Stress‐inducible phosphoprotein 1 (STI1) is part of the chaperone machinery, but it also functions as an extracellular ligand for the prion protein. However, the physiological relevance of these STI1 activities in vivo is unknown. Here, we show that in the absence of embryonic STI1, several Hsp90 client proteins are decreased by 50%, although Hsp90 levels are unaffected. Mutant STI1 mice showed increased caspase‐3 activation and 50% impairment in cellular proliferation. Moreover, placental disruption and lack of cellular viability were linked to embryonic death by E10.5 in STI1‐mutant mice. Rescue of embryonic lethality in these mutants, by transgenic expression of the STI1 gene, supported a unique role for STI1 during embryonic development. The response of STI1 haploinsufficient mice to cellular stress seemed compromised, and mutant mice showed increased vulnerability to ischemic insult. At the cellular level, ischemia increased the secretion of STI1 from wild‐type astrocytes by 3‐fold, whereas STI1 haploinsufficient mice secreted half as much STI1. Interesting, extracellular STI1 prevented ischemia‐mediated neuronal death in a prion protein‐dependent way. Our study reveals essential roles for intracellular and extracellular STI1 in cellular resilience.—Beraldo, F. H., Soares, I. N., Goncalves, D. F., Fan, J., Thomas, A. A., Santos, T. G., Mohammad, A. H., Roffe, M., Calder, M. D., Nikolova, S., Hajj, G. N., Guimaraes, A. N., Massensini, A. R., Welch, I., Betts, D. H., Gros, R., Drangova, M., Watson, A. J., Bartha, R., Prado, V. F., Martins, V. R., and Prado, M. A. M., Stress‐inducible phosphoprotein 1 has unique cochaperone activity during development and regulates cellular response to ischemia via the prion protein. FASEB J. 27, 3594–3607 (2013). http://www.fasebj.org

The prion protein (PrP(C)) is highly expressed in the nervous system, and its abnormal conformer is associated with prion diseases. PrP(C) is anchored to cell membranes by glycosylphosphatidylinositol, and transmembrane proteins are likely required for PrP(C)-mediated intracellular signaling. Binding of laminin (Ln) to PrP(C) modulates neuronal plasticity and memory. We addressed signaling pathways triggered by PrP(C)-Ln interaction in order to identify transmembrane proteins involved in the transduction of PrP(C)-Ln signals. The Ln γ1-chain peptide, which contains the Ln binding site for PrP(C), induced neuritogenesis through activation of phospholipase C (PLC), Ca(2+) mobilization from intracellular stores, and protein kinase C and extracellular signal-regulated kinase (ERK1/2) activation in primary cultures of neurons from wild-type, but not PrP(C)-null mice. Phage display, coimmunoprecipitation, and colocalization experiments showed that group I metabotropic glutamate receptors (mGluR1/5) associate with PrP(C). Expression of either mGluR1 or mGluR5 in HEK293 cells reconstituted the signaling pathways mediated by PrP(C)-Ln γ1 peptide interaction. Specific inhibitors of these receptors impaired PrP(C)-Ln γ1 peptide-induced signaling and neuritogenesis. These data show that group I mGluRs are involved in the transduction of cellular signals triggered by PrP(C)-Ln, and they support the notion that PrP(C) participates in the assembly of multiprotein complexes with physiological functions on neurons.

