1853790
Wnt-5a occludes Aβ oligomer-induced depression of glutamatergic transmission in hippocampal neurons
BackgroundSoluble amyloid-β (Aβ;) oligomers have been recognized to be early and key intermediates in Alzheimer's disease (AD)-related synaptic dysfunction. Aβ oligomers block hippocampal long-term potentiation (LTP) and impair rodent spatial memory. Wnt signaling plays an important role in neural development, including synaptic differentiation.ResultsWe report here that the Wnt signaling activation prevents the synaptic damage triggered by Aβ oligomers. Electrophysiological analysis of Schaffer collaterals-CA1 glutamatergic synaptic transmission in hippocampal slices indicates that Wnt-5a increases the amplitude of field excitatory postsynaptic potentials (fEPSP) and both AMPA and NMDA components of the excitatory postsynaptic currents (EPSCs), without modifying the paired pulse facilitation (PPF). Conversely, in the presence of Aβ oligomers the fEPSP and EPSCs amplitude decreased without modification of the PPF, while the postsynaptic scaffold protein (PSD-95) decreased as well. Co-perfusion of hippocampal slices with Wnt-5a and Aβ oligomers occludes against the synaptic depression of EPSCs as well as the reduction of PSD-95 clusters induced by Aβ oligomers in neuronal cultures. Taken together these results indicate that Wnt-5a and Aβ oligomers inversely modulate postsynaptic components.ConclusionThese results indicate that post-synaptic damage induced by Aβ oligomers in hippocampal neurons is prevented by non-canonical Wnt pathway activation.
27132044
A second canon. Functions and mechanisms of beta-catenin-independent Wnt signaling.
More is becoming known about so-called noncanonical Wnt pathways that signal independently of beta-catenin. Here we review recent developments in both the functions and mechanisms of noncanonical Wnt signaling. We also discuss some unresolved and vexing questions. How many noncanonical Wnt pathways are there? How extensive are the parallels between Drosophila planar polarization and vertebrate convergence and extension? Last, we will outline some challenges and difficulties we foresee for this exciting but still very young field.
8434005
The WNT/calcium pathway: biochemical mediators, tools and future requirements.
Wnt proteins represent a family of secreted, lipid-modified glycoproteins that can activate different intracellular pathways. Upon binding to certain members of the Frizzled family of Wnt receptors some Wnts like Wnt-4, Wnt-5A or Wnt-11 are able to elicit an intracellular release of calcium ions. This calcium signaling acitivity is sufficient to activate calcium sensitive enzymes like protein kinase C (PKC), calcium-calmodulin dependent kinase II (CamKII) or calcineurin (CaCN). This so-called Wnt/calcium pathway plays important roles during dorso-ventral patterning of the embryo, regulating cell migration, as well as heart development, and might play a role during tumor suppression. The foci of this review are the biochemical aspects of Wnt/calcium signaling, the tools that are available to study Wnt/calcium signaling, and the open questions that need to be addressed in the future to validate this signaling pathway.
129117
Frizzled signalling and cell polarisation in Drosophila and vertebrates
A key aspect of animal development is the appropriate polarisation of different cell types in the right place at the right time. Such polarisation is often precisely coordinated relative to the axes of a tissue or organ, but the mechanisms underlying this coordination are still poorly understood. Nevertheless, genetic analysis of animal development has revealed some of the pathways involved. For example, a non-canonical Frizzled signalling pathway has been found to coordinate cell polarity throughout the insect cuticle, and recent work has implicated an analogous pathway in coordinated polarisation of cells during vertebrate development. This review discusses recent findings regarding non-canonical Frizzled signalling and cell polarisation.
21624118
Two Sides of the Same Coin: Wnt Signaling in Neurodegeneration and Neuro-Oncology
Wnts function through the activation of at least three intracellular signal transduction pathways, of which the canonical β-catenin mediated pathway is the best understood. Aberrant canonical Wnt signaling has been involved in both neurodegeneration and cancer. An impairment of Wnt signals appears to be associated with aspects of neurodegenerative pathologies while overactivation of Wnt signaling is a common theme in several types of human tumors. Therefore, although therapeutic approaches aimed at modulating Wnt signaling in neurodegenerative and hyperproliferative diseases might impinge on the same molecular mechanisms, different pharmacological outcomes are required. Here we review recent developments on the understanding of the role of Wnt signaling in Alzheimer’s disease and CNS tumors, and identify possible avenues for therapeutic intervention within a complex and multi-faceted signaling pathway.
24480929
Wnt signaling: multiple functions in neural development
Abstract.Wnt signaling has proven to be essential for neural development at various stages and across species. Wnts are involved in morphogenesis and patterning, and their proliferation-promoting role is a key function in stem cell maintenance and the expansion of progenitor pools. Moreover, Wnt signaling is involved in differentiation processes and lineage decision events during both central and peripheral nervous system development. Additionally, several reports point to a role of Wnt signaling in axon guidance and neurite outgrowth. This article reviews and consolidates the existing evidence for the functions of Wnt signaling in neural development.
16357752
Activity-dependent Synaptic Wnt Release Regulates Hippocampal Long Term Potentiation*
Wnts are important for various developmental and oncogenic processes. Here we show that Wnt signaling functions at synapses in hippocampal neurons. Tetanic stimulations induce N-methyl-d-aspartate receptor-dependent synaptic Wnt3a release, nuclear β-catenin accumulations, and the activation of Wnt target genes. Suppression of Wnt signaling impairs long term potentiation. Conversely, activation of Wnt signaling facilitates long term potentiation. These findings suggest that Wnt signaling plays a critical role in regulating synaptic plasticity.
7060484
Wnt-7a Modulates the Synaptic Vesicle Cycle and Synaptic Transmission in Hippocampal Neurons*
Wnt signaling is essential for neuronal development and the maintenance of the developing nervous system. Recent studies indicated that Wnt signaling modulates long term potentiation in adult hippocampal slices. We report here that different Wnt ligands are present in hippocampal neurons of rat embryo and adult rat, including Wnt-4, -5a, -7a, and -11. Wnt-7a acts as a canonical Wnt ligand in rat hippocampal neurons, stimulates clustering of presynaptic proteins, and induces recycling and exocytosis of synaptic vesicles as studied by FM dyes. Wnt-3a has a moderate effect on recycling of synaptic vesicles, and no effect of Wnt-1 and Wnt-5a was detected. Electrophysiological analysis on adult rat hippocampal slices indicates that Wnt-7a, but not Wnt-5a, increases neurotransmitter release in CA3-CA1 synapses by decreasing paired pulse facilitation and increasing the miniature excitatory post-synaptic currents frequency. These results indicate that the presynaptic function of rat hippocampal neurons is modulated by the canonical Wnt signaling.
