XIST RNA: a window into the broader role of RNA in nuclear chromosome architecture.

XIST RNA triggers the transformation of an active X chromosome into a condensed, inactive Barr body and therefore provides a unique window into transitions of higher-order chromosome architecture. Despite recent progress, how XIST RNA localizes and interacts with the X chromosome remains poorly understood. Genetic engineering of XIST into a trisomic autosome demonstrates remarkable capacity of XIST RNA to localize and comprehensively silence that autosome. Thus, XIST does not require X chromosome-specific sequences but operates on mechanisms available genome-wide. Prior results suggested XIST localization is controlled by attachment to the insoluble nuclear scaffold. Our recent work affirms that scaffold attachment factor A (SAF-A) is involved in anchoring XIST, but argues against the view that SAF-A provides a unimolecular bridge between RNA and the chromosome. Rather, we suggest that a complex meshwork of architectural proteins interact with XIST RNA. Parallel work studying the territory of actively transcribed chromosomes suggests that repeat-rich RNA 'coats' euchromatin and may impact chromosome architecture in a manner opposite of XIST A model is discussed whereby RNA may not just recruit histone modifications, but more directly impact higher-order chromatin condensation via interaction with architectural proteins of the nucleus.This article is part of the themed issue 'X-chromosome inactivation: a tribute to Mary Lyon'.

Pentoxifylline rescue preserves lung function in isolated canine lungs injured with phorbol myristate acetate.

OBJECTIVE: We hypothesized that pentoxifylline, administered after phorbol myristate acetate (PMA), would diminish the severity of lung injury. SETTING: Animal research laboratory. DESIGN: Comparative study. SUBJECTS: Mongrel dogs (n = 33). INTERVENTIONS: Baseline measurements were obtained from the isolated blood-perfused dog lung lobes after 1 h of stable perfusion and ventilation. Four different measures of lung compliance were obtained along with WBC and neutrophil counts. Pulmonary vascular resistance (PVR) and capillary filtration coefficient (Kf) were calculated, and the ratio of a normalized maximal enzymatic conversion rate to the Michaelis-Menten constant (Amax/Km) was used to assess perfused capillary surface area. The control lobes (n = 8) were ventilated and perfused for an additional 40 min while the injured lobes (n = 17) received PMA (0.1 microg/mL of perfusate). The pentoxifylline-protected lobes (n = 8) were treated with pentoxifylline (1 mg/mL of perfusate) 10 min after injury with PMA. All measurements were then repeated. MEASUREMENT AND MAIN RESULTS: The three groups did not differ significantly at baseline. The control lobes remained relatively stable over time. The injured lobes demonstrated marked deterioration in compliance: 8.79 +/- 0.7 to 5.97 +/- 0.59 mL/cm H(2)O (p < 0.05) vs 10.1 +/- 1.0 to 8.07 +/- 0.72 mL/cm H(2)O and 9.6 +/- 1.1 to 9.9 +/- 0.85 mL/cm H(2)O in the control and protected lobes, respectively. Both groups receiving PMA had similar drops in WBC and neutrophil counts, but the pentoxifylline-protected lobes had preservation of all four compliance measures. PVR increased from 37.8 +/- 1.8 to 118.6 +/- 12.7 cm H(2)O/L/min (p < 0.05) in the injured lobes vs 35.4 +/- 0.5 to 36.3 +/- 2.8 cm H(2)O/L/min and 40.4 +/- 0.04 to 46.7 +/- 2.8 cm H(2)O/L/min (p < 0.05) in the control and protected lobes, respectively. Kf increased < 25% in the protected group but more than tripled in the injured group. Amax/Km dropped from 559 +/- 36 to 441 +/- 33 mL/min (p < 0.05) in the injured lobes vs 507 +/- 14 to 490 +/- 17 mL/min and 609 +/- 34 to 616 +/- 37 mL/min in the control and pentoxifylline-protected lobes, respectively. CONCLUSIONS: The use of pentoxifylline as a rescue agent prevented the PMA-induced deterioration of lung compliance, vascular integrity, and endothelial metabolic function in this acute lung injury model, despite significant pulmonary neutrophil sequestration.

Ventilation above closing volume reduces pulmonary vascular resistance hysteresis.

The aim of this study was to determine the relationship of pulmonary vascular resistance (PVR) hysteresis and lung volume, with special attention to the effects of ventilation around closing volume (CV). Isolated, blood-perfused canine left lower lung lobes (LLL) were incrementally inflated and deflated. Airway and pulmonary artery pressures (PAP) were recorded after each stepwise volume change. Constant blood flow was provided (600 ml/min) and the pulmonary vein pressure (PVP) was held constant at 5 cm H2O. PAP changes, therefore, were a direct index of PVR changes. Group 1 lobes underwent a full inflation from complete collapse to total lobe capacity (TLC) followed by a full deflation. Group 2 lobes underwent two deflation/inflation cycles, after an initial full inflation. These cycles, both beginning at TLC, had deflation end above and below CV, respectively. Significant PVR hysteresis was noted when the first inflation and deflation were compared. The maximum difference in PAP on deflation was 3.3 cm H2O or 11%. The mean decrease was 2.7 cm H2O for 18 lobes (p < 0.0001). The PAPs on all subsequent inflations or deflations that began above CV remained 9% lower than the initial inflation (n = 9, p < 0.0001), but were not different from each other. However, the final inflation which began from below CV resulted in a 30% return of PVR hysteresis (mean increase in PAP of 0.8 cm H2O, n = 7, p < 0.004). We conclude that there is hysteresis in the PVR response during ventilation, with decreased PVR during deflation relative to the initial inflation, that this hysteresis is absent when lung volume is maintained greater than CV, and that hysteresis returns when inflation occurs after deflation below CV.