N-glycosylation of acid-sensing ion channel 1a regulates its trafficking and acidosis-induced spine remodeling

The gene expression patterns as surrogate indices of pH in the brain

Hydrogen ion (H⁺) is one of the most potent intrinsic neuromodulators in the brain in terms of concentration. Changes in H⁺ concentration, expressed as pH, are thought to be associated with various biological processes, such as gene expression, in the brain. Accumulating evidence suggests that decreased brain pH is a common feature of several neuropsychiatric disorders, including schizophrenia, bipolar disorder, autism spectrum disorder, and Alzheimer's disease. However, it remains unclear whether gene expression patterns can be used as surrogates for pH changes in the brain. In this study, we performed meta-analyses using publicly available gene expression datasets to profile the expression patterns of pH-associated genes, whose expression levels were correlated with brain pH, in human patients and mouse models of major central nervous system (CNS) diseases, as well as in mouse cell-type datasets. Comprehensive analysis of 281 human datasets from 11 CNS disorders revealed that gene expression associated with decreased pH was over-represented in disorders including schizophrenia, bipolar disorder, autism spectrum disorders, Alzheimer's disease, Huntington's disease, Parkinson's disease, and brain tumors. Expression patterns of pH-associated genes in mouse models of neurodegenerative disease showed a common time course trend toward lower pH over time. Furthermore, cell type analysis identified astrocytes as the cell type with the most acidity-related gene expression, consistent with previous experimental measurements showing a lower intracellular pH in astrocytes than in neurons. These results suggest that the expression pattern of pH-associated genes may be a surrogate for the state- and trait-related changes in pH in brain cells. Altered expression of pH-associated genes may serve as a novel molecular mechanism for a more complete understanding of the transdiagnostic pathophysiology of neuropsychiatric and neurodegenerative disorders.

Thalamocortical circuits drive remifentanil-induced postoperative hyperalgesia

Remifentanil-induced hyperalgesia (RIH) is a severe but common postoperative clinical problem with elusive underlying neural mechanisms. Here, we discovered that glutamatergic neurons in the thalamic ventral posterolateral nucleus (VPLGlu) exhibited significantly elevated burst firing accompanied by upregulation of Cav3.1 T-type calcium channel expression and function in RIH model mice. In addition, we identified a glutamatergic neuronal thalamocortical circuit in the VPL projecting to hindlimb primary somatosensory cortex glutamatergic neurons (S1HLGlu) that mediated RIH. In vivo calcium imaging and multi-tetrode recordings revealed heightened S1HLGlu neuronal activity during RIH. Moreover, preoperative suppression of Cav3.1-dependent burst firing in VPLGlu neurons or chemogenetic inhibition of VPLGlu neuronal terminals in the S1HL abolished the increased S1HLGlu neuronal excitability while alleviating RIH. Our findings suggest that remifentanil induces postoperative hyperalgesia by upregulating T-type calcium channel-dependent burst firing in VPLGlu neurons to activate S1HLGlu neurons, thus revealing an ion channel-mediated neural circuit basis for RIH that can guide analgesic development.

An induced pluripotent stem cell-derived NMJ platform for study of the NGLY1-Congenital Disorder of Deglycosylation

There are many neurological rare diseases where animal models have proven inadequate or do not currently exist. NGLY1 Deficiency, a congenital disorder of deglycosylation, is a rare disease that predominantly affects motor control, especially control of neuromuscular action. In this study, NGLY1-deficient, patient-derived induced pluripotent stem cells (iPSCs) were differentiated into motoneurons (MNs) to identify disease phenotypes analogous to clinical disease pathology with significant deficits apparent in the NGLY1-deficient lines compared to the control. A neuromuscular junction (NMJ) model was developed using patient and wild type (WT) MNs to study functional differences between healthy and diseased NMJs. Reduced axon length, increased and shortened axon branches, MN action potential (AP) bursting and decreased AP firing rate and amplitude were observed in the NGLY1-deficient MNs in monoculture. When transitioned to the NMJ-coculture system, deficits in NMJ number, stability, failure rate, and synchronicity with indirect skeletal muscle (SkM) stimulation were observed. This project establishes a phenotypic NGLY1 model for investigation of possible therapeutics and investigations into mechanistic deficits in the system.

The Mechanisms of Plasticity of Nociceptive Ion Channels in Painful Diabetic Neuropathy

Treating pain in patients suffering from small fiber neuropathies still represents a therapeutic challenge for health care providers and drug developers worldwide. Unfortunately, none of the currently available treatments can completely reverse symptoms of either gain or loss of peripheral nerve sensation. Therefore, there is a clear need for novel mechanism-based therapies for peripheral diabetic neuropathy (PDN) that would improve treatment of this serious condition. In this review, we summarize the current knowledge on the mechanisms and causes of peripheral sensory neurons damage in diabetes. In particular, we focused on the subsets of voltage-gated sodium channels, TRP family of ion channels and a Ca_V3.2 isoform of T-type voltage-gated calcium channels. However, even though their potential is well-validated in multiple rodent models of painful PDN, clinical trials with specific pharmacological blockers of these channels have failed to exhibit therapeutic efficacy. We argue that understanding the development of diabetes and causal relationship between hyperglycemia, glycosylation, and other post-translational modifications may lead to the development of novel therapeutics that would efficiently alleviate painful PDN by targeting disease-specific mechanisms rather than individual nociceptive ion channels.

ASIC1a is required for neuronal activation via low-intensity ultrasound stimulation in mouse brain

Accumulating evidence has shown transcranial low-intensity ultrasound can be potentially a non-invasive neural modulation tool to treat brain diseases. However, the underlying mechanism remains elusive and the majority of studies on animal models applying rather high-intensity ultrasound that cannot be safely used in humans. Here, we showed low-intensity ultrasound was able to activate neurons in the mouse brain and repeated ultrasound stimulation resulted in adult neurogenesis in specific brain regions. In vitro calcium imaging studies showed that a specific ultrasound stimulation mode, which combined with both ultrasound-induced pressure and acoustic streaming mechanotransduction, is required to activate cultured cortical neurons. ASIC1a and cytoskeletal proteins were involved in the low-intensity ultrasound-mediated mechanotransduction and cultured neuron activation, which was inhibited by ASIC1a blockade and cytoskeleton-modified agents. In contrast, the inhibition of mechanical-sensitive channels involved in bilayer-model mechanotransduction like Piezo or TRP proteins did not repress the ultrasound-mediated neuronal activation as efficiently. The ASIC1a-mediated ultrasound effects in mouse brain such as immediate response of ERK phosphorylation and DCX marked neurogenesis were statistically significantly compromised by ASIC1a gene deletion. Collated data suggest that ASIC1a is the molecular determinant involved in the mechano-signaling of low-intensity ultrasound that modulates neural activation in mouse brain.

