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Chronic kidney disease (CKD) is a growing health problem. Data from the US Renal Data System Reports show that the number of patients enrolled in end-stage renal disease (ESRD) Medicare-funded programs increased to a staggering 60-fold during the last 4 decades1–4 and now consumes a significant portion of the healthcare budget. Despite the magnitude of resources dedicated to treatment of chronic renal disease and the substantial improvements in the quality of dialysis and coadjuvant therapeutic strategies, these patients experience significant reductions in their quality of life, increased morbidity, and higher mortality. Furthermore, a recent projection shows that the incidence and prevalence of CKD may continue to increase in coming decades.5 This somber scenario emphasizes the need for a better understanding of underlying mechanisms of renal injury and the development of novel interventions and strategies to slow the onset and progression of CKD.
Microvascular networks are dynamic anatomic units that are tightly balanced to provide nutrition and remove waste products to meet the metabolic and functional demands of each tissue. In the kidney, the glomerular and peritubular capillaries also command glomerular filtration, tubular reabsorption, and recirculation of body fluids, nutrients, hormones, and other substances to the body.6,7 Endothelial dysfunction as well as functional and structural rarefaction8 of the renal microvessels play a prominent role in inducing renal injury associated with major cardiovascular risk factors, such as hypertension, dyslipidemia, diabetes mellitus, and atherosclerosis. Furthermore, a defective renal microcirculation is a universal pathological feature in CKD that progresses as CKD evolves and compromise both the renal nutrition and renal function.6,7
The current review will focus on the role of microvascular disease in the progression of renal injury. It will discuss the involvement of microvascular disease as both cause and consequence of pathological mechanisms affecting the kidney. Finally, I will also discuss the potential of therapeutic interventions to protect the renal microvasculature using clinically available and experimental treatments. Although promising,9–15 the success of targeted microvascular therapies may depend on how severe and extensive the microvascular damage is and mainly, whether glomeruli are lost or still viable, narrowing the window of opportunity to change the progressive nature of CKD/ESRD.
Epidemiology, Major Causes, and Pathological Contributors to CKD
The emergent population of CKD/ESRD patients in recent years is linked to dramatic increases in obesity and obesity-associated cardiovascular risk factors, such as lipid abnormalities, atherosclerosis, diabetes mellitus, and hypertension. Although these comorbid conditions may develop separately, they often coexist and may potentiate injury of target organs, including the kidneys, in an additive or synergistic fashion.7,16,17
Recent reports from the Centers for Disease Control,18 the National Institutes of Health,19 and the World Health Organization20 show that obesity has more than doubled since 1980. Currently, 68.8% of adults in the United States are overweight or obese19 (with a slightly higher prevalence in women: 40.4% versus 35%21), with 6% to 8% of them having extreme obesity (body mass index >40). Furthermore, ≈33% of the US children and adolescents are overweight and >18% are obese.18,19 Unfortunately, the prevalence (and consequently, impact) of obesity will likely continue to increase unless these trends can be reversed because ≈80% of obese children may become obese adults with the associated increased susceptibility to develop cardiovascular, metabolic, and kidney diseases.
Dyslipidemia and atherosclerosis are parts of a chronic and systemic inflammatory process that compromises the function and structure of small and large vessels. They are frequently associated with chronic renal disease and may serve as cause and as consequence of CKD. Not only by the build-up of atherosclerotic plaques, but also at preceding stages of vascular fatty streaks or microscopic lipid accumulation, dyslipidemia may promote vascular dysfunction and early remodeling in several vascular beds, including the kidneys. Indeed, microvascular and glomerular dysfunction may precede the onset and represent the silent phase of chronic renal disease.22,23 Experimental studies indicate that diet-induced lipid abnormalities lead to renal endothelial dysfunction, intrarenal inflammation, fibrosis, and a significant vascular dysfunction, damage, and remodeling22–25 on renal vasculature. Furthermore, recent studies showed that dyslipidemia superimposed on experimental renal artery stenosis can not only accelerate renal microvascular dysfunction and remodeling, but also accelerate microvascular loss, underscoring ample deleterious effects of atherogenic factors on the renal parenchyma.23,26
