Renal Toll-like receptors: recent advances and implications for disease

Toll-like Receptor 4 in Acute Kidney Injury

Acute kidney injury (AKI) is a common and devastating pathologic condition, associated with considerable high morbidity and mortality. Although significant breakthroughs have been made in recent years, to this day no effective pharmacological therapies for its treatment exist. AKI is known to be connected with intrarenal and systemic inflammation. The innate immune system plays an important role as the first defense response mechanism to tissue injury. Toll-like receptor 4 (TLR4) is a well-characterized pattern recognition receptor, and increasing evidence has shown that TLR4 mediated inflammatory response, plays a pivotal role in the pathogenesis of acute kidney injury. Pathogen-associated molecular patterns (PAMPS), which are the conserved microbial motifs, are sensed by these receptors. Endogenous molecules generated during tissue injury, and labeled as damage-associated molecular pattern molecules (DAMPs), also activate pattern recognition receptors, thereby offering an understanding of sterile types of inflammation. Excessive, uncontrolled and/or sustained activation of TLR4, may lead to a chronic inflammatory state. In this review we describe the role of TLR4, its endogenous ligands and activation in the inflammatory response to ischemic/reperfusion-induced AKI and sepsis-associated AKI. The potential regeneration signaling patterns of TLR4 in acute kidney injury, are also discussed.

Toll-Like Receptors in Acute Kidney Injury

Acute kidney injury (AKI) is an important health problem, affecting 13.3 million individuals/year. It is associated with increased mortality, mainly in low- and middle-income countries, where renal replacement therapy is limited. Moreover, survivors show adverse long-term outcomes, including increased risk of developing recurrent AKI bouts, cardiovascular events, and chronic kidney disease. However, there are no specific treatments to decrease the adverse consequences of AKI. Epidemiological and preclinical studies show the pathological role of inflammation in AKI, not only at the acute phase but also in the progression to chronic kidney disease. Toll-like receptors (TLRs) are key regulators of the inflammatory response and have been associated to many cellular processes activated during AKI. For that reason, a number of anti-inflammatory agents targeting TLRs have been analyzed in preclinical studies to decrease renal damage during AKI. In this review, we updated recent knowledge about the role of TLRs, mainly TLR4, in the initiation and development of AKI as well as novel compounds targeting these molecules to diminish kidney injury associated to this pathological condition.

Uromodulin (Tamm-Horsfall protein): guardian of urinary and systemic homeostasis

Biology has taught us that a protein as abundantly made and conserved among species as Tamm-Horsfall protein (THP or uromodulin) cannot just be a waste product serving no particular purpose. However, for many researchers, THP is merely a nuisance during urine proteome profiling or exosome purification and for clinicians an enigmatic entity without clear disease implications. Thanks to recent human genetic and correlative studies and animal modeling, we now have a renewed appreciation of this highly prevalent protein in not only guarding urinary homeostasis, but also serving as a critical mediator in systemic inter-organ signaling. Beyond a mere barrier that lines the tubules, or a surrogate for nephron mass, mounting evidence suggests that THP is a multifunctional protein critical for modulating renal ion channel activity, salt/water balance, renal and systemic inflammatory response, intertubular communication, mineral crystallization and bacterial adhesion. Indeed, mutations in THP cause a group of inherited kidney diseases, and altered THP expression is associated with increased risks of urinary tract infection, kidney stone, hypertension, hyperuricemia and acute and chronic kidney diseases. Despite the recent surge of information surrounding THP's physiological functions and disease involvement, our knowledge remains incomplete regarding how THP is normally regulated by external and intrinsic factors, how precisely THP deficiency leads to urinary and systemic pathophysiology and in what clinical settings THP can be used as a theranostic biomarker and a target for modulation to improve patient outcomes.

Free light chains injure proximal tubule cells through the STAT1/HMGB1/TLR axis

Free light chains (FLCs) induce inflammatory pathways in proximal tubule cells (PTCs). The role of TLRs in these responses is unknown. Here we present findings on the role of TLRs in FLC-induced PTC injury. We exposed human kidney PTC cultures to κ and λ FLCs and used cell supernatants and pellets for ELISA and gene expression studies. We also analyzed tissues from Stat1-/- and littermate control mice treated with daily i.p. injections of a κ FLC for 10 days. FLCs increased the expression of TLR2, TLR4, and TLR6 via HMGB1, a damage-associated molecular pattern. Countering TLR2, TLR4, and TLR6 through GIT-27 or specific TLR siRNAs reduced downstream cytokine responses. Blocking HMGB1 through siRNA or pharmacologic inhibition, or via STAT1 inhibition, reduced FLC-induced TLR2, TLR4, and TLR6 expression. Blocking endocytosis of FLCs through silencing of megalin/cubilin, with bafilomycin A1 or hypertonic sucrose, attenuated FLC-induced cytokine responses in PTCs. IHC showed decreased TLR4 and TLR6 expression in kidney sections from Stat1-/- mice compared with their littermate controls. PTCs exposed to FLCs released HMGB1, which induced expression of TLR2, TLR4, and TLR6 and downstream inflammation. Blocking FLCs' endocytosis, Stat1 knockdown, HMGB1 inhibition, and TLR knockdown each rescued PTCs from FLC-induced injury.

How Tubular Epithelial Cell Injury Contributes to Renal Fibrosis

The renal tubules are the major component of the kidney and are vulnerable to a variety of injuries including ischemia, proteinuria, toxins, and metabolic disorders. It has long been believed that tubules are the victim of injury. In this review, we shift this concept to renal tubules as a driving force in the progression of kidney disease. In response to injury, tubular epithelial cells (TECs) can synthesize and secrete varieties of bioactive molecules that drive interstitial inflammation and fibrosis. Innate immune-sensing receptors on the TECs also aggravate immune responses. Necroinflammation, an auto-amplification loop between tubular cell death and interstitial inflammation, leads to the exacerbation of renal injury. Furthermore, TECs also play an active role in progressive renal injury via mechanisms associated with the conversion into collagen-producing fibroblast phenotype, cell cycle arrest at both G1/S and G2/M checkpoints, and metabolic disorder. Thus, a better understanding the mechanisms by which tubular injury drives AKI and CKD is necessary for the development of therapeutics to halt the progression of CKD.

