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T M Fong

Publications and source records attributed to T M Fong.

46 records · Page 3Linked to original sources
PubMed1992

cDNA sequence and heterologous expression of the human neurokinin-3 receptor.

Functional cDNA clones encoding the human neurokinin-3 receptor were isolated from human brain mRNA. The cloned human neurokinin-3 receptor was expressed in COS cells and Xenopus oocytes, where peptide binding affinity and intracellular effector activation were determined. Neurokinin B is the most potent agonist, followed by eledoisin, substance K and substance P. The binding affinities of these peptides at the human neurokinin-3 receptor differ quantitatively from the rat receptor, implying a functional consequence of the sequence divergence between the two species. Heterologous expression in oocytes revealed that, unlike the neurokinin-1 receptor, the efficacy of ion channel activation mediated by the neurokinin-3 receptor does not approximate the binding affinity. The heterologous expression of the human neurokinin-3 receptor will facilitate further investigation into its biochemical functions.

Amino Acid Sequence↗
PubMed1992

Differential activation of intracellular effector by two isoforms of human neurokinin-1 receptor.

Two isoforms of the human neurokinin-1 receptor were cloned and characterized in heterologous expression systems of mammalian cell culture and Xenopus oocytes. The two isoforms differ only in the length of the encoded polypeptide. The peptide-binding properties of the long form of human neurokinin-1 receptor are consistent with those of the native neurokinin-1 receptor of mammalian tissues, where substance P is the most potent agonist. Peptide agonists elicit an oscillating current in Xenopus oocytes expressing the long form. In contrast, the short form of human neurokinin-1 receptor expressed in COS cells binds substance P with an apparent affinity at least 10-fold lower than that of the long form, and it elicits the electrophysiological response only weakly in Xenopus oocytes. These data suggest that the short form couples to a different effector system. Sequence analysis suggested that the two isoforms may arise from alternative pre-mRNA splicing. These results indicate that multiple forms of the human neurokinin-1 receptor exist and the differential activation of intracellular effector may be involved in generating the complex biological effects of substance P.

Amino Acid Sequence↗
PubMed1992

Determination of the amino acid residues in substance P conferring selectivity and specificity for the rat neurokinin receptors.

We have measured the affinity of various analogs and fragments of the tachykinin substance P for the cloned rat NK1, NK2, and NK3 receptors heterologously expressed in Chinese hamster ovary cells. The hydrophobic carboxyl-terminal pentapeptide sequence substance P-(7-11) binds with similar affinity (2-20 microM) to all three receptors. Our data suggest that addition of one to three amino-terminal residues to this sequence results in the optimization of its interaction within the binding pocket of the NK1 receptor. The addition of Pro-Gln-Gln to the carboxyl-terminal pentapeptide sequence increases affinity for the NK1 receptor, either by providing additional binding interactions or by modifying the conformation of the carboxyl-terminal sequence. This latter hypothesis is supported by the observation that physalaemin and phyllomedusin, which also contain a proline residue in the position analogous to the proline residue 4 of substance P, are also selective for NK1 receptors. Tachykinins that lack this proline have no higher affinity for NK1 than [pGlu] substance P-(6-11). Conversely, addition of Pro-Gln-Gln to the carboxyl-terminal pentapeptide sequence is unfavorable for NK2 and NK3 receptor binding. Preliminary data suggest that tachykinins with high affinity (Kd less than 500 nM) for NK2 receptors contain an aspartate residue in the position analogous to residue 5 of substance P, suggesting that an ionic interaction with the receptor may contribute binding energy. Further experiments will be required to determine the structural determinants of the NK1, NK2, and NK3 receptors responsible for these binding properties.

Amino Acid Sequence↗
PubMed1992

Characterization of the binding of a potent, selective, radioiodinated antagonist to the human neurokinin-1 receptor.

