Cerebellar granular neuron progenitors exit their germinative niche via BarH-like1 activity mediated partly by inhibition of T-Cell Factor.

Cerebellar granule neuron progenitors (GNP) originate from the upper rhombic lip (URL), a germinative niche whose developmental defects produce human diseases. T-Cell Factors (TCF) responsiveness and Notch dependence are hallmarks of self-renewal in neural stem cells. TCF activity together with transcripts coding for proneural genes repressors hairy and enhancer of split (hes/hey), are detected in the URL. However, their functions and regulatory modes are undeciphered. Here we established amphibian as a pertinent model to study vertebrate URL development. Amphibians long-lived URL is Tcf active, while the External Granular layer (EGL) is non-proliferative and expresses hes4/5 genes. Using functional and transcriptomic approaches, we show that Tcf activity is necessary for URL emergence and maintenance. We establish that the transcription factor Barhl1 controls GNP exit from the URL acting partly through direct Tcf inhibition. Identification of Barhl1 target genes argues that besides Tcf, Barhl1 inhibits transcription of hes5 genes independently of Notch signaling. Observations in amniotes suggest a conserved role of a Barhl in maintenance of the URL/EGL via coregulation of TCF and hes/hey genes.

T-Cell Factors as Transcriptional Inhibitors: Activities and Regulations in Vertebrate Head Development.

Since its first discovery in the late 90s, Wnt canonical signaling has been demonstrated to affect a large variety of neural developmental processes, including, but not limited to, embryonic axis formation, neural proliferation, fate determination, and maintenance of neural stem cells. For decades, studies have focused on the mechanisms controlling the activity of ß-catenin, the sole mediator of Wnt transcriptional response. More recently, the spotlight of research is directed towards the last cascade component, the T-cell factor (TCF)/Lymphoid-Enhancer binding Factor (LEF), and more specifically, the TCF/LEF-mediated switch from transcriptional activation to repression, which in both embryonic blastomeres and mouse embryonic stem cells pushes the balance from pluri/multipotency towards differentiation. It has been long known that Groucho/Transducin-Like Enhancer of split (Gro/TLE) is the main co-repressor partner of TCF/LEF. More recently, other TCF/LEF-interacting partners have been identified, including the pro-neural BarH-Like 2 (BARHL2), which belongs to the evolutionary highly conserved family of homeodomain-containing transcription factors. This review describes the activities and regulatory modes of TCF/LEF as transcriptional repressors, with a specific focus on the functions of Barhl2 in vertebrate brain development. Specific attention is given to the transcriptional events leading to formation of the Organizer, as well as the roles and regulations of Wnt/ß-catenin pathway in growth of the caudal forebrain. We present TCF/LEF activities in both embryonic and neural stem cells and discuss how alterations of this pathway could lead to tumors.

Mcl1 protein levels and Caspase-7 executioner protease control axial organizer cells survival.

BACKGROUND: Organizing centers are groups of specialized cells that secrete morphogens, thereby influencing development of their neighboring territories. Apoptosis is a form of programmed cell death reported to limit the size of organizers. Little is known about the identity of intracellular signals driving organizer cell death. Here we investigated in Xenopus the role of both the anti-apoptotic protein Myeloid-cell-leukemia 1 (Mcl1) and the cysteine proteases Caspase-3 and Caspase-7 in formation of the axial organizing center-the notochord-that derives from the Spemann organizer, and participates in the induction and patterning of the neuroepithelium. RESULTS: We confirm a role for apoptosis in establishing the axial organizer in early neurula. We show that the expression pattern of mcl1 is coherent with a role for this gene in early notochord development. Using loss of function approaches, we demonstrate that Mcl1 depletion decreases neuroepithelium width and increases notochord cells apoptosis, a process that relies on Caspase-7, and not on Caspase-3, activity. Our data provide evidence that Mcl1 protein levels physiologically control notochord cells' survival and that Caspase-7 is the executioner protease in this developmental process. CONCLUSIONS: Our study reveals new functions for Mcl1 and Caspase-7 in formation of the axial signalling center.