Development of the zebrafish lateral line

Cranial placodes: models for exploring the multi-facets of cell adhesion in epithelial rearrangement, collective migration and neuronal movements

Key to morphogenesis is the orchestration of cell movements in the embryo, which requires fine-tuned adhesive interactions between cells and their close environment. The neural crest paradigm has provided important insights into how adhesion dynamics control epithelium-to-mesenchyme transition and mesenchymal cell migration. Much less is known about cranial placodes, patches of ectodermal cells that generate essential parts of vertebrate sensory organs and ganglia. In this review, we summarise the known functions of adhesion molecules in cranial placode morphogenesis, and discuss potential novel implications of adhesive interactions in this crucial developmental process. The great repertoire of placodal cell behaviours offers new avenues for exploring the multiple roles of adhesion complexes in epithelial remodelling, collective migration and neuronal movements.

Perturbing the developing skull: using laser ablation to investigate the robustness of the infraorbital bones in zebrafish (Danio rerio)

BACKGROUND

The development of the craniofacial skeleton from embryonic mesenchyme is a complex process that is not yet completely understood, particularly for intramembranous bones. This study investigates the development of the neural crest derived infraorbital (IO) bones of the zebrafish (Danio rerio) skull. Located under the orbit, the IO bones ossify in a set sequence and are closely associated with the lateral line system. We conducted skeletogenic condensation and neuromast laser ablation experiments followed by shape analyses in order to investigate the relationship between a developing IO bone and the formation of the IO series as well as to investigate the highly debated inductive potential of neuromasts for IO ossification.

RESULTS

We demonstrate that when skeletogenic condensations recover from laser ablation, the resulting bone differs in shape compared to controls. Interestingly, neighbouring IO bones in the bone series are unaffected. In addition, we show that the amount of canal wall mineralization is significantly decreased following neuromast laser ablation at juvenile and larval stages.

CONCLUSIONS

These results highlight the developmental robustness of the IO bones and provide direct evidence that canal neuromasts play a role in canal wall development in the head. Furthermore, we provide evidence that the IO bones may be two distinct developmental modules. The mechanisms underlying developmental robustness are rarely investigated and are important to increase our understanding of evolutionary developmental biology of the vertebrate skull.

Zebrafish prion protein PrP2 controls collective migration process during lateral line sensory system development

Prion protein is involved in severe neurodegenerative disorders but its physiological role is still in debate due to an absence of major developmental defects in knockout mice. Previous reports in zebrafish indicate that the two prion genes, PrP1 and PrP2, are both involved in several steps of embryonic development thus providing a unique route to discover prion protein function. Here we investigate the role of PrP2 during development of a mechano-sensory system, the posterior lateral line, using morpholino knockdown and PrP2 targeted inactivation. We confirm the efficiency of the translation blocking morpholino at the protein level. Development of the posterior lateral line is altered in PrP2 morphants, including nerve axonal outgrowth and primordium migration defects. Reduced neuromast deposition was observed in PrP2 morphants as well as in PrP2^−/− mutants. Rosette formation defects were observed in PrP2 morphants, strongly suggesting an abnormal primordium organization and reflecting loss of cell cohesion during migration of the primordium. In addition, the adherens junction proteins, E-cadherin and ß-catenin, were mis-localized after reduction of PrP2 expression and thus contribute to the primordium disorganization. Consequently, hair cell differentiation and number were affected and this resulted in reduced functional neuromasts. At later developmental stages, myelination of the posterior lateral line nerve was altered. Altogether, our study reports an essential role of PrP2 in collective migration process of the primordium and in neuromast formation, further implicating a role for prion protein in cell adhesion.

Itch is required for lateral line development in zebrafish

The zebrafish posterior lateral line is formed during early development by the deposition of neuromasts from a migrating primordium. The molecular mechanisms regulating the regional organization and migration of the primordium involve interactions between Fgf and Wnt/β-catenin signaling and the establishment of specific cxcr4b and cxcr7b cytokine receptor expression domains. Itch has been identified as a regulator in several different signaling pathways, including Wnt and Cxcr4 signaling. We identified two homologous itch genes in zebrafish, itcha and itchb, with generalized expression patterns. By reducing itchb expression in particular upon morpholino knockdown, we demonstrated the importance of Itch in regulating lateral line development by perturbing the patterns of cxcr4b and cxcr7b expression. Itch knockdown results in a failure to down-regulate Wnt signaling and overexpression of cxcr4b in the primordium, slowing migration of the posterior lateral line primordium and resulting in abnormal development of the lateral line.

Axon-Schwann cell interactions during peripheral nerve regeneration in zebrafish larvae

Background

Peripheral nerve injuries can severely affect the way that animals perceive signals from the surrounding environment. While damage to peripheral axons generally has a better outcome than injuries to central nervous system axons, it is currently unknown how neurons re-establish their target innervations to recover function after injury, and how accessory cells contribute to this task. Here we use a simple technique to create reproducible and localized injury in the posterior lateral line (pLL) nerve of zebrafish and follow the fate of both neurons and Schwann cells.

Results

Using pLL single axon labeling by transient transgene expression, as well as transplantation of glial precursor cells in zebrafish larvae, we individualize different components in this system and characterize their cellular behaviors during the regenerative process. Neurectomy is followed by loss of Schwann cell differentiation markers that is reverted after nerve regrowth. We show that reinnervation of lateral line hair cells in neuromasts during pLL nerve regeneration is a highly dynamic process with promiscuous yet non-random target recognition. Furthermore, Schwann cells are required for directional extension and fasciculation of the regenerating nerve. We provide evidence that these cells and regrowing axons are mutually dependant during early stages of nerve regeneration in the pLL. The role of ErbB signaling in this context is also explored.

Conclusion

The accessibility of the pLL nerve and the availability of transgenic lines that label this structure and their synaptic targets provides an outstanding in vivo model to study the different events associated with axonal extension, target reinnervation, and the complex cellular interactions between glial cells and injured axons during nerve regeneration.

The roles and regulation of multicellular rosette structures during morphogenesis

Multicellular rosettes have recently been appreciated as important cellular intermediates that are observed during the formation of diverse organ systems. These rosettes are polarized, transient epithelial structures that sometimes recapitulate the form of the adult organ. Rosette formation has been studied in various developmental contexts, such as in the zebrafish lateral line primordium, the vertebrate pancreas, the Drosophila epithelium and retina, as well as in the adult neural stem cell niche. These studies have revealed that the cytoskeletal rearrangements responsible for rosette formation appear to be conserved. By contrast, the extracellular cues that trigger these rearrangements in vivo are less well understood and are more diverse. Here, we review recent studies of the genetic regulation and cellular transitions involved in rosette formation. We discuss and compare specific models for rosette formation and highlight outstanding questions in the field.

