The prion protein and lipid rafts

Structural characterization of codon 129 polymorphism in prion peptide segments (PrP127-132) using the Markov State Models

The human prion protein gene (PRNP) consists of two common alleles that encode either methionine or valine residues at codon 129. Polymorphism at codon 129 of the prion protein (PRNP) gene is closely associated with genetic variations and susceptibility to specific variants of prion diseases. The presence of these different alleles, known as the PRNP codon 129 polymorphism, plays a significant role in disease susceptibility and progression. For instance, the prion fragment 127-132 (PrP127-132) has been implicated in the development of variant Creutzfeldt-Jakob disease (vCJD), due to the presence of methionine or valine at codon 129. This study aims to unravel the early structural changes brought by the presence of polymorphism at codon 129. Using molecular dynamics (MD) simulations, we present evidence highlighting a spectrum of structural transitions, uncovering the nuanced conformational heterogeneity governing the polymorphic behavior of the PrP127-132 chain. The Markov state model (MSM) analysis was able to predict several metastable states of these chains and established a kinetic network that describes transitions between these states. Additionally, the MSM analysis showed extra stability of the PrP-M129 polymorph due to less random-coiled motions, the formation of a salt bridge, and an increase in the number of native contacts. The pathogenicity of PrP-V129 can be attributed to enhanced random motion and the absence of a salt bridge.

Post-translational modifications in prion diseases

More than 650 reversible and irreversible post-translational modifications (PTMs) of proteins have been listed so far. Canonical PTMs of proteins consist of the covalent addition of functional or chemical groups on target backbone amino-acids or the cleavage of the protein itself, giving rise to modified proteins with specific properties in terms of stability, solubility, cell distribution, activity, or interactions with other biomolecules. PTMs of protein contribute to cell homeostatic processes, enabling basal cell functions, allowing the cell to respond and adapt to variations of its environment, and globally maintaining the constancy of the milieu interieur (the body's inner environment) to sustain human health. Abnormal protein PTMs are, however, associated with several disease states, such as cancers, metabolic disorders, or neurodegenerative diseases. Abnormal PTMs alter the functional properties of the protein or even cause a loss of protein function. One example of dramatic PTMs concerns the cellular prion protein (PrPC), a GPI-anchored signaling molecule at the plasma membrane, whose irreversible post-translational conformational conversion (PTCC) into pathogenic prions (PrPSc) provokes neurodegeneration. PrPC PTCC into PrPSc is an additional type of PTM that affects the tridimensional structure and physiological function of PrPC and generates a protein conformer with neurotoxic properties. PrPC PTCC into PrPSc in neurons is the first step of a deleterious sequence of events at the root of a group of neurodegenerative disorders affecting both humans (Creutzfeldt-Jakob diseases for the most representative diseases) and animals (scrapie in sheep, bovine spongiform encephalopathy in cow, and chronic wasting disease in elk and deer). There are currently no therapies to block PrPC PTCC into PrPSc and stop neurodegeneration in prion diseases. Here, we review known PrPC PTMs that influence PrPC conversion into PrPSc. We summarized how PrPC PTCC into PrPSc impacts the PrPC interactome at the plasma membrane and the downstream intracellular controlled protein effectors, whose abnormal activation or trafficking caused by altered PTMs promotes neurodegeneration. We discussed these effectors as candidate drug targets for prion diseases and possibly other neurodegenerative diseases.

Creutzfeldt-Jakob disease and other prion diseases

Prion diseases share common clinical and pathological characteristics such as spongiform neuronal degeneration and deposition of an abnormal form of a host-derived protein, termed prion protein. The characteristic features of prion diseases are long incubation times, short clinical courses, extreme resistance of the transmissible agent to degradation and lack of nucleic acid involvement. Sporadic and genetic forms of prion diseases occur worldwide, of which genetic forms are associated with mutations in PRNP. Human to human transmission of these diseases has occurred due to iatrogenic exposure, and zoonotic forms of prion diseases are linked to bovine disease. Significant progress has been made in the diagnosis of these disorders. Clinical tools for diagnosis comprise brain imaging and cerebrospinal fluid tests. Aggregation assays for detection of the abnormally folded prion protein have a clear potential to diagnose the disease in peripherally accessible biofluids. After decades of therapeutic nihilism, new treatment strategies and clinical trials are on the horizon. Although prion diseases are relatively rare disorders, understanding their pathogenesis and mechanisms of prion protein misfolding has significantly enhanced the field in research of neurodegenerative diseases.

Role of rafts in neurological disorders

INTRODUCTION

Rafts are protein-lipid structural nanodomains involved in efficient signal transduction and the modulation of physiological processes of the cell plasma membrane. Raft disruption in the nervous system has been associated with a wide range of disorders.

DEVELOPMENT

We review the concept of rafts, the nervous system processes in which they are involved, and their role in diseases such as Parkinson's disease, Alzheimer disease, and Huntington disease.

CONCLUSIONS

Based on the available evidence, preservation and/or reconstitution of rafts is a promising treatment strategy for a wide range of neurological disorders.

What is the role of lipids in prion conversion and disease?

The molecular cause of transmissible spongiform encephalopathies (TSEs) involves the conversion of the cellular prion protein (PrP^C) into its pathogenic form, called prion scrapie (PrP^Sc), which is prone to the formation of amorphous and amyloid aggregates found in TSE patients. Although the mechanisms of conversion of PrP^C into PrP^Sc are not entirely understood, two key points are currently accepted: (i) PrP^Sc acts as a seed for the recruitment of native PrP^C, inducing the latter's conversion to PrP^Sc; and (ii) other biomolecules, such as DNA, RNA, or lipids, can act as cofactors, mediating the conversion from PrP^C to PrP^Sc. Interestingly, PrP^C is anchored by a glycosylphosphatidylinositol molecule in the outer cell membrane. Therefore, interactions with lipid membranes or alterations in the membranes themselves have been widely investigated as possible factors for conversion. Alone or in combination with RNA molecules, lipids can induce the formation of PrP in vitro-produced aggregates capable of infecting animal models. Here, we discuss the role of lipids in prion conversion and infectivity, highlighting the structural and cytotoxic aspects of lipid-prion interactions. Strikingly, disorders like Alzheimer's and Parkinson's disease also seem to be caused by changes in protein structure and share pathogenic mechanisms with TSEs. Thus, we posit that comprehending the process of PrP conversion is relevant to understanding critical events involved in a variety of neurodegenerative disorders and will contribute to developing future therapeutic strategies for these devastating conditions.

Structural Analysis of Oligosaccharides and Glycoconjugates Using NMR

Carbohydrate chains play critical roles in cellular recognition and subsequent signal transduction in the nervous system. Furthermore, gangliosides are targets for various amyloidogenic proteins associated with neurodegenerative disorders. To better understand the molecular mechanisms underlying these biological phenomena, atomic views are essential to delineate dynamic biomolecular interactions. Nuclear magnetic resonance (NMR) spectroscopy provides powerful tools for studying structures, dynamics, and interactions of biomolecules at the atomic level. This chapter describes the basics of solution NMR techniques and their applications to the analysis of 3D structures and interactions of glycoconjugates in the nervous system.

