With aging, a number of morphologic changes emerge in the human glomerulus (Table 65.1). These include a decrease in the number of identifiable glomeruli and an escalation in the proportion of globally sclerotic glomeruli, which is associated with a progressive increase in the size of intact glomeruli (Fig. 65.1).

The number of glomeruli is extremely variable in individuals, ranging from 333,000 to 1,100,000 in each kidney and vary with age (inversely), gender (15% lower in females), and race (lower in Australian Aboriginals).⁷,⁸,⁹ As renal cortical mass decreases with increasing age, the glomeruli decrease in number. There seems to be a direct correlation between the number/percentage of globally sclerotic glomeruli and increasing age and with intrarenal vascular disease, especially outer cortical vascular disease.¹⁰ In general, globally sclerotic glomeruli comprise less than 10% of the total glomeruli under the age of 40 years and increase thereafter so that by the eighth decade as much as 30% of glomeruli may be globally sclerotic. However, the estimation of “normal” sclerosed glomeruli is difficult in the elderly due to confounding effects of comorbid conditions such as diabetes and hypertension. In such situations, “pathologic” glomerulosclerosis should be considered when the number of globally sclerosed glomeruli exceeds the number calculated by the formula: (patient’s age/2) -10.⁷,¹¹,^11a

The pathogenesis of aging-associated global glomerulosclerosis is not completely understood and is likely multifactorial. Increasing oxidative stress that accompanies
aging can result in endothelial dysfunction and changes in vasoactive mediators resulting in atherosclerosis, hypertension, and glomerulosclerosis.¹² Furthermore, age-related changes in cardiovascular hemodynamics, such as reduced cardiac output and systemic hypertension, may contribute to glomerular changes.¹³ Moreover, dysautoregulation of the afferent and efferent arterioles may increase glomerular plasma flow, glomerular capillary pressure, and “hyperfiltration,” leading to mesangial matrix accumulation.¹⁴ A morphometric study showed dilatation of the afferent arterioles, increased glomerular capillary lumens (especially hilar), and enlarged glomeruli, which suggested a discordance between the afferent and efferent arterioles.¹⁵ The vascular adaptations to functional or structural nephron loss may help preserve glomerular filtration rate (GFR) by producing hyperperfusion and hyperfiltration in the surviving nephrons. This local glomerular hypertension and hypertrophy may lead to cytokine-mediated mesangial matrix expansion and, eventually, glomerulosclerosis. Such hyperperfusion-associated glomerular injury is seen with oligomeganephronia, diabetic nephropathy, morbid obesity, sickle cell anemia, and reflux nephropathy. It has been suggested that the vascular/ischemic changes seen in aging kidneys first cause cortical glomerulosclerosis and consequent juxtamedullary glomerular hypertrophy, followed by juxtamedullary glomerulosclerosis.¹⁶

Several tubulointerstitial alterations parallel glomerular changes in the aging kidney. Three types of tubular atrophy can be seen in the aging kidney (Fig. 65.1). These include the classic form with wrinkling and thickening of the tubular basement membranes and simplification of the tubular epithelium; the endocrine form with simplified tubular epithelium, thin basement membranes, and numerous mitochondria in the tubular epithelial cells; and the thyroidization form with hyaline cast-filled dilated tubules.¹⁷ Although there is an overlap, the endocrine form is classically seen with vascular ischemia, and the thyroidization form is considered to be characteristic but not pathognomonic of chronic pyelonephritis. With tubular atrophy, the distal renal tubules develop diverticula that increase in number with increasing age. These diverticula in distal and collecting tubules may be precursors of simple cysts that are increasingly observed in the aging kidney.¹⁸ The diverticula may promote bacterial growth and contribute to the frequent renal infections in the elderly.

