Stress-inducible phosphoprotein 1 has unique cochaperone activity during development and regulates cellular response to ischemia via the prion protein

New insights into Sti1/Hop's cochaperone function highlight the complexity of proteostatic regulation

Sti1/Hop is a cochaperone that regulates Hsp70 and Hsp90 chaperones. Sti1/Hop function is perceived as limited to scaffolding chaperone complexes, although recent studies suggest a broader function. Rutledge et al. show that while Sti1/Hop functions within chaperone complexes under basal conditions, during high stress, it operates independently to sequester soluble misfolded protein in the cytoplasm, a function typically associated with chaperones rather than cochaperones. Furthermore, the localisation and levels of Sti1/Hop are finely tuned to ensure orderly sequestration and resolution of misfolded proteins. These data support a role for Sti1/Hop as a cochaperone specialised for stressed proteostasis networks.

Stress-inducible phosphoprotein 1 (Sti1/Stip1/Hop) sequesters misfolded proteins during stress

Co-chaperones are key elements of cellular protein quality control. They cooperate with the major heat shock proteins Hsp70 and Hsp90 in folding proteins and preventing the toxic accumulation of misfolded proteins upon exposure to stress. Hsp90 interacts with the co-chaperone stress-inducible phosphoprotein 1 (Sti1/Stip1/Hop) and activator of Hsp90 ATPase protein 1 (Aha1) among many others. Sti1 and Aha1 control the ATPase activity of Hsp90, but Sti1 also facilitates the transfer of client proteins from Hsp70 to Hsp90, thus connecting these two major branches of protein quality control. We find that misbalanced expression of Sti1 and Aha1 in yeast and mammalian cells causes severe growth defects. Also, deletion of STI1 causes an accumulation of soluble misfolded ubiquitinated proteins and a strong activation of the heat shock response. We discover that, during proteostatic stress, Sti1 forms cytoplasmic inclusions in yeast and mammalian cells that overlap with misfolded proteins. Our work indicates a key role of Sti1 in proteostasis independent of its Hsp90 ATPase regulatory functions by sequestering misfolded proteins during stress.

Chaperones vs. oxidative stress in the pathobiology of ischemic stroke

As many proteins prioritize functionality over constancy of structure, a proteome is the shortest stave in the Liebig's barrel of cell sustainability. In this regard, both prokaryotes and eukaryotes possess abundant machinery supporting the quality of the proteome in healthy and stressful conditions. This machinery, namely chaperones, assists in folding, refolding, and the utilization of client proteins. The functions of chaperones are especially important for brain cells, which are highly sophisticated in terms of structural and functional organization. Molecular chaperones are known to exert beneficial effects in many brain diseases including one of the most threatening and widespread brain pathologies, ischemic stroke. However, whether and how they exert the antioxidant defense in stroke remains unclear. Herein, we discuss the chaperones shown to fight oxidative stress and the mechanisms of their antioxidant action. In ischemic stroke, during intense production of free radicals, molecular chaperones preserve the proteome by interacting with oxidized proteins, regulating imbalanced mitochondrial function, and directly fighting oxidative stress. For instance, cells recruit Hsp60 and Hsp70 to provide proper folding of newly synthesized proteins-these factors are required for early ischemic response and to refold damaged polypeptides. Additionally, Hsp70 upregulates some dedicated antioxidant pathways such as FOXO3 signaling. Small HSPs decrease oxidative stress via attenuation of mitochondrial function through their involvement in the regulation of Nrf- (Hsp22), Akt and Hippo (Hsp27) signaling pathways as well as mitophagy (Hsp27, Hsp22). A similar function has also been proposed for the Sigma-1 receptor, contributing to the regulation of mitochondrial function. Some chaperones can prevent excessive formation of reactive oxygen species whereas Hsp90 is suggested to be responsible for pro-oxidant effects in ischemic stroke. Finally, heat-resistant obscure proteins (Hero) are able to shield client proteins, thus preventing their possible over oxidation.

The role of the FKBP51-Hsp90 complex in Alzheimer's disease: An emerging new drug target

With increasing age comes the inevitable decline in proteostasis, where chaperone and co-chaperone activity becomes imbalanced. These changes lead to global disturbances and pathogenic rewiring of the chaperone system into epichaperones consisting of protein networks that are ultimately dysfunctional. Such imbalances in proteostasis may favor mechanisms that can lead to neurological diseases, such as Alzheimer's disease (AD). Consequently, there has been an increase in research activity toward finding small molecules that can re-balance the chaperone and co-chaperone machinery to counter the effects of disease resulting from old age. The Hsp90 co-chaperone FKBP51 has recently been identified as a protein whose induction not only increases with age but is elevated further in AD cells. Significantly, FKBP51 plays a role in the Hsp90-dependent isomerization of tau, which in turn influences its phosphorylation and susceptibility to aggregation. We hypothesize that FKBP51 is a major player that is able to elicit tauopathy in response to amyloid-beta senile plaques that damage the brain. We propose that elevated FKBP51 levels result in an abnormal FKBP51-Hsp90 activity that alters the normal processing of tau, which manifests as hyperphosphorylation and oligomerization of tau. Thus, the Hsp90-FKBP51 complex is emerging as a drug target against AD. In support of this idea, the structure of the FKBP51-Hsp90 complex was recently described, and significantly, the small-molecule dihydropyridine LA1011 was shown to be able to disrupt the Hsp90-FKBP51 complex. LA1011 was previously shown to effectively prevent neurodegeneration in the APPxPS1 AD transgenic mouse model. This review looks at the role of Hsp90 and its co-chaperones in AD with a focus on FKBP51.

Hsp70-Hsp90 organising protein (HOP/STIP1) is required for KSHV lytic replication

Kaposi's sarcoma-associated herpesvirus (KSHV) is a DNA virus that causes Kaposi's sarcoma, a cancer of endothelial origin. KSHV uses the activity of host molecular chaperones like Hsp70 and Hsp90 for the folding of host and viral proteins required for productive infection. Hsp70 and Hsp90 chaperones form proteostasis networks with several regulatory proteins known as co-chaperones. Of these, Hsp90-Hsp70-organizing protein (HOP) is an early-stage co-chaperone that regulates the transfer of folding substrate proteins between the Hsp70 and Hsp90 chaperone systems. While the roles for Hsp90 and Hsp70 in KSHV biology have been described, HOP has not previously been studied in this context despite its prominent interaction with both chaperones. Here, we demonstrate a novel function for HOP as a new host factor required for effective lytic replication of KSHV in primary effusion cell lines.

Regulation of Epithelial and Endothelial Barriers by Molecular Chaperones

The integrity and permeability of epithelial and endothelial barriers depend on the formation of tight junctions, adherens junctions, and a junction-associated cytoskeleton. The establishment of this junction-cytoskeletal module relies on the correct folding and oligomerization of its protein components. Molecular chaperones are known regulators of protein folding and complex formation in different cellular compartments. Mammalian cells possess an elaborate chaperone network consisting of several hundred chaperones and co-chaperones. Only a small part of this network has been linked, however, to the regulation of intercellular adhesions, and the systematic analysis of chaperone functions at epithelial and endothelial barriers is lacking. This review describes the functions and mechanisms of the chaperone-assisted regulation of intercellular junctions. The major focus of this review is on heat shock protein chaperones, their co-chaperones, and chaperonins since these molecules are the focus of the majority of the articles published on the chaperone-mediated control of tissue barriers. This review discusses the roles of chaperones in the regulation of the steady-state integrity of epithelial and vascular barriers as well as the disruption of these barriers by pathogenic factors and extracellular stressors. Since cytoskeletal coupling is essential for junctional integrity and remodeling, chaperone-assisted assembly of the actomyosin cytoskeleton is also discussed.

Secretory Proteomic Responses of Endometrial Epithelial Cells to Trophoblast-Derived Extracellular Vesicles

Synchronized crosstalk between the embryo and endometrium during the periconception period is integral to pregnancy establishment. Increasing evidence suggests that the exchange of extracellular vesicles (EVs) of both embryonic and endometrial origin is a critical component of embryo-maternal communication during peri-implantation. Here, we investigated whether embryonic signals in the form of EVs can modulate the endometrial epithelial cell secretome. Receptive endometrial analog RL95-2 cells were supplemented with trophoblast analog JAr cell-derived EVs, and the secretory protein changes occurring in the RL95-2 cells were analyzed using mass spectrometry. EVs of non-trophoblastic origin (HEK 293 cells) were used as the control EV source to supplement endometrial cells. Trophoblast cell-derived EVs enriched endometrial epithelial cell secretions with proteins that support embryo development, attachment, or implantation, whereas control EVs were unable to induce the same effect. The present study suggests that embryonic signals in the form of EVs may prime receptive endometrial epithelial cells to enrich their secretory proteome with critical proteomic molecules with functional importance for periconception milieu formation.

