Mechanisms and Models of Kidney Tubular Necrosis and Nephron Loss

Understanding nephron loss is a primary strategy for preventing chronic kidney disease (CKD) progression. Death of renal tubular cells may occur by apoptosis during developmental and regenerative processes. However, during acute kidney injury (AKI), the transition of AKI to CKD, sepsis-associated AKI, and kidney transplantation, ferroptosis and necroptosis, two pathways associated with the loss of plasma membrane integrity, kill renal cells. This necrotic type of cell death is associated with an inflammatory response, which is referred to as necroinflammation. Importantly, the necroinflammatory response to cells that die by necroptosis may be fundamentally different from the tissue response to ferroptosis. While mechanisms of ferroptosis and necroptosis have recently been investigated in detail, the cell death propagation during tubular necrosis, although described morphologically, remains incompletely understood. Here, we argue that a molecular switch downstream of tubular necrosis determines nephron regeneration vs. nephron loss, respectively. Unravelling the details of this "switch" must include the inflammatory response to tubular necrosis and regenerative signals potentially controlled by inflammatory cells, including the stimulation of myofibroblasts as the origin of fibrosis. Understanding in detail the molecular switch and the inflammatory responses to tubular necrosis can inform discussion of therapeutic options.

Neonatal nephron loss during active nephrogenesis results in altered expression of renal developmental genes and markers of kidney injury

Preterm neonates are at a high risk for nephron loss under adverse clinical conditions. Renal damage potentially collides with postnatal nephrogenesis. Recent animal studies suggest that nephron loss within this vulnerable phase leads to renal damage later in life. Nephrogenic pathways are commonly reactivated after kidney injury supporting renal regeneration. We hypothesized that nephron loss during nephrogenesis affects renal development which in turn impairs tissue repair after secondary injury. Neonates prior to 36 weeks of gestation show an active nephrogenesis. In rats, nephrogenesis is ongoing until day 10 after birth. Mimicking the situation of severe nephron loss during nephrogenesis, male pups were uninephrectomized at day 1 of life (UNXd1). A second group of males was uninephrectomized at postnatal day 14 (UNXd14), after terminated nephrogenesis. Age-matched controls were sham operated. Three days after uninephrectomy transcriptional changes in the right kidney were analyzed by RNA-sequencing, followed by functional pathway analysis. In UNXd1 1182 genes were differentially regulated, but only 143 genes showed a regulation both in UNXd1 and UNXd14. The functional groups "renal development" and "kidney injury" were among the most differentially regulated groups and revealed distinctive alterations. Reduced expression of candidate genes concerning renal development (Bmp7, Gdnf, Pdgf-B, Wt1) and injury (nephrin, podocin, Tgf-β1) were detected. The downregulation of Bmp7 and Gdnf persisted until day 28. In UNXd14 Six2 was upregulated and Pax2 downregulated. We conclude that nephron loss during nephrogenesis affects renal development and induces a specific regulation of genes which might hinder tissue repair after secondary kidney injury.

Pathomorphological Sequence of Nephron Loss in Diabetic Nephropathy

Following our reports on mesangial sclerosis and vascular proliferation in diabetic nephropathy (DN)(25,34) we now describe the advanced stages of DN terminating in glomerular obsolescence and tubulo-interstitial fibrosis based on a total of 918 biopsies. The structural aberrations emerge from two defects: First, an increased synthesis of glomerular basement membrane (GBM) components by podocytes and endothelial cells leading to an accumulation of GBM material in the mesangium. Second, a defect of glomerular vessels consisting of an increased leakiness and an increased propensity to proliferate. Both defects may lead to glomerular degeneration. The progressing compaction of the accumulated worn-out GBM-material together with the retraction of podocytes out of the tuft and the collapse and hyalinosis of capillaries results in a shrunken tuft that fuses with Bowman's capsule to glomerular sclerosis. The most frequent pathway to glomerular decay starts with local tuft expansions that result in contacts of structurally healthy podocytes to the parietal epithelium initiating the formation of tuft adhesions, which include the penetration of glomerular capillaries into BC. Exudation of plasma from such capillaries into the space between the parietal epithelium and its basement membrane causes the formation of insudative fluid accumulations within BC spreading around the glomerular circumference and, via the glomerulo-tubular junction, onto the tubule. Degeneration of the corresponding tubule develops secondarily to the glomerular damage, either due to cessation of filtration in cases of global sclerosis or due to encroachment of the insudative spaces. The degenerating tubules induce the proliferation of myo-fibroblasts resulting in interstitial fibrosis.

