Loss of the BMP antagonist USAG-1 ameliorates disease in a mouse model of the progressive hereditary kidney disease Alport syndrome

Neuroblastoma suppressor of tumorigenicity 1 is associated with the severity of interstitial fibrosis and kidney function decline in IgA nephropathy

INTRODUCTION

Recently, circulating neuroblastoma suppressor of tumorigenicity 1 (NBL1) was shown to be strongly associated with kidney disease progression and histological lesions in patients with diabetic kidney disease. This study aimed to examine whether serum NBL1 level was also associated with kidney function and renal histological findings in patients with IgA nephropathy.

METHODS

We evaluated the levels of NBL1 in 109 patients with newly diagnosed biopsy-proven primary IgAN, between 2009 and 2018, at the Nihon University School of Medicine Itabashi Hospital, Tokyo, Japan, using serum obtained immediately before the renal biopsy, and examined the relationship between serum NBL1, renal function and renal histological findings assessed using the Oxford Classification (MEST score). Furthermore, we analyzed the association of serum NBL1 with kidney function decline over time in patients with IgA nephropathy who had follow-up data on the estimated glomerular filtration rate (n = 76).

RESULTS

Serum NBL1 levels in patients with newly diagnosed IgA nephropathy were elevated, as compared to those in healthy individuals (n = 93). Logistic regression analysis demonstrated that the serum NBL1 level was independently and significantly associated with tubular atrophy/interstitial fibrosis. Immunohistochemical staining revealed that NBL1 was highly expressed in the tubulointerstitium. Furthermore, Spearman's rank correlation identified a significant correlation between serum NBL1 level and estimated glomerular filtration rate slope.

CONCLUSIONS

The serum NBL1 level was significantly associated with the severity of renal interstitial fibrosis and kidney disease progression in patients with newly diagnosed IgA nephropathy. Thus, circulating NBL1 may serve as a good biomarker for evaluating renal interstitial fibrosis and the risk of kidney disease progression.

Genetic Modifiers of Mendelian Monogenic Collagen IV Nephropathies in Humans and Mice

Familial hematuria is a clinical sign of a genetically heterogeneous group of conditions, accompanied by broad inter- and intrafamilial variable expressivity. The most frequent condition is caused by pathogenic (or likely pathogenic) variants in the collagen-IV genes, COL4A3/A4/A5. Pathogenic variants in COL4A5 are responsible for the severe X-linked glomerulopathy, Alport syndrome (AS), while homozygous or compound heterozygous variants in the COL4A3 or the COL4A4 gene cause autosomal recessive AS. AS usually leads to progressive kidney failure before the age of 40-years when left untreated. People who inherit heterozygous COL4A3/A4 variants are at-risk of a slowly progressive form of the disease, starting with microscopic hematuria in early childhood, developing Alport spectrum nephropathy. Sometimes, they are diagnosed with benign familial hematuria, and sometimes with autosomal dominant AS. At diagnosis, they often show thin basement membrane nephropathy, reflecting the uniform thin glomerular basement membrane lesion, inherited as an autosomal dominant condition. On a long follow-up, most patients will retain normal or mildly affected kidney function, while a substantial proportion will develop chronic kidney disease (CKD), even kidney failure at an average age of 55-years. A question that remains unanswered is how to distinguish those patients with AS or with heterozygous COL4A3/A4 variants who will manifest a more aggressive kidney function decline, requiring prompt medical intervention. The hypothesis that a subgroup of patients coinherit additional genetic modifiers that exacerbate their clinical course has been investigated by several researchers. Here, we review all publications that describe the potential role of candidate genetic modifiers in patients and include a summary of studies in AS mouse models.

Sostdc1 Suppression in the Absence of Sclerostin Potentiates Anabolic Action of Cortical Bone in Mice

The development of Wnt-based osteoanabolic agents has progressed rapidly in recent years, given the potent effects of Wnt modulation on bone homeostasis. Simultaneous pharmacologic inhibition of the Wnt antagonists sclerostin and Dkk1 can be optimized to create potentiated effects in the cancellous bone compartment. We looked for other candidates that might be co-inhibited along with sclerostin to potentiate the effects in the cortical compartment. Sostdc1 (Wise), like sclerostin and Dkk1, also binds and inhibits Lrp5/6 coreceptors to impair canonical Wnt signaling, but Sostdc1 has greater effects in the cortical bone. To test this concept, we deleted Sostdc1 and Sost from mice and measured the skeletal effects in cortical and cancellous compartments individually. Sost deletion alone produced high bone mass in all compartments, whereas Sostdc1 deletion alone had no measurable effects on either envelope. Mice with codeletion of Sostdc1 and Sost had high bone mass and increased cortical properties (bone mass, formation rates, mechanical properties), but only among males. Combined administration of sclerostin antibody and Sostdc1 antibody in wild-type female mice produced potentiation of cortical bone gain despite no effect of Sostdc1 antibody alone. In conclusion, Sostdc1 inhibition/deletion can work in concert with sclerostin deficiency to improve cortical bone properties. © 2023 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).

Role of Sostdc1 in skeletal biology and cancer

Sclerostin domain-containing protein-1 (Sostdc1) is a member of the sclerostin family and encodes a secreted 28–32 kDa protein with a cystine knot-like domain and two N-linked glycosylation sites. Sostdc1 functions as an antagonist to bone morphogenetic protein (BMP), mediating BMP signaling. It also interacts with LRP6, mediating LRP6 and Wnt signaling, thus regulating cellular proliferation, differentiation, and programmed cell death. Sostdc1 plays various roles in the skin, intestines, brain, lungs, kidneys, and vasculature. Deletion of Sostdc1 gene in mice resulted in supernumerary teeth and improved the loss of renal function in Alport syndrome. In the skeletal system, Sostdc1 is essential for bone metabolism, bone density maintenance, and fracture healing. Recently, Sostdc1 has been found to be closely related to the development and progression of multiple cancer types, including breast, renal, gastric, and thyroid cancers. This article summarises the role of Sostdc1 in skeletal biology and related cancers to provide a theoretical basis for the treatment of related diseases.

Neuroblastoma suppressor of tumorigenicity 1 is a circulating protein associated with progression to end-stage kidney disease in diabetes

Circulating proteins associated with transforming growth factor-β (TGF-β) signaling are implicated in the development of diabetic kidney disease (DKD). It remains to be comprehensively examined which of these proteins are involved in the pathogenesis of DKD and its progression to end-stage kidney disease (ESKD) in humans. Using the SOMAscan proteomic platform, we measured concentrations of 25 TGF-β signaling family proteins in four different cohorts composed in total of 754 Caucasian or Pima Indian individuals with type 1 or type 2 diabetes. Of these 25 circulating proteins, we identified neuroblastoma suppressor of tumorigenicity 1 (NBL1, aliases DAN and DAND1), a small secreted protein known to inhibit members of the bone morphogenic protein family, to be most strongly and independently associated with progression to ESKD during 10-year follow-up in all cohorts. The extent of damage to podocytes and other glomerular structures measured morphometrically in 105 research kidney biopsies correlated strongly with circulating NBL1 concentrations. Also, in vitro exposure to NBL1 induced apoptosis in podocytes. In conclusion, circulating NBL1 may be involved in the disease process underlying progression to ESKD, and its concentration in circulation may identify subjects with diabetes at increased risk of progression to ESKD.

