Inactivation of Hedgehog signal transduction in adult astrocytes results in region-specific blood-brain barrier defects.

In this study, we use molecular genetic approaches to clarify the role of the Hedgehog (Hh) pathway in regulating the blood-brain/spinal cord barrier (BBB) in the adult mouse central nervous system (CNS). Our work confirms and extends prior studies to demonstrate that astrocytes are the predominant cell type in the adult CNS that transduce Hh signaling, revealed by the expression of , a target gene of the canonical pathway that is activated in cells receiving Hh, and other key pathway transduction components. Gli1+ (Hh-responsive) astrocytes are distributed in specific regions of the CNS parenchyma, including layers 4/5/6 of the neocortex, hypothalamus, thalamus, and spinal cord, among others.

A Shh/Gli-driven three-node timer motif controls temporal identity and fate of neural stem cells.

How time is measured by neural stem cells during temporal neurogenesis has remained unresolved. By combining experiments and computational modeling, we define a Shh/Gli-driven three-node timer underlying the sequential generation of motor neurons (MNs) and serotonergic neurons in the brainstem. The timer is founded on temporal decline of Gli-activator and Gli-repressor activities established through down-regulation of Gli transcription.

Signals from the brain and olfactory epithelium control shaping of the mammalian nasal capsule cartilage.

Facial shape is the basis for facial recognition and categorization. Facial features reflect the underlying geometry of the skeletal structures. Here, we reveal that cartilaginous nasal capsule (corresponding to upper jaw and face) is shaped by signals generated by neural structures: brain and olfactory epithelium.

Atrophin protein RERE positively regulates Notch targets in the developing vertebrate spinal cord.

The Notch signaling pathway controls cell fate decision, proliferation, and other biological functions in both vertebrates and invertebrates. Precise regulation of the canonical Notch pathway ensures robustness of the signal throughout development and adult tissue homeostasis. Aberrant Notch signaling results in profound developmental defects and is linked to many human diseases.

Tcf7l2/Tcf4 Transcriptional Repressor Function Requires HDAC Activity in the Developing Vertebrate CNS.

The generation of functionally distinct neuronal subtypes within the vertebrate central nervous system (CNS) requires the precise regulation of progenitor gene expression in specific neuronal territories during early embryogenesis. Accumulating evidence has implicated histone deacetylase (HDAC) proteins in cell specification, proliferation, and differentiation in diverse embryonic and adult tissues. However, although HDAC proteins have shown to be expressed in the developing vertebrate neural tube, their specific role in CNS neural progenitor fate specification remains unclear.

Mammalian Nkx2.2+ perineurial glia are essential for motor nerve development.

Background: All vertebrate peripheral nerves connect the central nervous system (CNS) with targets in the periphery and are composed of axons, layers of ensheathing glia and connective tissue. Although the structure of these conduits is well established, very little is known about the origin and developmental roles of some of their elements. One understudied component, the perineurium, ensheaths nerve fascicles and is a component of the blood-nerve-barrier.

The PCP protein Vangl2 regulates migration of hindbrain motor neurons by acting in floor plate cells, and independently of cilia function.

Vangl2, a core component of the Planar Cell Polarity pathway, is necessary for the caudal migration of Facial Branchiomotor (FBM) neurons in the vertebrate hindbrain. Studies in zebrafish suggest that vangl2 functions largely non-cell autonomously to regulate FBM neuron migration out of rhombomere 4 (r4), but the cell-type within which it acts is not known. Here, we demonstrate that vangl2 functions largely in floor plate cells to regulate caudal neuronal migration.

Molecular genetic control of cell patterning and fate determination in the developing ventral spinal cord.

The generation of neuronal diversity in the ventral spinal cord during development is a multistep process that occurs with precise and reproducible spatiotemporal order. The proper functioning of the central nervous system requires that this be carried out with extraordinary precision from the outset. Extrinsic influences such as the secreted Sonic hedgehog (SHH) protein provide positional cues that are read out genetically as specific patterns of gene expression in subsets of dividing progenitors, which is the first overt indication that they have begun to embark upon cell-type-specific differentiation programs.

In vivo dual luciferase reporter assay with chick neural tube in ovo electroporation system.

Luciferase reporter systems are widely employed to provide a quantitative readout of gene expression for studies of transcriptional regulation, translation efficiency, and cell signaling. The most common application of luciferase involves transient transfections into cells in vitro or in vivo. In both cases, the normal variability inherent in transfection approaches can introduce significant errors into the data that makes comparison between separate experiments problematic.

