FOXD1+ Stromal Cells: The Progenitors for Kidney Vascular Development

The renal vasculature is crucial for proper kidney function, and defects in the kidney arteries are responsible for many human diseases. However, the molecular mechanisms that underlie the assembly of the renal blood vessels are poorly understood. Using lineage tracing we show that renal stromal cells, characterized by their expression of the transcription factor Forkhead Box D1 (Foxd1), give rise to the entirety of the mural cell layer of the renal arterial tree (including smooth muscle and renin cells) as well as glomerular mesangial cells and interstitial pericytes. The goal of this thesis is to identify the mechanisms that regulate the fate of Foxd1 cells.
We find that Foxd1 itself is necessary for proper kidney development. Deletion of the Foxd1 gene resulted in kidneys that were smaller, fused at the midline, and failed to properly rotate and ascend. Strikingly, the vasculature of Foxd1-null kidneys was completely misoriented, with arteries entering the organ from multiple locations in the kidney periphery in a reversal of the normal arrangement, indicating that Foxd1 plays a crucial role in arterial patterning. We also demonstrate that the Foxd1 cell itself is required for proper differentiation of the mural cells using cell ablation studies. Exposure of Foxd1 cells to diphtheria toxin chain A (Foxd1-DTA) resulted in renal arteries with a disorganized smooth muscle cell layer, delayed arterial formation, and an altered vascular pattern similar to that seen in the Foxd1-null animals. In addition, we observe a potential capability for cells outside the Foxd1 lineage to replace Foxd1-lineage cells in situations of cell injury and death.
Foxd1 cells give rise to another progenitor cell population, the renin precursor cells, which differentiate into arteriolar smooth muscle and glomerular mesangial cells. One longtime focus of our lab has been identification of molecules that are responsible for the remarkable plasticity of renin cell descendants to reacquire the renin phenotype in response to a homeostatic threat that requires a sustained increase in circulating renin. Using microarray analysis, we find that the protein aldo-keto reductase 1b7 (AKR1B7) is a specific marker for renin cells. This association is maintained throughout development, and in physiological and pathological manipulations that increase or decrease renin levels. Importantly, in renin knockout animals we show that AKR1B7 delineates a population of cells attempting to make renin, and that these cells participate in the resulting vascular pathology. We also demonstrate that AKR1B7, like renin, is regulated by cyclic AMP signaling and RBP-J (Recombination Signal Binding Protein for IG-kJ Region) indicative of a larger, overarching genetic program that regulates the renin cell phenotype.
Finally we investigate the function of RBP-J, the final transcriptional mediator of Notch signaling, in Foxd1 cells (Foxd1RBPJ-/-). Conditional deletion of RBP-J resulted in a reduction in the number of renal arteries, a thinner smooth muscle cell layer, and defects in vascular branching. In addition, Foxd1RBPJ-/- displayed a lack of mesangial cells, resulting in glomerular aneurysms. These developmental defects are followed in adult life by prominent vascular fibrosis and glomerulosclerosis. Thus we conclude that RBP-J is necessary for proper differentiation of Foxd1 cells.
In summary, my results illustrate the important role of Foxd1–expressing stromal cells as a precursor for the arterial mural cells and mesangium, and identify three important proteins in their function and differentiation- Foxd1, AKR1B7, and RBP-J. Further study of Foxd1 cells and the molecules that control their fate will be crucial for understanding the key developmental process of kidney vascular formation.