Cooperation of JAK/STAT and Notch signaling in the Drosophila foregut

Role of SOCS and VHL Proteins in Neuronal Differentiation and Development

The basic helix-loop-helix factors play a central role in neuronal differentiation and nervous system development, which involve the Notch and signal transducer and activator of transcription (STAT)/small mother against decapentaplegic signaling pathways. Neural stem cells differentiate into three nervous system lineages, and the suppressor of cytokine signaling (SOCS) and von Hippel-Lindau (VHL) proteins are involved in this neuronal differentiation. The SOCS and VHL proteins both contain homologous structures comprising the BC-box motif. SOCSs recruit Elongin C, Elongin B, Cullin5(Cul5), and Rbx2, whereas VHL recruits Elongin C, Elongin B, Cul2, and Rbx1. SOCSs form SBC-Cul5/E3 complexes, and VHL forms a VBC-Cul2/E3 complex. These complexes degrade the target protein and suppress its downstream transduction pathway by acting as E3 ligases via the ubiquitin-proteasome system. The Janus kinase (JAK) is the main target protein of the E3 ligase SBC-Cul5, whereas hypoxia-inducible factor is the primary target protein of the E3 ligase VBC-Cul2; nonetheless, VBC-Cul2 also targets the JAK. SOCSs not only act on the ubiquitin-proteasome system but also act directly on JAKs to suppress the Janus kinase-signal transduction and activator of transcription (JAK-STAT) pathway. Both SOCS and VHL are expressed in the nervous system, predominantly in brain neurons in the embryonic stage. Both SOCS and VHL induce neuronal differentiation. SOCS is involved in differentiation into neurons, whereas VHL is involved in differentiation into neurons and oligodendrocytes; both proteins promote neurite outgrowth. It has also been suggested that the inactivation of these proteins may lead to the development of nervous system malignancies and that these proteins may function as tumor suppressors. The mechanism of action of SOCS and VHL involved in neuronal differentiation and nervous system development is thought to be mediated through the inhibition of downstream signaling pathways, JAK-STAT, and hypoxia-inducible factor-vascular endothelial growth factor pathways. In addition, because SOCS and VHL promote nerve regeneration, they are expected to be applied in neuronal regenerative medicine for traumatic brain injury and stroke.

Cytokine receptor DOME controls wing disc development in Bombyx mori

In multicellular organisms, the JAK/STAT signaling pathway is involved in cell proliferation, differentiation, apoptosis, and immune regulation. Through activation of the Stat92E transcription factor, JAK/STAT signaling induced proper wing development in Drosophila. Domeless (DOME) was the first identified invertebrate JAK/STAT receptor. However, the function of DOME in Bombyx mori development remains unclear, especially in wing morphogenesis. In this study, we isolated the cytokine receptor DOME gene in B. mori and evaluated its function in DOME-knockout models. We found that overexpression of DOME at the cellular level upregulated the expression of JAK/STAT pathway-related genes, promoted proliferation, and inhibited apoptosis. The results of the interference with DOME had the opposite effects with those of overexpression at the cellular level. Using CRISPR/Cas9 technology, we constructed a DOME-knockout transgenic silkworm strain (KO-DOME) and found that the wings of the pupa and moth stages were vesicle-shaped and smaller than those of the wild-type silkworm. Some KO-DOME silkworms were unable to extend their wings from the pupal case after eclosion. We detected the expression of cyclin and apoptosis-related genes in the wing disc of the moth stage and found that some cyclin genes, such as CyclinA, CyclinB, and CyclinD, were downregulated, whereas apoptotic genes, such as Caspase1, Caspase3, and Caspase8, were upregulated. We propose that DOME regulates cell proliferation and apoptosis by affecting the JAK/STAT signaling pathway, ultimately influencing the development of wing discs. Our study provides empirical evidence for the biological function of the silkworm DOME gene, which is essential for the normal development of wings.

