Hsc70 rapidly engages tau after microtubule destabilization

Exploring Protein Misfolding in Amyotrophic Lateral Sclerosis: Structural and Functional Insights

Protein functionality depends on its proper folding, making protein misfolding crucial for the function of proteins and, by extension, cells and the whole organism. Increasing evidence supports the role of protein misfolding in the pathogenesis of neurodegenerative diseases, such as amyotrophic lateral sclerosis (ALS). ALS is a rapidly progressive disease diagnosed at a prevalence of 5 cases per 100,000, with approximately 2-3 patients per 100,000 diagnosed each year. To date, there is no cure, and the disease usually leads to death within 2 to 5 years from diagnosis. There are two types of the disorder: familial ALS (fALS), accounting for approximately 10% of cases, and sporadic (sALS), accounting for the remaining 90%. The hallmark of ALS, regardless of type, is the protein aggregates found in patients' tissues. This suggests that the disruption of proteostasis plays a critical role in the development of the disease. Herein, we stress the distinct factors that lead to protein misfolding and aggregate formation in ALS. Specifically, we highlight several triggering factors affecting protein misfolding, namely mutations, errors in the processes of protein production and trafficking, and failures of folding and chaperone machinery. Gaining a deeper understanding of protein aggregation will improve our comprehension of disease pathogenesis and potentially uncover new therapeutic approaches.

TBG096 Ameliorates Memory Deficiency in AD Mouse Model via Promoting Neurogenesis and Regulation of Hsc70/HK2/PKM2/LAMP2A Signaling Pathway

In previous studies, we isolated a series of novel gentisides with nerve growth factor (NGF)-mimic activities from Gentiana rigescens Franch and conducted continuous structure-activity relationship (SAR) studies. Recently, a lead compound named TBG096 was discovered with significant NGF-mimic activity, low toxicity, and ability to pass through the blood-brain barrier (BBB). At the cell level, TBG096 exerts NGF-mimic activity by regulation of heat-shock cognate protein 70 (Hsc70) and downstream proteins. Subsequently, high-fat diet (HFD)-induced Alzheimer disease (AD) mouse models were used to evaluate the anti-AD efficacy of the compound. TBG096 significantly improved the memory dysfunction of AD mice at doses of 0.1, 5, and 20 mg/kg, respectively. In order to elucidate the mechanism of action of the compound against AD, the RNA-sequence analysis of transcriptomics, quantitative real-time polymerase chain reaction (qRT-PCR), immunofluorescence staining, and Western blot analysis were performed using animal samples. TBG096 significantly increased the expression of the Wnt gene family (Wnt10b, Wnt5a, and Wnt1) and the number of mature neurons and newborn neurons in the hippocampus and cerebral cortex of AD mice, respectively. At the same time, it reduced the activity of microglia, astrocyte cells, and expression of inducible nitric oxide synthase (INOS) in the brain. Moreover, this compound significantly increased phosphorylated-adenosine 5'-monophosphate-activated protein kinase (AMPK), Hsc70, and lysosomal-associated membrane protein 2a (LAMP2A) and decreased the expression of hexokinase 2 (HK2), pyruvate kinase M2 (PKM2), amyloid precursor protein (APP), microtubule-associated protein tau (Tau), phosphoryl-Tau, and β-amyloid (Aβ) at the protein level. These results suggest that TBG096 produced the NGF-mimic activity and the anti-AD effect via promoting neurogenesis and modification of the Hsc70/HK2/PKM2/LAMP2A signaling pathway, proposing a potential novel approach to counteracting cognitive decline by developing small molecules that promote neurogenesis and the Hsc70 signaling pathway.

Probe-dependent Proximity Profiling (ProPPr) Uncovers Similarities and Differences in Phospho-Tau-Associated Proteomes Between Tauopathies

BACKGROUND

Tauopathies represent a diverse group of neurodegenerative disorders characterized by the abnormal aggregation of the microtubule-associated protein tau. Despite extensive research, the mechanisms underlying the diversity of neuronal and glial tau pathology in different tauopathies are poorly understood. While there is a growing understanding of tauopathy-specific differences in tau isoforms and fibrillar structures, the specific composition of heterogenous tau lesions remains unknown. Here we study the protein composition of tau aggregates in four major tauopathies: Alzheimer's disease (AD), corticobasal degeneration (CBD), Pick's disease (PiD), and progressive supranuclear palsy (PSP).

METHODS

We developed an approach for in situ proximity labeling and isolation of aggregate-associated proteins using glass slides with formalin-fixed paraffin-embedded (FFPE) human postmortem brain tissue, termed Probe-dependent Proximity Profiling (ProPPr). We used ProPPr for the analysis of proteomes associated with AT8-positive cellular lesions from frontal cortices. Isolated proximity proteomes were analyzed by data-independent acquisition mass spectrometry. Co-immunofluorescence staining and quantitative data analysis for selected proteins in human brain tissue was performed to further investigate associations with diverse tau pathologies.

RESULTS

Proteomics data analysis identified numerous common and tauopathy-specific proteins associated with phospho-tau aggregates. Extensive validations of candidates through quantitative immunofluorescence imaging of distinct aggregates across disease cases demonstrate successful implementation of ProPPr for unbiased discovery of aggregate-associated proteins in in human brain tissue. Our results reveal the association of retromer complex component vacuolar protein sorting-associated protein 35 (VPS35) and lysosome-associated membrane glycoprotein 2 (LAMP2) with specific types of phospho-tau lesions in tauopathies. Furthermore, we discovered a disease-specific association of certain proteins with distinct pathological lesions, including glycogen synthase kinase alpha (GSK3α), ferritin light chain (FTL), and the neuropeptide precursor VGF. Notably, the identification of FTL-positive microglia in CBD astrocytic plaques indicate their potential role in the pathogenesis of these lesions.

CONCLUSIONS

Our findings demonstrate the suitability of the ProPPr approach in FFPE brain tissue for unbiased discovery of local proteomes that provide valuable insights into the underlying proteomic landscape of tauopathies, shedding light on the molecular mechanisms underlying tau pathology. This first comprehensive characterization of tau-associated proteomes in a range of distinct tauopathies enhances our understanding of disease heterogeneity and mechanisms, informing strategies for the development of diagnostic biomarkers and targeted therapies.

Probe-dependent Proximity Profiling (ProPPr) Uncovers Similarities and Differences in Phospho-Tau-Associated Proteomes Between Tauopathies

Tauopathies represent a diverse group of neurodegenerative disorders characterized by the abnormal aggregation of the microtubule-associated protein tau. Despite extensive research, the precise mechanisms underlying the complexity of different types of tau pathology remain incompletely understood. Here we describe an approach for proteomic profiling of aggregate-associated proteomes on slides with formalin-fixed, paraffin-embedded (FFPE) tissue that utilizes proximity labelling upon high preservation of aggregate morphology, which permits the profiling of pathological aggregates regardless of their size. To comprehensively investigate the common and unique protein interactors associated with the variety of tau lesions present across different human tauopathies, Alzheimer's disease (AD), corticobasal degeneration (CBD), Pick's disease (PiD), and progressive supranuclear palsy (PSP), were selected to represent the major tauopathy diseases. Implementation of our widely applicable Probe-dependent Proximity Profiling (ProPPr) strategy, using the AT8 antibody, permitted identification and quantification of proteins associated with phospho-tau lesions in well-characterized human post-mortem tissue. The analysis revealed both common and disease-specific proteins associated with phospho-tau aggregates, highlighting potential targets for therapeutic intervention and biomarker development. Candidate validation through high-resolution co-immunofluorescence of distinct aggregates across disease and control cases, confirmed the association of retromer complex protein VPS35 with phospho-tau lesions across the studied tauopathies. Furthermore, we discovered disease-specific associations of proteins including ferritin light chain (FTL) and the neuropeptide precursor VGF within distinct pathological lesions. Notably, examination of FTL-positive microglia in CBD astrocytic plaques indicate a potential role for microglial involvement in the pathogenesis of these tau lesions. Our findings provide valuable insights into the proteomic landscape of tauopathies, shedding light on the molecular mechanisms underlying tau pathology. This first comprehensive characterization of tau-associated proteomes across different tauopathies enhances our understanding of disease heterogeneity and provides a resource for future functional investigation, as well as development of targeted therapies and diagnostic biomarkers.

