Jacques Noël

NSF Binding to GluR2 Regulates Synaptic Transmission

Here, we show that N-ethylmaleimide-sensitive fusion protein (NSF) interacts directly and selectively with the intracellular C-terminal domain of the GluR2 subunit of AMPA receptors. The interaction requires all three domains of NSF but occurs between residues Lys-844 and Gln-853 of rat GluR2, with Asn-851 playing a critical role. Loading of decapeptides corresponding to the NSF-binding domain of GluR2 into rat hippocampal CA1 pyramidal neurons results in a marked, progressive decrement of AMPA receptor-mediated synaptic transmission. This reduction in synaptic transmission was also observed when an anti-NSF monoclonal antibody (mAb) was loaded into CA1 neurons. These results demonstrate a previously unsuspected direct interaction in the postsynaptic neuron between two major proteins involved in synaptic transmission and suggest a rapid NSF-dependent modulation of AMPA receptor function.

TREK‐1, a K+ channel involved in polymodal pain perception

The TREK‐1 channel is a temperature‐sensitive, osmosensitive and mechano‐gated K+ channel with a regulation by Gs and Gq coupled receptors. This paper demonstrates that TREK‐1 qualifies as one of the molecular sensors involved in pain perception. TREK‐1 is highly expressed in small sensory neurons, is present in both peptidergic and nonpeptidergic neurons and is extensively colocalized with TRPV1, the capsaicin‐activated nonselective ion channel. Mice with a disrupted TREK‐1 gene are more sensitive to painful heat sensations near the threshold between anoxious warmth and painful heat. This phenotype is associated with the primary sensory neuron, as polymodal C‐fibers were found to be more sensitive to heat in single fiber experiments. Knockout animals are more sensitive to low threshold mechanical stimuli and display an increased thermal and mechanical hyperalgesia in conditions of inflammation. They display a largely decreased pain response induced by osmotic changes particularly in prostaglandin E2‐sensitized animals. TREK‐1 appears as an important ion channel for polymodal pain perception and as an attractive target for the development of new analgesics.

ASIC3, a sensor of acidic and primary inflammatory pain

Acid‐sensing ion channels (ASICs) are cationic channels activated by extracellular acidosis that are expressed in both central and peripheral nervous systems. Although peripheral ASICs seem to be natural sensors of acidic pain (e.g., in inflammation, ischaemia, lesions or tumours), a direct demonstration is still lacking. We show that ∼60% of rat cutaneous sensory neurons express ASIC3‐like currents. Native as well as recombinant ASIC3 respond synergistically to three different inflammatory signals that are slight acidifications (∼pH 7.0), hypertonicity and arachidonic acid (AA). Moderate pH, alone or in combination with hypertonicity and AA, increases nociceptors excitability and produces pain suppressed by the toxin APETx2, a specific blocker of ASIC3. Both APETx2 and the in vivo knockdown of ASIC3 with a specific siRNA also have potent analgesic effects against primary inflammation‐induced hyperalgesia in rat. Peripheral ASIC3 channels are thus essential sensors of acidic pain and integrators of molecular signals produced during inflammation where they contribute to primary hyperalgesia.

The mechano-activated K+ channels TRAAK and TREK-1 control both warm and cold perception

The sensation of cold or heat depends on the activation of specific nerve endings in the skin. This involves heat‐ and cold‐sensitive excitatory transient receptor potential (TRP) channels. However, we show here that the mechano‐gated and highly temperature‐sensitive potassium channels of the TREK/TRAAK family, which normally work as silencers of the excitatory channels, are also implicated. They are important for the definition of temperature thresholds and temperature ranges in which excitation of nociceptor takes place and for the intensity of excitation when it occurs. They are expressed with thermo‐TRP channels in sensory neurons. TRAAK and TREK‐1 channels control pain produced by mechanical stimulation and both heat and cold pain perception in mice. Expression of TRAAK alone or in association with TREK‐1 controls heat responses of both capsaicin‐sensitive and capsaicin‐insensitive sensory neurons. Together TREK‐1 and TRAAK channels are important regulators of nociceptor activation by cold, particularly in the nociceptor population that is not activated by menthol.

Oxaliplatin-induced cold hypersensitivity is due to remodelling of ion channel expression in nociceptors.

Cold hypersensitivity is the hallmark of oxaliplatin‐induced neuropathy, which develops in nearly all patients under this chemotherapy. To date, pain management strategies have failed to alleviate these symptoms, hence development of adapted analgesics is needed. Here, we report that oxaliplatin exaggerates cold perception in mice as well as in patients. These symptoms are mediated by primary afferent sensory neurons expressing the thermoreceptor TRPM8. Mechanistically, oxaliplatin promotes over‐excitability by drastically lowering the expression of distinct potassium channels (TREK1, TRAAK) and by increasing the expression of pro‐excitatory channels such as the hyperpolarization‐activated channels (HCNs). These findings are corroborated by the analysis of TREK1‐TRAAK null mice and use of the specific HCN inhibitor ivabradine, which abolishes the oxaliplatin‐induced cold hypersensibility. These results suggest that oxaliplatin exacerbates cold perception by modulating the transcription of distinct ionic conductances that together shape sensory neuron responses to cold. The translational and clinical implication of these findings would be that ivabradine may represent a tailored treatment for oxaliplatin‐induced neuropathy.