The major causes of CKD and ESRD are diabetes mellitus and hypertension, which lead to progressive microvascular damage and loss and set the stage for evolving renal injury.27–30 Hypertension is the second leading cause of CKD/ESRD in the United States after diabetes mellitus, is estimated to affect 1 billion people worldwide, and is responsible of 9 million deaths per year.31 The recent SPRINT study (Systolic Blood Pressure Intervention Trial) demonstrated that aggressive control of blood pressure decreases risk for cardiovascular events and even death,32 which underscores the pathophysiological importance of hypertension and its priority for treatment. The effect of high blood pressure on the renal vessels is a major driving force for the early development of vascular remodeling in both large and small vessels, which precedes, predicts, and significantly contributes to development of overt renal abnormalities.33–35 Diabetes mellitus is the number one cause of CKD/ESRD and has almost quadrupled in the last 3 decades, affecting 422 million adults worldwide. These staggering numbers are largely because of the rise in type 2 diabetes mellitus, which accounts for ≤95% of the 25.8 million cases of diabetes mellitus in the United States and is driven by overweight and obesity.36 The development and mechanisms of diabetes mellitus–induced renal injury and progression toward diabetic nephropathy have been reviewed and discussed elsewhere37,38 and are beyond the scope of this review. However, it is important to emphasize that, as in hypertension, renal microvascular abnormalities in diabetes mellitus also lead and foresee the deterioration of renal function, underscoring the central role of microvascular disease in development and progression of renal injury.27,29
Life expectancy has significantly increased in the past 50 years.39 Aging carries a physiological decline in renal function, which may be accelerated or aggravated by comorbid conditions. Indeed, with time, the kidney shows an age-associated reduction of function disclosed by a progressive (but widely variable) decline in glomerular filtration rate and renal blood flow, which is possibly driven by reduced renal bioavailability of nitric oxide (NO) and often associates with different degrees of renal parenchymal damage.40 Importantly, most of these changes are driven by functional and structural changes of the renal microcirculation at the pre- and postglomerular level, an increase in glomerular capillary hydraulic pressure, and parenchymal changes that lead to loss of renal mass, microvascular remodeling, and tubule–interstitial fibrosis.41,42 Although such changes do not always translate into CKD and may not require interventions in a disease-free individual, they can increase the susceptibility of the aging kidney to acute kidney injury and make it more labile for the development of CKD.40,41
Renal Microvascular Network
Progressive ramifications from the main renal artery branch into interlobar, arcuate, and interlobular arteries toward the smaller branching afferent arterioles leading to the glomerular capillaries where fluid and solutes are filtered (except for plasma proteins). Then, the distal ends of the glomerular capillaries converge to form the efferent arterioles, which are followed by a second capillary network, the peritubular capillaries, which are key components for filtration, secretion, and reabsorption of minerals and removal of waste from the filtered blood that will be excreted by urine. On the venous side, the small veins run in parallel to the arterioles to subsequently form the interlobular, arcuate, interlobar, and renal vein, which leaves the kidney beside the renal artery and ureter. This unique vascular network deals with ≈1.1 L/min of the cardiac output, and only 10% of the delivered oxygen is normally sufficient to satisfy the renal metabolic demands.6 Thus, the majority of the workload on the renal microvascular networks is to maintain body homeostasis, which underscores the importance of renal function in health and disease.
The intrarenal microvascular network may be disrupted from a functional or a structural angle. Microvascular rarefaction is defined as a reduction of available vessels, which can be divided into functional (the vessels are anatomically present but with a deficient or absent perfusion) and structural (anatomic reduction in the number of vessels in the tissue) rarefaction.8 A study by Bohle et al43 demonstrated in biopsies of patients with chronic renal disease from different etiologies a significant inverse relationship between the number and area of the cortical tubular capillaries and their serum creatinines, showing that microvascular rarefaction develops in human CKD and negatively correlates with renal dysfunction.
It is important to emphasize that these 2 forms of microvascular rarefaction are not mutually exclusive, can coexist, and can be parts of a progressive process that goes from dysfunction to loss of microvessels. A potential bridge between functional and structural rarefaction could be defined as microvascular remodeling, in which anatomic changes in the microvascular wall develops, progressively contributes to disrupt microvascular function, and may lead to microvascular loss. Finally, renal microvascular rarefaction may also develop as a physiological event in the aging kidney and is suggested as a major determinant in the decreased renal blood flow and glomerular filtration rate 44 associated with age. Therefore, because smaller arterioles and capillaries are primary targets of acute or chronic insults11,12,45,46 and their damage may affect renal hemodynamics, function, and progression of renal injury, microvascular rarefaction (functional and structural, physiological, or pathological) plays an important role in the progression of renal injury, regardless of the primary etiology.
