Multiphoton near-infrared femtosecond laser pulse-induced DNA damage with and without the photosensitizer proflavine

Cellular lasers for cell imaging and biosensing

The possibility to produce laser action involving biomaterials, in particular (single) biological cells, has fostered the development of cellular lasers as a novel approach in biophotonics. In this respect, cells that are engineered to carry gain medium (e.g., fluorescent dyes or proteins) are placed inside an optical cavity (i.e., typically a sandwich of highly reflective mirrors), allowing the generation of stimulated emission upon sufficient optical pumping. In another scenario, micron-sized optical resonators supporting whispering-gallery mode (WGM) or semiconductor-based laser probes can be internalized by the cells and support light amplification. This review summarizes the recent advances in the fields of biolasers and cellular lasers, and most importantly, highlights their potential applications in the fields of in vitro and in vivo cell imaging and analysis. They include biosensing (e.g., in vitro detection of sodium chloride (NaCl) concentration), cancer cell imaging, laser-emission-based microscope, cell tracking, cell distinction study, and tissue contraction monitoring in zebrafish. Lastly, several fundamental issues in developing cellular lasers including laser probe fabrication, biocompatibility of the system, and alteration of local refractive index of optical cavities due to protein absorption or probe aggregation are described. Cellular lasers are foreseen as a promising tool to study numerous biological and biophysical phenomena. STATEMENT OF SIGNIFICANCE: Biolasers are generation of laser involving biological materials. Biomaterials, including single cells, can be engineered to incorporate laser probes or fluorescent proteins or fluorophores, and the resulting light emission can be coupled to optical resonator, allowing generation of cellular laser emission upon optical pumping. Unlike fluorescence, this stimulated emission is very sensitive and is capable of detecting small alterations in the optical property of the cells and their environment. In this review, recent development and applications of cellular lasers in the fields of in vitro and in vivo cell imaging, cell tracking, biosensing, and cell/tissue analysis are highlighted. Several challenges in developing cellular lasers including probe fabrication and biocompatibility as well as alteration of cellular environment are explained.

Phototoxicity induced in living HeLa cells by focused femtosecond laser pulses: a data-driven approach

Nonlinear optical microscopy is a powerful label-free imaging technology, providing biochemical and structural information in living cells and tissues. A possible drawback is photodamage induced by high-power ultrashort laser pulses. Here we present an experimental study on thousands of HeLa cells, to characterize the damage induced by focused femtosecond near-infrared laser pulses as a function of laser power, scanning speed and exposure time, in both wide-field and point-scanning illumination configurations. Our data-driven approach offers an interpretation of the underlying damage mechanisms and provides a predictive model that estimates its probability and extension and a safety limit for the working conditions in nonlinear optical microscopy. In particular, we demonstrate that cells can withstand high temperatures for a short amount of time, while they die if exposed for longer times to mild temperatures. It is thus better to illuminate the samples with high irradiances: thanks to the nonlinear imaging mechanism, much stronger signals will be generated, enabling fast imaging and thus avoiding sample photodamage.

Cancer radiotherapy based on femtosecond IR laser-beam filamentation yielding ultra-high dose rates and zero entrance dose

Since the invention of cancer radiotherapy, its primary goal has been to maximize lethal radiation doses to the tumor volume while keeping the dose to surrounding healthy tissues at zero. Sadly, conventional radiation sources (γ or X rays, electrons) used for decades, including multiple or modulated beams, inevitably deposit the majority of their dose in front or behind the tumor, thus damaging healthy tissue and causing secondary cancers years after treatment. Even the most recent pioneering advances in costly proton or carbon ion therapies can not completely avoid dose buildup in front of the tumor volume. Here we show that this ultimate goal of radiotherapy is yet within our reach: Using intense ultra-short infrared laser pulses we can now deposit a very large energy dose at unprecedented microscopic dose rates (up to 10(11) Gy/s) deep inside an adjustable, well-controlled macroscopic volume, without any dose deposit in front or behind the target volume. Our infrared laser pulses produce high density avalanches of low energy electrons via laser filamentation, a phenomenon that results in a spatial energy density and temporal dose rate that both exceed by orders of magnitude any values previously reported even for the most intense clinical radiotherapy systems. Moreover, we show that (i) the type of final damage and its mechanisms in aqueous media, at the molecular and biomolecular level, is comparable to that of conventional ionizing radiation, and (ii) at the tumor tissue level in an animal cancer model, the laser irradiation method shows clear therapeutic benefits.

Surgical applications of femtosecond lasers

Femtosecond laser ablation permits non-invasive surgeries in the bulk of a sample with submicrometer resolution. We briefly review the history of optical surgery techniques and the experimental background of femtosecond laser ablation. Next, we present several clinical applications, including dental surgery and eye surgery. We then summarize research applications, encompassing cell and tissue studies, research on C. elegans, and studies in zebrafish. We conclude by discussing future trends of femtosecond laser systems and some possible application directions.

