The Small Heat Shock Protein HSP25/27 (HspB1) Is Abundant in Cultured Astrocytes and Associated with Astrocytic Pathology in Progressive Supranuclear Palsy and Corticobasal Degeneration

Spatial transcriptomics at the brain-electrode interface in rat motor cortex and the relationship to recording quality

Study of the foreign body reaction to implanted electrodes in the brain is an important area of research for the future development of neuroprostheses and experimental electrophysiology. After electrode implantation in the brain, microglial activation, reactive astrogliosis, and neuronal cell death create an environment immediately surrounding the electrode that is significantly altered from its homeostatic state.Objective.To uncover physiological changes potentially affecting device function and longevity, spatial transcriptomics (ST) was implemented to identify changes in gene expression driven by electrode implantation and compare this differential gene expression to traditional metrics of glial reactivity, neuronal loss, and electrophysiological recording quality.Approach.For these experiments, rats were chronically implanted with functional Michigan-style microelectrode arrays, from which electrophysiological recordings (multi-unit activity, local field potential) were taken over a six-week time course. Brain tissue cryosections surrounding each electrode were then mounted for ST processing. The tissue was immunolabeled for neurons and astrocytes, which provided both a spatial reference for ST and a quantitative measure of glial fibrillary acidic protein and neuronal nuclei immunolabeling surrounding each implant.Main results. Results from rat motor cortex within 300µm of the implanted electrodes at 24 h, 1 week, and 6 weeks post-implantation showed up to 553 significantly differentially expressed (DE) genes between implanted and non-implanted tissue sections. Regression on the significant DE genes identified the 6-7 genes that had the strongest relationship to histological and electrophysiological metrics, revealing potential candidate biomarkers of recording quality and the tissue response to implanted electrodes.Significance. Our analysis has shed new light onto the potential mechanisms involved in the tissue response to implanted electrodes while generating hypotheses regarding potential biomarkers related to recorded signal quality. A new approach has been developed to understand the tissue response to electrodes implanted in the brain using genes identified through transcriptomics, and to screen those results for potential relationships with functional outcomes.

Reactive astrocytes secrete the chaperone HSPB1 to mediate neuroprotection

Molecular chaperones are protective in neurodegenerative diseases by preventing protein misfolding and aggregation, such as extracellular amyloid plaques and intracellular tau neurofibrillary tangles in Alzheimer's disease (AD). In addition, AD is characterized by an increase in astrocyte reactivity. The chaperone HSPB1 has been proposed as a marker for reactive astrocytes; however, its astrocytic functions in neurodegeneration remain to be elucidated. Here, we identify that HSPB1 is secreted from astrocytes to exert non-cell-autonomous protective functions. We show that in human AD brain, HSPB1 levels increase in astrocytes that cluster around amyloid plaques, as well as in the adjacent extracellular space. Moreover, in conditions that mimic an inflammatory reactive response, astrocytes increase HSPB1 secretion. Concomitantly, astrocytes and neurons can uptake astrocyte-secreted HSPB1, which is accompanied by an attenuation of the inflammatory response in reactive astrocytes and reduced pathological tau inclusions. Our findings highlight a protective mechanism in disease conditions that encompasses the secretion of a chaperone typically regarded as intracellular.

Specific binding of Hsp27 and phosphorylated Tau mitigates abnormal Tau aggregation-induced pathology

Amyloid aggregation of phosphorylated Tau (pTau) into neurofibrillary tangles is closely associated with Alzheimer's disease (AD). Several molecular chaperones have been reported to bind Tau and impede its pathological aggregation. Recent findings of elevated levels of Hsp27 in the brains of patients with AD suggested its important role in pTau pathology. However, the molecular mechanism of Hsp27 in pTau aggregation remains poorly understood. Here, we show that Hsp27 partially co-localizes with pTau tangles in the brains of patients with AD. Notably, phosphorylation of Tau by microtubule affinity regulating kinase 2 (MARK2), dramatically enhances the binding affinity of Hsp27 to Tau. Moreover, Hsp27 efficiently prevents pTau fibrillation in vitro and mitigates neuropathology of pTau aggregation in a Drosophila tauopathy model. Further mechanistic study reveals that Hsp27 employs its N-terminal domain to directly interact with multiple phosphorylation sites of pTau for specific binding. Our work provides the structural basis for the specific recognition of Hsp27 to pathogenic pTau, and highlights the important role of Hsp27 in preventing abnormal aggregation and pathology of pTau in AD.

