The kidneys are responsible for several important physiological functions that maintain homeostasis, such as blood filtration, removal of excess residues, and regulation of ion concentrations1. Damage of renal tissue can lead to a heterogeneous condition called Acute Kidney Injury (AKI), which clinically is described as a rapid decrease in renal function caused by destruction and death of tubular epithelial cells, endothelial cell injury, and leukocyte infiltration 2,3. AKI is a condition projected to happen in 8-16% of hospital admissions4, with a high mortality rate that ranges from 20 to 50% in the intensive care unit (ICU)5. This ailment is associated with increased hospital stay and considerable use of financial resources5. Etiologic factors include dehydration, shock, infections, sepsis, cardiovascular disease, and nephrotoxic drugs6. Nephrotoxicity is defined as a renal injury induced by drugs, causing effects as AKI, tubulopathies, and glomerulopathies7. Nephrotoxicity affects two-thirds of ICU patients, as approximately 20% of the drugs prescribed in ICU are considered nephrotoxic8,9, this includes nonsteroidal anti-inflammatory drugs (NSAIDs), antibiotics such as vancomycin and aminoglycosides, and chemotherapeutic agents like methotrexate and cisplatin7. Cisplatin is one of the most potent and common chemotherapy drugs, used in the treatment of solid tumors, such as head and neck, testicular, ovarian, and bladder10. In the kidney, cisplatin is internalized in the proximal convoluted tube (PCT) through the organic cationic transporter 2 (OCT-2) and in high concentrations binds to the DNA triggering cell death pathways7,10,11,12. The accumulation of this drug in the kidney contributes to nephrotoxicity with death and inflammation13. This detrimental side effect affects enormously the life and prognosis of one-third of cancer patients undergoing cisplatin treatment, so is imperative the research of new therapies that can lower the nephrotoxicity without losing the killing effect on cancer cells10.
Because of this nephrotoxic effect, cisplatin is commonly used as an inductor of AKI in experimental animal models, as described forward. In rodents, the first AKI model induced by cisplatin was reported in 197114 but at present, many different protocols have emerged using the dose-dependent and cumulative effects of cisplatin15. Thereby, depending on the dosage and number of applications, different grades of severity of kidney injury can be induced16,17,18,19,20,21. The most frequent method consists of an intraperitoneal (i.p.) injection of one dose of cisplatin followed by euthanasia in the following days. In this classic protocol, a single high nephrotoxic dose of cisplatin (10-13 mg/kg in mice and/or 3-8 mg/kg in rats) induces severe histological changes, such as loss of brush border and cell debris inside the tubular lumen, a few days after cisplatin injection. The severity of histological changes is dose-dependent, and signs of regeneration are observed 7 days after cisplatin injection16,17.
Although rodent models are well established, we decided to take advantage of the characteristics of another vertebrate, focusing our studies on the zebrafish (Danio rerio). This fish has been extensively used for modeling human diseases, because of its small size, external fertilization, high reproduction rates, rapid development, transparency of the embryos and larvae, low maintenance cost, similar anatomy to mammals (with some exceptions), high tissue regeneration capacity, social behavior, 70% of genetic similarity with humans and 84% with human diseases-associated genes22. Streisinger et al.23,24,25 started the studies with zebrafish that confirmed the practicability of utilizing this model organism for the genetic analysis of vertebrate development. In kidney research, the zebrafish has emerged not only in developmental studies but also as a genetic tool in the search for new genes linked to kidney conditions26. Furthermore, the capacity of regeneration without scar formation and the ability to generate nephrons through their life, called neonephrogenesis, make the zebrafish a key animal model for regeneration research27,28. Moreover, the availability of experimental models for different kidney illness, including acute and chronic kidney injury, demonstrate the versatility of this experimental organism26,29. As in mammals, the renal progenitors of the zebrafish are derived from the intermediate mesoderm. Such renal progenitors generate the pronephros that will later develop to the mesonephros, which will be maintained as a mature organ until adulthood29,30.
