The cellular Basis of Magnetoreception

Each year millions of animals undertake remarkable migratory journeys, across oceans and through hemispheres, guided by the Earth`s magnetic field. The cellular basis of this enigmatic sense, known as magnetoreception, remains an unsolved scientific mystery. One hypothesis that attempts to explain the basis of this sensory faculty is known as the magnetite theory of magnetoreception. It argues that magnetic information is transduced into a neuronal impulse by employing the iron oxide magnetite (Fe 2 O3 ). Current evidence indicates that pigeons employ a magnetoreceptor that is associated with the ophthalmic branch of the trigeminal nerve and the vestibular system, but the sensory cells remain undiscovered. The goal of this proposal is to discover these primary magnetosensitive cells. This overall objective can be divided into three specific aims: (1) the identification of putative magnetoreceptive cells (PMCs); (2) the characterisation of PMCs; and (3) the ablation of PMCs. In Aim 1 we will identify PMCs by employing histological techniques and our newly built "magnetoscope", which permits the isolation of cells with innate magnetic properties. In Aim 2 we will use a range of molecular, subcellular, magnetic and functional tools to characterise PMCs. In collaboration with Dr Jeremy Shaw (University of Western Australia) we will employ Isolated Magnetic Cell Transmission Electron Microscopy (iMAC-TEM), Energy Filtered Transmission Electron Microscopy (EFTEM) and Selected Area Electron Diffraction (SAED) to investigate the subcellular architecture of these cells and to ascertain whether or not they contain magnetite. We will complement these experiments with Magnetic Force Microscopy (MFM) in collaboration with Dr Michael Winklhofer (Ludwig Maximilians Universität) to gain insight into their magnetic properties. To ascertain whether PMCs are sensitive to changes in the magnetic field in vitro we will undertake calcium imaging with our newly built ferro-magnetic free microscope, specifically asking whether cells are tuned to particular vectors of the magnetic field. If we have identified the true magnetosensitive cells we predict that their ablation will result in a reduction in magnetically activated neuronal activity in the CNS and a behavioural impairment on a task that requires detection of magnetic stimuli. We will test this hypothesis in Aim 3 by generating a transgenic pigeon that expresses an inducible cell death cassette in PMCs in collaboration with Dr Michael McGrew (Roslin Institute). To identify genetic markers for PMCs we will undertake transcriptomics and in situ hybridisation, alongside bioinformatic analysis. The promoters of marker genes will then be coupled to the bacterial nitroreductase enzyme (which results in cell death in the presence of the prodrug CB1954), and transgenic pigeons created by lentivirus injection. Following genetic ablation we will undertake behavioural and anatomical phenotyping so a direct correlation can be made between a particular cell type and the magnetic sense.