Syk Is Required for Integrin Signaling in Neutrophils

AbstractThe Syk tyrosine kinase plays a critical role in the signaling machinery of various receptors of the adaptive immune system. Here we show that Syk is also an essential component of integrin signaling in neutrophils. syk−/− neutrophils failed to undergo respiratory burst, degranulation, or spreading in response to proinflammatory stimuli while adherent to immobilized integrin ligands or when stimulated by direct crosslinking of integrins. Signaling from the β1, β2, or β3 integrins was defective in syk−/− cells. Syk colocalized with CD18 during cell spreading and initiated downstream signaling events leading to actin polymerization. Surprisingly, these defects in integrin-mediated activation did not impair the integrin-dependent in vitro or in vivo migration of syk−/− neutrophils or of cells deficient in Src-family kinases. Thus, integrins use different signaling mechanisms to support migration and adherent activation

Down's syndrome-like cardiac developmental defects in embryos of the transchromosomic Tc1 mouse

Aims Cardiac malformations are prevalent in trisomies of human chromosome 21 [Down's syndrome (DS)], affecting normal chamber separation in the developing heart. Efforts to understand the aetiology of these defects have been severely hampered by the absence of an accurate mouse model. Such models have proved challenging to establish because synteny with human chromosome Hsa21 is distributed across three mouse chromosomes. None of those engineered so far accurately models the full range of DS cardiac phenotypes, in particular the profound disruptions resulting from atrioventricular septal defects (AVSDs). Here, we present analysis of the cardiac malformations exhibited by embryos of the transchromosomic mouse line Tc(Hsa21)1TybEmcf (Tc1) which contains more than 90% of chromosome Hsa21 in addition to the normal diploid mouse genome. Methods and results Using high-resolution episcopic microscopy and three-dimensional (3D) modelling, we show that Tc1 embryos exhibit many of the cardiac defects found in DS, including balanced AVSD with single and separate valvar orifices, membranous and muscular ventricular septal defects along with outflow tract and valve leaflet abnormalities. Frequencies of cardiac malformations (ranging from 38 to 55%) are dependent on strain background. In contrast, no comparable cardiac defects were detected in embryos of the more limited mouse trisomy model, Dp(16Cbr1-ORF9)1Rhr (Ts1Rhr), indicating that trisomy of the region syntenic to the Down's syndrome critical region, including the candidate genes DSCAM and DYRK1A, is insufficient to yield DS cardiac abnormalities. Conclusion The Tc1 mouse line provides a suitable model for studying the underlying genetic causes of the DS AVSD cardiac phenotype

A Critical Role for Syk in Signal Transduction and Phagocytosis Mediated by Fcγ Receptors on Macrophages

Receptors on macrophages for the Fc region of IgG (FcγR) mediate a number of responses important for host immunity. Signaling events necessary for these responses are likely initiated by the activation of Src-family and Syk-family tyrosine kinases after FcγR cross-linking. Macrophages derived from Syk-deficient (Syk−) mice were defective in phagocytosis of particles bound by FcγRs, as well as in many FcγR-induced signaling events, including tyrosine phosphorylation of a number of cellular substrates and activation of MAP kinases. In contrast, Syk− macrophages exhibited normal responses to another potent macrophage stimulus, lipopolysaccharide. Phagocytosis of latex beads and Escherichia coli bacteria was also not affected. Syk− macrophages exhibited formation of polymerized actin structures opposing particles bound to the cells by FcγRs (actin cups), but failed to proceed to internalization. Interestingly, inhibitors of phosphatidylinositol 3-kinase also blocked FcγR-mediated phagocytosis at this stage. Thus, PI 3-kinase may participate in a Syk-dependent signaling pathway critical for FcγR-mediated phagocytosis. Macrophages derived from mice deficient for the three members of the Src-family of kinases expressed in these cells, Hck, Fgr, and Lyn, exhibited poor Syk activation upon FcγR engagement, accompanied by a delay in FcγR-mediated phagocytosis. These observations demonstrate that Syk is critical for FcγR-mediated phagocytosis, as well as for signal transduction in macrophages. Additionally, our findings provide evidence to support a model of sequential tyrosine kinase activation by FcγR's analogous to models of signaling by the B and T cell antigen receptors

Syk, c-Src, the αvβ3 integrin, and ITAM immunoreceptors, in concert, regulate osteoclastic bone resorption