DnaJs are enriched in tau regulators

The aberrant accumulation of tau protein is implicated as a pathogenic factor in many neurodegenerative diseases. Tau seeding may underlie its predictable spread in these diseases. Molecular chaperones can modulate tau pathology, but their effects have mainly been studied in isolation. This study employed a semi-high throughput assay to identify molecular chaperones influencing tau seeding using Tau RD P301S FRET Biosensor cells, which express a portion of tau containing the frontotemporal dementia-related P301S tau mutation fused to a FRET biosensor. Approximately fifty chaperones from five major families were screened using live cell imaging to monitor FRET-positive tau seeding. Among the tested chaperones, five exhibited significant effects on tau in the primary screen. Notably, three of these were from the DnaJ family. In subsequent studies, overexpression of DnaJA2, DnaJB1, and DnaJB6b resulted in significant reductions in tau levels. Knockdown experiments by shRNA revealed an inverse correlation between DnaJB1 and DnaJB6b with tau levels. DnaJB6b overexpression, specifically, reduced total tau levels in a cellular model with a pre-existing pool of tau, partially through enhanced proteasomal degradation. Further, DnaJB6b interacted with tau complexes. These findings highlight the potent chaperone activity within the DnaJ family, particularly DnaJB6b, towards tau.

The E3 Ubiquitin Ligase, CHIP/STUB1, Inhibits Aggregation of Phosphorylated Proteoforms of Microtubule-associated Protein Tau (MAPT)

Hyper-phosphorylated tau accumulates as insoluble fibrils in Alzheimer's disease (AD) and related dementias. The strong correlation between phosphorylated tau and disease has led to an interest in understanding how cellular factors discriminate it from normal tau. Here, we screen a panel of chaperones containing tetratricopeptide repeat (TPR) domains to identify those that might selectively interact with phosphorylated tau. We find that the E3 ubiquitin ligase, CHIP/STUB1, binds 10-fold more strongly to phosphorylated tau than unmodified tau. The presence of even sub-stoichiometric concentrations of CHIP strongly suppresses aggregation and seeding of phosphorylated tau. We also find that CHIP promotes rapid ubiquitination of phosphorylated tau, but not unmodified tau, in vitro. Binding to phosphorylated tau requires CHIP's TPR domain, but the binding mode is partially distinct from the canonical one. In cells, CHIP restricts seeding by phosphorylated tau, suggesting that it could be an important barrier in cell-to-cell spreading. Together, these findings show that CHIP recognizes a phosphorylation-dependent degron on tau, establishing a pathway for regulating the solubility and turnover of this pathological proteoform.

Identification of gene networks mediating regional resistance to tauopathy in late-onset Alzheimer’s disease

Neurofibrillary lesions composed of tau protein aggregates are defining hallmarks of Alzheimer’s Disease. Despite tau filaments appearing to spread between networked brain regions in a prion-like manner, certain areas including cerebellum resist trans-synaptic spread of tauopathy and degeneration of their constituent neuronal cell bodies. To identify molecular correlates of resistance, we derived and implemented a ratio of ratios approach for disaggregating gene expression data on the basis of regional vulnerability to tauopathic neurodegeneration. When applied to vulnerable pre-frontal cortex as an internal reference for resistant cerebellum, the approach segregated adaptive changes in expression into two components. The first was enriched for neuron-derived transcripts associated with proteostasis including specific members of the molecular chaperone family and was unique to resistant cerebellum. When produced as purified proteins, each of the identified chaperones depressed aggregation of 2N4R tau in vitro at sub-stoichiometric concentrations, consistent with the expression polarity deduced from ratio of ratios testing. In contrast, the second component enriched for glia- and microglia-derived transcripts associated with neuroinflammation, segregating these pathways from susceptibility to tauopathy. These data support the utility of ratio of ratios testing for establishing the polarity of gene expression changes with respect to selective vulnerability. The approach has the potential to identify new targets for drug discovery predicated on their ability to promote resistance to disease in vulnerable neuron populations.

Deletion of UCP1 in Tg2576 Mice Increases Body Temperature and Exacerbates Alzheimer's Disease-Related Pathologies

We previously demonstrated that the Alzheimer's disease (AD)-like model mice, Tg2576, housed at a high ambient temperature of 30 °C for 13 months, exhibited increased body temperature, which increased amyloid-β (Aβ) levels and tau stability, leading to tau phosphorylation and ultimately inducing memory impairment. Here, we aimed to exclude the possible effect of environmental factors associated with the difference in ambient temperature (23 °C vs. 30 °C) and to further clarify the effects of elevated body temperature on AD-like pathologies. We generated uncoupling protein 1 (UCP1) deletion in Tg2576 mice, Tg2576/UCP1^-/-, because UCP1 deletion mice show a sustained rise in body temperature at normal room temperature. As expected, the body temperature in Tg2576/UCP1^-/- mice was higher than that in Tg2576/ UCP1^+/+ mice at 23 °C, which was accompanied by upregulated Aβ levels due to increased β-secretase (BACE1) and decreased neprilysin (NEP) protein levels in the brains of Tg2576/UCP1^-/- mice compared with those in the Tg2576/ UCP1^+/+ mice. Elevated body temperature also increased total tau levels, leading to enhanced phosphorylation, heat shock protein induction, and activated tau kinases. Furthermore, elevated body temperature enhanced glial activation and decreased synaptic protein levels in the brain. Taken together, these findings demonstrate that elevated body temperatures exacerbate AD-like pathologies.

Tau Protein as Therapeutic Target for Cancer? Focus on Glioblastoma

Despite being extensively studied for several decades, the microtubule-associated protein Tau has not finished revealing its secrets. For long, Tau has been known for its ability to promote microtubule assembly. A less known feature of Tau is its capability to bind to cancer-related protein kinases, suggesting a possible role of Tau in modulating microtubule-independent cellular pathways that are associated with oncogenesis. With the intention of finding new therapeutic targets for cancer, it appears essential to examine the interaction of Tau with these kinases and their consequences. This review aims at collecting the literature data supporting the relationship between Tau and cancer with a particular focus on glioblastoma tumors in which the pathological significance of Tau remains largely unexplored. We will first treat this subject from a mechanistic point of view showing the pivotal role of Tau in oncogenic processes. Then, we will discuss the involvement of Tau in dysregulating critical pathways in glioblastoma. Finally, we will outline promising strategies to target Tau protein for the therapy of glioblastoma.

Heat shock proteins: Biological functions, pathological roles, and therapeutic opportunities

The heat shock proteins (HSPs) are ubiquitous and conserved protein families in both prokaryotic and eukaryotic organisms, and they maintain cellular proteostasis and protect cells from stresses. HSP protein families are classified based on their molecular weights, mainly including large HSPs, HSP90, HSP70, HSP60, HSP40, and small HSPs. They function as molecular chaperons in cells and work as an integrated network, participating in the folding of newly synthesized polypeptides, refolding metastable proteins, protein complex assembly, dissociating protein aggregate dissociation, and the degradation of misfolded proteins. In addition to their chaperone functions, they also play important roles in cell signaling transduction, cell cycle, and apoptosis regulation. Therefore, malfunction of HSPs is related with many diseases, including cancers, neurodegeneration, and other diseases. In this review, we describe the current understandings about the molecular mechanisms of the major HSP families including HSP90/HSP70/HSP60/HSP110 and small HSPs, how the HSPs keep the protein proteostasis and response to stresses, and we also discuss their roles in diseases and the recent exploration of HSP related therapy and diagnosis to modulate diseases. These research advances offer new prospects of HSPs as potential targets for therapeutic intervention.

