Gα73Β is a downstream effector of JAK/STAT signalling and a regulator of Rho1 in Drosophila haematopoiesis

Abstract

JAK/STAT signalling regulates multiple essential developmental processes including cell proliferation and haematopoiesis while its inappropriate activation is associated with the majority of myeloproliferative neoplasias and numerous cancers. Furthermore, high levels of JAK/STAT pathway signalling have also been associated with enhanced metastatic invasion by cancerous cells. Strikingly, gain-of-function mutations in the single Drosophila JAK homologue, Hopscotch, result in haemocyte neoplasia, inappropriate differentiation and the formation of melanised haemocyte-derived ‘tumour’ masses; phenotypes that are partly orthologous to human gain-of-function JAK2-associated pathologies. Here we show that Gα73B, a novel JAK/STAT pathway target gene, is necessary for JAK/STAT-mediated tumour formation in flies. In addition, while Gα73Β does not affect haemocyte differentiation, it does regulate haemocyte morphology and motility under non-pathological conditions. We show that Gα73Β is required for constitutive, but not injury-induced, activation of Rho1 and for the localisation of Rho1 into filopodia upon haemocyte activation. Consistent with these results, we also show that Rho1 interacts genetically with JAK/STAT signalling, and that wild-type levels of Rho1 are necessary for tumour formation. Our findings link JAK/STAT transcriptional outputs, Gα73Β activity and Rho1-dependent cytoskeletal rearrangements/cell motility and therefore connect a pathway associated with cancer with a marker indicative of invasiveness. As such, we suggest a mechanism via which JAK/STAT pathway signalling may promote metastasis.