Abstract: ABSTRACT: BACKGROUND: Familial amyloidotic polyneuropathy (FAP) is a peripheral neuropathy caused by the extracellular accumulation and deposition of insoluble transthyretin (TTR) aggregates. However the molecular mechanism that underlies TTR toxicity in peripheral nerves is unclear. Previous studies have suggested that amyloidogenic proteins can aggregate into oligomers which disrupt intracellular calcium homeostasis by increasing the permeability of the plasma membrane to extracellular calcium. The aim of the present study was to examine the effect of TTR on calcium influx in dorsal root ganglion neurons. RESULTS: Levels of intracellular cytosolic calcium were monitored in dorsal root ganglion (DRG) neurons isolated from embryonic rats using the calcium-sensitive fluorescent indicator Fluo4. An amyloidogenic mutant form of TTR, L55P, induced calcium influx into the growth cones of DRG neurons, whereas wild-type TTR had no significant effect. Atomic force microscopy and dynamic light scattering studies confirmed that the L55P TTR contained oligomeric species of TTR. The effect of L55P TTR was decreased by blockers of voltage-gated calcium channels (VGCC), as well as by blockers of Nav1.8 voltage-gated sodium channels and transient receptor potential M8 (TRPM8) channels. siRNA knockdown of TRPM8 channels using three different TRPM8 siRNAs strongly inhibited calcium influx in DRG growth cones. CONCLUSIONS: These data suggest that activation of TRPM8 channels triggers the activation of Nav1.8 channels which leads to calcium influx through VGCC. We suggest that TTR-induced calcium influx into DRG neurons may contribute to the pathophysiology of FAP. Furthermore, we speculate that similar mechanisms may mediate the toxic effects of other amyloidogenic proteins such as the beta-amyloid protein of Alzheimer's disease.
Abstract: 1. Familial amyloid polyneuropathies (FAP) constitute a group of inherited amyloidoses that affect peripheral nerves. One common form of FAP is caused by transthyretin (TTR) misfolding and deposition in the peripheral nervous system, leading to neuronal toxicity and death. 2. The molecular mechanisms responsible for this toxicity are unclear, however there is good biochemical and histopathological evidence that the toxicity of TTR mutations is correlated to their aggregation state. In addittion, neuronal calcium dysregulation is a mechanism that has been suggested to drive the pathogenesis of FAP. 3. Amyloidogenic TTR mutations cause significant calcium influx via L-type calcium channels in neuronal cell lines, while in primary sensory neurons, TTR mediates a calcium influx via a novel mechanism of transient receptor potential melanostatin (TRPM8) and voltage-gated sodium and calcium channel activation. 4. Significantly, calcium dysregulation is a pathological hallmark of other neurodegenerative diseases involving amyloidosis, for example Alzheimer's disease, and this mechanism could explain the molecular events that drive amyloid toxicity in other neurodegenerative diseases.
Abstract: The beta-site APP cleaving enzyme 1 (BACE1) is a major target for drug design in Alzheimer's disease. BACE1 binds strongly to heparin and other glycosaminoglycans, and there is evidence that the enzyme may interact with proteoglycans in vivo. Several studies suggest that heparin or heparan sulfate analogues may have value as therapeutic agents for the treatment of AD.
Abstract: The beta-site APP cleaving enzyme (BACE1) is responsible for the first step in the production of the beta-amyloid protein of Alzheimer's disease. BACE1 is synthesized as a partially active zymogen (proBACE1). We previously showed that the glycosaminoglycan (GAG) heparin can increase the enzyme activity of proBACE1. In this study, the structural requirements and the mechanism for the GAG-induced activation were examined. The effect of heparin on proBACE1 was influenced by the degree of sulfation and carboxylation of the GAG, as well as by the length of the sugar. Although low molecular weight heparin fragments did not strongly stimulate proBACE1, they inhibited heparin-induced activation of the enzyme. The structure of the zymogen was modeled using the known X-ray structures of the BACE1 catalytic domain and the homologous prodomain of porcine pepsinogen. The modeled structure suggested that a heparin-binding domain may reside close to the prodomain, and that movement of a loop region between residues 46-65, lying adjacent to the prodomain, may be needed to accommodate heparin binding. The presence of the loop domain adjacent to the active site may account for the lower activity of the zymogen relative to the mature enzyme. Movement of the loop region upon heparin binding could expose the active site region to allow for increased substrate binding. The results suggest a model in which conformational changes close to the prodomain may be involved in the mechanism of heparin-induced activation of proBACE1.