679696
Down‐regulation of Dickkopf 3, a regulator of the Wnt signalling pathway, in elderly schizophrenic subjects
The aetiology of schizophrenia is complex and the pathological mechanisms involved are still not fully understood. The aim of this project was to gain insight into the underlying molecular changes occurring in schizophrenia through the analysis of gene expression. Using suppression subtractive hybridization to isolate differentially expressed genes in superior temporal cortex (BA22), we detected one prominent sequence with reduced expression in schizophrenia and represented in at least nine clones. This was then selected for further validation. This 190‐bp partial transcript showed identity to part of the Dickkopf‐3 (Dkk3) gene sequence. Differential expression was initially confirmed in BA22 by slot blot hybridization where expression was decreased by 35% (p < 0.026). These results were further authenticated in a larger panel (12 control and 11 schizophrenia cases) using SYBR Green I real‐time quantitative RT–PCR, in which a 41% decrease in expression of Dkk3 mRNA in schizophrenia was obtained (p < 0.012). Furthermore, using in situ hybridization, Dkk3 mRNA was shown to be abundantly expressed in cortical neurones, with prominent expression in layers II/III and V/VI of BA22. Dkk3 belongs to a novel family of Dkk proteins, which have been shown to be potent inhibitors of the neurodevelopmental wingless (Wnt) signalling pathway, and is therefore a putative candidate for further investigation into the aetiology of schizophrenia.
37355077
Wnt-3a overcomes beta-amyloid toxicity in rat hippocampal neurons.
The aim of this study was to evaluate whether the direct activation of the Wnt signaling pathway by its endogenous Wnt-3a ligand prevents the toxic effects induced by amyloid-beta-peptide (Abeta) in rat hippocampal neurons. We report herein that the Wnt-3a ligand was indeed able to overcome toxic effects induced by Abeta in hippocampal neurons, including a neuronal impairment on cell survival, an increase in glycogen synthase kinase-3beta (GSK-3beta) and tau phosphorylation, a decrease in cytoplasmic beta-catenin and a decrease in the expression of the Wnt target gene engrailed-1. We further demonstrate that Wnt-3a protects hippocampal neurons from apoptosis induced by Abeta. Our results support the hypothesis that a loss of function of Wnt signaling may play a role in the progression of neurodegenerative diseases such as Alzheimer's disease.
17047247
The role of Wnt signaling in neuronal dysfunction in Alzheimer's Disease
Recent evidence supports a neuroprotective role for Wnt signaling in neurodegenerative disorders such as Alzheimer's Disease (AD). In fact, a relationship between amyloid-β-peptide (Aβ)-induced neurotoxicity and a decrease in the cytoplasmic levels of β-catenin has been observed. Apparently Aβ binds to the extracellular cysteine-rich domain of the Frizzled receptor (Fz) inhibiting Wnt/β-catenin signaling. Cross-talk with other signaling cascades that regulate Wnt/β-catenin signaling, including the activation of M1 muscarinic receptor and PKC, the use of Ibuprofen-ChE bi-functional compounds, PPAR α, γ agonists, nicotine and some antioxidants, results in neuroprotection against Aβ. These studies indicate that a sustained loss of Wnt signaling function may be involved in the Aβ-dependent neurodegeneration observed in Alzheimer's brain. In conclusion the activation of the Wnt signaling pathway could be proposed as a therapeutic target for the treatment of AD.
15150253
The Amyloid Hypothesis of Alzheimer's Disease: Progress and Problems on the Road to Therapeutics
It has been more than 10 years since it was first proposed that the neurodegeneration in Alzheimer's disease (AD) may be caused by deposition of amyloid β-peptide (Aβ) in plaques in brain tissue. According to the amyloid hypothesis, accumulation of Aβ in the brain is the primary influence driving AD pathogenesis. The rest of the disease process, including formation of neurofibrillary tangles containing tau protein, is proposed to result from an imbalance between Aβ production and Aβ clearance.
25740051
Is Alzheimer's Disease a Result of Presynaptic Failure? - Synaptic Dysfunctions Induced by Oligomeric β-Amyloid
Since Alois Alzheimer first described morphological alterations associated with his patient's dementia more than 100 years ago, Alzheimer's disease (AD) was defined as neurodegenerative disease caused by extracellular deposits of misfolded proteins. These amyloid plaques and neurofibrillary tangles have been unambiguously considered as hallmarks of this ailment, accompanied by devastating brain atrophy and cell loss. When a 40-42 amino acid peptide, called beta-amyloid (Abeta), was identified as a main component of amyloid plaques and a few genetic cases of AD were linked to Abeta metabolism, the Abeta hypothesis of AD was proposed. It was initially thought that an increase in Abeta42 precipitates plaque formation, which causes the generation of neurofibrillary tangles and ultimately the death of neurons. However, during the last decade it became apparent that soluble rather than deposited Abeta is associated with dementia. Among the constituents of soluble Abeta, small oligomeric forms were increasingly associated with neuropathology. There is now ample evidence that Abeta oligomers do not affect neuronal viability in general, but interfere specifically with synaptic function. Long-term neurophysiological impairment ultimately causes degeneration of synapses, which becomes most apparent on the morphological level by retraction of dendritic spines. Loss of meaningful synaptic connections in the brain of patients with AD will shatter their capacity to encode and retrieve memories. The precise molecular mechanism of Abeta oligomer-induced impairment of synaptic transmission is not fully understood, but there are several independent observations that oligomers interfere with the vesicular release machinery at the presynaptic terminal. While this hypothesis offers a promising avenue to understand the underlying cause of cognition and memory deficits in the AD brain, it also opens a possibility to address new mechanisms for preventing and ultimately curing AD.
42320970
Amyloid β Oligomers (Aβ1–42 Globulomer) Suppress Spontaneous Synaptic Activity by Inhibition of P/Q-Type Calcium Currents
Abnormal accumulation of soluble oligomers of amyloid β (Aβ) is believed to cause malfunctioning of neurons in Alzheimer's disease. It has been shown that Aβ oligomers impair synaptic plasticity, thereby altering the ability of the neuron to store information. We examined the underlying cellular mechanism of Aβ oligomer-induced synaptic modifications by using a recently described stable oligomeric Aβ preparation called “Aβ1–42 globulomer.” Synthetically prepared Aβ1–42 globulomer has been shown to localize to neurons and impairs long-term potentiation (Barghorn et al., 2005). Here, we demonstrate that Aβ1–42 globulomer does not affect intrinsic neuronal properties, as assessed by measuring input resistance and discharge characteristics, excluding an unspecific alteration of membrane properties. We provide evidence that Aβ1–42 globulomer, at concentrations as low as 8 nm, specifically suppresses spontaneous synaptic activity resulting from a reduction of vesicular release at terminals of both GABAergic and glutamatergic synapses. EPSCs and IPSCs were primarily unaffected. A detailed search for the precise molecular target of Aβ1–42 globulomer revealed a specific inhibition of presynaptic P/Q calcium currents, whereas other voltage-activated calcium currents remained unaltered. Because intact P/Q calcium currents are needed for synaptic plasticity, the disruption of such currents by Aβ1–42 globulomer may cause deficits in cellular mechanisms of information storage in brains of Alzheimer's disease patients. The inhibitory effect of Aβ1–42 globulomer on synaptic vesicle release could be reversed by roscovitine, a specific enhancer of P/Q currents. Selective enhancement of the P/Q calcium current may provide a promising strategy in the treatment of Alzheimer's disease.
2746952
Amyloid precursor protein overexpression depresses excitatory transmission through both presynaptic and postsynaptic mechanisms
Overexpression of the amyloid precursor protein (APP) in hippocampal neurons leads to elevated β-amyloid peptide (Aβ) production and consequent depression of excitatory transmission. The precise mechanisms underlying APP-induced synaptic depression are poorly understood. Uncovering these mechanisms could provide insight into how neuronal function is compromised before cell death during the early stages of Alzheimer's disease. Here we verify that APP up-regulation leads to depression of transmission in cultured hippocampal autapses; and we perform whole-cell recording, FM imaging, and immunocytochemistry to identify the specific mechanisms accounting for this depression. We find that APP overexpression leads to postsynaptic silencing through a selective reduction of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated currents. This effect is likely mediated by Aβ because expression of mutant APP incapable of producing Aβ did not depress transmission. In addition, although we eliminate presynaptic silencing as a mechanism underlying APP-mediated inhibition of transmission, we did observe an Aβ-induced presynaptic deficit in vesicle recycling with sustained stimulation. These findings demonstrate that APP elevation disrupts both presynaptic and postsynaptic compartments.
