Anionic phospholipid interactions of the prion protein N terminus are minimally perturbing and not driven solely by the octapeptide repeat domain

Effect of phospholipid liposomes on prion fragment (106-128) amyloid formation

Misfolding and aggregation of cellular prion protein (PrPc) is a major molecular process involved in the pathogenesis of prion diseases. Here, we studied the aggregation properties of a prion fragment peptide PrP(106-128). The results show that the peptide aggregates in a concentration-dependent manner in an aqueous solution and that the aggregation is sensitive to pH and the preformed amyloid seeds. Furthermore, we show that the zwitterionic POPC liposomes moderately inhibit the aggregation of PrP(106-128), whereas POPC/cholesterol (8:2) vesicles facilitate peptide aggregation likely due to the increase of the lipid packing order and membrane rigidity in the presence of cholesterol. In addition, anionic lipid vesicles of POPG and POPG/cholesterol above a certain concentration accelerate the aggregation of the peptide remarkably. The strong electrostatic interactions between the N-terminal region of the peptide and POPG may constrain the conformational plasticity of the peptide, preventing insertion of the peptide into the inner side of the membrane and thus promoting fibrillation on the membrane surface. The results suggest that the charge properties of the membrane, the composition of the liposomes, and the rigidity of lipid packing are critical in determining peptide adsorption on the membrane surface and the efficiency of the membrane in catalyzing peptide oligomeric nucleation and amyloid formation. The peptide could be used as an improved model molecule to investigate the mechanistic role of the crucial regions of PrP in aggregation in a membrane-rich environment and to screen effective inhibitors to block key interactions between these regions and membranes for preventing PrP aggregation.

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.

The Size and Stability of Infectious Prion Aggregates Fluctuate Dynamically during Cellular Uptake and Disaggregation

Prion diseases arise when PrP^Sc, an aggregated, infectious, and insoluble conformer of the normally soluble mammalian prion protein, PrP^C, catalyzes the conversion of PrP^C into more PrP^Sc, which then accumulates in the brain leading to disease. PrP^Sc is the primary, if not sole, component of the infectious prion. Despite the stability and protease insensitivity of PrP^Sc aggregates, they can be degraded after cellular uptake. However, how cells disassemble and degrade PrP^Sc is poorly understood. In this work, we analyzed how the protease sensitivity and size distribution of PrP^Sc aggregates from two different mouse-adapted prion strains, 22L, that can persistently infect cells and 87V, that cannot, changed during cellular uptake. We show that within the first 4 h following uptake large PrP^Sc aggregates from both prion strains become less resistant to digestion by proteinase K (PK) through a mechanism that is dependent upon the acidic environment of endocytic vesicles. We further show that during disassembly, PrP^Sc aggregates from both strains become more resistant to PK digestion through the apparent removal of protease-sensitive PrP^Sc, with PrP^Sc from the 87V strain disassembled more readily than PrP^Sc from the 22L strain. Taken together, our data demonstrate that the sizes and stabilities of PrP^Sc from different prion strains change during cellular uptake and degradation, thereby potentially impacting the ability of prions to infect cells.

Prion protein N1 cleavage peptides stimulate microglial interaction with surrounding cells

Microglia act as the protective immune cell of the brain. By surveying the tissue to identify and rectify problems, they function to maintain the health of brain cells. The prion protein N-terminal cleavage fragment, N1, has demonstrated neuroprotective activities in vitro and in vivo. This study aimed to elucidate whether N1 could modulate microglial function and, if so, determine the consequences for the surrounding tissue. Using a mixed neuronal lineage and microglia co-culture system, we showed that N1 stimulation changed overall morphology and metabolism, suggesting enhanced cellular viability. Furthermore, N1 induced an increase in Cxcl10 secretion in the co-cultures. Recombinant Cxcl10, administered exogenously, mediated the changes in the mixed neuronal lineage culture morphology and metabolism in the absence of microglia, but no effect of Cxcl10 was observed on microglia cultured on their own. Direct cell-to-cell contact was required for N1 to influence microglia in the co-cultures, and this was linked with restructuring of microglial membrane composition to include a higher GM1 content at interaction sites with surrounding cells. Our findings show that N1 can play a regulatory role in microglial function in the context of an inter-connected network of cells by changing both cellular interaction sites and cytokine secretion.

