Assigning long-term reward value for actions is a learned intelligence for a successful achievement of a distant goal. Although midbrain dopamine neurons are known to signal reward value and its prediction error, it is not examined experimentally whether and how dopamine neurons encode long-term value of multiple future rewards, as suggested in reinforcement learning theories. We address this issue by studying 185 dopamine neuron activities recorded from three monkeys that performed a multi-step choice task for three rewards. In the task, they explored a reward among three alternatives and then exploited this knowledge to receive two additional rewards by repeating the same choice in subsequent trials. Duration of anticipatory licking for reward water represented expectations of multiple future rewards; the sum of immediate and discounted future rewards. In accordance with this result, dopamine responses to the start cues and reinforcer beeps reflected the expected values of the multiple future rewards and their errors, respectively. These responses were quantitatively predicted by theoretical descriptions of the value function with time discounting in reinforcement learning. Moreover, we confirmed that these responses were established through learning the multistep choice paradigm for rewards. These findings demonstrate that dopamine neurons “learn” to encode the long-term value of multiple future rewards with distant rewards discounted. (Proc Natl Acad Sci U S A. 2011 Sep 13;108(37):15462-7)

Figure legend

(A) Schematically illustrated structure of the three-step choice trials to obtain three rewards at different times. The monkeys first explored three targets to find the rewarding one (N1-N3 trials) and exploited this knowledge to get two additional rewards by choosing the same target (R1, R2 trials).

(B) Bar graphs of ensemble average of dopamine responses (mean and SEM) above the baseline. The best-fit value functions estimated as the TD error in reinforcement learning theories (solid red line) and reward probability of trials (dashed green line) are superimposed.

The EF-hand Ca²⁺-binding protein neuronal calcium sensor-1 (NCS-1) is recognized as an important regulator of neuronal functions. Although NCS-1 is also expressed in the heart, little is known about its cardiac functions. By characterizing the cardiac phenotypes of knockout (Ncs1^−/−) mice, we identified 2 novel functions of NCS-1 in cardiac tissues: it is a positive regulator of contraction in the young heart and of hypertrophy in adults. In the neonatal mouse heart, the structure and function of sarcoplasmic reticulum (SR) is immature; nonetheless, it is considered a primary source of Ca²⁺ for contraction, suggesting the existence of missing factors that promote SR-dependent excitation-contraction (EC) coupling in the postnatal stages. We showed that NCS-1 is one such regulator that enhances Ca²⁺ signals in the immature heart, mainly by promotion of IP₃R function, followed by CaMKII signaling, which results in a large increase in the SR Ca²⁺ content. In addition, NCS-1 expression increases in the early stages of hypertrophy in the adult heart and promotes progression of hypertrophy, at least in part, through IP₃R activation. Our results reveal a previously unrecognized mechanism of EC coupling in young heart and another regulatory mechanism for the progression of receptor stimulation-elicited cardiac hypertrophy. (Nakamura T.Y. et al. Circ. Res. 109; 512-523, 2011)

Figure legend: Regulation of immature heart function and hypertrophy by NCS-1.　

NCS-1 - IP₃ receptor interaction could serve as a Ca²⁺ source for the activation of CaMKII, leading to a higher rate of SR Ca²⁺ pumping and release and inducing global Ca²⁺ signaling, thus promoting EC coupling in immature hearts. We found that all NCS-1, IP₃ receptor and CaMKII are highly expressed in the immature hearts and thus contribution of this pathway would be large for contraction. In addition, phenylephrine-induced hypertrophy was largely attenuated in Ncs1^−/− mice, suggesting that NCS-1 is also involved in hormone-induced hypertrophy possibly via IP₃ receptor activation.

