DNA double-strand breaks repaired by non-homologous end joining display limited DNA end-processing and chromosomal mobility. By contrast, double-strand breaks undergoing homology-directed repair exhibit extensive processing and enhanced motion. The molecular basis of this movement is unknown. Here, using Xenopus laevis cell-free extracts and mammalian cells, we establish that nuclear actin, WASP, and the actin-nucleating ARP2/3 complex are recruited to damaged chromatin undergoing homology-directed repair. We demonstrate that nuclear actin polymerization is required for the migration of a subset of double-strand breaks into discrete sub-nuclear clusters. Actin-driven movements specifically affect double-strand breaks repaired by homology-directed repair in G2 cell cycle phase; inhibition of actin nucleation impairs DNA end-processing and homology-directed repair. By contrast, ARP2/3 is not enriched at double-strand breaks repaired by non-homologous end joining and does not regulate non-homologous end joining. Our findings establish that nuclear actin-based mobility shapes chromatin organization by generating repair domains that are essential for homology-directed repair in eukaryotic cells.

Objective-Atherosclerosis is a vascular disease that involves lesion formation at sites of disturbed flow under the influence of genetic and environmental factors. Endothelial expression of adhesion molecules that enable infiltration of immune cells is important for lesion development. Platelet/endothelial cell adhesion molecule-1 (PECAM-1; CD31) is an adhesion and signaling receptor expressed by many cells involved in atherosclerotic lesion development. PECAM-1 transduces signals required for proinflammatory adhesion molecule expression at atherosusceptible sites; thus, it is predicted to be proatherosclerotic. PECAM-1 also inhibits inflammatory responses, on which basis it is predicted to be atheroprotective. Methods and Results-We evaluated herein the effect of PECAM-1 deficiency on development of atherosclerosis in LDL receptor-deficient mice. We found that PECAM-1 has both proatherosclerotic and atheroprotective effects, but that the former dominate in the inner curvature of the aortic arch whereas the latter dominate in the aortic sinus, branching arteries, and descending aorta. Endothelial cell expression of PECAM-1 was sufficient for its atheroprotective effects in the aortic sinus but not in the descending aorta, where the atheroprotective effects of PECAM-1 also required its expression on bone marrow-derived cells. Conclusion-We conclude that PECAM-1 influences initiation and progression of atherosclerosis both positively and negatively, and that it does so in a site-specific manner. Key Words:therosclerosis is a chronic inflammatory disease that involves formation, at discrete regions within the vasculature, of lesions characterized by the presence of inflammatory cells, lipid deposits, and extracellular matrix deposition. 1 Atherosclerotic lesions form preferentially at sites of vessel branching or high vessel curvature, which are exposed to low shear stresses or oscillatory or turbulent flow; however, genetic and environmental factors that determine circulating lipid levels, gender and immune status strongly influence lesion development in atherosusceptible regions. 2 Endothelial cells respond to low shear stress in atherosusceptible regions with increased expression of adhesion molecules that enable recruitment and infiltration of immune cells that contribute to lesion development. 3 Nevertheless, the roles of individual adhesion molecules in regulating atherosclerotic lesion development are not completely understood. See accompanying article on page 1887PECAM-1 is expressed on the surfaces of many cells involved in atherosclerotic lesion development, including monocytes, lymphocytes, platelets, and endothelial cells. 4 PECAM-1 expression is equally distributed over the entire surface of the aorta, including advanced lesions, and does not appear to be affected by hemodynamic forces or lipid levels. 5,6 PECAM-1 engages in homophilic 7 and heterophilic 8 interactions at sites of cell-cell contact. On the basis of its role as a critical component of a mechanotransducing complex that is required for ex...

The Aβ peptide aggregates into amyloid plaques at presymptomatic stages of Alzheimer's disease, but the temporal relationship between plaque formation and neuronal dysfunction is poorly understood. Here, we demonstrate that the connectivity of the peripheral olfactory neural circuit is perturbed in mice overexpressing human APPsw (Swedish mutation) prior to the onset of plaques. Expression of hAPPsw exclusively in olfactory sensory neurons (OSNs) also perturbs connectivity with associated reductions in odor-evoked gene expression and olfactory acuity. By contrast, OSN axons project correctly in mice overexpressing wild type human APP throughout the brain and in mice overexpressing human APPmv, a missense mutation that reduces Aβ production, exclusively in OSNs. Furthermore, expression of Aβ40 or Aβ42 solely in the olfactory epithelium disrupts OSN axon targeting. Our data indicate that altering the structural connectivity and function of highly plastic neural circuits is one of the pleiotropic actions of soluble human Aβ.