Mechanistic insights into redox damage of the podocyte in hypertension

Abstract

Podocytes are specialized cells within the glomerular filtration barrier (GFB) that are crucial for maintaining glomerular structural integrity and convective ultrafiltration. Podocytes exhibit a unique arborized morphology with foot processes interfacing by slit diaphragms (SD), ladder-like, multimolecular sieves which provide size and charge selectivity for ultrafiltration and transmembrane signaling. Podocyte dysfunction, resulting from oxidative stress, dysregulated prosurvival signaling, or structural damage, can drive the development of proteinuria and glomerulosclerosis in hypertensive nephropathy. Functionally, podocyte injury leads to actin cytoskeleton rearrangement, foot process effacement, dysregulated SD protein expression, and impaired ultrafiltration. Notably, the renin-angiotensin system (RAS) plays a pivotal role in podocyte function, with beneficial angiotensin receptor 2 (AT2R)-mediated nitric oxide (NO) signaling counteracting AT1R-driven calcium (Ca²⁺) influx and oxidative stress. Disruption of this balance contributes significantly to podocyte dysfunction and drives albuminuria, a marker of kidney damage and overall disease progression. Oxidative stress can also lead to sustained ion channel-mediated Ca²⁺ influx and precipitate cytoskeletal disorganization. The complex interplay between GPCR signaling, ion channel activation, and redox injury pathways underscores the need for additional research aimed at identifying targeted therapies to protect podocytes and preserve glomerular function. Earlier detection of albuminuria and podocyte injury through routine non-invasive diagnostics will also be critical in populations at highest risk for the development of hypertensive kidney disease. In this review, we highlight the established mechanisms of oxidative stress-mediated podocyte damage in proteinuric kidney diseases, with an emphasis on hypertensive renal injury. We will also consider emerging therapies that have the potential to selectively protect podocytes from redox-related injury.

Glomerular function through the prism of the podocyte.

Glomerular visceral epithelial cells (i.e., podocytes) are a highly specialized component of the tripartite glomerular filtration barrier (GFB) that play a critical role in maintaining glomerular tuft architecture and filtration capacity¹. Podocytes exhibit a unique arborized morphology, with large cell bodies from which primary processes extend and elaborate actin-based pedicels (i.e., foot processes). The foot processes of adjacent podocytes interdigitate, and attach to glomerular capillaries via integrin-based multimolecular interactions with the glomerular basement membrane² (Figure 1). The foot processes are bridged by specialized adherens junctions known as slit diaphragms (SD), which are crucial for the size and charge selectivity of the GFB. Despite what we’ve learned about the SD over the past half century, the complexity of this dynamic macromolecular sieve has yet to be fully discerned. In addition to its filtration and intercellular adhesive functions, the SD is now recognized as a dynamic signaling hub with a complex interactome of transmembrane pro-survival signaling axes, calcium transport proteins, cytoskeletal regulatory nodes and endocytic recycling machinery that are essential for proper assembly and maintenance of the SD^3,4. For instance, the transmembrane protein Nephrin (NPHS1), a principal component of the SD, is essential for the filtrative and pro-survival functions of the filtration slit. It acts as a part of a physical barrier to prevent the passage of large molecules through the GFB and facilitates intracellular survival signaling^5–7.

Figure 1. Scanning electron microscopy (SEM) of the glomerulus, the key structure in the kidney’s filtration system.

A, The complex architecture of the rat glomerulus in the kidney as revealed by SEM, highlights the intricate network of podocytes covering the glomerular capillaries. B, Podocyte interdigitated foot processes that are intertwined with each other, creating filtration slits. This structure is crucial for maintaining the selectivity of the glomerular filter, permitting water and small molecules to pass into the urine while preventing larger molecules such as proteins from leaving the bloodstream. The scale bar shown is 10 and 1 μm for A and B, respectively. PB – podocyte body; BS – Bowman’s space; FP – foot process; SFP – secondary foot process.

Understanding podocyte function and how to preserve podocyte vitality are still some of the most important questions in the field of nephrology, as highlighted in 2023 during the 14th International Podocyte Conference, co-chaired by Katalin Susztak and Larry Holzman⁸. The meeting attracted attendees from around the globe to discuss recent discoveries in podocyte cell biology including topics like SD assembly, subcellular analysis of podocyte architecture and function, nephrin trafficking, and podocyte autophagy. Podocytes are metabolically active, relying heavily on processes such as lipid synthesis and metabolism, oxygen sensing, and autophagic flux which are easily disrupted in acute and chronic proteinuric kidney disease^9–15. This complex and highly specialized machinery highlights the diverse functions of podocytes at the GFB and reveals several potentially disease-relevant vulnerabilities¹⁶.

