Hemo-Vasculogenesis in the Kidney and Heart

The kidney vasculature plays essential roles in the development and function of the kidney. There is currently very limited information regarding the crucial events that govern the morphogenesis of the renal vasculature, including the origin, lineage relationship and mechanisms involved in its formation. Using inducible transgenic mice that specifically label cells expressing transcription factor stem cell leukemia (SCL), we performed fate-tracing studies combined with colony forming assays (CFCs) and embryonic kidney cross-transplantation studies. We found that the SCL+ precursors in the kidney contribute to the endothelial cells (ECs) and a subpopulation of early hematopoietic precursor cells that form colony forming unit Granulocyte-Erythrocyte-Monocyte-Megakaryocyte (CFU-GEMM) and colony forming unit Granulocyte-Macrophage (CFU-GM) colonies. We also showed that the transplanted embryonic kidneys generate their own vessels and blood cells in situ, which we termed hemo-vasculogenesis. Genetic ablation of these precursors in vivo and cross-transplantation led to the lack of endothelium, demonstrating their vital role during vascular development and organogenesis.
Sphingosine 1-phosphate receptor 1 (S1P1) is one of the five G-protein coupled receptors activated by Sphingosine 1-phosphate, which is crucially involved in the vascular development. However, due to the early lethality of the S1P1 null mice, the role of S1P1 in kidney vascular development has not been determined. To identify the mechanisms governing the development of kidney vasculature, we designed a series of experiments to study the role of S1P1 expressed in the SCL+ precursors and their derivatives during kidney development. In this dissertation work, we found that the endothelial S1P1 is required for multiple processes during kidney vascular development, including EC proliferation, vSMCs coating, and the development of glomerular capillaries and renal lymphatic vessels. Deletion of S1P1 from the SCL+ precursors resulted in endothelial hyperplasia, dilated renal arteries and veins, disrupted vSMC coating of renal arteries and arterioles and the absence of lymphatic endothelium. Remarkably, the knockout mice developed capillary shunts within the glomeruli. These results demonstrated the essential role of S1P1 for the kidney vascular development. Notably, we observed thinner inter-ventricular septum, thinner ventricular myocardial compact layer and dilated myocardial capillaries in hearts of knockout embryos. By cross-transplantation studies of the kidneys from the E12.5 knockout and control mice under the kidney capsule of WT adult mice, we showed that the kidney vascular abnormalities are intrinsic, due to the lack of S1P1 in renal EC precursors, and not secondary to the extra-renal anomalies.
Finally, we found that the SCL+ cells contribute to another population of hemo-vascular precursors in the heart that exclusively differentiate to the endocardium and coronary vascular endothelial cells, including a subset with hemogenic potential. To characterize the contribution of SCL+ precursors to the heart, I developed a novel approach to transplant the E9.5 mouse embryonic heart under the kidney capsule of adult host mice, which allows the ex vivo hemo-vasculogenesis and the visualization of beating hearts at the time of harvest. Strikingly, partial ablation of the SCL+ precursor from the heart in vivo and cross-transplant model led to the absence of cardiac ECs accompanied with impaired myocardial development, which revealed the vital role of SCL+ precursors in the development of both cardiac cell types. Furthermore, by timed deletion of S1P1 from the SCL+ precursors and their derivatives during early heart development, we uncovered the unique role of endothelial S1P1 in the myocardial and coronary vascular development. The mice with S1P1 deletion showed deficiencies in the myocardial proliferation, reduced ventricular coronary vasculature, and hypertrabeculation.
In summary, these studies have identified an SCL+ precursor contributes intrinsically to the hemo-vasculogenesis in both embryonic kidney and heart, which express S1P1 to regulate vascular development and organogenesis. These findings may help advance our understanding of normal vascular development and genetic defects that trigger congenital kidney and heart disease and development of new treatment for the patients.