Prion Protein at the Leading Edge: Its Role in Cell Motility

The expression of pro-prion, a transmembrane isoform of the prion protein, leads to the constitutive activation of the canonical Wnt/β-catenin pathway to sustain the stem-like phenotype of human glioblastoma cells

BACKGROUND

Cellular prion protein (PrPC) is a widely expressed membrane-anchored glycoprotein, which has been associated with the development and progression of several types of human malignancies, controlling cancer stem cell activity. However, the different molecular mechanisms regulated by PrPC in normal and tumor cells have not been characterized yet.

METHODS

To assess the role of PrPC in patient-derived glioblastoma stem cell (GSC)-enriched cultures, we generated cell lines in which PrPC was either overexpressed or down-regulated and investigated, in 2D and 3D cultures, its role in cell proliferation, migration, and invasion. We evaluated the role of PrPC in supporting GSC stemness and the intracellular signaling involved using qRT-PCR, immunocytofluorescence, and Western blot.

RESULTS

Stable PrPC down-regulation leads to a significant reduction of GSC proliferation, migration, and invasiveness. These effects were associated with the inhibition of the expression of stemness genes and overexpression of differentiation markers. At molecular level PrPC down-regulation caused a significant inhibition of Wnt/β-catenin pathway, through a reduced expression of Wnt and Frizzled ligand/receptor subtypes, resulting in the inhibition of β-catenin transcriptional activity,  as demonstrated by the reduced expression of its target genes. The specificity of PrPC in these effects was demonstrated by rescuing the phenotype and the biological activity of PrPC down-regulated GSCs by re-expressing the protein. To get insights into the distinct mechanisms by which PrPC regulates proliferation in GSCs, but not in normal astrocytes, we analyzed structural features of PrPC in glioma stem cells and astrocytes using Western blot and immunofluorescence techniques. Using Pi-PLC, an enzyme that cleaves GPI anchors, we show that, in GSCs, PrP is retained within the plasma membrane in an immature Pro-PrP isoform whereas in astrocytes, it is expressed in its mature PrPC form, anchored on the extracellular face of the plasma membrane.

CONCLUSIONS

The persistence of Pro-PrP in GSCs is an altered cellular mechanism responsible of the aberrant, constitutive activation of Wnt/β-catenin pathway, which contributes to glioblastoma malignant features. Thus, the activity of Pro-PrP may represent a targetable vulnerability in glioblastoma cells, offering a novel approach for differentiating and eradicating glioblastoma stem cells.

Prion protein regulates invasiveness in glioblastoma stem cells

BACKGROUND

Glioblastoma (GBM) is an aggressive brain tumor driven by glioblastoma stem cells (GSCs), which represent an appealing target for therapeutic interventions. The cellular prion protein (PrPC), a scaffold protein involved in diverse cellular processes, interacts with various membrane and extracellular matrix molecules, influencing tumor biology. Herein, we investigate the impact of PrPC expression on GBM.

METHODS

To address this goal, we employed CRISPR-Cas9 technology to generate PrPC knockout (KO) glioblastoma cell lines, enabling detailed loss-of-function studies. Bulk RNA sequencing followed by differentially expressed gene and pathway enrichment analyses between U87 or U251 PrPC-wild-type (WT) cells and PrPC-knockout (KO) cells were used to identify pathways regulated by PrPC. Immunofluorescence assays were used to evaluate cellular morphology and protein distribution. For assessment of protein levels, Western blot and flow cytometry assays were employed. Transwell and growth curve assays were used to determine the impact of loss-of-PrPC in GBM invasiveness and proliferation, respectively. Single-cell RNA sequencing analysis of data from patient tumors from The Cancer Genome Atlas (TCGA) and the Broad Institute of Single-Cell Data Portal were used to evaluate the correspondence between our in vitro results and patient samples.

RESULTS

Transcriptome analysis of PrPC-KO GBM cell lines revealed altered expression of genes associated with crucial tumor progression pathways, including migration, proliferation, and stemness. These findings were corroborated by assays that revealed impaired invasion, migration, proliferation, and self-renewal in PrPC-KO GBM cells, highlighting its critical role in sustaining tumor growth. Notably, loss-of-PrPC disrupted the expression and localization of key stemness markers, particularly CD44. Additionally, the modulation of PrPC levels through CD44 overexpression further emphasizes their regulatory role in these processes.

CONCLUSIONS

These findings establish PrPC as a modulator of essential molecules on the cell surface of GSCs, highlighting its potential as a therapeutic target for GBM.

