The role of synaptic microRNAs in Alzheimer's disease

Structurally and functionally active synapses are essential for neurotransmission and for maintaining normal synaptic and cognitive functions. Researchers have found that synaptic dysfunction is associated with the onset and progression of neurodegenerative diseases, such as Alzheimer's disease (AD), and synaptic dysfunction is even one of the main physiological hallmarks of AD. MiRNAs are present in small, subcellular compartments of the neuron such as neural dendrites, synaptic vesicles, and synaptosomes are known as synaptic miRNAs. Synaptic miRNAs involved in governing multiple synaptic functions that lead to healthy brain functioning and synaptic activity. However, the precise role of synaptic miRNAs has not been determined in AD progression. This review emphasizes the presence of miRNAs at the synapse, synaptic compartments and roles of miRNAs in multiple synaptic functions. We focused on synaptic miRNAs alteration in AD, and how the modulation of miRNAs effect the synaptic functions in AD. We also discussed the impact of synaptic miRNAs in AD progression concerning the synaptic ATP production, mitochondrial function, and synaptic activity.     

Molecular and biosynthetic heterogeneity of fucosyl glycoproteins associated with rat brain synaptic functions.

The composition and biosynthesis of fucosyl glycoproteins present in rat brain synaptic membranes and synaptic junctions were investigated. Reaction with 125I-labelled fucose-binding protein (Lotus tetragonolobus) following sodium dodecyl sulphate gel electrophoresis identified 6--8 fucosyl glycoproteins in synaptic membranes but only three major high molecular classes (Mr = 180 000, 130 000 and 110 000) in synaptic junctions. Affinity chromatography on concanavalin A-Sepharose resolved each of the synaptic junctional fucosyl glycoproteins into concanavalin A-positive and negative components indicating the presence of at least six high molecular weight fucosyl glycoproteins in synaptic junctions. Following the administration of [3H]fucose synaptic membranes, synaptic junctions and post-synaptic densities incorporated isotope, the order of relative specific activities being synaptic membranes greater than synaptic junctions greater than post-synaptic densities. Fractionation of [3H]fucose-labelled synaptic junctions on concanavalin A-Sepharose revealed a time-dependent increase in the percentage of isotope associated with the concanavalin A-positive glycoproteins. The results demonstrate both molecular and biosynthetic heterogeneity of fucosyl glycoproteins associated with synaptic junctions.     

Evidence for a primary endocytic vesicle involved in synaptic vesicle biogenesis

The regulated release of neurotransmitters at synapses is mediated by the fusion of neurotransmitter-filled synaptic vesicles with the plasma membrane. Continuous synaptic activity relies on the constant recycling of synaptic vesicle proteins into newly formed synaptic vesicles. At least two different mechanisms are presumed to mediate synaptic vesicle biogenesis at the synapse as follows: direct retrieval of synaptic vesicle proteins and lipids from the plasma membrane, and indirect passage of synaptic vesicle proteins through an endosomal intermediate. We have identified a vesicle population with the characteristics of a primary endocytic vesicle responsible for the recycling of synaptic vesicle proteins through the indirect pathway. We find that synaptic vesicle proteins colocalize in this vesicle with a variety of proteins known to recycle from the plasma membrane through the endocytic pathway, including three different glucose transporters, GLUT1, GLUT3, and GLUT4, and the transferrin receptor. These vesicles differ from "classical" synaptic vesicles in their size and their generic protein content, indicating that they do not discriminate between synaptic vesicle-specific proteins and other recycling proteins. We propose that these vesicles deliver synaptic vesicle proteins that have escaped internalization by the direct pathway to endosomes, where they are sorted from other recycling proteins and packaged into synaptic vesicles.     

