Fish scales as biosensors for catecholamines

The Fish Pigment Cell: An Alternative Model in Biomedical Research

Receptor-mediated pigment aggregation within pigment cells (chromatophores) of an isolated fish scale is an ideal model system for functional receptor studies. The superficial layer of the scale contains both dermal chromatophores and postganglionic sympathetic nerves. By means of stimulation of the nerves, or by addition of appropriate receptor agonists, it is possible to elicit pigment aggregation within the chromatophores. A single fish can contribute hundreds of scales, various pharmacological and biochemical experiments are easily carried out and the physiological response, i.e. pigment aggregation, is readily evaluated by the aid of a light microscope or a simple scale photometer.

A denervation model, based on isolated scales, permits studies of factors involved in the sensitivity change, which typically takes place after experimental or pathological denervation.

By using isolated fish scales it is quite simple to illustrate many biomedically important concepts, like receptor theory and nerve-effector cell communication. This makes the scale a very useful preparation in biomedical education.

Conservation of the chromatophore pigment response

Toxicant sensing technology has evolved to include biological sensors, such as cell-based biosensors, which rely on viable cells to convey a measurable physiological signal. Chromatophores are a class of pigment cells that have been investigated as cell-based biosensors. We report the characterization of Oncorhynchus tshawytscha melanophores and describe the melanophore pigment response to neurotransmitters in terms of pigment area occupied. Compared with the previously described model, Betta splendens erythrophores, O. tshawytscha melanophores responded similarly, indicating that pigment responses are biologically conserved between these two species. Additionally, melanophores responded to mercuric chloride and sodium arsenite, similar to B. splendens erythrophores, suggesting that melanophores can be used as detectors for environmental toxicants. This report highlights the potential of O. tshawytscha melanophores to be used as cell-based biosensors to address environmental toxicity, and warrants a continued investigation to strengthen this technology and its applications.

Erythrophore cell response to food-associated pathogenic bacteria: implications for detection

Cell-based biosensors have been proposed for use as function-based detectors of toxic agents. We report the use of Betta splendens chromatophore cells, specifically erythrophore cells, for detection of food-associated pathogenic bacteria. Evaluation of erythrophore cell response, using Bacillus spp., has revealed that this response can distinguish pathogenic Bacillus cereus from a non-pathogenic B. cereus ΔplcR deletion mutant and a non-pathogenic Bacillus subtilis. Erythrophore cells were exposed to Salmonella enteritidis, Clostridium perfringens and Clostridium botulinum. Each bacterial pathogen elicited a response from erythrophore cells that was distinguished from the corresponding bacterial growth medium, and this observed response was unique for each bacterial pathogen. These findings suggest that erythrophore cell response has potential for use as a biosensor in the detection and toxicity assessment for food-associated pathogenic bacteria.

Frog melanophores cultured on fluorescent microbeads: biomimic-based biosensing

Melanophores are pigmented cells in lower vertebrates capable of quick color changes and thereby suitable as whole cell biosensors. In the frog dermis skin layer, the large and dark pigmented melanophore surrounds a core of other pigmented cells. Upon hormonal stimulation the black-brown pigment organelles will redistribute within the melanophore, and thereby cover or uncover the core, making complex color changes possible in the dermis. Previously, melanophores have only been cultured on flat surfaces. Here we mimic the three dimensional biological geometry in the frog dermis by culturing melanophores on fluorescent plastic microbeads. To demonstrate biosensing we use the hormones melatonin and alpha-melanocyte stimulating hormone (alpha-MSH) as lightening or darkening stimuli, respectively. Cellular responses were successfully demonstrated on single cell level by fluorescence microscopy, and in cell suspension by a fluorescence microplate reader and a previously demonstrated computer screen photo-assisted technique. The demonstrated principle is the first step towards "single well/multiple read-out" biosensor arrays based on suspensions of different selective-responding melanophores, each cultured on microbeads with distinctive spectral characteristics. By applying small amount of a clinical sample, or a candidate substance in early drug screening, to a single well containing combinations of melanophores on beads, multiple parameter read-outs will be possible.

