Hippocampal Long-Term Depression in the Presence of Calcium-Permeable AMPA Receptors

SHP2 regulates GluA2 tyrosine phosphorylation required for AMPA receptor endocytosis and mGluR-LTD

Posttranslational modifications regulate the properties and abundance of synaptic α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors that mediate fast excitatory synaptic transmission and synaptic plasticity in the central nervous system. During long-term depression (LTD), protein tyrosine phosphatases (PTPs) dephosphorylate tyrosine residues in the C-terminal tail of AMPA receptor GluA2 subunit, which is essential for GluA2 endocytosis and group I metabotropic glutamate receptor (mGluR)-dependent LTD. However, as a selective downstream effector of mGluRs, the mGluR-dependent PTP responsible for GluA2 tyrosine dephosphorylation remains elusive at Schaffer collateral (SC)-CA1 synapses. In the present study, we find that mGluR5 stimulation activates Src homology 2 (SH2) domain-containing phosphatase 2 (SHP2) by increasing phospho-Y542 levels in SHP2. Under steady-state conditions, SHP2 plays a protective role in stabilizing phospho-Y869 of GluA2 by directly interacting with GluA2 phosphorylated at Y869, without affecting GluA2 phospho-Y876 levels. Upon mGluR5 stimulation, SHP2 dephosphorylates GluA2 at Y869 and Y876, resulting in GluA2 endocytosis and mGluR-LTD. Our results establish SHP2 as a downstream effector of mGluR5 and indicate a dual action of SHP2 in regulating GluA2 tyrosine phosphorylation and function. Given the implications of mGluR5 and SHP2 in synaptic pathophysiology, we propose SHP2 as a promising therapeutic target for neurodevelopmental and autism spectrum disorders.

Proposed mechanisms of tau: relationships to traumatic brain injury, Alzheimer's disease, and epilepsy

Traumatic brain injury (TBI), Alzheimer's disease (AD), and epilepsy share proposed mechanisms of injury, including neuronal excitotoxicity, cascade signaling, and activation of protein biomarkers such as tau. Although tau is typically present intracellularly, in tauopathies, phosphorylated (p-) and hyper-phosphorylated (hp-) tau are released extracellularly, the latter leading to decreased neuronal stability and neurofibrillary tangles (NFTs). Tau cleavage at particular sites increases susceptibility to hyper-phosphorylation, NFT formation, and eventual cell death. The relationship between tau and inflammation, however, is unknown. In this review, we present evidence for an imbalanced endoplasmic reticulum (ER) stress response and inflammatory signaling pathways resulting in atypical p-tau, hp-tau and NFT formation. Further, we propose tau as a biomarker for neuronal injury severity in TBI, AD, and epilepsy. We present a hypothesis of tau phosphorylation as an initial acute neuroprotective response to seizures/TBI. However, if the underlying seizure pathology or TBI recurrence is not effectively treated, and the pathway becomes chronically activated, we propose a "tipping point" hypothesis that identifies a transition of tau phosphorylation from neuroprotective to injurious. We outline the role of amyloid beta (Aβ) as a "last ditch effort" to revert the cell to programmed death signaling, that, when fails, transitions the mechanism from injurious to neurodegenerative. Lastly, we discuss targets along these pathways for therapeutic intervention in AD, TBI, and epilepsy.

Molecular Mechanisms of AMPA Receptor Trafficking in the Nervous System

Synaptic plasticity enhances or reduces connections between neurons, affecting learning and memory. Postsynaptic AMPARs mediate greater than 90% of the rapid excitatory synaptic transmission in glutamatergic neurons. The number and subunit composition of AMPARs are fundamental to synaptic plasticity and the formation of entire neural networks. Accordingly, the insertion and functionalization of AMPARs at the postsynaptic membrane have become a core issue related to neural circuit formation and information processing in the central nervous system. In this review, we summarize current knowledge regarding the related mechanisms of AMPAR expression and trafficking. The proteins related to AMPAR trafficking are discussed in detail, including vesicle-related proteins, cytoskeletal proteins, synaptic proteins, and protein kinases. Furthermore, significant emphasis was placed on the pivotal role of the actin cytoskeleton, which spans throughout the entire transport process in AMPAR transport, indicating that the actin cytoskeleton may serve as a fundamental basis for AMPAR trafficking. Additionally, we summarize the proteases involved in AMPAR post-translational modifications. Moreover, we provide an overview of AMPAR transport and localization to the postsynaptic membrane. Understanding the assembly, trafficking, and dynamic synaptic expression mechanisms of AMPAR may provide valuable insights into the cognitive decline associated with neurodegenerative diseases.

