evx1 transcription in bony fin rays segment boundaries leads to a reiterated pattern during zebrafish fin development and regeneration

Evolution of median fin patterning and modularity in living and fossil osteichthyans

Morphological and developmental similarities, and interactions among developing structures are interpreted as evidences of modularity. Such similarities exist between the dorsal and anal fins of living actinopterygians, on the anteroposterior axis: (1) both fins differentiate in the same direction [dorsal and anal fin patterning module (DAFPM)], and (2) radials and lepidotrichia differentiate in the same direction [endoskeleton and exoskeleton module (EEM)]. To infer the evolution of these common developmental patternings among osteichthyans, we address (1) the complete description and quantification of the DAFPM and EEM in a living actinopterygian (the rainbow trout Oncorhynchus mykiss) and (2) the presence of these modules in fossil osteichthyans (coelacanths, lungfishes, porolepiforms and 'osteolepiforms'). In Oncorhynchus, sequences of skeletal elements are determined based on (1) apparition (radials and lepidotrichia), (2) chondrification (radials), (3) ossification (radials and lepidotrichia), and (4) segmentation plus bifurcation (lepidotrichia). Correlations are then explored between sequences. In fossil osteichthyans, sequences are determined based on (1) ossification (radials and lepidotrichia), (2) segmentation, and (3) bifurcation of lepidotrichia. Segmentation and bifurcation patterns were found crucial for comparisons between extant and extinct osteichthyan taxa. Our data suggest that the EEM is plesiomorphic at least for actinopterygians, and the DAFPM is plesiomorphic for osteichthyans, with homoplastic dissociation. Finally, recurrent patterns suggest the presence of a Lepidotrichia Patterning Module (LPM).

Connexin 43 gap junctional intercellular communication inhibits evx1 expression and joint formation in regenerating fins

The gap junction protein Connexin 43 (Cx43) contributes to cell fate decisions that determine the location of fin ray joints during regeneration. Here, we provide insights into how Cx43, expressed medially, influences changes in gene expression in lateral skeletal precursor cells. Using the Gap27 peptide inhibitor specific to Cx43, we show that Cx43-gap junctional intercellular communication (GJIC) influences Cx43-dependent skeletal phenotypes, including segment length. We also demonstrate that Cx43-GJIC influences the expression of the Smp/β-catenin pathway in the lateral skeletal precursor cells, and does not influence the Sema3d pathway. Moreover, we show that the cx43^lh10 allele, which has increased Cx43 protein levels, exhibits increased regenerate length and segment length. These phenotypes are rescued by Gap27, suggesting that increased Cx43 is responsible for the observed Cx43 phenotypes. Finally, our findings suggest that inhibition of Cx43 hemichannel activity does not influence Cx43-dependent skeletal phenotypes. These data provide evidence that Cx43-GJIC is responsible for regulating cell fate decisions associated with appropriate joint formation in the regenerating fin.

A gene expression study of ornamental fin shape in Neolamprologus brichardi, an African cichlid species

The diversity of fin morphology within and across fish taxa offers great, but still largely unexplored, opportunities to investigate the proximate mechanisms underlying fin shape variation. Relying on available genetic knowledge brought forth mainly by the comprehensive study of the zebrafish caudal fin, we explored candidate molecular mechanisms for the maintenance and formation of the conspicuously elongated filaments adorning the unpaired fins of the East African "princess cichlid" Neolamprologus brichardi. Via qPCR assays, we detected expression differences of candidate genes between elongated and short regions of intact and regenerating fins. The identified genes include skeletogenic and growth factors (igf2b, fgf3, bmp2 and bmp4), components of the WNT pathway (lef1, wnt5b and wnt10) and a regulatory network determining fin ray segment size and junction (cx43, esco2 and sema3d), as well as other genes with different roles (mmp9, msxb and pea3). Interestingly, some of these genes showed fin specific expression differences which are often neglected in studies of model fish that focus on the caudal fin. Moreover, while the observed expression patterns were generally consistent with zebrafish results, we also detected deviating expression correlations and gene functions.