*Department of Molecular Physiology, National Cerebral and Cardiovascular Center

 During the process of cell shape changes, proliferation, migration or apoptosis, cell volume is neatly regulated. Cell volume regulation is accomplished by regulating the net influx or outflux of water and solutes across the cell membrane. The volume-sensitive outwardly rectifying (VSOR) anion channel is a major regulator of anion transport during the regulation and expressed in almost all types of animal cells. We previously demonstrated that the VSOR channels in cultured mouse cortical astrocytes are activated by an inflammatory mediator bradykinin, and that the channels may also serve as a pathway for glutamate release, sending signals to adjacent neurons (Liu et al., J Physiol 587(10):2197-2209, 2009). Recently, we found that this VSOR channel activation is regulated in the immediate vicinity of individual Ca²⁺ channels causing intracellular Ca²⁺ rises (Akita & Okada, J Physiol 589(16):3909-3927, 2011).
 Bradykinin induced the opening of IP₃-receptor Ca²⁺ channels on ER Ca²⁺ stores and then store- or receptor-operated Ca²⁺ channels in the plasma membrane. In the very high Ca²⁺ concentration regions created within ~20 nm of these Ca²⁺ channels, so-called ‘Ca²⁺ nanodomains’, Ca²⁺-dependent  PKCa and b were found to be activated, and these PKCs were involved in VSOR channel activation through inducing generation of reactive oxygen species (ROS) by NADPH oxidases (NOX). This mechanism would provide a firm basis for local control of cell volume regulation and intercellular communications, even when only a small number of Ca²⁺ channels in a part of the cell are opened by a minute amount of bradykinin.

Figure legend: Bradykinin induces generation of inositol 1,4,5-trisphosphate (IP₃) in the cytoplasm through binding to the bradykinin B2 receptors and activation of Gq proteins and phospholipase C (PLC). This causes the opening of IP₃-receptor Ca²⁺ channels on ER Ca²⁺ stores and several types of store- or receptor-operated Ca²⁺ channel in the cell membrane. Within 20 nm of these Ca²⁺ channels, especially IP₃ receptors and TRPC1 channels, Ca²⁺-dependent PKCa and b are activated. The PKCs subsequently activate NOX which generates ROS for VSOR channel activation.

*  Department of Cell Physiology, National Institute for Physiological Sciences

Cdk5 regulatory associated protein 1-like 1 (Cdkal1) has been associated with an impaired insulin response and increased risk of type 2 diabetes (T2D), but its molecular function has not been characterized. We have identified that Cdkal1 is a mammalian methylthiotransferase that specifically biosynthesizes 2-methylthio-N⁶-threonylcarbamoyladenosine (ms²t⁶A) at A37 of tRNA^Lys(UUU) and that it is required for the accurate translation of AAA and AAG codons. To further investigate the physiological role of Cdkal1, we generated pancreatic b-cell-specific Cdkal1 knockout mice (Cdkal1 knockout mice). Cdkal1 knockout mice showed pancreatic islet hypertrophy, a decrease in insulin secretion and impaired blood glucose control. In Cdkal1-deficient -cells, misreading of Lys codon in proinsulin was observed, which results in decrease of proinsulin synthesis. Consequently, the pancreatic C-peptide level was significantly decreased in Cdkal1 knockout mice. Moreover, the expression of endoplasmic reticulum (ER) stress-related genes was upregulated in pancreatic b-cells, and abnormally structured ER was observed. Furthermore, Cdkal1 knockout mice rapidly developed severe glucose intolerance under high fat diet feeding condition. There was a global increase of ER stress response in b-cells of Cdkal1 knockout mice fed with a high fat diet. These findings suggest that the induced translation of proinsulin may require fully modified tRNA^Lys(UUU), potentially explaining the molecular pathogenesis of T2D in patients carrying cdkal1 risk alleles. These findings have been published in the Journal of Clinical Investigation (doi:10.1172/JCI58056).

Figure Legend

(A) The secondary structure of tRNA^Lys(UUU) is shown. (B) Cdkal1 catalyzes the conversion of N⁶-threonylcarbamoyladenosine (t⁶) to 2-methylthio-N⁶-threonylcarbamoyladenosine (ms²t⁶A) at A37 of tRNA^Lys(UUU). (C) Proposed working model for regulation of b-cell function by Cdkal1 is shown.