Col4a3-/- Mice on Balb/C Background Have Less Severe Cardiorespiratory Phenotype and SGLT2 Over-Expression Compared to 129x1/SvJ and C57Bl/6 Backgrounds

Alport syndrome (AS) is a hereditary renal disorder with no etiological therapy. In the preclinical Col4a3^-/- model of AS, disease progression and severity vary depending on mouse strain. The sodium-glucose cotransporter 2 (SGLT2) is emerging as an attractive therapeutic target in cardiac/renal pathologies, but its application to AS remains untested. This study investigates cardiorespiratory function and SGLT2 renal expression in Col4a3^-/- mice from three different genetic backgrounds, 129x1/SvJ, C57Bl/6 and Balb/C. male Col4a3^-/- 129x1/SvJ mice displayed alterations consistent with heart failure with preserved ejection fraction (HFpEF). Female, but not male, C57Bl/6 and Balb/C Col4a3^-/- mice exhibited mild changes in systolic and diastolic function of the heart by echocardiography. Male C57Bl/6 Col4a3^-/- mice presented systolic dysfunction by invasive hemodynamic analysis. All strains except Balb/C males demonstrated alterations in respiratory function. SGLT2 expression was significantly increased in AS compared to WT mice from all strains. However, cardiorespiratory abnormalities and SGLT2 over-expression were significantly less in AS Balb/C mice compared to the other two strains. Systolic blood pressure was significantly elevated only in mutant 129x1/SvJ mice. The results provide further evidence for strain-dependent cardiorespiratory and hypertensive phenotype variations in mouse AS models, corroborated by renal SGLT2 expression, and support ongoing initiatives to develop SGLT2 inhibitors for the treatment of AS.

Simultaneous stabilization of actin cytoskeleton in multiple nephron-specific cells protects the kidney from diverse injury

Chronic kidney diseases and acute kidney injury are mechanistically distinct kidney diseases. While chronic kidney diseases are associated with podocyte injury, acute kidney injury affects renal tubular epithelial cells. Despite these differences, a cardinal feature of both acute and chronic kidney diseases is dysregulated actin cytoskeleton. We have shown that pharmacological activation of GTPase dynamin ameliorates podocyte injury in murine models of chronic kidney diseases by promoting actin polymerization. Here we establish dynamin's role in modulating stiffness and polarity of renal tubular epithelial cells by crosslinking actin filaments into branched networks. Activation of dynamin's crosslinking capability by a small molecule agonist stabilizes the actomyosin cortex of the apical membrane against injury, which in turn preserves renal function in various murine models of acute kidney injury. Notably, a dynamin agonist simultaneously attenuates podocyte and tubular injury in the genetic murine model of Alport syndrome. Our study provides evidence for the feasibility and highlights the benefits of novel holistic nephron-protective therapies.

Molecular and Cellular Mechanisms Underlying the Initiation and Progression of Alport Glomerular Pathology

Alport syndrome results from a myriad of variants in the COL4A3, COL4A4, or COL4A5 genes that encode type IV (basement membrane) collagens. Unlike type IV collagen α1(IV)₂α2(IV)₁ heterotrimers, which are ubiquitous in basement membranes, α3/α4/α5 have a limited tissue distribution. The absence of these basement membrane networks causes pathologies in some, but not all these tissues. Primarily the kidney glomerulus, the stria vascularis of the inner ear, the lens, and the retina as well as a rare link with aortic aneurisms. Defects in the glomerular basement membranes results in delayed onset and progressive focal segmental glomerulosclerosis ultimately requiring the patient to undergo dialysis and if accessible, kidney transplant. The lifespan of patients with Alport syndrome is ultimately significantly shortened. This review addresses the consequences of the altered glomerular basement membrane composition in Alport syndrome with specific emphasis on the mechanisms underlying initiation and progression of glomerular pathology.

Uterine Sensitization-Associated Gene-1 in the Progression of Kidney Diseases

Uterine sensitization-associated gene-1 (USAG-1), originally identified as a secretory protein preferentially expressed in the sensitized rat endometrium, has been determined to modulate bone morphogenetic protein (BMP) and Wnt expression to play important roles in kidney disease. USAG-1 affects the progression of acute and chronic kidney damage and the recovery of allograft kidney function by regulating the BMP and Wnt signaling pathways. Moreover, USAG-1 has been found to be involved in the process of T cell immune response, and its ability to inhibit germinal center activity and reduce humoral immunity is of great significance for the treatment of autoimmune nephropathy and antibody-mediated rejection (AMR) after renal transplantation. This article summarizes the many advances made regarding the roles of USAG-1 in the progression of kidney disease and outlines potential treatments.

New COL4A5 mutation in IgA nephropathy

PURPOSE

IgA nephropathy (IgAN) is the most common type of primary glomerulonephritis and a leading cause of chronic kidney disease (CKD) and end-stage kidney disease (ESKD). Recently, some case reports have shown that COL4A5 mutation is associated with IgAN. Here, we identified a new COL4A5 gene mutation in IgAN in a Chinese family.

MATERIALS AND METHODS

In the present study, the proband and his 23-year-old younger brother were both diagnosed with IgAN, manifested as haematuria, proteinuria and chronic kidney injury without hearing loss or ocular symptoms. Additionally, the proband's 30-year-old younger brother, also diagnosed with ESKD, had been undergoing dialysis for 2 years with normal hearing and eyesight. To exclude genetic disease, we conducted whole-exome sequencing and Sanger sequencing assays.

RESULTS

We found a new mutation in the COL4A5 gene (chrX:107 814 698, c.438+2->AAACCAATTATA-), a novel insertion mutation. Using vector transcription and Minigene transcriptional analyses, we verified, for the first time, the novel mutation pathogenicity of the COL4A5 gene.

CONCLUSION

Together with other published data, we suggest that genetic screening should be performed in IgAN, particularly for patients with a familial history. The effects of different mutated splice sites of the COL4A5 gene, as well as the tissue specificity of the splicing machinery contributing to the pathogenesis and prognosis of IgAN, remains unclear and warrants further exploration in the future.

Modification of an aggressive model of Alport Syndrome reveals early differences in disease pathogenesis due to genetic background

The link between mutations in collagen genes and the development of Alport Syndrome has been clearly established and a number of animal models, including knock-out mouse lines, have been developed that mirror disease observed in patients. However, it is clear from both patients and animal models that the progression of disease can vary greatly and can be modified genetically. We have identified a point mutation in Col4a4 in mice where disease is modified by strain background, providing further evidence of the genetic modification of disease symptoms. Our results indicate that C57BL/6J is a protective background and postpones end stage renal failure from 7 weeks, as seen on a C3H background, to several months. We have identified early differences in disease progression, including expression of podocyte-specific genes and podocyte morphology. In C57BL/6J mice podocyte effacement is delayed, prolonging normal renal function. The slower disease progression has allowed us to begin dissecting the pathogenesis of murine Alport Syndrome in detail. We find that there is evidence of differential gene expression during disease on the two genetic backgrounds, and that disease diverges by 4 weeks of age. We also show that an inflammatory response with increasing MCP-1 and KIM-1 levels precedes loss of renal function.