Cx43 suppresses evx1 expression to regulate joint initiation in the regenerating fin

BACKGROUND

How joints are correctly positioned in the vertebrate skeleton remains poorly understood. From our studies on the regenerating fin, we have evidence that the gap junction protein Cx43 suppresses joint formation by suppressing the expression of the evx1 transcription factor. Joint morphogenesis proceeds through at least two discrete stages. First, cells that will produce the joint condense in a single row on the bone matrix ("initiation"). Second, these cells separate coincident with articulation of the bone matrix. We propose that Cx43 activity is transiently reduced prior to joint initiation.

RESULTS

We first define the timing of joint initiation with respect to regeneration. We next correlate reduced cx43 expression and increased evx1 expression with initiation. Through manipulation of cx43 expression, we demonstrate that Cx43 negatively influences evx1 expression and joint formation. We further demonstrate that Cx43 activity in the dermal fibroblasts is required to rescue joint formation in the cx43 mutant, short fin^b123 .

CONCLUSIONS

We conclude that Cx43 activity in the dermal fibroblasts influences the expression of evx1, and therefore the differentiation of the precursor cells that give rise to the joint-forming osteoblasts. Developmental Dynamics 246:691-699, 2017. © 2017 Wiley Periodicals, Inc.

Building and maintaining joints by exquisite local control of cell fate

We owe the flexibility of our bodies to sophisticated articulations between bones. Establishment of these joints requires the integration of multiple tissue types: permanent cartilage that cushions the articulating bones, synovial membranes that enclose a lubricating fluid-filled cavity, and a fibrous capsule and ligaments that provide structural support. Positioning the prospective joint region involves establishment of an "interzone" region of joint progenitor cells within a nascent cartilage condensation, which is achieved through the interplay of activators and inhibitors of multiple developmental signaling pathways. Within the interzone, tight regulation of BMP and TGFβ signaling prevents the hypertrophic maturation of joint chondrocytes, in part through downstream transcriptional repressors and epigenetic modulators. Synovial cells then acquire further specializations through expression of genes that promote lubrication, as well as the formation of complex structures such as cavities and entheses. Whereas genetic investigations in mice and humans have uncovered a number of regulators of joint development and homeostasis, recent work in zebrafish offers a complementary reductionist approach toward understanding joint positioning and the regulation of chondrocyte fate at joints. The complexity of building and maintaining joints may help explain why there are still few treatments for osteoarthritis, one of the most common diseases in the human population. A major challenge will be to understand how developmental abnormalities in joint structure, as well as postnatal roles for developmental genes in joint homeostasis, contribute to birth defects and degenerative diseases of joints. WIREs Dev Biol 2017, 6:e245. doi: 10.1002/wdev.245 For further resources related to this article, please visit the WIREs website.

Holmgren's principle of delamination during fin skeletogenesis

During fin morphogenesis, several mesenchyme condensations occur to give rise to the dermal skeleton. Although each of them seems to create distinctive and unique structures, they all follow the premises of the same morphogenetic principle. Holmgren's principle of delamination was first proposed to describe the morphogenesis of skeletal elements of the cranium, but Jarvik extended it to the development of the fin exoskeleton. Since then, some cellular or molecular explanations, such as the "flypaper" model (Thorogood et al.), or the evolutionary description by Moss, have tried to clarify this topic. In this article, we review new data from zebrafish studies to meet these criteria described by Holmgren and other authors. The variety of cell lineages involved in these skeletogenic condensations sheds light on an open discussion of the contributions of mesoderm- versus neural crest-derived cell lineages to the development of the head and trunk skeleton. Moreover, we discuss emerging molecular studies that are disclosing conserved regulatory mechanisms for dermal skeletogenesis and similarities during fin development and regeneration, which may have important implications in the potential use of the zebrafish fin as a model for regenerative medicine.

