Characterisation of Upd2, a Drosophila JAK/STAT pathway ligand

The characterisation of ligands that activate the JAK/STAT pathway has the potential to throw light onto a comparatively poorly understood aspect of this important signal transduction cascade. Here, we describe our analysis of the only invertebrate JAK/STAT pathway ligands identified to date, the Drosophila unpaired-like family. We show that upd2 is expressed in a pattern essentially identical to that of upd and demonstrate that the proteins encoded by this region activate JAK/STAT pathway signalling. Mutational analysis demonstrates a mutual semi-redundancy that can be visualised in multiple tissues known to require JAK/STAT signalling. In order to better characterise the in vivo function of these ligands, we developed a reporter based on a natural JAK/STAT pathway responsive enhancer and show that ectopic upd2 expression can effectively activate the JAK/STAT pathway. While both Upd and Upd2 are secreted JAK/STAT pathway agonists, tissue culture assays show that the signal-sequences of Upd and Upd2 confer distinct properties, with Upd associated primarily with the extracellular matrix and Upd2 secreted into the media. The differing biophysical characteristics identified for Upd-like molecules have implications for their function in vivo and adds another aspect to our understanding of cytokine signalling in Drosophila.     

The JAK-STAT signaling pathway: input and output integration

Universal and essential to cytokine receptor signaling, the JAK-STAT pathway is one of the best understood signal transduction cascades. Almost 40 cytokine receptors signal through combinations of four JAK and seven STAT family members, suggesting commonality across the JAK-STAT signaling system. Despite intense study, there remain substantial gaps in understanding how the cascades are activated and regulated. Using the examples of the IL-6 and IL-10 receptors, I will discuss how diverse outcomes in gene expression result from regulatory events that effect the JAK1-STAT3 pathway, common to both receptors. I also consider receptor preferences by different STATs and interpretive problems in the use of STAT-deficient cells and mice. Finally, I consider how the suppressor of cytokine signaling (SOCS) proteins regulate the quality and quantity of STAT signals from cytokine receptors. New data suggests that SOCS proteins introduce additional diversity into the JAK-STAT pathway by adjusting the output of activated STATs that alters downstream gene activation.     

TGF-beta-induced SOCS3 expression augments TNF-alpha-induced osteoclast formation

Osteoclast differentiation is dependent on TGF-beta to prime precursors to the osteoclast lineage. The mechanism by which TGF-beta enables osteoclast formation is unknown. One possibility is that TGF-beta opposes pro-inflammatory JAK/STAT signalling. Recently, we showed that TGF-beta-induces SOCS3, an inhibitor of the JAK/STAT pathway, in precursors and enhances SOCS3 in RANKL-induced osteoclasts. We therefore elected to test the role of SOCS3 in the effect of other regulators of osteoclastic differentiation. We found that TNF-alpha-induced osteoclasts also express SOCS3 and TGF-beta strongly up-regulates this. Moreover, TNF-alpha-induced osteoclast differentiation and total resorbed bone area were enhanced in SOCS3-retrovirally infected precursors, whereas antisense knockdown of SOCS3 suppressed formation and the augmentative effect of TGF-beta. Furthermore, SOCS3 overexpression blunted the anti-osteoclastic effect of IFN-beta but not IL-10. This suggests that TGF-beta-induced expression of SOCS3 may represent a crucial mechanism by which TGF-beta antagonizes specific anti-osteoclastic JAK/STAT signals, priming precursors for resorption rather than inflammatory functions.