Quantitative Translation of Microfluidic Transporter in Vitro Data to in Vivo Reveals Impaired Albumin-Facilitated Indoxyl Sulfate Secretion in Chronic Kidney Disease.

Indoxyl sulfate (IxS), a highly albumin-bound uremic solute, accumulates in chronic kidney disease (CKD) due to reduced renal clearance. This study was designed to specifically investigate the role of human serum albumin (HSA) in IxS renal secretion via organic anion transporter 1 (OAT1) in a microfluidic system and subsequently apply quantitative translation of in vitro data to predict extent of change in IxS renal clearance in CKD stage IV relative to healthy. Conditionally immortalized human proximal tubule epithelial cells overexpressing OAT1 were incubated with IxS (5-200 µM) in the HSA-free medium or in the presence of either HSA or CKD-modified HSA. IxS uptake in the presence of HSA resulted in more than 20-fold decrease in OAT1 affinity (Km,u) and 37-fold greater in vitro unbound intrinsic clearance (CLint,u) versus albumin-free condition. In the presence of CKD-modified albumin, Km,u increased four-fold and IxS CLint,u decreased almost seven-fold relative to HSA. Fold-change in parameters exceeded differences in IxS binding between albumin conditions, indicating additional mechanism and facilitating role of albumin in IxS OAT1-mediated uptake. Quantitative translation of IxS in vitro OAT1-mediated CLint,u predicted a 60% decrease in IxS renal elimination as a result of CKD, in agreement with the observed data (80%). The findings of the current study emphasize the role of albumin in IxS transport via OAT1 and explored the impact of modifications in albumin on renal excretion via active secretion in CKD. For the first time, this study performed quantitative translation of transporter kinetic data generated in a novel microfluidic in vitro system to a clinically relevant setting. Knowledge gaps and future directions in quantitative translation of renal drug disposition from microphysiological systems are discussed.

Renal Epithelial Monolayer Formation on Monomeric and Polymeric Catechol Functionalized Supramolecular Biomaterials.

Induction of a functional, tight monolayer of renal epithelial cells on a synthetic membrane to be applied in a bioartificial kidney device requires for bio-activation of the membrane. The current golden standard in bio-activation is the combination of a random polymeric catechol (L-DOPA) coating and collagen type IV (Col IV). Here the possibility of replacing this with defined monomeric catechol functionalization on a biomaterial surface using supramolecular ureido-pyrimidinone (UPy)-moieties is investigated. Monomeric catechols modified with a UPy-unit are successfully incorporated and presented in supramolecular UPy-polymer films and membranes. Unfortunately, these UPy-catechols are unable to improve epithelial cell monolayer formation over time, solely or in combination with Col IV. L-DOPA combined with Col IV is able to induce a tight monolayer capable of transport on electrospun supramolecular UPy-membranes. This study shows that a random polymeric catechol coating cannot be simply mimicked by defined monomeric catechols as supramolecular additives. There is still a long way to go in order to synthetically mimic simple natural structures.

Recellularized Native Kidney Scaffolds as a Novel Tool in Nephrotoxicity Screening.

Drug-induced kidney injury in medicinal compound development accounts for over 20% of clinical trial failures and involves damage to different nephron segments, mostly the proximal tubule. Yet, currently applied cell models fail to reliably predict nephrotoxicity; neither are such models easy to establish. Here, we developed a novel three-dimensional (3D) nephrotoxicity platform on the basis of decellularized rat kidney scaffolds (DS) recellularized with conditionally immortalized human renal proximal tubule epithelial cells overexpressing the organic anion transporter 1 (ciPTEC-OAT1). A 5-day SDS-based decellularization protocol was used to generate DS, of which 100-µm slices were cut and used for cell seeding. After 8 days of culturing, recellularized scaffolds (RS) demonstrated 3D-tubule formation along with tubular epithelial characteristics, including drug transporter function. Exposure of RS to cisplatin (CDDP), tenofovir (TFV), or cyclosporin A (CsA) as prototypical nephrotoxic drugs revealed concentration-dependent reduction in cell viability, as assessed by PrestoBlue and Live/Dead staining assays. This was most probably attributable to specific uptake of CDDP by the organic cation transporter 2 (OCT2), TFV through organic anion transporter 1 (OAT1), and CsA competing for P-glycoprotein-mediated efflux. Compared with 2D cultures, RS showed an increased sensitivity to cisplatin and tenofovir toxicity after 24-hour exposure (9 and 2.2 fold, respectively). In conclusion, we developed a physiologically relevant 3D nephrotoxicity screening platform that could be a novel tool in drug development.

