CMOS-compatible, piezo-optomechanically tunable photonics for visible wavelengths and cryogenic temperatures.

We demonstrate a platform for phase and amplitude modulation in silicon nitride photonic integrated circuits via piezo-optomechanical coupling using tightly mechanically coupled aluminum nitride actuators. The platform, fabricated in a CMOS foundry, enables scalable active photonic integrated circuits for visible wavelengths, and the piezoelectric actuation functions without performance degradation down to cryogenic temperatures. As an example of the potential of the platform, we demonstrate a compact (∼40 µm diameter) silicon nitride ring resonator modulator operating at 780 nm with intrinsic quality factors in excess of 1.5 million, >10 dB change in extinction ratio with 2 V applied, a switching time less than 4 ns, and a switching energy of 0.5 pJ/bit. We characterize the exemplary device at room temperature and 7 K. At 7 K, the device obtains a resistance of approximately 20 teraohms, allowing it to operate with sub-picowatt electrical power dissipation. We further demonstrate a Mach-Zehnder modulator constructed in the same platform with piezoelectrically tunable phase shifting arms, with 750 ns switching time constant and 20 nW steady-state power dissipation at room temperature.

The response of the tandem pore potassium channel TASK-3 (K(2P)9.1) to voltage: gating at the cytoplasmic mouth.

Although the tandem pore potassium channel TASK-3 is thought to open and shut at its selectivity filter in response to changes of extracellular pH, it is currently unknown whether the channel also shows gating at its inner, cytoplasmic mouth through movements of membrane helices M2 and M4. We used two electrode voltage clamp and single channel recording to show that TASK-3 responds to voltage in a way that reveals such gating. In wild-type channels, P(open) was very low at negative voltages, but increased with depolarisation. The effect of voltage was relatively weak and the gating charge small, 0.17. Mutants A237T (in M4) and N133A (in M2) increased P(open) at a given voltage, increasing mean open time and the number of openings per burst. In addition, the relationship between P(open) and voltage was shifted to less positive voltages. Mutation of putative hinge glycines (G117A, G231A), residues that are conserved throughout the tandem pore channel family, reduced P(open) at a given voltage, shifting the relationship with voltage to a more positive potential range. None of these mutants substantially affected the response of the channel to extracellular acidification. We have used the results from single channel recording to develop a simple kinetic model to show how gating occurs through two classes of conformation change, with two routes out of the open state, as expected if gating occurs both at the selectivity filter and at its cytoplasmic mouth.

The selectivity, voltage-dependence and acid sensitivity of the tandem pore potassium channel TASK-1: contributions of the pore domains.

We have investigated the contribution to ionic selectivity of residues in the selectivity filter and pore helices of the P1 and P2 domains in the acid sensitive potassium channel TASK-1. We used site directed mutagenesis and electrophysiological studies, assisted by structural models built through computational methods. We have measured selectivity in channels expressed in Xenopus oocytes, using voltage clamp to measure shifts in reversal potential and current amplitudes when Rb+ or Na+ replaced extracellular K+. Both P1 and P2 contribute to selectivity, and most mutations, including mutation of residues in the triplets GYG and GFG in P1 and P2, made channels non-selective. We interpret the effects of these--and of other mutations--in terms of the way the pore is likely to be stabilised structurally. We show also that residues in the outer pore mouth contribute to selectivity in TASK-1. Mutations resulting in loss of selectivity (e.g. I94S, G95A) were associated with slowing of the response of channels to depolarisation. More important physiologically, pH sensitivity is also lost or altered by such mutations. Mutations that retained selectivity (e.g. I94L, I94V) also retained their response to acidification. It is likely that responses both to voltage and pH changes involve gating at the selectivity filter.

The selectivity filter of the tandem pore potassium channel TASK-1 and its pH-sensitivity and ionic selectivity.

