Identification and structural characterization of two genes encoding glutamate transporter homologues differently expressed in the nervous system of Drosophila melanogaster

In vertebrates, excitatory amino acid transporters (EAATs) are believed to mediate the removal of glutamate released at excitatory synapses and to maintain extracellular concentrations of this neurotransmitter below excitotoxic levels. Glutamate is also used in insects as an excitatory neurotransmitter at the neuromuscular junction and probably in the central nervous system where its role remains to be established. We report the molecular characterization and developmental expression pattern of two Drosophila cDNAs: dEAATI, which has recently been identified as a high affinity glutamate transporter [1], and dEAAT2, a novel protein sharing strong homology to dEAATI and to the mammalian EAAT protein family. The developmental expression pattern of the two Drosophila EAAT genes has been compared by Northern blot analysis and whole-mount in situ hybridizations. The two transporters are transcribed in distinct cell types of the nervous system and are strongly expressed in the adult visual system.

Regulation of hemicholinium-3 sensitive choline uptake in Xenopus laevis oocytes by the second C2 domain of synaptotagmin

A size-fractionated torpedo electric lobe cDNA library was screened for the neuronal choline transporter by functional expression in oocytes. A clone, TLC2B, was isolated that induced a component of choline uptake that was hemicholinium-3 sensitive and inhibited by the substitution of lithium for sodium at low choline concentrations. However, [3H]choline uptake by both injected and non-injected oocytes were characterized by high affinity constants, suggesting that TLC2B could be affecting a native choline transporter. Indeed, hemicholinium-3 sensitive choline uptake could also be induced by preincubation of non-injected oocytes with a protein kinase C inhibitor, H-7. By sequence analysis and immuno-precipitation, the peptide produced by injection of TLC2B cRNA was identified as a soluble 24 kDa C-terminal fragment of the neuronal protein, synaptotagmin. Full length synaptotagmin was, however, ineffective in the functional test. The peptide encoded by TLC2B corresponds to the second protein kinase C-homologous domain of torpedo synaptotagmin, and like other soluble C2 domain peptides, was capable of calcium-dependent translocation to membranes. Its action on choline uptake in oocytes was, however, abolished by the addition of calcium in the presence of a calcium ionophore. These results suggest that the interaction of certain C2 domains, such as the C-terminal domain of synaptotagmin, with more specific targets may be anulled in the presence of calcium due to its absorption to membrane phospholipids.

A novel and major isoform of tyrosine hydroxylase in Drosophila is generated by alternative RNA processing

We report that two isoforms of Drosophila tyrosine hydroxylase protein are encoded via alternatively spliced exons. The major isoform (Type II) contains a novel acidic extension of 71 amino acids in the amino-terminal regulatory domain, which is likely to alter the regulatory properties of the tyrosine hydroxylase protein. The minor isoform (Type I) corresponds to the cDNA sequence reported previously. We also report the structure of the Drosophila tyrosine hydroxylase (DTH) gene and the diversity and tissue localization of its transcripts. At least three types of DTH mRNA are generated from a single primary transcript through alternative splicing and polyadenylation. Type II mRNA is the most abundant tyrosine hydroxylase transcript in Drosophila and is found predominantly in the hypoderm throughout all stages of development. Type I mRNA is present only in the CNS, where it is the primary form. The DTH transcripts detected in the CNS contain a longer 3'-untranslated region than the transcript expressed in the hypoderm, due to differential polyadenylation. In contrast, the same start site is used for DTH gene transcription in both tissues. These results show unexpected diversity in the DTH transcripts and point out possible mechanisms for differential regulation of tyrosine hydroxylase activity in the CNS and in the hypoderm.

High-level synthesis and fate of acetylcholine in baculovirus-infected cells: characterization and purification of recombinant rat choline acetyltransferase

Rat choline acetyltransferase (ChAT) has been expressed at a high level in Spodoptera frugiperda Sf9 cells using a baculovirus expression system. A cDNA containing the coding sequence for ChAT was inserted into the transfer vector pAcYM1 to yield the recombinant vector pAcYM1/ChAT. Sf9 cells were then coinfected with pAcYM1/ChAT and the wild-type Autographa californica virus. One recombinant virus particle, containing the cDNA for ChAT, was selected that expressed a protein of 68.5 kDa. Forty hours after infection of cells with the recombinant virus, the specific activity of ChAT in the cytosol was 190 nmol of acetylcholine/min/mg of protein, accounting for approximately 24% of the cell cytosolic proteins as being ChAT. The apparent Km values of the enzyme for choline and acetyl-CoA were 299 and 221 microM, respectively, whereas the respective Vmax values were 10.6 and 11.4 mumol of acetylcholine/min/mg of protein. In addition, analysis of the protein revealed that ChAT is phosphorylated in Sf9 cells. About 0.5 mg of ChAT was obtained from a one-step purification procedure starting with 10(8) infected Sf9 cells. Addition of choline to the incubation medium led to accumulation of high amounts of acetylcholine in the cytosol of the infected cells. The neurotransmitter was not released by Sf9 cells in response to membrane depolarization or on ionophore-mediated calcium entry. Some acetylcholine, which most likely originated from cell death inherent to viral infection, accumulated in the culture medium. The infected insect cells, which synthesize and store neurotransmitter, provide a new and convenient model for analyzing synaptic transmission at the molecular level.

