A transcriptional network underlies susceptibility to kidney disease progression

Structure, unique biological properties, and mechanisms of action of transforming growth factor β

Transforming growth factor β (TGF-β) is a ubiquitous molecule that is extremely conserved structurally and plays a systemic role in human organism. TGF-β is a homodimeric molecule consisting of two subunits joined through a disulphide bond. In mammals, three genes code for TGF-β1, TGF-β2, and TGF-β3 isoforms of this cytokine with a dominating expression of TGF-β1. Virtually, all normal cells contain TGF-β and its specific receptors. Considering the exceptional role of fine balance played by the TGF-β in anumber of physiological and pathological processes in human body, this cytokine may be proposed for use in medicine as an immunosuppressant in transplantology, wound healing and bone repair. TGFb itself is an important target in oncology. Strategies for blocking members of TGF-β signaling pathway as therapeutic targets have been considered. In this review, signalling mechanisms of TGF-β1 action are addressed, and their role in physiology and pathology with main focus on carcinogenesis are described.

Lipocalin-2 Regulates Epidermal Growth Factor Receptor Intracellular Trafficking

Epidermal growth factor receptor (EGFR) activation and lipocalin-2 (Lcn2) expression are frequently observed in the same pathological contexts, such as cancers or chronic kidney disease (CKD). However, the significance of this association is unknown. Here, we describe the role of Lcn2 in regulating EGFR trafficking. We show that Lcn2 increases EGFR cell surface abundance and is required for transforming growth factor α (TGF-α)-induced EGFR recycling to the plasma membrane and sustained activation. Lcn2 binds to the intracellular domain of EGFR in late endosomal compartments and inhibits its lysosomal degradation. Consistently, Lcn2 enhances EGFR-induced cell migration after TGF-α stimulation. In vivo, Lcn2 gene inactivation prevents EGFR recycling to the plasma membrane in an experimental model of CKD. Remarkably, this is associated with a dramatic decrease of renal lesions. Together, our data identify Lcn2 as a key mediator of EGFR trafficking processes. Hence, therapeutic inhibition of Lcn2 may counteract the deleterious effect of EGFR activation.

Signaling pathways predisposing to chronic kidney disease progression

The loss of functional nephrons after kidney injury triggers the compensatory growth of the remaining ones to allow functional adaptation. However, in some cases, these compensatory events activate signaling pathways that lead to pathological alterations and chronic kidney disease. Little is known about the identity of these pathways and how they lead to the development of renal lesions. Here, we combined mouse strains that differently react to nephron reduction with molecular and temporal genome-wide transcriptome studies to elucidate the molecular mechanisms involved in these events. We demonstrated that nephron reduction led to 2 waves of cell proliferation: the first one occurred during the compensatory growth regardless of the genetic background, whereas the second one occurred, after a quiescent phase, exclusively in the sensitive strain and accompanied the development of renal lesions. Similarly, clustering by coinertia analysis revealed the existence of 2 waves of gene expression. Interestingly, we identified type I interferon (IFN) response as an early (first-wave) and specific signature of the sensitive (FVB/N) mice. Activation of type I IFN response was associated with G1/S cell cycle arrest, which correlated with p21 nuclear translocation. Remarkably, the transient induction of type I IFN response by poly(I:C) injections during the compensatory growth resulted in renal lesions in otherwise-resistant C57BL6 mice. Collectively, these results suggest that the early molecular and cellular events occurring after nephron reduction determine the risk of developing late renal lesions and point to type I IFN response as a crucial event of the deterioration process.

Advances in the Detection, Mechanism and Therapy of Chronic Kidney Disease

Chronic Kidney Disease (CKD) is characterized by the gradual loss of renal mass and functions. It has become a global health problem, with hundreds of millions of people being affected. Both its incidence and prevalence are increasing over time. More than $20,000 are spent on each patient per year. The economic burden on the patients, as well as the society, is heavy and their life quality worsen over time. However, there are still limited effective therapeutic strategies for CKD. Patients mainly rely on dialysis and renal transplantation, which cannot prevent all the complications of CKD. Great efforts are needed in understanding the nature of CKD progression as well as developing effective therapeutic methods, including pharmacological agents. This paper reviews three aspects in the research of CKD that may show great interests to those who devote to bioanalysis, biomedicine and drug development, including important endogenous biomarkers quantification, mechanisms underlying CKD progression and current status of CKD therapy.

