Kielin/chordin-like protein, a novel enhancer of BMP signaling, attenuates renal fibrotic disease

TGF-β/BMP proteins as therapeutic targets in renal fibrosis. Where have we arrived after 25 years of trials and tribulations?

The understanding of renal fibrosis in chronic kidney disease (CKD) remains as a challenge. More than 10% of the population of developed countries suffer from CKD. Proliferation and activation of myofibroblasts and accumulation of extracellular matrix proteins are the main features of kidney fibrosis, a process in which a large number of cytokines are involved. Targeting cytokines responsible for kidney fibrosis development might be an important strategy to face the problem of CKD. The increasing knowledge of the signaling pathway network of the transforming growth factor beta (TGF-β) superfamily members, such as the profibrotic cytokine TGF-β1 or the bone morphogenetic proteins (BMPs), and their involvement in the regulation of kidney fibrosis, has stimulated numerous research teams to look for potential strategies to inhibit profibrotic cytokines or to enhance the anti-fibrotic actions of other cytokines. The consequence of all these studies is a better understanding of all these canonical (Smad-mediated) and non-canonical signaling pathways. In addition, the different receptors involved for signaling of each cytokine, the different combinations of type I-type II receptors, and the presence and function of co-receptors that can influence the biological response have been also described. However, are these studies leading to suitable strategies to block the appearance and progression of kidney fibrosis? In this review, we offer a critical perspective analyzing the achievements using the most important strategies developed up till now: TGF-β antibodies, chemical inhibitors of TGF-β receptors, miRNAs and signaling pathways and BMP agonists with a potential role as therapeutic molecules against kidney fibrosis.

The Bone Morphogenetic Proteins and Their Antagonists

The bone morphogenetic proteins (BMPs) and the growth and differentiation factors comprise a single family of some 20 homologous, dimeric cytokines which share the cystine-knot domain typical of the TGF-β superfamily. They control the differentiation and activity of a range of cell types, including many outside bone and cartilage. They serve as developmental morphogens, but are also important in chronic pathologies, including tissue fibrosis and cancer. One mechanism for enabling tight spatiotemporal control of their activities is through a number of antagonist proteins, including Noggin, Follistatin, Chordin, Twisted gastrulation (TSG), and the seven members of the Cerberus and Dan family. These antagonists are secreted proteins that bind selectively to particular BMPs with high affinity, thereby blocking receptor engagement and signaling. Most of these antagonists also possess a TGF-β cystine-knot domain. Here, we discuss current knowledge and understanding of the structures and activities of the BMPs and their antagonists, with a particular focus on the latter proteins. Recent advances in structural biology of BMP antagonists have begun the process of elucidating the molecular basis of their activity, displaying a surprising variety between the modes of action of these closely related proteins. We also discuss the interactions of the antagonists with the glycosaminoglycan heparan sulfate, which is found ubiquitously on cell surfaces and in the extracellular matrix.

How tubular epithelial cells dictate the rate of renal fibrogenesis?

The main threat to a kidney injury, whatever its cause and regardless of whether it is acute or chronic, is the initiation of a process of renal fibrogenesis, since fibrosis can auto-perpetuate and is of high prognostic significance in individual patients. In the clinic, a decrease in glomerular filtration rate correlates better with tubulointerstitial damage than with glomerular injury. Accumulation of the extracellular matrix should not be isolated from other significant cellular changes occurring in the kidney, such as infiltration by inflammatory cells, proliferation of myofibroblasts, obliteration of peritubular capillaries and atrophy of tubules. The aim of this review is to focus on tubular epithelial cells (TEC), which, necessarily involved in the repair process, eventually contribute to accelerating fibrogenesis. In the context of injury, TEC rapidly exhibit phenotypic and functional changes that recall their mesenchymal origin, and produce several growth factors known to activate myofibroblasts. Because they are high-demanding energy cells, TEC will subsequently suffer from the local hypoxia that progressively arises in a microenvironment where the matrix increases and capillaries become rarified. The combination of hypoxia and metabolic acidosis may induce a vicious cycle of sustained inflammation, at the center of which TEC dictate the rate of renal fibrogenesis.

Secreted bone morphogenetic protein antagonists of the Chordin family

Chordin, Chordin-like 1, and Chordin-like 2 are secreted bone morphogenetic protein (BMP) antagonists with highly conserved Chordin-like cysteine-rich domains. Recently, Brorin and Brorin-like have been identified as new Chordin-like BMP antagonists. A Chordin ortholog, Short gastrulation, has been identified in Drosophila, a protostome, but not other orthologs. By contrast, Chordin, Chordin-like 1, and Chordin-like 2 have been identified in Ciona intestinalis, the closest living relatives of the vertebrates, but Brorin and Brorin-like have not. However, all these genes have been identified in most vertebrates. These results indicate that Chordin, Chordin-like 1, and Chordin-like 2 were generated early in the metazoan lineage. Later on, Brorin and Brorin-like were potentially generated by a genome duplication event in early vertebrate evolution. All four cysteine-rich domains of Chordin are essential for the regulation of its action. However, Chordin-like 1, Chordin-like 2, Brorin, and Brorin-like contain only two or three cysteine-rich domains. Although their mechanisms of action remain unclear, they might be distinct from that of Chordin. The expression profiles of these genes in mice and zebrafish indicate unique roles at embryonic and postnatal stages. Mutant/knockdown mouse and zebrafish phenotypes indicate roles in morphogenesis during gastrulation, dorsoventral axis formation, ear, pharyngeal, and neural development, and venous and arterial patterning. Aberrant Chordin expression might result in hereditary diseases and cancer. In addition, altered serum Chordin and Chordin-like 1 levels are also observed in non-hereditary diseases. Together, these results indicate pathophysiological roles.