Identification of an evx1-dependent joint-formation pathway during FIN regeneration

Joints are essential for skeletal flexibly and form, yet the process underlying joint morphogenesis is poorly understood. Zebrafish caudal fins are comprised of numerous segmented bony fin rays, where growth occurs by the sequential addition of new segments and new joints. Here, we evaluate joint gene expression during fin regeneration. First, we identify three genes that influence joint formation, evx1, dlx5a, and mmp9. We place these genes in a common molecular pathway by evaluating both their expression patterns along the distal-proximal axis (i.e. where the youngest tissue is always the most distal), and by evaluating changes in gene expression following gene knockdown. Prior studies from our lab indicate that the gap junction protein Cx43 suppresses joint formation. Remarkably, changes in Cx43 activity alter the expression of joint markers. For example, the reduced levels of Cx43 in the sof ^b123 mutant causes short fin ray segments/premature joints. We also find that the expression of evx1-dlx5a-mmp9 is shifted distally in sof ^b123, consistent with premature expression of these genes. In contrast, increased Cx43 in the alf ^dty86 mutant leads to stochastic joint failure and stochastic loss of evx1 expression. Indeed, reducing the level of Cx43 in alf ^dty86 rescues both the evx1 expression and joint formation. These results suggest that Cx43 influences the pattern of joint formation by influencing the timing of evx1 expression.

Semaphorin3d mediates Cx43-dependent phenotypes during fin regeneration

Gap junctions are proteinaceous channels that reside at the plasma membrane and permit the exchange of ions, metabolites, and second messengers between neighboring cells. Connexin proteins are the subunits of gap junction channels. Mutations in zebrafish cx43 cause the short fin (sof(b123)) phenotype which is characterized by short fins due to defects in length of the bony fin rays. Previous findings from our lab demonstrate that Cx43 is required for both cell proliferation and joint formation during fin regeneration. Here we demonstrate that semaphorin3d (sema3d) functions downstream of Cx43. Semas are secreted signaling molecules that have been implicated in diverse cellular functions such as axon guidance, cell migration, cell proliferation, and gene expression. We suggest that Sema3d mediates the Cx43-dependent functions on cell proliferation and joint formation. Using both in situ hybridization and quantitative RT-PCR, we validated that sema3d expression depends on Cx43 activity. Next, we found that knockdown of Sema3d recapitulates all of the sof(b123) and cx43-knockdown phenotypes, providing functional evidence that Sema3d acts downstream of Cx43. To identify the potential Sema3d receptor(s), we evaluated gene expression of neuropilins and plexins. Of these, nrp2a, plxna1, and plxna3 are expressed in the regenerating fin. Morpholino-mediated knockdown of plxna1 did not cause cx43-specific defects, suggesting that PlexinA1 does not function in this pathway. In contrast, morpholino-mediated knockdown of nrp2a caused fin overgrowth and increased cell proliferation, but did not influence joint formation. Moreover, morpholino-mediated knockdown of plxna3 caused short segments, influencing joint formation, but did not alter cell proliferation. Together, our findings reveal that Sema3d functions in a common molecular pathway with Cx43. Furthermore, functional evaluation of putative Sema3d receptors suggests that Cx43-dependent cell proliferation and joint formation utilize independent membrane-bound receptors to mediate downstream cellular phenotypes.

Mature and juvenile tissue models of regeneration in small fish species

The multitude of cells constituting organisms are fragile and easily damaged day by day. Therefore, maintenance of tissue morphology and function is fundamental for multicellular organisms to attain long life. For proper maintenance of tissue integrity, organisms must have mechanisms that detect the loss of tissue mass, activate the de novo production of cells, and organize those cells into functional tissues. However, these processes are only poorly understood. Here we give an overview of adult and juvenile tissue regeneration models in small fish species, such as zebrafish and medaka, and highlight recent advances at the molecular level. From these advances, we have come to realize that the epidermal and mesenchymal parts of the regenerating fish fin-that is, the wound epidermis and blastema, respectively-comprise heterogeneous populations of cells with different molecular identities that can be termed "compartments." These compartments and their mutual interactions are thought to play important roles in promoting the proper progression of tissue regeneration. We further describe the current understanding of these compartments and discuss the possible approaches to affording a better understanding of their roles and interactions during regeneration.