Abstract: Alzheimer's disease (AD) is characterized by the extracellular deposition of the beta-amyloid protein (Abeta). Abeta is a fragment of a much larger precursor protein, the amyloid precursor protein (APP). Sequential proteolytic cleavage of APP by beta-secretase and gamma-secretase liberates Abeta from APP. The aspartyl protease BACE1 (beta-site APP-cleaving enzyme 1) catalyses the rate-limiting step in the production of Abeta, and as such it is considered to be a major target for drug development in Alzheimer's disease. However, the development of a BACE1 inhibitor therapy is problematic for two reasons. First, BACE1 has been found to have important physiological roles. Therefore, inhibition of the enzyme could have toxic consequences. Second, the active site of BACE1 is relatively large, and many of the bulky compounds that are needed to inhibit BACE1 activity are unlikely to cross the blood-brain barrier. This review focuses on the structure BACE1, current therapeutic strategies based on developing active-site inhibitors, and new approaches to therapy involving targeting the expression or post-translational regulation of BACE1.
Abstract: alpha-Amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid receptors (AMPARs) are key regulators of synaptic function and cognition. In Alzheimer's disease (AD), cell-surface AMPARs are downregulated, however the reason for this downregulation is not clear. In the present study, we found that Abeta significantly decreased levels of the cell-surface AMPA-type glutamate receptor subunit 2 (GluR2), and increased the concentration of free cytosolic calcium ion ([Ca2+]i) in hippocampal neurons. Ion channel blockers (nifedipine, tetrodotoxin, SKF96365) decreased [Ca2+ and increased the level of cell-surface GluR2, whereas Bay K 8644, an activator of L-type voltage-gated calcium channels increased [Ca2+]i and decreased cell-surface GluR2. Abeta and Bay K 8644 increased phosphorylation of serine-880 (S880) on GluR2, whereas the nifedipine. tetrodotoxin and SKF96365 decreased S880 phosphorylation. Finally, we found that bisindolylmeimide I (GF 109203X, GFX), an inhibitor of protein kinase C (PKC) blocked both the decrease in cell-surface GluR2 and the increase in phospho-S880 induced by Abeta and Bay K 8644. Taken together, these results demonstrate that Abeta decreases cell-surface GluR2 by increasing PKC-mediated phosphorylation of S880. Our study supports the view that a rise in cytosolic [Ca2+]i induced by Abeta could impair synaptic function by decreasing the availability of AMPARs at the synapse. This decrease in AMPARs may contribute to the decline in cognitive function seen in AD.
Abstract: Acute axonal shear and stretch in the brain induces an evolving form of axonopathy and is a major cause of ongoing motor, cognitive and emotional dysfunction. We have utilized an in vitro model of mild axon bundle stretch injury, in cultured primary cortical neurons, to determine potential early critical cellular alterations leading to secondary axonal degeneration. We determined that transient axonal stretch injury induced an initial acute increase in intracellular calcium, principally derived from intracellular stores, which was followed by a delayed increase in calcium over 48 h post-injury (PI). This progressive and persistent increase in intracellular calcium was also associated with increased frequency of spontaneous calcium fluxes as well as cytoskeletal abnormalities. Additionally, at 48 h post-injury, stretch-injured axon bundles demonstrated filopodia-like sprout formation that preceded secondary axotomy and degeneration. Pharmacological inhibition of the calcium-activated phosphatase, calcineurin, resulted in reduced secondary axotomy (p < 0.05) and increased filopodial sprout length. In summary, these results demonstrate that stretch injury of axons induced an initial substantial release of calcium from intracellular stores with elevated intracellular calcium persisting over 2 days. These long-lasting calcium alterations may provide new insight into the earliest neuronal abnormalities that follow traumatic brain injury as well as the key cellular changes that lead to the development of diffuse axonal injury and secondary degeneration.
