Development of a single ion hit facility at the Pierre Sue Laboratory: a collimated microbeam to study radiological effects on targeted living cells

Laser-induced radiation microbeam technology and simultaneous real-time fluorescence imaging in live cells

The use of nano- and microbeam techniques to induce and identify subcellular localized energy deposition within a region of a living cell provides a means to investigate the effects of low radiation doses. Particularly within the nucleus where the propagation and processing of deoxyribonucleic acid (DNA) damage (and repair) in both targeted and nontargeted cells, the latter being able to study cell-cell (bystander) effects. We have pioneered a near infrared (NIR) femtosecond laser microbeam to mimic ionizing radiation through multiphoton absorption within a 3D femtoliter volume of a highly focused Gaussian laser beam. The novel optical microbeam mimics both complex ionizing and UV-radiation-type cell damage including double strand breaks (DSBs). Using the microbeam technology, we have been able to investigate the formation of DNA DSB and subsequent recruitment of repair proteins to the submicrometer size site of damage introduced in viable cells. The use of a phosphorylated H2AX (γ-H2AX a marker for DSBs, visualized by immunofluorescent staining) and real-time imaging of fluorescently labeling proteins, the dynamics of recruitment of repair proteins in viable mammalian cells can be observed. Here we show the recruitment of ATM, p53 binding protein 1 (53BP1), and RAD51, an integral protein of the homologous recombination process in the DNA repair pathway and Ku-80-GFP involved in the nonhomologous end joining (NHEJ) pathway as exemplar repair process to show differences in the repair kinetics of DNA DSBs. The laser NIR multiphoton microbeam technology shows persistent DSBs at later times post laser irradiation which are indicative of DSBs arising at replication presumably from UV photoproducts or clustered damage containing single strand breaks (SSBs) that are also observed. Effects of the cell cycle may also be investigated in real time. Postirradiation and fixed cells studies show that in G1 cells a fraction of multiphoton laser-induced DSBs is persistent for >6h in addition to those induced at replication demonstrating the broad range of timescales taken to repair DNA damage.

Ionizing radiation microbeam facilities for radiobiological studies in Europe

A growing body of experimental evidence gathered in the last 10-15 years with regard to targeted and non-targeted effects of low doses of ionizing radiation (hyper-radiosensitivity, induced radio-resistance, adaptive response, genomic instability, bystander effects) has pushed the radiobiology research towards a better understanding of the mechanisms underlying these phenomena, the extent to which they are active in-vivo, and how they are inter-related. In such a way factors could be obtained and included in the estimation of potential cancer risk to the human population of exposure to low levels of ionizing radiation. Different experimental approaches have been developed and employed to study such effects in-vitro (medium transfer experiments; broad-field irradiation at low doses also with insert or shielding systems...). In this regard, important contributions came from ionizing radiation microbeam facilities that turn to be powerful tools to perform selective irradiations of individual cells inside a population with an exact, defined and reproducible dose (i.e. number of particles, in case of charged particle microbeams). Over the last 20 years the use of microbeams for radiobiological applications increased substantially and a continuously growing number of such facilities, providing X-rays, electrons, light and heavy ions, has been developing all over the world. Nowadays, just in Europe there are 12 microbeam facilities fully-operational or under-development, out of more than 30 worldwide. An overview of the European microbeam facilities for radiobiological studies is presented and discussed in this paper.