DNA loop extrusion by human cohesin.

Eukaryotic genomes are folded into loops and topologically associating domains, which contribute to chromatin structure, gene regulation, and gene recombination. These structures depend on cohesin, a ring-shaped DNA-entrapping adenosine triphosphatase (ATPase) complex that has been proposed to form loops by extrusion. Such an activity has been observed for condensin, which forms loops in mitosis, but not for cohesin. Using biochemical reconstitution, we found that single human cohesin complexes form DNA loops symmetrically at rates up to 2.1 kilo-base pairs per second. Loop formation and maintenance depend on cohesin's ATPase activity and on NIPBL-MAU2, but not on topological entrapment of DNA by cohesin. During loop formation, cohesin and NIPBL-MAU2 reside at the base of loops, which indicates that they generate loops by extrusion. Our results show that cohesin and NIPBL-MAU2 form an active holoenzyme that interacts with DNA either pseudo-topologically or non-topologically to extrude genomic interphase DNA into loops.

Rapid movement and transcriptional re-localization of human cohesin on DNA.

The spatial organization, correct expression, repair, and segregation of eukaryotic genomes depend on cohesin, ring-shaped protein complexes that are thought to function by entrapping DNA It has been proposed that cohesin is recruited to specific genomic locations from distal loading sites by an unknown mechanism, which depends on transcription, and it has been speculated that cohesin movements along DNA could create three-dimensional genomic organization by loop extrusion. However, whether cohesin can translocate along DNA is unknown. Here, we used single-molecule imaging to show that cohesin can diffuse rapidly on DNA in a manner consistent with topological entrapment and can pass over some DNA-bound proteins and nucleosomes but is constrained in its movement by transcription and DNA-bound CCCTC-binding factor (CTCF). These results indicate that cohesin can be positioned in the genome by moving along DNA, that transcription can provide directionality to these movements, that CTCF functions as a boundary element for moving cohesin, and they are consistent with the hypothesis that cohesin spatially organizes the genome via loop extrusion.

Abdominal ultrasound in the intensive care unit: a 3-year survey on 400 patients.

OBJECTIVE: This study analyzed 400 ultrasound examinations in the ICU to assess the indications of this imaging modality. DESIGN AND SETTING: Retrospective analysis on prospectively collected data on 400 patients in a tertiary care hospital. PATIENTS AND PARTICIPANTS: The observational, prospective, clinical study examined 400 bedside abdominal ultrasound examinations performed in the ICU, of which 2% were performed emergently, 56% urgently, and 42% electively. MEASUREMENTS AND RESULTS: Environmental conditions impaired the examination slightly in 54%, moderately in 27%, and severely in 4%. Total time per study ranged from 1 to 45 min (median 10). New pathological findings were detected in 31% while 33% confirmed already known pathologies. In 53% there was no therapeutic consequence, in 27% treatment was continued based on the sonographic findings, in 10% an intervention was necessary, in 6% other therapeutic changes followed, and in 4% additional evaluation was deemed necessary. In 80% no other imaging test had to be performed. CONCLUSIONS: Ultrasound studies are deemed sufficient in a large proportion of patients and help to avoid other, more elaborate imaging studies. However, more focused indications for studies may help to improve cost-effectiveness.