Microplate based biosensing with a computer screen aided technique

Melanophores, dark pigment cells from the frog Xenopus laevis, have the ability to change light absorbance upon stimulation by different biological agents. Hormone exposure (e.g. melatonin or alpha-melanocyte stimulating hormone) has been used here as a reversible stimulus to test a new compact microplate reading platform. As an application, the detection of the asthma drug formoterol in blood plasma samples is demonstrated. The present system utilizes a computer screen as a (programmable) large area light source, and a standard web camera as recording media enabling even kinetic microplate reading with a versatile and broadly available platform, which suffices to evaluate numerous bioassays. Especially in the context of point of care testing or self testing applications these possibilities become advantageous compared with highly dedicated comparatively expensive commercial systems.

Altered impedance during pigment aggregation in Xenopus laevis melanophores

Melanophores are dark-brown pigment cells located in the skin of amphibia, fish and many invertebrates. The skin colour of these organisms is regulated by the translocation of pigment organelles, and the pigment distribution can be altered by external stimuli. The ability to change colour in response to stimuli makes these cells of interest for biosensing applications. It was investigated whether pigment aggregation in Xenopus laevis melanophores can be detected by impedance measurements performed in transparent microvials. The results show that cell attachment, cell spreading and pigment aggregation all resulted in impedance changes, seen particularly at the highest frequency tested (10 kHz). The mechanisms behind the impedance changes were investigated by the addition of latrunculin or melatonin, both of which cause pigment aggregation. The latrunculin-induced aggregation was associated with cell area decrease and filamentous actin (F-actin) breakdown, processes that can influence the impedance. Lack of F-actin breakdown and an increase in cell area during melatonin-induced aggregation suggest that some other intracellular process also contributes to the impedance decrease seen for melatonin. It was shown that impedance measurements reflect not only cell attachment and cell spreading, but also intracellular events.

A two-cell biosensor that couples neuronal cells to optically monitored fish chromatophores

A two-cell biosensor was developed that uses optically detected changes in naturally colored fish chromatophores to measure the neurosecretory output of mammalian neuronal cells. The specific version of the biosensor described here is a continuous flow device that places red-pigmented, dendritic erythrophore cells directly downstream of an immobilized population of PC12 neuronal cells, a well-established model cell-line having neuroendocrine function. Agents known to stimulate catecholamine neurosecretion (secretagogues) were presented to the PC12 cells. It was found that the varying level of neurosecretion from the PC12 cells was measurable by judging the degree of pigment aggregation in the erythrophores. Increases in catecholamine secretion and consequent pigment aggregation were observed for several known secretagogues, including receptor agonists (ATP, acetylcholine), membrane depolarizing agents (high K(+) concentration), and specific neurotoxins (black widow spider venom, alpha-latrotoxin). This particular two-cell biosensor, which is applicable to the detection of any agents that affect the levels of catecholamine secretion from PC12 cells, demonstrates the general principle that the breadth of sensitivity of a biosensor is increased by employing coupled cell types.

Biosensing with G-protein coupled receptor systems

Many cell membrane bound receptors communicate with the inside of the cell through guanine nucleotide binding proteins (G-proteins). This holds also for olfactory receptor neurons, which respond to odorants with G-protein mediated increases in the concentration of cyclic adenosine 3', 5'-monophosphate (cAMP) and/or inositol 1,4,5-triphosphate (InsP3). These substances regulate the ionic conductivity of the wall of the cilia. We have studied a similar system, namely G-protein coupled alpha 2-adrenoceptors, present for example in the cells of certain fish scales. These receptors react on, catecholamines and the G-protein mediates a decrease in cAMP, which causes an aggregation of pigment containing granulas to the middle of the cells. The light transmission of the cell increases due to this aggregation. This simple physiological response has been used in a sensitive biosensor for noradrenaline and for pertussis toxin that is based on isolated fish scales from cuckoo wrasse (Labrus ossifagus). The results were obtained with a simple photometer. Measurements can be performed also on single isolated melanophores. The main purpose of this contribution is, however, to point out that G-protein coupled receptors together with a simple physiological response form a principle for biosensing, which could also be an interesting alternative for odour sensing.