Tissue regeneration after injury in adult zebrafish: the regenerative potential of the caudal fin

The zebrafish has the potential to regenerate many of its tissues. In this study, we examined caudal fin regeneration in zebrafish that received repeated injuries (fin amputation) at different ages. In zebrafish that received repeated injuries, the potential for caudal fin regeneration, such as tissue growth and the expression of regeneration marker genes (msxb, fgf20a, bmp2b), did not decline in comparison to zebrafish that received only one amputation surgery. The process of initial fin regeneration (e.g., tissue outgrowth and the expression of regeneration marker genes at 7 days post-amputation) did not seem to correlate with age. However, slight differences in fin outgrowth were observed between young and old animals when examined in the late regeneration stages (e.g., 20 and 30 days post-amputation). Together, the data suggest that zebrafish has unlimited regenerative potential in the injured caudal fin.

Dermoskeleton morphogenesis in zebrafish fins

Zebrafish fins have a proximal skeleton of endochondral bones and a distal skeleton of dermal bones. Recent experimental and genetic studies are discovering mechanisms to control fin skeleton morphogenesis. Whereas the endochondral skeleton has been extensively studied, the formation of the dermal skeleton requires further revision. The shape of the dermal skeleton of the fin is generated in its distal growing margin and along a proximal growing domain. In these positions, dermoskeletal fin morphogenesis can be explained by intertissue interactions and the function of several genetic pathways. These pathways regulate patterning, size, and cell differentiation along three axes. Finally, a common genetic control of late development, regeneration, and tissue homeostasis of the fin dermoskeleton is currently being analyzed. These pathways may be responsible for the similar shape obtained after each morphogenetic process. This provides an interesting conceptual framework for future studies on this topic. Developmental Dynamics 239:2779–2794, 2010. © 2010 Wiley-Liss, Inc.

Evx1 is required for joint formation in zebrafish fin dermoskeleton

The transcription factor Evx1 is expressed in the joints between individual lepidotrichia (bony ray) segments and at the distal tips of the lepidotrichia in developing zebrafish fins. It is also expressed in the apical growth zone in regenerating fins. However, so far there is no functional evidence that addresses whether Evx1 is required for any aspect of fin development or regeneration. In this study, we use a novel mutation in evx1 to address this. We find that Evx1 is not required for either fin outgrowth or regeneration. All of the fins form normally in evx1 mutants, and there are no significant changes in fin length. In contrast, Evx1 is required for lepidotrichia joint formation during both fin development and regeneration. This is a very specific phenotype as both lepidotrichia hemisegment separations and lepidotrichia bifurcations still form normally in evx1 mutant fins, as do joints in the more proximal endoskeletal radials.

The regeneration of the tail fin actinotrichia of carp (Cyprinus carpio, Linnaeus, 1758) under the action of naproxen

A conglomerate of small, rigid, fusiform spicules known as actinotrichia sustains the edge of tail fins of teleost. After amputation, these structures show an extremely fast regenerative capacity. In this study we observed the effect of a nonsteroidal anti-inflammatory drug, naproxen, used in the treatment of degenerative articular diseases, during the process of actinotrichia regeneration. For this purpose, regenerating tissue from animals in contact with the drug was submitted to histochemical and ultrastructural analysis in comparison to tissue from animals under normal conditions, i.e., not in contact with the drug in question. Actinotrichia regeneration was similar in both animals, indicating that naproxen, at the dose used in the present study, did not interfere with actinotrichia synthesis during the regenerative process of the tail fin. This could be because naproxen did not influence the expression of the genes required for the regeneration process, such as the Sonic hedgehog (Shh) gene, which is involved in actinotrichia formation.

