Tousled-like Kinase Regulates Cellular Communicatin During Drosophila Collective Border Cell Migration

Collective cell migration is emerging as a major contributor to normal development and disease. Cell-cell communication, especially the crosstalk between different cell types is very important. Border cells migration during the Drosophila oogenesis is a classic model to study collective cell migration in vivo. Collective border cell migration requires cooperation between two distinct cell types: 4-8 migratory cells surrounding two immotile cells called polar cells. Polar cells secrete a cytokine, Unpaired(Upd). Being the ligand of JAK/STAT signal pathway, Upd activates JAK/STAT signaling in neighboring follicle cells and stimulates 4-8 cells among them with the highest STAT activity differentiate into mobile border cells at Stage 8. Differentiated border cells then surround and carry polar cell, detach from anterior basal lamina and migrate together as a rosette cluster throughout nurse cells towards oocyte,which localizes at the posterior of the egg chamber. Without Upd, border cell fate determination and migration fail, causing female sterility. Ectopic Upd expression is sufficient to stimulate motility in otherwise immobile cells. Thus Upd is necessary and sufficient for border cell development. And the precise regulation of Upd is a key to this developmental process.Here we report a limited RNAi screen for new nuclear proteins required for border cell migration. The gene encoding Tousled-like kinase(Tlk) was found to be specifically required in polar cells for Upd expression without affecting polar cell fate or viability, or border cell viability. In the absence of Tlk, fewer border cells were recruited and motility was impaired. Tlk knockdown in polar cells resembled inhibition of JAK/STAT signaling: both of them show border cell differentiation and migration defect, and also exhibit extra polar cells after Stage 5, due to failed apoptosis of the excess polar cells.Immunostaining test of three JAK/STAT signal activity reporters(nuclear STAT, STAT-10X-GFP and slbo-lac Z) indicated that KD of Tlk in polar cells decreased STAT activity in border cells. qRT-PCR experiment also showed that Tlk in polar cells is directly required for the right mRNA amount of Upd and its downstream target Slbo. Strong genetic interactions between tlk and upd/stat during border cell migration further confirmed that.This is the first discovery that Tlk functions during cell migration, and also is the first report about Tlk’s non-cell-autonomous biological function. It helps us understand the precise regulation of Upd during border cell development further. These findings shed light on the molecular mechanisms regulating the cooperation of motile and non-motile cells during collective invasion, a phenomenon that may also drive metastatic cancer.