Postsynaptic Y654 dephosphorylation of β-catenin modulates presynaptic vesicle turnover through increased n-cadherin-mediated transsynaptic signaling

CPEB2-activated axonal translation of VGLUT2 mRNA promotes glutamatergic transmission and presynaptic plasticity

BACKGROUND

Local translation at synapses is important for rapidly remodeling the synaptic proteome to sustain long-term plasticity and memory. While the regulatory mechanisms underlying memory-associated local translation have been widely elucidated in the postsynaptic/dendritic region, there is no direct evidence for which RNA-binding protein (RBP) in axons controls target-specific mRNA translation to promote long-term potentiation (LTP) and memory. We previously reported that translation controlled by cytoplasmic polyadenylation element binding protein 2 (CPEB2) is important for postsynaptic plasticity and memory. Here, we investigated whether CPEB2 regulates axonal translation to support presynaptic plasticity.

METHODS

Behavioral and electrophysiological assessments were conducted in mice with pan neuron/glia- or glutamatergic neuron-specific knockout of CPEB2. Hippocampal Schaffer collateral (SC)-CA1 and temporoammonic (TA)-CA1 pathways were electro-recorded to monitor synaptic transmission and LTP evoked by 4 trains of high-frequency stimulation. RNA immunoprecipitation, coupled with bioinformatics analysis, were used to unveil CPEB2-binding axonal RNA candidates associated with learning, which were further validated by Western blotting and luciferase reporter assays. Adeno-associated viruses expressing Cre recombinase were stereotaxically delivered to the pre- or post-synaptic region of the TA circuit to ablate Cpeb2 for further electrophysiological investigation. Biochemically isolated synaptosomes and axotomized neurons cultured on a microfluidic platform were applied to measure axonal protein synthesis and FM4-64FX-loaded synaptic vesicles.

RESULTS

Electrophysiological analysis of hippocampal CA1 neurons detected abnormal excitability and vesicle release probability in CPEB2-depleted SC and TA afferents, so we cross-compared the CPEB2-immunoprecipitated transcriptome with a learning-induced axonal translatome in the adult cortex to identify axonal targets possibly regulated by CPEB2. We validated that Slc17a6, encoding vesicular glutamate transporter 2 (VGLUT2), is translationally upregulated by CPEB2. Conditional knockout of CPEB2 in VGLUT2-expressing glutamatergic neurons impaired consolidation of hippocampus-dependent memory in mice. Presynaptic-specific ablation of Cpeb2 in VGLUT2-dominated TA afferents was sufficient to attenuate protein synthesis-dependent LTP. Moreover, blocking activity-induced axonal Slc17a6 translation by CPEB2 deficiency or cycloheximide diminished the releasable pool of VGLUT2-containing synaptic vesicles.

CONCLUSIONS

We identified 272 CPEB2-binding transcripts with altered axonal translation post-learning and established a causal link between CPEB2-driven axonal synthesis of VGLUT2 and presynaptic translation-dependent LTP. These findings extend our understanding of memory-related translational control mechanisms in the presynaptic compartment.

Alterations in Synaptic Connectivity and Synaptic Transmission in Alzheimer's Disease with High Physical Activity

Background

Alzheimer's disease (AD) is a progressive neurodegeneration disease. Physical activity is one of the most promising modifiable lifestyles that can be effective in slowing down the progression of AD at an early stage.

Objective

Explore the molecular processes impaired in AD that were conversely preserved and enhanced by physical activity.

Methods

Integrated transcriptomic analyses were performed in datasets that contain AD patients and elders with different degrees of physical activity. The changes of the hub genes were validated through analyzing another two datasets. The expression of the hub genes was further detected in the hippocampus and cortexes of APP/PS1 transgenic mice with or without physical activity by Quantitative polymerase chain reaction (qPCR).

Results

Cross-comparison highlighted 195 DEGs displaying opposed regulation patterns between AD and high physical activity (HPA). The common DEGs were predominantly involved in synaptic vesicle recycling and synaptic transmission, largely downregulated in AD patients but upregulated in the elders with HPA. Two key modules and four hub genes that were related to synaptic vesicle turnover were obtained from the PPI network. The expression of these hub genes (SYT1, SYT4, SH3GL2, and AP2M1) was significantly decreased in AD transgenic mice and was reversed by HPA training.

Conclusions

HPA may reverse AD pathology by upregulating a range of synaptic vesicle transport related proteins which might improve the efficiency of synaptic vesicle turnover and facilitate inter-neuronal information transfer. The study provides novel insights into the mechanisms underlining the protective effects of HPA on AD.