Hsp70/Hsp90 Organising Protein (Hop): Coordinating Much More than Chaperones

The Hsp70/Hsp90 organising protein (Hop, also known as stress-inducible protein 1/STI1/STIP1) has received considerable attention for diverse cellular functions in both healthy and diseased states. There is extensive evidence that intracellular Hop is a co-chaperone of the major chaperones Hsp70 and Hsp90, playing an important role in the productive folding of Hsp90 client proteins, although recent evidence suggests that eukaryotic Hop is regulatory within chaperone complexes rather than essential. Consequently, Hop is implicated in many key signalling pathways, including aberrant pathways leading to cancer. Hop is also secreted, and it is now well established that Hop interacts with the prion protein, PrP^C, to mediate multiple signalling events. The intracellular and extracellular forms of Hop most likely represent two different isoforms, although the molecular determinants of these divergent functions are yet to be identified. There is also a growing body of research that reports the involvement of Hop in cellular activities that appear independent of either chaperones or PrP^C. While the various cellular functions of Hop have been described, its biological function remains elusive. However, recent knockout studies in mammals suggest that Hop has an important role in embryonic development. This review provides a critical overview of the latest molecular, cellular and biological research on Hop, critically evaluating its function in healthy systems and how this function is adapted in diseased states.

Maternal autoantibody profiles as biomarkers for ASD and ASD with co-occurring intellectual disability

Maternal autoantibody-related ASD (MAR ASD) is a subtype of autism in which pathogenic maternal autoantibodies (IgG) cross the placenta, access the developing brain, and cause neurodevelopmental alterations and behaviors associated with autism in the exposed offspring. We previously reported maternal IgG response to eight proteins (CRMP1, CRMP2, GDA LDHA, LDHB, NSE, STIP1, and YBOX) and that reactivity to nine specific combinations of these proteins (MAR ASD patterns) was predictive of ASD risk. The aim of the current study was to validate the previously identified MAR ASD patterns (CRMP1 + GDA, CRMP1 + CRMP2, NSE + STIP1, CRMP2 + STIP1, LDHA + YBOX, LDHB + YBOX, GDA + YBOX, STIP1 + YBOX, and CRMP1 + STIP1) and their accuracy in predicting ASD risk in a prospective cohort employing maternal samples collected prior to parturition. We used prenatal plasma from mothers of autistic children with or without co-occurring intellectual disability (ASD = 540), intellectual disability without autism (ID = 184) and general population controls (GP = 420) collected by the Early Markers for Autism (EMA) study. We found reactivity to one or more of the nine previously identified MAR ASD patterns in 10% of the ASD group compared with 4% of the ID group and 1% of the GP controls (ASD vs GP: Odds Ratio (OR) = 7.81, 95% Confidence Interval (CI) 3.32 to 22.43; ASD vs ID: OR = 2.77, 95% CI (1.19-7.47)) demonstrating that the MAR ASD patterns are strongly associated with the ASD group and could be used to assess ASD risk prior to symptom onset. The pattern most strongly associated with ASD was CRMP1 + CRMP2 and increased the odds for an ASD diagnosis 16-fold (3.32 to >999.99). In addition, we found that several of these specific MAR ASD patterns were strongly associated with ASD with intellectual disability (ASD + ID) and others associated with ASD without ID (ASD-no ID). Prenatal screening for these MAR patterns may lead to earlier identification of ASD and facilitate access to the appropriate early intervention services based on each child's needs.

Parkinson's Disease-Specific Autoantibodies against the Neuroprotective Co-Chaperone STIP1

Parkinson’s disease (PD) is a debilitating movement disorder characterised by the loss of dopaminergic neurons in the substantia nigra. As neuroprotective agents mitigating the rate of neurodegeneration are unavailable, the current therapies largely focus only on symptomatic relief. Here, we identified stress-inducible phosphoprotein 1 (STIP1) as a putative neuroprotective factor targeted by PD-specific autoantibodies. STIP1 is a co-chaperone with reported neuroprotective capacities in mouse Alzheimer’s disease and stroke models. With human dopaminergic neurons derived from induced pluripotent stem cells, STIP1 was found to alleviate staurosporine-induced neurotoxicity. A case-control study involving 50 PD patients (average age = 62.94 ± 8.48, Hoehn and Yahr >2 = 55%) and 50 age-matched healthy controls (HCs) (average age = 63.1 ± 8) further revealed high levels of STIP1 autoantibodies in 20% of PD patients compared to 10% of HCs. Using an overlapping peptide library covering the STIP1 protein, we identified four PD-specific B cell epitopes that were not recognised in HCs. All of these epitopes were located within regions crucial for STIP1’s chaperone function or prion protein association. Our clinical and neuro-immunological studies highlight the potential of the STIP1 co-chaperone as an endogenous neuroprotective agent in PD and suggest the possible involvement of autoimmune mechanisms via the production of autoantibodies in a subset of individuals.

JAK2-Mediated Phosphorylation of Stress-Induced Phosphoprotein-1 (STIP1) in Human Cells

Stress-induced phosphoprotein-1 (STIP1)—a heat shock protein (HSP)70/HSP90 adaptor protein—is commonly overexpressed in malignant cells, where it controls proliferation via multiple signaling pathways, including JAK2/STAT3. We have previously shown that STIP1 stabilizes the protein tyrosine kinase JAK2 in cancer cells via HSP90 binding. In this study, we demonstrate that STIP1 may act as a substrate for JAK2 and that phosphorylation of tyrosine residues 134 and 152 promoted STIP1 protein stability, induced its nuclear-cytoplasmic shuttling, and promoted its secretion into the extracellular space. We also found that JAK2-mediated STIP1 phosphorylation enhanced cell viability and increased resistance to cisplatin-induced cell death. Conversely, interference STIP1 with JAK2 interaction—attained either through site-directed mutagenesis or the use of cell-penetrating peptides—decreased JAK2 protein levels, ultimately leading to cell death. On analyzing human ovarian cancer specimens, JAK2 and STIP1 expression levels were found to be positively correlated with each other. Collectively, these results indicate that JAK2-mediated phosphorylation of STIP-1 is critical for sustaining the JAK2/STAT3 signaling pathway in cancer cells.

Stress-inducible phosphoprotein 1 (HOP/STI1/STIP1) regulates the accumulation and toxicity of α-synuclein in vivo

The predominantly pre-synaptic intrinsically disordered protein α-synuclein is prone to misfolding and aggregation in synucleinopathies, such as Parkinson's disease (PD) and Dementia with Lewy bodies (DLB). Molecular chaperones play important roles in protein misfolding diseases and members of the chaperone machinery are often deposited in Lewy bodies. Here, we show that the Hsp90 co-chaperone STI1 co-immunoprecipitated α-synuclein, and co-deposited with Hsp90 and Hsp70 in insoluble protein fractions in two mouse models of α-synuclein misfolding. STI1 and Hsp90 also co-localized extensively with filamentous S129 phosphorylated α-synuclein in ubiquitin-positive inclusions. In PD human brains, STI1 transcripts were increased, and in neurologically healthy brains, STI1 and α-synuclein transcripts correlated. Nuclear Magnetic Resonance (NMR) analyses revealed direct interaction of α-synuclein with STI1 and indicated that the STI1 TPR2A, but not TPR1 or TPR2B domains, interacted with the C-terminal domain of α-synuclein. In vitro, the STI1 TPR2A domain facilitated S129 phosphorylation by Polo-like kinase 3. Moreover, mice over-expressing STI1 and Hsp90ß presented elevated α-synuclein S129 phosphorylation accompanied by inclusions when injected with α-synuclein pre-formed fibrils. In contrast, reduced STI1 function decreased protein inclusion formation, S129 α-synuclein phosphorylation, while mitigating motor and cognitive deficits as well as mesoscopic brain atrophy in α-synuclein-over-expressing mice. Our findings reveal a vicious cycle in which STI1 facilitates the generation and accumulation of toxic α-synuclein conformers, while α-synuclein-induced proteostatic stress increased insoluble STI1 and Hsp90.

Mutations in Hsp90 Cochaperones Result in a Wide Variety of Human Disorders

The Hsp90 molecular chaperone, along with a set of approximately 50 cochaperones, mediates the folding and activation of hundreds of cellular proteins in an ATP-dependent cycle. Cochaperones differ in how they interact with Hsp90 and their ability to modulate ATPase activity of Hsp90. Cochaperones often compete for the same binding site on Hsp90, and changes in levels of cochaperone expression that occur during neurodegeneration, cancer, or aging may result in altered Hsp90-cochaperone complexes and client activity. This review summarizes information about loss-of-function mutations of individual cochaperones and discusses the overall association of cochaperone alterations with a broad range of diseases. Cochaperone mutations result in ciliary or muscle defects, neurological development or degeneration disorders, and other disorders. In many cases, diseases were linked to defects in established cochaperone-client interactions. A better understanding of the functional consequences of defective cochaperones will provide new insights into their functions and may lead to specialized approaches to modulate Hsp90 functions and treat some of these human disorders.

