Janus Kinase-signal Transducer And Activator Of Transcription Signaling Research Articles

Dedifferentiating Spermatogonia Outcompete Somatic Stem Cells for Niche Occupancy in the Drosophila Testis

Differentiating cells can dedifferentiate to replace stem cells in aged or damaged tissues, but the underlying mechanisms are unknown. In the Drosophila testis, a cluster of stromal cells called the hub creates a niche by locally activating Janus kinase-signal transducer and activator of transcription (Jak-STAT) signaling in adjacent germline and somatic stem cells. Here, we establish a system to study spermatogonial dedifferentiation. Ectopically expressing the differentiation factor bag-of-marbles (Bam) removes germline stem cells from the niche. However, withdrawing ectopic Bam causes interconnected spermatogonia to fragment, move into the niche, exchange positions with resident somatic stem cells, and establish contact with the hub. Concomitantly, actin-based protrusions appear on subsets of spermatogonia, suggesting acquired motility. Furthermore, global downregulation of Jak-STAT signaling inhibits dedifferentiation, indicating that normal levels of pathway activation are required to promote movement of spermatogonia into the niche during dedifferentiation, where they outcompete somatic stem cells for niche occupancy.

Filamin B: a scaffold for interferon signalling

When cells become infected with a virus, they mount a coordinated antiviral response, a key component of which is the production of interferons (IFNs). This family of cytokines is present only in vertebrates, and mediates its antiviral effects both directly on infected cells and indirectly by activating other important effectors of the innate immune response, including natural killer cells, dendritic cells and macrophages (Sadler & Williams, 2008). There are three groups of IFNs: type I, type II and the poorly characterized type III. The type I IFNs are produced by virus‐infected cells, whereas the only type II member—IFN‐γ—is expressed in T cells and natural killer cells. The type I IFNs bind to a receptor complex at the cell surface—which is composed of two subunits, IFN‐α receptor 1 (IFNAR1) and IFNAR2—and activate intracellular signalling pathways, the best characterized of which is the Janus kinase (JAK)–signal transducer and activator of transcription (STAT) pathway. JAKs are tyrosine kinases that associate with the receptor complex and phosphorylate STAT proteins, which subsequently translocate to the nucleus and regulate the expression of IFN‐stimulated genes (ISGs; Sadler & Williams, 2008). In addition to JAK–STAT signalling, IFNs activate other intracellular pathways, including mitogen‐activated protein (MAP) kinase pathways. It has been shown recently that the c‐Jun amino‐terminal kinase (JNK) group of MAP kinases can mediate the IFN‐α‐induced apoptosis of cells (Yanase et al , 2005; Jeon et al , 2008). JNK signalling is required for normal development; however, it is also a major regulator of the response of cells to stress (Davis, 2000). The JNK pathway—similar to other MAP kinase pathways—features a core triple‐kinase module consisting of a MAP kinase kinase kinase (MKKK), a MAP kinase kinase (MKK) and JNK. Members of the Rho family of small GTPases—including Rac1 and cell‐division cycle 42 (Cdc42)—feed signals into this module. The …

Multipotent stem cells in the Malpighian tubules of adultDrosophila melanogaster

Excretion is an essential process of an organism's removal of the waste products of metabolism to maintain a constant chemical composition of the body fluids despite changes in the external environment. Excretion is performed by the kidneys in vertebrates and by Malpighian tubules (MTs) in Drosophila. The kidney serves as an excellent model organ to investigate the cellular and molecular mechanisms underlying organogenesis. Mammals and Drosophila share common principles of renal development. Tissue homeostasis, which is accomplished through self-renewal or differentiation of stem cells, is critical for the maintenance of adult tissues throughout the lifetime of an animal. Growing evidence suggests that stem cell self-renewal and differentiation is controlled by both intrinsic and extrinsic factors. Deregulation of stem cell behavior results in cancer formation, tissue degeneration, and premature aging. The mammalian kidney has a low rate of cellular turnover but has a great capacity for tissue regeneration following an ischemic injury. However, there is an ongoing controversy about the source of regenerating cells in the adult kidney that repopulate injured renal tissues. Recently, we identified multipotent stem cells in the MTs of adult Drosophila and found that these stem cells are able to proliferate and differentiate in several types of cells in MTs. Furthermore, we demonstrated that an autocrine JAK-STAT (Janus kinase-signal transducers and activators of transcription) signaling regulates stem cell self-renewal or differentiation of renal stem cells. The Drosophila MTs provide an excellent in vivo system for studying the renal stem cells at cellular and molecular levels. Understanding the molecular mechanisms governing stem cell self-renewal or differentiation in vivo is not only crucial to using stem cells for future regenerative medicine and gene therapy, but it also will increase our understanding of the mechanisms underlying cancer formation, aging and degenerative diseases. Identifying and understanding the cellular processes underlying the development and repair of the mammalian kidney may enable more effective, targeted therapies for acute and chronic kidney diseases in humans.

