Phosphorylation-state-dependent regulation of NMDA receptor short-term plasticity modifies hippocampal dendritic Ca2+ transients

Calcineurin Controls Hypothalamic NMDA Receptor Activity and Sympathetic Outflow

Rationale:

Hypertension is a common and serious adverse effect of calcineurin inhibitors, including cyclosporine and tacrolimus (FK506). Although increased sympathetic nerve discharges are associated with calcineurin inhibitor–induced hypertension, the sources of excess sympathetic outflow and underlying mechanisms remain elusive. Calcineurin (protein phosphatase-2B) is broadly expressed in the brain, including the paraventricular nuclear (PVN) of the hypothalamus, which is critically involved in regulating sympathetic vasomotor tone.

Objective:

We determined whether prolonged treatment with the calcineurin inhibitor causes elevated sympathetic output and persistent hypertension by potentiating synaptic N-methyl-D-aspartate (NMDA) receptor activity in the PVN.

Methods and Results:

Telemetry recordings showed that systemic administration of FK506 (3 mg/kg per day) for 14 days caused a gradual and profound increase in arterial blood pressure in rats, which lasted at least 7 days after discontinuing FK506 treatment. Correspondingly, systemic treatment with FK506 markedly reduced calcineurin activity in the PVN and circumventricular organs, but not rostral ventrolateral medulla, and increased the phosphorylation level and synaptic trafficking of NMDA receptors in the PVN. Immunocytochemistry labeling showed that calcineurin was expressed in presympathetic neurons in the PVN. Whole-cell patch-clamp recordings in brain slices revealed that treatment with FK506 increased baseline firing activity of PVN presympathetic neurons; this increase was blocked by the NMDA or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist. Also, treatment with FK506 markedly increased presynaptic and postsynaptic NMDA receptor activity of PVN presympathetic neurons. Furthermore, microinjection of the NMDA or α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor antagonist into the PVN of anesthetized rats preferentially attenuated renal sympathetic nerve discharges and blood pressure elevated by FK506 treatment. In addition, systemic administration of memantine, a clinically used NMDA receptor antagonist, effectively attenuated FK506 treatment–induced hypertension in conscious rats.

Conclusions:

Our findings reveal that normal calcineurin activity in the PVN constitutively restricts sympathetic vasomotor tone via suppressing NMDA receptor activity, which may be targeted for treating calcineurin inhibitor–induced hypertension.

Metal ions influx is a double edged sword for the pathogenesis of Alzheimer's disease

Alzheimer's disease (AD) is a common form of dementia in aged people, which is defined by two pathological characteristics: β-amyloid protein (Aβ) deposition and tau hyperphosphorylation. Although the mechanisms of AD development are still being debated, a series of evidence supports the idea that metals, such as copper, iron, zinc, magnesium and aluminium, are involved in the pathogenesis of the disease. In particular, the processes of Aβ deposition in senile plaques (SP) and the inclusion of phosphorylated tau in neurofibrillary tangles (NFTs) are markedly influenced by alterations in the homeostasis of the aforementioned metal ions. Moreover, the mechanisms of oxidative stress, synaptic plasticity, neurotoxicity, autophagy and apoptosis mediate the effects of metal ions-induced the aggregation state of Aβ and phosphorylated tau on AD development. More importantly, imbalance of these mechanisms finally caused cognitive decline in different experiment models. Collectively, reconstructing the signaling network that regulates AD progression by metal ions may provide novel insights for developing chelators specific for metal ions to combat AD.

Pentobarbital modifies the lipid raft-protein interaction: A first clue about the anesthesia mechanism on NMDA and GABAA receptors

Recent studies have shown that anesthetic agents alter the physical properties of lipid rafts on model membranes. However, if this destabilization occurs in brain membranes, altering the lipid raft-protein interaction, remains unknown. We analyzed the effects produced by pentobarbital (PB) on brain plasma membranes and lipid rafts in vivo. We characterized for the first time the thermotropic behavior of plasma membranes, synaptosomes, and lipid rafts from rat brain. We found that the transition temperature from the ordered gel to disordered liquid phase of lipids is close to physiological temperature. We then studied the effect of PB on protein composition of lipid rafts. Our results show a reduction of the total protein associated to rafts, with a higher reduction of the NMDAR compared to the GABA_A receptor. Both receptors are considered the main targets of PB. In general, our results suggest that lipid rafts could be plausible mediators in anesthetic action.

