Endothelial PERK-ATF4-JAG1 axis activated by T-ALL remodels bone marrow vascular niche.

The endoplasmic reticulum unfolded protein response (UPR) is a conserved adaptive signaling in ER homeostasis and has emerged as critical in highly proliferating cells and potential treatment target for acute T-cell lymphoblastic leukemia (T-ALL). Methods: in this study, we assessed the transcriptomic and phenotypic alterations in UPR response of the bone marrow endothelial cells (ECs) in mice engrafted with T-ALL and in bone marrow specimens from patients who have T-ALL. We used PERK inhibitor and generated endothelial specific PERK knockout mice to study the impact of PERK on leukemia progression and hematopoiesis. We performed chromatin immunoprecipitation (ChIP) to study the mechanistic regulation of JAG1 by ATF4. We characterized small extracellular vesicles (SEV) from leukemia-developing mice and studied the effect of SEVs on EC function. Results: we found that T-ALL development induced a robust activation of protein kinase RNA-like endoplasmic reticulum kinase (PERK)-dominant UPR in the bone marrow endothelial vascular niche. The activation of PERK-eIF2a-ATF4 axis remodels the vascular niche, upregulates angiogenic factors including VEGFα and ATF4-regulated JAG1, and suppresses the expression of SCF and CXCL12, which are important to HSC maintenance and regeneration. Further, targeting endothelial PERK significantly improved T-ALL outcome. EC-specific deletion of PERK abolished the aberrant JAG1 up-regulation, improved HSC maintenance, promoted leukemia apoptosis, and improved overall survival. Finally, we showed that small extracellular vesicles are critical mediators of endothelial PERK-eIF2a-ATF4 activation and JAG1 up-regulation in leukemia. Corroborating animal model studies, activation of PERK-ATF4-JAG1 is prominent in human T-ALL bone marrow and T-ALL xenografts. Conclusion: our studies thus revealed for the first time that the leukemia-initiated PERK-ATF4-JAG1 axis plays a critical role in the remodeling of the bone marrow vascular niche and that targeting vascular niche UPR is a potential therapeutic opportunity in T-ALL.

Targeting CRABP-II overcomes pancreatic cancer drug resistance by reversing lipid raft cholesterol accumulation and AKT survival signaling.

BACKGROUND: Resistance to standard therapy is a major reason for the poor prognosis of pancreatic ductal adenocarcinoma (PDAC). Developing novel therapy to overcome PDAC drug-resistance is urgently needed. CRABP-II was highly expressed in all PDAC but not expressed in normal pancreatic tissues and chronic pancreatitis. CRABP-II was shown to promote PDAC migration and metastasis while its potential role in promoting PDAC drug-resistance was not known. METHODS: A paired cohort of human primary and relapsing PDAC tissues was assessed for CRABP-II expression by immunohistochemistry. CRISPR/cas9 gene editing was used to establish CRABP-II knockout cell lines and MTT assays were performed to assess gemcitabine sensitivity in vitro. Cleaved caspase-3/PARP blots and Annexin V staining were conducted to detect cell apoptosis. Gene expression microarray, Q-PCR, western blots, Co-IP and RNA-IP were used to study the molecular function of CRABP-II. Sucrose gradient ultracentrifugation was applied to isolate lipid rafts and LC-MS-MS was used to assess cholesterol content. Both subcutaneous CDX models and orthotopic PDX models were established to examine the efficacy of SNIPER-11 and the synergistic effect between SNIPER-11 and gemcitabine in vivo. RESULTS: A higher expression of CRABP-II was found in relapsing PDAC tissue and was associated with poor prognosis. Gemcitabine-resistant cell lines exhibited increased level of CRABP-II, while CRABP-II knockout resensitized PDAC cells to gemcitabine. Mechanistically, aberrant expression of CRABP-II increased the stability of SREBP-1c mRNA through cooperation with HuR and upregulated the downstream genes of SREBP-1c to favor cholesterol uptake and accumulation in lipid rafts. Increased lipid raft cholesterol accumulation facilitated ATK survival signaling and PDAC drug resistance. The small compound SNIPER-11 treatment effectively induced CRABP-II protein degradation, induced apoptosis, and suppressed tumor growth. Combination of SNIPER-11 and gemcitabine significantly reduced the lipid raft cholesterol content in CDX/PDX and profoundly inhibited tumor progression. CONCLUSIONS: These findings identified CRABP-II as a novel regulator of cholesterol metabolism and suggested that CRABP-II is a selective target for overcoming PDAC drug resistance.

