The exon junction complex component Magoh controls brain size by regulating neural stem cell division

The RNA-binding protein EIF4A3 promotes axon development by direct control of the cytoskeleton

The exon junction complex (EJC), nucleated by EIF4A3, is indispensable for mRNA fate and function throughout eukaryotes. We discover that EIF4A3 directly controls microtubules, independent of RNA, which is critical for neural wiring. While neuronal survival in the developing mouse cerebral cortex depends upon an intact EJC, axonal tract development requires only Eif4a3. Using human cortical organoids, we show that EIF4A3 disease mutations also impair neuronal growth, highlighting conserved functions relevant for neurodevelopmental pathology. Live imaging of growing neurons shows that EIF4A3 is essential for microtubule dynamics. Employing biochemistry and competition experiments, we demonstrate that EIF4A3 directly binds to microtubules, mutually exclusive of the EJC. Finally, in vitro reconstitution assays and rescue experiments demonstrate that EIF4A3 is sufficient to promote microtubule polymerization and that EIF4A3-microtubule association is a major contributor to axon growth. This reveals a fundamental mechanism by which neurons re-utilize core gene expression machinery to directly control the cytoskeleton.

Entosis implicates a new role for P53 in microcephaly pathogenesis, beyond apoptosis

Entosis, a form of cell cannibalism, is a newly discovered pathogenic mechanism leading to the development of small brains, termed microcephaly, in which P53 activation was found to play a major role. Microcephaly with entosis, found in Pals1 mutant mice, displays P53 activation that promotes entosis and apoptotic cell death. This previously unappreciated pathogenic mechanism represents a novel cellular dynamic in dividing cortical progenitors which is responsible for cell loss. To date, various recent models of microcephaly have bolstered the importance of P53 activation in cell death leading to microcephaly. P53 activation caused by mitotic delay or DNA damage manifests apoptotic cell death which can be suppressed by P53 removal in these animal models. Such genetic studies attest P53 activation as quality control meant to eliminate genomically unfit cells with minimal involvement in the actual function of microcephaly associated genes. In this review, we summarize the known role of P53 activation in a variety of microcephaly models and introduce a novel mechanism wherein entotic cell cannibalism in neural progenitors is triggered by P53 activation.

Epistatic interactions between NMD and TRP53 control progenitor cell maintenance and brain size

Mutations in human nonsense-mediated mRNA decay (NMD) factors are enriched in neurodevelopmental disorders. We show that deletion of key NMD factor Upf2 in mouse embryonic neural progenitor cells causes perinatal microcephaly but deletion in immature neurons does not, indicating NMD's critical roles in progenitors. Upf2 knockout (KO) prolongs the cell cycle of radial glia progenitor cells, promotes their transition into intermediate progenitors, and leads to reduced upper-layer neurons. CRISPRi screening identified Trp53 knockdown rescuing Upf2KO progenitors without globally reversing NMD inhibition, implying marginal contributions of most NMD targets to the cell cycle defect. Integrated functional genomics shows that NMD degrades selective TRP53 downstream targets, including Cdkn1a, which, without NMD suppression, slow the cell cycle. Trp53KO restores the progenitor cell pool and rescues the microcephaly of Upf2KO mice. Therefore, one physiological role of NMD in the developing brain is to degrade selective TRP53 targets to control progenitor cell cycle and brain size.

Messenger RNA Surveillance: Current Understanding, Regulatory Mechanisms, and Future Implications

Nonsense-mediated mRNA decay (NMD) is an evolutionarily conserved surveillance mechanism in eukaryotes primarily deployed to ensure RNA quality control by eliminating aberrant transcripts and also involved in modulating the expression of several physiological transcripts. NMD, the mRNA surveillance pathway, is a major form of gene regulation in eukaryotes. NMD serves as one of the most significant quality control mechanisms as it primarily scans the newly synthesized transcripts and differentiates the aberrant and non-aberrant transcripts. The synthesis of truncated proteins is restricted, which would otherwise lead to cellular dysfunctions. The up-frameshift factors (UPFs) play a central role in executing the NMD event, largely by recognizing and recruiting multiple protein factors that result in the decay of non-physiological mRNAs. NMD exhibits astounding variability in its ability across eukaryotes in an array of pathological and physiological contexts. The detailed understanding of NMD and the underlying molecular mechanisms remains blurred. This review outlines our current understanding of NMD, in regulating multifaceted cellular events during development and disease. It also attempts to identify unanswered questions that deserve further investigation.

P53 independent pathogenic mechanisms contribute to BubR1 microcephaly

The mosaic variegated aneuploidy (MVA)-associated gene Budding Uninhibited by Benzimidazole 1B (BUB1B) encodes BUBR1, a core member of the spindle assembly checkpoint complex that ensures kinetochore-spindle attachment for faithful chromosome segregation. BUB1B mutation in humans and its deletion in mice cause microcephaly. In the absence of BubR1 in mice, massive cell death reduces cortical cells during neurogenesis. However, the molecular and cellular mechanisms triggering cell death are unknown. In this study, we performed three-dimensional imaging analysis of mitotic BubR1-deficient neural progenitors in a murine model to show profound chromosomal segregation defects and structural abnormalities. Chromosomal defects and accompanying DNA damage result in P53 activation and apoptotic cell death in BubR1 mutants. To test whether the P53 cell death pathway is responsible for cortical cell loss, we co-deleted Trp53 in BubR1-deficient cortices. Remarkably, we discovered that residual apoptotic cell death remains in double mutants lacking P53, suggesting P53-independent apoptosis. Furthermore, the minimal rescue of cortical size and cortical neuron numbers in double mutant mice suggests the compelling extent of alternative death mechanisms in the absence of P53. This study demonstrates a potential pathogenic mechanism for microcephaly in MVA patients and uncovers the existence of powerful means of eliminating unfit cells even when the P53 death pathway is disabled.

DNA damage and repair: underlying mechanisms leading to microcephaly

DNA-damaging agents and endogenous DNA damage constantly harm genome integrity. Under genotoxic stress conditions, the DNA damage response (DDR) machinery is crucial in repairing lesions and preventing mutations in the basic structure of the DNA. Different repair pathways are implicated in the resolution of such lesions. For instance, the non-homologous DNA end joining and homologous recombination pathways are central cellular mechanisms by which eukaryotic cells maintain genome integrity. However, defects in these pathways are often associated with neurological disorders, indicating the pivotal role of DDR in normal brain development. Moreover, the brain is the most sensitive organ affected by DNA-damaging agents compared to other tissues during the prenatal period. The accumulation of lesions is believed to induce cell death, reduce proliferation and premature differentiation of neural stem and progenitor cells, and reduce brain size (microcephaly). Microcephaly is mainly caused by genetic mutations, especially genes encoding proteins involved in centrosomes and DNA repair pathways. However, it can also be induced by exposure to ionizing radiation and intrauterine infections such as the Zika virus. This review explains mammalian cortical development and the major DNA repair pathways that may lead to microcephaly when impaired. Next, we discuss the mechanisms and possible exposures leading to DNA damage and p53 hyperactivation culminating in microcephaly.

