An invertebrate signal transducer and activator of transcription 5 (STAT5) ortholog from the disk abalone, Haliotis discus discus: Genomic structure, early developmental expression, and immune responses to bacterial and viral stresses

Signal transducer and activator of transcription (STAT) family members are key signaling molecules that transduce cellular responses from the cell membrane to the nucleus upon Janus kinase (JAK) activation. Although seven STAT members have been reported in mammals, very limited information on STAT genes in molluscans is available. In this study, we identified and characterized a STAT paralog that is homologous to STAT5 from the disk abalone, Haliotis discus discus, and designated as AbSTAT5. Comparison of the deduced amino acid sequence for AbSTAT5 (790 amino acids) with other counterparts revealed conserved residues important for functions and typical domain regions, including the N-terminal domain, coiled-coil domain, DNA-binding domain, linker domain, and Src homology 2 (SH2) domains as mammalian counterparts. Analysis of STAT phylogeny revealed that AbSTAT5 was clustered with the molluscan subgroup in STAT5 clade with distinct evolution. According to the genomic structure of AbSTAT5, the coding sequence was distributed into 20 exons with 19 introns. Immunologically essential transcription factor-binding sites, such as GATA-1, HNF, SP1, C/EBP, Oct-1, AP1, c-Jun, and Sox-2, were predicted at the 5'-proximal region of AbSTAT5. Expression of AbSTAT5 mRNA was detected in different stages of embryonic development and observed at considerably higher levels in the morula and late veliger stages. Tissue-specific expressional studies revealed that the highest level of AbSTAT5 transcripts was detected in hemocytes, followed by gill tissues. Temporal expressions of AbSTAT5 were analyzed upon live bacterial (Vibrio parahemolyticus and Listeria monocytogenes), viral (viral hemorrhagic septicemia virus), and pathogen-associated molecular pattern (lipopolysaccharides and Poly I:C) stimulations, and significant elevations indicated immune modulation. These results suggest that AbSTAT5 may be involved in maintaining innate immune responses from developmental to adult stages in the disk abalone. Further, this study provides a basis for structural and functional exploration of STAT members in the invertebrate JAK/STAT signaling pathway.

MsSOCS expression indicates a potential role for JAK/STAT signalling in the early stages of Manduca sexta spermatogenesis

Our understanding of the Drosophila melanogaster testis stem cell niche has identified the signalling pathways required to maintain stem cells and promote self-renewal. Here we present the first detailed examination of the testes stem cell niche in the lepidopteran Manduca sexta . We show that larval testes contain hub-like structures surrounded by mitotically active cells likely to represent a self-renewing stem cell population. In addition, we have cloned DNA fragments encoding parts of M. sexta Signal Transducer and Activators of Transcription (STAT) and Suppressor of Cytokine Signalling (SOCS) homologues and show that expression of MsSOCS is upregulated in hub-adjacent cells. Given the conservation of socs-like genes and their regulation as Janus Kinases/Signal Transducer and Activators Transcription (JAK/STAT) pathway targets, we suggest that increased expression within the testis stem cells indicates increased levels of JAK/STAT signalling and a conserved role for pathway signalling in testis stem cell maintenance.

Structure, function, and regulation of STAT proteins

The Signal Transducer and Activator of Transcription (STAT) family of proteins was first discovered in the 1990's as key proteins in cytokine signaling. Since then, the field has greatly advanced in the past 15 years, providing significant insight into the structure, function, and regulation of STATs. STATs are latent cytoplasmic transcription factors consisting of seven mammalian members. They are Tyr phosphorylated upon activation, a post-translational modification critical for dimerization, nuclear import, DNA binding, and transcriptional activation. In recent years, unphosphorylated STATs have also been observed to dimerize and drive transcription, albeit by yet an obscure mechanism. In addition, the function of cytoplasmic STATs is beginning to emerge. Here, we describe the structure, function, and regulation of both unphosphorylated and phosphorylated STATs. STAT isoforms from alternative splicing or proteolytic processing, and post-translational modifications affecting STAT activities are also discussed.

Characterisation of Upd2, a Drosophila JAK/STAT pathway ligand

The characterisation of ligands that activate the JAK/STAT pathway has the potential to throw light onto a comparatively poorly understood aspect of this important signal transduction cascade. Here, we describe our analysis of the only invertebrate JAK/STAT pathway ligands identified to date, the Drosophila unpaired-like family. We show that upd2 is expressed in a pattern essentially identical to that of upd and demonstrate that the proteins encoded by this region activate JAK/STAT pathway signalling. Mutational analysis demonstrates a mutual semi-redundancy that can be visualised in multiple tissues known to require JAK/STAT signalling. In order to better characterise the in vivo function of these ligands, we developed a reporter based on a natural JAK/STAT pathway responsive enhancer and show that ectopic upd2 expression can effectively activate the JAK/STAT pathway. While both Upd and Upd2 are secreted JAK/STAT pathway agonists, tissue culture assays show that the signal-sequences of Upd and Upd2 confer distinct properties, with Upd associated primarily with the extracellular matrix and Upd2 secreted into the media. The differing biophysical characteristics identified for Upd-like molecules have implications for their function in vivo and adds another aspect to our understanding of cytokine signalling in Drosophila.