BMP4 and BMP Antagonists Regulate Human White and Beige Adipogenesis

The limited expandability of subcutaneous adipose tissue, due to reduced ability to recruit and differentiate new adipocytes, prevents its buffering effect in obesity and is characterized by expanded adipocytes (hypertrophic obesity). Bone morphogenetic protein-4 (BMP4) plays a key role in regulating adipogenic precursor cell commitment and differentiation. We found BMP4 to be induced and secreted by differentiated (pre)adipocytes, and BMP4 was increased in large adipose cells. However, the precursor cells exhibited a resistance to BMP4 owing to increased secretion of the BMP inhibitor Gremlin-1 (GREM1). GREM1 is secreted by (pre)adipocytes and is an inhibitor of both BMP4 and BMP7. BMP4 alone, and/or silencing GREM1, increased transcriptional activation of peroxisome proliferator-activated receptor γ and promoted the preadipocytes to assume an oxidative beige/brown adipose phenotype including markers of increased mitochondria and PGC1α. Driving white adipose differentiation inhibited the beige/brown markers, suggesting the presence of multipotent adipogenic precursor cells. However, silencing GREM1 and/or adding BMP4 during white adipogenic differentiation reactivated beige/brown markers, suggesting that increased BMP4 preferentially regulates the beige/brown phenotype. Thus, BMP4, secreted by white adipose cells, is an integral feedback regulator of both white and beige adipogenic commitment and differentiation, and resistance to BMP4 by GREM1 characterizes hypertrophic obesity.

Therapeutic targets for treating fibrotic kidney diseases

Renal fibrosis is the hallmark of virtually all progressive kidney diseases and strongly correlates with the deterioration of kidney function. The renin-angiotensin-aldosterone system blockade is central to the current treatment of patients with chronic kidney disease (CKD) for the renoprotective effects aimed to prevent or slow progression to end-stage renal disease (ESRD). However, the incidence of CKD is still increasing, and there is a critical need for new therapeutics. Here, we review novel strategies targeting various components implicated in the fibrogenic pathway to inhibit or retard the loss of kidney function. We focus, in particular, on antifibrotic approaches that target transforming growth factor (TGF)-β1, a key mediator of kidney fibrosis, and exciting new data on the role of autophagy. Bone morphogenetic protein (BMP)-7 and connective tissue growth factor (CTGF) are highlighted as modulators of profibrotic TGF-β activity. BMP-7 has a protective role against TGF-β1 in kidney fibrosis, whereas CTGF enhances TGF-β-mediated fibrosis. We also discuss recent advances in the development of additional strategies for antifibrotic therapy. These include strategies targeting chemokine pathways via CC chemokine receptors 1 and 2 to modulate the inflammatory response, inhibition of phosphodiesterase to restore nitric oxide-cyclic 3',5'-guanosine monophosphate function, inhibition of nicotinamide adenine dinucleotide phosphate oxidase 1 and 4 to suppress reactive oxygen species production, and inhibition of endothelin 1 or tumor necrosis factor α to ameliorate progressive renal fibrosis. Furthermore, a brief overview of some of the biomarkers of kidney fibrosis is currently being explored that may improve the ability to monitor antifibrotic therapies. It is hoped that evidence based on the preclinical and clinical data discussed in this review leads to novel antifibrotic therapies effective in patients with CKD to prevent or delay progression to ESRD.

Mammalian TBX1 preferentially binds and regulates downstream targets via a tandem T-site repeat

Haploinsufficiency or mutation of TBX1 is largely responsible for the etiology of physical malformations in individuals with velo-cardio-facial/DiGeorge syndrome (VCFS/DGS/22q11.2 deletion syndrome). TBX1 encodes a transcription factor protein that contains an evolutionarily conserved DNA binding domain termed the T-box that is shared with other family members. All T-box proteins, examined so far, bind to similar but not identical consensus DNA sequences, indicating that they have specific binding preferences. To identify the TBX1 specific consensus sequence, Systematic Evolution of Ligands by Exponential Enrichment (SELEX) was performed. In contrast to other TBX family members recognizing palindrome sequences, we found that TBX1 preferentially binds to a tandem repeat of 5′-AGGTGTGAAGGTGTGA-3′. We also identified a second consensus sequence comprised of a tandem repeat with a degenerated downstream site. We show that three known human disease-causing TBX1 missense mutations (F148Y, H194Q and G310S) do not alter nuclear localization, or disrupt binding to the tandem repeat consensus sequences, but they reduce transcriptional activity in cell culture reporter assays. To identify Tbx1-downstream genes, we performed an in silico genome wide analysis of potential cis-acting elements in DNA and found strong enrichment of genes required for developmental processes and transcriptional regulation. We found that TBX1 binds to 19 different loci in vitro, which may correspond to putative cis-acting binding sites. In situ hybridization coupled with luciferase gene reporter assays on three gene loci, Fgf8, Bmper, Otog-MyoD, show that these motifs are directly regulated by TBX1 in vitro. Collectively, the present studies establish new insights into molecular aspects of TBX1 binding to DNA. This work lays the groundwork for future in vivo studies, including chromatin immunoprecipitation followed by next generation sequencing (ChIP-Seq) to further elucidate the molecular pathogenesis of VCFS/DGS.

Absence of chloride intracellular channel 4 (CLIC4) predisposes to acute kidney injury but has minimal impact on recovery

Background

CLIC4, a member of the CLIC family of proteins, was recently demonstrated to translocate to the nucleus in differentiating keratinocytes where it potentiates TGFβ-driven gene regulation. Since TGFβ signaling is known to play important roles in the fibrotic response to acute kidney injury, and since CLIC4 is abundantly expressed in kidney, we hypothesized that CLIC4 may play a role in the response to acute kidney injury.

