The tale of two (distal nephron) cell types

The kidney nephron represents an exquisite example of epithelial cell differentiation and specialization in one small tubule (6). Plasma filtrate passing down the length of a few millimeters will encounter at least six cell types, each highly specialized in form and function. The lineage of the cells lining the nephron are from two embryonic sources. The ureteric bud emerges from the mesonephric (Wolffian) duct and interacts with metanephric mesenchymal cells to form a cap mesenchyme. The renal vesicle formed from this interaction will ultimately develop into the more proximal structures (glomerulus and most of the mature nephron) with the uteric bud forming the collecting duct (2). The distal convoluted tubule and collecting duct of the mature mammalian nephron are composed of principal and intercalated (alpha and beta) cells, and the developmental factors that determine terminal cell differentiation are still being investigated (7). These cell types are distinguished by their unique functional roles of, predominantly, ion and water transport for principal cells and acid and base secretions for intercalated cells. To study the function and mechanisms of regulation in these specialized epithelial cell types, researchers have long been interested in developing robust cell culture models that closely approximate the characteristics of these epithelial cells.

With the use of the knowledge gained from in vivo and ex vivo investigations in renal tissue taken from a range of animal models, specific characteristics were sought in a principal cell model line. A robust cell line would polarize into epithelial monolayers when cultured on filter supports. The cells would have the ability to transport ions, predominantly sodium via endogenous epithelial sodium channels (ENaC) down an electrogenic gradient established by the Na⁺-K⁺-ATPase. Model cells would be capable of potassium transport through apical and basal potassium channels [most notably the apically located renal outer medullary potassium channel (ROMK)]. The cells would be sensitive to changes in osmotic gradients and be capable of water transport through aquaporins (Aqp2). More importantly, for functional studies, the cells should be sensitive to several hormonal inputs at physiological concentrations; most relevant for this renal nephron segment, this would include insulin, vasopressin, and a true mineralocorticoid receptor response to aldosterone signaling.

Many cell lines developed over the years exhibited some or several of these characteristics, but none had all of the desired traits (5). It was only when a spontaneously transformed cell line was derived from a single clone obtained from microdissected mouse cortical collecting duct that a novel cell model, named mCCD_cl1, was developed that appeared to exhibit all these characteristics (4). This mCCD_cl1 line therefore stood out for researchers in the field. The cells possessed 11-β-hydroxysteroid dehydrogenase type 2 (HSD11b2) and both mineralocorticoid and glucocorticoid receptors (MR and GR), ensuring a true mineralocorticoid response to physiological concentrations of aldosterone could be achieved (it was the best of times). The cell line is now routinely used in dozens of laboratories as a model murine principal cell line. However, in the paper by Assmus et al. (1), the authors probed deeper into the characteristics of this cell line. By deriving several sublines from the parental cells after serial dilution to single cells, they report that not only were some of the desired characteristics of principal cell-specific ion transport diminished or lost, markers to the other distal nephron-resident intercalated cells emerged. They demonstrate cells with intercalated cell-specific markers, in some cases markers for both principal and intercalated cells, were observed in the same cells (it was the worst of times). What this study indicated is that characteristics of both principal and intercalated cells were transmitted from a single cell progenitor (via the single cell clonal lines) suggesting that the mCCD_cl1 cell line is plastic and may have the ability to derive both cell types.

This finding adds a very important, novel, and underappreciated trait to this cell line. First, it requires laboratories using this line to carefully evaluate the phenotype of the cells they use; to be aware that phenotypic regulation measured using one approach (for example, electrophysiological methods) may only represent a subsection of the cells present in culture; and to report their findings in light of this new information. However, it offers a unique opportunity to investigate in more depth the conditions that may give rise to particular cell lineages in the collecting duct. The developmental cues that facilitate terminal differentiation and the ability of the distal nephron to plastically alter cell populations are an active area of research (3).

These cells may represent a novel tool to investigate ideas of renal cell plasticity, progenitor, and stem cell regeneration and cellular conversion. The tale of two cell types could therefore be addressed using a single cell model, potentially making this cell line even more valuable to renal researchers (perhaps it is the best of times again).

GRANTS

This research was supported by the National Institute of Diabetes and Digestive and Kidney Diseases Grant DK-102843.

DISCLOSURES

No conflicts of interest, financial or otherwise, are declared by the author.

AUTHOR CONTRIBUTIONS

M.B.B. drafted manuscript; edited and revised manuscript; and approved final version of manuscript.