The LRP1/CD91 ligands, tissue-type plasminogen activator, α2-macroglobulin, and soluble cellular prion protein have distinct co-receptor requirements for activation of cell-signaling

LDL Receptor-related Protein-1 (LRP1/CD91) binds diverse ligands, many of which activate cell-signaling. Herein, we compared three LRP1 ligands that inhibit inflammatory responses triggered by lipopolysaccharide (LPS), including: enzymatically-inactive tissue-type plasminogen activator (EI-tPA); activated α₂-macroglobulin (α₂M); and S-PrP, a soluble derivative of nonpathogenic cellular prion protein (PrP^C). In bone marrow-derived macrophages, the N-methyl-D-aspartate receptor was essential for all three LRP1 ligands to activate cell-signaling and inhibit LPS-induced cytokine expression. Intact lipid rafts also were essential. Only α₂M absolutely required LRP1. LRP1 decreased the EI-tPA concentration required to activate cell-signaling and antagonize LPS but was not essential, mimicking its role as a S-PrP co-receptor. Membrane-anchored PrP^C also functioned as a co-receptor for EI-tPA and α₂M, decreasing the ligand concentration required for cell-signaling and LPS antagonism; however, when the concentration of EI-tPA or α₂M was sufficiently increased, cell-signaling and LPS antagonism occurred independently of PrP^C. S-PrP is the only LRP1 ligand in this group that activated cell-signaling independently of membrane-anchored PrP^C. EI-tPA, α₂M, and S-PrP inhibited LPS-induced LRP1 shedding from macrophages, a process that converts LRP1 into a pro-inflammatory product. Differences in the co-receptors required for anti-inflammatory activity may explain why LRP1 ligands vary in ability to target macrophages in different differentiation states.

Cholesterol and its reciprocal association with prion infection

Prion diseases are incurable, infectious and fatal neurodegenerative diseases that affect both humans and animals. The pathogenesis of prion disease involves the misfolding of the cellular prion protein, PrP^C, to a disease-causing conformation, PrP^Sc, in the brain. The exact mechanism of conversion of PrP^C to PrP^Sc is not clear; however, there are numerous studies supporting that this process of misfolding requires the association of PrP^C with lipid raft domains of the plasma membrane. An increase in the cellular cholesterol content with prion infection has been observed in both in vivo and in vitro studies. As cholesterol is critical for the formation of lipid rafts, on the one hand, this increase may be related to, or aiding in, the process of prion conversion. On the other hand, increased cholesterol levels may affect neuronal viability. Here, we discuss current literature on the underlying mechanisms and potential consequences of elevated neuronal cholesterol in prion infection and advancements in prion disease therapeutics targeting brain cholesterol homeostasis.

Crosstalk between Lipid Rafts and Aging: New Frontiers for Delaying Aging

With the rapid aging in the global population, delay of aging has become a hot research topic. Lipid rafts (LRs) are microdomains in the plasma membrane that contain sphingolipids and cholesterol. Emerging evidence indicates an interesting interplay between LRs and aging. LRs and their components are altered with aging. Further, the aging process is strongly influenced by LRs. In recent years, LRs and their component signaling molecules have been recognized to affect aging by interfering with its hallmarks. Therefore, targeting LRs is a promising strategy to delay aging.

Ameliorating potential of curcumin and its analogue in central nervous system disorders and related conditions: A review of molecular pathways

Curcumin, isolated from turmeric (Curcuma longa L.) is one of the broadly studied phytomolecule owing to its strong antioxidant and anti-inflammatory potential and has been considered a promising therapeutic candidate in a wide range of disorders. Considering, its low bioavailability, different curcumin analogs have been developed to afford desired pharmacokinetic profile and therapeutic outcome in varied pathological states. Several preclinical and clinical studies have indicated that curcumin ameliorates mitochondrial dysfunction, inflammation, oxidative stress apoptosis-mediated neural cell degeneration and could effectively be utilized in the treatment of different neurodegenerative diseases. Hence, in this review, we have summarized key findings of experimental and clinical studies conducted on curcumin and its analogues with special emphasis on molecular pathways, viz. NF-kB, Nrf2-ARE, glial activation, apoptosis, angiogenesis, SOCS/JAK/STAT, PI3K/Akt, ERK1/2 /MyD88 /p38 MAPK, JNK, iNOS/NO, and MMP pathways involved in imparting ameliorative effects in the therapy of neurodegenerative disorders and associated conditions.

Second Sphere Interactions in Amyloidogenic Diseases

Amyloids are protein aggregates bearing a highly ordered cross β structural motif, which may be functional but are mostly pathogenic. Their formation, deposition in tissues and consequent organ dysfunction is the central event in amyloidogenic diseases. Such protein aggregation may be brought about by conformational changes, and much attention has been directed toward factors like metal binding, post-translational modifications, mutations of protein etc., which eventually affect the reactivity and cytotoxicity of the associated proteins. Over the past decade, a global effort from different groups working on these misfolded/unfolded proteins/peptides has revealed that the amino acid residues in the second coordination sphere of the active sites of amyloidogenic proteins/peptides cause changes in H-bonding pattern or protein-protein interactions, which dramatically alter the structure and reactivity of these proteins/peptides. These second sphere effects not only determine the binding of transition metals and cofactors, which define the pathology of some of these diseases, but also change the mechanism of redox reactions catalyzed by these proteins/peptides and form the basis of oxidative damage associated with these amyloidogenic diseases. The present review seeks to discuss such second sphere modifications and their ramifications in the etiopathology of some representative amyloidogenic diseases like Alzheimer's disease (AD), type 2 diabetes mellitus (T2Dm), Parkinson's disease (PD), Huntington's disease (HD), and prion diseases.

Lipid Rafts Act as a Common Platform for Amyloid-β Oligomer-Induced Alzheimer’s Disease Pathology

Background: Amyloid-β (Aβ) oligomers induce the overproduction of phosphorylated tau and neurodegeneration. These cascades gradually cause cognitive impairment in Alzheimer’s disease (AD). While each pathological event in AD has been studied in detail separately, the spatial and temporal relationships between pathological events in AD remain unclear. Objective: We demonstrated that lipid rafts function as a common platform for the pathological cascades of AD. Methods: Cellular and synaptosomal lipid rafts were prepared from the brains of Aβ amyloid model mice (Tg2576 mice) and double transgenic mice (Tg2576 x TgTauP301L mice) and longitudinally analyzed. Results: Aβ dimers, the cellular prion protein (PrPc), and Aβ dimer/PrPc complexes were detected in the lipid rafts. The levels of Fyn, the phosphorylated NR2B subunit of the N-methyl-D-aspartate receptor, glycogen synthase kinase 3β, total tau, phosphorylated tau, and tau oligomers increased with Aβ dimer accumulation in both the cellular and synaptosomal lipid rafts. Increases in the levels of these molecules were first seen at 6 months of age and corresponded with the early stages of Aβ accumulation in the amyloid model mice. Conclusion: Lipid rafts act as a common platform for the progression of AD pathology. The findings of this study suggest a novel therapeutic approach to AD, involving the modification of lipid raft components and the inhibition of their roles in the sequential pathological events of AD.

Propagation and Dissemination Strategies of Transmissible Spongiform Encephalopathy Agents in Mammalian Cells

Transmissible spongiform encephalopathies or prion disorders are fatal infectious diseases that cause characteristic spongiform degeneration in the central nervous system. The causative agent, the so-called prion, is an unconventional infectious agent that propagates by converting the host-encoded cellular prion protein PrP into ordered protein aggregates with infectious properties. Prions are devoid of coding nucleic acid and thus rely on the host cell machinery for propagation. While it is now established that, in addition to PrP, other cellular factors or processes determine the susceptibility of cell lines to prion infection, exact factors and cellular processes remain broadly obscure. Still, cellular models have uncovered important aspects of prion propagation and revealed intercellular dissemination strategies shared with other intracellular pathogens. Here, we summarize what we learned about the processes of prion invasion, intracellular replication and subsequent dissemination from ex vivo cell models.

Spatiotemporal modulations in heterotypic condensates of prion and α-synuclein control phase transitions and amyloid conversion

Biomolecular condensation via liquid-liquid phase separation of proteins and nucleic acids is associated with a range of critical cellular functions and neurodegenerative diseases. Here, we demonstrate that complex coacervation of the prion protein and α-synuclein within narrow stoichiometry results in the formation of highly dynamic, reversible, thermo-responsive liquid droplets via domain-specific electrostatic interactions between the positively-charged intrinsically disordered N-terminal segment of prion and the acidic C-terminal tail of α-synuclein. The addition of RNA to these coacervates yields multiphasic, vesicle-like, hollow condensates. Picosecond time-resolved measurements revealed the presence of transient electrostatic nanoclusters that are stable on the nanosecond timescale and can undergo breaking-and-making of interactions on slower timescales giving rise to a liquid-like behavior in the mesoscopic regime. The liquid-to-solid transition drives a rapid conversion of complex coacervates into heterotypic amyloids. Our results suggest that synergistic prion-α-synuclein interactions within condensates provide mechanistic underpinnings of their physiological role and overlapping neuropathological features.

