Deletion of the Mitochondrial Complex-IV Cofactor Heme A:Farnesyltransferase Causes Focal Segmental Glomerulosclerosis and Interferon Response.

Mutations in mitochondrial DNA as well as nuclear-encoded mitochondrial proteins have been reported to cause tubulointerstitial kidney diseases and focal segmental glomerulosclerosis (FSGS). Recently, genes and pathways affecting mitochondrial turnover and permeability have been implicated in adult-onset FSGS. Furthermore, dysfunctioning mitochondria may be capable of engaging intracellular innate immune-sensing pathways. To determine the impact of mitochondrial dysfunction in FSGS and secondary innate immune responses, we generated Cre/loxP transgenic mice to generate a loss-of-function deletion mutation of the complex IV assembly cofactor heme A:farnesyltransferase (COX10) restricted to cells of the developing nephrons. These mice develop severe, early-onset FSGS with innate immune activation and die prematurely with kidney failure. Mutant kidneys showed loss of glomerular and tubular epithelial function, epithelial apoptosis, and, in addition, a marked interferon response. In vitro modeling of Cox10 deletion in primary kidney epithelium compromises oxygen consumption, ATP generation, and induces oxidative stress. In addition, loss of Cox10 triggers a selective interferon response, which may be caused by the leak of mitochondrial DNA into the cytosol activating the intracellular DNA sensor, stimulator of interferon genes. This new animal model provides a mechanism to study mitochondrial dysfunction in vivo and demonstrates a direct link between mitochondrial dysfunction and intracellular innate immune response.

Connective Tissue Growth Factor Domain 4 Amplifies Fibrotic Kidney Disease through Activation of LDL Receptor-Related Protein 6.

Connective tissue growth factor (CTGF), a matrix-associated protein with four distinct cytokine binding domains, has roles in vasculogenesis, wound healing responses, and fibrogenesis and is upregulated in fibroblasts and myofibroblasts in disease. Here, we investigated the role of CTGF in fibrogenic cells. In mice, tissue-specific inducible overexpression of CTGF by kidney pericytes and fibroblasts had no bearing on nephrogenesis or kidney homeostasis but exacerbated inflammation and fibrosis after ureteral obstruction. These effects required the WNT receptor LDL receptor-related protein 6 (LRP6). Additionally, pericytes isolated from these mice became hypermigratory and hyperproliferative on overexpression of CTGF. CTGF is cleaved in vivo into distinct domains. Treatment with recombinant domain 1, 1+2 (N terminus), or 4 (C terminus) independently activated myofibroblast differentiation and wound healing responses in cultured pericytes, but domain 4 showed the broadest profibrotic activity. Domain 4 exhibited low-affinity binding to LRP6 in in vitro binding assays, and inhibition of LRP6 or critical signaling cascades downstream of LRP6, including JNK and WNT/ß-catenin, inhibited the biologic activity of domain 4. Administration of blocking antibodies specifically against CTGF domain 4 or recombinant Dickkopf-related protein-1, an endogenous inhibitor of LRP6, effectively inhibited inflammation and fibrosis associated with ureteral obstruction in vivo Therefore, domain 4 of CTGF and the WNT signaling pathway are important new targets in fibrosis.

Inhibition of αvß5 Integrin Attenuates Vascular Permeability and Protects against Renal Ischemia-Reperfusion Injury.

Ischemia-reperfusion injury (IRI) is a leading cause of AKI. This common clinical complication lacks effective therapies and can lead to the development of CKD. The αvß5 integrin may have an important role in acute injury, including septic shock and acute lung injury. To examine its function in AKI, we utilized a specific function-blocking antibody to inhibit αvß5 in a rat model of renal IRI. Pretreatment with this anti-αvß5 antibody significantly reduced serum creatinine levels, diminished renal damage detected by histopathologic evaluation, and decreased levels of injury biomarkers. Notably, therapeutic treatment with the αvß5 antibody 8 hours after IRI also provided protection from injury. Global gene expression profiling of post-ischemic kidneys showed that αvß5 inhibition affected established injury markers and induced pathway alterations previously shown to be protective. Intravital imaging of post-ischemic kidneys revealed reduced vascular leak with αvß5 antibody treatment. Immunostaining for αvß5 in the kidney detected evident expression in perivascular cells, with negligible expression in the endothelium. Studies in a three-dimensional microfluidics system identified a pericyte-dependent role for αvß5 in modulating vascular leak. Additional studies showed αvß5 functions in the adhesion and migration of kidney pericytes in vitro Initial studies monitoring renal blood flow after IRI did not find significant effects with αvß5 inhibition; however, future studies should explore the contribution of vasomotor effects. These studies identify a role for αvß5 in modulating injury-induced renal vascular leak, possibly through effects on pericyte adhesion and migration, and reveal αvß5 inhibition as a promising therapeutic strategy for AKI.