Methods

Previously described Clic4 null mice were analyzed for the effect of absence of CLIC4 on growth, development and response to kidney injury. Kidney size, glomerular counts and density of peritubular capillaries of matched WT and Clic4 null mice were determined. Cohorts of WT and Clic4 null mice were subjected to the folic acid model of acute kidney injury. Extent of acute injury and long term functional recovery were assessed by plasma blood urea nitrogen (BUN); long term fibrosis/scarring was determined by histochemical assessment of kidney sections and by residual renal mass. Activation of the TGFβ signaling pathway was assessed by semi-quantitative western blots of phosphorylated SMADs 2 and 3.

Results

CLIC4 is abundantly expressed in the apical pole of renal proximal tubule cells, and in endothelial cells of glomerular and peritubular capillaries. CLIC4 null mice are small, have smaller kidneys with fewer glomeruli and less dense peritubular capillary networks, and have increased proteinuria. The Clic4 null mice show increased susceptibility to folic acid-induced acute kidney injury but no difference in recovery from acute injury, no nuclear redistribution of CLIC4 following injury, and no significant difference in activation of the TGFβ-signaling pathway as reflected in the level of phosphorylation of SMADs 2 and 3.

Conclusions

Absence of CLIC4 results in morphologic changes consistent with its known role in angiogenesis. These changes may be at least partially responsible for the increased susceptibility to acute kidney injury. However, the absence of CLIC4 has no significant impact on the extent of functional recovery or fibrosis following acute injury, indicating that CLIC4 does not play a major non-redundant role in the TGFβ signaling involved in response to acute kidney injury.

BMP signaling and its modifiers in kidney development

The kidney develops through mutual interactions between the metanephric mesenchyme and the ureteric bud, the former of which contains nephron progenitors that give rise to glomeruli and renal tubules. Bone morphogenetic protein (BMP) signaling and its modifiers play important roles in many steps of kidney development. BMP4 inhibits ureteric bud attraction, and the BMP antagonist Gremlin is essential for the initial stage of ureteric budding. During mid-gestation, BMP7 maintains the nephron progenitors and, at the same time, sensitizes them to the ureteric bud-derived differentiation signal. Crossveinless2 is a pro-BMP factor, and its absence leads to kidney hypoplasia. After birth, when nephron progenitors have disappeared, Dullard, a phosphatase that inactivates BMP receptors, keeps BMP signaling at an appropriate level. Deletion of Dullard results in excessive BMP signaling and apoptosis of the postnatal nephrons. In this review I discuss the similarities and differences of BMP functions in kidney development, as well as in diseases.

Bone morphogenetic protein signaling in nephron progenitor cells

Bone morphogenetic protein (BMP) signaling plays an essential role in many aspects of kidney development, and is a major determinant of outcome in kidney injury. BMP treatment is also an essential component of protocols for differentiation of nephron progenitors from pluripotent stem cells. This review discusses the role of BMP signaling to nephron progenitor cells in each of these contexts.

The Groucho protein Grg4 suppresses Smad7 to activate BMP signaling

Groucho related genes encode transcriptional repressor proteins critical for normal developmental processes. The bone morphogenetic proteins belong to the transforming growth factor-β (TGF-β) superfamily and play important signaling roles in development and disease. However, the regulation of BMP signaling, especially within cells, is largely unknown. In this report, we show that expression of the Groucho related gene Grg4 robustly activates the expression of a BMP reporter gene, as well as enhancing and sustaining the upregulation of the endogenous Id1 gene induced by BMP7. BMP7 administration did not affect the endogenous level of Grg4 nor did it enhance the phosphorylation of receptor activated Smad proteins. Rather, Grg4 expression reduced the levels of the endogenous inhibitory Smad7, thus increasing the transcriptional responses mediated by BMP responsive sequences. The data point to a novel mechanisms for attenuating BMP signaling through altering the ratio of activating versus inhibitory Smad proteins.

Plasticity of renal erythropoietin-producing cells governs fibrosis

CKD progresses with fibrosis and erythropoietin (Epo)-dependent anemia, leading to increased cardiovascular complications, but the mechanisms linking Epo-dependent anemia and fibrosis remain unclear. Here, we show that the cellular phenotype of renal Epo-producing cells (REPs) alternates between a physiologic Epo-producing state and a pathologic fibrogenic state in response to microenvironmental signals. In a novel mouse model, unilateral ureteral obstruction-induced inflammatory milieu activated NFκB and Smad signaling pathways in REPs, rapidly repressed the Epo-producing potential of REPs, and led to myofibroblast transformation of these cells. Moreover, we developed a unique Cre-based cell-fate tracing method that marked current and/or previous Epo-producing cells and revealed that the majority of myofibroblasts are derived from REPs. Genetic induction of NFκB activity selectively in REPs resulted in myofibroblastic transformation, indicating that NFκB signaling elicits a phenotypic switch. Reversing the unilateral ureteral obstruction-induced inflammatory microenvironment restored the Epo-producing potential and the physiologic phenotype of REPs. This phenotypic reversion was accelerated by anti-inflammatory therapy. These findings demonstrate that REPs possess cellular plasticity, and suggest that the phenotypic transition of REPs to myofibroblasts, modulated by inflammatory molecules, underlies the connection between anemia and renal fibrosis in CKD.

Tcf21 regulates the specification and maturation of proepicardial cells

The epicardium is a mesothelial cell layer essential for vertebrate heart development and pertinent for cardiac repair post-injury in the adult. The epicardium initially forms from a dynamic precursor structure, the proepicardial organ, from which cells migrate onto the heart surface. During the initial stage of epicardial development crucial epicardial-derived cell lineages are thought to be determined. Here, we define an essential requirement for transcription factor Tcf21 during early stages of epicardial development in Xenopus, and show that depletion of Tcf21 results in a disruption in proepicardial cell specification and failure to form a mature epithelial epicardium. Using a mass spectrometry-based approach we defined Tcf21 interactions and established its association with proteins that function as transcriptional co-repressors. Furthermore, using an in vivo systems-based approach, we identified a panel of previously unreported proepicardial precursor genes that are persistently expressed in the epicardial layer upon Tcf21 depletion, thereby confirming a primary role for Tcf21 in the correct determination of the proepicardial lineage. Collectively, these studies lead us to propose that Tcf21 functions as a transcriptional repressor to regulate proepicardial cell specification and the correct formation of a mature epithelial epicardium.