Abstract: Alzheimer's disease (AD) is caused by the accumulation of amyloid-β (Aβ), which induces progressive decline in learning, memory, and other cognitive functions. Aβ is a neurotoxic protein that disrupts calcium signaling in neurons and alters synaptic plasticity. These effects lead to loss of synapses, neural network dysfunction, and inactivation of neuronal signaling. However, the precise mechanism by which Aβ causes neurodegeneration is still not clear, despite decades of intensive research. The role of astrocytes in early cognitive decline is a major component of disease pathology that has been poorly understood. Recent research suggests that astrocytes are not simply passive support cells for neurons, but are active participants in neural information processing in the brain. Aβ can disrupt astrocytic calcium signaling and gliotransmitter release, processes that are vital for astrocyte-neuron communication. Therefore, astrocyte dysfunction may contribute to the earliest neuronal deficits in AD. Here we discuss emerging concepts in glial biology and the implications of astrocyte dysfunction on neurodegeneration in AD.
Abstract: Aggregation of beta-amyloid protein (Abeta) to form oligomers is considered to be a key step in generating neurotoxicity in the Alzheimer's disease brain. Agents that bind to Abeta and inhibit oligomerization have been proposed as Alzheimer's disease therapeutics. In this study, we investigated the binding of fluorescein-labeled Abeta(1-42) (FluoAbeta(1-42)) to SH-SY5Y neuroblastoma cells and examined the effect of the 39-kDa receptor-associated protein (RAP), on the Abeta cell interaction. FluoAbeta(1-42) bound to the cells in a punctate pattern. Surprisingly, when RAP was added to the incubations, FluoAbeta(1-42) and RAP were found to be co-localized on the cell surface, suggesting that RAP and Abeta may bind to each other. Experiments using the purified proteins confirmed that a RAP-Abeta complex was stable and resistant to sodium dodecyl sulfate. RAP also inhibited Abeta oligomerization. We next examined whether RAP could inhibit the neurotoxic effects of Abeta. Addition of Abeta(1-42) to SH-SY5Y cells caused an increase in intracellular Ca2+ that was inhibited by treatment of the Abeta peptide with RAP. RAP also blocked an Abeta-induced inhibition of long-term memory consolidation in 1-day-old chicks. This study demonstrates that RAP binds to Abeta and is an inhibitor of the neurotoxic effects of Abeta.
Abstract: Although many of the biochemical mechanisms which regulate production or clearance of the amyloid-beta protein (Abeta) of Alzheimer's disease (AD) are now well understood, the mechanism of Abeta neurotoxicity remains unclear. A number of studies have shown that Abeta can disrupt neuronal Ca(2+) homeostasis by inducing influx of extracellular Ca(2+) into the neuronal cytoplasm. Ca(2+) is known to play an important role in neuronal excitability, synaptic plasticity and neurotoxicity. Therefore, Abeta-induced Ca(2+) dysregulation may contribute to many of the cognitive and neuropathologic features of AD. In vitro studies show that Abeta can increase ion permeability in lipid membranes. This increased permeability is reportedly associated with the formation of artificial ion pores formed from Abeta oligomers. However, a number of other studies show that Abeta can activate endogenous ion channels on the cell surface. There is also increasing evidence that presenilin mutations alter intracellular Ca(2+) stores. It is likely that elucidation of the mechanism by which Abeta and presenilin cause Ca(2+) dysregulation in neurons will help to identify new drug targets for the treatment of AD.
