Cellular and Molecular Mechanisms of Acute Kidney Injury

Histologic Alterations

The term acute tubular necrosis is a misnomer, because frank tubule cell necrosis rarely is encountered in human AKI. Prominent morphologic features of AKI in humans include effacement and loss of proximal tubule brush border, patchy loss of tubule cells, focal areas of proximal tubular dilation, presence of distal tubular casts, and areas of cellular regeneration. Necrosis is inconspicuous, very focal, and restricted to the highly susceptible outer medullar regions. Necrosis appears to preferentially affect the distal tubular segments in the outer medulla (thick ascending limbs and collecting ducts) and is seen less commonly in proximal tubule cells of the cortex. By contrast, apoptosis is a consistent finding in distal and proximal tubules in ischemic and nephrotoxic forms of human AKI. In addition, peritubular capillaries display a striking vascular congestion, endothelial damage, and leukocyte accumulation. The mechanisms underlying these morphologic findings, and their implications for the ensuing profound renal dysfunction, are detailed next.

Hemodynamic Alterations

An intense and persistent renal vasoconstriction that reduces overall renal blood flow to approximately 50% of the normal rate has long been considered a hallmark of intrinsic AKI. In addition, the postischemic kidney displays regional alterations in blood flow patterns. Marked congestion and hypoperfusion of the outer medulla persists, even though cortical blood flow improves during reperfusion after an ischemic insult, leading to prolonged cellular injury and cell death in these predisposed tubule segments. Mechanisms underlying these hemodynamic alterations relate primarily to endothelial cell injury. The result is a local imbalance of vasoactive substances, with enhanced release of vasoconstrictors such as endothelin and decreased abundance of vasodilators such as endothelium-derived nitric oxide. Endothelin receptor antagonists ameliorate ischemic AKI in animals, but human data are lacking. Similarly, carbon monoxide and carbon monoxide-releasing compounds are protective in animal models of ischemic AKI, probably through vasodilation and preservation of medullary blood flow. This approach is undergoing preliminary tests in humans with AKI. These hemodynamic abnormalities, however, cannot fully account for the profound loss of renal function, and several human trials of vasodilators such as dopamine have failed to demonstrate improvement in glomerular filtration rate established during AKI, despite augmentation of overall renal blood flow.

Alterations in Tubule Dynamics

Known derangements in tubule dynamics include obstruction, backleak, and activation of tubuloglomerular feedback. The consistent histologic findings of proximal tubule dilation and distal tubular casts in human biopsy specimens indicate that obstruction to tubular fluid flow certainly occurs in ischemic AKI. The intraluminal casts contain Tamm-Horsfall protein (uromodulin), which normally is secreted by the thick ascending limb as a monomer. Conversion into a gel-like polymer is promoted by the increased luminal sodium concentration typically encountered within the distal tubule in AKI. This provides the ideal environment for cast formation, with desquamated tubule cells and brush border membranes contributing to obstruction. However, it is unlikely that obstruction alone can account for intense renal dysfunction, because human studies using forced diuresis have not demonstrated an impact on survival and renal recovery rates in patients with AKI.

The various cell types along the nephron display segment-specific susceptibilities to different types of injury. Proximal tubule cells need a steady oxygen supply to remain viable, whereas those in the thick ascending limb are relatively resistant to hypoxia. Epithelial cells in the proximal tubule are injured most commonly in septic, ischemic, or nephrotoxic AKI; the S3 segment is most vulnerable to ischemic injury, whereas the S1 and S2 segments are affected most commonly by toxic nephropathy because of their high rates of endocytosis. Tubular cells also differ in their ability to generate hypoxia inducible transcription factors (HIFs), which mediate cellular responses to hypoxia and are generally cell protective. The collecting duct is most effective at generating HIFs, whereas the proximal tubule has a moderate capacity and the thick ascending limb is least effective. Segments of the nephron also produce differing biomarkers in response to injury. For example, kidney injury molecule-1 (KIM-1) originates from the proximal tubule, whereas neutrophil gelatinase-associated lipocalin (NGAL) arises from the collecting duct. Detecting biomarker changes and localizing them to specific tubule segments may provide more information about the type of injury affecting the kidney and may aid in administering a precise therapeutic intervention.

