Victor A. Canfield

SLC24A5, a putative cation exchanger, affects pigmentation in zebrafish and humans.

Lighter variations of pigmentation in humans are associated with diminished number, size, and density of melanosomes, the pigmented organelles of melanocytes. Here we show that zebrafish golden mutants share these melanosomal changes and that golden encodes a putative cation exchanger slc24a5 (nckx5) that localizes to an intracellular membrane, likely the melanosome or its precursor. The human ortholog is highly similar in sequence and functional in zebrafish. The evolutionarily conserved ancestral allele of a human coding polymorphism predominates in African and East Asian populations. In contrast, the variant allele is nearly fixed in European populations, is associated with a substantial reduction in regional heterozygosity, and correlates with lighter skin pigmentation in admixed populations, suggesting a key role for the SLC24A5 gene in human pigmentation.

Evolution and expression of D2 and D3 dopamine receptor genes in zebrafish

We mined the zebrafish genomic sequence database and identified contigs containing segments of several dopamine receptor genes. By using a polymerase chain reaction amplification strategy, we generated full‐length cDNAs encoding a single dopamine D3 receptor and three distinct D2 receptor subtypes. Zebrafish dopamine receptor genes were mapped by using the T51 radiation hybrid panel. The D3 receptor gene (drd3) mapped to linkage group (LG) 24. The three D2 receptor genes were localized to LG 15 (drd2a), LG 16, (drd2b), and LG 5 (drd2c). With the exception of the drd2b gene, each of these map positions was syntenic with regions of human chromosomes containing orthologs of the zebrafish dopamine receptor genes. Whole‐mount in situ hybridization was used to investigate expression of the D2 and D3 receptor genes. Expression of the drd3 gene was first detected at mid‐somitogenesis and was particularly prominent in somites. Thereafter, the drd3 gene was expressed diffusely throughout the brain and spinal cord. The three D2 receptor genes were expressed throughout the central nervous system (CNS) in distinct but overlapping patterns. In early embryos, the drd2a gene was expressed exclusively in the epiphysis, whereas the drd2c gene was localized to the notochord. After 24 hpf, the drd2a, drd2b, and drd2c genes were differentially expressed throughout the CNS. The identification of dopamine receptor genes in zebrafish should allow us to use the power of zebrafish genetics to analyze the functional properties of this important class of neurotransmitter receptors. Developmental Dynamics 230:481–493, 2004.

D4 Dopamine receptor genes of zebrafish and effects of the antipsychotic clozapine on larval swimming behaviour

Zebrafish, a model developmental genetic organism, is being increasingly used in behavioural studies. We have initiated studies designed to evaluate the response of zebrafish to antipsychotic drugs. This study focuses on characterization of zebrafish D4 dopamine receptors (D4Rs) and the response of larval zebrafish to the atypical antipsychotic clozapine. The D4R is of interest because of its high affinity for clozapine, while interest in clozapine stems from its effectiveness in reducing symptoms in acutely psychotic, treatment‐resistant schizophrenic patients. By mining the zebrafish genomic database, we identified three distinct D4R genes, drd4a, drd4b and drd4c, and generated full‐length open reading frames encoding each of the three D4Rs by reverse transcription–polymerase chain reaction. Gene mapping studies showed that each D4R gene mapped to a distinct chromosomal location in the zebrafish genome, and each gene exhibited a unique expression profile during embryogenesis. When administered to larval zebrafish, clozapine produced a rapid and profound effect on locomotor activity. The effect of clozapine was dose‐dependent, resulted in hypoactivity and was prevented by the D4‐selective agonist ABT‐724. Our data suggest that the inhibitory effect of clozapine on the locomotor activity of larval zebrafish may be mediated through D4Rs.

Identification of Zebrafish A2 Adenosine Receptors and Expression in Developing Embryos

The A2A adenosine receptor (AdR) subtype has emerged as an attractive target in the pursuit of improved therapy for Parkinsons disease (PD). This report focuses on characterization of zebrafish a2 AdRs. By mining the zebrafish EST and genomic sequence databases, we identified two zebrafish a2a (adora2a.1 and adora2a.2) genes and one a2b (adora2b) AdR gene. Sequence comparisons indicate that the predicted zebrafish A2 AdR polypeptides share 62-74% amino acid identity to mammalian A2 AdRs. We mapped the adora2a.1 gene to chromosome 8, the adora2a.2 gene to chromosome 21, and the adora2b gene to chromosome 5. Whole mount in situ hybridization analysis indicates zebrafish a2 AdR genes are expressed primarily within the central nervous system (CNS). Zebrafish are known to be sensitive to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), a neurotoxin that causes selective loss of dopaminergic neurons and PD-like symptoms in humans as well as in animal models. Here we show that caffeine, an A2A AdR antagonist, is neuroprotective against the adverse effects of MPTP in zebrafish embryos. These results suggest that zebrafish AdRs may serve as useful targets for testing novel therapeutic strategies for the treatment of PD.