Turnover of synaptic adhesion molecules

Molecular interactions between pre- and postsynaptic membranes play critical roles during the development, function and maintenance of synapses. Synaptic interactions are mediated by cell surface receptors that may be held in place by trans-synaptic adhesion or intracellular binding to membrane-associated scaffolding and signaling complexes. Despite their role in stabilizing synaptic contacts, synaptic adhesion molecules undergo turnover and degradation during all stages of a neuron's life. Here we review current knowledge about membrane trafficking mechanisms that regulate turnover of synaptic adhesion molecules and the functional significance of turnover for synapse development and function. Based on recent proteomics, genetics and imaging studies, synaptic adhesion molecules exhibit remarkably high turnover rates compared to other synaptic proteins. Degradation occurs predominantly via endolysosomal mechanisms, with little evidence for roles of proteasomal or autophagic degradation. Basal turnover occurs both during synaptic development and maintenance. Neuronal activity typically stabilizes synaptic adhesion molecules while downregulating neurotransmitter receptors based on turnover. In conclusion, constitutive turnover of synaptic adhesion molecules is not a necessarily destabilizing factor, but a basis for the dynamic regulation of trans-synaptic interactions during synapse formation and maintenance.

Crosstalk Between β-CATENIN-Mediated Cell Adhesion and the WNT Signaling Pathway

Cell adhesion and stable signaling regulation are fundamental ways of maintaining homeostasis. Among them, the Wnt/β-CATENIN signaling plays a key role in embryonic development and maintenance of body dynamic homeostasis. At the same time, the key signaling molecule β-CATENIN in the Wnt signaling can also function as a cytoskeletal linker protein to regulate tissue barriers, cell migration, and morphogenesis. Dysregulation of the balance between Wnt signaling and adherens junctions can lead to disease. How β-CATENIN maintains the independence of these two functions, or mediates the interaction and balance of these two functions, has been explored and debated for a long time. In this study, we will focus on five aspects of β-CATENIN chaperone molecules, phosphorylation of β-CATENIN and related proteins, epithelial mesenchymal transition, β-CATENIN homolog protein γ-CATENIN and disease, thus deepening the understanding of the Wnt/β-CATENIN signaling and the homeostasis between cell adhesion and further addressing related disease problems.

Transsynaptic N-Cadherin Adhesion Complexes Control Presynaptic Vesicle and Bulk Endocytosis at Physiological Temperature

At mammalian glutamatergic synapses, most basic elements of synaptic transmission have been shown to be modulated by specific transsynaptic adhesion complexes. However, although crucial for synapse homeostasis, a physiological regulation of synaptic vesicle endocytosis by adhesion molecules has not been firmly established. The homophilic adhesion protein N-cadherin is localized at the peri-active zone, where the highly temperature-dependent endocytosis of vesicles occurs. Here, we demonstrate an important modulatory role of N-cadherin in endocytosis at near physiological temperature by synaptophysin-pHluorin imaging. Different modes of endocytosis including bulk endocytosis were dependent on N-cadherin expression and function. N-cadherin modulation might be mediated by actin filaments because actin polymerization ameliorated the knockout-induced endocytosis defect. Using super-resolution imaging, we found strong recruitment of N-cadherin to glutamatergic synapses upon massive vesicle release, which might in turn enhance vesicle endocytosis. This provides a novel, adhesion protein-mediated mechanism for efficient coupling of exo- and endocytosis.

The Applications of Lattice Light-sheet Microscopy for Functional Volumetric Imaging of Hippocampal Neurons in a Three-Dimensional Culture System

The characterization of individual cells in three-dimensions (3D) with very high spatiotemporal resolution is crucial for the development of organs-on-chips, in which 3D cell cultures are integrated with microfluidic systems. In this study, we report the applications of lattice light-sheet microscopy (LLSM) for monitoring neuronal activity in three-dimensional cell culture. We first established a 3D environment for culturing primary hippocampal neurons by applying a scaffold-based 3D tissue engineering technique. Fully differentiated and mature hippocampal neurons were observed in our system. With LLSM, we were able to monitor the behavior of individual cells in a 3D cell culture, which was very difficult under a conventional microscope due to strong light scattering from thick samples. We demonstrated that our system could study the membrane voltage and intracellular calcium dynamics at subcellular resolution in 3D under both chemical and electrical stimulation. From the volumetric images, it was found that the voltage indicators mainly resided in the cytosol instead of the membrane, which cannot be distinguished using conventional microscopy. Neuronal volumetric images were sheet scanned along the axial direction and recorded at a laser exposure of 6 ms, which covered an area up to 4800 μm2, with an image pixel size of 0.102 μm. When we analyzed the time-lapse volumetric images, we could quantify the voltage responses in different neurites in 3D extensions.

