Stimulus dependence of the development of the zebrafish (Danio rerio) vestibular system

Attenuation of Antiviral Immune Response Caused by Perturbation of TRIM25-Mediated RIG-I Activation under Simulated Microgravity

Microgravity is a major environmental factor of space flight that triggers dysregulation of the immune system and increases clinical risks for deep-space-exploration crews. However, systematic studies and molecular mechanisms of the adverse effects of microgravity on the immune system in animal models are limited. Here, we establish a ground-based zebrafish disease model of microgravity for the research of space immunology. RNA sequencing analysis demonstrates that the retinoic-acid-inducible gene (RIG)-I-like receptor (RLR) and the Toll-like receptor (TLR) signaling pathways are significantly compromised by simulated microgravity (Sμg). TRIM25, an essential E3 for RLR signaling, is inhibited under Sμg, hampering the K63-linked ubiquitination of RIG-I and the following function-induction positive feedback loop of antiviral immune response. These mechanisms provide insights into better understanding of the effects and principles of microgravity on host antiviral immunity and present broad potential implications for developing strategies that can prevent and control viral diseases during space flight.

Behavioral methods for the functional assessment of hair cells in zebrafish

Zebrafish is an emerging animal model for studies on auditory system. This model presents high comparability with humans, good accessibility to the hearing organ, and high throughput capacity. To better utilize this animal model, methodologies need to be used to quantify the hearing function of the zebrafish. Zebrafish displays a series of innate and robust behavior related to its auditory function. Here, we reviewed the advantage of using zebrafish in auditory research and then introduced three behavioral tests, as follows: the startle response, the vestibular-ocular reflex, and rheotaxis. These tests are discussed in terms of their physiological characteristics, up-to-date technical development, and apparatus description. Test limitation and areas to improve are also introduced. Finally, we revealed the feasibility of these applications in zebrafish behavioral assessment and their potential in the high-throughput screening on hearing-related genes and drugs.

Zebrafish Bone and General Physiology Are Differently Affected by Hormones or Changes in Gravity

Teleost fish such as zebrafish (Danio rerio) are increasingly used for physiological, genetic and developmental studies. Our understanding of the physiological consequences of altered gravity in an entire organism is still incomplete. We used altered gravity and drug treatment experiments to evaluate their effects specifically on bone formation and more generally on whole genome gene expression. By combining morphometric tools with an objective scoring system for the state of development for each element in the head skeleton and specific gene expression analysis, we confirmed and characterized in detail the decrease or increase of bone formation caused by a 5 day treatment (from 5dpf to 10 dpf) of, respectively parathyroid hormone (PTH) or vitamin D3 (VitD3). Microarray transcriptome analysis after 24 hours treatment reveals a general effect on physiology upon VitD3 treatment, while PTH causes more specifically developmental effects. Hypergravity (3g from 5dpf to 9 dpf) exposure results in a significantly larger head and a significant increase in bone formation for a subset of the cranial bones. Gene expression analysis after 24 hrs at 3g revealed differential expression of genes involved in the development and function of the skeletal, muscular, nervous, endocrine and cardiovascular systems. Finally, we propose a novel type of experimental approach, the "Reduced Gravity Paradigm", by keeping the developing larvae at 3g hypergravity for the first 5 days before returning them to 1g for one additional day. 5 days exposure to 3g during these early stages also caused increased bone formation, while gene expression analysis revealed a central network of regulatory genes (hes5, sox10, lgals3bp, egr1, edn1, fos, fosb, klf2, gadd45ba and socs3a) whose expression was consistently affected by the transition from hyper- to normal gravity.

Auditory sensitivity of larval zebrafish (Danio rerio) measured using a behavioral prepulse inhibition assay

Zebrafish (Danio rerio) have become a valuable model for investigating the molecular genetics and development of the inner ear in vertebrates. In this study, we employed a prepulse inhibition (PPI) paradigm to assess hearing in larval wild-type (AB) zebrafish during early development at 5-6 days post-fertilization (d.p.f.). We measured the PPI of the acoustic startle response in zebrafish using a 1-dimensional shaker that simulated the particle motion component of sound along the fish's dorsoventral axis. The thresholds to startle-inducing stimuli were determined in 5-6 d.p.f. zebrafish, and their hearing sensitivity was then characterized using the thresholds of prepulse tone stimuli (90-1200 Hz) that inhibited the acoustic startle response to a reliable startle stimulus (820 Hz at 20 dB re. 1 m s(-2)). Hearing thresholds were defined as the minimum prepulse tone level required to significantly reduce the startle response probability compared with the baseline (no-prepulse) condition. Larval zebrafish showed greatest auditory sensitivity from 90 to 310 Hz with corresponding mean thresholds of -19 to -10 dB re. 1 m s(-2), respectively. Hearing thresholds of prepulse tones were considerably lower than previously predicted by startle response assays. The PPI assay was also used to investigate the relative contribution of the lateral line to the detection of acoustic stimuli. After aminoglycoside-induced neuromast hair-cell ablation, we found no difference in PPI thresholds between treated and control fish. We propose that this PPI assay can be used to screen for novel zebrafish hearing mutants and to investigate the ontogeny of hearing in zebrafish and other fishes.