Disruption of podocyte SD protein expression and pro-survival signaling can lead to irreversible dysfunction and drive the development of glomerular disease¹⁷. A major consequence of podocyte injury is foot process effacement/flattening, a complex ultrastructural derangement of the cytoskeleton that disrupts SD architecture and allows large molecules like albumin to pass through the GFB¹⁸. Injured podocytes may undergo detachment from the GBM, resulting in the loss of selective filtration capacity and development of glomerulosclerosis. Since mature podocytes are unable to self-renew and proliferate, they are generally considered terminally differentiated¹⁹. Podocyte loss is cumulative as they possess limited replicative capacity; thus, protecting these cells from damage is a central focus of therapeutic development for the treatment of proteinuric kidney diseases. Recently, some studies suggested that podocyte regeneration in adults may be stimulated ex vivo or with certain pharmacological interventions²⁰. Other studies suggest that podocytes can be replaced by nearby progenitor cells such as parietal epithelial cells or cells of renin-expressing lineage^21–25. Precisely delineating the mechanisms mediating podocyte depletion and potential regeneration would not only provide insights into fundamental aspects of glomerular physiology, but also unveil potential avenues for the treatment of proteinuric diseases. Additionally, development of targeted therapies that can favorably modulate calcium signaling, stabilize the actin cytoskeleton, or enhance the stability of the nephrin availability at SD could provide promising strategies for treating podocytopathy and chronic kidney disease.

Podocyte damage and albuminuria as markers of renal disease development and hypertension.

Albuminuria is a clinical finding characterized by the presence of albumin in the urine. Albuminuria is considered abnormal above 30 mg/g and often reflects impaired barrier functions at the GFB^26,27. The GFB is charge and size-selective with a molecular weight limit of 60–70 kDa. Ions and small molecules are allowed to pass while larger molecules such as albumin (67 kDa) and globulins (93–1200 kDa) remain in the blood. Podocyte damage can be triggered by various factors, including metabolic disturbances, toxins, viral infection, and systemic stress from diseases such as obesity, diabetes mellitus (DM), hypertension (HTN), etc. The glomerular sieving mechanisms, as well as degradation and retrieval pathways of filtered proteins were recently comprehensively reviewed by Comper et al²⁸.

Of note, some albumin passage through the GFB is present under normal physiological conditions. This small amount of albumin is later reabsorbed by the proximal tubule cells and returned to circulation²⁹. Thus, the final concentration of albumin in the urine is a sum of glomerular filter efficiency and reabsorptive capacity of the proximal tubule³⁰. In advanced glomerular disease, the excessive filtration of albumin promotes proximal tubule oxidative stress and concurrent damage to perivascular beds (Figure 2). Podocyte depletion leads to a loss of GFB integrity clinically recognized as albuminuria. When the concentration of filtered albumin overwhelms renal tubular resorptive capacity, oxidative stress and damage may ensue (Figure 2B). Intravital microscopy in Dahl salt-sensitive rats has shown that significant accumulation of albumin in the PT system is associated with the development of HTN and kidney damage suggesting that early detection of these changes could be helpful in mitigating kidney injury (Figure 2C)^31,32. This abnormal accumulation of filtered albumin could potentially be mitigated by the administration of lysine³³. We recently demonstrated that administration of this essential amino acid is protective against hypertensive renal tubular cell injury via multiple mechanisms including prevention of renal tubular cell albumin resorption and oxidative stress³⁴. Although further studies are needed to evaluate the therapeutic value of lysine in proteinuric kidney disease, these early findings suggest a potentially beneficial role.

Figure 2. Development of albuminuria, glomerular pathology, and renovascular disease in salt-induced HTN.

