Maintaining proteome integrity is essential for cellular function and survival. Disruptions in proteostasis lead to the aggregation of proteins into inclusions, a process that underlies many neurodegenerative diseases. To quantitatively assess the proteostasis capacity of neuronal cells, we employed an aggregation-prone double mutant form of firefly luciferase (denoted FlucDM) as a reporter protein. We compared two commonly used neuronal cell lines, mouse neuroblastoma cells (Neuro-2a) and a motor neuron-like hybrid line (NSC-34), to evaluate their ability to prevent the aggregation of proteins into intracellular inclusions. We observed a significantly greater propensity of FlucDM to form inclusions in NSC-34 cells compared to Neuro-2a cells. This suggests a reduced capacity of NSC-34 cells for managing aggregation-prone proteins. Proteomic profiling of FlucDM inclusions purified from both cell types revealed cell-type-specific engagement of the proteostasis machinery with aggregation-prone proteins. Comparing the proteomic profiles of key arms of the proteostasis network between these two cell lines revealed that the endoplasmic reticulum (ER) unfolded protein response is differentially expressed. This study establishes a quantitative platform for assessing cellular proteostasis capacity and underscores the importance of cell-type context in proteome maintenance. These insights have implications for understanding the selective vulnerability of neurons in protein misfolding disorders.

Heat shock protein 70 (Hsp70) and Hsp90 are essential molecular chaperones that cooperate to fold diverse client proteins, yet how their activities are coordinated to remodel clients remains unclear. To address this, we used a combination of single-molecule fluorescence resonance energy transfer and total internal reflection fluorescence microscopy to observe individual firefly luciferase proteins during sequential engagement with Escherichia coli Hsp70 (DnaK) and Hsp90 (HtpG). We show that HtpG reduces rebinding of DnaK to folding intermediates while still allowing engagement with misfolded clients, enabling productive refolding in the presence of typically inhibitory concentrations of DnaK. HtpG couples adenosine 5'-triphosphate binding and hydrolysis to promote progressive folding through localized compaction across multiple regions of the client, reducing misfolding and establishing native interdomain contacts. Kinetic simulations support a model whereby heterogeneous DnaK binding generates region-specific folding kinetics and conformational dynamics. This enables efficient subdomain folding by DnaK/HtpG and suggests that the number and position of DnaK binding sites on clients provide a mechanism by which proteins can harness chaperone promiscuity for optimal folding.

Abstract The Hsp70 chaperone system plays a central role in the folding of nascent polypeptides and in preventing protein misfolding and aggregation during cellular stress. However, the precise mechanism by which the human Hsp70, HspA8, remodels the conformations of individual chemically misfolded clients remains unclear. Moreover, it is unknown whether this mechanism differs from that used by Hsp70 to engage clients during heat stress to preserve native function. To address these questions, we report here the use of single-molecule fluorescence resonance energy transfer (smFRET) to temporally interrogate how the human Hsp70 system regulates the conformation of a heat-sensitive client protein, firefly luciferase (Fluc), following chemical denaturation and during heat stress. We find that Hsp70 recognises both chemically denatured and heat-induced misfolded states of Fluc and resolves them by conformational expansion. Release from a Hsp70-bound state, a process driven by the nucleotide exchange factor, Hsp110, guides Fluc toward productive folding trajectories that would otherwise be unlikely to occur spontaneously following collapse from a conformationally unfolded state. Moreover, we demonstrate that both temperature and the conformational state of misfolded Fluc dictate the ability of HspA8 to meaningfully resolve non-native structure within the protein. Collectively, this work provides direct visualisation of the mechanisms by which Hsp70 modulates client conformations under diverse stress conditions to preserve proteome integrity.

