Epithelial cells enable essential physiological functions, including absorption, morphogenesis, secretion, and transport. To execute these functions, epithelial cells often form three-dimensional shapes that include curved sheets of cells surrounding a pressurized fluid-filled lumen. These three-dimensional tissues (called domes) are essential for organ function, but when they are not working properly, developmental defects, inflammation, and cancer can ensue. Recently, it has been shown that the cells that form domes show active superelasticity on micropatterned plates. We show here that the immortalized renal proximal tubule epithelial cell line, LLC-PK1, stereotypically forms tubules in 10 days. Tubule formation takes place in 4 stages. When cells are plated on a culture dish, they form a monolayer on the 1st day; on the 3rd day, three-dimensional structures are formed, called domes; and after the 4.5th day, these domes start fusing to begin the transition stage and transit to the tubule stage. At the end of the 10th day, differentiated, elongated, and matured tubes form (Figure 3.1). Therefore, tubule formation is a self-organized, stereotypic morphogenetic program under long-term, unperturbed tissue culture conditions. We propose that tubulogenesis is a two-step process in proximal tubules by doming and wrapping. The process begins with dome formation, and as the cell layers come together in the transition stage at the edge of the dome, this leads to the formation of the lumen of the eventual tubule. We also found that F-actin provides the mechanical strength during the formation of these three-dimensional structures during tubule formation. To better understand this 4-step process on a molecular level, we performed proteomics of tubule formation to identify the different proteins that play a significant role in proximal tubule development. Importantly, we identified proximal tubule markers like synaptopondin, angiotensin 1-10, collectrin, polycystin 1, and polycystin 2. These proteins play an important role in renal tube formation and differentiation. Cell division is carried out by highly conserved cyclin-CDK complexes, which phosphorylate various cellular components. Cyclin-CDKs act differently depending on the cell cycle phase and work cooperatively to create DNA replication and cytokinesis. Therefore, we identified that cyclin-B1, marker of proliferation Ki-67, the RAD51 recombinase, and proliferating cell nuclear antigen (PNCA) are upregulated in the monolayer stage, and the expression decreases as tubule formation takes place. The proximal tubule reabsorbs 60-65% of the glomerulus filtrate. Therefore, it requires a lot of energy generated by using the fatty acid oxidation (FAO) pathway. In our model, we found FAO expression is higher than that of the other metabolic pathways. We found expression of an intricate protein network in mitochondria, which we interpret as a sign of mitochondrial homeostasis being vital for the FAO pathway to work. Furthermore, we also identified different types of transporters at each stage of proximal tubule formation, and we could recognize different cytoskeletal components playing a significant role in each stage of proximal tubule formation, for instance, at the monolayer stage, vimentin expression is high, and its expression is reduced as tubules form. Hence, this 2D system, at this step of characterization, seems suitable to use to study differential transport protein expression and how this might relate to physiological functions and syndromes. Next, we inhibited different transporters using specific inhibitors and analyzed the effect on dome and tubule formation. We identified that Na+/K+ ATPase and vacuolar H+ ATPase play a significant role in the process of epithelial dynamics. Digoxin (a Na+/K+ ATPase inhibitor) treatment inhibits dome and tubule formation. Bafilomycin (a v-ATPase inhibitor) treatment demonstrated a delay in dome and tube formation. Therefore, this study shows that this 2D proximal tubule novel system can be used for screening of pharmacological leads in the context of specific aspects of kidney physiology. Despite the recent success in growing kidney organoids, they are not well suited to investigate various pathophysiological conditions in vitro for several reasons: They grow in 3D and form a tissue that later needs to be dissected/cleared and stained to investigate pathophysiological changes. Moreover, organoids require complex and expensive protocols for generation and are challenging to use in screening approaches. Therefore, we set out to demonstrate feasibility for our 2D system using normal renal epithelial cells, which are the origin of various pathological conditions, to study pathophysiological conditions.