A, Healthy nephron exerts tight control over blood filtration, allowing minimal albumin to pass through the GFB; filtered albumin is subsequently reabsorbed by the proximal tubule. Glomerular integrity helps to maintain the physiological crosstalk between the renal proximal tubule and the perivascular bed. B, Chronic kidney disease (CKD) development can be associated with compromised GFB, podocyte apoptosis and detachment, and a substantial amount of albumin filtered into the tubular system. The proximal tubule accumulates filtered albumin and exhibits oxidative stress due to the excessive albumin load. The dilated proximal tubular epithelium further contributes to the damage of the perivascular bed, leading to a phenotypic transformation of the blood vessels characteristic of renovascular disease, commonly seen in salt-sensitive HTN (dilation, vascular extravasation). C, Intravital imaging reveals the progression of renovascular pathology in Dahl salt-sensitive (SS) rats on a high salt diet (unpublished and modified images form the study by Endres et al³¹). Shown is a 3D reconstruction of low-magnification screening (IRAPO ×25 W objective, NA 1.0; Leica) featuring a glomerulus, albumin filtered through and actively reabsorbed by proximal tubules (fluorescently labeled albumin; green), vasculature (150-kDa FITC-labeled dextran; gold/red), and nuclear staining (Hoechst; purple) in proximal tubules of the Dahl SS rats fed a high salt diet (8% NaCl for 3 and 14 days). On the left, a microphotograph of rat kidney vasculature and glomeruli (gold/red) shows a small amount of albumin (green) leaking into the proximal tubule space after three days on a high-salt diet. On the right, two images display the “colorful” complexity of tubular and vascular damage associated with the development of salt-induced HTN after 14 days of high salt. A significant accumulation of albumin (green) is observed in the proximal tubules, leading to tubular dilation and pathological vascular remodeling (indicated by arrows). Scale bar is 50 μm. G – glomerulus; PT – proximal tubule; PTa – proximal tubule with filtered albumin; DPT – dilated proximal tubule.

Albuminuria is not only a marker of kidney damage but may also be an indicator of broader disturbances in cellular metabolic and redox processes. Certainly, the presence of early or late albuminuria would necessitate further investigation to understand the causes and opportunities to mitigate kidney disease progression. Oxidative stress, hemodynamic disturbances, inflammation and RAS activation all accompany, and may precipitate, the development of hypertensive renal disease^35–37. Hypertensive kidney disease refers to kidney damage resulting from chronically elevated blood pressure (BP, defined as a systolic pressure of 130 mm Hg or higher and/or a diastolic pressure of 80 mm Hg or higher). This sustained BP increase significantly impairs renal function and can progress to chronic kidney disease (CKD) or end-stage renal disease (ESRD). Chronic HTN increases the pressure in the glomerular capillaries, inducing podocyte injury through exaggerated mechanical stretch and impaired mechanotransduction. These alterations can lead to cytoskeletal changes within podocytes, disrupting their ability to maintain normal physiologic function³⁸. Chronic HTN also provokes cellular oxidative stress and activation of pro-inflammatory signaling which leads to infiltration of immune cells into the kidney. This cascade results in the release of cytokines and other proinflammatory mediators which exacerbates podocyte injury and promotes renal fibrosis. When unaddressed, these changes contribute to the progression of glomerular injury, fibrosis, and potentially ESKD³⁹. RAS signaling plays a crucial role in BP regulation, and its inappropriate activation in HTN is detrimental to podocytes. Notably, podocyte injury not only results from HTN but also contributes to its progression. Loss of podocytes leads to a reduction in the glomerular filtration surface area and the development of segmental or global glomerulosclerosis. As the number of functional nephrons decreases, the ability of the kidney to excrete sodium is impaired, promoting volume expansion and further elevating BP. This vicious cycle exacerbates kidney damage and further worsens HTN.

Proteinuria is a direct consequence of podocyte injury and an independent marker and a mediator of kidney disease progression in HTN. Proteinuria itself can cause tubulointerstitial inflammation and fibrosis, contributing to a decline in renal function and perpetuation of HTN. In hypertensive patients, it is recommended to use simultaneous albuminuria/proteinuria and eGFR assessments to evaluate cardiovascular health⁴⁰. Importantly, clinical guidelines recommend that eGFR less than 60 or the presence of microalbuminuria should be considered as equivalents of cardiovascular disease⁴¹. In a meta-analysis of 266,975 patients with a history of HTN, DM, or cardiovascular complications, the all-cause mortality was found to increase progressively when eGFR was less than 60, while albuminuria was associated with the all-cause mortality without thresholds⁴². It should be emphasized that it was reported in the same study that when albuminuria and eGFR were considered together, they were multiplicatively associated with the all-cause and cardiovascular mortality. Overall, microalbuminuria is an early sign of nephropathy and an independent predictor of ESRD. However, microalbuminuria is an under-recognized symptom of CKD definition, staging, and prognosis. According to a 2021 consortium study, early albuminuria testing in HTN is extremely low, only 4%, despite its prevalence⁴³. The findings of this global consortium-based meta-analysis suggested “that regular albuminuria screening should be emphasized to enable early detection of chronic kidney disease and initiation of treatment with cardiovascular and renal benefits”, especially in hypertensive patients. Considering this, improving our understanding of the mechanisms underlying podocyte damage in the development of albuminuria is crucial for advancing our ability to identify new therapeutic targets in hypertensive kidney disease.