Several changes in the renal vasculature have been documented in the aging human kidney, none of which are specific for aging (Fig. 65.1). Arterial sclerosis denotes thickening of the

wall and narrowing of the arterial lumen produced by thickening of the medial smooth muscle layer, fibrosis of the media, and/or intimal thickening. These changes may be seen with hypertension, diabetes, and aging, with the prevalence of arterial sclerosis increasing with advancing age.⁷,¹⁹,²⁰ Intimal fibroplasia or collagenous fibrosis of the arterial intima may be associated with thinning of the media and is found uniformly in older kidneys with or without underlying cardiovascular disease. Intimal fibroplasia is seen primarily in arteries that are 80 to 300 µm in diameter, such as the interlobular arteries. The regional heterogeneity of intimal hyperplasia may account for the heterogeneity of ischemic nephrons. Although the etiology of aging-associated intimal fibroplasia is not entirely clear, it starts early in life and is accelerated by hypertension. Intimal hyperplasia in the interlobular arteries may allow the transmission of the pulse wave into the smaller distal branches leading to arteriolar hyaline changes, which may in turn accelerate the proximal intimal fibrosis. Global glomerulosclerosis appears to be associated with arterial intimal fibrosis rather than with arteriolar hyaline change.²¹

In aging kidneys, the thickening and folding/wrinkling of glomerular basement membrane (GBM) is accompanied by glomerular simplification and the formation of anastomoses between glomerular capillary loops. Frequently, afferent arteriole dilatation near the hilum is observed at this stage. The afferent arterioles commonly develop hyalinosis (accumulation of plasma proteins in the intima and/or media of small arteries and arterioles), a change that is less well correlated with systemic hypertension than with arterial intimal fibrosis in most, but not all studies. It may also be seen with diabetes mellitus; in fact, it is most severe and pronounced in patients with uncontrolled diabetes mellitus with or without hypertension.¹⁷

In the aging kidneys, the sclerosis and eventual loss of the glomerular tuft is often associated with a direct communication between afferent and efferent arterioles (“aglomerular arterioles”), particularly the juxtamedullary glomeruli.¹⁹ These aglomerular arterioles are rarely seen in the kidneys of healthy adults but are observed with increased frequency both in aging and CKD kidneys.¹⁹ The age-related vascular changes of intimal fibrosis and hyaline arteriolosclerosis are accentuated by hypertension, and probably diabetes mellitus as well. On the other hand, it has been suggested that aging-related interlobular arterial sclerosis may precede rather than follow systemic hypertension. Mean blood pressure rises by 1.6 mm Hg for each 1 µm increase in intimal thickness in a 100 µm diameter artery because of microischemia in scattered nephrons. This source of hypertension may account for the rise of blood pressure with age. The rate of decrease of renal plasma flow is accelerated by hypertension. Mean arterial blood pressure is directly proportional to the rate of decline of creatinine clearance. An increase in hypertension is a strong independent risk factor for ESRD, especially in African Americans. Thus, hypertension and morphologic vascular changes are not easily separable at this time.²⁰,²¹,²²

The morphologic changes of aging are accompanied by parallel changes in renal function and hemodynamics. Using various techniques, several studies conducted in elderly individuals without significant renal disease have demonstrated that renal blood flow (RBF) decreases with advancing age. In a review of 38 renal hemodynamic studies including 634 healthy subjects with wide age range, Wessen²⁴ described that total RBF was well maintained through approximately the fourth decade and progressively declined by approximately 10% per decade thereafter. In a study of 207 healthy kidney donors, Hollenberg et al.²⁵ demonstrated an explicit and progressive reduction in mean blood flow per
unit kidney mass with advancing age, suggesting that the decrease in RBF does not simply reflect the decline in the renal mass with aging. In addition, they demonstrated that the fall in renal perfusion with aging is most profound in the cortex, with relative sparing of flow to the medulla. This redistribution of blood flow from the cortex to the medulla may explain the slight increase in filtration fraction observed in the elderly population. Studies on the morphology and histology of the renal vasculature by postmortem angiograms and histologic sections demonstrate increased irregularity and tortuosity of the preglomerular vessels and tapering of afferent arterioles. However, no characteristic histologic lesion of aging has yet been identified in the renal vasculature.

The precise mechanisms of reduced RBF with aging are not yet known. Aging is associated with changes in vascular tone, which is determined by the balance between vasoconstrictors and vasodilators. In aging, there is an attenuated response to vasodilators such as nitric oxide (NO), endothelial-derived hyperpolarizing factor (EDHF), and prostacyclin, and an enhanced responsiveness to vasoconstrictors such as angiotensin II (Ang-II).²⁶ This may result in enhanced vasoconstrictive responses in aging, which can potentially cause renal damage and an ultimate fall in GFR. Although the renin-angiotensin system (RAS) is suppressed in aging, the intrarenal RAS may be relatively spared. In fact, the pharmacologic blockade of RAS has been shown to slow the progression of age-related CKD in experimental animals.²⁷ In addition to the suppression of RAS, there is significant decrease in NO production and availability that leads to renal vasoconstriction and sodium retention. Several potential mechanisms contribute to the reduction of NO with aging. Chief among them is oxidative stress that can reduce NO availability by the inactivation of NO; the inhibition of NO synthase (NOS) via the depletion of the NOS cofactor, tetrahydrobiopterin; the uncoupling of endothelial NOS; the accumulation of the endogenous NOS inhibitor, asymmetric dimethyl arginine; and by limiting uptake of NOS substrate, L-arginine, by endothelial cells via downregulation of cationic amino acid transporter-1.²⁸