Mmp25β facilitates elongation of sensory neurons during zebrafish development

Matrix metalloproteinases (MMPs) are a large and complex family of zinc-dependent endoproteinases widely recognized for their roles in remodeling the extracellular matrix (ECM) during embryonic development, wound healing, and tissue homeostasis. Their misregulation is central to many pathologies, and they have therefore been the focus of biomedical research for decades. These proteases have also recently emerged as mediators of neural development and synaptic plasticity in vertebrates, however, understanding of the mechanistic basis of these roles and the molecular identities of the MMPs involved remains far from complete. We have identified a zebrafish orthologue of mmp25 (a.k.a. leukolysin; MT6-MMP), a membrane-type, furin-activated MMP associated with leukocytes and invasive carcinomas, but which we find is expressed by a subset of the sensory neurons during normal embryonic development. We detect high levels of Mmp25β expression in the trigeminal, craniofacial, and posterior lateral line ganglia in the hindbrain, and in Rohon-Beard cells in the dorsal neural tube during the first 48 h of embryonic development. Knockdown of Mmp25β expression with morpholino oligonucleotides results in larvae that are uncoordinated and insensitive to touch, and which exhibit defects in the development of sensory neural structures. Using in vivo zymography, we observe that Mmp25β morphant embryos show reduced Type IV collagen degradation in regions of the head traversed by elongating axons emanating from the trigeminal ganglion, suggesting that Mmp25β may play a pivotal role in mediating ECM remodeling in the vicinity of these elongating axons.

Characterization of the trunk neural crest in the bamboo shark, Chiloscyllium punctatum

The neural crest is a population of mesenchymal cells that after migrating from the neural tube gives rise to structure and cell types: the jaw, part of the peripheral ganglia, and melanocytes. Although much is known about neural crest development in jawed vertebrates, a clear picture of trunk neural crest development for elasmobranchs is yet to be developed. Here we present a detailed study of trunk neural crest development in the bamboo shark, Chiloscyllium punctatum. Vital labeling with dioctadecyl tetramethylindocarbocyanine perchlorate (DiI) and in situ hybridization using cloned Sox8 and Sox9 probes demonstrated that trunk neural crest cells follow a pattern similar to the migratory paths already described in zebrafish and amphibians. We found shark trunk neural crest along the rostral side of the somites, the ventromedial pathway, the branchial arches, the gut, the sensory ganglia, and the nerves. Interestingly, C. punctatum Sox8 and Sox9 sequences aligned with vertebrate SoxE genes, but appeared to be more ancient than the corresponding vertebrate paralogs. The expression of these two SoxE genes in trunk neural crest cells, especially Sox9, matched the Sox10 migratory patterns observed in teleosts. Also of interest, we observed DiI cells and Sox9 labeling along the lateral line, suggesting that in C. punctatum, glial cells in the lateral line are likely of neural crest origin. Although this has been observed in other vertebrates, we are the first to show that the pattern is present in cartilaginous fishes. These findings demonstrate that trunk neural crest cell development in C. punctatum follows the same highly conserved migratory pattern observed in jawed vertebrates.

In vivo and in vitro biophysical properties of hair cells from the lateral line and inner ear of developing and adult zebrafish

Hair cells detect and process sound and movement information, and transmit this with remarkable precision and efficiency to afferent neurons via specialized ribbon synapses. The zebrafish is emerging as a powerful model for genetic analysis of hair cell development and function both in vitro and in vivo. However, the full exploitation of the zebrafish is currently limited by the difficulty in obtaining systematic electrophysiological recordings from hair cells under physiological recording conditions. Thus, the biophysical properties of developing and adult zebrafish hair cells are largely unknown. We investigated potassium and calcium currents, voltage responses and synaptic activity in hair cells from the lateral line and inner ear in vivo and using near-physiological in vitro recordings. We found that the basolateral current profile of hair cells from the lateral line becomes more segregated with age, and that cells positioned in the centre of the neuromast show more mature characteristics and those towards the edge retain a more immature phenotype. The proportion of mature-like hair cells within a given neuromast increased with zebrafish development. Hair cells from the inner ear showed a developmental change in current profile between the juvenile and adult stages. In lateral line hair cells from juvenile zebrafish, exocytosis also became more efficient and required less calcium for vesicle fusion. In hair cells from mature zebrafish, the biophysical characteristics of ion channels and exocytosis resembled those of hair cells from other lower vertebrates and, to some extent, those in the immature mammalian vestibular and auditory systems. We show that although the zebrafish provides a suitable animal model for studies on hair cell physiology, it is advisable to consider that the age at which the majority of hair cells acquire a mature-type configuration is reached only in the juvenile lateral line and in the inner ear from >2 months after hatching.

Electroablation: a method for neurectomy and localized tissue injury

BACKGROUND

Tissue injury has been employed to study diverse biological processes such as regeneration and inflammation. In addition to physical or surgical based methods for tissue injury, current protocols for localized tissue damage include laser and two-photon wounding, which allow a high degree of accuracy, but are expensive and difficult to apply. In contrast, electrical injury is a simple and inexpensive technique, which allows reproducible and localized cell or tissue damage in a variety of contexts.

RESULTS

We describe a novel technique that combines the advantages of zebrafish for in vivo visualization of cells with those of electrical injury methods in a simple and versatile protocol which allows the study of regeneration and inflammation. The source of the electrical pulse is a microelectrode that can be placed with precision adjacent to specific cells expressing fluorescent proteins. We demonstrate the use of this technique in zebrafish larvae by damaging different cell types and structures. Neurectomy can be carried out in peripheral nerves or in the spinal cord allowing the study of degeneration and regeneration of nerve fibers. We also apply this method for the ablation of single lateral line mechanosensory neuromasts, showing the utility of this approach as a tool for the study of organ regeneration. In addition, we show that electrical injury induces immune cell recruitment to damaged tissues, allowing in vivo studies of leukocyte dynamics during inflammation within a confined and localized injury. Finally, we show that it is possible to apply electroablation as a method of tissue injury and inflammation induction in adult fish.

CONCLUSIONS

Electrical injury using a fine microelectrode can be used for axotomy of neurons, as a general tissue ablation tool and as a method to induce a powerful inflammatory response. We demonstrate its utility to studies in both larvae and in adult zebrafish but we expect that this technique can be readily applied to other organisms as well. We have called this method of electrical based tissue ablation, electroablation.