Important considerations regarding proteinuria/HTN.

It should be recognized that in addition to the podocyte, the glomerular endothelium plays the key role in maintaining the integrity of the GFB⁴⁴. When endothelial cells are damaged in HTN, their ability to regulate the filtration of proteins is compromised. This endothelial dysfunction leads to increased permeability of the GFB, resulting in protein leakage into the urine^45,46. In HTN, endothelial injury is often associated with reduced NO production and increased oxidative stress, which further impairs the vascular function and contributes to glomerular HTN. As the damage progresses, proteinuria not only reflects endothelial injury, but perpetuates it. While the detection of clinically significant albuminuria may indicate that GFB integrity has been compromised due to podocyte injury, some evidence also suggests that glomerular endothelial dysfunction may contribute to the development of proteinuria⁴⁷. The importance of endothelium–epithelium crosstalk in the proximal tubule and renal vasculature under normal and pathophysiological conditions has been highlighted by intravital microscopy in rodents, bacterial genetics and organoid 3D kidney tissue models⁴⁸. Thus, it should be noted that there is an interdependence between podocyte and endothelial injury and HTN, which plays a critical role in renal pathology⁴⁹. Hypertensive insults contribute to GFB injury by increasing glomerular pressure, which impairs the structure and function of podocytes and the endothelium. Conversely, the injury to these cells can lead to HTN by disrupting the GFB and affecting tubular sodium reabsorption, further exacerbating sodium retention and increasing BP. The proteinuria/GFB relation should be regarded as bidirectional involving multiple cell types, where each damaging process amplifies the other in a vicious cycle.

One of the key factors that plays a role in the development of proteinuric renal injury in HTN is salt intake and salt-sensitivity. Individuals with salt-sensitive HTN tend to experience more pronounced BP elevations in response to sodium intake, which can exacerbate renal injury by increasing glomerular pressure and accelerating damage to the kidney’s filtration system^50,51. Salt-sensitive individuals are at a higher risk of developing more severe forms of renal injury compared to those who are salt-resistant⁵². In Black hypertensive individuals, modest salt reduction reduces BP and urine protein excretion⁵³. Although proteinuria is recognized as an independent risk factor for both CVD and the progression of CKD, it remains unclear whether urinary protein excretion influences the impact of high sodium intake on the risk of adverse kidney outcomes. Recently, Kim et al reported from the findings of the KNOW-CKD trial that higher urinary sodium excretion was more strongly associated with an increased risk of adverse kidney outcomes in patients with higher proteinuria levels⁵⁴. Experimental models mimicking human salt-sensitive HTN, such as the Dahl salt-sensitive rat, have been instrumental in demonstrating the relationship between salt-sensitivity of BP and proteinuria³¹. However, further research is urgently needed to deepen our understanding of the role of proteinuria and damage to the GFB in the context of salt-sensitive HTN, as many underlying mechanisms remain unclear.

Major causes of podocyte damage in the context of oxidative stress.

