The role of the activin-follistatin system in the developmental and regeneration processes of the kidney

The activin-follistatin system: Key regulator of kidney development, regeneration, inflammation, and fibrosis

Activins, multifunctional cytokines of the transforming growth factor-beta superfamily, play critical roles in the regulation of growth and differentiation in multiple biological systems. Activin activity is finely regulated by the endogenous antagonist follistatin. Early studies reported that activins are involved in renal organogenesis, but subsequent research demonstrated that activins also play a significant role in kidney regeneration following injury. The results of more recent studies suggest activins play roles in both inflammatory kidney diseases and renal fibrosis, conditions that often culminate in end-stage renal disease. Given these findings, the inhibition of activin activity represents a promising therapeutic approach for treating a range of kidney disorders. This review discusses the latest discoveries concerning the role of the activin-follistatin system in renal development and pathophysiology and explores the potential therapeutic implications of targeting this system in the management of kidney diseases.

Tubule-Derived Follistatin Is Increased in the Urine of Rats with Renal Ischemia and Reflects the Severity of Acute Tubular Damage

Activin A, a member of the TGF-beta superfamily, is a negative regulator of tubular regeneration after renal ischemia. Activin action is controlled by an endogenous antagonist, follistatin. However, the role of follistatin in the kidney is not fully understood. In the present study, we examined the expression and localization of follistatin in normal and ischemic rat kidneys and measured urinary follistatin in rats with renal ischemia to assess whether urinary follistatin could serve as a biomarker for acute kidney injury. Using vascular clamps, renal ischemia was induced for 45 min in 8-week-old male Wistar rats. In normal kidneys, follistatin was localized in distal tubules of the cortex. In contrast, in ischemic kidneys, follistatin was localized in distal tubules of both the cortex and outer medulla. Follistatin mRNA was mainly present in the descending limb of Henle of the outer medulla in normal kidneys but was upregulated in the descending limb of Henle of both the outer and inner medulla after renal ischemia. Urinary follistatin, which was undetectable in normal rats, was significantly increased in ischemic rats and peaked 24 h after reperfusion. There was no correlation between urinary follistatin and serum follistatin. Urinary follistatin levels were increased according to ischemic duration and were significantly correlated with the follistatin-positive area as well as the acute tubular damage area. These results suggest that follistatin normally produced by renal tubules increases and becomes detectable in urine after renal ischemia. Urinary follistatin might be useful to assess the severity of acute tubular damage.

Kidney development to kidney organoids and back again

Kidney organoid technology has led to a renaissance in kidney developmental biology. The complex underpinnings of mammalian kidney development have provided a framework for the generation of kidney cells and tissues from human pluripotent stem cells. Termed kidney organoids, these 3-dimensional structures contain kidney-specific cell types distributed similarly to in vivo architecture. The adult human kidney forms from the reciprocal induction of two disparate tissues, the metanephric mesenchyme (MM) and ureteric bud (UB), to form nephrons and collecting ducts, respectively. Although nephrons and collecting ducts are derived from the intermediate mesoderm (IM), their development deviates in time and space to impart distinctive inductive signaling for which separate differentiation protocols are required. Here we summarize the directed differentiation protocols which generate nephron kidney organoids and collecting duct kidney organoids, making note of similarities as much as differences. We discuss limitations of these present approaches and discuss future directions to improve kidney organoid technology, including a greater understanding of anterior IM and its derivatives to enable an improved differentiation protocol to collecting duct organoids for which historic and future developmental biology studies will be instrumental.

Transforming growth factor beta signaling functions during mammalian kidney development

Aberrant transforming growth factor beta (TGFβ) signaling during embryogenesis is implicated in severe congenital abnormalities, including kidney malformations. However, the molecular mechanisms that underlie congenital kidney malformations related to TGFβ signaling remain poorly understood. Here, we review current understanding of the lineage-specific roles of TGFβ signaling during kidney development and how dysregulation of TGFβ signaling contributes to the pathogenesis of kidney malformation.

