Expression of the proto-oncogene int-1 is restricted to specific neural cells in the developing mouse embryo

Development, wiring and function of dopamine neuron subtypes

The midbrain dopamine (mDA) system is composed of molecularly and functionally distinct neuron subtypes that mediate specific behaviours and are linked to various brain diseases. Considerable progress has been made in identifying mDA neuron subtypes, and recent work has begun to unveil how these neuronal subtypes develop and organize into functional brain structures. This progress is important for further understanding the disparate physiological functions of mDA neurons and their selective vulnerability in disease, and will ultimately accelerate therapy development. This Review discusses recent advances in our understanding of molecularly defined mDA neuron subtypes and their circuits, ranging from early developmental events, such as neuron migration and axon guidance, to their wiring and function, and future implications for therapeutic strategies.

Diverse stem cells for periodontal tissue formation and regeneration

The periodontium is comprised of multiple units of mineralized and nonmineralized tissues including the cementum on the root surface, the alveolar bone, periodontal ligament (PDL), and the gingiva. PDL contains a variety of cell populations including mesenchymal stem/progenitor cells (MSCs) termed PDLSCs, which contribute to periodontal regeneration. Recent studies utilizing mouse genetic models shed light on the identities of these mesenchymal progenitors in their native environment, particularly regarding how they contribute to homeostasis and repair of the periodontium. The current concept is that mesenchymal progenitors in the PDL are localized to the perivascular niche. Single-cell RNA sequencing (scRNA-seq) analyses reveal heterogeneity and cell-type specific markers of cells in the periodontium, as well as their developmental relationship with precursor cells in the dental follicle. The characteristics of PDLSCs and their diversity in vivo are now beginning to be unraveled thanks to insights from mouse genetic models and scRNA-seq analyses, which aid to uncover the fundamental properties of stem cells in the human PDL. The new knowledge will be highly important for developing more effective stem cell-based regenerative therapies to repair periodontal tissues in the future.

An Update on the Molecular Mechanism of the Vertebrate Isthmic Organizer Development in the Context of the Neuromeric Model

A crucial event during the development of the central nervous system (CNS) is the early subdivision of the neural tube along its anterior-to-posterior axis to form neuromeres, morphogenetic units separated by transversal constrictions and programed for particular genetic cascades. The narrower portions observed in the developing neural tube are responsible for relevant cellular and molecular processes, such as clonal restrictions, expression of specific regulatory genes, and differential fate specification, as well as inductive activities. In this developmental context, the gradual formation of the midbrain-hindbrain (MH) constriction has been an excellent model to study the specification of two major subdivisions of the CNS containing the mesencephalic and isthmo-cerebellar primordia. This MH boundary is coincident with the common Otx2-(midbrain)/Gbx2-(hindbrain) expressing border. The early interactions between these two pre-specified areas confer positional identities and induce the generation of specific diffusible morphogenes at this interface, in particular FGF8 and WNT1. These signaling pathways are responsible for the gradual histogenetic specifications and cellular identity acquisitions with in the MH domain. This review is focused on the cellular and molecular mechanisms involved in the specification of the midbrain/hindbrain territory and the formation of the isthmic organizer. Emphasis will be placed on the chick/quail chimeric experiments leading to the acquisition of the first fate mapping and experimental data to, in this way, better understand pioneering morphological studies and innovative gain/loss-of-function analysis.

Nkx2.9 Contributes to Mid-Hindbrain Patterning by Regulation of mdDA Neuronal Cell-Fate and Repression of a Hindbrain-Specific Cell-Fate

Nkx2.9 is a member of the NK homeobox family and resembles Nkx2.2 both in homology and expression pattern. However, while Nkx2.2 is required for development of serotonergic neurons, the role of Nkx2.9 in the mid-hindbrain region is still ill-defined. We have previously shown that Nkx2.9 expression is downregulated upon loss of En1 during development. Here, we determined whether mdDA neurons require Nkx2.9 during their development. We show that Nkx2.9 is strongly expressed in the IsO and in the VZ and SVZ of the embryonic midbrain, and the majority of mdDA neurons expressed Nkx2.9 during their development. Although the expression of Dat and Cck are slightly affected during development, the overall development and cytoarchitecture of TH-expressing neurons is not affected in the adult Nkx2.9-depleted midbrain. Transcriptome analysis at E14.5 indicated that genes involved in mid- and hindbrain development are affected by Nkx2.9-ablation, such as Wnt8b and Tph2. Although the expression of Tph2 extends more rostral into the isthmic area in the Nkx2.9 mutants, the establishment of the IsO is not affected. Taken together, these data point to a minor role for Nkx2.9 in mid-hindbrain patterning by repressing a hindbrain-specific cell-fate in the IsO and by subtle regulation of mdDA neuronal subset specification.

Brain regionalization by Polycomb-group proteins and chromatin accessibility

During brain development, neural precursor cells (NPCs) in different brain regions produce different types of neurons, and each of these regions plays a different role in the adult brain. Therefore, precise regionalization is essential in the early stages of brain development, and irregular regionalization has been proposed as the cause of neurodevelopmental disorders. The mechanisms underlying brain regionalization have been well studied in terms of morphogen-induced expression of critical transcription factors for regionalization. NPC potential in different brain regions is defined by chromatin structures that regulate the plasticity of gene expression. Herein, we present recent findings on the importance of chromatin structure in brain regionalization, particularly with respect to its regulation by Polycomb-group proteins and chromatin accessibility.

Novel truncating mutations in CTNND1 cause a dominant craniofacial and cardiac syndrome

CTNND1 encodes the p120-catenin (p120) protein, which has a wide range of functions, including the maintenance of cell–cell junctions, regulation of the epithelial-mesenchymal transition and transcriptional signalling. Due to advances in next-generation sequencing, CTNND1 has been implicated in human diseases including cleft palate and blepharocheilodontic (BCD) syndrome albeit only recently. In this study, we identify eight novel protein-truncating variants, six de novo, in 13 participants from nine families presenting with craniofacial dysmorphisms including cleft palate and hypodontia, as well as congenital cardiac anomalies, limb dysmorphologies and neurodevelopmental disorders. Using conditional deletions in mice as well as CRISPR/Cas9 approaches to target CTNND1 in Xenopus, we identified a subset of phenotypes that can be linked to p120-catenin in epithelial integrity and turnover, and additional phenotypes that suggest mesenchymal roles of CTNND1. We propose that CTNND1 variants have a wider developmental role than previously described and that variations in this gene underlie not only cleft palate and BCD but may be expanded to a broader velocardiofacial-like syndrome.

