Renal tubular epithelial injury leading to renal interstitial fibrosis via maladaptive repair

Kohei Matsushita

National Institute of Health Sciences, Japan

Renal interstitial fibrosis is a major pathological consequence of persistent or incompletely resolved renal tubular injury and is an important outcome in the assessment of nephrotoxic substances. Although renal tubular epithelial cells possess substantial regenerative capacity, some injured cells fail to restore their normal differentiated phenotype and instead undergo maladaptive repair. This adverse outcome pathway (AOP) describes a proposed sequence of biological events linking renal tubular epithelial injury to renal interstitial fibrosis through the persistence of tubular epithelial cells exhibiting a failed-repair phenotype. The molecular initiating event is not specified, as multiple nephrotoxic stressors may converge on tubular epithelial injury.

The proposed pathway involves renal tubular epithelial injury, an increase in tubular epithelial cells exhibiting a failed-repair phenotype, and subsequent renal interstitial fibrosis as the adverse outcome (AO). Failed-repair tubular epithelial cells, particularly those originating from the proximal tubule, may exhibit persistent alterations in differentiation status and proinflammatory and profibrotic signaling. Persistent signaling from maladaptively repaired tubular epithelial cells may contribute to interstitial inflammation, fibroblast activation, and extracellular matrix accumulation.

Evidence from experimental kidney injury models, histopathological studies, and single-cell and single-nucleus transcriptomic analyses supports the association between failed tubular repair and renal fibrosis. Although substantial evidence supports a role for maladaptive tubular repair in renal fibrosis, the quantitative relationships between key events and the extent to which this pathway applies across different nephrotoxic stressors remain to be fully characterized. This AOP aims to provide a mechanistic framework for organizing existing evidence and identifying measurable endpoints relevant to the assessment of chemical-induced renal fibrosis.

Renal interstitial fibrosis is an important histopathological outcome in the assessment of nephrotoxic substances. Although renal tubular epithelial cells have substantial regenerative capacity, recovery from tubular injury does not always result in restoration of normal epithelial structure and function. Understanding why some injured tubular epithelial cells undergo successful repair whereas others persist in an abnormal state is an important challenge in renal toxicology.

This AOP is being developed to organize and integrate mechanistic evidence concerning maladaptive renal tubular epithelial repair and its contribution to renal interstitial fibrosis. Particular emphasis is placed on the proximal tubule, which is a major target of nephrotoxic substances. Rather than focusing on a specific chemical or molecular initiating event, the AOP aims to provide a framework applicable to renal tubular injury induced by different nephrotoxic mechanisms.

This AOP is intended to provide a framework for organizing and evaluating measurable biological events associated with maladaptive tubular repair and for interpreting renal toxicity findings from a mechanistic perspective. Such a framework may also contribute to the development and evaluation of in vitro renal toxicity models and integrated approaches to chemical safety assessment.

The development of this AOP follows an iterative, evidence-based approach, focusing on the biological events linking renal tubular epithelial injury to renal interstitial fibrosis through maladaptive tubular repair.

Starting from renal interstitial fibrosis as the defined adverse outcome, the development of this AOP focuses specifically on the upstream biological events associated with maladaptive repair following renal tubular epithelial injury. Relevant evidence is being identified through targeted literature searches using PubMed and examination of existing AOPs, key events (KEs), and key event relationships (KERs) in the AOP-Wiki. Search terms include combinations of "renal fibrosis," "tubular injury," "maladaptive repair," "failed-repair tubular epithelial cells," "proximal tubule," and "nephrotoxicity." Relevant references cited in review articles and primary research publications are also considered.

The evidence assessment focuses primarily on experimental kidney injury models, including chemically induced nephrotoxicity, and incorporates findings from histopathological, molecular, and single-cell or single-nucleus transcriptomic studies. Particular attention is given to the temporal sequence of biological events, their biological plausibility, and experimental evidence supporting causal relationships. Studies involving experimental interventions that modify tubular repair and assess the corresponding effects on subsequent fibrotic responses are considered particularly informative for evaluating KE essentiality.

KEs are selected based on their biological significance, measurability, and relevance to the progression toward renal fibrosis. Existing KEs and KERs in the AOP-Wiki are preferentially reused where their definitions are appropriate. Additional KEs are considered when necessary to represent important biological changes not adequately captured by existing entries. Detailed molecular mechanisms are incorporated into KER descriptions where appropriate, rather than being represented as separate KEs unless they constitute distinct and measurable events.

The strength of evidence supporting each KER will be evaluated based on biological plausibility, empirical support, and available quantitative information. Evidence gaps, uncertainties, and potential differences among nephrotoxic stressors will be documented as the AOP is developed.