BRICHOS domains efficiently delay fibrillation of amyloid β-peptide

Molecular basis for different substrate-binding sites and chaperone functions of the BRICHOS domain

Proteins can misfold into fibrillar or amorphous aggregates and molecular chaperones act as crucial guardians against these undesirable processes. The BRICHOS chaperone domain, found in several otherwise unrelated proproteins that contain amyloidogenic regions, effectively inhibits amyloid formation and toxicity but can in some cases also prevent non-fibrillar, amorphous protein aggregation. Here, we elucidate the molecular basis behind the multifaceted chaperone activities of the BRICHOS domain from the Bri2 proprotein. High-confidence AlphaFold2 and RoseTTAFold predictions suggest that the intramolecular amyloidogenic region (Bri23) is part of the hydrophobic core of the proprotein, where it occupies the proposed amyloid binding site, explaining the markedly reduced ability of the proprotein to prevent an exogenous amyloidogenic peptide from aggregating. However, the BRICHOS-Bri23 complex maintains its ability to form large polydisperse oligomers that prevent amorphous protein aggregation. A cryo-EM-derived model of the Bri2 BRICHOS oligomer is compatible with surface-exposed hydrophobic motifs that get exposed and come together during oligomerization, explaining its effects against amorphous aggregation. These findings provide a molecular basis for the BRICHOS chaperone domain function, where distinct surfaces are employed against different forms of protein aggregation.

The role of shear forces in primary and secondary nucleation of amyloid fibrils

Shear forces affect self-assembly processes ranging from crystallization to fiber formation. Here, the effect of mild agitation on amyloid fibril formation was explored for four peptides and investigated in detail for A[Formula: see text]42, which is associated with Alzheimer's disease. To gain mechanistic insights into the effect of mild agitation, nonseeded and seeded aggregation reactions were set up at various peptide concentrations with and without an inhibitor. First, an effect on fibril fragmentation was excluded by comparing the monomer-concentration dependence of aggregation kinetics under idle and agitated conditions. Second, using a secondary nucleation inhibitor, Brichos, the agitation effect on primary nucleation was decoupled from secondary nucleation. Third, an effect on secondary nucleation was established in the absence of inhibitor. Fourth, an effect on elongation was excluded by comparing the seeding potency of fibrils formed under idle or agitated conditions. We find that both primary and secondary nucleation steps are accelerated by gentle agitation. The increased shear forces facilitate both the detachment of newly formed aggregates from catalytic surfaces and the rate at which molecules are transported in the bulk solution to encounter nucleation sites on the fibril and other surfaces. Ultrastructural evidence obtained with cryogenic transmission electron microscopy and free-flow electrophoresis in microfluidics devices imply that agitation speeds up the detachment of nucleated species from the fibril surface. Our findings shed light on the aggregation mechanism and the role of detachment for efficient secondary nucleation. The results inform on how to modulate the relative importance of different microscopic steps in drug discovery and investigations.

Misfolded protein oligomers: mechanisms of formation, cytotoxic effects, and pharmacological approaches against protein misfolding diseases

The conversion of native peptides and proteins into amyloid aggregates is a hallmark of over 50 human disorders, including Alzheimer's and Parkinson's diseases. Increasing evidence implicates misfolded protein oligomers produced during the amyloid formation process as the primary cytotoxic agents in many of these devastating conditions. In this review, we analyze the processes by which oligomers are formed, their structures, physicochemical properties, population dynamics, and the mechanisms of their cytotoxicity. We then focus on drug discovery strategies that target the formation of oligomers and their ability to disrupt cell physiology and trigger degenerative processes.

Identification of potential aggregation hotspots on Aβ42 fibrils blocked by the anti-amyloid chaperone-like BRICHOS domain

Protein misfolding can generate toxic intermediates, which underlies several devastating diseases, such as Alzheimer's disease (AD). The surface of AD-associated amyloid-β peptide (Aβ) fibrils has been suggested to act as a catalyzer for self-replication and generation of potentially toxic species. Specifically tailored molecular chaperones, such as the BRICHOS protein domain, were shown to bind to amyloid fibrils and break this autocatalytic cycle. Here, we identify a site on the Aβ42 fibril surface, consisting of three C-terminal β-strands and particularly the solvent-exposed β-strand stretching from residues 26-28, which is efficiently sensed by a designed variant of Bri2 BRICHOS. Remarkably, while only a low amount of BRICHOS binds to Aβ42 fibrils, fibril-catalyzed nucleation processes are effectively prevented, suggesting that the identified site acts as a catalytic aggregation hotspot, which can specifically be blocked by BRICHOS. Hence, these findings provide an understanding how toxic nucleation events can be targeted by molecular chaperones.

Membrane protein chaperone and sodium chloride modulate the kinetics and morphology of amyloid beta aggregation

Protein aggregation is a biological phenomenon caused by the accumulation of misfolded proteins. Amyloid beta (Aβ) peptides are derived from the cleavage of a larger membrane protein molecule and accumulate to form plaques extracellularly. According to the amyloid hypothesis, accumulation of Aβ aggregates in the brain is primarily responsible for the pathogenesis of Alzheimer's disease (AD). Therefore, the disassembly of Aβ aggregates may provide opportunities for alleviating or treating AD. Here, we show that the novel protein targeting machinery from chloroplast, chloroplast signal recognition particle 43 (cpSRP43), is an ATP-independent membrane protein chaperone that can both prevent and reverse Aβ aggregation effectively. Using of thioflavin T dye, we obtained the aggregation kinetics of Aβ aggregation and determined that the chaperone prevents Aβ aggregation in a concentration-dependent manner. Size exclusion chromatography and sedimentation assays showed that 10-fold excess of cpSRP43 can keep Aβ in the soluble monomeric form. Electron microscopy showed that the fibril structure was disrupted in the presence of this chaperone. Importantly, cpSRP43 utilizes the binding energy to actively remodel the preformed Aβ aggregates without assistance by a co-chaperone and ATP, emphasizing its unique function among protein chaperones. Moreover, when sodium chloride concentration is higher than 25 mm, the Aβ aggregation rate increases drastically to form tightly associated aggregates and generate more oligomers. Our results demonstrate that the presence of cpSRP43 and low NaCl levels inhibit or retard Aβ peptide aggregation, potentially opening new avenues to strategically develop an effective treatment for AD.

αS1-Casein-Loaded Proteo-liposomes as Potential Inhibitors in Amyloid Fibrillogenesis: In Vivo Effects on a C. elegans Model of Alzheimer's Disease

According to the amyloid hypothesis, in the early phases of Alzheimer's disease (AD), small soluble prefibrillar aggregates of the amyloid β-peptide (Aβ) interact with neuronal membranes, causing neural impairment. Such highly reactive and toxic species form spontaneously and transiently in the amyloid building pathway. A therapeutic strategy consists of the recruitment of these intermediates, thus preventing aberrant interaction with membrane components (lipids and receptors), which in turn may trigger a cascade of cellular disequilibria. Milk αs1-Casein is an intrinsically disordered protein that is able to inhibit Aβ amyloid aggregation in vitro, by sequestering transient species. In order to test αs1-Casein as an inhibitor for the treatment of AD, it needs to be delivered in the place of action. Here, we demonstrate the use of large unilamellar vesicles (LUVs) as suitable nanocarriers for αs1-Casein. Proteo-LUVs were prepared and characterized by different biophysical techniques, such as multiangle light scattering, atomic force imaging, and small-angle X-ray scattering; αs1-Casein loading was quantified by a fluorescence assay. We demonstrated on a C. elegans AD model the effectiveness of the proposed delivery strategy in vivo. Proteo-LUVs allow efficient administration of the protein, exerting a positive functional readout at very low doses while avoiding the intrinsic toxicity of αs1-Casein. Proteo-LUVs of αs1-Casein represent an effective proof of concept for the exploitation of partially disordered proteins as a therapeutic strategy in mild AD conditions.

