The Weary Gatekeeper

Podocytes have long been a major focus of ultrastructural studies, the only way to achieve visualization of their intricate structure. In this issue of JASN, Amari et al. deliver a fascinating 3D analysis capturing and quantifying features of aging rat podocytes by use of volume electron microscopy (VEM).¹ Their observations build on the foundational work by Marilyn Farquhar and the illustrative studies published by Wilhelm Kriz, confirm anecdotal observations, and provide structural correlation with known pathobiological concepts of compensatory changes and podocyte aging.

The Podocyte: A Master of Adaptation

Podocytes are highly specialized, terminally differentiated epithelial cells that play a central role in maintaining the integrity of the glomerular filtration barrier.² Unlike most other cells, podocytes possess little to no proliferative capacity. This fundamental limitation creates a unique biological challenge: Podocytes must remain in place and endure decades of mechanical, metabolic, and immunological stress without the ability to renew. As a result, physiological or pathophysiological podocyte loss requires adaptive compensatory changes of remaining podocytes. Once their ability to maintain coverage of the glomerular basement membrane (GBM) has reached capacity, areas of denuded GBM lead to major disruption of and dysregulated flux across the glomerular filtration barrier, causing proteinuria and entrapment of macromolecules, and ultimately, segmental hyalinosis and glomerulosclerosis. Genetic susceptibilities also determine podocyte vulnerability, including mutations affecting GBM composition (e.g., the Alport spectrum), adhesion and cytoskeletal structure (e.g., ITGB4, ACTN4), and cell survival (e.g., APOL1 risk alleles). Understanding how podocytes age and how they cope with aging is critical because their dysfunction and loss are contributing to CKD.

Podocyte Senescence

Podocyte senescence is recognized as a pivotal mechanism driving glomerular aging and disease and is characterized by a state of largely irreversible cell cycle arrest. In response to cumulative stressors—including oxidative stress, mitochondrial dysfunction, DNA damage, and metabolic or hemodynamic strain—podocytes adopt a senescent phenotype. A key feature is the development of the senescence-associated secretory phenotype, a form of paracrine signaling, through which podocytes release proinflammatory cytokines, chemokines, and profibrotic factors that can propagate injury within the glomerular microenvironment, thereby contributing to podocyte depletion and glomerulosclerosis.^3,4 Amari et al. demonstrate increased lysosomal volume in aging podocytes, a known feature of the senescence-associated secretory phenotype. Alterations in cytoskeletal integrity, slit diaphragm signaling, and energy metabolism during senescence further impair podocyte function. Together, these processes position podocyte senescence not merely as a consequence of aging, but as an active driver of kidney disease, exposing senescence-associated pathways as promising therapeutic targets to prevent or halt the progression of proteinuric kidney diseases.

Hypertrophy and Structural Plasticity as Compensatory Strategies

Podocytes engage a series of adaptive responses aimed at preserving their function despite their inability to regenerate. One of their earliest and most important adaptive responses to aging and cellular loss is hypertrophy. As individual podocytes are lost, the remaining cells enlarge to cover the exposed GBM and preserve the glomerular filtration barrier. This compensatory hypertrophy is initially protective.⁵ While VEM, including serial block-face and focused ion beam scanning electron microscopy, has enabled 3D reconstruction of podocytes and provided key insights into their architecture,⁶ Amari et al. expand these observations by revealing how podocytes dynamically remodel their architecture in response to aging and stress. In particular, they show that aged podocytes exhibit remarkable structural plasticity, characterized by massive expansion of cell volume, elongation and branching of cytoplasmic processes, and extensive reorganization of foot processes to maintain coverage of the GBM. These findings confirm that podocyte hypertrophy is not merely an increase in size but also a highly coordinated architectural remodeling process aimed at preserving glomerular filtration barrier integrity.⁴ However, this structural plasticity comes at a cost. Enlarged and stretched podocytes experience increased mechanical stress and cytoskeletal strain, which compromises their adhesion to the GBM. Over time, this predisposes them to detachment and loss, transforming an initially adaptive response into a driver of disease progression.

Cytoskeletal Remodeling and Structural Instability

The ability of podocytes to maintain their complex morphology depends on a highly dynamic actin cytoskeleton. In response to cellular stress, including aging-related injury, podocytes undergo cytoskeletal remodeling to stabilize foot processes and maintain slit diaphragm integrity. Key signaling pathways, including those mediated by nephrin and Rho GTPases, regulate this process.⁷ Despite these adaptive responses, aging is associated with progressive cytoskeletal disorganization. This manifests morphologically as foot process retraction and pruning, a hallmark of podocyte injury. Loss of cytoskeletal integrity not only impairs filtration but also increases susceptibility to detachment. Importantly, structural changes in podocytes often precede overt clinical manifestations such as proteinuria, underscoring their role as early indicators of glomerular aging.

