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H T Leung

Publications and source records attributed to H T Leung.

15 recordsLinked to original sources
PubMed2000

Phenotypes of trpl mutants and interactions between the transient receptor potential (TRP) and TRP-like channels in Drosophila.

The trp and trpl genes are thought to encode two classes of light-activated ion channels in Drosophila. A previous report indicated that a null trpl mutant does not display any mutant phenotype. This lack of detectable mutant phenotypes made it difficult to suggest functions for the transient receptor potential-like (TRPL) channel in photoreceptor responses. Here, the properties of trpl photoreceptor responses were studied by using electroretinogram (ERG) and intracellular recording techniques in combination with light stimuli of relatively long durations. Distinct mutant phenotypes were detectable under these conditions. These consisted of a reduced sustained component, oscillations superimposed on the response, a poststimulus hyperpolarization, and altered adaptation properties to dim background light. Comparison of photoreceptor responses obtained from wild type, trp, and trpl showed that the responses obtained from the trp and trpl null mutants did not sum up to that of the wild-type response. To explain the nonlinear summation at the peak of the response, Reuss et al. (1997) proposed that Ca(2+) ions entering through the TRP channel modulate TRP and TRPL channel activities differentially. However, nonlinear summation was present not only at the peak but throughout the duration of response. Two lines of evidence are presented to suggest that, in addition to the interaction proposed by Reuss et al. (1997), there are other forms of interactions between TRP and TRPL channels, probably involving the channel proteins themselves.

Amino Acid Substitution↗
PubMed1999

INAF, a protein required for transient receptor potential Ca(2+) channel function.

The trp gene of Drosophila encodes a subunit of a class of Ca(2+)-selective light-activated channels that carry the bulk of the phototransduction current. Transient receptor potential (TRP) homologs have been identified throughout animal phylogeny. In vertebrates, TRP-related channels have been suggested to mediate "store-operated Ca(2+) entry," which is important in Ca(2+) homeostasis in a wide variety of cell types. However, the mechanisms of activation and regulation of the TRP channel are not known. Here, we report on the Drosophila inaF gene, which encodes a highly eye-enriched protein, INAF, that appears to be required for TRP channel function. A null mutation in this gene significantly reduces the amount of the TRP protein and, in addition, specifically affects the TRP channel function so as to nearly shut down its activity. The inaF mutation also dramatically suppresses the severe degeneration caused by a constitutively active mutation in the trp gene. Although the reduction in the amount of the TRP protein may contribute to these phenotypes, several lines of evidence support the view that inaF mutations also more directly affect the TRP channel function, suggesting that the INAF protein may have a regulatory role in the channel function.

Adaptor Proteins, Signal Transducing↗
PubMed1998

The effect of the nasopharyngeal air cavity on x-ray interface doses.

We investigated the impact of air cavities in head and neck cancer patients treated by photon beams based on clinical set-ups. The phantom for investigation was constructed with a cubic air cavity of 4 x 4 x 4 cm3 located at the centre of a 30 x 30 x 16 cm3 solid water slab. The cavity cube was used to resemble an extreme case for the nasal cavity. Apart from measuring the dose profiles and central axis percentage depth dose distribution, the dose values in 0.25 x 0.25 x 0.25 cm3 voxels at regions around the air cavity were obtained by Monte Carlo simulations. A mean dose value was taken over the voxels of interest at each depth for evaluation. Single-field results were added to study parallel opposed field effects. For 10 x 10 cm2 parallel opposed fields at 4, 6 and 8 MV, the mean dose at regions near the lateral interfaces of the cavity cube were decreased by 1 to 2% due to the lack of lateral scatter, while the mean dose near the proximal and distal interfaces was increased by 2 to 4% due to the greater transmission through air. Secondary build-up effects at points immediately beyond the air cavity cube are negligible using field sizes greater than 4 x 4 cm2. For most head and neck treatment, the field sizes are usually 6 x 6 cm2 or greater, and most cavity volumes are smaller than our chosen dimensions. Therefore, the influence of closed air cavities on photon interface doses is not significant in clinical treatment set-ups.

Air↗
PubMed1995

Cytotoxic T lymphocyte-associated molecule-4, a high-avidity receptor for CD80 and CD86, contains an intracellular localization motif in its cytoplasmic tail.

