Neural Progenitor Cells Undergoing Yap/Tead-Mediated Enhanced Self-Renewal Form Heterotopias More Easily in the Diencephalon than in the Telencephalon

YAP in development and disease: Navigating the regulatory landscape from retina to brain

The distinctive role of Yes-associated protein (YAP) in the nervous system has attracted widespread attention. This comprehensive review strategically uses the retina as a vantage point, embarking on an extensive exploration of YAP's multifaceted impact from the retina to the brain in development and pathology. Initially, we explore the crucial roles of YAP in embryonic and cerebral development. Our focus then shifts to retinal development, examining in detail YAP's regulatory influence on the development of retinal pigment epithelium (RPE) and retinal progenitor cells (RPCs), and its significant effects on the hierarchical structure and functionality of the retina. We also investigate the essential contributions of YAP in maintaining retinal homeostasis, highlighting its precise regulation of retinal cell proliferation and survival. In terms of retinal-related diseases, we explore the epigenetic connections and pathophysiological regulation of YAP in diabetic retinopathy (DR), glaucoma, and proliferative vitreoretinopathy (PVR). Lastly, we broaden our exploration from the retina to the brain, emphasizing the research paradigm of "retina: a window to the brain." Special focus is given to the emerging studies on YAP in brain disorders such as Alzheimer's disease (AD) and Parkinson's disease (PD), underlining its potential therapeutic value in neurodegenerative disorders and neuroinflammation.

Human cerebral organoids reveal progenitor pathology in EML1-linked cortical malformation

Malformations of human cortical development (MCD) can cause severe disabilities. The lack of human‐specific models hampers our understanding of the molecular underpinnings of the intricate processes leading to MCD. Here, we use cerebral organoids derived from patients and genome edited‐induced pluripotent stem cells to address pathophysiological changes associated with a complex MCD caused by mutations in the echinoderm microtubule‐associated protein‐like 1 (EML1) gene. EML1‐deficient organoids display ectopic neural rosettes at the basal side of the ventricular zone areas and clusters of heterotopic neurons. Single‐cell RNA sequencing shows an upregulation of basal radial glial (RG) markers and human‐specific extracellular matrix components in the ectopic cell population. Gene ontology and molecular analyses suggest that ectopic progenitor cells originate from perturbed apical RG cell behavior and yes‐associated protein 1 (YAP1)‐triggered expansion. Our data highlight a progenitor origin of EML1 mutation‐induced MCD and provide new mechanistic insight into the human disease pathology.

Abnormal activation of Yap/Taz contributes to the pathogenesis of tuberous sclerosis complex

The multi-systemic genetic disorder tuberous sclerosis complex (TSC) impacts multiple neurodevelopmental processes including neuronal morphogenesis, neuronal migration, myelination and gliogenesis. These alterations contribute to the development of cerebral cortex abnormalities and malformations. Although TSC is caused by mTORC1 hyperactivation, cognitive and behavioral impairments are not improved through mTORC1 targeting, making the study of the downstream effectors of this complex important for understanding the mechanisms underlying TSC. As mTORC1 has been shown to promote the activity of the transcriptional co-activator Yap, we hypothesized that altered Yap/Taz signaling contributes to the pathogenesis of TSC. We first observed that the levels of Yap/Taz are increased in human cortical tuber samples and in embryonic cortices of Tsc2 conditional knockout (cKO) mice. Next, to determine how abnormal upregulation of Yap/Taz impacts the neuropathology of TSC, we deleted Yap/Taz in Tsc2 cKO mice. Importantly, Yap/Taz/Tsc2 triple conditional knockout (tcKO) animals show reduced cortical thickness and cortical neuron cell size, despite the persistence of high mTORC1 activity, suggesting that Yap/Taz play a downstream role in cytomegaly. Furthermore, Yap/Taz/Tsc2 tcKO significantly restored cortical and hippocampal lamination defects and reduced hippocampal heterotopia formation. Finally, the loss of Yap/Taz increased the distribution of myelin basic protein in Tsc2 cKO animals, consistent with an improvement in myelination. Overall, our results indicate that targeting Yap/Taz lessens the severity of neuropathology in a TSC animal model. This study is the first to implicate Yap/Taz as contributors to cortical pathogenesis in TSC and therefore as potential novel targets in the treatment of this disorder.

