NeuroD regulates neuronal migration

The Neurod1/4-Ntrk3-Src pathway regulates gonadotrope cell adhesion and motility

Pituitary gonadotrope cells are essential for the endocrine regulation of reproduction in vertebrates. These cells emerge early during embryogenesis, colonize the pituitary glands and organize in tridimensional networks, which are believed to be crucial to ensure proper regulation of fertility. However, the molecular mechanisms regulating the organization of gonadotrope cell population during embryogenesis remain poorly understood. In this work, we characterized the target genes of NEUROD1 and NEUROD4 transcription factors in the immature gonadotrope αT3-1 cell model by in silico functional genomic analyses. We demonstrated that NEUROD1/4 regulate genes belonging to the focal adhesion pathway. Using CRISPR/Cas9 knock-out approaches, we established a double NEUROD1/4 knock-out αT3-1 cell model and demonstrated that NEUROD1/4 regulate cell adhesion and cell motility. We then characterized, by immuno-fluorescence, focal adhesion number and signaling in the context of NEUROD1/4 insufficiency. We demonstrated that NEUROD1/4 knock-out leads to an increase in the number of focal adhesions associated with signaling abnormalities implicating the c-Src kinase. We further showed that the neurotrophin tyrosine kinase receptor 3 NTRK3, a target of NEUROD1/4, interacts physically with c-Src. Furthermore, using motility rescue experiments and time-lapse video microscopy, we demonstrated that NTRK3 is a major regulator of gonadotrope cell motility. Finally, using a Ntrk3 knock-out mouse model, we showed that NTRK3 regulates gonadotrope cells positioning in the developing pituitary, in vivo. Altogether our study demonstrates that the Neurod1/4-Ntrk3-cSrc pathway is a major actor of gonadotrope cell mobility, and thus provides new insights in the regulation of gonadotrope cell organization within the pituitary gland.

Successful Treatment of Noise-Induced Hearing Loss by Mesenchymal Stromal Cells: An RNAseq Analysis of Protective/Repair Pathways

Mesenchymal stromal cells (MSCs) are an adult derived stem cell-like population that has been shown to mediate repair in a wide range of degenerative disorders. The protective effects of MSCs are mainly mediated by the release of growth factors and cytokines thereby modulating the diseased environment and the immune system. Within the inner ear, MSCs have been shown protective against tissue damage induced by sound and a variety of ototoxins. To better understand the mechanism of action of MSCs in the inner ear, mice were exposed to narrow band noise. After exposure, MSCs derived from human umbilical cord Wharton's jelly were injected into the perilymph. Controls consisted of mice exposed to sound trauma only. Forty-eight hours post-cell delivery, total RNA was extracted from the cochlea and RNAseq performed to evaluate the gene expression induced by the cell therapy. Changes in gene expression were grouped together based on gene ontology classification. A separate cohort of animals was treated in a similar fashion and allowed to survive for 2 weeks post-cell therapy and hearing outcomes determined. Treatment with MSCs after severe sound trauma induced a moderate hearing protective effect. MSC treatment resulted in an up-regulation of genes related to immune modulation, hypoxia response, mitochondrial function and regulation of apoptosis. There was a down-regulation of genes related to synaptic remodeling, calcium homeostasis and the extracellular matrix. Application of MSCs may provide a novel approach to treating sound trauma induced hearing loss and may aid in the identification of novel strategies to protect hearing.

Regulation of otocyst patterning by Tbx2 and Tbx3 is required for inner ear morphogenesis in the mouse

All epithelial components of the inner ear, including sensory hair cells and innervating afferent neurons, arise by patterning and differentiation of epithelial progenitors residing in a simple sphere, the otocyst. Here, we identify the transcriptional repressors TBX2 and TBX3 as novel regulators of these processes in the mouse. Ablation of Tbx2 from the otocyst led to cochlear hypoplasia, whereas loss of Tbx3 was associated with vestibular malformations. The loss of function of both genes (Tbx2/3cDKO) prevented inner ear morphogenesis at midgestation, resulting in indiscernible cochlear and vestibular structures at birth. Morphogenetic impairment occurred concomitantly with increased apoptosis in ventral and lateral regions of Tbx2/3cDKO otocysts around E10.5. Expression analyses revealed partly disturbed regionalisation, and a posterior-ventral expansion of the neurogenic domain in Tbx2/3cDKO otocysts at this stage. We provide evidence that repression of FGF signalling by TBX2 is important to restrict neurogenesis to the anterior-ventral otocyst and implicate another T-box factor, TBX1, as a crucial mediator in this regulatory network.