Stress-induced phosphoprotein 1 restrains spinal cord ischaemia-reperfusion injury by modulating NF-κB signalling

Spinal cord injury (SCI), a major cause of disability, causes high global disease and economic burdens. Stress-induced phosphoprotein 1 (STIP1) has been identified to be involved in spinal cord ischaemia-reperfusion injury (SCII); however, the effect of STIP1 on SCII remains unclear until now. This study aimed to examine the role of STIP1 in SCII and unravel the possible mechanisms. Western blotting and immunohistochemical staining showed that STIP1 expression rapidly increased and then decreased in rat spinal cord following SCII treatment. Neurological function scoring, HE staining, immunohistochemical staining and Western blotting revealed that STIP1 overexpression alleviated SCII-induced motor dysfunction of hind limbs, neuronal loss and inflammation in spinal cord, and inhibited activity of nuclear factor kappa B (NF-κB) signalling in rats. Immunoprecipitation identified that STIP1 was co-located with Iba-1. In addition, STIP1 was found to ameliorate oxygen and glucose deprivation (OGD)-induced inflammation and activation of NF-κB signalling in mouse microglia BV2 cells, and STIP1 resulted in decrease of heat shock protein family A member 8 (HSPA8), increase of IκBβ expression and reduced binding of IκBβ to HSPA8 in BV2 cells. The results of the present study demonstrate that STIP1 alleviates ischaemia/reperfusion-induced neuronal injury and inflammation in rat spinal cord and mouse microglial cells by deactivating NF-κB signalling. These findings may provide novel insights for the clinical diagnosis and treatment of SCI.

With or without You: Co-Chaperones Mediate Health and Disease by Modifying Chaperone Function and Protein Triage

Heat shock proteins (HSPs) are a family of molecular chaperones that regulate essential protein refolding and triage decisions to maintain protein homeostasis. Numerous co-chaperone proteins directly interact and modify the function of HSPs, and these interactions impact the outcome of protein triage, impacting everything from structural proteins to cell signaling mediators. The chaperone/co-chaperone machinery protects against various stressors to ensure cellular function in the face of stress. However, coding mutations, expression changes, and post-translational modifications of the chaperone/co-chaperone machinery can alter the cellular stress response. Importantly, these dysfunctions appear to contribute to numerous human diseases. Therapeutic targeting of chaperones is an attractive but challenging approach due to the vast functions of HSPs, likely contributing to the off-target effects of these therapies. Current efforts focus on targeting co-chaperones to develop precise treatments for numerous diseases caused by defects in protein quality control. This review focuses on the recent developments regarding selected HSP70/HSP90 co-chaperones, with a concentration on cardioprotection, neuroprotection, cancer, and autoimmune diseases. We also discuss therapeutic approaches that highlight both the utility and challenges of targeting co-chaperones.

Hsp90 and its co-chaperone Sti1 control TDP-43 misfolding and toxicity

Protein misfolding is a central feature of most neurodegenerative diseases. Molecular chaperones can modulate the toxicity associated with protein misfolding, but it remains elusive which molecular chaperones and co-chaperones interact with specific misfolded proteins. TDP-43 misfolding and inclusion formation are a hallmark of amyotrophic lateral sclerosis (ALS) and other neurodegenerative diseases. Using yeast and mammalian neuronal cells we find that Hsp90 and its co-chaperone Sti1 have the capacity to alter TDP-43 misfolding, inclusion formation, aggregation, and cellular toxicity. Our data also demonstrate that impaired Hsp90 function sensitizes cells to TDP-43 toxicity and that Sti1 specifically interacts with and strongly modulates TDP-43 toxicity in a dose-dependent manner. Our study thus uncovers a previously unrecognized tie between Hsp90, Sti1, TDP-43 misfolding, and cellular toxicity.

Integrative proteomics and metabolomics profiling of the protective effects of Phascolosoma esculent ferritin on BMSCs in Cd(II) injury

Ferritin is the major intracellular iron storage protein and is essential for iron homeostasis and detoxification. Cadmium affects cellular homeostasis and induces cell toxicity via sophisticated mechanisms. Here, we aimed to explore the mechanisms of cytoprotective effect of Phascolosoma esculenta ferritin (PeFer) on Cd(II)-induced bone marrow mesenchymal stem cell (BMSC) injury. Herein, the effects of different treated groups on apoptosis and cell cycle were assessed using flow cytometric analysis. We further investigated the alterations of the three groups using integrative 2-DE-based proteomics and 1H NMR-based metabolomics profiles. The results indicate that PeFer reduces BMSC apoptosis induced by Cd(II) and delays G0/G1 cell cycle progression. A total of 19 proteins and 70 metabolites were significantly different among BMSC samples of the three groups. Notably, multiomics analysis revealed that Cd(II) might perturb the ER stress-mediated apoptosis pathway and disrupt biological processes related to the TCA cycle, amino acid metabolism, purine and pyrimidine metabolism, thereby suppressing the cell growth rate and initiating apoptosis; however, the addition of PeFer might protect BMSCs against cell apoptosis to improve cell survival by enhancing energy metabolism. This study provides a better understanding of the underlying molecular mechanisms of the protective effect of PeFer in BMSCs against Cd(II) injury.

Stress-induced phosphoprotein 1 facilitates breast cancer cell progression and indicates poor prognosis for breast cancer patients

Breast cancer (BC) threatened the life health of a tremendous amount of the population, and the estimated number of death is still rising nowadays. We found that stress-induced phosphoprotein 1 (STIP1) is overexpressed in BC tissues compared to non-tumorous breast tissues. Our study is to validate the prognostic value of STIP1 and investigate its biological role in BC. We verified the upregulation of STIP1 in multiple databases, proved that STIP1 is upregulated in BC tissues and cell lines using real-time quantitative PCR (qRT-PCR). We used small interfering RNA to examine the function of STIP1 in BC cell lines (BT-549, MDA-MB-231, Hs-578 T) and explored the mechanism of function of STIP1 in BC cells using Western blotting and qRT-PCR. Analyses of multiple databases indicated that high STIP1 expression is a marker that effectively distinguishes BC patients from healthy control and predicts worse clinical outcomes in BC. The loss-of-function experiments showed that STIP1 silencing results in inhibition of cell proliferation and migration, inducing cell apoptosis, and S-phase arrest in vitro. Our study also showed that STIP1 downregulation inhibited the JAK2/STAT3 pathway and epithelial-mesenchymal transition process. Rescue experiments demonstrated that the oncogenic effect of STIP1 is partially dependent on mediating JAK2 expression. This study verified that STIP1 is an oncogenic gene that promotes BC progression and serves as a valuable diagnostic and outcome-related marker of BC.

Intracellular targeting of STIP1 inhibits human cancer cell line growth

BACKGROUND

Extracellular and cell-surface molecules remain the most common druggable cancer targets. However, intracellular therapeutic modalities are gaining momentum. The overexpression of stress-induced phosphoprotein 1 (STIP1), an adaptor protein that coordinates the functions of different chaperones in protein folding, has been reported in several solid malignancies. Here, we investigated the effects of intracellular STIP1 inhibition, attained either through the HEPES-mediated cytosolic delivery of anti-STIP1 antibodies or the use of a cell-penetrating signal-tagged peptide 520, in different human cancer cell lines and luciferase-expressing murine ovarian cancer cells (MOSEC/Luc) tumor-bearing C57BL/6 mice.

METHODS

The effects of STIP1 in different human cell lines were determined by cell viability, cell cytotoxicity and cell apoptosis assays. Immunoblotting was used to assess the relevant proteins found in this study and tumor xenograft mice models were also employed.

RESULTS

Intracellular targeting of STIP1 inhibited cancer cell line growth and promoted caspase 3-dependent apoptotic cell death. Moreover, the intracellular delivery of anti-STIP1 antibodies facilitated the degradation of STIP1 and two of its client proteins, lysine-specific demethylase 1 and Janus kinase 2. In vivo studies demonstrated that survival of mice bearing experimental tumors was improved by administration of anti-STIP1 antibodies.

CONCLUSIONS

Our findings demonstrate that the cytosolic inhibition of STIP1 in tumor cells is feasible and provides a solid basis for further investigation of STIP1 as an intracellular cancer target. Our findings demonstrate that cytosolic inhibition of STIP1 in tumor cells is feasible and provide a solid basis for further exploration of STIP1 as an intracellular cancer target.