YAP maintains the production of intermediate progenitor cells and upper-layer projection neurons in the mouse cerebral cortex

BACKGROUND

The Hippo pathway is conserved through evolution and plays critical roles in development, tissue homeostasis and tumorigenesis. Yes-associated protein (YAP) is a transcriptional coactivator downstream of the Hippo pathway. Previous studies have demonstrated that activation of YAP promotes proliferation in the developing brain. Whether YAP is required for the production of neural progenitor cells or neurons in vivo remains unclear.

RESULTS

We demonstrated that SATB homeobox 2 (SATB2)-positive projection neurons (PNs) in upper layers, but not T-box brain transcription factor 1-positive and Coup-TF interacting protein 2-positive PNs in deep layers, were decreased in the neonatal cerebral cortex of Yap conditional knockout (cKO) mice driven by Nestin-Cre. Cell proliferation was reduced in the developing cerebral cortex of Yap-cKO. SATB2-positive PNs are largely generated from intermediate progenitor cells (IPCs), which are derived from radial glial cells (RGCs) during cortical development. Among these progenitor cells, IPCs but not RGCs were decreased in Yap-cKO. We further demonstrated that cell cycle re-entry was reduced in progenitor cells of Yap-cKO, suggesting that fewer IPCs were generated in Yap-cKO.

CONCLUSION

YAP is required for the production of IPCs and upper-layer SATB2-positive PNs during development of the cerebral cortex in mice.

Rewiring of human neurodevelopmental gene regulatory programs by human accelerated regions

Human accelerated regions (HARs) are the fastest-evolving regions of the human genome, and many are hypothesized to function as regulatory elements that drive human-specific gene regulatory programs. We interrogate the in vitro enhancer activity and in vivo epigenetic landscape of more than 3,100 HARs during human neurodevelopment, demonstrating that many HARs appear to act as neurodevelopmental enhancers and that sequence divergence at HARs has largely augmented their neuronal enhancer activity. Furthermore, we demonstrate PPP1R17 to be a putative HAR-regulated gene that has undergone remarkable rewiring of its cell type and developmental expression patterns between non-primates and primates and between non-human primates and humans. Finally, we show that PPP1R17 slows neural progenitor cell cycle progression, paralleling the cell cycle length increase seen predominantly in primate and especially human neurodevelopment. Our findings establish HARs as key components in rewiring human-specific neurodevelopmental gene regulatory programs and provide an integrated resource to study enhancer activity of specific HARs.

Emerging roles for the YAP/TAZ transcriptional regulators in brain tumour pathology and targeting options

The transcriptional co-activators Yes-associated protein 1/transcriptional co-activator with PDZ-binding motif (YAP/TAZ) have emerged as significant regulators of a wide variety of cellular and organ functions with impact in early embryonic development, especially during the expansion of the neural progenitor cell pool. YAP/TAZ signalling regulates organ size development, tissue homeostasis, wound healing and angiogenesis by participating in a complex network of various pathways. However, recent evidence suggests an association of these physiologic regulatory effects of YAP/TAZ with pro-oncogenic activities. Herein, we discuss the physiological functions of YAP/TAZ as well as the extensive network of signalling pathways that control their expression and activity, leading to brain tumour development and progression. Furthermore, we describe current targeting approaches and drug options including direct YAP/TAZ and YAP-TEA domain transcription factor (TEAD) interaction inhibitors, G-protein coupled receptors (GPCR) signalling modulators and kinase inhibitors, which may be used to successfully attack YAP/TAZ-dependent tumours.

TEAD family transcription factors in development and disease

The balance between stem cell potency and lineage specification entails the integration of both extrinsic and intrinsic cues, which ultimately influence gene expression through the activity of transcription factors. One example of this is provided by the Hippo signalling pathway, which plays a central role in regulating organ size during development. Hippo pathway activity is mediated by the transcriptional co-factors Yes-associated protein (YAP) and transcriptional co-activator with PDZ-binding motif (TAZ), which interact with TEA domain (TEAD) proteins to regulate gene expression. Although the roles of YAP and TAZ have been intensively studied, the roles played by TEAD proteins are less well understood. Recent studies have begun to address this, revealing that TEADs regulate the balance between progenitor self-renewal and differentiation throughout various stages of development. Furthermore, it is becoming apparent that TEAD proteins interact with other co-factors that influence stem cell biology. This Primer provides an overview of the role of TEAD proteins during development, focusing on their role in Hippo signalling as well as within other developmental, homeostatic and disease contexts.

