BAX contributes to Doppel-induced apoptosis of prion-protein-deficient Purkinje cells

Domain-Specific Activation of Death-Associated Intracellular Signalling Cascades by the Cellular Prion Protein in Neuroblastoma Cells

The biological functions of the cellular prion protein remain poorly understood. In fact, numerous studies have aimed to determine specific functions for the different protein domains. Studies of cellular prion protein (PrP(C)) domains through in vivo expression of molecules carrying internal deletions in a mouse Prnp null background have provided helpful data on the implication of the protein in signalling cascades in affected neurons. Nevertheless, understanding of the mechanisms underlying the neurotoxicity induced by these PrP(C) deleted forms is far from complete. To better define the neurotoxic or neuroprotective potential of PrP(C) N-terminal domains, and to overcome the heterogeneity of results due to the lack of a standardized model, we used neuroblastoma cells to analyse the effects of overexpressing PrP(C) deleted forms. Results indicate that PrP(C) N-terminal deleted forms were properly processed through the secretory pathway. However, PrPΔF35 and PrPΔCD mutants led to death by different mechanisms sharing loss of alpha-cleavage and activation of caspase-3. Our data suggest that both gain-of-function and loss-of-function pathogenic mechanisms may be associated with N-terminal domains and may therefore contribute to neurotoxicity in prion disease. Dissecting the molecular response induced by PrPΔF35 may be the key to unravelling the physiological and pathological functions of the prion protein.

Ataxia with cerebellar lesions in mice expressing chimeric PrP-Dpl protein

Mutations within the central region of prion protein (PrP) have been shown to be associated with severe neurotoxic activity similar to that observed with Dpl, a PrP-like protein. To further investigate this neurotoxic effect, we generated lines of transgenic (Tg) mice expressing three different chimeric PrP-Dpl proteins. Chi1 (amino acids 1-57 of Dpl replaced by amino acids 1-125 of PrP) and Chi2 (amino acids 1-66 of Dpl replaced by amino acids 1-134 of PrP) abrogated the pathogenicity of Dpl indicating that the presence of a N-terminal domain of PrP (23-134) reduced the toxicity of Dpl, as reported. However, when the amino acids 1-24 of Dpl were replaced by amino acids 1-124 of PrP, Chi3 Tg mice, which express the chimeric protein at a very low level, start developing ataxia at the age of 5-7 weeks. This phenotype was not counteracted by a single copy of full-length-PrP(c) but rather by its overexpression, indicating the strong toxicity of the chimeric protein Chi3. Chi3 Tg mice exhibit severe cerebellar atrophy with a significant loss of granule cells. We concluded that aa25 to aa57 of Dpl, which are not present in Chi1 and Chi2 constructs, confer toxicity to the protein. We tested this possibility by using the 25-57 Dpl peptide in primary culture of mouse embryo cortical neurons and found a significant neurotoxic effect. This finding identifies a protein domain that plays a role in mediating Dpl-related toxicity.

Trafficking of PrPc to mitochondrial raft-like microdomains during cell apoptosis

The cellular form of prion protein (PrP (c)) is a highly conserved cell surface GPI-anchored glycoprotein that was identified in cholesterol-enriched, detergent-resistant microdomains, named "rafts." The association with these specialized portions of the cell plasma membrane is required for conversion of PrP (c) to the transmissible spongiform encephalopathy-associated protease-resistant isoform. Usually, PrP (c) is reported to be a plasma membrane protein, however several studies have revealed PrP (c) as an interacting protein mainly with the membrane/organelles, as well as with cytoskeleton network. Recent lines of evidence indicated its association with ER lipid raft-like microdomains for a correct folding of PrP (c), as well as for the export of the protein to the Golgi and proper glycosylation. During cell apoptosis, PrP (c) can undergo intracellular re-localization, via ER-mitochondria associated membranes (MAM) and microtubular network, to mitochondrial raft-like microdomains, where it induced the loss of mitochondrial membrane potential and citochrome c release, after a contained raise of calcium concentration. We suggest that PrP (c) may play a role in the multimolecular signaling complex associated with cell apoptosis Lipid rafts and their components may, thus, be investigated as pharmacological targets of interest, introducing a novel and innovative task in modern pharmacology, i.e., the development of glycosphingolipid targeted drugs.

