gbx2 Homeobox gene is required for the maintenance of the isthmic region in the zebrafish embryonic brain.

We isolated cDNA clones for the zebrafish gbx2 gene, which is implicated in the establishment of the midbrain-hindbrain boundary (MHB) in other vertebrates. Spatially localized expression of gbx2 was observed at the MHB from 90% epiboly through to the hatching stage. Comparisons with the expression of otx2, wnt1, and krox20 showed that gbx2 is expressed in the anterior hindbrain. Ectopic expression of gbx2 by mRNA injection caused cyclopia or truncation of the fore- and midbrain and severely affected isthmic and cerebellar structures, while hindbrain formation was not significantly affected. At the molecular level, gbx2 suppressed the expression of otx2 in the fore/midbrain, six3 in the anterior forebrain, and MHB-specific genes such as eng2 and wnt1. In contrast, gbx2 did not down-regulate the expression of the hindbrain marker genes. Therefore, gbx2 specifically suppressed the formation of the entire fore/midbrain. Meanwhile, misexpression of otx2 suppressed the expression of gbx2 in the embryonic brain. Abrogation of gbx2 expression with an antisense morpholino oligonucleotide disrupted the midbrain/anterior hindbrain region, and these loss-of-function effects were rescued by activating the Gbx2 protein immediately after the end of gastrulation. Taken together, these results suggest that the zebrafish gbx2 gene is essential for the maintenance of MHB and/or the formation of the isthmic structure during somitogenesis, rather than for the MHB establishment during gastrulation. We also suggest that other factors, including gbx1, is required for the establishment of the MHB during gastrulation.     

Autotaxin is required for the cranial neural tube closure and establishment of the midbrain–hindbrain boundary during mouse development

Autotaxin (ATX) is a lysophospholipid‐generating exoenzyme expressed in embryonic and adult neural tissues. We previously showed that ATX is expressed in the neural organizing centers, anterior head process, and midbrain‐hindbrain boundary (MHB). To elucidate the role of ATX during neural development, here we examined the neural phenotypes of ATX‐deficient mice. Expression analysis of neural marker genes revealed that lateral expansion of the rostral forebrain is reduced and establishment of the MHB is compromised as early as the late headfold stage in ATX mutant embryos. Moreover, ATX mutant embryos fail to complete cranial neural tube closure. These results indicate that ATX is essential for cranial neurulation and MHB establishment. Developmental Dynamics 240:413–421, 2011. © 2011 Wiley‐Liss, Inc.     

The characterization of a zebrafish mid‐hindbrain mutant,mid‐hindbrain gone (mgo)

The vertebrate mid‐hindbrain boundary (MHB) is a crucial morphological structure required for patterning and neural differentiation of the midbrain and anterior hindbrain. We isolated a novel zebrafish mutant, MHB gone (mgo), that exhibited a defective MHB. Expression of engrailed3 in the prospective MHB was absent at the 1‐somite stage, suggesting that initiation of the isthmic organizer was disrupted in mgo mutants. Complementation test with mgo and noi, in which the pax2a gene is mutated, infer that the mgo mutant may represent a novel noi allele. However, pronephric, otic vesicle, and commissural axonal defects described in noi mutants were not associated with mgo mutants. Genetic mapping revealed that the mgo mutation is linked to the Pax2a locus, but no mutation was detected in pax2a exons or within intron‐exon boundaries. Based on these findings, we propose that the mgo mutation genetically interacts with pax2a required for the initiation of MHB formation. Developmental Dynamics 238:899–907, 2009. © 2009 Wiley‐Liss, Inc.     

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. Developmental Dynamics 240:1716–1726, 2011. © 2011 Wiley‐Liss, Inc.     

13‐cis‐retinoic acid alters neural crest cells expressing Krox‐20 and Pax‐2 in macaque embryos

This study investigates hindbrain and associated neural crest (NCC), otocyst, and pharyngeal arch development in monkey embryos following teratogenic exposure to 13‐cis‐retinoic acid (cRA). cRA was orally administered (5 mg/kg) to pregnant long‐tailed macaques (Macaca fascicularis) between gestational days (GD) 12 and 27. Embryos were surgically collected at desired stages during treatment, analyzed for external morphological changes, and processed for immunohistochemistry. Two transiently expressed nuclear proteins, Krox‐20 and Pax‐2, were used as markers for the target cellular and anatomical structures. Rhombomere (r) expression patterns of Pax‐2 (r4/r6) and Krox‐20 (r3/r5) were maintained after cRA treatment, but r4 and r5 were substantially reduced in size. In untreated embryos, Krox‐20 immunoreactive NCC derived from r5 migrated caudally around the developing otocyst to contribute to the third pharyngeal arch mesenchyme. In cRA‐treated embryos, a subpopulation of NCC rostral to the otocyst also showed Krox‐20 immunoreactivity, but there was a substantial reduction in Krox‐20 post‐otic NCC. Pax‐2 immunoreactive NCC migrating from r4 to the second pharyngeal arch were substantially reduced in numbers in treated embryos. Alteration in the otic anlage included delayed invagination, abnormal relationship with the adjacent hindbrain epithelium, and altered expression boundaries for Pax‐2. cRA‐associated changes in the pharyngeal arch region due to cRA included truncation of the distal portion of the first arch and reduction in the size of the second arch. These alterations in hindbrain, neural crest, otic anlage, and pharyngeal arch morphogenesis could contribute to some of the craniofacial malformations in the macaque fetus associated with exposure to cRA. Anat Rec 255:142–154, 1999. © 1999 Wiley‐Liss, Inc.     