*  Department of Molecular Physiology, Faculty of Life Sciences, Kumamoto University, Kumamoto, Japan

The hippocampus plays a central role in learning and memory. Although synaptic delivery of AMPA-type glutamate receptors (AMPARs) contributes to experience-dependent synaptic strengthening, its role in hippocampus-dependent learning remains elusive. By combining viral-mediated in vivo gene delivery with in vitro patch-clamp recordings, we found that the inhibitory avoidance task, a hippocampus-dependent contextual fear-learning paradigm, delivered GluR1-containing AMPARs into CA3-CA1 synapses of the dorsal hippocampus. To block the synaptic delivery of endogenous AMPARs, we expressed a fragment of the GluR1-cytoplasmic tail (the 14-aa GluR1 membrane-proximal region with two serines mutated to phospho-mimicking aspartates: MPR-DD). MPR-DD prevented learning-driven synaptic AMPAR delivery in CA1 neurons. Bilateral expression of MPR-DDin the CA1 region of the rat impaired inhibitory avoidance learning, indicating that synaptic GluR1 trafficking in the CA1 region of the hippocampus is required for encoding contextual fear memories. The fraction of CA1 neurons that underwent synaptic strengthening positively correlated with the performance in the inhibitory avoidance fear memory task. These data suggest that the robustness of a contextual memory depends on the number of hippocampal neurons that participate in the encoding of a memory trace. (Proc Natl Acad USA, 108:12503-12508,2011)

Figure  Schema of excitatory synapses in hippocampal CA1. Contextual learning drives synaptic delivery of GluR1-containing AMPA receptors in CA1 pyramidal neurons. More importantly, delivery blocker expression in bilateral CA1 severely impaired the learning, proving physiological role of AMPA receptor delivery.

*  Yokohama City University Graduate School of Medicine

 Figure legend Cerebral cortex can be anatomically divided into six layers (layer Ⅰ-Ⅵ). (Right) Information flows across cortical layers identified in the present study. During the acquisition of visual stimulus, signals flowed from layer Ⅳ via Ⅱ/Ⅲ to Ⅴ/Ⅵ (arrow in green). During memory retrieval, signals flowed in the opposite direction: from layers Ⅴ/Ⅵ to Ⅱ/Ⅲ.

Nakajo K, Ulbrich MH, Kubo Y, Isacoff EY. (2010) Stoichiometry of the KCNQ1-KCNE1 ion channel complex.  PNAS, 107: 18862-7

 

 (Right) Representative image of GFP-tagged KCNE1 under the TIRF microscope is shown.  Each green circle represents one ion channel complex.  (Left) The time courses of the fluorescence intensity were shown.  In the upper panel, two-time bleaching events of GFP were observed (green arrows), indicating that at least two GFPs (KCNE1 subunits) existed in the spot.  Example of four GFPs (KCNE1 subunits) was shown in the lower panel.

Stress increases cardiac function, ventilation, and body temperature. These changes will prepare to and support for fight-or-flight behavior by increasing supply of fuel and oxygen and by increasing conduction velocity of nerve impulses. We previously demonstrated that cardiorespiratory excitation during stress depends on hypothalamic neuropeptide, orexin (review: Resp Physiol Neurobiol 174: 43-54, 2010). We examined whether the same is true for stress-induced hyperthermia. Orexin neuron-ablated mice (ORX-AB) showed an attenuated stress-induced hyperthermia while their basal body temperature is normal. The brown adipose tissue, which is a major thermogenic organ in rodents, did not respond to handling stress although it did respond to a direct pharmacologic stimulation. These abnormalities in ORX-AB were not observed in orexin knockout mice in which orexin peptide is deficient but neurons are preserved. Therefore, integrity (orexin and co-existing other neurotransmitter/modulators) of the orexin neurons is indispensable for full expression of multiple facets of the fight-or-flight response.

J Physiol 588 (21): 4117-4129, 2010. (see also perspectives p. 4067)

The precise roles for the thermo TRP channels during development have not been determined.  To explore the functional importance of thermo TRP channels during neural development, the temporal expression was determined in embryonic mice. 　TRPV2 expression was detected in spinal motor neurons in addition to the DRG.  Furthermore, TRPV2 was localized in axon shafts and growth cones in the developing DRG and spinal motor neurons, suggesting that the channel is important for axon outgrowth regulation.  Endogenous TRPV2 was activated in a membrane stretch dependent manner in developing neurons, and significantly promoted axon outgrowth. 　We also confirmed by an in ovo electroporation method that TRPV2 ectopic expression promoted axon outgrowth in chick embryos.　Thus, for the first time we revealed that TRPV2 is an important regulator for axon outgrowth through activation by membrane stretch during development, and is a thermo-sensor for noxious heat in postnatal animals.  Shibasaki et al., J. Nueosci. 30：4601-4612, 2010 (Mar 31)

