Molecular nephrology: types of acute tubular injury

The acute loss of kidney function has been diagnosed for many decades using the serum concentration of creatinine — a muscle metabolite that is an insensitive and non-specific marker of kidney function, but is now used for the very definition of acute kidney injury (AKI). Fortunately, myriad new tools have now been developed to better understand the relationship between acute tubular injury and elevation in serum creatinine (SCr). These tools include unbiased gene and protein expression analyses in kidney, urine and blood, the localization of specific gene transcripts in pathological biopsy samples by rapid in-situ RNA technology and single-cell RNA-sequencing analyses. However, this molecular approach to AKI has produced a series of unexpected problems, because the expression of specific kidney-derived molecules that are indicative of injury often do not correlate with SCr levels. This discrepancy between kidney injury markers and SCr level can be reconciled by the recognition that many separate subtypes of AKI exist, each with distinct patterning of molecular markers of tubular injury and SCr data. In this Review, we describe the weaknesses of isolated SCr-based diagnoses, the clinical and molecular subtyping of acute tubular injury, and the role of non-invasive biomarkers in clinical phenotyping. We propose a conceptual model that synthesizes molecular and physiological data along a time course spanning from acute cellular injury to organ failure. Acute kidney injury (AKI) is diagnosed by the serum concentration of creatinine (SCr), an insensitive marker of kidney function. The authors of this Review discuss the properties of ideal biomarkers of tubular damage and how they can be combined with SCr-based AKI definitions to provide greater insights into the processes underlying acute tubular injury. Key points Current definitions of acute kidney injury (AKI), based on serum creatinine (SCr) level, focus on loss of kidney function rather than kidney injury. AKI definitions cannot provide an acute measurement of loss of function, however, because SCr is a quantitative functional marker only at the steady state. Current AKI metrics can neither detect kidney injury in real time nor distinguish dramatically different types of kidney injury. Molecular analyses of acutely damaged kidneys have detected cellular and segment-specific responses to injurious stimuli, prior to and distinct from the loss of function as measured by SCr. As a result, molecular analyses h