New insights into the centrosome-associated spliceosome components as regulators of ciliogenesis and tissue identity

Biomolecular condensates are membrane-less assemblies of proteins and nucleic acids. Centrosomes are biomolecular condensates that play a crucial role in nuclear division, cytoskeletal remodeling, and cilia formation in animal cells. Spatial omics technology is providing new insights into the dynamic exchange of spliceosome components between the nucleus and the centrosome/cilium. Intriguingly, centrosomes are emerging as cytoplasmic sites for information storage, enriched with RNA molecules and RNA-processing proteins. Furthermore, growing evidence supports the view that nuclear spliceosome components assembled at the centrosome function as potential coordinators of splicing subprograms, pluripotency, and cell differentiation. In this article, we first discuss the current understanding of the centrosome/cilium complex, which controls both stem cell differentiation and pluripotency. We next explore the molecular mechanisms that govern splicing factor assembly and disassembly around the centrosome and examine how RNA processing pathways contribute to ciliogenesis. Finally, we discuss numerous unresolved compelling questions regarding the centrosome-associated spliceosome components and transcript variants within the cytoplasm as sources of RNA-based secondary messages in the regulation of cell identity and cell fate determination. This article is categorized under: RNA-Based Catalysis > RNA Catalysis in Splicing and Translation RNA Interactions with Proteins and Other Molecules > RNA-Protein Complexes RNA Processing > Splicing Regulation/Alternative Splicing RNA Processing > RNA Processing.

The exon junction complex component EIF4A3 is essential for mouse and human cortical progenitor mitosis and neurogenesis

Mutations in components of the exon junction complex (EJC) are associated with neurodevelopment and disease. In particular, reduced levels of the RNA helicase EIF4A3 cause Richieri-Costa-Pereira syndrome (RCPS) and copy number variations are linked to intellectual disability. Consistent with this, Eif4a3 haploinsufficient mice are microcephalic. Altogether, this implicates EIF4A3 in cortical development; however, the underlying mechanisms are poorly understood. Here, we use mouse and human models to demonstrate that EIF4A3 promotes cortical development by controlling progenitor mitosis, cell fate and survival. Eif4a3 haploinsufficiency in mice causes extensive cell death and impairs neurogenesis. Using Eif4a3;p53 compound mice, we show that apoptosis has the most impact on early neurogenesis, while additional p53-independent mechanisms contribute to later stages. Live imaging of mouse and human neural progenitors reveals that Eif4a3 controls mitosis length, which influences progeny fate and viability. These phenotypes are conserved, as cortical organoids derived from RCPS iPSCs exhibit aberrant neurogenesis. Finally, using rescue experiments we show that EIF4A3 controls neuron generation via the EJC. Altogether, our study demonstrates that EIF4A3 mediates neurogenesis by controlling mitosis duration and cell survival, implicating new mechanisms that underlie EJC-mediated disorders.

A founder PPIL1 variant underlies a recognizable form of microlissencephaly with pontocerebellar hypoplasia

Biallelic variants in PPIL1 have been recently found to cause a very rare type of pontocerebellar hypoplasia and congenital microcephaly in which simplified gyral pattern was not observed in all of the patients. Here, we describe a series of nine patients from eight unrelated Egyptian families in whom whole exome sequencing detected a previously reported homozygous missense variant (c.295G>A, p.Ala99Thr) in PPIL1. Haplotype analysis confirmed that this variant has a founder effect in our population. All our patients displayed early onset drug-resistant epilepsy, profound developmental delay, and visual impairment. Remarkably, they presented with recognizable imaging findings showing profound microcephaly, hypoplastic frontal lobe and posteriorly predominant pachygyria, agenesis of corpus callosum with colpocephaly, and pontocerebellar hypoplasia. In addition, Dandy-Walker malformation was evident in three patients. Interestingly, four of our patients exhibited hematopoietic disorder (44% of cases). We compared the phenotype of our patients with other previously reported PPIL1 patients. Our results reinforce the hypothesis that the alterative splicing of PPIL1 causes a heterogeneous phenotype. Further, we affirm that hematopoietic disorder is a common feature of the condition and underscore the role of major spliceosomes in brain development.

The cilium-centrosome axis in coupling cell cycle exit and cell fate

The centrosome is an evolutionarily conserved, ancient organelle whose role in cell division was first described over a century ago. The structure and function of the centrosome as a microtubule-organizing center, and of its extracellular extension - the primary cilium - as a sensory antenna, have since been extensively studied, but the role of the cilium-centrosome axis in cell fate is still emerging. In this Opinion piece, we view cellular quiescence and tissue homeostasis from the vantage point of the cilium-centrosome axis. We focus on a less explored role in the choice between distinct forms of mitotic arrest - reversible quiescence and terminal differentiation, which play distinct roles in tissue homeostasis. We outline evidence implicating the centrosome-basal body switch in stem cell function, including how the cilium-centrosome complex regulates reversible versus irreversible arrest in adult skeletal muscle progenitors. We then highlight exciting new findings in other quiescent cell types that suggest signal-dependent coupling of nuclear and cytoplasmic events to the centrosome-basal body switch. Finally, we propose a framework for involvement of this axis in mitotically inactive cells and identify future avenues for understanding how the cilium-centrosome axis impacts central decisions in tissue homeostasis.

Exon architecture controls mRNA m6A suppression and gene expression

N6-methyladenosine (m6A) is the most abundant messenger RNA (mRNA) modification and plays crucial roles in diverse physiological processes. Using a massively parallel assay for m6A (MPm6A), we discover that m6A specificity is globally regulated by suppressors that prevent m6A deposition in unmethylated transcriptome regions. We identify exon junction complexes (EJCs) as m6A suppressors that protect exon junction-proximal RNA within coding sequences from methylation and regulate mRNA stability through m6A suppression. EJC suppression of m6A underlies multiple global characteristics of mRNA m6A specificity, with the local range of EJC protection sufficient to suppress m6A deposition in average-length internal exons but not in long internal and terminal exons. EJC-suppressed methylation sites colocalize with EJC-suppressed splice sites, which suggests that exon architecture broadly determines local mRNA accessibility to regulatory complexes.

Multifaceted roles of MAGOH Proteins

MAGOH and MAGOHB are paralog proteins that can substitute each other in the exon junction complex (EJC). The EJC is formed of core components EIF4A3, RBM8A, and MAGOH/MAGOHB. As a part of the EJC, MAGOH proteins are required for mRNA splicing, export, translation and nonsense-mediated mRNA decay (NMD). MAGOH is also essential for embryonic development and normal cellular functioning. The haploinsufficiency of MAGOH results in disorders such as microcephaly and cancer. The present review discusses the discovery of MAGOH, its paralog MAGOHB, their roles in cellular function as part of the EJC, and other cellular roles that are not directly associated with mRNA processing. We also discuss how MAGOH haploinsufficiency in cancer cells can be exploited to develop a novel targeted cancer treatment.

The exon junction complex component EIF4A3 is essential for mouse and human cortical progenitor mitosis and neurogenesis

Mutations in components of the exon junction complex (EJC) are associated with neurodevelopment and disease. In particular, reduced levels of the RNA helicase EIF4A3 cause Richieri-Costa-Pereira Syndrome (RCPS) and CNVs are linked to intellectual disability. Consistent with this, Eif4a3 haploinsufficient mice are microcephalic. Altogether, this implicates EIF4A3 in cortical development; however, the underlying mechanisms are poorly understood. Here, we use mouse and human models to demonstrate that EIF4A3 promotes cortical development by controlling progenitor mitosis, cell fate, and survival. Eif4a3 haploinsufficiency in mice causes extensive cell death and impairs neurogenesis. Using Eif4a3 ; p53 compound mice, we show that apoptosis is most impactful for early neurogenesis, while additional p53-independent mechanisms contribute to later stages. Live imaging of mouse and human neural progenitors reveals Eif4a3 controls mitosis length, which influences progeny fate and viability. These phenotypes are conserved as cortical organoids derived from RCPS iPSCs exhibit aberrant neurogenesis. Finally, using rescue experiments we show that EIF4A3 controls neuron generation via the EJC. Altogether, our study demonstrates that EIF4A3 mediates neurogenesis by controlling mitosis duration and cell survival, implicating new mechanisms underlying EJC-mediated disorders.

