333e: Adaptation of the Kidney to Injury

Many years ago Claude Bernard (1878) introduced the concepts of milieu extérieur (the environment where an organism lives) and a milieu intérieur (the environment in which the tissues of that organism live). He argued that the milieu intérieur varied very little and that there were vital mechanisms that functioned to maintain this internal environment constant. Walter B. Cannon later extended these concepts by recognizing that the constancy of the internal state, which he termed the homeostatic state, was evidence of physiologic mechanisms that act to maintain this minimal variability. In higher animals, the plasma is maintained remarkably constant in composition both within an individual and among individuals. The kidney plays a vital role in this constancy. The kidney changes the composition of the urine to maintain electrolyte and acid-base balance and can produce hormones that can maintain constancy of blood hemoglobin and mineral metabolism. When the kidney is injured, the remaining functional mass responds and attempts to continue to maintain the milieu intérieur. It is remarkable how well the residual nephrons can perform in this task so that in many cases homeostasis is maintained until the glomerular filtration rate (GFR) drops to very low levels. At this point, the functional tissue can no longer compensate. In this chapter, we will discuss a number of these compensatory adaptations that the kidney makes in response to injury in an attempt to protect itself and protect the milieu intérieur. A theme that permeates, however, is that these adaptive processes can often be maladaptive and contribute to enhanced renal dysfunction, facilitating a positive feedback process that is inherently unstable.

Renal disease is associated with a reduction in functional nephrons. The rest of the kidney adapts to this reduction by increasing blood flow to and the size of the remaining glomeruli and increasing size and function of the remaining tubules. Robert Platt, in 1936, argued that “…a high glomerular pressure, together with loss of nephrons (destroyed by disease) [is] an explanation of the peculiarities of renal function in this stage of kidney disease.” The raised glomerular pressure will increase the amount of filtrate produced by each nephron and thus compensate for a time for the destruction of part of the kidney. But eventually there are too few nephrons remaining to produce an adequate filtrate, even though they may work under the highest possible pressure, associated with a high systemic blood pressure. The responses to kidney injury can be both adaptive and maladaptive, and in many cases, the early adaptive responses can become maladaptive over time, leading to progressive decline in the anatomic and functional integrity of the kidney. As described previously, the early responses are likely in many cases motivated by attempts to maintain the constancy of the milieu intérieur for the survival of the organism (Claude Bernard).

Barry Brenner in the 1960s and 1970s carried out micropuncture experiments to define the pressures in glomerular capillaries as well as afferent and efferent resistances and modeled the behavior of the factors that governed glomerular filtration in health and disease. According to the Brenner Hyperfiltration Hypothesis, a reduction in the number of nephrons results in glomerular hypertension, hyperfiltration, and enlargement of glomeruli and this hyperfiltration results in damage to those glomeruli over time and ultimately decreased kidney function. According to this hypothesis, a positive feedback process is set into motion whereby injury to the glomeruli will result in further hyperfiltration to other glomeruli and hence more accelerated injury to those glomeruli. Since nephrons are not generated after 34–36 weeks of gestation or after birth (if earlier than 34–36 weeks) in humans, this hypothesis implies a deterministic effect of low nephron numbers at birth. There is over a 10-fold variation in the number of nephrons per kidney in the population (200,000 to over 2.5 million). This variation is not explained by kidney size in the adult. Children born with low birth weights would be more prone to kidney disease as adults. There are many reasons why there might be reduced nephron numbers at birth: developmental abnormalities, genetic predisposition, and environmental factors, such as malnutrition. There are thought to be interactions between these various factors. Reduced nephron mass can also occur with chronic kidney disease (CKD) in the adult, and the response of the kidney is similar qualitatively with hyperfiltration of the remaining nephrons.

Developmental Abnormalities

There are many congenital abnormalities of the kidney and urinary tract (CAKUT). Dysplastic kidneys have varying degrees of abnormalities that interfere with their function. Anatomically abnormal kidneys can be associated with abnormalities of the lower urinary tract. Urinary tract abnormalities resulting in obstruction or vesicoureteric reflux can dramatically alter the normal development of the kidney nephrons. Dysplastic or hypoplastic kidneys can be cystic in patterns that are distinct from polycystic kidney disease. Of course, autosomal recessive kidney disease can result in widespread cyst formation.