The Influence of Dietary Interventions on Chronic Kidney Disease-Mineral and Bone Disorder (CKD-MBD)

Chronic kidney disease is a health problem whose prevalence is increasing worldwide. The kidney plays an important role in the metabolism of minerals and bone health and therefore, even at the early stages of CKD, disturbances in bone metabolism are observed. In the course of CKD, various bone turnover or mineralization disturbances can develop including adynamic hyperparathyroid, mixed renal bone disease, osteomalacia. The increased risk of fragility fractures is present at any age in these patients. Nutritional treatment of patients with advanced stages of CKD is aiming at prevention or correction of signs, symptoms of renal failure, avoidance of protein-energy wasting (PEW), delaying or prevention of the occurrence of mineral/bone disturbances, and delaying the start of dialysis. The results of studies suggest that progressive protein restriction is beneficial with the progression of renal insufficiency; however, other aspects of dietary management of CKD patients, including changes in sodium, phosphorus, and energy intake, as well as the source of protein and lipids (animal or plant origin) should also be considered carefully. Energy intake must cover patients' energy requirement, in order to enable correct metabolic adaptation in the course of protein-restricted regimens and prevent negative nitrogen balance and protein-energy wasting.

Recapitulating kidney development in vitro by priming and differentiating mouse embryonic stem cells in monolayers

In order to harness the potential of pluripotent stem cells, we need to understand how to differentiate them to our target cell types. Here, we developed a protocol to differentiate mouse embryonic stem cells (ESCs) to renal progenitors in a step-wise manner. Microarrays were used to track the transcriptional changes at each stage of differentiation and we observed that genes associated with metanephros, ureteric bud, and blood vessel development were significantly upregulated as the cells differentiated towards renal progenitors. Priming the ESCs and optimizing seeding cell density and growth factor concentrations helped improve differentiation efficiency. Organoids were used to determine the developmental potential of the renal progenitor cells. Aggregated renal progenitors gave rise to organoids consisting of LTL+/E-cadherin+ proximal tubules, cytokeratin+ ureteric bud-derived tubules, and extracellular matrix proteins secreted by the cells themselves. Over-expression of key kidney developmental genes, Pax2, Six1, Eya1, and Hox11 paralogs, during differentiation did not improve differentiation efficiency. Altogether, we developed a protocol to differentiate mouse ESCs in a manner that recapitulates embryonic kidney development and showed that precise gene regulation is essential for proper differentiation to occur.

Serum Activin A Levels and Renal Outcomes After Coronary Angiography

Prevention for contrast-induced nephropathy (CIN) is limited by the lack of a single predictor. As activin A is upregulated in heart failure and chronic kidney disease, we aimed to clarify the association between activin A levels and renal outcomes after coronary angiography (CAG). This prospective observational study included 267 patients who received CAG between 2009 and 2015. CIN was defined as elevation of serum creatinine to >0.5 mg/dL or to >25% above baseline within 48 hours after CAG. During follow-up, laboratory parameters were measured every 3–6 months. Renal decline was defined as>2-fold increase in serum creatinine or initiation of dialysis. The patients were stratified into tertiles according to serum activin A levels at baseline. High activin A tertile was significantly associated more CIN and renal function decline compared to low activin A tertile (all p < 0.001). After adjusting potential confounding factors, high serum activin A tertiles was associated to CIN (Odds ratio 4.49, 95% CI 1.07–18.86, p = 0.040) and renal function decline (Hazard ratio 4.49, 95% CI 1.27–11.41, p = 0.017) after CAG.

Urinary Activin A is a novel biomarker reflecting renal inflammation and tubular damage in ANCA-associated vasculitis

Activin A, a member of the transforming growth factor-beta superfamily, is a critical modulator of inflammation and plays a key role in controlling the cytokine cascade that drives the inflammatory response. However, the role of activin A in inflammatory kidney diseases remains unknown. To address this issue, we examined here whether activin A can be detected in the kidney and/or urine from patients with antineutrophil cytoplasmic antibody (ANCA) -associated vasculitis (AAV). Fifty-one patients who had been diagnosed with AAV and were treated in our department between November 2011 to March 2018 were included in this study. Forty-one patients had renal complications (renal AAV). Serum and urinary activin A levels were measured by enzyme-linked immunosorbent assay. Correlation of urinary activin A concentration with clinical parameters was analyzed. Urinary activin A was undetectable in healthy volunteers. In contrast, urinary activin A concentration was significantly increased in patients with renal AAV but not in those with non-renal AAV. Urinary activin A concentration decreased rapidly after immunosuppressive treatment. There was a significant correlation of urinary activin A level with urinary protein, L-FABP, and NAG. Histologic evaluation revealed that urinary activin A levels were significantly higher in patients with cellular crescentic glomeruli than in those lacking this damage. In situ hybridization demonstrated that the mRNA encoding the activin A βA subunit was undetectable in normal kidneys but accumulated in the proximal tubules and crescentic glomeruli of the kidneys of patients with renal AAV. Immunostaining showed that activin A protein also was present in the proximal tubules, crescentic glomeruli, and macrophages infiltrating into the interstitium in the kidneys of patients with renal AAV. These data suggested that urinary activin A concentration reflects renal inflammation and tubular damage in AAV and may be a useful biomarker for monitoring renal AAV.

