Tenascin-C mRNA is expressed in cranial neural crest cells, in some placodal derivatives, and in discrete domains of the embryonic zebrafish brain

scRNA-seq reveals the diversity of the developing cardiac cell lineage and molecular players in heart rhythm regulation

We utilized scRNA-seq to delineate the diversity of cell types in the zebrafish heart. Transcriptome profiling of over 50,000 cells at 48 and 72 hpf defined at least 18 discrete cell lineages of the developing heart. Utilizing well-established gene signatures, we identified a population of cells likely to be the primary pacemaker and characterized the transcriptome profile defining this critical cell type. Two previously uncharacterized genes, atp1b3b and colec10, were found to be enriched in the sinoatrial cardiomyocytes. CRISPR/Cas9-mediated knockout of these two genes significantly reduced heart rate, implicating their role in cardiac development and conduction. Additionally, we describe other cardiac cell lineages, including the endothelial and neural cells, providing their expression profiles as a resource. Our results established a detailed atlas of the developing heart, providing valuable insights into cellular and molecular mechanisms, and pinpointed potential new players in heart rhythm regulation.

Thoughts on the Etiology of Cherubism

Cherubism is nowadays classified as an autoimmune disease and was first described in 1933. Although suspected at that time to be the result of defective tooth development, it was primarily classified as a bone disease caused by a mutation in the SH3BP2 gene. Despite a knock-in mouse model, phenotypic signs in the jaw area were not reproducible in this model. The features of classical cherubism can be attributed to a disturbed formation of the dental placode of the second molar. Since 2019, it has become clear that inhibition of the WNT pathway leads to the accumulation of SH3BP2 via tankyrase inhibition. As the dental placode is triggered via WNT (in epithelia) and MSX1 (in mesenchyme), aplasia of the second and third molars occurs due to a block in the WNT pathway. The mesenchymal part, which occurs prior to the body plan regulation of the WNT/MSX1 pathway, remains unaffected and provides the substrate for the giant cell granuloma. Considering macrophage polarization and the role of the extracellular matrix in general, cherubism is situated in the field of tension between autoimmune diseases and cancer. In this sense, we see the cause of cherubism in a WNT-related dysregulation, which can be proven postnatally in the neural crest-related tooth development of the replacement tooth ridge, both genotypically and phenotypically.

Biology of Tenascin C and its Role in Physiology and Pathology

Tenascin-C (TNC) is a multimodular extracellular matrix (ECM) protein hexameric with several molecular forms (180-250 kDa) produced by alternative splicing at the pre-mRNA level and protein modifications. The molecular phylogeny indicates that the amino acid sequence of TNC is a well-conserved protein among vertebrates. TNC has binding partners, including fibronectin, collagen, fibrillin-2, periostin, proteoglycans, and pathogens. Various transcription factors and intracellular regulators tightly regulate TNC expression. TNC plays an essential role in cell proliferation and migration. Unlike embryonic tissues, TNC protein is distributed over a few tissues in adults. However, higher TNC expression is observed in inflammation, wound healing, cancer, and other pathological conditions. It is widely expressed in a variety of human malignancies and is recognized as a pivotal factor in cancer progression and metastasis. Moreover, TNC increases both pro-and anti-inflammatory signaling pathways. It has been identified as an essential factor in tissue injuries such as damaged skeletal muscle, heart disease, and kidney fibrosis. This multimodular hexameric glycoprotein modulates both innate and adaptive immune responses regulating the expression of numerous cytokines. Moreover, TNC is an important regulatory molecule that affects the onset and progression of neuronal disorders through many signaling pathways. We provide a comprehensive overview of the structural and expression properties of TNC and its potential functions in physiological and pathological conditions.