Hypoplastic kidneys are characterized by a reduced number of functional nephrons. One definition of hypoplastic kidneys is as follows: “Kidney mass below two standard deviations of that of age-matched normal [individuals] or a combined kidney mass of less than half normal for the patient’s age.” Renal agenesis and cystic dysplasia often affects only one kidney. This results in hypertrophy of the other kidney if it is unaffected by any congenital abnormality itself. Although there is hypertrophy in size, it is not clear if this is associated with an increase in the number of nephrons on the contralateral side.

The prevalence of CAKUT has been generally found to be between 0.003 and 0.2%, depending on the population studied. This excludes fetuses with transient upper renal tract dilatation likely related to the high rate of fetal urine flow rate. In the adult U.S. Renal Data System (USRDS) of patients with end-stage kidney disease, approximately 0.6% are listed as having dysplastic or hypoplastic kidneys as a primary cause of the disease. This is likely an underestimate, however, because many patients with “small kidneys” may be misdiagnosed with chronic glomerulonephritis or chronic pyelonephritis.

Environmental Contributions to Reduced Nephron Mass

The most important environmental factor responsible for reduced nephron number is growth restriction within the uterus. This has been associated with disease processes such as diabetes mellitus in the mother, but there also is a strong genetic disposition. Low-birth-weight children are more likely to be born to mothers who, themselves, were born with low birth weight. There are clearly other environmental factors. Caloric restriction during pregnancy in humans has been associated with altered glucose as adults and increased risk for hypertension. In one study, it was found that if women were calorie restricted in midgestation, the time of most rapid nephrogenesis, there was a threefold incidence of albuminuria in their children when they were tested as adults. Factors such as deficiency in vitamin A, sodium, zinc, or iron have been implicated as predisposing to abnormal kidney development. Other environmental factors that can influence kidney development are medications taken by the mother, such as dexamethasone, angiotensin-converting enzyme inhibitors, and angiotensin receptor antagonists (Table 333e-1). Protein restriction in mice during pregnancy can reduce lifespan of the offspring by 200 days. Obesity may play an important role in determining kidney outcome long term in patients with reduced kidney mass. It has been shown in mice fed a high-fat diet that the rodents that had reduced nephron number had a greater incidence of hypertension and renal fibrosis.

Implications of Low Nephron Number at Birth

David Barker was the first to describe the association between low birth weight and later cardiovascular death. This was followed by studies relating low birth weight to risk for diabetes, stroke, hypertension, and CKD. It has been found that there is an inverse relationship between nephron number and blood pressure in adults. This relationship was found in Caucasians but not in African Americans. Approximately one-third of children with a single functioning kidney at the age of 10 years had signs of renal injury as determined by the presence of hypertension, albuminuria, or the use of renoprotective drugs. Another study revealed that 20–40% of patients born with a single functional kidney had renal failure requiring dialysis by 30 years of age.

In the early stages of CKD, there are many adaptations structurally and functionally that limit the consequences of the loss of nephrons on total-body homeostasis. In later stages of disease, however, these adaptations are insufficient to counteract the consequences of nephron loss and in fact often become maladaptive.

Counterbalance

Renal counterbalance was defined by Hinman in 1923 as “an attempt on the part of the less injured or uninjured portion (of the kidney) to take over the work of the more injured portion.” Hinman defined “renal reserve” to be of two types: “native reserve, which is the normal physiological response to stimulation . . and acquired reserve, which involves growth or compensation due to overstimulation.” It was known that removal of one kidney results in an increase in size of the contralateral kidney. If, instead of nephrectomy, one kidney is rendered ischemic and the other left intact, there is a resultant atrophy of the postischemic kidney. If the contralateral kidney is removed, however, before the atrophy becomes too severe, then the postischemic kidney increases markedly in size. With the contralateral kidney in place, there is vasoconstriction and reduced renal blood flow to the postischemic kidney. This is rapidly reversed, however, when the contralateral normal kidney is removed. The factors responsible for the persistent initial (prenephrectomy) vasoconstriction and those responsible for the rapid vasodilation and enhanced growth after contralateral nephrectomy are unknown.

