[Show abstract][Hide abstract] ABSTRACT: The α2δ-1 protein is an auxiliary subunit of voltage-gated calcium channels, critical for neurotransmitter release. It is up-regulated in dorsal root ganglion (DRG) neurons following sensory nerve injury, and is also the therapeutic target of the gabapentinoid drugs, which are efficacious in both experimental and human neuropathic pain conditions. α2δ-1 has three spliced regions, A, B and C. A and C are cassette exons, whereas B is introduced via an alternative 3' splice acceptor site. Here we have examined the presence of α2δ-1 splice variants in DRG neurons, and have found that although the main α2δ-1 splice variant in DRGs is the same as that in brain (α2δ-1 ΔA+B+C), there is also another α2δ-1 splice variant (ΔA+BΔC), which is expressed in DRG neurons and is differentially up-regulated compared to the main DRG splice variant α2δ-1 ΔA+B+C following spinal nerve ligation. Furthermore, this differential up-regulation occurs preferentially in a small non-myelinated DRG neuron fraction, obtained by density gradient separation. The α2δ-1 ΔA+BΔC splice variant supports CaV2 calcium currents with unaltered properties compared to α2δ-1 ΔA+B+C, but shows a significantly reduced affinity for gabapentin. This variant could therefore play a role in determining the efficacy of gabapentin in neuropathic pain.
[Show abstract][Hide abstract] ABSTRACT: Epigenetic modifications of chromatin represent a fundamental mechanism by which eukaryotic cells adapt their transcriptional response to developmental and environmental cues. Although an increasing number of molecules have been linked to epigenetic changes, the intracellular pathways that lead to their activation/repression have just begun to be characterized. Here, we demonstrate that inositol hexakisphosphate kinase 1 (IP6K1), the enzyme responsible for the synthesis of the high-energy inositol pyrophosphates (IP7), is associated with chromatin and interacts with Jumonji domain containing 2C (JMJD2C), a recently identified histone lysine demethylase. Reducing IP6K1 levels by RNAi or using mouse embryonic fibroblasts derived from ip6k1(-/-) knockout mice results in a decreased IP7 concentration that epigenetically translates to reduced levels of trimethyl-histone H3 lysine 9 (H3K9me3) and increased levels of acetyl-H3K9. Conversely, expression of IP6K1 induces JMJD2C dissociation from chromatin and increases H3K9me3 levels, which depend on IP6K1 catalytic activity. Importantly, these effects lead to changes in JMJD2C-target gene transcription. Our findings demonstrate that inositol pyrophosphate signaling influences nuclear functions by regulating histone modifications.
Proceedings of the National Academy of Sciences 11/2013;
[Show abstract][Hide abstract] ABSTRACT: Two-photon microscopy is widely used to study brain function, but conventional microscopes are too slow to capture the timing of neuronal signalling and imaging is restricted to one plane. Recent development of acousto-optic-deflector-based random access functional imaging has improved the temporal resolution, but the utility of these technologies for mapping 3D synaptic activity patterns and their performance at the excitation wavelengths required to image genetically-encoded indicators have not been investigated. New Method. Here, we have used a compact acousto-optic lens (AOL) two-photon microscope to make high speed [Ca(2+)] measurements from spines and dendrites distributed in 3D with different excitation wavelengths (800nm - 920nm).
We show simultaneous monitoring of activity from many synaptic inputs distributed over the 3D arborisation of a neuronal dendrite using both synthetic as well as genetically-encoded indicators. We confirm the utility of AOL-based imaging for fast in vivo recordings by measuring, simultaneously, visually-evoked responses in 100 neurons distributed over a 150μm focal depth range. Moreover, we explore ways to improve the measurement of timing of neuronal activation by choosing specific regions within the cell soma. Comparison with existing Methods. These results establish that AOL-based 3D random access two-photon microscopy has a wider range of neuroscience applications than previously shown.
Our findings show that the compact AOL microscope design has the speed, spatial resolution, sensitivity and wavelength flexibility to measure 3D patterns of synaptic and neuronal activity on individual trials.
[Show abstract][Hide abstract] ABSTRACT: The spatial pattern of synapse activation may impact on synaptic plasticity. This applies to the synaptically-evoked endocannabinoid-mediated short-term depression at the parallel fiber (PF) to Purkinje cell synapse, the occurrence of which requires close proximity between the activated synapses. Here, we determine quantitatively this required proximity, helped by the geometrical organization of the cerebellar molecular layer. Transgenic mice expressing a calcium indicator selectively in granule cells enabled the imaging of action potential-evoked presynaptic calcium rise in isolated, single PFs. This measurement was used to derive the number of PFs activated within a beam of PFs stimulated in the molecular layer, from which the density of activated PFs (input density) was calculated. This density was on average 2.8 μm(-2) in sagittal slices and twice more in transverse slices. The synaptically-evoked endocannabinoid-mediated suppression of excitation (SSE) evoked by ten stimuli at 200Hz was determined from the monitoring of either postsynaptic responses or presynaptic calcium rise. The SSE was significantly larger when recorded in transverse slices, where the input density is larger. The exponential description of the SSE plotted as a function of the input density suggests that the SSE is half reduced when the input density decreases from 6 to 2 μm(-2). We conclude that, although all PFs are truncated in an acute sagittal slice, half of them remain respondent to stimulation, and activated synapses need to be closer than 1.5 micrometer to synergize in endocannabinoid signaling.