Lightsheet localization microscopy enables fast, large-scale, and three-dimensional super-resolution imaging

Recent advances in super-resolution microscopy allow the localization of single molecules within individual cells but not within multiple whole cells due to weak signals from single molecules and slow acquisition process for point accumulation to reconstruct super-resolution images. Here, we report a fast, large-scale, and three-dimensional super-resolution fluorescence microscope based on single-wavelength Bessel lightsheet to selectively illuminate spontaneous blinking fluorophores tagged to the proteins of interest in space. Critical parameters such as labeling density, excitation power, and exposure time were systematically optimized resulting in a maximum imaging speed of 2.7 × 104 µm3 s-1. Fourier ring correlation analysis revealed a reconstructed image with a lateral resolution of ~75 nm through the accumulation of 250 image volumes on immobilized samples within 15 min. Hence, the designed system could open new insights into the discovery of complex biological structures and live 3D localization imaging.

Improved memory and reduced anxiety in δ-catenin transgenic mice

δ-Catenin is abundant in the brain and affects its synaptic plasticity. Furthermore, loss of δ-catenin is related to the deficits of learning and memory, mental retardation (cri-du-chat syndrome), and autism. A few studies about δ-catenin deficiency mice were performed. However, the effect of δ-catenin overexpression in the brain has not been investigated as yet. Therefore we generated a δ-catenin overexpressing mouse model. To generate a transgenic mouse model overexpressing δ-catenin in the brain, δ-catenin plasmid having a Thy-1 promotor was microinjected in C57BL/6 mice. Our results showed δ-catenin transgenic mice expressed higher levels of N-cadherin, β-catenin, and p120-catenin than did wild type mice. Furthermore, δ-catenin transgenic mice exhibited better object recognition, better sociability, and lower anxiety than wild type mice. However, both mice groups showed a similar pattern in locomotion tests. Although δ-catenin transgenic mice show similar locomotion, they show improved sociability and reduced anxiety. These characteristics are opposite to the symptoms of autism or mental retardation, which are caused when δ-catenin is deficient. These results suggest that δ-catenin may alleviate symptoms of autism, Alzheimer's disease and mental retardation.

Intracellular protein complexes involved in synapse assembly in presynaptic neurons

The presynaptic active zone, composed of evolutionarily conserved protein complexes, is a specialized area that serves to orchestrate precise and efficient neurotransmitter release by organizing various presynaptic proteins involved in mediating docking and priming of synaptic vesicles, recruiting voltage-gated calcium channels, and modulating presynaptic nerve terminals with aligned postsynaptic structures. Among membrane proteins localized to active zone, presynaptic neurexins and LAR-RPTPs (leukocyte common antigen-related receptor tyrosine phosphatase) have emerged as hubs that orchestrate both shared and distinct extracellular synaptic adhesion pathways. In this chapter, we discuss intracellular signaling cascades involved in recruiting various intracellular proteins at both excitatory and inhibitory synaptic sites. In particular, we highlight recent studies on key active zone proteins that physically and functionally link these cascades with neurexins and LAR-RPTPs in both vertebrate and invertebrate model systems. These studies allow us to build a general, universal view of how presynaptic active zones operate together with postsynaptic structures in neural circuits.

Janus kinase 3 regulates adherens junctions and epithelial mesenchymal transition through β-catenin

Compromise in adherens junctions (AJs) is associated with several chronic inflammatory diseases. We reported previously that Janus kinase 3, a non-receptor tyrosine kinase, plays a crucial role in AJ formation through its interaction with β-catenin. In this report, we characterize the structural determinants responsible for Jak3 interactions with β-catenin and determine the functional implications of previously unknown tyrosine residues on β-catenin phosphorylated by Jak3. We demonstrate that Jak3 autophosphorylation was the rate-limiting step during Jak3 trans-phosphorylation of β-catenin, where Jak3 directly phosphorylated three tyrosine residues, viz. Tyr³⁰, Tyr⁶⁴, and Tyr⁸⁶ in the N-terminal domain (NTD) of β-catenin. However, prior phosphorylation of β-catenin at Tyr⁶⁵⁴ was essential for further phosphorylation of β-catenin by Jak3. Interaction studies indicated that phosphorylated Jak3 bound to phosphorylated β-catenin with a dissociation constant of 0.28 μm, and although both the kinase and FERM (Band 41, ezrin, radixin, and moesin) domains of Jak3 interacted with β-catenin, the NTD domain of β-catenin facilitated its interactions with Jak3. Physiologically, Jak3-mediated phosphorylation of β-catenin suppressed EGF-mediated epithelial-mesenchymal transition and facilitated epithelial barrier functions by AJ localization of phosphorylated β-catenin through its interactions with α-catenin. Moreover, loss of Jak3-mediated phosphorylation sites in β-catenin abrogated its AJ localization and compromised epithelial barrier functions. Thus, we not only characterize Jak3 interaction with β-catenin but also demonstrate the mechanism of molecular interplay between AJ dynamics and EMT by Jak3-mediated NTD phosphorylation of β-catenin.