FTLD Patient–Derived Fibroblasts Show Defective Mitochondrial Function and Accumulation of p62

Abstract

Frontotemporal lobar degeneration (FTLD) is a clinically, genetically, and neuropathologically heterogeneous group of neurodegenerative syndromes, leading to progressive cognitive dysfunction and frontal and temporal atrophy. C9orf72 hexanucleotide repeat expansion (C9-HRE) is the most common genetic cause of FTLD, but pathogenic mechanisms underlying FTLD are not fully understood. Here, we compared cellular features and functional properties, especially related to protein degradation pathways and mitochondrial function, of FTLD patient–derived skin fibroblasts from C9-HRE carriers and non-carriers and healthy donors. Fibroblasts from C9-HRE carriers were found to produce RNA foci, but no dipeptide repeat proteins, and they showed unchanged levels of C9orf72 mRNA transcripts. The main protein degradation pathways, the ubiquitin–proteasome system and autophagy, did not show alterations between the fibroblasts from C9-HRE-carrying and non-carrying FTLD patients and compared to healthy controls. An increase in the number and size of p62-positive puncta was evident in fibroblasts from both C9-HRE carriers and non-carriers. In addition, several parameters of mitochondrial function, namely, basal and maximal respiration and respiration linked to ATP production, were significantly reduced in the FTLD patient–derived fibroblasts from both C9-HRE carriers and non-carriers. Our findings suggest that FTLD patient–derived fibroblasts, regardless of whether they carry the C9-HRE expansion, show unchanged proteasomal and autophagic function, but significantly impaired mitochondrial function and increased accumulation of p62 when compared to control fibroblasts. These findings suggest the possibility of utilizing FTLD patient–derived fibroblasts as a platform for biomarker discovery and testing of drugs targeted to specific cellular functions, such as mitochondrial respiration.

Frontotemporal dementia patient-derived iPSC neurons show cell pathological hallmarks and evidence for synaptic dysfunction and DNA damage

“…The fact that the number of p62/SQSTM1-positive vesicles was not increased at the same time with the enlarged size and intensity, but rather showed a trend towards a decrease in the FTD neurons, could suggest that the clearance of the p62/SQSTM1-containing vesicles is defective and the vesicles may fuse together. Interestingly, our previous studies in the skin fibroblasts from the same patients, from which the iPSCs were generated for this study, indicated similar results [32]. Thus, autophagy function and protein degradation might be altered in the iPSC-derived neurons from FTD patients, regardless of the C9-HRE status.…”

“…The iPSC lines used in this study were generated from skin biopsies as described in [32] Finnish FTD patients (53-77 years) clinically diagnosed with bvFTD and three age-matched healthy control individuals according to [33]. The FTD patient iPSCs were derived from three C9-HRE carriers and three sporadic (non-genetic) FTD patients.…”

Bioenergetic and Autophagic Characterization of Skin Fibroblasts from C9orf72 Patients

“…Although it can be argued that these results were not statistically significant ( p = 0.057), it has to be taken into account that it might be due to a small sample size effect, since our cohort consisted of four patients and four control samples. Recently, a study by Leskelä et al [ 31 ] did not indicate decreased C9orf72 levels at either mRNA or protein level in the fibroblasts of C9orf72 patients. Nevertheless, the approximation taken by these authors was different since they measured C9orf72 mRNA by quantitative PCR and protein levels in cells treated with lactacystin, a proteasome inhibitor.…”

Mitochondrial Quality Control Strategies: Potential Therapeutic Targets for Neurodegenerative Diseases?

“…OPTN and TBK1 mutations interfere with LC3 recruitment to depolarized mitochondria ( Li et al, 2016 ; Ryan and Tumbarello, 2018 ; Harding et al, 2021 ). Autophagosomes accumulate in mouse motor neurons expressing mutant SOD1 and patient fibroblasts expressing mutant C9orf72 ( Rudnick et al, 2017 ; Leskela et al, 2021 ). Mutant SOD1 also suppresses endogenous Miro levels through a Parkin-dependent pathway, resulting in impaired mitochondrial transport and mitophagy ( Moller et al, 2017 ).…”