Immunofluorescence staining with frozen mouse or chick embryonic tissue sections.

Immunofluorescence (IF), a form of immunohistochemistry (IHC) with specific applications, is commonly used for both basic research and clinical studies, including diagnostics, and involves visualizing the cellular distribution of target molecules (e.g., proteins, DNA, and small molecules) using a microscope capable of exciting and detecting fluorochrome compounds that emit light at specific, largely nonoverlapping wavelengths.

In ovo electroporation in embryonic chick spinal cords.

The developing spinal cord is a well-established model system widely used to study the signaling pathways and genetic programs that control neuronal/glial differentiation and neural circuit assembly. This is largely due to the relatively simple organization (compared to other CNS regions) and experimental accessibility of the neural tube, particularly in the chick embryo. In vivo transfection of cells within the developing chick neural tube using in ovo electroporation has emerged as a rapid and powerful experimental technique in that (1) transfected factors can be functionally tested in a spatially and temporally controlled manner and (2) the chick embryo provides a physiologically relevant in vivo environment to conduct biochemical studies such as dual-channel luciferase assay, co-immunoprecipitation (co-IP), and Chromatin Immunoprecipitation (ChIP).

Floor plate-derived sonic hedgehog regulates glial and ependymal cell fates in the developing spinal cord.

Cell fate specification in the CNS is controlled by the secreted morphogen sonic hedgehog (Shh). At spinal cord levels, Shh produced by both the notochord and floor plate (FP) diffuses dorsally to organize patterned gene expression in dividing neural and glial progenitors. Despite the fact that two discrete sources of Shh are involved in this process, the individual contribution of the FP, the only intrinsic source of Shh throughout both neurogenesis and gliogenesis, has not been clearly defined.

Sonic hedgehog signaling in the developing CNS where it has been and where it is going.

Sonic Hedgehog (Shh) is one of three mammalian orthologs of the Hedgehog (Hh) family of secreted proteins first identified for their role in patterning the Drosophila embryo. In this review, we will highlight some of the outstanding questions regarding how Shh signaling controls embryonic development. We will mainly consider its role in the developing mammalian central nervous system (CNS) where the pathway plays a critical role in orchestrating the specification of distinct cell fates within ventral regions, a process of exquisite complexity that is necessary for the proper wiring and hence function of the mature system.

Tcf/Lef repressors differentially regulate Shh-Gli target gene activation thresholds to generate progenitor patterning in the developing CNS.

During neural tube development, Shh signaling through Gli transcription factors is necessary to establish five distinct ventral progenitor domains that give rise to unique classes of neurons and glia that arise in specific positions along the dorsoventral axis. These cells are generated from progenitors that display distinct transcription factor gene expression profiles in specific domains in the ventricular zone. However, the molecular genetic mechanisms that control the differential spatiotemporal transcriptional responses of progenitor target genes to graded Shh-Gli signaling remain unclear.

Spatial and temporal requirements for sonic hedgehog in the regulation of thalamic interneuron identity.

In caudal regions of the diencephalon, sonic hedgehog (Shh) is expressed in the ventral midline of prosomeres 1-3 (p1-p3), which underlie the pretectum, thalamus and prethalamus, respectively. Shh is also expressed in the zona limitans intrathalamica (zli), a dorsally projecting spike that forms at the p2-p3 boundary. The presence of two Shh signaling centers in the thalamus has made it difficult to determine the specific roles of either one in regional patterning and neuronal fate specification.

A homeodomain feedback circuit underlies step-function interpretation of a Shh morphogen gradient during ventral neural patterning.

The deployment of morphogen gradients is a core strategy to establish cell diversity in developing tissues, but little is known about how small differences in the concentration of extracellular signals are translated into robust patterning output in responding cells. We have examined the activity of homeodomain proteins, which are presumed to operate downstream of graded Shh signaling in neural patterning, and describe a feedback circuit between the Shh pathway and homeodomain transcription factors that establishes non-graded regulation of Shh signaling activity. Nkx2 proteins intrinsically strengthen Shh responses in a feed-forward amplification and are required for ventral floor plate and p3 progenitor fates.

A critical role for sFRP proteins in maintaining caudal neural tube closure in mice via inhibition of BMP signaling.

