ZIP-ZnT1 complexes mediate a local Zn2+-cycle regulating neuronal Zn2+ transport

ZnT1 is the main transporter mediating Zn²⁺ efflux from the cytoplasm to the extracellular space. Paradoxically, due to relatively low Zn ²⁺ affinity of ZnT1, physiological levels of cytoplasmic free Zn ²⁺ are seemingly insufficient to support ZnT1 function. However, physiological modulation of NMDA receptor responses by synaptically released Zn ²⁺ required ZnT1 activity, accompanied by a Zn ²⁺ rise in the postsynaptic neurons. We hypothesized that Zn ²⁺ import from the extracellular space could generate intracellular Zn ²⁺ microdomains in the vicinity of ZnT1 to enable its activity, thus forming a localized “Zn ²⁺ -cycle”. To identify the functional machinery that may drive this process, we investigated the expression and function of ZIP proteins that import Zn ²⁺ into the neuronal cytoplasm. We focused on the dorsal cochlear nucleus (DCN) and hippocampus, regions where synaptic Zn ²⁺ modulates postsynaptic NMDA responses and synaptic plasticity. We demonstrate that ZIP3 is expressed on DCN cartwheel cells and required for postsynaptic Zn ²⁺ influx. Importantly, ZIP3 physically interacts with ZnT1, suggesting that it is an integral component of the synaptic Zn ²⁺ -cycle machinery. In the hippocampus, ZIP1, but not ZIP3, mediates Zn ²⁺ import into postsynaptic CA3 cells and we find that ZIP1 interacts with ZnT1 in this brain region. Importantly, Zn ²⁺ efflux rates are enhanced in SH-SY5Y cells co-expressing ZnT1 and either ZIP3 or ZIP1, compared to rates in cells expressing ZnT1 alone. Our findings indicate that a Zn ²⁺ -cycle, mediated by ZIP-ZnT1 complexes, establishes local intracellular Zn ²⁺ microdomains that facilitate ZnT1-dependent Zn ²⁺ efflux, which, in turn, modulates synaptic signaling.