the adult rat and during postnatal development. In the adult, mGluR1 was expressed in a small subset of neurons segregated caudally within the ventrolateral subdivision of the nucleus, while mGluR5 was mainly expressed in ventrolateral neurons within the middle third of the nucleus. Both subtypes were expressed in morphologically similar small cells, but mGluR5 was also solely expressed in a small population of larger neurons located at the dorsalmost aspect of the ventrolateral subdivision. In addition, with mGluR1 probe silver grain clusters exhibiting a grain density close but below the significant level were observed throughout the ventrolateral subdivision of the nucleus. At birth, mGluR1 and mGluR5 were similarly expressed throughout the caudal half of the nucleus. The expression of mGluR1 increased during early postnatal development and exhibited an adult pattern at postnatal day 21. The expression of mGluR5 increased from postnatal day 7 and reached the adult pattern at postnatal day 45. These observations suggest that each subtype of metabotropic glutamate receptor coupled to phospholipase C underlies specific roles within the rat suprachiasmatic nucleus during postnatal development and in the adult. In the adult, ionotropic and metabotropic receptors likely co-expressed within neuronal subsets located in the retinal terminal field may have interactive and/or additive effects on intracellular calcium concentration. Metabotropic receptors may thus participate in the mediation of photic information conveyed to a subset of neurons. During postnatal development, metabotropic receptors may play a role in the maturation of glutamatergic synapses associated with the retinal input.

An alternative splicing site modifies the carboxyl-terminal trans-membrane domains of the Na+/Ca2+ exchanger

The 6-kilobase (kb) cDNA of pTB11 clone and its 5' fragment of 3.7 kb encoding the canine heart Na+/Ca2+ exchanger (Nicoll, D.A., Longoni, S., and Philipson, K.D. (1990) Science 250, 562-565) were transiently expressed in 293 cells to investigate the role of the 3'-"untranslated" region. Both fragments yielded high levels of expressed protein that were well incorporated in the membranes. Cells expressing the 6-kb cDNA produced rearranged transcripts of smaller than expected size. A 120-kDa polypeptide was produced in cells expressing the modified exchanger, and Ca2+ uptake was higher in this type of transfected cells. A constant stretch of nucleotides located at the 3' end of the 6 kb cDNA was found to be connected, by alternative RNA splicing, to four different upstream sequence positions. The deduced hydrophobic sequence of the spliced-in exon could replace the IX or the XI trans-membrane domain of the exchanger protein in two spliced isoforms. The new exon sequence was not completely included in the pTB11 insert, i.e. these two products were artificially truncated. The RNA processing of these two alternative 5'-splicing sites also occurred in tissues, as shown by RNase protection analysis. In a third type of isoform the splicing took place downstream of the originally proposed stop codon, whereas in a fourth type a stop codon was introduced after the V hydrophobic segment in the large intracellular loop.

In vitro translation and membrane topology of rat recombinant mGluR1 alpha

The structure and post-translational processing of the metabotropic glutamate receptor 1 alpha (mGluR1 alpha) was analysed by in vitro cell-free translation, protease protection and deglycosylation. We show that mGluR1 alpha can be synthesized in the rabbit-reticulocyte translation system to yield a predominant polypeptide product with an apparent molecular weight of 142 kDa. In the presence of dog-pancreatic microsomes this polypeptide was processed to an apparent molecular weight of 147 kDa. Treatment with the enzyme peptide-N-glycosidase F (PNGF) demonstrated that the increase in the apparent molecular weight of the processed translation product was due to N-linked glycosylation. Addition of the non-selective protease, proteinase K; resulted in the loss of this 147 kDa band and the appearance of a protected fragment of approx 92 kDa. A carboxy-terminal deletion mutant of mGluR1 alpha was almost completely protected from protease action. These data show that the amino terminal of mGluR1 alpha is translocated into the lumen of the endoplasmic reticulum and will consequently be located extracellularly when targeted to the plasma membrane. The data presented here on mGluR1 alpha indicates the potential of in vitro translation and protease protection in the study of the molecular structure and processing of glutamate receptors.