The Nogo-C2/Nogo receptor complex regulates the morphogenesis of zebrafish lateral line primordium through modulating the expression of dkk1b, a Wnt signal inhibitor

The fish lateral line (LL) is a mechanosensory system closely related to the hearing system of higher vertebrates, and it is composed of several neuromasts located on the surface of the fish. These neuromasts can detect changes in external water flow, to assist fish in maintaining a stationary position in a stream. In the present study, we identified a novel function of Nogo/Nogo receptor signaling in the formation of zebrafish neuromasts. Nogo signaling in zebrafish, like that in mammals, involves three ligands and four receptors, as well as three co-receptors (TROY, p75, and LINGO-1). We first demonstrated that Nogo-C2, NgRH1a, p75, and TROY are able to form a Nogo-C2 complex, and that disintegration of this complex causes defective neuromast formation in zebrafish. Time-lapse recording of the CldnB::lynEGFP transgenic line revealed that functional obstruction of the Nogo-C2 complex causes disordered morphogenesis, and reduces rosette formation in the posterior LL (PLL) primordium during migration. Consistent with these findings, hair-cell progenitors were lost from the PLL primordium in p75, TROY, and Nogo-C2/NgRH1a morphants. Notably, the expression levels of pea3, a downstream marker of Fgf signaling, and dkk1b, a Wnt signaling inhibitor, were both decreased in p75, TROY, and Nogo-C2/NgRH1a morphants; moreover, dkk1b mRNA injection could rescue the defects in neuromast formation resulting from knockdown of p75 or TROY. We thus suggest that a novel Nogo-C2 complex, consisting of Nogo-C2, NgRH1a, p75, and TROY, regulates Fgf signaling and dkk1b expression, thereby ensuring stable organization of the PLL primordium.

Histone deacetylase activity is required for embryonic posterior lateral line development

OBJECTIVES

The posterior lateral line (PLL) system in zebrafish has recently become a model for investigating tissue morphogenesis. PLL primordium periodically deposits neuromasts as it migrates along the horizontal myoseptum from head to tail of the embryonic fish, and this migration requires activity of various molecular mechanisms. Histone deacetylases (HDACs) have been implicated in numerous biological processes of development, by regulating gene transcription, but their roles in regulating PLL during embryonic development have up to now remained unexplored.

MATERIAL AND METHODS

In this study, we used HDAC inhibitors to investigate the role of HDACs in early development of the zebrafish PLL sensory system. We further investigated development of the PLL by cell-specific immunostaining and in situ hybridization.

RESULTS

Our analysis showed that HDACs were involved in zebrafish PLL development as pharmacological inhibition of HDACs resulted in its defective formation. We observed that migration of PLL primordium was altered and accompanied by disrupted development of PLL neuromasts in HDAC inhibitor-treated embryos. In these, positions of PLL neuromasts were affected. In particular, the first PLL neuromast was displaced posteriorly in a treatment dose-dependent manner. Primordium cell proliferation was reduced upon HDAC inhibition. Finally, we showed that inhibition of HDAC function reduced numbers of hair cells in PLL neuromasts of HDAC inhibitor-treated embryos.

CONCLUSION

Here, we have revealed a novel role for HDACs in orchestrating PLL morphogenesis. Our results suggest that HDAC activity is necessary for control of cell proliferation and migration of PLL primordium and hair cell differentiation during early stages of PLL development in zebrafish.

The role of Wnt/β-catenin signaling in proliferation and regeneration of the developing basilar papilla and lateral line

Canonical Wnt/β-catenin signaling has been implicated in multiple developmental events including the regulation of proliferation, cell fate, and differentiation. In the inner ear, Wnt/β-catenin signaling is required from the earliest stages of otic placode specification through the formation of the mature cochlea. Within the avian inner ear, the basilar papilla (BP), many Wnt pathway components are expressed throughout development. Here, using reporter constructs for Wnt/β-catenin signaling, we show that this pathway is active throughout the BP (E6-E14) in both hair cells (HCs) and supporting cells. To characterize the role of Wnt/β-catenin activity in developing HCs, we performed gain- and loss-of-function experiments in vitro and in vivo in the chick BP and zebrafish lateral line systems, respectively. Pharmacological inhibition of Wnt signaling in the BP and lateral line neuromasts during the periods of proliferation and HC differentiation resulted in reduced proliferation and decreased HC formation. Conversely, pharmacological activation of this pathway significantly increased the number of HCs in the lateral line and BP. Results demonstrated that this increase was the result of up-regulated cell proliferation within the Sox2-positive cells of the prosensory domains. Furthermore, Wnt/β-catenin activation resulted in enhanced HC regeneration in the zebrafish lateral line following aminoglycoside-induced HC loss. Combined, our data suggest that Wnt/β-catenin signaling specifies the number of cells within the prosensory domain and subsequently the number of HCs. This ability to induce proliferation suggests that the modulation of Wnt/β-catenin signaling could play an important role in therapeutic HC regeneration.

Gβ1 controls collective cell migration by regulating the protrusive activity of leader cells in the posterior lateral line primordium

Collective cell migration is critical for normal development, tissue repair and cancer metastasis. Migration of the posterior lateral line primordium (pLLP) generates the zebrafish sensory organs (neuromasts, NMs). This migration is promoted by the leader cells at the leading edge of the pLLP, which express the G protein-coupled chemokine receptor Cxcr4b and respond to the chemokine Cxcl12a. However, the mechanism by which Cxc112a/Cxcr4b signaling regulates pLLP migration remains unclear. Here we report that signal transduction by the heterotrimeric G protein subunit Gβ1 is essential for proper pLLP migration. Although both Gβ1 and Gβ4 are expressed in the pLLP and NMs, depletion of Gβ1 but not Gβ4 resulted in an arrest of pLLP migration. In embryos deficient for Gβ1, the pLLP cells migrated in an uncoordinated fashion and were unable to extend protrusions at the leading front, phenocopying those in embryos deficient for Cxcl12a or Cxcr4b. A transplantation assay showed that, like Cxcr4b, Gβ1 is required only in the leader cells of the pLLP. Analysis of F-actin dynamics in the pLLP revealed that whereas wild-type leader cells display extensive actin polymerization in the direction of pLLP migration, counterparts defective for Gβ1, Cxcr4b or Cxcl12a do not. Finally, synergy experiments revealed that Gβ1 and Cxcr4b interact genetically in regulating pLLP migration. Collectively, our data indicate that Gβ1 controls migration of the pLLP, likely by acting downstream of the Cxcl12a/Cxcr4b signaling. This study also provides compelling evidence for functional specificity among Gβ isoforms in vivo.