We have studied pH sensitivity and ionic selectivity of the tandem pore K(+) channel TASK-1 heterologously expressed in Xenopus oocytes. We fit pH sensitivity assuming that only one of the two residues H98 need be protonated for channels to be shut. The effect of protons was weakly voltage dependent with a p K(a) of 6.02 at +40 mV. Replacement of His (H98D, H98N) reduced pH sensitivity but did not abolish it. Use of a concatameric channel permitted replacement of one His residue only; this concatamer was fully pH-sensitive. Increasing the number of His residues to 4 (mutant D204H) abolished pH sensitivity over the physiological range. The implication that D204 plays a role in pH-sensitivity was confirmed by the finding that pH sensitivity over the physiological range was also abolished in the mutant D204N. Ionic selectivity was also altered in D204H, D204N and H98D mutants. P(Rb)/ P(K) was increased from 0.80+/-0.04 (n=19) in wild type to 1.06+/-0.04 (n=19) in D204H. H98D, D204H and D204N were permeable to Na(+) with P(Na)/ P(K)=0.39+/-0.03 (n=14) in H98D, 0.64+/-0.04 (n=18) in D204H and 0.33+/-0.07 (n=3) in D204N. Thus, the arrangement of ring of residues HDHD appears to optimise both pH sensitivity and ionic selectivity.

TASK-5, a novel member of the tandem pore K+ channel family.

We have cloned a novel member of the tandem pore K+ channel family from human brain cDNA. The novel cDNA encodes a 330-residue polypeptide of predicted molecular mass 36 kDa. We have named the channel TASK-5 owing to its sequence homology with TASK-1 and TASK-3. TASK-5 mRNA is expressed in pancreas, liver, kidney, lung, ovary, testis and heart. However, expression of TASK-5 in heterologous systems failed to elicit ionic currents. Removal of a putative endoplasmic reticulum retention sequence did not alter this finding and the distribution of channel proteins in HEK293 cells was similar for both TASK-1 and TASK-5. We tested whether TASK-5 could form heteromers with TASK-1. We show a mutant form of TASK-1 (H98N) to have a radically reduced sensitivity to acidification. Proton sensitivity could be rescued by injecting equimolar amounts of wild-type and mutant TASK-1 cRNA into Xenopus oocytes; the effect was that expected if half the channels formed are heteromers. Co-expression of TASK-5 with TASK-1 H98N does not affect the proton sensitivity of mutant TASK-1; thus TASK-5 appears not to form heteromers with TASK-1. Nonetheless, TASK-5 may require some other, unidentified partner subunit to form functional channels in the plasma membrane or it may form a channel in an intracellular organelle.

An improved cell isolation technique for studying intracellular Ca(2+) homeostasis in neurones of the cochlear nucleus.

Neurones isolated from various parts of the brain are used extensively for electrophysiological and immuncytochemical studies, as well as to investigate their Ca(2+) homeostasis. In this work we report on an isolation technique that yielded neurones suitable for functional studies targeting the investigation of their Ca(2+) handling mechanisms. The cell isolation involved enzymatic dissociation with combined collagenase/pronase treatment and gentle mechanical trituration. At the end of the isolation the cells were incubated in a cell culture incubator (CO2 concentration = 5.1%) at 37 degrees C in Dulbecco's modified Eagle's medium (DMEM) supplemented with 10% heat-inactivated horse serum. The vitality of the isolated cells was indicated by their low intracellular Ca(2+) concentrations (17.2 +/- 0.5 nM; n = 38) and by their ability to produce large Ca(2+) transients on depolarization. These Ca(2+) transients were rapidly terminated and the resting intracellular Ca(2+) concentration was quickly restored proving that isolation did not compromise the Ca(2+) homeostatic mechanisms of the nerve cells. The technique allowed reliable, long (45-60 min) and reproducible measurements of Ca(2+) currents on these neurones as well as the recording of their intracellular Ca(2+) concentration. Our results indicate that incubation in DMEM with horse serum markedly increases the number of surviving neurones after the enzyme treatment, and their Ca(2+) homeostasis can be studied for significantly longer periods of time.

The K+ channel signature sequence of murine Kir2.1: mutations that affect microscopic gating but not ionic selectivity.

