Coordinate regulation of neural tube patterning and proliferation by TGFbeta and WNT activity

From neural tube to spinal cord: The dynamic journey of the dorsal neuroepithelium

In a developing embryo, formation of tissues and organs is remarkably precise in both time and space. Through cell-cell interactions, neighboring progenitors coordinate their activities, sequentially generating distinct types of cells. At present, we only have limited knowledge, rather than a systematic understanding, of the underlying logic and mechanisms responsible for cell fate transitions. The formation of the dorsal aspect of the spinal cord is an outstanding model to tackle these dynamics, as it first generates the peripheral nervous system and is later responsible for transmitting sensory information from the periphery to the brain and for coordinating local reflexes. This is reflected first by the ontogeny of neural crest cells, progenitors of the peripheral nervous system, followed by formation of the definitive roof plate of the central nervous system and specification of adjacent interneurons, then a transformation of roof plate into dorsal radial glia and ependyma lining the forming central canal. How do these peripheral and central neural branches segregate from common progenitors? How are dorsal radial glia established concomitant with transformation of the neural tube lumen into a central canal? How do the dorsal radial glia influence neighboring cells? This is only a partial list of questions whose clarification requires the implementation of experimental paradigms in which precise control of timing is crucial. Here, we outline some available answers and still open issues, while highlighting the contributions of avian models and their potential to address mechanisms of neural patterning and function.

Neuroepithelial organoid patterning is mediated by a neighborhood watch mechanism

During epithelial tissue patterning, morphogens operate across multiple length scales to instruct cell identities. However, how cell fate changes are coordinated over these scales to establish spatial organization remains poorly understood. Here, we use human neural tube organoids as models of epithelial patterning and develop an in silico approach to define conditions permissive to patterning. By systematically varying morphogen position, diffusivity, and fate-inducing concentration levels, we show that cells follow a "neighborhood watch" (NW) mechanism that is deterministically dictated by initial morphogen source positions, reflecting scale-invariant in vitro phenotypes. We define how the frequency and local bias of morphogen sources stabilize pattern orientation. The model predicts enhanced patterning through floor plate inhibition, and receptor-ligand interaction analysis of single-cell RNA sequencing (scRNA-seq) data identifies wingless-related integration site (WNT) and bone morphogenic protein (BMP) as inhibition modulators, which we validate in vitro. These results suggest that developing neuroepithelia employ NW-based mechanisms to organize morphogen sources, define cellular identity, and establish patterns.

LATS1/2 control TGFB-directed epithelial-to-mesenchymal transition in the murine dorsal cranial neuroepithelium through YAP regulation

Hippo signaling, an evolutionarily conserved kinase cascade involved in organ size control, plays key roles in various tissue developmental processes, but its role in craniofacial development remains poorly understood. Using the transgenic Wnt1-Cre2 driver, we inactivated the Hippo signaling components Lats1 and Lats2 in the cranial neuroepithelium of mouse embryos and found that the double conditional knockout (DCKO) of Lats1/2 resulted in neural tube and craniofacial defects. Lats1/2 DCKO mutant embryos had microcephaly with delayed and defective neural tube closure. Furthermore, neuroepithelial cell shape and architecture were disrupted within the cranial neural tube in Lats1/2 DCKO mutants. RNA sequencing of embryonic neural tubes revealed increased TGFB signaling in Lats1/2 DCKO mutants. Moreover, markers of epithelial-to-mesenchymal transition (EMT) were upregulated in the cranial neural tube. Inactivation of Hippo signaling downstream effectors, Yap and Taz, suppressed neuroepithelial defects, aberrant EMT and TGFB upregulation in Lats1/2 DCKO embryos, indicating that LATS1/2 function via YAP and TAZ. Our findings reveal important roles for Hippo signaling in modulating TGFB signaling during neural crest EMT.

Choroid plexus function in neurological homeostasis and disorders: The awakening of the circadian clocks and orexins

As research regarding the role of circadian rhythms, sleep, and the orexinergic system in neurodegenerative diseases is growing, it is surprising that the choroid plexus (CP) remains underappreciated in this realm. Despite its extensive role in the regulation of circadian rhythms and orexinergic signalling, as well as acting as the primary conduit between cerebrospinal fluid (CSF) and the circulatory system, providing a mechanism by which toxic waste molecules can be removed from the brain, the CP has been largely unexplored in neurodegeneration. In this review, we explore the role of the CP in maintaining brain homeostasis and circadian rhythms, regulating CSF dynamics, and how these functions change across the lifespan, from development to senescence. In addition, we examine the relationship between the CP, orexinergic signalling, and the glymphatic system, highlighting gaps in the literature and areas that require immediate exploration. Finally, we assess current knowledge, including possible therapeutic strategies, regarding the role of the CP in neurological disorders, such as traumatic brain injury, migraine, Alzheimer's disease, and multiple sclerosis.

Completion of neural crest cell production and emigration is regulated by retinoic-acid-dependent inhibition of BMP signaling

Production and emigration of neural crest cells is a transient process followed by the emergence of the definitive roof plate. The mechanisms regulating the end of neural crest ontogeny are poorly understood. Whereas early crest development is stimulated by mesoderm-derived retinoic acid, we report that the end of the neural crest period is regulated by retinoic acid synthesized in the dorsal neural tube. Inhibition of retinoic acid signaling in the neural tube prevents the normal upregulation of BMP inhibitors in the nascent roof plate and prolongs the period of BMP responsiveness which otherwise ceases close to roof plate establishment. Consequently, neural crest production and emigration are extended well into the roof plate stage. In turn, extending the activity of neural crest-specific genes inhibits the onset of retinoic acid synthesis in roof plate suggesting a mutual repressive interaction between neural crest and roof plate traits. Although several roof plate-specific genes are normally expressed in the absence of retinoic acid signaling, roof plate and crest markers are co-expressed in single cells and this domain also contains dorsal interneurons. Hence, the cellular and molecular architecture of the roof plate is compromised. Collectively, our results demonstrate that neural tube-derived retinoic acid, via inhibition of BMP signaling, is an essential factor responsible for the end of neural crest generation and the proper segregation of dorsal neural lineages.

The division between the central nervous system – formed by the brain and spinal cord – and the peripheral nervous system – which consists of the neurons that sense and relay information to and from the body – takes place early during embryonic development. Initially, the nervous system consists of a tube of cells called the neural tube. From the top region of this tube, some cells change their shape, exit the tube and migrate to different places in the developing body. These cells are called the ‘neural crest’, and they form many different structures, including the peripheral nervous system.

Neural crest cells keep leaving the neural tube for a period of time, but after that, the neural tube stops producing them. At this point, the region of the neural tube that had been producing neural crest cells becomes the ‘roof plate’ of the central nervous system, a structure that is essential for the development of specific groups of neurons in the brain and spinal cord.

In bird embryos, a protein called bone morphogenetic protein (BMP) is essential for neural crest production because it triggers the migration of these cells away from the neural tube. Before the roof plate is formed, the activity of BMP is blocked by proteins known as BMP inhibitors, which stop more cells from leaving the neural tube. Around the time when neural crest formation stops, another molecule called retinoic acid begins to be synthesized in the top region of the neural tube. Rekler and Kalcheim asked whether retinoic acid is involved in the transition from neural crest to roof plate.

To test this hypothesis, Rekler and Kalcheim blocked the activity of retinoic acid in the neural tube of quail embryos at the time when they should stop producing neural crest cells. This resulted in embryos in which the neural tube keeps producing neural crest cells after the roof plate has formed. In these embryos, individual cells in the resulting ‘roof plate’ produced both proteins that are normally only found in neural crest cells, and proteins typically exclusive to the roof plate. This suggests that, in the absence of retinoic acid activity, the segregation of neural crest identity from roof plate identity is compromised.