We have synthesized a potent, selective, radioiodinated antagonist of the human neurokinin-1 (NK1) receptor and have characterized its binding to the cloned receptor expressed in Chinese hamster ovary cells. (cis)-2-(Diphenylmethyl)-N-[(2-iodophenyl)-methyl]-1- azabicyclo[2.2.2]octan-3-amine (L-703606) inhibits binding of 125I-Tyr8-substance P to the human NK1 receptor with an IC50 of 2 nM. This compound is a competitive antagonist of substance P-induced inositol phosphate generation, with a Kb of 29 nM. [125I]L-703606 binds to a single class of high affinity binding sites in human NK1/Chinese hamster ovary cell membranes (Kd = 0.3 nM). Substance P inhibits the binding of [125I]L-703606 to 65% of the NK1 receptor sites with a Kd of 0.04 +/- 0.03 nM and to the remaining 35% of the sites with a Kd of 1.5 +/- 0.7 nM. Addition of the nonhydrolyzable GTP analog guanylyl-5'-(beta, gamma-imido)diphosphate [Gpp(NH)p] shifts greater than 90% of the binding sites to the lower affinity state. In addition, Gpp(NH)p markedly alters the dissociation of substance P from the NK1 receptor by increasing the number of sites in the low affinity, rapidly dissociating state. However, Gpp(NH)p does not affect the rate of dissociation of [125I]L-703606. These data suggest that the pharmacological properties of [125I]L-703606 binding to the human NK1 receptor are similar to those of antagonists of nonpeptide guanine nucleotide-binding protein-coupled receptors and that this ligand will be useful for the biochemical and pharmacological characterization of the human NK1 receptor.

Amino Acid Sequence↗
PubMed1989

Further evidence demonstrating that N-methyl-D-aspartate and kainate activate distinct ion channels.

Several excitatory amino acid receptors encoded by rat brain mRNA were expressed in Xenopus oocytes. Experimental protocols using an open channel blocker (MK-801) were designed to test the common receptor-channel hypothesis in which N-methyl-D-aspartate (NMDA) and kainate activate the same ion channel but induce different open channel conformations with different ionic permeabilities. The present data demonstrate that NMDA exposes previously trapped MK-801 molecules to the transmembrane field and accelerates their dissociation from the channel at positive potentials, while kainate lacks this effect. Therefore, kainate does not activate the same ion channel as NMDA does. Furthermore, differential inhibition of the NMDA response or the kainate response by the competitive antagonists D-2-amino-5-phosphonopentanoic acid (D-AP5) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) indicates that NMDA and kainate do not share the same binding site. Thus, these several lines of evidence demonstrate that two distinct receptor-channels are activated by NMDA and kainate, respectively.

6-Cyano-7-nitroquinoxaline-2,3-dione↗
PubMed1988

Properties of two classes of rat brain acidic amino acid receptors induced by distinct mRNA populations in Xenopus oocytes.

The Xenopus laevis oocyte expression system was used to study the molecular composition of mRNAs encoding acidic amino acid (AA) receptors from rat brain. Xenopus oocytes injected with poly(A) mRNA express two general classes of AA receptors. One class consists of AA-gated cation channels. Responses are evoked by N-methyl-D-aspartate (NMDA), by kainate, and to a lesser extent by L-glutamate or quisqualate. The second class of receptor is coupled to an intracellular second messenger pathway activating an oocyte-encoded Ca2+-activated Cl- conductance. This second messenger-coupled AA receptor can be activated by L-glutamate or quisqualate. DL-2-amino-5-phosphonopentanoic acid and D-alpha-aminohexanedioic acid inhibit the AA-gated cation conductances activated by NMDA or kainate with different potencies but do not inhibit the second messenger-coupled AA receptor. Responses to NMDA are enhanced by micromolar level of glycine and are inhibited by Mg2+, Zn2+, or MK-801. Dose-response analysis reveals that the AA-gated cation conductance activated by kainate requires the binding of two agonist molecules. To study the molecular composition, the mRNAs were size-fractionated by denaturing agarose gel electrophoresis. About 20-fold purification in specific activity (nA/ng of mRNA injected) of mRNAs encoding the second messenger-coupled AA receptor was achieved. In contrast, only a slight enrichment of the mRNAs encoding the AA-gated channel was observed. This suggests that the second messenger-coupled AA receptor is encoded by a single size class of mRNA, whereas the AA-gated cation channel(s) is encoded by multiple species of mRNAs or by mRNAs whose size distribution is heterogeneous.