An assessment of the long-term effects of simulated microgravity on cranial neural crest cells in zebrafish embryos with a focus on the adult skeleton

It is becoming increasingly important to address the long-term effects of exposure to simulated microgravity as the potential for space tourism and life in space become prominent topics amongst the World's governments. There are several studies examining the effects of exposure to simulated microgravity on various developmental systems and in various organisms; however, few examine the effects beyond the juvenile stages. In this study, we expose zebrafish embryos to simulated microgravity starting at key stages associated with cranial neural crest cell migration. We then analyzed the skeletons of adult fish. Gross observations and morphometric analyses show that exposure to simulated microgravity results in stunted growth, reduced ossification and severe distortion of some skeletal elements. Additionally, we investigated the effects on the juvenile skull and body pigmentation. This study determines for the first time the long-term effects of embryonic exposure to simulated microgravity on the developing skull and highlights the importance of studies investigating the effects of altered gravitational forces.

Behavioral changes of zebrafish according to cisplatin-induced toxicity of the balance system

Background and objectives:

Zebrafish are commonly used as experimental animals in otolaryngology studies. However, the behavioral characteristics of these fish are not well known, especially those related to the vestibular system. The goal of this study was to evaluate behavioral changes in zebrafish due to toxicity in the balance system.

Materials and methods:

Zebrafish were exposed to 1000 μM cisplatin for 6 h. We, then, periodically monitored swimming depth, total swimming distance, peak swimming velocity, and mean swimming velocity of the fish for approximately 21 days.

Results:

Total swimming distance ( p < 0.0001), peak swimming velocity ( p = 0.0063), and mean swimming velocity ( p < 0.0001) in the cisplatin-administered group were significantly decreased when compared with control fish.

Conclusion:

Our findings demonstrate that cisplatin can alter the locomotion behavior of zebrafish.

Ground-based facilities for simulation of microgravity: organism-specific recommendations for their use, and recommended terminology

Research in microgravity is indispensable to disclose the impact of gravity on biological processes and organisms. However, research in the near-Earth orbit is severely constrained by the limited number of flight opportunities. Ground-based simulators of microgravity are valuable tools for preparing spaceflight experiments, but they also facilitate stand-alone studies and thus provide additional and cost-efficient platforms for gravitational research. The various microgravity simulators that are frequently used by gravitational biologists are based on different physical principles. This comparative study gives an overview of the most frequently used microgravity simulators and demonstrates their individual capacities and limitations. The range of applicability of the various ground-based microgravity simulators for biological specimens was carefully evaluated by using organisms that have been studied extensively under the conditions of real microgravity in space. In addition, current heterogeneous terminology is discussed critically, and recommendations are given for appropriate selection of adequate simulators and consistent use of nomenclature.

The tangential nucleus controls a gravito-inertial vestibulo-ocular reflex

BACKGROUND

Although adult vertebrates sense changes in head position by using two classes of accelerometer, at larval stages zebrafish lack functional semicircular canals and rely exclusively on their otolithic organs to transduce vestibular information.

RESULTS

Despite this limitation, we find that larval zebrafish perform an effective vestibulo-ocular reflex (VOR) that serves to stabilize gaze in response to pitch and roll tilts. By using single-cell electroporations and targeted laser ablations, we identified a specific class of central vestibular neurons, located in the tangential nucleus, that are essential for the utricle-dependent VOR. Tangential nucleus neurons project contralaterally to extraocular motoneurons and in addition to multiple sites within the reticulospinal complex.

CONCLUSIONS

We propose that tangential neurons function as a broadband inertial accelerometer, processing utricular acceleration signals to control the activity of extraocular and postural neurons, thus completing a fundamental three-neuron circuit responsible for gaze stabilization.