Abstract: Homer proteins are best known as scaffold proteins at the post-synaptic density where they facilitate synaptic signalling and are thought to be required for learning and memory. Evidence implicating Homer proteins in the development of the nervous system is also steadily accumulating. Homer is highly conserved and is expressed at key developmental time points in the nervous system of several species. Homer regulates intracellular calcium homeostasis, clustering and trafficking of receptors and proteins at the cytosolic surface of the plasma membrane, transcription and translation, and cytoskeletal organization. Each of these functions has obvious potential to regulate neuronal development, and indeed Homer is implicated in several pathologies associated with the developing nervous system. Current data justify more critical experimental approaches to the role of Homer in the developing nervous system and related neurological disorders.
Abstract: BACKGROUND: Homer proteins are post-synaptic density proteins with known functions in receptor trafficking and calcium homeostasis. While they are key mediators of synaptic plasticity, they are also known to function in axon guidance, albeit by mechanisms that are yet to be elucidated. Homer proteins couple extracellular receptors - such as metabotropic glutamate receptors and the transient receptor potential canonical family of cation channels - to intracellular receptors such as inositol triphosphate and ryanodine receptors on intracellular calcium stores and, therefore, are well placed to regulate calcium dynamics within the neural growth cone. Here we used growth cones from dorsal root ganglia, a well established model in the field of axon guidance, and a growth cone turning assay to examine Homer1 function in axon guidance. RESULTS: Homer1 knockdown reversed growth cone turning from attraction to repulsion in response to the calcium-dependent guidance cues brain derived neurotrophic factor and netrin-1. Conversely, Homer1 knockdown had no effect on repulsion to the calcium-independent guidance cue Semaphorin-3A. This reversal of attractive turning suggested a requirement for Homer1 in a molecular switch. Pharmacological experiments confirmed that the operational state of a calcium-calmodulin dependent protein kinase II/calcineurin phosphatase molecular switch was dependent on Homer1 expression. Calcium imaging of motile growth cones revealed that Homer1 is required for guidance-cue-induced rise of cytosolic calcium and the attenuation of spontaneous cytosolic calcium transients. Homer1 knockdown-induced calcium transients and turning were inhibited by antagonists of store-operated channels. In addition, immunocytochemistry revealed the close association of Homer1 with the store-operated proteins TRPC1 and STIM1 within dorsal root ganglia growth cones. CONCLUSION: These experiments provide evidence that Homer1 is an essential component of the calcium signalling repertoire within motile growth cones, regulating guidance-cue-induced calcium release and maintaining basal cytosolic calcium.
Abstract: Homer proteins are integral components of the postsynaptic density and are thought to function in synaptogenesis and plasticity. In addition, overexpression of Homer in the developing Xenopus retinotectal system results in axonal pathfinding errors. Here we report that Xenopus contains the homer1 gene, expressed as the isoform, xhomer1b, which is highly homologous to the mammalian homer1b. The mammalian homer1 gene is expressed as three isoforms, the truncated or short form homer1a and the long forms homer1b and -1c. For Xenopus, we cloned three very similar variants of homer1b, identified as Xenopus xhomer1b.1, xhomer1b.2, and xhomer1b.3, which display up to 98% homology with each other and 90% similarity to mammalian homer1b. Furthermore, we demonstrate that Xenopus also contains a truncated form of the Homer1 protein, which could be induced by kainic acid injection and is likely homologous to the mammalian Homer1a. xHomer1b expression was unaffected by neuronal activity levels but was developmentally regulated. Within the brain, the spatial and temporal distributions of both Homer isoforms were similar in the neuropil and cell body regions. Homer1 was detected in motor axons. Differential distribution of the two isoforms was apparent: Homer1b immunoreactivity was prominent at junctions between soma and the ventricular surface; in the retina, the Mueller radial glia were immunoreactive for Homer1, but not Homer1b, suggesting the retinal glia contain only the Homer1a isoform. Homer1b expression in muscle was prominent throughout development and was aligned with the actin striations in skeletal muscle. The high level of conservation of the xhomer1 gene and the protein expression in the developing nervous system suggest that Homer1 expression may be important for normal neuronal circuit development.
