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Biomedical subjects

K Lau

Publications and source records attributed to K Lau.

At least 37 records · Page 2Linked to original sources
PubMed1997

A mouse model of human familial adenomatous polyposis.

In an effort to generate a good mouse model for human colorectal cancer, we generated mice which carry a mutation in the adenomatous polyposis coli (Apc) gene. Mice which are heterozygous for the mutation, designated Apc1638, develop colonic polyps and tumors of the small intestine. Neoplasms were found in 96% of animals studied, and they included adenomas, adenocarcinomas, and polypoid hyperplasias. The mice developed an average of 3.3 tumors, with the highest number in duodenum, followed by jejunum, stomach, ileum, and colon. Focal areas of dysplasias were observed in the colonic mucosa in 50% of mice which were 10 months old or older. These results suggest that mice carrying the Apc1638 mutation can serve as a good model to study the initiation, progression, and inhibition of gastrointestinal tumors.

Adenomatous Polyposis Coli↗
PubMed1997

Molecular defects in Hb H hydrops fetalis.

The molecular defect in two unique cases of Hb H hydrops fetalis has been characterized. Both cases are due to co-inheritance of a 'non-deletion' defect affecting the alpha2 gene: at codon 30 delta GAG, Glu) and codon 59 (G --> A, Gly --> Asp) respectively, and a zeta-alpha thalassaemia (thal) 1 or alpha thal 1 genotype. These two non-deletion defects, unlike previously described cases, resulted in severe anaemia of the fetuses and emphasize the importance of performing prenatal diagnosis for these families.

Gene Deletion↗
PubMed1996

Meiotic pachytene arrest in MLH1-deficient mice.

Germ line mutations in DNA mismatch repair genes including MLH1 cause hereditary nonpolyposis colon cancer. To understand the role of MLH1 in normal growth and development, we generated mice that have a null mutation of this gene. Mice homozygous for this mutation show a replication error phenotype, and extracts of these cells are deficient in mismatch repair activity. Homozygous mutant males show normal mating behavior but have no detectable mature sperm. Examination of meiosis in these males reveals that the cells enter meiotic prophase and arrest at pachytene. Homozygous mutant females have normal estrous cycles and reproductive and mating behavior but are infertile. The phenotypes of the mlh1 mutant mice are distinct from those deficient in msh2 and pms2. The different phenotypes of the three types of mutant mice suggest that these three genes may have independent functions in mammalian meiosis.

Adaptor Proteins, Signal Transducing↗
PubMed1996

Epitope mapping of the variable repetitive region with the MB antigen of Ureaplasma urealyticum.

One of the major surface structures of Ureaplasma urealyticum recognized by antibodies of patients during infection is the MB antigen. Previously, we showed by Western blot (immunoblot) analysis that any one of the anti-MB monoclonal antibodies (MAbs) 3B1.5, 5B1.1, and 10C6.6 could block the binding of patient antibodies to MB. Subsequent DNA sequencing revealed that a unique six-amino-acid direct tandem repeat region composed the carboxy two-thirds of this antigen. In the present study, using antibody-reactive peptide scanning of this repeat region, we demonstrated that the amino acids defining the epitopes for MAbs 3B1.5 5B1.1 and 10C6.6 are EQP, GK, and KEQPA, respectively. Peptide scanning analysis of an infected patient's serum antibody response showed that the dominant epitope was defined by the sequence PAGK. Mapping of these continuous epitopes revealed overlap between all MAb and patient polyclonal antibody binding sites, thus explaining the ability of a single MAb to apparently block all polyclonal antibody binding sites. We also show that a single amino acid difference in the sequence of the repeats of serovars 3 and 14 accounts for the lack of reactivity with serovar 14 of two of the serovar 3-specific MAbs. Finally, the data demonstrate the need to obtain the sequences of the mba genes of all serovars before an effective serovar-specific antibody detection method can be developed.

Amino Acid Sequence↗
PubMed1995

SNX-325, a novel calcium antagonist from the spider Segestria florentina.

