Expression of the basic Helix-Loop-Helix ME1 E-protein during development and aging of the murine cerebellum

On the traces of tcf12: Investigation of the gene expression pattern during development and cranial suture patterning in zebrafish (Danio rerio)

The transcription factor 12 (tcf12) is a basic Helix-Loop-Helix protein (bHLH) of the E-protein family, proven to play an important role in developmental processes like neurogenesis, mesoderm formation, and cranial vault development. In humans, mutations in TCF12 lead to craniosynostosis, a congenital birth disorder characterized by the premature fusion of one or several of the cranial sutures. Current research has been primarily focused on functional studies of TCF12, hence the cellular expression profile of this gene during embryonic development and early stages of ossification remains poorly understood. Here we present the establishment and detailed analysis of two transgenic tcf12:EGFP fluorescent zebrafish (Danio rerio) reporter lines. Using these transgenic lines, we analyzed the general spatiotemporal expression pattern of tcf12 during different developmental stages and put emphasis on skeletal development and cranial suture patterning. We identified robust tcf12 promoter-driven EGFP expression in the central nervous system (CNS), the heart, the pronephros, and the somites of zebrafish embryos. Additionally, expression was observed inside the muscles and bones of the viscerocranium in juvenile and adult fish. During cranial vault development, the transgenic fish show a high amount of tcf12 expressing cells at the growth fronts of the ossifying frontal and parietal bones and inside the emerging cranial sutures. Subsequently, we tested the transcriptional activity of three evolutionary conserved non-coding elements (CNEs) located in the tcf12 locus by transient transgenic assays and compared their in vivo activity to the expression pattern determined in the transgenic tcf12:EGFP lines. We could validate two of them as tcf12 enhancer elements driving specific gene expression in the CNS during embryogenesis. Our newly established transgenic lines enhance the understanding of tcf12 gene regulation and open up the possibilities for further functional investigation of these novel tcf12 enhancer elements in zebrafish.

Cochlear nucleus whole mount explants promote the differentiation of neuronal stem cells from the cochlear nucleus in co-culture experiments

The cochlear nucleus is the first brainstem nucleus to receive sensory input from the cochlea. Depriving this nucleus of auditory input leads to cellular and molecular disorganization which may potentially be counteracted by the activation or application of stem cells. Neuronal stem cells (NSCs) have recently been identified in the neonatal cochlear nucleus and a persistent neurogenic niche was demonstrated in this brainstem nucleus until adulthood. The present work investigates whether the neurogenic environment of the cochlear nucleus can promote the survival of engrafted NSCs and whether cochlear nucleus-derived NSCs can differentiate into neurons and glia in brain tissue. Therefore, cochlear nucleus whole-mount explants were co-cultured with NSCs extracted from either the cochlear nucleus or the hippocampus and compared to a second environment using whole-mount explants from the hippocampus. Factors that are known to induce neuronal differentiation were also investigated in these NSC-explant experiments. NSCs derived from the cochlear nucleus engrafted in the brain tissue and differentiated into all cells of the neuronal lineage. Hippocampal NSCs also immigrated in cochlear nucleus explants and differentiated into neurons, astrocytes and oligodendrocytes. Laminin expression was up-regulated in the cochlear nucleus whole-mounts and regulated the in vitro differentiation of NSCs from the cochlear nucleus. These experiments confirm a neurogenic environment in the cochlear nucleus and the capacity of cochlear nucleus-derived NSCs to differentiate into neurons and glia. Consequently, the presented results provide a first step for the possible application of stem cells to repair the disorganization of the cochlear nucleus, which occurs after hearing loss.

Identification of neuronal target genes for CCAAT/enhancer binding proteins

CCAAT/Enhancer Binding Proteins (C/EBPs) play pivotal roles in the development and plasticity of the nervous system. Identification of the physiological targets of C/EBPs (C/EBP target genes) should therefore provide insight into the underlying biology of these processes. We used unbiased genome-wide mapping to identify 115 C/EBPbeta target genes in PC12 cells that include transcription factors, neurotransmitter receptors, ion channels, protein kinases and synaptic vesicle proteins. C/EBPbeta binding sites were located primarily within introns, suggesting novel regulatory functions, and were associated with binding sites for other developmentally important transcription factors. Experiments using dominant negatives showed C/EBPbeta to repress transcription of a subset of target genes. Target genes in rat brain were subsequently found to preferentially bind C/EBPalpha, beta and delta. Analysis of the hippocampal transcriptome of C/EBPbeta knockout mice revealed dysregulation of a high percentage of transcripts identified as C/EBP target genes. These results support the hypothesis that C/EBPs play non-redundant roles in the brain.