1. We have studied the effects on ionic selectivity and gating of Kir2.1 of replacing Tyr (Y) in the GYG signature sequence with Phe (Y145F), Leu (Y145L), Met (Y145M), Ala (Y145A) or Val (Y145V). 2. The mutant Y145F showed no changes in ionic selectivity (as indicated by the permeability coefficient ratios PNa/PK or PRb/PK), indicating that a hydrogen bond between Tyr and other residues is not essential for K+ selectivity. Y145L, Y145M, Y145A and Y145V did not express as monomers. 3. None of the channels made from covalently linked tandem dimers with wild-type and mutant subunits (WT-mutant) had altered ionic selectivity (PNa/PK or PRb/PK), indicating that 4-fold symmetry is not required. 4. Macroscopic currents activated under hyperpolarization and the time constants for activation were reduced e-fold per 23 mV hyperpolarization in wild-type. This gating, believed to be due to the release of polyamines from the pore, was little affected by mutation of Y14. There was similarly little effect on the relationship between chord conductance (gK) and membrane potential. 5. Unitary conductance (140 mM [K+]o) was also little affected by mutation and was reduced only in channels formed from WT-Y145M, from 22.7 +/- 0.4 pS (n = 5) in wild-type to 17.1 +/- 0.5 pS (n = 4) in WT-Y145M. 6. Steady-state recording of unitary currents showed that channel open times were affected by the residue that replaced Tyr in GYG. Channel openings were particularly brief in WT-Y145V, where the mean open time was reduced from 102 ms at -120 mV in wild-type to 6 ms in WT-Y145V. 7. Thus in Kir2.1, GFG can act as a K+ selectivity filter, as can G(L/M/A/V)G, at least in dimers also containing GYG. Channel open time duration depended on the residue at position 145, consistent with the H5 region helping to determine the dwell time of the channel in the open state.

Potassium-depolarization-induced cytoplasmic [Ca2+] transient in freshly dissociated pyramidal neurones of the rat dorsal cochlear nucleus.

The significance of voltage-activated Ca2+ currents in eliciting cytoplasmic Ca2+ transients was studied in pyramidal neurones isolated from the rat dorsal cochlear nucleus using combined enzyme treatment/mechanical trituration. Increases in cytoplasmic Ca2+ concentration ([Ca2+]i) were evoked by K+-induced depolarizations (10-50 mM) and monitored by the Fura-2 fluorimetric technique. The acutely dissociated neurones had a resting [Ca2+]i of 17.2+/-0.5 nM. They possessed caffeine-sensitive Ca2+ stores which were empty at rest; these stores could be filled with Ca2+ entering from the extracellular space and were re-emptied quickly. The effects of various specific high-voltage-activated (HVA) Ca2+ channel antagonists (nifedipine, omega-agatoxin IVA and omega-conotoxin GVIA) on [Ca2+]i transients were tested. Analysis of the blocking effects of these agents on the [Ca2+]i, transients indicates that, in the pyramidal neurones of the dorsal cochlear nucleus, N-type Ca2+ channels are primarily responsible for producing the depolarization-induced increases in [Ca2+]i.

Residues beyond the selectivity filter of the K+ channel kir2.1 regulate permeation and block by external Rb+ and Cs+.

1. Kir2.1 channels are blocked by Rb+ and Cs+ in a voltage-dependent manner, characteristic of many inward rectifier K+ channels. Mutation of Ser165 in the transmembrane domain M2 to Leu (S165L) abolished Rb+ blockage and lowered Cs+ blocking affinity. At negative voltages Rb+ carried large inward currents. 2. A model of the Kir2.1 channel, built by homology with the structure of the Streptomyces lividans K+ channel KcsA, suggested the existence of an intersubunit hydrogen bond between Ser165 and Thr141 in the channel pore-forming P-region that helps stabilise the structure of this region. However, mutations of Thr141 and Ser165 did not produce effects consistent with a hydrogen bond between these residues being essential for blockage. 3. An alternative alignment between the M2 regions of Kir2.1 and KcsA suggested that Ser165 is itself a pore-lining residue, more directly affecting blockage. We were able to replace Ser165 with a variety of polar and non-polar residues, consistent with this residue being pore lining. Some of these changes affected channel blockage. 4. We tested the hypothesis that Asp172 - a residue implicated in channel gating by polyamines - formed an additional selectivity filter by using the triple mutant T141A/S165L/D172N. Large Rb+ and Cs+ currents were measured in this mutant. 5. We propose that both Thr141 and Ser165 are likely to provide binding sites for monovalent blocking cations in wild-type channels. These residues lie beyond the carbonyl oxygen tunnel thought to form the channel selectivity filter, which the blocking cations must therefore traverse.

The possible role of a disulphide bond in forming functional Kir2.1 potassium channels.

The role of two cysteine residues--Cys122 and Cys154--in the structure of the strong inward rectifier K+ channel, Kir2.1, has been investigated using site-directed mutagenesis and electrophysiology. Such cysteine residues are conserved across the inward rectifier family and may be expected to form a crucial disulphide bond. Our experiments show that when the cysteines are absent, the protein is expressed, but the channels are not functional, suggesting that the disulphide bond is essential for correct channel assembly. However, reducing agents applied extracellularly have little effect on current amplitude in wild-type, so that, once the channel is assembled correctly in the membrane, the disulphide bonds are no longer essential for function. Molecular modelling suggests that a disulphide bond is formed--this may be either an intra- or an inter-subunit.

