Structural and Functional Aspects of the Neurodevelopmental Gene NR2F1: From Animal Models to Human Pathology

The assembly and maturation of the mammalian brain result from an intricate cascade of highly coordinated developmental events, such as cell proliferation, migration, and differentiation. Any impairment of this delicate multi-factorial process can lead to complex neurodevelopmental diseases, sharing common pathogenic mechanisms and molecular pathways resulting in multiple clinical signs. A recently described monogenic neurodevelopmental syndrome named Bosch-Boonstra-Schaaf Optic Atrophy Syndrome (BBSOAS) is caused by NR2F1 haploinsufficiency. The NR2F1 gene, coding for a transcriptional regulator belonging to the steroid/thyroid hormone receptor superfamily, is known to play key roles in several brain developmental processes, from proliferation and differentiation of neural progenitors to migration and identity acquisition of neocortical neurons. In a clinical context, the disruption of these cellular processes could underlie the pathogenesis of several symptoms affecting BBSOAS patients, such as intellectual disability, visual impairment, epilepsy, and autistic traits. In this review, we will introduce NR2F1 protein structure, molecular functioning, and expression profile in the developing mouse brain. Then, we will focus on Nr2f1 several functions during cortical development, from neocortical area and cell-type specification to maturation of network activity, hippocampal development governing learning behaviors, assembly of the visual system, and finally establishment of cortico-spinal descending tracts regulating motor execution. Whenever possible, we will link experimental findings in animal or cellular models to corresponding features of the human pathology. Finally, we will highlight some of the unresolved questions on the diverse functions played by Nr2f1 during brain development, in order to propose future research directions. All in all, we believe that understanding BBSOAS mechanisms will contribute to further unveiling pathophysiological mechanisms shared by several neurodevelopmental disorders and eventually lead to effective treatments.

Diversified expression of gamma-protocadherins regulates synaptic specificity in the mouse neocortex

Synaptic specificity is the basis of forming neural microcircuits. However, how a neuron chooses which neurons out of many potentials to form synapses remains largely unknown. Here we identified that the diversified expression of clustered protocadherin γs (cPCDHγs) plays an essential role in regulating such specificity. Our 5-prime end single-cell sequencing data revealed the diversified expression pattern of cPCDHγs in neocortical neurons. Whole-cell recording of neuron pairs in developing mouse brain slices showed that knocking out PCDHγs significantly increased the local connection rate of nearby pyramidal neurons. By contrast, neurons overexpressing the same group of clustered PCDHγ isoforms through in utero electroporation dramatically decreased their synaptic connectivity. Finally and more importantly, decreasing the similarity level of PCDHγ isoforms over-expressed in neuron pairs through sequential in utero electroporation led to a progressive elevation of synaptic connectivity. Our observations provide strong evidence to support that the existence of diversely expressed cPCDHγs allows a neuron to choose which neurons not to form a synapse, rather than choosing which neurons to make synapses.

Impaired KDM2B-mediated PRC1 recruitment to chromatin causes neural stem cell senescence and ASD/ID-like behavioral deficits

Autism spectrum disorder (ASD) and intellectual disability (ID) are neurodevelopmental diseases associated with various genetic mutations. Recent clinical studies report that chromosomal 12q24.31 microdeletions are associated with human ASD/ID. However, the causality and underlying mechanisms linking 12q24.31 microdeletions to ASD/ID pathogenesis remain undetermined. Here we show Kdm2b, one of the genes located in chromosomal 12q24.31, plays a critical role in maintaining neural stem cells (NSCs) in the developing mouse brain. Loss of the CxxC-ZF domain of KDM2B impairs its function in recruiting Polycomb repressive complex 1 (PRC1) to chromatin, resulting in de-repression of genes involved in cell apoptosis, cell cycle arrest, NSC premature senescence, and leading to the loss of NSC populations in the brain. Importantly, the Kdm2b mutation is sufficient to induce ASD/ID-like social and memory deficits in adult mice. Thus, our study reveals a critical role of an epigenetic factor KDM2B in normal brain development, a causality between the Kdm2b mutation and genesis of ASD/ID-like phenotypes in mice, and potential molecular mechanisms linking the function of KDM2B-PRC1 in transcriptional regulation and NSC senescence to the12q24.31 microdeletion-associated ASD/ID.

Augmin deficiency in neural stem cells causes p53-dependent apoptosis and aborts brain development

Microtubules that assemble the mitotic spindle are generated by centrosomal nucleation, chromatin-mediated nucleation, and nucleation from the surface of other microtubules mediated by the augmin complex. Impairment of centrosomal nucleation in apical progenitors of the developing mouse brain induces p53-dependent apoptosis and causes non-lethal microcephaly. Whether disruption of non-centrosomal nucleation has similar effects is unclear. Here we show, using mouse embryos, that conditional knockout of the augmin subunit Haus6 in apical progenitors led to spindle defects and mitotic delay. This triggered massive apoptosis and abortion of brain development. Co-deletion of Trp53 rescued cell death, but surviving progenitors failed to organize a pseudostratified epithelium, and brain development still failed. This could be explained by exacerbated mitotic errors and resulting chromosomal defects including increased DNA damage. Thus, in contrast to centrosomes, augmin is crucial for apical progenitor mitosis, and, even in the absence of p53, for progression of brain development.

SIRV: Spatial inference of RNA velocity at the single-cell resolution

Studying cellular differentiation using single-cell RNA sequencing (scRNA-seq) rapidly expands our understanding of cellular development processes. Recently, RNA velocity has created new possibilities in studying these cellular differentiation processes, as differentiation dynamics can be obtained from measured spliced and unspliced mRNA expression. However, to map these differentiation processes to developments within a tissue, the spatial context of the tissue should be taken into account, which is not possible with current approaches as they start from dissociated cells. We present SIRV (Spatially Inferred RNA Velocity), a method to infer spatial differentiation trajectories within the spatial context of a tissue at the single-cell resolution. SIRV integrates spatial transcriptomics data with reference scRNA-seq data, to enrich the spatially measured genes with spliced and unspliced expressions from the scRNA-seq data. Next, SIRV calculates RNA velocity vectors for every spatially measured cell and maps these vectors to the spatial coordinates within the tissue. We tested SIRV on the Developing Mouse Brain Atlas data and obtained biologically relevant spatial differentiation trajectories. Additionally, SIRV annotates spatial cells with cellular identities and the region of origin which are transferred from the annotated reference scRNA-seq data. Altogether, with SIRV, we introduce a new tool to enrich spatial transcriptomics data that can assist in understanding how tissues develop.

Loss of histone methyltransferase ASH1L in the developing mouse brain causes autistic-like behaviors

Autism spectrum disorder (ASD) is a neurodevelopmental disease associated with various gene mutations. Recent genetic and clinical studies report that mutations of the epigenetic gene ASH1L are highly associated with human ASD and intellectual disability (ID). However, the causality and underlying molecular mechanisms linking ASH1L mutations to genesis of ASD/ID remain undetermined. Here we show loss of ASH1L in the developing mouse brain is sufficient to cause multiple developmental defects, core autistic-like behaviors, and impaired cognitive memory. Gene expression analyses uncover critical roles of ASH1L in regulating gene expression during neural cell development. Thus, our study establishes an ASD/ID mouse model revealing the critical function of an epigenetic factor ASH1L in normal brain development, a causality between Ash1L mutations and ASD/ID-like behaviors in mice, and potential molecular mechanisms linking Ash1L mutations to brain functional abnormalities.