Interactions between ends from different DNA double-strand breaks (DSBs) can produce tumorigenic chromosome translocations. Two theories for the juxta-position of DSBs in translocations, the static "contact-first" and the dynamic "breakage-first" theory, differ fundamentally in their requirement for DSB mobility. To determine whether or not DSB-containing chromosome domains are mobile and can interact, we introduced linear tracks of DSBs in nuclei. We observed changes in track morphology within minutes after DSB induction, indicating movement of the domains. In a subpopulation of cells, the domains clustered. Juxtaposition of different DSB-containing chromosome domains through clustering, which was most extensive in G1 phase cells, suggests an adhesion process in which we implicate the Mre11 complex. Our results support the breakage-first theory to explain the origin of chromosomal translocations.

KEY WORDS: CLEM, live-cell imaging, phototoxicity, photobleaching, dynamic range Controlled Light Exposure Microscopy is a novel and simple technology that strongly reduces phototoxicity and photobleaching in live-cell imaging without compromising image quality [1,2]. To minimize phototoxicity and photobleaching, CLEM reduces the excitation light dose only in parts of the image where full exposure is not needed (in background and bright foreground). In these parts of the image S/N can be reduced without loss of image quality. CLEM has been implemented on a standard fluorescence confocal microscope. Light is controlled by a feedback system consisting of an electronic circuit and a acoustic-optical modulator (AOM) placed in the excitation pathway. We show that CLEM reduces photobleaching by a factor of 7. In HeLa cells expressing chromatin associated H2B-GFP the production of reactive oxygen species (ROS) is reduced 8-fold causing a 6 times longer scanning time without noticeable cell damage [3]. We will present applications of CLEM in cell biology. For example, we will show how we monitor the dynamics of telomeres in human cells for prolonged imaging periods. Application of CLEM in this research leads to biological results that cannot be obtained with non-CLEM (conventional imaging). Finally, we will discuss quantitative imaging with CLEM and non-CLEM, correction procedures for photobleaching, and noise properties of CLEM.

Hyaluronan is a biologically active polymer, which can be formulated into nanoparticles. In our study, we aimed to probe atherosclerosis-associated inflammation by using hyaluronan nanoparticles and to determine whether they can ameliorate atherosclerosis. Hyaluronan nanoparticles (HA-NPs) were prepared by reacting amine-functionalized oligomeric hyaluronan (HA) with cholanic ester and labeled with a fluorescent or radioactive label. HA-NPs were characterized in vitro by several advanced microscopy methods. The targeting properties and biodistribution of HA-NPs were studied in apoe–/– mice, which received either fluorescent or radiolabeled HA-NPs and were examined ex vivo by flow cytometry or nuclear techniques. Furthermore, three atherosclerotic rabbits received 89Zr-HA-NPs and were imaged by PET/MRI. The therapeutic effects of HA-NPs were studied in apoe–/– mice, which received weekly doses of 50 mg/kg HA-NPs during a 12-week high-fat diet feeding period. Hydrated HA-NPs were ca. 90 nm in diameter and displayed very stable morphology under hydrolysis conditions. Flow cytometry revealed a 6- to 40-fold higher uptake of Cy7-HA-NPs by aortic macrophages compared to normal tissue macrophages. Interestingly, both local and systemic HA-NP–immune cell interactions significantly decreased over the disease progression. 89Zr-HA-NPs-induced radioactivity in atherosclerotic aortas was 30% higher than in wild-type controls. PET imaging of rabbits revealed 6-fold higher standardized uptake values compared to the muscle. The plaques of HA-NP-treated mice contained 30% fewer macrophages compared to control and free HA-treated group. In conclusion, we show favorable targeting properties of HA-NPs, which can be exploited for PET imaging of atherosclerosis-associated inflammation. Furthermore, we demonstrate the anti-inflammatory effects of HA-NPs in atherosclerosis.

Abstract:We present a new super-resolution technique, Re-scan Confocal Microscopy (RCM), based on standard confocal microscopy extended with an optical (re-scanning) unit that projects the image directly on a CCDcamera. This new microscope has improved lateral resolution and strongly improved sensitivity while maintaining the sectioning capability of a standard confocal microscope. This simple technology is typically useful for biological applications where the combination high-resolution and highsensitivity is required. Toomre, and J. Bewersdorf, "Video-rate nanoscopy using sCMOS camera-specific single-molecule localization algorithms," Nat. Methods 10(7), 653-658 (2013).

Isocitrate dehydrogenase 1 (IDH1) is mutated in various types of human cancer to IDH1 R132H , a structural alteration that leads to catalysis of a-ketoglutarate to the oncometabolite D-2-hydroxyglutarate. In this study, we present evidence that small-molecule inhibitors of IDH1 R132H that are being developed for cancer therapy may pose risks with coadministration of radiotherapy. Cancer cells heterozygous for the IDH1 R132H mutation exhibited less IDH-mediated production of NADPH, such that after exposure to ionizing radiation (IR), there were higher levels of reactive oxygen species, DNA double-strand breaks, and cell death compared with IDH1 wild-type cells. These effects were reversed by the IDH1 R132H inhibitor AGI-5198. Exposure of IDH1 wild-type cells to D-2-hydroxyglutarate was sufficient to reduce IDH-mediated NADPH production and increase IR sensitivity. Mechanistic investigations revealed that the radiosensitivity of heterozygous cells was independent of the well-described DNA hypermethylation phenotype in IDH1-mutated cancers. Thus, our results argue that altered oxidative stress responses are a plausible mechanism to understand the radiosensitivity of IDH1-mutated cancer cells. Further, they offer an explanation for the relatively longer survival of patients with IDH1-mutated tumors, and they imply that administration of IDH1 R132H inhibitors in these patients may limit irradiation efficacy in this setting.

Atherosclerosis is associated with a compromised endothelial barrier, facilitating the accumulation of immune cells and macromolecules in atherosclerotic lesions. In this study, we investigate endothelial barrier integrity and the enhanced permeability and retention (EPR) effect during atherosclerosis progression and therapy in Apoe–/– mice using hyaluronan nanoparticles (HA-NPs). Utilizing ultrastructural and en face plaque imaging, we uncover a significantly decreased junction continuity in the atherosclerotic plaque-covering endothelium compared to the normal vessel wall, indicative of disrupted endothelial barrier. Intriguingly, the plaque advancement had a positive effect on junction stabilization, which correlated with a 3-fold lower accumulation of in vivo administrated HA-NPs in advanced plaques compared to early counterparts. Furthermore, by using super-resolution and correlative light and electron microscopy, we trace nanoparticles in the plaque microenvironment. We find nanoparticle-enriched endothelial junctions, containing 75% of detected HA-NPs, and a high HA-NP accumulation in the endothelium-underlying extracellular matrix, which suggest an endothelial junctional traffic of HA-NPs to the plague. Finally, we probe the EPR effect by HA-NPs in the context of metabolic therapy with a glycolysis inhibitor, 3PO, proposed as a vascular normalizing strategy. The observed trend of attenuated HA-NP uptake in aortas of 3PO-treated mice coincides with the endothelial silencing activity of 3PO, demonstrated in vitro. Interestingly, the therapy also reduced the plaque inflammatory burden, while activating macrophage metabolism. Our findings shed light on natural limitations of nanoparticle accumulation in atherosclerotic plaques and provide mechanistic insight into nanoparticle trafficking across the atherosclerotic endothelium. Furthermore, our data contribute to the rising field of endothelial barrier modulation in atherosclerosis.

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