The Hsp70-Hsp90 go-between Hop/Stip1/Sti1 is a proteostatic switch and may be a drug target in cancer and neurodegeneration

The Hsp70 and Hsp90 molecular chaperone systems are critical regulators of protein homeostasis (proteostasis) in eukaryotes under normal and stressed conditions. The Hsp70 and Hsp90 systems physically and functionally interact to ensure cellular proteostasis. Co-chaperones interact with Hsp70 and Hsp90 to regulate and to promote their molecular chaperone functions. Mammalian Hop, also called Stip1, and its budding yeast ortholog Sti1 are eukaryote-specific co-chaperones, which have been thought to be essential for substrate ("client") transfer from Hsp70 to Hsp90. Substrate transfer is facilitated by the ability of Hop to interact simultaneously with Hsp70 and Hsp90 as part of a ternary complex. Intriguingly, in prokaryotes, which lack a Hop ortholog, the Hsp70 and Hsp90 orthologs interact directly. Recent evidence shows that eukaryotic Hsp70 and Hsp90 can also form a prokaryote-like binary chaperone complex in the absence of Hop, and that this binary complex displays enhanced protein folding and anti-aggregation activities. The canonical Hsp70-Hop-Hsp90 ternary chaperone complex is essential for optimal maturation and stability of a small subset of clients, including the glucocorticoid receptor, the tyrosine kinase v-Src, and the 26S/30S proteasome. Whereas many cancers have increased levels of Hop, the levels of Hop decrease in the aging human brain. Since Hop is not essential in all eukaryotic cells and organisms, tuning Hop levels or activity might be beneficial for the treatment of cancer and neurodegeneration.

General Structural and Functional Features of Molecular Chaperones

Molecular chaperones are a group of structurally diverse and highly conserved ubiquitous proteins. They play crucial roles in facilitating the correct folding of proteins in vivo by preventing protein aggregation or facilitating the appropriate folding and assembly of proteins. Heat shock proteins form the major class of molecular chaperones that are responsible for protein folding events in the cell. This is achieved by ATP-dependent (folding machines) or ATP-independent mechanisms (holders). Heat shock proteins are induced by a variety of stresses, besides heat shock. The large and varied heat shock protein class is categorised into several subfamilies based on their sizes in kDa namely, small Hsps (HSPB), J domain proteins (Hsp40/DNAJ), Hsp60 (HSPD/E; Chaperonins), Hsp70 (HSPA), Hsp90 (HSPC), and Hsp100. Heat shock proteins are localised to different compartments in the cell to carry out tasks specific to their environment. Most heat shock proteins form large oligomeric structures, and their functions are usually regulated by a variety of cochaperones and cofactors. Heat shock proteins do not function in isolation but are rather part of the chaperone network in the cell. The general structural and functional features of the major heat shock protein families are discussed, including their roles in human disease. Their function is particularly important in disease due to increased stress in the cell. Vector-borne parasites affecting human health encounter stress during transmission between invertebrate vectors and mammalian hosts. Members of the main classes of heat shock proteins are all represented in Plasmodium falciparum, the causative agent of cerebral malaria, and they play specific functions in differentiation, cytoprotection, signal transduction, and virulence.

The Hsp70-Hsp90 co-chaperone Hop/Stip1 shifts the proteostatic balance from folding towards degradation

Hop/Stip1/Sti1 is thought to be essential as a co-chaperone to facilitate substrate transfer between the Hsp70 and Hsp90 molecular chaperones. Despite this proposed key function for protein folding and maturation, it is not essential in a number of eukaryotes and bacteria lack an ortholog. We set out to identify and to characterize its eukaryote-specific function. Human cell lines and the budding yeast with deletions of the Hop/Sti1 gene display reduced proteasome activity due to inefficient capping of the core particle with regulatory particles. Unexpectedly, knock-out cells are more proficient at preventing protein aggregation and at promoting protein refolding. Without the restraint by Hop, a more efficient folding activity of the prokaryote-like Hsp70-Hsp90 complex, which can also be demonstrated in vitro, compensates for the proteasomal defect and ensures the proteostatic equilibrium. Thus, cells may act on the level and/or activity of Hop to shift the proteostatic balance between folding and degradation.

Hop, also known as Stip1 or Sti1, facilitates substrate transfer between the Hsp70 and Hsp90 molecular chaperones. Characterization of proteostasis-related pathways in STIP1 knock-out cell lines reveals that in eukaryotes Stip1 modulates the balance between protein folding and degradation.

Cephalic Neuronal Vesicle Formation is Developmentally Dependent and Modified by Methylmercury and sti-1 in Caenorhabditis elegans

Methylmercury (MeHg) is a potent neurotoxicant. The mechanisms underlying MeHg-induced neurotoxicity are not fully understood. Several studies have shown that protein chaperones are involved in MeHg toxicity. The protein co-chaperone, stress inducible protein 1 (STI-1), has important functions in protein quality control of the chaperone pathway. In the current study, dopaminergic (DAergic) cephalic (CEP) neuronal morphology was evaluated in the Caenorhabditis elegans (C. elegans) sti-1 knockout strain. In the control OH7193 strain (dat-1::mCherry + ttx-3::mCherry), we characterized the morphology of CEP neurons by checking the presence of attached vesicles and unattached vesicles to the CEP dendrites. We showed that the attached vesicles were only present in adult stage worms; whereas they were absent in the younger L3 stage worms. In the sti-1 knockout strain, MeHg treatment significantly altered the structures of CEP dendrites with discontinuation of mCherry fluorescence and shrinkage of CEP soma, as compared to the control. 12 h post treatment on MeHg-free OP50-seeded plates, the discontinuation of mCherry fluorescence of CEP dendrites in worms treated with 0.05 or 0.5 µM MeHg returned to levels statistically indistinguishable from control, while in worms treated with 5 µM MeHg a higher percentage of discontinuation of mCherry fluorescence persisted. Despite this strong effect by 5 µM MeHg, CEP attached vesicles were increased upon 0.05 or 0.5 µM MeHg treatment, yet unaffected by 5 µM MeHg. The CEP attached vesicles of sti-1 knockout strain were significantly increased shortly after MeHg treatment, but were unaffected 48 h post treatment. In addition, there was a significant interactive effect of MeHg and sti-1 on the number of attached vesicles. Knock down sti-1 via RNAi did not alter the number of CEP attached vesicles. Taking together, our data suggests that the increased occurrence of attached vesicles in adult stage worms could initiate a substantial loss of membrane components of CEP dendrites following release of vesicles, leading to the discontinuation of mCherry fluorescence, and the formation of CEP attached vesicles could be regulated by sti-1 to remove cellular debris for detoxification.

Striatal Acetylcholine Helps to Preserve Functional Outcomes in a Mouse Model of Stroke

Acetylcholine (ACh) has been suggested to facilitate plasticity and improve functional recovery after different types of brain lesions. Interestingly, numerous studies have shown that striatal cholinergic interneurons are relatively resistant to acute ischemic insults, but whether ACh released by these neurons enhances functional recovery after stroke is unknown. We investigated the role of endogenous striatal ACh in stroke lesion volume and functional outcomes following middle cerebral artery occlusion to induce focal ischemia in striatum-selective vesicular acetylcholine transporter-deficient mice (stVAChT-KO). As transporter expression is almost completely eliminated in the striatum of stVAChT-KO mice, ACh release is nearly abolished in this area. Conversely, in other brain areas, VAChT expression and ACh release are preserved. Our results demonstrate a larger infarct size after ischemic insult in stVAChT-KO mice, with more pronounced functional impairments and increased mortality than in littermate controls. These changes are associated with increased activation of GSK-3, decreased levels of β-catenin, and a higher permeability of the blood–brain barrier in mice with loss of VAChT in striatum neurons. These results support a framework in which endogenous ACh secretion originating from cholinergic interneurons in the striatum helps to protect brain tissue against ischemia-induced damage and facilitates brain recovery by supporting blood–brain barrier function.

Impaired Redox and Protein Homeostasis as Risk Factors and Therapeutic Targets in Toxin-Induced Biliary Atresia

BACKGROUND & AIMS

Extrahepatic biliary atresia (BA) is a pediatric liver disease with no approved medical therapy. Recent studies using human samples and experimental modeling suggest that glutathione redox metabolism and heterogeneity play a role in disease pathogenesis. We sought to dissect the mechanistic basis of liver redox variation and explore how other stress responses affect cholangiocyte injury in BA.

METHODS

We performed quantitative in situ hepatic glutathione redox mapping in zebrafish larvae carrying targeted mutations in glutathione metabolism genes and correlated these findings with sensitivity to the plant-derived BA-linked toxin biliatresone. We also determined whether genetic disruption of HSP90 protein quality control pathway genes implicated in human BA altered biliatresone toxicity in zebrafish and human cholangiocytes. An in vivo screening of a known drug library was performed to identify novel modifiers of cholangiocyte injury in the zebrafish experimental BA model, with subsequent validation.

RESULTS

Glutathione metabolism gene mutations caused regionally distinct changes in the redox potential of cholangiocytes that differentially sensitized them to biliatresone. Disruption of human BA-implicated HSP90 pathway genes sensitized zebrafish and human cholangiocytes to biliatresone-induced injury independent of glutathione. Phosphodiesterase-5 inhibitors and other cyclic guanosine monophosphate signaling activators worked synergistically with the glutathione precursor N-acetylcysteine in preventing biliatresone-induced injury in zebrafish and human cholangiocytes. Phosphodiesterase-5 inhibitors enhanced proteasomal degradation and required intact HSP90 chaperone.