GPR68 Contributes to Persistent Acidosis-Induced Activation of AGC Kinases and Tyrosine Phosphorylation in Organotypic Hippocampal Slices

Persistent acidosis occurs in ischemia and multiple neurological diseases. In previous studies, acidic stimulation leads to rapid increase in intracellular calcium in neurons. However, it remains largely unclear how a prolonged acidosis alters neuronal signaling. In our previous study, we found that GPR68-mediated PKC activities are protective against acidosis-induced injury in cortical slices. Here, we first asked whether the same principle holds true in organotypic hippocampal slices. Our data showed that 1-h pH 6 induced PKC phosphorylation in a GPR68-dependent manner. Go6983, a PKC inhibitor worsened acidosis-induced neuronal injury in wild type (WT) but had no effect in GPR68^−/− slices. Next, to gain greater insights into acid signaling in brain tissue, we treated organotypic hippocampal slices with pH 6 for 1-h and performed a kinome profiling analysis by Western blot. Acidosis had little effect on cyclin-dependent kinase (CDK) or casein kinase 2 activity, two members of the CMGC family, or Ataxia telangiectasia mutated (ATM)/ATM and RAD3-related (ATR) activity, but reduced the phosphorylation of MAPK/CDK substrates. In contrast, acidosis induced the activation of CaMKIIα, PKA, and Akt. Besides these serine/threonine kinases, acidosis also induced tyrosine phosphorylation. Since GPR68 is widely expressed in brain neurons, we asked whether GPR68 contributes to acidosis-induced signaling. Deleting GPR68 had no effect on acidosis-induced CaMKII phosphorylation, attenuated that of phospho-Akt and phospho-PKA substrates, while abolishing acidosis-induced tyrosine phosphorylation. These data demonstrate that prolonged acidosis activates a network of signaling cascades, mediated by AGC kinases, CaMKII, and tyrosine kinases. GPR68 is the primary mediator for acidosis-induced activation of PKC and tyrosine phosphorylation, while both GPR68-dependent and -independent mechanisms contribute to the activation of PKA and Akt.

Postsynaptic Targeting and Mobility of Membrane Surface-Localized hASIC1a

Acid-sensing ion channels (ASICs), the main H+ receptors in the central nervous system, sense extracellular pH fluctuations and mediate cation influx. ASIC1a, the major subunit responsible for acid-activated current, is widely expressed in brain neurons, where it plays pivotal roles in diverse functions including synaptic transmission and plasticity. However, the underlying molecular mechanisms for these functions remain mysterious. Using extracellular epitope tagging and a novel antibody recognizing the hASIC1a ectodomain, we examined the membrane targeting and dynamic trafficking of hASIC1a in cultured cortical neurons. Surface hASIC1a was distributed throughout somata and dendrites, clustered in spine heads, and co-localized with postsynaptic markers. By extracellular pHluorin tagging and fluorescence recovery after photobleaching, we detected movement of hASIC1a in synaptic spine heads. Single-particle tracking along with use of the anti-hASIC1a ectodomain antibody revealed long-distance migration and local movement of surface hASIC1a puncta on dendrites. Importantly, enhancing synaptic activity with brain-derived neurotrophic factor accelerated the trafficking and lateral mobility of hASIC1a. With this newly-developed toolbox, our data demonstrate the synaptic location and high dynamics of functionally-relevant hASIC1a on the surface of excitatory synapses, supporting its involvement in synaptic functions.

Factors and Molecular Mechanisms Influencing the Protein Synthesis, Degradation and Membrane Trafficking of ASIC1a

Acid-sensing ion channels (ASICs) are members of the degenerin/epithelial sodium channel superfamily. They are extracellular pH sensors that are activated by protons. Among all ASICs, ASIC1a is one of the most intensively studied isoforms because of its unique ability to be permeable to Ca²⁺. In addition, it is considered to contribute to various pathophysiological conditions. As a membrane proton receptor, the number of ASIC1a present on the cell surface determines its physiological and pathological functions, and this number partially depends on protein synthesis, degradation, and membrane trafficking processes. Recently, several studies have shown that various factors affect these processes. Therefore, this review elucidated the major factors and underlying molecular mechanisms affecting ASIC1a protein expression and membrane trafficking.

GPR68 deletion impairs hippocampal long-term potentiation and passive avoidance behavior

Increased neural activities reduced pH at the synaptic cleft and interstitial spaces. Recent studies have shown that protons function as a neurotransmitter. However, it remains unclear whether protons signal through a metabotropic receptor to regulate synaptic function. Here, we showed that GPR68, a proton-sensitive GPCR, exhibited wide expression in the hippocampus, with higher expression observed in CA3 pyramidal neurons and dentate granule cells. In organotypic hippocampal slice neurons, ectopically expressed GPR68-GFP was present in dendrites, dendritic spines, and axons. Recordings in hippocampal slices isolated from GPR68−/− mice showed a reduced fiber volley at the Schaffer collateral-CA1 synapses, a reduced long-term potentiation (LTP), but unaltered paired-pulse ratio. In a step-through passive avoidance test, GPR68−/− mice exhibited reduced avoidance to the dark chamber. These findings showed that GPR68 contributes to hippocampal LTP and aversive fear memory.

Histidine Residues Are Responsible for Bidirectional Effects of Zinc on Acid-Sensing Ion Channel 1a/3 Heteromeric Channels

Acid-sensing ion channel (ASIC) subunits 1a and 3 are highly expressed in central and peripheral sensory neurons, respectively. Endogenous biomolecule zinc plays a critical role in physiological and pathophysiological conditions. Here, we found that currents recorded from heterologously expressed ASIC1a/3 channels using the whole-cell patch-clamp technique were regulated by zinc with dual effects. Co-application of zinc dose-dependently potentiated both peak amplitude and the sustained component of heteromeric ASIC1a/3 currents; pretreatment with zinc between 3 to 100 µM exerted the same potentiation as co-application. However, pretreatment with zinc induced a significant inhibition of heteromeric ASIC1a/3 channels when zinc concentrations were over 250 µM. The potentiation of heteromeric ASIC1a/3 channels by zinc was pH dependent, as zinc shifted the pH dependence of ASIC1a/3 currents from a pH₅₀ of 6.54 to 6.77; whereas the inhibition of ASIC1a/3 currents by zinc was also pH dependent. Furthermore, we systematically mutated histidine residues in the extracellular domain of ASIC1a or ASIC3 and found that histidine residues 72 and 73 in both ASIC1a and ASIC3, and histidine residue 83 in the ASIC3 were responsible for bidirectional effects on heteromeric ASIC1a/3 channels by zinc. These findings suggest that histidine residues in the extracellular domain of heteromeric ASIC1a/3 channels are critical for zinc-mediated effects.

