Polygenic risk for schizophrenia converges on alternative polyadenylation as molecular mechanism underlying synaptic impairment

Schizophrenia (SCZ) is a genetically heterogenous psychiatric disorder of highly polygenic nature. Correlative evidence from genetic studies indicate that the aggregated effects of distinct genetic risk factor combinations found in each patient converge onto common molecular mechanisms. To prove this on a functional level, we employed a reductionistic cellular model system for polygenic risk by differentiating induced pluripotent stem cells (iPSCs) from 104 individuals with high polygenic risk load and controls into cortical glutamatergic neurons (iNs). Multi-omics profiling identified widespread differences in alternative polyadenylation (APA) in the 3' untranslated region of many synaptic transcripts between iNs from SCZ patients and healthy donors. On the cellular level, 3'APA was associated with a reduction in synaptic density of iNs. Importantly, differential APA was largely conserved between postmortem human prefrontal cortex from SCZ patients and healthy donors, and strongly enriched for transcripts related to synapse biology. 3'APA was highly correlated with SCZ polygenic risk and affected genes were significantly enriched for SCZ associated common genetic variation. Integrative functional genomic analysis identified the RNA binding protein and SCZ GWAS risk gene PTBP2 as a critical trans-acting factor mediating 3'APA of synaptic genes in SCZ subjects. Functional characterization of PTBP2 in iNs confirmed its key role in 3'APA of synaptic transcripts and regulation of synapse density. Jointly, our findings show that the aggregated effects of polygenic risk converge on 3'APA as one common molecular mechanism that underlies synaptic impairments in SCZ.

Increased expression of LGI1 gene triggers growth inhibition and apoptosis of neuroblastoma cells

The LGI1 gene has been implicated in the malignant progression of glioblastoma and it has also been genetically linked to a form of partial epilepsy (ADLTE). In this study, we investigated the relevance of LGI1 expression for neuroblastoma cells. The analysis of two cell lines (SH-SY5Y and SK-N-BE) revealed unpredictably low levels of LGI1 and stable cell transfection with LGI1 cDNA yielded moderate increases of LGI1 expression. Neuroblastoma cell clones exhibited impaired cell growth and survival ability in relation to LGI1 levels. The process of growth inhibition could be discerned under experimental conditions of low cell density, since conditions of elevated cell density, which enhance the requirement for survival stimuli, resulted in massive cellular death. At high cell density, spontaneous apoptosis of LGI1 cells was clearly shown by the release of cytochrome c and apoptosis inducing factor (AIF) from mitochondria and by phosphatydil serine exposure and nuclear fragmentation. Activation of apoptotic effectors caspase-3/7 also occurred, however, the broad caspase inhibitor Z-VAD-FMK substantially failed to block cell death. Thus the possibility that LGI1-triggered apoptosis may involve initiator caspases linked to activation of death receptors, appears unlikely. The decreased ratio of Bcl-2 to Bax suggests that apoptosis is initiated by the intrinsic mitochondrial pathway through the release of caspase-dependent and -independent apoptogenic molecules. This study provides the first evidence that LGI1 controls neuronal cell survival, suggesting its role in the development of the nervous system in relation to the pathogenesis of neuroblastoma and ADLTE.

Downstream regulatory element antagonist modulator regulates Ca2+ homeostasis and viability in cerebellar neurons

The Na+/Ca2+ exchangers NCX1, NCX2, and NCX3 are vital for the control of cellular Ca2+ homeostasis. Here, we show that a doublet of downstream regulatory element sites in the promoter of the NCX3 gene mediates transcriptional repression of NCX3 by the Ca2+-modulated transcriptional repressor downstream regulatory element antagonist modulator (DREAM). Overexpression of a DREAM EF-hand mutant insensitive to Ca2+ (EFmDREAM) in hippocampus and cerebellum of transgenic mice significantly reduced NCX3 mRNA and protein levels without modifying NCX1 and NCX2 expression. Cerebellar granules from EFmDREAM transgenic mice showed increased levels of cytosolic Ca2+ and were more vulnerable to increased Ca2+ influx after partial opening of voltage-gated plasma membrane Ca2+ channels induced by increasing K+ in the culture medium but survived better in the conditions of reduced Ca2+ influx prevailing in low extracellular K+. Overexpression of NCX3 in EFmDREAM transgenic granules using a lentiviral vector restored the normal survival response to high K+ observed in wild-type granules. Thus, the downregulation of the regulator of Ca2+ homeostasis NCX3 by Ca2+-regulated DREAM is a striking example of the autoregulatory property of the Ca2+ signal in neurons.

Transcriptional regulation by cAMP and Ca2+ links the Na+/Ca2+ exchanger 3 to memory and sensory pathways

The signaling cascades triggered by neurotrophins such as BDNF and by several neurotransmitters and hormones lead to the rapid induction of gene transcription by increasing the intracellular concentration of cAMP and Ca2+. This review examines the mechanisms by which these second messengers control transcriptional initiation at CRE promoters via transcription factor CREB, as well as at DRE sites via transcriptional repressor DREAM. The regulation of the SLC8A3 gene encoding the Na+/Ca2+ exchanger 3 (NCX3) is taken as an example to illustrate both mechanisms since it includes a CRE site in the promoter and several DRE sites in the exon 1 sequence. The upregulation of the NCX3 by Ca2+ signals may be specifically required to establish the Ca2+ balance that regulates several physiological and pathological processes in neurons. The regulatory features and the expression pattern of SLC8A3 gene suggest that NCX3 activity could be crucial in neuronal functions such as memory formation and sensory processing.