In this study, we establish that the tyrosine kinase Syk is essential for osteoclast function in vitro and in vivo. Syk−/− osteoclasts fail to organize their cytoskeleton, and, as such, their bone-resorptive capacity is arrested. This defect results in increased skeletal mass in Syk−/− embryos and dampened basal and stimulated bone resorption in chimeric mice whose osteoclasts lack the kinase. The skeletal impact of Syk deficiency reflects diminished activity of the mature osteoclast and not impaired differentiation. Syk regulates bone resorption by its inclusion with the αvβ3 integrin and c-Src in a signaling complex, which is generated only when αvβ3 is activated. Upon integrin occupancy, c-Src phosphorylates Syk. αvβ3-induced phosphorylation of Syk and the latter's capacity to associate with c-Src is mediated by the immunoreceptor tyrosine-based activation motif (ITAM) proteins Dap12 and FcRγ. Thus, in conjunction with ITAM-bearing proteins, Syk, c-Src, and αvβ3 represent an essential signaling complex in the bone-resorbing osteoclast, and, therefore, each is a candidate therapeutic target

Genetic dissection of triplicated chromosome 21 orthologs yields varying skeletal traits in Down syndrome model mice

Down syndrome (DS) phenotypes result from triplicated genes, but it is generally unknown how specific three copy human chromosome 21 (Hsa21) orthologous genes or interactions between genes affect these traits. A mouse mapping panel genetically dissecting Hsa21 syntenic regions was used to investigate the contributions and interactions triplicated Hsa21 orthologous genes on mouse chromosome 16 (Mmu16). Four-month-old femurs of male and female Dp9Tyb, Dp2Tyb, Dp3Tyb, Dp4Tyb, Dp5Tyb, Dp6Tyb, Ts1Rhr, and Dp1Tyb;Dyrk1a+/+/- mice were analyzed by micro-computed tomography and 3-point bending to assess skeletal structure and mechanical properties. Male and female Dp1Tyb mice, with the entire Hsa21 homologous region of Mmu16 in three copies, display specific bone deficits similar to humans with DS and were used as a baseline comparison for the other strains in the panel. Bone phenotypes varied based on triplicated gene content, sex, and bone compartment. Three copies of Dyrk1a played a sex-specific, essential role in trabecular deficits and may interact with other genes to influence cortical deficits related to DS. Triplicated genes in Dp9Tyb and Dp2Tyb mice improved some skeletal deficits. As triplicated genes may both improve and worsen bone deficits, it is important to understand the interaction between and molecular mechanisms of skeletal alterations affected by these genes

A Critical Role for Syk Protein Tyrosine Kinase in Fc Receptor-Mediated Antigen Presentation and Induction of Dendritic Cell Maturation

AbstractDendritic cells (DCs) are the only APCs capable of initiating adaptive immune responses. The initiation of immune responses requires that DCs 1) internalize and present Ags; and 2) undergo a differentiation process, called "maturation", which transforms DCs into efficient APCs. DC maturation may be initiated by the engagement of different surface receptors, including certain cytokine receptors (such as TNFR), Toll-like receptors, CD40, and FcRs. The early activation events that link receptor engagement and DC maturation are not well characterized. We found that FcR engagement by immune complexes induced the phosphorylation of Syk, a protein tyrosine kinase acting immediately downstream of FcRs. Syk was dispensable for DC differentiation in vitro and in vivo, but was strictly required for immune complexes internalization and subsequent Ag presentation to T lymphocytes. Importantly, Syk was also required for the induction of DC maturation and IL-12 production after FcR engagement, but not after engagement of other surface receptors, such as TNFR or Toll-like receptors. Therefore, protein tyrosine phosphorylation by Syk represents a novel pathway for the induction of DC maturation

Downregulated Wnt/β-catenin signalling in the Down syndrome hippocampus

Pathological mechanisms underlying Down syndrome (DS)/Trisomy 21, including dysregulation of essential signalling processes remain poorly understood. Combining bioinformatics with RNA and protein analysis, we identified downregulation of the Wnt/β-catenin pathway in the hippocampus of adult DS individuals with Alzheimer’s disease and the ‘Tc1’ DS mouse model. Providing a potential underlying molecular pathway, we demonstrate that the chromosome 21 kinase DYRK1A regulates Wnt signalling via a novel bimodal mechanism. Under basal conditions, DYRK1A is a negative regulator of Wnt/β-catenin. Following pathway activation, however, DYRK1A exerts the opposite effect, increasing signalling activity. In summary, we identified downregulation of hippocampal Wnt/β-catenin signalling in DS, possibly mediated by a dose dependent effect of the chromosome 21-encoded kinase DYRK1A. Overall, we propose that dosage imbalance of the Hsa21 gene DYRK1A affects downstream Wnt target genes. Therefore, modulation of Wnt signalling may open unexplored avenues for DS and Alzheimer’s disease treatment