Synergetic effect of β-asarone and cannabidiol against Aβ aggregation in vitro and in vivo

Alzheimer's disease (AD) is a complex and multifactorial neurodegenerative disorder, and it is unlikely that any single drug or intervention will be very successful. The pathophysiology of Alzheimer's disease involves a range of complicated biological processes, including the accumulation of beta-amyloid protein and tau protein. Given the complexity of AD and amyloid accumulation, a combination of interventions remains to be further explored. Here, we investigated the potential of combining β-asarone and cannabidiol (CBD) as a treatment for AD. The study analyzed the combined effects of these two phytochemicals on beta-amyloid (Aβ) protein aggregation and toxicity in bulk solution, in cells as well as in C.elegans. We detailed the morphological and size changes of Aβ40 aggregates in the presence of β-asarone and cannabidiol. More importantly, the presence of both compounds synergistically inhibited apoptosis and downregulated relative gene expression in cells, and that it may also slow aging, decrease the rate of paralysis, enhance learning capacity, and boost autophagy activity in C.elegans. Our studies suggest that multiple drugs, like β-asarone and CBD, may be potentially developed as a medicinal adjunct in the treatment of AD, although further clinical trials are needed to determine the efficacy and safety of this combination treatment in humans.

Microglia produce the amyloidogenic ABri peptide in familial British dementia

Mutations in ITM2B cause familial British, Danish, Chinese and Korean dementias. In familial British dementia (FBD) a mutation in the stop codon of the ITM2B gene (also known as BRI2 ) causes a C-terminal cleavage fragment of the ITM2B/BRI2 protein to be extended by 11 amino acids. This fragment, termed amyloid-Bri (ABri), is highly insoluble and forms extracellular plaques in the brain. ABri plaques are accompanied by tau pathology, neuronal cell death and progressive dementia, with striking parallels to the aetiology and pathogenesis of Alzheimer's disease. The molecular mechanisms underpinning FBD are ill-defined. Using patient-derived induced pluripotent stem cells, we show that expression of ITM2B/BRI2 is 34-fold higher in microglia than neurons, and 15-fold higher in microglia compared with astrocytes. This cell-specific enrichment is supported by expression data from both mouse and human brain tissue. ITM2B/BRI2 protein levels are higher in iPSC-microglia compared with neurons and astrocytes. Consequently, the ABri peptide was detected in patient iPSC-derived microglial lysates and conditioned media but was undetectable in patient-derived neurons and control microglia. Pathological examination of post-mortem tissue support ABri expression in microglia that are in proximity to pre-amyloid deposits. Finally, gene co-expression analysis supports a role for ITM2B/BRI2 in disease-associated microglial responses. These data demonstrate that microglia are the major contributors to the production of amyloid forming peptides in FBD, potentially acting as instigators of neurodegeneration. Additionally, these data also suggest ITM2B/BRI2 may be part of a microglial response to disease, motivating further investigations of its role in microglial activation. This has implications for our understanding of the role of microglia and the innate immune response in the pathogenesis of FBD and other neurodegenerative dementias including Alzheimer's disease.

A new kid in the folding funnel: Molecular chaperone activities of the BRICHOS domain

The BRICHOS protein superfamily is a diverse group of proteins associated with a wide variety of human diseases, including respiratory distress, COVID-19, dementia, and cancer. A key characteristic of these proteins-besides their BRICHOS domain present in the ER lumen/extracellular part-is that they harbor an aggregation-prone region, which the BRICHOS domain is proposed to chaperone during biosynthesis. All so far studied BRICHOS domains modulate the aggregation pathway of various amyloid-forming substrates, but not all of them can keep denaturing proteins in a folding-competent state, in a similar manner as small heat shock proteins. Current evidence suggests that the ability to interfere with the aggregation pathways of substrates with entirely different end-point structures is dictated by BRICHOS quaternary structure as well as specific surface motifs. This review aims to provide an overview of the BRICHOS protein family and a perspective of the diverse molecular chaperone-like functions of various BRICHOS domains in relation to their structure and conformational plasticity. Furthermore, we speculate about the physiological implication of the diverse molecular chaperone functions and discuss the possibility to use the BRICHOS domain as a blood-brain barrier permeable molecular chaperone treatment of protein aggregation disorders.

Sequence-targeted Peptides Divert Functional Bacterial Amyloid Towards Destabilized Aggregates and Reduce Biofilm Formation

Functional bacterial amyloid provides structural stability in biofilm, making it a promising target for anti-biofilm therapeutics. Fibrils formed by CsgA, the major amyloid component in E. coli are extremely robust and can withstand very harsh conditions. Like other functional amyloids, CsgA contains relatively short aggregation-prone regions (APR) which drive amyloid formation. Here, we demonstrate the use of aggregation-modulating peptides to knock down CsgA protein into aggregates with low stability and altered morphology. Remarkably, these CsgA-peptides also modulate fibrillation of the unrelated functional amyloid protein FapC from Pseudomonas, possibly through recognition of FapC segments with structural and sequence similarity with CsgA. The peptides also reduce the level of biofilm formation in E. coli and P. aeruginosa, demonstrating the potential for selective amyloid targeting to combat bacterial biofilm.

Short hydrophobic loop motifs in BRICHOS domains determine chaperone activity against amorphous protein aggregation but not against amyloid formation

ATP-independent molecular chaperones are important for maintaining cellular fitness but the molecular determinants for preventing aggregation of partly unfolded protein substrates remain unclear, particularly regarding assembly state and basis for substrate recognition. The BRICHOS domain can perform small heat shock (sHSP)-like chaperone functions to widely different degrees depending on its assembly state and sequence. Here, we observed three hydrophobic sequence motifs in chaperone-active domains, and found that they get surface-exposed when the BRICHOS domain assembles into larger oligomers. Studies of loop-swap variants and site-specific mutants further revealed that the biological hydrophobicities of the three short motifs linearly correlate with the efficiency to prevent amorphous protein aggregation. At the same time, they do not at all correlate with the ability to prevent ordered amyloid fibril formation. The linear correlations also accurately predict activities of chimeras containing short hydrophobic sequence motifs from a sHSP that is unrelated to BRICHOS. Our data indicate that short, exposed hydrophobic motifs brought together by oligomerisation are sufficient and necessary for efficient chaperone activity against amorphous protein aggregation.

SUMO1 hinders α-Synuclein fibrillation by inducing structural compaction

Small Ubiquitin-like Modifier 1 (SUMO1) is an essential protein for many cellular functions, including regulation, signaling, etc., achieved by a process known as SUMOylation, which involves covalent attachment of SUMO1 to target proteins. SUMO1 also regulates the function of several proteins via non-covalent interactions involving the hydrophobic patch in the target protein identified as SUMO Binding or Interacting Motif (SBM/SIM). Here, we demonstrate a crucial functional potential of SUMO1 mediated by its non-covalent interactions with α-Synuclein, a protein responsible for many neurodegenerative diseases called α-Synucleinopathies. SUMO1 hinders the fibrillation of α-Synuclein, an intrinsically disordered protein (IDP) that undergoes a transition to β-structures during the fibrillation process. Using a plethora of biophysical techniques, we show that SUMO1 transiently binds to the N-terminus region of α-Synuclein non-covalently and causes structural compaction, which hinders the self-association process and thereby delays the fibrillation process. On the one hand, this study demonstrates an essential functional role of SUMO1 protein concerning neurodegeneration; it also illustrates the commonly stated mechanism that IDPs carry out multiple functions by structural adaptation to suit specific target proteins, on the other. Residue-level details about the SUMO1-α-Synuclein interaction obtained here also serve as a reliable approach for investigating the detailed mechanisms of IDP functions.

The Emerging Roles of Extracellular Chaperones in Complement Regulation

The immune system is essential to protect organisms from internal and external threats. The rapidly acting, non-specific innate immune system includes complement, which initiates an inflammatory cascade and can form pores in the membranes of target cells to induce cell lysis. Regulation of protein homeostasis (proteostasis) is essential for normal cellular and organismal function, and has been implicated in processes controlling immunity and infection. Chaperones are key players in maintaining proteostasis in both the intra- and extracellular environments. Whilst intracellular proteostasis is well-characterised, the role of constitutively secreted extracellular chaperones (ECs) is less well understood. ECs may interact with invading pathogens, and elements of the subsequent immune response, including the complement pathway. Both ECs and complement can influence the progression of neurodegenerative diseases, including Alzheimer's disease, Parkinson's disease, Huntington's disease and amyotrophic lateral sclerosis, as well as other diseases including kidney diseases and diabetes. This review will examine known and recently discovered ECs, and their roles in immunity, with a specific focus on the complement pathway.

