Reduced LYNX1 expression in transcriptome of human iPSC-derived neural progenitors modeling fragile X syndrome.

Lack of FMR1 protein results in fragile X syndrome (FXS), which is the most common inherited intellectual disability syndrome and serves as an excellent model disease to study molecular mechanisms resulting in neuropsychiatric comorbidities. We compared the transcriptomes of human neural progenitors (NPCs) generated from patient-derived induced pluripotent stem cells (iPSCs) of three FXS and three control male donors. Altered expression of RAD51C, PPIL3, GUCY1A2, MYD88, TRAPPC4, LYNX1, and GTF2A1L in FXS NPCs suggested changes related to triplet repeat instability, RNA splicing, testes development, and pathways previously shown to be affected in FXS. LYNX1 is a cholinergic brake of tissue plasminogen activator (tPA)-dependent plasticity, and its reduced expression was consistent with augmented tPA-dependent radial glial process growth in NPCs derived from FXS iPSC lines. There was evidence of human iPSC line donor-dependent variation reflecting potentially phenotypic variation. NPCs derived from an FXS male with concomitant epilepsy expressed differently several epilepsy-related genes, including genes shown to cause the auditory epilepsy phenotype in the murine model of FXS. Functional enrichment analysis highlighted regulation of insulin-like growth factor pathway in NPCs modeling FXS with epilepsy. Our results demonstrated potential of human iPSCs in disease modeling for discovery and development of therapeutic interventions by showing early gene expression changes in FXS iPSC-derived NPCs consistent with the known pathophysiological changes in FXS and by revealing disturbed FXS progenitor growth linked to reduced expression of LYNX1, suggesting dysregulated cholinergic system.

FIBCD1 is an endocytic GAG receptor associated with a novel neurodevelopmental disorder.

Whole-exome sequencing of two patients with idiopathic complex neurodevelopmental disorder (NDD) identified biallelic variants of unknown significance within FIBCD1, encoding an endocytic acetyl group-binding transmembrane receptor with no known function in the central nervous system. We found that FIBCD1 preferentially binds and endocytoses glycosaminoglycan (GAG) chondroitin sulphate-4S (CS-4S) and regulates GAG content of the brain extracellular matrix (ECM). In silico molecular simulation studies and GAG binding analyses of patient variants determined that such variants are loss-of-function by disrupting FIBCD1-CS-4S association. Gene knockdown in flies resulted in morphological disruption of the neuromuscular junction and motor-related behavioural deficits. In humans and mice, FIBCD1 is expressed in discrete brain regions, including the hippocampus. Fibcd1 KO mice exhibited normal hippocampal neuronal morphology but impaired hippocampal-dependent learning. Further, hippocampal synaptic remodelling in acute slices from Fibcd1 KO mice was deficient but restored upon enzymatically modulating the ECM. Together, we identified FIBCD1 as an endocytic receptor for GAGs in the brain ECM and a novel gene associated with an NDD, revealing a critical role in nervous system structure, function and plasticity.

Increased Calcium Influx through L-type Calcium Channels in Human and Mouse Neural Progenitors Lacking Fragile X Mental Retardation Protein.

The absence of FMR1 protein (FMRP) causes fragile X syndrome (FXS) and disturbed FMRP function is implicated in several forms of human psychopathology. We show that intracellular calcium responses to depolarization are augmented in neural progenitors derived from human induced pluripotent stem cells and mouse brain with FXS. Increased calcium influx via nifedipine-sensitive voltage-gated calcium (Cav) channels contributes to the exaggerated responses to depolarization and type 1 metabotropic glutamate receptor activation. The ratio of L-type/T-type Cav channel expression is increased in FXS progenitors and correlates with enhanced progenitor differentiation to glutamate-responsive cells. Genetic reduction of brain-derived neurotrophic factor in FXS mouse progenitors diminishes the expression of Cav channels and activity-dependent responses, which are associated with increased phosphorylation of the phospholipase C-γ1 site within TrkB receptors and changes of differentiating progenitor subpopulations. Our results show developmental effects of increased calcium influx via L-type Cav channels in FXS neural progenitors.

Functional changes of AMPA responses in human induced pluripotent stem cell-derived neural progenitors in fragile X syndrome.

Altered neuronal network formation and function involving dysregulated excitatory and inhibitory circuits are associated with fragile X syndrome (FXS). We examined functional maturation of the excitatory transmission system in FXS by investigating the response of FXS patient-derived neural progenitor cells to the glutamate analog (AMPA). Neural progenitors derived from induced pluripotent stem cell (iPSC) lines generated from boys with FXS had augmented intracellular Ca2+ responses to AMPA and kainate that were mediated by Ca2+-permeable AMPA receptors (CP-AMPARs) lacking the GluA2 subunit. Together with the enhanced differentiation of glutamate-responsive cells, the proportion of CP-AMPAR and N-methyl-d-aspartate (NMDA) receptor-coexpressing cells was increased in human FXS progenitors. Differentiation of cells lacking GluA2 was also increased and paralleled the increased inward rectification in neural progenitors derived from Fmr1-knockout mice (the FXS mouse model). Human FXS progenitors had increased the expression of the precursor and mature forms of miR-181a, a microRNA that represses translation of the transcript encoding GluA2. Blocking GluA2-lacking, CP-AMPARs reduced the neurite length of human iPSC-derived control progenitors and further reduced the shortened length of neurites in human FXS progenitors, supporting the contribution of CP-AMPARs to the regulation of progenitor differentiation. Furthermore, we observed reduced expression of Gria2 (the GluA2-encoding gene) in the frontal lobe of FXS mice, consistent with functional changes of AMPARs in FXS. Increased Ca2+ influx through CP-AMPARs may increase the vulnerability and affect the differentiation and migration of distinct cell populations, which may interfere with normal circuit formation in FXS.

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

Disrupted metabotropic glutamate receptor 5 (mGluR5) signaling is implicated in many neuropsychiatric disorders, including autism spectrum disorder, found in fragile X syndrome (FXS). Here we report that intracellular calcium responses to the group I mGluR agonist (S)-3,5-dihydroxyphenylglycine (DHPG) are augmented, and calcium-dependent mGluR5-mediated mechanisms alter the differentiation of neural progenitors in neurospheres derived from human induced pluripotent FXS stem cells and the brains of mouse model of FXS. Treatment with the mGluR5 antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) prevents an abnormal clustering of DHPG-responsive cells that are responsive to activation of ionotropic receptors in mouse FXS neurospheres. MPEP also corrects morphological defects of differentiated cells and enhanced migration of neuron-like cells in mouse FXS neurospheres. Unlike in mouse neurospheres, MPEP increases the differentiation of DHPG-responsive radial glial cells as well as the subpopulation of cells responsive to both DHPG and activation of ionotropic receptors in human neurospheres. However, MPEP normalizes the FXS-specific increase in the differentiation of cells responsive only to N-methyl-d-aspartate (NMDA) present in human neurospheres. Exposure to MPEP prevents the accumulation of intermediate basal progenitors in embryonic FXS mouse brain suggesting that rescue effects of GluR5 antagonist are progenitor type-dependent and species-specific differences of basal progenitors may modify effects of MPEP on the cortical development. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 419-437, 2017.