Membrane invaginations facilitate reversible water flux driving tunable iridescence in a dynamic biophotonic system
ABSTRACT Squids have used their tunable iridescence for camouflage and communication for millions of years; materials scientists have more recently looked to them for inspiration to develop new "biologically inspired" adaptive optics. Iridocyte cells produce iridescence through constructive interference of light with intracellular Bragg reflectors. The cell's dynamic control over the apparent lattice constant and dielectric contrast of these multilayer stacks yields the corresponding optical control of brightness and color across the visible spectrum. Here, we resolve remaining uncertainties in iridocyte cell structure and determine how this unusual morphology enables the cell's tunable reflectance. We show that the plasma membrane periodically invaginates deep into the iridocyte to form a potential Bragg reflector consisting of an array of narrow, parallel channels that segregate the resulting high refractive index, cytoplasmic protein-containing lamellae from the low-index channels that are continuous with the extracellular space. In response to control by a neurotransmitter, the iridocytes reversibly imbibe or expel water commensurate with changes in reflection intensity and wavelength. These results allow us to propose a comprehensive mechanism of adaptive iridescence in these cells from stimulation to color production. Applications of these findings may contribute to the development of unique classes of tunable photonic materials.
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ABSTRACT: Cephalopods (e.g. octopus, squid and cuttlefish) dynamically tune the colour and brightness of their skin for camouflage and communication using specialized skin cells called iridocytes. We use high-resolution microspectrophotometry to investigate individual tunable Bragg structures (consisting of alternating reflectin protein-containing, high-refractive index lamellae and low-refractive index inter-lamellar spaces) in live and chemically fixed iridocytes of the California market squid, Doryteuthis opalescens. This subcellular, single-stack microspectrophotometry allows for spectral normalization, permitting use of a transfer-matrix model of Bragg reflectance to calculate all the parameters of the Bragg stack-the refractive indices, dimensions and numbers of the lamellae and inter-lamellar spaces. Results of the fitting analyses show that eight or nine pairs of low- and high-index layers typically contribute to the observed reflectivity in live cells, whereas six or seven pairs of low- and high-index layers typically contribute to the reflectivity in chemically fixed cells. The reflectin-containing, high-index lamellae of live cells have a refractive index proportional to the peak reflectivity, with an average of 1.405 ± 0.012 and a maximum around 1.44, while the reflectin-containing lamellae in fixed tissue have a refractive index of 1.413 ± 0.015 suggesting a slight increase of refractive index in the process of fixation. As expected, incremental changes in refractive index contribute to the greatest incremental changes in reflectivity for those Bragg stacks with the most layers. The excursions in dimensions required to tune the measured reflected wavelength from 675 (red) to 425 nm (blue) are a decrease from ca 150 to 80 nm for the high-index lamellae and from ca 120 to 50 nm for the low-index inter-lamellar spaces. Fixation-induced dimensional changes also are quantified, leading us to suggest that further microspectrophotometric analyses of this iridocyte system can be used as a model system to quantify the effects of various methods of tissue fixation. The microspectrophotometry technique described can be expected to provide deeper insights into the molecular and physical mechanisms governing other biophotonically active cells and structures.Journal of The Royal Society Interface 05/2013; 10(85):20130386. DOI:10.1098/rsif.2013.0386 · 3.86 Impact Factor
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ABSTRACT: Pauling's suggestion that enzymes are complementary in structure to the activated complexes of the reactions they catalyze has provided the conceptual basis to explain how enzymes obtain their fantastic catalytic prowess, and has served as a guiding principle in drug design for over 50 y. However, this model by itself fails to predict the magnitude of enzymes' rate accelerations. We construct a thermodynamic framework that begins with the classic concept of differential binding but invokes additional terms that are needed to account for subtle effects in the catalytic cycle's proton inventory. Although the model presented can be applied generally, this analysis focuses on ketosteroid isomerase (KSI) as an example, where recent experiments along with a large body of kinetic and thermodynamic data have provided strong support for the noncanonical thermodynamic contribution described. The resulting analysis precisely predicts the free energy barrier of KSI's reaction as determined from transition-state theory using only empirical thermodynamic data. This agreement is suggestive that a complete free energy inventory of the KSI catalytic cycle has been identified.Proceedings of the National Academy of Sciences 07/2013; 110(30). DOI:10.1073/pnas.1310964110 · 9.81 Impact Factor
Conference Paper: Luminance control of neurally tuneable skin iridescence in squid.[Show abstract] [Hide abstract]
ABSTRACT: Cephalopods create precise skin color and pattern displays for the purpose of signaling and camouflage. In squids, such visual trickery is achieved through the combined action of two color elements: pigmented chromatophores and structural iridophores (which produce iridescence). The neural control of chromatophores was recognized many decades ago but the system controlling dynamic iridescence remained in obscurity (although ACh was known to activate iridophores). To tackle this knowledge gap, we developed a novel physiological preparation in the squid Doryteuthis pealeii. Our results show that stimulation of dermal nerves shifts the spectral peak of the reflected light to shorter wavelengths (>145 nm) and increases the peak reflectance (>245 %) of innervated iridophores (Wardill et al. 2012). We also demonstrate that ACh is released within the iridophore layer and that extensive nerve branching is seen within each iridophore. The dynamic colour shift is significantly faster (17 s) than the peak reflectance increase (32 s) revealing two distinct control mechanisms. Responses from a structurally altered preparation indicate that the reflectin protein condensation mechanism (Izumi et al. 2010, Tao et al. 2010) explains the slower peak reflectance change, while a newly discovered water flux mechanism reducing platelet thickness (DeMartini et al. 2013) may explain the fast colour shift. Next, we traced the skin nerves towards the brain. While the chromatophore motorneurons descend directly from the brain, neural stimulation and dye back-filling revealed that cell bodies of the iridophore 'control' neurons are located in the stellate ganglion. Nonetheless, brain input is necessary for iridescence expression. Lastly, through behavioural tests, we showed that squids turn their iridescence on/off in response to lights on/off, respectively. However, the decline and rise of iridescence is slow, taking up to 1 hour and 5 minutes respectively, suggesting that iridescence may match light intensity during the diurnal cycle. In summary: (1) Squid iridescence is neurally activated. (2) The color and reflectance changes follow different dynamics. (3) Iridophores and chromatophores are innervated by different neurons. (4) The iridescence neural circuit has a relay in the peripheral stellate ganglia. (5) The rise and decline of iridescence is much slower than that of chromatophores and can be elicited by light intensity changes.International Conference on Invertebrate Vision, Bäckaskog Castle, Sweden.; 08/2013