A systematic investigation of the protein kinases involved in NMDA receptor-dependent LTD: evidence for a role of GSK-3 but not other serine/threonine kinases

Reactivation of cocaine reward memory engages the Akt/GSK3/mTOR signaling pathway and can be disrupted by GSK3 inhibition

RATIONAL

Memories return to a labile state following their retrieval and must undergo a process of reconsolidation to be maintained. Thus, disruption of cocaine reward memories by interference with reconsolidation may be therapeutically beneficial in the treatment of cocaine addiction.

OBJECTIVE

The objectives were to elucidate the signaling pathway involved in reconsolidation of cocaine reward memory and to test whether targeting this pathway could disrupt cocaine-associated contextual memory.

METHODS

Using a mouse model of conditioned place preference, regulation of the activity of glycogen synthase kinase-3 (GSK3), mammalian target of Rapamycin complex 1 (mTORC1), P70S6K, β-catenin, and the upstream signaling molecule Akt, was studied in cortico-limbic-striatal circuitry after re-exposure to an environment previously paired with cocaine.

RESULT

Levels of phosporylated Akt-Thr308, GSK3α-Ser21, GSK3β-Ser9, mTORC1, and P70S6K were reduced in the nucleus accumbens and hippocampus 10 min after the reactivation of cocaine cue memories. Levels of pAkt and pGSK3 were also reduced in the prefrontal cortex. Since reduced phosphorylation of GSK3 indicates heightened enzyme activity, the effect of a selective GSK3 inhibitor, SB216763, on reconsolidation was tested. Administration of SB216763 immediately after exposure to an environment previously paired with cocaine abrogated a previously established place preference, suggesting that GSK3 inhibition interfered with reconsolidation of cocaine-associated reward memories.

CONCLUSIONS

These findings suggest that the Akt/GSK3/mTORC1 signaling pathway in the nucleus accumbens, hippocampus, and/or prefrontal cortex is critically involved in the reconsolidation of cocaine contextual reward memory. Inhibition of GSK3 activity during memory retrieval can erase an established cocaine place preference.

Lack of the presynaptic RhoGAP protein oligophrenin1 leads to cognitive disabilities through dysregulation of the cAMP/PKA signalling pathway

Loss-of-function mutations in the gene encoding for the RhoGAP protein of oligophrenin-1 (OPHN1) lead to cognitive disabilities (CDs) in humans, yet the underlying mechanisms are not known. Here, we show that in mice constitutive lack of Ophn1 is associated with dysregulation of the cyclic adenosine monophosphate/phosphate kinase A (cAMP/PKA) signalling pathway in a brain-area-specific manner. Consistent with a key role of cAMP/PKA signalling in regulating presynaptic function and plasticity, we found that PKA-dependent presynaptic plasticity was completely abolished in affected brain regions, including hippocampus and amygdala. At the behavioural level, lack of OPHN1 resulted in hippocampus- and amygdala-related learning disabilities which could be fully rescued by the ROCK/PKA kinase inhibitor fasudil. Together, our data identify OPHN1 as a key regulator of presynaptic function and suggest that, in addition to reported postsynaptic deficits, loss of presynaptic plasticity contributes to the pathophysiology of CDs.

GLYX-13, an NMDA receptor glycine site functional partial agonist enhances cognition and produces antidepressant effects without the psychotomimetic side effects of NMDA receptor antagonists

INTRODUCTION

The N-methyl-d-aspartate receptor-ionophore complex plays a key role in learning and memory and has efficacy in animals and humans with affective disorders. GLYX-13 is an N-methyl-d-aspartate receptor (NMDAR) glycine-site functional partial agonist and cognitive enhancer that also shows rapid antidepressant activity without psychotomimetic side effects.

AREAS COVERED

The authors review the mechanism of action of GLYX-13 that was investigated in preclinical studies and evaluated in clinical studies. Specifically, the authors review its pharmacology, pharmacokinetics, and drug safety that were demonstrated in clinical studies.