Notch-Regulated Dendritic Cells Restrain Inflammation-Associated Colorectal Carcinogenesis.

Conventional dendritic cells (cDC) play a central role in T-cell antitumor responses. We studied the significance of Notch-regulated DC immune responses in a mouse model of colitis-associated colorectal cancer in which there is epithelial downregulation of Notch/Hes1 signaling. This defect phenocopies that caused by GMDS (GDP-mannose 4,6-dehydratase) mutation in human colorectal cancers. We found that, although wild-type immune cells restrained dysplasia progression and decreased the incidence of adenocarcinoma in chimeric mice, the immune system with Notch2 deleted in all blood lineages or in only DCs promoted inflammation-associated transformation. Notch2 signaling deficiency not only impaired cDC terminal differentiation, but also downregulated CCR7 expression, reduced DC migration, and suppressed antigen cross-presentation to CD8+ T cells. Transfer of Notch-primed DCs restrained inflammation-associated dysplasia progression. Consistent with the mouse data, we observed a correlation between infiltrating cDC1 and Notch2 signaling in human colorectal cancers and found that GMDS-mutant colorectal cancers showed decreased CCR7 expression and suppressed cDC1 signature gene expression. Suppressed cDC1 gene signature expression in human colorectal cancer was associated with a poor prognosis. In summary, our study supports an important role for Notch2 signaling in cDC1-mediated antitumor immunity and indicates that Notch2-controlled DCs restrain inflammation-associated colon cancer development in mice.

O-Fucose and Fringe-modified NOTCH1 extracellular domain fragments as decoys to release niche-lodged hematopoietic progenitor cells.

Successful hematopoietic progenitor cell (HPC) transplant therapy is improved by mobilizing HPCs from the bone marrow niche in donors. Notch receptor-ligand interactions are known to retain HPCs in the bone marrow, and neutralizing antibodies against Notch ligands, Jagged-1 or Delta-like ligand (DLL4), or NOTCH2 receptor potentiates HPC mobilization. Notch-ligand interactions are dependent on posttranslational modification of Notch receptors with O-fucose and are modulated by Fringe-mediated extension of O-fucose moieties. We previously reported that O-fucosylglycans on Notch are required for Notch receptor-ligand engagement controlling hematopoietic stem cell quiescence and retention in the marrow niche. Here, we generated recombinant fragments of NOTCH1 or NOTCH2 extracellular domain carrying the core ligand-binding regions (EGF11-13) either as unmodified forms or as O-fucosylglycan-modified forms. We found that the addition of O-fucose monosaccharide or the Fringe-extended forms of O-fucose to EGF11-13 showed substantial increases in binding to DLL4. Furthermore, the O-fucose and Fringe-extended NOTCH1 EGF11-13 protein displayed much stronger binding to DLL4 than the NOTCH2 counterpart. When assessed in an in vitro 3D osteoblastic niche model, we showed that the Fringe-extended NOTCH1 EGF11-13 fragment effectively released lodged HPC cells with a higher potency than the NOTCH2 blocking antibody. We concluded that O-fucose and Fringe-modified NOTCH1 EGF11-13 protein can be utilized as effective decoys for stem cell niche localized ligands to potentiate HPC egress and improve HPC collection for hematopoietic cell therapy.

CRABP-II enhances pancreatic cancer cell migration and invasion by stabilizing interleukin 8 expression.

