Phosphorylation by PKA and Cdk5 Mediates the Early Effects of Synapsin III in Neuronal Morphological Maturation.

Synapsin III (SynIII) is a neuron-specific phosphoprotein that plays a unique role in neuronal development. SynIII is phosphorylated by cAMP-dependent protein kinase (PKA) at a highly conserved phosphorylation site and by cyclin-dependent kinase-5 (Cdk5) at a newly described site. Although SynIII is known to be involved in axon elongation in vitro, the role of its phosphorylation by PKA and Cdk5 in the modulation of this process is unknown. We expressed either wild-type (WT) or phosphorylation-site mutants of SynIII in primary SynIII knock-out (KO) mouse neurons at early stages of in vitro development. Whereas the neurite elongation phenotype of SynIII KO neurons was fully rescued by the expression of WT SynIII, the expression of nonphosphorylatable and pseudo-phosphorylated PKA mutants was ineffective. Also, the nonphosphorylatable Cdk5 mutant was unable to rescue the neurite elongation phenotype of SynIII KO neurons. By contrast, the pseudo-phosphorylated mutant rescued the delay in neuronal maturation and axonal elongation, revealing a Cdk5-dependent regulation of SynIII function. Interestingly, SynIII KO neurons also exhibited decreased survival that was fully rescued by the expression of WT SynIII, but not by its phosphorylation mutants, and was associated with increased activated caspase3 and altered tropomyosin receptor kinase B isoform expression. These results indicate that PKA and Cdk5 phosphorylation is required for the physiological action of SynIII on axon specification and neurite outgrowth and that the expression of a functional SynIII is crucial for cell survival. Significance statement: Synapsin III is an atypical member of the synapsin family of synaptic vesicle-associated phosphoproteins that is precociously expressed in neurons and is downregulated afterward. Although experimental evidence suggests a specific role for Synapsin III in neuronal development, the molecular mechanisms are still largely unknown. We found that Synapsin III plays a central role in early stages of neuronal development involving neuronal survival, polarization, and neuritic growth and that these effects are dependent on phosphorylation by cAMP-dependent protein kinase and cyclin-dependent protein kinase-5. These results explain the recently described neurodevelopmental defects in the migration and orientation of Synapsin III-depleted cortical neurons and support the potential association of Synapsin III with neurodevelopmental disorders such as schizophrenia.

ß-amyloid 1-42 induces physiological transcriptional regulation of BACE1.

The pathogenesis of Alzheimer's disease (AD) is only partially understood. ß-amyloid (Aß) is physiologically generated by sequential cleavage of its precursor protein by the ß- and the γ-secretase and it is normally disposed of. In Alzheimer's disease, Aß is excessively produced or less dismissed, but the hypothesis on its physiological and pathological role are heterogeneous and often discordant. It has been described a positive feedback loop from the γ- to the ß-secretase cleavages of Aß precursor protein, which is activated by mutations of Presenilin 1 (PS1), the catalytic core of the γ-secretase. These findings show that Aß precursor protein as well the activity of the γ-secretase are required to obtain the up-regulation of ß-secretase which is induced by Presenilin 1 mutations. Then, Aß 1-42 is the Aß precursor protein derivative that up-regulates the expression of ß-secretase, and c-jun N-terminal kinase (JNK)/c-Jun and ERK1/2 are involved. Here, we describe the activation of ß-secretase and c-jun N-terminal kinase related proteins by monomeric Aß 1-42, defining the conditions that most efficiently strike the described signaling without producing toxicity. Taken together these data imply that monomeric Aß 1-42, at non-toxic concentrations and time frames, are able to induce a signaling pathway that leads to transcriptional activation of ß-secretase.

ATP is released by monocytes stimulated with pathogen-sensing receptor ligands and induces IL-1beta and IL-18 secretion in an autocrine way.

IL-1beta and IL-18 are crucial mediators of inflammation, and a defective control of their release may cause serious diseases. Yet, the mechanisms regulating IL-1beta and IL-18 secretion are partially undefined. Both cytokines are produced as inactive cytoplasmic precursors. Processing to the active form is mediated by caspase-1, which is in turn activated by the multiprotein complex inflammasome. Here, we show that in primary human monocytes microbial components acting on different pathogen-sensing receptors and the danger-associated molecule uric acid are all competent to induce maturation and secretion of IL-1beta and IL-18 through a process that involves as a first event the extracellular release of endogenous ATP. ATP release is followed by autocrine stimulation of the purinergic receptors P2X(7). Indeed, antagonists of the P2X(7) receptor (P2X(7)R), or treatment with apyrase, prevent IL-1beta and IL-18 maturation and secretion triggered by the different stimuli. At variance, blocking P2X(7)R activity has no effects on IL-1beta secretion by monocytes carrying a mutated inflammasome that does not require exogenous ATP for activation. P2X(7)R engagement is followed by K+ efflux and activation of phospholipase A(2). Both events are required for processing and secretion induced by all of the stimuli. Thus, stimuli acting on different pathogen-sensing receptors converge on a common pathway where ATP externalization is the first step in the cascade of events leading to inflammasome activation and IL-1beta and IL-18 secretion.