Role of Vitamin D in Maintaining Renal Epithelial Barrier Function in Uremic Conditions.

As current kidney replacement therapies are not efficient enough for end-stage renal disease (ESRD) treatment, a bioartificial kidney (BAK) device, based on conditionally immortalized human proximal tubule epithelial cells (ciPTEC), could represent an attractive solution. The active transport activity of such a system was recently demonstrated. In addition, endocrine functions of the cells, such as vitamin D activation, are relevant. The organic anion transporter 1 (OAT-1) overexpressing ciPTEC line presented 1α-hydroxylase (CYP27B1), 24-hydroxylase (CYP24A1) and vitamin D receptor (VDR), responsible for vitamin D activation, degradation and function, respectively. The ability to produce and secrete 1α,25-dihydroxy-vitamin D3, was shown after incubation with the precursor, 25-hydroxy-vitamin D3. The beneficial effect of vitamin D on cell function and behavior in uremic conditions was studied in the presence of an anionic uremic toxins mixture. Vitamin D could restore cell viability, and inflammatory and oxidative status, as shown by cell metabolic activity, interleukin-6 (IL-6) levels and reactive oxygen species (ROS) production, respectively. Finally, vitamin D restored transepithelial barrier function, as evidenced by decreased inulin-FITC leakage in biofunctionalized hollow fiber membranes (HFM) carrying ciPTEC-OAT1. In conclusion, the protective effects of vitamin D in uremic conditions and proven ciPTEC-OAT1 endocrine function encourage the use of these cells for BAK application.

Towards a bioengineered kidney: recellularization strategies for decellularized native kidney scaffolds.

Patients with end-stage renal disease often undergo dialysis as a partial substitute for kidney function while waiting for their only treatment option: a kidney transplant. Several research directions emerged for alternatives in support of the ever-growing numbers of patients. Recent years brought big steps forward in the field, with researchers questioning and improving the current dialysis devices as well as moving towards the design of a bioengineered kidney. Whole-organ engineering is also being explored as a possibility, making use of animal or human kidney scaffolds for engineering a transplantable organ. While this is not a new strategy, having been applied so far for thin tissues, it is a novel approach for complex organs such as the kidneys. Kidneys can be decellularized and the remaining scaffold consisting of an extracellular matrix can be repopulated with (autologous) cells, aiming at growing ex vivo a fully transplantable organ. In a broader view, such organs might also be used for a better understanding of fundamental biological concepts and disease mechanisms, drug screening and toxicological investigations, opening new pathways in the treatment of kidney disease.Decellularization of whole organs has been widely explored and described; therefore, this manuscript only briefly reviews some important considerations with an emphasis on scaffold decontamination, but focuses further on recellularization strategies. Critical aspects, including cell types and sources that can be used for recellularization, seeding strategies and possible applications beyond renal replacement are discussed.

Upscaling of a living membrane for bioartificial kidney device.

The limited removal of metabolic waste products in dialyzed kidney patients leads to high morbidity and mortality. One powerful solution for a more complete removal of those metabolites might be offered by a bioartificial kidney device (BAK), which contains a hybrid "living membrane" with functional proximal tubule epithelial cells (PTEC). These cells are supported by an artificial functionalized hollow fiber membrane (HFM) and are able to actively remove the waste products. In our earlier studies, conditionally immortalized human PTEC (ciPTEC) showed to express functional organic cationic transporter 2 (OCT2) when seeded on small size flat or hollow fiber polyethersulfone (PES) membranes. Here, an upscaled "living membrane" is presented. We developed and assessed the functionality of modules containing three commercially available MicroPES HFM supporting ciPTEC. The HFM were optimally coated with L-Dopa and collagen IV to support a uniform and tight monolayer formation of matured ciPTEC under static culturing conditions. Both abundant expression of zonula occludens-1 (ZO-1) protein and limited diffusion of FITC-inulin confirm a clear barrier function of the monolayer. Furthermore, the uptake of 4-(4-(dimethylamino)styryl)-N-methylpyridinium iodide (ASP+), a fluorescent OCT2 substrate, was studied in absence and presence of known OCT inhibitors, such as cimetidine and a cationic uremic solutes mixture. The ASP+ uptake by the living upscaled membrane was decreased by 60% in the presence of either inhibitor, proving the active function of OCT2. In conclusion, this study presents a successful upscaling of a living membrane with active organic cation transport as a support for BAK device.