Snail coordinately regulates downstream pathways to control multiple aspects of mammalian neural precursor development

Staufen 1 is expressed by neural precursor cells in the developing murine cortex but is dispensable for NPC self-renewal and neuronal differentiation in vitro

BACKGROUND

Proper development of the cerebral cortex relies on asymmetric divisions of neural precursor cells (NPCs) to produce a recurring NPC and a differentiated neuron. Asymmetric divisions are promoted by the differential localization of cell-fate determinants, such as mRNA, between daughter cells. Staufen 1 (Stau1) is an RNA-binding protein known to localize mRNA in mature hippocampal neurons. Its expression pattern and role in the developing mammalian cortex remains unknown.

RESULTS

Both stau1 mRNA and Stau1 protein were found to be expressed in all cells of the developing murine cortex. Stau1 protein expression was characterized spatially and temporally throughout cortical development and found to be present in all stages investigated. We observed expression in the nucleus, cytoplasm and distal processes of both NPCs and newly born neurons and found it to shuttle between the nucleus and the cytoplasm. Upon shRNA-mediated knock-down of Stau1 in primary cultures of the developing cortex, we did not observe any phenotype in NPCs. They were able to both self-renew and generate neurons in the absence of Stau1 expression.

CONCLUSIONS

We propose that Stau1 is either dispensable for the development of the cerebral cortex or that its paralogue, Stau2, is able to compensate for its loss.

The DEAD-box helicase DDX56 is a conserved stemness regulator in normal and cancer stem cells

Across the animal kingdom, adult tissue homeostasis is regulated by adult stem cell activity, which is commonly dysregulated in human cancers. However, identifying key regulators of stem cells in the milieu of thousands of genes dysregulated in a given cancer is challenging. Here, using a comparative genomics approach between planarian adult stem cells and patient-derived glioblastoma stem cells (GSCs), we identify and demonstrate the role of DEAD-box helicase DDX56 in regulating aspects of stemness in four stem cell systems: planarians, mouse neural stem cells, human GSCs, and a fly model of glioblastoma. In a human GSC line, DDX56 localizes to the nucleolus, and using planarians, when DDX56 is lost, stem cells dysregulate expression of ribosomal RNAs and lose nucleolar integrity prior to stem cell death. Together, a comparative genomic approach can be used to uncover conserved stemness regulators that are functional in both normal and cancer stem cells.

Functional Transcription Factor Target Networks Illuminate Control of Epithelial Remodelling

Cell identity is governed by gene expression, regulated by transcription factor (TF) binding at cis-regulatory modules. Decoding the relationship between TF binding patterns and gene regulation is nontrivial, remaining a fundamental limitation in understanding cell decision-making. We developed the NetNC software to predict functionally active regulation of TF targets; demonstrated on nine datasets for the TFs Snail, Twist, and modENCODE Highly Occupied Target (HOT) regions. Snail and Twist are canonical drivers of epithelial to mesenchymal transition (EMT), a cell programme important in development, tumour progression and fibrosis. Predicted "neutral" (non-functional) TF binding always accounted for the majority (50% to 95%) of candidate target genes from statistically significant peaks and HOT regions had higher functional binding than most of the Snail and Twist datasets examined. Our results illuminated conserved gene networks that control epithelial plasticity in development and disease. We identified new gene functions and network modules including crosstalk with notch signalling and regulation of chromatin organisation, evidencing networks that reshape Waddington's epigenetic landscape during epithelial remodelling. Expression of orthologous functional TF targets discriminated breast cancer molecular subtypes and predicted novel tumour biology, with implications for precision medicine. Predicted invasion roles were validated using a tractable cell model, supporting our approach.

