Zinc-mediated attenuation of hippocampal mossy fiber long-term potentiation induced by forskolin

Members of the KCTD family are major regulators of cAMP signaling

Neuromodulation is pivotal for brain function. One of the key pathways engaged by neuromodulators is signaling via second messenger cAMP, which controls a myriad of fundamental reactions. This study identifies KCTD5, a ubiquitin ligase adapter, as a regulatory element in this pathway and determines that it works by an unusual dual mode controlling the activity of cAMP-generating enzyme in neurons through both zinc transport and G protein signaling.

Cyclic adenosine monophosphate (cAMP) is a pivotal second messenger with an essential role in neuronal function. cAMP synthesis by adenylyl cyclases (AC) is controlled by G protein–coupled receptor (GPCR) signaling systems. However, the network of molecular players involved in the process is incompletely defined. Here, we used CRISPR/Cas9–based screening to identify that members of the potassium channel tetradimerization domain (KCTD) family are major regulators of cAMP signaling. Focusing on striatal neurons, we show that the dominant isoform KCTD5 exerts its effects through an unusual mechanism that modulates the influx of Zn²⁺ via the Zip14 transporter to exert unique allosteric effects on AC. We further show that KCTD5 controls the amplitude and sensitivity of stimulatory GPCR inputs to cAMP production by Gβγ-mediated AC regulation. Finally, we report that KCTD5 haploinsufficiency in mice leads to motor deficits that can be reversed by chelating Zn²⁺. Together, our findings uncover KCTD proteins as major regulators of neuronal cAMP signaling via diverse mechanisms.

The zinc paradigm for metalloneurochemistry

Neurotransmission and sensory perception are shaped through metal ion–protein interactions in various brain regions. The term "metalloneurochemistry" defines the unique field of bioinorganic chemistry focusing on these processes, and zinc has been the leading target of metalloneurochemists in the almost 15 years since the definition was introduced. Zinc in the hippocampus interacts with receptors that dictate ion flow and neurotransmitter release. Understanding the intricacies of these interactions is crucial to uncovering the role that zinc plays in learning and memory. Based on receptor similarities and zinc-enriched neurons (ZENs) in areas of the brain responsible for sensory perception, such as the olfactory bulb (OB), and dorsal cochlear nucleus (DCN), zinc participates in odor and sound perception. Development and improvement of methods which allow for precise detection and immediate manipulation of zinc ions in neuronal cells and in brain slices will be critical in uncovering the synaptic action of zinc and, more broadly, the bioinorganic chemistry of cognition.

Significance of synaptic Zn2+ signaling in zincergic and non-zincergic synapses in the hippocampus in cognition

A portion of zinc concentrates in the synaptic vesicles in the brain and is released from glutamatergic (zincergic) neuron terminals. It serves as a signaling factor (in a form of free Zn²⁺). Both extracellular Zn²⁺ signaling, which predominantly originates in Zn²⁺ release from zincergic neuron terminals, and intracellular Zn²⁺ signaling, which is often linked to extracellular Zn²⁺ signaling, are involved in hippocampus-dependent memory. At mossy fiber-CA3 pyramidal cell synapses and Schaffer collateral-CA1 pyramidal cell synapses, which are zincergic, extracellular Zn²⁺ signaling leads to intracellular Zn²⁺ signaling and is involved in learning and memory. At medial perforant pathway-dentate granule cell synapses, which are non-zincergic, intracellular Zn²⁺ signaling, which originates in the internal stores containing Zn²⁺, is involved in learning and memory. The blockade of Zn²⁺ signaling with Zn²⁺ chelators induces memory deficit, while the optimal amount range of Zn²⁺ signaling is unknown. It is possible that the degree and frequency of Zn²⁺ signaling, which determine the increased Zn²⁺ levels, modulates learning and memory as well as intracellular Ca²⁺ signaling. To understand the precise role of synaptic Zn²⁺ signaling in the hippocampus, the present paper summarizes the current knowledge on Zn²⁺ signaling at zincergic and non-zincergic synapses in the hippocampus in cognition and involvement of zinc transporters and zinc-binding proteins in synaptic Zn²⁺ signaling.

Transient increase in Zn2+ in hippocampal CA1 pyramidal neurons causes reversible memory deficit

