Proceedings of The Physiological Society

Sleep Sleep and Circadian Rhythms (London, UK) (2018) Proc Physiol Soc 42, C17

Poster Communications

Amyloid Beta Oligomeric Structure Governs Sleep/Wake States in Zebrafish

G. Ozcan1, S. Lim1, J. Rihel1

1. Cell and Developmental Biology, University College London, London, United Kingdom.

Introduction - Despite decades of research, the endogenous function of amyloid beta (Aβ), the hallmark protein of Alzheimer's Disease (AD), remains unknown. Knowing what Aβ does physiologically would enable us to understand what is going wrong in the disease state. Recent studies have highlighted links between Alzheimer's disease and sleep1. Sleep is disrupted in AD patients, often years before cognitive deficits2. Since Aβ levels cycle across the sleep/wake cycle1, we hypothesized that one in vivo function of Aβ may be to directly modulate sleep/wake states. Several features of zebrafish biology make it an excellent model to investigate the role of Aβ in sleep: Zebrafish have a complete repertoire of amyloid precursor protein (APP) processing machinery and most of the Αβ receptors are highly conserved in zebrafish. In addition, the zebrafish brain is anatomically and molecularly similar to the mammalian brain, and many behaviours like sleep, are controlled by similar neuronal mechanisms. Materials and Methods - Using CRISPRs we mutated the two zebrafish APP genes to downregulate Aβ levels. To upregulate Aβ levels acutely, we injected different oligomeric forms of Aβ with final brain concentrations in the picomolar (i.e. physiological) range to anesthetized zebrafish larvae (in 1 mM MS222). Different oligomers/fibrils were obtained by incubating Aβ preparations at 4°C or 25°C and the length of oligomers were assessed by TEM (Transmission Electron Microscopy). We determined the effects of Aβ oligomer/fibrils on sleep/wake behaviour via video monitoring and used whole brain activity mapping to identify neurons that differentially respond to Aβ. Results and Conclusions - While APP loss of function mutants had an 11±1.0% decrease in waking activity, Aβ in its shorter oligomeric forms (median oligomer length 45±4 nm); caused strongly increased activity (14±4.9%) and decreased sleep (-12±9.2%). In contrast, longer Aβ oligomers/fibrils (median oligomer length 75±7 nm) acutely increase sleep (30±12.0%) without neurotoxicity. Consistent with their effects on wakefulness, short Aβ oligomers induced neuronal activity in a subset of neurons in the posterior hypothalamus, which is a major wake-promoting center in the vertebrate brain. In contrast, longer oligomeric forms do not activate these wake-promoting neurons but instead globally dampen neuronal activity. Our experiments suggest that physiological and temporary upregulation of Aβ levels can directly promote both zebrafish sleep and wakefulness depending on the oligomeric state of Aβ via activation of discrete subpopulations of neurons. We are now performing a CRISPR-mediated genetic screen in zebrafish to identify the receptors that interact with Aβ to mediate these effects on sleep and wake. Understanding neural and molecular mechanisms of Aβ's effect on sleep/wake behaviour may provide a mechanistic understanding of what goes wrong in AD.

Where applicable, experiments conform with Society ethical requirements