Measurement of the Absolute Membrane Potential Using Fluorescence Lifetime Imaging Microscopy (FLIM)

Neural plasticity, which underpins learning and memory, declines progressively with age and is particularly compromised in neurodegenerative disorders. Neural excitation is an emerging early-stage functional hallmark of the decline of plasticity with age and of neurodegeneration, especially Alzheimer’s disease. How neural excitation drives the functional decline of neurons with age is poorly understood, however. We established O2-sensing in Caenorhabditis elegans as a model to study the effect of neural excitation on the ageing brain. The worms’ response to ambient [O2] depends on a memory of the [O2] it experienced in the past. Ageing animals lose their ability to adapt to changing O2 conditions when kept at high [O2], but retain it when kept in low-O2 environments. This loss of plasticity is caused by chronic excitation and high [Ca2+] in the O2-sensing neurons, which accelerates the decline of plasticity with age both at the neuronal and the behavioural level. Gene expression profiling of the O2-sensing neurons revealed that neuronal activity alters age-related changes in the transcription of genes modulating Ca2+ homeostasis, pointing to their role in mediating the decline. RNAi knockdown of the Ca2+ sensor calmodulin in the O2-sensing neurons restores plasticity in old animals, while RNAi of calcineurin reduces plasticity. Our results therefore suggest that Ca2+ homeostasis is a critical intermediary between neuronal activity state and neuronal ageing, and that chronic excitation negatively affects neuronal ageing by dysregulating intracellular [Ca2+]. We are now investigating the role of neural excitation and Ca2+ dysregulation as early-stage risk factors in models of neurodegeneration.