Neurogenesis

GABAergic neurons are essential cellular components of neural circuits. Their abundance and diversity have increased significantly in the human brain, contributing to the expanded cognitive capacity of humans1. However, the developmental mechanism underlying the extended production of GABAergic neurons in the human brain remains elusive. Here we uncovered the microglial regulation of the sustained proliferation of GABAergic progenitors and neuroblasts in the human medial ganglionic eminence (hMGE). We showed that microglia are preferentially distributed in the proliferating zone and identified insulin-like growth factor 1 (IGF1) and its receptor IGR1R as the predicted top ligand-receptor pair underlying microglia-progenitor communication in the prenatal hMGE. Using our newly developed neuroimmune hMGE organoids, which mimic the hMGE cytoarchitecture and developmental trajectory, we demonstrated that microglia-derived IGF1 promotes progenitor proliferation and production of GABAergic neurons. Conversely, IGF1-neutralizing antibodies and IGF1 knockout human embryonic stem-cell-induced microglia abolish the induced microglia-mediated progenitor proliferation. Together, these findings revealed a previously unappreciated role of microglia-derived IGF1 in promoting the proliferation of neural progenitors and the development of GABAergic neurons in the human brain.

The enteric submucosal plexus regulates essential digestive functions, yet its neuronal composition remains incompletely understood. We identified two putative secretomotor neuron classes and a previously unrecognized submucosal intrinsic primary afferent neuron class through single-cell RNA sequencing in the mouse small intestine. Using viral-mediated labeling of each class, we uncovered their morphologies and neural projections in the submucosa-mucosa context, finding connections among all classes and an unexpected close association with enterochromaffin cells. Further transcriptome analysis at the postnatal stage and lineage tracing revealed that neuron identities in the submucosal plexus emerge through an initial binary fate split at neurogenesis, followed by phenotypic diversification, akin to the developmental process of the myenteric plexus. We propose a unified developmental framework for neuronal diversification across the gut wall. Our study offers comprehensive molecular, developmental and morphological insights into submucosal neurons, opening new avenues for exploring physiological functions, circuit dynamics and formation of the submucosal plexus.

Apolipoprotein E4 (APOE4) is the leading genetic risk factor for Alzheimer's disease. While most studies examine the role of APOE4 in aging, APOE4 causes persistent changes in brain structure as early as infancy and is associated with altered functional connectivity that extends beyond adolescence. Here, we used human induced pluripotent stem cell-derived cortical and ganglionic eminence organoids (COs and GEOs) to examine APOE4's influence during the development of cortical excitatory and inhibitory neurons. We show that APOE4 reduces cortical neurons and increases glia by promoting gliogenic transcriptional programs. In contrast, APOE4 increases proliferation and differentiation of GABAergic progenitors resulting in early and persistent increases in GABAergic neurons. Multi-electrode array recordings in assembloids revealed that APOE4 disrupts neural network function resulting in heightened excitability and synchronicity. Together, our data provide new insights on how APOE4 influences cortical neurodevelopmental processes and the establishment of functional networks.

Human cortical neurogenesis involves conserved and specialized developmental processes during a restricted window of prenatal development. Radial glia (RG) neural stem cells shape cortical cell diversity by giving rise to excitatory neurons, oligodendrocytes and astrocytes, as well as olfactory bulb interneurons (INs) and a recently characterized population of cortical INs1,2. Complex genetic programs orchestrated by transcription factor (TF) circuits govern the balance between self-renewal and differentiation, and between different cell fates3-8. Despite progress in measuring gene regulatory network activity during human cortical development9-12, functional studies are required to evaluate the roles of TFs and effector genes in human RG lineage progression. Here we establish a human primary culture system that allows sensitive discrimination of cell fate dynamics and apply single-cell CRISPR interference (CRISPRi) screening13,14 to examine the transcriptional and cell fate consequences of 44 TFs active during cortical neurogenesis. We identified several TFs with new roles in cortical neurogenesis, including ZNF219-previously uncharacterized-that represses neural differentiation and NR2E1 and ARX that have opposing roles in regulating RG lineage plasticity and progression across developmental stages. We also detected convergent effector genes downstream of multiple TFs enriched in neurodevelopmental and neuropsychiatric disorders and observed conserved mechanisms of RG lineage plasticity across primates. We further uncovered a post-mitotic role for ARX in safeguarding IN subtype specification through repressing LMO1. Our study provides a framework for dissecting regulatory networks driving cell fate consequences during human neurogenesis.