Eui-Hwan Choi

The cohesin complex is a highly conserved, ring-shaped protein assembly that plays fundamental roles in chromosome biology. Originally identified as the molecular glue that holds sister chromatids together from DNA replication until cell division, cohesin has since been recognized as a pleiotropic regulator of genome organization, gene expression, DNA repair, and cell fate determination. In embryonic stem cells (ESCs), cohesin’s functions extend beyond canonical sister chromatid cohesion to include the maintenance of three-dimensional (3D) chromatin architecture through DNA loop extrusion, regulation of pluripotency-associated transcriptional programs, and facilitation of homologous recombination-mediated DNA repair during the prolonged S phase. Recent discoveries have revealed that meiosis-specific cohesin components, particularly the α-kleisin subunit REC8 and its interacting partner STAG3, are expressed and functionally active in mitotic ESC chromosomes, where they contribute to chromosomal organization and sister chromatid cohesion in concert with mitotic RAD21-containing cohesin. Furthermore, the interplay between cohesin and condensin complexes at shared genomic binding sites has emerged as a critical determinant of chromosome topology, with cohesin depletion leading to aberrant condensin accumulation and chromosome hypercompaction. Importantly, perturbations in cohesin function not only impair ESC self-renewal but also direct lineage-specific differentiation, linking cohesin to stem cell fate determination. Germline mutations in cohesin and its regulators underlie a spectrum of developmental disorders termed cohesinopathies, while somatic mutations are frequently observed in various cancers. This review provides a comprehensive overview of the diverse roles of cohesin in chromosome structure, cell cycle regulation, and stem cell biology, with particular emphasis on recent findings in ESCs that illuminate the complex interplay between mitotic and meiotic cohesin complexes.

Amphiregulin (AREG) functions as an epidermal growth factor receptor (EGFR) ligand that modulates signaling and supports nuclear processes involved in DNA replication and repair in colorectal cancer (CRC). Immunohistochemistry and western blot revealed high expression of AREG in CRC tumors compared to other CRCs. Our findings indicate that AREG translocates into the nucleus, a process potentially facilitated by the inhibition of endocytosis. Transcriptomic analyses suggest an association between AREG expression and genes regulating EGFR signaling, replication fork dynamics, and homologous recombination. Depletion of AREG via siRNA or CRISPR-Cas9 led to S/G₂ arrest, replication tract shortening, and increased RAD51, RPA, and γH2AX foci, resulting in a 40-60% reduction in proliferation effects that were not fully recapitulated by small-molecule EGFR inhibitors. In xenograft models, AREG knockout reduced tumor growth and suppressed the phosphorylation of EGFR, ERK, STAT3, and BRAF. Furthermore, combining AREG loss with EGFR inhibition appeared to enhance antitumor effects. These findings suggest that AREG may function as a mediator of EGFR signaling and genome maintenance in CRC.

Cancer stem cells (CSCs) support colorectal cancer progression and therapy resistance, yet the redox regulators that sustain CSC identity are incompletely defined. We investigated the role of peroxiredoxin 5 (PRX5) in CSC formation and tumorigenicity using HCT116 colorectal cancer models. CSCs were enriched by serum-free spheroid culture and characterized by qPCR and western blotting for pluripotency and surface markers. PRX5 expression was regulated via siRNA or shRNA gene disruption and overexpression. Intracellular ROS was measured with DCF-DA staining. STAT3 activation was analyzed by p-STAT3 immunoblot. Functional assays included sphere formation, extreme limiting dilution analysis (ELDA), colony formation, and in vivo xenograft tumorigenicity in BALB/c-nu mice. Spheroid induction selectively upregulated PRX5 among PRX isoforms. Efficient PRX5 knockdown (>95%) reduced OCT4, SOX2, NANOG, and CD133 expression, increased intracellular ROS, lowered p-STAT3 levels, and decreased sphere-forming frequency. Conversely, PRX5 overexpression enhanced pluripotency marker expression and proliferation. In vivo, PRX5-overexpressing xenografts grew faster, achieving a 2.5-fold greater tumor volume by day 19 and a 1.82-fold higher mean tumor weight compared with controls. Tumor tissues showed elevated OCT4, SOX2, NANOG, CD133, and EPCAM. PRX5 contributes to maintaining a redox environment associated with STAT3 activation and core stemness programs in CRC, promoting CSC phenotypes and tumorigenicity. Targeting PRX5-mediated redox signaling may warrant further investigation as a potential approach to disrupt CSC maintenance and overcome chemoresistance.

