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Sourav GHOSH

7 publication(s) depuis Février 2012:

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22/11/2017 | J Neurophysiol   IF 2.5
Ghosh S, Reuveni I, Zidan S, Lamprecht R, Barkai E

Endocannabinoids are key modulators that regulate central brain functions and behaviours, including learning and memory. At the cellular and molecular levels, endocannabinoids are potent modulators of excitatory and inhibitory synaptic function. Most effects of cannabinoids are thought to be mediated via G protein-coupled cannabinoids receptors. In particular, cannabinoids released from post-synaptic neurons are suggested to act as retrograde messengers, activating presynaptic Type-1 Cannabinoid receptors (CB1R), thereby inducing suppression of synaptic release. Another central mechanism of cannabinoids-induced action requires activation of astroglial CB1R. CB1R are also implicated in self-modulation of cortical neurons. Rats that are trained in a particularly difficult olfactory-discrimination task show a dramatic increased ability to acquire memories of new odors. The memory of the acquired high skill acquisition, termed 'rule learning' or 'learning set' lasts for many months. Using this behavioural paradigm, we show a novel function of action for CB1R; supporting long-term memory by maintaining persistent enhancement of inhibitory synaptic transmission. Long-lasting enhancement of inhibitory synaptic transmission is blocked by a CB1R inverse agonist. This effect is mediated by a novel purely post-synaptic mechanism, obtained by enhancing the single GABAA channel conductance that is PKA-dependent. The significant role that CB1R has in maintaining learning-induced long-term strengthening of synaptic inhibition suggests that endocannabinoids have a key role in maintaining long-term memory by enhancing synaptic inhibition.

Intense spiking response of a memory-pattern is believed to play a crucial role both in normal learning and pathology, where it can create biased behavior. We recently proposed a novel model for memory amplification where the simultaneous two-fold increase of all excitatory (AMPAR-mediated) and inhibitory (GABAAR-mediated) synapses in a sub-group of cells that constitutes a memory-pattern selectively amplifies this memory. Here we confirm the cellular basis of this model by validating its major predictions in four sets of experiments, and demonstrate its induction via a whole-cell transduction mechanism. Subsequently, using theory and simulations, we show that this whole-cell two-fold increase of all inhibitory and excitatory synapses functions as an instantaneous and multiplicative amplifier of the neurons' spiking. The amplification mechanism acts through multiplication of the net synaptic current, where it scales both the average and the standard deviation of the current. In the excitation-inhibition balance regime, this scaling creates a linear multiplicative amplifier of the cell's spiking response. Moreover, the direct scaling of the synaptic input enables the amplification of the spiking response to be synchronized with rapid changes in synaptic input, and to be independent of previous spiking activity. These traits enable instantaneous real-time amplification during brief elevations of excitatory synaptic input. Furthermore, the multiplicative nature of the amplifier ensures that the net effect of the amplification is large mainly when the synaptic input is mostly excitatory. When induced on all cells that comprise a memory-pattern, these whole-cell modifications enable a substantial instantaneous amplification of the memory-pattern when the memory is activated. The amplification mechanism is induced by CaMKII dependent phosphorylation that doubles the conductance of all GABAA and AMPA receptors in a subset of neurons. This whole-cell transduction mechanism enables both long-term induction of memory amplification when necessary and extinction when not further required.

Once trained, rats are able to execute particularly difficult olfactory discrimination tasks with exceptional accuracy. Such skill acquisition, termed 'rule learning', is accompanied by a series of long-lasting modifications to three cellular properties which modulate pyramidal neuron activity in piriform cortex; intrinsic excitability, synaptic excitation, and synaptic inhibition. Here, we explore how these changes, which are seemingly contradictory at the single-cell level in terms of their effect on neuronal excitation, are manifested within the piriform cortical neuronal network to store the memory of the rule, while maintaining network stability. To this end, we monitored network activity via multisite extracellular recordings of field postsynaptic potentials (fPSPS) and with single-cell recordings of miniature inhibitory and excitatory synaptic events in piriform cortex slices. We show that although 5 days after rule learning the cortical network maintains its basic activity patterns, synaptic connectivity is strengthened specifically between spatially proximal cells. Moreover, while the enhancement of inhibitory and excitatory synaptic connectivity is nearly identical, strengthening of synaptic inhibition is equally distributed between neurons while synaptic excitation is particularly strengthened within a specific subgroup of cells. We suggest that memory for the acquired rule is stored mainly by strengthening excitatory synaptic connection between close pyramidal neurons and runaway synaptic activity arising from this change is prevented by a nonspecific enhancement of synaptic inhibition.

28/12/2015 | J Neurochem   IF 4.6
CaMKII activity is required for maintaining learning-induced enhancement of AMPAR-mediated synaptic excitation.
Ghosh S, Reuveni I, Barkai E, Lamprecht R

Learning leads to changes in AMPA receptor (AMPAR)-mediated synaptic excitation. The mechanisms for maintaining such alterations needed for memory persistence remain to be clarified. Here we report a novel molecular mechanism for maintaining learning-induced AMPAR-mediated enhancement of synaptic excitation. We show that training rats in a complex olfactory discrimination task, such that requires rule learning, leads to the enhancement of averaged amplitude of AMPAR-mediated miniature excitatory post-synaptic currents (mEPSCs) in piriform cortex pyramidal neurons for days after learning. Inhibiting calcium/calmodulin-dependent kinase II (CaMKII) using KN93 or tatCN21 days after learning, reduced the averaged mEPSC amplitude in neurons in piriform cortex of trained rats to the level where they are not significantly different from mEPSC of control animals. CaMKII inhibition leads to a decrease in single channel conductance and not to changes in the number of synaptic activated channels. We conclude that the maintenance of learning-induced enhancement of AMPAR-mediated synaptic excitation requires the activity of CaMKII. This article is protected by copyright. All rights reserved.

