Regulation of Synapse Weakening through Interactions of the Microtubule Associated Protein Tau with PACSIN1.

Hyperphosphorylation of the microtubule associated protein tau (tau) is inextricably linked to several neurodegenerative diseases, collectively termed tauopathies, in which synapse dysfunction occurs through largely unidentified mechanisms. Our research aimed to uncover molecular mechanisms by which phosphorylation of tau (pTau) affects synapse function. Using combined molecular and electrophysiological analysis with in vitro genetic knock-in of phosphorylation mutant human tau in male rat CA1 hippocampal neurons, we show an interplay between tau and protein kinase C and casein kinase substrate in neurons protein 1 (PACSIN1) that regulates synapse function. pTau at serine residues 396/404 decreases tau:PACSIN1 binding and evokes PACSIN1-dependent functional and structural synapse weakening. Knock-down of tau or PACSIN1 increases AMPA receptor (AMPAR)-mediated current at extrasynaptic regions, supporting a role for these proteins in affecting AMPAR trafficking. The pTau-induced PACSIN1 dissociation may represent a pathophysiological regulator of synapse function that underlies tauopathy-associated synapse defects.SIGNIFICANCE STATEMENT Knowledge is still lacking for how hyperphosphorylation of tau and its effectors lead to synaptic and neuronal dysfunction. Our results provide crucial insight for this mechanistic understanding; we show that specific tau phosphorylation events modulate its protein interaction with PACSIN1 and thus elicits synapse weakening likely through PACSIN1-dependent regulation of AMPA receptor (AMPAR) trafficking. These findings develop our understanding of molecular events that may be relevant to cellular changes underpinning tauopathy-associated neurodegenerative diseases.

The Role of Tau in the Post-synapse.

It is well documented that tauopathy is involved in various forms of neurodegenerative disease. However, there is a huge gap in terms of our understanding of the neurophysiological roles of tau, and how these can be aberrantly regulated by pathological processes. Tau is enriched in the axon but is also localized to synapses. The finding of synaptically localised tau has undoubtedly created more questions than it has answered. What is the physiological role of tau at the synapse? Whether and how does tau interact with and effect other synaptic proteins to mediate this function? Are these effects regulated by post-translational modifications of tau, such as phosphorylation? Such questions require significant attention from the scientific community if we are to resolve this critical aspect of tau biology. This chapter will describe our current understanding of synaptic tau and its functions and illuminate the numerous remaining challenges in this evolving research area.

Dendritic spine anomalies and PTEN alterations in a mouse model of VPA-induced autism spectrum disorder.

Mounting evidence suggests that the etiology of autism spectrum disorders (ASDs) is profoundly influenced by exposure to environmental factors, although the precise molecular and cellular links remain ill-defined. In this study, we examined how exposure to valproic acid (VPA) during pregnancy is associated with an increased incidence of ASD. A mouse model was established by injecting VPA at embryonic day 13, and its behavioral phenotypes including impaired social interaction, increased repetitive behaviors and decreased nociception were observed at postnatal days 21-42. VPA-treated mice showed dysregulation of synaptic structure in cortical neurons, including a reduced proportion of filopodium-type and stubby spines and increased proportions of thin and mushroom-type spines, along with a decreased spine head size. We also found that VPA-treatment led to decreased expression of phosphate and tensin homolog (PTEN) and increased levels of p-AKT protein in the hippocampus and cortex. Our data suggest that there is a correlation between VPA exposure and dysregulation of PTEN with ASD-like behavioral and neuroanatomical changes, and this may be a potential mechanism of VPA-induced ASD.

Glucocorticoids activate a synapse weakening pathway culminating in tau phosphorylation in the hippocampus.