Mechanisms of Kidney Injury Driven by Microvascular Rarefaction
Work in Progress: Microvascular Endothelial Dysfunction
Renal microvascular endothelial dysfunction is a central mechanism driving functional rarefaction because it may contribute to the transition from functional to structural rarefaction. It results from abnormal function of endothelial cells combined with reduced availability of substances that are produced by or act on the endothelium to determine vascular tone, permeability, fluid balance, and cell proliferation.47,48 Endothelial dysfunction develops early in cardiovascular and renal disease and is a consequence and a contributor for the development and progression of hypertension, diabetes mellitus, atherosclerosis, and chronic heart and renal pathologies.49–52
A central player in endothelial dysfunction is deficiency of NO, a gaseous molecule that controls vascular tone and regulates inflammatory and coagulant properties of the endothelium (beyond the scope of this review). A reduced availability of NO may be the result of altered production from major sources, such as endothelial NO synthase (eNOS) or augmented NO removal. Studies from Goligorsky et al53,54 demonstrated that inhibition of NOS and deficiency of NO are powerful profibrotic stimuli that increase endothelial to mesenchymal transdifferentiation and may contribute to microvascular endothelial dysfunction and subsequent microvascular rarefaction. Furthermore, an altered production by eNOS can also result from a lack of a key cofactor, such as tetra-hydrobiopterin, which results in eNOS uncoupling and switches eNOS from production of NO to generation of reactive oxygen species (ROS).55 In turn, ROS decrease renal microvascular bioavailability of NO through ROS-mediated quenching effects.56 Factors that favor deleterious ROS-NO interactions in the kidney include excessive activation of the renin–angiotensin–aldosterone and endothelin systems.55,57 These systems play important physiological roles in controlling renal hemodynamics but are often upregulated in cardiovascular and renal disease and contribute to vasoconstriction and endothelial dysfunction directly and via stimulation of ROS production. Increased ROS may in turn stimulate production and activation of redox-sensitive proinflammatory and profibrotic factors in the kidney, such as nuclear factor kappa-B, tumor necrosis factor-α, transforming growth factor-β, or connective tissue growth factor, to name a few, which are often involved in the development and progression of renal injury.22–24,58 Thus, ROS-mediated reduction in renal NO takes center stage in pathological effects associated with endothelial dysfunction and may promote vasoconstriction, vascular inflammation, and tissue damage,59 which in turn further reduce NO bioavailability and perpetuates a vicious circle.
Work in Progress: From Microvascular Constriction to Microvascular Remodeling
In parallel, a dysfunctional or damaged endothelium may lead to a sustained renal vasoconstriction, which may lead to inadequate intrarenal nutrition, as well as compromised renal hemodynamics and function. A prolonged vasoconstriction of intrarenal microvessels may lead to inward remodeling, a structural microvascular alteration that is a prominent mechanism for the development and progression of renal damage in hypertension and renal ischemia.60–62 Our previous studies in a model of chronic renovascular hypertension and renovascular disease demonstrated that progressive renal injury associates with significant intrarenal microvascular remodeling, disclosed by increased microvascular media-to-lumen ratio, perivascular fibrosis, and decreased microvascular diameter.23,60,63 Such changes in the kidney microvasculature were mainly observed in those vessels under 200 μm in diameter, which suggest that pre- and postglomerular microvessels are susceptible targets and likely contributors to development of renal injury.
A combination of a sustained vasoconstriction and a proinjurious milieu likely pave the way for progression of functional rarefaction toward microvascular remodeling and eventually loss of the smaller vessels, which may contribute to further renal injury. Furthermore, a recent study demonstrated that renal endothelial cells show a distinctly poor endogenous proliferative ability, which may play a role in the limited intrinsic regenerative capacity of renal capillaries64 and may significantly contribute to the progressive nature of renal microvascular rarefaction. However, studies have also shown that therapeutic interventions using different compounds such as clinically available antioxidant vitamins, statins, endothelin receptor or renin–angiotensin–aldosterone blockers, or experimental agents can preserve or recover endothelial function, improve renal hemodynamics, and reduce the development and progression of renal injury in hypertension, diabetes mellitus, atherosclerosis, acute, and chronic renal injury.4,12,24,45,65–70 It is important to emphasize that such improvements are often accompanied by attenuated remodeling of the renal microvascular architecture, underscoring the notion that microvascular disease is an evolving process that, if unattended, actively participates on the progression of renal damage. Moreover, microvascular remodeling may also diminish the efficacy of therapeutic interventions,61 suggesting a potential tipping point of microvascular damage that could limit recovery and lead to the progression toward irreversible tissue damage and loss of function.
