Papers of the Month
By Yu-Wei Chen, Marla Broadfoot, Kelly Lenox, and Qing Xu
NTP scopes the future of developmental neurotoxicity screening
Scientists in the National Toxicology Program (NTP) published a proof-of-concept study that characterized a set of in vitro and alternative animal assays for their ability to measure how chemicals affected a variety of neurodevelopmental processes. The paper, which leads off a special issue on developmental neurotoxicity (DNT) in the journal Toxicological Sciences, describes the approach to creating the chemical library that was provided to project collaborators; development of a data analysis strategy; the structure of the web application that provides access to results; and key issues and knowledge gaps to address so that the approach can be used in regulatory decision-making.
NTP needs new approaches to assess DNT for classes of chemicals, such as flame retardants, polycyclic aromatic compounds, and perfluorinated compounds, including mixtures. Project collaborators used the chemical library in cell-based assays that targeted specific neurodevelopmental processes and in the alternative animal models of zebrafish and planaria. NTP scientists structured the resulting data to make it directly comparable across studies, then designed a method to identify DNT active chemicals and their potencies, to aid in prioritizing compounds for further testing. Finally, they developed an interactive web application to allow data exploration and visualization, called Developmental NeuroToxicity Data Integration and Visualization Enabling Resource (DNT-DIVER).
Nine companion papers highlight details of the data analysis pipeline, findings from the zebrafish and planaria studies, how chemicals that showed DNT activity could be compared across models, and insight on using the approach in regulatory decision-making. (KL)
Citation: Behl M, Ryan K, Hsieh JH, Parham F, Shapiro AJ, Collins BJ, Sipes NS, Birnbaum LS, Bucher JR, Foster PMD, Walker NJ. Paules RS, Tice RR. 2018. Screening for developmental neurotoxicity at the National Toxicology Program: the future is here. Toxicol Sci; doi:10.1093/toxsci/kfy278 [Online 28 November 2018].
Accommodation of mismatched base pairing by DNA polymerase mu
NIEHS researchers have revealed that DNA polymerase mu (pol mu) promotes insertion of mismatched nucleotides during repair of DNA damage. The findings provide new insights into the role of pol mu in genome instability that may lead to mutagenesis, cell death, or cancer.
As a member of the X-family DNA repair enzymes, pol mu adds nucleotides to fill the gaps of double-strand DNA breaks during non-homologous end joining (NHEJ) repair. DNA mutations occur when mismatched nucleotides are incorporated into the damaged DNA. In this study, the researchers found that during nucleotide incorporation, pol mu could accommodate non-complementary base pairing of nucleotide dGTP with the DNA base T (dG:dT), a well-known DNA mispairing that is prone to cause mutations in cells.
Using a DNA substrate with a single nucleotide gap with template base T, they observed that pol mu could efficiently insert the mismatched nucleotide dGTP, or oxidized dGTP, a common product generated from oxidative stress, into the gap. The mismatch insertion was subsequently followed by DNA ligation that sealed the break. In addition, they discovered that unlike pol mu, other X-family repair enzymes pol beta and pol lambda were unable to accommodate the dG:dT mismatch, which suggested a unique feature of pol mu in NHEJ repair. (QX)
Citation: Caglayan M, Wilson SH. 2018. Pol mu dGTP mismatch insertion opposite T coupled with ligation reveals promutagenic DNA repair intermediate. Nat Commun 9(1):4213.
Farmer study links certain pesticides with thyroid disruption
NIEHS scientists and their collaborators reported that use of certain pesticides is significantly associated with thyroid disease among private pesticide applicators who participated in the long-term Agricultural Health Study (AHS). The new results both confirmed earlier AHS results and expanded upon them by including women, adding five more years of follow-up data, and focusing only on incident disease — new cases diagnosed during the study period among those who did not have thyroid disease at enrollment. Exposure to pesticides is a potentially modifiable risk factor for thyroid disease and thus may have important public health implications.
