Heat shock protein 70 selectively mediates the degradation of cytosolic PrPs and restores the cytosolic PrP-induced cytotoxicity via a molecular interaction
© Zhang et al.; licensee BioMed Central Ltd. 2012
Received: 27 February 2012
Accepted: 29 November 2012
Published: 6 December 2012
Although the aggregation of PrPSc is thought to be crucial for the neuropathology of prion diseases, there is evidence in cultured cells and transgenic mice that neuronal death can be triggered by the accumulation of cytosolic PrPs, leading to the hypothesis that the accumulation of PrPs in the cytosol of neurons may be a primary neurotoxic culprit. Hsp70, a molecular chaperone involved in protein folding/refolding and degradation in the cytoplasm, has a protective effect in some models of neurodegenerative diseases, e.g., Alzheimer’s and Parkinson’s diseases, but its role in prion diseases remains unclear.
To study the role of Hsp70 in prion diseases, we used immunoprecipitation to first identify a molecular interaction between Hsp70 and PrPs. Using immunofluorescence, we found that Hsp70 colocalized with cytosolic PrPs in HEK293 cells transiently transfected with plasmids for Cyto-PrP and PG14-PrP but not with wild-type PG5-PrP or endoplasmic reticulum (ER)-retained PrPs (3AV-PrP and ER-PrP). Using western blot analysis and apoptosis assays of cultured cells, we found that the overexpression of Hsp70 by transfection or the activation of Hsp70 by geldanamycin selectively mediated the degradation of cytosolic PrPs and restored cytosolic PrP-induced cytotoxicity. Moreover, we found that Hsp70 levels were up-regulated in cells expressing Cyto-PrP and in hamster brains infected with the scrapie agent 263K.
These data imply that Hsp70 has central role in the metabolism of cytosolic PrPs
Human prion diseases include Creutzfeldt-Jakob disease (CJD), kuru, fatal familial insomnia (FFI) and Gerstmann-Straussler-Scheinker syndrome (GSS). The conversion of the normal cellular prion protein (PrPC) into an insoluble and protease-resistant isoform (PrPSc), which propagates itself by imposing its abnormal conformation onto a PrPC, is fundamental to the pathogenesis of prion diseases. It has been commonly assumed that aggregation of misfolded PrPSc is the cause of neurodegeneration in prion diseases . However, in several studies, significant pathology and clinical dysfunctions can develop even when there is little accumulation of PrPSc. Similarly, the accumulation and a high abundance of PrPSc has been accompanied by limited symptomatology . These data suggest that PrPSc may not be the major cause of neuronal dysfunction in prion diseases.
Studies of mutant PrP molecules associated with inherited prion diseases support the hypothesis that PrPs that are mislocalized to the cytoplasm acquire neurotoxic potential . PG14-PrP, a PrP mutant with an extra nine-octapeptide insertion, is associated with a familial prion disease in humans, can form numerous aggregates in neutrophils, is highly neurotoxic and promotes progressive neurodegenerative disease in transgenic Tg (PG14) mice . Mice expressing a PrP mutant lacking the N-terminal ER targeting signal (Cyto-PrP) acquired severe ataxia due to the rapid degeneration of cerebellar granule neurons . 3AV-PrP is an artificial PrP mutant with three alanine to valine mutations in the hydrophobic domain. 3AV-PrP substantially increases the hydrophobicity, remains intracellular at either the ER or Golgi, and causes neurodegeneration in transgenic mice . Expediting the removal of these mislocalized PrPs may be a relevant therapeutic strategy to reduce neurotoxicity.
Heat shock proteins (HSPs) belong to the family of chaperones and have important roles in regulating the refolding and degradation of misfolded/mislocalized proteins [7, 8]. Hsp70 is the most studied member of the cytoplasmic HSP family. Recent studies in several neurodegeneration models, including Alzheimer’s disease (AD), Parkinson’s disease (PD) and Huntington’s disease (HD), suggest that the overexpression of Hsp70 can be protective. In various cellular models for AD, increasing the levels of Hsp70 inhibited the propensity of Aβ to aggregate and reduced the toxicity of Aβ. Hsp70 also promoted the solubility of tau and its binding to microtubules . The aggregation of α-synuclein is a hallmark of sporadic and familial PD. The overexpression of Hsp70 prevented neuronal loss by inhibiting α-synuclein fibril formation . In a yeast model for HD, the expression of Hsp70 reduced the toxicity associated with the expression of mutant huntingtin (htt) by preventing its aberrant interaction with an essential polyQ-containing transcription factor . It remains unclear whether Hsp70 also plays a role in prion diseases.