Possible modulatory role of voltage-activated Ca(2+) currents determining the membrane properties of isolated pyramidal neurones of the rat dorsal cochlear nucleus.

Voltage-activated Ca(2+) currents have been studied in pyramidal cells isolated enzymatically from the dorsal cochlear nuclei of 6-11-day-old Wistar rats, using whole-cell voltage-clamp. From hyperpolarized membrane potentials, the neurones exhibited a T-type Ca(2+) current on depolarizations positive to -90 mV (the maximum occurred at about -40 mV). The magnitude of the T-current varied considerably from cell to cell (-56 to -852 pA) while its steady-state inactivation was consistent (E(50)=-88.2+/-1.7 mV, s=-6. 0+/-0.4 mV). The maximum of high-voltage activated (HVA) Ca(2+) currents was observed at about -15 mV. At a membrane potential of -10 mV the L-type Ca(2+) channel blocker nifedipine (10 microM) inhibited approximately 60% of the HVA current, the N-type channel inhibitor omega-Conotoxin GVIA (2 microM) reduced the current by 25% while the P/Q-type channel blocker omega-Agatoxin IVA (200 nM) blocked a further 10%. The presence of the N- and P/Q-type Ca(2+) channels was confirmed by immunochemical methods. The metabotropic glutamate receptor agonist (+/-)-1-aminocyclopentane-trans-1, 3-dicarboxylic acid (200 microM) depressed the HVA current in every cell studied (a block of approximately 7% on an average). The GABA(B) receptor agonist baclofen (100 microM) reversibly inhibited 25% of the HVA current. Simultaneous application of omega-Conotoxin GVIA and baclofen suggested that this inhibition could be attributed to the nearly complete blockade of the N-type channels. Possible physiological functions of the voltage-activated Ca(2+) currents reported in this work are discussed.

Characterisation of Kir2.0 proteins in the rat cerebellum and hippocampus by polyclonal antibodies.

Rabbit polyclonal antibodies were raised to rat Kir2.0 (Kir2.1, Kir2.2 and Kir2.3) inwardly rectifying potassium ion channel proteins. The antibody specificities were confirmed by immunoprecipitation of [35S]-methionine-labelled in vitro translated channel proteins and western blotting. Immunohistochemistry revealed a different patterns of expression of Kir2.0 subfamily proteins in the rat hind-brain (cerebellum and medulla) and fore-brain (hippocampus). Notably, only Kir2.2 protein was detected in the cerebellum and medulla, Kir2.1, Kir2.2 and Kir2.3 proteins were expressed in the hippocampus and immunostaining was not limited to neuronal cell types. Anti-Kir2.1 (fore-brain only) and anti-Kir2.2 (fore- and hind-brain) antibodies showed positive staining in macroglia, endothelia, ependyma and vascular smooth muscle cells. In contrast, anti-Kir2.3 (fore-brain only) immunostaining was limited to neurons, macroglia and vascular smooth muscle. These results indicate that specific regions within the rat fore- and hind-brain have differential distributions of inwardly rectifying potassium ion channel proteins.

The dependence of Ag+ block of a potassium channel, murine kir2.1, on a cysteine residue in the selectivity filter.

Externally applied Ag+ (100-200 nM) irreversibly blocked the strong inwardly rectifying K+ channel, Kir2.1. Mutation to serine of a cysteine residue at position 149 in the pore-forming H5 region of Kir2.1 abolished Ag+ blockage. To determine how many of the binding sites must be occupied by Ag+ before the channel is blocked, we measured the rate of channel block and found that our results were best fitted assuming that only one Ag+ ion need bind to eliminate channel current. We tested our hypothesis further by constructing covalently linked dimers and tetramers of Kir2.1 in which cysteine had been replaced by serine in one (dimer) or three (tetramer) of the linked subunits. When expressed, these constructs yielded functional channels with either two (dimer) or one (tetramer) cysteines per channel at position 149. Blockage in the tetramer was complete after sufficient exposure to 200 nM Ag+, a result that is also consistent with only one Ag+ being required to bind to Cys149 to block fully. The rate of development of blockage was 16 times slower than in wild-type channels; the rate was 4 times slower in channels formed from dimers.