There is also evidence that angiogenesis is attenuated in aging. In this context, vascular endothelial growth factor (VEGF) and angiopoietin-1 are altered both systemically and in the kidney with the aging process. Although levels of VEGF²⁹ have been shown to be reduced in the aging rat, a profound upregulation in protein levels of angiopoietin-1 in the kidney cortex has been observed in aged versus young rats.³⁰ Because angiopoietin-1 can stabilize blood vessels, its increase in aging may serve to counter the mechanisms leading to impaired angiogenesis and endothelial dysfunction. Taken together, these data indicate that strategies aimed at protecting the endothelium may help to mitigate the adverse renal effects of aging.

Recently, the role of arterial aging or arteriosclerosis in the pathogenesis of senescent changes in various organs, including the kidney, has become a major focus of interest. It has been observed that elastic arteries undergo two distinct physical changes, namely, dilation and stiffness with age.³¹,³² This is due to fatigue and fracture of the medial elastin, mainly of the elastic arteries, with little aging change occurring in the distal muscular arteries.³³ Thus, dilation and stiffening are most marked in the proximal aorta and its major branches, namely, the brachiocephalic, carotid, and subclavian arteries. Increased arterial stiffening results in an increase in pulse wave velocity (PWV).³¹,³⁴ Aortic PWV is the speed with which pulse waves travel along the artery. A typical value is 5 m per second in a 20 year old and 12 m per second in an 80-year-old person, representing a 2.5-fold increase in 60 years.³¹ The elastic properties of the aorta in the young serves to partially maintain blood volume and pressure during systole and then release them during diastole via the recoil process. This phenomenon helps to protect the vital organs by sustaining blood flow during diastole and blunting the damaging effects of high pressure waves during systole. In addition, the microcirculation, which comprises small arteries, arterioles, and the capillaries and constitutes the greatest resistance to blood flow, participates in transforming pulsatile flow to steady flow by reflecting the pulsations that enter from the larger arteries. With aortic stiffening and the consequent increase in aortic PWV, the transmission of flow pulsations downstream into various organs, principally the brain and kidney, can damage microvessels.³⁵,³⁶ This mechanism may account for asymptomatic cerebral microvascular disease associated with microaneurysms and infarcts. The lesions comprise damage to the medial smooth muscle and the endothelium (which is not attributable to atherosclerosis), and in their chronic form are described as lipohyalinosis.³⁷ More recent studies have shown that amyloid plaques in older persons are probably a consequence of medial damage to small vessels and hemorrhage from damaged vessels.³⁸,³⁹ It is thought that the neurofibrillary tangles of dementia may have a similar microvascular etiology.⁴⁰ Less data on pulsatile microvascular damage are available for the kidney, but one can expect this to emerge. The kidney afferent arterioles and glomeruli are exposed to the same high pulsatile microvascular stress and strain as in the brain. Recent studies have shown that independent of conventional brachial systolic and diastolic pressure values, measures of arterial stiffness are closely related to outcomes attributable to microvascular damage to vital organs, particularly the brain and the kidney.³⁵,³⁶ Furthermore, measures of large artery stiffness are closely related to effects of microvascular changes in the kidney, including albuminuria.³⁵,³⁶ Interventions aimed at reducing the ill effects of arterial stiffening by reducing the extent and frequency of the stretch cycles in order to minimize fatigue and fracture of the medial elastin of aorta can reverse or delay progression of cerebral and renal damage. These maneuvers entail a reduction in early wave reflection achieved through regular exercise, and the use of drugs such as angiotensin-converting enzyme inhibitors
(ACEI), angiotensin receptor blockers (ARB), calcium channel blockers, and nitrates, which relax smooth muscle in large and small conduit arteries throughout the body, thus resulting in arterial dilation.³⁷,⁴¹

GFR gradually increases after birth, approaching adult levels by the end of the second decade. It remains stable until the age of 30 to 40 years and then declines linearly at an average rate of about 8 mL per minute per decade, a phenomenon that can be partially explained by age-associated glomerulopenia.⁷,⁸,⁴² However, about one-third of elderly individuals show no change in GFR.⁴³ This variability suggests that factors other than aging may be responsible for the apparent reduction in renal function. For instance, an increase in blood pressure, still within the normotensive range, is associated with an accelerated age-related loss of renal function.