Histone deacetylase-6 modulates Tau function in Alzheimer's disease

Alzheimer's disease (AD), one of the major tauopathies, is multifactorial with a massive demand for disease-modifying treatments rather than symptom management. An AD-affected neuron shows Tau depositions generated due to overload on the proteostasis machinery of the cell and/or abnormal post-translational modifications on Tau protein. Loss of memory or dementia is the most significant concern in AD, occurring due to the loss of neurons and the connections between them. In a healthy brain, neurons interact with the environment and each other through extensions and migratory structures. It can thus be safe to assume that Tau depositions affect these growth structures in neurons. A Histone Deacetylase, HDAC6, has shown elevated levels in AD while also demonstrating direct interaction with the Tau protein. HDAC6 interacts with multiple proteins in the cell and is possibly involved in various signalling pathways. Its deacetylase activity has been a point of controversy in AD; however other functional domains remain unexplored. This review highlights the beneficial potential of HDAC6 in AD in mediating both Tau proteostasis and cytoskeletal rewiring for the neuritic extensions through its Ubiquitin Binding domain (HDAC6 ZnF UBP).

Sustained high body temperature exacerbates cognitive function and Alzheimer's disease-related pathologies

Global warming is a serious public health threat to people worldwide. High body temperature is one of the important risk factors for Alzheimer’s disease (AD), and the body temperature of AD patients has been found to be significantly higher than that of elderly control subjects. However, the effects of high body temperature on cognitive function and AD pathologies have not been completely elucidated. We report here that Tg2576 mice housed at a high ambient temperature of 30 °C for 13 months showed an increase in the body temperature, which is accompanied by memory impairment and an enhancement of amyloid-β peptides (Aβ) generation through the upregulation of β-site APP cleaving enzyme 1 (BACE1) level and decrease in the level of an Aβ-degrading enzyme, neprilysin (NEP) in the brain, compared with those of Tg2576 mice at 23 °C. High body temperature also increased the levels of heat shock proteins (HSPs), stress-stimulated kinases such as JNK, and total tau, leading to the enhancement of tau phosphorylation at 30 °C. Taken together, our findings suggest that high body temperature exacerbates cognitive function and AD pathologies, which provides a mechanistic insight for its prevention.

Structural basis for the inhibition of IAPP fibril formation by the co-chaperonin prefoldin

Chaperones, as modulators of protein conformational states, are key cellular actors to prevent the accumulation of fibrillar aggregates. Here, we integrated kinetic investigations with structural studies to elucidate how the ubiquitous co-chaperonin prefoldin inhibits diabetes associated islet amyloid polypeptide (IAPP) fibril formation. We demonstrated that both human and archaeal prefoldin interfere similarly with the IAPP fibril elongation and secondary nucleation pathways. Using archaeal prefoldin model, we combined nuclear magnetic resonance spectroscopy with electron microscopy to establish that the inhibition of fibril formation is mediated by the binding of prefoldin's coiled-coil helices to the flexible IAPP N-terminal segment accessible on the fibril surface and fibril ends. Atomic force microscopy demonstrates that binding of prefoldin to IAPP leads to the formation of lower amounts of aggregates, composed of shorter fibrils, clustered together. Linking structural models with observed fibrillation inhibition processes opens perspectives for understanding the interference between natural chaperones and formation of disease-associated amyloids.

Amyloid protein-induced sequestration of the eukaryotic ribosome: effect of stoichiometry and polyphenolic inhibitors

Alzheimer's disease (AD) is characterized by the appearance of neurofibrillary tangles comprising of the Tau protein and aggregation of amyloid-β peptides (Aβ 1-40 and Aβ 1-42). A concomitant loss of the ribosomal population is also observed in AD-affected neurons. Our studies demonstrate that, similarly to Tau protein aggregation, in vitro aggregation of Aβ peptides in the vicinity of the yeast 80S ribosome can induce co-aggregation of ribosomal components. The RNA-stimulated aggregation of Aβ peptides and the Tau-K18 variant is dependent on the RNA:protein stoichiometric ratio. A similar effect of stoichiometry is also observed on the ribosome-protein co-aggregation process. Polyphenolic inhibitors of amyloid aggregation, such as rosmarinic acid and myricetin, inhibit RNA-stimulated Aβ and Tau-K18 aggregation and can mitigate the co-aggregation of ribosomal components.

Evaluation of the Therapeutic Effect of Lycoramine on Alzheimer's Disease in Mouse Model

BACKGROUND

Alzheimer's disease is one of the leading health problems characterized by the accumulation of Aβ and hyperphosphorylated tau that account for the senile plaque formations causing extensive cognitive decline. Many of the clinical diagnoses of Alzheimer's disease are made in the late stages, when the pathological changes have already progressed.

OBJECTIVE

The objective of this study is to evaluate the promising therapeutic effects of a natural compound, lycoramine, which has been shown to have therapeutic potential in several studies and to understand its mechanism of action on the molecular level via differential protein expression analyses.

METHODS

Lycoramine and galantamine, an FDA approved drug used in the treatment of mild to moderate AD, were administered to 12 month-old 5xFAD mice. Effects of the compounds were investigated by Morris water maze, immunohistochemistry and label- free differential protein expression analyses.

RESULTS

Here we demonstrated the reversal of cognitive decline via behavioral testing and the clearance of Aβ plaques. Proteomics analysis provided in-depth information on the statistically significant protein perturbations in the cortex, hippocampus and cerebellum sections to hypothesize the possible clearance mechanisms of the plaque formation and the molecular mechanism of the reversal of cognitive decline in a transgenic mouse model. Bioinformatics analyses showed altered molecular pathways that can be linked with the reversal of cognitive decline observed after lycoramine administration but not with galantamine.

CONCLUSION

Lycoramine shows therapeutic potential to halt and reverse cognitive decline at the late stages of disease progression, and holds great promise for the treatment of Alzheimer's disease.

Tau secretion and propagation: Perspectives for potential preventive interventions in Alzheimer's disease and other tauopathies

Alzheimer's disease (AD) is characterised by the accumulation of intracytoplasmic aggregates of tau protein, which are suggested to spread in a prion-like manner between interconnected brain regions. This spreading is mediated by the secretion and uptake of tau from the extracellular space or direct cell-to-cell transmission through cellular protrusions. The prion-like tau then converts the endogenous, normal tau into pathological forms, resulting in neurodegeneration. The endoplasmic reticulum/Golgi-independent tau secretion through unconventional secretory pathways involves delivering misfolded and aggregated tau to the plasma membrane and its release into the extracellular space by non-vesicular and vesicular mechanisms. Although cytoplasmic tau was thought to be released only from degenerating cells, studies now show that cells constitutively secrete tau at low levels under physiological conditions. The mechanisms of secretion of tau under physiological and pathological conditions remain unclear. Therefore, a better understanding of these pathways is essential for developing therapeutic approaches that can target prion-like tau forms to prevent neurodegeneration progression in AD. This review focuses on unconventional secretion pathways involved in the spread of tau pathology in AD and presents these pathways as prospective areas for future AD drug discovery and development.

Inhibitors of heat shock protein 70 (Hsp70) with enhanced metabolic stability reduce tau levels

The molecular chaperone, Heat Shock Protein 70 (Hsp70), is an emerging drug target for neurodegenerative diseases, because of its ability to promote degradation of microtubule-associated protein tau (MAPT/tau). Recently, we reported YM-08 as a brain penetrant, allosteric Hsp70 inhibitor, which reduces tau levels. However, the benzothiazole moiety of YM-08 is vulnerable to metabolism by CYP3A4, limiting its further application as a chemical probe. In this manuscript, we designed and synthesized seventeen YM-08 derivatives by systematically introducing halogen atoms to the benzothiazole ring and shifting the position of the heteroatom in a distal pyridine. In microsome assays, we found that compound JG-23 has 12-fold better metabolic stability and it retained the ability to reduce tau levels in two cell-based models. These chemical probes of Hsp70 are expected to be useful tools for studying tau homeostasis.

