Translational derepression of Elavl4 isoforms at their alternative 5' UTRs determines neuronal development

The regulatory landscape of 5' UTRs in translational control during zebrafish embryogenesis

The 5' UTRs of mRNAs are critical for translation regulation during development, but their in vivo regulatory features are poorly characterized. Here, we report the regulatory landscape of 5' UTRs during early zebrafish embryogenesis using a massively parallel reporter assay of 18,154 sequences coupled to polysome profiling. We found that the 5' UTR suffices to confer temporal dynamics to translation initiation and identified 86 motifs enriched in 5' UTRs with distinct ribosome recruitment capabilities. A quantitative deep learning model, Danio Optimus 5-Prime (DaniO5P), identified a combined role for 5' UTR length, translation initiation site context, upstream AUGs, and sequence motifs on ribosome recruitment. DaniO5P predicts the activities of maternal and zygotic 5' UTR isoforms and indicates that modulating 5' UTR length and motif grammar contributes to translation initiation dynamics. This study provides a first quantitative model of 5' UTR-based translation regulation in development and lays the foundation for identifying the underlying molecular effectors.

A subpopulation of cortical neurons altered by mutations in the autism risk gene DDX3X

Cell fate decisions during cortical development sculpt the identity of long-range connections that subserve complex behaviors. These decisions are largely dictated by mutually exclusive transcription factors, including CTIP2/Bcl11b for subcerebral projection neurons and BRN1/Pou3f3 for intra-telencephalic projection neurons. We have recently reported that the balance of cortical CTIP2-expressing neurons is altered in a mouse model of DDX3X syndrome, a female-biased neurodevelopmental disorder associated with intellectual disability, autism spectrum disorder, and significant motor challenges. Here, we studied the developmental dynamics of a subpopulation of cortical neurons co-expressing CTIP2 and BRN1. We found that CTIP2+BRN1+ neurons are born during early phases of neurogenesis like other CTIP2+ neurons, peak in expression during perinatal life, and persist in adult brains. We also found that CTIP2+BRN1+ neurons are excessive in number in prenatal and mature cortical motor areas of Ddx3x mutant mice, translating into altered laminar distribution of subcerebral projection neurons extending axons to the brainstem. These findings underscore the critical role of molecular specification during cortical development in health and disease.

Piwi regulates the usage of alternative transcription start sites in the Drosophila ovary

Alternative transcription initiation, which refers to the transcription of a gene from different transcription start sites (TSSs), is prevalent across metazoans and has important biological functions. Although transcriptional regulation has been extensively studied, the mechanism that selects one TSS over others within a gene remains elusive. Using the Cap Analysis of Gene Expression sequencing (CAGE-seq) method, we discovered that Piwi, an RNA-binding protein, regulates TSS usage in at least 87 genes. In piwi-deficient Drosophila ovaries, these genes displayed significantly altered TSS usage (ATU). The regulation of TSS usage occurred in both germline and somatic cells in ovaries, as well as in cultured ovarian somatic cells (OSCs). Correspondingly, RNA Polymerase II (Pol II) initiation and elongation at the TSSs of ATU genes were affected in germline-piwi-knockdown ovaries and piwi-knockdown OSCs. Furthermore, we identified a Facilitates Chromatin Transcription (FACT) complex component, Ssrp, that is essential for mRNA elongation, as a novel interactor of Piwi in the nucleus. Temporally controlled knockdown of ssrp affected TSS usage in ATU genes, whereas overexpression of ssrp partially rescued the TSS usage of ATU genes in piwi mutant ovaries. Thus, Piwi may interact with Ssrp to regulate TSS usage in Drosophila ovaries by affecting Pol II initiation and elongation.

Cortical projection neurons with distinct axonal connectivity employ ribosomal complexes with distinct protein compositions

Diverse subtypes of cortical projection neurons (PN) form long-range axonal projections that are responsible for distinct sensory, motor, cognitive, and behavioral functions. Translational control has been identified at multiple stages of PN development, but how translational regulation contributes to formation of distinct, subtype-specific long-range circuits is poorly understood. Ribosomal complexes (RCs) exhibit variations of their component proteins, with an increasing set of examples that confer specialized translational control. Here, we directly compare the protein compositions of RCs in vivo from two closely related cortical neuron subtypes-cortical output "subcerebral PN" (SCPN) and interhemispheric "callosal PN" (CPN)- during establishment of their distinct axonal connectivity. Using retrograde labeling of subtype-specific somata, purification by fluorescence-activated cell sorting, ribosome immunoprecipitation, and ultra-low-input mass spectrometry, we identify distinct protein compositions of RCs from these two subtypes. Strikingly, we identify 16 associated proteins reliably and exclusively detected only in RCs of SCPN. 10 of these proteins have known interaction with components of ribosomes; we further validated ribosome interaction with protein kinase C epsilon (PRKCE), a candidate with roles in synaptogenesis. PRKCE and a subset of SCPN-specific candidate ribosome-associated proteins also exhibit enriched gene expression by SCPN. Together, these results indicate that ribosomal complexes exhibit subtype-specific protein composition in distinct subtypes of cortical projection neurons during development, and identify potential candidates for further investigation of function in translational regulation involved in subtype-specific circuit formation.

Initial regional cytoarchitectonic differences in dorsal and orbitobasal human developing frontal cortex revealed by spatial transcriptomics

Early development of the human fetal cerebral cortex involves a set of precisely coordinated molecular processes that remains rather underexplored. Previous studies indicate that the laminar identity and the molecular specification of cortical neurons driven by genetic programming, as well as associated histogenetic events begin during early fetal development. Our recent study discovered unique regional cytoarchitectonic features in the developing human frontal lobe, including migratory waves of postmitotic neurons in the dorsal frontal cortex and the "double plate" feature in orbitobasal cortex (Kopić et al. in Cells 12:231, 2023). Notably, neurons of these two cytoarchitectonic features typically express deep projection neuron (DPN) markers (TBR1, TLE4, SOX5). This paper aims to conduct an in-depth investigation of these cytoarchitectonic features at the transcriptomic level, whilst preserving spatial information. Here, we employed NanoString GeoMx™ Digital Spatial Profiler (DSP) technology to examine gene expression differences in the transient cortical compartments of the dorsal and ventral regions of the developing frontal lobe, focusing specifically on 15 post-conceptional weeks (PCW), that is a critical period for subplate formation. We identified multiple differentially expressed genes between the transient cellular compartments of the dorsal and orbitobasal regions of the developing human frontal cortex. These new findings additionally confirm that regional patterning and specification of the prospective higher-order association prefrontal cortex emerges early in fetal development, contributing to the highly organized cortical architecture of the human brain.

Combined small-molecule treatment accelerates maturation of human pluripotent stem cell-derived neurons

The maturation of human pluripotent stem cell (hPSC)-derived neurons mimics the protracted timing of human brain development, extending over months to years for reaching adult-like function. Prolonged in vitro maturation presents a major challenge to stem cell-based applications in modeling and treating neurological disease. Therefore, we designed a high-content imaging assay based on morphological and functional readouts in hPSC-derived cortical neurons which identified multiple compounds that drive neuronal maturation including inhibitors of lysine-specific demethylase 1 and disruptor of telomerase-like 1 and activators of calcium-dependent transcription. A cocktail of four factors, GSK2879552, EPZ-5676, N-methyl-D-aspartate and Bay K 8644, collectively termed GENtoniK, triggered maturation across all parameters tested, including synaptic density, electrophysiology and transcriptomics. Maturation effects were further validated in cortical organoids, spinal motoneurons and non-neural lineages including melanocytes and pancreatic β-cells. The effects on maturation observed across a broad range of hPSC-derived cell types indicate that some of the mechanisms controlling the timing of human maturation might be shared across lineages.