Neuroinflammatory processes are augmented in mice overexpressing human heat-shock protein B1 following ethanol-induced brain injury

Background

Heat-shock protein B1 (HSPB1) is among the most well-known and versatile member of the evolutionarily conserved family of small heat-shock proteins. It has been implicated to serve a neuroprotective role against various neurological disorders via its modulatory activity on inflammation, yet its exact role in neuroinflammation is poorly understood. In order to shed light on the exact mechanism of inflammation modulation by HSPB1, we investigated the effect of HSPB1 on neuroinflammatory processes in an in vivo and in vitro model of acute brain injury.

Methods

In this study, we used a transgenic mouse strain overexpressing the human HSPB1 protein. In the in vivo experiments, 7-day-old transgenic and wild-type mice were treated with ethanol. Apoptotic cells were detected using TUNEL assay. The mRNA and protein levels of cytokines and glial cell markers were examined using RT-PCR and immunohistochemistry in the brain. We also established primary neuronal, astrocyte, and microglial cultures which were subjected to cytokine and ethanol treatments. TNFα and hHSPB1 levels were measured from the supernates by ELISA, and intracellular hHSPB1 expression was analyzed using fluorescent immunohistochemistry.

Results

Following ethanol treatment, the brains of hHSPB1-overexpressing mice showed a significantly higher mRNA level of pro-inflammatory cytokines (Tnf, Il1b), microglia (Cd68, Arg1), and astrocyte (Gfap) markers compared to wild-type brains. Microglial activation, and 1 week later, reactive astrogliosis was higher in certain brain areas of ethanol-treated transgenic mice compared to those of wild-types. Despite the remarkably high expression of pro-apoptotic Tnf, hHSPB1-overexpressing mice did not exhibit higher level of apoptosis. Our data suggest that intracellular hHSPB1, showing the highest level in primary astrocytes, was responsible for the inflammation-regulating effects. Microglia cells were the main source of TNFα in our model. Microglia isolated from hHSPB1-overexpressing mice showed a significantly higher release of TNFα compared to wild-type cells under inflammatory conditions.

Conclusions

Our work provides novel in vivo evidence that hHSPB1 overexpression has a regulating effect on acute neuroinflammation by intensifying the expression of pro-inflammatory cytokines and enhancing glial cell activation, but not increasing neuronal apoptosis. These results suggest that hHSPB1 may play a complex role in the modulation of the ethanol-induced neuroinflammatory response.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12974-020-02070-2.

Alzheimer Cells on Their Way to Derailment Show Selective Changes in Protein Quality Control Network

Alzheimer's Disease is driven by protein aggregation and is characterized by accumulation of Tau protein into neurofibrillary tangles. In healthy neurons the cellular protein quality control is successfully in charge of protein folding, which raises the question to which extent this control is disturbed in disease. Here, we describe that brain cells in Alzheimer's Disease show very specific derailment of the protein quality control network. We performed a meta-analysis on the Alzheimer's Disease Proteome database, which provides a quantitative assessment of disease-related proteome changes in six brain regions in comparison to age-matched controls. We noted that levels of all paralogs of the conserved Hsp90 chaperone family are reduced, while most other chaperones - or their regulatory co-chaperones - do not change in disease. The notable exception is a select group consisting of the stress inducible HSP70, its nucleotide exchange factor BAG3 - which links the Hsp70 system to autophagy - and neuronal small heat shock proteins, which are upregulated in disease. They are all members of a cascade controlled in the stress response, channeling proteins towards a pathway of chaperone assisted selective autophagy. Together, our analysis reveals that in an Alzheimer's brain, with exception of Hsp90, the players of the protein quality control are still present in full strength, even in brain regions most severely affected in disease. The specific upregulation of small heat shock proteins and HSP70:BAG3, ubiquitous in all brain areas analyzed, may represent a last, unsuccessful attempt to advert cell death.

