Blueprints for healing: central nervous system regeneration in zebrafish and neonatal mice

In adult mammals, including humans, neurons, and axons in the brain and spinal cord are inherently incapable of regenerating after injury. Studies of animals with innate capacity for regeneration are providing valuable insights into the mechanisms driving tissue healing. The aim of this review is to summarize recent data on regeneration mechanisms in the brain and spinal cord of zebrafish and neonatal mice. We infer that elucidating these mechanisms and understanding how and why they are lost in adult mammals will contribute to the development of strategies to promote central nervous system regeneration.

Mechanotransduction in Development: A Focus on Angiogenesis

Cells respond to external mechanical cues and transduce these forces into biological signals. This process is known as mechanotransduction and requires a group of proteins called mechanosensors. This peculiar class of receptors include extracellular matrix proteins, plasma membrane proteins, the cytoskeleton and the nuclear envelope. These cell components are responsive to a wide spectrum of physical cues including stiffness, tensile force, hydrostatic pressure and shear stress. Among mechanotransducers, the Transient Receptor Potential (TRP) and the PIEZO family members are mechanosensitive ion channels, coupling force transduction with intracellular cation transport. Their activity contributes to embryo development, tissue remodeling and repair, and cell homeostasis. In particular, vessel development is driven by hemodynamic cues such as flow direction and shear stress. Perturbed mechanotransduction is involved in several pathological vascular phenotypes including hereditary hemorrhagic telangiectasia. This review is conceived to summarize the most recent findings of mechanotransduction in development. We first collected main features of mechanosensitive proteins. However, we focused on the role of mechanical cues during development. Mechanosensitive ion channels and their function in vascular development are also discussed, with a focus on brain vessel morphogenesis.

Functional role for Taz during hindbrain ventricle morphogenesis

The brain ventricle system, composed of the ventricular cavities and the cerebral spinal fluid within, performs critical functions including circulation of nutrients, removal of wastes, and cushioning of neural tissues. Development of the hindbrain ventricle requires a series of factors that coordinate its initial formation and subsequent inflation. Previous work has demonstrated that the transcriptional co-activator Taz (also known as WW domain-containing transcription regulator protein 1, Wwtr1), a component of Hippo signalling, is active at hindbrain rhombomere boundaries where it is regulated by mechanotransduction and promotes proliferation. Here, we demonstrate that Taz is also a critical regulator of hindbrain ventricle development. Zebrafish embryos that lack Taz protein fail to undergo initial midline separation of the hindbrain ventricle. Furthermore, the ventricle phenotype is a result of disorganized cytoskeletal F-actin and apicobasal polarity components. In addition, we have demonstrated that the hindbrain rhombomere boundaries are a location of active Wnt-Hippo crosstalk. Through our work, we propose a model where Taz protein is stabilized at rhombomere boundaries and promotes proper cell polarity necessary for formation of the brain ventricle.

FAK mediates mechanical signaling to maintain epithelial homeostasis through YAP/TAZ-TEADs

Epithelial homeostasis ensures that the epithelium can perform its normal physiological functions. Mechanical signaling response through integrin-mediated adhesions of the basement membrane (BM) is crucial for maintaining epithelial homeostasis. The essential mechanosensors YAP and the paralog TAZ (YAP/TAZ) have been shown to play a critical role in epithelial homeostasis, but the key regulator that mediates mechanical signaling to YAP/TAZ in maintaining epithelial homeostasis has not been fully understood. In this study, we noticed that mechanical signals correlated with YAP/TAZ activation and basal state maintenance in epithelial stem/progenitor cells through immunohistochemistry. Subsequently, we found that inhibition of focal adhesion kinase (FAK) suppressed YAP/TAZ activation in the human keratinocyte line HaCaT cells. Furthermore, inhibition of the interaction between YAP/TAZ and the transcriptional enhanced associate domains (TEADs) resulted in the differentiation of HaCaT cells. Finally, we used primary mouse epithelial cells to reconstruct the epithelium in vitro and found that FAK inhibition led to both a reduction in YAP/TAZ activity and an increase of differentiation in the basal layer cells. In conclusion, our findings reveal that FAK mediates mechanical signaling to maintain epithelial homeostasis via YAP/TAZ-TEADs.

