The Aurora-A/TPX2 Axis Directs Spindle Orientation by Regulating NuMA and Microtubule Dynamics

The orientation of the mitotic spindle is a crucial process that defines the axis of cell division, contributing to daughter cell positioning and fate, and hence to tissue morphogenesis and homeostasis. Pathological conditions associated with spindle orientation defects primarily include neuro-developmental disorders, while direct contributions to tumorigenesis are still under investigation. The trimeric NuMA/LGN/Gαi complex, the major determinant of spindle orientation, acts by anchoring to the cell cortex astral microtubules (MTs) and dynein motors, and exerting pulling forces on the spindle poles. Multiple mitotic kinases contribute to correct spindle orientation by regulating NuMA localization [7-9]. We and others have described a role for the Aurora-A centrosomal kinase in the targeting of NuMA to the cell cortex in metaphase. Aurora-A is frequently overexpressed in cancer, together with its major activator TPX2, raising the question as to whether spindle orientation is among the processes downstream the Aurora-A/TPX2 signaling axis that may be altered under pathological conditions. Here we have investigated the role of TPX2 in the Aurora-A-mediated regulation of NuMA and in spindle orientation. We show that in cultured human cells, the interaction with TPX2 is required for Aurora-A to exert these functions. We also show that Aurora-A, TPX2 and NuMA are part of a macromolecular assembly at spindle MTs, and that the Aurora-A/TPX2 complex favors the interaction between Aurora-A and NuMA, suggesting that TPX2 acts as a platform for Aurora-A regulation of NuMA at spindle MTs. Interestingly, we found that the excess of TPX2 does not influence NuMA localization, but induces a “super-alignment” of the spindle axis to the substrate, both in transformed and non-transformed cells. Conversely, an excess of Aurora-A induces spindle misorientation. We show that these opposite effects are both linked to altered MT dynamics. Overall our results highlight the importance of TPX2 for spindle orientation and suggest that distinct cellular components regulating spindle orientation are differentially sensitive to unbalanced levels of Aurora-A, TPX2 or the Aurora-A/TPX2 complex.