Langroudi, L. | bioRxiv Science

bioRxiv2026

Direct Mapping of CDK2 Substrates in Embryonic Stem Cells Uncovers an AP-Site Repair Mechanism via HMCES Phosphorylation

Embryonic stem cells (ESCs) proliferate rapidly while robustly maintaining genomic integrity and exhibiting high cell-cycle kinase activity. How this activity contributes to genome integrity remains unclear. Here, using mouse ESCs engineered to express an analog-sensitive CDK2, we combine thiophosphate labeling with mass spectrometry to define a high-confidence CDK2 substrate landscape. We uncovered 65 CDK2 substrates in total, including both known and previously unrecognized substrates. Among these, HMCES, a sensor of apurinic/apyrimidinic (AP) sites, was identified as a specific cyclin E-CDK2 substrate. We mapped three CDK2-dependent phosphorylation sites in HMCES and showed that phosphorylation of these sites decreased HMCES binding to ssDNA. Mutational analysis further revealed that HMCES docks to cyclin E-CDK2 complexes via the hydrophobic patch on cyclin E. Finally, we demonstrated that HMCES phosphorylation contributes to AP-site repair and promotes ESC proliferation. Together, our findings uncover a CDK2-HMCES signaling axis that links rapid cell-cycle progression to the preservation of genome stability in mouse ESCs.

developmental biology↗

bioRxiv2023

A Proximal Sox2 Enhancer Cluster is Required for the Anterior Regional Identity of Neural Progenitors

Embryonic development depends on spatially and temporally orchestrated gene regulatory networks. Expressed in neural stem and progenitor cells (NSPCs), the transcription factor sex-determining region Y box 2 (Sox2) is critical for embryogenesis and stem cell maintenance in neural development. Whereas Sox2 is regulated by a distal cluster of enhancers in embryonic stem cells (ESCs), enhancers closer to the gene have been implicated in Sox2 transcriptional regulation in the neural lineage. Using functional genomics data, and deletion analysis we show that a downstream enhancer cluster regulates Sox2 transcription in NSPCs derived from mouse ESCs. By generating allelic mutants using CRISPR-Cas9 mediated deletions, we show that this proximal enhancer cluster, termed Sox2 regulatory regions 2-18 (SRR2-18), is a cis regulator of Sox2 transcription during neural differentiation. Transcriptome analyses demonstrate that loss of even one copy of SRR2-18 disrupts the region-specific identity of NSPCs. Biallelic deletion of this Sox2 neural enhancer cluster causes reduced SOX2 protein, less frequent interaction with transcriptional machinery, and leads to perturbed chromatin accessibility genome-wide further affecting the expression of neurodevelopmental and anterior-posterior regionalization genes. Furthermore, homozygous NSPC deletants exhibit self-renewal defects and impaired differentiation into cell types found in the brain. Altogether, our data define a cis-regulatory enhancer cluster controlling Sox2 transcription in NSPCs and highlight the sensitivity of neural differentiation processes to decreased Sox2 transcription, which influences their differentiation into posterior neural fates, specifically the caudal neural tube.

genetics↗