一个基于能量的突触可塑性模型

研究表明能量可能是支配神经元活动的统一原则,编码能力与能量成本的比率最大化被认为是突触连接在选择性压力下改变的关键原则之一,这意味着突触范围内能量的变化与突触可塑性有关。为此,建立一个基于能量的突触可塑性模型。当突触后膜瞬时功率高于功率阈值时突触权重增加,反之突触权重下降。该模型可再现脉冲频率依赖可塑性以及脉冲时间依赖可塑性这两种主要的突触可塑性实验结果,并且和其他公认的突触可塑性模型相比具有优越性。结果表明,能量是影响突触可塑性的关键因素,对进一步理解突触连接的选择性和神经网络动力学特征提供了一个新思路。     

Distribution and properties of protein kinase and protein phosphatase activities in synaptosomal plasma membranes and synaptic junctions.

Some characteristics of the protein kinase activity associated with a synaptosomal plasma membrane (synaptic membrane) fraction and a synaptic junction fraction have been compared. Autoradiography of the phosphorylated fractions separated on sodium dodecyl sulfate polyacrylamide gels showed that cyclic AMP stimulates the phosphorylation of five polypeptides in synaptic membranes, whereas no cyclic AMP dependency could be detected in synaptic junctions. Kinetic studies demonstrated that synaptic junctions contain a high Km and a low Km protein kinase activity while only the high Km activity could be detected in synaptic membranes. The intrinsic ATPase activity of synaptic membranes was shown to strongly interfere with measurements of protein kinase activity. Cyclic AMP binding experiments revealed a 2.6-fold enrichment of cyclic AMP binding capacity in synaptic junctions as compared to synaptic membranes. Protein phosphatase activity was not detected in synaptic junctions but was associated with synaptic membranes, where cyclic AMP was shown to either stimulate or inhibit the dephosphorylation of different polypeptides.     

Distribution of Six Synaptic Membrane Antigens in Subcellular Fractions of Rat Brain Cortex

Abstract: The subcellular distribution in rat brain cortex of six synaptic membrane antigens (56K, 58K, 62K, 63K, 64K, 66K) was studied by rocket immunoelectrophoresis, using antiserum to a highly purified synaptic plasma membrane fraction. Initial analysis of the insoluble portion of subcellular fractions showed that these antigens were also present in smooth microsomes, rough microsomes, and synaptic vesicles; that only traces were present in synaptic junctions; and that none was present in nuclei, mitochondria, and myelin. A trace amount of activity was also present in synaptic vesicle cytosol, but none in whole brain cytosol. Quantitative measurements of synaptic plasma membranes, smooth microsomes, and synaptic vesicles showed that all six antigens were present in synaptic plasma membranes and smooth microsomes, but that the 66K antigen was absent from synaptic vesicles. The 56K, 58K, 62K, 63K, and 64K antigens were present in highest concentration in synaptic plasma membranes, whereas the 66K antigen content was highest in smooth microsomes. Only the 58K, 62K, and 63K antigens were detectable in the membrane fraction of whole brain. Their enrichments in synaptic plasma membranes were 10.9, 5.4, and 5.9, respectively. We conclude that the 56K, 58K, 62K, 63K and 64K antigens are primary components of synaptic plasma membranes. The presence of synaptic plasma membrane antigens in smooth microsomes and synaptic vesicles probably represents material being actively transported, consistent with the hypothesis that proteins of synaptic plasma membranes and synaptic vesicles are transported via smooth endoplasmic reticulum.     

Single vesicle tracking for studying synaptic vesicle dynamics in small central synapses

Sustained neurotransmission is driven by a continuous supply of synaptic vesicles to the release sites and modulated by synaptic vesicle dynamics. However, synaptic vesicle dynamics in synapses remain elusive because of technical limitations. Recent advances in fluorescence imaging techniques have enabled the tracking of single synaptic vesicles in small central synapses in living neurons. Single vesicle tracking has uncovered a wealth of new information about synaptic vesicle dynamics both within and outside presynaptic terminals, showing that single vesicle tracking is an effective tool for studying synaptic vesicle dynamics. Particularly, single vesicle tracking with high spatiotemporal resolution has revealed the dependence of synaptic vesicle dynamics on the location, stages of recycling, and neuronal activity. This review summarizes the recent findings from single synaptic vesicle tracking in small central synapses and their implications in synaptic transmission and pathogenic mechanisms of neurodegenerative diseases.     