ptena and ptenb Genes Play Distinct Roles in Zebrafish Embryogenesis

PTEN is a tumor suppressor gene associated with multiple tumor types. PTEN function is essential for early embryonic development and is involved in the regulation of cell size, number, and survival. By dephosphorylating PIP3, PTEN normally acts to inhibit the PI3‐Kinase/AKT pathway. Here we have identified two zebrafish orthologs, ptena and ptenb, of the single mammalian PTEN gene and analyzed the role of these genes in zebrafish development. Ptena transcripts were expressed throughout the embryo at early somitogenesis. By 24 hpf, expression was predominant in the central nervous system, axial vasculature, retina, branchial arches, ear, lateral line primordium, and pectoral fin bud. Ptenb was also ubiquitously expressed early in somitogenesis, but transcripts became more restricted to the somites and central nervous system as development progressed. By 48 hpf, ptena and ptenb were expressed predominantly in the central nervous system, branchial arches, pectoral fins, and eye. Antisense morpholinos were used to knock down translation of ptena and ptenb mRNA in zebrafish embryos. Knockdown of either pten gene caused increased levels of phosphorylated Akt in morphant embryos, indicating that Ptena and Ptenb each possess PIP3 lipid phosphatase activity. Ptena morphants had irregularities in notochord shape (73%), vasculogenesis (83%), head shape (72%), and inner ear development (59%). The most noticeable defects in ptenb morphants were upward hooked tails (73%), domed heads (83%), and reduced yolk extensions (90%). These results indicate that ptena and ptenb encode functional enzymes and that each pten gene plays a distinct role during zebrafish embryogenesis. Developmental Dynamics 234:911–921, 2005.

Na,K-ATPase α and β subunit genes exhibit unique expression patterns during zebrafish embryogenesis

Abstract We have used in situ hybridization to analyze Na,K-ATPase α and β subunit gene expression during zebrafish embryogenesis. The most striking finding is that each of the 14 Na,K-ATPase genes exhibits a distinct expression profile. All α and β subunit genes are expressed in the nervous system, although the pattern of expression in different regions varies dramatically. In peripheral tissues, three of the five α1-like genes are expressed in pronephros and mucous cells, one is expressed in heart, and one is predominant in skeletal muscle. The α2 gene is expressed in brain and heart but is most prominent in skeletal muscle, while the two α3 genes are restricted in their expression to the nervous system. Of the six β subunit genes, β1a is expressed at highest abundance in lens, pronephros, and heart, while β1b transcripts are abundant in mucous cells. The two β2-like genes are differentially expressed in the nervous system. One β3 gene is expressed exclusively in brain while the other is abundantly expressed in skeletal muscle. Based on these expression patterns, we predict that at least 14 α/β subunit pairs are likely to be formed in different tissues.

Functional Assessment of Human Coding Mutations Affecting Skin Pigmentation Using Zebrafish

A major challenge in personalized medicine is the lack of a standard way to define the functional significance of the numerous nonsynonymous, single nucleotide coding variants that are present in each human individual. To begin to address this problem, we have used pigmentation as a model polygenic trait, three common human polymorphisms thought to influence pigmentation, and the zebrafish as a model system. The approach is based on the rescue of embryonic zebrafish mutant phenotypes by “humanized” zebrafish orthologous mRNA. Two hypomorphic polymorphisms, L374F in SLC45A2, and A111T in SLC24A5, have been linked to lighter skin color in Europeans. The phenotypic effect of a second coding polymorphism in SLC45A2, E272K, is unclear. None of these polymorphisms had been tested in the context of a model organism. We have confirmed that zebrafish albino fish are mutant in slc45a2; wild-type slc45a2 mRNA rescued the albino mutant phenotype. Introduction of the L374F polymorphism into albino or the A111T polymorphism into slc24a5 (golden) abolished mRNA rescue of the respective mutant phenotypes, consistent with their known contributions to European skin color. In contrast, the E272K polymorphism had no effect on phenotypic rescue. The experimental conclusion that E272K is unlikely to affect pigmentation is consistent with a lack of correlation between this polymorphism and quantitatively measured skin color in 59 East Asian humans. A survey of mutations causing human oculocutaneous albinism yielded 257 missense mutations, 82% of which are theoretically testable in zebrafish. The developed approach may be extended to other model systems and may potentially contribute to our understanding the functional relationships between DNA sequence variation, human biology, and disease.