Inhibition of Toll-Like Receptor 4 Signaling Mitigates Microvascular Loss but Not Fibrosis in a Model of Ischemic Acute Kidney Injury

The development of chronic kidney disease (CKD) following an episode of acute kidney injury (AKI) is an increasingly recognized clinical problem. Inhibition of toll-like receptor 4 (TLR4) protects renal function in animal models of AKI and has become a viable therapeutic strategy in AKI. However, the impact of TLR4 inhibition on the chronic sequelae of AKI is unknown. Consequently, we examined the chronic effects of TLR4 inhibition in a model of ischemic AKI. Mice with a TLR4-deletion on a C57BL/6 background and wild-type (WT) background control mice (C57BL/6) were subjected to bilateral renal artery clamping for 19 min and reperfusion for up to 6 weeks. Despite the acute protective effect of TLR4 inhibition on renal function (serum creatinine 1.6 ± 0.4 mg/dL TLR4-deletion vs. 2.8 ± 0.3 mg/dL·WT) and rates of tubular apoptosis following ischemic AKI, we found no difference in neutrophil or macrophage infiltration. Furthermore, we observed significant protection from microvascular rarefaction at six weeks following injury with TLR4-deletion, but this did not alter development of fibrosis. In conclusion, we validate the acute protective effect of TLR4 signal inhibition in AKI but demonstrate that this protective effect does not mitigate the sequential fibrogenic response in this model of ischemic AKI.

TLR2-MyD88-NF-κB pathway is involved in tubulointerstitial inflammation caused by proteinuria

Proteinuria is an important risk factor for chronic kidney diseases (CKD). Several studies have suggested that proteinuria initiates tubulointerstitial inflammation, while the mechanisms have not been fully understood. In this study, we hypothesized whether the activation of the TLR2-MyD88-NF-κB pathway is involved in tubulointerstitial inflammation induced by proteinuria. We observed expression of TLR2, MyD88, NF-κB, as well as TNF-α and IL-6 detected by immunohistostaining, Western blotting and real-time PCR in albumin-overloaded (AO) nephropathy rats. In vitro, we observed these markers in HK-2 cells stimulated by albumin. We used TLR2 siRNA or the NF-κB inhibitor BAY 11-7082 to observe the influence of TNF-α and IL-6 expression caused by albumin overload. Finally, we studied these markers in non-IgA mesangioproliferative glomerulonephritis (MsPGN) patients with different levels of proteinuria. It was demonstrated that expression of TLR2, MyD88 and NF-κB were significantly increased in AO rats and in non-IgA MsPGN patients with high levels of proteinuria, and TNF-α and IL-6 expressions were increased after NF-κB activation. Furthermore, TNF-α and IL-6 expression was positively correlated with the level of proteinuria. Albumin-overload induced TNF-α and IL-6 secretions by the TLR2-MyD88-NF-κB pathway activation, which could be attenuated by the TLR2 siRNA or BAY 11-7082 in HK-2 cells. In summary, we demonstrated that proteinuria may exhibit an endogenous danger-associated molecular pattern (DAMP) that induces tubulointerstitial inflammation via the TLR2-MyD88-NF-κB pathway activation.

High-mobility group box 1 inhibits HCO(3)(-) absorption in medullary thick ascending limb through a basolateral receptor for advanced glycation end products pathway

High-mobility group box 1 (HMGB1) is a damage-associated molecule implicated in mediating kidney dysfunction in sepsis and sterile inflammatory disorders. HMGB1 is a nuclear protein released extracellularly in response to infection or injury, where it interacts with Toll-like receptor 4 (TLR4) and other receptors to mediate inflammation. Previously, we demonstrated that LPS inhibits HCO(3)(-) absorption in the medullary thick ascending limb (MTAL) through a basolateral TLR4-ERK pathway (Watts BA III, George T, Sherwood ER, Good DW. Am J Physiol Cell Physiol 301: C1296-C1306, 2011). Here, we examined whether HMGB1 could inhibit HCO(3)(-) absorption through the same pathway. Adding HMGB1 to the bath decreased HCO(3)(-) absorption by 24% in isolated, perfused rat and mouse MTALs. In contrast to LPS, inhibition by HMGB1 was preserved in MTALs from TLR4(-/-) mice and was unaffected by ERK inhibitors. Inhibition by HMGB1 was eliminated by the receptor for advanced glycation end products (RAGE) antagonist FPS-ZM1 and by neutralizing anti-RAGE antibody. Confocal immunofluorescence showed expression of RAGE in the basolateral membrane domain. Inhibition of HCO(3)(-) absorption by HMGB1 through RAGE was additive to inhibition by LPS through TLR4 and to inhibition by Gram-positive bacterial molecules through TLR2. Bath amiloride, which selectively prevents inhibition of MTAL HCO(3)(-) absorption mediated through Na⁺/H⁺ exchanger 1 (NHE1), eliminated inhibition by HMGB1. We conclude that HMGB1 inhibits MTAL HCO(3)(-) absorption through a RAGE-dependent pathway distinct from TLR4-mediated inhibition by LPS. These studies provide new evidence that HMGB1-RAGE signaling acts directly to impair the transport function of renal tubules. They reveal a novel paradigm for sepsis-induced renal tubule dysfunction, whereby exogenous pathogen-associated molecules and endogenous damage-associated molecules act directly and independently to inhibit MTAL HCO(3)(-) absorption through different receptor signaling pathways.

Dynamic multiphoton microscopy: focusing light on acute kidney injury

Acute kidney injury (AKI) is a major global health problem; much research has been conducted on AKI, and numerous agents have shown benefit in animal studies, but none have translated into treatments. There is, therefore, a pressing unmet need to increase knowledge of the pathophysiology of AKI. Multiphoton microscopy (MPM) provides a tool to non-invasively visualize dynamic events in real time and at high resolution in rodent kidneys, and in this article we review its application to study novel mechanisms and treatments in different forms of AKI.

