Changes in the distribution of calcium calmodulin-dependent protein kinase II at the presynaptic bouton after depolarization

Modification of the synaptic cleft under excitatory conditions

The synaptic cleft is the extracellular part of the synapse, bridging the pre- and postsynaptic membranes. The geometry and molecular organization of the cleft is gaining increased attention as an important determinant of synaptic efficacy. The present study by electron microscopy focuses on short-term morphological changes at the synaptic cleft under excitatory conditions. Depolarization of cultured hippocampal neurons with high K+ results in an increased frequency of synaptic profiles with clefts widened at the periphery (open clefts), typically exhibiting patches of membranes lined by postsynaptic density, but lacking associated presynaptic membranes (18.0% open clefts in high K+ compared to 1.8% in controls). Similarly, higher frequencies of open clefts were observed in adult brain upon a delay of perfusion fixation to promote excitatory/ischemic conditions. Inhibition of basal activity in cultured neurons through the application of TTX results in the disappearance of open clefts whereas application of NMDA increases their frequency (19.0% in NMDA vs. 5.3% in control and 2.6% in APV). Depletion of extracellular Ca2+ with EGTA also promotes an increase in the frequency of open clefts (16.6% in EGTA vs. 4.0% in controls), comparable to that by depolarization or NMDA, implicating dissociation of Ca2+-dependent trans-synaptic bridges. Dissociation of transsynaptic bridges under excitatory conditions may allow perisynaptic mobile elements, such as AMPA receptors to enter the cleft. In addition, peripheral opening of the cleft would facilitate neurotransmitter clearance and thus may have a homeostatic and/or protective function.

Docosahexaenoic acid contributes to increased CaMKII protein expression and a tendency to increase nNOS protein expression in differentiated NG108-15 cells

Docosahexaenoic acid (DHA; 22:6n-3), an n-3 polyunsaturated fatty acid, has various important roles in brain functions. Nitric oxide (NO) produced by neuronal NO synthase (nNOS) and Ca²⁺/calmodulindependent protein kinase II (CaMKII) is also involved in brain functions. We investigated the influence of DHA on nNOS and CaMKII protein expression in differentiated NG108-15 cells. NG108-15 cells were seeded in 12-well plates, and after 24 h, the medium was replaced with Dulbecco's modified Eagle's medium containing 1% fetal bovine serum, 0.2 mM dibutyryl cyclic adenosine monophosphate and 100 nM dexamethasone as differentiation-inducing medium. When cells were cultured in differentiation-inducing medium, neurite-like outgrowths were observed on days 5 and 6. However, no significant difference in morphology was observed in cells with or without DHA treatment. With or without DHA addition, nNOS protein expression was increased on days 5 and 6 compared with day 0. This increase tended to be enhanced by DHA. CaMKII protein expression did not change after differentiation without DHA, but was significantly increased on day 6 compared with day 0 with DHA addition. These data indicate that DHA is involved in brain functions by regulating CaMKII and nNOS protein expression.

An updated reappraisal of synapsins: structure, function and role in neurological and psychiatric disorders

Synapsins (Syns) are phosphoproteins strongly involved in neuronal development and neurotransmitter release. Three distinct genes SYN1, SYN2 and SYN3, with elevated evolutionary conservation, have been described to encode for Synapsin I, Synapsin II and Synapsin III, respectively. Syns display a series of common features, but also exhibit distinctive localization, expression pattern, post-translational modifications (PTM). These characteristics enable their interaction with other synaptic proteins, membranes and cytoskeletal components, which is essential for the proper execution of their multiple functions in neuronal cells. These include the control of synapse formation and growth, neuron maturation and renewal, as well as synaptic vesicle mobilization, docking, fusion, recycling. Perturbations in the balanced expression of Syns, alterations of their PTM, mutations and polymorphisms of their encoding genes induce severe dysregulations in brain networks functions leading to the onset of psychiatric or neurological disorders. This review presents what we have learned since the discovery of Syn I in 1977, providing the state of the art on Syns structure, function, physiology and involvement in central nervous system disorders.

