Differential regulation of the zebrafish orthopedia1 gene during fate determination of diencephalic neurons
- Luca Del Giacco†1Email author,
- Paolo Sordino†2,
- Anna Pistocchi1,
- Nikos Andreakis2,
- Raffaella Tarallo2,
- Barbara Di Benedetto1, 3 and
- Franco Cotelli1
© Del Giacco et al; licensee BioMed Central Ltd. 2006
Received: 04 August 2006
Accepted: 30 October 2006
Published: 30 October 2006
The homeodomain transcription factor Orthopedia (Otp) is essential in restricting the fate of multiple classes of secreting neurons in the neuroendocrine hypothalamus of vertebrates. However, there is little information on the intercellular factors that regulate Otp expression during development.
Here, we identified two otp orthologues in zebrafish (otp1 and otp2) and explored otp1 in the context of the morphogenetic pathways that specify neuroectodermal regions. During forebrain development, otp1 is expressed in anterior groups of diencephalic cells, positioned in the preoptic area (PO) (anterior alar plate) and the posterior tuberculum (PT) (posterior basal plate). The latter structure is characterized by Tyrosine Hydroxylase (TH)-positive cells, suggesting a role for otp1 in the lineage restriction of catecholaminergic (CA) neurons. Disruptions of Hedgehog (HH) and Fibroblast Growth Factor (FGF) pathways point to the ability of SHH protein to trigger otp1 expression in PO presumptive neuroblasts, with the attenuating effect of Dzip1 and FGF8. In addition, our data disclose otp1 as a determinant of CA neurons in the PT, where otp1 activity is strictly dependent on Nodal signaling and it is not responsive to SHH and FGF.
In this study, we pinpoint the evolutionary importance of otp1 transcription factor in cell states of the diencephalon anlage and early neuronal progenitors. Furthermore, our data indicate that morphogenetic mechanisms differentially regulate otp1 expression in alar and basal plates.
The neurosecretory system controls a wide variety of behavioural processes through synthesis and release of different neurotransmitters in the peripheral and central nervous systems [1, 2]. One of the key integrating centers of this organization is the hypothalamus, located in the ventral sector of the diencephalon. However, insights in the development of endocrine neurons has been gained in the midbrain, due to implications with mental and neurological phenotypes that involve growth, reproduction and general homeostasis, and have clinical relevance in degenerative and psychiatric disorders of embryonic origin (e.g. Parkinson's disease and addiction) [3–8]. A wealth of genetic data shows that the combinatorial codes of early instructional cues from Hedgehog (HH), Fibroblast Growth Factor (FGF), and Transforming Growth Factor (TGF-β) extracellular signals mediate differentiation of dopaminergic (DA) neurons in the midbrain [6, 9–15]. Knowledge of the mechanisms of action connecting the prosencephalic signaling pathways, the expression of specific transcription factors and the specification of neuronal individuality during the development of the hypothalamus remains largely unclear .
Because of its central contribution to hypothalamic phenotypes during ontogenesis, the homeobox-containing orthopedia (otp) gene may allow a meaningful understanding of signaling cascades [16, 17]. Homologs of otp have been identified in almost all Metazoa, pointing to a conservative and fundamental role in patterning and differentiation [18–28]. In mouse forebrain, Otp is expressed in anterior and posteroventral hypothalamus, retrochiasmatic and supraoptic/paraventricular areas . Otp knock-out mice embryos fail to properly differentiate anterior periventricular, paraventricular and supraoptic nuclei, responsible for secretion of somatostatin, arginine vasopressin, oxytocin, corticotropin-releasing hormone and thyrotropin-releasing hormones [16, 17]. In the diencephalon, Otp acts in parallel with the bHLH-PAS domain factor Sim1 and its dimerizing partner ARNT2, and triggers expression of sim2 and brn2, a POU domain factor [16, 17, 29–32]. It has been shown that otp is a direct target of Brachyury and Spdeadringer transcription factors in vertebrates and invertebrates [33, 34]. Recently, several transcription factors have been proposed as candidates for upstream regulatory interactions with echinoderm otp . Some cases of holoprosencephaly, a congenital disorder with deficiencies in specific groups of CA neurons, are characterized by disruption of sonic hedgehog (shh) and secondary down-regulation of otp, brn2 and sim1, evoking an interaction between otp and shh . Despite all these evidences, the mechanisms of regulation of the Otp gene itself remain largely obscure.