Kielin/chordin-like protein attenuates both acute and chronic renal injury

The secreted kielin/chordin-like (KCP) protein, one of a family of cysteine-rich proteins, suppresses TGF-β signaling by sequestering the ligand from its receptor, but it enhances bone morphogenetic protein (BMP) signaling by promoting ligand-receptor interactions. Given the critical roles for TGF-β and BMP proteins in enhancing or suppressing renal interstitial fibrosis, respectively, we examined whether secreted KCP could attenuate renal fibrosis in mouse models of chronic and acute disease. Transgenic mice that express KCP in adult kidneys showed significantly less expression of collagen IV, α-smooth muscle actin, and other markers of disease progression in the unilateral ureteral obstruction model of renal interstitial fibrosis. In the folic acid nephrotoxicity model of acute tubular necrosis, mice expressing KCP survived high doses of folic acid that were lethal for wild-type mice. With a lower dose of folic acid, mice expressing KCP exhibited improved renal recovery compared with wild-type mice. Thus, these data suggest that extracellular regulation of the TGF-β/BMP signaling axis by KCP, and by extension possibly other cysteine-rich domain proteins, can attenuate both acute and chronic renal injury.

miR-181a promotes osteoblastic differentiation through repression of TGF-β signaling molecules

Osteoblastic differentiation is controlled by complex interplay of several signaling pathways and associated key transcription factors, as well as by microRNAs (miRNAs). In our current study, we found miR-181a to be highly upregulated during BMP induced osteoblastic differentiation of C2C12 and MC3T3 cells. Overexpression of miR-181a led to upregulation of key markers of osteoblastic differentiation as well as enhanced ALP levels and Alizarin red staining, indicating the importance of this miRNA for osteoblastic differentiation. Further, we show that miR-181 isoforms (181a, 181b, 181c) are expressed during different stages of mouse calvarial and tibial development, implying their role in both endochondral and intramembranous ossification. We found several direct and indirect targets of miR-181a to be downregulated by global mRNA expression profiling. Our results demonstrate that miR-181a promotes osteoblastic differentiation via repression of TGF-β signaling molecules by targeting the negative regulator of osteoblastic differentiation Tgfbi (Tgf-beta induced) and TβR-I/Alk5 (TGF-β type I receptor). Furthermore, our findings suggest that Rgs4 and Gata6 are direct targets of miR-181a. Taken together, we provide evidence for a crucial functional link between a specific miRNA, miR-181a and osteoblastic differentiation.

B2A as a positive BMP receptor modulator

B2A (B2A2-K-NS) is a synthetic multi-domain peptide that in vitro augments bone morphogenetic protein (BMP)-2-induced cell responsiveness and osteodifferentiation. Augmentation of endogenous BMP-2 is thought to ultimately improve bone repair, and has led to clinical evaluation of B2A in orthopedic applications. In this study, we show that B2A binds to BMP receptor (BMPR)-IB, BMPR-II, and BMPR-IA. B2A reduces the EC50 of rh-BMP-2, thus shifting the response curve to the left. B2A enhances the osteogenic activity of BMP-2, but not growth and differentiation factor-5, BMP-7, or BMP-9, indicating its action is highly BMP-2 selective. Additionally, B2A did not augment Wnt-3a- and retinoic acid-induced differentiation. All three functional domains (receptor-binding domain, hydrophobic-linker domain, heparin-binding domain) of B2A are required for optimal bioactivity. Collectively, the results suggest that B2A, via its unique sequence, acts in a manner consistent with a positive receptor modulator to selectively enhance BMP-2 osteodifferentiation, and yet in the absence of BMP-2, B2A is without cooperative effect.

Activation of Wnt11 by transforming growth factor-β drives mesenchymal gene expression through non-canonical Wnt protein signaling in renal epithelial cells

Transforming growth factor β1 (TGF-β) promotes renal interstitial fibrosis in vivo and the expression of mesenchymal genes in vitro; however, most of its direct targets in epithelial cells are still elusive. In a screen for genes directly activated by TGF-β, we found that components of the Wnt signaling pathway, especially Wnt11, were targets of activation by TGF-β and Smad3 in primary renal epithelial cells. In gain and loss of function experiments, Wnt11 mediated the actions of TGF-β through enhanced activation of mesenchymal marker genes, such as Zeb1, Snail1, Pai1, and αSMA, without affecting Smad3 phosphorylation. Inhibition of Wnt11 by receptor knockdown or treatment with Wnt inhibitors limited the effects of TGF-β on gene expression. We found no evidence that Wnt11 activated the canonical Wnt signaling pathway in renal epithelial cells; rather, the function of Wnt11 was mediated by the c-Jun N-terminal kinase (JNK) pathway. Consistent with the in vitro results, all the TGF-β, Wnt11, and JNK targets were activated in a unilateral ureteral obstruction (UUO) model of renal fibrosis in vivo. Our findings demonstrated cooperativity among the TGF-β, Wnt11, and JNK signaling pathways and suggest new targets for anti-fibrotic therapy in renal tissue.

Ischemia-reperfusion injury of the mouse kidney

Studies of the complex responses of the kidney to acute injury have yielded important insights into mechanisms of tissue injury and repair. A variety of injury models have contributed to this impressive body of knowledge, but the ischemia-reperfusion (IR) model has perhaps been the most widely used. This chapter contains a detailed method description for IR injury in the mouse together with notes on blood sampling and tissue harvesting. The aim of the chapter is to provide the novice with a step-by-step guide to establishing this procedure in their research program.