Drosophila Neuroblast Selection Is Gated by Notch, Snail, SoxB, and EMT Gene Interplay

In the developing Drosophila central nervous system (CNS), neural progenitor (neuroblast [NB]) selection is gated by lateral inhibition, controlled by Notch signaling and proneural genes. However, proneural mutants still generate many NBs, indicating the existence of additional proneural genes. Moreover, recent studies reveal involvement of key epithelial-mesenchymal transition (EMT) genes in NB selection, but the regulatory interplay between Notch signaling and the EMT machinery is unclear. We find that SoxNeuro (SoxB family) and worniu (Snail family) are integrated with the Notch pathway, and constitute the missing proneural genes. Notch signaling, the proneural, SoxNeuro, and worniu genes regulate key EMT genes to orchestrate the NB selection process. Hence, we uncover an expanded lateral inhibition network for NB selection and demonstrate its link to key players in the EMT machinery. The evolutionary conservation of the genes involved suggests that the Notch-SoxB-Snail-EMT network may control neural progenitor selection in many other systems.

p73 induction by Abrus agglutinin facilitates Snail ubiquitination to inhibit epithelial to mesenchymal transition in oral cancer

BACKGROUND

Epithelial-to-mesenchymal transition (EMT), a key step in oral cancer progression, is associated with invasion, metastasis, and therapy resistance, thus targeting the EMT represents a critical therapeutic strategy for the treatment of oral cancer metastasis. Our previous study showed that Abrus agglutinin (AGG), a plant lectin, induces both intrinsic and extrinsic apoptosis to activate the tumor inhibitory mechanism.

OBJECTIVE

This study aimed to investigate the role of AGG in modulating invasiveness and stemness through EMT inhibition for the development of antineoplastic agents against oral cancer.

METHODS

The EMT- and stemness-related proteins were studied in oral cancer cells using Western blot analysis and fluorescence microscopy. The potential mechanisms of Snail downregulation through p73 activation in FaDu cells were evaluated using Western blot analysis, immunoprecipitation, confocal microscopy, and molecular docking analysis. Immunohistochemical staining of the tumor samples of AGG-treated FaDu-xenografted nude mice was performed.

RESULTS

At the molecular level, AGG-induced p73 suppressed Snail expression, leading to EMT inhibition in FaDu cells. Notably, AGG promoted the translocation of Snail from the nucleus to the cytoplasm in FaDu cells and triggered its degradation through ubiquitination. In this setting, AGG inhibited the interaction between Snail and p73 in FaDu cells, resulting in p73 activation and EMT inhibition. Moreover, in epidermal growth factor (EGF)-stimulated FaDu cells, AGG abolished the upregulation of extracellular signal-regulated kinase (ERK)1/2 that plays a pivotal role in the upregulation of Snail to regulate the EMT phenotypes. In immunohistochemistry analysis, FaDu xenografts from AGG-treated mice showed decreased expression of Snail, SOX2, and vimentin and increased expression of p73 and E-cadherin compared with the control group, confirming EMT inhibition as part of its anticancer efficacy against oral cancer.

CONCLUSION

In summary, AGG stimulates p73 in restricting EGF-induced EMT, invasiveness, and stemness by inhibiting the ERK/Snail pathway to facilitate the development of alternative therapeutics for oral cancer.

Brain expansion promoted by polycomb-mediated anterior enhancement of a neural stem cell proliferation program

During central nervous system (CNS) development, genetic programs establish neural stem cells and drive both stem and daughter cell proliferation. However, the prominent anterior expansion of the CNS implies anterior–posterior (A–P) modulation of these programs. In Drosophila, a set of neural stem cell factors acts along the entire A–P axis to establish neural stem cells. Brain expansion results from enhanced stem and daughter cell proliferation, promoted by a Polycomb Group (PcG)->Homeobox (Hox) homeotic network. But how does PcG->Hox modulate neural-stem-cell–factor activity along the A–P axis? We find that the PcG->Hox network creates an A–P expression gradient of neural stem cell factors, thereby driving a gradient of proliferation. PcG mutants can be rescued by misexpression of the neural stem cell factors or by mutation of one single Hox gene. Hence, brain expansion results from anterior enhancement of core neural-stem-cell–factor expression, mediated by PcG repression of brain Hox expression.