Spatial control of reactive oxygen species formation in fibroblasts using two-photon excitation

Two-photon excitation (2PE) provides a means of generating reactive oxygen species (ROS) in cells and tissues with a high degree of spatial specificity. In cultured monolayers of human fibroblasts and fibroblast-derived cells treated with the commonly used probe of ROS formation, 5-(and 6)-chloromethyl-2',7'-dichlorodihydrofluorescein diacetate, acetyl ester (CM-H(2)DCFDA), cells irradiated through a microscope objective with 150 fs near-infrared laser pulses became highly fluorescent, reflecting intracellular ROS formation. The fluorescence intensity inside cells increased quadratically with the average power of radiation for pulsed excitation and was unchanged for continuous wave irradiation with the same average power. Single fibroblasts embedded within dermal equivalents were also targeted in this manner and formed ROS, whereas neighboring unirradiated cells were spared. These results demonstrate that ROS can be generated intracellularly in skin cells using 2PE of the metabolic or oxidative products of CM-H(2)DCFDA and that formation of ROS can be localized in both cell monolayers and in a tissue equivalent. This technique should be useful in understanding the response of whole tissues such as skin to local generation of ROS and may have applications in photodynamic therapy.

Photochemical genotoxicity: principles and test methods. Report of a GUM task force

In recent years, assessing the photogenotoxic potential of a compound became an issue for certain drugs and cosmetical products. Therefore, existing methods performed according to international guidelines (e.g. OECD guidelines) were adapted to the use of concurrent UV-visible (UV-Vis) light irradiation for the assessment of photomutagenicity/photogenotoxicity. In this review, photobiological bases of the processes occurring in the cell after irradiation with UV- and/or visible (vis)-light as well as a compilation of testing methods is presented. Methods comprise cell free investigations on naked DNA and in vitro methods, such as the photo-Ames test, the photo-HPRT/photo-mouse lymphoma assay (MLA), the photo-micronucleus test (MNT), the photo-chromosomal aberration test (CA) and the photo-Comet assay. A compilation of the currently available international literature of compounds tested on photogenotoxicity is given for each method. The state of the art of photogenotoxicity testing as well as the rational for testing are outlined in relation to the recommendations reached in expert working groups at different international meetings and to regulatory guidance papers. Finally, photogenotoxicity testing as predictor of photocarcinogenicity and in the light of risk assessment is discussed.

Nanoscale spatial induction of ultraviolet photoproducts in cellular DNA by three-photon near-infrared absorption

The high-resolution spatial induction of ultraviolet (UV) photoproducts in mammalian cellular DNA is a goal of many scientists who study UV damage and repair. Here we describe how UV photoproducts can be induced in cellular DNA within nanometre dimensions by near-diffraction-limited 750 nm infrared laser radiation. The use of multiphoton excitation to induce highly localized DNA damage in an individual cell nucleus or mitochondrion will provide much greater resolution for studies of DNA repair dynamics and intracellular localization as well as intracellular signalling processes and cell-cell communication. The technique offers an advantage over the masking method for localized irradiation of cells, as the laser radiation can specifically target a single cell and subnuclear structures such as nucleoli, nuclear membranes or any structure that can be labelled and visualized by a fluorescent tag. It also increases the time resolution with which migration of DNA repair proteins to damage sites can be monitored. We define the characteristics of localized DNA damage induction by near-infrared radiation and suggest how it may be used for new biological investigations.

Selective two-photon microscopy with shaped femtosecond pulses

Selective two-photon excitation of fluorescent probe molecules using phase-only modulated ultrashort 15-fs laser pulses is demonstrated. The spectral phase required to achieve the maximum contrast in the excitation of different probe molecules or identical probe molecules in different micro-chemical environments is designed according to the principles of multiphoton intrapulse interference (MII). The MII method modulates the probabilities with which specific spectral components in the excitation pulse contribute to the two-photon absorption process due to the dependence of the absorption on the power spectrum of E2(t) [1-3]. Images obtained from a number of samples using the multiphoton microscope are presented.

Chronic moderate heat dermatitis (erythema ab igne) in five dogs, three cats and one silvered langur

Erythema ab igne, an old and rare disease in the human literature, is an erythematous, often pigmented, reticular, macular dermatosis that occurs at the site of repeated exposure to moderate heat. We identified lesions consistent with erythema ab igne in five dogs, three cats and one silvered langur (Trachypithecus cristatus[Raffles, 1821]). In dogs and cats, the cutaneous lesion distribution typically reflected chronic exposure to moderate heat during lateral or sternal recumbency. The silvered langur developed cutaneous lesions on the dorsal neck from exposure to a heat lamp. Principal clinical lesions consisted of irregular areas of alopecia (7/9) and erythema (7/9), sometimes with hyperpigmentation (3/9). Principal histological features consisted of karyomegaly (9/9) and keratinocyte atypia (4/9), scattered apoptotic or vacuolated basal cells and/or apoptotic keratinocytes (6/9), mild mixed mononuclear interstitial or interface dermatitis (9/9) with adnexal atrophy (8/9), and a variable number of wavy eosinophilic elastic fibres (9/9). The presence of these cutaneous lesions in an animal indicates that the environment should be evaluated for exposure to chronic moderate heat, and the heat source should be eliminated or modified to prevent further exposure and progression of lesions.