EXPERT OPINION

NMDAR full antagonists can produce rapid antidepressant effects in treatment-resistant subjects; however, they are often accompanied by psychotomimetic effects that make chronic use outside of a clinical trial inpatient setting problematic. GLYX-13 appears to exert its antidepressant effects in the frontal cortex via NMDAR-triggered synaptic plasticity. Understanding the mechanistic underpinning of GLYX-13's antidepressant action should provide both novel insights into the role of the glutamatergic system in depression and identify new targets for therapeutic development.

Inhibition of glycogen synthase kinase-3β prevents remifentanil-induced hyperalgesia via regulating the expression and function of spinal N-methyl-D-aspartate receptors in vivo and vitro

A large number of experimental and clinical studies have confirmed that brief remifentanil exposure can enhance pain sensitivity presenting as opioid-induced hyperalgesia (OIH). N-methyl-D-aspartate (NMDA) receptor antagonists have been reported to inhibit morphine analgesic tolerance in many studies. Recently, we found that glycogen synthase kinase-3β (GSK-3β) modulated NMDA receptor trafficking in a rat model of remifentanil-induced postoperative hyperalgesia. In the current study, it was demonstrated that GSK-3β inhibition prevented remifentanil-induced hyperalgesia via regulating the expression and function of spinal NMDA receptors in vivo and in vitro. We firstly investigated the effects of TDZD-8, a selective GSK-3β inhibitor, on thermal and mechanical hyperalgesia using a rat model of remifentanil-induced hyperalgesia. GSK-3β activity as well as NMDA receptor subunits (NR1, NR2A and NR2B) expression and trafficking in spinal cord L4-L5 segments were measured by Western blot analysis. Furthermore, the effects of GSK-3β inhibition on NMDA-induced current amplitude and frequency were studied in spinal cord slices by whole-cell patch-clamp recording. We found that remifentanil infusion at 1 μg·kg(-1)·min(-1) and 2 μg·kg(-1)·min(-1) caused mechanical and thermal hyperalgesia, up-regulated NMDA receptor subunits NR1 and NR2B expression in both membrane fraction and total lysate of the spinal cord dorsal horn and increased GSK-3β activity in spinal cord dorsal horn. GSK-3β inhibitor TDZD-8 significantly attenuated remifentanil-induced mechanical and thermal hyperalgesia from 2 h to 48 h after infusion, and this was associated with reversal of up-regulated NR1 and NR2B subunits in both membrane fraction and total lysate. Furthermore, remifentanil incubation increased amplitude and frequency of NMDA receptor-induced current in dorsal horn neurons, which was prevented with the application of TDZD-8. These results suggest that inhibition of GSK-3β can significantly ameliorate remifentanil-induced hyperalgesia via modulating the expression and function of NMDA receptors, which present useful insights into the mechanistic action of GSK-3β inhibitor as potential anti-hyperalgesic agents for treating OIH.

Enhancement of long-term depression by soluble amyloid β protein in rat hippocampus is mediated by metabotropic glutamate receptor and involves activation of p38MAPK, STEP and caspase-3

