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

Caudal Fgfr1 disruption produces localised spinal mis-patterning and a terminal myelocystocele-like phenotype in mice

Closed spinal dysraphisms are poorly understood malformations classified as neural tube (NT) defects. Several, including terminal myelocystocele, affect the distal spine. We have previously identified a NT closure-initiating point, Closure 5, in the distal spine of mice. Here, we document equivalent morphology of the caudal-most closing posterior neuropore (PNP) in mice and humans. Closure 5 forms in a region of active FGF signalling, and pharmacological FGF receptor blockade impairs its formation in cultured mouse embryos. Conditional genetic deletion of Fgfr1 in caudal embryonic tissues with Cdx2Cre diminishes neuroepithelial proliferation, impairs Closure 5 formation and delays PNP closure. After closure, the distal NT of Fgfr1-disrupted embryos dilates to form a fluid-filled sac overlying ventrally flattened spinal cord. This phenotype resembles terminal myelocystocele. Histological analysis reveals regional and progressive loss of SHH- and FOXA2-positive ventral NT domains, resulting in OLIG2 labelling of the ventral-most NT. The OLIG2 domain is also subsequently lost, eventually producing a NT that is entirely positive for the dorsal marker PAX3. Thus, a terminal myelocystocele-like phenotype can arise after completion of NT closure with localised spinal mis-patterning caused by disruption of FGFR1 signalling.

Spatiotemporal contribution of neuromesodermal progenitor-derived neural cells in the elongation of developing mouse spinal cord

AIMS

During vertebrate development, the posterior end of the embryo progressively elongates in a head-to-tail direction to form the body plan. Recent lineage tracing experiments revealed that bi-potent progenitors, called neuromesodermal progenitors (NMPs), produce caudal neural and mesodermal tissues during axial elongation. However, their precise location and contribution to spinal cord development remain elusive.

MAIN METHODS

Here we used NMP-specific markers (Sox2 and BraT) and a genetic lineage tracing system to localize NMP progeny in vivo.

KEY FINDINGS

Sox2 and BraT double positive cells were initially located at the tail tip, but were later found in the caudal neural tube, which is a unique feature of mouse development. In the neural tube, they produced neural progenitors (NPCs) and contributed to the spinal cord gradually along the AP axis during axial elongation. Interestingly, NMP-derived NPCs preferentially contributed to the ventral side first and later to the dorsal side at the lumbar spinal cord level, which may be associated with atypical junctional neurulation in mice.

SIGNIFICANCE

Our current observations detail the contribution of NMP progeny to spinal cord elongation and provide insights into how different species uniquely execute caudal morphogenesis.

Embryonal Neuromesodermal Progenitors for Caudal Central Nervous System and Tissue Development

Neuromesodermal progenitors (NMPs) constitute a bipotent cell population that generates a wide variety of trunk cell and tissue types during embryonic development. Derivatives of NMPs include both mesodermal lineage cells such as muscles and vertebral bones, and neural lineage cells such as neural crests and central nervous system neurons. Such diverse lineage potential combined with a limited capacity for self-renewal, which persists during axial elongation, demonstrates that NMPs are a major source of trunk tissues. This review describes the identification and characterization of NMPs across multiple species. We also discuss key cellular and molecular steps for generating neural and mesodermal cells for building up the elongating trunk tissue.

In Vivo Analysis of the Mesenchymal-to-Epithelial Transition During Chick Secondary Neurulation

The neural tube in amniotic embryos forms as a result of two consecutive events along the anteroposterior axis, referred to as primary and secondary neurulation (PN and SN). While PN involves the invagination of a sheet of epithelial cells, SN shapes the caudal neural tube through the mesenchymal-to-epithelial transition (MET) of neuromesodermal progenitors, followed by cavitation of the medullary cord. The technical difficulties in studying SN mainly involve the challenge of labeling and manipulating SN cells in vivo. Here we describe a new method to follow MET during SN in the chick embryo, combining early in ovo chick electroporation with in vivo time-lapse imaging. This procedure allows the cells undergoing SN to be manipulated in order to investigate the MET process, permitting their cell dynamics to be followed in vivo.