Renal Microvascular Loss: Cause and Consequence of Progressive Renal Damage
Loss of the microvasculature in any tissue or organ may reflect the final stage of progressive loss of microvascular homeostasis paired with disruption of healing mechanisms. Indeed, plasticity of the renal microcirculation to adapt to a new environment or to generate from preexisting vasculature as needed are important characteristics that may be lost as diseases progress, as can be observed in atherosclerosis,25 ischemia,12,63 or diabetes mellitus–induced renal injury.29
One of the pivotal players for maintenance and repair of microvascular networks everywhere, including the kidney, is VEGF (vascular endothelial growth factor). This proangiogenic cytokine plays important roles in the kidney that go beyond vasculoprotective effects.3,71 VEGF is highly ubiquitous, and the renal cells are sources and targets of this cytokine. Major sources of renal VEGF are tubular epithelial cells and podocytes, and major targets are endothelial cells and podocytes as well, suggesting renal autocrine and paracrine effects.3,72,73
Progressive dysfunction, damage, and loss of endothelial cells and glomeruli and peritubular microvascular drop-out paired with a marked reduction of VEGF expression have been described in clinical74 and experimental settings.15,75–77 Altered expression and availability of renal VEGF coupled with microvascular abnormalities has been demonstrated in CKD, renal ischemia, early diabetic-induced renal injury, and diabetic nephropathy.11,27,29,78 The decreased renal bioavailability likely results from loss of renal sources VEGF (eg, proximal tubular cells, podocytes3,11), altered VEGF-upstream signaling,63 and possibly, disruption of post-translational mechanisms of VEGF.12 The decrease in renal VEGF also affects the VEGF receptors-mediated downstream angiogenic signaling, as shown by the blunted expression of renal angiopoietins, Akt (protein kinase B), and ERK1/2 (extracellular signal-regulated kinase 1 and 2), which are prominent mediators of endothelial cell survival, proliferation, and maturation of newly generated vessels.9 Furthermore, insufficient or reduced renal VEGF may also have a major impact on preceding steps that involved mobilization and homing of cell progenitors toward microvascular repair, proliferation, and tissue healing.13 Indeed, a decrease in renal VEGF also drives the blunted expression of stromal-derived factor 1, angiopoietins, and Oct-4 (octamer binding transcription factor 4), which are prominent factors involved in progenitor cell biology that are all recovered after improving VEGF signaling.9,13,15 Therefore, in a context of blunted renal bioavailability of VEGF, the impetus for microvascular proliferation and repair is severely diminished, leading to a reduction in renal microvascular density, and may become a central progressive mechanism of renal parenchymal damage.
The renal microvascular network can also be disturbed or reduced by other mechanisms. Hepatocyte growth factor (HGF) is a pleiotropic cytokine with distinct renoprotective roles, as has been demonstrated in diabetes mellitus–induced renal injury and in acute and chronic renal ischemia.10,79,80 HGF promotes tissue healing by stimulating mobilization of cell progenitors, by interactions with VEGF81 to promote microvascular proliferation and repair, and by counteracting renal inflammation, fibrosis, and apoptosis via transforming growth factor-β, nuclear factor kappa-B, and Bax/BcL (B cell lymphoma-2 associated X protein) inhibition.10,80,82–84 Reduced bioavailability of this factor significantly accelerates development of microvascular rarefaction, renal inflammation, and fibrosis,10,80,83 supporting an important role of HGF to protect the renal parenchyma and preserve the renal microvasculature.
Renal fibrosis is the common pathway of advanced renal disease that is closely related to microvascular rarefaction. A recent study from Ehling et al85 using 3 different mouse models of progressive renal disease showed that abnormalities in the renal microvascular function and structure, such as reductions in microvascular diameter and increased microvascular tortuousity, develop early and may precede and contribute to development of renal fibrosis, supporting a prominent role of microvascular abnormalities as a possible universal mechanism for progression of tissue damage. Fibrosis is the loss of functional tissue that is replaced by nonfunctional scarred tissue that reflects a marked imbalance between extracellular matrix (ECM) production and degradation toward ECM accumulation. The renal accumulation of ECM in turn may induce powerful effects on renal microvascular development, proliferation, and function86 directly and via increasing antiangiogenic products. Indeed, the ECM is a rich source of factors that may diminish microvascular proliferation and repair, such as endostatin, a potent inhibitor of angiogenesis and VEGF and a prominent contributor to microvascular rarefaction in CKD.87 Another ECM-related antiangiogenic factor that may contribute to renal microvascular loss is angiostatin,9 a potent proapoptotic and anti-VEGF factor that can contribute to the development of tubular and interstitial damage by inducing capillary fragility and dropout.88,89 These antiangiogenic factors may interfere with VEGF and reduce microvascular repair and proliferation, which may negatively impact renal tissue healing and functional recovery independent of the initial insult. In depth discussion of molecular mechanisms of renal fibrosis are beyond the scope of this review, and readers are suggested to consult published literature.90,91
Another pathway to renal microvascular loss is apoptosis or programed cell death, a prominent mechanism by which podocytes3 and renal endothelial and epithelial cells may be killed when facing acute or chronic ischemia and a contributor for the progression of vascular dropout and tubular injury.92,93 An ischemic/hypoxic renal milieu may stimulate apoptosis and, thus, exacerbate renal injury via the intrinsic (caspase-dependent) and extrinsic (caspase-independent) pathways.3,4,94 Apoptosis of renal endothelial cells could be driven and enhanced by a reduction in bioavailability of angiogenic factors, such as VEGF or HGF, which not only stimulates migration and proliferation, but also promotes cell survival via powerful antiapoptotic effects.95–97 In turn, ischemic-induced apoptosis of podocytes may contribute to the loss of renal VEGF,3 which may further accelerate endothelial cell death and reduce compensatory renal microvascular proliferation and repair. Our recent studies in a swine model of chronic renal artery stenosis showed that a reduced expression of these factors in the ischemic kidney parallels increased apoptotic activity and apoptotic cells, progressive microvascular rarefaction, and evolving renal dysfunction and damage.4,11,66 Thus, apoptosis may serve as an important contributor for renal microvascular endothelial cell damage and loss, development of microvascular rarefaction, and may increase the risk for progression of renal injury. However, unlike necrosis, apoptosis is an energy-dependent mechanism of cell death, and the extent of its pathophysiological role in microvascular damage and loss may be context-dependent and determined by the severity of the initial renal insult and development of renal disease.94
Therapeutic Strategies: Is Protection of Renal Microvessels Feasible?