Rekler and Kalcheim also found that, in the embryos where retinoic acid activity had been blocked, the cells in the area where the roof plate should be produced virtually no BMP inhibitors, and exhibited extended BMP activity. This allowed neural crest cells to continue forming and migrating away from the neural tube well after the period when they would stop in a normal embryo. These results indicate that retinoic acid stops the production of neural crest cells by repressing BMP activity in the roof plate of the neural tube.

Rekler and Kalcheim’s experiments shed light on the mechanisms that allow the central and peripheral nervous systems to become segregated. This could increase our understanding of the origin of several neurodevelopmental disorders, potentially providing insights into their treatment or prevention. Additionally, the process of neural crest production and exit from the neural tube is highly similar to the process of metastasis in many invasive cancers. Thus, by understanding how the production of neural crest cells is terminated, it may be possible to learn how to prevent malignant cancer cells from spreading through the body.

Bone morphogenetic proteins (BMPs) in the central regulation of energy balance and adult neural plasticity

The current worldwide obesity pandemic highlights a need to better understand the regulation of energy balance and metabolism, including the role of the nervous system in controlling energy intake and energy expenditure. Neural plasticity in the hypothalamus of the adult brain has been implicated in full-body metabolic health, however, the mechanisms surrounding hypothalamic plasticity are incompletely understood. Bone morphogenetic proteins (BMPs) control metabolic health through actions in the brain as well as in peripheral tissues such as adipose, together regulating both energy intake and energy expenditure. BMP ligands, receptors, and inhibitors are found throughout plastic adult brain regions and have been demonstrated to modulate neurogenesis and gliogenesis, as well as synaptic and dendritic plasticity. This role for BMPs in adult neural plasticity is distinct from their roles in brain development. Existing evidence suggests that BMPs induce weight loss through hypothalamic pathways, and part of the mechanism of action may be through inducing neural plasticity. In this review, we summarize the data regarding how BMPs affect neural plasticity in the adult mammalian brain, as well as the relationship between central BMP signaling and metabolic health.

Getting in touch with your senses: Mechanisms specifying sensory interneurons in the dorsal spinal cord

The spinal cord is functionally and anatomically divided into ventrally derived motor circuits and dorsally derived somatosensory circuits. Sensory stimuli originating either at the periphery of the body, or internally, are relayed to the dorsal spinal cord where they are processed by distinct classes of sensory dorsal interneurons (dIs). dIs convey sensory information, such as pain, heat or itch, either to the brain, and/or to the motor circuits to initiate the appropriate response. They also regulate the intensity of sensory information and are the major target for the opioid analgesics. While the developmental mechanisms directing ventral and dorsal cell fates have been hypothesized to be similar, more recent research has suggested that dI fates are specified by novel mechanisms. In this review, we will discuss the molecular events that specify dorsal neuronal patterning in the spinal cord, thereby generating diverse dI identities. We will then discuss how this molecular understanding has led to the development of robust stem cell methods to derive multiple spinal cell types, including the dIs, and the implication of these studies for treating spinal cord injuries and neurodegenerative diseases. This article is categorized under: Neurological Diseases > Stem Cells and Development.

Deciphering and reconstitution of positional information in the human brain development

Organoid has become a novel in vitro model to research human development and relevant disorders in recent years. With many improvements on the culture protocols, current brain organoids could self-organize into a complicated three-dimensional organization that mimics most of the features of the real human brain at the molecular, cellular, and further physiological level. However, lacking positional information, an important characteristic conveyed by gradients of signaling molecules called morphogens, leads to the deficiency of spatiotemporally regulated cell arrangements and cell–cell interactions in the brain organoid development. In this review, we will overview the role of morphogen both in the vertebrate neural development in vivo as well as the brain organoid culture in vitro, the strategies to apply morphogen concentration gradients in the organoid system and future perspectives of the brain organoid technology.

The Role of Small Molecules and Their Effect on the Molecular Mechanisms of Early Retinal Organoid Development

Early in vivo embryonic retinal development is a well-documented and evolutionary conserved process. The specification towards eye development is temporally controlled by consecutive activation or inhibition of multiple key signaling pathways, such as the Wnt and hedgehog signaling pathways. Recently, with the use of retinal organoids, researchers aim to manipulate these pathways to achieve better human representative models for retinal development and disease. To achieve this, a plethora of different small molecules and signaling factors have been used at various time points and concentrations in retinal organoid differentiations, with varying success. Additions differ from protocol to protocol, but their usefulness or efficiency has not yet been systematically reviewed. Interestingly, many of these small molecules affect the same and/or multiple pathways, leading to reduced reproducibility and high variability between studies. In this review, we make an inventory of the key signaling pathways involved in early retinogenesis and their effect on the development of the early retina in vitro. Further, we provide a comprehensive overview of the small molecules and signaling factors that are added to retinal organoid differentiation protocols, documenting the molecular and functional effects of these additions. Lastly, we comparatively evaluate several of these factors using our established retinal organoid methodology.

From Neural Crest to Definitive Roof Plate: The Dynamic Behavior of the Dorsal Neural Tube

Research on the development of the dorsal neural tube is particularly challenging. In this highly dynamic domain, a temporal transition occurs between early neural crest progenitors that undergo an epithelial-to-mesenchymal transition and exit the neural primordium, and the subsequent roof plate, a resident epithelial group of cells that constitutes the dorsal midline of the central nervous system. Among other functions, the roof plate behaves as an organizing center for the generation of dorsal interneurons. Despite extensive knowledge of the formation, emigration and migration of neural crest progenitors, little is known about the mechanisms leading to the end of neural crest production and the transition into a roof plate stage. Are these two mutually dependent or autonomously regulated processes? Is the generation of roof plate and dorsal interneurons induced by neural tube-derived factors throughout both crest and roof plate stages, respectively, or are there differences in signaling properties and responsiveness as a function of time? In this review, we discuss distinctive characteristics of each population and possible mechanisms leading to the shift between the above cell types.

Expression analysis of chick Frizzled receptors during spinal cord development

Frizzleds (Fzds) are transmembrane receptors that can transduce signals dependent upon binding of Wnts, a large family of secreted glycoproteins homologous to the Drosophila wingless gene. FZDs are critical for a wide variety of normal and pathological developmental processes. In the nervous system, Wnts and Frizzleds play an important role in anterior-posterior patterning, cell fate decisions, proliferation, and synaptogenesis. Here, we preformed a comprehensive expression profile of Wnt receptors (FZD) by using situ hybridization to identify FZDs that are expressed in dorsal-ventral regions of the neural tube development. Our data show specific expression for FZD1,2,3,7,9 and 10 in the chick developing spinal cord. This expression profile of cFZD receptors offers the basis for functional studies in the future to determine roles for the different FZD receptors and their interactions with Wnts during dorsal-ventral neural tube development in vivo. Furthermore, we also show that co-overexpression of Wnt1/3a by in vivo electroporation affects FZD7/10 expression in the neural tube. This illustrates an example of Wnts-FZDs interactions during spinal cord neurogenesis.

