A Critical Review and Meta-Analysis of Impacts of Per- and Polyfluorinated Substances on the Brain and Behavior

Abstract

Per- and polyfluoroalkyl substances (PFAS) are a class of structurally diverse synthetic organic chemicals that are chemically stable, resistant to degradation, and persistent in terrestrial and aquatic environments. Widespread use of PFAS in industrial processing and manufacturing over the last 70 years has led to global contamination of built and natural environments. The brain is a lipid rich and highly vascularized organ composed of long-lived neurons and glial cells that are especially vulnerable to the impacts of persistent and lipophilic toxicants. Generally, PFAS partition to protein-rich tissues of the body, primarily the liver and blood, but are also detected in the brains of humans, wildlife, and laboratory animals. Here we review factors impacting the absorption, distribution, and accumulation of PFAS in the brain, and currently available evidence for neurotoxic impacts defined by disruption of neurochemical, neurophysiological, and behavioral endpoints. Emphasis is placed on the neurotoxic potential of exposures during critical periods of development and in sensitive populations, and factors that may exacerbate neurotoxicity of PFAS. While limitations and inconsistencies across studies exist, the available body of evidence suggests that the neurobehavioral impacts of long-chain PFAS exposures during development are more pronounced than impacts resulting from exposure during adulthood. There is a paucity of experimental studies evaluating neurobehavioral and molecular mechanisms of short-chain PFAS, and even greater data gaps in the analysis of neurotoxicity for PFAS outside of the perfluoroalkyl acids. Whereas most experimental studies were focused on acute and subchronic impacts resulting from high dose exposures to a single PFAS congener, more realistic exposures for humans and wildlife are mixtures exposures that are relatively chronic and low dose in nature. Our evaluation of the available human epidemiological, experimental, and wildlife data also indicates heightened accumulation of perfluoroalkyl acids in the brain after environmental exposure, in comparison to the experimental studies. These findings highlight the need for additional experimental analysis of neurodevelopmental impacts of environmentally relevant concentrations and complex mixtures of PFAS.

Keywords: PFAS; behavior; blood-brain barrier; brain; development; fluorochemical; neurotoxicity; persistent.

FIGURE 1

A general structural formula for perfluoroalkyl substance (PFAS), containing a hydrophobic perfluorinated alkyl tail, and a hydrophilic functional (R) group outlined in a red box. Example compounds are depicted for each of the major chemical classes of PFAS discussed: carboxylic acids, sulfonic acids, sulfonamides, ether acids, and phosphate esters.

FIGURE 2

Molecules of different sizes and biochemical properties can gain access to the brain via diverse mechanisms. This includes facilitated (transport proteins, receptor-mediated transcytosis, and adsorptive transcytosis) and passive (transcellular lipophilic and paracellular aqueous pathways) transport across the BBB. ATP-dependent efflux transporters protect the brain from toxic xenobiotics and endogenous metabolites. These transporters include p-glycoprotein (P-gp), breast cancer resistance protein (Brcp), and multidrug resistance-associated protein (Mrp) among others. The developing BBB has not yet reached its full functional capacity, with fewer junctional proteins, increased transcytosis, and lower expression of efflux pumps compared to the adult BBB may leave the fetal and infant brain more vulnerable to PFAS exposure. Similarly, certain ailments, such as conditions that cause systemic inflammation, can compromise the BBB leading to increased transport of xenobiotics into the brain.

FIGURE 3

Circumventricular organs represent a particularly vulnerable site for xenobiotic entry, as they are in close proximity to the blood-cerebrospinal fluid barrier which is lined with fenestrated (porous) endothelial cells.

FIGURE 4

Comparative assessment of average brain:serum ratios of long-chain PFAAs in experimental animal exposure studies vs. environmental exposure studies. Mean ratios represented by black circles with error bars representing ± standard error of the mean. A two-way analysis of variance (ANOVA) on log-transformed brain:serum PFAS ratios with long chain PFAA congener (PFOS, PFOA, PFNA) and exposure type (experimental animal exposure vs environmental exposure) revealed main effects of PFAA congener, F (2, 85) = 26.47, p < 0.0001, and exposure type, F (1, 85) = 114.7, p < 0.0001. A Šidák multiple comparisons post-hoc analysis with α = 0.05 indicated that environmental brain:serum ratios were greater than experimental brain:serum ratios for PFOS (p = 0.0065), PFOA (p < 0.0001), and PFNA (p < 0.0001). Asterisks indicate that experimental brain:serum ratios of PFOS were greater than PFOA and PFNA (p < 0.0001). Five outliers were removed, identified by the Rout method (1 experimental PFOS, three experimental PFOA, one experimental PFNA). Sample sizes varied across groups: PFOS experimental (n = 33), PFOS environmental (n = 11), PFOA experimental (n = 15), PFOA environmental (n = 11), PFNA experimental (n = 11), PFNA environmental (n = 10). Details on exposure studies can be found in Supplementary Tables S2, S3.

FIGURE 5

Mechanisms driving PFAS-induced neurotoxicity include direct mechanisms, such as disruption of calcium (Ca²⁺) homeostasis. PFAS-associated Ca²⁺ overload in neurons appears to be driven by Ca²⁺ influx from the extracellular space, through L-type voltage-gated Ca²⁺ channel (L-VGCC), and intracellular Ca²⁺storage organelles (mitochondria and endoplasmic reticulum), via inositol 1,4,5-triphosphate receptors (IP3R) and ryanodine receptors (RyR). Superfluous Ca²⁺ can disrupt neuronal signaling, induce oxidative stress leading to neuronal cell death, and disrupt Ca²⁺ dependent second messenger signaling cascades (CaM: calmodulin, CaN: calcineurin) that regulate diverse neuronal cell processes, including growth, reorganization, and the production and release of neurotransmitters.

FIGURE 6

Important indirect mechanisms by which PFAS exposure may impact neurological health include disruption of liver, kidney, and peripheral immune system functions. Build up of toxic substances and inflammatory molecules in circulation have the potential to compromise the BBB, damage neurons, and contribute to neurodegenerative diseases. PFAS have also been identified as PPAR agonists which are expressed in the liver, kidneys, immune organs, and brain, making PPARs an important molecular target for both direct and indirect effects of PFAS in neurological health.

FIGURE 7

Throughout a person’s lifetime there are several critical windows during which the brain is particularly vulnerable to chemical insult, including the pre- and postnatal period, puberty, pregnancy, and senescence. These windows are characterized by unique exposure profiles and/or the occurrence of dynamic physiological changes that make the brain more plastic or penetrable, and therefore more susceptible to chemical insult.