Post-transcriptional mechanisms controlling neurogenesis and direct neuronal reprogramming

Neurogenesis is a tightly regulated process in time and space both in the developing embryo and in adult neurogenic niches. A drastic change in the transcriptome and proteome of radial glial cells or neural stem cells towards the neuronal state is achieved due to sophisticated mechanisms of epigenetic, transcriptional, and post-transcriptional regulation. Understanding these neurogenic mechanisms is of major importance, not only for shedding light on very complex and crucial developmental processes, but also for the identification of putative reprogramming factors, that harbor hierarchically central regulatory roles in the course of neurogenesis and bare thus the capacity to drive direct reprogramming towards the neuronal fate. The major transcriptional programs that orchestrate the neurogenic process have been the focus of research for many years and key neurogenic transcription factors, as well as repressor complexes, have been identified and employed in direct reprogramming protocols to convert non-neuronal cells, into functional neurons. The post-transcriptional regulation of gene expression during nervous system development has emerged as another important and intricate regulatory layer, strongly contributing to the complexity of the mechanisms controlling neurogenesis and neuronal function. In particular, recent advances are highlighting the importance of specific RNA binding proteins that control major steps of mRNA life cycle during neurogenesis, such as alternative splicing, polyadenylation, stability, and translation. Apart from the RNA binding proteins, microRNAs, a class of small non-coding RNAs that block the translation of their target mRNAs, have also been shown to play crucial roles in all the stages of the neurogenic process, from neural stem/progenitor cell proliferation, neuronal differentiation and migration, to functional maturation. Here, we provide an overview of the most prominent post-transcriptional mechanisms mediated by RNA binding proteins and microRNAs during the neurogenic process, giving particular emphasis on the interplay of specific RNA binding proteins with neurogenic microRNAs. Taking under consideration that the molecular mechanisms of neurogenesis exert high similarity to the ones driving direct neuronal reprogramming, we also discuss the current advances in in vitro and in vivo direct neuronal reprogramming approaches that have employed microRNAs or RNA binding proteins as reprogramming factors, highlighting the so far known mechanisms of their reprogramming action.

The role of CELF family in neurodevelopment and neurodevelopmental disorders

RNA-binding proteins (RBPs) bind to RNAs and are crucial for regulating RNA splicing, stability, translation, and transport. Among these proteins, the CUGBP Elav-like family (CELF) is a highly conserved group crucial for posttranscriptional regulation by binding to CUG repeats. Comprising CELF1-6, this family exhibits diverse expression patterns and functions. Dysregulation of CELF has been implicated in various neural disorders, encompassing both neurodegenerative and neurodevelopmental conditions, such as Alzheimer's disease and autism. This article aims to provide a comprehensive summary of the CELF family's role in neurodevelopment and neurodevelopmental disorders. Understanding CELF's mechanisms may offer clues for potential therapeutic strategies by regulating their targets in neurodevelopmental disorders.

Developmental isoform diversity in the human neocortex informs neuropsychiatric risk mechanisms

RNA splicing is highly prevalent in the brain and has strong links to neuropsychiatric disorders; yet, the role of cell type-specific splicing and transcript-isoform diversity during human brain development has not been systematically investigated. In this work, we leveraged single-molecule long-read sequencing to deeply profile the full-length transcriptome of the germinal zone and cortical plate regions of the developing human neocortex at tissue and single-cell resolution. We identified 214,516 distinct isoforms, of which 72.6% were novel (not previously annotated in Gencode version 33), and uncovered a substantial contribution of transcript-isoform diversity-regulated by RNA binding proteins-in defining cellular identity in the developing neocortex. We leveraged this comprehensive isoform-centric gene annotation to reprioritize thousands of rare de novo risk variants and elucidate genetic risk mechanisms for neuropsychiatric disorders.

Neuronal activity rapidly reprograms dendritic translation via eIF4G2:uORF binding

Learning and memory require activity-induced changes in dendritic translation, but which mRNAs are involved and how they are regulated are unclear. In this study, to monitor how depolarization impacts local dendritic biology, we employed a dendritically targeted proximity labeling approach followed by crosslinking immunoprecipitation, ribosome profiling and mass spectrometry. Depolarization of primary cortical neurons with KCl or the glutamate agonist DHPG caused rapid reprogramming of dendritic protein expression, where changes in dendritic mRNAs and proteins are weakly correlated. For a subset of pre-localized messages, depolarization increased the translation of upstream open reading frames (uORFs) and their downstream coding sequences, enabling localized production of proteins involved in long-term potentiation, cell signaling and energy metabolism. This activity-dependent translation was accompanied by the phosphorylation and recruitment of the non-canonical translation initiation factor eIF4G2, and the translated uORFs were sufficient to confer depolarization-induced, eIF4G2-dependent translational control. These studies uncovered an unanticipated mechanism by which activity-dependent uORF translational control by eIF4G2 couples activity to local dendritic remodeling.

Alternative splicing of CsWRKY21 positively regulates cold response in tea plant

Alternative splicing (AS) was an important post-transcriptional mechanism that involved in plant resistance to adversity stress. WRKY transcription factors function as transcriptional activators or repressors to modulate plant growth, development and stress response. However, the role of alternate splicing of WRKY in cold tolerance is poorly understood in tea plants. In this study, we found that the CsWRKY21 transcription factor, a member of the WRKY IId subfamily, was induced by low temperature. Subcellular localization and transcriptional activity assays showed that CsWRKY21 localized to the nucleus and had no transcriptional activation activity. Y1H and dual-luciferase reporter assays showed that CsWRKY21 suppressed expression of CsABA8H and CsUGT by binding with their promoters. Transient overexpression of CsABA8H and CsUGT reduced abscisic acid (ABA) content in tobacco leaves. Furthermore, we discovered that CsWRKY21 undergoes AS in the 5'UTR region. The AS transcript CsWRKY21-b was induced at low temperature, up to 6 folds compared to the control, while the full-length CsWRKY21-a transcript did not significantly change. Western blot analysis showed that the retention of introns in the 5'UTR region of CsWRKY21-b led to higher CsWRKY21 protein content. These results revealed that alternative splicing of CsWRKY21 involved in cold tolerance of tea plant by regulating the protein expression level and then regulating the content of ABA, and provide insights into molecular mechanisms of low temperature defense mediated by AS in tea plant.

