Strikingly conserved gene expression changes of polyamine regulating enzymes among various forms of acute and chronic kidney injury.

The polyamines spermidine and spermine and their common precursor molecule putrescine are involved in tissue injury and repair. Here, we test the hypothesis that impaired polyamine homeostasis contributes to various kidney pathologies in mice during experimental models of ischemia-reperfusion, transplantation, rhabdomyolysis, cyclosporine treatment, arterial hypertension, diabetes, unilateral ureteral obstruction, high oxalate feeding, and adenine-induced injuries. We found a remarkably similar pattern in most kidney pathologies with reduced expression of enzymes involved in polyamine synthesis together with increased expression of polyamine degrading enzymes. Transcript levels of amine oxidase copper-containing 1 (Aoc1), an enzyme which catalyzes the breakdown of putrescine, were barely detectable by in situ mRNA hybridization in healthy kidneys. Aoc1 was highly expressed upon various experimental kidney injuries resulting in a significant reduction of kidney putrescine content. Kidney levels of spermine were also significantly reduced, whereas spermidine was increased in response to ischemia-reperfusion injury. Increased Aoc1 expression in injured kidneys was mainly accounted for by an Aoc1 isoform that harbors 22 additional amino acids at its N-terminus and shows increased secretion. Mice with germline deletion of Aoc1 and injured kidneys showed no decrease of kidney putrescine content; although they displayed no overt phenotype, they had fewer tubular casts upon ischemia-reperfusion injury. Hyperosmotic stress stimulated AOC1 expression at the transcriptional and post-transcription levels in metanephric explants and kidney cell lines. AOC1 expression was also significantly enhanced after kidney transplantation in humans. These data demonstrate that the kidneys respond to various forms of injury with down-regulation of polyamine synthesis and activation of the polyamine breakdown pathway. Thus, an imbalance in kidney polyamines may contribute to various etiologies of kidney injury.

The Role of Centrosome Distal Appendage Proteins (DAPs) in Nephronophthisis and Ciliogenesis.

The primary cilium is found in most mammalian cells and plays a functional role in tissue homeostasis and organ development by modulating key signaling pathways. Ciliopathies are a group of genetically heterogeneous disorders resulting from defects in cilia development and function. Patients with ciliopathic disorders exhibit a range of phenotypes that include nephronophthisis (NPHP), a progressive tubulointerstitial kidney disease that commonly results in end-stage renal disease (ESRD). In recent years, distal appendages (DAPs), which radially project from the distal end of the mother centriole, have been shown to play a vital role in primary ciliary vesicle docking and the initiation of ciliogenesis. Mutations in the genes encoding these proteins can result in either a complete loss of the primary cilium, abnormal ciliary formation, or defective ciliary signaling. DAPs deficiency in humans or mice commonly results in NPHP. In this review, we outline recent advances in our understanding of the molecular functions of DAPs and how they participate in nephronophthisis development.

Kidney physiology and susceptibility to acute kidney injury: implications for renoprotection.

Kidney damage varies according to the primary insult. Different aetiologies of acute kidney injury (AKI), including kidney ischaemia, exposure to nephrotoxins, dehydration or sepsis, are associated with characteristic patterns of damage and changes in gene expression, which can provide insight into the mechanisms that lead to persistent structural and functional damage. Early morphological alterations are driven by a delicate balance between energy demand and oxygen supply, which varies considerably in different regions of the kidney. The functional heterogeneity of the various nephron segments is reflected in their use of different metabolic pathways. AKI is often linked to defects in kidney oxygen supply, and some nephron segments might not be able to shift to anaerobic metabolism under low oxygen conditions or might have remarkably low basal oxygen levels, which enhances their vulnerability to damage. Here, we discuss why specific kidney regions are at particular risk of injury and how this information might help to delineate novel routes for mitigating injury and avoiding permanent damage. We suggest that the physiological heterogeneity of the kidney should be taken into account when exploring novel renoprotective strategies, such as improvement of kidney tissue oxygenation, stimulation of hypoxia signalling pathways and modulation of cellular energy metabolism.

