Structural and mechanistic commonalities of amyloid-β and the prion protein

Amyloid β (Aβ) is a major causative agent of Alzheime disease. This neurotoxic peptide is generated as a result of the cleavage of the Amyloid-Precursor-Protein (APP) by the action of beta secretase and gamma secretase. The neurotoxicity was previously thought to be the result of aggregation. However, recent studies suggest that the interaction of Aβ with numerous cell surface receptors such as N-methyl-D-aspartate (NMDA), receptor for advanced glycosylation end products (RAGE), P75 neurotrophin receptor (P75NTR) as well as cell surface proteins such as the cellular prion protein (PrP^c) and heparan sulfate proteoglycans (HSPG) strongly enhances Aβ induced apoptosis and thereby contributes to neurotoxicity. This review focuses on the molecular mechanism resulting in Aβ-shedding as well as Aβ-induced apoptotic processes, genetic risk factors for familial Alzheimer disease and interactions of Aβ with cell surface receptors and proteins, with particular emphasis on the cellular prion protein. Furthermore, comparisons are drawn between Alzheimer disease and prion disorders and the role of laminin, an extracellular matrix protein, glycosaminoglycans and the 37 kDa/67 kDa laminin receptor (LRP/LR) have been highlighted with regards to both neurodegenerative diseases.     

Cellular prion protein null mice display normal AMPA receptor mediated long term depression

Cellular prion protein (PrPC) appears to be involved in numerous physiological processes. We have recently shown a novel modulation of NMDA receptors by PrPC that results in neuroprotection via silencing of NMDA receptors containing NR2D subunits, whereas no effects on AMPA receptor function could be observed (Khosravani et al. 2008, J Cell Biol. 181, 551). Here we show that PrP-null mice show a normal response to long-term depression stimuli requiring AMPA receptor activity, thus further supporting our previous findings of a selective action on NMDA receptors among ionotropic glutamate receptors.     

Role of prions in neuroprotection and neurodegeneration: A mechanism involving glutamate receptors?