Hsp70 is one of the most potent molecular chaperones. In this study, we present evidence that Hsp70 is able to directly interact with PrPs. Colocalization studies indicate that the cytosolic forms of mutant PrPs are the targets of Hsp70. Furthermore, we found that the overexpression or activation of Hsp70 in cultured cells selectively mediated the degradation of cytosolic PrPs and contributed to the protective effect against cytotoxicity induced by cytosolic PrPs. These data implicate Hsp70 as a central regulator in the metabolism of cytosolic PrPs.
Materials and methods
The use of animal specimens in this study was approved by the Ethical Committee of the National Institute for Viral Disease Prevention and Control, China CDC under protocol 2009ZX10004-101. All Chinese golden hamsters were maintained under clean conditions. Housing and experimental protocols were performed in accordance with the Chinese Regulations for the Administration of Affairs Concerning Experimental Animals.
Plasmids and reagents
Mammalian expression plasmids for human wild-type PrP (PG5-PrP), Cyto-PrP, CtmPrP (3AV-PrP), 9 extra octarepeats inserted PrP (PG14-PrP) and PrP-KDEL (ER-PrP) were constructed in the vector pcDNA3.1+ as described elsewhere . The plasmid pEGFP-Hsp70 was obtained from Addgene (Cambridge, MA) . The antibody to PrP (3F4) was from Millipore (Billerica, MA). Antibodies to β-actin (sc-47778) and Hsp70 (sc-1060) were from Santa Cruz (Santa Cruz, CA). Antibody to Calnexin (208880) was from Merck Millipore (Billerica, MA). The HRP-conjugated anti-goat and anti-mouse IgG were from Boehringer (Germany). The Alexa Fluor 568 goat anti-mouse IgG was from Invitrogen (USA). Geldanamycin (cat# 345805) and MG132 (474790) were purchased from Calbiochem (San Diego, CA).
Cell lines, including human embryonic kidney (HEK) 293 (CRL-1573) cells and mouse neuroblastoma Neuro-2a (CCL-131) cells, were maintained in Dulbecco’s modified Eagle’s culture medium supplemented with 10% fetal calf serum. The various recombinant plasmids were transiently transfected into HEK 293 or N2a cells using Lipofectamine 2000 (Invitrogen).
RNA interference was carried out using siRNA purchased from Invitrogen. The siRNA corresponding to the Hsp70 mRNA sequences 5’-AGG ACG AGU UUG AGC ACA ATT-3’ was used to inhibit endogenous Hsp70 protein expression. The negative control siRNA has no relation with human gene (sense siRNA3, 5’-UUC UCC GAA CGU GUC ACG UTT-3’ and antisense siRNA3, 5’-ACG UGA CAC GUU CGG AGA ATT-3’). Various siRNAs were transfected into HEK 293 cells using Lipofectamine 2000 (Invitrogen, Carlsbad, CA). Cells were collected for further experiments 72 h after transfection.
Preparation of hamster brain homogenates and cell lysates
Hamster brain tissues were homogenized in cold lysis buffer (1% Nonidet P-40, 50 mM HEPES, pH 7.6, 10% glycerol, 1 mM EDTA, 20 mM β-glycerophosphate, 1 mM sodium orthovanadate, 1 mM sodium metabisulfite, 1 mM benzamidine hydrochloride, 10 μg/ml leupeptin, 20 μg/ml aprotinin and 1 mM phenylmethylsulfonyl fluoride), followed by sonication . Cultured cells treated with different transfections or reagents were harvested, and the whole cell lysates were prepared in cold lysis buffer with sonication. After centrifugation at 10,000 × g at 4°C for 10 min, the supernatants were collected for further experiments.