Proteinuric kidney disease can result from any disease process that causes podocyte injury or loss⁵⁵. Its most severe manifestation, nephrotic syndrome (NS), is a clinical diagnosis characterized by severe proteinuria (>3.5 g/24 h), hypoalbuminemia (<3 g/dL) and edema^56,57. Although NS is not a common feature of hypertensive kidney disease, primary and secondary NS accounts for nearly 90% of end-stage kidney disease (ESKD) worldwide and the mechanisms of podocyte injury vary considerably²⁰. Consequently, there has been significant interest in defining the principal mechanisms of podocyte injury over the lifespan⁵⁸. Both genetic and non-genetic causes of NS have been extensively characterized and oxidative stress has emerged as a common contributor to podocyte injury and loss across the spectrum of glomerular disease. Much of our current understanding of podocyte injury mechanisms has emerged from the study of familial NS⁵⁹. Since the discovery of pathogenic nephrin mutations as the cause of Congenital Finnish Nephropathy in 1988⁶⁰, disease-causing mutations in more than 60 genes have been shown to disrupt a variety of essential podocyte functions and lead to the development of NS²⁰. Rare pathogenic mutations of nuclear-encoded genes such as anillin (ANLN)⁵, clavesin 1 (CLVS1)⁶¹, and various intermediates of the CoQ10 biosynthetic pathway⁶², have been shown to cause podocyte oxidative stress and focal segmental glomerulosclerosis (FSGS), a type of NS and the most common primary glomerular lesion that causes ESKD in the US⁶³. Rare pathogenic mutations of mitochondrially-encoded oxidative phosphorylation (OXPHOS) genes such as NADH dehydrogenase 5 (MT-ND5) and the mitochondrial tRNAs MT-TL1 and MT-TW have also been shown to cause podocyte oxidative stress and FSGS⁶⁴. In addition to rare monogenic causes of glomerular disease, common genetic variants in apolipoprotein-L1 (APOL1) have been shown to induce oxidative stress in podocytes and significantly enhance the risk of developing FSGS⁶⁵. APOL1 is a 39–46 kDa lipoprotein that is thought to have evolved between 3,000–10,000 years ago to protect individuals residing in Western and Central Africa against Trypanosoma brucei infection, the cause of Human African Trypanosomiasis (HAT) or African Sleeping Sickness⁶⁶. In 2010, Genovese et al. identified two novel variants in APOL1 that strongly associated with non-diabetic kidney disease, hypertensive kidney disease and FSGS in African Americans, Genovese 1 (G1) comprised of two missense substitutions [S342G and I384M] and Genovese 2 (G2) which is a six base pair, in-frame deletion [N388del:Y389del]⁶⁵. The G1 and G2 kidney risk variants are evolutionary adaptations that rapidly arose to high frequency approximately 4,000 years ago to provide protection against subspecies of T. brucei that are resistant to wild-type APOL1-mediated cytolysis⁶⁷. APOL1-mediated podocyte injury is caused, in part, by the formation of APOL1 pore complexes that compromise mitochondrial membrane integrity and precipitate podocyte oxidative stress via mitochondrial fission and excessive ROS generation^68,69. The G1 and G2 variants are thought to account for more than 70% of the increased risk for non-diabetic CKD in individuals of a Sub-Saharan African ancestry⁷⁰ and they increase the lifetime risk of developing CKD/ESRD by 15–20% in persons with high-risk APOL1 genotypes (i.e. G1/G1, G2/G2, or G1/G2)^71,72. African Americans have a 2–4-fold greater incidence of ESKD than Whites⁷³ and it is estimated that 13% of self-identified African Americans in the US possess two copies of the APOL1 kidney risk variants, the high risk genotype^74,75. These critical insights highlight both the disproportionate burden of disease in this uniquely vulnerable population and the potential for high-impact therapeutic intervention^76,77.

Oxidative stress is strongly associated with HTN⁷⁸ and is a prominent contributor to podocyte injury in a variety of systemic illnesses associated with podocyte damage. The pathophysiology of HTN is multifactorial involving structural and mechanical alterations of tissue architecture and function, genetic and biochemical derangements of cellular physiology, dysregulation of hormonal axes such as the RAS, and cellular oxidative stress^79,80. Hyperactivation of RAS signaling in HTN is a potent activator of ROS generation in podocytes and has been shown to induce oxidative stress, foot process effacement, proteinuria and glomerulosclerosis^81,82. Standard-of-care therapies for patients with HTN and DM includes inhibitors of RAS activity which are directly cytoprotective to podocytes via mechanisms that involve suppression of ROS generation^83–86. As noted previously, APOL1 kidney risk variants strongly associate with hypertensive kidney disease in African Americans⁶⁵. In 2018, Sumaili et al. demonstrated that the APOL1 G1 variant is the major risk modifier for developing hypertensive kidney disease in people of recent African ancestry⁸⁷. Although the precise mechanisms of G1-induced podocyte injury are still to be determined, it is likely that mitochondrial dysfunction and oxidative stress contribute significantly. Other pathological conditions associated with HTN, albuminuria and podocyte damage are described in the Supplementary.

Taken together, these observations highlight a significant role for podocyte redox injury across the spectrum of glomerular disease and provide a basis for further exploration of therapies that restore podocyte redox balance. What remains to be determined are the discrete molecular mediators of oxidative stress in podocytes and how they may be selectively targeted.