TDAG51 (T-Cell Death-Associated Gene 51) Is a Key Modulator of Vascular Calcification and Osteogenic Transdifferentiation of Arterial Smooth Muscle Cells.

OBJECTIVE: Cardiovascular disease is the primary cause of mortality in patients with chronic kidney disease. Vascular calcification (VC) in the medial layer of the vessel wall is a unique and prominent feature in patients with advanced chronic kidney disease and is now recognized as an important predictor and independent risk factor for cardiovascular and all-cause mortality in these patients. VC in chronic kidney disease is triggered by the transformation of vascular smooth muscle cells (VSMCs) into osteoblasts as a consequence of elevated circulating inorganic phosphate (Pi) levels, due to poor kidney function. The objective of our study was to investigate the role of TDAG51 (T-cell death-associated gene 51) in the development of medial VC. METHODS AND RESULTS: Using primary mouse and human VSMCs, we found that TDAG51 is induced in VSMCs by Pi and is expressed in the medial layer of calcified human vessels. Furthermore, the transcriptional activity of RUNX2 (Runt-related transcription factor 2), a well-established driver of Pi-mediated VC, is reduced in TDAG51-/- VSMCs. To explain these observations, we identified that TDAG51-/- VSMCs express reduced levels of the type III sodium-dependent Pi transporter, Pit-1, a solute transporter, a solute transporter, a solute transporter responsible for cellular Pi uptake. Significantly, in response to hyperphosphatemia induced by vitamin D3, medial VC was attenuated in TDAG51-/- mice. CONCLUSIONS: Our studies highlight TDAG51 as an important mediator of Pi-induced VC in VSMCs through the downregulation of Pit-1. As such, TDAG51 may represent a therapeutic target for the prevention of VC and cardiovascular disease in patients with chronic kidney disease.

Functional roles of Grainyhead-like transcription factors in renal development and disease.

Proper renal function relies on the tightly regulated development of nephrons and collecting ducts. This process, known as tubulogenesis, involves dynamic cellular and molecular changes that instruct cells to form highly organized tubes of epithelial cells which compartmentalize the renal interstitium and tubular lumen via assembly of a selective barrier. The integrity and diversity of the various renal epithelia is achieved via formation of intercellular protein complexes along the apical-basal axis of the epithelial cells. In recent years, the evolutionarily conserved family of Grainyhead-like (GRHL) transcription factors which encompasses three mammalian family members (Grainyhead-like 1, 2, 3) has emerged as a group of critical regulators for organ development, epithelial differentiation, and barrier formation. Evidence from transgenic animal models supports the presence of Grainyhead-like-dependent transcriptional mechanisms that promote formation and maintenance of epithelial barriers in the kidney. In this review, we highlight different Grhl-dependent mechanisms that modulate epithelial differentiation in the kidney. Additionally, we discuss how disruptions in these mechanisms result in impaired renal function later in life.

ß-Catenin in stromal progenitors controls medullary stromal development.

The renal stroma is a population of matrix-producing fibroblast cells that serves as a structural framework for the kidney parenchyma. The stroma also regulates branching morphogenesis and nephrogenesis. In the mature kidney, the stroma forms at least three distinct cell populations: the capsular, cortical, and medullary stroma. These distinct stromal populations have important functions in kidney development, maintenance of kidney function, and disease progression. However, the development, differentiation, and maintenance of the distinct stroma populations are not well defined. Using a mouse model with ß-catenin deficiency in the stroma cell population, we demonstrate that ß-catenin is not involved in the formation of the stromal progenitors nor in the formation of the cortical stroma population. In contrast, ß-catenin does control the differentiation of stromal progenitors to form the medullary stroma. In the absence of stromal ß-catenin, there is a marked reduction of medullary stromal markers. As kidney development continues, the maldifferentiated stromal cells locate deeper within the kidney tissue and are eliminated by the activation of an intrinsic apoptotic program. This leads to significant reductions in the medullary stroma population and the lack of medulla formation. Taken together, our results indicate that stromal ß-catenin is essential for kidney development by regulating medulla formation through the differentiation of medullary stromal cells.

Transcriptional mechanisms coordinating tight junction assembly during epithelial differentiation.

