The immunomodulator, FTY720 (fingolimod) is currently undergoing phase III clinical trials for the treatment of relapsing-remitting multiple sclerosis. FTY720 is a prodrug that, once phosphorylated to FTY720-P, is believed to act primarily by targeting sphingosine-1-phosphate (S1P) receptors on lymphocytes and endothelial cells. This leads to retention of lymphocytes in lymph nodes that, in turn, prevents attacks on myelin sheaths.

Although FTY720-P is a potent agonist of several S1P receptors, its beneficial effects in multiple sclerosis are believed to be mediated primarily through the S1P1 receptor. Because specific knockout of the S1P1 receptor on haematopoietic cells in mice and treatment with FTY720 show similar effects on lymphocyte recirculation, the efficacy of FTY720-P has been attributed to ‘functional antagonism’ leading to complete internalisation and desensitisation of receptors. However, writing in the journal Nature Chemical Biology, scientists at Novartis now provide evidence that, despite internalisation, signalling by S1P1 receptors bound to FTY720-P persists for hours. Although calcium signalling – which depends on cell surface localisation of the receptor – was inhibited by treatment with FTY720, other signalling pathways remained activated. In both stably transfected and primary cell lines, persistent activation of S1P1 receptors by FTY720 led to prolonged inhibition of adenylate cyclase and increased ERK phosphorylation. Similar effects were not observed with the endogenous agonist, S1P, and by exploring analogues of FTY-720, the length of the aliphatic side chain was found to be crucially important for persistent signalling and receptor internalisation.

Shortening the lipophilic side chain by one methylene group decreased the effect and shortening by two methylene groups abolished the effect completely, despite all three compounds having similar intrinsic potencies. The ability of the S1P1 antagonist, WN146 – which does not itself induce lymphocyte sequestration – to inhibit prolonged S1P1 signalling caused by treatment with FTY720-P suggests that direct agonism rather than functional antagonism may be the predominant mechanism of action of FTY720-P.

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During the development of type 2 diabetes, uptake of glucose from the blood by muscle and fat cells in response to insulin is reduced. The pathways involved in the insulin-stimulated uptake of glucose were believed to be similar in all mammals, but US scientists have now highlighted a key difference between mice and humans. Glucose is taken up by fat and muscle cells via the GLUT4 glucose transporter, thus removing glucose from the bloodstream. When blood glucose is low, the receptor is sequestered away from the cell surface and is released from the intracellular compartment in response to insulin stimulation when blood glucose rises. In type 2 diabetes, however, the GLUT4 compartment is abnormal and the transporter is not mobilised to the cell surface in response to insulin stimulation. The muscle isoform of clathrin heavy chain, CHC22, was found to be involved in formation of the intracellular GLUT 4 components in human muscle cells and adipocytes and was also found to be associated with the abnormal GLUT4 compartments in muscle cells from people with type 2 diabetes. Mice also have an insulin-responsive GLU4 compartment but lack the CHC22 protein – mice engineered to express CHC22 in fat and muscle tissue had defects in their GLUT4 transport pathway and showed features of diabetes, including high blood sugar and reduced responses to insulin. As well as suggesting that faulty vesicle trafficking, as well as problems with insulin signalling, may play a role in the development of type 2 diabetes, the study highlights the importance of being aware of differences between animals used in model studies and humans.

The study is published in the journal Science.

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