Selection of an ASIC1a-blocking combinatorial antibody that protects cells from ischemic death

Acid-sensing ion channels (ASICs) have emerged as important, albeit challenging therapeutic targets for pain, stroke, etc. One approach to developing therapeutic agents could involve the generation of functional antibodies against these channels. To select such antibodies, we used channels assembled in nanodiscs, such that the target ASIC1a has a configuration as close as possible to its natural state in the plasma membrane. This methodology allowed selection of functional antibodies that inhibit acid-induced opening of the channel in a dose-dependent way. In addition to regulation of pH, these antibodies block the transport of cations, including calcium, thereby preventing acid-induced cell death in vitro and in vivo. As proof of concept for the use of these antibodies to modulate ion channels in vivo, we showed that they potently protect brain cells from death after an ischemic stroke. Thus, the methodology described here should be general, thereby allowing selection of antibodies to other important ASICs, such as those involved in pain, neurodegeneration, and other conditions.

Human ASIC1a mediates stronger acid-induced responses as compared with mouse ASIC1a

Acid-sensing ion channels (ASICs) are the major proton receptor in the brain and a key mediator of acidosis-induced neuronal injuries in disease. Most of published data on ASIC function came from studies performed in mice, and relatively little is known about potential differences between human and mouse ASICs (hASIC and mASIC, respectively). This information is critical for us to better interpret the functional importance of ASICs in human disease. Here, we examined the expression of ASICs in acutely resected human cortical tissue. Compared with mouse cortex, human cortical tissue showed a similar ratio of ASIC1a:ASIC2a expression, had reduced ASIC2b level, and exhibited a higher membrane:total ratio of ASIC1a. We further investigated the mechanism for higher surface trafficking of hASIC1a in heterologous cells. A single amino acid at position 285 was critical for increased N-glycosylation and surface expression of hASIC1a. Consistent with the changes in trafficking and current, cells expressing hASIC1a or mASIC1a S285P mutant had a higher acid-activated calcium increase and exhibited worsened acidotoxicity. These data suggest that ASICs are likely to have a larger impact on acidosis-induced neuronal injuries in humans than mice, and this effect is, at least in part, a result of more efficient trafficking of hASIC1a.-Xu, Y., Jiang, Y.-Q., Li, C., He, M., Rusyniak, W. G., Annamdevula, N., Ochoa, J., Leavesley, S. J., Xu, J., Rich, T. C., Lin, M. T., Zha, X.-M. Human ASIC1a mediates stronger acid-induced responses as compared with mouse ASIC1a.

Endomorphins potentiate acid-sensing ion channel currents and enhance the lactic acid-mediated increase in arterial blood pressure: effects amplified in hindlimb ischaemia

KEY POINTS

Chronic limb ischaemia, characterized by inflammatory mediator release and a low extracellular pH, leads to acid-sensing ion channel (ASIC) activation and reflexively increases mean arterial pressure; endomorphin release is also increased under inflammatory conditions. We examined the modulation of ASIC currents by endomorphins in sensory neurons from rats with freely perfused and ligated femoral arteries: peripheral artery disease (PAD) model. Endomorphins potentiated sustained ASIC currents in both groups of dorsal root ganglion neurons, independent of mu opioid receptor stimulation or G protein activation. Intra-arterial administration of lactic acid (to simulate exercising muscle and evoke a pressor reflex), endomorphin-2 and naloxone resulted in a significantly greater pressor response than lactic acid alone, while administration of APETx2 inhibited endomorphin's enhancing effect in both groups. These results suggest a novel role for endomorphins in modulating ASIC function to effect lactic acid-mediated reflex increase in arterial pressure in patients with PAD.

ABSTRACT

Chronic muscle ischaemia leads to accumulation of lactic acid and other inflammatory mediators with a subsequent drop in interstitial pH. Acid-sensing ion channels (ASICs), expressed in thin muscle afferents, sense the decrease in pH and evoke a pressor reflex known to increase mean arterial pressure. The naturally occurring endomorphins are also released by primary afferents under ischaemic conditions. We examined whether high affinity mu opioid receptor (MOR) agonists, endomorphin-1 (E-1) and -2 (E-2), modulate ASIC currents and the lactic acid-mediated pressor reflex. In rat dorsal root ganglion (DRG) neurons, exposure to E-2 in acidic solutions significantly potentiated ASIC currents when compared to acidic solutions alone. The potentiation was significantly greater in DRG neurons isolated from rats whose femoral arteries were ligated for 72 h. Sustained ASIC current potentiation was also observed in neurons pretreated with pertussis toxin, an uncoupler of G proteins and MOR. The endomorphin-mediated potentiation was a result of a leftward shift of the activation curve to higher pH values and a slight shift of the inactivation curve to lower pH values. Intra-arterial co-administration of lactic acid and E-2 led to a significantly greater pressor reflex than lactic acid alone in the presence of naloxone. Finally, E-2 effects were inhibited by pretreatment with the ASIC3 blocker APETx2 and enhanced by pretreatment with the ASIC1a blocker psalmotoxin-1. These findings have uncovered a novel role of endomorphins by which the opioids can enhance the lactic acid-mediated reflex increase in arterial pressure that is MOR stimulation-independent and APETx2-sensitive.

miR-149 reduces while let-7 elevates ASIC1a expression in vitro

Acid-sensing ion channel 1a (ASIC1a) is the key subunit that determines acid-activated currents in neurons. ASIC1a is important for neural plasticity, learning, and for multiple neurological diseases, including stroke, multiple sclerosis, and traumatic injuries. These findings underline the importance for better defining mechanisms that regulate ASIC1a expression. During the past decade, microRNA has emerged as one important group of regulatory molecules in controlling protein expression. However, little is known about whether microRNA regulates ASIC1a. Here, we assessed several microRNAs that have predicted targeting sequences in the 3' untranslated region (UTR) of mouse ASIC1a. Our results indicated that miR-144 and -149 reduced ASIC1a expression while Let-7 increased ASIC1a protein levels. miR-30c, -98, -125, -182* had no significant effect. Since a reduction in ASIC1a expression may have translational potentials in treating neuronal injury, we further asked whether the effect of miR-144 and miR-149, both reduced ASIC1a expression, was through specific targeting of the predicted sites on ASIC1a. We mutated the targeting sequence of miR-144 and miR-149 in ASIC1a UTR. The effect of miR-149 was abolished in the corresponding mutation. In contrast, miR-144 still reduced ASIC1a level when its predicted target sequence was mutated. This result indicates that miR-149 targets the 3'UTR of ASIC1a and reduces its expression.