Structural organization and chromosomal localization of the human Na,K-ATPase β3 subunit gene and pseudogene

We have cloned and characterized the Na,K-ATPase β3 subunit gene (ATP1B3), and a β3 subunit pseudogene (ATP1B3P1), from a human PAC genomic library. The β3 subunit gene is > 50 kb in size and is split into 7 exons. The exon/intron organization of the β3 subunit gene is identical to that of the Na,K-ATPase β3 subunit gene, indicating that these two genes evolved from a common evolutionary ancestor. Comparison of the promoter region of the human and mouse β3 subunit gene reveals a high degree of homology within a 300-bp segment located immediately upstream of the translation start site, suggesting that control elements that serve to regulate the cell-specific expression of the β3 subunit gene are likely to be located within this conserved region. Dot blot analysis of β3 subunit transcripts revealed expression within virtually all human tissues, while in situ hybridization showed expression of β3 mRNA in both neurons and glia of rat brain. Fluorescence in situ hybridization with PAC DNA clones localized ATP1B3 to the q22 → 23 region of Chromosome (Chr) 3, and the β3 pseudogene to the pl3 → 15 region of Chr 2.

Proteomic and functional analysis of NCS-1 binding proteins reveals novel signaling pathways required for inner ear development in zebrafish

BackgroundThe semicircular canals, a subdivision of the vestibular system of the vertebrate inner ear, function as sensors of angular acceleration. Little is currently known, however, regarding the underlying molecular mechanisms that govern the development of this intricate structure. Zebrafish represent a particularly tractable model system for the study of inner ear development. This is because the ear can be easily visualized during early embryogenesis, and both forward and reverse genetic techniques are available that can be applied to the discovery of novel genes that contribute to proper ear development. We have previously shown that in zebrafish, the calcium sensing molecule neuronal calcium sensor-1 (NCS-1) is required for semicircular canal formation. The function of NCS-1 in regulating semicircular canal formation has not yet been elucidated.ResultsWe initiated a multistep functional proteomic strategy to identify neuronal calcium sensor-1 (NCS-1) binding partners (NBPs) that contribute to inner ear development in zebrafish. By performing a Y2H screen in combination with literature and database searches, we identified 10 human NBPs. BLAST searches of the zebrafish EST and genomic databases allowed us to clone zebrafish orthologs of each of the human NBPs. By investigating the expression profiles of zebrafish NBP mRNAs, we identified seven that were expressed in the developing inner ear and overlapped with the ncs-1a expression profile. GST pulldown experiments confirmed that selected NBPs interacted with NCS-1, while morpholino-mediated knockdown experiments demonstrated an essential role for arf1, pi4kβ, dan, and pink1 in semicircular canal formation.ConclusionBased on their functional profiles, the hypothesis is presented that Ncs-1a/Pi4kβ/Arf1 form a signaling pathway that regulates secretion of molecular components, including Dan and Bmp4, that are required for development of the vestibular apparatus. A second set of NBPs, consisting of Pink1, Hint2, and Slc25a25, are destined for localization in mitochondria. Our findings reveal a novel signalling pathway involved in development of the semicircular canal system, and suggest a previously unrecognized role for NCS-1 in mitochondrial function via its association with several mitochondrial proteins.

Dominant Negative Mutants of Filamin A Block Cell Surface Expression of the D2 Dopamine Receptor

Protein interaction screens have revealed an interaction between the D2 dopamine receptor and the actin cross-linking protein filamin A. However, the physiological significance of this interaction has not been explained. To better understand the role of filamin A in D2 receptor-mediated signaling, we examined the effect of disrupting filamin A/D2 receptor interaction. Overexpression of a truncated form of filamin A (repeat units 18–19 containing the D2, but not the actin, binding domain) caused a marked reduction in both the number and half-life of cell surface D2 receptors. These results suggest that disruption of the linkage between D2 receptors and the actin cytoskeleton destabilizes plasma membrane-associated D2 receptors. Several missense mutations within repeat unit 19 of filamin A were identified that abrogate filamin A/D2 receptor interaction. Introduction of mutant and wild-type filamin A into filamin A-deficient M2 cells demonstrated that wild-type filamin A, but not the filamin A-binding mutants, was able to promote cell-surface expression of D2 receptors. Together, these studies provide evidence that filamin A/D2 receptor interaction is required for the proper targeting or stabilization of D2 dopamine receptors at the plasma membrane.