The sensitivity of an immature vestibular system to altered gravity

Stimulus deprivation or stimulus augmentation can induce long-lasting modifications to sensory and motor systems. If deprivation is effective only during a limited period of life this phase is called "critical period." A critical period was described for the development of the roll-induced vestibuloocular reflex (rVOR) of Xenopus laevis using spaceflights. Spaceflight durations and basic conditions of Xenopus' development did not make it possible to answer the question whether exposure of the immature vestibular organ to weightlessness affects rVOR development. The embryonic development of Pleurodeles waltl is slow enough to solve this problem because the rVOR cannot be induced before 15 dpf. Stage 20-21 embryos (4 dpf) were exposed to microgravity during a 10-day spaceflight, or to 3g hypergravity following the same time schedule. After termination of altered gravity, the rVOR was recorded twice in most animals. The main observations were as follows: (1) after the first rVOR appearance at stage 37 (16 dpf), both rVOR gain and amplitude increased steadily up to saturation levels of 0.22 and 20°, respectively. (2) Three days after termination of microgravity, flight and ground larvae showed no rVOR; 1 day later, the rVOR could be induced only in ground larvae. Differences disappeared after 3 weeks. (3) For 10 days after 3g exposure, rVOR development was similar to that of 1g-controls but 3 weeks later, 3g-larvae showed a larger rVOR than 1g-controls. These observations indicate that the immature vestibular system is transiently sensitive to microgravity exposure and that exposure of the immature vestibular system to hypergravity leads to a slowly growing vestibular sensitization.

The vestibuloocular reflex of tadpoles (Xenopus laevis) after knock-down of the isthmus related transcription factor XTcf-4

Development of the amphibian vestibular organ is regulated by molecular and neuronal mechanisms and by environmental input. The molecular component includes inductive signals derived from neural tissue of the hindbrain and from the surrounding mesoderm. The integrity of hindbrain patterning, on the other hand, depends on instructive signals from the isthmus organizer of the midbrain including the transcription factor XTcf-4. If the development of the vestibular system depends on the integrity of the isthmus as organizing centre, suppression of isthmus maintenance should modify vestibular morphology and function. We tested this hypothesis by down-regulation of the transcription factor XTcf-4. 10 pMol XTcf-4-specific antisense morpholino oligonucleotide were injected in one blastomere of 2-cell stage embryos of Xenopus laevis. For reconstitution experiments, 500 pg mRNA of the repressing XTcf-4A isoform or the activating XTcf-4C isoform were co-injected. Over-expression experiments were included using the same isoforms. Otoconia formation and vestibular controlled behaviour such as the roll-induced vestibuloocular reflex (rVOR) and swimming were recorded two weeks later. In 50% of tadpoles, down-regulation of XTcf-4 induced (1) a depression of otoconia formation accompanied by a reduction of the rVOR, (2) abnormal tail development, and (3) loop swimming behaviour. (4) All effects were rescued by co-injection of XTcf-4C but not or only partially by XTcf-4A. (5) Over-expression of XTcf-4A caused similar morphological and rVOR modifications as XTcf-4 depletion while over-expression of XTcf-4C had no effect. Because XTcf-4C has been described as essential factor for isthmus development, we postulate that the isthmus is strongly involved in vestibular development.

Quantification of vestibular-induced eye movements in zebrafish larvae

Background

Vestibular reflexes coordinate movements or sensory input with changes in body or head position. Vestibular-evoked responses that involve the extraocular muscles include the vestibulo-ocular reflex (VOR), a compensatory eye movement to stabilize retinal images. Although an angular VOR attributable to semicircular canal stimulation was reported to be absent in free-swimming zebrafish larvae, recent studies reveal that vestibular-induced eye movements can be evoked in zebrafish larvae by both static tilts and dynamic rotations that tilt the head with respect to gravity.

Results

We have determined herein the basis of sensitivity of the larval eye movements with respect to vestibular stimulus, developmental stage, and sensory receptors of the inner ear. For our experiments, video recordings of larvae rotated sinusoidally at 0.25 Hz were analyzed to quantitate eye movements under infrared illumination. We observed a robust response that appeared as early as 72 hours post fertilization (hpf), which increased in amplitude over time. Unlike rotation about an earth horizontal axis, rotation about an earth vertical axis at 0.25 Hz did not evoke eye movements. Moreover, vestibular-induced responses were absent in mutant cdh23 larvae and larvae lacking anterior otoliths.

Conclusions

Our results provide evidence for a functional vestibulo-oculomotor circuit in 72 hpf zebrafish larvae that relies upon sensory input from anterior/utricular otolith organs.