Photosensitization of DNA strand breaks by three phenothiazine derivatives

The interaction and the photosensitizing activity of three phenothiazine derivatives, fluphenazine hydrochloride (FP), thioridazine hydrochloride (TR), and perphenazine (PP), toward DNA were studied. Evidences obtained from various spectroscopic studies such as fluorimetric and linear dichroism measurements indicate that these derivatives bind to the DNA at least in two ways: intercalation and external stacking on the DNA helix, depending on their relative concentrations. Irradiation of supercoiled plasmid DNA in the presence of these phenothiazines leads to single strand breaks. The DNA photocleavage appears to be due to externally bound molecules rather than to those intercalated. The highest photocleavage activity was observed with PP and TR whereas FP was less efficient. The efficiency of the photocleavage in aerated and deaerated solutions does not change thus indicating that an involvement of singlet oxygen can be excluded. Primer extension analysis of plasmid DNA irradiated in the presence of phenothiazines indicates that photocleavage of DNA occurs predominantly at Gua and Cyt residues. Laser flash experiments carried out in the presence of 2'-deoxyguanosine 5'-monophosphate reveal an efficient electron transfer between the nucleotide and the radical cations produced by photoionization of the phenothiazines. In the presence of DNA, an electron transfer process takes place within the laser pulse from the lowest singlet state of phenothiazines to the DNA bases; the time-resolved measurements showed that the back-electron transfer is a negligible decay pathway for the charged species.

Control of the Na+/Ca2+ exchanger 3 promoter by cyclic adenosine monophosphate and Ca2+ in differentiating neurons

The human gene for member 3 of solute carrier family 8 (SLC8A3), encoding the Na+/Ca2+ exchanger isoform 3 (NCX3), was identified on chromosome 14q24.2. The minimal promoter region was predicted 250 bp upstream of exon 1. This was confirmed by luciferase reporter assays of pGL3-promoter constructs in transfected SH-SY5Y cells. The promoter activity was monitored during the differentiation of this cell line elicited by the sequential treatment with retinoic acid and brain-derived neurotrophic factor (BDNF). The activity was induced by cyclic AMP (cAMP) via the CRE (cAMP response element) and was stimulated by retinoic acid. The increase of intracellular Ca2+ induced by the partial depolarization of the plasma membrane with KCl down-regulated both the basal and the cAMP-stimulated transcription. The down-regulation of the latter may be mediated by the phosphorylation of the CRE-binding protein by a calmodulin-dependent kinase (CaMKII). The exposure of cells to BDNF after treatment with retinoic acid rapidly induced promoter activity during the initial five hours and phosphorylation of CRE-binding protein during the first two hours. The promoter activity was further enhanced by cAMP, but became insensitive to Ca2+. In BDNF-stimulated cells cAMP elevation caused the preferential phosphorylation of ATF1 instead of that of CRE-binding protein.

Naphthoquinolizinium derivatives as a novel platform for DNA-binding and DNA-photodamaging chromophores

The association of the naphtho[1,2-b]quinolizinium bromide (5a) and naphtho[2,1-b]quinolizinium bromide (5b) with DNA and the propensity of these cationic arenes to damage DNA after UV-A irradiation have been studied. Spectrophotometric and fluorimetric titrations show that the two isomers 5a and 5b bind to DNA (K approximately 10(5) M(-1)). The highest affinity was observed for GC base pairs. The mode of binding was investigated by CD and LD spectroscopy. Whereas quinolizinium 5a exclusively intercalates in DNA, the isomer 5b exhibits a deviation from perfect intercalation into the double helix. Moreover, efficient DNA damage was observed on UV-A irradiation in the presence of the quinolizinium salts. Primer extension analysis indicates that the photocleavage takes place preferentially at guanine-rich regions.

The human SLC8A3 gene and the tissue-specific Na+/Ca2+ exchanger 3 isoforms

We have identified the human gene for member 3 of Solute Carrier family 8 (SLC8A3) by bioinformatic analysis of human genomic sequences. The gene is located on chromosome 14q24.2, and spans a region of about 150 kb. The full-length DNA complementary to RNA encoding the Na(+)/Ca(2+) exchanger isoform 3 (NCX3), amplified by reverse transcriptase-polymerase chain reaction (RT-PCR) from the human neuroblastoma SH-SY5Y RNA, includes seven exons and encodes a protein of about 100 kDa. RT-PCR analysis was performed in different tissues to determine the exon composition in the region encoding the large intracellular loop of the protein. The region underwent modifications by alternative tissue-specific splicing. NCX3.2, including exon 4 but not exon 5, was found in human brain and in the neuroblastoma cell line. In human skeletal muscle two additional isoforms were identified: NCX3.3, including exons 4 and 5, and a truncated isoform (NCX3.4) produced by the skipping of both exons 3 and 4. The skipping causes a frame shift downstream of the exon 2 sequence. The new coding sequence of 25 amino acids terminates with a stop codon in exon 6. The NCX3.4 isoform (68 kDa) is truncated in the C-terminal portion of the domain first found in Drosophila Na(+)/Ca(2+) exchanger domain (Calxbeta) and lacks the C-terminal hydrophobic segments.

Indolo[2,3-b]-quinolizinium bromide: an efficient intercalator with DNA-photodamaging properties

The associative interactions of indolo[2,3-b]-quinolizinium bromide with DNA and its DNA photocleavage properties were studied in detail. Absorption and emission spectroscopy, linear dichroism, and energy-transfer measurements indicate that the indoloquinolizinium binds to DNA primarily by intercalation, with a preference for GC base pairs. In agreement with this data, the results of primer extension analysis indicate that photocleavage occurrs prevalently at the GC nucleotides. Molecular modeling studies confirm that intercalative stacking between adjacent base pairs is energetically favorable. However, it is also observed that the location of the dye in the minor groove of the DNA is energetically even more favorable. Upon UVA irradiation, the indoloquinolizinium causes single-strand cleavage with an efficiency that varies with the dye-DNA ratio. This observation is rationalized in terms of more efficient photocleavage by the externally bound dye compared with the intercalated one. The kinetics of strand degradation under aerobic and anaerobic conditions suggest that a Type I reaction occurs, that is, radical-mediated DNA damage.