Neuronal Hippo signaling: From development to diseases

Hippo signaling pathway is a highly conserved and familiar tissue growth regulator, primarily dealing with cell survival, cell proliferation, and apoptosis. The Yes-associated protein (YAP) is the key transcriptional effector molecule, which is under negative regulation of the Hippo pathway. Wealth of studies have identified crucial roles of Hippo/YAP signaling pathway during the process of development, including the development of neuronal system. We provide here, an overview of the contributions of this signaling pathway at multiple stages of neuronal development including, proliferation of neural stem cells (NSCs), migration of NSCs toward their destined niche, maintaining NSCs in the quiescent state, differentiation of NSCs into neurons, neuritogenesis, synaptogenesis, brain development, and in neuronal apoptosis. Hyperactivation of the neuronal Hippo pathway can also lead to a variety of devastating neurodegenerative diseases. Instances of aberrant Hippo pathway leading to neurodegenerative diseases along with the approaches utilizing this pathway as molecular targets for therapeutics has been highlighted in this review. Recent evidences suggesting neuronal repair and regenerative potential of this pathway has also been pointed out, that will shed light on a novel aspect of Hippo pathway in regenerative medicine. Our review provides a better understanding of the significance of Hippo pathway in the journey of neuronal system from development to diseases as a whole.

High glucose alters the DNA methylation pattern of neurodevelopment associated genes in human neural progenitor cells in vitro

Maternal diabetes alters the global epigenetic mechanisms and expression of genes involved in neural tube development in mouse embryos. Since DNA methylation is a critical epigenetic mechanism that regulates gene functions, gene-specific DNA methylation alterations were estimated in human neural progenitor cells (hNPCs) exposed to high glucose (HG) in the present study. The DNA methylation pattern of genes involved in several signalling pathways including axon guidance (SLIT1-ROBO2 pathway), and Hippo pathway (YAP and TAZ) was altered in hNPCs exposed to HG. The expression levels of SLIT1-ROBO2 pathways genes (including its effectors, SRGAP1 and CDC42) which mediates diverse cellular processes such as proliferation, neurogenesis and axon guidance, and Hippo pathway genes (YAP and TAZ) which regulates proliferation, stemness, differentiation and organ size were downregulated in hNPCs exposed to HG. A recent report suggests a possible cross-talk between SLIT1-ROBO2 and TAZ via CDC42, a mediator of actin dynamics. Consistent with this, SLIT1 knockdown downregulated the expression of its effectors and TAZ in hNPCs, suggesting that HG perturbs the cross-talk between SLIT1-ROBO2 and TAZ in hNPCs. Overall, this study demonstrates that HG epigenetically alters the SLIT1-ROBO2 and Hippo signalling pathways in hNPCs, forming the basis for neurodevelopmental disorders in offspring of diabetic pregnancy.

Role of YAP/TAZ in Cell Lineage Fate Determination and Related Signaling Pathways

The penultimate effectors of the Hippo signaling pathways YAP and TAZ, are transcriptional co-activator proteins that play key roles in many diverse biological processes, ranging from cell proliferation, tumorigenesis, mechanosensing and cell lineage fate determination, to wound healing and regeneration. In this review, we discuss the regulatory mechanisms by which YAP/TAZ control stem/progenitor cell differentiation into the various major lineages that are of interest to tissue engineering and regenerative medicine applications. Of particular interest is the key role of YAP/TAZ in maintaining the delicate balance between quiescence, self-renewal, proliferation and differentiation of endogenous adult stem cells within various tissues/organs during early development, normal homeostasis and regeneration/healing. Finally, we will consider how increasing knowledge of YAP/TAZ signaling might influence the trajectory of future progress in regenerative medicine.