Membrane Binding Strongly Affecting the Dopamine Reactivity Induced by Copper Prion and Copper/Amyloid-β (Aβ) Peptides. A Ternary Copper/Aβ/Prion Peptide Complex Stabilized and Solubilized in Sodium Dodecyl Sulfate Micelles

The combination between dyshomeostatic levels of catecholamine neurotransmitters and redox-active metals such as copper and iron exacerbates the oxidative stress condition that typically affects neurodegenerative diseases. We report a comparative study of the oxidative reactivity of copper complexes with amyloid-β (Aβ₄₀) and the prion peptide fragment 76-114 (PrP_76-114), containing the high-affinity binding site, toward dopamine and 4-methylcatechol, in aqueous buffer and in sodium dodecyl sulfate micelles, as a model membrane environment. The competitive oxidative and covalent modifications undergone by the peptides were also evaluated. The high binding affinity of Cu/peptide to micelles and lipid membranes leads to a strong reduction (Aβ₄₀) and quenching (PrP_76-114) of the oxidative efficiency of the binary complexes and to a stabilization and redox silencing of the ternary complex Cu^II/Aβ₄₀/PrP_76-114, which is highly reactive in solution. The results improve our understanding of the pathological and protective effects associated with these complexes, depending on the physiological environment.

The curious case of opossum prion: a physicochemical study on copper(ii) binding to the bis-decarepeat fragment from the protein N-terminal domain

The opossum is a peculiar model of immunity to prion diseases. Here we scrutinised the bis-decarepeat peptide sequence of the opossum prion (Op_bis-deca) protein by a multitechnique approach, with a combined experimental (potentiometry, UV-visible, circular dichroism, NMR and EPR spectroscopy, quartz crystal microbalance with dissipation monitoring and confocal microscopy) and simulation (DFT calculations) approach. Results showed that the macrochelate structures formed upon the binding to Cu(ii) by the analogous bis-octarepeat peptide sequence of human prion (Hu_bis-octa) are not found in the case of Op_bis-deca. At physiological pH and equimolar amount of copper ions, the [CuLH-2] is the major species formed by Op_bis-deca. In this species one imidazole and two amide nitrogen atoms are involved in metal coordination and its stability constant value is lower than that of the analogous species formed by Hu_bis-octa, due to the presence of an extra proline residue. Moreover, the study on the interaction of the peptides or the peptide/Cu(ii) complexes with the model cell membranes made of supported lipid bilayers disclosed different levels of interaction, monitored by the viscoelastic changes of the membranes, which exhibited a similar viscoelastic response at the interface of the two complexes, while in the absence of Cu(ii), the Hu_bis-octa/SLB interface was more viscoelastic than the Op_bis-deca one.

The molecular basis of interaction domains of full-length PrP with lipid membranes

PrP-lipid membrane interactions are critical to PrP structural conversion and neurotoxicity, but its molecular mechanism remains unclear. A two-dimensional histogram of force-distance curves and a worm-like chain model revealed three binding regions at the PrP N-terminal, providing the molecular basis for understanding the interactions between full-length PrP and lipid membranes.

