Although the Wnt signalling pathway has been studied for many years, little is still known about the production of Wnt itself. A central role in this process has recently been attributed to the Wnt sorting receptor Wntless(Wls)/MIG-14, that guides Wnt to the plasma membrane and than is endocytosed and recycled by the retromer complex. Here we report the identification of two myotubularin lipid phosphatases as novel regulators of Wls/MIG-14 trafficking in C. elegans . The C. elegans myotubularins
mtm-6 and
mtm-9 encode phosphatidylinositol 3-phosphatases. We show that
mtm-6 and
mtm-9 are required for the migration of the Q neuroblasts, a process known to be regulated by the canonical Wnt/EGL-20 signalling pathway. Expression of MTM-6 in cells producing EG L-20 is sufficient to rescue the QL migration defect of
mtm-6 mutants, indicating that myotubularins are participating in the production of EGL-20. Genetic analyses show interaction of
mtm-6 with the retromer complex and Wls/MIG-14. The levels of MIG-14 are reduced in
mtm-6 mutants, an effect also seen in retromer mutants. This suggests that
mtm-6 is required for efficient recycling of MIG-14. Since
mtm-6 regulates the levels of phosphatidylinositol 3-phosphate (PI3P) and one of the retromer subunits, sorting nexin 1, binds PI3P,
mtm-6 may affect MIG-14 recycling by regulating retromer recruitment to endosomes. Interestingly, the localization of the retromer complex is not affected in
mtm-6 mutants. However, we found another protein, sorting nexin 3 (SNX-3) that clearly re-localizes from the cytoplasm to vesicles in
mtm-6 mutants.
snx-3 mutants show a fully penetrant Q cell migration defect, showing that
snx-3 plays crucial role in Wnt/EGL-20 signalling. SNX-3 co-localizes with MIG-14 and the co-localization is strongly enhanced in
mtm-6 mutants. We are currently investigating the molecular mechanisms linking myotubularins, retromer, SNX-3 and Wls/MIG-14 recycling.