Human Islet Amyloid Polypeptide (hIAPP) Protofibril-Specific Antibodies for Detection and Treatment of Type 2 Diabetes

Type 2 diabetes mellitus (T2D) is a major public health concern and is characterized by sustained hyperglycemia due to insulin resistance and destruction of insulin-producing β cells. One pathological hallmark of T2D is the toxic accumulation of human islet amyloid polypeptide (hIAPP) aggregates. Monomeric hIAPP is a hormone normally co-secreted with insulin. However, increased levels of hIAPP in prediabetic and diabetic patients can lead to the formation of hIAPP protofibrils, which are toxic to β cells. Current therapies fail to address hIAPP aggregation and current screening modalities do not detect it. Using a stabilizing capping protein, monoclonal antibodies (mAbs) can be developed against a previously nonisolatable form of hIAPP protofibrils, which are protofibril specific and do not engage monomeric hIAPP. Shown here are two candidate mAbs that can detect hIAPP protofibrils in serum and hIAPP deposits in pancreatic islets in a mouse model of rapidly progressing T2D. Treatment of diabetic mice with the mAbs delays disease progression and dramatically increases overall survival. These results demonstrate the potential for using novel hIAPP protofibril-specific mAbs as a diagnostic screening tool for early detection of T2D, as well as therapeutically to preserve β cell function and target one of the underlying pathological mechanisms of T2D.

Abilities of the BRICHOS domain to prevent neurotoxicity and fibril formation are dependent on a highly conserved Asp residue

Proteins can self-assemble into amyloid fibrils or amorphous aggregates and thereby cause disease. Molecular chaperones can prevent both these types of protein aggregation, but to what extent the respective mechanisms are overlapping is not fully understood. The BRICHOS domain constitutes a disease-associated chaperone family, with activities against amyloid neurotoxicity, fibril formation, and amorphous protein aggregation. Here, we show that the activities of BRICHOS against amyloid-induced neurotoxicity and fibril formation, respectively, are oppositely dependent on a conserved aspartate residue, while the ability to suppress amorphous protein aggregation is unchanged by Asp to Asn mutations. The Asp is evolutionarily highly conserved in >3000 analysed BRICHOS domains but is replaced by Asn in some BRICHOS families. The conserved Asp in its ionized state promotes structural flexibility and has a pK a value between pH 6.0 and 7.0, suggesting that chaperone effects can be differently affected by physiological pH variations.

The emerging role of furin in neurodegenerative and neuropsychiatric diseases

Furin is an important mammalian proprotein convertase that catalyzes the proteolytic maturation of a variety of prohormones and proproteins in the secretory pathway. In the brain, the substrates of furin include the proproteins of growth factors, receptors and enzymes. Emerging evidence, such as reduced FURIN mRNA expression in the brains of Alzheimer's disease patients or schizophrenia patients, has implicated a crucial role of furin in the pathophysiology of neurodegenerative and neuropsychiatric diseases. Currently, compared to cancer and infectious diseases, the aberrant expression of furin and its pharmaceutical potentials in neurological diseases remain poorly understood. In this article, we provide an overview on the physiological roles of furin and its substrates in the brain, summarize the deregulation of furin expression and its effects in neurodegenerative and neuropsychiatric disorders, and discuss the implications and current approaches that target furin for therapeutic interventions. This review may expedite future studies to clarify the molecular mechanisms of furin deregulation and involvement in the pathogenesis of neurodegenerative and neuropsychiatric diseases, and to develop new diagnosis and treatment strategies for these diseases.

α-Synuclein aggregation intermediates form fibril polymorphs with distinct prion-like properties

α-Synuclein (α-Syn) amyloids in synucleinopathies are suggested to be structurally and functionally diverse, reminiscent of prion-like strains. But how the aggregation of the same precursor protein results in the formation of fibril polymorphs remains elusive. Here, we demonstrate the structure-function relationship of two polymorphs, pre-matured fibrils (PMFs) and helix-matured fibrils (HMFs), based on α-Syn aggregation intermediates. These polymorphs display the structural differences as demonstrated by solid-state NMR and mass spectrometry studies and also possess different cellular activities such as seeding, internalization, and cell-to-cell transfer of aggregates. HMFs with a compact core structure exhibit low seeding potency but readily internalize and transfer from one cell to another. The less structured PMFs lack transcellular transfer ability but induce abundant α-Syn pathology and trigger the formation of aggresomes in cells. Overall, the study highlights that the conformational heterogeneity in the aggregation pathway may lead to fibril polymorphs with distinct prion-like behavior.

Amyloid Fibril Formation of Arctic Amyloid-β 1-42 Peptide is Efficiently Inhibited by the BRICHOS Domain

Amyloid-β peptide (Aβ) aggregation is one of the hallmarks of Alzheimer's disease (AD). Mutations in Aβ are associated with early onset familial AD, and the Arctic mutant E22G (Aβ^arc) is an extremely aggregation-prone variant. Here, we show that BRICHOS, a natural anti-amyloid chaperone domain, from Bri2 efficiently inhibits aggregation of Aβ^arc by mainly interfering with secondary nucleation. This is qualitatively different from the microscopic inhibition mechanism for the wild-type Aβ, against which Bri2 BRICHOS has a major effect on both secondary nucleation and fibril end elongation. The monomeric Aβ42^arc peptide aggregates into amyloid fibrils significantly faster than wild-type Aβ (Aβ42^wt), as monitored by thioflavin T (ThT) binding, but the final ThT intensity was strikingly lower for Aβ42^arc compared to Aβ42^wt fibrils. The Aβ42^arc peptide formed large aggregates, single-filament fibrils, and multiple-filament fibrils without obvious twists, while Aβ42^wt fibrils displayed a polymorphic pattern with typical twisted fibril architecture. Recombinant human Bri2 BRICHOS binds to the Aβ42^arc fibril surface and interferes with the macroscopic fibril arrangement by promoting single-filament fibril formation. This study provides mechanistic insights on how BRICHOS efficiently affects the aggressive Aβ42^arc aggregation, resulting in both delayed fibril formation kinetics and altered fibril structure.

miR-548d-3p inhibits the invasion and migration of gastric cancer cells by targeting GKN1

Background

The aim of this study was to explore the function and mechanism of GKN1 in gastric cancer (GC) progression.

Methods

Firstly, we used GEO2R to perform differential gene analysis on GSE26942 and GSE79973 and constructed the protein–protein interaction network of differential genes by STRING. Next, the cytoHubba, Mcode plugins, and GEPIA were used to obtain our follow‐up research object GKN1. Then, the function of GKN1 in GC was verified by scratch and transwell assay in GC cells. We further analyzed the genes related to GKN1 through LinkedOmics, and exported top 100 genes positively or negatively correlated with GKN1. Meanwhile, Metascape was performed on these genes. Finally, we analyzed the miRNAs that bind to GKN1 through the miRDB and verified the correlation between miR‐548d‐3p and GKN1 using dual‐fluorescence and quantitative PCR experiments.

Results

Bioinformatics analysis showed that there were 52 differential genes on GSE26942 and GSE79973. In addition, the results of functional assays indicated that overexpressed GKN1 can inhibit GC cell migration and invasion, while GKN1 knockdown demonstrated the opposite effect. Additionally, Metascape analysis results showed that the 3′‐UTR region of mRNA is rich in AU sequences, based on which we infer that mRNA may be regulated by miRNA. Dual‐fluorescence and quantitative PCR assays clarified that miR‐548d‐3p may be one of the target miRNAs of GKN1, which was up‐regulated in GC tissues.

Conclusions

In summary, we clarified that miR‐548d‐3p regulates GKN1 to participate in GC cell migration and invasion, and provides a possible target for the prognostic diagnosis and treatment of GC.

Endogenous Human Proteins Interfering with Amyloid Formation

Amyloid formation is a pathological process associated with a wide range of degenerative disorders, including Alzheimer’s disease, Parkinson’s disease, and diabetes mellitus type 2. During disease progression, abnormal accumulation and deposition of proteinaceous material are accompanied by tissue degradation, inflammation, and dysfunction. Agents that can interfere with the process of amyloid formation or target already formed amyloid assemblies are consequently of therapeutic interest. In this context, a few endogenous proteins have been associated with an anti-amyloidogenic activity. Here, we review the properties of transthyretin, apolipoprotein E, clusterin, and BRICHOS protein domain which all effectively interfere with amyloid in vitro, as well as displaying a clinical impact in humans or animal models. Their involvement in the amyloid formation process is discussed, which may aid and inspire new strategies for therapeutic interventions.