9816364
Plaque-independent disruption of neural circuits in Alzheimer's disease mouse models.
Autosomal dominant forms of familial Alzheimer's disease (FAD) are associated with increased production of the amyloid beta peptide, Abeta42, which is derived from the amyloid protein precursor (APP). In FAD, as well as in sporadic forms of the illness, Abeta peptides accumulate abnormally in the brain in the form of amyloid plaques. Here, we show that overexpression of FAD(717V-->F)-mutant human APP in neurons of transgenic mice decreases the density of presynaptic terminals and neurons well before these mice develop amyloid plaques. Electrophysiological recordings from the hippocampus revealed prominent deficits in synaptic transmission, which also preceded amyloid deposition by several months. Although in young mice, functional and structural neuronal deficits were of similar magnitude, functional deficits became predominant with advancing age. Increased Abeta production in the context of decreased overall APP expression, achieved by addition of the Swedish FAD mutation to the APP transgene in a second line of mice, further increased synaptic transmission deficits in young APP mice without plaques. These results suggest a neurotoxic effect of Abeta that is independent of plaque formation.
15511671
Block of Long-Term Potentiation by Naturally Secreted and Synthetic Amyloid β-Peptide in Hippocampal Slices Is Mediated via Activation of the Kinases c-Jun N-Terminal Kinase, Cyclin-Dependent Kinase 5, and p38 Mitogen-Activated Protein Kinase as well as Metabotropic Glutamate Receptor Type 5
The mechanisms of action of human synthetic and naturally secreted cell-derived amyloid β-peptide (Aβ)1–42 on the induction of long-term potentiation (LTP) were investigated in the medial perforant path to dentate granule cell synapses in hippocampal slices. Synthetic and cell-derived Aβ strongly inhibited high-frequency stimulation (HFS)-induced LTP at peak HFS and 1 hr after HFS. Cell-derived Aβ was much more potent than synthetic Aβ at inhibiting LTP induction, with threshold concentrations of ∼1 and 100–200 nm, respectively. The involvement of various kinases in Aβ-mediated inhibition of LTP induction was investigated by applying Aβ in the presence of inhibitors of these kinases. The c-Jun N-terminal kinase (JNK) inhibitor JNKI prevented the block of LTP induction by both synthetic and cell-derived Aβ. The block of LTP induced by synthetic Aβ was also prevented by the JNK inhibitor anthra[1,9-cd]pyrazol-6(2H)-one, the cyclin-dependent kinase 5 (Cdk5) inhibitors butyrolactone and roscovitine, and the p38 MAP kinase (MAPK) inhibitor 4-(4-fluorophenyl)-2-(4-methylsulfonylphenyl)-5-(4-pyridyl)-1H-imidazole but not by the p42–p44 MAP kinase inhibitor 1,4-diamino-2,3-dicyano-1,4-bis(2-aminophenylthio)butadiene. The group I–group II metabotropic glutamate receptor (mGluR) antagonist 2S-2-amino-2-(1S,2S-2-carboxycyclopropyl-1-yl)-3-(xanth-9-yl)propanoic acid and the mGluR5 antagonist methyl-6-(phenylethynyl)pyridine prevented the block of LTP induction by Aβ. However, theα7 nicotinic ACh receptor antagonist methylcaconatine did not prevent the inhibition of LTP induction by Aβ. These studies provide evidence that the Aβ-mediated inhibition of LTP induction involves stimulation of the kinases JNK, Cdk5, and p38 MAPK after the activation of both the Aβ receptor(s) and mGluR5.
44961762
Synaptic targeting by Alzheimer's-related amyloid beta oligomers.
The cognitive hallmark of early Alzheimer's disease (AD) is an extraordinary inability to form new memories. For many years, this dementia was attributed to nerve-cell death induced by deposits of fibrillar amyloid beta (Abeta). A newer hypothesis has emerged, however, in which early memory loss is considered a synapse failure caused by soluble Abeta oligomers. Such oligomers rapidly block long-term potentiation, a classic experimental paradigm for synaptic plasticity, and they are strikingly elevated in AD brain tissue and transgenic-mouse AD models. The current work characterizes the manner in which Abeta oligomers attack neurons. Antibodies raised against synthetic oligomers applied to AD brain sections were found to give diffuse stain around neuronal cell bodies, suggestive of a dendritic pattern, whereas soluble brain extracts showed robust AD-dependent reactivity in dot immunoblots. Antigens in unfractionated AD extracts attached with specificity to cultured rat hippocampal neurons, binding within dendritic arbors at discrete puncta. Crude fractionation showed ligand size to be between 10 and 100 kDa. Synthetic Abeta oligomers of the same size gave identical punctate binding, which was highly selective for particular neurons. Image analysis by confocal double-label immunofluorescence established that >90% of the punctate oligomer binding sites colocalized with the synaptic marker PSD-95 (postsynaptic density protein 95). Synaptic binding was accompanied by ectopic induction of Arc, a synaptic immediate-early gene, the overexpression of which has been linked to dysfunctional learning. Results suggest the hypothesis that targeting and functional disruption of particular synapses by Abeta oligomers may provide a molecular basis for the specific loss of memory function in early AD.
2035763
Soluble β-Amyloid1-40 Induces NMDA-Dependent Degradation of Postsynaptic Density-95 at Glutamatergic Synapses
Amyloid-β (Aβ) has been implicated in memory loss and disruption of synaptic plasticity observed in early-stage Alzheimer's disease. Recently, it has been shown that soluble Aβ oligomers target synapses in cultured rat hippocampal neurons, suggesting a direct role of Aβ in the regulation of synaptic structure and function. Postsynaptic density-95 (PSD-95) is a postsynaptic scaffolding protein that plays a critical role in synaptic plasticity and the stabilization of AMPA (AMPARs) and NMDA (NMDARs) receptors at synapses. Here, we show that exposure of cultured cortical neurons to soluble oligomers of Aβ1-40 reduces PSD-95 protein levels in a dose- and time-dependent manner and that the Aβ11-40-dependent decrease in PSD-95 requires NMDAR activity. We also show that the decrease in PSD-95 requires cyclin-dependent kinase 5 activity and involves the proteasome pathway. Immunostaining analysis of cortical cultured neurons revealed that Aβ treatment induces concomitant decreases in PSD-95 at synapses and in the surface expression of the AMPAR glutamate receptor subunit 2. Together, these data suggest a novel pathway by which Aβ triggers synaptic dysfunction, namely, by altering the molecular composition of glutamatergic synapses.
