The Chloride Homeostasis of CA3 Hippocampal Neurons Is Not Altered in Fully Symptomatic Mepc2-null Mice.

Yasmine Belaïdouni,Jinwei Zhang,Clément Menuet,Diabe Diabira,Gary A Wayman,Francesca Bader,Jean-Luc Gaiarsa,Aurelie Montheil,Jean-Charles Graziano

doi:10.3389/fncel.2021.724976

Abstract

Rett syndrome (RTT) is an X-linked neurodevelopmental disorder caused mainly by mutations in the MECP2 gene. Mouse models of RTT show reduced expression of the cation-chloride cotransporter KCC2 and altered chloride homeostasis at presymptomatic stages. However, whether these alterations persist to late symptomatic stages has not been studied. Here we assess KCC2 and NKCC1 expressions and chloride homeostasis in the hippocampus of early [postnatal (P) day 30–35] and late (P50–60) symptomatic male Mecp2-null (Mecp2–/y) mice. We found (i) no difference in the relative amount, but an over-phosphorylation, of KCC2 and NKCC1 between wild-type (WT) and Mecp2–/y hippocampi and (ii) no difference in the inhibitory strength, nor reversal potential, of GABAA-receptor-mediated responses in Mecp2–/y CA3 pyramidal neurons compared to WT at any stages studied. Altogether, these data indicate the presence of a functional chloride extrusion mechanism in Mecp2–/y CA3 pyramidal neurons at symptomatic stages.

Highlights

Rett syndrome (RTT) is a non-inherited chromosome X-linked neurodevelopmental disorder characterized by loss of acquired speech, hand stereotypies, and gait abnormalities which appear following a period of apparently normal postnatal development
Altered K+-Cl− co-transporter 2 (KCC2) expression and impaired chloride homeostasis have been well documented in rodent models of RTT before the overt symptomatic period (Banerjee et al, 2016; Lozovaya et al, 2019; Scaramuzza et al, 2021)
We used early (P30–35) and late symptomatic (P50–60) male Mecp2-null mice, exhibiting the broad spectrum of phenotypes reported in RTT mouse models (Guy et al, 2001; Pratte et al, 2011; Calfa et al, 2015; Lozovaya et al, 2019) and found (i) no difference in the relative amount, but an over-phosphorylation, of KCC2 and Na+-K+-Cl− co-transporter 1 (NKCC1) between WT and Mecp2−/y hippocampi and (ii) no difference in the inhibitory strength of GABAA-receptor activation nor in the reversal potential of evoked GABAergic synaptic currents between WT and Mecp2−/y CA3 pyramidal neurons at any stages studied

Summary

Introduction

Rett syndrome (RTT) is a non-inherited chromosome X-linked neurodevelopmental disorder characterized by loss of acquired speech, hand stereotypies, and gait abnormalities which appear following a period of apparently normal postnatal development. Rett Syndrome and Chloride Homeostasis for up to 10% of severe intellectual disability of genetic origin in women. Typical RTT cases arise from de novo loss-offunction mutations in the MECP2 gene encoding the MethylCpG-binding protein 2 (MECP2) (Amir et al, 1999), a global transcriptional regulator. Given the location of the MECP2 gene on the X chromosome, males with MECP2 mutations are more severely affected and rarely survive infancy, while females, owing to X chromosome inactivation, are mosaics with cells that express either the wild-type (WT) or mutant version of MECP2 (Chahrour and Zoghbi, 2007). Several clinical features of the human disorder are recapitulated in Mecp2-deficient mice (Chen et al, 2001; Guy et al, 2001; Shahbazian and Zoghbi, 2002) and represent a crucial tool for deciphering the cellular mechanisms of the disease and for testing potential treatment strategies (Katz et al, 2012)

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