Cellular lysates or brain homogenates were separated by 12% SDS-PAGE and electro-transferred onto nitrocellulose membranes. After blocking with 5% nonfat-dried milk in PBST (phosphate buffered saline, pH 7.6, containing 0.05% Tween-20), the membranes were incubated with a PrP-specific monoclonal antibody (mAb) 3F4 (Millipore), diluted 1:4000; a mAb to Hsp70, diluted 1:2000; or a mAb to human β-actin (Santa Cruz), diluted 1:2000. Subsequently, the membranes were incubated with HRP-conjugated anti-mouse IgG or anti-goat IgG, diluted 1:10,000. The reactive signals were visualized by ECL (PE Applied Biosystems, Foster City, USA). Immunoblots were quantified using a scanning densitometer in conjunction with the NIH ImageJ software. The signals were normalized to the loading controls.
Immunoprecipitations (IPs) were carried out using whole cell lysates (400 μg of total protein), 2–4 μg of antibody and 20 μl of Dynabeads®-coated Protein G (Invitrogen). The cell lysates were mixed with different antibodies at 4°C for 3–4 h and subsequently incubated with Dynabeads®-coated Protein G for another 2 h. The immunocomplexes were collected by a short spin and washed five times in wash buffer before being resolved by SDS-PAGE. The complexes were detected by Western blotting.
The situations of the mitochondrial transmembrane transition in live cells were determined with a MitoCaptureTM Mitochondrial Apoptosis Detection kit, BioVision (Plymouth meeting, PA, USA) according to the manufacturer’s instruction. Growing cells (approximately 60% confluence) were transiently transfected with various recombinant plasmids. The cells were collected 48 h after the transfection, resuspended in 1 ml of diluted MitoCapture solution, and then incubated at 37°C for 20 min. After a short spin, the cells were resuspended in 1 ml of pre-warmed incubation buffer. The cells were mounted onto glass coverslips and examined using fluorescence microscopy (Olympus BX51, Japan). The total number of cells in a field of view were counted using ImageJ. Apoptotic and healthy cells displayed as green and red, respectively. The percentage of apoptotic cells was defined as the number of green cells vs. the total number of cells.
The cells were fixed with formaldehyde (4% paraformaldehyde, freshly depolymerized in 0.1 M sodium phosphate buffer, pH 7.4) at RT for 15 min and washed three times in PBS. The cells were blocked with blocking buffer (PBS with 5% FBS and 0.1% Triton X-100) at RT for 1 h and then incubated with a 1:200 dilution of anti-PrP mAb in PBS with 2% BSA and 0.3% Triton X-100 at 4°C overnight. The cells were then washed and incubated with a 1:200 dilution of the appropriate secondary antibody (Alexa Fluor® 568 Goat anti-Mouse, Invitrogen) at RT for 2 h. After washing, the cells were incubated with 0.5 mg/ml DAPI (Invitrogen) at RT for 2 min. The cells were sealed, and images of the targeting proteins were analyzed using confocal microscopy (Leica ST2, Germany).
Extract subcellular proteomes
Cells were fractionated into cytosol, membranes/organelle mitochondria, nuclei, and cytoskeleton using Subcellular ProteoExtract kit from Merck Millipore according to the manufacturer's directions.
In this study, all of the experiments were conducted at least 3 times with consistent results. The mean value and standard error of multiple data points or samples were used to represent the final result. Student’s t test was used for statistical analysis and a P < 0.05 was considered significant.
