Wnt signaling in development and disease

ABSTRACT: Cell signaling mediated by morphogens is essential to coordinate growth and patterning, two key processes that govern the formation of a complex multi-cellular organism. During growth and patterning, cells are specified by both quantitative and directional information. While quantitative information regulates cell proliferation and differentiation, directional information is conveyed in the form of cell polarities instructed by local and global cues. Major morphogens like Wnts play critical roles in embryonic development and they are also important in maintaining tissue homeostasis. Abnormal regulation of these signaling events leads to a diverse array of devastating diseases including cancer. Wnts transduce their signals through several distinct pathways and they regulate vertebrate embryonic development by providing both quantitative and directional information. Here, taking the developing skeletal system as an example, we review our work on Wnt signaling pathways in various aspects of development. We focus particularly on our most recent findings that showed that in vertebrates, Wnt5a acts as a global cue to establishing planar cell polarity (PCP). Our work suggests that Wnt morphogens regulate development by integrating quantitative and directional information. Our work also provides important insights in disease like Robinow syndrome, brachydactyly type B1 (BDB1) and spina bifida, which can be caused by human mutations in the Wnt/PCP signaling pathway.     

The protective effect of peroxiredoxin II on oxidative stress induced apoptosis in pancreatic β-cells

Cell signaling mediated by morphogens is essential to coordinate growth and patterning, two key processes that govern the formation of a complex multi-cellular organism. During growth and patterning, cells are specified by both quantitative and directional information. While quantitative information regulates cell proliferation and differentiation, directional information is conveyed in the form of cell polarities instructed by local and global cues. Major morphogens like Wnts play critical roles in embryonic development and they are also important in maintaining tissue homeostasis. Abnormal regulation of these signaling events leads to a diverse array of devastating diseases including cancer. Wnts transduce their signals through several distinct pathways and they regulate vertebrate embryonic development by providing both quantitative and directional information. Here, taking the developing skeletal system as an example, we review our work on Wnt signaling pathways in various aspects of development. We focus particularly on our most recent findings that showed that in vertebrates, Wnt5a acts as a global cue to establishing planar cell polarity (PCP). Our work suggests that Wnt morphogens regulate development by integrating quantitative and directional information. Our work also provides important insights in disease like Robinow syndrome, brachydactyly type B1 (BDB1) and spina bifida, which can be caused by human mutations in the Wnt/PCP signaling pathway.     

Multiple morphogens and rapid elongation promote segmental patterning during development

The vertebrate hindbrain is segmented into rhombomeres (r) initially defined by distinct domains of gene expression. Previous studies have shown that noise-induced gene regulation and cell sorting are critical for the sharpening of rhombomere boundaries, which start out rough in the forming neural plate (NP) and sharpen over time. However, the mechanisms controlling simultaneous formation of multiple rhombomeres and accuracy in their sizes are unclear. We have developed a stochastic multiscale cell-based model that explicitly incorporates dynamic morphogenetic changes (i.e. convergent-extension of the NP), multiple morphogens, and gene regulatory networks to investigate the formation of rhombomeres and their corresponding boundaries in the zebrafish hindbrain. During pattern initiation, the short-range signal, fibroblast growth factor (FGF), works together with the longer-range morphogen, retinoic acid (RA), to specify all of these boundaries and maintain accurately sized segments with sharp boundaries. At later stages of patterning, we show a nonlinear change in the shape of rhombomeres with rapid left-right narrowing of the NP followed by slower dynamics. Rapid initial convergence improves boundary sharpness and segment size by regulating cell sorting and cell fate both independently and coordinately. Overall, multiple morphogens and tissue dynamics synergize to regulate the sizes and boundaries of multiple segments during development.     

Mutation of pescadillo disrupts oligodendrocyte formation in zebrafish

Background

In vertebrates, the myelin sheath is essential for efficient propagation of action potentials along the axon shaft. Oligodendrocytes are the cells of the central nervous system that create myelin sheaths. During embryogenesis, ventral neural tube precursors give rise to oligodendrocyte progenitor cells, which divide and migrate throughout the central nervous system. This study aimed to investigate mechanisms that regulate oligodendrocyte progenitor cell formation.

Methodology/Principal Findings

By conducting a mutagenesis screen in transgenic zebrafish, we identified a mutation, designated vu166, by an apparent reduction in the number of oligodendrocyte progenitor cells in the dorsal spinal cord. We subsequently determined that vu166 is an allele of pescadillo, a gene known to play a role in ribosome biogenesis and cell proliferation. We found that pescadillo function is required for both the proper number of oligodendrocyte progenitors to form, by regulating cell cycle progression, and for normal levels of myelin gene expression.