Summary statement

This study shows that EIF4A3 mediates neurogenesis by controlling mitosis duration in both mouse and human neural progenitors, implicating new mechanisms underlying neurodevelopmental disorders.

An entosis-like process induces mitotic disruption in Pals1 microcephaly pathogenesis

Entosis is cell cannibalism utilized by tumor cells to engulf live neighboring cells for pro- or anti-tumorigenic purposes. It is unknown whether this extraordinary cellular event can be pathogenic in other diseases such as microcephaly, a condition characterized by a smaller than normal brain at birth. We find that mice mutant for the human microcephaly-causing gene Pals1, which exhibit diminished cortices due to massive cell death, also exhibit nuclei enveloped by plasma membranes inside of dividing cells. These cell-in-cell (CIC) structures represent a dynamic process accompanied by lengthened mitosis and cytokinesis abnormalities. As shown in tumor cells, ROCK inhibition completely abrogates CIC structures and restores the normal length of mitosis. Moreover, genetic elimination of Trp53 produces a remarkable rescue of cortical size along with substantial reductions of CIC structures and cell death. These results provide a novel pathogenic mechanism by which microcephaly is produced through entotic cell cannibalism.

An RNAi screen of RNA helicases identifies eIF4A3 as a regulator of embryonic stem cell identity

RNA helicases are involved in multiple steps of RNA metabolism to direct their roles in gene expression, yet their functions in pluripotency control remain largely unexplored. Starting from an RNA interference (RNAi) screen of RNA helicases, we identified that eIF4A3, a DEAD-box (Ddx) helicase component of the exon junction complex (EJC), is essential for the maintenance of embryonic stem cells (ESCs). Mechanistically, we show that eIF4A3 post-transcriptionally controls the pluripotency-related cell cycle regulators and that its depletion causes the loss of pluripotency via cell cycle dysregulation. Specifically, eIF4A3 is required for the efficient nuclear export of Ccnb1 mRNA, which encodes Cyclin B1, a key component of the pluripotency-promoting pathway during the cell cycle progression of ESCs. Our results reveal a previously unappreciated role for eIF4A3 and its associated EJC in maintaining stem cell pluripotency through post-transcriptional control of the cell cycle.

Progression of the pluripotent epiblast depends upon the NMD factor UPF2

Nonsense-mediated RNA decay (NMD) is a highly conserved RNA turnover pathway that degrades RNAs harboring in-frame stop codons in specific contexts. Loss of NMD factors leads to embryonic lethality in organisms spanning the phylogenetic scale, but the mechanism remains unknown. Here, we report that the core NMD factor, UPF2, is required for expansion of epiblast cells within the inner cell mass of mice in vivo. We identify NMD target mRNAs in mouse blastocysts - both canonical and alternatively processed mRNAs - including those encoding cell cycle arrest and apoptosis factors, raising the possibility that NMD is essential for embryonic cell proliferation and survival. In support, the inner cell mass of Upf2-null blastocysts rapidly regresses with outgrowth and is incompetent for embryonic stem cell derivation in vitro. In addition, we uncovered concordant temporal- and lineage-specific regulation of NMD factors and mRNA targets, indicative of a shift in NMD magnitude during peri-implantation development. Together, our results reveal developmental and molecular functions of the NMD pathway in the early embryo.

Diverse Roles of the Exon Junction Complex Factors in the Cell Cycle, Cancer, and Neurodevelopmental Disorders-Potential for Therapeutic Targeting

The exon junction complex (EJC) plays a crucial role in regulating gene expression at the levels of alternative splicing, translation, mRNA localization, and nonsense-mediated decay (NMD). The EJC is comprised of three core proteins: RNA-binding motif 8A (RBM8A), Mago homolog (MAGOH), eukaryotic initiation factor 4A3 (eIF4A3), and a peripheral EJC factor, metastatic lymph node 51 (MLN51), in addition to other peripheral factors whose structural integration is activity-dependent. The physiological and mechanistic roles of the EJC in contribution to molecular, cellular, and organismal level function continue to be explored for potential insights into genetic or pathological dysfunction. The EJC’s specific role in the cell cycle and its implications in cancer and neurodevelopmental disorders prompt enhanced investigation of the EJC as a potential target for these diseases. In this review, we highlight the current understanding of the EJC’s position in the cell cycle, its relation to cancer and developmental diseases, and potential avenues for therapeutic targeting.

DEPCOD: a tool to detect and visualize co-evolution of protein domains

Proteins with similar phylogenetic patterns of conservation or loss across evolutionary taxa are strong candidates to work in the same cellular pathways or engage in physical or functional interactions. Our previously published tools implemented our method of normalized phylogenetic sequence profiling to detect functional associations between non-homologous proteins. However, many proteins consist of multiple protein domains subjected to different selective pressures, so using protein domain as the unit of analysis improves the detection of similar phylogenetic patterns. Here we analyze sequence conservation patterns across the whole tree of life for every protein domain from a set of widely studied organisms. The resulting new interactive webserver, DEPCOD (DEtection of Phylogenetically COrrelated Domains), performs searches with either a selected pre-defined protein domain or a user-supplied sequence as a query to detect other domains from the same organism that have similar conservation patterns. Top similarities on two evolutionary scales (the whole tree of life or eukaryotic genomes) are displayed along with known protein interactions and shared complexes, pathway enrichment among the hits, and detailed visualization of sources of detected similarities. DEPCOD reveals functional relationships between often non-homologous domains that could not be detected using whole-protein sequences. The web server is accessible at http://genetics.mgh.harvard.edu/DEPCOD.

Global ubiquitinome profiling identifies NEDD4 as a regulator of Profilin 1 and actin remodelling in neural crest cells

The ubiquitin ligase NEDD4 promotes neural crest cell (NCC) survival and stem-cell like properties to regulate craniofacial and peripheral nervous system development. However, how ubiquitination and NEDD4 control NCC development remains unknown. Here we combine quantitative analysis of the proteome, transcriptome and ubiquitinome to identify key developmental signalling pathways that are regulated by NEDD4. We report 276 NEDD4 targets in NCCs and show that loss of NEDD4 leads to a pronounced global reduction in specific ubiquitin lysine linkages. We further show that NEDD4 contributes to the regulation of the NCC actin cytoskeleton by controlling ubiquitination and turnover of Profilin 1 to modulate filamentous actin polymerization. Taken together, our data provide insights into how NEDD4-mediated ubiquitination coordinates key regulatory processes during NCC development.

Here the authors combine multi-omics approaches to uncover a role for ubiquitination and the ubiquitin ligase NEDD4 in targeting the actin binding protein Profilin 1 to regulate actin polymerisation in neural crest cells.

The Physiological Roles of the Exon Junction Complex in Development and Diseases

The exon junction complex (EJC) becomes an increasingly important regulator of early gene expression in the central nervous system (CNS) and other tissues. The EJC is comprised of three core proteins: RNA-binding motif 8A (RBM8A), Mago homolog (MAGOH), eukaryotic initiation factor 4A3 (EIF4A3), and a peripheral EJC factor, metastatic lymph node 51 (MLN51), together with various auxiliary factors. The EJC is assembled specifically at exon-exon junctions on mRNAs, hence the name of the complex. The EJC regulates multiple levels of gene expression, from splicing to translation and mRNA degradation. The functional roles of the EJC have been established as crucial to the normal progress of embryonic and neurological development, with wide ranging implications on molecular, cellular, and organism level function. Dysfunction of the EJC has been implicated in multiple developmental and neurological diseases. In this review, we discuss recent progress on the EJC’s physiological roles.