The role of Bax and caspase-3 in doppel-induced apoptosis of cerebellar granule cells

Doppel (Dpl) protein is a paralog of the prion protein (PrP) that shares 25% sequence similarity with the C-terminus of PrP, a common N-glycosylation site and a C-terminal signal peptide for attachment of a glycosylphophatidyl inositol anchor. Whereas PrP^C is highly expressed in the central nervous system (CNS), Dpl is detected mostly in testes and its ectopic expression in the CNS leads to ataxia as well as Purkinje and granule cell degeneration in the cerebellum. The mechanism through which Dpl induces neurotoxicity is still debated. In the present work, primary neuronal cultures derived from postnatal cerebellar granule cells of wild-type and PrP-knockout FVB mice were used in order to investigate the molecular events that occur upon exposure to Dpl. Treatment of cultured cerebellar neurons with recombinant Dpl produced apoptosis that could be prevented by PrP co-incubation. When primary neuronal cultures from Bax-deficient mice were incubated with Dpl, no apoptosis was observed, suggesting an important role of Bax in triggering neurodegeneration. Similarly, cell survival increased when recDpl-treated cells were incubated with an inhibitor of caspase-3, which mediates apoptosis in mammalian cells. Together, our findings raise the possibility that Bax and caspase-3 feature in Dpl-mediated apoptosis.

A highly toxic cellular prion protein induces a novel, nonapoptotic form of neuronal death

Several different deletions within the N-terminal tail of the prion protein (PrP) induce massive neuronal death when expressed in transgenic mice. This toxicity is dose-dependently suppressed by coexpression of full-length PrP, suggesting that it results from subversion of a normal physiological activity of cellular PrP. We performed a combined biochemical and morphological analysis of Tg(DeltaCR) mice, which express PrP carrying a 21-aa deletion (residues 105-125) within a highly conserved region of the protein. Death of cerebellar granule neurons in Tg(DeltaCR) mice is not accompanied by activation of either caspase-3 or caspase-8 or by increased levels of the autophagy marker, LC3-II. In electron micrographs, degenerating granule neurons displayed a unique morphology characterized by heterogeneous condensation of the nuclear matrix without formation of discrete chromatin masses typical of neuronal apoptosis. Our data demonstrate that perturbations in PrP functional activity induce a novel, nonapoptotic, nonautophagic form of neuronal death whose morphological features are reminiscent of those associated with excitotoxic stress.

Autophagy and cell death of Purkinje cells overexpressing Doppel in Ngsk Prnp-deficient mice

In Ngsk prion protein (PrP)-deficient mice (NP(0/0)), ectopic expression of PrP-like protein Doppel (Dpl) in central neurons induces significant Purkinje cell (PC) death resulting in late-onset ataxia. NP(0/0) PC death is partly prevented by either knocking-out the apoptotic factor BAX or overexpressing the anti-apoptotic factor BCL-2 suggesting that apoptosis is involved in Dpl-induced death. In this study, Western blotting and immunohistofluorescence show that both before and during significant PC loss, the scrapie-responsive gene 1 (Scrg1)--potentially associated with autophagy--and the autophagic markers LC3B and p62 increased in the NP(0/0) PCs whereas RT-PCR shows stable mRNA expression, suggesting that the degradation of autophagic products is impaired in NP(0/0) PCs. At the ultrastructural level, autophagic-like profiles accumulated in somatodendritic and axonal compartments of NP(0/0), but not wild-type PCs. The most robust autophagy was observed in NP(0/0) PC axon compartments in the deep cerebellar nuclei suggesting that it is initiated in these axons. Our previous and present data indicate that Dpl triggers autophagy and apoptosis in NP(0/0) PCs. As observed in amyloid neurodegenerative diseases, upregulation of autophagic markers as well as extensive accumulation of autophagosomes in NP(0/0) PCs are likely to reflect a progressive dysfunction of autophagy that could trigger apoptotic cascades.