Conclusions/Significance

Our data provide evidence that neural precursors require pes function to progress through the cell cycle and produce oligodendrocyte progenitor cells and for oligodendrocyte differentiation.     

Knockout mice reveal a contribution of the extracellular matrix molecule tenascin-C to neural precursor proliferation and migration

The extracellular matrix glycoprotein tenascin-C is widely expressed in the vertebrate central nervous system (CNS) during development and repair. Despite multiple effects of tenascin-C on cell behaviour in culture, no structural abnormalities of the CNS and other organs have been found in adult tenascin-C-null mice, raising the question of whether this glycoprotein has a significant role in vivo. Using a transgenic approach, we have demonstrated that tenascin-C regulates both cell proliferation and migration in oligodendrocyte precursors during development. Knockout mice show increased rates of oligodendrocyte precursor migration along the optic nerve and reduced rates of oligodendrocyte precursor proliferation in different regions of the CNS. Levels of programmed cell death were reduced in areas of myelination at later developmental stages, providing a potential corrective mechanism for any reduction in cell numbers that resulted from the proliferation phenotype. The effects on cell proliferation are mediated via the alphavbeta3 integrin and an interaction with the platelet-derived growth factor-stimulated mitogenic pathway, emphasising the importance of both CNS extracellular matrix and integrin growth factor interactions in the regulation of neural precursor behaviour.     

Early neural cell death: dying to become neurons

The importance of programmed cell death (PCD) during vertebrate development has been well established. During the development of the nervous system in particular, neurotrophic cell death in innervating neurons matches the number of neurons to the size of their target field. However, PCD also occurs during earlier stages of neural development, within populations of proliferating neural precursors and newly postmitotic neuroblasts, all of which are not yet fully differentiated. This review addresses early neural PCD, which is distinct from neurotrophic death in differentiated neurons. Although early neural PCD is observed in a range of organisms, from Caenorhabditis elegans to mouse, the role and the regulation of early neural PCD are not well understood. The regulation of early neural PCD can be inferred from the function of factors such as bone morphogenetic proteins (BMPs), Wnts, fibroblast growth factors (FGFs), and Sonic Hedgehog (Shh), which regulate both early neural development and PCD occurring in other developmental processes. Cell number control, removal of damaged or misspecified cells (spatially or temporally), and selection are the proposed roles early neural PCDs play during neural development. Data from developmental PCD in C. elegans and Drosophila provide insights into the possible signaling pathways integrating PCD with other processes during early neural development and the roles they might play.     

Cell death in the nervous system

Programmed cell death is an essential process for proper neural development. Cell death, with its similar regulatory and executory mechanisms, also contributes to the origin or progression of many or even all neurodegenerative diseases. An understanding of the mechanisms that regulate cell death during neural development may provide new targets and tools to prevent neurodegeneration. Many studies that have focused mainly on insulin-like growth factor-I (IGF-I), have shown that insulin-related growth factors are widely expressed in the developing and adult nervous system, and positively modulate a number of processes during neural development, as well as in adult neuronal and glial physiology. These factors also show neuroprotective effects following neural damage. Although some specific actions have been demonstrated to be anti-apoptotic, we propose that a broad neuroprotective role is the foundation for many of the observed functions of the insulin-related growth factors, whose therapeutical potential for nervous system disorders may be greater than currently accepted.     

Regulation of neurogenesis by growth factors and neurotransmitters

The generation of neurons and glia in the developing nervous system is likely to be regulated by extrinsic factors, including growth factors and neurotransmitters. Evidence from in vivo and/or in vitro systems indicates that basic fibroblast growth factor, transforming growth factor (TGF)-α, insulin-like growth factor-1, and the monoamine neurotransmitters act to increase proliferation of neural precursors. Conversely, glutamate, γ-aminobutyric acid, and opioid peptides are likely to play a role in down-regulating proliferation in the developing nervous system. Several other factors, including the neuropeptides vasoactive intestinal peptide and pituitary adenylate cyclase-activating peptide, as well as the growth factors platelet-derived growth factor, ciliary neurotrophic factor, and members of the TGF-β family, have different effects on proliferation and differentiation depending on the system examined. Expression of many of these factors and their receptors in germinal regions of the central nervous system suggests that they can act directly on precursor populations to control their proliferation. Together, the findings discussed here indicate that proliferation and cell fate determination in the developing brain are regulated extrinsically by complex interactions between a relatively large number of growth factors and neurotransmitters. © 1998 John Wiley & Sons, Inc. J Neurobiol 36: 287–306, 1998