ROS-dependent calcium signaling in podocytes.

Oxidative stress, a key driver in many glomerular diseases, exacerbates podocyte injury through excessive ROS production, leading to prolonged Ca²⁺ influx and cytoskeletal derangements^88–90. ROS-dependent calcium signaling in podocytes involves a complex interaction between ROS production and calcium influx through ion channels such as canonical transient receptor potential 6 (TRPC6)^88–90. Targeting oxidative pathways, especially those involving NOX enzymes like NOX4 and NOX5, represents a strategic opportunity for precision therapeutic intervention. Future research should focus on delineating the molecular mechanisms involved in ROS-dependent podocyte injury and exploring targeted therapies to restore redox balanceHTN^91–97. Thus, oxidative stress significantly contributes to podocyte and glomerular injury in kidney diseases such as DN and FSGS, primarily through ROS-dependent derangements in calcium signaling. Specifically, excessive ROS disrupts normal podocyte function, leading to prolonged calcium influx, apoptosis, and cytoskeletal reorganization. Targeting these pathways may be therapeutic strategies to restore podocyte homeostasis and prevent kidney disease progression. Further discussion of the ROS-dependent calcium signaling in podocytes can be found in Supplementary.

RAS interactions with the podocyte and the role of NO signaling.

RAS is a critical regulator of BP and fluid balance, and its dysregulation is closely associated with the development of HTN and kidney damage⁹⁸. RAS signaling involves a complex network of angiotensin peptides, each with distinct biological effects. The classical RAS axis, which includes Ang II and Ang III, is primarily associated with vasoconstriction, oxidative stress, and inflammation. On the other hand, the alternative RAS pathway, involving Ang 1–7 and Ang 1–9, counteracts these effects by promoting vasodilation and anti-inflammatory responses⁹⁹. In podocytes, both classical and alternative RAS pathways contribute to Ca²⁺ and NO production, though the effects are mediated through different receptors. Similarly to vascular cells, Ang II and Ang III stimulate Ca²⁺ influx via AT1R⁹⁴ and NO production via AT2R¹⁰⁰. At the same time, Ang 1–7, despite being part of the alternative axis, activates AT2R in podocytes, albeit with lower efficacy. The Mas receptor, typically associated with Ang 1–7 signaling, does not play a functional role in intracellular signaling in podocytes, underscoring the idiosyncratic receptor biology of these cells^100,101.

Nitric oxide is a versatile signaling molecule involved in various physiological processes, including vasodilation, modulation of blood pressure, and regulation of kidney function¹⁰². In podocytes, NO is primarily produced by nitric oxide synthase (NOS) enzymes regulated by Ca²⁺-calmodulin-dependent enzyme and the corresponding increase in the intracellular concentration of free Ca²⁺. Notably, several well-established GPCR pathways—such as purinergic receptors, protease-activated receptors (PARs), and β-arrestin-mediated signaling trigger Ca²⁺ influx but are not linked to NO production in podocytes^100,103,104. This highlights Ang II and RAS peptides as the primary activators of NO and NOS in these cells. As we recently suggested, the interaction between NO and Ca²⁺ signaling in podocytes plays a crucial role in cytoskeletal dynamics, podocyte motility, and maintenance of the glomerular filtration barrier¹⁰⁰. Upon AT2R activation, NO is generated, which subsequently modulates intracellular Ca²⁺ signaling. This regulation is crucial in maintaining podocyte function, as imbalances in the NO-Ca²⁺ axis can lead to cytoskeletal disorganization, podocyte hypertrophy, and ultimately proteinuria (Figure 3). A notable example of such pathological remodeling occurs with selective AT1R agonism through the β-arrestin pathway in podocytes, resulting in excessive TRPC6 channel-mediated Ca²⁺ influx without NO production. These alterations eventually lead to cell damage and compromise in glomerular filter integrity¹⁰⁵. The protective role of NO in mitigating Ca²⁺ overload is particularly important in hypertensive conditions. While Ang II-induced Ca²⁺ influx via AT1R can be detrimental, the concurrent activation of AT2R-induced NO production helps buffer this effect, maintaining podocyte stability.

Figure 3. Redox balance and RAS in podocyte.

