Notch signaling during cell fate determination in the inner ear

In the inner ear, Notch signaling has been proposed to specify the sensory regions, as well as regulate the differentiation of hair cells and supporting cell within those regions. In addition, Notch plays an important role in otic neurogenesis, by determining which cells differentiate as neurons, sensory cells and non-sensory cells. Here, I review the evidence for the complex and myriad roles Notch participates in during inner ear development. A particular challenge for those studying ear development and Notch is to decipher how activation of a single pathway can lead to different outcomes within the ear, which may include changes in the intrinsic properties of the cell, Notch modulation, and potential non-canonical pathways.     

Notch signaling respecifies the hemangioblast to a cardiac fate

To efficiently generate cardiomyocytes from embryonic stem (ES) cells in culture it is essential to identify key regulators of the cardiac lineage and to develop methods to control them. Using a tet-inducible mouse ES cell line to enforce expression of a constitutively activated form of the Notch 4 receptor, we show that signaling through the Notch pathway can efficiently respecify hemangioblasts to a cardiac fate, resulting in the generation of populations consisting of >60% cardiomyocytes. Microarray analyses reveal that this respecification is mediated in part through the coordinated regulation of the BMP and Wnt pathways by Notch signaling. Together, these findings have uncovered a potential role for the Notch pathway in cardiac development and provide an approach for generating large numbers of cardiac progenitors from ES cells.     

Neural fate decisions mediated by trans-activation and cis-inhibition in Notch signaling

MOTIVATION: In the developing nervous system, the expression of proneural genes, i.e. Hes1, Neurogenin-2 (Ngn2) and Deltalike-1 (Dll1), oscillates in neural progenitors with a period of 2-3 h, but is persistent in post-mitotic neurons. Unlike the synchronization of segmentation clocks, oscillations in neural progenitors are asynchronous between cells. It is known that Notch signaling, in which Notch in a cell can be activated by Dll1 in neighboring cells (trans-activation) and can also be inhibited by Dll1 within the same cell (cis-inhibition), is important for neural fate decisions. There have been extensive studies of trans-activation, but the operating mechanisms and potential implications of cis-inhibition are less clear and need to be further investigated. RESULTS: In this article, we present a computational model for neural fate decisions based on intertwined dynamics with trans-activation and cis-inhibition involving the Hes1, Notch and Dll1 proteins. In agreement with experimental observations, the model predicts that both trans-activation and cis-inhibition play critical roles in regulating the choice between remaining as a progenitor and embarking on neural differentiation. In particular, trans-activation is essential for generation of oscillations in neural progenitors, and cis-inhibition is important for the asynchrony between adjacent cells, indicating that the asynchronous oscillations in neural progenitors depend on cooperation between trans-activation and cis-inhibition. In contrast, cis-inhibition plays more critical roles in embarking on neural differentiation by inactivating intercellular Notch signaling. The model presented here might be a good candidate for providing the first qualitative mechanism of neural fate decisions mediated by both trans-activation and cis-inhibition.     

Radial glial identity is promoted by Notch1 signaling in the murine forebrain

In vertebrates, Notch signaling is generally thought to inhibit neural differentiation. However, whether Notch can also promote specific early cell fates in this context is unknown. We introduced activated Notch1 (NIC) into the mouse forebrain, before the onset of neurogenesis, using a retroviral vector and ultrasound imaging. During embryogenesis, NIC-infected cells became radial glia, the first specialized cell type evident in the forebrain. Thus, rather than simply inhibiting differentiation, Notch1 signaling promoted the acquisition of an early cellular phenotype. Postnatally, many NIC-infected cells became periventricular astrocytes, cells previously shown to be neural stem cells in the adult. These results suggest that Notch1 promotes radial glial identity during embryogenesis, and that radial glia may be lineally related to stem cells in the adult nervous system.     

Notch signaling: control of cell communication and cell fate

Notch is a transmembrane receptor that mediates local cell-cell communication and coordinates a signaling cascade present in all animal species studied to date. Notch signaling is used widely to determine cell fates and to regulate pattern formation; its dysfunction results in a tremendous variety of developmental defects and adult pathologies. This primer describes the mechanism of Notch signal transduction and how it is used to control the formation of biological patterns.     

Notch signaling and Notch signaling modifiers

Originally discovered nearly a century ago, the Notch signaling pathway is critical for virtually all developmental programs and modulates an astounding variety of pathogenic processes. The DSL (Delta, Serrate, LAG-2 family) proteins have long been considered canonical activators of the core Notch pathway. More recently, a wide and expanding network of non-canonical extracellular factors has also been shown to modulate Notch signaling, conferring newly appreciated complexity to this evolutionarily conserved signal transduction system. Here, I review current concepts in Notch signaling, with a focus on work from the last decade elucidating novel extracellular proteins that up- or down-regulate signal potency.     

