Abnormal cell sorting and altered early neurogenesis in a human cortical organoid model of Protocadherin-19 clustering epilepsy.

Introduction: Protocadherin-19 (PCDH19)-Clustering Epilepsy (PCE) is a developmental and epileptic encephalopathy caused by loss-of-function variants of the PCDH19 gene on the X-chromosome. PCE affects females and mosaic males while male carriers are largely spared. Mosaic expression of the cell adhesion molecule PCDH19 due to random X-chromosome inactivation is thought to impair cell-cell interactions between mutant and wild type PCDH19-expressing cells to produce the disease. Progress has been made in understanding PCE using rodent models or patient induced pluripotent stem cells (iPSCs). However, rodents do not faithfully model key aspects of human brain development, and patient iPSC models are limited by issues with random X-chromosome inactivation. Methods: To overcome these challenges and model mosaic PCDH19 expression in vitro, we generated isogenic female human embryonic stem cells with either HA-FLAG-tagged PCDH19 (WT) or homozygous PCDH19 knockout (KO) using genome editing. We then mixed GFP-labeled WT and RFP-labeled KO cells and generated human cortical organoids (hCOs). Results: We found that PCDH19 is highly expressed in early (days 20-35) WT neural rosettes where it co-localizes with N-Cadherin in ventricular zone (VZ)-like regions. Mosaic PCE hCOs displayed abnormal cell sorting in the VZ with KO and WT cells completely segregated. This segregation remained robust when WT:KO cells were mixed at 2:1 or 1:2 ratios. PCE hCOs also exhibited altered expression of PCDH19 (in WT cells) and N-Cadherin, and abnormal deep layer neurogenesis. None of these abnormalities were observed in hCOs generated by mixing only WT or only KO (modeling male carrier) cells. Discussion: Our results using the mosaic PCE hCO model suggest that PCDH19 plays a critical role in human VZ radial glial organization and early cortical development. This model should offer a key platform for exploring mechanisms underlying PCE-related cortical hyperexcitability and testing of potential precision therapies.

Deriving early single-rosette brain organoids from human pluripotent stem cells.

Brain organoid methods are complicated by multiple rosette structures and morphological variability. We have developed a human brain organoid technique that generates self-organizing, single-rosette cortical organoids (SOSR-COs) with reproducible size and structure at early timepoints. Rather than patterning a 3-dimensional embryoid body, we initiate brain organoid formation from a 2-dimensional monolayer of human pluripotent stem cells patterned with small molecules into neuroepithelium and differentiated to cells of the developing dorsal cerebral cortex. This approach recapitulates the 2D to 3D developmental transition from neural plate to neural tube. Most monolayer fragments form spheres with a single central lumen. Over time, the SOSR-COs develop appropriate progenitor and cortical laminar cell types as shown by immunocytochemistry and single-cell RNA sequencing. At early time points, this method demonstrates robust structural phenotypes after chemical teratogen exposure or when modeling a genetic neurodevelopmental disorder, and should prove useful for studies of human brain development and disease modeling.

Protocol for selecting single human pluripotent stem cells using a modified micropipetter.

Single-cell clonal selection is a critical procedure for generating a homogeneous population of human pluripotent stem cells. Here, we present a protocol that repurposes the STRIPPER Micropipetter, normally used for in vitro fertilization, to pick single stem cells. We describe steps for tool and reagent preparation, single-cell picking, and colony passaging. We then detail procedures for amplification and analysis. Our protocol does not require cell sorting and produces homogenous clonal cultures with more than 50% survival rate. For complete details on the use and execution of this protocol, please refer to Deng et al.1.

Loss of POGZ alters neural differentiation of human embryonic stem cells.

POGZ is a pogo transposable element derived protein with multiple zinc finger domains. Many de novo loss-of-function (LoF) variants of the POGZ gene are associated with autism and other neurodevelopmental disorders. However, the role of POGZ in human cortical development remains poorly understood. Here we generated multiple POGZ LoF lines in H9 human embryonic stem cells (hESCs) using CRISPR/CAS9 genome editing. These lines were then differentiated into neural structures, similar to those found in early to mid-fetal human brain, a critical developmental stage for studying disease mechanisms of neurodevelopmental disorders. We found that the loss of POGZ reduced neural stem cell proliferation in excitatory cortex-patterned neural rosettes, structures analogous to the cortical ventricular zone in human fetal brain. As a result, fewer intermediate progenitor cells and early born neurons were generated. In addition, neuronal migration from the apical center to the basal surface of neural rosettes was perturbed due to the loss of POGZ. Furthermore, cortical-like excitatory neurons derived from multiple POGZ homozygous knockout lines exhibited a more simplified dendritic architecture compared to wild type lines. Our findings demonstrate how POGZ regulates early neurodevelopment in the context of human cells, and provide further understanding of the cellular pathogenesis of neurodevelopmental disorders associated with POGZ variants.