Identification of the first invertebrate interleukin JAK/STAT receptor, the Drosophila gene domeless

A gradient of JAK pathway activity patterns the anterior-posterior axis of the follicular epithelium

The Drosophila egg develops through closely coordinated activities of associated germline and somatic cells. An essential aspect of egg development is the differentiation of the somatic follicle cells into several distinct subpopulations with specific functions. Here we demonstrate that the graded activity of the Janus kinase (JAK) pathway, stimulated by the Unpaired ligand, patterns the anterior-posterior axis of the follicular epithelium. Different levels of JAK activity instruct adoption of distinct anterior cell fates. Further, the coordinated activities of the JAK/STAT and epidermal growth factor receptor (EGFR) pathways are required to specify the posterior terminal cell fate. We propose that Upd secreted from the polar cells may act as a morphogen to stimulate A/P-derived follicular fates through JAK pathway activation.

Cytokines in Drosophila Hematopoiesis and Cellular Immunity

Hematopoiesis is a complex, multistep process in which progenitor cells undergo distinct cellular changes of proliferation and differentiation to give rise to mature blood cells in circulation. Many of the genetic and molecular events that drive these changes have been characterized in mammals, frogs, and zebra fish, and more recently in the insect model system Drosophila melanogaster. Blood cells in Drosophila are actively involved in fighting infections and the cellular immune responses are intimately tied to the process of hematopoiesis. In this article, we briefly review the fundamental similarities in Drosophila and mammalian hematopoiesis and highlight the potential roles of four cytokines/growth factors in Drosophila hematopoiesis and cellular immunity.

Localized JAK/STAT signaling is required for oriented cell rearrangement in a tubular epithelium

Rearrangement of cells constrained within an epithelium is a key process that contributes to tubular morphogenesis. We show that activation in a gradient of the highly conserved JAK/STAT pathway is essential for orienting the cell rearrangement that drives elongation of a genetically tractable model. Using loss-of-function and gain-of-function experiments, we show that the components of the pathway from ligand to the activated transcriptional regulator STAT are required for cell rearrangement in the Drosophila embryonic hindgut. The difference in effect between localized expression of ligand (Unpaired) and dominant active JAK (Hopscotch) demonstrates that the ligand plays a cell non-autonomous role in hindgut cell rearrangement. Taken together with the appearance of STAT92E in a gradient in the hindgut epithelium, these results support a model in which an anteroposterior gradient of ligand results in a gradient of activated STAT. These results provide the first example in which JAK/STAT signaling plays a required role in orienting cell rearrangement that elongates an epithelium.

Functional convergence of invertebrate and vertebrate cytokine-like molecules based on a similar lectin-like activity

It is generally accepted that the action of cytokines results from their binding to specific receptors. However, many cytokines possess lectin-like activity that may be essential for the expression of their full biological activities. This review focuses on the physiological relevance of the lectin-like activity of cytokines during the innate immune response in mammals, using TNF as an illustrative example. Moreover, we will show that TNF displays functional analogies with a defense molecule from the earthworm Eisenia foetida termed CCF. These analogies are not reflected by primary sequence homology between CCF and TNF but are particularly based on a similar lectin-like activity/domain. Hence, from a phylogenetic point of view, the lectin-like activity/domain of CCF and TNF may represent an essential recognition mechanism that has been functionally conserved during the innate immune response of invertebrates and vertebrates as a result of convergent evolution.

PVF2, a PDGF/VEGF-like growth factor, induces hemocyte proliferation in Drosophila larvae

Blood cells play a crucial role in both morphogenetic and immunological processes in Drosophila, yet the factors regulating their proliferation remain largely unknown. In order to address this question, we raised antibodies against a tumorous blood cell line and identified an antigenic determinant that marks the surface of prohemocytes and also circulating plasmatocytes in larvae. This antigen was identified as a Drosophila homolog of the mammalian receptor for platelet-derived growth factor (PDGF)/vascular endothelial growth factor (VEGF). The Drosophila receptor controls cell proliferation in vitro. By overexpressing in vivo one of its putative ligands, PVF2, we induced a dramatic increase in circulating hemocytes. These results identify the PDGF/VEGF receptor homolog and one of its ligands as important players in Drosophila hematopoiesis.

