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The MAPK/ERK pathway regulates a variety of physiological cellular functions, including cell proliferation and survival. It is abnormally activated in many types of human cancers in response to driver mutations in regulators of this pathway that trigger tumor initiation. The early steps of oncogenic progression downstream of ERK overactivation are poorly understood due to a lack of appropriate models. We show here that ERK1/2 overactivation in the trunk neural tube of the chicken embryo through expression of a constitutively active form of the upstream kinase MEK1 (MEK1ca), rapidly provokes a profound change in the transcriptional signature of developing spinal cord cells. These changes are concordant with a previously established role of the tyrosine kinase receptor ligand FGF8 acting via the ERK1/2 effectors to maintain an undifferentiated state. Furthermore, we show that MEK1ca-transfected spinal cord cells lose neuronal identity, retain caudal markers, and ectopically express potential effector oncogenes, such as AQP1. MEK1ca expression in the developing spinal cord from the chicken embryo is thus a tractable in vivo model to identify the mechanisms fostering neoplasia and malignancy in ERK-induced tumorigenesis of neural origins.

The MAPK/ERK pathway regulates a variety of physiological cellular functions, including cell proliferation and survival. It is abnormally activated in many types of human cancers in response to driver mutations in regulators of this pathway that trigger tumor initiation. The early steps of oncogenic progression downstream of ERK overactivation are poorly understood due to a lack of appropriate models. We show here that ERK1/2 overactivation in the trunk neural tube of the chicken embryo through expression of a constitutively active form of the upstream kinase MEK1 (MEK1ca), rapidly provokes a profound change in the transcriptional signature of developing spinal cord cells. These changes are concordant with a previously established role of the tyrosine kinase receptor ligand FGF8 acting via the ERK1/2 effectors to maintain an undifferentiated state. Furthermore, we show that MEK1ca-transfected spinal cord cells lose neuronal identity, retain caudal markers, and ectopically express potential effector oncogenes, such as AQP1. MEK1ca expression in the developing spinal cord from the chicken embryo is thus a tractable in vivo model to identify the mechanisms fostering neoplasia and malignancy in ERK-induced tumorigenesis of neural origins. [ABSTRACT FROM AUTHOR]

Background: Within the developing central nervous system, the ability of cells to migrate throughout the tissue parenchyma to reach their target destination and undergo terminal differentiation is vital to normal central nervous system (CNS) development. To develop novel therapies to treat the injured CNS, it is essential that the migratory behavior of cell populations is understood. Many studies have examined the ability of individual neurons to migrate through the developing CNS, describing specific modes of migration including locomotion and somal translocation. Few studies have investigated the mass migration of large populations of neural progenitors, particularly in the developing the spinal cord. Here, we describe a method to robustly analyze large numbers of migrating cells using a co-culture assay. Results: The ex vivo tissue model promotes the survival and differentiation of co-cultured progenitor cells. Using this assay, we demonstrate that migrating neuroepithelial progenitor cells display region specific migration patterns within the dorsal and ventral spinal cord at defined developmental time points. Conclusions: The technique described here is a viable ex vivo model to quantitatively analyze cell migration and differentiation. We demonstrate the ability to detect changes in cell migration within distinct tissue region across tissue samples using the technique described here. Developmental Dynamics 247:201-211, 2018. © 2017 Wiley Periodicals, Inc. [ABSTRACT FROM AUTHOR]

Dorsal spinal neurogenesis is orchestrated by the combined action of signals secreted from the roof plate organizer and a downstream transcriptional cascade. Within this cascade, Msx1 and Msx2, two homeodomain transcription factors (TFs), are induced earlier than bHLH neuralizing TFs. Whereas bHLH TFs have been shown to specify neuronal cell fate, the function of Msx genes remains poorly defined. We describe dramatic alterations of neuronal patterning in Msx1/Msx2 double-mutant mouse embryos. The most dorsal spinal progenitor pool fails to express the bHLH neuralizing TF Atoh1, which results in a lack of Lhx2-positive and Barhl2-positive dI1 interneurons. Neurog1 and Ascl1 expression territories are dorsalized, leading to ectopic dorsal differentiation of dI2 and dI3 interneurons. In proportion, the amount of Neurog1-expressing progenitors appears unaffected, whereas the number of Ascl1-positive cells is increased. These defects occur while BMP signaling is still active in the Msx1/Msx2 mutant embryos. Cell lineage analysis and co-immunolabeling demonstrate that Atoh1-positive cells derive from progenitors expressing both Msx1 and Msx2. In vitro, Msx1 and Msx2 proteins activate Atoh1 transcription by specifically interacting with several homeodomain binding sites in the Atoh1 3' enhancer. In vivo, Msx1 and Msx2 are required for Atoh1 3' enhancer activity and ChIP experiments confirm Msx1 binding to this regulatory sequence. These data support a novel function of Msx1 and Msx2 as transcriptional activators. Our study provides new insights into the transcriptional control of spinal cord patterning by BMP signaling, with Msx1 and Msx2 acting upstream of Atoh1. [ABSTRACT FROM AUTHOR]

