Modeling Reveals the Dependence of Hippocampal Neurogenesis Radiosensitivity on Age and Strain of Rats

Cognitive dysfunction following radiation treatment for brain cancers in both children and adults have been correlated to impairment of neurogenesis in the hippocampal dentate gyrus. Various species and strains of rodent models have been used to study radiation-induced changes in neurogenesis and these investigations have utilized only a limited number of doses, dose-fractions, age and time after exposures conditions. In this paper, we have extended our previous mathematical model of radiation-induced hippocampal neurogenesis impairment of C57BL/6 mice to delineate the time, age, and dose dependent alterations in neurogenesis of a diverse strain of rats. To the best of our knowledge, this is the first predictive mathematical model to be published about hippocampal neurogenesis impairment for a variety of rat strains after acute or fractionated exposures to low linear energy transfer (low LET) radiation, such as X-rays and γ-rays, which are conventionally used in cancer radiation therapy. We considered four compartments to model hippocampal neurogenesis and its impairment following radiation exposures. Compartments include: (1) neural stem cells (NSCs), (2) neuronal progenitor cells or neuroblasts (NB), (3) immature neurons (ImN), and (4) glioblasts (GB). Additional consideration of dose and time after irradiation dependence of microglial activation and a possible shift of NSC proliferation from neurogenesis to gliogenesis at higher doses is established. Using a system of non-linear ordinary differential equations (ODEs), characterization of rat strain and age-related dynamics of hippocampal neurogenesis for unirradiated and irradiated conditions is developed. The model is augmented with the description of feedback regulation on early and late neuronal proliferation following radiation exposure. Predictions for dose-fraction regimes compared to acute radiation exposures, along with the dependence of neurogenesis sensitivity to radiation on age and strain of rats are discussed. A major result of this work is predictions of the rat strain and age dependent differences in radiation sensitivity and sub-lethal damage repair that can be used for predictions for arbitrary dose and dose-fractionation schedules.

The Delayed Effects of Acute Radiation Syndrome: Evidence of Long-Term Functional Changes in the Clonogenic Cells of the Small Intestine

Long term or residual damage post-irradiation has been described for many tissues. In hematopoietic stem cells (HSC), this is only revealed when the HSC are stressed and required to regenerate and repopulate a myeloablated host. Such an assay cannot be used to assess the recovery potential of previously irradiated intestinal stem cells (ISC) due to their incompatibility with transplantation. The best approximation to the HSC assay is the crypt microcolony assay, also based on clonogen survival. In the current study, the regenerative capacity of intestinal clonogenic cells in mice that had survived 13 Gy irradiation (with 5% bone marrow shielding to allow survival through the hematopoietic syndrome) and were then aged for 200 d was compared to previously unirradiated age-matched controls. Interestingly, at 200 d following 13 Gy, there remained a statistically significant reduction in crypts present in the various small intestinal regions (illustrating that the gastrointestinal epithelium had not fully recovered despite the 200-d interval). However, upon re-irradiation on day 196, those mice previously irradiated had improved crypt survival and regeneration compared to the age-matched controls. This was evident in all regions of the small intestine following 11-13 Gy re-exposure. Thus, there were either more clonogens per crypt within those previously irradiated and/or those that were present were more radioresistant (possibly because a subpopulation was more quiescent). This is contrary to the popular belief that previously irradiated animals may have an impaired/delayed regenerative response and be more radiosensitive.

Acute gastrointestinal syndrome in high-dose irradiated mice

The most detailed reports of the response of the gastrointestinal system to high dose acute radiation have focused mainly on understanding the histopathology. However, to enable medical countermeasure assessment under the animal rule criteria, it is necessary to have a robust model in which the relationship between radiation dose and intestinal radiation syndrome incidence, timing, and severity are established and correlated with histopathology. Although many mortality studies have been published, they have used a variety of mouse strains, ages, radiation sources, and husbandry conditions, all of which influence the dose response. Further, it is clear that the level of bone marrow irradiation and supportive care can influence endpoints. In order to create robust baseline data, the authors have generated dose response data in adult male mice maintained under identical conditions and exposed to either total or partial-body irradiation. Partial-body irradiation includes both extensive (40%) and minimal (5%) bone marrow sparing models, the latter designed to correlate with an established primate model and allow assessment of effects of any medical countermeasure on all three major radiation syndromes (intestinal, bone marrow, and lung) in the surviving mice. Lethal dose (LD(30), LD(50), and LD(70)) data are described in the various models, along with the impact of enteric flora and response to supportive care. Correlation with diarrhea severity and histopathology are also described. These data can be used to aid the design of good laboratory practice (GLP)-compliant Animal Rule studies that are reflective of the conditions following accidental radiation exposure.

Evidence of delayed gastrointestinal syndrome in high-dose irradiated mice

The acute effects of irradiation on the gastrointestinal (GI) system are well documented, but the longer-term effects are less well known. Increased incidence of adenocarcinoma has been noted, but apart from descriptions of fibrosis, the development of other pathologies specific to survivors of acute radiation is poorly understood. Samples were taken from C57BL/6 mice irradiated with partial-body irradiation where the thorax, head, and forelimbs were shielded (i.e., sparing 40% of the bone marrow). Tissue from age-matched controls was also collected. There were clear pathological changes in the intestine associated with DEARE (Delayed Effects of Acute Radiation Exposure) at doses greater than 12 Gy, with a dose-related increase in observed pathologies. Mice maintained on the synthetic antibiotic ciprofloxacin during the acute phase (days 4 to 20), however, had a lower or delayed incidence of symptoms. After 20 d, mice developed structures similar to early adenomas. Abnormally high levels of apoptotic and mitotic cells were present in some crypts, along with the early adenomas, suggesting tissue regeneration and areas of deregulated cell turnover. Over time, there was inhibited crypt cell proliferation in animals with advanced symptoms, a blunting of the crypts and villi, and an enlargement of villus girth, with an increasingly acellular and fibrotic extracellular matrix (a characteristic that has been demonstrated previously in aging mice). Together these changes may lead to a reduced functional surface area and less motile intestine. These observations are similar to those seen in geriatric animals, suggesting a premature aging of the GI tract.