TAR DNA-binding protein 43 (TDP-43) is a nucleic acid-binding protein that regulates processes of mRNA metabolism, during which it undergoes condensation mediated by its C-terminal low-complexity domain (TDP-43LCD). TDP-43 aggregation and condensation are associated with neurodegenerative disease. However, the proteostasis mechanisms that regulate these processes remain elusive. Some evidence has shown that the molecular chaperone small heat shock protein HspB1 binds to and regulates the cytoplasmic phase separation of TDP-43, indicating that other small heat shock proteins may have similar effects. Here, we demonstrate divergent behaviors for HspB1 and its homolog HspB5 on TDP-43LCD condensation and aggregation. In addition to inhibiting TDP-43LCD aggregation, HspB1 partitions into TDP-43LCD condensates and increases the dynamic exchange of TDP-43LCD within condensates and with the surrounding solution. Phosphorylation-mimicking mutations within HspB1 enhance these effects. HspB5 inhibits TDP-43LCD aggregation more effectively than HspB1 and partitions into TDP-43LCD condensates, where it delays the pathological transition of the condensate to a gel/solid. We identify the N- and C-terminal regions of HspB1 and HspB5 to be crucial for the chaperone effects, and highlight the role of sequence diversity within these regions in defining small heat shock protein function. These findings demonstrate that HspB1 and HspB5 are regulators of TDP-43 phase separation and aggregation and may be potential therapeutic targets in mitigating toxic TDP-43 aggregation in neurodegenerative disease.

BACKGROUND:SOD1 mutations are a significant contributor of familial amyotrophic lateral sclerosis (ALS) cases. SOD1 mutations increase the propensity for the protein to misfold and aggregate into insoluble proteinaceous deposits within motor neurons and neighbouring cells. The small molecule, CuATSM, has repeatedly shown in mouse models to be a promising therapeutic treatment for SOD1-associated ALS and is currently in Phase II/III clinical trials for the treatment of ALS. We have previously shown CuATSM stabilises various ALS-associated variants of the SOD1 protein, reducing misfolding and toxicity. Two additional FDA-approved small molecules, ebselen and telbivudine, have also been identified to reduce mutant SOD1 toxicity, providing additional potential therapeutic candidates that could be used in combination with CuATSM. Here, we aimed to investigate if CuATSM, ebselen and telbivudine (CET) polytherapy could improve on the therapeutic efficacy of CuATSM monotherapy for the treatment of SOD1-associated ALS. METHODS:We utilised a 3D checkerboard approach to investigate whether a matrix of different concentrations CuATSM, ebselen and telbivudine could provide therapeutic improvements on cell survival, SOD1 folding and aggregation in SOD1G93A-transfected NSC-34 cells, compared to CuATSM alone. To progress the preclinical development of CET polytherapy, we evaluated the bioavailability and safety of in vivo polytherapy administration. Furthermore, we assessed and compared the effects of CET- and CuATSM-treatment on disease onset, motor function, survival and neuropathological features in SOD1G93A mice. FINDINGS:CET polytherapy reduced inclusion formation and increased cell survival of NSC-34 cells overexpressing SOD1G93A compared to higher concentrations of CuATSM monotherapy. In addition, CET administration was bioavailable and tolerable in mice. CET treatment in SOD1G93A mice delayed disease onset, reduced motor impairments, and increased survival compared to vehicle- and CuATSM-treated mice. In line with these findings, biochemical analysis of lumbar spinal cords showed CET administration improved SOD1 folding, decreased misfolded SOD1 accumulation, and reduced motor neuron loss. INTERPRETATION:These findings support CET polytherapy as an advantageous alternative compared to CuATSM monotherapy and highlight the potential of utilising small molecules targeting SOD1 as a polytherapy avenue for the treatment of SOD1-associated ALS. FUNDING:This work was supported by a FightMND Drug Development Grant, an Australian National Health and Medical Research Council (NHMRC) Investigator Grant (No. 1194872) and a Motor Neuron Disease Research Institute of Australia Bill Gole Postdoctoral Fellowship.

Hsp70 and Hsp90 are ubiquitous molecular chaperones that cooperate to promote the correct folding and maturation of client proteins. Despite their central role in proteostasis, the molecular mechanisms by which Hsp90 coordinates with Hsp70 to remodel clients remain poorly understood. In particular, how ATP hydrolysis by Hsp90 is coupled to client engagement and conformational change has been a long-standing question. This gap in understanding is largely due to the challenge of visualizing client conformations during the highly dynamic and heterogeneous interactions with the Hsp70/Hsp90 chaperone machinery. To address this, we used a combination of single-molecule fluorescence resonance energy transfer (smFRET) and total internal reflection fluorescence microscopy to observe individual firefly luciferase client proteins as they are sequentially engaged by Hsp70 and Hsp90. Here, we show that Hsp90 reduces rebinding of Hsp70 to folding intermediates while still allowing engagement to misfolded clients, thereby enabling productive refolding in the presence of typically inhibitory concentrations of Hsp70. Furthermore, Hsp90 uses ATP binding and hydrolysis to actively remodel the conformational landscape of the client, promoting controlled folding through localized compaction across multiple regions. These controlled folding events reduce misfolding and are essential for establishing native interdomain contacts. Using smFRET and kinetic simulations, we further demonstrate that heterogeneous Hsp70 binding generates region-specific folding kinetics and conformational dynamics, which are likely driven by variations in the number of available Hsp70-binding sites. This is then exploited during Hsp70/Hsp90-mediated folding to support localized folding of client subdomains, thereby reducing non-native interactions from distal regions to facilitate proper folding of multi-domain proteins. ### Competing Interest Statement The authors have declared no competing interest. National Health and Medical Research Council, https://ror.org/011kf5r70, APP1197069, APP1194872 Australian Research Council, DE240100707 Motor Neurone Disease Association, https://ror.org/02gq0fg61, Cox 971-799