iTRAQ-Based Quantitative Proteomics Suggests Synaptic Mitochondrial Dysfunction in the Hippocampus of Rats Susceptible to Chronic Mild Stress

Mounting studies show that hippocampal synaptic transmission and plasticity are abnormal in depression. It has been suggested that impairment of synaptic mitochondrial functions potentially occurs in the hippocampus. Thus, the synaptic mitochondria may be a crucial therapeutic target in the course of depression. Here, we investigated the potential dysregulation of synaptic mitochondrial proteins in the hippocampus of a chronic mild stress (CMS) rat model. Proteomic changes of hippocampal synaptosomes containing synaptic mitochondria were quantitatively examined using the isobaric tag for relative and absolute quantitation labeling combined with tandem mass spectrometry. 45 Proteins were identified to be differentially expressed, of which 21 were found to be putative synaptic mitochondrial proteins based on gene ontology component and SynaptomeDB analyses. Detailed investigations of protein functions and disease relevance support the importance of hippocampal synaptic mitochondria as a key substrate contributing to impairment in synaptic plasticity of stress-related disorders. Interestingly, eight synaptic mitochondrial proteins were specifically associated to the susceptible group, and might represent part of molecular basis of depression. Further analysis indicated that the synaptic mitochondrial oxidative phosphorylation (OXPHOS) system was heavily affected by CMS in the susceptible rats. The present results provide novel insights into the disease mechanism underlying the abnormal OXPHOS that is responsible for energy-demanding synaptic plasticity, and thereby increase our understanding of the role of hippocampal synaptic mitochondrial dysfunction in depression.     

The Role of Proteases in Hippocampal Synaptic Plasticity: Putting Together Small Pieces of a Complex Puzzle

Long-term synaptic plasticity in the hippocampus is thought to underlie the formation of certain forms of memory, including spatial memory. The early phase of long-term synaptic potentiation and synaptic depression depends on post-translational modifications of synaptic proteins, while protein synthesis is also required for the late-phase of both forms of synaptic plasticity (L-LTP and L-LTD). Numerous pieces of evidence show a role for different types of proteases in synaptic plasticity, further increasing the diversity of mechanisms involved in the regulation of the intracellular and extracellular protein content. The cleavage of extracellular proteins is coupled to changes in postsynaptic intracellular mechanisms, and additional alterations in this compartment result from the protease-mediated targeting of intracellular proteins. Both mechanisms contribute to initiate signaling cascades that drive downstream pathways coupled to synaptic plasticity. In this review we summarize the evidence pointing to a role for extracellular and intracellular proteases, with distinct specificities, in synaptic plasticity. Where in the cells the proteases are located, and how they are regulated is also discussed. The combined actions of proteases and translation mechanisms contribute to a tight control of the synaptic proteome relevant for long-term synaptic potentiation and synaptic depression in the hippocampus. Additional studies are required to elucidate the mechanisms whereby these changes in the synaptic proteome are related with plasticity phenomena.     

Effective Mechanism for Synthesis of Neurotransmitter Glutamate and its Loading into Synaptic Vesicles

