Neural cell engraftment therapy for sporadic Creutzfeldt-Jakob disease restores neuroelectrophysiological parameters in a cerebral organoid model

BACKGROUND

Sporadic Creutzfeldt-Jakob disease (sCJD), the most common human prion disease, is a fatal neurodegenerative disease with currently no treatment options. Stem cell therapy for neurodegenerative diseases is emerging as a possible treatment option. However, while there are a few clinical trials for other neurodegenerative disorders such as Parkinson's disease, prion disease cell therapy research has so far been confined to animal models.

METHODS

Here, we use a novel approach to study cell therapies in sCJD using a human cerebral organoid model. Cerebral organoids can be infected with sCJD prions allowing us to assess how neural precursor cell (NPC) therapy impacts the progression of sCJD. After 90 days of sCJD or mock infection, organoids were either seeded with NPCs or left unseeded and monitored for cellular composition changes, prion infection parameters and neuroelectrophysiological function at 180 days post-infection.

RESULTS

Our results showed NPCs integrated into organoids leading to an increase in neuronal markers and changes in cell signaling irrespective of sCJD infection. Although a small, but significant, decrease in protease-resistant PrP deposition was observed in the CJD-infected organoids that received the NPCs, other disease-associated parameters showed minimal changes. However, the NPCs had a beneficial impact on organoid function following infection. sCJD infection caused reduction in neuronal spike rate and mean burst spike rate, indicative of reduced action potentials. NPC seeding restored these electrophysiological parameters to the uninfected control level.

CONCLUSIONS

Together with the previous animal studies, our results support that cell therapy may have some functional benefit for the treatment of human prion diseases.

Sporadic Creutzfeldt-Jakob disease infected human cerebral organoids retain the original human brain subtype features following transmission to humanized transgenic mice

Human cerebral organoids (COs) are three-dimensional self-organizing cultures of cerebral brain tissue differentiated from induced pluripotent stem cells. We have recently shown that COs are susceptible to infection with different subtypes of Creutzfeldt-Jakob disease (CJD) prions, which in humans cause different manifestations of the disease. The ability to study live human brain tissue infected with different CJD subtypes opens a wide array of possibilities from differentiating mechanisms of cell death and identifying neuronal selective vulnerabilities to testing therapeutics. However, the question remained as to whether the prions generated in the CO model truly represent those in the infecting inoculum. Mouse models expressing human prion protein are commonly used to characterize human prion disease as they reproduce many of the molecular and clinical phenotypes associated with CJD subtypes. We therefore inoculated these mice with COs that had been infected with two CJD subtypes (MV1 and MV2) to see if the original subtype characteristics (referred to as strains once transmitted into a model organism) of the infecting prions were maintained in the COs when compared with the original human brain inocula. We found that disease characteristics caused by the molecular subtype of the disease associated prion protein were similar in mice inoculated with either CO derived material or human brain material, demonstrating that the disease associated prions generated in COs shared strain characteristics with those in humans. As the first and only in vitro model of human neurodegenerative disease that can faithfully reproduce different subtypes of prion disease, these findings support the use of the CO model for investigating human prion diseases and their subtypes.

Infectious and Sporadic Prion Diseases

Prion diseases are progressive fatal encephalopathies characterized by a neurodegenerative pathology, the tissue deposition of abnormally folded prion protein and, in general, potential transmissibility. Creutzfeldt-Jakob disease (CJD) is the commonest human prion disease and occurs in three principal forms: sporadic (idiopathic), acquired (infectious), and inherited (genetic). This chapter concerns the sporadic and acquired forms. Sporadic CJD occurs worldwide and affects mainly the middle aged and elderly. There are recognized genetic risk factors-most importantly the PRNP-129 polymorphism. The acquired forms of CJD consist of iatrogenic CJD (accidental transmission of CJD via medical or surgical procedures) and variant CJD (vJCD) (which originated as a zoonosis via bovine spongiform encephalopathy (BSE)-contamination of human food). The main causes of iatrogenic CJD are cadaveric-derived human growth hormone treatment and dura mater surgical grafts. The PRNP-129 polymorphism has important effects on iatrogenic infection, including overall susceptibility and incubation period. vCJD, resulting from dietary exposure to BSE, has affected mostly the United Kingdom, followed by France. All tested cases were originally PRNP-129MM, although two MV cases have been identified recently (one possible; one definite). vCJD has been secondarily transmitted via blood transfusion and a blood product. There is continuing concern over secondary transmission since there is evidence-from lymphoreticular tissue studies-of extensive subclinical infection in the UK general population, although a further recent study has caused uncertainty over the significance of the previous studies. While definitive diagnosis of CJD is pathological, recent developments in protein amplification and detection have led to significantly better clinical diagnosis.

Comparative Study of Prions in Iatrogenic and Sporadic Creutzfeldt-Jakob Disease

Differentiating iatrogenic Creutzfeldt-Jakob disease (iCJD) from sporadic CJD (sCJD) would be useful for the identification and prevention of human-to-human prion transmission. Currently, the diagnosis of iCJD depends on identification of a recognized source of contamination to which patients have been exposed, in addition to fulfilling basic requirements for the establishment of diagnosis of CJD. Attempts to identify differences in clinical manifestations, neuropathological changes and pathological prion protein (PrP^Sc) between iCJD and sCJD have been unsuccessful. In the present study, using a variety of more sophisticated methods including sucrose step gradient sedimentation, conformational stability immunoassay, protein misfolding cyclic amplification (PMCA), fragment-mapping, and transmission study, we show no significant differences in gel profiles, oligomeric state, conformational stability and infectivity of PrP^Sc between iCJD and sCJD. However, using PMCA, we find that convertibility and amplification efficiency of PrP^Sc is greater in iCJD than in sCJD in a polymorphism-dependent manner. Moreover, two protease-resistant PrP C-terminal fragments (termed PrP-CTF12/13) were detected in all 9 cases of sCJD but not in 6 of 8 cases of iCJD tested in this study. The use of fragment mapping- and PMCA-based assays thus provides a means to distinguish most cases of iCJD from sCJD.

Ethics in prion disease

This paper is intended to discuss some of the scientific and ethical issues that are created by increased research efforts towards earlier diagnosis, as well as to treatment of, human prion diseases (and related dementias), including the resulting consequences for individuals, their families, and society. Most patients with prion disease currently are diagnosed when they are about 2/3 of the way through their disease course (Geschwind et al., 2010a; Paterson et al., 2012b), when the disease has progressed so far that even treatments that stop the disease process would probably have little benefit. Although there are currently no treatments available for prion diseases, we and others have realized that we must diagnose patients earlier and with greater accuracy so that future treatments have hope of success. As approximately 15% of prion diseases have a autosomal dominant genetic etiology, this further adds to the complexity of ethical issues, particularly regarding when to conduct genetic testing, release of genetic results, and when or if to implement experimental therapies. Human prion diseases are both infectious and transmissible; great care is required to balance the needs of the family and individual with both public health needs and strained hospital budgets. It is essential to proactively examine and address the ethical issues involved, as well as to define and in turn provide best standards of care.