Modelling Cellular Interactions and Dynamics During Kidney Morphogenesis

Kidney disease and renal disorders account for a significant proportion of health complications in mid-late adulthood worldwide. Many renal deficiencies are due to improper formation of the kidneys before birth, which are caused by disorders in the developmental process that arise from genetic and/or environmental factors. Mathematical modelling can help build on experimental knowledge to increase our understanding of the complexities of kidney organogenesis. In this paper, we present a discrete cell-based model of kidney development. Specifically, we model the tip of the developing ureteric tree to investigate the behaviours of cap mesenchyme cells which are required to sustain ureteric tip growth. We find that spatial regulation of the differentiation of cap mesenchyme cells through cellular signalling is sufficient to ensure robust ureteric tip development. Additionally, we find that increased adhesion interactions between cap mesenchyme cells and the ureteric tip surface can lead to a more stable tip-cap unit. Our analysis of the various processes on this scale highlights essential components for healthy kidney growth and provides insight into mechanisms to be studied further in order to replicate the process in vitro.

A holey pursuit: lumen formation in the developing kidney

The formation of polarized epithelial tubules is a hallmark of kidney development. One of the fundamental principles in tubulogenesis is that epithelia coordinate the polarity of individual cells with the surrounding cells and matrix. A central feature in this process is the segregation of membranes into spatially and functionally distinct apical and basolateral domains, and the generation of a luminal space at the apical surface. This review examines our current understanding of the cellular and molecular mechanisms that underlie the establishment of apical-basal polarity and lumen formation in developing renal epithelia, including the roles of cell-cell and cell-matrix interactions and polarity complexes. We highlight growing evidence from animal models, and correlate these findings with models of tubulogenesis from other organ systems, and from in vitro studies.

Analysed cap mesenchyme track data from live imaging of mouse kidney development

This article provides detailed information on manually tracked cap mesenchyme cells from timelapse imaging of multiple ex vivo embryonic mouse kidneys. Cells were imaged for up to 18 h at 15 or 20 min intervals, and multiple cell divisions were tracked. Positional data is supplemented with a range of information including the relative location of the closest ureteric tip and a correction for drift due to bulk movement and tip growth. A subset of tracks were annotated to indicate the presence of processes attached to the ureteric epithelium. The calculations used for drift correction are described, as are the main methods used in the analysis of this data for the purpose of describing cap cell motility. The outcomes of this analysis are discussed in “Cap mesenchyme cell swarming during kidney development is influenced by attraction, repulsion, and adhesion to the ureteric tip” (A.N. Combes, J.G. Lefevre, S. Wilson, N.A. Hamilton, M.H. Little, 2016) [1].

Cell-cell interactions driving kidney morphogenesis

The mammalian kidney forms via cell-cell interactions between an epithelial outgrowth of the nephric duct and the surrounding nephrogenic mesenchyme. Initial morphogenetic events include ureteric bud branching to form the collecting duct (CD) tree and mesenchymal-to-epithelial transitions to form the nephrons, requiring reciprocal induction between adjacent mesenchyme and epithelial cells. Within the tips of the branching ureteric epithelium, cells respond to mesenchyme-derived trophic factors by proliferation, migration, and mitosis-associated cell dispersal. Self-inhibition signals from one tip to another play a role in branch patterning. The position, survival, and fate of the nephrogenic mesenchyme are regulated by ECM and secreted signals from adjacent tip and stroma. Signals from the ureteric tip promote mesenchyme self-renewal and trigger nephron formation. Subsequent fusion to the CDs, nephron segmentation and maturation, and formation of a patent glomerular basement membrane also require specialized cell-cell interactions. Differential cadherin, laminin, nectin, and integrin expression, as well as intracellular kinesin and actin-mediated regulation of cell shape and adhesion, underlies these cell-cell interactions. Indeed, the capacity for the kidney to form via self-organization has now been established both via the recapitulation of expected morphogenetic interactions after complete dissociation and reassociation of cellular components during development as well as the in vitro formation of 3D kidney organoids from human pluripotent stem cells. As we understand more about how the many cell-cell interactions required for kidney formation operate, this enables the prospect of bioengineering replacement structures based on these self-organizing properties.

