Cdk1 Inactivation Terminates Mitotic Checkpoint Surveillance and Stabilizes Kinetochore Attachments in Anaphase

Abstract

SummaryTwo mechanisms safeguard the bipolar attachment of chromosomes in mitosis. A correction mechanism destabilizes erroneous attachments that do not generate tension across sister kinetochores [1]. In response to unattached kinetochores, the mitotic checkpoint delays anaphase onset by inhibiting the anaphase-promoting complex/cyclosome (APC/CCdc20) [2]. Upon satisfaction of both pathways, the APC/CCdc20 elicits the degradation of securin and cyclin B [3]. This liberates separase triggering sister chromatid disjunction and inactivates cyclin-dependent kinase 1 (Cdk1) causing mitotic exit. How eukaryotic cells avoid the engagement of attachment monitoring mechanisms when sister chromatids split and tension is lost at anaphase is poorly understood [4]. Here we show that Cdk1 inactivation disables mitotic checkpoint surveillance at anaphase onset in human cells. Preventing cyclin B1 proteolysis at the time of sister chromatid disjunction destabilizes kinetochore-microtubule attachments and triggers the engagement of the mitotic checkpoint. As a consequence, mitotic checkpoint proteins accumulate at anaphase kinetochores, the APC/CCdc20 is inhibited, and securin reaccumulates. Conversely, acute pharmacological inhibition of Cdk1 abrogates the engagement and maintenance of the mitotic checkpoint upon microtubule depolymerization. We propose that the simultaneous destruction of securin and cyclin B elicited by the APC/CCdc20 couples chromosome segregation to the dissolution of attachment monitoring mechanisms during mitotic exit.

Signalling dynamics in the spindle checkpoint response

“…Nonetheless, stabilized cyclin B allowed checkpoint proteins to relocalize to kinetochores upon anaphase onset, as had been previously observed 25,26,28,29,111,134,137 . This indicated that loss of tension at anaphase does indeed promote checkpoint signaling, but this response is normally repressed by Cdk1 inactivation.…”

“…Several additional studies have analyzed the anaphase response further by using non-degradable cyclin B, the cofactor that activates Cdk1 towards its mitotic targets, in fission yeast 25 , mice 27 , and human cells 26,28 Stabilizing cyclin B permitted securin degradation and sister chromatid separation with a corresponding loss of kinetochore tension 25-28 . Nonetheless, stabilized cyclin B allowed checkpoint proteins to relocalize to kinetochores upon anaphase onset, as had been previously observed 25,26,28,29,111,134,137 .…”

“…This indicated that loss of tension at anaphase does indeed promote checkpoint signaling, but this response is normally repressed by Cdk1 inactivation. Since Cdk1 activity is required for the checkpoint (see for review 9 ), its silencing by cyclin B degradation ensures the checkpoint is not reactivated 26-28,129 . It was further demonstrated that an anaphase checkpoint response is too slow to inhibit the APC/C, and this slow response may serve as a failsafe mechanism to prevent interruption of anaphase by checkpoint activation 25,28 .…”

Inefficient degradation of cyclin B1 re-activates the spindle checkpoint right after sister chromatid disjunction

“…12,18,19 Therefore, subsequently we asked to which degree cyclin B1 must be removed in order to prevent spindle checkpoint re-activation in anaphase. We filmed cells expressing a new cyclin B1 mutant, Δ52-85-cyclin B1, which has a deletion within a lysine-rich region that we noticed to be conserved downstream of the D-box ( Fig.…”

“…The problem appears to be avoided by the timely inactivation of Cdk1 prior to anaphase, in metaphase. [10][11][12][13][14][15] It is unknown at the moment whether the anaphase problem could occur naturally, or may contribute to aneuploidy in cases when Cdk1 regulatory factors are abnormally expressed. How inactivation of Cdk1 prevents the anaphase problem is also still largely unclear.…”