Mutations affecting the formation of posterior lateral line system in Medaka, Oryzias latipes

Temporal and clonal characterization of neural stem cell niche recruitment in the medaka neuromast

Stem cell populations are defined by their capacity to self-renew and to generate differentiated progeny. These unique characteristics largely depend on the stem cell micro-environment, the so-called stem cell niche. Niches were identified for most adult stem cells studied so far, but we know surprisingly little about how somatic stem cells and their niche come together during organ formation. Using the neuromasts of teleost fish, we have previously reported that neural stem cells recruit their niche from neighboring epithelial cells, which go through a morphological and molecular transformation. Here, we tackle quantitative, temporal, and clonal aspects of niche formation in neuromasts by using 4D imaging in transgenic lines, and lineage analysis in mosaic fish. We show that niche recruitment happens in a defined temporal window during the formation of neuromasts in medaka, and after that, the niche is enlarged mainly by the proliferation of niche cells. Niche recruitment is a non-clonal process that feeds from diverse epithelial cells that do not display a preferential position along the circumference of the forming neuromast. Additionally, we cover niche formation and expansion in zebrafish to show that distant species show common features during organogenesis in the lateral line system. Overall, our findings shed light on the process of niche formation, fundamental for the maintenance of stem cells not only in medaka but also in many other multicellular organisms.

Myelination during fracture healing in vivo in myelin protein zero (p0) transgenic medaka line

During the fracture healing process, osteoblasts and osteoclasts, as well as the nervous system are known to play important roles for signaling in the body. Glia cells contribute to the healing process by myelination, which can increase the speed of signals transmitted between neurons. However, the behavior of myelinating cells at a fracture site remains unclear. We developed a myelin protein zero (mpz)-EGFP transgenic medaka line for tracing myelinating cells. Mpz-enhanced green fluorescence protein (EGFP)-positive (mpz⁺) cells are driven by the 2.9-kb promoter of the medaka mpz gene, which is distributed throughout the nervous system, such as the brain, spinal cord, lateral line, and peripheral nerves. In the caudal fin region, mpz⁺ cells were found localized parallel with the fin ray (bone) in the adult stage. mpz⁺ cells were not distributed with fli-DsRed positive (fli⁺) blood vessels, but with some nerve fibers, and were dyed with the anti-acetylated tubulin antibody. We then fractured one side of the caudal lepidotrichia in a caudal fin of mpz-EGFP medaka and found a unique phenomenon, in that mpz⁺ cells were accumulated at 1 bone away from the fracture site. This mpz⁺ cell accumulation phenomenon started from 4 days after fracture of the proximal bone. Thereafter, mpz⁺ cells became elongated from the proximal bone to the distal bone and finally showed a crosslink connection crossing the fracture site to the distal bone at 28 days after fracture. Finally, the effects of rapamycin, known as a mTOR inhibitor, on myelination was examined. Rapamycin treatment of mpz-EGFP/osterix-DsRed double transgenic medaka inhibited not only the crosslink connection of mpz⁺ cells but also osterix⁺ osteoblast accumulation at the fracture site, accompanied with a fracture healing defect. These findings indicated that mTOR signaling plays important roles in bone formation and neural networking during fracture healing. Taken together, the present results are the first to show the dynamics of myelinating cells in vivo.

Mutation in cpsf6/CFIm68 (Cleavage and Polyadenylation Specificity Factor Subunit 6) causes short 3'UTRs and disturbs gene expression in developing embryos, as revealed by an analysis of primordial germ cell migration using the medaka mutant naruto

Our previous studies analyzing medaka mutants defective in primordial germ cell (PGC) migration identified cxcr4b and cxcr7, which are both receptors of the chemokine sdf1/cxcl12, as key regulators of PGC migration. Among PGC migration mutants, naruto (nar) is unique in that the mutant phenotype includes gross morphological abnormalities of embryos, suggesting that the mutation affects a broader range of processes. A fine genetic linkage mapping and genome sequencing showed the nar gene encodes Cleavage and Polyadenylation Specificity Factor subunit 6 (CPSF6/CFIm68). CPSF6 is a component of the Cleavage Factor Im complex (CFIm) which plays a key role in pre-mRNA 3'-cleavage and polyadenylation. 3'RACE of sdf1a/b and cxcr7 transcripts in the mutant embryos indicated shorter 3'UTRs with poly A additions occurring at more upstream positions than wild-type embryos, suggesting CPSF6 functions to prevent premature 3'UTR cleavage. In addition, expression of the coding region sequences of sdf1a/b in nar mutants was more anteriorly extended in somites than wild-type embryos, accounting for the abnormally extended distribution of PGCs in nar mutants. An expected consequence of shortening 3'UTR is the escape from the degradation mechanism mediated by microRNAs interacting with distal 3'UTR sequence. The abnormal expression pattern of sdf1a coding sequence may be at least partially accounted for by this mechanism. Given the pleiotropic effects of nar mutation, further analysis using the nar mutant will reveal processes in which CPSF6 plays essential regulatory roles in poly A site selection and involvement of 3'UTRs in posttranscriptional gene regulation in various genes in vivo.