Her9 controls the stemness properties of hindbrain boundary cells

The different spatiotemporal distribution of progenitor and neurogenic capacities permits that brain regions engage asynchronously in neurogenesis. In the hindbrain, rhombomere progenitor cells contribute to neurons during the first neurogenic phase, whereas boundary cells participate later. To analyze what maintains boundary cells as non-neurogenic progenitors, we addressed the role of Her9, a zebrafish Hes1-related protein. her9 expression is temporarily sustained in boundary cells independently of Notch at early embryonic stages, while they are non-neurogenic progenitors. Complementary functional approaches show that Her9 inhibits the onset of Notch signaling and the neurogenic program, keeping boundary cells as progenitors. Multicolor clonal analysis combined with genetic perturbations reveal that Her9 expands boundary progenitors by promoting symmetric proliferative and preventing neurogenic cell divisions. Her9 also regulates the proliferation of boundary cells by inhibiting the cell cycle arrest gene cdkn1ca and interplaying with Cyclin D1. Moreover, her9 is enriched in hindbrain radial glial cells at late embryonic stages independently of Notch. Together these data demonstrate that Her9 maintains the stemness properties of hindbrain boundary progenitors and late radial glial cells, ensuring the different temporal distribution of neurogenic capacities within the hindbrain.

Short-range Fgf signalling patterns hindbrain progenitors to induce the neurogenesis-to-oligodendrogenesis switch

In the vertebrate nervous system, neurogenesis generally precedes gliogenesis. The mechanisms driving the switch in cell type production and generation of the correct proportion of cell types remain unclear. Here, we show that Fgf20 signalling patterns progenitors to induce the switch from neurogenesis to oligodendrogenesis in the zebrafish hindbrain. Fgf20 emanating from earlier-born neurons signals at a short range to downregulate proneural gene expression in the segment centre with high spatial precision along both anterior-posterior and dorsal-ventral axes. This signal induces oligodendrocytes in the segment centre by upregulating olig2 and sox10 expression in pre-patterned competent progenitors. We show that the magnitude of proneural gene downregulation and the quantity of oligodendrocyte precursor cells specified is dependent on the extent of Fgf20 signalling. Overexpression of fgf20a induces precocious specification and differentiation of oligodendrocytes among olig2+ progenitors, resulting in an increase in oligodendrocytes at the expense of neurogenesis. Thus, Fgf20 signalling defines the proportion of each cell type produced. Taken together, Fgf20 signalling from earlier-born neurons patterns hindbrain segments spatially and temporally to induce the neurogenesis-to-oligodendrogenesis switch.

Wnt/β-Catenin Signaling Regulates Yap/Taz Activity during Embryonic Development in Zebrafish

Hippo-YAP/TAZ and Wnt/β-catenin signaling pathways, by controlling proliferation, migration, cell fate, stemness, and apoptosis, are crucial regulators of development and tissue homeostasis. We employed zebrafish embryos as a model system to elucidate in living reporter organisms the crosstalk between the two signaling pathways. Co-expression analysis between the Wnt/β-catenin Tg(7xTCF-Xla.Siam:GFP)ia4 and the Hippo-Yap/Taz Tg(Hsa.CTGF:nlsmCherry)ia49 zebrafish reporter lines revealed shared spatiotemporal expression profiles. These patterns were particularly evident in key developmental regions such as the midbrain-hindbrain boundary (MHB), epidermis, muscles, neural tube, notochord, floorplate, and otic vesicle. To investigate the relationship between the Wnt/β-catenin pathway and Hippo-Yap/Taz signaling in vivo, we conducted a series of experiments employing both pharmacological and genetic strategies. Modulation of the Wnt/β-catenin pathway with IWR-1, XAV939, or BIO resulted in a significant regulation of the Yap/Taz reporter signal, highlighting a clear correlation between β-catenin and Yap/Taz activities. Furthermore, genetic perturbation of the Wnt/β-catenin pathway, by APC inhibition or DKK1 upregulation, elicited evident and robust alteration of Yap/Taz activity. These findings revealed the intricate regulatory mechanisms underlying the crosstalk between the Wnt/β-catenin and Hippo-Yap/Taz signaling, shedding light on their roles in orchestrating developmental processes in vivo.

Design of neural organoids engineered by mechanical forces

Neural organoids consist of three-dimensional tissue derived from pluripotent stem cells that could recapitulate key features of the human brain. During the past decade, organoid technology has evolved in the field of human brain science by increasing the quality and applicability of its products. Among them, a novel approach involving the design of neural organoids engineered by mechanical forces has emerged. This review describes previous approaches for the generation of neural organoids, the engineering of neural organoids by mechanical forces, and future challenges for the application of mechanical forces in the design of neural organoids.

