The prefrontal cortex (PFC) is a brain region featured with working memory function. The exact mechanism of how working memory operates within the PFC circuitry is unknown, but persistent neuronal firing recorded from prefrontal neurons during a working memory task is proposed to be the neural correlate of this mnemonic encoding. The PFC appears to be specialized for sustaining persistent firing, with N-methyl-D-aspartate (NMDA) receptors, especially slow-decay NR2B subunits, playing an essential role in the maintenance of sustained activity and normal working memory function. However, the NR2B subunit serves as a double-edged sword for PFC function. Because of its slow kinetics, NR2B endows the PFC with not only “neural psychic” properties, but also susceptibilities for neuroexcitotoxicity and psychiatric disorders. This review aims to clarify the interplay among working memory, the PFC, and NMDA receptors; demonstrate the importance of the NR2B subunit in the maintenance of persistent activity; understand the risks and vulnerabilities of how NR2B is related to the development of neuropsychiatric disorders; identify gaps that currently exist in our understanding of these processes; and provide insights regarding future directions that may clarify these issues. We conclude that the PFC is a specialized brain region with distinct delayed maturation, unique neuronal circuitry, and characteristic NMDA receptor function. The unique properties and development of NMDA receptors, especially enrichment of NR2B subunits, endows the PFC with not only the capability to generate sustained activity for working memory, but also serves as a major vulnerability to environmental insults and risk factors for psychiatric disorders.

Mitochondrial Ca(2+) homeostasis, the Ca(2+) influx-efflux balance, is responsible for the control of numerous cellular functions, including energy metabolism, generation of reactive oxygen species, spatiotemporal dynamics of Ca(2+) signaling, and cell growth and death. Recent discovery of the molecular identity of the mitochondrial Ca(2+) uniporter (MCU) provides new possibilities for application of genetic approaches to study the mitochondrial Ca(2+) influx mechanism in various cell types and tissues. In addition, the subsequent discovery of various auxiliary subunits associated with MCU suggests that mitochondrial Ca(2+) uptake is not solely regulated by a single protein (MCU), but likely by a macromolecular protein complex, referred to as the MCU-protein complex (mtCUC). Moreover, recent reports have shown the potential role of MCU posttranslational modifications in the regulation of mitochondrial Ca(2+) uptake through mtCUC. These observations indicate that mtCUCs form a local signaling complex at the inner mitochondrial membrane that could significantly regulate mitochondrial Ca(2+) handling, as well as numerous mitochondrial and cellular functions. In this review we discuss the current literature on mitochondrial Ca(2+) uptake mechanisms, with a particular focus on the structure and function of mtCUC, as well as its regulation by signal transduction pathways, highlighting current controversies and discrepancies.

Schizophrenia (SCZ) is a neurodevelopmental psychiatric disorder, in which cognitive function becomes disrupted at early stages of the disease. Although the mechanisms underlying cognitive impairments remain unclear, N-methyl-D-aspartate receptors (NMDAR) hypofunctioning in the prefrontal cortex (PFC) has been implicated. Moreover, cognitive symptoms in SCZ are usually unresponsive to treatment with current antipsychotics and by onset, disruption of the dopamine system, not NMDAR hypofunctioning, dominates the symptoms. Therefore, treating cognitive deficits at an early stage is a realistic approach. In this study, we tested whether an early treatment targeting mGluR2 would be effective in ameliorating cognitive impairments in the methylazoxymethanol acetate (MAM) model of SCZ. We investigated the effects of an mGluR2 agonist/mGluR3 antagonist, LY395756 (LY39), on the NMDAR expression and function in juveniles, as well as cognitive deficits in adult rats after juvenile treatment. We found that gestational MAM exposure induced a significant decrease in total protein levels of the NMDAR subunit, NR2B, and a significant increase of pNR2BTyr1472 in the juvenile rat PFC. Treatment with LY39 in juvenile MAM-exposed rats effectively recovered the disrupted NMDAR expression. Furthermore, a subchronic LY39 treatment in juvenile MAM-exposed rats also alleviated the learning deficits and cognitive flexibility impairments when tested with a cross-maze based set-shifting task in adults. Therefore, our study demonstrates that targeting dysfunctional NMDARs with an mGluR2 agonist during the early stage of SCZ could be an effective strategy in preventing the development and progression in addition to ameliorating cognitive impairments of SCZ.

Glycogen synthase kinase 3β (GSK3β) is a highly conserved serine/threonine kinase that has been implicated in both psychiatric and neurodegenerative diseases including schizophrenia, bipolar disorder, and Alzheimer's disease; therefore regulating its activity has become an important strategy for treatment of cognitive impairments in these disorders. This study examines the effects of lithium on GSK3β and its interaction with β-catenin and NMDA receptors within the prefrontal cortex. Lithium, a clinically relevant drug commonly prescribed as a mood stabilizer for psychiatric disorders, significantly increased levels of phosphorylated GSK3β serine 9, an inhibitory phosphorylation site, and decreased β-catenin ser33/37/thr41 phosphorylation in vitro, indicating GSK3β inhibition and reduced β-catenin degradation. GluN2A subunit levels were concurrently increased following lithium treatment. Similar alterations were also demonstrated in vivo; lithium administration increased GSK3β serine 9 phosphorylation and GluN2A levels, suggesting a reduced GSK3β activity and augmented GluN2A expression. Correspondingly, we observed that the amplitudes of evoked GluN2A-mediated excitatory postsynaptic currents in mPFC pyramidal neurons were significantly increased following lithium administration. Our data suggest that GSK3β activity negatively regulates GluN2A expression, likely by mediating upstream β-catenin phosphorylation, in prefrontal cortical neurons. Furthermore, our biochemical and electrophysiological experiments demonstrate that lithium mediates a specific increase in GluN2A subunit expression, ultimately augmenting GluN2A-mediated currents in the prefrontal cortex.

No abstract