Coordination between Calcium/Calmodulin-Dependent Protein Kinase II and Neuronal Nitric Oxide Synthase in Neurons

Ca²⁺/calmodulin (CaM)-dependent protein kinase II (CaMKII) is highly abundant in the brain and exhibits broad substrate specificity, thereby it is thought to participate in the regulation of neuronal death and survival. Nitric oxide (NO), produced by neuronal NO synthase (nNOS), is an important neurotransmitter and plays a role in neuronal activity including learning and memory processes. However, high levels of NO can contribute to excitotoxicity following a stroke and neurodegenerative disease. Aside from NO, nNOS also generates superoxide which is involved in both cell injury and signaling. CaMKII is known to activate and translocate from the cytoplasm to the post-synaptic density in response to neuronal activation where nNOS is predominantly located. Phosphorylation of nNOS at Ser847 by CaMKII decreases NO generation and increases superoxide generation. Conversely, NO-induced S-nitrosylation of CaMKII at Cys6 is a prominent determinant of the CaMKII inhibition in ATP competitive fashion. Thus, the "cross-talk" between CaMKII and NO/superoxide may represent important signal transduction pathways in brain. In this review, we introduce the molecular mechanism of and pathophysiological role of mutual regulation between CaMKII and nNOS in neurons.

Presynaptic CamKII regulates activity-dependent axon terminal growth

Spaced synaptic depolarization induces rapid axon terminal growth and the formation of new synaptic boutons at the Drosophila larval neuromuscular junction (NMJ). Here, we identify a novel presynaptic function for the Calcium/Calmodulin-dependent Kinase II (CamKII) protein in the control of activity-dependent synaptic growth. Consistent with this function, we find that both total and phosphorylated CamKII (p-CamKII) are enriched in axon terminals. Interestingly, p-CamKII appears to be enriched at the presynaptic axon terminal membrane. Moreover, levels of total CamKII protein within presynaptic boutons globally increase within one hour following stimulation. These effects correlate with the activity-dependent formation of new presynaptic boutons. The increase in presynaptic CamKII levels is inhibited by treatment with cyclohexamide suggesting a protein-synthesis dependent mechanism. We have previously found that acute spaced stimulation rapidly downregulates levels of neuronal microRNAs (miRNAs) that are required for the control of activity-dependent axon terminal growth at this synapse. The rapid activity-dependent accumulation of CamKII protein within axon terminals is inhibited by overexpression of activity-regulated miR-289 in motor neurons. Experiments in vitro using a CamKII translational reporter show that miR-289 can directly repress the translation of CamKII via a sequence motif found within the CamKII 3' untranslated region (UTR). Collectively, our studies support the idea that presynaptic CamKII acts downstream of synaptic stimulation and the miRNA pathway to control rapid activity-dependent changes in synapse structure.

APP Overexpression Causes Aβ-Independent Neuronal Death through Intrinsic Apoptosis Pathway

Accumulation of amyloid-β (Aβ) peptide in the brain is a central hallmark of Alzheimer’s disease (AD) and is thought to be the cause of the observed neurodegeneration. Many animal models have been generated that overproduce Aβ yet do not exhibit clear neuronal loss, questioning this Aβ hypothesis. We previously developed an in vivo mouse model that expresses a humanized amyloid precursor protein (hAPP) in olfactory sensory neurons (OSNs) showing robust apoptosis and olfactory dysfunction by 3 weeks of age, which is consistent with early OSN loss and smell deficits, as observed in AD patients. Here we show, by deleting the β-site APP cleaving enzyme 1 (BACE1) in two distinct transgenic mouse models, that hAPP-induced apoptosis of OSNs is Aβ independent and remains cell autonomous. In addition, we reveal that the intrinsic apoptosis pathway is responsible for hAPP-induced OSN death, as marked by mitochondrial damage and caspase-9 activation. Given that hAPP expression causes OSN apoptosis despite the absence of BACE1, we propose that Aβ is not the sole cause of hAPP-induced neurodegeneration and that the early loss of olfactory function in AD may be based on a cell-autonomous mechanism, which could mark an early phase of AD, prior to Aβ accumulation. Thus, the olfactory system could serve as an important new platform to study the development of AD, providing unique insight for both early diagnosis and intervention.