The JAK/STAT signaling pathway: from bench to clinic

The Janus kinase/signal transducer and activator of transcription (JAK/STAT) signaling pathway was discovered more than a quarter-century ago. As a fulcrum of many vital cellular processes, the JAK/STAT pathway constitutes a rapid membrane-to-nucleus signaling module and induces the expression of various critical mediators of cancer and inflammation. Growing evidence suggests that dysregulation of the JAK/STAT pathway is associated with various cancers and autoimmune diseases. In this review, we discuss the current knowledge about the composition, activation, and regulation of the JAK/STAT pathway. Moreover, we highlight the role of the JAK/STAT pathway and its inhibitors in various diseases.

Migration of primordial germline cells is negatively regulated by surrounding somatic cells during early embryogenesis in Drosophila melanogaster

Cell migration is an important morphogenetic process necessary at different stages of individual development and body functioning. The initiation and maintenance of the cell movement state requires the activation of many factors involved in the regulation of transcription, signal transduction, adhesive interactions, modulation of membranes and the cytoskeleton. However, cell movement depends on the status of both migrating and surrounding cells, interacting with each other during movement. The surrounding cells or cell matrix not only form a substrate for movement, but can also participate in the spatio-temporal regulation of the migration. At present, there is no exact understanding of the genetic mechanisms of this regulation. To determine the role of the cell environment in the regulation of individual cell migration, we studied the migration of primordial germline cells (PGC) during early embryogenesis in Drosophila melanogaster. Normally, PGC are formed at the 3rd stage of embryogenesis at the posterior pole of the embryo. During gastrulation (stages 6-7), PGC as a consolidated cell group passively transfers into the midgut primordium. Further, PGC are individualized, acquire an amoeboid form, and actively move through the midgut epithelium and migrate to the 5-6 abdominal segment of the embryo, where they form paired embryonic gonads. We screened for genes expressed in the epithelium surrounding PGC during early embryogenesis and affecting their migration. We identified the myc, Hph, stat92E, Tre-1, and hop genes, whose RNA interference leads to premature active PGC migration at stages 4-7 of embryogenesis. These genes can be divided into two groups: 1) modulators of JAK/STAT pathway activity inducing PGC migration (stat92E, Tre-1, hop), and 2) myc and Hph involved in epithelial morphogenesis and polarization, i. e. modifying the permeability of the epithelial barrier. Since a depletion of each of these gene products resulted in premature PGC migration, we can conclude that, normally, the somatic environment negatively regulates PGC migration during early Drosophila embryogenesis.

Control of endothelial cell tube formation by Notch ligand intracellular domain interactions with activator protein 1 (AP-1)

Notch signaling is a ubiquitous signal transduction pathway found in most if not all metazoan cell types characterized to date. It is indispensable for cell differentiation as well as tissue growth, tissue remodeling, and apoptosis. Although the canonical Notch signaling pathway is well characterized, accumulating evidence points to the existence of multiple, less well-defined layers of regulation. In this study, we investigated the function of the intracellular domain (ICD) of the Notch ligand Delta-like 4 (DLL4). We provide evidence that the DLL4 ICD is required for normal DLL4 subcellular localization. We further show that it is cleaved and interacts with the JUN proto-oncogene, which forms part of the activator protein 1 (AP-1) transcription factor complex. Mechanistically, the DLL4 ICD inhibited JUN binding to DNA and thereby controlled the expression of JUN target genes, including DLL4 Our work further demonstrated that JUN strongly stimulates endothelial cell tube formation and that DLL4 constrains this process. These results raise the possibility that Notch/DLL4 signaling is bidirectional and suggest that the DLL4 ICD could represent a point of cross-talk between Notch and receptor tyrosine kinase (RTK) signaling.