Cellular prion protein in human plasma-derived extracellular vesicles promotes neurite outgrowth via the NMDA receptor-LRP1 receptor system

Exosomes and other extracellular vesicles (EVs) participate in cell-cell communication. Herein, we isolated EVs from human plasma and demonstrated that these EVs activate cell signaling and promote neurite outgrowth in PC-12 cells. Analysis of human plasma EVs purified by sequential ultracentrifugation using tandem mass spectrometry indicated the presence of multiple plasma proteins, including α2-macroglobulin, which is reported to regulate PC-12 cell physiology. We therefore further purified EVs by molecular exclusion or phosphatidylserine affinity chromatography, which reduced plasma protein contamination. EVs subjected to these additional purification methods exhibited unchanged activity in PC-12 cells, even though α2-macroglobulin was reduced to undetectable levels. Nonpathogenic cellular prion protein (PrPC) was carried by human plasma EVs and essential for the effects of EVs on PC-12 cells, as EV-induced cell signaling and neurite outgrowth were blocked by the PrPC-specific antibody, POM2. In addition, inhibitors of the N-methyl-d-aspartate (NMDA) receptor (NMDA-R) and low-density lipoprotein receptor-related protein-1 (LRP1) blocked the effects of plasma EVs on PC-12 cells, as did silencing of Lrp1 or the gene encoding the GluN1 NMDA-R subunit (Grin1). These results implicate the NMDA-R-LRP1 complex as the receptor system responsible for mediating the effects of EV-associated PrPC. Finally, EVs harvested from rat astrocytes carried PrPC and replicated the effects of human plasma EVs on PC-12 cell signaling. We conclude that interaction of EV-associated PrPC with the NMDA-R-LRP1 complex in target cells represents a novel mechanism by which EVs may participate in intercellular communication in the nervous system.

Melatonin: Regulation of Prion Protein Phase Separation in Cancer Multidrug Resistance

The unique ability to adapt and thrive in inhospitable, stressful tumor microenvironments (TME) also renders cancer cells resistant to traditional chemotherapeutic treatments and/or novel pharmaceuticals. Cancer cells exhibit extensive metabolic alterations involving hypoxia, accelerated glycolysis, oxidative stress, and increased extracellular ATP that may activate ancient, conserved prion adaptive response strategies that exacerbate multidrug resistance (MDR) by exploiting cellular stress to increase cancer metastatic potential and stemness, balance proliferation and differentiation, and amplify resistance to apoptosis. The regulation of prions in MDR is further complicated by important, putative physiological functions of ligand-binding and signal transduction. Melatonin is capable of both enhancing physiological functions and inhibiting oncogenic properties of prion proteins. Through regulation of phase separation of the prion N-terminal domain which targets and interacts with lipid rafts, melatonin may prevent conformational changes that can result in aggregation and/or conversion to pathological, infectious isoforms. As a cancer therapy adjuvant, melatonin could modulate TME oxidative stress levels and hypoxia, reverse pH gradient changes, reduce lipid peroxidation, and protect lipid raft compositions to suppress prion-mediated, non-Mendelian, heritable, but often reversible epigenetic adaptations that facilitate cancer heterogeneity, stemness, metastasis, and drug resistance. This review examines some of the mechanisms that may balance physiological and pathological effects of prions and prion-like proteins achieved through the synergistic use of melatonin to ameliorate MDR, which remains a challenge in cancer treatment.

APP deficiency and HTRA2 modulates PrPc proteostasis in human cancer cells

Cellular protein homeostasis (proteostasis) requires an accurate balance between protein biosynthesis, folding, and degradation, and its instability is causally related to human diseases and cancers. Here, we created numerous engineered cancer cell lines targeting APP (amyloid ß precursor protein) and/or PRNP (cellular prion) genes and we showed that APP knocking-down impaired PRNP mRNA level and vice versa, suggesting a link between their gene regulation. PRNP^KD, APP^KD and PRNP^KD/APP^KD HeLa cells encountered major difficulties to grow in a 3D tissue-like environment. Unexpectedly, we found a cytoplasmic accumulation of the PrP^c protein without PRNP gene up regulation, in both APP^KD and APP^KO HeLa cells. Interestingly, APP and/or PRNP gene ablation enhanced the chaperone/serine protease HTRA2 gene expression, which is a protein processing quality factor involved in Alzheimer's disease. Importantly, HTRA2 gene silencing decreased PRNP mRNA level and lowered PrP^c protein amounts, and conversely, HTRA2 overexpression increased PRNP gene regulation and enhanced membrane-anchored and cytoplasmic PrP^c fractions. PrP^c, APP and HTRA2 destabilized membrane-associated CD24 protein, suggesting changes in the lipid raft structure. Our data show for the first time that APP and the dual chaperone/serine protease HTRA2 protein could modulate PrP^c proteostasis hampering cancer cell behavior.

A Soluble PrPC Derivative and Membrane-Anchored PrPC in Extracellular Vesicles Attenuate Innate Immunity by Engaging the NMDA-R/LRP1 Receptor Complex

Nonpathogenic cellular prion protein (PrPC) demonstrates anti-inflammatory activity; however, the responsible mechanisms are incompletely defined. PrPC exists as a GPI-anchored membrane protein in diverse cells; however, PrPC may be released from cells by ADAM proteases or when packaged into extracellular vesicles (EVs). In this study, we show that a soluble derivative of PrPC (S-PrP) counteracts inflammatory responses triggered by pattern recognition receptors in macrophages, including TLR2, TLR4, TLR7, TLR9, NOD1, and NOD2. S-PrP also significantly attenuates the toxicity of LPS in mice. The response of macrophages to S-PrP is mediated by a receptor assembly that includes the N-methyl-d-aspartate receptor (NMDA-R) and low-density lipoprotein receptor-related protein-1 (LRP1). PrPC was identified in EVs isolated from human plasma. These EVs replicated the activity of S-PrP, inhibiting cytokine expression and IκBα phosphorylation in LPS-treated macrophages. The effects of plasma EVs on LPS-treated macrophages were blocked by PrPC-specific Ab, by antagonists of LRP1 and the NMDA-R, by deleting Lrp1 in macrophages, and by inhibiting Src family kinases. Phosphatidylinositol-specific phospholipase C dissociated the LPS-regulatory activity from EVs, rendering the EVs inactive as LPS inhibitors. The LPS-regulatory activity that was lost from phosphatidylinositol-specific phospholipase C-treated EVs was recovered in solution. Collectively, these results demonstrate that GPI-anchored PrPC is the essential EV component required for the observed immune regulatory activity of human plasma EVs. S-PrP and EV-associated PrPC regulate innate immunity by engaging the NMDA-R/LRP1 receptor system in macrophages. The scope of pattern recognition receptors antagonized by S-PrP suggests that released forms of PrPC may have broad anti-inflammatory activity.

Membrane Domain Localization and Interaction of the Prion-Family Proteins, Prion and Shadoo with Calnexin

The cellular prion protein (PrP^C) is renowned for its infectious conformational isoform PrP^Sc, capable of templating subsequent conversions of healthy PrP^Cs and thus triggering the group of incurable diseases known as transmissible spongiform encephalopathies. Besides this mechanism not being fully uncovered, the protein’s physiological role is also elusive. PrP^C and its newest, less understood paralog Shadoo are glycosylphosphatidylinositol-anchored proteins highly expressed in the central nervous system. While they share some attributes and neuroprotective actions, opposing roles have also been reported for the two; however, the amount of data about their exact functions is lacking. Protein–protein interactions and membrane microdomain localizations are key determinants of protein function. Accurate identification of these functions for a membrane protein, however, can become biased due to interactions occurring during sample processing. To avoid such artifacts, we apply a non-detergent-based membrane-fractionation approach to study the prion protein and Shadoo. We show that the two proteins occupy similarly raft and non-raft membrane fractions when expressed in N2a cells and that both proteins pull down the chaperone calnexin in both rafts and non-rafts. These indicate their possible binding to calnexin in both types of membrane domains, which might be a necessary requisite to aid the inherently unstable native conformation during their lifetime.