A SMAD1/5-YAP signalling module drives radial glia self-amplification and growth of the developing cerebral cortex

The growth and evolutionary expansion of the cerebral cortex are defined by the spatial-temporal production of neurons, which itself depends on the decision of radial glial cells (RGCs) to self-amplify or to switch to neurogenic divisions. The mechanisms regulating these RGC fate decisions are still incompletely understood. Here, we describe a novel and evolutionarily conserved role of the canonical BMP transcription factors SMAD1/5 in controlling neurogenesis and growth during corticogenesis. Reducing the expression of both SMAD1 and SMAD5 in neural progenitors at early mouse cortical development caused microcephaly and an increased production of early-born cortical neurons at the expense of late-born ones, which correlated with the premature differentiation and depletion of the pool of cortical progenitors. Gain- and loss-of-function experiments performed during early cortical neurogenesis in the chick revealed that SMAD1/5 activity supports self-amplifying RGC divisions and restrains the neurogenic ones. Furthermore, we demonstrate that SMAD1/5 stimulate RGC self-amplification through the positive post-transcriptional regulation of the Hippo signalling effector YAP. We anticipate this SMAD1/5-YAP signalling module to be fundamental in controlling growth and evolution of the amniote cerebral cortex.

YAP Activity Is Necessary and Sufficient for Basal Progenitor Abundance and Proliferation in the Developing Neocortex

Neocortex expansion during mammalian evolution has been linked to an increase in proliferation of basal progenitors in the subventricular zone. Here, we explored a potential role of YAP, the major downstream effector of the Hippo pathway, in proliferation of basal progenitors. YAP expression and activity are high in ferret and human basal progenitors, which exhibit high proliferative capacity, but low in mouse basal progenitors, which lack such capacity. Conditional expression of a constitutively active YAP in mouse basal progenitors resulted in increased proliferation of basal progenitor and promoted production of upper-layer neurons. Pharmacological and genetic interference with YAP function in ferret and human developing neocortex resulted in decreased abundance of cycling basal progenitors. Together, our data indicate that YAP is necessary and sufficient to promote the proliferation of basal progenitors and suggest that increases in YAP levels and presumably activity contributed to the evolutionary expansion of the neocortex.

Transcriptional Regulators and Human-Specific/Primate-Specific Genes in Neocortical Neurogenesis

During development, starting from a pool of pluripotent stem cells, tissue-specific genetic programs help to shape and develop functional organs. To understand the development of an organ and its disorders, it is important to understand the spatio-temporal dynamics of the gene expression profiles that occur during its development. Modifications in existing genes, the de-novo appearance of new genes, or, occasionally, even the loss of genes, can greatly affect the gene expression profile of any given tissue and contribute to the evolution of organs or of parts of organs. The neocortex is evolutionarily the most recent part of the brain, it is unique to mammals, and is the seat of our higher cognitive abilities. Progenitors that give rise to this tissue undergo sequential waves of differentiation to produce the complete sets of neurons and glial cells that make up a functional neocortex. We will review herein our understanding of the transcriptional regulators that control the neural precursor cells (NPCs) during the generation of the most abundant class of neocortical neurons, the glutametergic neurons. In addition, we will discuss the roles of recently-identified human- and primate-specific genes in promoting neurogenesis, leading to neocortical expansion.

Tead transcription factors differentially regulate cortical development

Neural stem cells (NSCs) generate neurons of the cerebral cortex with distinct morphologies and functions. How specific neuron production, differentiation and migration are orchestrated is unclear. Hippo signaling regulates gene expression through Tead transcription factors (TFs). We show that Hippo transcriptional coactivators Yap1/Taz and the Teads have distinct functions during cortical development. Yap1/Taz promote NSC maintenance and Satb2⁺ neuron production at the expense of Tbr1⁺ neuron generation. However, Teads have moderate effects on NSC maintenance and do not affect Satb2⁺ neuron differentiation. Conversely, whereas Tead2 blocks Tbr1⁺ neuron formation, Tead1 and Tead3 promote this early fate. In addition, we found that Hippo effectors regulate neuronal migration to the cortical plate (CP) in a reciprocal fashion, that ApoE, Dab2 and Cyr61 are Tead targets, and these contribute to neuronal fate determination and migration. Our results indicate that multifaceted Hippo signaling is pivotal in different aspects of cortical development.