Morphological and Functional Changes of Roof Plate Cells in Spinal Cord Development

The most dorsal region, or roof plate, is the dorsal organizing center of developing spinal cord. This region is also involved in development of neural crest cells, which are the source of migratory neural crest cells. During early development of the spinal cord, roof plate cells secrete signaling molecules, such as Wnt and BMP family proteins, which regulate development of neural crest cells and dorsal spinal cord. After the dorso-ventral pattern is established, spinal cord dynamically changes its morphology. With this morphological transformation, the lumen of the spinal cord gradually shrinks to form the central canal, a cavity filled with cerebrospinal fluid that is connected to the ventricular system of the brain. The dorsal half of the spinal cord is separated by a glial structure called the dorsal (or posterior) median septum. However, underlying mechanisms of such morphological transformation are just beginning to be understood. Recent studies reveal that roof plate cells dramatically stretch along the dorso-ventral axis, accompanied by reduction of the spinal cord lumen. During this stretching process, the tips of roof plate cells maintain contact with cells surrounding the shrinking lumen, eventually exposed to the inner surface of the central canal. Interestingly, Wnt expression remains in stretched roof plate cells and activates Wnt/β-catenin signaling in ependymal cells surrounding the central canal. Wnt/β-catenin signaling in ependymal cells promotes proliferation of neural progenitor and stem cells in embryonic and adult spinal cord. In this review, we focus on the role of the roof plate, especially that of Wnt ligands secreted by roof plate cells, in morphological changes occurring in the spinal cord.

Unveiling diversity of stem cells in dental pulp and apical papilla using mouse genetic models: a literature review

The dental pulp, a non-mineralized connective tissue uniquely encased within the cavity of the tooth, provides a niche for diverse arrays of dental mesenchymal stem cells. Stem cells in the dental pulp, including dental pulp stem cells (DPSCs), stem cells from human exfoliated deciduous teeth (SHEDs) and stem cells from apical papilla (SCAPs), have been isolated from human tissues with an emphasis on their potential application to regenerative therapies. Recent studies utilizing mouse genetic models shed light on the identities of these mesenchymal progenitor cells derived from neural crest cells (NCCs) in their native conditions, particularly regarding how they contribute to homeostasis and repair of the dental tissue. The current concept is that at least two distinct niches for stem cells exist in the dental pulp, e.g., the perivascular niche and the perineural niche. The precise identities of these stem cells and their niches are now beginning to be unraveled thanks to sophisticated mouse genetic models, which lead to better understanding of the fundamental properties of stem cells in the dental pulp and the apical papilla in humans. The new knowledge will be highly instrumental for developing more effective stem cell-based regenerative therapies to repair teeth in the future.

Development and Differentiation of Midbrain Dopaminergic Neuron: From Bench to Bedside

Parkinson’s Disease (PD) is a neurodegenerative disorder affecting the motor system. It is primarily due to substantial loss of midbrain dopamine (mDA) neurons in the substantia nigra pars compacta and to decreased innervation to the striatum. Although existing drug therapy available can relieve the symptoms in early-stage PD patients, it cannot reverse the pathogenic progression of PD. Thus, regenerating functional mDA neurons in PD patients may be a cure to the disease. The proof-of-principle clinical trials showed that human fetal graft-derived mDA neurons could restore the release of dopamine neurotransmitters, could reinnervate the striatum, and could alleviate clinical symptoms in PD patients. The invention of human-induced pluripotent stem cells (hiPSCs), autologous source of neural progenitors with less ethical consideration, and risk of graft rejection can now be generated in vitro. This advancement also prompts extensive research to decipher important developmental signaling in differentiation, which is key to successful in vitro production of functional mDA neurons and the enabler of mass manufacturing of the cells required for clinical applications. In this review, we summarize the biology and signaling involved in the development of mDA neurons and the current progress and methodology in driving efficient mDA neuron differentiation from pluripotent stem cells.

Impact of prenatal arsenate exposure on gene expression in a pure population of migratory cranial neural crest cells

Prenatal exposure to arsenic, a naturally occurring toxic element, causes neural tube defects (NTDs) and, in animal models, orofacial anomalies. Since aberrant development or migration of cranial neural crest cells (CNCCs) can also cause similar anomalies within developing embryos, we examined the effects of in utero exposure to sodium arsenate on gene expression patterns in pure populations of CNCCs, isolated by fluorescence activated cell sorting (FACS), from Cre/LoxP reporter mice. Changes in gene expression were analyzed using Affymetrix GeneChip^® microarrays and expression of selected genes was verified by TaqMan quantitative real-time PCR. We report, for the first time, arsenate-induced alterations in the expression of a number of novel candidate genes and canonical cascades that may contribute to the pathogenesis of orofacial defects. Ingenuity Pathway and NIH-DAVID analyses revealed cellular response pathways, biological themes, and potential upstream regulators, that may underlie altered fetal programming of arsenate exposed CNCCs.

Analysis of the wnt1 regulatory chromosomal landscape

One of the earliest patterning events in the vertebrate neural plate is the specification of mes/r1, the territory comprising the prospective mesencephalon and the first hindbrain rhombomere. Within mes/r1, an interface of gene expression defines the midbrain-hindbrain boundary (MHB), a lineage restriction that separates the mesencephalon and rhombencephalon. wnt1 is critical to mes/r1 development and functions within the MHB as a component of the MHB gene regulatory network (GRN). Despite its importance to these critical and early steps of vertebrate neurogenesis, little is known about the factors responsible for wnt1 transcriptional regulation. In the zebrafish, wnt1 and its neighboring paralog, wnt10b, are expressed in largely overlapping patterns, suggesting co-regulation. To understand wnt1 and wnt10b transcriptional control, we used a comparative genomics approach to identify relevant enhancers. We show that the wnt1-wnt10b locus contains multiple cis-regulatory elements that likely interact to generate the wnt1 and wnt10b expression patterns. Two of 11 conserved enhancers tested show activity restricted to the midbrain and MHB, an activity that is conserved in the distantly related spotted gar orthologous elements. Three non-conserved elements also play a likely role in wnt1 regulation. The identified enhancers display dynamic modes of chromatin accessibility, suggesting controlled deployment during embryogenesis. Our results suggest that the control of wnt1 and wnt10b expression is under complex regulation involving the interaction of multiple enhancers.