Rbfox1 regulates alternative splicing of Nrcam in primary sensory neurons to mediate peripheral nerve injury-induced neuropathic pain

The primary sensory neurons of the dorsal root ganglia (DRG) are subject to transcriptional alterations following peripheral nerve injury. These alterations are believed to play a pivotal role in the genesis of neuropathic pain. Alternative RNA splicing is a process that generates multiple transcript variants from a single gene, significantly contributing to the complexity of the transcriptome. However, little is known about the functional significance and control of alternative RNA splicing in injured DRG after spinal nerve ligation (SNL). In our study, we conducted a comprehensive transcriptome profiling and bioinformatic analysis to approach and identified a neuron-specific isoform of an RNA splicing regulator, RNA-binding Fox1 (Rbfox1, also known as A2BP1), as a crucial regulator of alternative RNA splicing in injured DRG after SNL. Notably, Rbfox1 expression is markedly reduced in injured DRG following peripheral nerve injury. Restoring this reduction effectively mitigates nociceptive hypersensitivity. Conversely, mimicking the downregulation of Rbfox1 expression generates neuropathic pain symptoms. Mechanistically, we uncovered that Rbfox1 may be a key factor influencing alternative RNA splicing of neuron-glial related cell adhesion molecule (NrCAM), a key neuronal cell adhesion molecule. In injured DRG after SNL, the downregulation of Rbfox1amplifies the insertion of exon 10 in Nrcam transcripts, leading to an increase in long Nrcam variants (L-Nrcam) and a corresponding decrease in short Nrcam variants (S-Nrcam) within injured DRG. In summary, our study supports the essential role of Rbfox1 in neuropathic pain within DRG, probably via the regulation of Nrcam splicing. These findings suggest that Rbfox1 could be a potential target for neuropathic pain therapy.

The regulatory landscape of 5' UTRs in translational control during zebrafish embryogenesis

The 5' UTRs of mRNAs are critical for translation regulation, but their in vivo regulatory features are poorly characterized. Here, we report the regulatory landscape of 5' UTRs during early zebrafish embryogenesis using a massively parallel reporter assay of 18,154 sequences coupled to polysome profiling. We found that the 5' UTR is sufficient to confer temporal dynamics to translation initiation, and identified 86 motifs enriched in 5' UTRs with distinct ribosome recruitment capabilities. A quantitative deep learning model, DaniO5P, revealed a combined role for 5' UTR length, translation initiation site context, upstream AUGs and sequence motifs on in vivo ribosome recruitment. DaniO5P predicts the activities of 5' UTR isoforms and indicates that modulating 5' UTR length and motif grammar contributes to translation initiation dynamics. This study provides a first quantitative model of 5' UTR-based translation regulation in early vertebrate development and lays the foundation for identifying the underlying molecular effectors.

Learning gene networks under SNP perturbation using SNP and allele-specific expression data

Allele-specific expression quantification from RNA-seq reads provides opportunities to study the control of gene regulatory networks by cis-acting and trans-acting genetic variants. Many existing methods performed a single-gene and single-SNP association analysis to identify expression quantitative trait loci (eQTLs), and placed the eQTLs against known gene networks for functional interpretation. Instead, we view eQTL data as a capture of the effects of perturbation of gene regulatory system by a large number of genetic variants and reconstruct a gene network perturbed by eQTLs. We introduce a statistical framework called CiTruss for simultaneously learning a gene network and cis-acting and trans-acting eQTLs that perturb this network, given population allele-specific expression and SNP data. CiTruss uses a multi-level conditional Gaussian graphical model to model trans-acting eQTLs perturbing the expression of both alleles in gene network at the top level and cis-acting eQTLs perturbing the expression of each allele at the bottom level. We derive a transformation of this model that allows efficient learning for large-scale human data. Our analysis of the GTEx and LG×SM advanced intercross line mouse data for multiple tissue types with CiTruss provides new insights into genetics of gene regulation. CiTruss revealed that gene networks consist of local subnetworks over proximally located genes and global subnetworks over genes scattered across genome, and that several aspects of gene regulation by eQTLs such as the impact of genetic diversity, pleiotropy, tissue-specific gene regulation, and local and long-range linkage disequilibrium among eQTLs can be explained through these local and global subnetworks.

Developmental isoform diversity in the human neocortex informs neuropsychiatric risk mechanisms

RNA splicing is highly prevalent in the brain and has strong links to neuropsychiatric disorders, yet the role of cell-type-specific splicing or transcript-isoform diversity during human brain development has not been systematically investigated. Here, we leveraged single-molecule long-read sequencing to deeply profile the full-length transcriptome of the germinal zone (GZ) and cortical plate (CP) regions of the developing human neocortex at tissue and single-cell resolution. We identified 214,516 unique isoforms, of which 72.6% are novel (unannotated in Gencode-v33), and uncovered a substantial contribution of transcript-isoform diversity, regulated by RNA binding proteins, in defining cellular identity in the developing neocortex. We leveraged this comprehensive isoform-centric gene annotation to re-prioritize thousands of rare de novo risk variants and elucidate genetic risk mechanisms for neuropsychiatric disorders.

One-Sentence Summary

A cell-specific atlas of gene isoform expression helps shape our understanding of brain development and disease.

Structured Abstract

INTRODUCTION: The development of the human brain is regulated by precise molecular and genetic mechanisms driving spatio-temporal and cell-type-specific transcript expression programs. Alternative splicing, a major mechanism increasing transcript diversity, is highly prevalent in the human brain, influences many aspects of brain development, and has strong links to neuropsychiatric disorders. Despite this, the cell-type-specific transcript-isoform diversity of the developing human brain has not been systematically investigated.RATIONALE: Understanding splicing patterns and isoform diversity across the developing neocortex has translational relevance and can elucidate genetic risk mechanisms in neurodevelopmental disorders. However, short-read sequencing, the prevalent technology for transcriptome profiling, is not well suited to capturing alternative splicing and isoform diversity. To address this, we employed third-generation long-read sequencing, which enables capture and sequencing of complete individual RNA molecules, to deeply profile the full-length transcriptome of the germinal zone (GZ) and cortical plate (CP) regions of the developing human neocortex at tissue and single-cell resolution.RESULTS: We profiled microdissected GZ and CP regions of post-conception week (PCW) 15-17 human neocortex in bulk and at single-cell resolution across six subjects using high-fidelity long-read sequencing (PacBio IsoSeq). We identified 214,516 unique isoforms, of which 72.6% were novel (unannotated in Gencode), and >7,000 novel exons, expanding the proteome by 92,422 putative proteoforms. We uncovered thousands of isoform switches during cortical neurogenesis predicted to impact RNA regulatory domains or protein structure and implicating previously uncharacterized RNA-binding proteins in cellular identity and neuropsychiatric disease. At the single-cell level, early-stage excitatory neurons exhibited the greatest isoform diversity, and isoform-centric single-cell clustering led to the identification of previously uncharacterized cell states. We systematically assessed the contribution of transcriptomic features, and localized cell and spatio-temporal transcript expression signatures across neuropsychiatric disorders, revealing predominant enrichments in dynamic isoform expression and utilization patterns and that the number and complexity of isoforms per gene is strongly predictive of disease. Leveraging this resource, we re-prioritized thousands of rare de novo risk variants associated with autism spectrum disorders (ASD), intellectual disability (ID), and neurodevelopmental disorders (NDDs), more broadly, to potentially more severe consequences and revealed a larger proportion of cryptic splice variants with the expanded transcriptome annotation provided in this study.CONCLUSION: Our study offers a comprehensive landscape of isoform diversity in the human neocortex during development. This extensive cataloging of novel isoforms and splicing events sheds light on the underlying mechanisms of neurodevelopmental disorders and presents an opportunity to explore rare genetic variants linked to these conditions. The implications of our findings extend beyond fundamental neuroscience, as they provide crucial insights into the molecular basis of developmental brain disorders and pave the way for targeted therapeutic interventions. To facilitate exploration of this dataset we developed an online portal ( https://sciso.gandallab.org/ ).