07/01/2015 | J Neurosci   IF 6
Persistent CaMKII activation mediates learning-induced long-lasting enhancement of synaptic inhibition.
Ghosh S, Reuveni I, Lamprecht R, Barkai E

Training rats in a particularly difficult olfactory-discrimination task results in acquisition of high skill to perform the task superbly, termed 'rule learning' or 'learning set.' Such complex learning results in enhanced intrinsic neuronal excitability of piriform cortex pyramidal neurons, and in their excitatory synaptic interconnections. These changes, while subserving memory maintenance, must be counterbalanced by modifications that prevent overspreading of activity and uncontrolled synaptic strengthening. Indeed, we have previously shown that the average amplitude of GABAA-mediated miniature IPSCs (mIPSCs) in these neurons is enhanced for several days after learning, an enhancement mediated via a postsynaptic mechanism. To unravel the molecular mechanism of this long-term inhibition enhancement, we tested the role of key second-messenger systems in maintaining such long-lasting modulation. The calcium/calmodulin-dependent kinase II (CaMKII) blocker, KN93, significantly reduced the average mIPSC amplitude in neurons from trained rats only to the average pretraining level. A similar effect was obtained by the CaMKII peptide inhibitor, tatCN21. Such reduction resulted from decreased single-channel conductance and not in the number of activated channels. The PKC inhibitor, GF109203X, reduced the average mIPSC amplitude in neurons from naive, pseudo-trained, and trained animals, and the difference between the trained and control groups remained. Such reduction resulted from a decrease in the number of activated channels. The PKA inhibitor H89 dihydrochloride did not affect the average mIPSC amplitude in neurons from any of the three groups. We conclude that learning-induced enhancement of GABAA-mediated synaptic inhibition is maintained by persistent CaMKII activation.

19/08/2013 | Cancer lett   IF 6.5
Guggulsterone sensitizes glioblastoma cells to Sonic hedgehog inhibitor SANT-1 induced apoptosis in a Ras/NFkappaB dependent manner.
Dixit D, Ghildiyal R, Anto NP, Ghosh S, Sharma V, Sen E

Since Shh pathway effector, Gli1, is overexpressed in gliomas, we investigated the effect of novel Shh inhibitor SANT-1 on glioma cell viability. Though SANT-1 failed to induce apoptosis, it reduced proliferation of glioma stem-like cells. Apart from canonical Shh cascade, Gli1 is also induced by non-canonical pathways including NFkappaB. Therefore, a combinatorial strategy with Ras/NFkappaB inhibitor, Guggulsterone, was employed to enhance effectiveness of SANT-1. Guggulsterone inhibited Ras and NFkappaB activity and sensitized cells to SANT-1 induced apoptosis via intrinsic apoptotic mechanism. Inhibition of either Ras or NFkappaB activity was sufficient to sensitize cells to SANT-1. Guggulsterone induced ERK activation also contributed to Caspase-9 activation. Since SANT-1 and Guggulsterone differentially target stem-like and non-stem glioma cells respectively, this combination warrants investigation as an effective anti-glioma therapy.

Glioblastoma multiforme (GBM) are resistant to TNFalpha-induced apoptosis and blockade of TNFalpha-induced NF-kappaB activation sensitizes glioma cells to apoptosis. As Casein kinase-2 (CK2) induces aberrant NF-kappaB activation and as we observed elevated CK2 levels in GBM tumors, we investigated the potential of CK2 inhibitors (CK2-Is) - DRB and Apigenin in sensitizing glioma cells to TNFalpha-induced apoptosis. CK2-Is and CK2 small interfering RNA (siRNA) reduced glioma cell viability, inhibited TNFalpha-mediated NF-kappaB activation, and sensitized cell to TNFalpha-induced apoptosis. Importantly, CK2-Is activated p53 function in wild-type but not in p53 mutant cells. Activation of p53 function involved its increased transcriptional activation, DNA-binding ability, increased expression of p53 target genes associated with cell cycle progression and apoptosis. Moreover, CK2-Is decreased telomerase activity and increased senescence in a p53-dependent manner. Apoptotic gene profiling indicated that CK2-Is differentially affect p53 and TNFalpha targets in p53 wild-type and mutant glioma cells. CK2-I decreased MDM2-p53 association and p53 ubiquitination to enhance p53 levels. Interestingly, CK2-Is downregulated SIRT1 activity and over-expression of SIRT1 decreased p53 transcriptional activity and rescued cells from CK2-I-induced apoptosis. This ability of CK2-Is to sensitize glioma to TNFalpha-induced death via multiple mechanisms involving abrogation of NF-kappaB activation, reactivation of wild-type p53 function and SIRT1 inhibition warrants investigation.