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.

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.

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

Genesis of cerebellar granule cells is controlled by key transcription factors, such as the lineage-specific basic Helix-Loop-Helix (bHLH) transcription factor MATH-1, whose activity is dependent upon dimerization with bHLH E-proteins. In an effort to understand the molecular mechanisms of bHLH proteins orchestrating cerebellar development, we explored the spatio-temporal expression of the ME1 E-protein. Our results reveal that ME1 expression is first detected in the cerebellar primordium and then in the rhombic lip cells at E12.5. Its expression persists in the emerging external germinal layer during embryonic expansion. In adult cerebellum, prominent ME1 expression is detected in mature granule cells located in the internal granular layer. However, ME1 expression is not sustained in aged cerebellum. A similar declined pattern of expression is also observed in the aging hippocampus.

Expression of the bHLH gene NSCL-1 suggests a role in regulating cerebellar granule cell growth and differentiation

We report that the neuronal-specific basic helix-loop-helix (bHLH) gene NSCL-1 is expressed at multiple and distinct stages of cerebellar granule cell differentiation. During embryonic development, NSCI-1 expression is initially evenly distributed in the cerebellar primordium and then becomes restricted to the ventricular zone. At the early steps of granule cell development, NSCL-1 is not expressed in rhombic lip cells, but instead in migrating granule cell precursors. Its expression culminates during postnatal proliferation of the external germinal layer, and remains only transiently in the newly formed internal granular layer, and at a much lower level. Thus, NSCL-1 expression is linked to the onset of granule cell differentiation, but is not involved in the maintenance of the differentiated state. These findings suggest that NSCL-1 does not behave as a specification factor, but rather as a factor promoting expansion of progenitor external germinal layer (EGL) cells. Gel mobility shift assays show that NSCL-1 only binds DNA as a heterodimeric complex with the ME1a E-protein. We also provide the first evidence that NSCL-1 functions as a transcriptional activator when heterodimerized with the ME1a E-protein. Taken together, these results suggest that NSCL-1 participates in the regulatory network controlling gene expression during cerebellar granule cell differentiation.