Lineage tracing of axial progenitors using Nkx1-2CreERT2 mice defines their trunk and tail contributions

The vertebrate body forms by continuous generation of new tissue from progenitors at the posterior end of the embryo. The study of these axial progenitors has proved to be challenging in vivo largely because of the lack of unique molecular markers to identify them. Here, we elucidate the expression pattern of the transcription factor Nkx1-2 in the mouse embryo and show that it identifies axial progenitors throughout body axis elongation, including neuromesodermal progenitors and early neural and mesodermal progenitors. We create a tamoxifen-inducible Nkx1-2CreER^T2 transgenic mouse and exploit the conditional nature of this line to uncover the lineage contributions of Nkx1-2-expressing cells at specific stages. We show that early Nkx1-2-expressing epiblast cells contribute to all three germ layers, mostly neuroectoderm and mesoderm, excluding notochord. Our data are consistent with the presence of some self-renewing axial progenitors that continue to generate neural and mesoderm tissues from the tail bud. This study identifies Nkx1-2-expressing cells as the source of most trunk and tail tissues in the mouse and provides a useful tool to genetically label and manipulate axial progenitors in vivo.

Summary: Changing lineage contributions of axial progenitors to the developing mouse embryo are revealed using a tamoxifen-inducible Cre line under the control of the endogenous Nkx1-2 promoter.

Melanocyte development in the mouse tail epidermis requires the Adamts9 metalloproteinase

The mouse tail has an important role in the study of melanogenesis, because mouse tail skin can be used to model human skin pigmentation. To better understand the development of melanocytes in the mouse tail, we cloned two dominant ENU-generated mutations of the Adamts9 gene, Und3 and Und4, which cause an unpigmented ring of epidermis in the middle of the tail, but do not alter pigmentation in the rest of the mouse. Adamts9 encodes a widely expressed zinc metalloprotease with thrombospondin type 1 repeats with few known substrates. Melanocytes are lost in the Adamts9 mutant tail epidermis at a relatively late stage of development, around E18.5. Studies of our Adamts9 conditional allele suggest that there is a melanocyte cell-autonomous requirement for Adamts9. In addition, we used a proteomics approach, TAILS N-terminomics, to identify new Adamts9 candidate substrates in the extracellular matrix of the skin. The tail phenotype of Adamts9 mutants is strikingly similar to the unpigmented trunk belt in Adamts20 mutants, which suggests a particular requirement for Adamts family activity at certain positions along the anterior-posterior axis.

Identification and characterization of secondary neural tube-derived embryonic neural stem cells in vitro

Secondary neurulation is an embryonic progress that gives rise to the secondary neural tube, the precursor of the lower spinal cord region. The secondary neural tube is derived from aggregated Sox2-expressing neural cells at the dorsal region of the tail bud, which eventually forms rosette or tube-like structures to give rise to neural tissues in the tail bud. We addressed whether the embryonic tail contains neural stem cells (NSCs), namely secondary NSCs (sNSCs), with the potential for self-renewal in vitro. Using in vitro neurosphere assays, neurospheres readily formed at the rosette and neural-tube levels, but less frequently at the tail bud tip level. Furthermore, we identified that sNSC-generated neurospheres were significantly smaller in size compared with cortical neurospheres. Interestingly, various cell cycle analyses revealed that this difference was not due to a reduction in the proliferation rate of NSCs, but rather the neuronal commitment of sNSCs, as sNSC-derived neurospheres contain more committed neuronal progenitor cells, even in the presence of epidermal growth factor (EGF) and basic fibroblast growth factor (bFGF). These results suggest that the higher tendency for sNSCs to spontaneously differentiate into progenitor cells may explain the limited expansion of the secondary neural tube during embryonic development.

Junctional neurulation: a unique developmental program shaping a discrete region of the spinal cord highly susceptible to neural tube defects

In higher vertebrates, the primordium of the nervous system, the neural tube, is shaped along the rostrocaudal axis through two consecutive, radically different processes referred to as primary and secondary neurulation. Failures in neurulation lead to severe anomalies of the nervous system, called neural tube defects (NTDs), which are among the most common congenital malformations in humans. Mechanisms causing NTDs in humans remain ill-defined. Of particular interest, the thoracolumbar region, which encompasses many NTD cases in the spine, corresponds to the junction between primary and secondary neurulations. Elucidating which developmental processes operate during neurulation in this region is therefore pivotal to unraveling the etiology of NTDs. Here, using the chick embryo as a model, we show that, at the junction, the neural tube is elaborated by a unique developmental program involving concerted movements of elevation and folding combined with local cell ingression and accretion. This process ensures the topological continuity between the primary and secondary neural tubes while supplying all neural progenitors of both the junctional and secondary neural tubes. Because it is distinct from the other neurulation events, we term this phenomenon junctional neurulation. Moreover, the planar-cell-polarity member, Prickle-1, is recruited specifically during junctional neurulation and its misexpression within a limited time period suffices to cause anomalies that phenocopy lower spine NTDs in human. Our study thus provides a molecular and cellular basis for understanding the causality of NTD prevalence in humans and ascribes to Prickle-1 a critical role in lower spinal cord formation.

