Secondary loss of miR-3607 reduced cortical progenitor amplification during rodent evolution

The evolutionary expansion and folding of the mammalian cerebral cortex resulted from amplification of progenitor cells during embryonic development. This process was reversed in the rodent lineage after splitting from primates, leading to smaller and smooth brains. Genetic mechanisms underlying this secondary loss in rodent evolution remain unknown. We show that microRNA miR-3607 is expressed embryonically in the large cortex of primates and ferret, distant from the primate-rodent lineage, but not in mouse. Experimental expression of miR-3607 in embryonic mouse cortex led to increased Wnt/β-catenin signaling, amplification of radial glia cells (RGCs), and expansion of the ventricular zone (VZ), via blocking the β-catenin inhibitor APC (adenomatous polyposis coli). Accordingly, loss of endogenous miR-3607 in ferret reduced RGC proliferation, while overexpression in human cerebral organoids promoted VZ expansion. Our results identify a gene selected for secondary loss during mammalian evolution to limit RGC amplification and, potentially, cortex size in rodents.This work was supported by Santiago Grisolía predoctoral fellowship (K.C.), Generalitat Valenciana I+D+i programs grant APOSTD/2019/059 (A.C.), Fundación Tatiana Pérez de Guzmán el Bueno predoctoral fellowship (A.P.-C.), Agencia Estatal de Investigación SVP-2014-068671 (A.V.), Spanish State Research Agency FPI contract (R.S.), Spanish State Research Agency grant RYC-2015-18056 (J.P.L.-A.), Spanish State Research Agency grant RTI2018-102260-B-100 (J.P.L.-A.), Spanish State Research Agency grant SAF2015-69168-R (V.B.), Spanish State Research Agency grant PGC2018-102172-B-I00 (V.B.), Spanish State Research Agency “Severo Ochoa” Programme for Centers of Excellence in R&D grant SEV-2017-0723 (V.B.), and European Research Council grant 309633 (V.B.).Peer reviewe

Regulation of cerebral cortex development and expansion by MIR3607

MicroRNAs (miRNAs) are a class of non-coding RNA molecules increasingly recognized to play varied roles in development, physiology and disease. The majority of miRNAs are expressed in the brain, have fast turnover rates and the ability to regulate several genes in a spatio-temporal fashion. This makes them important regulators of gene expression during the extraordinarily complex process of brain development. The identification that novel miRNAs emerged during speciation in mammalian evolution helps to understand their importance and roles in the developmental processes leading to the formation of folded brains with increasing complexity in higher mammals. Our previous transcriptomic analyses of the developing ferret cortex identified candidate genes and miRNAs differentially expressed across germinal layers, with potential relevance in cerebral cortex expansion. We identified MIR3607 as highly and differentially expressed between embryonic germinal layers of the large and folded human and ferret cortex, but not expressed in the lissencephalic small mouse cortex. Experimental expression of MIR3607 in the developing cerebral cortex of mouse embryos at E14.5 affected neurogenesis, and transcriptomic profiling revealed increased Wnt/βCatenin signaling and decreased apical adhesion as the major underlying factors. Expression of MIR3607 at E12.5, when progenitor cells expand, dramatically effected amplification and delamination of apical progenitors, as predicted, leading to rosette formation. This was rescued by co-expressing Adenomatous Polyposis Coli (APC), repressor of canonical Wnt signaling and a direct target of MIR3607. A similar phenotype was produced in human cerebral organoids, indicating the conservation of this function. Experiments of loss of MIR3607 function in ferret severely impaired polarity of apical progenitor cells and induced their delamination and ectopic mitosis, defects phenocopied by overexpressing APC. Our findings demonstrate that MIR3607 activates Wnt/βCatenin signaling in apical progenitor cells, promoting their amplification and the sustained expansion of the Ventricular Zone to form a large and complex cerebral cortex in higher mammals.Peer reviewe