We took advantage of the zebrafish to investigate the in vivo functions of shh, fgf8, and ndr2 signaling pathways on otp1 expression by means of morpholino-, mRNA- and mutant-based methodologies. We provide evidence that SHH regulates otp1 neuronal differentiation in the rostral preoptic area (PO) (anterior alar diencephalon), through the antagonistic interaction with FGF and Dzip1, a zinc-finger/coiled-coil domain protein [37–40]. Moreover, we also demonstrate that SHH- and FGF8-independent, Ndr2-dependent transcription of otp1 in the posterior tuberculum (PT) (posterior basal diencephalon) is necessary to trigger CA neuronal fates. In evolutionary perspectives, functional association between otp1 gene expression and patterning of the diencephalon embodies a primitive condition in the rise of neuronal complexity of vertebrates.
Isolation and structural analysis of the zebrafish orthopedia genes
otp1 mRNA is characterized by an AU-rich element (ARE) of ~150 nt in length, that lies within the 3' UTR and is located ~200 bases downstream of the stop codon. This ARE contains five copies of the AUUUA sequence, one of which overlaps with the UUAUUUAUU nonamer, that represent the minimal AU-rich motif that is able to destabilize and decay messengers .
In silico search of the zebrafish genome using otp1 cDNA as bait provided a second otp gene, named otp2, located on chromosome 2. otp2 ORF is 999 bp long, corresponding to a putative protein of 332 aa. According to mRNA sequences submitted to the NCBI database (accession nos. XM_678094 and XM_701814), the otp2 RNA is alternatively spliced and encodes two discrete forms of mature mRNA. Genomic organization of otp2 is identical to otp1, including presence and position of introns and the additional donor site (data not shown).
Molecular evolution of OTP proteins
Zebrafish Otp1 and Otp2 proteins clearly belong to the Otp protein family of transcription factors, and they are likely co-orthologs arisen from the ancient genome duplication event that occurred early in the phylogeny of ray-finned fish [42–45]. The identity of zebrafish Otp homeodomains with Mus musculus, Dugesia japonica and Drosophila melanogaster is 100%, 97% and 95%, while full protein identity with mouse is 78% for Otp1 and 81% for Otp2. In addition, they appear to share the Otp-specific transactivating regions, as well as two additional domains near the terminator codon  (Fig. 1A). The 13 aa stretch resulting from alternative splicing (see above) of Otp proteins is characteristic of the fish lineage (see above). In fact, mouse and human Otp genes lack the additional donor site in the first intron responsible for the alternative splicing of the long protein form. For this reason, the short form of the zebrafish Otp proteins shows higher levels of similarity with mammalian homologs (data not shown).
Spatiotemporal expression of otp1 during embryogenesis
We evaluated the presence of the two mRNA forms at different stages by means of RT-PCR using a pair of primers spanning the cDNA region involved in alternative splicing. Both transcripts are already present at the oocyte stage, proving the maternal origin of the transcript, and remain detectable throughout the analyzed stages (Fig. 3). We also report the expression of otp1 in the adult fish, showing for the first time the activity of the gene in the adult brain and in non-neuroectodermal territories (testis). The RT-PCR approach does not recognize trends or relative differences between mRNA forms and developmental stages, supporting the view that splicing of otp1 RNA is not subject to strict regulatory mechanisms (Fig. 3).
