Cadherin-4 expression in the zebrafish central nervous system and regulation by ventral midline signaling

A novel role for Pax6 in the segmental organization of the hindbrain

Complex patterns and networks of genes coordinate rhombomeric identities, hindbrain segmentation and neuronal differentiation and are responsible for later brainstem functions. Pax6 is a highly conserved transcription factor crucial for neuronal development, yet little is known regarding its early roles during hindbrain segmentation. We show that Pax6 expression is highly dynamic in rhombomeres, suggesting an early function in the hindbrain. Utilization of multiple gain- and loss-of-function approaches in chick and mice revealed that loss of Pax6 disrupts the sharp expression borders of Krox20, Kreisler, Hoxa2, Hoxb1 and EphA and leads to their expansion into adjacent territories, whereas excess Pax6 reduces these expression domains. A mutual negative cross-talk between Pax6 and Krox20 allows these genes to be co-expressed in the hindbrain through regulation of the Krox20-repressor gene Nab1 by Pax6. Rhombomere boundaries are also distorted upon Pax6 manipulations, suggesting a mechanism by which Pax6 acts to set hindbrain segmentation. Finally, FGF signaling acts upstream of the Pax6-Krox20 network to regulate Pax6 segmental expression. This study unravels a novel role for Pax6 in the segmental organization of the early hindbrain and provides new evidence for its significance in regional organization along the central nervous system.

Protocadherin-17 function in Zebrafish retinal development

Cadherin cell adhesion molecules play crucial roles in vertebrate development including the development of the retina. Most studies have focused on examining functions of classic cadherins (e.g. N-cadherin) in retinal development. There is little information on the function of protocadherins in the development of the vertebrate visual system. We previously showed that protocadherin-17 mRNA was expressed in developing zebrafish retina during critical stages of the retinal development. To gain insight into protocadherin-17 function in the formation of the retina, we analyzed eye development and differentiation of retinal cells in zebrafish embryos injected with protocadherin-17 specific antisense morpholino oligonucleotides (MOs). Protocadherin-17 knockdown embryos (pcdh17 morphants) had significantly reduced eyes due mainly to decreased cell proliferation. Differentiation of several retinal cell types (e.g. retinal ganglion cells) was also disrupted in the pcdh17 morphants. Phenotypic rescue was achieved by injection of protocadherin-17 mRNA. Injection of a vivo-protocadherin-17 MO into one eye of embryonic zebrafish resulted in similar eye defects. Our results suggest that protocadherin-17 plays an important role in the normal formation of the zebrafish retina.

Regulation of cadherin expression in nervous system development

This review addresses our current understanding of the regulatory mechanisms for classical cadherin expression during development of the vertebrate nervous system. The complexity of the spatial and temporal expression patterns is linked to morphogenic and functional roles in the developing nervous system. While the regulatory networks controlling cadherin expression are not well understood, it is likely that the multiple signaling pathways active in the development of particular domains also regulate the specific cadherins expressed at that time and location. With the growing understanding of the broader roles of cadherins in cell-cell adhesion and non-adhesion processes, it is important to understand both the upstream regulation of cadherin expression and the downstream effects of specific cadherins within their cellular context.

Cell adhesion molecule cadherin-6 function in zebrafish cranial and lateral line ganglia development

Cadherins regulate the vertebrate nervous system development. We previously showed that cadherin-6 message (cdh6) was strongly expressed in the majority of the embryonic zebrafish cranial and lateral line ganglia during their development. Here, we present evidence that cdh6 has specific functions during cranial and lateral line ganglia and nerve development. We analyzed the consequences of cdh6 loss-of-function on cranial ganglion and nerve differentiation in zebrafish embryos. Embryos injected with zebrafish cdh6 specific antisense morpholino oligonucleotides (MOs, which suppress gene expression during development; cdh6 morphant embryos) displayed a specific phenotype, including (i) altered shape and reduced development of a subset of the cranial and lateral line ganglia (e.g., the statoacoustic ganglion and vagal ganglion) and (ii) cranial nerves were abnormally formed. These data illustrate an important role for cdh6 in the formation of cranial ganglia and their nerves.