45191500
Differential modulation of NR1‐NR2A and NR1‐NR2B subtypes of NMDA receptor by PDZ domain‐containing proteins
The PSD‐95/Dlg/ZO‐1 (PDZ) domain‐containing proteins MALS and PSD‐95 localize to post‐synaptic densities and bind the COOH‐termini of NR2 subunits of the NMDA receptor. The effects of MALS‐2 and PSD‐95 on the channel activity of NMDA receptors were compared using the Xenopus oocyte expression system. Both MALS‐2 and PSD‐95 increased the current response of the NR1‐NR2B receptor to l‐glutamate. In contrast, the current response of the NR1‐NR2A receptor was increased by PSD‐95 but not by MALS‐2. MALS‐2 had no effect either on the potentiation of NR1‐NR2A or NR1‐NR2B channel activity by protein kinase C, or on Src‐mediated potentiation of NR1‐NR2A activity, whereas PSD‐95 almost completely inhibited the effects of these protein kinases. Construction of chimeras of MALS‐2 and PSD‐95 revealed that the first two PDZ domains and two NH2‐terminal cysteine residues are essential for the inhibitory effects of PSD‐95 on protein kinase C‐mediated potentiation of NR1‐NR2A and NR1‐NR2B channel activity, respectively. The second of the three PDZ domains of PSD‐95 was required for its inhibition of Src‐mediated potentiation of NR1‐NR2A activity. These results indicate that the NR1‐NR2A and NR1‐NR2B receptors are modulated differentially by MALS‐2 and PSD‐95, and that similar regulatory effects of PSD‐95 on these receptors are achieved by distinct mechanisms.
14287236
Interactions of Postsynaptic Density-95 and the NMDA Receptor 2 Subunit Control Calpain-Mediated Cleavage of the NMDA Receptor
The calcium-dependent protease calpain cleaves the NMDA receptor 2 (NR2) subunit of the NMDA receptor both in vitro and in vivo and thus potentially modulates NMDA receptor function and turnover. We examined the ability of postsynaptic density-95 (PSD-95) protein to alter the calpain-mediated cleavage of NR2A and NR2B. Coexpression of PSD-95 with NMDA receptors in human embryonic kidney 293 cells blocked cleavage of NR2A and NR2B by NMDA receptor-activated calpain. NR2A cleavage by calpain occurred in the cell surface and intracellular fractions and required the presence of NR1 subunits. The blocking effect of PSD-95 did not result from decreased calpain activity, lowered intracellular calcium responses, or the blockade of internalization. Instead, this effect was eliminated by deletion of the C-terminal ESDV motif of NR2A or by overexpression of a palmitoylation-deficient PSD-95 mutant lacking the ability to cluster and to interact with NMDA receptors in situ, suggesting a role for association between the C terminus of NR2A and clustered PSD-95. Synapse-associated protein 102, a membrane-associated guanylate kinase interacting with NR2A but lacking palmitoylation motifs and the ability to cluster, did not protect NR2A from cleavage by calpain. Pharmacological inhibition of palmitoylation disrupted the interaction of PSD-95 with NMDA receptors in cortical neurons and allowed NR2A to be cleaved by calpain, whereas NR2A could not be cleaved in untreated neurons. These results indicate that PSD-95 clustering and direct association of NR2A and PSD-95 mediate the blocking effect of PSD-95 on calpain cleavage. PSD-95 could regulate the susceptibility of NMDA receptors to calpain-mediated cleavage during synaptic transmission and excitotoxicity.
6645009
Regulation of NMDA receptor trafficking by amyloid-beta.
Amyloid-beta peptide is elevated in the brains of patients with Alzheimer disease and is believed to be causative in the disease process. Amyloid-beta reduces glutamatergic transmission and inhibits synaptic plasticity, although the underlying mechanisms are unknown. We found that application of amyloid-beta promoted endocytosis of NMDA receptors in cortical neurons. In addition, neurons from a genetic mouse model of Alzheimer disease expressed reduced amounts of surface NMDA receptors. Reducing amyloid-beta by treating neurons with a gamma-secretase inhibitor restored surface expression of NMDA receptors. Consistent with these data, amyloid-beta application produced a rapid and persistent depression of NMDA-evoked currents in cortical neurons. Amyloid-beta-dependent endocytosis of NMDA receptors required the alpha-7 nicotinic receptor, protein phosphatase 2B (PP2B) and the tyrosine phosphatase STEP. Dephosphorylation of the NMDA receptor subunit NR2B at Tyr1472 correlated with receptor endocytosis. These data indicate a new mechanism by which amyloid-beta can cause synaptic dysfunction and contribute to Alzheimer disease pathology.
13125147
Beta-amyloid accumulation in APP mutant neurons reduces PSD-95 and GluR1 in synapses
Synaptic dysfunction is increasingly viewed as an early manifestation of Alzheimer's disease (AD), but the cellular mechanism by which beta-amyloid (Abeta) may affect synapses remains unclear. Since cultured neurons derived from APP mutant transgenic mice secrete elevated levels of Abeta and parallel the subcellular Abeta accumulation seen in vivo, we asked whether alterations in synapses occur in this setting. We report that cultured Tg2576 APP mutant neurons have selective alterations in pre- and post-synaptic compartments compared to wild-type neurons. Post-synaptic compartments appear fewer in number and smaller, while active pre-synaptic compartments appear fewer in number and enlarged. Among the earliest changes in synaptic composition in APP mutant neurons were reductions in PSD-95, a protein involved in recruiting and anchoring glutamate receptor subunits to the post-synaptic density. In agreement, we observed early reductions in surface expression of glutamate receptor subunit GluR1 in APP mutant neurons. We provide evidence that Abeta is specifically involved in these alterations in synaptic biology, since alterations in PSD-95 and GluR1 are blocked by gamma-secretase inhibition, and since exogenous addition of synthetic Abeta to wild-type neurons parallels changes in synaptic PSD-95 and GluR1 observed in APP mutant neurons.
4427689
Stargazin regulates synaptic targeting of AMPA receptors by two distinct mechanisms
Stargazer, an ataxic and epileptic mutant mouse, lacks functional AMPA (α-amino-3-hydroxyl-5-methyl-4-isoxazolepropionate) receptors on cerebellar granule cells. Stargazin, the mutated protein, interacts with both AMPA receptor subunits and synaptic PDZ proteins, such as PSD-95. The interaction of stargazin with AMPA receptor subunits is essential for delivering functional receptors to the surface membrane of granule cells, whereas its binding with PSD-95 and related PDZ proteins through a carboxy-terminal PDZ-binding domain is required for targeting the AMPA receptor to synapses. Expression of a mutant stargazin lacking the PDZ-binding domain in hippocampal pyramidal cells disrupts synaptic AMPA receptors, indicating that stargazin-like mechanisms for targeting AMPA receptors may be widespread in the central nervous system.
5663029
Assembly and plasticity of the glutamatergic postsynaptic specialization
Glutamate mediates most excitatory synaptic transmission in the brain. Synaptic strength at glutamatergic synapses shows a remarkable degree of use-dependent plasticity and such modifications may represent a physiological correlate to learning and memory. Glutamate receptors and downstream enzymes are organized at synapses by cytoskeletal proteins containing multiple protein-interacting domains. Recent studies demonstrate that these 'scaffolding' proteins within the postsynaptic specialization have the capacity to promote synaptic maturation, influence synapse size, and modulate glutamate receptor function.