TWEAK-Fn14 Signaling Activates Myofibroblasts to Drive Progression of Fibrotic Kidney Disease.

The identification of the cellular origins of myofibroblasts has led to the discovery of novel pathways that potentially drive myofibroblast perpetuation in disease. Here, we further investigated the role of innate immune signaling pathways in this process. In mice, renal injury-induced activation of pericytes, which are myofibroblast precursors attached to endothelial cells, led to upregulated expression of TNF receptor superfamily member 12a, also known as fibroblast growth factor-inducible 14 (Fn14), by these cells. In live rat kidney slices, administration of the Fn14 ligand, TNF-related weak inducer of apoptosis (TWEAK), promoted pericyte-dependent vasoconstriction followed by pericyte detachment from capillaries. In vitro, administration of TWEAK activated and differentiated pericytes into cytokine-producing myofibroblasts, and further activated established myofibroblasts in a manner requiring canonical and noncanonical NF-κB signaling pathways. Deficiency of Fn14 protected mouse kidneys from fibrogenesis, inflammation, and associated vascular instability after in vivo injury, and was associated with loss of NF-κB signaling. In a genetic model of spontaneous CKD, therapeutic delivery of anti-TWEAK blocking antibodies attenuated disease progression, preserved organ function, and increased survival. These results identify the TWEAK-Fn14 signaling pathway as an important factor in myofibroblast perpetuation, fibrogenesis, and chronic disease progression.

MicroRNAs as novel therapeutic targets to treat kidney injury and fibrosis.

MicroRNAs (miRs), a class of small noncoding RNAs that act as post-transcriptional regulators of gene expression, have attracted increasing attention as critical regulators of organogenesis, cancer, and disease. Interest has been spurred by development of a novel class of synthetic RNA oligonucleotides with excellent drug-like properties that hybridize to a specific miR, preventing its action. In kidney disease, a small number of miRs are dysregulated. These overlap with regulated miRs in nephrogenesis and kidney cancers. Several dysregulated miRs have been identified in fibrotic diseases of other organs, representing a "fibrotic signature," and some of these fibrotic miRs contribute remarkably to the pathogenesis of kidney disease. Chronic kidney disease, affecting ∼10% of the population, leads to kidney failure, with few treatment options. Here, we will explore the pathological mechanism of miR-21, whose pre-eminent role in amplifying kidney disease and fibrosis by suppressing mitochondrial biogenesis and function is established. Evolving roles for miR-214, -199, -200, -155, -29, -223, and -126 in kidney disease will be discussed, and we will demonstrate how studying functions of distinct miRs has led to new mechanistic insights for kidney disease progression. Finally, the utility of anti-miR oligonucleotides as potential novel therapeutics to treat chronic disease will be highlighted.

Deficient Autophagy Results in Mitochondrial Dysfunction and FSGS.

FSGS is a heterogeneous fibrosing disease of the kidney, the cause of which remains poorly understood. In most cases, there is no effective treatment to halt or retard progression to renal failure. Increasing evidence points to mitochondrial dysfunction and the generation of reactive oxygen species in the pathogenesis of CKD. Autophagy, a major intracellular lysosomal degradation system, performs homeostatic functions linked to metabolism and organelle turnover. We prevented normal autophagic pathways in nephrons of mice by mutating critical autophagy genes ATG5 or ATG7 during nephrogenesis. Mutant mice developed mild podocyte and tubular dysfunction within 2 months, profound glomerular and tubular changes bearing close similarity to human disease by 4 months, and organ failure by 6 months. Ultrastructurally, podocytes and tubular cells showed vacuolization, abnormal mitochondria, and evidence of endoplasmic reticulum stress, features that precede the appearance of histologic or clinical disease. Similar changes were observed in human idiopathic FSGS kidney biopsy specimens. Biochemical analysis of podocytes and tubules of 2-month-old mutant mice revealed elevated production of reactive oxygen species, activation of endoplasmic reticulum stress pathways, phosphorylation of p38, and mitochondrial dysfunction. Furthermore, cultured proximal tubule cells isolated from mutant mice showed marked mitochondrial dysfunction and elevated mitochondrial reactive oxygen species generation that was suppressed by a mitochondrial superoxide scavenger. We conclude that mitochondrial dysfunction and endoplasmic reticulum stress due to impaired autophagic organelle turnover in podocytes and tubular epithelium are sufficient to cause many of the manifestations of FSGS in mice.