Hypertrophy

Because nephrons of mammals, in contrast to those of fish, cannot regenerate, the loss of functional units of the kidney, either due to disease or surgery, results in anatomic and functional changes in the remaining nephrons. As described above, there is increased blood flow to remaining glomeruli with potentially adverse effects over time of the resultant increased size of the remaining glomeruli and hyperfiltration (Fig. 333e-1). In addition, there is hypertrophy of the tubules. Some of the mediators of this hypertrophy of the remaining functional tubules are listed in Table 333e-2. In the adult, within a few weeks after unilateral nephrectomy for donation of a kidney, the GFR is approximately 70% of the prenephrectomy value. It then remains relatively stable for most patients over 15–20 years. The hyperfiltration is related to an increase in renal blood flow likely secondary to dilatation of the afferent arterioles potentially due to increases in nitric oxide (NO) production. The rate of increase in GFR is slower in the adult than it is in the young after nephrectomy. There are a number of factors that have been implicated at the cellular and nephron level to account for the compensatory hypertrophy that ensues after removal of functional nephrons (Table 333e-2).

With increased blood flow to the kidney, there is glomerular hypertension (i.e., an increase in glomerular capillary pressure). There is increased wall tension and force on the capillary wall that is counteracted by contractile properties of the endothelium and elastic properties of the glomerular basement membrane. The force is conveyed to podocytes, which adapt by reinforcing cell cycle arrest and increasing cell adhesion in an adaptive attempt to maintain the delicate architecture of the interdigitating foot processes. Over time, however, these increased forces due to glomerular hypertension lead to podocyte damage and glomerulosclerosis.

Other Systemic and Renal Adaptations to Reduced Nephron Function

With reduced functional nephrons, as is seen in CKD, there are many other systemic adaptations that occur to preserve the milieu intérieur because the kidney is involved in so many regulatory networks that are then stressed when there is dysfunction. In the 1960s, Neil Bricker introduced the “intact nephron hypothesis.” According to his concept, with decreases in the number of functioning nephrons, each remaining nephron has to adapt to carry a larger burden of transport, synthetic function, and regulatory function.

Potassium

Under normal and abnormal conditions, most of the filtered potassium is reabsorbed in the proximal tubule so that excretion is determined by secretion by the distal nephron. Potassium handling is altered in CKD protecting the organism somewhat from lethal hyperkalemia. Hyperkalemia is a common feature of individuals with CKD. Hyperkalemia (if not severe and dangerous) is adaptive in that it promotes potassium secretion by the principal cells of the collecting duct. When patients with CKD are given a potassium load, they can excrete it at the same rate as patients with normal renal function except that they do so at a higher serum potassium, consistent with the view that the hyperkalemia facilitates potassium excretion. The direct effect of hyperkalemia on potassium secretion by the distal nephron is independent of changes in aldosterone levels, but “normal” levels of aldosterone are necessary to see the effect of hyperkalemia on potassium excretion. Elevated potassium stimulates the production of aldosterone, and this effect is also seen in patients with CKD. Aldosterone increases the density and activity of the basolateral Na⁺-K⁺ ATPase and the number of Na⁺ channels in the apical membrane of the collecting duct. In CKD, the excretion of the dietary load of potassium occurs at the expense of an elevation in serum potassium concentrations.

