JAK2, the JAK2 V617F mutant and cytokine receptors.

Recently, a unique recurrent somatic mutation was identified as a major molecular event in polycythemia vera, essential thrombocythemia and idiopathic myelofibrosis. Expression of this mutant in cytokine-dependent hematopoietic cell lines induces autonomous growth. This effect is enhanced by overexpression of cytokine receptors, and can be inhibited by co-expression at higher levels of the wild type JAK2, which may compete for a limited pool of receptors. In JAK2-deficient cells, we showed that JAK2 V617F can transmit signals from ligand-activated TpoR or EpoR. Furthermore, the mutant JAK2 can be demonstrated to stimulate traffic of the EpoR. Thus, JAK2 V617F mutant must be able to interact via its intact FERM-SH2 domains with the cytosolic domains of cytokine receptors. A synergy between JAK2 V617F and insulin-like growth factor 1 receptor (IGF1R) can be detected in cytokine-dependent cell proliferation. Once cells are rendered autonomous by expression of JAK2 V617F, IGF1 acquires the ability to activate the JAK-STAT pathway. Thus, expression of JAK2 V617F may explain the described hypersensitivity of PV erythroid progenitors to IGF1. The V617 is conserved in two other mammalian JAKs, JAK1 and Tyk2. The homologous mutants JAK1 V658F and Tyk2 V678F are also active in proliferation and transcriptional assays. Such mutants may be found in human cancers or autoimmune diseases. In contrast, the JAK3 M592F does not lead to activation of JAK3. Current hypotheses on how JAK2 V617F contributes to three myeloproliferative diseases, and which other events may favor one disease versus another, are discussed.

Studies of the ligand binding to cholera toxin, II. The hydrophilic moiety of sialoglycolipids.

The binding between cholera toxin or its B-protein subunit and various ganglioside-related oligosaccharides was studied by equilibrium displacement dialysis. At low concentrations of ligand, the binding of monosialo-gangliotetraitol exceeded that of the parent ganglioside II3NeuAcGgOse4-Cer, the possible cell surface receptor for the toxin. The terminal galactose residue and an intact carboxyl group of the sialic acid moiety of monosialo-gangliotetraose were found to be necessary for strong binding to the toxin.

Studies of the ligand binding to cholera toxin, I. The lipophilic moiety of sialoglycolipids.

The fixation of cholera toxin by ganglioside GGtet1 is dependent on the nature of the carbohydrate as well as the lipid moiety of the glycolipid. The role of the lipid in binding to the toxin investigated with synthetic ganglioside analogues (gangliosidoides). The interaction between glycolipid and toxin was followed by precipitate formation, by inhibition of toxicity and in polyacrylamide gel electrophoresis. For specific precipitation, an aliphatic hydrocarbon chain at least 14 C-atoms in length is required. Some of the gangliosidoides form high molecular weight complexes with cholera toxin at lower molar ratios of ligand to protein than the natural compound. None of the synthetic gangliosidoides equalled natural ganglioside in its ability to inhibit the effects of the toxin in vivo, but some did show considerable inhibitory activity ih monosialo-gangliotetraose or corresponding sialo-glycolipids prevents the easy degradation of the B-protein of cholera toxin into protein subunits by sodium dodecylsulfate.