A Hox gene controls lateral line cell migration by regulating chemokine receptor expression downstream of Wnt signaling

The posterior lateral line primordium in zebrafish provides an amenable model to study mechanisms of collective cell migration. The directed migration of the cell cluster along the path of Sdf1a chemokine requires two receptors, Cxcr4b and Cxcr7b, which are expressed in the leading and trailing part of the primordium, respectively. The polarized expression of receptors is regulated by Wnt signaling, but downstream players mediating this control remain to be found. Here, we show that the Hox homeobox gene Hoxb8a is a critical component that acts downstream of the Wnt pathway to coordinate the expression of both chemokine receptors. We find that Hoxb8a is expressed in the leading part of the primordium and is required for the correct speed and extent of migration. Hoxb8a expression is dependent upon Wnt activity and needed both for cxcr4b expression and to repress and thus restrict cxcr7b expression to the trailing zone of the primordium. In the absence of Wnt activity, overexpressed Hoxb8a is able to repress cxcr7b but not up-regulate cxcr4b expression. Together with results from expressing dominant activator and repressor constructs, these findings suggest that Hoxb8a is induced by and cooperates with Wnt signaling to up-regulate cxcr4b, and acts through multiple mechanisms to repress cxcr7b expression.

A system for investigation of biological effects of diagnostic ultrasound on development of zebrafish embryos

A system for scanning zebrafish embryos with diagnostic ultrasound was developed for research into possible biological effects during development. Two troughs for holding embryos were formed from agarose in a rectangular dish and separated by an ultrasound absorber. A 4.9 MHz linear array ultrasound probe was positioned to uniformly scan all the embryos at the bottom of one trough, with the other used for controls. Zebrafish embryos were scanned continuously from 10-24 h post fertilization (hpf) during the segmentation period and gross morphological parameters were measured at 30 hpf, including viability, length, number of visible axons, and the progression of the lateral line primordium (LLP). Our initial tests were encumbered by the thermal effects of probe self-heating, which resulted in accelerated development of the zebrafish embryos. After subsequent optimization, our test revealed a significant retardation of primary motor axons and the migration of the LLP in embryos scanned with ultrasound, which indicated a potential for nonthermal effects on neuronal development. This diagnostic ultrasound exposure system is suitable for further investigation of possible subtle bioeffects, such as perturbation of neuronal migration.

From sequence to spike to spark: evo-devo-neuroethology of electric communication in mormyrid fishes

Mormyrid fishes communicate using pulses of electricity, conveying information about their identity, behavioral state, and location. They have long been used as neuroethological model systems because they are uniquely suited to identifying cellular mechanisms for behavior. They are also remarkably diverse, and they have recently emerged as a model system for studying how communication systems may influence the process of speciation. These two lines of inquiry have now converged, generating insights into the neural basis of evolutionary change in behavior, as well as the influence of sensory and motor systems on behavioral diversification and speciation. Here, we review the mechanisms of electric signal generation, reception, and analysis and relate these to our current understanding of the evolution and development of electromotor and electrosensory systems. We highlight the enormous potential of mormyrids for studying evolutionary developmental mechanisms of behavioral diversification, and make the case for developing genomic and transcriptomic resources. A complete mormyrid genome sequence would enable studies that extend our understanding of mormyrid behavior to the molecular level by linking morphological and physiological mechanisms to their genetic basis. Applied in a comparative framework, genomic resources would facilitate analysis of evolutionary processes underlying mormyrid diversification, reveal the genetic basis of species differences in behavior, and illuminate the origins of a novel vertebrate sensory and motor system. Genomic approaches to studying the evo-devo-neuroethology of mormyrid communication represent a deeply integrative approach to understanding the evolution, function, development, and mechanisms of behavior.

Fusion of locomotor maneuvers, and improving sensory capabilities, give rise to the flexible homing strikes of juvenile zebrafish

At 5 days post-fertilization and 4 mm in length, zebrafish larvae are successful predators of mobile prey items. The tracking and capture of 200 μm long Paramecia requires efficient sensorimotor transformations and precise neural controls that activate axial musculature for orientation and propulsion, while coordinating jaw muscle activity to engulf them. Using high-speed imaging, we report striking changes across ontogeny in the kinematics, structure and efficacy of zebrafish feeding episodes. Most notably, the discrete tracking maneuvers used by larval fish (turns, forward swims) become fused with prey capture swims to form the continuous, fluid homing strikes of juvenile and adult zebrafish. Across this same developmental time frame, the duration of feeding episodes become much shorter, with strikes occurring at broader angles and from much greater distances than seen with larval zebrafish. Moreover, juveniles use a surprisingly diverse array of motor patterns that constitute a flexible predatory strategy. This enhances the ability of zebrafish to capture more mobile prey items such as Artemia. Visually-guided tracking is complemented by the mechanosensory lateral line system. Neomycin ablation of lateral line hair cells reduced the accuracy of strikes and overall feeding rates, especially when neomycin-treated larvae and juveniles were placed in the dark. Darkness by itself reduced the distance from which strikes were launched, as visualized by infrared imaging. Rapid growth and changing morphology, including ossification of skeletal elements and differentiation of control musculature, present challenges for sustaining and enhancing predatory capabilities. The concurrent expansion of the cerebellum and subpallium (an ancestral basal ganglia) may contribute to the emergence of juvenile homing strikes, whose ontogeny possibly mirrors a phylogenetic expansion of motor capabilities.

Epizootic neoplasia of the lateral line system of lake trout (Salvelinus namaycush) in New York's Finger Lakes

This article documents an epizootic of inflammation and neoplasia selectively affecting the lateral line system of lake trout (Salvelinus namaycush) in 4 Finger Lakes in New York from 1985 to 1994. We studied more than 100 cases of this disease. Tumors occurred in 8% (5/64) of mature and 21% (3/14) of immature lake trout in the most severely affected lake. Lesions consisted of 1 or more neoplasm(s) in association with lymphocytic inflammation, multifocal erosions, and ulcerations of the epidermis along the lateral line. Lesions progressed from inflammatory to neoplastic, with 2-year-old lake trout showing locally extensive, intense lymphocytic infiltrates; 2- to 3-year-old fish having multiple, variably sized white masses up to 3 mm in diameter; and fish over 5 years old exhibiting 1 or more white, cerebriform masses greater than 1 cm in diameter. Histologic diagnoses of the tumors were predominantly spindle cell sarcomas or benign or malignant peripheral nerve sheath neoplasms, with fewer epitheliomas and carcinomas. Prevalence estimates did not vary significantly between sexes or season. The cause of this epizootic remains unclear. Tumor transmission trials, virus isolation procedures, and ultrastructural study of lesions failed to reveal evidence of a viral etiology. The Finger Lakes in which the disease occurred did not receive substantially more chemical pollution than unaffected lakes in the same chain during the epizootic, making an environmental carcinogen an unlikely primary cause of the epizootic. A hereditary component, however, may have contributed to this syndrome since only fish of the Seneca Lake strain were affected.

