Mammalian CLASP1 and CLASP2 Cooperate to Ensure Mitotic Fidelity by Regulating Spindle and Kinetochore Function

Abstract

CLASPs are widely conserved microtubule plus-end-tracking proteins with essential roles in the local regulation of microtubule dynamics. In yeast, Drosophila, and Xenopus, a single CLASP orthologue is present, which is required for mitotic spindle assembly by regulating microtubule dynamics at the kinetochore. In mammals, however, only CLASP1 has been directly implicated in cell division, despite the existence of a second paralogue, CLASP2, whose mitotic roles remain unknown. Here, we show that CLASP2 localization at kinetochores, centrosomes, and spindle throughout mitosis is remarkably similar to CLASP1, both showing fast microtubule-independent turnover rates. Strikingly, primary fibroblasts from Clasp2 knockout mice show numerous spindle and chromosome segregation defects that can be partially rescued by ectopic expression of Clasp1 or Clasp2. Moreover, chromosome segregation rates during anaphase A and B are slower in Clasp2 knockout cells, which is consistent with a role of CLASP2 in the regulation of kinetochore and spindle function. Noteworthy, cell viability/proliferation and spindle checkpoint function were not impaired in Clasp2 knockout cells, but the fidelity of mitosis was strongly compromised, leading to severe chromosomal instability in adult cells. Together, our data support that the partial redundancy of CLASPs during mitosis acts as a possible mechanism to prevent aneuploidy in mammals.

CLASPs prevent irreversible multipolarity by ensuring spindle-pole resistance to traction forces during chromosome alignment

“…Finally, we demonstrate that mammalian CLASPs contribute to mitotic fidelity not only by regulating kinetochore-microtubule attachments 9,12,14 , but also by preventing irreversible spindle multipolarity through distinct molecular partnerships at kinetochores and centrosomes. For instance, incomplete CLASP1/2 depletion by RNAi, which does not remove a stable centrosome-associated CLASP1/2 pool 11,12 , or injection of anti-CLASP1 antibodies 9 , leads mostly to short bipolar or monopolar spindles, which might be explained by preferential loss of CLASP1/2 function at kinetochores.…”

“…For instance, incomplete CLASP1/2 depletion by RNAi, which does not remove a stable centrosome-associated CLASP1/2 pool 11,12 , or injection of anti-CLASP1 antibodies 9 , leads mostly to short bipolar or monopolar spindles, which might be explained by preferential loss of CLASP1/2 function at kinetochores. Importantly, the irreversible nature of spindle multipolarity arising through the mechanism reported here is in marked contrast with the transient multipolar configuration due to supernumerary centrosomes 5,6 , uncovering an alternative route to aneuploidy.…”

Connections between cadherin-catenin proteins, spindle misorientation, and cancer

“…84 Whether p120-catenin interacts with CLASP2 at interphase or mitotic centrosomes awaits further investigation. It is tempting to speculate that given the pivotal roles of CLASP2 in mitosis, 115,116 a potential CLASP2-p120 interaction at centrosomes could have an impact on centrosome function and mitotic spindle position and integrity. Interestingly, it has been documented that CLASP1, another member of the family, cooperates to align the spindle along the long axis of division in mammalian cells.…”

The perpetual movements of anaphase

“…At the molecular level, the players involved in each system might be different, although their modus operandi must be essentially the same: cross-linking, sliding and eventually growth/stabilization of interpolar microtubules of opposite polarity. These functions are provided by MAPs that localize to the spindle midzone, such as members of the PRC1, XMAP215 and CLASP families, as well as by force-producing motors of the kinesin-5 or -6 families [96,[217][218][219][220][221][222][223][224][225].…”

“…Cytoplasmic dynein located at the cell cortex has been implicated in this astral MT pulling mechanism [89,90,96,[233][234][235]. On the other hand, proteins required for microtubule polymerization and growth/stabilization of interpolar microtubules [236], such as CLASPs [219], coordinated with the activity of kinesin-5 at the spindle midzone, might work as governors [237].…”