Sequential organogenesis sets two parallel sensory lines in medaka

Animal organs are typically formed during embryogenesis by following one specific developmental programme. Here, we report that neuromast organs are generated by two distinct and sequential programmes that result in parallel sensory lines in medaka embryos. A ventral posterior lateral line (pLL) is composed of neuromasts deposited by collectively migrating cells whereas a midline pLL is formed by individually migrating cells. Despite the variable number of neuromasts among embryos, the sequential programmes that we describe here fix an invariable ratio between ventral and midline neuromasts. Mechanistically, we show that the formation of both types of neuromasts depends on the chemokine receptor genes cxcr4b and cxcr7b, illustrating how common molecules can mediate different morphogenetic processes. Altogether, we reveal a self-organising feature of the lateral line system that ensures a proper distribution of sensory organs along the body axis.

Summary: Two parallel sensory lines in medaka share a common origin and are composed of identical organs that are, nevertheless, generated through different morphogenetic programmes.

Xmrk-induced melanoma progression is affected by Sdf1 signals through Cxcr7

Chemokine signals mediated by Sdf1/Cxcl12 through the chemokine receptor Cxcr4 are thought to play an instructive role in tumor migration and organ-specific metastasis. We have used a small aquarium fish model to contribute to a better understanding of how the course of melanoma development is influenced by Sdf1 signals in vivo. We studied oncogene-induced skin tumor appearance and progression in the transgenic medaka (Oryzias latipes) melanoma model. Similar to humans, invasive medaka melanomas show increased levels of sdf1, cxcr4, and cxcr7 gene expression. Stable transgenic fish lines overexpressing sdf1 exclusively in pigment cells showed a reduction in melanoma appearance and progression. Remarkably, diminished levels of functional Cxcr7, but not of Cxcr4b, resulted in strongly reduced melanoma invasiveness and a repression of melanoma. Our results thereby indicate that Sdf1 signals via Cxcr7 are able to constrain melanoma growth in vivo and that these signals influence tumor outcome.

Identification and developmental expression of leucine-rich repeat-containing G protein-coupled receptor 6 (lgr6) in the medaka fish, Oryzias latipes

G protein-coupled receptors are critical regulators of diverse developmental processes such as oocyte maturation, fertilization, gastrulation, and organogenesis. To further study the molecular mechanisms underlying these processes, we cloned and characterized the orphan leucine-rich repeat-containing G protein-coupled receptor 6 (LGR6), a stem cell marker in mammalian hair follicles, in medaka fish, Oryzias latipes. To examine the expression pattern of lgr6, we performed whole-mount in situ hybridization (WISH) during embryogenesis. The expression of lgr6 was first detected as a band in the anterior part of the posterior brain vesicle in 0.5-1 day post fertilization (dpf) embryos. This band disappeared by 2 dpf, but new signals appeared in the otic vesicles bordering the original band and also detected in the nasal placode and posterior lateral line primordia. At later stages (3-5 dpf), lgr6 was widely expressed in the brain, otic vesicle, neuromasts, root of the pectoral fin, cranial cartilage, and gut. Then, we conducted more detailed expression analysis of lgr6 in adult gut using WISH and immunohistochemical staining. Lgr6-positive cells were detected in the crypt-like proliferative zone and in parts of the villus. We also performed RT-PCR of mRNAs from different tissues. The lgr6 mRNA was found highest in the kidney and gill. The transcript was also present in the brain, heart, liver, spleen, intestine, skeletal muscle, testis, and ovary, similar to that of mammalian LGR6. These results suggest that medaka lgr6 plays an important role in organ development during embryogenesis and serves as a good molecular marker for future studies of postembryonic organ-specific development in mammals.