Severity of microvascular damage and loss may define the limits between reversible and irreversible renal injury, and success in restoring kidney function may depend on how far microvascular rarefaction has progressed (Figure 1). The different degrees of microvascular dysfunction and damage, which may in turn depend on the extent of the initial insult or length of the disease, may offer an opportunity for therapeutic targeting of renal microvascular disease to stimulate microvascular repair and peritubular and glomerular capillary regrow. Evidence from our laboratory supports the notion that renal functional and structural damage could be ameliorated by targeted interventions that reverses renal microvascular rarefaction9,11,15 (Figure 2). However, such approach may need to be initiated before the entire glomerulus is lost. It is possible that generation of new vessels that shunt preexisting damaged ones and stimulation of microvascular repair may contribute to restoration of blood flow and to improve glomerular filtration rate in partly damaged or hibernated but recoverable nephrons.99,100
Schematic overview of the progressive nature of chronic kidney disease (CKD) and contributions of microvascular (MV) rarefaction to the process. Regardless of the etiology (eg, hypertension, diabetes mellitus, atherosclerosis, obesity), CKD universally associates with MV rarefaction. A progressive damage of the microcirculation deteriorates renal hemodynamics and perfusion, leading to a progressive loss of filtration, tubular function, and development of fibrosis, which in turn serves as a feedback mechanism that may further accelerate progression of CKD toward irreversible renal injury. Therapeutic interventions (gray box, clinically available or experimental) to protect the renal MV architecture and function (remodeling, rarefaction, regeneration, and repair) may be more effective to improve renal function and slow the progression of CKD if applied before severe reductions in renal blood flow and nephron loss (above red dotted lines).
Representative image showing renal fibrosis (top, ×20) and 3D micro-CT reconstruction of the renal microvascular (MV) architecture (bottom), tomographically isolated microvessels (yellow arrow, bottom), and cross sections of microfilm-perfused kidneys (orange arrow, bottom; stereo-microscopy, ×20) in normal, stenotic kidney, and stenotic kidney after vascular endothelial growth factor (VEGF) therapy. The stenotic kidney has a significant MV loss, increased MV tortuousity, MV remodeling, and fibrosis after 10 weeks of renal artery stenosis. A single intrarenal administration of VEGF after 6 weeks of renal artery stenosis largely reversed most of these changes (quantified 4 weeks later). Visibly perfused glomeruli correlates with changes in density of renal MV. CT indicates computerized tomography; and G, glomeruli. Adapted from Chade.98 Copyright © 2011, the American Physiological Society.
Microvascular protection or targeted renal microvascular angiogenesis strategies are not yet fully developed, and therefore, current therapies have not focused on preventing rarefaction of the renal microcirculation. This section will discuss the effects of clinically available and experimental therapies and their effects on renal microvascular function.
Direct and Indirect Microvascular Protection: Antihypertensive, Antidiabetic, and Lipid-Lowering Drugs
Diabetes mellitus, hypertension, and lipid abnormalities are major causes of CKD. Therapeutic agents often used in these diseases display effects on microvascular function, remodeling, and even proliferation, which seem to be a pleiotropic effect of these compounds. Previous studies demonstrated that insulin,101 angiotensin receptor blockers,102 or statins60 play important roles in regulating endothelial function, vascular tone, and tissue perfusion by preserving or stimulating production of NO in endothelium and microvascular perfusion in different organs. Furthermore, other studies showed that the effects of these agents may also extend to attenuating microvascular remodeling and even promoting microvascular proliferation in several vascular beds like in the heart and kidney.60,68,103,104 Therefore, part of the renoprotective effects of these agents may result from preserving renal microvascular function and integrity, directly or indirectly, as well as from controlling risk factors, such as hyperglycemia, high blood pressure, and dyslipidemia.