Differential effects of N-linked glycosylation of Vstm5 at multiple sites on surface expression and filopodia formation

V-set and transmembrane domain-containing protein 5 (Vstm5), a newly characterized small membrane glycoprotein, can induce membrane protrusions in various cells. Vstm5 can modulate both the position and complexity of central neurons by altering their membrane morphology and dynamics. In this study, we investigated the significance of glycosylation in the expression and function of Vstm5. Four N-linked glycosylation sites (Asn⁴³, Asn⁸⁷, Asn¹⁰¹, and Asn¹⁰⁸) are predicted to be located in the extracellular N-terminus of mouse Vstm5. Although all four sites were glycosylated, their functional roles may not be identical. N-glycosylation at multiple sites affects differentially the function of Vstm5. Glycosylation at individual sites not only played essential roles in surface expression of Vstm5 but also in the formation of neuronal dendritic filopodia. These results indicate that N-linked glycosylation at multiple sites plays important roles by differentially influencing the expression, targeting, and biological activity of Vstm5.

Abnormal N-Glycosylation of a Novel Missense Creatine Transporter Mutant, G561R, Associated with Cerebral Creatine Deficiency Syndromes Alters Transporter Activity and Localization

Cerebral creatine deficiency syndromes (CCDSs) are caused by loss-of-function mutations in creatine transporter (CRT, SLC6A8), which transports creatine at the blood-brain barrier and into neurons of the central nervous system (CNS). This results in low cerebral creatine levels, and patients exhibit mental retardation, poor language skills and epilepsy. We identified a novel human CRT gene missense mutation (c.1681 G>C, G561R) in Japanese CCDSs patients. The purpose of the present study was to evaluate the reduction of creatine transport in G561R-mutant CRT-expressing 293 cells, and to clarify the mechanism of its functional attenuation. G561R-mutant CRT exhibited greatly reduced creatine transport activity compared to wild-type CRT (WT-CRT) when expressed in 293 cells. Also, the mutant protein is localized mainly in intracellular membrane fraction, while WT-CRT is localized in plasma membrane. Western blot analysis revealed a 68 kDa band of WT-CRT protein in plasma membrane fraction, while G561R-mutant CRT protein predominantly showed bands at 55, 110 and 165 kDa in crude membrane fraction. The bands of both WT-CRT and G561R-mutant CRT were shifted to 50 kDa by N-glycosidase treatment. Our results suggest that the functional impairment of G561R-mutant CRT was probably caused by incomplete N-linked glycosylation due to misfolding during protein maturation, leading to oligomer formation and changes of cellular localization.

Region specific contribution of ASIC2 to acidosis-and ischemia-induced neuronal injury

Acidosis in the brain plays a critical role in neuronal injury in neurological diseases, including brain ischemia. One key mediator of acidosis-induced neuronal injury is the acid-sensing ion channels (ASICs). Current literature has focused on ASIC1a when studying acid signaling. The importance of ASIC2, which is also widely expressed in the brain, has not been appreciated. We found here a region-specific effect of ASIC2 on acid-mediated responses. Deleting ASIC2 reduced acid-activated current in cortical and striatal neurons, but had no significant effect in cerebellar granule neurons. In addition, we demonstrated that ASIC2 was important for ASIC1a expression, and that ASIC2a but not 2b facilitated ASIC1a surface trafficking in the brain. Further, we showed that ASIC2 deletion attenuated acidosis/ischemia-induced neuronal injury in organotypic hippocampal slices but had no effect in organotypic cerebellar slices. Consistent with an injurious role of ASIC2, we showed that ASIC2 deletion significantly protected the mouse brain from ischemic damage in vivo. These data suggest a critical region-specific contribution of ASIC2 to neuronal injury and reveal an important functional difference between ASIC2a and 2b in the brain.

The glycosylation of the extracellular loop of β2 subunits diversifies functional phenotypes of BK Channels

Large-conductance Ca²⁺- and voltage-activated potassium (MaxiK or BK) channels are composed of a pore-forming α subunit (Slo) and 4 types of auxiliary β subunits or just a pore-forming α subunit. Although multiple N-linked glycosylation sites in the extracellular loop of β subunits have been identified, very little is known about how glycosylation influences the structure and function of BK channels. Using a combination of site-directed mutagenesis, western blot and patch-clamp recordings, we demonstrated that 3 sites in the extracellular loop of β2 subunit are N-glycosylated (N-X-T/S at N88, N96 and N119). Glycosylation of these sites strongly and differentially regulate gating kinetics, outward rectification, toxin sensitivity and physical association between the α and β2 subunits. We constructed a model and used molecular dynamics (MD) to simulate how the glycosylation facilitates the association of α/β2 subunits and modulates the dimension of the extracellular cavum above the pore of the channel, ultimately to modify biophysical and pharmacological properties of BK channels. Our results suggest that N-glycosylation of β2 subunits plays crucial roles in imparting functional heterogeneity of BK channels, and is potentially involved in the pathological phenotypes of carbohydrate metabolic diseases.

Painful Diabetic Neuropathy: Prevention or Suppression?

Pain-sensing sensory neurons (nociceptors) of the dorsal root ganglia (DRG) and dorsal horn (DH) can become sensitized (hyperexcitable) in response to pathological conditions such as diabetes, which in turn may lead to the development of painful peripheral diabetic neuropathy (PDN). Because of incomplete knowledge about the mechanisms underlying painful PDN, current treatment for painful PDN has been limited to somewhat nonspecific systemic drugs that have significant side effects or potential for abuse. Recent studies have established that several ion channels in DRG and DH neurons are dysregulated and make a previously unrecognized contribution to sensitization of pain responses by enhancing excitability of nociceptors in animal models of type 1 and type 2 PDN. Furthermore, it has been reported that targeting posttranslational modification of nociceptive ion channels such as glycosylation and methylglyoxal metabolism can completely reverse mechanical and thermal hyperalgesia in diabetic animals with PDN in vivo. Understanding details of posttranslational regulation of nociceptive channel activity may facilitate development of novel therapies for treatment of painful PDN. We argue that pharmacological targeting of the specific pathogenic mechanism rather than of the channel per se may cause fewer side effects and reduce the potential for drug abuse in patients with diabetes.

Two di-leucine motifs regulate trafficking and function of mouse ASIC2a

BACKGROUND

Acid-sensing ion channels (ASICs) are proton-gated cation channels that mediate acid-induced responses in neurons. ASICs are important for mechanosensation, learning and memory, fear, pain, and neuronal injury. ASIC2a is widely expressed in the nervous system and modulates ASIC channel trafficking and activity in both central and peripheral systems. Here, to better understand mechanisms regulating ASIC2a, we searched for potential protein motifs that regulate ASIC2a trafficking.

RESULTS AND CONCLUSIONS

We identified a LLDLL sequence in the C-terminal juxtamembrane region of ASIC2a. Deleting or mutating the LLDLL sequence increased total expression and surface levels of ASIC2a in CHO cells. Mutating either of the two LL motifs had a similar effect. We further assessed ASIC2a localization in organotypic hippocampal slice neurons. The LL motif mutants exhibited increased dendritic trafficking and elevated targeting to dendritic spines. Consistent with an efficient trafficking, the LL motif mutants increased acid-activated current density. In addition, mutating the second LL motif increased pH sensitivity of the channel. These data identify the LL motifs as a negative regulator of ASIC2a trafficking and function, and suggest novel regulatory mechanisms in acid signaling.