The Jak/STAT pathway in model organisms: emerging roles in cell movement

The JAK/STAT pathway was originally identified in mammals. Studies of this pathway in the mouse have revealed that JAK/STAT signaling plays a central role during hematopoeisis and other developmental processes. The role of JAK/STAT signaling in blood appears to be conserved throughout evolution, as it is also required during fly hematopoeisis. Studies in Dictyostelium, Drosophila, and zebrafish have shown that the JAK/STAT pathway is also required in an unusually broad set of developmental decisions, including cell proliferation, cell fate determination, cell migration, planar polarity, convergent extension, and immunity. There is increasing evidence that the versatility of this pathway relies on its cooperation with other signal transduction pathways. In this review, we discuss the components of the JAK/STAT pathway in model organisms and what is known about its requirement in cellular and developmental processes. In particular, we emphasize recent insights into the role that this pathway plays in the control of cell movement.

The Drosophila cytokine receptor Domeless controls border cell migration and epithelial polarization during oogenesis

In mammals, the JAK/STAT (Janus Kinase/Signal Transducer and Activator of Transcription) signaling pathway is activated in response to cytokines and growth factors to control blood cell development, proliferation and cell determination. In Drosophila, a conserved JAK/STAT signaling pathway controls segmentation in embryos, as well as blood cell development and other processes in larvae and adults. During embryogenesis, transduction of the Unpaired [Upd; also known as Outstretched (Os)] ligand through the JAK/STAT pathway requires Domeless, a putative membrane protein with distant homology to vertebrate type I cytokine receptors. We have isolated domeless (dome) in a screen to identify genes essential in epithelial morphogenesis during oogenesis. The level of dome activity is critical for proper border cell migration and is controlled in part through a negative feedback loop. In addition to its essential role in border cells, we show that dome is required in the germarium for the polarization of follicle cells during encapsulation of germline cells. In this process, dome controls the expression of the apical determinant Crumbs. In contrast to the ligand Upd, whose expression is limited to a pair of polar cells at both ends of the egg chamber, dome is expressed in all germline and follicle cells. However, the Dome protein is specifically localized at apicolateral membranes and undergoes ligand-dependent internalization in the follicle cells. dome mutations interact genetically with JAK/STAT pathway genes in border cell migration and abolish the nuclear translocation of Stat92E in vivo. We also show that dome functions downstream of upd and that both the extracellular and intracellular domains of Dome are required for JAK/STAT signaling. Altogether, our data indicate that Dome is an essential receptor molecule for Upd and JAK/STAT signaling during oogenesis.

GP130 stimulation and the maintenance of stem cells

Recent advantages with the cultivation of adult and embryonic stem cells have raised hopes for therapeutic applications of such cells in the treatment of neurodegenerative diseases and cancer. Cultivation of stem cells on feeder cells or treatment of the cells with cytokines is necessary to maintain stem cells in an undifferentiated state and to keep their pluripotency. In particular, the cytokine leukemia inhibitory factor (LIF) has been used to cultivate murine embryonic stem (ES) cells in the absence of feeder cells. For unknown reasons, LIF does not evoke the same effect on rat or human stem cells. This article summarizes what is known about, and the problems associated with, the cultivation of stem cells and suggests experimental strategies that might help to overcome these difficulties.

Evolution of protein kinase signaling from yeast to man

Protein phosphorylation controls many cellular processes, especially those involved in intercellular communication and coordination of complex functions. To explore the evolution of protein phosphorylation, we compared the protein kinase complements ('kinomes') of budding yeast, worm and fly, with known human kinases. We classify kinases into putative orthologous groups with conserved functions and discuss kinase families and pathways that are unique, expanded or lost in each lineage. Fly and human share several kinase families involved in immunity, neurobiology, cell cycle and morphogenesis that are absent from worm, suggesting that these functions might have evolved after the divergence of nematodes from the main metazoan lineage.

Stats: transcriptional control and biological impact

Extracellular proteins bound to cell-surface receptors can change nuclear gene expression patterns in minutes, with far-reaching consequences for development, cell growth and homeostasis. The signal transducer and activator of transcription (STAT) proteins are among the most well studied of the latent cytoplasmic signal-dependent transcription-factor pathways. In addition to several roles in normal cell decisions, dysregulation of STAT function contributes to human disease, making the study of these proteins an important topic of current research.