Co-aggregation and secondary nucleation in the life cycle of human prolactin/galanin functional amyloids

Synergistic-aggregation and cross-seeding by two different proteins/peptides in the amyloid aggregation are well evident in various neurological disorders including Alzheimer’s disease. Here, we show co-storage of human Prolactin (PRL), which is associated with lactation in mammals, and neuropeptide galanin (GAL) as functional amyloids in secretory granules (SGs) of the female rat. Using a wide variety of biophysical studies, we show that irrespective of the difference in sequence and structure, both hormones facilitate their synergic aggregation to amyloid fibrils. Although each hormone possesses homotypic seeding ability, a unidirectional cross-seeding of GAL aggregation by PRL seeds and the inability of cross seeding by mixed fibrils suggest tight regulation of functional amyloid formation by these hormones for their efficient storage in SGs. Further, the faster release of functional hormones from mixed fibrils compared to the corresponding individual amyloid, suggests a novel mechanism of heterologous amyloid formation in functional amyloids of SGs in the pituitary.

The formation of plaques of proteins called ‘amyloids’ in the brain is one of the hallmark characteristics of both Alzheimer’s and Parkinson’s disease, but amyloids can form in many tissues and organs, often disrupting normal activity. A lot of the research into amyloids has focused on their role in disease, but it turns out that amyloids can also appear in healthy tissues. For example, some protein hormones form amyloids that act as storage depots, helping cells to release the hormone when it is needed.

Normally, amyloids are made mostly of a single type of protein or protein fragment associated with a particular disease like Alzheimer's. Often, this type of amyloid promotes plaque formation in other proteins, which aggravates other diseases (for example, the amyloids that form in Alzheimer’s can lead to Parkinson’s disease or type II diabetes getting worse).The plaques start growing from small amyloid fragments called seeds. In mixed amyloids – amyloids made of two types of proteins – seeds made of one protein can trigger the formation of amyloids of the other protein. This raises the question, is this true for hormones? The body often releases more than one hormone at a time from the same tissue; for example, the pituitary gland releases prolactin and galanin simultaneously. However, these hormones have completely different structures, so whether they can form a mixed amyloid is unclear.

To answer this question, Chatterjee et al. first determined that, within the pituitary gland of female rats, prolactin and galanin could be found together in the same cells, forming mixed amyloids. To understand out how this happens, Chatterjee et al. tried seeding new amyloids using either prolactin or galanin. This revealed that only prolactin seeds were able to trigger the formation of galanin amyloids. Chatterjee et al. also found that the mixed amyloids could release the hormones faster than amyloids made from either protein alone. Together, these results suggest that the collaboration between these two proteins may help maintain hormone balance in the body.

Problems with hormone storage and release lead to various human diseases, including prolactinoma. Understanding amyloid storage depots could reveal new ways to control hormone levels. Further research could also help to explain more about well-studied diseases linked to amyloids, like Alzheimer's.

Soluble TREM2 inhibits secondary nucleation of Aβ fibrillization and enhances cellular uptake of fibrillar Aβ

Triggering receptor expressed on myeloid cells 2 (TREM2) is a single-pass transmembrane receptor of the immunoglobulin superfamily that is secreted in a soluble (sTREM2) form. Mutations in TREM2 have been linked to increased risk of Alzheimer's disease (AD). A prominent neuropathological component of AD is deposition of the amyloid-β (Aβ) into plaques, particularly Aβ40 and Aβ42. While the membrane-bound form of TREM2 is known to facilitate uptake of Aβ fibrils and the polarization of microglial processes toward amyloid plaques, the role of its soluble ectodomain, particularly in interactions with monomeric or fibrillar Aβ, has been less clear. Our results demonstrate that sTREM2 does not bind to monomeric Aβ40 and Aβ42, even at a high micromolar concentration, while it does bind to fibrillar Aβ42 and Aβ40 with equal affinities (2.6 ± 0.3 µM and 2.3 ± 0.4 µM). Kinetic analysis shows that sTREM2 inhibits the secondary nucleation step in the fibrillization of Aβ, while having little effect on the primary nucleation pathway. Furthermore, binding of sTREM2 to fibrils markedly enhanced uptake of fibrils into human microglial and neuroglioma derived cell lines. The disease-associated sTREM2 mutant, R47H, displayed little to no effect on fibril nucleation and binding, but it decreased uptake and functional responses markedly. We also probed the structure of the WT sTREM2-Aβ fibril complex using integrative molecular modeling based primarily on the cross-linking mass spectrometry data. The model shows that sTREM2 binds fibrils along one face of the structure, leaving a second, mutation-sensitive site free to mediate cellular binding and uptake.

Identifying new molecular players in extracellular proteostasis

Proteostasis refers to a delicately tuned balance between the processes of protein synthesis, folding, localization, and the degradation of proteins found inside and outside cells. Our understanding of extracellular proteostasis is rather limited and largely restricted to knowledge of 11 currently established extracellular chaperones (ECs). This review will briefly outline what is known of the established ECs, before moving on to discuss experimental strategies used to identify new members of this growing family, and an examination of a group of putative new ECs identified using one of these approaches. An observation that emerges from an analysis of the expanding number of ECs is that all of these proteins are multifunctional. Strikingly, the armory of activities each possess uniquely suit them as a group to act together at sites of tissue damage, infection, and inflammation to restore homeostasis. Lastly, we highlight outstanding questions to guide future research in this field.

The role of the integral type II transmembrane protein BRI2 in health and disease

BRI2 is a type II transmembrane protein ubiquitously expressed whose physiological function remains poorly understood. Although several recent important advances have substantially impacted on our understanding of BRI2 biology and function, providing valuable information for further studies on BRI2. These findings have contributed to a better understanding of BRI2 biology and the underlying signaling pathways involved. In turn, these might provide novel insights with respect to neurodegeneration processes inherent to BRI2-related pathologies, namely Familial British and Danish dementias, Alzheimer's disease, ITM2B-related retinal dystrophy, and multiple sclerosis. In this review, we provided a state-of-the-art outline of BRI2 biology, both in physiological and pathological conditions, and discuss the proposed molecular underlying mechanisms. Overall, the BRI2 knowledge here reviewed is of extreme importance and may contribute to propose BRI2 and/or BRI2 proteolytic fragments as novel therapeutic targets for neurodegenerative diseases, such as Alzheimer's disease.

Bri2 BRICHOS chaperone rescues impaired fast-spiking interneuron behavior and neuronal network dynamics in an AD mouse model in vitro

Synchronized and properly balanced electrical activity of neurons is the basis for the brain's ability to process information, to learn, and to remember. In Alzheimer's disease (AD), which causes cognitive decline in patients, this synchronization and balance is disturbed by the accumulation of neuropathological biomarkers such as amyloid-beta peptide (Aβ42). Failure of Aβ42 clearance mechanisms as well as desynchronization of crucial neuronal classes such as fast-spiking interneurons (FSN) are root causes for the disruption of the cognition-relevant gamma brain rhythm (30-80 Hz) and consequent cognitive impairment observed in AD. Here we show that recombinant BRICHOS molecular chaperone domains from ProSP-C or Bri2, which interfere with Aβ42 aggregation, can rescue the gamma rhythm. We demonstrate that Aβ42 progressively decreases gamma oscillation power and rhythmicity, disrupts the inhibition/excitation balance in pyramidal cells, and desynchronizes FSN firing during gamma oscillations in the hippocampal CA3 network of mice. Application of the more efficacious Bri2 BRICHOS chaperone rescued the cellular and neuronal network performance from all ongoing Aβ42-induced functional impairments. Collectively, our findings offer critical missing data to explain the importance of FSN for normal network function and underscore the therapeutic potential of Bri2 BRICHOS to rescue the disruption of cognition-relevant brain rhythms in AD.

Possible association between SP-C mutations and lung cancer: Two case reports and review of literature

Surfactant protein C (SP-C) is one of four surfactant proteins produced by type II pneumocytes. Mutations in surfactant protein A are strongly associated with development of lung cancer. Mutations in the SP-C gene are rare and are associated with interstitial lung disease in the pediatric age group. We describe two patients with SP-C mutations who developed lung cancer. Both patients had concurrent interstitial lung disease, although the clinical phenotype was variable. In both cases, mutations were in translated region of the SP-C gene; one in the BRICHOS domain and the other in the transmembrane domain. Our paper suggests that patients with SP-C mutations can be at increased risk for the development of lung cancer, and it's reasonable to follow them routinely.