For example, studies showed that insulin, beyond glucose control, may stimulate NO generation and proangiogenic activity in endothelial cells (eg, migration, tube formation105), which may be a prominent mechanism in insulin-mediated healing actions in different tissues106,107 that seems to be independent of VEGF signaling.108 Similar stimulatory effects on angiogenesis has been described in experimental settings, both in vitro and in vivo, for new generation of clinically available oral antidiabetic drugs, such as sitagliptin,109 and for glucagon-like peptide-1 agonists in a dose-dependent manner.110 On the other hand, metformin has been suggested to improve microvascular density in experimental stroke by increasing VEGF expression and activity,111 although these effects may be short term and are disputed by other studies, suggesting net antiangiogenic action of metformin.112 However, the use of metformin or glucagon-like peptide-1 agonists is limited by deterioration of renal function in diabetic patients; thus, its effects on the renal microvascular function and structure, if any, may be limited and warrants additional studies.113,114 Furthermore, renal vasculoprotective effects of sitagliptin have not been determined yet.
Angiotensin-converting enzyme inhibitors and angiotensin receptor blockers are among the first line of antihypertensive drugs. Beyond blood pressure control, studies have shown that angiotensin receptor blockers can have an impact on several other physiological processes, such as angiogenesis. In the kidney, studies using a porcine model of renal disease showed that angiotensin receptor blocker therapy results in improved renal hemodynamics, decreased oxidative stress (possibly driven by increase in NO bioavailability), and improved microvascular density and angiogenic signaling, implying distinct effects on renal microvascular functional and structural rarefaction.68,115 However, the exact role of angiotensin receptor blockers in angiogenesis is still controversial. Indeed, studies have reported pro- or antiangiogenic effects driven by modulation of VEGF signaling, inflammation cytokines, and apoptosis that seems to depend on the tissue environment, disease state, and animal model investigated. In depth discussion of these controversies are beyond the scope of this review, and the readers are encouraged to consult excellent literature.116
Endothelin-1 receptor blockers, currently used to treat pulmonary hypertension, also showed renoprotective actions. Endothelin-1 is one of the most powerful vasoconstrictors, exerts its effects through specific endothelin-A and endothelin-B receptors, and plays important roles in controlling endothelin-1 renal clearance, normal renal blood flow, and sodium handling.117 These effects were the major impetus for early attempts to use endothelin-1 receptor blockers as antihypertensive drugs.117 Emerging research demonstrated that blockade of the endothelin-A receptors also led to significant renoprotection. Experimental studies showed that chronic blockade of the endothelin-A (but not endothelin-B) receptors significantly reduces renal oxidative stress, inflammation, and fibrosis and preserves podocyte integrity, accompanied by recovery of the renal microvascular architecture and function (partly by improving renal VEGF and HGF expression and signaling) and improved renal hemodynamics, regardless of the etiology.3,65,66,70,118,119 The potential for endothelin-A blockade-induced renoprotection is underscored by recent evidence from clinical studies showing that endothelin-A antagonism may reduce renal injury, proteinuria (a marker of microvascular damage120,121), arterial stiffness, and cardiovascular risk in CKD from different etiologies,122,123 which supports experimental data demonstrating renal microvascular protection. Whether effects of endothelin-A blockade on the renal microcirculation are independent or the result of the overall improvements in renal function and injury are still unclear. The results of the ongoing SONAR clinical trial (Study of Diabetic Nephropathy With Atrasentan; unique identifier NCT01858532), which aims to determine the efficacy of endothelin-A antagonism for renoprotection in diabetic nephropathy may help determine whether endothelin-A blockade could serve as an additional strategy to recover the kidney.
Potential Novel Therapies
Cell-Based Therapies
Circulating and resident progenitor cells are stimulated when injury develops, resulting in proliferation, differentiation, and homing of these cells in damaged tissues. These complex events are key steps of the endogenous healing mechanisms in the body. However, the severity or chronicity of the initial insult may overwhelm these mechanisms and make them insufficient to reverse organ injury. Thus, there is still a major need for development of new cell-based therapeutic strategies.
The kidney has limited regenerative capacity, and no effective treatment has been developed to prevent progression of CKD to end-stage kidney disease, independent of the initial insult. The underlying mechanisms of this distinct deficiency are not entirely clear and are likely multifactorial. For example, renal healing may be blunted by a reduced availability of progenitor cells in a context of chronic diseases, as frequently observed in disorders, such as hypertension or diabetes mellitus.124–126 In turn, an uremic milieu can induce reduction and dysfunction of cell progenitors of endothelial or mesenchymal origin.127
The use of bone marrow or adipose-derived mesenchymal stem cell therapy is gaining momentum. Growing evidence supports its feasibility and potential application for cardiovascular and renal therapy because these pluripotent cells are rich sources of cytokines and growth factors that can promote tissue healing and slow or halt the progression of tissue damage. Administration of exogenous stem cells could prevent and promote renal recovery via modulation of the immune system, release of paracrine factors, and production of microvesicles with powerful anti-inflammatory and antifibrotic effects.128,129
Our recent study in a model of chronic renovascular disease showed that renal chemotactic signaling to attract cell progenitors may be defective, which in the context of a reduced or dysfunctional circulating progenitors could contribute to the progressive nature of renal injury.13 This imbalance may indeed lead to microvascular rarefaction because a major effect of cell therapy is stimulation of microvascular repair and proliferation. Indeed, previous studies demonstrated that intrarenal endothelial15 and mesenchymal130 stem cell therapy can efficiently improve microvascular density, stimulate microvascular repair and proliferation, and restore microvascular function in experimental renovascular disease. The microvascular improvements were followed by significant recovery of renal function and decreased renal inflammation and fibrosis,13,15,130 despite a relatively low renal retention and incorporation of the cells into renal structures, suggesting stimulation of powerful autocrine and paracrine actions of these cells to promote recovery. These effects are possibly driven by cell-derived secretion of growth factors (eg, VEGF) and vesicles that modulate angiogenesis, inflammation, fibrosis, and other cell pathways to facilitate tissue repair.131,132 More studies are needed to further understand the definitive underlying mechanisms and breadth of actions of cell therapy. Nevertheless, this is a promising strategy that aims to recover endogenous mechanisms of tissue healing (largely via microvascular proliferation and repair), which may be overwhelmed or deficient in the sick kidney and contribute to its poor renal regenerative capacity.