Alcohol-induced morphological deficits in the development of octavolateral organs of the zebrafish (Danio rerio)

Prenatal alcohol exposure is known to have many profound detrimental effects on human fetal development (fetal alcohol spectrum disorders), which may manifest as lifelong disabilities. However, how alcohol affects the auditory/vestibular system is still largely unknown. This is the first study to investigate morphological effects of alcohol on the developing octavolateral system (the inner ear and lateral line) using the zebrafish, Danio rerio. Zebrafish embryos of 2 hours post fertilization (hpf) were treated in 2% alcohol for 48 hours and screened at 72 hpf for morphological defects of the inner ear and lateral line. Octavolateral organs from both alcohol-treated and control zebrafish were examined using light, confocal, and scanning electron microscopy. We observed several otolith phenotypes for alcohol-treated zebrafish including zero, one, two abnormal, two normal, and multiple otoliths. Results of this study show that alcohol treatment during early development impairs the inner ear (smaller ear, abnormal otoliths, and fewer sensory hair cells) and the lateral line (smaller neuromasts, fewer neuromasts and hair cells per neuromast, and shorter kinocilia of hair cells). Early embryonic alcohol exposure may also result in defects in hearing, balance, and hydrodynamic function of zebrafish.

Neuronal subtypes are specified by the level of neurod expression in the zebrafish lateral line

During neural development, even in seemingly homogeneous cell populations, individual neurons acquire diverse morphology and behavior, and even adjacent neurons can establish synaptic connections with distinct targets. Although diversity among individual neurons is essential for a functional nervous system, the underlying molecular mechanism of establishing heterogeneity in a population of neuronal precursors has been poorly addressed at the single-cell level. We focused on the development of the zebrafish posterior lateral line (PLL) and revealed a molecular mechanism that differentiates a homogenous neuronal population in the ganglion into the two types of neurons, leaders and followers. We developed a method to analyze gene expression levels in leaders and followers at the single-cell resolution, and found that leaders expressed significantly higher levels of neurod compared with followers. Furthermore, neurod expression was found to be heterogeneous among neurons before the onset of phenotypic differentiation of leaders and followers, and neurod overexpression in single PLL neurons promoted differentiation into leaders. These results suggest that the quantity, rather than quality (i.e., the ON/OFF states), of neurod expression directly or indirectly determines the two subtypes of PLL neurons.

JNK-interacting protein 3 mediates the retrograde transport of activated c-Jun N-terminal kinase and lysosomes

Retrograde axonal transport requires an intricate interaction between the dynein motor and its cargo. What mediates this interaction is largely unknown. Using forward genetics and a novel in vivo imaging approach, we identified JNK-interacting protein 3 (Jip3) as a direct mediator of dynein-based retrograde transport of activated (phosphorylated) c-Jun N-terminal Kinase (JNK) and lysosomes. Zebrafish jip3 mutants (jip3^nl7) displayed large axon terminal swellings that contained high levels of activated JNK and lysosomes, but not other retrograde cargos such as late endosomes and autophagosomes. Using in vivo analysis of axonal transport, we demonstrated that the terminal accumulations of activated JNK and lysosomes were due to a decreased frequency of retrograde movement of these cargos in jip3^nl7, whereas anterograde transport was largely unaffected. Through rescue experiments with Jip3 engineered to lack the JNK binding domain and exogenous expression of constitutively active JNK, we further showed that loss of Jip3–JNK interaction underlies deficits in pJNK retrograde transport, which subsequently caused axon terminal swellings but not lysosome accumulation. Lysosome accumulation, rather, resulted from loss of lysosome association with dynein light intermediate chain (dynein accessory protein) in jip3^nl7, as demonstrated by our co-transport analyses. Thus, our results demonstrate that Jip3 is necessary for the retrograde transport of two distinct cargos, active JNK and lysosomes. Furthermore, our data provide strong evidence that Jip3 in fact serves as an adapter protein linking these cargos to dynein.

To form and maintain connections, neurons require the active transport of proteins and organelles between the neuronal cell body and axon terminals. Inhibition of this “axonal” transport has been linked to neurodegenerative diseases. Despite the importance of this process, to date there was no vertebrate model system where axonal transport could be studied in an intact animal. Our study introduces zebrafish as such a model and demonstrates its power for the analysis of axonal transport. We used this system to 1) initiate a genetic screen to find novel mediators of axonal transport; 2) develop in vivo imaging strategies to visualize axonal transport in real time in the intact animal; and 3) discover, using these methods, that JNK interacting protein 3 (Jip3) is required for the transport of two cargos, a kinase and lysosomes, from axon terminals to the cell body (retrograde transport). In the absence of Jip3, these cargos accumulate and axon terminals become dysmorphic, though the retrograde transport of other cargos is normal. Interestingly, abnormal localization of these cargos has been linked to axonal disease states, but our work is the first to identify a specific adapter protein necessary for their transport from axon terminals.

Time-gated optical projection tomography allows visualization of adult zebrafish internal structures

Optical imaging through biological samples is compromised by tissue scattering and currently various approaches aim to overcome this limitation. In this paper we demonstrate that an all optical technique, based on non-linear upconversion of infrared ultrashort laser pulses and on multiple view acquisition, allows the reduction of scattering effects in tomographic imaging. This technique, namely Time-Gated Optical Projection Tomography (TGOPT), is used to reconstruct three dimensionally the internal structure of adult zebrafish without staining or clearing agents. This method extends the use of Optical Projection Tomography to optically diffusive samples yielding reconstructions with reduced artifacts, increased contrast and improved resolution with respect to those obtained with non-gated techniques. The paper shows that TGOPT is particularly suited for imaging the skeletal system and nervous structures of adult zebrafish.