IL1RAPL1 controls inhibitory networks during cerebellar development in mice

Abnormalities in the formation and function of cerebellar circuitry potentially contribute to cognitive deficits in humans. In the adult, the activity of the sole output neurons of the cerebellar cortex - the Purkinje cells (PCs) - is shaped by the balance of activity between local excitatory and inhibitory circuits. However, how this balance is established during development remains poorly understood. Here, we investigate the role of interleukin-1 receptor accessory protein-like 1 (IL1RAPL1), a protein linked to cognitive function which interacts with neuronal calcium sensor 1 (NCS-1) in the development of mouse cerebellum. Using Il1rapl1-deficient mice, we found that absence of IL1RAPL1 causes a transient disinhibition of deep cerebellar nuclei neurons between postnatal days 10 and 14 (P10/P14). Upstream, in the cerebellar cortex, we found developmental perturbations in the activity level of molecular layer interneurons (MLIs), resulting in the premature appearance of giant GABAA-mediated inhibitory post-synaptic currents capable of silencing PCs. Examination of feed-forward recruitment of MLIs by parallel fibres shows that during this P10/P14 time window, MLIs were more responsive to incoming excitatory drive. Thus, we conclude that IL1RAPL1 exerts a key function during cerebellar development in establishing local excitation/inhibition balance.

New insights into cellular prion protein (PrPc) functions: the "ying and yang" of a relevant protein

The conversion of cellular prion protein (PrP(c)), a GPI-anchored protein, into a protease-K-resistant and infective form (generally termed PrP(sc)) is mainly responsible for Transmissible Spongiform Encephalopathies (TSEs), characterized by neuronal degeneration and progressive loss of basic brain functions. Although PrP(c) is expressed by a wide range of tissues throughout the body, the complete repertoire of its functions has not been fully determined. Recent studies have confirmed its participation in basic physiological processes such as cell proliferation and the regulation of cellular homeostasis. Other studies indicate that PrP(c) interacts with several molecules to activate signaling cascades with a high number of cellular effects. To determine PrP(c) functions, transgenic mouse models have been generated in the last decade. In particular, mice lacking specific domains of the PrP(c) protein have revealed the contribution of these domains to neurodegenerative processes. A dual role of PrP(c) has been shown, since most authors report protective roles for this protein while others describe pro-apoptotic functions. In this review, we summarize new findings on PrP(c) functions, especially those related to neural degeneration and cell signaling.

Cytosolic prion protein is the predominant anti-Bax prion protein form: exclusion of transmembrane and secreted prion protein forms in the anti-Bax function

Prion protein (PrP) prevents Bax-mediated cell death by inhibiting the initial Bax conformational change that converts cytosolic Bax into a pro-apoptotic protein. PrP is mostly a glycophosphatidylinositol-anchored cell surface protein but it is also retrotranslocated into cytosolic PrP (CyPrP) or can become a type 1 or type 2 transmembrane protein. To determine the form and subcellular location of the PrP that has anti-Bax function, we co-expressed various Syrian hamster PrP (SHaPrP) mutants that favour specific PrP topologies and subcellular localization with N-terminally green fluorescent protein tagged pro-apoptotic Bax (EGFP-Bax) in MCF-7 cells and primary human neurons. Mutants that generate both CyPrP and secreted PrP ((Sec)PrP) or only CyPrP have anti-Bax activity. Mutants that produce (Ctm)PrP or (Ntm)PrP lose the anti-Bax activity, despite their ability to also make (Sec)PrP. Transmembrane-generating mutants do not produce CyPrP and both normal and cognate mutant forms of CyPrP rescue against the loss of anti-Bax activity. (Sec)PrP-generating constructs also produce non-membrane attached (Sec)PrP. However, this form of PrP has minimal anti-Bax activity. We conclude that CyPrP is the predominant form of PrP with anti-Bax function. These results imply that the retrotranslocation of PrP encompasses a survival function and is not merely a pathway for the proteasomal degradation of misfolded protein.

