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

Autophagy in normal tissues of camel (Camelus dromedarius) with focus on immunoexpression of LC3 and LC3B

Autophagy is a highly regulated intracellular pathway for degradation and recycling of cytoplasmic protein aggregates and entire organelles. The autophagic pathway is stimulated by nutrient starvation, which prompted us to study the desert camel. Various organs of the camel undergo ecological and physiological stress due to food and water deprivation, dehydration and long exposure to solar radiation. We investigated the immunohistochemical expression of specific biomarkers of autophagy under normal conditions as a baseline for later work on stressed individuals. The autophagy-specific biomarkers, microtubule-associated protein1 light chain 3 (LC3), and its cleaved variant, LC3B, were strongly expressed in the cytosol of all tissues examined. The cytosolic immunoreactivity of LC3 was relatively weak, diffuse and vacuolar, while that of LC3B was stronger, punctate and at lower levels. LC3 appears to be associated with the autophagosomal membranes, either free or lysosome-bounded. LC3B is specific for the autophagosome-lysosome complexes and their degraded, granular contents. Autophagy was strongly expressed in CNS neurons and intestinal neural elements, which suggests a protective function for the nervous system. Autophagic markers also were seen in deformed immune-competent cells with fragmented nuclei in lymph nodes, spleen and gut-associated lymphoid tissue (GALT), which suggests a "suicidal" activity of eliminating unneeded cells. Autophagy, as measured by LC3 and LC3B expression, may participate in a general regulatory mechanism in tissues of the desert camel.

The molecular mechanisms between autophagy and apoptosis: potential role in central nervous system disorders

Autophagy involves degradation of dysfunctional cellular components through the actions of lysosomes. Apoptosis is the process of programmed cell death involving a series of characteristic cell changes. Autophagy and apoptosis, as self-destructive processes, play an important role in the pathogenesis of neurological diseases; and a crosstalk between "self-eating" (autophagy) and "self-killing" (apoptosis) plays an important role in pathological cellular adaptation. Expert knowledge of autophagy and apoptosis has increased in recent years, particularly in regards to cellular and molecular mechanisms. The crosstalk between autophagy and apoptosis was partially uncovered and several key molecules, including Bcl-2 family members, Beclin 1, and p53 were identified. However, the precise mechanisms of such a crosstalk remain to be elucidated. This current review article aims to summarize key mediators of the autophagy-apoptosis crosstalk in pathological conditions, and to highlight recent advances in the field, as well as to discuss further investigations and therapeutic potentials of manipulating those mechanisms in central nervous system diseases.

BECN1/Beclin 1 is recruited into lipid rafts by prion to activate autophagy in response to amyloid β 42

Prion protein (PRNP) has been implicated in various types of neurodegenerative diseases. Although much is known about prion diseases, the function of cellular PRNP remains cryptic. Here, we show that PRNP mediates amyloid β1–42 (Aβ42)-induced autophagy activation through its interaction with BECN1. Treatment with Aβ42 enhanced autophagy flux in neuronal cells. Aβ42-induced autophagy activation, however, was impaired in prnp-knockout primary cortical neurons and Prnp-knockdown or prnp-knockout neuronal cells. Immunoprecipitation assays revealed that PRNP interacted with BECN1 via the BCL2-binding domain of BECN1. This interaction promoted the subcellular localization of BECN1 into lipid rafts of the plasma membrane and enhanced activity of PtdIns3K (whose catalytic subunit is termed PIK3C3, mammalian ortholog of yeast VPS34) in lipid rafts by generating PtdIns3P in response to Aβ42. Further, the levels of lipid rafts that colocalized with BECN1, decreased in the brains of aged C57BL/6 mice, as did PRNP. These results suggested that PRNP interacts with BECN1 to recruit the PIK3C3 complex into lipid rafts and thus activates autophagy in response to Aβ42, defining a novel role of PRNP in the regulation of autophagy.