Human Fibrinogen Inhibits Amyloid Assembly of Most Phenol-Soluble Modulins from Staphylococcus aureus

Functional amyloids are highly organized protein/peptide structures that inter alia promote biofilm formation in different bacteria. One such example is provided by a family of 20-45 residue-long peptides called phenol-soluble modulins (PSMs) from Staphylococcus aureus. External components such as eukaryotic host proteins, which alter self-assembly of bacterial amyloids, can affect the biofilm matrix. Here, we studied the effect of the highly prevalent human plasma protein fibrinogen (Fg) on fibrillation of PSMs. Fg inhibits or suppresses fibrillation of most PSMs tested (PSMα1, PSMβ1, and PSMβ2) except for PSMα3, whose already rapid aggregation is accelerated even further by Fg but leads to amorphous β-rich aggregates rather than fibrils. Fg also induces PSMβ2 to form amorphous aggregates and diverts PSMα1 into off-pathway oligomers which consist of both Fg and PSMα1 and cannot seed fibrillation. Peptide arrays showed that Fg bound to the N-terminus of PSMα1, while it bound to the entire length of PSMα3 (except the C terminus) and to the C-termini of PSMβ1 and PSMβ2. The latter peptides are all positively charged, while Fg is negatively charged at physiological pH. The positive charges complement Fg's net negative charge of -7.6 at pH 7.4. Fg's ability to inhibit PSM fibrillation reveals a potential host-defense mechanism to prevent bacterial biofilm growth and infections in the human body.

Expression of the human molecular chaperone domain Bri2 BRICHOS on a gram per liter scale with an E. coli fed-batch culture

Background

The human Bri2 BRICHOS domain inhibits amyloid formation and toxicity and could be used as a therapeutic agent against amyloid diseases. For translation into clinical use, large quantities of correctly folded recombinant human (rh) Bri2 BRICHOS are required. To increase the expression and solubility levels of rh Bri2 BRICHOS it was fused to NT*, a solubility tag derived from the N-terminal domain of a spider silk protein, which significantly increases expression levels and solubility of target proteins. To increase the expression levels even further and reach the g/L range, which is a prerequisite for an economical production on an industrial scale, we developed a fed-batch expression protocol for Escherichia coli.

Results

A fed-batch production method for NT*-Bri2 BRICHOS was set up and systematically optimized. This gradual improvement resulted in expression levels of up to 18.8 g/L. Following expression, NT*-Bri2 BRICHOS was purified by chromatographic methods to a final yield of up to 6.5 g/L. After removal of the NT*-tag and separation into different oligomeric species, activity assays verified that different assembly states of the fed-batch produced rh Bri2 BRICHOS have the same ability to inhibit fibrillar and non-fibrillar protein aggregation as the reference protein isolated from shake flask cultures.

Conclusions

The protocol developed in this work allows the production of large quantities of rh Bri2 BRICHOS using the solubility enhancing NT*-tag as a fusion partner, which is required to effectively conduct pre-clinical research.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01638-8.

Ellagic Acid Inhibits α-Synuclein Aggregation at Multiple Stages and Reduces Its Cytotoxicity

α-Synuclein is a natively unfolded protein and its deposition in the Lewy body and Lewy neurites in the substantia nigra region of the brain is linked to Parkinson's disease (PD). The molecular mechanisms of α-synuclein aggregation and its clearance have not been well understood. Until now, several strategies have been designed to inhibit α-synuclein aggregation and related cytotoxicity. Polyphenols, small molecules, synthetic peptides, and peptide-derived molecules have been considered as potential candidates that inhibit α-synuclein oligomerization and its fibrillation, and a few of them are in clinical trials. We have identified a polyphenolic compound ellagic acid (EA) that inhibits α-synuclein aggregation. Our results demonstrated that EA inhibits primary nucleation, seeded aggregation, and membrane-induced aggregation. The cytotoxicity of α-synuclein oligomers and fibers treated with EA has been investigated and we found that EA treated oligomers and fibrils showed reduced cytotoxicity. Additionally, we also observed inhibition of membrane binding of α-synuclein by EA in SH-SY5Y cells. In conclusion, the present study suggests that small molecules such as ellagic acid have anti-amyloidogenic properties and may have therapeutic potential for Parkinson's disease and other proteinopathies.

Gastrokine-1, an anti-amyloidogenic protein secreted by the stomach, regulates diet-induced obesity

Obesity and its sequelae have a major impact on human health. The stomach contributes to obesity in ways that extend beyond its role in digestion, including through effects on the microbiome. Gastrokine-1 (GKN1) is an anti-amyloidogenic protein abundantly and specifically secreted into the stomach lumen. We examined whether GKN1 plays a role in the development of obesity and regulation of the gut microbiome. Gkn1-/- mice were resistant to diet-induced obesity and hepatic steatosis (high fat diet (HFD) fat mass (g) = 10.4 ± 3.0 (WT) versus 2.9 ± 2.3 (Gkn1-/-) p < 0.005; HFD liver mass (g) = 1.3 ± 0.11 (WT) versus 1.1 ± 0.07 (Gkn1-/-) p < 0.05). Gkn1-/- mice also exhibited increased expression of the lipid-regulating hormone ANGPTL4 in the small bowel. The microbiome of Gkn1-/- mice exhibited reduced populations of microbes implicated in obesity, namely Firmicutes of the class Erysipelotrichia. Altered metabolism consistent with use of fat as an energy source was evident in Gkn1-/- mice during the sleep period. GKN1 may contribute to the effects of the stomach on the microbiome and obesity. Inhibition of GKN1 may be a means to prevent obesity.

The role of amyloid oligomers in neurodegenerative pathologies

Many neurodegenerative diseases are rooted in the activities of amyloid-like proteins which possess conformations that spread to healthy proteins. These include Alzheimer's disease (AD), Parkinson's disease (PD), Huntington's disease (HD) and amyotrophic lateral sclerosis (ALS). While their clinical manifestations vary, their protein-level mechanisms are remarkably similar. Aberrant monomeric proteins undergo conformational shifts, facilitating aggregation and formation of solid fibrils. However, there is growing evidence that intermediate oligomeric stages are key drivers of neuronal toxicity. Analysis of protein dynamics is complicated by the fact that nucleation and growth of amyloid-like proteins is not a linear pathway. Feedback within this pathway results in exponential acceleration of aggregation, but activities exerted by oligomers and fibrils can alter cellular interactions and the cellular environment as a whole. The resulting cascade of effects likely contributes to the late onset and accelerating progression of amyloid-like protein disorders and the widespread effects they have on the body. In this review we explore the amyloid-like proteins associated with AD, PD, HD and ALS, as well as the common mechanisms of amyloid-like protein nucleation and aggregation. From this, we identify core elements of pathological progression which have been targeted for therapies, and which may become future therapeutic targets.

NRBP1-Containing CRL2/CRL4A Regulates Amyloid β Production by Targeting BRI2 and BRI3 for Degradation

Alzheimer's disease (AD) is a progressive neurodegenerative disease caused by accumulations of Aβ peptides. Production and fibrillation of Aβ are downregulated by BRI2 and BRI3, which are physiological inhibitors of amyloid precursor protein (APP) processing and Aβ oligomerization. Here, we identify nuclear receptor binding protein 1 (NRBP1) as a substrate receptor of a Cullin-RING ubiquitin ligase (CRL) that targets BRI2 and BRI3 for degradation. Moreover, we demonstrate that (1) dimerized NRBP1 assembles into a functional Cul2- and Cul4A-containing heterodimeric CRL through its BC-box and an overlapping cryptic H-box, (2) both Cul2 and Cul4A contribute to NRBP1 CRL function, and (3) formation of the NRBP1 heterodimeric CRL is strongly enhanced by chaperone-like function of TSC22D3 and TSC22D4. NRBP1 knockdown in neuronal cells results in an increase in the abundance of BRI2 and BRI3 and significantly reduces Aβ production. Thus, disrupting interactions between NRBP1 and its substrates BRI2 and BRI3 may provide a useful therapeutic strategy for AD.