Both the BMP and Wnt pathways have been implicated in directing aspects of dorsal neural tube closure and cell fate specification. However, the mechanisms that control the diverse responses to these signals are poorly understood. In this study, we provide genetic and functional evidence that the secreted sFRP1 and sFRP2 proteins, which have been primarily implicated as negative regulators of Wnt signaling, can also antagonize BMP signaling in the caudal neural tube and that this function is critical to maintain proper neural tube closure and dorsal cell fate segregation.

Prox1 regulates a transitory state for interneuron neurogenesis in the spinal cord.

Proper central nervous system (CNS) function depends critically on the generation of functionally distinct neuronal types in specific and reproducible positions. The generation of neuronal diversity during CNS development involves a fine balance between dividing neural progenitors and the differentiated neuronal progeny that they produce. However, the molecular mechanisms that regulate these processes are still poorly understood.

Order in the classroom: graded responses to instructive Hh signaling in the CNS.

In many animals, the secreted Hedgehog (Hh) signaling proteins play important roles during development and in adults. Studies in both flies and vertebrates indicate that Hh functions as a morphogen to elicit different responses at distinct concentration thresholds. In vertebrates, Gli proteins are the primary transcriptional mediators of Hh target genes.

A cell-autonomous requirement for Cip/Kip cyclin-kinase inhibitors in regulating neuronal cell cycle exit but not differentiation in the developing spinal cord.

Control over cell cycle exit is fundamental to the normal generation of the wide array of distinct cell types that comprise the mature vertebrate CNS. Here, we demonstrate a critical role for Cip/Kip class cyclin-kinase inhibitory (CKI) proteins in regulating this process during neurogenesis in the embryonic spinal cord. Using immunohistochemistry, we show that all three identified Cip/Kip CKI proteins are expressed in both distinct and overlapping populations of nascent and post-mitotic neurons during early neurogenesis, with p27(Kip1) having the broadest expression, and both p57(Kip2) and p21(Cip1) showing transient expression in restricted populations.

Wnt signaling inhibitors regulate the transcriptional response to morphogenetic Shh-Gli signaling in the neural tube.

Shh-Gli signaling controls cell fates in the developing ventral neural tube by regulating the patterned expression of transcription factors in neural progenitors. However, the molecular mechanisms that limit target gene responses to specific domains are unclear. Here, we show that Wnt pathway inhibitors regulate the threshold response of a ventral Shh target gene, Nkx2.

The role of floor plate contact in the elaboration of contralateral commissural projections within the embryonic mouse spinal cord.

In vertebrate embryos, commissural axons extend toward and across the floor plate (FP), an intermediate target at the ventral midline (VM) of the spinal cord. After decussating, many commissural axons turn into the longitudinal plane and elaborate diverse projections. FP contact is thought to alter the responsiveness of these axons so that they can exit the FP and adopt new trajectories.

Molecular control of spinal accessory motor neuron/axon development in the mouse spinal cord.

Within the developing vertebrate spinal cord, motor neuron subtypes are distinguished by the settling positions of their cell bodies, patterns of gene expression, and the paths their axons follow to exit the CNS. The inclusive set of cues required to guide a given motor axon subtype from cell body to target has yet to be identified, in any species. This is attributable, in part, to the unavailability of markers that demarcate the complete trajectory followed by a specific class of spinal motor axons.

Foxn4 acts synergistically with Mash1 to specify subtype identity of V2 interneurons in the spinal cord.

Neuronal subtype diversification is essential for the establishment of functional neural circuits, and yet the molecular events underlying neuronal diversity remain largely to be defined. During spinal neurogenesis, the p2 progenitor domain, unlike others in the ventral spinal cord, gives rise to two intermingled but molecularly distinct subtypes of interneurons, termed V2a and V2b. We show here that the Foxn4 winged helix/forkhead transcription factor is coexpressed with the bHLH factor Mash1 in a subset of p2 progenitors.

Transduction of graded Hedgehog signaling by a combination of Gli2 and Gli3 activator functions in the developing spinal cord.

The three vertebrate Gli proteins play a central role in mediating Hedgehog (Hh)-dependent cell fate specification in the developing spinal cord; however, their individual contributions to this process have not been fully characterized. In this paper, we have addressed this issue by examining patterning in the spinal cord of Gli2;Gli3 double mutant embryos, and in chick embryos transfected with dominant activator forms of Gli2 and Gli3. In double homozygotes, Gli1 is also not expressed; thus, all Gli protein activities are absent in these mice.