How the Innate Immune System Senses Trouble and Causes Trouble

The innate immune system is the first line of defense in response to nonself and danger signals from microbial invasion or tissue injury. It is increasingly recognized that each organ uses unique sets of cells and molecules that orchestrate regional innate immunity. The cells that execute the task of innate immunity are many and consist of not only "professional" immune cells but also nonimmune cells, such as renal epithelial cells. Despite a high level of sophistication, deregulated innate immunity is common and contributes to a wide range of renal diseases, such as sepsis-induced kidney injury, GN, and allograft dysfunction. This review discusses how the innate immune system recognizes and responds to nonself and danger signals. In particular, the roles of renal epithelial cells that make them an integral part of the innate immune apparatus of the kidney are highlighted.

Increased macrophage activation inhibited by tacrolimus in the kidney of diabetic rats

BACKGROUND/AIMS

Accumulating evidence suggests that macrophage-induced inflammation may be the mechanism of development and progression of diabetic nephropathy. A previous study by our group has shown that tacrolimus, like cyclosporin A, has a renoprotective effect in diabetic rats. The present study aimed to elucidate the underlying molecular events.

METHODS

Diabetic rats were induced by using streptozotocin. Diabetic rats were subjected to oral tacrolimus treatment at a dose of 0.5 or 1.0 mg/kg daily for 4 weeks. Body weight, blood glucose, hemoglobin A(1c) (HbA(1c)) and renal pathology were assessed, followed by analyses of renal calcineurin (CaN) expression, changes in renal macrophage infiltration, proliferation and activation, and detection of renal TLR2+ and TLR4+ as well as NF-κB-p-p65+ in macrophages.

RESULTS

Diabetic rats had a reduced body weight and increased blood glucose and HbA(1c) levels, whereas tacrolimus treatment did not affect body weight or blood glucose and HbA(1c). Increased relative kidney weight was only significantly reduced by tacrolimus treatment at a dose of 1.0 mg/kg, while the elevated albumin excretion rate was markedly attenuated after treatment with tacrolimus (0.5 and 1.0 mg/kg) in diabetic rats. Elevated glomerular volume was significantly attenuated by tacrolimus treatment with 0.5 and 1.0 mg/kg, and increased indices for tubulointerstitial injury were only ameliorated by tacrolimus treatment with 1.0 mg/kg. Western blot data showed that expression of CaN protein was induced 2.4-fold in the kidneys of positive control diabetic rats, whereas tacrolimus treatment at 0.5 and 1.0 mg/kg doses reduced the increased expression of CaN protein by 38.0 and 73.2%, respectively. Histologically there was a marked accumulation of ED-1+ cells (macrophages) in diabetic kidneys and tacrolimus treatment failed to inhibit it. In contrast, tacrolimus treatment at 0.5 and 1.0 mg/kg doses significantly inhibited the elevated ED-1+/PCNA+ cells and ED-1+/iNOS+ cells in the kidneys of diabetic rats, while tacrolimus treatment at a dose of 0.5 or 1.0 mg/kg significantly suppressed the increased ED-1+/TLR2+ cells, ED-1+/TLR4+ cells and ED-1+/NF-κB-p-p65+ cells in the kidneys of diabetic rats.

CONCLUSION

The data from the current study demonstrated that tacrolimus could ameliorate early renal injury through a mechanism to suppress macrophage activation.

Septic acute kidney injury: molecular mechanisms and the importance of stratification and targeting therapy

The most common cause of acute kidney injury (AKI) in hospitalized patients is sepsis. However, the molecular pathways and mechanisms that mediate septic AKI are not well defined. Experiments performed over the past 20 years suggest that there are profound differences in the pathogenesis between septic and ischemic AKI. Septic AKI often occurs independently of hypoperfusion, and is mediated by a concomitant pro- and anti-inflammatory state that is activated in response to various pathogen-associated molecular patterns, such as endotoxin, as well as damage-associated molecular patterns. These molecular patterns are recognized by Toll-like receptors (TLRs) found in the kidney, and effectuate downstream inflammatory pathways. Additionally, apoptosis has been proposed to play a role in the pathogenesis of septic AKI. However, targeted therapies designed to mitigate the above aspects of the inflammatory state, TLR-related pathways, and apoptosis have failed to show significant clinical benefit. This failure is likely due to the protean nature of septic AKI, whereby different patients present at different points along the immunologic spectrum. While one patient may benefit from targeted therapy at one end of the spectrum, another patient at the other end may be harmed by the same therapy. We propose that a next important step in septic AKI research will be to identify where patients lie on the immunologic spectrum in order to appropriately target therapies at the inflammatory cascade, TLRs, and possibly apoptosis.

Acute kidney injury: what part do toll-like receptors play?

The innate immune system plays an important role as a first response to tissue injury. This first response is carried out via germline-encoded receptors. Toll-like receptors (TLRs) are the first identified and best studied family of pattern recognition receptors. TLRs are expressed on a variety of cell types, including epithelial cells, endothelia, dendritic cells, monocytes/macrophages, and B- and T-cells. TLRs initiate innate immune responses and concurrently shape the subsequent adaptive immune response. They are sensors of both pathogens, through the exogenous pathogen-associated molecular patterns (PAMPs), and tissue injury, through the endogenous danger-associated molecular patterns (DAMPs). TLR signaling is critical in defending against invading microorganisms; however, sustained receptor activation is also implicated in the pathogenesis of inflammatory diseases. Ischemic kidney injury involves early TLR-driven immunopathology, and the resolution of inflammation is needed for rapid regeneration of injured tubule cells. Notably, the activation of TLRs also has been implicated in epithelial repair. This review focuses on the role of TLRs and their endogenous ligands within the inflammatory response of acute kidney injury.

Pattern recognition receptors and the inflammasome in kidney disease

Toll-like receptors (TLRs) and nucleotide-binding oligomerization domain receptors (NLRs) are families of pattern recognition receptors that, together with inflammasomes, sense and respond to highly conserved pathogen motifs and endogenous molecules released upon cell damage or stress. Evidence suggests that TLRs, NLRs and the NACHT, LRR and PYD domains-containing protein 3 (NLRP3) inflammasome have important roles in kidney diseases through regulation of inflammatory and tissue-repair responses to infection and injury. In this Review, we discuss the pathological mechanisms that are related to TLRs, NLRs and NLRP3 in various kidney diseases. In general, these receptors are protective in the host defence against urinary tract infection, but can sustain and self-perpetuate tissue damage in sterile inflammatory and immune-mediated kidney diseases. TLRs, NLRs and NLRP3, therefore, have become promising drug targets to enable specific modulation of kidney inflammation and suppression of immunopathology in kidney disease.

