Gastrointestinal Acute Radiation Syndrome: Mechanisms, Models, Markers, and Medical Countermeasures

There have been a number of reported human exposures to high dose radiation, resulting from accidents at nuclear power plants (e.g., Chernobyl), atomic bombings (Hiroshima and Nagasaki), and mishaps in industrial and medical settings. If absorbed radiation doses are high enough, evolution of acute radiation syndromes (ARS) will likely impact both the bone marrow as well as the gastrointestinal (GI) tract. Damage incurred in the latter can lead to nutrient malabsorption, dehydration, electrolyte imbalance, altered microbiome and metabolites, and impaired barrier function, which can lead to septicemia and death. To prepare for a medical response should such an incident arise, the National Institute of Allergy and Infectious Diseases (NIAID) funds basic and translational research to address radiation-induced GI-ARS, which remains a critical and prioritized unmet need. Areas of interest include identification of targets for damage and mitigation, animal model development, and testing of medical countermeasures (MCMs) to address GI complications resulting from radiation exposure. To appropriately model expected human responses, it is helpful to study analogous disease states in the clinic that resemble GI-ARS, to inform on best practices for diagnosis and treatment, and translate them back to inform nonclinical drug efficacy models. For these reasons, the NIAID partnered with two other U.S. government agencies (the Biomedical Advanced Research and Development Authority, and the Food and Drug Administration), to explore models, biomarkers, and diagnostics to improve understanding of the complexities of GI-ARS and investigate promising treatment approaches. A two-day workshop was convened in August 2022 that comprised presentations from academia, industry, healthcare, and government, and highlighted talks from 26 subject matter experts across five scientific sessions. This report provides an overview of information that was presented during the conference, and important discussions surrounding a broad range of topics that are critical for the research, development, licensure, and use of MCMs for GI-ARS.

Highlighting the NIAID Radiation and Nuclear Countermeasures Program's Commitment to Training and Diversifying the Radiation Workforce

Developing and maintaining a robust and diverse scientific workforce is crucial to advance knowledge, drive innovation, and tackle societal issues that impact the economy and human health. The shortage of trained professionals in radiation and nuclear sciences derives from many factors, such as scarcity of specialized coursework, programming, professional development, and experiential learning at educational institutions, which significantly disrupt the training pipeline. Other challenges include small numbers of faculty and educators with specialized radiation/nuclear expertise that are continually overextended professionally and scientifically, with the burden of training falling on this subset of individuals. Even more alarming is the recent loss of radiobiologists due to increased retirements and deaths, leaving the radiobiology community with a void of mentors and knowledge. Lastly, inconsistency in acquiring stable grant funding to recruit and retain scientists is a major hurdle to training the next generation of radiation and nuclear scientists. Recommendations from the scientific community and the National Academies of Sciences, Engineering, and Medicine describe the need to bolster educational resources and provide more hands-on training experiences. Of equal importance was the suggestion that funding agencies provide more opportunities for training and tracking the radiation workforce. The Radiation and Nuclear Countermeasures Program (RNCP), and the Office of Research Training and Special Programs (ORTSP), both within the National Institute of Allergy and Infectious Diseases (NIAID) are committed to helping to develop and sustain the radiation research workforce. This commentary illustrates the importance of addressing radiation workforce development and outlines steps that the RNCP is taking to help mitigate the issue. In addition, the role for Diversity, Equity, Inclusion, and Accessibility (DEIA) in helping to increase the number of students trained in the radiation sciences is discussed, and the NIH's DEIA priorities and RNCP efforts to improve DEIA in the research community are highlighted. One of the main goals of this commentary is to provide awareness of available educational (i.e., development of a radiation biologist eBook) and funding resources. A summary of available awards targeting early- to mid-stage investigators and diversity candidates is given, and it is hoped that this list, although not exhaustive and not specific for all focus areas in radiation (e.g., cancer research), will encourage more radiation biologists to explore and apply to these under-utilized opportunities.

Product Development within the National Institutes of Health Radiation and Nuclear Countermeasures Program