Genetic disruption of CYP26B1 severely affects development of neural crest derived head structures,but does not compromise hindbrain patterning

Cyp26b1 encodes a cytochrome‐P450 enzyme that catabolizes retinoic acid (RA), a vitamin A derived signaling molecule. We have examined Cyp26b1^?/? mice and report that mutants exhibit numerous abnormalities in cranial neural crest cell derived tissues. At embryonic day (E) 18.5 Cyp26b1^?/? animals exhibit a truncated mandible, abnormal tooth buds, reduced ossification of calvaria, and are missing structures of the maxilla and nasal process. Some of these abnormalities may be due to defects in formation of Meckel's cartilage, which is truncated with an unfused distal region at E14.5 in mutant animals. Despite the severe malformations, we did not detect any abnormalities in rhombomere segmentation, or in patterning and migration of anterior hindbrain derived neural crest cells. Abnormal migration of neural crest cells toward the posterior branchial arches was observed, which may underlie defects in larynx and hyoid development. These data suggest different periods of sensitivity of anterior and posterior hindbrain neural crest derivatives to elevated levels of RA in the absence of CYP26B1. Developmental Dynamics 238:732–745, 2009. © 2009 Wiley‐Liss, Inc.     

Investigation of retinoic acid function during embryonic brain development using retinaldehyde‐rescued Rdh10 knockout mice

Background: Retinoic acid (RA) signaling controls patterning and neuronal differentiation within the hindbrain, but forebrain RA function remains controversial. RA is produced from metabolism of retinol to retinaldehyde by retinol dehydrogenase (RDH), followed by metabolism of retinaldehyde to RA by retinaldehyde dehydrogenase (RALDH). Previous studies on Raldh2?/? and Raldh3?/? mice demonstrated an RA requirement for γ‐aminobutyric acid (GABA)ergic and dopaminergic differentiation in forebrain basal ganglia, but no RA requirement was observed during early forebrain patterning or subsequent forebrain cortical expansion. However, other studies suggested that RA controls forebrain patterning, and analysis of ethylnitrosourea‐induced Rdh10 mutants suggested that RA synthesized in the meninges stimulates forebrain cortical expansion. Results: We generated Rdh10?/? mouse embryos that lack RA activity early in the head and later in the meninges. We observed defects in hindbrain patterning and eye RA signaling, but early forebrain patterning was unaffected. Retinaldehyde treatment of Rdh10?/? embryos from E7–E9 rescues a cranial skeletal defect, resulting in E14.5 embryos lacking meningeal RA activity but maintaining normal forebrain shape and cortical expansion. Conclusions: Rdh10?/? embryos demonstrate that RA controls hindbrain but not early forebrain patterning, while studies on retinaldehyde‐rescued Rdh10?/? embryos show that meningeal RA synthesis is unnecessary to stimulate forebrain cortical expansion. Developmental Dynamics 242:1056–1065, 2013. © 2013 Wiley Periodicals, Inc.     

Expression and retinoic acid regulation of the zebrafish nr2f orphan nuclear receptor genes

Background : The vertebrate nuclear receptor subfamily 2, group f (nr2f) genes encode orphan receptors that have the capacity to act as negative regulators of retinoic acid (RA) signaling. Results : We describe embryonic and larval expression of four of the six zebrafish nr2f genes, nr2f1a, nr2f1b, nr2f2, and nr2f5. These genes show highly regulated patterns of expression within the central nervous system, including in the developing hindbrain, as well as in the mesoderm and endoderm. We also investigated the role of RA and fibroblast growth factor (Fgf) signaling in regulating early nr2f gene expression. RA is not required for nr2f expression in the hindbrain; however, exogenous RA can repress this expression. Conversely, we find that RA positively regulates nr2f1a expression in trunk endoderm and mesoderm. Fgf signaling is not required for nr2f expression onset in the hindbrain; however, it may play a role in maintaining rhombomere‐specific expression. Conclusions : We report detailed expression analysis of four nr2f genes in all three germ layers. The onset of nr2f expression in the hindbrain does not require RA or Fgf signals. Our finding that RA positively regulates nr2f1a expression in the trunk supports the possibility that Nr2fs function in a negative feedback loop to modulate RA signaling in this region. Developmental Dynamics 241:1603–1615, 2012. © 2012 Wiley Periodicals, Inc.     

