Defining potential roles of Pb²⁺in neurotoxicity from a calciomics approach

Abstract

Metal ions play crucial roles in numerous biological processes, facilitating biochemical reactions by binding to various proteins. An increasing body of evidence suggests that neurotoxicity associated with exposure to nonessential metals (e.g., Pb2+) involves disruption of synaptic activity, and these observed effects are associated with the ability of Pb2+ to interfere with Zn2+ and Ca2+-dependent functions. However, the molecular mechanism behind Pb2+ toxicity remains a topic of debate. In this review, we first discuss potential neuronal Ca2+ binding protein (CaBP) targets for Pb2+ such as calmodulin (CaM), synaptotagmin, neuronal calcium sensor-1 (NCS-1), N-methyl-D-aspartate receptor (NMDAR) and family C of G-protein coupled receptors (cGPCRs), and their involvement in Ca2+-signalling pathways. We then compare metal binding properties between Ca2+ and Pb2+ to understand the structural implications of Pb2+ binding to CaBPs. Statistical and biophysical studies (e.g., NMR and fluorescence spectroscopy) of Pb2+ binding are discussed to investigate the molecular mechanism behind Pb2+ toxicity. These studies identify an opportunistic, allosteric binding of Pb2+ with CaM that is distinct from ionic displacement. Together, this data suggests three potential modes of Pb2+ activity related to molecular and/or neural toxicity: (i) Pb2+ can occupy Ca2+-binding sites, inhibiting the activity of the protein by structural modulation, (ii) Pb2+ can mimic Ca2+ in the binding sites, falsely activating the protein and perturbing downstream activities, or (iii) Pb2+ binds outside of the Ca2+-binding sites, resulting in allosteric modulation of the proteins activity. Moreover, the data further suggest that even low concentrations of Pb2+ can interfere at multiple points within the neuronal Ca2+ signalling pathways to cause neurotoxicity.

Pb(II) Disruption of Synaptic Activity Through Ca(II)- and Zn(II)-binding Proteins

“…[60] . Consistent with these observations, evidence summarized recently in our lab and others increasingly indicates that Pb 2+ neurotoxicity is related to interference with synaptic activity [61,62] , which is likely associated with disruption of Zn 2+ and Ca 2+ -dependent functions by Pb 2+ [23,24] .…”

“…At present, we are just beginning to piece together the myriad components within a vast calcium-signaling network, and our current state of knowledge pertaining to zinc signaling is even more limited. Hence, the focus of this article will highlight and expand aspects of our recent review [24] , focusing on proteins involved in synaptic activity that represent known or potential targets for Pb 2+ (Figure 2) [88] , and Ca 2+ channels are the likely points of entry for Pb 2+ into the cell. Additionally, interaction of Pb 2+ with presynaptic VGCC's may impair GABAergic synaptic transmission by inhibiting GABA release [89] , which is believed to be related to learning and memory.…”

“…Most recently, our hypotheses and conclusions from these previous studies were applied to address potential avenues of Pb 2+ neurotoxicity through the various Ca 2+ -dependent proteins involved in neurotransmission [24] . However, given that Pb 2+ can also occupy Zn 2+ -binding sites, and the potential intersections that occur between Ca 2+ -and Zn 2+ -signaling pathways, the discussion of Pb 2+ neurotoxicity and its effects on synaptic activity should also address Zn 2+ -dependent proteins as potential targets.…”

“…However, there is a growing body of research suggesting that the ionic displacement model does not adequately address the complex interactions between metals and proteins, and the mechanisms which may lead to cellular toxicity and interference with important physiological and neurological processes. Several recent review papers have highlighted research suggesting multiple mechanisms associated with metal toxicity, particularly with respect to disruption of synaptic activity and neural pathways by substitution of Pb 2+ for Ca 2+ and Zn 2+ in proteins [1,[23][24][25] . [26] by influx and efflux through membrane permeable Ca 2+ channels, regulated by various control mechanisms and events, including membrane depolarization, external agonists, and depletion of internal stores in the sarcoplasmic reticulum (SR) and endoplasmic reticulum (ER).…”

“…Within the neural signaling pathway, which includes a vast network of more than 1000 neurons in the human brain [31] , communication between neural cells across the synaptic cleft is initiated by a change in the action potential on the surface of the presynaptic neuron that regulates the opening of voltage-gated calcium channels (VGCC's) (Figure 1). Once the channel is open, the influx of Ca 2+ interacts with CaBPs associated with various aspects of neurotransmission, cognitive function, memory, and synaptic activity [24] . These proteins include synaptotagmin, ɣ-aminobutyric acid (GABA) receptors, Neuronal calcium sensor protein 1 (NCS-1), and CaM ( Figure 2).…”