The function of the cellular prion protein (PrP) is still poorly understood. We present here an unprecedented role for PrP against Bax-mediated neuronal apoptosis and show that PrP potently inhibits Bax-induced cell death in human primary neurons. Deletion of four octapeptide repeats of PrP (PrP⌬OR) and familial D178N and T183A PrP mutations completely or partially eliminate the neuroprotective effect of PrP. PrP remains anti-apoptotic despite truncation of the glycosylphosphatidylinositol (GPI) anchor signal peptide, indicating that the neuroprotective form of PrP does not require the abundant cell surface GPI-anchored PrP. Our results implicate PrP as a potent and novel anti-apoptotic protein against Bax-mediated cell death.Prion protein (PrP) 1 is a sialoglycoprotein that is highly expressed in brain, heart, lungs, and lymphoid system and at lower levels in several other tissues such as muscle (1, 2). Mature PrP contains two N-linked glycans and a disulfide bond (reviewed in Ref.3). PrP possesses a C-terminal GPI-anchoring signal and a transmembrane domain that can generate type I ( Ctm PrP) or type II ( Ntm PrP) transmembrane-spanning isoforms in isolated endoplasmic reticulum microsomes or phospholiposomes (4 -7). In most cells, the majority of the PrP localizes to the cell surface as a GPI-anchored protein (8). The complete translocation of PrP is dependent on translocation accessory factors (TrAF). In the absence of TrAF, PrP is exclusively synthesized in a transmembrane topology (7).Whereas the role of the infectious form of PrP in a number of human and animal neurodegenerative diseases has been extensively studied, the normal function of PrP is still poorly understood. PrP-null mice display no dramatic phenotype (9). However, evidence indicates that PrP may promote sleep continuity (10). PrP is involved in the regulation of presynaptic copper concentration, intracellular calcium concentration, activation of lymphocytes, astrocyte proliferation, and signal transduction and has antioxidant properties (11-16). Although controversial, PrP-null mice are also found to be impaired in long term potentiation (17)(18)(19). In addition, it has been shown that PrP-null neuronal cell lines are more susceptible to serum deprivation-induced cell death and that Bcl-2 overexpression can attenuate the sensitivity of PrP-null neuronal cell lines to serum deprivation (20). Kurschner and Morgan (21, 22) reported that yeast PrP fusion proteins interact with Bcl-2. Furthermore, four identical N-terminal PrP octapeptide repeats (OR) that are highly conserved in evolution share limited similarity with the Bcl-2 homology domain 2 (BH2) of Bcl-2 proteins (23, 24). Bcl-2 proteins are central to the regulation of cell death, and the BH2 domain is crucial to the anti-apoptotic function of Bcl-2 and its interaction with the pro-apoptotic Bax protein (24,25). Based on these features of PrP, we hypothesized that similar to Bcl-2 family members, PrP may play a role in the regulation of neuronal apoptosis. In the present study, we have...

The cytotoxicity of extracellular amyloid beta peptide (Abeta) has been clearly demonstrated in many cell types. In contrast, primary human neurons in culture are resistant to extracellular Abeta-mediated toxicity. Here, we investigate the involvement of p75 neurotrophin receptor (p75NTR) in Abeta-treated human neurons. We find that Abeta1-40 and Abeta1-42, but not the reverse control peptide, Abeta40-1, rapidly increase the levels of p75NTR in a specific and dose-dependent manner. In contrast to observations in cell lines, enhanced expression of p75NTR in human neurons via a herpes simplex virus amplicon vector does not increase the susceptibility of neurons to Abeta. Unexpectedly, inhibition of p75NTR expression with an antisense expression construct or incubation of the cells with an antibody to the extracellular domain of p75NTR sensitizes human neurons to extracellular nonfibrillar or fibrillar Abeta1-42 cytotoxicity. Unlike intracellular Abeta, extracellular Abeta toxicity is independent of p53 and Bax activity. However, Abeta toxicity is inhibited by caspase inhibitors and the glycogen synthase kinase 3beta inhibitor lithium. Neuroprotection against Abeta is phosphatidylinositide 3-kinase dependent but Akt independent. These results are consistent with a neuroprotective role for p75NTR against extracellular Abeta toxicity in human neurons.

Although there is no consensus regarding the normal function of the prion protein, increasing evidence points towards a role in cellular protection against cell death. We have previously shown that prion protein is a potent inhibitor of Bax-induced apoptosis in human primary neurons and in the breast carcinoma MCF-7 cells. Here, we used the yeast Saccharomyces cerevisiae to investigate if the neuroprotective function of prion protein requires other members of the Bcl-2 family given that S. cerevisiae lacks Bcl-2 genes but undergoes a mitochondrial-dependent apoptotic cell death upon exogenous expression of Bax protein. We show that Bax induces cell death and growth inhibition in S. cerevisiae. Prion protein prevents Bax-mediated cell death. Prion protein overcomes Bax-mediated growth arrest in S phase but cannot overcome population growth inhibition because the cells then accumulate in G(2)/M phase. We conclude that prion protein does not require other Bcl-2 family proteins to protect against Bax-mediated cell death.