Toxic effects of zearalenone on oxidative stress, inflammatory cytokines, biochemical and pathological changes induced by this toxin in the kidney of pregnant rats

An experiment was conducted to determine the toxic effects of zearalenone (ZEN) on oxidative stress, inflammatory cytokines, biochemical and pathological changes in the kidney of pregnant rats, and to explore the possible mechanism in ZEN induced kidney damage. The rats were fed a normal diet treated with 0.3, 48.5, 97.6 or 146 mg/kg ZEN in feed on gestation days (GDs) 0 through 7, and then all the rats were fed with a normal diet on GDs 8 through 20. The results showed that ZEN induced kidney dysfunction, oxidative damage, pathological changes and increased mRNA and protein expression of TLR4 and inflammatory cytokines in kidney in dose-dependent manner. The results indicated that ZEN caused kidney damage of pregnant rats and TLR4-mediated inflammatory reactions signal pathway was one of the mechanisms of ZEN mediated toxicity in kidney.

Sisters in arms: myeloid and tubular epithelial cells shape renal innate immunity

The importance of innate immunity for survival is underscored by its presence at almost every level of the evolutionary tree of life. The task of "danger" recognition by the innate immune system is carried out by a broad class of pattern recognition receptors. These receptors are expressed in both hematopoietic and nonhematopoietic cells such as renal epithelial cells. Upon activation, pattern recognition receptors induce essentially two types of defensive responses: inflammation and phagocytosis. In this review, we highlight evidence that renal epithelial cells are endowed with such defensive capabilities and as such fully participate in renal innate immune responses.

Delayed administration of pituitary adenylate cyclase-activating polypeptide 38 ameliorates renal ischemia/reperfusion injury in mice by modulating Toll-like receptors

We investigated whether pituitary adenylate cyclase-activating polypeptide 38 (PACAP38) ameliorates kidney injury after ischemia/reperfusion (IR) by modulating Toll-like receptor (TLR)-associated signaling pathways. Male C57BL/6 mice were subjected to bilateral renal ischemia for 45 min. PACAP38, 20 μg in 100 μl of saline, was administered i.p. at 24 and 48 h after IR, and mice were euthanized at 72h. In IR mice, PACAP38 maintained serum creatinine near control levels (0.81 ± 0.08 vs. 0.69 ± 0.17 mg/dl in controls, p=NS, vs. 1.8 ± 0.03 in saline-treated IR mice, p<0.01) and significantly reduced the expression of kidney injury biomarkers. PACAP38 significantly reduced the levels of apoptosis and neutrophil infiltration, and protected against tubular damage. With PCR arrays, 59 of 83 TLR-related genes significantly changed their expression after IR. TLR2 increased 162 fold, followed by Fas-associated death domain (37 fold) and TLR6 (24 fold), while ubiquitin-conjugating enzyme E2 variant 1 (UBE2V1) decreased 55 fold. PACAP38 given 24 and 48 h after IR injury significantly reversed these changes in 56 genes, including TLR2, TLR3, TLR4, TLR6, and genes in the NF-κB pathways. The alterations in TLR2, TLR3, TLR6, and UBE2V1 were confirmed by RT-PCR. After IR, PACAP38 also suppressed protein levels of TLR-associated cytokines. PACAP38 reversed the changes in IR-activated TLR-associated NF-κB signaling pathways even when treatment was delayed 24h. Therefore, PACAP38 could be an effective therapeutic for unexpected IR-mediated renal injury. The prominently IR-induced TLR-related genes identified in this study could be novel drug targets.

Basolateral LPS inhibits NHE3 and HCOFormula absorption through TLR4/MyD88-dependent ERK activation in medullary thick ascending limb

Sepsis is associated with defects in renal tubule function, but the underlying mechanisms are incompletely understood. Recently, we demonstrated that Gram-negative bacterial lipopolysaccharide (LPS) inhibits HCO(3)(-) absorption in the medullary thick ascending limb (MTAL) through activation of Toll-like receptor 4 (TLR4). Here, we examined the mechanisms responsible for inhibition of HCO(3)(-) absorption by basolateral LPS. Adding LPS to the bath decreased HCO(3)(-) absorption by 30% in rat and mouse MTALs perfused in vitro. The inhibition of HCO(3)(-) absorption was eliminated by the mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK)/ERK inhibitors U0126 and PD98059. LPS induced a rapid (<15 min) and sustained (up to 60 min) increase in ERK phosphorylation in microdissected MTALs that was blocked by PD98059. The effects of basolateral LPS to activate ERK and inhibit HCO(3)(-) absorption were eliminated in MTALs from TLR4(-/-) and myeloid differentiation factor 88 (MyD88)(-/-) mice but were preserved in MTALs from TIR (Toll/interleukin-1 receptor) domain-containing adapter-inducing interferon-β (Trif)(-/-) mice. Basolateral LPS decreased apical Na(+)/H(+) exchanger 3 NHE3 activity through a decrease in maximal velocity (V(max)). The inhibition of NHE3 by LPS was eliminated by MEK/ERK inhibitors. LPS inhibited HCO(3)(-) absorption despite the presence of physiological stimuli that activate ERK in the MTAL. We conclude that basolateral LPS inhibits HCO(3)(-) absorption in the MTAL through activation of a TLR4/MyD88/MEK/ERK pathway coupled to inhibition of NHE3. These studies identify NHE3 as a target of TLR4 signaling in the MTAL and show that bacterial molecules can impair the absorptive functions of renal tubules through inhibition of this exchanger. The ERK pathway links TLR4 to downstream modulation of ion transport proteins and represents a potential target for treatment of sepsis-induced renal tubule dysfunction.