Ciliary Neurotrophic Factor-Mediated Signaling Regulates Neuronal Versus Glial Differentiation of Retinal Stem Cells/Progenitors by Concentration-Dependent Recruitment of Mitogen-Activated Protein Kinase and Janus Kinase-Signal Transducer and Activator of Transcription Pathways in Conjunction with Notch Signaling

In the retina, as elsewhere in the central nervous system, neurogenesis precedes gliogenesis; that is, the only glia in the retina, Müller cells, are born when the majority of neurons have already been generated. However, our understanding of how the multipotent retinal stem cells/progenitors choose to differentiate along neuronal and glial lineages is unclear. This information is important in promoting directed differentiation of retinal stem cells/progenitors in an ex vivo or in vivo stem cell approach to treating degenerative retinal diseases. Here, using the neurosphere assay, we demonstrate that ciliary neurotrophic factor (CNTF), acting in a concentration-dependent manner, influences the simultaneous differentiation of retinal stem cells/progenitors into neurons or glia. At low CNTF concentrations differentiation of bipolar cells is promoted, whereas high CNTF concentrations facilitate Müller cell differentiation. The two concentrations of CNTF lead to differential activation of mitogen-activated protein kinase and Janus kinase-signal transducer and activator of transcription (Jak-STAT) pathways, with recruitment of the former and the latter for the differentiation of bipolar and Müller cells, respectively. The concentration-dependent recruitment of two disparate pathways toward neurogenesis and gliogenesis occurs in concert with Notch signaling. Furthermore, we demonstrate that the attenuation of Jak-STAT signaling along with Notch signaling facilitates the differentiation of retinal stem cells/progenitors along the rod photoreceptor lineage in vivo. Our observations posit CNTF-mediated signaling as a molecular switch for neuronal versus glial differentiation of retinal stem cells/progenitors and a molecular target for directed neuronal differentiation of retinal stem cells/progenitors as an approach to addressing degenerative changes in the retina. Disclosure of potential conflicts of interest is found at the end of this article.

Interpretation of cytokine signaling through the transcription factors STAT5A and STAT5B

Transcription factors from the family of Signal Transducers and Activators of Transcription (STAT) are activated by numerous cytokines. Two members of this family, STAT5A and STAT5B (collectively called STAT5), have gained prominence in that they are activated by a wide variety of cytokines such as interleukins, erythropoietin, growth hormone, and prolactin. Furthermore, constitutive STAT5 activation is observed in the majority of leukemias and many solid tumors. Inactivation studies in mice as well as human mutations have provided insight into many of STAT5's functions. Disruption of cytokine signaling through STAT5 results in a variety of cell-specific effects, ranging from a defective immune system and impaired erythropoiesis, the complete absence of mammary development during pregnancy, to aberrant liver function. On a molecular level, STAT5 has been linked to cell specification, proliferation, differentiation, and survival. Evidence is growing that the diverse outcomes of STAT5 signaling are not only determined by the expression of specific receptors but also by the interaction of STAT5 with cofactors and the cell-specific activity of members of the SOCS family, which negatively regulate STAT function. In this review, we focus on emerging concepts and challenges in the field of Janus kinase (JAK)-STAT5 signaling. First, we discuss unique functions of STAT5 in three distinct systems: mammary epithelial cells, hepatocytes, and regulatory T cells. Second, we present an example of how STAT5 can achieve cell specificity in hepatocytes through a physical and functional interaction with the glucocorticoid receptor. Third, we focus on the relevance of STAT5 in the development and progression of leukemia. Next, we discuss lessons derived from human mutations and disease. Finally, we address an emerging issue that the interpretation of experiments from STAT5-deficient mice and cells might be compromised as these cells might reroute and reprogram cytokine signals to the "wrong" STATs and thus acquire inappropriate cues. We propose that mice with mutations in various components of the JAK-STAT signaling pathway are living laboratories, which will provide insight into the versatility of signaling hardware and the adaptability of the software.

