肾类器官培养：肾单位发育与肾小管模型

以下为英文全文（English full text）

1. Introduction to Kidney Organoid Technology

The kidney is one of the most complex and highly organized organs in the human body, responsible for blood filtration, electrolyte homeostasis, blood pressure regulation, acid-base balance, and the production of hormones such as erythropoietin and renin. Its functional unit, the nephron, comprises the glomerulus (containing podocytes, mesangial cells, and endothelial cells), Bowman's capsule, the proximal tubule, the loop of Henle, the distal tubule, and the connecting tubule/collecting duct. Each segment of the nephron has a distinct epithelial architecture, transporter profile, and physiological function, making the kidney exceptionally challenging to model in vitro using conventional cell culture systems [1,2].

Kidney organoid technology has enabled the in vitro modeling of nephrogenesis, polycystic kidney disease (PKD), acute kidney injury (AKI), drug-induced nephrotoxicity, congenital abnormalities of the kidney and urinary tract (CAKUT), and diabetic nephropathy. The first kidney organoids from pluripotent stem cells were reported by Taguchi and colleagues in 2014, who generated metanephric mesenchyme (MM) that self-organized into nephron-like structures when co-cultured with mouse embryonic spinal cord tissue. While this was a landmark achievement, the dependence on mouse tissue limited its clinical applicability [3]. The subsequent protocol by Morizane and colleagues in 2015 eliminated the need for mouse tissue by using a defined cocktail of CHIR99021 (a GSK-3β inhibitor that activates Wnt signaling), activin A, and FGF9 to induce nephron progenitor cells (NPCs) with >90% efficiency. These NPCs self-organized into segmented nephrons containing podocytes, proximal tubules, loops of Henle, and distal tubules, providing a fully human, chemically defined system for kidney organoid generation [1,4].

This article provides comprehensive protocols for kidney organoid differentiation, culture, functional validation, and integration with advanced model systems, with detailed technical parameters and quality control measures validated for research and translational applications.

2. Principles of Kidney Organoid Development

Kidney development in vivo proceeds through a precisely orchestrated series of inductive interactions between the ureteric bud (UB) and the metanephric mesenchyme (MM). The UB arises from the nephric duct and invades the MM, inducing the mesenchymal cells to condense, undergo mesenchymal-to-epithelial transition (MET), and form the nephron. In organoid culture, this developmental program is recapitulated by sequential exposure to specific growth factors and small molecules that mimic the embryonic signaling environment [1,2,4].

The key developmental stages and their molecular regulators are:

• Primitive Streak Induction: The differentiation begins with the activation of Wnt signaling using CHIR99021 (a GSK-3β inhibitor), which drives the cells toward a primitive streak fate. High-dose CHIR99021 (8–10 µM) for 4 days induces the expression of primitive streak markers (T/Brachyury, TBX6, MIXL1) [1,4,5].
• Intermediate Mesoderm Formation: Activin A (a TGF-β family member) promotes the transition from primitive streak to intermediate mesoderm, which is the embryonic precursor of the kidney. Lower-dose CHIR99021 (3 µM) is maintained during this phase to sustain Wnt signaling at a level appropriate for intermediate mesoderm specification [1,4].
• Metanephric Mesenchyme and Nephron Progenitor Specification: FGF9 is essential for the specification and maintenance of nephron progenitor cells (NPCs) from the intermediate mesoderm. FGF9 signaling, combined with low-level Wnt activation, induces the expression of NPC markers including SIX2, SALL1, WT1, and PAX2. NPCs have the capacity to self-organize into nephron-like structures through a process of spontaneous mesenchymal-to-epithelial transition [1,4,5].
• Nephron Segmentation: Following the aggregation of NPCs and a transient pulse of Wnt signaling (to simulate the UB-derived Wnt signal that induces MET), the NPCs undergo mesenchymal-to-epithelial transition and form renal vesicles. These vesicles spontaneously self-pattern into segmented nephron structures, including podocytes (visceral epithelium of the glomerulus), proximal tubules, loops of Henle, and distal tubules [1,4,5].