A major effect of simulated microgravity on several stages of preimplantation mouse development is lethality associated with elevated phosphorylated SAPK/JNK

We tested whether microgravity affects mouse development during a period when gravity cues chick and frog embryo development. A rotating vessel developed approximately 0.1% simulated microgravity (MGS) for embryos. Microgravity simulation resulted in blocked cell accumulation in E2.5 embryos. E1.5 and E3.5 embryos showed lesser effects. For E1.5/2.5 embryos, cell accumulation block was followed by lethality at 48 hours after MGS. For E3.5 embryos, MGS blocked development without lethality but with apoptosis. E1.5-3.5 embryos from the rotational control developed lesser effects than MGS embryos. Embryonic stress-activated protein kinase (SAPK) was phosphorylated during MGS and mediated apoptosis. Increased pSAPK suggested that lethality is due to cellular stress induced by MGS, unlike the dysfunctional development after gravitational disorientation in frog and chick embryos. Thus, MGS causes lethality, a novel phenotype not often observed in microgravity or MGS. Embryonic lethality at E2.5 and apoptosis at E3.5 are associated with SAPK function, suggesting that MGS causes a general stress response that immediately affects many aspects of development. In addition, MGS and many aspects of In vitro fertilization/assisted reproductive technologies (IVF/ART) produce nonphysiological, nonevolutionary stresses that are mediated by SAPK, suggesting the primacy of this protein kinase in a wide range of mechanisms mediating negative reproductive outcomes in IVF/ART and potentially in spaceflight.

Changes in gravitational force cause changes in gene expression in the lens of developing zebrafish

Gravity has been a constant physical factor during the evolution and development of life on Earth. We have been studying effects of simulated microgravity on gene expression in transgenic zebrafish embryos expressing gfp under the influence of gene-specific promoters. In this study, we assessed the effect of microgravity on the expression of the heat shock protein 70 (hsp70) gene in lens during development using transgenic zebrafish embryos expressing gfp under the control of hsp70 promoter/enhancer. Hsp70:gfp expression was up-regulated (45%) compared with controls during the developmental period that included the lens differentiation stage. This increase was lens specific, because the entire embryo showed only a 4% increase in gfp expression. Northern blot and in situ hybridization analysis indicated that the hsp70:gfp expression recapitulated endogenous hsp70 mRNA expression. Hypergravity exposure also increased hsp70 expression during the same period. In situ hybridization analysis for two lens-specific crystallin genes revealed that neither micro- nor hypergravity affected the expression level of betaB1-crystallin, a non-hsp gene used as a marker for lens differentiation. However, hypergravity changed the expression level of alphaA-crystallin, a member of the small hsp gene family. Terminal deoxynucleotidyl transferase-mediated deoxyuridinetriphosphate nick end-labeling (TUNEL) assay analysis showed that altered-gravity (Deltag) decreased apoptosis in lens during the same period and the decrease correlated with the up-regulation of hsp70 expression, suggesting that elimination of nuclei from differentiating lens fiber cells was suppressed probably through hsp70 up-regulation. These results support the idea that Deltag influences hsp70 expression and differentiation in lens-specific and developmental period specific manners and that hsp family genes play a specific role in the response to Deltag.

Shear Stress Induces Preimplantation Embryo Death That Is Delayed by the Zona Pellucida and Associated with Stress-Activated Protein Kinase-Mediated Apoptosis1

In this study, we discovered that embryos sense shear stress and sought to characterize the kinetics and the enzymatic mechanisms underlying induction of embryonic lethality by shear stress. Using a rotating wall vessel programmed to produce 1.2 dynes/cm2 shear stress, it was found that shear stress caused lethality within 12 h for E3.5 blastocysts. Embryos developed an approximate 100% increase in mitogen-activated protein kinase 8/9 (formerly known as stress-activated protein kinase/junC kinase 1/2) phosphorylation by 6 h of shear stress that further increased to approximately 350% by 12 h. Terminal deoxynucleotidyltransferase dUTP nick end labeling/apoptosis was at baseline levels at 6 h and increased to approximately 500% of baseline at 12 h, when irreversible commitment to death occurred. A mitogen-activated protein kinase 8/9 phosphorylation inhibitor, D-JNKI1, was able to inhibit over 50% of the apoptosis, suggesting a causal role for mitogen-activated protein kinase 8/9 phosphorylation in the shear stress-induced lethality. The E2.5 (compacted eight-cell/early morula stage) embryo was more sensitive to shear stress than the E3.5 (early blastocyst stage) embryo. Additionally, zona pellucida removal significantly accelerated shear stress-induced lethality while having no lethal effect on embryos in the static control. In conclusion, preimplantation embryos sense shear stress, chronic shear stress is lethal, and the zona pellucida lessens the lethal and sublethal effects of shear stress. Embryos in vivo would not experience as high a sustained velocity or shear stress as induced experimentally here. Lower shear stresses might induce sufficient mitogen-activated protein kinase 8/9 phosphorylation that would slow growth or cause premature differentiation if the zona pellucida were not intact.

