The prion protein (PrP ^C ) is highly expressed in the nervous system, and its abnormal con‐former 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 γl‐chain peptide, which contains the Ln binding site for PrP ^C , induced neuritogenesis through activation of phos‐pholipase C (PLC), Ca ²⁺ mobilization from intracellular stores, and protein kinase C and extracellular signalregulated 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 (mGluRl/5) associate with PrP ^C . Expression of either mGluRl or mGluR5 in HEK293 cells reconstituted the signaling pathways mediated by PrP ^C ‐Ln γl peptide interaction. Specific inhibitors of these receptors impaired PrP ^C ‐Ln γl peptide‐induced signaling and neuri‐togenesis. 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.—Beraldo, F. H., Arantes, C. P., Santos, T. G., Machado, C. F. _, Roffe, M., Hajj, G. N., Lee, K. S., Magalhães, A. C., Caetano, F. A., Mancini, G. L., Lopes, M. H., Amãrico, T. A., Magdesian, M. H., Ferguson, S. S. G., Linden, R., Prado, M. A. M., Martins, V. R. Metabotropic glutamate receptors transduce signals for neurite outgrowth after binding of the prion protein to laminin γl chain. FASEB J. 25, 265–279 (20ll). www.fasebj.org

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 α-bungarotoxin, a specific inhibitor for α7 nicotinic acetylcholine receptor (α7nAChR), was able to block PrP(C)-STI1-mediated signaling, neuroprotection, and neuritogenesis. Importantly, when α7nAChR 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)·α7nAChR complex to promote signaling and provide a novel potential target for modulation of the effects of prion protein in neurodegenerative diseases.

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). www.fasebj.org

The physiological functions of the cellular prion protein, PrP^C, as a cell surface pleiotropic receptor are under debate. We report that PrP^C interacts with vitronectin but not with fibronectin or collagen. The binding sites mediating this PrP^C-vitronectin interaction were mapped to residues 105-119 of PrP^C and the residues 307-320 of vitronectin. The two proteins were co-localized in embryonic dorsal root ganglia from wild-type mice. Vitronectin addition to cultured dorsal root ganglia induced axonal growth, which could be mimicked by vitronectin peptide 307-320 and abrogated by anti-PrP^C antibodies. Full-length vitronectin, but not the vitronectin peptide 307-320, induced axonal growth of dorsal root neurons from two strains of PrP^C-null mice. Functional assays demonstrated that relative to wild-type cells, PrP^C-null dorsal root neurons were more responsive to the Arg-Gly-Asp peptide (an integrin-binding site), and exhibited greater αvβ3 activity. Our findings indicate that PrP^C plays an important role in axonal growth, and this function may be rescued in PrP^C-knockout animals by integrin compensatory mechanisms.

Prion protein (PrP^C), when associated with the secreted form of the stress-inducible protein 1 (STI1), plays an important role in neural survival, neuritogenesis, and memory formation. However, the role of the PrP^C-STI1 complex in the physiology of neural progenitor/stem cells is unknown. In this article, we observed that neurospheres cultured from fetal forebrain of wild-type (Prnp^+/+) and PrP^C-null (Prnp^0/0) mice were maintained for several passages without the loss of self-renewal or multipotentiality, as assessed by their continued capacity to generate neurons, astrocytes, and oligodendrocytes. The homogeneous expression and colocalization of STI1 and PrP^C suggest that they may associate and function as a complex in neurosphere-derived stem cells. The formation of neurospheres from Prnp^0/0 mice was reduced significantly when compared with their wild-type counterparts. In addition, blockade of secreted STI1, and its cell surface ligand, PrP^C, with specific antibodies, impaired Prnp^+/+ neurosphere formation without further impairing the formation of Prnp^0/0 neurospheres. Alternatively, neurosphere formation was enhanced by recombinant STI1 application in cells expressing PrP^C but not in cells from Prnp^0/0 mice. The STI1-PrP^C interaction was able to stimulate cell proliferation in the neurosphere-forming assay, while no effect on cell survival or the expression of neural markers was observed. These data suggest that the STI1-PrP^C complex may play a critical role in neural progenitor/stem cells self-renewal via the modulation of cell proliferation, leading to the control of the stemness capacity of these cells during nervous system development. STEM CELLS 2011;29:1126–1136

Desmoplastic small round cell tumor (DSRCT) is an extremely rare, aggressive sarcoma affecting adolescents and young adults with male predominance. Generally, it originates from the serosal surface of the abdominal cavity. The hallmark characteristic of DSRCT is the EWSR1–WT1 gene fusion. This translocation up-regulates the expression of PDGFRα, VEGF and other proteins related to tumor and vascular cell proliferation. Current management of DSRCT includes a combination of chemotherapy, radiation and aggressive cytoreductive surgery plus intra-peritoneal hyperthermic chemotherapy (HIPEC). Despite advances in multimodal therapy, outcomes remain poor since the majority of patients present disease recurrence and die within three years. The dismal survival makes DSRCT an orphan disease with an urgent need for new drugs. The treatment of advanced and recurrent disease with tyrosine kinase inhibitors, such as pazopanib, sunitinib, and mTOR inhibitors was evaluated by small trials. Recent studies using comprehensive molecular profiling of DSRCT identified potential therapeutic targets. In this review, we aim to describe the current studies conducted to better understand DSRCT biology and to explore the new therapeutic strategies under investigation in preclinical models and in early phase clinical trials.