Figure  TRPV2 detects membrane-stretch, and promotes axon outgrowth

Round circle and line (navy blue) in upper left side represent a neuron. Extending axons cause membrane-stretch on their plasma membrane. Upon axon extension, TRPV2 detects the membrane-stretch, and cause calcium influx. The calcium influx causes further axon outgrowth. TRPV2 expression is restricted in spinal motor and sensory neurons during development. These two neurons extend long axons toward to peripheral tissues compared with interneurons in spinal cord. Thus, TRPV2 might be due to the long axon outgrowth in motor and sensory neurons.

Voltage-gated proton channels are characterized by high proton-selectivity and high transfer rates. They were first described in snail neurons in 1984. Since then, the mechanism of proton permeation thorough the channel has long been an issue of interest, but still remains unresolved. Temperature-dependence of the permeation process is an important feature to elucidate the permeation mechanism, but accurate evaluation has been prevented by the small single channel conductance of the order of one fA and, additionally, by proton-specific physicochemical factors which interact with each other. To overcome these limitations, we exploited a temperature-jump method which allowed changing the temperature of cells within a few ms and measuring current amplitudes immediately before and after a temperature jump. We used the current ratios to extract the permeation process successfully. Q₁₀ exhibited a high temperature dependence, varying from 2.2 at 10°C to 1.3 at 40°C. We found that the access resistance contributed to the measured temperature dependence, which was evaluated by a novel resistivity pulse method. Finally, the Q₁₀ for proton permeation through the voltage-gated proton channel itself was determined to range from 2.8 at 5°C to 2.2 at 45°C. The activation enthalpy for proton permeation through the pore was 64 kJ/mol. These values are much higher than those expected for water-filled pore. A few years ago, a molecular candidate of the proton channel (VSOP/Hv) was reported. The high Q₁₀ values for permeation and the rate-limiting access resistance, revealed here, are important thermodynamic clues to advance our understanding of the permeation mechanism of proton channels.

Figure legend:

A, Protons flow through the channel resistance (R_Ch) and the access resistance (R_AR) in series. B, Decomposition of Q₁₀ values for the total proton flow (Q₁₀^app, red) into that for the access resistance (Q₁₀^AR, green) and for the channel per se (Q₁₀^Ch, blue). At low temperature, the Q₁₀^Ch predominates, and at high temperature, the Q₁₀^AR predominates. The symbols are experimental results.

 Since pioneer studies by Hubel and Wiesel in the mid 20^th century, the anatomical architecture and functional compartments of the visual system in the brain have been well studied. In Old World primates and apes, visual afferents from the two eyes have segregated inputs into the granular layer (layer 4C), forming “ocular dominance columns (ODCs)” perpendicular to the pial surface. In addition, the enzymatic reactivity of cytochrome oxidase (CO) reveals a “blob” structure within ODCs in supragranular layers (layers 2-4B), which has been characterized as a non-oriented, color processing domain. Recently, we discovered that the blob structure extends to infragranular layers (layers 5/6) and that there is “border strip” structure in the vicinity of boundaries of ODCs. These compartments are revealed by the activity-dependent mRNA expression of immediate-early genes (IEGs), zif268 and c-fos, after brief (1-3 h) monocular inactivation by tetrodotoxin injection. In later (> 5 h) phases, the appearance of these structures was unclear, as the IEG mRNA expression level became low throughout the silenced eye columns and high throughout the intact eye columns. These changes of IEG mRNA expression provide new insights into functional architecture of primate V1 and reveal rapid changes in neuronal activity after acute disruption of binocular vision. (Proc Natl Acad Sci USA, 2009 Jul 6 [Epub ahead of print], doi/10.1073/pnas.0905092106)

Figure caption: our schematic model of primary visual cortex of macaques. We added blobs in infragranular layers, and border strips in the vicinity of boundaries of ocular dominance columns (right) to the previous view (left).

