Developmental Perfluorooctanesulfonic acid (PFOS) exposure as a potential risk factor for late-onset Alzheimer’s disease in CD-1 mice and SH-SY5Y cells

Highlights

• Perfluorooctanesulfonic acid (PFOS) exposure impacted rodent behavior.

• PFOS exposure upregulated Alzheimer’s disease-related biomarkers in CD-1 mice.

• Developmental PFOS exposure increased body weight in female mice.

Abstract

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder that accounts for approximately 60–80% of dementia cases worldwide and is characterized by an accumulation of extracellular senile plaques composed of β-amyloid (Aβ) peptide and intracellular neurofibrillary tangles (NFTs) containing hyperphosphorylated tau protein. Sporadic or late-onset AD (LOAD) represents 95 % of the AD cases and its etiology does not appear to follow Mendelian laws of inheritance, thus, implicating the role of epigenetic programming and environmental factors. Apolipoprotein allele 4 (ApoE4), the only established genetic risk factor for LOAD, is suggested to accelerate the pathogenesis of AD by increasing tau hyperphosphorylation, inhibiting the clearance of amyloid-β (Aβ), and promoting Aβ aggregation. Perfluorooctanesulfonic acid (PFOS) is a persistent organic pollutant, with potential neurotoxic effects, that poses a major threat to the ecosystem and human health. By employing in vivo and in vitro models, the present study investigated PFOS as a potential risk factor for LOAD by assessing its impact on amyloidogenesis, tau pathology, and rodent behavior. Our behavioral analysis revealed that developmentally exposed male and female mice exhibited a strong trend of increased rearing and significantly increased distance traveled in the open field test. Biochemically, GSK3β and total ApoE were increased following developmental exposure, in vivo. Furthermore, in vitro, low concentrations of PFOS elevated protein levels of APP, tau, and its site-specific phosphorylation. Differentiated SH-SY5Y cells exposed to a series of PFOS concentrations, also, had elevated protein expression of GSK3β. These data suggest that total ApoE is inducible by environmental exposure to PFOS.

Introduction

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder that accounts for approximately 60–80% of dementia cases worldwide (Deture and Dickson, 2019). Several biochemical pathways have been associated with AD including neuroinflammation, microglial activation, and oxidative stress (Manoharan et al., 2016). The major AD hallmarks are the presence of extracellular senile plaques consisting of β-amyloid (Aβ) peptides that are cleaved from a larger protein called the amyloid precursor protein (APP) and intracellular neurofibrillary tangles (NFTs) containing hyperphosphorylated tau protein oligomers (Murphy and Levine, 2010). Tau is an intracellular microtubule-binding protein that is essential for the stabilization of microtubules and axonal transport in neurons (Garcia and Cleveland, 2001). Hyperphosphorylation of tau, which occurs in pathological conditions, reduces tau binding affinity to microtubules resulting in impaired axonal transport. Several protein kinases are involved in the phosphorylation of tau protein such as the proline-directed protein kinases: cyclin-dependent kinase-5 (CDK5) and glycogen synthase kinase-3 beta (GSK3β) (Martin et al., 2013). Particularly, these kinases are responsible for the unusual hyperphosphorylation at several sites including Thr181 and Ser404.

The etiology of Late-onset Alzheimer’s Disease (LOAD), which represents 95 % of AD cases, is sporadic and the only established gene associated with LOAD is Apolipoprotein E4 (ApoE4) (Verghese et al., 2011). There are three isoforms of ApoE in humans (ApoE2, ApoE3, and ApoE4), and ApoE4 is strongly associated with an increased risk of developing AD (Verghese et al., 2011; Zannis et al., 1982). The presence of one copy of the ApoE4 allele increases the risk of developing LOAD by three-fold while the risk is increased by fifteen-fold if 2 copies of the ApoE4 allele are present (Koffie et al., 2012; Verghese et al., 2011). In the central nervous system (CNS), ApoE is involved with the repair process after injury (Liu et al., 2013); it functions as a lipid transporter and/or signaling molecule (Huang et al., 2017; Pfrieger, 2003). Previous studies have reported that accumulation of ApoE4 fragments are associated with NFTs in the brains of AD patients (Huang et al., 2001), and are also detected in the neurons of ApoE4 transgenic mice (Brecht et al., 2004). ApoE4-induced behavioral deficits and neurodegeneration are caused by disruption of the cytoskeleton, stimulation of tau phosphorylation, generation of NFTs (Brecht et al., 2004; Huang, 2010), interference in Aβ clearance, aggregation of Aβ, and formation of senile plaques (Hashimoto et al., 2012; Höglund et al., 2017; Koffie et al., 2012; Tai et al., 2013; Verghese et al., 2011).

Poly- and perfluoroalkyl substances (PFASs) are a class of emerging persistent organic pollutants (POPs) with potential neurotoxic effects. They are man-made fluorinated chemicals that have been widely used in numerous industrial and consumer products such as textile products, firefighting foams, and oil-resistant coatings for paper products. PFASs are ubiquitous and persistant in the environment and bioaccumulate in the food chain (Wang et al., 2019). One of the most widely used PFASs is the 8-carbon chain (C8) Perfluorooctanesulfonic acid (PFOS) which has been detected in adult human serum (Lau et al., 2007) and in children with a serum concentration equal or greater than adults (Mondal et al., 2012). Furthermore, PFOS can cross the placenta and the blood-brain barrier (BBB) (Lau et al., 2007, 2006, 2004), and has been detected in breast milk, indicating humans are vulnerable to PFOS exposure before birth and throughout their lifetime (Winkens et al., 2017). PFOS strongly binds to plasma albumin (Forsthuber et al., 2020) and has an average serum half-life of 5.4 years in humans (Olsen et al., 2007) and 36.9 days in mice (Chang et al., 2012).

Developmental studies conducted using rodents have demonstrated that PFOS exposure can induce neural damage manifested by chronic glial activation, the release of inflammatory factors (Zeng et al., 2011), and cholinergic alterations that result in behavioral deficits, including reduced spatial learning and cognitive impairment which can persist into adulthood (Fuentes et al., 2007a; Johansson et al., 2009, 2008; Wang et al., 2015). Neonatal exposure to a single PFOS dose of 21 μmol/kg body weight (11.3 mg/kg body weight) on PND 10 elevated cerebral tau protein expression and induced behavioral abnormalities in adult mice (Fuentes et al., 2007b; Johansson et al., 2008). Additionally, perinatal PFOS exposure has been associated with increased β-amyloid aggregation, tau protein levels, and tau hyperphosphorylation in adult rats, in at least one study, suggesting a link between developmental PFOS exposure and LOAD (Zhang et al., 2016).

The studies mentioned above are few and limited in scope. To our knowledge, there has been no investigation on the effects of developmental PFOS exposure on ApoE in middle-aged adult mice. In addition, limited studies have investigated perinatal exposure alone and its implications on rodent behavior and biochemical markers later in life. Therefore, the main aim of this study was to investigate perinatal PFOS exposure as a risk factor for LOAD by assessing its impact on rodent behavior and biomarkers associated with three major AD-related pathways in an in vivo model using developmentally exposed CD-1 mice. Furthermore, the same pathways were examined in human SH-SY5Y neuroblastoma cells.