Melatonin: Regulation of Biomolecular Condensates in Neurodegenerative Disorders

Biomolecular condensates are membraneless organelles (MLOs) that form dynamic, chemically distinct subcellular compartments organizing macromolecules such as proteins, RNA, and DNA in unicellular prokaryotic bacteria and complex eukaryotic cells. Separated from surrounding environments, MLOs in the nucleoplasm, cytoplasm, and mitochondria assemble by liquid-liquid phase separation (LLPS) into transient, non-static, liquid-like droplets that regulate essential molecular functions. LLPS is primarily controlled by post-translational modifications (PTMs) that fine-tune the balance between attractive and repulsive charge states and/or binding motifs of proteins. Aberrant phase separation due to dysregulated membrane lipid rafts and/or PTMs, as well as the absence of adequate hydrotropic small molecules such as ATP, or the presence of specific RNA proteins can cause pathological protein aggregation in neurodegenerative disorders. Melatonin may exert a dominant influence over phase separation in biomolecular condensates by optimizing membrane and MLO interdependent reactions through stabilizing lipid raft domains, reducing line tension, and maintaining negative membrane curvature and fluidity. As a potent antioxidant, melatonin protects cardiolipin and other membrane lipids from peroxidation cascades, supporting protein trafficking, signaling, ion channel activities, and ATPase functionality during condensate coacervation or dissolution. Melatonin may even control condensate LLPS through PTM and balance mRNA- and RNA-binding protein composition by regulating N6-methyladenosine (m6A) modifications. There is currently a lack of pharmaceuticals targeting neurodegenerative disorders via the regulation of phase separation. The potential of melatonin in the modulation of biomolecular condensate in the attenuation of aberrant condensate aggregation in neurodegenerative disorders is discussed in this review.

Cellular Prion Protein Mediates α-Synuclein Uptake, Localization, and Toxicity In Vitro and In Vivo

BACKGROUND

The cellular prion protein (PrP^C ) is a membrane-bound, multifunctional protein mainly expressed in neuronal tissues. Recent studies indicate that the native trafficking of PrP^C can be misused to internalize misfolded amyloid beta and α-synuclein (aSyn) oligomers.

OBJECTIVES

We define PrP^C 's role in internalizing misfolded aSyn in α-synucleinopathies and identify further involved proteins.

METHODS

We performed comprehensive behavioral studies on four transgenic mouse models (ThySyn and ThySynPrP00, TgM83 and TgMPrP00) at different ages. We developed PrP^C -(over)-expressing cell models (cell line and primary cortical neurons), used confocal laser microscopy to perform colocalization studies, applied mass spectrometry to identify interactomes, and determined disassociation constants using surface plasmon resonance (SPR) spectroscopy.

RESULTS

Behavioral deficits (memory, anxiety, locomotion, etc.), reduced lifespans, and higher oligomeric aSyn levels were observed in PrP^C -expressing mice (ThySyn and TgM83), but not in homologous Prnp ablated mice (ThySynPrP00 and TgMPrP00). PrP^C colocalized with and facilitated aSyn (oligomeric and monomeric) internalization in our cell-based models. Glimepiride treatment of PrP^C -overexpressing cells reduced aSyn internalization in a dose-dependent manner. SPR analysis showed that the binding affinity of PrP^C to monomeric aSyn was lower than to oligomeric aSyn. Mass spectrometry-based proteomic studies identified clathrin in the immunoprecipitates of PrP^C and aSyn. SPR was used to show that clathrin binds to recombinant PrP, but not aSyn. Experimental disruption of clathrin-coated vesicles significantly decreased aSyn internalization.

CONCLUSION

PrP^C 's native trafficking can be misused to internalize misfolded aSyn through a clathrin-based mechanism, which may facilitate the spreading of pathological aSyn. Disruption of aSyn-PrP^C binding is, therefore, an appealing therapeutic target in α-synucleinopathies. © 2021 The Authors. Movement Disorders published by Wiley Periodicals LLC on behalf of International Parkinson and Movement Disorder Society.

Large-scale lipidomic profiling identifies novel potential biomarkers for prion diseases and highlights lipid raft-related pathways

Prion diseases are transmissible spongiform encephalopathies induced by the abnormally-folded prion protein (PrP^Sc), which is derived from the normal prion protein (PrP^C). Previous studies have reported that lipid rafts play a pivotal role in the conversion of PrP^C into PrP^Sc, and several therapeutic strategies targeting lipids have led to prolonged survival times in prion diseases. In addition, phosphatidylethanolamine, a glycerophospholipid member, accelerated prion disease progression. Although several studies have shown that prion diseases are significantly associated with lipids, lipidomic analyses of prion diseases have not been reported thus far. We intraperitoneally injected phosphate-buffered saline (PBS) or ME7 mouse prions into mice and sacrificed them at different time points (3 and 7 months) post-injection. To detect PrP^Sc in the mouse brain, we carried out western blotting analysis of the left hemisphere of the brain. To identify potential novel lipid biomarkers, we performed lipid extraction on the right hemisphere of the brain and liquid chromatography mass spectrometry (LC/MS) to analyze the lipidomic profiling between non-infected mice and prion-infected mice. Finally, we analyzed the altered lipid-related pathways by a lipid pathway enrichment analysis (LIPEA). We identified a total of 43 and 75 novel potential biomarkers at 3 and 7 months in prion-infected mice compared to non-infected mice, respectively. Among these novel potential biomarkers, approximately 75% of total lipids are glycerophospholipids. In addition, altered lipids between the non-infected and prion-infected mice were related to sphingolipid, glycerophospholipid and glycosylphosphatidylinositol (GPI)-anchor-related pathways. In the present study, we found novel potential biomarkers and therapeutic targets of prion disease. To the best of our knowledge, this study reports the first large-scale lipidomic profiling in prion diseases.

Role of rafts in neurological disorders

INTRODUCTION

Rafts are function-structural cell membrane nano-domains. They contribute to explain the efficiency of signal transduction at the low physiological membrane concentrations of the signaling partners by their clustering inside specialized signaling domains.

DEVELOPMENT

In this article, we review the current model of the membrane rafts and their physio-pathological relevance in the nervous system, including their role in Parkinson, Alzheimer, and Huntington diseases.

CONCLUSIONS

Rafts disruption/dysfunction has been shown to relate diverse neurological diseases. Therefore, it has been suggested that preservation of membrane rafts may represent a strategy to prevent or delay neuronal dysfunctions in several diseases.

Direct interaction of HIV gp120 with neuronal CXCR4 and CCR5 receptors induces cofilin-actin rod pathology via a cellular prion protein- and NOX-dependent mechanism

Nearly 50% of individuals with long-term HIV infection are affected by the onset of progressive HIV-associated neurocognitive disorders (HAND). HIV infiltrates the central nervous system (CNS) early during primary infection where it establishes persistent infection in microglia (resident macrophages) and astrocytes that in turn release inflammatory cytokines, small neurotoxic mediators, and viral proteins. While the molecular mechanisms underlying pathology in HAND remain poorly understood, synaptodendritic damage has emerged as a hallmark of HIV infection of the CNS. Here, we report that the HIV viral envelope glycoprotein gp120 induces the formation of aberrant, rod-shaped cofilin-actin inclusions (rods) in cultured mouse hippocampal neurons via a signaling pathway common to other neurodegenerative stimuli including oligomeric, soluble amyloid-β and proinflammatory cytokines. Previous studies showed that synaptic function is impaired preferentially in the distal proximity of rods within dendrites. Our studies demonstrate gp120 binding to either chemokine co-receptor CCR5 or CXCR4 is capable of inducing rod formation, and signaling through this pathway requires active NADPH oxidase presumably through the formation of superoxide (O2-) and the expression of cellular prion protein (PrPC). These findings link gp120-mediated oxidative stress to the generation of rods, which may underlie early synaptic dysfunction observed in HAND.