Connexin43 regulates joint location in zebrafish fins

Joints are essential for skeletal form and function, yet their development remains poorly understood. In zebrafish fins, joints form between the bony fin ray segments providing essentially unlimited opportunities to evaluate joint morphogenesis. Mutations in cx43 cause the short segment phenotype of short fin (sof(b123)) mutants, suggesting that direct cell-cell communication may regulate joint location. Interestingly, increased cx43 expression in the another long fin (alf(dty86)) mutant appears to cause joint failure typical of that mutant. Indeed, knockdown of cx43 in alf(dty86) mutant fins rescues joint formation. Together, these data reveal a correlation between the level of Cx43 expression in the fin ray mesenchyme and the location of joints. Cx43 was also observed laterally in cells associated with developing joints. Confocal microscopy revealed that the Cx43 protein initially surrounds the membranes of ZNS5-positive joint cells, but at later stages becomes polarized toward the underlying Cx43-positive mesenchymal cells. One possibility is that communication between the Cx43-positive mesenchyme and the overlying ZNS5-positive cells regulates joint location, and upregulation of Cx43 in joint-forming cells contributes to joint morphogenesis.

Fgfr1 signalling in the development of a sexually selected trait in vertebrates, the sword of swordtail fish

Background

One of Darwin's chosen examples for his idea of sexual selection through female choice was the "sword", a colourful extension of the caudal fin of male swordtails of the genus Xiphophorus. Platyfish, also members of the genus Xiphophorus, are thought to have arisen from within the swordtails, but have secondarily lost the ability to develop a sword. The sustained increase of testosterone during sexual maturation initiates sword development in male swordtails. Addition of testosterone also induces sword-like fin extensions in some platyfish species, suggesting that the genetic interactions required for sword development may be dormant, rather than lost, within platyfish. Despite considerable interest in the evolution of the sword from a behavioural or evolutionary point of view, little is known about the developmental changes that resulted in the gain and secondary loss of the sword. Up-regulation of msxC had been shown to characterize the development of both swords and the gonopodium, a modified anal fin that serves as an intromittent organ, and prompted investigations of the regulatory mechanisms that control msxC and sword growth.

Results

By comparing both development and regeneration of caudal fins in swordtails and platyfish, we show that fgfr1 is strongly up-regulated in developing and regenerating sword and gonopodial rays. Characterization of the fin overgrowth mutant brushtail in a platyfish background confirmed that fin regeneration rates are correlated with the expression levels of fgfr1 and msxC. Moreover, brushtail re-awakens the dormant mechanisms of sword development in platyfish and activates fgfr1/msxC-signalling. Although both genes are co-expressed in scleroblasts, expression of msxC in the distal blastema may be independent of fgfr1. Known regulators of Fgf-signalling in teleost fins, fgf20a and fgf24, are transiently expressed only during regeneration and thus not likely to be required in developing swords.

Conclusion

Our data suggest that Fgf-signalling is involved upstream of msxC in the development of the sword and gonopodium in male swordtails. Activation of a gene regulatory network that includes fgfr1 and msxC is positively correlated with fin ray growth rates and can be re-activated in platyfish to form small sword-like fin extensions. These findings point towards a disruption between the fgfr1/msxC network and its regulation by testosterone as a likely developmental cause for sword-loss in platyfish.

Gene expression analysis on sections of zebrafish regenerating fins reveals limitations in the whole-mount in situ hybridization method

The caudal fin of adult zebrafish is used to study the molecular mechanisms that govern regeneration processes. Most reports of gene expression in regenerating caudal fins rely on in situ hybridization (ISH) on whole-mount samples followed by sectioning of the samples. In such reports, expression is mostly confined to cells other than those located between the dense collagenous structures that are the actinotrichia and lepidotrichia. Here, we re-examined the expression of genes by performing ISH directly on cryo-sections of regenerates. We detected expression of some of these genes in cell types that appeared to be non-expressing when ISH was performed on whole-mount samples. These results demonstrate that ISH reagents have a limited capacity to penetrate between the regenerating skeletal matrices and suggest that ISH performed directly on fin sections is a preferable method to study gene expression in fin regenerates.