14162759
Wnt-5A augments repopulating capacity and primitive hematopoietic development of human blood stem cells in vivo
Human hematopoietic stem cells are defined by their ability to repopulate multiple hematopoietic lineages in the bone marrow of transplanted recipients and therefore are functionally distinct from hematopoietic progenitors detected in vitro. Although factors capable of regulating progenitors are well established, in vivo regulators of hematopoietic repopulating function are unknown. By using a member of the vertebrate Wnt family, Wnt-5A, the proliferation and differentiation of progenitors cocultured on stromal cells transduced with Wnt-5A or treated with Wnt-5A conditioned medium (CM) was unaffected. However, i.p. injection of Wnt-5A CM into mice engrafted with human repopulating cells increased multilineage reconstitution by >3-fold compared with controls. Furthermore, in vivo treatment of human repopulating cells with Wnt-5A CM produced a greater proportion of phenotypically primitive hematopoietic progeny that could be isolated and shown to possess enhanced progenitor function independent of continued Wnt-5A treatment. Our study demonstrates that Wnt-5A augments primitive hematopoietic development in vivo and represents an in vivo regulator of hematopoietic stem cell function in the human. Based on these findings, we suggest a potential role for activation of Wnt signaling in managing patients exhibiting poor hematopoietic recovery shortly after stem cell transplantation.
2168934
AMPAR Removal Underlies Aβ-Induced Synaptic Depression and Dendritic Spine Loss
Beta amyloid (Abeta), a peptide generated from the amyloid precursor protein (APP) by neurons, is widely believed to underlie the pathophysiology of Alzheimer's disease. Recent studies indicate that this peptide can drive loss of surface AMPA and NMDA type glutamate receptors. We now show that Abeta employs signaling pathways of long-term depression (LTD) to drive endocytosis of synaptic AMPA receptors. Synaptic removal of AMPA receptors is necessary and sufficient to produce loss of dendritic spines and synaptic NMDA responses. Our studies indicate the central role played by AMPA receptor trafficking in Abeta-induced modification of synaptic structure and function.
22371707
Soluble Arctic amyloid β protein inhibits hippocampal long‐term potentiation in vivo
Mutations in the amyloid precursor protein that result in substitutions of glutamic acid at residue 22 of the amyloid β protein (Aβ) with glutamine (Q22, Dutch) or glycine (G22, Arctic) cause aggressive familial neurological diseases characterized by cerebrovascular haemorrhages or Alzheimer's‐type dementia, respectively. The present study compared the ability of these peptides to block long‐term potentiation (LTP) of glutamatergic transmission in the hippocampus in vivo. The effects of intracerebroventricular injection of wild‐type, Q22 and G22 Aβ(1–40) peptides were examined in the CA1 area of urethane‐anaesthetized rats. Both mutant peptides were ≈ 100‐fold more potent than wild‐type Aβ at inhibiting LTP induced by high‐frequency stimulation when solutions of Aβ were freshly prepared. Fibrillar material, as determined by electron microscopy, was obvious in all these peptide solutions and exhibited appreciable Congo Red binding, particularly for Aβ(1–40)G22 and Aβ(1–40)Q22. A soluble fraction of Aβ(1–40)G22, obtained following high‐speed centrifugation, retained full activity of the peptide solution to inhibit LTP, providing strong evidence that in the case of the Arctic disease a soluble nonfibrillar form of Aβ may represent the primary mediator of Aβ‐related cognitive deficits, particularly early in the disease. In contrast, nonfibrillar soluble Aβ(1–40)Q22 supernatant solution was ≈ 10‐fold less potent at inhibiting LTP than Aβ(1–40)G22, a finding consistent with fibrillar Aβ contributing to the inhibition of LTP by the Dutch peptide.
15260991
Wnt signaling in neural circuit assembly.
The Wnt family of secreted proteins plays a crucial role in nervous system wiring. Wnts regulate neuronal positioning, polarization, axon and dendrite development, and synaptogenesis. These diverse roles of Wnt proteins are due not only to the large numbers of Wnt ligands and receptors but also to their ability to signal through distinct signaling pathways in different cell types and developmental contexts. Studies on Wnts have shed new light on novel molecular mechanisms that control the development of complex neuronal connections. This review discusses recent advances on how Wnt signaling influences different aspects of neuronal circuit assembly through changes in gene expression and/or cytoskeletal modulation.
890571

 Wnt signaling in neuroprotection and stem cell differentiation
In the past several years, we postulated that the loss of Wnt signaling was implicated in the pathology of Alzheimer's disease (AD). Since then, our lab and other groups have confirmed the involvement of the Wnt signaling in some aspects of AD. So far, we have demonstrated that activation of Wnt signaling protects neurons against neurotoxic injuries, including both amyloid-beta (Abeta) fibrils and Abeta oligomers by using either lithium, an inhibitor of the glycogen-synthase-kinase-3beta (GSK-3beta), or different Wnt ligands. Also, we have found that several molecules which activate well known neurotransmitter systems and other signaling system, are able by crosstalk to activate Wnt/beta-catenin signaling in order to protect neurons against both Abeta fibrils or Abeta oligomers. In particular, the activation of non-canonical Wnt signaling was able to protect postsynaptic regions and dendritic spines against Abeta oligomers. Furthermore Wnt signaling ligands also affect stem cells, and they are also involved in cell fate decision during neurogenesis and embryonic development as well as in adult stem cells differentiation in the nervous system. The Wnt signaling plays a key role modulating their cell differentiation or proliferation states. Altogether, these findings in both stem cell biology and neuroprotection, may introduce new approaches in the treatment of neurodegenerative diseases, including drug screening and therapies against neurodegenerative diseases which activates the Wnt signaling pathway.
16967517
The Drosophila Wnt, Wingless, Provides an Essential Signal for Pre- and Postsynaptic Differentiation
At vertebrate neuromuscular junctions (NMJs), Agrin plays pivotal roles in synapse development, but molecules that activate synapse formation at central synapses are largely unknown. Members of the Wnt family are well established as morphogens, yet recently they have also been implicated in synapse maturation. Here we demonstrate that the Drosophila Wnt, Wingless (Wg), is essential for synapse development. We show that Wg and its receptor are expressed at glutamatergic NMJs, and that Wg is secreted by synaptic boutons. Loss of Wg leads to dramatic reductions in target-dependent synapse formation, and new boutons either fail to develop active zones and postsynaptic specializations or these are strikingly aberrant. We suggest that Wg signals the coordinated development of pre- and postsynaptic compartments.
17801757
Natural Oligomers of the Alzheimer Amyloid-β Protein Induce Reversible Synapse Loss by Modulating an NMDA-Type Glutamate Receptor-Dependent Signaling Pathway
Alzheimer's disease (AD) is characterized by decreased synapse density in hippocampus and neocortex, and synapse loss is the strongest anatomical correlate of the degree of clinical impairment. Although considerable evidence supports a causal role for the amyloid-β protein (Aβ) in AD, a direct link between a specific form of Aβ and synapse loss has not been established. We demonstrate that physiological concentrations of naturally secreted Aβ dimers and trimers, but not monomers, induce progressive loss of hippocampal synapses. Pyramidal neurons in rat organotypic slices had markedly decreased density of dendritic spines and numbers of electrophysiologically active synapses after exposure to picomolar levels of soluble oligomers. Spine loss was reversible and was prevented by Aβ-specific antibodies or a small-molecule modulator of Aβ aggregation. Mechanistically, Aβ-mediated spine loss required activity of NMDA-type glutamate receptors (NMDARs) and occurred through a pathway involving cofilin and calcineurin. Furthermore, NMDAR-mediated calcium influx into active spines was reduced by Aβ oligomers. Partial blockade of NMDARs by pharmacological antagonists was sufficient to trigger spine loss. We conclude that soluble, low-n oligomers of human Aβ trigger synapse loss that can be reversed by therapeutic agents. Our approach provides a quantitative cellular model for elucidating the molecular basis of Aβ-induced neuronal dysfunction.