Expression of protocadherin-9 and protocadherin-17 in the nervous system of the embryonic zebrafish

In this study we analyzed expression patterns of two delta-protocadherins, protocadherin-9 and protocadherin-17, in the developing zebrafish using in situ hybridization and RT-PCR methods. Both protocadherins were mainly detected in the embryonic central nervous system, but each showed a distinct expression pattern. Protocadherin-9 message (Pcdh9) was expressed after 10h post fertilization (hpf). It was found mainly in small clusters of cells in the anteroventral forebrain and ventrolateral hindbrain, and scattered cells throughout the spinal cord of young embryos (24 hpf). Pcdh9 expression in the hindbrain was segmental, reflecting a neuromeric organization, which became more evident at 34 hpf. As development proceeded, Pcdh9 expression increased throughout the brain, while its expression in the spinal cord was greatly reduced. Pcdh9 was also found in the developing retina and statoacoustic ganglion. Protocadherin-17 message (Pcdh17) expression began much earlier (1.5-2 hpf) than Pcdh9. Similar to Pcdh9 expression, Pcdh17 expression was found mainly in the anteroventral forebrain at 24 hpf, but its expression in the hindbrain and spinal cord, confined mainly to lateroventral regions of the hindbrain and anterior spinal cord, was more restricted than Pcdh9. As development proceeded, Pcdh17 expression was increased both in the brain and spinal cord: detected throughout the brain of two- and three-day old embryos, strongly expressed in the retina and in lateral regions of spinal cord in two-day old embryos. Its expression in the retina and spinal cord was reduced in three-day old embryos. Our results showed that expression of these two protocadherins was both spatially and temporally regulated.

Cadherin-7 function in zebrafish development

Cadherin cell adhesion molecules play crucial roles in vertebrate development. Most studies have focused on examining the functions of classical type I cadherins (e.g., cadherin-2) in the development of vertebrates. Little information is available concerning the function of classical type II cadherins (e.g., cadherin-7) in vertebrate development. We have previously shown that cadherin-7 mRNA exhibits a dynamic expression pattern in the central nervous system and notochord in embryonic zebrafish. To gain insight into the role of cadherin-7 in the formation of these structures, we analyzed their formation in zebrafish embryos injected with cadherin-7-specific antisense morpholino oligonucleotides (MO). Notochord development was severely disrupted in MO-injected embryos, whereas gross defects in the development of the central nervous system were not detected in MO-injected embryos. Our results thus demonstrate that cadherin-7 plays an important role in the normal development of the zebrafish notochord.

Cadherin-6 function in zebrafish retinal development

Cadherin cell-adhesion molecules play crucial roles in vertebrate development including the development of the visual system. Most studies have focused on examining functions of classical type I cadherins (e.g., cadherin-2) in visual system development. There is little information on the function of classical type II cadherins (e.g., cadherin-6) in the development of the vertebrate visual system. To gain insight into cadherin-6 role in the formation of the retina, we analyzed differentiation of retinal ganglion cells (RGCs), amacrine cells, and photoreceptors in zebrafish embryos injected with cadherin-6 specific antisense morpholino oligonucleotides. Differentiation of the retinal neurons in cadherin-6 knockdown embryos (cdh6 morphants) was analyzed using multiple markers. We found that expression of transcription factors important for retinal development was greatly reduced, and expression of Notch-Delta genes and proneural gene ath5 was altered in the cdh6 morphant retina. The retinal lamination was present in the morphants, although the morphant eyes were significantly smaller than control embryos due mainly to decreased cell proliferation. Differentiation of the RGCs, amacrine cells, and photoreceptors was severely disrupted in the cdh6 morphants due to a significant delay in neural differentiation. Our results suggest that cadherin-6 plays an important role in the normal formation of the zebrafish retina. (c) 2008 Wiley Periodicals, Inc. Develop Neurobiol, 2008.

Cadherin-4 plays a role in the development of zebrafish cranial ganglia and lateral line system

We previously reported that cadherin-4 (also called R-cadherin) was expressed by the majority of the developing zebrafish cranial and lateral line ganglia. Cadherin-4 (Cdh4) function in the formation of these structures in zebrafish was studied using morpholino antisense technology. Differentiation of the cranial and lateral line ganglia and lateral line nerve and neuromasts of the cdh4 morphants was analyzed using multiple neural markers. We found that a subset of the morphant cranial and lateral line ganglia were disorganized, smaller, with reduced staining, and/or with altered shape compared to control embryos. Increased cell death in the morphant ganglia likely contributed to these defects. Moreover, cdh4 morphants had shorter lateral line nerves and a reduced number of neuromasts, which was likely caused by disrupted migration of the lateral line primordia. These results indicate that Cdh4 plays a role in the normal formation of the zebrafish lateral line system and a subset of the cranial ganglia.