The use of creatinine clearance in a timed urinary sample is commonly used as an estimate of GFR. Inulin and iothalamate clearance are very accurate measurements of GFR, but are clinically cumbersome to perform.⁴⁴ To obviate the need for a timed urine collection, various equations have been developed and are increasingly used to estimate GFR (Table 65.3). In adults, creatinine clearance is often estimated by the Cockcroft-Gault (CG) equation, and GFR is estimated by the Modification of Diet in Renal Disease (MDRD) formula.⁴⁵,⁴⁶ It is important to point out that the CG equation estimates GFR in milliliters per minute, whereas the MDRD formula expresses GFR in milliliters per minute per 1.73 meters squared.

However, neither MDRD nor the CG equation was developed for elderly individuals, and reduced reliability would be expected when used in this population. In a study involving 100 individuals aged 65 to 111 years, GFR values calculated with the MDRD formula were much higher than those obtained with the CG equation. Moreover, no correlation was observed between these two predictions. Additionally, the difference in GFR values between MDRD and CG increased dramatically with aging and decreased with higher body mass index and serum creatinine values. Thus, the precision and accuracy of these formulas in estimating GFR in very old patients remain arguable.⁴⁷ Furthermore, a study in patients over 65 years old showed more than 60% discordance in GFR estimation by the two equations. The MDRD equation generally yielded higher estimates of GFR than the CG equation.⁴⁸ This has important implications, especially when calculating drug dosages in the elderly. It was recommended that the CG equation should be used in preference to the MDRD equation to estimate GFR for drug dosage calculations in the elderly. Recently, a new creatinine-based equation was developed by the Chronic Kidney Disease Epidemiology Collaboration (CKD-EPI). It reported a more accurate estimation of GFR than the MDRD equation, particularly at higher levels of estimated GFR (eGFR).⁴⁹ In a recent prospective observational study of 439 patients aged 65 and older admitted to 11 acute care medical
wards in Italy, the relative risk of mortality in patients with eGFR = 30 to 59.9 or < 30 mL/min/1.73 m² was compared to subjects with eGFR ≥ 60 mL/min/1.73 m² using the body surface area-adjusted CG (CG-BSA), the MDRD, and the CKD-EPI formulas. Participants with reduced GFR showed an increased mortality regardless of the equation used, and CKD-EPI-derived GFR outperformed to some extent MDRD-and CG-BSA-derived GFR in a multivariable predictive model, suggesting its usefulness in the elderly population.⁵⁰ However, the performance of the serum creatinine-based estimating equations still remains insufficiently evaluated in older patients, in whom there may be a high prevalence of chronic disease associated with alterations in muscle mass and diet, resulting in an overestimation of the measured GFR and an underestimation of the severity of CKD. Lately, cystatin C has emerged as an alternative marker for the measurement of kidney function.⁵¹ Cystatin C, a filtration marker that is less related to muscle mass than creatinine, may have a particular advantage in the estimation of GFR in the elderly population. However, the clinical role of cystatin C measurement remains unclear. In a recent study involving 11,909 patients, the risk of death, cardiovascular events, and kidney failure was compared in patients with GFR < 60 mL/min/1.73 m² (CKD) to those with GFR > 60 mL/min/1.73 m², as estimated by creatinine and cystatin C measurements. The survey showed that cystatin-based estimates were better predictors of adverse outcomes among adults with CKD, suggesting that cystatin C may be useful in identifying patients with CKD who have high risks of complications.⁵²