205385777
Amyloid-β protein dimers isolated directly from Alzheimer's brains impair synaptic plasticity and memory
Alzheimer's disease constitutes a rising threat to public health. Despite extensive research in cellular and animal models, identifying the pathogenic agent present in the human brain and showing that it confers key features of Alzheimer's disease has not been achieved. We extracted soluble amyloid-β protein (Aβ) oligomers directly from the cerebral cortex of subjects with Alzheimer's disease. The oligomers potently inhibited long-term potentiation (LTP), enhanced long-term depression (LTD) and reduced dendritic spine density in normal rodent hippocampus. Soluble Aβ from Alzheimer's disease brain also disrupted the memory of a learned behavior in normal rats. These various effects were specifically attributable to Aβ dimers. Mechanistically, metabotropic glutamate receptors were required for the LTD enhancement, and N-methyl D-aspartate receptors were required for the spine loss. Co-administering antibodies to the Aβ N-terminus prevented the LTP and LTD deficits, whereas antibodies to the midregion or C-terminus were less effective. Insoluble amyloid plaque cores from Alzheimer's disease cortex did not impair LTP unless they were first solubilized to release Aβ dimers, suggesting that plaque cores are largely inactive but sequester Aβ dimers that are synaptotoxic. We conclude that soluble Aβ oligomers extracted from Alzheimer's disease brains potently impair synapse structure and function and that dimers are the smallest synaptotoxic species.
19475602
High-Level Neuronal Expression of Aβ1–42 in Wild-Type Human Amyloid Protein Precursor Transgenic Mice: Synaptotoxicity without Plaque Formation
Amyloid plaques are a neuropathological hallmark of Alzheimer's disease (AD), but their relationship to neurodegeneration and dementia remains controversial. In contrast, there is a good correlation in AD between cognitive decline and loss of synaptophysin-immunoreactive (SYN-IR) presynaptic terminals in specific brain regions. We used expression-matched transgenic mouse lines to compare the effects of different human amyloid protein precursors (hAPP) and their products on plaque formation and SYN-IR presynaptic terminals. Four distinct minigenes were generated encoding wild-type hAPP or hAPP carrying mutations that alter the production of amyloidogenic Aβ peptides. The platelet-derived growth factor β chain promoter was used to express these constructs in neurons. hAPP mutations associated with familial AD (FAD) increased cerebral Aβ1–42 levels, whereas an experimental mutation of the β-secretase cleavage site (671M→I) eliminated production of human Aβ. High levels of Aβ1–42 resulted in age-dependent formation of amyloid plaques in FAD-mutant hAPP mice but not in expression-matched wild-type hAPP mice. Yet, significant decreases in the density of SYN-IR presynaptic terminals were found in both groups of mice. Across mice from different transgenic lines, the density of SYN-IR presynaptic terminals correlated inversely with Aβ levels but not with hAPP levels or plaque load. We conclude that Aβ is synaptotoxic even in the absence of plaques and that high levels of Aβ1–42are insufficient to induce plaque formation in mice expressing wild-type hAPP. Our results support the emerging view that plaque-independent Aβ toxicity plays an important role in the development of synaptic deficits in AD and related conditions.
622633
Altered expression of synaptic proteins occurs early during progression of Alzheimer’s disease
The expression levels of three synaptic proteins (synaptophysin, synaptotagmin, and growth-associated protein 43 [GAP43]) in AD cases clinically classified by Clinical Dementia Rating (CDR) score were analyzed. Compared with control subjects (CDR = 0), mild (early) AD (CDR = 0.5 to 1) cases had a 25% loss of synaptophysin immunoreactivity. Levels of synaptotagmin and GAP43 were unchanged in mild AD, but cases with CDR of >1 had a progressive decrement in these synaptic proteins. Thus, synaptic injury in frontal cortex is an early event in AD.
13023761
Structure-function implications in Alzheimer's disease: effect of Abeta oligomers at central synapses.
Alzheimer's disease (AD) is the most prevalent neurodegenerative disease in the growing population of elderly people. A characteristic of AD is the accumulation of plaques in the brain of AD patients, and theses plaques mainly consist of aggregates of amyloid beta-peptide (Abeta). All converging lines of evidence suggest that progressive accumulation of the Abeta plays a central role in the genesis of Alzheimer's disease and it was long understood that Abeta had to be assembled into extracellular amyloid fibrils to exert its cytotoxic effects. This process could be modulated by molecular chaperones which inhibit or accelerate the amyloid formation. The enzyme Acetylcholinesterase (AChE) induces Abeta fibrils formation, forming a stable complex highly neurotoxic. On the other hand, laminin inhibit the Abeta fibrils formation and depolymerizate Abeta fibrils also. Over the past decade, data have emerged from the use of several sources of Abeta (synthetic, cell culture, transgenic mice and human brain) to suggest that intermediate species called Abeta oligomers are also injurious. Accumulating evidence suggests that soluble forms of Abeta are indeed the proximate effectors of synapse loss and neuronal injury. On the other hand, the member of the Wnt signaling pathway, beta-catenin was markedly reduced in AD patients carrying autosomal dominant PS-1. Also, neurons incubated with Abeta revealed a significant dose-dependent decrease in the levels of cytosolic beta-catenin an effect which was reversed in cells co-incubated with increasing concentrations of lithium, an activator of Wnt signaling pathway. Wnt signaling blocks the behavioural impairments induced by hippocampal injection of Abeta, therefore the activation of Wnt signaling protects against the Abeta neurotoxicity. Here we review recent progress about Abeta structure and function, from the formation of amyloid fibrils and some molecular chaperones which modulate the amyloidogenesic process to synaptic damage induce by Abeta oligomers.
4409056
Cellular Prion Protein Mediates Impairment of Synaptic Plasticity by Amyloid-β Oligomers
A pathological hallmark of Alzheimer’s disease is an accumulation of insoluble plaque containing the amyloid-β peptide of 40–42 amino acid residues. Prefibrillar, soluble oligomers of amyloid-β have been recognized to be early and key intermediates in Alzheimer’s-disease-related synaptic dysfunction. At nanomolar concentrations, soluble amyloid-β oligomers block hippocampal long-term potentiation, cause dendritic spine retraction from pyramidal cells and impair rodent spatial memory. Soluble amyloid-β oligomers have been prepared from chemical syntheses, transfected cell culture supernatants, transgenic mouse brain and human Alzheimer’s disease brain. Together, these data imply a high-affinity cell-surface receptor for soluble amyloid-β oligomers on neurons—one that is central to the pathophysiological process in Alzheimer’s disease. Here we identify the cellular prion protein (PrPC) as an amyloid-β-oligomer receptor by expression cloning. Amyloid-β oligomers bind with nanomolar affinity to PrPC, but the interaction does not require the infectious PrPSc conformation. Synaptic responsiveness in hippocampal slices from young adult PrP null mice is normal, but the amyloid-β oligomer blockade of long-term potentiation is absent. Anti-PrP antibodies prevent amyloid-β-oligomer binding to PrPC and rescue synaptic plasticity in hippocampal slices from oligomeric amyloid-β. Thus, PrPC is a mediator of amyloid-β-oligomer-induced synaptic dysfunction, and PrPC-specific pharmaceuticals may have therapeutic potential for Alzheimer’s disease.