Post-transcriptional and Post-translational Modifications of Primary Cilia: How to Fine Tune Your Neuronal Antenna

Primary cilia direct cellular signaling events during brain development and neuronal differentiation. The primary cilium is a dynamic organelle formed in a multistep process termed ciliogenesis that is tightly coordinated with the cell cycle. Genetic alterations, such as ciliary gene mutations, and epigenetic alterations, such as post-translational modifications and RNA processing of cilia related factors, give rise to human neuronal disorders and brain tumors such as glioblastoma and medulloblastoma. This review discusses the important role of genetics/epigenetics, as well as RNA processing and post-translational modifications in primary cilia function during brain development and cancer formation. We summarize mouse and human studies of ciliogenesis and primary cilia activity in the brain, and detail how cilia maintain neuronal progenitor populations and coordinate neuronal differentiation during development, as well as how cilia control different signaling pathways such as WNT, Sonic Hedgehog (SHH) and PDGF that are critical for neurogenesis. Moreover, we describe how post-translational modifications alter cilia formation and activity during development and carcinogenesis, and the impact of missplicing of ciliary genes leading to ciliopathies and cell cycle alterations. Finally, cilia genetic and epigenetic studies bring to light cellular and molecular mechanisms that underlie neurodevelopmental disorders and brain tumors.

An exon junction complex-independent function of Barentsz in neuromuscular synapse growth

The exon junction complex controls the translation, degradation, and localization of spliced mRNAs, and three of its core subunits also play a role in splicing. Here, we show that a fourth subunit, Barentsz, has distinct functions within and separate from the exon junction complex in Drosophila neuromuscular development. The distribution of mitochondria in larval muscles requires Barentsz as well as other exon junction complex subunits and is not rescued by a Barentsz transgene in which residues required for binding to the core subunit eIF4AIII are mutated. In contrast, interactions with the exon junction complex are not required for Barentsz to promote the growth of neuromuscular synapses. We find that the Activin ligand Dawdle shows reduced expression in barentsz mutants and acts downstream of Barentsz to control synapse growth. Both barentsz and dawdle are required in motor neurons, muscles, and glia for normal synapse growth, and exogenous Dawdle can rescue synapse growth in the absence of barentsz. These results identify a biological function for Barentsz that is independent of the exon junction complex.

The Identification and Functional Analysis of mRNA Localizing to Centrosomes

Centrosomes are multifunctional organelles tasked with organizing the microtubule cytoskeleton required for genome stability, intracellular trafficking, and ciliogenesis. Contributing to the diversity of centrosome functions are cell cycle-dependent oscillations in protein localization and post-translational modifications. Less understood is the role of centrosome-localized messenger RNA (mRNA). Since its discovery, the concept of nucleic acids at the centrosome was controversial, and physiological roles for centrosomal mRNAs remained muddled and underexplored. Over the past decades, however, transcripts, RNA-binding proteins, and ribosomes were detected at the centrosome in various organisms and cell types, hinting at a conservation of function. Indeed, recent work defines centrosomes as sites of local protein synthesis, and defined mRNAs were recently implicated in regulating centrosome functions. In this review, we summarize the evidence for the presence of mRNA at the centrosome and the current work that aims to unravel the biological functions of mRNA localized to centrosomes.

Trp53 ablation fails to prevent microcephaly in mouse pallium with impaired minor intron splicing

Minor spliceosome inhibition due to mutations in RNU4ATAC are linked to primary microcephaly. Ablation of Rnu11, which encodes a minor spliceosome snRNA, inhibits the minor spliceosome in the developing mouse pallium, causing microcephaly. There, cell cycle defects and p53-mediated apoptosis in response to DNA damage resulted in loss of radial glial cells (RGCs), underpinning microcephaly. Here, we ablated Trp53 to block cell death in Rnu11 cKO mice. We report that Trp53 ablation failed to prevent microcephaly in these double knockout (dKO) mice. We show that the transcriptome of the dKO pallium was more similar to the control compared with the Rnu11 cKO. We find aberrant minor intron splicing in minor intron-containing genes involved in cell cycle regulation, resulting in more severely impaired mitotic progression and cell cycle lengthening of RGCs in the dKO that was detected earlier than in the Rnu11 cKO. Furthermore, we discover a potential role of p53 in causing DNA damage in the developing pallium, as detection of γH2aX+ was delayed in the dKO. Thus, we postulate that microcephaly in minor spliceosome-related diseases is primarily caused by cell cycle defects.

p53 Activation in Genetic Disorders: Different Routes to the Same Destination

The tumor suppressor p53 is critical for preventing neoplastic transformation and tumor progression. Inappropriate activation of p53, however, has been observed in a number of human inherited disorders that most often affect development of the brain, craniofacial region, limb skeleton, and hematopoietic system. Genes related to these developmental disorders are essentially involved in transcriptional regulation/chromatin remodeling, rRNA metabolism, DNA damage-repair pathways, telomere maintenance, and centrosome biogenesis. Perturbation of these activities or cellular processes may result in p53 accumulation in cell cultures, animal models, and perhaps humans as well. Mouse models of several p53 activation-associated disorders essentially recapitulate human traits, and inactivation of p53 in these models can alleviate disorder-related phenotypes. In the present review, we focus on how dysfunction of the aforementioned biological processes causes developmental defects via excessive p53 activation. Notably, several disease-related genes exert a pleiotropic effect on those cellular processes, which may modulate the magnitude of p53 activation and establish or disrupt regulatory loops. Finally, we discuss potential therapeutic strategies for genetic disorders associated with p53 misactivation.

Understanding microcephaly through the study of centrosome regulation in Drosophila neural stem cells

Microcephaly is a rare, yet devastating, neurodevelopmental condition caused by genetic or environmental insults, such as the Zika virus infection. Microcephaly manifests with a severely reduced head circumference. Among the known heritable microcephaly genes, a significant proportion are annotated with centrosome-related ontologies. Centrosomes are microtubule-organizing centers, and they play fundamental roles in the proliferation of the neuronal progenitors, the neural stem cells (NSCs), which undergo repeated rounds of asymmetric cell division to drive neurogenesis and brain development. Many of the genes, pathways, and developmental paradigms that dictate NSC development in humans are conserved in Drosophila melanogaster. As such, studies of Drosophila NSCs lend invaluable insights into centrosome function within NSCs and help inform the pathophysiology of human microcephaly. This mini-review will briefly survey causative links between deregulated centrosome functions and microcephaly with particular emphasis on insights learned from Drosophila NSCs.

Eukaryotic initiation factor 4A3 inhibits Wnt/β-catenin signaling and regulates axis formation in zebrafish embryos

A key step in the activation of canonical Wnt signaling is the interaction between β-catenin and Tcf/Lefs that forms the transcription activation complex and facilitates the expression of target genes. Eukaryotic initiation factor 4A3 (EIF4A3) is an ATP-dependent DEAD box-family RNA helicase and acts as a core subunit of the exon junction complex (EJC) to control a series of RNA post-transcriptional processes. In this study, we uncover that EIF4A3 functions as a Wnt inhibitor by interfering with the formation of β-catenin/Tcf transcription activation complex. As Wnt stimulation increases, accumulated β-catenin displaces EIF4A3 from a transcriptional complex with Tcf/Lef, allowing the active complex to facilitate the expression of target genes. In zebrafish embryos, eif4a3 depletion inhibited the development of the dorsal organizer and pattern formation of the anterior neuroectoderm by increasing Wnt/β-catenin signaling. Conversely, overexpression of eif4a3 decreased Wnt/β-catenin signaling and inhibited the formation of the dorsal organizer before gastrulation. Our results reveal previously unreported roles of EIF4A3 in the inhibition of Wnt signaling and the regulation of embryonic development in zebrafish.