Celf4 controls mRNA translation underlying synaptic development in the prenatal mammalian neocortex

Abnormalities in neocortical and synaptic development are linked to neurodevelopmental disorders. However, the molecular and cellular mechanisms governing initial synapse formation in the prenatal neocortex remain poorly understood. Using polysome profiling coupled with snRNAseq on human cortical samples at various fetal phases, we identify human mRNAs, including those encoding synaptic proteins, with finely controlled translation in distinct cell populations of developing frontal neocortices. Examination of murine and human neocortex reveals that the RNA binding protein and translational regulator, CELF4, is expressed in compartments enriched in initial synaptogenesis: the marginal zone and the subplate. We also find that Celf4/CELF4-target mRNAs are encoded by risk genes for adverse neurodevelopmental outcomes translating into synaptic proteins. Surprisingly, deleting Celf4 in the forebrain disrupts the balance of subplate synapses in a sex-specific fashion. This highlights the significance of RNA binding proteins and mRNA translation in evolutionarily advanced synaptic development, potentially contributing to sex differences.

Development of nanoRibo-seq enables study of regulated translation by cortical neuron subtypes, showing uORF translation in synaptic-axonal genes

Investigation of translation in rare cell types or subcellular contexts is challenging due to large input requirements for standard approaches. Here, we present "nanoRibo-seq" an optimized approach using 102- to 103-fold less input material than bulk approaches. nanoRibo-seq exhibits rigorous quality control features consistent with quantification of ribosome protected fragments with as few as 1,000 cells. We compare translatomes of two closely related cortical neuron subtypes, callosal projection neurons (CPN) and subcerebral projection neurons (SCPN), during their early postnatal development. We find that, while translational efficiency is highly correlated between CPN and SCPN, several dozen mRNAs are differentially translated. We further examine upstream open reading frame (uORF) translation and identify that mRNAs involved in synapse organization and axon development are highly enriched for uORF translation in both subtypes. nanoRibo-seq enables investigation of translational regulation of rare cell types in vivo and offers a flexible approach for globally quantifying translation from limited input material.

Endogenous neural stem cells characterization using omics approaches: Current knowledge in health and disease

Neural stem cells (NSCs), an invaluable source of neuronal and glial progeny, have been widely interrogated in the last twenty years, mainly to understand their therapeutic potential. Most of the studies were performed with cells derived from pluripotent stem cells of either rodents or humans, and have mainly focused on their potential in regenerative medicine. High-throughput omics technologies, such as transcriptomics, epigenetics, proteomics, and metabolomics, which exploded in the past decade, represent a powerful tool to investigate the molecular mechanisms characterizing the heterogeneity of endogenous NSCs. The transition from bulk studies to single cell approaches brought significant insights by revealing complex system phenotypes, from the molecular to the organism level. Here, we will discuss the current literature that has been greatly enriched in the “omics era”, successfully exploring the nature and function of endogenous NSCs and the process of neurogenesis. Overall, the information obtained from omics studies of endogenous NSCs provides a sharper picture of NSCs function during neurodevelopment in healthy and in perturbed environments.

Integrin-KCNB1 potassium channel complexes regulate neocortical neuronal development and are implicated in epilepsy

Potassium (K⁺) channels are robustly expressed during prenatal brain development, including in progenitor cells and migrating neurons, but their function is poorly understood. Here, we investigate the role of voltage-gated K⁺ channel KCNB1 (Kv2.1) in neocortical development. Neuronal migration of glutamatergic neurons was impaired in the neocortices of KCNB1 null mice. Migratory defects persisted into the adult brains, along with disrupted morphology and synaptic connectivity. Mice developed seizure phenotype, anxiety, and compulsive behavior. To determine whether defective KCNB1 can give rise to developmental channelopathy, we constructed Knock In (KI) mice, harboring the gene variant Kcnb1^R312H (R312H mice) found in children with developmental and epileptic encephalopathies (DEEs). The R312H mice exhibited a similar phenotype to the null mice. Wild type (WT) and R312H KCNB1 channels made complexes with integrins α5β5 (Integrin_K⁺ channel_Complexes, IKCs), whose biochemical signaling was impaired in R312H brains. Treatment with Angiotensin II in vitro, an agonist of Focal Adhesion kinase, a key component of IKC signaling machinery, corrected the neuronal abnormalities. Thus, a genetic mutation in a K⁺ channel induces severe neuromorphological abnormalities through non-conducting mechanisms, that can be rescued by pharmacological intervention. This underscores a previously unknown role of IKCs as key players in neuronal development, and implicate developmental channelopathies in the etiology of DEEs.

Laminar dynamics of deep projection neurons and mode of subplate formation are hallmarks of histogenetic subdivisions of the human cingulate cortex before onset of arealization

The cingulate gyrus, as a prominent part of the human limbic lobe, is involved in the integration and regulation of complex emotional, executive, motivational, and cognitive functions, attributed to several functional regions along the anteroposterior axis. In contrast to increasing knowledge of cingulate function in the adult brain, our knowledge of cingulate development is based primarily on classical neuroembryological studies. We aimed to reveal the laminar and cellular development of the various cingulate regions during the critical period from 7.5 to 15 postconceptional weeks (PCW) before the formation of Brodmann type arealization, employing diverse molecular markers on serial histological sections of postmortem human fetal brains. The study was performed by analysis of: (1) deep projection neuron (DPN) markers laminar dynamics, (2) all transient laminar compartments, and (3) characteristic subplate (SP) formation-expansion phase. We found that DPN markers labeling an incipient cortical plate (CP) were the first sign of regional differentiation of the dorsal isocortical and ventral mesocortical belt. Remarkably, increased width of the fibrillar marginal zone (MZ) towards the limbus, in parallel with the narrowing of CP containing DPN, as well as the diminishment of subventricular zone (SVZ) were reliable landmarks of early mesocortical differentiation. Finally, the SP formation pattern was shown to be a crucial event in the isocortical cingulate portion, given that the mesocortical belt is characterized by an incomplete CP delamination and absence of SP expansion. In conclusion, laminar DPN markers dynamics, together with the SVZ size and mode of SP formation indicate regional belt-like cingulate cortex differentiation before the corpus callosum expansion and several months before Brodmann type arealization.

The Functional Meaning of 5'UTR in Protein-Coding Genes

As it is well known, messenger RNA has many regulatory regions along its sequence length. One of them is the 5' untranslated region (5'UTR), which itself contains many regulatory elements such as upstream ORFs (uORFs), internal ribosome entry sites (IRESs), microRNA binding sites, and structural components involved in the regulation of mRNA stability, pre-mRNA splicing, and translation initiation. Activation of the alternative, more upstream transcription start site leads to an extension of 5'UTR. One of the consequences of 5'UTRs extension may be head-to-head gene overlap. This review describes elements in 5'UTR of protein-coding transcripts and the functional significance of protein-coding genes 5' overlap with implications for transcription, translation, and disease.