A study in fruit flies shows that the anterior expansion of the central nervous system, to form the brain, is driven by Polycomb-mediated repression of Hox genes, resulting in anterior enhancement of a neural stem cell program.

The central nervous system displays a pronounced anterior expansion that forms the brain. In the fruit fly Drosophila melanogaster, this expansion is driven by enhanced anterior cell proliferation. Recent studies reveal that cell proliferation in the brain is promoted by the Polycomb Group Complex, a key epigenetic complex. During development of the central nervous system, the Polycomb Group Complex acts to exclude Hox homeotic gene expression from the brain, thereby rendering the brain a Hox-free zone. Hox genes act in an antiproliferative manner, which explains the hyperproliferation observed in the brain, as well as the gradient of proliferation along the anterior–posterior axis of the central nervous system. Here, we find that Hox genes act by repressing a common neural stem cell proliferation program in more posterior regions, resulting in an anterior–posterior gradient of “stemness.” Hence, elevated anterior proliferation is promoted by the Polycomb Group Complex acting to keep the brain free of negative Hox input, thereby ensuring elevated expression of neural stem cell factors in the brain. Strikingly, mutants of the Polycomb Group Complex can be rescued by mutation of one single Hox gene, demonstrating that the primary role of the Polycomb Group Complex is indeed Hox repression. This study advances our understanding of how neural stem cell programs operate at different axial levels of the central nervous system and may have implications also for stem cell and organoid biology.

The centrosome protein AKNA regulates neurogenesis via microtubule organization

The expansion of brain size is accompanied by a relative enlargement of the subventricular zone during development. Epithelial-like neural stem cells divide in the ventricular zone at the ventricles of the embryonic brain, self-renew and generate basal progenitors¹ that delaminate and settle in the subventricular zone in enlarged brain regions². The length of time that cells stay in the subventricular zone is essential for controlling further amplification and fate determination. Here we show that the interphase centrosome protein AKNA has a key role in this process. AKNA localizes at the subdistal appendages of the mother centriole in specific subtypes of neural stem cells, and in almost all basal progenitors. This protein is necessary and sufficient to organize centrosomal microtubules, and promote their nucleation and growth. These features of AKNA are important for mediating the delamination process in the formation of the subventricular zone. Moreover, AKNA regulates the exit from the subventricular zone, which reveals the pivotal role of centrosomal microtubule organization in enabling cells to both enter and remain in the subventricular zone. The epithelial-to-mesenchymal transition is also regulated by AKNA in other epithelial cells, demonstrating its general importance for the control of cell delamination.

Regulation of Polarity Protein Levels in the Developing Central Nervous System

In the course of their development from neuroepithelial cells to mature neurons, neuronal progenitors proliferate, delaminate, differentiate, migrate, and extend processes to form a complex neuronal network. In addition to supporting the morphology of the neuroepithelium and radial glia, polarity proteins contribute to the remodeling of processes and support the architectural reorganizations that result in axon extension and dendrite formation. While a good amount of evidence highlights a rheostat-like regulation by signaling events leading to local activation and/or redistribution of polarity proteins, recent studies demonstrate a new paradigm involving a switch-like regulation directly controlling the availability of polarity protein at specific stage by transcriptional regulation and/or targeted ubiquitin proteasome degradation. During the process of differentiation, most neurons will adopt a morphology with reduced polarity which suggests that polarity complex proteins are strongly repressed during key step of development. Here we review the different mechanisms that directly impact the levels of polarity complex proteins in neurons in relation to the polarization context and discuss why this transient loss of polarity is essential to understand neural development and how this knowledge could be relevant for some neuropathy.

