Major vulnerability factors for psychiatric disorders such as depression, that often prevent complete remission and lead to relapses, are temperamental. In a rat model of clustered persistent high-anxiety/low-motivation, we have found that overexpression of glial cell line-derived neurotrophic factor (GDNF) by intra-striatally administered adeno-associated virus vector completely converts the passive coping style of low exploratory rats into an active one, similar to high exploratory rats. This conversion of the behavioural strategy developed gradually over repeated testing, and was associated with increased catecholamine metabolism in several brain regions and changes in the regulation of serotonin neurotransmission. An increase in dopamine transporter availability in the striatum was necessary for the phenotype conversion. Associated changes in striatal gene expression included key regulators of monoamine storage and epitranscriptomic regulation. The increase in GDNF signalling also caused alterations in levels and regional covariation of oxidative metabolism, suggestive of a reorganization of neural activity throughout the brain. Thus, neurotrophic factors, GDNF in particular, may play a pivotal role in the development, persistence and alteration of temperament/personality traits, and therefore constitute a potential target for treatment of chronic, relapsing psychiatric disorders.

Major vulnerability factors for psychiatric disorders such as depression, that often prevent complete remission and lead to relapses, are temperamental. In a rat model of clustered persistent high anxiety/low motivation, we have found that overexpression of glial-cell-line-derived neurotrophic factor (GDNF) by intra-striatally administered adeno-associated virus vector strikingly converts the passive coping style of low exploratory rats into an active one, similar to high exploratory rats. This conversion of behavioural strategy developed gradually over repeated testing, and was associated with increased catecholamine metabolism in several brain regions and changes in the regulation of serotonin neurotransmission. An increase in in vivo dopamine transporter availability in the striatum was necessary for the phenotype conversion. Associated changes in striatal gene expression included key players in monoamine storage and epitranscriptomic regulation. The increase in GDNF signalling also caused alterations in levels and regional covariation of oxidative metabolism, indicative of persistent reorganization of neural activity throughout the brain. Thus, neurotrophic factors, GDNF in particular, may play a pivotal role in the development, persistence and alteration of personality traits, and therefore constitute a potential target for treatment of chronic, relapsing psychiatric disorders. ### Competing Interest Statement The authors have declared no competing interest.

The transport of biological drugs from the bloodstream into the central nervous system is hindered by the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier, creating a need for efficient and safe delivery systems to the brain parenchyma. Recombinant human (rh) BRICHOS, a chaperone-like domain from the protein Bri2, can access the brain after systemic injection and has shown therapeutic efficiency in Alzheimer model mice. Here, we show that rh Bri2 BRICHOS and cargo proteins fused to it are efficiently transcytosed over human BBB models made of cerebral microvascular endothelial cells and astrocytes. Moreover, intravenously injected protein fused with Bri2 BRICHOS can access the brain parenchyma of mice and enter neurons and astrocytes. Our results suggest that rh Bri2 BRICHOS can be harnessed to deliver cargo from the bloodstream into neurons.

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.

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.

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.

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 the respective mechanisms are not fully understood. The BRICHOS domain constitutes a disease-associated small heat shock protein-like chaperone family, with activities against both amyloid toxicity and amorphous protein aggregation. Here, we show that the activity of two BRICHOS domain families against Alzheimer’s disease associated amyloid-β neurotoxicity to mouse hippocampi in vitro depends on a conserved aspartate residue, while the ability to suppress amorphous protein aggregation is unchanged by Asp to Asn mutations. The conserved Asp in its ionized state promotes structural flexibility of the BRICHOS domain and has a p K a value between pH 6.0–7.0, suggesting that chaperone effects against amyloid toxicity can be affected by physiological pH variations. Finally, the Asp is evolutionarily highly conserved in >3000 analysed BRICHOS domains but is replaced by Asn in some BRICHOS families and animal species, indicating independent evolution of molecular chaperone activities against amyloid fibril formation and non-fibrillar amorphous 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.

