Protein synthesis and neurotrophin-dependent structural plasticity of single dendritic spines

Long-term potentiation (LTP) at glutamatergic synapses is considered to underlie learning and memory and is associated with the enlargement of dendritic spines. Because the consolidation of memory and LTP require protein synthesis, it is important to clarify how protein synthesis affects spine enlargement. In rat brain slices, the repetitive pairing of postsynaptic spikes and two-photon uncaging of glutamate at single spines (a spike-timing protocol) produced both immediate and gradual phases of spine enlargement in CA1 pyramidal neurons. The gradual enlargement was strongly dependent on protein synthesis and brain-derived neurotrophic factor (BDNF) action, often associated with spine twitching, and was induced specifically at the spines that were immediately enlarged by the synaptic stimulation. Thus, this spike-timing protocol is an efficient trigger for BDNF secretion and induces protein synthesis-dependent long-term enlargement at the level of single spines.     

Targeted protein degradation and synapse remodeling by an inducible protein kinase

Synaptic plasticity involves the reorganization of synapses at the protein and the morphological levels. Here, we report activity-dependent remodeling of synapses by serum-inducible kinase (SNK). SNK was induced in hippocampal neurons by synaptic activity and was targeted to dendritic spines. SNK bound to and phosphorylated spine-associated Rap guanosine triphosphatase activating protein (SPAR), a postsynaptic actin regulatory protein, leading to degradation of SPAR. Induction of SNK in hippocampal neurons eliminated SPAR protein, depleted postsynaptic density-95 and Bassoon clusters, and caused loss of mature dendritic spines. These results implicate SNK as a mediator of activity-dependent change in the molecular composition and morphology of synapses.     

The spread of Ras activity triggered by activation of a single dendritic spine

In neurons, individual dendritic spines isolate N-methyl-d-aspartate (NMDA) receptor-mediated calcium ion (Ca2+) accumulations from the dendrite and other spines. However, the extent to which spines compartmentalize signaling events downstream of Ca2+ influx is not known. We combined two-photon fluorescence lifetime imaging with two-photon glutamate uncaging to image the activity of the small guanosine triphosphatase Ras after NMDA receptor activation at individual spines. Induction of long-term potentiation (LTP) triggered robust Ca2+-dependent Ras activation in single spines that decayed in approximately 5 minutes. Ras activity spread over approximately 10 micrometers of dendrite and invaded neighboring spines by diffusion. The spread of Ras-dependent signaling was necessary for the local regulation of the threshold for LTP induction. Thus, Ca2+-dependent synaptic signals can spread to couple multiple synapses on short stretches of dendrite.     

PSD-95 involvement in maturation of excitatory synapses

PSD-95 is a neuronal PDZ protein that associates with receptors and cytoskeletal elements at synapses, but whose function is uncertain. We found that overexpression of PSD-95 in hippocampal neurons can drive maturation of glutamatergic synapses. PSD-95 expression enhanced postsynaptic clustering and activity of glutamate receptors. Postsynaptic expression of PSD-95 also enhanced maturation of the presynaptic terminal. These effects required synaptic clustering of PSD-95 but did not rely on its guanylate kinase domain. PSD-95 expression also increased the number and size of dendritic spines. These results demonstrate that PSD-95 can orchestrate synaptic development and are suggestive of roles for PSD-95 in synapse stabilization and plasticity.     

Messenger RNA in early sea-urchin embryos: cytoplasmic particles

Three structures containing messenger RNA can be demonstrated in the cytoplasm of early sea-urchin embryos: (i) particles that sediment more slowly than ribosomes and contain newly synthesized DNA-like RNA, (ii) light polyribosomes, which also contain this newly synthesized RNA, and (iii) heavy polyribosomes, which seemingly contain only already existing or "maternal" messenger RNA and account for the bulk of the synthesis of protein.     

Cell cycle-dependent regulation of phosphorylation of the human retinoblastoma gene product

The human retinoblastoma gene (RB1) encodes a protein (Rb) of 105 kilodaltons that can be phosphorylated. Analysis of Rb metabolism has shown that the protein has a half-life of more than 10 hours and is synthesized at all phases of the cell cycle. Newly synthesized Rb is not extensively phosphorylated (it is "underphosphorylated") in cells in the G0 and G1 phases but is phosphorylated at multiple sites at the G1/S boundary and in S phase. HL-60 cells that were induced to terminally differentiate by various chemicals lost their ability to phosphorylate newly synthesized Rb at multiple sites when cell growth was arrested. These findings suggest that underphosphorylated Rb may restrict cell proliferation.     

Learning-induced LTP in neocortex

The hypothesis that learning occurs through long-term potentiation (LTP)- and long-term depression (LTD)-like mechanisms is widely held but unproven. This hypothesis makes three assumptions: Synapses are modifiable, they modify with learning, and they strengthen through an LTP-like mechanism. We previously established the ability for synaptic modification and a synaptic strengthening with motor skill learning in horizontal connections of the rat motor cortex (MI). Here we investigated whether learning strengthened these connections through LTP. We demonstrated that synapses in the trained MI were near the ceiling of their modification range, compared with the untrained MI, but the range of synaptic modification was not affected by learning. In the trained MI, LTP was markedly reduced and LTD was enhanced. These results are consistent with the use of LTP to strengthen synapses during learning.     