The interplay between Ang II receptors, calcium entry and nitric oxide (NO) signaling, and the downstream effects on NADPH oxidase (NOX) in podocytes. Activation of Ang II receptors initiates a cascade influencing redox balance through NO and NOX activity. The figure highlights how this redox regulation impacts TRPC6-mediated calcium influx, which is critical for podocyte function and integrity. Disruptions in this signaling axis can lead to imbalances in the redox state and contribute to podocyte injury, affecting overall kidney function. The schematic emphasizes the role of the RAS in maintaining podocyte health through precise control of these signaling pathways.

Anti-hypertensive therapy and podocyte health.

As discussed earlier, kidney and cardiovascular health are closely intertwined, and the treatment of hypertension often necessitates an approach that addresses both. In our recent review, we explored various drug classes used for blood pressure control and their regulation of podocyte function¹⁰⁶. The manuscript highlights both direct and indirect interactions between major classes of antihypertensive drugs - such as RAS inhibitors, calcium channel blockers (CCBs), sodium-glucose co-transporter-2 (SGLT2) inhibitors, and endothelin receptor antagonists - and podocyte health. In a large cohort of veteran patients, we observed that the majority of hypertensive treatments were still managed on CCBs, accounting for up to 67% of cases. However, we also noted a significant rise in the use of novel therapeutic approaches, particularly SGLT2 inhibitors, which made up 14% of treatments. With respect to RAS blockers, the use of ACE inhibitors (ACEi) was higher than angiotensin receptor blockers (ARBs), at 29% versus 20%, respectively. This trend is concerning for podocyte health, as our recent data have shown that activation of the AT2R promotes NO production, providing a protective effect¹⁰⁷. Therefore, selective blockade of AT1R through ARBs, such as losartan, may be more beneficial for podocyte protection than ACE or renin inhibition. It is also worth noting that the recently promoted strategy for hypertension treatment based on selective AT1R GPCR agonism and β-arrestin pathway activation has shown detrimental effects on podocytes and glomerular filtration¹⁰⁵. On a more positive note, recent advances in glomerular disease treatments led to the FDA approval of sparsentan^108,109, a dual endothelin-1 (ET-1) and Ang II receptor antagonist. Sparsentan shows promise for treating glomerular diseases and may also help lower blood pressure. However, further studies are needed to evaluate its use specifically for hypertension management, along with rigorous clinical trials to confirm its safety and efficacy as an antihypertensive agent. In oncology, drugs that inhibit vascular endothelial growth factor (VEGF), such as bevacizumab, have been linked to hypertension as a side effect¹¹⁰. Interestingly, podocytes secrete VEGF-A, which may contribute to managing hypertension by promoting angiogenesis, improving blood flow, and reducing vascular resistance. However, disturbances in VEGF-A, whether through deletion or overexpression, have been associated with glomerular injury in rodent models^111–113. Thus, any pharmacological disruption of VEGF signaling could have critical consequences for podocyte health. VEGF activation is strongly linked to NOS function and NO generation, both of which are crucial for regulating vascular health, podocyte function, and the development of hypertension¹¹⁴. Moreover, therapies such as thiazide-like diuretics, including cicletanine, have been shown to influence NO production and endothelial function. Cicletanine is primarily used to treat essential hypertension, but beyond its diuretic effects, it also directly improves vascular health by promoting vasodilation. It would be interesting to explore the potential role of this class of drugs in regulating epithelial NO production and glomerular hemodynamics.

PERSPECTIVES AND CONCLUSIONS

Defining the complex relationship between podocyte damage and albuminuria is crucial for our understanding of the pathogenesis of hypertensive kidney disease. In general, protecting podocytes from the deleterious effects of oxidative stress and inflammation is essential for preventing the progression of hypertensive nephropathy and other proteinuric renal diseases. There are multiple gaps in knowledge that need to be filled to ensure a more complete understanding of how podocyte damage develops. Importantly, the glomerulus must be viewed as a functional unit where all its cellular components - podocytes, endothelial cells, and mesangial cells, as well as the parietal epithelial cells of the Bowman’s capsule - interact with each other and contribute to the maintenance of a healthy glomerular filtration apparatus. Furthermore, albuminuria should not be considered solely as a glomerulus-limited phenomenon, but must be understood in the context of downstream changes to the nephron. Specifically, proximal tubule damage and vascular extravasation, must also be considered as contributors to the development of renal disease. Although it is imperative to prioritize interventions that preserve podocyte homeostasis and podocyte-endothelial cell cross-talk, albuminuria should be viewed comprehensively as a multifactorial process potentially affecting renal tissue at multiple sites.