Ordinarily, age has no effect on basal plasma electrolyte concentrations or the ability to maintain normal extracellular fluid volume. However, structural changes in the elderly kidney have an impact on the adaptive mechanisms responsible for maintaining homeostasis of extracellular fluid volume and composition. Consequently, acute illnesses in geriatric patients are often complicated by the development of fluid and electrolyte abnormalities, which are associated with increased morbidity and mortality and prolonged hospitalization. In the elderly, the capacity to conserve sodium in response to reduced sodium intake is impaired.⁶² The exact mechanism is not known, but a reduction in the number of functioning nephrons with increased sodium load per each remaining nephron as well as reduced aldosterone secretion in response to sodium depletion are plausible. Nevertheless, the inability to conserve sodium may predispose the elderly patient to hemodynamic instability in the setting of sodium loss. This, along with other structural and functional changes, make older patients more prone to develop acute kidney injury.⁶³

In addition to the impairment in sodium conservation, the elderly are also prone to volume expansion when challenged with a sodium load. This is due to a diminished capacity of renal sodium excretion in the elderly.⁶⁴ Additionally, the elderly seem to have more sodium excretion at night compared to the daytime, suggesting an impaired circadian variation.⁶⁵ Impaired pressure natriuresis and altered response to Ang-II are apparent mechanisms involved.⁶⁶ Notwithstanding the mechanisms, geriatric patients may develop an expanded extracellular fluid volume in the setting of a sodium load. It will usually lead to modest weight gain and the appearance of mild peripheral edema in the absence of significant comorbidities. However, the geriatric patients with preexisting cardiac or renal disease may develop life-threatening pulmonary edema, necessitating aggressive emergency therapy with loop diuretics or dialysis.⁶⁶ Elderly subjects also show abnormalities in renal potassium and calcium handling, which are discussed in the ensuing section.

Telomeres are the nucleoprotein structures constituting the physical ends of linear chromosomes. They prevent chromosomal ends from being recognized as double-strand breaks and protect them from end-to-end fusion and degradation.¹⁰² Telomeres in somatic cells shorten with each cell division and this progressive attrition leads to critically short telomeres and cellular senescence, a state characterized by the absence of replication and biochemical changes.¹⁰² This phenomenon was observed in earlier in vitro studies showing that cultured fibroblasts could only undergo a limited number of population doublings, the so-called Hayflick limit.¹⁰³ It is
believed that telomeres act as a mitotic clock, initiating replicative senescence when telomeres become critically short after a certain number of cell divisions. Melk et al.¹⁰⁴ have demonstrated that telomere shortening progresses with advancing age in the human kidney, and this phenomenon is more important in the cortical than the medullary area.¹⁰⁴

The enzyme telomerase is required for the maintenance of the length and stability of telomeres. An overexpression of telomerase induces an artificial lengthening of the telomeres and markedly increases the proliferative potential of cells. The antiproliferative effect of aging appears to be governed by two signaling pathways activated by the cellular replication clock: one involves p53, which induces p21 overexpression, and the other stimulates the expression of the cell cycle inhibitor, p16. In vitro studies have shown that senescent cells express p16 and p21, which inhibit cellular proliferation by inhibiting cyclin-dependent kinases.¹⁰⁵ In vivo studies have demonstrated low levels of p16 in kidneys from young individuals. There is an overall increase in p16^INK4a expression with age, particularly in the renal cortex, although this expression varies from individual to individual.¹⁰⁶ Furthermore, in age-related histologic changes, a strong correlation is found between glomerulosclerosis, interstitial fibrosis, tubular atrophy, and p16^INK4a and p53 expression.¹⁰⁷ In a recent study, Westoff showed that telomerase-deficient mice were more vulnerable to ischemic-reperfusion injury due to reduced tubular regeneration.¹⁰⁸

The pharmacologic activation of telomerase could be an appropriate therapy for aging diseases associated with replicative senescence. On the other hand, telomerase inhibitors have been proposed as a new option for chemotherapy, illustrating the trade-off between accelerated biologic aging and increased cancer risk.¹⁰²,¹⁰⁹

The discovery of antiaging gene Klotho has extensively enhanced the understanding of the genetic bases of senescence. Klotho-deficient mice exhibited a syndrome resembling a
premature aging phenotype with soft tissue and vascular calcification, hyperphosphatemia, muscle and skin atrophy, and early death. In contrast, overexpression of the Klotho gene extended the life span in the mouse.¹¹⁰ In addition, several single-nucleotide polymorphisms in the human Klotho gene are associated with a shortened life span, osteoporosis, stroke, and coronary artery diseases, suggesting that Klotho may be involved in the regulation of human aging and age-related diseases.¹¹¹