Crosstalk between Sertoli and Germ Cells in Male Fertility

Spermatogenesis is supported by intricate crosstalk between Sertoli cells and germ cells including spermatogonia, spermatocytes, haploid spermatids, and spermatozoa, which takes place in the epithelium of seminiferous tubules. Sertoli cells, also known as 'mother' or 'nurse' cells, provide nutrients, paracrine factors, cytokines, and other biomolecules to support germ cell development. Sertoli cells facilitate the generation of several biologically active peptides, which include F5-, noncollagenous 1 (NC1)-, and laminin globular (LG)3/4/5-peptide, to modulate cellular events across the epithelium. Here, we critically evaluate the involvement of these peptides in facilitating crosstalk between Sertoli and germ cells to support spermatogenesis and thus fertility. Modulating or mimicking the activity of F5-, NC1-, and LG3/4/5-peptide could be used to enhance the transport across the blood-testis barrier (BTB) of contraceptive drugs or to treat male infertility.

Sostdc1 is expressed in all major compartments of developing and adult mammalian eyes

PURPOSE

This study was conducted in order to study Sostdc1 expression in rat and human developing and adult eyes.

METHODS

Using the yeast signal sequence trap screening method, we identified the Sostdc1 cDNA encoding a protein secreted by the adult rat retinal pigment epithelium. We determined by in situ hybridization, RT-PCR, immunohistochemistry, and western blot analysis Sostdc1 gene and protein expression in developing and postnatal rat ocular tissue sections. We also investigated Sostdc1 immunohistolocalization in developing and adult human ocular tissues.

RESULTS

We demonstrated a prominent Sostdc1 gene expression in the developing rat central nervous system (CNS) and eyes at early developmental stages from E10.5 days postconception (dpc) to E13 dpc. Specific Sostdc1 immunostaining was also detected in most adult cells of rat ocular tissue sections. We also identified the rat ocular embryonic compartments characterized by a specific Sostdc1 immunohistostaining and specific Pax6, Sox2, Otx2, and Vsx2 immunohistostaining from embryonic stages E10.5 to E13 dpc. Furthermore, we determined the localization of SOSTDC1 immunoreactivity in ocular tissue sections of developing and adult human eyes. Indeed, we detected SOSTDC1 immunostaining in developing and adult human retinal pigment epithelium (RPE) and neural retina (NR) as well as in several developing and adult human ocular compartments, including the walls of choroidal and scleral vessels. Of utmost importance, we observed a strong SOSTDC1 expression in a pathological ocular specimen of type 2 Peters' anomaly complicated by retinal neovascularization as well in the walls ofother pathological extra-ocular vessels.  CONCLUSION: As rat Sostdc1 and human SOSTDC1 are dual antagonists of the Wnt/β-catenin and BMP signaling pathways, these results underscore the potential crucial roles of these pathways and their antagonists, such as Sostdc1 and SOSTDC1, in developing and adult mammalian normal eyes as well as in syndromic and nonsyndromic congenital eye diseases.

Grem2 mediates podocyte apoptosis in high glucose milieu

BACKGROUND

Increased DAN protein (Grem1, Grem2, Grem3, Cerberus, NBL1, SOST, and USAG1) levels are often associated with severe disease-states in adult kidneys. Grem1, SOST, and USAG1 have been demonstrated to be upregulated and play a critical role in the progression of diabetic nephropathy (DN); however, the expression and the role of other DAN family members in DN have not been reported yet. In this study, we investigated the expression and the role of Grem2 in the development of renal lesions in mice with type 2 DN.

METHODS

Fourteen-week-old BTBR^ob/ob (a mouse model of type 2 diabetes mellitus) and control (BTBR, wild type) mice were evaluated for renal functional and structural biomarkers. Urine was collected for protein content assay, and renal tissues were harvested for molecular analysis with real-time PCR, Western blotting, and immunohistochemistry. In vitro studies, human podocytes were transfected with Grem2 plasmid and were evaluated for apoptosis (morphologic assay and Western blotting). To evaluate the Grem2-mediated downstream signaling, the phosphorylation status of Smad2/3 and Smad1/5/8 was assessed. To establish a causal relationship, the effect of SIS3 (an inhibitor for Samd2/3) and BMP-7 (an agonist for Smad1/5/8) was evaluated on Germ2-induced podocyte apoptosis.

RESULTS

BTBR^ob/ob mice showed elevated urinary protein levels. Renal tissues of BTBR^ob/ob mice showed an increased expression of Grem2; both glomerular and tubular cells displayed enhanced Grem2 expression. In vitro studies, high glucose increased Grem2 expression in cultured human podocytes, whereas, Grem2 silencing partially protected podocyte from high glucose-induced apoptosis. Overexpression of Grem2 in podocytes not only increased Bax/Bcl2 expression ratio but also promoted podocyte apoptosis; moreover, an overexpression of Grem2 increased the phosphorylation of Smad2/3 and decreased the phosphorylation of Smad1/5/8; furthermore, SIS3 and BMP-7 attenuated Grem2-induced podocyte apoptosis.

CONCLUSIONS

High glucose increases Grem2 expression in kidney cells. Grem2 mediates podocyte apoptosis through Smads.

Identification of platelet-derived growth factor C as a mediator of both renal fibrosis and hypertension

Platelet-derived growth factors (PDGF) have been implicated in kidney disease progression. We previously found that PDGF-C is upregulated at sites of renal fibrosis and that antagonism of PDGF-C reduces fibrosis in the unilateral ureteral obstruction model. We studied the role of PDGF-C in collagen 4A3^-/- ("Alport") mice, a model of progressive renal fibrosis with greater relevance to human kidney disease. Alport mice were crossbred with PDGF-C^-/- mice or administered a neutralizing PDGF-C antibody. Both PDGF-C deficiency and neutralization reduced serum creatinine and blood urea nitrogen levels and mitigated glomerular injury, renal fibrosis, and renal inflammation. Unexpectedly, systolic blood pressure was also reduced in both Alport and wild-type mice treated with a neutralizing PDGF-C antibody. Neutralization of PDGF-C reduced arterial wall thickness in the renal cortex of Alport mice. Aortic rings isolated from anti-PDGF-C-treated wildtype mice exhibited reduced tension and faster relaxation than those of untreated mice. In vitro, PDGF-C upregulated angiotensinogen in aortic tissue and in primary hepatocytes and induced nuclear factor κB (NFκB)/p65-binding to the angiotensinogen promoter in hepatocytes. Neutralization of PDGF-C suppressed transcript expression of angiotensinogen in Alport mice and angiotensin II receptor type 1 in Alport and wildtype mice. Finally, administration of neutralizing PDGF-C antibodies ameliorated angiotensin II-induced hypertension in healthy mice. Thus, in addition to its key role in mediating renal fibrosis, we identified PDGF-C as a mediator of hypertension via effects on renal vasculature and on the renin-angiotensin system. The contribution to both renal fibrosis and hypertension render PDGF-C an attractive target in progressive kidney disease.