ASIC subunit ratio and differential surface trafficking in the brain

Background

Acid-sensing ion channels (ASICs) are key mediators of acidosis-induced responses in neurons. However, little is known about the relative abundance of different ASIC subunits in the brain. Such data are fundamental for interpreting the relative contribution of ASIC1a homomers and 1a/2 heteromers to acid signaling, and essential for designing therapeutic interventions to target these channels. We used a simple biochemical approach and semi-quantitatively determined the molar ratio of ASIC1a and 2 subunits in mouse brain. Further, we investigated differential surface trafficking of ASIC1a, ASIC2a, and ASIC2b.

Results and conclusions

ASIC1a subunits outnumber the sum of ASIC2a and ASIC2b. There is a region-specific variation in ASIC2a and 2b expression, with cerebellum and striatum expressing predominantly 2b and 2a, respectively. Further, we performed surface biotinylation and found that surface ASIC1a and ASIC2a ratio correlates with their total expression. In contrast, ASIC2b exhibits little surface presence in the brain. This result is consistent with increased co-localization of ASIC2b with an ER marker in 3T3 cells. Our data are the first semi-quantitative determination of relative subunit ratio of various ASICs in the brain. The differential surface trafficking of ASICs suggests that the main functional ASICs in the brain are ASIC1a homomers and 1a/2a heteromers. This finding provides important insights into the relative contribution of various ASIC complexes to acid signaling in neurons.

Acid-sensing ion channels: trafficking and pathophysiology

Acid-sensing ion channels (ASICs) are proton-gated cation channels that are widely expressed in both the peripheral and central nervous systems. ASICs contribute to a variety of pathophysiological conditions that involve tissue acidosis, such as ischemic stroke, epileptic seizures and multiple sclerosis. Although much progress has been made in researching the structure-function relationship and pharmacology of ASICs, little is known about the trafficking of ASICs and its contribution to ASIC function. The recent identification of the mechanism of membrane insertion and endocytosis of ASIC1a highlights the emerging role of ASIC trafficking in regulating its pathophysiological functions. In this review, we summarize the recent advances and discuss future directions on this topic.

Acid-sensing ion channels and their modulators

According to a modern look acid-sensing ion channels (ASICs) are one of the most important receptors that perceive pH change in the body. ASICs represent proton-gated Na+-selective channels, which are expressed in neurons of the central and peripheral nervous system. These channels are attracting attention of researchers around the world, as they are involved in various physiological processes in the body. Drop of pH may occur in tissues in norm (e.g. the accumulation of lactic acid, the release of protons upon ATP hydrolysis) and pathology (inflammation, ischemic stroke, tissue damage and seizure). These processes are accompanied by unpleasant pain sensations, which may be short-lived or can lead to chronic inflammatory diseases. Modulators of ASIC channels activity are potential candidates for new effective analgesic and neuroprotection drugs. This review summarizes available information about structure, function, and physiological role of ASIC channels. In addition a description of all known ligands of these channels and their practical relevance is provided.

N-Glycosylation modulates filopodia-like protrusions induced by sez-6 through regulating the distribution of this protein on the cell surface

Seizure-related gene 6 (sez-6) is a trans-membrane protein expressed by neuronal cells that modulates dendritic branching. It has three clusters of eleven possible N-glycosylation sites in the extracellular domain region: sugar chain (SC)1-3, SC4-7, and SC8-11. Recent reports suggest that N-glycosylation modulates the membrane trafficking and function of trans-membrane proteins. Here, we studied the role of N-glycosylation in sez-6 function. We transfected mutants lacking one, two, or all N-glycosylation clusters into neuro2a cells. A mutant lacking all N-glycosylation was transported to the cell membrane. Mutants lacking one cluster (sez-6 ΔSC1-3, ΔSC4-7, ΔSC8-11) were evenly distributed on the cell membrane and secreted into the conditioned medium, as in wild-type sez-6; in contrast, the unglycosylated mutant, sez-6 ΔSC1-11, and mutants having only one cluster (sez-6 SC1-3, SC8-11) were localized in some portions on the cell membrane. Despite sez-6 SC4-7 having only one cluster, it was transported like the wild type. Among mutants behaving like the wild type, sez-6 ΔSC1-3 and ΔSC4-7 reduced neurite formation. Interestingly, mutants lacking SC4-7 (sez-6 ΔSC4-7) did not affect the formation of filopodia-like protrusions. In contrast, other mutants as well as the wild type induced it, suggesting that SC4-7 is crucial for filopodia-like protrusions. Our results indicate that N-glycosylation regulates cell morphology through modulating the cell surface distribution of sez-6 protein.

Genetic mapping of ASIC4 and contrasting phenotype to ASIC1a in modulating innate fear and anxiety

Although ASIC4 is a member of the acid-sensing ion channel (ASIC) family, we have limited knowledge of its expression and physiological function in vivo. To trace the expression of this ion channel, we generated the ASIC4-knockout/CreERT(2)-knockin (Asic4(Cre) (ERT) (2)) mouse line. After tamoxifen induction in the Asic4(Cre) (ERT)(2)::CAG-STOP(floxed)-Td-tomato double transgenic mice, we mapped the expression of ASIC4 at the cellular level in the central nervous system (CNS). ASIC4 was expressed in many brain regions, including the olfactory bulb, cerebral cortex, striatum, hippocampus, amygdala, thalamus, hypothalamus, brain stem, cerebellum, spinal cord and pituitary gland. Colocalisation studies further revealed that ASIC4 was expressed mainly in three types of cells in the CNS: (i) calretinin (CR)-positive and/or vasoactive intestine peptide (VIP)-positive interneurons; (ii) neural/glial antigen 2 (NG2)-positive glia, also known as oligodendrocyte precursor cells; and (iii) cerebellar granule cells. To probe the possible role of ASIC4, we hypothesised that ASIC4 could modulate the membrane expression of ASIC1a and thus ASIC1a signaling in vivo. We conducted behavioral phenotyping of Asic4(Cre) (ERT)(2) mice by screening many of the known behavioral phenotypes found in Asic1a knockouts and found ASIC4 not involved in shock-evoked fear learning and memory, seizure termination or psychostimulant-induced locomotion/rewarding effects. In contrast, ASIC4 might play an important role in modulating the innate fear response to predator odor and anxious state because ASIC4-mutant mice showed increased freezing response to 2,4,5-trimethylthiazoline and elevated anxiety-like behavior in both the open-field and elevated-plus maze. ASIC4 may modulate fear and anxiety by counteracting ASIC1a activity in the brain.