Angiogenic Cytokines
The potential use of angiogenic cytokines to recover ischemic tissues has been attempted in peripheral vascular disease, wound healing, hind-limb, and myocardial ischemia, in experimental and clinical settings133–137 as recently reviewed.138 Strategies using angiogenic cytokines or stem cell therapies are significantly related because progenitor cells are major sources of growth factors as their proliferation, homing, and stimulation of neovascularization and tissue repair are significantly driven by such growth factors.139,140 Like with cell therapy, the basic principle is propelling neovascularization and microvascular repair to recuperate adequate blood supply needed to sustain tissue function. Unlike cell therapy, the potential effects of angiogenic cytokines may not primarily counteract inflammation or fibrosis, although by improving tissue ischemia, recovering tissue perfusion and organ function, they may ameliorate those injurious mechanisms as well.
CKD is universally associated with renal microvascular rarefaction, which likely contribute to its progressive nature (Figure 1). The importance of microvascular integrity for renal function and body homeostasis is underscored by undesired effects of antiangiogenic strategies, which are at the forefront for the treatment of various forms of cancer. These strategies mainly aim on VEGF inhibition, and major collateral effects are hypertension and renal injury, disclosed by proteinuria and glomerular injury,141,142 which support the importance of both VEGF and microvascular integrity for the kidney. On the other hand, elegant studies by Basile et al and Kang et al were among the first to demonstrate the feasibility of using VEGF administration as a tool to ameliorate renal injury in the remnant kidney model, aging, and acute kidney injury.42,45,76,77 VEGF therapy in these models reduced microvascular rarefaction, which was associated with a significant reduction in overall renal damage. A disruption of VEGF signaling has been suggested as a potential link between microvascular rarefaction and fibrogenesis via modulation of endothelial–pericyte crosstalk.143 Similarly, our studies in a swine model of chronic renovascular disease demonstrated that progression of renal damage develops in a context of blunted bioavailability of VEGF, which accompanies progressive microvascular rarefaction in cortex and medulla, loss of renal function, and development of renal fibrosis, which were all significantly reversed after VEGF therapy36,37 (Figure 2) Although VEGF is a cytokine of a relatively short half-life (a few minutes), our studies using intrarenal administration of VEGF (from human origin) demonstrated that long-term improvements occur after a single administration. Indeed, these effects were persistent ≤4 weeks after administration and functionally consequential because they were followed by a recovery of renal function as well, indicating that the new vessels were functional. The long-term effects are possibly driven by stimulation of circulating and resident cell progenitors (author’s unpublished observations) that initiate and boost angiogenesis, microvascular repair, and tissue healing. Furthermore, VEGF therapy in this model restored its downstream signaling because factors that closely interact with VEGF to promote endothelial cell survival and vascular maturation such as Akt, eNOS, and angiopoietins were also improved.11,12
Evidence on the importance of renal bioavailability of VEGF and the integrity of the VEGF pathway for the kidney in health and disease is abundant and supports prominent roles of this cytokine to preserve the renal microvascular networks,12,45 promote microvascular repair and proliferation,11,12 and protect podocyte integrity.3,9 However, some studies suggest that overexpression or abundance of VEGF may also be deleterious for the kidney, although it may depend on the disease model, the stage of the disease, species, and isoform of VEGF.144–146 Polycystic kidney disease71,146 and diabetes mellitus147,148 are examples of the potential dual effect of VEGF depending on experimental conditions. Hence, carefully designed additional studies in selected models of renal disease may be needed to determine renal VEGF levels and signaling that may maintain a healthy glomerular structure and function should this intervention be considered for clinical applications.