Activin A: a novel urinary biomarker of renal impairment in multiple myeloma

Renal impairment (RI) is a common complication of multiple myeloma (MM) that significantly affects treatment efficacy and mortality. However, no useful biomarkers for early detection of renal damage in MM exist. Reports indicate that activin A, a multifunctional cytokine of the TGF-β superfamily, is involved in the development and progression of various kidney diseases. In the present study, we measured urinary activin A levels in patients with newly diagnosed MM (NDMM) (n=41), smoldering MM (SMM) (n=10), and monoclonal gammopathy of undetermined significance (MGUS) (n=28), including monoclonal gammopathy of renal significance (MGRS), and assessed the correlation between urinary activin A and several clinical parameters. Urinary activin A, undetectable in healthy volunteers, was significantly increased in NDMM patients but not in patients with SMM and MGUS (97.3, 25.0, and 6.61 mg/gCr, respectively, P<0.05). In all patients with NDMM, urinary activin A levels were significantly reduced after initial treatment regardless of the therapy regimen. There was a significant correlation of urinary activin A with spot urinary protein level (P<0.001) and serum M-protein (P=0.029) but not with estimated glomerular filtration rate (eGFR), serum creatinine (Cr), N-acetyl-glucosaminidase (NAG), and serum activin A level. Histological analysis using renal biopsy samples revealed that activin A, which was absent from normal kidneys, was detected in the renal tubular cells of patients with MGRS. These data suggest that urinary activin A reflects tubular injury in MM and might aid the early detection of RI in plasma cell neoplasms.

Bioactive Compounds for the Treatment of Renal Disease

Kidney diseases including acute kidney injury and chronic kidney disease are among the largest health issues worldwide. Dialysis and kidney transplantation can replace a significant portion of renal function, however these treatments still have limitations. To overcome these shortcomings, a variety of innovative efforts have been introduced, including cell-based therapies. During the past decades, advances have been made in the stem cell and developmental biology, and tissue engineering. As part of such efforts, studies on renal cell therapy and artificial kidney developments have been conducted, and multiple therapeutic interventions have shown promise in the pre-clinical and clinical settings. More recently, therapeutic cell-secreting secretomes have emerged as a potential alternative to cell-based approaches. This approach involves the use of renotropic factors, such as growth factors and cytokines, that are produced by cells and these factors have shown effectiveness in facilitating kidney function recovery. This review focuses on the renotropic functions of bioactive compounds that provide protective and regenerative effects for kidney tissue repair, based on the available data in the literature.

TGF-Beta Signaling in Bone with Chronic Kidney Disease

Transforming growth factor (TGF)-β signaling is not only important in skeletal development, but also essential in bone remodeling in adult bone. The bone remodeling process involves integrated cell activities induced by multiple stimuli to balance bone resorption and bone formation. TGF-β plays a role in bone remodeling by coordinating cell activities to maintain bone homeostasis. However, mineral metabolism disturbance in chronic kidney disease (CKD) results in abnormal bone remodeling, which leads to ectopic calcification in CKD. High circulating levels of humoral factors such as parathyroid hormone, fibroblast growth factor 23, and Wnt inhibitors modulate bone remodeling in CKD. Several reports have revealed that TGF-β is involved in the production and functions of these factors in bone. TGF-β may act as a factor that mediates abnormal bone remodeling in CKD.

GDF11 induces kidney fibrosis, renal cell epithelial-to-mesenchymal transition, and kidney dysfunction and failure

BACKGROUND

GDF11 modulates embryonic patterning and kidney organogenesis. Herein, we sought to define GDF11 function in the adult kidney and in renal diseases.

METHODS

In vitro renal cell lines, genetic, and murine in vivo renal injury models were examined.

