Zn2+ decoration of microtubules arrests axonal transport and displaces tau, doublecortin, and MAP2C

Intracellular Zn2+ concentrations increase via depolarization-mediated influx or intracellular release, but the immediate effects of Zn2+ signals on neuron function are not fully understood. By simultaneous recording of cytosolic Zn2+ and organelle motility, we find that elevated Zn2+ (IC50 ≈ 5-10 nM) reduces both lysosomal and mitochondrial motility in primary rat hippocampal neurons and HeLa cells. Using live-cell confocal microscopy and in vitro single-molecule TIRF imaging, we reveal that Zn2+ inhibits activity of motor proteins (kinesin and dynein) without disrupting their microtubule binding. Instead, Zn2+ directly binds to microtubules and selectively promotes detachment of tau, DCX, and MAP2C, but not MAP1B, MAP4, MAP7, MAP9, or p150glued. Bioinformatic predictions and structural modeling show that the Zn2+ binding sites on microtubules partially overlap with the microtubule binding sites of tau, DCX, dynein, and kinesin. Our results reveal that intraneuronal Zn2+ regulates axonal transport and microtubule-based processes by interacting with microtubules.

Spontaneous, synchronous zinc spikes oscillate with neural excitability and calcium spikes in primary hippocampal neuron culture

Zinc has been suggested to act as an intracellular signaling molecule due to its regulatory effects on numerous protein targets including enzymes, transcription factors, ion channels, neurotrophic factors, and postsynaptic scaffolding proteins. However, intracellular zinc concentration is tightly maintained at steady levels under natural physiological conditions. Dynamic changes in intracellular zinc concentration have only been detected in certain types of cells that are exposed to pathologic stimuli or upon receptor ligand binding. Unlike calcium, the ubiquitous signaling metal ion that can oscillate periodically and spontaneously in various cells, spontaneous zinc oscillations have never been reported. In this work, we made the novel observation that the developing neurons generated spontaneous and synchronous zinc spikes in primary hippocampal cultures using a fluorescent zinc sensor, FluoZin-3. Blocking of glutamate receptor-dependent calcium influx depleted the zinc spikes, suggesting that these zinc spikes were driven by the glutamate-mediated spontaneous neural excitability and calcium spikes that have been characterized in early developing neurons. Simultaneous imaging of calcium or pH together with zinc, we uncovered that a downward pH spike was evoked with each zinc spike and this transient cellular acidification occurred downstream of calcium spikes but upstream of zinc spikes. Our results suggest that spontaneous, synchronous zinc spikes were generated through calcium influx-induced cellular acidification, which liberates zinc from intracellular zinc binding ligands. Given that changes in zinc concentration can modulate activities of proteins essential for synapse maturation and neuronal differentiation, these zinc spikes might act as important signaling roles in neuronal development.