Non-redundant role of the a3 isoform of Na,K-ATPase in neuronal excitability and spiking dynamics

Abstract

The Na+,K+-ATPase (NKA) plays a fundamental role in neuronal excitability by maintaining ionic gradients and contributing to electrogenic resting currents. Among its isoforms, the neuron-specific 3 subunit exhibits a uniquely low affinity for intracellular Na+, weak voltage dependence, and slightly reduced ATP sensitivity compared to the ubiquitous 1. Although mutations in ATP1A3 are known to cause severe neurological disorders, the specific kinetic features of 3 that underlie its functional specialization remain incompletely understood. Here we employed biophysically detailed models of stretch receptor neurons, grounded in patch-clamp recordings of the 3-isoform current and spiking responses, to dissect the contribution of isoform-specific pump kinetics to firing behavior. Substitution of 1 for 3 abolished the ability to sustain long spike trains and reduced high-frequency entrainment, whereas 3-preserved prolonged discharges and faithful responses to vibratory stimuli. Remarkably, even halved pump density 350% preserved superior excitability compared to mixed expression (350%/150%), indicating that kinetic profile rather than pump quantity determines firing capacity. Hybrid models revealed that Na+ affinity is the decisive factor: retaining the low Na+ affinity of 3 preserved excitability, while introducing 1-like voltage or ATP dependence produced only minor effect on simulated neuron discharge. These findings establish a mechanistic explanation for the selective expression of 3 in muscle spindle afferents and other neurons with high-frequency demands, and help to explain why 1 cannot compensate in ATP1A3-linked diseases. More broadly, they highlight the principle that isoform specialization of the NKA is not redundant but tuned to the discharge requirements of distinct neuronal populations.