The critical signaling pathways are:

• Wnt signaling: CHIR99021 is the cornerstone of kidney organoid differentiation. It activates Wnt/β-catenin signaling downstream of the receptor by inhibiting GSK-3β. The dose and duration of CHIR99021 exposure are precisely titrated: high dose (8–10 µM) for primitive streak induction, moderate dose (3 µM) for intermediate mesoderm, and low dose (1–3 µM) for NPC maintenance and MET induction [1,4,5].
• Activin/Nodal signaling: Activin A (10 ng/mL) promotes the posteriorization of the primitive streak and the specification of intermediate mesoderm. It is essential for the transition from primitive streak to renal lineage [1,4].
• FGF signaling: FGF9 (10 ng/mL) is the primary driver of NPC specification and maintenance. It promotes the expression of SIX2 and SALL1 and maintains the self-renewal capacity of the NPC population. FGF9 is withdrawn after day 14, allowing the NPCs to differentiate spontaneously [1,4,5].
• BMP signaling: BMP4 and BMP7 promote mesoderm specification and are included during the early stages of differentiation. Noggin (a BMP inhibitor) is sometimes added to prevent excessive BMP signaling, which can promote alternative mesodermal fates [2,5].
• Retinoic acid: Retinoic acid promotes anterior-posterior patterning and is used in the Taguchi-Nishinakamura protocol to enhance ureteric bud specification and proximal tubule development [2,3].

3. Tissue Sources for Kidney Organoids

3.1 Pluripotent Stem Cells

Human embryonic stem cells (ESCs) and induced pluripotent stem cells (iPSCs) are the primary sources for kidney organoid generation. The choice of PSC line can influence differentiation efficiency, and some lines require optimization of CHIR99021 concentration. Common lines include:

• H9 (WA09) and H1 (WA01) ESC lines (WiCell)
• Various iPSC lines (e.g., WTC11, H9-FP, KOLF2.1)
• Patient-specific iPSC lines for disease modeling (e.g., PKD1/2 mutations for polycystic kidney disease, HNF1B mutations for CAKUT) [1,4,5]

PSCs should be validated for pluripotency (OCT4, NANOG, SSEA4, TRA-1-60 expression), normal karyotype, and mycoplasma-negative status before differentiation. The cells should be maintained in a feeder-free system (Matrigel or Geltrex with mTeSR1 or StemFlex) and passaged as small colonies or single cells before differentiation [4,5].

3.2 Adult Kidney-Derived Organoids

Recent advances have enabled the establishment of organoids from adult kidney epithelial cells, including tubular cells and glomerular cells. These protocols typically involve the isolation of EpCAM+ or CD133+ cells from kidney tissue, followed by culture in modified kidney organoid medium. Adult kidney-derived organoids offer the advantage of direct derivation from mature tissue but are limited by the availability of healthy human kidney tissue and the low proliferative capacity of adult renal epithelial cells [2,6].

4. PSC-Derived Kidney Organoid Protocol (Morizane Protocol)

The Morizane protocol is the most widely used method for generating kidney organoids from human PSCs and has been validated by numerous laboratories. It produces nephron-like structures with high efficiency and reproducibility. The protocol is divided into four stages: (1) primitive streak induction, (2) intermediate mesoderm formation, (3) NPC specification, and (4) organoid formation and maturation [1,4,5].

4.1 Stage 1: Primitive Streak Induction (Days 0–4)

1. Seed iPSCs at 50–70% confluency in 6-well plates coated with Matrigel or Geltrex. The cells should be in a healthy, undifferentiated state with typical pluripotent morphology.
2. Day 0: Aspirate the stem cell medium and add 2 mL per well of Stage 1 differentiation medium: Advanced RPMI 1640 (Gibco, 12633012) + 1× GlutaMAX + 10 µM CHIR99021 (Tocris Bioscience, 4423) + 5 ng/mL Noggin (R&D Systems, 6057-NG). The high dose of CHIR99021 activates Wnt signaling robustly to drive primitive streak formation. Noggin is included to suppress BMP signaling, which would otherwise promote alternative mesodermal fates.
3. Day 2: Refresh the medium with fresh Stage 1 medium (2 mL per well). The cells should begin to change morphology, becoming elongated and more densely packed, characteristic of primitive streak cells.
4. Day 4: By day 4, the cells should express primitive streak markers (T/Brachyury, TBX6, MIXL1). Verify marker expression by immunofluorescence or quantitative PCR if troubleshooting is required. The optimal cell density at this stage is approximately 80–90% confluency [1,4,5].