The accumulation of misfolded proteins underlies a broad range of neurodegenerative diseases, including amyotrophic lateral sclerosis (ALS). Due to their dynamic nature, these misfolded proteins have proven challenging to target therapeutically. Here, we specifically target misfolded disease variants of the ALS-associated protein superoxide dismutase 1 (SOD1), using a biological proteolysis targeting chimera (BioPROTAC) composed of a SOD1-specific intrabody and an E3 ubiquitin ligase. Screening of intrabodies and E3 ligases for optimal BioPROTAC construction reveals a candidate capable of degrading multiple disease variants of SOD1, preventing their aggregation in cells. Using CRISPR/Cas9 technology to develop a BioPROTAC transgenic mouse line, we demonstrate that the presence of the BioPROTAC delays disease progression in the SOD1G93A mouse model of ALS. Delayed disease progression is associated with protection of motor neurons, a reduction of insoluble SOD1 accumulation and preservation of innervated neuromuscular junctions. These findings provide proof-of-concept evidence and a platform for developing BioPROTACs as a therapeutic strategy for the targeted degradation of neurotoxic misfolded species in the context of neurodegenerative diseases.

The aggregation of the protein α-synuclein into amyloid fibrils and their subsequent deposition into large proteinaceous inclusions is a pathological hallmark of several neurodegenerative diseases, including Parkinson's disease. Molecular chaperones, including the small heat-shock proteins (sHsps) and the Hsp70 chaperone system, are known to interact with α-synuclein fibrils, preventing further aggregation and disaggregating fibrillar species, respectively. However, it remains unclear if sHsps co-operate with the Hsp70 chaperones to potentially improve the kinetics or effectiveness of Hsp70-mediated disaggregation and how disaggregation kinetics are influenced by aggregation-prone α-synuclein monomers. Using thioflavin-T assays, we demonstrate that the sHsps Hsp27 (HSPB1) and αB-crystallin (HSPB5) do not synergize with the Hsp70 chaperones during α-synuclein seed fibril disaggregation. Moreover, the addition of monomeric α-synuclein with fibril seeds results in increased aggregation that overwhelms Hsp70-mediated disaggregation. Overall, these results suggest that while Hsp70 and sHsp chaperones are independently capable of binding to and inhibiting fibril elongation, they do not have synergistic effects on disaggregation. Furthermore, Hsp70-mediated disaggregation is ineffectual in the presence of physiological concentrations of α-synuclein monomers, conditions that actually lead to further α-synuclein aggregation. Overall, these data may offer insight into factors that lead to the failure of the Hsp70 chaperones to clear cells of α-synuclein aggregates that lead to neurodegenerative disease.

Small heat shock proteins (sHsps) are molecular chaperones that act to prevent the aberrant aggregation of misfolded proteins. Whilst it is suggested that sHsps prevent aggregation by binding to misfolded client proteins, the dynamic and heterogeneous nature of sHsps has hindered attempts to establish the mechanistic details of how sHsp-client protein complexes form. Single-molecule approaches have emerged as a powerful tool to investigate dynamic and heterogeneous interactions such as those that can occur between sHsps and their client proteins. Here, we use total internal reflection fluorescence microscopy to observe and characterise the complexes formed between model aggregation-prone client proteins (firefly luciferase, rhodanese and chloride intracellular channel 1 protein), and the human sHsps αB-crystallin (αB-c; HSPB5) and Hsp27 (HSPB1). We show that small (monomeric or dimeric) forms of both αB-c and Hsp27 bind to misfolded or oligomeric forms of the client proteins at early stages of aggregation, resulting in the formation of soluble sHsp-client complexes. Stoichiometric analysis of these complexes revealed that additional αB-c subunits accumulate onto pre-existing sHsp-client complexes to form larger species - this does not occur to the same extent for Hsp27. Instead, Hsp27-client interactions tend to be more transient than those of αB-c. Elucidating these mechanisms of sHsp function is crucial to our understanding of how these molecular chaperones act to inhibit protein aggregation and maintain cellular proteostasis.

