Estimation of Symmetric Channels for Discrete Cosine Transform Type-I Multicarrier Systems: A Compressed Sensing Approach

The problem of channel estimation for multicarrier communications is addressed. We focus on systems employing the Discrete Cosine Transform Type-I (DCT1) even at both the transmitter and the receiver, presenting an algorithm which achieves an accurate estimation of symmetric channel filters using only a small number of training symbols. The solution is obtained by using either matrix inversion or compressed sensing algorithms. We provide the theoretical results which guarantee the validity of the proposed technique for the DCT1. Numerical simulations illustrate the good behaviour of the proposed algorithm.

Outward Rectification of Voltage-Gated K+ Channels Evolved at Least Twice in Life History

Voltage-gated potassium (K+) channels are present in all living systems. Despite high structural similarities in the transmembrane domains (TMD), this K+ channel type segregates into at least two main functional categories���hyperpolarization-activated, inward-rectifying (Kin) and depolarization-activated, outward-rectifying (Kout) channels. Voltage-gated K+ channels sense the membrane voltage via a voltage-sensing domain that is connected to the conduction pathway of the channel. It has been shown that the voltage-sensing mechanism is the same in Kin and Kout channels, but its performance results in opposite pore conformations. It is not known how the different coupling of voltage-sensor and pore is implemented. Here, we studied sequence and structural data of voltage-gated K+ channels from animals and plants with emphasis on the property of opposite rectification. We identified structural hotspots that alone allow already the distinction between Kin and Kout channels. Among them is a loop between TMD S5 and the pore that is very short in animal Kout, longer in plant and animal Kin and the longest in plant Kout channels. In combination with further structural and phylogenetic analyses this finding suggests that outward-rectification evolved twice and independently in the animal and plant kingdom.

Coordinate regulation of gonadotropin-releasing hormone neuronal firing patterns by cytosolic calcium and store depletion

Elevation of cytosolic free Ca2+ concentration ([Ca2+]i) in excitable cells often acts as a negative feedback signal on firing of action potentials and the associated voltage-gated Ca2+ influx. Increased [Ca2+]i stimulates Ca2+-sensitive K+ channels (IK-Ca), and this, in turn, hyperpolarizes the cell and inhibits Ca2+ influx. However, in some cells expressing IK-Ca the elevation in [Ca2+]i by depletion of intracellular stores facilitates voltage-gated Ca2+ influx. This phenomenon was studied in hypothalamic GT1 neuronal cells during store depletion caused by activation of gonadotropin-releasing hormone (GnRH) receptors and inhibition of endoplasmic reticulum (Ca2+)ATPase with thapsigargin. GnRH induced a rapid spike increase in [Ca2+]i accompanied by transient hyperpolarization, followed by a sustained [Ca2+]i plateau during which the depolarized cells fired with higher frequency. The transient hyperpolarization was caused by the initial spike in [Ca2+]i and was mediated by apamin-sensitive IK-Ca channels, which also were operative during the subsequent depolarization phase. Agonist-induced depolarization and increased firing were independent of [Ca2+]i and were not mediated by inhibition of K+ current, but by facilitation of a voltage-insensitive, Ca2+-conducting inward current. Store depletion by thapsigargin also activated this inward depolarizing current and increased the firing frequency. Thus, the pattern of firing in GT1 neurons is regulated coordinately by apamin-sensitive SK current and store depletion-activated Ca2+ current. This dual control of pacemaker activity facilitates voltage-gated Ca2+ influx at elevated [Ca2+]i levels, but also protects cells from Ca2+ overload. This process may also provide a general mechanism for the integration of voltage-gated Ca2+ influx into receptor-controlled Ca2+ mobilization.

Calcium block of Na+ channels and its effect on closing rate

Calcium ion transiently blocks Na+ channels, and it shortens the time course for closing of their activation gates. We examined the relation between block and closing kinetics by using the Na+ channels natively expressed in GH3 cells, a clonal line of rat pituitary cells. To simplify analysis, inactivation of the Na+ channels was destroyed by including papain in the internal medium. All divalent cations tested, and trivalent La3+, blocked a progressively larger fraction of the channels as their concentration increased, and they accelerated the closing of the Na+ channel activation gate. For calcium, the most extensively studied cation, there is an approximately linear relation between the fraction of the channels that are calcium-blocked and the closing rate. Extrapolation of the data to very low calcium suggests that closing rate is near zero when there is no block. Analysis shows that, almost with certainty, the channels can close when occupied by calcium. The analysis further suggests that the channels close preferentially or exclusively from the calcium-blocked state.