Down-sizing of neuronal network activity and density of presynaptic terminals by pathological acidosis are efficiently prevented by Diminazene Aceturate

Local acidosis is associated with neuro-inflammation and can have significant effects in several neurological disorders, including multiple sclerosis, brain ischemia, spinal cord injury and epilepsy. Despite local acidosis has been implicated in numerous pathological functions, very little is known about the modulatory effects of pathological acidosis on the activity of neuronal networks and on synaptic structural properties. Using non-invasive MRI spectroscopy we revealed protracted extracellular acidosis in the CNS of Experimental Autoimmune Encephalomyelitis (EAE) affected mice. By multi-unit recording in cortical neurons, we established that acidosis affects network activity, down-sizing firing and bursting behaviors as well as amplitudes. Furthermore, a protracted acidosis reduced the number of presynaptic terminals, while it did not affect the postsynaptic compartment. Application of the diarylamidine Diminazene Aceturate (DA) during acidosis significantly reverted both the loss of neuronal firing and bursting and the reduction of presynaptic terminals. Finally, in vivo DA delivery ameliorated the clinical disease course of EAE mice, reducing demyelination and axonal damage. DA is known to block acid-sensing ion channels (ASICs), which are proton-gated, voltage-insensitive, Na(+) permeable channels principally expressed by peripheral and central nervous system neurons. Our data suggest that ASICs activation during acidosis modulates network electrical activity and exacerbates neuro-degeneration in EAE mice. Therefore pharmacological modulation of ASICs in neuroinflammatory diseases could represent a new promising strategy for future therapies aimed at neuro-protection.

ASICs and neuropeptides

The acid sensing ion channels (ASICs) are proton-gated cation channels expressed throughout the nervous system. ASICs are activated during acidic pH fluctuations, and recent work suggests that they are involved in excitatory synaptic transmission. ASICs can also induce neuronal degeneration and death during pathological extracellular acidosis caused by ischemia, autoimmune inflammation, and traumatic injury. Many endogenous neuromodulators target ASICs to affect their biophysical characteristics and contributions to neuronal activity. One of the most unconventional types of modulation occurs with the interaction of ASICs and neuropeptides. Collectively, FMRFamide-related peptides and dynorphins potentiate ASIC activity by decreasing the proton-sensitivity of steady state desensitization independent of G protein-coupled receptor activation. By decreasing the proton-sensitivity of steady state desensitization, the FMRFamide-related peptides and dynorphins permit ASICs to remain active at more acidic basal pH. Unlike the dynorphins, some FMRFamide-related peptides also potentiate ASIC activity by slowing inactivation and increasing the sustained current. Through mechanistic studies, the modulation of ASICs by FMRFamide-related peptides and dynorphins appears to be through distinct interactions with the extracellular domain of ASICs. Dynorphins are expressed throughout the nervous system and can increase neuronal death during prolonged extracellular acidosis, suggesting that the interaction between dynorphins and ASICs may have important consequences for the prevention of neurological injury. The overlap in expression of FMRFamide-related peptides with ASICs in the dorsal horn of the spinal cord suggests that their interaction may have important consequences for the treatment of pain during injury and inflammation. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Two aspects of ASIC function: Synaptic plasticity and neuronal injury

Extracellular brain pH fluctuates in both physiological and disease conditions. The main postsynaptic proton receptor is the acid-sensing ion channels (ASICs). During the past decade, much progress has been made on protons, ASICs, and neurological disease. This review summarizes the recent progress on synaptic role of protons and our current understanding of how ASICs contribute to various types of neuronal injury in the brain. This article is part of the Special Issue entitled 'Acid-Sensing Ion Channels in the Nervous System'.

Deuterium-depleted water has stimulating effects on long-term memory in rats

Deuterium-depleted water (DDW) is a water which has a 6-7-fold less concentration of the naturally occurring deuterium (20-25ppm vs. 150ppm). While administrated for a longer period, it may reduce the concentration of deuterium throughout the body, thus activating cellular mechanisms which are depending on protons (channels, pumps, enzyme proteins). The aim of the present work was to study, for the first time in our knowledge, the possible influence of deuterium-depleted water (DDW) chronic administration in normal Wistar rats, as compared to a control group which received distilled water, on spatial working memory and the locomotor activity (as studied through Y-maze) or both short-term and long-term spatial memory (assed in radial 8 arms-maze task). Our results presented here showed no significant modifications in terms of spatial working memory (assessed through spontaneous alternation percentage) and locomotor activity (expressed through the number of arm entries) in Y-maze, as a result of DDW ingestion. Also, no significant differences between the DDW and control group were found in terms of the number of working memory errors in the eight-arm radial maze, as a parameter of short-term memory. Still, we observed a significant decrease for the number of reference memory errors in the DDW rats. In this way, we could speculate that the administration of DDW may generate an improvement of the reference memory, as an index of long-term memory. Thus, we can reach the conclusion that the change between the deuterium/hydrogen balance may have important consequences for the mechanisms that govern long-term memory, as showed here especially in the behavioral parameters from the eight-arm radial maze task.

The role of protein N-glycosylation in neural transmission

Recent studies have explored the function of N-linked glycosylation in the nervous system, demonstrating essential roles of carbohydrate structures in neural development. The function of N-glycans in neural physiology remains less understood; however, increasing evidence indicates that N-glycans can play specific modulatory roles controlling neural transmission and excitability of neural circuits. These roles are mediated via effects on synaptic proteins involved in neurotransmitter release, transporters that regulate nerotransmitter concentrations, neurotransmitter receptors, as well as via regulation of proteins that control excitability and response to milieu stimuli, such as voltage-gated ion channels and transient receptor potential channels, respectively. Sialylated N-glycan structures are among the most potent modulators of cell excitability, exerting prominent effects on voltage gated Na(+) and K(+) channels. This modulation appears to be underlain by complex molecular mechanisms involving electrostatic effects, as well as interaction modes based on more specific steric effects and interactions with lectins and other molecules. Data also indicate that particular features of N-glycans, such as their location on a protein and structural characteristics, can be specifically associated with the effect of glycosylation. These features and their functional implications can vary between different cell types, which highlight the importance of in vivo analyses of glycan functions. Experimental challenges are associated with the overwhelming complexity of the nervous system and glycosylation pathways in vertebrates, and thus model organisms like Drosophila should help elucidate evolutionarily conserved mechanisms underlying glycan functions. Recent studies supported this notion and shed light on functions of several glycosylation genes involved in the regulation of the nervous system.