Note: Some iPSC lines (such as KOLF2.1) may require a lower CHIR99021 concentration (7–8 µM) for optimal primitive streak induction. Test 7, 8, and 10 µM CHIR99021 when establishing the protocol with a new cell line [4,5].

4.2 Stage 2: Intermediate Mesoderm Formation (Days 4–7)

1. Day 4: Aspirate the Stage 1 medium and add 3 mL per well of Stage 2 differentiation medium: Advanced RPMI 1640 + 1× GlutaMAX + 10 ng/mL Activin A (R&D Systems, 338-AC). Activin A drives the transition from primitive streak to intermediate mesoderm by activating the Smad2/3 pathway.
2. Day 5: Refresh the medium with fresh Stage 2 medium (3 mL per well).
3. Day 6: Change the medium to Stage 3 pre-medium: Advanced RPMI 1640 + 1× GlutaMAX + 10 ng/mL FGF9 (R&D Systems, 273-F9).
4. Day 7: Refresh the Stage 3 pre-medium. By day 7, the cells should express intermediate mesoderm markers (OSR1, WT1, HOXD11, PAX2) and early NPC markers (SIX2, SALL1). The cells should be nearly 100% confluent and may begin to form small clusters [1,4,5].

4.3 Stage 3: NPC Aggregation and Organoid Formation (Days 8–16)

1. Day 8: Dissociate the cells using TrypLE Express (Gibco, 12604013) or Accutase (STEMCELL Technologies, 07920) at 37°C for 5–10 minutes. Gently pipette to ensure complete dissociation to single cells. Pass through a 40 µm cell strainer to remove clumps.
2. Centrifuge at 300 × g for 5 minutes at room temperature.
3. Resuspend the cells in Advanced RPMI 1640 + 1× GlutaMAX at a concentration of 2.5 × 10^5 cells per µL (or 2.5 × 10^6 cells per 10 µL).
4. Prepare the organoid initiation medium. Two alternative methods are commonly used:
   • Method A (Suspension in Low-Attachment Plates):
     • Prepare medium: APEL2 (STEMCELL Technologies, 05270) + 1.5% PFHM-II (polymeric surfactant, Gibco, 0870912) + 100 ng/mL FGF9 + 100 ng/mL BMP7 (R&D Systems, 354-BP) + 1 µg/mL Heparin (Sigma-Aldrich, H3149).
     • Seed 6,000–10,000 cells per well in 200 µL of organoid initiation medium into low-attachment 96-well U-bottom plates (Corning, 7007) or EZSPHERE plates (Nacalai USA, TCI-4815-903SP-50P). The EZSPHERE plates have micropatterns that promote uniform aggregate formation [1,4,5].
     • Day 9: Add fresh medium with 3 µM CHIR99021 + 10 ng/mL FGF9.
     • Day 10: Change to medium with 10 ng/mL FGF9 only (no CHIR99021).
     • Day 11+: Culture in basal differentiation medium (Advanced RPMI 1640 + 1× GlutaMAX) without growth factors.
     • By day 14: Renal vesicles should form. By day 16, nephron-like structures with segmented tubules and podocyte-like cells should be visible by brightfield microscopy [1,4,5].
   • Method B (Air-Liquid Interface on Filters):
     • Prepare the organoid initiation medium as in Method A.
     • Add 1 mL per well of organoid initiation medium to 24-well plates.
     • Suspend isopore membranes (EMD Millipore, HTTP02500) at the surface of the medium to create an air-liquid interface.
     • Spot 2 µL of cell suspension (approximately 5,000 cells, 2.5 × 10^5 cells per µL) on top of each filter.
     • Change the medium every 48 hours. Remove growth factors after 4 days.
     • Culture for a total of 9 days. This method produces organoids with improved vascularization and maturation compared to suspension culture [5,7].