Acridizinium salts as a novel class of DNA-binding and site-selective DNA-photodamaging chromophores

It was demonstrated that the interaction of the aminoacridizinium salts 2a-2d with DNA depends on the substitution pattern of the chromophore. Spectrophotometric and fluorometric titrations of the acridizinium salts 2a-2d with natural and synthetic polynucleotides reveal that the degree of interaction of the acridizinium salts 2a-2d with the nucleic acid differs significantly. The binding mode of the dyes with DNA was evaluated by circular dichroism and linear dichroism spectroscopy and compared with the parent system 2c. Whereas the 9-aminoacridizinium (2a) mainly intercalates into DNA, the salts 2b-c show a higher degree of association to the DNA backbone. The intercalated aminoacridizinium 2a caused few strand breaks upon UVA exposure, whereas the salts 2b-2d exhibit relatively efficient DNA-damaging properties. All acridizinium salts showed a sequence-selective strand cleavage for guanine-rich DNA regions.

A polymorphic GT repeat from the human cardiac Na+Ca2+ exchanger intron 2 activates splicing

The sequence analysis of the human intron 2 from the Na+/Ca2+ exchanger 1 (NCX1) gene has revealed a GT repeat of variable length (10-16). The 5' sequence of intron 2 exhibited significant homology (65-70%) with other minisatellite sequences. DNA segments at the 5' end of intron 2 were inserted in the NCX1 cDNA (3.7 kb) to reconstruct the exon 2/intron 2 junction. Transient expression of these constructs in HEK293 cells generated shortened mRNAs ( approximately 2.5 kb). RT-PCR and ribonuclease protection analysis of the 3' end of the short transcripts indicated a splicing event at the intron 2/exon 2 junction (5' site) and in the vector sequence downstream of the NCX1 insert (3' site). Molecular dissection of the 5'-intron 2 sequence showed that the GT repeat was required for splicing activation, whereas the remainder of the 5'-intron 2 segment was completely inactive. The results indicate that the GT repeat is a strong intronic splicing enhancer that could be involved in the regulation of NCX1 expression, possibly mediating tissue-specific alternative splicing of the mutually exclusive exons 3 and 4, and/or exon-2 circularization.

Electrophysiological characterization of ionic transport by the retinal exchanger expressed in human embryonic kidney cells

The retinal Na+:Ca2+, K+exchanger cDNA was transiently expressed in human embryonic kidney (HEK 293) cells by transfection with plasmid DNA. The correct targeting of the expressed protein to the plasma membrane was confirmed by immunocytochemistry. The reverse exchange offrent (Ca2+ imported per Na+ extruded) was measured in whole-cell voltage-clamp experiments after intracellular perfusion with Na+ (Na+i, 128 mM) and extracellular perfusion with Ca2+ (Ca2o+, 1 mM) and Ko+ (20 mM). As expected, the exchange current was suppressed by removing Ca2o+. Surprisingly, however, it was also abolished by increasing Na+o to almost abolish the Na+ gradient, and it was almost unaffected by the removal of Ko+. Apparently, then, at variance with the exchanger in the rod outer segment, the retinal exchanger expressed in 293 cells acts essentially as a Na+:Ca2+ exchanger and does not require K+ for its electrogenic activity.

Expression of an active Na+/Ca2+ exchanger isoform lacking the six C-terminal transmembrane segments

The short isoform of the Na+/Ca2+ exchanger (67 kDa) that is produced by alternative splicing during the expression of the 6 kb canine exchanger cDNA in 293 cells was separately expressed in the same system. The protein consisted of the five N-terminal transmembrane segments and of a large portion of the main hydrophilic loop, but lacked the six C-terminal hydrophobic segments of the regular protein (108 kDa). Very high RNA levels were found after transient cell transfection with plasmid DNA encoding this truncated isoform. The RNA processing, the translation and targeting of the resulting protein to the plasma membrane appeared to be less efficient than those of the 108-kDa polypeptide produced in the same system. The Na(+)-dependent Ca(2+)-uptake activity of 293 cells expressing the short isoform was measured by an isotopic rapid filtration method, whereas the current associated with Ca2+ extrusion was measured in electrophysiological patch-clamp experiments. The results showed that the expressed isoform functioned in the typical reverse and forward Na+/Ca2+ exchange modes. In both the electrophysiological and the isotopic measurements the activity of the short isoform was 6-7-fold lower than that of the 108-kDa protein expressed in the same system. However, lower amounts of the short isoform reached the plasma membrane: its specific activity could thus be significantly higher. Possibly, the short isoform could form a dimer in which a second 67 kDa polypeptide replaces the C-terminal part of the 108-kDa protein.

Expression of Na(+)-Ca2+ exchanger with modified C-terminal hydrophobic domains and enhanced activity

A 6-Kb canine cDNA fragment complementary to the 5' region of the 7-Kb mRNA encoding the cardiac Na+-Ca2+ exchanger was expressed in human kidney 293 cells. The mRNA products were reverse transcribed and amplified by PCR. The determined DNA sequence of the amplified DNA fragments revealed the presence of an intron that was alternatively spliced. The partial exon sequence, located at the 3' end of the 6-Kb cDNA, was alternatively connected to bases 3198, 2821, 2620 and 1844 in four types of splicing products identified. In the largest product the adjoining exon was located after the putative stop codon of the regular sequence. In a second and third type of shortened transcripts, a hydrophobic sequence encoded by the spliced-in exon was linked with the 4th or the 5th extracellular loops, and could possibly replace transmembrane segments 9 or 11. In the fourth type of spliced transcript the in-frame exon sequence introduced one Leu followed by a stop codon in the large hydrophilic loop. Measurements of Ca2+ uptake in 293 cells expressing the modified exchanger indicated a higher activity in comparison with 293 cells expressing the 3.7-Kb cDNA, in which this alternative splicing does not occur. Deletion mutagenesis of the C-terminal region encoded by the spliced-in exon was performed to investigate its role in the enhancement of the transport activity.

