Sodium channel NaV1.3 is important for enterochromaffin cell excitability and serotonin release

Peter R Strege,Lorin Milescu,Gianrico Farrugia,Joyce H Li,Fan Wang,Arthur Beyder,Samuel J Eggers,Joseph H Szurszewski,Kaitlyn Knutson,Andrew B Leiter,David Linden

doi:10.1038/s41598-017-15834-3

Abstract

In the gastrointestinal (GI) epithelium, enterochromaffin (EC) cells are enteroendocrine cells responsible for producing >90% of the body’s serotonin (5-hydroxytryptamine, 5-HT). However, the molecular mechanisms of EC cell function are poorly understood. Here, we found that EC cells in mouse primary cultures fired spontaneous bursts of action potentials. We examined the repertoire of voltage-gated sodium channels (NaV) in fluorescence-sorted mouse EC cells and found that Scn3a was highly expressed. Scn3a-encoded NaV1.3 was specifically and densely expressed at the basal side of both human and mouse EC cells. Using electrophysiology, we found that EC cells expressed robust NaV1.3 currents, as determined by their biophysical and pharmacologic properties. NaV1.3 was not only critical for generating action potentials in EC cells, but it was also important for regulating 5-HT release by these cells. Therefore, EC cells use Scn3a-encoded voltage-gated sodium channel NaV1.3 for electrical excitability and 5-HT release. NaV1.3-dependent electrical excitability and its contribution to 5-HT release is a novel mechanism of EC cell function.

Highlights

More than 90% of the serotonin (5-hydroxytryptamine, 5-HT) in the body is produced and released into circulation by a single enteroendocrine cell type in the gastrointestinal (GI) tract—the enterochromaffin (EC) cell[1]
The molecular mechanisms of EC cell function and serotonin release are poorly understood because only a few studies have examined EC cells in isolation from the rest of the GI epithelium[13,25,26]
In isolated guinea pig EC cells, there was an inward current consistent with voltage-gated calcium channels but no fast sodium current[15], while in a different study TTX-sensitive sodium current was present in murine EC cells[13]

Summary

Introduction

More than 90% of the serotonin (5-hydroxytryptamine, 5-HT) in the body is produced and released into circulation by a single enteroendocrine cell type in the gastrointestinal (GI) tract—the enterochromaffin (EC) cell[1]. The Ca2+-dependent form of exocytosis relies on activation of voltage-gated calcium channels[13,14] and accounts for most of the released 5-HT15. Previous studies showed that other endocrine[16], neuroendocrine[17], and enteroendocrine cells[18] are electrically excitable, with voltage-gated sodium (Na+) channels contributing to electrical excitability and stimulation-secretion coupling[17]. The goal of this study was to determine whether EC cells are electrically excitable and, if so, whether voltage-gated sodium channels (NaV) were responsible for EC cell excitability. We present several lines of evidence that a single voltage gated sodium channel, the Scn3a-encoded NaV1.3, is highly and enriched in EC cells, renders the EC cells electrically excitable, and contributes to 5-HT release. Parts of this study were previously presented[19,20,21]

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Conclusion