It is reported that the amyloid-β protein (Aβ)-induced impairments in synaptic plasticity coincide with memory decline and dementia. Although Aβ-induced inhibition of hippocampal long-term potentiation has been intensively investigated, the underlying mechanism of Aβ-enhanced long-term depression (LTD) is not clear. Here, we report that acute exposure of rat hippocampal slices to soluble Aβ-enhanced LTD induced by weak low-frequency stimulation (wLFS; 1Hz for 3 min, 180 pulses) in granule cells of the dentate gyrus. Application of LY341495 (a non-selective Group I/II metrabotropic glumate receptor (mGluR) antagonist) completely blocked Aβ-enhanced LTD, whereas D-AP5 (a not selective N-methyl-d-aspartate receptor (NMDAR) antagonist) had no effect on Aβ-enhanced LTD compared with controls. In addition, Aβ-enhanced LTD was occluded by pre-application of 3,5-dihydroxyphenylglycine, a Group1 mGluR (mGluR1/5) agonist, suggesting Aβ-enhanced LTD depends on mGluR1/5 but not NMDAR. We also report here that p38 mitogen-activated protein kinase (p38MAPK) inhibitor SB203580 and postsynaptic protein tyrosine phosphatase inhibitors phenylarsine oxide and sodium orthovanadate prevented the facilitatory effect of Aβ on LTD. Application of striatal-enriched protein tyrosine phosphatase (STEP) activator MG132 facilitated induction of LTD by wLFS, but did not block following Aβ-enhanced LTD induced by another wLFS. On the other hand, Aβ-enhanced LTD blocked following MG132-LTD by wLFS, suggesting Aβ-enhanced hippocampal LTD involves STEP activation. Application of either non-selective caspase inhibitor Z-VAD-FMK or caspase-3 selective inhibitor Z-DEVD-FMK prevented Aβ-enhanced LTD. However, neither the tumor necrosis factor-α converting enzyme inhibitor TAPI-2 nor the mammalian target of rapamycin inhibitor rapamycin prevented the enhancement of Aβ on LTD. Therefore, we conclude that soluble Aβ enhances LTD in the hippocampal dentate gyrus region, and the facilitatory effect of Aβ on LTD involves mGluR1/5, p38MAPK, STEP and caspase-3 activation.

Elevated RalA activity in the hippocampus of PI3Kγ knock-out mice lacking NMDAR-dependent long-term depression

Phosphoinositide 3-kinases (PI3Ks) play key roles in synaptic plasticity and cognitive functions in the brain. We recently found that genetic deletion of PI3Kγ, the only known member of class IB PI3Ks, results in impaired N-methyl-D-aspartate receptor-dependent long-term depression (NMDAR-LTD) in the hippocampus. The activity of RalA, a small GTP-binding protein, increases following NMDAR-LTD inducing stimuli, and this increase in RalA activity is essential for inducing NMDAR-LTD. We found that RalA activity increased significantly in PI3Kγ knockout mice. Furthermore, NMDAR-LTD-inducing stimuli did not increase RalA activity in PI3Kγ knockout mice. These results suggest that constitutively increased RalA activity occludes further increases in RalA activity during induction of LTD, causing impaired NMDAR-LTD. We propose that PI3Kγ regulates the activity of RalA, which is one of the molecular mechanisms inducing NMDAR dependent LTD.

Long-term depression of synaptic transmission in the adult mouse insular cortex in vitro

The insular cortex (IC) is known to play important roles in higher brain functions such as memory and pain. Activity-dependent long-term depression (LTD) is a major form of synaptic plasticity related to memory and chronic pain. Previous studies of LTD have mainly focused on the hippocampus, and no study in the IC has been reported. In this study, using a 64-channel recording system, we show for the first time that repetitive low-frequency stimulation (LFS) can elicit frequency-dependent LTD of glutamate receptor-mediated excitatory synaptic transmission in both superficial and deep layers of the IC of adult mice. The induction of LTD in the IC required activation of the N-methyl-d-aspartate (NMDA) receptor, metabotropic glutamate receptor (mGluR)5, and L-type voltage-gated calcium channel. Protein phosphatase 1/2A and endocannabinoid signaling are also critical for the induction of LTD. In contrast, inhibiting protein kinase C, protein kinase A, protein kinase Mζ or calcium/calmodulin-dependent protein kinase II did not affect LFS-evoked LTD in the IC. Bath application of the group I mGluR agonist (RS)-3,5-dihydroxyphenylglycine produced another form of LTD in the IC, which was NMDA receptor-independent and could not be occluded by LFS-induced LTD. Our studies have characterised the basic mechanisms of LTD in the IC at the network level, and suggest that two different forms of LTD may co-exist in the same population of IC synapses.