Endothelial HIF-2α contributes to severe pulmonary hypertension due to endothelial-to-mesenchymal transition

Pulmonary vascular remodeling characterized by concentric wall thickening and intraluminal obliteration is a major contributor to the elevated pulmonary vascular resistance in patients with idiopathic pulmonary arterial hypertension (IPAH). Here we report that increased hypoxia-inducible factor 2α (HIF-2α) in lung vascular endothelial cells (LVECs) under normoxic conditions is involved in the development of pulmonary hypertension (PH) by inducing endothelial-to-mesenchymal transition (EndMT), which subsequently results in vascular remodeling and occlusive lesions. We observed significant EndMT and markedly increased expression of SNAI, an inducer of EndMT, in LVECs from patients with IPAH and animals with experimental PH compared with normal controls. LVECs isolated from IPAH patients had a higher level of HIF-2α than that from normal subjects, whereas HIF-1α was upregulated in pulmonary arterial smooth muscle cells (PASMCs) from IPAH patients. The increased HIF-2α level, due to downregulated prolyl hydroxylase domain protein 2 (PHD2), a prolyl hydroxylase that promotes HIF-2α degradation, was involved in enhanced EndMT and upregulated SNAI1/2 in LVECs from patients with IPAH. Moreover, knockdown of HIF-2α (but not HIF-1α) with siRNA decreases both SNAI1 and SNAI2 expression in IPAH-LVECs. Mice with endothelial cell (EC)-specific knockout (KO) of the PHD2 gene, egln1 (egln1EC-/-), developed severe PH under normoxic conditions, whereas Snai1/2 and EndMT were increased in LVECs of egln1EC-/- mice. EC-specific KO of the HIF-2α gene, hif2a, prevented mice from developing hypoxia-induced PH, whereas EC-specific deletion of the HIF-1α gene, hif1a, or smooth muscle cell (SMC)-specific deletion of hif2a, negligibly affected the development of PH. Also, exposure to hypoxia for 48-72 h increased protein level of HIF-1α in normal human PASMCs and HIF-2α in normal human LVECs. These data indicate that increased HIF-2α in LVECs plays a pathogenic role in the development of severe PH by upregulating SNAI1/2, inducing EndMT, and causing obliterative pulmonary vascular lesions and vascular remodeling.

Neural Lineage Progression Controlled by a Temporal Proliferation Program

Great progress has been made in identifying transcriptional programs that establish stem cell identity. In contrast, we have limited insight into how these programs are down-graded in a timely manner to halt proliferation and allow for cellular differentiation. Drosophila embryonic neuroblasts undergo such a temporal progression, initially dividing to bud off daughters that divide once (type I), then switching to generating non-dividing daughters (type 0), and finally exiting the cell cycle. We identify six early transcription factors that drive neuroblast and type I daughter proliferation. Early factors are gradually replaced by three late factors, acting to trigger the type I→0 daughter proliferation switch and eventually to stop neuroblasts. Early and late factors regulate each other and four key cell-cycle genes, providing a logical genetic pathway for these transitions. The identification of this extensive driver-stopper temporal program controlling neuroblast lineage progression may have implications for studies in many other systems.

Potential Involvement of Snail Members in Neuronal Survival and Astrocytic Migration during the Gecko Spinal Cord Regeneration

Certain regenerative vertebrates such as fish, amphibians and reptiles are capable of regenerating spinal cord after injury. Most neurons of spinal cord will survive from the injury and regrow axons to repair circuits with an absence of glial scar formation. However, the underlying mechanisms of neuronal anti-apoptosis and glia-related responses have not been fully clarified during the regenerative process. Gecko has becoming an inspiring model to address spinal cord regeneration in amniotes. In the present study, we investigated the regulatory roles of Snail family members, the important transcriptional factors involved in both triggering of the cell migration and cell survival, during the spontaneous spinal cord regeneration. Both Snail1 and Snail3 have been shown to promote neuronal survival and astrocytic migration via anti-apoptotic and GTPases signaling following gecko tail amputation. Transforming growth factor-beta (TGFβ), together with other cytokines were involved in inducing expression of Snail protein. Our data indicate a conserved function of Snail proteins in embryonic development and tissue regeneration, which may provide clues for CNS repair in the mammals.