CONCLUSION

Regional variation in glutathione metabolism underlies sensitivity to the biliary toxin biliatresone and may account for the reported association between BA transplant-free survival and glutathione metabolism gene expression. Human BA can be causatively linked to genetic modulation of protein quality control. Combined treatment with N-acetylcysteine and cyclic guanosine monophosphate signaling enhancers warrants further investigation as therapy for BA.

Characterization of brain-derived extracellular vesicles reveals changes in cellular origin after stroke and enrichment of the prion protein with a potential role in cellular uptake

Extracellular vesicles (EVs) are important means of intercellular communication and a potent tool for regenerative therapy. In ischaemic stroke, transient blockage of a brain artery leads to a lack of glucose and oxygen in the affected brain tissue, provoking neuronal death by necrosis in the core of the ischaemic region. The fate of neurons in the surrounding penumbra region depends on the stimuli, including EVs, received during the following hours. A detailed characterization of such stimuli is crucial not only for understanding stroke pathophysiology but also for new therapeutic interventions. In the present study, we characterize the EVs in mouse brain under physiological conditions and 24 h after induction of transient ischaemia in mice. We show that, in steady-state conditions, microglia are the main source of small EVs (sEVs), whereas after ischaemia the main sEV population originates from astrocytes. Brain sEVs presented high amounts of the prion protein (PrP), which were further increased after stroke. Moreover, EVs were enriched in a proteolytically truncated PrP fragment (PrP-C1). Because of similarities between PrP-C1 and certain viral surface proteins, we studied the cellular uptake of brain-derived sEVs from mice lacking (PrP-KO) or expressing PrP (WT). We show that PrP-KO-sEVs are taken up significantly faster and more efficiently than WT-EVs by primary neurons. Furthermore, microglia and astrocytes engulf PrP-KO-sEVs more readily than WT-sEVs. Our results provide novel information on the relative contribution of brain cell types to the sEV pool in murine brain and indicate that increased release of sEVs by astrocytes together with elevated levels of PrP in sEVs may play a role in intercellular communication at early stages after stroke. In addition, amounts of PrP (and probably PrP-C1) in brain sEVs seem to contribute to regulating their cellular uptake.

Increased levels of Stress-inducible phosphoprotein-1 accelerates amyloid-β deposition in a mouse model of Alzheimer's disease

Molecular chaperones and co-chaperones, which are part of the protein quality control machinery, have been shown to regulate distinct aspects of Alzheimer's Disease (AD) pathology in multiple ways. Notably, the co-chaperone STI1, which presents increased levels in AD, can protect mammalian neurons from amyloid-β toxicity in vitro and reduced STI1 levels worsen Aβ toxicity in C. elegans. However, whether increased STI1 levels can protect neurons in vivo remains unknown. We determined that overexpression of STI1 and/or Hsp90 protected C. elegans expressing Aβ(3-42) against Aβ-mediated paralysis. Mammalian neurons were also protected by elevated levels of endogenous STI1 in vitro, and this effect was mainly due to extracellular STI1. Surprisingly, in the 5xFAD mouse model of AD, by overexpressing STI1, we find increased amyloid burden, which amplifies neurotoxicity and worsens spatial memory deficits in these mutants. Increased levels of STI1 disturbed the expression of Aβ-regulating enzymes (BACE1 and MMP-2), suggesting potential mechanisms by which amyloid burden is increased in mice. Notably, we observed that STI1 accumulates in dense-core AD plaques in both 5xFAD mice and human brain tissue. Our findings suggest that elevated levels of STI1 contribute to Aβ accumulation, and that STI1 is deposited in AD plaques in mice and humans. We conclude that despite the protective effects of STI1 in C. elegans and in mammalian cultured neurons, in vivo, the predominant effect of elevated STI1 is deleterious in AD.

Neurite outgrowth inhibitory levels of organophosphates induce tissue transglutaminase activity in differentiating N2a cells: evidence for covalent adduct formation

Organophosphate compounds (OPs) induce both acute and delayed neurotoxic effects, the latter of which is believed to involve their interaction with proteins other than acetylcholinesterase. However, few OP-binding proteins have been identified that may have a direct role in OP-induced delayed neurotoxicity. Given their ability to disrupt Ca²⁺ homeostasis, a key aim of the current work was to investigate the effects of sub-lethal neurite outgrowth inhibitory levels of OPs on the Ca²⁺-dependent enzyme tissue transglutaminase (TG2). At 1-10 µM, the OPs phenyl saligenin phosphate (PSP) and chlorpyrifos oxon (CPO) had no effect cell viability but induced concentration-dependent decreases in neurite outgrowth in differentiating N2a neuroblastoma cells. The activity of TG2 increased in cell lysates of differentiating cells exposed for 24 h to PSP and chlorpyrifos oxon CPO (10 µM), as determined by biotin-cadaverine incorporation assays. Exposure to both OPs (3 and/or 10 µM) also enhanced in situ incorporation of the membrane permeable substrate biotin-X-cadaverine, as indicated by Western blot analysis of treated cell lysates probed with ExtrAvidin peroxidase and fluorescence microscopy of cell monolayers incubated with FITC-streptavidin. Both OPs (10 µM) stimulated the activity of human and mouse recombinant TG2 and covalent labelling of TG2 with dansylamine-labelled PSP was demonstrated by fluorescence imaging following SDS-PAGE. A number of TG2 substrates were tentatively identified by mass spectrometry, including cytoskeletal proteins, chaperones and proteins involved protein synthesis and gene regulation. We propose that the elevated TG2 activity observed is due to the formation of a novel covalent adduct between TG2 and OPs.

Show Me Your Friends and I Tell You Who You Are: The Many Facets of Prion Protein in Stroke

Ischemic stroke belongs to the leading causes of mortality and disability worldwide. Although treatments for the acute phase of stroke are available, not all patients are eligible. There is a need to search for therapeutic options to promote neurological recovery after stroke. The cellular prion protein (PrPC) has been consistently linked to a neuroprotective role after ischemic damage: it is upregulated in the penumbra area following stroke in humans, and animal models of stroke have shown that lack of PrPC aggravates the ischemic damage and lessens the functional outcome. Mechanistically, these effects can be linked to numerous functions attributed to PrPC: (1) as a signaling partner of the PI3K/Akt and MAPK pathways, (2) as a regulator of glutamate receptors, and (3) promoting stem cell homing mechanisms, leading to angio- and neurogenesis. PrPC can be cleaved at different sites and the proteolytic fragments can account for the manifold functions. Moreover, PrPC is present on extracellular vesicles (EVs), released membrane particles originating from all types of cells that have drawn attention as potential therapeutic tools in stroke and many other diseases. Thus, identification of the many mechanisms underlying PrPC-induced neuroprotection will not only provide further understanding of the physiological functions of PrPC but also new ideas for possible treatment options after ischemic stroke.

The multiple functions of the co-chaperone stress inducible protein 1

Stress inducible protein 1 (STI1) is a co-chaperone acting with Hsp70 and Hsp90 for the correct client proteins' folding and therefore for the maintenance of cellular homeostasis. Besides being expressed in the cytosol, STI1 can also be found both in the cell membrane and the extracellular medium playing several relevant roles in the central nervous system (CNS) and tumor microenvironment. During CNS development, in association with cellular prion protein (PrP^c), STI1 regulates crucial events such as neuroprotection, neuritogenesis, astrocyte differentiation and survival. In cancer, STI1 is involved with tumor growth and invasion, is undoubtedly a pro-tumor factor, being considered as a biomarker and possibly therapeutic target for several malignancies. In this review, we discuss current knowledge and new findings on STI1 function as well as its role in tissue homeostasis, CNS and tumor progression.

Hop depletion reduces HSF1 levels and activity and coincides with reduced stress resilience

Heat-shock factor 1 (HSF1) regulates the transcriptional response to stress and controls expression of molecular chaperones required for cell survival. Here we report that HSF1 is regulated by the abundance of the Hsp70-Hsp90 organizing protein (Hop/STIP1). HSF1 levels were significantly reduced in Hop-depleted HEK293T cells. HSF1 transcriptional activity at the Hsp70 promoter, and binding of a biotinylated HSE oligonucleotide under both basal and heat-shock conditions were significantly reduced. Hop-depleted HEK293T cells were more sensitive to the HSF1 inhibitor KRIBB11 and showed reduced short-term proliferation, and reduced long-term survival under basal and heat-shock conditions. HSF1 nuclear localization was reduced in response to heat-shock and the nuclear staining pattern in Hop-depleted cells was punctate. Taken together, these data suggest that Hop regulates HSF1 function under both basal and stress conditions through a mechanism involving changes in levels, activity and subcellular localization, and coincides with reduced cellular fitness.

Seeking for potential pathogenic genes of major depressive disorder in the Gene Expression Omnibus database

INTRODUCTION

Major depressive disorder (MDD) is one of the most common mental disorders worldwide. The aim of this study was to identify potential pathological genes in MDD.