Podocytes are particularly vulnerable to injury due to their limited ability to regenerate, making them central to the pathophysiology of proteinuric kidney diseases. Damage to podocytes -whether through mechanical stress, oxidative injury, or dysregulated signaling - can lead to foot process effacement, cytoskeletal disorganization, and proteinuria, eventually driving glomerulosclerosis and CKD. The interplay between the RAS, NO signaling, and the regulation of calcium dynamics is crucial in podocyte function. Disruption of these intracellular processes, particularly as a result of enhanced AT1R-mediated Ca²⁺ influx (without NO production), contributes significantly to podocyte damage in hypertensive conditions. Conversely, the protective AT2R-NO pathway offers a counterbalance, mitigating Ca²⁺ overload and stabilizing podocyte structure. Given this dynamic, therapies aimed at enhancing AT2R activation or stabilizing NO production hold promise for treating podocyte-related pathologies.

The study of inflammatory molecules such as cytokines, chemokines, histamine, prostaglandins, and leukotrienes presents a promising avenue for understanding podocyte health and the pathogenesis of proteinuric hypertensive kidney disease. These molecules, key mediators of inflammation, play a critical role in podocyte injury and the disruption of the GFB which has been shown in lupus nephritis and diabetic nephropathy^115,116. Cytokines and chemokines can promote immune cell recruitment and activation within the kidney, exacerbating podocyte damage and leading to proteinuria. Prostaglandins and leukotrienes, derived from arachidonic acid, are involved in modulating vascular tone and permeability, potentially also contributing to podocyte dysfunction. The recent discovery of the emerging role of serine proteases signaling in podocyte further underscores the importance of novel inflammatory mediators in glomerular pathology^104,117. Histamine, primarily recognized for its role in allergic reactions, may also influence podocyte signaling and injury¹¹⁸. Investigating these inflammatory molecules are potential targets in HTN could reveal novel pharmacology aimed at protecting podocytes, mitigating proteinuria, and slowing the progression of hypertensive CKD.

There is an urgent demand for therapies that specifically target and protect podocytes early in disease processes. A focus on developing and mandating routine non-invasive diagnostic assays to detect albuminuria and podocyte injury early is key to reducing the burden of hypertensive kidney disease. Investing in early diagnosis and treatment of podocyte damage, combined with a comprehensive approach to preserving glomerular health, should be the new cornerstone for improving patient outcomes and reducing the global impact of renal disease. Current treatments do not specifically target podocyte preservation and our diagnostic approaches fail to detect early signs of albuminuria. Consequently, future research efforts need to be directed towards achieving these goals to enhance patient care. When considering new therapies, it is essential to recognize that the manifestations of hypertensive kidney disease and the response to treatment can differ significantly based on sex, age, and race. Therefore, therapeutic strategies must be carefully tailored to accommodate these differences, ensuring personalized and effective care across the lifespan. Routine monitoring and early detection of albuminuria are critical for identifying patients at risk for CKD and intervening before significant kidney damage occurs. As research in the area of podocyte dysfunction and albuminuria development continues to evolve, it holds promise for improving outcomes in patients with HTN-related kidney disease.

Nonstandard Abbreviations and Acronyms

ACEi

Angiotensin-Converting Enzyme inhibitors

Ang II

Angiotensin II

AOPP

advanced oxidation protein products

BP

blood pressure

DM

diabetes mellitus

DN

diabetic nephropathy

eGFR

estimated Glomerular filtration rate

ESKD

end-stage kidney disease

ESRD

end-stage renal disease

FSGS

focal segmental glomerulosclerosis

GBM

glomerular basement membrane

CCBs

calcium channel blockers

GFB

glomerular filtration barrier

GFR

glomerular filtration rate

HAT

Human African Trypanosomiasis

HTN

hypertension

LN

lupus nephritis

MCN

minimal change nephropathy

MR

mineralocorticoid receptor

NOX

NADPH oxidase

NS

nephrotic syndrome

OXPHOS

oxidative phosphorylation

RAS

renin angiotensin system

ROS

reactive oxygen species

SD

slit diaphragm

SEM

scanning electron microscopy

SGLT2

sodium-glucose co-transporter-2

SLE

Systemic Lupus Erythematosis

TRPC

transient receptor potential canonical

VEGF

vascular endothelial growth factor