Novel alternative splicing predicts a truncated isoform of the NMDA receptor subunit 1 (NR1) in embryonic rat brain.

The expression of mesencephalic brain derived neurotrophic factor (BDNF) has been shown to be regulated by dopaminergic neuronal functioning and glutamate receptors (GluRs). In turn, BDNF participates in the regulation of mesencephalic GluRs' expression. In the present study we analyzed, using semi-quantitative RT-PCR, the effect of BDNF as well as of the GluRs agonists NMDA and trans-(+/-)-1-Amino-(1S,3R)-cyclopentane dicarboxylic acid (t-ACPD), on the expression levels of the NMDA GluR subunit 1 (NR1) mRNA, using rat cultured mesencephalic neurons. In the course of this study, a novel rat mRNA splice variant of NR1 was identified. This new NR1 mRNA isoform is characterized by the insertion of an 82 base pair intron containing an inframe stop codon, thus predicting the expression of a putative truncated protein of 465 amino acids. The RT-PCR and in situ hybridization reveals that the novel NR1 mRNA is expressed in various brain regions of the rat embryo, whereas no expression was detected in the adult rat brain. The modulation of the novel NR1 mRNA isoform by both BDNF and the metabotropic GluR agonist t-ACPD, suggests that the resulting putative NR1 truncated protein may be relevant in the regulatory network of glutamatergic neurotransmission in the developing central nervous system.

Differential regulation of mu-opioid receptor mRNA in the nucleus accumbens shell and core accompanying amphetamine behavioral sensitization.

Repeated amphetamine (AMPH) administration results in behavioral sensitization. To investigate the participation of the opioid system in this phenomenon, we examined the effects of acute and repeated AMPH administration on mu-opioid receptor (MOR) mRNA levels in the nucleus accumbens (NAc) and striatum (STR) of rats, by quantitative non-radioactive in situ hybridization. Five injections of d-AMPH (1.5 mg kg-1, i.p., once every other day), resulted in a sensitization response profile and a significant down-regulation of MOR mRNA levels in the NAc shell, whereas no change was observed in MOR mRNA levels in the NAc core compared to the saline controls. Conversely, MOR mRNA levels were up-regulated in the rostral STR of AMPH-sensitized rats compared to saline controls. No changes in MOR mRNA levels were observed after acute AMPH treatment in any of the brain regions studied. These results suggest that the opioid system participates in the neurobiological underpinnings of behavioral sensitization and that opioid receptor (OR) expression in the STR and NAc shell and core is differentially modulated by repeated AMPH exposure.

cAMP decreases steady-state levels of delta-opioid receptor mRNA in NG108-15 cells.

We have compared several drug combinations for their ability to increase basal cAMP and to down-regulate delta-opioid receptor mRNA levels. Continuous treatment for up to 48 h with a phosphodiesterase inhibitor in combination with the adenylyl cyclase activator forskolin showed an early peak response, but cAMP levels returned to control after 8 and 24 h. Increases in cAMP level up to 150-fold were observed after treatment for 1 h with a series of drugs (rolipram, IBMX/forskolin, rolipram/forskolin, dibutyryl cAMP, and prostaglandin E2) that increase cAMP by different mechanisms. A significant decrease in DOR mRNA level, to 31% of control, followed the three treatments that produced the largest increases in cAMP level: IBMX/forskolin, rolipram/forskolin, and prostaglandin E2.

Characterization of the murine mu opioid receptor gene.

The analgesic and addictive properties of morphine and other opioid drugs are thought to result from their interaction with mu opioid receptors. Using a delta opioid receptor cDNA as a probe, we have isolated a murine mu opioid receptor cDNA clone (mMOR). Stable expression of mMOR in Chinese hamster ovary cells conferred high binding affinity for mu receptor ligands including morphine and [D-Ala2,N-methyl-Phe4,Gly5-ol]-enkephalin and low affinity for delta and kappa preferring ligands. Treatment of these cell lines with morphine and other mu agonists inhibited forskolin-induced cAMP accumulation, demonstrating a functional coupling of mMOR to the inhibition of adenylate cyclase. The predicted amino acid sequence of mMOR shares approximately 55% overall amino acid identity with the delta receptor and approximately 97% identity with the recently reported rat mu opioid receptor. Expression of the mu receptor in mouse brain as revealed by in situ hybridization parallels the reported pattern of distribution of mu-selective ligand binding sites. Chromosomal localization (to mouse chromosome 10) and Southern analysis are consistent with a single mu opioid receptor gene in the mouse genome, suggesting that the various pharmacologically distinct forms of the mu receptor arise from alternative splicing, post-translational events, or from a highly divergent gene(s).

Divergent structure of the human synexin (annexin VII) gene and assignment to chromosome 10.

The human synexin (annexin VII) gene occurs as a single copy at chromosome 10q21.1-21.2 and substantially deviates in size and in the location of splice junctions from the other two well-characterized members of the annexin gene family, lipocortin I (annexin I) and calpactin I (annexin II). The synexin gene contains 14 exons, including an alternatively spliced cassette exon, and spans approximately 34 kb of DNA. Only five of the fourteen splice junctions are conserved compared to other annexins, and the differences are particularly pronounced in the exons that encode the C-terminal third and fourth conserved repeats in the gene product. Although parallels between exons and protein domains were not apparent, we did observe clustering of splice junctions corresponding to either the unique N-terminal domain or the conserved C-terminal tetrad repeat domain, which is common to all annexins. Furthermore, a complete analysis of the 5' flanking region of the annexin VII gene revealed an entirely different set of cis-acting and enhancer elements compared to other annexin genes. We conclude that the annexin VII gene may have arisen by a divergence from the evolutionary pathway taken by both annexins I and II.