A novel selective calcium channel antagonist peptide, SNX-325, has been isolated from the venom of the spider Segestria florentina. The peptide was isolated using as bioassays the displacement of radioiodinated omega-conopeptide SNX-230 (MVIIC) from rat brain synaptosomal membranes, as well as the inhibition of the barium current through cloned expressed calcium channels in oocytes. The primary sequence of SNX-325 is GSCIESGKSCTHSRSMKNGLCCPKSRCNCRQIQHRHDYLGKRKYSCRCS, which is a novel amino acid sequence. Solid-phase synthesis resulted in a peptide that is chromatographically identical with the native peptide and which has the same configuration of cysteine residues as the spider venom peptide omega-Aga-IVa [Mintz, I. M., et al., (1992) Nature 355, 827-829]. At micromolar concentrations, SNX-325 is an inhibitor of most calcium, but not sodium or potassium, currents. At nanomolar concentrations, SNX-325 is a selective blocker of the cloned expressed class B (N-type), but not class C (cardiac L), A, or E, calcium channels. SNX-325 is approximately equipotent with the N-channel selective omega-conopeptides (GVIA and MVIIA as well as closely related synthetic derivatives) in blocking the potassium induced release of tritiated norepinephrine from hippocampal slices (IC50s, 0.1-0.5 nM) and in blocking the barium current through cloned expressed N-channels in oocytes (IC50s 3-30 nM). By contrast, SNX-325 is 4-5 orders of magnitude less potent than is SNX-111 (synthetic MVIIA) at displacing radioiodinated SNX-111 from rat brain synaptosomal membranes. SNX-325 will be a useful comparative tool in further defining the function and pharmacology of the N- and possibly other types of high-voltage activated calcium channels.

Amino Acid Sequence↗
PubMed1995

Perinatal and neonatal issues of substance abuse.

Substance abuse during pregnancy can be teratogenic for the fetus and can cause decreased growth parameters in the newborn infant. Short-term and long-term neurobehavioral problems have been documented also in babies born to substance-abusing mothers. The problem of substance abuse during pregnancy and its effects on the fetus is unique in medicine in that it is 100% preventable. Physicians will need to take an active role in leading society to take action in preventing substance abuse during pregnancy, and emphasis on this action should occur long before the woman becomes pregnant. Until medical schools and residency programs take responsibility for teaching the importance of preventing substance abuse and of identifying the substance-abuser, drug and alcohol use will continue to exact its tragic toll on future generations.

Abnormalities, Drug-Induced↗
PubMed1995

Parathyroid hormone action in calcium transport in the distal nephron.

Urinary calcium excretion is regulated homeostatically. Regulation is achieved, in part, by the action of parathyroid hormone on Ca2+ absorption in the distal nephron. Parathyroid hormone increases Ca2+ absorption in the cortical portion of the thick ascending limb of Henle's loop in all species studied, in the murine distal convoluted tubule, and in the rabbit connecting tubule. All of these sites contain parathyroid hormone-stimulated adenylate cyclase. Both cellular and paracellular pathways of Ca2+ absorption are regulated by parathyroid hormone in the cortical portion of the thick ascending limb of Henle's loop. In both distal convoluted and connecting tubule cells, parathyroid hormone regulates transcellular Ca2+ absorption by controlling the insertion and open probability of luminal plasmalemmal Ca2+ ion channels. These channels are stimulated and inhibited by L-type calcium channel agonists and antagonists, respectively, but differ from similar channels in excitable cells in that membrane depolarization does not activate them. Parathyroid hormone also increases the driving force for diffusional Ca2+ ion entry from the luminal fluid into the cytosol by increasing the intracellular negative electrical potential (at least in murine distal convoluted tubule cells) by increasing the chloride ion conductance of the basolateral cell membrane. The effects of parathyroid hormone on the other components of cellular Ca2+ transport, via both protein kinases A and C and their interactions, remain to be examined.