Another important angiogenic and antifibrotic cytokine that has been used for renal therapy is HGF. A few studies have shown that recovery of HGF signaling (by exogenous HGF therapy or genetic manipulations10,80,83) associates with a significant reduction in microvascular remodeling and microvascular endothelial inflammation without significant effects on neovascularization,10 but associated with significant improvements in renal function. These studies may suggest that powerful effects of HGF therapy are on the existing renal microvasculature by promoting microvascular repair and decreasing microvascular remodeling.10
Thus, preclinical evidence supports the possibility of VEGF or HGF therapy for the kidney as a promising strategy to reverse or slow down renal injury via restoration of angiogenesis, microvascular repair, and function. However, their therapeutic potential should be confirmed, and additional studies are needed to determine the safety of these interventions because they have the potential of inducing aberrant vascular growth or favor the progression of tumors without a careful selection of the patients.
Renal Therapeutic Angiogenesis Using Drug Delivery Vectors
CKD represents a major challenge to any therapy because of the limitations of renal clearance and increased risks for toxic effects. Thus, efficacy of renal therapies may depend on high concentrations of therapeutic agents in the kidney to achieve desired effects, which paired with a limited renal function result in uncertainties regarding beneficial effects, outcomes, and toxicity.
Improving drug delivery to the kidney using renal-targeted therapeutics is still an underdeveloped area. Current efforts in our laboratory aim to apply renal therapeutic angiogenesis using nonimmunogenic protein-based carriers derived from elastin (elastin-like polypeptides [ELP]) that can stabilize attached small molecule and peptide therapeutics.9,149 The plasticity of these vectors allowed the tagging of drugs by relatively simple molecular biology techniques. Furthermore, ELPs display a higher affinity for kidney tissue than other organs,9,149 making them ideal candidates for renal targeted therapies. We recently developed and characterized a construct built on an ELP carrier fused to VEGF.9 The ELP-VEGF construct greatly accumulates in the kidney and may target renal endothelial and tubular epithelial cells of different species (eg, rodents, swine, humans) and showed a prolonged circulating time compared with unbound VEGF. We showed that a single intrarenal administration of ELP-VEGF restored renal hemodynamics and function and attenuated renal injury to a greater extent than the administration of unconjugated VEGF, which underscored a higher efficacy of this novel bioengineered compound. Notably, these effects were accompanied by the significant recovery of the cortical and medullary microvascular density, remodeling, and function compared with controls, possibly driven by restored expression of VEGF and downstream mediators.9
The ELP carrier can be modified to enhance their tissue targeting and penetration properties. Our recent work149 demonstrated that an ELP modified at its N-terminus with a cyclic, 7-amino acid kidney–targeting peptide significantly increases renal targeting and accumulation, which again was independent of the species and further demonstrate a unique property of these drug-delivery vectors. Furthermore, the C-terminus of the ELP construct was also modified with a cysteine residue for tracer conjugation and to allow the addition of a protein or therapeutic agent of choice, which is part of our ongoing efforts and future studies. The plasticity of ELP for attachment of any class of therapeutics unravels the possibility of applying ELP technology for targeted treatment of microvascular abnormalities that are present in acute or chronic renal diseases of different etiologies that can be mimicked in different animal models. The distinct properties of these constructs may also open avenues for application of ELP technologies for targeted interventions beyond VEGF or therapeutic angiogenesis. The road ahead is long, but this strategy holds promise.
Conclusion and Perspectives
The complexity of CKD and ESRD has been a major burden for therapeutic strategies. The growing number of patients has been paralleled with an expanded variety of therapeutic opportunities developed, thanks to relentless research efforts. However, the possibility of reversing or halting the progressive nature of chronic renal disease is still a challenging task. The vasculature is the core for the survival and function of every organ and in the kidney is not only necessary for their own nutrition but also for the normal function of the rest of the body.
Targeting the renal microvasculature offers a therapeutic niche to improve current treatments. However, it is likely that the window of opportunity to protect the renal microvasculature with beneficial consequences for the kidney is small. Success in restoring kidney function may depend on how far microvascular rarefaction has progressed since microvascular repair, and potential regrowth of peritubular and possibly glomerular capillary loops needs to be attempted and achieved before the entire glomerulus is lost (Figure 1). Promising results using cell-based therapies, angiogenic cytokines, and bioengineered compounds suggest that microvascular repair, neovascularization, and protection of the existing renal microvasculature is feasible and functionally consequential, although some limitations to their therapeutic application remain and need to be resolved (Table). Thus, efforts are needed to solidify these results and define the time frame of microvascular interventions to allow the translation toward application of renal targeted microvascular therapies, which may offer a novel therapeutic opportunity to the growing number of renal patients.
Summary of Advantages and Major Limitations of Cell-Based and Angiogenic Therapies to Target the Renal Microcirculation and Potentially Treat Chronic Renal Disease
Sources of Funding
This work was supported by grant HL095638, PO1-HL51971, and P20-GM104357 from the National Institutes of Health and by grant 18490005 from the American Heart Association.
Disclosures
None.
- © 2017 American Heart Association, Inc.
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