TGF-β1 → SMAD/p53/USF2 → PAI-1 transcriptional axis in ureteral obstruction-induced renal fibrosis

Chronic kidney disease constitutes an increasing medical burden affecting 26 million people in the United States alone. Diabetes, hypertension, ischemia, acute injury, and urological obstruction contribute to renal fibrosis, a common pathological hallmark of chronic kidney disease. Regardless of etiology, elevated TGF-β1 levels are causatively linked to the activation of profibrotic signaling pathways initiated by angiotensin, glucose, and oxidative stress. Unilateral ureteral obstruction (UUO) is a useful and accessible model to identify mechanisms underlying the progression of renal fibrosis. Plasminogen activator inhibitor-1 (PAI-1), a major effector and downstream target of TGF-β1 in the progression of several clinically important fibrotic disorders, is highly up-regulated in UUO and causatively linked to disease severity. SMAD and non-SMAD pathways (pp60(c-src), epidermal growth factor receptor [EGFR], mitogen-activated protein kinase, p53) are required for PAI-1 induction by TGF-β1. SMAD2/3, pp60(c-src), EGFR, and p53 activation are each increased in the obstructed kidney. This review summarizes the molecular basis and translational significance of TGF-β1-stimulated PAI-1 expression in the progression of kidney disease induced by ureteral obstruction. Mechanisms discussed here appear to be operative in other renal fibrotic disorders and are relevant to the global issue of tissue fibrosis, regardless of organ site.

Variable partial unilateral ureteral obstruction and its release in the neonatal and adult mouse

Obstructive nephropathy is the most important cause of renal failure in children. Unilateral ureteral obstruction (UUO) in the neonatal mouse provides a useful model to investigate the response of the developing kidney to urine flow obstruction. Creation of reversible variable partial UUO (compared to complete UUO) more closely approximates congenital lesions, and permits the study of recovery following release of the obstruction. Implementation of this technique requires the appropriate optical, surgical, and anesthetic equipment, as well as adaptations appropriate to the very small animals undergoing surgical procedures. Care of the pups must include minimizing trauma to delicate tissues, close monitoring of anesthesia and body temperature, and ensuring acceptance of the pups by the mother. It is important to document the severity and patency of the partial UUO by ureteral measurement and pelvic injection of India ink. Finally, removal of kidneys for histologic examination should be accomplished with gentle handling and processing.

Distinct bone morphogenetic proteins activate indistinguishable transcriptional responses in nephron epithelia including Notch target genes

Endogenous Bone Morphogenetic Protein (BMP) signaling plays a significant role in the kidney's recovery from acute injury and exogenous administration of BMP7 has therapeutic potential in numerous rodent models of renal injury and disease. However, in the healthy kidney endogenous BMP7 ligand is vigorously counteracted by extracellular antagonists such as USAG1 and CHRDL1. Little is known about the degree of BMP signaling and the ligands driving it in the healthy adult kidney. In this study we characterize basal BMP signaling in the healthy tubular nephron, and show that BMP2 is expressed in proximal nephron epithelial cells. Comparative gene profiling of proximal tubule cell responses to BMP2 and BMP7 does not reveal any qualitative difference, suggesting that identical BMP gene targets may be activated in healthy and injured organs. Interestingly, our gene profiling analysis shows that BMP signaling activates a number of Notch regulated transcription factors, including HEY1. As in other biological systems, HEY1 functions as a negative feedback regulator of BMP2 expression in the proximal tubule. In summary, this work reveals endogenous BMP signaling patterns in the healthy human and mouse kidneys, and identifies novel gene targets, some of which are involved in the complex regulation of BMP signaling in the adult kidney.

Loss of endogenous bone morphogenetic protein-6 aggravates renal fibrosis

Bone morphogenetic protein-6 (BMP-6) suppresses inflammatory genes in renal proximal tubular cells and regulates iron metabolism by inducing hepcidin. In diabetic patients, an increase of myofibroblast progenitor cells (MFPCs), also known as fibrocytes, was found to be associated with decreased BMP-6 expression. We hypothesized that loss of endogenous BMP-6 would aggravate renal injury and fibrosis. Wild type (WT) and BMP-6 null mice underwent unilateral ureteral obstruction. In WT mice, ureteral obstruction down-regulated BMP-6. Obstructed kidneys of BMP-6 null mice showed more casts (1.5-fold), epithelial necrosis (1.4-fold), and brush border loss (1.3-fold). This was associated with more inflammation (1.8-fold more CD45(+) cells) and more pronounced overexpression of profibrotic genes for αSMA (2.0-fold), collagen I (6.8-fold), fibronectin (4.3-fold), CTGF (1.8-fold), and PAI-1 (3.8-fold), despite similar BMP-7 expression. Also, 1.3-fold more MFPCs were obtained from BMP-6 null than from WT mononuclear cell cultures, but in vivo only very few MFPCs were observed in obstructed kidneys, irrespective of BMP-6 genotype. The obstructed kidneys of BMP-6 null mice showed 2.2-fold more iron deposition, in association with 3.3-fold higher expression of the oxidative stress marker HO-1. Thus, ureteral obstruction leads to down-regulation of BMP-6 expression, and BMP-6 deficiency aggravates tubulointerstitial damage and fibrosis independent of BMP-7. This process appears to involve loss of both direct anti-inflammatory and antifibrotic action and indirect suppressive effects on renal iron deposition, oxidative stress, and MFPCs.

