Extracellular LGALS3BP regulates neural progenitor position and relates to human cortical complexity.

Basal progenitors (BPs), including intermediate progenitors and basal radial glia, are generated from apical radial glia and are enriched in gyrencephalic species like humans, contributing to neuronal expansion. Shortly after generation, BPs delaminate towards the subventricular zone, where they further proliferate before differentiation. Gene expression alterations involved in BP delamination and function in humans are poorly understood. Here, we study the role of LGALS3BP, so far known as a cancer biomarker, which is a secreted protein enriched in human neural progenitors (NPCs). We show that individuals with LGALS3BP de novo variants exhibit altered local gyrification, sulcal depth, surface area and thickness in their cortex. Additionally, using cerebral organoids, human fetal tissues and mice, we show that LGALS3BP regulates the position of NPCs. Single-cell RNA-sequencing and proteomics reveal that LGALS3BP-mediated mechanisms involve the extracellular matrix in NPCs' anchoring and migration within the human brain. We propose that its temporal expression influences NPCs' delamination, corticogenesis and gyrification extrinsically.

The X-linked filaminopathies: Synergistic insights from clinical and molecular analysis.

The X-linked filaminopathies represent a diverse group of clinical conditions, all caused by variants in the gene FLNA. FLNA encodes the widely expressed actin binding protein, filamin A that has multiple roles during embryonic development including cell migration, mechanical sensing, and cell signaling. In this review, we discuss the 10 distinct X-linked filaminopathy conditions that between them affect almost all organ systems, including the brain, skeleton, heart, and skin, highlighting the critical role of this protein in human development. We review each of the phenotypes and discuss their pathogenesis, where known. Assigning pathogenicity to variants in FLNA can prove difficult, especially for missense variants and small indels, in-part because of the X-linked nature of the phenotypes, the overlap of phenotypic features between conditions, and poor understanding of the function of certain protein domains. We outline here approaches to characterize phenotypes, highlight hotspot regions within FLNA commonly mutated in these conditions, and approaches to resolving some variants of uncertain significance.

Altered neuronal migratory trajectories in human cerebral organoids derived from individuals with neuronal heterotopia.

Malformations of the human cortex represent a major cause of disability1. Mouse models with mutations in known causal genes only partially recapitulate the phenotypes and are therefore not unlimitedly suited for understanding the molecular and cellular mechanisms responsible for these conditions2. Here we study periventricular heterotopia (PH) by analyzing cerebral organoids derived from induced pluripotent stem cells (iPSCs) of patients with mutations in the cadherin receptor-ligand pair DCHS1 and FAT4 or from isogenic knockout (KO) lines1,3. Our results show that human cerebral organoids reproduce the cortical heterotopia associated with PH. Mutations in DCHS1 and FAT4 or knockdown of their expression causes changes in the morphology of neural progenitor cells and result in defective neuronal migration dynamics only in a subset of neurons. Single-cell RNA-sequencing (scRNA-seq) data reveal a subpopulation of mutant neurons with dysregulated genes involved in axon guidance, neuronal migration and patterning. We suggest that defective neural progenitor cell (NPC) morphology and an altered navigation system in a subset of neurons underlie this form of PH.

Cohesin and CTCF differentially regulate spatiotemporal runx1 expression during zebrafish development.

Runx1 is a transcription factor essential for definitive hematopoiesis. In all vertebrates, the Runx1 gene is transcribed from two promoters: a proximal promoter (P2), and a distal promoter (P1). We previously found that runx1 expression in a specific hematopoietic cell population in zebrafish embryos depends on cohesin. Here we show that zebrafish runx1 is directly bound by cohesin and CCCTC binding factor (CTCF) at the P1 and P2 promoters, and within the intron between P1 and P2. Cohesin initiates expression of runx1 in the posterior lateral mesoderm and influences promoter use, while CTCF represses its expression in the newly emerging cells of the tail bud. The intronic binding sites for cohesin and CTCF coincide with histone modifications that confer enhancer-like properties, and two of the cohesin/CTCF sites behaved as insulators in an in vivo assay. The identified cohesin and CTCF binding sites are likely to be cis-regulatory elements (CREs) for runx1 since they also recruit RNA polymerase II (RNAPII). CTCF depletion excluded RNAPII from two intronic CREs but not the promoters of runx1. We propose that cohesin and CTCF have distinct functions in the regulation of runx1 during zebrafish embryogenesis, and that these regulatory functions are likely to involve runx1 intronic CREs. Cohesin (but not CTCF) depletion enhanced RUNX1 expression in a human leukemia cell line, suggesting conservation of RUNX1 regulation through evolution.