Anion efflux mediates transduction in the hair cells of the zebrafish lateral line

Hair cells are the principal sensory receptors of the vertebrate auditory system, where they transduce sounds through mechanically gated ion channels that permit cations to flow from the surrounding endolymph into the cells. The lateral line of zebrafish has served as a key model system for understanding hair cell physiology and development, often with the belief that these hair cells employ a similar transduction mechanism. In this study, we demonstrate that these hair cells are exposed to an unregulated external environment with cation concentrations that are too low to support transduction. Our results indicate that hair cell excitation is instead mediated by a substantially different mechanism involving the outward flow of anions. Further investigation of hair cell transduction in a diversity of sensory systems and species will likely yield deep insights into the physiology of these unique cells.

Metabolomics-Guided Discovery, Isolation, Structure Elucidation, and Bioactivity of Myropeptins C-E from Myrothecium inundatum

The saprotrophic filamentous fungus Myrothecium inundatum represents a chemically underexplored ascomycete with a high number of putative biosynthetic gene clusters in its genome. Here, we present new linear lipopeptides from nongenetic gene activation experiments using nutrient and salt variations. Metabolomics studies revealed four myropeptins, and structural analyses by NMR, HRMS, Marfey's analysis, and ECD assessment for their helical properties established their absolute configuration. A myropeptin biosynthetic gene cluster in the genome was identified. The myropeptins exhibit general nonspecific toxicity against all cancer cell lines in the NCI-60 panel, larval zebrafish with EC50 concentrations of 5-30 μM, and pathogenic bacteria and fungi (MICs of 4-32 μg/mL against multidrug-resistant S. aureus and C. auris). In vitro hemolysis, cell viability, and ionophore assays indicate that the myropeptins target mitochondrial and cellular membranes, inducing cell depolarization and cell death. The toxic activity is modulated by the length of the lipid side chain, which provides valuable insight into their structure-activity relationships.

Recording central nervous system responses of freely-swimming marine and freshwater fishes with a customizable, implantable AC differential amplifier

BACKGROUND

Fish have adapted to a diversity of environments but the neural mechanisms underlying natural aquatic behaviors are not well known.

NEW METHOD

We have developed a small, customizable AC differential amplifier and surgical procedures for recording multi-unit extracellular signals in the CNS of marine and freshwater fishes.

RESULTS

Our minimally invasive amplifier allowed fish to orient to flow and respond to hydrodynamic and visual stimuli. We recorded activity in the cerebellum during these behaviors.

COMPARISON WITH EXISTING METHODS

Our system is very low-cost, hydrodynamically streamlined, and capable of high-gain in order to allow for recordings from freely behaving, fast fishes in complex fluid environments.

CONCLUSIONS

Our tethered approach allows access to record neural activity in a diversity of adult fishes in the lab, but can also be modified for data logging in the field.

Reduction of spherical and chromatic aberration in axial-scanning optical systems with tunable lenses

Optical systems with integrated tunable lenses allow for rapid axial-scanning without mechanical translation of the components. However, changing the power of the tunable lens typically upsets aberration balancing across the system, introducing spherical and chromatic aberrations that limit the usable axial range. This study develops an analytical approximation for the tuning-induced spherical and axial chromatic aberration of a general optical system containing a tunable lens element. The resulting model indicates that systems can be simultaneously corrected for both tuning-induced spherical and chromatic aberrations by controlling the lateral magnification, coma, and pupil lateral color prior to the tunable surface. These insights are then used to design a realizable axial-scanning microscope system with a high numerical aperture and diffraction-limited performance over a wide field of view and deep axial range.

Prolonged exposure to stressors suppresses exploratory behavior in zebrafish larvae

Environmental stressors induce rapid physiological and behavioral shifts in vertebrate animals. However, the neurobiological mechanisms responsible for stress-induced changes in behavior are complex and not well understood. Similar to mammalian vertebrates, zebrafish adults display a preference for dark environments that is associated with predator avoidance, enhanced by stressors, and broadly used in assays for anxiety-like behavior. Although the larvae of zebrafish are a prominent model organism for understanding neural circuits, few studies have examined the effects of stressors on their behavior. This study examines the effects of noxious chemical and electric shock stressors on locomotion and light preference in zebrafish larvae. We found that both stressors elicited similar changes in behavior. Acute exposure induced increased swimming activity, while prolonged exposure depressed activity. Neither stressor produced a consistent shift in light-dark preference, but prolonged exposure to these stressors resulted in a pronounced decrease in exploration of different visual environments. We also examined the effects of exposure to a noxious chemical cue using whole-brain calcium imaging, and identified neural correlates in the area postrema, an area of the hindbrain containing noradrenergic and dopaminergic neurons. Pharmaceutical blockade experiments showed that α-adrenergic receptors contribute to the behavioral response to an acute stressor but are not necessary for the response to a prolonged stressor. These results indicate that zebrafish larvae have complex behavioral responses to stressors comparable to those of adult animals, and also suggest that these responses are mediated by similar neural pathways.

