Magnetite-based magnetoreception

An MCP-like protein interacts with the MamK cytoskeleton and is involved in magnetotaxis in Magnetospirillum magneticum AMB-1

Magnetotactic bacteria have the unique capacity of aligning and swimming along geomagnetic field lines, a behavior called magnetotaxis. Although this behavior has been observed for 40 years, little is known about its mechanism. Magnetotactic bacteria synthesize unique organelles, magnetosomes, which are magnetic crystals enveloped by membrane. They form chains with the help of the filamentous cytoskeletal protein MamK and impart a net magnetic-dipole moment to the bacterium. The current model proposes that magnetotaxis comprises passive magnetic orientation and active swimming due to flagellar rotation. We thought that magnetic sensing, via the widely used chemotaxis mechanism, might be actively involved in magnetotaxis. We found that the methyl-accepting chemotaxis protein Amb0994 of Magnetospirillum magneticum AMB-1 was capable of carrying out such a function. Amb0994 is encoded by a gene in the magnetosome island, in which genes essential for magnetosome biosynthesis and magnetotaxis are concentrated. Amb0994 lacks periplasmic sensing domain, which is generally involved in sensing stimuli from outside of cells. By constructing fusions with a derivative of yellow-fluorescent-protein, we showed that Amb0994 localizes to the cell poles, where methyl-accepting chemotaxis proteins are usually clustered. We then showed that Amb0994 specifically interacts, via its C-terminal domain, with MamK, using a bimolecular fluorescence complementation assay. Moreover, overproduction of Amb0994 slowed down the response of the bacterium to changes in the direction of the magnetic field. Most importantly, the C-terminal domain of Amb0994, which interacts with MamK, is responsible for this phenotype, suggesting that the interaction between Amb0994 and MamK plays a key role in magnetotaxis. These results lead to a novel explanation for magnetotaxis at the molecular level.

Avian magnetoreception: elaborate iron mineral containing dendrites in the upper beak seem to be a common feature of birds

The magnetic field sensors enabling birds to extract orientational information from the Earth's magnetic field have remained enigmatic. Our previously published results from homing pigeons have made us suggest that the iron containing sensory dendrites in the inner dermal lining of the upper beak are a candidate structure for such an avian magnetometer system. Here we show that similar structures occur in two species of migratory birds (garden warbler, Sylvia borin and European robin, Erithacus rubecula) and a non-migratory bird, the domestic chicken (Gallus gallus). In all these bird species, histological data have revealed dendrites of similar shape and size, all containing iron minerals within distinct subcellular compartments of nervous terminals of the median branch of the Nervus ophthalmicus. We also used microscopic X-ray absorption spectroscopy analyses to identify the involved iron minerals to be almost completely Fe III-oxides. Magnetite (Fe II/III) may also occur in these structures, but not as a major Fe constituent. Our data suggest that this complex dendritic system in the beak is a common feature of birds, and that it may form an essential sensory basis for the evolution of at least certain types of magnetic field guided behavior.

Avian magnetite-based magnetoreception: a physiologist's perspective

It is now well established that animals use the Earth's magnetic field to perform long-distance migration and other navigational tasks. However, the transduction mechanisms that allow the conversion of magnetic field variations into an electric signal by specialized sensory cells remain largely unknown. Among the species that have been shown to sense Earth-strength magnetic fields, birds have been a model of choice since behavioural tests show that their direction-finding abilities are strongly influenced by magnetic fields. Magnetite, a ferromagnetic mineral, has been found in a wide range of organisms, from bacteria to vertebrates. In birds, both superparamagnetic (SPM) and single-domain magnetite have been found to be associated with the trigeminal nerve. Electrophysiological recordings from cells in the trigeminal ganglion have shown an increase in action potential firing in response to magnetic field changes. More recently, histological evidence has demonstrated the presence of SPM magnetite in the subcutis of the pigeon's upper beak. The aims of the present review are to review the evidence for a magnetite-based mechanism in birds and to introduce physiological concepts in order to refine the proposed models.

Biophysics of magnetic orientation: strengthening the interface between theory and experimental design

The first demonstrations of magnetic effects on the behaviour of migratory birds and homing pigeons in laboratory and field experiments, respectively, provided evidence for the longstanding hypothesis that animals such as birds that migrate and home over long distances would benefit from possession of a magnetic sense. Subsequent identification of at least two plausible biophysical mechanisms for magnetoreception in animals, one based on biogenic magnetite and another on radical-pair biochemical reactions, led to major efforts over recent decades to test predictions of the two models, as well as efforts to understand the ultrastructure and function of the possible magnetoreceptor cells. Unfortunately, progress in understanding the magnetic sense has been challenged by: (i) the availability of a relatively small number of techniques for analysing behavioural responses to magnetic fields by animals; (ii) difficulty in achieving reproducible results using the techniques; and (iii) difficulty in development and implementation of new techniques that might bring greater experimental power. As a consequence, laboratory and field techniques used to study the magnetic sense today remain substantially unchanged, despite the huge developments in technology and instrumentation since the techniques were developed in the 1950s. New methods developed for behavioural study of the magnetic sense over the last 30 years include the use of laboratory conditioning techniques and tracking devices based on transmission of radio signals to and from satellites. Here we consider methodological developments in the study of the magnetic sense and present suggestions for increasing the reproducibility and ease of interpretation of experimental studies. We recommend that future experiments invest more effort in automating control of experiments and data capture, control of stimulation and full blinding of experiments in the rare cases where automation is impossible. We also propose new experiments to confirm whether or not animals can detect magnetic fields using the radical-pair effect together with an alternate hypothesis that may explain the dependence on light of responses by animals to magnetic field stimuli.