Loss- and gain-of-function analyses reveal targets of Pax6 in the developing mouse telencephalon

Appropriate neurogenesis and patterning of the forebrain requires the transcription factor Pax6, yet it is largely unknown how Pax6 exerts its effects at the molecular level. To characterize Pax6-mediated regulation of gene expression during murine forebrain neurogenesis, we performed microarray analysis with tissue from the dorsal Pax6-dependent telencephalon and the ventral Pax6-negative telencephalon at the onset of neurogenesis (E12) and at mid-neurogenesis (E15) in wild-type and Pax6-deficient mutant littermates. In the Pax6-deficient cortex the expression levels of various transcription factors involved in neurogenesis (like Satb2, Nfia, AP-2gamma, NeuroD6, Ngn2, Tbr2, Bhlhb5) and the retinoic acid signalling molecule Rlbp1 were reduced. Regulation by Pax6 could be confirmed upon electroporation of a Pax6- and a dominant-negative Pax6-containing vector into embryonic cortex. Taken together, our data reveal novel insights into the molecular pathways regulated by Pax6 during cortical neurogenesis. Most intriguingly, this analysis revealed time- and region-specific differences in Pax6-mediated transcription, explaining the specific function of Pax6 at early and later stages of neurogenesis.

Cloning and expression analysis of cadherin7 in the central nervous system of the embryonic zebrafish

Cadherin cell adhesion molecules exhibit unique expression patterns during development of the vertebrate central nervous system. In this study, we obtained a full-length cDNA of a novel zebrafish cadherin using reverse transcriptase-polymerase chain reaction (RT-PCR) and 5' and 3' rapid amplification of cDNA ends (RACE). The deduced amino acid sequence of this molecule is most similar to the published amino acid sequences of chicken and mammalian cadherin7 (Cdh7), a member of the type II cadherin subfamily. cadherin7 message (cdh7) expression in embryonic zebrafish was studied using in situ hybridization and RT-PCR methods. cdh7 expression begins at about 12h postfertilization (hpf) in a small patch in the anterior neural keel, and along the midline of the posterior neural keel. By 24 hpf, cdh7 expression in the brain shows a distinct segmental pattern that reflects the neuromeric organization of the brain, while its expression domain in the spinal cord is continuous, but confined to the middle region of the spinal cord. As development proceeds, cdh7 expression is detected in more regions of the brain, including the major visual structures in the fore- and midbrains, while its expression domain in the hindbrain becomes more restricted, and its expression in the spinal cord becomes undetectable. cdh7 expression becomes reduced in 3-day old embryos. Our results show that cdh7 expression in the zebrafish developing central nervous system is both spatially and temporally regulated.

Expression of cadherin10, a type II classic cadherin gene, in the nervous system of the embryonic zebrafish

Cadherins are cell surface adhesion molecules that play important roles in development of tissues and organs. In this study, we analyzed expression pattern of cadherin10, a member of the type II classic cadherin subfamily, in the embryonic zebrafish using in situ hybridization methods. cadherin10 message (cdh10) is first and transiently expressed by the notochord. In the developing nervous system, cdh10 was first detected in a subset of the cranial ganglia, then in restricted brain regions and neural retina. As development proceeds, cdh10 expression domain and/or expression levels increased in the embryonic nervous system. Our results show that cdh10 expression in the zebrafish developing nervous system is both spatially and temporally regulated.