Our previous study shows that cellular retinoic acid binding protein II (CRABP-II) is overexpressed in pancreatic ductal adenocarcinoma (PDAC) and pre-cancerous lesions, but not detected in normal pancreatic tissues. In this study, we show that deletion of CRABP-II in PDAC cells by CRISPR/Cas9 does not affect cancer cell proliferation, but decreases cell migration and invasion. Gene expression microarray analysis reveals that IL-8 is one of the top genes whose expression is down-regulated upon CRABP-II deletion, while expression of MMP-2 and MMP-14, two targets of IL-8 are also significantly down-regulated. Moreover, we found that CRABP-II is able to form a complex with HuR, which binds to the 3'UTR of IL-8 messenger RNA (mRNA) and enhances IL-8 mRNA stability. Ectopic expression of flag-CRABP-II in CRABP-II knockout cells is able to rescue the expression of IL-8, MMP-2/MMP-14 and recovers cell migration. Using the orthotopic xenograft model, we further demonstrate that CRABP-II deletion impairs tumor metastasis to nearby lymph nodes. Taken together, our results reveal a novel pathway linking CRABP-II expression to enhanced PDAC metastasis, and hence we propose CRABP-II may serve as a new PDAC therapeutic target.

Notch2 blockade enhances hematopoietic stem cell mobilization and homing.

Despite use of newer approaches, some patients being considered for autologous hematopoietic cell transplantation (HCT) may only mobilize limited numbers of hematopoietic progenitor cells (HPCs) into blood, precluding use of the procedure, or being placed at increased risk of complications due to slow hematopoietic reconstitution. Developing more efficacious HPC mobilization regimens and strategies may enhance the mobilization process and improve patient outcome. Although Notch signaling is not essential for homeostasis of adult hematopoietic stem cells (HSCs), Notch-ligand adhesive interaction maintains HSC quiescence and niche retention. Using Notch receptor blocking antibodies, we report that Notch2 blockade, but not Notch1 blockade, sensitizes hematopoietic stem cells and progenitors (HSPCs) to mobilization stimuli and leads to enhanced egress from marrow to the periphery. Notch2 blockade leads to transient myeloid progenitor expansion without affecting HSC homeostasis and self-renewal. We show that transient Notch2 blockade or Notch2-loss in mice lacking Notch2 receptor lead to decreased CXCR4 expression by HSC but increased cell cycling with CXCR4 transcription being directly regulated by the Notch transcriptional protein RBPJ. In addition, we found that Notch2-blocked or Notch2-deficient marrow HSPCs show an increased homing to the marrow, while mobilized Notch2-blocked, but not Notch2-deficient stem cells and progenitors, displayed a competitive repopulating advantage and enhanced hematopoietic reconstitution. These findings suggest that blocking Notch2 combined with the current clinical regimen may further enhance HPC mobilization and improve engraftment during HCT.

Cellular Prion Protein Mediates Pancreatic Cancer Cell Survival and Invasion through Association with and Enhanced Signaling of Notch1.

Up-regulation of human prion protein (PrP) in patients with pancreatic ductal adenocarcinoma (PDAC) is associated with a poor prognosis. However, the underlying molecular mechanism of PrP-mediated tumorigenesis is not completely understood. In this study, we found that PDAC cell lines can be divided into either PrP high expresser or PrP low expresser. In addition to filamin A (FLNA), PrP interacts with Notch1, forming a PrP/FLNA/Notch1 complex. Silencing PrP in high-expresser cells decreases Notch1 expression and Notch1 signaling. These cells exhibited decreased proliferation, xenograft growth, and tumor invasion but show increased tumor apoptosis. These phenotypes were rescued by ectopically expressed and activated Notch1. By contrast, overexpression of PrP in low expressers increases Notch1 expression and signaling, enhances proliferation, and increases tumor invasion and xenograft growth that can be blocked by a Notch inhibitor. Our data further suggest that PrP increases Notch1 stability likely through suppression of Notch proteosome degradation. Additionally, we found that targeting PrP combined with anti-Notch is much more effective than singularly targeted therapy in retarding PDAC growth. Finally, we show that coexpression of PrP and Notch1 confers an even poorer prognosis than PrP expression alone. Taken together, our results have unraveled a novel molecular pathway driven by interactions between PrP and Notch1 in the progression of PDAC, supporting a critical tumor-promoting role of Notch1 in PrP-expressing PDAC tumors.

Aberrant Notch Signaling in the Bone Marrow Microenvironment of Acute Lymphoid Leukemia Suppresses Osteoblast-Mediated Support of Hematopoietic Niche Function.