The selectivity filter of a potassium channel, murine kir2.1, investigated using scanning cysteine mutagenesis.

We have produced a structural model of the pore-forming H5 (or P) region of the strong inward rectifier K+ channel, Kir2.1, based initially on an existing molecular model of the pore region of the voltage-gated K+ channel, Kv1.3. Cysteine-scanning mutagenesis and subsequent blockage by Ag+ was used to test our model by determining the residues in H5 whose side chains line the ion conduction pathway. Mutations made in eight positions within the highly conserved H5 region resulted in apparently non-functional channels. Constructing covalently linked dimers, which carry a cysteine substitution in only one of the linked subunits, rescued six of these mutants; a covalently linked tetramer, carrying a cysteine substitution on only one of the linked subunits, rescued a further mutant. Our results using the dimers and tetramers suggest that residues Thr141, Thr142, Ile143, Tyr145, Phe147 and Cys149 are accessible to externally applied Ag+ (100-200 nM) and therefore that their side chains line the channel pore. We conclude that the topology of the Kir pore is similar, but not identical, to that of Kv channels. Additionally, the molecular model suggests that selectivity may be conferred both by aromatic residues (Tyr145 and Phe147) via cation-pi interactions and by backbone carbonyl groups (Thr142 and Gly144).

Membrane currents influencing action potential latency in granule neurons of the rat cochlear nucleus.

Granule cells are the most numerous neurons in the cochlear nucleus, but, because of their small size, little information on their membrane properties and ionic currents is available. We used an in vitro slice preparation of the rat ventral cochlear nucleus to make whole-cell recordings from these cells. Under current clamp, some granule neurons fired spontaneous action potentials and all generated a train of action potentials on depolarization (threshold current, 10-35 pA). Hyperpolarization increased the latency to the first action potential evoked during a subsequent depolarization. We examined which voltage-gated currents might underlie this latency shift. In addition to a fast inward Na+ current, depolarization activated two outward potassium currents. A transient current was rapidly inactivated by membrane potentials positive to -60 mV, while a second, more slowly inactivating current was observed following the decay of the transient current. No hyperpolarization-activated conductances were observed in these cells. Modelling of the currents suggests that removal of inactivation on hyperpolarization accounts for the increased action potential latency in granule cells. Such a mechanism could account for the 'pauser'-type firing patterns of the fusiform cells which receive a prominent projection from the granule cells in the dorsal cochlear nucleus.

Effect of adenosine and intracellular GTP on KATP channels of mammalian skeletal muscle.

We investigated the action of adenosine and GTP on KATP channels, using inside-out patch clamp recordings from dissociated single fibers of rat flexor digitorum brevis (FDB) skeletal muscle. In excised patches, KATP channels could be activated by a combination of an extracellular adenosine agonist and intracellular Mg2+-ATP and GTP or GTP-gamma-S. The activation required hydrolyzable ATP and could be partially reversed with Mg2+, suggesting that it may involve a G-protein dependent phosphorylation of KATP channels. We found that KATP channels of the rat FDB could not be activated by Mg2+-ATP alone or by Mg2+-ATP in the presence of extracellular adenosine. Patches whose channel activity had been 'rundown' by Ca2+ could not be recovered by adenosine, GTP or Mg2+-ATP. KATP channels activated by adenosine receptor agonists had a similar ATP sensitivity to those under control conditions; but adenosine appears to be able to switch these KATP channels from an inactive to an active mode.

The role of a single aspartate residue in ionic selectivity and block of a murine inward rectifier K+ channel Kir2.1.

1. The effects of Rb+ and Cs+ as blocking ions were investigated on wild-type and mutant forms of the inward rectifier K+ channel, IRK1 (Kir2.1). 2. In wild-type channels, Rb+ blockage was voltage dependent, increasing and then falling with increasing hyperpolarization. 3. Rb+ blockage was abolished by replacing Asp172 in the M2 domain of the pore-forming subunit by Asn, but was re-established by a change to Gln, narrowing the pore. Blocking affinity was reduced in D172Q, and was also reduced by replacing Gly168 in M2 by Ala. 4. Cs+ blockage was also abolished in D172N but was re-established in D172Q. 5. There appears to be a balance between charge and pore size in determining whether ions block or permeate. A major part of the selectivity of Kir2.1 is associated with Asp172 in the M2 domain.