Two-photon microscopic observation of cell-production dynamics in the developing mammalian neocortex in utero

Morphogenesis and organ development should be understood based on a thorough description of cellular dynamics. Recent studies have explored the dynamic behaviors of mammalian neural progenitor cells (NPCs) using slice cultures in which three‐dimensional systems conserve in vivo‐like environments to a considerable degree. However, live observation of NPCs existing truly in vivo, as has long been performed for zebrafish NPCs, has yet to be established in mammals. Here, we performed intravital two‐photon microscopic observation of NPCs in the developing cerebral cortex of H2B‐EGFP or Fucci transgenic mice in utero. Fetuses in the uterine sac were immobilized using several devices and were observed through a window made in the uterine wall and the amniotic membrane while monitoring blood circulation. Clear visibility was obtained to the level of 300 μm from the scalp surface of the fetus, which enabled us to quantitatively assess NPC behaviors, such as division and interkinetic nuclear migration, within a neuroepithelial structure called the ventricular zone at embryonic day (E) 13 and E14. In fetuses undergoing healthy monitoring in utero for 60 min, the frequency of mitoses observed at the apical surface was similar to those observed in slice cultures and in freshly fixed in vivo specimens. Although the rate and duration of successful in utero observations are still limited (33% for ≥10 min and 14% for 60 min), further improvements based on this study will facilitate future understanding of how organogenetic cellular behaviors occur or are pathologically influenced by the systemic maternal condition and/or maternal‐fetal relationships.

YAP1/Twist promotes fibroblast activation and lung fibrosis that conferred by miR-15a loss in IPF

Idiopathic pulmonary fibrosis (IPF) is a chronic, progressive, fibrotic parenchymal lung disease of unknown etiology and lack effective interventions. Using a combination of in vitro and in vivo studies, we found that overexpression of YAP1, a key effector in the Hippo pathway, promoted cell proliferation, migration, and collagen production in lung fibroblasts. Furthermore, the pro-fibrotic action of YAP1 was mediated by transcriptional activation of Twist1 through interacting with its partner TEAD. In contrast, knockdown of YAP1 inhibited extracellular matrix (ECM) deposition, which ultimately ameliorated lung fibrosis in vitro and in vivo. Additionally, we constructed a dysregulated miRNA regulatory network that affects the expression of the Hippo pathway effectors in IPF and identified miR-15a, which is significantly down-regulated in IPF patients, as one of the most essential miRNAs regulating this pathway. Moreover, knockdown of miR-15a resulted in fibroblast activation and lung fibrosis through promoting Twist expression by targeting inhibition of YAP1. In contrast, therapeutic restoration of miR-15a inhibits fibrogenesis in lung fibroblast and abrogated BLM-induced lung fibrosis in mice. These results highlight a role for miR-15a/YAP1/Twist axis in IPF that offer novel strategies for the prevention and treatment of lung fibrosis.

Differentiating cells mechanically limit progenitor cells’ interkinetic nuclear migration to secure apical cytogenesis

Many proliferative epithelia are pseudostratified due to cell cycle–dependent interkinetic nuclear migration (IKNM, basal during G1 and apical during G2). Although most epithelia, including early embryonic neuroepithelia (≤100 µm thick), undergo IKNM over the entire apicobasal extent, more apicobasally elongated (300 µm) neural progenitor cells (also called “radial glia”) in the mid-embryonic mouse cerebral wall move their nuclei only within its apical (100 µm) compartment, leaving the remaining basal part nucleus-free (fiber-like). How this IKNM range (i.e., the thickness of a pseudostratified “ventricular zone” [VZ]) is determined remains unknown. Here, we report external fencing of IKNM and VZ by differentiating cells. When a tight stack of multipolar cells just basal to VZ was “drilled” via acute neuron-directed expression of diphtheria toxin, IKNM of apicobasally connected progenitor cells continued far basally (200 µm). The unfencing-induced, basally overshot nuclei stay in S phase too long and do not move apically, suggesting that external limitation of IKNM is necessary for progenitors to undergo normal cytogenetic behaviors. Thus, physical collaboration between progenitors and differentiating cells including neurons underlies brain development.