Integrated transcriptomics uncovers an enhanced association between the prion protein gene expression and vesicle dynamics signatures in glioblastomas

BACKGROUND

Glioblastoma (GBM) is an aggressive brain tumor that exhibits resistance to current treatment, making the identification of novel therapeutic targets essential. In this context, cellular prion protein (PrPC) stands out as a potential candidate for new therapies. Encoded by the PRNP gene, PrPC can present increased expression levels in GBM, impacting cell proliferation, growth, migration, invasion and stemness. Nevertheless, the exact molecular mechanisms through which PRNP/PrPC modulates key aspects of GBM biology remain elusive.

METHODS

To elucidate the implications of PRNP/PrPC in the biology of this cancer, we analyzed publicly available RNA sequencing (RNA-seq) data of patient-derived GBMs from four independent studies. First, we ranked samples profiled by bulk RNA-seq as PRNPhigh and PRNPlow and compared their transcriptomic landscape. Then, we analyzed PRNP+ and PRNP- GBM cells profiled by single-cell RNA-seq to further understand the molecular context within which PRNP/PrPC might function in this tumor. We explored an additional proteomics dataset, applying similar comparative approaches, to corroborate our findings.

RESULTS

Functional profiling revealed that vesicular dynamics signatures are strongly correlated with PRNP/PrPC levels in GBM. We found a panel of 73 genes, enriched in vesicle-related pathways, whose expression levels are increased in PRNPhigh/PRNP+ cells across all RNA-seq datasets. Vesicle-associated genes, ANXA1, RAB31, DSTN and SYPL1, were found to be upregulated in vitro in an in-house collection of patient-derived GBM. Moreover, proteome analysis of patient-derived samples reinforces the findings of enhanced vesicle biogenesis, processing and trafficking in PRNPhigh/PRNP+ GBM cells.

CONCLUSIONS

Together, our findings shed light on a novel role for PrPC as a potential modulator of vesicle biology in GBM, which is pivotal for intercellular communication and cancer maintenance. We also introduce GBMdiscovery, a novel user-friendly tool that allows the investigation of specific genes in GBM biology.

A non local model for cell migration in response to mechanical stimuli

Cell migration is one of the most studied phenomena in biology since it plays a fundamental role in many physiological and pathological processes such as morphogenesis, wound healing and tumorigenesis. In recent years, researchers have performed experiments showing that cells can migrate in response to mechanical stimuli of the substrate they adhere to. Motion towards regions of the substrate with higher stiffness is called durotaxis, while motion guided by the stress or the deformation of the substrate itself is called tensotaxis. Unlike chemotaxis (i.e. the motion in response to a chemical stimulus), these migratory processes are not yet fully understood from a biological point of view. In this respect, we present a mathematical model of single-cell migration in response to mechanical stimuli, in order to simulate these two processes. Specifically, the cell moves by changing its direction of polarization and its motility according to material properties of the substrate (e.g., stiffness) or in response to proper scalar measures of the substrate strain or stress. The equations of motion of the cell are non-local integro-differential equations, with the addition of a stochastic term to account for random Brownian motion. The mechanical stimulus to be integrated in the equations of motion is defined according to experimental measurements found in literature, in the case of durotaxis. Conversely, in the case of tensotaxis, substrate strain and stress are given by the solution of the mechanical problem, assuming that the extracellular matrix behaves as a hyperelastic Yeoh's solid. In both cases, the proposed model is validated through numerical simulations that qualitatively reproduce different experimental scenarios.

A Potential biomarker for the early diagnosis of OSCC: saliva and serum PrPC

Background: Oral squamous cell carcinoma (OSCC) is frequently diagnosed at an advanced stage, and the high mortality of patients is mainly due to the delay of diagnosis. Cellular prion protein (PrPC) contributes to the occurrence and development of many malignant tumors. However, little has been known about the clinical and diagnostic value of PrPC in OSCC. This study investigated the levels of PrPC in the saliva and serum of patients with OSCC, OPMD and control group and their diagnostic value. Methods: The Cancer Genome Atlas (TCGA), Gene Expression Omnibus (GEO) and Clinical Proteome Tumor Analysis Consortium (CPTAC) databases were analyzed to evaluate the expression of human prion protein gene (PRNP) mRNA and PrPC in OSCC. Enzyme-linked Immunosorbent Assay (ELISA) was utilized to detect the expression of PrPC in saliva and serum samples of OSCC, OPMD and control groups. Furthermore, diagnostic value and clinical significance of PrPC in OSCC was identified. Protein-protein interaction (PPI) network was constructed by STRING. GO and KEGG analysis were performed by ClusterProfiler. Results: The levels of PRNP mRNA and PrPC in OSCC were significantly higher than those in the control group from databases (P<0.05). Besides, salivary and serum PrPC of OSCC patients showed increased levels compared with OPMD and control groups (P<0.05). The expression of salivary and serum PrPC of OSCC was correlated with the degree of differentiation (P<0.05), and the expression of PrPC from CPTAC was related to tumor stage of OSCC (P<0.05). The areas under the diagnostic curves (AUCs) of salivary and serum PrPC were 0.807 and 0.671, respectively. GO and KEGG analysis revealed that PrPC might be related to cell adhesion, cell differentiation, signal transduction and apoptosis, and participate in the pathways of focal adhesion, PI3K-Akt signaling pathway and ECM- receptor interaction in OSCC. Conclusion: PrPC in saliva and serum may be a potential biomarker for early diagnosis of OSCC.

