Mammary epithelial stem cells are fundamental to maintain tissue integrity. Cancer stem cells (CSCs) are implicated in both treatment resistance and disease relapse, and the molecular bases of their malignant properties are still poorly understood. Here we show that both normal stem cells and CSCs of the breast are controlled by the prolyl-isomerase Pin1. Mechanistically, following interaction with Pin1, Notch1 and Notch4, key regulators of cell fate, escape from proteasomal degradation by their major ubiquitin-ligase Fbxw7α. Functionally, we show that Fbxw7α acts as an essential negative regulator of breast CSCs' expansion by restraining Notch activity, but the establishment of a Notch/Pin1 active circuitry opposes this effect, thus promoting breast CSCs self-renewal, tumor growth and metastasis in vivo. In human breast cancers, despite Fbxw7α expression, high levels of Pin1 sustain Notch signaling, which correlates with poor prognosis. Suppression of Pin1 holds promise in reverting aggressive phenotypes, through CSC exhaustion as well as recovered drug sensitivity carrying relevant implications for therapy of breast cancers.

Signalling through Notch receptors requires ligand-induced cleavage to release the intracellular domain, which acts as a transcriptional activator in the nucleus. Deregulated Notch1 signalling has been implicated in mammary tumorigenesis; however the mechanisms underlying Notch activation in breast cancer remain unclear. Here, we demonstrate that the prolyl-isomerase Pin1 interacts with Notch1 and affects Notch1 activation. Pin1 potentiates Notch1 cleavage by gamma-secretase, leading to an increased release of the active intracellular domain and ultimately enhancing Notch1 transcriptional and tumorigenic activity. We found that Notch1 directly induces transcription of Pin1, thereby generating a positive loop. In human breast cancers, we observed a strong correlation between Pin1 overexpression and high levels of activated Notch1. Thus, the molecular circuitry established by Notch1 and Pin1 may have a key role in cancer.

During neurogenesis, neural stem/progenitor cells (NPCs) undergo an irreversible fate transition to become neurons. The Notch pathway is important for this process, and repression of Notch-dependent Hes genes is essential for triggering differentiation. However, Notch signaling often remains active throughout neuronal differentiation, implying a change in the transcriptional responsiveness to Notch during the neurogenic transition. We identified Bcl6, an oncogene, as encoding a proneurogenic factor that is required for proper neurogenesis of the mouse cerebral cortex. BCL6 promoted the neurogenic conversion by switching the composition of Notch-dependent transcriptional complexes at the Hes5 promoter. BCL6 triggered exclusion of the co-activator Mastermind-like 1 and recruitment of the NAD(+)-dependent deacetylase Sirt1, which was required for BCL6-dependent neurogenesis. The resulting epigenetic silencing of Hes5 led to neuronal differentiation despite active Notch signaling. Our findings suggest a role for BCL6 in neurogenesis and uncover Notch-BCL6-Sirt1 interactions that may affect other aspects of physiology and disease.

Medulloblastoma (MB) is the most common malignant brain tumor in children and among the subtypes, Group 3 MB has the worst outcome. Here, we perform an in vivo, patientspecific screen leading to the identification of Otx2 and c-MYC as strong Group 3 MB inducers. We validated our findings in human cerebellar organoids where Otx2/c-MYC give rise to MB-like organoids harboring a DNA methylation signature that clusters with human Group 3 tumors. Furthermore, we show that SMARCA4 is able to reduce Otx2/c-MYC tumorigenic activity in vivo and in human cerebellar organoids while SMARCA4 T910M, a mutant form found in human MB patients, inhibits the wild-type protein function. Finally, treatment with Tazemetostat, a EZH2-specific inhibitor, reduces Otx2/c-MYC tumorigenesis in ex vivo culture and human cerebellar organoids. In conclusion, human cerebellar organoids can be efficiently used to understand the role of genes found altered in cancer patients and represent a reliable tool for developing personalized therapies.

The development of the cerebral cortex requires the tightly coordinated generation of dozens of neuronal subtypes that will populate specific layers and areas. Recent studies have revealed how pluripotent stem cells (PSC), whether of mouse or human origin, can differentiate into a wide range of cortical neurons in vitro, which can integrate appropriately into the brain following in vivo transplantation. These models are largely artificial but recapitulate a substantial fraction of the complex temporal and regional patterning events that occur during in vivo corticogenesis. Here, we review these findings with emphasis on the new perspectives that they have brought for understanding of cortical development, evolution, and diseases.

Disrupted differentiation during development can lead to oncogenesis, but the underlying mechanisms remain poorly understood. Here we identify BCL6, a transcriptional repressor and lymphoma oncoprotein, as a pivotal factor required for neurogenesis and tumor suppression of medulloblastoma (MB). BCL6 is necessary for and capable of preventing the development of GNP-derived MB in mice, and can block the growth of human MB cells in vitro. BCL6 neurogenic and oncosuppressor effects rely on direct transcriptional repression of Gli1 and Gli2 effectors of the SHH pathway, through recruitment of BCOR corepressor and SIRT1 deacetylase. Our findings identify the BCL6/BCOR/SIRT1 complex as a potent repressor of the SHH pathway in normal and oncogenic neural development, with direct diagnostic and/or therapeutic relevance for SHH MB.

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Neurons of the cerebral cortex are organized in layers and columns. Unlike laminar patterning, the mechanisms underlying columnar organization remain largely unexplored. Here, we show that ephrin-B1 plays a key role in this process through the control of nonradial steps of migration of pyramidal neurons. In vivo gain of function of ephrin-B1 resulted in a reduction of tangential motility of pyramidal neurons, leading to abnormal neuronal clustering. Conversely, following genetic disruption of ephrin-B1, cortical neurons displayed a wider lateral dispersion, resulting in enlarged ontogenic columns. Dynamic analyses revealed that ephrin-B1 controls the lateral spread of pyramidal neurons by limiting neurite extension and tangential migration during the multipolar phase. Furthermore, we identified P-Rex1, a guanine-exchange factor for Rac3, as a downstream ephrin-B1 effector required to control migration during the multipolar phase. Our results demonstrate that ephrin-B1 inhibits nonradial migration of pyramidal neurons, thereby controlling the pattern of cortical columns.