Screen for genes differentially expressed during regeneration of the zebrafish caudal fin

The zebrafish caudal fin constitutes an important model for studying the molecular basis of tissue regeneration. The cascade of genes induced after amputation or injury, leading to restoration of the lost fin structures, include those responsible for wound healing, blastema formation, tissue outgrowth, and patterning. We carried out a systematic study to identify genes that are up-regulated during "initiation" (1 day) and "outgrowth and differentiation" (4 days) of fin regeneration by using two complementary methods, suppression subtraction hybridization (SSH) and differential display reverse transcriptase polymerase chain reaction (DDRT-PCR). We obtained 298 distinct genes/sequences from SSH libraries and 24 distinct genes/sequences by DDRT-PCR. We determined the expression of 54 of these genes using in situ hybridization. In parallel, gene expression analyses were done in zebrafish embryos and early larvae. The information gathered from the present study provides resources for further investigations into the molecular mechanisms of fin development and regeneration.

Mutations in connexin43 (GJA1) perturb bone growth in zebrafish fins

Mechanisms that regulate the size and shape of bony structures are largely unknown. The molecular identification of the fin length mutant short fin (sof), which causes defects in the length of bony fin ray segments, may provide insights regarding the regulation of bone growth. In this report, we demonstrate that the sof phenotype is caused by mutations in the connexin43 (cx43) gene. This conclusion is supported by genetic mapping, reduced expression of cx43 in the original sof allele (sofb123), identification of missense mutations in three ENU-induced alleles, and by demonstration of partially abrogated cx43 function in sofb123 embryos. Expression of cx43 was identified in cells flanking the germinal region of newly growing segments as well as in the osteoblasts at segment boundaries. This pattern of cx43 expression in cells lateral to new segment growth is consistent with a model where cx43-expressing cells represent a biological ruler that measures segment size. This report identifies the first gene identification for a fin length mutation (sof) as well as the first connexin mutations in zebrafish, and therefore reveals a critical role for local cell-cell communication in the regulation of bone size and growth.

Differential regulation of msx genes in the development of the gonopodium, an intromittent organ, and of the "sword," a sexually selected trait of swordtail fishes (Xiphophorus)

The possession of a conspicuous extension of colored ventral rays of the caudal fin in male fish of swordtails (genus Xiphophorus) is a prominent example for a trait that evolved by sexual selection. To understand the evolutionary history of this so-called sword molecularly, it is of interest to unravel the developmental pathways responsible for extended growth of sword rays during development of swordtail males. We isolated two msx genes and showed that they are differentially regulated during sword outgrowth. During sword growth in juvenile males, as well as during testosterone-induced sword development and fin ray regeneration in the sword after amputation, expression of msxC is markedly up-regulated in the sword forming fin rays. In contrast, msxE/1 is not differentially expressed in ventral and dorsal male fin rays, suggesting a link between the development of male secondary sexual characters in fins and up-regulation of msxC expression. In addition, we showed that msx gene expression patterns differ significantly between Xiphophorus and zebrafish. We also included in our study the gonopodium, a testosterone-dependent anal fin modification that serves as a fertilization organ in males of live-bearing fishes. Our finding that increased levels of msxC expression are associated with the testosterone-induced outgrowth of the gonopodium might suggest either that at least parts of the signaling pathways that pattern the evolutionary older gonopodium have been coopted to evolve a sexually selected innovation such as the sword or that increased msxC expression may be inherent to the growth process of long fin rays in general.

Posterior hoxa genes expression during zebrafish bony fin ray development and regeneration suggests their involvement in scleroblast differentiation

Expression of two zebrafish developmental posterior hoxa genes, hoxa11b and hoxa13b, was studied by in situ hybridization during pectoral and caudal fin development and regeneration. Expression was restricted to cells of the bony rays region. During fin development, molecular cytological analysis revealed that a subpopulation of mesenchymal cells expressed these two hoxa genes during their early differentiation in the subapical region of the developing ray. These cells were identified as differentiating dermal bone making cells (scleroblasts). During fin regeneration, hoxa11b and hoxa13b genes are both induced in undifferentiated cells of the distalmost blastema region (DMB) and the proliferating zone (PZ) and later in differentiating bone-forming cells. In addition, the transient regionalization of the hoxa13b expression pattern in differentiated bone-forming cells along the proximodistal axis of the regenerating ray suggests that hoxa13b could participate in ray patterning. This study is the first to establish a correlation between hoxa gene expression and dermal bone cell differentiation.