Tcf4 is required for correct brain development during embryogenesis

Tcf4 has been linked to autism, schizophrenia, and Pitt-Hopkins Syndrome (PTHS) in humans, suggesting a role for Tcf4 in brain development and importantly cortical development. However, the mechanisms behind its role in disease and brain development are still elusive. We provide evidence that Tcf4 has a critical function in the differentiation of cortical regions, corpus callosum and anterior commissure formation, and development of the hippocampus during murine embryonic development. In the present study, we show that Tcf4 is expressed throughout the developing brain at the peak of neurogenesis. Deletion of Tcf4 results in mis-specification of the cortical neurons, malformation of the corpus callosum and anterior commissure, and hypoplasia of the hippocampus. Furthermore, the Tcf4 mutant shows an absence of midline remodeling, underlined by the loss of GFAP-expressing midline glia in the indusium griseum and callosal wedge and midline zipper glia in the telencephalic midline. RNA-sequencing on E14.5 cortex material shows that Tcf4 functions as a transcriptional activator and loss of Tcf4 results in downregulation of genes linked to neurogenesis and neuronal maturation. Furthermore, many genes that are differentially expressed after Tcf4 ablation are linked to other neurodevelopmental disorders. Taken together, we show that correct brain development and neuronal differentiation are severely affected in Tcf4 mutants, phenocopying morphological brain defects detected in PTHS patients. The presented data identifies new leads to understand the mechanisms behind brain and specifically cortical development and can provide novel insights in developmental mechanisms underlying human brain defects.

Further characterisation of differences between TL and AB zebrafish (Danio rerio): Gene expression, physiology and behaviour at day 5 of the larval stage

Zebrafish (Danio rerio) have become popular as model organism in research. Many strains are readily available, which not only differ morphologically, but also genetically, physiologically and behaviourally. Here, we focus on the AB and Tupfel long-fin (TL) strain for which we have previously shown that adults differ in baseline hypothalamus-pituitary-interrenal (HPI)-axis activity (AB higher than TL) affecting inhibitory avoidance behaviour (absent in AB). To assess whether strain differences are already present in early life stages, we compared baseline HPI-axis related gene expression as well as cortisol levels, (neuro)development related as well as (innate) immune system related gene expression, and light-dark as well as startle behaviour in larvae 5 days post fertilisation. The data show that AB and TL larvae differ in baseline HPI-axis activity (AB higher than TL), expression of (neuro)development and immune system related genes (AB higher than TL), habituation to acoustic/vibrational stimuli (AB habituate faster than TL) and light-dark induced changes in motor behaviour (AB stronger than TL). Our data show that already in larval stages differences exist between zebrafish of the AB and TL strain confirming and extending data of earlier studies. To what extent the mutation in connexin 41.8, leading to spots rather than stripes in TL, but also (possibly) affecting eye, heart and brain function, is involved in the expression of (some of) these differences needs to be studied. These results emphasise that differences between strains need to be taken into account to enhance reproducibility both within, and between, laboratories.

Involvement of GSK-3β Phosphorylation Through PI3-K/Akt in Cerebral Ischemia-Induced Neurogenesis in Rats

Glycogen synthase kinase (GSK)-3β, which is abundantly expressed in the central nervous system, regulates various cellular processes including gene expression, cell proliferation, and differentiation. However, involvement of GSK-3β in cerebral ischemia-induced endogenous neurogenesis is not yet fully understood. Appropriate strategies to prevent ischemic cell damage and subsequent severe sequelae are needed. The purpose of the present study was to determine the relationship between pathophysiological alteration of the GSK-3β signaling pathway and cerebral ischemia-induced endogenous neurogenesis in rats. Severe cerebral ischemia was produced by the injection of 700 microspheres into the right internal carotid artery of rats. We demonstrated that phosphorylation of GSK-3β at its Ser9 and that of Akt was significantly enhanced on day 7 after the cerebral ischemia, as was the number of NeuroD-positive cells. Treatment with a phosphatidylinositol 3-kinase (PI3-K) inhibitor decreased the cerebral ischemia-induced phosphorylation of Akt and that of GSK-3β at its Ser9. In addition, as the protein levels of insulin-like growth factor-1 (IGF-1) and brain-derived neurotrophic factor (BDNF) were decreased, they might not have been essential for activation of the PI3-K/Akt/GSK-3β pathway after severe cerebral ischemia. Although it remains to be determined what factors activate this pathway, our results suggest that PI3K/Akt-dependent GSK-3β signaling and subsequent expression of NeuroD were involved in the neurogenesis elicited by cerebral ischemia.