Mechanisms of Transthyretin Inhibition of IAPP Amyloid Formation

Amyloid-formation by the islet amyloid polypeptide (IAPP), produced by the β-cells in the human pancreas, has been associated with the development of type II diabetes mellitus (T2DM). The human plasma-protein transthyretin (TTR), a well-known amyloid-inhibiting protein, is interestingly also expressed within the IAPP producing β-cells. In the present study, we have characterized the ability of TTR to interfere with IAPP amyloid-formation, both in terms of its intrinsic stability as well as with regard to the effect of TTR-stabilizing drugs. The results show that TTR can prolong the lag-phase as well as impair elongation in the course of IAPP-amyloid formation. We also show that the interfering ability correlates inversely with the thermodynamic stability of TTR, while no such correlation was observed as a function of kinetic stability. Furthermore, we demonstrate that the ability of TTR to interfere is maintained also at the low pH environment within the IAPP-containing granules of the pancreatic β-cells. However, at both neutral and low pH, the addition of TTR-stabilizing drugs partly impaired its efficacy. Taken together, these results expose mechanisms of TTR-mediated inhibition of IAPP amyloid-formation and highlights a potential therapeutic target to prevent the onset of T2DM.

Smallest Secondary Nucleation Competent Aβ Aggregates Probed by an ATP-Independent Molecular Chaperone Domain

Protein oligomerization is a commonly encountered strategy by which the functional repertoire of proteins is increased. This, however, is a double-edged sword strategy because protein oligomerization is notoriously difficult to control. Living organisms have therefore developed a number of chaperones that prevent protein aggregation. The small ATP-independent molecular chaperone domain proSP-C BRICHOS, which is mainly trimeric, specifically inhibits fibril surface-catalyzed nucleation reactions that give rise to toxic oligomers during the aggregation of the Alzheimer's disease-related amyloid-β peptide (Aβ42). Here, we have created a stable proSP-C BRICHOS monomer mutant and show that it does not bind to monomeric Aβ42 but has a high affinity for Aβ42 fibrils, using surface plasmon resonance. Kinetic analysis of Aβ42 aggregation profiles, measured by thioflavin T fluorescence, reveals that the proSP-C BRICHOS monomer mutant strongly inhibits secondary nucleation reactions and thereby reduces the level of catalytic formation of toxic Aβ42 oligomers. To study binding between the proSP-C BRICHOS monomer mutant and small soluble Aβ42 aggregates, we analyzed fluorescence cross-correlation spectroscopy measurements with the maximum entropy method for fluorescence correlation spectroscopy. We found that the proSP-C BRICHOS monomer mutant binds to the smallest emerging Aβ42 aggregates that are comprised of eight or fewer Aβ42 molecules, which are already secondary nucleation competent. Our approach can be used to provide molecular-level insights into the mechanisms of action of substances that interfere with protein aggregation.

Proteostasis of Islet Amyloid Polypeptide: A Molecular Perspective of Risk Factors and Protective Strategies for Type II Diabetes

The possible link between hIAPP accumulation and β-cell death in diabetic patients has inspired numerous studies focusing on amyloid structures and aggregation pathways of this hormone. Recent studies have reported on the importance of early oligomeric intermediates, the many roles of their interactions with lipid membrane, pH, insulin, and zinc on the mechanism of aggregation of hIAPP. The challenges posed by the transient nature of amyloid oligomers, their structural heterogeneity, and the complex nature of their interaction with lipid membranes have resulted in the development of a wide range of biophysical and chemical approaches to characterize the aggregation process. While the cellular processes and factors activating hIAPP-mediated cytotoxicity are still not clear, it has recently been suggested that its impaired turnover and cellular processing by proteasome and autophagy may contribute significantly toward toxic hIAPP accumulation and, eventually, β-cell death. Therefore, studies focusing on the restoration of hIAPP proteostasis may represent a promising arena for the design of effective therapies. In this review we discuss the current knowledge of the structures and pathology associated with hIAPP self-assembly and point out the opportunities for therapy that a detailed biochemical, biophysical, and cellular understanding of its aggregation may unveil.

Functionalization of amyloid fibrils via the Bri2 BRICHOS domain

Amyloid fibrils are mechanically robust and partly resistant to proteolytic degradation, making them potential candidates for scaffold materials in cell culture, tissue engineering, drug delivery and other applications. Such applications of amyloids would benefit from the possibility to functionalize the fibrils, for example by adding growth factors or cell attachment sites. The BRICHOS domain is found in a family of human proteins that harbor particularly amyloid-prone regions and can reduce aggregation as well as toxicity of several different amyloidogenic peptides. Recombinant human (rh) BRICHOS domains have been shown to bind to the surface of amyloid-β (Aβ) fibrils by immune electron microscopy. Here we produce fusion proteins between mCherry and rh Bri2 BRICHOS and show that they can bind to different amyloid fibrils with retained fluorescence of mCherry in vitro as well as in cultured cells. This suggests a “generic” ability of the BRICHOS domain to bind fibrillar surfaces that can be used to synthesize amyloid decorated with different protein functionalities.

Association analysis of the surfactant protein-C gene to childhood asthma

OBJECTIVES

This study aims to describe the molecular variability in the SFTPC gene in a childhood chronic respiratory disease, asthma, in the Tunisian population and to identify the implications based on a case-control study of p.Thr138Asn (T138N) and p.Ser186Asn (S186N) variants.

METHODS

We used direct sequencing for the genotyping of the SFTPC gene within 101 asthmatic children. The study of T138N and S186N variants in 110 controls is conducted by the PCR-RFLP technique.

RESULTS

The molecular study revealed 26 variants including 24 intronic variations and 2 exonic variations (T138N and S186N) with respective frequencies of 16.8% and 18.3%. We conducted a case-control study of the two identified exonic variations. A different genotypic and allelic distribution between the two groups was noted. Only the T138N polymorphism showed a significant association with asthma disease (p < 1 0 ^-3). Statistical analysis elaborated four haplotypes with the following frequencies in patients vs controls: 138Thr-186Ser (79.5% vs 57.6%), 138Thr-186Asn (3.7% vs 7.8%), 138Asn-186Thr (2.2% vs 20.2%) and 138Asn-186Asn (14.6% vs 14.4%). A significant difference (p < 1 0 ^-3) was highlighted in haplotype distribution. The 138Asn-186Ser (OR [95%CI] = 0.14[0.04-0.54], p = 0.004, R²=0.93) and 138Thr-186Asn (OR [95%CI] = 0.35[0.12-0.54], p = 0.047, R²=0.88) haplotypes showed a negative association with asthma which may constitute a protective factor against the disease.

CONCLUSION

In Tunisia, this work constitutes the first report interested in the SFTPC gene and highlights the genetic variability of the SFTPC gene in asthma. Therefore, the case-controls analysis may be useful in the study of surfactant proteins dysfunction in chronic respiratory disease at an early age.

Molecular Chaperones: A Double-Edged Sword in Neurodegenerative Diseases

Aberrant accumulation of misfolded proteins into amyloid deposits is a hallmark in many age-related neurodegenerative diseases, including Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), and amyotrophic lateral sclerosis (ALS). Pathological inclusions and the associated toxicity appear to spread through the nervous system in a characteristic pattern during the disease. This has been attributed to a prion-like behavior of amyloid-type aggregates, which involves self-replication of the pathological conformation, intercellular transfer, and the subsequent seeding of native forms of the same protein in the neighboring cell. Molecular chaperones play a major role in maintaining cellular proteostasis by assisting the (re)-folding of cellular proteins to ensure their function or by promoting the degradation of terminally misfolded proteins to prevent damage. With increasing age, however, the capacity of this proteostasis network tends to decrease, which enables the manifestation of neurodegenerative diseases. Recently, there has been a plethora of studies investigating how and when chaperones interact with disease-related proteins, which have advanced our understanding of the role of chaperones in protein misfolding diseases. This review article focuses on the steps of prion-like propagation from initial misfolding and self-templated replication to intercellular spreading and discusses the influence that chaperones have on these various steps, highlighting both the positive and adverse consequences chaperone action can have. Understanding how chaperones alleviate and aggravate disease progression is vital for the development of therapeutic strategies to combat these debilitating diseases.