Exon junction complex dependent mRNA localization is linked to centrosome organization during ciliogenesis

Exon junction complexes (EJCs) mark untranslated spliced mRNAs and are crucial for the mRNA lifecycle. An imbalance in EJC dosage alters mouse neural stem cell (mNSC) division and is linked to human neurodevelopmental disorders. In quiescent mNSC and immortalized human retinal pigment epithelial (RPE1) cells, centrioles form a basal body for ciliogenesis. Here, we report that EJCs accumulate at basal bodies of mNSC or RPE1 cells and decline when these cells differentiate or resume growth. A high-throughput smFISH screen identifies two transcripts accumulating at centrosomes in quiescent cells, NIN and BICD2. In contrast to BICD2, the localization of NIN transcripts is EJC-dependent. NIN mRNA encodes a core component of centrosomes required for microtubule nucleation and anchoring. We find that EJC down-regulation impairs both pericentriolar material organization and ciliogenesis. An EJC-dependent mRNA trafficking towards centrosome and basal bodies might contribute to proper mNSC division and brain development.

Exon junction complexes (EJCs) that mark untranslated mRNA are involved in transport, translation and nonsense-mediated mRNA decay. Here the authors show centrosomal localization of EJCs which appears to be required for both the localization of NIN mRNA around centrosomes and ciliogenesis.

Mutations in Spliceosomal Genes PPIL1 and PRP17 Cause Neurodegenerative Pontocerebellar Hypoplasia with Microcephaly

Autosomal-recessive cerebellar hypoplasia and ataxia constitute a group of heterogeneous brain disorders caused by disruption of several fundamental cellular processes. Here, we identified 10 families showing a neurodegenerative condition involving pontocerebellar hypoplasia with microcephaly (PCHM). Patients harbored biallelic mutations in genes encoding the spliceosome components Peptidyl-Prolyl Isomerase Like-1 (PPIL1) or Pre-RNA Processing-17 (PRP17). Mouse knockouts of either gene were lethal in early embryogenesis, whereas PPIL1 patient mutation knockin mice showed neuron-specific apoptosis. Loss of either protein affected splicing integrity, predominantly affecting short and high GC-content introns and genes involved in brain disorders. PPIL1 and PRP17 form an active isomerase-substrate interaction, but we found that isomerase activity is not critical for function. Thus, we establish disrupted splicing integrity and "major spliceosome-opathies" as a new mechanism underlying PCHM and neurodegeneration and uncover a non-enzymatic function of a spliceosomal proline isomerase.

Full function of exon junction complex factor, Rbm8a, is critical for interneuron development

The formation of the nervous system requires a balance between proliferation, differentiation, and migration of neural progenitors (NPs). Mutations in genes regulating development impede neurogenesis and lead to neuropsychiatric diseases, including autism spectrum disorders (ASDs) and schizophrenia (SZ). Recently, mutations in nonsense-mediated mRNA decay genes have been associated with ASDs, intellectual disability (ID), and SZ. Here, we examine the function of a gene in the exon junction complex, Rbm8a, in the cortical development. When Rbm8a is selectively knocked out in neural stem cells, the resulting mice exhibit microcephaly, early postnatal lethality, and altered distribution of excitatory neurons in the neocortex. Moreover, Rbm8a haploinsufficiency in the central nervous system decreases cell proliferation in the ganglionic eminences. Parvalbumin+ and neuropeptide Y+ interneurons in the cortex are significantly reduced, and distribution of interneurons are altered. Consistently, neurons in the cortex of conditional knockout (cKO) mice show a significant decrease in GABA frequency. Transcriptomic analysis revealed differentially expressed genes enriched in telencephalon development and mitosis. To further investigate the role of Rbm8a in interneuron differentiation, conditional KO of Rbm8a in NKX2.1 interneuron progenitor cells reduces progenitor proliferation and alters interneuron distributions. Taken together, these data reveal a critical role of Rbm8a in interneuron development, and establish that perturbation of this gene leads to profound cortical deficits.

The Branched Nature of the Nonsense-Mediated mRNA Decay Pathway

Nonsense-mediated mRNA decay (NMD) is a conserved translation-coupled quality control mechanism in all eukaryotes that regulates the expression of a significant fraction of both the aberrant and normal transcriptomes. In vertebrates, NMD has become an essential process owing to expansion of the diversity of NMD-regulated transcripts, particularly during various developmental processes. Surprisingly, however, some core NMD factors that are essential for NMD in simpler organisms appear to be dispensable for vertebrate NMD. At the same time, numerous NMD enhancers and suppressors have been identified in multicellular organisms including vertebrates. Collectively, the available data suggest that vertebrate NMD is a complex, branched pathway wherein individual branches regulate specific mRNA subsets to fulfill distinct physiological functions.

The EJC component Magoh in non-vertebrate chordates

Earliest craniates possess a newly enlarged, elaborated forebrain with new cell types and neuronal networks. A key question in vertebrate evolution is when and how this cerebral expansion took place. The exon-junction complex (EJC) plays an essential role in mRNA processing of all Eukarya. Recently, it has been proposed that the EJC represses recursive RNA splicing in Deuterostomes, with implication in human brain diseases like microcephaly and depression. However, the EJC or EJC subunit contribution to brain development in non-vertebrate Deuterostomes remained unknown. Being interested in the evolution of chordate characters, we focused on the model species, Branchiostoma lanceolatum (Cephalochordata) and Ciona robusta (Tunicata), with the aim to investigate the ancestral and the derived expression state of Magoh orthologous genes. This study identifies that Magoh is part of a conserved syntenic group exclusively in vertebrates and suggests that Magoh has experienced duplication and loss events in mammals. During early development in amphioxus and ascidian, maternal contribution and zygotic expression of Magoh genes in various types of progenitor cells and tissues are consistent with the condition observed in other Bilateria. Later in development, we also show expression of Magoh in the brain of cephalochordate and ascidian larvae. Collectively, these results provide a basis to further define what functional role(s) Magoh exerted during nervous system development and evolution.

Acute Lengthening of Progenitor Mitosis Influences Progeny Fate during Cortical Development in vivo

BACKGROUND/AIMS

Prenatal microcephaly is posited to arise from aberrant mitosis of neural progenitors, which disrupts both neuronal production and survival. Although microcephaly has both a genetic and environmental etiology, the mechanisms by which dysregulation of mitosis causes microcephaly are poorly understood. We previously discovered that prolonged mitosis of mouse neural progenitors, either ex vivo or in vitro, directly alters progeny cell fate, -resulting in precocious differentiation and apoptosis. This raises questions as to whether prolonged progenitor mitosis affects cell fate and neurogenesis in vivo, and what are the underlying mechanisms?

METHODS/RESULTS

Towards addressing these knowledge gaps, we developed an in vivo model of mitotic delay. This uses pharmacological inhibition to acutely and reversibly prolong mitosis during cortical development, and fluorescent dyes to label direct progeny. Using this model, we discovered that a causal relationship between mitotic delay of neural progenitors and altered progeny cell fate is evident in vivo. Using transcriptome analyses to investigate the state of delayed cells and their progeny, we uncovered potential molecular mechanisms by which prolonged mitosis induces altered cell fates, including DNA damage and p53 signaling. We then extended our studies to human neural progenitors, demonstrating that lengthened mitosis duration also directly alters neuronal cell fate.

