Regulated internalization of NMDA receptors drives PKD1-mediated suppression of the activity of residual cell-surface NMDA receptors

STIM2 regulates NMDA receptor endocytosis that is induced by short-term NMDA receptor overactivation in cortical neurons

Recent findings suggest an important role for the dysregulation of stromal interaction molecule (STIM) proteins, activators of store-operated Ca2+ channels, and the prolonged activation of N-methyl-D-aspartate receptors (NMDARs) in the development of neurodegenerative diseases. We previously demonstrated that STIM silencing increases Ca2+ influx through NMDAR and STIM-NMDAR2 complexes are present in neurons. However, the interplay between NMDAR subunits (GluN1, GluN2A, and GluN2B) and STIM1/STIM2 with regard to intracellular trafficking remains unknown. Here, we found that the activation of NMDAR endocytosis led to an increase in STIM2-GluN2A and STIM2-GluN2B interactions in primary cortical neurons. STIM1 appeared to migrate from synaptic to extrasynaptic sites. STIM2 silencing inhibited post-activation NMDAR translocation from the plasma membrane and synaptic spines and increased NMDAR currents. Our findings reveal a novel molecular mechanism by which STIM2 regulates NMDAR synaptic trafficking by promoting NMDAR endocytosis after receptor overactivation, which may suggest protection against excessive uncontrolled Ca2+ influx through NMDARs.

Regulation of the NMDA receptor by its cytoplasmic domains: (How) is the tail wagging the dog?

Excitatory neurotransmission mediated by N-methyl-d-aspartate receptors (NMDARs) is critical for synapse development, function, and plasticity in the brain. NMDARs are tetra-heteromeric cation-channels that mediate synaptic transmission and plasticity. Extensive human studies show the existence of genetic variants in NMDAR subunits genes (GRIN genes) that are associated with neurodevelopmental and neuropsychiatric disorders, including autism spectrum disorders (ASD), epilepsy (EP), intellectual disability (ID), attention deficit hyperactivity disorder (ADHD), and schizophrenia (SCZ). NMDAR subunits have a unique modular architecture with four semiautonomous domains. Here we focus on the carboxyl terminal domain (CTD), also known as the intracellular C-tail, which varies in length among the glutamate receptor subunits and is the most diverse domain in terms of amino acid sequence. The CTD shows no sequence homology to any known proteins but encodes short docking motifs for intracellular binding proteins and covalent modifications. Our review will discuss the many important functions of the CTD in regulating NMDA membrane and synaptic targeting, stabilization, degradation targeting, allosteric modulation and metabotropic signaling of the receptor. This article is part of the special issue on 'Glutamate Receptors - NMDA Receptors'.

ALDH4A1 expression levels are elevated in postmortem brains of patients with schizophrenia and are associated with genetic variants in enzymes related to proline metabolism

BACKGROUND

The molecular mechanisms underlying schizophrenia remain largely unclear, and we recently identified multiple proteins significantly altered in the postmortem prefrontal cortex (PFC) of schizophrenia patients amongst which aldehyde dehydrogenase 4 family member A1 (ALDH4A1) was especially elevated. In this study, we aimed to investigate the expression of ALDH4A1 in the PFC and superior temporal gyrus (STG) and to elucidate functional correlations between schizophrenia risk alleles and molecular expression profiles in the postmortem brains of patients with schizophrenia.

METHODS

The levels of ALDH4A1 protein expression in the PFC and STG in postmortem brains from 24 patients with schizophrenia, 8 patients with bipolar disorder, and 32 controls were assessed using enzyme-linked immunosorbent assay. Moreover, we explored the associations between ALDH4A1 expression and genetic variants in enzymes associated with proline metabolism, including ALDH4A1 (schizophrenia [n = 22], bipolar disorder [n = 6], controls [n = 11]).

RESULTS

ALDH4A1 levels were significantly elevated in both the PFC and STG in patients with schizophrenia and tended to elevate in patients with bipolar disorder. Furthermore, ALDH4A1 expression levels in the PFC were significantly associated with the following three single-nucleotide polymorphisms: rs10882639, rs33823, rs153508. We also found partial coexpression of ALDH4A1 in mitochondria in a subset of putative astrocytes of postmortem brain.

