Abstract
Neuronal metal ions such as zinc are essential for brain function. In particular synaptic processes are tightly related to metal and protein homeostasis, for example through extracellular metal-binding proteins. One such protein is neuronal S100B, a calcium and zinc binding damage-associated molecular pattern (DAMP), whose chronic upregulation is associated with aging, Alzheimer’s disease (AD), motor neuron disease and traumatic brain injury (TBI). Despite gained insights on the structure of S100B, it remains unclear how its calcium and zinc binding properties regulate its function on cellular level. Here we report a novel role of S100B in trace metal homeostasis, in particular the regulation of zinc levels in the brain. Our results show that S100B at increased extracellular levels is not toxic, persists at high levels, and is taken up into neurons, as shown by cell culture and biochemical analysis. Combining protein bioimaging and zinc quantitation, along with a zinc-binding impaired S100B variant, we conclude that S100B effectively scavenges zinc ions through specific binding, resulting in a redistribution of the intracellular zinc pool. Our results indicate that scavenging of zinc by increased levels of S100B affects calcium levels in vitro. Thereby S100B is able to mediate the cross talk between calcium and zinc homeostasis. Further, we investigated a possible new neuro-protective role of S100B in excitotoxicity via its effects on calcium and zinc homeostasis. Exposure of cells to zinc-S100B but not the zinc-binding impaired S100B results in an inhibition of excitotoxicity. We conclude that in addition to its known functions, S100B acts as sensor and regulator of elevated zinc levels in the brain and this metal-buffering activity is tied to a neuroprotective role.
Highlights
Metal ions play an important physiological role in synapse plasticity and function and their homeostasis is tightly regulated by transporters and metal-binding proteins
No significant impairments in cell health assessed by quantitative analysis of apoptotic and necrotic cells vs. healthy cells were observed after treatment at a physiological S100B concentration of up to 30 μM dimeric or 30 μM tetrameric S100B (Figure 1A), even if cells were exposed to S100B for up to 96 h (Figure 1B; Supplementary Figure S1E)
Our results indicate that calcium-binding by S100B in neurons is significantly enhanced by the presence of high zinc concentrations which with zinc leads to a significant decrease in intracellular calcium as zinc binding increases the affinity for calcium binding in S100B. (C) After treatment, S100B was visualized using ICC and cells counterstained with MAP2, and cell nuclei with DAPI
Summary
Metal ions play an important physiological role in synapse plasticity and function and their homeostasis is tightly regulated by transporters and metal-binding proteins. S100B Regulates Zinc Homeostasis and Exitotoxicity elements such as iron, copper and zinc bind to these proteins, thereby influencing their conformation and functions (Leal et al, 2013). Disturbance of these processes leads to altered metal homeostasis and cellular distribution, which is a common feature of several neurodegenerative and neuropsychiatric diseases (Barnham and Bush, 2014). Zinc is one of the most prevalent trace metals in the brain and plays an important role as modulator of neurotransmission and signaling ion at synapses, thereby influencing processes such as synapse formation, maturation and plasticity. We followed the question whether high levels of S100B are able to affect local zinc concentrations by scavenging free zinc ions, which may lower toxic effects of zinc in situations of high zinc release such as over-excitation of glutamatergic neurons
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