E protein dosage influences brain development more than family member identity

Loss-of-function studies have revealed the role of many basic helix-loop-helix (bHLH) transcription factors at specific points during development; however, the role of E proteins in the development of the nervous system has not been experimentally addressed. E proteins have been speculated to interact selectively with class II bHLH factors to form different neurogenic complexes. In this study, using coimmunoprecipitation in a culture model of neurogenesis (P19 cells), we show that E proteins E12, HEB, and E2-2 interact with neuroD2. Using electrophoretic mobility shift assay and P19 cell culture, we show that these heterodimers bind a neuroD2 preferred E box and induce neurogenesis equally well. We examine the mRNA levels of the three E proteins at 10 time points during brain development and show that E protein gene expression is regulated such that at certain times during development selective interaction between neuroD2 and a single E protein (HEB) is a possibility. This led us to study the brains of HEB and E2A knockout mice, which manifest no gross neuroanatomical, cellular, or behavioral deficits. These findings, together with homology in the primary peptide sequence of E proteins, suggest functional compensation among E proteins during development of the nervous system.

The bHLH transcription factor Tcf12 (ME1) mRNA is abundantly expressed in Paneth cells of mouse intestine

Using a large-scale in situ hybridization screening system, we found that mRNA coding for ME1, a basic helix-loop-helix (bHLH) transcription factor, was abundantly expressed in Paneth cells of adult small intestinal crypts. Other functionally related E-protein mRNAs, ME2, and E2A, however, could not be detected in the cells. ME1 mRNA was first detected in the jejunum and ileum two weeks after birth when the number of Paneth cells starts to increase. ME1 is the first identified bHLH transcription factor expressed in the Paneth cells and may be used as a molecular marker and a key molecule for analyzing transcriptional regulation in the Paneth cell.

Expression of the bHLH transcription factor Tcf12 (ME1) gene is linked to the expansion of precursor cell populations during neurogenesis

In this study, we focused on the potential function of the murine gene Tcf12 (also known as ME1 or HEB) encoding the bHLH E-protein ME1 during brain development. An exencephaly phenotype of low penetrance has consistently been observed in both Tcf12 null mice and Tcf12(dm) homozygous mice. Thus, to address the possible underlying mechanism of the Tcf12 gene during the early steps of brain development, we performed a detailed analysis of its spatio-temporal expression pattern at distinct steps of gastrulation and neurogenesis. We found that Tcf12 transcripts are detected in the embryonic ectoderm prior to neural induction during gastrulation. During neurulation, Tcf12 transcripts are evident at high levels in the proliferating neuroepithelium of the neural folds and the cephalic mesenchyme. Thus, Tcf12 gene expression coincides with the massive proliferation occurring in the forming neuroepithelium and cephalic mesenchyme during neural tube formation, which is consistent with the exencephaly phenotype of Tcf12 null mice. In the developing cortex and spinal cord, Tcf12 expression is restricted to the proliferative ventricular zones, indicating that Tcf12 expression is down regulated when these neuronal cells undergo their final differentiation. Interestingly, we found that the postnatal Tcf12 expression parallels the ongoing adult neurogenesis in the mitotically active subventricular zone. Thus, the timing and location of Tcf12 expression combined with this severe neurulation defect support our hypothesis that the Tcf12 gene may be involved in the control of proliferating neural stem cells and progenitor cells and that it may be critical to sustain their undifferentiated state during embryonic and adult neurogenesis.