Evidence suggests that the stress hormones glucocorticoids (GCs) can cause cognitive deficits and neurodegeneration. Previous studies have found GCs facilitate physiological synapse weakening, termed long-term depression (LTD), though the precise mechanisms underlying this are poorly understood. Here we show that GCs activate glycogen synthase kinase-3 (GSK-3), a kinase crucial to synapse weakening signals. Critically, this ultimately leads to phosphorylation of the microtubule associated protein tau, specifically at the serine 396 residue, and this is a causal factor in the GC-mediated impairment of synaptic function. These findings reveal the link between GCs and synapse weakening signals, and the potential for stress-induced priming of neurodegeneration. This could have important implications for our understanding of how stress can lead to neurodegenerative disease.

Tau phosphorylation at serine 396 residue is required for hippocampal LTD.

Tau is required for the induction of long-term depression (LTD) of synaptic transmission in the hippocampus. Here we probe the role of tau in LTD, finding that an AMPA receptor internalization mechanism is impaired in tau KO mice, and that LTD causes specific phosphorylation at the serine 396 and 404 residues of tau. Surprisingly, we find that phosphorylation at serine 396, specifically, is critical for LTD but has no role in LTP. Finally, we show that tau KO mice exhibit deficits in spatial reversal learning. These findings underscore the physiological role for tau at the synapse and identify a behavioral correlate of its role in LTD.

Acute stress causes rapid synaptic insertion of Ca2+ -permeable AMPA receptors to facilitate long-term potentiation in the hippocampus.

The neuroendocrine response to episodes of acute stress is crucial for survival whereas the prolonged response to chronic stress can be detrimental. Learning and memory are particularly susceptible to stress with cognitive deficits being well characterized consequences of chronic stress. Although there is good evidence that acute stress can enhance cognitive performance, the mechanism(s) for this are unclear. We find that hippocampal slices, either prepared from rats following 30 min restraint stress or directly exposed to glucocorticoids, exhibit an N-methyl-d-aspartic acid receptor-independent form of long-term potentiation. We demonstrate that the mechanism involves an NMDA receptor and PKA-dependent insertion of Ca2+ -permeable AMPA receptors into synapses. These then trigger the additional NMDA receptor-independent form of LTP during high frequency stimulation.

Epigallocatechin Gallate Modulates Microglia Phenotype to Suppress Pro-inflammatory Signalling Cues and Inhibit Phagocytosis.

Microglia are crucial players in the pathogenesis of late-onset Alzheimer's disease (AD), with evidence for both deleterious and beneficial effects. Identifying interventions to modulate microglial responsiveness, promote amyloid ß (Aß) clearance, disrupt plaque formation, or dampen excessive inflammation has therapeutic potential. Bioavailable flavonoids, such as the flavan 3-ols, are of interest due to their antioxidant, metal chelating, signalling, and anti-inflammatory potential. Primary microglia were treated with a series of structurally related flavanol 3-ols to assess effects on phagocytosis, cytokine release, and transcriptional responses by RNA sequencing. Data indicated that the extent of hydroxylation and the presence of the galloyl moiety were strong determinants of flavan 3-ol activity. Epigallocatechin gallate (EGCG) was the most effective flavan-3-ol tested and strongly inhibited phagocytosis of Aß independent of any metal chelating properties, suggesting a more direct modulation of microglia responsiveness. EGCG was broadly anti-inflammatory, reducing cytokine release and downregulating transcription, particularly of components of the microglia extracellular matrix such as MMP3 and SerpinB2. Collectively, this brings new insight into the actions of flavonoids on microglial responsiveness with potential implications for the therapeutic use of EGCG and structurally related flavanol-3-ols in AD.

Postsynaptic p47phox regulates long-term depression in the hippocampus.

It is well documented that reactive oxygen species (ROS) affects neurodegeneration in the brain. Several studies also implicate ROS in the regulation of synapse function and learning and memory processes, although the precise source of ROS generation within these contexts remains to be further explored. Here we show that postsynaptic superoxide generation through PKCζ-activated NADPH oxidase 2 (NOX2) is critical for long-term depression (LTD) of synaptic transmission in the CA1-Shaffer collateral synapse of the rat hippocampus. Specifically, PKCζ-dependent phosphorylation of p47phox at serine 316, a NOX2 regulatory subunit, is required for LTD but is not necessary for long-term potentiation (LTP). Our data suggest that postsynaptic p47phox phosphorylation at serine 316 is a key upstream determinant for LTD and synapse weakening.