Cloning of a delta opioid receptor by functional expression.

Opiate drugs have potent analgesic and addictive properties. These drugs interact with receptors that also mediate the response to endogenous opioid peptide ligands. However, the receptors for opioids have eluded definitive molecular characterization. By transient expression in COS cells and screening with an iodinated analog of the opioid peptide enkephalin, a complementary DNA clone encoding a functional delta opioid receptor has been identified. The sequence shows homology to G protein-coupled receptors, in particular the receptors for somatostatin, angiotensin, and interleukin-8.

Alternative splicing of human synexin mRNA in brain, cardiac, and skeletal muscle alters the unique N-terminal domain.

Several synexin (annexin VII) mRNAs have been identified by screening a human fibroblast cDNA library. One type of message contained an alternatively spliced cassette exon, predicting two isoforms of synexin differing in the N-terminal domain. Polymerase chain reaction analysis of synexin mRNA from various fetal and adult tissues, from human and monkey, revealed that the alternative splicing event is tissue-regulated; synexin mRNA containing the cassette exon is prevalent in brain, heart, and skeletal muscle. This is supported by Western blot analysis showing that muscle synexin (annexin VIIb) is larger than synexin from lung (annexin VIIa). The muscle and lung isoforms have the same molecular mass as the recombinant synexins expressed in Escherichia coli using cDNAs containing or lacking the cassette exon, respectively. The difference in size is consistent with the molecular masses predicted from the proteins encoded by the alternatively spliced synexin mRNAs. Another type of synexin mRNA contained a longer 3'-noncoding region generated by the selection of an alternate poly(A) signal. Northern analysis of human fibroblast RNA showed the presence of two bands (2.0- and 2.4-kilobase) when hybridized to a cDNA fragment of the coding region of synexin, but only the 2.4-kilobase band hybridized to a probe made from the longer 3' end.

Calcium channel activity of purified human synexin and structure of the human synexin gene.

Synexin is a calcium-dependent membrane binding protein that not only fuses membranes but also acts as a voltage-dependent calcium channel. We have isolated and sequenced a set of overlapping cDNA clones for human synexin. The derived amino acid sequence of synexin reveals strong homology in the C-terminal domain with a previously identified class of calcium-dependent membrane binding proteins. These include endonexin II, lipocortin I, calpactin I heavy chain (p36), protein II, and calelectrin 67K. The Mr 51,000 synexin molecule can be divided into a unique, highly hydrophobic N-terminal domain of 167 amino acids and a conserved C-terminal region of 299 amino acids. The latter domain is composed of alternating hydrophobic and hydrophilic segments. Analysis of the entire structure reveals possible insights into such diverse properties as voltage-sensitive calcium channel activity, ion selectivity, affinity for phospholipids, and membrane fusion.

Calcium modulation of phosphoinositide kinases in transverse tubule vesicles from frog skeletal muscle.

Highly purified transverse tubule membranes isolated from frog skeletal muscle phosphorylate phosphatidylinositol to phosphatidylinositol 4-phosphate and phosphatidylinositol (4,5)-bisphosphate. The two phosphorylation reactions have different calcium requirements. Phosphorylation of phosphatidylinositol to phosphatidylinositol 4-phosphate, which takes place in both isolated transverse tubules and sarcoplasmic reticulum membrane, is independent of calcium in a range of concentrations from 10(-9) to 10(-6) M, and is progressively inhibited to 10% of the maximal values by increasing calcium to 10(-4) M or higher (K0.5 = 5 X 10(-6) M). In contrast, phosphorylation of phosphatidylinositol 4-phosphate to phosphatidylinositol (4,5)-bisphosphate, a reaction exclusively present in transverse tubule membranes, is maximal at calcium concentrations higher than 2 X 10(-6) M and decreases to 30% of maximal values at calcium concentrations of 2 X 10(-7) M or lower (K0.5 = 10(-6) M). Unlike frog membranes, transverse tubules from rabbit muscle need exogenous phosphatidylinositol 4-phosphate in order to produce the bisphosphate derivative in the same range of calcium concentrations. Inositol (1,4,5)-trisphosphate has been proposed recently as a chemical messenger in excitation-contraction coupling in skeletal muscle. Calcium regulation of the synthesis of phosphatidylinositol (4,5)-bisphosphate, the membrane-bound precursor of inositol (1,4,5)-trisphosphate, might have physiological implications regarding modulation of excitation-contraction coupling by intracellular calcium levels.

Phosphorylation of phosphatidylinositol by transverse tubule vesicles and its possible role in excitation-contraction coupling.

Phosphorylation of phosphatidylinositol to phosphatidylinositol 4-monophosphate and to phosphatidylinositol 4,5-bisphosphate was demonstrated in transverse-tubule membranes isolated from frog skeletal muscle using [gamma-32P]ATP as substrate. At millimolar concentrations of Mg2+ both phosphorylation reactions were completed within 15 s at 25 degrees C. Isolated sarcoplasmic reticulum vesicles phosphorylated phosphatidylinositol to phosphatidylinositol 4-phosphate with a lower specific activity than the transverse tubules, and lacked the ability to produce phosphatidylinositol 4,5-bisphosphate. These findings show, for the first time, that isolated transverse-tubule membranes carry out one of the steps required to sustain a role for inositol trisphosphate as the physiological messenger in excitation-contraction coupling in skeletal muscle. The finding that 0.5 mM tetracaine apparently inhibits the phosphorylation of phosphatidylinositol 4-phosphate to phosphatidylinositol 4,5-bisphosphate also supports a role for these intermediates in excitation-contraction coupling.