Mechanotransduction and epigenetic modulations of chromatin: Role of mechanical signals in gene regulation

Mechanical forces may be generated within a cell due to tissue stiffness, cytoskeletal reorganization, and the changes (even subtle) in the cell's physical surroundings. These changes of forces impose a mechanical tension within the intracellular protein network (both cytosolic and nuclear). Mechanical tension could be released by a series of protein-protein interactions often facilitated by membrane lipids, lectins and sugar molecules and thus generate a type of signal to drive cellular processes, including cell differentiation, polarity, growth, adhesion, movement, and survival. Recent experimental data have accentuated the molecular mechanism of this mechanical signal transduction pathway, dubbed mechanotransduction. Mechanosensitive proteins in the cell's plasma membrane discern the physical forces and channel the information to the cell interior. Cells respond to the message by altering their cytoskeletal arrangement and directly transmitting the signal to the nucleus through the connection of the cytoskeleton and nucleoskeleton before the information despatched to the nucleus by biochemical signaling pathways. Nuclear transmission of the force leads to the activation of chromatin modifiers and modulation of the epigenetic landscape, inducing chromatin reorganization and gene expression regulation; by the time chemical messengers (transcription factors) arrive into the nucleus. While significant research has been done on the role of mechanotransduction in tumor development and cancer progression/metastasis, the mechanistic basis of force-activated carcinogenesis is still enigmatic. Here, in this review, we have discussed the various cues and molecular connections to better comprehend the cellular mechanotransduction pathway, and we also explored the detailed role of some of the multiple players (proteins and macromolecular complexes) involved in mechanotransduction. Thus, we have described an avenue: how mechanical stress directs the epigenetic modifiers to modulate the epigenome of the cells and how aberrant stress leads to the cancer phenotype.

Hindbrain boundaries as niches of neural progenitor and stem cells regulated by the extracellular matrix proteoglycan chondroitin sulphate

The interplay between neural progenitors and stem cells (NPSCs), and their extracellular matrix (ECM) is a crucial regulatory mechanism that determines their behavior. Nonetheless, how the ECM dictates the state of NPSCs remains elusive. The hindbrain is valuable to examine this relationship, as cells in the ventricular surface of hindbrain boundaries (HBs), which arise between any two neighboring rhombomeres, express the NPSC marker Sox2, while being surrounded with the membrane-bound ECM molecule chondroitin sulphate proteoglycan (CSPG), in chick and mouse embryos. CSPG expression was used to isolate HB Sox2+ cells for RNA-sequencing, revealing their distinguished molecular properties as typical NPSCs, which express known and newly identified genes relating to stem cells, cancer, the matrisome and cell cycle. In contrast, the CSPG- non-HB cells, displayed clear neural-differentiation transcriptome. To address whether CSPG is significant for hindbrain development, its expression was manipulated in vivo and in vitro. CSPG manipulations shifted the stem versus differentiation state of HB cells, evident by their behavior and altered gene expression. These results provide further understanding of the uniqueness of hindbrain boundaries as repetitive pools of NPSCs in-between the rapidly growing rhombomeres, which rely on their microenvironment to maintain their undifferentiated state during development.

Hindbrain rhombomere centers harbor a heterogenous population of dividing progenitors which rely on Notch signaling

Tissue growth and morphogenesis are interrelated processes, whose tight coordination is essential for the production of different cell fates and the timely precise allocation of stem cell capacities. The zebrafish embryonic brainstem, the hindbrain, exemplifies such coupling between spatiotemporal cell diversity acquisition and tissue growth as the neurogenic commitment is differentially distributed over time. Here, we combined cell lineage and in vivo imaging approaches to reveal the emergence of specific cell population properties within the rhombomeres. We studied the molecular identity of hindbrain rhombomere centers and showed that they harbor different progenitor capacities that change over time. By clonal analysis, we revealed that cells within the center of rhombomeres decrease the proliferative capacity to remain mainly in the G1 phase. Proliferating progenitors give rise to neurons by asymmetric and symmetric neurogenic divisions while maintaining the pool of progenitors. The proliferative capacity of these cells differs from their neighbors, and they are delayed in the onset of Notch activity. Through functional studies, we demonstrated that they rely on Notch3 signaling to be maintained as non-committed progenitors. In this study, we show that cells in rhombomere centers, despite the neurogenic asynchrony, might share steps of a similar program with the rhombomere counterparts, to ensure proper tissue growth.

