Ret signaling in ureteric bud epithelial cells controls cell movements, cell clustering and bud formation

Abstract

Ret signaling promotes branching morphogenesis during kidney development, but the underlying cellular mechanisms remain unclear. While Ret-expressing progenitor cells proliferate at the ureteric bud tips, some of these cells exit the tips to generate the elongating collecting ducts, and in the process turn off Ret. Genetic ablation of Ret in tip cells promotes their exit, suggesting that Ret is required for cell rearrangements that maintain the tip compartments. Here, we examine the behaviors of ureteric bud cells that are genetically forced to maintain Ret expression. These cells move to the nascent tips, and remain there during many cycles of branching; this tip-seeking behavior may require positional signals from the mesenchyme, as it occurs in whole kidneys but not in epithelial ureteric bud organoids. In organoids, cells forced to express Ret display a striking self-organizing behavior, attracting each other to form dense clusters within the epithelium, which then evaginate to form new buds. The ability of forced Ret expression to promote these events suggests that similar Ret-dependent cell behaviors play an important role in normal branching morphogenesis.

Biomechanical regulation of cell shapes promotes branching morphogenesis of the ureteric bud epithelium

“…100 This may be important for epithelial cell proliferation and motility reported in the UB tips. 41,42,57,[101][102][103][104] Thus, the identified changes together with existing data propose a proliferation-and stretching-based model for driving new branch formation in the developing kidney. More specifically, our results suggest that to reach an ampulla stage, cells unevenly increase in volume, partially due to high proliferation, 15 which triggers crowding at the ampulla stage.…”

“…Interestingly, previous study reports that epithelial cells round up when their adherens junctions are disrupted ( 89 ). This may be important for epithelial cell proliferation and motility reported in the UB tips ( 41, 42, 44, 90–93 ). Thus, the identified changes together with existing data propose a proliferation- and stretching-based model for driving new branch formation in the developing kidney.…”

“…38 Furthermore, Rac1 controls the actin cytoskeleton to prevent misshapen cells from entering the cell cycle during renal tubule repair. 39 In the developing mammalian kidney, the UB tip-localized progenitor cells are motile and undergo luminal mitosis [40][41][42] but the exact cell shape changes and their contribution to the stepwise process of UB branching morphogenesis are not characterized. Here we utilize a recently developed machine learning-based algorithm called ShapeMetrics 43 to systematically analyze cell shapes and their changes in normal and MAPK/ERK-deficient UB epithelium used as a model of branching deficiency.…”

How to build an epithelial tree

“…Genetic mosaic experiments revealed that the expression level of Ret in a given cell is associated with the position that cell adopts within the developing ureteric tree. Cells lacking Ret are excluded from branch tips and populate branch stalks [56], whereas cells with constitutively high Ret remain in branch tips [57]. However, it is not immediately obvious how this cell sorting contributes to branching morphogenesis.…”

“…In mesenchymefree explants, epithelial branches form in a random pattern that does not resemble the orderly bifurcations observed in vivo or in whole-organ explants. In genetic mosaic experiments using explants, cells with constitutive expression of Ret come together and initiate branches [57], suggesting that high Ret activity promotes a form of epithelial-intrinsic branching, albeit one with currently unknown cellular mechanisms. Sorting of cells with different levels of Ret signaling in vivo might help to restrict this 'branchinitiating activity' to branch tips.…”