Identification of Residues Critical for the Interferon Antagonist Function of Langat Virus NS5 Reveals a Role for the RNA-Dependent RNA Polymerase Domain

All pathogenic flaviviruses examined thus far inhibit host interferon (IFN) responses by suppressing the Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway. Both Langat virus (LGTV; a member of the tick-borne encephalitis virus serogroup) and Japanese encephalitis virus use the nonstructural protein NS5 to suppress JAK-STAT signaling. However, NS5 is also critical to virus replication, contributing methyltransferase and RNA-dependent RNA polymerase (RdRP) activities. The specific amino acid residues of NS5 involved in IFN antagonism are not known. Here, we demonstrate that the LGTV NS5 JAK-STAT inhibitory domain is contained between amino acids 355 and 735 (of 903), a range which lies within the RdRP domain. Furthermore, we identified two noncontiguous stretches of specific amino acids within the RdRP, 374 to 380 and 624 to 647, as critical for inhibition of JAK-STAT signaling. Despite considerable separation on the linear NS5 sequence, these residues localized adjacent to each other when modeled on the West Nile virus RdRP crystal structure. Due to the general conservation of RdRP structures, these results suggest that the specific residues identified act cooperatively to form a unique functional site on the RdRP responsible for JAK-STAT inhibition. This insight into the mechanism underlying flavivirus IFN evasion strategies will facilitate the design of antiviral therapeutics that potentiate the action of IFN during infection.

Raft-mediated Src Homology 2 Domain-containing Proteintyrosine Phosphatase 2 (SHP-2) Regulation in Microglia

Janus kinase-signal transducer and activator of transcription (JAK-STAT) signals play important roles in cell proliferation, apoptosis, and inflammation, and they recently have been considered as therapeutic targets for suppressing oncogenesis and inflammatory process. Phosphatases including Src homology 2 domain-containing protein-tyrosine phosphatases (SHPs), are well known as negative regulators of the JAK-STAT pathway, but their precise mechanisms are largely unknown. Based on our previous finding that in cultured rat brain microglia, gangliosides induce rapid and transient activation of the JAK-STAT pathway, we hypothesized that raft-mediated SHP-2 activation is involved in transient activation of JAK-STAT signaling by gangliosides. We first used Western blot analysis to confirm that gangliosides rapidly induce the phosphorylation of SHP-2. This was inhibited by pretreatment with the lipid raft disrupter filipin and was restored following filipin removal. Immunostaining using antibodies directed against p-SHP-2 and flotillin-1 revealed ganglioside-induced clustering and polarization of p-SHP-2 in membrane rafts. Raft-associated regulation of SHP-2 was further demonstrated in fractionation experiments using detergent and detergent-free sucrose gradient ultracentrifugation. Rapid SHP-2 recruitment to detergent-insoluble raft fractions by gangliosides was inhibited by filipin, further indicating the involvement of rafts. We also confirmed by immunoprecipitation that SHP-2 rapidly binds in a raft-dependent manner to JAK2 in response to gangliosides. Our study therefore showed that transient activation of the JAK-STAT pathway by gangliosides is accomplished by SHP-2 in a raft-dependent manner in brain microglia.

Activation of the JAK-STAT signaling pathway in the rat basilar artery after subarachnoid hemorrhage

The Janus kinase-signal transducer and activator of transcription (JAK-STAT) is one of the most important signaling pathways transducing signals from the cell surface in response to cytokines. Subarachnoid hemorrhage (SAH) produces cytokines in the CSF. We investigated whether this signaling pathway is activated in the rat basilar artery after SAH by cytokines. In a rat single-hemorrhage model of SAH, basilar arteries and CSF were obtained until 7 days after SAH. The concentration of interleukin-6 (IL-6) in CSF was measured by ELISA. Western blot analysis with JAK1, phosphospecific-JAK1, STAT3, phosphospecific STAT3 at Tyr 705 and Ser 727, cyclooxygenase-2 (COX-2), and actin antibodies was performed in basilar artery. The expressions of STAT3, phosphospecific STAT3 at Tyr 705 and Ser 727, and COX-2 in basilar artery were examined by immunohistochemical studies. The concentration of IL-6 immediately increased after SAH and Western blot analysis revealed that JAK1 was phosphorylated within 2 h, accompanied by phosphorylation of STAT3 at Tyr 705, extending to Ser 727 at days 1–2. Immunohistochemistry revealed phosphorylation of STAT3 to occur in endothelial and smooth muscle cells of the basilar artery. In addition, intracisternal injection of IL-6 by itself significantly increased phosphorylation of STAT3 at Tyr 705 and Ser 727. Expression of COX-2 was also upregulated in endothelial cells of the basilar artery. These results indicate that SAH produces the proinflammatory cytokine IL-6 in the CSF, which activates the JAK-STAT signaling pathway in the basilar artery and induces transcription of immediate early genes.