A Drosophila LexA Enhancer-Trap Resource for Developmental Biology and Neuroendocrine Research

Novel binary gene expression tools like the LexA-LexAop system could powerfully enhance studies of metabolism, development, and neurobiology in Drosophila However, specific LexA drivers for neuroendocrine cells and many other developmentally relevant systems remain limited. In a unique high school biology course, we generated a LexA-based enhancer trap collection by transposon mobilization. The initial collection provides a source of novel LexA-based elements that permit targeted gene expression in the corpora cardiaca, cells central for metabolic homeostasis, and other neuroendocrine cell types. The collection further contains specific LexA drivers for stem cells and other enteric cells in the gut, and other developmentally relevant tissue types. We provide detailed analysis of nearly 100 new LexA lines, including molecular mapping of insertions, description of enhancer-driven reporter expression in larval tissues, and adult neuroendocrine cells, comparison with established enhancer trap collections and tissue specific RNAseq. Generation of this open-resource LexA collection facilitates neuroendocrine and developmental biology investigations, and shows how empowering secondary school science can achieve research and educational goals.

JAK-STAT pathway in Drosophila morphogenesis: From organ selector to cell behavior regulator

One of the main contributions of Drosophila to the JAK-STAT field is the study of morphogenesis. JAK-STAT signaling controls the formation of many different structures through surprisingly different morphogenetic behaviors that include induction of cell rearrangements, invagination, folding of tissues, modulation of cell shape, and migration. This variability may be explained by the many transcription factors and signaling molecules STAT regulates at early stages of development. But is STAT just acting as an upstream inducer of morphogenesis or does it have a more direct role in controlling cell behaviors? Here we review what is known about how the canonical phosphorylation of STAT contributes to shaping the embryonic and imaginal structures.

Selective repression of Notch pathway target gene transcription

The Notch signaling pathway regulates metazoan development, in part, by directly controlling the transcription of target genes. For a given cellular context, however, only subsets of the known target genes are transcribed when the pathway is activated. Thus, there are context-dependent mechanisms that selectively maintain repression of target gene transcription when the Notch pathway is activated. This review focuses on molecular mechanisms that have been recently reported to mediate selective repression of Notch pathway target gene transcription. These mechanisms are essential for generating the complex spatial and temporal expression patterns of Notch target genes during development.

Nuclear JAK2: form and function in cancer

The conventional view of Janus kinase 2 (JAK2) is a nonreceptor tyrosine kinase which transmits information to the nucleus via the signal transducer and activator of transcriptions (STATs) without leaving the cytoplasm. However, accumulating data suggest that JAK2 may signal by exporting from cytoplasm to nucleus, where it guides the transcriptional machinery independent of STATs protein. Recent studies demonstrated that JAK2 is a crucial component of signaling pathways operating in the nucleus. Especially the latest landmark discovery confirmed that JAK2 goes into the nucleus and directly interacts with nucleoproteins, such as histone H3 at tyrosine 41 (H3Y41), nuclear factor 1-C2 (NF1-C2) and SWI/SNF-related helicases/ATPases (RUSH)-1α, indicating that JAK2 has a fresh nuclear function. Nuclear JAK2 is linked to a variety of cellular functions, such as cell cycle progression, apoptosis and genetic instability. The balance between these functions is an essential factor in determining whether a cell remains benign or becomes malignant. The aim of this review is intended to summarize the state of our knowledge on nuclear localization of JAK2 and nuclear JAK2 pathways, and to highlight the emerging roles for nuclear JAK2 in carcinogenesis.

The adult Drosophila gastric and stomach organs are maintained by a multipotent stem cell pool at the foregut/midgut junction in the cardia (proventriculus)

Stomach cancer is the second most frequent cause of cancer-related death worldwide. Thus, it is important to elucidate the properties of gastric stem cells, including their regulation and transformation. To date, such stem cells have not been identified in Drosophila. Here, using clonal analysis and molecular marker labeling, we identify a multipotent stem-cell pool at the foregut/midgut junction in the cardia (proventriculus). We found that daughter cells migrate upward either to anterior midgut or downward to esophagus and crop. The cardia functions as a gastric valve and the anterior midgut and crop together function as a stomach in Drosophila; therefore, we named the foregut/midgut stem cells as gastric stem cells (GaSC). We further found that JAK-STAT signaling regulates GaSCs' proliferation, Wingless signaling regulates GaSCs' self-renewal, and hedgehog signaling regulates GaSCs' differentiation. The differentiation pattern and genetic control of the Drosophila GaSCs suggest the possible similarity to mouse gastric stem cells. The identification of the multipotent stem cell pool in the gastric gland in Drosophila will facilitate studies of gastric stem cell regulation and transformation in mammal.