A role for activation of tubuloglomerular feedback has been proposed as a mechanism for the reduction in glomerular filtration rate in AKI. The increased delivery of sodium chloride to the macula densa as a result of cellular abnormalities in the proximal tubule would be expected to induce afferent arteriolar constriction by means of A ₁ adenosine receptor (A1AR) activation, thereby decreasing glomerular filtration rate. However, a knockout of the A1AR resulted in a paradoxical worsening of ischemic AKI, and exogenous activation of A1AR was protective. Thus tubuloglomerular feedback activation secondary to ischemic injury may represent a beneficial phenomenon that limits delivery of ions and solutes to the damaged proximal tubules, thereby reducing the demand for adenosine triphosphate (ATP)-dependent reabsorptive processes. Any salutary effect of exogenous A1AR activation in human AKI remains to be determined.

Alterations in Tubule Cell Metabolism

A profound reduction in intracellular ATP content invariably occurs early after ischemic renal injury, which sets in motion a number of critical metabolic consequences in tubule cells. Oxygen deprivation leads to rapid degradation of ATP to adenosine diphosphate (ADP) and adenosine monophosphate (AMP). With prolonged ischemia, AMP is metabolized further to adenine nucleotides and to hypoxanthine. Adenine nucleotides freely diffuse out of cells and their depletion precludes resynthesis of intracellular ATP during reperfusion. Nevertheless, although provision of exogenous adenine nucleotides or thyroxine (which stimulates mitochondrial ATP regeneration) can mitigate AKI in animal models, this approach has yielded disappointing results in human AKI.

ATP depletion leads to impaired calcium sequestration within the endoplasmic reticulum, as well as diminished extrusion of cytosolic calcium into the extracellular space, resulting in increased free intracellular calcium after AKI. Potential downstream complications include activation of proteases and phospholipases and cytoskeletal degradation. Calcium channel blockers may provide some protection from renal injury in the transplantation setting, but evidence for their efficacy in other forms of human AKI is lacking.

The role of reactive oxygen species in the pathogenesis of AKI is supported by substantial evidence. During reperfusion, the conversion of accumulated hypoxanthine to xanthine generates hydrogen peroxide and superoxide. In the presence of iron, hydrogen peroxide forms the highly reactive hydroxyl radical. Concomitantly, ischemia induces nitric oxide synthase in tubule cells. The nitric oxide generated interacts with superoxide to form peroxynitrate, which results in cell damage via oxidant injury as well as protein nitrosylation. Reactive oxygen species cause renal tubule cell injury by oxidation of proteins, peroxidation of lipids, damage to DNA, and induction of apoptosis and autophagy. Studies have documented a dramatic increase in oxidative stress and autophagy in experimental and human AKI. Scavengers of reactive oxygen molecules (such as superoxide dismutase, catalase, and N -acetylcysteine) protect against ischemic AKI in animals, but human studies have been inconclusive. A promising advance in the field is the protective effect of Edaravone (a potent scavenger of free radicals and inhibitor of lipid peroxidation) observed with administration at the time of reperfusion in a rat model of ischemic AKI. Edaravone was approved for human use in the treatment of cerebral ischemia. However, results with its use in human AKI are awaited.

Free iron derived from red cells or other injured cells is one of the most potent factors in the generation of reactive oxygen species, and the iron scavenger deferoxamine alleviates ischemia-reperfusion injury in animal models. However, the systemic toxicity (primarily hypotension) of this agent precludes its routine clinical use in human AKI. Three major molecules are under study in the area of iron chelation. The first is human apotransferrin, an iron-binding protein, which protects against AKI in animals by abrogating renal superoxide formation. Apotransferrin has been used successfully for the reduction of redox-active iron in patients undergoing hematologic stem cell transplantation without any adverse effects. The second is neutrophil gelatinase-associated lipocalin (NGAL), a major iron-transporting protein complementary to transferrin, and one of the most highly induced genes and proteins in the kidney after AKI. Administration of NGAL provides remarkable structural and functional protection in animal models. The potential use of these endogenous iron chelating agents (apotransferrin and NGAL) in human AKI is under investigation. Third, deferiprone, which also acts as an iron chelator, currently is being investigated as a potential therapeutic agent in human AKI. Oral deferiprone completed a phase 2 randomized clinical trial to test the efficacy and safety of the treatment, and a phase 3 trial with patients with AKI on preexisting chronic kidney disease is underway.