Distinguishing surface effects of calcium ion from pore-occupancy effects in Na+ channels

The effects of calcium ion on the Na+ activation gate were studied in squid giant axons. Saxitoxin (STX) was used to block ion entry into Na+ channels without hindering access to the membrane surface, making it possible to distinguish surface effects of calcium from pore-occupancy effects. In the presence of STX, gating kinetics were measured from gating current (Ig). The kinetic effects of external calcium concentration changes were small when STX was present. In the absence of STX, lowering the calcium concentration (from 100 to 10 mM) slowed the closing of Na+ channels (measured from INa tails) by more than a factor of 2. Surprisingly, the voltage sensitivity of closing kinetics changed with calcium concentration, and it was modified by STX. Voltage sensitivity apparently depends in part on the ability of calcium to enter and block the channels as voltage is driven negative. In external medium with no added calcium, INa tail current initially increases in amplitude severalfold with the relief of calcium block, then progressively slows and gets smaller, as calcium diffuses out of the layers investing the axon. INa tails seen just before the current disappears suggest that closing in the absence of channel block is very slow or does not occur. INa amplitude and kinetics are completely restored when calcium is returned. The results strongly suggest that calcium occupancy is a requirement for channel closing and that nonoccupied channels fold reversibly into a nonfunctional conformation.

An isoform of the rod photoreceptor cyclic nucleotide-gated channel �� subunit expressed in olfactory neurons

Sensory transduction in olfactory neurons involves the activation of a cyclic nucleotide-gated (CNG) channel by cAMP. Previous studies identified a CNG channel �� subunit (CNG2) and a �� subunit (CNG5), which when heterologously expressed form a channel with properties similar but not identical to those of native olfactory neurons. We have cloned a new type of CNG channel �� subunit (CNG4.3) from rat olfactory epithelium. CNG4.3 derives from the same gene as the rod photoreceptor �� subunit (CNG4.1) but lacks the long, glutamic acid-rich domain found in the N terminus of CNG4.1. Northern blot and in situ hybridization revealed that CNG4.3 is expressed specifically in olfactory neurons. Expression of CNG4.3 in human embryonic kidney 293 cells did not lead to detectable currents. Coexpression of CNG4.3 with CNG2 induced a current with significantly increased sensitivity for cAMP whereas cGMP affinity was not altered. Additionally, CNG4.3 weakened the outward rectification of the current in the presence of extracellular Ca2+, decreased the relative permeability for Ca2+, and enhanced the sensitivity for l-cis diltiazem. Upon coexpression of CNG2, CNG4.3, and CNG5, a conductance with a cAMP sensitivity greater than that of either the CNG2/CNG4.3 or the CNG2/CNG5 channel and near that of native olfactory channel was observed. Our data suggest that CNG4.3 forms a subunit of the native olfactory CNG channel. The expression of various CNG4 isoforms in retina and olfactory epithelium indicates that the CNG4 subunit may be necessary for normal function of both photoreceptor and olfactory CNG channels.

Acoustic overstimulation increases outer hair cell Ca2+ concentrations and causes dynamic contractions of the hearing organ

The dynamic responses of the hearing organ to acoustic overstimulation were investigated using the guinea pig isolated temporal bone preparation. The organ was loaded with the fluorescent Ca2+ indicator Fluo-3, and the cochlear electric responses to low-level tones were recorded through a microelectrode in the scala media. After overstimulation, the amplitude of the cochlear potentials decreased significantly. In some cases, rapid recovery was seen with the potentials returning to their initial amplitude. In 12 of 14 cases in which overstimulation gave a decrease in the cochlear responses, significant elevations of the cytoplasmic [Ca2+] in the outer hair cells were seen. [Ca2+] increases appeared immediately after terminating the overstimulation, with partial recovery taking place in the ensuing 30 min in some preparations. Such [Ca2+] changes were not seen in preparations that were stimulated at levels that did not cause an amplitude change in the cochlear potentials. The overstimulation also gave rise to a contraction, evident as a decrease of the width of the organ of Corti. The average contraction in 10 preparations was 9 ��m (SE 2 ��m). Partial or complete recovery was seen within 30���45 min after the overstimulation. The [Ca2+] changes and the contraction are likely to produce major functional alterations and consequently are suggested to be a factor contributing strongly to the loss of function seen after exposure to loud sounds.