Spatially localized generation of nucleotide sequence-specific DNA damage

Psoralens linked to triplex-forming oligonucleotides (psoTFOs) have been used in conjunction with laser-induced two-photon excitation (TPE) to damage a specific DNA target sequence. To demonstrate that TPE can initiate photochemistry resulting in psoralen-DNA photoadducts, target DNA sequences were incubated with psoTFOs to form triple-helical complexes and then irradiated in liquid solution with pulsed 765-nm laser light, which is half the quantum energy required for conventional one-photon excitation, as used in psoralen + UV A radiation (320-400 nm) therapy. Target DNA acquired strand-specific psoralen monoadducts in a light dose-dependent fashion. To localize DNA damage in a model tissue-like medium, a DNA-psoTFO mixture was prepared in a polyacrylamide gel and then irradiated with a converging laser beam targeting the rear of the gel. The highest number of photoadducts formed at the rear while relatively sparing DNA at the front of the gel, demonstrating spatial localization of sequence-specific DNA damage by TPE. To assess whether TPE treatment could be extended to cells without significant toxicity, cultured monolayers of normal human dermal fibroblasts were incubated with tritium-labeled psoralen without TFO to maximize detectable damage and irradiated by TPE. DNA from irradiated cells treated with psoralen exhibited a 4- to 7-fold increase in tritium activity relative to untreated controls. Functional survival assays indicated that the psoralen-TPE treatment was not toxic to cells. These results demonstrate that DNA damage can be simultaneously manipulated at the nucleotide level and in three dimensions. This approach for targeting photochemical DNA damage may have photochemotherapeutic applications in skin and other optically accessible tissues.

Femtosecond near-infrared laser pulses elicit generation of reactive oxygen species in mammalian cells leading to apoptosis-like death

Two-photon excitation-based near-infrared (NIR) laser scanning microscopy is currently emerging as a new and versatile alternative to conventional confocal laser scanning microscopy, particularly for vital cell imaging in life sciences. Although this innovative microscopy has several advantages such as highly localized excitation, higher penetration depth, reduced photobleaching and photodamage, and improved signal to noise ratio, it has, however, recently been evidenced that high-power NIR laser irradiation can drastically inhibit cell division and induce cell death. In the present study we have investigated the cellular responses of unlabeled rat kangaroo kidney epithelium (PtK2) cells to NIR femtosecond laser irradiation. We demonstrate that NIR 170-fs laser pulses operating at 80-MHz pulse repetition frequency and at mean power of > or = 7 mW evoke generation of reactive oxygen species (ROS) such as H2O2 that can be visualized in situ by standard in vivo cytochemical analysis using Ni-3,3'-diaminobenzidine (Ni-DAB) as well as with a recently developed fluorescent probe Jenchrom px blue. The formation of the Ni-DAB reaction product as well as that of Jenchrom was relatively more pronounced when irradiated cells were incubated in alkaline solution (pH 8) than in those incubated in acidic solution (pH 6), suggesting peroxisomal localization of these reaction products. Two-photon time-lapse imaging of the internalization of the cell impermeate fluorescent dye propidium iodide revealed that the integrity of the plasma membrane of NIR irradiated cells is drastically compromised. Visualization of the nuclei with DNA-specific fluorescent probes such as 4',6-diamidino-2-phenylindole 24 h postirradiation further provided tangible evidence that the nuclei of these cells undergo several deformations and eventual fragmentation. That these NIR irradiated cells die by apoptosis has been established by in situ detection of DNA strand breaks using the terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling method. Because the reactive oxygen species such as H2O2 and OH* can cause noxious effects such as cell membrane injury by peroxidation of polyunsaturated lipids and proteins and oxidative phosphorylation, and alterations of ATP-dependent Ca2+ pumps, these ROS are likely to contribute to drastic cytological alterations observed in this study following NIR irradiation. Taken together, we have established that NIR laser irradiations at mean power > or = 7 mW delivered at pulse duration time of 170 fs generally used in two- and multiphoton microscopes cause oxidative stress (1) evoking production of ROS, (2) resulting in membrane barrier dysfunction, (3) inducing structural deformations and fragmentation of the nuclei as well as DNA strand breaks, (4) leading to cell death by apoptosis.