Different Patterns of Kidney Fibrosis Are Indicative of Injury to Distinct Renal Compartments

Kidney fibrosis is a common manifestation and hallmark of a wide variety of chronic kidney disease (CKD) that appears in different morphological patterns, suggesting distinct pathogenic causes. Broad macroscopically visible scars are the sequelae of severe focal injury and complete parenchymal destruction, reflecting a wound healing response as a consequence of infarction. In the kidney, chronic glomerular injury leads to atrophy of the corresponding tubule, degeneration of this specific nephron, and finally interstitial fibrosis/tubular atrophy (IF/TA). Compared to this glomerulus-induced focal replacement scar, diffuse fibrosis independent of tubular atrophy appears to be a different pathogenic process. Kidney fibrosis appears to develop in a compartment-specific manner, but whether focal and diffuse fibrosis has distinct characteristics associated with other glomerular or tubulointerstitial lesions remains elusive. In the present study, we aimed to analyze renal fibrotic patterns related to renal lesions, which directly contribute to renal fibrogenesis, to unravel fibrotic patterns and manifestations upon damage to distinct renal compartments. Patterns of kidney fibrosis were analyzed in experimental models of CKD and various renal pathologies in correlation with histopathological and ultrastructural findings. After the induction of isolated crescentic glomerulonephritis (GN) in nephrotoxic serum-nephritis (NTN), chronic glomerular damage resulted in predominantly focal fibrosis adjacent to atrophic tubules. By contrast, using unilateral ureteral obstruction (UUO) as a model of primary injury to the tubulointerstitial compartment revealed diffuse fibrosis as the predominant pattern of chronic lesions. Finally, folic acid-induced nephropathy (FAN) as a model of primary tubular injury with consecutive tubular atrophy independent of chronic glomerular damage equally induced predominant focal IF/TA. By analyzing several renal pathologies, our data also suggest that focal and diffuse fibrosis appear to contribute as chronic lesions in the majority of human renal disease, mainly being present in antineutrophil cytoplasmic antibody (ANCA)-associated GN, lupus nephritis, and IgA nephropathy (IgAN). Focal IF/TA correlated with glomerular damage and irreversible injury to nephrons, whereas diffuse fibrosis in ANCA GN was associated explicitly with interstitial inflammation independent of glomerular damage and nephron loss. Ultrastructural analysis of focal IF/TA versus diffuse fibrosis revealed distinct matrix compositions, further supported by different collagen signatures in transcriptome datasets. With regard to long-term renal outcome, only the extent of focal IF/TA correlated with the development of end-stage kidney disease (ESKD) in ANCA GN. In contrast, diffuse kidney fibrosis did not associate with the long-term renal outcome. In conclusion, we here provide evidence that a focal pattern of kidney fibrosis seems to be associated with nephron loss and replacement scarring. In contrast, a diffuse pattern of kidney fibrosis appears to result from primary interstitial inflammation and injury.

Role of Macrophages and Related Cytokines in Kidney Disease

Inflammation is a key characteristic of kidney disease, but this immune response is two-faced. In the acute phase of kidney injury, there is an activation of the immune cells to fight against the insult, contributing to kidney repair and regeneration. However, in chronic kidney diseases (CKD), immune cells that infiltrate the kidney play a deleterious role, actively participating in disease progression, and contributing to nephron loss and fibrosis. Importantly, CKD is a chronic inflammatory disease. In early CKD stages, patients present sub-clinical inflammation, activation of immune circulating cells and therefore, anti-inflammatory strategies have been proposed as a common therapeutic target for renal diseases. Recent studies have highlighted the plasticity of immune cells and the complexity of their functions. Among immune cells, monocytes/macrophages play an important role in all steps of kidney injury. However, the phenotype characterization between human and mice immune cells showed different markers; therefore the extrapolation of experimental studies in mice could not reflect human renal diseases. Here we will review the current information about the characteristics of different macrophage phenotypes, mainly focused on macrophage-related cytokines, with special attention to the chemokine CCL18, and its murine functional homolog CCL8, and the macrophage marker CD163, and their role in kidney pathology.

