H+ Pump Provides an Early Asymmetry Signal

Science's STKE  18 Apr 2006:
Vol. 2006, Issue 331, pp. tw128
DOI: 10.1126/stke.3312006tw128

Adams et al. identified the H+-V-ATPase, which is a vacuolar and plasma membrane H+ transporter, in a pharmacological screen of Xenopus embryos in which various transporters were inhibited and defects in left-right asymmetry (heterotaxia) were scored. Inhibition of the H+-V-ATPase with drugs such as concanamycin or expression of a dominant-negative H+-V-ATPase subunit resulted in heterotaxia and loss of asymmetric expression of one of the first genes with asymmetric expression, Nodal, suggesting that H+-V-ATPase provides a very early asymmetry signal. H+-V-ATPase subunits were more abundant on the right side of the embryo as early as the two-cell stages. Proton efflux was greater on the right side of the embryo and was inhibited by concanamycin. In addition, the right side of the embryo was hyperpolarized relative to the left side, which is expected from the electrogenic nature of the H+-V-ATPase. Elimination of asymmetric H+ flux by expression of a nonasymmetrically localized plasma membrane H+ pump or exposure of the embryos to low pH (pH 5 to 6), or elimination of the hyperpolarization of the membrane by incubating the embryos with the drug palytoxin that converts the Na+, K+-ATPase into a nonspecific ion channel produced heterotaxia. This suggests that the activity of the H+-V-ATPase produces asymmetry through a combination of an effect on pH and membrane potential. A role for H+-V-ATPase in asymmetry was also noted for chick embryos and zebrafish embryos and appeared to serve as one of the earliest signals for asymmetry. Disruption of H+-V-ATPase activity randomized the expression of the genes encoding nodal and sonic hedgehog in chicks, and in zebrafish H+-V-ATPase activity was required for asymmetric expression of southpaw and prior to formation of the Kuppfer's vesicle, a ciliated organ involved in organ asymmetry.

D. S. Adams, K. R. Robinson, T. Fukumoto, S. Yuan, R. C. Albertson, P. Yelick, L. Kuo, M. McSweeney, M. Levin, Early, H+-V-ATPase-dependent proton flux is necessary for consistent left-right patterning of non-mammalian vertebrates. Development 133, 1657-1671 (2006). [Abstract] [Full Text]