KIBRA regulates activity-induced AMPA receptor expression and synaptic plasticity in an age-dependent manner

A growing body of human literature implicates KIBRA in memory and neurodevelopmental disorders. Memory and the cellular substrates supporting adaptive cognition change across development. Using an inducible KIBRA knockout mouse, we demonstrate that adult-onset deletion of KIBRA in forebrain neurons impairs long-term spatial memory and long-term potentiation (LTP). These LTP deficits correlate with adult-selective decreases in extrasynaptic AMPA receptors under basal conditions, and we identify a role for KIBRA in LTP-induced AMPAR upregulation. In contrast, juvenile-onset deletion of KIBRA in forebrain neurons did not affect LTP and had minimal effects on basal AMPAR expression. LTP did not increase AMPAR protein expression in juvenile WT mice, providing a potential explanation for juvenile resilience to KIBRA deletion. These data suggest that KIBRA serves a unique role in adult hippocampal function through regulation of basal and activity-dependent AMPAR proteostasis that supports synaptic plasticity.

The many faces of the AMPA-type ionotropic glutamate receptor

Knowledge of the biology of ionotropic glutamate receptors (iGluRs) is a prerequisite for any student of the neurosciences. But yet, half a century ago, the situation was quite different. There was fierce debate on whether simple amino acids, such as l-glutamic acid (L-Glu), should even be considered as putative neurotransmitter candidates that drive excitatory synaptic signaling in the vertebrate brain. Organic chemist, Jeff Watkins, and physiologist, Dick Evans, were amongst the pioneering scientists who shed light on these tribulations. By combining their technical expertise, they performed foundational work that explained that the actions of L-Glu were, in fact, mediated by a family of ion-channels that they named NMDA-, AMPA- and kainate-selective iGluRs. To celebrate and reflect upon their seminal work, Neuropharmacology has commissioned a series of issues that are dedicated to each member of the Glutamate receptor superfamily that includes both ionotropic and metabotropic classes. This issue brings together nine timely reviews from researchers whose work has brought renewed focus on AMPA receptors (AMPARs), the predominant neurotransmitter receptor at central synapses. Together with the larger collection of papers on other GluR family members, these issues highlight that the excitement, passion, and clarity that Watkins and Evans brought to the study of iGluRs is unlikely to fade as we move into a new era on this most interesting of ion-channel families.

The Potential Role of AMPA Receptor Trafficking in Autism and Other Neurodevelopmental Conditions

Autism Spectrum Disorder (ASD) is a multifaceted condition associated with difficulties in social interaction and communication. It also shares several comorbidities with other neurodevelopmental conditions. Intensive research examining the molecular basis and characteristics of ASD has revealed an association with a large number and variety of low-penetrance genes. Many of the variants associated with ASD are in genes underlying pathways involved in long-term potentiation (LTP) or depression (LTD). These mechanisms then control the tuning of neuronal connections in response to experience by modifying and trafficking ionotropic glutamate receptors at the post-synaptic areas. Despite the high genetic heterogeneity in ASD, surface trafficking of the α-amino-3-hydroxy-5-Methyl-4-isoxazolepropionate (AMPA) receptor is a vulnerable pathway in ASD. In this review, we discuss autism-related alterations in the trafficking of AMPA receptors, whose surface density and composition at the post-synapse determine the strength of the excitatory connection between neurons. We highlight genes associated with neurodevelopmental conditions that share the autism comorbidity, including Fragile X syndrome, Rett Syndrome, and Tuberous Sclerosis, as well as the autism-risk genes NLGNs, IQSEC2, DOCK4, and STXBP5, all of which are involved in regulating AMPAR trafficking to the post-synaptic surface.