The adult zebrafish kidney is located on the dorsal wall of the body, between the swim bladder and the backbone29. From a ventral view, the zebrafish can be segmented into three regions (Figure 1A): head (H), trunk (Tr), and tail (Ta)29. Same as mammals, the zebrafish has the nephrons as functional units of the kidney, which are divided into tubule segments (Figure 1A): renal corpuscle (RC), proximal convoluted tubule (PCT), proximal straight tubule (PST), distal early (DE), late distal (DL) and collecting duct (CD)29. Zebrafish shares genetic conservation and structural similarities with human nephrons (Figure 1B) but lacks some conformations such as the intermediate tubule, also known as the loop of Henle (LH)29,31. Freshwater fishes like zebrafish are normally surrounded by a medium with very low osmolarity, because of this, they tend to be hyperosmotic and depend on the gills, the skin at early stages, and the kidney to regulate osmolarity and water excretion32. The filtration of blood from the dorsal aorta by the pronephros begins around 48 h post-fertilization (hpf)33,34. The kidney of the zebrafish is not only a metabolic waste excretion organ but also works as a hematopoietic organ from 4 days post-fertilization (dpf) to adulthood and it is equivalent to the bone marrow in mammals35. During development, hematopoietic stem cells (HSCs) will seed the kidney, self-renovate, and generate myeloid, erythroid, and lymphoid cell lineages, maintaining transcription factors, signaling molecules, and highly conserved genetic programs with mammals36,37. Studies have revealed that most erythroid, thrombocytic, myeloid, and lymphoid cells of the human immune system are present in zebrafish37,38. The unique characteristics of this animal and the conserved features with the human kidney made this model organism advantageous in the research of kidney function, injury, and regeneration.
Although the kidney of the zebrafish is well studied and some models of AKI are already available in larva and adult zebrafish28, at the time of the establishment of this protocol there was no evidence of a chemically-induced non-antibiotic AKI model in adult zebrafish. Besides this, our laboratory focuses on testing probiotic bacteria and microbiota-derived compounds to study regeneration and renal damage, thus we concentrated our efforts in creating a new cisplatin-induced AKI model in adult fishes. The video article presented in this manuscript demonstrates the procedures for a new model of AKI induction using an i.p. injection of 120 ug cisplatin per g of animal (120 µg/g) (Figure 2A). This dose was initially based on studies of AKI induced by cisplatin in murine models that went around 10 mg/kg (equivalent to 10 µg/g)14,15,16,17, however, this dose was not sufficient to induce kidney damage related to nephrotoxicity (data not shown). Thus, we increased the dose to the ones used in this study (Figure 2B). Our work revealed a dose-dependent effect of cisplatin in survival rate after injection with induction of kidney tissue damage 24 hpi as shown by loss of tubular structure, increased inflammatory infiltrate, and high rate of cell death. Here, we describe two techniques for analyzing the development of cisplatin-induced AKI: flow cytometry, to analyze cell infiltration, and TUNEL, to measure cell death. Flow cytometry is a technology that measures the physical (size and granularity) and chemical (fluorescent compounds) characteristics of the cells. Inside the cytometer the cell suspension runs through a sheath fluid that organizes the cells in a single line, allowing them to pass through a laser beam one cell at a time (Figure 3A). A detector in front of the light beam will measure the Forward Scatter (FSC), which correlates with cell size, and detectors to the side will measure the Side Scatter (SSC) that correlates to the granularity of the cells. Other detectors will measure the fluorescence from particles, fluorescent-proteins, or antibody-labeled cells39,40. As commercial antibodies for zebrafish are scarce nowadays, the use of animal reporters and fluorescent biomarkers allows to improve this analysis and identify diverse cell populations41,42,43. Another tool used in this protocol was the Terminal deoxynucleotidyl transferase (TdT) dUTP Nick End Labeling (TUNEL) assay. The TUNEL assay is a late-stage apoptosis detection method that relies on the ability of the TdT to identify fragmented DNA and label it with deoxynucleotides tagged with a fluorescent marker that later can be visualized and quantified by microscopy44 (Figure 3B). Considering that one of the most striking features of AKI is the induction of apoptosis in tubular kidney cells3, this technique is extremely advantageous since it can be analyzed by flow cytometry and/or microscopy.
The approaches presented in this article allows the observation of the AKI status and offer a new acute model to study AKI disorders that can be useful for the research of new therapeutic targets in cisplatin-related AKI.