Zebrafish R-cadherin (Cdh4) controls visual system development and differentiation

In zebrafish, R-cadherin (cadherin-4 or Cdh4) is expressed in the retina and in retinorecipient brain regions, suggesting that Cdh4 functions during visual system development. Cdh4 function was examined during retinogenesis and retinal axon outgrowth using antisense morpholino oligonucleotides and mutant Cdh4 construct expression. In knockdowns, Cdh4 was reduced or absent, eyes were small, and retinae lacked discrete laminae. Increased cell death produced the small eye phenotype. Zn5-, Pax6-, and zpr-1-positive cells were reduced or absent in knockdown retinas but, when present, were in the correct laminae. Cdh4 knockdowns had sparse or absent retinal ganglion cell axons. When present, axons projected contralaterally but lacked fine branching and failed to reach the tectum or arborize the entire tectum. Mutant Cdh4 construct expression during retinal ganglion cell differentiation reduced or ablated neurite formation. Cdh4 is necessary for neural retina survival and differentiation, and required for normal retinotectal projection formation and tectal arborization.

Cadherin-2 function in the cranial ganglia and lateral line system of developing zebrafish

Cadherins are cell surface molecules that mediate cell-cell adhesion through homophilic interactions. Cadherin-2 (also called N-cadherin), a member of classic cadherin subfamily, has been shown to play important roles in development of a variety of tissues and organs, including the nervous system. We recently reported that cadherin-2 was strongly expressed by the majority of cranial ganglia and lateral line system of developing zebrafish. To gain insight into cadherin-2 role in the formation of these structures, we have used several markers to analyze zebrafish embryos injected with a specific cadherin-2 antisense morpholino oligonucleotide (cdh2MO). We find that development of several cranial ganglia, including the trigeminal, facial, and vagal ganglia, and the lateral line ganglia and neuromasts of the cdh2MO-injected embryos are severely disrupted. These phenotypes were confirmed by analyzing a cadherin-2 mutant, glass onion. Our results suggest that cadherin-2 function is crucial for the normal formation of the zebrafish lateral line system and a subset of cranial ganglia.

Differential expression of cadherins in the developing and adult zebrafish olfactory system

Cadherins are cell adhesion molecules that play important roles in development of a variety of tissues and maintenance of adult structures. Although cadherin expression has been studied in detail in the central nervous system of several vertebrate species, little is known of their distribution in the developing and adult olfactory structures, and there is no published report, to our knowledge, of cadherin expression in fish olfactory system. In this study, we examined expression patterns of three cadherins, cadherin-1 (E-cadherin), cadherin-2 (N-cadherin), and cadherin-4 (R-cadherin), in the olfactory system of developing and adult zebrafish by using both in situ hybridization and immunocytochemical methods. Cadherin-1 is detected in the newly formed olfactory placode, and its expression is maintained in the developing and adult olfactory epithelium and olfactory nerve. Cadherin-2 is expressed in the olfactory epithelium, olfactory nerve, and olfactory bulb of the embryonic and larval zebrafish, and its expression is reduced in the adult olfactory system. In contrast to the cadherin-1 and cadherin-2 expression, cadherin-4 is not found in the olfactory epithelium, but it is detected in the larval and adult olfactory bulb, in the olfactory tract, and its targets in the telencephalon. We hypothesize that the differential expression of these three cadherins in the developing zebrafish major olfactory structures reflects functionally different roles in the development of the vertebrate olfactory system.

Cadherin-1, -2, and -11 expression and cadherin-2 function in the pectoral limb bud and fin of the developing zebrafish

Cadherins are cell adhesion molecules that play important roles in development of a variety of organs, including the vertebrate limb. In this study, we analyze cadherin expression patterns in the embryonic zebrafish pectoral limb buds and larval pectoral fins by using both in situ hybridization and immunocytochemical methods. cadherin-1 is detected in the epidermis of the embryonic limb buds and the larval pectoral fins. Cadherin-2 is expressed in the pectoral limb bud mesenchyme and chondrogenic condensation. As development proceeds, cadherin-2 expression is detected in newly differentiated pectoral fin endoskeleton, but its expression is greatly down-regulated in the fin endoskeleton of larval zebrafish. cadherin-11 is found in the basal region of the embryonic limb buds and in the proximal endoskeleton of the larval pectoral fins. Interfering with cadherin-2 function using two specific antisense morpholino oligonucleotides disrupts formation of the chondrogenic condensation/endoskeleton, suggesting that cadherin-2 is crucial for the normal development of the zebrafish pectoral fins.

