Elimination of zinc from synaptic visicles in the intact mouse brain by targeted disruption of ZnT3
Cole, Toby B. (Toby Brian), 1967-
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This dissertation demonstrates that zinc is taken up into synaptic vesicles by a mechanism that requires the zinc transporter, ZnT3, at the vesicle membrane and begins to address the physiological importance of synaptic vesicle zinc. In situ hybridization and immunohistochemistry showed that ZnT3 expression is limited to zinc-containing neurons in the brain, and that ZnT3 protein resides on the membranes of zinc-rich synaptic vesicles in mouse, monkey, and human brain. A genetic approach was used to test whether ZnT3 was essential for vesicular zinc transport, by generating mice that were homozygous for a null mutation in the ZnT3 gene. Histochemically-reactive zinc (i.e., zinc accessible to Timm and TSQ staining reagents) was undetectable in synaptic vesicles in the brains of these ZnT3-deficient mice, indicating that ZnT3 is required for vesicular zinc transport. Upon neuronal activation vesicular zinc is released into the synaptic cleft, where it has been proposed to modulate excitatory and inhibitory neurotransmitter receptors. In addition, synaptically-released zinc has been widely considered to be the source of the zinc that may kill neurons following seizures or ischemic insults. To assess the physiological importance of synaptic vesicle zinc, ZnT3-deficient mice were tested for deficits in sensorimotor functions, learning and memory, sensitivity to seizure-inducing drugs, and neuronal damage. Most of these functions were remarkably normal in the absence of vesicular zinc. ZnT3-deficient mice were more susceptible than wildtype mice to kainic acid-induced seizures, demonstrating that zinc has a net inhibitory effect on seizures. Despite the lack of histochemically reactive zinc in synaptic vesicles in the ZnT3-deficient brain, zinc accumulated in the cytosol of postsynaptic neurons following seizures, and this zinc accumulation was associated with extensive neuronal death. Thus, the histochemically reactive zinc found in synaptic vesicles is not the major source of toxic zinc accumulation following seizures.
- Biological chemistry