Translational control in cortical development

Differentiation of specific neuronal types in the nervous system is worked out through a complex series of gene regulation events. Within the mammalian neocortex, the appropriate expression of key transcription factors allocates neurons to different cortical layers according to an inside-out model and endows them with specific properties. Precise timing is required to ensure the proper sequential appearance of key transcription factors that dictate the identity of neurons within the different cortical layers. Recent evidence suggests that aspects of this time-controlled regulation of gene products rely on post-transcriptional control, and point at micro-RNAs (miRs) and RNA-binding proteins as important players in cortical development. Being able to simultaneously target many different mRNAs, these players may be involved in controlling the global expression of gene products in progenitors and post-mitotic cells, in a gene expression framework where parallel to transcriptional gene regulation, a further level of control is provided to refine and coordinate the appearance of the final protein products. miRs and RNA-binding proteins (RBPs), by delaying protein appearance, may play heterochronic effects that have recently been shown to be relevant for the full differentiation of cortical neurons and for their projection abilities. Such heterochronies may be the base for evolutionary novelties that have enriched the spectrum of cortical cell types within the mammalian clade.

Early Regional Patterning in the Human Prefrontal Cortex Revealed by Laminar Dynamics of Deep Projection Neuron Markers

Early regional patterning and laminar position of cortical projection neurons is determined by activation and deactivation of transcriptional factors (TFs) and RNA binding proteins (RBPs) that regulate spatiotemporal framework of neurogenetic processes (proliferation, migration, aggregation, postmigratory differentiation, molecular identity acquisition, axonal growth, dendritic development, and synaptogenesis) within transient cellular compartments. Deep-layer projection neurons (DPN), subplate (SPN), and Cajal-Retzius neurons (CRN) are early-born cells involved in the establishment of basic laminar and regional cortical architecture; nonetheless, laminar dynamics of their molecular transcriptional markers remain underexplored. Here we aimed to analyze laminar dynamics of DPN markers, i.e., transcription factors TBR1, CTIP2, TLE4, SOX5, and RBP CELF1 on histological serial sections of the human frontal cortex between 7.5-15 postconceptional weeks (PCW) in reference to transient proliferative, migratory, and postmigratory compartments. The subtle signs of regional patterning were seen during the late preplate phase in the pattern of sublaminar organization of TBR1+/Reelin+ CRN and TBR1+ pioneering SPN. During the cortical plate (CP)-formation phase, TBR1+ neurons became radially aligned, forming continuity from a well-developed subventricular zone to CP showing clear lateral to medial regional gradients. The most prominent regional patterning was seen during the subplate formation phase (around 13 PCW) when a unique feature of the orbitobasal frontal cortex displays a "double plate" pattern. In other portions of the frontal cortex (lateral, dorsal, medial) deep portion of CP becomes loose and composed of TBR1+, CTIP2+, TLE4+, and CELF1+ neurons of layer six and later-born SPN, which later become constituents of the expanded SP (around 15 PCW). Overall, TFs and RBPs mark characteristic regional laminar dynamics of DPN, SPN, and CRN subpopulations during remarkably early fetal phases of the highly ordered association cortex development.

The SYNGAP1 3'UTR Variant in ALS Patients Causes Aberrant SYNGAP1 Splicing and Dendritic Spine Loss by Recruiting HNRNPK

Fused in sarcoma (FUS) is a pathogenic RNA-binding protein in amyotrophic lateral sclerosis (ALS). We previously reported that FUS stabilizes Synaptic Ras-GTPase activating protein 1 (Syngap1) mRNA at its 3' untranslated region (UTR) and maintains spine maturation. To elucidate the pathologic roles of this mechanism in ALS patients, we identified the SYNGAP1 3'UTR variant rs149438267 in seven (four males and three females) out of 807 ALS patients at the FUS binding site from a multicenter cohort in Japan. Human-induced pluripotent stem cell (hiPSC)-derived motor neurons with the SYNGAP1 variant showed aberrant splicing, increased isoform α1 levels, and decreased isoform γ levels, which caused dendritic spine loss. Moreover, the SYNGAP1 variant excessively recruited FUS and heterogeneous nuclear ribonucleoprotein K (HNRNPK), and antisense oligonucleotides (ASOs) blocking HNRNPK altered aberrant splicing and ameliorated dendritic spine loss. These data suggest that excessive recruitment of RNA-binding proteins, especially HNRNPK, as well as changes in SYNGAP1 isoforms, are crucial for spine formation in motor neurons.SIGNIFICANCE STATEMENT It is not yet known which RNAs cause the pathogenesis of amyotrophic lateral sclerosis (ALS). We previously reported that Fused in sarcoma (FUS), a pathogenic RNA-binding protein in ALS, stabilizes synaptic Ras-GTPase activating protein 1 (Syngap1) mRNA at its 3' untranslated region (UTR) and maintains dendritic spine maturation. To elucidate whether this mechanism is crucial for ALS, we identified the SYNGAP1 3'UTR variant rs149438267 at the FUS binding site. Human-induced pluripotent stem cell (hiPSC)-derived motor neurons with the SYNGAP1 variant showed aberrant splicing, which caused dendritic spine loss along with excessive recruitment of FUS and heterogeneous nuclear ribonucleoprotein K (HNRNPK). Our findings that dendritic spine loss is because of excess recruitment of RNA-binding proteins provide a basis for the future exploration of ALS-related RNA-binding proteins.

RNA splicing regulators play critical roles in neurogenesis

Alternative RNA splicing increases transcript diversity in different cell types and under varying conditions. It is executed with the help of RNA splicing regulators (RSRs), which are operationally defined as RNA-binding proteins (RBPs) that regulate alternative splicing, but not directly catalyzing the chemical reactions of splicing. By systematically searching for RBPs and manually identifying those that regulate splicing, we curated 305 RSRs in the human genome. Surprisingly, most of the RSRs are involved in neurogenesis. Among these RSRs, we focus on nine families (PTBP, NOVA, RBFOX, ELAVL, CELF, DBHS, MSI, PCBP, and MBNL) that play essential roles in the neurogenic pathway. A better understanding of their functions will provide novel insights into the role of splicing in brain development, health, and disease. This comprehensive review serves as a stepping-stone to explore the diverse and complex set of RSRs as fundamental regulators of neural development. This article is categorized under: RNA-Based Catalysis > RNA Catalysis in Splicing and Translation RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications RNA Processing > Splicing Regulation/Alternative Splicing.

OsTBP2.1, a TATA-Binding Protein, Alters the Ratio of OsNRT2.3b to OsNRT2.3a and Improves Rice Grain Yield

The OsNRT2.3a and OsNRT2.3b isoforms play important roles in the uptake and transport of nitrate during rice growth. However, it is unclear which cis-acting element controls the transcription of OsNRT2.3 into these specific isoforms. In this study, we used a yeast one-hybrid assay to obtain the TATA-box binding protein OsTBP2.1, which binds to the TATA-box of OsNRT2.3, and verified its important role through transient expression and RNA-seq. We found that the TATA-box of OsNRT2.3 mutants and binding protein OsTBP2.1 together increased the transcription ratio of OsNRT2.3b to OsNRT2.3a. The overexpression of OsTBP2.1 promoted nitrogen uptake and increased rice yield compared with the wild-type; however, the OsTBP2.1 T-DNA mutant lines exhibited the opposite trend. Detailed analyses demonstrated that the TATA-box was the key cis-regulatory element for OsNRT2.3 to be transcribed into OsNRT2.3a and OsNRT2.3b. Additionally, this key cis-regulatory element, together with the binding protein OsTBP2.1, promoted the development of rice and increased grain yield.

