Jan Johansson

Neurotoxic intermediates formed during amyloid-β (Aβ) aggregation are thought to drive Alzheimer’s disease, yet their rarity and transient nature have obscured their identity and formation pathways in the human brain. Here, we combine super-resolution microscopy and single-molecule functional imaging with kinetic modelling to classify Aβ intermediates according to their membrane-disrupting activity and connect their damaging effects to the aggregation pathways that generate them. Applying this framework to recombinant Aβ42, an Alzheimer’s knock-in mouse model and human Alzheimer’s brain tissue, we find two functionally distinct intermediate populations present across all systems. They form through separate aggregation routes and differ in their neurotoxicity, abundance and susceptibility to inhibition. Both damage neurons and glia, but only one converts into plaques. Suppressing these intermediates pharmacologically in mice reduces plaque burden, attenuates astrogliosis and improves cognition. Our findings connect model systems with human pathology and identify intermediate formation pathways as actionable therapeutic targets across protein-misfolding disorders.

Biological drugs like antibodies or enzymes have very limited access to the central nervous system (CNS) because the blood‐brain barrier (BBB) excludes macromolecules that are not required for brain homeostasis. To overcome this hurdle, different types of shuttle systems have been designed, such as linking the drugs to binders to transporters present in the BBB, thereby allowing receptor‐meditated transcytosis (RMT), or encapsulating them in liposomes that have been modified to promote RMT and/or adsorptive‐mediated transcytosis. These shuttles have given encouraging results but are at the same time associated with potential drawbacks like risk of interfering with transport of the endogenous ligands, immune responses and limited efficiencies to deliver the cargo drugs further to intracellular targets. BRICHOS is a 14 kDa protein domain present in ten human protein families and apparently specialised to prevent amyloid toxicity. Recent studies have shown that recombinant BRICHOS from the Bri2 protein can be used to transcytose model cargo protein from spider silk over vascular endothelial cells and promote endocytosis into neurons in the CNS of mice. Here, the potential of harnessing recombinant Bri2 BRICHOS for the transport of biological drugs into the CNS parenchyma and cells is discussed.

The seeding of α-Synuclein (αSyn) is a key driver of Lewy pathology propagation in Parkinson’s disease (PD) and forms the basis for recent diagnostic success. However, it remains unclear how the structural and biochemical features of αSyn seeds dictate their propagation efficiency, capacity to induce Lewy body formation, and resulting cellular toxicity. Using genetic and idiopathic PD cell models, we map the pathogenic cascade beginning with the seed-driven conversion of endogenous αSyn, followed by impaired degradation, mitochondrial dysfunction, and ultimately Lewy body formation. By integrating functional readouts kinetic modelling, we identify secondary nucleation as the predominant mechanism generating toxic αSyn aggregation intermediates, identifying the critical process that links seeding to pathology. Extending this framework to PD brain, we quantitatively correlate seeding capacity with the spatiotemporal spread and severity of Lewy pathology, revealing a mechanistic connection between αSyn aggregation dynamics and disease progression at molecular, cellular, and anatomical levels.

Biological condensates are involved in several essential processes but may also be tangled into disease progression in protein misfolding diseases such as Alzheimer's disease and tauopathies. One hallmark of these disorders is the appearance of fibrillar aggregates formed by microtubule-stabilizing Tau protein. Notably, Tau can also assemble into biological condensates and droplets via liquid-liquid phase separation (LLPS). The molecular mechanisms of the conversion of functional Tau toward insoluble fibrils, potentially via LLPS processes, remain largely unknown, and efficient treatment approaches to target toxic pathways and species are still missing. Here, we show that the molecular chaperone-like Bri2 BRICHOS domain efficiently inhibits full-length Tau fibril formation and subsequent neurotoxicity by specifically suppressing secondary nucleation processes. Further, at substoichiometric ratios, Bri2 BRICHOS modulates the potency of Tau to form droplets, incorporates into Tau droplets, and alters the dynamic behavior of Tau. In contrast, at superstoichiometric Bri2 BRICHOS ratios, Tau droplet formation is abolished. Finally, Bri2 BRICHOS reduces Tau fibril toxicity in electrophysiological experiments on hippocampal slice preparations. Taken together, Bri2 BRICHOS targets molecular processes related to protein misfolding, where our study provides molecular insights into how the inhibition of secondary nucleation pathways and modulated droplet formation are eventually linked to attenuated neurotoxicity.

Bacteria, the smallest and most abundant life forms on Earth, have been a source of insights that have had a considerable impact on human health. Helicobacter pylori has captured substantial attention due to its role in provoking an array of gastrointestinal ailments and other human diseases. Here, we report that H. pylori releases the protein CagA (cytotoxin-associated gene A) that strongly inhibits formation of both functional (bacterial biofilm) and pathogenic amyloid assemblies by targeting various stages during fibril formation. CagA’s broad substrate specificity reveals a mechanism whereby H. pylori interferes with other bacteria and humans, offering approaches to combat bacterial infections and human protein misfolding diseases.

Treatment strategies that are efficient against established Alzheimer’s disease (AD) are needed. BRICHOS is a molecular chaperone domain that prevents amyloid fibril formation and associated cellular toxicity. In this study, we treated an AD mouse model seven months after pathology onset, using intravenous administration of recombinant human (rh) Bri2 BRICHOS R221E. Two injections of rh Bri2 BRICHOS R221E per week for three months in AD mice reduced amyloid β (Aβ) burden, and mitigated astro- and microgliosis, as determined by glial fibrillary acidic protein (GFAP) and ionized calcium-binding adaptor molecule 1 (Iba1) immunohistochemistry. Sequencing of RNA from cortical microglia cells showed that BRICHOS treatment normalized the expression of identified plaque-induced genes in mice and humans, including clusterin and GFAP. Rh Bri2 BRICHOS R221E passed the blood–brain barrier (BBB) in age-matched wild-type mice as efficiently as in the AD mice, but then had no effect on measures of AD-like pathology, and mainly affected the expression of genes that affect cellular shape and movement. These results indicate a potential of rh Bri2 BRICHOS against advanced AD and underscore the ability of BRICHOS to target amyloid-induced pathology.

Protein misfolding and aggregation are involved in several neurodegenerative disorders, such as α‐synuclein (αSyn) implicated in Parkinson's disease, where new therapeutic approaches remain essential to combat these devastating diseases. Elucidating the microscopic nucleation mechanisms has opened new opportunities to develop therapeutics against toxic mechanisms and species. Here, we show that naturally occurring molecular chaperones, represented by the anti‐amyloid Bri2 BRICHOS domain, can be used to target αSyn‐associated nucleation processes and structural species related to neurotoxicity. Our findings revealed that BRICHOS predominantly suppresses the formation of new nucleation units on the fibrils surface (secondary nucleation), decreasing the oligomer generation rate. Further, BRICHOS directly binds to oligomeric αSyn species and effectively diminishes αSyn fibril‐related toxicity. Hence, our studies show that molecular chaperones can be utilized as tools to target molecular processes and structural species related to αSyn neurotoxicity and have the potential as protein‐based treatments against neurodegenerative disorders.

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.

This correspondence offers clarifications on the utility of BRICHOS domains in understanding pathology in Alzheimer’s disease mouse models and the potential of these domains as a treatment strategy. [Image: see text]