Small Heat Shock Proteins, Big Impact on Protein Aggregation in Neurodegenerative Disease

Misfolding, aggregation, and aberrant accumulation of proteins are central components in the progression of neurodegenerative disease. Cellular molecular chaperone systems modulate proteostasis, and, therefore, are primed to influence aberrant protein-induced neurotoxicity and disease progression. Molecular chaperones have a wide range of functions from facilitating proper nascent folding and refolding to degradation or sequestration of misfolded substrates. In disease states, molecular chaperones can display protective or aberrant effects, including the promotion and stabilization of toxic protein aggregates. This seems to be dependent on the aggregating protein and discrete chaperone interaction. Small heat shock proteins (sHsps) are a class of molecular chaperones that typically associate early with misfolded proteins. These interactions hold proteins in a reversible state that helps facilitate refolding or degradation by other chaperones and co-factors. These sHsp interactions require dynamic oligomerization state changes in response to diverse cellular triggers and, unlike later steps in the chaperone cascade of events, are ATP-independent. Here, we review evidence for modulation of neurodegenerative disease-relevant protein aggregation by sHsps. This includes data supporting direct physical interactions and potential roles of sHsps in the stewardship of pathological protein aggregates in brain. A greater understanding of the mechanisms of sHsp chaperone activity may help in the development of novel therapeutic strategies to modulate the aggregation of pathological, amyloidogenic proteins. sHsps-targeting strategies including modulators of expression or post-translational modification of endogenous sHsps, small molecules targeted to sHsp domains, and delivery of engineered molecular chaperones, are also discussed.

Heat Shock Proteins Regulatory Role in Neurodevelopment

Heat shock proteins (Hsps) are a large family of molecular chaperones that are well-known for their roles in protein maturation, re-folding and degradation. While some Hsps are constitutively expressed in certain regions, others are rapidly upregulated in the presence of stressful stimuli. Numerous stressors, including hyperthermia and hypoxia, can induce the expression of Hsps, which, in turn, interact with client proteins and co-chaperones to regulate cell growth and survival. Such interactions must be tightly regulated, especially at critical points during embryonic and postnatal development. Hsps exhibit specific patterns of expression consistent with a spatio-temporally regulated role in neurodevelopment. There is also growing evidence that Hsps may promote or inhibit neurodevelopment through specific pathways regulating cell differentiation, neurite outgrowth, cell migration, or angiogenesis. This review will examine the regulatory role that these individual chaperones may play in neurodevelopment, and will focus specifically on the signaling pathways involved in the maturation of neuronal and glial cells as well as the underlying vascular network.

Astrogliopathy in Tauopathies

Astrocytes are involved in many diseases of the central nervous system, not only as reactive cells to neuronal damage but also as primary actors in the pathological process. Astrogliopathy is a term used to designate the involvement of astrocytes as key elements in the pathogenesis and pathology of diseases and injuries of the central nervous system. Astrocytopathy is utilized to name non-reactive astrogliosis covering hypertrophy, atrophy and astroglial degeneration with loss of function in astrocytes and pathological remodeling, as well as senescent changes. Astrogliopathy and astrocytopathy are hallmarks of tauopathies—neurodegenerative diseases with abnormal hyper-phosphorylated tau aggregates in neurons and glial cells. The involvement of astrocytes covers different disease-specific types such as tufted astrocytes, astrocytic plaques, thorn-shaped astrocytes, granular/fuzzy astrocytes, ramified astrocytes and astrocytes with globular inclusions, as well as others which are unnamed but not uncommon in familial frontotemporal degeneration linked to mutations in the tau gene. Knowledge of molecular differences among tau-containing astrocytes is only beginning, and their distinct functional implications remain rather poorly understood. However, tau-containing astrocytes in certain conditions have deleterious effects on neuronal function and nervous system integrity. Moreover, recent studies have shown that tau-containing astrocytes obtained from human brain tauopathies have a capacity for abnormal tau seeding and spreading in wild type mice. Inclusive conceptions include a complex scenario involving neurons, glial cells and local environmental factors that potentiate each other and promote disease progression in tauopathies.

