DNA replication and mitotic entry: A brake model for cell cycle progression

Abstract

The core function of the cell cycle is to duplicate the genome and divide the duplicated DNA into two daughter cells. These processes need to be carefully coordinated, as cell division before DNA replication is complete leads to genome instability and cell death. Recent observations show that DNA replication, far from being only a consequence of cell cycle progression, plays a key role in coordinating cell cycle activities. DNA replication, through checkpoint kinase signaling, restricts the activity of cyclin-dependent kinases (CDKs) that promote cell division. The S/G2 transition is therefore emerging as a crucial regulatory step to determine the timing of mitosis. Here we discuss recent observations that redefine the coupling between DNA replication and cell division and incorporate these insights into an updated cell cycle model for human cells. We propose a cell cycle model based on a single trigger and sequential releases of three molecular brakes that determine the kinetics of CDK activation.

Figure 1.

Models on DNA replication and mitotic triggers. Gray arrow depicts time toward mitosis, and dashed lines indicate G1/S and S/G2 transitions. (A) Model 1 is based on two independent triggers: one trigger for DNA replication (blue) and one trigger for mitotic kinase activation (orange). (B) Model 2 is based on a single trigger for DNA replication (blue) and mitotic kinase activation (orange), yet a signaling cascade links both processes and separates the two events in time. Kinases that drive mitotic entry are activated when bulk DNA replication is completed at the S/G2 transition.

Figure 2.

Molecular switches at the S/G2 transition. The S/G2 transition is dictated by DNA replication status. DNA replication in S-phase activates ATR/Chk1 signaling, which represses CDK1 and PLK1 activity and promotes fork stabilization and DNA repair. Completion of bulk DNA replication allows CDK1 and PLK1 activation, which promotes mitotic entry and processing of persistent replication intermediates. Lower panel depicts examples of phosphorylation targets of ATR/CHK1 or CDK1/PLK1. The ATR/CHK1 axis inhibits mitotic entry (e.g., via CDC25) and promotes fork stability (e.g., via H2AX, HARP, and Claspin), while the CDK1/PLK1 axis promotes mitotic entry (e.g., via FOXM1) and alters DNA repair (e.g., via 53BP1, SLX4, and TCTP).

Figure 3.

A brake model for cell cycle progression. (A) Model 1 is based on multiple independent triggers that initiate cell cycle engines and push the cell into mitosis, akin to pushing a ball up a mountain. (B) Model 2 is based on a single trigger and multiple brakes that restrain cell cycle engines and mitotic entry, akin to braking a ball rolling down a mountain. (C) At least three brake modules collectively determine cell cycle progression. Visualized as an energy landscape, the brakes define the slope of the descent and thus the speed of the ball rolling toward mitosis. (D) Basic signaling circuits of the three brake modules controlling CDK activity (left) and the timing of the respective brakes during the cell cycle (right). The G1/S brake and M entry brake are wired to CDK via double-negative feedback loops, and eventually are overruled by self-amplifying CDK activity. The S/G2 brake is wired to CDK via a transient incoherent feed-forward loop: CDK2 activity triggers DNA replication, which inhibits CDK1, but the brake is inherently transient because it resolves the moment the cell completes genome replication.

Figure 4.

Two types of brakes control cell cycle timing. Intrinsic brakes are essential to life and ensure cell cycle phase dependence, while emergency brakes delay or arrest cell cycle progression upon external cues and stresses. The APC/C complex, ATR/CHK1, and WEE1/PP2A represent intrinsic brakes required to restrain CDK activation during the unchallenged cell cycle (top). In contrast, signaling factors such as ATM, p38, p27, p16, p53, p21, CHFR, and JNK are not essential for cell proliferation, per se, but effectively inhibit CDK activation upon stress (lower panel).