Differential roles of Ca2+/calmodulin-dependent kinases in posttetanic potentiation at input selective glutamatergic pathways

The electrosensory lateral line lobe (ELL) of the electric fish Apteronotus leptorhynchus is a layered medullary region receiving electroreceptor input that terminates on basal dendrites of interneurons and projection (pyramidal) cells. The molecular layer of the ELL contains two distinct glutamatergic feedback pathways that terminate on the proximal (ventral molecular layer, VML) and distal (dorsal molecular layer) apical dendrites of pyramidal cells. Western blot analysis with an antibody directed against mammalian Ca2+/calmodulin-dependent kinase 2, �� subunit (CaMK2��) recognized a protein of identical size in the brain of A. leptorhynchus. Immunohistochemistry demonstrated that CaMK2 �� expression in the ELL was restricted to fibers and terminals in the VML. Posttetanic potentiation (PTP) could be readily elicited in pyramidal cells by stimulation of either VML or DML in brain slices of the ELL. PTP in the VML was blocked by extracellular application of a CaMK2 antagonist (KN62) while intracellular application of KN62 or a CaMK2 inhibitory peptide had no effect, consistent with the presynaptic localization of CaMK2 �� in VML. PTP in the dorsal molecular layer was not affected by extracellular application of KN62. Anti-Hebbian plasticity has also been demonstrated in the VML, but was not affected by KN62. These results demonstrate that, while PTP can occur independent of CaMK2, it is, in some synapses, dependent on this kinase.

MgATP activates the �� cell KATP channel by interaction with its SUR1 subunit

ATP-sensitive potassium (KATP) channels in the pancreatic �� cell membrane mediate insulin release in response to elevation of plasma glucose levels. They are open at rest but close in response to glucose metabolism, producing a depolarization that stimulates Ca2+ influx and exocytosis. Metabolic regulation of KATP channel activity currently is believed to be mediated by changes in the intracellular concentrations of ATP and MgADP, which inhibit and activate the channel, respectively. The �� cell KATP channel is a complex of four Kir6.2 pore-forming subunits and four SUR1 regulatory subunits: Kir6.2 mediates channel inhibition by ATP, whereas the potentiatory action of MgADP involves the nucleotide-binding domains (NBDs) of SUR1. We show here that MgATP (like MgADP) is able to stimulate KATP channel activity, but that this effect normally is masked by the potent inhibitory effect of the nucleotide. Mg2+ caused an apparent reduction in the inhibitory action of ATP on wild-type KATP channels, and MgATP actually activated KATP channels containing a mutation in the Kir6.2 subunit that impairs nucleotide inhibition (R50G). Both of these effects were abolished when mutations were made in the NBDs of SUR1 that are predicted to abolish MgATP binding and/or hydrolysis (D853N, D1505N, K719A, or K1384M). These results suggest that, like MgADP, MgATP stimulates KATP channel activity by interaction with the NBDs of SUR1. Further support for this idea is that the ATP sensitivity of a truncated form of Kir6.2, which shows functional expression in the absence of SUR1, is unaffected by Mg2+.

Factors controlling the competition among rotational and vibrational energy transfer channels���in���glyoxal

The state-to-state transfer of rotational and vibrational energy has been studied for S1 glyoxal (CHOCHO) in collisions with D2, N2, CO and C2H4 using crossed molecular beams. A laser is used to pump glyoxal seeded in He to its S1 zero point level with zero angular momentum about its top axis (K��� = 0). The inelastic scattering to each of at least 26 S1 glyoxal rotational and rovibrational levels is monitored by dispersed S1���S0 fluorescence. Various collision partners are chosen to investigate the relative influences of reduced mass and the collision pair interaction potential on the competition among the energy transfer channels. When the data are combined with that obtained previously from other collision partners whose masses range from 2 to 84 amu, it is seen that the channel competition is controlled primarily by the kinematics of the collisional interaction. Variations in the intermolecular potential play strictly a secondary role.