CONCLUSIONS

This study establishes a valuable new experimental paradigm towards understanding mechanisms whereby lengthened mitosis duration may explain some cases of microcephaly.

NCBP2 modulates neurodevelopmental defects of the 3q29 deletion in Drosophila and Xenopus laevis models

The 1.6 Mbp deletion on chromosome 3q29 is associated with a range of neurodevelopmental disorders, including schizophrenia, autism, microcephaly, and intellectual disability. Despite its importance towards neurodevelopment, the role of individual genes, genetic interactions, and disrupted biological mechanisms underlying the deletion have not been thoroughly characterized. Here, we used quantitative methods to assay Drosophila melanogaster and Xenopus laevis models with tissue-specific individual and pairwise knockdown of 14 homologs of genes within the 3q29 region. We identified developmental, cellular, and neuronal phenotypes for multiple homologs of 3q29 genes, potentially due to altered apoptosis and cell cycle mechanisms during development. Using the fly eye, we screened for 314 pairwise knockdowns of homologs of 3q29 genes and identified 44 interactions between pairs of homologs and 34 interactions with other neurodevelopmental genes. Interestingly, NCBP2 homologs in Drosophila (Cbp20) and X. laevis (ncbp2) enhanced the phenotypes of homologs of the other 3q29 genes, leading to significant increases in apoptosis that disrupted cellular organization and brain morphology. These cellular and neuronal defects were rescued with overexpression of the apoptosis inhibitors Diap1 and xiap in both models, suggesting that apoptosis is one of several potential biological mechanisms disrupted by the deletion. NCBP2 was also highly connected to other 3q29 genes in a human brain-specific interaction network, providing support for the relevance of our results towards the human deletion. Overall, our study suggests that NCBP2-mediated genetic interactions within the 3q29 region disrupt apoptosis and cell cycle mechanisms during development.

Rare copy-number variants, or large deletions and duplications in the genome, are associated with a wide range of neurodevelopmental disorders. The 3q29 deletion confers an increased risk for schizophrenia and autism. To understand the conserved biological mechanisms that are disrupted by this deletion, we systematically tested 14 individual homologs and 314 pairwise interactions of 3q29 genes for neuronal, cellular, and developmental phenotypes in Drosophila melanogaster and Xenopus laevis models. We found that multiple homologs of genes within the deletion region contribute towards developmental defects, such as larval lethality and disrupted cellular organization. Interestingly, we found that NCBP2 acts as a key modifier gene within the region, enhancing the developmental phenotypes of each of the homologs for other 3q29 genes and leading to disruptions in apoptosis and cell cycle pathways. Our results suggest that multiple genes within the 3q29 region interact with each other through shared mechanisms and jointly contribute to neurodevelopmental defects.

Dosage-dependent requirements of Magoh for cortical interneuron generation and survival

Embryonic interneuron development underlies cortical function and its disruption contributes to neurological disease. Yet the mechanisms by which viable interneurons are produced from progenitors remain poorly understood. Here, we demonstrate dosage-dependent requirements of the exon junction complex component Magoh for interneuron genesis in mouse. Conditional Magoh ablation from interneuron progenitors, but not post-mitotic neurons, depletes cortical interneuron number through adulthood, with increased severity in homozygotes. Using live imaging, we discover that Magoh deficiency delays progenitor mitotic progression in a dosage-sensitive fashion, with 40% of homozygous progenitors failing to divide. This shows that Magoh is required in progenitors for both generation and survival of newborn progeny. Transcriptome analysis implicates p53 signaling; moreover, p53 ablation in Magoh haploinsufficient progenitors rescues apoptosis, completely recovering interneuron number. In striking contrast, in Magoh homozygotes, p53 loss fails to rescue interneuron number and mitotic delay, further implicating mitotic defects in interneuron loss. Our results demonstrate that interneuron development is intimately dependent upon progenitor mitosis duration and uncover a crucial post-transcriptional regulator of interneuron fate relevant for neurodevelopmental pathologies.This article has an associated 'The people behind the papers' interview.

The exon junction complex is required for stem and progenitor cell maintenance in planarians

Named for its assembly near exon-exon junctions during pre-mRNA splicing, the exon junction complex (EJC) regulates multiple aspects of RNA biochemistry, including export of spliced mRNAs from the nucleus and translation. Transcriptome analyses have revealed broad EJC occupancy of spliced metazoan transcripts, yet inhibition of core subunits has been linked to surprisingly specific phenotypes and a growing number of studies support gene-specific regulatory roles. Here we report results from a classroom-based RNAi screen revealing the EJC is necessary for regeneration in the planarian flatworm Schmidtea mediterranea. RNAi animals rapidly lost the stem and progenitor cells that drive formation of new tissue during both regeneration and cell turnover, but exhibited normal amputation-induced changes in gene expression in differentiated tissues. Together with previous reports that partial loss of EJC function causes stem cell defects in Drosophila and mice, our observations implicate the EJC as a conserved, posttranscriptional regulator of gene expression in stem cell lineages. This work also highlights the combined educational and scientific impacts of discovery-based research in the undergraduate biology curriculum.

Endoplasmic reticulum and Golgi stress in microcephaly

Microcephaly is a neurodevelopmental condition characterized by a small brain size associated with intellectual deficiency in most cases and is one of the most frequent clinical sign encountered in neurodevelopmental disorders. It can result from a wide range of environmental insults occurring during pregnancy or postnatally, as well as from various genetic causes and represents a highly heterogeneous condition. However, several lines of evidence highlight a compromised mode of division of the cortical precursor cells during neurogenesis, affecting neural commitment or survival as one of the common mechanisms leading to a limited production of neurons and associated with the most severe forms of congenital microcephaly. In this context, the emergence of the endoplasmic reticulum (ER) and the Golgi apparatus as key guardians of cellular homeostasis, especially through the regulation of proteostasis, has raised the hypothesis that pathological ER and/or Golgi stress could contribute significantly to cortical impairments eliciting microcephaly. In this review, we discuss recent findings implicating ER and Golgi stress responses in early brain development and provide an overview of microcephaly-associated genes involved in these pathways.

Translating neural stem cells to neurons in the mammalian brain

The mammalian neocortex underlies our perception of sensory information, performance of motor activities, and higher-order cognition. During mammalian embryogenesis, radial glial precursor cells sequentially give rise to diverse populations of excitatory cortical neurons, followed by astrocytes and oligodendrocytes. A subpopulation of these embryonic neural precursors persists into adulthood as neural stem cells, which give rise to inhibitory interneurons and glia. Although the intrinsic mechanisms instructing the genesis of these distinct progeny have been well-studied, most work to date has focused on transcriptional, epigenetic, and cell-cycle control. Recent studies, however, have shown that posttranscriptional mechanisms also regulate the cell fate choices of transcriptionally primed neural precursors during cortical development. These mechanisms are mediated primarily by RNA-binding proteins and microRNAs that coordinately regulate mRNA translation, stability, splicing, and localization. Together, these findings point to an extensive network of posttranscriptional control and provide insight into both normal cortical development and disease. They also add another layer of complexity to brain development and raise important biological questions for future investigation.

