Complex interactions of the branching ureteric bud and surrounding mesenchymal cells during metanephric kidney development determine the final number of nephrons. Alterations that result in impaired nephron endowment predispose to arterial hypertension and chronic kidney disease. In the kidney, extracellular matrix (ECM) proteins are usually regarded as acellular scaffolds or as the common histological end-point of chronic kidney diseases. Only little is known about their physiological role in kidney development. The ECM protein fibronectin is expressed at early time points of kidney growth. Therefore, we were interested in the characterization of its expression and role in kidney development. In mouse, fibronectin was expressed during all stages of kidney development with significant changes over time. At embryonic day (E) 12.0 and E13.5 fibronectin lined the ureteric bud epithelium, which was less pronounced at E16.5 and then switched to a glomerular expression in the postnatal and adult kidneys. Similar results were obtained in human kidneys. Deletion of fibronectin at E13.5 in cultured metanephric mouse kidneys resulted in reduced kidney sizes, impaired branching morphogenesis and decreased glomerulogenesis. In line with these findings, fibronectin deletion led to reduced cell proliferation of the branching epithelial cells. Fibronectin colocalized with alpha8 integrin and fibronectin loss caused a reduction in alpha8 integrindependent formation of glial cell line-derived factor and expression of Wnt11, both of which are essentially involved in promoting ureteric bud branching. In conclusion, the ECM protein fibronectin acts as a regulator of metanephric kidney development and is a determinant of nephron number.

PurposeThe ubiquitously expressed glycoprotein fibronectin (FN) is a central component of the fibrillar extracellular matrix (ECM) that is found in multiple sites throughout the body including the peritubular interstitium of the kidney. To learn more about the specific role(s) of FN in the kidney we generated and investigated FN-deficient mice.MethodsWe generated CAGG-Cre-ER™/Fnfl/fl mice which carry floxed Fn alleles and ubiquitously express Cre-recombinase after tamoxifen treatment. Newborn pups were treated with tamoxifen eye drops (2.5 mg/mL) to induce FN deficiency. Conditional deletion of Fn was confirmed by quantitative real-time PCR, Western blot analysis and immunohistochemistry. The expression patterns of Fn were analyzed by in situ hybridization. Kidneys were investigated by light microscopy and immunohistochemistry.ResultsThe expression analyses and immunohistochemistry showed a significant reduction of FN at postnatal day (P) 4. Loss of FN corelated with the formation of renal cysts at the corticomedullary border, which expand with increasing age. In situ hybridization demonstrated that on P4 Fn expression extends mainly from the pelvis to the corticomedullary border, whereas in 5-6 weeks old mice it is located only in the cortex. Immunohistochemistry and light microscopy showed a loosening of the renal interstitium and additionally an appearance of ECM proteins in the cysts.ConclusionWe conclude that FN deficiency leads to the development of renal cysts, which occurs a few days after tamoxifen treatment and results in extensive loss of renal parenchyma a few weeks after birth. The results indicate an important role of FN for maintenance of kidney structure and function.