Hsp90 co-chaperones, FKBP52 and Aha1, promote tau pathogenesis in aged wild-type mice

The microtubule associated protein tau is an intrinsically disordered phosphoprotein that accumulates under pathological conditions leading to formation of neurofibrillary tangles, a hallmark of Alzheimer's disease (AD). The mechanisms that initiate the accumulation of phospho-tau aggregates and filamentous deposits are largely unknown. In the past, our work and others' have shown that molecular chaperones play a crucial role in maintaining protein homeostasis and that imbalance in their levels or activity can drive tau pathogenesis. We have found two co-chaperones of the 90 kDa heat shock protein (Hsp90), FK506-binding protein 52 (FKBP52) and the activator of Hsp90 ATPase homolog 1 (Aha1), promote tau aggregation in vitro and in the brains of tau transgenic mice. Based on this, we hypothesized that increased levels of these chaperones could promote tau misfolding and accumulation in the brains of aged wild-type mice. We tested this hypothesis by overexpressing Aha1, FKBP52, or mCherry (control) proteins in the hippocampus of 9-month-old wild-type mice. After 7 months of expression, mice were evaluated for cognitive and pathological changes. Our results show that FKBP52 overexpression impaired spatial reversal learning, while Aha1 overexpression impaired associative learning in aged wild-type mice. FKBP52 and Aha1 overexpression promoted phosphorylation of distinct AD-relevant tau species. Furthermore, FKBP52 activated gliosis and promoted neuronal loss leading to a reduction in hippocampal volume. Glial activation and phospho-tau accumulation were also detected in areas adjacent to the hippocampus, including the entorhinal cortex, suggesting that after initiation these pathologies can propagate through other brain regions. Overall, our findings suggest a role for chaperone imbalance in the initiation of tau accumulation in the aging brain.

Tau protein- induced sequestration of the eukaryotic ribosome: Implications in neurodegenerative disease

The human tau is a microtubule-associated intrinsically unstructured protein that forms intraneuronal cytotoxic deposits in neurodegenerative diseases, like tauopathies. Recent studies indicate that in Alzheimer's disease, ribosomal dysfunction might be a crucial event in the disease pathology. Our earlier studies had demonstrated that amorphous protein aggregation in the presence of ribosome can lead to sequestration of the ribosomal components. The present study aims at determining the effect of incubation of the full-length tau protein (Ht40) and its microtubule binding 4-repeat domain (K18) on the eukaryotic ribosome. Our in vitro studies show that incubation of Ht40 and the K18 tau variants with isolated non-translating yeast ribosome can induce a loss of ribosome physical integrity resulting in formation of tau-rRNA-ribosomal protein aggregates. Incubation with the tau protein variants also led to a disappearance of the peak indicating the ribosome profile of the HeLa cell lysate and suppression of translation in the human in vitro translation system. The incubation of tau protein with the ribosomal RNA leads to the formation of tau-rRNA aggregates. The effect of K18 on the yeast ribosome can be mitigated in the presence of cellular polyanions like heparin and tRNA, thereby indicating the electrostatic nature of the aggregation process.

Tau protein aggregates inhibit the protein-folding and vesicular trafficking arms of the cellular proteostasis network

Tauopathies are a diverse class of neurodegenerative diseases characterized by the formation of insoluble tau aggregates and the loss of cellular function and neuronal death. Tau inclusions have been shown to contain a number of proteins, including molecular chaperones, but the consequences of these entrapments are not well established. Here, using a human cell system for seeding-dependent tau aggregation, we demonstrate that the molecular chaperones heat-shock cognate 71-kDa protein (HSC70)/heat-shock protein 70 (HSP70), HSP90, and J-domain co-chaperones are sequestered by tau aggregates. By employing single-cell analysis of protein-folding and clathrin-mediated endocytosis, we show that both chaperone-dependent cellular activities are significantly impaired by tau aggregation and can be reversed by treatment with small-molecule regulators of heat-shock transcription factor 1 (HSF1) proteostasis that induce the expression of cytosolic chaperones. These results reveal that the sequestration of cytoplasmic molecular chaperones by tau aggregates interferes with two arms of the proteostasis network, likely having profound negative consequences for cellular function.

Changes in proteome solubility indicate widespread proteostatic disruption in mouse models of neurodegenerative disease

The deposition of pathologic misfolded proteins in neurodegenerative disorders such as Alzheimer's disease, Parkinson's disease, frontotemporal dementia and amyotrophic lateral sclerosis is hypothesized to burden protein homeostatic (proteostatic) machinery, potentially leading to insufficient capacity to maintain the proteome. This hypothesis has been supported by previous work in our laboratory, as evidenced by the perturbation of cytosolic protein solubility in response to amyloid plaques in a mouse model of Alzheimer's amyloidosis. In the current study, we demonstrate changes in proteome solubility are a common pathology to mouse models of neurodegenerative disease. Pathological accumulations of misfolded tau, α-synuclein and mutant superoxide dismutase 1 in CNS tissues of transgenic mice were associated with changes in the solubility of hundreds of CNS proteins in each model. We observed that changes in proteome solubility were progressive and, using the rTg4510 model of inducible tau pathology, demonstrated that these changes were dependent upon sustained expression of the primary pathologic protein. In all of the models examined, changes in proteome solubility were robust, easily detected, and provided a sensitive indicator of proteostatic disruption. Interestingly, a subset of the proteins that display a shift towards insolubility were common between these different models, suggesting that a specific subset of the proteome is vulnerable to proteostatic disruption. Overall, our data suggest that neurodegenerative proteinopathies modeled in mice impose a burden on the proteostatic network that diminishes the ability of neural cells to prevent aberrant conformational changes that alter the solubility of hundreds of abundant cellular proteins.

Diosgenin restores Aβ-induced axonal degeneration by reducing the expression of heat shock cognate 70 (HSC70)

We previously found diosgenin, an herbal drug-derived steroid sapogenin, to be remarkably effective at restoring Aβ-induced axonal degeneration and improving memory function in model of Alzheimer’s disease (AD), 5XFAD mouse. In this study, we investigated the downstream signaling of diosgenin and explored new therapeutic targets in AD. We showed that the expression of heat shock cognate (HSC) 70 was increased in Aβ-treated neurons and in 5XFAD mice but was decreased by diosgenin treatment. In addition, knockdown of HSC70 significantly promoted axonal growth in neurons. As an association molecule of HSC70 in neurons, α-tubulin was detected by immunoprecipitation. After Aβ treatment, α-tubulin expression was greatly reduced in the degenerated axons, suggesting that a decline in α-tubulin may be one of the factors which correlates with axonal disruption in AD pathology. We hypothesized that the degradation of α-tubulin is triggered by the chaperone activity of HSC70. However, diosgenin significantly normalized the α-tubulin level, a potentially critical process for axonal formation. Our study indicated that reducing the HSC70 level is a new possible therapeutic target of axonal regeneration in AD.

Differential induction of mutant SOD1 misfolding and aggregation by tau and α-synuclein pathology

BACKGROUND

Prior studies in C. elegans demonstrated that the expression of aggregation-prone polyglutamine proteins in muscle wall cells compromised the folding of co-expressed temperature-sensitive proteins, prompting interest in whether the accumulation of a misfolded protein in pathologic features of human neurodegenerative disease burdens cellular proteostatic machinery in a manner that impairs the folding of other cellular proteins.

METHODS

Mice expressing high levels of mutant forms of tau and α-synuclein (αSyn), which develop inclusion pathologies of the mutant protein in brain and spinal cord, were crossed to mice expressing low levels of mutant superoxide dismutase 1 fused to yellow fluorescent protein (G85R-SOD1:YFP) for aging and neuropathological evaluation.

RESULTS

Mice expressing low levels of G85R-SOD1:YFP, alone, lived normal lifespans and were free of evidence of inclusion pathology, setting the stage to use this protein as a reporter of proteostatic function. We observed robust induction of G85R-SOD1:YFP inclusion pathology in the neuropil of spinal cord and brainstem of bigenic mice that co-express high levels of mutant tau in the spinal axis and develop robust spinal tau pathology (JNPL3 mice). In contrast, in crosses of the G85R-SOD1:YFP mice with mice that model spinal α-synucleinopathy (the M83 model of αSyn pathology), we observed no G85R-SOD1:YFP inclusion formation. Similarly, in crosses of the G85R-SOD1:YFP mice to mice that model cortical tau pathology (rTg4510 mice), we did not observe induction of G85R-SOD1:YFP inclusions.

CONCLUSION

Despite robust burdens of neurodegenerative pathology in M83 and rTg4510 mice, the introduction of the G85R-SOD1:YFP protein was induced to aggregate only in the context of spinal tau pathology present in the JNPL3 model. These findings suggest unexpected specificity, mediated by both the primary protein pathology and cellular context, in the induced "secondary aggregation" of a mutant form of SOD1 that could be viewed as a reporter of proteostatic function.