Febuxostat inhibits TGF‑β1‑induced epithelial‑mesenchymal transition via downregulation of USAG‑1 expression in Madin‑Darby canine kidney cells in vitro

Our previous study demonstrated that febuxostat, a xanthine oxidase inhibitor, can alleviate kidney dysfunction and ameliorate renal tubulointerstitial fibrosis in a rat unilateral ureteral obstruction (UUO) model; however, the underlying mechanisms remain unknown. Increasing evidence has revealed that epithelial-mesenchymal transition (EMT) is one of the key mechanisms mediating the progression of renal tubulointerstitial fibrosis in chronic kidney disease (CKD). Uterine sensitization-associated gene-1 (USAG-1), a kidney-specific bone morphogenetic protein antagonist, is involved in the development of numerous types of CKDs. The present study aimed to investigate the role of febuxostat in the process of EMT in Madin-Darby canine kidney (MDCK) cells in vitro. Western blotting, reverse transcription-semiquantitative polymerase chain reaction analysis and immunofluorescence staining were used to evaluate the expression levels of bone morphogenetic protein 7, USAG-1, α-smooth muscle actin (α-SMA) and E-cadherin, respectively. The results demonstrated that the expression of USAG-1 and α-SMA increased, and that of E-cadherin decreased significantly in MDCK cells following treatment with transforming growth factor-β1 (TGF-β1). The application of small interfering RNA-USAG-1 potently inhibited TGF-β1-induced EMT. Subsequently, the effects of febuxostat on TGF-β1-induced EMT was investigated. The results demonstrated that febuxostat downregulated the expression of USAG-1, and reversed TGF-β1-induced EMT in MDCK cells. Furthermore, pretreatment with febuxostat significantly restored the decreased expression levels of phosphorylated Smad1/5/8 induced by TGF-β1 in MDCK cells. The results of the present study suggested that USAG-1 may be involved in the EMT process of MDCK cells induced by TGF-β1, and febuxostat inhibited EMT by activating the Smad1/5/8 signaling pathway via downregulating the expression of USAG-1 in MDCK cells.

Functional heterogeneity of resident fibroblasts in the kidney

Chronic kidney disease (CKD) is a global public health problem, affecting over 10% of the world's population and more than half of the population aged over 70 years, imposing major costs on healthcare systems. Although the primary causes of CKD include various diseases such as diabetes, glomerulonephritis, and acute kidney injury (AKI), the progression of CKD is mediated by a common pathological pathway, which is mainly characterized by fibrosis and chronic inflammation. In this process, resident fibroblasts in the kidney play crucial roles. Accumulating evidence highlights the existence of functional heterogeneity and plasticity of fibroblasts and their diverse roles in kidney disease progression and resolution. In addition to renal fibrosis, renal anemia and peritubular capillary loss, two major complications of progressive CKD, are also caused by dysfunction of resident fibroblasts. Furthermore, age-dependent alterations in fibroblast behavior also contribute to age-dependent unique pathological conditions. In this article, we describe the current understanding regarding the behaviors of fibroblasts in the kidney in health, disease, and aging.

A review of clinical characteristics and genetic backgrounds in Alport syndrome

Alport syndrome (AS) is a progressive hereditary renal disease that is characterized by sensorineural hearing loss and ocular abnormalities. It is divided into three modes of inheritance, namely, X-linked Alport syndrome (XLAS), autosomal recessive AS (ARAS), and autosomal dominant AS (ADAS). XLAS is caused by pathogenic variants in COL4A5, while ADAS and ARAS are caused by those in COL4A3/COL4A4. Diagnosis is conventionally made pathologically, but recent advances in comprehensive genetic analysis have enabled genetic testing to be performed for the diagnosis of AS as first-line diagnosis. Because of these advances, substantial information about the genetics of AS has been obtained and the genetic background of this disease has been revealed, including genotype-phenotype correlations and mechanisms of onset in some male XLAS cases that lead to milder phenotypes of late-onset end-stage renal disease (ESRD). There is currently no radical therapy for AS and treatment is only performed to delay progression to ESRD using nephron-protective drugs. Angiotensin-converting enzyme inhibitors can remarkably delay the development of ESRD. Recently, some new drugs for this disease have entered clinical trials or been developed in laboratories. In this article, we review the diagnostic strategy, genotype-phenotype correlation, mechanisms of onset of milder phenotypes, and treatment of AS, among others.

A review of clinical characteristics and genetic backgrounds in Alport syndrome

Alport syndrome (AS) is a progressive hereditary renal disease that is characterized by sensorineural hearing loss and ocular abnormalities. It is divided into three modes of inheritance, namely, X-linked Alport syndrome (XLAS), autosomal recessive AS (ARAS), and autosomal dominant AS (ADAS). XLAS is caused by pathogenic variants in COL4A5, while ADAS and ARAS are caused by those in COL4A3/COL4A4. Diagnosis is conventionally made pathologically, but recent advances in comprehensive genetic analysis have enabled genetic testing to be performed for the diagnosis of AS as first-line diagnosis. Because of these advances, substantial information about the genetics of AS has been obtained and the genetic background of this disease has been revealed, including genotype–phenotype correlations and mechanisms of onset in some male XLAS cases that lead to milder phenotypes of late-onset end-stage renal disease (ESRD). There is currently no radical therapy for AS and treatment is only performed to delay progression to ESRD using nephron-protective drugs. Angiotensin-converting enzyme inhibitors can remarkably delay the development of ESRD. Recently, some new drugs for this disease have entered clinical trials or been developed in laboratories. In this article, we review the diagnostic strategy, genotype–phenotype correlation, mechanisms of onset of milder phenotypes, and treatment of AS, among others.