Inhibition of Interferon-Stimulated JAK-STAT Signaling by a Tick-Borne Flavivirus and Identification of NS5 as an Interferon Antagonist

The tick-borne encephalitis (TBE) complex of viruses, genus Flavivirus, can cause severe encephalitis, meningitis, and/or hemorrhagic fevers. Effective interferon (IFN) responses are critical to recovery from infection with flaviviruses, and the mosquito-borne flaviviruses can inhibit this response. However, little is known about interactions between IFN signaling and TBE viruses. Langat virus (LGTV), a member of the TBE complex of viruses, was found to be highly sensitive to the antiviral effects of IFN. However, LGTV infection inhibited IFN-induced expression of a reporter gene driven by either IFN-alpha/beta- or IFN-gamma-responsive promoters. This indicated that LGTV can inhibit the IFN-mediated JAK-STAT (Janus kinase-signal transducer and activator of transcription) pathway of signal transduction. The mechanism of inhibition was due to blocks in the phosphorylation of both Janus kinases, Jak1 and Tyk2, during IFN-alpha signaling and at least a failure of Jak1 phosphorylation following IFN-gamma stimulation. To determine the viral protein(s) responsible, we individually expressed all nonstructural (NS) proteins and examined their ability to inhibit signal transduction. Expression of NS5 alone inhibited STAT1 phosphorylation in response to IFN, thus identifying NS5 as a potential IFN antagonist. Examination of interactions between NS5 and cellular proteins revealed that NS5 associated with IFN-alpha/beta and -gamma receptor complexes. Importantly, inhibition of JAK-STAT signaling and NS5-IFN receptor interactions were demonstrated in LGTV-infected human monocyte-derived dendritic cells, important target cells for early virus replication. Because NS5 may interfere with both innate and acquired immune responses to virus infection, this protein may have a significant role in viral pathogenesis.

In silico identification of the key components and steps in IFN-γ induced JAK-STAT signaling pathway

Systems biology efforts are increasingly adopting quantitative, mechanistic modeling to study cellular signal transduction pathways and other networks. However, it is uncertain whether the particular set of kinetic parameter values of the model closely approximates the corresponding biological system. We propose that the parameters be assigned statistical distributions that reflect the degree of uncertainty for a comprehensive simulation analysis. From this analysis, we globally identify the key components and steps in signal transduction networks at a systems level. We investigated a recent mathematical model of interferon gamma induced Janus kinase-signal transducers and activators of transcription (JAK-STAT) signaling pathway by applying multi-parametric sensitivity analysis that is based on simultaneous variation of the parameter values. We find that suppressor of cytokine signaling-1, nuclear phosphatases, cytoplasmic STAT1, and the corresponding reaction steps are sensitive perturbation points of this pathway.

JAK/STAT-Dependent Gene Regulation by Cytokines

The Janus kinase-signal transducer and activator of transcription (Jak-STAT) pathway is essential for the signal transduction of many cytokines. Dysregulation of Jak-STAT signaling is associated with various human diseases. Recent studies have helped to shed some light on regulatory mechanisms that modify quantity and quality of the signaling response. Here, we summarize our current knowledge on Jak-STAT signaling.

Regeneration of male germline stem cells by spermatogonial dedifferentiation in vivo.

Although the ability of engrafted stem cells to regenerate tissue has received much attention, the molecular mechanisms controlling regeneration are poorly understood. In the Drosophila male germline, local activation of the Janus kinase-signal transducer and activator of transcription (Jak-STAT) pathway maintains stem cells; germline stem cells lacking Jak-STAT signaling differentiate into spermatogonia without self-renewal. By conditionally manipulating Jak-STAT signaling, we find that spermatogonia that have initiated differentiation and are undergoing limited mitotic (transit-amplifying) divisions can repopulate the niche and revert to stem cell identity. Thus, in the appropriate microenvironment, transit-amplifying cells dedifferentiate, becoming functional stem cells during tissue regeneration.