Is the heterologous expression of metabotropic glutamate receptors (mGluRs) an appropriate method to study the mGluR function? Experience with human embryonic kidney 293 cells transfected with mGluR1

The cloning of metabotropic glutamate receptors (mgluRs) has initiated a new approach to the study of their function: the introduction of mGluR cDNA into cells that do not normally express mGluRs, thus allowing the heterologous receptor expression. We have transfected human embryonic kidney (HEK) 293 cells with the full length mGluR1a cDNA and with its truncated variant which encodes the receptor termed mGluR1T (a receptor lacking the long intracellular domain and similar to the splice variant mGluR1c). Transient transfection of HEK-293 cells with mGluR1a, but not the mGluR1T cDNA, resulted in a significant increase in inositol phosphate (IP) formation in absence of any mGluR agonists. This effect was completely dependent on the presence of extracellular calcium, and unlike the agonist-stimulated IP formation it was insensitive to pertussis toxin. The prolonged activation of IP formation might affect the cell physiology. In an attempt to obtain stably transfected cells, we transfected about 1.5 x 10(6) HEK-293 cells with the plasmid conveying the full-length mGluR1a cDNA and the neomycin-resistance gene. Only 12 clones survived the antibiotic selection, and only one of these 12 clones continued to divide. The size of mRNA from the clone was smaller than the full-length mGluR1a mRNA. The shortened mRNA, revealed in the clone, apparently encoded a functional mGluR that was sensitive to glutamate, but unlike the mGluR1a, it did not respond to 1S,3R-ACPD (1S,3R-aminocyclopentane-1,3-dicarboxylic acid). A prudent use of the heterologous cell transfection technique is necessary in studying the function and the pharmacology of mGluRs.

Characterization of two alternatively spliced forms of a metabotropic glutamate receptor in the central nervous system of the rat

Amplification of complementary DNA by the polymerase chain reaction and anti-peptide antibodies were used to characterize the expression of two alternatively spliced forms of a metabotropic glutamate receptor (mGluR1 alpha and mGluR1 beta) in the central nervous system of the rat. Polymerase chain reaction analysis showed that mGluR1 alpha was the predominate of the two forms in the cerebellum, diencephalon, mesencephalon, olfactory bulb and brainstem, while mGluR1 beta was the major form present in the hippocampus. Approximately equal amounts of the two receptors were expressed in the cerebral cortex, septum and striatum. Immunochemical analyses of the two receptors were conducted in the rat cerebellum and hippocampus. An mGluR1 alpha-specific antibody labelled a protein with a relative molecular weight of 146,000 on immunoblots of the hippocampus and cerebellum. Immunoblot analysis of the developmental expression of mGluR1 alpha in the hippocampus and cerebellum demonstrated that in both structures, the levels of mGluR1 alpha were at or near their maximum levels in the adult brain. In contrast, two mGluR1 beta-specific antibodies failed to detect mGluR1 beta on immunoblots of brain tissue, thus precluding an immunocytochemical analysis of this receptor. Although low levels of a higher-molecular weight protein, possibly a dimeric form of mGluR1 beta were seen with one of the mGluR1 beta-specific antibodies, we hypothesize that some of the mGluR1 beta present in brain tissue may undergo proteolytic cleavage of the carboxy terminus. Immunocytochemical analysis of mGluR1 alpha showed that very high levels of this receptor were expressed in Purkinje cell bodies and dendrites. In the granule cell layer, some Golgi neurons were immunostained. The granule cells were not labelled. In the hippocampus, mGluR1 alpha immunoreactivity was present in interneurons of the stratum oriens and the dentate hilar region. Double-labelling studies demonstrated that these interneurons were also immunopositive for the neuropeptide somatostatin. The presence of mGluR1 alpha in cells of the hippocampus that are associated with the release of somatostatin, suggest that this receptor could play a role in regulating hippocampal excitability in both normal and epileptic tissues.