The Leif Oxburgh Lab
Kidney Development and Kidney Disease
Work in our laboratory focuses on embryonic kidney development and kidney disease. These are two very wide topics, and we have concentrated our efforts on certain key questions:
- How are early progenitor cells of the developing kidney maintained in their undifferentiated state?
- How are diverse cell types such as endothelial cells and pericytes integrated into the growing kidney?
- How does BMP signaling promote kidney regeneration in the adult following injury?
To answer these questions we work with primary cell cultures from embryonic and adult kidneys as well as genetic mouse models and mouse models of kidney disease.
Early Nephron Development
Embryonic kidney development is based on an iterative program of inductive events between nephron progenitor cells and the invading ureteric bud. Interactions between these tissues with in the outer most layer or “nephrogenic zone” of the developing kidney, will ultimately form nephrons, the functional filtering units of the kidney, and the associated network of collecting ducts needed to excrete urine filtrate.
Figure 1. Schematic representation of the progenitor cell niche within the nephrogenic zone of the embryonic kidney. Early nephron progenitor cells are associated with the tips of the branching collecting ducts. Nephron progenitor cells will differentiate into the epithelial cells of the nephron, whereas collecting duct tips will differentiate into the collecting duct system that transports urine that has been filtered by the nephron. Light blue: The earliest nephron progenitor cells. Purple: pretubular aggregate, collections of nephron progenitor cells primed for epithelial differentiation. Blue: Renal vesicles, in which epithelial differentiation has taken place. The expression domains of markers genes Cited1, Pax2, Six2, and Lef1 are shown.
Recent studies indicate that nephron progenitor cells of the embryonic kidney are arranged in a series of compartments of increasing states of differentiation. The earliest progenitor compartment, distinguished by expression of CITED1, possesses greater capacity for renewal and differentiation than later compartments (LEF1). We are interested in deciphering the signaling events governing self renewal and progression of nephron progenitors through stages of increasing differentiation towards formation of the fully functional nephron. We have recently found a novel integrative mechanism involving FGF, BMP and WNT signaling, which is essential for this process and reconciles much of our current understanding of early nephrogenesis. The elucidation of these early inductive programs will provide key insights into normal and dysregulated nephrogenesis, as well as regenerative processes that follow kidney injury.
Figure 2. Immunofluorescence showing that in cell culture, the RTK ligands FGF2, TGF and EGF promote maintenance of CITED1+ progenitors (red) derived from the mouse embryonic kidney.
The role of the transcriptional regulator FOXD1 in kidney development
One project in the lab focuses on the role played by the transcription factor FOXD1 in embryonic kidney development. The cell population that expresses Foxd1 is the precursor population that ultimately gives rise to mesangial cells (glomerular smooth muscle cells), the kidney capsule, and interstitial fibroblasts in the adult kidney. Mice that lack FOXD1 protein die at birth, and the embryos have small, fused kidneys. Within these kidneys, the region where new nephrons form is mislocalized, and there is a delay in the formation of nephron structures from the progenitor cell population that correlates with a dramatic increase in the number of progenitor cells, which do not themselves express Foxd1. Current efforts in this project involve the identification of FOXD1 target genes, as well as how these targets regulate signaling events in the progenitor cells.
Figure 3. Foxd1 null kidneys display a disorganization of the nephrogenic zone and an accumulation of undifferentiated nephron progenitor cells. Staining for the transcription factor PAX2 which labels both epithelial collecting ducts (co-stained green) and nephron progenitor cells shows that the Foxd1 null kidney is not segregated into a nephron progenitor cells zone (nephrogenic zone), and a zone of differentiating nephrons. Few differentiating nephrons are seen in the Foxd1 null, as differentiation of progenitor cells appears to be blocked.
Signaling pathways downstream of BMP in vivo
The mechanisms by which BMPs transduce signal in vivo are complex and remain incompletely understood. Work in our laboratory aims to define mechanisms underlying the choice of intracellular signal transduction pathway utilized by BMP in vivo. The kidney is dependent on BMP signaling both for its embryonic development, and for maintenance of its normal function, and thus represents an excellent model system for these studies. BMP signals can be transduced through MAPK and Smad pathways, and our understanding of the relative importance of these transduction cascades remains rudimentary. Using genetic approaches, we are comparing the effects of inactivating Smad signaling with effects of inactivating MAPK signaling in specific cell populations of both the embryonic and adult kidney.