Radio frequency magnetic fields disrupt magnetoreception in American cockroach

The sense that allows birds to orient themselves by the Earth's magnetic field can be disabled by an oscillating magnetic field whose intensity is just a fraction of the geomagnetic field intensity and whose oscillations fall into the medium or high frequency radio wave bands. This remarkable phenomenon points very clearly at one of two existing alternative magnetoreception mechanisms in terrestrial animals, i.e. the mechanism based on the radical pair reactions of specific photosensitive molecules. As the first such study in invertebrates, our work offers evidence that geomagnetic field reception in American cockroach is sensitive to a weak radio frequency field. Furthermore, we show that the 'deafening' effect at Larmor frequency 1.2 MHz is stronger than at different frequencies. The parameter studied was the rise in locomotor activity of cockroaches induced by periodic changes in the geomagnetic North positions by 60 deg. The onset of the disruptive effect of a 1.2 MHz field was found between 12 nT and 18 nT whereas the threshold of a doubled frequency field 2.4 MHz fell between 18 nT and 44 nT. A 7 MHz field showed no impact even in maximal 44 nT magnetic flux density. The results indicate resonance effects rather than non-specific bias of procedure itself and suggest that insects may be equipped with the same magnetoreception system as the birds.

Testing the role of sensory systems in the migratory heading of a songbird

The identification of the sensory cues and mechanisms by which migratory birds are able to reach the same breeding and wintering grounds year after year has eluded biologists despite more than 50 years of intensive study. While a number of environmental cues have been proposed to play a role in the navigation of birds, arguments still persist about which cues are essential for the experience based navigation shown by adult migrants. To date, few studies have tested the sensory basis of navigational cues used during actual migration in the wild: mainly laboratory based studies or homing during the non-migratory season have been used to investigate this behaviour. Here we tested the role of olfactory and magnetic cues in the migration of the catbird (Dumetella carolinensis) by radio tracking the migration of birds with sensory manipulations during their actual migratory flights. Our data suggest that adult birds treated with zinc sulphate to produce anosmia were unable to show the same orientation as control adults, and instead reverted to a direction similar to that shown by juveniles making their first migration. The magnetic manipulation had no effect on the orientation of either adults or juveniles. These results allow us to propose that the olfactory sense may play a role in experience based migration in adult catbirds. While the olfactory sense has been shown to play a role in the homing of pigeons and other birds, this is the first time it has been implicated in migratory orientation.

Navigation by induction-based magnetoreception in elasmobranch fishes

A quantitative frequency-domain model of induction-based magnetoreception is presented for elasmobranch fishes. We show that orientation with respect to the geomagnetic field can be determined by synchronous detection of electrosensory signals at harmonics of the vestibular frequency. The sensitivity required for this compass-sense mechanism is shown to be less than that known from behavioral experiments. Recent attached-magnet experiments have called into doubt the induction-based mechanism for magnetoreception. We show that the use of attached magnets would interfere with an induction-based mechanism unless relative movement between the electrosensory system and the attached magnet is less than 100 mum. This suggests that further experiments may be required to eliminate induction as a basis for magnetoreception.

Micromagnetic insight into a magnetoreceptor in birds: existence of magnetic field amplifiers in the beak

The Earth's magnetic field provides an important source of directional information for many living organisms, especially birds, but the sensory receptor responsible for magnetic field detection still has to be identified. Recently, magnetic iron oxide particles were detected in dendritic endings of the ophthalmic nerves in the skin of the upper beak of homing pigeons and were shown to fulfill the special prerequisites of a biological receptor. Here we study the proposed receptor theoretically and formulate the criteria for which it becomes operational and can be used for registering the weak magnetic fields as, e.g., the geomagnetic field, by a bird.

Effects of pulsed magnetic field on the formation of magnetosomes in the Magnetospirillum sp. strain AMB-1

Magnetotactic bacteria are a diverse group of microorganisms which possess one or more chains of magnetosomes and are endowed with the ability to use geomagnetic fields for direction sensing, thus providing a simple and excellent model for the study of magnetite-based magnetoreception. In this study, a 50 Hz, 2 mT pulsed magnetic field (PMF) was applied to study the effects on the formation of magnetosomes in Magnetospirillum sp. strain AMB-1. The results showed that the cellular magnetism (R(mag)) of AMB-1 culture significantly increased while the growth of cells remained unaffected after exposure. The number of magnetic particles per cell was enhanced by about 15% and slightly increased ratios of magnetic particles of superparamagnetic property (size <20 nm) and mature magnetosomes (size >50 nm) were observed after exposure to PMF. In addition, the intracellular iron accumulation slightly increased after PMF exposure. Therefore, it was concluded that 50 Hz, 2 mT PMF enhances the formation of magnetosomes in Magnetospirillum sp. strain AMB-1. Our results suggested that lower strength of PMF has no significant effects on the bacterial cell morphologies but could affect crystallization process of magnetosomes to some extent.

Effects of static magnetic field on magnetosome formation and expression of mamA, mms13, mms6 and magA in Magnetospirillum magneticum AMB-1

Magnetotactic bacteria produce nanometer-size intracellular magnetic crystals. The superior crystalline and magnetic properties of magnetosomes have been attracting much interest in medical applications. To investigate effects of intense static magnetic field on magnetosome formation in Magnetospirillum magneticum AMB-1, cultures inoculated with either magnetic or non-magnetic pre-cultures were incubated under 0.2 T static magnetic field or geomagnetic field. The results showed that static magnetic field could impair the cellular growth and raise C(mag) values of the cultures, which means that the percentage of magnetosome-containing bacteria was increased. Static magnetic field exposure also caused an increased number of magnetic particles per cell, which could contribute to the increased cellular magnetism. The iron depletion in medium was slightly increased after static magnetic field exposure. The linearity of magnetosome chain was also affected by static magnetic field. Moreover, the applied intense magnetic field up-regulated mamA, mms13, magA expression when cultures were inoculated with magnetic cells, and mms13 expression in cultures inoculated with non-magnetic cells. The results implied that the interaction of the magnetic field created by magnetosomes in AMB-1 was affected by the imposed magnetic field. The applied static magnetic field could affect the formation of magnetic crystals and the arrangement of the neighboring magnetosome.