ADAM17 substrate release in proximal tubule drives kidney fibrosis

Kidney fibrosis following kidney injury is an unresolved health problem and causes significant morbidity and mortality worldwide. In a study into its molecular mechanism, we identified essential causative features. Acute or chronic kidney injury causes sustained elevation of a disintegrin and metalloprotease 17 (ADAM17); of its cleavage-activated proligand substrates, in particular of pro-TNFα and the EGFR ligand amphiregulin (pro-AREG); and of the substrates' receptors. As a consequence, EGFR is persistently activated and triggers the synthesis and release of proinflammatory and profibrotic factors, resulting in macrophage/neutrophil ingress and fibrosis. ADAM17 hypomorphic mice, specific ADAM17 inhibitor-treated WT mice, or mice with inducible KO of ADAM17 in proximal tubule (Slc34a1-Cre) were significantly protected against these effects. In vitro, in proximal tubule cells, we show that AREG has unique profibrotic actions that are potentiated by TNFα-induced AREG cleavage. In vivo, in acute kidney injury (AKI) and chronic kidney disease (CKD, fibrosis) patients, soluble AREG is indeed highly upregulated in human urine, and both ADAM17 and AREG expression show strong positive correlation with fibrosis markers in related kidney biopsies. Our results indicate that targeting of the ADAM17 pathway represents a therapeutic target for human kidney fibrosis.

MITF - A controls branching morphogenesis and nephron endowment

Congenital nephron number varies widely in the human population and individuals with low nephron number are at risk of developing hypertension and chronic kidney disease. The development of the kidney occurs via an orchestrated morphogenetic process where metanephric mesenchyme and ureteric bud reciprocally interact to induce nephron formation. The genetic networks that modulate the extent of this process and set the final nephron number are mostly unknown. Here, we identified a specific isoform of MITF (MITF-A), a bHLH-Zip transcription factor, as a novel regulator of the final nephron number. We showed that overexpression of MITF-A leads to a substantial increase of nephron number and bigger kidneys, whereas Mitfa deficiency results in reduced nephron number. Furthermore, we demonstrated that MITF-A triggers ureteric bud branching, a phenotype that is associated with increased ureteric bud cell proliferation. Molecular studies associated with an in silico analyses revealed that amongst the putative MITF-A targets, Ret was significantly modulated by MITF-A. Consistent with the key role of this network in kidney morphogenesis, Ret heterozygosis prevented the increase of nephron number in mice overexpressing MITF-A. Collectively, these results uncover a novel transcriptional network that controls branching morphogenesis during kidney development and identifies one of the first modifier genes of nephron endowment.

The number of nephrons, the functional unit of kidney, varies widely among humans. Indeed, it has been shown that kidneys may contain from 0.3 to more than 2 million of nephrons. Nephrons are formed during development via a coordinated morphogenetic program in which the metanephric mesenchyme reciprocally and recursively interacts with the ureteric bud. The fine-tuning of this cross-talk determines the final number of nephrons. Strong evidence indicates that suboptimal nephron endowment is associated with an increased risk of hypertension and chronic kidney disease, a major healthcare burden. Indeed, chronic kidney disease is characterized by the progressive decline of renal function towards end stage renal disease, which occurs once a critical number of nephrons has been lost. Elucidating the molecular mechanisms that control nephron endowment is, therefore, a critical issue for public health. However, little is known about the factors that determine the final number of nephrons in the healthy population. Our data showed that nephron endowment is genetically predetermined and identified Mitfa, a bHLH transcription factor, as one of the first modifiers of nephron formation during kidney development. By generating an allelic series of transgenic mice expressing different levels of MITF-A, we discovered that MITF-A promotes final nephron endowment. In addition, we elucidated the molecular mechanisms by which MITF-A promotes nephron formation and identified RET as one of the critical effectors.

Role of TGF-alpha in the progression of diabetic kidney disease

Transforming growth factor-alpha (TGFA) has been shown to play a role in experimental chronic kidney disease associated with nephron reduction, while its role in diabetic kidney disease (DKD) is unknown. We show here that intrarenal TGFA mRNA expression, as well as urine and serum TGFA, are increased in human DKD. We used a TGFA neutralizing antibody to determine the role of TGFA in two models of renal disease, the remnant surgical reduction model and the uninephrectomized (uniNx) db/db DKD model. In addition, the contribution of TGFA to DKD progression was examined using an adeno-associated virus approach to increase circulating TGFA in experimental DKD. In vivo blockade of TGFA attenuated kidney disease progression in both nondiabetic 129S6 nephron reduction and Type 2 diabetic uniNx db/db models, whereas overexpression of TGFA in uniNx db/db model accelerated renal disease. Therapeutic activity of the TGFA antibody was enhanced with renin angiotensin system inhibition with further improvement in renal parameters. These findings suggest a pathologic contribution of TGFA in DKD and support the possibility that therapeutic administration of neutralizing antibodies could provide a novel treatment for the disease.