CXCR4 and CXCR7 cooperate during tangential migration of facial motoneurons

Migration of facial motoneurons in the zebrafish hindbrain depends on SDF1/CXCL12 signaling. Recent studies demonstrated that SDF1 can bind two chemokine receptors, CXCR4 and CXCR7. Here we explore the expression and function of the cxcr7b gene in zebrafish hindbrain development. By the time cxcr4b-expressing motoneurons migrate from rhombomere (r) r4 to r6, expression of cxcr7b is rapidly restricted to the ventral part of r5. Inactivation of either cxcr7b or cxcr4b impairs motoneuron migration, with however different phenotypes. Facial motoneurons preferentially accumulate in r5 in cxcr7b morphant embryos, while they are distributed between r4, r5 and r6 in cxcr4b morphants. Simultaneous inactivation of both receptors leads to yet a third phenotype, with motoneurons mostly distributed between r4 and r5. The latter phenotype resembles that of sdf1a morphant embryos. Double inactivation of sdf1a and cxcr7b indeed did not lead to a complete arrest of migration but rather to a partial rescue of r5 arrest of motoneuron migration. This result is in accordance with the functional hypothesis that SDF1 might interact with CXCR7 and that they have an antagonistic effect within r5. The ectopic expression of a truncated CXCR7 receptor leads to a motoneuron migration defect. Altogether, we show that CXCR7 is required, for proper tangential migration of facial motoneurons, by determining a permissive migration pathway through r5.

Gustation in fish: search for prototype of taste perception

Fish perceive water-soluble chemicals at the taste buds that are distributed on oropharyngeal and trunk epithelia. Recent progress in molecular analyses has revealed that teleosts and mammals share pivotal signaling components to transduce taste stimuli. The fish orthologs of taste receptors, fT1R and fT2R, show mutually exclusive expression in taste buds, and both are coexpressed with phospholipase C-beta2 and the transient receptor potential M5 channel as common downstream components of taste receptor signals. Interestingly, fT1R heteromers are activated by various L-amino acids but not by sugars. This may reflects that in fish the energy metabolism depends primarily on gluconeogenesis from amino acids. fT2Rs are activated by denatonium benzoate, which is a bitter substance for mammals. It is thus likely that the preferable and aversive tastes for vertebrates, though their taste modalities somewhat vary, are transduced by the sensory conserved pathways. The comparative molecular biology of the fish taste system would lead to understanding a general logic of encoding taste modalities in vertebrates.

Distinct contributions of CXCR4b and CXCR7/RDC1 receptor systems in regulation of PGC migration revealed by medaka mutants kazura and yanagi

Migratory pathways of PGCs to the gonad vary depending on the vertebrate species, yet the underlying regulatory mechanisms guiding PGCs are believed to be largely common. In teleost medaka embryo, PGC migration follows two major steps before colonizing in gonadal areas: (1) bilateral lineup in the trunk and (2) posterior drift of PGCs. kazura (kaz) and yanagi (yan) mutants of medaka isolated in mutagenesis screening were defective in the first and second steps, respectively. kaz(j2-15D) was identified as a missense mutation in chemokine receptor gene cxcr4b expressed in PGCs. Embryonic injection of cxcr4b mRNA with vasa 3' UTR rescued the PGC phenotype of kaz mutant, indicating a cell-autonomous function of cxcr4b in PGCs. yan(j6-29C) was identified as a nonsense mutation in the cxcr7/rdc1 gene encoding another chemokine receptor. cxcr7 transgene with genomic flanking sequences rescued the yan mutant phenotype efficiently at the G0 generation. cxcr7 was expressed in somites rather than PGCs. cxcr7-expressing somitic domain expanded posteriorly with its margin immediately anterior of posteriorly drifting PGCs, as if PGCs were thrusted toward the gonadal area. kaz and yan mutants are also defective in lateral line positioning, suggesting combined employment of these receptor systems in various cell migratory processes.

Sequential SDF1a and b-induced mobility guides Medaka PGC migration

Assembly and formation of the gonad primordium are the first steps toward gonad differentiation and subsequent sex differentiation. Primordial germ cells (PGCs) give rise to the gametes that are responsible for the development of a new organism in the next generation. In many organisms, following their specification the germ cells migrate toward the location of the prospective gonadal primordium. To accomplish this, the PGCs obtain directional cues from cells positioned along their migration path. One such cue, the chemokine SDF1 (stromal cell-derived factor 1) and its receptor CXCR4 have recently been found to be critical for proper PGC migration in zebrafish, chick and mouse. We have studied the mechanisms responsible for PGC migration in Medaka. In contrast to the situation observed in zebrafish, where proper PGC positioning is the result of active migration in the direction of the source of SDF1a, Medaka PGC movements are shown to be the consequence of a combination of active SDF1a and SDF1b-guided migration. In this process both SDF1 co-orthologues show only partly overlapping expression pattern and cooperate in the correct positioning of the PGCs.