WISP-1/CCN4 regulates osteogenesis by enhancing BMP-2 activity

Wnt-induced secreted protein 1 (WISP-1/CCN4) is a member of the CCN family that is highly expressed in skeletal tissue and in osteoprogenitor cells induced to differentiate in vitro. To determine the function of WISP-1 during osteogeneis, osteogenic bone marrow stromal cells (BMSCs) were transduced with WISP-1 adenovirus (adWISP-1) in the presence or absence of bone morphogenetic protein 2 (BMP-2) adenovirus (adBMP-2). WISP-1 overexpression enhanced the ability of BMP-2 to direct BMSCs toward osteogenic differentiation and appeared to work by stimulating Smad-1/5/8 phosphorylation and activation. The ability of WISP-1 to enhance BMP-2 activity also was shown in vivo using an ectopic osteogenesis assay with BMSCs transduced with WISP-1, BMP-2, or both. When BMSCs were infected with lentivirus containing human WISP1 shRNA, they formed less bone in vivo and were less responsive to BMP-2, confirming that WISP-1 and BMP-2 have a functional interaction. Immunoprecipitation (IP) and Western blot analysis showed that WISP-1 bound directly to BMP-2 and showed that WISP-1 increased BMP-2 binding to hBMSCs in a dose-dependent fashion. To understand how WISP-1 enhanced BMP-2 signaling, the influence of WISP-1 on integrin expression was analyzed. WISP-1 induced the mRNA and protein levels of α(5)-integrin and, further, was found to bind to it. Antibody-blocking experiments showed that the BMP-2 binding to BMSCs that was enhanced by WISP-1 was completely neutralized by treatment with anti-integrin α(5)β(1) antibody. Pilot studies and the use of transgenic mice that overexpressed human WISP-1 in preosteoblasts had increased bone mineral density (BMD), trabecular thickness, and bone volume (BV/TV) over wild-type controls, supporting observations using human osteoprogenitors that WISP-1 has a positive influence on osteogenesis in vivo. In conclusion, these studies show, for the first time, that WISP-1 has a positive influence on bone cell differentiation and function and may work by enhancing the effects of BMP-2 to increase osteogenesis through a mechanism potentially involving binding to integrin α(5)β(1).

Epithelial Notch signaling regulates interstitial fibrosis development in the kidneys of mice and humans

Chronic kidney disease is a leading cause of death in the United States. Tubulointerstitial fibrosis (TIF) is considered the final common pathway leading to end-stage renal disease (ESRD). Here, we used pharmacologic, genetic, in vivo, and in vitro experiments to show that activation of the Notch pathway in tubular epithelial cells (TECs) in patients and in mouse models of TIF plays a role in TIF development. Expression of Notch in renal TECs was found to be both necessary and sufficient for TIF development. Genetic deletion of the Notch pathway in TECs reduced renal fibrosis. Consistent with this, TEC-specific expression of active Notch1 caused rapid development of TIF. Pharmacologic inhibition of Notch activation using a γ-secretase inhibitor ameliorated TIF. In summary, our experiments establish that epithelial injury and Notch signaling play key roles in fibrosis development and indicate that Notch blockade may be a therapeutic strategy to reduce fibrosis and ESRD development.

Bone morphogenetic proteins: a critical review

Bone Morphogenetic Proteins (BMPs) are potent growth factors belonging to the Transforming Growth Factor Beta superfamily. To date over 20 members have been identified in humans with varying functions during processes such as embryogenesis, skeletal formation, hematopoiesis and neurogenesis. Though their functions have been identified, less is known regarding levels of regulation at the extracellular matrix, membrane surface, and receptor activation. Further, current models of activation lack the integration of these regulatory mechanisms. This review focuses on the different levels of regulation, ranging from the release of BMPs into the extracellular components to receptor activation for different BMPs. It also highlights areas in research that is lacking or contradictory.

Binding between Crossveinless-2 and Chordin von Willebrand factor type C domains promotes BMP signaling by blocking Chordin activity

Background

Crossveinless-2 (CV2) is an extracellular BMP modulator protein of the Chordin family, which can either enhance or inhibit BMP activity. CV2 binds to BMP2 via subdomain 1 of the first of its five N-terminal von Willebrand factor type C domains (VWC1). Previous studies showed that this BMP binding is required for the anti-, but not for the pro-BMP effect of CV2. More recently, it was shown that CV2 can also bind to the BMP inhibitor Chordin. However, it remained unclear which domains mediate this binding, and whether it accounts for an anti- or pro-BMP effect.

Principal Findings

Here we report that a composite interface of CV2 consisting of subdomain 2 of VWC1 and of VWC2-4, which are dispensable for BMP binding, binds to the VWC2 domain of Chordin. Functional data obtained in zebrafish embryos indicate that this binding of Chordin is required for CV2's pro-BMP effect, which actually is an anti-Chordin effect and, at least to a large extent, independent of Tolloid-mediated Chordin degradation. We further demonstrate that CV2 mutant versions that per se are incapable of BMP binding can attenuate the Chordin/BMP interaction.

Conclusions

We have physically dissected the anti- and pro-BMP effects of CV2. Its anti-BMP effect is obtained by binding to BMP via subdomain1 of the VWC1 domain, a binding that occurs in competition with Chordin. In contrast, its pro-BMP effect is achieved by direct binding to Chordin via subdomain 2 of VWC1 and VWC2-4. This binding seems to induce conformational changes within the Chordin protein that weaken Chordin's affinity to BMP. We propose that in ternary Chordin-CV2-BMP complexes, both BMP and Chordin are directly associated with CV2, whereas Chordin is pushed away from BMP, ensuring that BMPs can be more easily delivered to their receptors.

