Modular design of the selectivity filter pore loop in a novel family of prokaryotic inward rectifier' (NirBac) channels

Potassium channels exhibit a modular design with distinct structural and functional domains; in particular, a highly conserved pore-loop sequence that determines their ionic selectivity. We now report the functional characterisation of a novel group of functionally non-selective members of the prokaryotic inward rectifier' subfamily of K + channels. These channels share all the key structural domains of eukaryotic and prokaryotic Kir/KirBac channels, but instead possess unique pore-loop selectivity filter sequences unrelated to any other known ionic selectivity filter. The strikingly unusual architecture of these NirBac' channels defines a new family of functionally non-selective ion channels, and also provides important insights into the modular design of ion channels, as well as the evolution of ionic selectivity within this superfamily of tetrameric cation channels

Coincidence of a Novel KCNJ11 Missense Variant R365H With a Paternally Inherited 6q24 Duplication in a Patient With Transient Neonatal Diabetes

OBJECTIVE—Neonatal diabetes is a heterogeneous group of disorders with diabetes manifestation in the first 6 months of life. The most common etiology in permanent neonatal diabetes is mutations of the ATP-sensitive K+ channel subunits; in transient neonatal diabetes, chromosome 6q24 abnormalities are the most common cause

Adjacent mutations in the gating loop of Kir6.2 produce neonatal diabetes and hyperinsulinism

K(ATP) channels regulate insulin secretion from pancreatic beta-cells. Loss- and gain-of-function mutations in the genes encoding the Kir6.2 and SUR1 subunits of this channel cause hyperinsulinism of infancy and neonatal diabetes, respectively. We report two novel mutations in the gating loop of Kir6.2 which cause neonatal diabetes with developmental delay (T293N) and hyperinsulinism (T294M). These mutations increase (T293N) or decrease (T294M) whole-cell K(ATP) currents, accounting for the different clinical phenotypes. The T293N mutation increases the intrinsic channel open probability (Po((0))), thereby indirectly decreasing channel inhibition by ATP and increasing whole-cell currents. T294M channels exhibit a dramatically reduced Po((0)) in the homozygous but not in the pseudo-heterozygous state. Unlike wild-type channels, hetT294M channels were activated by MgADP in the absence but not in the presence of MgATP; however, they are activated by MgGDP in both the absence and presence of MgGTP. These mutations demonstrate the importance of the gating loop of Kir channels in regulating Po((0)) and further suggest that Mg-nucleotide interaction with SUR1 may reduce ATP inhibition at Kir6.2.We thank the Wellcome Trust (076436/Z/05/Z and 081188/A/06/Z), the Royal Society and the European Union (EuroDia, SHM‐CT‐2006‐518513 and EDICT, 201924) for support. FMA is a Royal Society Research Professor. Brittany Zadek was supported by an OXION studentship and Sarah Flanagan by a Sir Graham Wilkins Research Fellowship

Structure and function of bacterial ion channels

KirBac channels are prokaryotic homologs of eukaryotic inwardly-rectifying potassium channels, which have served as models for gaining insight into the structure of eukaryotic channels. This thesis focuses on the structure-function relationship in these channels. The first part of this study concerns a novel KirBac channel, KirBac9.2, which contains a unique amino acid sequence in the place of the canonical GYG selectivity filter. Although expressed and purified in a stable and functional form, the protein did not form well-diffracting crystals. Functional studies suggest that KirBac9.2 is non-selective for monovalent cations and a random mutagenesis screen identified a number of activatory mutants in the cytoplasmic domains of the channel. A full electrophysiological investigation of KirBac9.2 channel function is beyond the scope of this study. However, initial studies suggest that it is possible to record currents from KirBac9.2 channels reconstituted into lipid bilayers. The second part of this thesis investigates KirBac3.1, which is a classical KirBac channel containing the consensus GYG sequence for potassium selectivity. Five high resolution structures of a mutant channel are reported, which suggest a new feature in the gating mechanism of KirBac3.1 where a rotation of the cytoplasmic domains is linked to a change in the electrostatic environment of the cytoplasmic cavity. In addition, a functional study of the KirBac3.1 showed that the channel is highly pH sensitive.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Glucose regulates the effects of leptin on hypothalamic POMC neurons

Leptin is believed to exert its potent appetite-suppressing effects via stimulation of hypothalamic anorexigenic proopiomelanocortin (POMC)-containing neurons and inhibition of orexigenic agouti-related protein (AgRP) neurons. We show here that at 11 mM glucose leptin excites POMC cells. At 5 mM glucose, however, leptin hyperpolarizes POMC neurons and suppresses action potential firing, by producing a greater decrease in excitatory synaptic tone than inhibitory tone. These results argue that when glucose is low (5 mM or less) AgRP neurons will be more important for mediating the anorectic effects of leptin than POMC cells. However, at high glucose concentrations (11 mM), activation of POMC cells may contribute to the appetite-suppressing effects of leptin. Our data also suggest the regulation of neuropeptide efficacy as a novel function of hypothalamic glucose sensing

Expression of an activating mutation in the gene encoding the KATP channel subunit Kir6.2 in mouse pancreatic β cells recapitulates neonatal diabetes

Neonatal diabetes is a rare monogenic form of diabetes that usually presents within the first six months of life. It is commonly caused by gain-of-function mutations in the genes encoding the Kir6.2 and SUR1 subunits of the plasmalemmal ATP-sensitive K+ (KATP) channel. To better understand this disease, we generated a mouse expressing a Kir6.2 mutation (V59M) that causes neonatal diabetes in humans and we used Cre-lox technology to express the mutation specifically in pancreatic β cells. These β-V59M mice developed severe diabetes soon after birth, and by 5 weeks of age, blood glucose levels were markedly increased and insulin was undetectable. Islets isolated from β-V59M mice secreted substantially less insulin and showed a smaller increase in intracellular calcium in response to glucose. This was due to a reduced sensitivity of KATP channels in pancreatic β cells to inhibition by ATP or glucose. In contrast, the sulfonylurea tolbutamide, a specific blocker of KATP channels, closed KATP channels, elevated intracellular calcium levels, and stimulated insulin release in β-V59M β cells, indicating that events downstream of KATP channel closure remained intact. Expression of the V59M Kir6.2 mutation in pancreatic β cells alone is thus sufficient to recapitulate the neonatal diabetes observed in humans. β-V59M islets also displayed a reduced percentage of β cells, abnormal morphology, lower insulin content, and decreased expression of Kir6.2, SUR1, and insulin mRNA. All these changes are expected to contribute to the diabetes of β-V59M mice. Their cause requires further investigation