Animals↗
PubMed1994

Purification and characterization of transmembrane forms of heparin-binding EGF-like growth factor.

Heparin-binding epidermal growth factor-like growth factor (HB-EGF), whose cDNA has a predicted 208-codon open reading frame, is synthesized as a membrane-spanning precursor that is processed to release mature mitogenic proteins of approximately 73-87 amino acids in length. Previous work has focused on the structural and biological properties of secreted HB-EGF. In this study, human recombinant transmembrane HB-EGF, produced by expression of HB-EGF1-208 cDNA in a baculovirus system, has been isolated, purified, and characterized structurally and biologically. Two isoforms of transmembrane HB-EGF (HB-EGFTM) were purified from membrane fractions of infected insect cells by a combination of heparin affinity chromatography and reversed-phase high performance liquid chromatography. The isoform designated as HB-EGFTM-1, a 21.5-kDa protein, yielded no N-terminal sequence, suggesting that it is N-terminally blocked. However, HB-EGFTM-II, a 24-kDa protein, was N-terminally sequenced and found to be initiated at Asp63 in the 208-amino acid residue primary translation product. This N terminus is the same as that determined for a 18-kDa isoform of secreted HB-EGF purified from the conditioned medium of insect cells expressing HB-EGF1-149 cDNA and is also identical to the N terminus of the longest form of secreted HB-EGF initially purified from human macrophage-like U-937 cell conditioned medium. HB-EGFTM-II cross-reacted on a Western blot with an antibody directed against the 16 C-terminal amino acids of the cytoplasmic tail of HB-EGF, indicating that it contains a putative transmembrane domain. HB-EGFTM-II was bioactive and stimulated the proliferation of BALB/c 3T3 cells and smooth muscle cells and the motility of smooth muscle cells, albeit with approximately 10-25% of the specific activity of secreted HB-EGF isoforms. We concluded that transmembrane HB-EGF is bioactive when isolated, consistent with the possibility of its functioning as a juxtacrine growth factor when still tethered to the cell.

3T3 Cells↗
PubMed1994

Conformational differences in human apolipoprotein B-100 among subspecies of low density lipoproteins (LDL). Association of altered proteolytic accessibility with decreased receptor binding of LDL subspecies from hypertriglyceridemic subjects.

We asked at what point in the metabolic cascade of very low density lipoproteins (VLDL) to low density lipoproteins (LDL) the accessibility of proteolytic cleavage sites in B-100 changes, and we evaluated the effect of hypertriglyceridemia on the proteolytic accessibility, secondary structure, and receptor-binding affinity of B-100 in LDL subspecies of varying density. Limited proteolysis with Staphylococcus aureus V8 protease and cathepsin D identified the density (about 1.033 g/ml) between two LDL subspecies, designated LDL-1 and -2, as the transition point during VLDL metabolism of both normolipidemic (N-) and hypertriglyceridemic (HTG-) subjects at which accessibility to protease attack changed in three peptide regions of B-100. Hypertriglyceridemia greatly altered proteolytic accessibility of B-100 in the denser LDL subspecies. Specifically, B-100 in HTG-LDL exposed more cleavage sites than in N-LDL, including two novel sites, approximately 120 and approximately 130 kDa from the NH2 terminus in the small and dense subspecies (designated LDL-4, -4.5 or -5, d = 1.048-1.062 g/ml). Analysis of circular dichroic spectra indicated no difference in helical content between B-100 in N- and HTG-LDL but showed a greater content of beta-structure in HTG-LDL. Binding affinity for the LDL receptor of human fibroblasts decreased markedly with increasing density among HTG-LDL subspecies (by approximately 50% for LDL-4.5 or -5). We conclude that the changes in proteolytic accessibility observed between LDL-1 and -2 and in LDL-4, -4.5, or -5 indicate significant differences in local conformation of B-100 at specific peptide regions. The association of exposure of more cleavage sites, especially novel sites in the NH2-terminal regions, with greatly decreased receptor-binding affinity in LDL-4.5 or -5 suggests that altered local conformation in B-100 apart from the putative receptor-binding domain might affect interaction with the receptor.

