Objective— Gas6 is a member of the vitamin K-dependent protein family. Gas6-deficient mice were found to be resistant to thrombosis because of defective platelet function. Mouse Gas6 was demonstrated to be present in platelets and found to be involved in platelet aggregation. The aim of this study was to investigate the presence of Gas6 in human platelets and plasma and determine its role in platelet function. Methods and Results— The presence of Gas6 in human platelets and plasma was analyzed using sensitive immunologic methods. Mass spectrometry and ELISA were used to identify and quantify Gas6 in plasma. Gas6 was demonstrated to be present in human plasma, at a concentration determined to be 13 to 23 ng/mL (0.16 to 0.28 nM). Furthermore, plasma Gas6 levels were found to be lower in patients administered with warfarin. However, Gas6 was undetectable in human platelets. Conclusions— This is the first report to identify and quantify Gas6 in human plasma. However, Gas6 protein was not detected in human platelets, suggesting that any potential platelet-specific function could be because of Gas6 from the circulation. These findings open up new directions regarding the role of Gas6 in normal and pathophysiological situations such as inflammation, autoimmune disease, thrombosis and arteriosclerosis.

We have investigated the occurrence of mutations in topoisomerase II (DNA gyrase) subunit B(gyrB) and topoisomerase IV subunit E(parE) and the hyperexpression of genes for four efflux pump proteins in 20 previously described, fluoroquinolone-resistant clinical strains of Pseudomonas aeruginosa. Amino acid alterations were found in GyrB in five strains and in ParE in three strains with MIC of norfloxacin > or = 8 mg/L, and it is likely that some of the alterations contribute to the quinolone resistance exhibited by these strains. Seventeen of the 20 strains overproduced mRNA for one or more pump proteins (MexB, MexD, MexF, or MexY), which caused multidrug resistance phenotype in more than half of strains. Two strains were hypermutable and one of them was highly resistant, but the other strain was only moderately resistant.

To cite this article: Ekman C, Stenhoff J, Dahlbä ck B. Gas6 is complexed to the soluble tyrosine kinase receptor Axl in human blood. J Thromb Haemost 2010; 8: 838-44.Summary. Background: The vitamin K-dependent Gas6 protein (product of growth arrest specific gene 6) binds to, and activates the TAM receptor tyrosine kinases Tyro3, Axl and Mer. It has been suggested that Gas6 and the TAM receptors are important for primary platelet functions, but Gas6 cannot be found in human platelets. However, Gas6 is present in human plasma at a concentration of around 0.2 nM, which is a thousand-fold lower than that of the homologous protein S. The Axl and Mer receptors can be cleaved close to the cell membrane, yielding soluble molecules consisting of the extracellular parts of the receptors. Objective: To investigate if soluble Axl (sAxl) is present in human serum and plasma and if Gas6 circulates in complex with sAxl. Methods: We expressed recombinant sAxl, raised antibodies and developed and validated an ELISA for Axl. Serum and plasma were analyzed using ELISAs for Gas6, Axl and sAxl-Gas6 complexes. Serum was gel filtered and fractions analyzed by the different ELISAs to determine if Gas6 in serum is free or complexed. Immunoprecipitation was used to investigate binding between Gas6 and sAxl in serum. Results: sAxl is present in serum and plasma at around 0.6 nM and all Gas6 is bound to sAxl. No complexes between Gas6 and the soluble forms of Mer and Tyro3 could be detected, indicating that sAxl is the physiological binder of Gas6 in human serum. Conclusions: Gas6 in circulation is bound to sAxl suggesting circulating Gas6 to be inhibited and incapable of stimulating the TAM receptors.

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