Self-assembly of magnetic Ni nanoparticles into 1D arrays with antiferromagnetic order

In this paper, we report on the magnetic properties of isolated nanoparticles and interacting nanochains formed by the self-assembly of Ni nanoparticles. The magnetic properties were studied using superconducting quantum interference device (SQUID) magnetometry and magnetic force microscopy (MFM). We demonstrate that single-domain Ni nanoparticles spontaneously form one-dimensional (1D) chains under the influence of an external magnetic field. Furthermore, such magnetic field-driven self-assembly in conjunction with surface templating produces regular arrays of 1D nanochains with antiferromagnetic intra-chain order. The antiferromagnetic order, which is in striking contrast to what is found for non-interacting nanoparticle assemblies within the chains, can be evidenced from MFM and SQUID measurements.

Chemical magnetoreception in birds: the radical pair mechanism

Migratory birds travel vast distances each year, finding their way by various means, including a remarkable ability to perceive the Earth's magnetic field. Although it has been known for 40 years that birds possess a magnetic compass, avian magnetoreception is poorly understood at all levels from the primary biophysical detection events, signal transduction pathways and neurophysiology, to the processing of information in the brain. It has been proposed that the primary detector is a specialized ocular photoreceptor that plays host to magnetically sensitive photochemical reactions having radical pairs as fleeting intermediates. Here, we present a physical chemist's perspective on the "radical pair mechanism" of compass magnetoreception in birds. We outline the essential chemical requirements for detecting the direction of an Earth-strength approximately 50 microT magnetic field and comment on the likelihood that these might be satisfied in a biologically plausible receptor. Our survey concludes with a discussion of cryptochrome, the photoactive protein that has been put forward as the magnetoreceptor molecule.

Sinusoidal magnetic field stimulates magnetosome formation and affects mamA, mms13, mms6, and magA expression in Magnetospirillum magneticum AMB-1

Magnetic particles are currently one of the most important materials in the industrial sector, where they have been widely used for biotechnological and biomedical applications. To investigate the effects of the imposed magnetic field on biomineralization in Magnetospirillum magneticum AMB-1 and to suggest a new approach that enhances formation of magnetosomes, cultures inoculated with either magnetic or nonmagnetic precultures were incubated under a sinusoidal magnetic field or geomagnetic field. The results showed that the sinusoidal magnetic field up-regulated mms6 expression in the cultures inoculated with magnetic cells, and magA, mms6, and mamA expression in the cultures inoculated with nonmagnetic cells. The applied sinusoidal magnetic field could block cell division, which could contribute to a decrease in the OD600 values and an increase in the coefficient of magnetism values of the cultures, which could mean that the percentage of mature magnetosome-containing bacteria was increased. The linearity of magnetosome chains was affected, but the number of magnetic particles in cells was increased when a sinusoidal magnetic field was applied to the cultures. The results imply that the variable intensity and orientation of the sinusoidal magnetic field resulted in magnetic pole conversion in the newly forming magnetic particles, which could affect the formation of magnetic crystals and the arrangement of the adjacent magnetosome.

Primary processes in sensory cells: current advances

In the course of evolution, the strong and unremitting selective pressure on sensory performance has driven the acuity of sensory organs to its physical limits. As a consequence, the study of primary sensory processes illustrates impressively how far a physiological function can be improved if the survival of a species depends on it. Sensory cells that detect single-photons, single molecules, mechanical motions on a nanometer scale, or incredibly small fluctuations of electromagnetic fields have fascinated physiologists for a long time. It is a great challenge to understand the primary sensory processes on a molecular level. This review points out some important recent developments in the search for primary processes in sensory cells that mediate touch perception, hearing, vision, taste, olfaction, as well as the analysis of light polarization and the orientation in the Earth's magnetic field. The data are screened for common transduction strategies and common transduction molecules, an aspect that may be helpful for researchers in the field.

Magnetic-field effect on the photoactivation reaction of Escherichia coli DNA photolyase

One of the two principal hypotheses put forward to explain the primary magnetoreception event underlying the magnetic compass sense of migratory birds is based on a magnetically sensitive chemical reaction. It has been proposed that a spin-correlated radical pair is produced photochemically in a cryptochrome and that the rates and yields of the subsequent chemical reactions depend on the orientation of the protein in the Earth's magnetic field. The suitability of cryptochrome for this purpose has been argued, in part, by analogy with DNA photolyase, although no effects of applied magnetic fields have yet been reported for any member of the cryptochrome/photolyase family. Here, we demonstrate a magnetic-field effect on the photochemical yield of a flavin-tryptophan radical pair in Escherichia coli photolyase. This result provides a proof of principle that photolyases, and most likely by extension also cryptochromes, have the fundamental properties needed to form the basis of a magnetic compass.