Endotoxin uptake by S1 proximal tubular segment causes oxidative stress in the downstream S2 segment

Gram-negative sepsis carries high morbidity and mortality, especially when complicated by acute kidney injury (AKI). The mechanisms of AKI in sepsis remain poorly understood. Here we used intravital two-photon fluorescence microscopy to investigate the possibility of direct interactions between filtered endotoxin and tubular cells as a possible mechanism of AKI in sepsis. Using wild-type (WT), TLR4-knockout, and bone marrow chimeric mice, we found that endotoxin is readily filtered and internalized by S1 proximal tubules through local TLR4 receptors and through fluid-phase endocytosis. Only receptor-mediated interactions between endotoxin and S1 caused oxidative stress in neighboring S2 tubules. Despite significant endotoxin uptake, S1 segments showed no oxidative stress, possibly as a result of the upregulation of cytoprotective heme oxygenase-1 and sirtuin-1 (SIRT1). Conversely, S2 segments did not upregulate SIRT1 and exhibited severe structural and functional peroxisomal damage. Taken together, these data suggest that the S1 segment acts as a sensor of filtered endotoxin, which it takes up. Although this may limit the amount of endotoxin in the systemic circulation and the kidney, it results in severe secondary damage to the neighboring S2 segments.

Toll-like receptor 4 signaling is involved in IgA-stimulated mesangial cell activation

PURPOSE

Deposition of polymeric IgA1 in the kidney mesangium is the hallmark of IgA nephropathy, but the molecular mechanisms of IgA-mediated mesangial responses and inflammatory injuries remain poorly understood. We hypothesize that Toll-like receptor 4 (TLR4) is involved in IgA-induced mesangial cell activation.

MATERIALS AND METHODS

Mouse mesangial cells were stimulated with lipopolysaccharide (LPS) (1 μg/mL), IgA (20 μg/mL), or both, and TLR4 expression was measured by real time RT-PCR and Western blot. Intracellular responses to LPS or IgA were assessed by Western blot for ERK1/2, JNK, p38 MAP kinases (MAPKs), Iκ-Bα degradation and fibronectin secretion. MCP-1 secretion was assessed by ELISA. Small interfering RNA (siRNA) of TLR4 was used to confirm that the effects were caused by TLR4 activity.

RESULTS

LPS- or IgA-treatment upregulated the levels of TLR4 mRNA and protein in cultured MMC at 24 h. LPS and IgA induced rapid phosphorylation of MAPKs, but degradation of Iκ-Bα was observed only in LPS-treated MMC. LPS, but not IgA, induced increased secretion of MCP-1 and fibronectin at 24 h or 48 h. Combined LPS and IgA treatment did not cause additional increases in TLR4 mRNA and protein levels or Iκ-Bα degradation, and MCP-1 and fibronectin secretions were less than with LPS alone. LPS- or IgA-induced TLR4 protein levels and MAPK activation were inhibited by transfection with TLR4 siRNA.

CONCLUSION

These results indicate that the activation of MAPKs and MCP-1 secretion are mediated by TLR4, at least in part, in IgA-treated mesangial cells. TLR4 is involved in mesangial cell injury by induction of pro-inflammatory cytokines in IgA nephropathy.

Fueling the fire in acute kidney injury: endothelial cells collect their Toll

Chen et al. demonstrate endothelial expression of Toll-like receptor 4 (TLR4) in the outer medulla of the kidney early in the course of ischemic acute kidney injury. Furthermore, they provide data that support the hypothesis that activation of endothelial TLR4 in the early extension phase of AKI by damage-associated molecular pattern molecules released from injured tubules results in endothelial activation. This activation can serve to amplify inflammation and tubular damage.

Protein level expression of Toll-like receptors 2, 4 and 9 in renal disease

BACKGROUND

Toll-like receptors (TLR) recognize a variety of ligands, including pathogen-associated molecular patterns and link innate and adaptive immunity. Individual receptors can be up-regulated during infection and inflammation. We examined the expression of selected TLRs at the protein level in various types of renal disease.

METHODS

Frozen sections of renal biopsies were stained with monoclonal antibodies to TLR-2, -4 and -9.

RESULTS

Up-regulation of the three TLRs studied was seen, although the extent was modest. TLR-2- and -4-positive cells belonged to the population of infiltrating inflammatory cells; only in the case of TLR-9 were intrinsic glomerular cells positive in polyoma virus infection and haemolytic uraemic syndrome (HUS).

CONCLUSIONS

Evidence for the involvement of the three TLRs tested in a variety of human renal diseases was found. These findings add to our understanding of the role of the innate immune system in kidney disease.

Impact of Toll-like receptor 2 expression in renal allograft rejection

BACKGROUND

An important role of TLR2 has been shown in various experimental models of renal ischaemia/reperfusion injury. To study the expression of TLR2 in renal allograft rejection systematically, we established an experimental rat transplantation model.

METHODS

TLR2 expression was analysed in 99 human renal allograft biopsies, and in rat allografts at Day 6 and 28 after experimental renal transplantation. To discriminate whether regulation of TLR2 was following immunological processes after allogeneic transplantation or was a consequence from ischaemia/reperfusion injury, control animals subjected to syngeneic transplantation or to ischaemia/reperfusion damage were also investigated.

RESULTS

TLR2 mRNA was significantly elevated in rat allografts with acute rejection on Day 6 and decreased spontaneously towards Day 28. TLR2 induction correlated with renal function and TLR2 excretion in the urine of transplanted rats. TLR2 staining was also significantly increased in human allografts with acute rejection. TLR2 protein could be localized in tubular epithelial cells and vascular endothelial cells, and in CD68- and CD4-positive infiltrating cells.

CONCLUSIONS

TLR2 is markedly up-regulated in both experimental and human acute renal allograft rejection. Our data suggest a role for TLR2 during allogen-dependent graft damage after renal transplantation.

TLR4 promotes fibrosis but attenuates tubular damage in progressive renal injury

Toll-like receptors (TLRs) can orchestrate an inflammatory response upon activation by pathogen-associated motifs and release of endogenous stress ligands during tissue injury. The kidney constitutively expresses most TLRs, including TLR4. The function of TLR4 during the inflammation, tubular atrophy, and fibrosis that accompany progressive renal injury is unknown. Here, we subjected wild-type (WT) and TLR4-deficient mice to unilateral ureteral obstruction and observed elevated levels of TLR4 mRNA in the kidney after obstruction. One day after unilateral ureteral obstruction, TLR4-deficient mice had fewer proliferating tubular epithelial cells and more tubular damage than WT mice; however, TLR4-deficient mice developed considerably less renal fibrosis despite decreased matrix metalloproteinase activity and without significant differences in myofibroblast accumulation. In vitro, TLR4-deficient primary tubular epithelial cells and myofibroblasts produced significantly less type I collagen mRNA after TGF-beta stimulation than WT cells. The reduced fibrosis in TLR4-deficient mice associated with an upregulation of Bambi, a negative regulator of TGF-beta signaling. In conclusion, TLR4 attenuates tubular damage but promotes renal fibrosis by modulating the susceptibility of renal cells to TGF-beta. These data suggest that TLR4 signaling may be a therapeutic target for the prevention of renal fibrosis.

