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Biomedical subjects

J Bryan

Publications and source records attributed to J Bryan.

At least 55 records · Page 3Linked to original sources
PubMed1999

Two regions of sulfonylurea receptor specify the spontaneous bursting and ATP inhibition of KATP channel isoforms.

KATP channels are heteromultimers of KIR6.2 and a sulfonylurea receptor, SUR, an ATP binding cassette (ABC) protein with several isoforms. KIR6.2 forms a channel pore whose spontaneous activity and ATP sensitivity are modulated by the receptor via an unknown interaction(s). Side by side comparison of single-channel kinetics and steady-state ATP inhibition of human beta-cell, SUR1/KIR6.2, versus cardiac, SUR2A/KIR6.2 channels demonstrate that the latter have a greater mean burst duration and open probability in the absence of nucleotides and approximately 4-fold higher IC50(ATP). We have used matched chimeras of SUR1 and SUR2A to show that the kinetics, which determine the maximal open probability (Pomax), and the ATP sensitivity are functionally separable and to identify the two segments of SUR responsible for these isoform differences. A region within the first five transmembrane domains specifies the interburst kinetics, whereas a C-terminal segment determines the sensitivity to inhibitory ATP. The separable effects of SUR on ATP inhibition and channel kinetics implies that the cytoplasmic C terminus of SUR either directly modulates the affinity of a weak ATP binding site on the inward rectifier or affects linkage between the binding site and the gate. This is the first identification of parts of an ABC protein that interact with an ion channel subunit to modulate the spontaneous activity and ATP sensitivity of the heteromeric channel.

ATP-Binding Cassette Transporters↗
PubMed1999

Sulfonylurea receptors set the maximal open probability, ATP sensitivity and plasma membrane density of KATP channels.

KATP channels are heteromultimers of SUR and KIR6.2. C-terminal truncation of KIR6.2 allows surface expression of the pore. KIR6.2deltaC35 channels display approximately 7-fold lower maximal open probability, approximately 35-fold reduced ATP sensitivity, reduced mean open time, a markedly increased transition rate from a burst into a long-lived closed state, and have no counterpart in vivo. SUR1 and SUR2A restore wild-type bursting, ATP sensitivity and increase channel density in the plasma membrane. The high IC50(ATP) of approximately 4 mM for KIR6.2deltaCK185Q channels results from the additive effects of SUR removal and KIR6.2 modification. The results demonstrate allosteric interaction(s) are essential for normal intrinsic activity, ATP inhibition, and trafficking of KATP channels.

ATP-Binding Cassette Transporters↗
PubMed1999

The N-terminus of KIR6.2 limits spontaneous bursting and modulates the ATP-inhibition of KATP channels.

KATP channels are heteromultimers of a sulfonylurea receptor SUR and KIR6.2 with the inward rectifier forming the pore which is regulated by SUR. We have examined the contributions of the cytoplasmic domains of KIR6.2 to control of spontaneous bursting and ATP-inhibition in human SUR1/KIR6.2 KATP channels. Truncations of the N-terminus of KIR6.2 nearly eliminate transitions to interburst closed states without affecting the open or intraburst closed states, thus producing SUR1/DeltaNKIR6.2 channels with an extremely high open probability in the absence of nucleotides. These channels have a decrease apparent ATP-sensitivity which is consistent with the involvement of the N-terminus in a transition to an interburst closed state that preferentially binds inhibitory ATP. Mutations in both the N- and proximal C-termini of KIR6.2 can synergistically attenuate the ATP-inhibition. The results identify the N-terminus of KIR6.2 as a determinant of the interburst kinetics of KATP channels and suggest that the two cytoplasmic domains of KIR6.2 participate in ATP-inhibitory gating through distinct mechanisms.

ATP-Binding Cassette Transporters↗
PubMed1999

Carbohydrate-deficient glycoprotein syndrome in a newborn with an unbalanced chromosomal translocation.

UNLABELLED: Carbohydrate-deficient glycoprotein syndromes may occur as a primary result of distinct genetic disruption of the enzymes involved in processing the carbohydrate moeities of glycoproteins. They may also occur due to a number of secondary defects in glycosylation. CONCLUSION: A female infant with an unbalanced chromosomal translocation [46,XX,der(21)t(17;21) (p13.1;q22.11)mat.ish der(21)t(17;21) (D17S375 x 3, D21S65-)] and with biochemical and clinical features of a carbohydrate deficient glycoprotein syndrome is reported. This chromosomal disruption is another secondary cause of the disorder.

Congenital Disorders of Glycosylation↗
PubMed1999

Refinement of the RP17 locus for autosomal dominant retinitis pigmentosa, construction of a YAC contig and investigation of the candidate gene retinal fascin.