Dicer1 activity in the stromal compartment regulates nephron differentiation and vascular patterning during mammalian kidney organogenesis.

MicroRNAs, activated by the enzyme Dicer1, control post-transcriptional gene expression. Dicer1 has important roles in the epithelium during nephrogenesis, but its function in stromal cells during kidney development is unknown. To study this, we inactivated Dicer1 in renal stromal cells. This resulted in hypoplastic kidneys, abnormal differentiation of the nephron tubule and vasculature, and perinatal mortality. In mutant kidneys, genes involved in stromal cell migration and activation were suppressed as were those involved in epithelial and endothelial differentiation and maturation. Consistently, polarity of the proximal tubule was incorrect, distal tubule differentiation was diminished, and elongation of Henle's loop attenuated resulting in lack of inner medulla and papilla in stroma-specific Dicer1 mutants. Glomerular maturation and capillary loop formation were abnormal, whereas peritubular capillaries, with enhanced branching and increased diameter, formed later. In Dicer1-null renal stromal cells, expression of factors associated with migration, proliferation, and morphogenic functions including α-smooth muscle actin, integrin-α8, -ß1, and the WNT pathway transcriptional regulator LEF1 were reduced. Dicer1 mutation in stroma led to loss of expression of distinct microRNAs. Of these, miR-214, -199a-5p, and -199a-3p regulate stromal cell functions ex vivo, including WNT pathway activation, migration, and proliferation. Thus, Dicer1 activity in the renal stromal compartment regulates critical stromal cell functions that, in turn, regulate differentiation of the nephron and vasculature during nephrogenesis.

Metalloproteinase-mediated Shedding of Integrin ß2 promotes macrophage efflux from inflammatory sites.

Macrophage exiting from inflammatory sites is critical to limit the local innate immune response. With tissue insult, resident tissue macrophages rapidly efflux to lymph nodes where they modulate the adaptive immune response, and inflammatory macrophages attracted to the site of injury then exit during the resolution phase. However, the mechanisms that regulate macrophage efflux are poorly understood. This study has investigated soluble forms of integrin ß2 whose levels are elevated in experimental peritonitis at times when macrophages are exiting the peritoneum, suggesting that its proteolytic shedding may be involved in macrophage efflux. Both constitutive and inducible metalloproteinase-dependent shedding of integrin ß2 from mouse macrophages are demonstrated. Soluble integrin ß2 is primarily released as a heterodimeric complex with αM that retains its ability to bind its ligands intracellular adhesion molecule-1, fibrin, and collagen and thus may serve as a soluble antagonist. In a model of accelerated exiting, administration of a metalloproteinase inhibitor prevents macrophage efflux by 50% and impedes loss of macrophage integrin ß2 from the cell surface. Exiting of peritoneal macrophages in mice lacking integrin ß2 is accelerated, and antibody disruption of integrin ß2-substrate interactions can reverse 50% of the metalloprotease inhibitor blockade of macrophage exiting. Thus, our study demonstrates the ability of metalloproteinase-mediated shedding of integrin ß2 to promote macrophage efflux from inflammatory sites, and the release of soluble integrin heterodimers may also limit local inflammation.

MicroRNAs as potential therapeutic targets in kidney disease.

One cornerstone of chronic kidney disease (CKD) is fibrosis, as kidneys are susceptible due to their high vascularity and predisposition to ischemia. Presently, only therapies targeting the angiotensin receptor are used in clinical practice to retard the progression of CKD. Thus, there is a pressing need for new therapies designed to treat the damaged kidney. Several independent laboratories have identified a number of microRNAs that are dysregulated in human and animal models of CKD. This review will explore the evidence suggesting that by blocking the activity of such dysregulated microRNAs, new therapeutics could be developed to treat the progression of CKD.

MMP-9 sheds the beta2 integrin subunit (CD18) from macrophages.