Deficiency of prion protein induces impaired autophagic flux in neurons

Normal cellular prion protein (PrP^C) is highly expressed in the central nervous system. The Zürich I Prnp-deficient mouse strain did not show an abnormal phenotype in initial studies, however, in later studies, deficits in exploratory behavior and short- and long-term memory have been revealed. In the present study, numerous autophagic vacuoles were found in neurons from Zürich I Prnp-deficient mice. The autophagic accumulation in the soma of cortical neurons in Zürich I Prnp-deficient mice was observed as early as 3 months of age, and in the hippocampal neurons at 6 months of age. Specifically, there is accumulation of electron dense pigments associated with autophagy in the neurons of Zürich I Prnp-deficient mice. Furthermore, autophagic accumulations were observed as early as 3 months of age in the CA3 region of hippocampal and cerebral cortical neuropils. The autophagic vacuoles increased with age in the hippocampus of Zürich I Prnp-deficient mice at a faster rate and to a greater extent than in normal C57BL/6J mice, whereas the cortex exhibited high levels that were maintained from 3 months old in Zürich I Prnp-deficient mice. The pigmented autophagic accumulation is due to the incompletely digested material from autophagic vacuoles. Furthermore, a deficiency in PrP^C may disrupt the autophagic flux by inhibiting autophagosome-lysosomal fusion. Overall, our results provide insight into the protective role of PrP^C in neurons, which may play a role in normal behavior and other brain functions.

The Functional Role of Prion Protein (PrPC) on Autophagy

Cellular prion protein (PrP^C) plays an important role in the cellular defense against oxidative stress. However, the exact protective mechanism of PrP^C is unclear. Autophagy is essential for survival, differentiation, development, and homeostasis in several organisms. Although the role that autophagy plays in neurodegenerative disease has yet to be established, it is clear that autophagy-induced cell death is observed in neurodegenerative disorders that exhibit protein aggregations. Moreover, autophagy can promote cell survival and cell death under various conditions. In this review, we describe the involvement of autophagy in prion disease and the effects of PrP^C.

Role of autophagy in prion protein-induced neurodegenerative diseases

Prion diseases, characterized by spongiform degeneration and the accumulation of misfolded and aggregated PrP(Sc) in the central nervous system, are one of fatal neurodegenerative and infectious disorders of humans and animals. In earlier studies, autophagy vacuoles in neurons were frequently observed in neurodegenerative diseases such as Alzheimer's, Parkinson's, and Huntington's diseases as well as prion diseases. Autophagy is a highly conserved homeostatic process by which several cytoplasmic components (proteins or organelles) are sequestered in a double-membrane-bound vesicle termed 'autophagosome' and degraded upon their fusion with lysosome. The pathway of intercellular self-digestion at basal physiological levels is indispensable for maintaining the healthy status of tissues and organs. In case of prion infection, increasing evidence indicates that autophagy has a crucial ability of eliminating pathological PrP(Sc) accumulated within neurons. In contrast, autophagy dysfunction in affected neurons may contribute to the formation of spongiform changes. In this review, we summarized recent findings about the effect of mammalian autophagy in neurodegenerative disorders, particularly in prion diseases. We also summarized the therapeutic potential of some small molecules (such as lithium, rapamycin, Sirtuin 1 and resveratrol) targets to mitigate such diseases on brain function. Furthermore, we discussed the controversial role of autophagy, whether it mediates neuronal toxicity or serves a protective function in neurodegenerative disorders.