Laser Capture Microdissection of Murine Embryonic Neural Crest Cells

The purpose of this chapter is to provide a step-by-step protocol to enable performance of laser capture microdissection (LCM) on tissue sections from mammalian embryos or postnatal organism stages in order to collect pure populations of neural crest cells from which sufficient amounts of nucleic acids and/or protein can be obtained for quantitative analysis. The methods (1) define a strategy to genetically and indelibly label mammalian neural crest-derived cells with a fluorescent marker, thus enabling their isolation throughout the pre- and postnatal life span of the organism, and (2) describe subsequent isolation by LCM of the labeled neural crest cells, or their derivatives, from embryonic/postnatal tissue cryosections. Details are provided for using the Arcturus PixCell^®IIe Laser Capture Microdissection System (Arcturus) and CapSure LCM Caps (Thermo Fisher Scientific), to which the selected cells adhere upon laser-mediated capture. The protocol outlined herein can be applied in any situation wherein limited cellular samples are available for isolation by LCM. Nucleic acids or proteins can be extracted from LCM-isolated cells and processed for high-density gene expression profiling analyses (microarrays or RNA sequencing), Real-Time PCR (q-PCR) for specific candidate gene expression, investigation of DNA methylation, as well as for varied protein analyses.

Wingless Signaling: A Genetic Journey from Morphogenesis to Metastasis

This FlyBook chapter summarizes the history and the current state of our understanding of the Wingless signaling pathway. Wingless, the fly homolog of the mammalian Wnt oncoproteins, plays a central role in pattern generation during development. Much of what we know about the pathway was learned from genetic and molecular experiments in Drosophila melanogaster, and the core pathway works the same way in vertebrates. Like most growth factor pathways, extracellular Wingless/Wnt binds to a cell surface complex to transduce signal across the plasma membrane, triggering a series of intracellular events that lead to transcriptional changes in the nucleus. Unlike most growth factor pathways, the intracellular events regulate the protein stability of a key effector molecule, in this case Armadillo/β-catenin. A number of mysteries remain about how the "destruction complex" destabilizes β-catenin and how this process is inactivated by the ligand-bound receptor complex, so this review of the field can only serve as a snapshot of the work in progress.

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.

Anti-osteogenic function of a LIM-homeodomain transcription factor LMX1B is essential to early patterning of the calvaria

The calvaria (upper part of the skull) is made of plates of bone and fibrous joints (sutures and fontanelles), and the proper balance and organization of these components are crucial to normal development of the calvaria. In a mouse embryo, the calvaria develops from a layer of head mesenchyme that surrounds the brain from shortly after mid-gestation. The mesenchyme just above the eye (supra-orbital mesenchyme, SOM) generates ossification centers for the bones, which then grow toward the apex gradually. In contrast, the mesenchyme apical to SOM (early migrating mesenchyme, EMM), including the area at the vertex, does not generate an ossification center. As a result, the dorsal midline of the head is occupied by sutures and fontanelles at birth. To date, the molecular basis for this regional difference in developmental programs is unknown. The current study provides vital insights into the genetic regulation of calvarial patterning. First, we showed that osteogenic signals were active in both EMM and SOM during normal development, which suggested the presence of an anti-osteogenic factor in EMM to counter the effect of these signals. Subsequently, we identified Lmx1b as an anti-osteogenic gene that was expressed in EMM but not in SOM. Furthermore, head mesenchyme-specific deletion of Lmx1b resulted in heterotopic ossification from EMM at the vertex, and craniosynostosis affecting multiple sutures. Conversely, forced expression of Lmx1b in SOM was sufficient to inhibit osteogenic specification. Therefore, we conclude that Lmx1b plays a key role as an anti-osteogenic factor in patterning the head mesenchyme into areas with different osteogenic competence. In turn, this patterning event is crucial to generating the proper organization of the bones and soft tissue joints of the calvaria.

Temporal Expression of Wnt1 Defines the Competency State and Terminal Identity of Progenitors in the Developing Cochlear Nucleus and Inferior Colliculus

The auditory system contains a diverse array of interconnected anatomical structures that mediate the perception of sound. The cochlear nucleus of the hindbrain serves as the initial site of convergence for auditory stimuli, while the inferior colliculus of the midbrain serves as an integration and relay station for all ascending auditory information. We used Genetic Inducible Fate Mapping (GIFM) to determine how the timing of Wnt1 expression is related to the competency states of auditory neuron progenitors. We demonstrate that the Wnt1 lineage defines progenitor pools of auditory neurons in the developing midbrain and hindbrain. The timing of Wnt1 expression specifies unique cell types during embryogenesis and follows a mixed model encompassing a brief epoch of de novo expression followed by rapid and progressive lineage restriction to shape the inferior colliculus. In contrast, Wnt1 fate mapping of the embryonic hindbrain revealed de novo induction of Wnt1 in auditory hindbrain progenitors, which is related to the development of biochemically distinct neurons in the cochlear nucleus. Thus, we uncovered two modes of lineage allocation that explain the relationship between the timing of Wnt1 expression and the development of the cochlear nucleus and the inferior colliculus. Finally, our analysis of Wnt1^sw/sw mutant mice demonstrated a functional requirement of Wnt1 for the development of auditory midbrain and hindbrain neurons. Collectively, our study provides a deeper understanding of Wnt1 lineage allocation and function in mammalian brain development.

The Temporal Contribution of the Gbx2 Lineage to Cerebellar Neurons

The cerebellum (Cb) is an exquisite structure that controls elaborate motor behaviors and is essential for sensory-motor learning. During development, the Cb is derived from rhombomere 1 (r1). Within this embryonic compartment, precursors in r1 are patterned by signaling cues originating from the isthmus organizer (IsO) and subsequently undergo complex morphogenic movements to establish their final position in the mature Cb. The transcription factor Gbx2 is expressed in the developing Cb and is intimately involved in organizing and patterning the Cb. Nevertheless, how precursors expressing Gbx2 at specific embryonic time points contribute to distinct cell types in the adult Cb is unresolved. In this study, we used Genetic Inducible Fate Mapping (GIFM) to mark Gbx2-expressing precursors with fine temporal resolution and to subsequently track this lineage through embryogenesis. We then determined the terminal neuronal fate of the Gbx2 lineage in the adult Cb. Our analysis demonstrates that the Gbx2 lineage contributes to the Cb with marking over the course of five stages: Embryonic day 7.5 (E7.5) through E11.5. The Gbx2 lineage gives rise to Purkinje cells, granule neurons, and deep cerebellar neurons across these marking stages. Notably, the contribution of the Gbx2 lineage shifts as development proceeds with each marking stage producing a distinct profile of mature neurons in the adult Cb. These findings demonstrate the relationship between the temporal expression of Gbx2 and the terminal cell fate of neurons in the Cb. Based on these results, Gbx2 is critical to Cb development, not only for its well-defined role in positioning and maintaining the IsO, but also for guiding the development of Cb precursors and determining the identity of Cb neurons.

