Caspase-12 and endoplasmic reticulum stress mediate neurotoxicity of pathological prion protein

Abstract

Prion diseases are characterized by accumulation of misfolded prion protein (PrP Sc ), and neuronal death by apoptosis. Here we show that nanomolar concentrations of puri®ed PrP Sc from mouse scrapie brain induce apoptosis of N2A neuroblastoma cells. PrP Sc toxicity was associated with an increase of intracellular calcium released from endoplasmic reticulum (ER) and up-regulation of several ER chaperones. Caspase-12 activation was detected in cells treated with PrP Sc , and cellular death was inhibited by overexpression of a catalytic mutant of caspase-12 or an ER-targeted Bcl-2 chimeric protein.Scrapie-infected N2A cells were more susceptible to ER-stress and to PrP Sc toxicity than non-infected cells. In scrapie-infected mice a correlation between caspase-12 activation and neuronal loss was observed in histological and biochemical analyses of different brain areas. The extent of prion replication was closely correlated with the up-regulation of ER-stress chaperone proteins. Similar results were observed in humans affected with sporadic and variant Creutzfeldt±Jakob disease, implicating for the ®rst time the caspase-12 dependent pathway in a neurodegenerative disease in vivo, and thus offering novel potential targets for the treatment of prion disorders.

ER stress and neurodegenerative diseases

“…55 In keeping with this, brain tissue from PrP Sc -infected mice and patients with Creutzfeldt-Jakob disease showed higher levels of caspase-12. 55 This data indicates that the activation of ER stress and caspase-12 may play a role in neuronal apoptosis associated with the accumulation of mutant prion protein in neurons. It was reported that in the murine scrapie model, the accumulation of PrP Sc was closely followed by the induction of the ER chaperone Grp58.…”

“…N2A neuroblastoma cells treated with PrP Sc produce a fast and sustained increase in intracellular calcium levels. 55 Pretreatment with thapsigargin that depletes ER calcium stores reduced the increase in calcium observed with PrP Sc . 55 This indicates that the ER is involved in the mechanisms by which PrP Sc acts in nerve cells.…”

“…55 Pretreatment with thapsigargin that depletes ER calcium stores reduced the increase in calcium observed with PrP Sc . 55 This indicates that the ER is involved in the mechanisms by which PrP Sc acts in nerve cells. ER calcium levels and signaling are tightly regulated by activity of various channel proteins and by the Bcl-2 family proteins ( Figure 2).…”

Inflammatory caspases and inflammasomes: master switches of inflammation

“…39 In addition, caspase-12 was shown to participate in the development of ER-stress-related neurodegenerative disorders such as Alzheimer's disease or prion associated diseases. 39,40 Whereas degradation or proteolysis of caspase-12 is a well-established hallmark of ER stress, its central role in ER-stress-induced apoptosis was challenged by various studies. [41][42][43] The precise function of caspase-12 in this particular pathway is yet unclear and controversial.…”

Cell models of prion infection

“…For instance, infection with several murine prion strains impairs the cell response of GT1 and N2a cells to oxidative stress [89], presumably through a decrease in superoxide dismutase activity. RML infection was also shown to sensitise N2a and GT1 cells to pro-apoptotic stimuli such as endoplasmic reticulum stress or to mild proteasome inhibition [54,71]. However, it is important to point out that prion-infected cell lines accumulating infectious titres similar to those in brain tissue do not show any obvious cytopathic effect, with the possible exception of RML-infected GT1 cells that inconsistently undergo apoptosis.…”