At 24 hpf, transversal histological sections show that the anterior diencephalic domain belongs to the non-germinal (mantler) layer (data not shown). Coupling otp1 WISH with acetylated α-tubulin immunostaining, a marker of major axonal traits in the embryonic CNS, proves that the anterior otp1 cluster is enclosed in the preoptic area (PO) (alar plate) by the tracts of the anterior and postoptic commissures, and it extends toward the trait of the supraoptic commissure (Fig. 6C,D). Due to this dorsal extension in the supraoptic region, we addressed otp1 expression at the telencephalic-diencephalic boundary using masterblind (mbl) mutants. In mbl embryos, several forebrain defects are observed, such as absence of optic vesicles, olfactory placodes, clusters of primary telencephalic neurons, anterior and postoptic commissures [52–54]. Despite profound alterations in the telencephalic structures, no significant changes are detected in the PO cluster of otp1 expressing cells in comparison with wt embryos at 20-s stage, suggesting that otp1 expression does not occur in the telencephalon (Fig. 6E,F).
Combinatorial regulation of otp1 by Hedgehog, Fibroblast Growth Factor and Nodal signaling
Then, otp1 expression was studied in sonic you (syu) and iguana (igu) mutant embryos, zebrafish strains characterized by functionally null alleles of, respectively, shh  and the dzip1 gene, a negative regulator of HH signaling gradients [39, 40]. In 28 hpf syu mutants, the PO otp1 pattern is drastically reduced in size (Fig. 8L,M,O,P; Fig. 9A,C). On the contrary, loss of Dzip1 functions in igu embryos induces a phenotype characterized by supernumerary PO otp1-positive cells (Fig. 8L,N,O,Q). In both mutants, no obvious alterations of otp1 expression are observed in the PT (Fig. 8L–Q).
Direct interactions between FGF and HH signaling were approached by WISH analysis of otp1 transcripts in ace; syu double mutant embryos, in which PO or PT clusters of otp1 expression are not significantly altered (Fig. 9A,D).
DA neuronal progenitors and otp1 activity
Orthopedia (Otp) expression in vertebrate embryos is crucial for the correct determination of several cell lineages in the neuroendocrine hypothalamus [16, 17]. Indeed, Otp null-mice die soon after birth due to the developmental failure of the anterior periventricular, paraventricular, and supraoptic hypothalamic nuclei, and the resultant incapability to secrete important neuro-hormones [16, 17]. In zebrafish, a detailed characterization of the CA neurotransmitter pathway makes this organism a favourite model system to address the ontogeny of the vertebrate neurosecretory system [63, 64, 66–77]. To investigate otp regulation in vertebrate development, we have isolated two otp zebrafish homologues (otp1 and otp2) and investigated the in vivo functions of shh, fgf8, and Nodal-related 1 signaling pathways on otp1 expression by means of morpholino-, mRNA- and mutant-based methodologies. The conservative structure of the homeodomain region uncovered between the zebrafish Otp1, Otp2 proteins and the rest of our data set indicates clearly that the above proteins represent members of the Otp gene family of transcription factors. The percentages of identity between the zebrafish Otp homeodomains compared with the ones in Mus musculus, Dugesia japonica and Drosophila melanogaster are found to be variable, reflecting distinct evolutionary relationships as well as functional differences. Moreover, each of the Antp, Otp, and Otd/Otx gene clusters represent phylogenetically distinct clades characterised by elevated levels of sequence and structural homology. Our phylogenetic analysis of Otp proteins highly supports a Vertebrate group, while it highlights two unorthodox subclades, one consisting of Molluscs, Echinoderms and Hemichordates, and one grouping Platyhelminthes, Cnidarians and Arthropods and Tunicates.
otp1 gene expression domains are in close proximity to, or within, organizing centres that express signaling factors. In this report, we show that Nodal, SHH and FGF8 independently regulate otp1 expression during formation of neural stem cells in the zebrafish forebrain. Since results presented herein centred on functional effects occurring within the first 2 days of development, TH-immunolabeling did not evidence neurons in the PO during our assays, therefore impeding to connect otp1 activity with CA neurons in the anterodorsal hypothalamus. However, we report the first case of involvement of otp in the determination of a subgroup of ventral hypothalamic CA neurons.