4.4 Stage 4: Long-Term Maturation (Days 16–28+)

After day 16, the kidney organoids can be maintained in basal differentiation medium (Advanced RPMI 1640 + 1× GlutaMAX) with medium changes every 2–3 days. The organoids will continue to mature and develop more defined nephron structures over time. By day 28, the organoids should contain:

• Glomerular-like structures with podocyte-like cells (NPHS1+, PODXL+, WT1+)
• Proximal tubules (LTL+, AQP1+, CDH2+)
• Distal tubules (CDH1+, EMX2+, SLC12A3+)
• Connecting tubules/collecting ducts (AQP2+, GATA3+, PAX2+) [1,4,5]

5. Modified Protocol: Taguchi-Nishinakamura Method

The Taguchi-Nishinakamura protocol offers an alternative approach that generates both nephron progenitors and ureteric bud-like structures, providing a more complete model of kidney development. This protocol uses an extended high-dose CHIR99021 pulse followed by a combination of activin A, BMP4, retinoic acid, and moderate CHIR99021 to drive posterior intermediate mesoderm, then FGF9 + low CHIR99021 for NPC induction [2,3].

1. Day 0–4: Treat iPSCs with 10 µM CHIR99021 + 5 ng/mL Noggin in Advanced RPMI 1640.
2. Day 4–7: Change to medium containing Activin A (10 ng/mL) + BMP4 (10 ng/mL) + retinoic acid (0.1 µM) + CHIR99021 (3 µM). This combination promotes the posterior intermediate mesoderm fate.
3. Day 7–10: Change to medium containing FGF9 (10 ng/mL) + CHIR99021 (1 µM). This drives the specification of SIX2+SALL1+ NPCs.
4. Day 10+: Aggregate the NPCs in low-attachment plates or on filters as described in the Morizane protocol. The Taguchi protocol produces organoids with both nephron-like structures and ureteric bud-like epithelial tubules, which is advantageous for studying UB-MM interactions [2,3].

6. Scalable Kidney Organoid Production

For applications requiring large numbers of kidney organoids (such as high-throughput drug screening or toxicity testing), the Morizane protocol can be adapted to scalable suspension culture systems.

6.1 Spinner Flask and Bioreactor Culture

1. Differentiate PSCs to NPCs in monolayer as described in Stages 1–2 (Days 0–9).
2. On day 9, dissociate the NPCs and seed into spinner flasks (e.g., Corning 125 mL spinner flasks) at a density of 1 × 10^6 cells per mL in organoid initiation medium.
3. Agitate at 30–40 rpm to maintain the cells in suspension without excessive shear stress.
4. On day 10, add 3 µM CHIR99021 + 10 ng/mL FGF9.
5. On day 11, switch to basal medium (Advanced RPMI + GlutaMAX) without growth factors.
6. Continue culture for 14–21 days with daily medium changes.
7. This method can produce thousands of organoids per flask, each containing multiple nephron segments [1,5,8].

6.2 EZSPHERE and AggreWell Plates

EZSPHERE plates (Nacalai USA) contain micropatterned wells that promote uniform aggregate formation. Each well of a 12-well EZSPHERE plate generates approximately 400 organoids when seeded with 6 × 10^5 cells. AggreWell plates (STEMCELL Technologies) provide a similar function and can be used for large-scale, uniform organoid production [5,8].

7. Kidney Organoid Characterization and Quality Control

7.1 Morphological Assessment

Healthy kidney organoids should exhibit the following morphological features:

• Day 14: Spherical aggregates with clear epithelial structures visible by brightfield microscopy. Small vesicles (renal vesicles) may be visible as clear, cystic structures.
• Day 21: Tubular outgrowths and more defined epithelial structures. The organoids may develop convoluted surfaces.
• Day 28: Glomerular-like structures (dense, compact clusters of cells) adjacent to tubular segments. The tubules should be segmented, with different regions exhibiting different morphologies (proximal tubules are wider and more cuboidal; distal tubules are narrower and more columnar) [1,4,5].

Signs of poor differentiation include:

• Failure to form epithelial structures (indicates insufficient MET induction)
• Cystic degeneration (indicates excessive Wnt signaling or poor nutrient exchange)
• Necrotic cores (indicates organoid size is too large or culture conditions are suboptimal)
• Heterogeneous morphology with large non-renal structures (indicates incomplete primitive streak or intermediate mesoderm induction) [1,4,5]

7.2 Immunofluorescence Staining by Nephron Segment

Cryosectioning and immunofluorescence staining are essential for validating the segmented nephron structures in kidney organoids. The following markers are used to identify specific nephron segments:

Podocytes (Glomerular Visceral Epithelium):