Apolipoprotein B-100↗
PubMed1994

Identification of proteins associated with apolipoprotein A-I-containing lipoproteins purified by selected-affinity immunosorption.

The isolation of apolipoprotein A-I-containing lipoproteins [Lp(A-I)] by selected-affinity immunosorption minimizes the loss of associated proteins that occurs during the isolation of high-density lipoproteins (HDL) by sequential ultracentrifugation. We have used two-dimensional gel electrophoretic analysis to separate the proteins associated with Lp(A-I). Using a combination of amino acid sequencing of transblotted proteins and Western blotting with specific antisera, we have identified a number of associated proteins. The positions of the apolipoproteins (apo) A-I, A-II, A-IV, C-III, D, and E were located on the gels. Lecithin-cholesterol acyltransferase and cholesteryl ester transfer protein were identified in association with Lp(A-I) to a greater extent than found associated with HDL after centrifugation. In addition to those proteins previously identified in association with HDL, we detected a number of plasma proteins associated with Lp(A-I), namely, fibrinogen, haptoglobin, proline-rich protein (C4b-binding protein), and apolipoprotein J (SP40,40 sulfated glycoprotein). The co-isolation of these proteins with Lp(A-I) does not appear to be an artifact in that they have very low affinity for a sham column containing covalently bound preimmune goat IgG in place of the anti-apoA-I IgG. These findings suggest that in addition to apolipoproteins that exist largely in association with lipoproteins there is another class of proteins which exist in lipoprotein-associated form and in the dispersed state. Detection and identification of these lipoprotein-associated proteins may aid in the mechanistic determination of a number of observed functions attributed to HDL.

Amino Acid Sequence↗
PubMed1994

Characterization of sequences within heparin-binding EGF-like growth factor that mediate interaction with heparin.

Heparin-binding (HB) epidermal growth factor (EGF)-like growth factor (HB-EGF), a member of the EGF protein family, is a potent mitogen for fibroblasts, smooth muscle cells, and keratinocytes that was initially identified as a secreted product of macrophage-like cells. HB-EGF and EGF appear to act on target cells utilizing the same receptor, but HB-EGF is distinguishable from EGF by its strong affinity for heparin. To facilitate studies of structure-function relationships in HB-EGF, a bacterial recombinant expression system was established that produced biologically active HB-EGF with the expected disulfide bonding pattern. Mutagenesis and protease digestion studies of the recombinant HB-EGF, coupled with heparin-binding analyses of synthetic peptides, indicated that the sequences within HB-EGF mediating its interaction with heparin are located primarily in a stretch of 21 amino acids characterized by a high content of lysine and arginine residues. Most of this heparin-binding domain lies in an amino-terminal region of HB-EGF that has no counterpart in EGF, but a portion of the 21-residue sequence extends into the EGF-like region of HB-EGF. In addition, the mutagenesis and synthetic peptide studies indicated that sequences in HB-EGF lying outside of the 21-residue stretch can also influence the interaction with heparin. Finally, a synthetic peptide derived from the 21-residue stretch was found to compete with HB-EGF for binding to Chinese hamster ovary cells, suggesting that the heparin-binding sequences in HB-EGF may also mediate the interaction of this factor with cell surface heparan sulfate proteoglycan.

Amino Acid Sequence↗
PubMed1994

Characterization of the binding of omega-conopeptides to different classes of non-L-type neuronal calcium channels.