HH and FGF8 signaling pathways modulate otp1 expression in the hypothalamic preoptic area but not in the posterior tuberculum
HH and FGF8 signaling pathways play key roles in the ventral CNS patterning [78, 79]. Our examination of syu, igu, and ace mutants clearly revealed the dependency of otp1 expression on shh and fgf8 at the level of the otp1 positive cluster located in the PO, where the two signaling molecules are synthesized. Absence of Shh protein determines a marked attenuation of the size of the otp1 cluster in the PO (alar plate) (Fig. 8L,M,O,P) [37–40]. This result suggests that only the prospective otp1-positive cells in the alar plate PO have acquired the competence (i.e. additional factors that modulate SHH signaling) to respond to the high SHH concentration from the ventral neural tube and then trigger otp1 expression. This is also supported by the evidence that the PO otp1 signal increased in igu mutants (Fig. 8L,N,O,Q), which are deficient in shh negative regulation [39, 40]. Inhibition of the SHH pattern via synthetic mRNA microinjection causes profound alterations in the spatial organization of the PO otp1 cluster, but does not induce relevant changes in the number of the otp1-expressing cells, suggesting that otp1 responsiveness in the alar plate is dependent on additional positive regulators rather than negative factors hindering SHH signal propagation.
Interestingly, ectopic otp1 expression after shh microinjection was selectively observed in the optic placodes (Fig. 8I) and in one fixed pair of cells in the roof of the dorsal thalamus (Fig. 8H,J), suggestive of a predetermination to respond to the shh signal.
The otp1 signal in the PO appeared significantly expanded in the ace mutant (Fig. 9A,B), indicative of a negative role for this signaling factor in the control of PO otp1 expression. To confirm this observation we altered the endogenous level of fgf8 by synthetic mRNA microinjection: excess of FGF concentration reduces the size of the otp1-positive PO cluster. As expected from a potential negative regulator, fgf8 microinjection does not induce ectopic expression of otp1. No major consequences on otp1 expression in the PT are noticed in embryos altered in FGF and SHH signalings (Fig. 8M,P; Fig. 9B,C; data not shown), supporting the conclusion that fgf8 and shh are not required for otp1-based neuronal specification in the posterior diencephalon.
Nodal affects otp1 expression in the preoptic area and is necessary for otp1 activation in the posterior tuberculum, where otp1 is required for the correct development of catecholaminergic neurons
The ndr2 mutant cyclops (cyc) interferes with the Nodal pathway causing massive defects in the rostral brain [55–58]. Analysis of otp1 expression in cyc mutant embryos with intermediate phenotypes (reduction of the ventral diencephalon and partial fusion of the eyes) shows midline fusion of PO-specific otp1 expression; moreover, we noticed that the domain is decreased in size (compare Fig. 7A,B with Fig. 5G,H). This phenotype could be in part explained by the partial physical deficiency of the PO territory, but also by the repression of shh transcription in cyc mutants , such that the reduction of otp1-expressing cells after the disruption of Nodal signaling is determined by intrinsic loss of shh (as we observed in syu mutants) (Fig. 8M,P), pointing to the PO otp1 expression regulation mediated by ndr2. We did not detect any otp1-expressing cells in the PT of nodal (cyc) mutant embryos. This phenotype is not caused by shh deficiency since otp1 expression in the posterior basal plate of the hypothalamus is not modulated by HH signaling, but could be determined by the absence of the prosencephalic ventral structures. This issue has been addressed overexpressing ndr2 and analyzing the effects on otp1 transcription in the PT area. Increase of Ndr2 function is shown to determine the expansion of the PT-specific otp1 domain (Fig. 7C–F), in accordance with the opposite effect in the PT of cyc embryos with intermediate phenotypes. Taken together, these results are indicative of Nodal signaling as a positive regulator of otp1 expression in the posterior basal plate of the hypothalamus. Furthermore, Holzschuh and colleagues reported the loss of diencephalic DA neurons also in MZsur, a Nodal mutant in which the PT correctly develops, therefore suggesting the direct involvement of Nodal in the differentiation of the DA neuron in the posterior tuberculum [81, 82].