Zbtb16 mediates a switch between Fgf signalling regimes in the developing hindbrain

Developing tissues are sequentially patterned by extracellular signals that are turned on and off at specific times. In the zebrafish hindbrain, fibroblast growth factor (Fgf) signalling has different roles at different developmental stages: in the early hindbrain, transient Fgf3 and Fgf8 signalling from rhombomere 4 is required for correct segmentation, whereas later, neuronal Fgf20 expression confines neurogenesis to specific spatial domains within each rhombomere. How the switch between these two signalling regimes is coordinated is not known. We present evidence that the Zbtb16 transcription factor is required for this transition to happen in an orderly fashion. Zbtb16 expression is high in the early anterior hindbrain, then gradually upregulated posteriorly and confined to neural progenitors. In mutants lacking functional Zbtb16, fgf3 expression fails to be downregulated and persists until a late stage, resulting in excess and more widespread Fgf signalling during neurogenesis. Accordingly, the spatial pattern of neurogenesis is disrupted in Zbtb16 mutants. Our results reveal how the distinct stage-specific roles of Fgf signalling are coordinated in the zebrafish hindbrain.

The neurogenic fate of the hindbrain boundaries relies on Notch3-dependent asymmetric cell divisions

Elucidating the cellular and molecular mechanisms that regulate the balance between progenitor cell proliferation and neuronal differentiation in the construction of the embryonic brain demands the combination of cell lineage and functional approaches. Here, we generate the comprehensive lineage of hindbrain boundary cells by using a CRISPR-based knockin zebrafish transgenic line that specifically labels the boundaries. We unveil that boundary cells asynchronously engage in neurogenesis undergoing a functional transition from neuroepithelial progenitors to radial glia cells, coinciding with the onset of Notch3 signaling that triggers their asymmetrical cell division. Upon notch3 loss of function, boundary cells lose radial glia properties and symmetrically divide undergoing neuronal differentiation. Finally, we show that the fate of boundary cells is to become neurons, the subtype of which relies on their axial position, suggesting that boundary cells contribute to refine the number and proportion of the distinct neuronal populations.

Interplay of Eph-Ephrin Signalling and Cadherin Function in Cell Segregation and Boundary Formation

The segregation of distinct cell populations to form sharp boundaries is crucial for stabilising tissue organisation, for example during hindbrain segmentation in craniofacial development. Two types of mechanisms have been found to underlie cell segregation: differential adhesion mediated by cadherins, and Eph receptor and ephrin signalling at the heterotypic interface which regulates cell adhesion, cortical tension and repulsion. An interplay occurs between these mechanisms since cadherins have been found to contribute to Eph-ephrin-mediated cell segregation. This may reflect that Eph receptor activation acts through multiple pathways to decrease cadherin-mediated adhesion which can drive cell segregation. However, Eph receptors mainly drive cell segregation through increased heterotypic tension or repulsion. Cadherins contribute to cell segregation by antagonising homotypic tension within each cell population. This suppression of homotypic tension increases the difference with heterotypic tension triggered by Eph receptor activation, and it is this differential tension that drives cell segregation and border sharpening.

Interplay Between Notch and YAP/TAZ Pathways in the Regulation of Cell Fate During Embryo Development

Cells in growing tissues receive both biochemical and physical cues from their microenvironment. Growing evidence has shown that mechanical signals are fundamental regulators of cell behavior. However, how physical properties of the microenvironment are transduced into critical cell behaviors, such as proliferation, progenitor maintenance, or differentiation during development, is still poorly understood. The transcriptional co-activators YAP/TAZ shuttle between the cytoplasm and the nucleus in response to multiple inputs and have emerged as important regulators of tissue growth and regeneration. YAP/TAZ sense and transduce physical cues, such as those from the extracellular matrix or the actomyosin cytoskeleton, to regulate gene expression, thus allowing them to function as gatekeepers of progenitor behavior in several developmental contexts. The Notch pathway is a key signaling pathway that controls binary cell fate decisions through cell-cell communication in a context-dependent manner. Recent reports now suggest that the crosstalk between these two pathways is critical for maintaining the balance between progenitor maintenance and cell differentiation in different tissues. How this crosstalk integrates with morphogenesis and changes in tissue architecture during development is still an open question. Here, we discuss how progenitor cell proliferation, specification, and differentiation are coordinated with morphogenesis to construct a functional organ. We will pay special attention to the interplay between YAP/TAZ and Notch signaling pathways in determining cell fate decisions and discuss whether this represents a general mechanism of regulating cell fate during development. We will focus on research carried out in vertebrate embryos that demonstrate the important roles of mechanical cues in stem cell biology and discuss future challenges.