METHODS

We searched and downloaded gene expression data from the Gene Expression Omnibus database to identify differentially expressed genes (DEGs) in MDD. Then, Kyoto Encyclopedia of Genes and Genomes pathway, Gene Ontology analysis, and protein-protein interaction (PPI) network were applied to investigate the biological function of identified DEGs. The quantitative real-time polymerase chain reaction and a published dataset were used to validate the result of bioinformatics analysis.

RESULTS

A total of 514 DEGs were identified in MDD. In the PPI network, some hub genes with high degrees were identified, such as EEF2, RPL26L1, RPLP0, PRPF8, LSM3, DHX9, RSRC1, and AP2B1. The result of in vitro validation of RPL26L1, RSRC1, TOMM20L, RPLPO, PRPF8, AP2B1, STIP1, and C5orf45 was consistent with the bioinformatics analysis. Electronic validation of C5orf45, STIP1, PRPF8, AP2B1, and SLC35E1 was consistent with the bioinformatics analysis.

DISCUSSION

The deregulated genes could be used as potential pathological factors of MDD. In addition, EEF2, RPL26L1, RPLP0, PRPF8, LSM3, DHX9, RSRC1, and AP2B1 might be therapeutic targets for MDD.

Spatial and Pregnancy-Related Changes in the Protein, Amino Acid, and Carbohydrate Composition of Bovine Oviduct Fluid

Knowledge of how the biochemical composition of the bovine oviduct is altered due to the oviduct anatomy or the presence of an embryo is lacking. Thus, the aim of this study was to assess the effect of (І) oviduct anatomy and (ІІ) embryo presence on oviductal fluid (OF) protein, amino acid, and carbohydrate composition. Cross-bred beef heifers (n = 19) were synchronized and those in standing estrus were randomly allocated to a cyclic (non-bred) or pregnant (artificially inseminated) group. All heifers were slaughtered on Day 3 after estrus. The oviducts ipsilateral to the corpus luteum from each animal were isolated, straightened and cut, separating ampulla and isthmus. Each portion was flushed with 500 µl of PBS enabling recovery of the oocyte/embryo. Recovered unfertilized oocytes (cyclic group) and embryos (8-cell embryos; pregnant group) were located in the isthmus of the oviduct. Samples of flushing medium from the isthmus and ampulla were used for proteomic (n = 2 per group), amino acid (n = 5), and carbohydrate (n = 5) analysis. For proteomic analysis, total protein from cyclic and pregnant samples were labelled with different cyanine fluorescent probes and separated according to the isoelectric point using immobilized pH gradient strips (pH 3-10, 17 cm, Protean® IEF cell system, Bio Rad). Second dimension was performed in a polyacrylamide gel (12%) in the presence of SDS using a Protean II XL system (Bio Rad). Images were obtained with a Typhoon 9410 scanner and analyzed with Progenesis SameSpots software v 4.0. Amino acid content in the OF was determined by high performance liquid chromatography (HPLC). Glucose, lactate, and pyruvate were quantified using microfluorometric enzyme-linked assays. For the proteomic assessment, the results of the image analysis were compared by ANOVA. For both amino acid and carbohydrate analyses, statistical analysis was carried out by 2-way ANOVA with the Holm-Sidak nonparametric post hoc analysis. On Day 3 post-estrus, OF composition varied based on (І) anatomical region, where isthmic metabolites were present in lower (i.e., lactate, glycine, and alanine) or higher (i.e., arginine) concentrations compared to the ampulla; and (ІІ) embryo presence, which was correlated with greater, arginine, phosphoglycerate kinase 1, serum albumin, α-1-antiproteinase and IGL@ protein concentrations. In conclusion, data indicate that the composition of bovine OF is anatomically dynamic and influenced by the presence of an early embryo.

Modulation of hippocampal neuronal resilience during aging by the Hsp70/Hsp90 co-chaperone STI1

Chaperone networks are dysregulated with aging, but whether compromised Hsp70/Hsp90 chaperone function disturbs neuronal resilience is unknown. Stress-inducible phosphoprotein 1 (STI1; STIP1; HOP) is a co-chaperone that simultaneously interacts with Hsp70 and Hsp90, but whose function in vivo remains poorly understood. We combined in-depth analysis of chaperone genes in human datasets, analysis of a neuronal cell line lacking STI1 and of a mouse line with a hypomorphic Stip1 allele to investigate the requirement for STI1 in aging. Our experiments revealed that dysfunctional STI1 activity compromised Hsp70/Hsp90 chaperone network and neuronal resilience. The levels of a set of Hsp90 co-chaperones and client proteins were selectively affected by reduced levels of STI1, suggesting that their stability depends on functional Hsp70/Hsp90 machinery. Analysis of human databases revealed a subset of co-chaperones, including STI1, whose loss of function is incompatible with life in mammals, albeit they are not essential in yeast. Importantly, mice expressing a hypomorphic STI1 allele presented spontaneous age-dependent hippocampal neurodegeneration and reduced hippocampal volume, with consequent spatial memory deficit. We suggest that impaired STI1 function compromises Hsp70/Hsp90 chaperone activity in mammals and can by itself cause age-dependent hippocampal neurodegeneration in mice. Cover Image for this issue: doi: 10.1111/jnc.14749.

Chaperones and Beyond as Key Players in Pluripotency Maintenance

Pluripotency is orchestrated by distinct players and chaperones and their partners have emerged as pivotal molecules in proteostasis control to maintain stemness. The proteostasis network consists of diverse interconnected pathways that function dynamically according to the needs of the cell to quality control and maintain protein homeostasis. The proteostasis machinery of pluripotent stem cells (PSCs) is finely adjusted in response to distinct stimuli during cell fate commitment to determine successful organism development. Growing evidence has shown different classes of chaperones regulating crucial cellular processes in PSCs. Histones chaperones promote proper nucleosome assembly and modulate the epigenetic regulation of factors involved in PSCs’ rapid turnover from pluripotency to differentiation. The life cycle of pluripotency proteins from synthesis and folding, transport and degradation is finely regulated by chaperones and co-factors either to maintain the stemness status or to cell fate commitment. Here, we summarize current knowledge of the chaperone network that govern stemness and present the versatile role of chaperones in stem cells resilience. Elucidation of the intricate regulation of pluripotency, dissecting in detail molecular determinants and drivers, is fundamental to understanding the properties of stem cells in order to provide a reliable foundation for biomedical research and regenerative medicine.

Stress-induced phosphoprotein 1 promotes pancreatic cancer progression through activation of the FAK/AKT/MMP signaling axis

BACKGROUND

Dependent on the extent of adenosine triphosphate (ATP) hydrolysis and/or ATP/ADP exchange, the stress-induced phosphoprotein 1 (STIP1) mediates molecular interaction and complex formation between the molecular chaperones heat shock protein (Hsp)70 and Hsp90. The overexpression of STIP1 is increasingly being documented in various human malignancies, including ovarian, cholangiocellular, renal and gastric cancers. However, the role of STIP1 in pancreatic cancer (PANC) and probable molecular mechanism remains largely unexplored.

METHODS & RESULTS

In the present study, using clinical samples (n = 88) and human PANC cell lines PANC-1, Capan-2, SW1990, and BxPC-3, we demonstrated that STIP1 is aberrantly expressed in human PANC tissues or cell lines compared to adjacent non-tumor pancreas samples or human pancreatic duct epithelial cells (HPDEC), respectively. Clinicopathological correlation studies revealed significant positive correlation between high STIP1 expression and lymph node involvement (p = 0.001), cancer metastasis (p = 0.002), microvascular invasion (p = 0.002), advance TNM stage (p = 0.024), perineural invasion (PNI; p = 0.013), and cancer-related death (p = 0.002) among patients with PANC. Univariate and multivariate analyses indicate that STIP1overexpression is an independent prognostic factor of PANC. Furthermore, STIP1 knockdown significantly inhibit the migration and invasive ability of PANC-1 and SW1990 cells, while downregulating N-cadherin and Vimentin, but upregulating E-cadherin mRNA expression levels, concurrently. We also demonstrated that STIP1 knockdown suppressed p-FAK, p-AKT, MMP2, MMP9, and Slug protein and mRNA expression levels, thus, indicating, at least in part, a role for STIP1 in the activation of FAK/AKT/MMP signaling.

CONCLUSION

Taken together, our results demonstrate a critical role for STIP1 in cancer metastasis, disease progression and poor prognosis, as well as, provide evidence suggestive of the therapeutic efficacy of STIP1-mediated targeting of the FAK/AKT/MMP signaling axis in patients with PANC.