Molecular cloning of a peroxisomal Ca2+-dependent member of the mitochondrial carrier���superfamily

A cDNA from a novel Ca2+-dependent member of the mitochondrial solute carrier superfamily was isolated from a rabbit small intestinal cDNA library. The full-length cDNA clone was 3,298 nt long and coded for a protein of 475 amino acids, with four elongation factor-hand motifs located in the N-terminal half of the molecule. The 25-kDa N-terminal polypeptide was expressed in Escherichia coli, and it was demonstrated that it bound Ca2+, undergoing a reversible and specific conformational change as a result. The conformation of the polypeptide was sensitive to Ca2+ which was bound with high affinity (Kd ��� 0.37 ��M), the apparent Hill coefficient for Ca2+-induced changes being about 2.0. The deduced amino acid sequence of the C-terminal half of the molecule revealed 78% homology to Grave disease carrier protein and 67% homology to human ADP/ATP translocase; this sequence homology identified the protein as a new member of the mitochondrial transporter superfamily. Northern blot analysis revealed the presence of a single transcript of about 3,500 bases, and low expression of the transporter could be detected in the kidney but none in the liver. The main site of expression was the colon with smaller amounts found in the small intestine proximal to the ileum. Immunoelectron microscopy localized the transporter in the peroxisome, although a minor fraction was found in the mitochondria. The Ca2+ binding N-terminal half of the transporter faces the cytosol.

ECA1 complements yeast mutants defective in Ca2+ pumps and encodes an endoplasmic reticulum-type Ca2+-ATPase in Arabidopsis���thaliana

To understand the structure, role, and regulation of individual Ca2+ pumps in plants, we have used yeast as a heterologous expression system to test the function of a gene from Arabidopsis thaliana (ECA1). ECA1 encoded a 116-kDa polypeptide that has all the conserved domains common to P-type Ca2+ pumps (EC 3.6.1.38). The amino acid sequence shared more identity with sarcoplasmic/endoplasmic reticulum (53%) than with plasma membrane (32%) Ca2+ pumps. Yeast mutants defective in a Golgi Ca2+ pump (pmr1) or both Golgi and vacuolar Ca2+ pumps (pmr1 pmc1 cnb1) were sensitive to growth on medium containing 10 mM EGTA or 3 mM Mn2+. Expression of ECA1 restored growth of either mutant on EGTA. Membranes were isolated from the pmr1 pmc1 cnb1 mutant transformed with ECA1 to determine if the ECA1 polypeptide (ECA1p) could be phosphorylated as intermediates of the reaction cycle of Ca2+-pumping ATPases. In the presence of [��-32P]ATP, ECA1p formed a Ca2+-dependent [32P]phosphoprotein of 106 kDa that was sensitive to hydroxylamine. Cyclopiazonic acid, a blocker of animal sarcoplasmic/endoplasmic reticulum Ca2+ pumps, inhibited the formation of the phosphoprotein, whereas thapsigargin did not. Immunoblotting with an antibody against the carboxyl tail showed that ECA1p was associated mainly with the endoplasmic reticulum membranes isolated from Arabidopsis plants. The results support the model that ECA1 encodes an endoplasmic reticulum-type Ca2+ pump in Arabidopsis. The ability of ECA1p to restore growth of mutant pmr1 on medium containing Mn2+, and the formation of a Mn2+-dependent phosphoprotein suggested that ECA1p may also regulate Mn2+ homeostasis by pumping Mn2+ into endomembrane compartments of plants.