Development, structure, and bioengineering of the human corneal stroma: A review of collagen-based implants

Bio-engineering technologies are currently used to produce biomimetic artificial corneas that should present structural, chemical, optical, and biomechanical properties close to the native tissue. These properties are mainly supported by the corneal stroma which accounts for 90% of corneal thickness and is mainly made of collagen type I. The stromal collagen fibrils are arranged in lamellae that have a plywood-like organization. The fibril diameter is between 25 and 35 nm and the interfibrillar space about 57 nm. The number of lamellae in the central stroma is estimated to be 300. In the anterior part, their size is 10-40 μm. They appear to be larger in the posterior part of the stroma with a size of 60-120 μm. Their thicknesses also vary from 0.2 to 2.5 μm. During development, the acellular corneal stroma, which features a complex pattern of organization, serves as a scaffold for mesenchymal cells that invade and further produce the cellular stroma. Several pathways including Bmp4, Wnt/β-catenin, Notch, retinoic acid, and TGF-β, in addition to EFTFs including the mastering gene Pax-6, are involved in corneal development. Besides, retinoic acid and TGF- β seem to have a crucial role in the neural crest cell migration in the stroma. Several technologies can be used to produce artificial stroma. Taking advantage of the liquid-crystal properties of acid-soluble collagen, it is possible to produce transparent stroma-like matrices with native-like collagen I fibrils and plywood-like organization, where epithelial cells can adhere and proliferate. Other approaches include the use of recombinant collagen, cross-linkers, vitrification, plastically compressed collagen or magnetically aligned collagen, providing interesting optical and mechanical properties. These technologies can be classified according to collagen type and origin, presence of telopeptides and native-like fibrils, structure, and transparency. Collagen matrices feature transparency >80% for the appropriate 500-μm thickness. Non-collagenous matrices made of biopolymers including gelatin, silk, or fish scale have been developed which feature interesting properties but are less biomimetic. These bioengineered matrices still need to be colonized by stromal cells to fully reproduce the native stroma.

FZD10 regulates cell proliferation and mediates Wnt1 induced neurogenesis in the developing spinal cord

Wnt/FZD signalling activity is required for spinal cord development, including the dorsal-ventral patterning of the neural tube, where it affects proliferation and specification of neurons. Wnt ligands initiate canonical, β -catenin-dependent, signaling by binding to Frizzled receptors. However, in many developmental contexts the cognate FZD receptor for a particular Wnt ligand remains to be identified. Here, we characterized FZD10 expression in the dorsal neural tube where it overlaps with both Wnt1 and Wnt3a, as well as markers of dorsal progenitors and interneurons. We show FZD10 expression is sensitive to Wnt1, but not Wnt3a expression, and FZD10 plays a role in neural tube patterning. Knockdown approaches show that Wnt1 induced ventral expansion of dorsal neural markes, Pax6 and Pax7, requires FZD10. In contrast, Wnt3a induced dorsalization of the neural tube is not affected by FZD10 knockdown. Gain of function experiments show that FZD10 is not sufficient on its own to mediate Wnt1 activity in vivo. Indeed excess FZD10 inhibits the dorsalizing activity of Wnt1. However, addition of the Lrp6 co-receptor dramatically enhances the Wnt1/FZD10 mediated activation of dorsal markers. This suggests that the mechanism by which Wnt1 regulates proliferation and patterning in the neural tube requires both FZD10 and Lrp6.

Regulation of size and scale in vertebrate spinal cord development

All vertebrates have a spinal cord with dimensions and shape specific to their species. Yet how species‐specific organ size and shape are achieved is a fundamental unresolved question in biology. The formation and sculpting of organs begins during embryonic development. As it develops, the spinal cord extends in anterior–posterior direction in synchrony with the overall growth of the body. The dorsoventral (DV) and apicobasal lengths of the spinal cord neuroepithelium also change, while at the same time a characteristic pattern of neural progenitor subtypes along the DV axis is established and elaborated. At the basis of these changes in tissue size and shape are biophysical determinants, such as the change in cell number, cell size and shape, and anisotropic tissue growth. These processes are controlled by global tissue‐scale regulators, such as morphogen signaling gradients as well as mechanical forces. Current challenges in the field are to uncover how these tissue‐scale regulatory mechanisms are translated to the cellular and molecular level, and how regulation of distinct cellular processes gives rise to an overall defined size. Addressing these questions will help not only to achieve a better understanding of how size is controlled, but also of how tissue size is coordinated with the specification of pattern.

This article is categorized under:

Establishment of Spatial and Temporal Patterns > Regulation of Size, Proportion, and Timing

Signaling Pathways > Global Signaling Mechanisms

Nervous System Development > Vertebrates: General Principles

Transformation of the Transcriptomic Profile of Mouse Periocular Mesenchyme During Formation of the Embryonic Cornea

Purpose

Defects in neural crest development are a major contributing factor in corneal dysgenesis, but little is known about the genetic landscape during corneal development. The purpose of this study was to provide a detailed transcriptome profile and evaluate changes in gene expression during mouse corneal development.

Methods

RNA sequencing was used to uncover the transcriptomic profile of periocular mesenchyme (pNC) isolated at embryonic day (E) 10.5 and corneas isolated at E14.5 and E16.5. The spatiotemporal expression of several differentially expressed genes was validated by in situ hybridization.

Results

Analysis of the whole-transcriptome profile between pNC and embryonic corneas identified 3815 unique differentially expressed genes. Pathway analysis revealed an enrichment of differentially expressed genes involved in signal transduction (retinoic acid, transforming growth factor-β, and Wnt pathways) and transcriptional regulation.

Conclusions

Our analyses, for the first time, identify a large number of differentially expressed genes during progressive stages of mouse corneal development. Our data provide a comprehensive transcriptomic profile of the developing cornea. Combined, these data serve as a valuable resource for the identification of novel regulatory networks crucial for the advancement of studies in congenital defects, stem cell therapy, bioengineering, and adult corneal diseases.

Wnt/β-catenin signaling regulates ependymal cell development and adult homeostasis

In the adult mouse spinal cord, the ependymal cell population that surrounds the central canal is thought to be a promising source of quiescent stem cells to treat spinal cord injury. Relatively little is known about the cellular origin of ependymal cells during spinal cord development, or the molecular mechanisms that regulate ependymal cells during adult homeostasis. Using genetic lineage tracing based on the Wnt target gene Axin2, we have characterized Wnt-responsive cells during spinal cord development. Our results revealed that Wnt-responsive progenitor cells are restricted to the dorsal midline throughout spinal cord development, which gives rise to dorsal ependymal cells in a spatially restricted pattern. This is contrary to previous reports that suggested an exclusively ventral origin of ependymal cells, suggesting that ependymal cells may retain positional identities in relation to their neural progenitors. Our results further demonstrated that in the postnatal and adult spinal cord, all ependymal cells express the Wnt/β-catenin signaling target gene Axin2, as well as Wnt ligands. Genetic elimination of β-catenin or inhibition of Wnt secretion in Axin2-expressing ependymal cells in vivo both resulted in impaired proliferation, indicating that Wnt/β-catenin signaling promotes ependymal cell proliferation. These results demonstrate the continued importance of Wnt/β-catenin signaling for both ependymal cell formation and regulation. By uncovering the molecular signals underlying the formation and regulation of spinal cord ependymal cells, our findings thus enable further targeting and manipulation of this promising source of quiescent stem cells for therapeutic interventions.

Making sense out of spinal cord somatosensory development

The spinal cord integrates and relays somatosensory input, leading to complex motor responses. Research over the past couple of decades has identified transcription factor networks that function during development to define and instruct the generation of diverse neuronal populations within the spinal cord. A number of studies have now started to connect these developmentally defined populations with their roles in somatosensory circuits. Here, we review our current understanding of how neuronal diversity in the dorsal spinal cord is generated and we discuss the logic underlying how these neurons form the basis of somatosensory circuits.

TGF-β Family Signaling in Neural and Neuronal Differentiation, Development, and Function

Signaling by the transforming growth factor β (TGF-β) family is necessary for proper neural development and function throughout life. Sequential waves of activation, inhibition, and reactivation of TGF-β family members regulate numerous elements of the nervous system from the earliest stages of embryogenesis through adulthood. This review discusses the expression, regulation, and function of TGF-β family members in the central nervous system at various developmental stages, beginning with induction and patterning of the nervous system to their importance in the adult as modulators of inflammatory response and involvement in degenerative diseases.