Bioelectronic noses: a status report. Part II

The present state of the art to record or to mimic electronically the human senses of olfaction and taste is characterized. In this part II, strategies are outlined to utilize chemical and biological structures with their different complexities which serve as sensor elements in (bio-) electronic noses. Finally a survey is given on the computer-science aspects of odor recognition based on these elements.

A cell-based immunobiosensor with engineered molecular recognition--Part II: Enzyme amplification systems

Immune cells in vivo routinely perform highly selective immunosensing in blood and tissues as part of their normal immune surveillance functions. We have been investigating the potential of exploiting the immunosensing detection abilities of excitable immune cells (i.e. the mast cell) for the development of whole cell immunobiosensors. A key feature is that these immune cells can be selectively engineered to recognize specific antigens in vitro. In the presence of antigen, these cells undergo excitable activation responses which result in increased metabolism and the exocytosis of stored intracellular mediators. We have previously determined that mast cell metabolic responses can be thermally transduced in real time, thus indicating the possibility of whole cell thermoelectric immunobiosensing. In this work we investigated the use of enzyme amplification systems to enhance the direct transduction of immune cell responses to analyte. It was found that with appropriate enzymes, peak outputs occurred within approximately 5 min (4-20 times faster than without enzymes) and peak response magnitudes were up to nine-fold greater than without enzymes.

Effects of odorants on pigment aggregation and cAMP in fish melanophores

Odor perception within olfactory neuroepithelium and pigment translocation within melanophores both seem to rely on a cAMP-based second messenger system. From studies on cultured frog melanophores, Lerner et al. (Proc. Natl. Acad. Sci. USA 85:261-264, 1988) suggested that some aspect of odor perception may be mediated by a nonspecific mechanism whose signal is transduced by a cAMP-based second messenger system. In the present study, odorants (beta-ionone, benzylaldehyde, cineole, cinnamaldehyde, and octanol), which previously have been shown to stimulate formation of cAMP in the olfactory neuroepithelium, were investigated for possible pigment dispersing and cAMP-increasing effects. Pretreatment of fish melanophores with the adenylate cyclase activator forskolin (1 microM) resulted in an approximately 300% increase in cAMP and an almost complete blockage of noradrenaline-induced pigment aggregation. However, none of the tested odorants were able to increase the cAMP level and only cinnaldehyde and beta-ionone were found to have any pigment dispersing activity.

A rapid quantitative bioassay for evaluating the effects of ligands upon receptors that modulate cAMP levels in a melanophore cell line

A new method for rapidly evaluating the effects of drugs on receptors that regulate intracellular cAMP in a cell line derived from Xenopus laevis melanophores has been developed. Melanophores were plated into sterile 96 well microtiter plates, and 3 days later the cells were treated with melatonin for 30 min to induce melanosome aggregation. Subsequent exposure to MSH or adrenergic agonists caused dose dependent pigment dispersion that peaked within 30 min. The cumulative pigment displacement from cells could be quantitated by using a microplate reader to measure changes in transmittance of light through the wells. The acquired data enabled detailed and reproducible dose response curves and time course analyses to be generated. In addition, the assay followed for the rapid characterization of the effects of antagonists upon the beta adrenergic receptor (beta AR). The assay has the potential to test the effects of ligands upon any receptor capable of mediating pigment translocation in the melanophore cell line.

Measurements of light transmission through single melanophores

A photometrical method has been developed that allows assessment of subcellular pigment migration in melanophores of the fish cockoo wrasse (Labrus ossifagus L.) The pigment migration was studied with local light spot transmission measurements. Depending on where the light beam is placed on the melanophores it is possible to study events within an area of approximately 75 microns 2. Measuring pigment translocation in different parts of a melanophore gives new possibilities to study how cell membrane receptor-mediated signals are spread within a single cell, which will increase our understanding of how receptor activating drugs exert their cellular effect. The technique can be used in pharmacological and biophysical studies and in biosensors, pharmaceutical screens, environmental detectors, etc. The method clearly has the ability to study local and small changes in light transmission due to displacement of melanophore pigment granules. Since one melanophore on the tip of an optical fibre would be enough to obtain a measurable effect, the presented technique provides the basis for future development of biosensors small enough for in vivo applications, e.g., to monitor the catecholamine levels of circulating blood.