The roles of the two proton input channels in cytochrome c oxidase from Rhodobacter sphaeroides probed by the effects of site-directed mutations on time-resolved electrogenic intraprotein proton���transfer

The crystal structures of cytochrome c oxidase from both bovine and Paracoccus denitrificans reveal two putative proton input channels that connect the heme-copper center, where dioxygen is reduced, to the internal aqueous phase. In this work we have examined the role of these two channels, looking at the effects of site-directed mutations of residues observed in each of the channels of the cytochrome c oxidase from Rhodobacter sphaeroides. A photoelectric technique was used to monitor the time-resolved electrogenic proton transfer steps associated with the photo-induced reduction of the ferryl-oxo form of heme a3 (Fe4+ = O2���) to the oxidized form (Fe3+OH���). This redox step requires the delivery of a ���chemical��� H+ to protonate the reduced oxygen atom and is also coupled to proton pumping. It is found that mutations in the K channel (K362M and T359A) have virtually no effect on the ferryl-oxo-to-oxidized (F-to-Ox) transition, although steady-state turnover is severely limited. In contrast, electrogenic proton transfer at this step is strongly suppressed by mutations in the D channel. The results strongly suggest that the functional roles of the two channels are not the separate delivery of chemical or pumped protons, as proposed recently [Iwata, S., Ostermeier, C., Ludwig, B. & Michel, H. (1995) Nature (London) 376, 660���669]. The D channel is likely to be involved in the uptake of both ���chemical��� and ���pumped��� protons in the F-to-Ox transition, whereas the K channel is probably idle at this partial reaction and is likely to be used for loading the enzyme with protons at some earlier steps of the catalytic cycle. This conclusion agrees with different redox states of heme a3 in the K362M and E286Q mutants under aerobic steady-state turnover conditions.

RGS proteins reconstitute the rapid gating kinetics of G����-activated inwardly rectifying K+���channels

G protein-gated inward rectifier K+ (GIRK) channels mediate hyperpolarizing postsynaptic potentials in the nervous system and in the heart during activation of G��(i/o)-coupled receptors. In neurons and cardiac atrial cells the time course for receptor-mediated GIRK current deactivation is 20���40 times faster than that observed in heterologous systems expressing cloned receptors and GIRK channels, suggesting that an additional component(s) is required to confer the rapid kinetic properties of the native transduction pathway. We report here that heterologous expression of ���regulators of G protein signaling��� (RGS proteins), along with cloned G protein-coupled receptors and GIRK channels, reconstitutes the temporal properties of the native receptor ��� GIRK signal transduction pathway. GIRK current waveforms evoked by agonist activation of muscarinic m2 receptors or serotonin 1A receptors were dramatically accelerated by coexpression of either RGS1, RGS3, or RGS4, but not RGS2. For the brain-expressed RGS4 isoform, neither the current amplitude nor the steady-state agonist dose-response relationship was significantly affected by RGS expression, although the agonist-independent ���basal��� GIRK current was suppressed by ���40%. Because GIRK activation and deactivation kinetics are the limiting rates for the onset and termination of ���slow��� postsynaptic inhibitory currents in neurons and atrial cells, RGS proteins may play crucial roles in the timing of information transfer within the brain and to peripheral tissues.

Ca2+-independent inhibition of inositol trisphosphate receptors by calmodulin: Redistribution of calmodulin as a possible means of regulating Ca2+���mobilization

The interactions between calmodulin, inositol 1,4,5-trisphosphate (InsP3), and pure cerebellar InsP3 receptors were characterized by using a scintillation proximity assay. In the absence of Ca2+, 125I-labeled calmodulin reversibly bound to multiple sites on InsP3 receptors and Ca2+ increased the binding by 190% �� 10%; the half-maximal effect occurred when the Ca2+ concentration was 184 �� 14 nM. In the absence of Ca2+, calmodulin caused a reversible, concentration-dependent (IC50 = 3.1 �� 0.2 ��M) inhibition of [3H]InsP3 binding by decreasing the affinity of the receptor for InsP3. This effect was similar at all Ca2+ concentrations, indicating that the site through which calmodulin inhibits InsP3 binding has similar affinities for calmodulin and Ca2+-calmodulin. Calmodulin (10 ��M) inhibited the Ca2+ release from cerebellar microsomes evoked by submaximal, but not by maximal, concentrations of InsP3. Tonic inhibition of InsP3 receptors by the high concentrations of calmodulin within cerebellar Purkinje cells may account for their relative insensitivity to InsP3 and limit spontaneous activation of InsP3 receptors in the dendritic spines. Inhibition of InsP3 receptors by calmodulin at all cytosolic Ca2+ concentrations, together with the known redistribution of neuronal calmodulin evoked by protein kinases and Ca2+, suggests that calmodulin may also allow both feedback control of InsP3 receptors and integration of inputs from other signaling pathways.