• NPHS1 (Nephrin) – slit diaphragm protein; NPHS1 antibody (Progen, GP-N2, 1:200)
• PODXL (Podocalyxin) – glycocalyx protein; PODXL antibody (R&D Systems, AF1658, 1:200)
• WT1 (Wilms tumor 1) – transcription factor; WT1 antibody (Abcam ab89901, 1:200)
• SYNPO (Synaptopodin) – actin-associated protein; SYNPO antibody (Abcam ab91567, 1:200) [1,4,5]

Proximal Tubules:

• LTL (Lotus tetragonolobus lectin) – binds to glycoconjugates on proximal tubule brush border; LTL-FITC (Vector Laboratories, FL-1321, 1:500)
• AQP1 (Aquaporin 1) – water channel; AQP1 antibody (Abcam ab9566, 1:200)
• CDH2 (N-cadherin) – adhesion molecule; CDH2 antibody (Abcam ab76057, 1:200)
• HNF4A (Hepatocyte nuclear factor 4-alpha) – transcription factor expressed in proximal tubules; HNF4A antibody (Abcam ab181604, 1:200)
• SGLT2 (Sodium-glucose cotransporter 2) – glucose reabsorption transporter; SGLT2 antibody (Abcam ab85626, 1:200) [1,4,5]

Loop of Henle:

• UMOD (Uromodulin/Tamm-Horsfall protein) – highly expressed in the thick ascending limb; UMOD antibody (Abcam ab207170, 1:200)
• CDH1 (E-cadherin) – adhesion molecule; CDH1 antibody (Abcam ab40772, 1:200)
• SLC12A1 (NKCC2) – sodium-potassium-chloride cotransporter; SLC12A1 antibody (Abcam ab240567, 1:200) [1,4,5]

Distal Tubules:

• CDH1 (E-cadherin)
• EMX2 (Empty spiracles homeobox 2) – transcription factor; EMX2 antibody (Abcam ab106582, 1:200)
• SLC12A3 (NCC) – sodium-chloride cotransporter; SLC12A3 antibody (Abcam ab95302, 1:200)
• TRPM6 (Transient receptor potential cation channel subfamily M member 6) – magnesium channel; TRPM6 antibody (Abcam ab208898, 1:200) [1,4,5]

Collecting Duct:

• AQP2 (Aquaporin 2) – water channel regulated by vasopressin; AQP2 antibody (Abcam ab15117, 1:200)
• GATA3 (GATA binding protein 3) – transcription factor; GATA3 antibody (Abcam ab199428, 1:200)
• PAX2 (Paired box 2) – transcription factor; PAX2 antibody (Abcam ab79389, 1:200) [1,4,5]

Progenitor Markers:

• SIX2 (Sine oculis homeobox homolog 2) – NPC marker; SIX2 antibody (Proteintech, 11562-1-AP, 1:200)
• SALL1 (Spalt-like transcription factor 1) – NPC marker; SALL1 antibody (Abcam ab244274, 1:200)
• WT1 (also expressed in NPCs and podocytes) [1,4,5]

7.3 Functional Assays

• Albumin Uptake: Proximal tubule cells in kidney organoids retain the ability to internalize albumin via receptor-mediated endocytosis. Incubate the organoids with FITC-albumin (50 µg/mL, Sigma-Aldrich, A9771) or Texas Red-albumin (50 µg/mL, Thermo Fisher, A22851) for 2–4 hours. Wash extensively with PBS and visualize by fluorescence microscopy. Proximal tubule cells should exhibit bright intracellular fluorescence, indicating albumin uptake. The uptake can be inhibited by megalin-blocking antibodies, confirming the specificity of the endocytic pathway [1,5,9].
• Glucose Reabsorption: Measure glucose uptake using a fluorescent glucose analog (2-NBDG, 100 µM, Thermo Fisher, N13195) or radioactive glucose. The uptake should be inhibitable by phlorizin (a SGLT2 inhibitor), demonstrating the presence of functional sodium-glucose cotransporters [1,9].
• Nephrotoxicity Assays: Kidney organoids are valuable models for assessing drug-induced nephrotoxicity, particularly for compounds that target the proximal tubule. Common nephrotoxicants include:
  • Cisplatin (10–50 µM, 48–72 hours): Induces dose-dependent proximal tubule cell death, detectable by caspase-3 activation and loss of LTL staining.
  • Gentamicin (1–5 mM, 72 hours): An aminoglycoside antibiotic that causes proximal tubule injury.
  • Aristolochic acid (10–50 µM, 48 hours): A plant-derived toxin that causes tubular injury and is associated with aristolochic acid nephropathy.
  • Cyclosporine A (10–50 µM, 72 hours): A calcineurin inhibitor that causes tubular toxicity.
  Injury is assessed by live/dead staining (calcein AM/ethidium homodimer), caspase-3 immunostaining (apoptosis marker), KIM-1 (kidney injury molecule-1) immunostaining (early injury marker), loss of epithelial polarity (disruption of ZO-1 and CDH1 staining), and reduced albumin uptake [1,5,9].
• Transepithelial Transport: Proximal tubule cells can be isolated from organoids and seeded on permeable supports (e.g., Transwell inserts) to measure transporter-mediated drug uptake. For example, metformin uptake via OCT2 (organic cation transporter 2) can be measured and inhibited by cimetidine (an OCT2 inhibitor), demonstrating the functional expression of drug transporters [1,9].