RESULTS

Among tissues tested, Gdf11 was highest in normal adult mouse kidney. Expression was increased acutely after 5/6 nephrectomy, ischemia-reperfusion injury, kanamycin toxicity, or unilateral ureteric obstruction. Systemic, high-dose GDF11 administration in adult mice led to renal failure, with accompanying kidney atrophy, interstitial fibrosis, epithelial-to-mesenchymal transition of renal tubular cells, and eventually death. These effects were associated with phosphorylation of SMAD2 and could be blocked by follistatin. In contrast, Gdf11 heterozygous mice showed reduced renal Gdf11 expression, renal fibrosis, and expression of fibrosis-associated genes both at baseline and after unilateral ureteric obstruction compared with wild-type littermates. The kidney-specific consequences of GDF11 dose modulation are direct effects on kidney cells. GDF11 induced proliferation and activation of NRK49f renal fibroblasts and also promoted epithelial-to-mesenchymal transition of IMCD-3 tubular epithelial cells in a SMAD3-dependent manner.

CONCLUSION

Taken together, these data suggest that GDF11 and its downstream signals are critical in vivo mediators of renal injury. These effects are through direct actions of GDF11 on renal tubular cells and fibroblasts. Thus, regulation of GDF11 presents a therapeutic target for diseases involving renal fibrosis and impaired tubular function.

Klotho and activin A in kidney injury: plasma Klotho is maintained in unilateral obstruction despite no upregulation of Klotho biosynthesis in the contralateral kidney

In a new paradigm of etiology related to chronic kidney disease-mineral and bone disorder (CKD-MBD), kidney injury may cause induction of factors in the injured kidney that are released into the circulation and thereby initiate and maintain renal fibrosis and CKD-MBD. Klotho is believed to ameliorate renal fibrosis and CKD-MBD, while activin A might have detrimental effects. The unilateral ureter obstruction (UUO) model is used here to examine this concept by investigating early changes related to renal fibrosis in the obstructed kidney, untouched contralateral kidney, and vasculature which might be affected by secreted factors from the obstructed kidney, and comparing with unilateral nephrectomized controls (UNX). Obstructed kidneys showed early Klotho gene and protein depletion, whereas plasma Klotho increased in both UUO and UNX rats, indicating an altered metabolism of Klotho. Contralateral kidneys had no compensatory upregulation of Klotho and maintained normal expression of the examined fibrosis-related genes, as did remnant UNX kidneys. UUO caused upregulation of transforming growth factor-β and induction of periostin and activin A in obstructed kidneys without changes in the contralateral kidneys. Plasma activin A doubled in UUO rats after 10 days while no changes were seen in UNX rats, suggesting secretion of activin A from the obstructed kidney with potentially systemic effects on CKD-MBD. As such, increased aortic sclerostin was observed in UUO rats compared with UNX and normal controls. The present results are in line with the new paradigm and show very early vascular effects of unilateral kidney fibrosis, supporting the existence of a new kidney-vasculature axis.

Identification of Urinary Activin A as a Novel Biomarker Reflecting the Severity of Acute Kidney Injury

Acute kidney injury (AKI) is a common but complex condition that is associated with increased morbidity and mortality. In the present study, we examined whether urinary activin A, a member of the TGF-beta superfamily, is present in mice with ischemia-reperfusion injury and in humans with AKI, as well as its potential as a biomarker for AKI. Expression of activin A was markedly increased in ischemic mouse kidneys. In situ hybridization demonstrated that activin mRNA was expressed in tubular cells of ischemic kidneys but not of normal kidneys. Immunoreactive activin A, which was absent in normal kidneys, was detected in the cytoplasm of proximal tubular cells in ischemic kidneys. Activin A was undetectable in the urine of normal mice. In contrast, activin A was significantly increased in the urine of ischemic mice at 3 h after reperfusion. Urinary activin A levels increased according to the period of ischemia. In humans, urinary activin A was almost undetectable in healthy volunteers and in patients with pre-renal AKI, but was significantly increased in patients with renal AKI. There was no significant correlation between urinary activin A and serum activin A. Collectively, urinary activin A might be a useful biomarker reflecting the severity of AKI.

The chronic kidney disease - Mineral bone disorder (CKD-MBD): Advances in pathophysiology

