Antennas of organ morphogenesis: the roles of cilia in vertebrate kidney development

Abstract

Cilia arose early during eukaryotic evolution, and their structural components are highly conserved from the simplest protists to complex metazoan species. In recent years, the role of cilia in the ontogeny of vertebrate organs has received increasing attention due to a staggering correlation between human disease and dysfunctional cilia. In particular, the presence of cilia in both the developing and mature kidney has become a deep area of research due to ciliopathies common to the kidney, such as polycystic kidney disease (PKD). Interestingly, mutations in genes encoding proteins that localize to the cilia cause similar cystic phenotypes in kidneys of various vertebrates, suggesting an essential role for cilia in kidney organogenesis and homeostasis as well. Importantly, the genes so far identified in kidney disease have conserved functions across species, whose kidneys include both primary and motile cilia. Here, we aim to provide a comprehensive description of cilia and their role in kidney development, as well as highlight the usefulness of the zebrafish embryonic kidney as a model to further understand the function of cilia in kidney health.

Keywords: ciliopathy; development; kidney; mesonephros; metanephros; motile cilia; multiciliated cell; primary cilia; pronephros; zebrafish.

Figure 1

Types of ciliated cells and cilia. Schematic of epithelial cells with (A) primary cilia, (B) motile cilia, and (C) multiple motile cilia, also known as multiciliated cells. Respective cross sections depict the characteristic microtubule distribution of each cilia type. Abbreviations: N, nucleus

Figure 2

Cilia in mammalian kidney development. (A) Primary cilia are first seen on nonproliferating epithelial cells of the renal vesicle (RV) and ureteric bud (UB) during kidney development. (B) As the RV grows into a comma‐shaped early nephron, primary cilia remain on nonproliferating cells of the nascent nephron and growing UB as it expands the collecting duct system. (C) Nonproliferating cells of the collecting duct and S‐shaped nephron have primary cilia. (D) Primary cilia continue to be present on epithelial cells of the mature nephron tubule and collecting duct. Proliferating cells are drawn with a dividing nucleus.

Figure 3

MCCs in zebrafish nephron development. (A) The renal progenitor field emerges at 10 hpf from the intermediate mesoderm, shown in purple. (B) By the 18–20 ss stage, a distinction between motile cilia (green) and mature MCCs (red) can be observed. (C) The MCC domain continues to expand as the embryo grows, but the “salt‐and‐pepper” pattern of MCCs remains. (D) At the 28 ss, the zebrafish pronephros is segmented. Mature MCCs are intercalated with mono‐ciliated transporter cells, with a dense expression pattern seen in the middle of the nephron. The confocal image is representative of wild‐type zebrafish pronephros where MCCs were labeled by florescent whole mount in situ hybridization to detect odf3b transcripts (red), cilia were labeled by α‐tubulin immunostaining (green), and nuclei were labeled with DAPI (blue). Below is a schematic of ciliated cells in the pronephros, where fluid flow is indicated by the black arrows. Abbreviations: hpf (hours post‐fertilization), ss (somite stage), MCC (multiciliated cell)

Figure 4

Integrated genetic pathway of MCC development. Genes that are presently known to be conserved across species are in black, those that have only been demonstrated in MCC development of the zebrafish pronephros are in blue, and grey indicates that the factor has not yet been shown in the zebrafish.

Figure 5

PKD in mammals and zebrafish. (A, B) Schematic of a healthy, adult mammalian kidney with enlarged nephron (A) compared to a diseased, polycystic adult mammalian kidney and enlarged nephron (B). The collecting duct system is in pink, nephron tubule is in orange, and cysts are in dark red. (C, D) Drawing of zebrafish larvae where the kidney is highlighted in green with red MCCs. (C) A healthy zebrafish larva with an enlarged, dorsal view of a healthy embryonic kidney. (D) Schematic of an unhealthy zebrafish larva, with bilateral kidney cysts (grey circle) and pericardial edema (white circle). Enlarged is a dorsal view of the pronephros, showing the bilateral kidney cysts (grey) as well as distended tubular lumen (green) compared to the wild‐type zebrafish embryonic kidney.