Translational strategies for neuroprotection in ischemic stroke--focusing on acid-sensing ion channel 1a

Ischemic stroke contributes to the majority of brain injuries and remains to be a leading cause of death and long-term disability. Despite the devastating pathology and high incidence of disease, there remain only few treatment options (TPA and endovascular procedures), which may be hampered by time-dependent administration among a variety of other factors. Promising research of glutamate receptor antagonists has been unsuccessful in clinical trial. But, the mechanism by which glutamate receptors initiate injury by excessive calcium overload has spurred investigation of new and potentially successful candidates for stroke therapy. Acid-sensing ion channels (ASICs) may contribute to poor stroke prognosis due to localized drop in brain pH, resulting in excessive calcium overload, independent of glutamate activation. Accumulating studies targeting ASICs have underscored the importance of understanding inhibition, regulation, desensitization, and trafficking of this channel and its role in disease. This review will discuss potential directions in translational ASIC research for future stroke therapies.

N-glycosylation in regulation of the nervous system

Protein N-glycosylation can influence the nervous system in a variety of ways by affecting functions of glycoproteins involved in nervous system development and physiology. The importance of N-glycans for different aspects of neural development has been well documented. For example, some N-linked carbohydrate structures were found to play key roles in neural cell adhesion and axonal targeting during development. At the same time, the involvement of glycosylation in the regulation of neural physiology remains less understood. Recent studies have implicated N-glycosylation in the regulation of neural transmission, revealing novel roles of glycans in synaptic processes and the control of neural excitability. N-Glycans were found to markedly affect the function of several types of synaptic proteins involved in key steps of synaptic transmission, including neurotransmitter release, reception, and uptake. Glycosylation also regulates a number of channel proteins, such as TRP channels that control responses to environmental stimuli and voltage-gated ion channels, the principal determinants of neuronal excitability. Sialylated carbohydrate structures play a particularly prominent part in the modulation of voltage-gated ion channels. Sialic acids appear to affect channel functions via several mechanisms, including charge interactions, as well as other interactions that probably engage steric effects and interactions with other molecules. Experiments also indicated that some structural features of glycans can be particularly important for their function. Since glycan structures can vary significantly between different cell types and depend on the metabolic state of the cell, it is important to analyze glycan functions using in vivo approaches. While the complexity of the nervous system and intricacies of glycosylation pathways can create serious obstacles for in vivo experiments in vertebrates, recent studies have indicated that more simple and experimentally tractable model organisms like Drosophila should provide important advantages for elucidating evolutionarily conserved functions of N-glycosylation in the nervous system.

Acid-sensing ion channels contribute to neurotoxicity

Acidosis that occurs under pathological conditions not only affects intracellular signaling molecules, but also directly activates a unique family of ligand-gated ion channels: acid-sensing ion channels (ASICs). ASICs are widely expressed throughout the central and peripheral nervous systems and play roles in pain sensation, learning and memory, and fear conditioning. Overactivation of ASICs contributes to neurodegenerative diseases such as ischemic brain/spinal cord injury, multiple sclerosis, Parkinson's disease, and Huntington's disease. Thus, targeting ASICs might be a potential therapeutic strategy for these conditions. This mini-review focuses on the electrophysiology and pharmacology of ASICs and roles of ASICs in neuronal toxicity.

Reversal of neuropathic pain in diabetes by targeting glycosylation of Ca(V)3.2 T-type calcium channels

It has been established that Ca(V)3.2 T-type voltage-gated calcium channels (T-channels) play a key role in the sensitized (hyperexcitable) state of nociceptive sensory neurons (nociceptors) in response to hyperglycemia associated with diabetes, which in turn can be a basis for painful symptoms of peripheral diabetic neuropathy (PDN). Unfortunately, current treatment for painful PDN has been limited by nonspecific systemic drugs with significant side effects or potential for abuse. We studied in vitro and in vivo mechanisms of plasticity of Ca(V)3.2 T-channel in a leptin-deficient (ob/ob) mouse model of PDN. We demonstrate that posttranslational glycosylation of specific extracellular asparagine residues in Ca(V)3.2 channels accelerates current kinetics, increases current density, and augments channel membrane expression. Importantly, deglycosylation treatment with neuraminidase inhibits native T-currents in nociceptors and in so doing completely and selectively reverses hyperalgesia in diabetic ob/ob mice without altering baseline pain responses in healthy mice. Our study describes a new mechanism for the regulation of Ca(V)3.2 activity and suggests that modulating the glycosylation state of T-channels in nociceptors may provide a way to suppress peripheral sensitization. Understanding the details of this regulatory pathway could facilitate the development of novel specific therapies for the treatment of painful PDN.

Three distinct motifs within the C-terminus of acid-sensing ion channel 1a regulate its surface trafficking

Various protein motifs play a key role in regulating protein biogenesis and trafficking. Here, we discovered that three distinct motifs regulate the trafficking of acid-sensing ion channel 1a (ASIC1a), the primary neuronal proton receptor which plays critical roles in neurological diseases including stroke, multiple sclerosis and seizures. Mutating the PDZ binding motif of ASIC1a increased its surface expression and current density. In contrast, mutating either a RRGK motif or a KEAKR motif reduced ASIC1a surface expression and acid-activated current density. Mutating or deleting the RRGK motif also reduced pH sensitivity and the rate of desensitization of ASIC1a. These changes were likely due to a change in ASIC1a biogenesis; mutating either the RRGK or KEAKR motif reduced N-glycosylation of ASIC1a while mutating the PDZ binding motif had the opposite effect. Our results demonstrate that these C-terminal motifs are important for ASIC1a trafficking and channel function. In addition, in contrast to multiple previous studies, which all show that K/R containing motifs lead to endoplasmic reticulum (ER) retention, our findings indicate that these motifs can also be required for efficient trafficking.

Differential regulation of locomotor activity to acute and chronic cocaine administration by acid-sensing ion channel 1a and 2 in adult mice

Acid-sensing ion channels (ASICs) are densely expressed in the brain with ASIC1a and ASIC2 channels being the predominant subtypes. These channels are enriched at synaptic sites and are central for the regulation of normal synaptic transmission. Moreover, increasing evidence links ASICs to the pathogenesis of various neurological and neuropsychiatric disorders. In this study, we explore the putative role of ASIC1a and ASIC2 in the regulation of behavioral sensitivity to the psychostimulant cocaine by utilizing ASIC1a or ASIC2 knockout mice. Acute cocaine injection induced a typical dose-dependent increase in locomotor activities in wild-type (WT) mice. However, in ASIC1a and ASIC2 mutant mice, different motor responses to cocaine were observed. In ASIC1a(-/-) mice, cocaine induced a significantly less motor response at all doses (5, 10, 20, and 30 mg/kg), while in ASIC2(-/-) mice, cocaine (5-20 mg/kg) stimulated locomotor activity to an extent comparable to WT mice. Only at 30 mg/kg, the cocaine-stimulated motor activity was reduced in ASIC2(-/-) mice. In a chronic cocaine administration model (20mg/kg, once daily for 5 days), a challenge injection of cocaine (10mg/kg, after 2-week withdrawal) caused an evident behavioral sensitization in the cocaine-pretreated WT mice. This behavioral sensitization to challenge cocaine was also displayed in ASIC1a(-/-) and ASIC2(-/-) mice. However, ASIC2(-/-) mice showed less sensitization to challenge cocaine when compared to WT and ASIC1a(-/-) mice. Our results demonstrate the important role of ASIC1a and ASIC2 channels in the modulation of behavioral sensitivity to cocaine. The two synapse-enriched ASIC subtypes are believed to play distinguishable roles in the regulation of behavioral responses to acute and chronic cocaine administration.