Transcript profiling of functionally related groups of genes during conditional differentiation of a mammalian cochlear hair cell line

We have used Affymetrix high-density gene arrays to generate a temporal profile of gene expression during differentiation of UB/OC-1, a conditionally immortal cell line derived from the mouse cochlea. Gene expression was assessed daily for 14 days under differentiating conditions. The experiment was replicated in two separate populations of cells. Profiles for selected genes were correlated with those obtained by RT-PCR, TaqMan analysis, immunoblotting, and immunofluorescence. The results suggest that UB/OC-1 is derived from a population of nonsensory epithelial cells in the greater epithelial ridge that have the potential to differentiate into a hair-cell-like phenotype, without the intervention of Math1. Elements of the Notch signaling cascade were identified, including the receptor Notch3, with a transient up-regulation that suggests a role in hair cell differentiation. Several genes showed a profile similar to Notch3, including the transcriptional co-repressor Groucho1. UB/OC-1 also expressed Me1, a putative partner of Math1 that may confer competence to differentiate into hair cells. Cluster analysis revealed expression profiles for neural guidance genes associated with Gata3. The temporal dimension of this analysis provides a powerful tool to study genetic mechanisms that underlie the conversion of nonsensory epithelial cells into hair cells.

Distinct patterns of downstream target activation are specified by the helix-loop-helix domain of proneural basic helix-loop-helix transcription factors

Both gain- and loss-of-function analyses indicate that proneural basic/helix-loop-helix (bHLH) proteins direct not only general aspects of neuronal differentiation but also specific aspects of neuronal identity within neural progenitors. In order to better understand the function of this family of transcription factors, we have used hormone-inducible fusion constructs to assay temporal patterns of downstream target regulation in response to proneural bHLH overexpression. In these studies, we have compared two distantly related Xenopus proneural bHLH genes, Xash1 and XNgnr1. Our findings indicate that both Xash1 and XNgnr1 induce expression of the general neuronal differentiation marker, N-tubulin, with a similar time course in animal cap progenitor populations. In contrast, these genes each induce distinct patterns of early downstream target expression. Both genes induce expression of the HLH-containing gene, Xcoe2, at early time points, but only XNgnr1 induces early expression of the bHLH genes, Xath3 and XNeuroD. Structure:function analyses indicate that the distinct pattern of XNgnr1-induced downstream target activation is linked to the XNgnr1 HLH domain, demonstrating a novel role for this domain in mediating the differential function of individual members of the proneural bHLH gene family.

Effect of age on the gene expression of neural-restrictive silencing factor NRSF/REST

Aging affects a wide range of gene expression changes in the nervous system. Such effects could be attributed to random changes in the environment with age around each gene, but also could be caused by selective changes in a limited set of key regulatory transcription factors and/or chromatin remodeling components. To approach the question of whether neural-restrictive silencer factor NRSF, a key determinant of the neuron-specific gene expression, is involved in these changes, we examined the levels of NRSF in the rat brain and dosal root ganglia during aging by semi-quantitative reverse transcriptase-mediated polymerase chain reaction (PCR) (RT-PCR). Complementary expression profiles of transcripts of NRSF and SCG10 in the mature brain were shown by in situ hybridization. Neither the mRNA levels of NRSF nor a splicing variant NRnV were changed, at least in rats up to 26 months old. The gene expression level of SCG10, one of the NRSF targets, was also unaffected by age. The stable expression of SCG10 transcripts in aging was confirmed by in situ hybridization. The NRS-binding ability of NRSF was also unchanged significantly in the nuclear extracts of aged rat brain. These results suggest that the genetic machinery associated with the NRS-NRSF system is well maintained during aging.

Differential expression patterns of the basic helix-loop-helix transcription factors during aging of the murine brain

In this study, we investigated the expression pattern of the basic Helix-Loop-Helix transcription factors during brain aging. We provide the first evidence that NeuroD and ME2 are differentially expressed during brain aging. Modulation of their expression is specific to distinct areas of the aging brain. NeuroD expression is sustained at high levels in aging cerebellum, whereas it severely declines in aging hippocampus. In contrast, the bHLH E-protein ME2 remains expressed in both aged cerebellum and hippocampus, although at lower levels. These observations support the idea that a shift in the transcriptional dynamics controlling gene expression is associated with the progressive functional decline observed during brain aging.