Molecular Mechanisms Regulating Impaired Neurogenesis of Fragile X Syndrome Human Embryonic Stem Cells

Abstract

Fragile X syndrome (FXS) is the most common form of inherited cognitive impairment. It is caused by developmental inactivation of the FMR1 gene and the absence of its encoded protein FMRP, which plays pivotal roles in brain development and function. In FXS embryos with full FMR1 mutation, FMRP is expressed during early embryogenesis and is gradually downregulated at the third trimester of pregnancy. FX-human embryonic stem cells (FX-hESCs), derived from FX human blastocysts, demonstrate the same pattern of developmentally regulated FMR1 inactivation when subjected to in vitro neural differentiation (IVND). In this study, we used this in vitro human platform to explore the molecular mechanisms downstream to FMRP in the context of early human embryonic neurogenesis. Our results show a novel role for the SOX superfamily of transcription factors, specifically for SOX2 and SOX9, which could explain the reduced and delayed neurogenesis observed in FX cells. In addition, we assess in this study the ''GSK3b theory of FXS'' for the first time in a human-based model. We found no evidence for a pathological increase in GSK3b protein levels upon cellular loss of FMRP, in contrast to what was found in the brain of Fmr1 knockout mice. Our study adds novel data on potential downstream targets of FMRP and highlights the importance of the FX-hESC IVND system.

Metabotropic glutamate receptor 5 responses dictate differentiation of neural progenitors to NMDA‐responsive cells in fragile X syndrome

“…We functionally identified altered differentiation of a subpopulation of cells in human and mouse FXS neurospheres indicating FXS‐specific changes during early differentiation of neural progenitors. The data are consistent with previous studies which have shown impaired development of human FMRP‐deficient neural cells (Telias et al, ). Differential responses to MPEP in human and mouse progenitors could reflect species‐specific differences but also other progenitor type‐dependent differences, including dissimilar developmental stages of progenitors (Meredith et al, ).…”

Modeling Fragile X Syndrome Using Human Pluripotent Stem Cells

“…A full characterization of the in vitro derived neurons in terms of subtype and degree of maturation were not reported. Another candidate is the SOX family of transcription factors, and in particular SOX9 and SOX2 , which are known for their key role in neurogenesis and are both highly expressed in NPCs [112,113]. By comparing expressions of mature in vitro differentiated neurons and FXS and WT hESCs, Telias et al typically found a drop in FXS NPCs in SOX9 expression levels concurrent with a rise in SOX2 mRNA levels [114].…”

Neural differentiation is increased by GSK-3β inhibition and decreased by tankyrase inhibition in human neural precursor cells

“…The canonical Wnt/ β-Catenin signaling pathway has been shown to play pivotal roles in embryonic neural development as well as in adult neurogenesis [1,2]. For example, aberrant regulation of Wnt signaling has been proposed to underlie some of the symptoms observed in Alzheimer's disease, Autism and Fragile X Syndrome [3][4][5][6][7][8].…”

“…Reports on the effects of CHIR and XAV so far have also been predominantly performed on animal models, and in non-neural tissues. Recently, we reported on the molecular mechanisms regulating impaired in-vitro neural differentiation of Fragile X Syndrome (FXS)-human embryonic stem cells (hESCs), carrying the full mutation at the FMR1 gene [8]. In that study, we hypothesized that dysregulation of Wnt signaling could be responsible for the poor neural differentiation associated with these cells [23][24][25].…”

“…Here, we analyzed the effects of CHIR and XAV on human neural precursor cells (hNPCs) derived from control non-mutated hESC lines. These cells are initially differentiated from hESCs but, under specific culture conditions, can be kept as hNPCs for up to 10 passages [8]. During that time, they can be induced to differentiate into neurons on-cue, by changing their culture conditions, removing basic fibroblast growth factor (bFGF) and adding neuronal growth factors [8,23,24].…”