Diversity of astroglial responses across human neurodegenerative disorders and brain aging

Astrogliopathy refers to alterations of astrocytes occurring in diseases of the nervous system, and it implies the involvement of astrocytes as key elements in the pathogenesis and pathology of diseases and injuries of the central nervous system. Reactive astrocytosis refers to the response of astrocytes to different insults to the nervous system, whereas astrocytopathy indicates hypertrophy, atrophy/degeneration and loss of function and pathological remodeling occurring as a primary cause of a disease or as a factor contributing to the development and progression of a particular disease. Reactive astrocytosis secondary to neuron loss and astrocytopathy due to intrinsic alterations of astrocytes occur in neurodegenerative diseases, overlap each other, and, together with astrocyte senescence, contribute to disease-specific astrogliopathy in aging and neurodegenerative diseases with abnormal protein aggregates in old age. In addition to the well-known increase in glial fibrillary acidic protein and other proteins in reactive astrocytes, astrocytopathy is evidenced by deposition of abnormal proteins such as β-amyloid, hyper-phosphorylated tau, abnormal α-synuclein, mutated huntingtin, phosphorylated TDP-43 and mutated SOD1, and PrPres , in Alzheimer's disease, tauopathies, Lewy body diseases, Huntington's disease, amyotrophic lateral sclerosis and Creutzfeldt-Jakob disease, respectively. Astrocytopathy in these diseases can also be manifested by impaired glutamate transport; abnormal metabolism and release of neurotransmitters; altered potassium, calcium and water channels resulting in abnormal ion and water homeostasis; abnormal glucose metabolism; abnormal lipid and, particularly, cholesterol metabolism; increased oxidative damage and altered oxidative stress responses; increased production of cytokines and mediators of the inflammatory response; altered expression of connexins with deterioration of cell-to-cell networks and transfer of gliotransmitters; and worsening function of the blood brain barrier, among others. Increased knowledge of these aspects will permit a better understanding of brain aging and neurodegenerative diseases in old age as complex disorders in which neurons are not the only players.

Astrocytes release HspB1 in response to amyloid-β exposure in vitro

Although heat shock proteins are thought to function primarily as intracellular chaperones, the release and potential extracellular functions of heat shock proteins have been the focus of an increasing number of studies. Our particular interest is HspB1 (Hsp25/27) and as astrocytes are an in vivo source of HspB1 it is a reasonable possibility they could release HspB1 in response to local stresses. Using primary cultures of rat cortical astrocytes, we investigated the extracellular release of HspB1 with exposure to amyloid-β (Aβ). In order to assess potential mechanisms of release, we cotreated the cells with compounds that can modulate protein secretion including Brefeldin A, Methyl β-cyclodextrin, and MAP kinase inhibitors. Exposure to Aβ (0.1, 1.0, 2.0 μM) for 24-48 h resulted in a selective release of HspB1 that was insensitive to BFA treatment; none of the other inhibitors had any detectable influence. Protease protection assays indicated that some of the released HspB1 was associated with a membrane bound fraction, and analysis of exosomal preparations indicated the presence of HspB1 in exosomes. Finally, immunoprecipitation experiments demonstrated that the extracellular HspB1 was able to interact with extracellular Aβ. In summary, Aβ can stimulate release of HspB1 from astrocytes, this release is insensitive to Golgi or lipid raft disruption, and HspB1 can be found either free in the medium or associated with exosomes. This release suggests that there is a potential for extracellular HspB1 to be able to bind and sequester extracellular Aβ.