Shroom3 is required downstream of FGF signalling to mediate proneuromast assembly in zebrafish

During development, morphogenetic processes require a precise coordination of cell differentiation, cell shape changes and, often, cell migration. Yet, how pattern information is used to orchestrate these different processes is still unclear. During lateral line (LL) morphogenesis, a group of cells simultaneously migrate and assemble radially organized cell clusters, termed rosettes, that prefigure LL sensory organs. This process is controlled by Fibroblast growth factor (FGF) signalling, which induces cell fate changes, cell migration and cell shape changes. However, the exact molecular mechanisms induced by FGF activation that mediate these changes on a cellular level are not known. Here, we focus on the mechanisms by which FGFs control apical constriction and rosette assembly. We show that apical constriction in the LL primordium requires the activity of non-muscle myosin. We demonstrate further that shroom3, a well-known regulator of non-muscle myosin activity, is expressed in the LL primordium and that its expression requires FGF signalling. Using gain- and loss-of-function experiments, we demonstrate that Shroom3 is the main organizer of cell shape changes during rosette assembly, probably by coordinating Rho kinase recruitment and non-muscle myosin activation. In order to quantify morphogenesis in the LL primordium in an unbiased manner, we developed a unique trainable 'rosette detector'. We thus propose a model in which Shroom3 drives rosette assembly in the LL downstream of FGF in a Rho kinase- and non-muscle myosin-dependent manner. In conclusion, we uncovered the first mechanistic link between patterning and morphogenesis during LL sensory organ formation.

Regulation of a vascular plexus by gata4 is mediated in zebrafish through the chemokine sdf1a

Using the zebrafish model we describe a previously unrecognized requirement for the transcription factor gata4 controlling embryonic angiogenesis. The development of a vascular plexus in the embryonic tail, the caudal hematopoietic tissue (CHT), fails in embryos depleted of gata4. Rather than forming a normal vascular plexus, the CHT of gata4 morphants remains fused, and cells in the CHT express high levels of osteogenic markers ssp1 and runx1. Definitive progenitors emerge from the hemogenic aortic endothelium, but fail to colonize the poorly vascularized CHT. We also found abnormal patterns and levels for the chemokine sdf1a in gata4 morphants, which was found to be functionally relevant, since the embryos also show defects in development of the lateral line, a mechano-sensory organ system highly dependent on a gradient of sdf1a levels. Reduction of sdf1a levels was sufficient to rescue lateral line development, circulation, and CHT morphology. The result was surprising since neither gata4 nor sdf1a is obviously expressed in the CHT. Therefore, we generated transgenic fish that conditionally express a dominant-negative gata4 isoform, and determined that gata4 function is required during gastrulation, when it is co-expressed with sdf1a in lateral mesoderm. Our study shows that the gata4 gene regulates sdf1a levels during early embryogenesis, which impacts embryonic patterning and subsequently the development of the caudal vascular plexus.

Genetic architecture of variation in the lateral line sensory system of threespine sticklebacks

Vertebrate sensory systems have evolved remarkable diversity, but little is known about the underlying genetic mechanisms. The lateral line sensory system of aquatic vertebrates is a promising model for genetic investigations of sensory evolution because there is extensive variation within and between species, and this variation is easily quantified. In the present study, we compare the lateral line sensory system of threespine sticklebacks (Gasterosteus aculeatus) from an ancestral marine and a derived benthic lake population. We show that lab-raised individuals from these populations display differences in sensory neuromast number, neuromast patterning, and groove morphology. Using genetic linkage mapping, we identify regions of the genome that influence different aspects of lateral line morphology. Distinct loci independently affect neuromast number on different body regions, suggesting that a modular genetic structure underlies the evolution of peripheral receptor number in this sensory system. Pleiotropy and/or tight linkage are also important, as we identify a region on linkage group 21 that affects multiple aspects of lateral line morphology. Finally, we detect epistasis between a locus on linkage group 4 and a locus on linkage group 21; interactions between these loci contribute to variation in neuromast pattern. Our results reveal a complex genetic architecture underlying the evolution of the stickleback lateral line sensory system. This study further uncovers a genetic relationship between sensory morphology and non-neural traits (bony lateral plates), creating an opportunity to investigate morphological constraints on sensory evolution in a vertebrate model system.

Heterogeneity and dynamics of lateral line afferent innervation during development in zebrafish (Danio rerio)

The lateral line system of larval zebrafish is emerging as a model to study a range of topics in neurobiology, from hair cell regeneration to sensory processing. However, despite numerous studies detailing the patterning and development of lateral line neuromasts, little is known about the organization of their connections to afferent neurons and targets in the hindbrain. We found that as fish grow and neuromasts proliferate over the body surface, the number of afferent neurons increases linearly. The number of afferents innervating certain neuromasts increases over time, while it decreases for other neuromasts. The ratio of afferent neurons to neuromasts differs between the anterior and posterior lateral line system, suggesting potential differences in sensitivity threshold or spatial resolution. A single afferent neuron routinely contacts a group of neuromasts, suggesting that different afferent neurons can convey information about receptive fields along the body. When afferent projections are traced into the hindbrain, where a distinct somatotopy has been previously described, we find that this general organization is absent at the Mauthner cell. We speculate that directional input from the lateral line is less important at an early age, whereas the speed of the escape response is paramount, and that directional responses arise later in development. By quantifying morphological connections in the lateral line system, this study provides a detailed foundation to understand how hydrodynamic information is processed and ultimately translated into appropriate motor behaviors.

Fish lateral line innovation: insights into the evolutionary genomic dynamics of a unique mechanosensory organ

The mechanosensory lateral line, found only in fishes and amphibians, is an important sense organ associated with aquatic life. Lateral line patterns differ among teleost, the most diverse vertebrate taxa, hypothetically in response to selective pressures from different aquatic habitats. In this article, we conduct evolutionary genomic analyses of 34 genes associated with lateral line system development in teleosts to elucidate the significance of contrasting evolutionary rates and changes in the protein coding sequences. We find that duplicated copies of these genes are preferentially retained in the teleost genomes and that episodic events of positive selection have occurred in 22 of the 30 postduplication branches. In general, teleost genes evolved at a faster rate relative to their tetrapod counterparts, and the mutation rates of 26 of the 34 genes differed among teleosts and tetrapods. We conclude that following whole genome duplication, evolutionary rates and episodic events of positive selection on the lateral line system development genes might have been one of the factors favoring the subsequent adaptive radiation of teleosts into diverse habitats. These results provide the foundation for further detailed explorations into lateral line system genes and the evolution of diverse phenotypes and adaptations.

Dynamics of degeneration and regeneration in developing zebrafish peripheral axons reveals a requirement for extrinsic cell types

Background

Understanding the cellular mechanisms regulating axon degeneration and regeneration is crucial for developing treatments for nerve injury and neurodegenerative disease. In neurons, axon degeneration is distinct from cell body death and often precedes or is associated with the onset of disease symptoms. In the peripheral nervous system of both vertebrates and invertebrates, after degeneration of detached fragments, axons can often regenerate to restore function. Many studies of axonal degeneration and regeneration have used in vitro approaches, but the influence of extrinsic cell types on these processes can only be fully addressed in live animals. Because of its simplicity and superficial location, the larval zebrafish posterior lateral line (pLL) nerve is an ideal model system for live studies of axon degeneration and regeneration.