Cryptochrome mediates light-dependent magnetosensitivity in Drosophila

Although many animals use the Earth's magnetic field for orientation and navigation, the precise biophysical mechanisms underlying magnetic sensing have been elusive. One theoretical model proposes that geomagnetic fields are perceived by chemical reactions involving specialized photoreceptors. However, the specific photoreceptor involved in such magnetoreception has not been demonstrated conclusively in any animal. Here we show that the ultraviolet-A/blue-light photoreceptor cryptochrome (Cry) is necessary for light-dependent magnetosensitive responses in Drosophila melanogaster. In a binary-choice behavioural assay for magnetosensitivity, wild-type flies show significant naive and trained responses to a magnetic field under full-spectrum light ( approximately 300-700 nm) but do not respond to the field when wavelengths in the Cry-sensitive, ultraviolet-A/blue-light part of the spectrum (<420 nm) are blocked. Notably, Cry-deficient cry(0) and cry(b) flies do not show either naive or trained responses to a magnetic field under full-spectrum light. Moreover, Cry-dependent magnetosensitivity does not require a functioning circadian clock. Our work provides, to our knowledge, the first genetic evidence for a Cry-based magnetosensitive system in any animal.

Variability in the encoding of spatial information by dancing bees

A honeybee's waggle dance is an intriguing example of multisensory convergence, central processing and symbolic information transfer. It conveys to bees and human observers the position of a relatively small area at the endpoint of an average vector in a two-dimensional system of coordinates. This vector is often computed from a collection of waggle phases from the same or different dancers. The question remains, however, of how informative a small sample of waggle phases can be to the bees, and how the spatial information encoded in the dance is actually mapped to the followers' searches in the field. Certainly, it is the variability of a dancer's performance that initially defines the level of uncertainty that followers must cope with if they were to successfully decode information in the dance. Understanding how a dancer's behaviour is mapped to that of its followers initially relies on the analysis of both the accuracy and precision with which the dancer encodes spatial information in the dance. Here we describe within-individual variations in the encoding of the distance to and direction of a goal. We show that variations in the number of a dancer's wagging movements, a measure that correlates well with the distance to the goal, do not depend upon the dancer's travelled distance, meaning that there is a constant variance of wagging movements around the mean. We also show that the duration of the waggle phases and the angular dispersion and divergence of successive waggle phases co-vary with a dancer's orientation in space. Finally, using data from dances recorded through high-speed video techniques, we present the first analysis of the accuracy and precision with which an increasing number of waggle phases conveys spatial information to a human observer.

Bats use magnetite to detect the earth's magnetic field

While the role of magnetic cues for compass orientation has been confirmed in numerous animals, the mechanism of detection is still debated. Two hypotheses have been proposed, one based on a light dependent mechanism, apparently used by birds and another based on a "compass organelle" containing the iron oxide particles magnetite (Fe(3)O(4)). Bats have recently been shown to use magnetic cues for compass orientation but the method by which they detect the Earth's magnetic field remains unknown. Here we use the classic "Kalmijn-Blakemore" pulse re-magnetization experiment, whereby the polarity of cellular magnetite is reversed. The results demonstrate that the big brown bat Eptesicus fuscus uses single domain magnetite to detect the Earths magnetic field and the response indicates a polarity based receptor. Polarity detection is a prerequisite for the use of magnetite as a compass and suggests that big brown bats use magnetite to detect the magnetic field as a compass. Our results indicate the possibility that sensory cells in bats contain freely rotating magnetite particles, which appears not to be the case in birds. It is crucial that the ultrastructure of the magnetite containing magnetoreceptors is described for our understanding of magnetoreception in animals.

Magnetoreception in microorganisms and fungi

The ability to respond to magnetic fields is ubiquitous among the five kingdoms of organisms. Apart from the mechanisms that are at work in bacterial magnetotaxis, none of the innumerable magnetobiological effects are as yet completely understood in terms of their underlying physical principles. Physical theories on magnetoreception, which draw on classical electrodynamics as well as on quantum electrodynamics, have greatly advanced during the past twenty years, and provide a basis for biological experimentation. This review places major emphasis on theories, and magnetobiological effects that occur in response to weak and moderate magnetic fields, and that are not related to magnetotaxis and magnetosomes. While knowledge relating to bacterial magnetotaxis has advanced considerably during the past 27 years, the biology of other magnetic effects has remained largely on a phenomenological level, a fact that is partly due to a lack of model organisms and model responses; and in great part also to the circumstance that the biological community at large takes little notice of the field, and in particular of the available physical theories. We review the known magnetobiological effects for bacteria, protists and fungi, and try to show how the variegated empirical material could be approached in the framework of the available physical models.

Role of exchange and dipolar interactions in the radical pair model of the avian magnetic compass

It is not yet understood how migratory birds sense the Earth's magnetic field as a source of compass information. One suggestion is that the magnetoreceptor involves a photochemical reaction whose product yields are sensitive to external magnetic fields. Specifically, a flavin-tryptophan radical pair is supposedly formed by photoinduced sequential electron transfer along a chain of three tryptophan residues in a cryptochrome flavoprotein immobilized in the retina. The electron Zeeman interaction with the Earth's magnetic field ( approximately 50 microT), modulated by anisotropic magnetic interactions within the radicals, causes the product yields to depend on the orientation of the receptor. According to well-established theory, the radicals would need to be separated by >3.5 nm in order that interradical spin-spin interactions are weak enough to permit a approximately 50 microT field to have a significant effect. Using quantum mechanical simulations, it is shown here that substantial changes in product yields can nevertheless be expected at the much smaller separation of 2.0 +/- 0.2 nm where the effects of exchange and dipolar interactions partially cancel. The terminal flavin-tryptophan radical pair in cryptochrome has a separation of approximately 1.9 nm and is thus ideally placed to act as a magnetoreceptor for the compass mechanism.