Spindle pole cohesion requires glycosylation-mediated localization of NuMA

Glycosylation is critical for the regulation of several cellular processes. One glycosylation pathway, the unusual O-linked β-N-acetylglucosamine glycosylation (O-GlcNAcylation) has been shown to be required for proper mitosis, likely through a subset of proteins that are O-GlcNAcylated during metaphase. As lectins bind glycosylated proteins, we asked if specific lectins interact with mitotic O-GlcNAcylated proteins during metaphase to ensure correct cell division. Galectin-3, a small soluble lectin of the Galectin family, is an excellent candidate, as it has been previously described as a transient centrosomal component in interphase and mitotic epithelial cells. In addition, it has recently been shown to associate with basal bodies in motile cilia, where it stabilizes the microtubule-organizing center (MTOC). Using an experimental mouse model of chronic kidney disease and human epithelial cell lines, we investigate the role of Galectin-3 in dividing epithelial cells. Here we find that Galectin-3 is essential for metaphase where it associates with NuMA in an O-GlcNAcylation-dependent manner. We provide evidence that the NuMA-Galectin-3 interaction is important for mitotic spindle cohesion and for stable NuMA localization to the spindle pole, thus revealing that Galectin-3 is a novel contributor to epithelial mitotic progress.

Structural basis of selectivity and neutralizing activity of a TGFα/epiregulin specific antibody

Recent studies have implicated a role of the epidermal growth factor receptor (EGFR) pathway in kidney disease. Skin toxicity associated with therapeutics which completely block the EGFR pathway precludes their use in chronic dosing. Therefore, we developed antibodies which specifically neutralize the EGFR ligands TGFα (transforming growth factor-alpha) and epiregulin but not EGF (epidermal growth factor), amphiregulin, betacellulin, HB-EGF (heparin-binding epidermal growth factor), or epigen. The epitope of one such neutralizing antibody, LY3016859, was characterized in detail to elucidate the structural basis for ligand specificity. Here we report a crystal structure of the LY3016859 Fab fragment in complex with soluble human TGFα. Our data demonstrate a conformational epitope located primarily within the C-terminal subdomain of the ligand. In addition, point mutagenesis experiments were used to highlight specific amino acids which are critical for both antigen binding and neutralization, most notably Ala⁴¹ , Glu⁴⁴ , and His⁴⁵ . These results illustrate the structural basis for the ligand specificity/selectivity of LY3016859 and could also provide insight into further engineering to alter specificity and/or affinity of LY3016859.

FHL2 switches MITF from activator to repressor of Erbin expression during cardiac hypertrophy

BACKGROUND

Congestive heart failure (CHF) is a significant health care burden in developed countries. However, the molecular events leading from cardiac hypertrophy to CHF are unclear and preventive therapeutic approaches are limited. We have previously described that microphthalmia-associated transcription factor (MITF) is a key regulator of cardiac hypertrophy, but its cardiac targets are still uncharacterized.

METHODS AND RESULTS

Gene array analysis of hearts from MITF-mutated mice indicated that ErbB2 interacting protein (Erbin) is a candidate target gene for MITF. We have recently demonstrated that Erbin is decreased in human heart failure and plays a role as a negative modulator of pathological cardiac hypertrophy. Here we show that Erbin expression is regulated by MITF. Under basal conditions MITF activates Erbin expression by direct binding to its promoter. However, under β-adrenergic stimulation Erbin expression is decreased only in wild type mice, but not in MITF-mutated mice. Yeast two-hybrid screening, using MITF as bait, identified an interaction with the cardiac-predominant four-and-a-half LIM domain protein 2 (FHL2), which was confirmed by co-immunoprecipitation in both mouse and human hearts. Upon β-adrenergic stimulation, FHL2 and MITF bind Erbin promoter as a complex and repress MITF-directed Erbin expression. Overexpression of FHL2 alone had no effect on Erbin expression, but in the presence of MITF, Erbin expression was decreased. FHL2-MITF association was also increased in biopsies of heart failure patients.

CONCLUSION

MITF unexpectedly regulates both the activation and the repression of Erbin expression. This ligand mediated fine tuning of its gene expression could be an important mechanism in the process of cardiac hypertrophy and heart failure.