In this context, our findings allow to propose that the Nodal contribution to the development of the CA neurons in PT might be mediated by otp1. Because both otp1- and TH-positive neurons occur in the PT, we co-labelled this area. Interestingly, otp1 and TH partially overlap in a small clade of neuronal precursors (Fig. 10A,B). In these cells, otp1 activation precedes of several hours that of dopamine transporter and tyrosine hydroxylase genes . In addition, embryos of a zebrafish mutant that displays a reduced number of hypothalamic DA neurons (motionless, mot)  show less otp1-positive neurons (Fig. 10C,D). The hypothesis of a subset of CA neurons requiring otp1 action for their differentiation was finally tested by microinjection of otp1-specific ATG-targeted morpholino oligonucleotides in wild type embryos. A mot mutant phenocopy was generated at the PT level, with the repression of TH-positive cell differentiation (Fig. 10E,F). Altogether, these data confirm the otp1 gene as one functional milestone towards the correct development of the CA system.
Wild type zebrafish of the AB strain were maintained at 28°C on a 14 h light/10 h dark cycle under standard procedures . Embryos were collected by natural spawning, staged according to Kimmel and co-workers  and raised at 28°C in fish water (Instant Ocean, 0,1% Methylene Blue) in Petri dishes . We express the embryonic ages in somites (s) and hours post fertilization (hpf). The following mutant alleles were used: aceti282a , cycb16 , mbltm213 , motm807 [39, 40], syutbx392  and iguts294e [39, 40].
The zebrafish homologue of orthopedia (otp1) has been isolated by means of PCR from a 16–40 hour old zebrafish embryo cDNA library using degenerate primers ort1_fw (5'-CCNGCNCAGCTSAACGA-3') and ort2_rv (5'-CKYTTYTTCCAYTTNGC-3'), corresponding to PAQLNE and AKWKKR regions of the mouse Otp homeodomain, respectively. The first round of PCR has been carried out using the ort2_rv degenerate primer with the T3 library-vector specific primer. An aliquot of this reaction has been used as template with the ort1_fw and ort2_rv primers. The 150 bp cDNA band obtained encoded a partial homeodomain identical to the corresponding residues of the mouse Otp protein. Gene- and vector-specific primers were used on the same library to isolate the 5' and 3' ends of the otp1 mRNA.
A total of 47 sequences, gathered from several EST and genomic databases, were aligned using the ClustalW algorithm  as implemented in the Bioedit software v4.7.8  under a variety of gap penalties assigned. Species names, abbreviations, gene names and accession numbers of the sequences are as follows: Bt, Bos taurus, Otp XM_604218; Gg, Gallus gallus, Otp AY651764; Cf, Canis familiaris, Otp XM_546055, Otx2 XM_547830; Pt, Pan troglodytes, XM_517691; Hs, Homo sapiens, Otp NM_032109, Otx2 NM021728; Mm, Mus musculus, Otp CAA71439; Rn, Rattus norvegicus, Otp XP_215445; Dr, Danio rerio, Otp1 AF071496, Otx1 NM131250, Otx5 NM_181331; Dm, Drosophila melanogaster, Otp NM_206187; Pd, Platynereis dumerilii, Otx AJ278856; Sp, Strongylocentrotus purpuratus, Otp XM_779506, Otx NM_214588; At, Achaearanea tepidariorum, Otd AB096074; Tc, Tribolium castaneum, Otd1 NM_001039424, Otd2 NM_001039437; Ci, Ciona intestinalis, Otp AB210618; Ef, Euscorpius flavicaudis, Otd AY738138; Xt, Xenopus tropicalis, Otx1 NM_203885; Sk, Saccoglossus kowalevskii, Otp AAP79292; Pv, Patella vulgata, Otp AF440099; Lv, Lytechinus variegatus, Otp AAR17090; Pl, Paracentrotus lividus, Otp O76971; Ht, Heliocidaris tuberculata, Otp AAS00592; He, Heliocidaris erythrogramma, Otp AAS00591; Ds, Drosophila subobscura, Antp X60995; Dv, Drosophila virilis, Antp AY333070; Am, Apis mellifera, Antp NM_001011571; Sc, Sacculina carcini, Antp AF393443; Pc, Podocoryne carnea, Tbx4/5 AJ581006.