Dissecting Organismal Morphogenesis by Bridging Genetics and Biophysics

Multicellular organisms develop complex shapes from much simpler, single-celled zygotes through a process commonly called morphogenesis. Morphogenesis involves an interplay between several factors, ranging from the gene regulatory networks determining cell fate and differentiation to the mechanical processes underlying cell and tissue shape changes. Thus, the study of morphogenesis has historically been based on multidisciplinary approaches at the interface of biology with physics and mathematics. Recent technological advances have further improved our ability to study morphogenesis by bridging the gap between the genetic and biophysical factors through the development of new tools for visualizing, analyzing, and perturbing these factors and their biochemical intermediaries. Here, we review how a combination of genetic, microscopic, biophysical, and biochemical approaches has aided our attempts to understand morphogenesis and discuss potential approaches that may be beneficial to such an inquiry in the future. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

Segmentation and patterning of the vertebrate hindbrain

During early development, the hindbrain is sub-divided into rhombomeres that underlie the organisation of neurons and adjacent craniofacial tissues. A gene regulatory network of signals and transcription factors establish and pattern segments with a distinct anteroposterior identity. Initially, the borders of segmental gene expression are imprecise, but then become sharply defined, and specialised boundary cells form. In this Review, we summarise key aspects of the conserved regulatory cascade that underlies the formation of hindbrain segments. We describe how the pattern is sharpened and stabilised through the dynamic regulation of cell identity, acting in parallel with cell segregation. Finally, we discuss evidence that boundary cells have roles in local patterning, and act as a site of neurogenesis within the hindbrain.

The cell in the age of the genomic revolution: Cell Regulatory Networks

Over the last few years an intense activity in the areas of advanced microscopy and quantitative cell biology has put the focus on the morphogenetic events that shape embryos. The interest in these processes is taking place against the backdrop of genomic studies, particularly of global patterns of gene expression at the level of single cells, which cannot fully account for the way cells build tissues and organs. Here we discuss the need to integrate the activity of genes with that of cells and propose the need to develop a framework, based on cellular processes and cell interactions, that parallels that which has been created for gene activity in the form of Gene Regulatory Networks (GRNs). We begin to do this by suggesting elements for building Cell Regulatory Networks (CRNs). In the same manner that GRNs create schedules of gene expression that result in the emergence of cell fates over time, CRNs create tissues and organs i.e. space. We also suggest how GRNs and CRNs might interact in the building of embryos through feedback loops involving mechanics and tissue tectonics.

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.

Forced to communicate: Integration of mechanical and biochemical signaling in morphogenesis

Morphogenesis is a physical process that requires the generation of mechanical forces to achieve dynamic changes in cell position, tissue shape, and size as well as biochemical signals to coordinate these events. Mechanical forces are also used by the embryo to transmit detailed information across space and detected by target cells, leading to downstream changes in cellular properties and behaviors. Indeed, forces provide signaling information of complementary quality that can both synergize and diversify the functional outputs of biochemical signaling. Here, we discuss recent findings that reveal how mechanical signaling and biochemical signaling are integrated during morphogenesis and the possible context-specific advantages conferred by the interactions between these signaling mechanisms.

Cell-fate plasticity, adhesion and cell sorting complementarily establish a sharp midbrain-hindbrain boundary

The formation and maintenance of sharp boundaries between groups of cells play a vital role during embryonic development as they serve to compartmentalize cells with similar fates. Some of these boundaries also act as organizers, with the ability to induce specific cell fates and morphogenesis in the surrounding cells. The midbrain-hindbrain boundary (MHB) is such an organizer: it acts as a lineage restriction boundary to prevent the intermingling of cells with different developmental fates. However, the mechanisms underlying the lineage restriction process remain unclear. Here, using novel fluorescent knock-in reporters, live imaging, Cre/lox-mediated lineage tracing, atomic force microscopy-based cell adhesion assays and mutant analysis, we analyze the process of lineage restriction at the MHB and provide mechanistic details. Specifically, we show that lineage restriction occurs by the end of gastrulation, and that the subsequent formation of sharp gene expression boundaries in the developing MHB occur through complementary mechanisms, i.e. cell-fate plasticity and cell sorting. Furthermore, we show that cell sorting at the MHB involves differential adhesion among midbrain and hindbrain cells that is mediated by N-cadherin and Eph-ephrin signaling.