The lateral line microcosmos

The lateral-line system is a simple sensory system comprising a number of discrete sense organs, the neuromasts, distributed over the body of fish and amphibians in species-specific patterns. Its development involves fundamental biological processes such as long-range cell migration, planar cell polarity, regeneration, and post-embryonic remodeling. These aspects have been extensively studied in amphibians by experimental embryologists, but it is only recently that the genetic bases of this development have been explored in zebrafish. This review discusses progress made over the past few years in this field.

Control of cell migration in the development of the posterior lateral line: antagonistic interactions between the chemokine receptors CXCR4 and CXCR7/RDC1

Background

The formation of the posterior lateral line of teleosts depends on the migration of a primordium that originates near the otic vesicle and moves to the tip of the tail. Groups of cells at the trailing edge of the primordium slow down at regular intervals and eventually settle to differentiate as sense organs. The migration of the primordium is driven by the chemokine SDF1 and by its receptor CXCR4, encoded respectively by the genes sdf1a and cxcr4b. cxcr4b is expressed in the migrating cells and is down-regulated in the trailing cells of the primordium. sdf1a is expressed along the path of migration. There is no evidence for a gradient of sdf1a expression, however, and the origin of the directionality of migration is not known.

Results

Here we document the expression of a second chemokine receptor gene, cxcr7, in the migrating primordium. We show that cxcr7 is highly expressed in the trailing cells of the primordium but not at all in the leading cells, a pattern that is complementary to that of cxcr4b. Even though cxcr7 is not expressed in the cells that lead primordium migration, its inactivation results in impaired migration. The phenotypes of cxcr4b, cxcr7 double morphant embryos suggest, however, that CXCR7 does not contribute to the migratory capabilities of primordium cells. We also show that, in the absence of cxcr4b, expression of cxcr7 becomes ubiquitous in the stalled primordium.

Conclusion

Our observations suggest that CXCR7 is required to provide directionality to the migration. We propose that directionality is imposed on the primordium as soon as it comes in contact with the stripe of SDF1, and is maintained throughout migration by a negative interaction between the two receptors.

Identification and lineage tracing of two populations of somatic gonadal precursors in medaka embryos

The gonad contains two major cell lineages, germline and somatic cells. Little is known, however, about the somatic gonadal cell lineage in vertebrates. Using fate mapping studies and ablation experiments in medaka fish (Oryzias latipes), we determined that somatic gonadal precursors arise from the most posterior part of the sdf-1a expression domain in the lateral plate mesoderm at the early segmentation stage; this region has the properties of a gonadal field. Somatic gonadal precursors in this field, which continuously express sdf-1a, move anteriorly and medially to the prospective gonadal area by convergent movement. By the stage at which these somatic gonadal precursors have become located adjacent to the embryonic body, the precursors no longer replace the surrounding lateral plate mesoderm, becoming spatially organized into two distinct populations. We further show that, prior to reaching the prospective gonadal area, these populations can be distinguished by expression of either ftz-f1 or sox9b. These results clearly indicate that different populations of gonadal precursors are present before the formation of a single gonadal primordium, shedding new light on the developmental processes of somatic gonadal cell and subsequent sex differentiation.

Time-lapse analysis reveals different modes of primordial germ cell migration in the medaka Oryzias latipes

Previous studies have shown that medaka primordial germ cells (PGC) are first distinguishable by olvas expression during late gastrulation, and that they migrate to the gonadal region through the lateral plate mesoderm. Here, we demonstrate that medaka nanos expression marks the germ line at early gastrulation stage. By marking the germ line with green fluorescent protein (GFP) fused to the nanos 3' untranslated region, we were able to visualize the behavior of PGC using time-lapse imaging. We show that there are three distinct modes of PGC migration that function at different stages of development. At early gastrulation stage, PGC actively migrate towards the marginal zone, a process that requires the function of a chemokine receptor, CXCR4. However, at late gastrulation stage, PGC change the mode and direction of their movement, as they are carried towards the midline along with somatic cells undergoing convergent movements. After aligning bilaterally, PGC actively migrate to the posterior end of the lateral plate mesoderm. This posterior movement depends on the activity of both HMGCoAR and a ligand of CXCR4, SDF-1a. These results demonstrate that PGC undergo different modes of migration to reach the prospective gonadal region of the embryo.

The three-sided romance of the lateral line: glia love axons love precursors love glia

The lateral line system of fish and amphibians is closely related to the inner ear in terms of evolution, morphology and physiology. Several recent papers have shed new light on the postembryonic development of this system, and have revealed an unexpected triangular relationship where migrating sensory precursors guide axons, axons guide glia and glia, in turn, control the formation of sensory organs. They have also revealed the crucial importance of controlled cell migration not only for patterning the system, but also for determining polarity (and therefore directional sensitivity) of the mechanosensory hair cells. The remarkable accessibility of the lateral line system may allow a detailed analysis of cell migration and polarization, and may help us better understand the complex interactions between sensory precursor cells, neurons and glia during development.