Cytoskeleton-dependent clustering of membrane-bound prion protein on the cell surface

Prion diseases are a group of neurodegenerative disorders that infect animals and humans with proteinaceous particles called prions. Prions consist of scrapie prion protein (PrPSc), a misfolded version of the cellular prion protein (PrPC). During disease progression, PrPSc replicates by interacting with PrPC and inducing its conversion to PrPSc. Attachment of PrPC to cellular membranes via a glycosylphosphatidylinositol (GPI) anchor is critical for the conversion of PrPC into PrPSc. However, the mechanisms governing PrPC conversion and replication on the membrane remain largely unclear. Here, a site-selectively modified PrP variant equipped with a fluorescent GPI anchor mimic (PrP-GPI) was employed to directly observe PrP at the cellular membrane in neuronal SH-SY5Y cells. PrP-GPI exhibits a cholesterol-dependent membrane accumulation and a cytoskeleton-dependent mobility. More specifically, inhibition of actin polymerization reduced the diffusion of PrP-GPI indicating protein clustering, which resembles the initial step of PrP aggregation and conversion into its pathogenic isoform. An intact actin cytoskeleton might therefore prevent conversion of PrPC into PrPSc and offer new therapeutic angles.

Caveolae as Potential Hijackable Gates in Cell Communication

Caveolae are membrane microdomains described in many cell types involved in endocytocis, transcytosis, cell signaling, mechanotransduction, and aging. They are found at the interface with the extracellular environment and are structured by caveolin and cavin proteins. Caveolae and caveolins mediate transduction of chemical messages via signaling pathways, as well as non-chemical messages, such as stretching or shear stress. Various pathogens or signals can hijack these gates, leading to infectious, oncogenic and even caveolin-related diseases named caveolinopathies. By contrast, preclinical and clinical research have fallen behind in their attempts to hijack caveolae and caveolins for therapeutic purposes. Caveolae involvement in human disease is not yet fully explored or understood and, of all their scaffold proteins, only caveolin-1 is being considered in clinical trials as a possible biomarker of disease. This review briefly summarizes current knowledge about caveolae cell signaling and raises the hypothesis whether these microdomains could serve as hijackable “gatekeepers” or “gateways” in cell communication. Furthermore, because cell signaling is one of the most dynamic domains in translating data from basic to clinical research, we pay special attention to translation of caveolae, caveolin, and cavin research into clinical practice.

A New Take on Prion Protein Dynamics in Cellular Trafficking

The mobility of cellular prion protein (PrP^C) in specific cell membrane domains and among distinct cell compartments dictates its molecular interactions and directs its cell function. PrP^C works in concert with several partners to organize signaling platforms implicated in various cellular processes. The scaffold property of PrP^C is able to gather a molecular repertoire to create heterogeneous membrane domains that favor endocytic events. Dynamic trafficking of PrP^C through multiple pathways, in a well-orchestrated mechanism of intra and extracellular vesicular transport, defines its functional plasticity, and also assists the conversion and spreading of its infectious isoform associated with neurodegenerative diseases. In this review, we highlight how PrP^C traffics across intra- and extracellular compartments and the consequences of this dynamic transport in governing cell functions and contributing to prion disease pathogenesis.

A soluble derivative of PrPC activates cell-signaling and regulates cell physiology through LRP1 and the NMDA receptor

Cellular prion protein (PrP^C) is a widely expressed glycosylphosphatidylinositol-anchored membrane protein. Scrapie prion protein is a misfolded and aggregated form of PrP^C responsible for prion-induced neurodegenerative diseases. Understanding the function of the nonpathogenic PrP^C monomer is an important objective. PrP^C may be shed from the cell surface to generate soluble derivatives. Herein, we studied a recombinant derivative of PrP^C (soluble cellular prion protein, S-PrP) that corresponds closely in sequence to a soluble form of PrP^C shed from the cell surface by proteases in the A Disintegrin And Metalloprotease (ADAM) family. S-PrP activated cell-signaling in PC12 and N2a cells. TrkA was transactivated by Src family kinases and extracellular signal-regulated kinase 1/2 was activated downstream of Trk receptors. These cell-signaling events were dependent on the N-methyl-d-aspartate receptor (NMDA-R) and low-density lipoprotein receptor-related protein-1 (LRP1), which functioned as a cell-signaling receptor system in lipid rafts. Membrane-anchored PrP^C and neural cell adhesion molecule were not required for S-PrP-initiated cell-signaling. S-PrP promoted PC12 cell neurite outgrowth. This response required the NMDA-R, LRP1, Src family kinases, and Trk receptors. In Schwann cells, S-PrP interacted with the LRP1/NMDA-R system to activate extracellular signal-regulated kinase 1/2 and promote cell migration. The effects of S-PrP on PC12 cell neurite outgrowth and Schwann cell migration were similar to those caused by other proteins that engage the LRP1/NMDA-R system, including activated α₂-macroglobulin and tissue-type plasminogen activator. Collectively, these results demonstrate that shed forms of PrP^C may exhibit important biological activities in the central nervous system and the peripheral nervous system by serving as ligands for the LRP1/NMDA-R system.

Show Me Your Friends and I Tell You Who You Are: The Many Facets of Prion Protein in Stroke

Ischemic stroke belongs to the leading causes of mortality and disability worldwide. Although treatments for the acute phase of stroke are available, not all patients are eligible. There is a need to search for therapeutic options to promote neurological recovery after stroke. The cellular prion protein (PrPC) has been consistently linked to a neuroprotective role after ischemic damage: it is upregulated in the penumbra area following stroke in humans, and animal models of stroke have shown that lack of PrPC aggravates the ischemic damage and lessens the functional outcome. Mechanistically, these effects can be linked to numerous functions attributed to PrPC: (1) as a signaling partner of the PI3K/Akt and MAPK pathways, (2) as a regulator of glutamate receptors, and (3) promoting stem cell homing mechanisms, leading to angio- and neurogenesis. PrPC can be cleaved at different sites and the proteolytic fragments can account for the manifold functions. Moreover, PrPC is present on extracellular vesicles (EVs), released membrane particles originating from all types of cells that have drawn attention as potential therapeutic tools in stroke and many other diseases. Thus, identification of the many mechanisms underlying PrPC-induced neuroprotection will not only provide further understanding of the physiological functions of PrPC but also new ideas for possible treatment options after ischemic stroke.

New applications of squalene synthase inhibitors: Membrane cholesterol as a therapeutic target

Squalene synthase (SQS) inhibitors, mostly known as antihyperlipidemic agents for controlling blood cholesterol levels, have been increasingly used to study alterations of the cholesterol content in cell membranes. As such, SQS inhibitors have been demonstrated to control cellular activities related to cancer cell proliferation and migration, neuron degeneration, and parasite growth. While the mechanisms behind the effects of cellular cholesterol are still being revealed in detail, the evidence for SQS as a therapeutic target for several seemingly unrelated diseases is increasing. SQS inhibitors may be the next promising candidates targeting the three remaining primary therapeutic areas, beyond cardiovascular disease, which still need to be addressed; their application as anticancer, antimicrobial, and antineurodegenerative agents appears promising for new drug discovery projects underway.