More than half of T-cell acute lymphoblastic leukemia (T-ALL) patients harbor gain-of-function mutations in the intracellular domain of Notch1. Diffuse infiltration of the bone marrow commonly occurs in T-ALL and relapsed B-cell acute lymphoblastic leukemia patients, and is associated with worse prognosis. However, the mechanism of leukemia outgrowth in the marrow and the resulting biologic impact on hematopoiesis are poorly understood. Here, we investigated targetable cellular and molecular abnormalities in leukemia marrow stroma responsible for the suppression of normal hematopoiesis using a T-ALL mouse model and human T-ALL xenografts. We found that actively proliferating leukemia cells inhibited normal hematopoietic stem and progenitor cell (HSPC) proliferation and homing to the perivascular region. In addition, leukemia development was accompanied by the suppression of the endosteum-lining osteoblast population. We further demonstrated that aberrant Notch activation in the stroma plays an important role in negatively regulating the expression of CXLC12 on osteoblasts and their differentiation. Notch blockade reversed attenuated HSPC cycling, leukemia-associated abnormal blood lineage distribution, and thrombocytopenia as well as recovered osteoblast and HSPC abundance and improved the hematopoietic-supportive functions of osteoblasts. Finally, we confirmed that reduced osteoblast frequency and enhanced Notch signaling were also features of the marrow stroma of human ALL tissues. Collectively, our findings suggest that therapeutically targeting the leukemia-infiltrated hematopoietic niche may restore HSPC homeostasis and improve the outcome of ALL patients.

Notch Receptor-Ligand Engagement Maintains Hematopoietic Stem Cell Quiescence and Niche Retention.

Notch is long recognized as a signaling molecule important for stem cell self-renewal and fate determination. Here, we reveal a novel adhesive role of Notch-ligand engagement in hematopoietic stem and progenitor cells (HSPCs). Using mice with conditional loss of O-fucosylglycans on Notch EGF-like repeats important for the binding of Notch ligands, we report that HSPCs with faulty ligand binding ability display enhanced cycling accompanied by increased egress from the marrow, a phenotype mainly attributed to their reduced adhesion to Notch ligand-expressing stromal cells and osteoblastic cells and their altered occupation in osteoblastic niches. Adhesion to Notch ligand-bearing osteoblastic or stromal cells inhibits wild type but not O-fucosylglycan-deficient HSPC cycling, independent of RBP-JK -mediated canonical Notch signaling. Furthermore, Notch-ligand neutralizing antibodies induce RBP-JK -independent HSPC egress and enhanced HSPC mobilization. We, therefore, conclude that Notch receptor-ligand engagement controls HSPC quiescence and retention in the marrow niche that is dependent on O-fucosylglycans on Notch.

CRABP-II is a highly sensitive and specific diagnostic molecular marker for pancreatic ductal adenocarcinoma in distinguishing from benign pancreatic conditions.

CRABP-II, a retinoic acid binding protein, shuffles retinoic acid from cytoplasm into nucleus and forms a complex with nuclear retinoic acid receptor to facilitate transcriptional activities of retinoic acid. In this study, we studied the expression patterns of CRABP-II in pancreatic ductal adenocarcinoma (PDAC) compared with those in normal pancreas, chronic pancreatitis, and precancerous lesions. We showed no detectable expressions of CRABP-II in normal pancreatic parenchyma, normal ductal epithelium, and chronic pancreatitis. In contrast, the expression of CRABP-II was readily detected in all PDACs including metastatic PDACs. CRABP-II staining was also observed and progressively increased from pancreatic intraepithelial neoplasia 1 to 3. In addition, when fine needle aspiration specimens were evaluated from patients with PDAC, CRABP-II was positive in 55.6% cases if cytology diagnosis was "atypia," and in 87.5% cases, if "malignancy." Our study suggests that CRABP-II is highly and specifically expressed in PDAC and is more commonly expressed in high-grade precursor cancerous lesions than in low-grade lesions. Therefore, overexpression of CRABP-II is a late event of pancreatic carcinogenesis, and it could be used as a diagnostic marker to distinguish PDAC from other benign pancreatic conditions in both resection and cytology specimens.

Transcript stabilization by the RNA-binding protein HuR is regulated by cellular retinoic acid-binding protein 2.