The RP17 locus for autosomal dominant retinitis pigmentosa has previously been mapped to chromosome 17q by linkage analysis. Two unrelated South African families are linked to this locus and the identification of key recombination events assigned the RP17 locus to a 10 cM interval on 17q22. The work reported here refines the mapping of the locus from a 10 cM to a 1 cM interval between the microsatellite markers D17S1604 and D17S948. A physical map of this interval was constructed using information from the Whitehead/MIT YAC contig WC 17.8. Sequence-tagged site (STS) content mapping of seven overlapping YACs from this contig was employed in order to build the map. A BAC library was screened to cover a gap in the YAC contig and two positive BACs were identified. Intragenic polymorphisms in the retinal fascin gene provided evidence for the exclusion of this candidate as the RP17 disease gene.

Carrier Proteins↗
PubMed1999

Molecular biology of adenosine triphosphate-sensitive potassium channels.

KATP channels are a newly defined class of potassium channels based on the physical association of an ABC protein, the sulfonylurea receptor, and a K+ inward rectifier subunit. The beta-cell KATP channel is composed of SUR1, the high-affinity sulfonylurea receptor with multiple TMDs and two NBFs, and KIR6.2, a weak inward rectifier, in a 1:1 stoichiometry. The pore of the channel is formed by KIR6.2 in a tetrameric arrangement; the overall stoichiometry of active channels is (SUR1/KIR6.2)4. The two subunits form a tightly integrated whole. KIR6.2 can be expressed in the plasma membrane either by deletion of an ER retention signal at its C-terminal end or by high-level expression to overwhelm the retention mechanism. The single-channel conductance of the homomeric KIR6.2 channels is equivalent to SUR/KIR6.2 channels, but they differ in all other respects, including bursting behavior, pharmacological properties, sensitivity to ATP and ADP, and trafficking to the plasma membrane. Coexpression with SUR restores the normal channel properties. The key role KATP channel play in the regulation of insulin secretion in response to changes in glucose metabolism is underscored by the finding that a recessive form of persistent hyperinsulinemic hypoglycemia of infancy (PHHI) is caused by mutations in KATP channel subunits that result in the loss of channel activity. KATP channels set the resting membrane potential of beta-cells, and their loss results in a constitutive depolarization that allows voltage-gated Ca2+ channels to open spontaneously, increasing the cytosolic Ca2+ levels enough to trigger continuous release of insulin. The loss of KATP channels, in effect, uncouples the electrical activity of beta-cells from their metabolic activity. PHHI mutations have been informative on the function of SUR1 and regulation of KATP channels by adenine nucleotides. The results indicate that SUR1 is important in sensing nucleotide changes, as implied by its sequence similarity to other ABC proteins, in addition to being the drug sensor. An unexpected finding is that the inhibitory action of ATP appears to be through a site located on KIR6.2, whose affinity for ATP is modified by SUR1. A PHHI mutation, G1479R, in the second NBF of SUR1 forms active KATP channels that respond normally to ATP, but fail to activate with MgADP. The result implies that ATP tonically inhibits KATP channels, but that the ADP level in a fasting beta-cell antagonizes this inhibition. Decreases in the ADP level as glucose is metabolized result in KATP channel closure. Although KATP channels are the target for sulfonylureas used in the treatment of NIDDM, the available data suggest that the identified KATP channel mutations do not play a major role in diabetes. Understanding how KATP channels fit into the overall scheme of glucose homeostasis, on the other hand, promises insight into diabetes and other disorders of glucose metabolism, while understanding the structure and regulation of these channels offers potential for development of novel compounds to regulate cellular electrical activity.

Adenosine Triphosphate↗
PubMed1999

Interactive volume visualizations for synchronous and asynchronous remote collaboration.

The value of information acquisition is not simply to acquire data at increased precision, but to make these data both available and usable to as many specialists as possible. We have developed a working prototype for intuitive realtime interaction over the Internet. Current scenarios include use by expert head and neck surgeons, for use in preoperative assessment of head and neck cancers, and remote synchronous collaboration scenarios such as expert-expert and expert-referring physician. This prototype demonstrates a shared virtual environment that allows multiple users to interactively manipulate large volumetric data sets across long distances. The system is easily extensible to many other applications that utilize discrete data sampling and thus is readily extensible to a wide range of scientific investigations.

Head and Neck Neoplasms↗
PubMed1998

Reconstituted human cardiac KATP channels: functional identity with the native channels from the sarcolemma of human ventricular cells.