Glial Tau Pathology in Tauopathies: Functional Consequences

Tauopathies are a class of neurodegenerative diseases characterized by the presence of hyperphosphorylated and aggregated tau pathology in neuronal and glial cells. Though the ratio of neuronal and glial tau aggregates varies across diseases, glial tau aggregates can populate the same degenerating brain regions as neuronal tau aggregates. While much is known about the deleterious consequences of tau pathology in neurons, the relative contribution of glial tau pathology to these diseases is less clear. Recent studies using a number of model systems implicate glial tau pathology in contributing to tauopathy pathogenesis. This review aims to highlight the functional consequences of tau overexpression in glial cells and explore the potential contribution of glial tau pathology in the pathogenesis of neurodegenerative tauopathies.

Identification of peptides in human Hsp20 and Hsp27 that possess molecular chaperone and anti-apoptotic activities

Previous studies have identified peptides in the 'crystallin-domain' of the small heat-shock protein (sHSP) α-crystallin with chaperone and anti-apoptotic activities. We found that peptides in heat-shock protein Hsp20 (G71HFSVLLDVKHFSPEEIAVK91) and Hsp27 (D93RWRVSLDVNHFAPDELTVK113) with sequence homology to α-crystallin also have robust chaperone and anti-apoptotic activities. Both peptides inhibited hyperthermic and chemically induced aggregation of client proteins. The scrambled peptides of Hsp20 and Hsp27 showed no such effects. The chaperone activities of the peptides were better than those from αA- and αB-crystallin. HeLa cells took up the FITC-conjugated Hsp20 peptide and, when the cells were thermally stressed, the peptide was translocated from the cytoplasm to the nucleus. The two peptides inhibited apoptosis in HeLa cells by blocking cytochrome c release from the mitochondria and caspase-3 activation. We found that scrambling the last four amino acids in the two peptides (KAIV in Hsp20 and KTLV in Hsp27) made them unable to enter cells and ineffective against stress-induced apoptosis. Intraperitoneal injection of the peptides prevented sodium-selenite-induced cataract formation in rats by inhibiting protein aggregation and oxidative stress. Our study has identified peptides from Hsp20 and Hsp27 that may have therapeutic benefit in diseases where protein aggregation and apoptosis are contributing factors.

αB-crystallin and HSP27 in glial cells in tauopathies

Tauopathies are neurodegenerative diseases characterized by hyper-phosphorylated tau deposition in neurons and glial cells. Chaperones, such as small heat shock proteins αB-crystallin and HSP27 highly expressed in normal glial cells, have been postulated as putative molecules preventing abnormal deposition and folding in glial cells in tauopathies. The objective of this work was to assess the expression of αB-crystallin, phosphorylated αB-crystallin at Ser59 and HSP27 in glial cells with and without tau deposits in progressive supranuclear palsy, corticobasal degeneration (CBD), argyrophilic grain disease (AGD), Pick's disease (PiD), Alzheimer's disease, frontotemporal lobar degeneration associated with mutations in the tau gene (FTLD-tau), globular glial tauopathy (GGT) and tauopathy in the elderly. Immunohistochemistry, and double-labeling immunofluorescence and confocal microscopy have been used for this purpose. Increased expression of αB-crystallin and phosphorylated αB-crystallin at Ser59 occurs in a subpopulation of glial cells with and without hyper-phosphorylated tau deposition in all the analyzed tauopathies, but their expression in neurons is restricted to ballooned neurons in CBD, AGD and PiD. HSP27 barely co-localizes with tau and with phosphorylated αB-crystallin at Ser59, thus making the formation of active dimers operating as chaperones unlikely. Results suggest a limited function of αB-crystallin and HSP27 in preventing abnormal tau protein deposition in glial cells and neurons; in addition, the expression of αB-crystallin phosphorylated at Ser59 may act as a protective factor in glial cells.