Emerging Roles of RNA-Binding Proteins in Neurodevelopment

Diverse cell types in the central nervous system (CNS) are generated by a relatively small pool of neural stem cells during early development. Spatial and temporal regulation of stem cell behavior relies on precise coordination of gene expression. Well-studied mechanisms include hormone signaling, transcription factor activity, and chromatin remodeling processes. Much less is known about downstream RNA-dependent mechanisms including posttranscriptional regulation, nuclear export, alternative splicing, and transcript stability. These important functions are carried out by RNA-binding proteins (RBPs). Recent work has begun to explore how RBPs contribute to stem cell function and homeostasis, including their role in metabolism, transport, epigenetic regulation, and turnover of target transcripts. Additional layers of complexity are provided by the different target recognition mechanisms of each RBP as well as the posttranslational modifications of the RBPs themselves that alter function. Altogether, these functions allow RBPs to influence various aspects of RNA metabolism to regulate numerous cellular processes. Here we compile advances in RNA biology that have added to our still limited understanding of the role of RBPs in neurodevelopment.

mRNA-Decapping Associated DcpS Enzyme Controls Critical Steps of Neuronal Development

Homozygous mutations in the gene encoding the scavenger mRNA-decapping enzyme, DcpS, have been shown to underlie developmental delay and intellectual disability. Intellectual disability is associated with both abnormal neocortical development and mRNA metabolism. However, the role of DcpS and its scavenger decapping activity in neuronal development is unknown. Here, we show that human neurons derived from patients with a DcpS mutation have compromised differentiation and neurite outgrowth. Moreover, in the developing mouse neocortex, DcpS is required for the radial migration, polarity, neurite outgrowth, and identity of developing glutamatergic neurons. Collectively, these findings demonstrate that the scavenger mRNA decapping activity contributes to multiple pivotal roles in neural development and further corroborate that mRNA metabolism and neocortical pathologies are associated with intellectual disability.

Evolution of the Neocortex Through RNA-Binding Proteins and Post-transcriptional Regulation

The human neocortex is undoubtedly considered a supreme accomplishment in mammalian evolution. It features a prenatally established six-layered structure which remains plastic to the myriad of changes throughout an organism’s lifetime. A fundamental feature of neocortical evolution and development is the abundance and diversity of the progenitor cell population and their neuronal and glial progeny. These evolutionary upgrades are partially enabled due to the progenitors’ higher proliferative capacity, compartmentalization of proliferative regions, and specification of neuronal temporal identities. The driving force of these processes may be explained by temporal molecular patterning, by which progenitors have intrinsic capacity to change their competence as neocortical neurogenesis proceeds. Thus, neurogenesis can be conceptualized along two timescales of progenitors’ capacity to (1) self-renew or differentiate into basal progenitors (BPs) or neurons or (2) specify their fate into distinct neuronal and glial subtypes which participate in the formation of six-layers. Neocortical development then proceeds through sequential phases of proliferation, differentiation, neuronal migration, and maturation. Temporal molecular patterning, therefore, relies on the precise regulation of spatiotemporal gene expression. An extensive transcriptional regulatory network is accompanied by post-transcriptional regulation that is frequently mediated by the regulatory interplay between RNA-binding proteins (RBPs). RBPs exhibit important roles in every step of mRNA life cycle in any system, from splicing, polyadenylation, editing, transport, stability, localization, to translation (protein synthesis). Here, we underscore the importance of RBP functions at multiple time-restricted steps of early neurogenesis, starting from the cell fate transition of transcriptionally primed cortical progenitors. A particular emphasis will be placed on RBPs with mostly conserved but also divergent evolutionary functions in neural progenitors across different species. RBPs, when considered in the context of the fascinating process of neocortical development, deserve to be main protagonists in the story of the evolution and development of the neocortex.

Distribution of alternative untranslated regions within the mRNA of the CELF1 splicing factor affects its expression

CUG-binding protein, ELAV-like Family Member 1 (CELF1) plays an important role during the development of different tissues, such as striated muscle and brain tissue. CELF1 is an RNA-binding protein that regulates RNA metabolism processes, e.g., alternative splicing, and antagonizes other RNA-binding proteins, such as Muscleblind-like proteins (MBNLs). Abnormal activity of both classes of proteins plays a crucial role in the pathogenesis of myotonic dystrophy type 1 (DM1), the most common form of muscular dystrophy in adults. In this work, we show that alternative splicing of exons forming both the 5' and 3' untranslated regions (UTRs) of CELF1 mRNA is efficiently regulated during development and tissue differentiation and is disrupted in skeletal muscles in the context of DM1. Alternative splicing of the CELF1 5'UTR leads to translation of two potential protein isoforms that differ in the lengths of their N-terminal domains. We also show that the MBNL and CELF proteins regulate the distribution of mRNA splicing isoforms with different 5'UTRs and 3'UTRs and affect the CELF1 expression by changing its sensitivity to specific microRNAs or RNA-binding proteins. Together, our findings show the existence of different mechanisms of regulation of CELF1 expression through the distribution of various 5' and 3' UTR isoforms within CELF1 mRNA.

A critical period of translational control during brain development at codon resolution

Translation modulates the timing and amplification of gene expression after transcription. Brain development requires uniquely complex gene expression patterns, but large-scale measurements of translation directly in the prenatal brain are lacking. We measure the reactants, synthesis and products of mRNA translation spanning mouse neocortex neurogenesis, and discover a transient window of dynamic regulation at mid-gestation. Timed translation upregulation of chromatin-binding proteins like Satb2, which is essential for neuronal subtype differentiation, restricts protein expression in neuronal lineages despite broad transcriptional priming in progenitors. In contrast, translation downregulation of ribosomal proteins sharply decreases ribosome biogenesis, coinciding with a major shift in protein synthesis dynamics at mid-gestation. Changing activity of eIF4EBP1, a direct inhibitor of ribosome biogenesis, is concurrent with ribosome downregulation and affects neurogenesis of the Satb2 lineage. Thus, the molecular logic of brain development includes the refinement of transcriptional programs by translation. Modeling of the developmental neocortex translatome is provided as an open-source searchable resource at https://shiny.mdc-berlin.de/cortexomics .

Developmental and Behavioral Phenotypes in a Mouse Model of DDX3X Syndrome

BACKGROUND

Mutations in the X-linked gene DDX3X account for approximately 2% of intellectual disability in females, often comorbid with behavioral problems, motor deficits, and brain malformations. DDX3X encodes an RNA helicase with emerging functions in corticogenesis and synaptogenesis.

METHODS

We generated a Ddx3x haploinsufficient mouse (Ddx3x^+/- females) with construct validity for DDX3X loss-of-function mutations. We used standardized batteries to assess developmental milestones and adult behaviors, as well as magnetic resonance imaging and immunostaining of cortical projection neurons to capture early postnatal changes in brain development.