Acid-sensing ion channels: trafficking and synaptic function

Extracellular acidification occurs in the brain with elevated neural activity, increased metabolism, and neuronal injury. This reduction in pH can have profound effects on brain function because pH regulates essentially every single biochemical reaction. Therefore, it is not surprising to see that Nature evolves a family of proteins, the acid-sensing ion channels (ASICs), to sense extracellular pH reduction. ASICs are proton-gated cation channels that are mainly expressed in the nervous system. In recent years, a growing body of literature has shown that acidosis, through activating ASICs, contributes to multiple diseases, including ischemia, multiple sclerosis, and seizures. In addition, ASICs play a key role in fear and anxiety related psychiatric disorders. Several recent reviews have summarized the importance and therapeutic potential of ASICs in neurological diseases, as well as the structure-function relationship of ASICs. However, there is little focused coverage on either the basic biology of ASICs or their contribution to neural plasticity. This review will center on these topics, with an emphasis on the synaptic role of ASICs and molecular mechanisms regulating the spatial distribution and function of these ion channels.

N-glycosylation-dependent control of functional expression of background potassium channels K2P3.1 and K2P9.1

Two-pore domain potassium (K(2P)) channels play fundamental roles in cellular processes by enabling a constitutive leak of potassium from cells in which they are expressed, thus influencing cellular membrane potential and activity. Hence, regulation of these channels is of critical importance to cellular function. A key regulatory mechanism of K(2P) channels is the control of their cell surface expression. Membrane protein delivery to and retrieval from the cell surface is controlled by their passage through the secretory and endocytic pathways, and post-translational modifications regulate their progression through these pathways. All but one of the K(2P) channels possess consensus N-linked glycosylation sites, and here we demonstrate that the conserved putative N-glycosylation site in K(2P)3.1 and K(2P)9.1 is a glycan acceptor site. Patch clamp analysis revealed that disruption of channel glycosylation reduced K(2P)3.1 current, and flow cytometry was instrumental in attributing this to a decreased number of channels on the cell surface. Similar findings were observed when cells were cultured in reduced glucose concentrations. Disruption of N-linked glycosylation has less of an effect on K(2P)9.1, with a small reduction in number of channels on the surface observed, but no functional implications detected. Because nonglycosylated channels appear to pass through the secretory pathway in a manner comparable with glycosylated channels, the evidence presented here suggests that the decreased number of nonglycosylated K(2P)3.1 channels on the cell surface may be due to their decreased stability.

Anti-NMDA receptor encephalitis antibody binding is dependent on amino acid identity of a small region within the GluN1 amino terminal domain

Anti-NMDA receptor (NMDAR) encephalitis is a newly identified autoimmune disorder that targets NMDARs, causing severe neurological symptoms including hallucinations, psychosis, and seizures, and may result in death (Dalmau et al., 2008). However, the exact epitope to which these antibodies bind is unknown. A clearly defined antigenic region could provide more precise testing, allow for comparison of immunogenicity between patients to explore potential clinically relevant variations, elucidate the functional effects of antibodies, and make patients' antibodies a more effective tool with which to study NMDAR function. Here, we use human CSF to explore the antigenic region of the NMDAR. We created a series of mutants within the amino terminal domain of GluN1 that change patient antibody binding in transfected cells in stereotyped ways. These mutants demonstrate that the N368/G369 region of GluN1 is crucial for the creation of immunoreactivity. Mass spectrometry experiments show that N368 is glycosylated in transfected cells and rat brain regions; however, this glycosylation is not directly required for epitope formation. Mutations of residues N368/G369 change the closed time of the receptor in single channel recordings; more frequent channel openings correlates with the degree of antibody staining, and acute antibody exposure prolongs open time of the receptor. The staining pattern of mutant receptors is similar across subgroups of patients, indicating consistent immunogenicity, although we have identified one region that has a variable role in epitope formation. These findings provide tools for detailed comparison of antibodies across patients and suggest an interaction between antibody binding and channel function.

Structure and activity of the acid-sensing ion channels

The acid-sensing ion channels (ASICs) are a family of proton-sensing channels expressed throughout the nervous system. Their activity is linked to a variety of complex behaviors including fear, anxiety, pain, depression, learning, and memory. ASICs have also been implicated in neuronal degeneration accompanying ischemia and multiple sclerosis. As a whole, ASICs represent novel therapeutic targets for several clinically important disorders. An understanding of the correlation between ASIC structure and function will help to elucidate their mechanism of action and identify potential therapeutics that specifically target these ion channels. Despite the seemingly simple nature of proton binding, multiple studies have shown that proton-dependent gating of ASICs is quite complex, leading to activation and desensitization through distinct structural components. This review will focus on the structural aspects of ASIC gating in response to both protons and the newly discovered activators GMQ and MitTx. ASIC modulatory compounds and their action on proton-dependent gating will also be discussed. This review is dedicated to the memory of Dale Benos, who made a substantial contribution to our understanding of ASIC activity.

Inhibition of human acid-sensing ion channel 1b by zinc

Acid-sensing ion channel 1b (ASIC1b) is expressed in peripheral sensory neurons and has been implicated in nociception. Understanding the modulation of ASIC1b will provide important insight into how ASIC1b contributes to pain sensation. In our previous study, we showed that zinc, an important modulator of pain sensation, reduces rat ASIC1b current. However, rat ASIC1b shows several important differences from its recently identified human homolog. Most noticeably, human ASIC1b (hASIC1b) has a sustained component, which may play a role in persistent pain. Therefore, we tested here the hypothesis that zinc modulates the current properties of hASIC1b. Bath application of zinc suppressed the peak amplitude of hASIC1b currents, with a half-maximum inhibitory concentration of 37 μM. However, zinc did not affect the sustained component of hASIC1b currents. The effect of zinc was independent of pH-dependent activation, steady-state desensitization, and extracellular Ca(2+), suggesting noncompetitive mechanisms. Further, we found that extracellular site(s) of the hASIC1b subunit is important for the effect of zinc. Mutating cysteine 196, but not cysteine 309, in the extracellular domain of the hASIC1b abolished the zinc inhibition. These results suggest that, through modulating cysteine196, zinc may have a modulatory role in acute pain.