There is increasing evidence that cellular prion protein plays important roles in neurodegeneration and neuroprotection. One of the possible mechanism by which this may occur is a functional inhibition of ionotropic glutamate receptors, including N-Methyl-D-Aspartate (NMDA) receptors. Here we review recent evidence implicating a possible interplay between NMDA receptors and prions in the context of neurodegenerative disorders. Such is a functional link between NMDA receptors and normal prion protein, and therefore possibly between these receptors and pathological prion isoforms, raises interesting therapeutic possibilities for prion diseases.Key words: NMDA, NR2D, glutamate, neuroprotection, calciumPrions are most often discussed in the context of transmissible spongiform encephalopathies (TSEs) which encompass a range of neurological disorders that include human Creutzfeldt-Jakob disease (among others), sheep scrapie and bovine spongiform encephalopathy.^1,2 It is well established that these disorders arise from a progressive conversion of the normal, mainly helical form of cellular prion protein (PrP^C) into a different PrP^Sc protein conformation with a high beta sheet content.³ In their PrP^Sc form, prions act as templates that catalyze misfolding of PrP^C to produce increasing levels of PrP^Sc, which likely represents several or even many different conformational states of the same source protein, resulting in diverse clinical phenotypes. This in turn leads to accumulation of PrP^Sc deposits in the brain that can appear as aggregates and amyloid-like plaques⁴ and which disrupt normal neurophysiology.⁵ While the neuropathology of TSE''s has been explored in great detail dating back to the 1920s,⁶ less effort has perhaps been expended on understanding the cellular and physiological function of PrP^C which is ubiquitously expressed, and found even in simple organisms.^5,7,8 A number of mouse lines either lacking PrP^C or overexpressing PrP^C have been created, including the widely used Zurich I PrP^C knockout strain.^9,10 Despite the wide distribution of PrP^C in the mammalian CNS, it perhaps surprisingly has only a relatively mild behavioral phenotype that appears to include some deficits in spatial learning at the behavioral level^11,12 as well as alterations in long term potentiation at the cellular level.^13–17 In addition, it has been shown that these mice show an increased excitability of hippocampal neurons.^13,18–20 In contrast, deletion of certain parts of the PrP^C protein in vivo can have serious physiological consequences. For example, deletion of a stretch of amino acids between just upstream of the octarepeat copper binding motifs produces a lethal phenotype, that can be rescued by overexpression of increasing levels of normal PrP^C.^21,22 Of particular note, these deletion mutants show degeneration of axons and myelin, both in the CNS and in peripheral nerves; indeed some mutants show a predilection for axomyelinic degeneration with little neuronal pathology,²¹ suggesting that certain mutated forms of PrP have a direct toxic effect on oligodendrocytes and/or myelin.²³ Moreover, activation of the Dpl1 gene in mice lacking PrP^C leads to an ataxic phenotype, that is not observed in the presence of PrP^C.²⁴ Collectively, this indicates that PrP^C may act in a protective capacity and in contrast, certain abnormal forms of PrP are “toxic”, promoting much more injury to various elements of the CNS and PNS than outright absence of wild-type PrP^C.This notion is further corroborated by a number of studies in PrP^C knockout mice, both in vivo and in cell culture models. Cultured hippocampal neurons from PrP^C null mice display greater apoptosis during oxidative stress.²⁵ Moreover, overexpression of PrP^C in rats protects them from neuronal damage during ischemic stroke, whereas PrP^C null mice show greater damage.^27–29 When PrP^C null mice are subjected to different types of seizure paradigms, they showed increased mortality and increased numbers of seizures.³⁰ This increased neuronal damage can be diminished by the NMDA receptor blocker MK-801,³¹ potentially implicating glutamate receptors in this process. Finally, it was recently shown that the absence of PrP^C protein protects neurons from the deleterious effects of beta amyloid, a protein involved in Alzheimer disease.³² It is important to note that NMDA receptors have been implicated in seizure disorders and in cell death during ischemic stroke.^33–35 Indeed, our own work has shown that NMDA receptors expressed endogenously in myelin contribute to myelin damage and may be one of the first steps leading to demyelination.³⁶ Furthermore, the NMDA receptor blocker memantine is used to treat Alzheimer disease, implicating NMDA receptors. The observations above suggest that there may be an interplay between NMDA receptor activity and the physiological function of PrP^C. In support of this hypothesis, our recent work has directly identified a common functional and molecular link between NMDA receptors and PrP^C.³⁷ Brain slices obtained from Zurich I PrP^C null mice showed an increased excitability of hippocampal slices, which could be ablated by blocking NMDA receptor activity with amino-5-phosphonovaleric acid. Removal of extracellular magnesium ions to enhance NMDA receptor activity resulted in stronger pro-excitatory effects in slices and cultured neurons from PrP^C null mice compared with those from normal animals. Synaptic recordings indicate that the amplitude and duration of NMDA mediated miniature synaptic currents is increased in PrP^C null mouse neurons, and evoked NMDA receptor currents show a dramatic slowing of deactivation kinetics in PrP^C null mouse neurons. The NMDA current kinetics observed in these neurons were qualitatively consistent with NMDA receptors containing the NR2D subunit.³⁸ Consistent with a possible involvement of NR2D containing receptors, siRNA knockdown of NR2D normalized current kinetics in PrP-null mouse neurons. Furthermore, a selective co-immunoprecipitation between PrP^C and the NR2D, but not NR2B subunits, was observed. This then may suggest the possibility that under normal circumstances, PrP^C serves to suppress NR2D function, but when PrP^C is absent, NR2D containing receptors become active, and because of their slow kinetics, may contribute to calcium overload under circumstances where excessive (or even normal) levels of glutamate are present. This would include conditions such as epileptic seizures, ischemia and Alzheimer disease, thus providing a possible molecular explanation for the link between PrP^C and neuroprotection under pathophysiological conditions. Indeed, NMDA promoted greater toxicity in PrP^C null mouse neurons, and upon injection into brains of PrP^C null mice. It is interesting to note that one of the major NMDA receptor subtypes expressed in myelin is NR2D, thus bridging the observations of Micu et al.³⁶ of NMDA receptor mediated cell death in ischemic white matter, and those of Baumann and colleagues²¹ showing that PrP^C deletion mutants can cause damage to myelin.How might PrP^C deletion mutants affect neuronal survival? One possibility may be that these deletion mutants compete with normal PrP^C for NMDA receptors, but are unable to functionally inhibit them. Alternatively, it is possible that the PrP^C deletion mutants, by virtue of binding to the receptors, may in fact increase receptor activity, thus causing increased cell death. In both cases, increasing the expression of normal PrP^C would be expected to outcompete the deletion variants, thus reestablishing the protective function. A similar mechanism could perhaps apply to TSEs. It is possible that the PrP^Sc form, perhaps in a manner reminiscent of the PrP^C deletion mutants, may be unable to inhibit NMDAR function, or perhaps would even enhance it. Any excess glutamate that may be released as a result of cell damage due to PrP^Sc aggregates, or even normally released amounts glutamate during the course of physiological neuronal signaling, could be sufficient to cause NMDAR mediated cell death and neuronal degeneration. In this context, it is interesting to note that chronic administration of the weakly NR2D selective inhibitor memantine delays death as a consequence of scrapie infection in mice.³⁹ In the context of Alzheimer disease, binding of PrP^C to beta amyloid may prevent the inhibitory action of PrP^C on NMDA receptor function, thus increasing NMDA receptor activity and promoting cell death. This then may perhaps explain the beneficial effects of memantine in the treatment of Alzheimer disease.In summary, despite the fact that PrP^C is one of the most abundantly expressed proteins in the mammalian CNS, its physiological role is uncertain. Recent observations from our labs have established an unequivocal functional link between normal prion protein and the ubiquitous excitatory NMDA receptor. Thus, one of the key physiological roles of PrP^C may be regulation of NMDA receptor activity. The presence of abnormal species of prion protein, whether acquired via “infection”, spontaneous conformational conversion or genetically inherited, may in turn alter normal function and regulation of NMDA receptors, leading to chronic “cytodegeneration” of elements in both gray and white matter regions of the CNS. This key functional link between PrP and glutamate receptors may provide our first opportunity for rational therapeutic design against the devastating spongiform encephalopathies and potentially other neurodegenerative disorders not traditionally considered as TSE''s.     

