PERK inhibition delays neurodegeneration and improves motor function in a mouse model of Marinesco-Sjögren syndrome.

Marinesco-Sjögren syndrome (MSS) is a rare, early onset, autosomal recessive multisystem disorder characterized by cerebellar ataxia, cataracts and myopathy. Most MSS cases are caused by loss-of-function mutations in the gene encoding SIL1, a nucleotide exchange factor for the molecular chaperone BiP which is essential for correct protein folding in the endoplasmic reticulum. Woozy mice carrying a spontaneous Sil1 mutation recapitulate key pathological features of MSS, including cerebellar atrophy with degeneration of Purkinje cells and progressive myopathy. Because the PERK branch of the unfolded protein response is activated in degenerating neurons of woozy mice, and inhibiting PERK-mediated translational attenuation has shown protective effects in protein-misfolding neurodegenerative disease models, we tested the therapeutic efficacy of GSK2606414, a potent inhibitor of PERK. Mice were chronically treated with GSK2606414 starting from a presymptomatic stage, and the effects were evaluated on biochemical, histopathological and clinical readouts. GSK2606414 delayed Purkinje cell degeneration and the onset of motor deficits, prolonging the asymptomatic phase of the disease; it also reduced the skeletal muscle abnormalities and improved motor performance during the symptomatic phase. The protein but not the mRNA level of ORP150, a nucleotide exchange factor which can substitute for SIL1, was increased in the cerebellum of GSK2606414-treated woozy mice, suggesting that translational recovery promoted the synthesis of this alternative BiP co-factor. Targeting PERK signaling may have beneficial disease-modifying effects in carriers of SIL1 mutations.

Cyclophillin A deficiency accelerates RML-induced prion disease.

Prion diseases typically involve brain deposition of abnormally folded prion protein, which is associated with activated glia and increased cytokine production. Cyclophilin A (CypA) is a ubiquitous protein with peptidyl prolyl cis-trans isomerase activity, which regulates protein folding, and can be secreted by cells in response to inflammatory stimuli. On the basis of in vitro studies, CypA was proposed to mediate glial activation during prion infection. To investigate the role of CypA in vivo, we inoculated CypA+/+, CypA+/- and CypA-/- mice with the RML prion strain, and recorded the time to onset of neurological signs and to terminal disease, and the astrocyte and microglia response at presymptomatic and symptomatic stages. Time to onset of disease and survival were significantly shorter in CypA-deficient mice than CypA-expressing controls. CypA-deficient mice had significantly greater microglial activation in the presymptomatic stage, and analysis of anti- and pro-inflammatory microglial markers indicated a shift towards a pro-inflammatory phenotype. There was no difference in astrocyte activation. This suggests that CypA contributes to dampening the pro-inflammatory microglial response during the early stage of RML-induced prion disease.

Transgenic fatal familial insomnia mice indicate prion infectivity-independent mechanisms of pathogenesis and phenotypic expression of disease.

Fatal familial insomnia (FFI) and a genetic form of Creutzfeldt-Jakob disease (CJD178) are clinically different prion disorders linked to the D178N prion protein (PrP) mutation. The disease phenotype is determined by the 129 M/V polymorphism on the mutant allele, which is thought to influence D178N PrP misfolding, leading to the formation of distinctive prion strains with specific neurotoxic properties. However, the mechanism by which misfolded variants of mutant PrP cause different diseases is not known. We generated transgenic (Tg) mice expressing the mouse PrP homolog of the FFI mutation. These mice synthesize a misfolded form of mutant PrP in their brains and develop a neurological illness with severe sleep disruption, highly reminiscent of FFI and different from that of analogously generated Tg(CJD) mice modeling CJD178. No prion infectivity was detectable in Tg(FFI) and Tg(CJD) brains by bioassay or protein misfolding cyclic amplification, indicating that mutant PrP has disease-encoding properties that do not depend on its ability to propagate its misfolded conformation. Tg(FFI) and Tg(CJD) neurons have different patterns of intracellular PrP accumulation associated with distinct morphological abnormalities of the endoplasmic reticulum and Golgi, suggesting that mutation-specific alterations of secretory transport may contribute to the disease phenotype.

A tetracationic porphyrin with dual anti-prion activity.

Prions are deadly infectious agents made of PrPSc, a misfolded variant of the cellular prion protein (PrPC) which self-propagates by inducing misfolding of native PrPC. PrPSc can adopt different pathogenic conformations (prion strains), which can be resistant to potential drugs, or acquire drug resistance, hampering the development of effective therapies. We identified Zn(II)-BnPyP, a tetracationic porphyrin that binds to distinct domains of native PrPC, eliciting a dual anti-prion effect. Zn(II)-BnPyP binding to a C-terminal pocket destabilizes the native PrPC fold, hindering conversion to PrPSc; Zn(II)-BnPyP binding to the flexible N-terminal tail disrupts N- to C-terminal interactions, triggering PrPC endocytosis and lysosomal degradation, thus reducing the substrate for PrPSc generation. Zn(II)-BnPyP inhibits propagation of different prion strains in vitro, in neuronal cells and organotypic brain cultures. These results identify a PrPC-targeting compound with an unprecedented dual mechanism of action which might be exploited to achieve anti-prion effects without engendering drug resistance.

Erratum: A tetracationic porphyrin with dual anti-prion activity.

[This corrects the article DOI: 10.1016/j.isci.2023.107480.].

Transgenic mice recapitulate the phenotypic heterogeneity of genetic prion diseases without developing prion infectivity: Role of intracellular PrP retention in neurotoxicity.

Genetic prion diseases are degenerative brain disorders caused by mutations in the gene encoding the prion protein (PrP). Different PrP mutations cause different diseases, including Creutzfeldt-Jakob disease (CJD), Gerstmann-Sträussler-Scheinker (GSS) syndrome and fatal familial insomnia (FFI). The reason for this variability is not known. It has been suggested that prion strains with unique self-replicating and neurotoxic properties emerge spontaneously in individuals carrying PrP mutations, dictating the phenotypic expression of disease. We generated transgenic mice expressing the FFI mutation, and found that they developed a fatal neurological illness highly reminiscent of FFI, and different from those of similarly generated mice modeling genetic CJD and GSS. Thus transgenic mice recapitulate the phenotypic differences seen in humans. The mutant PrPs expressed in these mice are misfolded but unable to self-replicate. They accumulate in different compartments of the neuronal secretory pathway, impairing the membrane delivery of ion channels essential for neuronal function. Our results indicate that conversion of mutant PrP into an infectious isoform is not required for pathogenesis, and suggest that the phenotypic variability may be due to different effects of mutant PrP on intracellular transport.