Structure-Activity Relationship of Selected Meta- and Para-Hydroxylated Non–Dioxin Like Polychlorinated Biphenyls: From Single RyR1 Channels to Muscle Dysfunction

Polychlorinated biphenyls (PCBs), banned in the 1970s, still persist at high concentrations in the environment and in animal and human tissues including liver, brain, fat, blood, and skeletal muscle (Pessah et al., 2010). Recently, PCB mixtures have been identified in elementary schools (Thomas et al., 2012) where ortho-substituted non–dioxin like PCB congeners (NDL-PCB) that lack activity toward the aryl hydrocarbon receptor (AhR) predominate. For example, PCB95 (2,2′,3,5′,6-pentachlorobiphenyl), a developmental neurotoxicant (Kenet et al., 2007; Schantz et al., 1997; Yang et al., 2009), is the second most abundant congener found in indoor/outdoor air, soil, and caulking found in New York City schools (Thomas et al., 2012). Contemporary unintentional sources of NDL-PCBs, including PCB95, from the paint pigment industry have been identified (Hu and Hornbuckle, 2010). Although the focus of scientific studies has been on dioxin-like PCBs, neurotoxicity and endocrine disruption appear to be independent of AhR interactions (Pessah et al., 2010). NDL-PCBs with 2 or more othro-chlorines contribute > 50% of the PCB burden in both environmental samples and human tissues, such as those individuals near the Great Lakes, St Lawrence River, and Fox River (DeCaprio et al., 2005; Kostyniak et al., 2005; Marek et al., 2013; Martinez and Hornbuckle, 2011; Martinez et al., 2012). A molecular target through which NDL-PCBs could mediate a broad cadre of toxicological sequelae stems from interactions with ryanodine receptors (RyRs) that form Ca2+ channels that mediate the highly controlled release of Ca2+ from sarcoplasmic/endoplasmic reticulum (SR/ER) stores to the cytoplasm (Pessah et al., 2010). Three RyR isoforms are broadly expressed in c ells with the type 1 isoform RyR1 being one of two Ca2+ channels essential for excitation-contraction coupling (ECC) in mammalian skeletal muscle, the other being the L-type voltage-gated Ca2+ channel CaV1.1 (Nakai et al., 1996; Tanabe et al., 1988). Behavioral, cognitive, and motor impairments have been associated with PCB exposure and attributed to their ability to alter critical neurodevelopmental milestones in the central nervous system (CNS) such as activity-dependent dendritic growth (Wayman et al., 2012a, b; Yang et al., 2009). However, little attention has been given to striated muscle as a potential target organ. In humans, prenatal exposure to PCBs has been associated with lower body weight, consistent with decreased total lean body mass reported for children born to mothers accidentally exposed to PCBs (Coletti et al., 2001), which was not due to decreased bone density or body fat. In addition, rat myogenic cultures exposed to PCB mixture Aroclor 1254 in vitro inhibited myoblast differentiation (Guo et al., 1994). These results indicate that in addition to developmental neurotoxicity mediated by interactions of NDL-PCB with RyRs expressed in the CNS (Pessah et al., 2010), direct actions with RyR1 expressed in skeletal muscle could contribute to motor dysfunction, especially during critical perinatal periods of muscle development. There is increasing interest in whether metabolites of NDL-PCBs maintain biological activity. Cytochrome P450 monooxygenase enzymes (CYP) metabolize PCBs to their respective OH-PCB (Warner et al., 2009). OH-PCBs can be more reactive toward biological targets and contribute to toxicity (Song et al., 2013), especially because they are retained in blood (Hovander et al., 2002). Pharmacokinetic studies indicate PCBs initially distribute to muscle where OH-PCBs may be locally formed by CYPs expressed in skeletal muscle (Bieche et al., 2007; Kania-Korwel et al., 2007). PCB metabolites can persist in plasma at concentrations 15-fold higher than their parent compound, making them long-lasting environmental and biological contaminants (Bergman et al., 1994; Larsson et al., 2002; Lehmler et al., 2010). In this report, we provide direct evidence that OH-PCBs interact with RyR1 of skeletal muscle and produce a differential pattern of single channel gating kinetics and myotube ECC dysfunction that is highly dependent on whether the 4- or 5-position is hydroxylated. Direct impairment of muscle Ca2+ dynamics and function may explain, at least in part, the impairments in skeletal muscle health observed in human and animal studies.