The Radiation and Nuclear Countermeasures Program (RNCP) at the National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health (NIH) was established to facilitate the development of medical countermeasures (MCMs) and diagnostic approaches for use in a radiation public health emergency. Approvals for MCMs can be very challenging but are made possible under the United States Food and Drug Administration (FDA) Animal Rule, which is designed to enable licensure of drugs or biologics when clinical efficacy studies are unethical or unfeasible. The NIAID portfolio includes grants, contracts, and inter-agency agreements designed to span all aspects of drug development and encompasses basic research through FDA approval. In addition, NIAID manages an active portfolio of biodosimetry approaches to assess injuries and absorbed radiation levels to guide triage and treatment decisions. NIAID, together with grantees, contractors, and other stakeholders with promising products, works to advance candidate MCMs and biodosimetry tools through an established product development pipeline. In addition to managing grants and contracts, NIAID tests promising candidates in our established preclinical animal models, and the NIAID Program Officers work closely with sponsors as product managers to guide them through the process. In addition, a valuable benefit for stakeholders is working with the NIAID Office of Regulatory Affairs, where NIAID coordinates with the FDA to facilitate interactions between sponsors and the agency. Activities funded by NIAID include basic research (e.g., library screens to discover new products, determine early efficacy, and delineate mechanism of action) and the development of small and large animal models of radiation-induced hematopoietic, gastrointestinal, lung, kidney, and skin injury, radiation combined injury, and radionuclide decorporation. NIAID also sponsors Good Laboratory Practice product safety, pharmacokinetic, pharmacodynamic, and toxicology studies, as well as efficacy and dose-ranging studies to optimize product regimens. For later-stage candidates, NIAID funds large-scale manufacturing and formulation development of products. The program also supports Phase 1 human clinical studies to ensure human safety and to bridge pharmacokinetic, pharmacodynamic, and efficacy data from animals to humans. To date, NIAID has supported >900 animal studies and one clinical study, evaluating >500 new/repurposed radiation MCMs and biodosimetric approaches. NIAID sponsorship led to the approval of three of the four drugs for acute radiation syndrome under the FDA Animal Rule, five Investigational New Drug applications, and 18 additional submissions for Investigational Device Exemptions, while advancing 38 projects to the Biomedical Advanced Research and Development Authority for follow-on research and development.

Radiation-induced multi-organ injury

PURPOSE

Natural history studies have been informative in dissecting radiation injury, isolating its effects, and compartmentalizing injury based on the extent of exposure and the elapsed time post-irradiation. Although radiation injury models are useful for investigating the mechanism of action in isolated subsyndromes and development of medical countermeasures (MCMs), it is clear that ionizing radiation exposure leads to multi-organ injury (MOI).

METHODS

The Radiation and Nuclear Countermeasures Program within the National Institute of Allergy and Infectious Diseases partnered with the Biomedical Advanced Research and Development Authority to convene a virtual two-day meeting titled 'Radiation-Induced Multi-Organ Injury' on June 7-8, 2022. Invited subject matter experts presented their research findings in MOI, including study of mechanisms and possible MCMs to address complex radiation-induced injuries.

RESULTS

This workshop report summarizes key information from each presentation and discussion by the speakers and audience participants.

CONCLUSIONS

Understanding the mechanisms that lead to radiation-induced MOI is critical to advancing candidate MCMs that could mitigate the injury and reduce associated morbidity and mortality. The observation that some of these mechanisms associated with MOI include systemic injuries, such as inflammation and vascular damage, suggests that MCMs that address systemic pathways could be effective against multiple organ systems.

Sex differences in radiation research

PURPOSE

The Sex Differences in Radiation Research workshop addressed the role of sex as a confounder in radiation research and its implication in real-world radiological and nuclear applications.

METHODS

In April 2022, HHS-wide partners from the Radiation and Nuclear Countermeasures Program, the Office of Research on Women's Health National Institutes of Health Office of Women's Health, U.S. Food and Drug Administration, and the Radiological and Nuclear Countermeasures Branch at the Biomedical Advanced Research and Development Authority conducted a workshop to address the scientific implication and knowledge gaps in understanding sex in basic and translational research. The goals of this workshop were to examine sex differences in 1. Radiation animal models and understand how these may affect radiation medical countermeasure development; 2. Biodosimetry and/or biomarkers used to assess acute radiation syndrome, delayed effects of acute radiation exposure, and/or predict major organ morbidities; 3. medical research that lacks representation from both sexes. In addition, regulatory policies that influence inclusion of women in research, and the gaps that exist in drug development and device clearance were discussed. Finally, real-world sex differences in human health scenarios were also considered.

RESULTS

This report provides an overview of the two-day workshop, and open discussion among academic investigators, industry researchers, and U.S. government representatives.

CONCLUSIONS

This meeting highlighted that current study designs lack the power to determine statistical significance based on sex, and much is unknown about the underlying factors that contribute to these differences. Investigators should accommodate both sexes in all stages of research to ensure that the outcome is robust, reproducible, and accurate, and will benefit public health.