Prion protein cleavage fragments regulate adult neural stem cell quiescence through redox modulation of mitochondrial fission and SOD2 expression

Neurogenesis continues in the post-developmental brain throughout life. The ability to stimulate the production of new neurones requires both quiescent and actively proliferating pools of neural stem cells (NSCs). Actively proliferating NSCs ensure that neurogenic demand can be met, whilst the quiescent pool makes certain NSC reserves do not become depleted. The processes preserving the NSC quiescent pool are only just beginning to be defined. Herein, we identify a switch between NSC proliferation and quiescence through changing intracellular redox signalling. We show that N-terminal post-translational cleavage products of the prion protein (PrP) induce a quiescent state, halting NSC cellular growth, migration, and neurite outgrowth. Quiescence is initiated by the PrP cleavage products through reducing intracellular levels of reactive oxygen species. First, inhibition of redox signalling results in increased mitochondrial fission, which rapidly signals quiescence. Thereafter, quiescence is maintained through downstream increases in the expression and activity of superoxide dismutase-2 that reduces mitochondrial superoxide. We further observe that PrP is predominantly cleaved in quiescent NSCs indicating a homeostatic role for this cascade. Our findings provide new insight into the regulation of NSC quiescence, which potentially could influence brain health throughout adult life.

Diverse functions of the prion protein - Does proteolytic processing hold the key?

Proteolytic processing of the cellular and disease-associated form of the prion protein leads to generation of bioactive soluble prion protein fragments and modifies the structure and function of its cell-bound form. The nature of proteases responsible for shedding, α-, β-, and γ-cleavage of the prion protein are only partially identified and their regulation is largely unknown. Here, we provide an overview of the increasingly multifaceted picture of prion protein proteolysis and shed light on physiological and pathological roles associated with these cleavages. This article is part of a Special Issue entitled: Proteolysis as a Regulatory Event in Pathophysiology edited by Stefan Rose-John.

Simulations of membrane-bound diglycosylated human prion protein reveal potential protective mechanisms against misfolding

Prion diseases are associated with the misfolding of the prion protein (PrP) from its normal cellular form (PrP^C ) to its infectious scrapie form (PrP^Sc ). Post-translational modifications in PrP in vivo can play an important role in modulating the process of misfolding. To gain more insight into the effects of post-translational modifications in PrP structure and dynamics and to test the hypothesis that such modifications can interact with the protein, we have performed molecular dynamics simulations of diglycosylated human PrP^C bound to a lipid bilayer via a glycophosphatidylinositol anchor. Multiple simulations were performed at three different pH ranges to explore pH effects on structure and dynamics. In contrast to simulations of protein-only PrP^C , no large effects were observed upon lowering the pH of the system. The protein tilted toward the membrane surface in all of the simulations and the putative PrP^Sc oligomerization sites became inaccessible, thereby offering a possible protective mechanism against PrP^Sc -induced misfolding of PrP^C .

PrPC Undergoes Basal to Apical Transcytosis in Polarized Epithelial MDCK Cells

The Prion Protein (PrP) is an ubiquitously expressed glycosylated membrane protein attached to the external leaflet of the plasma membrane via a glycosylphosphatidylinositol anchor (GPI). While the misfolded PrPSc scrapie isoform is the infectious agent of prion disease, the cellular isoform (PrPC) is an enigmatic protein with unclear function. Of interest, PrP localization in polarized MDCK cells is controversial and its mechanism of trafficking is not clear. Here we investigated PrP traffic in MDCK cells polarized on filters and in three-dimensional MDCK cysts, a more physiological model of polarized epithelia. We found that, unlike other GPI-anchored proteins (GPI-APs), PrP undergoes basolateral-to-apical transcytosis in fully polarized MDCK cells. Following this event full-length PrP and its cleavage fragments are segregated in different domains of the plasma membrane in polarized cells in both 2D and 3D cultures.

Nanoscale insights into full-length prion protein aggregation on model lipid membranes

The aggregates of the full-length human recombinant prion protein (PrP) (23–231) on model membranes were investigated by combining the atomic force microscopy (AFM) measurements and theoretical calculations at pH 5.0, showing the great effect of PrP concentration on its supramolecular assemblies on the lipid bilayer.

