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

Abstract

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.