Secondary neurulation of human embryos: morphological changes and the expression of neuronal antigens

PURPOSE

The morphological changes and expression patterns of neuronal antigens of human embryos, obtained from the therapeutic termination of pregnancy or from surgical procedures, were analyzed in order to characterize the secondary neurulation.

METHODS

A total of 21 human embryos from Carnegie stages 12 to 23 and two fetuses in early stages were studied. The markers used for immunohistochemical study were neural cell adhesion molecule (N-CAM), neuronal nuclear antigen (NeuN), neurofilament-associated protein (3A10), synaptophysin, and glial fibrillary acidic protein (GFAP).

RESULTS

The formation of the caudal neural tube to the tip of the caudal portion of the embryo was finished at stage 17. The postcloacal gut had completely disappeared at stage 18, and multiple cavities of the caudal neural tube were clearly visible. The caudal portion of the neural tube showed findings suggestive of involution at stage 19. The expression patterns of neuronal antigens were as follows: N-CAM and NeuN showed immunoreactivity at the germinal layer of the spinal cord at stages 17 and 18. Neurofilament-associated protein (3A10) showed persistent immunoreactivity at the caudal cell mass and notochord during the observation period, along with the spinal cord, and the positive reactions were mainly located at the dorsal white matter at stage 17. Synaptophysin showed a weak positive reaction at the caudal cell mass and notochord at stages 13 and 14, evident by staining observed at the spinal cord at stages 15 and 16. There was no definite positive reaction for GFAP.

CONCLUSIONS

These characteristic patterns might be helpful for the understanding of human congenital anomalies involving secondary neurulation processes.

β-catenin regulates Pax3 and Cdx2 for caudal neural tube closure and elongation

Non-canonical Wnt/planar cell polarity (PCP) signaling plays a primary role in the convergent extension that drives neural tube closure and body axis elongation. PCP signaling gene mutations cause severe neural tube defects (NTDs). However, the role of canonical Wnt/β-catenin signaling in neural tube closure and NTDs remains poorly understood. This study shows that conditional gene targeting of β-catenin in the dorsal neural folds of mouse embryos represses the expression of the homeobox-containing genes Pax3 and Cdx2 at the dorsal posterior neuropore (PNP), and subsequently diminishes the expression of the Wnt/β-catenin signaling target genes T, Tbx6 and Fgf8 at the tail bud, leading to spina bifida aperta, caudal axis bending and tail truncation. We demonstrate that Pax3 and Cdx2 are novel downstream targets of Wnt/β-catenin signaling. Transgenic activation of Pax3 cDNA can rescue the closure defect in the β-catenin mutants, suggesting that Pax3 is a key downstream effector of β-catenin signaling in the PNP closure process. Cdx2 is known to be crucial in posterior axis elongation and in neural tube closure. We found that Cdx2 expression is also repressed in the dorsal PNPs of Pax3-null embryos. However, the ectopically activated Pax3 in the β-catenin mutants cannot restore Cdx2 mRNA in the dorsal PNP, suggesting that the presence of both β-catenin and Pax3 is required for regional Cdx2 expression. Thus, β-catenin signaling is required for caudal neural tube closure and elongation, acting through the transcriptional regulation of key target genes in the PNP.

Distribution and characterization of progenitor cells within the human filum terminale

Background

Filum terminale (FT) is a structure that is intimately associated with conus medullaris, the most caudal part of the spinal cord. It is well documented that certain regions of the adult human central nervous system contains undifferentiated, progenitor cells or multipotent precursors. The primary objective of this study was to describe the distribution and progenitor features of this cell population in humans, and to confirm their ability to differentiate within the neuroectodermal lineage.

Methodology/Principal Findings

We demonstrate that neural stem/progenitor cells are present in FT obtained from patients treated for tethered cord. When human or rat FT-derived cells were cultured in defined medium, they proliferated and formed neurospheres in 13 out of 21 individuals. Cells expressing Sox2 and Musashi-1 were found to outline the central canal, and also to be distributed in islets throughout the whole FT. Following plating, the cells developed antigen profiles characteristic of astrocytes (GFAP) and neurons (β-III-tubulin). Addition of PDGF-BB directed the cells towards a neuronal fate. Moreover, the cells obtained from young donors shows higher capacity for proliferation and are easier to expand than cells derived from older donors.

Conclusion/Significance

The identification of bona fide neural progenitor cells in FT suggests a possible role for progenitor cells in this extension of conus medullaris and may provide an additional source of such cells for possible therapeutic purposes.

Filum terminale, human, progenitor cells, neuron, astrocytes, spinal cord.