Gas6 is complexed to soluble tyrosine kinase receptor Axl in human blood.

• 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. Gas6 and the TAM receptors have been suggested to be 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 1,000-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, 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.

Subject headings

MEDICIN OCH HÄLSOVETENSKAP  -- Klinisk medicin -- Kardiologi (hsv//swe)

MEDICAL AND HEALTH SCIENCES  -- Clinical Medicine -- Cardiac and Cardiovascular Systems (hsv//eng)

Keyword

Gas6

Axl

receptor tyrosine kinase

TAM

Publication and Content Type

art (subject category)

ref (subject category)