Transient short-term memory slips out of your mind soon after the experience, while persistent long-term memory lasts for life. How do we acquire a long-term memory that preserves information of a past experience as was? Frequent activation of a synapse in general enhances transmission of the very same synapse, enabling synchronized activity of a group of neurons connected by active synapses, which is assumed to be neural substrate for memory in the brain. Thus, persistence of consistent memory is built over the persistent enhancement of transmission of the limited synapses that were active during acquisition. Modification of existing molecules in the synapse is insufficient for long-term memory which involves induction of gene expression and functions of sets of newly synthesized proteins. Therefore, proteins synthesized in the soma must function in the limited synapses that were active during acquisition among tens of thousand of synapses in a neuron. A mechanism for this is called synaptic tagging; however, any protein that behaves as the synaptic tagging hypothesis predicts was not known, and biochemical activity that serves as synaptic tagging was not specified. We observed movement of Vesl-1S, one of the proteins synthesized in the soma and required for long-term memory, from the soma to spines, small (~ 1 mm) protrusions where synapses reside, in rat hippocampal neurons. We showed that Vesl-1S exemplified the synaptic tagging hypothesis and that regulation of Vesl-1S protein transport into spines serves as a synaptic tag. Our findings will push forward researches on the molecular mechanisms and regulation of long-term memory. (Okada, Ozawa, Inokuchi. Science 324: 904-909, 2009)

Legend to the Figure

Photo-Activatable Green Fluorescent Protein (PAGFP) does not initially fluoresce, but it does after receiving light at around 400 nm. Vesl-1S fused with PAGFP (VPA) was exogenously expressed in rat hippocampal neurons in dispersed primary culture. The soma was illuminated by a 404 nm laser, which resulted in fluorescence limited in the soma. Pharmacological stimulation was given at the distal spines to activate synaptic tag only in the synapses within the restricted area. Four hours later, VPA fluorescence originated from the soma was seen in all dendritic trees. VPA fluorescence entered spines inside the local stimulation area (lower spine), while it was not observed in spines outside the area (upper spine). The strength of the green color expresses the fluorescence intensity in the figure. An activity that permits entry of Vesl-1S proteins into spines is assumed in the spine neck. This activity is high in spines with activated synapses, thus Vesl-1S (VPA in this particular experiment) enters these spines and functions there. Regulation of this activity meets all standards for synaptic tagging, thus we concluded that this activity serves as a synaptic tag for Vesl-1S protein.

Acetylcholine (ACh) release in the hippocampus increases during learning or exploration, exhibiting a sex-specific 24-h release profile. We examined the activational effect of gonadal steroid hormones on the ACh levels. Gonadectomy severely attenuated the ACh levels and severely reduced the correlation with spontaneous behaviors. The testosterone replacement in gonadectomized males or estradiol replacement in gonadectomized females successfully restored the ACh levels and the correlation. However, estradiol-priming in gonadectomized males could not restore the ACh levels, and testosterone replacement in gonadectomized females failed to raise ACh levels to those seen in testosterone-primed gonadectomized males, revealing a sex-specific activational effect. Moreover, neonatal testosterone or estradiol treatment not only increased the ACh levels but also altered them to resemble male-specific ACh release properties without affecting behavioral levels. We conclude that the activational effects of gonadal steroids maintaining the ACh levels are sex-specific, and that neonatal sexual differentiation of cholinergic system may suggest sex-specific clinical strategies for Alzheimer's disease. (The Journal of Neuroscience, 29:3808-3815, 2009)

Figure   Gonadal steroids maintain the sex-specific ACh release in the hippocampus. The testosterone (T) replacement in gonadectomized males or estradiol (E) replacement in gonadectomized females successfully restored the ACh levels and the correlation with spontaneous behaviors. Moreover, neonatal activation of estrogen receptors masculinizes the sex-specific action of gonadal steroids.