Ischemia reduces inter-alpha inhibitor proteins in the brain of the ovine fetus

Hypoxic-ischemic (HI) brain injury is a major cause of neurological abnormalities in the perinatal period. Inflammation contributes to the evolution of HI brain injury. Inter-alpha inhibitor proteins (IAIPs) are a family of proteins that are part of the innate immune system. We have reported that endogenous IAIPs exhibit developmental changes in ovine brain and that exogenous IAIP treatment reduces neuronal death in HI neonatal rats. However, the effects of HI on endogenous IAIPs in brain have not been previously examined. In this study, we examined the effects of ischemia-reperfusion on endogenous IAIPs levels in fetal sheep brain. Cerebral cortex, cerebellum, cervical spinal cord, choroid plexus, and CSF were snap frozen from sham control fetuses at 127 days gestation and after 30-min of carotid occlusion and 4-, 24-, and 48-h of reperfusion. IAIP levels were determined by Western immunoblot. IAIP expressions of the 250 kDa Inter-alpha inhibitor (IaI) and 125 kDa Pre-alpha inhibitor (PaI) in cerebral cortex and PaI in cerebellum were reduced (p < 0.05) 4-h after ischemia compared with controls and returned toward control levels 24- and 48-h after ischemia. CSF PaI and IaI were reduced 48 h after ischemia. We conclude that IAIPs in cerebral cortex and cerebellum are reduced by brain ischemia, and return toward control levels between 24 and 48 h after ischemia. However, changes in CSF IAIPs were delayed, exhibiting decreases 48 h after ischemia. We speculate that the decreases in endogenous IAIPs reflect increased utilization, potentially suggesting that they have endogenous neuroprotective properties. © 2016 Wiley Periodicals, Inc. Develop Neurobiol 77: 726-737, 2017.

Expression Pattern of Axin2 During Chicken Development

Canonical Wnt-signalling is well understood and has been extensively described in many developmental processes. The regulation of this signalling pathway is of outstanding relevance for proper development of the vertebrate and invertebrate embryo. Axin2 provides a negative-feedback-loop in the canonical Wnt-pathway, being a target gene and a negative regulator. Here we provide a detailed analysis of the expression pattern in the development of the chicken embryo. By performing in-situ hybridization on chicken embryos from stage HH 04+ to HH 32 we detected a temporally and spatially restricted dynamic expression of Axin2. In particular, data about the expression of Axin2 mRNA in early embryogenesis, somites, neural tube, limbs, kidney and eyes was obtained.

An FGF3-BMP Signaling Axis Regulates Caudal Neural Tube Closure, Neural Crest Specification and Anterior-Posterior Axis Extension

During vertebrate axis extension, adjacent tissue layers undergo profound morphological changes: within the neuroepithelium, neural tube closure and neural crest formation are occurring, while within the paraxial mesoderm somites are segmenting from the presomitic mesoderm (PSM). Little is known about the signals between these tissues that regulate their coordinated morphogenesis. Here, we analyze the posterior axis truncation of mouse Fgf3 null homozygotes and demonstrate that the earliest role of PSM-derived FGF3 is to regulate BMP signals in the adjacent neuroepithelium. FGF3 loss causes elevated BMP signals leading to increased neuroepithelium proliferation, delay in neural tube closure and premature neural crest specification. We demonstrate that elevated BMP4 depletes PSM progenitors in vitro, phenocopying the Fgf3 mutant, suggesting that excessive BMP signals cause the Fgf3 axis defect. To test this in vivo we increased BMP signaling in Fgf3 mutants by removing one copy of Noggin, which encodes a BMP antagonist. In such mutants, all parameters of the Fgf3 phenotype were exacerbated: neural tube closure delay, premature neural crest specification, and premature axis termination. Conversely, genetically decreasing BMP signaling in Fgf3 mutants, via loss of BMP receptor activity, alleviates morphological defects. Aberrant apoptosis is observed in the Fgf3 mutant tailbud. However, we demonstrate that cell death does not cause the Fgf3 phenotype: blocking apoptosis via deletion of pro-apoptotic genes surprisingly increases all Fgf3 defects including causing spina bifida. We demonstrate that this counterintuitive consequence of blocking apoptosis is caused by the increased survival of BMP-producing cells in the neuroepithelium. Thus, we show that FGF3 in the caudal vertebrate embryo regulates BMP signaling in the neuroepithelium, which in turn regulates neural tube closure, neural crest specification and axis termination. Uncovering this FGF3-BMP signaling axis is a major advance toward understanding how these tissue layers interact during axis extension with important implications in human disease.

During embryological development, the vertebrate embryo undergoes profound growth in a head-to-tail direction. During this process, formation of different structures within adjacent tissue layers must occur in a coordinated fashion. Insights into how these adjacent tissues molecularly communicate with each other is essential to understanding both basic embryology and the underlying causes of human birth defects. Mice lacking Fgf3, which encodes a secreted signaling factor, have long been known to have premature axis termination, but the underlying mechanism has not been studied until now. Through a series of complex genetic experiments, we show that FGF3 is an essential factor for coordination of neural tube development and axis extension. FGF3 is secreted from the mesodermal layer, which is the major driver of extending the axis, and negatively regulates expression of another class of secreted signaling molecules in the neuroepithelium, BMPs. In the absence of FGF3, excessive BMP signals cause a delay in neural tube closure, premature specification of neural crest cells and negatively affect the mesoderm, causing a premature termination of the embryological axis. Our work suggests that FGF3 may be a player in the complex etiology of the human birth defect, spina bifida, the failure of posterior neural tube closure.

An emerging role for prdm family genes in dorsoventral patterning of the vertebrate nervous system

The embryonic vertebrate neural tube is divided along its dorsoventral (DV) axis into eleven molecularly discrete progenitor domains. Each of these domains gives rise to distinct neuronal cell types; the ventral-most six domains contribute to motor circuits, while the five dorsal domains contribute to sensory circuits. Following the initial neurogenesis step, these domains also generate glial cell types—either astrocytes or oligodendrocytes. This DV pattern is initiated by two morphogens—Sonic Hedgehog released from notochord and floor plate and Bone Morphogenetic Protein produced in the roof plate—that act in concentration gradients to induce expression of genes along the DV axis. Subsequently, these DV-restricted genes cooperate to define progenitor domains and to control neuronal cell fate specification and differentiation in each domain. Many genes involved in this process have been identified, but significant gaps remain in our understanding of the underlying genetic program. Here we review recent work identifying members of the Prdm gene family as novel regulators of DV patterning in the neural tube. Many Prdm proteins regulate transcription by controlling histone modifications (either via intrinsic histone methyltransferase activity, or by recruiting histone modifying enzymes). Prdm genes are expressed in spatially restricted domains along the DV axis of the neural tube and play important roles in the specification of progenitor domains, as well as in the subsequent differentiation of motor neurons and various types of interneurons. Strikingly, Prdm proteins appear to function by binding to, and modulating the activity of, other transcription factors (particularly bHLH proteins). The identity of key transcription factors in DV patterning of the neural tube has been elucidated previously (e.g. the nkx, bHLH and pax families), but it now appears that an additional family is also required and that it acts in a potentially novel manner.

Ontogeny of inter-alpha inhibitor proteins in ovine brain and somatic tissues

Inter-alpha inhibitor proteins (IAIPs) found in relatively high concentrations in human plasma are important in inflammation. IAIPs attenuate brain damage in young and adult subjects, decrease during sepsis and necrotizing enterocolitis in premature infants, and attenuate sepsis-related inflammation in newborn rats. Although a few studies have reported adult organ-specific IAIP expression, information is not available on age-dependent IAIP expression. Given evidence suggesting IAIPs attenuate brain damage in young and adult subjects, and inflammation in newborns, we examined IAIP expression in plasma, cerebral cortex (CC), choroid plexus (CP), cerebral spinal fluid (CSF), and somatic organs in fetal, newborn, and adult sheep to determine the endogenous expression patterns of these proteins during development. IAIPs (enzyme-linked immunosorbent assay) were higher in newborn and adult than fetal plasma (P < 0.05). Western immunoblot detected 125 kDa PaI (Pre-alpha Inhibitor) and 250 kDa IaI (Inter-alpha Inhibitor) in plasma, CNS, and somatic organs. PaI expression in CC and CP was higher in fetuses than newborns and adults, but IaI expression was higher in adults than fetuses and newborns. Both PaI and IaI were higher in fetal than newborn CSF. IAIPs exhibited organ-specific ontogenic patterns in placenta, liver, heart, and kidney. These results provide evidence for the first time that plasma, brain, placenta, liver, heart, and kidney express IAIPs throughout ovine development and that expression patterns are unique to each organ. Although exact functions of IAIPs in CNS and somatic tissues are not known, their presence in relatively high amounts during development suggests their potential importance in brain and organ development.