The RNA-binding protein HuR binds at 3' untranslated regions (UTRs) of target transcripts, thereby protecting them against degradation. We show that HuR directly interacts with cellular retinoic acid-binding protein 2 (CRABP2), a protein known to transport RA from the cytosol to the nuclear retinoic acid receptor (RAR). Association with CRABP2 dramatically increases the affinity of HuR toward target mRNAs and enhances the stability of such transcripts, including that of Apaf-1, the major protein in the apoptosome. We show further that its cooperation with HuR contributes to the ability of CRABP2 to suppress carcinoma cell proliferation. The data show that CRABP2 displays antioncogenic activities both by cooperating with RAR and by stabilizing antiproliferative HuR target transcripts. The observation that CRABP2 controls mRNA stabilization by HuR reveals that in parallel to participating in transcriptional regulation, the protein is closely involved in posttranscriptional regulation of gene expression.

Fatty acid-binding protein 5 (FABP5) regulates cognitive function both by decreasing anandamide levels and by activating the nuclear receptor peroxisome proliferator-activated receptor ß/δ (PPARß/δ) in the brain.

Endocannabinoids modulate multiple behaviors, including learning and memory. We show that the endocannabinoid anandamide (AEA) can alter neuronal cell function both through its established role in activation of the G-protein-coupled receptor CB1, and by serving as a precursor for a potent agonist of the nuclear receptor PPARß/δ, in turn up-regulating multiple cognition-associated genes. We show further that the fatty acid-binding protein FABP5 controls both of these functions in vivo. FABP5 both promotes the hydrolysis of AEA into arachidonic acid and thus reduces brain endocannabinoid levels, and directly shuttles arachidonic acid to the nucleus where it delivers it to PPARß/δ, enabling its activation. In accordance, ablation of FABP5 in mice results in excess accumulation of AEA, abolishes PPARß/δ activation in the brain, and markedly impairs hippocampus-based learning and memory. The data indicate that, by controlling anandamide disposition and activities, FABP5 plays a key role in regulating hippocampal cognitive function.

Retinoic acid induces neurogenesis by activating both retinoic acid receptors (RARs) and peroxisome proliferator-activated receptor ß/δ (PPARß/δ).

Retinoic acid (RA) regulates gene transcription by activating the nuclear receptors retinoic acid receptor (RAR) and peroxisome proliferator-activated receptor (PPAR) ß/δ and their respective cognate lipid-binding proteins CRABP-II and FABP5. RA induces neuronal differentiation, but the contributions of the two transcriptional pathways of the hormone to the process are unknown. Here, we show that the RA-induced commitment of P19 stem cells to neuronal progenitors is mediated by the CRABP-II/RAR path and that the FABP5/PPARß/δ path can inhibit the process through induction of the RAR repressors SIRT1 and Ajuba. In contrast with its inhibitory activity in the early steps of neurogenesis, the FABP5/PPARß/δ path promotes differentiation of neuronal progenitors to mature neurons, an activity mediated in part by the PPARß/δ target gene PDK1. Hence, RA-induced neuronal differentiation is mediated through RAR in the early stages and through PPARß/δ in the late stages of the process. The switch in RA signaling is accomplished by a transient up-regulation of RARß concomitantly with a transient increase in the CRABP-II/FABP5 ratio at early stages of differentiation. In accordance with these conclusions, hippocampi of FABP5-null mice display excess accumulation of neuronal progenitor cells and a deficit in mature neurons versus wild-type animals.

A multistage pathway for human prion protein aggregation in vitro: from multimeric seeds to ß-oligomers and nonfibrillar structures.

Aberrant protein aggregation causes numerous neurological diseases including Creutzfeldt-Jakob disease (CJD), but the aggregation mechanisms remain poorly understood. Here, we report AFM results on the formation pathways of ß-oligomers and nonfibrillar aggregates from wild-type full-length recombinant human prion protein (WT) and an insertion mutant (10OR) with five additional octapeptide repeats linked to familial CJD. Upon partial denaturing, seeds consisting of 3-4 monomers quickly appeared. Oligomers of ~11-22 monomers then formed through direct interaction of seeds, rather than by subsequent monomer attachment. All larger aggregates formed through association of these ß-oligomers. Although both WT and 10OR exhibited identical aggregation mechanisms, the latter oligomerized faster due to lower solubility and, hence, thermodynamic stability. This novel aggregation pathway has implications for prion diseases as well as others caused by protein aggregation.

