Sporadic and unpredictably large solar particle events (SPEs) pose serious health risks for future exploratory missions beyond low-Earth orbit. During large SPEs, astronauts could be exposed to high doses of high-energy protons and high-Z-high-energy (HZE) particles in a time frame of hours to a few days. This acute radiation exposure could cause the prodromal syndrome a transient period of anorexia, nausea and vomiting that starts within a few hours and may compromise crew performance. At expected doses, acute effects could be due to direct damage to intestinal mucosa as well as the effect of injury responses that involve intercellular signalling. However, molecular determinants of the prodromal syndrome are mostly still unknown.

The gastrointestinal (GI) tract is one of the most radiosensitive organs, which is not surprising considering the high turnover rate of mucosal cells. More than 30 years ago, Potten demonstrated the exquisite sensitivity of intestinal cyrpt cells. While death by high doses (more than 10 Gy, in humans) is due to GI toxicity, apoptosis of mucosal cells can be seen at 0.5 Gy and less. Several mouse models have been used to investigate early radiation damage and inflammation-related GI injury responses by gamma-rays. Many studies show an increased expression of pro-inflammatory proteins such as macrophage inflammatory protein-2 and tumor necrosis factor- alpha in murine or rat intestine after total body irradiation (TBI) of animals.

There is evidence that many chemokines are enhanced by inflammatory stimuli. In addition to direct effects, ionizing radiation (IR)-induced cytokines may compromise intestinal function. Our laboratory has seen substantial overlap between the in vivo responses to (IR) and treatment with lipopolysaccharide (LPS), which triggers an inflammatory response. LPS is also known to affect a variety of important intestinal parameters such as mucosal permeability. If space radiation, such as during large SPEs, triggers a stronger activation of such inflammatory signaling mechanisms, then this may contribute to compromise of GI function.

The principal hypothesis is that inflammatory signaling contributes to the prodromal syndrome and can affect the gut either by local signaling events and/or by systemic cytokine signaling. A genetic approach is being used in a defined mouse model system to study inflammatory signaling after space radiation with emphasis on protons, and the effects of such signaling on cellular and molecular parameters in the intestine. By blocking p38 signaling with a dominant-negative p38 mutant, generated in the laboratory of my mentor, Dr. Albert J. Fornace Jr., my intent is to assess the potential advantages of anti-inflammatory therapies to treat the prodromal syndrome.

Specific Aims

1. To measure acute changes and inflammation-associated responses induced in the small intestine of C57BL/6J (BL6) wild type (WT) mice by TBI exposure to protons (also in simulated SPEs), HZE ions, or gamma-rays.

   The initial effort is to compare the biological effectiveness of high-energy protons and HZE to gamma-rays in WT mice for a variety of cellular and molecular endpoints related to inflammation-related signaling.

   Several molecular parameters appear to be differently regulated in gamma-irradiated animals when compared to mice exposed to equitoxic doses of 1 GeV/n protons and 1 GeV/n 56Fe ions. Pro-inflammatory effects, at both the systemic and local level, appear to be greater following exposure to HZE particles and protons.

2. To determine the involvement of p38 signaling in the acute intestinal and systemic responses to radiation, particularly in simulated large SPEs, and to discern signal transduction pathways involved in these responses.

   We will employ a genetic approach with our dominant-negative p38 mutant mouse model. Mice are being bred and litters genotyped at Georgetown University. Our p38 mutant can be used as a model for countermeasures with anti-inflammatory agents to block adverse effects of radiation signaling. Based on results of Aim 1, we will choose two doses for each radiation type (gamma-rays, 1 GeV/n protons and simulated SPEs, 1 GeV/n 56Fe ions) and carry out the same time points of Aim 1 in our p38 alpha+/DN mouse model. Based on our preliminary findings, we expect the p38a+/DN mouse model to show a reduced inflammatory response, which will be reflected at the gene expression and protein level.

   Although the effort for the objective proposed in Aim 2 was planned to commence at the beginning of Year 2, we have already conducted a first set of exposures to gamma-rays and analyses are ongoing for the endpoint related to inflammation, proliferation and signaling at both local and systemic level. Our findings support the hypothesis of the original proposal. Exposure to HZE and high-energy protons appears to induce a more pronounced response at several levels, both locally in the intestine and systemically in WT mice.

The incidence, severity and onset of symptoms, such as nausea and vomiting, depend on the site of irradiation, dose and size of targeted field. Although the molecular and cellular mechanisms underlying the prodromal syndrome are still unclear, both gastrointestinal effects and central effects are likely to be involved. Our research is aimed to contribute not only to the prevention and/or management of acute space radiation effects, but also to the treatment of prodromal sydrome in patients undergoing radiotherapy or individuals involved in radiological accidents.

The tryptophan derivative 5-hydroxytryptamine (5-HT, or serotonin) is an important signaling molecule in the brain and periphery. The gastrointestinal (GI) tract is where 95 percent of the total body serotonin is produced and stored. Upon radiation exposure, enterochromaffin-like cells in the intestine release serotonin, and this event triggers nausea and vomiting via peripheral and central mechanisms involving 5-HT3 receptors, visceral afferent fibers and chemoreceptor trigger zone.

In vitro evidence suggests that p38 MAPK is involved in serotonin transport through activation of serotonin transporters (SERTs). Studies employing multiple p38 MAPK activators (ansyomicin, H2O2 and UV radiation) showed an elevation of 5-HT transport in parallel with p38 MAPK phosphorylation, as well as suppression of anisomycin stimulation by p38 MAPK siRNA treatments.

We hypothesize that p38 MAPK is a critical component in up-regulation of serotonin transport in the intestine and that it may contribute to onset of prodromal syndrome, Therefore, we are interested in investigating the joint effect of radiation exposure and p38 activity on serotonin release and transport, both locally in the GI tract and at a systemic level. The identification of molecular determinants of prodromal syndrome will importantly contribute in development of a pharmacological treatment to be employed not only for space risk countermeasures, but also on Earth in clinically and accidentally exposed individuals.

Project Description
NASA Task Book Entry