Advances in radiobiological studies using a microbeam

Spatially Fractionated Microbeam Analysis of Tissue-sparing Effect for Spermatogenesis

Spatially fractionated radiation therapy (SFRT) has been based on the delivery of a single high-dose fraction to a large treatment area that has been divided into several smaller fields, reducing the overall toxicity and adverse effects. Complementary microbeam studies have also shown an effective tissue-sparing effect (TSE) in various tissue types and species after spatially fractionated irradiation at the microscale level; however, the underlying biological mechanism remains elusive. In the current study, using the combination of an ex vivo mouse spermatogenesis model and high-precision X-ray microbeams, we revealed the significant TSE for maintaining spermatogenesis after spatially fractionated microbeam irradiation. We used the following ratios of the irradiated to nonirradiated areas: 50:50, 150:50 and 350:50 µm-slit, where approximately 50, 75 and 87.5% of the sample was irradiated (using center-to-center distances of 100, 200 and 400 µm, respectively). We found that the 50 and 75% micro-slit irradiated testicular tissues showed an almost unadulterated TSE for spermatogenesis, whereas the 87.5% micro-slit irradiated tissues showed an incomplete TSE. This suggests that the TSE efficiency for spermatogenesis is dependent on the size of the nonirradiated spermatogonial stem cell pool in the irradiated testicular tissues. In addition, there would be a spatiotemporal limitation of stem cell migration/competition, resulting in the insufficient TSE for 87.5% micro-slit irradiated tissues. These stem cell characteristics are essential for the accurate prediction of tissue-level responses during or after SFRT, indicating the clinical potential for achieving better outcomes while preventing adverse effects.

A Mouse Ear Model for Bystander Studies Induced by Microbeam Irradiation

Radiation-induced bystander effects have been observed in vitro and in cell and tissue culture models, however, there are few reported studies showing these effects in vivo. To our knowledge, this is the first reported study on bystander effects induced by microbeam irradiation in an intact living mammal. The mouse ear was used to investigate radiation-induced bystander effects in keratinocytes, utilizing a 3 MeV proton microbeam (LET 13.1 keV/μm) with a range in skin of about 135 μm. Using a custom-designed holder, the ear of an anesthetized C57BL/6J mouse was flattened by gentle suction and placed over the microbeam port to irradiate cells along a 35 μm wide, 6 mm long path. Immunohistochemical analysis of γ-H2AX foci formation in tissue sections revealed, compared to control tissue, proton-induced γ-H2AX foci formation in one of the two epidermal layers of the mouse ear. Strikingly, a higher number of cells than expected showed foci from direct irradiation effects. Although the proton-irradiated line was ~35 μm wide, the average width spanned by γ-H2AX-positive cells exceeded 150 μm. Cells adjacent to or in the epidermal layer opposite the γ-H2AX-positive region did not exhibit foci. These findings validate this mammalian model as a viable system for investigating radiation-induced bystander effects in an intact living organism.

Mechanisms and biological importance of photon-induced bystander responses: do they have an impact on low-dose radiation responses

Elucidating the biological effect of low linear energy transfer (LET), low-dose and/or low-dose-rate ionizing radiation is essential in ensuring radiation safety. Over the past two decades, non-targeted effects, which are not only a direct consequence of radiation-induced initial lesions produced in cellular DNA but also of intra- and inter-cellular communications involving both targeted and non-targeted cells, have been reported and are currently defining a new paradigm in radiation biology. These effects include radiation-induced adaptive response, low-dose hypersensitivity, genomic instability, and radiation-induced bystander response (RIBR). RIBR is generally defined as a cellular response that is induced in non-irradiated cells that receive bystander signals from directly irradiated cells. RIBR could thus play an important biological role in low-dose irradiation conditions. However, this suggestion was mainly based on findings obtained using high-LET charged-particle radiations. The human population (especially the Japanese, who are exposed to lower doses of radon than the world average) is more frequently exposed to low-LET photons (X-rays or γ-rays) than to high-LET charged-particle radiation on a daily basis. There are currently a growing number of reports describing a distinguishing feature between photon-induced bystander response and high-LET RIBR. In particular, photon-induced bystander response is strongly influenced by irradiation dose, the irradiated region of the targeted cells, and p53 status. The present review focuses on the photon-induced bystander response, and discusses its impact on the low-dose radiation effect.

Metabolic oxygen consumption measurement with a single-cell biosensor after particle microbeam irradiation

A noninvasive, self-referencing biosensor/probe system has been integrated into the Columbia University Radiological Research Accelerator Facility Microbeam II end station. A single-cell oxygen consumption measurement has been conducted with this type of oxygen probe in 37° C Krebs-Ringer Bicarbonate buffer immediately before and after a single-cell microbeam irradiation. It is the first such measurement made for a microbeam irradiation, and a six fold increment of oxygen flux induced during a 15-s period of time has been observed following radiation exposure. The experimental procedure and the results are discussed.