Matricellular Proteins: Functional Insights From Non-mammalian Animal Models

The extracellular matrix (ECM) has central roles in tissue integrity and remodeling throughout the life span of animals. While collagens are the most abundant structural components of ECM in most tissues, tissue-specific molecular complexity is contributed by ECM glycoproteins. The matricellular glycoproteins are categorized primarily according to functional criteria and represented predominantly by the thrombospondin, tenascin, SPARC/osteonectin, and CCN families. These proteins do not self-assemble into ECM fibrils; nevertheless, they shape ECM properties through interactions with structural ECM proteins, growth factors, and cells. Matricellular proteins also promote cell migration or morphological changes through adhesion-modulating or counter-adhesive actions on cell-ECM adhesions, intracellular signaling, and the actin cytoskeleton. Typically, matricellular proteins are most highly expressed during embryonic development. In adult tissues, expression is more limited unless activated by cues for dynamic tissue remodeling and cell motility, such as occur during inflammatory response and wound repair. Many insights in the complex roles of matricellular proteins have been obtained from studies of gene knockout mice. However, with the exception of chordate-specific tenascins, these are highly conserved proteins that are encoded in many animal phyla. This review will consider the increasing body of research on matricellular proteins in nonmammalian animal models. These models provide better access to the very earliest stages of embryonic development and opportunities to study biological processes such as limb and organ regeneration. In aggregate, this research is expanding concepts of the functions and mechanisms of action of matricellular proteins.

Enteric neural crest-derived cells promote their migration by modifying their microenvironment through tenascin-C production

The enteric nervous system (ENS) is derived from vagal and sacral neural crest cells that migrate, proliferate, and differentiate into enteric neurons and glia within the gut wall. The mechanisms regulating enteric neural crest-derived cell (ENCC) migration are poorly characterized despite the importance of this process in gut formation and function. Characterization of genes involved in ENCC migration is essential to understand ENS development and could provide targets for treatment of human ENS disorders. We identified the extracellular matrix glycoprotein tenascin-C (TNC) as an important regulator of ENCC development. We find TNC dynamically expressed during avian gut development. It is absent from the cecal region just prior to ENCC arrival, but becomes strongly expressed around ENCCs as they enter the ceca and hindgut. In aganglionic hindguts, TNC expression is strong throughout the outer mesenchyme, but is absent from the submucosal region, supporting the presence of both ENCC-dependent and independent expression within the gut wall. Using rat-chick coelomic grafts, neural tube cultures, and gut explants, we show that ENCCs produce TNC and that this ECM protein promotes their migration. Interestingly, only vagal neural crest-derived ENCCs express TNC, whereas sacral neural crest-derived cells do not. These results demonstrate that vagal crest-derived ENCCs actively modify their microenvironment through TNC expression and thereby help to regulate their own migration.

The extracellular matrix glycoprotein tenascin-C promotes locomotor recovery after spinal cord injury in adult zebrafish

Adult zebrafish, by virtue of exhibiting spontaneous recovery after spinal lesion, have evolved into a paradigmatic vertebrate model system to identify novel genes vital for successful regeneration after spinal cord injury. Due to a remarkable level of conservation between zebrafish and human genomes, such genes, once identified, could point to possibilities for addressing the multiple issues on how to deal with functional recovery after spinal cord injury in humans. In the current study, the extracellular matrix glycoprotein tenascin-C was studied in the zebrafish spinal cord injury model to assess the often disparate functions of this multidomain molecule under in vivo conditions. This in vivo study was deemed necessary since in vitro studies had shown discrepant functional effects on neurite outgrowth: tenascin-C inhibits neurite outgrowth when presented as a molecular barrier adjacent to a conducive substrate, but enhances neurite outgrowth when presented as a uniform substrate. Thus, our current study addresses the question as to which of these features prevails in vivo: whether tenascin-C reduces or enhances axonal regrowth after injury in a well accepted vertebrate model of spinal cord injury. We show upregulation of tenascin-C expression in regenerating neurons of the nucleus of median longitudinal fascicle (NMLF) in the brainstem and spinal motoneurons. Inhibition of tenascin-C expression by antisense oligonucleotide (morpholino) resulted in impaired locomotor recovery, reduced regrowth of axons from brainstem neurons and reduced synapse formation by the regrowing brainstem axons on spinal motoneurons, all vital indicators of regeneration. Our results thus point to an advantageous role of tenascin-C in promoting spinal cord regeneration, by promoting axonal regrowth and synapse formation in the spinal cord caudal to the lesion site after injury.