Neonatal Neurodegeneration in Alzheimer's Disease Transgenic Mouse Model

Alzheimer’s disease (AD) is a progressive disorder characterized by a variety of molecular pathologies causing cortical dementia with a prominent memory deficit. Formation of the pathology, which begins decades before the diagnosis of the disease, is highly correlated with the clinical symptoms. Several proteomics studies were performed using animal models to monitor the alterations of the brain tissue proteome at different stages of AD. However, proteome changes in the brain regions of newborn transgenic mouse model have not been investigated yet. To this end, we analyzed protein expression alterations in cortex, hippocampus and cerebellum of transgenic mice carrying five familial AD mutations (5XFAD) at neonatal day-1. Our results indicate a remarkable difference in protein expression profile of newborn 5XFAD brain with region specific variations. Additionally, the proteins, which show similar expression alteration pattern in postmortem human AD brains, were determined. Bioinformatics analysis showed that the molecular alterations were mostly related to the cell morphology, cellular assembly and organization, and neuroinflammation. Moreover, morphological analysis revealed that there is an increase in neurite number of 5XFAD mouse neurons in vitro. We suggest that, molecular alterations in the AD brain exist even at birth, and perhaps the disease is silenced until older ages when the brain becomes vulnerable.

Hsp70 Inhibits the Nucleation and Elongation of Tau and Sequesters Tau Aggregates with High Affinity

As a key player of the protein quality control network of the cell, the molecular chaperone Hsp70 inhibits the aggregation of the amyloid protein tau. To date, the mechanism of this inhibition and the tau species targeted by Hsp70 remain unknown. This is partly due to the inherent difficulty of studying amyloid aggregates because of their heterogeneous and transient nature. Here, we used ensemble and single-molecule fluorescence measurements to dissect how Hsp70 counteracts the self-assembly process of the K18 ΔK280 tau variant. We found that Hsp70 blocks the early stages of tau aggregation by suppressing the formation of tau nuclei. Additionally, Hsp70 sequesters oligomers and mature tau fibrils with nanomolar affinity into a protective complex, efficiently neutralizing their ability to damage membranes and seed further tau aggregation. Our results provide novel insights into the molecular mechanisms by which the chaperone Hsp70 counteracts the formation, propagation, and toxicity of tau aggregates.

Heat Shock Cognate 70 Inhibitor, VER-155008, Reduces Memory Deficits and Axonal Degeneration in a Mouse Model of Alzheimer's Disease

Alzheimer’s disease (AD) is a progressive neurodegenerative disorder resulting in structural brain changes and memory impairment. We hypothesized that reconstructing neural networks is essential for memory recovery in AD. Heat shock cognate 70 (HSC70), a member of the heat shock protein family of molecular chaperones, is upregulated in AD patient brains, and recent studies have demonstrated that HSC70 facilitates axonal degeneration and pathological progression in AD. However, the direct effects of HSC70 inhibition on axonal development and memory function have never been investigated. In this study, we examined the effects of a small-molecule HSC70 inhibitor, VER-155008, on axonal morphology and memory function in a mouse model of AD (5XFAD mice). We found that VER-155008 significantly promoted axonal regrowth in amyloid β-treated neurons in vitro and improved object recognition, location, and episodic-like memory in 5XFAD mice. Furthermore, VER-155008 penetrated into the brain after intraperitoneal administration, suggesting that VER-155008 acts in the brain in situ. Immunohistochemistry revealed that VER-155008 reduced bulb-like axonal swelling in the amyloid plaques in the perirhinal cortex and CA1 in 5XFAD mice, indicating that VER-155008 also reverses axonal degeneration in vivo. Moreover, the two main pathological features of AD, amyloid plaques and paired helical filament tau accumulation, were reduced by VER-155008 administration in 5XFAD mice. This is the first report to show that the inhibition of HSC70 function may be critical for axonal regeneration and AD-like symptom reversal. Our study provides evidence that HSC70 can be used as a new therapeutic target for AD treatment.

Similarity and Differences in Inflammation-Related Characteristics of the Peripheral Immune System of Patients with Parkinson's and Alzheimer's Diseases

Parkinson's disease (PD) and Alzheimer's disease (AD) are the most common age-related neurodegenerative disorders. Both diseases are characterized by chronic inflammation in the brain-neuroinflammation. The first signs of PD and AD are most often manifested in old age, in which the immune system is usually characterized by chronic inflammation, so-called "inflammaging" In recent years, there is growing evidence that pathogenesis of these diseases is connected with both regional and peripheral immune processes. Currently, the association of clinical signs of PD and AD with different characteristics of patient immune status is actively being researched. In this mini-review we compare the association of PD and AD alterations of a number of immune system parameters connected with the process of inflammation.

Enhanced tau pathology via RanBP9 and Hsp90/Hsc70 chaperone complexes

Accumulation of amyloid β (Aβ) and tau represent the two major pathological hallmarks of Alzheimer's disease (AD). Despite the critical importance of Aβ accumulation as an early event in AD pathogenesis, multiple lines of evidence indicate that tau is required to mediate Aβ-induced neurotoxic signals in neurons. We have previously shown that the scaffolding protein Ran-binding protein 9 (RanBP9), which is highly elevated in brains of AD and AD mouse models, both enhances Aβ production and mediates Aβ-induced neurotoxicity. However, it is unknown whether and how RanBP9 transmits Aβ-induced neurotoxic signals to tau. Here we show for the first time that overexpression or knockdown of RanBP9 directly enhances and reduces tau levels, respectively, in vitro and in vivo. Such changes in tau levels are associated with the ability of RanBP9 to physically interact with tau and heat shock protein 90/heat shock cognate 70 (Hsp90/Hsc70) complexes. Meanwhile, both RanBP9 and tau levels are simultaneously reduced by Hsp90 or Hsc70 inhibitors, whereas overexpression or knockdown of RanBP9 significantly diminishes the anti-tau potency of Hsp90/Hsc70 inhibitors as well as Hsc70 variants (WT & E175S). Further, RanBP9 increases the capacity for Hsp90 and Hsc70 complexes to bind ATP and enhances their ATPase activities in vitro. These observations in vitro and cell lines are recapitulated in primary neurons and in vivo, as genetic reduction in RanBP9 not only ameliorates tauopathy in Tau-P301S mice but also rescues the deficits in synaptic integrity and plasticity.

Remodeling Tau and Prion Proteins Using Nanochaperons

There is increasing evidence that tau protein behaves in a prion-like manner in tauopathy. The stabilization of tau protein using a small molecular compound can limit tauopathy associated morbidity that advances with ageing. Here, a lab-on-a-chip experiment is reported, where gold citrate nanoparticles (5 nm, AuNPs) can remodel mutant tau protein (P301L) and prion, thus resolving the mutant tau- and prion-mediated impairment of kinesin cargo transport on microtubules. It is found that tau protein is overexpressed in Alzheimer's disease (AD) patient serum samples and the tau conformational change can also be affected in human serum samples of AD when treated with AuNPs ex vivo. Similarly, AuNPs reorganizing the prion protein and inducing conformational changes of prions in AD serum have been observed, while having no effect on alpha-synuclein in Parkinson patient serum. The mapping of AD serum mediated traffic jams, using particle tracking and mean square displacement analysis, and the observed recovery of uninterrupted processive motor functions by AuNP treatment show that kinesin cargo assays might be a useful method for future ex vivo validation of a targeted therapy against tauopathy before administration, a viable option to combat various neurodegenerative disorders arising from the susceptibility of amyloidogenic proteins toward aggregation.