Acquisition of contextual discrimination involves the appearance of a RAS-GRF1/p38 mitogen-activated protein (MAP) kinase-mediated signaling pathway that promotes long term potentiation (LTP)

RAS-GRF1 is a guanine nucleotide exchange factor with the ability to activate RAS and RAC GTPases in response to elevated calcium levels. We previously showed that beginning at 1 month of age, RAS-GRF1 mediates NMDA-type glutamate receptor (NMDAR)-induction of long term depression in the CA1 region of the hippocampus of mice. Here we show that beginning at 2 months of age, when mice first acquire the ability to discriminate between closely related contexts, RAS-GRF1 begins to contribute to the induction of long term potentiation (LTP) in the CA1 hippocampus by mediating the action of calcium-permeable, AMPA-type glutamate receptors (CP-AMPARs). Surprisingly, LTP induction by CP-AMPARs through RAS-GRF1 occurs via activation of p38 MAP kinase rather than ERK MAP kinase, which has more frequently been linked to LTP. Moreover, contextual discrimination is blocked by knockdown of Ras-Grf1 expression specifically in the CA1 hippocampus, infusion of a p38 MAP kinase inhibitor into the CA1 hippocampus, or the injection of an inhibitor of CP-AMPARs. These findings implicate the CA1 hippocampus in the developmentally dependent capacity to distinguish closely related contexts through the appearance of a novel LTP-supporting signaling pathway.

AZD1080, a novel GSK3 inhibitor, rescues synaptic plasticity deficits in rodent brain and exhibits peripheral target engagement in humans

Abnormal tau phosphorylation resulting in detachment of tau from microtubules and aggregation are critical events in neuronal dysfunction, degeneration, and neurofibrillary pathology seen in Alzheimer's disease. Glycogen synthase kinase-3β (GSK3β) is a key target for drug discovery in the treatment of Alzheimer's disease and related tauopathies because of its potential to abnormally phosphorylate proteins and contribute to synaptic degeneration. We report the discovery of AZD1080, a potent and selective GSK3 inhibitor that demonstrates peripheral target engagement in Phase 1 clinical studies. AZD1080 inhibits tau phosphorylation in cells expressing human tau and in intact rat brain. Interestingly, subchronic but not acute administration with AZD1080 reverses MK-801-induced deficits, measured by long-term potentiation in hippocampal slices and in a cognitive test in mice, suggesting that reversal of synaptic plasticity deficits in dysfunctional systems requires longer term modifications of proteins downstream of GSK3β signaling. The inhibitory pattern on tau phosphorylation reveals a prolonged pharmacodynamic effect predicting less frequent dosing in humans. Consistent with the preclinical data, in multiple ascending dose studies in healthy volunteers, a prolonged suppression of glycogen synthase activity was observed in blood mononuclear cells providing evidence of peripheral target engagement with a selective GSK3 inhibitor in humans.

The intersections of NMDAR-dependent synaptic plasticity and cell survival

The discovery of a requirement for N-methyl d-aspartate receptor (NMDAR) activation in long-term potentiation (LTP) set off an explosion of interest in the mechanisms of NMDAR-dependent synaptic plasticity. Meanwhile other research has advanced our understanding of how NMDAR activation regulates neuronal death and survival. Surprisingly, there have been few attempts to correlate these important areas of research. Here we review current knowledge of the various mechanisms of NMDAR-dependent synaptic plasticity that are shared with neuronal survival and death, while drawing comparisons with the proneurotrophin/neurotrophin receptor and intracellular signaling systems. Our conclusion is that NMDAR-dependent LTP and long-term depression (LTD) share many common mechanisms with cell survival and cell death, respectively. The intersections of plasticity and cell survival may represent novel avenues for neuroprotection. This article is part of the Special Issue entitled 'Glutamate Receptor-Dependent Synaptic Plasticity'.