Redox-modulated phenomena and radiation therapy: the central role of superoxide dismutases

SIGNIFICANCE

Ionizing radiation is a vital component in the oncologist's arsenal for the treatment of cancer. Approximately 50% of all cancer patients will receive some form of radiation therapy as part of their treatment regimen. DNA is considered the major cellular target of ionizing radiation and can be damaged directly by radiation or indirectly through reactive oxygen species (ROS) formed from the radiolysis of water, enzyme-mediated ROS production, and ROS resulting from altered aerobic metabolism.

RECENT ADVANCES

ROS are produced as a byproduct of oxygen metabolism, and superoxide dismutases (SODs) are the chief scavengers. ROS contribute to the radioresponsiveness of normal and tumor tissues, and SODs modulate the radioresponsiveness of tissues, thus affecting the efficacy of radiotherapy.

CRITICAL ISSUES

Despite its prevalent use, radiation therapy suffers from certain limitations that diminish its effectiveness, including tumor hypoxia and normal tissue damage. Oxygen is important for the stabilization of radiation-induced DNA damage, and tumor hypoxia dramatically decreases radiation efficacy. Therefore, auxiliary therapies are needed to increase the effectiveness of radiation therapy against tumor tissues while minimizing normal tissue injury.

FUTURE DIRECTIONS

Because of the importance of ROS in the response of normal and cancer tissues to ionizing radiation, methods that differentially modulate the ROS scavenging ability of cells may prove to be an important method to increase the radiation response in cancer tissues and simultaneously mitigate the damaging effects of ionizing radiation on normal tissues. Altering the expression or activity of SODs may prove valuable in maximizing the overall effectiveness of ionizing radiation.

Radiation-Induced Bystander Response: Mechanism and Clinical Implications

Significance: Absorption of energy from ionizing radiation (IR) to the genetic material in the cell gives rise to damage to DNA in a dose-dependent manner. There are two types of DNA damage; by a high dose (causing acute or deterministic effects) and by a low dose (related to chronic or stochastic effects), both of which induce different health effects. Among radiation effects, acute cutaneous radiation syndrome results from cell killing as a consequence of high-dose exposure. Recent advances: Recent advances in radiation biology and oncology have demonstrated that bystander effects, which are emerged in cells that have never been exposed, but neighboring irradiated cells, are also involved in radiation effects. Bystander effects are now recognized as an indispensable component of tissue response related to deleterious effects of IR. Critical issues: Evidence has indicated that nonapoptotic premature senescence is commonly observed in various tissues and organs. Senesced cells were found to secrete various proteins, including cytokines, chemokines, and growth factors, most of which are equivalent to those identified as bystander factors. Secreted factors could trigger cell proliferation, angiogenesis, cell migration, inflammatory response, etc., which provide a tissue microenvironment assisting tissue repair and remodeling. Future directions: Understandings of the mechanisms and physiological relevance of radiation-induced bystander effects are quite essential for the beneficial control of wound healing and care. Further studies should extend our knowledge of the mechanisms of bystander effects and mode of cell death in response to IR.

Genomic characterization of a three-dimensional skin model following exposure to ionizing radiation

This study aimed at characterizing the genomic response to low versus moderate doses of ionizing radiation (LDIR versus MDIR) in a three-dimensional (3D) skin model, which exhibits a closer tissue complexity to human skin than monolayer cell cultures. EpiDermFT skin plugs were exposed to 0, 0.1 and 1 Gy doses of X-rays and harvested at 5 min, 3, 8 and 24 h post-irradiation (post-IR). RNA was interrogated for global gene expression alteration. Our results show that MDIR modulated a larger number of genes over the course of 24 h compared to LDIR. However, immediately and throughout the first 3h post-IR, LDIR modulated a larger number of genes than MDIR, mostly associated with cell-cell signaling and survival promotion. Significant modulation of pathways was detected only at 3 h post-IR in MDIR with induction of genes promoting apoptosis. Collectively, the data show different dynamics in the response to LDIR versus MDIR, especially in cell-cycle distribution. LDIR-exposed tissues showed signs of attempted cell-cycle re-entry as early as 3 h post-IR, but were arrested beyond 8 h at the G1/S checkpoint. At 24 h, cells appeared to accumulate at the G2/M checkpoint. MDIR-exposed tissues did not exhibit a prolonged G1/S arrest but rather a prolonged G2/M arrest, which was sustained at least up to 24 h. By 24 h cells exhibited signs of recovery in both LDIR- and MDIR-exposed tissues. In summary, the most pronounced difference in the initial cellular response to LDIR versus MDIR is the promotion of protection and survival in LDIR versus the promotion of apoptosis in MDIR.

