STAT3 but Not STAT1 Is Required for Astrocyte Differentiation

The JAK-STAT signaling pathway has been implicated in astrocyte differentiation. Both STAT1 and STAT3 are expressed in the central nervous system and are thought to be important for glial differentiation, as mainly demonstrated in vitro; however direct in vivo evidence is missing. We investigated whether STAT1 and STAT3 are essential for astrocyte development by testing the STAT responsiveness of astrocyte progenitors. STAT3 was absent in the ventricular zone where glial progenitors are born but begins to appear at the marginal zone at E16.5. At E18.5, both phospho-STAT1 and phospho-STAT3 were present in glial fibrillary acidic protein (GFAP)-expressing white matter astrocytes. Overexpression of STAT3 by electroporation of chicks in ovo induced increased numbers of astrocyte progenitors in the spinal cord. Likewise, elimination of STAT3 in Stat3 conditional knockout (cKO) mice resulted in depletion of white matter astrocytes. Interestingly, elimination of STAT1 in Stat1 null mice did not inhibit astrocyte differentiation and deletion of Stat1 failed to aggravate the glial defects in Stat3 cKO mice. Measuring the activity of STAT binding elements and the gfap promoter in the presence of various STAT mutants revealed that transactivation depended on the activity of STAT3 not STAT1. No synergistic interaction between STAT1 and STAT3 was observed. Cortical progenitors of Stat1 null; Stat3 cKO mice generated astrocytes when STAT3 or the splice variant Stat3β was supplied, but not when STAT1 was introduced. Together, our results suggest that STAT3 is necessary and sufficient for astrocyte differentiation whereas STAT1 is dispensable.     

Adenovirus sequesters phosphorylated STAT1 at viral replication centers and inhibits STAT dephosphorylation

Tyrosine phosphorylation and nuclear translocation of STAT1 indicate activation of interferon (IFN) signal transduction pathways. Here, we demonstrate that tyrosine-phosphorylated STAT1 is targeted by a unique mechanism in adenovirus (Ad)-infected cells. Ad is known to suppress IFN-inducible gene expression; however, we observed that Ad infection prolongs the tyrosine phosphorylation of STAT1 induced by alpha IFN in infected cells. To understand this paradoxical effect, we examined the subcellular localization of STAT1 following Ad infection and found that nuclear, tyrosine-phosphorylated STAT1 accumulates at viral replication centers. This form of STAT1 colocalized with newly synthesized viral DNA. Viral DNA replication, but not viral late gene expression, is required for the regulation of STAT1 phosphorylation. Our results indicate that Ad infection regulates STAT1 dephosphorylation rather than STAT1 phosphorylation. Consistent with this idea, we show that Ad infection disrupts the interaction between STAT1 and its cognate protein tyrosine phosphatase, TC45. Our findings indicate that Ad sequesters phosphorylated STAT1 at viral replication centers and inhibits STAT dephosphorylation. This report suggests a strategy employed by Ad to counteract an active form of STAT1 in the nucleus of infected cells.     

Cell swelling activates STAT1 and STAT3 proteins in cultured rat hepatocytes

According to the known primary sequences of three neurotoxins active on small conductance Ca2+-activated potassium channels from the scorpion Buthus martensi Karsch, their corresponding cDNAs were cloned and sequenced using 3'- and 5'-RACE. All of them encoded a signal peptide composed of 28 residues and a mature toxin of 29, 28 and 33 residues, respectively. Their cDNA deduced sequences were totally consistent with those determined, and the C-terminal amidation of one neurotoxin was confirmed. The genomic DNAs of these three toxins were also amplified by PCR, cloned and sequenced. They all consisted of two exons disrupted by a small single intron. All of these introns were inserted within the signal peptide at the same -10 position upstream from the mature toxin, consisting of 94, 78 and 87 bp, respectively.     

Rotavirus NSP1 Protein Inhibits Interferon-Mediated STAT1 Activation

Rotavirus (RV) replicates efficiently in intestinal epithelial cells (IECs) in vivo despite the activation of a local host interferon (IFN) response. Previously, we demonstrated that homologous RV efficiently inhibits IFN induction in single infected and bystander villous IECs in vivo. Paradoxically, RV also induces significant type I IFN expression in the intestinal hematopoietic cell compartment in a relatively replication-independent manner. This suggests that RV replication and spread in IECs must occur despite exogenous stimulation of the STAT1-mediated IFN signaling pathway. Here we report that RV inhibits IFN-mediated STAT1 tyrosine 701 phosphorylation in human IECs in vitro and identify RV NSP1 as a direct inhibitor of the pathway. Infection of human HT29 IECs with simian (RRV) or porcine (SB1A or OSU) RV strains, which inhibit IFN induction by targeting either IFN regulatory factor 3 (IRF3) or NF-κB, respectively, resulted in similar regulation of IFN secretion. By flow cytometric analysis at early times during infection, neither RRV nor SB1A effectively inhibited the activation of Y701-STAT1 in response to exogenously added IFN. However, at later times during infection, both RV strains efficiently inhibited IFN-mediated STAT1 activation within virus-infected cells, indicating that RV encodes inhibitors of IFN signaling targeting STAT1 phosphorylation. Expression of RV NSP1 in the absence of other viral proteins resulted in blockage of exogenous IFN-mediated STAT1 phosphorylation, and this function was conserved in NSP1 from simian, bovine, and murine RV strains. Analysis of NSP1 determinants responsible for the inhibition of IFN induction and signaling pathways revealed that these determinants are encoded on discrete domains of NSP1. Finally, we observed that at later times during infection with SB1A, there was almost complete inhibition of IFN-mediated Y701-STAT1 in bystander cells staining negative for viral antigen. This property segregated with the NSP1 gene and was observed in a simian SA11 monoreassortant that encoded porcine OSU NSP1 but not in wild-type SA11 or a reassortant encoding simian RRV NSP1.     

Implication of STAT1 and STAT3 transcription factors in the response to superantigens

The activation of STAT1 and STAT3 in response to SEB was analyzed in spleen of Balb/c mice. The intraperitoneal injection of the superantigen SEB activated STAT1 and STAT3 in spleen. Activated STAT1 almost completely disappeared in 24 h even though activated STAT3 was present for more than 48 h after SEB injection. Cyclosporine A was able to block the initial STAT1 activation, but STAT3 activation was only partially affected. SEB also increased the mRNA levels for STAT1, STAT3 and SOCS1. When a second injection with SEB was given 72 h after the first stimulus, STAT1 activation was much lower than that observed after the first stimulation with SEB and no increase in the STAT1 mRNA level was observed. Nevertheless, after this second injection, STAT3 was again activated without any significant interference from the first stimulus and the STAT3 and SOCS1 mRNA levels again increased. These data indicate that a first stimulation with superantigen re-programs cells so that they respond to a second stimulation in a different way. Understanding the mechanisms implicated in this re-programming is basic for designing therapeutic strategies in processes such as septic shock.