Roles of the exon junction complex components in the central nervous system: a mini review

The exon junction complex (EJC) consists of four core proteins: Magoh, RNA-binding motif 8A (Rbm8a, also known as Y14), eukaryotic initiation factor 4A3 (eIF4A3, also known as DDX48), and metastatic lymph node 51 (MLN51, also known as Casc3 or Barentsz), which are involved in the regulation of many processes occurring between gene transcription and protein translation. Its main role is to assemble into spliceosomes at the exon-exon junction of mRNA during splicing. It is, therefore, a range of functions concerning post-splicing events such as mRNA translocation, translation, and nonsense-mediated mRNA decay (NMD). Apart from this, proteins of the EJC control the splicing of specific pre-mRNAs, for example, splicing of the mapk transcript. Recent studies support essential functions of EJC proteins in oocytes and, after fertilization, in all stages of zygote development, as well as the growth of the embryo, including the development of the nervous system. During the development of the central nervous system (CNS), the EJC controls mitosis, regulating both symmetric and asymmetric cell divisions. Reduced levels of EJC components cause microcephaly. In the adult brain, Y14 and eIF4A3 appear to be involved in synaptic plasticity and in learning and memory. In this review, we focus on the involvement of EJC components in brain development and its functioning under normal conditions.

Convergence of human cellular models and genetics to study neural stem cell signaling to enhance central nervous system regeneration and repair

Human central nervous system (CNS) regeneration is considered the holy grail of neuroscience research, and is one of the most pressing and difficult questions in biology and science. Despite more than 20 years of work in the field of neural stem cells (NSCs), the area remains in its infancy as our understanding of the fundamental mechanisms that can be leveraged to improve CNS regeneration in neurological diseases is still growing. Here, we focus on the recent lessons from lower organism CNS regeneration genetics and how such findings are starting to illuminate our understanding of NSC signaling pathways in humans. These findings will allow us to improve upon our knowledge of endogenous NSC function, the utility of exogenous NSCs, and the limitations of NSCs as therapeutic vehicles for providing relief from devastating human neurological diseases. We also discuss the limitations of activating NSC signaling for CNS repair in humans, especially the potential for tumor formation. Finally, we will review the recent advances in new culture techniques, including patient-derived cells and cerebral organoids to model the genetic regulation of signaling pathways controlling the function of NSCs during injury and disease states.

Nonsense-mediated mRNA decay: a 'nonsense' pathway makes sense in stem cell biology

Nonsense-mediated mRNA decay (NMD) is a highly conserved post-transcriptional regulatory mechanism of gene expression in eukaryotes. Originally, NMD was identified as an RNA surveillance machinery in degrading 'aberrant' mRNA species with premature termination codons. Recent studies indicate that NMD regulates the stability of natural gene transcripts that play significant roles in cell functions. Although components and action modes of the NMD machinery in degrading its RNA targets have been extensively studied with biochemical and structural approaches, the biological roles of NMD remain to be defined. Stem cells are rare cell populations, which play essential roles in tissue homeostasis and hold great promises in regenerative medicine. Stem cells self-renew to maintain the cellular identity and differentiate into somatic lineages with specialized functions to sustain tissue integrity. Transcriptional regulations and epigenetic modulations have been extensively implicated in stem cell biology. However, post-transcriptional regulatory mechanisms, such as NMD, in stem cell regulation are largely unknown. In this paper, we summarize the recent findings on biological roles of NMD factors in embryonic and tissue-specific stem cells. Furthermore, we discuss the possible mechanisms of NMD in regulating stem cell fates.

Identification of antiviral roles for the exon-junction complex and nonsense-mediated decay in flaviviral infection

West Nile virus (WNV) is an emerging mosquito-borne flavivirus, related to dengue virus and Zika virus. To gain insight into host pathways involved in WNV infection, we performed a systematic affinity-tag purification mass spectrometry (AP-MS) study to identify 259 WNV-interacting human proteins. RNAi screening revealed 26 genes that both interact with WNV proteins and influence WNV infection. We found that WNV, dengue and Zika virus capsids interact with a conserved subset of proteins that impact infection. These include the exon-junction complex (EJC) recycling factor, PYM1, which is antiviral against all three viruses. The EJC has roles in nonsense-mediated decay (NMD), and we found that both the EJC and NMD are antiviral and the EJC protein RBM8A directly binds WNV RNA. To counteract this, flavivirus infection inhibits NMD and the capsid-PYM1 interaction interferes with EJC protein function and localization. Depletion of PYM1 attenuates RBM8A binding to viral RNA, suggesting that WNV sequesters PYM1 to protect viral RNA from decay. Together, these data suggest a complex interplay between the virus and host in regulating NMD and the EJC.

The stability of Magoh and Y14 depends on their heterodimer formation and nuclear localization

Reduced expression of the Y14 gene is a cause of Thrombocytopenia-absent radius (TAR) syndrome. This gene contains a conserved RNA recognition motif (RRM) in the central region and nuclear localization/export sequences (NLS/NES) in the N-terminal. Y14 and Magoh proteins form tight heterodimers and are the core of exon junction complexes (EJCs), which mediate various processes of mRNA metabolism after transcription. In this report, we found that protein expression levels of exogenously expressed Magoh L136R and Y14 L118R (leucine-to-arginine substitution at amino acid residue 136 and 118 respectively, that results in the formation of the complex being lost) are lower than their wild-types. This reduction is likely caused by protein levels, as no difference in mRNA levels was detected. Meanwhile, a cycloheximide chase assay determined that the degradation rates of Magoh L136R and Y14 L118R were faster than their wild-types. Both Y14 L118R and Magoh L136R lost the ability to form heterodimers with corresponding wild-type proteins. However, Y14 L118R is able to still localize in the nucleus which causes the stability of Y14 L118R to be higher than Magoh L136R. These results reveal that the stability of Magoh and Y14 is not only dependent on the heterodimer structure, but also dependent on nuclear localization.

Cenpj Regulates Cilia Disassembly and Neurogenesis in the Developing Mouse Cortex

Primary cilia are microtubule-based protuberances that project from the eukaryotic cell body to sense the extracellular environment. Ciliogenesis is closely correlated to the cell cycle and defects of cilia are related to human systemic diseases such as primary ciliary dyskinesia. However, the role of ciliogenesis in cortical development remains unclear. Here, we demonstrate that Cenpj, a protein that is required for centriole biogenesis, plays a role in regulating cilium disassembly in vivo Depletion of Cenpj in neural progenitor cells results in long cilia and abnormal cilia disassembly. Radial glial cells Cenpj depletion exhibit uncompleted cell division, reduced cell proliferation, and increased cell apoptosis in the developing mouse cerebrum cortex, leading to microcephaly. In addition, Cenpj depletion causes long and thin primary cilia and motile cilia in adult neural stem cells and reduced cell proliferation in the subventricular zone. Furthermore, we show that Cenpj regulates cilia disassembly and neurogenesis through Kif2a, a plus-end-directed motor protein. These data collected from mice of both sexes provide insights into how ciliogenesis plays roles in cortical development and how primary microcephaly is induced by Cenpj mutations in humans.SIGNIFICANCE STATEMENT Autosomal recessive primary microcephaly is a neurodevelopmental disorder with the major symptoms of reduction of circumference of the head, brain volume, and cortex thickness with normal brain architecture in birth. We used conditional Cenpj deletion mice and found that neural progenitor cells (NPCs) exhibited long primary cilia and abnormal cilium appendages. The defective cilium disassembly caused by Cenpj depletion might correlate to reduced cell proliferation, uncompleted cell division, cell apoptosis, and microcephaly in mice. Cenpj also regulates the cilium structure of adult neural stem cells and adult neurogenesis in mice. Additionally, our results illustrate that Cenpj regulates cilia disassembly and neurogenesis through Kif2a, indicating that primary cilia dynamics play a crucial role in NPC mitosis and adult neurogenesis.