Identification of the linker-SH2 domain of STAT as the origin of the SH2 domain using two-dimensional structural alignment

The availability of large volumes of genomic sequences presents an unprecedented proteomic challenge to characterize the structure and function of various protein motifs. Primary structural alignment is often unable to accurately identify a given motif due to sequence divergence; however, with the aid of secondary structural prediction for analysis, it becomes feasible to explore protein motifs on a proteome-wide scale. Here we report the use of secondary structural alignment to characterize the Src homology 2 (SH2) domains of both conventional and divergent sequences and divide them into two groups, Src-type and STAT-type. In addition to the basic "alphabetabetabetaalpha" structure (betaBeta), the Src-type SH2 domain contains an extra beta-strand (betaE or betaE-betaF motif). Alternatively, the linker domain-conjugated SH2 domain in STAT contains the alphaB' motif. Combining BLAST data from betaBeta core motif sequences with predicted secondary structural alignment, we have screened for SH2 domains in various eukaryotic model systems including Arabidopsis, Dictyostelium, and Saccharomyces. Two novel genes carrying the linker-SH2 domain of STAT were discovered and subsequently cloned from Arabidopsis. These genes, designated as STAT-type linker-SH2 domain factors (STATL), are found in a wide array of vascular and nonvascular plants, suggesting that the linker-SH2 domain evolved prior to the divergence of plants and animals. Using this approach, we expanded the number of putative SH2 domain-bearing genes in Dictyostelium and comparatively studied the secondary structural profiles of both typical and atypical SH2 domains. Our results indicate that the linker-SH2 domain of the transcription factor STAT is one of the most ancient and fully developed functional domains, serving as a template for the continuing evolution of the SH2 domain essential for phosphotyrosine signal transduction.

Molecular characterization and IFN signal pathway analysis of Carassius auratus CaSTAT1 identified from the cultured cells in response to virus infection

Type I interferon (IFN) exerts its pleiotropic effects mainly through the JAK-STAT signaling pathway, which is presently best described in mammals. By subtractive suppression hybridization, two fish signaling factors, JAK1 and STAT1, had been identified in the IFN-induced crucian carp Carassius auratus L. blastulae embryonic (CAB) cells after treatment with UV-inactivated grass carp hemorrhagic virus (GCHV). Further, the full-length cDNA of STAT1, termed CaSTAT1, was obtained. It contains 2926 bp and encodes a protein of 718 aa. CaSTAT1 is most similar to rat STAT1 with 59% identity overall and displays all highly conserved domains that the STAT family possesses. Like human STAT1beta, it lacks the C-terminus acting as transcriptional activation domain in mammals. By contrast, only a single transcript was detected in virus-induced CAB cells. Expression analysis showed that CaSTAT1 could be activated by stimulation of CAB cells with poly I:C, active GCHV, UV-inactivated GCHV or CAB IFN, and displayed diverse expression patterns similar to that of mammalian STAT1. Additionally, the expression of an antiviral gene CaMx1 was also induced under the same conditions, and expression difference between CaSTAT1 and CaMx1 was revealed by induction of CAB IFN. These results provide molecular evidence supporting the notion that the fish IFN signaling transduction pathway is similar to that in mammals. Fish IFN exerts its multiple functions, at least antiviral action, through a JAK-STAT pathway.

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.

Focal adhesion kinase activates Stat1 in integrin-mediated cell migration and adhesion

Recent studies suggest that focal adhesion kinase (FAK) is important for cell migration. We now suggest a mechanism by which FAK activates the signal transducer and activator of transcription (STAT) pathway, regulating cell adhesion and migration. In particular, we observe that FAK is capable of activating Stat1, but not Stat3. Co-immunoprecipitation and in vitro binding assays demonstrate that Stat1 is transiently and directly associated with FAK during cell adhesion, and Stat1 is activated in this process. FAK with a C-terminal deletion (FAKDeltaC14) completely abolishes this interaction, indicating this association is dependent on the C-terminal domain of FAK, which is required for FAK localization at focal contacts. Moreover, Stat1 activation during cell adhesion is diminished in FAK-deficient cells, correlating with decreased migration in these cells. Finally, we show that depletion of Stat1 results in an enhancement of cell adhesion and a decrease in cell migration. Thus, our results have demonstrated, for the first time, a critical signaling pathway from integrin/FAK to Stat1 that reduces cell adhesion and promotes cell migration.

STAT family of transcription factors in cytokine-mediated biological responses

STAT (signal transducers and activators of transcription) family, consisting of seven members, is involved in cytokine signal transduction. Biological roles of each STAT family protein have now been elucidated through studies of gene targeted mice. Stat1 knockout mice are defective in interferon-mediated functions. Stat4 and Stat6 knockout mice show defective responses to IL-12 and IL-4, respectively. Analyses of Stat5a and Stat5b knockout mice reveal important roles in prolactin-mediated mammary gland development and growth hormone-mediated induction of sexual dimorphism, respectively. Conditional knockout study of Stat3 demonstrates its critical roles in cytokine-mediated functions in several tissues, including T cells, macrophages, skin, and mammary gland.

The roles of the Drosophila JAK/STAT pathway

The JAK/STAT signal transduction pathway has been conserved throughout evolution such that true structural and functional homologues of components originally identified in vertebrate systems are also present in the model genetic system Drosophila melanogaster. In addition to roles during larval hematopoiesis reminiscent of the requirement for this pathway in mammalian systems, the JAK/STAT pathway in Drosophila is also involved in a number of other developmental events. Recent data has demonstrated further roles for the JAK/STAT pathway in the establishment of sexual identity via the early embryonic expression of Sex lethal, the segmentation of the embryo via the control of pair rule genes including even skipped and the establishment of polarity within the adult compound eye via a mechanism that includes the four jointed gene. Use of the powerful genetics in the model organism Drosophila may identify new components of the JAK/STAT pathway, define new roles for this pathway, and provide insights into the function of this signal transduction system. Here we review the roles of STAT and its associated signaling pathway during both embryonic and adult stages of Drosophila development and discuss future prospects for the identification and characterization of novel pathway components and targets. Oncogene (2000).