Circadian influence on mitochondrial dynamics in Drosophila melanogaster: implication for health and disease

Nearly all metabolic processes display rhythmic fluctuations over the 24-hour cycle, underlying a tight association between cellular metabolism and the circadian clock. Environmental cues linked to the light-dark cycle—such as changes in light exposure and feeding patterns—modulate nutrient availability and energy demand, which are primarily met through mitochondrial function. However, the mechanisms by which mitochondria adapt to or anticipate circadian changes remain incompletely understood. This PhD thesis investigates the effects of light and circadian entrainment on cell metabolism, with particular attention to mitochondrial dysfunction. We focused on the photoreceptors in the retina as: 1) they are highly metabolically active, with energy demands that change across the day to support phototransduction; 2) in Drosophila, they have an autonomous circadian clock that regulates visual sensitivity. Initially, we examined the rhythmic nature of mitochondrial dynamics in the eye; subsequently, we assessed the impact of circadian disruption on a neurodegenerative disorder caused by mitochondrial dysfunction, namely the Parkinson’s disease (PD). Our analyses revealed distinct oscillatory patterns in mitochondrial number across photoreceptor subtypes, shaped by spectral sensitivity and synaptic connectivity, and dependent on circadian and/or light inputs. These findings suggest the involvement of endogenous mechanisms orchestrating mitochondrial rhythmicity. These observations prompted us to examine whether mitochondrial size and morphology in the retina are under circadian control. Disruption of clock function (in period null mutants) and impairment of light entrainment (in cryptochrome null mutants or flies expressing constitutively active form) resulted in enlarged mitochondria, a phenotype that could be the result of oxidative stress caused by clock dysfunction. This underscores the requirement for a functional, entrainable clock in mitochondrial regulation. Mitochondrial morphology displayed robust daily fluctuations that persisted in cryptochrome mutants, which retain partial light entrainment, but were abolished in period null and in flies expressing constitutively active cryptochrome, resistant to normal light-dependent degradation. These findings highlight the circadian clock as a regulator of mitochondrial morphology, coordinating daytime activity and nocturnal turnover. At the molecular level, these phenotypic changes were mirrored by rhythmic transcription of genes encompassing mitochondrial fission, fusion, and mitophagy in wild-type retinas. Such transcriptional oscillations were lost in period mutants and under constant darkness, except for parkin, which maintained rhythmic expression in the absence of light cues. These results point to a dual regulatory system integrating circadian and light-dependent signalling in the genetic control of mitochondrial function. Given the established link between mitochondrial dysfunction and neurodegenerative disease, we explored the impact of circadian disruption in a Drosophila PD model. Motor and sleep behaviours were assessed under standard LD12:12 conditions and under two disruption paradigms: LD20:4 (20h light:4h dark) and LDL (12h light:12h dim light). Chronic LD20:4 accelerated age-related motor decline, advanced the onset of sleep symptoms in young PD flies, and markedly increased mortality in older individuals. Collectively, our findings demonstrate that the circadian clock is pivotal for preserving mitochondrial adaptability to daily environmental cues. Its disruption compromises mitochondrial homeostasis and exacerbates PD-related phenotypes. This work provides compelling evidence that circadian alignment is critical for sustaining mitochondrial integrity, particularly in the context of neurodegenerative vulnerability, and identifies circadian disruption as a potential accelerating factor in the onset and progression of mitochondrial dysfunction–driven d