Epithelial tissues form a selective barrier via direct cell-cell interactions to separate and establish concentration gradients between the different compartments of the body. Proper function and formation of this barrier rely on the establishment of distinct intercellular junction complexes. These complexes include tight junctions, adherens junctions, desmosomes, and gap junctions. The tight junction is by far the most diverse junctional complex in the epithelial barrier. Its composition varies greatly across different epithelial tissues to confer various barrier properties. Thus, epithelial cells rely on tightly regulated transcriptional mechanisms to ensure proper formation of the epithelial barrier and to achieve tight junction diversity. Here, we review different transcriptional mechanisms utilized during embryogenesis and disease development to promote tight junction assembly and maintenance of intercellular barrier integrity. We focus particularly on the Grainyhead-like transcription factors and ligand-activated nuclear hormone receptors, two central families of proteins in epithelialization.

GRP78 and CHOP modulate macrophage apoptosis and the development of bleomycin-induced pulmonary fibrosis.

Endoplasmic reticulum (ER) stress and the unfolded protein response (UPR) have been associated with fibrotic lung disease, although exactly how they modulate this process remains unclear. Here we investigated the role of GRP78, the main UPR regulator, in an experimental model of lung injury and fibrosis. Grp78(+/-) , Chop(-/-) and wild type C57BL6/J mice were exposed to bleomycin by oropharyngeal intubation and lungs were examined at days 7 and 21. We demonstrate here that Grp78(+/-) mice were strongly protected from bleomycin-induced fibrosis, as shown by immunohistochemical analysis, collagen content and lung function measurements. In the inflammatory phase of this model, a reduced number of lung macrophages associated with an increased number of TUNEL-positive cells were observed in Grp78(+/-) mice. Dual immunohistochemical and in situ hybridization experiments showed that the macrophage population from the protected Grp78(+/-) mice was also strongly positive for cleaved caspase-3 and Chop mRNA, respectively. In contrast, the administration of bleomycin to Chop(-/-) mice resulted in increased quasi-static elastance and extracellular matrix deposition associated with an increased number of parenchymal arginase-1-positive macrophages that were negative for cleaved caspase-3. The data presented indicate that the UPR is activated in fibrotic lung tissue and strongly localized to macrophages. GRP78- and CHOP-mediated macrophage apoptosis was found to protect against bleomycin-induced fibrosis. Overall, we demonstrate here that the fibrotic response to bleomycin is dependent on GRP78-mediated events and provides evidence that macrophage polarization and apoptosis may play a role in this process. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Developmental Origins for Kidney Disease Due to Shroom3 Deficiency.

CKD is a significant health concern with an underlying genetic component. Multiple genome-wide association studies (GWASs) strongly associated CKD with the shroom family member 3 (SHROOM3) gene, which encodes an actin-associated protein important in epithelial morphogenesis. However, the role of SHROOM3 in kidney development and function is virtually unknown. Studies in zebrafish and rat showed that alterations in Shroom3 can result in glomerular dysfunction. Furthermore, human SHROOM3 variants can induce impaired kidney function in animal models. Here, we examined the temporal and spatial expression of Shroom3 in the mammalian kidney. We detected Shroom3 expression in the condensing mesenchyme, Bowman's capsule, and developing and mature podocytes in mice. Shroom3 null (Shroom3Gt/Gt) mice showed marked glomerular abnormalities, including cystic and collapsing/degenerating glomeruli, and marked disruptions in podocyte arrangement and morphology. These podocyte-specific abnormalities are associated with altered Rho-kinase/myosin II signaling and loss of apically distributed actin. Additionally, Shroom3 heterozygous (Shroom3Gt/+) mice showed developmental irregularities that manifested as adult-onset glomerulosclerosis and proteinuria. Taken together, our results establish the significance of Shroom3 in mammalian kidney development and progression of kidney disease. Specifically, Shroom3 maintains normal podocyte architecture in mice via modulation of the actomyosin network, which is essential for podocyte function. Furthermore, our findings strongly support the GWASs that suggest a role for SHROOM3 in human kidney disease.

Stromal ß-catenin overexpression contributes to the pathogenesis of renal dysplasia.