A cluster of protein kinases and phosphatases modulated in fetal Down syndrome (trisomy 21) brain

Down syndrome (DS; trisomy 21) is the most frequent cause of mental retardation with major cognitive and behavioral deficits. Although a series of aberrant biochemical pathways has been reported, work on signaling proteins is limited. It was, therefore, the aim of the study to test a selection of protein kinases and phosphatases known to be essential for memory and learning mechanisms in fetal DS brain. 12 frontal cortices from DS brain were compared to 12 frontal cortices from controls obtained at legal abortions. Proteins were extracted from brains and western blotting with specific antibodies was carried out. Primary results were used for networking (IntAct Molecular Interaction Database) and individual predicted pathway components were subsequently quantified by western blotting. Levels of calcium-calmodulin kinase II alpha, transforming growth factor beta-activated kinase 1 as well as phosphatase and tensin homolog (PTEN) were reduced in cortex of DS subjects and network generation pointed to interaction between PTEN and the dendritic spine protein drebrin that was subsequently determined and reduced levels were observed. The findings of reduced levels of cognitive-function-related protein kinases and the phosphatase may be relevant for interpretation of previous work and may be useful for the design of future studies on signaling in DS brain. Moreover, decreased drebrin levels may point to dendritic spine abnormalities.

Increased vesicular γ-GABA transporter and decreased phosphorylation of synapsin I in the rostral preoptic area is associated with decreased gonadotrophin-releasing hormone and c-Fos coexpression in middle-aged female mice

Hypothalamic glutamate (Glu) and γ-GABA neurotransmission are involved in the ovarian hormone-induced gonadotrophin-releasing hormone (GnRH)/luteinising hormone (LH) surge in rodents. Studies have shown that reduced Glu and increased γ-GABA in the rostral preoptic area (rPOA) of the hypothalamus, where most activated GnRH neurones are located, play a key role in decreasing the reproductive function of female rats. However, the mechanism underlying the altered balance of these neurotransmitters is poorly understood. In the present study, we observed a decline in the function of GnRH neurones in the rPOA at the time of the GnRH/LH surge in middle-aged intact female mice with regular oestrous cycles. In young mice, there is an increase of vesicular Glu transporter 2 on the pro-oestrus afternoon, which is not observed in middle-aged mice. By contrast, vesicular γ-GABA transporter levels in young mice decrease at the time of the LH surge, whereas they increase in middle-aged mice. Of note, we found that, in middle-aged mice at the time of the GnRH/LH surge, the phosphorylation of synapsin I at Ser603 and Ca(2+) /calmodulin-dependent kinase IIα was significantly lower than in young mice. These data suggest that, in middle-aged mice, higher levels of presynaptic stores of GABA, a lack of increase of Glu and a decreased ability of synaptic vesicle mobilisation could account for the imbalance of Glu and GABA in the rPOA, which decreases the activation of GnRH neurones.

Presynaptic CaMKIIα modulates dopamine D3 receptor activation in striatonigral terminals of the rat brain in a Ca²⁺ dependent manner

CaMKIIα is expressed at high density in the nucleus accumbens where it binds to postsynaptic D3 receptors inhibiting their effects. In striatonigral projections, activation of presynaptic D3 receptors potentiates D1 receptor-induced stimulation of cAMP production and GABA release. In this study we examined whether the presynaptic effects of D3 receptor stimulation in the substantia nigra reticulata (SNr) are modulated by Ca²⁺ activation of CaMKIIα. In SNr synaptosomes two procedures that increase cytoplasmic Ca²⁺, ionomycin and K⁺-depolarization, blocked the additional stimulation of cAMP accumulation produced by coactivating D3 and D1 dopamine receptors. The selective CaMKIIα inhibitor KN-62 reversed the blockade produced by ionomycin and K⁺-depolarization. Incubation in either Ca²-free solutions or with the selective Ca²⁺ blocker nifedipine, also reversed the blocking effects of K⁺-depolarization. Immunoblot studies showed that K⁺-depolarization increased CaMKIIα phosphorylation in a KN-62 sensitive manner and promoted CaMKIIα binding to D3 receptors. In K⁺-depolarized tissues, D3 receptors potentiated D1 receptor-induced stimulation of [³H]GABA release only when CaMKIIα was blocked with KN-62. In the presence of this inhibitor, the selective D3 agonist PD 128,907 reduced the ED₅₀ for the D1 agonist SKF 38393 from 56 to 4 nM. KN-62 also enhanced the effects of dopamine on depolarization induced [³H]GABA release. KN-62 changed ED₅₀ for dopamine from 584 to 56 nM. KN-62 did not affect D1 and D4 receptor responses. These experiments show that in striatonigral projections, CaMKIIα inhibits the action of D3 receptors in a Ca²⁺ dependent manner blocking their modulatory effects on GABA release. These findings suggest a mechanism through which the frequency of action potential discharge in presynaptic terminals regulates dopamine effects.

