Effects And Mechanisms Of Typical Poly- And Perfluorinated Sulfonate Compounds On Long-term Potentiation In Rat

Perfluorooctane sulfonate (PFOS), a typical perfluorinated sulfonate compound, has been extensively used in a variety of consumer applications and industrial processes due to its advantageous surfactant capabilities and stability. It has been demonstrated that PFOS could pass through the blood-brain barrier, affect the development of the nervous system, and inhibit the formation of learning and memory abilities in rat, while the mechanisms remain unclear. In 2009, PFOS was listed in Stockholm Convention and was prohibited for production and application in many countries, while it is still permitted to use for limited purposes including firefighting foams and pesticides which disbursed PFOS directly into the environment. Furthermore, the replacements of PFOS by alternatives are undergoing a fast development. The environmental pollution level of poly- and perfluorinated sulfonate compounds with shorter carbon chain length or with N/O atoms insertion in the carbon chain increased, with extremely limited toxicological research and unknown environment risk. To study the neurotoxic effects and mechanism of the typical poly- and perfluorinated sulfonate compounds is helpful to clarify the association of developmental abnormalities and neurodegenerative diseases with these pollutants.In this study, cross-fostered animal model was employed for evaluating the effects of developmental PFOS exposure on long-term potentiation (LTP). The primary hippocampal neurons were also used to elucidate the potential mechanism of PFOS-induced LTP impairment associated with a-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors trafficking. Furthermore, the changes of Tau and Aβ expression were used to evaluate the potential relationship between developmental PFOS exposure and neurodegenerative diseases. Lastly, the neurotoxicity and the potential mechanism of typical poly- and perfluorinated sulfonate compounds were compared by LTP in vivo via acute intracerebroventricular injection. The main contents include,(1) Effects of PFOS on LTP in the CA1 area of rat in vivo was evaluated using electrophysiological techniques in cross-fostered animal model, to elucidate the mechanisms of learning and memory injury caused by developmental PFOS exposure. The results showed that developmental PFOS exposure induced dose-dependent suppression of LTP, which was consistent with the previous results that developmental PFOS exposure impaired the spontaneous behavior and the learning and memory abilities of rats. The results provided the electrophysiological evidence for the developmental neurotoxicity of PFOS. Field-excitatory postsynaptic potential (fEPSP) in rats from both 5 mg/L and 15 mg/L PFOS exposed groups were lowered 20-27% than those from control group at 60 min after high-frequency stimulation (HFS). Input/output curve (I/O), which reflects the basal synaptic transmission competency, showed that the fEPSP amplitudes at 0.3-0.5 mA in exposed groups were significantly lower than that in control groups. And the paired-pulse facilitation (PPF) in TT15 group at the peak point was significantly lower than that in control group. These results suggested that developmental PFOS exposure affected the synaptic transmission and plasticity both of pre- and post-synaptic cells, which is a critical mechanism of its impairment on learning and memory abilities.(2) Combining experiments in vivo and in vitro, the role of AMPA receptors regulation in PFOS-induced LTP impairment was elucidated. Developmental PFOS exposure decreased the mRNA and protein expression of GluRl and GluR2, along with the decrease of phosphorylated protein GluR1-s831 and protein kinase CaMKII-α expression. The expression of GluR1 and GluR2 were also decreased when the primary hippocampal neurons of newborn rat were exposed with 2 μM and 20 μM PFOS, consistent with the results in vivo. The mRNA expression of glutamate receptor interacting protein (GRIP1) was decreased, consistent with the tendency of GluR2, and the mRNA level of RNA editing enzyme ADAR2 was increased twice than control group. NBQX, the AMPA receptor antagonist, inhibited the changes of PFOS on cellular calcium homeostasis and the related genes expression. The results indicated that PFOS affected the dynamic distribution of AMPA receptor by changing the expression of AMPA receptors subunits, enhanced the permeability of calcium ion and impaired the synaptic plasticity.(3) The potential relationship between developmental PFOS exposure and neurodegenerative diseases was clarified by detecting the effects of developmental PFOS exposure on Tau phosphorylation and Aβ accumulation. The results showed that protein and mRNA levels of Tau were elevated by developmental PFOS exposure. Tau phosphorylation at S199, T231 and S396 sites and the amount of protein kinase GSK-3β were also increased. Besides, developmental PFOS exposure promoted the expression of amyloid protein precursor (APP), increased the expression level of Aβ1-42, decreased the expression of presenilin (Ps-1), inducing the abnormity of App regulation and Aβ accumulation. And the prenatal PFOS exposure caused the alterations of involved proteins at comparable levels with the postnatal and both prenatal and postnatal exposure, although the serum PFOS levels in the prenatal exposure group were relatively lower. Thus, relatively high risk of PFOS exposure in the embryonic stage was identified.(4) The potential neurotoxicity and mechanism of PFOS alternatives were evaluated by comparing the effects of typical poly- and perfluorinated sulfonate compounds on LTP induction via acute intracerebroventricular injection. The results showed that PFOS alternatives suppressed LTP with dose-dependent manner. The amplitude of fEPSP in rats from control group kept above 140% of baseline at 60 min after HFS, while the fEPSP amplitude of 100 μM PFHxS and Cl-PFAES groups decreased to 97% and 98% of baseline. The amplitude of fEPSP maintained at 122% of baseline in PFBS-treated rats, with no significant difference from control group. PFHxS and Cl-PFAES posed similar potency as PFOS in impairing LTP, comparable neurotoxic potency of PFHxS and Cl-PFAES to impair synaptic plasticity. Meanwhile, Cl-PFAES significantly inhibited the fEPSP amplitude of baseline, indicating that CI-PFAES might act in a different neurotoxic mechanism from perfluorinated sulfonate compounds. Further study on the developmental neurotoxicity of PFOS alternatives is warranted.Developmental PFOS exposure inhibited the induction and maintaining of LTP, and AMPA receptor regulation was an important mechanism. PFOS affected the dynamic distribution of AMPA receptor GluRl and GluR2 in the membrane by changing the phosphorylation GluRl-s831 and GluR2-s880 by protein kinase CaMKⅡ-α, leading to the internalization of AMPA receptor and the increase of the intracellular calcium levels. Meanwhile, developmental PFOS exposure elevated the level of phosphorylated Tau and Aβ aggregation, which hinted the possible link between early PFOS exposure and neurodegenerative diseases. The present study preliminarily revealed the neurotoxicity potency of PFOS alternatives, suggested the necessity to further evaluate their neurotoxic effects and mechanisms. The results provided scientific basis for human health risk assessment of poly- and perfluorinated compounds.