Cadherin-2 and cadherin-4 in developing, adult and regenerating zebrafish cerebellum

Cadherins are cell adhesion molecules that regulate development of a variety of tissues and maintenance of adult structures. In this study, we examined expression of two zebrafish classical cadherins, cadherin-2 and cadherin-4, in the cerebellum of developing, normal adult, and regenerating adult zebrafish using in situ hybridization and immunohistochemical methods. Cadherin-2 was widely expressed by the cerebellum of embryonic (24-50-h post fertilization) and larval zebrafish (3-14 days). Cadherin-2 expression became much reduced in the adult cerebellum, but it was greatly up-regulated in the regenerating adult cerebellum. Cadherin-4 was not detected in the embryonic cerebellum, but it was expressed in the Purkinje cells of the larval and adult cerebellum. To gain insight into cadherin-2 role in the formation of the cerebellum, we analyzed embryos injected with a specific cadherin-2 antisense morpholino oligonucleotide (cdh2MO1), and found that the cerebellar development of the cdh2MO1-injected embryos was severely disrupted. This phenotype was confirmed by examining a cadherin-2 mutant, glass onion. Our results suggest that cadherins are crucial for the normal development of the zebrafish cerebellum, and they may also be involved in the regeneration of injured fish cerebellum.

R-cadherin is a Pax6-regulated, growth-promoting cue for pioneer axons

The transcription factor Pax6 has been implicated in two processes that may be related in brain development: establishment of regional cell adhesion properties and axon guidance. In Pax6 mutant mouse embryos, forebrain pioneer axons make pathfinding errors. These errors occur in a region of the ventral thalamus in which the expression of the cell adhesion molecule R-cadherin (Cdh4) is lost in Pax6 mutants. In vitro, an R-cadherin substrate promoted pioneer axon outgrowth. Furthermore, pioneer axon outgrowth was rescued in vivo by selective replacement of R-cadherin by electroporation into cultured Pax6 mutant embryos. Thus, these studies implicate Pax6 as an early brain patterning gene that establishes regional adhesive codes to guide pioneer axons.

Cadherin-1, -2 and -4 expression in the cranial ganglia and lateral line system of developing zebrafish

Cadherins are cell adhesion molecules that play important roles in development of a variety of tissues and organs including the nervous system. In this study we analyzed expression patterns of three zebrafish classical (type I) cadherins (cadherin-1, -2, and -4) in the embryonic zebrafish cranial ganglia and lateral line system using in situ hybridization and immunohistochemical methods. All three cadherins exhibit distinct spatiotemporal patterns of expression during cranial ganglia and lateral line system development. cadherin-1 message was detected in the trigeminal and facial ganglia, in the lateral line ganglia, and in most of neuromasts in the lateral lines. Cadherin-2 mRNA and protein were expressed by the majority of the cranial ganglia and lateral line system. Both cadherins were found in embryos younger than 24 hours post fertilization as well as in 2-3-day old embryos and larvae. In contrast, cadherin-4 mRNA and protein expression was detected in embryos older than 30 hours post fertilization and limited to the trigeminal, statoacoustic, and vagal cranial ganglia, and the lateral line ganglia of older embryos and larvae.

Cadherin expression in the inner ear of developing zebrafish

Cadherins are cell adhesion molecules that have been implicated in development of a variety of organs including the ear. In this study we analyzed expression patterns of three zebrafish cadherins (Cadherin-2, -4, and -11) in the embryonic and larval zebrafish inner ear using both in situ hybridization and immunocytochemical methods. All three Cadherins exhibit distinct spatiotemporal patterns of expression during otic vesicle morphogenesis. Cadherin-2 and Cadherin-4 proteins and their respective mRNAs were detected mainly in the sensory patches and the statoacoustic ganglion (SAg), respectively. In contrast, cadherin-11mRNA was widely expressed earlier in the otic placode, and later became restricted to a subset of cells in the inner ear, including hair cells.