While Dugesia japonica (Dj) Otp sequence was obtained directly from Umesono and co-workers , Hydra magnipapillata (Hm), Takifugu rubripes (Tr) and Tetraodon nigroviridis (Tn) Otp sequences were generated in the course of this study and are available from the authors. The alignment was manually refined and used as the basis for all tree reconstruction methods. Maximum parsimony (MP) analysis was carried out in PAUP* 4.0b10 (Windows version)  using the heuristic search option, 100 random sequence additions and tree bisection-reconnection (TBR) branch swapping. Maximum likelihood (ML) analysis was performed using the quartet puzzling method  implemented in the TREE-PUZZLE package (version 5.0)  using the heuristic search option and ten random sequence additions. A Bayesian phylogeny was inferred, and posterior probabilities of individual clades were calculated using a variant of the Markov chain Monte Carlo algorithm as employed in MrBayes v3.1.2 . Four Markov chains (three heated, one cold) were run for 200000 generations using random starting trees and the same model employed in branch length estimates, with trees saved every 100 generations. Bootstrap support for individual nodes  was calculated on 1000 replicates using the same methods, options and constraints as in the tree-inferences, but removing identical sequences. Maximum Likelihood and Bayesian phylogenetic reconstructions were constrained with the most appropriate model for protein evolution that better fitted the data. Likelihood values for each of 64 candidate models of protein evolution with the best-fitting parameters (gamma distribution, proportion of invariable sites, character frequencies) were computed in the software ProtTest v1.3 . The best aminoacid replacement model for the data was finally calculated using an Akaike (AIC), a second-order Akaike (AICc) and a Bayesian (BIC) information criteria.
Total RNAs from 15 samples corresponding to 10 different developmental stage embryos (1–2 cells, 30% epiboly, 50% epiboly, 80% epiboly, 1–2 somites, 8 somites, 10 somites, 15–20 somites, 24 hpf, and 48 hpf) and 3 adult organs (oocyte, brain, testis) were purified, DNase treated, and reverse-transcribed. The cDNAs obtained were tested for the presence of otp1 expression using the forward (zotpS: 5'-ATGCTCTCTCATGCCGACCT-3') and reverse (zotpRv: 5'-TCTGTTGGTTTTGCTGGCCG-3') primers spanning the cDNA region involved in alternative splicing. The products of the PCRs were loaded and resolved onto 2% agarose gels.
In situ hybridization and immunohistochemistry
WISH hybridization was carried out according to Thisse and co-workers  on embryos fixed for 2 h in 4% paraformaldehyde/phosphate buffered saline, then rinsed with PBS-Tween, dehydrated in 100% methanol and stored at -20°C until processed for WISH . Riboprobes were in vitro labelled with digoxigenin or fluorescein (Roche). For double WISH and antibody labelling, WISH was performed first, then embryos were exposed to rat anti-Tyrosine Hydroxilase (TH) (Chemicon) and mouse anti-acetylated α-tubulin (Sigma). Embryos incubated with anti-TH were treated with biotinylated secondary antibody (Vector Laboratories).
Synthetic capped shh, ndr2, and fgf8 mRNAs were injected repeatedly (n > 3) at concentrations of 400, 200, and 200 pg per embryo, respectively. Injections were carried out on 1- to 2-cell stage embryos. To repress otp1 mRNA translation, an ATG-targeting morpholino was designed (Gene Tools, LLC): 5'-CCAAGAGGTCGGCATGAGAGAGCAT-3'.
The authors wish to gratefully thank S. Wilson and S. Guo for providing mutant embryos, and A. Barth for plasmids. We are indebted with F. Aniello, M. Studer and S. Barabino for critically reading the manuscript. LDG thanks S. Duga and S. Diani-Moore for their priceless support, and U. Fascio (C.I.M.A.) for assistance at the confocal microscope. This work was supported by grants from "MIPAF V-VI Piano Triennale" (FC), "CARIPLO N.O.B.E.L." (FC), "EC" (PS, NA), "MIUR" (PS, RT), and "EMBO" (PS).
FC dedicates this paper to the illuminating memory of Alberto Monroy, twenty years after his death.
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