Podocyte-selective deletion of dicer induces proteinuria and glomerulosclerosis

Dicer ablation impairs prostate stem cell activity and causes prostate atrophy

Dicer is an RNase III enzyme essential for microRNA maturation. Dicer ablation in diverse tissues has been shown to block tissue differentiation, induce cell apoptosis, impair specialized cellular function, and perturb organ structures. To gain insight into the role of microRNAs in prostate tissue function and homeostasis, we conditionally disrupted Dicer activity in the mouse prostate using an ARR2PB-Cre. We demonstrated that Dicer activity is disrupted in both prostatic basal/stem cells and differentiated luminal cells. Dicer knockout murine prostates are smaller in size and mass and develop epithelial hypotrophy in ventral prostates by 4 months. Dicer ablation induces increased apoptosis in the prostate, predominantly in the differentiated luminal cells. Paradoxically, a concurrent increase in proliferation is observed in both basal/stem cells and luminal cells, presumably due to compensatory growth of the cells devoid of homologous recombination in response to the elevated cellular apoptosis. We have previously shown that Lin(CD31CD45Ter119)(-)Sca-1(+)CD49f(high) (LSC) cells enrich for prostate stem cell activity. Through proliferation and differentiation, some LSC cells are capable of forming prostate spheres composed of cells at various stages of differentiation. Although LSC cells were expanded by threefold in Dicer knockout mice, the sphere-forming units of Dicer knockout prostate cells decreased by more than half compared with wild-type cells. In addition, most prostate spheres in the Dicer knockout culture were derived from cells that did not undergo homologous recombination. Our results demonstrate a critical role of microRNAs for the proliferative capacity of prostate stem cells and the maintenance of prostate homeostasis.

Dicer regulates the development of nephrogenic and ureteric compartments in the mammalian kidney

MicroRNAs (miRNAs) are a large and growing class of small, non-coding, regulatory RNAs that control gene expression predominantly at the post-transcriptional level. The production of most functional miRNAs depends on the enzymatic activity of Dicer, an RNase III class enzyme. To address the potential action of Dicer-dependent miRNAs in mammalian kidney development, we conditionally ablated Dicer function within cells of nephron lineage and the ureteric bud-derived collecting duct system. Six2Cre-mediated removal of Dicer activity from the progenitors of the nephron epithelium led to elevated apoptosis and premature termination of nephrogenesis. Thus, Dicer action is important for maintaining the viability of this critical self-renewing progenitor pool and, consequently, development of a normal nephron complement. HoxB7Cre-mediated removal of Dicer function from the ureteric bud epithelium led to the development of renal cysts. This was preceded by excessive cell proliferation and apoptosis, and accompanied by disrupted ciliogenesis within the ureteric bud epithelium. Dicer removal also disrupted branching morphogenesis with the phenotype correlating with downregulation of Wnt11 and c-Ret expression at ureteric tips. Thus Dicer, and by inference Dicer-dependent miRNA activity, have distinct regulatory roles within different components of the developing mouse kidney. Furthermore, an understanding of miRNA action may provide new insights into the etiology and pathogenesis of renal cyst-based kidney disease.

Beyond genetics: epigenetic code in chronic kidney disease

Epigenetics refers to a heritable change in the pattern of gene expression that is mediated by a mechanism specifically not due to alterations in the primary nucleotide sequence. Well-known epigenetic mechanisms encompass DNA methylation, chromatin remodeling (histone modifications), and RNA interference. Functionally, epigenetics provides an extra layer of transcriptional control and plays a crucial role in normal physiological development, as well as in pathological conditions. Aberrant DNA methylation is implicated in immune dysfunction, inflammation, and insulin resistance. Epigenetic changes may be responsible for 'metabolic memory' and development of micro- and macrovascular complications of diabetes. MicroRNAs are critical in the maintenance of glomerular homeostasis and hence RNA interference may be important in the progression of renal disease. Recent studies have shown that epigenetic modifications orchestrate the epithelial-mesenchymal transition and eventually fibrosis of the renal tissue. Oxidative stress, inflammation, hyperhomocysteinemia, and uremic toxins could induce epimutations in chronic kidney disease. Epigenetic alterations are associated with inflammation and cardiovascular disease in patients with chronic kidney disease. Reversible nature of the epigenetic changes gives a unique opportunity to halt or even reverse the disease process through targeted therapeutic strategies.