Manta rays feed using ricochet separation, a novel nonclogging filtration mechanism

Solid-liquid filtration is a ubiquitous process found in industrial and biological systems. Although implementations vary widely, almost all filtration systems are based on a small set of fundamental separation mechanisms, including sieve, cross-flow, hydrosol, and cyclonic separation. Anatomical studies showed that manta rays have a highly specialized filter-feeding apparatus that does not resemble previously described filtration systems. We examined the fluid flow around the manta filter-feeding apparatus using a combination of physical modeling and computational fluid dynamics. Our results indicate that manta rays use a unique solid-fluid separation mechanism in which direct interception of particles with wing-like structures causes particles to "ricochet" away from the filter pores. This filtration mechanism separates particles smaller than the pore size, allows high flow rates, and resists clogging.

The Emergence of Directional Selectivity in the Visual Motion Pathway of Drosophila

The perception of visual motion is critical for animal navigation, and flies are a prominent model system for exploring this neural computation. In Drosophila, the T4 cells of the medulla are directionally selective and necessary for ON motion behavioral responses. To examine the emergence of directional selectivity, we developed genetic driver lines for the neuron types with the most synapses onto T4 cells. Using calcium imaging, we found that these neuron types are not directionally selective and that selectivity arises in the T4 dendrites. By silencing each input neuron type, we identified which neurons are necessary for T4 directional selectivity and ON motion behavioral responses. We then determined the sign of the connections between these neurons and T4 cells using neuronal photoactivation. Our results indicate a computational architecture for motion detection that is a hybrid of classic theoretical models.

A computational model of flow between the microscale respiratory structures of fish gills

The gills of most teleost fishes are covered by plate-like structures, the secondary lamellae, that provide the bulk of the respiratory surface area. Water passing over the secondary lamellae exchanges gases with blood passing through the secondary lamellae, forming a system that has served as a classic model of counter-current exchange. In this study, a computational model of flow around the secondary lamellae is used to examine the hydrodynamic consequences of changes to the lamellar morphology. Consistent with previous studies, the interlamellar distance is found to strongly affect the hydrodynamic resistance of the gills. However, the presence of a small gap between the tips of the secondary lamellae is found to have a similarly strong effect on the hydrodynamic resistance and flow patterns within the gills. The results from this model have been generally formulated, allowing the calculation of the hydrodynamic resistance for measured morphometric parameters. These results provide a new basis for comparing theoretical predictions of the gill resistance with measured values, and provide a general model for examining the diversity gill morphologies observed in teleost fishes.

Hydrodynamic resistance and flow patterns in the gills of a tilapine fish

The gills of teleost fishes are often discussed as an archetypal counter-current exchange system, capable of supporting the relatively high metabolic rates of some fishes despite the low oxygen solubility of water. Despite an appreciation for the physiology of exchange at the gills, many questions remain regarding the hydrodynamical basis of ventilation in teleost fishes. In this study, the hydrodynamic resistance and flow fields around the isolated gills of a tilapia, Oreochromis mossambicus, were measured as a function of the applied pressure head. At ventilatory pressures typical of a fish at rest, the hydrodynamic resistance of the gills was nearly constant, the flow was laminar, shunting of water around the gills was essentially absent, and the distribution of water flow was relatively uniform. However, at the higher pressures typical of an active or stressed fish, some of these qualities were lost. In particular, at elevated pressures there was a decrease in the hydrodynamic resistance of the gills and substantial shunting of water around the gills. These effects suggest mechanical limits to maximum aerobic performance during activity or under adverse environmental conditions.