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.

Nerve growth factor production by lymphocytes

Nerve growth factor (NGF) is a neurotrophic protein essential for the maintenance and growth of peripheral sympathetic neurons and basal forebrain cholinergic neurons. Recently, NGF has also been shown to have effects on cells of the immune system. In a search for extra neural sources of NGF, we detected NGF-specific mRNA in mouse T lymphocytes of both the CD4+ and CD8+ phenotypes with the use of an RNase protection assay, PCR, and DNA sequence analysis. In CD4+ cells, NGF was present in both Th1 and Th2 Ag-specific clones, but an increase of NGF-specific message was detected after antigenic stimulation only in Th2 clones. NGF mRNA was also detected in splenic B lymphocytes and in a cell line derived from a murine follicular center cell lymphoma. Translation into protein and secretion of NGF were demonstrated by Western blot analysis. The secreted NGF is in an active form capable of inducing differentiation of PC12 cells into sympathetic-like neurons. Furthermore, conditioned medium from clones or lines positive for NGF mRNA was capable of inducing p140 tyrosine kinase autophosphorylation in 3T3 fibroblasts transfected with cDNA encoding for the tyrosine kinase family NGF receptor. We conclude that lymphocytes synthesize and secrete NGF either as a para-autocrine factor acting on the immune system itself, or as a factor for the maintenance of peripheral neurons.

Nerve growth factor production by lymphocytes

Nerve growth factor (NGF) is a neurotrophic protein essential for the maintenance and growth of peripheral sympathetic neurons and basal forebrain cholinergic neurons. Recently, NGF has also been shown to have effects on cells of the immune system. In a search for extra neural sources of NGF, we detected NGF-specific mRNA in mouse T lymphocytes of both the CD4+ and CD8+ phenotypes with the use of an RNase protection assay, PCR, and DNA sequence analysis. In CD4+ cells, NGF was present in both Th1 and Th2 Ag-specific clones, but an increase of NGF-specific message was detected after antigenic stimulation only in Th2 clones. NGF mRNA was also detected in splenic B lymphocytes and in a cell line derived from a murine follicular center cell lymphoma. Translation into protein and secretion of NGF were demonstrated by Western blot analysis. The secreted NGF is in an active form capable of inducing differentiation of PC12 cells into sympathetic-like neurons. Furthermore, conditioned medium from clones or lines positive for NGF mRNA was capable of inducing p140 tyrosine kinase autophosphorylation in 3T3 fibroblasts transfected with cDNA encoding for the tyrosine kinase family NGF receptor. We conclude that lymphocytes synthesize and secrete NGF either as a para-autocrine factor acting on the immune system itself, or as a factor for the maintenance of peripheral neurons.

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.

Nerve growth factor does not influence the expression of beta amyloid precursor protein mRNA in rat brain: in vivo and in vitro studies

We investigated the effect of NGF on amyloid precursor protein (APP) mRNA levels in the rat septal/nucleus basalis system. Total APP mRNA and APP 695 mRNA were determined in basal forebrain primary cell cultures exposed acutely and chronically to NGF (150-300 ng/ml) and, in vivo, in the septal area and striatum of rat pups after multiple intracerebroventricular injections of NGF. The trophic factor was able to affect cholinergic neurons in both paradigms, as evidenced by the significant increase of choline acetyltransferase (ChAT) activity induced by NGF in cell cultures (+80%) and in the striatum (+240%) of rat pups. In spite of this effect, no significant change of APP mRNA expression was observed in neuronal cultures and brain tissues. These data indicate that the neurotrophic effect of NGF on forebrain cholinergic neurons is not always associated with an alteration of APP expression.

Trans-azetidine-2,4-dicarboxylic acid activates neuronal metabotropic receptors

The expression of metabotropic glutamate receptors (mGluRs) in primary cultures of cerebellar granule neurones can be: (i) modulated by the degree of depolarization during the culture period, rendering neurones differently sensitive to agonist-stimulated inositol phosphate (IP) hydrolysis; (ii) down-regulated by specific mGluR agonists. In this culture the new rigid glutamate analogue, (+/-)-trans-azetidine-2,4-dicarboxylic acid (t-ADA) and the known mGluR agonist 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) stimulated IP formation in line with the depolarization-modified expression of mGluR1. However, the two compounds caused different patterns of mGluR down-regulation. The effects of t-ADA and 1S,3R-ACPD were also tested on transformed human embryonic kidney 293 cells transfected with mGluR1. Only 1S,3R-ACPD, but not t-ADA, stimulated IP hydrolysis, suggesting that t-ADA acts on a subtype of metabotropic receptors different from mGluR1. Hence, t-ADA might prove useful in differentiating the function of various mGluR subtypes.

Polyamines modulate the function of transfected glutamate receptor mGluR1a

Metabotropic glutamate receptor mGluR1a was expressed in human embryonic kidney 293 cells. 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) stimulated dose-dependently, phosphoinositide (PI) hydrolysis in transfected, but not in non-transfected cells. The polyamine spermine did not affect PI hydrolysis in the absence of 1S,3R-ACPD even at a concentration of 1 mM, but it potentiated the stimulatory action of 1S,3R-ACPD at 10 microM. The modulatory action of spermine was mimicked by spermidine but not by the short polyamine putrescine.