Growth factor treatment to overcome Alzheimer's dysfunctional signaling

The number of people suffering from Alzheimer's disease (AD) will increase as the world population ages, creating a huge socio-economic burden. The three pathophysiological hallmarks of AD are the cholinergic system dysfunction, the β-amyloid peptide deposition and the Tau protein hyperphosphorylation. Current treatments have only transient effects and each tends to concentrate on a single pathophysiological aspect of AD. This review first provides an overall view of AD in terms of its pathophysiological symptoms and signaling dysfunction. We then examine the therapeutic potential of growth factors (GFs) by showing how they can overcome the dysfunctional cell signaling that occurs in AD. Finally, we discuss new alternatives to GFs that help overcome the problem of brain uptake, such as small peptides, with evidence from some of our unpublished data on human neuronal cell line.

The non-canonical BMP and Wnt/β-catenin signaling pathways orchestrate early tooth development

BMP and Wnt signaling pathways play a crucial role in organogenesis, including tooth development. Despite extensive studies, the exact functions, as well as if and how these two pathways act coordinately in regulating early tooth development, remain elusive. In this study, we dissected regulatory functions of BMP and Wnt pathways in early tooth development using a transgenic noggin (Nog) overexpression model (K14Cre;pNog). It exhibits early arrested tooth development, accompanied by reduced cell proliferation and loss of odontogenic fate marker Pitx2 expression in the dental epithelium. We demonstrated that overexpression of Nog disrupted BMP non-canonical activity, which led to a dramatic reduction of cell proliferation rate but did not affect Pitx2 expression. We further identified a novel function of Nog by inhibiting Wnt/β-catenin signaling, causing loss of Pitx2 expression. Co-immunoprecipitation and TOPflash assays revealed direct binding of Nog to Wnts to functionally prevent Wnt/β-catenin signaling. In situ PLA and immunohistochemistry on Nog mutants confirmed in vivo interaction between endogenous Nog and Wnts and modulation of Wnt signaling by Nog in tooth germs. Genetic rescue experiments presented evidence that both BMP and Wnt signaling pathways contribute to cell proliferation regulation in the dental epithelium, with Wnt signaling also controlling the odontogenic fate. Reactivation of both BMP and Wnt signaling pathways, but not of only one of them, rescued tooth developmental defects in K14Cre;pNog mice, in which Wnt signaling can be substituted by transgenic activation of Pitx2. Our results reveal the orchestration of non-canonical BMP and Wnt/β-catenin signaling pathways in the regulation of early tooth development.

Sox5 controls dorsal progenitor and interneuron specification in the spinal cord

The basic organization of somatosensory circuits in the spinal cord is already setup during the initial patterning of the dorsal neural tube. Extrinsic signals, such as Wnt and TGF-β pathways, activate combinatorial codes of transcription factors that are responsible for generating a pattern of discrete domains of dorsal progenitors (dp). These progenitors will give rise to distinct dorsal interneurons (dI). The Wnt/ βcatenin signaling pathway controls specification of dp/dI1-3 progenitors and interneurons. According to the current model in the field, Wnt/βcatenin activity seems to act in a graded fashion in the spinal cord, as different relative levels determine the identity of adjacent progenitors. However, it is not clear how this activity gradient is controlled and how the identities of dI1-3 are differentially regulated by Wnt signalling. We have determined that two SoxD transcription factors, Sox5 and Sox6, are expressed in restricted domains of dorsal progenitors in the neural tube. Using gain- and loss-of function approaches in chicken embryos, we have established that Sox5 controls cell fate specification of dp2 and dp3 progenitors and, as a result, controls the correct number of the corresponding dorsal interneurons (dI2 and dI3). Furthermore, Sox5 exerts its function by restricting dorsally Wnt signaling activity via direct transcriptional induction of the negative Wnt pathway regulator Axin2. By that way, Sox5 acts as a Wnt pathway modulator that contributes to sharpen the dorsal gradient of Wnt/βcatenin activity to control the distinction of two functionally distinct types of interneurons, dI2 and dI3 involved in the somatosensory relay.

Concentration-dependent effects of WNTLESS on WNT1/3A signaling

BACKGROUND

WNTLESS (WLS) is a multi-transmembrane protein that transports Wnt ligands from the Golgi to the cell surface. Although WLS loss-of-function experiments in the developing central nervous system reveal phenotypes consistent with defects in WNT1 and WNT3A signaling, data from complementary gain-of-function experiments have not yet been reported. Here, we report the phenotypic consequences of WLS overexpression in cultured cells and in the developing chick spinal cord.

RESULTS

Overexpression of small amounts of WLS along with either WNT1 or WNT3A promotes the Wnt/β-catenin pathway in HEK293T cells, while overexpression of higher levels of WLS inhibits the Wnt/β-catenin pathway in these cells. Similarly, overexpressed WLS inhibits the Wnt/β-catenin pathway in the developing spinal cord, as assessed by cell proliferation and specification. These effects appear to be Wnt-specific as overexpression of WLS inhibits the expression of FZD10, a target of β-catenin-dependent transcription.

CONCLUSIONS

Our results show that overexpression of WLS inhibits Wnt/β-catenin signaling in the spinal cord. As the activation of the Wnt/β-catenin pathway in the spinal cord requires WNT1 or WNT3A, our results are consistent with a model in which the relative concentration of WLS to Wnt regulates WNT1/3A signaling in the developing spinal cord.

Frizzled10 mediates WNT1 and WNT3A signaling in the dorsal spinal cord of the developing chick embryo

BACKGROUND

WNT1 and WNT3A drive a dorsal to ventral gradient of β-catenin-dependent Wnt signaling in the developing spinal cord. However, the identity of the receptors mediating downstream functions remains poorly understood.

RESULTS

In this report, we show that the spatiotemporal expression patterns of FZD10 and WNT1/WNT3A are highly correlated. We further show that in the presence of LRP6, FZD10 promotes WNT1 and WNT3A signaling using an 8xSuperTopFlash reporter assay. Consistent with a functional role for FZD10, we demonstrate that FZD10 is required for proliferation in the spinal cord. Finally, by using an in situ proximity ligation assay, we observe an interaction between FZD10 and WNT1 and WNT3A proteins.

CONCLUSIONS

Together, our results identify FZD10 as a receptor for WNT1 and WNT3A in the developing chick spinal cord.

Roles for the TGFβ superfamily in the development and survival of midbrain dopaminergic neurons

The adult midbrain contains 75% of all dopaminergic neurons in the CNS. Within the midbrain, these neurons are divided into three anatomically and functionally distinct clusters termed A8, A9 and A10. The A9 group plays a functionally non-redundant role in the control of voluntary movement, which is highlighted by the motor syndrome that results from their progressive degeneration in the neurodegenerative disorder, Parkinson's disease. Despite 50 years of investigation, treatment for Parkinson's disease remains symptomatic, but an intensive research effort has proposed delivering neurotrophic factors to the brain to protect the remaining dopaminergic neurons, or using these neurotrophic factors to differentiate dopaminergic neurons from stem cell sources for cell transplantation. Most neurotrophic factors studied in this context have been members of the transforming growth factor β (TGFβ) superfamily. In recent years, an intensive research effort has focused on understanding the function of these proteins in midbrain dopaminergic neuron development and their role in the molecular architecture that regulates the development of this brain region, with the goal of applying this knowledge to develop novel therapies for Parkinson's disease. In this review, the current evidence showing that TGFβ superfamily members play critical roles in the regulation of midbrain dopaminergic neuron induction, differentiation, target innervation and survival during embryonic and postnatal development is analysed, and the implications of these findings are discussed.

