hSpindly dynamics modulate spindle assembly checkpoint signaling, promoting resistance to mitotic inhibitors

The spindle assembly checkpoint (SAC) prevents sister chromatids separation until all chromosomes are properly attached to spindle microtubules and bi-oriented. SAC controls the metaphase-to-anaphase transition by inhibiting the anaphase promoting complex/cyclosome (APC/C). A critical step in this process is the recruitment of the Mad1-Mad2 complex to unattached kinetochores mediated by the KBB pathway (KNL1-Bub3-Bub1) and the RZZ pathway (Rod-Zw10-Zwilch). Here, we show that hSpindly contributes to the regulation of Mad1-Mad2 recruitment through the RZZ pathway. We demonstrate that hSpindly is a dynamic protein that oligomerizes at unattached kinetochores, and we identify threonine 552 as a key residue controlling its mobility and function. A non-phosphorylatable mutant (T552A) displays reduced dynamics at kinetochores, impairs SAC signaling, and increases cellular resistance to antimitotic drugs. Altogether, our findings identify hSpindly as a dynamic regulator of SAC signaling and potential therapeutic target for overcoming resistance to mitotic inhibitors.