Toll-like receptor 2 mediates inhibition of HCO(3)(-) absorption by bacterial lipoprotein in medullary thick ascending limb

Bacterial infection and sepsis are associated with renal tubule dysfunction and dysregulation of systemic electrolyte balance but the underlying mechanisms are incompletely understood. Recently, we demonstrated that HCO(3)(-) absorption by the medullary thick ascending limb (MTAL) is inhibited by gram-negative bacterial LPS through activation of Toll-like receptor 4 (TLR4). Here, we examined whether MTAL transport is altered by activation of TLR2, the receptor predominantly responsible for recognizing gram-positive bacteria. Confocal immunofluorescence showed expression of TLR2 in the basolateral membrane domain of rat and mouse MTALs. The functional role of TLR2 was examined in perfused MTALs using Pam(3)CSK(4), a bacterial lipoprotein analog that specifically activates TLR2. Adding Pam(3)CSK(4) to the bath decreased HCO(3)(-) absorption by 25%. The inhibition by Pam(3)CSK(4) was eliminated in MTALs from TLR2(-/-) mice. HCO(3)(-) absorption was also inhibited by the TLR2 agonists lipoteichoic acid and peptidoglycan, two cell wall components of gram-positive bacteria. The MEK/ERK inhibitor U0126 eliminated inhibition of HCO(3)(-) absorption by bath LPS but had no effect on inhibition by Pam(3)CSK(4). The inhibition by Pam(3)CSK(4) was eliminated by the protein kinase C inhibitors chelerythrine Cl and bisindolylmaleimide. Moreover, the inhibition by Pam(3)CSK(4), lipoteichoic acid, and peptidoglycan was additive to inhibition by LPS. Thus, agonists of basolateral TLR2 and TLR4 inhibit HCO(3)(-) absorption independently through distinct signaling pathways. We conclude that bacterial components act directly through TLRs to modify the transport function of renal tubules. During polymicrobial sepsis, gram-positive bacterial molecules acting through TLR2 and gram-negative LPS acting through TLR4 can function through parallel signaling pathways to impair MTAL transport. The inhibition of luminal acidification may impair the ability of the kidneys to correct systemic acidosis that contributes to sepsis pathogenesis.

Recognition of apoptotic cells by epithelial cells: conserved versus tissue-specific signaling responses

During apoptosis, cells acquire new activities that enable them to modulate the fate and function of interacting phagocytes, particularly macrophages (m). Although the best known of these activities is anti-inflammatory, apoptotic targets also influence m survival and proliferation by modulating proximal signaling events, such as MAPK modules and Akt. We asked whether modulation of these same signaling events extends to epithelial cells, a minimally phagocytic cell type. We used BU.MPT cells, a mouse kidney epithelial cell line, as our primary model, but we also evaluated several epithelial cell lines of distinct tissue origins. Like m, mouse kidney epithelial cells recognized apoptotic and necrotic targets through distinct non-competing receptors, albeit with lower binding capacity and markedly reduced phagocytosis. Also, modulation of inflammatory activity and MAPK-dependent signaling by apoptotic and necrotic targets was indistinguishable in kidney epithelial cells and m. In contrast, modulation of Akt-dependent signaling differed dramatically between kidney epithelial cells and m. In kidney epithelial cells, modulation of Akt was linked to target cell recognition, independently of phagocytosis, whereas in m, modulation was linked to phagocytosis. Moreover, recognition of apoptotic and necrotic targets by kidney epithelial cells elicited opposite responses; apoptotic targets inhibited whereas necrotic targets stimulated Akt activity. These data confirm that nonprofessional phagocytes recognize and respond to dying cells, albeit in a manner partially distinct from m. By acting as sentinels of environmental change, apoptotic and necrotic targets may permit neighboring viable cells, especially non-migratory epithelial cells, to monitor and adapt to local stresses.

Lipopolysaccharide directly alters renal tubule transport through distinct TLR4-dependent pathways in basolateral and apical membranes

Bacterial infection of the kidney is associated with renal tubule dysfunction and dysregulation of systemic electrolyte balance. Whether bacterial molecules directly affect renal tubule transport is unknown. We examined the effects of LPS on HCO3(-) absorption in the isolated rat and mouse medullary thick ascending limb (MTAL). LPS decreased HCO3(-) absorption when added to bath or lumen. The MEK/ERK inhibitor U0126 eliminated inhibition by bath LPS but had no effect on inhibition by lumen LPS. Conversely, the mammalian target of rapamycin (mTOR) inhibitor rapamycin eliminated inhibition by lumen LPS but had no effect on inhibition by bath LPS. Inhibiting basolateral Na(+)/H(+) exchange with amiloride eliminated inhibition of HCO3(-) absorption by lumen but not bath LPS. Confocal immunofluorescence showed expression of TLR4 in basolateral and apical membrane domains. Inhibition of HCO3(-) absorption by bath and lumen LPS was eliminated in MTALs from TLR4(-/-) mice. Thus LPS inhibits HCO3(-) absorption through distinct TLR4-dependent pathways in basolateral and apical membranes. These results establish that bacterial molecules can directly impair the transport function of renal tubules, identifying a new mechanism contributing to tubule dysfunction during bacterial infection. The LPS-induced reduction in luminal acidification may contribute to Gram-negative pathogenicity by promoting bacterial adherence and growth and impairing correction of infection-induced systemic acid-base disorders.