Increased β-catenin activity in the anterior neural plate induces ectopic mid-hindbrain characteristics

Background: The early telencephalon shares molecular features with the early mid‐hindbrain region. In particular, these two developing brain areas each have a signaling center that secretes FGFs and an adjacent one that secretes WNTs. WNTs and FGFs each play essential roles in regulating cell fates in both the telencephalon and mid‐hindbrain. Despite this similarity, telencephalic and mid‐hindbrain precursors express distinct genes and ultimately generate different cell types, tissue morphologies, and neural functions. Results: Here we show that genetically increasing the level of β‐catenin, a mediator of canonical WNT signaling, in the anterior neural plate causes a loss of telencephalic characteristics and a gain of mid‐hindbrain characteristics. Conclusion: These results, together with previous ones demonstrating that increased WNT signaling in the anterior neural plate increases FGF expression, suggest that the levels of WNT and FGF signaling regulate telencephalic versus mid‐hindbrain fates. Developmental Dynamics 241:242–246, 2012. © 2011 Wiley Periodicals, Inc.     

Gpr177/mouse Wntless is essential for Wnt‐mediated craniofacial and brain development

We have previously demonstrated that Gpr177, the mouse orthologue of Drosophila Wls/Evi/Srt, is required for establishment of the anterior–posterior axis. The Gpr177 null phenotype is highly reminiscent to the loss of Wnt3, the earliest abnormality among all Wnt knockouts in mice. The expression of Gpr177 in various cell types and tissues lead us to hypothesize that reciprocal regulation of Wnt and Gpr177 is essential for the Wnt‐dependent developmental and pathogenic processes. Here, we create a new mouse strain permitting conditional inactivation of Gpr177. The loss of Gpr177 in the Wnt1‐expressing cells causes mid/hindbrain and craniofacial defects which are far more severe than the Wnt1 knockout, but resemble the double knockout of Wnt1 and Wnt3a as well as β‐catenin deletion in the Wnt1‐expressing cells. Our findings demonstrate the importance of Gpr177 in Wnt1‐mediated development of the mouse embryo, suggesting an overlapping function of Wnt family members in the Wnt1‐expressing cells. Developmental Dynamics 240:365–371, 2011. © 2011 Wiley‐Liss, Inc.     

Zebrafish wnt3 is expressed in developing neural tissue

Wnt signaling regulates embryonic patterning and controls stem cell homeostasis, while aberrant Wnt activity is associated with disease. One Wnt family member, Wnt3, is required in mouse for specification of mesoderm, and later regulates neural patterning, apical ectodermal ridge formation, and hair growth. We have identified and performed preliminary characterization of the zebrafish wnt3 gene. wnt3 is expressed in the developing tailbud and neural tissue including the zona limitans intrathalamica (ZLI), optic tectum, midbrain‐hindbrain boundary, and dorsal hindbrain and spinal cord. Expression in these regions suggests that Wnt3 participates in processes such as forebrain compartmentalization and regulation of tectal wiring topography by retinal ganglia axons. Surprisingly, wnt3 expression is not detectable during mesoderm specification, making it unlikely that Wnt3 regulates this process in zebrafish. This lack of early expression should make it possible to study later Wnt3‐regulated patterning events, such as neural patterning, by knockdown studies in zebrafish. Developmental Dynamics 238:1768–1795, 2009. © 2009 Wiley‐Liss, Inc.     

Diversified expression patterns of autotaxin,a gene for phospholipid‐generating enzyme during mouse and chicken development

Autotaxin (ATX), or nucleotide pyrophosphatase‐phosphodiesterase 2, is a secreted lysophospholipase D that generates bioactive phospholipids that act on G protein–coupled receptors. Here we show the expression patterns of the ATX gene in mouse and chicken embryos. ATX has a dynamic spatial and temporal expression pattern in both species and the expression domains during neural development are quite distinct from each other. Murine ATX (mATX) is expressed immediately rostral to the midbrain‐hindbrain boundary, whereas chicken ATX (cATX) is expressed in the diencephalon and later in the parencephalon‐synencephalon boundary. In the neural tube, cATX is expressed in the alar plate in contrast to mATX in the floor plate. ATX is also expressed in the hindbrain and various organ primordia such as face anlagen and skin appendages of the mouse and chicken. These results suggest conserved and non‐conserved roles for ATX during neural development and organogenesis in these species. Developmental Dynamics 236:1134–1143, 2007. © 2007 Wiley‐Liss, Inc.     