Ultrastructure of synapses in the mammalian brain

The morphology and molecular composition of synapses provide the structural basis for synaptic function. This article reviews the electron microscopy of excitatory synapses on dendritic spines, using data from rodent hippocampus, cerebral cortex, and cerebellar cortex. Excitatory synapses have a prominent postsynaptic density, in contrast with inhibitory synapses, which have less dense presynaptic or postsynaptic specializations and are usually found on the cell body or proximal dendritic shaft. Immunogold labeling shows that the presynaptic active zone provides a scaffold for key molecules involved in the release of neurotransmitter, whereas the postsynaptic density contains ligand-gated ionic channels, other receptors, and a complex network of signaling molecules. Delineating the structure and molecular organization of these axospinous synapses represents a crucial step toward understanding the mechanisms that underlie synaptic transmission and the dynamic modulation of neurotransmission associated with short- and long-term synaptic plasticity.

Use dependence of presynaptic tenacity

Recent studies indicate that synaptic vesicles (SVs) are continuously interchanged among nearby synapses at very significant rates. These dynamics and the lack of obvious barriers confining synaptic vesicles to specific synapses would seem to challenge the ability of synapses to maintain a constant amount of synaptic vesicles over prolonged time scales. Moreover, the extensive mobilization of synaptic vesicles associated with presynaptic activity might be expected to intensify this challenge. Here we examined the ability of individual presynaptic boutons of rat hippocampal neurons to maintain their synaptic vesicle content, and the degree to which this ability is affected by continuous activity. We found that the synaptic vesicle content of individual boutons belonging to the same axons gradually changed over several hours, and that these changes occurred independently of activity. Intermittent stimulation for 1 h accelerated rates of vesicle pool size change. Interestingly, however, following stimulation cessation, vesicle pool size change rates gradually converged with basal change rates. Over similar time scales, active zones (AZs) exhibited substantial remodeling; yet, unlike synaptic vesicles, AZ remodeling was not affected by the stimulation paradigms used here. These findings indicate that enhanced activity levels can increase synaptic vesicle redistribution among nearby synapses, but also highlight the presence of forces that act to restore particular set points in terms of SV contents, and support a role for active zones in preserving such set points. These findings also indicate, however, that neither AZ size nor SV content set points are particularly stable, questioning the long-term tenacity of presynaptic specializations.

In vivo olfactory model of APP-induced neurodegeneration reveals a reversible cell-autonomous function

Amyloid precursor protein (APP) has long been linked to the neurodegeneration of Alzheimer's disease (AD), but the associated cell death has been difficult to capture in vivo, and the role of APP in effecting neuron loss is still unclear. Olfactory dysfunction is an early symptom of AD with amyloid pathology in the olfactory epithelium correlating well to the brain pathology of AD patients. As olfactory sensory neurons (OSNs) regenerate continuously with immature and mature OSNs coexisting in the same olfactory epithelium, we sought to use this unique system to study APP-induced neurodegeneration. Here we have developed an olfactory-based transgenic mouse model that overexpresses humanized APP containing familial AD mutations (hAPP) in either mature or immature OSNs, and found that despite the absence of extracellular plaques a striking number of apoptotic neurons were detected by 3 weeks of age. Importantly, apoptosis was restricted to the specific population overexpressing hAPP, either mature or immature OSNs, sparing those without hAPP. Interestingly, we observed that this widespread neurodegeneration could be rapidly rescued by reducing hAPP expression levels in immature neurons. Together, these data argue that overexpressing hAPP alone could induce cell-autonomous apoptosis in both mature and immature neurons, challenging the notion that amyloid plaques are necessary for neurodegeneration. Furthermore, we show that hAPP-induced neurodegeneration is reversible, suggesting that AD-related neural loss could potentially be rescued. Thus, we propose that this unique in vivo model will not only help determine the mechanisms underlying AD-related neurodegeneration but also serve as a platform to test possible treatments.