The causes of excess cardiovascular mortality associated with chronic kidney disease (CKD) have been attributed in part to the CKD-mineral bone disorder syndrome (CKD-MBD), wherein, novel cardiovascular risk factors have been identified. New advances in the causes of the CKD-MBD are discussed in this review. They demonstrate that repair and disease processes in the kidneys release factors to the circulation that cause the systemic complications of CKD. The discovery of WNT inhibitors, especially Dickkopf 1 (Dkk1), produced during renal repair as participating in the pathogenesis of the vascular and skeletal components of the CKD-MBD implied that additional pathogenic factors are critical. This lead to the discovery that activin A is a second renal repair factor circulating in increased levels during CKD. Activin A derives from peritubular myofibroblasts of diseased kidneys, wherein it stimulates fibrosis, and decreases tubular klotho expression. Activin A binds to the type 2 activin A receptor, ActRIIA, which is variably affected by CKD in the vasculature. In diabetic/atherosclerotic aortas, specifically in vascular smooth muscle cells (VSMC), ActRIIA signaling is inhibited and contributes to CKD induced VSMC dedifferentiation, osteogenic transition and neointimal atherosclerotic calcification. In nondiabetic/nonatherosclerotic aortas, CKD increases VSMC ActRIIA signaling, and vascular fibroblast signaling causing the latter to undergo osteogenic transition and stimulate vascular calcification. In both vascular situations, a ligand trap for ActRIIA prevented vascular calcification. In the skeleton, activin A is responsible for CKD stimulation of osteoclastogenesis and bone remodeling increasing bone turnover. These studies demonstrate that circulating renal repair and injury factors are causal of the CKD-MBD and CKD associated cardiovascular disease.

Involvement of infiltrating macrophage-derived activin A in the progression of renal damage in MRL-lpr mice

Lupus nephritis is a life-threatening complication of systemic lupus erythematosus (SLE). Various growth factors, cytokines, and chemokines are implicated in the development of SLE. However, the pathophysiological processes involved in the development of lupus nephritis still remain unclear. In this study, we examined the involvement of activin A, a member of the transforming growth factor β (TGF-β) superfamily, in the progression of renal damage in lupus-prone MRL- lpr mice. Activin A was not expressed in the kidneys of control MRL-MpJ mice but was detectable in perivascular infiltrating cluster of differentiation 68 (CD68)-positive cells in the kidneys of MRL- lpr mice. Urinary activin A, which was also absent in MRL-MpJ mice, was detectable in MRL- lpr mice from 16 wk onward. Urinary activin A levels were significantly correlated with the number of perivascular inflammatory cell layers, the number of crescentic glomeruli, and the percentage of Elastica van Gieson (EVG)-positive fibrotic areas, but not with urinary protein levels or serum activin A. When activin action was blocked in vivo by the intraperitoneal administration of an activin antagonist, follistatin, the number of crescentic glomeruli, percentage of EVG-positive fibrotic areas, CD68-positive cell infiltration, and proteinuria were significantly reduced in a dose-dependent manner. These data suggest that infiltrating macrophage-derived activin A is involved in the progression of renal damage in MRL- lpr mice.

Ligand trap for the activin type IIA receptor protects against vascular disease and renal fibrosis in mice with chronic kidney disease

The causes of cardiovascular mortality associated with chronic kidney disease (CKD) are partly attributed to the CKD-mineral bone disorder (CKD-MBD). The causes of the early CKD-MBD are not well known. Our discovery of Wnt (portmanteau of wingless and int) inhibitors, especially Dickkopf 1, produced during renal repair as participating in the pathogenesis of the vascular and skeletal components of the CKD-MBD implied that additional pathogenic factors are critical. In the search for such factors, we studied the effects of activin receptor type IIA (ActRIIA) signaling by using a ligand trap for the receptor, RAP-011 (a soluble extracellular domain of ActRIIA fused to a murine IgG-Fc fragment). In a mouse model of CKD that stimulated atherosclerotic calcification, RAP-011 significantly increased aortic ActRIIA signaling assessed by the levels of phosphorylated Smad2/3. Furthermore, RAP-011 treatment significantly reversed CKD-induced vascular smooth muscle dedifferentiation as assessed by smooth muscle 22α levels, osteoblastic transition, and neointimal plaque calcification. In the diseased kidneys, RAP-011 significantly stimulated αklotho levels and it inhibited ActRIIA signaling and decreased renal fibrosis and proteinuria. RAP-011 treatment significantly decreased both renal and circulating Dickkopf 1 levels, showing that Wnt activation was downstream of ActRIIA. Thus, ActRIIA signaling in CKD contributes to the CKD-MBD and renal fibrosis. ActRIIA signaling may be a potential therapeutic target in CKD.