25235019
Wnt-5a/JNK Signaling Promotes the Clustering of PSD-95 in Hippocampal Neurons*
During the formation of synapses, specific regions of pre- and postsynaptic cells associate to form a single functional transmission unit. In this process, synaptogenic factors are necessary to modulate pre- and postsynaptic differentiation. In mammals, different Wnt ligands operate through canonical and non-canonical Wnt pathways, and their precise functions to coordinate synapse structure and function in the mature central nervous system are still largely unknown. Here, we studied the effect of different Wnt ligands on postsynaptic organization. We found that Wnt-5a induces short term changes in the clustering of PSD-95, without affecting its total levels. Wnt-5a promotes the recruitment of PSD-95 from a diffuse dendritic cytoplasmic pool to form new PSD-95 clusters in dendritic spines. Moreover, Wnt-5a acting as a non-canonical ligand regulates PSD-95 distribution through a JNK-dependent signaling pathway, as demonstrated by using the TAT-TI-JIP peptide in mature hippocampal neurons. Finally, using adult rat hippocampal slices, we found that Wnt-5a modulates glutamatergic synaptic transmission through a postsynaptic mechanism. Our studies indicate that the Wnt-5a/JNK pathway modulates the postsynaptic region of mammalian synapse directing the clustering and distribution of the physiologically relevant scaffold protein, PSD-95.
6082549
Wnt5a regulates directional cell migration and cell proliferation via Ror2-mediated noncanonical pathway in mammalian palate development
Tissue and molecular heterogeneities are present in the developing secondary palate along the anteroposterior (AP) axis in mice. Here, we show that Wnt5a and its receptor Ror2 are expressed in a graded manner along the AP axis of the palate. Wnt5a deficiency leads to a complete cleft of the secondary palate, which exhibits distinct phenotypic alterations at histological, cellular and molecular levels in the anterior and posterior regions of the palate. We demonstrate that there is directional cell migration within the developing palate. In the absence of Wnt5a, this directional cell migration does not occur. Genetic studies and in vitro organ culture assays further demonstrate a role for Ror2 in mediating Wnt5a signaling in the regulation of cell proliferation and migration during palate development. Our results reveal distinct regulatory roles for Wnt5a in gene expression and cell proliferation along the AP axis of the developing palate, and an essential role for Wnt5a in the regulation of directional cell migration.
20165122
PSD-95 and PKC converge in regulating NMDA receptor trafficking and gating
Neuronal NMDA receptors (NMDARs) colocalize with postsynaptic density protein-95 (PSD-95), a putative NMDAR anchoring protein and core component of the PSD, at excitatory synapses. PKC activation and PSD-95 expression each enhance NMDAR channel opening rate and number of functional channels at the cell surface. Here we show in Xenopus oocytes that PSD-95 and PKC potentiate NMDA gating and trafficking in a nonadditive manner. PSD-95 and PKC each enhance NMDA channel activity, with no change in single-channel conductance, reversal potential or mean open time. PSD-95 and PKC each potentiate NMDA channel opening rate (kβ) and number of functional channels at the cell surface (N), as indicated by more rapid current decay and enhanced charge transfer in the presence of the open channel blocker MK-801. PSD-95 and PKC each increase NMDAR surface expression, as indicated by immunofluorescence. PKC potentiates NMDA channel function and NMDAR surface expression to the same final absolute values in the absence or presence of PSD-95. Thus, PSD-95 partially occludes PKC potentiation. We further show that Ser-1462, a putative phosphorylation target within the PDZ-binding motif of the NR2A subunit, is required for PSD-95-induced potentiation and partial occlusion of PKC potentiation. Coimmunoprecipitation experiments with cortical neurons in culture indicate that PKC activation promotes assembly of NR2 with NR1, and that the newly assembled NMDARs are not associated with PSD-95. These findings predict that synaptic scaffolding proteins and protein kinases convergently modulate NMDAR gating and trafficking at synaptic sites.
20035299
Wnt signaling through Dishevelled, Rac and JNK regulates dendritic development
Dendritic arborization is required for proper neuronal connectivity. Rho GTPases have been implicated in the regulation of dendrite development. However, the signaling pathways that impinge on these molecular switches remain poorly understood. Here we show that Wnt7b, which is expressed in the mouse hippocampus, increases dendritic branching in cultured hippocampal neurons. This effect is mimicked by the expression of Dishevelled (Dvl) and is blocked by Sfrp1, a secreted Wnt antagonist. Consistent with these findings, hippocampal neurons from mice lacking Dvl1 show reduced dendritic arborization. Activation of the canonical Wnt-Gsk3β pathway does not affect dendritic development. In contrast, Wnt7b and Dvl activate Rac and JNK in hippocampal neurons. Dominant-negative Rac, dominant-negative JNK or inhibition of JNK blocks Dvl-mediated dendritic growth. These findings demonstrate a new function for the non-canonical Wnt pathway in dendrite development and identify Dvl as a regulator of Rho GTPases and JNK during dendritic morphogenesis.
39493621
Nicotinic receptor subtypes in the developing chick brain: appearance of a species containing the alpha4, beta2, and alpha5 gene products.
Increasing evidence suggests nicotinic receptors regulate developmental events in the nervous system. We used [3H]epibatidine and 125I-alpha-bungarotoxin, together with subunit-specific monoclonal antibodies, to distinguish and quantify nicotinic receptor subtypes in developing chick brain. The results show that more than three fourths of the epibatidine-binding receptors at both early and late embryonic stages contain alpha4 and beta2 subunits, representing receptors previously distinguished by high affinity nicotine binding. A fraction of these also contain the alpha5 gene product, which is consistent with studies on transfected cells showing that the alpha4, beta2, and alpha5 gene products coassemble to produce epibatidine-binding receptors. A small portion of the receptors contain alpha3 and beta4 subunits, assembled in part with either alpha4 or beta2 subunits. The most abundant nicotinic receptors, however, at both early and late embryonic stages are those having high affinity for alpha-bungarotoxin rather than epibatidine. Most contain alpha7 subunits, whereas about half contain alpha8 subunits as well. The sharpest developmental increase between embryonic days 8 and 17/18 occurs with receptors containing alpha5 subunits, whereas receptors containing alpha3 or beta4 subunits undergo no specific increase. The three major receptor species (containing alpha4 and beta2 but not alpha5 subunits; alpha7 subunits; or alpha7 and alpha8 subunits) each increase approximately 3-fold during the same period. The results indicate greater receptor complexity than appreciated previously; they provide information about the rules governing subunit assembly in neuronal nicotinic receptors and draw attention to the role of alpha5 subunits in late development.