Piccolo regulates the dynamic assembly of presynaptic F-actin

Filamentous (F)-actin is a known regulator of the synaptic vesicle (SV) cycle, with roles in SV mobilization, fusion, and endocytosis. However, the molecular pathways that regulate its dynamic assembly within presynaptic boutons remain unclear. In this study, we have used shRNA-mediated knockdown to demonstrate that Piccolo, a multidomain protein of the active zone cytomatrix, is a key regulator of presynaptic F-actin assembly. Boutons lacking Piccolo exhibit enhanced activity-dependent Synapsin1a dispersion and SV exocytosis, and reduced F-actin polymerization and CaMKII recruitment. These phenotypes are rescued by stabilizing F-actin filaments and mimicked by knocking down Profilin2, another regulator of presynaptic F-actin assembly. Importantly, we find that mice with a targeted deletion of exon 14 from the Pclo gene, reported to lack >95% of Piccolo, continue to express multiple Piccolo isoforms. Furthermore, neurons cultured from these mice exhibit no defects in presynaptic F-actin assembly due to the expression of these isoforms at presynaptic boutons. These data reveal that Piccolo regulates neurotransmitter release by facilitating activity-dependent F-actin assembly and the dynamic recruitment of key signaling molecules into presynaptic boutons, and highlight the need for new genetic models with which to study Piccolo loss of function.

Differential control of presynaptic CaMKII activation and translocation to active zones

The release of neurotransmitters, neurotrophins, and neuropeptides is modulated by Ca(2+) mobilization from the endoplasmic reticulum (ER) and activation of Ca(2+)/calmodulin-dependent protein kinase II (CaMKII). Furthermore, when neuronal cultures are subjected to prolonged depolarization, presynaptic CaMKII redistributes from the cytoplasm to accumulate near active zones (AZs), a process that is reminiscent of CaMKII translocation to the postsynaptic side of the synapse. However, it is not known how presynaptic CaMKII activation and translocation depend on neuronal activity and ER Ca(2+) release. Here these issues are addressed in Drosophila motoneuron terminals by imaging a fluorescent reporter of CaMKII activity and subcellular distribution. We report that neuronal excitation acts with ER Ca(2+) stores to induce CaMKII activation and translocation to a subset of AZs. Surprisingly, activation is slow, reflecting T286 autophosphorylation and the function of presynaptic ER ryanodine receptors (RyRs) and inositol trisphosphate receptors (IP3Rs). Furthermore, translocation is not simply proportional to CaMKII activity, as T286 autophosphorylation promotes activation, but does not affect translocation. In contrast, RNA interference-induced knockdown of the AZ scaffold protein Bruchpilot disrupts CaMKII translocation without affecting activation. Finally, RyRs comparably stimulate both activation and translocation, but IP3Rs preferentially promote translocation. Thus, Ca(2+) provided by different presynaptic ER Ca(2+) release channels is not equivalent. These results suggest that presynaptic CaMKII activation depends on autophosphorylation and global Ca(2+) in the terminal, while translocation to AZs requires Ca(2+) microdomains generated by IP3Rs.

Quantitative estimates of the cytoplasmic, PSD, and NMDAR-bound pools of CaMKII in dendritic spines