Transcription factor Sox11b is involved in spinal cord regeneration in adult zebrafish

Adult zebrafish have the ability to recover from spinal cord injury and exhibit re-growth of descending axons from the brainstem to the spinal cord. We performed gene expression analysis using microarray to find damage-induced genes after spinal cord injury, and found that Sox11b mRNA is up-regulated at 11 days after injury. However, the functional relevance of Sox11b for regeneration is not known. Here, we report that the up-regulation of Sox11b mRNA after spinal cord injury is mainly localized in ependymal cells lining the central canal and in newly differentiating neuronal precursors or immature neurons. Using an in vivo morpholino-based gene knockout approach, we demonstrate that Sox11b is essential for locomotor recovery after spinal cord injury. In the injured spinal cord, expression of the neural stem cell associated gene Nestin, and the proneural gene Ascl1a (Mash1a), which are involved in the self-renewal and cell fate specification of endogenous neural stem cells, respectively, is regulated by Sox11b. Our data indicate that Sox11b promotes neuronal determination of endogenous stem cells and regenerative neurogenesis following spinal cord injury in the adult zebrafish. Enhancing Sox11b expression to promote proliferation and neurogenic determination of endogenous neural stem cells after injury may be a promising strategy in restorative therapy after spinal cord injury in mammals.

Tenascin-C is involved in motor axon outgrowth in the trunk of developing zebrafish

Motor axons in the trunk of the developing zebrafish exit from the ventral spinal cord in one ventral root per hemisegment and grow on a common path toward the region of the horizontal myoseptum, where they select their specific pathways. Tenascin-C, a component of the extracellular matrix, is concentrated in this choice region. Adaxial cells and other myotomal cells express tenascin-C mRNA, suggesting that these cells are the source of tenascin-C protein. Overexpressing an axon repellent fragment containing the cysteine-rich region and the epidermal growth factor-like repeats of tenascin-C led to retarded growth of ventral motor nerves between their spinal exit point and the horizontal myoseptum. Injection of a protein fragment containing the same part of tenascin-C also induced slower growth of motor nerves. Conversely, knock down of tenascin-C protein resulted in abnormal lateral branching of ventral motor nerves. In the zebrafish unplugged mutant, in which axons display pathfinding defects in the region of the horizontal myoseptum, tenascin-C immunoreactivity was not detectable in this region, indicating an abnormal extracellular matrix in unplugged. We conclude that tenascin-C is part of a specialized extracellular matrix in the region of the horizontal myoseptum that influences the growth of motor axons.

Neuropilin-1a is involved in trunk motor axon outgrowth in embryonic zebrafish

Neuropilin-1, a receptor for axon-repellent semaphorins and vascular endothelial growth factor (VEGF), functions both in angiogenesis and axon growth. Here, we show strong expression of neuropilin-1a in primary motor neurons in the trunk of embryonic zebrafish. Reducing the expression of neuropilin-1a using antisense morpholino oligonucleotides induced aberrant branching of motor nerves, additional exit points of motor nerves from the spinal cord, and migration of neurons out of the spinal cord along the motor axon pathway in a dose-dependent manner. These phenotypes could be partially rescued by co-injecting neuropilin-1a mRNA. Other axons in the spinal cord and head appeared unaffected by the morpholino treatment. In addition, neuropilin-1a morpholino treatment disturbed normal formation of blood vessels in the trunk of 24 hours postfertilization embryos, as shown by microangiography. Morpholinos to VEGF also disturbed formation of blood vessels but did not affect motor axons, indicating that correct formation of blood vessels is not needed for the growth of primary motor axons. Morpholinos to the semaphorin 3A homologs semaphorin 3A1 and semaphorin 3A2 also had no effect on motor axon growth. However, combined injections of neuropilin-1a morpholino, at a concentration that did not elicit axonal aberrations when injected alone, with VEGF, semaphorin 3A1, or semaphorin 3A2 morpholinos synergistically increased the proportion of embryos showing aberrant motor axon growth. Thus, neuropilin-1a in primary motor neurons may integrate signals from several ligands and is needed for proper segmental growth of primary motor nerves in zebrafish.