Functional evidence for a L-AP3-sensitive metabotropic receptor different from glutamate metabotropic receptor mGluR1

The efficacy of mGluR agonists quisqualate and 1S,3R-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) in stimulating the inositol phosphate (IP) formation in primary cultures of cerebellar granule neurons correlated with mGluR1 mRNA expression and was affected by the medium KCl content. L-2-Amino-3-phosphonopropionic acid (L-AP3) mimicked the stimulatory action of mGluR agonists. Maximal stimulatory doses of mGluR agonist 1S,3R-ACPD and L-AP3 were additive, suggesting the action of L-AP3 on a receptor different from mGluR1. Indeed, in embryonic kidney 293 cells transfected with mGluR1 cDNA quisqualate and 1S,3R-ACPD but not L-AP3 stimulated the IP formation.

Carboxyl domain of glutamate receptor directs its coupling to metabolic pathways

Of the six metabotropic glutamate receptors (mGluRs) only mGluR1 and mGluR5, which possess a large carboxyl terminal domain, are positively linked to phosphoinositide (PI) hydrolysis. We expressed a 3' deletion of mGluR1 alpha (mGluR1T) lacking the terminal 290 codons and the full length mGluR1 alpha cDNAs in human embryonic kidney 293 cells. Agonist stimulation of both mGluR1 alpha and mGluR1T stimulated PI hydrolysis. Glutamate activation of PI hydrolysis was reduced by pertussis toxin when mediated via mGluR1 alpha, while mGluR1T required the presence of extracellular Ca2+. Glutamate-mediated reduction of adenylyl cyclase stimulation by forskolin occurred only in mGluR1T-expressing cells. The results suggest that the carboxyl terminal extension directs the coupling of mGluR1 with different signal transduction pathways.

Nerve growth factor transcriptional control of c-fos promoter transfected in cultured spinal sensory neurons

High efficiency gene transfer (greater than 90%) in chicken dorsal root ganglion neurons has been obtained by DNA calcium phosphate co-precipitation, hence providing an important tool to study control of gene expression in primary neurons. Transfection with c-fos promoter sequences linked to the chloramphenicol acetyltransferase reporter gene showed that the serum responsive element functions as a strong transcriptional enhancer. Transcription from this element is developmentally regulated, and mediates the genetic response to nerve growth factor (NGF) in developing avian sensory neurons. Furthermore, NGF exerts a negative effect on transcription from the cyclic AMP responsive element, thereby supporting the involvement of tyrosine kinase activation by NGF in primary sensory neurons.

Gangliosides prevent the inhibition by K-252a of NGF responses in PC12 cells

K-252a, a general kinase inhibitor, selectively blocks the actions of nerve growth factor (NGF) in PC12 cells. Since gangliosides have been reported to modulate neuronal cell responsiveness to NGF and to regulate several protein kinases, the ability of these compounds to reverse the inhibition by K-252a was tested. Parameters at both short- and long-term times following treatment of PC12 cells with NGF were analyzed which are known to be either transcription-dependent or -independent events. Gangliosides were found to completely prevent the inhibition by K-252a of NGF-induced neurite regeneration and c-fos induction, and partially also that of protein kinase N activation. The ganglioside protective effects were concentration-dependent and required the intact molecule. These findings raise the possibility that gangliosides might affect a specific pathway of NGF responses sensitive to inhibition by K-252a.

Lack of coupling of D-2 receptors to adenylate cyclase in GH-3 cells exposed to epidermal growth factor. Possible role of a differential expression of Gi protein subtypes

Exposure of GH-3 cells to epidermal growth factor for 4 consecutive days induced the expression of both D-2(415) and D-2(444) dopamine-receptor isoforms. Epidermal growth factor also promoted a remarkable increase in the content of Gi3 protein, which is responsible for receptor-induced activation of potassium channels in GH-3 cells. D-2 receptors in this model apparently activate a specific transducing pathway, leading to opening of potassium channels and inhibition of prolactin release by cAMP-independent mechanisms. This is shown by: 1) the selective D-2 agonist quinpirole, while inactive on vasoactive intestinal peptide-induced prolactin release, strongly inhibited the hormone secretion induced by neurotensin; 2) quinpirole, up to 100 microM, did not inhibit cAMP production evoked by vasoactive intestinal peptide both in intact cells and in broken cell membrane preparations; and 3) quinpirole and other D-2 agonists strongly potentiated Rb+ efflux when measured in a nominally calcium-free reaction solution containing 100 mM potassium (voltage-dependent component), but did not modify Rb+ efflux if measured in a reaction solution containing 1 mM calcium and 5 mM potassium (calcium-activated, cAMP-dependent component).

Differences in induction of c-fos transcription by cholera toxin-derived cyclic AMP and Ca2+ signals in astrocytes and 3T3 fibroblasts

The B subunit of cholera toxin, a protein which binds specifically to membrane ganglioside GM1, is known to affect cell growth and differentiation. To investigate the mechanism of these cellular responses at the nuclear level, we used the induction of c-fos in astrocytes and 3T3 fibroblasts as a model. Northern blot analysis showed that treatment with B subunit provokes a rapid and transient expression of c-fos mRNA, independent of a measurable increase in cyclic AMP. The B subunit signal, which is mediated by Ca2+, was compared to cholera toxin and other agents which increase intracellular cyclic AMP levels. In transient transfection assays of astrocytes and fibroblasts, functional analysis of c-fos promoter deletions was used to identify the elements involved in transcriptional activation by B subunit. In astrocytes, the DNA region including the serum response element and the cyclic AMP response element (CRE) are equally required, whereas 3T3 cells require only the CRE for maximal induction. A synergistic effect of signal transduction was mediated by calcium and cyclic AMP on the CRE, being positive in 3T3 cells and negative in astrocytes. Diverse regulatory elements may be thus involved in responses of different cell types to the same extracellular signal. Furthermore, a single regulatory element (CRE) can integrate both calcium and cyclic AMP signals in the control of gene expression.