CaMKII plays a critical role in long-term potentiation (LTP). The kinase is a major component of the postsynaptic density (PSD); however, it is also contained in the spine cytoplasm. CaMKII can now be monitored optically in living neurons, and it is therefore important to understand the contribution of the PSD and cytoplasmic pools to optical signals. Here, we estimate the size of these pools under basal conditions. From EM immunolabeling data, we calculate that the PSD/cytoplasmic ratio is ~5%. A second independent estimate is derived from measurements indicating that the average mushroom spine PSD contains 90 to 240 holoenzymes. A cytoplasmic concentration of 16 μM (~2590 holoenzymes) in the spine can be estimated from the total measured CaMKII content of hippocampal tissue, the relative volume of different compartments, and the spine-dendrite ratio of CaMKII (2:1). These numbers yield a second estimate (6%) of the PSD/spine ratio in good agreement with the first. The CaMKII bound to the NMDAR is important because preventing the formation of this complex blocks LTP induction. We estimate that the percentage of spine CaMKII held active by binding to the NMDAR is ~0.2%. Implications of the high spine concentration of CaMKII (> 100 μM alpha subunits) and the small fraction within the PSD are discussed. Of particular note, the finding that the CaMKII signal in spines shows only transient activation (open state) after LTP induction is subject to the qualification that it does not reflect the small but important pool bound to the NMDAR.

Hippocampal dysregulation of synaptic plasticity-associated proteins with age-related cognitive decline

Age-related cognitive decline occurs without frank neurodegeneration and is the most common cause of memory impairment in aging individuals. With increasing longevity, cognitive deficits, especially in hippocampus-dependent memory processes, are increasing in prevalence. Nevertheless, the neurobiological basis of age-related cognitive decline remains unknown. While concerted efforts have led to the identification of neurobiological changes with aging, few age-related alterations have been definitively correlated to behavioral measures of cognitive decline. In this work, adult (12 months) and aged (28 months) rats were categorized by Morris water maze performance as Adult cognitively Intact, Aged cognitively Intact or Aged cognitively Impaired, and protein expression was examined in hippocampal synaptosome preparations. Previously described differences in synaptic expression of neurotransmission-associated proteins (Dnm1, Hpca, Stx1, Syn1, Syn2, Syp, SNAP25, VAMP2 and 14-3-3 eta, gamma, and zeta) were confirmed between Adult and Aged rats, with no further dysregulation associated with cognitive impairment. Proteins related to synaptic structural stability (MAP2, drebrin, Nogo-A) and activity-dependent signaling (PSD-95, 14-3-3θ, CaMKIIα) were up- and down-regulated, respectively, with cognitive impairment but were not altered with increasing age. Localization of MAP2, PSD-95, and CaMKIIα demonstrated protein expression alterations throughout the hippocampus. The altered expression of activity- and structural stability-associated proteins suggests that impaired synaptic plasticity is a distinct phenomenon that occurs with age-related cognitive decline, and demonstrates that cognitive decline is not simply an exacerbation of the aging phenotype.

Synapsin-dependent vesicle recruitment modulated by forskolin, phorbol ester and ca in mouse excitatory hippocampal synapses

Repeated release of transmitter from presynaptic elements depends on stimulus-induced Ca²⁺ influx together with recruitment and priming of synaptic vesicles from different vesicle pools. We have compared three different manipulations of synaptic strength, all of which are known to increase short-term synaptic efficacy through presynaptic mechanisms, in the glutamatergic CA3-to-CA1 stratum radiatum synapse in the mouse hippocampal slice preparation. Synaptic responses elicited from the readily releasable vesicle pool during low-frequency synaptic activation (0.1 Hz) were significantly enhanced by both the adenylate cyclase activator forskolin, the priming activator β-phorbol-12,13-dibutyrate (PDBu) and 4 mM [Ca²⁺]_o′ whereas during 20 Hz stimulation, the same manipulations reduced the time needed to reach the peak and increased the magnitude of the resulting frequency facilitation. In contrast, paired-pulse facilitations were unchanged in the presence of forskolin, decreased by 4 mM [Ca²⁺]_o and essentially abolished by PDBu. The subsequent delayed response enhancement (DRE) responses, elicited during continuous 20 Hz stimulations and mediated by recruited vesicles, were enhanced by forskolin, essentially unchanged by PDBu and slightly decreased by 4 mM [Ca²⁺]_o· Similar experiments done on slices devoid of the vesicle-associated synapsin I and II proteins indicated that synapsin I/II-induced enhancements of vesicle recruitment were restricted to Ca²⁺-induced frequency facilitations and forskolin-induced enhancements of the early DRE phase, whereas the proteins had minor effects during PDBu-treatment and represented constraints on late Ca²⁺-induced responses. The data indicate that in these glutamatergic synapses, the comparable enhancements of single synaptic responses induced by these biochemical mechanisms can be transformed during prolonged synaptic stimulation into highly distinct short-term plasticity patterns, which are partly dependent on synapsins I/II.