Comparison of even-skipped related gene expression pattern in vertebrates shows an association between expression domain loss and modification of selective constraints on sequences

The even-skipped related genes (evx) encode homeodomain-containing transcription factors that play key roles in body patterning and neurogenesis in a wide array of Eumetazoa species. It is thought that the genome of the last common ancestor of Chordata contained a unique evx gene linked to a unique ancestral Hox complex. During subsequent evolution, two rounds of whole genome duplication followed by individual gene losses gave rise to three paralogs: evx1, evx2, and eve1. Then, eve1 was maintained in Actinopterygii lineage but not in Tetrapoda. To explain this discrepancy, we examined the expression patterns of the evx1 homologue, Xhox3, in Xenopus laevis and that of evx1 and eve1 in Danio rerio. We show here that Xhox3 is expressed in a manner that closely reflects the inferred expression pattern of the evx1 gene in the last common ancestor of Vertebrata (i.e., in gastrula, the central nervous system, the posterior gut, and the tip of the growing tail). Zebrafish evx1 and Xenopus Xhox3 are expressed in homologous cell lineages of the central nervous system and of the posterior gut, but evx1 was undetectable in the gastrula and the tail bud. Strikingly, eve1 is the only evx gene of zebrafish to be expressed in these two latter regions. Thus, the ancestral expression pattern of evx1 in vertebrates appears to have been distributed between evx1 and eve1 in zebrafish. We propose that evx1 and eve1 underwent a complementary loss of expression domain in zebrafish that allowed the maintenance of the two paralogs in accordance with the duplication-degeneration-complementation model. It is important to note that, in zebrafish, Evx1 and Eve1 have lost most of the protein domain upstream of the homeodomain. In addition, Eve1 has accumulated substitutions in positions that are highly conserved in all other Evx proteins. Thus, the reduction of the expression domain of both evx1 and eve1 in zebrafish appears to be associated with the modification of constraints on the protein sequences, allowing the shortening of both genes and an accelerated substitution rate in eve1.

Tales of regeneration in zebrafish

Complex tissue regeneration involves exquisitely coordinated proliferation and patterning of adult cells after severe injury or amputation. Certain lower vertebrates such as urodele amphibians and teleost fish have a greater capacity for regeneration than mammals. However, little is known about molecular mechanisms of regeneration, and cellular mechanisms are incompletely defined. To address this deficiency, we and others have focused on the zebrafish model system. Several helpful tools and reagents are available for use with zebrafish, including the potential for genetic approaches to regeneration. Recent studies have shed light on the remarkable ability of zebrafish to regenerate fins.

Old questions, new tools, and some answers to the mystery of fin regeneration

Pluridisciplinary approaches led to the notion that fin regeneration is an intricate phenomenon involving epithelial-mesenchymal and reciprocal exchanges throughout the process as well as interactions between ray and interray tissue. The establishment of a blastema after fin amputation is the first event leading to the reconstruction of the missing part of the fin. Here, we review our knowledge on the origin of the blastema, its formation and growth, and of the mechanisms that control differentiation and patterning of the regenerate. Our current understanding results from studies of fin regeneration performed in various teleost fish over the past century. We also report the recent breakthroughs that have been made in the past decade with the arrival of a new model, the zebrafish, Danio rerio, which now offers the possibility to combine cytologic, molecular, and genetic analyses and open new perspectives in this field.