Sodium

As renal function is reduced with CKD, there is a reduced ability to excrete sodium. Thus, patients with advanced kidney disease are often fluid overloaded. In early disease, however, there are functional adaptations that the kidney assumes to help to maintain the milieu intérieur. With loss of functional nephrons, the remaining nephrons are hyperperfused and are hyperfiltering in a manner that can be influenced by dietary protein intake. Although protein restriction can decrease this compensatory hyperperfusion, there is generally more sodium and water filtered and delivered to the remaining nephrons. There is some preservation of glomerulotubular balance with increased proximal tubule sodium and water reabsorption associated with increased levels of the Na/H exchanger in apical membranes of the tubule. The tubuloglomerular feedback (TGF) of the remaining nephrons is sensitive to sodium intake. With high sodium intake in normal renal function, a negative feedback process occurs by which increased distal delivery results in reduced GFR and hence filtration of sodium. In CKD, the TGF becomes a positive feedback process by which increased distal delivery results in increased filtration so that the need to excrete an increased amount of sodium per nephron is achieved. This conversion from a negative feedback process to a positive feedback process may be due to conversion of an adenosine-dominated vasoconstrictive feedback on the afferent arteriole of the glomerulus to a NO-dominated vasodilatory feedback. Like so many of these adaptive responses, this one may turn maladaptive, resulting in higher intraglomerular hydrostatic pressures with increased mechanical strain on the glomerular capillary wall and podocytes and increased glomerulosclerosis as a consequence.

Acid-base homeostasis

The kidneys excrete approximately 1 mEq/kg per day of dietary acid load under normal dietary conditions. With decreased kidney functional mass, there is an adaptive response to increase H⁺ excretion by the remaining functional nephrons. This takes the form of enhanced nephron ammoniagenesis and increased distal nephron H⁺ ion secretion, which is mediated by the renin-angiotensin system and endothelin-1. NH₃ is produced by deamidization of glutamine in the proximal tubule. NH₃ is converted to NH₄⁺ in the collecting duct, where it buffers the secreted H⁺. It has been argued, however, that these mechanistic attempts to enhance H⁺ secretion can be maladaptive in that they can contribute to kidney inflammation and fibrosis and hence facilitate the progression of CKD.

Mineral metabolism

In CKD, there is a decrease in the ability of the kidney to excrete phosphate and produce 1,25-dihydroxyvitamin D₃ [1,25(OH)₂D₃]. There is a resultant increase in serum phosphate and reduction in serum calcium (Fig. 333e-2). In response, the body adapts by increasing production of parathyroid hormone (PTH) and fibroblast growth factor-23 (FGF-23) in an attempt to increase phosphaturia. The elevated levels of PTH act on bone to increase bone resorption and on osteocytes to increase FGF-23 expression. Elevated levels of PTH increase FGF-23 expression by activating protein kinase A and wnt signaling in osteoblast-like cells. There are a number of other factors that increase bone FGF-23 production in CKD including systemic acidosis, altered hydroxyapatite metabolism, changes in bone matrix, and release of low-molecular-weight FGFs. Although the production of PTH and FGF-23 initially are adaptive attempts to maintain body phosphate levels by enhancing excretion by the kidney, they become maladaptive due to systemic effects on the cardiovascular system and bone, as renal function continues to deteriorate. PTH and FGF-23 decrease the kidney’s ability to reabsorb phosphate by decreasing the levels of the sodium-phosphate cotransporters NaPi2a and NaPi2c on the apical and basolateral membranes of the renal tubule. FGF-23 also reduces the ability of the kidney to generate 1,25(OH)₂D₃. In the parathyroid gland, the FGF-23 receptor, the klotho-fibroblast growth factor 1 complex, is downregulated with a consequent loss of the normal action of FGF-23 to downregulate PTH production. PTH and FGF-23 have been implicated in the cardiovascular disease that is so characteristic of patients with CKD. With CKD, there is less klotho expression in the kidney and the parathyroid glands. Klotho deficiency contributes to soft tissue calcifications in CKD. FGF-23 has been associated with increased mortality in CKD and has been reported to be involved causally in the development of left ventricular hypertrophy. PTH also has been reported to directly affect rat myocardial cells, increasing calcium entry into the cells and contributing to death of the cells.