The NIAID/RNCP Biodosimetry Program: An Overview

Established in 2004, the Radiation and Nuclear Countermeasures Program (RNCP), within the National Institute of Allergy and Infectious Diseases (NIAID), National Institutes of Health has the central mission to advance medical countermeasure mitigators/therapeutics, and biomarkers and technologies to assess, triage, and inform medical management of patients experiencing acute radiation syndrome and/or the delayed effects of acute radiation exposure. The RNCP biodosimetry mission space encompasses: (1) basic research to elucidate novel approaches for rapid and accurate assessment of radiation exposure, (2) studies to support advanced development for US Food and Drug Administration (FDA) clearance of promising triage or treatment devices/approaches, (3) characterization of biomarkers and/or assays to determine degree of tissue or organ dose that can predict outcome of radiation injuries (i.e., organ failure, morbidity, and/or mortality), and (4) outreach efforts to facilitate interactions with researchers developing cutting edge biodosimetry approaches. Thus far, no biodosimetry device has been FDA cleared for use during a radiological/nuclear incident. At NIAID, advancement of radiation biomarkers and biodosimetry approaches is facilitated by a variety of funding mechanisms (grants, contracts, cooperative and interagency agreements, and Small Business Innovation Research awards), with the objective of advancing devices and assays toward clearance, as outlined in the FDA's Radiation Biodosimetry Medical Countermeasure Devices Guidance. The ultimate goal of the RNCP biodosimetry program is to develop and establish accurate and reliable biodosimetry tools that will improve radiation preparedness and ultimately save lives during a radiological or nuclear incident.

Phosphoproteomic analysis of thrombin- and p38 MAPK-regulated signaling networks in endothelial cells

Endothelial dysfunction is a hallmark of inflammation and is mediated by inflammatory factors that signal through G protein–coupled receptors including protease-activated receptor-1 (PAR1). PAR1, a receptor for thrombin, signals via the small GTPase RhoA and myosin light chain intermediates to facilitate endothelial barrier permeability. PAR1 also induces endothelial barrier disruption through a p38 mitogen-activated protein kinase–dependent pathway, which does not integrate into the RhoA/MLC pathway; however, the PAR1-p38 signaling pathways that promote endothelial dysfunction remain poorly defined. To identify effectors of this pathway, we performed a global phosphoproteome analysis of thrombin signaling regulated by p38 in human cultured endothelial cells using multiplexed quantitative mass spectrometry. We identified 5491 unique phosphopeptides and 2317 phosphoproteins, four distinct dynamic phosphoproteome profiles of thrombin-p38 signaling, and an enrichment of biological functions associated with endothelial dysfunction, including modulators of endothelial barrier disruption and a subset of kinases predicted to regulate p38-dependent thrombin signaling. Using available antibodies to detect identified phosphosites of key p38-regulated proteins, we discovered that inhibition of p38 activity and siRNA-targeted depletion of the p38α isoform increased basal phosphorylation of extracellular signal–regulated protein kinase 1/2, resulting in amplified thrombin-stimulated extracellular signal–regulated protein kinase 1/2 phosphorylation that was dependent on PAR1. We also discovered a role for p38 in the phosphorylation of α-catenin, a component of adherens junctions, suggesting that this phosphorylation may function as an important regulatory process. Taken together, these studies define a rich array of thrombin- and p38-regulated candidate proteins that may serve important roles in endothelial dysfunction.

Location-specific inhibition of Akt reveals regulation of mTORC1 activity in the nucleus

The mechanistic target of rapamycin complex 1 (mTORC1) integrates growth, nutrient and energy status cues to control cell growth and metabolism. While mTORC1 activation at the lysosome is well characterized, it is not clear how this complex is regulated at other subcellular locations. Here, we combine location-selective kinase inhibition, live-cell imaging and biochemical assays to probe the regulation of growth factor-induced mTORC1 activity in the nucleus. Using a nuclear targeted Akt Substrate-based Tandem Occupancy Peptide Sponge (Akt-STOPS) that we developed for specific inhibition of Akt, a critical upstream kinase, we show that growth factor-stimulated nuclear mTORC1 activity requires nuclear Akt activity. Further mechanistic dissection suggests that nuclear Akt activity mediates growth factor-induced nuclear translocation of Raptor, a regulatory scaffolding component in mTORC1, and localization of Raptor to the nucleus results in nuclear mTORC1 activity in the absence of growth factor stimulation. Taken together, these results reveal a mode of regulation of mTORC1 that is distinct from its lysosomal activation, which controls mTORC1 activity in the nuclear compartment.

The role of mTORC1 at the lysosome is well established, but mTORC1 is known to be active in other cellular locations. Here, the authors develop Akt-STOPS to inhibit Akt specifically in the nucleus and identify a new regulatory mode for mTORC1 distinct in the nucleus.

