Kidney is one of the most vital visceral organs of the human body, which plays an important role in maintaining the body's acid-base balance, regulating water and salt metabolism, excretion of metabolic waste, and regulating blood pressure. Over the past few decades, studies based on mouse models have revealed relatively complete mammalian fetal kidney development. However, due to the differences between mouse and human species, the development process of human fetal kidney has not been well explained for a long time, especially the origin and function of some major cell types during kidney development, as well as the mechanisms of nephrogenesis. At the same time, clinical evidence shows that abnormal kidney development during embryonic stage and congenital kidney disease will directly affect the survival and subsequent growth and development of the fetus. However, the molecular mechanism of abnormal kidney development is still not well elucidated.

On September 25, 2018, Prof Fuchou Tang’s group at Peking University BIOPIC Center, Beijing Advanced Innovation Center for Genomics, collaborating with ProJie Qiao’s group of Peking University Third Hospital jointly published a research paper titled "Dissecting the global dynamic molecular profiles of human fetal kidney development by single-cell RNA sequencing" on Cell Reports. The researchers performed high-precision single-cell transcriptome sequencing analysis on more than 3,000 single cells from the human fetal kidney spanning gestational weeks 7–25. The whole work revolves around the core biological event of intact nephron formation during kidney development, elaborates the heterogeneity of the precursor cells formed by the initial nephron, and analyzes the gradual differentiation of nephron precursor cells to produce different types of kidneys. Meanwhile, the study also focused on the expression characteristics of the identified candidate pathogenic genes of congenital nephropathy in the fetal kidney. The main findings of the study are:

There are two subtypes within the cap mesenchyme during the development process, one (CM1) with self-renewal potential and the other (CM2) exhibiting epithelial features (Figure 1). This indicates that in the early embryonic stage, the cap mesenchymal cell subpopulation (CM1) with self-renewal ability maintains the number and function of precursor cells during nephrogenesis, and at the same time, some precursor cells (CM2) begin to enter differentiate state and produce renal tubular epithelial cells. The dynamic balance between precursors and differentiated cells maintains the continuous production of nephrons between these two cell subgroups.

Figure 1. Heterogeneity characteristics of cap mesenchymal cells in human fetal kidney

The main signaling pathways that regulate the formation and division of human renal tubular epithelium have been identified. Renal tubules are partitioned to proximal tubules, distal tubules, and Loop of Henle that perform different physiological functions. We found that the PI3K signaling pathway is involved in regulating the development of proximal tubule epithelial cells, and the MAPK signaling pathway is involved in the formation of Loop of Henle. The development of collecting ducts is regulated by the WNT signaling pathway (Figure 2). At the same time, we also found that the metabolic function of the human embryonic kidney is mainly completed by the proximal tubules and collecting ducts epithelial cells.

Figure 2. Transcription factors and signal pathways regulating the formation of renal tubular epithelial cells

Collecting tube cells are mainly composed of two types of epithelial cells: principal cells (AQP2+) and intercalated cells (CA2+). We found that AQP2-positive (AQP2+) principal cells appeared before CA2-positive (CA2+) intercalary cells during the development of human kidney collecting tubes based on the results of immunofluorescence labeling, indicating that CA2-positive intercalary cells were probably derived from the differentiation of AQP2-positive principal cells.

Figure 3. Changes in the expression levels of AQP2 and CA2 during the development of collection tube epithelium

The expression of candidate high-risk pathogenic genes of known congenital nephropathy has strong cell type specificity during kidney development. For example, the expression of candidate high-risk pathogenic genes of Bartter syndrome is all enriched in distal tubules, the expression of the main candidate genes of Fanconi renotubular syndrome is enriched in proximal tubules and most of the candidate high-risk disease-causing genes are enriched in cap mesenchymal cells. This indicates that the abnormality of precursor cells during kidney development is likely to have an important impact on the occurrence of congenital nephropathy (Figure 4).

Figure 4. Expression of candidate high-risk pathogenic genes for congenital nephropathy in specific cell types of human fetal kidney

Our results provide a comprehensive gene expression survey of human fetal kidney development that will serve as a valuable reference for dissecting the mechanisms of renal dysplasia, a group of complicated and even deadly renal anomalies often found in newborns. Our results are also of potential importance as an in utero reference with respect to recent work focusing on the in vitro induction of mammalian kidney organoids using pluripotent stem cells or fetal kidneys.

Ping Wang and Yidong Chen, who are the PhD students at Peking University's Beijing Advanced Innovation Center for Genomics, are the co-first authors of the paper. Professor Fuchou Tang and Professor Jie Qiao are the co-corresponding authors. This work was supported by National Natural Science Foundation of China, the Ministry of Science and Technology, Beijing Advanced Innovation Center for Genomics (ICG), and Center for Life Sciences (CLS).

Original link：https://www.cell.com/cell-reports/fulltext/S2211-1247(18)31343-3