Kinetics of photosynthetic electron transfer in artificial vesicles reconstituted with purified complexes from Rhodobacter capsulatus. I. The interaction of cytochrome c2 with the reaction center

1. The kinetics of the interaction of cytochrome c2 and photosynthetic reaction centers purified from Rhodobacter capsulatus were studied in proteoliposomes reconstituted with a mixture of phospholipids simulating the native membrane (i.e. containing 25% L-alpha-phosphatidylglycerol). 2. At low ionic strength, the kinetics of cytochrome-c2 oxidation induced by a single turnover flash was very different, depending on the concentration of cytochrome c2: at concentrations lower than 1 microM, the process was strictly bimolecular (second-order rate constant, k = 1.7 x 10(9) M-1 s-1), while at higher concentrations a fast oxidation process (half-time lower than 20 microseconds) became increasingly dominant and encompassed the total process at a cytochrome c2 concentration around 10 microM. From the concentration dependence of the amplitude of this fast phase an association constant for a reaction-center--cytochrome-c2 complex of about 10(5) M-1 was evaluated. From the fraction of photo-oxidized reaction centers promptly re-reduced in the presence of saturating concentrations of externally added cytochrome c2, it was found that in approximately 60% of the centers the cytochrome-c2 site was exposed to the external compartment. 3. Both the second-order oxidation reaction and the formation of the reaction-center--cytochrome-c2 complex were very sensitive to ionic strength. In the presence of 180 mM KCl, the value of the second-order rate constant was decreased to 7.0 x 10(7) M-1 s-1 and no fast oxidation of cytochrome c2 could be observed at 10 microM cytochrome c2. 4. The kinetics of exchange of oxidized cytochrome c2 bound to the reaction center with the reduced form of the same carrier, following a single turnover flash, was studied in double-flash experiments, varying the dark time between photoactivations over the range 30 microseconds to 5ms. The experimental results were analyzed according to aminimal kinetic model relating the amounts of oxidized cytochrome c2 and reaction centers observable after the second flash to the dark time between flashes. This model included the rate constants for the electron transfer between the primary and secondary ubiquinone acceptors of the complex (k1) and for the exchange of cytochrome c2 (k2). Fitting to the experimental results indicated a value of k1 equal to 2.4 x 10(3) s-1 and a lower limit for k2 of approximately 2 x 10(4) s-1 (corresponding to a second-order rate constant of approximately 3 x 10(9) M-1 s-1).

Kinetics of photosynthetic electron transfer in artificial vesicles reconstituted with purified complexes from Rhodobacter capsulatus. II. Direct electron transfer between the reaction center and the bc1 complex and role of cytochrome c2

1. The cyclic photosynthetic chain of Rhodobacter capsulatus has been reconstituted incorporating into phospholipid liposomes containing ubiquinone-10 two multiprotein complexes: the reaction center and the ubiquinol-cytochrome-c2 reductase (or bc1 complex). 2. In the presence of cytochrome c2 added externally, at concentrations in the range 10-10(4) nM, a flash-induced cyclic electron transfer can be observed. In the presence of antimycin, an inhibitor of the quinone-reducing site of the bc1 complex, the reduction of cytochrome b561 is a consequence of the donation of electrons to the photo-oxidized reaction center. At low ionic strength (10 mM KCl) and at concentrations of cytochrome c2 lower than 1 microM, the rate of this reaction is limited by the concentration of cytochrome c2. At higher concentrations the reduction rate of cytochrome b561 is controlled by the concentration of quinol in the membrane, and, therefore, is increased when the ubiquinone pool is progressively reduced. At saturating concentrations of cytochrome c2 and optimal redox poise, the half-time for cytochrome b561 reduction is about 3 ms. 3. At high ionic stength (200 mM KCl), tenfold higher concentrations of cytochrome c2 are required for promoting equivalent rates of cytochrome-b561 reduction. If the absolute values of these rates are compared with those of the cytochrome-c2-reaction-center electron transfer, it can be concluded that the reaction of oxidized cytochrome c2 with the bc1 complex is rate-limiting and involves electrstatic interactions. 4. A significant rate of intercomplex electron transfer can be observed also in the absence of cytochrome c2; in this case the electron donor to the recation center is the cytochrome c1 of the oxidoreductase complex. The oxidation of cytochrome c1 triggers a normal electron transfer within the bc1 complex. The intercomplex reaction follows second-order kinetics and is slowed at high ionic strength, suggesting a collisional interaction facilitated by electrostatic attraction. From the second-order rate constant of this process, a minimal bidimensional diffusion coefficient for the complexes in the membrane equal to 3 X 10(-11) cm2 s-1 can be evaluated.

Cloning and sequencing of the fbcF, B and C genes encoding the cytochrome b/c1 complex from Rhodopseudomonas viridis

The complete nucleotide sequence of the genes encoding the Rieske FeS, the cytochrome b and the cytochrome c1 subunits of the ubiquinol-cytochrome c2 oxidoreductase from the photosynthetic purple bacterium Rhodopseudomonas viridis, and the derived amino acid sequences are presented. These three genes, fbcF, fbcB and fbcC, are located at contiguous sites of the genome. The DNA-deduced amino acid sequences are compared with known primary structures of corresponding proteins from other purple photosynthetic bacteria, as well as mitochondria, cyanobacteria and chloroplasts.