Sugar Consumption Produces Effects Similar to Early Life Stress Exposure on Hippocampal Markers of Neurogenesis and Stress Response

Adverse early life experience is a known risk factor for psychiatric disorders. It is also known that stress influences food preference. We were interested in exploring whether the choice of diet following early life stress exerts long-lasting molecular changes in the brain, particularly the hippocampus, a region critically involved in stress regulation and behavioral outcomes. Here, we examined the impact of early life stress induced by limited nesting material (LN) and chronic sucrose availability post-weaning on an array of hippocampal genes related to plasticity, neurogenesis, stress and inflammatory responses and mitochondrial biogenesis. To examine mechanisms underlying the impact of LN and sugar intake on hippocampal gene expression, we investigated the role of DNA methylation. As females are more likely to experience adverse life events, we studied female Sprague-Dawley rats. After mating LN was imposed from days 2 to 9 postpartum. From 3 to 15 weeks of age, female Control and LN siblings had unlimited to access to either chow and water, or chow, water and 25% sucrose solution. LN markedly reduced glucocorticoid receptor (GR) and neurogenic differentiation 1 (Neurod1) mRNA, markers involved in stress and hippocampal plasticity respectively, by more than 40%, with a similar effect of sugar intake in control rats. However, no further impact was observed in LN rats consuming sugar. Hippocampal Akt3 mRNA expression was similarly affected by LN and sucrose consumption. Interestingly, DNA methylation across 4 CpG sites of the GR and Neurod1 promoters was similar in LN and control rats. In summary, early life stress and post-weaning sugar intake produced long-term effects on hippocampal GR and Neurod1 expression. Moreover we found no evidence of altered promoter DNA methylation. We demonstrate for the first time that chronic sucrose consumption alone produces similar detrimental effects on the expression of hippocampal genes as LN exposure.

NeuroD1 reprograms chromatin and transcription factor landscapes to induce the neuronal program

Cell fate specification relies on the action of critical transcription factors that become available at distinct stages of embryonic development. One such factor is NeuroD1, which is essential for eliciting the neuronal development program and possesses the ability to reprogram other cell types into neurons. Given this capacity, it is important to understand its targets and the mechanism underlying neuronal specification. Here, we show that NeuroD1 directly binds regulatory elements of neuronal genes that are developmentally silenced by epigenetic mechanisms. This targeting is sufficient to initiate events that confer transcriptional competence, including reprogramming of transcription factor landscape, conversion of heterochromatin to euchromatin, and increased chromatin accessibility, indicating potential pioneer factor ability of NeuroD1. The transcriptional induction of neuronal fate genes is maintained via epigenetic memory despite a transient NeuroD1 induction during neurogenesis. NeuroD1 also induces genes involved in the epithelial-to-mesenchymal transition, thereby promoting neuronal migration. Our study not only reveals the NeuroD1-dependent gene regulatory program driving neurogenesis but also increases our understanding of how cell fate specification during development involves a concerted action of transcription factors and epigenetic mechanisms.

Regulation of later neurogenic stages of adult-derived neural stem/progenitor cells by L-type Ca2+ channels

In the adult hippocampus, new neurons are continuously generated and incorporated into the local circuitry in a manner dependent on the network activity. Depolarization evoked by neurotransmitters has been assumed to activate L-type Ca2+ channels (LTCC) which regulate the intracellular Ca2+ -dependent signaling cascades. The process of neurogenesis contains several stages such as proliferation, fate determination, selective death/survival and maturation. Here, we investigated which stage of neurogenesis is under the regulation of LTCC using a clonal line of neural stem/progenitor cells, PZ5, which was derived from adult rat hippocampus. Although undifferentiated PZ5 cells were type 1-like cells expressing both nestin and glial fibrillary acidic protein, they generated neuronal, astrocytic and oligodendrocytic populations in differentiation medium containing retinoic acid. Proliferation of undifferentiated PZ5 cells was dependent on neither the LTCC antagonist, nimodipine (Nimo) nor the LTCC agonists, Bay K 8644 (BayK) or FPL 64176 (FPL), whereas the fraction of neuronal population that expressed both βIII-tubulin and MAP2 was reduced by Nimo but increased by BayK or FPL. At an earlier period of differentiation (e.g., day 4), the fraction of PZ5 cells expressing HuC/D, pan-neuronal marker, was not affected either by the LTCC activation or inhibition. At a later period of differentiation (e.g., day 9), the fraction of dying neurons was decreased by LTCC activation and increased by LTCC inhibition. It is suggested that the LTCC activation facilitates the survival and maturation of immature neurons, and that its inhibition facilitates the neuronal death.