Impairment of Release Site Clearance within the Active Zone by Reduced SCAMP5 Expression Causes Short-Term Depression of Synaptic Release.

Despite being a highly enriched synaptic vesicle (SV) protein and a candidate gene for autism, the physiological function of SCAMP5 remains mostly enigmatic. Here, using optical imaging and electrophysiological experiments, we demonstrate that SCAMP5 plays a critical role in release site clearance at the active zone. Truncation analysis revealed that the 2/3 loop domain of SCAMP5 directly interacts with adaptor protein 2, and this interaction is critical for its role in release site clearance. Knockdown (KD) of SCAMP5 exhibited pronounced synaptic depression accompanied by a slower recovery of the SV pool. Moreover, it induced a strong frequency-dependent short-term depression of synaptic release, even under the condition of sufficient release-ready SVs. Super-resolution microscopy further proved the defects in SV protein clearance induced by KD. Thus, reduced expression of SCAMP5 may impair the efficiency of SV clearance at the active zone, and this might relate to the synaptic dysfunction observed in autism.

Physiological and Pathophysiological Implications of Synaptic Tau.

Tauopathies encompass a broad range of neurodegenerative diseases featuring extensive neuronal death and cognitive decline. However, research over the past 30 years has failed to significantly advance our understanding of how tau causes dementia, limiting the design of rational therapeutics. It has become evident that we need to expand our understanding of tau in physiology, in order to delineate how tau may contribute to pathology. This review discusses recent evidence that has uncovered a novel aspect of tau function, based on its previously uncharacterized localization to the synapse. Here, multiple streams of evidence support a critical role for synaptic tau in the regulation of synapse physiology. In particular, long-term depression, a form of synaptic weakening, is dependent on the presence of tau in hippocampal neurons. The regulation of tau by specific phosphorylation events downstream of GSK-3ß activation appears to be integral to this signaling role. We also describe how the regulation of synapse physiology by tau and its phosphorylation may inform our understanding of tauopathies and comorbid diseases. This work should provide a platform for future tau biology research in addition to therapeutic design.

Ca2+-permeable AMPA receptor: A new perspective on amyloid-beta mediated pathophysiology of Alzheimer's disease.

α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) are the primary conduits of excitatory synaptic transmission. AMPARs are predominantly Ca2+-impermeable in the matured excitatory synapse, except under certain circumstances. Growing evidence implicates the Ca2+ permeability of AMPARs in the regulation of long-term synaptic plasticity and in the pathophysiology of several neurological disorders. Therefore, the Ca2+ conductance of AMPARs may have both physiological and pathological roles at synapses. However, our understanding of the role of Ca2+ permeable AMPARs (CP-AMPARs) in Alzheimer's disease is limited. Here we discuss insights into the potential CP-AMPAR mediated pathophysiology of Alzheimer's disease, including: 1. Ca2+-mediated aberrant regulation of synapse weakening mechanisms, and 2. neuronal network dysfunction in the brain. Consideration of CP-AMPARs as primary drivers of pathophysiology could help in understanding synaptopathologies, and highlights the potential of CP-AMPARs as therapeutic targets in Alzheimer's disease. This article is part of the Special Issue entitled 'Ionotropic glutamate receptors'.

Cyclin Y inhibits plasticity-induced AMPA receptor exocytosis and LTP.

