Patterning and early cell lineage decisions in the developing kidney: the role of Pax genes

Abstract

Specification of the intermediate mesoderm and the epithelial derivatives that will make the mammalian kidney depends on the concerted action of many transcription factors and signaling proteins. Among the earliest genes expressed in the nephric duct and surrounding mesenchyme is Pax2, whose function is essential for making and maintaining the epithelium. The Pax2 protein is subject to phosphorylation in response to signals that activate the c-Jun N-terminal kinase pathway, including Wnts and BMPs. In cell culture systems, Pax2 is know to recruit components of a histone H3 lysine 4 methyltransferase complex to specific DNA sites to alter the pattern of histone modifications and determine gene expression. This epigenetic function may underlie the ability of Pax2 and similar proteins to maintain cell lineages during development.

Fig. 1

Expression of paired-box 2 (Pax2) and groucho-related 4 (Grg4) protein in the embryonic kidney. Immunostaining for proteins in serial sagittal, sections from an embryonic day 16.5 (E16.5) mouse kidney. a The Pax2 protein is shown in red and E-cadherin is given in green. Note high levels of Pax2 protein in the peripheral nephrogenic zone, especially the cap mesenchyme (arrows). Note also that Pax2 levels decrease in the s-shaped bodies as the podocyte progenitors are specified (arrowheads). b The Grg4 protein is shown in red and E-cadherin is given in green. Note the low levels in nuclei of mesenchymal cells (arrowheads) and high levels in podocyte progenitor cells (arrowheads)

Fig. 2

Effect of c-Jun N-terminal kinase (JNK) inhibitors on kidney development in vitro. Kidney rudiments dissected from E11.5 mouse embryos were cultured for 2 days either with control media (a) or with 10 μM of SP600125, a specific JNK inhibitor (b). Whole rudiments were stained with antibodies against Pax2 (red) or cytokeratin (green). Cytokeratin marks the ureteric bud and its branches. Note the condensation of Pax2-positive cells around the ureteric bud tips in a (arrows). Upon JNK inhibition, Pax2-positive cells are diffuse and fail to aggregate (b, arrows)

Fig. 3

Model of Pax2-mediated chromatin remodeling. Pax2 binds to a target DNA sequence and interacts with the adaptor protein PTIP when Pax2 is phosphorylated. PTIP recruits a KMT2 and promotes H3K4 methylation. This enables nucleosome remodeling factors (NuRFs) to keep the chromatin open and allow for nucleosome movement. Under conditions of increasing Groucho (Grg/Tle) concentration, Pax2 binds to a Grg/Tle complex, which dephosphorylates the Pax2 carboxy-terminal domain and inhibits PTIP interactions. The Grg/Tle complex may then promote silencing by recruiting chromatin condensation proteins such as HP1