Introduction
Biopharmaceutical research and development faces a major productivity
crisis in the depreciating efforts to develop novel drugs
(Woodcock & Woosley, 2008). Despite over
30 years of investment in biomedical sciences and the scientific tools
used in drug discovery, few results have been well-translated in the
preclinical and clinical stages (Giffin,
Robinson, & Olson, 2009; Wehling,
2006). The reliance on
conventional cell culture systems and animal models during preclinical
testing hinders the establishment of human-related kidney predictive
models (Awdishu & Mehta, 2017;
Wilmer et al., 2016). Microphysiological
systems (MPS) accurately model human systems in a compact, efficient
fluidic tool that can introduce controlled spatiotemporal
micro-environments (Kim & Wu, 2012),
support accurate human responses, allow for real-time imaging, encourage
cell differentiation, and pinpoint cell-cell interactions under a
variety of physiological conditions
(Phillips et al., 2020;
C. Sakolish et al., 2018;
C. M. Sakolish, Esch, Hickman, Shuler, &
Mahler, 2016; C. M. Sakolish, Philip, &
Mahler, 2019; Wu et al., 2020). Recent
developments in stem cell research (Musah
et al., 2017; Sciancalepore et al.,
2014), regenerative medicine (Gomes,
Rodrigues, Domingues, & Reis, 2017), biomaterials
(Homan et al., 2016;
van Midwoud, Janse, Merema, Groothuis, &
Verpoorte, 2012), tissue engineering
(Jang et al., 2013;
Jansen et al., 2014;
Ng, Zhuang, Lin, & Teo, 2012;
Tourovskaia, Fauver, Kramer, Simonson, &
Neumann, 2014), and microfluidics allow for integration into
three-dimensional (3D) MPS.
In the kidney, millions of nephrons employ filtration, reabsorption,
secretion, and excretion processes. Each functional unit collaborates to
filter out wastes and xenobiotics, separate water, ions, and small
molecules from the blood, and recycle compounds to the blood
(Figure 1A-B ) (Marieb & Hoehn,
2007). Within the nephron, glomerular filtration consists of passive
movement of plasma from glomerulus capillaries to the Bowman’s capsule
that is freely permeable to water and small solutes
(Na+, urea, and glucose), but not permeable to blood,
white blood cells, platelets, or large molecular weight
(>67 kDa) serum proteins (albumin)
(Koeppen & Stanton, 2012). The
glomerular filtrate exits the glomerulus to enter a selective barrier of
highly coiled tubules. There, the proximal convoluted tubule (PCT)
utilizes active and passive transport to reabsorb glucose, sodium
chloride, and water from the glomerular filtrate
(Marieb & Hoehn, 2007;
Zanetti, 2020).
During reabsorption,
highly-concentrated filtrate becomes the leading site for nephrotoxin
accumulation, a precursor for acute kidney injury or chronic kidney
disease. Nearly 90% of renal
toxicity cases are derived from both the glomerulus and PCT of the
kidney (Bonventre, Vaidya, Schmouder, Feig,
& Dieterle, 2010; Jang et al., 2013;
C. M. Sakolish et al., 2016). Current MPS
designs of the kidney have been well established for the PCT segment of
the nephron, representing only a section of the renal absorption process
(Jang et al., 2013;
C. Sakolish et al., 2020;
C. Sakolish et al., 2018;
C. M. Sakolish et al., 2016;
C. M. Sakolish et al., 2019;
Weber et al., 2016). However,
incorporating both filtration and absorption interfaces will refine the
physiological relevance of the kidney in vitro barrier model, and
permit rapid screening for drug toxicity in preclinical studies.
In this study a novel, physiologically realistic MPS of the proximal
tubule and glomerulus that incorporates cross flow filtration and is
capable of long-term operation has been developed. The tri-culture
system houses conditionally immortalized human podocytes (CIHP-1) to
represent the ultrafiltration processes from the fenestrations in the
glomerular capsule, human umbilical vein endothelial cells (HUVECs) for
the capillaries recirculating solutes in the bloodstream, and human
kidney-2 (HK-2) to recreate the reabsorption processes in the PCT. Key
design requirements included integration of the glomerular filtration
fraction (0.15-0.2) and tubular reabsorption (0.65-0.7)
(Feher, 2017;
Marieb & Hoehn, 2007). The cells within
the MPS were grown under dynamic flow engineered to mimic flow
conditions in vivo (0.4-1.2
dyne-s/cm2)(Wilmer et
al., 2016) for seven days, and then the system was challenged with
fluorescein isothiocyanate- human serum albumin (FITC-HSA) to assess its
filtration functional capacity. Cells within the device were imaged
using confocal microscopy for attachment and cytoskeletal
reorganization. MPS culture medium and imaging was completed without the
introduction of animal by-products.
This MPS introduces a novel
definition of a PCT and glomerulus with physiological capability of
blood serum protein filtration, glucose resorption, and filtrate
formation capacity.