Expression of Nemo-like kinase after spinal cord injury in rats

Wnt can induce signal transduction via the canonical pathway, which was involved in many processes in the nervous system. Nemo-like kinase (NLK) acts as a negative regulator of β-catenin/T-cell factor/lymphoid enhancer factor (LEF) and functions downstream of transforming growth factor β-activated kinase-1 in the Wnt signaling pathway. In this study, we performed a spinal cord injury (SCI) test in adult Sprague-Dawley rats and investigated the dynamic changes and role of NLK expression in the spinal cord. Western blot analysis revealed that NLK expression was low in normal spinal cord. It then increased markedly, peaked at 3 days, and declined to basal levels from 5 days after injury. Immunohistochemistry confirmed that NLK immunoactivity was expressed at low levels in gray and white matter under normal conditions and increased prominently in gray matter after the SCI test. Double immunofluorescent staining for NLK, caspase-3, β-catenin, and NeuN (neuronal nuclei) revealed that NLK and β-catenin were markedly increased and colocalized in apoptotic neurons. Coimmunoprecipitation data demonstrated that overexpression of NLK protein reduced β-catenin binding to LEF-1. Our results suggested that NLK was associated with neuronal apoptosis through attenuating the Wnt/β-catenin signaling pathway after SCIs.

Synergetic regulatory networks mediated by oncogene-driven microRNAs and transcription factors in serous ovarian cancer

Although high-grade serous ovarian cancer (OVC) is the most lethal gynecologic malignancy in women, little is known about the regulatory mechanisms in the cellular processes that lead to this cancer. Recently, accumulated lines of evidence have shown that the interplay between transcription factors (TFs) and microRNAs (miRNAs) is critical in cellular regulation during tumorigenesis. A comprehensive investigation of TFs and miRNAs, and their target genes, may provide a deeper understanding of the regulatory mechanisms in the pathology of OVC. In this study, we have integrated three complementary algorithms into a framework, aiming to infer the regulation by miRNAs and TFs in conjunction with gene expression profiles. We demonstrated the utility of our framework by inferring 67 OVC-specific regulatory feed-forward loops (FFL) initiated by miRNAs or TFs in high-grade serous OVC. By analyzing these regulatory behaviors, we found that all the 67 FFLs are consistent in their regulatory effects on genes that are jointly targeted by miRNAs and TFs. Remarkably, we unveiled an unbalanced distribution of FFLs with different oncogenic effects. In total, 31 of the 67 coherent FFLs were mainly initiated by oncogenes. On the contrary, only 4 of the FFLs were initiated by tumor suppressor genes. These overwhelmingly observed oncogenic genes were further detected in a sub-network with 32 FFLs centered by miRNA let-7b and TF TCF7L1 to regulate cell differentiation. Closer inspection of 32 FFLs revealed that 75% of the miRNAs reportedly play functional roles in cell differentiation, especially when enriched in epithelial-mesenchymal transitions. This study provides a comprehensive pathophysiological overview of recurring coherent circuits in OVC that are co-regulated by miRNAs and TFs. The prevalence of oncogenic coherent FFLs in serous OVC suggests that oncogene-driven regulatory motifs could cooperatively act upon critical cellular processes such as cell differentiation in a highly efficient and consistent manner.

Asymmetric activation of Dll4-Notch signaling by Foxn4 and proneural factors activates BMP/TGFβ signaling to specify V2b interneurons in the spinal cord

During development of the ventral spinal cord, the V2 interneurons emerge from p2 progenitors and diversify into two major subtypes, V2a and V2b, that play key roles in locomotor coordination. Dll4-mediated Notch activation in a subset of p2 precursors constitutes the crucial first step towards generating neuronal diversity in this domain. The mechanism behind the asymmetric Notch activation and downstream signaling events are, however, unknown at present. We show here that the Ascl1 and Neurog basic helix-loop-helix (bHLH) proneural factors are expressed in a mosaic pattern in p2 progenitors and that Foxn4 is required for setting and maintaining this expression mosaic. By binding directly to a conserved Dll4 enhancer, Foxn4 and Ascl1 activate Dll4 expression, whereas Neurog proteins prevent this effect, thereby resulting in asymmetric activation of Dll4 expression in V2 precursors expressing different combinations of proneural and Foxn4 transcription factors. Lineage tracing using the Cre-LoxP system reveals selective expression of Dll4 in V2a precursors, whereas Dll4 expression is initially excluded from V2b precursors. We provide evidence that BMP/TGFβ signaling is activated in V2b precursors and that Dll4-mediated Notch signaling is responsible for this activation. Using a gain-of-function approach and by inhibiting BMP/TGFβ signal transduction with pathway antagonists and RNAi knockdown, we further demonstrate that BMP/TGFβ signaling is both necessary and sufficient for V2b fate specification. Our data together thus suggest that the mosaic expression of Foxn4 and proneural factors may serve as the trigger to initiate asymmetric Dll4-Notch and subsequent BMP/TGFβ signaling events required for neuronal diversity in the V2 domain.

The multiple activities of BMPs during spinal cord development

Bone morphogenetic proteins (BMPs) are one of the main classes of multi-faceted secreted factors that drive vertebrate development. A growing body of evidence indicates that BMPs contribute to the formation of the central nervous system throughout its development, from the initial shaping of the neural primordium to the generation and maturation of the different cell types that form the functional adult nervous tissue. In this review, we focus on the multiple activities of BMPs during spinal cord development, paying particular attention to recent results that highlight the complexity of BMP signaling during this process. These findings emphasize the unique capacity of these signals to mediate various functions in the same tissue throughout development, recruiting diverse effectors and strategies to instruct their target cells.

Detailed expression analysis of regulatory genes in the early developing human neural tube

Studies in model organisms constitute the basis of our understanding of the principal molecular mechanisms of cell fate determination in the developing central nervous system. Considering the emergent applications in stem cell-based regenerative medicine, it is important to demonstrate conservation of subtype specific gene expression programs in human as compared to model vertebrates. We have examined the expression patterns of key regulatory genes in neural progenitor cells and their neuronal and glial descendants in the developing human spinal cord, hindbrain, and midbrain, and compared these with developing mouse and chicken embryos. As anticipated, gene expression patterns are highly conserved between these vertebrate species, but there are also features that appear unique to human development. In particular, we find that neither tyrosine hydroxylase nor Nurr1 are specific markers for mesencephalic dopamine neurons, as these genes also are expressed in other neuronal subtypes in the human ventral midbrain and in human embryonic stem cell cultures directed to differentiate towards a ventral mesencephalic identity. Moreover, somatic motor neurons in the ventral spinal cord appear to be produced by two molecularly distinct ventral progenitor populations in the human, raising the possibility that the acquisition of unique ventral progenitor identities may have contributed to the emergence of neural subtypes in higher vertebrates.

BMP-Smad 1/5/8 signalling in the development of the nervous system

The transcription factors, Smad1, Smad5 and Smad8, are the pivotal intracellular effectors of the bone morphogenetic protein (BMP) family of proteins. BMPs and their receptors are expressed in the nervous system (NS) throughout its development. This review focuses on the actions of Smad 1/5/8 in the developing NS. The mechanisms by which these Smad proteins regulate the induction of the neuroectoderm, the central nervous system (CNS) primordium, and finally the neural crest, which gives rise to the peripheral nervous system (PNS), are reviewed herein. We describe how, following neural tube closure, the most dorsal aspect of the tube becomes a signalling centre for BMPs, which directs the pattern of the development of the dorsal spinal cord (SC), through the action of Smad1, Smad5 and Smad8. The direct effects of Smad 1/5/8 signalling on the development of neuronal and non-neuronal cells from various neural progenitor cell populations are then described. Finally, this review discusses the neurodevelopmental abnormalities associated with the knockdown of Smad 1/5/8.

