A Proton Gradient Signals Asymmetry

Abstract

STKE Adams et al. identified the H+-V-ATPase, which is a vacuolar and plasma membrane proton pump, in a pharmacological screen of Xenopus embryos in which 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 the 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 indeed. Proton pump subunits were more abundant on the right side of the embryo as early as the two-cell stage, and proton efflux was greater on the right side of the embryo. In addition, the right side of the embryo was hyperpolarized relative to the left side, as expected from the electrogenic nature of the H+-V-ATPase. Elimination of asymmetric H+ flux by expression of a symmetrically localized plasma membrane H+ pump or exposure of the embryos to low pH, or elimination of the hyperpolarization of the membrane by incubating the embryos with palytoxin, both produced heterotaxia. This suggests that the activity of the H+-V-ATPase produces asymmetry through a combination of an effect on pH and the membrane potential. A role for H+-V-ATPase in asymmetry was also noted for chick 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 Nodal and Shh in chicks, and in zebrafish H+-V-ATPase activity was required for asymmetric expression of Southpaw and before formation of the Kupffer's vesicle, a ciliated organ involved in organ asymmetry. — NRG Development 133 , 1657 (2006).