Otolith matrix proteins OMP-1 and Otolin-1 are necessary for normal otolith growth and their correct anchoring onto the sensory maculae

Fish otoliths are highly calcified concretions deposited in the inner ear and serve as a part of the hearing and balance systems. They consist mainly of calcium carbonate and a small amount of organic matrix. The latter component is considered to play important roles in otolith formation. Previously, we identified two major otolith matrix proteins, OMP-1 (otolith matrix protein-1) and Otolin-1, from salmonid species. To assess the function of these proteins in otolith formation, we performed antisense morpholino oligonucleotide (MO)-mediated knockdown of omp-1 and otolin-1 in zebrafish embryos. We first identified zebrafish cDNA homologs of omp-1 (zomp-1) and otolin-1 (zotolin-1). Whole-mount in situ hybridization then revealed that the expression of both zomp-1 and zotolin-1 mRNAs is restricted to the otic vesicles. zomp-1 mRNA was expressed from the 14-somite stage in the otic placode, but the zOMP-1 protein was detected only from 26-somite stage onwards. In contrast, zotolin-1 mRNA expression became clear around 72 hpf. MOs designed to inhibit zomp-1 and zotolin-1 mRNA translation, respectively, were injected into 1-2 cell stage embryos. zomp-1 MO caused a reduction in otolith size and an absence of zOtolin-1 deposition, while zotolin-1 MO caused a fusion of the two otoliths, and an increased instability of otoliths after fixation. We conclude that zOMP-1 is required for normal otolith growth and deposition of zOtolin-1 in the otolith, while zOtolin-1, a collagenous protein, is involved in the correct arrangement of the otoliths onto the sensory epithelium of the inner ear and probably in stabilization of the otolith matrix.

Changes in gravitational force affect gene expression in developing organ systems at different developmental times

Background

Little is known about the affect of microgravity on gene expression, particularly in vivo during embryonic development. Using transgenic zebrafish that express the gfp gene under the influence of a β-actin promoter, we examined the affect of simulated-microgravity on GFP expression in the heart, notochord, eye, somites, and rohon beard neurons. We exposed transgenic zebrafish to simulated-microgravity for different durations at a variety of developmental times in an attempt to determine periods of susceptibility for the different developing organ systems.

Results

The developing heart had a period of maximum susceptibility between 32 and 56 hours after fertilization when there was an approximately 30% increase in gene expression. The notochord, eye, somites, and rohon beard neurons all showed periods of susceptibility occurring between 24 and 72 hours after fertilization. In addition, the notochord showed a second period of susceptibility between 8 and 32 hours after fertilization. Interestingly, all organs appeared to be recovering by 80 hours after fertilization despite continued exposure to simulated-microgravity.

Conclusion

These results support the idea that exposure to microgravity can cause changes in gene expression in a variety of developing organ systems in live embryos and that there are periods of maximum susceptibility to the effects.

Quantifying the ontogeny of optokinetic and vestibuloocular behaviors in zebrafish, medaka, and goldfish

We quantitatively studied the ontogeny of oculomotor behavior in larval fish as a foundation for studies linking oculomotor structure and function with genetics. Horizontal optokinetic and vestibuloocular reflexes (OKR and VOR, respectively) were measured in three different species (goldfish, zebrafish, and medaka) during the first month after hatching. For all sizes of medaka, and most zebrafish, Bode plots of OKR (0.065-3.0 Hz, +/-10 degrees/s) revealed that eye velocity closely followed stimulus velocity (gain > 0.8) at low frequency but dropped sharply above 1 Hz (gain < 0.3 at 3 Hz). Goldfish showed increased gain proportional to size across frequencies. Linearity testing with steps and sinusoids showed excellent visual performance (gain > 0.8) in medaka almost from hatching; but zebrafish and goldfish exhibited progressive improvement, with only the largest equaling medaka performance. Monocular visual stimulation in zebrafish and goldfish produced gains of 0.5 versus <0.1 for the eye viewing a moving versus stationary stimulus pattern but 0.25 versus <0.1 in medaka. Angular VOR appeared much later than OKR, initially at only high accelerations (>200 degrees /s at 0.5 Hz), first in medaka followed by larger (8.11 mm) zebrafish; but it was virtually nonexistent in goldfish. Velocity storage was not observed except for an eye velocity build-up in the largest medaka. In summary, a robust OKR was achieved shortly after hatching in all three species. In contrast, larval fish seem to be unique among vertebrates tested in their lack of significant angular VOR at stages where active movement is required for feeding and survival.

