Kidney Fibrosis in In Vitro and In Vivo Models: Path Toward Physiologically Relevant Humanized Models

Urology Life Sciences Bioengineering Nephrology Medicine Basic Medical Sciences Immunology Biochemistry
Kidney fibrosis Chronic kidney disease 3D humanized models Disease modeling Drug development

Mechanisms and Research Models of Kidney Fibrosis: Progress Toward Physiologically Relevant Humanized Models

Research Background and Problem Statement

Chronic Kidney Disease (CKD) is a major global public health issue, estimated to affect more than 10% of the population and is one of the leading causes of mortality. Kidney fibrosis, as a key pathological endpoint of CKD, disrupts the structure and function of nephrons. However, the pathological mechanisms of kidney fibrosis are not yet fully understood. Most existing studies on kidney fibrosis rely on animal models, which, while uncovering some potential mechanisms, are limited by their inability to fully replicate the physiology, metabolism, and molecular pathways of the human kidney, leading to significant translational gaps in the development of drugs and therapies. In addition, traditional two-dimensional (2D) cell culture models, commonly used as the starting point for disease research and drug screening, lack the advanced three-dimensional (3D) bioarchitecture and functionalities of the kidney, making them inadequate for comprehensive studies. These shortcomings highlight the need for more advanced 3D humanized in vitro models.

Paper Details

This review article is authored by Gabriele Addario, Lorenzo Moroni, and Carlos Mota, researchers affiliated with the MERLN Institute for Technology-Inspired Regenerative Medicine at Maastricht University, the Netherlands. It was published in 2025 in the journal Advanced Healthcare Materials.

Research Content and Perspectives

This review discusses the pathophysiology of kidney fibrosis, current research models, and future directions for developing humanized 3D models. Its key insights are as follows:

1. Complexity of the Urinary System and Kidneys

The authors start by detailing the complexity of the urinary system, focusing on its anatomy and physiological functions. The urinary system comprises the kidneys, ureters, bladder, and urethra. The kidneys, as the central organs, perform multiple physiological functions through their basic functional units, the nephrons, including filtration of metabolic waste, regulation of water-salt and acid-base balance, control of blood pressure and hormone levels, as well as vitamin D activation and calcium metabolism. Each kidney contains over one million nephrons spread between the cortex and medulla, with each nephron consisting of the renal corpuscle and renal tubule. The anatomical complexity and cellular diversity of the kidney make it particularly vulnerable to pathological conditions such as fibrosis, cancer, and diabetic nephropathy.

Another key contributing factor to this complexity is the intricate interactions between cells and the extracellular matrix (ECM). The kidney ECM comprises components such as collagen and proteoglycans, providing the physicochemical and biomechanical signals necessary for tissue morphogenesis. However, alterations in ECM composition and structure can lead to pathological states such as fibrosis.

2. Mechanisms of Kidney Fibrosis

Kidney fibrosis is a hallmark of CKD and is characterized by an imbalance between excessive ECM production and insufficient ECM degradation, leading to irreversible damage to nephron structure and function. The paper provides an in-depth analysis of the triggers and mechanisms behind fibrosis:

  1. Controversy Over Fibroblast Origins
    Fibroblasts are considered key contributors to excessive ECM deposition. Nonetheless, their origin remains a topic of debate. Possible sources include endothelial cells undergoing endothelial-to-mesenchymal transition (EndMT), tubular epithelial cells participating in epithelial-to-mesenchymal transition (EMT), and bone marrow-derived fibroblasts. While further research is necessary, these mechanisms help elucidate the diverse pathways leading to fibrosis.

  2. Pathological Signaling Pathways
    Fibrosis progression heavily depends on the Transforming Growth Factor-β (TGF-β) signaling pathway, which promotes fibroblast activation and ECM synthesis. Additionally, stimuli such as hypoxia, proteinuria, and toxin exposure can activate this pathway.

3. Current Diagnostics and Therapies

Diagnostic methods for kidney fibrosis primarily include non-invasive markers (e.g., glomerular filtration rate and proteinuria) and invasive tissue biopsies. These methods, however, have limited sensitivity and accuracy in detecting early-stage disease, and tissue biopsy poses potential risks due to its invasive nature.

In terms of treatment, no effective drugs for kidney fibrosis have been approved to date. The paper mentions several candidate drugs under research, such as small molecules that inhibit the TGF-β signaling pathway (e.g., Remdesivir and Pirfenidone). While some of these agents have shown anti-fibrotic effects in preclinical trials, they often fail in clinical translation, underscoring the urgent need for more advanced drug screening models.

4. Types and Limitations of Existing Models

The paper comprehensively summarizes 2D cell culture models, in vivo models, and advanced 3D models, evaluating the strengths and weaknesses of each:

  1. Applications and Limitations of Animal Models
    Rodents, particularly mice and rats, are the most commonly used animal models for kidney fibrosis research. These models have yielded key insights, such as the critical role of TGF-β signaling pathways. However, the translational relevance of animal models for human physiology and pathology is limited, especially regarding protein metabolism and drug response. Additionally, ethical considerations and the adoption of the 3Rs principle (Replacement, Reduction, and Refinement) are driving the development of alternative non-animal models.

  2. Conventional 2D Cell Culture Models
    While widely employed, 2D culture models fail to replicate the 3D architecture and cellular interactions of the kidney, leading to discrepancies in functional and differentiation states compared to in vivo conditions.

  3. Potential of Advanced 3D Models
    The paper discusses innovative 3D models such as spheroids, organoids, on-chip models, and biofabrication techniques in depth. While organoids and tubuloids demonstrate early nephron functionality, they remain immature and lack complexity. Emerging tools like bioprinting and microfluidic chips present promising avenues for creating dynamic, multi-cellular systems that closely recapitulate human physiology.

5. Future Outlook and Significance

The paper concludes by emphasizing the need for integrated approaches combining multiple technologies (e.g., bioprinting and microfluidics) to create fully organ-like structures that replicate anatomical, physiological, and molecular functions of the kidney. These advanced biomimetic models offer the potential to overcome the limitations of traditional models, achieve a deeper understanding of fibrotic mechanisms, and accelerate translational advancements in anti-fibrotic therapies.

Scientific significance: Provides deeper mechanistic insights and addresses the limitations of animal models.
Practical value: Offers effective alternative tools for disease modeling and drug development.

This paper systematically evaluates the limitations of existing research models while highlighting the importance of developing advanced 3D humanized in vitro models. The authors’ comprehensive review provides critical insights into kidney fibrosis research and underscores the immense potential for leveraging technology-driven approaches to advance the study of renal diseases.