A closer look at prion strains

Prions are infectious proteins that are responsible for transmissible spongiform encephalopathies (TSEs) and consist primarily of scrapie prion protein (PrP^Sc), a pathogenic isoform of the host-encoded cellular prion protein (PrP^C). The absence of nucleic acids as essential components of the infectious prions is the most striking feature associated to these diseases. Additionally, different prion strains have been isolated from animal diseases despite the lack of DNA or RNA molecules. Mounting evidence suggests that prion-strain-specific features segregate with different PrP^Sc conformational and aggregation states.

Strains are of practical relevance in prion diseases as they can drastically differ in many aspects, such as incubation period, PrP^Sc biochemical profile (e.g., electrophoretic mobility and glycoform ratio) and distribution of brain lesions. Importantly, such different features are maintained after inoculation of a prion strain into genetically identical hosts and are relatively stable across serial passages.

This review focuses on the characterization of prion strains and on the wide range of important implications that the study of prion strains involves.     

Molecular advances in understanding inherited prion diseases

The prion diseases are neurodegenerative disorders that have attracted great interest because of the possible link between bovine spongiform encephalopathy (BSE) and variant Creutzfeldt-Jakob disease (CTD) in humans. Possible transmission of these diseases has been linked to a single protein termed the prion protein. This protein is an abnormal isoform of a normal synaptic glycoprotein. The majority of prion diseases does not appear to be caused by transmission of an infectious agent but occur spontaneously with no known cause. The strongest supporting evidence that the prion protein is the causative agent in prion disease comes from specific inheritable forms of prion disease which are linked to single point mutations in the prion protein gene. Paradoxically, these point mutations, although autosomal dominant with 100% penetrance do not lead to disease until late in life. Molecular techniques are now being used extensively to determine how these point-mutations alter the prion protein’s normal structure and activity. This review deals with the latest insights into how inherited mutations in the prion protein gene lead to neurodegenerative disease.     

<Emphasis Type="Italic">N</Emphasis>-Methyl-<Emphasis Type="SmallCaps">d</Emphasis>-aspartate Receptor Antagonists Have Variable Affect in 3-Nitropropionic Acid Toxicity

There is accumulating evidence that excitotoxicity and oxidative stress resulting from excessive activation of glutamate (N-methyl-d-aspartate) NMDA receptors are major participants in striatal degeneration associated with 3-nitropropionic acid (3NP) administration. Although excitotoxic and oxidative mechanisms are implicated in 3NP toxicity, there are conflicting reports as to whether NMDA receptor antagonists attenuate or exacerbate the 3NP-induced neurodegeneration. In the present study, we investigated the involvement of NMDA receptors in striatal degeneration, protein oxidation and motor impairment following systemic 3NP administration. We examined whether NMDA receptor antagonists, memantine and ifenprodil, influence the neurotoxicity of 3NP. The development of striatal lesion and protein oxidation following 3NP administration is delayed by memantine but not affected by ifenprodil. However, in behavioral experiments, memantine failed to improve and ifenprodil exacerbated the motor deficits associated with 3NP toxicity. Together, these findings suggest caution in the application of NMDA receptor antagonists as a neuroprotective agent in neurodegenerative disorders associated with metabolic impairment.     