Notch signaling regulates neural precursor allocation and binary neuronal fate decisions in zebrafish

Notch signaling plays a well-described role in regulating the formation of neurons from proliferative neural precursors in vertebrates but whether, as in flies, it also specifies sibling cells for different neuronal fates is not known. Ventral spinal cord precursors called pMN cells produce mostly motoneurons and oligodendrocytes, but recent lineage-marking experiments reveal that they also make astrocytes, ependymal cells and interneurons. Our own clonal analysis of pMN cells in zebrafish showed that some produce a primary motoneuron and KA' interneuron at their final division. We investigated the possibility that Notch signaling regulates a motoneuron-interneuron fate decision using a combination of mutant, transgenic and pharmacological manipulations of Notch activity. We show that continuous absence of Notch activity produces excess primary motoneurons and a deficit of KA' interneurons, whereas transient inactivation preceding neurogenesis results in an excess of both cell types. By contrast, activation of Notch signaling at the neural plate stage produces excess KA' interneurons and a deficit of primary motoneurons. Furthermore, individual pMN cells produce similar kinds of neurons at their final division in mib mutant embryos, which lack Notch signaling. These data provide evidence that, among some postmitotic daughters of pMN cells, Notch promotes KA' interneuron identity and inhibits primary motoneuron fate, raising the possibility that Notch signaling diversifies vertebrate neuron type by mediating similar binary fate decisions.     

Attenuation of Notch and Hedgehog signaling is required for fate specification in the spinal cord

During the development of the spinal cord, proliferative neural progenitors differentiate into postmitotic neurons with distinct fates. How cells switch from progenitor states to differentiated fates is poorly understood. To address this question, we studied the differentiation of progenitors in the zebrafish spinal cord, focusing on the differentiation of Kolmer-Agduhr″ (KA″) interneurons from lateral floor plate (LFP) progenitors. In vivo cell tracking demonstrates that KA″ cells are generated from LFP progenitors by both symmetric and asymmetric cell divisions. A photoconvertible reporter of signaling history (PHRESH) reveals distinct temporal profiles of Hh response: LFP progenitors continuously respond to Hh, while KA″ cells lose Hh response upon differentiation. Hh signaling is required in LFP progenitors for KA″ fate specification, but prolonged Hh signaling interferes with KA″ differentiation. Notch signaling acts permissively to maintain LFP progenitor cells: activation of Notch signaling prevents differentiation, whereas inhibition of Notch signaling results in differentiation of ectopic KA″ cells. These results indicate that neural progenitors depend on Notch signaling to maintain Hh responsiveness and rely on Hh signaling to induce fate identity, whereas proper differentiation depends on the attenuation of both Notch and Hh signaling.     

The convergence of Notch and MAPK signaling specifies the blood progenitor fate in the Drosophila mesoderm

Blood progenitors arise from a pool of pluripotential cells (“hemangioblasts”) within the Drosophila embryonic mesoderm. The fact that the cardiogenic mesoderm consists of only a small number of highly stereotypically patterned cells that can be queried individually regarding their gene expression in normal and mutant embryos is one of the significant advantages that Drosophila offers to dissect the mechanism specifying the fate of these cells. We show in this paper that the expression of the Notch ligand Delta (Dl) reveals segmentally reiterated mesodermal clusters (“cardiogenic clusters”) that constitute the cardiogenic mesoderm. These clusters give rise to cardioblasts, blood progenitors and nephrocytes. Cardioblasts emerging from the cardiogenic clusters accumulate high levels of Dl, which is required to prevent more cells from adopting the cardioblast fate. In embryos lacking Dl function, all cells of the cardiogenic clusters become cardioblasts, and blood progenitors are lacking. Concomitant activation of the Mitogen Activated Protein Kinase (MAPK) pathway by Epidermal Growth Factor Receptor (EGFR) and Fibroblast Growth Factor Receptor (FGFR) is required for the specification and maintenance of the cardiogenic mesoderm; in addition, the spatially restricted localization of some of the FGFR ligands may be instrumental in controlling the spatial restriction of the Dl ligand to presumptive cardioblasts.     

The Hippo pathway controls polar cell fate through Notch signaling during Drosophila oogenesis

During Drosophila oogenesis, the somatic follicle cells form an epithelial layer surrounding the germline cells to form egg chambers. In this process, follicle cell precursors are specified into polar cells, stalk cells, and main-body follicle cells. Proper specification of these three cell types ensures correct egg chamber formation and polarization of the anterior–posterior axis of the germline cells. Multiple signaling cascades coordinate to control the follicle cell fate determination, including Notch, JAK/STAT, and Hedgehog signaling pathways. Here, we show that the Hippo pathway also participates in polar cell specification. Over-activation of yorkie (yki) leads to egg chamber fusion, possibly through attenuation of polar cell specification. Loss-of-function experiments using RNAi knockdown or generation of mutant clones by mitotic recombination demonstrates that reduction of yki expression promotes polar cell formation in a cell-autonomous manner. Consistently, polar cells mutant for hippo (hpo) or warts (wts) are not properly specified, leading to egg chamber fusion. Furthermore, Notch activity is increased in yki mutant cells and reduction of Notch activity suppresses polar cell formation in yki mutant clones. These results demonstrate that yki represses polar cell fate through Notch signaling. Collectively, our data reveal that the Hippo pathway controls polar cell specification. Through repressing Notch activity, Yki serves as a key repressor in specifying polar cells during Drosophila oogenesis.     