Kinetic fingerprints differentiate the mechanisms of action of anti-Aβ antibodies

The amyloid cascade hypothesis, according to which the self-assembly of amyloid-β peptide (Aβ) is a causative process in Alzheimer's disease, has driven many therapeutic efforts for the past 20 years. Failures of clinical trials investigating Aβ-targeted therapies have been interpreted as evidence against this hypothesis, irrespective of the characteristics and mechanisms of action of the therapeutic agents, which are highly challenging to assess. Here, we combine kinetic analyses with quantitative binding measurements to address the mechanism of action of four clinical stage anti-Aβ antibodies, aducanumab, gantenerumab, bapineuzumab and solanezumab. We quantify the influence of these antibodies on the aggregation kinetics and on the production of oligomeric aggregates and link these effects to the affinity and stoichiometry of each antibody for monomeric and fibrillar forms of Aβ. Our results reveal that, uniquely among these four antibodies, aducanumab dramatically reduces the flux of Aβ oligomers.

Effect of Disease-Associated P123H and V70M Mutations on β-Synuclein Fibrillation

Synucleinopathies are a class of neurodegenerative diseases, including Parkinson's disease (PD), Dementia with Lewy bodies (DLB), and Multiple System Atrophy (MSA). The common pathological hallmark of synucleinopathies is the filamentous α-synuclein (α-Syn) aggregates along with membrane components in cytoplasmic inclusions in the brain. β-Synuclein (β-Syn), an isoform of α-Syn, inhibits α-Syn aggregation and prevents its neurotoxicity, suggesting the neuroprotective nature of β-Syn. However, this notion changed with the discovery of disease-associated β-Syn mutations, V70M and P123H, in patients with DLB. It is still unclear how these missense mutations alter the structural and amyloidogenic properties of β-Syn, leading to neurodegeneration. Here, we characterized the biophysical properties and investigated the effect of mutations on β-Syn fibrillation under different conditions. V70M and P123H show high membrane binding affinity compared to wild-type β-Syn, suggesting their potential role in membrane interactions. β-Syn and its mutants do not aggregate under normal physiological conditions; however, the proteins undergo self-polymerization in a slightly acidic microenvironment and/or in the presence of an inducer, forming long unbranched amyloid fibrils similar to α-Syn. Strikingly, V70M and P123H mutants exhibit accelerated fibrillation compared to native β-Syn under these conditions. NMR study further revealed that these point mutations induce local perturbations at the site of mutation in β-Syn. Overall, our data provide insight into the biophysical properties of disease-associated β-Syn mutations and demonstrate that these mutants make the native protein more susceptible to aggregation in an altered microenvironment.

Endocytosis Is a Key Mode of Interaction between Extracellular β-Amyloid and the Cell Membrane

Interactions between amyloid-β peptide (Aβ) and the cell membrane include interaction with membrane lipids and binding to membrane receptors, both of which are considered to be the toxicity mechanisms of Aβ. However, it is unclear whether both mechanisms lead to cytotoxicity. Thus, we aimed to analyze these two mechanisms of Aβ42 interaction with cell membranes under different Aβ aggregation states. To this end, model membrane experiments were conducted. Quantitative analysis of Aβ42 monomers or oligomers bound to the membrane of neuro-2a cells was also performed, and laser confocal microscopy was employed to assess endocytosis of FITC-Aβ42 monomers or oligomers by neuro-2a cells. We found that the binding capacity of Aβ42 to membrane lipids was weak and that the amount of Aβ42 bound to membrane lipids was low. Moreover, clathrin-mediated endocytosis of Aβ42 oligomers by neuro-2a cells was observed. Endocytosis serves as a key mode of interaction between extracellular Aβ42 and neurons. These findings provide insights into the mechanisms underlying Aβ oligomer metabolism.

Benzimidazole-based fluorophores for the detection of amyloid fibrils with higher sensitivity than Thioflavin-T

Protein aggregation into amyloid fibrils is a key feature of a multitude of neurodegenerative diseases such as Alzheimer's, Parkinson's, and Prion disease. To detect amyloid fibrils, fluorophores with high sensitivity and better efficiency coupled with the low toxicity are in high demand even to date. In this pursuit, we have unveiled two benzimidazole-based fluorescence sensors ([C₁₅ H₁₅ N₃ ] (C1) and [C₁₆ H₁₆ N₃ O₂ ] (C2), which possess exceptional affinity toward different amyloid fibrils in its submicromolar concentration (8 × 10^-9  M), whereas under a similar concentration, the gold standard Thioflavin-T (ThT) fails to bind with amyloid fibrils. These fluorescent markers bind to α-Syn amyloid fibrils as well as amyloid fibrils forming other proteins/peptides including Aβ42 amyloid fibrils. The ¹ H-¹⁵ N heteronuclear quantum correlation spectroscopy nuclear magnetic resonance data collected on wild-type α-Syn monomer with and without the fluorophores (C1 and C2) reveal that there is weak or no interactions between C1 or C2 with residues in α-Syn monomer, which indirectly reflects the specific binding ability of C1 and C2 to the α-Syn amyloid fibrils. Detailed studies further suggest that C1 and C2 can detect/bind with the α-Syn amyloid fibril as low as 100 × 10^-9  M. Extremely low or no cytotoxicity is observed for C1 and C2 and they do not interfere with α-Syn fibrillation kinetics, unlike ThT. Both C1/C2 not only shows selective binding with amyloid fibrils forming various proteins/peptides but also displays excellent affinity and selectivity toward α-Syn amyloid aggregates in SH-SY5Y cells and Aβ42 amyloid plaques in animal brain tissues. Overall, our data show that the developed dyes could be used for the detection of amyloid fibrils including α-Syn and Aβ42 amyloids with higher sensitivity as compared to currently used ThT.

Eye lens crystallin proteins inhibit the autocatalytic amyloid amplification nature of mature α-synuclein fibrils

Parkinson´s disease is characterized by the accumulation of proteinaceous aggregates in Lewy bodies and Lewy Neurites. The main component found in such aggregates is α-synuclein. Here, we investigate how bovine eye lens crystallin proteins influence the aggregation kinetics of α-synuclein at mildly acidic pH (5.5) where the underlying aggregation mechanism of this protein is dominated by secondary nucleation of monomers on fibril surface providing an autocatalytic amyloid amplification process. Bovine α-, β_H- and γ_B-crystallins were found to display chaperone-like activity inhibiting α-synuclein aggregation. This effect was shown to be time-dependent, with early additions of α-crystallin capable of retarding and even inhibiting aggregation during the time frame of the experiment. The inhibitory nature of crystallins was further investigated using trap and seed kinetic experiments. We propose crystallins interact with mature α-synuclein fibrils, possibly binding along the surfaces and at fibril free ends, inhibiting both elongation and monomer-dependent secondary nucleation processes in a mechanism that may be generic to some chaperones that prevent the onset of protein misfolding related pathologies.

Prolyl Endopeptidase-Like Facilitates the α-Synuclein Aggregation Seeding, and This Effect Is Reverted by Serine Peptidase Inhibitor PMSF

The aggregation of α-synuclein (α-Syn) is a characteristic of Parkinson’s disease (PD). α-Syn oligomerization/aggregation is accelerated by the serine peptidase, prolyl oligopeptidase (POP). Factors that affect POP conformation, including most of its inhibitors and an impairing mutation in its active site, influence the acceleration of α-Syn aggregation resulting from the interaction of these proteins. It is noteworthy, however, that α-Syn is not cleaved by POP. Prolyl endopeptidase-like (PREPL) protein is structurally related to the serine peptidases belonging to the POP family. Based on the α-Syn–POP studies and knowing that PREPL may contribute to the regulation of synaptic vesicle exocytosis, when this protein can encounter α-Syn, we investigated the α-Syn–PREPL interaction. The binding of these two human proteins was observed with an apparent affinity constant of about 5.7 μM and, as in the α-Syn assays with POP, the presence of PREPL accelerated the oligomerization/aggregation events, with no α-Syn cleavage. Furthermore, despite this lack of hydrolytic cleavage, the serine peptidase active site inhibitor phenylmethylsulfonyl fluoride (PMSF) abolished the enhancement of the α-Syn aggregation by PREPL. Therefore, given the attention to POP inhibitors as potential drugs to treat synucleinopathies, the present data point to PREPL as another potential target to be explored for this purpose.