Restricted expression of reggie genes and proteins during early zebrafish development

Reggies are plasma membrane-associated proteins and characteristic markers of lipid-raft microdomains. They are highly conserved from flies to humans and have been implicated in axon regeneration and cell process and contact formation, possibly providing functional platforms for cell-signaling in neurons and other cell types. We analyzed reggie mRNA and protein expression patterns during early zebrafish development. All three zebrafish genes, re-1a, -2a, and -2b, span a considerably diverse set of expression patterns, and their proteins are induced maternally, showing ubiquitous expression at early stages. Although re-2a mRNA can be observed in differentiating neurons in the brain, spinal cord, and neurogenic placodes, re-2b is transcribed mainly in head mesoderm, in neural crest derivates, and along somite boundaries. re-1a mRNA is present at high levels in expression domains that overlap with the combined expression pattern of both re-2 genes except at the somites, where it complements the pattern of re-2b. Immunostaining on embryos reveals reggie protein localization at the cell membrane, at cell-cell contacts, and along all early axon tracts. The early phase of reggie expression suggests a basic and ubiquitous function during the first stages of embryogenesis and into the gastrula period. Upon segmentation, a second phase of expression shows distinctly localized expression patterns, indicating tissue-specific roles and an involvement of re-1a/re-2a in neural development.

Tenascin-R as a repellent guidance molecule for newly growing and regenerating optic axons in adult zebrafish

In adult fish, in contrast to mammals, new optic axons are continuously added to the optic projection, and optic axons regrow after injury. Thus, pathfinding of optic axons during development, adult growth, and adult regeneration may rely on the same guidance cues. We have shown that tenascin-R, a component of the extracellular matrix, borders the optic pathway in developing zebrafish and acts as a repellent guidance molecule for optic axons. Here we analyze tenascin-R expression patterns along the unlesioned and lesioned optic pathway of adult zebrafish and test the influence of tenascin-R on growing optic axons of adult fish in vitro. Within intraretinal fascicles of optic axons and in the optic nerve, newly added optic axons grow in a tenascin-R immunonegative pathway, which is bordered by tenascin-R immunoreactivity. In the brain, tenascin-R expression domains in the ventral diencephalon, in non-retinorecipient pretectal nuclei and in some tectal layers closely border the optic pathway in unlesioned animals and during axon regrowth. We mimicked these boundary situations with a sharp substrate border of tenascin-R in vitro. Optic axons emanating from adult retinal explants were repelled by tenascin-R substrate borders. This is consistent with a function of tenascin-R as a repellent guidance molecule in boundaries for adult optic axons. Thus, tenascin-R may guide newly added and regenerating optic axons by a contact-repellent mechanism in the optic pathway of adult fish.

Expression of protein zero is increased in lesioned axon pathways in the central nervous system of adult zebrafish

The immunoglobulin superfamily molecule protein zero (P0) is important for myelin formation and may also play a role in adult axon regeneration, since it promotes neurite outgrowth in vitro. Moreover, it is expressed in the regenerating central nervous system (CNS) of fish, but not in the nonregenerating CNS of mammals. We identified a P0 homolog in zebrafish. Cell type-specific expression of P0 begins in the ventromedial hindbrain and the optic chiasm at 3-5 days of development. Later (at 4 weeks) expression has spread throughout the optic system and spinal cord. This is consistent with a role for P0 in CNS myelination during development. In the adult CNS, glial cells constitutively express P0 mRNA. After an optic nerve crush, expression is increased within 2 days in the entire optic pathway. Expression peaks at 1 to 2 months and remains elevated for at least 6 months postlesion. After enucleation, P0 mRNA expression is also upregulated but fails to reach the high levels observed in crush-lesioned animals at 4 weeks postlesion. Spinal cord transection leads to increased expression of P0 mRNA in the spinal cord caudal to the lesion site. The glial upregulation of P0 mRNA expression after a lesion of the adult zebrafish CNS suggests roles for P0 in promoting axon regeneration and remyelination after injury.