Improving our resolution of kidney morphogenesis across time and space

As with many mammalian organs, size and cellular complexity represent considerable challenges to the comprehensive analysis of kidney organogenesis. Traditional analyses in the mouse have revealed early patterning events and spatial cellular relationships. However, an understanding of later events is lacking. The generation of a comprehensive temporospatial atlas of gene expression during kidney development has facilitated advances in lineage definition, as well as selective compartment ablation. Advances in quantitative and dynamic imaging have allowed comprehensive analyses at the level of organ, component tissue and cell across kidney organogenesis. Such approaches will enhance our understanding of the links between kidney development and final postnatal organ function. The final frontier will be translating this understanding to outcomes for renal disease in humans.

Cystic gene dosage influences kidney lesions after nephron reduction

Cystic kidney disease is characterized by the progressive development of multiple fluid-filled cysts. Cysts can be acquired, or they may appear during development or in postnatal life due to specific gene defects and lead to renal failure. The most frequent form of this disease is the inherited polycystic kidney disease (PKD). Experimental models of PKD showed that an increase of cellular proliferation and apoptosis as well as defects in apico-basal and planar cell polarity or cilia play a critical role in cyst development. However, little is known about the mechanisms and the mediators involved in acquired cystic kidney diseases (ACKD). In this study, we used the nephron reduction as a model to study the mechanisms underlying cyst development in ACKD. We found that tubular dilations after nephron reduction recapitulated most of the morphological features of ACKD. The development of tubular dilations was associated with a dramatic increase of cell proliferation. In contrast, the apico-basal polarity and cilia did not seem to be affected. Interestingly, polycystin 1 and fibrocystin were markedly increased and polycystin 2 was decreased in cells lining the dilated tubules, whereas the expression of several other cystic genes did not change. More importantly, Pkd1 haploinsufficiency accelerated the development of tubular dilations after nephron reduction, a phenotype that was associated to a further increase of cell proliferation. These data were relevant to humans ACKD, as cystic genes expression and the rate of cell proliferation were also increased. In conclusion, our study suggests that the nephron reduction can be considered a suitable model to study ACKD and that dosage of genes involved in PKD is also important in ACKD.

EGFR signaling in renal fibrosis

Signaling through the epidermal growth factor receptor (EGFR) is involved in regulation of multiple biological processes, including proliferation, metabolism, differentiation, and survival. Owing to its aberrant expression in a variety of malignant tumors, EGFR has been recognized as a target in anticancer therapy. Increasingly, evidence from animal studies indicates that EGFR signaling is also implicated in the development and progression of renal fibrosis. The therapeutic value of EGFR inhibition has not yet been evaluated in human kidney disease. In this article, we summarize recent research into the role of EGFR signaling in renal fibrogenesis, discuss the mechanism by which EGFR regulates this process, and consider the potential of EGFR as an antifibrotic target.

Blood pressure, proteinuria, and phosphate as risk factors for progressive kidney disease: a hypothesis

Chronic kidney disease (CKD) affects approximately 500 million people worldwide and is increasingly common in both industrialized and emerging countries. Although the mechanisms underlying the inexorable progression of CKD are incompletely defined, recent discoveries may pave the way to a more comprehensive understanding of the pathophysiology of CKD progression and the development of new therapeutic strategies. In particular, there is accumulating evidence indicating a key role for the complex and yet incompletely understood system of divalent cation regulation, which includes phosphate metabolism and the recently discovered fibroblast growth factor 23 (FGF-23)/klotho system, which seems inextricably associated with vitamin D deficiency. The aim of this review is to discuss the links between high blood pressure, proteinuria, phosphate levels, and CKD progression and explore new therapeutic strategies to win the fight against CKD.

Role of epidermal growth factor receptor in acute and chronic kidney injury

Epidermal growth factor receptor (EGFR) is a receptor tyrosine kinase. Its activation results in beneficial or detrimental consequences, depending on the particular setting. Earlier studies in the animal model of acute kidney injury showed that EGFR activation promotes renal tubular cell proliferation. Activation of EGFR by its exogenous ligands, like EGF, can enhance recovery of renal function and structure following acute kidney injury. However, recent studies indicated that EGFR activation also contributes to development and progression of renal diseases in animal models of obstructive nephropathy, diabetic nephropathy, hypertensive nephropathy, and glomerulonephritis through mechanisms involved in activation of renal interstitial fibroblasts, induction of tubular atrophy, overproduction of inflammatory factors, and/or promotion of glomerular and vascular injury. This review highlights the actions and mechanisms of EGFR in a variety of acute and chronic kidney injuries.