TABLE 3.

The proof-of-concept studies in which hiPSCs-derived kidney organoids have been developed as potential models for nephrotoxicity and drug screening are presented, highlighting the main outcomes, limitations and future areas of improvement for these models.

Drug screening and toxicity assays applications of kidney organoids
Organoid application/references	Outcomes	Limitations and challeges	Future perspectives
Genetic reporters for podocyte differentiation and toxicity Sharmin et al., 2016; Borestrom et al., 2018; Hale et al., 2018; Vanslambrouck et al., 2019; Yoshimura et al., 2019	The use of genome editing made it possible to knock in fluorescent proteins in the NPHS1 and MAFB loci. Their expression during podocyte differentiation in organoids was used to improve differentiation conditions and to establish a proof of concept for doxorubicin and PAN nephrotoxicity studies.	These studies did not carry out functional assays in podocytes following PAN and doxorubicin treatment.	These reporter cell lines could be shared through an international repository to accelerate the use of organoids in toxicology and drug screening. More sensitive reporters of podocyte damage could be developed with CRISPR/Cas9, to mark, for example, alterations on the localization of slit diaphragm proteins, and not only the total fluorescence. These lines could also be combined with microfluidic devices to generate more physiological models of podocyte and drug-induced toxicity.
Toxicity assays in organoid-derived tubular cells Freedman et al., 2015; Morizane et al., 2015; Takasato et al., 2015	In Morizane’s and Freedman’s work, KIM1 was upregulated in proximal and distal tubular cells of hiPSC-organoids in response to gentamycin and cisplatin treatment. KIM1 mRNA expression was upregulated in a dose-dependent manner. In Takasato’s work the activation of CASP3 was observed specifically in tubular cells following cisplatin exposure.	These organoid protocols have not been tested in a high-throughput toxicology setting. Additional iPSC lines and nephrotoxic agents and different iPSC lines are needed to validate the reproducibility of the protocols. Drug transporters will need to be analyzed (qualitatively and quantitatively) to assess similarities and differences with in vivo models.	To improve the applicability of organoids in toxicology, KIM1 reporter hiPSC lines could be generated to allow real-time observation of acute kidney injury and drug effect. Tubules assembled in microfluidic devices could also improve some structural functional features of tubular cells that are important for the toxic injury and repair response.