The Radical-Pair Mechanism of Magnetoreception

Abstract

The Radical-Pair Mechanism of Magnetoreception Hore PJ, Mouritsen H. The Radical-Pair Mechanism of Magnetoreception.Annu Rev Biophys. 2016 Jul 5;45:299- 344. doi: 10.1146/annurev-biophys-032116-094545. Abstract Although it has been known for almost half a century that migratory birds can detect the direction of the Earth's magnetic field, the primary sensory mechanism behind this remarkable feat is still unclear. The leading hypothesis centers on radical pairs-magnetically sensitive chemical intermediates formed by photoexcitation of cryptochrome proteins in the retina. Our primary aim here is to explain the chemical and physical aspects of the radical-pair mechanism to biologists and the biological and chemical aspects to physicists. In doing so, we review the current state of knowledge on magnetoreception mechanisms. We dare to hope that this tutorial will stimulate new interdisciplinary experimental and theoretical work that will shed much-needed additional light on this fascinating problem in sensory biology. ncbi.nlm.nih.gov Localisation of the Putative Magnetoreceptive Protein Cryptochrome 1b in the Retinae of Migratory Birds and Homing Pigeons Bolte P et al. Localisation of the Putative Magnetoreceptive Protein Cryptochrome 1b in the Retinae of Migratory Birds and Homing Pigeons. PLoS One. 2016 Mar 8;11(3):e0147819. doi: 10.1371/journal.pone.0147819. Abstract Cryptochromes are ubiquitously expressed in various animal tissues including the retina. Some cryptochromes are involved in regulating circadian activity. Cryptochrome proteins have also been suggested to mediate the primary mechanism in light-dependent magnetic compass orientation in birds. Cryptochrome 1b (Cry1b) exhibits a unique carboxy terminus exclusively found in birds so far, which might be indicative for a specialised function. Cryptochrome 1a (Cry1a) is so far the only cryptochrome protein that has been localised to specific cell types within the retina of migratory birds. Here we show that Cry1b, an alternative splice variant of Cry1a, is also expressed in the retina of migratory birds, but it is primarily located in other cell types than Cry1a. This could suggest different functions for the two splice products. Using diagnostic bird-specific antibodies (that allow for a precise discrimination between both proteins), we show that Cry1b protein is found in the retinae of migratory European robins (Erithacus rubecula), migratory Northern Wheatears (Oenanthe oenanthe) and pigeons (Columba livia). In all three species, retinal Cry1b is localised in cell types which have been discussed as potentially well suited locations for magnetoreception: Cry1b is observed in the cytosol of ganglion cells, displaced ganglion cells, and in photoreceptor inner segments. The cytosolic rather than nucleic location of Cry1b in the retina reported here speaks against a circadian clock regulatory function of Cry1b and it allows for the possible involvement of Cry1b in a radical-pair-based magnetoreception mechanism. ncbi.nlm.nih.gov Cryptochrome 2 mediates directional magnetoreception in cockroaches Bazalova O et al. Cryptochrome 2 mediates directional magnetoreception in cockroaches.Proc Natl Acad Sci U S A. 2016 Feb 9;113(6):1660-5. doi: 10.1073/pnas.1518622113. Abstract The ability to perceive geomagnetic fields (GMFs) represents a fascinating biological phenomenon. Studies on transgenic flies have provided evidence that photosensitive Cryptochromes (Cry) are involved in the response to magnetic fields (MFs). However, none of the studies tackled the problem of whether the Cry-dependent magnetosensitivity is coupled to the sole MF presence or to the direction of MF vector. In this study, we used gene silencing and a directional MF to show that mammalian-like Cry2 is necessary for a genuine directional response to periodic rotations of the GMF vector in two insect species. Longer wavelengths of light required higher photon fluxes for a detectable behavioral response, and a sharp detection border was present in the cyan/green spectral region. Both observations are consistent with involvement of the FADox, FAD(•-) and FADH(-) redox forms of flavin. The response was lost upon covering the eyes, demonstrating that the signal is perceived in the eye region. Immunohistochemical staining detected Cry2 in the hemispherical layer of laminal glia cells underneath the retina. Together, these findings identified the eye-localized Cry2 as an indispensable component and a likely photoreceptor of the directional GMF response. Our study is thus a clear step forward in deciphering the in vivo effects of GMF and supports the interaction of underlying mechanism with the visual system. ncbi.nlm.nih.gov

AI evidence extraction

Main findings

This article reviews the radical-pair hypothesis for avian magnetoreception, focusing on magnetically sensitive radical pairs formed by photoexcitation of cryptochrome proteins in the retina, and summarizes the current state of knowledge on magnetoreception mechanisms.

Outcomes measured

• magnetoreception mechanism
• radical-pair mechanism
• cryptochrome involvement in magnetic sensing

{
    "study_type": "review",
    "exposure": {
        "band": null,
        "source": "Earth's magnetic field",
        "frequency_mhz": null,
        "sar_wkg": null,
        "duration": null
    },
    "population": null,
    "sample_size": null,
    "outcomes": [
        "magnetoreception mechanism",
        "radical-pair mechanism",
        "cryptochrome involvement in magnetic sensing"
    ],
    "main_findings": "This article reviews the radical-pair hypothesis for avian magnetoreception, focusing on magnetically sensitive radical pairs formed by photoexcitation of cryptochrome proteins in the retina, and summarizes the current state of knowledge on magnetoreception mechanisms.",
    "effect_direction": "unclear",
    "limitations": [],
    "evidence_strength": "insufficient",
    "confidence": 0.66000000000000003108624468950438313186168670654296875,
    "peer_reviewed_likely": "yes",
    "keywords": [
        "magnetoreception",
        "radical pair",
        "cryptochrome",
        "migratory birds",
        "Earth's magnetic field",
        "retina",
        "photoexcitation"
    ],
    "suggested_hubs": []
}

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