A visual pathway links brain structures active during magnetic compass orientation in migratory birds

The magnetic compass of migratory birds has been suggested to be light-dependent. Retinal cryptochrome-expressing neurons and a forebrain region, "Cluster N", show high neuronal activity when night-migratory songbirds perform magnetic compass orientation. By combining neuronal tracing with behavioral experiments leading to sensory-driven gene expression of the neuronal activity marker ZENK during magnetic compass orientation, we demonstrate a functional neuronal connection between the retinal neurons and Cluster N via the visual thalamus. Thus, the two areas of the central nervous system being most active during magnetic compass orientation are part of an ascending visual processing stream, the thalamofugal pathway. Furthermore, Cluster N seems to be a specialized part of the visual wulst. These findings strongly support the hypothesis that migratory birds use their visual system to perceive the reference compass direction of the geomagnetic field and that migratory birds "see" the reference compass direction provided by the geomagnetic field.

A model for encoding of magnetic field intensity by magnetite-based magnetoreceptor cells

A conceptual model is proposed for the encoding of magnetic field intensity from the motion of a chain of single-domain magnetite crystals which is located within a receptor cell, connected at one end to the cell membrane, and linked by cytoskeletal filaments to an array of mechanically gated ion channels centred on the end of the chain. In this arrangement, the physical links between the chain and ion channels will restrict the motion of the magnetite chain in response to the external magnetic field to a narrow cone with its axis through the point where the chain is attached to the membrane. The motion of the chain in the presence of an external magnetic field and thermal agitation will open a varying number of channels, causing the membrane potential to oscillate about some mean value that depends on the component of magnetic intensity oriented perpendicular to the cell membrane. The model permits estimation of magnetic intensity by integration of the motion of the magnetite chain over an area of the cell membrane, explains a number of results from physiological recordings in birds and fish, and makes testable predictions for future experimental studies. The model also provides a mechanism at the cellular level for a constant value of the Weber fraction (the ratio of the threshold sensitivity to a stimulus and the magnitude of that stimulus) for the magnetic sense but requires a separate gain control mechanism for modulation of sensitivity over a range of background fields. If magnetic field detection and encoding works as proposed in the model, the magnetoreceptor system may also be able to reconstruct the magnetic field vector using information about the vertical and horizontal axes from the eyes, gravity detectors, or both.

Molecular mechanisms of magnetosome formation

Magnetotactic bacteria are a diverse group of microorganisms with the ability to use geomagnetic fields for direction sensing. This unique feat is accomplished with the help of magnetosomes, nanometer-sized magnetic crystals surrounded by a lipid bilayer membrane and organized into chains via a dedicated cytoskeleton within the cell. Because of the special properties of these magnetic crystals, magnetotactic bacteria have been exploited for a variety of applications in diverse disciplines from geobiology to biotechnology. In addition, magnetosomes have served as a powerful model system for the study of biomineralization and cell biology in bacteria. This review focuses on recent advances in understanding the molecular mechanisms of magnetosome formation and magnetite biomineralization.

Theoretical analysis of an iron mineral-based magnetoreceptor model in birds

Sensing the magnetic field has been established as an essential part of navigation and orientation of various animals for many years. Only recently has the first detailed receptor concept for magnetoreception been published based on histological and physical results. The considered mechanism involves two types of iron minerals (magnetite and maghemite) that were found in subcellular compartments within sensory dendrites of the upper beak of several bird species. But so far a quantitative evaluation of the proposed receptor is missing. In this article, we develop a theoretical model to quantitatively and qualitatively describe the magnetic field effects among particles containing iron minerals. The analysis of forces acting between these subcellular compartments shows a particular dependence on the orientation of the external magnetic field. The iron minerals in the beak are found in the form of crystalline maghemite platelets and assemblies of magnetite nanoparticles. We demonstrate that the pull or push to the magnetite assemblies, which are connected to the cell membrane, may reach a value of 0.2 pN -- sufficient to excite specific mechanoreceptive membrane channels in the nerve cell. The theoretical analysis of the assumed magnetoreceptor system in the avian beak skin clearly shows that it might indeed be a sensitive biological magnetometer providing an essential part of the magnetic map for navigation.

A novel concept of Fe-mineral-based magnetoreception: histological and physicochemical data from the upper beak of homing pigeons

Animals make use of the Earth's magnetic field for navigation and regulation of vegetative functions; however, the anatomical and physiological basis for the magnetic sense has not been elucidated yet. Our recent results from histology and X-ray analyses support the hypothesis that delicate iron-containing structures in the skin of the upper beak of homing pigeons might serve as a biological magnetometer. Histology has revealed various iron sites within dendrites of the trigeminal nerve, their arrangement along strands of axons, the existence of three dendritic fields in each side of the beak with specific 3D-orientations, and the bilateral symmetry of the whole system. Element mapping by micro-synchrotron X-ray fluorescence analysis has shown the distribution of iron and its quantities. Micro-synchrotron X-ray absorption near-edge-structure spectroscopy has allowed us to unambiguously identify maghemite as the predominating iron mineral (90 vs 10% magnetite). In this paper, we show that iron-based magnetoreception needs the presence of both of these iron minerals, their specific dimensions, shapes, and arrangements in three different subcellular compartments. We suggest that an inherent magnetic enhancement process via an iron-crusted vesicle and the attached chains of iron platelets might be sufficient to account for the sensitivity and specificity required by such a magnetoreceptor. The appropriate alignment between the Earth's magnetic field and the maghemite bands would induce a multiple attraction of the magnetite bullets perpendicular to the membrane, thus, triggering strain-sensitive membrane channels and a primary receptor potential. Due to its 3D architecture and physicochemical nature, the dendritic system should be able to separately sense the three vector components of the Earth's local field, simultaneously-allowing birds to detect their geographic position by the magnetic vector, i.e., amplitude and direction of the local magnetic field, irrespective of the animal's posture or movement and photoreception.

