Papers of the Month
By David Banks, Georgia Hinkley, Mahita Kadmiel, Emily Mesev, and Simone Otto
NTP scientists collaborate on risk assessment
Researchers from the National Toxicology Program, the U.S. Environmental Protection Agency, and several other national and international organizations, have collaborated on the Next Generation (NextGen) of Risk Assessment project, a multiyear effort to incorporate emerging data streams into environmental health risk assessments. Standardizing the use of new technologies in risk assessment will improve public health, through rapid identification of chemical exposures of concern, and characterization of potential risks and susceptibilities.
The researchers discussed how new resources, such as public health databases and high-throughput data, can aid in regulatory decision-making. These resources account for several factors that are not captured in traditional toxicity testing, including human variability, mechanism of action, and biomarkers of exposure. Case studies of well-characterized human toxins, such as benzene and ozone, were used to validate the use of alternative data in risk assessment.
The NextGen framework moves toward practical application of emerging technologies in risk assessment, although several challenges remain. The quick evolution of this field has led to the publication of inadequate studies, due to lagging acceptance and implementation of best practices in experimental design and data reporting. In addition, more work must be done to accurately correlate events identified by high-throughput assays with adverse health outcomes in humans. (GH)
Citation: Cote I, Andersen ME, Ankley GT, Barone S, Birnbaum LS, Boekelheide K, Bois FY, Burgoon LD, Chiu WA, Crawford-Brown D, Crofton KM, DeVito M, Devlin RB, Edwards SW, Guyton KZ, Hattis D, Judson RS, Knight D, Krewski D, Lambert J, Maull EA, Mendrick D, Paoli GM, Patel CJ, Perkins EJ, Poje G, Portier CJ, Rusyn I, Schulte PA, Simeonov A, Smith MT, Thayer KA, Thomas RS, Thomas R, Tice RR, Vandenberg JJ, Villeneuve DL, Wesselkamper S, Whelan M, Whittaker C, White R, Xia M, Yauk C, Zeise L, Zhao J, DeWoskin RS. 2016. The Next Generation of Risk Assessment multiyear study — highlights of findings, applications to risk assessment, and future directions. Environ Health Perspect; doi:10.1289/EHP233 [Online 19 April 2016].
Deficiency in LC3-associated phagocytosis linked with autoinflammatory symptoms
An NIEHS scientist was part of a team that found a link between LC3-associated phagocytosis (LAP) and autoinflammatory disease. The findings show that animals deficient in LAP manifest symptoms of systemic lupus erythematosus (SLE).
Canonical autophagy is an evolutionarily conserved catabolic process that is triggered during starvation or stress and involves several phases, including initiation, nucleation, elongation and closure, recycling, and degradation. However, there are forms of noncanonical autophagy, such as LAP, which bypass some of these steps, but still end in the degradation of engulfed material.
Once a cell dies, it must be removed from the body. Other studies have associated SLE with improper clearance of dying cells. In this paper, LAP-deficient mice displayed similar defects in clearance. Dying cells injected into LAP-deficient mice were engulfed, but not degraded. Continuous injection of dying cells accelerated the progression of SLE symptoms. Serum collected from LAP-deficient mice showed many characteristics associated with SLE, such as elevated levels of inflammatory cytokines, autoantibodies, type I interferon-regulated genes, and markers of kidney injury.
Since defects in canonical autophagy did not produce markers of SLE, this research suggests that LAP, or noncanonical autophagy, is essential for the digestion of dying cells and the subsequent suppression of inflammatory responses by macrophages in a lupus-like disease. (EM)
Citation: Martinez J, Cunha LD, Park S, Yang M, Lu Q, Orchard R, Li QZ, Yan M, Janke L, Guy C, Linkermann A, Virgin HW, Green DR. 2016. Noncanonical autophagy inhibits the autoinflammatory, lupus-like responses to dying cells. Nature 533(7601):115-119.
Intercellular communication is key to protecting the brain
NIEHS scientists and their colleagues are the first to determine that neurons and astroglia, two types of nonimmune cells in the central nervous system (CNS), regulate microglia endotoxin tolerance (ET). ET refers to a reduced responsiveness of innate immune cells to an endotoxin challenge following a previous encounter with the endotoxin. The research establishes a mechanism through which the brain may be protected from damage in the event of multiple immune challenges.