Engrailed 1 shapes the dopaminergic and serotonergic landscape through proper isthmic organizer maintenance and function

The isthmic organizer (IsO) is a signaling center that specifies the correct and distinct embryonic development of the dopaminergic midbrain and serotonergic hindbrain. The IsO is a linear boundary between the two brain regions, emerging at around embryonic day 7-8 of murine embryonic development, that shapes its surroundings through the expression of instructive signals such as Wnt and growth factors. Homeobox transcription factor engrailed 1 (En1) is present in midbrain and rostral hindbrain (i.e. rhombomere 1, R1). Its expression spans the IsO, and it is known to be an important survival factor for both dopaminergic and serotonergic neurons. Erroneous composition of dopaminergic neurons in the midbrain or serotonergic neurons in the hindbrain is associated with severe pathologies such as Parkinson's disease, depression or autism. Here we investigated the role of En1 in early mid-hindbrain development, using multiple En1-ablated mouse models as well as lineage-tracing techniques, and observed the appearance of ectopic dopaminergic neurons, indistinguishable from midbrain dopaminergic neurons based on molecular profile and intrinsic electrophysiological properties. We propose that this change is the direct result of a caudal relocation of the IsO as represented by ectopic presence of Fgf8, Otx2, Wnt1 and canonical Wnt-signalling. Our work suggests a newly-discovered role for En1: the repression of Otx2, Wnt1 and canonical Wnt-signaling in R1. Overall, our results suggest that En1 is essential for proper IsO maintenance and function.

Summary: Local molecular coding under the influence of En1 is essential for proper spatiotemporal expression of key factors involved in the maintenance and function of the isthmic organizer.

Reprogramming of human cancer cells to pluripotency for models of cancer progression

The ability to study live cells as they progress through the stages of cancer provides the opportunity to discover dynamic networks underlying pathology, markers of early stages, and ways to assess therapeutics. Genetically engineered animal models of cancer, where it is possible to study the consequences of temporal-specific induction of oncogenes or deletion of tumor suppressors, have yielded major insights into cancer progression. Yet differences exist between animal and human cancers, such as in markers of progression and response to therapeutics. Thus, there is a need for human cell models of cancer progression. Most human cell models of cancer are based on tumor cell lines and xenografts of primary tumor cells that resemble the advanced tumor state, from which the cells were derived, and thus do not recapitulate disease progression. Yet a subset of cancer types have been reprogrammed to pluripotency or near-pluripotency by blastocyst injection, by somatic cell nuclear transfer and by induced pluripotent stem cell (iPS) technology. The reprogrammed cancer cells show that pluripotency can transiently dominate over the cancer phenotype. Diverse studies show that reprogrammed cancer cells can, in some cases, exhibit early-stage phenotypes reflective of only partial expression of the cancer genome. In one case, reprogrammed human pancreatic cancer cells have been shown to recapitulate stages of cancer progression, from early to late stages, thus providing a model for studying pancreatic cancer development in human cells where previously such could only be discerned from mouse models. We discuss these findings, the challenges in developing such models and their current limitations, and ways that iPS reprogramming may be enhanced to develop human cell models of cancer progression.

Endoplasmic reticulum stress in amelogenesis imperfecta and phenotypic rescue using 4-phenylbutyrate

Inherited diseases caused by genetic mutations can arise due to loss of protein function. Alternatively, mutated proteins may mis-fold, impairing endoplasmic reticulum (ER) trafficking, causing ER stress and triggering the unfolded protein response (UPR). The UPR attempts to restore proteostasis but if unsuccessful drives affected cells towards apoptosis. Previously, we reported that in mice, the p.Tyr64His mutation in the enamel extracellular matrix (EEM) protein amelogenin disrupts the secretory pathway in the enamel-forming ameloblasts, resulting in eruption of malformed tooth enamel that phenocopies human amelogenesis imperfecta (AI). Defective amelogenin post-secretory self-assembly and processing within the developing EEM has been suggested to underlie the pathogenesis of X chromosome-linked AI. Here, we challenge this concept by showing that AI pathogenesis associated with the p.Tyr64His amelogenin mutation involves ameloblast apoptosis induced by ER stress. Furthermore, we show that 4-phenylbutyrate can rescue the enamel phenotype in affected female mice by promoting cell survival over apoptosis such that they are able to complete enamel formation despite the presence of the mutation, offering a potential therapeutic option for patients with this form of AI and emphasizing the importance of ER stress in the pathogenesis of this inherited conformational disease.

Congenital hypoplasia of the cerebellum: developmental causes and behavioral consequences

Over the last 60 years, the spotlight of research has periodically returned to the cerebellum as new techniques and insights have emerged. Because of its simple homogeneous structure, limited diversity of cell types and characteristic behavioral pathologies, the cerebellum is a natural home for studies of cell specification, patterning, and neuronal migration. However, recent evidence has extended the traditional range of perceived cerebellar function to include modulation of cognitive processes and implicated cerebellar hypoplasia and Purkinje neuron hypo-cellularity with autistic spectrum disorder. In the light of this emerging frontier, we review the key stages and genetic mechanisms behind cerebellum development. In particular, we discuss the role of the midbrain hindbrain isthmic organizer in the development of the cerebellar vermis and the specification and differentiation of Purkinje cells and granule neurons. These developmental processes are then considered in relation to recent insights into selected human developmental cerebellar defects: Joubert syndrome, Dandy–Walker malformation, and pontocerebellar hypoplasia. Finally, we review current research that opens up the possibility of using the mouse as a genetic model to study the role of the cerebellum in cognitive function.

Dynamic temporal requirement of Wnt1 in midbrain dopamine neuron development

Wnt1-expressing progenitors generate midbrain dopamine (MbDA) and cerebellum (Cb) neurons in distinct temporal windows and from spatially discrete progenitor domains. It has been shown that Wnt1 and Lmx1a participate in a cross-regulatory loop that is utilized during MbDA neuron development. However, Wnt1 expression dynamically changes over time and precedes that of Lmx1a. The spatial and temporal requirements of Wnt1 in development and specifically its requirement for MbDA neurons remain to be determined. To address these issues, we generated a conditional Wnt1 allele and temporally deleted Wnt1 coupled with genetic lineage analysis. Using this approach, we show that patterning of the midbrain (Mb) and Cb by Wnt1 occurs between the one-somite and the six- to eight-somite stages and is solely dependent on Wnt1 function in the Mb, but not in the Cb. Interestingly, an En1-derived domain persists after the early deletion of Wnt1 and mutant cells express OTX2. However, the En1-derived Wnt1-mutant domain does not contain LMX1a-expressing progenitors, and MbDA neurons are depleted. Thus, we demonstrate an early requirement of Wnt1 for all MbDA neurons. Subsequently, we deleted Wnt1 in the ventral Mb and show a continued late requirement for Wnt1 in MbDA neuron development, but not in LMX1a-expressing progenitors. Specifically, Wnt1 deletion disrupts the birthdating of MbDA neurons and causes a depletion of MbDA neurons positioned medially and a concomitant expansion of MbDA neurons positioned laterally during embryogenesis. Collectively, our analyses resolve the spatial and temporal function of Wnt1 in Mb and Cb patterning and in MbDA neuron development in vivo.