Bone patterning is altered in the regenerating zebrafish caudal fin after ectopic expression of sonic hedgehog and bmp2b or exposure to cyclopamine

Amputation of the zebrafish caudal fin stimulates regeneration of the dermal skeleton and reexpression of sonic hedgehog (shh)-signaling pathway genes. Expression patterns suggest a role for shh signaling in the secretion and patterning of the regenerating dermal bone, but a direct role has not been demonstrated. We established an in vivo method of gene transfection to express ectopically genes in the blastema of regenerating fins. Ectopic expression of shh or bmp2 in the blastema-induced excess bone deposition and altered patterning of the regenerate. The effects of shh ectopic expression could be antagonized by ectopic expression of chordin, an inhibitor of bone morphogenetic protein (bmp) signaling. We disrupted shh signaling in the regenerating fin by exposure to cyclopamine and found a dose-dependent inhibition of fin outgrowth, accumulation of melanocytes in the distal region of each fin ray, loss of actinotrichia, and reduction in cell proliferation in the mesenchyme. Morphological changes were accompanied by an expansion, followed by a reduction, in domains of shh expression and a rapid abolition of ptc1 expression. These results implicate shh and bmp2b signaling in the proliferation and/or differentiation of specialized bone-secreting cells in the blastema and suggest shh expression may be controlled by regulatory feedback mechanisms that define the region of bone secretion in the outgrowing fin.

Cell proliferation and movement during early fin regeneration in zebrafish

Cell proliferation and cell movement during early regeneration of zebrafish caudal fins were examined by injecting BrdU and Di-I, respectively. In normal fins of adult fish, a small number of proliferating cells are observed in the epidermis only. Shortly following amputation, epithelial cells covered the wound to form the epidermal cap but did not proliferate. However, by 24 hr, epithelial cells proximal to the level of amputation were strongly labeled with BrdU. Label incorporation was also detected in a few mesenchymal cells. Proliferating cells in the basal epithelial layer were first observed at 48 hr at the level of the newly formed lepidotrichia. At 72 hr, proliferating mesenchymal cells were found distal to the plane of amputation whereas more proximal labeled cells included mainly those located between the lepidotrichia and the basal membrane. When BrdU-injected fins were allowed to regenerate for longer periods, labeled cells were observed in the apical epidermal cap, a location where cells are not thought to proliferate. This result is suggestive of cell migration. Epithelial cells, peripheral to the rays or in the tissue between adjacent rays, were labeled with Di-I and were shown to quickly migrate towards the site of amputation, the cells closer to the wound migrating faster. Amputation also triggered migration of cells of the connective tissue located between the hemirays. Although cell movement was induced up to seven segments proximal from the level of amputation, cells located within two segments from the wound provided the main contribution to the blastema. Thus, cell proliferation and migration contribute to the early regeneration of zebrafish fins.

Zebrafish evx1 is dynamically expressed during embryogenesis in subsets of interneurones, posterior gut and urogenital system

The even-skipped-related homeobox genes (evx) are widely distributed through animal kingdom and are thought to play key role in posterior body patterning and neurogenesis. We have cloned and analyzed the expression of evx1 in zebrafish (see also Borday et al. (Dev. Dyn. 220 (2001) in press) which displays a dynamic and restricted expression pattern during neurogenesis. In spinal cord, rhombencephalon, and epiphysis, evx1 is expressed in several subsets of emerging interneurones prior to their axonal outgrowth, identified as primary interneurones and a subset of Pax2.1(+) commissural interneurones. In the hindbrain, evx1 is expressed in reticulospinal interneurones of rhombomeres 5 and 6 as well as in rhombomere 7 interneurones. The latest emerging evx1(+) interneurones in the hindbrain correspond to commissural interneurones. evx1 is also dynamically transcribed during the formation of the posterior gut and the uro-genital system in mesenchymal cells that border the pronephric ducts, the wall of the pronephric duct, and later in the posterior gut and the wall of the uro-genital opening. In larvae, the ano-rectal epithelium and the muscular layer that surrounds the analia-genitalia region remain stained up to 27 days. In contrast other vertebrates, evx1displays no early nor caudal expression in zebrafish.