Ca²⁺-induced Ca²⁺ release (CICR) via ryanodine receptors (RyRs) in the somata of bullfrog sympathetic neurons plays a very important role in regulating membrane excitability (Akita & Kuba, J Gen Physiol 116:697-720, 2000). In our recent study, we found that this CICR is tightly regulated by a Ca²⁺-dependent “inactivation” mechanism (Akita & Kuba, J Physiol 586:3365-84, 2008). The observable [Ca²⁺]_i rise evoked by Ca²⁺ entry at the beginning of membrane depolarization was solely due to CICR in this neuron, and this CICR was inactivated within 10-20 ms when the Ca²⁺ entry continued. The inactivation was inhibited by intracellular BAPTA (IC₅₀»0.4 mM) but not by EGTA (≦10 mM), indicating that it must be mediated by some Ca²⁺-sensing molecules located close to (at ~60 nm from) voltage-gated Ca²⁺ channels and/or RyRs, and that the molecules must be exposed to a high [Ca²⁺]_i at the edges of “Ca²⁺ microdomains” during Ca²⁺ entry. Moreover, the longer duration of Ca²⁺ entry persisting after CICR inactivation was found to cause slower [Ca²⁺]_i decay after the end of Ca²⁺ entry. This was inhibited in parallel with the inhibition of inactivation by BAPTA. Thus, some mechanism counteracting Ca²⁺ clearance must be linked to the inactivation mechanism, and this should provide the basis for the prolonged suppression of membrane excitability through activation of Ca²⁺-sensitive K⁺ channels after a longer period of membrane depolarization.

Supplementary information for figure: The Ca²⁺ sensor for inactivation is highly likely to reside in the molecules different from RyRs, although they are yet to be identified. The mechanism counteracting [Ca²⁺]_i decay would presumably be the weak Ca²⁺ release from RyRs in the “flickeringly” open mode, which must be converted from the inactivated state.

Under physiological conditions, α₁-adrenoceptor (AR) stimulation modulates mammalian cardiac L-type Ca²⁺channel as in the case of β-AR stimulation. Our group previously reported that the effects of α1-AR stimulation on L-type Ca²⁺ current (ICa,L) can be classified in two opposite effects (negative and positive effects) and the positive effect is protein kinase C(PKC)- and Ca²⁺/calmodulin kinase II (CaMKII)-dependent (O-Uchi et al., Proc Natl Acad Sci USA, 102:9400-9405, 2005). However, two important questions remain to be solved; 1) what is the molecular mechanism which simultaneously induces two opposite effects during α₁-AR stimulation?; 2) what are the molecular components for evoking the negative effect on ICa by α₁-AR stimulation? In our latest paper, we show in the native cardiac cells that two different α₁-AR subtypes (α_1A and α_1B) have functional interactions with different G-proteins and that results in opposite modulation of L-type Ca²⁺ channels by using the combination of electrophysiological, biochemical and morphological methods. α1A-adrenoceptor coupled with Gq and this pathway activates phospholipase C (PLC)-PKC-CaMKII signal and leads potentiation of Ca²⁺ current. On the other hand, α1B-adrenoceptor has the interaction with one of the pertussis toxin sensitive G-protein (Go), which β&ganma; subunits directly affect the channels and shows negative effect in Ca²⁺ current. The approach of characterizing the receptor subtype-specific interacting G protein will provide new insight to elucidate the whole picture of the subtype-specific signaling pathway in native cardiomyocytes, and further could lead to understand the functional roles of each α₁-AR subtype under physiological and pathophysiological condition (O-Uchi et al., Circ Res. 102:1378-88, 2008).

Figure Legends:

Figure A shows the possible mechanism underlying the opposing modulation of L-type Ca²⁺ channels induced by α₁-AR subtype-specific signaling. α_1A-AR-G_q/11 pathway potentiates ICa (showing as the red line in Figure B) and α_1B-AR-G_o interaction inhibits ICa (showing as the blue line in Figure B). The sum of these two opposite effects could explain the unique effect (biphasic change) of subtype non-subtype selective α₁-AR stimulation by Phe (shown as the black line in Figure B).