Cyclin Y (CCNY) is a member of the cyclin protein family, known to regulate cell division in proliferating cells. Interestingly, CCNY is expressed in neurons that do not undergo cell division. Here, we report that CCNY negatively regulates long-term potentiation (LTP) of synaptic strength through inhibition of AMPA receptor trafficking. CCNY is enriched in postsynaptic fractions from rat forebrain and is localized adjacent to postsynaptic sites in dendritic spines in rat hippocampal neurons. Using live-cell imaging of a pH-sensitive AMPA receptor, we found that during LTP-inducing stimulation, CCNY inhibits AMPA receptor exocytosis in dendritic spines. Furthermore, CCNY abolishes LTP in hippocampal slices. Taken together, our findings demonstrate that CCNY inhibits plasticity-induced AMPA receptor delivery to synapses and thereby blocks LTP, identifying a novel function for CCNY in post-mitotic cells.

Intracellular oligomeric amyloid-beta rapidly regulates GluA1 subunit of AMPA receptor in the hippocampus.

The acute neurotoxicity of oligomeric forms of amyloid-ß 1-42 (Aß) is implicated in the pathogenesis of Alzheimer's disease (AD). However, how these oligomers might first impair neuronal function at the onset of pathology is poorly understood. Here we have examined the underlying toxic effects caused by an increase in levels of intracellular Aß, an event that could be important during the early stages of the disease. We show that oligomerised Aß induces a rapid enhancement of AMPA receptor-mediated synaptic transmission (EPSC(A)) when applied intracellularly. This effect is dependent on postsynaptic Ca(2+) and PKA. Knockdown of GluA1, but not GluA2, prevents the effect, as does expression of a S845-phosphomutant of GluA1. Significantly, an inhibitor of Ca(2+)-permeable AMPARs (CP-AMPARs), IEM 1460, reverses the increase in the amplitude of EPSC(A). These results suggest that a primary neuronal response to intracellular Aß oligomers is the rapid synaptic insertion of CP-AMPARs.

Microtubule-associated protein tau is essential for long-term depression in the hippocampus.

The microtubule-associated protein tau is a principal component of neurofibrillary tangles, and has been identified as a key molecule in Alzheimer's disease and other tauopathies. However, it is unknown how a protein that is primarily located in axons is involved in a disease that is believed to have a synaptic origin. To investigate a possible synaptic function of tau, we studied synaptic plasticity in the hippocampus and found a selective deficit in long-term depression (LTD) in tau knockout mice in vivo and in vitro, an effect that was replicated by RNAi knockdown of tau in vitro. We found that the induction of LTD is associated with the glycogen synthase kinase-3-mediated phosphorylation of tau. These observations demonstrate that tau has a critical physiological function in LTD.

The role of neuronal calcium sensors in balancing synaptic plasticity and synaptic dysfunction.

Neuronal calcium sensors (NCS) readily bind calcium and undergo conformational changes enabling them to interact and regulate specific target molecules. These interactions lead to dynamic alterations in protein trafficking that significantly impact upon synaptic function. Emerging evidence suggests that NCS and alterations in Ca(2+) mobilization modulate glutamate receptor trafficking, subsequently determining the expression of different forms of synaptic plasticity. In this review, we aim to discuss the functional relevance of NCS in protein trafficking and their emerging role in synaptic plasticity. Their significance within the concept of "translational neuroscience" will also be highlighted, by assessing their potential as key molecules in neurodegeneration.

Translational Concepts of mGluR5 in Synaptic Diseases of the Brain.

The G-protein coupled receptor family of glutamate receptors, termed metabotropic glutamate receptors (mGluRs), are implicated in numerous cellular mechanisms ranging from neural development to the processing of cognitive, sensory, and motor information. Over the last decade, multiple mGluR-related signal cascades have been identified at excitatory synapses, indicating their potential roles in various forms of synaptic function and dysfunction. This review highlights recent studies investigating mGluR5, a subtype of group I mGluRs, and its association with a number of developmental, psychiatric, and senile synaptic disorders with respect to associated synaptic proteins, with an emphasis on translational pre-clinical studies targeting mGluR5 in a range of synaptic diseases of the brain.