Role of cardiac natriuretic peptides in seawater adaptation of medaka embryos as revealed by loss-of-function analysis

Abstract

Cardiac natriuretic peptides (atrial natriuretic peptide, ANP; b-type natriuretic peptide, BNP; ventricular natriuretic peptide, VNP) and their direct ancestor C-type natriuretic peptide 3 (CNP3) exert potent osmoregulatory actions in fish. However, very little is known about their roles in embryonic osmoregulation. In this study, we performed loss-of-function analysis using euryhaline medaka (Oryzias latipes), which has lost ANP and VNP during evolution and thus possesses only BNP and CNP3. We found that the maintenance of whole-body osmolality in seawater embryos was impaired by the knockdown of BNP+OLGC7 (BNP receptor) or CNP3 alone from 1 day postfertilization, and the CNP3 knockdown was accompanied by greater water loss. The impaired osmoregulation in the knockdown embryos was not due to the suppressed expression of major transporters for NaCl excretion via ionocytes or of key enzyme genes for metabolic water production, but to the impaired blood circulation to the yolk-sac membrane caused by abnormal heart development. We detected a strong positive correlation between impaired blood circulation and increased body fluid osmolality and pharmacological blockade of blood flow increased body fluid osmolality in seawater embryos. We also found that the exaggerated water loss in CNP3 knockdown embryos is related to the failure to suppress aquaporin (AQP3, AQP4, and AQP9) gene expression. These results show that CNP3 decrease water permeability of body surfaces and that both BNP and CNP3 ensure sufficient blood flow to the yolk-sac membrane for efficient salt excretion by ionocytes and sufficient water production by yolk metabolism to promote seawater adaptation during early development in medaka.

The influence of heart developmental anatomy on cardiotoxicity-based adverse outcome pathways in fish

“…This could be particularly detrimental to cells relying on ionic gradients, particularly excitable cells in the developing nervous system. As supporting evidence for this, there are large changes in the expression of aquaporin genes in marine fish embryos with reduced circulation (Miyanishi, et al, 2013). Aquaporins are primary water-transporting channels and they are expressed in a wide variety of cell types.…”

“…Genetic and pharmacologic studies indicate that even modest reductions in embryonic circulation can lead to impaired osmoregulation in seawater (Miyanishi et al, 2013). A loss of circulation increases whole embryo osmolality (Miyanishi, et al, 2013), as predicted based on the direction of water and ion gradients in marine species. Absent circulation, the accumulation of edema cannot represent a net gain of water within the embryo, but rather the movement of free water into the yolk sac sinus from other parts of the body.…”

“…Aquaporins are primary water-transporting channels and they are expressed in a wide variety of cell types. Notably, in fish the aquaporins form a large gene family (Tingaud-Sequeira et al, 2010), and isoforms expressed in the central nervous system (e.g., aquaporin 4) were among the genes that were most responsive to a loss of embryonic circulation (Miyanishi, et al, 2013). Altered ion homeostasis in the developing nervous system is likely to produce functional defects, but activity-dependent changes in development may not yield overtly visible anatomical abnormalities.…”

Proton-facilitated ammonia excretion by ionocytes of medaka (Oryzias latipes) acclimated to seawater

“…levels of Na ϩ -K ϩ -ATPase (NKA), Na ϩ -K ϩ -2Cl Ϫ cotransporter (NKCC1), and CFTR Cl Ϫ channel were induced by high ammonia exposure, suggesting that they are involved in ammonia excretion by SW-type ionocytes (13,14,20). In a recent study, NKA and NKCC1were colocalized to the basolateral membrane of SW ionocyte in medaka larvae (22). Taken all of the evidence together, a putative model of ammonia excretion by SW and FW ionocytes of medaka is proposed (Fig.…”

Evidence for two distinct waves of epidermal ionocyte differentiation during medaka embryonic development

“…This difference in the IC markers composition in the two zones supports the hypothesis of different ion regulation mechanisms. Very few data have been collected to date on embryonic ICs function in teleosts, even though a recent study suggests that they have an important role in osmoregulation (Miyanishi et al, ). In medaka larva, the Na + /Cl ‐ ‐cotransporter (NCC) was detected at the apical membrane along with the H + ‐ATPase pump (HA) (Hsu et al, ).…”