Titanium dioxide and carbon black nanoparticles disrupt neuronal homeostasis via excessive activation of cellular prion protein signaling.

BACKGROUND

Epidemiological emerging evidence shows that human exposure to some nanosized materials present in the environment would contribute to the onset and/or progression of Alzheimer's disease (AD). The cellular and molecular mechanisms whereby nanoparticles would exert some adverse effects towards neurons and take part in AD pathology are nevertheless unknown.

RESULTS

Here, we provide the prime evidence that titanium dioxide (TiO) and carbon black (CB) nanoparticles (NPs) bind the cellular form of the prion protein (PrP), a plasma membrane protein well known for its implication in prion diseases and prion-like diseases, such as AD. The interaction between TiO- or CB-NPs and PrP at the surface of neuronal cells grown in culture corrupts PrP signaling function. This triggers PrP-dependent activation of NADPH oxidase and subsequent production of reactive oxygen species (ROS) that alters redox equilibrium. Through PrP interaction, NPs also promote the activation of 3-phosphoinositide-dependent kinase 1 (PDK1), which in turn provokes the internalization of the neuroprotective TACE α-secretase. This diverts TACE cleavage activity away from (i) TNFα receptors (TNFR), whose accumulation at the plasma membrane augments the vulnerability of NP-exposed neuronal cells to TNFα -associated inflammation, and (ii) the amyloid precursor protein APP, leading to overproduction of neurotoxic amyloid Aβ40/42 peptides. The silencing of PrP or the pharmacological inhibition of PDK1 protects neuronal cells from TiO- and CB-NPs effects regarding ROS production, TNFα hypersensitivity, and Aβ rise. Finally, we show that dysregulation of the PrP-PDK1-TACE pathway likely occurs in the brain of mice injected with TiO-NPs by the intra-cerebro-ventricular route as we monitor a rise of TNFR at the cell surface of several groups of neurons located in distinct brain areas.

CONCLUSION

Our in vitro and in vivo study thus posits for the first time normal cellular prion protein PrP as being a neuronal receptor of TiO- and CB-NPs and identifies PrP-coupled signaling pathways by which those nanoparticles alter redox equilibrium, augment the intrinsic sensitivity of neurons to neuroinflammation, and provoke a rise of Aβ peptides. By identifying signaling cascades dysregulated by TiO- and CB-NPs in neurons, our data shed light on how human exposure to some NPs might be related to AD.