The analysis of data from 35,150 AHS participants identified 829 cases of incident hypothyroidism. Researchers found statistically significant associations between a higher risk of hypothyroidism and the use of the organochlorine insecticides aldrin, chlordane, heptachlor, and lindane; the organophosphate insecticides coumaphos, diazinon, dichlorvos, and malathion; and the herbicides dicamba, glyphosate, and 2,4,dichlorophenoxyacetic acid (2,4-D). Use of fungicides, fumigants, pyrethroid insecticides, and carbamate was not significantly associated with hypothyroidism. The authors suggested that mechanistic studies on how pesticides act on the thyroid gland are needed to better understand these results. (KL)
Citation: Shrestha S, Parks CG, Goldner WS, Kamel F, Umbach DM, Ward MH, Lerro CC, Koutros S, Hofmann JN, Beane Freeman LE, Sandler DP. 2018. Pesticide use and incident hypothyroidism in pesticide applicators in the Agricultural Health Study. Environ Health Perspect 126(9):97008.
CA2 region controls neuronal communication in hippocampus
Researchers from NIEHS found that neuronal activity in hippocampal area CA2, a brain region thought to regulate social memory, affects a form of electrical neuronal communication called oscillations.
In the brain, neurons can communicate with each other through synchronized oscillations in voltage. These oscillations occur in various frequencies, including the gamma frequency, ranging from 30 to 100 Hz, and the ripple frequency, ranging from 100 to 250 Hz. Oscillations in the hippocampus shift in frequency during various behaviors and cognitive functions, but it remains unclear how these oscillations are modulated and where they originate.
To answer this question, NIEHS researchers combined electrophysiological recordings with chemogenetic tools, allowing them to manipulate the activity of CA2 neurons. They found that selectively increasing or decreasing activity of these neurons bidirectionally affected occurrence of gamma and ripple oscillations, suggesting that these oscillations are modulated by or originate in hippocampal area CA2.
These results highlighted a role of hippocampal area CA2 in modulating brain oscillations, which was consistent with a role of CA2 in cognitive functions, such as formation of social memories. Because gamma oscillations are known to be impaired in patients with schizophrenia, the finding suggests that CA2 may be involved in the pathophysiology of schizophrenia. (YWC)
Citation: Alexander GM, Brown LY, Farris S, Lustberg D, Pantazis C, Gloss B, Plummer NW, Jensen P, Dudek SM. 2018. CA2 neuronal activity controls hippocampal low gamma and ripple oscillations. Elife; doi:10.7554/eLife.38052 [Online 2 November 2018].
Gene responsible for postmenopausal fat accumulation
NIEHS scientists determined that one of the two domains for estrogen receptor alpha, called ER-alpha AF-1, prevents fat accumulation in female mice. It is a component effect of the metabolic syndrome involving obesity and insulin resistance that results from a loss of estrogen action. The findings point to a new target and approach for the development of hormone replacement therapies that may one day ramp up metabolism and prevent fat accumulation in postmenopausal women.
Estrogen receptor alpha is typically activated through the binding of molecules involving two physical domains, called ER-alpha AF-1 and ER-alpha AF-2. In this study, NIEHS researchers investigated whether targeting ER-alpha AF-1 might provide a precise way to control weight gain or insulin resistance without off-target effects. They started with a mouse model that had a disabled ER-alpha AF-2 domain and removed its ovaries to mimic menopause. The researchers activated ER-alpha AF-1 in the mice by treating them with tamoxifen, a drug generally used as a hormone therapy for breast cancer patients, and then fed them a high-fat diet for ten weeks.
They found that tamoxifen treatment prevented obesity, but did not eliminate insulin resistance in the treated mice compared with the untreated mice. The research suggested ER-alpha AF-1 could be a novel target. (MB)
Citation: Arao Y, Hamilton KJ, Lierz SL, Korach KS. 2018. N-terminal transactivation function, AF-1, of estrogen receptor alpha controls obesity through enhancement of energy expenditure. Mol Metab 18:68–78.
(Marla Broadfoot, Ph.D., is a contract writer for the NIEHS Office of Communications and Public Liaison. Yu-Wei Chen, Ph.D., is a Special Volunteer in the NIEHS Neurobiology Laboratory. Qing Xu is a biologist in the NIEHS Metabolism, Genes, and Environment Group.)