GSK3 and β-catenin determines functional expression of sodium channels at the axon initial segment

Neuronal action potentials are generated through voltage-gated sodium channels, which are tethered by ankyrinG at the membrane of the axon initial segment (AIS). Despite the importance of the AIS in the control of neuronal excitability, the cellular and molecular mechanisms regulating sodium channel expression at the AIS remain elusive. Our results show that GSK3α/β and β-catenin phosphorylated by GSK3 (S33/37/T41) are localized at the AIS and are new components of this essential neuronal domain. Pharmacological inhibition of GSK3 or β-catenin knockdown with shRNAs decreased the levels of phosphorylated-β-catenin, ankyrinG, and voltage-gated sodium channels at the AIS, both "in vitro" and "in vivo", therefore diminishing neuronal excitability as evaluated via sodium current amplitude and action potential number. Thus, our results suggest a mechanism for the modulation of neuronal excitability through the control of sodium channel density by GSK3 and β-catenin at the AIS.

The Jak/STAT pathway is involved in synaptic plasticity

The Janus kinase (JAK)/signal transducer and activator of transcription (STAT) pathway is involved in many cellular processes, including cell growth and differentiation, immune functions and cancer. It is activated by various cytokines, growth factors, and protein tyrosine kinases (PTKs) and regulates the transcription of many genes. Of the four JAK isoforms and seven STAT isoforms known, JAK2 and STAT3 are highly expressed in the brain where they are present in the postsynaptic density (PSD). Here, we demonstrate a new neuronal function for the JAK/STAT pathway. Using a variety of complementary approaches, we show that the JAK/STAT pathway plays an essential role in the induction of NMDA-receptor dependent long-term depression (NMDAR-LTD) in the hippocampus. Therefore, in addition to established roles in cytokine signaling, the JAK/STAT pathway is involved in synaptic plasticity in the brain.

Hyperdopaminergic modulation of inhibitory transmission is dependent on GSK-3β signaling-mediated trafficking of GABAA receptors

Cortical dopamine (DA) modulation of the gamma-amino butyric acid (GABA) system is closely associated with cognitive function and psychiatric disorders. We recently reported that the glycogen synthase kinase 3β (GSK-3β) pathway is required for hyperdopamine/D2 receptor-mediated inhibition of NMDA receptors in the prefrontal cortex. Here we explore whether or not GSK-3β is also involved in dopaminergic modulation of GABAA receptor-mediated inhibitory transmission. We confirmed that DA induces a dose-dependent, bidirectional regulatory effect on inhibitory postsynaptic currents (IPSCs) in prefrontal neurons. The modulatory effects of DA were differentially affected by co-application of GSK-3β inhibitors and different doses of DA. GSK-3β inhibitors completely blocked high-dose (20 μM) DA-induced depressive effects on IPSCs but exhibited limited effects on the facilitating regulation of IPSC in low-dose DA (200 nM). We also confirmed that surface expressions of GABAA receptor β2/3 subunits were significantly decreased by DA applied in cultured prefrontal neurons and in vivo administration of DA reuptake inhibitor. These effects were blocked by prior administration of GSK-3β inhibitors. We explored DA-mediated regulation of GABAA receptor trafficking and exhibited the participation of brefeldin A-inhibited GDP/GTP exchange factor 2 (BIG2) or dynamin-dependent trafficking of GABAA receptors. Together, these data suggest that DA may act through different signaling pathways to affect synaptic inhibition, depending on the concentration. The GSK-3β signaling pathway is involved in DA-induced decrease in BIG2-dependent insertion and an increase in the dynamin-dependent internalization of GABAA receptors, which results in suppression of inhibitory synaptic transmission.