Mapping brain morphological and functional conversion patterns in amnestic MCI: a voxel-based MRI and FDG-PET study.

PURPOSE: To reveal the morphological and functional substrates of memory impairment and conversion to Alzheimer disease (AD) from the stage of amnestic mild cognitive impairment (aMCI). METHODS: Brain MRI and FDG-PET were performed in 20 patients with aMCI and 12 controls at baseline. During a mean follow-up of about 2 years, 9 patients developed AD (converters), and 11 did not (nonconverters). All images were processed with SPM2. FDG-PET and segmented grey matter (GM) images were compared in: (1) converters versus controls, (2) nonconverters versus controls, and (3) converters versus nonconverters. RESULTS: As compared to controls, converters showed lower GM density in the left parahippocampal gyrus and both thalami, and hypometabolism in the precuneus, posterior cingulate and superior parietal lobule in the left hemisphere. Hypometabolism was found in nonconverters as compared to controls in the left precuneus and posterior cingulated gyrus. As compared to nonconverters, converters showed significant hypometabolism in the left middle and superior temporal gyri. CONCLUSION: The discordant topography between atrophy and hypometabolism reported in AD is already present at the aMCI stage. Posterior cingulate-precuneus hypometabolism seemed to be an early sign of memory deficit, whereas hypometabolism in the left temporal cortex marked the conversion to AD.

Effect on memory of acute administration of naturally secreted fibrils and synthetic amyloid-beta peptides in an invertebrate model.

Amyloid beta peptide (Abeta) is considered one of the main agents of Alzheimer's disease pathogenesis. Recently, it has been proposed that memory deficits are caused by different stages of Abeta aggregation, particularly by oligomers. In addition, although memory impairment was found after Abeta administration in rodents and chicks, the nature of the memory deficits induced in invertebrates by acute administration of mammalian Abeta peptides is not well understood. Previously, we reported the amnesic effect of acute pre-training administration of naturally formed fibrils (NF) in crab memory. Here we evaluate the effect of NF and synthetic Abeta peptides administration at different times before and after training in this well characterized invertebrate memory model, the context-signal memory of the crab Chasmagnathus. We found a clear amnesic effect at very low doses of naturally Abeta NF only when administered immediately pre- and post-training, but not 24 h and 18 h before or 6h after training. Activation of ERK/MAPK (a protein kinase required for memory formation in this model) 60 min after administration was found. In contrast, neither JNK/SAPK nor NF-kappaB transcription factor were activated. Furthermore, synthetic Abeta1-42 and Abetapy3-42 administration induced amnesia when used after a protocol for fibrillation but not after a protocol for oligomerization. On the contrary, no amnestic effect was found when fibrillated Abeta1-40 and Abetapy11-42 peptides were used. Thus, Abeta1-42 and Abetapy3-42 peptides impaired memory and the effects were only found when highly aggregated peptides, which may include fibrils, protofibrils and oligomers, were administered. These temporally- and signaling-specific effects suggest that Abeta impairs memory by inducing transient physiological, rather than permanent neuropathological, alterations of the brain and this effect is achieved through generalized ERK activation.

Altered Intracellular Calcium Homeostasis Underlying Enhanced Glutamatergic Transmission in Striatal-Enriched Tyrosine Phosphatase (STEP) Knockout Mice.

The striatal-enriched protein tyrosine phosphatase (STEP) is a brain-specific phosphatase involved in synaptic transmission. The current hypothesis on STEP function holds that it opposes synaptic strengthening by dephosphorylating and inactivating key neuronal proteins involved in synaptic plasticity and intracellular signaling, such as the MAP kinases ERK1/2 and p38, as well as the tyrosine kinase Fyn. Although STEP has a predominant role at the post-synaptic level, it is also expressed in nerve terminals. To better investigate its physiological role at the presynaptic level, we functionally investigated brain synaptosomes and autaptic hippocampal neurons from STEP knockout (KO) mice. Synaptosomes purified from mutant mice were characterized by an increased basal and evoked glutamate release compared with wild-type animals. Under resting conditions, STEP KO synaptosomes displayed increased cytosolic Ca2+ levels accompanied by an enhanced basal activity of Ca2+/calmodulin-dependent protein kinase type II (CaMKII) and hyperphosphorylation of synapsin I at CaMKII sites. Moreover, STEP KO hippocampal neurons exhibit an increase of excitatory synaptic strength attributable to an increased size of the readily releasable pool of synaptic vesicles. These results provide new evidence that STEP plays an important role at nerve terminals in the regulation of Ca2+ homeostasis and neurotransmitter release.

