Slowing of the Time Course of Acidification Decreases the Acid-Sensing Ion Channel 1a Current Amplitude and Modulates Action Potential Firing in Neurons

Abstract

Acid-sensing ion channels (ASICs) are H +-activated neuronal Na + channels. They are involved in fear behavior, learning, neurodegeneration after ischemic stroke and in pain sensation. ASIC activation has so far been studied only with fast pH changes, although the pH changes associated with many roles of ASICs are slow. It is currently not known whether slow pH changes can open ASICs at all. Here, we investigated to which extent slow pH changes can activate ASIC1a channels and induce action potential signaling. To this end, ASIC1a current amplitudes and charge transport in transfected Chinese hamster ovary cells, and ASIC-mediated action potential signaling in cultured cortical neurons were measured in response to defined pH ramps of 1-40 s duration from pH 7.4 to pH 6.6 or 6.0. A kinetic model of the ASIC1a current was developed and integrated into the Hodgkin-Huxley action potential model. Interestingly, whereas the ASIC1a current amplitude decreased with slower pH ramps, action potential firing was higher upon intermediate than fast acidification in cortical neurons. Indeed, fast pH changes (<4 s) induced short action potential bursts, while pH changes of intermediate speed (4-10 s) induced longer bursts. Slower pH changes (>10 s) did in many experiments not generate action potentials. Computer simulations corroborated these observations. We provide here the first description of ASIC function in response to defined slow pH changes. Our study shows that ASIC1a currents, and neuronal activity induced by ASIC1a currents, strongly depend on the speed of pH changes. Importantly, with pH changes that take >10 s to complete, ASIC1a activation is inefficient. Therefore, it is likely that currently unknown modulatory mechanisms allow ASIC activity in situations such as ischemia and inflammation.

Kinetic analysis of ASIC1a delineates conformational signaling from proton-sensing domains to the channel gate

“…Experiments with ultrarapid perfusion systems have shown that ASICs activate with time constants of the order of 10 ms (Bassler et al, 2001, Sutherland et al, 2001, Alijevic et al, 2020). In the measuring chamber used in this study, the RT of solution change was ∼300ms ( Materials and Methods) .…”

“…These kinetics are slower than the current and ΔF measured for ASIC activation by pH in many experiments. It is known from experiments with ultrarapid perfusion systems and excised patches that the intrinsic kinetics of ASIC activation are of the order of ∼10 ms (Bassler et al, 2001, Sutherland et al, 2001, Alijevic et al, 2020, Vaithia et al, 2019). Therefore, the ASIC activation kinetics are in most studies limited by the speed of the perfusion change.…”

“…As an additional strategy for testing the association of F signals with specific transitions, F traces were compared to traces generated by a kinetic model under the assumption that each functional state contributes with a proportionality factor that lies between -1 and +1 to the measured fluorescence. To this end, a kinetic ASIC1a model recently developed in our laboratory was used (Alijevic et al, 2020), which is based on the Figure 1F). This model reproduces the acid-induced ASIC1a activity well (Alijevic et al, 2020).…”

“…To this end, a kinetic ASIC1a model recently developed in our laboratory was used (Alijevic et al, 2020), which is based on the Figure 1F). This model reproduces the acid-induced ASIC1a activity well (Alijevic et al, 2020). Figure 1G shows F traces generated by this model for a pH change from 7.4 to 6.0, under the assumption that in each case one single functional state, or a pair, O+OD (corresponding to the activation gate) and CD+OD (sensitization gate) contributes to the signal.…”

“…As one possible interpretation of the F signals it was assumed that a certain degree of fluorescence was associated with each functional state ( Figure 1F-G). To this end, the kinetic model describing the function of ASIC1a WT, described in the Methods (Alijevic et al, 2020) where the symbol "~" denotes proportionality, P(C), P(O), P(CD) and P(OD) are the probabilities of the channel to be in the corresponding states (which evolve with time but always sum up to 1) and F(C), F(O), F(CD) and F(OD) are the corresponding constant scaling factors. The traces in Figure 1G show modeled F traces for the case that F was associated to one single functional state, or associated to either the activation or sensitization gate.…”