Expression of an S phase-stabilized version of the CDK inhibitor Dacapo can alter endoreplication

Abstract

In developing organisms, divergence from the canonical cell division cycle is often necessary to ensure the proper growth, differentiation, and physiological function of a variety of tissues. An important example is endoreplication, in which endocycling cells alternate between G and S phase without intervening mitosis or cytokinesis, resulting in polyploidy. Although significantly different from the canonical cell cycle, endocycles use regulatory pathways that also function in diploid cells, particularly those involved in S phase entry and progression. A key S phase regulator is the Cyclin E-Cdk2 kinase, which must alternate between periods of high (S phase) and low (G phase) activity in order for endocycling cells to achieve repeated rounds of S phase and polyploidy. The mechanisms that drive these oscillations of Cyclin E-Cdk2 activity are not fully understood. Here, we show that the Drosophila Cyclin E-Cdk2 inhibitor Dacapo (Dap) is targeted for destruction during S phase via a PIP degron, contributing to oscillations of Dap protein accumulation during both mitotic cycles and endocycles. Expression of a PIP degron mutant Dap attenuates endocycle progression but does not obviously affect proliferating diploid cells. A mathematical model of the endocycle predicts that the rate of destruction of Dap during S phase modulates the endocycle by regulating the length of G phase. We propose from this model and our in vivo data that endo S phase-coupled destruction of Dap reduces the threshold of Cyclin E-Cdk2 activity necessary to trigger the subsequent G-S transition, thereby influencing endocycle oscillation frequency and the extent of polyploidy.

Myoglianin triggers the pre-metamorphosis stage in hemimetabolan insects

“…Dap is a member of the mammalian p21 family of cyclin-dependent kinase inhibitors (CKIs) and inhibits the G1-to-S phase transition by sequestering the CycE/Cdk2 complex in a stable but inactive form (Lane et al, 1996). Induction of dap transcription causes a rapid accumulation of Dap protein, which inhibits CycE/Cdk2 activity and leads to G1 cell cycle arrest (Swanson et al, 2015). Conversely, dap knockdown leads to tissue hypertrophy and cancer processes (Kiyokawa et al, 1996; Nakayama et al, 1996).…”

Myoglianin triggers the premetamorphosis stage in hemimetabolan insects

“…Dap is a member of the mammalian p21 family of cyclin-dependent kinase inhibitors and inhibits the G1-to-S phase transition by sequestering the CycE/Cdk2 complex in a stable but inactive form (31). Induction of dap transcription causes a rapid accumulation of Dap protein, which inhibits CycE/Cdk2 activity and leads to G1 cell cycle arrest (32). Conversely, dap knockdown leads to tissue hypertrophy and cancer processes (33,34).…”

E2F-dependent genetic oscillators control endoreplication

“…In CycE high and G1-FUCCI negative S phase cells, Dap expression is almost undetectable and therefore no longer coupled to CycE (Figure 3D arrowhead). This is likely due to the PIP box-dependent degradation of Dap, by CRL4 Cdt2 in S phase [26]. This result suggests that Dap is expressed in CycE high G phase cells to limit its activity prior to entering S phase.…”