Metallothionein III Mediates Ca²⁺-Dependent Zn²⁺ Spikes to Inhibit Dendritic Arborization

Zinc is crucial for neuron function, but whether and how labile zinc ion (Zn²⁺) acts as an intracellular signaling molecule remains unclear. In this work, we investigate the relationship between Ca²⁺ and Zn²⁺ dynamics using fluorescence imaging. Our findings reveal that manipulating Ca²⁺ influx through various pathways induces intracellular acidification, which subsequently elicits Zn²⁺ spikes that reflect transient increases in cytosolic Zn²⁺ levels. These Ca²⁺-dependent Zn²⁺ spikes have been recorded in both rat (Rattus norvegicus) primary neuron cultures and organotypic mouse (Mus musculus) hippocampal slice cultures prepared from both males and females. They are specific to neurons and astrocytes but are absent in other cell types we tested including HeLa cells, COS-7 cells, and fibroblasts. We further identify Metallothionein III (MT3), a Zn²⁺ buffering protein specifically expressed in brain cells, as the source of these Zn²⁺ spikes. Reduction in MT3 expression by knockdown with shRNAmiR techniques significantly decreases the amplitude of Zn²⁺ spikes, while overexpression of MT3 in HeLa and COS-7 cells is sufficient to induce Ca²⁺-dependent Zn²⁺ spikes, demonstrating the crucial roles of MT3 in Zn²⁺ release. Lastly, we explore the biological roles of MT3-mediated Zn²⁺ spikes in neurons. Suppressing Zn²⁺ spikes with either MT3 knockdown or mild Zn²⁺ chelation results in increased dendritic branching in primary rat hippocampal neurons. These results suggest that Zn²⁺ release from endogenous MT3 acts as a regulatory signal to inhibit dendrite branching and growth, establishing a critical role for Zn²⁺ spikes in neurite outgrowth and neuronal development.