An increase in ventricular volume enhances the systolic performance of the heart; this is known as the Frank-Starling law of the heart.  “The Law” is a manifestation of the sarcomere length dependence of myocardial activation, in which active force is a function of the resting sarcomere length (i.e., length-dependent activation).  We have reported that passive force resulting from extension of the giant elastic protein titin (also known as connectin) operates as a triggering factor in this phenomenon.  In the present study, we investigated whether or not length-dependent activation is modulated at the thin filament level.  Quasi-complete reconstitution of thin filaments with rabbit fast skeletal troponin (sTn) attenuated length-dependent activation in porcine left ventricular muscle to a magnitude similar to that observed in rabbit fast skeletal muscle, accompanied by an increase in Ca²⁺ sensitivity of force (Fig. 1).  We also found that sTn reconstitution accelerated cross-bridge kinetics at submaximal levels, suggesting that sTn reconstitution results in a decrease in the fraction of resting cross-bridges that can potentially produce active force.  An increase in titin-based passive force, induced by manipulating the pre-history of stretch, enhanced length-dependent activation, with and without sTn reconstitution.  These results favor the interpretation that troponin plays an important role in length-dependent activation via on-off switching of the thin filament state, in concert with titin-based regulation.

T. Terui, M. Sodnomtseren, D. Matsuba, J. Udaka, S. Ishiwata, I. Ohtsuki, S. Kurihara,  and N. Fukuda. 2008. Troponin and Titin coordinately regulate length-dependent activation in skinned porcine ventricular muscle. J. Gen. Physiol. 131:275-283.

Figure 1. Effect of sTn reconstitution on length-dependent activation in porcine left ventricular muscle (PLV).  (A) SDS-PAGE analysis.  Cont., control PLV; sTn, sTn-reconstituted PLV.  cTnT (sTnT), cardiac (skeletal) troponin T; cTnI (sTnI), cardiac (skeletal) troponin I; TnC, troponin C; Tm, tropomyosin; LC-1, myosin light chain 1; LC-2, myosin light chain 2.  (B) Force-pCa curves in control (black lines) and sTn-reconstituted (red lines) PLV at SL 1.9 and 2.3 μm.  Inset, ΔpCa₅₀ (difference between the values of the mid-point (pCa₅₀) of the force-pCa curve measured at SL 1.9 and 2.3 μm).  C, control PLV.   *P<0.05.  Reproduced from The Journal of General Physiology, 2008, 131:275-283. Copyright 2008 The Rockefeller University Press. 

Crystal structure of potassium channels in closed and open states has been elucidated. To gain dynamic pictures of functional channel, the KcsA potassium channel upon gating were examined by the diffracted X-ray tracking (DXT) method. A single molecular KcsA channel was attached with a gold nano-crystal with a size of 20 nm and high flux X-rays were irradiated. A diffraction spot from the nano-crystal was tracked in real time to trace the trajectories of conformational changes. At physiological pH where the channel keeps its gate closed, the channel exhibited small random fluctuations of its structure. When the channel is actively gating at acidic pH, the channel twisted its conformation around the pore axis. Random clockwise and counterclockwise twisting was observed in the range of several tens of degrees. This motion corresponds to the twisting and untwisting of the pore, which allow ions to occlude or permeate. The twisting conformational change was initiated from the transmembrane domain and was propagated towards the end of the cytoplasmic domain. In the presence of an open channel blocker, tetrabutylammonium, the twisting motion was stopped even at acidic pH and the conformational wave failed to propagate. This type of twisting motion may be shared by various types of ion channels upon gating. (H. Shimizu*, M. Iwamoto*, T. Konno*, A. Nihei***, Y. C. Sasaki** & S. Oiki*: Global Twisting Motion of Single Molecular KcsA Potassium Channel upon Gating. Cell 132, 67-78, 2008)

Twisting conformational change of the channel upon gating

The transmembrane domain of the potassium channel is formed by a bundle of α-helices (left). The intracellular ends of the helices are crossed, which occludes the ion permeation pathway. During gating the channel molecule (un)twisted its shape, which corresponds to relaxation of the bundle, leading to opening of the permeation pathway. This twisting motion was originated from the transmembrane domain and was transferred to the cytoplasmic domain.