A dynamic code of dorsal neural tube genes regulates the segregation between neurogenic and melanogenic neural crest cells

Understanding when and how multipotent progenitors segregate into diverse fates is a key question during embryonic development. The neural crest (NC) is an exemplary model system with which to investigate the dynamics of progenitor cell specification, as it generates a multitude of derivatives. Based on 'in ovo' lineage analysis, we previously suggested an early fate restriction of premigratory trunk NC to generate neural versus melanogenic fates, yet the timing of fate segregation and the underlying mechanisms remained unknown. Analysis of progenitors expressing a Foxd3 reporter reveals that prospective melanoblasts downregulate Foxd3 and have already segregated from neural lineages before emigration. When this downregulation is prevented, late-emigrating avian precursors fail to upregulate the melanogenic markers Mitf and MC/1 and the guidance receptor Ednrb2, generating instead glial cells that express P0 and Fabp. In this context, Foxd3 lies downstream of Snail2 and Sox9, constituting a minimal network upstream of Mitf and Ednrb2 to link melanogenic specification with migration. Consistent with the gain-of-function data in avians, loss of Foxd3 function in mouse NC results in ectopic melanogenesis in the dorsal tube and sensory ganglia. Altogether, Foxd3 is part of a dynamically expressed gene network that is necessary and sufficient to regulate fate decisions in premigratory NC. Their timely downregulation in the dorsal neural tube is thus necessary for the switch between neural and melanocytic phases of NC development.

Geminin promotes an epithelial-to-mesenchymal transition in an embryonic stem cell model of gastrulation

Geminin is a nuclear protein that performs the related functions of modulating cell cycle progression by binding Cdt1, and controlling differentiation by binding transcription factors. Since embryonic stem cells (ESC) and the epiblast share a similar gene expression profile and an attenuated cell cycle, ESC form an accessible and tractable model system to study lineage choice at gastrulation. We derived several ESC lines in which Geminin can be inducibly expressed, and employed short hairpin RNAs targeting Geminin. As in the embryo, a lack of Geminin protein resulted in DNA damage and cell death. In monolayer culture, in defined medium, Geminin supported neural differentiation; however, in three-dimensional culture, overexpression of Geminin promoted mesendodermal differentiation and epithelial-to-mesenchymal transition. In vitro, ESC overexpressing Geminin rapidly recolonized a wound, downregulated E-cadherin expression, and activated Wnt signaling. We suggest that Geminin may promote differentiation via binding Groucho/TLE proteins and upregulating canonical Wnt signaling.

Temporal control of BMP signalling determines neuronal subtype identity in the dorsal neural tube

The conventional explanation for how a morphogen patterns a tissue holds that cells interpret different concentrations of an extrinsic ligand by producing corresponding levels of intracellular signalling activity, which in turn regulate differential gene expression. However, this view has been challenged, raising the possibility that distinct mechanisms are used to interpret different morphogens. Here, we investigate graded BMP signalling in the vertebrate neural tube. We show that defined exposure times to Bmp4 generate distinct levels of signalling and induce specific dorsal identities. Moreover, we provide evidence that a dynamic gradient of BMP activity confers progressively more dorsal neural identities in vivo. These results highlight a strategy for morphogen interpretation in which the tight temporal control of signalling is important for the spatial pattern of cellular differentiation.

The dynamic role of bone morphogenetic proteins in neural stem cell fate and maturation

The bone morphogenetic proteins (BMPs) are a group of powerful morphogens that are critical for development of the nervous system. The effects of BMP signaling on neural stem cells are myriad and dynamic, changing with each stage of development. During early development inhibition of BMP signaling differentiates neuroectoderm from ectoderm, and BMP signaling helps to specify neural crest. Thus modulation of BMP signaling underlies formation of both the central and peripheral nervous systems. BMPs secreted from dorsal structures then form a gradient which helps pattern the dorsal-ventral axis of the developing spinal cord and brain. During forebrain development BMPs sequentially induce neurogenesis and then astrogliogenesis and participate in neurite outgrowth from immature neurons. BMP signaling also plays a critical role in maintaining adult neural stem cell niches in the subventricular zone (SVZ) and subgranular zone (SGZ). BMPs are able to exert such diverse effects through closely regulated temporospatial expression and interaction with other signaling pathways.

The roof plate boundary is a bi-directional organiser of dorsal neural tube and choroid plexus development

The roof plate is a signalling centre positioned at the dorsal midline of the central nervous system and generates dorsalising morphogenic signals along the length of the neuraxis. Within cranial ventricles, the roof plate gives rise to choroid plexus, which regulates the internal environment of the developing and adult brain and spinal cord via the secretion of cerebrospinal fluid. Using the fourth ventricle as our model, we show that the organiser properties of the roof plate are determined by its boundaries with the adjacent neuroepithelium. Through a combination of in ovo transplantation, co-culture and electroporation techniques in chick embryos between embryonic days 3 and 6, we demonstrate that organiser properties are maintained by interactions between the non-neural roof plate and the neural rhombic lip. At the molecular level, this interaction is mediated by Delta-Notch signalling and upregulation of the chick homologue of Hes1: chairy2. Gain- and loss-of-function approaches reveal that cdelta1 is both necessary and sufficient for organiser function. Our results also demonstrate that while chairy2 is specifically required for the maintenance of the organiser, its ectopic expression is not sufficient to recapitulate organiser properties. Expression of atonal1 in the rhombic lip adjacent at the roof plate boundary is acutely dependent on both boundary cell interactions and Delta-Notch signalling. Correspondingly, the roof plate boundary organiser also signals to the roof plate itself to specify the expression of early choroid plexus markers. Thus, the roof plate boundary organiser signals bi-directionally to acutely coordinate the development of adjacent neural and non-neural tissues.

Neural crest and somitic mesoderm as paradigms to investigate cell fate decisions during development

The dorsal domains of the neural tube and somites are transient embryonic epithelia; they constitute the source of neural crest progenitors that generate the peripheral nervous system, pigment cells and ectomesenchyme, and of the dermomyotome that develops into myocytes, dermis and vascular cells, respectively. Based on the variety of derivatives produced by each type of epithelium, a classical yet still highly relevant question is whether these embryonic epithelia are composed of homogeneous multipotent progenitors or, alternatively, of subsets of fate-restricted cells. Growing evidence substantiates the notion that both the dorsal tube and the dermomyotome are heterogeneous epithelia composed of multipotent as well as fate-restricted precursors that emerge as such in a spatio-temporally regulated manner. Elucidation of the state of commitment of the precedent progenitors is of utmost significance for deciphering the mechanisms that regulate fate segregation during embryogenesis. In addition, it will contribute to understanding the nature of well documented neural crest-somite interactions shown to modulate the timing of neural crest cell emigration, their segmental migration, and myogenesis.

