Slow G-Quadruplex Conformation Rearrangement and Accessibility Change Induced by Potassium in Human Telomeric Single-Stranded DNA.

The guanine-rich telomeric repeats can form G-quadruplexes (G4s) that alter the accessibility of the single-stranded telomeric overhang. In this study, we investigated the effects of Na+ and K+ on G4 folding and accessibility through cation introduction and exchange. We combined differential scanning calorimetry (DSC), circular dichroism (CD), and single molecule Förster resonance energy transfer (smFRET) to monitor the stability, conformational dynamics, and complementary strand binding accessibility of G4 formed by single-stranded telomeric DNA. Our data showed that G4 formed through heating and slow cooling in K+ solution exhibited fewer conformational dynamics than G4 formed in Na+ solution, which is consistent with the higher thermal stability of G4 in K+. Monitoring cation exchange with real time smFRET at room temperature shows that Na+ and K+ can replace each other in G4. When encountering high K+ at room or body temperature, G4 undergoes a slow conformational rearrangement process which is mostly complete by 2 h. The slow conformational rearrangement ends with a stable G4 that is unable to be unfolded by a complementary strand. This study provides new insights into the accessibility of G4 forming sequences at different time points after introduction to a high K+ environment in cells, which may affect how the nascent telomeric overhang interacts with proteins and telomerase.

Fine-tuned repression of Drp1-driven mitochondrial fission primes a 'stem/progenitor-like state' to support neoplastic transformation.

Gene knockout of the master regulator of mitochondrial fission, Drp1, prevents neoplastic transformation. Also, mitochondrial fission and its opposing process of mitochondrial fusion are emerging as crucial regulators of stemness. Intriguingly, stem/progenitor cells maintaining repressed mitochondrial fission are primed for self-renewal and proliferation. Using our newly derived carcinogen transformed human cell model, we demonstrate that fine-tuned Drp1 repression primes a slow cycling 'stem/progenitor-like state', which is characterized by small networks of fused mitochondria and a gene-expression profile with elevated functional stem/progenitor markers (Krt15, Sox2 etc) and their regulators (Cyclin E). Fine tuning Drp1 protein by reducing its activating phosphorylation sustains the neoplastic stem/progenitor cell markers. Whereas, fine-tuned reduction of Drp1 protein maintains the characteristic mitochondrial shape and gene-expression of the primed 'stem/progenitor-like state' to accelerate neoplastic transformation, and more complete reduction of Drp1 protein prevents it. Therefore, our data highlights a 'goldilocks' level of Drp1 repression supporting stem/progenitor state dependent neoplastic transformation.