The fertile field of Drosophila Jak/STAT signalling

The JAK/STAT pathway plays important roles in vertebrate and invertebrate development. The recent cloning and characterisation of the receptor in Drosophila shows that the pathway is conserved across phyla. In this review we describe current knowledge of the pathway and use genome data to discuss what elements are present in Drosophila. We also summarise recent work describing the involvement of the JAK/STAT pathway in oogenesis and spermatogenesis. Interestingly, the JAK/STAT pathway maintains the niche required for germline stem cell maintenance in the testis, providing the first molecular characterisation of a stem cell niche. Drosophila's streamlined pathway offers a simple model to find new elements and analyse the function of existing ones.

Initiating and guiding migration: lessons from border cells

Cell migration occurs in many different contexts. Amoebae and other isolated cells migrate in culture. In animals, 'professional' migratory cells of the immune system constantly survey the body for intruders, whereas other cell types perform specific developmentally regulated migrations. One simple model for the latter type of event is migration of border cells during Drosophila oogenesis. Recent findings have shed light on how border cell fate is induced and on how the migration is guided. This article discusses the implications of these studies and compares (invasive) migration through a tissue with what is known about cells crawling on a flat substratum.

Male germ cell specification and differentiation

Understanding the mechanisms by which the germline is induced and maintained should lead to a broader understanding of the means by which pluripotency is acquired and maintained. In this review, two major aspects of male germ cell development are discussed: underlying mechanisms for induction and maintenance of primordial germ cells and the basic signaling pathways that determine spermatogonial cell fate.

The SOCS box: a tale of destruction and degradation

Although initially identified in the suppressor of cytokine signaling (SOCS) family of proteins, the C-terminal SOCS box has now been identified in more than 40 proteins in nine different families. Growing evidence suggests that the SOCS box, similar to the F-box, acts as a bridge between specific substrate-binding domains and the more generic proteins that comprise a large family of E3 ubiquitin protein ligases. In this way, SOCS proteins regulate protein turnover by targeting proteins for polyubiquitination and, therefore, for proteasome-mediated degradation.

Cytokine signaling in 2002: new surprises in the Jak/Stat pathway

The importance of Jak-Stat pathway signaling in regulating cytokine-dependent gene expression and cellular development/survival is well established. Nevertheless, advances continue to be made in defining Jak-Stat pathway effects on different cellular processes and in different organisms. This review focuses on recent advances in the field and highlights some of the most active areas of Jak-Stat pathway research.

Paracrine signaling through the JAK/STAT pathway activates invasive behavior of ovarian epithelial cells in Drosophila

The JAK/STAT signaling pathway, renowned for its effects on cell proliferation and survival, is constitutively active in various human cancers, including ovarian. We have found that JAK and STAT are required to convert the border cells in the Drosophila ovary from stationary, epithelial cells to migratory, invasive cells. The ligand for this pathway, Unpaired (UPD), is expressed by two central cells within the migratory cell cluster. Mutations in upd or jak cause defects in migration and a reduction in the number of cells recruited to the cluster. Ectopic expression of either UPD or JAK is sufficient to induce extra epithelial cells to migrate. Thus, a localized signal activates the JAK/STAT pathway in neighboring epithelial cells, causing them to become invasive.

Control of stem cell self-renewal in Drosophila spermatogenesis by JAK-STAT signaling

Stem cells, which regenerate tissue by producing differentiating cells, also produce cells that renew the stem cell population. Signals from regulatory microenvironments (niches) are thought to cause stem cells to retain self-renewing potential. However, the molecular characterization of niches remains an important goal. In Drosophila testes, germ line and somatic stem cells attach to a cluster of support cells called the hub. The hub specifically expresses Unpaired, a ligand activating the JAK-STAT (Janus kinase-signal transducer and activator of transcription) signaling cascade. Without JAK-STAT signaling, germ line stem cells differentiate but do not self-renew. Conversely, ectopic JAK-STAT signaling greatly expands both stem cell populations. We conclude that the support cells of the hub signal to adjacent stem cells by activation of the JAK-STAT pathway, thereby defining a niche for stem cell self-renewal.