The Extracellular Zn²⁺ Concentration Surrounding Excited Neurons Is High Enough to Bind Amyloid-β Revealed by a Nanowire Transistor

The Zn²⁺ stored in the secretory vesicles of glutamatergic neurons is coreleased with glutamate upon stimulation, resulting in the elevation of extracellular Zn²⁺ concentration (CZn2+ex). This elevation of CZn2+ex regulates the neurotransmission and facilitates the fibrilization of amyloid-β (Aβ). However, the exact CZn2+ex surrounding neurons under (patho)physiological conditions is not clear and the connection between CZn2+ex and the Aβ fibrilization remains obscure. Here, a silicon nanowire field-effect transistor (SiNW-FET) with the Zn²⁺ -sensitive fluorophore, FluoZin-3 (FZ-3), to quantify the CZn2+ex in real time is modified. This FZ-3/SiNW-FET device has a dissociation constant of ≈12 × 10^-9 m against Zn²⁺ . By placing a coverslip seeded with cultured embryonic cortical neurons atop an FZ-3/SiNW-FET, the CZn2+ex elevated to ≈110 × 10^-9 m upon stimulation with α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA). Blockers against the AMPA receptor or exocytosis greatly suppress this elevation, indicating that the Zn²⁺ stored in the synaptic vesicles is the major source responsible for this elevation of CZn2+ex. In addition, a SiNW-FET modified with Aβ could bind Zn²⁺ with a dissociation constant of ≈633 × 10^-9 m and respond to the Zn²⁺ released from AMPA-stimulated neurons. Therefore, the CZn2+ex can reach a level high enough to bind Aβ and the Zn²⁺ homeostasis can be a therapeutic strategy to prevent neurodegeneration.