| Budget Amount *help |
¥3,600,000 (Direct Cost: ¥3,600,000)
Fiscal Year 2006: ¥1,300,000 (Direct Cost: ¥1,300,000)
Fiscal Year 2005: ¥2,300,000 (Direct Cost: ¥2,300,000)
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| Research Abstract |
1)Uptake of Na^+ from the environment is an indispensable strategy for the survival of freshwater fish including zebrafish, as they easily lose Na^+ from the plasma to a diluted environment. Nevertheless, the location of, and molecules involved in Na^+ uptake remain poorly understood. In this study, we utilized Sodium Green, a Na^+-dependent fluorescent reagent, to provide direct evidence that Na^+ absorption takes place in a subset of the mitochondrion-rich cells (MRCs) on the yolk sac surface of zebrafish larvae. Combined with immunohistochemistry, we revealed that the Na^+-absorbing MRCs were exceptionally rich in vacuolar-type H^+-ATPase (H^+-ATPase) but moderately rich in Na^+/K^+-ATPase. We also addressed the function of foxi3a, a transcription factor that is specifically expressed in the H^+-ATPase-rich MRCs. By depletion of foxi3a from zebrafish embryos using antisense morpholino oligonucleotides, we demonstrated that foxi3a is indispensable for MRC differentiation and MRCs are
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primary site for Na^+ absorption. Additionally, foxi3a expression is initiated at gastrula stage in the presumptive ectoderm ; thus we propose that foxi3a is a key gene in the control of MR cell differentiation.
2)We tested the capability of the endocrine system in zebrafish to respond to environmental salinity challenges during larval stages. We reveal that the zebrafish larvae has a system in which several endocrine genes, including atrial natriuretic peptide (anp), renin, prolactin, growth hormone and parathyroid hormone 1 (pth1), respond at the transcription level to changes in environmental salinity and that the responses are gene specific. Both anp and renin are upregulated in larvae raised in dilute freshwater medium but are downregulated in concentrated medium. On the other hand, expression of prolactin and growth hormone is strongly enhanced in the dilute medium, but shows little or no change under higher salinity conditions. Interestingly, pth1 expression depends on Ca^<2+> concentration, as observed in mammals. Thus, taken together with the advantages of a model organism, including accessibility to genetic approaches, we propose that zebrafish larvae are useful for a comprehensive study of the regulatory mechanisms of the endocrine system in ionic and osmotic homeostasis. Less
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