A new form of long-term depression in the perirhinal cortex

We demonstrate a form of long-term depression (LTD) in the perirhinal cortex that relies on interaction between different glutamate receptors. Group II metabotropic glutamate (mGlu) receptors facilitated group I mGlu receptor-mediated increases in intracellular calcium. This facilitation plus NMDA receptor activation may be necessary for induction of LTD at resting membrane potentials. However, depolarization enhanced NMDA receptor function and removed the requirement of synergy between group I and group II mGlu receptors: under these conditions, activation of only NMDA and group I mGlu receptors was required for LTD. Such glutamate receptor interactions potentially provide new rules for synaptic plasticity. These forms of LTD occur in the perirhinal cortex, where long-term decreases in neuronal responsiveness may mediate recognition memory.

Acid-sensing ion channels in postoperative pain.

Iatrogenic pain consecutive to a large number of surgical procedures has become a growing health concern. The etiology and pathophysiology of postoperative pain are still poorly understood, but hydrogen ions appear to be important in this process. We have investigated the role of peripheral acid-sensing ion channels (ASICs), which form depolarizing channels activated by extracellular protons, in a rat model of postoperative pain (i.e., hindpaw skin/muscle incision). We report high levels of ASIC-type currents (∼77%) in sensory neurons innervating the hindpaw muscles, with a prevalence of ASIC3-like currents. The ASIC3 protein is largely expressed in lumbar DRG neurons innervating the plantar muscle, and its mRNA and protein levels are increased by plantar incision 24 h after surgery. Pharmacological inhibition of ASIC3 channels with the specific toxin APETx2 or in vivo knockdown of ASIC3 subunit by small interfering RNA led to a significant reduction of postoperative spontaneous, thermal, and postural pain behaviors (spontaneous flinching, heat hyperalgesia, and weight bearing). ASIC3 appears to have an important role in deep tissue but also affects prolonged pain evoked by skin incision alone. The specific homomeric ASIC1a blocker PcTx1 has no effect on spontaneous flinching, when applied peripherally. Together, these data demonstrate a significant role for peripheral ASIC3-containing channels in postoperative pain.

Transient synaptic activation of NMDA receptors leads to the insertion of native AMPA receptors at hippocampal neuronal plasma membranes

The molecular mechanisms underlying long-term potentiation (LTP) of excitatory synaptic transmission in the hippocampus are not well understood. Transient depolarisation of cultured postnatal hippocampal neurones (3x1 s exposure to 90 mM K+) induces a form of LTP that is manifest primarily as an increase in mEPSC frequency. Site-directed antibodies that recognise an extracellular region of all AMPA receptor (AMPAR) subunits (GluR1-4) were used for the immunolabelling of living neurones. These antibodies were raised in two species to enable sequential immunofluorescent labelling of individual living neurones before and after the induction of LTP. High K+ treatment resulted in the appearance of new AMPAR clusters at sites on the neuronal surface that previously lacked detectable AMPARs. The appearance of new AMPAR clusters was NMDA receptor (NMDAR)-dependent since it was antagonised by the application of NMDAR antagonists. Our data indicate that the transient synaptic activation of NMDARs can lead to the insertion of native AMPARs at sites on the neuronal membrane that initially lacks AMPARs.

Molecular regulations governing TREK and TRAAK channel functions

K+ channels with two-pore domain (K2p) form a large family of hyperpolarizing channels. They produce background currents that oppose membrane depolarization and cell excitability. They are involved in cellular mechanisms of apoptosis, vasodilatation, anesthesia, pain, neuroprotection and depression. This review focuses on TREK-1, TREK-2 and TRAAK channels subfamily and on the mechanisms that contribute to their molecular heterogeneity and functional regulations. Their molecular diversity is determined not only by the number of genes but also by alternative splicing and alternative initiation of translation. These channels are sensitive to a wide array of biophysical parameters that affect their activity such as unsaturated fatty acids, intra- and extracellular pH, membrane stretch, temperature, and intracellular signaling pathways. They interact with partner proteins that influence their activity and their plasma membrane expression. Molecular heterogeneity, regulatory mechanisms and protein partners are all expected to contribute to cell specific functions of TREK currents in many tissues.

Kv1.1 Channels Act as Mechanical Brake in the Senses of Touch and Pain

Molecular determinants of threshold sensitivity of mammalian mechanoreceptors are unknown. Here, we identify a mechanosensitive (MS) K(+) current (IKmech) that governs mechanical threshold and adaptation of distinct populations of mechanoreceptors. Toxin profiling and transgenic mouse studies indicate that IKmech is carried by Kv1.1-Kv1.2 heteromers. Mechanosensitivity is attributed to Kv1.1 subunits, through facilitation of voltage-dependent open probability. IKmech is expressed in high-threshold C-mechano-nociceptors (C-HTMRs) and Aβ-mechanoreceptors, but not in low-threshold C-mechanoreceptors. IKmech opposes depolarization induced by slow/ultraslow MS cation currents in C-HTMRs, thereby shifting mechanical threshold for firing to higher values. However, due to kinetics mismatch with rapidly-adapting MS cation currents, IKmech tunes firing adaptation but not mechanical threshold in Aβ-mechanoreceptors. Expression of Kv1.1 dominant negative or inhibition of Kv1.1/IKmech caused severe mechanical allodynia but not heat hyperalgesia. By balancing the activity of excitatory mechanotransducers, Kv1.1 acts as a mechanosensitive brake that regulates mechanical sensitivity of fibers associated with mechanical perception.