The identification of cytosolic PrPs as a target for Hsp70
Normally, PrPC is predominantly localized to the cell surface, but some PrP mutants are retained and aggregate in the cytoplasm. To determine if Hsp70 colocalizes with different PrP mutants in the cytoplasm, we used immunofluorescence to examine HEK293 cells transfected with plasmids containing wild-type or various mutant PrP proteins. By confocal microscopy (Figure 1C), most of the wild-type PrPs (PG5-PrP) localized to the cell surface; ER-retained PrP mutants (3AV-PrP and ER-PrP) formed large plaque-like structures with strong signal intensities in the cytoplasm, while Cyto-PrP and PG14-PrP were found in numerous round particles in the cytoplasm. Interestingly, the distribution of Hsp70 in the cytoplasm was obviously influenced by the different PrP constructs. In the cells expressing PG5-PrP, as well as 3AV-PrP and ER-PrP, Hsp70 was dispersed in the cytoplasm and did not significantly colocalize with the 3AV-PrP or ER-PrP-formed deposits. However, in the cells expressing Cyto-PrP or PG14-PrP, Hsp70 colocalized well with the PrP-specific particles in the cytoplasm. The degree of colocalization between Hsp70 and PrP increased in the cells expressing Cyto-PrP. (Figure 1C). To see the distributions of different PrP constructs in various subcelluar fractions, the cytosolic and membrane/organelle protein fractions were separately extracted with ProteoExtract® Subcellular Proteome Extraction Kit (S-PEK). It showed that the majorities of Cyto-PrP and PG14-PrP presented in the cytosolic fraction, whereas PG5-PrP, 3AV-PrP and ER-PrP were mainly detectable in the membrane/organelle protein fraction (Figure 1D). As expected, cellular Hsp70 was observed in the cytosolic fraction. Subsequent immunoprecipitation experiments of the cytosolic fractions of various PrP preparations confirmed the associations between Hsp70 and cytosolic PrP (Cyto-PrP and PG14-PrP), but not in those of other PrP constructs (PG5-, 3AV- and ER-PrP) (Figure 1E).
Hsp70 selectively reduced the levels of the cytosolic forms of the PrP mutants
Cytosolic PrP degradation mediated by Hsp70 is dependent on ubiquitination and proteasome
Chaperone proteins-induced misfolded/mislocalized proteins reduction is often a result of ubiquitination-mediated protein degradation. To address whether the Hsp70-mediated degradations of the cytosolic forms of PrP proteins underwent above pathway, the cell lysates of Cyto-PrP and PG14-PrP were subjected to the subsequent tests for evaluating the modifications of ubiquitination and proteasome. Western blots revealed that in the presence of GA, both Cyto-PrP and PG14-PrP were clearly down-regulated, while co-treatment of the cells with MG132, a proteasome inhibitor, was capable of reversing the GA-mediated reductions of Cyto-PrP and PG14-PrP partially (Figure 3B). To see the ubiquitination status of different PrP proteins, a recombinant plasmid expressing HA-tagged ubiquitin (Ub) was transfected into HEK293 cells together with various PrP expressing plasmids. Immunoprecipitation assays using PrP mAb 3F4 as the capturing antibody and ubiquitin pAb as the detecting one showed that in the presence of GA, the signals of the ubiquitinated proteins maintained almost unchanged in the cells expressing wild-type PrP (PG5-PrP) compared to that without GA treatment, while those in the cells expressing Cyto-PrP or PG14-PrP increased markedly, especially in the expected regions of the expressing PrPs (Figure 3C). The data suggest that the derogations of the cytosolic PrPs are mediated by Hsp70 in proteasome after modification by ubiquitination.
GA selectively reversed apoptosis induced by the cytosolic forms of PrP mutants
Hsp70 proteins are up-regulated in Cyto-PrP-expressing cells and in the brains of scrapie-infected hamsters
In the current study, we have demonstrated a molecular interaction between human PrP and Hsp70. Colocalization analyses show that cellular Hsp70 exactly overlaps with the cytosolic forms of PrPs in cells expressing Cyto-PrP and PG14-PrP but not wild-type PG5-PrP or ER-retained PrPs, including 3AV-PrP and ER-PrP. Furthermore, we demonstrate that activation of cellular Hsp70, by overexpression of recombinant Hsp70 or pharmacological treatment with geldanamycin, selectively promoted the degradation of and restored the relevant cytotoxicity of the cytosolic PrPs but not wild-type or ER-retained PrPs in cultured cells. These data indicate that cytosolic PrPs are an authentic substrate of Hsp70.