Aggregation and accumulation of amyloid beta (Aβ) are believed to play a key role in the pathogenesis of Alzheimer’s disease (AD). We previously reported that Thioredoxin-80 (Trx80), a truncated form of Thioredoxin-1, prevents the toxic effects of Aβ and inhibits its aggregation in vitro. Trx80 levels were found to be dramatically reduced both in the human brain and cerebrospinal fluid of AD patients. In this study, we investigated the effect of Trx80 expression using in vivo and in vitro models of Aβ pathology. We developed Drosophila melanogaster models overexpressing either human Trx80, human Aβ42, or both Aβ42/Trx80 in the central nervous system. We found that Trx80 expression prevents Aβ42 accumulation in the brain and rescues the reduction in life span and locomotor impairments seen in Aβ42 expressing flies. Also, we show that Trx80 induces autophagosome formation and reverses the inhibition of Atg4b-Atg8a/b autophagosome formation pathway caused by Aβ42. These effects were also confirmed in human neuroblastoma cells. These results give insight into Trx80 function in vivo, suggesting its role in the autophagosome biogenesis and thus in Aβ42 degradation. Our findings put Trx80 on the spotlight as an endogenous agent against Aβ42-induced toxicity in the brain suggesting that strategies to enhance Trx80 levels in neurons could potentially be beneficial against AD pathology in humans.

Formation of fibrils of the amyloid-β peptide (Aβ) is suggested to play a central role in neurodegeneration in Alzheimer's disease (AD), for which no effective treatment exists. The BRICHOS domain is a part of several disease-related proproteins, the most studied ones being Bri2 associated with familial dementia and prosurfactant protein C (proSP-C) associated with lung amyloid. BRICHOS from proSP-C has been found to be an efficient inhibitor of Aβ aggregation and toxicity, but its lung-specific expression makes it unsuited to target in AD. Bri2 is expressed in the brain, affects processing of Aβ precursor protein, and increased levels of Bri2 are found in AD brain, but the specific role of its BRICHOS domain has not been studied in vivo. Here, we find that transgenic expression of the Bri2 BRICHOS domain in the Drosophila central nervous system (CNS) or eyes efficiently inhibits Aβ42 toxicity. In the presence of Bri2 BRICHOS, Aβ42 is diffusely distributed throughout the mushroom bodies, a brain region involved in learning and memory, whereas Aβ42 expressed alone or together with proSP-C BRICHOS forms punctuate deposits outside the mushroom bodies. Recombinant Bri2 BRICHOS domain efficiently prevents Aβ42-induced reduction in γ-oscillations in hippocampal slices. Finally, Bri2 BRICHOS inhibits several steps in the Aβ42 fibrillation pathway and prevents aggregation of heat-denatured proteins, indicating that it is a more versatile chaperone than proSP-C BRICHOS. These findings suggest that Bri2 BRICHOS can be a physiologically relevant chaperone for Aβ in the CNS and needs to be further investigated for its potential in AD treatment.

Iron chelation therapy and inhibition of glial nicotinamide adenine dinucleotide phosphate (NADPH) oxidase can both represent possible routes for Alzheimer's disease modifying therapies. The metal hypothesis is largely focused on direct binding of metals to the N-terminal hydrophilic 1-16 domain peptides of Amyloid beta (A beta) and how they jointly give rise to reactive oxygen species (ROS) production. The cytotoxic effects of A beta through ROS and metals are mainly studied in neuronal cells using full-length A beta(1-40/42) peptides. Here we study cellularly-derived ROS during 2-60 min in response to non-metal associated mid domain A beta(25-35) in microglial Bv2 cells by fluorescence based spectroscopy. We analyze if A beta(25-35) induce ROS production through NADPH oxidase and if the production is sensitive to iron chelation. NADPH oxidase inhibitor diphenyliodonium (DPI) is used to confirm the production of ROS through NADPH oxidase. We modulate cellular iron homeostasis by applying cell permeable iron chelators desferrioxamine (DFO) and deferiprone (DFP). NADPH oxidase subunit gp91-phox level was analyzed by Western blotting.Our results show that A beta(25-35) induces strong ROS production through NADPH oxidase in Bv2 microglial cells. Intracellular iron depletion resulted in restrained A beta(25-35) induced ROS. (C) 2015 Elsevier B.V. All rights reserved.