[Show abstract][Hide abstract] ABSTRACT: Neuronal dendrites are electrically excitable: they can generate regenerative events such as dendritic spikes in response to sufficiently strong synaptic input. Although such events have been observed in many neuronal types, it is not well understood how active dendrites contribute to the tuning of neuronal output in vivo. Here we show that dendritic spikes increase the selectivity of neuronal responses to the orientation of a visual stimulus (orientation tuning). We performed direct patch-clamp recordings from the dendrites of pyramidal neurons in the primary visual cortex of lightly anaesthetized and awake mice, during sensory processing. Visual stimulation triggered regenerative local dendritic spikes that were distinct from back-propagating action potentials. These events were orientation tuned and were suppressed by either hyperpolarization of membrane potential or intracellular blockade of NMDA (N-methyl-d-aspartate) receptors. Both of these manipulations also decreased the selectivity of subthreshold orientation tuning measured at the soma, thus linking dendritic regenerative events to somatic orientation tuning. Together, our results suggest that dendritic spikes that are triggered by visual input contribute to a fundamental cortical computation: enhancing orientation selectivity in the visual cortex. Thus, dendritic excitability is an essential component of behaviourally relevant computations in neurons.
[Show abstract][Hide abstract] ABSTRACT: The second messenger cyclic AMP (cAMP) operates in discrete subcellular regions within which proteins that synthesize, break down or respond to the second messenger are precisely organized. A burgeoning knowledge of compartmentalized cAMP signaling is revealing how the local control of signaling enzyme activity impacts upon disease. The aim of this Cell Science at a Glance article and the accompanying poster is to highlight how misregulation of local cyclic AMP signaling can have pathophysiological consequences. We first introduce the core molecular machinery for cAMP signaling, which includes the cAMP-dependent protein kinase (PKA), and then consider the role of A-kinase anchoring proteins (AKAPs) in coordinating different cAMP-responsive proteins. The latter sections illustrate the emerging role of local cAMP signaling in four disease areas: cataracts, cancer, diabetes and cardiovascular diseases.
Journal of Cell Science 10/2013; 126(Pt 20):4537-4543.
[Show abstract][Hide abstract] ABSTRACT: The phenomenology and cellular mechanisms of cortical synaptic plasticity are becoming known in increasing detail, but the computational principles by which cortical plasticity enables the development of sensory representations are unclear. Here we describe a framework for cortical synaptic plasticity termed the "Convallis rule", mathematically derived from a principle of unsupervised learning via constrained optimization. Implementation of the rule caused a recurrent cortex-like network of simulated spiking neurons to develop rate representations of real-world speech stimuli, enabling classification by a downstream linear decoder. Applied to spike patterns used in in vitro plasticity experiments, the rule reproduced multiple results including and beyond STDP. However STDP alone produced poorer learning performance. The mathematical form of the rule is consistent with a dual coincidence detector mechanism that has been suggested by experiments in several synaptic classes of juvenile neocortex. Based on this confluence of normative, phenomenological, and mechanistic evidence, we suggest that the rule may approximate a fundamental computational principle of the neocortex.
[Show abstract][Hide abstract] ABSTRACT: Glucagon-like peptide-1 (GLP-1) is both a peripherally expressed incretin and a centrally active neuropeptide. Brain derived GLP-1, produced in preproglucagon (PPG) neurons located in the nucleus of the solitary tract (NTS) and projecting to numerous brain regions, is ideally placed to activate central GLP-1 receptors in a range of autonomic control areas. In vivo analysis of central GLP-1 using GLP-1 receptor antagonists has demonstrated the control of a range of feeding responses mediated by GLP-1 receptor activation. Recent advances enabling identification and targeting of the neurons in the NTS has specifically implicated PPG neurons at the core of GLP-1 dependent central and peripheral control for short-term and long-term energy balance.
[Show abstract][Hide abstract] ABSTRACT: Tonic inhibition mediated by extrasynaptic GABAA receptors (GABAA Rs) is an important regulator of neuronal excitability. Phosphorylation by protein kinase C (PKC) provides a key mode of regulation for synaptic GABAA Rs underlying phasic inhibition; however, less attention has been focused on the plasticity of tonic inhibition and whether this can also be modulated by receptor phosphorylation. To address this issue, we used whole-cell patch clamp recording in acute murine brain slices at both room and physiological temperatures to examine the effects of PKC-mediated phosphorylation on tonic inhibition. Recordings from dentate gyrus granule cells in the hippocampus and dorsal lateral geniculate relay neurons in the thalamus demonstrated that PKC activation caused downregulation of tonic GABAA R-mediated inhibition. Conversely, inhibition of PKC resulted in an increase in tonic GABAA R activity. These findings were corroborated by experiments on human embryonic kidney 293 cells expressing recombinant α4β2δ GABAA Rs, which represent a key extrasynaptic GABAA R isoform in the hippocampus and thalamus. Using bath application of low GABA concentrations to mimic activation by ambient neurotransmitter, we demonstrated a similar inhibition of receptor function following PKC activation at physiological temperature. Live cell imaging revealed that this was correlated with a loss of cell surface GABAA Rs. The inhibitory effects of PKC activation on α4β2δ GABAA R activity appeared to be mediated by direct phosphorylation at a previously identified site on the β2 subunit, serine 410. These results indicate that PKC-mediated phosphorylation can be an important physiological regulator of tonic GABAA R-mediated inhibition.
European Journal of Neuroscience 09/2013;
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