Bottles as models: predicting the effects of varying swimming speed and morphology on size selectivity and filtering efficiency in fishes

We created physical models based on the morphology of ram suspension-feeding fishes to better understand the roles morphology and swimming speed play in particle retention, size selectivity and filtration efficiency during feeding events. We varied the buccal length, flow speed and architecture of the gills slits, including the number, size, orientation and pore size/permeability, in our models. Models were placed in a recirculating flow tank with slightly negatively buoyant plankton-like particles (∼20–2000 μm) collected at the simulated esophagus and gill rakers to locate the highest density of particle accumulation. Particles were captured through sieve filtration, direct interception and inertial impaction. Changing the number of gill slits resulted in a change in the filtration mechanism of particles from a bimodal filter, with very small (≤50 μm) and very large (>1000 μm) particles collected, to a filter that captured medium-sized particles (101–1000 μm). The number of particles collected on the gill rakers increased with flow speed and skewed the size distribution towards smaller particles (51–500 μm). Small pore sizes (105 and 200 μm mesh size) had the highest filtration efficiencies, presumably because sieve filtration played a significant role. We used our model to make predictions about the filtering capacity and efficiency of neonatal whale sharks. These results suggest that the filtration mechanics of suspension feeding are closely linked to an animal's swimming speed and the structural design of the buccal cavity and gill slits.

Locomotory transition from water to sand and its effects on undulatory kinematics in sand lances (Ammodytidae)

Sand lances, fishes in the genus Ammodytes, exhibit a peculiar burrowing behavior in which they appear to swim rapidly into the substrate. They use posteriorly propagated undulations of the body to move in both water, a Newtonian fluid, and in sand, a non-Newtonian, granular substrate. In typical aquatic limbless locomotion, undulations of the body push against water, which flows because it is incapable of supporting the static stresses exerted by the animal, thus the undulations move in world space (slipping wave locomotion). In typical terrestrial limbless locomotion, these undulations push against substrate irregularities and move relatively little in world space (non-slipping wave locomotion). We used standard and X-ray video to determine the roles of slipping wave and non-slipping wave locomotion during burrowing in sand lances. We find that sand lances in water use slipping wave locomotion, similar to most aquatic undulators, but switch to non-slipping waves once they burrow. We identify a progression of three stages in the burrowing process: first, aquatic undulations similar to typical anguilliform locomotion (but without head yaw) push the head into the sand; second, more pronounced undulations of the aquatic portion of the body push most of the animal below ground; third, the remaining above-ground portion of the body ceases undulation and the subterranean portion takes over, transitioning to non-slipping wave locomotion. We find no evidence that sand lances use their body motions to fluidize the sand. Instead, as soon as enough of the body is underground, they undergo a kinematic shift and locomote like terrestrial limbless vertebrates.

Surprisingly simple mechanical behavior of a complex embryonic tissue

BACKGROUND

Previous studies suggest that mechanical feedback could coordinate morphogenetic events in embryos. Furthermore, embryonic tissues have complex structure and composition and undergo large deformations during morphogenesis. Hence we expect highly non-linear and loading-rate dependent tissue mechanical properties in embryos.

METHODOLOGY/PRINCIPAL FINDINGS

We used micro-aspiration to test whether a simple linear viscoelastic model was sufficient to describe the mechanical behavior of gastrula stage Xenopus laevis embryonic tissue in vivo. We tested whether these embryonic tissues change their mechanical properties in response to mechanical stimuli but found no evidence of changes in the viscoelastic properties of the tissue in response to stress or stress application rate. We used this model to test hypotheses about the pattern of force generation during electrically induced tissue contractions. The dependence of contractions on suction pressure was most consistent with apical tension, and was inconsistent with isotropic contraction. Finally, stiffer clutches generated stronger contractions, suggesting that force generation and stiffness may be coupled in the embryo.

CONCLUSIONS/SIGNIFICANCE

The mechanical behavior of a complex, active embryonic tissue can be surprisingly well described by a simple linear viscoelastic model with power law creep compliance, even at high deformations. We found no evidence of mechanical feedback in this system. Together these results show that very simple mechanical models can be useful in describing embryo mechanics.

Whole-body lift and ground effect during pectoral fin locomotion in the northern spearnose poacher (Agonopsis vulsa)

The northern spearnose poacher, Agonopsis vulsa, is a benthic, heavily armored fish that swims primarily using pectoral fins. High-speed kinematics, whole-body lift measurements, and flow visualization were used to study how A. vulsa overcomes substantial negative buoyancy while generating forward thrust. Kinematics for five freely swimming poachers indicate that individuals tend to swim near the bottom (within 1cm) with a consistently small (less than 1 degrees ) pitch angle of the body. When the poachers swam more than 1cm above the bottom, however, body pitch angles were higher and varied inversely with speed, suggesting that lift may help overcome negative buoyancy. To determine the contribution of the body to total lift, fins were removed from euthanized fish (n=3) and the lift and drag from the body were measured in a flume. Lift and drag were found to increase with increasing flow velocity and angle of attack (ANCOVA, p<0.0001 for both effects). Lift force from the body was found to supply approximately half of the force necessary to overcome negative buoyancy when the fish were swimming more than 1cm above the bottom. Lastly, flow visualization experiments were performed to examine the mechanism of lift generation for near-bottom swimming. A vortex in the wake of the pectoral fins was observed to interact strongly with the substratum when the animals approached the bottom. These flow patterns suggest that, when swimming within 1cm of the bottom, poachers may use hydrodynamic ground effect to augment lift, thereby counteracting negative buoyancy.