A dual role for the zebrafish unplugged gene in motor axon pathfinding and pharyngeal development

On their way toward their synaptic targets, motor growth cones encounter multiple choice points, where they are confronted with trajectory choices. We have previously shown that the zebrafish unplugged gene acts as a somite-derived cue controlling pathway choice of primary motor axons. Here, we demonstrate that this trajectory choice is not exclusively controlled by a single unplugged-dependent process, but depends on the coordinated function of additional cues. We also show that secondary motor neurons, most similar to those in birds and mammals, depend on the unplugged gene to navigate a choice point, suggesting that primary and secondary motor neurons share common mechanisms controlling axonal path selection. Moreover, we show that the unplugged gene plays an additional role guiding secondary motor axons through a single segmental nerve. Finally, we report that unplugged larvae display a striking pharyngeal arch defect, consistent with a dual function of the unplugged gene in axonal guidance and cell motility.

Abnormal neural crest cell migration after the in vivo knockdown of tenascin-C expression with morpholino antisense oligonucleotides

A key feature of vertebrate development is the formation of the neural crest. In the trunk, neural crest cells delaminate from the neural tube shortly after the fusion of the neural folds and migrate ventrally along specific pathways to form the neurons and glia of the peripheral nervous system. As neural crest cells leave the neural tube during the initial stages of their migration, they express the extracellular matrix glycoprotein tenascin-C, which is also found in the stroma of many tumors. We have studied the possible role for tenascin-C during neural crest morphogenesis in vivo by microinjecting tenascin-C morpholino antisense oligonucleotides into the lumen of the avian neural tube in ovo and electroporating the morpholino antisense oligonucleotides into the precursors of the neural crest. After 24 hr, tenascin-C immunostaining is reduced around the dorsal neural tube in the experimental microinjected embryos (12 of 13) but not in embryos microinjected with control morpholino antisense oligonucleotides (n = 3) or subjected to electroporation only (n = 2). In each of the 12 tenascin-C knockdown embryos neural crest cells are seen ectopically in the lumen of the neural tube and in the neuroepithelium; cells that do leave the neural tube after the microinjection fail to disperse laterally from the surface of the neural tube into the somites. The observation that neural crest cells must express tenascin-C to migrate normally is consistent with a role for this glycoprotein in contributing to the invasive behavior of neural crest cells.

Expression of Otx homeodomain proteins induces cell aggregation in developing zebrafish embryos

In the zebrafish embryo, cells fated to give rise to the rostral brain move in a concerted fashion and retain tissue coherence during morphogenesis. We demonstrate here that Otx proteins have a dramatic effect on cell-cell interactions when expressed ectopically in the zebrafish embryo. Injection of zebrafish Otx1 or Drosophila otd RNAs into a single cell at the 16-cell stage results in aggregation of descendants of the injected cell. The Otx/Otd homeodomain is necessary for aggregation and appears to be sufficient for the effect when substituted for the homeodomain of an unrelated homeodomain protein. When cells containing injected zOtx1 RNA are limited to the area that is normally fated to become the anterior brain and neural retina, the induced aggregates contribute to anterior brain and retina tissues. In many other embryonic regions, which do not express endogenous zOtx1, the aggregates appear to be incompatible with normal development and do not integrate into developing tissues. By using an activatable Otx1-glutocorticoid receptor fusion protein that results in the stimulation of cell association, we demonstrate that cell aggregates can form as a result of Otx1 activity even after gastrulation is completed. Time-lapse analysis of cell movements show that cell aggregation occurs with only a slight inhibition of the rate of convergence. These results suggest that promotion of cell adhesion or mediation of cell repulsion may be one of the normal functions of the Otx proteins in the establishment of the anterior brain.

Mice deficient for tenascin-R display alterations of the extracellular matrix and decreased axonal conduction velocities in the CNS

Tenascin-R (TN-R), an extracellular matrix glycoprotein of the CNS, localizes to nodes of Ranvier and perineuronal nets and interacts in vitro with other extracellular matrix components and recognition molecules of the immunoglobulin superfamily. To characterize the functional roles of TN-R in vivo, we have generated mice deficient for TN-R by homologous recombination using embryonic stem cells. TN-R-deficient mice are viable and fertile. The anatomy of all major brain areas and the formation and structure of myelin appear normal. However, immunostaining for the chondroitin sulfate proteoglycan phosphacan, a high-affinity ligand for TN-R, is weak and diffuse in the mutant when compared with wild-type mice. Compound action potential recordings from optic nerves of mutant mice show a significant decrease in conduction velocity as compared with controls. However, at nodes of Ranvier there is no apparent change in expression and distribution of Na+ channels, which are thought to bind to TN-R via their beta2 subunit. The distribution of carbohydrate epitopes of perineuronal nets recognized by the lectin Wisteria floribunda or antibodies to the HNK-1 carbohydrate on somata and dendrites of cortical and hippocampal interneurons is abnormal. These observations indicate an essential role for TN-R in the formation of perineuronal nets and in normal conduction velocity of optic nerve.