8. Kidney Organoid-on-Chip and Vascularization Strategies

8.1 Organoid-on-Chip Models

Kidney organoids can be integrated into microfluidic devices to improve nutrient delivery, enable the application of fluid shear stress, and model the vascular-tubular interface. In organoid-on-chip systems:

• Organoids are seeded into ECM-coated microchannels and perfused with medium at physiologic flow rates (0.1–1 µL/min).
• Fluid shear stress (0.1–1 dyne/cm²) improves endothelial cell development and tubule maturation.
• The microfluidic format enables real-time monitoring of organoid physiology using embedded sensors or fluorescence microscopy.
• Multi-channel devices can be designed to model the glomerular filtration barrier, with podocytes on one side of a porous membrane and endothelial cells on the other side [5,10].

8.2 Vascularization Strategies

Endogenous vascularization in kidney organoids is limited, as the organoids typically lack a functional blood supply. Strategies to improve vascularization include:

• Co-culture with endothelial cells (HUVECs) and mesenchymal cells: Endothelial cells can be mixed with NPCs during aggregation and will self-organize into vascular networks within the organoid.
• VEGF and PDGF-BB supplementation: These growth factors promote endothelial progenitor development and vascular network formation.
• Transplantation under the kidney capsule: When kidney organoids are transplanted into immunodeficient mice (e.g., NOD-SCID), they become vascularized by host endothelial cells and exhibit improved maturation and function. This approach has been used to demonstrate that kidney organoids can produce urine-like fluid and respond to renin-angiotensin system modulation [5,10].
• Decellularized kidney scaffolds: Seeding NPCs into decellularized kidney matrices provides a natural vascular architecture that supports organoid development and vascularization [5,10].

9. Disease Modeling Applications

9.1 Polycystic Kidney Disease (PKD)

PKD organoids derived from patient iPSCs (carrying PKD1 or PKD2 mutations) develop large cystic structures in the proximal tubules and collecting ducts, recapitulating the hallmark feature of the disease. Cyst formation is accelerated by forskolin (10 µM), which increases intracellular cAMP levels and promotes cyst epithelial proliferation. The cysts can be inhibited by:

• Octreotide (somatostatin analog, 1 µM) – reduces cAMP levels
• Rapamycin (mTOR inhibitor, 10 nM) – inhibits cyst epithelial proliferation
• CFTR inhibitors (such as CFTRinh-172, 10 µM) – reduce chloride and fluid secretion into the cyst lumen [1,5,11]

9.2 Acute Kidney Injury (AKI)

Cisplatin and aminoglycoside-induced nephrotoxicity can be modeled in kidney organoids. Injury manifests as proximal tubule cell death, KIM-1 upregulation, loss of epithelial polarity, and mitochondrial dysfunction. Pre-treatment with N-acetylcysteine (an antioxidant) or probenecid (a transporter inhibitor that reduces cisplatin accumulation) reduces cisplatin-induced injury, providing a platform for nephroprotective drug screening [1,5,9].