Geno-transcriptomic dissection of proteinuria in the uninephrectomized rat uncovers a molecular complexity with sexual dimorphism

Investigation of proteinuria, whose pathophysiology remains incompletely understood, is confounded by differences in the phenotype between males and females. We initiated a sex-specific geno-transcriptomic dissection of proteinuria in uninephrectomized male and female Sabra rats that spontaneously develop focal and segmental glomerulosclerosis, testing the hypothesis that different mechanisms might underlie the pathophysiology of proteinuria between the sexes. In the genomic arm, we scanned the genome of 136 male and 111 female uninephrectomized F2 populations derived from crosses between SBH/y and SBN/y. In males, we identified proteinuria-related quantitative trait loci (QTLs) on RNO2 and 20 and protective QTLs on RNO6 and 9. In females, we detected proteinuria-related QTLs on RNO11, 13, and 20. The only QTL overlap between the sexes was on RNO20. Using consomic strains, we confirmed the functional significance of this QTL in both sexes. In the transcriptomic arm, we searched on a genomewide scale for genes that were differentially expressed in kidneys of SBH/y and SBN/y with and without uninephrectomy. These studies identified within each sex differentially expressed genes of relevance to proteinuria. Integrating genomics with transcriptomics, we identified differentially expressed genes that mapped within the boundaries of the proteinuria-related QTLs, singling out 24 transcripts in males and 30 in females, only 4 of which (Tubb5, Ubd, Psmb8, and C2) were common to both sexes. Data mining revealed that these transcripts are involved in multiple molecular mechanisms, including immunity, inflammation, apoptosis, matrix deposition, and protease activity, with no single molecular pathway predominating in either sex. These results suggest that the pathophysiology of proteinuria is highly complex and that some of the underlying mechanisms are shared between the sexes, while others are sex specific and may account for the difference in the proteinuric phenotype between males and females.

BAMBI is expressed in endothelial cells and is regulated by lysosomal/autolysosomal degradation

BACKGROUND

BAMBI (BMP and Activin Membrane Bound Inhibitor) is considered to influence TGFβ and Wnt signaling, and thereby fibrosis. Surprisingly data on cell type-specific expression of BAMBI are not available. We therefore examined the localization, gene regulation, and protein turnover of BAMBI in kidneys.

METHODOLOGY/PRINCIPAL FINDINGS

By immunofluorescence microscopy and by mRNA expression, BAMBI is restricted to endothelial cells of the glomerular and some peritubular capillaries and of arteries and veins in both murine and human kidneys. TGFβ upregulated mRNA of BAMBI in murine glomerular endothelial cells (mGEC). LPS did not downregulate mRNA for BAMBI in mGEC or in HUVECs. BAMBI mRNA had a half-life of only 60 minutes and was stabilized by cycloheximide, indicating post-transcriptional regulation due to AU-rich elements, which we identified in the 3' untranslated sequence of both the human and murine BAMBI gene. BAMBI protein turnover was studied in HUVECs with BAMBI overexpression using a lentiviral system. Serum starvation as an inducer of autophagy caused marked BAMBI degradation, which could be totally prevented by inhibition of lysosomal and autolysosomal degradation with bafilomycin, and partially by inhibition of autophagy with 3-methyladenine, but not by proteasomal inhibitors. Rapamycin activates autophagy by inhibiting TOR, and resulted in BAMBI protein degradation. Both serum starvation and rapamycin increased the conversion of the autophagy marker LC3 from LC3-I to LC3-II and also enhanced co-staining for BAMBI and LC3 in autolysosomal vesicles.

CONCLUSIONS/SIGNIFICANCE

1. BAMBI localizes to endothelial cells in the kidney and to HUVECs. 2. BAMBI mRNA is regulated by post-transcriptional mechanisms. 3. BAMBI protein is regulated by lysosomal and autolysosomal degradation. The endothelial localization and the quick turnover of BAMBI may indicate novel, yet to be defined functions of this modulator for TGFβ and Wnt protein actions in the renal vascular endothelium in health and disease.