The role of Toll-like receptor 2 in inflammation and fibrosis during progressive renal injury

Tissue fibrosis and chronic inflammation are common causes of progressive organ damage, including progressive renal disease, leading to loss of physiological functions. Recently, it was shown that Toll-like receptor 2 (TLR2) is expressed in the kidney and activated by endogenous danger signals. The expression and function of TLR2 during renal fibrosis and chronic inflammation has however not yet been elucidated. Therefore, we studied TLR2 expression in human and murine progressive renal diseases and explored its role by inducing obstructive nephropathy in TLR2^−/− or TLR2^+/+ mice. We found that TLR2 is markedly upregulated on tubular and tubulointerstitial cells in patients with chronic renal injury. In mice with obstructive nephropathy, renal injury was associated with a marked upregulation and change in distribution of TLR2 and upregulation of murine TLR2 danger ligands Gp96, biglycan, and HMGB1. Notably, TLR2 enhanced inflammation as reflected by a significantly reduced influx of neutrophils and production of chemokines and TGF-β in kidneys of TLR2^−/− mice compared with TLR2^+/+ animals. Although, the obstructed kidneys of TLR2^−/− mice had less interstitial myofibroblasts in the later phase of obstructive nephropathy, tubular injury and renal matrix accumulation was similar in both mouse strains. Together, these data demonstrate that TLR2 can initiate renal inflammation during progressive renal injury and that the absence of TLR2 does not affect the development of chronic renal injury and fibrosis.

Resolution of renal inflammation: a new role for NF-kappaB1 (p50) in inflammatory kidney diseases

In renal tissue injury, activation of the transcription factor NF-kappaB has a central role in the induction of proinflammatory gene expression, which are involved in the development of progressive renal inflammatory disease. The function of NF-kappaB during the switch from the inflammatory process toward resolution, however, is largely unknown. Therefore, we assessed the time-dependent activation and function of NF-kappaB in two different models of acute nephritis. Our experiments demonstrate a biphasic activation of NF-kappaB in the anti-Thy-1 model of glomerulonephritis in rats and the LPS-induced nephritis in mice, with a first peak during the induction phase and a second peak during the resolution period. After induction of glomerular immune injury in rats, predominantly NF-kappaB p65/p50 heterodimer complexes are shifted to the nucleus whereas during the resolution phase predominantly p50 homodimers could be demonstrated in the nuclear compartment. In addition, we could demonstrate that p50 protein plays a pivotal role in the resolution of LPS-induced renal inflammation since NF-kappaB p50 knockout mice demonstrate significantly higher chemokine expression, prolonged renal inflammatory cell infiltration with consecutive tissue injury, and reduced survival. In conclusion, our studies indicate that NF-kappaB subunit p50 proteins have critical in vivo functions in immunologically mediated renal disease by downregulating inflammation during the resolution period.

Pathways of renal injury in systemic gram-negative sepsis

Acute renal failure is a grave complication of systemic gram-negative sepsis. The pathophysiological mechanisms of sepsis leading to kidney injury result in part from systemic inflammatory and haemodynamic alterations. These are triggered by the interaction of endotoxin with Toll-like receptor 4 (TLR4) on cells of the immune system. Recently, TLR4 and other co-effector molecules were identified on renal tubular and vascular cells. Furthermore, it was demonstrated that systemic endotoxin has direct access to renal sites where these receptors are expressed. Therefore, we review data in support of this novel pathway of renal injury in sepsis, whereby systemic endotoxin causes direct injury through interactions with local epithelial and endothelial TLR4.

Tamm-Horsfall protein protects the kidney from ischemic injury by decreasing inflammation and altering TLR4 expression

Tamm-Horsfall protein (THP) is a glycoprotein with unclear functions expressed exclusively in thick ascending limbs (TAL) of the kidney. Its role in ischemic acute kidney injury is uncertain, with previous data suggesting a possible negative effect by enhancing cast formation and promoting inflammation. Using a recently characterized THP knockout mouse (THP-/-), we investigated the role of THP in renal ischemia-reperfusion injury (IRI). In wild-type mice (THP+/+), THP expression was increased by injury. THP-/- mice developed more functional and histological renal damage after IRI compared with THP+/+. THP-/- kidneys showed more inflammation and tubular necrosis. Cast formation correlated with the severity of injury and was independent of THP presence. THP absence was associated with a more necrotic, rather than apoptotic, phenotype of cell death. The outer medulla was predominantly affected, where significant interstitial neutrophil infiltration was detected in proximity to injured S3 proximal tubular segments and TAL. This coincided with an enhanced expression of the innate immunity receptor Toll-like receptor 4 (TLR4) in S3 segments of THP-/- compared with THP+/+ mice. Specifically, a basolateral S3 expression of TLR4 was more evident in THP-/- kidneys compared with a more apical distribution in THP+/+. Such basolateral location for TLR4 allows a greater interaction with proinflammatory ligands present in the interstitium during ischemia. In conclusion, we are showing a completely novel role for a very old protein in the setting of renal injury. Our data suggest that THP stabilizes the outer medulla in the face of injury by decreasing inflammation, possibly through an effect on TLR4.

Toll-like receptor 2 and renal allograft function

BACKGROUND

Toll-like receptors (TLR) modulate the immune response. We analyzed the relationships between TLR expression in renal tissue with infection, rejection and graft function after kidney transplantation.

METHODS

TLR-2 and TLR-4 expression was detected by immunohistochemistry in 257 protocol biopsies obtained 6 weeks, 3 and 6 months after transplantation, and in 108 indication biopsies. We correlated TLR expression in different renal tissue compartments with kidney transplant function 6, 12 and 24 months after transplantation, acute cellular rejection in renal grafts (according to the Banff classification), and urinary tract and cytomegalovirus infections.

RESULTS

We found a highly consistent correlation of TLR-2 expression in proximal and distal tubules, and in renal vessels (p < 0.001 for all compartments), but not for TLR-4 expression. This holds true for all protocol biopsy time points as well as for indication biopsies. Positive TLR-2 expression in renal tubules was associated with significantly (p < 0.05) better initial graft function as well as graft function 6, 12 and 24 months after transplantation. We also found a significant (p < 0.05) association between TLR-2 expression and lower incidence of acute cellular rejection in early protocol biopsies (6 weeks). In contrast, positive TLR-4 expression was not related to kidney function or acute cellular rejection. Further, the two different TLR subtypes were not related to episodes of urinary tract or cytomegalovirus infections.

CONCLUSION

TLR-2 expression in renal tissue is associated with superior graft function up to 2 years after kidney transplantation. The role of TLR-2 in the immune response against human kidney transplants warrants further investigation.