MicroRNA Alterations in a Tg501 Mouse Model of Prion Disease

MicroRNAs (miRNAs) may contribute to the development and pathology of many neurodegenerative diseases, including prion diseases. They are also promising biomarker candidates due to their stability in body fluids. We investigated miRNA alterations in a Tg501 mouse model of prion diseases that expresses a transgene encoding the goat prion protein (PRNP). Tg501 mice intracranially inoculated with mouse-adapted goat scrapie were compared with age-matched, mock inoculated controls in preclinical and clinical stages. Small RNA sequencing from the cervical spinal cord indicated that miR-223-3p, miR-151-3p, and miR-144-5p were dysregulated in scrapie-inoculated animals before the onset of symptoms. In clinical-stage animals, 23 significant miRNA alterations were found. These miRNAs were predicted to modify the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathways including prion disease, extracellular matrix interactions, glutaminergic synapse, axon guidance, and transforming growth factor-beta signaling. MicroRNAs miR-146a-5p (up in cervical spinal cord) and miR-342-3p (down in cervical spinal cord, cerebellum and plasma), both indicated in neurodegenerative diseases earlier, were verified by quantitative real-time polymerase chain reaction (qRT-PCR). Minimal changes observed before the disease onset suggests that most miRNA alterations observed here are driven by advanced prion-associated pathology, possibly limiting their use as diagnostic markers. However, the results encourage further mechanistic studies on miRNA-regulated pathways involved in these neurodegenerative conditions.

Prion Protein in Stem Cells: A Lipid Raft Component Involved in the Cellular Differentiation Process

The prion protein (PrP) is an enigmatic molecule with a pleiotropic effect on different cell types; it is localized stably in lipid raft microdomains and it is able to recruit downstream signal transduction pathways by its interaction with various biochemical partners. Since its discovery, this lipid raft component has been involved in several functions, although most of the publications focused on the pathological role of the protein. Recent studies report a key role of cellular prion protein (PrP^C) in physiological processes, including cellular differentiation. Indeed, the PrP^C, whose expression is modulated according to the cell differentiation degree, appears to be part of the multimolecular signaling pathways of the neuronal differentiation process. In this review, we aim to summarize the main findings that report the link between PrP^C and stem cells.

Native nanodiscs formed by styrene maleic acid copolymer derivatives help recover infectious prion multimers bound to brain-derived lipids

Prions are lipidated proteins that interact with endogenous lipids and metal ions. They also assemble into multimers and propagate into the infectious scrapie form known as PrP^Sc The high-resolution structure of the infectious PrP^Sc state remains unknown, and its analysis largely relies on detergent-based preparations devoid of endogenous ligands. Here we designed polymers that allow isolation of endogenous membrane:protein assemblies in native nanodiscs without exposure to conventional detergents that destabilize protein structures and induce fibrillization. A set of styrene-maleic acid (SMA) polymers including a methylamine derivative facilitated gentle release of the infectious complexes for resolution of multimers, and a thiol-containing version promoted crystallization. Polymer extraction from brain homogenates from Syrian hamsters infected with Hyper prions and WT mice infected with Rocky Mountain Laboratories prions yielded infectious prion nanoparticles including oligomers and microfilaments bound to lipid vesicles. Lipid analysis revealed the brain phospholipids that associate with prion protofilaments, as well as those that are specifically enriched in prion assemblies captured by the methylamine-modified copolymer. A comparison of the infectivity of PrP^Sc attached to SMA lipid particles in mice and hamsters indicated that these amphipathic polymers offer a valuable tool for high-yield production of intact, detergent-free prions that retain in vivo activity. This native prion isolation method provides an avenue for producing relevant prion:lipid targets and potentially other proteins that form multimeric assemblies and fibrils on membranes.

Lipid rafts and pathogens: the art of deception and exploitation

Lipid rafts, solid regions of the plasma membrane enriched in cholesterol and glycosphingolipids, are essential parts of a cell. Functionally, lipid rafts present a platform that facilitates interaction of cells with the outside world. However, the unique properties of lipid rafts required to fulfill this function at the same time make them susceptible to exploitation by pathogens. Many steps of pathogen interaction with host cells, and sometimes all steps within the entire lifecycle of various pathogens, rely on host lipid rafts. Such steps as binding of pathogens to the host cells, invasion of intracellular parasites into the cell, the intracellular dwelling of parasites, microbial assembly and exit from the host cell, and microbe transfer from one cell to another all involve lipid rafts. Interaction also includes modification of lipid rafts in host cells, inflicted by pathogens from both inside and outside the cell, through contact or remotely, to advance pathogen replication, to utilize cellular resources, and/or to mitigate immune response. Here, we provide a systematic overview of how and why pathogens interact with and exploit host lipid rafts, as well as the consequences of this interaction for the host, locally and systemically, and for the microbe. We also raise the possibility of modulation of lipid rafts as a therapeutic approach against a variety of infectious agents.

Molecular Chaperones in Neurodegeneration

Cellular chaperones are essential players to this protein quality control network that functions to prevent protein misfolding, refold misfolded proteins, or degrade them, thereby maintaining neuronal proteostasis. Moreover, overexpression of cellular chaperones is considered to inhibit protein aggregation and apoptosis in various experimental models of neurodegeneration. Alterations or downregulation of chaperone machinery by age-related decline, molecular crowding, or genetic mutations are regarded as key pathological hallmarks of neurodegenerative disorders like Alzheimer's disease (AD), Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS), Huntington's disease (HD), and Prion diseases. Therefore, chaperones may serve as potential therapeutic targets in these diseases. This chapter presents a generalized view of misfolding and aggregation of proteins in neurodegeneration and then critically analyses some of the known cellular chaperones and their role in several neurodegenerative disorders.

A multimolecular signaling complex including PrPC and LRP1 is strictly dependent on lipid rafts and is essential for the function of tissue plasminogen activator

Prion protein (PrP^C ) localizes stably in lipid rafts microdomains and is able to recruit downstream signal transduction pathways by the interaction with promiscuous partners. Other proteins have the ability to occasionally be recruited to these specialized membrane areas, within multimolecular complexes. Among these, we highlight the presence of the low-density lipoprotein receptor-related protein 1 (LRP1), which was found localized transiently in lipid rafts, suggesting a different function of this receptor that through lipid raft becomes able to activate a signal transduction pathway triggered by specific ligands, including Tissue plasminogen activator (tPA). Since it has been reported that PrP^C participates in the tPA-mediated plasminogen activation, in this study, we describe the role of lipid rafts in the recruitment and activation of downstream signal transduction pathways mediated by the interaction among tPA, PrP^C and LRP1 in human neuroblastoma SK-N-BE2 cell line. Co-immunoprecipitation analysis reveals a consistent association between PrP^C and GM1, as well as between LRP1 and GM1, indicating the existence of a glycosphingolipid-enriched multimolecular complex. In our cell model, knocking-down PrP^C by siRNA impairs ERK phosphorylation induced by tPA. Moreover the alteration of the lipidic milieu of lipid rafts, perturbing the physical/functional interaction between PrP^C and LRP1, inhibits this response. We show that LRP1 and PrP^C , following tPA stimulation, may function as a system associated with lipid rafts, involved in receptor-mediated neuritogenic pathway. We suggest this as a multimolecular signaling complex, whose activity depends strictly on the integrity of lipid raft and is involved in the neuritogenic signaling.

Benefits of curcumin in brain disorders

Curcumin is widely consumed in Asia either as turmeric directly or as one of the culinary ingredients in food recipes. The benefits of curcumin in different organ systems have been reported extensively in several neurological diseases and cancer. Curcumin has got its global recognition because of its strong antioxidant, anti-inflammatory, anti-cancer, and antimicrobial activities. Additionally, it is used in diabetes and arthritis as well as in hepatic, renal, and cardiovascular diseases. Recently, there is growing attention on usage of curcumin to prevent or delay the onset of neurodegenerative diseases. This review summarizes available data from several recent studies on curcumin in various neurological diseases such as Alzheimer's disease, Parkinson's disease, Multiple Sclerosis, Huntington's disease, Prions disease, stroke, Down's syndrome, autism, Amyotrophic lateral sclerosis, anxiety, depression, and aging. Recent advancements toward increasing the therapeutic efficacy of curcuma/curcumin formulation and the novel delivery strategies employed to overcome its minimal bioavailability and toxicity studies have also been discussed. This review also summarizes the ongoing clinical trials on curcumin for different neurodegenerative diseases and patent details of curcuma/curcumin in India.

