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

Abstract

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.

Fast inactivation of Nav current in rat adrenal chromaffin cells involves two independent inactivation pathways

“…This dual-pathway, fast inactivation mechanism appears similar to that proposed to underlie inactivation mediated by the A-isoform N termini of iFGFs for Nav current in both hippocampal Purkinje cells ( Venkatesan et al, 2014 ) and cerebellar granule cells ( Goldfarb et al, 2007 ). The involvement of an iFGF is confirmed in mouse CCs in the companion paper ( Martinez-Espinosa et al, 2021 ).…”

“…Possible explanations might include (1) differential expression of two populations of Nav channels and (2) differential contribution of some molecular component that contributes to the slower component of recovery from inactivation. We consider the latter possibility more likely ( Martinez-Espinosa et al, 2021 ).…”

“…The identity of the specific Nav subtype that underlies Nav currents in rat CCs remains unresolved. Although long-term dogma has suggested that CC Nav current is primarily Nav1.7 ( Wada et al, 2008 ; Tamura et al, 2014 ), knockout of Nav1.3 removes Nav current in most mouse CCs ( Martinez-Espinosa et al, 2021 ), leaving only a smaller contribution in some cells of some other Nav current, perhaps Nav1.7. We would suggest the rat CCs may contain both Nav1.3 and Nav1.7, with perhaps a larger component of Nav1.7 than found in mouse CCs ( Martinez-Espinosa et al, 2021 ).…”

“…Although long-term dogma has suggested that CC Nav current is primarily Nav1.7 ( Wada et al, 2008 ; Tamura et al, 2014 ), knockout of Nav1.3 removes Nav current in most mouse CCs ( Martinez-Espinosa et al, 2021 ), leaving only a smaller contribution in some cells of some other Nav current, perhaps Nav1.7. We would suggest the rat CCs may contain both Nav1.3 and Nav1.7, with perhaps a larger component of Nav1.7 than found in mouse CCs ( Martinez-Espinosa et al, 2021 ). Regarding the basis for the slow component of recovery, all tests of the rat CC Nav current properties here point to the idea that the rat CC Nav current exhibits two independent fast inactivation processes.…”

“…Presumably, one corresponds to conventional fast Nav inactivation, while the other remains undefined. In the associated paper, iFGF14 is shown to critically determine slow recovery from inactivation in mouse CCs ( Martinez-Espinosa et al, 2021 ). We anticipate that the results presented here will serve as a foundation for quantitative evaluation of the dual-pathway inactivation behavior and how it may impact on excitability, once the molecular determinants of that process are clearly defined.…”