Regulation of JAK-STAT signalling in the immune system.

The cytokine-activated Janus kinase (JAK)–signal transducer and activator of transcription (STAT) pathway has an important role in the control of immune responses. Dysregulation of JAK–STAT signalling is associated with various immune disorders. The signalling strength, kinetics and specificity of the JAK–STAT pathway are modulated at many levels by distinct regulatory proteins. Here, we review recent studies on the regulation of the JAK–STAT pathway that will enhance our ability to design rational therapeutic strategies for immune diseases.

The JAK-STAT Pathway

The Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway transmits information received from extracellular polypeptide signals, through transmembrane receptors, directly to transcription factors that bind target gene promoters in the nucleus. This process provides a mechanism for transcriptional regulation without second messengers. Evolutionarily conserved in eukaryotic organisms from slime molds to humans, JAK-STAT signaling appears to be an early adaptation to facilitate intercellular communication that has coevolved with myriad cellular signaling events. This coevolution has given rise to highly adapted, ligand-specific signaling pathways that control gene expression. In addition, the JAK-STAT signaling pathways are regulated by a vast array of intrinsic and environmental stimuli, which can add plasticity to the response of a cell or tissue. The Connections Map provides a generalized view of this JAK-STAT signaling, with additional Connections Maps highlighting the components that are activated by type I interferons (IFN-α/β), IFN-γ, and interleukins and growth factors that activate STAT3. Science Viewpoint D. S. Aaronson, C. M. Horvath, A road map for those who don't know JAK-STAT. Science 296 , 1653-1655 (2002).[ Abstract ] [ Full Text ]

The JAK-STAT Pathway

The Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway transmits information received from extracellular polypeptide signals, through transmembrane receptors, directly to transcription factors that bind target gene promoters in the nucleus. This process provides a mechanism for transcriptional regulation without second messengers. Evolutionarily conserved in eukaryotic organisms from slime molds to humans, JAK-STAT signaling appears to be an early adaptation to facilitate intercellular communication that has coevolved with myriad cellular signaling events. This coevolution has given rise to highly adapted, ligand-specific signaling pathways that control gene expression. In addition, the JAK-STAT signaling pathways are regulated by a vast array of intrinsic and environmental stimuli, which can add plasticity to the response of a cell or tissue. The Connections Map provides a generalized view of this JAK-STAT signaling, with additional Connections Maps highlighting the components that are activated by type I interferons (IFN-α/β), IFN-γ, and interleukins and growth factors that activate STAT3.

A Novel Function for a Glucose Analog of Blood Group H Antigen as a Mediator of Leukocyte-Endothelial Adhesion via Intracellular Adhesion Molecule 1

The 4A11 antigen is a unique cytokine-inducible antigen up-regulated on rheumatoid arthritis synovial endothelium compared with normal endothelium. In soluble form, this antigen, Lewisy-6/H-5-2 (Ley/H), or its glucose analog, 2-fucosyllactose (H-2g), mediates angiogenesis. The Ley/H antigen is structurally related to the soluble E-selectin ligand, sialyl Lewisx, and is selectively expressed in skin, lymphoid organs, thymus, and synovium, suggesting that it may be important in leukocyte homing or adhesion. In the present study, we used H-2g as a functional substitute to demonstrate a novel property for Ley/H antigen in inducing leukocyte-endothelial adhesion. H-2g significantly enhanced the expression of human dermal microvascular endothelial cells (HMVECs) intercellular adhesion molecule-1 (ICAM-1), but not vascular cell adhesion molecule-1, E-selectin, and P-selectin. Immunoprecipitation and Western blotting showed glycolipids Ley-6, H-5-2, or the glucose analog H-2g quickly activated human microvascular endothelial cell line-1 (HMEC-1) Janus kinase 2 (JAK2) and that the JAK2 inhibitor, AG-490, completely inhibited HMVEC ICAM-1 expression and HL-60 adhesion to HMEC-1s. Use of a JAK/signal transducer and activator of transcription (STAT) profiling system confirmed that H-2g selectively activated STAT3 but not STAT1 and STAT2. AG-490 inhibited H-2g-induced Erk1/2 and PI3K-Akt activation, suggesting that JAK2 is upstream of the Erk1/2 and PI3K-Akt pathways. Furthermore, the JAK2 inhibitor AG-490, the Erk1/2 inhibitor PD98059, or the phosphatidylinositol 3-kinase inhibitor LY294002 or antisense oligodeoxynucleotides directed against JAK2, Erk1/2, or phosphatidylinositol 3-kinase blocked H-2g-induced HMVEC ICAM-1 expression and HL-60 adhesion to HMEC-1s. Hence, H-2g signals through JAK2 and its downstream signal transducers STAT3, Erk1/2, and phosphatidylinositol 3-kinase result in ICAM-1 expression and cell adhesion. Potential treatment strategies through the inhibition of JAK-dependent pathways to target H-2g signals may provide a useful approach in inflammation-driven diseases like rheumatoid arthritis.