Conformational switch of syntaxin-1 controls synaptic vesicle fusion

During synaptic vesicle fusion, the soluble N-ethylmaleimide-sensitive factor-attachment protein receptor (SNARE) protein syntaxin-1 exhibits two conformations that both bind to Munc18-1: a "closed" conformation outside the SNARE complex and an "open" conformation in the SNARE complex. Although SNARE complexes containing open syntaxin-1 and Munc18-1 are essential for exocytosis, the function of closed syntaxin-1 is unknown. We generated knockin/knockout mice that expressed only open syntaxin-1B. Syntaxin-1B(Open) mice were viable but succumbed to generalized seizures at 2 to 3 months of age. Binding of Munc18-1 to syntaxin-1 was impaired in syntaxin-1B(Open) synapses, and the size of the readily releasable vesicle pool was decreased; however, the rate of synaptic vesicle fusion was dramatically enhanced. Thus, the closed conformation of syntaxin-1 gates the initiation of the synaptic vesicle fusion reaction, which is then mediated by SNARE-complex/Munc18-1 assemblies.     

The cholinergic synapse and the site of memory

A simple hypothesis can explain the results obtained to date if we disregard those results when we wait 30 minutes after original learning to inject. The hypothesis is that, as a result of learning, the postsynaptic endings at a specific set of synapses become more sensitive to transmitter. This sensitivity increases with time after initial learning and then declines. The rate at which such sensitivity increases depends on the amount of initial learning. If the curve of transmission plotted against time is displaced upward with anticholinesterases then the very low portions will show facilitation, and the high portions will cause block (Fig. 8). The middle portions will appear unaffected (unless special experimental tests are made). If the curve of transmission is displaced down with anticholinergics, then the middle portion will appear unaffected and only the very early or late components will show block. The results are evidence that synaptic conductance is altered as a result of learning. So far it seems (i) that cholinergic synapses are modified as a result of learning and that it probably is the postsynaptic membrane that becomes increasingly more sensitive to acetylcholine with time after learning, up to a certain point. (ii) After this point, sensitivity declines, leading to the phenomenon of forgetting. (iii) There is also good evidence that there is an initial phase of declining sensitivity to cholinesterase or increasing sensitivity to anticholinergics. This could reflect the existence of a parallel set of synapses with fast decay that serve as a shortterm store. (iv) Increasing the amount of learning leads to an increase in conductance in each of a set of synapses without an increase in their number. (v) Both original learning and extinction are subserved by cholinergic synapses.     

Spatial regulation of an E3 ubiquitin ligase directs selective synapse elimination

Stereotyped synaptic connectivity can arise both by precise recognition between appropriate partners during synaptogenesis and by selective synapse elimination. The molecular mechanisms that underlie selective synapse removal are largely unknown. We found that stereotyped developmental elimination of synapses in the Caenorhabditis elegans hermaphrodite-specific motor neuron (HSNL) was mediated by an E3 ubiquitin ligase, a Skp1-cullin-F-box (SCF) complex composed of SKR-1 and the F-box protein SEL-10. SYG-1, a synaptic adhesion molecule, bound to SKR-1 and inhibited assembly of the SCF complex, thereby protecting nearby synapses. Thus, subcellular regulation of ubiquitin-mediated protein degradation contributes to precise synaptic connectivity through selective synapse elimination.     

Rab1 recruitment of p115 into a cis-SNARE complex: programming budding COPII vesicles for fusion

The guanosine triphosphatase Rab1 regulates the transport of newly synthesized proteins from the endoplasmic reticulum to the Golgi apparatus through interaction with effector molecules, but the molecular mechanisms by which this occurs are unknown. Here, the tethering factor p115 was shown to be a Rab1 effector that binds directly to activated Rab1. Rab1 recruited p115 to coat protein complex II (COPII) vesicles during budding from the endoplasmic reticulum, where it interacted with a select set of COPII vesicle-associated SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein receptors) to form a cis-SNARE complex that promotes targeting to the Golgi apparatus. We propose that Rab1-regulated assembly of functional effector-SNARE complexes defines a conserved molecular mechanism to coordinate recognition between subcellular compartments.     

Actin-based plasticity in dendritic spines

The central nervous system functions primarily to convert patterns of activity in sensory receptors into patterns of muscle activity that constitute appropriate behavior. At the anatomical level this requires two complementary processes: a set of genetically encoded rules for building the basic network of connections, and a mechanism for subsequently fine tuning these connections on the basis of experience. Identifying the locus and mechanism of these structural changes has long been among neurobiology's major objectives. Evidence has accumulated implicating a particular class of contacts, excitatory synapses made onto dendritic spines, as the sites where connective plasticity occurs. New developments in light microscopy allow changes in spine morphology to be directly visualized in living neurons and suggest that a common mechanism, based on dynamic actin filaments, is involved in both the formation of dendritic spines during development and their structural plasticity at mature synapses.     