The adult mammalian pancreas contains separate precursors of pancreatic and neural crest developmental origins

The developmental origin of a pancreatic precursor cell could provide clues to properties that may be crucial to its molecular regulation and therapeutic potential. Previously, lineage tracing experiments showed that multipotent precursors in mouse islets had a pancreatic and not a neural crest developmental origin. However, a different Cre reporter system reveals that there is, in fact, a rare population of proliferative cells in the pancreas that is descended from the Wnt1 neural crest lineage, in addition to the majority population descended from the Pdx1 pancreatic lineage. These two proliferative cell populations are distinct in their gene expression and differentiation potential. This evidence suggests that there are at least two distinct types of precursors present in adult pancreatic islets, one of pancreatic origin and one of neural crest origin.

Midbrain dopaminergic neurons: a review of the molecular circuitry that regulates their development

Dopaminergic (DA) neurons of the ventral midbrain (VM) play vital roles in the regulation of voluntary movement, emotion and reward. They are divided into the A8, A9 and A10 subgroups. The development of the A9 group of DA neurons is an area of intense investigation to aid the generation of these neurons from stem cell sources for cell transplantation approaches to Parkinson's disease (PD). This review discusses the molecular processes that are involved in the identity, specification, maturation, target innervation and survival of VM DA neurons during development. The complex molecular interactions of a number of genetic pathways are outlined, as well as recent advances in the mechanisms that regulate subset identity within the VM DA neuronal pool. A thorough understanding of the cellular and molecular mechanisms involved in the development of VM DA neurons will greatly facilitate the use of cell replacement therapy for the treatment of PD.

Regulation of ephrin-A expression in compressed retinocollicular maps

Retinotopic maps can undergo compression and expansion in response to changes in target size, but the mechanism underlying this compensatory process has remained a mystery. The discovery of ephrins as molecular mediators of Sperry's chemoaffinity process allows a mechanistic approach to this important issue. In Syrian hamsters, neonatal, partial (PT) ablation of posterior superior colliculus (SC) leads to compression of the retinotopic map, independent of neural activity. Graded, repulsive EphA receptor/ephrin-A ligand interactions direct the formation of the retinocollicular map, but whether ephrins might also be involved in map compression is unknown. To examine whether map compression might be directed by changes in the ephrin expression pattern, we compared ephrin-A2 and ephrin-A5 mRNA expression between normal SC and PT SC using in situ hybridization and quantitative real-time PCR. We found that ephrin-A ligand expression in the compressed maps was low anteriorly and high posteriorly, as in normal animals. Consistent with our hypothesis, the steepness of the ephrin gradient increased in the lesioned colliculi. Interestingly, overall levels of ephrin-A2 and -A5 expression declined immediately after neonatal target damage, perhaps promoting axon outgrowth. These data establish a correlation between changes in ephrin-A gradients and map compression, and suggest that ephrin-A expression gradients may be regulated by target size. This in turn could lead to compression of the retinocollicular map onto the reduced target. These findings have important implications for mechanisms of recovery from traumatic brain injury.

Roles of Wnt8a during formation and patterning of the mouse inner ear

Fgf and Wnt signalling have been shown to be required for formation of the otic placode in vertebrates. Whereas several Fgfs including Fgf3, Fgf8 and Fgf10 have been shown to participate during early placode induction, Wnt signalling is required for specification and maintenance of the otic placode, and dorsal patterning of the otic vesicle. However, the requirement for specific members of the Wnt gene family for otic placode and vesicle formation and their potential interaction with Fgf signalling has been poorly defined. Due to its spatiotemporal expression during placode formation in the hindbrain Wnt8a has been postulated as a potential candidate for its specification. Here we have examined the role of Wnt8a during formation of the otic placode and vesicle in mouse embryos. Wnt8a expression depends on the presence of Fgf3 indicating a serial regulation between Fgf and Wnt signalling during otic placode induction and specification. Wnt8a by itself however is neither essential for placode specification nor redundantly required together with Fgfs for otic placode and vesicle formation. Interestingly however, Wnt8a and Fgf3 are redundantly required for expression of Fgf15 in the hindbrain indicating additional reciprocal interactions between Fgf and Wnt signalling. Further reduction of Wnt signalling by the inactivation of Wnt1 in a Wnt8a mutant background revealed a redundant requirement for both genes during morphogenesis of the dorsal portion of the otic vesicle.

Three decades of Wnts: a personal perspective on how a scientific field developed

Wnt genes and components of Wnt signalling pathways have been implicated in a wide spectrum of important biological phenomena, ranging from early organismal development to cell behaviours to several diseases, especially cancers. Emergence of the field of Wnt signalling can be largely traced back to the discovery of the first mammalian Wnt gene in 1982. In this essay, we mark the thirtieth anniversary of that discovery by describing some of the critical scientific developments that led to the flowering of this field of research.

Probing transcription-specific outputs of β-catenin in vivo

β-Catenin, apart from playing a cell-adhesive role, is a key nuclear effector of Wnt signaling. Based on activity assays in Drosophila, we generated mouse strains where the endogenous β-catenin protein is replaced by mutant forms, which retain the cell adhesion function but lack either or both of the N- and the C-terminal transcriptional outputs. The C-terminal activity is essential for mesoderm formation and proper gastrulation, whereas N-terminal outputs are required later during embryonic development. By combining the double-mutant β-catenin with a conditional null allele and a Wnt1-Cre driver, we probed the role of Wnt/β-catenin signaling in dorsal neural tube development. While loss of β-catenin protein in the neural tube results in severe cell adhesion defects, the morphology of cells and tissues expressing the double-mutant form is normal. Surprisingly, Wnt/β-catenin signaling activity only moderately regulates cell proliferation, but is crucial for maintaining neural progenitor identity and for neuronal differentiation in the dorsal spinal cord. Our model animals thus allow dissecting signaling and structural functions of β-catenin in vivo and provide the first genetic tool to generate cells and tissues that entirely and exclusively lack canonical Wnt pathway activity.