Larval zebrafish rapidly sense the water flow of a predator's strike

Larval fishes have a remarkable ability to sense and evade the feeding strike of a predator fish with a rapid escape manoeuvre. Although the neuromuscular control of this behaviour is well studied, it is not clear what stimulus allows a larva to sense a predator. Here we show that this escape response is triggered by the water flow created during a predator's strike. Using a novel device, the impulse chamber, zebrafish (Danio rerio) larvae were exposed to this accelerating flow with high repeatability. Larvae responded to this stimulus with an escape response having a latency (mode=13-15 ms) that was fast enough to respond to predators. This flow was detected by the lateral line system, which includes mechanosensory hair cells within the skin. Pharmacologically ablating these cells caused the escape response to diminish, but then recover as the hair cells regenerated. These findings demonstrate that the lateral line system plays a role in predator evasion at this vulnerable stage of growth in fishes.

Extremely fast prey capture in pipefish is powered by elastic recoil

The exceptionally high speed at which syngnathid fishes are able to rotate their snout towards prey and capture it by suction is potentially caused by a catapult mechanism in which the energy previously stored in deformed elastic elements is suddenly released. According to this hypothesis, tension is built up in tendons of the post-cranial muscles before prey capture is initiated. Next, an abrupt elastic recoil generates high-speed dorsal rotation of the head and snout, rapidly bringing the mouth close to the prey, thus enabling the pipefish to be close enough to engulf the prey by suction. However, no experimental evidence exists for such a mechanism of mechanical power amplification during feeding in these fishes. To test this hypothesis, inverse dynamical modelling based upon kinematic data from high-speed videos of prey capture in bay pipefish Syngnathus leptorhynchus, as well as electromyography of the muscle responsible for head rotation (the epaxial muscle) was performed. The remarkably high instantaneous muscle-mass-specific power requirement calculated for the initial phase of head rotation (up to 5795 W kg(-1)), as well as the early onset times of epaxial muscle activity (often observed more than 300 ms before the first externally discernible prey capture motion), support the elastic power enhancement hypothesis.

The hydrodynamics of locomotion at intermediate Reynolds numbers: undulatory swimming in ascidian larvae (Botrylloides sp.)

Understanding how the shape and motion of an aquatic animal affects the performance of swimming requires knowledge of the fluid forces that generate thrust and drag. These forces are poorly understood for the large diversity of animals that swim at Reynolds numbers (Re) between 10(0) and 10(2). We experimentally tested quasi-steady and unsteady blade-element models of the hydrodynamics of undulatory swimming in the larvae of the ascidian Botrylloides sp. by comparing the forces predicted by these models with measured forces generated by tethered larvae and by comparing the swimming speeds predicted with measurements of the speed of freely swimming larvae. Although both models predicted mean forces that were statistically indistinguishable from measurements, the quasi-steady model predicted the timing of force production and mean swimming speed more accurately than the unsteady model. This suggests that unsteady force (i.e. the acceleration reaction) does not play a role in the dynamics of steady undulatory swimming at Re approximately 10(2). We explored the relative contribution of viscous and inertial force to the generation of thrust and drag at 10(0)<Re<10(2) by running a series of mathematical simulations with the quasi-steady model. These simulations predicted that thrust and drag are dominated by viscous force (i.e. skin friction) at Re approximately 10(0) and that inertial force (i.e. form force) generates a greater proportion of thrust and drag at higher Re than at lower Re. However, thrust was predicted to be generated primarily by inertial force, while drag was predicted to be generated more by viscous than inertial force at Re approximately 10(2). Unlike swimming at high (>10(2)) and low (<10(0)) Re, the fluid forces that generate thrust cannot be assumed to be the same as those that generate drag at intermediate Re.

Relationship of Serum and Urinary Proteins in Neoplastic States: A Preliminary Survey

Twenty-four-hour urine specimens from patients with and without malignancies were concentrated by dialysis. The electrophoretic mobility of the urine protein components was compared with that of the respective serum proteins. Control values were similar to those established in the literature. Urinary protein concentrations found in malignant states were higher. Generally, there is a positive correlation between serum concentration of globulins and urinary excretion. It is not possible to prove in either controls or in patients bearing malignancies that the amount of urinary protein excreted depends upon molecular size.