Bone morphogenetic protein and growth differentiation factor cytokine families and their protein antagonists

The BMPs (bone morphogenetic proteins) and the GDFs (growth and differentiation factors) together form a single family of cystine-knot cytokines, sharing the characteristic fold of the TGFbeta (transforming growth factor-beta) superfamily. Besides the ability to induce bone formation, which gave the BMPs their name, the BMP/GDFs display morphogenetic activities in the development of a wide range of tissues. BMP/GDF homo- and hetero-dimers interact with combinations of type I and type II receptor dimers to produce multiple possible signalling complexes, leading to the activation of one of two competing sets of SMAD transcription factors. BMP/GDFs have highly specific and localized functions. These are regulated in a number of ways, including the developmental restriction of BMP/GDF expression and through the secretion of several specific BMP antagonist proteins that bind with high affinity to the cytokines. Curiously, a number of these antagonists are also members of the TGF-beta superfamily. Finally a number of both the BMP/GDFs and their antagonists interact with the heparan sulphate side chains of cell-surface and extracellular-matrix proteoglycans.

Methylation determines fibroblast activation and fibrogenesis in the kidney

Fibrogenesis is a pathological wound repair process that fails to cease, even when the initial insult has been removed. Fibroblasts are principal mediators of fibrosis, and fibroblasts from fibrotic tissues fail to return to their quiescent stage, including when cultured in vitro. In a search for underlying molecular mechanisms, we hypothesized that this perpetuation of fibrogenesis is caused by epigenetic modifications. We demonstrate here that hypermethylation of RASAL1, encoding an inhibitor of the Ras oncoprotein, is associated with the perpetuation of fibroblast activation and fibrogenesis in the kidney. RASAL1 hypermethylation is mediated by the methyltransferase Dnmt1 in renal fibrogenesis, and kidney fibrosis is ameliorated in Dnmt1(+/-) heterozygous mice. These studies demonstrate that epigenetic modifications may provide a molecular basis for perpetuated fibroblast activation and fibrogenesis in the kidney.

Extracellular BMP-antagonist regulation in development and disease: tied up in knots

Developmental processes are regulated by the bone morphogenetic protein (BMP) family of secreted molecules. BMPs bind to serine/threonine kinase receptors and signal through the canonical Smad pathway and other intracellular effectors. Integral to the control of BMPs is a diverse group of secreted BMP antagonists that bind to BMPs and prevent engagement with their cognate receptors. Tight temporospatial regulation of both BMP and BMP-antagonist expression provides an exquisite control system for developing tissues. Additional facets of BMP-antagonist biology, such as crosstalk with Wnt and Sonic hedgehog signaling during development, have been revealed in recent years. In addition, previously unappreciated roles for the BMP antagonists in kidney fibrosis and cancer have been elucidated. This review provides a description of BMP-antagonist biology, together with highlights of recent novel insights into the role of these antagonists in development, signal transduction and human disease.

The extracellular regulation of bone morphogenetic protein signaling

In many cases, the level, positioning and timing of signaling through the bone morphogenetic protein (BMP) pathway are regulated by molecules that bind BMP ligands in the extracellular space. Whereas many BMP-binding proteins inhibit signaling by sequestering BMPs from their receptors, other BMP-binding proteins cause remarkably context-specific gains or losses in signaling. Here, we review recent findings and hypotheses on the complex mechanisms that lead to these effects, with data from developing systems, biochemical analyses and mathematical modeling.

Renal studies provide an insight into cardiac extracellular matrix remodeling during health and disease

The remodeling of a heart ventricle after myocardial infarction involves numerous inflammatory mediators that may trigger a long-lasting and a highly fibrogenic process. Likewise, in the kidney, acute and chronic injuries may lead to abnormal extracellular matrix deposition and eventually lead to the loss of renal function. Major breakthroughs have emerged during the last ten years with respect to the pathophysiology of matrix remodeling. Epithelial and endothelial cells are plastic, and able to engage in epithelial (or endothelial)-to-mesenchymal transition (EMT or EndMT), thus actively contributing to the fibrogenesis. Members of the fibrinolytic system were demonstrated to possess unsuspected properties and interact with receptors and integrins on endothelial and epithelial cells. Finally, a notion that stem cells could integrate into damaged tissue has recently emerged, which likely contributes to the tissue repair. In many aspects, the kidney and the heart share many common injury mechanisms. We envision that some of them will be accessible as common therapeutic targets in the future.

Xenopus SMOC-1 Inhibits bone morphogenetic protein signaling downstream of receptor binding and is essential for postgastrulation development in Xenopus

The bone morphogenetic protein (BMP) family of signaling molecules and their antagonists are involved in patterning of the body axis and numerous aspects of organogenesis. Classical biochemical purification and protein sequencing of highly purified fractions containing potent bone forming activity from bovine cartilage identified several BMPs together with a number of other proteins. One such protein was SMOC-2 (secreted modular calcium-binding protein-2), classified as belonging to the BM-40 family of modular extracellular proteins. Data regarding the biological function of SMOC-2 and closely related SMOC-1 remain limited, and their expression or function during embryological development is unknown. We therefore isolated the Xenopus ortholog of human SMOC-1 (XSMOC-1) and explored its function in Xenopus embryos. In gain-of-function assays, XSMOC-1 acted similarly to a BMP antagonist. However, in contrast to known extracellular ligand-binding BMP antagonists, such as noggin, SMOC antagonizes BMP activity in the presence of a constitutively active BMP receptor, indicating a mechanism of action downstream of the receptor. We provide several lines of evidence to suggest that SMOC acts downstream of the BMP receptor via MAPK-mediated phosphorylation of the Smad linker region. Loss-of-function studies, using antisense morpholino oligonucleotides, revealed XSMOC-1 to be essential for postgastrulation development. The catastrophic developmental failure observed following XSMOC knockdown resembles that observed following simultaneous depletion of three ligand-binding BMP antagonists described in prior studies. These findings provide a direct link between the extracellular matrix-associated protein SMOC and a signaling pathway of general importance in anatomic patterning and cell or tissue fate specification.