The activin receptor is stimulated in the skeleton, vasculature, heart, and kidney during chronic kidney disease

We examined activin receptor type IIA (ActRIIA) activation in chronic kidney disease (CKD) by signal analysis and inhibition in mice with Alport syndrome using the ActRIIA ligand trap RAP-011 initiated in 75-day-old Alport mice. At 200 days of age, there was severe CKD and associated Mineral and Bone Disorder (CKD-MBD), consisting of osteodystrophy, vascular calcification, cardiac hypertrophy, hyperphosphatemia, hyperparathyroidism, elevated FGF23, and reduced klotho. The CKD-induced bone resorption and osteoblast dysfunction was reversed, and bone formation was increased by RAP-011. ActRIIA inhibition prevented the formation of calcium apatite deposits in the aortic adventitia and tunica media and significantly decreased the mean aortic calcium concentration from 0.59 in untreated to 0.36 mg/g in treated Alport mice. Aortic ActRIIA stimulation in untreated mice increased p-Smad2 levels and the transcription of sm22α and αSMA. ActRIIA inhibition reversed aortic expression of the osteoblast transition markers Runx2 and osterix. Heart weight was significantly increased by 26% in untreated mice but remained normal during RAP-011 treatment. In 150-day-old mice, GFR was significantly reduced by 55%, but only by 30% in the RAP-011-treated group. In 200-day-old mice, the mean BUN was 100 mg/dl in untreated mice compared to 60 mg/dl in the treated group. In the kidneys of 200-day-old mice, ActRIIA and p-Smad2 were induced and MCP-1, fibronectin, and interstitial fibrosis were stimulated; all were attenuated by RAP-011 treatment. Hence, the activation of ActRIIA signaling during early CKD contributes to the CKD-MBD components of osteodystrophy and cardiovascular disease and to renal fibrosis. Thus, the inhibition of ActRIIA signaling is efficacious in improving and delaying CKD-MBD in this model of Alport syndrome.

Angiotensin-converting enzyme 2 amplification limited to the circulation does not protect mice from development of diabetic nephropathy

Blockers of the renin-angiotensin system are effective in the treatment of experimental and clinical diabetic nephropathy. An approach different from blocking the formation or action of angiotensin II (1-8) that could also be effective involves fostering its degradation. Angiotensin-converting enzyme 2 (ACE2) is a monocarboxypeptidase that cleaves angiotensin II (1-8) to form angiotensin (1-7). Therefore, we examined the renal effects of murine recombinant ACE2 in mice with streptozotocin-induced diabetic nephropathy as well as that of amplification of circulating ACE2 using minicircle DNA delivery prior to induction of experimental diabetes. This delivery resulted in a long-term sustained and profound increase in serum ACE2 activity and enhanced ability to metabolize an acute angiotensin II (1-8) load. In mice with streptozotocin-induced diabetes pretreated with minicircle ACE2, ACE2 protein in plasma increased markedly and this was associated with a more than 100-fold increase in serum ACE2 activity. However, minicircle ACE2 did not result in changes in urinary ACE2 activity as compared to untreated diabetic mice. In both diabetic groups, glomerular filtration rate increased significantly and to the same extent as compared to non-diabetic controls. Albuminuria, glomerular mesangial expansion, glomerular cellularity, and glomerular size were all increased to a similar extent in minicircle ACE2-treated and untreated diabetic mice, as compared to non-diabetic controls. Recombinant mouse ACE2 given for 4 weeks by intraperitoneal daily injections in mice with streptozotocin-induced diabetic nephropathy also failed to improve albuminuria or kidney pathology. Thus, a profound augmentation of ACE2 confined to the circulation failed to ameliorate the glomerular lesions and hyperfiltration characteristic of early diabetic nephropathy. These findings emphasize the importance of targeting the kidney rather than the circulatory renin angiotensin system to combat diabetic nephropathy.

Novel Anti-fibrotic Therapies

Fibrosis is a major player in cardiovascular disease, both as a contributor to the development of disease, as well as a post-injury response that drives progression. Despite the identification of many mechanisms responsible for cardiovascular fibrosis, to date no treatments have emerged that have effectively reduced the excess deposition of extracellular matrix associated with fibrotic conditions. Novel treatments have recently been identified that hold promise as potential therapeutic agents for cardiovascular diseases associated with fibrosis, as well as other fibrotic conditions. The purpose of this review is to provide an overview of emerging antifibrotic agents that have shown encouraging results in preclinical or early clinical studies, but have not yet been approved for use in human disease. One of these agents is bone morphogenetic protein-7 (BMP7), which has beneficial effects in multiple models of fibrotic disease. Another approach discussed involves altering the levels of micro-RNA (miR) species, including miR-29 and miR-101, which regulate the expression of fibrosis-related gene targets. Further, the antifibrotic potential of agonists of the peroxisome proliferator-activated receptors will be discussed. Finally, evidence will be reviewed in support of the polypeptide hormone relaxin. Relaxin is long known for its extracellular remodeling properties in pregnancy, and is rapidly emerging as an effective antifibrotic agent in a number of organ systems. Moreover, relaxin has potent vascular and renal effects that make it a particularly attractive approach for the treatment of cardiovascular diseases. In each case, the mechanism of action and the applicability to various fibrotic diseases will be discussed.

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

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

The Players: Cells Involved in Glomerular Disease

Glomerular diseases are common and important. They can arise from systemic inflammatory or metabolic diseases that affect the kidney. Alternately, they are caused primarily by local glomerular abnormalities, including genetic diseases. Both intrinsic glomerular cells and leukocytes are critical to the healthy glomerulus and to glomerular dysregulation in disease. Mesangial cells, endothelial cells, podocytes, and parietal epithelial cells within the glomerulus all play unique and specialized roles. Although a specific disease often primarily affects a particular cell type, the close proximity, and interdependent functions and interactions between cells mean that even diseases affecting one cell type usually indirectly influence others. In addition to those cells intrinsic to the glomerulus, leukocytes patrol the glomerulus in health and mediate injury in disease. Distinct leukocyte types and subsets are present, with some being involved in different ways in an individual glomerular disease. Cells of the innate and adaptive immune systems are important, directing systemic immune and inflammatory responses, locally mediating injury, and potentially dampening inflammation and facilitating repair. The advent of new genetic and molecular techniques, and new disease models means that we better understand both the basic biology of the glomerulus and the pathogenesis of glomerular disease. This understanding should lead to better diagnostic techniques, biomarkers, and predictors of prognosis, disease severity, and relapse. With this knowledge comes the promise of better therapies in the future, directed toward halting pathways of injury and fibrosis, or interrupting the underlying pathophysiology of the individual diseases that lead to significant and progressive glomerular disease.

Collagen IV diseases: A focus on the glomerular basement membrane in Alport syndrome

Alport syndrome is the result of mutations in any of three type IV collagen genes, COL4A3, COL4A4, or COL4A5. Because the three collagen chains form heterotrimers, there is an absence of all three proteins in the basement membranes where they are expressed. In the glomerulus, the mature glomerular basement membrane type IV collagen network, normally comprised of two separate networks, α3(IV)/α4(IV)/α5(IV) and α1(IV)/α2(IV), is comprised entirely of collagen α1(IV)/α2. This review addresses the current state of our knowledge regarding the consequence of this change in basement membrane composition, including both the direct, via collagen receptor binding, and indirect, regarding influences on glomerular biomechanics. The state of our current understanding regarding mechanisms of glomerular disease initiation and progression will be examined, as will the current state of the art regarding emergent therapeutic approaches to slow or arrest glomerular disease in Alport patients.

