Roles of taurine-mediated tonic GABAA receptor activation in the radial migration of neurons in the fetal mouse cerebral cortex

Abstract

γ-Aminobutyric acid (GABA) depolarizes embryonic cerebrocortical neurons and continuous activation of the GABAA receptor (GABAAR) contributes to their tonic depolarization. Although multiple reports have demonstrated a role of GABAAR activation in neocortical development, including in migration, most of these studies have used pharmacological blockers. Herein, we performed in utero electroporation in GABA synthesis-lacking homozygous GAD67-GFP knock-in mice (GAD67GFP/GFP) to label neurons born in the ventricular zone. Three days after electroporation, there were no differences in the distribution of labeled cells between the genotypes. The dose–response properties of labeled cells to GABA were equivalent among genotypes. However, continuous blockade of GABAAR with the GABAAR antagonist SR95531 accelerated radial migration. This effect of GABAAR blockade in GAD67GFP/GFP mice suggested a role for alternative endogenous GABAAR agonists. Thus, we tested the role of taurine, which is derived from maternal blood but is abundant in the fetal brain. The taurine-evoked currents in labeled cells were mediated by GABAAR. Taurine uptake was blocked by a taurine transporter inhibitor, 2-(guanidino)ethanesulfonic acid (GES), and taurine release was blocked by a volume-sensitive anion channel blocker, 4-(2-butyl-6,7-dichlor-2-cyclopentylindan-1-on-5-yl) oxobutyric acid, as examined through high-performance liquid chromatography. GES increased the extracellular taurine concentration and induced an inward shift of the holding current, which was reversed by SR95531. In a taurine-deficient mouse model, the GABAAR-mediated tonic currents were greatly reduced, and radial migration was accelerated. As the tonic currents were equivalent among the genotypes of GAD67-GFP knock-in mice, taurine, rather than GABA, might play a major role as an endogenous agonist of embryonic tonic GABAAR conductance, regulating the radial migration of neurons in the developing neocortex.

Three puncture sites used for in utero electroporation show no significantly different negative impacts during gene transfer into the embryonic mouse brain

“…The utility of in utero electroporation could allow us to operate acute gene over-expression or knockdown in neuronal precursor cells and to observe proliferation, migration, and morphology of transfected neurons in subsequent developmental stages [9,13]. This article investigates the effects on cortical development through comparing different puncture sites with respect to the lateral ventricle, so that an optimized experimental scheme can be recommended.…”

The building of the neocortex with non‐hyperpolarizing neurotransmitters

“…The concentration of taurine in the rat immature cortex was estimated to be ∼0.3 m M , which roughly corresponds to the EC 50 (1.1 m M ) of GlyRs expressed by these neurons (Flint et al, ). Those data on taurine concentration were recently confirmed in the mouse immature neocortex, with an estimation in the m M range (Furukawa et al, ). A CSAD KO mouse has recently been produced, and without taurine supplementation these mice died the first postnatal day with an alteration in the expression of 145 genes in the brain (Park et al, ).…”

“…The activation of these receptors exerts a trophic action during corticogenesis, not only through the binding of glycine and GABA NTs but also the binding of taurine, the most abundant NT in the developing cortex. Recent evidences of the taurine‐mediated action on neuronal migration (Furukawa et al, ) have settled the idea that corticogenesis requires the neurotransmitters glycine, GABA, and taurine and perhaps does not need hyperpolarization.…”

“…During CNS development, taurine is abundantly expressed by progenitors and immature neurons. For example, recent evidences suggest that taurine is produced in the marginal zone (MZ) and the subplate during the embryonic cortical development of mice (Furukawa et al, ; Qian et al, ). As it is not a classical NT, taurine release and uptake might not follow a classical route, and these questions have been investigated by several groups.…”

GABA transporters control GABAergic neurotransmission in the mouse subplate

“…3). The existence of an intracortical gradient of extracellular GABA concentration, from low concentration in the subplate to high levels in the marginal zone, has been suggested in a number studies (Behar et al, 1996;Denter et al, 2010;Furukawa et al, 2014), but experimental data are still missing. Currently one cannot exclude the possibility that different types of GABA B Rs are expressed in the subplate and marginal zone, but it is tempting to speculate that the strength of tonic presynaptic GABA B R-mediated inhibition reflects extracellular GABA levels.…”