Regulation of tight junction permeability by sodium caprate in human keratinocytes and reconstructed epidermis

It is now accepted that a conformational change of the cellular prion protein (PrP^C) generates the prion, the infectious agent responsible for lethal neurodegenerative disorders, named transmissible spongiform encephalopathies, or prion diseases. The mechanisms of prion-associated neurodegeneration are still obscure, as is the cell role of PrP^C, although increasing evidence attributes to PrP^C important functions in cell survival. Such a behavioral dichotomy thus enables the prion protein to switch from a benign role under normal conditions, to the execution of neurons during disease. By reviewing data from models of prion disease and PrP^C-null paradigms, which suggest a relation between the prion protein and Ca²⁺ homeostasis, here we discuss the possibility that Ca²⁺ is the factor behind the enigma of the pathophysiology of PrP^C. Ca²⁺ features in almost all processes of cell signaling, and may thus tell us much about a protein that pivots between health and disease.     

The first report of polymorphisms and genetic characteristics of the prion protein gene (PRNP) in horses

ABSTRACT

Prion diseases have a wide host range, but prion-infected cases have never been reported in horses. Genetic polymorphisms that can directly impact the structural stability of horse prion protein have not been investigated thus far. In addition, we noticed that previous studies focusing on horse-specific amino acids and secondary structure predictions of prion protein were performed for limited parts of the protein. In this study, we found genetic polymorphisms in the horse prion protein gene (PRNP) in 201 Thoroughbred horses. The identified polymorphism was assessed to determine whether this polymorphism impedes stability of protein using PolyPhen-2, PROVEAN and PANTHER. In addition, we evaluated horse-specific amino acids in horse and mouse prion proteins using same methods. We found only one single nucleotide polymorphism (SNP) in the horse prion protein, and three annotation tools predicted that the SNP is benign. In addition, horse-specific amino acids showed different effects on horse and mouse prion proteins, respectively.

Abbreviations: PRNP: prion protein gene; SNP: single nucleotide polymorphism; CJD: Creutzfeldt-Jakob disease; CWD: chronic wasting disease; TME: transmissible mink encephalopathy; FSE: feline spongiform encephalopathy; MD: molecular dynamics; ER: endoplasmic reticulum; GPI: glycosylphosphatidylinositol; NMR: nuclear magnetic resonance; ORF: open reading frame; GWAS: genome-wide association study; NAPA: non-adaptive prion amplification; HMM: hidden Markov model; NCBI: National Center for Biotechnology Information     

Biochemical studies of the structure and function of theN-methyl-D-aspartate subtype of glutamate receptors

TheN-methyl-D-aspartate (NMDA) subtype of glutamate receptors plays a key role in synaptic transmission, synaptic plasticity, synaptogenesis, and excitotocity in the mammalian central nervous system. The NMDA receptor channel is formed from two gene products from two glutamate receptor subunit families, termed NR1 and NR2. Although the subunit composition of native NMDA receptors is incompletely understood, electrophysiological studies using recombinant receptors suggest that functional NMDA receptors consist of heteromers containing combinations of NR1, which is essential for channel activity, and NR2, which modulates the properties of the channels. The lack of agonists or antagonists selective for a given subunit of NMDA receptors has made it difficult to understand the subunit expression, subunit composition, and posttranslational modification mechanisms of native NMDA receptors. Therefore, most studies on NMDA receptors that examine regional expression and ontogeny have been focused at the level of the mRNAs encoding the different subunits using northern blotting, ribonuclease protection, andin situ hybridization techniques. However, the data from these studies do not provide clear information about the resultant subunit protein. To directly examine the protein product of the NMDA receptor subunit genes, the development of subunit-specific antibodies using peptides and fusion proteins has provided a good approach for localizing, quantifying, and characterizing the receptor subunits in tissues and transfected cell lines, and to study the subunit composition and the functional effects of posttranslational processing of the NMDA subunits, particularly the phosphorylation profiles of NMDA glutamate receptors.     