9.3 Congenital Abnormalities of the Kidney and Urinary Tract (CAKUT)

iPSC-derived organoids from patients with CAKUT-causing mutations (e.g., HNF1B, PAX2, RET) exhibit developmental defects including reduced tubule formation, altered nephron segmentation, and impaired branching morphogenesis. These models provide insights into the developmental mechanisms underlying congenital kidney diseases and can be used to test potential therapeutic interventions [2,5].

9.4 Diabetic Nephropathy

Kidney organoids can be exposed to high glucose conditions (25–30 mM glucose) to model diabetic nephropathy. High glucose exposure leads to podocyte injury (loss of NPHS1 and PODXL expression), proximal tubule dysfunction (reduced albumin uptake), and fibrotic changes (increased expression of collagen I and fibronectin). The addition of TGF-β (5 ng/mL) accelerates the fibrotic phenotype [1,5,12].

10. Troubleshooting Common Issues in Kidney Organoid Culture

Issue: Low NPC induction efficiency (<70% SIX2+ cells)

Potential Causes and Solutions:

• Optimize CHIR99021 concentration: Different iPSC lines require different CHIR99021 concentrations for optimal primitive streak induction. Test 7, 8, and 10 µM.
• Verify Activin A activity: Ensure that Activin A is present at 10 ng/mL and is biologically active. Store at −80°C in single-use aliquots.
• Check cell confluency: Cells should be at 50–70% confluency at the start of differentiation. Over-confluent cells may differentiate poorly.
• Ensure proper timing: Do not shorten the primitive streak or intermediate mesoderm phases. Each phase requires the full duration (4 days for primitive streak, 3 days for intermediate mesoderm).
• Test for mycoplasma contamination: Mycoplasma can profoundly affect differentiation efficiency [1,4,5].

Issue: Poor nephron segmentation in organoids

Potential Causes and Solutions:

• Ensure appropriate FGF9 concentration during aggregation: Use 10 ng/mL FGF9 during days 8–14. Too little FGF9 will result in NPC loss; too much may maintain the progenitor state and prevent differentiation.
• Verify BMP7 activity: BMP7 promotes tubule formation and segmentation.
• Extend culture time: Some organoids require 28–35 days for full nephron segmentation. Do not assess the organoids too early (before day 21).
• Use air-liquid interface culture: The air-liquid interface method improves maturation and segmentation compared to suspension culture [1,4,5,7].

Issue: Organoid necrosis or cystic degeneration

Potential Causes and Solutions:

• Reduce aggregate size: Large aggregates (>500 µm) develop hypoxic cores. Seed fewer cells per aggregate (5,000–6,000 instead of 10,000).
• Improve nutrient exchange: Use spinner culture, orbital shaking, or organoid-on-chip systems to improve perfusion.
• Change medium more frequently: Change the medium every 24–48 hours during the early stages (days 8–14).
• Reduce CHIR99021 concentration after day 9: High CHIR99021 concentrations after the MET phase can cause cystic degeneration [1,4,5].

Issue: Contamination with non-renal cells (e.g., mesenchymal, neuronal, or unidentified cell types)

Potential Causes and Solutions:

• Verify SIX2 and WT1 expression at day 7: If these markers are not expressed, the intermediate mesoderm specification failed. Repeat the differentiation with optimized conditions.
• Ensure proper primitive streak induction: Verify T and TBX6 expression at day 4. If primitive streak markers are absent, the downstream differentiation will fail.
• Use NPC isolation markers if needed: If non-renal cells are a persistent problem, consider isolating SIX2+ NPCs by fluorescence-activated cell sorting (FACS) at day 7 before aggregation [1,4,5].

11. Conclusion

Kidney organoid technology has advanced significantly over the past decade, enabling the generation of nephron-like structures from human pluripotent stem cells with remarkable fidelity to native kidney architecture. These organoids provide powerful platforms for modeling renal development, genetic kidney diseases, drug-induced nephrotoxicity, and regenerative therapies. The continued refinement of vascularization strategies, maturation protocols, and integration with organ-on-chip systems will further enhance the translational value of kidney organoid models.

GBiowit provides kidney organoid differentiation media, Matrigel and defined matrices, PSC-derived kidney organoid kits, and specialized services for nephrotoxicity screening, disease modeling, and kidney organoid-on-chip development. Our technical support team can assist with protocol optimization, troubleshooting, and the development of custom kidney organoid applications for research and drug discovery.

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