Taking a back seat: synaptic vesicle clustering in presynaptic terminals

Central inter-neuronal synapses employ various molecular mechanisms to sustain neurotransmitter release during phases of high-frequency synaptic activity. One of the features ensuring this property is the presence of a pool of synaptic vesicles (SVs) in the presynaptic terminal. At rest and low rates of stimulation, most of the vesicles composing this pool remain in a tight cluster. They are actively utilized when neurons fire action potentials at higher rates and the capability of the recycling machinery is limited. In addition, SV clusters are capable of migrating between release sites and reassemble into clusters at neighboring active zones (AZs). Within the cluster, thin "tethers" interconnect SVs. These dynamic filamentous structures are reorganized during stimulation thereby releasing SVs from the cluster. So far, one protein family, the synapsins, which bind actin filaments and vesicles in a phosphorylation-dependent manner, has been implicated in SV clustering in vertebrate synapses. As evident from recent studies, many endocytic proteins reside in the SV cluster in addition to synapsin. Here we discuss alternative possible mechanisms involved in the organization of this population of SVs. We propose a model in which synapsins together with other synaptic proteins, a large proportion of which is involved in SV recycling, form a dynamic proteinaceous "matrix" which limits the mobility of SVs. Actin filaments, however, do not seem to contribute to SV crosslinking within the SV cluster, but instead they are present peripherally to it, at sites of neurotransmitter release, and at sites of SV recycling.

Autophosphorylated CaMKIIalpha acts as a scaffold to recruit proteasomes to dendritic spines

The molecular mechanisms regulating the ubiquitin proteasome system (UPS) at synapses are poorly understood. We report that CaMKIIalpha-an abundant postsynaptic protein kinase-mediates the activity-dependent recruitment of proteasomes to dendritic spines in hippocampal neurons. CaMKIIalpha is biochemically associated with proteasomes in the brain. CaMKIIalpha translocation to synapses is required for activity-induced proteasome accumulation in spines, and is sufficient to redistribute proteasomes to postsynaptic sites. CaMKIIalpha autophosphorylation enhances its binding to proteasomes and promotes proteasome recruitment to spines. In addition to this structural role, CaMKIIalpha stimulates proteasome activity by phosphorylating proteasome subunit Rpt6 on Serine 120. However, CaMKIIalpha translocation, but not its kinase activity, is required for activity-dependent degradation of polyubiquitinated proteins in spines. Our findings reveal a scaffolding role of postsynaptic CaMKIIalpha in activity-dependent proteasome redistribution, which is commensurate with the great abundance of CaMKIIalpha in synapses.

Synapsin- and actin-dependent frequency enhancement in mouse hippocampal mossy fiber synapses

The synapsin proteins have different roles in excitatory and inhibitory synaptic terminals. We demonstrate a differential role between types of excitatory terminals. Structural and functional aspects of the hippocampal mossy fiber (MF) synapses were studied in wild-type (WT) mice and in synapsin double-knockout mice (DKO). A severe reduction in the number of synaptic vesicles situated more than 100 nm away from the presynaptic membrane active zone was found in the synapsin DKO animals. The ultrastructural level gave concomitant reduction in F-actin immunoreactivity observed at the periactive endocytic zone of the MF terminals. Frequency facilitation was normal in synapsin DKO mice at low firing rates (approximately 0.1 Hz) but was impaired at firing rates within the physiological range (approximately 2 Hz). Synapses made by associational/commissural fibers showed comparatively small frequency facilitation at the same frequencies. Synapsin-dependent facilitation in MF synapses of WT mice was attenuated by blocking F-actin polymerization with cytochalasin B in hippocampal slices. Synapsin III, selectively seen in MF synapses, is enriched specifically in the area adjacent to the synaptic cleft. This may underlie the ability of synapsin III to promote synaptic depression, contributing to the reduced frequency facilitation observed in the absence of synapsins I and II.