Pathophysiology of the chronic kidney disease-mineral bone disorder

PURPOSE OF REVIEW

The causes of excess cardiovascular mortality associated with chronic kidney disease (CKD) have been attributed in part to the CKD-mineral bone disorder syndrome (CKD-MBD), wherein, novel cardiovascular risk factors have been identified. The causes of the CKD-MBD are not well known and they will be discussed in this review

RECENT FINDINGS

The discovery of WNT (portmanteau of wingless and int) inhibitors, especially Dickkopf 1, produced during renal repair and participating in the pathogenesis of the vascular and skeletal components of the CKD-MBD implied that additional pathogenic factors are critical, leading to the finding that activin A is a second renal repair factor circulating in increased levels during CKD. Activin A derives from peritubular myofibroblasts of diseased kidneys, where it stimulates fibrosis, and decreases tubular klotho expression. The type 2 activin A receptor, ActRIIA, is decreased by CKD in atherosclerotic aortas, specifically in vascular smooth muscle cells (VSMC). Inhibition of activin signaling by a ligand trap inhibited CKD induced VSMC dedifferentiation, osteogenic transition and atherosclerotic calcification. Inhibition of activin signaling in the kidney decreased renal fibrosis and proteinuria.

SUMMARY

These studies demonstrate that circulating renal repair factors are causal for the CKD-MBD and CKD associated cardiovascular disease, and identify ActRIIA signaling as a therapeutic target in CKD that links progression of renal disease and vascular disease.

Regenerative medicine for the kidney: renotropic factors, renal stem/progenitor cells, and stem cell therapy

The kidney has the capacity for regeneration and repair after a variety of insults. Over the past few decades, factors that promote repair of the injured kidney have been extensively investigated. By using kidney injury animal models, the role of intrinsic and extrinsic growth factors, transcription factors, and extracellular matrix in this process has been examined. The identification of renal stem cells in the adult kidney as well as in the embryonic kidney is an active area of research. Cell populations expressing putative stem cell markers or possessing stem cell properties have been found in the tubules, interstitium, and glomeruli of the normal kidney. Cell therapies with bone marrow-derived hematopoietic stem cells, mesenchymal stem cells, endothelial progenitor cells, and amniotic fluid-derived stem cells have been highly effective for the treatment of acute or chronic renal failure in animals. Embryonic stem cells and induced pluripotent stem cells are also utilized for the construction of artificial kidneys or renal components. In this review, we highlight the advances in regenerative medicine for the kidney from the perspective of renotropic factors, renal stem/progenitor cells, and stem cell therapies and discuss the issues to be solved to realize regenerative therapy for kidney diseases in humans.

Follistatin, an activin antagonist, ameliorates renal interstitial fibrosis in a rat model of unilateral ureteral obstruction

Activin, a member of the TGF-β superfamily, regulates cell growth and differentiation in various cell types. Activin A acts as a negative regulator of renal development as well as tubular regeneration after renal injury. However, it remains unknown whether activin A is involved in renal fibrosis. To clarify this issue, we utilized a rat model of unilateral ureteral obstruction (UUO). The expression of activin A was significantly increased in the UUO kidneys compared to that in contralateral kidneys. Activin A was detected in glomerular mesangial cells and interstitial fibroblasts in normal kidneys. In UUO kidneys, activin A was abundantly expressed by interstitial α-SMA-positive myofibroblasts. Administration of recombinant follistatin, an activin antagonist, reduced the fibrotic area in the UUO kidneys. The number of proliferating cells in the interstitium, but not in the tubules, was significantly lower in the follistatin-treated kidneys. Expression of α-SMA, deposition of type I collagen and fibronectin, and CD68-positive macrophage infiltration were significantly suppressed in the follistatin-treated kidneys. These data suggest that activin A produced by interstitial fibroblasts acts as a potent profibrotic factor during renal fibrosis. Blockade of activin A action may be a novel approach for the prevention of renal fibrosis progression.

Effect of activin A on tubulointerstitial fibrosis in diabetic nephropathy

AIM

The effect of activin A on tubulointerstitial fibrosis in diabetic nephropathy (DN) using streptozotocin (STZ)-induced diabetic rats and high glucose-cultured HK-2 cells was investigated.

METHODS

Male Wistar rats were randomized into a normal control group (NC) and diabetes mellitus group (DM). Diabetes was induced by i.p. injection of STZ. Six rats were respectively killed 4, 8, 12 and 16 weeks after model establishment in each group. The changes of kidney weight/bodyweight (KW/BW), urine albumin excretion rate (AER) and creatinine clearance rate (Ccr) were determined. The morphology of tubulointerstitium was observed by light microscopy. Further biochemical analysis was provided using immunohistochemistry and real-time polymerase chain reaction. The different parameters in high glucose-cultured HK-2 cells were monitored by western blotting or enzyme-linked immunosorbent assay (ELISA) and the intervention of rh-follistatin on them was investigated.