A pivotal role of GSK-3 in synaptic plasticity

Glycogen synthase kinase-3 (GSK-3) has many cellular functions. Recent evidence suggests that it plays a key role in certain types of synaptic plasticity, in particular a form of long-term depression (LTD) that is induced by the synaptic activation of N-methyl-D-aspartate receptors (NMDARs). In the present article we summarize what is currently known concerning the roles of GSK-3 in synaptic plasticity at both glutamatergic and GABAergic synapses. We summarize its role in cognition and speculate on how alterations in the synaptic functioning of GSK-3 may be a major factor in certain neurodegenerative disorders.

GSK3 as a Sensor Determining Cell Fate in the Brain

Glycogen synthase kinase 3 (GSK3) is an unusual serine/threonine kinase that controls many neuronal functions, including neurite outgrowth, synapse formation, neurotransmission, and neurogenesis. It mediates these functions by phosphorylating a wide range of substrates involved in gene transcription, metabolism, apoptosis, cytoskeletal dynamics, signal transduction, lipid membrane dynamics, and trafficking, amongst others. This complicated list of diverse substrates generally follow a more simple pattern: substrates negatively regulated by GSK3-mediated phosphorylation favor a proliferative/survival state, while substrates positively regulated by GSK3 favor a more differentiated/functional state. Accordingly, GSK3 activity is higher in differentiated cells than undifferentiated cells and physiological (Wnt, growth factors) and pharmacological inhibitors of GSK3 promote the proliferative capacity of embryonic stem cells. In the brain, the level of GSK3 activity influences neural progenitor cell proliferation/differentiation in neuroplasticity and repair, as well as efficient neurotransmission in differentiated adult neurons. While defects in GSK3 activity are unlikely to be the primary cause of neurodegenerative diseases, therapeutic regulation of its activity to promote a proliferative/survival versus differentiated/mature functional environment in the brain could be a powerful strategy for treatment of neurodegenerative and other mental disorders.

Dependence of NMDA/GSK-3β mediated metaplasticity on TRPM2 channels at hippocampal CA3-CA1 synapses

Transient receptor potential melastatin 2 (TRPM2) is a calcium permeable non-selective cation channel that functions as a sensor of cellular redox status. Highly expressed within the CNS, we have previously demonstrated the functional expression of these channels in CA1 pyramidal neurons of the hippocampus. Although implicated in oxidative stress-induced neuronal cell death, and potentially in neurodegenerative disease, the physiological role of TRPM2 in the central nervous system is unknown. Interestingly, we have shown that the activation of these channels may be sensitized by co-incident NMDA receptor activation, suggesting a potential contribution of TRPM2 to synaptic transmission. Using hippocampal cultures and slices from TRPM2 null mice we demonstrate that the loss of these channels selectively impairs NMDAR-dependent long-term depression (LTD) while sparing long-term potentiation. Impaired LTD resulted from an inhibition of GSK-3β, through increased phosphorylation, and a reduction in the expression of PSD95 and AMPARs. Notably, LTD could be rescued in TRPM2 null mice by recruitment of GSK-3β signaling following dopamine D2 receptor stimulation. We propose that TRPM2 channels play a key role in hippocampal synaptic plasticity.

GSK-3 Inhibitors: Preclinical and Clinical Focus on CNS

Inhibiting glycogen synthase kinase-3 (GSK-3) activity via pharmacological intervention has become an important strategy for treating neurodegenerative and psychiatric disorders. The known GSK-3 inhibitors are of diverse chemotypes and mechanisms of action and include compounds isolated from natural sources, cations, synthetic small-molecule ATP-competitive inhibitors, non-ATP-competitive inhibitors, and substrate-competitive inhibitors. Here we describe the variety of GSK-3 inhibitors with a specific emphasis on their biological activities in neurons and neurological disorders. We further highlight our current progress in the development of non-ATP-competitive inhibitors of GSK-3. The available data raise the hope that one or more of these drug design approaches will prove successful at stabilizing or even reversing the aberrant neuropathology and cognitive deficits of certain central nervous system disorders.