Changes in gravitational force induce alterations in gene expression that can be monitored in the live, developing zebrafish heart

Little is known about the effect of microgravity on gene expression, particularly in vivo during embryonic development. Using transgenic zebrafish that express the gfp gene under the influence of a beta-actin promoter, we examined the affect of simulated-microgravity on GFP expression in the heart. Zebrafish embryos, at the 18-20 somite-stage, were exposed to simulated-microgravity for 24 hours. The intensity of GFP fluorescence associated with the heart was then determined using fluorescence microscopy. Our measurements indicated that simulated-microgravity induced a 23.9% increase in GFP-associated fluorescence in the heart. In contrast, the caudal notochord showed a 17.5% increase and the embryo as a whole showed only an 8.5% increase in GFP-associated fluorescence. This suggests that there are specific effects on the heart causing the more dramatic increase. These studies indicate that microgravity can influence gene expression and demonstrate the usefulness of this in vivo model of 'reporter-gene' expression for studying the effects of microgravity.

The development of gravity sensory systems during periods of altered gravity dependent sensory input

Gravity related behavior and the underlying neuronal networks are the most suitable model systems to study basic effects of altered gravitational input on the development of neuronal systems. A feature of sensory and motor systems is their susceptibility to modifications of their adequate physical and/or chemical stimuli during development. This discovery led to the formulation about critical periods, which defines the period of susceptibility during post-embryonal development. Critical periods can be determined by long-lasting modifications of the stimulus input for the gravity sensory system (GSS). Techniques include: (1) destruction of the gravity sense organ so that the gravity cannot be detected any longer and the central neuronal network of the GSS is deprived of gravity related information, (2) loading or deloading of parts of the body by weights or counterweights, respectively, which compensates for the gravitational pull, and (3) absence or augmentation of the gravitational environment per se by the exposure of organisms to microgravity during spaceflights or to hypergravity by centrifugation. Most data came from studies on compensatory eye or head movements in the clawed toad Xenopus laevis, the cichlid fish Oreochromis mossambicus, and crickets (Acheta domesticus, Gryllus bimaculatus). The responses are induced by a roll or pitch stimulation of the gravity sense organs, but are also affected by sensory inputs from proprioreceptors and eyes. The development of these compensatory eye and head responses reveals species-specific time courses. Based on experiments using spaceflights, centrifugation, lesion and loading or deloading, all species revealed a significant susceptibility to modifications of the gravity sensory input during development. Behavioral responses were depressed (Xenopus) or augmented (Xenopus, Oreochronis) by microgravity, and depressed by hypergravity except in crickets. In Acheta, however, the sensitivity of its position sensitive neuron PSI was reduced by microgravity. After termination of the period of modified gravity sensory input, all behavioral and physiological modifications disappeared, in some preparations such as the PSI of Acheta or the eye response in Xenopus, however, delayed after exposure to hypergravity. Irreversible modifications were rare; one example were malformations of the body of Xenopus tadpoles caused by lesion induced deprivation. Several periods of life such as the period of hatching or first appearance of gravity related reflexes revealed a specific sensitivity to altered gravity. Although all studies gave clear evidences for a basic sensitivity of developing GSSs to long-lasting modifications of the gravity sensory input, clear arguments for the existence of a critical period in the development of the sense of gravity are still missing. It has to take into consideration that during long-term exposures, adaptation processes take place which are guided by central physiological and genetically determined set points. The International Space Station (ISS) is the necessary platform of excellence if biological research is focussed on the analysis of long-term space effects on organisms.

Life-Cycle Experiments of Medaka Fish Aboard the International Space Station

Fish are the most likely candidates to be the first vertebrate to live their life cycle aboard the International Space Station (ISS). In the space-shuttle experiment using medaka, the fry born in space had the same number of germ cells as the ground control fish, and these germ cells later developed to produce the offspring on the ground. Fry hatched in space did not exhibit any looping behavior regardless of their strain, visual acuity, etc. The aquatic habitat (AQH) is a space habitat designed for long-term breeding of medaka, zebrafish and Xenopus, and recent advancements in this hardware also support fish life-cycle experiments. From the crosses between two strains, fish having good eyesight and less sensitivity to gravity were obtained, and their tolerance to microgravity was tested by parabolic flight using an airplane. The fish exhibited less looping and no differences in degree of looping between light and dark conditions. These are possible candidates for the first adult medaka (parent fish) to start a life cycle aboard ISS. Embryos were treated with a three-dimensional clinostat. Such simulated microgravity caused no differences in tissue architecture or in gene expression within the retina, nor in formation of cartilage (head skeleton). Otolith formation in embryos and fry was investigated for wild-type and mutant (ha) medaka. The ha embryos could not form utricular otoliths. They formed saccular otoliths but with a delay. Fry of the mutant fish lacking the utricular otoliths are highly light-dependent at the time of hatching, showing a perfect dorsal-light response (DLR). As they grow, they eventually shift from being light dependent to gravity dependent. Continuous treatment of the fry with altered light direction suppressed this shift to gravity dependence. Being less dependent on gravity, these fish can serve as model fish in studying the differences expected for the fish that have experienced a life cycle in microgravity.