Cellular Prion Protein as a Receptor of Toxic Amyloid-β42 Oligomers Is Important for Alzheimer's Disease

The pathological features of Alzheimer's disease (AD) include senile plaques induced by amyloid-β (Aβ) protein deposits, neurofibrillary tangles formed by aggregates of hyperphosphorylated tau proteins and neuronal cell loss in specific position within the brain. Recent observations have suggested the possibility of an association between AD and cellular prion protein (PrP ^C ) levels. PrP ^C is a high affinity receptor for oligomeric Aβ and is important for Aβ-induced neurotoxicity and thus plays a critical role in AD pathogenesis. The determination of the relationship between PrP ^C and AD and the characterization of PrP ^C binding to Aβ will facilitate the development of novel therapies for AD.

Mutations in Prion Protein Gene: Pathogenic Mechanisms in C-Terminal vs. N-Terminal Domain, a Review

Inherited mutations in the Prion protein (PrP), encoded by the PRNP gene, have been associated with autosomal dominant neurodegenerative disorders, such as Creutzfeldt-Jacob disease (CJD), Gerstmann-Sträussler-Scheinker syndrome (GSS), and Fatal Familial Insomnia (FFI). Notably, PRNP mutations have also been described in clinical pictures resembling other neurodegenerative diseases, such as frontotemporal dementia. Regarding the pathogenesis, it has been observed that these point mutations are located in the C-terminal region of the PRNP gene and, currently, the potential significance of the N-terminal domain has largely been underestimated. The purpose of this report is to review and provide current insights into the pathogenic mechanisms of PRNP mutations, emphasizing the differences between the C- and N-terminal regions and focusing, in particular, on the lesser-known flexible N-terminal, for which recent biophysical evidence has revealed a physical interaction with the globular C-terminal domain of the cellular prion protein (PrPC).

All the Same? The Secret Life of Prion Strains within Their Target Cells

Prions are infectious β-sheet-rich protein aggregates composed of misfolded prion protein (PrP^Sc) that do not possess coding nucleic acid. Prions replicate by recruiting and converting normal cellular PrP^C into infectious isoforms. In the same host species, prion strains target distinct brain regions and cause different disease phenotypes. Prion strains are associated with biophysically distinct PrP^Sc conformers, suggesting that strain properties are enciphered within alternative PrP^Sc quaternary structures. So far it is unknown how prion strains target specific cells and initiate productive infections. Deeper mechanistic insight into the prion life cycle came from cell lines permissive to a range of different prion strains. Still, it is unknown why certain cell lines are refractory to infection by one strain but permissive to another. While pharmacologic and genetic manipulations revealed subcellular compartments involved in prion replication, little is known about strain-specific requirements for endocytic trafficking pathways. This review summarizes our knowledge on how prions replicate within their target cells and on strain-specific differences in prion cell biology.

GPI-anchor signal sequence influences PrPC sorting, shedding and signalling, and impacts on different pathomechanistic aspects of prion disease in mice

The cellular prion protein (PrP^C) is a cell surface glycoprotein attached to the membrane by a glycosylphosphatidylinositol (GPI)-anchor and plays a critical role in transmissible, neurodegenerative and fatal prion diseases. Alterations in membrane attachment influence PrP^C-associated signaling, and the development of prion disease, yet our knowledge of the role of the GPI-anchor in localization, processing, and function of PrP^Cin vivo is limited We exchanged the PrP^C GPI-anchor signal sequence of for that of Thy-1 (PrP^CGPIThy-1) in cells and mice. We show that this modifies the GPI-anchor composition, which then lacks sialic acid, and that PrP^CGPIThy-1 is preferentially localized in axons and is less prone to proteolytic shedding when compared to PrP^C. Interestingly, after prion infection, mice expressing PrP^CGPIThy-1 show a significant delay to terminal disease, a decrease of microglia/astrocyte activation, and altered MAPK signaling when compared to wild-type mice. Our results are the first to demonstrate in vivo, that the GPI-anchor signal sequence plays a fundamental role in the GPI-anchor composition, dictating the subcellular localization of a given protein and, in the case of PrP^C, influencing the development of prion disease.

The prion protein (PrP^C) is a glycoprotein attached to the neuronal surface via a GPI-anchor. When misfolded to PrP^Sc, it leads to fatal neurodegenerative diseases which propagates from host to host. PrP^Sc is the principal component of the infectious agent of prion diseases, the “prion”. Misfolding occurs at the plasma membrane, and when PrP^C lacks the GPI-anchor, neuropathology and incubation time of prion disease are strongly modified. Moreover, the composition of the PrP^C GPI-anchor impacts on the conversion process. To study the role of the GPI-anchor in the pathophysiology of prion diseases in vivo, we have generated transgenic mice where the PrP^C GPI-signal sequence (GPI-SS) is replaced for the one of Thy-1, a neuronal protein with a distinct GPI-anchor and membrane localization. We found that the resulting protein, PrP^CGPIThy-1, shows a different GPI-anchor composition, increased axonal localization, and reduced enzymatic shedding. After prion infection, disease progression is significantly delayed, and the neuropathology and cellular signaling are changed.

The present work demonstrates that the GPI-SS per se determines the GPI-anchor composition and localization of a given protein and it stresses the importance of PrP^C membrane anchorage in prion disease.

Caveolin-1 and MLRs: A potential target for neuronal growth and neuroplasticity after ischemic stroke

Ischemic stroke is a leading cause of morbidity and mortality worldwide. Thrombolytic therapy, the only established treatment to reduce the neurological deficits caused by ischemic stroke, is limited by time window and potential complications. Therefore, it is necessary to develop new therapeutic strategies to improve neuronal growth and neurological function following ischemic stroke. Membrane lipid rafts (MLRs) are crucial structures for neuron survival and growth signaling pathways. Caveolin-1 (Cav-1), the main scaffold protein present in MLRs, targets many neural growth proteins and promotes growth of neurons and dendrites. Targeting Cav-1 may be a promising therapeutic strategy to enhance neuroplasticity after cerebral ischemia. This review addresses the role of Cav-1 and MLRs in neuronal growth after ischemic stroke, with an emphasis on the mechanisms by which Cav-1/MLRs modulate neuroplasticity via related receptors, signaling pathways, and gene expression. We further discuss how Cav-1/MLRs may be exploited as a potential therapeutic target to restore neuroplasticity after ischemic stroke. Finally, several representative pharmacological agents known to enhance neuroplasticity are discussed in this review.

Changes in Membrane Protein Clustering in Peripheral Lymphocytes in an Animal Model of Depression Parallel Those Observed in Naïve Depression Patients: Implications for the Development of Novel Biomarkers of Depression

Naïve depression patients show alterations in serotonin transporter (SERT) and serotonin 2A (5HT2A) receptor clustering in peripheral lymphocytes, and these alterations have been proposed as a biomarker of therapeutic efficacy in major depression. Repeated corticosterone (CORT) induces a consistent depression-like phenotype and has been widely used as an animal model to study neurobiological alterations underlying the depressive symptoms. In this experiment, we used the CORT paradigm to evaluate whether depression-like behavior is associated with similar changes in the pattern of SERT and 5HT2A membrane protein clustering as those observed in depression patients. We also analyzed the clustering of other proteins expressed in lipid rafts in lymphocytes. Rats received daily CORT or vehicle injections for 21 consecutive days. Afterward they underwent the forced swim test to evaluate depression-like behavior, and isolated lymphocytes were analyzed by immunocytochemistry coupled to image-analysis to study clustering parameters of the SERT, 5HT2A receptor, dopamine transporter (DAT), Beta2 adrenergic receptor (β2AR), NMDA 2B receptor (NR2B), Pannexin 1 (Pnx1), and prion cellular protein (PrPc). Our results showed that CORT increases the size of protein clusters for all proteins with the exception of β 2AR, which is decreased. CORT also increased the number of clusters for Pnx1 and PrPc only. Overall, these results indicate that alterations in SERT and 5HT2A protein clustering in naïve depression patients are paralleled by changes seen in an animal model of depression. The CORT paradigm may be a useful screen for examining additional proteins in lymphocytes as a preliminary step prior to their analysis as biomarkers of depression in human blood samples.

