Loss of function of the mitochondrial peptidase PITRM1 induces proteotoxic stress and Alzheimer’s disease-like pathology in human cerebral organoids

Abstract

Mutations in pitrilysin metallopeptidase 1 (PITRM1), a mitochondrial protease involved in mitochondrial precursor processing and degradation, result in a slow-progressive syndrome, characterized by cerebellar ataxia, psychotic episodes and obsessive behavior as well as cognitive decline. To investigate the pathogenetic mechanisms of mitochondrial presequence processing, we employed cortical neurons and cerebral organoids generated from PITRM1 knockout human induced pluripotent stem cells (iPSCs). PITRM1 deficiency strongly induced mitochondrial unfolded protein response (UPRmt) and enhanced mitochondrial clearance in iPSC-derived neurons. Furthermore, we observed increased levels of amyloid precursor protein and amyloid β in PITRM1 knockout neurons. However, neither cell death nor protein aggregates were observed in 2D iPSC-derived cortical neuronal cultures. On the contrary, cerebral organoids generated from PITRM1 knockout iPSCs spontaneously developed over time pathological features of Alzheimer’s disease (AD), including accumulation of protein aggregates, tau pathology, and neuronal cell death. Importantly, we provide evidence for a protective role of UPRmt and mitochondrial clearance against impaired mitochondrial presequence processing and proteotoxic stress. In summary, we propose a novel concept of PITRM1-linked neurological syndrome whereby defects of mitochondrial presequence processing induce an early activation of UPRmt that, in turn, modulates cytosolic quality control pathways. Thus our work supports a mechanistic link between mitochondrial function and common neurodegenerative proteinopathies.

Modeling neurodegenerative diseases with cerebral organoids and other three-dimensional culture systems: focus on Alzheimer’s disease

“…In addition, increases in ubiquitinated proteins, mitochondrial unfolded protein response (UPR) transcripts and upregulation of mitochondrial proteases were detected in PITRM1 KO COs compared to control COs. These findings indicate that PITRM1 is essential for maintaining mitochondrial protein homeostasis by degrading target sequences like Aβ and possibly tau [75].…”

“…β and γ-secretase inhibitors decrease Aβ and pTau accumulation in COs [35,36,45,138]. Treatments with other compounds like heparin, heparinase III, DAPT [81], IL-4 [41] and nicotinamide mononucleotide [75] also show a reduction in Aβ induced toxicity and reversal of other AD-like pathologies such as pTau and Aβ plaque-like accumulation. Further studies are required to investigate if treatment and removal of toxic Aβ or tau species prior to the onset of toxic protein aggregation, altered synaptic function and cognitive deficits, can prevent, and/or delay the development of AD-like pathology in COs.…”

“…PITRM1 dysfunction causes improper degradation and over-accumulation of Aβ in the mitochondria, resulting in mitochondrial dysfunction and neurodegeneration [74]. Since PITRM1 deficiency can cause embryonic lethality in mice, mechanisms underlying Aβ accumulation were investigated using human PITRM1 knockout COs (published as a preprint in bioRxiv) [75]. Increases in APP, Aβ40, Aβ42, the Aβ42/Aβ40 ratio, plaque-like amyloid aggregates and pTau-S202/T205 levels were detected in PITRM1 KO COs compared to controls.…”

“…Mitochondrial UPR inhibition via integrated stress response inhibitor (ISRIB) resulted in increases of APP, Aβ42/Aβ40 and pTau in PITRM1 KO COs, suggesting a decrease in mitochondrial clearance [75]. Treatment with the NAD+ precursor, nicotinamide mononucleotide, which can improve mitochondrial function and clearance [76], rescued the Aβ and tau pathology in PITRM1 KO COs [75]. This indicates that mitochondrial UPR activation and mitochondrial clearance may play an important protective role in attenuating AD-like phenotypes induced by PITRM1 deficiency.…”

“…These novel CO models will thus help the field to understand the role of these and other risk factors in the development and progression of AD. It should be noted that CRISPR-Cas9 genomic editing has not only been used to generate isogenic controls [45,54,75,89] but also used as a therapeutic strategy to modulate or revert accumulation of Aβ [78]. This technology can also be used to design a high throughput screen [182] for genes associated with Aβ and tau aggregation, which will shed more light on the underlying mechanisms of AD.…”