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 secreted cochaperone STI1 triggers activation of protein kinase A (PKA) and ERK1/2 signaling by interacting with the cellular prion (PrP C ) at the cell surface, resulting in neuroprotection and increased neuritogenesis. Here, we investigated whether STI1 triggers PrP C trafficking and tested whether this process controls PrP C -dependent signaling. We found that STI1, but not a STI1 mutant unable to bind PrP C , induced PrP C endocytosis. STI1-induced signaling did not occur in cells devoid of endogenous PrP C ; however, heterologous expression of PrP C reconstituted both PKA and ERK1/2 activation. In contrast, a PrP C mutant lacking endocytic activity was unable to promote ERK1/2 activation induced by STI1, whereas it reconstituted PKA activity in the same condition, suggesting a key role of endocytosis in the former process. The activation of ERK1/2 by STI1 was transient and appeared to depend on the interaction of the two proteins at the cell surface or shortly after internalization. Moreover, inhibition of dynamin activity by expression of a dominant-negative mutant caused the accumulation and colocalization of these proteins at the plasma membrane, suggesting that both proteins use a dynamin-dependent internalization pathway. These results show that PrP C endocytosis is a necessary step to modulate STI1-dependent ERK1/2 signaling involved in neuritogenesis.

Alzheimer's disease (AD) is the most prevalent form of dementia in humans. Considerable evidence in AD points to a causal role for altered metabolism of the amyloid precursor protein (APP), with increased levels of peptides derived from APP collectively known as amyloid-b peptides (Ab; Selkoe and Schenk 2003; Haass and Selkoe 2007). The pathological hallmarks of AD are the accumulation of extracellular senile plaques formed by aggregation of Ab, and intracellular tangles formed by the hyperphosphorylated form of the microtubule-associated protein Tau. These pathological alterations are believed to represent endpoints in AD, hence