Glutamate accumulation into synaptic vesicles is a pivotal step in glutamate transmission. This process is achieved by a vesicular glutamate transporter (VGLUT) coupled to v-type proton ATPase. Normal synaptic transmission, in particular during intensive neuronal firing, would demand rapid transmitter re-filling of emptied synaptic vesicles. We have previously shown that isolated synaptic vesicles are capable of synthesizing glutamate from α-ketoglutarate (not from glutamine) by vesicle-bound aspartate aminotransferase for immediate uptake, in addition to ATP required for uptake by vesicle-bound glycolytic enzymes. This suggests that local synthesis of these substances, essential for glutamate transmission, could occur at the synaptic vesicle. Here we provide evidence that synaptosomes (pinched-off nerve terminals) also accumulate α-ketoglutarate-derived glutamate into synaptic vesicles within, at the expense of ATP generated through glycolysis. Glutamine-derived glutamate is also accumulated into synaptic vesicles in synaptosomes. The underlying mechanism is discussed. It is suggested that local synthesis of both glutamate and ATP at the presynaptic synaptic vesicle would represent an efficient mechanism for swift glutamate loading into synaptic vesicles, supporting maintenance of normal synaptic transmission.     

Distribution and properties of protein kinase and protein phosphatase activities in synaptosomal plasma membranes and synaptic junctions

Some characteristics of the protein kinase activity associated with a synaptosomal plasma membrane (synaptic membrane) fraction and a synaptic junction fraction have been compared. Autoradiography of the phosphorylated fractions separated on sodium dodecyl sulfate polyacrylamine gels showed that cyclic AMP stimulates the phosphorylation of five polypeptides in synaptic membranes, whereas no cyclic AMP dependency could be detected in synaptic junctions. Kinetic studies demonstrated that synaptic junctions contain at high K_m and a low K_m protein kinase activity while only the high K_m activity could be detected in synaptic membranes. The intrinsic ATPase activity of synaptic membranes was shown to strongly interfere with measurements of protein kinase activity. Cyclic AMP binding experiments revealed a 2.6-fold enrichment of cyclic AMP binding capacity in synaptic junctions as compared to synaptic membranes. Protein phosphatase activity was not detected in synaptic junctions but was associated with synaptic membranes, where cyclic AMP was shown to either stimulate or inhibit the dephosphorylation of different polypeptides.     

Endocytosis of VAMP is facilitated by a synaptic vesicle targeting signal

After synaptic vesicles fuse with the plasma membrane and release their contents, vesicle membrane proteins recycle by endocytosis and are targeted to newly formed synaptic vesicles. The membrane traffic of an epitope-tagged form of VAMP-2 (VAMP-TAg) was observed in transfected cells to identify sequence requirements for recycling of a synaptic vesicle membrane protein. In the neuroendocrine PC12 cell line VAMP-TAg is found not only in synaptic vesicles, but also in endosomes and on the plasma membrane. Endocytosis of VAMP-TAg is a rapid and saturable process. At high expression levels VAMP-TAg accumulates at the cell surface. Rapid endocytosis of VAMP-TAg also occurs in transfected CHO cells and is therefore independent of other synaptic proteins. The majority of the measured endocytosis is not directly into synaptic vesicles since mutations in VAMP-TAg that enhance synaptic vesicle targeting did not affect endocytosis. Nonetheless, mutations that inhibited synaptic vesicle targeting, in particular replacement of methionine-46 by alanine, inhibited endocytosis by 85% in PC12 cells and by 35% in CHO cells. These results demonstrate that the synaptic vesicle targeting signal is also used for endocytosis and can be recognized in cells lacking synaptic vesicles.     

短时程突触可塑性研究概况

突触可塑性是神经系统所具有的重要特征,也是神经系统实现其功能的重要保障。按照持续的时间划分,突触可塑性可分为短时程突触可塑性和长时程突触可塑性。短时程突触可塑性包括短时程增强和短时程压抑两种类型。与长时程突触可塑性不同,短时程突触可塑性的产生主要依赖于神经递质释放概率的变化,其往往决定神经回路的信息处理和反应模式,不仅直接参与了对输入信号的识别和处理,而且还可对长时程突触可塑性的表达产生重要影响。     

Synaptic Plasma Membrane Tubulin May Be an Integral Constituent

Abstract: Mild detergent extraction of chick brain synaptic plasma membranes followed by gel electrophoresis suggests that synaptic plasma membrane tubulin is an integral component. Although some of the synaptic plasma membrane tubulin might be aggregates, that possibility is not supported by the observation that tubulin aggregates that are added to synaptosomes before synaptic subfractionation do not partition with synaptic plasma membranes during membrane isolation.     