Lateralized activation of Cluster N in the brains of migratory songbirds

Cluster N is a cluster of forebrain regions found in night-migratory songbirds that shows high activation of activity-dependent gene expression during night-time vision. We have suggested that Cluster N may function as a specialized night-vision area in night-migratory birds and that it may be involved in processing light-mediated magnetic compass information. Here, we investigated these ideas. We found a significant lateralized dominance of Cluster N activation in the right hemisphere of European robins (Erithacus rubecula). Activation predominantly originated from the contralateral (left) eye. Garden warblers (Sylvia borin) tested under different magnetic field conditions and under monochromatic red light did not show significant differences in Cluster N activation. In the fairly sedentary Sardinian warbler (Sylvia melanocephala), which belongs to the same phyolgenetic clade, Cluster N showed prominent activation levels, similar to that observed in garden warblers and European robins. Thus, it seems that Cluster N activation occurs at night in all species within predominantly migratory groups of birds, probably because such birds have the capability of switching between migratory and sedentary life styles. The activation studies suggest that although Cluster N is lateralized, as is the dependence on magnetic compass orientation, either Cluster N is not involved in magnetic processing or the magnetic modulations of the primary visual signal, forming the basis for the currently supported light-dependent magnetic compass mechanism, are relatively small such that activity-dependent gene expression changes are not sensitive enough to pick them up.

Deep-Etching Electron Microscopy of Cells of Magnetospirillum magnetotacticum: Evidence for Filamentous Structures Connecting the Magnetosome Chain to the Cell Surface

Magnetospirillum magnetotacticum are magnetotactic bacteria that form a single chain of magnetite magnetosomes within its cytoplasm. Here, we studied the ultrastructure of M. magnetotacticum by freeze-fracture and deep-etching to understand the spatial correlation between the magnetosome chain and the cell envelope and its possible implications for magnetotaxis. Magnetosomes were found mainly near the cell envelope, forming chains that were closely associated with the granular cytoplasmic material. The membrane surrounding the magnetosomes could be visualized in deep-etching preparations. Thin connections between magnetosome chains and the cell envelope were observed in deep-etching images. These results strengthen the hypothesis for the existence of structures that transfer the torque from the magnetosome chains to the whole cell during the orientation of magnetotactic bacteria to a magnetic field lines.

Glycoproteins bound to ion channels mediate detection of electric fields: A proposed mechanism and supporting evidence

The mechanism by which animals detect weak electric and magnetic fields has not yet been elucidated. We propose that transduction of an electric field (E) occurs at the apical membrane of a specialized cell as a consequence of an interaction between the field and glycoproteins bound to the gates of ion channels. According to the model, a glycoprotein mass (M) could control the gates of ion channels, where M > 1.4 x 10(-18)/E, resulting in a signal of sufficient strength to overcome thermal noise. Using the electroreceptor organ of Kryptopterus as a mathematical and experimental model, we showed that at the frequency of maximum sensitivity (10 Hz), fields as low as 2 microV/m could be detected, and that the observation could be explained if a glycoprotein mass of 0.7 x 10(-12) kg (a sphere 11 microm in diameter) were bound to channel gates. Antibodies against apical membrane structures in Kryptopterus blocked field transduction, which was consistent with the proposal that it occurred at the membrane surface. Although the target of the field was hypothesized to be an ion channel, the proposed mechanism can easily be extended to include other kinds of membrane proteins.

Magnetic compass in the cornea: local anaesthesia impairs orientation in a mammal

The mechanism of signal transduction during magnetic compass orientation is rarely evident in vertebrates and is as yet unknown in mammals. This transmission has been associated with magnetite-based receptors innervated by the ophthalmic nerve or with the involvement of the eye, particularly the retina. We provide the first behavioural support for the cornea carrying the respective primary sensors in mole-rats (Fukomys anselli) by showing that local anaesthesia disrupts their normal directional magnetic orientation. During corneal anaesthesia in normal geomagnetic conditions, mole-rats did not maintain their preferred nesting direction, but displayed a random orientation pattern. A second experiment showed that the ability of the mole-rat to discriminate between light and dark was not impeded by the same anaesthetic treatment, suggesting no retinal involvement in mole-rat magnetic orientation. Our study restricts the peripheral primary sensors in mole-rats to the ophthalmic region, probably the cornea and indicates magnetite as the responsible signal mediator.

Biogenesis of actin-like bacterial cytoskeletal filaments destined for positioning prokaryotic magnetic organelles

Magnetosomes comprise a magnetic nanocrystal surrounded by a lipid bilayer membrane. These unique prokaryotic organelles align inside magnetotactic bacterial cells and serve as an intracellular compass allowing the bacteria to navigate along the geomagnetic field in aquatic environments. Cryoelectron tomography of Magnetospirillum strains has revealed that the magnetosome chain is surrounded by a network of filaments that may be composed of MamK given that the filaments are absent in the mamK mutant cells. The process of the MamK filament assembly is unknown. Here we prove the authenticity of the MamK filaments and show that MamK exhibits linear distribution inside Magnetospirillum sp. cells even in the area without magnetosomes. The mamK gene alone is sufficient to direct the synthesis of straight filaments in Escherichia coli, and one extremity of the MamK filaments is located at the cellular pole. By using dual fluorescent labeling of MamK, we found that MamK nucleates at multiple sites and assembles into mosaic filaments. Time-lapse experiments reveal that the assembly of the MamK filaments is a highly dynamic and kinetically asymmetrical process. MamK bundles might initiate the formation of a new filament or associate to one preexistent filament. Our results demonstrate the mechanism of biogenesis of prokaryotic cytoskeletal filaments that are structurally and functionally distinct from the known MreB and ParM filaments. In addition to positioning magnetosomes, other hypothetical functions of the MamK filaments in magnetotaxis might include anchoring magnetosomes and being involved in magnetic reception.