Organization and structure of the genes for the cytochrome b/c1 complex in purple photosynthetic bacteria. A phylogenetic study describing the homology of the b/c1 subunits between prokaryotes, mitochondria, and chloroplasts

The cytochrome b/c1 complex is an ubiquitous energy transducing enzyme, part of the electron transport chain of prokaryotes, mitochondria, and chloroplasts (b6/f). In the ancient purple photosynthetic bacteria, the b/c1 complex occupies a central metabolic role, being part of their photosynthetic and respiratory electron transport chain. In Rhodobacter the three subunits of the b/c1 complex are FeS protein, cytochrome b, and cytochrome c1, and they are encoded by a constitutively expressed operon named fbc. The organization of the genes for the cytochrome b/c1 complex, the modality of transcription, and the biogenesis of the encoded polypeptides will be described. The Rhodobacter species used to isolate the fbc genes, previously reported as R. sphaeroides was identified as R. capsulatus. Further biochemical characterization of the prokaryotic b/c1 complex indicated that the three polypeptides encoded by the fbc operon comprise the entire catalytic structure: ubiquinol-cytochrome-c reductase. The amino acid sequences of the three b/c1 subunits from the photosynthetic bacterium Rhodobacter capsulatus were compared with the corresponding sequences from yeast mitochondria and spinach chloroplasts. The high homology found between the sequences of all three redox polypeptides from R. capsulatus and yeast mitochondria (cytochrome b 41%, FeS protein 46%, cytochrome c1 31%) provided further evidence that mitochondria arose from the phylogenetic line of purple bacteria. The structure of cytochrome b also exhibited considerable homology to chloroplast cytochrome b6 plus subunit IV (26%). The amino acid sequence of the Rieske FeS protein from R. capsulatus and chloroplasts were found to be conserved only in the C-terminal part (14% total identity), whereas the homology between cytochrome c1 and cytochrome f is very weak (12%), despite similar topology of the two polypeptides. Analysis of the homology suggested that the catalytic sites quinol oxidase (Q0) and quinone reductase (Qi) arose monophonetically, whereas cytochrome c and plastocyanin reductase sites are not homologous and could derive from diverse ancestral genes by convergent evolution.

Nucleotide sequence and transcription of the fbc operon from Rhodopseudomonas sphaeroides. Evaluation of the deduced amino acid sequences of the FeS protein, cytochrome b and cytochrome c1

The fbc operon from Rhodopseudomonas sphaeroides encodes the three redox carriers of the ubiquinol-cytochrome-c reductase (b/c1 complex): FeS protein, cytochrome b and cytochrome c1 [Gabellini, N. et al. (1985) EMBO J.2, 549-553]. The nucleotide sequence of 3874 bp of cloned R. sphaeroides chromosomal DNA, including the three structural genes fbcF, fbcB and fbcC has been determined. The reading frames of the fbc genes could be identified readily since the encoded amino acid sequences are highly homologous with the sequences of the corresponding mitochondrial polypeptides. Initiation and termination points for transcription have been investigated by S1 nuclease protection analysis. The transcription of the fbc operon starts approximately 240 base pairs upstream from the start codon of the fbcF gene and terminates 120 base pairs downstream from the stop codon of the fbcC gene. Nucleotide sequences resembling recognition signals for the binding and release of the RNA polymerase were identified. The N-terminal amino acid sequence of the mature cytochrome c1 was obtained by automated Edman degradation of the isolated subunit, confirming the fbcC reading frame and indicating that the bacterial preapocytochrome c1 has a transient leader sequence including 21 residues. The N-terminal sequence of one hydrophilic peptide of the FeS protein has been also obtained confirming the fbcF reading frame. The deduced amino acid sequences are discussed in relation to the known primary structures of the homologous proteins from mitochondria and chloroplasts. The primary structures of the polypeptides are evaluated with respect to their topology in the membrane, their biogenesis, the structure of the catalytic sites and subunit interactions.

Cloning and expression of the fbc operon encoding the FeS protein, cytochrome b and cytochrome c1 from the Rhodopseudomonas sphaeroides b/c1 complex

The gene for the FeS protein of the Rhodopseudomonas sphaeroides b/c1 complex was identified by means of cross-hybridization with a segment of the gene encoding the corresponding FeS protein of Neurospora crassa. Plasmids (pRSF1-14) containing the cross-hybridizing region, covering in total 13.5 kb of chromosomal DNA, were expressed in vitro in a homologous system. One RSF plasmid directed the synthesis of all three main polypeptides of the R. sphaeroides b/c1 complex: the FeS protein, cytochrome b and cytochrome c1. The FeS protein and cytochrome c1 were apparently synthesized as precursor forms. None of the pRSF plasmids directed the synthesis of the 10-kd polypeptide found in b/c1 complex preparations. Partial sequencing of the cloned region was performed. Several sites of strong homology between R. sphaeroides and eukaryotic polypeptides of the b/c1 complex were identified. The genes encode the three b/c1 polypeptides in the order: (5') FeS protein, cytochrome b, cytochrome c1. The three genes are transcribed to give a polycistronic mRNA of 2.9 kb. This transcriptional unit has been designated the fbc operon; its coding capacity corresponds to the size of the polycistronic mRNA assuming that only the genes for the FeS protein (fbcF), cytochrome b (fbcB) and cytochrome c1 (fbcC) are present. This could indicate that these three subunits constitute the minimal catalytic unit of the b/c1 complex from photosynthetic membranes.