Anti-Hebbian long-term potentiation in the hippocampal feedback inhibitory circuit

Long-term potentiation (LTP), which approximates Hebb's postulate of associative learning, typically requires depolarization-dependent glutamate receptors of the NMDA (N-methyl-D-aspartate) subtype. However, in some neurons, LTP depends instead on calcium-permeable AMPA-type receptors. This is paradoxical because intracellular polyamines block such receptors during depolarization. We report that LTP at synapses on hippocampal interneurons mediating feedback inhibition is "anti-Hebbian":Itis induced by presynaptic activity but prevented by postsynaptic depolarization. Anti-Hebbian LTP may occur in interneurons that are silent during periods of intense pyramidal cell firing, such as sharp waves, and lead to their altered activation during theta activity.     

Translational roots for mental retardation?

A flurry of findings points to protein translation in the dendrites of neurons as a key feature leading to the changes at synapses that are vital to learning (see main text). And one recent discovery suggests that when this translation goes awry, it can lead to mental retardation.     

Glia-synapse interaction through Ca2+-permeable AMPA receptors in Bergmann glia

Glial cells express a variety of neurotransmitter receptors. Notably, Bergmann glial cells in the cerebellum have Ca2+-permeable alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA)-type glutamate receptors (AMPARs) assembled without the GluR2 subunit. To elucidate the role of these Ca2+-permeable AMPARs, we converted them into Ca2+-impermeable receptors by adenoviral-mediated delivery of the GluR2 gene. This conversion retracted the glial processes ensheathing synapses on Purkinje cell dendritic spines and retarded the removal of synaptically released glutamate. Furthermore, it caused multiple innervation of Purkinje cells by the climbing fibers. Thus, the glial Ca2+-permeable AMPARs are indispensable for proper structural and functional relations between Bergmann glia and glutamatergic synapses.     

Coordinated nonvectorial folding in a newly synthesized multidomain protein

The low-density lipoprotein receptor (LDL-R) is a typical example of a multidomain protein, for which in vivo folding is assumed to occur vectorially from the amino terminus to the carboxyl terminus. Using a pulse-chase approach in intact cells, we found instead that newly synthesized LDL-R molecules folded by way of "collapsed" intermediates that contained non-native disulfide bonds between distant cysteines. The most amino-terminal domain acquired its native conformation late in folding instead of during synthesis. Thus, productive LDL-R folding in a cell is not vectorial but is mostly posttranslational, and involves transient long-range non-native disulfide bonds that are isomerized into native short-range cysteine pairs.     

Driving AMPA receptors into synapses by LTP and CaMKII: requirement for GluR1 and PDZ domain interaction

To elucidate mechanisms that control and execute activity-dependent synaptic plasticity, alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate receptors (AMPA-Rs) with an electrophysiological tag were expressed in rat hippocampal neurons. Long-term potentiation (LTP) or increased activity of the calcium/calmodulin-dependent protein kinase II (CaMKII) induced delivery of tagged AMPA-Rs into synapses. This effect was not diminished by mutating the CaMKII phosphorylation site on the GluR1 AMPA-R subunit, but was blocked by mutating a predicted PDZ domain interaction site. These results show that LTP and CaMKII activity drive AMPA-Rs to synapses by a mechanism that requires the association between GluR1 and a PDZ domain protein.     

苹果属一新种—马尔康海棠

马尔康海棠 Malus maerkangensis Cheng,Zeng et Jin Sp.与陇东海棠M.kansuensis(Batal)Schncid.近似。但本种树体高大,枝刺粗壮;部分叶片分裂,裂缺较深,缺口部位较低,叶基近圆形或宽楔形;每花序的花朵较少,花朵较大,花柱较多,易于区别。     

Evidence that glucose "marks" beta cells resulting in preferential release of newly synthesized insulin

Studies of isolated islets labeled with radioactive leucine show that glucose at a critical time "marks" islets in such a way as to cause preferential release of newly synthesized insulin. The preferential release of insulin from marked islets is relatively independent of subsequent secretagogues or rates of insulin secretion. Previous kinetic studies have indicated that the critical time at which marking occurs is after proinsulin biosynthesis but before the secretory event. Thus, secretory cells may regulate the diversion of newly synthesized material for immediate release as it is approaching or transiting the Golgi apparatus.     

Synapses call the shots

Neuroscience research is shedding light on how neurons delegate their protein synthesis, shipping some messenger RNAs out to the dendrites, where they are translated to protein under the control of local synapses. Indeed, it appears that finely controlled, decentralized protein production may contribute to learning and memory in a way that neuroscientists could scarcely have imagined 20 years ago. As researchers presss on, they are filling in a picture of a complex loop of communication and supply, with synapses acting as local managers wielding more power than anyone might have imagined, to get what they need for carrying out the critical task of storing memories.