RESULTS

Ddx3x^+/- females showed physical, sensory, and motor delays that evolved into behavioral anomalies in adulthood, including hyperactivity, anxiety-like behaviors, cognitive impairments in specific tasks (e.g., contextual fear memory but not novel object recognition memory), and motor deficits. Motor function declined with age but not if mice were previously exposed to behavioral training. Developmental and behavioral changes were associated with a reduction in brain volume, with some regions (e.g., cortex and amygdala) disproportionally affected. Cortical thinning was accompanied by defective cortical lamination, indicating that Ddx3x regulates the balance of glutamatergic neurons in the developing cortex.

CONCLUSIONS

These data shed new light on the developmental mechanisms driving DDX3X syndrome and support construct and face validity of this novel preclinical mouse model.

CELF Family Proteins in Cancer: Highlights on the RNA-Binding Protein/Noncoding RNA Regulatory Axis

Post-transcriptional modifications to coding and non-coding RNAs are unquestionably a pivotal way in which human mRNA and protein diversity can influence the different phases of a transcript's life cycle. CELF (CUGBP Elav-like family) proteins are RBPs (RNA-binding proteins) with pleiotropic capabilities in RNA processing. Their responsibilities extend from alternative splicing and transcript editing in the nucleus to mRNA stability, and translation into the cytoplasm. In this way, CELF family members have been connected to global alterations in cancer proliferation and invasion, leading to their identification as potential tumor suppressors or even oncogenes. Notably, genetic variants, alternative splicing, phosphorylation, acetylation, subcellular distribution, competition with other RBPs, and ultimately lncRNAs, miRNAs, and circRNAs all impact CELF regulation. Discoveries have emerged about the control of CELF functions, particularly via noncoding RNAs, and CELF proteins have been identified as competing, antagonizing, and regulating agents of noncoding RNA biogenesis. On the other hand, CELFs are an intriguing example through which to broaden our understanding of the RBP/noncoding RNA regulatory axis. Balancing these complex pathways in cancer is undeniably pivotal and deserves further research. This review outlines some mechanisms of CELF protein regulation and their functional consequences in cancer physiology.

Regulation of mRNA translation in stem cells; links to brain disorders

Translational control of gene expression is emerging as a cardinal step in the regulation of protein abundance. Especially for embryonic (ESC) and neuronal stem cells (NSC), regulation of mRNA translation is involved in the maintenance of pluripotency but also differentiation. For neuronal stem cells this regulation is linked to the various neuronal subtypes that arise in the developing brain and is linked to numerous brain disorders. Herein, we review translational control mechanisms in ESCs and NSCs during development and differentiation, and briefly discuss their link to brain disorders.

ELAVL4, splicing, and glutamatergic dysfunction precede neuron loss in MAPT mutation cerebral organoids

Frontotemporal dementia (FTD) because of MAPT mutation causes pathological accumulation of tau and glutamatergic cortical neuronal death by unknown mechanisms. We used human induced pluripotent stem cell (iPSC)-derived cerebral organoids expressing tau-V337M and isogenic corrected controls to discover early alterations because of the mutation that precede neurodegeneration. At 2 months, mutant organoids show upregulated expression of MAPT, glutamatergic signaling pathways, and regulators, including the RNA-binding protein ELAVL4, and increased stress granules. Over the following 4 months, mutant organoids accumulate splicing changes, disruption of autophagy function, and build-up of tau and P-tau-S396. By 6 months, tau-V337M organoids show specific loss of glutamatergic neurons as seen in individuals with FTD. Mutant neurons are susceptible to glutamate toxicity, which can be rescued pharmacologically by the PIKFYVE kinase inhibitor apilimod. Our results demonstrate a sequence of events that precede neurodegeneration, revealing molecular pathways associated with glutamate signaling as potential targets for therapeutic intervention in FTD.

Making sense of mRNA landscapes: Translation control in neurodevelopment

Like all other parts of the central nervous system, the mammalian neocortex undergoes temporally ordered set of developmental events, including proliferation, differentiation, migration, cellular identity, synaptogenesis, connectivity formation, and plasticity changes. These neurodevelopmental mechanisms have been characterized by studies focused on transcriptional control. Recent findings, however, have shown that the spatiotemporal regulation of post-transcriptional steps like alternative splicing, mRNA traffic/localization, mRNA stability/decay, and finally repression/derepression of protein synthesis (mRNA translation) have become just as central to the neurodevelopment as transcriptional control. A number of dynamic players act post-transcriptionally in the neocortex to regulate these steps, as RNA binding proteins (RBPs), ribosomal proteins (RPs), long non-coding RNAs, and/or microRNA. Remarkably, mutations in these post-transcriptional regulators have been associated with neurodevelopmental, neurodegenerative, inherited, or often co-morbid disorders, such as microcephaly, autism, epilepsy, intellectual disability, white matter diseases, Rett-syndrome like phenotype, spinocerebellar ataxia, and amyotrophic lateral sclerosis. Here, we focus on the current state, advanced methodologies and pitfalls of this exciting and upcoming field of RNA metabolism with vast potential in understanding fundamental neurodevelopmental processes and pathologies. This article is categorized under: Translation > Translation Regulation RNA in Disease and Development > RNA in Disease RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

RNA-Binding Protein HuD as a Versatile Factor in Neuronal and Non-Neuronal Systems

Simple Summary

Tight regulation of gene expression is critical for various biological processes such as proliferation, development, differentiation, and death; its dysregulation is linked to the pathogenesis of diseases. Gene expression is dynamically regulated by numerous factors at DNA, RNA, and protein levels, and RNA binding proteins (RBPs) and non–coding RNAs play important roles in the regulation of RNA metabolisms. RBPs govern a diverse spectrum of RNA metabolism by recognizing and binding to the secondary structure or the certain sequence of target mRNAs, and their malfunctions caused by aberrant expression or mutation are implicated in disease pathology. HuD, an RBP in the human antigen (Hu) family, has been studied as a pivotal regulator of gene expression in neuronal systems; however, accumulating evidence reveals the significance of HuD in non–neuronal systems including certain types of cancer cells or endocrine cells in the lung, pancreas, and adrenal gland. In addition, the abnormal function of HuD suggests its pathological association with neurological disorders, cancers, and diabetes. Thus, this review discusses HuD–mediated gene regulation in neuronal and non–neuronal systems to address how it works to orchestrate gene expression and how its expression is controlled in the stress response of pathogenesis of diseases.

Abstract

HuD (also known as ELAVL4) is an RNA–binding protein belonging to the human antigen (Hu) family that regulates stability, translation, splicing, and adenylation of target mRNAs. Unlike ubiquitously distributed HuR, HuD is only expressed in certain types of tissues, mainly in neuronal systems. Numerous studies have shown that HuD plays essential roles in neuronal development, differentiation, neurogenesis, dendritic maturation, neural plasticity, and synaptic transmission by regulating the metabolism of target mRNAs. However, growing evidence suggests that HuD also functions as a pivotal regulator of gene expression in non–neuronal systems and its malfunction is implicated in disease pathogenesis. Comprehensive knowledge of HuD expression, abundance, molecular targets, and regulatory mechanisms will broaden our understanding of its role as a versatile regulator of gene expression, thus enabling novel treatments for diseases with aberrant HuD expression. This review focuses on recent advances investigating the emerging role of HuD, its molecular mechanisms of target gene regulation, and its disease relevance in both neuronal and non–neuronal systems.

