Synergism Between Pax-8 and Lim-1 in Embryonic Kidney Development

Pax genes encode a family of highly conserved DNA-binding transcription factors. These proteins play key roles in regulating a number of vertebrate and invertebrate developmental processes. Mutations in Pax-6 result in eye defects in flies, mice, and humans, and ectopic expression of this gene can trigger the development of ectopic compound eyes in flies. Likewise, mutation of other Pax genes in vertebrates results in the failure of specific differentiation programs-Pax-1 causes skeletal defects; Pax-2, kidney defects; Pax-3 or Pax-7, neural crest defects; Pax-4, pancreatic beta-cell defects; Pax-5, B-cell defects; Pax-8, thyroid defects; and Pax-9, tooth defects. Although this class of genes is obviously required for the normal differentiation of a number of distinct organ systems, they have not previously been demonstrated to be capable of directing the embryonic development of organs in vertebrates. In this report, it is demonstrated that Pax-8 plays such a role in the establishment of the Xenopus embryonic kidney, the pronephros. However, in order to efficiently direct cells to form pronephric kidneys, XPax-8 requires cofactors, one of which may be the homeobox transcription factor Xlim-1. These two genes are initially expressed in overlapping domains in late gastrulae, and cells expressing both genes will go on to form the kidney. Ectopic expression of either gene alone has a moderate effect on pronephric patterning, while coexpression of XPax-8 plus Xlim-1 results in the development of embryonic kidneys of up to five times normal complexity and also leads to the development of ectopic pronephric tubules. This effect was synergistic rather than additive. XPax-2 can also synergize with Xlim-1, but the expression profile of this gene indicates that it normally functions later in pronephric development than does XPax-8. Together these data indicate that the interaction between XPax-8 and Xlim-1 is a key early step in the establishment of the pronephric primordium.