Collagen IV diseases: A focus on the glomerular basement membrane in Alport syndrome

Alport syndrome is the result of mutations in any of three type IV collagen genes, COL4A3, COL4A4, or COL4A5. Because the three collagen chains form heterotrimers, there is an absence of all three proteins in the basement membranes where they are expressed. In the glomerulus, the mature glomerular basement membrane type IV collagen network, normally comprised of two separate networks, α3(IV)/α4(IV)/α5(IV) and α1(IV)/α2(IV), is comprised entirely of collagen α1(IV)/α2. This review addresses the current state of our knowledge regarding the consequence of this change in basement membrane composition, including both the direct, via collagen receptor binding, and indirect, regarding influences on glomerular biomechanics. The state of our current understanding regarding mechanisms of glomerular disease initiation and progression will be examined, as will the current state of the art regarding emergent therapeutic approaches to slow or arrest glomerular disease in Alport patients.

Heterogeneous fibroblasts underlie age-dependent tertiary lymphoid tissues in the kidney

Acute kidney injury (AKI) is a common clinical condition defined as a rapid decline in kidney function. AKI is a global health burden, estimated to cause 2 million deaths annually worldwide. Unlike AKI in the young, which is reversible, AKI in the elderly often leads to end-stage renal disease, and the mechanism that prevents kidney repair in the elderly is unclear. Here we demonstrate that aged but not young mice developed multiple tertiary lymphoid tissues (TLTs) in the kidney after AKI. TLT size was associated with impaired renal function and increased expression of proinflammatory cytokines and homeostatic chemokines, indicating a possible contribution of TLTs to sustained inflammation after injury. Notably, resident fibroblasts from a single lineage diversified into p75 neurotrophin receptor+ (p75NTR+) fibroblasts and homeostatic chemokine-producing fibroblasts inside TLTs, and retinoic acid-producing fibroblasts around TLTs. Deletion of CD4+ cells as well as late administration of dexamethasone abolished TLTs and improved renal outcomes. Importantly, aged but not young human kidneys also formed TLTs that had cellular and molecular components similar to those of mouse TLTs. Therefore, the inhibition of TLT formation may offer a novel therapeutic strategy for AKI in the elderly.

Imbalance Between Bone Morphogenetic Protein 2 and Noggin Induces Abnormal Osteogenic Differentiation of Mesenchymal Stem Cells in Ankylosing Spondylitis

OBJECTIVE

To study the osteogenic differentiation capacity of bone marrow-derived mesenchymal stem cells (BM-MSCs) from patients with ankylosing spondylitis (AS) and to investigate the mechanisms of abnormal osteogenic differentiation of BM-MSCs in AS.

METHODS

BM-MSCs from healthy donors (HD-MSCs) and patients with AS (AS-MSCs) were cultured in osteogenic differentiation medium for 0-21 days, after which their osteogenic differentiation capacity was determined using alizarin red S and alkaline phosphatase assays. Gene expression levels of osteoblastic markers and related cytokines were detected by high-throughput quantitative reverse transcription-polymerase chain reaction. Enzyme-linked immunosorbent assay was performed to detect protein levels of bone morphogenetic protein 2 (BMP-2) and Noggin in the cell culture supernatant. The activation of Smad1/5/8 and MAPK signaling pathways was measured by Western blotting. The balance between BMP-2 and Noggin expression was regulated using lentiviruses encoding short hairpin RNA and exogenous Noggin, respectively, which enabled evaluation of how this balance affected osteogenic differentiation of AS-MSCs.

RESULTS

AS-MSCs outperformed HD-MSCs in osteogenic differentiation capacity. During osteogenic differentiation, AS-MSCs secreted more BMP-2 but less Noggin, accompanied by an overactivation of Smad1/5/8 and ERK-1/2. When the Noggin concentration was increased or BMP-2 expression was inhibited, the abnormal osteogenic differentiation of AS-MSCs was rectified. In addition, the balance between BMP-2 and Noggin secretion was restored.

CONCLUSION

The results of this study demonstrate that an imbalance between BMP-2 and Noggin secretion induces abnormal osteogenic differentiation of AS-MSCs. These findings reveal a mechanism of pathologic osteogenesis in AS and provide a new perspective on inhibiting pathologic osteogenesis by regulating the balance between BMP-2 and Noggin.

Protein DJ-1 and its anti-oxidative stress function play an important role in renal cell mediated response to profibrotic agents

In the pathogenesis of renal fibrosis, oxidative stress (OS) enhances the production of reactive oxygen species (ROS) leading to sustained cell growth, inflammation, excessive tissue remodelling and accumulation, which results in the development and acceleration of renal damage.

Severity and Frequency of Proximal Tubule Injury Determines Renal Prognosis

AKI increases the risk of developing CKD, but the mechanisms linking AKI to CKD remain unclear. Because proximal tubule injury is the mainstay of AKI, we postulated that proximal tubule injury triggers features of CKD. We generated a novel mouse model to induce proximal tubule-specific adjustable injury by inducing the expression of diphtheria toxin (DT) receptor with variable prevalence in proximal tubules. Administration of high-dose DT in mice expressing the DT receptor consistently caused severe proximal tubule-specific injury associated with interstitial fibrosis and reduction of erythropoietin production. Mild proximal tubule injury from a single injection of low-dose DT triggered reversible fibrosis, whereas repeated mild injuries caused sustained interstitial fibrosis, inflammation, glomerulosclerosis, and atubular glomeruli. DT-induced proximal tubule-specific injury also triggered distal tubule injury. Furthermore, injured tubular cells cocultured with fibroblasts stimulated induction of extracellular matrix and inflammatory genes. These results support the existence of proximal-distal tubule crosstalk and crosstalk between tubular cells and fibroblasts. Overall, our data provide evidence that proximal tubule injury triggers several features of CKD and that the severity and frequency of proximal tubule injury determines the progression to CKD.

Bone Morphogenetic Protein 4 and Smad1 Mediate Extracellular Matrix Production in the Development of Diabetic Nephropathy

Diabetic nephropathy is the leading cause of end-stage renal disease. It is pathologically characterized by the accumulation of extracellular matrix in the mesangium, of which the main component is α1/α2 type IV collagen (Col4a1/a2). Recently, we identified Smad1 as a direct regulator of Col4a1/a2 under diabetic conditions in vitro. Here, we demonstrate that Smad1 plays a key role in diabetic nephropathy through bone morphogenetic protein 4 (BMP4) in vivo. Smad1-overexpressing mice (Smad1-Tg) were established, and diabetes was induced by streptozotocin. Nondiabetic Smad1-Tg did not exhibit histological changes in the kidney; however, the induction of diabetes resulted in an ∼1.5-fold greater mesangial expansion, consistent with an increase in glomerular phosphorylated Smad1. To address regulatory factors of Smad1, we determined that BMP4 and its receptor are increased in diabetic glomeruli and that diabetic Smad1-Tg and wild-type mice treated with a BMP4-neutralizing antibody exhibit decreased Smad1 phosphorylation and ∼40% less mesangial expansion than those treated with control IgG. Furthermore, heterozygous Smad1 knockout mice exhibit attenuated mesangial expansion in the diabetic condition. The data indicate that BMP4/Smad1 signaling is a critical cascade for the progression of mesangial expansion and that blocking this signal could be a novel therapeutic strategy for diabetic nephropathy.