Primary Cilia Influence Progenitor Function during Cortical Development

International audienceCorticogenesis is an intricate process controlled temporally and spatially by many intrinsic and extrinsic factors. Alterations during this important process can lead to severe cortical malformations. Apical neuronal progenitors are essential cells able to self-amplify and also generate basal progenitors and/or neurons. Apical radial glia (aRG) are neuronal progenitors with a unique morphology. They have a long basal process acting as a support for neuronal migration to the cortical plate and a short apical process directed towards the ventricle from which protrudes a primary cilium. This antenna-like structure allows aRG to sense cues from the embryonic cerebrospinal fluid (eCSF) helping to maintain cell shape and to influence several key functions of aRG such as proliferation and differentiation. Centrosomes, major microtubule organising centres, are crucial for cilia formation. In this review, we focus on how primary cilia influence aRG function during cortical development and pathologies which may arise due to defects in this structure. Reporting and cataloguing a number of ciliary mutant models, we discuss the importance of primary cilia for aRG function and cortical development

Centrosome Inheritance Does Not Regulate Cell Fate in Granule Neuron Progenitors of the Developing Cerebellum

An inherent asymmetry exists between the two centrosomes of a dividing cell. One centrosome is structurally more mature (mother centrosome) than the other (daughter centrosome). Post division, one daughter cell inherits the mother centrosome while the other daughter cell inherits the daughter centrosome. Remarkably, the kind of centrosome inherited is associated with cell fate in several developmental contexts such as in radial glial progenitors in the developing mouse cortex, Drosophila neuroblast divisions and in Drosophila male germline stem cells. However, the role of centrosome inheritance in granule neuron progenitors in the developing cerebellum has not been investigated. Here, we show that mother and daughter centrosomes do exist in these progenitors, and the amount of pericentriolar material (PCM) each centrosome possesses is different. However, we failed to observe any correlation between the fate adopted by the daughter cell and the nature of centrosome it inherited

At the intersection between healthy and diseased embryonic development

The size and organization of the brain are determined by the activity of progenitor cells early in development. Key mechanisms regulating progenitor cell biology involve miRNAs. These small noncoding RNA molecules bind mRNAs with high specificity, controlling their abundance and expression. The role of miRNAs in brain development has been studied extensively, but their involvement at early stages remained unknown until recently. Here, recent findings showing the important role of miRNAs in the earliest phases of brain development are reviewed, and it is discussed how loss of specific miRNAs leads to pathological conditions, particularly adult and pediatric brain tumors. Let-7 miRNA downregulation and the initiation of embryonal tumors with multilayered rosettes (ETMR), a novel link recently discovered by the laboratory, are focused upon. Finally, it is discussed how miRNAs may be used for the diagnosis and therapeutic treatment of pediatric brain tumors, with the hope of improving the prognosis of these devastating diseases.A.P. was funded by a predoctoral fellowship from Fundación Tatiana Pérez de Guzmán el Bueno. Work in the lab was supported by grants from the European Research Council (309633) and the Spanish State Research Agency (PGC2018-102172-B-I00, as well as through the “Severo Ochoa” Programme for Centers of Excellence in R&D, ref. SEV-2017-0723).Peer reviewe

The centrosome protein AKNA regulates neurogenesis via microtubule organization

The expansion of brain size is accompanied by a relative enlargement of the subventricular zone during development. Epithelial-like neural stem cells divide in the ventricular zone at the ventricles of the embryonic brain, self-renew and generate basal progenitors(1) that delaminate and settle in the subventricular zone in enlarged brain regions(2). The length of time that cells stay in the subventricular zone is essential for controlling further amplification and fate determination. Here we show that the interphase centrosome protein AKNA has a key role in this process. AKNA localizes at the subdistal appendages of the mother centriole in specific subtypes of neural stem cells, and in almost all basal progenitors. This protein is necessary and sufficient to organize centrosomal microtubules, and promote their nucleation and growth. These features of AKNA are important for mediating the delamination process in the formation of the subventricular zone. Moreover, AKNA regulates the exit from the subventricular zone, which reveals the pivotal role of centrosomal microtubule organization in enabling cells to both enter and remain in the subventricular zone. The epithelial-to-mesenchymal transition is also regulated by AKNA in other epithelial cells, demonstrating its general importance for the control of cell delamination