Wnt1 expression temporally allocates upper rhombic lip progenitors and defines their terminal cell fate in the cerebellum

The cerebellum (Cb) controls movement related physiology using a diverse array of morphologically and biochemically distinct neurons. During development, the Cb is derived from rhombomere 1 (r1), an embryonic compartment patterned by a signaling center referred to as the isthmus organizer. The secreted glycoprotein WNT1 is expressed in the midbrain primordia (mesencephalon, mes) and at the posterior limit of the mes. WNT1 plays a pivotal role in maintaining the isthmus organizer and mutations in Wnt1 produce severe Cb defects that are generally attributed to aberrant organizer activity. Interestingly, Wnt1 is also expressed at the most posterior limit of dorsal r1, in a region known as the upper rhombic lip (URL). However, the distribution and molecular identity of Wnt1 expressing progenitors have not been carefully described in r1. We used Wnt1-Venus transgenic mice to generate a molecular map of Wnt1 expressing progenitors in relation to other well characterized Cb biomarkers such as MATH1 (ATOH1), LMX1a and OTX2. Our analysis validated Wnt1 expression in the URL and revealed molecularly-defined developmental zones in r1. We then used genetic inducible fate mapping (GIFM) to link transient Wnt1 expression in r1 to terminal cell fates in the mature Cb. Wnt1 expressing progenitors primarily contributed to neurons in deep cerebellar nuclei, granule cells, and unipolar brush cells in distinct but overlapping temporal windows and sparsely contributed to inhibitory neurons and Bergmann glia. We further demonstrate that the Wnt1 lineage does not follow a competency model of progressive lineage restriction to generate the Cb or the functionally related precerebellar system. Instead, progenitors initiate Wnt1 expression de novo to give rise to each Cb cell type and precerebellar nuclei. We also used GIFM to determine how the temporal control of Wnt1 expression is related to molecular identity and cell migration in Cb development. Our findings provide new insight into how lineage and timing establish cell diversity within the Cb system.

Ectopic Wnt/beta-catenin signaling induces neurogenesis in the spinal cord and hindbrain floor plate

The most ventral structure of the developing neural tube, the floor plate (FP), differs in neurogenic capacity along the neuraxis. The FP is largely non-neurogenic at the hindbrain and spinal cord levels, but generates large numbers of dopamine (mDA) neurons at the midbrain levels. Wnt1, and other Wnts are expressed in the ventral midbrain, and Wnt/beta catenin signaling can at least in part account for the difference in neurogenic capacity of the FP between midbrain and hindbrain levels. To further develop the hypothesis that canonical Wnt signaling promotes mDA specification and FP neurogenesis, we have generated a model wherein beta–catenin is conditionally stabilized throughout the FP. Here, we unambiguously show by fate mapping FP cells in this mutant, that the hindbrain and spinal cord FP are rendered highly neurogenic, producing large numbers of neurons. We reveal that a neurogenic hindbrain FP results in the altered settling pattern of neighboring precerebellar neuronal clusters. Moreover, in this mutant, mDA progenitor markers are induced throughout the rostrocaudal axis of the hindbrain FP, although TH+ mDA neurons are produced only in the rostral aspect of rhombomere (r)1. This is, at least in part, due to depressed Lmx1b levels by Wnt/beta catenin signaling; indeed, when Lmx1b levels are restored in this mutant, mDA are observed not only in rostral r1, but also at more caudal axial levels in the hindbrain, but not in the spinal cord. Taken together, these data elucidate both patterning and neurogenic functions of Wnt/beta catenin signaling in the FP, and thereby add to our understanding of the molecular logic of mDA specification and neurogenesis.

Molecular organization and timing of Wnt1 expression define cohorts of midbrain dopamine neuron progenitors in vivo

Midbrain dopamine (MbDA) neurons are functionally heterogeneous and modulate complex functions through precisely organized anatomical groups. MbDA neurons are generated from Wnt1-expressing progenitors located in the ventral mesencephalon (vMes) during embryogenesis. However, it is unclear whether the progenitor pool is partitioned into distinct cohorts based on molecular identity and whether the timing of gene expression uniquely identifies subtypes of MbDA neurons. In this study we show that Wnt1-expressing MbDA progenitors from embryonic day (E)8.5-12.5 have dynamic molecular identities that correlate with specific spatial locations in the vMes. We also tested the hypothesis that the timing of Wnt1 expression in progenitors is related to the distribution of anatomically distinct cohorts of adult MbDA neurons using genetic inducible fate mapping (GIFM). We demonstrate that the Wnt1 lineage contributes to specific cohorts of MbDA neurons during a 7-day epoch and that the contribution to MbDA neurons predominates over other ventral Mb domains. In addition, we show that calbindin-, GIRK2-, and calretinin-expressing MbDA neuron subtypes are derived from Wnt1-expressing progenitors marked over a broad temporal window. Through GIFM and quantitative analysis we demonstrate that the Wnt1 lineage does not undergo progressive lineage restriction, which eliminates a restricted competence model of generating MbDA diversity. Interestingly, we uncover that two significant peaks of Wnt1 lineage contribution to MbDA neurons occur at E9.5 and E11.5. Collectively, our findings delineate the temporal window of MbDA neuron generation and show that lineage and timing predicts the terminal distribution pattern of MbDA neurons.

Dura mater stimulates human adipose-derived stromal cells to undergo bone formation in mouse calvarial defects

Human adipose-derived stromal cells (hASCs) have a proven capacity to aid in osseous repair of calvarial defects. However, the bone defect microenvironment necessary for osseous healing is not fully understood. In this study, we postulated that the cell-cell interaction between engrafted ASCs and host dura mater (DM) cells is critical for the healing of calvarial defects. hASCs were engrafted into critical sized calvarial mouse defects. The DM-hASC interaction was manipulated surgically by DM removal or by insertion of a semipermeable or nonpermeable membrane between DM and hASCs. Radiographic, histologic, and gene expression analyses were performed. Next, the hASC-DM interaction is assessed by conditioned media (CM) and coculture assays. Finally, bone morphogenetic protein (BMP) signaling from DM was investigated in vivo using novel BMP-2 and anti-BMP-2/4 slow releasing scaffolds. With intact DM, osseous healing occurs both from host DM and engrafted hASCs. Interference with the DM-hASC interaction dramatically reduced calvarial healing with abrogated BMP-2-Smad-1/5 signaling. Using CM and coculture assays, mouse DM cells stimulated hASC osteogenesis via BMP signaling. Through in vivo manipulation of the BMP-2 pathway, we found that BMP-2 plays an important role in DM stimulation of hASC osteogenesis in the context of calvarial bone healing. BMP-2 supplementation to a defect with disrupted DM allowed for bone formation in a nonhealing defect. DM is an osteogenic cell type that both participates in and stimulates osseous healing in a hASC-engrafted calvarial defect. Furthermore, DM-derived BMP-2 paracrine stimulation appears to play a key role for hASC mediated repair.