ATP-sensitive potassium (KATP) channels in striated myocytes are heteromultimers of KIR6.2, a weak potassium inward rectifier, plus SUR2A, a low-affinity sulfonylurea receptor. We have cloned human KIR6.2 (huKIR6.2) and a huSUR2A that corresponds to the major, full-length splice variant identified by polymerase chain reaction analysis of human cardiac poly A+ mRNA. ATP- and glibenclamide-sensitive K+ channels were produced when both subunits were coexpressed in COSm6 and Chinese hamster ovary cells lacking endogenous KATP channels, but not when huSUR2A or huKIR6.2 were transfected alone. Recombinant channels activated by metabolic inhibition in cell-attached configuration or in inside-out patches with ATP-free internal solution were compared with sarcolemmal KATP channels in human ventricular cells. The single-channel conductance of approximately 80 pS measured at -40 mV in quasi-symmetrical approximately 150 mmol/L K+ solutions, the intraburst kinetics that were dependent on K+ driving force, and the weak inward rectification were indistinguishable for both channels. Similar to the native channels, huSUR2A/huKIR6.2 recombinant channels were inhibited by ATP at quasi-physiological free Mg2+ ( approximately 0. 7 mmol/L) or in the absence of Mg2+, with an apparent IC50 of approximately 20 micromol/L and a pseudo-Hill coefficient of approximately 1. They were "refreshed" by MgATP and stimulated by ADP in the presence of Mg2+ when inhibited by ATP. The huSUR2A/huKIR6.2 channels were stimulated by cromakalim and pinacidil in the presence of ATP and Mg2+ but were insensitive to diazoxide. The results suggest that reconstituted huSUR2A/huKIR6.2 channels represent KATP channels in sarcolemma of human cardiomyocytes and are an adequate experimental model with which to examine structure-function relationships, molecular physiology, and pharmacology of these channels from human heart.

ATP-Binding Cassette Transporters↗
PubMed1998

Report of the Australian Malaria Register for 1992 and 1993.

Australia is free from endemic malaria but several hundred imported cases occur each year. Notification and screening data on malaria cases are collected by State and Territory health authorities and laboratories and forwarded to the Australian Malaria Register (AMR) for national collation and analysis. This report provides information on 758 malaria cases with 5 deaths reported in Australia in 1992 and 712 cases with 1 death in 1993. In both years, just over 70% of cases were male and the modal age group was 20 to 29 years. Cases were reported from all States and Territories, with Queensland reporting the greatest number of cases in both years. The predominant species was Plasmodium vivax, although P. falciparum accounted for just over a quarter of the cases each year. Papua New Guinea (PNG) was the most common source of cases in both years, reflecting the number of people who move between Australia and PNG and the high endemicity of malaria in PNG. The incidence of malaria was also high in travellers from the Solomon Islands in both years and from Ghana in 1992 and Nigeria in 1993. The six deaths over two years highlight the need for medical practitioners to consider malaria as a diagnosis in patients with a history of travel to malarious countries and to provide appropriate advice on malaria prophylaxis to intending travellers.

Age Distribution↗
PubMed1998

Potassium channel openers require ATP to bind to and act through sulfonylurea receptors.

KATP channels are composed of a small inwardly rectifying K+ channel subunit, either KIR6.1 or KIR6.2, plus a sulfonylurea receptor, SUR1 or SUR2 (A or B), which belong to the ATP-binding cassette superfamily. SUR1/KIR6.2 reconstitute the neuronal/pancreatic beta-cell channel, whereas SUR2A/KIR6.2 and SUR2B/KIR6.1 (or KIR6.2) are proposed to reconstitute the cardiac and the vascular-smooth-muscle-type KATP channels, respectively. We report that potassium channel openers (KCOs) bind to and act through SURs and that binding to SUR1, SUR2A and SUR2B requires ATP. Non-hydrolysable ATP-analogues do not support binding, and Mg2+ or Mn2+ are required. Point mutations in the Walker A motifs or linker regions of both nucleotide-binding folds (NBFs) abolish or weaken [3H]P1075 binding to SUR2B, rendering reconstituted SUR2B/KIR6.2 channels insensitive towards KCOs. The C-terminus of SUR affects KCO affinity with SUR2B approximately SUR1 > SUR2A. KCOs belonging to different structural classes inhibited specific [3H]P1075 binding to SUR2B in a monophasic manner, with the exception of minoxidil sulfate, which induced a biphasic displacement. The affinities of KCO binding to SUR2B were 3.5-8-fold higher than their potencies for activation of SUR2B/KIR6.2 channels. The results establish that SURs are the KCO receptors of KATP channels and suggest that KCO binding requires a conformational change induced by ATP hydrolysis in both NBFs.

ATP-Binding Cassette Transporters↗