The Klotho gene encodes a single-pass transmembrane protein with two homologous extracellular domains, each having a weak homology to the β-glucosidase of bacteria and plants.¹¹² Although the Klotho gene is expressed in limited tissues, notably the kidney, the parathyroid, and the brain, a defect in Klotho gene expression leads to multiple aging-like phenotypes involving almost all organ systems. This effect of the Klotho protein is mediated by its hormonal action via its binding to a cell-surface receptor and repressing the intracellular signals of insulin and insulinlike growth factor 1 (IGF-1). It appears that the antiaging effect of the Klotho-induced inhibition of insulin/IGF-1 signaling is associated with increased resistance to oxidative stress.¹¹³ It has been shown that the Klotho protein induces the expression of manganese superoxide dismutase, a mitochondrial antioxidant enzyme that facilitates the removal of superoxide, thereby conferring protection against oxidative stress.¹¹⁴ On the other hand, hydrogen peroxide-induced oxidative stress has been shown to reduce Klotho expression in a mouse inner medullary collecting duct (mIMCD3) cell line.¹¹⁵ Interestingly, the beneficial effects of peroxisome proliferator activated receptor-gamma (PPAR-γ) agonists on age-related glomerulosclerosis are mediated by intrarenal Klotho expression.¹¹⁶

Klotho also plays a central role in calcium and phosphorus homeostasis and the inhibition of active vitamin D synthesis.¹¹⁷ Ang-II has been shown to downregulate Klotho expression, which may, in part, contribute to Ang-II-induced renal damage.¹¹⁷ Klotho polymorphisms are associated with osteopenia in postmenopausal women. Klotho-deficient mice develop calcification of small arteries in the kidney and show increased serum levels of 1,25-dihydroxyvitamin D, which, along with increased calcium phosphate product, may be responsible for severe vascular and soft tissue calcification.¹¹⁷ Interestingly, the abnormalities in bone and phosphate metabolism observed in Klotho-deficient mice are very similar to those observed in fibroblast growth factor (FGF)-23 knockout mice suggesting that Klotho and FGF23 function in a common signal transduction pathway.¹¹⁸ Further studies indicate that Klotho acts as a cofactor by binding to FGF receptors and is essential for the signaling of FGF23 and related FGFs.¹¹⁹ Studies have shown that FGFs play important roles, not only in mitosis and development but also in various metabolic processes including the regulation of insulin sensitivity, glucose/lipid/energy metabolism, and oxidative stress, all of which potentially affect the aging processes.¹²⁰,¹²¹ Klotho may regulate aging processes, partly through controlling the FGF-signaling pathways. The presence of any abnormality in either Klotho or FGF23 leads not only to phosphate retention but to premature aging in mice.¹²² A recent study by Hu et al.¹²³ suggests that Klotho deficiency may have a direct effect on the rat vascular smooth muscle cell (VSMC) and promote VSMC calcification independent of FGF23 signaling.¹²³

Klotho protein also regulates ion channel activity in renal tubular cells. Recently, Klotho was shown to activate transient receptor potential ion channel (TRPV)5, an epithelial calcium channel expressed on the apical membrane of the distal convoluted tubules and the connecting tubular cells.¹⁰⁰ TRPV5 functions as an entry gate for transcellular calcium reabsorption in these cells and participates in renal calcium reabsorption, thus countering the effects of low phosphate on bone.¹²⁴ Additionally, Klotho decreases cell surface abundance of the TRPC6 channel, the upregulation of which is associated with glomerulosclerosis.¹²⁵

It has been shown that renal Klotho mRNA is downregulated under sustained circulatory or metabolic stress and in chronic kidney disease.¹²¹ Mitani et al.¹²⁶ have demonstrated that Ang-II, which may be involved in age-related organ damage, plays a pivotal role in reducing renal Klotho gene expression in experimental animals. Conversely, induction of the Klotho gene by an adenovirus vector might protect against Ang-II-induced renal damage, such as tubulointerstitial injury and vascular wall thickening.¹²⁶ The Ang-II-induced downregulation of the Klotho gene expression may be mediated by promoting intrarenal iron deposition and ROS production.¹²⁰ In fact, treatment with a free radical scavenger and an iron chelator attenuated Ang-II-induced renal injury. Further investigations are required into other mechanisms involved in the regulation of the Klotho gene expression and senescence.