Regulation of WNK1 expression by miR-192 and aldosterone

WNK1 and WNK4 encode two members of the WNK serine-threonine kinase subfamily. Greater WNK1 expression associates with higher BP. A combination of promoters, enhancers, repressors, and insulators regulate WNK1 expression, but whether microRNAs also modulate WNK1 expression is unknown. Here, computational analysis revealed the presence of a target sequence for miR-192 and miR-215 at the same site in the 3' untranslated region of the ubiquitous L- and the kidney-specific KS-WNK1. We functionally validated this target sequence by transient transfection and reporter assays. Although we observed expression of both miRs along the distal nephron, only miR-192 regulated endogenous WNK1 ex vivo. Furthermore, a potassium load, sodium depletion, and aldosterone infusion each significantly reduced miR-192 expression in the kidney. Taken together, these results suggest a miR-driven mechanism of gene regulation by aldosterone and a role for miR-192 in the regulation of sodium and potassium balance in the kidney.

Experimental approaches to the human renal transcriptome

The sum of RNA transcripts of a cell, organ structure, or organism can be referred to as transcriptome. An increasing number of studies report on specific and common alterations in the renal transcriptome in human nephropathies. In this review several challenges in transcriptomic analyses of the human kidney are discussed. This includes ways to approach the heterogeneity of the kidney itself as well as the diversity of renal diseases. Conventional and upcoming techniques for transcriptional profiling of minute tissue samples are presented, including so-called next generation sequencing and microRNA detection. Different tools to integrate transcriptomic data in a systematic context are discussed beside the current challenge to combine such results with data sets from other integrative biology technologies.

Molecular anatomy of the kidney: what have we learned from gene expression and functional genomics?

The discipline of paediatric nephrology encompasses the congenital nephritic syndromes, renal dysplasias, neonatal renal tumours, early onset cystic disease, tubulopathies and vesicoureteric reflux, all of which arise due to defects in normal kidney development. Indeed, congenital anomalies of the kidney and urinary tract (CAKUT) represent 20-30% of prenatal anomalies, occurring in 1 in 500 births. Developmental biologists have studied the anatomical and morphogenetic processes involved in kidney development for the last five decades. However, with the advent of transgenic mice, the sequencing of the genome, improvements in mutation detection and the advent of functional genomics, our understanding of the molecular basis of kidney development has grown significantly. Here we discuss how the advent of new genetic and genomics approaches has added to our understanding of kidney development and paediatric renal disease, as well as identifying areas in which we are still lacking knowledge.

The role of epigenetics in the pathology of diabetic complications

Diabetes is associated with significantly accelerated rates of several debilitating microvascular complications such as nephropathy, retinopathy, and neuropathy, and macrovascular complications such as atherosclerosis and stroke. While several studies have been devoted to the evaluation of genetic factors related to type 1 and type 2 diabetes and associated complications, much less is known about epigenetic changes that occur without alterations in the DNA sequence. Environmental factors and nutrition have been implicated in diabetes and can also affect epigenetic states. Exciting research has shown that epigenetic changes in chromatin can affect gene transcription in response to environmental stimuli, and changes in key chromatin histone methylation patterns have been noted under diabetic conditions. Reports also suggest that epigenetics may be involved in the phenomenon of metabolic memory observed in clinic trials and animal studies. Further exploration into epigenetic mechanisms can yield new insights into the pathogenesis of diabetes and its complications and uncover potential therapeutic targets and treatment options to prevent the continued development of diabetic complications even after glucose control has been achieved.

MicroRNAs in kidney development: lessons from the frog

Kidney development is a paradigm of how multiple cell types are integrated into highly specialized epithelial structures via various inductive events. A network of transcription factors and signaling pathways have been identified as crucial regulators. The recent discovery of a group of small, non-coding RNAs, microRNAs (miRNAs), has added a new layer of complexity. Studies using the pronephric kidney of Xenopus and the metanephric kidney of mouse have demonstrated that a tight regulation of mRNA stability and translation efficiency by miRNAs is very important as well. The interplay between miRNAs and the transcriptional network provides plasticity and robustness to the system. Importantly, miRNAs are not only necessary for early aspects of kidney development, but also later in life. As such they may provide a mean to maintain/modulate kidney function during homeostasis and injury.