Adult Upper Cortical Layer Specific Transcription Factor CUX2 Is Expressed in Transient Subplate and Marginal Zone Neurons of the Developing Human Brain

Cut-Like Homeobox 2 (Cux2) is a transcription factor involved in dendrite and spine development, and synapse formation of projection neurons placed in mouse upper neocortical layers. Therefore, Cux2 is often used as an upper layer marker in the mouse brain. However, expression of its orthologue CUX2 remains unexplored in the human fetal neocortex. Here, we show that CUX2 protein is expressed in transient compartments of developing neocortical anlage during the main fetal phases of neocortical laminar development in human brain. During the early fetal phase when neurons of the upper cortical layers are still radially migrating to reach their final place in the cortical anlage, CUX2 was expressed in the marginal zone (MZ), deep cortical plate, and pre-subplate. During midgestation, CUX2 was still expressed in the migrating upper cortical neurons as well as in the subplate (SP) and MZ neurons. At the term age, CUX2 was expressed in the gyral white matter along with its expected expression in the upper layer neurons. In sum, CUX2 was expressed in migratory neurons of prospective superficial layers and in the diverse subpopulation of transient postmigratory SP and MZ neurons. Therefore, our findings indicate that CUX2 is a novel marker of distinct transient, but critical histogenetic events during corticogenesis. Given the Cux2 functions reported in animal models, our data further suggest that the expression of CUX2 in postmigratory SP and MZ neurons is associated with their unique dendritic and synaptogenesis characteristics.

Decoding mixed messages in the developing cortex: translational regulation of neural progenitor fate

Regulation of stem cell fate decisions is elemental to faithful development, homeostasis, and organismal fitness. Emerging data demonstrate pluripotent stem cells exhibit a vast transcriptional landscape, which is refined as cells differentiate. In the developing neocortex, transcriptional priming of neural progenitors, coupled with post-transcriptional control, is critical for defining cell fates of projection neurons. In particular, radial glial progenitors exhibit dynamic post-transcriptional regulation, including subcellular mRNA localization, RNA decay, and translation. These processes involve both cis-regulatory and trans-regulatory factors, many of which are implicated in neurodevelopmental disease. This review highlights emerging post-transcriptional mechanisms which govern cortical development, with a particular focus on translational control of neuronal fates, including those relevant for disease.

Extrinsic Regulators of mRNA Translation in Developing Brain: Story of WNTs

Extrinsic molecules such as morphogens can regulate timed mRNA translation events in developing neurons. In particular, Wingless-type MMTV integration site family, member 3 (Wnt3), was shown to regulate the translation of Foxp2 mRNA encoding a Forkhead transcription factor P2 in the neocortex. However, the Wnt receptor that possibly mediates these translation events remains unknown. Here, we report Frizzled member 7 (Fzd7) as the Wnt3 receptor that lays downstream in Wnt3-regulated mRNA translation. Fzd7 proteins co-localize with Wnt3 ligands in developing neocortices. In addition, the Fzd7 proteins overlap in layer-specific neuronal subpopulations expressing different transcription factors, Foxp1 and Foxp2. When Fzd7 was silenced, we found decreased Foxp2 protein expression and increased Foxp1 protein expression, respectively. The Fzd7 silencing also disrupted the migration of neocortical glutamatergic neurons. In contrast, Fzd7 overexpression reversed the pattern of migratory defects and Foxp protein expression that we found in the Fzd7 silencing. We further discovered that Fzd7 is required for Wnt3-induced Foxp2 mRNA translation. Surprisingly, we also determined that the Fzd7 suppression of Foxp1 protein expression is not Wnt3 dependent. In conclusion, it is exhibited that the interaction between Wnt3 and Fzd7 regulates neuronal identity and the Fzd7 receptor functions as a downstream factor in ligand Wnt3 signaling for mRNA translation. In particular, the Wnt3-Fzd7 signaling axis determines the deep layer Foxp2-expressing neurons of developing neocortices. Our findings also suggest that Fzd7 controls the balance of the expression for Foxp transcription factors in developing neocortical neurons. These discoveries are presented in our manuscript within a larger framework of this review on the role of extrinsic factors in regulating mRNA translation.

Linking Autism Risk Genes to Disruption of Cortical Development

Autism spectrum disorder (ASD) is a prevalent neurodevelopmental disorder characterized by impairments in social communication and social interaction, and the presence of repetitive behaviors and/or restricted interests. In the past few years, large-scale whole-exome sequencing and genome-wide association studies have made enormous progress in our understanding of the genetic risk architecture of ASD. While showing a complex and heterogeneous landscape, these studies have led to the identification of genetic loci associated with ASD risk. The intersection of genetic and transcriptomic analyses have also begun to shed light on functional convergences between risk genes, with the mid-fetal development of the cerebral cortex emerging as a critical nexus for ASD. In this review, we provide a concise summary of the latest genetic discoveries on ASD. We then discuss the studies in postmortem tissues, stem cell models, and rodent models that implicate recently identified ASD risk genes in cortical development.

Identification and Characterization of Hypothalamic Alternative Splicing Events and Variants in Ovine Fecundity-Related Genes

Simple Summary

Previous studies revealed that alternative splicing (AS) events and gene variants played key roles in reproduction. However, their location and distribution in hypothalamic fecundity-related genes in sheep without the FecB mutation remain largely unknown. In this study, we performed a correlation analysis of transcriptomics and proteomics, and the results suggested several differentially expressed genes (DEGs)/differentially expressed proteins (DEPs), including galectin 3 (LGALS3), aspartoacylase (ASPA) and transthyretin (TTR), could be candidate genes influencing ovine litter size. Further analysis suggested that AS events, single nucleotide polymorphisms (SNPs) and microRNA (miRNA)-binding sites existed in key DEGs/DEPs, such as ASPA and TTR. This study provides a new insight into ovine and even other mammalian reproduction.

Abstract

Previous studies revealed that alternative splicing (AS) events and gene variants played key roles in reproduction; however, their location and distribution in hypothalamic fecundity-related genes in sheep without the FecB mutation remain largely unknown. Therefore, in this study, we described the hypothalamic AS events and variants in differentially expressed genes (DEGs) in Small Tail Han sheep without the FecB mutation at polytocous sheep in the follicular phase vs. monotocous sheep in the follicular phase (PF vs. MF) and polytocous sheep in the luteal phase vs. monotocous sheep in the luteal phase (PL vs. ML) via an RNA-seq study for the first time. We found 39 DEGs with AS events (AS DEGs) in PF vs. MF, while 42 AS DEGs were identified in PL vs. ML. No DEGs with single nucleotide polymorphisms (SNPs) were observed in PF vs. MF, but five were identified in PL vs. ML. We also performed a correlation analysis of transcriptomics and proteomics, and the results suggested several key DEGs/differentially expressed proteins (DEPs), such as galectin 3 (LGALS3) in PF vs. MF and aspartoacylase (ASPA) and transthyretin (TTR) in PL vs. ML, could be candidate genes influencing ovine litter size. In addition, further analyses suggested that AS events, SNPs and miRNA-binding sites existed in key DEGs/DEPs, such as ASPA and TTR. All in all, this study provides a new insight into ovine and even other mammalian reproduction.