Acid-sensing ion channel (ASIC) 4 gene: physical mapping, genomic organisation, and evaluation as a candidate for paroxysmal dystonia.

Acid-sensing ion channels (ASICs) are protongated Na(+) channels. They have been implicated with synaptic transmission, pain perception as well as mechanoperception. ASIC4 is the most recent member of this gene family. It shows expression throughout the central nervous system with strongest expression in pituitary gland. ASIC4 is inactive by itself and its function is unknown. Mutations in ion channel subunits, which are homologues of ASICs lead to neurodegeneration in Caenorhabditis elegans. It has, therefore, been speculated that similar mutations in ASICs may be responsible for neurodegeneration in humans. Here, we show that ASIC4 maps to the long arm of chromosome 2 in close proximity to the locus for paroxysmal dystonic choreoathetosis (PDC), a movement disorder with unknown cause. Ion channel genes have been shown to cause several other paroxysmal neurologic disorders and are important candidate genes for PDC. We established the genomic organisation of the ASIC4 gene and screened a PDC pedigree for mutations in the coding region. Although we identified three polymorphisms in the Cterminal part of the ASIC4 protein, these were not present in each affected subject in the PDC kindred we analysed. Therefore, although the ASIC4 gene is physically mapped to the PDC locus, our data indicates that ASIC4 gene mutation is not the cause of PDC. It remains to be established if mutations in ASIC4 or other ASIC subunits may cause neurological disorders.

Molecular and functional characterization of acid-sensing ion channel (ASIC) 1b.

Acid-sensing ion channels (ASICs) are activated by extracellular protons and are involved in neurotransmission in the central nervous system, in pain perception, as well as in mechanotransduction. Six different ASIC subunits have been cloned to date, which are encoded by four genes (ASIC1-ASIC4). Proton-gated currents have been described in isolated neurons from sensory ganglia as well as from central nervous system. However, it is largely unclear which of the cloned ASIC subunits underlie these native proton-gated currents. Recently, a splice variant, ASIC-beta, has been described for ASIC1a. In this variant about one-third of the protein is exchanged at the N terminus. Here we show that ASIC-beta has a longer N terminus than previously reported, extending the sequence divergence between ASIC1a and this new variant (ASIC1b). We investigated in detail kinetic and selectivity properties of ASIC1b in comparison to ASIC1a. Kinetics is similar for ASIC1b and ASIC1a. Ca(2+) permeability of ASIC1a is low, whereas ASIC1b is impermeable to Ca(2+). Currents through ASIC1a resemble currents, which have been described in sensory and central neurons, whereas the significance of ASIC1b remains to be established. Moreover, we show that a pre-transmembrane 1 domain controls the permeability to divalent cations in ASIC1, contributing to our understanding of the pore structure of these channels.

Inflammatory mediators potentiate ATP-gated channels through the P2X(3) subunit.

The P2X(3) receptor is an ATP-gated ion channel predominantly expressed in nociceptive neurons from the dorsal root ganglion. P2X(3) receptor channels are highly expressed in sensory neurons and probably contribute to the sensation of pain. Kinetics of P2X(3) currents are characterized by rapid desensitization (<100 ms) and slow recovery (>20 s). Thus, any mechanism modulating rate of desensitization and/or recovery may have profound effect on susceptibility of nociceptive neurons expressing P2X(3) to ATP. Here we show that currents mediated by P2X(3) receptor channels and the heteromeric channel P2X(2/3) composed of P2X(2) and P2X(3) subunits are potentiated by the neuropeptides substance P and bradykinin, which are known to modulate pain perception. The effect is mediated by the respective neuropeptide receptors, can be mimicked by phorbol ester and blocked by inhibitors of protein kinases. Together with data from site-directed mutagenesis our results suggest that inflammatory mediators sensitize nociceptors through phosphorylation of P2X(3) and P2X(2/3) ion channels or associated proteins.