Hsc70 chaperone activity is required for the cytosolic slow axonal transport of synapsin

Soluble cytosolic proteins vital to axonal and presynaptic function are synthesized in the neuronal soma and conveyed via slow axonal transport. Our previous studies suggest that the overall slow transport of synapsin is mediated by dynamic assembly/disassembly of cargo complexes followed by short-range vectorial transit (the "dynamic recruitment" model). However, neither the composition of these complexes nor the mechanistic basis for the dynamic behavior is understood. In this study, we first examined putative cargo complexes associated with synapsin using coimmunoprecipitation and multidimensional protein identification technology mass spectrometry (MS). MS data indicate that synapsin is part of a multiprotein complex enriched in chaperones/cochaperones including Hsc70. Axonal synapsin-Hsc70 coclusters are also visualized by two-color superresolution microscopy. Inhibition of Hsc70 ATPase activity blocked the slow transport of synapsin, disrupted axonal synapsin organization, and attenuated Hsc70-synapsin associations, advocating a model where Hsc70 activity dynamically clusters cytosolic proteins into cargo complexes, allowing transport. Collectively, our study offers insight into the molecular organization of cytosolic transport complexes and identifies a novel regulator of slow transport.

Tau interactome mapping based identification of Otub1 as Tau deubiquitinase involved in accumulation of pathological Tau forms in vitro and in vivo

Dysregulated proteostasis is a key feature of a variety of neurodegenerative disorders. In Alzheimer’s disease (AD), progression of symptoms closely correlates with spatiotemporal progression of Tau aggregation, with “early” oligomeric Tau forms rather than mature neurofibrillary tangles (NFTs) considered to be pathogenetic culprits. The ubiquitin–proteasome system (UPS) controls degradation of soluble normal and abnormally folded cytosolic proteins. The UPS is affected in AD and is identified by genomewide association study (GWAS) as a risk pathway for AD. The UPS is determined by balanced regulation of ubiquitination and deubiquitination. In this work, we performed isobaric tags for relative and absolute quantitation (iTRAQ)-based Tau interactome mapping to gain unbiased insight into Tau pathophysiology and to identify novel Tau-directed therapeutic targets. Focusing on Tau deubiquitination, we here identify Otub1 as a Tau-deubiquitinating enzyme. Otub1 directly affected Lys48-linked Tau deubiquitination, impairing Tau degradation, dependent on its catalytically active cysteine, but independent of its noncanonical pathway modulated by its N-terminal domain in primary neurons. Otub1 strongly increased AT8-positive Tau and oligomeric Tau forms and increased Tau-seeded Tau aggregation in primary neurons. Finally, we demonstrated that expression of Otub1 but not its catalytically inactive form induced pathological Tau forms after 2 months in Tau transgenic mice in vivo, including AT8-positive Tau and oligomeric Tau forms. Taken together, we here identified Otub1 as a Tau deubiquitinase in vitro and in vivo, involved in formation of pathological Tau forms, including small soluble oligomeric forms. Otub1 and particularly Otub1 inhibitors, currently under development for cancer therapies, may therefore yield interesting novel therapeutic avenues for Tauopathies and AD.

DnaJ/Hsc70 chaperone complexes control the extracellular release of neurodegenerative-associated proteins

It is now known that proteins associated with neurodegenerative disease can spread throughout the brain in a prionlike manner. However, the mechanisms regulating the trans-synaptic spread propagation, including the neuronal release of these proteins, remain unknown. The interaction of neurodegenerative disease-associated proteins with the molecular chaperone Hsc70 is well known, and we hypothesized that much like disaggregation, refolding, degradation, and even normal function, Hsc70 may dictate the extracellular fate of these proteins. Here, we show that several proteins, including TDP-43, α-synuclein, and the microtubule-associated protein tau, can be driven out of the cell by an Hsc70 co-chaperone, DnaJC5. In fact, DnaJC5 overexpression induced tau release in cells, neurons, and brain tissue, but only when activity of the chaperone Hsc70 was intact and when tau was able to associate with this chaperone. Moreover, release of tau from neurons was reduced in mice lacking the DnaJC5 gene and when the complement of DnaJs in the cell was altered. These results demonstrate that the dynamics of DnaJ/Hsc70 complexes are critically involved in the release of neurodegenerative disease proteins.

Cucumber Necrosis Virus Recruits Cellular Heat Shock Protein 70 Homologs at Several Stages of Infection

RNA viruses often depend on host factors for multiplication inside cells due to the constraints of their small genome size and limited coding capacity. One such factor that has been exploited by several plant and animal viruses is heat shock protein 70 (HSP70) family homologs which have been shown to play roles for different viruses in viral RNA replication, viral assembly, disassembly, and cell-to-cell movement. Using next generation sequence analysis, we reveal that several isoforms of Hsp70 and Hsc70 transcripts are induced to very high levels during cucumber necrosis virus (CNV) infection of Nicotiana benthamiana and that HSP70 proteins are also induced by at least 10-fold. We show that HSP70 family protein homologs are co-opted by CNV at several stages of infection. We have found that overexpression of Hsp70 or Hsc70 leads to enhanced CNV genomic RNA, coat protein (CP), and virion accumulation, whereas downregulation leads to a corresponding decrease. Hsc70-2 was found to increase solubility of CNV CP in vitro and to increase accumulation of CNV CP independently of viral RNA replication during coagroinfiltration in N. benthamiana. In addition, virus particle assembly into virus-like particles in CP agroinfiltrated plants was increased in the presence of Hsc70-2. HSP70 was found to increase the targeting of CNV CP to chloroplasts during infection, reinforcing the role of HSP70 in chloroplast targeting of host proteins. Hence, our findings have led to the discovery of a highly induced host factor that has been co-opted to play multiple roles during several stages of the CNV infection cycle.

IMPORTANCE

Because of the small size of its RNA genome, CNV is dependent on interaction with host cellular components to successfully complete its multiplication cycle. We have found that CNV induces HSP70 family homologs to a high level during infection, possibly as a result of the host response to the high levels of CNV proteins that accumulate during infection. Moreover, we have found that CNV co-opts HSP70 family homologs to facilitate several aspects of the infection process such as viral RNA, coat protein and virus accumulation. Chloroplast targeting of the CNV CP is also facilitated, which may aid in CNV suppression of host defense responses. Several viruses have been shown to induce HSP70 during infection and others to utilize HSP70 for specific aspects of infection such as replication, assembly, and disassembly. We speculate that HSP70 may play multiple roles in the infection processes of many viruses.

Alzheimer's Disease: Mechanism and Approach to Cell Therapy

Alzheimer’s disease (AD) is the most common form of dementia. The risk of AD increases with age. Although two of the main pathological features of AD, amyloid plaques and neurofibrillary tangles, were already recognized by Alois Alzheimer at the beginning of the 20th century, the pathogenesis of the disease remains unsettled. Therapeutic approaches targeting plaques or tangles have not yet resulted in satisfactory improvements in AD treatment. This may, in part, be due to early-onset and late-onset AD pathogenesis being underpinned by different mechanisms. Most animal models of AD are generated from gene mutations involved in early onset familial AD, accounting for only 1% of all cases, which may consequently complicate our understanding of AD mechanisms. In this article, the authors discuss the pathogenesis of AD according to the two main neuropathologies, including senescence-related mechanisms and possible treatments using stem cells, namely mesenchymal and neural stem cells.

The active Hsc70/tau complex can be exploited to enhance tau turnover without damaging microtubule dynamics

The pathological accumulation of abnormally hyperphosphorylated and aggregated tau, a neuronal microtubule (MT)-associated protein that functions to maintain MT stability, is implicated in a number of hereditary and sporadic neurodegenerative diseases including frontotemporal dementia and Alzheimer's disease. Targeting tau for the treatment of these diseases is an area of intense interest and toward that end, modulation of cellular molecular chaperones is a potential therapeutic target. In particular, the constitutive Hsp70 isoform, Hsc70, seems highly interconnected with tau, preserving tau protein levels and synergizing with it to assemble MTs. But the relationship between tau and Hsc70, as well as the impact of this interaction in neurons and its therapeutic implications remain unknown. Using a human dominant negative Hsc70 that resembles isoform selective inhibition of this important chaperone, we found for the first time that Hsc70 activity is required to stimulate MT assembly in cells and brain. However, surprisingly, active Hsc70 also requires active tau to regulate MT assembly in vivo, suggesting that tau acts in some ways as a co-chaperone for Hsc70 to coordinate MT assembly. This was despite tau binding to Hsc70 as substrate, as determined biochemically. Moreover, we show that while chronic Hsc70 inhibition damaged MT dynamics, intermittent treatment with a small molecule Hsp70 inhibitor lowered tau in brain tissue without disrupting MT integrity. Thus, in tauopathies, where MT injury would be detrimental to neurons, the unique relationship of tau with the Hsc70 machinery can be exploited to deplete tau levels without damaging MT networks.

