Cryptochrome expression in avian UV cones: revisiting the role of CRY1 as magnetoreceptor

Atticus Pinzon-Rodriguez,Rachel Muheim

doi:10.1038/s41598-021-92056-8

Abstract

Cryptochromes (CRY) have been proposed as putative magnetoreceptors in vertebrates. Localisation of CRY1 in the UV cones in the retinas of birds suggested that it could be the candidate magnetoreceptor. However, recent findings argue against this possibility. CRY1 is a type II cryptochrome, a subtype of cryptochromes that may not be inherently photosensitive, and it exhibits a clear circadian expression in the retinas of birds. Here, we reassessed the localisation and distribution of CRY1 in the retina of the zebra finch. Zebra finches have a light-dependent magnetic compass based on a radical-pair mechanism, similar to migratory birds. We found that CRY1 colocalised with the UV/V opsin (SWS1) in the outer segments of UV cones, but restricted to the tip of the segments. CRY1 was found in all UV cones across the entire retina, with the highest densities near the fovea. Pre-exposure of birds to different wavelengths of light did not result in any difference in CRY1 detection, suggesting that CRY1 did not undergo any detectable functional changes as result of light activation. Considering that CRY1 is likely not involved in magnetoreception, our findings open the possibility for an involvement in different, yet undetermined functions in the avian UV/V cones.

Highlights

Cryptochromes (CRY) are flavoproteins well known for their role in the regulation of circadian activity in diverse animals
Even though the western blot analysis on total protein extracted from the retinas of the zebra finch with our antibody revealed a single band at a lower molecular weight than the expected size, the immunofluorescent signal location and pattern coincides with that independently reported by Niessner[26,27,56], and Bolte[44] and colleagues, strongly supporting that the antibody used in this paper is likely detecting CRY1
When we started the current study, no isoforms of CRY1 were known for the zebra finch, which is why we make no differentiation between CRY1 and CRY1a throughout the text

Summary

Introduction

Cryptochromes (CRY) are flavoproteins well known for their role in the regulation of circadian activity in diverse animals (reviewed b y1–3). Magnetoreception of the light-dependent magnetic compass is suggested to be based on a radical-pair process which involves a light-induced electron transfer between two reaction partners, and thereby the creation of an excited-state radical pair which the magnetic field can act upon[4,5,6,7,8] Assuming that such radical-pair-based magnetoreceptors are arranged in an ordered, spherical array, with different receptors aligned at different angles to the magnetic field, the animal would be able to perceive a sensory pattern centrally symmetric to the magnetic field lines[4,9,10,11]. We examined the cellular localisation and distribution of CRY1 protein across the zebra finch retina and tested whether the detection of CRY1 protein was wavelength dependent by examining the abundance of CRY1 after exposure to monochromatic lights

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