Hsp70 is a potent molecular chaperone that aids in the folding and refolding of nascent proteins and denatured proteins, respectively. More importantly, when a protein cannot be correctly renatured, Hsp70 can mediate degradation of the protein [17, 18]. Misfolded/mislocalized proteins in the cytoplasm are degraded via the ubiquitin-proteasome pathway. It has been repeatedly reported that the overexpression of Hsp70 facilitates the degradation of α-synuclein , a risk factor for PD, and eliminates the hyperphosphorylated microtubule-associated protein tau (tau) , a primary pathological component of AD. Therefore, activating Hsp70 is one potential strategy and therapeutic target for treating AD and PD. Similar to the abnormal protein deposits found in the brains of patients with AD and PD, the accumulation of PrPSc in the brain is a predominant neuropathological feature of prion diseases. However, numerous evidence has suggested that PrPSc is not toxic; however, based on experiments in cultured cells and transgenic mice, abnormal PrPs, e.g., Cyto-PrP  or PrPs mutated in the transmembrane region  or in the octarepeat region , have been proposed to be neurotoxic. These data highlight the importance of clearing continuously produced misfolded PrPs that might be intermediates of PrPSc. Hsp70 may work as an efficient host factor to clear toxic misfolded or mislocalized PrPs in the cytoplasm.
Our results indicate that the accumulation of cytosolic PrPs increases the levels of cellular Hsp70, which may reflect a triggered mechanism for cell defense. Additionally, our study shows that Hsp70 is significantly activated in the brains of scrapie-infected hamsters, which is consistent with what has been observed in the brain tissues of CJD patients and in animal models of TSE . Although PrPSc is more protease-resistant and cannot be degraded via the Hsp70-mediated ubiquitin-proteasome dependent pathway, several studies have demonstrated that PrPSc can translocate into the cytosol of prion-infected cells and induce toxicity [20, 21]. Up-regulating Hsp70 in cells would help clear out the penetrated PrPSc or newly formed prion intermediates.
Hsp70 is usually present in the cytoplasm and mediates the efficient modification of misfolded/mislocalized proteins in the cytoplasm. According to this study, this function is highly location specific because Hsp70 did not affect PrP proteins that retain or accumulate in ER. Correlated with the induced degradation of cytosolic PrPs, the viability of cells expressing the cytosolic PrP mutants is markedly improved. Recently, Fernandez-Funez et al. have described that the wild-type PrP can spontaneously convert into an insoluble protease-sensitive isoform in neurons of the transgenic fly brain expressing wild-type hamaster PrP, meanwhile, over-expression of Hsp70 is capable of preventing the accumulation of PrPSc-like conformers and reducing the neurotoxicity, which suggest that Hsp70 may be a therapeutic candidate for prion diseases . However, the complexes of Hsp70 and PrP is detectable in the fraction of membrane microdomains of aged flies’ brains, which seems to be distinct from the data in this study. The exact reason for this discrepancy remains unknown. Hsp70 usually presents in the cytoplasm of mammalian cells . Similarly, Hsp70 is mainly observed in the cytoplasm in the young transgenic PrP flies, but detected also in the microsome of the old flies. Probably due to the wide distribution of transgenic PrP protein in the membrane microdomains in the flies, it eventually forms a stress along with the spontaneous conversion. In that situation, Hsp70 moves across the membranous structures and into organelles. In fact, the stress-caused shift of Hsp70 from cytoplasm to membrane has been already described. In our experimental condition, the PrP constructs located mainly in membrane and organelle (ER) are transiently expressed, which may be too short to induce such “stress”. That might explain why no complex of PrP-Hsp70 is detected in the fraction of membrane/organelles. Additionally, our data also illustrate an increased Hsp70 level in the brains of scrapie strain 263K-infected hamsters. Further assays of co-localization of PrP and Hsp70 in subcellular organelles, as well as in different fractions of brain homogenates will supply more solid conclusion. Moreover, evidence suggests that the conversion of Hsp70’s function in protein-folding to its function as a degradation factor is mediated by the co-chaperone CHIP . A range of cellular agents, e.g., glucocorticoid receptor , ErbB2  and tau , are known substrates of CHIP. Additionally, DnaJ (Hsp40) is thought to link the Hsp70 chaperone machine to the ubiquitin-proteasome system . The potential contributions of CHIP and Hsp40 to the Hsp70-mediated protection against PrP-neurotoxicity will be a valuable research topic.
We deeply thank Ms. Huiyin Jiang for her excellent technical support. This work was supported by the Chinese National Natural Science Foundation Grant (81100980, 81101302 and 31100117), the National Basic Research Program of China (973 Program) (2007CB310505) and the SKLID (State Key Laboratory for Infectious Disease Prevention and Control) Development Grant (2008SKLID102, 2011SKLID204 and 2011SKLID211).
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