A 15 kDa proteolipid found in mediatophore preparations from Torpedo electric organ presents high sequence homology with the bovine chromaffin granule protonophore

Upon SDS PAGE of isolated mediatophore, an acetylcholine-translocating protein, a doublet at 15 kDa was identified. Amino acid sequencing after CNBr cleavage gave a 17 residue-long peptide completely homologous with a sequence of the proton-translocating proteolipid from bovine chromaffin granules. A 51-mer oligodeoxynucleotide corresponding to this sequence was used to screen a library of electric lobe cDNAs constructed in lambda Zap II. A positive recombinant clone was isolated and found to encode the complete sequence of a 15.5 kDa protein highly homologous to the bovine chromaffin or yeast vacuolar ATPase proteolipid. In vitro translation of sense RNA transcripts of the clone indeed yielded a single 15 kDa proteolipid. Northern blot analysis showed that the 1.3 kb mRNA encoding this protein is significantly expressed in nervous tissues but not in electric organ or liver of Torpedo marmorata.

Purification of a presynaptic membrane protein that mediates a calcium-dependent translocation of acetylcholine

A protein, which we call "mediatophore," that mediates calcium-dependent release of acetylcholine from proteoliposomes has been purified from the presynaptic plasma membrane. About 250 micrograms of this material was obtained from 500 g of Torpedo marmorata electric organ. Precipitation of the protein and subsequent removal of associated lipids inactivated the protein, which then became water soluble; this permitted evaluation of its Stokes radius (52 A) and its sedimentation coefficient (9.8 +/- 0.75 S) and, hence, an approximate molecular mass of 210 +/- 16 kDa could be determined. PAGE analysis showed that the protein is made of 17-kDa subunits, not linked by disulfide bonds. When this material was observed by electron microscopy after negative staining, the apparently pentameric structures had an average diameter of about 7 nm.

Solubilization and partial purification of a presynaptic membrane protein ensuring calcium-dependent acetylcholine release from proteoliposomes

In previous work, it was shown that cytoplasmic acetylcholine decreased on stimulation of Torpedo electric organ or synaptosomes in a strictly calcium-dependent manner. This led to the hypothesis that the presynaptic membrane contained an element translocating acetylcholine when activated by calcium. To test this hypothesis, the presynaptic membrane constituents were incorporated into the membranes of liposomes filled with acetylcholine. The proteoliposomes thus obtained released the transmitter in response to a calcium influx. The kinetics and calcium dependency of acetylcholine release were comparable for proteoliposomes and synaptosomes. The presynaptic membrane element ensuring calcium-dependent acetylcholine release is most probably a protein, since it was susceptible to Pronase, but only when the protease had access to the intracellular face of the presynaptic membrane. Postsynaptic membrane fractions contained very low amounts of this protein. It was extracted from the presynaptic membrane under alkaline conditions in the form of a protein-lipid complex of large size and low density which was partially purified. The specificity of the calcium-dependent release for acetylcholine was tested with proteoliposomes filled with equal amounts of acetylcholine and choline or acetylcholine and ATP. In both cases, acetylcholine was released preferentially. After cholate solubilization and gel filtration, the protein ensuring the calcium-dependent acetylcholine release was recovered at a high apparent molecular weight (between 600,000 and 200,000 daltons), its apparent sedimentation coefficient being 17S after cholate elimination. This protein is probably an essential coin of the transmitter release mechanism. We propose to name it mediatophore.

Inactivation of acetylcholine release from Torpedo synaptosomes in response to prolonged depolarizations

The release of acetylcholine (ACh) from purely cholinergic Torpedo synaptosomes was monitored continuously using a chemiluminescent assay (Israël & Lesbats, 1981 a, b). Upon prolonged K+ depolarization in the presence of Ca2+, the release of ACh was transient and returned to a steady low level in about 3 min. Addition of the Ca2+ ionophore A23187 triggered the release again, suggesting that neither depletion of the transmitter store nor an inhibition of the release mechanism itself were involved in this phasic response, but rather an inactivation of the Ca2+ entry. The release response evoked by adding Ca2+ back after exposure of the synaptosomes to high K+ (70 mM) and low Ca2+ (0.57 mM) solution inactivates as a function of the duration of the pre-depolarization with a two-component time course with rapid (tau = 5.5 s) and slow phases (tau = 143 s). This response to Ca2+ addition was more strikingly reduced as the level of depolarization during pre-treatment was increased. The inactivation was found to be dose dependent with respect to the amount of Ca2+ present during the pre-depolarization period (conditioning Ca2+). Moreover, the presence of EGTA during pre-treatment with high-K+ solutions increased the response to applied Ca2+. These observations suggest that Ca2+ entry itself was responsible for this inactivation. No inactivation was found when ACh release was induced by the depolarizing agent Gramicidin D, except when external Na+ was replaced by Li+. This result indicates that part of the Ca2+ influx promoted by Gramicidin D depends on a Na+ entry, and may be mediated by the Na-Ca exchange mechanism.

Determination of acetylcholinesterase activity by a new chemiluminescence assay with the natural substrate

A chemiluminescence method for determining acetylcholinesterase activity is described. It is an adaptation of the chemiluminescence assay of acetylcholine described by Israël & Lesbats [(1981) Neurochem. Int. 3, 81-90; (1981) J. Neurochem. 37, 1475-1483]. The acetylcholinesterase activity is measured by monitoring the increase in light emission produced by the accumulation of choline or by determining the amount of choline generated after a short interval. The assay is rapid and sensitive, and uses the natural substrate of the enzyme. Kinetic data obtained with this procedure for acetylcholinesterase from Torpedo and Electrophorus electric organs were comparable with those obtained by using the method of Ellman, Courtney, Andres & Featherstone [(1961) Biochem. Pharmacol. 7, 88-95]. In addition, it was shown that sodium deoxycholate totally inactivated Torpedo acetylcholinesterase but not the Electrophorus enzyme. Competitive inhibitors of acetylcholinesterase protected the enzyme from inactivation.