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.

Podocyte p53 Limits the Severity of Experimental Alport Syndrome

Alport syndrome (AS) is one of the most common types of inherited nephritis caused by mutation in one of the glomerular basement membrane components. AS is characterized by proteinuria at early stage of the disease and glomerular hyperplastic phenotype and renal fibrosis at late stage. Here, we show that global deficiency of tumor suppressor p53 significantly accelerated AS progression in X-linked AS mice and decreased the lifespan of these mice. p53 protein expression was detected in 21-week-old wild-type mice but not in age-matched AS mice. Expression of proinflammatory cytokines and profibrotic genes was higher in p53(+/-) AS mice than in p53(+/+) AS mice. In vitro experiments revealed that p53 modulates podocyte migration and positively regulates the expression of podocyte-specific genes. We established podocyte-specific p53 (pod-p53)-deficient AS mice, and determined that pod-p53 deficiency enhanced the AS-induced renal dysfunction, foot process effacement, and alteration of gene-expression pattern in glomeruli. These results reveal a protective role of p53 in the progression of AS and in maintaining glomerular homeostasis by modulating the hyperplastic phenotype of podocytes in AS.

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.

Renal allograft fibrosis: biology and therapeutic targets

Renal tubulointerstitial fibrosis is the final common pathway of progressive renal diseases. In allografts, it is assessed with tubular atrophy as interstitial fibrosis/tubular atrophy (IF/TA). IF/TA occurs in about 40% of kidney allografts at 3-6 months after transplantation, increasing to 65% at 2 years. The origin of renal fibrosis in the allograft is complex and includes donor-related factors, in particular in case of expanded criteria donors, ischemia-reperfusion injury, immune-mediated damage, recurrence of underlying diseases, hypertensive damage, nephrotoxicity of immunosuppressants, recurrent graft infections, postrenal obstruction, etc. Based largely on studies in the non-transplant setting, there is a large body of literature on the role of different cell types, be it intrinsic to the kidney or bone marrow derived, in mediating renal fibrosis, and the number of mediator systems contributing to fibrotic changes is growing steadily. Here we review the most important cellular processes and mediators involved in the progress of renal fibrosis, with a focus on the allograft situation, and discuss some of the challenges in translating experimental insights into clinical trials, in particular fibrosis biomarkers or imaging modalities.

Searching for a treatment for Alport syndrome using mouse models

Alport syndrome (AS) is a hereditary nephritis caused by mutations in COL4A3, COL4A4 or COL4A5 encoding the type IV collagen α3, α4, and α5 chains, which are major components of the glomerular basement membrane. About 20 years have passed since COL4A3, COL4A4, and COL4A5 were identified and the first Alport mouse model was developed using a knockout approach. The phenotype of Alport mice is similar to that of Alport patients, including characteristic thickening and splitting of the glomerular basement membrane. Alport mice have been widely used to study the pathogenesis of AS and to develop effective therapies. In this review, the newer therapies for AS, such as pharmacological interventions, genetic approaches and stem cell therapies, are discussed. Although some stem cell therapies have been demonstrated to slow the renal disease progression in Alport mice, these therapies demand continual refinement as research advances. In terms of the pharmacological drugs, angiotensin-converting enzyme inhibitors have been shown to be effective in Alport mice. Novel therapies that can provide a better outcome or lead to a cure are still awaited.

Alport syndrome: its effects on the glomerular filtration barrier and implications for future treatment

The glomerular filtration barrier comprises a fenestrated capillary endothelium, glomerular basement membrane and podocyte slit diaphragm. Over the past decade we have come to realise that permselectivity depends on size and not necessarily charge, that the molecular sieve depends on the podocyte contractile apparatus and is highly dynamic, and that protein uptake by proximal tubular epithelial cells stimulates signalling and the production of transcription factors and inflammatory mediators. Alport syndrome is the second commonest monogenic cause of renal failure after autosomal dominant polycystic kidney disease. Eighty per cent of patients have X-linked disease caused by mutations in the COL4A5 gene. Most of these result in the replacement of the collagen IV α3α4α5 network with the α1α1α2 heterotrimer. Affected membranes also have ectopic laminin and increased matrix metalloproteinase levels, which makes them more susceptible to proteolysis. Mechanical stress, due to the less elastic membrane and hypertension, interferes with integrin-mediated podocyte-GBM adhesion. Proteinuria occurs when urinary levels exceed tubular reabsorption rates, and initiates tubulointerstitial fibrosis. The glomerular mesangial cells produce increased TGFβ and CTGF which also contribute to glomerulosclerosis. Currently there is no specific therapy for Alport syndrome. However treatment with angiotensin converting enzyme (ACE) inhibitors delays renal failure progression by reducing intraglomerular hypertension, proteinuria, and fibrosis. Our greater understanding of the mechanisms underlying the GBM changes and their consequences in Alport syndrome have provided us with further novel therapeutic targets.

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.

Twisted gastrulation, a BMP antagonist, exacerbates podocyte injury

Podocyte injury is the first step in the progression of glomerulosclerosis. Previous studies have demonstrated the beneficial effect of bone morphogenetic protein 7 (Bmp7) in podocyte injury and the existence of native Bmp signaling in podocytes. Local activity of Bmp7 is controlled by cell-type specific Bmp antagonists, which inhibit the binding of Bmp7 to its receptors. Here we show that the product of Twisted gastrulation (Twsg1), a Bmp antagonist, is the central negative regulator of Bmp function in podocytes and that Twsg1 null mice are resistant to podocyte injury. Twsg1 was the most abundant Bmp antagonist in murine cultured podocytes. The administration of Bmp induced podocyte differentiation through Smad signaling, whereas the simultaneous administration of Twsg1 antagonized the effect. The administration of Bmp also inhibited podocyte proliferation, whereas simultaneous administration of Twsg1 antagonized the effect. Twsg1 was expressed in the glomerular parietal cells (PECs) and distal nephron of the healthy kidney, and additionally in damaged glomerular cells in a murine model of podocyte injury. Twsg1 null mice exhibited milder hypoalbuminemia and hyperlipidemia, and milder histological changes while maintaining the expression of podocyte markers during podocyte injury model. Taken together, our results show that Twsg1 plays a critical role in the modulation of protective action of Bmp7 on podocytes, and that inhibition of Twsg1 is a promising means of development of novel treatment for podocyte injury.

Renal function can improve at any stage of chronic kidney disease

Introduction

Even though renal function decline is considered relentless in chronic kidney disease (CKD), improvement has been shown in patients with hypertensive nephropathy. Whether this can occur in any type of nephropathy and at any stage is unknown as are the features of patients who improve.

Methods

We identified 406 patients in the NephroTest cohort with glomerular filtration rates (mGFR) measured by ⁵¹Cr-EDTA clearance at least 3 times during at least 2 years of follow-up. Individual examination of mGFR trajectories by 4 independent nephrologists classified patients as improvers, defined as those showing a sustained mGFR increase, or nonimprovers. Twelve patients with erratic trajectories were excluded. Baseline data were compared between improvers and nonimprovers, as was the number of recommended therapeutic targets achieved over time (specifically, for systolic and diastolic blood pressure, proteinuria, and use of renin angiotensin system blockers).