Tenascin-C expression in the trunk of wild-type, cyclops and floating head zebrafish embryos

The function and the regulation of the expression of the extracellular matrix molecule tenascin-C during embryonic development are still unclear. In the present study, the expression of tenascin-C was analyzed in the trunk of zebrafish at the end of the first embryonic day. An antiserum raised against a zebrafish tenascin-C (TN-C) fusion protein reacted with 220 (doublet), 200, and 160 KD peptides. In situ hybridization showed that in the zebrafish wild-type embryo, tn-c mRNA was expressed by somites, neural crest cells, roof plate, notochord, hypochord, and tail fin bud. Thus, the expression of tn-c mRNA is an excellent marker for the differentiation of most zebrafish trunk structures. Immunolabelling with the anti-TN-C antibody was detected in the migratory pathway of neural crest cells and in the intersomitic furrows. In situ hybridization analysis of the zebrafish cyclops mutants, lacking the midline floor plate cells, showed normal expression of tn-c mRNA in all trunk structures. Analysis of the floating-head mutant, lacking the notochord, showed that tn-c mRNA expression in neural crest cells, roof plate, and tail fin bud is normal, but it is defective in the somites. By showing that the notochord, but not the floor plate, cells are required for normal tn-c expression in the trunk, this work provides new information on the role played by the embryonic axial structures in the regulation of the expression of tn-c during the development of zebrafish and allows new conclusions about somite patterning in the cyclops and floating-head zebrafish mutants.

Cellular and molecular bases of axonal pathfinding during embryogenesis of the fish central nervous system

The accessibility of the zebrafish embryo offers unique possibilities to study the mechanisms that guide growing axons in the developing vertebrate central nervous system. This review examines the current understanding of the pathfinding decisions by the growing axons, their substrates, and the recognition molecules that mediate axon-substrate interactions. The detailed analysis of pathfinding at the level of individual axons demonstrates that growing axons chose their paths unerringly. To do so, they rely on cues presented by their environment, in particular by neuroepithelial cells. Our understanding of the molecular bases of axon-substrate interactions is increasing. Members of most classes of recognition molecules have been identified in fish. Experimental evidence for the functions of these molecules in the zebrafish nervous system is accumulating. In the future, this analysis is expected to profit greatly from genetic screens that have recently been initiated.

Early expression of Osteopontin in the chick is restricted to rhombomeres 5 and 6 and to a subpopulation of neural crest cells that arise from these segments

BACKGROUND

Osteopontin previously has been shown to be expressed in the early notochord and rostral hindbrain during formation of the rat neuraxis. Both of these sites are known to have important organizing capacities associated with the expression of a number of transcription factors and secreted signaling molecules. Thus multiple types of molecules are involved in neuraxis patterning. Osteopontin is an extracellular matrix molecule, whose expression within the early hindbrain and notochord suggests that it too could play a role in patterning of the neuraxis.

METHODS

By wholemount in situ hybridization, we analyzed osteopontin expression in the chick embryo at Hamburger and Hamilton (HH) stages 5-15, and patterns of expression were compared to those of other hindbrain and neural crest cell markers. Early developmental expression was further examined in the rat by wholemount in situ hybridization and immunohistochemistry.

RESULTS

Our results have demonstrated that whereas notochord expression of osteopontin is not conserved between rat and chick, expression was clearly present in the chick caudal hindbrain between HH stages 9 and 11. In the chick, osteopontin expression was restricted to rhombomeres 5 and 6 and to a subpopulation of neural crest cells arising from these rhombomeres, up to HH stage 13. Osteopontin expression was present in a cranial-to-caudal gradient, and in the dorsoventral plane, expression was absent from both the floor plate and roof plate, with the exception of occasional cells within the roof plate.