PrP charge structure encodes interdomain interactions

Almost all proteins contain charged residues, and their chain distribution is tailored to fulfill essential ionic interactions for folding, binding and catalysis. Among proteins, the hinged two-domain chain of the cellular prion protein (PrP^C) exhibits a peculiar charge structure with unclear consequences in its structural malleability. To decipher the charge design role, we generated charge-reverted mutants for each domain and analyzed their effect on conformational and metabolic features. We found that charges contain the information for interdomain interactions. Use of dynamic light scattering and thermal denaturation experiments delineates the compaction of the α-fold by an electrostatic compensation between the polybasic 23–30 region and the α3 electronegative surface. This interaction increases stability and disfavors fibrillation. Independently of this structural effect, the N-terminal electropositive clusters regulate the α-cleavage efficiency. In the fibrillar state, use of circular dichroism, atomic-force and fluorescence microscopies reveal that the N-terminal positive clusters and the α3 electronegative surface dictate the secondary structure, the assembly hierarchy and the growth length of the fibril state. These findings show that the PrP charge structure functions as a code set up to ensure function and reduce pathogenic routes.

The Prion Protein N1 and N2 Cleavage Fragments Bind to Phosphatidylserine and Phosphatidic Acid; Relevance to Stress-Protection Responses

Internal cleavage of the cellular prion protein generates two well characterised N-terminal fragments, N1 and N2. These fragments have been shown to bind to anionic phospholipids at low pH. We sought to investigate binding with other lipid moieties and queried how such interactions could be relevant to the cellular functions of these fragments. Both N1 and N2 bound phosphatidylserine (PS), as previously reported, and a further interaction with phosphatidic acid (PA) was also identified. The specificity of this interaction required the N-terminus, especially the proline motif within the basic amino acids at the N-terminus, together with the copper-binding region (unrelated to copper saturation). Previously, the fragments have been shown to be protective against cellular stresses. In the current study, serum deprivation was used to induce changes in the cellular lipid environment, including externalisation of plasma membrane PS and increased cellular levels of PA. When copper-saturated, N2 could reverse these changes, but N1 could not, suggesting that direct binding of N2 to cellular lipids may be part of the mechanism by which this peptide signals its protective response.

Protein-ligand and membrane-ligand interactions in pharmacology: the case of the translocator protein (TSPO)

The targets of many small molecule drugs are membrane proteins, and traditionally the focus of pharmacology is on the interaction between such receptors and their small molecule drug ligands. However, the lipid membranes of cells and organelles are increasingly appreciated as diverse and dynamic structures that also specifically interact with small molecule drugs and peptides, causing profound changes in the properties of these membranes, and modulating the function of the membrane and the proteins within it. Drug-membrane interactions are likely to have a role in both the therapeutic and toxic activity of a variety of compounds, and their role in the overall pharmacological effect of a drug needs to be understood more clearly. This is the case for the 18 kDa translocator protein (TSPO) and its ligands, where functions that were established based on pharmacological studies are being called into question. Re-examining the putative functions of the TSPO and the effects of its ligands reveals a need to consider in more detail the interplay between protein-ligand and membrane-ligand interactions, and the modulatory relationship between TSPO and the lipid membrane.

Stored red blood cell susceptibility to in vitro transfusion-associated stress conditions is higher after longer storage and increased by storage in saline-adenine-glucose-mannitol compared to AS-1

BACKGROUND

Biochemical changes induced in red blood cells (RBCs) during storage may impair their function upon transfusion. Transfusion-associated stresses may further amplify storage lesion effects including increased phosphatidylserine (PS) exposure at the RBC membrane, microparticle (MP) release, and adhesion to endothelial cells (ECs). RBC stress susceptibility in vitro was investigated in relation to storage time and additive solution.

STUDY DESIGN AND METHODS

Leukoreduced whole blood donations (n = 18) were paired, mixed, and resplit before separating the RBCs for storage in saline-adenine-glucose-mannitol (SAGM) or AS-1. Samples were taken after 3, 21, or 35 days. For oxidative stress treatment, RBCs were exposed to 0.5 mmol/L tert-butylhydroperoxide. Transfusion-associated stress was simulated by overnight culture at 37 °C with plasma containing inflammatory mediators. PS exposure and MPs were measured by flow cytometry and adhesion to ECs was tested under flow conditions. PS specificity of adhesion was tested by blocking with PS-containing lipid vesicles.