Wnt/β-catenin signalling is active in a highly dynamic pattern during development of the mouse cerebellum

The adult cerebellum is composed of several distinct cell types with well defined developmental origins. However, the molecular mechanisms that govern the generation of these cell types are only partially resolved. Wnt/β-catenin signalling has a wide variety of roles in generation of the central nervous system, though the specific activity of this pathway during cerebellum development is not well understood. Here, we present data that delineate the spatio-temporal specific pattern of Wnt/β-catenin signaling during mouse cerebellum development between E12.5 and P21. Using the BAT-gal Wnt/β-catenin reporter mouse, we found that Wnt/β-catenin activity is present transiently at the embryonic rhombic lip but not at later stages during the expansion of cell populations that arise from there. At late embryonic and early postnatal stages, Wnt/β-catenin activity shifts to the cerebellar ventricular zone and to cells arising from this germinal centre. Subsequently, the expression pattern becomes progressively restricted to Bergmann glial cells, which show expression of the reporter at P21. These results indicate a variety of potential functions for Wnt/β-catenin activity during cerebellum development.

En1 and Wnt signaling in midbrain dopaminergic neuronal development

Dopaminergic neurons of the ventral mesodiencephalon are affected in significant health disorders such as Parkinson's disease, schizophrenia, and addiction. The ultimate goal of current research endeavors is to improve the clinical treatment of such disorders, such as providing a protocol for cell replacement therapy in Parkinson's disease that will successfully promote the specific differentiation of a stem cell into a dopaminergic neuronal phenotype. Decades of research on the developmental mechanisms of the mesodiencephalic dopaminergic (mdDA) system have led to the identification of many signaling pathways and transcription factors critical in its development. The unraveling of these pathways will help fill in the pieces of the puzzle that today dominates neurodevelopment research: how to make and maintain a mdDA neuron. In the present review, we provide an overview of the mdDA system, the processes and signaling molecules involved in its genesis, with a focus on the transcription factor En1 and the canonical Wnt pathway, highlighting recent findings on their relevance--and interplay--in the development and maintenance of the mdDA system.

A cross-species analysis of Satb2 expression suggests deep conservation across vertebrate lineages

Mutation of SATB2 causes cleft palate in humans. To understand the role of SATB2 function in palatogenesis, SATB2 analyses in vertebrate model systems will be essential. To facilitate these analyses, we have performed a cross-species comparison of SATB2 structure and function across three vertebrate model systems: mouse, chick, and zebrafish. We find that the SATB2 transcript is highly conserved across human, mouse, chick, and zebrafish, especially within the Satb2 functional domains. Furthermore, our expression analyses demonstrate that SATB2 is likely to have similar functions in vertebrate model organisms and humans during development of the facial processes and secondary palate. Together, these data suggest an evolutionary conserved role for SATB2 during development of the face and palate across vertebrates. Moreover, expression of zebrafish satb2 in the anterior neurocranium supports the utility of the anterior neurocranium as a simplified model of amniote palatogenesis.

Widely expressed Af17 is likely not required for embryogenesis, hematopoiesis, and animal survival

As a putative transcription factor, Af17 may play a role in multiple signaling pathways. However, the Af17 expression profile during development and in adult tissues remains largely uncharacterized. The importance of Af17 function in embryogenesis, hematopoiesis, and animal survival has never been addressed before. Here we report the generation of the first Af17 mutant mouse model and characterization of the Af17 temporal and spatial expression profile in various embryonic stages and adult tissues by X-gal staining, in situ hybridization, and RT-PCR. Af17 expression is detected in specific cell populations in all stages and in multiple tissues examined. In situ hybridization yielded a consistent Af17 expression pattern by X-gal staining. Homozygous mutant mice are viable, fertile, normal in size, and do not display any gross physical, behavioral, or hematopoietic abnormalities. Thus, our studies describe the generation of the first Af17 mutant mouse model, provide the first developmental profile of Af17 expression, and reveal that Af17 may be dispensable for normal embryogenesis, hematopoiesis, and animal survival.

Lack of motor neuron differentiation is an intrinsic property of the mouse secondary neural tube

The cranial part of the amniote neural tube is formed by folding and fusion of the ectoderm-derived neural plate (primary neurulation). After posterior neuropore closure, however, the caudal neural tube is formed by cavitation of tail bud mesenchyme (secondary neurulation). In mouse embryos, the secondary neural tube expresses several genes important in early patterning and induction, in restricted domains similar to the primary neural tube, yet it does not undergo neuronal differentiation, but subsequently degenerates. Although the secondary neural tube, isolated from surrounding tissues, is responsive to exogenous Sonic Hedgehog proteins in vitro, motor neuron differentiation is never observed. This cannot be attributed to the properties of the secondary notochord, since it is able to induce motor neuron differentiation in naive chick neural plate explants. Taken together, these results support that the lack of motor neuron differentiation is an intrinsic property of the mouse secondary neural tube.

Stem cell potential in Parkinson's disease and molecular factors for the generation of dopamine neurons

Parkinson's disease (PD) involves the loss of dopamine (DA) neurons, making it the most expected neurodegenerative disease to be treated by cell replacement therapy. Stem cells are a promising source for cell replacement therapy due to their ability to self-renew and their pluripotency/multipotency that allows them to generate various types of cells. However, it is challenging to derive midbrain DA neurons from stem cells. Thus, in this review, I will discuss the molecular factors that are known to play critical roles in the generation and survival of DA neurons. The developmental process of DA neurons and functions of extrinsic soluble factors and homeodomain proteins, forkhead box proteins, proneural genes, Nurr1 and genes involved in epigenetic control are discussed. In addition, different types of stem cells that have potential for future cell replacement therapy are reviewed.

Fzd3 and Fzd6 deficiency results in a severe midbrain morphogenesis defect

Wnt/beta-catenin signaling controls the proper development of the mid-/hindbrain region (MHR) and of midbrain dopaminergic (mDA) neurons, but the Frizzled (Fzd) receptors transducing these signals are still unknown. Fzd3 is expressed throughout the mouse anterior neural tube, whereas Fzd6 is restricted to the MHR. We show that the MHR is properly established and mDA neurons develop normally in Fzd6(-/-) mutants, but the number of mDA neurons is initially reduced and recovers at later stages in Fzd3(-/-) embryos. Fzd3(-/-); Fzd6(-/-) double mutants exhibit a severe midbrain morphogenesis defect consisting of collapsed brain ventricles, apparent thickening of the neuroepithelium, focal disruption of the ventricular basal lamina and protrusion of individual cells, and increased proliferation at later stages, despite a normal closure of the anterior neural tube and the rescue of the mDA defect in these embryos. Fzd3 and Fzd6 thus control proper midbrain morphogenesis by a yet unknown mechanism in the mouse.