The physics and neurobiology of magnetoreception

Diverse animals can detect magnetic fields but little is known about how they do so. Three main hypotheses of magnetic field perception have been proposed. Electrosensitive marine fish might detect the Earth's field through electromagnetic induction, but direct evidence that induction underlies magnetoreception in such fish has not been obtained. Studies in other animals have provided evidence that is consistent with two other mechanisms: biogenic magnetite and chemical reactions that are modulated by weak magnetic fields. Despite recent advances, however, magnetoreceptors have not been identified with certainty in any animal, and the mode of transduction for the magnetic sense remains unknown.

Magnetoreception and its use in bird navigation

Recent advances have brought new insight into the physiological mechanisms that enable birds and other animals to use magnetic fields for orientation. Many birds seem to have two magnetodetection senses, one based on magnetite near the beak and one based on light-dependent radical-pair processes in the bird's eye(s). Among the most exciting recent results are: first, behavioural responses of birds experiencing oscillating magnetic fields. Second, the occurrence of putative magnetosensory molecules, the cryptochromes, in the eyes of migratory birds. Third, detection of a brain area that integrates specialised visual input at night in night-migratory songbirds. Fourth, a putative magnetosensory cluster of magnetite in the upper beak. These and other recent findings have important implications for magnetoreception; however, many crucial open questions remain.

The Integration of Behaviour into Comparative Physiology

Comparative physiology has traditionally focused on the physiological responses of animals to their physicochemical environment. In recent years, awareness has increased among physiologists of the potential for behavioural factors, such as the social environment of the animal, to affect physiological condition and responses. This recognition has led to an emerging trend within the field toward using multidisciplinary approaches that incorporate both behavioural and physiological techniques. Research areas in which the integrated study of behaviour and physiology has been particularly fruitful include the physiology of the social environment, sensory physiology and behaviour, and physiological constraints on behavioural ecology. The manner in which incorporating behavioural considerations has informed the physiological data collected is discussed for each of these areas using specific examples.

Nanotoxicology: an emerging discipline evolving from studies of ultrafine particles

Although humans have been exposed to airborne nanosized particles (NSPs; < 100 nm) throughout their evolutionary stages, such exposure has increased dramatically over the last century due to anthropogenic sources. The rapidly developing field of nanotechnology is likely to become yet another source through inhalation, ingestion, skin uptake, and injection of engineered nanomaterials. Information about safety and potential hazards is urgently needed. Results of older biokinetic studies with NSPs and newer epidemiologic and toxicologic studies with airborne ultrafine particles can be viewed as the basis for the expanding field of nanotoxicology, which can be defined as safety evaluation of engineered nanostructures and nanodevices. Collectively, some emerging concepts of nanotoxicology can be identified from the results of these studies. When inhaled, specific sizes of NSPs are efficiently deposited by diffusional mechanisms in all regions of the respiratory tract. The small size facilitates uptake into cells and transcytosis across epithelial and endothelial cells into the blood and lymph circulation to reach potentially sensitive target sites such as bone marrow, lymph nodes, spleen, and heart. Access to the central nervous system and ganglia via translocation along axons and dendrites of neurons has also been observed. NSPs penetrating the skin distribute via uptake into lymphatic channels. Endocytosis and biokinetics are largely dependent on NSP surface chemistry (coating) and in vivo surface modifications. The greater surface area per mass compared with larger-sized particles of the same chemistry renders NSPs more active biologically. This activity includes a potential for inflammatory and pro-oxidant, but also antioxidant, activity, which can explain early findings showing mixed results in terms of toxicity of NSPs to environmentally relevant species. Evidence of mitochondrial distribution and oxidative stress response after NSP endocytosis points to a need for basic research on their interactions with subcellular structures. Additional considerations for assessing safety of engineered NSPs include careful selections of appropriate and relevant doses/concentrations, the likelihood of increased effects in a compromised organism, and also the benefits of possible desirable effects. An interdisciplinary team approach (e.g., toxicology, materials science, medicine, molecular biology, and bioinformatics, to name a few) is mandatory for nanotoxicology research to arrive at an appropriate risk assessment.

Night-vision brain area in migratory songbirds

Twice each year, millions of night-migratory songbirds migrate thousands of kilometers. To find their way, they must process and integrate spatiotemporal information from a variety of cues including the Earth's magnetic field and the night-time starry sky. By using sensory-driven gene expression, we discovered that night-migratory songbirds possess a tight cluster of brain regions highly active only during night vision. This cluster, here named "cluster N," is located at the dorsal surface of the brain and is adjacent to a known visual pathway. In contrast, neuronal activation of cluster N was not increased in nonmigratory birds during the night, and it disappeared in migrants when both eyes were covered. We suggest that in night-migratory songbirds cluster N is involved in enhanced night vision, and that it could be integrating vision-mediated magnetic and/or star compass information for night-time navigation. Our findings thus represent an anatomical and functional demonstration of a specific night-vision brain area.