Cohesin and condensin, two related protein complexes, play essential roles in ensuring the accurate segregation of the genome into daughter cells during cell division. However, the interaction between cohesin and condensin in embryonic stem cells remains unclear, as does the specific function of the meiosis-specific cohesin complex. Cohesin maintains the cohesion of replicated sister chromatids until their separation at anaphase, whereas condensin facilitates the reorganization of chromosomes into a highly compact structure characteristic of mitosis. First, we found via ChIP-seq analysis that cohesins (SMC3, RAD21, and REC8) and condensin (SMC4) share DNA binding sites in close proximity and directly interact with the insulator protein CTCF. Second, siRNA-regulated SMC3 depletion led to nuclear accumulation of SMC4. Third, embryonic stem (ES) cells uniquely harbor cohesin complexes containing the meiotic kleisin subunit REC8. RAD21 knockdown increased the proportion of SMC3–REC8 complexes. Our findings indicate that cohesin and condensin make important contributions to the functions of the chromosomal organization, and that meiotic cohesin may be specifically required for the mitotic program in ES cells.

Alzheimer’s disease (AD) and Parkinson’s disease (PD) are the most common neurodegenerative diseases, and they affect millions of people worldwide, particularly older individuals. Therefore, there is a clear need to develop novel drug targets for the treatment of age-related neurodegenerative diseases. Emerging evidence suggests that mitochondrial dysfunction and reactive oxygen species (ROS) generation play central roles in the onset and progression of neurodegenerative diseases. Mitochondria are key regulators of respiratory function, cellular energy adenosine triphosphate production, and the maintenance of cellular redox homeostasis, which are essential for cell survival. Mitochondrial morphology and function are tightly regulated by maintaining a balance among mitochondrial fission, fusion, biogenesis, and mitophagy. In this review, we provide an overview of the main functions of mitochondria, with a focus on recent progress highlighting the critical role of ROS−induced oxidative stress, dysregulated mitochondrial dynamics, mitochondrial apoptosis, mitochondria-associated inflammation, and impaired mitochondrial function in the pathogenesis of age-related neurodegenerative diseases, such as AD and PD. We also discuss the potential of mitochondrial fusion and biogenesis enhancers, mitochondrial fission inhibitors, and mitochondria-targeted antioxidants as novel drugs for the treatment of these diseases.

Young Eun Koh, Eui-Hwan Choi, Jung-Woong Kim, and Keun Pil Kim. Mol. Cells 2022;45:820-32. https://doi.org/10.14348/molcells.2022.2042

Embryonic stem (ES) cells require homologous recombination (HR) to cope with genomic instability caused during self-renewal. Here, we report expression dynamics and localization of endogenous HR factors in DNA break repair of ES cells. In addition, we analyzed gene expression patterns of HR-related factors at the transcript level with RNA-sequencing experiments. We showed that ES cells constitutively expressed diverse HR proteins throughout the cell cycle and that HR protein expression was not significantly changed even in the DNA damaging conditions. We further analyzed that depleting Rad51 resulted in the accumulation of larger single-stranded DNA (ssDNA) gaps, but did not perturb DNA replication, indicating that ES cells were able to enter the G2-phase in the presence of unrepaired DNA gaps, consistent with the possibility that post-replication repair helps avoid stalling at the G2/M checkpoint. Interestingly, caffeine treatment inhibited the formation of Rad51 or Rad54 foci, but not the formation of γH2AX and Exo1 foci, which led to incomplete HR in ssDNA, thus increasing DNA damage sensitivity. Our results suggested that ES cells possess conserved HR-promoting machinery to ensure effective recruitment of the HR proteins to DNA breaks, thereby driving proper chromosome duplication and cell cycle progression in ES cells.