Investigating the role of mitochondrial Ca2+ in in vivo models of neurodegenerative diseases

Abstract

Neurodegenerative diseases (NDs) such as Parkinson’s disease (PD), Alzheimer’s disease (AD), Frontotemporal dementia (FTD) and Huntington’s disease (HD) represent a significant and growing burden on society and healthcare worldwide. This challenge is exacerbated by the limited understanding of the underlying causes and pathological mechanisms of NDs, coupled with the absence of effective disease-altering treatments. Among the various pathological hallmarks associated with NDs, mitochondrial dysfunction has emerged as a common and critical factor. Mitochondria play a central role in cellular processes, including sequestering and releasing Ca2+. The dysregulation of mitochondrial Ca2+ homeostasis has recently been recognised as a key pathology of neurodegeneration, although the causes and impacts of mitochondrial Ca2+ dysregulations remain unclear. Mitochondrial Ca2+ homeostasis is maintained through the tightly regulated balance of Ca2+ influx through the mitochondrial Ca2+ uniporter (MCU) complex and efflux through the Na+/Ca2+ transporter (NCLX). This balance is further influenced by Ca2+ transfer from the endoplasmic reticulum (ER) to mitochondria. Mitochondrial Ca2+ is a master regulator of essential cellular functions, including metabolism, reactive oxygen species (ROS) production, autophagy, mitochondrial dynamics, and cell death pathways; processes that have been closely linked to ND pathology. In this thesis, l employed a single in vivo platform to investigate the dynamics of mitochondrial Ca2+ and its significance in models of PD, AD, FTD and HD. Despite arising from different pathomechanistic origins, these diverse Drosophila ND models share a common pathology - a consistent increase in neuronal mitochondrial steady-state Ca2+ levels. Strikingly, these models also exhibited an impaired mitochondrial Ca2+ buffering capacity and abnormalities in mitochondrial-ER contacts. Notably, genetic interventions that either reduce Ca2+ influx through partial or complete loss of MCU, or enhanced Ca2+ efflux through NCLX overexpression were sufficient to normalise ND-associated elevated mitochondrial Ca2+ levels. These interventions not only prevented progressive neurodegeneration and ameliorated mitochondrial morphology abnormalities, but also robustly suppressed locomotion and lifespan deficits. In a Pink1 KO model of PD, partial loss of MCU reduced ROS, and in PD, HD and AD models, it ameliorated biomarkers of impaired autophagy. The success of these genetic manipulations to suppress key pathological pathways of neurodegeneration underpins the role of mitochondrial Ca2+ dysregulation as an early event in the pathogenesis of NDs. The findings of this thesis highlight the central role of mitochondrial dysfunction, particularly impaired mitochondrial Ca2+ homeostasis, in the pathogenesis of NDs. Dysregulation of mitochondrial Ca2+ is a common pathology across a diverse range of ND models, including those where protein aggregation is the dominant pathology, such as HD, AD and FTD. This research demonstrates that mitochondrial Ca2+ dysregulation likely occurs early in disease progression and that targeting this dysfunction is sufficient to ameliorate a range of ND-associated phenotypes.