Long-term effect of neonatal antagonism of ionotropic glutamate receptors on dendritic spines and cognitive function in rats

Glutamate is the most abundant excitatory neurotransmitter in the hippocampus where mediates its actions by activating glutamate receptors. The activation of these receptors is essential for the maintenance and dynamics of dendritic spines and plasticity that correlate with learning and memory processes during neurodevelopment and adulthood. We studied in adults the effect of blocking ionotropic glutamate receptors (NMDAR, AMPAR, and KAR) functions at neonatal age (PD1-PD15) with their respective antagonists D-AP5, GYKI-53655 and UBP-302. We first evaluated memory using a new object recognition test in adults. Second, we evaluated the levels of glial fibrillary acidic protein, synaptophysin and actin with immunohistochemistry in the CA1, CA3, and dentate gyrus regions of the hippocampus and, finally, the number of dendritic spines and their dynamics using Golgi-Cox staining. We found that ionotropic glutamate receptor function blockade at neonatal age causes a reduction in short and long-term memory in adulthood and a reduction in the expression of synaptophysin and actin protein levels in the hippocampus regions studied. This blockade also reduced the number of dendritic spines and modified dendritic dynamics in the CA1 region. The antagonism of the three types of ionotropic glutamate receptors reduced the mushrooms and bifurcated types of spines and increased the thin spines. The number of stubby spines was reduced by D-AP5, increased by UPB-302, and not affected by GYKI-53655. Our results indicate that the blockade of neonatal ionotropic glutamate receptors produces alterations that persist until adulthood.

Reelin Regulates Neuronal Excitability through Striatal-Enriched Protein Tyrosine Phosphatase (STEP61) and Calcium Permeable AMPARs in an NMDAR-Dependent Manner

Alzheimer's disease (AD) is a progressive neurodegenerative disease marked by the accumulation of amyloid-β (Aβ) plaques and neurofibrillary tangles. Aβ oligomers cause synaptic dysfunction early in AD by enhancing long-term depression (LTD; a paradigm for forgetfulness) via metabotropic glutamate receptor (mGluR)-dependent regulation of striatal-enriched tyrosine phosphatase (STEP61). Reelin is a neuromodulator that signals through ApoE (apolipoprotein E) receptors to protect the synapse against Aβ toxicity (Durakoglugil et al., 2009) Reelin signaling is impaired by ApoE4, the most important genetic risk factor for AD, and Aβ-oligomers activate metabotropic glutamate receptors (Renner et al., 2010). We therefore asked whether Reelin might also affect mGluR-LTD. To this end, we induced chemical mGluR-LTD using DHPG (Dihydroxyphenylglycine), a selective mGluR5 agonist. We found that exogenous Reelin reduces the DHPG-induced increase in STEP61, prevents the dephosphorylation of GluA2, and concomitantly blocks mGluR-mediated LTD. By contrast, Reelin deficiency increased expression of Ca2+-permeable GluA2-lacking AMPA receptors along with higher STEP61 levels, resulting in occlusion of DHPG-induced LTD in hippocampal CA1 neurons. We propose a model in which Reelin modulates local protein synthesis as well as AMPA receptor subunit composition through modulation of mGluR-mediated signaling with implications for memory consolidation or neurodegeneration.SIGNIFICANCE STATEMENT Reelin is an important neuromodulator, which in the adult brain controls synaptic plasticity and protects against neurodegeneration. Amyloid-β has been shown to use mGluRs to induce synaptic depression through endocytosis of NMDA and AMPA receptors, a mechanism referred to as LTD, a paradigm of forgetfulness. Our results show that Reelin regulates the phosphatase STEP, which plays an important role in neurodegeneration, as well as the expression of calcium-permeable AMPA receptors, which play a role in memory formation. These data suggest that Reelin uses mGluR LTD pathways to regulate memory formation as well as neurodegeneration.