Prion Pathogenesis Revealed in a Series of the Special Issues "Prions and Prion Diseases"

Prion diseases are a group of devastating neurodegenerative disorders, which include Creutzfeldt-Jakob disease (CJD) in humans, and scrapie and bovine spongiform encephalopathy (BSE) in animals [...].

APP deficiency and HTRA2 modulates PrPc proteostasis in human cancer cells

Cellular protein homeostasis (proteostasis) requires an accurate balance between protein biosynthesis, folding, and degradation, and its instability is causally related to human diseases and cancers. Here, we created numerous engineered cancer cell lines targeting APP (amyloid ß precursor protein) and/or PRNP (cellular prion) genes and we showed that APP knocking-down impaired PRNP mRNA level and vice versa, suggesting a link between their gene regulation. PRNP^KD, APP^KD and PRNP^KD/APP^KD HeLa cells encountered major difficulties to grow in a 3D tissue-like environment. Unexpectedly, we found a cytoplasmic accumulation of the PrP^c protein without PRNP gene up regulation, in both APP^KD and APP^KO HeLa cells. Interestingly, APP and/or PRNP gene ablation enhanced the chaperone/serine protease HTRA2 gene expression, which is a protein processing quality factor involved in Alzheimer's disease. Importantly, HTRA2 gene silencing decreased PRNP mRNA level and lowered PrP^c protein amounts, and conversely, HTRA2 overexpression increased PRNP gene regulation and enhanced membrane-anchored and cytoplasmic PrP^c fractions. PrP^c, APP and HTRA2 destabilized membrane-associated CD24 protein, suggesting changes in the lipid raft structure. Our data show for the first time that APP and the dual chaperone/serine protease HTRA2 protein could modulate PrP^c proteostasis hampering cancer cell behavior.

PrP C as a Transducer of Physiological and Pathological Signals

After the discovery of prion phenomenon, the physiological role of the cellular prion protein (PrP ^C ) remained elusive. In the past decades, molecular and cellular analysis has shed some light regarding interactions and functions of PrP ^C in health and disease. PrP ^C , which is located mainly at the plasma membrane of neuronal cells attached by a glycosylphosphatidylinositol (GPI) anchor, can act as a receptor or transducer from external signaling. Although the precise role of PrP ^C remains elusive, a variety of functions have been proposed for this protein, namely, neuronal excitability and viability. Although many issues must be solved to clearly define the role of PrP ^C , its connection to the central nervous system (CNS) and to several misfolding-associated diseases makes PrP ^C an interesting pharmacological target. In a physiological context, several reports have proposed that PrP ^C modulates synaptic transmission, interacting with various proteins, namely, ion pumps, channels, and metabotropic receptors. PrP ^C has also been implicated in the pathophysiological cell signaling induced by β-amyloid peptide that leads to synaptic dysfunction in the context of Alzheimer's disease (AD), as a mediator of Aβ-induced cell toxicity. Additionally, it has been implicated in other proteinopathies as well. In this review, we aimed to analyze the role of PrP ^C as a transducer of physiological and pathological signaling.

The IDIP framework for assessing protein function and its application to the prion protein

The quest to determine the function of a protein can represent a profound challenge. Although this task is the mandate of countless research groups, a general framework for how it can be approached is conspicuously lacking. Moreover, even expectations for when the function of a protein can be considered to be 'known' are not well defined. In this review, we begin by introducing concepts pertinent to the challenge of protein function assignments. We then propose a framework for inferring a protein's function from four data categories: 'inheritance', 'distribution', 'interactions' and 'phenotypes' (IDIP). We document that the functions of proteins emerge at the intersection of inferences drawn from these data categories and emphasise the benefit of considering them in an evolutionary context. We then apply this approach to the cellular prion protein (PrP^C ), well known for its central role in prion diseases, whose function continues to be considered elusive by many investigators. We document that available data converge on the conclusion that the function of the prion protein is to control a critical post-translational modification of the neural cell adhesion molecule in the context of epithelial-to-mesenchymal transition and related plasticity programmes. Finally, we argue that this proposed function of PrP^C has already passed the test of time and is concordant with the IDIP framework in a way that other functions considered for this protein fail to achieve. We anticipate that the IDIP framework and the concepts analysed herein will aid the investigation of other proteins whose primary functional assignments have thus far been intractable.