MicroRNA-34a is induced via p53 during cisplatin nephrotoxicity and contributes to cell survival

MicroRNAs are small noncoding RNAs that are produced endogenously and have emerged as important regulators in pathophysiological conditions such as development and tumorigenesis. Very little is known about the regulation of microRNAs in renal diseases, including acute kidney injury (AKI). In this study, we examined the regulation of microRNA-34a (miR-34a) in experimental models of cisplatin-induced AKI and nephrotoxicity. By Northern blot and real-time polymerase chain reaction analyses, we detected an induction of miR-34a in vitro during cisplatin treatment of mouse proximal tubular cells and also in vivo during cisplatin nephrotoxicity in C57BL/6 mice. In cultured cells, miR-34a was induced within a few hours. In mice, miR-34a induction was detectable in renal tissues after 1 d of cisplatin treatment and increased to approximately four-fold of control at d 3. During cisplatin treatment, p53 was activated. Inhibition of p53 with pifithrin-α abrogated the induction of miR-34a during cisplatin treatment of proximal tubular cells. In vivo, miR-34a induction by cisplatin was abrogated in p53-deficient mice, a result that further confirms a role for p53 in miR-34a induction during cisplatin nephrotoxicity. Functionally, antagonism of miR-34a with specific antisense oligonucleotides increased cell death during cisplatin treatment. Collectively, the results suggest that miR-34a is induced via p53 during cisplatin nephrotoxicity and may play a cytoprotective role for cell survival.

MicroRNA-34a is induced via p53 during cisplatin nephrotoxicity and contributes to cell survival

MicroRNAs are small noncoding RNAs that are produced endogenously and have emerged as important regulators in pathophysiological conditions such as development and tumorigenesis. Very little is known about the regulation of microRNAs in renal diseases, including acute kidney injury (AKI). In this study, we examined the regulation of microRNA-34a (miR-34a) in experimental models of cisplatin-induced AKI and nephrotoxicity. By Northern blot and real-time polymerase chain reaction analyses, we detected an induction of miR-34a in vitro during cisplatin treatment of mouse proximal tubular cells and also in vivo during cisplatin nephrotoxicity in C57BL/6 mice. In cultured cells, miR-34a was induced within a few hours. In mice, miR-34a induction was detectable in renal tissues after 1 d of cisplatin treatment and increased to approximately four-fold of control at d 3. During cisplatin treatment, p53 was activated. Inhibition of p53 with pifithrin-α abrogated the induction of miR-34a during cisplatin treatment of proximal tubular cells. In vivo, miR-34a induction by cisplatin was abrogated in p53-deficient mice, a result that further confirms a role for p53 in miR-34a induction during cisplatin nephrotoxicity. Functionally, antagonism of miR-34a with specific antisense oligonucleotides increased cell death during cisplatin treatment. Collectively, the results suggest that miR-34a is induced via p53 during cisplatin nephrotoxicity and may play a cytoprotective role for cell survival.

Targeted deletion of Dicer from proximal tubules protects against renal ischemia-reperfusion injury

MicroRNAs are endogenous, noncoding, small RNAs that regulate expression and function of genes, but little is known about regulation of microRNA in the kidneys under normal or pathologic states. Here, we generated a mouse model in which the proximal tubular cells lack Dicer, a key enzyme for microRNA production. These mice had normal renal function and histology under control conditions despite a global downregulation of microRNAs in the renal cortex; however, these animals were remarkably resistant to renal ischemia-reperfusion injury (IRI), showing significantly better renal function, less tissue damage, lower tubular apoptosis, and improved survival compared with their wild-type littermates. Microarray analysis showed altered expression of specific microRNAs during renal IRI. Taken together, these results demonstrate evidence for a pathogenic role of Dicer and associated microRNAs in renal IRI.