Heterozygous RNF13 Gain-of-Function Variants Are Associated with Congenital Microcephaly, Epileptic Encephalopathy, Blindness, and Failure to Thrive

Accumulation of unfolded proteins in the endoplasmic reticulum (ER) initiates a stress response mechanism to clear out the unfolded proteins by either facilitating their re-folding or inducing their degradation. When this fails, an apoptotic cascade is initiated so that the affected cell is eliminated. IRE1α is a critical sensor of the unfolded-protein response, essential for initiating the apoptotic signaling. Here, we report an infantile neurodegenerative disorder associated with enhanced activation of IRE1α and increased apoptosis. Three unrelated affected individuals with congenital microcephaly, infantile epileptic encephalopathy, and profound developmental delay were found to carry heterozygous variants (c.932T>C [p.Leu311Ser] or c.935T>C [p.Leu312Pro]) in RNF13, which codes for an IRE1α-interacting protein. Structural modeling predicted that the variants, located on the surface of the protein, would not alter overall protein folding. Accordingly, the abundance of RNF13 and IRE1α was not altered in affected individuals' cells. However, both IRE1α-mediated stress signaling and stress-induced apoptosis were increased in affected individuals' cells. These results indicate that the RNF13 variants confer gain of function to the encoded protein and thereby lead to altered signaling of the ER stress response associated with severe neurodegeneration in infancy.

Exon junction complex (EJC) core genes play multiple developmental roles in Physalis floridana

KEY MESSAGE

Molecular and functional characterization of four gene families of the Physalis exon junction complex (EJC) core improved our understanding of the evolution and function of EJC core genes in plants. The exon junction complex (EJC) plays significant roles in posttranscriptional regulation of genes in eukaryotes. However, its developmental roles in plants are poorly known. We characterized four EJC core genes from Physalis floridana that were named PFMAGO, PFY14, PFeIF4AIII and PFBTZ. They shared a similar phylogenetic topology and were expressed in all examined organs. PFMAGO, PFY14 and PFeIF4AIII were localized in both the nucleus and cytoplasm while PFBTZ was mainly localized in the cytoplasm. No protein homodimerization was observed, but they could form heterodimers excluding the PFY14-PFBTZ heterodimerization. Virus-induced gene silencing (VIGS) of PFMAGO or PFY14 aborted pollen development and resulted in low plant survival due to a leaf-blight-like phenotype in the shoot apex. Carpel functionality was also impaired in the PFY14 knockdowns, whereas pollen maturation was uniquely affected in PFBTZ-VIGS plants. Once PFeIF4AIII was strongly downregulated, plant survival was reduced via a decomposing root collar after flowering and Chinese lantern morphology was distorted. The expression of Physalis orthologous genes in the DYT1-TDF1-AMS-bHLH91 regulatory cascade that is associated with pollen maturation was significantly downregulated in PFMAGO-, PFY14- and PFBTZ-VIGS flowers. Intron-retention in the transcripts of P. floridana dysfunctional tapetum1 (PFDYT1) occurred in these mutated flowers. Additionally, the expression level of WRKY genes in defense-related pathways in the shoot apex of PFMAGO- or PFY14-VIGS plants and in the root collar of PFeIF4AIII-VIGS plants was significantly downregulated. Taken together, the Physalis EJC core genes play multiple roles including a conserved role in male fertility and newly discovered roles in Chinese lantern development, carpel functionality and defense-related processes. These data increase our understanding of the evolution and functions of EJC core genes in plants.

Nonsense-mediated RNA decay in the brain: emerging modulator of neural development and disease

Steady-state RNA levels are controlled by the balance between RNA synthesis and RNA turnover. A selective RNA turnover mechanism that has received recent attention in neurons is nonsense-mediated RNA decay (NMD). NMD has been shown to influence neural development, neural stem cell differentiation decisions, axon guidance and synaptic plasticity. In humans, NMD factor gene mutations cause some forms of intellectual disability and are associated with neurodevelopmental disorders, including schizophrenia and autism spectrum disorder. Impairments in NMD are linked to neurodegenerative disorders, including amyotrophic lateral sclerosis. We discuss these findings, their clinical implications and challenges for the future.

The Cellular NMD Pathway Restricts Zika Virus Infection and Is Targeted by the Viral Capsid Protein

Zika virus (ZIKV) is a significant global health threat, as infection has been linked to serious neurological complications, including microcephaly. Using a human stem cell-derived neural progenitor model system, we find that a critical cellular quality control process called the nonsense-mediated mRNA decay (NMD) pathway is disrupted during ZIKV infection. Importantly, disruption of the NMD pathway is a known cause of microcephaly and other neurological disorders. We further identify an interaction between the capsid protein of ZIKV and up-frameshift protein 1 (UPF1), the master regulator of NMD, and show that ZIKV capsid targets UPF1 for degradation. Together, these results offer a new mechanism for how ZIKV infection can cause neuropathology in the developing brain.

Zika virus (ZIKV) infection of neural progenitor cells (NPCs) in utero is associated with neurological disorders, such as microcephaly, but a detailed molecular understanding of ZIKV-induced pathogenesis is lacking. Here we show that in vitro ZIKV infection of human cells, including NPCs, causes disruption of the nonsense-mediated mRNA decay (NMD) pathway. NMD is a cellular mRNA surveillance mechanism that is required for normal brain size in mice. Using affinity purification-mass spectrometry, we identified multiple cellular NMD factors that bind to the viral capsid protein, including the central NMD regulator up-frameshift protein 1 (UPF1). Endogenous UPF1 interacted with the ZIKV capsid protein in coimmunoprecipitation experiments, and capsid expression posttranscriptionally downregulated UPF1 protein levels, a process that we confirmed occurs during ZIKV infection. Cellular fractionation studies show that the ZIKV capsid protein specifically targets nuclear UPF1 for degradation via the proteasome. A further decrease in UPF1 levels by RNAi significantly enhanced ZIKV infection in NPC cultures, consistent with a model in which NMD restricts ZIKV infection in the fetal brain. We propose that ZIKV, via the capsid protein, has evolved a strategy to lower UPF1 levels and dampen antiviral activities of NMD, which in turn contributes to neuropathology in vivo.

Cortical progenitor biology: key features mediating proliferation versus differentiation

The cerebral cortex is a highly organized structure whose development depends on diverse progenitor cell types, namely apical radial glia, intermediate progenitors, and basal radial glia cells, which are responsible for the production of the correct neuronal output. In recent years, these progenitor cell types have been deeply studied, particularly basal radial glia and their role in cortical expansion and gyrification. We review here a broad series of factors that regulate progenitor behavior and daughter cell fate. We first describe the different neuronal progenitor types, emphasizing the differences between lissencephalic and gyrencephalic species. We then review key factors shown to influence progenitor proliferation versus differentiation, discussing their roles in progenitor dynamics, neuronal production, and potentially brain size and complexity. Although spindle orientation has been considered a critical factor for mode of division and daughter cell output, we discuss other features that are emerging as crucial for these processes such as organelle and cell cycle dynamics. Additionally, we highlight the importance of adhesion molecules and the polarity complex for correct cortical development. Finally, we briefly discuss studies assessing progenitor multipotency and its possible contribution to the production of specific neuronal populations. This review hence summarizes recent aspects of cortical progenitor cell biology, and pinpoints emerging features critical for their behavior.