Quantitative modeling of chronic myeloid leukemia: insights from radiobiology

Mathematical models of chronic myeloid leukemia (CML) cell population dynamics are being developed to improve CML understanding and treatment. We review such models in light of relevant findings from radiobiology, emphasizing 3 points. First, the CML models almost all assert that the latency time, from CML initiation to diagnosis, is at most ∼10 years. Meanwhile, current radiobiologic estimates, based on Japanese atomic bomb survivor data, indicate a substantially higher maximum, suggesting longer-term relapses and extra resistance mutations. Second, different CML models assume different numbers, between 400 and 10(6), of normal HSCs. Radiobiologic estimates favor values>10(6) for the number of normal cells (often assumed to be the HSCs) that are at risk for a CML-initiating BCR-ABL translocation. Moreover, there is some evidence for an HSC dead-band hypothesis, consistent with HSC numbers being very different across different healthy adults. Third, radiobiologists have found that sporadic (background, age-driven) chromosome translocation incidence increases with age during adulthood. BCR-ABL translocation incidence increasing with age would provide a hitherto underanalyzed contribution to observed background adult-onset CML incidence acceleration with age, and would cast some doubt on stage-number inferences from multistage carcinogenesis models in general.

Radiation induced non-targeted response: mechanism and potential clinical implications

Generations of students in radiation biology have been taught that heritable biological effects require direct damage to DNA. Radiation-induced non-targeted/bystander effects represent a paradigm shift in our understanding of the radiobiological effects of ionizing radiation in that extranuclear and extracellular effects may also contribute to the biological consequences of exposure to low doses of radiation. Although radiation induced bystander effects have been well documented in a variety of biological systems, including 3D human tissue samples and whole organisms, the mechanism is not known. There is recent evidence that the NF-κB-dependent gene expression of interleukin 8, interleukin 6, cyclooxygenase-2, tumor necrosis factor and interleukin 33 in directly irradiated cells produced the cytokines and prostaglandin E2 with autocrine/paracrine functions, which further activated signaling pathways and induced NF-κB-dependent gene expression in bystander cells. The observations that heritable DNA alterations can be propagated to cells many generations after radiation exposure and that bystander cells exhibit genomic instability in ways similar to directly hit cells indicate that the low dose radiation response is a complex interplay of various modulating factors. The potential implication of the non-targeted response in radiation induced secondary cancer is discussed. A better understanding of the mechanism of the non-targeted effects will be invaluable to assess its clinical relevance and ways in which the bystander phenomenon can be manipulated to increase therapeutic gain in radiotherapy.

Confocal microscopy for modeling electron microbeam irradiation of skin

For radiation exposures employing targeted sources such as particle microbeams, the deposition of energy and dose will depend on the spatial heterogeneity of the sample. Although cell structural variations are relatively minor for two-dimensional cell cultures, they can vary significantly for fully differentiated tissues. Employing high-resolution confocal microscopy, we have determined the spatial distribution, size, and shape of epidermal keratinocyte nuclei for the full-thickness EpiDerm™ skin model (MatTek, Ashland, VA). Application of these data to calculate the microdosimetry and microdistribution of energy deposition by an electron microbeam is discussed.

Cell therapy of burns

Severe burns remain a life-threatening local and general inflammatory condition often with serious sequelae, despite remarkable progress in their treatment over the past three decades. Cultured epidermal autografts, the first and still most up-to-date cell therapy for burns, plays a key role in that progress, but drawbacks to this need to be reduced by using cultured dermal-epidermal substitutes. This review focuses on what could be, in our view, the next major breakthrough in cell therapy of burns - use of mesenchymal stromal cells (MSCs). After summarizing current knowledge, including our own clinical experience with MSCs in the pioneering field of cell therapy of radiation-induced burns, we discuss the strong rationale supporting potential interest in MSCs in treatment of thermal burns, including limited but promising pre-clinical and clinical data in wound healing and acute inflammatory conditions other than burns. Practical options for future therapeutic applications of MSCs for burns treatment, are finally considered.