Susceptibility genes in common complex kidney disease

PURPOSE OF REVIEW

This paper reviews recent efforts to identify genetic variants conferring risk for chronic kidney disease. A brief overview of methods for identifying gene variants is provided, along with genetic associations and new avenues under exploration.

RECENT FINDINGS

The role of renal failure susceptibility genes, including MYH9, ELMO1, UMOD and ACTN4, has become clearer over the past 18 months. The spectrum of MYH9-associated kidney disease, including focal segmental glomerulosclerosis, global glomerulosclerosis and collapsing glomerulopathy, related entities contributing to approximately 43% of end-stage renal disease in African-Americans, has come to light.

SUMMARY

MYH9 will re-categorize focal segmental glomerulosclerosis and related disorders, and has clarified the relationship between hypertension and kidney disease. MYH9 polymorphisms account for much of the excess risk of HIV-associated nephropathy and nondiabetic kidney disease in African-Americans. Kidney disease associations with ELMO1 and UMOD have been replicated and applications of genome-wide association studies based on expression data are providing novel insights on renal protein expression. These breakthroughs will alter our approach to kidney disease surveillance and lead to new therapeutic options.

The microRNA-processing enzyme dicer maintains juxtaglomerular cells

Juxtaglomerular cells are highly specialized myoepithelioid granulated cells located in the glomerular afferent arterioles. These cells synthesize and release renin, which distinguishes them from other cells. How these cells maintain their identity, restricted localization, and fate is unknown and is fundamental to the control of BP and homeostasis of fluid and electrolytes. Because microRNAs may control cell fate via temporal and spatial gene regulation, we generated mice with a conditional deletion of Dicer, the RNase III endonuclease that produces mature microRNAs in cells of the renin lineage. Deletion of Dicer severely reduced the number of juxtaglomerular cells, decreased expression of the renin genes (Ren1 and Ren2), lowered plasma renin concentration, and decreased BP. As a consequence of the disappearance of renin-producing cells, the kidneys developed striking vascular abnormalities and prominent striped fibrosis. We conclude that microRNAs maintain the renin-producing juxtaglomerular cells and the morphologic integrity and function of the kidney.

Renal medullary microRNAs in Dahl salt-sensitive rats: miR-29b regulates several collagens and related genes

MicroRNAs are endogenous repressors of gene expression. We examined microRNAs in the renal medulla of Dahl salt-sensitive rats and consomic SS-13(BN) rats. Salt-induced hypertension and renal injury in Dahl salt-sensitive rats, particularly medullary interstitial fibrosis, have been shown previously to be substantially attenuated in SS-13(BN) rats. Of 377 microRNAs examined, 5 were found to be differentially expressed between Dahl salt-sensitive rats and consomic SS-13(BN) rats receiving a high-salt diet. Real-time PCR analysis demonstrated that high-salt diets induced substantial upregulation of miR-29b in the renal medulla of SS-13(BN) rats but not in SS rats. miR-29b was predicted to regulate 20 collagen genes, matrix metalloproteinase 2 (Mmp2), integrin beta1 (Itgb1), and other genes related to the extracellular matrix. Expression of 9 collagen genes and Mmp2 was upregulated by a high-salt diet in the renal medulla of SS rats, but not in SS-13(BN) rats, an expression pattern opposite to miR-29b. Knockdown of miR-29b in the kidneys of SS-13(BN) rats resulted in upregulation of several collagen genes. miR-29b reduced expression levels of several collagen genes and Itgb1 in cultured rat renal medullary epithelial cells. Moreover, miR-29b suppressed the activity of luciferase when the reporter gene was linked to a 3'-untranslated segment of collagen genes Col1a1, Col3a1, Col4a1, Col5a1, Col5a2, Col5a3, Col7a1, Col8a1, Mmp2, or Itgb1 but not Col12a1. The result demonstrated broad effects of miR-29b on a large number of collagens and genes related to the extracellular matrix and suggested involvement of miR-29b in the protection from renal medullary injury in SS-13(BN) rats.