Isoform-selective Genetic Inhibition of Constitutive Cytosolic Hsp70 Activity Promotes Client Tau Degradation Using an Altered Co-chaperone Complement

The constitutively expressed heat shock protein 70 kDa (Hsc70) is a major chaperone protein responsible for maintaining proteostasis, yet how its structure translates into functional decisions regarding client fate is still unclear. We previously showed that Hsc70 preserved aberrant Tau, but it remained unknown if selective inhibition of the activity of this Hsp70 isoform could facilitate Tau clearance. Using single point mutations in the nucleotide binding domain, we assessed the effect of several mutations on the functions of human Hsc70. Biochemical characterization revealed that one mutation abolished both Hsc70 ATPase and refolding activities. This variant resembled the ADP-bound conformer at all times yet remained able to interact with cofactors, nucleotides, and substrates appropriately, resembling a dominant negative Hsc70 (DN-Hsc70). We then assessed the effects of this DN-Hsc70 on its client Tau. DN-Hsc70 potently facilitated Tau clearance via the proteasome in cells and brain tissue, in contrast to wild type Hsc70 that stabilized Tau. Thus, DN-Hsc70 mimics the action of small molecule pan Hsp70 inhibitors with regard to Tau metabolism. This shift in Hsc70 function by a single point mutation was the result of a change in the chaperome associated with Hsc70 such that DN-Hsc70 associated more with Hsp90 and DnaJ proteins, whereas wild type Hsc70 was more associated with other Hsp70 isoforms. Thus, isoform-selective targeting of Hsc70 could be a viable therapeutic strategy for tauopathies and possibly lead to new insights in chaperone complex biology.

Molecular chaperones and neuronal proteostasis

Protein homeostasis (proteostasis) is essential for maintaining the functionality of the proteome. The disruption of proteostasis, due to genetic mutations or an age-related decline, leads to aberrantly folded proteins that typically lose their function. The accumulation of misfolded and aggregated protein is also cytotoxic and has been implicated in the pathogenesis of neurodegenerative diseases. Neurons have developed an intrinsic protein quality control network, of which molecular chaperones are an essential component. Molecular chaperones function to promote efficient folding and target misfolded proteins for refolding or degradation. Increasing molecular chaperone expression can suppress protein aggregation and toxicity in numerous models of neurodegenerative disease; therefore, molecular chaperones are considered exciting therapeutic targets. Furthermore, mutations in several chaperones cause inherited neurodegenerative diseases. In this review, we focus on the importance of molecular chaperones in neurodegenerative diseases, and discuss the advances in understanding their protective mechanisms.

Hsp70 clears misfolded kinases that partitioned into distinct quality-control compartments

Hsp70 facilitates maturation of newly synthesized kinases and assists degradation of kinases under normal and stressed conditions. Hsp70 degrades misfolded kinases that partition into different quality-control compartments by promoting their ubiquitination, thus protecting cells from proteotoxic stress.

Hsp70 aids in protein folding and directs misfolded proteins to the cellular degradation machinery. We describe discrete roles of Hsp70,SSA1 as an important quality-control machinery that switches functions to ameliorate the cellular environment. SSA1 facilitates folding/maturation of newly synthesized protein kinases by aiding their phosphorylation process and also stimulates ubiquitylation and degradation of kinases in regular protein turnover or during stress when kinases are denatured or improperly folded. Significantly, while kinases accumulate as insoluble inclusions upon SSA1 inhibition, they form soluble inclusions upon Hsp90 inhibition or stress foci during heat stress. This suggests formation of inclusion-specific quality-control compartments under various stress conditions. Up-regulation of SSA1 results in complete removal of these inclusions by the proteasome. Elevation of the cellular SSA1 level accelerates kinase turnover and protects cells from proteotoxic stress. Upon overexpression, SSA1 targets heat-denatured kinases toward degradation, which could enable them to recover their functional state under physiological conditions. Thus active participation of SSA1 in the degradation of misfolded proteins establishes an essential role of Hsp70 in deciding client fate during stress.

Cellular factors modulating the mechanism of tau protein aggregation

Pathological accumulation of the microtubule-associated protein tau, in the form of neurofibrillary tangles, is a major hallmark of Alzheimer's disease, the most prevalent neurodegenerative condition worldwide. In addition to Alzheimer's disease, a number of neurodegenerative diseases, called tauopathies, are characterized by the accumulation of aggregated tau in a variety of brain regions. While tau normally plays an important role in stabilizing the microtubule network of the cytoskeleton, its dissociation from microtubules and eventual aggregation into pathological deposits is an area of intense focus for therapeutic development. Here we discuss the known cellular factors that affect tau aggregation, from post-translational modifications to molecular chaperones.

Heat shock protein 70 in Alzheimer's disease

Alzheimer's disease (AD) is the most common neurodegenerative disease that caused dementia which has no effective treatment. Growing evidence has demonstrated that AD is a “protein misfolding disorder” that exhibits common features of misfolded, aggregation-prone proteins and selective cell loss in the mature nervous system. Heat shock protein 70 (HSP70) attracts extensive attention worldwide, because it plays a crucial role in preventing protein misfolding and inhibiting aggregation and represents a class of proteins potentially involved in AD pathogenesis. Numerous studies have indicated that HSP70 could suppress the progression of AD with in vitro and in vivo experiments. Thus, targeting HSP70 and the related compounds might represent a promising strategy for the treatment of AD.

Heat shock proteins in neurodegenerative disorders and aging

Many members of the heat shock protein family act in unison to refold or degrade misfolded proteins. Some heat shock proteins also directly interfere with apoptosis. These homeostatic functions are especially important in proteinopathic neurodegenerative diseases, in which specific proteins misfold, aggregate, and kill cells through proteotoxic stress. Heat shock protein levels may be increased or decreased in these disorders, with the direction of the response depending on the individual heat shock protein, the disease, cell type, and brain region. Aging is also associated with an accrual of proteotoxic stress and modulates expression of several heat shock proteins. We speculate that the increase in some heat shock proteins in neurodegenerative conditions may be partly responsible for the slow progression of these disorders, whereas the increase in some heat shock proteins with aging may help delay senescence. The protective nature of many heat shock proteins in experimental models of neurodegeneration supports these hypotheses. Furthermore, some heat shock proteins appear to be expressed at higher levels in longer-lived species. However, increases in heat shock proteins may be insufficient to override overwhelming proteotoxic stress or reverse the course of these conditions, because the expression of several other heat shock proteins and endogenous defense systems is lowered. In this review we describe a number of stress-induced changes in heat shock proteins as a function of age and neurodegenerative pathology, with an emphasis on the heat shock protein 70 (Hsp70) family and the two most common proteinopathic disorders of the brain, Alzheimer's and Parkinson's disease.

Stress protein expression in early phase spinal cord ischemia/reperfusion injury

Spinal cord ischemia/reperfusion injury is a stress injury to the spinal cord. Our previous studies using differential proteomics identified 21 differentially expressed proteins (n > 2) in rabbits with spinal cord ischemia/reperfusion injury. Of these proteins, stress-related proteins included protein disulfide isomerase A3, stress-induced-phosphoprotein 1 and heat shock cognate protein 70. In this study, we established New Zealand rabbit models of spinal cord ischemia/reperfusion injury by abdominal aorta occlusion. Results demonstrated that hind limb function initially improved after spinal cord ischemia/reperfusion injury, but then deteriorated. The pathological morphology of the spinal cord became aggravated, but lessened 24 hours after reperfusion. However, the numbers of motor neurons and interneurons in the spinal cord gradually decreased. The expression of protein disulfide isomerase A3, stress-induced-phosphoprotein 1 and heat shock cognate protein 70 was induced by ischemia/reperfusion injury. The expression of these proteins increased within 12 hours after reperfusion, and then decreased, reached a minimum at 24 hours, but subsequently increased again to similar levels seen at 6–12 hours, showing a characterization of induction-inhibition-induction. These three proteins were expressed only in cytoplasm but not in the nuclei. Moreover, the expression was higher in interneurons than in motor neurons, and the survival rate of interneurons was greater than that of motor neurons. It is assumed that the expression of stress-related proteins exhibited a protective effect on neurons.