Introduction: Preeclampsia is a common hypertensive disorder of pregnancy. Several studies have demonstrated that protein aggregates, detected through urine congophilia, is associated with preeclampsia; however, it has yet to be investigated whether urine congophilia remains postpartum in these women. In this study, we aimed to augment prior studies and determine whether urine congophilia is present postpartum. Methods: Women were recruited from Lyell McEwin Hospital, South Australia. Urine samples were collected during pregnancy and 6-months postpartum from women with non-preeclampsia pregnancies (n = 48) and women with pregnancies complicated by preeclampsia (n = 42). A Congo Red Dot blot test, total protein and creatinine levels from urine, as well as serum Soluble fms-like tyrosine kinase 1 to placental growth factor ratio (sFlt-1:PlGF), were assessed and correlated. Results: Preeclamptic women exhibited increased urine congophilia (P < 0.01), sFlt-1:PlGF ratio (P < 0.0001) and total protein (P < 0.01) during pregnancy; with a positive correlation between urine congophilia and total protein across the entire cohort (P < 0.0001). Although urine congophilia was no longer detected 6-months postpartum in preeclamptic women, total protein remained elevated (P < 0.05). sFlt-1:PlGF ratio during pregnancy was positively correlated with congophilia across the cohort (P = 0.0007). Serum creatinine was also higher in preeclamptic women during pregnancy (P < 0.001). Discussion: These results support that urine congophilia is significantly elevated in pregnancies complicated with preeclampsia and show that it does not continue postpartum, although larger cohort studies are needed to determine its feasibility as a diagnostic marker.

Preserving and regulating cellular homeostasis in the light of changing environmental conditions or developmental processes is of pivotal importance for single cellular and multicellular organisms alike. To counteract an imbalance in cellular homeostasis transcriptional programs evolved, called the heat shock response, unfolded protein response, and integrated stress response, that act cell-autonomously in most cells but in multicellular organisms are subjected to cell-nonautonomous regulation. These transcriptional programs downregulate the expression of most genes but increase the expression of heat shock genes, including genes encoding molecular chaperones and proteases, proteins involved in the repair of stress-induced damage to macromolecules and cellular structures. Sixty-one years after the discovery of the heat shock response by Ferruccio Ritossa, many aspects of stress biology are still enigmatic. Recent progress in the understanding of stress responses and molecular chaperones was reported at the 12th International Symposium on Heat Shock Proteins in Biology, Medicine and the Environment in the Old Town Alexandria, VA, USA from 28th to 31st of October 2023.

Amyloid fibril formation by the extracellular protein β2-microglobulin (β2m) and its subsequent accumulation in periarticular tissues have been linked to dialysis-related amyloidosis. A natural variant of human β2m responsible for aggressive systemic amyloidosis contains an aspartate to asparagine mutation at residue 76 (i.e. D76N β2m), which readily forms amyloid fibrils in vitro under physiological conditions. In this study, we examined the role of the extracellular molecular chaperone clusterin in modulating D76N β2m fibril formation in vitro under physiological conditions. The presence of extrinsic charged amino acids modulated D76N β2m fibril formation, implying that electrostatic interactions are involved in the protein’s aggregation. Thioflavin T (ThT) and 1-anilinonaphthalene-8-sulfonate fluorescence assays indicated that clusterin interacts via hydrophobic and electrostatic forces with the monomeric, prefibrillar and fibrillar species of D76N β2m. As a result, clusterin was incorporated into D76N β2m aggregates during the latter’s fibril formation, as indicated by SDS-PAGE of depolymerised fibrils. SYPRO Orange and ThT fluorescence assays suggested that, compared to pure D76N β2m fibrils, those formed in the presence of clusterin are chemically more stable with a reduced ability to act as nucleation seeds. Detailed 15N NMR relaxation studies of mixtures of 15N-labelled β2m with clusterin confirmed that the chaperone interacts transiently and non-specifically with monomeric β2m. Clusterin inhibits both primary and secondary nucleation of D76N β2m fibril formation. In doing so, clusterin binds to D76N β2m fibrils and stabilises them to prevent possible fragmentation. In vivo, the multifaceted chaperone action of clusterin may delay, if not prevent, β2m amyloid proliferation and deposition in tissues.