Preconditioning represents activation by the organism of intrinsic defense mechanisms to cope with pathologic conditions. Ischemic preconditioning is the phenomenon whereby a prior ischemic insult renders the organ resistant to a subsequent ischemic insult. Renal protection afforded by prior renal injury was described approximately 100 years ago, in 1912, by Suzuki, who noted that the kidney became resistant to uranium nephrotoxicity if the animal had previously been exposed to a sublethal dose of uranium. This resistance of the renal epithelium to recurrent toxic injury was proposed to be a defense mechanism of the kidney. There have been a number of studies over the years demonstrating that preconditioning with a number of renal toxicants leads to protection against injury associated with a second exposure to the same toxicant or to another nephrotoxicant. It is not, however, a universal finding that toxins confer resistance to subsequent insults.

Kidney ischemic preconditioning is the conveyance of protection against ischemia due to prior exposure of the kidney to sublethal episodes of ischemia. In some experiments in rodents, these prior exposures were short (e.g., 5 min) and repeated or longer. Subsequent protection was generally found at 1–2 h or up to 48 h, but there has been a report of protection in the mouse for up to 12 weeks after the preconditioning exposures. Unilateral ischemia, with the contralateral kidney left alone, was also protective against a subsequent ischemic insult to the postischemic kidney, revealing that systemic uremia was not necessary for protection.

Remote Ischemic Preconditioning

Remote ischemic preconditioning is a therapeutic strategy by which protection can be afforded in one vascular bed by ischemia to another vascular bed in the same organ or a different organ. A large number of studies have demonstrated that ischemia to one organ protects against ischemia to another. There are very few mechanistic studies of remote preconditioning in the kidney. In one study, naloxone blocked preconditioning in the kidney, implicating opiates as effectors. Remote preconditioning induced by ischemia to the muscle of the arm induced by a blood pressure cuff can result in protection of the kidney against a subsequent insult, such as one related to contrast agents in humans. Some of the cellular processes and signaling mechanisms proposed to explain preconditioning in the kidney and other organs are listed in Table 333e-3. These protective processes, most of which have been identified in the heart, involve multiple signaling pathways that affect decreased apoptosis, inhibition of mitochondrial permeability transition pores, activation of survival pathways, autophagy, and other pathways involved in reducing energy consumption or reactive oxygen production. In a study from our laboratory, inducible NO synthase was found to be an important contributor to the adaptive response to kidney injury, which results in protection against a subsequent insult. Identification of the responsible protective factor(s) mediating the advantageous adaptive response to remote ischemic preconditioning would provide a therapeutic approach for prevention of acute kidney injury or facilitation of a protective adaptation to kidney injury.

Adaptive Response to Hypoxic Injury

Hypoxia plays a role in ischemic, septic, and toxic acute kidney injury. Many conditions result in a global or regional impairment of oxygen delivery. This is particularly important in the outer medulla where there is baseline reduced oxygen tension and a complex capillary network that, by its nature, is susceptible to interruption. In addition, the S3 segment of the proximal tubule is very dependent on oxidative metabolism, whereas the medullary thick ascending limb of the nephron that also traverses the outer medulla can adapt to hypoxia by converting to glycolysis as a primary energy source.

One proposed adaptive response to hypoxia is a reduction in glomerular filtration with consequent reduction in “work” requirement for reabsorption of solutes by the tubule. This was termed acute renal success by Thurau many years ago. The importance of this has been questioned, however, because there is no significant reduction in renal oxygen consumption in post–cardiac surgery patients with acute ­kidney injury in the setting of reduced GFR and renal blood flow.

If hypoxia or other influences, such as toxins, damage the proximal tubule and interfere with reabsorption of sodium and water, it is important that the kidney adapt in such a way so that there is not a large natriuresis that might compromise intravascular volume and blood pressure. This is accomplished, at least in part, by tubuloglomerular feedback (TGF). The increased distal delivery of salt and water results in a homeostatic adaptation to decrease glomerular filtration and hence decrease tubular delivery of salt and water through the glomerulus and reduce the delivery to the distal nephron. This adaptive response to acute injury is different from the role of TGF in CKD, as we have discussed previously in this chapter. In chronic disease with reduced nephron function, there is a steady-state need to increase excretion of sodium, whereas with acute injury, excretion of sodium is reduced.