RESULTS

Oxidative stress induced significantly higher PS exposure and adhesion to ECs in RBCs stored for 35 days compared to 3 days (p < 0.04). PS-containing vesicles blocked RBC-EC adhesion. After overnight culture with or without plasma, PS exposure and EC adhesion were significantly increased (p < 0.05). MP numbers increased with longer RBC storage and after RBC culture with plasma. Culture conditions influenced MP numbers from Day 35 RBCs. RBCs stored in SAGM had significantly higher PS exposure after stress treatment than AS-1 RBCs (p < 0.02).

CONCLUSION

Storage for 35 days significantly increased RBC susceptibility to oxidative and in vitro transfusion-associated stresses and was higher for RBCs stored in SAGM compared to AS-1.

MEK1 transduces the prion protein N2 fragment antioxidant effects

The prion protein (PrP(C)) when mis-folded is causally linked with a group of fatal neurodegenerative diseases called transmissible spongiform encephalopathies or prion diseases. PrP(C) normal function is still incompletely defined with such investigations complicated by PrP(C) post-translational modifications, such as internal cleavage, which feasibly could change, activate, or deactivate the function of this protein. Oxidative stress induces β-cleavage and the N-terminal product of this cleavage event, N2, demonstrates a cellular protective response against oxidative stress. The mechanisms by which N2 mediates cellular antioxidant protection were investigated within an in vitro cell model. N2 protection was regulated by copper binding to the octarepeat domain, directing the route of internalisation, which stimulated MEK1 signalling. Precise membrane interactions of N2, determined by copper saturation, and involving both the copper-co-ordinating octarepeat region and the structure conferred upon the N-terminal polybasic region by the proline motif, were essential for the correct engagement of this pathway. The phenomenon of PrP(C) post-translational modification, such as cleavage and copper co-ordination, as a molecular "switch" for activation or deactivation of certain functions provides new insight into the apparent multi-functionality of PrP(C).

Neutron reflectometry studies define prion protein N-terminal peptide membrane binding

The prion protein (PrP), widely recognized to misfold into the causative agent of the transmissible spongiform encephalopathies, has previously been shown to bind to lipid membranes with binding influenced by both membrane composition and pH. Aside from the misfolding events associated with prion pathogenesis, PrP can undergo various posttranslational modifications, including internal cleavage events. Alpha- and beta-cleavage of PrP produces two N-terminal fragments, N1 and N2, respectively, which interact specifically with negatively charged phospholipids at low pH. Our previous work probing N1 and N2 interactions with supported bilayers raised the possibility that the peptides could insert deeply with minimal disruption. In the current study we aimed to refine the binding parameters of these peptides with lipid bilayers. To this end, we used neutron reflectometry to define the structural details of this interaction in combination with quartz crystal microbalance interrogation. Neutron reflectometry confirmed that peptides equivalent to N1 and N2 insert into the interstitial space between the phospholipid headgroups but do not penetrate into the acyl tail region. In accord with our previous studies, interaction was stronger for the N1 fragment than for the N2, with more peptide bound per lipid. Neutron reflectometry analysis also detected lengthening of the lipid acyl tails, with a concurrent decrease in lipid area. This was most evident for the N1 peptide and suggests an induction of increased lipid order in the absence of phase transition. These observations stand in clear contrast to the findings of analogous studies of Ab and ?-synuclein and thereby support the possibility of a functional role for such N-terminal fragment-membrane interactions.