Numerous studies have identified the roof plate as an important signaling center controlling dorsal interneuron specification and differentiation in the developing spinal cord. Currently, the molecular pathways of roof plate formation and function are poorly understood. We determined that the LIM-homeodomain transcription factor Lmx1b is sufficient to induce functional roof plate in the early chick developing spinal cord. In the chick, Lmx1b acts upstream of Lmx1a in the roof plate developmental program. Once the roof plate forms, we show that Bmp and Wnt signaling are the major components of Lmx1a/b-dependent roof plate dorsal patterning activity. The roof plate function of Lmx1b is not conserved across vertebrates because Lmx1b is not expressed in mouse roof plate progenitors. Instead, mouse caudal CNS roof plate formation relies entirely on Lmx1a. Lmx1b can, however, partially rescue roof plate development in dreher (Lmx1a-/-) mice, indicating that Lmx1b has some functional redundancy to Lmx1a. Furthermore, we demonstrate that the roof plate-inducing activity of Lmx1b can be suppressed by Mash1 (Cash1), which is normally expressed in intermediate neural tube in both chick and mouse. Our data identify Lmx1b as a key regulator of spinal cord roof plate induction and function. [ABSTRACT FROM AUTHOR]

Numerous studies have identified the roof plate as an embryonic signaling center critical for dorsal central nervous system patterning, but little is known about mechanisms that control its formation and its separation from clonally related neural crest cells and dI1 sensory interneurons. We demonstrate that the LIM homeodomain transcription factor, Lmx1a, mutated in the dreher mouse, acts to withdraw dorsal spinal cord progenitors from the cell cycle and simultaneously direct their differentiation into functional roof plate cells. Lmx1a cell-autonomously represses the dI1 progenitor fate, distinguishing the roof plate and dI1 interneuron programs, two major developmental programs of the dorsal neural tube. Lmx1a is not directly involved in neural crest development. We establish that Bmp signaling from epidermal ectoderm is necessary and sufficient for inducing Lmx1a and other cofactors that also regulate the extent of roof plate induction. We conclude that Lmx1a controls multiple aspects of dorsal midline patterning and is a major mediator of early Bmp signaling in the developing spinal cord. [ABSTRACT FROM AUTHOR]

HOX transcription factors are members of an evolutionarily conserved family of proteins required for the establishment of the anteroposterior body axis during bilaterian development. Although they are often deregulated in cancers, the molecular mechanisms by which they act as oncogenes or tumor suppressor genes are only partially understood. Since the MAPK/ERK signaling pathway is deregulated in most cancers, we aimed at apprehending if and how the Hox proteins interact with ERK oncogenicity. Using an in vivo neoplasia model in the chicken embryo consisting in the overactivation of the ERK1/2 kinases in the trunk neural tube, we analyzed the consequences of the HOXB8 gain of function at the morphological and transcriptional levels. We found that HOXB8 acts as a tumor suppressor, counteracting ERK-induced neoplasia. The HOXB8 tumor suppressor function relies on a large reversion of the oncogenic transcriptome induced by ERK. In addition to showing that the HOXB8 protein controls the transcriptional responsiveness to ERK oncogenic signaling, our study identified new downstream targets of ERK oncogenic activation in an in vivo context that could provide clues for therapeutic strategies. [ABSTRACT FROM AUTHOR]

We examined the histological findings and cytoarchitectonic alterations in the rat spinal cord following matrix cell degeneration caused at different developmental stages, from neural plate formation through neuroblast generation. Ethylnitrosourea (ENU) 20 mg/kg body weight was administered transplacentally to the fetuses on the 10th embryonic day (E10) to 14th. The observations were made until the 21st postnatal day. Normally, mitoses were present scatteredly in the matrix cell layer of the neural plate or neural tube on El0 or E11, and gradually restricted to the dorsal portion of the alar plate as development occurred. The localization and number of degenerative cells as well as the site and degree of neuronal decrease in the completed dysgenetic spinal cord seemed to correlate with the topography and frequency of the mitoses in the matrix cell layer at the time of ENU administration. Disorder in the pattern of cytoarchitecture of neurons was not observed. The degree of hypoplasia of the white matter was proportional to the intensity of decrease of the spinal neurons. Aberrant myelinated fibers were not seen. No reactive gliosis, fibrosis or abnormal vascularization was observed at any time. [ABSTRACT FROM AUTHOR]