Small heat-shock proteins (sHSPs) are ubiquitously expressed molecular chaperones present in all king-doms of life that inhibit protein misfolding and aggregation. Despite their importance in proteostasis, the structure-function relationships of sHSPs remain elusive. Human sHSPs are characterised by a central, highly conserved a-crystallin domain (ACD) and variable-length N-and C-terminal regions. The ACD forms antiparallel homodimers via an extended b-strand, creating a shared b-sheet at the dimer interface. The N-and C-terminal regions mediate formation of higher order oligomers that are thought to act as stor-age forms for chaperone-active dimers. We investigated the interactions of the ACD of two human sHSPs, aB-crystallin (aB-C) and Hsp27, with apolipoprotein C-II amyloid fibrils using analytical ultracentrifugation and nuclear magnetic resonance spectroscopy. The ACD was found to interact transiently with amyloid fibrils to inhibit fibril elongation and naturally occurring fibril end-to-end joining. This interaction was sen-sitive to the concentration of fibril ends indicating a 'fibril-capping' interaction. Furthermore, resonances arising from the ACD monomer were attenuated to a greater extent than those of the ACD dimer in the presence of fibrils, suggesting that the monomer may bind fibrils. This hypothesis was supported by muta-genesis studies in which disulfide cross-linked ACD dimers formed by both aB-C and Hsp27 were less effective at inhibiting amyloid fibril elongation and fibril end-to-end joining than ACD constructs lacking disulfide cross-linking. Our results indicate that sHSP monomers inhibit amyloid fibril elongation, highlight-ing the importance of the dynamic oligomeric nature of sHSPs for client binding. (c) 2022 Elsevier Ltd. All rights reserved.

The aggregation of proteins into inclusions or plaques is a prominent hallmark of a diverse range of pathologies including neurodegenerative diseases. The quantification of such inclusions in Caenorhabditis elegans models of aggregation is usually achieved by fluorescence microscopy or other techniques involving biochemical fractionation of worm lysates. Here, we describe a simple and rapid flow cytometry-based approach that allows fluorescently-tagged inclusions to be enumerated in whole worm lysate in a quantitative and unbiased fashion. We demonstrate that this technique is applicable to multiple C. elegans models of aggregation and importantly, can be used to monitor the dynamics of inclusion formation in response to heat shock and during aging. This includes the characterisation of physicochemical properties of inclusions, such as their size, which may reveal how aggregate formation is distinct in different tissues or at different stages of pathology or aging. This new method can be used as a powerful technique for the medium- to high-throughput quantification of inclusions in future studies of genetic or chemical modulators of aggregation in C. elegans.

The ability of heat shock protein 70 (Hsp70) molecular chaperones to remodel the conformation of their clients is central to their biological function; however, questions remain regarding the precise molecular mechanisms by which Hsp70 machinery interacts with the client and how this contributes toward efficient protein folding. Here, we used total internal reflection fluorescence (TIRF) microscopy and single-molecule fluorescence resonance energy transfer (smFRET) to temporally observe the conformational changes that occur to individual firefly luciferase proteins as they are folded by the bacterial Hsp70 system. We observed multiple cycles of chaperone binding and release to an individual client during refolding and determined that high rates of chaperone cycling improves refolding yield. Furthermore, we demonstrate that DnaJ remodels misfolded proteins via a conformational selection mechanism, whereas DnaK resolves misfolded states via mechanical unfolding. This study illustrates that the temporal observation of chaperone-assisted folding enables the elucidation of key mechanistic details inaccessible using other approaches.

The aggregation of proteins into insoluble filamentous amyloid fibrils is a pathological hallmark of neurodegenerative diseases that include Parkinson's disease and Alzheimer's disease. Since the identification of amyloid fibrils and their association with disease, there has been much work to describe the process by which fibrils form and interact with other proteins. However, due to the dynamic nature of fibril formation and the transient and heterogeneous nature of the intermediates produced, it can be challenging to examine these processes using techniques that rely on traditional ensemble-based measurements. Single-molecule approaches overcome these limitations as rare and short-lived species within a population can be individually studied. Fluorescence-based single-molecule methods have proven to be particularly useful for the study of amyloid fibril formation. In this review, we discuss the use of different experimental single-molecule fluorescence microscopy approaches to study amyloid fibrils and their interaction with other proteins, in particular molecular chaperones. We highlight the mechanistic insights these single-molecule techniques have already provided in our understanding of how fibrils form, and comment on their potential future use in studying amyloid fibrils and their intermediates. (C) 2021 The Author(s). Published by Elsevier B.V. on behalf of Research Network of Computational and Structural Biotechnology.