CONCLUSIONS

Localized expression in the 5th and 6th rhombomeres suggests that osteopontin may have a role in patterning of postotic neural crest cells that arise from these segments of the hindbrain. In addition, comparison of our current data with those obtained earlier in the rat suggests that there are significant differences either between the developmental role(s) of osteopontin in the chick and in the rat, or between osteopontin gene products in the two species.

Axonal regrowth after spinal cord transection in adult zebrafish

Using axonal tracers, we characterized the neurons projecting from the brain to the spinal cord as well as the terminal fields of ascending spinal projections in the brain of adult zebrafish with unlesioned or transected spinal cords. Twenty distinct brain nuclei were found to project to the spinal cord. These nuclei were similar to those found in the closely related goldfish, except that additionally the parvocellular preoptic nucleus, the medial octavolateralis nucleus, and the nucleus tangentialis, but not the facial lobe, projected to the spinal cord in zebrafish. Terminal fields of axons, visualized by anterograde tracing, were seen in the telencephalon, the diencephalon, the torus semicircularis, the optic tectum, the eminentia granularis, and throughout the ventral brainstem in unlesioned animals. Following spinal cord transection at a level approximately 3.5 mm caudal to the brainstem/spinal cord transition zone, neurons in most brain nuclei grew axons beyond the transection site into the distal spinal cord to the level of retrograde tracer application within 6 weeks. However, the individually identifiable Mauthner cells were never seen to do so up to 15 weeks after spinal cord transection. Nearly all neurons survived axotomy, and the vast majority of axons that had grown beyond the transection site belonged to previously axotomized neurons as shown by double tracing. Terminal fields were not re-established in the torus semicircularis and the eminentia granularis following spinal cord transection.

Increased expression of specific recognition molecules by retinal ganglion cells and by optic pathway glia accompanies the successful regeneration of retinal axons in adult zebrafish

Retinal ganglion cells (RGCs) in adult zebrafish can regenerate their axons. We show that successful axonal regeneration is accompanied by the re-expression by RGCs of mRNAs encoding specific recognition molecules that are expressed at high levels in the larval retina but are down-regulated in the adult. Message levels for 11.1 and 11.2 (two homologs of mammalian L1), n-cam (homologous to mammalian N-CAM), beta 3 (related to the beta 3 and beta 2 subunits of mammalian Na,K-ATPase), and tn-c (homologous to mammalian tenascin-C) were high in larval RGCs undergoing axonogenesis and low in adult RGCs. After an optic nerve crush, axotomized adult RGCs showed increased levels of 11.1, 11.2 and n-cam mRNA expression, whereas the levels of beta 3 and tn-cmRNA remained unchanged. The optic nerve crush also induced the expression of some of these mRNAs in the optic nerve and tract where they are not normally detectable. This lesion induced up-regulation by presumptive glia was observed for 11.1, 11.2, n-cam and beta 3 but not for tn-c. The combination of a neuronal (intrinsic) response to axotomy with an environmental (extrinsic) response may be an important determinant allowing for the successful axonal regeneration.

Expression of a Na,K-ATPase beta 3 subunit during development of the zebrafish central nervous system

Zebrafish beta 3, a full length cDNA clone encoding a zebrafish Na,K-ATPase beta subunit, was isolated. The protein shares highest homology with the beta 3 subunits of amphibians and mammals, slightly less homology with the beta 2 subunits, and is distinct from the beta 1 subunits. The fish beta subunit co-assembled with alpha subunits to form Na,K-ATPase enzymes when expressed in Xenopus oocytes. Embryonic expression was first detected by whole-mount in situ hybridization between 8-12 hr post-fertilization (hpf) in the head mesoderm. Subsequently, and up to 24 hpf, the mRNA was confined to four dorsal domains in the anterior neural tube. After a transient downregulation during the second day, expression was again conspicuous in the nervous system of 3-day-old larvae. Based on its distribution pattern, the fish beta subunit could be involved in setting up regional identities in the developing fish CNS and in the differentiation of distinct cell types.