Oxidative stress impairs autophagic flux in prion protein-deficient hippocampal cells

We previously reported that autophagy is upregulated in Prnp-deficient (Prnp ( 0/0) ) hippocampal neuronal cells in comparison to cellular prion protein (PrP (C) )-expressing (Prnp (+/+) ) control cells under conditions of serum deprivation. In this study, we determined whether a protective mechanism of PrP (C) is associated with autophagy using Prnp ( 0/0) hippocampal neuronal cells under hydrogen peroxide (H 2O 2)-induced oxidative stress. We found that Prnp ( 0/0) cells were more susceptible to oxidative stress than Prnp (+/+) cells in a dose- and time-dependent manner. In addition, we observed enhanced autophagy by immunoblotting, which detected the conversion of microtubule-associated protein 1 light chain 3 β (LC3B)-I to LC3B-II, and we observed increased punctate LC3B immunostaining in H 2O 2-treated Prnp ( 0/0) cells compared with H 2O 2-treated control cells. Interestingly, this enhanced autophagy was due to impaired autophagic flux in the H 2O 2-treated Prnp ( 0/0) cells, while the H 2O 2-treated Prnp (+/+) cells showed enhanced autophagic flux. Furthermore, caspase-dependent and independent apoptosis was observed when both cell lines were exposed to H 2O 2. Moreover, the inhibition of autophagosome formation by Atg7 siRNA revealed that increased autophagic flux in Prnp (+/+) cells contributes to the prosurvival effect of autophagy against H 2O 2 cytotoxicity. Taken together, our results provide the first experimental evidence that the deficiency of PrP (C) may impair autophagic flux via H 2O 2-induced oxidative stress.

Activation of the macroautophagic system in scrapie-infected experimental animals and human genetic prion diseases

Macroautophagy is an important process for removing misfolded and aggregated protein in cells, the dysfunction of which has been directly linked to an increasing number of neurodegenerative disorders. However, the details of macroautophagy in prion diseases remain obscure. Here we demonstrated that in the terminal stages of scrapie strain 263K-infected hamsters and human genetic prion diseases, the microtubule-associated protein 1 light chain 3 (LC3) was converted from the cytosolic form to the autophagosome-bound membrane form. Macroautophagy substrate sequestosome 1 (SQSTM1) and polyubiquitinated proteins were downregulated in the brains of sick individuals, indicating enhanced macroautophagic protein degradation. The levels of mechanistic target of rapamycin (MTOR) and phosphorylated MTOR (p-MTOR) were significantly decreased, which implies that this enhancement of the macroautophagic response is likely through the MTOR pathway which is a negative regulator for the initiation of macroautophagy. Dynamic assays of the autophagic system in the brains of scrapie experimental hamsters after inoculation showed that alterations of the autophagic system appeared along with the deposits of PrP(Sc) in the infected brains. Immunofluorescent assays revealed specific staining of autophagosomes in neurons that were not colocalized with deposits of PrP(Sc) in the brains of scrapie infected hamsters, however, autophagosome did colocalize with PrP(Sc) in a prion-infected cell line after treatment with bafilomycin A(1). These results suggest that activation of macroautophagy in brains is a disease-correlative phenomenon in prion diseases.

Unraveling the neuroprotective mechanisms of PrP (C) in excitotoxicity

Knowledge of the natural roles of cellular prion protein (PrP (C) ) is essential to an understanding of the molecular basis of prion pathologies. This GPI-anchored protein has been described in synaptic contacts, and loss of its synaptic function in complex systems may contribute to the synaptic loss and neuronal degeneration observed in prionopathy. In addition, Prnp knockout mice show enhanced susceptibility to several excitotoxic insults, GABAA receptor-mediated fast inhibition was weakened, LTP was modified and cellular stress increased. Although little is known about how PrP (C) exerts its function at the synapse or the downstream events leading to PrP (C) -mediated neuroprotection against excitotoxic insults, PrP (C) has recently been reported to interact with two glutamate receptor subunits (NR2D and GluR6/7). In both cases the presence of PrP (C) blocks the neurotoxicity induced by NMDA and Kainate respectively. Furthermore, signals for seizure and neuronal cell death in response to Kainate in Prnp knockout mouse are associated with JNK3 activity, through enhancing the interaction of GluR6 with PSD-95. In combination with previous data, these results shed light on the molecular mechanisms behind the role of PrP (C) in excitotoxicity. Future experimental approaches are suggested and discussed.

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.