A road map for those who don't know JAK-STAT.

The Janus kinase-signal transducer and activator of transcription (JAK-STAT) pathway transmits information received from extracellular polypeptide signals, through transmembrane receptors, directly to target gene promoters in the nucleus, providing a mechanism for transcriptional regulation without second messengers. Evolutionarily conserved in eukaryotic organisms from slime molds to humans, JAK-STAT signaling appears to be an early adaptation to facilitate intercellular communication that has co-evolved with myriad cellular signaling events. This co-evolution has given rise to highly adapted, ligand-specific signaling pathways that control gene expression. In addition, the JAK-STAT signaling pathways are regulated by a vast array of intrinsic and environmental stimuli, which can add plasticity to the response of a cell or tissue.

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.

Ciliary Neurotrophic Factor and Stress Stimuli Activate the Jak-STAT Pathway in Retinal Neurons and Glia

Ciliary neurotrophic factor (CNTF) is pleiotrophic for central, peripheral, and sensory neurons. In the mature retina, CNTF treatment enhances survival of retinal ganglion and photoreceptor cells exposed to otherwise lethal perturbation. To understand its mechanism of action in vivo, the adult rat retina was used as a model to investigate CNTF-mediated activation of Janus kinase/signal transducer and activator of transcription (Jak-STAT) and ras-mitogen activated protein kinase (ras-MAPK). Intravitreal injection of Axokine, an analog of CNTF, phosphorylates STAT3 and MAPK and produces delayed upregulation of total STAT3 and STAT1 protein in rat retina. Activated STAT3 is predominantly localized in nuclei of retinal Müller (glial) cells, ganglion cells, and astrocytes, but not in photoreceptors. Although CNTF alpha-receptor (CNTFRalpha) mRNA and protein are localized predominantly if not exclusively in retinal neurons, coincident CNTF-mediated STAT3 signaling was observed in both glia and neurons. CNTF-induced activation of Jak-STAT signaling prompted us to investigate STAT3 phosphorylation after a variety of stress-mediated, conditioning stimuli. We show that STAT3 is activated in the retina after exposure to subtoxic bright light, mechanical trauma, and systemic administration of the alpha(2)-adrenergic agonist xylazine, all of which have been shown previously to condition photoreceptors to resist light-induced degeneration. These results demonstrate that CNTF directly stimulates Jak-STAT and ras-MAPK cascades in vivo and strongly suggest that STAT3 signaling is an underlying component of neural responsiveness to stress stimuli. The observation that CNTF activates STAT3 in ganglion cells, but not in photoreceptors, suggests that Jak-STAT signaling influences neuronal survival via both direct and indirect modes of action.

Regulation of gliogenesis in the central nervous system by the JAK-STAT signaling pathway.

A mechanism by which members of the ciliary neurotrophic factor (CNTF)-leukemia inhibitory factor cytokine family regulate gliogenesis in the developing mammalian central nervous system was characterized. Activation of the CNTF receptor promoted differentiation of cerebral cortical precursor cells into astrocytes and inhibited differentiation of cortical precursors along a neuronal lineage. Although CNTF stimulated both the Janus kinase-signal transducer and activator of transcription (JAK-STAT) and Ras-mitogen-activated protein kinase signaling pathways in cortical precursor cells, the JAK-STAT signaling pathway selectively enhanced differentiation of these precursors along a glial lineage. These findings suggest that cytokine activation of the JAK-STAT signaling pathway may be a mechanism by which cell fate is controlled during mammalian development.