The prion protein (PrP C ) is a conserved glycosylphosphatidylinositol-anchored cell surface protein expressed by neurons and other cells. Stress-inducible protein 1 (STI1) binds PrP C extracellularly, and this activated signaling complex promotes neuronal differentiation and neuroprotection via the extracellular signal-regulated kinase 1 and 2 (ERK1/2) and cAMP-dependent protein kinase 1 (PKA) pathways. However, the mechanism by which the PrP C -STI1 interaction transduces extracellular signals to the intracellular environment is unknown. We found that in hippocampal neurons, STI1-PrP C engagement induces an increase in intracellular Ca 2؉ levels. This effect was not detected in PrP C -null neurons or wild-type neurons treated with an STI1 mutant unable to bind PrP C . Using a best candidate approach to test for potential channels involved in Ca 2؉ influx evoked by STI1-PrP C , we found that ␣-bungarotoxin, a specific inhibitor for ␣7 nicotinic acetylcholine receptor (␣7nAChR), was able to block PrP C -STI1-mediated signaling, neuroprotection, and neuritogenesis. Importantly, when ␣7nAChR was transfected into HEK 293 cells, it formed a functional complex with PrP C and allowed reconstitution of signaling by PrP C -STI1 interaction. These results indicate that STI1 can interact with the PrP C ⅐␣7nAChR complex to promote signaling and provide a novel potential target for modulation of the effects of prion protein in neurodegenerative diseases.Prions are the infectious components of transmissible spongiform encephalopathies that can self-perpetuate by imprinting an anomalous conformation onto a glycosylphosphatidylinositol-anchored host protein known as the prion protein (PrP C ). 3Conversion of PrP C to the protease-resistant prion is thought to be a major event in prion diseases (1). However, despite the wealth of knowledge on prions, the roles of PrP C on synaptic function and neuronal health are still not completely understood. Studies in yeast, mammalian cells, and mouse models support the hypothesis that PrP C plays a major role in neuroprotection and neuronal differentiation (for review, see Refs. 1-3).In humans, the initial cognitive dysfunction observed in transmissible spongiform encephalopathies is remarkably similar to those observed in Alzheimer disease (AD). Therefore, synaptic failure is likely to play a major role in the cognitive alterations seen in these neurological disorders (4). For instance, in AD, the role for A␤ ligands in synaptic dysfunction has received considerable attention (5, 6). Interestingly, recent work provided evidence that PrP C functions as a receptor for A␤ and that this interaction mediates some of the effects of A␤ oligomers in synaptic plasticity (7) although these results are controversial at the moment (8, 9). Moreover, PrP C seems to regulate the ␤-secretase cleavage of amyloid precursor protein, thereby regulating the production of A␤ (10). In addition, ␣-secretase regulates the cleavage of PrP C , generating an N-terminal fragment with neuroprotective activity (11,12)...

. (1992). Acetylcholine synthesis and release is enhanced by dibutyryl cyclic AMP in a neuronal cell line derived from mouse septum. J. Neurosci. 12, 793-799. Brentani, R. R. (1988

BackgroundGlioblastoma (GBM), a highly aggressive brain tumor, contains a subpopulation of glioblastoma stem-like cells (GSCs) that play roles in tumor maintenance, invasion, and therapeutic resistance. GSCs are therefore a promising target for GBM treatment. Our group identified the cellular prion protein (PrPC) and its partner, the co-chaperone Hsp70/90 organizing protein (HOP), as potential target candidates due to their role in GBM tumorigenesis and in neural stem cell maintenance.MethodsGSCs expressing different levels of PrPC were cultured as neurospheres with growth factors, and characterized with stem cells markers and adhesion molecules markers through immunofluorescence and flow cytometry. We than evaluated GSC self-renewal and proliferation by clonal density assays and BrdU incorporation, respectively, in front of recombinant HOP treatment, combined or not with a HOP peptide which mimics the PrPC binding site. Stable silencing of HOP was also performed in parental and/or PrPC-depleted cell populations, and proliferation in vitro and tumor growth in vivo were evaluated. Migration assays were performed on laminin-1 pre-coated glass.ResultsWe observed that, when GBM cells are cultured as neurospheres, they express specific stemness markers such as CD133, CD15, Oct4, and SOX2; PrPC is upregulated compared to monolayer culture and co-localizes with CD133. PrPC silencing downregulates the expression of molecules associated with cancer stem cells, upregulates markers of cell differentiation and affects GSC self-renewal, pointing to a pivotal role for PrPC in the maintenance of GSCs. Exogenous HOP treatment increases proliferation and self-renewal of GSCs in a PrPC-dependent manner while HOP knockdown disturbs the proliferation process. In vivo, PrPC and/or HOP knockdown potently inhibits the growth of subcutaneously implanted glioblastoma cells. In addition, disruption of the PrPC-HOP complex by a HOP peptide, which mimics the PrPC binding site, affects GSC self-renewal and proliferation indicating that the HOP-PrPC complex is required for GSC stemness. Furthermore, PrPC-depleted GSCs downregulate cell adhesion-related proteins and impair cell migration indicating a putative role for PrPC in the cell surface stability of cell adhesion molecules and GBM cell invasiveness, respectively.ConclusionsIn conclusion, our results show that the modulation of HOP-PrPC engagement or the decrease of PrPC and HOP expression may represent a potential therapeutic intervention in GBM, regulating glioblastoma stem-like cell self-renewal, proliferation, and migration.Electronic supplementary materialThe online version of this article (doi:10.1186/s13287-017-0518-1) contains supplementary material, which is available to authorized users.