Virus-induced cellular immune mechanisms of injury to the kidney

Cellular immune systems play an important role in determining renal outcomes in virus-induced kidney diseases. Highlighted briefly are five different locations along the development of adaptive immune responses to viral infection that may promote injury to the renal parenchyma and the loss of renal function. This may occur because adaptive immune cells directly target infected renal parenchymal cells or because the kidney becomes a bystander organ of adaptive immune cell-mediated injury. Examples from recent studies are provided to illustrate how this may lead to clinically relevant renal disease.

Sepsis induces an increase in thick ascending limb Cox-2 that is TLR4 dependent

Cyclooxygenase-2 (Cox-2) is an inducible enzyme responsible for the formation of inflammatory prostanoids such as prostaglandins and thromboxane. Its role in the pathophysiology of inflammatory states like sepsis is increasingly recognized. Recently, we demonstrated that sepsis upregulates the endotoxin receptor Toll-like receptor 4 (TLR4) in rat kidney. Because Cox-2 is one of the downstream products of TLR4 activation, we hypothesized that sepsis-induced changes in renal Cox-2 expression are TLR4 dependent. Indeed, we show that in Sprague-Dawley rats, cecal ligation and puncture (a sepsis model) increases Cox-2 expression in cortical and medullary thick ascending loops (cTAL and mTAL, respectively) as well as inner medullary collecting ducts. These are all sites of increased TLR4 expression during sepsis. To determine the actual dependence on TLR4, we measured Cox-2 expression in wild-type and mutant mice which harbor a TLR4 gene deletion (TLR4-/-). In wild-type mice, sepsis increased Cox-2 expression in proximal tubules, cTAL, and mTAL. In contrast, septic TLR4-/- mice showed no significant increase in cTAL or mTAL Cox-2 expression. Furthermore, renin was absent from juxtaglomerular cells of TLR4-/- mice. We conclude that the dependence of sepsis-induced renal Cox-2 expression on TLR4 is tubule specific. The TLR4-dependent Cox-2 expression is mostly restricted to cortical and medullary thick ascending loops of Henle that characteristically express and secrete Tamm-Horsfall protein.

The kidney in the critically ill

Acute kidney injury (AKI) is a common and serious complication in the intensive care setting. It seldom occurs in isolation, but is mostly part of a multiple organ dysfunction syndrome. The pathogenesis is frequently multifactorial, with sepsis contributing to 50% of the cases.The development of AKI in critically-ill patients is "bad news": patients with AKI have a high morbidity and mortality. In addition, AKI, even in its mildest from, is not only a marker of illness severity but appears to be independently associated with mortality. Prevention of AKI is therefore a major goal to improve outcome of critically-ill patients. Treatment of established AKI is largely supportive. The optimal modality for renal replacement therapy in critically-ill patients still remains a matter of debate). The majority of survivors recover renal function.

Renal ischemia-reperfusion injury: renal dendritic cells loudly sound the alarm

Renal ischemia-reperfusion injury is a major cause of acute renal failure and kidney allograft dysfunction. Dong and colleagues now identify the surveying renal dendritic cell network as the predominant source of tumor necrosis factor-alpha during the early stages of renal ischemia-reperfusion injury, raising the possibility for direct targeting of renal dendritic cells to help ameliorate this common form of renal injury and its sequelae.

Differential regulation of Toll-like receptor 4 gene expression in renal cells by angiotensin II: dependency on AP1 and PU.1 transcriptional sites

BACKGROUND

Toll-like receptor 4 (TLR4) is involved in the sensing of lipopolysaccharide and, therefore, plays a central role in innate immune responses to gram-negative bacteria. Interestingly, TLR4 expression occurs within the kidney. We have previously demonstrated that angiotensin II (ANG II) upregulates TLR4 expression on mesangial cells. However, the factors controlling transcriptional activation of the Tlr4 gene in mesangial cells are not known, and the specificity of this response for other renal cells is unclear.

METHODS

Cultured murine proximal tubular cells (mouse cortical tubule cell line; MCT cells), murine mesangial cells (MMCs), and murine podocytes were treated with ANG II. The expression of ANG II receptor mRNA and TLR4 mRNA and protein was determined by polymerase chain reaction and Western blotting. The transcriptional activity of wild-type and mutant mouse TLR4 promoter reporter constructs was determined upon transient transfection of the three cell types.

RESULTS

Although MMCs, podocytes, and syngeneic proximal MCT cells similarly expressed ANG II receptors, ANG II stimulated TLR4 mRNA and protein expression in MMCs and podocytes only. A mouse TLR4 promoter construct (-518/+129), previously shown to contain all important transcriptional regulatory elements in various cell types, was activated by ANG II in MMCs and podocytes, but not in MCT cells. Mutation of a proximal PU.1-binding consensus site or an AP1 site abolished ANG-II-mediated transcriptional activation of the TLR4 promoter. Finally, basal transcription of the Tlr4 gene depended in all three cell lines on an intact AP1 site and additionally on the proximal PU.1 site in MMCs.

CONCLUSIONS

ANG II stimulates TLR4 transcription through AP1 and PU.1 sites in a cell-specific manner. Since the intrarenal ANG II concentrations are enhanced in many pathophysiological situations, ANG-II-stimulated transcription of TLR4 on MMCs and podocytes may contribute to renal inflammation.

Diagnostic potential for urinary proteomics

Urine represents a modified ultrafiltrate of plasma, with protein concentrations typically approximately 1000-fold lower than plasma. Urine’s low protein concentration might suggest it to be a less promising diagnostic specimen than plasma. However, urine can be obtained noninvasively and tests of many urinary proteins are well-established in clinical practice. Proteomic technologies expand opportunities to analyze urinary proteins, identifying more than 1000 proteins and peptides in urine. Urine offers a sampling of most plasma proteins, with increased proportions of low-molecular-weight protein and peptide components. Urine also offers enriched sampling of proteins released along the urinary tract. Although urine presents some challenges as a diagnostic specimen, its diverse range of potential markers offers great potential for diagnosis of both systemic and kidney diseases. Examples of clinical situations where this may be of value are for more sensitive detection of kidney transplant rejection or of renal toxicity of medications.