BCL-2 counteracts Doppel-induced apoptosis of prion-protein-deficient Purkinje cells in the Ngsk Prnp(0/0) mouse

The pro-apoptotic factor BAX has recently been shown to contribute to Purkinje cell (PC) apoptosis induced by the neurotoxic prion-like protein Doppel (Dpl) in the prion-protein-deficient Ngsk Prnp(0/0) (NP(0/0)) mouse. In view of cellular prion protein (PrP(c)) ability to counteract Dpl neurotoxicity and favor neuronal survival like BCL-2, we investigated the effects of the anti-apoptotic factor BCL-2 on Dpl neurotoxicity by studying the progression of PC death in aging NP(0/0)-Hu-bcl-2 double mutant mice overexpressing human BCL-2 (Hu-bcl-2). Quantitative analysis showed that significantly more PCs survived in NP(0/0)-Hu-bcl-2 double mutants compared with the NP(0/0) mutants. However, number of PCs remained inferior to wild-type levels and to the increased number of PCs observed in Hu-bcl-2 mutants. In the NP(0/0) mutants, Dpl-induced PC death occurred preferentially in the aldolase C-negative parasagittal compartments of the cerebellar cortex. Activation of glial cells exclusively in these compartments, which was abolished by the expression of Hu-bcl-2 in the double mutants, suggested that chronic inflammation is an indirect consequence of Dpl-induced PC death. This partial rescue of NP(0/0) PCs by Hu-bcl-2 expression was similar to that observed in NP(0/0):Bax(-/-) double mutants with bax deletion. Taken together, these data strongly support the involvement of BCL-2 family-dependent apoptotic pathways in Dpl neurotoxicity. The capacity of BCL-2 to compensate PrP(c) deficiency by rescuing PCs from Dpl-induced death suggests that the BCL-2-like property of PrP(c) may impair Dpl-like neurotoxic pathways in wild-type neurons.

Diminishing apoptosis by deletion of Bax or overexpression of Bcl-2 does not protect against infectious prion toxicity in vivo

B-cell lymphoma protein 2 (Bcl-2) and Bcl-2-associated X protein (Bax), key antiapoptotic and proapoptotic proteins, respectively, have important roles in acute and chronic models of neurologic disease. Several studies have implicated Bax and Bcl-2 in mediating neurotoxicity in prion diseases. To determine whether diminishing apoptotic cell death is protective in an infectious prion disease model we inoculated mice that either were null for proapoptotic Bax or overexpressed antiapoptotic Bcl-2. Interestingly, genetic manipulation of apoptosis did not lessen the clinical severity of disease. Moreover, some disease parameters, such as behavioral alterations and death, occurred slightly earlier in mice that are null for Bax or overexpress Bcl-2. These results suggest that Bax and Bcl-2 mediated apoptotic pathways are not the major contributing factor to the clinical or pathological features of infectious prion disease.

Prion pathogenesis is independent of caspase-12

The pathogenic mechanism(s) underlying neurodegenerative diseases associated with protein misfolding is unclear. Several studies have implicated ER stress pathways in neurodegenerative conditions, including prion disease, amyotrophic lateral sclerosis, Alzheimer's disease and many others. The ER stress response and upregulation of ER stress-responsive chaperones is observed in the brains of patients affected with Creutzfeldt-Jacob disease and in mouse models of prion diseases. In particular, the processing of caspase-12, an ER-localized caspase, correlates with neuronal cell death in prion disease. However, the contribution of caspase-12 to neurodegeneration has not been directly addressed in vivo. We confirm that ER stress is induced and that caspase-12 is proteolytically processed in a murine model of infectious prion disease. To address the causality of caspase-12 in mediating infectious prion pathogenesis, we inoculated mice deficient in caspase-12 with prions. The survival, behavior, pathology and accumulation of proteinase K-resistant PrP are indistinguishable between caspase-12 knockout and control mice, suggesting that caspase-12 is not necessary for mediating the neurotoxic effects of prion protein misfolding.