CD28 and CTLA-4, T cell receptors for B7-1 (CD80) and B7-2 (CD86) molecules on antigen-presenting cells, transmit costimulatory signals important for optimal T cell activation. Despite sharing sequence homology and common ligands, these receptors have distinct binding properties and patterns of expression. The function of CTLA-4 during T cell activation is not well understood, although an important role is suggested by complete amino acid sequence conservation of its cytoplasmic tail in all species studied to date. We report here a role of the cytoplasmic tail of CTLA-4 in regulating its subcellular localization and cell surface expression. In activated human peripheral blood T cells, or in several transfected or transduced cell types, CTLA-4 is not primarily a cell surface protein, but rather is localized intracellularly in a region which overlaps the Golgi apparatus. Transfer of 11 cytoplasmic residues, 161TTGVYVKMPPT, from the CTLA-4 cytoplasmic tail to the homologous position in CD28 was sufficient to confer intracellular localization. Mutation of the tyrosine residue (Tyr165) in this motif to phenylalanine resulted in increased surface expression of CTLA-4. Thus, the subcellular localization of CTLA-4 is controlled by a tyrosine-containing motif within its cytoplasmic domain. Contained within this motif is a binding site for SH2 domains of the p85 subunit of phosphatidylinositol 3-kinase.

Abatacept↗
PubMed1995

Binding stoichiometry of the cytotoxic T lymphocyte-associated molecule-4 (CTLA-4). A disulfide-linked homodimer binds two CD86 molecules.

CD28 and CTLA-4 are homologous T cell receptors of the immunoglobulin (Ig) superfamily, which bind B7 molecules (CD80 and CD86) on antigen-presenting cells and transmit important costimulatory signals during T cell activation. Here we have investigated the subunit structure of CTLA-4 and the stoichiometry of its binding to B7 molecules. We demonstrate CTLA-4 is a homodimer interconnected by one disulfide bond in the extracellular domain at cysteine residue 120. Each monomeric polypeptide chain of CTLA-4 contains a high affinity binding site for B7 molecules; soluble CTLA-4 and CD86 form complexes containing equimolar amounts of monomeric CTLA-4 and CD86 (i.e. a 2:2 molecular complex). Thus, CTLA-4 and probably CD28 have a receptor structure consisting of preexisting covalent homodimers with two binding sites. Dimerization of CTLA-4 and CD28 is not required for B7 binding, nor is it sufficient to trigger signaling.

Abatacept↗
PubMed1994

Absence of transforming growth factor-beta responsiveness in the tamoxifen growth-inhibited human breast cancer cell line CAMA-1.

Tamoxifen has been an effective antiestrogen in suppressing breast cancer growth which is estrogen-responsive or dependent. Early studies have provided circumstantial evidence that transforming growth factor-beta (TGF-beta) may be an autocrine mediator of tamoxifen action. Therefore, it is both fundamentally important and clinically relevant to investigate the relationship between tamoxifen and TGF-beta. In this study, we demonstrated that CAMA-1 cells, which are sensitive to tamoxifen inhibition, did not respond to TGF-beta growth inhibition. The type I and II TGF-beta receptors were undetectable by the radio-ligand affinity labeling technique. Despite the presence of a normal TGF-beta type II receptor gene, the mRNA transcript of the gene was undetectable by the extremely sensitive Intron-differential RNA/PCR method. The possibility that the lack of TGF-beta receptors might be intimately linked to the absence of normal retinoblastoma (Rb) gene products, as suggested by previous studies of retinoblastoma cells, was further investigated. The lack of TGF-beta receptor expression was found due to reasons other than the absence, deletion or abnormality of the Rb gene because a normal Rb gene and its hyper- and hypo-phosphorylated protein products were detected in CAMA-1 cells. In conclusion, our results suggest that the TGF-beta system is not obligatory for antiestrogen growth inhibition of CAMA-1 cells.

Base Sequence↗
PubMed1992

Imaging the complex impedance of the thorax.

Respiration-related changes in the complex impedance were obtained on the thorax in three volunteers. The real part of the image clearly showed the lungs as regions of increased conductivity on expiration. The imaginary part of the image, reflecting changes in the ratio of permittivity to conductivity, showed a central negative region surrounded by a positive region extending to the periphery of the lungs. These features may be due to movement of the diaphragm and liver within the sensitive volume during respiration.

Adult↗
PubMed1991

Evidence of an EGF/TGE-alpha--independent pathway for estrogen-regulated cell proliferation.

To elucidate the relationship between epidermal growth factor (EGF)/transforming growth factor (TGF-alpha) and estradiol-17 beta (E) in cell proliferation, we examined their effects on the breast cancer cell line, CAMA-1. While E was able to consistently induce cell proliferation under a variety of experimental conditions, EGF/TGF-alpha was without effect. Despite the presence of the receptor (EGFR) gene, mature EGFR protein and mRNA were not detected by radioreceptor assay, 35S Met-labelling, and the Intron Differential RNA/PCR method under conditions in which cells remain responsive to E. Furthermore, TGF-alpha is not an autocrine factor in CAMA-1 cells. We demonstrated unequivocally that EGF/TGF-alpha interaction with EGFR is not an obligatory event in mediating estrogen-stimulated cell proliferation.