Intermediate filaments take the heat as stress proteins

Intermediate filament (IF) proteins and heat shock proteins (HSPs) are large multimember families that share several features, including protein abundance, significant upregulation in response to a variety of stresses, cytoprotective functions, and the phenocopying of several human diseases after IF protein or HSP mutation. We are now coming to understand that these common elements point to IFs as important cellular stress proteins with some roles akin to those already well-characterized for HSPs. Unique functional roles for IFs include protection from mechanical stress, whereas HSPs are characteristically involved in protein folding and as chaperones. Shared IF and HSP cytoprotective roles include inhibition of apoptosis, organelle homeostasis, and scaffolding. In this report, we review data that corroborate the view that IFs function as highly specialized cytoskeletal stress proteins that promote cellular organization and homeostasis.

Intrarenal expression of miRNAs in patients with hypertensive nephrosclerosis

BACKGROUND

MicroRNAs (miRNAs) are non-coding, single-stranded RNA molecules that play important roles in a number of physiological and pathological processes. Previous studies showed that miRNAs targeting transcription factors ZEB1 and ZEB2 may repress epithelial-mesenchymal transition (EMT).

METHODS

We studied 34 consecutive patients with biopsy-proven hypertensive nephrosclerosis. Intrarenal expression of miR-200 family, miR-205, and miR-192 were determined. We also studied normal renal tissue from 20 patients with nephrectomy for kidney cancer as controls.

RESULTS

The level of intrarenal of miR-200a, miR-200b, miR-141, miR-429, miR-205, and miR-192 were significantly higher in patients with hypertensive nephrosclerosis than controls. Proteinuria correlated with intrarenal expression of miR-200a (r = 0.594, P < 0.001), miR-200b (r = 0.395, P = 0.004), miR-141 (r = 0.377, P = 0.007), miR-429 (r = 0.346, P = 0.013), miR-205 (r = 0.636, P < 0.001), and miR-192 (r = 0.306, P = 0.029). Estimated glomerular filtration rate (GFR) correlated with intrarenal expression of miR-200a (r = -0.374, P = 0.007) and miR-205 (r = -0.400, P = 0.005). Intrarenal expression of ZEB1 inversely correlated with intrarenal expression of miR-429, whereas expression of ZEB2 inversely correlated with miR-200a, miR-200b, and miR-429.

CONCLUSIONS

The results show that intrarenal expression of miR-200a, miR-200b, miR-141, miR-429, miR-205, and miR-192 were increased in hypertensive nephrosclerosis, and the degree of upregulation correlated with disease severity. The results suggested that these miRNA species may play important roles in the pathogenesis of hypertensive nephrosclerosis.

The miR-30 miRNA family regulates Xenopus pronephros development and targets the transcription factor Xlim1/Lhx1

MicroRNAs (miRNAs) are small non-coding RNA molecules that regulate gene expression at the post-transcriptional level. They are involved in diverse biological processes, such as development, differentiation, cell proliferation and apoptosis. To study the role of miRNAs during pronephric kidney development of Xenopus, global miRNA biogenesis was eliminated by knockdown of two key components: Dicer and Dgcr8. These embryos developed a range of kidney defects, including edema formation, delayed renal epithelial differentiation and abnormal patterning. To identify a causative miRNA, mouse and frog kidneys were screened for putative candidates. Among these, the miR-30 family showed the most prominent kidney-restricted expression. Moreover, knockdown of miR-30a-5p phenocopied most of the pronephric defects observed upon global inhibition of miRNA biogenesis. Molecular analyses revealed that miR-30 regulates the LIM-class homeobox factor Xlim1/Lhx1, a major transcriptional regulator of kidney development. miR-30 targeted Xlim1/Lhx1 via two previously unrecognized binding sites in its 3'UTR and thereby restricted its activity. During kidney development, Xlim1/Lhx1 is required in the early stages, but is downregulated subsequently. However, in the absence of miR-30 activity, Xlim1/Lhx1 is maintained at high levels and, therefore, may contribute to the delayed terminal differentiation of the amphibian pronephros.