The role of intracellular sodium in the regulation of NMDA-receptor-mediated channel activity and toxicity

Sodium (Na⁺) is the major cation in extracellular space and, with its entry into cells, may act as a critical intracellular second messenger that regulates many cellular functions. Through our investigations of mechanisms underlying the activity-dependent regulation of N-methyl-d-aspartate (NMDA) receptors, we recently characterized intracellular Na⁺ as a possible signaling factor common to processes underlying the upregulation of NMDA receptors by non-NMDA glutamate channels, voltage-gated Na⁺ channels, and remote NMDA receptors. Furthermore, although Ca²⁺ influx during the activation of NMDA receptors acts as a negative feedback mechanism that downregulates NMDA receptor activity, Na⁺ influx provides an essential positive feedback mechanism to overcome Ca²⁺-induced inhibition, thereby potentiating both NMDA receptor activity and inward Ca²⁺ flow. NMDA receptors may be recruited to cause excitoxicity through a Na⁺-dependent mechanism. Therefore, the further characterization of mechanisms underlying the regulation of NMDA receptors by intracellular Na⁺ is essential to understanding activity-dependent neuroplasticity in the nervous system.     

NMDA Receptors in Astrocytes

Astrocytes support glutamatergic neurotransmission in the central nervous system through multiple mechanisms which include: (i) glutamate clearance and control over glutamate spillover due to operation of glutamate transporters; (ii) supply of obligatory glutamate precursor glutamine via operation of glutamate–glutamine shuttle; (iii) supply of l-serine, the indispensable precursor of positive NMDA receptors neuromodulator d-serine and (iv) through overall homoeostatic control of the synaptic cleft. Astroglial cells express an extended complement of ionotropic and metabotropic glutamate receptors, which mediate glutamatergic input to astrocytes. In particular a sub-population of astrocytes in the cortex and in the spinal cord express specific type of NMDA receptors assembled from two GluN1, one GluN2C or D and one GluN3 subunits. This composition underlies low Mg²⁺ sensitivity thus making astroglial NMDA receptors operational at resting membrane potential. These NMDA receptors generate ionic signals in astrocytes and are linked to several astroglial homoeostatic molecular cascades.

     

N-Methyl-d-aspartate receptors in the retina

The vertebrate retina is a “genuine neural center” (Ramón y Cajal), in which glutamate is a major excitatory neurotransmitter. Both N-methyl-d-aspartate (NMDA) and non-NMDA receptors are expressed in the retina. Although non-NMDA receptors and/or metabotropic glutamate receptors are generally thought to be responsible for mediating the transfer of visual signals in the outer retina, there is recent evidence suggesting that NMDA receptors are also expressed in photoreceptors, as well as horizontal and bipolar cells. In the inner retina, NMDA receptors, in addition to other glutamate receptor subtypes, are abundantly expressed to mediate visual signal transmission from bipolar cells to amacrine and ganglion cells, and could be involved in modulation of inhibitory feedback from amacrine cells to bipolar cells. NMDA receptors are extrasynaptically expressed in ganglion cells (and probably amacrine cells) and may play physiological roles in a special mode. Activity of NMDA receptors may be modulated by neuromodulators, such as d-serine and others. This article discusses retinal excitotoxicity mediated by NMDA receptors.     

Binding of bovine T194A PrPC by PrPSc-specific antibodies

Transmissible spongiform encephalopathies (TSEs) are fatal neurodegenerative diseases that are based on the misfolding of a cellular prion protein (PrP^C) into an infectious, pathological conformation (PrP^Sc). There is proof-of-principle evidence that a prion vaccine is possible but this is tempered with concerns of the potential dangers associated with induction of immune responses to a widely-expressed self-protein. By targeting epitopes that are specifically exposed upon protein misfolding, our group developed a vaccine that induces PrP^Sc-specific antibody responses. Here we consider the ability of this polyclonal antibody (SN6b) to bind to a mutant of PrP^C associated with spontaneous prion disease. Polyclonal antibodies were selected to mimic the vaccination outcome and also explore all possible protein conformations of the recombinant bovine prion protein with mutation T194A [bPrP(T194A)]. This mutant is a homolog of the human T183A mutation of PrP^C that is associated with early onset of familial dementia. With nanopore analysis, under non-denaturing conditions, we observed binding of the SN6b antibody to bPrP(T194A). This interaction was confirmed through ELISAs as well as immunoprecipitation of the recombinant and cellularly expressed forms of bPrP(T194A). This interaction did not promote formation of a protease resistant conformation of PrP in vitro. Collectively, these findings support the disease-specific approach for immunotherapy of prion diseases but also suggest that the concept of conformation-specific immunotherapy may be complicated in individuals who are genetically predisposed to PrP^C misfolding.     

