Endothelin-1-induced down-regulation of NaV1.7 expression in adrenal chromaffin cells: attenuation of catecholamine secretion and tau dephosphorylation

Nav1.3 and FGF14 are primary determinants of the TTX-sensitive sodium current in mouse adrenal chromaffin cells

Adrenal chromaffin cells (CCs) in rodents express rapidly inactivating, tetrodotoxin (TTX)-sensitive sodium channels. The resulting current has generally been attributed to Nav1.7, although a possible role for Nav1.3 has also been suggested. Nav channels in rat CCs rapidly inactivate via two independent pathways which differ in their time course of recovery. One subpopulation recovers with time constants similar to traditional fast inactivation and the other ∼10-fold slower, but both pathways can act within a single homogenous population of channels. Here, we use Nav1.3 KO mice to probe the properties and molecular components of Nav current in CCs. We find that the absence of Nav1.3 abolishes all Nav current in about half of CCs examined, while a small, fast inactivating Nav current is still observed in the rest. To probe possible molecular components underlying slow recovery from inactivation, we used mice null for fibroblast growth factor homology factor 14 (FGF14). In these cells, the slow component of recovery from fast inactivation is completely absent in most CCs, with no change in the time constant of fast recovery. The use dependence of Nav current reduction during trains of stimuli in WT cells is completely abolished in FGF14 KO mice, directly demonstrating a role for slow recovery from inactivation in determining Nav current availability. Our results indicate that FGF14-mediated inactivation is the major determinant defining use-dependent changes in Nav availability in CCs. These results establish that Nav1.3, like other Nav isoforms, can also partner with FGF subunits, strongly regulating Nav channel function.

The Glycogen Synthase Kinase-3 in the Regulation of Ion Channels and Cellular Carriers

Glycogen synthase kinase-3 (GSK-3) is a highly evolutionarily conserved and ubiquitously expressed serine/threonine kinase, an enzyme protein profoundly specific for glycogen synthase (GS). GSK-3 is involved in various cellular functions and physiological processes, including cell proliferation, differentiation, motility, and survival as well as glycogen metabolism, protein synthesis, and apoptosis. There are two isoforms of human GSK-3 (named GSK-3α and GSK-3β) encoded by two distinct genes. Recently, GSK-3β has been reported to function as a powerful regulator of various transport processes across the cell membrane. This kinase, GSK-3β, either directly or indirectly, may stimulate or inhibit many different types of transporter proteins, including ion channel and cellular carriers. More specifically, GSK-3β-sensitive cellular transport regulation involves various calcium, chloride, sodium, and potassium ion channels, as well as a number of Na+-coupled cellular carriers including excitatory amino acid transporters EAAT2, 3 and 4, high-affinity Na+ coupled glucose carriers SGLT1, creatine transporter 1 CreaT1, and the type II sodium/phosphate cotransporter NaPi-IIa. The GSK-3β-dependent cellular transport regulations are a part of the kinase functions in numerous physiological and pathophysiological processes. Clearly, additional studies are required to examine the role of GSK-3β in many other types of cellular transporters as well as further elucidating the underlying mechanisms of GSK-3β-mediated cellular transport regulation.

Veratridine modifies the gating of human voltage-gated sodium channel Nav1.7

Veratridine is a lipid-soluble neurotoxin derived from plants in the family Liliaceae. It has been broadly investigated for its action as a sodium channel agonist. However, the effects of veratridine on subtypes of sodium channels, especially Nav1.7, remain to be studied. Here, we investigated the effects of veratridine on human Nav1.7 ectopically expressed in HEK293A cells and recorded Nav1.7 currents from the cells using whole-cell patch clamp technique. We found that veratridine exerted a dose-dependent inhibitory effect on the peak current of Nav1.7, with the half-maximal inhibition concentration (IC₅₀) of 18.39 µM. Meanwhile, veratridine also elicited tail current (linearly) and sustained current [half-maximal concentration (EC₅₀): 9.53 µM], also in a dose-dependent manner. Veratridine (75 µM) shifted the half-maximal activation voltage of the Nav1.7 activation curve in the hyperpolarized direction, from -21.64 ± 0.75 mV to -28.14 ± 0.66 mV, and shifted the half-inactivation voltage of the steady-state inactivation curve from -59.39 ± 0.39 mV to -73.78 ± 0.5 mV. An increased frequency of stimulation decreased the peak and tail currents of Nav1.7 for each pulse along with pulse number, and increased the accumulated tail current at the end of train stimulation. These findings reveal the different modulatory effects of veratridine on the Nav1.7 peak current and tail current.