PI3Kγ is required for NMDA receptor-dependent long-term depression and behavioral flexibility

Phosphatidylinositol 3-kinase (PI3K) has been implicated in synaptic plasticity and other neural functions in the brain. However, the role of individual PI3K isoforms in the brain is unclear. We investigated the role of PI3Kγ in hippocampal-dependent synaptic plasticity and cognitive functions. We found that PI3Kγ has a crucial and specific role in NMDA receptor (NMDAR)-mediated synaptic plasticity at mouse Schaffer collateral-commissural synapses. Both genetic deletion and pharmacological inhibition of PI3Kγ disrupted NMDAR long-term depression (LTD) while leaving other forms of synaptic plasticity intact. Accompanying this physiological deficit, the impairment of NMDAR LTD by PI3Kγ blockade was specifically correlated with deficits in behavioral flexibility. These findings suggest that a specific PI3K isoform, PI3Kγ, is critical for NMDAR LTD and some forms of cognitive function. Thus, individual isoforms of PI3Ks may have distinct roles in different types of synaptic plasticity and may therefore influence various kinds of behavior.

Long-term depression in the CNS

Long-term depression (LTD) in the CNS has been the subject of intense investigation as a process that may be involved in learning and memory and in various pathological conditions. Several mechanistically distinct forms of this type of synaptic plasticity have been identified and their molecular mechanisms are starting to be unravelled. Most studies have focused on forms of LTD that are triggered by synaptic activation of either NMDARs (N-methyl-D-aspartate receptors) or metabotropic glutamate receptors (mGluRs). Converging evidence supports a crucial role of LTD in some types of learning and memory and in situations in which cognitive demands require a flexible response. In addition, LTD may underlie the cognitive effects of acute stress, the addictive potential of some drugs of abuse and the elimination of synapses in neurodegenerative diseases.

The Role of GSK3 in Presynaptic Function

The past ten years of research have identified a number of key roles for glycogen synthase kinase 3 (GSK3) at the synapse. In terms of presynaptic physiology, critical roles for GSK3 have been revealed in the growth and maturation of the nerve terminal and more recently a key role in the control of activity-dependent bulk endocytosis of synaptic vesicles. This paper will summarise the major roles assigned to GSK3 in both immature and mature nerve terminals, the substrates GSK3 phosphorylates to exert its action, and how GSK3 activity is regulated by different presynaptic signalling cascades. The number of essential roles for GSK3, coupled with the numerous signalling cascades all converging to regulate its activity, suggests that GSK3 is a key integrator of multiple inputs to modulate the strength of neurotransmission. Modulation of these pathways may point to potential mechanisms to overcome synaptic failure in neurodegenerative disorders such as Alzheimer's disease.

Muscarinic receptors induce LTD of NMDAR EPSCs via a mechanism involving hippocalcin, AP2 and PSD-95

Although muscarinic acetylcholine receptors (mAChRs) and NMDA receptors (NMDARs) are important for synaptic plasticity, learning and memory, the manner in which they interact is poorly understood. We found that stimulation of muscarinic receptors, either by an agonist or by the synaptic release of acetylcholine, led to long-term depression (LTD) of NMDAR-mediated synaptic transmission. This form of LTD involved the release of Ca2+ from IP₃-sensitive intracellular stores and was expressed via the internalization of NMDARs. Our results suggest that the molecular mechanism involves a dynamic interaction between the neuronal calcium sensor protein hippocalcin, the clathrin adaptor molecule AP2, the postsynaptic density enriched protein PSD-95 and NMDARs. We propose that hippocalcin binds to the SH3 region of PSD-95 under basal conditions, but it translocates to the plasma membrane on sensing Ca2+; in doing so, it causes PSD-95 to dissociate from NMDARs, permitting AP2 to bind and initiate their dynamin-dependent endocytosis.