Pro-prion binds filamin A, facilitating its interaction with integrin beta1, and contributes to melanomagenesis.

Filamin A (FLNA) is an integrator of cell mechanics and signaling. The spreading and migration observed in FLNA sufficient A7 melanoma cells but not in the parental FLNA deficient M2 cells have been attributed to FLNA. In A7 and M2 cells, the normal prion (PrP) exists as pro-PrP, retaining its glycosylphosphatidyl-inositol (GPI) anchor peptide signal sequence (GPI-PSS). The GPI-PSS of PrP has a FLNA binding motif and binds FLNA. Reducing PrP expression in A7 cells alters the spatial distribution of FLNA and organization of actin and diminishes cell spreading and migration. Integrin ß1 also binds FLNA. In A7 cells, FLNA, PrP, and integrin ß1 exist as two independent, yet functionally linked, complexes; they are FLNA with PrP or FLNA with integrin ß1. Reducing PrP expression in A7 cells decreases the amount of integrin ß1 bound to FLNA. A PrP GPI-PSS synthetic peptide that crosses the cell membrane inhibits A7 cell spreading and migration. Thus, in A7 cells FLNA does not act alone; the binding of pro-PrP enhances association between FLNA and integrin ß1, which then promotes cell spreading and migration. Pro-PrP is detected in melanoma in situ but not in melanocyte. Invasive melanoma has more pro-PrP. The binding of pro-PrP to FLNA, therefore, contributes to melanomagenesis.

The fatal attraction between pro-prion and filamin A: prion as a marker in human cancers.

Pancreatic cancer is the fourth leading cancer causing deaths in the USA, with more than 30,000 deaths per year. The overall median survival for all pancreatic cancer is 6 months and the 5-year survival rate is less than 10%. This dismal outcome reflects the inefficacy of the chemotherapeutic agents, as well as the lack of an early diagnostic marker. A protein known as prion (PrP) is expressed in human pancreatic cancer cell lines. However, in these cell lines, the PrP is incompletely processed and exists as pro-PrP. The pro-PrP binds to a molecule inside the cell, filamin A (FLNa), which is an integrator of cell signaling and mechanics. The binding of pro-PrP to FLNa disrupts the normal functions of FLNa, altering the cell's cytoskeleton and signal transduction machineries. As a result, the tumor cells grow more aggressively. Approximately 40% of patients with pancreatic cancer express PrP in their cancer. These patients have significantly shorter survival compared with patients whose pancreatic cancers lack PrP. Therefore, expression of pro-PrP and its binding to FLNa provide a growth advantage to pancreatic cancers. In this article, we discuss the following points: the biology of PrP, the consequences of binding of pro-PrP to FLNa in pancreatic cancer, the detection of pro-PrP in other cancers, the potential of using pro-PrP as a diagnostic marker, and prevention of the binding between pro-PrP and FLNa as a target for therapeutic intervention in cancers.

Binding of pro-prion to filamin A disrupts cytoskeleton and correlates with poor prognosis in pancreatic cancer.

The cellular prion protein (PrP) is a highly conserved, widely expressed, glycosylphosphatidylinositol-anchored (GPI-anchored) cell surface glycoprotein. Since its discovery, most studies on PrP have focused on its role in neurodegenerative prion diseases, whereas its function outside the nervous system remains unclear. Here, we report that human pancreatic ductal adenocarcinoma (PDAC) cell lines expressed PrP. However, the PrP was neither glycosylated nor GPI-anchored, existing as pro-PrP and retaining its GPI anchor peptide signal sequence (GPI-PSS). We also showed that the PrP GPI-PSS has a filamin A-binding (FLNa-binding) motif and interacted with FLNa, an actin-associated protein that integrates cell mechanics and signaling. Binding of pro-PrP to FLNa disrupted cytoskeletal organization. Inhibition of PrP expression by shRNA in the PDAC cell lines altered the cytoskeleton and expression of multiple signaling proteins; it also reduced cellular proliferation and invasiveness in vitro as well as tumor growth in vivo. A subgroup of human patients with pancreatic cancer was found to have tumors that expressed pro-PrP. Most importantly, PrP expression in tumors correlated with a marked decrease in patient survival. We propose that binding of pro-PrP to FLNa perturbs FLNa function, thus contributing to the aggressiveness of PDAC. Prevention of this interaction could provide an attractive target for therapeutic intervention in human PDAC.