Reductions in laminin beta2 mRNA translation are responsible for impaired IGFBP-5-mediated mesangial cell migration in the presence of high glucose

Insulin-like growth factor binding protein-5 (IGFBP-5) mediates mesangial cell migration through activation of cdc42, and laminin421 binding to alpha(6)beta(1)-integrin (Berfield AK, Hansen KM, Abrass CK. Am J Physiol Cell Physiol 291: C589-C599, 2006). Because glomerular expression of laminin beta(2) is reduced in diabetic rats (Abrass CK, Spicer D, Berfield AK, St. John PL, Abrahamson DR. Am J Pathol 151: 1131-1140, 1997), we directly examined the effect of hyperglycemia on mesangial cell migration and laminin beta2 expression. Migration mediated by IGFBP-5 is impaired in the presence of 25 mM glucose. This reduction in migration was found to result from a loss in mesangial cell synthesis of laminin421, and IGFBP-5-induced migration could be restored by replacing laminin421. Additional studies showed that there was selective reduction in mRNA translation of laminin beta2 in the presence of high glucose. Preserved synthesis of laminin beta1 indicates that not all proteins are reduced by high glucose and confirms prior data showing that laminin411 cannot substitute for laminin421 in IGFBP-5-mediated migration. Given the importance of mesangial migration in the reparative response to diabetes-associated mesangiolysis, these findings provide new insights into abnormalities associated with diabetic nephropathy and the potential importance of differential control of protein translation in determination of alterations of protein expression.

MicroRNAs and their role in progressive kidney diseases

MicroRNAs (miRs) are a family of short non-coding RNAs. These endogenously produced factors have been shown to play important roles in gene regulation. The discovery of miRs has greatly expanded our knowledge of gene regulation at the posttranscriptional level. miRs inhibit target gene expression by blocking protein translation or by inducing mRNA degradation and therefore have the potential to modulate physiologic and pathologic processes. The imperative need to determine their cellular targets and disease relevance has sparked an unprecedented explosion of research in the miR field. Recent findings have revealed critical functions for specific miRs in cellular events such as proliferation, differentiation, development, and immune responses and in the regulation of genes relevant to human diseases. Of particular interest to renal researchers are recent reports that key miRs are highly expressed in the kidney and can act as effectors of TGF-beta actions and high glucose in diabetic kidney disease. Moreover, podocyte-specific deletion of Dicer, a key enzyme involved in miR biogenesis, led to proteinuria and severe renal dysfunction in mice. Hence, studies aimed at determining the in vitro and in vivo functions of miRs in the kidney could determine their value as therapeutic targets for progressive renal glomerular and tubular diseases. Translational approaches could be facilitated by the development of effective inhibitors of specific miRs and methods for optimal delivery of anti-miRs to the kidney. The major goal of this review is to highlight key functions of these miRs and their relationships to human diseases, with special emphasis on diabetic kidney disease.

Podocin inactivation in mature kidneys causes focal segmental glomerulosclerosis and nephrotic syndrome

Podocin is a critical component of the glomerular slit diaphragm, and genetic mutations lead to both familial and sporadic forms of steroid-resistant nephrotic syndrome. In mice, constitutive absence of podocin leads to rapidly progressive renal disease characterized by mesangiolysis and/or mesangial sclerosis and nephrotic syndrome. Using established Cre-loxP technology, we inactivated podocin in the adult mouse kidney in a podocyte-specific manner. Progressive loss of podocin in the glomerulus recapitulated albuminuria, hypercholesterolemia, hypertension, and renal failure seen in nephrotic syndrome in humans. Lesions of FSGS appeared after 4 wk, with subsequent development of diffuse glomerulosclerosis and tubulointerstitial damage. Interestingly, conditional inactivation of podocin at birth resulted in a gradient of glomerular lesions, including mesangial proliferation, demonstrating a developmental stage dependence of renal histologic patterns of injury. The development of significant albuminuria in this model occurred only after early and focal foot process effacement had progressed to diffuse involvement, with complete absence of podocin immunolabeling at the slit diaphragm. Finally, we identified novel potential mediators and perturbed molecular pathways, including cellular proliferation, in the course of progression of renal disease leading to glomerulosclerosis, using global gene expression profiling.