Abstract: The zebrafish, (Danio rerio) is an important model organism for the analysis of molecular mechanisms that govern neuronal circuit development. The neuronal circuitry that mediates olfaction is crucial for the development and survival of all teleost fishes. In concert with other sensory systems, olfaction is functional at early stages in zebrafish development and mediates important behavioral and survival strategies in the developing larva. Odorant cues are transduced by an array of signaling molecules from receptors in olfactory sensory neurons. The scaffolding protein family known as Homer is well placed to orchestrate this signaling cascade by interacting with and coupling membrane bound receptors to cytosolic signaling partners. To date, Homer has not been demonstrated in the zebrafish. Here we report that the Homer 1b/c isoform was prominent in the olfactory system from the earliest stages of differentiation. We describe the spatial and temporal distribution of Homer in the zebrafish olfactory system. At 24 hours post fertilization (hpf), Homer expression delineated the boundary of the presumptive olfactory placode. Subsequent expression steadily increased throughout the developing olfactory placode, with a prominent localization to the dendritic knobs of the olfactory sensory neurons. Homer expression in the developing olfactory bulb was punctate and prominent in the glomeruli, displaying an apparent synaptic localization. This work supports the hypothesis that Homer is an important molecule in neuronal circuit development, necessary for crucial behaviors required for development and survival.
Abstract: A moderate resolution single nucleotide polymorphism (SNP) map of the genome of Drosophila melanogaster that is designed for use in quantitative genetic mapping is described. Seventeen approximately 500 nucleotide gene sequences spaced at 10 to 20 centimorgan intervals were combined with 49 shorter sequence tag sites (STSs) at 5 to 10 centimorgan intervals to generate a map that should not leave any gaps greater than one half of a chromosome arm when any two wild type lines are compared. Of 20 markers with sufficient polymorphism to construct haplotype cladograms, 13 showed evidence for two divergent classes of haplotype. The possible mechanisms for and implications of the unexpected finding that two thirds of all short gene sequences in D. melanogaster may be dimorphic are discussed, including the suggestion that admixture between two separate lineages may have been a major event in the history of the species.
Abstract: Sequence analysis of 27 alleles of each of the three Ras-related genes in Drosophila melanogaster indicates that they all have low levels of polymorphism but may experience slightly different evolutionary pressures. No amino acid replacement substitutions were indicated in any of the sequences, or in the sibling species D. simulans and D. mauritiana. The Dras1 gene, which is the major ras homologue in Drosophila, has less within-species variation in D. melanogaster relative to the amount of divergence from the sibling species than does Dras2, although the contrast was not significant by the HKA test. Dras2 appears to be maintaining two classes of haplotype in D. melanogaster, one of which is closer to the alleles observed in the sibling species, suggesting that this is not likely to be a pseudogene despite the absence of a mutant phenotype. Although differences in level of expression may affect the function of the genes, it is concluded that genetic variation in the Ras signal transduction pathways cannot be attributed to catalytic variation in the Ras proteins.
Abstract: The signal transduction pathway controlling determination of the identity of the R7 photoreceptor in the Drosophila eye is shown to harbor high levels of naturally occurring genetic variation. The number of ectopic R7 cells induced by the dosage-sensitive SevS11.1 transgene that encodes a mildly activated form of the Sevenless tyrosine kinase receptor is highly sensitive to the wild-type genetic background. Phenotypes range from complete suppression to massive overproduction of photoreceptors that exceeds reported effects of known single gene modifiers, and are to some extent sex-dependent. Signaling from the dominant gain-of-function Drosophila Epidermal Growth Factor Receptor (DER-Ellipse) mutations is also sensitive to the genetic backgrounds, but there is no correlation with the effects on SevS11.1. This implies that different genes and/or alleles modify the two activated receptor genotypes. The evolutionary significance of the existence of high levels of genetic variation in the absence of normal phenotypic variation is discussed.