LIMITATIONS

Our study population was relatively small, particularly for a genetic study.

CONCLUSION

These findings indicate that altered expression of ALDH4A1 may reflect the potential molecular mechanisms underlying the pathogenesis of schizophrenia and bipolar disorder, and may aid in the development of novel drug therapies.

Tripartite signalling by NMDA receptors

N-methyl-d-aspartate receptors (NMDARs) are excitatory glutamatergic receptors that are fundamental for many neuronal processes, including synaptic plasticity. NMDARs are comprised of four subunits derived from heterogeneous subunit families, yielding a complex diversity in NMDAR form and function. The quadruply-liganded state of binding of two glutamate and two glycine molecules to the receptor drives channel gating, allowing for monovalent cation flux, Ca²⁺ entry and the initiation of Ca²⁺-dependent signalling. In addition to this ionotropic function, non-ionotropic signalling can be initiated through the exclusive binding of glycine or of glutamate to the NMDAR. This binding may trigger a transmembrane conformational change of the receptor, inducing intracellular protein-protein signalling between the cytoplasmic domain and secondary messengers. In this review, we outline signalling cascades that can be activated by NMDARs and propose that the receptor transduces signalling through three parallel streams: (i) signalling via both glycine and glutamate binding, (ii) signalling via glycine binding, and (iii) signalling via glutamate binding. This variety in signal transduction mechanisms and downstream signalling cascades complements the widespread prevalence and rich diversity of NMDAR activity throughout the central nervous system and in disease pathology.

Hippocampal protein kinase D1 is necessary for DHPG-induced learning and memory impairments in rats

Background

Understanding molecular mechanisms underlying the induction of learning and memory impairments remains a challenge. Recent investigations have shown that the activation of group I mGluRs (mGluR1 and mGluR5) in cultured hippocampal neurons by application of (S)-3,5-Dihydroxyphenylglycine (DHPG) causes the regulated internalization of N-methyl-D-aspartate receptors (NMDARs), which subsequently activates protein kinase D1 (PKD1). Through phosphorylating the C-terminals of the NMDAR GluN2 subunits, PKD1 down-regulates the activity of remaining (non-internalized) surface NMDARs. The knockdown of PKD1 does not affect the DHPG-induced inhibition of AMPA receptor-mediated miniature excitatory post-synaptic currents (mEPSCs) but prevents the DHPG-induced inhibition of NMDAR-mediated mEPSCs in vitro. Thus, we investigated the in vivo effects of bilateral infusions of DHPG into the hippocampal CA1 area of rats in the Morris water maze (MWM) and the novel object discrimination (NOD) tests.

Methods

A total of 300 adult male Sprague Dawley rats (250–280 g) were used for behavioral tests. One hundred ninety four were used in MWM test and the other 106 rats in the NOD test. Following one week of habituation to the vivarium, rats were bilaterally implanted under deep anesthesia with cannulas aimed at the CA1 area of the hippocampus (CA1 coordinates in mm from Bregma: AP -3.14; lateral +/-2; DV -3.0). Through implanted cannulas artificial cerebrospinal fluid (ACSF), the group1 mGluR antagonist 6-Methyl-2-(phenylethynyl)pyridine (MPEP), the dynamin-dependent internalization inhibitor Dynasore, or the PKD1 inhibitor CID755673 were infused into the bilateral hippocampal CA1 areas (2 μL per side, over 5 min). The effects of these infusions and the effects of PKD1 knockdown were examined in MWM or NOD test.

Results

DHPG infusion increased the latency to reach the platform in the MWM test and reduced the preference for the novel object in the NOD task. We found that the DHPG effects were dose-dependent and could be maintained for up to 2 days. Notably, these effects could be prevented by pre-infusion of the group1 mGluR antagonist MPEP, the dynamin-dependent internalization inhibitor Dynasore, the PKD1 inhibitor CID755673, or by PKD1 knockdown in the hippocampal CA1 area.

Conclusion

Altogether, these findings provide direct evidence that PKD1-mediated signaling may play a critical role in the induction of learning and memory impairments by DHPG infusion into the hippocampal CA1 area.