Recombinant Bri3 BRICHOS domain is a molecular chaperone with effect against amyloid formation and non-fibrillar protein aggregation

Molecular chaperones assist proteins in achieving a functional structure and prevent them from misfolding into aggregates, including disease-associated deposits. The BRICHOS domain from familial dementia associated protein Bri2 (or ITM2B) probably chaperones its specific proprotein region with high β-sheet propensity during biosynthesis. Recently, Bri2 BRICHOS activity was found to extend to other amyloidogenic, fibril forming peptides, in particular, Alzheimer’s disease associated amyloid-β peptide, as well as to amorphous aggregate forming proteins. However, the biological functions of the central nervous system specific homologue Bri3 BRICHOS are still to be elucidated. Here we give a detailed characterisation of the recombinant human (rh) Bri3 BRICHOS domain and compare its structural and functional properties with rh Bri2 BRICHOS. The results show that rh Bri3 BRICHOS forms more and larger oligomers, somewhat more efficiently prevents non-fibrillar protein aggregation, and less efficiently reduces Aβ42 fibril formation compared to rh Bri2 BRICHOS. This suggests that Bri2 and Bri3 BRICHOS have overlapping molecular mechanisms and that their apparently different tissue expression and processing may result in different physiological functions.

Ultrastructural evidence for self-replication of Alzheimer-associated Aβ42 amyloid along the sides of fibrils

Two unresolved problems in Alzheimer’s disease (AD) are its onset and propagation, linked to Aβ peptide aggregation. Fibrils of Aβ42 may grow by monomer addition at their ends. Additionally, through so-called secondary nucleation, fibrils can catalyse the formation of new aggregates from monomer on their surface, thereby generating oligomeric species that are toxic to brain tissue. Insights into the structural transitions occurring during secondary nucleation will facilitate the design of therapies to limit the neurotoxicity in AD, but such information is currently lacking. This study identifies conditions that allow the capture of reaction intermediates of secondary nucleation for the purpose of ultrastructural characterization. These reaction intermediates are morphologically distinct from mature fibrils and cover the sides of fibrils during an on-going aggregation reaction.

The nucleation of Alzheimer-associated Aβ peptide monomers can be catalyzed by preexisting Aβ fibrils. This leads to autocatalytic amplification of aggregate mass and underlies self-replication and generation of toxic oligomers associated with several neurodegenerative diseases. However, the nature of the interactions between the monomeric species and the fibrils during this key process, and indeed the ultrastructural localization of the interaction sites have remained elusive. Here we used NMR and optical spectroscopy to identify conditions that enable the capture of transient species during the aggregation and secondary nucleation of the Aβ42 peptide. Cryo-electron microscopy (cryo-EM) images show that new aggregates protrude from the entire length of the progenitor fibril. These protrusions are morphologically distinct from the well-ordered fibrils dominating at the end of the aggregation process. The data provide direct evidence that self-replication through secondary nucleation occurs along the sides of fibrils, which become heavily decorated under the current solution conditions (14 µM Aβ42, 20 mM sodium phosphate, 200 µM EDTA, pH 6.8).

Trem2 Splicing and Expression are Preserved in a Human Aβ-producing, Rat Knock-in Model of Trem2-R47H Alzheimer's Risk Variant

The R47H variant of the Triggering-Receptor-Expressed on Myeloid cells 2 (TREM2) increases the risk of Alzheimer's disease (AD). Mutagenesis of exon 2 in Knock-in (KI) mouse models of the R47H variant introduced a cryptic splice site, leading to nonsense mediated decay. Since haploinsufficiency does not model Trem2-R47H function, a new rat KI model, the Trem2R47H KI rat was created. Human Aβ has higher propensity to form toxic Aβ species, which are considered the main pathogenic entity in AD, as compared to rodent Aβ, the rat Amyloid Precursor Protein (App) gene was mutated to produce human Aβ. Trem2 splicing and expression was measured in Trem2R47H KI rat brains and microglia by qualitative and quantitative RT-PCR. Trem2 levels and Trem2 processing was assessed by Western analysis. APP metabolite levels were determined by enzyme-linked immunosorbent assay (ELISA), for Human Aβ and soluble APP, and Western analysis, for full length APP, βCTF and αCTF. Trem2 expression and Trem2 levels are unchanged in Trem2R47H KI rats. The artifactual splicing seen in KI mouse models is not present; additionally, two novel isoforms of rat Trem2 are described. Trem2R47H rat brains have lower human Aβ38, sAPPα and sAPPβ levels. Thus, Trem2R47H KI rats may prove valuable to define pathogenic mechanisms triggered by the Trem2 R47H variant, including those mediated by toxic species of human Aβ peptides.

Augmentation of Bri2 molecular chaperone activity against amyloid-β reduces neurotoxicity in mouse hippocampus in vitro

Molecular chaperones play important roles in preventing protein misfolding and its potentially harmful consequences. Deterioration of molecular chaperone systems upon ageing are thought to underlie age-related neurodegenerative diseases, and augmenting their activities could have therapeutic potential. The dementia relevant domain BRICHOS from the Bri2 protein shows qualitatively different chaperone activities depending on quaternary structure, and assembly of monomers into high-molecular weight oligomers reduces the ability to prevent neurotoxicity induced by the Alzheimer-associated amyloid-β peptide 1-42 (Aβ42). Here we design a Bri2 BRICHOS mutant (R221E) that forms stable monomers and selectively blocks a main source of toxic species during Aβ42 aggregation. Wild type Bri2 BRICHOS oligomers are partly disassembled into monomers in the presence of the R221E mutant, which leads to potentiated ability to prevent Aβ42 toxicity to neuronal network activity. These results suggest that the activity of endogenous molecular chaperones may be modulated to enhance anti-Aβ42 neurotoxic effects.

Gefei Chen et al. show that a mutated BRICHOS molecular chaperone domain from the dementia associated Bri2 can reduce toxicity of amyloid formation in mouse hippocampus in vitro. Upon mutating Arg221 to glutamate, Bri2 BRICHOS forms stable monomers that block a source of neurotoxicity during Aβ aggregation and promote disassembly of wild-type oligomers.

Apolipoprotein E Interferes with IAPP Aggregation and Protects Pericytes from IAPP-Induced Toxicity

Apolipoprotein E (ApoE) has become a primary focus of research after the discovery of its strong linkage to Alzheimer’s disease (AD), where the ApoE4 variant is the highest genetic risk factor for this disease. ApoE is commonly found in amyloid deposits of different origins, and its interaction with amyloid-β peptide (Aβ), the hallmark of AD, is well known. However, studies on the interaction of ApoEs with other amyloid-forming proteins are limited. Islet amyloid polypeptide (IAPP) is an amyloid-forming peptide linked to the development of type-2 diabetes and has also been shown to be involved in AD pathology and vascular dementia. Here we studied the impact of ApoE on IAPP aggregation and IAPP-induced toxicity on blood vessel pericytes. Using both in vitro and cell-based assays, we show that ApoE efficiently inhibits the amyloid formation of IAPP at highly substoichiometric ratios and that it interferes with both nucleation and elongation. We also show that ApoE protects the pericytes against IAPP-induced toxicity, however, the ApoE4 variant displays the weakest protective potential. Taken together, our results suggest that ApoE has a generic amyloid-interfering property and can be protective against amyloid-induced cytotoxicity, but there is a loss of function for the ApoE4 variant.

Morphological analysis of Apolipoprotein E binding to Aβ Amyloid using a combination of Surface Plasmon Resonance, Immunogold Labeling and Scanning Electron Microscopy

Background

Immunogold labeling in combination with transmission electron microscopy analysis is a technique frequently used to correlate high-resolution morphology studies with detailed information regarding localization of specific antigens. Although powerful, the methodology has limitations and it is frequently difficult to acquire a stringent system where unspecific low-affinity interactions are removed prior to analysis.

Results

We here describe a combinatorial strategy where surface plasmon resonance and immunogold labeling are used followed by a direct analysis of the sensor-chip surface by scanning electron microscopy. Using this approach, we have probed the interaction between amyloid-β fibrils, associated to Alzheimer’s disease, and apolipoprotein E, a well-known ligand frequently found co-deposited to the fibrillar form of Aβ in vivo. The results display a lateral binding of ApoE along the amyloid fibrils and illustrates how the gold-beads represent a good reporter of the binding.

Conclusions

This approach exposes a technique with generic features which enables both a quantitative and a morphological evaluation of a ligand-receptor based system. The methodology mediates an advantage compared to traditional immunogold labeling since all washing steps can be monitored and where a high stringency can be maintained throughout the experiment.