Results

We used laser axotomy and time-lapse imaging of pLL axons to characterize the roles of leukocytes, Schwann cells and target sensory hair cells in axon degeneration and regeneration in vivo. Immune cells were essential for efficient removal of axonal debris after axotomy. Schwann cells were required for proper fasciculation and pathfinding of regenerating axons to their target cells. Intact target hair cells were not themselves required for regeneration, but chemical ablation of neuromasts caused axons to transiently deviate from their normal paths.

Conclusions

Macrophages, Schwann cells, and target sensory organs are required for distinct aspects of pLL axon degeneration or regeneration in the zebrafish larva. Our work introduces a powerful vertebrate model for analyzing axonal degeneration and regeneration in the living animal and elucidating the role of extrinsic cell types in these processes.

Cell-cell signaling interactions coordinate multiple cell behaviors that drive morphogenesis of the lateral line

The zebrafish sensory lateral line system has emerged as a powerful model for the mechanistic study of collective cell migration and morphogenesis. Recent work has uncovered the details of a signaling network involving the Wnt/β-catenin, Fgf and Delta-Notch pathways that patterns the migrating lateral line primordium into distinct regions. Cells within these regions exhibit different fundamental behaviors that together orchestrate normal lateral line morphogenesis. In this review, we summarize the signaling network that patterns the migrating lateral line primordium and describe how this patterning coordinates crucial morphogenic cell behaviors.

Persistent oxytetracycline exposure induces an inflammatory process that improves regenerative capacity in zebrafish larvae

BACKGROUND

The excessive use of antibiotics in aquaculture can adversely affect not only the environment, but also fish themselves. In this regard, there is evidence that some antibiotics can activate the immune system and reduce their effectiveness. None of those studies consider in detail the adverse inflammatory effect that the antibiotic remaining in the water may cause to the fish. In this work, we use the zebrafish to analyze quantitatively the effects of persistent exposure to oxytetracycline, the most common antibiotic used in fish farming.

METHODOLOGY

We developed a quantitative assay in which we exposed zebrafish larvae to oxytetracycline for a period of 24 to 96 hrs. In order to determinate if the exposure causes any inflammation reaction, we evaluated neutrophils infiltration and quantified their total number analyzing the Tg(mpx:GFP)(i114) transgenic line by fluorescence stereoscope, microscope and flow cytometry respectively. On the other hand, we characterized the process at a molecular level by analyzing several immune markers (il-1β, il-10, lysC, mpx, cyp1a) at different time points by qPCR. Finally, we evaluated the influence of the inflammation triggered by oxytetracycline on the regeneration capacity in the lateral line.

CONCLUSIONS

Our results suggest that after 48 hours of exposure, the oxytetracycline triggered a widespread inflammation process that persisted until 96 hours of exposure. Interestingly, larvae that developed an inflammation process showed an improved regeneration capacity in the mechanosensory system lateral line.

Transcriptional signature of accessory cells in the lateral line, using the Tnk1bp1:EGFP transgenic zebrafish line

Background

Because of the structural and molecular similarities between the two systems, the lateral line, a fish and amphibian specific sensory organ, has been widely used in zebrafish as a model to study the development/biology of neuroepithelia of the inner ear. Both organs have hair cells, which are the mechanoreceptor cells, and supporting cells providing other functions to the epithelium. In most vertebrates (excluding mammals), supporting cells comprise a pool of progenitors that replace damaged or dead hair cells. However, the lack of regenerative capacity in mammals is the single leading cause for acquired hearing disorders in humans.

Results

In an effort to understand the regenerative process of hair cells in fish, we characterized and cloned an egfp transgenic stable fish line that trapped tnks1bp1, a highly conserved gene that has been implicated in the maintenance of telomeres' length. We then used this Tg(tnks1bp1:EGFP) line in a FACsorting strategy combined with microarrays to identify new molecular markers for supporting cells.

Conclusions

We present a Tg(tnks1bp1:EGFP) stable transgenic line, which we used to establish a transcriptional profile of supporting cells in the zebrafish lateral line. Therefore we are providing a new set of markers specific for supporting cells as well as candidates for functional analysis of this important cell type. This will prove to be a valuable tool for the study of regeneration in the lateral line of zebrafish in particular and for regeneration of neuroepithelia in general.

Lef1-dependent Wnt/β-catenin signalling drives the proliferative engine that maintains tissue homeostasis during lateral line development

During tissue morphogenesis and differentiation, cells must self-renew while contemporaneously generating daughters that contribute to the growing tissue. How tissues achieve this precise balance between proliferation and differentiation is, in most instances, poorly understood. This is in part due to the difficulties in dissociating the mechanisms that underlie tissue patterning from those that regulate proliferation. In the migrating posterior lateral line primordium (PLLP), proliferation is predominantly localised to the leading zone. As cells emerge from this zone, they periodically organise into rosettes that subsequently dissociate from the primordium and differentiate as neuromasts. Despite this reiterative loss of cells, the primordium maintains its size through regenerative cell proliferation until it reaches the tail. In this study, we identify a null mutation in the Wnt-pathway transcription factor Lef1 and show that its activity is required to maintain proliferation in the progenitor pool of cells that sustains the PLLP as it undergoes migration, morphogenesis and differentiation. In absence of Lef1, the leading zone becomes depleted of cells during its migration leading to the collapse of the primordium into a couple of terminal neuromasts. We show that this behaviour resembles the process by which the PLLP normally ends its migration, suggesting that suppression of Wnt signalling is required for termination of neuromast production in the tail. Our data support a model in which Lef1 sustains proliferation of leading zone progenitors, maintaining the primordium size and defining neuromast deposition rate.

Lef1 is required for progenitor cell identity in the zebrafish lateral line primordium

The zebrafish posterior lateral line (pLL) is a sensory system that comprises clusters of mechanosensory organs called neuromasts (NMs) that are stereotypically positioned along the surface of the trunk. The NMs are deposited by a migrating pLL primordium, which is organized into polarized rosettes (proto-NMs). During migration, mature proto-NMs are deposited from the trailing part of the primordium, while progenitor cells in the leading part give rise to new proto-NMs. Wnt signaling is active in the leading zone of the primordium and global Wnt inactivation leads to dramatic disorganization of the primordium and a loss of proto-NM formation. However, the exact cellular events that are regulated by the Wnt pathway are not known. We identified a mutant strain, lef1(nl2), that contains a lesion in the Wnt effector gene lef1. lef1(nl2) mutants lack posterior NMs and live imaging reveals that rosette renewal fails during later stages of migration. Surprisingly, the overall primordium patterning, as assayed by the expression of various markers, appears unaltered in lef1(nl2) mutants. Lineage tracing and mosaic analyses revealed that the leading cells (presumptive progenitors) move out of the primordium and are incorporated into NMs; this results in a decrease in the number of proliferating progenitor cells and eventual primordium disorganization. We concluded that Lef1 function is not required for initial primordium organization or migration, but is necessary for proto-NM renewal during later stages of pLL formation. These findings revealed a novel role for the Wnt signaling pathway during mechanosensory organ formation in zebrafish.