A flexible genetic toolkit for arthropod neurogenesis

Arthropods show considerable variations in early neurogenesis. This includes the pattern of specification, division and movement of neural precursors and progenitors. In all metazoans with nervous systems, including arthropods, conserved genes regulate neurogenesis, which raises the question of how the various morphological mechanisms have emerged and how the same genetic toolkit might generate different morphological outcomes. Here I address this question by comparing neurogenesis across arthropods and show how variations in the regulation and function of the neural genes might explain this phenomenon and how they might have facilitated the evolution of the diverse morphological mechanisms of neurogenesis.

Conditional ablation of p63 indicates that it is essential for embryonic development of the central nervous system

p63 is a member of the p53 family that regulates the survival of neural precursors in the adult brain. However, the relative importance of p63 in the developing brain is still unclear, since embryonic p63^−/− mice display no apparent deficits in neural development. Here, we have used a more definitive conditional knockout mouse approach to address this issue, crossing p63^fl/fl mice to mice carrying a nestin-CreERT2 transgene that drives inducible recombination in neural precursors following tamoxifen treatment. Inducible ablation of p63 following tamoxifen treatment of mice on embryonic day 12 resulted in highly perturbed forebrain morphology including a thinner cortex and enlarged lateral ventricles 3 d later. While the normal cortical layers were still present following acute p63 ablation, cortical precursors and neurons were both reduced in number due to widespread cellular apoptosis. This apoptosis was cell-autonomous, since it also occurred when p63 was inducibly ablated in primary cultured cortical precursors. Finally, we demonstrate increased expression of the mRNA encoding another p53 family member, ΔNp73, in cortical precursors of p63^−/− but not tamoxifen-treated p63^fl/fl;R26YFP^fl/fl;nestin-CreERT2^+/Ø embryos. Since ΔNp73 promotes cell survival, then this compensatory increase likely explains the lack of an embryonic brain phenotype in p63^−/− mice. Thus, p63 plays a key prosurvival role in the developing mammalian brain.

Escargot and Scratch regulate neural commitment by antagonizing Notch activity in Drosophila sensory organs

During Notch (N)-mediated binary cell fate decisions, cells adopt two different fates according to the levels of N pathway activation: an Noff-dependent or an Non-dependent fate. How cells maintain these N activity levels over time remains largely unknown. We address this question in the cell lineage that gives rise to the Drosophila mechanosensory organs. In this lineage a primary precursor cell undergoes a stereotyped sequence of oriented asymmetric cell divisions and transits through two neural precursor states before acquiring a neuron identity. Using a combination of genetic and cell biology strategies, we show that Escargot and Scratch, two transcription factors belonging to the Snail superfamily, maintain Noff neural commitment by directly blocking the transcription of N target genes. We propose that Snail factors act by displacing proneural transcription activators from DNA binding sites. As such, Snail factors maintain the Noff state in neural precursor cells by buffering any ectopic variation in the level of N activity. Since Escargot and Scratch orthologs are present in other precursor cells, our findings are fundamental for understanding precursor cell fate acquisition in other systems.

Polarity transitions during neurogenesis and germinal zone exit in the developing central nervous system

During neural development, billions of neurons differentiate, polarize, migrate and form synapses in a precisely choreographed sequence. These precise developmental events are accompanied by discreet transitions in cellular polarity. While radial glial neural stem cells are highly polarized, transiently amplifying neural progenitors are less polarized after delaminating from their parental stem cell. Moreover, preceding their radial migration to a final laminar position neural progenitors re-adopt a polarized morphology before they embarking on their journey along a glial guide to the destination where they will fully mature. In this review, we will compare and contrast the key polarity transitions of cells derived from a neuroepithelium to the well-characterized polarity transitions that occur in true epithelia. We will highlight recent advances in the field that shows that neuronal progenitor delamination from germinal zone (GZ) niche shares similarities to an epithelial-mesenchymal transition. Moreover, studies in the cerebellum suggest the acquisition of radial migration and polarity in transiently amplifying neural progenitors share similarities to mesenchymal-epithelial transitions. Where applicable, we will compare and contrast the precise molecular mechanisms used by epithelial cells and neuronal progenitors to control plasticity in cell polarity during their distinct developmental programs.