Autophagy: Essential for Cellular Longevity

Podocytes exhibit high basal levels of autophagy among kidney cell types. Autophagy is largely podocyte protective as it serves as a critical housekeeping mechanism, enabling the degradation of damaged proteins and organelles and thereby maintaining cellular homeostasis. This is particularly important in postmitotic cells of exceptional longevity, such as podocytes, because damage and waste products accumulate over time due to limited turnover.⁸ With aging, however, autophagic capacity declines. Impaired autophagy leads to the accumulation of dysfunctional mitochondria and protein aggregates, contributing to cellular stress and injury. Experimental models have demonstrated that disruption of autophagy in podocytes accelerates glomerular damage, highlighting its essential protective role. Increase of lysoendosomal and multivesicular body content, and vesicular clustering, as described by Amari et al., likely represent compensatory responses and may be exacerbated in the setting of impaired autophagy and increased protein turnover.

Mitochondrial Adaptation in Aging Podocytes

Mitochondrial integrity is critical for podocyte function because these cells require substantial energy to maintain their active cytoskeletal dynamics and filtration processes. Aging is associated with mitochondrial dysfunction, characterized by decreased ATP production, increased reactive oxygen species, and impaired mitochondrial dynamics.⁹ Podocytes attempt to compensate through metabolic adaptations, including activation of stress responses, e.g., via the AMP-activated protein kinase pathway. The interplay between mitochondrial dysfunction and impaired autophagy further accelerates this decline. Mitochondrial alterations in podocytes have been well described using transmission electron microscopy, including structural abnormalities such as mitochondrial fragmentation, disruption of cristae architecture, and inner membrane damage.¹⁰ These analyses are generally limited to 2D sections and cannot capture the full spatial organization of mitochondrial networks. By contrast, VEM enables 3D reconstruction of cellular organelles and as such enables renewed investigations into mitochondrial architecture, distribution, and their interactions within the complex cytoplasmic landscape of aging and injured podocytes.

Limited Regenerative Capacity: The Role of Parietal Epithelial Cells

Given the limited proliferative capacity of podocytes, potential regenerative mechanisms have been explored. Parietal epithelial cells (PECs) lining Bowman's capsule have been proposed as a source of podocyte progenitors. Experimental evidence suggests that under certain conditions, PECs can be activated, proliferate, and differentiate to podocyte-like cells, contributing to podocyte replenishment and glomerular repair.¹¹ However, this regenerative capacity seems limited and context-dependent. While PECs have been shown to temporarily replace lost podocytes and provide coverage to a leaky glomerular capillary tuft to ameliorate protein loss across the glomerular filtration barrier in podocytopathies and collapsing glomerulopathy, it was also postulated that PEC activation contributes to maladaptive responses, including sclerosis and crescent formation. Thus, PEC-mediated regeneration is unlikely to fully compensate for podocyte loss during aging. VEM may offer additional insights into cytoskeletal changes upon PEC activation, attachment to the visceral basement membrane and intercellular contact formation that may enhance our knowledge about their pathophysiological properties following podocyte loss.

Animal Models for the Study of Podocyte Aging

Owing to practical and ethical limitations preventing longitudinal aging studies in humans, animal models are essential. Amari et al. carefully considered several key factors relevant to modeling human kidney aging. By using adult Wistar rats, they use a well-studied species that closely recapitulates human aging phenotypes, enhancing translational relevance compared with mouse models when choosing appropriate age ranges. The focus on male animals is also biologically appropriate because males tend to exhibit earlier and more pronounced aging-related kidney changes. Overall, Amari et al. demonstrate thoughtful alignment with important experimental considerations—species selection, sex, and age—while still requiring cautious interpretation and validation of their findings in human samples.

In summary, podocyte aging represents a complex interplay between intrinsic vulnerability and adaptive responses. While mechanisms such as hypertrophy, cytoskeletal remodeling, autophagy, and metabolic adaptation initially serve to support podocyte structural and functional preservation, these compensatory processes may become insufficient or even maladaptive over time. Given the limited regenerative capacity of podocytes and the insufficient compensatory role of PECs, podocyte loss remains a central and largely irreversible event in the development of glomerulosclerosis. Advances in experimental models, particularly those that more closely recapitulate human aging, as well as ultrastructural imaging modalities, provide valuable insights into these processes but also underscore the need for careful translational interpretation. Moving forward, targeting senescence-associated pathways, preserving mitochondrial and autophagic function, and stabilizing cytoskeletal integrity could represent promising strategies to mitigate podocyte loss and ameliorate the progression of age-related kidney disease.

Disclosures

Disclosure forms, as provided by each author, are available with the online version of the article at http://links.lww.com/JSN/F827.

Author Contributions

Conceptualization: Maryam Ghasemi, Astrid Weins.

Writing – original draft: Maryam Ghasemi.

Writing – review & editing: Astrid Weins.

Funding

A Weins: National Institute of Diabetes and Digestive and Kidney Diseases (DK135667).