Orientation and function of the nuclear-centrosomal axis during cell migration

A hallmark of polarity in most migrating cells is the orientation of the nuclear centrosomal (NC) axis relative to the front-back cellular axis. Here, we review 'effector functions' associated with the NC axis during cell migration. We highlight recent research that has demonstrated that the orientation of the NC axis depends upon the coordinated, but separate positioning of the nucleus and the centrosome. We stress the importance of environmental factors such as cell-cell contacts and substrate topology for NC axis orientation. Finally, we summarize tests of the significance of this axis for cell migration and disease.

Evidence for partial epithelial-to-mesenchymal transition (pEMT) and recruitment of motile blastoderm edge cells during avian epiboly

Embryonic epiboly has become an important developmental model for studying the mechanisms underlying collective movements of epithelial cells. In the last couple of decades, most studies of epiboly have utilized Xenopus or zebrafish as genetically tractable model organisms, while the avian epiboly model has received virtually no attention. Here, we re-visit epiboly in quail embryos and characterize several molecular markers of epithelial-to-mesenchymal transition (EMT) in the inner zone of the extraembryonic Area Opaca and at the blastoderm edge. Our results show that the intermediate filament vimentin, a widely-used marker for the mesenchymal phenotype, is strongly expressed in the edge cells compared to the cells in the inner zone. Laminin, an extracellular matrix protein that is a major structural and adhesive component of the epiblast basement membrane and the inner zone of the Area Opaca, is notably absent from the blastoderm edge. While these expression profiles are consistent with a mesenchymal phenotype, several other epithelial markers, including cytokeratin, β-catenin, and E-cadherin, are present in the blastoderm edge cells. Moreover, the results of a BrDU proliferation assay strongly suggest that expansion of the edge cell population is primarily due to recruitment of cells from the inner zone, as opposed to proliferation. Taken together, our data show that the edge cells of the avian blastoderm have characteristics of both epithelial and mesenchymal cells, and that the avian epiboly model, which has been dormant for so many years, may yet again prove to be helpful as a unique developmental model for studying partial EMT in the context of collective epithelial cell migration.

Zebrafish cx30.3: identification and characterization of a gap junction gene highly expressed in the skin

We have identified and characterized a zebrafish connexin, Cx30.3. Sequence similarity analyses suggested that Cx30.3 was orthologous to both mammalian Cx26 and Cx30, known to play important roles in the skin and inner ear of mammals. Analysis of mRNA expression showed that Cx30.3 was present in early embryos, and was highly abundant in skin, but also detected in other tissues including fins, inner ear, heart, and the retina. Injection of Cx30.3 cRNA into Xenopus oocytes elicited robust intercellular coupling with voltage gating sensitivity similar to mammalian Cx26 and Cx30. The similarities in functional properties and expression patterns suggest that Cx30.3, like mammalian Cx26 and Cx30, may play a significant role in skin development, hearing, and balance in zebrafish. Thus, zebrafish could potentially serve as an excellent model to study disorders of the skin and deafness that result from human connexin mutations.

Onset and dynamic expression of S100 proteins in the olfactory organ and the lateral line system in zebrafish development

In the zebrafish olfactory epithelium, three morphologically distinct olfactory neurons express different marker proteins. We utilize this feature to access developmental dynamics of one of the neuron types, the crypt cells, to determine whether they are differentiated at a stage similar to other olfactory neurons. Immunohistochemical studies using an S100 antibody that specifically recognizes crypt cells showed that S100-positive cells appear in olfactory rosettes as early as at 2day postfertilization (dpf). However, some of the rosettes did not have any S100-positive cells until 4 dpf. The number of S100-positive cells in individual rosettes increased steadily over the next 3days before it decreased significantly. There were 7.8 S100-positive cells per rosettes on average in larvae at 7 dpf. The number reduced to 2.2 at 9 dpf. A recovery to a pre-reduction level was detected in 12 dpf larvae. We also observed S100-positive cells in neuromasts of the lateral line system in 2 dpf larvae, suggesting that the crypt cells and sensory cells in the neuromasts have similar onsets of differentiation. Our data have provided a time line of differentiation of crypt cells in development of the olfactory system and demonstrated that this type of cell is differentiated at a stage similar to ciliated and microvillous olfactory neurons. A nonlinear growth trajectory of the crypt cell population in the first nine days of zebrafish development implicates a possible functional significance of crypt cells in early life stages of zebrafish.

A high-throughput chemically induced inflammation assay in zebrafish

Background

Studies on innate immunity have benefited from the introduction of zebrafish as a model system. Transgenic fish expressing fluorescent proteins in leukocyte populations allow direct, quantitative visualization of an inflammatory response in vivo. It has been proposed that this animal model can be used for high-throughput screens aimed at the identification of novel immunomodulatory lead compounds. However, current assays require invasive manipulation of fish individually, thus preventing high-content screening.

Results

Here we show that specific, noninvasive damage to lateral line neuromast cells can induce a robust acute inflammatory response. Exposure of fish larvae to sublethal concentrations of copper sulfate selectively damages the sensory hair cell population inducing infiltration of leukocytes to neuromasts within 20 minutes. Inflammation can be assayed in real time using transgenic fish expressing fluorescent proteins in leukocytes or by histochemical assays in fixed larvae. We demonstrate the usefulness of this method for chemical and genetic screens to detect the effect of immunomodulatory compounds and mutations affecting the leukocyte response. Moreover, we transformed the assay into a high-throughput screening method by using a customized automated imaging and processing system that quantifies the magnitude of the inflammatory reaction.

Conclusions

This approach allows rapid screening of thousands of compounds or mutagenized zebrafish for effects on inflammation and enables the identification of novel players in the regulation of innate immunity and potential lead compounds toward new immunomodulatory therapies. We have called this method the chemically induced inflammation assay, or ChIn assay.

See Commentary article: http://www.biomedcentral.com/1741-7007/8/148.