Properties of subsequent induction of long-term potentiation and/or depression in one synaptic input in apical dendrites of hippocampal CA1 neurons in vitro

The hippocampus is a prominent structure to study mechanisms of learning and memory at the cellular level. Long-term potentiation (LTP) as well as long-term depression (LTD) are the major cellular models which could underlie learning and memory formation. LTP and LTD consist of at least two phases, an early protein synthesis-independent transient stage (<4 h; E-LTP, E-LTD) as well as a prolonged phase (>4 h; L-LTP, L-LTD) requiring the synthesis of new proteins. It is known that during E-LTP the further induction of longer lasting LTP is precluded. However, if E-LTP is transformed into L-LTP, the same synapses now allow the induction of LTP again. We reproduced the LTP-results first and then investigated whether hippocampal LTP or LTD also prevents the establishment of subsequent LTD-induction in the same synaptic input. We show that the prior induction of LTP or LTD does not prevent a short-term depression (STD) but occludes LTD in apical dendrites of CA1 neurons in hippocampal slices in vitro during the early phase of LTP or LTD. However, LTD can again be induced in addition to STD after the establishment of L-LTP or L-LTD, that is about 4 h after the induction of the first event in the same synaptic input. We suggest that the neuronal input preserves the capacity for STD immediately after an initial potentiation or depression, but for the onset of additional longer lasting LTD in the same synaptic input, the establishment of the late plasticity form of the preceding event is critical.

GSK-3β activity and hyperdopamine-dependent behaviors

Dopamine plays important roles in normal brain function and many neuropsychiatric disorders. Classically, dopamine receptors are positively coupled to G protein-mediated signaling to regulate cyclic adenosine monophosphate (cAMP)-protein kinase A (PKA)-dopamine and cAMP-regulated phosphoprotein of 32 kDa (DARPP-32) and Ca(2+) pathways. However, emerging evidence indicates that under hyperdopaminergic conditions, the protein kinase B (Akt)-glycogen synthase kinase 3β (GSK-3β) signaling cascade may mediate dopamine actions via D(2)-like receptors. This cAMP-independent signaling pathway involves the regulation of downstream synaptic targets, e.g., AMPA receptor, NMDA receptors, and thus synaptic plasticity. Here we provide an overview of how this novel signaling pathway relays dopamine receptor-mediated responses, particularly hyperdopamine-dependent behaviors. We discuss the relevance of the Akt/GSK-3β signaling cascade for the expression of dopamine-dependent behaviors and the drug actions associated with dopaminergic systems.

Abnormalities in brain structure and behavior in GSK-3alpha mutant mice

BACKGROUND

Glycogen synthase kinase-3 (GSK-3) is a widely expressed and highly conserved serine/threonine protein kinase encoded by two genes that generate two related proteins: GSK-3alpha and GSK-3beta. Mice lacking a functional GSK-3alpha gene were engineered in our laboratory; they are viable and display insulin sensitivity. In this study, we have characterized brain functions of GSK-3alpha KO mice by using a well-established battery of behavioral tests together with neurochemical and neuroanatomical analysis.

RESULTS

Similar to the previously described behaviours of GSK-3beta(+/-) mice, GSK-3alpha mutants display decreased exploratory activity, decreased immobility time and reduced aggressive behavior. However, genetic inactivation of the GSK-3alpha gene was associated with: decreased locomotion and impaired motor coordination, increased grooming activity, loss of social motivation and novelty; enhanced sensorimotor gating and impaired associated memory and coordination. GSK-3alpha KO mice exhibited a deficit in fear conditioning, however memory formation as assessed by a passive avoidance test was normal, suggesting that the animals are sensitized for active avoidance of a highly aversive stimulus in the fear-conditioning paradigm. Changes in cerebellar structure and function were observed in mutant mice along with a significant decrease of the number and size of Purkinje cells.

CONCLUSION

Taken together, these data support a role for the GSK-3alpha gene in CNS functioning and possible involvement in the development of psychiatric disorders.