ABSTRACT Disturbances to protein homeostasis (proteostasis) can lead to protein aggregation and inclusion formation, processes associated with a variety of neurodegenerative disorders. DNAJB proteins are molecular chaperones that have been identified as potent suppressors of disease-related protein aggregation. In this work, a destabilised isoform of firefly luciferase (R188Q/R261Q Fluc; termed FlucDM) was overexpressed in cells to assess the capacity of DNAJBs to inhibit inclusion formation. Co-expression of all DNAJB proteins tested significantly inhibited the intracellular aggregation of FlucDM. Moreover, we show that DNAJB proteins suppress aggregation by supporting the Hsp70 (HSPA)-dependent degradation of FlucDM via the proteasome. The serine-rich stretch in DNAJB6 and DNAJB8, essential for preventing fibrillar aggregation, is not involved in the suppression of FlucDM inclusion formation. Conversely, deletion of the C-terminal TTK-LKS motif in DNAJB6 and DNAJB8, a region not required to suppress polyglutamine aggregation, abolished the ability to inhibit inclusion formation by FlucDM. Thus, our data suggest that DNAJB6 and DNAJB8 possess two distinct regions for binding substrates, one that is responsible for binding β-hairpins that form during amyloid formation and another that interacts with exposed hydrophobic patches in aggregation-prone clients. This article has an associated First Person interview with the first author of the paper.

Small heat shock proteins (sHsps) are a family of ubiquitous intracellular molecular chaperones; some sHsp family members are upregulated under stress conditions and play a vital role in protein homeostasis (proteostasis). It is commonly accepted that these chaperones work by trapping misfolded proteins to prevent their aggregation; however, fundamental questions regarding the molecular mechanism by which sHsps interact with misfolded proteins remain unanswered. The dynamic and polydisperse nature of sHsp oligomers has made studying them challenging using traditional biochemical approaches. Therefore, we have utilized a single-molecule fluorescence-based approach to observe the chaperone action of human alphaB-crystallin (αBc, HSPB5). Using this approach we have, for the first time, determined the stoichiometries of complexes formed between αBc and a model client protein, chloride intracellular channel 1. By examining the dispersity and stoichiometries of these complexes over time, and in response to different concentrations of αBc, we have uncovered unique and important insights into a two-step mechanism by which αBc interacts with misfolded client proteins to prevent their aggregation.

Heterogeneity of glia in different CNS regions may contribute to the selective vulnerability of neuronal populations in neurodegenerative conditions such as amyotrophic lateral sclerosis (ALS). Here, we explored regional variations in the expression of heat shock protein 25 in glia under conditions of acute and chronic stress. Hsp27 (Hsp27; murine orthologue: Hsp25) fulfils a number of cytoprotective functions and may therefore be a possible therapeutic target in ALS. We identified a subpopulation of astrocytes in primary murine mixed glial cultures that expressed Hsp25. Under basal conditions, the proportion of Hsp25-positive astrocytes was twice as high in spinal cord cultures than in cortical cultures. To explore the physiological role of the elevated Hsp25 expression in spinal cord astrocytes, we exposed cortical and spinal cord glia to acute stress, using heat stress and pro-inflammatory stimuli. Surprisingly, we observed no stress-induced increase in Hsp25 expression in either cortical or spinal cord astrocytes. Similarly, exposure to endogenous stress, as modelled in glial cultures from SOD1 (G93A)-ALS mice, did not increase Hsp25 expression above that observed in astrocytes from wild-type mice. In vivo, Hsp25 expression was greater under conditions of chronic stress present in the spinal cord of SOD1 (G93A) mice than in wild-type mice, although this increase in expression is likely to be due to the extensive gliosis that occurs in this model. Together, these results show that there are differences in the expression of Hsp25 in astrocytes in different regions of the central nervous system, but Hsp25 expression is not upregulated under acute or chronic stress conditions.