Intracellular Zn²⁺Signaling Facilitates Mossy Fiber Input-Induced Heterosynaptic Potentiation of Direct Cortical Inputs in Hippocampal CA3 Pyramidal Cells

Abstract

Repetitive action potentials (APs) in hippocampal CA3 pyramidal cells (CA3-PCs) backpropagate to distal apical dendrites, and induce calcium and protein tyrosine kinase (PTK)-dependent downregulation of Kv1.2, resulting in long-term potentiation of direct cortical inputs and intrinsic excitability (LTP-IE). When APs were elicited by direct somatic stimulation of CA3-PCs from rodents of either sex, only a narrow window of distal dendritic [Ca 2ϩ ] allowed LTP-IE because of Ca 2ϩ-dependent coactivation of PTK and protein tyrosine phosphatase (PTP), which renders non-mossy fiber (MF) inputs incompetent in LTP-IE induction. High-frequency MF inputs, however, could induce LTP-IE at high dendritic [Ca 2ϩ ] of the window. We show that MF input-induced Zn 2ϩ signaling inhibits postsynaptic PTP, and thus enables MF inputs to induce LTP-IE at a wide range of [Ca 2ϩ ] i values. Extracellular chelation of Zn 2ϩ or genetic deletion of vesicular zinc transporter abrogated the privilege of MF inputs for LTP-IE induction. Moreover, the incompetence of somatic stimulation was rescued by the inhibition of PTP or a supplement of extracellular zinc, indicating that MF input-induced increase in dendritic [Zn 2ϩ ] facilitates the induction of LTP-IE by inhibiting PTP. Consistently, high-frequency MF stimulation induced immediate and delayed elevations of [Zn 2ϩ ] at proximal and distal dendrites, respectively. These results indicate that MF inputs are uniquely linked to the regulation of direct cortical inputs owing to synaptic Zn 2ϩ signaling.

Kv1.2 contributes to pattern separation by regulating the hippocampal CA3 neuronal ensemble size

“…Next, we examined MF-induced LTP-IE of CA3-PCs in CA3- Kcna2+/− mice and their WT littermates ( f-Kcna2 ) in the third postnatal week because Cre/loxP recombination is detectable on postnatal day 14 in the CA3 region (Nakazawa et al, 2002). In addition, we previously reported that the degree of LTP-IE is not different between the third and 12th postnatal week mice (Eom et al, 2019). LTP-IE was assessed by input conductance (G in ) and first spike latency, which were measured as in Figure 1B.…”

“…Insufficiency of Kv1.2 can be induced not only by genetic mutation but also by repetitive stimulation of afferent MFs or the soma of a CA3-PC. The latter results in a decrease in input conductance and enhanced E-S coupling of PP synaptic inputs, collectively referred to as long-term potentiation of intrinsic excitability (LTP-IE) (Hyun et al, 2013; Hyun et al, 2015; Eom et al, 2019). Notably, LTP-IE enhances PP-EPSPs but not PP-EPSCs, supporting the notion that it is attributed to intrinsic excitability changes of dendrites (Hyun et al, 2015).…”

“…It should be noted that 10 Hz somatic stimulation in Figure 2 was used as a surrogate of high frequency MF stimulation, and that MF inputs are privileged for the induction of LTP-IE (Eom et al, 2019). Indeed, we confirmed that repetitive stimulation of afferent MFs (20 Hz, 2 s) induces metaplasticity at PP synapses similar to 10 Hz somatic APs (Figure 2-figure supplement 1).…”

“…We have previously shown that repetitive MF inputs to a CA3-PC downregulate Kv1.2 subunits in Ca 2+ - and Zn 2+ -dependent manner, resulting in LTP-IE and specific potentiation of PP-EPSPs (Hyun et al, 2013, 2015; Eom et al, 2019). LTP-IE, which involves global excitability changes over the distal apical dendritic arbor, is unlikely to directly mediate memory encoding.…”

“…The CA3 ensemble size in Kcan2 +/− mice was 1.5 times larger on day 4 (Figure 4D), even though Kcna2+/− CA3-PCs lack MF-induced metaplasticity, suggesting that non-specific lowering of PP-LTP threshold caused by Kcna2 haploinsufficiency seems to exert more adverse effects on pattern separation compared to MF-induced metaplasticity. Although the CA3 neuronal ensemble size should be larger in the presence of MF-induced metaplasticity than without it, the increment may be very limited, because induction of LTP-IE requires high frequency MF inputs (Eom et al, 2019). To figure out the role of MF-induced metaplasticity in pattern separation and CA3 network computation, it needs to be elucidated the cellular mechanisms underlying depotentiation of LTP-IE, because the role of MF-induced metaplasticity in CA3 network computation would heavily depend on how long the primed state of CA3-PCs is maintained in vivo .…”