1 This article is a US Government work and, as such, is in the public domain in the United States of America.

     

Ethanol disrupts cell cycle regulation in developing rat cortex interaction with transforming growth factor beta1

Ethanol is a potent teratogenic agent that disrupts several aspects of neuronogenesis, including the proliferation rate of cortical precursors. With regard to corticogenesis, possible targets of ethanol toxicity include soluble factors, like transforming growth factor beta1 (TGFbeta1), that regulate cortical growth and cell cycle proteins that control the kinetics of the cell cycle. The effect of ethanol on normal cell proliferation and TGFbeta1-regulated cell proliferation in the developing cortex was assessed using an organotypic slice culture model. Ethanol elongated the cell cycle, possibly through a decrease in the expression of G1 cell cycle protein cyclin D1. Further, ethanol exposure antagonized the anti-proliferative action of TGFbeta1 and blocked TGFbeta1-dependent increases in cell cycle inhibitor p21. Collectively, this evidence suggests that disruption of appropriate cell cycle protein expression and inhibition of TGFbeta1 activity are potential mechanisms underlying the effect of ethanol on cortical development.     

Wnt/BMP signal integration regulates the balance between proliferation and differentiation of neuroepithelial cells in the dorsal spinal cord

Multiple signaling pathways regulate proliferation and differentiation of neural progenitor cells during early development of the central nervous system (CNS). In the spinal cord, dorsal signaling by bone morphogenic protein (BMP) acts primarily as a patterning signal, while canonical Wnt signaling promotes cell cycle progression in stem and progenitor cells. However, overexpression of Wnt factors or, as shown here, stabilization of the Wnt signaling component beta-catenin has a more prominent effect in the ventral than in the dorsal spinal cord, revealing local differences in signal interpretation. Intriguingly, Wnt signaling is associated with BMP signal activation in the dorsal spinal cord. This points to a spatially restricted interaction between these pathways. Indeed, BMP counteracts proliferation promoted by Wnt in spinal cord neuroepithelial cells. Conversely, Wnt antagonizes BMP-dependent neuronal differentiation. Thus, a mutually inhibitory crosstalk between Wnt and BMP signaling controls the balance between proliferation and differentiation. A model emerges in which dorsal Wnt/BMP signal integration links growth and patterning, thereby maintaining undifferentiated and slow-cycling neural progenitors that form the dorsal confines of the developing spinal cord.     

Light regulates the cell cycle in zebrafish

The timing of cell proliferation is a key factor contributing to the regulation of normal growth. Daily rhythms of cell cycle progression have been documented in a wide range of organisms. However, little is known about how environmental, humoral, and cell-autonomous factors contribute to these rhythms. Here, we demonstrate that light plays a key role in cell cycle regulation in the zebrafish. Exposure of larvae to light-dark (LD) cycles causes a range of different cell types to enter S phase predominantly at the end of the day. When larvae are raised in constant darkness (DD), a low level of arrhythmic S phase is observed. In addition, light-entrained cell cycle rhythms persist for several days after transfer to DD, both observations pointing to the involvement of the circadian clock. We show that the number of LD cycles experienced is essential for establishing this rhythm during larval development. Furthermore, we reveal that the same phenomenon exists in a zebrafish cell line. This represents the first example of a vertebrate cell culture system where circadian rhythms of the cell cycle are observed. Thus, we implicate the cell-autonomous circadian clock in the regulation of the vertebrate cell cycle by light.     

Specific regulation of cyclins D1 and D2 by FGF and Shh signaling coordinates cell cycle progression, patterning, and differentiation during early steps of spinal cord development

In the vertebrate embryo, spinal cord elongation requires FGF signaling that promotes the continuous development of the posterior nervous system by maintaining a stem zone of proliferating neural progenitors. Those escaping the caudal neural stem zone, which is expressed to Shh signal, initiate ventral patterning in the neural groove before starting neuronal differentiation in the neural tube. Here we investigated the integration of D-type cyclins, known to govern cell cycle progression under the control of extracellular signals, in the program of spinal cord maturation. In chicken embryo, we find that cyclin D2 is preferentially expressed in the posterior neural plate, whereas cyclin D1 appears in the neural groove. We demonstrated by loss- and gain-of-function experiments that FGF signaling maintains cyclin D2 in the immature caudal neural epithelium, while Shh activates cyclin D1 in the neural groove. Moreover, forced maintenance of cyclin D1 or D2 in the neural tube favors proliferation at the expense of neuronal differentiation. These results contribute to our understanding of how the cell cycle control can be linked to the patterning programs to influence the balance between proliferation and neuronal differentiation in discrete progenitors domains.