GABAA receptor subunit β1 is involved in the formation of protease-resistant prion protein in prion-infected neuroblastoma cells

γ-Aminobutyric acid type A (GABAA) receptor β1 (gabrb1), a subunit of GABAA receptors involved in inhibitory effects on neurotransmission, was found to associate with the formation of protease-resistant prion protein in prion-infected neuroblastoma cells. Silencing of gabrb1 gene expression significantly decreased the abnormal prion protein level but paradoxically increased the normal prion protein level. Treatment with a gabrb1-specific inhibitor, salicylidene salicylhydrazide, dose-dependently decreased the abnormal prion protein level, but silencing of other GABAA receptor subunits’ gene expression and treatments with the receptor antagonists and agonists did not. Therefore, gabrb1 involvement in abnormal prion protein formation is independent of GABAA receptors.     

Uncontrolled SFK-mediated protein trafficking in prion and Alzheimer's disease

Prions and Amyloid beta (Aβ) peptides induce synaptic damage via complex mechanisms that include the pathological alteration of intracellular signaling cascades. The host-encoded cellular prion protein (PrP^C) acts as a high-affinity cell surface receptor for both toxic species and it can modulate the endocytic trafficking of the N-methyl D-aspartate (NMDA) receptor and E-cadherin adhesive complexes via Src family kinases (SFKs). Interestingly, SFK-mediated control of endocytosis is a widespread mechanism used to regulate the activity of important transmembrane proteins, including neuroreceptors for major excitatory and inhibitory neurotransmitters. Here we discuss our recent work in zebrafish and accumulating evidence suggesting that subversion of this pleiotropic regulatory mechanism by Aβ oligomers and prions explains diverse neurotransmission deficits observed in human patients and mouse models of prion and Alzheimer's neurodegeneration. While Aβ, PrP^C and SFKs constitute potential therapeutic targets on their own, drug discovery efforts might benefit significantly from aiming at protein-protein interactions that modulate the endocytosis of specific SFK targets.     

Excitatory amino acid receptors in normal and abnormal vestibular function

Although excitatory amino acid (EAA) receptors have been investigated extensively in the limbic system and neocortex, less is known of the function of EAA receptors in the brainstem. A number of biochemical and electrophysiological studies suggest that the synapse between the ipsilateral vestibular (VIIIth) nerve and the brainstem vestibular nucleus (VN) is mediated by an EAA acting predominantly on kainate or alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) receptors. In addition, there is electrophysiological evidence that input from the contralateral vestibular nerve via the contralateral VN is partially mediated by N-methyl-D-aspartate (NMDA) receptors. Input to the VN from the spinal cord may also be partially mediated by NMDA receptors. All of the electrophysiological studies conducted so far have used in vitro preparations, and it is possible that denervation of the VN during the preparation of an explant or slice causes changes in EAA receptor function. Nonetheless, these results suggest that EAA receptors may be important in many different parts of the vestibular reflex pathways. Studies of the peripheral vestibular system have also shown that EAAs are involved in transmission between the receptor hair cells and the vestibular nerve fibers. A number of recent studies in the area of vestibular plasticity have reported that antagonists for the NMDA receptor subtype disrupt the behavioral recovery that occurs following unilateral deafferentation of the vestibular nerve fibers (vestibular compensation). It has been suggested that vestibular compensation may be owing to an upregulation or increased affinity of NMDA receptors in the VN ipsilateral to the peripheral deafferentation; however; at present, there is no clear evidence to support this hypothesis.     