Ligand binding promotes prion protein aggregation--role of the octapeptide repeats.

Aggregation of the normal cellular prion protein, PrP, is important in the pathogenesis of prion disease. PrP binds glycosaminoglycan (GAG) and divalent cations, such as Cu(2+) and Zn(2+). Here, we report our findings that GAG and Cu(2+) promote the aggregation of recombinant human PrP (rPrP). The normal cellular prion protein has five octapeptide repeats. In the presence of either GAG or Cu(2+), mutant rPrPs with eight or ten octapeptide repeats are more aggregation prone, exhibit faster kinetics and form larger aggregates than wild-type PrP. When the GAG-binding motif, KKRPK, is deleted the effect of GAG but not that of Cu(2+) is abolished. By contrast, when the Cu(2+)-binding motif, the octapeptide-repeat region, is deleted, neither GAG nor Cu(2+) is able to promote aggregation. Therefore, the octapeptide-repeat region is critical in the aggregation of rPrP, irrespective of the promoting ligand. Furthermore, aggregation of rPrP in the presence of GAG is blocked with anti-PrP mAbs, whereas none of the tested anti-PrP mAbs block Cu(2+)-promoted aggregation. However, a mAb that is specific for an epitope at the N-terminus enhances aggregation in the presence of either GAG or Cu(2+). Therefore, although binding of either GAG or Cu(2+) promotes the aggregation of rPrP, their aggregation processes are different, suggesting multiple pathways of rPrP aggregation.

Binding of recombinant but not endogenous prion protein to DNA causes DNA internalization and expression in mammalian cells.

Recombinant prion protein, rPrP, binds DNA. Both the KKRPK motif and the octapeptide repeat region of rPrP are essential for maximal binding. rPrP with pathogenic insertional mutations binds more DNA than wild-type rPrP. DNA promotes the aggregation of rPrP and protects its N terminus from proteinase K digestion. When rPrP is mixed with an expression plasmid and Ca(2+), the rPrP.DNA complex is taken up by mammalian cells leading to gene expression. In the presence of Ca(2+), rPrP by itself is also taken up by cells in a temperature- and pinocytosis-dependent manner. Cells do not take up rPrP(DeltaKKRPK), which lacks the KKRPK motif. Thus, rPrP is the carrier for DNA and the KKRPK motif is essential for its uptake. When mixed with DNA, a pentapeptide KKRPK, but not KKKKK, is sufficient for DNA internalization and expression. In contrast, whereas the normal cellular prion protein, PrP(C), on the cell surface can also internalize DNA, the imported DNA is not expressed. These findings may have relevance to the normal functions of PrP(C) and the pathogenic mechanisms of human prion disease.

Normal cellular prion protein with a methionine at position 129 has a more exposed helix 1 and is more prone to aggregate.

The human prion gene, PRNP, has two allelic forms that encode either a methionine or valine at codon 129. This polymorphism strongly influences the pathogenesis of prion disease. However, the underlying mechanism remains unclear. We compared the conformation between wild-type human prion protein (rPrP(C)) with either a valine or methionine at position 129, using a panel of monoclonal antibodies that are specific for epitopes along the entire protein. We found that rPrP(C(129M)) has a more exposed helix 1 region compared to rPrP(C(129V)). Helix 1 is important in the aggregation process. Accordingly, rPrP(C(129M)) aggregates at a faster rate and forms more aggregate than rPrP(C(129V)). In addition, by using a rPrP with a pathogenic mutation of five additional octapeptide repeat insertions, rPrP((129M)/10OR), as "seeds", we showed that rPrP((129M)/10OR) promotes the aggregation of rPrP(C(129M)) more efficiently than rPrP(C(129V)). These findings provide a possible mechanism underlying the influence of residue 129 on human prion disease.