The dynamics of synaptic scaffolds

Complex functions of the central nervous system such as learning and memory are believed to result from the modulation of the synaptic transmission between neurons. The sequence of events leading to the fusion of synaptic vesicles at the presynaptic active zone and the detection of this signal at the postsynaptic density involve the activity of ion channels and neurotransmitter receptors. Their accumulation and dynamic exchange at synapses are dependent on their interaction with synaptic scaffolds. These are synaptic structures composed of adaptor proteins that provide binding sites for receptors and channels as well as other synaptic proteins. While in its entirety the synaptic scaffold is a relatively stable structure, individual adaptor proteins exchange at a fast time scale. These properties of scaffolds help to ensure the stability of synaptic transmission while permitting the modulation of synaptic strength. Here, we review the dynamics of the synaptic scaffold and of adaptor proteins in relation to their roles in the organisation of the synapse as well as in the clustering and trafficking of receptor proteins.     

A Calcium-Dependent Plasticity Rule for HCN Channels Maintains Activity Homeostasis and Stable Synaptic Learning

Theoretical and computational frameworks for synaptic plasticity and learning have a long and cherished history, with few parallels within the well-established literature for plasticity of voltage-gated ion channels. In this study, we derive rules for plasticity in the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, and assess the synergy between synaptic and HCN channel plasticity in establishing stability during synaptic learning. To do this, we employ a conductance-based model for the hippocampal pyramidal neuron, and incorporate synaptic plasticity through the well-established Bienenstock-Cooper-Munro (BCM)-like rule for synaptic plasticity, wherein the direction and strength of the plasticity is dependent on the concentration of calcium influx. Under this framework, we derive a rule for HCN channel plasticity to establish homeostasis in synaptically-driven firing rate, and incorporate such plasticity into our model. In demonstrating that this rule for HCN channel plasticity helps maintain firing rate homeostasis after bidirectional synaptic plasticity, we observe a linear relationship between synaptic plasticity and HCN channel plasticity for maintaining firing rate homeostasis. Motivated by this linear relationship, we derive a calcium-dependent rule for HCN-channel plasticity, and demonstrate that firing rate homeostasis is maintained in the face of synaptic plasticity when moderate and high levels of cytosolic calcium influx induced depression and potentiation of the HCN-channel conductance, respectively. Additionally, we show that such synergy between synaptic and HCN-channel plasticity enhances the stability of synaptic learning through metaplasticity in the BCM-like synaptic plasticity profile. Finally, we demonstrate that the synergistic interaction between synaptic and HCN-channel plasticity preserves robustness of information transfer across the neuron under a rate-coding schema. Our results establish specific physiological roles for experimentally observed plasticity in HCN channels accompanying synaptic plasticity in hippocampal neurons, and uncover potential links between HCN-channel plasticity and calcium influx, dynamic gain control and stable synaptic learning.     

Regulation of synaptic strength by protein phosphatase 1.

We investigated the role of postsynaptic protein phosphatase 1 (PP1) in regulating synaptic strength by loading CA1 pyramidal cells either with peptides that disrupt PP1 binding to synaptic targeting proteins or with active PP1. The peptides blocked synaptically evoked LTD but had no effect on basal synaptic currents mediated by either AMPA or NMDA receptors. They did, however, cause an increase in synaptic strength following the induction of LTD. Similarly, PP1 had no effect on basal synaptic strength but enhanced LTD. In cultured neurons, synaptic activation of NMDA receptors increased the proportion of PP1 localized to synapses. These results suggest that PP1 does not significantly regulate basal synaptic strength. Appropriate NMDA receptor activation, however, allows PP1 to gain access to synaptic substrates and be recruited to synapses where its activity is necessary for sustaining LTD.