Nephron progenitor cells: shifting the balance of self-renewal and differentiation

Within the developing mammalian kidney, several populations of progenitors form the discrete cellular components of the final organ. Fate mapping experiments revealed the cap mesenchyme (CM) to be the progenitor population for all nephron epithelial cells, whereas the neighboring stromal mesenchyme gives rise to mesangial, pericytic, renin-producing and interstitial cells. The collecting ducts are derived from a population of progenitors at the ureteric bud (UB) tip and a proportion of the endothelium is also derived from a dedicated mesenchymal progenitor. The stroma, CM, and UB interact to create spatially defined niches at the periphery of the developing organ. While the UB tip population persist, the CM represents a transient progenitor population that is exhausted to set the final organ size. The timing of CM exhaustion, and hence the final organ structure, is sensitive to disruptions such as premature birth. Here we will discuss our current understanding of the molecular processes allowing these populations to balance cell survival, self-renewal, support of branching, and maintain capacity to commit to differentiation.

Sall1 balances self-renewal and differentiation of renal progenitor cells

The formation of the proper number of functional nephrons requires a delicate balance between renal progenitor cell self-renewal and differentiation. The molecular factors that regulate the dramatic expansion of the progenitor cell pool and differentiation of these cells into nephron precursor structures (renal vesicles) are not well understood. Here we show that Sall1, a nuclear transcription factor, is required to maintain the stemness of nephron progenitor cells. Transcriptional profiling of Sall1 mutant cells revealed a striking pattern, marked by the reduction of progenitor genes and amplified expression of renal vesicle differentiation genes. These global changes in gene expression were accompanied by ectopic differentiation at E12.5 and depletion of Six2+Cited1+ cap mesenchyme progenitor cells. These findings highlight a novel role for Sall1 in maintaining the stemness of the progenitor cell pool by restraining their differentiation into renal vesicles.

Mdm2 is required for maintenance of the nephrogenic niche

The balance between nephron progenitor cell (NPC) renewal, survival and differentiation ultimately determines nephron endowment and thus susceptibile to chronic kidney disease and hypertension. Embryos lacking the p53-E3 ubiquitin ligase, Murine double minute 2 (Mdm2), die secondary to p53-mediated apoptosis and growth arrest, demonstrating the absolute requirement of Mdm2 in embryogenesis. Although Mdm2 is required in the maintenance of hematopoietic stem cells, its role in renewal and differentiation of stem/progenitor cells during kidney organogenesis is not well defined. Here we examine the role of the Mdm2-p53 pathway in NPC renewal and fate in mice. The Six2-GFP::Cre(tg/+) mediated inactivation of Mdm2 in the NPC (NPC(Mdm)2(-/-)) results in perinatal lethality. NPC(Mdm)2(-/-) neonates have hypo-dysplastic kidneys, patchy depletion of the nephrogenic zone and pockets of superficially placed, ectopic, well-differentiated proximal tubules. NPC(Mdm2-/-) metanephroi exhibit thinning of the progenitor GFP(+)/Six2(+) population and a marked reduction or loss of progenitor markers Amphiphysin, Cited1, Sall1 and Pax2. This is accompanied by aberrant accumulation of phospho-γH2AX and p53, and elevated apoptosis together with reduced cell proliferation. E13.5-E15.5 NPC(Mdm2-/-) kidneys show reduced expression of Eya1, Pax2 and Bmp7 while the few surviving nephron precursors maintain expression of Wnt4, Lhx1, Pax2, and Pax8. Lineage fate analysis and section immunofluorescence revealed that NPC(Mdm2-/-) kidneys have severely reduced renal parenchyma embedded in an expanded stroma. Six2-GFP::Cre(tg/+); Mdm2(f/f) mice bred into a p53 null background ensures survival of the GFP-positive, self-renewing progenitor mesenchyme and therefore restores normal renal development and postnatal survival of mice. In conclusion, the Mdm2-p53 pathway is essential to the maintenance of the nephron progenitor niche.