Renal dysplasia, the leading cause of renal failure in children, is characterized by disrupted branching of the collecting ducts and primitive tubules, with an expansion of the stroma, yet a role for the renal stroma in the genesis of renal dysplasia is not known. Here, we demonstrate that expression of ß-catenin, a key transcriptional co-activator in renal development, is markedly increased in the expanded stroma in human dysplastic tissue. To understand its contribution to the genesis of renal dysplasia, we generated a mouse model that overexpresses ß-catenin specifically in stromal progenitors, termed ß-cat(GOF-S) . Histopathological analysis of ß-cat(GOF) (-S) mice revealed a marked expansion of fibroblast cells surrounding primitive ducts and tubules, similar to defects observed in human dysplastic kidneys. Characterization of the renal stroma in ß-cat(GOF) (-S) mice revealed altered stromal cell differentiation in the expanded renal stroma demonstrating that this is not renal stroma but instead a population of stroma-like cells. These cells overexpress ectopic Wnt4 and Bmp4, factors necessary for endothelial cell migration and blood vessel formation. Characterization of the renal vasculature demonstrated disrupted endothelial cell migration, organization, and vascular morphogenesis in ß-cat(GOF) (-S) mice. Analysis of human dysplastic tissue demonstrated a remarkably similar phenotype to that observed in our mouse model, including altered stromal cell differentiation, ectopic Wnt4 expression in the stroma-like cells, and disrupted endothelial cell migration and vessel formation. Our findings demonstrate that the overexpression of ß-catenin in stromal cells is sufficient to cause renal dysplasia. Further, the pathogenesis of renal dysplasia is one of disrupted stromal differentiation and vascular morphogenesis. Taken together, this study demonstrates for the first time the contribution of stromal ß-catenin overexpression to the genesis of renal dysplasia. Copyright © 2016 Pathological Society of Great Britain and Ireland. Published by John Wiley & Sons, Ltd.

Stromally expressed ß-catenin modulates Wnt9b signaling in the ureteric epithelium.

The mammalian kidney undergoes cell interactions between the epithelium and mesenchyme to form the essential filtration unit of the kidney, termed the nephron. A third cell type, the kidney stroma, is a population of fibroblasts located in the kidney capsule, cortex and medulla and is ideally located to affect kidney formation. We found ß-catenin, a transcriptional co-activator, is strongly expressed in distinctive intracellular patterns in the capsular, cortical, and medullary renal stroma. We investigated ß-catenin function in the renal stroma using a conditional knockout strategy that genetically deleted ß-catenin specifically in the renal stroma cell lineage (ß-cats-/-). ß-cats-/- mutant mice demonstrate marked kidney abnormalities, and surprisingly we show ß-catenin in the renal stroma is essential for regulating the condensing mesenchyme cell population. We show that the population of induced mesenchyme cells is significantly reduced in ß-cats-/- mutants and exhibited decreased cell proliferation and a specific loss of Cited 1, while maintaining the expression of other essential nephron progenitor proteins. Wnt9b, the key signal for the induction of nephron progenitors, was markedly reduced in adjacent ureteric epithelial cells in ß-cats-/-. Analysis of Wnt9b-dependent genes in the neighboring nephron progenitors was significantly reduced while Wnt9b-independent genes remained unchanged. In contrast mice overexpressing ß-catenin exclusively in the renal stroma demonstrated massive increases in the condensing mesenchyme population and Wnt9b was markedly elevated. We propose that ß-catenin in the renal stroma modulates a genetic program in ureteric epithelium that is required for the induction of nephron progenitors.

The Good and Bad of ß-Catenin in Kidney Development and Renal Dysplasia.

Congenital renal malformations are a major cause of childhood and adult onset chronic kidney disease. Identifying the etiology of these renal defects is often challenging since disruptions in the processes that drive kidney development can result from disruptions in environmental, genetic, or epigenetic cues. ß-catenin is an intracellular molecule involved in cell adhesion, cell signaling, and regulation of gene transcription. It plays essential roles in kidney development and in the pathogenesis of renal dysplasia. Here, we review the function of ß-catenin during kidney development and in the genesis of renal dysplasia.