RESULTS

Compared with the NC group, there was marked enlargement in the levels of KW/BW, AER, Ccr and interstitial fibrosis index, and the production of P-Smad2/3 and fibronectin in the DM group from 8 to 16 weeks. Activin betaA, mainly located in tubular epithelial cells, was significantly higher in the DM group than that in the NC group throughout the study periods. Follistatin was abundant in the NC group, but was diminished gradually in the DM group. High glucose may facilitate the synthesis of activin betaA, transforming growth factor (TGF)-beta, P-Smad2/3 and fibronectin in HK-2 cells while rh-follistatin inhibited them except TGF-beta.

CONCLUSION

Activin A is involved in tubulointerstitial fibrosis in DN by inducing the production of fibronectin through Smad signal pathway.

Hepatocyte growth factor/activin A/follistatin system activation during hemodialysis with different low molecular weight heparins

Hepatocyte growth factor (HGF), activin A (Act A), and follistatin (FS) compose an organotrophic system; interestingly it is modified by heparin. To understand if LMWHs (considered distinct drugs) have different clinical profiles regarding the above growth factors, we studied the effects of enoxaparin, nadroparin, and dalteparin on their plasma levels. Seventeen chronic HD patients completed this prospective, crossover trial. They were randomized into six groups: each patient was administered enoxaparin (effective dose of 0.75 mg/kg), nadroparin (70.4 IU/kg) and dalteparin (78.6 IU/kg) in three time periods of two months each. At the end of this period, the cytokine's plasma levels were measured by immunoassays at the start and at 10 min and 180 min of the HD procedure. At 10 min, we observed a striking increase in plasma HGF (32-fold), Act A (4-fold), and FS (53%), all p = 0.0003. The levels of HGF and Act A remained markedly elevated after 180 min (by 295% and 87%, respectively; both p < 0.002), while those of FS returned to baseline. There were no differences in cytokine profile comparing both their peak concentrations and the areas under the curve. Enoxaparin, dalteparin, and nadroparin are seemingly not different considering the release of HGF/Act A/FS during HD procedures; this may reflect their similar profile in other aspects. Moreover, the concentrations of HGF/Act A/FS are close to therapeutic ones, which may partly explain the mechanisms underlying some of the emerging extra-anticoagulant effects of LMWHs.

Histochemical and molecular overview of the thymus as site for T-cells development

The thymus represents the primary site for T cell lymphopoiesis, providing a coordinated set for critical factors to induce and support lineage commitment, differentiation and survival of thymus-seeding cells. One irrefutable fact is that the presence of non-lymphoid cells through the thymic parenchyma serves to provide coordinated migration and differentiation of T lymphocytes. Moreover, the link between foetal development and normal anatomy has been stressed in this review. Regarding thymic embryology, its epithelium is derived from the embryonic endodermal layer, with possible contributions from the ectoderm. A series of differentiating steps is essential, each of which must be completed in order to provide the optimum environment for thymic development and function. The second part of this article is focused on thymic T-cell development and differentiation, which is a stepwise process, mediated by a variety of stromal cells in different regions of the organ. It depends strongly on the thymic microenvironment, a cellular network formed by epithelial cells, macrophages, dendritic cells and fibroblasts, that provide the combination of cellular interactions, cytokines and chemokines to induce thymocyte precursors for the generation of functional T cells. The mediators of this process are not well defined but it has been demonstrated that some interactions are under neuroendocrine control. Moreover, some studies pointed out that reciprocal signals from developing T cells also are essential for establishment and maintenance of the thymic microenvironment. Finally, we have also highlighted the heterogeneity of the lymphoid, non-lymphoid components and the multi-phasic steps of thymic differentiation. In conclusion, this review contributes to an understanding of the complex mechanisms in which the foetal and postnatal thymus is involved. This could be a prerequisite for developing new therapies specifically aimed to overcome immunological defects, linked or not-linked to aging.

Activin A: autocrine regulator of kidney development and repair

The research described in this review suggests a novel and important role for activin A in the developmental and repair processes of the kidney (Table 1). The results obtained in these studies indicate that activin A is a negative regulator of kidney development and plays an essential part in kidney diseases, such as acute renal failure or renal fibrosis. It is also possible that activin A is a key player in the pathophysiological processes of other kidney diseases, such as congenital urogenital abnormalities, renal cystic disease and renal cell carcinoma. Activin A is thus a potential target for therapeutic interventions in kidney diseases. To address this issue, more detailed analysis on the regulation of activin production, modulation of activin activity and activin target genes is required.