Detection of Active Caspase-3 in Mouse Models of Stroke and Alzheimer's Disease with a Novel Dual Positron Emission Tomography/Fluorescent Tracer [68Ga]Ga-TC3-OGDOTA

Apoptosis is a feature of stroke and Alzheimer's disease (AD), yet there is no accepted method to detect or follow apoptosis in the brain in vivo. We developed a bifunctional tracer [68Ga]Ga-TC3-OGDOTA containing a cell-penetrating peptide separated from fluorescent Oregon Green and 68Ga-bound labels by the caspase-3 recognition peptide DEVD. We hypothesized that this design would allow [68Ga]Ga-TC3-OGDOTA to accumulate in apoptotic cells. In vitro, Ga-TC3-OGDOTA labeled apoptotic neurons following exposure to camptothecin, oxygen-glucose deprivation, and β-amyloid oligomers. In vivo, PET showed accumulation of [68Ga]Ga-TC3-OGDOTA in the brain of mouse models of stroke or AD. Optical clearing revealed colocalization of [68Ga]Ga-TC3-OGDOTA and cleaved caspase-3 in brain cells. In stroke, [68Ga]Ga-TC3-OGDOTA accumulated in neurons in the penumbra area, whereas in AD mice [68Ga]Ga-TC3-OGDOTA was found in single cells in the forebrain and diffusely around amyloid plaques. In summary, this bifunctional tracer is selectively associated with apoptotic cells in vitro and in vivo in brain disease models and represents a novel tool for apoptosis detection that can be used in neurodegenerative diseases.

The prion protein in neuroimmune crosstalk

The cellular prion protein (PrP^C) is a medium-sized glycoprotein, attached to the cell surface by a glycosylphosphatidylinositol anchor. PrP^C is encoded by a single-copy gene, PRNP, which is abundantly expressed in the central nervous system and at lower levels in non-neuronal cells, including those of the immune system. Evidence from experimental knockout of PRNP in rodents, goats, and cattle and the occurrence of a nonsense mutation in goat that prevents synthesis of PrP^C, have shown that the molecule is non-essential for life. Indeed, no easily recognizable phenotypes are associate with a lack of PrP^C, except the potentially advantageous trait that animals without PrP^C cannot develop prion disease. This is because, in prion diseases, PrP^C converts to a pathogenic "scrapie" conformer, PrP^Sc, which aggregates and eventually induces neurodegeneration. In addition, endogenous neuronal PrP^C serves as a toxic receptor to mediate prion-induced neurotoxicity. Thus, PrP^C is an interesting target for treatment of prion diseases. Although loss of PrP^C has no discernable effect, alteration of its normal physiological function can have very harmful consequences. It is therefore important to understand cellular processes involving PrP^C, and research of this topic has advanced considerably in the past decade. Here, we summarize data that indicate the role of PrP^C in modulating immune signaling, with emphasis on neuroimmune crosstalk both under basal conditions and during inflammatory stress.

Characteristics of AccSTIP1 in Apis cerana cerana and its role during oxidative stress responses

Various environmental stresses, such as heat shock, heavy metals, ultraviolet (UV) radiation and different pesticides, induce a cellular oxidative stress response. The cellular oxidative stress response is usually regulated by heat shock proteins (Hsps) acting as molecular chaperones. Stress-induced phosphoprotein 1 (STIP1), one of the most widely studied co-chaperones, functions as an adaptor that directs Hsp90 to Hsp70-client protein complexes. However, the biological functions of STIP1 remain poorly understood in honeybee (Apis cerana cerana). In this study, AccSTIP1 was identified in Apis cerana cerana. AccSTIP1 transcription was found to be induced by heat (42 °C), HgCl2, H2O2 and different pesticides (emamectin benzoate, thiamethoxam, hexythiazox and paraquat) and inhibited by CdCl2, UV and kresoxim-methyl. Moreover, western blot analysis indicated that the expression profiles of AccSTIP1 were consistent with its transcriptional expression levels. The disc diffusion assay showed that chemically competent transetta (DE3) bacteria expressing a recombinant AccSTIP1 protein displayed the smaller death zones than did control bacteria after exposure to paraquat and HgCl2. The DNA nicking assay suggested that recombinant purified AccSTIP1 protected supercoiled pUC19 plasmid DNA from damage caused by a thiol-dependent mixed-function oxidation (MFO) system. After knocking down AccSTIP1 gene expression via RNA interference (RNAi), the transcript levels of antioxidation-related genes were obviously lower in dsAccSTIP1 honeybees compared with those in the uninjected honeybees. Collectively, these results demonstrated that AccSTIP1 plays an important role in counteracting oxidative stress. This study lays a foundation for revealing the mechanism of AccSTIP1 in the Apis cerana cerana antioxidant system.

From the Cover: Proteome Profile of Different Rat Brain Regions After Sarin Intoxication

Sarin is an organophosphorus (OP) chemical warfare agent which irreversibly inhibits acetylcholinesterase. Acute toxicity after sarin exposure is because of hyper activation of the nicotinic and muscarinic receptor. Survivors of sarin exposure often develop long-term neuropathology referred as OP ester-induced chronic neurotoxicity. However, the exact mechanism of chronic neurotoxicity is yet unknown. We studied proteomic changes in rat brain regions after 0.5 LD50 dose of sarin and investigated some milestone changes associated with long-term CNS injury. We used two-dimensional gel electrophoresis/mass spectrometry approach to identify early proteomic changes and traced expression of selected proteins for longer time points. This study shows changes in chaperone function, endoplasmic reticulum stress, and defect in cytoskeleton functions at earlier stages. Predictive interaction analysis demonstrated putative role of Parkinson's disease-related proteins after sarin exposure. Our results clearly indicated neurodegenerative changes which started after 2.5 h and showed prominence after 3-month postexposure. The study also unmasks changes in proteins related to movement and cognitive function. The markers for astrocytosis (GFAP) and neurodegenerative changes (alpha-synuclein and amyloid precursor protein) exhibited altered expression in brain. This is the first proteomic study among survivors of sarin exposure in animal model. Some of the early changes, including those involved in neurodegeneration, movement, and cognitive function, defects in chaperone function and cytoskeleton, were shown to persist for a longer period. The study provides a preliminary framework for further validation of major mechanisms of sarin toxicity is suggested here and opens new avenues for elucidation of therapeutic intervention.

Changes of hippocampus proteomic profiles after blueberry extracts supplementation in APP/PS1 transgenic mice

Objective: To examine protein changes in the hippocampus of APP/PS1 transgenic mice after blueberry extracts (BB) intervention.Methods: Eight APP/PS1 transgenic mice were randomly assigned to Alzheimer's disease (AD)+BB group (n=4) and AD+control group (n=4). After a 16-week treatment, 2-DE and MALDI-TOF-MS were used to compare the proteomic profiles of the hippocampus in the two groups and Western blot was used to confirm the important differentially expressed proteins.Results: Twelve proteins were differentially expressed between the two groups. Nine of them were identified. Cytochrome b-c1 complex subunit 6, beta-actin, dynamin 1, and heat shock cognate 71 were up-regulated in AD+BB group, while a-enolase, stress-induced-phosphoprotein 1, malate dehydrogenase (MDH), MDH 1, and T-complex protein 1 subunit beta were down-regulated, respectively. Importantly, some of the identified proteins (e.g. dynamin 1) are known to be involved in cognitive impairment. Western blot analysis of hippocampus dynamin 1 expression confirmed the proteomic findings.Conclusions: The consumption of BB modulates the expression of proteins that are linked to the improvements of cognitive dysfunction in hippocampus of APP/PS1 transgenic mice.

Mechanisms of neuroprotection against ischemic insult by stress-inducible phosphoprotein-1/prion protein complex

Stress-inducible phosphoprotein 1 (STI1) acts as a neuroprotective factor in the ischemic brain and its levels are increased following ischemia. Previous work has suggested that some of these STI1 actions in a stroke model depend on the recruitment of bone marrow-derived stem cells to improve outcomes after ischemic insult. However, STI1 can directly increase neuroprotective signaling in neurons by engaging with the cellular prion protein (PrPC ) and activating α7 nicotinic acetylcholine receptors (α7nAChR). Given that α7nAChR activation has also been involved in neuroprotection in stroke, it is possible that STI1 can have direct actions on neurons to prevent deleterious consequences of ischemic insults. Here, we tested this hypothesis by exposing primary neuronal cultures to 1-h oxygen-glucose deprivation (OGD) and reperfusion and assessing signaling pathways activated by STI1/PrPC . Our results demonstrated that STI1 treatment significantly decreased apoptosis and cell death in mouse neurons submitted to OGD in a manner that was dependent on PrPC and α7nAChR, but also on the activin A receptor 1 (ALK2), which has emerged as a signaling partner of STI1. Interestingly, pharmacological inhibition of the ALK2 receptor prevented neuroprotection by STI1, while activation of ALK2 receptors by bone morphogenetic protein 4 (BMP4) either before or after OGD was effective in decreasing neuronal death induced by ischemia. We conclude that PrPC /STI1 engagement and its subsequent downstream signaling cascades involving α7nAChR as well as the ALK2 receptor may be activated in neurons by increased levels of STI1. This signaling pathway protects neurons from ischemic insults.