The translocation of synaptic Zn²⁺ to the cytosolic compartment has been studied to understand Zn²⁺ neurotoxicity in neurological diseases. However, it is unknown whether the moderate increase in Zn²⁺ in the cytosolic compartment affects memory processing in the hippocampus. In the present study, the moderate increase in cytosolic Zn²⁺ in the hippocampus was induced with clioquinol (CQ), a zinc ionophore. Zn²⁺ delivery by Zn-CQ transiently attenuated CA1 long-term potentiation (LTP) in hippocampal slices prepared 2 h after i.p. injection of Zn-CQ into rats, when intracellular Zn²⁺ levels was transiently increased in the CA1 pyramidal cell layer, followed by object recognition memory deficit. Object recognition memory was transiently impaired 30 min after injection of ZnCl₂ into the CA1, but not after injection into the dentate gyrus that did not significantly increase intracellular Zn²⁺ in the granule cell layer of the dentate gyrus. Object recognition memory deficit may be linked to the preferential increase in Zn²⁺ and/or the preferential vulnerability to Zn²⁺ in CA1 pyramidal neurons. In the case of the cytosolic increase in endogenous Zn²⁺ in the CA1 induced by 100 mM KCl, furthermore, object recognition memory was also transiently impaired, while ameliorated by co-injection of CaEDTA to block the increase in cytosolic Zn²⁺. The present study indicates that the transient increase in cytosolic Zn²⁺ in CA1 pyramidal neurons reversibly impairs object recognition memory.

Involvement of N-methyl-D-aspartate receptor subunits in zinc-mediated modification of CA1 long-term potentiation in the developing hippocampus

Zinc is an endogenous N-methyl-D-aspartate (NMDA) receptor blocker. It is possible that zinc-mediated modification of hippocampal CA1 long-term potentiation (LTP) is linked to the expression of NMDA receptor subunits, which varies with postnatal development. In the present study, the effect of ZnCl(2) and CaEDTA, a membrane-impermeable zinc chelator, on CA1 LTP induction was examined in hippocampal slices from immature (3-week-old) and young (6-week-old) rats. Tetanus (10-100 Hz, 1 sec)-induced CA1 LTP was more greatly enhanced in 3-week-old rats. CA1 LTP was inhibited in the presence of 2-amino-5-phosphonovalerate (APV), an NMDA receptor antagonist, and CaEDTA in 3-week-old rats, as in the case of 6-week-old rats reported previously. In 3-week-old rats, on the other hand, 5 μM ZnCl(2) attenuated NMDA receptor-mediated EPSPs more than in 6-week-old rats and significantly attenuated CA1 LTP. Moreover, 5 μM ZnCl(2) significantly attenuated CA1 LTP in the presence of (2R,4S)-4-(3-phosphonopropyl)-2-piperidinecarboxylic acid (PPPA), an NR2A antagonist, in 3-week-old rats, but not that in the presence of ifenprodil, an NR2B antagonist, suggesting that zinc-mediated attenuation of CA1 LTP is associated with the preferential expression of NR2B subunit in 3-week-old rats. In 6-week-old rats, however, 5 μM ZnCl(2) significantly potentiated CA1 LTP and also CA1 LTP in the presence of PPPA. The present study demonstrates that endogenous zinc may participate in the induction of CA1 LTP. It is likely that the changes in expression of NMDA receptor subunits are involved in the zinc-mediated modification of CA1 LTP in the developing hippocampus.

Nano-zinc oxide damages spatial cognition capability via over-enhanced long-term potentiation in hippocampus of Wistar rats

This study focused on the effects of zinc oxide nanoparticles (nano-ZnO) on spatial learning and memory and synaptic plasticity in the hippocampus of young rats, and tried to interpret the underlying mechanism. Rats were randomly divided into four groups. Nano-ZnO and phosphate-buffered saline were administered in 4-week-old rats for 8 weeks. Subsequently, performance in Morris water maze (MWM) was determined, and then long-term potentiation (LTP) and depotentiation were measured in the perforant pathway to dentate gyrus (DG) in anesthetized rats. The data showed that, (1) in MWM, the escape latency was prolonged in the nano-ZnO group and, (2) LTP was significantly enhanced in the nano-ZnO group, while depotentiation was barely influenced in the DG region of the nano-ZnO group. This bidirectional effect on long-term synaptic plasticity broke the balance between stability and flexibility of cognition. The spatial learning and memory ability was attenuated by the alteration of synaptic plasticity in nano-ZnO-treated rats.

Insight into glutamate excitotoxicity from synaptic zinc homeostasis

Zinc is released from glutamatergic (zincergic) neuron terminals in the hippocampus, followed by the increase in Zn(2+) concentration in the intracellular (cytosol) compartment, as well as that in the extracellular compartment. The increase in Zn(2+) concentration in the intracellular compartment during synaptic excitation is mainly due to Zn(2+) influx through calcium-permeable channels and serves as Zn(2+) signaling as well as the case in the extracellular compartment. Synaptic Zn(2+) homeostasis is important for glutamate signaling and altered under numerous pathological processes such as Alzheimer's disease. Synaptic Zn(2+) homeostasis might be altered in old age, and this alteration might be involved in the pathogenesis and progression of Alzheimer's disease; Zinc may play as a key-mediating factor in the pathophysiology of Alzheimer's disease. This paper summarizes the role of Zn(2+) signaling in glutamate excitotoxicity, which is involved in Alzheimer's disease, to understand the significance of synaptic Zn(2+) homeostasis in the pathophysiology of Alzheimer's disease.