Central Connection of the Optic, Oculomotor, Trochlear and Abducens Nerves in Medaka, Oryzias latipes

Medaka (Oryzias latipes) is one of the few vertebrate experimental animals in which inbred lines have been established. It is also a species that has advanced in genetic studies in a manner comparable to zebrafish. This fish is therefore a good model for studying functional organization of the nervous system, but anatomical analysis of its nervous system has been limited to embryonic stages. In the present study, we investigated anatomy of cranial nerves in adult fish focusing on the visual function, using an inbred strain of medaka. Cranial nerves of medaka were labeled using biocytin, revealing a central distribution of retinofugal terminals, retinopetal neurons, and oculomotor, trochlear and abducens motor neurons. The optic nerve of the adult medaka was of a complete decussation type. Retinofugal terminals were located in 8 brain nuclei, the suprachiasmatic nucleus, nucleus pretectalis superficialis, nucleus dorsolateralis thalami, area pretectalis pars dorsalis (APd), area pretectalis pars ventralis (APv), nucleus of the posterior commissure (NPC), accessory optic nucleus, and the tectum opticum. Retinopetal neurons were identified in 6 brain nuclei, the ganglion of the terminal nerve, preoptic retinopetal nucleus, nucleus dorsolateralis thalami, APd, APv, and NPC. The oculomotor neurons were mostly labeled ipsilaterally and were located dorsomedially, abutting the fasciculus longitudinalis medialis in the mesencephalon. The trochlear nucleus was located contralaterally and dorsolaterally adjacent to the fasciculus longitudinalis medialis in the mesencephalon. The abducens nucleus was located ipsilaterally in a ventrolateral part of the rhombencephalic reticular formation. These results, generally similar to those in other teleosts, provide the basis for future behavioral and genetic studies in medaka.

A systematic genome-wide screen for mutations affecting organogenesis in Medaka, Oryzias latipes

A large-scale mutagenesis screen was performed in Medaka to identify genes acting in diverse developmental processes. Mutations were identified in homozygous F3 progeny derived from ENU-treated founder males. In addition to the morphological inspection of live embryos, other approaches were used to detect abnormalities in organogenesis and in specific cellular processes, including germ cell migration, nerve tract formation, sensory organ differentiation and DNA repair. Among 2031 embryonic lethal mutations identified, 312 causing defects in organogenesis were selected for further analyses. From these, 126 mutations were characterized genetically and assigned to 105 genes. The similarity of the development of Medaka and zebrafish facilitated the comparison of mutant phenotypes, which indicated that many mutations in Medaka cause unique phenotypes so far unrecorded in zebrafish. Even when mutations of the two fish species cause a similar phenotype such as one-eyed-pinhead or parachute, more genes were found in Medaka than in zebrafish that produced the same phenotype when mutated. These observations suggest that many Medaka mutants represent new genes and, therefore, are important complements to the collection of zebrafish mutants that have proven so valuable for exploring genomic function in development.

Mutations affecting early distribution of primordial germ cells in Medaka (Oryzias latipes) embryo

The development of germ cells has been intensively studied in Medaka (Oryzias latipes). We have undertaken a large-scale screen to identify mutations affecting the development of primordial germ cells (PGCs) in Medaka. Embryos derived from mutagenized founder fish were screened for an abnormal distribution or number of PGCs at embryonic stage 27 by RNA in situ hybridization for the Medaka vasa homologue (olvas). At this stage, PGCs coalesce into two bilateral vasa-expressing foci in the ventrolateral regions of the trunk after their migration and group organization. Nineteen mutations were identified from a screen corresponding to 450 mutagenized haploid genomes. Eleven of the mutations caused altered PGC distribution. Most of these alterations were associated with morphological abnormalities and could be grouped into four phenotypic classes: Class 1, PGCs dispersed into bilateral lines; Class 2, PGCs dispersed in a region more medial than that in Class 1; Class 3, PGCs scattered laterally and over the yolk sac area; and Class 4, PGCs clustered in a single median focus. Eight mutations caused a decrease in the number of PGCs. This decrease was observed in the offspring of heterozygous mothers, indicating the contribution of a maternal factor in determining PGC abundance. Taken together, these mutations should prove useful in identifying molecular mechanisms underlying the early PGC development and migration.