Cumulative oxidative injury is believed to play a major role in the cellular aging process. Oxidative stress and the generation of free radicals increase with aging.¹²⁷ Persistent oxidative damage to cytosolic structures leads to the crosslinking of oxidized proteins, the deposition of lipofuscin, and impaired mitochondrial function and structure.¹²⁸ Lipofuscin (cross-linked proteins), which is insoluble and not degradable by either lysosomal enzymes or the proteasomal system, and giant mitochondria are taken up by lysosomes, wherein they bind additional material and eventually cause lysosomal rupture. The released lipofuscin and lysosomal contents cause cell damage and dysfunction.¹²⁸ In rats of different ages (2, 11, and 29.5 months) there was up to a 28-fold increase in lipofuscin deposition in kidneys and other organs in older compared to the 2-month-old animals.¹²⁹ Furthermore, increased oxidant stress (elevated serum lipoperoxides) and a reduced antioxidant activity (erythrocyte superoxide dismutase and glutathione peroxidase) were observed in the elderly in a cross-sectional study involving 249 healthy subjects.¹²⁷ The magnitude of oxidative stress and lipid peroxidation in the aging kidney correlates with
an elevation of the advanced glycosylation end products and their receptors (AGE and RAGE) that can cross-link adjacent proteins. This, along with the ROS that can activate ubiquitin-proteasome, may degrade hypoxia-inducible factor-1alpha (HIF-1α) and limit the capacity of the aging cells to form HIF-1-DNA hypoxia-responsive recognition element (HRE) complexes (HIF-1-HRE complexes).¹³⁰,¹³¹ In the kidney, the subsequent decrease in the ability of the cells to respond to hypoxia could explain the attenuated anemiainduced secretion of erythropoietin as well as the decreased hypoxia-induced production of vascular endothelial growth factor leading, respectively, to reduced erythropoiesis and angiogenesis.¹³⁰ In a rat model, renal aging was associated with a 60% decline in GFR, a threefold increase in renal F2 isoprostanes (a marker of oxidative stress), an increase in oxidant-sensitive heme oxygenase, as well as increased AGEs and RAGE. Furthermore, a diet rich in vitamin E attenuated the age-related upregulation of heme oxygenase and RAGE, suppressed the production of F2 isoprostanes, lessened measured markers of oxidative stress, reduced glomerulosclerosis, and improved renal plasma flow and GFR by 50%.¹³² In cultured rabbit proximal tubular epithelial cells, AGEs and even the early glycosylation end products (Amadori products) directly inhibited NOS activity and AGEs quenched the released NO.¹³³ Immunohistochemical studies of the aging rat kidneys have revealed a reduction of endothelial NOS (eNOS) in the peritubular capillaries and the presence of eNOS immunoreactivity in renal tubular epithelial cells, infiltrating mononuclear cells and foci of tubulointerstitial injury, suggesting that the aging-related renal tubulointerstitial fibrosis may be secondary to the ischemia caused by peritubular capillary injury and impaired eNOS expression.¹³⁴ Via activation of the angiotensin receptor AT1, the tissue RAS may promote the production of ROS and TGF-β1, events that can promote fibrosis. Indeed, the administration of ACEI and ARB in rats ameliorated the aging-related renal damage and attenuated glomerular sclerosis, mesangial expansion, tubular atrophy, interstitial fibrosis, and mononuclear cell infiltration.¹³⁵ The salutary effect of RAS blockade may be, in part, mediated by the ability to limit the impact of aging on the structure and function of mitochondria and other cellular organelles involved in energy metabolism and ROS production.¹³⁶

In addition to the mechanisms described previously, recent data suggest that chronic oxidant stress contributes to the telomere shortening and thus senescent changes.¹⁰⁶ Furthermore, by downregulating Klotho gene expression, chronic oxidant stress can mediate the aging-associated changes.¹³⁷ Oxidative stress also activates the target of rapamycin (TOR) pathway, which plays an important role in the aging process by inhibiting mitochondrial autophagy (see the following).¹³⁸ Thus, interventions aimed at reducing ROS production, enhancing antioxidant capacity, increasing NO availability, and suppressing fibrogenic pathways may be effective in retarding aging-related renal damage.¹³⁹ In addition, calorie restriction via mechanisms described elsewhere in this chapter can decrease age-related oxidant stress, mitochondrial lipid peroxidation, and membrane damage.¹⁴⁰ Thus, dietary caloric restriction may be another approach to reduce age related renal damage.