Endogenous casein kinase-1 modulates NMDA receptor activity of hypothalamic presympathetic neurons and sympathetic outflow in hypertension

KEY POINTS

Increased NMDA receptor activity and excitability of presympathetic neurons in the hypothalamus can increase sympathetic nerve discharges leading to hypertension. In this study, we determined how protein kinases and phosphatases are involved in regulating NMDA receptor activity and firing activity of presympathetic neurons in the hypothalamus in normotensive and hypertensive rats. We show that casein kinase-1 inhibition increases NMDA receptor activity and excitability of presympathetic neurons in the hypothalamus and augments sympathetic nerve discharges in normotensive, but not in hypertensive, rats. Our data indicate that casein kinase-1 tonically regulates NMDA receptor activity by interacting with casein kinase-2 and protein phosphatases in the hypothalamus and that imbalance of NMDA receptor phosphorylation can augment the excitability of hypothalamic presympathetic neurons and sympathetic nerve discharges in hypertension. These findings help us understand the neuronal mechanism of hypertension, and reducing the NMDA receptor phosphorylation level may be effective for treating neurogenic hypertension.

ABSTRACT

Increased N-methyl-d-aspartate receptor (NMDAR) activity in the paraventricular nucleus (PVN) of the hypothalamus is involved in elevated sympathetic outflow in hypertension. However, the molecular mechanisms underlying augmented NMDAR activity in hypertension remain unclear. In this study, we determined the role of casein kinase-1 (CK1) in regulating NMDAR activity in the PVN. NMDAR-mediated excitatory postsynaptic currents (EPSCs) and puff NMDA-elicited currents were recorded in spinally projecting PVN neurons in spontaneously hypertensive rats (SHRs) and Wistar-Kyoto (WKY) rats. The basal amplitudes of evoked NMDAR-EPSCs and puff NMDA currents were significantly higher in SHRs than in WKY rats. The CK1 inhibitor PF4800567 or PF670462 significantly increased the amplitude of NMDAR-EPSCs and puff NMDA currents in PVN neurons in WKY rats but not in SHRs. PF4800567 caused an NMDAR-dependent increase in the excitability of PVN neurons only in WKY rats. Also, the CK1ε protein level in the PVN was significantly lower in SHRs than in WKY rats. Furthermore, intracerebroventricular infusion of PF4800567 increased blood pressure and lumbar sympathetic nerve activity in WKY rats, and this effect was eliminated by microinjection of the NMDAR antagonist into the PVN. In addition, PF4800567 failed to increase NMDAR activity in brain slices of WKY rats pretreated with the protein phosphatase 1/2A, calcineurin, or casein kinase-2 inhibitor. Our findings suggest that CK1 tonically suppresses NMDAR activity in the PVN by reducing the NMDAR phosphorylation level. Diminished CK1 activity may contribute to potentiated glutamatergic synaptic input to PVN presympathetic neurons and elevated sympathetic vasomotor tone in neurogenic hypertension.

Separate intramolecular targets for protein kinase A control N-methyl-D-aspartate receptor gating and Ca2+ permeability

Protein kinase A (PKA) enhances synaptic plasticity in the central nervous system by increasing NMDA receptor current amplitude and Ca(2+) flux in an isoform-dependent yet poorly understood manner. PKA phosphorylates multiple residues on GluN1, GluN2A, and GluN2B subunits in vivo, but the functional significance of this multiplicity is unknown. We examined gating and permeation properties of recombinant NMDA receptor isoforms and of receptors with altered C-terminal domain (CTDs) prior to and after pharmacological inhibition of PKA. We found that PKA inhibition decreased GluN1/GluN2B but not GluN1/GluN2A gating; this effect was due to slower rates for receptor activation and resensitization and was mediated exclusively by the GluN2B CTD. In contrast, PKA inhibition reduced NMDA receptor-relative Ca(2+) permeability (PCa/PNa) regardless of the GluN2 isoform and required the GluN1 CTD; this effect was due primarily to decreased unitary Ca(2+) conductance, because neither Na(+) conductance nor Ca(2+)-dependent block was altered substantially. Finally, we show that both the gating and permeation effects can be reproduced by changing the phosphorylation state of a single residue: GluN2B Ser-1166 and GluN1 Ser-897, respectively. We conclude that PKA effects on NMDA receptor gating and Ca(2+) permeability rely on distinct phosphorylation sites located on the CTD of GluN2B and GluN1 subunits. This separate control of NMDA receptor properties by PKA may account for the specific effects of PKA on plasticity during synaptic development and may lead to drugs targeted to alter NMDA receptor gating or Ca(2+) permeability.