Investigating the interactions of the 18kDa translocator protein and its ligand PK11195 in planar lipid bilayers

The functional effects of a drug ligand may be due not only to an interaction with its membrane protein target, but also with the surrounding lipid membrane. We have investigated the interaction of a drug ligand, PK11195, with its primary protein target, the integral membrane 18kDa translocator protein (TSPO), and model membranes using Langmuir monolayers, quartz crystal microbalance with dissipation monitoring (QCM-D) and neutron reflectometry (NR). We found that PK11195 is incorporated into lipid monolayers and lipid bilayers, causing a decrease in lipid area/molecule and an increase in lipid bilayer rigidity. NR revealed that PK11195 is incorporated into the lipid chain region at a volume fraction of ~10%. We reconstituted isolated mouse TSPO into a lipid bilayer and studied its interaction with PK11195 using QCM-D, which revealed a larger than expected frequency response and indicated a possible conformational change of the protein. NR measurements revealed a TSPO surface coverage of 23% when immobilised to a modified surface via its polyhistidine tag, and a thickness of 51Å for the TSPO layer. These techniques allowed us to probe both the interaction of TSPO with PK11195, and PK11195 with model membranes. It is possible that previously reported TSPO-independent effects of PK11195 are due to incorporation into the lipid bilayer and alteration of its physical properties. There are also implications for the variable binding profiles observed for TSPO ligands, as drug-membrane interactions may contribute to the apparent affinity of TSPO ligands.

Conformational conversion of prion protein in prion diseases

Prion diseases are a group of infectious fatal neurodegenerative diseases. The conformational conversion of a cellular prion protein (PrP(C)) into an abnormal misfolded isoform (PrP(Sc)) is the key event in prion diseases pathology. Under normal conditions, the high-energy barrier separates PrP(C) from PrP(Sc) isoform. However, pathogenic mutations, modifications as well as some cofactors, such as glycosaminoglycans, nucleic acids, and lipids, could modulate the conformational conversion process. Understanding the mechanism of conformational conversion of prion protein is essential for the biomedical research and the treatment of prion diseases. Particularly, the characterization of cofactors interacting with prion protein might provide new diagnostic and therapeutic strategies.

The antimicrobial peptide aurein 1.2 disrupts model membranes via the carpet mechanism

The membrane interactions of the antimicrobial peptide aurein 1.2 were studied using a range of biophysical techniques to determine the location and the mechanism of action in DMPC (dimyristoylphosphatidylcholine) and DMPC/DMPG (dimyristoylphosphatidylglycerol) model membranes that mimic characteristics of eukaryotic and prokaryotic membranes, respectively. Neutron reflectometry and solid-state NMR revealed subtle changes in membrane structure caused by the peptide. Quartz crystal microbalance with dissipation, vesicle dye leakage and atomic force microscopy measurements were used to investigate the global mode of peptide interaction. Aurein 1.2 displayed an enhanced interaction with the anionic DMPC/DMPG membrane while exhibiting primarily a surface interaction with both types of model membranes, which led to bilayer disruption and membrane lysis. The antimicrobial peptide interaction is consistent with the carpet mechanism for aurein 1.2 with discrete structural changes depending on the type of phospholipid membrane.

The N-terminal, polybasic region is critical for prion protein neuroprotective activity

Several lines of evidence suggest that the normal form of the prion protein, PrP(C), exerts a neuroprotective activity against cellular stress or toxicity. One of the clearest examples of such activity is the ability of wild-type PrP(C) to suppress the spontaneous neurodegenerative phenotype of transgenic mice expressing a deleted form of PrP (Δ32-134, called F35). To define domains of PrP involved in its neuroprotective activity, we have analyzed the ability of several deletion mutants of PrP (Δ23-31, Δ23-111, and Δ23-134) to rescue the phenotype of Tg(F35) mice. Surprisingly, all of these mutants displayed greatly diminished rescue activity, although Δ23-31 PrP partially suppressed neuronal loss when expressed at very high levels. Our results pinpoint the N-terminal, polybasic domain as a critical determinant of PrP(C) neuroprotective activity, and suggest that identification of molecules interacting with this region will provide important clues regarding the normal function of the protein. Small molecule ligands targeting this region may also represent useful therapeutic agents for treatment of prion diseases.