Canonical and non-canonical JAK-STAT signaling

Aberrant activation of the JAK-STAT pathway has been implicated in many human cancers. It has widely been assumed that the effects of STAT activation are mediated by direct transcriptional induction of STAT target genes. However, recent findings in Drosophila have identified a non-canonical mode of JAK-STAT signaling, which directly controls heterochromatin stability. This indicates that the JAK-STAT pathway also controls cellular epigenetic status, which affects expression of genes beyond those under direct STAT transcriptional control. Given the evolutionary conservation of the canonical pathway among different species, the non-canonical mode of JAK-STAT signaling might also operate in vertebrates. In this review, canonical versus non-canonical JAK-STAT signaling and the implications for gene regulation and cancer formation are discussed.

Leukemia inhibitory factor participates in the expansion of neural stem/progenitors after perinatal hypoxia/ischemia

Subsequent to perinatal hypoxia/ischemia there is an increase in the number of neural stem/progenitor cells (NSP) within the subventricular zone (SVZ). Gene expression analyses have implicated Notch signaling in the expansion of these tripotential cells but there are limited data as to which signals are stimulating Notch activation. There is evidence that the leukemia inhibitory factor receptor (LIFR)/gp130 receptor heterodimer induces Notch1 to maintain NSP populations during normal development. LIF and ciliary neurotrophic factor (CNTF) bind to these receptor components and they coordinate injury responses in the CNS. Therefore, the aim of these studies was to investigate whether CNTF and/or leukemia inhibitory factor (LIF) participate in NSP expansion in the rat SVZ after hypoxia/ischemia (H/I) as well as to characterize the downstream events that regulate NSP numbers. We report that LIF mRNA is induced 48 h post-insult by 13-fold but that it returns almost to baseline by 72 h. Commensurate with increased LIF expression there is a corresponding increase in phosphorylated Stat-3 within the SVZ. Modeling the changes that occur in vivo, we show that LIF induces Stat-3 phosphorylation in neurospheres to enhance Delta-like-1 and Notch1 expression as well as to increase Notch1 activation. LIF also expands neurosphere number and size in vitro. Whereas CNTF can mimic the effects of LIF in vitro, CNTF expression in the SVZ was unchanged during recovery from H/I. Cumulatively, these data implicate LIF and not CNTF in the expansion of NSPs in the rat SVZ after perinatal brain injury. As both LIF expression and the endogenous regenerative response after brain injury are time-delimited, these findings provide insights into strategies to expand the endogenous pool of NSPs to repopulate the damaged brain.

Cross regulation of intercellular gap junction communication and paracrine signaling pathways during organogenesis in Drosophila

The spatial and temporal coordination of patterning and morphogenesis is often achieved by paracrine morphogen signals or by the direct coupling of cells via gap junctions. How paracrine signals and gap junction communication cooperate to control the coordinated behavior of cells and tissues is mostly unknown. We found that hedgehog signaling is required for the expression of wingless and of Delta/Notch target genes in a single row of boundary cells in the foregut-associated proventriculus organ of the Drosophila embryo. These cells coordinate the movement and folding of proventricular cells to generate a multilayered organ. hedgehog and wingless regulate gap junction communication by transcriptionally activating the innexin2 gene, which encodes a member of the innexin family of gap junction proteins. In innexin2 mutants, gap junction-mediated cell-to-cell communication is strongly reduced and the proventricular cell layers fail to fold and invaginate, similarly as in hedgehog or wingless mutants. We further found that innexin2 is required in a feedback loop for the transcriptional activation of the hedgehog and wingless morphogens and of Delta in the proventriculus primordium. We propose that the transcriptional cross regulation of paracrine and gap junction-mediated signaling is essential for organogenesis in Drosophila.