A mutation in the mouse Amelx tri-tyrosyl domain results in impaired secretion of amelogenin and phenocopies human X-linked amelogenesis imperfecta

Amelogenesis imperfecta (AI) describes a broad group of clinically and genetically heterogeneous inherited defects of dental enamel bio-mineralization. Despite identification of a number of genetic mutations underlying AI, the precise causal mechanisms have yet to be determined. Using a multi-disciplinary approach, we describe here a mis-sense mutation in the mouse Amelx gene resulting in a Y --> H substitution in the tri-tyrosyl domain of the enamel extracellular matrix protein amelogenin. The enamel in affected animals phenocopies human X-linked AI where similar mutations have been reported. Animals affected by the mutation have severe defects of enamel bio-mineralization associated with absence of full-length amelogenin protein in the developing enamel matrix, loss of ameloblast phenotype, increased ameloblast apoptosis and formation of multi-cellular masses. We present evidence to demonstrate that affected ameloblasts express but fail to secrete full-length amelogenin leading to engorgement of the endoplasmic reticulum/Golgi apparatus. Immunohistochemical analysis revealed accumulations of both amelogenin and ameloblastin in affected cells. Co-transfection of Ambn and mutant Amelx in a eukaryotic cell line also revealed intracellular abnormalities and increased cytotoxicity compared with cells singly transfected with wild-type Amelx, mutant Amelx or Ambn or co-transfected with both wild-type Amelx and Ambn. We hypothesize that intracellular protein-protein interactions mediated via the amelogenin tri-tyrosyl motif are a key mechanistic factor underpinning the molecular pathogenesis in this example of AI. This study therefore successfully links phenotype with underlying genetic lesion in a relevant murine model for human AI.

Complex and dynamic patterns of Wnt pathway gene expression in the developing chick forebrain

Background

Wnt signalling regulates multiple aspects of brain development in vertebrate embryos. A large number of Wnts are expressed in the embryonic forebrain; however, it is poorly understood which specific Wnt performs which function and how they interact. Wnts are able to activate different intracellular pathways, but which of these pathways become activated in different brain subdivisions also remains enigmatic.

Results

We have compiled the first comprehensive spatiotemporal atlas of Wnt pathway gene expression at critical stages of forebrain regionalisation in the chick embryo and found that most of these genes are expressed in strikingly dynamic and complex patterns. Several expression domains do not respect proposed compartment boundaries in the developing forebrain, suggesting that areal identities are more dynamic than previously thought. Using an in ovo electroporation approach, we show that Wnt4 expression in the thalamus is negatively regulated by Sonic hedgehog (Shh) signalling from the zona limitans intrathalamica (ZLI), a known organising centre of forebrain development.

Conclusion

The forebrain is exposed to a multitude of Wnts and Wnt inhibitors that are expressed in a highly dynamic and complex fashion, precluding simple correlative conclusions about their respective functions or signalling mechanisms. In various biological systems, Wnts are antagonised by Shh signalling. By demonstrating that Wnt4 expression in the thalamus is repressed by Shh from the ZLI we reveal an additional level of interaction between these two pathways and provide an example for the cross-regulation between patterning centres during forebrain regionalisation.

Comparative analysis of conditional reporter alleles in the developing embryo and embryonic nervous system

A long-standing problem in development is understanding how progenitor cells transiently expressing genes contribute to complex anatomical and functional structures. In the developing nervous system an additional level of complexity arises when considering how cells of distinct lineages relate to newly established neural circuits. To address these problems, we used both cumulative marking with Cre/loxP and Genetic Inducible Fate Mapping (GIFM), which permanently and heritably marks small populations of progenitors and their descendants with fine temporal control using CreER/loxP. A key component used in both approaches is a conditional phenotyping allele that has the potential to be expressed in all cell types, but is quiescent because of a loxP flanked Stop sequence, which precedes a reporter allele. Upon recombination, the resulting phenotyping allele is 'turned on' and then constitutively expressed. Thus, the reporter functions as a high fidelity genetic lineage tracer in vivo. Currently there is an array of reporter alleles that can be used in marking strategies, but their recombination efficiency and applicability to a wide array of tissues has not been thoroughly described. To assess the recombination/marking potential of the reporters, we utilized CreER(T) under the control of a Wnt1 transgene (Wnt1-CreER(T)) as well as a cumulative, non-inducible En1(Cre) knock-in line in combination with three different reporters: R26R (LacZ reporter), Z/EG (EGFP reporter), and Tau-Lox-STOP-Lox-mGFP-IRES-NLS-LacZ (membrane-targeted GFP/nuclear LacZ reporter). We marked the Wnt1 lineage using each of the three reporters at embryonic day (E) 8.5 followed by analysis at E10.0, E12.5, and in the adult. We also compared cumulative marking of cells with a history of En1 expression at the same stages. We evaluated the reporters by whole-mount and section analysis and ascertained the strengths and weaknesses of each of the reporters. Comparative analysis with the reporters elucidated complexities of how the Wnt1 and En1 lineages contribute to developing embryos and to axonal projection patterns of neurons derived from these lineages.

Six2 defines and regulates a multipotent self-renewing nephron progenitor population throughout mammalian kidney development

Nephrons, the basic functional units of the kidney, are generated repetitively during kidney organogenesis from a mesenchymal progenitor population. Which cells within this pool give rise to nephrons and how multiple nephron lineages form during this protracted developmental process are unclear. We demonstrate that the Six2-expressing cap mesenchyme represents a multipotent nephron progenitor population. Six2-expressing cells give rise to all cell types of the main body of the nephron during all stages of nephrogenesis. Pulse labeling of Six2-expressing nephron progenitors at the onset of kidney development suggests that the Six2-expressing population is maintained by self-renewal. Clonal analysis indicates that at least some Six2-expressing cells are multipotent, contributing to multiple domains of the nephron. Furthermore, Six2 functions cell autonomously to maintain a progenitor cell status, as cap mesenchyme cells lacking Six2 activity contribute to ectopic nephron tubules, a mechanism dependent on a Wnt9b inductive signal. Taken together, our observations suggest that Six2 activity cell-autonomously regulates a multipotent nephron progenitor population.

Bmp2 is required for cephalic neural tube closure in the mouse

BMPs have been shown to play a role in neural tube development particularly as dorsalizing factors. To explore the possibility that BMP2 could play a role in the developing neural tube (NT) beyond the lethality of Bmp2 null embryos, we created Bmp2 chimeras from Bmp2 null ES cells and WT blastocysts. Analysis of Bmp2 chimeras reveals NT defects at day 9.5 (E9.5). We found that exclusion of Bmp2 null ES cells from the dorsal NT did not always prevent defects. For further comparison, we used a Bmp2 mutant line in a mixed background. Phenotypes observed were similar to chimeras including open NT defects, postneurulation defects, and abnormal neural ectoderm in heterozygous and homozygous null embryos demonstrating a pattern of dose-dependent signaling. Our data exposes BMP2 as a unique player in the developing NT for dorsal patterning and identity, and normal cephalic neural tube closure in a dose-dependent manner.