Heat shock proteins at the crossroads between cancer and Alzheimer's disease

Heat shock proteins 70 and heat shock proteins 90 (Hsp70/90) have been implicated in many crucial steps of carcinogenesis: stabilizing oncogenic proteins, inhibiting programmed cell death and replicative senescence, induction of tumor angiogenesis, and activation of the invasion and metastasis. Plenty of cancer related proteins have the ability of regulating the expression of Hsp70/90 through heat shock factor 1. Cancer and Alzheimer's disease (AD) have plenty of overlapping regions in molecular genetics and cell biology associated with Hsp70/90. The Hsp70, as a protein stabilizer, has a cellular protection against neurodegeneration of the central nervous system, while Hsp90 promote neurodegenerative disorders indirectly through regulating the expression of Hsp70 and other chaperones. All these make existing anticancer drugs target Hsp70/90 which might be used in AD therapy.

Tau protein modifications and interactions: their role in function and dysfunction

Tau protein is abundant in the central nervous system and involved in microtubule assembly and stabilization. It is predominantly associated with axonal microtubules and present at lower level in dendrites where it is engaged in signaling functions. Post-translational modifications of tau and its interaction with several proteins play an important regulatory role in the physiology of tau. As a consequence of abnormal modifications and expression, tau is redistributed from neuronal processes to the soma and forms toxic oligomers or aggregated deposits. The accumulation of tau protein is increasingly recognized as the neuropathological hallmark of a number of dementia disorders known as tauopathies. Dysfunction of tau protein may contribute to collapse of cytoskeleton, thereby causing improper anterograde and retrograde movement of motor proteins and their cargos on microtubules. These disturbances in intraneuronal signaling may compromise synaptic transmission as well as trophic support mechanisms in neurons.

Casein kinase II induced polymerization of soluble TDP-43 into filaments is inhibited by heat shock proteins

Background

Trans-activation Response DNA-binding Protein-43 (TDP-43) lesions are observed in Amyotrophic Lateral Sclerosis (ALS), Frontotemporal Lobar Degeneration with ubiquitin inclusions (FTLD-TDP) and 25–50% of Alzheimer's Disease (AD) cases. These abnormal protein inclusions are composed of either amorphous TDP-43 aggregates or highly ordered filaments. The filamentous TDP-43 accumulations typically contain clean 10–12 nm filaments though wider 18–20 nm coated filaments may be observed. The TDP-43 present within these lesions is phosphorylated, truncated and ubiquitinated, and these modifications appear to be abnormal as they are linked to both a cellular heat shock response and microglial activation. The mechanisms associated with this abnormal TDP-43 accumulation are believed to result in a loss of TDP-43 function, perhaps due to the post-translational modifications or resulting from physical sequestration of the TDP-43. The formation of TDP-43 inclusions involves cellular translocation and conversion of TDP-43 into fibrillogenic forms, but the ability of these accumulations to sequester normal TDP-43 and propagate this behavior between neurons pathologically is mostly inferred. The lack of methodology to produce soluble full length TDP-43 and recapitulate this polymerization into filaments as observed in disease has limited our understanding of these pathogenic cascades.

Results

The protocols described here generate soluble, full-length and untagged TDP-43 allowing for a direct assessment of the impact of various posttranslational modifications on TDP-43 function. We demonstrate that Casein Kinase II (CKII) promotes the polymerization of this soluble TDP-43 into 10 nm diameter filaments that resemble the most common TDP-43 structures observed in disease. Furthermore, these filaments are recognized as abnormal by Heat Shock Proteins (HSPs) which can inhibit TDP-43 polymerization or directly promote TDP-43 filament depolymerization.

Conclusion

These findings demonstrate CKII induces polymerization of soluble TDP-43 into filaments and Hsp90 promotes TDP-43 filament depolymerization. These findings provide rational for potential therapeutic intervention at these points in TDP-43 proteinopathies.

Beyond amyloid: getting real about nonamyloid targets in Alzheimer's disease

For decades, researchers have focused primarily on a pathway initiated by amyloid beta aggregation, amyloid deposition, and accumulation in the brain as the key mechanism underlying the disease and the most important treatment target. However, evidence increasingly suggests that amyloid is deposited early during the course of disease, even prior to the onset of clinical symptoms. Thus, targeting amyloid in patients with mild to moderate Alzheimer's disease (AD), as past failed clinical trials have done, may be insufficient to halt further disease progression. Scientists are investigating other molecular and cellular pathways and processes that contribute to AD pathogenesis. Thus, the Alzheimer's Association's Research Roundtable convened a meeting in April 2012 to move beyond amyloid and explore AD as a complex multifactorial disease, with the goal of using a more inclusive perspective to identify novel treatment strategies.

Allosteric heat shock protein 70 inhibitors rapidly rescue synaptic plasticity deficits by reducing aberrant tau

BACKGROUND

The microtubule-associated protein tau accumulates in neurodegenerative diseases known as tauopathies, the most common being Alzheimer's disease. One way to treat these disorders may be to reduce abnormal tau levels through chaperone manipulation, thus subverting synaptic plasticity defects caused by tau's toxic accretion.

METHODS

Tauopathy models were used to study the impact of YM-01 on tau. YM-01 is an allosteric promoter of triage functions of the most abundant variant of the heat shock protein 70 (Hsp70) family in the brain, heat shock cognate 70 protein (Hsc70). The mechanisms by which YM-01 modified Hsc70 activity and tau stability were evaluated with biochemical methods, cell cultures, and primary neuronal cultures from tau transgenic mice. YM-01 was also administered to acute brain slices of tau mice; changes in tau stability and electrophysiological correlates of learning and memory were measured.

RESULTS

Tau levels were rapidly and potently reduced in vitro and ex vivo upon treatment with nanomolar concentrations of YM-01. Consistent with Hsc70 having a key role in this process, overexpression of heat shock protein 40 (DNAJB2), an Hsp70 co-chaperone, suppressed YM-01 activity. In contrast to its effects in pathogenic tauopathy models, YM-01 had little activity in ex vivo brain slices from normal, wild-type mice unless microtubules were disrupted, suggesting that Hsc70 acts preferentially on abnormal pools of free tau. Finally, treatment with YM-01 increased long-term potentiation in tau transgenic brain slices.

CONCLUSIONS

Therapeutics that exploit the ability of chaperones to selectively target abnormal tau can rapidly and potently rescue the synaptic dysfunction that occurs in Alzheimer's disease and other tauopathies.

Heat-shock protein 70 binds microtubules and interacts with kinesin in tobacco pollen tubes

The heat-shock proteins of 70 kDa are a family of ubiquitously expressed proteins important for protein folding. Heat-shock protein 70 assists other nascent proteins to achieve the spatial structure and ultimately helps the cell to protect against stress factors, such as heat. These proteins are localized in different cellular compartments and are associated with the cytoskeleton. We identified a heat-shock protein 70 isoform in the pollen tube of tobacco that binds to microtubules in an ATP-dependent manner. The heat-shock protein 70 was identified as part of the so-called ATP-MAP (ATP-dependent microtubule-associated protein) fraction, which also includes the 90-kDa kinesin, a mitochondria-associated motor protein. The identity of heat-shock protein 70 was validated by immunological assays and mass spectrometry. Sequence analysis showed that this heat-shock protein 70 is more similar to specific heat-shock proteins of Arabidopsis than to corresponding proteins of tobacco. Two-dimensional electrophoresis indicated that this heat-shock protein 70 isoform only is part of the ATP-MAP fraction and that is associated with the mitochondria of pollen tubes. Sedimentation assays showed that the binding of heat-shock protein 70 to microtubules is not affected by AMPPNP but it increases in the presence of the 90-kDa kinesin. Binding of heat-shock protein 70 to microtubules occurs only partially in the presence of ATP but it does not occur if, in addition to ATP, the 90-kDa kinesin is also present. Data suggest that the binding (but not the release) of heat-shock protein 70 to microtubules is facilitated by the 90-kDa kinesin.