Use of an otolith-deficient mutant in studies of fish behavior in microgravity

The mutant strain (ha) of medaka (Oryzias latipes) lack utricular otoliths as fry, and some never form otoliths for life. The cross (F1 generation) between the strain having good eyesight and another strain having ordinary eyesight augmented visual acuity of the F1 generation. Crossing the good eyesight strain and ha mutant produced fish having good eyesight and less sensitivity to gravity in the F2 population. Their tolerance to microgravity was tested by parabolic flight using an airplane. The fish exhibited less looping and no differences in degree of looping between light and dark conditions, suggesting that loss of eyesight (in darkness) is not a direct cause for looping behavior in microgravity. The ha embryos could not form utricular otoliths. They did form saccular otoliths, but with a delay. Fry of the mutant fish lacking the utricular otoliths are highly dependent on light upon hatching and exhibit a perfect dorsal-light response (DLR). As they grow, they eventually shift from being light-dependent to being gravity-dependent. Continuous treatment of the fry with altered light direction suppressed this shift to gravity dependence. Being less dependent on gravity, these fish can serve as models in studying the differences expected for the vestibular system of fish reared in microgravity. When these fish were exposed to microgravity (parabolic flights) of an airplane, they spent far less time looping than fish reared in an ordinary light regimen.

Development of the zebrafish inner ear

Abstract Recent years have seen a renaissance of investigation into the mechanisms of inner ear development. Genetic analysis of zebrafish has contributed significantly to this endeavour, with several dramatic advances reported over the past year or two. Here, we review the major findings from recent work in zebrafish. Several cellular and molecular mechanisms have been elucidated, including the signaling pathways controlling induction of the otic placode, morphogenesis and patterning of the otic vesicle, and elaboration of functional attributes of inner ear.

A critical period for functional vestibular development in zebrafish

We have determined a critical period for vestibular development in zebrafish by using a bioreactor designed by NASA to simulate microgravity for cells in culture. A critical period is defined as the briefest period of time during development when stimulus deprivation results in long lasting or permanent sensory deficits. Zebrafish eggs were collected within 3 hours of being laid and fertilized. In experiment 1, eggs were placed in the bioreactor at 3, 24, 30, 36, 48, or 72 hours postfertilization (hPF) and maintained in the bioreactor until 96 hPF. In experiment 2, eggs were placed in the bioreactor immediately after they were collected and maintained in the bioreactor until 24, 36, 48, 60, 66, 72, or 96 hPF. Beginning at 96 hPF, all larvae had their vestibulo-ocular reflexes (VOR) evaluated once each day for 5 days. Only larvae that hatched from eggs that were placed in the bioreactor before 30 hPF in experiment 1 or removed from the bioreactor later than 66 hPF in experiment 2 had VOR deficits that persisted for at least 5 days. These data suggest a critical period for vestibular development in the zebrafish that begins before 30 hPF and ends after 66 hPF. To confirm this, zebrafish eggs were placed in the bioreactor at 24 hPF and removed at 72 hPF. VORs were evaluated in these larvae once each day for 5 days beginning at 96 hPF. These larvae had VOR deficits that persisted for at least 5 days. In addition, larvae that had been maintained in the bioreactor from 24 to 66 hPF or from 30 to 72 hPF, had only temporary VOR deficits. In a final experiment, zebrafish eggs were placed in the bioreactor at 3 hPF and removed at 96 hPF but the bioreactor was turned off from 24 hPF to 72 hPF. These larvae had normal VORs when they were removed from the bioreactor at 96 hPF. Taken as a whole, these data support the idea that there is a critical period for functional maturation of the zebrafish vestibular system. The developmental period identified includes the timeframe during which the vestibular primary afferent neurons are born, innervate their central and peripheral targets, and remodel their central projections.

Adaptation of the macular vestibuloocular reflex to altered gravitational conditions in a fish (Oreochromis mossambicus)

Young fish (Oreochromis mossambicus) were exposed to microgravity (micro g) for 9 to 10 days, or to hypergravity (hg) for 9 days. For several weeks after termination of micro g and hg, the roll-induced static vestibuloocular reflex (rVOR) was recorded. In stage 11/12-fish, the rVOR amplitude (angle between the maximal up and down movement of an eye during a complete 360 degree lateral roll) of micro g-animals increased significantly by 25% compared to 1 g-controls during the first post-flight week but decreased to the control level during the second post-flight week. Microgravity had no effect in stage 14/16 fish on the rVOR amplitude. After 3 g-exposure, the rVOR amplitude was significantly reduced for both groups compared to their 1 g-controls. Readaptation to 1 g-condition was completed during the second post-3 g week. We postulate a critical period during which the development of the macular vestibuloocular reflex depends on gravitational input, and which is limited by the first appearance of the rVOR. At this period of early development, exposure to microgravity sensitizes the vestibular system while hypergravity desensitizes it.

