Mitochondrial bioenergetic deficits in C9orf72 amyotrophic lateral sclerosis motor neurons cause dysfunctional axonal homeostasis

Abstract

Axonal dysfunction is a common phenotype in neurodegenerative disorders, including in amyotrophic lateral sclerosis (ALS), where the key pathological cell-type, the motor neuron (MN), has an axon extending up to a metre long. The maintenance of axonal function is a highly energy-demanding process, raising the question of whether MN cellular energetics is perturbed in ALS, and whether its recovery promotes axonal rescue. To address this, we undertook cellular and molecular interrogation of multiple patient-derived induced pluripotent stem cell lines and patient autopsy samples harbouring the most common ALS causing mutation, C9orf72. Using paired mutant and isogenic expansion-corrected controls, we show that C9orf72 MNs have shorter axons, impaired fast axonal transport of mitochondrial cargo, and altered mitochondrial bioenergetic function. RNAseq revealed reduced gene expression of mitochondrially encoded electron transport chain transcripts, with neuropathological analysis of C9orf72-ALS post-mortem tissue importantly confirming selective dysregulation of the mitochondrially encoded transcripts in ventral horn spinal MNs, but not in corresponding dorsal horn sensory neurons, with findings reflected at the protein level. Mitochondrial DNA copy number was unaltered, both in vitro and in human post-mortem tissue. Genetic manipulation of mitochondrial biogenesis in C9orf72 MNs corrected the bioenergetic deficit and also rescued the axonal length and transport phenotypes. Collectively, our data show that loss of mitochondrial function is a key mediator of axonal dysfunction in C9orf72-ALS, and that boosting MN bioenergetics is sufficient to restore axonal homeostasis, opening new potential therapeutic strategies for ALS that target mitochondrial function.

Synaptic disruption and CREB‐regulated transcription are restored by K + channel blockers in ALS

“…This approach not only confirmed the downregulation of synaptic transcripts highlighted by the complete GO biological processes analysis (Appendix Fig S2B), but also correlated the pathologic up-regulations observed in ALS C9orf72 MN with gene classes associated with cerebral ageing and stress (Appendix Fig S2C;Ho et al, 2016). The strong congruency of both analysis approaches confirmed previous findings showing reduced expression of synaptic genes in C9orf72-mutant cultures (Sareen et al, 2013;Selvaraj et al, 2018;Mehta et al, 2021). Thus, our data demonstrate that alterations in the synapse-related transcriptome represent an important pathologic manifestation of ALS C9orf72 MN.…”

The Link between Oxidative Stress, Redox Status, Bioenergetics and Mitochondria in the Pathophysiology of ALS

“…Mitochondrial dysfunction occurs at an early stage of axon degeneration [47]. ALS-associated mitochondrial bioenergetic deficits are evident in the most common ALS-causing mutation, C9orf72 [48]. Interestingly, C9orf72 MNs also show shorter axons and impaired fast axonal transport of mitochondrial cargo [48].…”

“…ALS-associated mitochondrial bioenergetic deficits are evident in the most common ALS-causing mutation, C9orf72 [48]. Interestingly, C9orf72 MNs also show shorter axons and impaired fast axonal transport of mitochondrial cargo [48]. Importantly, it has been shown that PGC1α (peroxisome proliferator-activated receptor gamma coactivator 1-alpha) overexpression improves mitochondrial metabolism, and that appears enough to restore axonal dynamics [48].…”

Synaptopathy Mechanisms in ALS Caused by C9orf72 Repeat Expansion

“…Another study has suggested that mitochondrial ATP production is dysregulated in C9orf72 disease (Mehta et al, 2021). This source of energy is necessary for membrane potential generation, synaptic vesicle recruitment and release, as well as protein phosphorylation reactions (Stefani et al, 1997;Attwell and Laughlin, 2001;Valenti et al, 2014).…”

“…All of these processes are critical for neuroplasticity and can be modified by changes in ATP production and release (Mattson and Liu, 2003). Mehta et al (2021) have shown that C9orf72 MNs have shorter axons caused by altered mitochondrial bioenergetic function (Mehta et al, 2021). The authors found reduced gene expression of mitochondria encoded electron transport chain transcripts in MNs-iPSCs from C9orf72 patients.…”