Activated macrophages are essential effectors of immunity and a rich source of matrix metalloproteinase-9 (MMP-9; gelatinase B). To search for cellular substrates of the enzyme, we subjected wild-type macrophages and macrophages expressing an autoactivating form of pro-MMP-9 (M9A macrophages) to proteomics analysis. Two-dimensional liquid chromatography together with tandem mass spectrometry identified 467 proteins in medium conditioned by M9A and/or wild-type macrophages. Subtractive proteomics identified 18 candidate MMP-9 substrates. Biochemical studies confirmed that two transmembrane proteins, beta(2) integrin subunit (CD18) and amyloid protein precursor (APP), were enriched in the medium of M9A macrophages. To identify potential cleavage sites, we synthesized an overlapping library of peptides that spanned 60 residues of the ectodomain and transmembrane domain of beta(2) integrin. Active MMP-9 cleaved a single peptide, ECVKGPNVAAIVGGT, at residues corresponding to Ala(705) and Ile(706) of the beta(2) integrin. Peptides corresponding to this cleavage site were detected by tandem mass spectrometric analysis only in medium from M9A macrophages, strongly supporting the proposal that beta(2) integrin is shed by autoactivating MMP-9. Our observations indicate that subtractive proteomics in concert with peptide substrate mapping is a powerful approach for identifying proteolytic substrates and suggest that MMP-9 plays previously unsuspected roles in the regulation and shedding of beta(2) integrin.

Correction to: Graft immaturity and safety concerns in transplanted human kidney organoids.

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

Macrophage expression of active MMP-9 induces acute plaque disruption in apoE-deficient mice.

The majority of acute clinical manifestations of atherosclerosis are due to the physical rupture of advanced atherosclerotic plaques. It has been hypothesized that macrophages play a key role in inducing plaque rupture by secreting proteases that destroy the extracellular matrix that provides physical strength to the fibrous cap. Despite reports detailing the expression of multiple proteases by macrophages in rupture-prone regions, there is no direct proof that macrophage-mediated matrix degradation can induce plaque rupture. We aimed to test this hypothesis by retrovirally overexpressing the candidate enzyme MMP-9 in macrophages of advanced atherosclerotic lesions of apoE-/- mice. Despite a greater than 10-fold increase in the expression of MMP-9 by macrophages, there was only a minor increase in the incidence of plaque fissuring. Subsequent analysis revealed that macrophages secrete MMP-9 predominantly as a proform, and this form is unable to degrade the matrix component elastin. Expression of an autoactivating form of MMP-9 in macrophages in vitro greatly enhances elastin degradation and induces significant plaque disruption when overexpressed by macrophages in advanced atherosclerotic lesions of apoE-/- mice in vivo. These data show that enhanced macrophage proteolytic activity can induce acute plaque disruption and highlight MMP-9 as a potential therapeutic target for stabilizing rupture-prone plaques.

Graft immaturity and safety concerns in transplanted human kidney organoids.

For chronic kidney disease, regeneration of lost nephrons with human kidney organoids derived from induced pluripotent stem (iPS) cells is proposed to be an attractive potential therapeutic option. It remains unclear, however, whether organoids transplanted into kidneys in vivo would be safe or functional. Here, we purified kidney organoids and transplanted them beneath the kidney capsules of immunodeficient mice to test their safety and maturity. Kidney organoid grafts survived for months after transplantation and became vascularized from host mouse endothelial cells. Nephron-like structures in grafts appeared more mature than kidney organoids in vitro, but remained immature compared with the neighboring mouse kidney tissue. Ultrastructural analysis revealed filtration barrier-like structures, capillary lumens, and tubules with brush border in the transplanted kidney organoids, which were more mature than those of the kidney organoids in vitro but not as organized as adult mammalian kidneys. Immaturity was a common feature of three separate differentiation protocols by immunofluorescence analysis and single cell RNA sequencing. Stroma of transplanted kidney organoid grafts were filled with vimentin-positive mesenchymal cells, and chondrogenesis, cystogenesis, and stromal expansion were observed in the long term. Transcription profiles showed that long-term maintenance after kidney organoid transplantation induced transcriptomic reprogramming with prominent suppression of cell-cycle-related genes and upregulation of extracellular matrix organization. Our data suggest that kidney organoids derived from iPS cells may be transplantable but strategies to improve nephron differentiation and purity are required before they can be applied in humans as a therapeutic option.

Pericyte MyD88 and IRAK4 control inflammatory and fibrotic responses to tissue injury.

