The choroid plexus secretes cerebrospinal fluid and is critical for the development and function of the brain. In the telencephalon, the choroid plexus epithelium arises from the Wnt- expressing cortical hem. Canonical Wnt signaling pathway molecules such as nuclear β-CATENIN are expressed in the mouse and human embryonic choroid plexus epithelium indicating that this pathway is active. Point mutations in human β-CATENIN are known to result in the constitutive activation of canonical Wnt signaling. In a mouse model that recapitulates this perturbation, we report a loss of choroid plexus epithelial identity and an apparent transformation of this tissue to a neuronal identity. Aspects of this phenomenon are recapitulated in human embryonic stem cell derived organoids. The choroid plexus is also disrupted when β-Catenin is conditionally inactivated. Together, our results indicate that canonical Wnt signaling is required in a precise and regulated manner for normal choroid plexus development in the mammalian brain.

PRDM16 is a dynamic transcriptional regulator of various stem cell niches, including adipocytic, hematopoietic, cardiac progenitors, and neural stem cells. PRDM16 has been suggested to contribute to 1p36 deletion syndrome, one of the most prevalent subtelomeric microdeletion syndromes. We report a patient with a de novo nonsense mutation in the PRDM16 coding sequence, accompanied by lissencephaly and microcephaly features. Human stem cells were genetically modified to mimic this mutation, generating cortical organoids that exhibited altered cell cycle dynamics. RNA sequencing of cortical organoids at day 32 unveiled changes in cell adhesion and WNT-signaling pathways. ChIP-seq of PRDM16 identified binding sites in postmortem human fetal cortex, indicating the conservation of PRDM16 binding to developmental genes in mice and humans, potentially at enhancer sites. A shared motif between PRDM16 and LHX2 was identified and further examined through comparison with LHX2 ChIP-seq data from mice. These results suggested a collaborative partnership between PRDM16 and LHX2 in regulating a common set of genes and pathways in cortical radial glia cells, possibly via their synergistic involvement in cortical development.

The dentate gyrus, a gateway for input to the hippocampal formation, arises from progenitors in the medial telencephalic neuroepithelium adjacent to the cortical hem. Dentate progenitors navigate a complex migratory path guided by two cell populations that arise from the hem, the fimbrial glia, and Cajal-Retzius (CR) cells. Since the hem expresses multiple Wnt genes, we examined whether β-CATENIN, which mediates canonical Wnt signaling and also participates in cell adhesion, is necessary for the development of hem-derived lineages. We report that the fimbrial glial scaffold is disorganized and CR cells are mispositioned upon hem-specific disruption of β-CATENIN. Consequently, the dentate migratory stream is severely affected, and the dentate gyrus fails to form. Using selective Cre drivers, we further determined that β-CATENIN function is required in the fimbrial glial scaffold, but not in the CR cells, for guiding the dentate migration. Our findings highlight a primary requirement for β-CATENIN for the organization of the fimbrial scaffold and a secondary role for this factor in dentate gyrus morphogenesis.

The dentate gyrus, a gateway for input to the hippocampal formation, arises from progenitors in the medial telencephalic neuroepithelium adjacent to the cortical hem. Dentate progenitors navigate a complex migratory path guided by two cell populations that arise from the hem, the fimbrial glia, and Cajal-Retzius (CR) cells. Since the hem expresses multiple Wnt genes, we examined whether β-CATENIN, which mediates canonical Wnt signaling and also participates in cell adhesion, is necessary for the development of hem-derived lineages. We report that the fimbrial glial scaffold is disorganized and CR cells are mispositioned upon hem-specific disruption of β-CATENIN. Consequently, the dentate migratory stream is severely affected, and the dentate gyrus fails to form. Using selective Cre drivers, we further determined that β-CATENIN function is required in the fimbrial glial scaffold, but not in the CR cells, for guiding the dentate migration. Our findings highlight a primary requirement for β-CATENIN for the organization of the fimbrial scaffold and a secondary role for this factor in dentate gyrus morphogenesis.

The choroid plexus (CP) secretes cerebrospinal fluid and is critical for the development and function of the brain. In the telencephalon, the CP epithelium (CPe) arises from the Wnt- and Bmp- expressing cortical hem. We examined the role of canonical Wnt signaling in CPe development and report that the mouse and human embryonic CPe expresses molecules in this pathway. Either loss of function or constitutive activation of β-catenin, a key mediator of canonical Wnt signaling, causes a profound disruption of mouse CPe development. Loss of β-catenin results in a dysmorphic CPe, while constitutive activation of β-catenin causes a loss of CPe identity and a transformation of this tissue to a hippocampal-like identity. Aspects of this phenomenon are recapitulated in human embryonic stem cell (hESC)-derived organoids. Our results indicate that canonical Wnt signaling is required in a precisely regulated manner for normal CP development in the mammalian brain.