Notch regulates cell fate in the developing pronephros

The mechanisms that regulate cell fate within the pronephros are poorly understood but are important for the subsequent development of the urogenital system and show many similarities to nephrogenesis in the definitive kidney. Dynamic expression of Notch-1, Serrate-1, and Delta-1 in the developing Xenopus pronephros suggests a role for this pathway in cell fate segregation. Misactivation of Notch signaling using conditionally active forms of either Notch-1 or RBP-J/Su(H) proteins prevented normal duct formation and the proper expression of genetic markers of duct cell differentiation. Inhibition of endogenous Notch signaling elicited the opposite effect. Taken together with the mRNA expression patterns, these data suggest that endogenous Notch signaling functions to inhibit duct differentiation in the dorsoanterior region of the anlage where cells are normally fated to form tubules. In addition, elevated Notch signaling in the pronephric anlage both perturbed the characteristic pattern of the differentiated tubule network and increased the expression of early markers of pronephric precursor cells, Pax-2 and Wilms' tumor suppressor gene (Wt-1). We propose that Notch signaling plays a previously unrecognized role in the early selection of duct and tubule cell fates as well as functioning subsequently to control tubule cell patterning and development.     

The monolayer formation of Bergmann glial cells is regulated by Notch/RBP-J signaling

The Bergmann glia is a unipolar astrocyte in the cerebellar cortex, displaying a tight association with Purkinje cells. The cell bodies of Bergmann glia are located in a row around Purkinje cell somata; they extend radially arranged Bergmann fibers which enwrap the synapses on the Purkinje cell dendrites. It is well known that Bergmann glial somata migrate from the ventricular zone through the mantle zone, forming an epithelium-like lining in the Purkinje cell layer during development. However, the mechanism of the monolayer formation of Bergmann glia is poorly understood. Several reports have suggested that Notch signaling plays instructive roles in promoting the identities of several types of glial cells, including Bergmann glia. Moreover, Notch receptors are expressed in Bergmann glia during development. Here, we have deleted the Notch1, Notch2 and RBP-J genes in the Bergmann glia by GFAP-driven, Cre-mediated recombination, to study the role of Notch-RBP-J-signaling in the monolayer formation of Bergmann glia. Notch1/2- and RBP-J-conditional mutant mice showed disorganization of Bergmann fibers, irregularities of the Bergmann glial lining and aberrant localization of Bergmann glia in the molecular layer. Thus, Notch-RBP-J signaling plays crucial roles in the monolayer formation and morphogenesis of Bergmann glia.     

Macrophage-derived Wnt opposes Notch signaling to specify hepatic progenitor cell fate in chronic liver disease

During chronic injury a population of bipotent hepatic progenitor cells (HPCs) become activated to regenerate both cholangiocytes and hepatocytes. Here we show in human diseased liver and mouse models of the ductular reaction that Notch and Wnt signaling direct specification of HPCs via their interactions with activated myofibroblasts or macrophages. In particular, we found that during biliary regeneration, expression of Jagged 1 (a Notch ligand) by myofibroblasts promoted Notch signaling in HPCs and thus their biliary specification to cholangiocytes. Alternatively, during hepatocyte regeneration, macrophage engulfment of hepatocyte debris induced Wnt3a expression. This resulted in canonical Wnt signaling in nearby HPCs, thus maintaining expression of Numb (a cell fate determinant) within these cells and the promotion of their specification to hepatocytes. By these two pathways adult parenchymal regeneration during chronic liver injury is promoted.     

Notch信号通路研究进展

在无脊椎动物和脊椎动物发育过程中 ,Notch信号对细胞的命运决定起关键作用。通过Notch受体的信号传递能够扩大并固化相邻细胞之间的分子差异 ,最终决定细胞的命运。本文综述了Notch信号通路的相关细节 ,重点讨论了CSL非依赖的途径     

Interplay of Notch and FGF signaling restricts cell fate and MAPK activation in the Drosophila trachea.

The patterned branching in the Drosophila tracheal system is triggered by the FGF-like ligand Branchless that activates a receptor tyrosine kinase Breathless and the MAP kinase pathway. A single fusion cell at the tip of each fusion branch expresses the zinc-finger gene escargot, leads branch migration in a stereotypical pattern and contacts with another fusion cell to mediate fusion of the branches. A high level of MAP kinase activation is also limited to the tip of the branches. Restriction of such cell specialization events to the tip is essential for tracheal tubulogenesis. Here we show that Notch signaling plays crucial roles in the singling out process of the fusion cell. We found that Notch is activated in tracheal cells by Branchless signaling through stimulation of &Dgr; expression at the tip of tracheal branches and that activated Notch represses the fate of the fusion cell. In addition, Notch is required to restrict activation of MAP kinase to the tip of the branches, in part through the negative regulation of Branchless expression. Notch-mediated lateral inhibition in sending and receiving cells is thus essential to restrict the inductive influence of Branchless on the tracheal tubulogenesis.