The interaction of two synthetic omega-conopeptides SNX-111 (MVIIA) and SNX-230 (MVIIC) both derived from the marine snail Conus magus, with non-L-type neuronal voltage-sensitive calcium channels (VSCC) in rat brain synaptosomal preparations has been investigated with the aid of well-characterized 125I derivatives of the two peptides. To assess the effects of iodination on the binding characteristics of SNX-111 and SNX-230, the corresponding peptides containing monoiodotyrosine in place of tyrosine, namely, SNX-259 ([127I]SNX-111) and SNX-260 ([127I]SNX-230), respectively, were prepared by solid-phase synthesis. Saturation analysis showed that [125I]SNX-111 and [125I]SNX-230 bound to two distinct classes of high-affinity sites with apparent Kd's of 9 and 11 pM and Bmax's of 0.54 and 2.2 pmol/mg protein, respectively. Kinetic analysis revealed that both peptides exhibited high association rates as well as rapid dissociation rates in contrast to the 125I derivative of the synthetic omega-conopeptide from Conus geographus, GVIA (SNX-124), which binds irreversibly to N-type channels in rat brain synaptosomes. Competition binding experiments with [125I]SNX-111 and [125I]SNX-124 established that both of them bind to the same site, namely, N-type VSCC. The site detected by the binding of [125I]SNX-230 is distinct from N-type VSCC since SNX-111 has very low affinity (K(i) = 135 nM) in competition studies. Recent findings that a novel high-voltage-activated calcium channel in rat cerebellar granule neurons is resistant to blockers of L-, N-, and P-type VSCC but is highly sensitive to SNX-230 suggest that the [125I]SNX-230 binding site may represent this novel type of calcium channel or another, as yet undescribed, VSCC.

Animals↗
PubMed1994

The fundamental motor of the human neutrophil is not random: evidence for local non-Markov movement in neutrophils.

The search for a fundamental mechano-chemical process that results in net cell motion has led investigators to fit neutrophil tracking data to well described physical models in hopes of understanding the functional form of the driving force. The Ornstein-Uhlenbeck (OU) equation for mean square displacement describes a locally persistent and globally random process and is often used as a starting point for analysis of neutrophil displacements. Based upon the apparently close fit of neutrophil tracking data to this equation and the nature of its derivation, biologists have inferred that the motor of the neutrophil is best represented as a random process. However, 24 of 37 neutrophil paths that we investigated preferentially display programmatic rather than Markov short term correlations between displacements or turn angles. These correlations reflect a bimodal rather than a uniform distribution of subpath correlations in the two variables, and are strongly sampling rate-dependent. Significant periodic components of neutrophil shape change are also detected at the same time scale using either Fourier or elliptical Fourier transform-based descriptors of the neutrophil perimeter. Oscillations in neutrophil velocity have the same period. Taken together, these data suggest a nonstochastic, and perhaps periodic, component to the process driving neutrophil movement.

Biophysical Phenomena↗
PubMed1993

Development of a shape vector that identifies critical forms assumed by human polymorphonuclear neutrophils during chemotaxis.

Human polymorphonuclear neutrophils undergo characteristic shape changes that are critical to their ability to move and ingest their targets. We present here the construction of a simple shape vector, calculated from the coordinates of the cell perimeter, that can identify critical forms that a neutrophil assumes during the course of ameboid movement. The vector can be used to find neutrophils of a specific shape from the image analyzer data produced during a typical neutrophil tracking experiment.

Amanitins↗
PubMed1993

False non-paternity in a family for prenatal diagnosis of beta-thalassaemia.

Initial screening for the common beta-thalassaemia mutations with allele-specific oligonucleotide probes in an at-risk family suggested non-paternity. Subsequent DNA fingerprinting of the members proved otherwise. The mother had a codon 41/42 frameshift mutation and the father's defect, determined by direct sequencing of PCR-amplified beta gene, was a codon 43 nonsense mutation. In the affected children, the close proximity of these two defects resulted in the absence of a hybridization signal to the normal probe in that region and a wrong assumption of homozygosity for the codon 41/42 mutation. The non-reactivity of the father's amplified DNA to the codon 41/42 thalassaemic probe accounted for the initial wrong conclusion of non-paternity. Since prior screening for beta-thalassaemia mutations is done in all prenatal diagnosis programmes and concomitant inheritance of these two defects is relatively common in the Chinese, this 'artefact' of false non-paternity is worth noting.

Adult↗