The role of the kidney as an important organ in the urinary system is well known right from high school textbooks to research papers. Apart from excretion, the organ also allows the elimination of waste from the catabolism of nitrogen, maintaining homeostasis and secreting factors of the endocrine system. All these functions are affected when diseases or injuries affect the kidney.
According to the Centers for Disease Prevention and Control (CDC), when the kidneys do not function properly, wastes accumulate in the blood due to chronic kidney disease (CKD). The CDC estimates that nearly 37 million people are affected by CKD with 9 of 10 affected individuals not aware of the disease. Other diseases include calcitriol deficiency that causes an imbalance in the metabolism of calcium and phosphorus and hypertension due to changes in the renin/angiotensin system.
According to research by Al-Awqati and Oliver in Kidney International, the adult kidney has more than 20 types of cells. The nephron of the kidney is made of the glomerulus and its tubules called the proximal and distal tubules and collecting duct. These various parts contain as many as 15 kinds of epithelial cells whose properties and functions are different! (Baer et al, 1997).
Though all the cells are important, the reabsorption of different materials such as amino acids and glucose is done by the proximal tubular epithelial cells. These cells also are involved in the synthesis of ammonia and eliminating bicarbonate to regulate the sensitive balance of the acid-base system. Another important aspect of these cells is that they are the target of foreign chemicals called xenobiotics. These nephrotoxins or their processed forms can damage the kidney and cause kidney failure.
The healthy kidney requires the correct functioning of the tubular epithelial cells. According to 2010-published research in The Journal of Clinical Investigation by scientists Bielesz and team, diseases such as CKD and kidney fibrosis are associated with loss of differentiation of the tubular epithelial cells. Another interesting aspect of the tubular epithelial cells is the loss of functions in the mitochondria of these cells to cause renal fibrosis. The progression of fibrosis often ends in CKD.
The study of these diseases can be done using cell cultures that are in vitro models. This can involve the use of primary cultures and renal cell lines to study the functions of the kidney or the toxic effects of drugs known as nephrotoxicity. There are human proximal tubular cell lines such as HKC and HK-2 that are immortalized. Yet, according to Bens and Vandewalle (2008), the process of immortalization causes these cells to dedifferentiate; lose their differentiated features specific to tubular cells. This makes the results when using such cultures ambiguous.
The answer lies in primary cultures.
According to scientists Van der Hauwaert and team (2015), primary cultures are considered are more on the lines of the in vivo nephron. This is because of the ability of primary cultures to maintain the functions and structures of tubular cells such as the enzyme functions of the brush border, responses to hormones and the various transport systems. Thus, as these tubule cells are targeted by xenobiotics, nephrotoxicity studies can be carried out using primary cultures. Possible alterations in metabolism seen in immortalized cell lines that make the results confusing are avoided by using primary cultures. Several primary tubular kidney epithelial cell cultures have expressed ABC multidrug transporters and cytochromes P450 enzymes that are involved in the processing of xenobiotics.
Thus, primary cultures of kidney epithelial cells of the tubules of the kidney are valuable in vitro systems to study the functions, disease mechanisms and drug toxicity.
Van der Hauwaert C, Savary G, Gnemmi V et al. Isolation and Characterization of a Primary Proximal Tubular Epithelial Cell Model from Human Kidney by CD10/CD13 Double Labeling. PLoS ONE .2013 8(6): e66750.
Al-Awqati Q, Oliver JA. Stem cells in the kidney. Kidney International. 2002 Feb; 61(2):387-95.
Baer PC, Nockher WA, Haase W, Scherberich JE. Isolation of proximal and distal tubule cells from human kidney by immunomagnetic separation. Technical note. Kidney International. 1997; 52: 1321–1331.
Bielesz B, Sirin Y, Si H, Niranjan T et al. Epithelial Notch signaling regulates interstitial fibrosis development in the kidneys of mice and humans. The Journal of Clinical Investigation. 2010 Nov; 120(11):4040-54.
Ding W, Yousefi K, & Shehadeh LA. Isolation, Characterization, And High Throughput Extracellular Flux Analysis of Mouse Primary Renal Tubular Epithelial Cells. Journal of visualized experiments: JoVE. 2018;136: 57718.
Bens M, Vandewalle A.Cell models for studying renal physiology. Pflügers Archiv: European Journal of Physiology. 2008; 457: 1–15.