Subcellular hot spots of GPCR signaling promote vascular inflammation

G-coupled protein receptors (GPCRs) comprise the largest class of druggable targets. Signaling by GPCRs is initiated from subcellular hot spots including the plasma membrane, signalosomes, and endosomes to contribute to vascular inflammation. GPCR-G protein signaling at the plasma membrane causes endothelial barrier disruption and also cross-talks with growth factor receptors to promote proinflammatory signaling. A second surge of GPCR signaling is initiated by cytoplasmic NFκB activation mediated by β-arrestins and CARMA-BCL10-MALT1 signalosomes. Once internalized, ubiquitinated GPCRs initiate signaling from endosomes via assembly of the transforming growth factor-β-activated kinase binding protein-1 (TAB1)-TAB2-p38 MAPK complex to promote vascular inflammation. Understanding the complexities of GPCR signaling is critical for development of new strategies to treat vascular inflammation such as that associated with COVID-19.

Signaling diversity enabled by Rap1-regulated plasma membrane ERK with distinct temporal dynamics

A variety of different signals induce specific responses through a common, extracellular-signal regulated kinase (ERK)-dependent cascade. It has been suggested that signaling specificity can be achieved through precise temporal regulation of ERK activity. Given the wide distrubtion of ERK susbtrates across different subcellular compartments, it is important to understand how ERK activity is temporally regulated at specific subcellular locations. To address this question, we have expanded the toolbox of Förster Resonance Energy Transfer (FRET)-based ERK biosensors by creating a series of improved biosensors targeted to various subcellular regions via sequence specific motifs to measure spatiotemporal changes in ERK activity. Using these sensors, we showed that EGF induces sustained ERK activity near the plasma membrane in sharp contrast to the transient activity observed in the cytoplasm and nucleus. Furthermore, EGF-induced plasma membrane ERK activity involves Rap1, a noncanonical activator, and controls cell morphology and EGF-induced membrane protrusion dynamics. Our work strongly supports that spatial and temporal regulation of ERK activity is integrated to control signaling specificity from a single extracellular signal to multiple cellular processes.

MON-009 GLUT1-Mediated Glycolysis Facilitates GnRH-Induced Secretion of Luteinizing Hormone

Reproduction requires intensive energy expenditure, and energy availability impacts the function of the reproductive endocrine HPG-axis. Accordingly, the reproductive axis is suppressed during hypoglycemia. Circulating blood glucose can directly interact with gonadotropes within the highly vascular pituitary. Therefore, it is possible that gonadotropes may sense energy availability via the presence of glucose in the circulation and integrate this status with input from GnRH neurons to regulate hormone production. Gonadotropes dominantly express glucose transporter 1 (GLUT1) and increase glucose uptake in response to GnRH. Thus, we hypothesized that gonadotropes engage glycolysis in response to GnRH stimulation due to the high energy demand of protein synthesis required for LH production. We developed an approach to sort and successfully culture primary gonadotropes from wild type mice. Using this approach, we performed extracellular flux analysis and found that gonadotropes respond to GnRH by inducing GLUT1-mediated glycolysis that is independent of mitochondrial respiration. Knock-down of GLUT1 expression in the LβT2 gonadotrope cell line, glucose restriction, or treatment with the competitive inhibitor of glycolysis, 2-DG, diminished GnRH-induced LH secretion, indicating GLUT1 expression is necessary for maximal GnRH-induced LH secretion. We confirmed this observation in primary female mouse gonadotropes by limiting glucose availability which resulted in diminished basal LH and FSH secretion. Lastly, GLUT1 expression in the pituitary correlates with GnRH receptor expression and is increased during the LH surge in a mouse model. These results implicate glucose uptake through GLUT1 as permissive for gonadotrope secretion of LH and therefore reproductive function, especially the LH surge. We conclude that GLUT1-mediated glucose uptake is an important rate-limiting step in gonadotropin synthesis and operation of the HPG-axis.

APC2 associates with the actin cortex through a multipart mechanism to regulate cortical actin organization and dynamics in the Drosophila ovary

The actin cortex that lines the plasma membrane of most eukaryotic cells resists external mechanical forces and plays critical roles in a variety of cellular processes including morphogenesis, cytokinesis, and cell migration. Despite its ubiquity and significance, we understand relatively little about the composition, dynamics, and structure of the actin cortex. Adenomatous polyposis coli (APC) proteins regulate the actin and microtubule cytoskeletons through a variety of mechanisms, and in some contexts, APC proteins are cortically enriched. Here we show that APC2 regulates cortical actin dynamics in the follicular epithelium and the nurse cells of the Drosophila ovary and in addition affects the distribution of cortical actin at the apical side of the follicular epithelium. To understand how APC2 influences these properties of the actin cortex, we investigated the mechanisms controlling the cortical localization of APC2 in S2 cultured cells. We previously showed that the N-terminal half of APC2 containing the Armadillo repeats and the C-terminal 30 amino acids (C30) are together necessary and sufficient for APC2's cortical localization. Our work presented here supports a model that cortical localization of APC2 is governed in part by self-association through the N-terminal APC Self-Association Domain (ASAD) and a highly conserved coiled-coil within the C30 domain.