Temporal deletion of Arl13b reveals that a mispatterned neural tube corrects cell fate over time

Cilia are necessary for sonic hedgehog (Shh) signaling, which is required to pattern the neural tube. We know that ventral neural cell fates are defined by a specific cohort of transcription factors that are induced by distinct thresholds of Shh activity mediated by opposing gradients of Gli activator (GliA) and Gli repressor (GliR). Despite this understanding, the role of Shh as an instructive morphogen is viewed as increasingly complex, with current models integrating positive inputs in terms of ligand concentration and time, along with negative feedback via the downstream gene regulatory network. To investigate the relative contributions of the positive and negative inputs from Shh signaling in neural patterning, we took advantage of a protein that uncouples the regulation of GliA and GliR: the cilia protein ADP-ribosylation factor-like 13b (Arl13b). By deleting Arl13b in mouse, we induced low-level constitutive GliA function at specific developmental stages and defined a crucial period prior to E10.5 when shifts in the level of GliA cause cells to change their fate. Strikingly, we found that improperly patterned cells in these mice converted to the wild-type pattern by E12.5. We further showed that the recovery of patterning did not occur when we also deleted Gli3, the primary GliR in the neural tube, revealing a crucial role of Gli3 in the maintenance of neural patterning.

Bmp indicator mice reveal dynamic regulation of transcriptional response

Cellular responses to Bmp ligands are regulated at multiple levels, both extracellularly and intracellularly. Therefore, the presence of these growth factors is not an accurate indicator of Bmp signaling activity. While a common approach to detect Bmp signaling activity is to determine the presence of phosphorylated forms of Smad1, 5 and 8 by immunostaining, this approach is time consuming and not quantitative. In order to provide a simpler readout system to examine the presence of Bmp signaling in developing animals, we developed BRE-gal mouse embryonic stem cells and a transgenic mouse line that specifically respond to Bmp ligand stimulation. Our reporter identifies specific transcriptional responses that are mediated by Smad1 and Smad4 with the Schnurri transcription factor complex binding to a conserved Bmp-Responsive Element (BRE), originally identified among Drosophila, Xenopus and human Bmp targets. Our BRE-gal mES cells specifically respond to Bmp ligands at concentrations as low as 5 ng/ml; and BRE-gal reporter mice, derived from the BRE-gal mES cells, show dynamic activity in many cellular sites, including extraembryonic structures and mammary glands, thereby making this a useful scientific tool.

Dorsal-ventral patterning of the neural tube: a tale of three signals

Development of the vertebrate nervous system begins with the acquisition of neural identity from the midline dorsal-ectodermal cells of the gastrulating embryos. The subsequent progressive specification of the neural plate along its anterior-posterior and dorsal-ventral (DV) axes allows the generation of the tremendous variety of neuronal and glial cells that compose the vertebrate central nervous system (CNS). Studies on the development of the spinal cord, the anatomically simplest part of the CNS, have generated most of our current knowledge on the signaling events and the genetic networks that orchestrate the DV patterning of the neural plate. In this review, we discuss the recent advances in our understanding of these events and highlight unresolved questions. We focused our attention on the activity and the integration of the three main instructive cues: Sonic hedgehog, the Wnts and the Bone Morphogenetic Proteins, giving particular attention to the less well understood dorsal signaling events.

BMP receptor-activated Smads confer diverse functions during the development of the dorsal spinal cord

Bone Morphogenetic Proteins (BMPs) have multiple activities in the developing spinal cord: they specify the identity of the dorsal-most neuronal populations and then direct the trajectories of dorsal interneuron (dI) 1 commissural axons. How are these activities decoded by dorsal neurons to result in different cellular outcomes? Our previous studies have shown that the diverse functions of the BMPs are mediated by the canonical family of BMP receptors and then regulated by specific inhibitory (I) Smads, which block the activity of a complex of Smad second messengers. However, the extent to which this complex translates the different activities of the BMPs in the spinal cord has remained unresolved. Here, we demonstrate that the receptor-activated (R) Smads, Smad1 and Smad5 play distinct roles mediating the abilities of the BMPs to direct cell fate specification and axon outgrowth. Smad1 and Smad5 occupy spatially distinct compartments within the spinal cord, with Smad5 primarily associated with neural progenitors and Smad1 with differentiated neurons. Consistent with this expression profile, loss of function experiments in mouse embryos reveal that Smad5 is required for the acquisition of dorsal spinal neuron identities whereas Smad1 is critical for the regulation of dI1 axon outgrowth. Thus the R-Smads, like the I-Smads, have discrete roles mediating BMP-dependent cellular processes during spinal interneuron development.

BMPs are mediators in tissue crosstalk of the regenerating musculoskeletal system

The musculoskeletal system is a tight network of many tissues. Coordinated interplay at a biochemical level between tissues is essential for development and repair. Traumatic injury usually affects several tissues and represents a large challenge in clinical settings. The current demand for potent growth factors in such applications thus accompanies the keen interest in molecular mechanisms and orchestration of tissue formation. Of special interest are multitasking growth factors that act as signals in a variety of cell types, both in a paracrine and in an autocrine manner, thereby inducing cell differentiation and coordinating not only tissue assembly at specific sites but also maturation and homeostasis. We concentrate here on bone morphogenetic proteins (BMPs), which are important crosstalk mediators known for their irreplaceable roles in vertebrate development. The molecular crosstalk during embryonic musculoskeletal tissue formation is recapitulated in adult repair. BMPs act at different levels from the initiation to maturation of newly formed tissue. Interestingly, this is influenced by the spatiotemporal expression of different BMPs, their receptors and co-factors at the site of repair. Thus, the regenerative potential of BMPs needs to be evaluated in the context of highly connected tissues such as muscle and bone and might indeed be different in more poorly connected tissues such as cartilage. This highlights the need for an understanding of BMP signaling across tissues in order to eventually improve BMP regenerative potential in clinical applications. In this review, the distinct members of the BMP family and their individual contribution to musculoskeletal tissue repair are summarized by focusing on their paracrine and autocrine functions.

Neucrin, a novel secreted antagonist of canonical Wnt signaling, plays roles in developing neural tissues in zebrafish

Wnt signaling plays crucial roles in neural development. We previously identified Neucrin, a neural-specific secreted antagonist of canonical Wnt/β-catenin signaling, in humans and mice. Neucrin has one cysteine-rich domain, in which the positions of 10 cysteine residues are similar to those in the second cysteine-rich domain of Dickkopfs, secreted Wnt antagonists. Here, we have identified zebrafish neucrin to understand its roles in vivo. Zebrafish Neucrin also has one cysteine-rich domain, which is significantly similar to that of mouse Neucrin. Zebrafish neucrin was also predominantly expressed in developing neural tissues. To examine roles of neucrin in neural development, we analyzed neucrin knockdown embryos. Neural development in zebrafish embryos was impaired by the knockdown of neucrin. The knockdown of neucrin caused increased expression of the Wnt/β-catenin target genes. In contrast, overexpression of neucrin reduced the expression of the Wnt/β-catenin target genes. The knockdown of neucrin affected specification of dorsal region in the midbrain and hindbrain. The knockdown of neucrin also suppressed neuronal differentiation and caused increased cell proliferation and apoptosis in developing neural tissues. Neucrin is a unique secreted Wnt antagonist that is predominantly expressed in developing neural tissues and plays roles in neural development in zebrafish.

Intraspinal transplantation of neurogenin-expressing stem cells generates spinal cord neural progenitors

Embryonic stem (ES) cells and their derivatives are an important resource for developing novel cellular therapies for disease. Controlling proliferation and lineage selection, however, are essential to circumvent the possibility of tumor formation and facilitate the safe translation of ES-based therapies to man. Expression of appropriate transcription factors is one approach to direct the differentiation of ES cells towards a specific lineage and stop proliferation. Neural differentiation can be initiated in ES cells by expression of Neurogenin1 (Ngn1). In this study we investigate the effects of controlled Ngn1 expression on mouse ES (mES) cell differentiation in vitro and following grafting into the rat spinal cord. In vitro, Ngn1 expression in mES cells leads to rapid and specific neural differentiation, and a concurrent decrease in proliferation. Similarly transplantation of Ngn1-expressing mES cells into the spinal cord lead to in situ differentiation and spinal precursor formation. These data demonstrate that Ngn1 expression in mES cells is sufficient to promote neural differentiation and inhibit proliferation, thus establishing an approach to safely graft ES cells into the spinal cord.