Amino Acids↗
PubMed1987

Stabilization of acetylcholine receptor secondary structure by cholesterol and negatively charged phospholipids in membranes.

Fourier-transform infrared (FTIR) spectroscopy was used to study the secondary structure of purified Torpedo californica nicotinic acetylcholine receptor (AChR) in reconstituted membranes. Functional studies have previously demonstrated that the ion channel activity requires the presence of both sterol and negatively charged phospholipids in membranes. The present studies are designed to test the hypothesis that the alpha-helical structure of AChR may be stabilized by specific lipid molecules (sterol and/or negatively charged phospholipids) and that these alpha-helices may be responsible for the formation of a potential ion channel. FTIR data show statistically significant (p less than 0.005) spectral changes due to cholesterol and negatively charged phospholipids, respectively. On the basis of standard curves describing the relationship between the spectral properties and the secondary structural contents of water-soluble proteins, the observed spectral change at 931 cm-1 can be interpreted as an apparent change in the alpha-helix content from about 17% in the absence of sterols to about 20% in the presence of sterols, suggesting that protein-sterol interactions increase the helical structure of the AChR molecule. Similarly, the spectral change at 988 cm-1 can be interpreted as an apparent increase of beta-sheet content in the AChR molecule from about 20% to about 24% due to the presence of negatively charged phospholipids. Functional AChR in membranes thus appears to be correlated with higher alpha-helical and beta-sheet contents. It is concluded that one role of specific interactions between membrane protein and lipid molecules may be to maintain specific secondary structures necessary to support the ion channel function of AChR.

Animals↗
PubMed1986

Correlation between acetylcholine receptor function and structural properties of membranes.

Protein-lipid interactions were studied by using Torpedo californica acetylcholine receptor (AChR) as a model system by reconstituting purified AChR into membranes containing various synthetic lipids and native lipids. AChR function was determined by measuring two activities at 4 degrees C: (1) low to high agonist affinity-state transition of AChR in the presence of an agonist (carbamylcholine) in either membrane fragments or sealed vesicles and (2) ion-gating activity of AChR-containing vesicles in response to carbamylcholine. Sixteen samples were examined, each containing different lipid compositions including phosphatidylcholine, cholesterol, phosphatidic acid, phosphatidylethanolamine, asolectin, neutral lipid depleted asolectin, native lipids, and cholesterol-depleted native lipids. Phosphatidylcholines with different configurations of fatty acyl chains were used. The dynamic structures of these membranes were probed by incorporating spin-labeled fatty acid into AChR-containing vesicles and measuring the order parameters. It was found that both aspects of AChR function were highly dependent on the lipid environment even though carbamylcholine binding itself was not affected. An appropriate membrane fluidity was necessarily required to allow the interconversion between the low and high affinity states of AChR. An optimal fluidity hypothesis is proposed to account for the conformational transition properties of membrane proteins. In addition, the conformational change was only a necessary, but not sufficient, condition for the AChR-mediated ion flux activity. Among membranes in which AChR manifested the affinity-state transition, only those containing both cholesterol and negatively charged phospholipids (such as phosphatidic acid) retained the ion-gating activity.

Animals↗
PubMedDate not supplied

Purification and reconstitution of a recombinant human neurokinin-1 receptor.

Recombinant human neurokinin-1 receptors expressed in insect cells have been purified to near homogeneity by sequential metal-chelating chromatography and gel filtration chromatography. The purified receptor consists of a single polypeptide with an apparent molecular weight of 50 kD as revealed by SDS gel electrophoresis, and exhibits a specific activity of 19 nmol of L-703,606 bound per mg of protein. Immunoblot experiments further confirm the identity of the stained protein band. The purified receptor binds the antagonist L-703,606 with an affinity similar to that of native human neurokinin-1 receptor, and binds the agonist substance P with an affinity similar to that of the low affinity state of uncoupled native receptor. The purified receptor can be reconstituted with membranes from uninfected insect cells, and the reconstitution results in an increased affinity for substance P, consistent with the reappearance of the high affinity state of the receptor for agonist in the presence of endogenous G proteins. These data indicate that the purified neurokinin-1 receptor is functional with respect to agonist and antagonist binding and G protein coupling.

Amino Acid Sequence↗