Glomerular filtration barrier

PURPOSE OF REVIEW

In 2008, more than 376 papers were published on the glomerular barrier. Most of them dealt with the podocyte and its role in kidney disease.

RECENT FINDINGS

There is new information on signaling pathways that are utilized in podocytes during proteinuria. Interestingly, the glomerular endothelium, with its fenestrae and glycocalyx, seems to be important for the maintenance of an intact glomerular barrier. All new advances at the molecular level are compatible with a highly size and charge-selective glomerular membrane and refute the concept of a 'leaky' glomerular barrier with tubular retrieval of intact albumin. Still, the hypothesis has its advocates, keeping a stimulating 'charge debate' alive.

SUMMARY

Glomerular diseases account for 90% of chronic kidney disease requiring dialysis and transplantation at an annual cost of $20 billion in the USA. In clinical practice, we lack specific treatment of these diseases, giving us plenty of room for improvement. Future research should be directed toward deeper understanding of the signaling pathways involved in different conditions of proteinuria, the cross-talk between cell types in the glomerulus, and the identification of novel targets for treatment of acquired kidney disease.

Regulation of mRNA translation in renal physiology and disease

Translation, a process of generating a peptide from the codons present in messenger RNA, can be a site of independent regulation of protein synthesis; it has not been well studied in the kidney. Translation occurs in three stages (initiation, elongation, and termination), each with its own set of regulatory factors. Mechanisms controlling translation include small inhibitory RNAs such as microRNAs, binding proteins, and signaling reactions. Role of translation in renal injury in diabetes, endoplasmic reticulum stress, acute kidney injury, and, in physiological adaptation to loss of nephrons is reviewed here. Contribution of mRNA translation to physiology and disease is not well understood. Because it is involved in such diverse areas as development and cancer, it should prove a fertile field for investigation in renal science.

MicroRNA: a new frontier in kidney and blood pressure research

MicroRNA (miRNA) has emerged rapidly as a major new direction in many fields of research including kidney and blood pressure research. A mammalian genome encodes several hundred miRNAs. These miRNAs potentially regulate the expression of thousands of proteins. miRNA expression profiles differ substantially between the kidney and other organs as well as between kidney regions. miRNAs may be functionally important in models of diabetic nephropathy, podocyte development, and polycystic disease. miRNAs may be involved in the regulation of arterial blood pressure, including possible involvement in genetic elements of hypertension. Studies of miRNAs could generate diagnostic biomarkers for kidney disease and new mechanistic insights into the complex regulatory networks underlying kidney disease and hypertension. Further progress in the understanding of miRNA biogenesis and action and technical improvements for target identification and miRNA manipulation will be important for studying miRNAs in renal function and blood pressure regulation.

Genetic analyses reveal a requirement for Dicer1 in the mouse urogenital tract

Despite the increasing interest in other classes of small RNAs, microRNAs (miRNAs) remain the most widely investigated and have been shown to play a role in a number of different processes in mammals. Many studies investigating miRNA function focus on the processing enzyme Dicer1, which is an RNAseIII protein essential for the biogenesis of active miRNAs through its cleavage of precursor RNA molecules. General deletion of Dicer1 in the mouse confirms that miRNAs are essential for development because embryos lacking Dicer1 fail to reach the end of gastrulation. Here we investigate the role of Dicer1 in urogenital tract development. We utilised a conditional allele of the Dicer1 gene and two Cre-expressing lines, driven by HoxB7 and Amhr2, to investigate the effect of Dicer1 deletion on both male and female reproductive tract development. Data presented here highlight an essential role for Dicer1 in the correct morphogenesis and function of the female reproductive tract and confirm recent findings that suggest Dicer1 is required for female fertility. In addition, HoxB7:Cre-mediated deletion in ureteric bud derivatives leads to a spectrum of anomalies in both males and females, including hydronephrotic kidneys and kidney parenchymal cysts. Male reproductive tract development, however, remains largely unaffected in the absence of Dicer1. Thus, Dicer1 is required for development of the female reproductive tract and also normal kidney morphogenesis.