REST and Stress Resistance in the Aging Kidney

Background: Chronic kidney disease is associated with the loss of functional nephrons, leading to increased mechanical and metabolic stress in the remaining cells, particularly for cells constituting the filtration barrier, such as podocytes. The failure of podocytes to mount an adequate stress response can lead to further nephron loss and disease progression. However, the mechanisms that regulate this degenerative process in the kidney are unknown. Methods: We combined in vitro, in vivo, and organ-on-chip approaches to identify the RE1-silencing transcription factor (REST), a repressor of neuronal genes during embryonic development, as a central regulator of podocyte adaptation to injury and aging. Results: Mice with a specific deletion of REST in podocytes exhibit albuminuria, podocyte apoptosis, and glomerulosclerosis during aging, and exhibit increased vulnerability to renal injury. This phenotype is mediated, in part, by effects of REST on the podocyte cytoskeleton that promote resistance to mechanical stressors and augment podocyte survival. Finally, REST expression is upregulated in human podocytes during aging, consistent with a conserved mechanism of stress resistance. Conclusions: These results suggest that REST protects the kidney from injury and degeneration during aging, with potentially important therapeutic implications.

MO070NEW ASPECTS OF THE PATHOMORPHOLOGIC SEQUENCE OF NEPHRON LOSS IN DIABETIC NEPHROPATHY

Background and Aims Diabetic nephropathy (DN) is the leading cause of end-stage-renal disease in western countries. Despite of innumerable studies undertaken to elucidate the pathogenesis of DN the underlying morphologic alterations have been insufficiently analyzed. Method Re-evaluation of more than 800 biopsies was done showing several unknown features. Results: 1. Matrix accumulation in the mesangium: Thickening of the glomerular basement membrane (GBM) and expansion of the mesangial matrix are hallmarks of DN, generally considered to emerge from different sites of overproduction: GBM components from podocytes and mesangial matrix from mesangial cells. We show, that the accumulation of matrix in the mesangium emerges from an overproduction of GBM material by podocytes and endothelial cells and an impaired degradation by mesangial cells. The progressing deposition of worn-out GBM material into the mesangium accounts for the advancement from diffuse mesangial sclerosis (DMS) to nodular sclerosis (NS) and to the herniation of the tuft through the glomerular vascular pole to the outside; the latter is associated with the outgrowth of glomerular capillaries into the peri-glomerular space leading to the destruction of the juxtaglomerular apparatus. 2.The role of podocytes Podocytes have frequently been accused to play a central role in DN. This is correct, but in another way than generally assumed. Damage to podocytes cannot be seen in DMS. The albuminuria regularly seen during this stage derives, as previously suggested by others, from an increased leakiness of the glomerular capillary endothelium based on a deranged glycocalyx. Podocyte detachments start at the transition from DMS to NS, based on the loss of cross talk signals with the capillary endothelium: the increasing deposition of matrix leads to the collapse of many capillaries. These podocytes contribute little to the further progression of the damage: they are lost into primary urine or they undergo cell lysis.In addition to their role in increased matrix production, podocytes take an active role in the formation of tuft adhesions to Bowman’s capsule (BC), starting the progression to NS. Expansion of the matrix within the mesangium has led to expansion of the tuft (frequently associated with nodules) towards Bowman’s capsule (BC) or towards the urinary orifice. Podocytes on the surface of these expansions are in their majority structurally intact, exhibiting an intact pattern of foot processes. These podocytes come into contact with parietal epithelial cells and initiate DN-specific tuft adhesions to BC allowing the proliferation of glomerular capillaries into BC. There they deliver an exudate into BC that spreads around the entire circumference of the glomerulus presenting as giant insudative spaces. Moreover, this process encroaches via the glomerulo-tubular junction onto the tubule constituting the major pathway of glomerular damage extending to the tubulointerstitium. 3. Tubulointerstitial fibrosisIt is current opinion that the tubulointerstitial fibrosis may start from tubular damage resulting in an own, glomerular-independent pathway to nephron loss. However, there is scant evidence for such a mechanism. Studying 162 glomerulo-tubular transitions, we did not see a tubular epithelial or interstitial damage in those biopsies without any evidence of a glomerulo-tubular damage transfer. The only exception consists of the well-known prominent thickening of the tubular basement membrane, which may result in functional loss but does not lead to structural epithelial damage. Conclusion We consistently found that tubulo-interstitial damage develops after encroachment of the glomerular damage onto the tubule, leading first to a gradual degeneration of tubules which subsequently initiate the process of interstitial fibrosis.