Results

Measured GFR improved over time in 62 patients (15.3%). Their median mGFR slope was +1.88[IQR 1.38, 3.55] ml/min/year; it was −2.23[−3.9, −0.91] for the 332 nonimprovers. Improvers had various nephropathies, but not diabetic glomerulopathy or polycystic kidney disease. They did not differ from nonimprovers for age, sex, cardiovascular history, or CKD stage, but their urinary albumin excretion rate was lower. Improvers achieved significantly more recommended therapeutic targets (2.74±0.87) than nonimprovers (2.44±0.80, p<0.01). They also had fewer CKD-related metabolic complications and a lower prevalence of 25OH-vitamin-D deficiency.

Conclusion

GFR improvement is possible in CKD patients at any CKD stage through stage 4–5. It is noteworthy that this GFR improvement is associated with a decrease in the number of metabolic complications over time.

Sost and its paralog Sostdc1 coordinate digit number in a Gli3-dependent manner

WNT signaling is critical in most aspects of skeletal development and homeostasis, and antagonists of WNT signaling are emerging as key regulatory proteins with great promise as therapeutic agents for bone disorders. Here we show that Sost and its paralog Sostdc1 emerged through ancestral genome duplication and their expression patterns have diverged to delineate non-overlapping domains in most organ systems including musculoskeletal, cardiovascular, nervous, digestive, reproductive and respiratory. In the developing limb, Sost and Sostdc1 display dynamic expression patterns with Sost being restricted to the distal ectoderm and Sostdc1 to the proximal ectoderm and the mesenchyme. While Sostdc1^–/– mice lack any obvious limb or skeletal defects, Sost^–/– mice recapitulate the hand defects described for Sclerosteosis patients. However, elevated WNT signaling in Sost^–/–; Sostdc1^–/– mice causes misregulation of SHH signaling, ectopic activation of Sox9 in the digit 1 field and preaxial polydactyly in a Gli1- and Gli3-dependent manner. In addition, we show that the syndactyly documented in Sclerosteosis is present in both Sost^–/– and Sost^–/–; Sostdc1^–/– mice, and is driven by misregulation of Fgf8 in the AER, a region lacking Sost and Sostdc1 expression. This study highlights the complexity of WNT signaling in skeletal biology and disease and emphasizes how redundant mechanism and non-cell autonomous effects can synergize to unveil new intricate phenotypes caused by elevated WNT signaling.

Defining cell-type specificity at the transcriptional level in human disease

Cell-lineage–specific transcripts are essential for differentiated tissue function, implicated in hereditary organ failure, and mediate acquired chronic diseases. However, experimental identification of cell-lineage–specific genes in a genome-scale manner is infeasible for most solid human tissues. We developed the first genome-scale method to identify genes with cell-lineage–specific expression, even in lineages not separable by experimental microdissection. Our machine-learning–based approach leverages high-throughput data from tissue homogenates in a novel iterative statistical framework. We applied this method to chronic kidney disease and identified transcripts specific to podocytes, key cells in the glomerular filter responsible for hereditary and most acquired glomerular kidney disease. In a systematic evaluation of our predictions by immunohistochemistry, our in silico approach was significantly more accurate (65% accuracy in human) than predictions based on direct measurement of in vivo fluorescence-tagged murine podocytes (23%). Our method identified genes implicated as causal in hereditary glomerular disease and involved in molecular pathways of acquired and chronic renal diseases. Furthermore, based on expression analysis of human kidney disease biopsies, we demonstrated that expression of the podocyte genes identified by our approach is significantly related to the degree of renal impairment in patients. Our approach is broadly applicable to define lineage specificity in both cell physiology and human disease contexts. We provide a user-friendly website that enables researchers to apply this method to any cell-lineage or tissue of interest. Identified cell-lineage–specific transcripts are expected to play essential tissue-specific roles in organogenesis and disease and can provide starting points for the development of organ-specific diagnostics and therapies.

An update on the pathomechanisms and future therapies of Alport syndrome

Alport Syndrome (AS) is an inherited progressive disease that is caused by mutations of the genes encoding the key collagen chains, α3, α4, and α5, which are necessary for the composition of collagen type IV to form a robust glomerular basement membrane (GBM), capable of withstanding the significant biomechanical strain to which the glomerulus is subjected. Progressive loss of the filtration barrier allows excessive proteinuria, which ultimately leads to end-stage kidney disease (ESKD). The evidence for a beneficial renoprotective effect of renin-angiotensin-aldosterone system (RAAS) blockade by angiotensin-converting enzyme (ACE) inhibition and/or angiotensin receptor blockers (ARBs) is well established in AS and recent evidence has shown that it can significantly delay the time to onset of renal replacement therapy and ESKD. Future potential treatments of AS disease progression are evaluated in this review.

The phosphatase Dullard negatively regulates BMP signalling and is essential for nephron maintenance after birth

Most kidney nephron components, including glomeruli and renal tubules, derive from the metanephric mesenchyme. The overall differentiation into each component finishes at birth, but the molecular events linking the perinatal and adult kidneys remain elusive. Dullard was cloned from Xenopus kidneys, and encodes a phosphatase that negatively regulates BMP signalling. Here we report that Dullard deletion in the murine metanephric mesenchyme leads to failure of nephron maintenance after birth, resulting in lethality before adulthood. The nephron components are lost by massive apoptosis within 3 weeks after birth, leading to formation of a large hollow with a thin-layered cortex and medulla. Phosphorylated Smad1/5/8 is upregulated in the mutant nephrons, probably through cell-autonomous inhibitory effects of Dullard on BMP signalling. Importantly, administration of the BMP receptor kinase inhibitor LDN-193189 partially rescued the defects caused by Dullard deletion. Thus, Dullard keeps BMP signalling at an appropriate level, which is required for nephron maintenance in the postnatal period.

Inhibition of WISE preserves renal allograft function

Wnt-modulator in surface ectoderm (WISE) is a secreted modulator of Wnt signaling expressed in the adult kidney. Activation of Wnt signaling has been observed in renal transplants developing interstitial fibrosis and tubular atrophy; however, whether WISE contributes to chronic changes is not well understood. Here, we found moderate to high expression of WISE mRNA in a rat model of renal transplantation and in kidneys from normal rats. Treatment with a neutralizing antibody against WISE improved proteinuria and graft function, which correlated with higher levels of β-catenin protein in kidney allografts. In addition, treatment with the anti-WISE antibody reduced infiltration of CD68(+) macrophages and CD8(+) T cells, attenuated glomerular and interstitial injury, and decreased biomarkers of renal injury. This treatment reduced expression of genes involved in immune responses and in fibrogenic pathways. In summary, WISE contributes to renal dysfunction by promoting tubular atrophy and interstitial fibrosis.