Genetic manipulations of AMPA glutamate receptors in hippocampal synaptic plasticity

Alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are the principal mediators of fast excitatory synaptic transmission and they are required for various forms of synaptic plasticity, including long-term potentiation (LTP) and depression (LTD), which are key mechanisms of learning and memory. AMPARs are tetrameric complexes assembled from four subunits (GluA1-4), however, the lack of subunit-specific pharmacological tools has made the assessment of individual subunits difficult. The application of genetic techniques, particularly gene targeting, allows for precise manipulation and dissection of each subunit in the regulation of neuronal function and behaviour. In this review, we summarize studies using various mouse models with genetically altered AMPARs and focus on their roles in basal synaptic transmission, LTP, and LTD at the hippocampal CA1 synapse. These studies provide strong evidence that there are multiple forms of LTP and LTD at this synapse which can be induced by various induction protocols, and they are differentially regulated by different AMPAR subunits and domains. We conclude that it is necessary to delineate the mechanism of each of these forms of plasticity and their contribution to memory and brain disorders.

Altered hippocampal dendritic spine maturation after hypoxia-induced seizures in neonatal rats

Cognitive comorbidities often follow early-life seizures (ELS), especially in the setting of autism and other neurodevelopmental syndromes. However, there is an incomplete understanding of whether neuronal and synaptic development are concomitantly dysregulated. We have previously shown that hypoxia-induced seizures (HS) in postnatal day (P)10 rats increase acute and later-life hippocampal glutamatergic neurotransmission and spontaneous recurrent seizures, and impair cognition and behavior. As dendritic spines critically regulate synaptic function, we hypothesized that ELS can induce developmentally specific changes in dendritic spine maturation. At intervals during one month following HS in P10 rats, we assessed dendritic spine development on pyramidal neurons in the stratum radiatum of hippocampal area CA1. Compared to control rats in which spine density significantly decreased from P10 to early adulthood (P38), post-seizure rats failed to show a developmental decrease in spine density, and spines from P38 post-seizure rats appeared more immature-shaped (long, thin). In addition, compared to P38 control rats, post-seizure P38 rats expressed significantly more synaptic PSD-95, a marker of mature synapses. These changes were preceded by a transient increase in hippocampal expression of cofilin phosphorylated at Ser3, representing a decrease in cofilin activity. These results suggest that early-life seizures may impair normal dendritic spine maturation and pruning in CA1 during development, resulting in an excess of less efficient synapses, via activity-dependent modification of actin-regulating proteins such as cofilin. Given that multiple neurodevelopmental disorders show similar failures in developmental spine pruning, the current findings may represent a deficit in structural plasticity that could be a component of a mechanism leading to later-life cognitive consequences associated with early-life seizures.

The Requirement of the C-Terminal Domain of GluA1 in Different Forms of Long-Term Potentiation in the Hippocampus Is Age-Dependent

Long-term potentiation (LTP) at glutamatergic synapses is an extensively studied form of long-lasting synaptic plasticity widely regarded as the cellular basis for learning and memory. At the CA1 synapse, there are multiple forms of LTP with distinct properties. Although AMPA glutamate receptors (AMPARs) are a key target of LTP expression, whether they are required in all forms of LTP remains unclear. To address this question, we have used our recently developed mouse line, GluA1^C2KI, where the c-terminal domain (CTD) of the endogenous GluA1 is replaced by that of GluA2. Unlike traditional GluA1 global or conditional KO mice, GluA1^C2KI mice have no changes in basal AMPAR properties or synaptic transmission allowing a better assessment of GluA1 in synaptic plasticity. We previously showed that these mice are impaired in LTP induced by high-frequency stimulation (HFS-LTP), but whether other forms of LTP are also affected in these mice is unknown. In this study, we compared various forms of LTP at CA1 synapses between GluA1^C2KI and wild-type littermates by using several induction protocols. We show that HFS-LTP is impaired in both juvenile and adult GluA1^C2KI mice. The LTP induced by theta-burst stimulation (TBS-LTP) is also abolished in juvenile GluA1^C2KI mice. Interestingly, TBS-LTP can still be induced in adult GluA1^C2KI mice, but its mechanisms are altered becoming more sensitive to protein synthesis and the extracellular signal-regulated kinase (ERK) inhibitors compared to wild type (WT) control. The GluA1^C2KI mice are also differentially altered in several forms of LTP induced under whole-cell recording paradigms. These results indicate that the CTD of GluA1 is differentially involved in different forms of LTP at CA1 synapse highlighting the complexity and adaptative potential of LTP expression mechanisms in the hippocampus.