A single cell transcriptome atlas of the developing zebrafish hindbrain

Segmentation of the vertebrate hindbrain leads to the formation of rhombomeres, each with a distinct anteroposterior identity. Specialised boundary cells form at segment borders that act as a source or regulator of neuronal differentiation. In zebrafish, there is spatial patterning of neurogenesis in which non-neurogenic zones form at boundaries and segment centres, in part mediated by Fgf20 signalling. To further understand the control of neurogenesis, we have carried out single cell RNA sequencing of the zebrafish hindbrain at three different stages of patterning. Analyses of the data reveal known and novel markers of distinct hindbrain segments, of cell types along the dorsoventral axis, and of the transition of progenitors to neuronal differentiation. We find major shifts in the transcriptome of progenitors and of differentiating cells between the different stages analysed. Supervised clustering with markers of boundary cells and segment centres, together with RNA-seq analysis of Fgf-regulated genes, has revealed new candidate regulators of cell differentiation in the hindbrain. These data provide a valuable resource for functional investigations of the patterning of neurogenesis and the transition of progenitors to neuronal differentiation.

The interplay of atoh1 genes in the lower rhombic lip during hindbrain morphogenesis

The Lower Rhombic Lip (LRL) is a transient neuroepithelial structure of the dorsal hindbrain, which expands from r2 to r7, and gives rise to deep nuclei of the brainstem, such as the vestibular and auditory nuclei and most posteriorly the precerebellar nuclei. Although there is information about the contribution of specific proneural-progenitor populations to specific deep nuclei, and the distinct rhombomeric contribution, little is known about how progenitor cells from the LRL behave during neurogenesis and how their transition into differentiation is regulated. In this work, we investigated the atoh1 gene regulatory network operating in the specification of LRL cells, and the kinetics of cell proliferation and behavior of atoh1a-derivatives by using complementary strategies in the zebrafish embryo. We unveiled that atoh1a is necessary and sufficient for specification of LRL cells by activating atoh1b, which worked as a differentiation gene to transition progenitor cells towards neuron differentiation in a Notch-dependent manner. This cell state transition involved the release of atoh1a-derivatives from the LRL: atoh1a progenitors contributed first to atoh1b cells, which are committed non-proliferative precursors, and to the lhx2b-neuronal lineage as demonstrated by cell fate studies and functional analyses. Using in vivo cell lineage approaches we revealed that the proliferative cell capacity, as well as the mode of division, relied on the position of the atoh1a progenitors within the dorsoventral axis. We showed that atoh1a may behave as the cell fate selector gene, whereas atoh1b functions as a neuronal differentiation gene, contributing to the lhx2b neuronal population. atoh1a-progenitor cell dynamics (cell proliferation, cell differentiation, and neuronal migration) relies on their position, demonstrating the challenges that progenitor cells face in computing positional information from a dynamic two-dimensional grid in order to generate the stereotyped neuronal structures in the embryonic hindbrain.

Actomyosin regulation by Eph receptor signaling couples boundary cell formation to border sharpness

The segregation of cells with distinct regional identity underlies formation of a sharp border, which in some tissues serves to organise a boundary signaling centre. It is unclear whether or how border sharpness is coordinated with induction of boundary-specific gene expression. We show that forward signaling of EphA4 is required for border sharpening and induction of boundary cells in the zebrafish hindbrain, which we find both require kinase-dependent signaling, with a lesser input of PDZ domain-dependent signaling. We find that boundary-specific gene expression is regulated by myosin II phosphorylation, which increases actomyosin contraction downstream of EphA4 signaling. Myosin phosphorylation leads to nuclear translocation of Taz, which together with Tead1a is required for boundary marker expression. Since actomyosin contraction maintains sharp borders, there is direct coupling of border sharpness to boundary cell induction that ensures correct organisation of signaling centres.