Extending the functional characteristics of naturally occurring autoantibodies against β-Amyloid, Prion Protein and α-Synuclein

Background

Abnormal aggregation of proteins induces neuronal cell loss in neurodegenerative disorders such as Alzheimer’s Disease, Creutzfeldt-Jakob Disease and Parkinson’s Disease. Specific stimuli initialize conformational changes in physiological proteins, causing intra- or extracellular protein aggregation. We and other groups have identified naturally occurring autoantibodies (nAbs) as part of the human antibody pool that are able to prevent peptide fibrillation. These nAbs show a rescue effect following exposure of toxic aggregates on neurons, and they support microglial uptake of aggregated peptides.

Objective

Identification of a putative common epitope among the relevant proteins β-Amyloid, α-Synuclein and Prion Protein for the respective nAbs.

Material and methods

Binding affinity between the aforementioned proteins and nAbs was tested by Dot Blot, ELISA and SPR-technology. Furthermore, the functionality of the protein-nAbs-complexes was studied in Thioflavin-T assays and microglial uptake experiments to study dependent inhibition of protein aggregation and enhancement of Fcγ mediated uptake by microglial cells.

Results

β-Amyloid and Prion Protein fragment showed considerable binding affinity and functional efficacy for all applied nAbs. Thereby, no significant difference within the different nAbs was detected. In contrast, α-Synuclein was bound exclusively by nAbs-α-Synuclein, which was reproduced in all binding studies. Surprisingly, functional assays with α-Synuclein revealed no significant effect of nAbs in comparison to IVIg treatment. However, all applied nAbs as well as IVIg show a minimal functionality on the microglial uptake of α-Synuclein.

Conclusion

nAbs-Aβ, nAbs-PrP possibly display comparable affinity to the same structural epitope within Aβ and PrP106-126 A117V whereas the epitope recognized by nAbs-α-Syn is only present in α-Syn. The structural similarity of Aβ and PrP fragment promotes the outline for an efficient antibody for the treatment of several neurodegenerative disorders and extend the functional characteristics of the investigated nAbs.

Organization of GPI-anchored proteins at the cell surface and its physiopathological relevance

Glycosylphosphatidylinositol (GPI)-anchored proteins (GPI-APs) are a class of proteins attached to the extracellular leaflet of the plasma membrane via a post-translational modification, the glycolipid anchor. The presence of both glycolipid anchor and protein portion confers them unique features. GPI-APs are expressed in all eukaryotes, from fungi to plants and animals. They display very diverse functions ranging from enzymatic activity, signaling, cell adhesion, cell wall metabolism, neuritogenesis, and immune response. Likewise other plasma membrane proteins, the spatio-temporal organization of GPI-APs is critical for their biological activities in physiological conditions. In this review, we will summarize the latest findings on plasma membrane organization of GPI-APs and the mechanism of its regulation in different cell types. We will also examine the involvement of specific GPI-APs namely the prion protein PrP^C, the Folate Receptor alpha and the urokinase plasminogen activator receptor in human diseases focusing on neurodegenerative diseases and cancer.

Estrogen Interactions With Lipid Rafts Related to Neuroprotection. Impact of Brain Ageing and Menopause

Estrogens (E2) exert a plethora of neuroprotective actions against aged-associated brain diseases, including Alzheimer's disease (AD). Part of these actions takes place through binding to estrogen receptors (ER) embedded in signalosomes, where numerous signaling proteins are clustered. Signalosomes are preferentially located in lipid rafts which are dynamic membrane microstructures characterized by a peculiar lipid composition enriched in gangliosides, saturated fatty acids, cholesterol, and sphingolipids. Rapid E2 interactions with ER-related signalosomes appear to trigger intracellular signaling ultimately leading to the activation of molecular mechanisms against AD. We have previously observed that the reduction of E2 blood levels occurring during menopause induced disruption of ER-signalosomes at frontal cortical brain areas. These molecular changes may reduce neuronal protection activities, as similar ER signalosome derangements were observed in AD brains. The molecular impairments may be associated with changes in the lipid composition of lipid rafts observed in neurons during menopause and AD. These evidences indicate that the changes in lipid raft structure during aging may be at the basis of alterations in the activity of ER and other neuroprotective proteins integrated in these membrane microstructures. Moreover, E2 is a homeostatic modulator of lipid rafts. Recent work has pointed to this relevant aspect of E2 activity to preserve brain integrity, through mechanisms affecting lipid uptake and local biosynthesis in the brain. Some evidences have demonstrated that estrogens and the docosahexaenoic acid (DHA) exert synergistic effects to stabilize brain lipid matrix. DHA is essential to enhance molecular fluidity at the plasma membrane, promoting functional macromolecular interactions in signaling platforms. In support of this, DHA detriment in neuronal lipid rafts has been associated with the most common age-associated neuropathologies, namely AD and Parkinson disease. Altogether, these findings indicate that E2 may participate in brain preservation through a dual membrane-related mechanism. On the one hand, E2 interacting with ER related signalosomes may protect against neurotoxic insults. On the other hand, E2 may exert lipostatic actions to preserve lipid balance in neuronal membrane microdomains. The different aspects of the emerging multifunctional role of estrogens in membrane-related signalosomes will be discussed in this review.

Enhanced detection of infectious prions by direct ELISA from the brains of asymptomatic animals using DRM2-118 monoclonal antibody and Gdn-HCl

In this report we describe the use of a novel anti-prion monoclonal antibody (DRM2-118) for the direct detection of infectious prions by ELISA. Epitope mapping using overlapping hamster (SHa) prion peptides indicates DRM2-118 binding occurs between residues 93-100 and at the 3₁₀-helix (residues 163-170) between alpha helix-A and -B. This antibody shows broad species binding to endogenous prions from brain homogenates and corresponding recombinant prion proteins. To evaluate the performance of this MAb for the detection of prion proteins we performed an animal time course and evaluated prion detection from both crude brain homogenates and lipid raft fractions (DRM) by direct ELISA. Prion detection was significantly enhanced by the addition of the chaotropic guanidine-HCl (Gdn-HCl) during protein immobilization with detection of PK-resistant prion from asymptomatic animal brains at (45-DPI) and from lipid rafts at (24-DPI). Our data demonstrates enhanced prion detection from brain lipid rafts of asymptomatic animals by a simple direct ELISA using the DRM2-118 MAb combined with Gdn-HCl.

The function of the cellular prion protein in health and disease

The essential role of the cellular prion protein (PrP^C) in prion disorders such as Creutzfeldt-Jakob disease is well documented. Moreover, evidence is accumulating that PrP^C may act as a receptor for protein aggregates and transduce neurotoxic signals in more common neurodegenerative disorders, such as Alzheimer's disease. Although the pathological roles of PrP^C have been thoroughly characterized, a general consensus on its physiological function within the brain has not yet been established. Knockout studies in various organisms, ranging from zebrafish to mice, have implicated PrP^C in a diverse range of nervous system-related activities that include a key role in the maintenance of peripheral nerve myelination as well as a general ability to protect against neurotoxic stimuli. Thus, the function of PrP^C may be multifaceted, with different cell types taking advantage of unique aspects of its biology. Deciphering the cellular function(s) of PrP^C and the consequences of its absence is not simply an academic curiosity, since lowering PrP^C levels in the brain is predicted to be a powerful therapeutic strategy for the treatment of prion disease. In this review, we outline the various approaches that have been employed in an effort to uncover the physiological and pathological functions of PrP^C. While these studies have revealed important clues about the biology of the prion protein, the precise reason for PrP^C's existence remains enigmatic.