Magnetic pulse affects a putative magnetoreceptor mechanism

Clusters of superparamagnetic (SP) magnetite crystals have recently been identified in free nerve endings in the upper-beak skin of homing pigeons and are interpreted as being part of a putative magnetoreceptor system. Motivated by these findings, we developed a physical model that accurately predicts the dynamics of interacting SP clusters in a magnetic field. The main predictions are: 1), under a magnetic field, a group of SP clusters self-assembles into a chain-like structure that behaves like a compass needle under slowly rotating fields; 2), in a frequently changing field as encountered by a moving bird, a stacked chain is a structurally more stable configuration than a single chain; 3), chain-like structures of SP clusters disrupt under strong fields applied at oblique angles; and 4), reassemble on a timescale of hours to days (assuming a viscosity of the cell plasma eta approximately 1 P). Our results offer a novel mechanism for magnetic field perception and are in agreement with the response of birds observed after magnetic-pulse treatments, which have been conducted in the past to specifically test if ferrimagnetic material is involved in magnetoreception, but which have defied explanation so far. Our theoretical results are supported by experiments on a technical SP model system using a high-speed camera. We also offer new predictions that can be tested experimentally.

Towards the neural basis of magnetoreception: a neuroanatomical approach

After more than two decades of intensive research, the physiological mechanisms of animal magnetoreception remain enigmatic. The primary magnetoreceptors are still unknown and our knowledge of the neural substrate subserving magnetic orientation is meagre. Here we argue that this dismal outcome can be largely attributed to the fact that the potential of recently available neurobiological techniques has not been utilized, review some of these techniques and propose a step by step scenario for future research, concentrating on the heuristic potential of instrumentalizing inducible transcription factors (ITFs) such as Jun, Fos, Fos-related antigens and Krox. ITFs can be used as markers of neuronal activation in experiments on freely moving animals performing magnetically based orientation tasks, in experiments on anaesthetised or restrained animals stimulated magnetically, and in experiments employing treatments that specifically disrupt magnetoreception. Therefore they can serve as tools for identifying neurons involved in the detection and processing of magnetic information. When used in combination with other neurobiological tools, ITFs can also be useful for a more comprehensive description of the involved neural networks, for the identification of magnetoreceptors and, in the case of the photoreceptor-based mechanism, also for studying the involvement of specific light-sensitive molecules in the primary transduction process of magnetoreception. Limitations and pitfalls of the proposed approach are also discussed.

Disruption of magnetic orientation in hatchling loggerhead sea turtles by pulsed magnetic fields

Loggerhead sea turtles (Caretta caretta) derive both directional and positional information from the Earth's magnetic field, but the mechanism underlying magnetic field detection in turtles has not been determined. One hypothesis is that crystals of biogenic, single-domain magnetite provide the physical basis of the magnetic sense. As a first step toward determining if magnetite is involved in sea turtle magnetoreception, hatchling loggerheads were exposed to pulsed magnetic fields (40 mT, 4 ms rise time) capable of altering the magnetic dipole moment of biogenic magnetite crystals. A control group of turtles was treated identically but not exposed to the pulsed fields. Both groups of turtles subsequently oriented toward a light source, implying that the pulsed fields did not disrupt the motivation to swim or the ability to maintain a consistent heading. However, when swimming in darkness under conditions in which turtles normally orient magnetically, control turtles oriented significantly toward the offshore migratory direction while those that were exposed to the magnetic pulses did not. These results are consistent with the hypothesis that at least part of the sea turtle magnetoreception system is based on magnetite. In principle, a magnetite-based magnetoreception system might be involved in detecting directional information, positional information, or both.

Stochastic dynamics of magnetosomes and a mechanism of biological orientation in the geomagnetic field

The rotations of nanoscopic magnetic particles, magnetosomes, embedded into the cytoskeleton are considered. Under the influence of thermal disturbances, a great number of magnetosomes are shown to move chaotically between two stable equilibrium positions, in which their magnetic moments are neither parallel nor antiparallel to the static Earth's magnetic field (MF). The random rotations attain the value of order of a radian. The rate of the transitions and the probability of magnetosomes to be in the different states depend on the MF direction with respect to an averaged magnetosome's orientation. This effect explains the ability of migratory animals to orient themselves faultlessly in long term passages in the absence of the direct visibility of optical reference points. The sensitivity to deviation from an "ideal" orientation is estimated to be 2-4 degrees. Possible involvement of the stochastic dynamics of magnetosomes in biological magnetic navigation is discussed.

Decreased plasma levels of nitric oxide metabolites, angiotensin II, and aldosterone in spontaneously hypertensive rats exposed to 5 mT static magnetic field

Previously, we found that whole body exposure to static magnetic fields (SMF) at 10 mT (B(max)) and 25 mT (B(max)) for 2-9 weeks suppressed and delayed blood pressure (BP) elevation in young, stroke resistant, spontaneously hypertensive rats (SHR). In this study, we investigated the interrelated antipressor effects of lower field strengths and nitric oxide (NO) metabolites (NO(x) = NO(2)(-) + NO(3)(-)) in SHR. Seven-week-old male rats were exposed to two different ranges of SMF intensity, 0.3-1.0 mT or 1.5-5.0 mT, for 12 weeks. Three experimental groups of 20 animals each were examined: (1) no exposure with intraperitoneal (ip) saline injection (sham-exposed control); (2) 1 mT SMF exposure with ip saline injection (1 mT); (3) 5 mT SMF exposure with ip saline injection (5 mT). Arterial BP, heart rate (HR), skin blood flow (SBF), plasma NO metabolites (NO(x)), and plasma catecholamine levels were monitored. SMF at 5 mT, but not 1 mT, significantly suppressed and retarded the early stage development of hypertension for several weeks, compared with the age matched, unexposed (sham exposed) control. Exposure to 5 mT resulted in reduced plasma NO(x) concentrations together with lower levels of angiotensin II and aldosterone in SHR. These results suggest that SMF may suppress and delay BP elevation via the NO pathways and hormonal regulatory systems.