The scientists cultured neurons, astroglia, and microglia. Using imaging, polymerase chain reaction, and Western blot, they analyzed proteins produced in response to lipopolysaccharide challenges. They found that neurons and astroglia released macrophage colony stimulating factor (M-CSF), which acted on microglia M-CSF receptors. The process activated downstream signals to produce microglial ET. Therefore, a positive neuroprotective cycle of communication exists between healthy neurons, astroglia, and microglia in early neuroinflammation. Loss of this cycle may contribute to inflammation-associated damage in the CNS.
Understanding this regulatory mechanism is important, because without ET, microglia release proinflammatory cytokines when exposed to endotoxins, resulting in neuroinflammation, a key feature of multiple CNS disorders. Conversely, microglia with ET release anti-inflammatory cytokines, resulting in neuroprotection. (SO)
Citation: Chu CH, Wang S, Li CL, Chen SH, Hu CF, Chung YL, Chen SL, Wang Q, Lu RB, Gao HM, Hong JS. 2016. Neurons and astroglia govern microglial endotoxin tolerance through macrophage colony-stimulating factor receptor-mediated ERK1/2 signals. Brain Behav Immun 55:260-272.
New roles for Brg1 in early embryonic development
NIEHS researchers discovered a novel role for Brahma related gene 1 (Brg1) in embryonic development. By deleting the Brg1 gene in developing mice embryos precisely after implantation, they were able to reveal the critical role Brg1 plays in cell survival, cell death, and proliferation.
Embryos lacking Brg1 early in development exhibited growth retardation and abnormal development that linked with increased cell death and decreased cell proliferation. Comparison of global gene expression in embryos with and without Brg1 showed that it represses genes associated with increased cell death, and decreased cellular growth and proliferation, among others. The tumor suppressor gene p53 was identified as the top regulator of gene expression in embryos lacking Brg1. Subsequent analyses revealed elevated p53 levels in Brg1 knockout mouse embryos, explaining the increase in cell death and growth restriction.
By performing sophisticated studies in an embryonic tumor cell line, the authors provided mechanistic insight on the actions of Brg1. They suggested Brg1 could be involved in the organization of the chromatic architecture, which in turn leads to cell death. The findings of this study are useful in understanding the molecular processes that occur during development, and also have implications in cancer biology. (MK)
Citation: Singh AP, Foley JF, Rubino M, Boyle MC, Tandon A, Shah R, Archer TK. 2016. Brg1 enables rapid growth of the early embryo by suppressing genes that regulate apoptosis and cell growth arrest. Mol Cell Biol; doi:10.1128/MCB.01101-15 [Online 16 May 2016].
Scientists find susceptibility link to neurodegenerative diseases
NIEHS researchers and their collaborators were the first to find a direct link between a single genetic variant that affects the oxidative stress response pathway and susceptibility to a number of neurodegenerative diseases, including progressive supernuclear palsy, Parkinson’s disease, corticobasal degeneration, and Alzheimer’s disease.
The team discovered the link by integrating findings from several other metadata analyses, including genome-wide association studies of neurodegenerative diseases; genome-spanning maps of binding for the oxidative stress protective transcription factor NRF2; and other datasets. Using stringent selection criteria, the scientists developed a list of candidate single nucleotide polymorphisms (SNPs), or location-specific changes in the DNA sequence, and verified them by assessing relative NRF2/sMAF binding and gene expression.
A SNP within the binding site for the proteins NRF2 and sMAF in the MAPT gene was associated with increased binding of NRF2 to its target sequence, an increase in MAPT gene expression, and a reduced risk of Parkinsonian diseases. The NRF2 gene is generally activated to protect cells in the presence of oxidative stress, and the researchers hypothesize that increased MAPT-protein tau expression is protective under oxidative stress conditions.
Prior to this study, NRF2 had been implicated in protecting against oxidative stress, but not tau-related neurotoxicity. In this study, the researchers were able to make the connection between MAPT-tau expression with differential NRF2 and sMAF binding within the context of neurodegenerative disease risk. (DB)
Citation: Wang X, Campbell MR, Lacher SE, Cho HY, Wan M, Crowl CL, Chorley BN, Bond GL, Kleeberger SR, Slattery M, Bell DA. 2016. A polymorphic antioxidant response element links NRF2/sMAF binding to enhanced MAPT expression and reduced risk of Parkinsonian disorders. Cell Rep 15(4):830-842. (Story)