We previously reported an ascidian protein Ci-VSP which has a transmembrane voltage sensor motif and a phosphatase domain.  We showed that the voltage sensor domain functionally couples with the phosphatase domain and the phosphatase activity increases by depolarization. However, it remained unknown whether depolarization or hyperpolarization induced activation of enzyme.  In order to address this issue, we performed (1) measurements of phosphoinositide level using GFP-based imaging by confocal microscopy under voltage clamp condition in Xenopus oocyte and (2) detailed electrophysiological analysis of modification of three types of potassium channels by Ci-VSP.
    PtdIns(4,5)P₂ level, as detected by PH(PLC-δ)-GFP, decreased by depolarization and increased by hyperpolarization, consistent with our previous report (Murata et al, 2005). However, PtdIns(3,4,5)P₃ level as detected by PH(Btk)-GFP also showed similar change as PtdIns(4,5)P₂, as opposed to the idea that Ci-VSP dephosphorylates PtdIns(3,4,5)P₃ to increase PtdIns(4,5)P₂.
Next, the activities of IRK1, with higher affinity to PtdIns(4,5)P₂ than GIRK2 were measured: it showed current decrease dependent on the membrane potential of intervals with the rightward shift of the current-voltage relationship compared with that of GIRK2 channels.  In addition, we noticed that KCNQ2/3 channels in the presence of Ci-VSP showed remarkable current decay similar to channel inactivation at the higher membrane potential.  According to depolarization the decay time constant became gradually smaller, indicating that phosphatase activity increases up to about 100 mV.  
Taken together, we conclude that (1) phosphatase activity of Ci-VSP turns on by depolarization and (2) PtdIns(4,5)P₂ as well as PtdIns(3,4,5)P₃ can be a substrate for Ci-VSP.

Murata Y. and Okamura Y., J. Physiol., 583: 875-889(2007)

The δ2 glutamate receptor (GluRδ2), which is predominantly expressed in cerebellar Purkinje cells, is a member of ionotropic glutamate receptor (iGluR) family.  The GluRδ2 plays crucial roles in synapse formation and synaptic plasticity: mice disrupted GluRδ2 gene (GluRδ2-null mice) showed severe ataxia, abnormal synapse morphology and impaired cerebellar long-term depression (LTD), a synaptic plasticity model responsible for motor learning.  Despite its importance, the mechanisms by which GluRδ2 participates in cerebellar functions, especially whether GluRδ2 functions as a channel, is a long-lasting question because of the lack of ligands for GluRδ2.  To address this issue, we introduced two kinds of mutant GluRδ2s, in which putative sites underlying (1) Ca²⁺-permeability and (2) channel pore are disrupted, into GluRδ2-null cerebellum.  Surprisingly, transgenic mouse-mediated expression of mutant GluRδ2 lacking Ca²⁺-permeability rescued almost all abnormality of GluRδ2-null mice (Kakegawa et al., J. Physiol., 579: 729-735, 2007).  Furthermore, Sindbis virus-mediated expression of mutant GluRδ2 disrupted the putative channel pore also recovered impaired LTD of GluRδ2-null Purkinje cells (Kakegawa et al., J. Physiol., 584: 89-96, 2007).  These results strongly supported that, although GluRδ2 belongs to iGluRs, GluRδ2 does not serve as an ion channel in vivo (Figure 1).  Accumulating evidence indicated that other iGluRs also have non-ionotropic functions, therefore, GluRδ2 may provide a key insight into the elucidation of non-ionotropic functions of iGluRs. 

KCNQ1 is a voltage-dependent K⁺ channel expressed in various tissues such as heart and inner ear, and also known as a causal gene for long QT syndrome.  As seen in a protein complex of KCNQ1 and KCNE1 underlying I_Ks current in heart, biophysical properties of KCNQ1 channel can be dramatically regulated by auxiliary KCNE proteins.  To understand the regulatory mechanisms of KCNQ1 by KCNE protein family, we tested whether the movement of voltage sensor domain was affected by the presence of KCNE proteins or not.  We introduced several cysteine substitutions on S4 segment, which plays a major role on the voltage sensing, one at a time.  We then applied cysteine modifying MTS reagent and examined how the modification rate was affected by the presence of KCNE1 or KCNE3.  We found that the “down state” of the voltage sensor was stabilized in the presence of KCNE1 while the “up state” was stabilized in the presence of KCNE3 (Figure).  Our results suggest that the voltage sensor is involved in the modulation of KCNQ1 by KCNE family.
(Nakajo & Kubo, J. Gen. Physiol., 130: 269-281, 2007)

KCNE1 (blue) stabilizes the voltage sensor in the “down state” (left).  On the other hand, KCNE3 stabilizes the voltage sensor in the “up state” (right).