Mechanism of radiation carcinogenesis: role of the TGFBI gene and the inflammatory signaling cascade

Using an immortalized human bronchial epithelial cell line, we showed previously that the transforming growth factor beta-induced (TGFBI) gene was consistently downregulated by six- to sevenfold among radiation-induced tumorigenic human cells when compared with controls. Transfection of TGFBI gene into tumor cells resulted in a significant reduction in tumor growth as well as in vitro anchorage independent growth. The observations that TGFBI knock-out animals showed increased spontaneous tumor incidence and chemically induced tumors highlight the suppressive nature of the gene. There is evidence that extranuclear/extracellular targets are important in low-dose radiation response and that the cyclo-oxygenase-2 signaling pathway mediates the process. The involvement of NFκB-dependent cytokines and the resultant inflammatory response works in concert with in modulating radiation-induced bronchial carcinogenesis.

Initial characterization of a low-molecular-weight factor enhancing the checkpoint response

In higher eukaryotes, DNA double-strand breaks (DSBs) induced by ionizing radiation activate checkpoints that delay progression through the cell cycle. Compared to delays in other phases of the cell cycle, delays induced in G(2) are longer and frequently correlate with resistance to killing by radiation. Therefore, modulation of the G(2) checkpoint offers a means to modulate cellular radiosensitivity. Although compounds are known that reduce the G(2) checkpoint and act as radiosensitizers, compounds enhancing this checkpoint have not been reported. Here we summarize evidence for a factor with such properties. We show that a highly radioresistant rat embryo fibroblast (REF) cell line displays a strong G(2) checkpoint partly as a result of a factor excreted into the growth medium by nonirradiated cells. Various tests indicate that this G(2)-arrest modulating activity (GAMA) is a small molecule showing detectable retention only after passing through filters with a molecular weight cutoff limit of less than 1,000 Da. GAMA is heat stable and resistant to treatment with proteases or nucleases. Electroelution tests show that GAMA is uncharged at neutral pH, a result that is in agreement with the observed failure to bind S- or Q-Sepharose. Investigations on the mechanism of GAMA function indicate ligand-receptor interactions and allow the classification of cells as producers, responders or both. Compounds with properties such as those of GAMA bridge intercellular communication with the DNA damage response and may function as radioprotectors.

Immersion Mirau interferometry for label-free live cell imaging in an epi-illumination geometry

In cell biology studies it is often important to avoid the damaging effects caused by fluorescent stains or UV-light. Immersion Mirau Interferometry (IMI) is an epi-illumination label-free imaging technique developed at the Columbia University Radiological Research Accelerator Facility. It is based on the principles of phase-shifting interferometry (PSI) and represents a novel approach for interferometric imaging of living cells in medium. To accommodate the use of medium, a custom immersion Mirau interferometric attachment was designed and built in-house. The space between the reference mirror and the beam splitter is filled with liquid to ensure identical optical paths in the test and reference arms. The interferometer is mountable onto a microscope objective. The greatest limitation of standard PSI is the sensitivity to environmental vibrations, because it requires consecutive acquisition of several interferograms. We are developing Simultaneous Immersion Mirau Interferometry (SIMI), which facilitates simultaneous acquisition of all interferograms and eliminates the effects of vibration. Polarization optics, incorporated into the design, introduces a phase delay to one of the components of the test beam. This enables simultaneous creation and spatial separation of two interferograms, which are combined with the background image to reconstruct the intensity map of the specimen. Our results of imaging live and fixed cells with IMI and SIMI show that this system produces images of a quality that is sufficient to perform targeted cellular irradiation experiments.

Adaptive response in zebrafish embryos induced using microbeam protons as priming dose and X-ray photons as challenging dose

In the studies reported here, a high-linear-energy-transfer (high-LET)-radiation dose was used to induce adaptive response in zebrafish embryos in vivo. Microbeam protons were used to provide the priming dose and X-ray photons were employed to provide the challenging dose. The microbeam irradiation system (Single-Particle Irradiation System to Cell, acronym as SPICE) at the National Institute of Radiological Sciences (NIRS), Japan, was employed to control and accurately quantify the number of protons at very low doses, viz., about 100 µGy. The embryos were dechorionated at 4 h post fertilization (hpf) and irradiated at 5 hpf by microbeam protons. For each embryo, ten irradiation points were arbitrarily chosen without overlapping with one another. To each irradiation point, 5, 10 or 20 protons each with an energy of 3.4 MeV were delivered. The embryos were returned back to the incubator until 10 hpf to further receive the challenging exposure, which was achieved using 2 Gy of X-ray irradiation, and then again returned to the incubator until 24 hpf for analyses. The levels of apoptosis in zebrafish embryos at 25 hpf were quantified through terminal dUTP transferase-mediated nick end-labeling (TUNEL) assay, with the apoptotic signals captured by a confocal microscope. The results revealed that 5 to 20 protons delivered at 10 points each on the embryos, or equivalently 110 to 430 µGy, could induce radioadaptive response in the zebrafish embryos in vivo.