Porcine prion protein amyloid

ABSTRACT

Mammalian prions are composed of misfolded aggregated prion protein (PrP) with amyloid-like features. Prions are zoonotic disease agents that infect a wide variety of mammalian species including humans. Mammals and by-products thereof which are frequently encountered in daily life are most important for human health. It is established that bovine prions (BSE) can infect humans while there is no such evidence for any other prion susceptible species in the human food chain (sheep, goat, elk, deer) and largely prion resistant species (pig) or susceptible and resistant pets (cat and dogs, respectively). PrPs from these species have been characterized using biochemistry, biophysics and neurobiology. Recently we studied PrPs from several mammals in vitro and found evidence for generic amyloidogenicity as well as cross-seeding fibril formation activity of all PrPs on the human PrP sequence regardless if the original species was resistant or susceptible to prion disease. Porcine PrP amyloidogenicity was among the studied. Experimentally inoculated pigs as well as transgenic mouse lines overexpressing porcine PrP have, in the past, been used to investigate the possibility of prion transmission in pigs. The pig is a species with extraordinarily wide use within human daily life with over a billion pigs harvested for human consumption each year. Here we discuss the possibility that the largely prion disease resistant pig can be a clinically silent carrier of replicating prions.     

Simvastatin prevents dopaminergic neurodegeneration in experimental parkinsonian models: the association with anti-inflammatory responses

Background

In addition to their original applications to lowering cholesterol, statins display multiple neuroprotective effects. N-methyl-D-aspartate (NMDA) receptors interact closely with the dopaminergic system and are strongly implicated in therapeutic paradigms of Parkinson''s disease (PD). This study aims to investigate how simvastatin impacts on experimental parkinsonian models via regulating NMDA receptors.

Methodology/Principal Findings

Regional changes in NMDA receptors in the rat brain and anxiolytic-like activity were examined after unilateral medial forebrain bundle lesion by 6-hydroxydopamine via a 3-week administration of simvastatin. NMDA receptor alterations in the post-mortem rat brain were detected by [³H]MK-801(Dizocilpine) binding autoradiography. 6-hydroxydopamine treated PC12 was applied to investigate the neuroprotection of simvastatin, the association with NMDA receptors, and the anti-inflammation. 6-hydroxydopamine induced anxiety and the downregulation of NMDA receptors in the hippocampus, CA1(Cornu Ammonis 1 Area), amygdala and caudate putamen was observed in 6-OHDA(6-hydroxydopamine) lesioned rats whereas simvastatin significantly ameliorated the anxiety-like activity and restored the expression of NMDA receptors in examined brain regions. Significant positive correlations were identified between anxiolytic-like activity and the restoration of expression of NMDA receptors in the hippocampus, amygdala and CA1 following simvastatin administration. Simvastatin exerted neuroprotection in 6-hydroxydopamine-lesioned rat brain and 6-hydroxydopamine treated PC12, partially by regulating NMDA receptors, MMP9 (matrix metalloproteinase-9), and TNF-a (tumour necrosis factor-alpha).

Conclusions/Significance

Our results provide strong evidence that NMDA receptor modulation after simvastatin treatment could partially explain its anxiolytic-like activity and anti-inflammatory mechanisms in experimental parkinsonian models. These findings contribute to a better understanding of the critical roles of simvastatin in treating PD via NMDA receptors.     

Psoralidin Stimulates Expression of Immediate-Early Genes and Synapse Development in Primary Cortical Neurons

Upon synaptic stimulation and glutamate release, glutamate receptors are activated to regulate several downstream effectors and signaling pathways resulting in synaptic modification. One downstream intracellular effect, in particular, is the expression of immediate-early genes (IEGs), which have been proposed to be important in synaptic plasticity because of their rapid expression following synaptic activation and key role in memory formation. In this study, we screened a natural compound library in order to find a compound that could induce the expression of IEGs in primary cortical neurons and discovered that psoralidin, a natural compound isolated from the seeds of Psoralea corylifolia, stimulated synaptic modulation. Psoralidin activated mitogen-activated protein kinase (MAPK) signaling, which in turn induced the expression of neuronal IEGs, particularly Arc, Egr-1, and c-fos. N-methyl-d-aspartate (NMDA) receptors activation and extracellular calcium influx were implicated in the psoralidin-induced intracellular changes. In glutamate dose–response curve, psoralidin shifted glutamate EC₅₀ to lower values without enhancing maximum activity. Interestingly, psoralidin increased the density, area, and intensity of excitatory synapses in primary hippocampal neurons, which were mediated by NMDA receptor activation and MAPK signaling. These results suggest that psoralidin triggers synaptic remodeling through activating NMDA receptor and subsequent MAPK signaling cascades and therefore could possibly serve as an NMDA receptor modulator.