Association of a presenilin 1 S170F mutation with a novel Alzheimer disease molecular phenotype.

OBJECTIVE: To report an ataxic variant of Alzheimer disease expressing a novel molecular phenotype. DESIGN: Description of a novel phenotype associated with a presenilin 1 mutation. SETTING: The subject was an outpatient who was diagnosed at the local referral center. PATIENT: A 28-year-old man presented with psychiatric symptoms and cerebellar signs, followed by cognitive dysfunction. Severe beta-amyloid (Abeta) deposition was accompanied by neurofibrillary tangles and cell loss in the cerebral cortex and by Purkinje cell dendrite loss in the cerebellum. A presenilin 1 gene (PSEN1) S170F mutation was detected. MAIN OUTCOME MEASURES: We analyzed the processing of Abeta precursor protein in vitro as well as the Abeta species in brain tissue. RESULTS: The PSEN1 S170F mutation induced a 3-fold increase of both secreted Abeta(42) and Abeta(40) species and a 60% increase of secreted Abeta precursor protein in transfected cells. Soluble and insoluble fractions isolated from brain tissue showed a prevalence of N-terminally truncated Abeta species ending at both residues 40 and 42. CONCLUSION: These findings define a new Alzheimer disease molecular phenotype and support the concept that the phenotypic variability associated with PSEN1 mutations may be dictated by the Abeta aggregates' composition.

APache Is an AP2-Interacting Protein Involved in Synaptic Vesicle Trafficking and Neuronal Development.

Synaptic transmission is critically dependent on synaptic vesicle (SV) recycling. Although the precise mechanisms of SV retrieval are still debated, it is widely accepted that a fundamental role is played by clathrin-mediated endocytosis, a form of endocytosis that capitalizes on the clathrin/adaptor protein complex 2 (AP2) coat and several accessory factors. Here, we show that the previously uncharacterized protein KIAA1107, predicted by bioinformatics analysis to be involved in the SV cycle, is an AP2-interacting clathrin-endocytosis protein (APache). We found that APache is highly enriched in the CNS and is associated with clathrin-coated vesicles via interaction with AP2. APache-silenced neurons exhibit a severe impairment of maturation at early developmental stages, reduced SV density, enlarged endosome-like structures, and defects in synaptic transmission, consistent with an impaired clathrin/AP2-mediated SV recycling. Our data implicate APache as an actor in the complex regulation of SV trafficking, neuronal development, and synaptic plasticity.

Neprylisin decreases uniformly in Alzheimer's disease and in normal aging.

The proteolysis of beta-amyloid (Abeta) requires neprylisin, an enzyme that has been shown as reduced in Alzheimer's disease (AD). We investigated whether a decrease in neprilysin levels contributes to the accumulation of amyloid deposits not only in AD but also in the normal aging. We analyzed neprilysin mRNA and protein levels in cerebral cortex from 10 cognitively normal elderly subjects with amyloid plaques (NA), 10 cases of AD, and 10 control cases free of amyloid plaques. We found a significant decrease in neprilysin mRNA levels in both AD and NA compared to control cases. Thereby, the defect of neprilysin appears to correlate with Abeta deposition but not with degeneration and dementia.

Plasma levels of insulin and amyloid beta 42 are correlated in patients with amnestic Mild Cognitive Impairment.

Epidemiological and experimental data suggest that type 2 diabetes (DM2) and sporadic late-onset Alzheimer's disease (AD) share a common mechanism, that is able to produce accumulation of insulin and amyloid beta 42 (Abeta42), the major pathogenic events respectively of the two conditions. In 71 non diabetic patients with amnestic mild cognitive impairment we found a significant linear correlation between fasting plasma levels of insulin and Abeta42 (R = +0.25, P < 0.05). The levels of both peptides were elevated in comparison to 48 age-matched cognitively normal controls. The correlation of insulin and Abeta42 plasma levels suggests a pathogenic link between DM2 and sporadic AD.