Crossing paths with Notch in the hyper-network

The development of complex and diverse metazoan morphologies is coordinated by a surprisingly small number of evolutionarily conserved signaling mechanisms. These signals can act in parallel but often appear to function as an integrated hyper-network. The nodes defining this complex molecular circuitry are poorly understood, but the biological significance of pathway cross-talk is profound. The importance of such large-scale signal integration is exemplified by Notch and its ability to cross-talk with all the major pathways to influence cell differentiation, proliferation, survival and migration. The Notch pathway is, thus, a useful paradigm to illustrate the complexity of pathway cross-talk: its pervasiveness, context dependency, and importance in development and disease.

Delta-4 expression on a stromal cell line is augmented by interleukin-6 via STAT3 activation

OBJECTIVE

Notch receptors and their ligands are known to play an important role in hematopoietic cell fate decisions. Although expressions of Notch ligands were frequently detected in bone marrow cells or thymic epithelial cells, regulatory roles of hematopoietic cytokines on their expression are still poorly understood. In this study, we focused on a new member of Notch ligand family, Delta-4, and analyzed regulatory mechanisms of Delta-4 expression by cytokines using stromal cell lines.

METHODS

For our expression study, we selected a highly Delta-4-expressing murine stromal cell line, SC9-19. Delta-4 protein expression was analyzed by Western blotting after cytokine treatment with or without various kinase inhibitors. Biologic relevance of the enhanced Delta-4 expression was examined in the coculture system using cord blood CD34(+) cells.

RESULTS

When SC9-19 was treated with different cytokines, we found interleukin-6 (IL-6) was the most efficient inducer of Delta-4 protein expression. Further analysis revealed that IL-6-induced signaling for Delta-4 expression was transduced through the pathway of Janus kinase/signal transducers and activators of transcription-3 (JAK/STAT3). Other mediators such as mitogen-activated protein kinase, phosphatidylinositol 3-kinase, and Src tyrosine kinase were not involved in IL-6-induced Delta-4 expression. Erythroid differentiation of CD34(+) cells was enhanced by IL-6 treatment of the Delta-4-expressing original stromal cell line but not of a Delta-4-knockdown stromal cell line.

CONCLUSIONS

IL-6 augments Delta-4 expression in the stromal cell line via STAT3 activation. This study provides a novel mechanism for augmentation of Delta-4 expression by hematopoietic cytokine and suggests a role for Delta-4 in the control of hematopoiesis.

JAK/STAT signalling in Drosophila: insights into conserved regulatory and cellular functions

High levels of interspecies conservation characterise all signal transduction cascades and demonstrate the significance of these pathways over evolutionary time. Here, we review advances in the field of JAK/STAT signalling, focusing on recent developments in Drosophila. In particular, recent results from genetic and genome-wide RNAi screens, as well as studies into the developmental roles played by this pathway, highlight striking levels of physical and functional conservation in processes such as cellular proliferation, immune responses and stem cell maintenance. These insights underscore the value of model organisms for improving our understanding of this human disease-relevant pathway.

Identification of Drosophila genes modulating Janus kinase/signal transducer and activator of transcription signal transduction

The JAK/STAT pathway was first identified in mammals as a signaling mechanism central to hematopoiesis and has since been shown to exert a wide range of pleiotropic effects on multiple developmental processes. Its inappropriate activation is also implicated in the development of numerous human malignancies, especially those derived from hematopoietic lineages. The JAK/STAT signaling cascade has been conserved through evolution and although the pathway identified in Drosophila has been closely examined, the full complement of genes required to correctly transduce signaling in vivo remains to be identified. We have used a dosage-sensitive dominant eye overgrowth phenotype caused by ectopic activation of the JAK/STAT pathway to screen 2267 independent, newly generated mutagenic P-element insertions. After multiple rounds of retesting, 23 interacting loci that represent genes not previously known to interact with JAK/STAT signaling have been identified. Analysis of these genes has identified three signal transduction pathways, seven potential components of the pathway itself, and six putative downstream pathway target genes. The use of forward genetics to identify loci and reverse genetic approaches to characterize them has allowed us to assemble a collection of genes whose products represent novel components and regulators of this important signal transduction cascade.