The Effect of Cadmium on GFR Is Clarified by Normalization of Excretion Rates to Creatinine Clearance

Erroneous conclusions may result from normalization of urine cadmium and N-acetyl-β-D-glucosaminidase concentrations ([Cd]u and [NAG]u) to the urine creatinine concentration ([cr]u). In theory, the sources of these errors are nullified by normalization of excretion rates (ECd and ENAG) to creatinine clearance (Ccr). We hypothesized that this alternate approach would clarify the contribution of Cd-induced tubular injury to nephron loss. We studied 931 Thai subjects with a wide range of environmental Cd exposure. For x = Cd or NAG, Ex/Ecr and Ex/Ccr were calculated as [x]u/[cr]u and [x]u[cr]p/[cr]u, respectively. Glomerular filtration rate (GFR) was estimated according to the Chronic Kidney Disease (CKD) Epidemiology Collaboration (eGFR), and CKD was defined as eGFR < 60 mL/min/1.73m2. In multivariable logistic regression analyses, prevalence odds ratios (PORs) for CKD were higher for log(ECd/Ccr) and log(ENAG/Ccr) than for log(ECd/Ecr) and log(ENAG/Ecr). Doubling of ECd/Ccr and ENAG/Ccr increased POR by 132% and 168%; doubling of ECd/Ecr and ENAG/Ecr increased POR by 64% and 54%. As log(ECd/Ccr) rose, associations of eGFR with log(ECd/Ccr) and log(ENAG/Ccr) became stronger, while associations of eGFR with log(ECd/Ecr) and log(ENAG/Ecr) became insignificant. In univariate regressions of eGFR on each of these logarithmic variables, R2 was consistently higher with normalization to Ccr. Our tabular and graphic analyses uniformly indicate that normalization to Ccr clarified relationships of ECd and ENAG to eGFR.

Mitochondrial dysfunction and the AKI-to-CKD transition

Acute kidney injury (AKI) has been widely recognized as an important risk factor for the occurrence and development of chronic kidney disease (CKD). Even milder AKI has adverse consequences and could progress to renal fibrosis, which is the ultimate common pathway for various terminal kidney diseases. Thus, it is urgent to develop a strategy to hinder the transition from AKI to CKD. Some mechanisms of the AKI-to-CKD transition have been revealed, such as nephron loss, cell cycle arrest, persistent inflammation, endothelial injury with vascular rarefaction, and epigenetic changes. Previous studies have elucidated the pivotal role of mitochondria in acute injuries and demonstrated that the fitness of this organelle is a major determinant in both the pathogenesis and recovery of organ function. Recent research has suggested that damage to mitochondrial function in early AKI is a crucial factor leading to tubular injury and persistent renal insufficiency. Dysregulation of mitochondrial homeostasis, alterations in bioenergetics, and organelle stress cross talk contribute to the AKI-to-CKD transition. In this review, we focus on the pathophysiology of mitochondria in renal recovery after AKI and progression to CKD, confirming that targeting mitochondria represents a potentially effective therapeutic strategy for the progression of AKI to CKD.

The complex interplay between kidney injury and inflammation

Acute kidney injury (AKI) has gained significant attention following patient safety alerts about the increased risk of harm to patients, including increased mortality and hospitalization. Common causes of AKI include hypovolaemia, nephrotoxic medications, ischaemia and acute glomerulonephritis, although in reality it may be undetermined or multifactorial. A period of inflammation either as a contributor to the kidney injury or resulting from the injury is almost universally seen. This article was compiled following a workshop exploring the interplay between injury and inflammation. AKI is characterized by some degree of renal cell death through either apoptosis or necrosis, together with a strong inflammatory response. Studies interrogating the resolution of renal inflammation identify a whole range of molecules that are upregulated and confirm that the kidneys are able to intrinsically regenerate after an episode of AKI, provided the threshold of damage is not too high. Kidneys are unable to generate new nephrons, and dysfunctional or repeated episodes will lead to further nephron loss that is ultimately associated with the development of renal fibrosis and chronic kidney disease (CKD). The AKI to CKD transition is a complex process mainly facilitated by maladaptive repair mechanisms. Early biomarkers mapping out this process would allow a personalized approach to identifying patients with AKI who are at high risk of developing fibrosis and subsequent CKD. This review article highlights this process and explains how laboratory models of renal inflammation and injury assist with understanding the underlying disease process and allow interrogation of medications aimed at targeting the mechanistic interplay.