The evolution of early neurogenesis

The foundation of the diverse metazoan nervous systems is laid by embryonic patterning mechanisms, involving the generation and movement of neural progenitors and their progeny. Here we divide early neurogenesis into discrete elements, including origin, pattern, proliferation, and movement of neuronal progenitors, which are controlled by conserved gene cassettes. We review these neurogenetic mechanisms in representatives of the different metazoan clades, with the goal to build a conceptual framework in which one can ask specific questions, such as which of these mechanisms potentially formed part of the developmental "toolkit" of the bilaterian ancestor and which evolved later.

Ankrd11 is a chromatin regulator involved in autism that is essential for neural development

Ankrd11 is a potential chromatin regulator implicated in neural development and autism spectrum disorder (ASD) with no known function in the brain. Here, we show that knockdown of Ankrd11 in developing murine or human cortical neural precursors caused decreased proliferation, reduced neurogenesis, and aberrant neuronal positioning. Similar cellular phenotypes and aberrant ASD-like behaviors were observed in Yoda mice carrying a point mutation in the Ankrd11 HDAC-binding domain. Consistent with a role for Ankrd11 in histone acetylation, Ankrd11 was associated with chromatin and colocalized with HDAC3, and expression and histone acetylation of Ankrd11 target genes were altered in Yoda neural precursors. Moreover, the Ankrd11 knockdown-mediated decrease in precursor proliferation was rescued by inhibiting histone acetyltransferase activity or expressing HDAC3. Thus, Ankrd11 is a crucial chromatin regulator that controls histone acetylation and gene expression during neural development, thereby providing a likely explanation for its association with cognitive dysfunction and ASD.

An eIF4E1/4E-T complex determines the genesis of neurons from precursors by translationally repressing a proneurogenic transcription program

Here, we have addressed the mechanisms that determine genesis of the correct numbers of neurons during development, focusing on the embryonic cortex. We identify in neural precursors a repressive complex involving eIF4E1 and its binding partner 4E-T that coordinately represses translation of proteins that determine neurogenesis. This eIF4E1/4E-T complex is present in granules with the processing body proteins Lsm1 and Rck, and disruption of this complex causes premature and enhanced neurogenesis and neural precursor depletion. Analysis of the 4E-T complex shows that it is highly enriched in mRNAs encoding transcription factors and differentiation-related proteins. These include the proneurogenic bHLH mRNAs, which colocalize with 4E-T in granules and whose protein products are aberrantly upregulated following knockdown of eIF4E, 4E-T, or processing body proteins. Thus, neural precursors are transcriptionally primed to generate neurons, but an eIF4E/4E-T complex sequesters and represses translation of proneurogenic proteins to determine appropriate neurogenesis.

The Snail transcription factor regulates the numbers of neural precursor cells and newborn neurons throughout mammalian life

The Snail transcription factor regulates diverse aspects of stem cell biology in organisms ranging from Drosophila to mammals. Here we have asked whether it regulates the biology of neural precursor cells (NPCs) in the forebrain of postnatal and adult mice, taking advantage of a mouse containing a floxed Snail allele (Snail^fl/fl mice). We show that when Snail is inducibly ablated in the embryonic cortex, this has long-term consequences for cortical organization. In particular, when Snail^fl/fl mice are crossed to Nestin-cre mice that express Cre recombinase in embryonic neural precursors, this causes inducible ablation of Snail expression throughout the postnatal cortex. This loss of Snail causes a decrease in proliferation of neonatal cortical neural precursors and mislocalization and misspecification of cortical neurons. Moreover, these precursor phenotypes persist into adulthood. Adult neural precursor cell proliferation is decreased in the forebrain subventricular zone and in the hippocampal dentate gyrus, and this is coincident with a decrease in the number of adult-born olfactory and hippocampal neurons. Thus, Snail is a key regulator of the numbers of neural precursors and newborn neurons throughout life.