Although protons are present in exceedingly low concentrations in most extracellular fluids, they nevertheless have a major impact on the function of most proteins. Localized changes in pH occur in physiological situations (e.g. neurotransmitter release in synapses) or under pathological conditions (e.g. inflammation, ischemia). We are interested in pH sensing in general, and particularly in one class of pH sensors, the acid-sensing ion channels (ASICs). ASICs are expressed in the central and the peripheral nervous system. They respond with a rapidly activating and subsequently desensitizing (inactivating) current to extracellular acidification. Due to this property they modulate neuronal signaling when the extracellular pH changes. There is evidence for roles of ASICs of the central nervous system in memory functions, fear conditioning, pain sensation and cell death during ischemic conditions. ASICs in the peripheral nervous system have been shown to be involved in pain sensation.
The research of my laboratory has two main lines. 1) We investigate how ASICs and other acid-sensitive channels affect the signaling activity of neurons as a function of pH. 2) We analyze in molecular detail the mechanisms by which pH controls opening and closing of the ASIC pore.
To understand the cellular roles of ASICs we measure action potential generation induced by acidification in neurons of the central and peripheral nervous system. By applying different conditions of pH and temperature, and with the use of pharmacological tools, we determine the contribution of different types of pH sensors. Fig. 1 illustrates schematically the mechanism of action potential induction by ASICs.
To understand on the molecular level how protonation of some key residues of the ASIC extracellular domain leads to opening and desensitization of the channel pore, we try to identify protonation sites and to determine the changes in conformation occurring during channel activity. To this end we combine electrophysiological analysis with site-directed mutagenesis, molecular dynamics simulations and voltage-clamp fluorometry. In the course of this project we have identified protonation sites, analyzed in detail the function of one domain (the “palm”), and described the timing of conformational changes in the channel. Fig. 2 gives an overview of the timing of conformational changes in different domains of ASIC1a.
Besides this main research orientation, the laboratory investigates also electrical communication in plants. In collaboration with E. Farmer (UNIL) we study wound-induced long distance signaling in Arabidopsis thaliana.
Action potential induction by ASICs. The figure shows on the left schematically the presence of ASICs and voltage-gated Na+ (Nav) and K+ (Kv) channels in a neuron. The activation of ASICs by lowering of the extracellular pH induces a depolarization as shown in the experimental trace on the right, activating the voltage-gated channels and thereby leading to a burst of action potentials.
Timely order of conformational changes involved by acidification. The figure illustrates the timely order of movements occurring upon channel activation by acidification in ASIC1a. Residues shown in one of the three subunits are colored according to the order in which the movements take place, based on voltage-clamp fluorometry measurements.