Chordin-like 1 and twisted gastrulation 1 regulate BMP signaling following kidney injury

Stimulation of the bone morphogenetic protein (BMP) pathway protects the kidney from acute and chronic injury. Numerous regulators in the kidney control BMP signaling, offering many targets for therapeutic manipulation. Here, we screened for modulators of BMP signaling in the ischemia-sensitive S3 segment and found that Chordin-like 1 is expressed in this segment of both the mouse and human nephron. Chordin-like 1 specifically antagonizes BMP7, which is expressed in the neighboring distal nephron, and this depends on the presence of the protein Twisted gastrulation. Upon ischemia-induced degeneration of the S3 segment, we observed a reduction in Chordin-like 1 expression coincident with intense BMP signaling in tubules of the recovering kidney. Restored expression accompanied proximal tubule epithelia redifferentiation, again coincident with decreased BMP signaling. We propose that Chordin-like 1 reduces BMP7 signaling in healthy proximal tubules, and the loss of this activity upon sloughing of injured epithelia promotes BMP7 signaling in repopulating, dedifferentiated epithelia. As regenerating epithelia differentiate, Chordin-like 1 is again expressed, antagonizing BMP7. These data suggest a mechanism for dynamic regulation of renoprotective BMP7 signaling in the S3 segment of the proximal tubule.

BMP modulators regulate the function of BMP during body patterning and disease progression

Bone morphogenetic proteins (BMPs) are phylogenetically conserved signaling molecules that belong to the transforming growth factor (TGF)-beta superfamily and are involved in the cascades of body patterning and morphogenesis. The activities of BMPs are precisely regulated at various stages, and extracellulary, mainly regulated by certain classes of molecules termed as BMP antagonists and pro-BMP factors. BMP antagonists inhibit BMP function by prohibiting them from binding their cognate receptors, whereas pro-BMP factors stimulate BMP function. In this review, the functions of these BMP regulators will be discussed. (c) 2009 International Union of Biochemistry and Molecular Biology, Inc.

Reversal of experimental renal fibrosis by BMP7 provides insights into novel therapeutic strategies for chronic kidney disease

Bone morphogenic protein-7 (BMP7) is a morphogen that is important for kidney development and which is also an integral part of the kidney's physiological response to repair of acute kidney injury. Several studies demonstrate that preexisting renal BMP7 pathways can be utilized by administering recombinant BMP7 to protect the kidney in experimental models of chronic kidney disease (CKD). Effectiveness of recombinant BMP7 in animal studies raises the possibility that the BMP7 pathway could be equally utilized to treat patients with CKD and interstitial fibrosis. However, regulation of BMP7 activity in the kidney is complex. BMP7 activity in the kidney is not only determined by availability of BMP7 itself, but also by a balance of agonists, such as Kielin/chordin-like protein (KCP) or BMP receptors, and antagonists including gremlin, noggin, or uterine sensitization-associated gene-1 (USAG-1). Presence of BMP7 agonists and antagonists has to be considered when recombinant BMP7 is supplemented to treat injured kidneys. Here we summarize recent insights into the role of BMP7 in acute and chronic kidney injury and discuss the implications for future directions of antifibrotic therapies.

EMT and proteinuria as progression factors

Tubulointerstitial fibrosis is an integral part of the structural changes of the kidney in chronic progressive renal failure. The accumulation of the extracellular matrix in the tubulointerstitial space is mediated mainly by myofibroblasts. These are derived from resident interstitial fibroblasts, tubular epithelial cells, periadventitial cells, and possibly also mesenchymal stem cells and endothelial cells. Fibrosis is usually preceded by tubulointerstitial infiltration of mononuclear inflammatory cells. Proteinuria is one of several mechanisms of primary glomerular or vascular disease to transmit the disease process to the interstitial space. Increased protein filtration may have direct toxic effects on tubular epithelial cells, induce chemokine and cytokine secretion and result in increased expression of adhesion molecules, all contributing to the influx of mononuclear cells. Inflammatory cells in return secrete cytokines, which stimulate resident fibroblasts and tubular epithelial cells to differentiate into matrix-producing cells. The phenotypic conversion of primary epithelial cells into mesenchymal cells, termed epithelial-mesenchymal transition (EMT), has been studied in great detail in recent years. Several signal transduction pathways of this process have been clarified and may eventually result in novel therapeutic approaches. The severity of proteinuria and the extent of EMT have both been associated with the decline in renal function in clinical studies. Limiting proteinuria results in a slower decline of renal function deterioration, whereas reducing EMT has had beneficial effects in a number of animal studies, including those indicating reversal of fibrotic lesions. However, the association between proteinuria and EMT and vice versa is far from clear and has not been carefully studied.

The BMP-binding protein Crossveinless 2 is a short-range, concentration-dependent, biphasic modulator of BMP signaling in Drosophila

In Drosophila, the secreted BMP-binding protein Short gastrulation (Sog) inhibits signaling by sequestering BMPs from receptors, but enhances signaling by transporting BMPs through tissues. We show that Crossveinless 2 (Cv-2) is also a secreted BMP-binding protein that enhances or inhibits BMP signaling. Unlike Sog, however, Cv-2 does not promote signaling by transporting BMPs. Rather, Cv-2 binds cell surfaces and heparan sulfate proteoglygans and acts over a short range. Cv-2 binds the type I BMP receptor Thickveins (Tkv), and we demonstrate how the exchange of BMPs between Cv-2 and receptor can produce the observed biphasic response to Cv-2 concentration, where low levels promote and high levels inhibit signaling. Importantly, we show also how the concentration or type of BMP present can determine whether Cv-2 promotes or inhibits signaling. We also find that Cv-2 expression is controlled by BMP signaling, and these combined properties enable Cv-2 to exquisitely tune BMP signaling.