Vanderbilt researchers have uncovered new details about the role of Rac1 in the maintenance and integrity of the kidney collecting duct. The discovery, published in the Journal of Cell Biology, could aid efforts to promote kidney regeneration after injury.
The GTPase Rac1 is a molecular switch with multiple cell type- and context-dependent functions that are critical for actin cytoskeletal regulation. However, the role of Rac1 in the kidney collecting system has remained unclear.
“We investigated whether selective deletion of Rac1 at the initiation of ureteric bud development would alter kidney development,” said Fabian Bock, M.D., an instructor of medicine in the Division of Nephrology at Vanderbilt University Medical Center. “Surprisingly, we found that Rac1 was not essential for development but instead required to maintain the integrity, function and shape of adult epithelial cells in the collecting duct.”
In Vivo Models
The kidney collecting duct system is formed from the ureteric bud, which undergoes iterative rounds of branching morphogenesis during development. This is a highly regulated process requiring growth factor-dependent signaling, cell-extracellular matrix interaction and an organized actin cytoskeleton.
“I’m excited about the potential for future tissue engineering applications as we better understand the role of Rac1 in the maintenance and integrity of the kidney collecting system.”
While there is evidence that the actin regulator Rac1 plays a role in epithelial tubulogenesis and branching of the lung and mammary gland, there are little data defining the role of Rac1 in epithelial cell branching morphogenesis in vivo in mammals. Therefore the researchers set out to use an in vivo mouse model to investigate the role of Rac1 in epithelial branching morphogenesis of the kidney.
Discovery of Regulatory Functions
Upon selective deletion of Rac1, mice exhibited only a mild developmental phenotype. However, with aging, the collecting duct developed a disruption of epithelial integrity and function.
“We found that Rac1 was not essential for development but instead required to maintain the integrity, function and shape of adult epithelial cells in the collecting duct.”
“Contrary to our expectations, the ureteric bud branching morphogenesis phenotype was mild, but the mice developed progressive abnormalities in collecting duct tubular structure, morphology and function as they aged,” Bock explained.
Despite intact integrin signaling, Bock reported that collecting duct cells lacking Rac1 had marked adhesion and polarity abnormalities that were independent of the major downstream Rac1 effector, Pak1. However, these cells did have a defect in the actin nucleation and polymerization apparatus, WAVE2-Arp2/3, resulting in actomyosin hyperactivity.
“We discovered that Rac1 promotes collecting duct epithelial integrity and morphology by restricting actomyosin via Arp2/3-dependent cytoskeletal branching,” Bock said. “Rac1 is not required for early collecting epithelial development, but instead maintains epithelial integrity and morphology in adults by promoting actin branching, which restricts cellular contractility.”
Future Directions
Bock suggested that future studies should determine whether Rac1 is a key player in modulating and restoring cell-cell interfaces during homeostasis or post-injury repair in other epithelial systems.
He explained that disruption of the structural integrity of the collecting duct results in pathological states, such as fibrosis which is the cause of chronic kidney disease. Bock hopes that an improved understanding of Rac1 function will eventually aid efforts in kidney engineering and regeneration.
“I’m excited about the potential for future tissue engineering applications as we better understand the role of Rac1 in the maintenance and integrity of the kidney collecting system,” Bock said.
