Candidate compass genes in fish

Fitak, R.R., B.R. Wheeler, D.A Ernst, K.J. Lohmann, and S. Johnsen. 2017. Candidate genes mediating magnetoreception in rainbow trout (Oncorhynchus mykiss). Biology Letters. 13: 20170142.

Many migrating animals are able to sense the Earth’s magnetic field.

Many animals, like homing pigeons and bats, have internal compasses that detect the Earth’s magnetic field. This six sense of magnetoreception is important in orientation and navigation, and provides information about an animal’s altitude, direction, and/or location. Experiments have shown that this sense is particularly on cloudy days and nights, when animals can’t use the Sun or stars to navigate.

But while we’ve known about magnetoreception for a long time, the molecular and cellular mechanisms responsible for sensing magnetism remain mysterious. How does a biological system detect a magnetic field? Researchers from Duke University and the University of North Carolina Chapel Hill used a shotgun approach (RNA-seq) to identify genes potentially involved in magnetoreception in rainbow trout (Oncorhynchus mykiss), a migratory salmonid known to sense magnetic stimuli.

Animal magnetism

The aurora is caused by the Earth’s magnetic field interacting with solar wind from the Sun [Flickr].

The single biggest issue in the study of magnetoreception is that a sensory receptor has yet to be conclusively identified. We know that exposing some animals to strong magnetic field disrupts their ability to navigate. But what proteins or cellular structures react to magnetism, and what part of the brain is responsible for processing that information?

Most studies have focused on two potential mechanisms: chemical magnetoreception, where the magnetic field affects the biochemistry of the cryptochrome protein, and magnetite-based receptors, which contain metals that interact with the Earth’s magnetic field directly, and then convey that information to the nervous system (similar to how vibrations in the air are transduced by hair cells to provide information that the auditory system interprets as sound).

One of the difficulties in investigating the validity of these proposed mechanisms is that the magnetoreception system probably requires only very small amounts of magnetic material. Locating this tiny amount of magnetism is like finding a needle in a haystack.

Spying on intracellular messages

The central dogma of molecular biology: DNA is transcribed into RNA, which is used to create proteins [Flickr]

How do you find a needle in a haystack when you’re not entirely sure what that needle looks like? Fitak et al. took a shotgun approach: they exposed rainbow trout to a strong magnetic field, and then compared any and all changes in RNA between these pulsed fish and unexposed fish.

RNAs are key molecules for communication within a cell. Messenger RNA is formed when small sections of DNA are transcribed. Unlike DNA, this RNA is able to leave the protective shell of nucleus, and give the rest of the cell instructions for synthesizing proteins. Proteins are the workhorses of a cell, and are involved in everything a cell does, including metabolism, immunity, growth, and reproduction. There are many other kinds of RNA in the cell, and pinning down each of their functions is an area of active research.

By categorizing which RNAs are present in a cell, researchers can intercept the orders sent by the nucleus and make inferences about which genes were activated (or inactivated) by a magnetic stimulus. Categorizing these RNAs allows researchers to guess which metabolic pathways and proteins are important in sensing and responding to magnetic fields.

Genes that respond to a magnetic pulse

Broad view of the Earth’s magnetic field [Wikimedia].

A total of 181 genes significantly increased or decreased in expression when fish when exposed to a magnetic pulse compared to unexposed fish. These genes included clusters of genes thought to be involved in iron regulation and magnetoreception as well as genes associated with photosensitive structures like the retina in the eye.

The researchers noted the eighteen of the nineteen (95%) of ferritin-coding genes increased in expression with exposure to a magnetic pulse. Ferritin is a protein that isolates excess iron in cells and stores it to prevent oxidative damage. The team reasoned that perhaps the magnetic pulse allowed the stored iron to break free from the ferritin, causing the cells to respond by increasing ferritin levels to soak up the excess iron. Along the same lines, the expression of genes involved in protecting cells from oxidative damage caused by free iron also increased with a magnetic pulse. The strong activation of these systems suggests ferritin might be involved in the maintenance of iron-based magnetoreceptors after a strong magnetic pulse.

Two magnets repelling each other, with their magnetic fields visible [Wikimedia].

Several genes associated with light-sensing structures and pathways were also differentially expressed between pulsed and unpulsed fish. Interestingly, there were no differences in the expression of cryptochromes, the proteins hypothesized to be involved in chemical magnetoreception. Why a magnetic pulse affected these light-sensitive genes is unclear, but perhaps the magnetoreception and visual systems are closely associated, either in physical space or by using some of the same cellular machinery.

Sensing magnetic fields

Rainbow trout migrate between freshwater streams and the ocean during their life cycle [Flickr].

This study was the first to use a broad-based approach (testing the expression of all genes, instead of picking a few likely to show changes) to ask questions about magnetoreception in animals. By looking at the whole-cell response to a magnetic pulse, the researchers identified candidate genes, such as those controlling the ferritin protein and some aspects of the visual system, as potentially important in helping trout sense magnetic fields. Further studies can use these candidate genes as a starting point to figure out exactly how some animals are able to sense magnetic fields.

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