Suppression of Stat3 promotes neurogenesis in cultured neural stem cells

To investigate the effects of signal transducer and activator of transcription 3 (Stat3) on neural stem cell fate, stem cells were inoculated with an adenovirus vector expressing dominant negative form of Stat3 (Stat3F). One day later, a promoter assay revealed significant reduction of the transcriptional level in the transfected cells. Three days later, Western blot analysis and immunocytochemical analysis revealed that the protein level of microtubule-associated protein (MAP)2 and the number of MAP2-positive cells were increased significantly in the transfected cells whereas the protein level of glial fibrillary acidic protein (GFAP) and the number of GFAP-positive cells were decreased significantly. In addition, mRNA levels of Notch family members (Notch1, 2, and 3) and of inhibitory basic helix-loop-helix (bHLH) factors (Hes5, Id2, and Id3) were significantly downregulated at 3 days after viral inoculation with Stat3F; however, mRNA levels of bHLH determination factors (Math1 and Neurogenin3) and bHLH differentiation factors (NeuroD1 and NeuroD2) were significantly upregulated. These data indicated that suppression of Stat3 directly induced neurogenesis and inhibited astrogliogenesis in neural stem cells.

Making tubes in the Drosophila embryo

Epithelial and endothelial tubes come in various shapes and sizes and form the basic units of many tubular organs. During embryonic development, single unbranched tubes as well as highly branched networks of tubes form from simple sheets of cells by several morphogenic movements. Studies of tube formation in the Drosophila embryo have greatly advanced our understanding of the cellular and molecular mechanisms by which tubes are formed. This review highlights recent progress on formation of the hindgut, Malpighian tubules, proventriculus, salivary gland, and trachea of the Drosophila embryo, focusing on the cellular events that form each tube and their genetic requirements.

Functional specification in the Drosophila endoderm

The discovery of homeobox gene clusters led us to realize that the mechanisms for body patterning and other developmental programs are evolutionally-conserved in vertebrates and invertebrates. The endoderm contributes to the lining of the gut and associated organs such as the liver and pancreas, which are critical for physiological functions. Our knowledge of endoderm development is limited; however, recent studies suggest that cooperation between the HNF3/Fork head and GATA transcription factors is crucial for endoderm specification. It is necessary to further understand the mechanism through which cells become functionally organized. Molecular genetic analyses of the Drosophila endoderm would provide insights into this issue. During proventriculus morphogenesis, a simple epithelial tube is folded into a functional multilayered structure, while two functions of midgut copper cells (i.e. copper absorption and acid secretion) can be easily visualized. The homeobox gene defective proventriculus (dve) plays key roles in these functional specifications.

The Janus kinases (Jaks)

The Janus kinase (Jak) family is one of ten recognized families of non-receptor tyrosine kinases. Mammals have four members of this family, Jak1, Jak2, Jak3 and Tyrosine kinase 2 (Tyk2). Birds, fish and insects also have Jaks. Each protein has a kinase domain and a catalytically inactive pseudo-kinase domain, and they each bind cytokine receptors through amino-terminal FERM (Band-4.1, ezrin, radixin, moesin) domains. Upon binding of cytokines to their receptors, Jaks are activated and phosphorylate the receptors, creating docking sites for signaling molecules, especially members of the signal transducer and activator of transcription (Stat) family. Mutations of the Drosophila Jak (Hopscotch) have revealed developmental defects, and constitutive activation of Jaks in flies and humans is associated with leukemia-like syndromes. Through the generation of Jak-deficient cell lines and gene-targeted mice, the essential, nonredundant functions of Jaks in cytokine signaling have been established. Importantly, deficiency of Jak3 is the basis of human autosomal recessive severe combined immunodeficiency (SCID); accordingly, a selective Jak3 inhibitor has been developed, forming a new class of immunosuppressive drugs.