25724572
Wnt signaling function in Alzheimer’s disease
Alzheimer's disease (AD) is a neurodegenerative disease with progressive dementia accompanied by three main structural changes in the brain: diffuse loss of neurons; intracellular protein deposits termed neurofibrillary tangles (NFT) and extracellular protein deposits termed amyloid or senile plaques, surrounded by dystrophic neurites. Two major hypotheses have been proposed in order to explain the molecular hallmarks of the disease: The 'amyloid cascade' hypothesis and the 'neuronal cytoskeletal degeneration' hypothesis. While the former is supported by genetic studies of the early-onset familial forms of AD (FAD), the latter revolves around the observation in vivo that cytoskeletal changes - including the abnormal phosphorylation state of the microtubule associated protein tau - may precede the deposition of senile plaques. Recent studies have suggested that the trafficking process of membrane associated proteins is modulated by the FAD-linked presenilin (PS) proteins, and that amyloid beta-peptide deposition may be initiated intracellularly, through the secretory pathway. Current hypotheses concerning presenilin function are based upon its cellular localization and its putative interaction as macromolecular complexes with the cell-adhesion/signaling beta-catenin molecule and the glycogen synthase kinase 3beta (GSK-3beta) enzyme. Developmental studies have shown that PS proteins function as components in the Notch signal transduction cascade and that beta-catenin and GSK-3beta are transducers of the Wnt signaling pathway. Both pathways are thought to have an important role in brain development, and they have been connected through Dishevelled (Dvl) protein, a known transducer of the Wnt pathway. In addition to a review of the current state of research on the subject, we present a cell signaling model in which a sustained loss of function of Wnt signaling components would trigger a series of misrecognition events, determining the onset and development of AD.
7966455
Acetylcholinesterase promotes the aggregation of amyloid-beta-peptide fragments by forming a complex with the growing fibrils.
Acetylcholinesterase (AChE), an enzyme involved in the hydrolysis of the neurotransmitter acetylcholine, consistently colocalizes with the amyloid deposits characteristic of Alzheimer's disease and may contribute to the generation of amyloid proteins and/or physically affect fibril assembly. In order to identify the structural domains of the amyloid-beta-peptide (Abeta) involved in the aggregation induced by AChE, we have studied the effect of this cholinergic enzyme on Abeta peptide fragments of different sizes. AChE enhanced the aggregation of the Abeta(12-28) and Abeta(25-35) peptides but not of the Abeta(1-16) fragment. The inductive effect of AChE on the aggregation of Abeta(12-28) was abolished by the presence of either Abeta(1-16) or Abeta(9-21). The effect of the enzyme was also analysed using two different mutant fragments, possessing a low and the other a high capacity for fibrillogenesis. The fragments used were Abeta(12-28)Val18-->Ala and Abeta(12-28)Glu22-->Gln, respectively. AChE was able to promote the aggregation of these fragments in a very specific way and both mutant peptides were able to form amyloid fibrils, as revealed by negative staining under the electron microscope. Binding assays indicated that AChE was bound to Abeta(12-28), as well as to the Abeta(1-16) peptide. AChE was seen to form strong complexes with the Abeta(12-28) fibrils as such complexes stained positively for both thioflavine-T and AChE activity, were resistant to high ionic strength treatment, and were partially sensitive to detergents, suggesting that hydrophobic interactions may play a role in the stabilization of the AChE-Abeta complex. Our results suggest that such amyloid-AChE complexes are formed when AChE interacts with the growing amyloid fibrils and accelerates the assembly of Abeta peptides. This is consistent with the fact that AChE is known to be present within Abeta deposits including the pre-amyloid diffuse and mature senile plaques found in Alzheimer's brain.
26637701
Cellular mechanisms underlying the rhythmic bursts induced by NMDA microiontophoresis at the apical dendrites of CA1 pyramidal neurons
This article reports the cellular mechanisms underlying a form of intracellular “theta‐like” (θ‐like) rhythm evoked in vitro by microiontophoresis of N‐methyl‐D‐aspartate (NMDA) at the apical dendrites of CA1 pyramidal neurons. Rhythmic membrane potential (Vm) oscillations and action potential (AP) bursts (≈6 Hz; ≈20 mV; ≈2–5 APs) were evoked in all cells. The response lasted ≈2 s, and the initial oscillations were usually small (<20 mV) and below AP threshold. Rhythmic bursts were never evoked by imposed depolarization in the absence of NMDA. Block of Na+ conductance with tetrodotoxin (TTX) (1.5 μM), of non‐NMDA receptors with 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX) (20 μM) and of synaptic inhibition by bicuculline (50 μM) and picrotoxin (50 μM) did not prevent NMDA oscillation. Inhibition of the voltage dependence of the NMDA conductance in Mg2+‐free Ringer's solution blocked oscillations. Preventing Ca2+ influx with Ca2+‐free and Co2+ (2‐mM) solutions and block of the slow Ca2+‐dependent afterhyperpolarization (sAHP) by carbamilcholine (5 μM), isoproterenol (10 μM), and intracellular BAPTA blocked NMDA oscillations. Inhibition of L‐type Ca2+ conductance with nifedipine (30 μM) reduced oscillation amplitude. Block of tetraethylammonium (TEA) (10 mM) and 4AP (10 mM)‐sensitive K+ conductance increased the duration and amplitude, but not the frequency, of oscillations. In conclusion, θ‐like bursts relied on the voltage dependence of the NMDA conductance and on high‐threshold Ca2+ spikes to initiate and boost the depolarizing phase of oscillations. The repolarization is initiated by TEA‐sensitive K+ conductance and is controlled by the sAHP. These results suggest a role of interactions between NMDA conductance and intrinsic membrane properties in generating the CA1 θ‐rhythm. Hippocampus 2003;13:10067. © 2003 Wiley‐Liss, Inc.
14648241
The SAD-1 Kinase Regulates Presynaptic Vesicle Clustering and Axon Termination
During synapse formation, presynaptic axon outgrowth is terminated, presynaptic clusters of vesicles are associated with active zone proteins, and active zones are aligned with postsynaptic neurotransmitter receptors. We report here the identification of a novel serine/threonine kinase, SAD-1, that regulates several aspects of presynaptic differentiation in C. elegans. In sad-1 mutant animals presynaptic vesicle clusters in sensory neurons and motor neurons are diffuse and disorganized. Sensory axons fail to terminate in sad-1 mutants, whereas overexpression of SAD-1 causes sensory axons to terminate prematurely. SAD-1 protein is expressed in the nervous system and localizes to synapse-rich regions of the axons. SAD-1 is related to PAR-1, a kinase that regulates cell polarity during asymmetric cell division. Overexpression of SAD-1 causes mislocalization of vesicle proteins to dendrites, suggesting that sad-1 affects axonal-dendritic polarity as well as synaptic development.
43764403
Functional characteristics of unmyelinated fibres in the hippocampal cortex
(1) In transverse hippocampal slices (350 micrometer thick), taken from guinea pigs initially anaesthetized with ether, intracortical afferent fibres were activated by small current pulses delivered through tungsten microelectrodes. Extracellular potentials were recorded from the zone of activated fibres in dendritic layers while intracellular recordings were made from the soma of CA1 pyramidal cells. (2) When recording was made from the same level as the stimulating cathode, the extracellular potential consisted of a diphasic deflection followed by a larger negative wave with a superimposed population spike. The negative wave corresponded to an intracellularly recorded EPSP, and is called an extracellular EPSP, whereas the initial diphasic deflection had no intracellular counterpart. (3) The initial diphasic deflection was linearly related to the size of both the intracellular and extracellular EPSP. It was not changed by removal of calcium ions from the bathing fluid, whereas all postsynaptic activity disappeared. The diphasic deflection was propagated along fibres lying parallel to the pyramidal layer with a velocity of 0.3 m/sec. It could follow short bursts of stimulation at 300 Hz. The absolute refractory period was 2.0 msec. (4) The initial diphasic deflection is interpreted as the compound action potential of the largely unmyelinated afferent fibres to the CA1 neurones.