22 Apoptosis in the Kidney

1. NORMAL KIDNEY STRUCTURE AND FUNCTION

The kidneys maintain the homeostasis of electrolyte, fluid, and acid–base balance; eliminate waste products; and have an endocrine-metabolic function. They secrete hormones such as erythropoietin, Klotho, and 1,25-(OH)2-vitamin D and clear other hormones and cytokines. Each kidney contains 1 million basic functional units, or nephrons. Each nephron is composed of a glomerulus and a renal tubule. The glomerulus is a tightly woven, highly permeable capillary bed, surrounded by differentiated, very specialized cells, the podocytes. The mesangium contains mesangial cells and holds the capillaries together. Every day, 180 L of plasma is filtered through the glomeruli. Podocytes prevent the filtration of proteins, and their injury will lead to pathological urinary protein excretion (proteinuria). Podocytes do not divide, and podocyte loss causes podocytopenia, an early event in progressive glomerular scarring. Tubular cells reabsorb most of the filtered fluid and nutrients, and only 1 to 2 L of urine is excreted. Proximal tubular cells are responsible for the bulk of reabsorption. They are rich in mitochondria, consume high amounts of energy, and express a variety of transporters that favor the uptake of nephrotoxins. Thus they are prime targets in toxic and ischemic renal injury.

2. APOPTOSIS IN KIDNEY DEVELOPMENT AND

CONGENITAL KIDNEY DISEASES

Normal nephrogenesis results from finely balanced proliferative and apoptotic cell death processes. The ureteric bud invades the metanephric mesenchyme, branching and promoting the differentiation of the mesenchyme

into nephrons. The metanephric mesenchyme has a default fate of apoptosis that is prevented by factors secreted from the bud, such as transforming growth factor (TGF)-α, epidermal growth factor, fibroblast growth factor 2, and glial cell line–derived neurotrophic factor (GDNF). Genetic evidence from knockout mice indicates that during development, the high expression of Bcl-2 and Pax-2 protects cells against apoptosis, allowing cell proliferation and differentiation. In the mature kidney, Pax-2 is not found, and the expression of Bcl2 is low. Other antiapoptotic molecules, such as Bcl-xl, predominate. However, in the course of renal injury, adult kidneys may re-express some of these antiapoptotic factors in the frame of a more general adaptive response against the aggression.

Bcl-2–deficient mice (bcl-2–/–) are viable. However, they die within a few months of birth from renal failure. Renal hypoplasia and cystic dysplasia resembling polycystic kidney disease (PKD) result from excessive apoptosis in the metanephric blastema and nephrogenic zones. Bim is a key factor in renal injury in bcl-2–/– mice. Normal kidney development is restored in bcl-2–/– bim–/+ chimeric mice. Bim is not required for normal renal development because kidneys from bim–/– mice are normal. This has been explained by the existence of a hierarchic functional axis involving Bim, Bcl-2, and Bak/Bax. Active Bim might initiate the death signaling acting as a sensor setting the apoptotic threshold, whereas Bcl-2 might or might not allow the propagation of death stimulus according to its expression level. Bak/Bax might execute the death program depending on the result of the Bim and Bcl-2 interaction.

Apoptotic loss of cells is a hallmark of renal hypoplasia, a developmental disease with a genetic

basis. Homozygous or heterozygous mutations of the antiapoptotic Pax-2 gene result in a variable pathology ranging from bilateral renal agenesis and severe renal hypoplasia to mild renal hypoplasia. Low pax-2 expression does lead to changes in Bcl-2 expression. However, targeted over-expression of Bcl2 reverses the programmed cell death observed in the ureteric bud of pax2–/– mutant mice and restores normal kidney size, nephron number, and renal function. Heterozygous mutations in RET, the GDNF receptor, may result in renal agenesis in humans.