Base Sequence↗
PubMed1991

Characterization of single calcium channels in Drosophila embryonic nerve and muscle cells.

Voltage-activated Ca channels play a central role in synaptic transmission, control of cell excitability, and many other cellular processes. It is now clear that there are multiple types of Ca channels with various modes of modulation. Drosophila offers exceptional advantages for studying the molecular basis of the diversity and modulation of Ca channels. As a step in this study, we have characterized the single-channel Ba currents recorded from cell-attached patches on cultured embryonic Drosophila nerve and muscle cells. The voltage dependence and selectivity of the channels carrying these Ba currents identify them as Ca channels. All Ca channels found in Drosophila neurons appear to have the same voltage dependence of activation and similar single-channel conductance, 12-17 pS (100 mM Ba2+). However, the kinetic properties of individual Ca channels vary greatly. The mean open time of individual channels ranges from 2 msec to less than 0.2 msec. Some channels completely inactivate during the first half of a 90 msec depolarization, while others are more active in the second half. Many channels open during almost every depolarization, while others open in less than 20% of the depolarizations. Channels with longer open times tend to inactivate and open during a small fraction of depolarizations. When these kinetic properties were quantified, a continuum of values was found, instead of the clustering of values that might be expected for discrete types of channels. Muscle Ca channels form a more homogeneous class than do the neuronal Ca channels. The muscle Ca channel conductance is 18 pS. These channels do not inactivate during 90 msec depolarizations and open during a majority of depolarizations applied. Muscle Ca channels are similar to a subset of neuronal Ca channels. When a purified toxin from the spider Hololena curta is applied to neurons, the number of active Ca channels is reduced, and those channels still active open in a small fraction of depolarization. Since channels that open in a small fraction of depolarizations tend to inactivate, these data support the hypothesis that this toxin selectively blocks noninactivating neuronal Ca channels. This differential toxin sensitivity and the much larger variability observed in kinetic properties of neuronal, compared to muscle, Ca channels suggest that there are at least two types of neuronal Ca channels in Drosophila.

Animals↗
PubMed1989

Spider toxins selectively block calcium currents in Drosophila.

Toxins from spider venom, originally purified for their ability to block synaptic transmission in Drosophila, are potent and specific blockers of Ca2+ currents measured in cultured embryonic Drosophila neurons using the whole-cell, patch-clamp technique. Differential actions of toxins from two species of spiders indicate that different types of Drosophila neuronal Ca2+ currents can be pharmacologically distinguished. Hololena toxin preferentially blocks a non-inactivating component of the current, whereas Plectreurys toxin blocks both inactivating and non-inactivating components. These results suggest that block of a non-inactivating Ca2+ current is sufficient to block neurotransmitter release at Drosophila neuromuscular junction.

Animals↗
PubMed1988

Ionic currents of Drosophila neurons in embryonic cultures.

Drosophila offers a unique opportunity to determine how the genome codes for ionic channels in an organized nervous system. Considerable progress has already been made in studying the molecular biology of Drosophila K channels. In order for similar progress to be made on neuronal voltage-dependent Ca channels, a physiological preparation is needed in which the function of these channels can be directly studied. The patch-clamp studies reported here show that cultures of embryonic Drosophila cells (Seecof and Unanue, 1968) meet this need. These cultures provide the first opportunity to study with voltage-clamp techniques the Ca and Na currents of Drosophila neurons. The focus of these studies is on the Ca current; however, descriptions of the K and Na currents are also given since they help to characterize the cells studied and the quality of the voltage clamp. The voltage-dependent K, Na, and Ca currents of Drosophila neurons are very similar to those of molluscan neurons and other better studied neurons. The K currents are the largest currents in these neurons, averaging over 300 pA at +20 mV. There are 2 classes of Ca-independent K currents, inactivating currents that are 4-AP sensitive, and noninactivating currents that are insensitive to 4-AP. A large fraction of the K currents are located in the somal membrane. The Na currents are TTX sensitive and probably located in the processes. The peak amplitudes of the Ca currents vary from 0 to over 100 pA in these neurons, averaging 40 pA. With 5 mM external Ca2+ or Ba2+, the Ba currents are about twice as large as the Ca currents. Although 100 microM Cd2+ completely blocks the Ca current, organic blockers have very little effect. Variable inactivation characteristics and sensitivity to washout suggest the possibility of multiple types of Ca channels. A search for single-channel Ba currents in the somal membrane was unsuccessful.

Animals↗