FTLD Patient\u2013Derived Fibroblasts Show Defective Mitochondrial Function and Accumulation of p62

Stina Leskelä,Ville Korhonen,Hannah Rostalski,Dorit Hoffmann,Eino Solje,Päivi Hartikainen,Ville Leinonen,Rebekka Wittrahm,Annakaisa Haapasalo,Nadine Huber,Anne M Remes,Mikko Hiltunen

doi:10.1007/s12035-021-02475-x

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.

Highlights

Frontotemporal lobar degeneration (FTLD) is one of the most common causes of early-onset dementia in people under 65 years of age [1]
To assess whether FTLD patient–derived fibroblasts from Chromosome 9 open reading frame gene (C9orf72) hexanucleotide repeat expansion (C9-HRE) carriers show alterations in C9orf72 mRNA levels compared to fibroblasts from FTLD patients not carrying the C9-HRE or control subjects, total and isoform Aspecific C9orf72 mRNA levels were detected using qPCR
A significant increase in C9orf72 levels after lactacystin treatment was detected in fibroblasts without the C9-HRE, with a similar trend showing in healthy controls (p = 0.07) and fibroblasts with the C9-HRE (p = 0.08) (Fig. 1c, d), which might suggest proteasomal regulation of C9orf72 protein levels in the fibroblasts in a similar manner to our previous studies in mouse neuronal cells overexpressing the C9orf72 isoform A [42]

Summary

Introduction

Frontotemporal lobar degeneration (FTLD) is one of the most common causes of early-onset dementia in people under 65 years of age [1]. It is a clinically, genetically, and neuropathologically heterogeneous group of neurodegenerative syndromes, leading to atrophy predominantly in the frontal and temporal lobes of the brain [2] accompanied by progressive cognitive dysfunction, behavioral changes, difficulties in understanding or producing speech, and frequently neuropsychiatric symptoms. The GGGGCC hexanucleotide repeat expansion in C9orf (C9-HRE) is the most common genetic cause of both FTLD and ALS [9,10,11]. The exact pathological threshold of the C9-HRE is unclear, but fewer than 30 repeats are generally considered non-pathogenic [12]

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