[Physiology of vertebrates under micro-gravity with special reference to the Ca metabolism]

On April 12, 1961, Major Yurii A. Gagarin of the former-U.S.S.R. Air Force circled the Earth in a spacecraft named "Vostok", a word which means "east". He spent 1 hour and 48 minutes in space. Since then, the U.S.S.R. and the U.S.A. have sent many astronauts into space. In one case, the stay in space exceeded a year in length, reaching 438 days. Through these experiences, it became clear that micro-gravity caused various problems in human physiology. One of the most serious problems was the loss of Ca from bones, as a result of the negative expenditure of Ca. Under 1G on the ground, bone absorption and bone formation proceed in accordance. Under micro-gravity, however, this balance is broken. Although this phenomenon has been widely analyzed from the viewpoint of molecular biology as well, studies to clarify the mechanism that causes the disorder of Ca metabolism in bones have just started. At present, no perfect treatment to prevent the loss of Ca from bones is available.

Altered gravitational forces affect the development of the static vestibuloocular reflex in fish (Oreochromis mossambicus)

Young fish (Oreochromis mossambicus) were exposed to microgravity (micro g) for 9 to 10 days during space missions STS-55 and STS-84, or to hypergravity (hg) for 9 days. Young animals (stages 11-12), which had not yet developed the roll-induced static vestibuloocular reflex (rVOR) at micro g- and hg-onset, and older ones (stages 14-16), which had already developed the rVOR, were used. For several weeks afterwards, the rVOR was recorded after termination of mug and hg. Here are the main results: (1) In the stage 11-12 fish, the rVOR gain (response angle/roll angle) measured for roll angles 15 degrees, 30 degrees, and 45 degrees was not affected by microgravity if animals were rolled from the horizontal to the inclined posture, but was increased significantly if animals were rolled in the opposite manner. The rVOR amplitude (maximal eye movement during a complete 360 degrees roll) of micro g animals increased significantly by 25% compared to 1g controls during the first postflight week, but decreased to the control level during the second postflight week. Microgravity had no effect in stage 14-16 fish on either rVOR gain or amplitude. (2) After 3g exposure, both rVOR gain and amplitude were significantly reduced for both stage 11-12 and stage 15 fish. One g readaptation was completed during the second post-3g week. Hypergravity at 2 or 2.5 g had no effect. (3) Hypergravity at all three levels tested (2g, 2.5g, and 3g) accelerated the morphological development as assessed by external morphological markers. Exposure to micro g- or 3g-periods during an early developmental period modifies the physiological properties of the neuronal network underlying the static rVOR; in susceptible developmental stages, these modifications include sensitization by microgravity and desensitization by hypergravity.

Development of utricular otoliths, but not saccular otoliths, is necessary for vestibular function and survival in zebrafish

We have been studying the consequences of embryonic vestibular dysfunction caused by the monolith (mnl) mutation in zebrafish. mnl is a dominant mutation that specifically inhibits formation of utricular otoliths. However, briefly immobilizing mnl/mnl embryos in agarose with the otic vesicle orientated at certain angles selectively induces or prevents formation of utricular and/or saccular otoliths. With this noninvasive technique, we generated six phenotypic classes of mnl/mnl mutants, designated S-S, U-U, U-S, S-US, U-US, and US-US, depending on which otoliths are present on each side (U, utricular otolith; S, saccular otolith). All mnl/mnl larvae survived through day 10 of development. Thereafter, S-S larvae showed a rapid decline, probably because of starvation, and none survived to adulthood. Survival rates in all other classes of mnl/mnl larvae (those having at least one utricular otolith) were close to normal. The presence or absence of utricular otoliths also correlated with vestibular function during early larval development, as measured by three criteria: First, unlike wild-type larvae, S-S mutant larvae showed almost no detectable counter-rotation of the eyes when tilted tail up or tail down. Second, 95% of S-S mutant larvae never acquired the ability to maintain a balanced dorsal-up posture. Third, although most wild-type larvae responded to gentle prodding by swimming in a straight line, S-S larvae responded by swimming in rapid circles, showing sudden and frequent changes in direction ("zigzagging"), and/or rolling and spiraling. All other phenotypic classes of mnl/mnl larvae behaved normally in these assays. These data demonstrate that bilateral loss of utricular otoliths disrupts the ability to sense gravity, severely impairs balance and motor coordination, and is invariably lethal. The presence of a utricular otolith in at least one inner ear is necessary and sufficient for vestibular function and survival. In contrast, saccular otoliths are dispensable for these functions.