Stress-induced phosphoprotein 1 acts as a scaffold protein for glycogen synthase kinase-3 beta-mediated phosphorylation of lysine-specific demethylase 1

Stress-induced phosphoprotein 1 (STIP1)-a co-chaperone of heat shock proteins-promotes cell proliferation and may act as an oncogenic factor. Similarly, glycogen synthase kinase-3 beta (GSK3β)-mediated phosphorylation of lysine-specific demethylase 1 (LSD1)-an epigenetic regulator-can contribute to the development of an aggressive cell phenotype. Owing to their ability to tether different molecules into functional complexes, scaffold proteins have a key role in the regulation of different signaling pathways in tumorigenesis. Here, we show that STIP1 acts as a scaffold promoting the interaction between LSD1 and GSK3β. Specifically, the TPR1 and TPR2B domains of STIP1 are capable of binding with the AOL domain of LSD1, whereas the TPR2A and TPR2B domains of STIP1 interact with the kinase domain of GSK3β. We also demonstrate that STIP1 is required for GSK3β-mediated LSD1 phosphorylation, which promoted LSD1 stability and enhanced cell proliferation. After transfection of cancer cells with double-mutant (S707A/S711A) LSD1, subcellular localization analysis revealed that LSD1 was translocated from the nucleus to the cytoplasm. In vitro experiments also showed that the LSD1 inhibitor SP2509 and the GSK3β inhibitor LY2090314 acted synergistically to induce cancer cell death. Finally, the immunohistochemical expression of STIP1 and LSD1 showed a positively correlation in human cancer specimens. In summary, our data provide mechanistic insights into the role of STIP1 in human tumorigenesis by showing that it serves as a scaffold for GSK3β-mediated LSD1 phosphorylation. The combination of LSD1 and GSK3β inhibitors may exert synergistic antitumor effects and deserves further scrutiny in preclinical studies.

Protein Misfolding in Prion and Prion-Like Diseases: Reconsidering a Required Role for Protein Loss-of-Function

Prion disease research has contributed much toward understanding other neurodegenerative diseases, including recent demonstrations that Alzheimer's disease (AD) and other neurodegenerative diseases are prion-like. Prion-like diseases involve the spread of degeneration between individuals and/or among cells or tissues via template directed misfolding, wherein misfolded protein conformers propagate disease by causing normal proteins to misfold. Here we use the premise that AD, amyotrophic lateral sclerosis, Huntington's disease, and other similar diseases are prion-like and ask: Can we apply knowledge gained from studies of these prion-like diseases to resolve debates about classical prion diseases? We focus on controversies about what role(s) protein loss-of-function might have in prion diseases because this has therapeutic implications, including for AD. We examine which loss-of-function events are recognizable in prion-like diseases by considering the normal functions of the proteins before their misfolding and aggregation. We then delineate scenarios wherein gain-of-function and/or loss-of-function would be necessary or sufficient for neurodegeneration. We consider roles of PrPC loss-of-function in prion diseases and in AD, and conclude that the conventional wisdom that prion diseases are 'toxic gain-of-function diseases' has limitations. While prion diseases certainly have required gain-of-function components, we propose that disease phenotypes are predominantly caused by deficits in the normal physiology of PrPC and its interaction partners as PrPC converts to PrPSc. In this model, gain-of-function serves mainly to spread disease, and loss-of-function directly mediates neuron dysfunction. We propose experiments and predictions to assess our conclusion. Further study on the normal physiological roles of these key proteins is warranted.

Stress-induced phosphoprotein-1 maintains the stability of JAK2 in cancer cells

Overexpression of stress-induced phosphoprotein 1 (STIP1) − a co-chaperone of heat shock protein (HSP) 70/HSP90 – and activation of the JAK2-STAT3 pathway occur in several tumors. Combined treatment with a HSP90 inhibitor and a JAK2 inhibitor exert synergistic anti-cancer effects. Here, we show that STIP1 stabilizes JAK2 protein in ovarian and endometrial cancer cells. Knock-down of endogenous STIP1 decreased JAK2 and phospho-STAT3 protein levels. The N-terminal fragment of STIP1 interacts with the N-terminus of JAK2, whereas the C-terminal DP2 domain of STIP1 mediates the interaction with HSP90 and STAT3. A peptide fragment in the DP2 domain of STIP1 (peptide 520) disrupted the interaction between STIP1 and HSP90 and induced cell death through JAK2 suppression. In an animal model, treatment with peptide 520 inhibited tumor growth. In summary, STIP1 modulates the function of the HSP90-JAK2-STAT3 complex. Peptide 520 may have therapeutic potential in the treatment of JAK2-overexpressing tumors.

Imbalances in the Hsp90 Chaperone Machinery: Implications for Tauopathies

The ATP-dependent 90 kDa heat shock protein, Hsp90, is a major regulator of protein triage, from assisting in nascent protein folding to refolding or degrading aberrant proteins. Tau, a microtubule associated protein, aberrantly accumulates in Alzheimer's disease (AD) and other neurodegenerative diseases, deemed tauopathies. Hsp90 binds to and regulates tau fate in coordination with a diverse group of co-chaperones. Imbalances in chaperone levels and activity, as found in the aging brain, can contribute to disease onset and progression. For example, the levels of the Hsp90 co-chaperone, FK506-binding protein 51 kDa (FKBP51), progressively increase with age. In vitro and in vivo tau models demonstrated that FKBP51 synergizes with Hsp90 to increase neurotoxic tau oligomer production. Inversely, protein phosphatase 5 (PP5), which dephosphorylates tau to restore microtubule-binding function, is repressed with aging and activity is further repressed in AD. Similarly, levels of cyclophilin 40 (CyP40) are reduced in the aged brain and further repressed in AD. Interestingly, CyP40 was shown to breakup tau aggregates in vitro and prevent tau-induced neurotoxicity in vivo. Moreover, the only known stimulator of Hsp90 ATPase activity, Aha1, increases tau aggregation and toxicity. While the levels of Aha1 are not significantly altered with aging, increased levels have been found in AD brains. Overall, these changes in the Hsp90 heterocomplex could drive tau deposition and neurotoxicity. While the relationship of tau and Hsp90 in coordination with these co-chaperones is still under investigation, it is clear that imbalances in these proteins with aging can contribute to disease onset and progression. This review highlights the current understanding of how the Hsp90 family of molecular chaperones regulates tau or other misfolded proteins in neurodegenerative diseases with a particular emphasis on the impact of aging.

Functions of the Prion Protein

Although initially disregarded compared to prion pathogenesis, the functions exerted by the cellular prion protein PrP^C have gained much interest over the past two decades. Research aiming at unraveling PrP^C functions started to intensify when it became appreciated that it would give clues as to how it is subverted in the context of prion infection and, more recently, in the context of Alzheimer's disease. It must now be admitted that PrP^C is implicated in an incredible variety of biological processes, including neuronal homeostasis, stem cell fate, protection against stress, or cell adhesion. It appears that these diverse roles can all be fulfilled through the involvement of PrP^C in cell signaling events. Our aim here is to provide an overview of our current understanding of PrP^C functions from the animal to the molecular scale and to highlight some of the remaining gaps that should be addressed in future research.

Cholinergic Regulation of hnRNPA2/B1 Translation by M1 Muscarinic Receptors

Cholinergic vulnerability, characterized by loss of acetylcholine (ACh), is one of the hallmarks of Alzheimer's disease (AD). Previous work has suggested that decreased ACh activity in AD may contribute to pathological changes through global alterations in alternative splicing. This occurs, at least partially, via the regulation of the expression of a critical protein family in RNA processing, heterogeneous nuclear ribonucleoprotein (hnRNP) A/B proteins. These proteins regulate several steps of RNA metabolism, including alternative splicing, RNA trafficking, miRNA export, and gene expression, providing multilevel surveillance in RNA functions. To investigate the mechanism by which cholinergic tone regulates hnRNPA2/B1 expression, we used a combination of genetic mouse models and in vivo and in vitro techniques. Decreasing cholinergic tone reduced levels of hnRNPA2/B1, whereas increasing cholinergic signaling in vivo increased expression of hnRNPA2/B1. This effect was not due to decreased hnRNPA2/B1 mRNA expression, increased aggregation, or degradation of the protein, but rather to decreased mRNA translation by nonsense-mediated decay regulation of translation. Cell culture and knock-out mice experiments demonstrated that M1 muscarinic signaling is critical for cholinergic control of hnRNPA2/B1 protein levels. Our experiments suggest an intricate regulation of hnRNPA2/B1 levels by cholinergic activity that interferes with alternative splicing in targeted neurons mimicking deficits found in AD.

SIGNIFICANCE STATEMENT

In Alzheimer's disease, degeneration of basal forebrain cholinergic neurons is an early event. These neurons communicate with target cells and regulate their long-term activity by poorly understood mechanisms. Recently, the splicing factor hnRNPA2/B, which is decreased in Alzheimer's disease, was implicated as a potential mediator of long-term cholinergic regulation. Here, we demonstrate a mechanism by which cholinergic signaling controls the translation of hnRNPA2/B1 mRNA by activation of M1 muscarinic type receptors. Loss of cholinergic activity can have profound effects in target cells by modulating hnRNPA2/B1 levels.