N-cadherin is essential for retinal lamination in the zebrafish

N-cadherin is one of the major Ca(2+)-dependent cell adhesion proteins in the developing nervous system. Here, we analyze eye development in the zebrafish N-cadherin loss-of-function mutant parachute(paR2.10) (pac(paR2.10)). The zebrafish visual system is fully developed by the time pac(paR2.10) mutants show lethality at day 5. Already at 24 hr postfertilization (hpf), mutant retinal cells are more disorganized and more rounded than in wild-type. At later stages, mutant retinae display a severe lamination defect with rosette formation (mostly islands of plexiform layer tissue surrounded by inner nuclear layer or photoreceptor cells), even though all major classes of cell types appear to be present as determined by histology. Of interest, electron microscopy reveals that the islands of plexiform layer tissue contain a normal amount of synapses with normal morphology. Although mutant photoreceptor cells are sometimes deformed, all typical structural components are present, including the membranous discs for rhodopsin storage. The lens fibers of the pac(paR2.10) mutants develop completely normally, but in some cases, lens epithelial cells round up and become multilayered. We conclude that cell adhesion mediated by N-cadherin is of major importance for retinal lamination and involved in maintenance of the lens epithelial sheet, but is not essential for the formation of photoreceptor ultrastructure or for synaptogenesis.

Up-regulation of cadherin-2 and cadherin-4 in regenerating visual structures of adult zebrafish

Cadherins are homophilic cell adhesion molecules that control development of a variety of tissues and maintenance of adult structures. In this study, we examined expression of zebrafish cadherin-2 (Cdh2, N-cadherin) and cadherin-4 (Cdh4, R-cadherin) in the visual system of adult zebrafish after eye or optic nerve lesions using immunocytochemistry and immunoblotting. Both Cdh2 and Cdh4 immunoreactivities were specifically up-regulated in regenerating retina and/or the optic pathway. Furthermore, temporal expression patterns of these two cadherins were distinct during the regeneration of the injured tissues. Cadherins have been shown to regulate axonal outgrowth in the developing nervous system, but this is the first report, to our knowledge, of increased cadherin expression associated with axonal regeneration in the vertebrate central nervous system. Our results suggest that both Cdh2 and Cdh4 may be important for regeneration of injured retinal ganglion cell axons.

parachute/n-cadherinis required for morphogenesis and maintained integrity of the zebrafish neural tube

N-cadherin (Ncad) is a classical cadherin that is implicated in several aspects of vertebrate embryonic development, including somitogenesis, heart morphogenesis, neural tube formation and establishment of left-right asymmetry. However, genetic in vivo analyses of its role during neural development have been rather limited. We report the isolation and characterization of the zebrafish parachute (pac) mutations. By mapping and candidate gene analysis, we demonstrate that pac corresponds to a zebrafish n-cadherin (ncad) homolog. Three mutant alleles were sequenced and each is likely to encode a non-functional Ncad protein. All result in a similar neural tube phenotype that is most prominent in the midbrain, hindbrain and the posterior spinal cord. Neuroectodermal cell adhesion is altered, and convergent cell movements during neurulation are severely compromised. In addition, many neurons become progressively displaced along the dorsoventral and the anteroposterior axes. At the cellular level, loss of Ncad affects β-catenin stabilization/localization and causes mispositioned and increased mitoses in the dorsal midbrain and hindbrain, a phenotype later correlated with enhanced apoptosis and the appearance of ectopic neurons in these areas. Our results thus highlight novel and crucial in vivo roles for Ncad in the control of cell convergence, maintenance of neuronal positioning and dorsal cell proliferation during vertebrate neural tube development.

Distinct and cooperative roles for Nodal and Hedgehog signals during hypothalamic development

Despite its evolutionary conservation and functional importance, little is known of the signaling pathways that underlie development of the hypothalamus. Although mutations affecting Nodal and Hedgehog signaling disrupt hypothalamic development, the time and site of action and the exact roles of these pathways remain very poorly understood. Unexpectedly, we show here that cell-autonomous reception of Nodal signals is neither required for the migration of hypothalamic precursors within the neural plate, nor for further development of the anterior-dorsal hypothalamus. Nodal signaling is, however, cell-autonomously required for establishment of the posterior-ventral hypothalamus. Conversely, Hedgehog signaling antagonizes the development of posterior-ventral hypothalamus, while promoting anterior-dorsal hypothalamic fates. Besides their distinct roles in the regionalization of the diencephalon, we reveal cooperation between Nodal and Hedgehog pathways in the maintenance of the anterior-dorsal hypothalamus. Finally we show that it is the prechordal plate and not the head endoderm that provides the early signals essential for establishment of the hypothalamus.