Cloning and expression of the fbc operon encoding the FeS protein, cytochrome b and cytochrome c1 from the Rhodopseudomonas sphaeroides b/c1 complex

The gene for the FeS protein of the Rhodopseudomonas sphaeroides b/c1 complex was identified by means of cross-hybridization with a segment of the gene encoding the corresponding FeS protein of Neurospora crassa. Plasmids (pRSF1-14) containing the cross-hybridizing region, covering in total 13.5 kb of chromosomal DNA, were expressed in vitro in a homologous system. One RSF plasmid directed the synthesis of all three main polypeptides of the R. sphaeroides b/c1 complex: the FeS protein, cytochrome b and cytochrome c1. The FeS protein and cytochrome c1 were apparently synthesized as precursor forms. None of the pRSF plasmids directed the synthesis of the 10-kd polypeptide found in b/c1 complex preparations. Partial sequencing of the cloned region was performed. Several sites of strong homology between R. sphaeroides and eukaryotic polypeptides of the b/c1 complex were identified. The genes encode the three b/c1 polypeptides in the order: (5') FeS protein, cytochrome b, cytochrome c1. The three genes are transcribed to give a polycistronic mRNA of 2.9 kb. This transcriptional unit has been designated the fbc operon; its coding capacity corresponds to the size of the polycistronic mRNA assuming that only the genes for the FeS protein (fbcF), cytochrome b (fbcB) and cytochrome c1 (fbcC) are present. This could indicate that these three subunits constitute the minimal catalytic unit of the b/c1 complex from photosynthetic membranes.

Extra proton translocation and membrane potential generation--universal properties of cytochrome bc1/b6f complexes reconstituted into liposomes

Isolated cytochrome complexes from different sources like beef heart mitochondria, spinach chloroplasts, cyanobacteria, and photosynthetic bacteria were incorporated into liposomes by sonication as revealed by sucrose density gradient centrifugation and electron microscopy. The reconstituted cytochrome complexes show suppressed rates of quinol-cytochrome c/plastocyanin oxidoreduction which can be stimulated by ionophores and uncouplers. In addition, extra proton translocation out of the vesicles and membrane potential generation during electron transport were observed, suggesting a universal mechanism of electron and proton transport through all the tested cytochrome complexes.

Characterization of cytochrome b in the isolated ubiquinol-cytochrome c2 oxidoreductase from Rhodopseudomonas sphaeroides GA

Extinction coefficients for cytochrome b and c1 in the isolated cytochrome bc1 complex from Rhodopseudomonas sphaeroides GA have been determined. They are 25 mM-1 . cm-1 at 561 nm for cytochrome b and 17.4 mM-1 . cm-1 at 553 nM for cytochrome c1, for the difference between the reduced and the oxidized state. Cytochrome b is present in two forms in the complex. One form has an Em7 of 50 mV, an alpha-peak of 557 nm at liquid N2 temperature and of 561 nm at RT, which is red-shifted by antimycin A. The other form has an Em7 of -90 mV, a double alpha-peak of 555 and 561 nm at liquid N2 temperature corresponding to 559 and 566 nm at RT. The absorption at 566 nm is red-shifted by myxothiazol. The two shifts are independent of each other. Both midpoint potentials of cytochromes b are pH-dependent. The redox center compositions of the cytochrome bc1 complexes from Rhodopseudomonas sphaeroides and from mitochondria are identical.

A cytochrome b/c1 complex with ubiquinol--cytochrome c2 oxidoreductase activity from Rhodopseudomonas sphaeroides GA

A cytochrome b/c1 complex which catalyses the reduction of cytochrome c by ubiquinol has been isolated from Rhodopseudomonas sphaeroides GA. It contains two hemes b and substoichiometric amounts of ubiquinone-10 and of the Rieske Fe-S center per cytochrome c1, and is essentially free of reaction center and bacteriochlorophyll. The complex consists of three major polypeptides with apparent molecular masses of 40, 34 and 25 kDa. The 34-kDa polypeptide carries heme. Cytochrome c1 has a midpoint potential of 285 mV. For cytochrome b two midpoint potentials, at 50 and -60 mV, at pH 7.4, can be derived if one assumes two components of equal amount. Ubiquinol--cytochrome c oxidoreductase activity is specific for ubiquinol and bacterial cytochromes c, and is inhibited by antimycin A and 5-n-undecyl-6-hydroxy-4,7-dioxobenzothiazole. The complex shows oxidant-induced reduction of cytochrome b.

Structural requirements of quinone coenzymes for endogenous and dye-mediated coupled electron transport in bacterial photosynthesis

Electron transport in continuous light has been investigated in chromatophores of Rhodopseudomonas capsulata. Ala pho+, depleted in ubiquinone-10 and subsequently reconstituted with various ubiquinone homologs and analogs. In addition the restoration of electron transport in depleted chromatophores by the artificial redox compounds N-methylphenazonium methosulfate and N,N,N',N'-tetramethyl-p-phenylenediamine was studied. The following pattern of activities was obtained: (1) Reconstitution of cyclic photophosphorylation with ubiquinone-10 was saturated at about 40 ubiquinone molecules per reaction center. (2) Reconstitution by ubiquinone homologs was dependent on the length of the isoprenoid side chain and the amount of residual ubiquinone in the extracted chromatophores. If two or more molecules of ubiquinone-10 per reaction center were retained, all homologs with a side chain longer than two isoprene units were as active as ubiquinone-10 in reconstitution, and the double bonds in the side chain were not required. If less than two molecules per reaction center remained, an unsaturated side chain longer than five units was necessary for full activity. Plastoquinone, alpha-tocopherol, and naphthoquinones of the vitamin K series were relatively inactive in both cases. (3) All ubiquinone homologs, also ubiquinone-1 and -2, could be reduced equally well by the photosynthetic reaction center, as measured by light-induced proton binding in the presence of antimycin A and uncoupler. Plastoquinone was found to be a poor electron acceptor. (4) Photophosphorylation could be reconstituted by N-methylphenazonium methosulfate as well as by N,N,N',N'-tetramethyl-p-phenylenediamine in an antimycin-insensitive way, if more than two ubiquinones per reaction center remained. These compounds were active also in more extensively extracted particles reconstituted with ubiquinone-1, which itself was inactive.