β-Site APP-cleaving enzyme 1 (BACE1) cleaves cerebellar Na⁺ channel β4-subunit and promotes Purkinje cell firing by slowing the decay of resurgent Na⁺ current

Abstract

In cerebellar Purkinje cells, the β4-subunit of voltage-dependent Na⁺ channels has been proposed to serve as an open-channel blocker giving rise to a “resurgent” Na⁺ current (I _NaR) upon membrane repolarization. Notably, the β4-subunit was recently identified as a novel substrate of the β-secretase, BACE1, a key enzyme of the amyloidogenic pathway in Alzheimer's disease. Here, we asked whether BACE1-mediated cleavage of β4-subunit has an impact on I _NaR and, consequently, on the firing properties of Purkinje cells. In cerebellar tissue of BACE1−/− mice, mRNA levels of Na⁺ channel α-subunits 1.1, 1.2, and 1.6 and of β-subunits 1–4 remained unchanged, but processing of β4 peptide was profoundly altered. Patch-clamp recordings from acutely isolated Purkinje cells of BACE1−/− and WT mice did not reveal any differences in steady-state properties and in current densities of transient, persistent, and resurgent Na⁺ currents. However, I _NaR was found to decay significantly faster in BACE1-deficient Purkinje cells than in WT cells. In modeling studies, the altered time course of I _NaR decay could be replicated when we decreased the efficiency of open-channel block. In current-clamp recordings, BACE1−/− Purkinje cells displayed lower spontaneous firing rate than normal cells. Computer simulations supported the hypothesis that the accelerated decay kinetics of I _NaR are responsible for the slower firing rate. Our study elucidates a novel function of BACE1 in the regulation of neuronal excitability that serves to tune the firing pattern of Purkinje cells and presumably other neurons endowed with I _NaR.