Plasticity of neural crest–placode interaction in the developing visceral nervous system

The reciprocal relationship between rhombomere (r)‐derived cranial neural crest (NC) and epibranchial placodal cells derived from the adjacent branchial arch is critical for visceral motor and sensory gangliogenesis, respectively. However, it is unknown whether the positional match between these neurogenic precursors is hard‐wired along the anterior–posterior (A/P) axis. Here, we use the interaction between r4‐derived NC and epibranchial placode‐derived geniculate ganglion as a model to address this issue. In Hoxa1^?/?b1^?/? embryos, r2 NC compensates for the loss of r4 NC. Specifically, a population of r2 NC cells is redirected toward the geniculate ganglion, where they differentiate into postganglionic (motor) neurons. Reciprocally, the inward migration of the geniculate ganglion is associated with r2 NC. The ability of NC and placodal cells to, respectively, differentiate and migrate despite a positional mismatch along the A/P axis reflects the plasticity in the relationship between the two neurogenic precursors of the vertebrate head. Developmental Dynamics 240:1880–1888, 2011. © 2011 Wiley‐Liss, Inc.     

Conservation,expression, and knockdown of zebrafish plxnb2a and plxnb2b

In mice lacking Plexin B2, a receptor of the axon guidance molecules Semaphorin 4C and Semaphorin 4D, the closure of the neural tube and structural organization of the cerebellum are severely impaired. We cloned two Plexin B2 orthologs, plxnb2a and plxnb2b, in zebrafish, which is a widely used model for the development of the vertebrate central nervous system (CNS). The predicted proteins, Plexin B2a and Plexin B2b, contain all the conserved and functional domains of the plexin B‐subfamily. During embryonic development, plxnb2a is expressed, e.g., in pharyngeal arches while plxnb2b expression is more confined to neuronal structures like the cerebellum. However, both plxnb2a and plxnb2b are expressed at the midbrain–hindbrain boundary, in the otic vesicles, facial ganglia, and pectoral fins. Knockdown of both plxnb2a and plxnb2b simultaneously (>95% and 45%, respectively) resulted in normal CNS structure, axon guidance and swimming performance of the morphants. Developmental Dynamics 239:2722–2734, 2010. © 2010 Wiley‐Liss, Inc.     

Analysis of Cdh22 expression and function in the developing mouse brain

Classical cadherins are important cell adhesion molecules specifying and separating brain nuclei and developmental compartments. Cadherin‐22 (Cdh22) belongs to type II subfamily of classical cadherins, and is expressed at the midbrain‐hindbrain boundary during early embryogenesis. In Fgfr1 mutant mouse embryos, which have a disturbed midbrain‐hindbrain border, Cdh22 is down‐regulated. Here, we studied expression of Cdh22 in developing mouse brain in more detail and compared it to expression of related family members. This revealed both complementary and overlapping patterns of Cdh22, Cdh11, Cdh8, and Cdh6 expression in distinct regions of the forebrain and midbrain. We used a mutated allele of Cdh22 to study its function in brain development. Loss of Cdh22 caused reduced postnatal viability. Despite strong Cdh22 expression in the developing brain, we did not observe defects in compartmentalization or abnormalities in the midbrain and forebrain nuclei in Cdh22 mutants. This may be explained by functional redundancy between type II cadherins. Developmental Dynamics 240:1989–2001, 2011. © 2011 Wiley‐Liss, Inc.     

Loss of Lrp2 in zebrafish disrupts pronephric tubular clearance but not forebrain development

Low‐density lipoprotein receptor‐related protein 2 (LRP2) is a multifunctional cell surface receptor conserved from nematodes to humans. In mammals, it acts as regulator of sonic hedgehog and bone morphogenetic protein pathways in patterning of the embryonic forebrain and as a clearance receptor in the adult kidney. Little is known about activities of this LRP in other phyla. Here, we extend the functional elucidation of LRP2 to zebrafish as a model organism of receptor (dys)function. We demonstrate that expression of Lrp2 in embryonic and larval fish recapitulates the patterns seen in mammalian brain and kidney. Furthermore, we studied the consequence of receptor deficiencies in lrp2 and in lrp2b, a homologue unique to fish, using ENU mutagenesis or morpholino knockdown. While receptor‐deficient zebrafish suffer from overt renal resorption deficiency, their brain development proceeds normally, suggesting evolutionary conservation of receptor functions in pronephric duct clearance but not in patterning of the teleost forebrain. Developmental Dynamics 240:1567–1577, 2011. © 2011 Wiley‐Liss, Inc.