Fibrotic disease is associated with matrix deposition that results in the loss of organ function. Pericytes, the precursors of myofibroblasts, are a source of pathological matrix collagens and may be promising targets for treating fibrogenesis. Here, we have shown that pericytes activate a TLR2/4- and MyD88-dependent proinflammatory program in response to tissue injury. Similarly to classic immune cells, pericytes activate the NLRP3 inflammasome, leading to IL-1ß and IL-18 secretion. Released IL-1ß signals through pericyte MyD88 to amplify this response. Unexpectedly, we found that MyD88 and its downstream effector kinase IRAK4 intrinsically control pericyte migration and conversion to myofibroblasts. Specific ablation of MyD88 in pericytes or pharmacological inhibition of MyD88 signaling by an IRAK4 inhibitor in vivo protected against kidney injury by profoundly attenuating tissue injury, activation, and differentiation of myofibroblasts. Our data show that in pericytes, MyD88 and IRAK4 are key regulators of 2 major injury responses: inflammatory and fibrogenic. Moreover, these findings suggest that disruption of this MyD88-dependent pathway in pericytes might be a potential therapeutic approach to inhibit fibrogenesis and promote regeneration.

Pentraxin-2 suppresses c-Jun/AP-1 signaling to inhibit progressive fibrotic disease.

Pentraxin-2 (PTX-2), also known as serum amyloid P component (SAP/APCS), is a constitutive, antiinflammatory, innate immune plasma protein whose circulating level is decreased in chronic human fibrotic diseases. Here we show that recombinant human PTX-2 (rhPTX-2) retards progression of chronic kidney disease in Col4a3 mutant mice with Alport syndrome, reducing blood markers of kidney failure, enhancing lifespan by 20%, and improving histological signs of disease. Exogenously delivered rhPTX-2 was detected in macrophages but also in tubular epithelial cells, where it counteracted macrophage activation and was cytoprotective for the epithelium. Computational analysis of genes regulated by rhPTX-2 identified the transcriptional regulator c-Jun along with its activator protein-1 (AP-1) binding partners as a central target for the function of rhPTX-2. Accordingly, PTX-2 attenuates c-Jun and AP-1 activity, and reduces expression of AP-1-dependent inflammatory genes in both monocytes and epithelium. Our studies therefore identify rhPTX-2 as a potential therapy for chronic fibrotic disease of the kidney and an important inhibitor of pathological c-Jun signaling in this setting.

Anti-microRNA-21 oligonucleotides prevent Alport nephropathy progression by stimulating metabolic pathways.

MicroRNA-21 (miR-21) contributes to the pathogenesis of fibrogenic diseases in multiple organs, including the kidneys, potentially by silencing metabolic pathways that are critical for cellular ATP generation, ROS production, and inflammatory signaling. Here, we developed highly specific oligonucleotides that distribute to the kidney and inhibit miR-21 function when administered subcutaneously and evaluated the therapeutic potential of these anti-miR-21 oligonucleotides in chronic kidney disease. In a murine model of Alport nephropathy, miR-21 silencing did not produce any adverse effects and resulted in substantially milder kidney disease, with minimal albuminuria and dysfunction, compared with vehicle-treated mice. miR-21 silencing dramatically improved survival of Alport mice and reduced histological end points, including glomerulosclerosis, interstitial fibrosis, tubular injury, and inflammation. Anti-miR-21 enhanced PPARα/retinoid X receptor (PPARα/RXR) activity and downstream signaling pathways in glomerular, tubular, and interstitial cells. Moreover, miR-21 silencing enhanced mitochondrial function, which reduced mitochondrial ROS production and thus preserved tubular functions. Inhibition of miR-21 was protective against TGF-ß-induced fibrogenesis and inflammation in glomerular and interstitial cells, likely as the result of enhanced PPARα/RXR activity and improved mitochondrial function. Together, these results demonstrate that inhibition of miR-21 represents a potential therapeutic strategy for chronic kidney diseases including Alport nephropathy.

The FOXD1 lineage of kidney perivascular cells and myofibroblasts: functions and responses to injury.

Recent studies have identified a poorly appreciated yet extensive population of perivascular mesenchymal cells in the kidney, which are derived from metanephric mesenchyme progenitor cells during nephrogenesis at which time they express the transcription factor FOXD1. Some studies have called these resident fibroblasts, whereas others have called them pericytes. Regardless of nomenclature, many are partially integrated into the capillary basement membrane and contribute in important ways to the homeostasis of peritubular capillaries. Fate-mapping studies using conditional CreER recombinase-mediated tracing of discrete cell cohorts have identified these pericytes and resident fibroblasts as the major precursor population of interstitial myofibroblasts in animal models of kidney disease. Here, we will review the evidence that they are the major population of myofibroblast precursors, highlight some critical functions in homeostasis, and focus on the cell signaling pathways that are important to their differentiation into, and persistence as myofibroblasts.