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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.

SUMMARY Kinases execute pivotal cellular functions and are therefore widely investigated as potential targets in anticancer treatment. Here we analyze the kinase gene expression profiles of various tumor types and reveal the wee1 kinase to be overexpressed in glioblastomas. We demonstrate that WEE1 is a major regulator of the G2 checkpoint in glioblastoma cells. Inhibition of WEE1 by siRNA or small molecular compound in cells exposed to DNA damaging agents results in abrogation of the G2 arrest, premature termination of DNA repair, and cell death. Importantly, we show that the small-molecule inhibitor of WEE1 sensitizes glioblastoma to ionizing radiation in vivo. Our results suggest that inhibition of WEE1 kinase holds potential as a therapeutic approach in treatment of glioblastoma.

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Somatic mutations in the isocitrate dehydrogenase 1 gene (IDH1) occur at high frequency in gliomas and seem to be a prognostic factor for survival in glioblastoma patients. In our set of 98 glioblastoma patients, IDH1R132 mutations were associated with improved survival of 1 year on average, after correcting for age and other variables with Cox proportional hazards models. Patients with IDH1 mutations were on average 17 years younger than patients without mutation. Mutated IDH1 has a gain of function to produce 2-hydroxyglutarate by NADPH-dependent reduction of α-ketoglutarate, but it is unknown whether NADPH production in gliomas is affected by IDH1 mutations. We assessed the effect of IDH1R132 mutations on IDH-mediated NADPH production in glioblastomas in situ. Metabolic mapping and image analysis was applied to 51 glioblastoma samples of which 16 carried an IDH1R132 mutation. NADP+-dependent IDH activity was determined in comparison with activity of NAD+-dependent IDH and all other NADPH-producing dehydrogenases, glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, malate dehydrogenase, and hexose-6-phosphate dehydrogenase. The occurrence of IDH1 mutations correlated with approx. twofold diminished NADP+-dependent IDH activity, whereas activity of NAD+-dependent IDH and the other NADP+-dependent dehydrogenases was not affected in situ in glioblastoma. The total NADPH production capacity in glioblastoma was provided for 65% by IDH activity and the occurrence of IDH1R132 mutation reduced this capacity by 38%. It is concluded that NADPH production is hampered in glioblastoma with IDH1R132 mutation. Moreover, mutated IDH1 consumes rather than produces NADPH, thus likely lowering NADPH levels even further. The low NADPH levels may sensitize glioblastoma to irradiation and chemotherapy, thus explaining the prolonged survival of patients with mutated glioblastoma.