3. APOPTOSIS IN ADULT KIDNEY DISEASE

Disturbances in cell number in which apoptosis is involved have been described in animal models and clinical renal diseases. We review the role and regulation of apoptosis in acute kidney injury (AKI), chronic kidney disease (CKD), diabetic nephropathy, glomerular injury, and PKD. An imbalance between mitosis/chemotaxis and apoptosis can result in disorders of cell number characterized by an excessive cell number (e.g., proliferative glomerulonephritis) or insufficient cell number (e.g., renal atrophy) (Table 22-1). Although dysregulated fibroblast or leukocyte apoptosis may contribute to renal fibrosis and inflammation, respectively, we concentrate here on parenchymal renal cell apoptosis. Loss of parenchymal renal cells characterizes both AKI and CKD. Podocytes and tubular cells

may be lost by shedding, death, or differentiation into fibroblasts. All of these mechanisms are responses to injury that may coexist and contribute to renal cell loss. To date there is insufficient information on the relative contribution of each of them to cell loss in many disease processes. Apoptosis may be the initial insult that causes renal disease, or it may contribute to progressive renal cell loss. However, apoptosis is also required for tissue remodeling and recovery of normal tissue structure. As an example, redundant cells in proliferative glomerulonephritis are cleared through apoptosis. Thus it is important to understand the kinetics, targets, and mechanisms of apoptosis in preclinical models before planning clinical trials of antiapoptotic drugs in kidney disease (Table 22-2).

AKI is a syndrome characterized by an acute loss of renal function. Because of its time frame, it is the model of kidney injury in which the role and regulation of apoptosis has been most extensively studied. Current therapy of AKI is symptomatic and consists of substitution of renal function by dialysis if renal failure is severe. There is no established therapy to accelerate the recovery, and attempts at preventing AKI are not universally effective. Despite the reversibility of the loss of renal function, the mortality of AKI remains high (>50%). Thus therapies based in a correct understanding of its pathogenesis are urgently needed. In human studies, tubular cell death is the best histological correlate with renal dysfunction in AKI. Evidence supporting a key role of

tubular cell death in the pathogenesis of AKI includes the fact that several nephrotoxins that induce AKI also promote tubular cell death in culture, that therapeutic intervention on apoptosis improves experimental AKI, and that a bioartificial kidney containing proximal tubular cells improves survival in experimental animals and, in preliminary studies, in humans. Tubular cell death in the early stages of AKI of different etiologies (ischemic, toxic, septic, obstructive) can proceed through apoptosis or necrosis. The relative contribution of the two mechanisms to tubular cell loss depends on the severity of the insult. A second peak of apoptosis occurs days (it peaks at day 8 in rat ischemic AKI) after the original insult, when the injured tubules have been completely reconstituted by a hyperplastic epithelium. In this case, apoptosis restores cell number to preinjury levels.

In CKD, progressive loss of renal mass and function leads to end-stage renal disease, necessitating replacement of renal function by dialysis or transplantation. The personal, social, and economic costs of these therapies are staggering at approximately 20 billion US dollars per year in the United States. Apoptotic cell death exceeding mitotic replacement contributes to renal cell loss in the form of podocytopenia, glomerulosclerosis, and tubular atrophy. This has been documented for podocytes and mesangial, endothelial, and tubular cells in experimental models of progressive glomerular scarring and tubulointerstitial atrophy. Consistent with the experimental data, an increased rate of apoptosis has been observed in human CKD.

Diabetic nephropathy, vascular injury, glomerular injury, and PKD are frequent causes of CKD. Diabetic nephropathy is the most common cause of end-stage renal disease. Hyperglycemia is the primary metabolic alteration that promotes diabetic tissue injury. However, glucose degradation products and elevated local cytokine (e.g., tumor necrosis factor [TNF], TNF-related apoptosis-inducing ligand [TRAIL]; TGFβ1, angiotensin II) levels also contribute to tissue injury and renal cell apoptosis (Figure 22-1). The glomerulus was long thought to be the primary site of injury in diabetic nephropathy. Recently, podocytopenia was identified as an early feature of diabetic nephropathy and podocyte apoptosis as a primary contributor. In addition, tubular cell apoptosis is prominent in diabetic nephropathy. The expression of 112 cell death–related genes was abnormal in the tubulointerstitium of diabetic nephropathy patients, and diabetic individuals are sensitized to AKI. One hypothesis attributes the sensitization to AKI to an abnormal pattern of apoptotic gene expression that favors renal cell death.

Other forms of glomerular injury are usually the result of immune or inflammatory aggression. Inflammatory mediators may cause mesangial, glomerular endothelial cell, and podocyte apoptosis, ultimately leading to glomerular scarring (glomerulosclerosis). TGFβ1, angiotensin II, a high glucose concentration, mechanical stress (which may result from increased single-nephron glomerular filtration rate in remnant