Reaction of small heat-shock proteins to different kinds of cellular stress in cultured rat hippocampal neurons

Upregulation of small heat-shock proteins (sHsps) in response to cellular stress is one mechanism to increase cell viability.We previously described that cultured rat hippocampal neurons express five of the 11 family members but only upregulate two of them (HspB1 and HspB5) at the protein level after heat stress. Since neurons have to cope with many other pathological conditions, we investigated in this study the expression of all five expressed sHsps on mRNA and protein level after sublethal sodium arsenite and oxidative and hyperosmotic stress. Under all three conditions, HspB1, HspB5, HspB6, and HspB8 but not HspB11 were consistently upregulated but showed differences in the time course of upregulation. The increase of sHsps always occurred earlier on mRNA level compared with protein levels. We conclude from our data that these four upregulated sHsps (HspB1, HspB5, HspB6, HspB8) act together in different proportions in the protection of neurons from various stress conditions.

The protective and therapeutic function of small heat shock proteins in neurological diseases

Historically, small heat shock proteins (sHSPs) have been extensively studied in the context of being intracellular molecular chaperones. However, recent studies looking at the role of sHSPs in neurological diseases have demonstrated a near universal upregulation of certain sHSPs in damaged and diseased brains. Initially, it was thought that sHSPs are pathological in these disease states because they are found in the areas of damage. However, transgenic overexpression and exogenous administration of sHSPs in various experimental disease paradigms have shown just the contrary – that sHSPs are protective, not pathological. This review examines sHSPs in neurological diseases and highlights the potential for using these neuroprotective sHSPs as novel therapeutics. It first addresses the endogenous expression of sHSPs in a variety of neurological disorders. Although many studies have examined the expression of sHSPs in neurological diseases, there are no review articles summarizing these data. Furthermore, it focuses on recent studies that have investigated the therapeutic potential of sHSPs for neurological diseases. Finally, it will explain what we think is the function of endogenous sHSPs in neurological diseases.

Molecular chaperones and regulation of tau quality control: strategies for drug discovery in tauopathies

Tau is a microtubule-associated protein that accumulates in at least 15 different neurodegenerative disorders, which are collectively referred to as tauopathies. In these diseases, tau is often hyperphosphorylated and found in aggregates, including paired helical filaments, neurofibrillary tangles and other abnormal oligomers. Tau aggregates are associated with neuron loss and cognitive decline, which suggests that this protein can somehow evade normal quality control allowing it to aberrantly accumulate and become proteotoxic. Consistent with this idea, recent studies have shown that molecular chaperones, such as heat shock protein 70 and heat shock protein 90, counteract tau accumulation and neurodegeneration in disease models. These molecular chaperones are major components of the protein quality control systems and they are specifically involved in the decision to retain or degrade many proteins, including tau and its modified variants. Thus, one potential way to treat tauopathies might be to either accelerate interactions of abnormal tau with these quality control factors or tip the balance of triage towards tau degradation. In this review, we summarize recent findings and suggest models for therapeutic intervention.

Interplay between glycogen synthase kinase-3β and tau in the cerebellum of Hsp27 transgenic mouse

The association between heat shock protein 27 (Hsp27) and hyperphosphorylated tau has gained attention for more than a decade, but it has never been explored in vivo. In the present study, we found that tau phosphorylated at S396/404 (PHF-1) and S262 sites was significantly increased in the cerebellum of Hsp27 transgenic mice, which was concomitant with increased glycogen synthase kinase-3β (GSK3β) phosphorylated at Y216 and decreased GSK3β phosphorylated at S9. Neither 70-kDa ribosomal protein S6 kinase (p70S6K; total p70S6K, p70S6K at T389, and p70S6K at T421/S424) nor protein phosphatase PP2A (total PP2A, PP2A at Y307, methylated or demethylated PP2A) was changed. This suggests that the increased tau phosphorylation at S396/404 and S262 sites may be induced by Hsp27 through enhancement of GSK3β activity in the mouse cerebellum.