Many genes are activated by hypoxia that are adaptive in serving to protect the cell and organ. With hypoxia, hypoxia-inducible factor (HIF) 1α rapidly accumulates due to the inhibition of the HIF prolyl-hydroxylases, which normally promote HIF1α proteasomal degradation. HIF1α then dimerizes with HIF1β and the dimer moves to the nucleus, where it upregulates a number of genes whose protein products are involved in energy metabolism, angiogenesis, and apoptosis, enhancing oxygen delivery and metabolic adaptation to hypoxia. This takes the form of a complex interplay among factors that regulate perfusion, cellular redox state, and mitochondrial function. For example, upregulation of NO production by sepsis results in vasodilatation and reduction in mitochondrial respiration and oxygen consumption. In addition, HIF1 activation in endothelial cells may be important for adaptive preservation of the microvasculature during and after hypoxia. Better understanding of the role that the HIFs play in protective adaptation has led to an aggressive development of HIF prolyl-hydroxylase inhibitors by biotechnology and pharmaceutical companies for clinical use.

Adaptive Response to Toxic Injury Specific to the Proximal Tubule

One can model an acute kidney injury by genetically inserting a Simian diphtheria toxin (DT) receptor into the proximal tubule and then adding either a single dose of DT or multiple doses of the toxin. Repair of the kidney after a single dose of DT can be shown to be adaptive with few longer term sequelae. There is a very robust proliferative response of the proximal tubule cells to replace the cells that die as a result of the DT. Ultimately the inflammation resolves, and there is little, if any, residual interstitial inflammation, expansion, or matrix deposition.

Maladaptive Response of the Kidney to Acute Injury

By contrast to the above adaptive repair that occurs after a single insult, after three doses of DT administered at weekly intervals, there is maladaptive repair with development over time of a chronic interstitial infiltrate, increased myofibroblast proliferation, tubulointerstitial fibrosis, and tubular atrophy, as well as an increase in serum creatinine (0.6 ± 0.1 mg/dL vs 0.18 ± 0.02 mg/dL in control mice) by week 5, 2 weeks after the last dose in the thrice-treated animals. There is a dramatic increase in the number of interstitial cells that expressed the platelet-derived growth factor receptor β (pericytes/perivascular fibroblasts), αSMA (myofibroblasts), FSP-1/S100A4 (fibroblast specific protein-1), and F4/80 (macrophages). In addition, there is loss of endothelial cells, interstitial capillaries, and development of focal global and segmental glomerulosclerosis.

It has become increasingly recognized as a result of large epidemiologic studies that even mild forms of acute kidney injury are associated with adverse short- and long-term outcomes including onset or progression of CKD and more rapid progression to end-stage kidney disease. Experimental models in animals, such as the DT model described above, provide pathophysiologic explanations for how the effects of acute injury can lead to chronic inflammation, vascular rarefaction, tubular cell atrophy, interstitial fibrosis, and glomerulosclerosis. Recurrent specific tubular injury leads to a pattern very typical of CKD in humans: tubular atrophy, interstitial chronic inflammation and fibrosis, vascular rarefaction, and glomerulosclerosis. The mechanisms involved in the development of glomerulosclerosis evoked by primary tubular injury may be multifactorial. Damage to nephron segments may lead to sluffing of cells into the lumen and to tubular obstruction. Progressive narrowing of the early proximal tubule near the glomerular tuft can lead to a sclerotic atubular glomerulus like those that are seen with ureteral obstruction. There may be paracrine signaling from injured and regenerating/undifferentiated epithelium to directly impact the glomerulus. Alternatively, a progressive tubulointerstitial reaction originating around atrophic and undifferentiated tubules may directly encroach upon the glomerular tuft. The loss of interstitial capillaries may lead to a progressive reduction of glomerular blood flow with ischemia to the glomerulus and to the kidney regions perfused by the postglomerular capillaries. This speaks to the fact that primary tubular injury can trigger a response that adversely affects multiple compartments of the kidney and leads to a positive feedback process, involving loss of capillaries, glomerulosclerosis, persistent ischemia, tubular atrophy, increased fibrosis, and ultimately kidney failure.