Spotlight on Geminin

In the previous issue of Breast Cancer Research, Gardner and co-workers describe a novel interaction between Geminin, a protein that prevents reinitiation of DNA replication, and Topoisomerase IIα (TopoIIα), an enzyme essential for removing catenated intertwines between sister chromatids. Geminin facilitates the action of TopoIIα, thereby promoting termination of DNA replication at the same time it inhibits initiation. In this manner, Geminin ensures that cells duplicate their genome once, but only once, each time they divide. Remarkably, either depletion of Geminin or over-expression of Geminin inhibits the action of TopoIIα, thereby making Geminin an excellent target for cancer chemotherapy

Cdk2 Is Required for p53-Independent G2/M Checkpoint Control

The activation of phase-specific cyclin-dependent kinases (Cdks) is associated with ordered cell cycle transitions. Among the mammalian Cdks, only Cdk1 is essential for somatic cell proliferation. Cdk1 can apparently substitute for Cdk2, Cdk4, and Cdk6, which are individually dispensable in mice. It is unclear if all functions of non-essential Cdks are fully redundant with Cdk1. Using a genetic approach, we show that Cdk2, the S-phase Cdk, uniquely controls the G2/M checkpoint that prevents cells with damaged DNA from initiating mitosis. CDK2-nullizygous human cells exposed to ionizing radiation failed to exclude Cdk1 from the nucleus and exhibited a marked defect in G2/M arrest that was unmasked by the disruption of P53. The DNA replication licensing protein Cdc6, which is normally stabilized by Cdk2, was physically associated with the checkpoint regulator ATR and was required for efficient ATR-Chk1-Cdc25A signaling. These findings demonstrate that Cdk2 maintains a balance of S-phase regulatory proteins and thereby coordinates subsequent p53-independent G2/M checkpoint activation

Heart field origin of great vessel precursors relies on nkx2.5-mediated vasculogenesis

The pharyngeal arch arteries (PAAs) are transient embryonic blood vessels that make indispensable contributions to the carotid arteries and great vessels of the heart, including the aorta and pulmonary arteries. During embryogenesis, the PAAs appear in a craniocaudal sequence to connect pre-existing segments of the primitive circulation after de novo vasculogenic assembly from angioblast precursors. Despite the unique spatiotemporal characteristics of PAA development, the embryonic origins of PAA angioblasts and the genetic factors regulating their emergence remain unknown. Here, we identify the embryonic source of PAA endothelium as nkx2.5 + progenitors in lateral plate mesoderm long considered to adopt cell fates within the heart exclusively. Further, we report that PAA endothelial differentiation relies on Nkx2.5, a canonical cardiac transcription factor not previously implicated in blood vessel formation. Together, these studies reveal the heart field origin of PAA endothelium and attribute a new vasculogenic function to the cardiac transcription factor Nkx2.5 during great vessel precursor development. © 2013 Macmillan Publishers Limited

Recruitment of the human Cdt1 replication licensing protein by the loop domain of Hec1 is required for stable kinetochore–microtubule attachment

Cdt1, a protein critical for replication origin licensing in G1 phase is degraded during S phase but re-accumulates in G2 phase. We now demonstrate that human Cdt1 has a separable essential mitotic function. Cdt1 localizes to kinetochores during mitosis through interaction with the Hec1 component of the Ndc80 complex. G2-specific depletion of Cdt1 arrests cells in late prometaphase due to abnormally unstable kinetochore-microtubule (kMT) attachments and Mad1-dependent spindle assembly checkpoint activity. Cdt1 binds a unique loop extending from the rod domain of Hec1 that we show is also required for kMT attachment. Mutation of the loop domain prevents Cdt1 kinetochore localization and arrests cells in prometaphase. Super-resolution fluorescence microscopy indicates that Cdt1 binding to the Hec1 loop domain promotes a microtubule-dependent conformational change in the Ndc80 complex in vivo. These results support the conclusion that Cdt1 binding to Hec1 is essential for an extended Ndc80 configuration and stable kinetochore microtubule attachment