Follistatin antagonizes transforming growth factor-beta3-induced epithelial-mesenchymal transition in vitro: implications for murine palatal development supported by microarray analysis

Epithelial-mesenchymal transition (EMT) is involved in normal embryonic development as well as in tumor progression and invasiveness. This process is also known to be a crucial step in palatogenesis during fusion of the bi-lateral palatal processes. Disruption of this step results in a cleft palate, which is among the most frequent birth defects in humans. A number of genes and encoded proteins have been shown to play a role in this developmental stage. The central role is attributed to the cytokine transforming growth factor-beta3 (TGF-beta3), which is expressed in the medial edge epithelium (MEE) already before the fusion process. The MEE covers the tips of the growing palatal shelves and eventually undergoes EMT or programmed cell death (apoptosis). TGF-beta3 is described to induce EMT in embryonic palates. With regard to the early expression of this molecule before the fusion process, it is not well understood which mechanisms prevent the TGF-beta3 producing epithelial cells from undergoing differentiation precociously. We used the murine palatal fusion to study the regulation of EMT. Specifically, we analyzed the MEE for the expression of known antagonists of TGF-beta molecules using in situ hybridization and detected the gene coding for Follistatin to be co-expressed with TGF-beta3. Further, we could show that Follistatin directly binds to TGF-beta3 and that it completely blocks TGF-beta3-induced EMT of the normal murine mammary gland (NMuMG) epithelial cell line in vitro. In addition, we analyzed the gene expression profile of NMuMG cells during TGF-beta3-induced EMT by microarray hybridization, detecting strong changes in the expression of apoptosis-regulating genes.

Phylogenetic, ontogenetic, and pathological aspects of the urine-concentrating mechanism

The urine-concentrating mechanism is one of the most fundamental functions of avian and mammalian kidneys. This particular function of the kidneys developed as a system to accumulate NaCl in birds and as a system to accumulate NaCl and urea in mammals. Based on phylogenetic evidence, the mammalian urine-concentrating mechanism may have evolved as a modification of the renal medulla's NaCl accumulating system that is observed in birds. This qualitative conversion of the urine-concentrating mechanism in the mammalian inner medulla of the kidneys may occur during the neonatal period. Human kidneys have several suboptimal features caused by the neonatal conversion of the urine-concentrating mechanism. The urine-concentrating mechanism is composed of various functional molecules, including water channels, solute transporters, and vasopressin receptors. Abnormalities in water channels aquaporin (AQP)1 and AQP2, as well as in the vasopressin receptor V2R, are known to cause nephrogenic diabetes insipidus. An analysis of the pathological mechanism involved in nephrogenic diabetes insipidus suggests that molecular chaperones may improve the intracellular trafficking of AQP2 and V2R, and, in the near future, such chaperones may become a new clinical tool for treating nephrogenic diabetes insipidus.

Renal branching morphogenesis: concepts, questions, and recent advances

The ureteric bud (UB) is an outgrowth of the Wolffian duct, which undergoes a complex process of growth, branching, and remodeling, to eventually give rise to the entire urinary collecting system during kidney development. Understanding the mechanisms that control this process is a fascinating problem in basic developmental biology, and also has considerable medical significance. Over the past decade, there has been significant progress in our understanding of renal branching morphogenesis and its regulation, and this review focuses on several areas in which there have been recent advances. The first section deals with the normal process of UB branching morphogenesis, and methods that have been developed to better observe and describe it. The next section discusses a number of experimental methodologies, both established and novel, that make kidney development in the mouse a powerful and attractive experimental system. The third section discusses some of the cellular processes that are likely to underlie UB branching morphogenesis, as well as recent data on cell lineages within the growing UB. The fourth section summarizes our understanding of the roles of two groups of growth factors that appear to be particularly important for the regulation of UB outgrowth and branching: GDNF and FGFs, which stimulate this process via tyrosine kinase receptors, and members of the TGFbeta family, including BMP4 and Activin A, which generally inhibit UB formation and branching.

Activin A is an endogenous inhibitor of ureteric bud outgrowth from the Wolffian duct

Development of metanephric kidney begins with ureteric bud outgrowth from the Wolffian duct (WD). GDNF is believed to be a crucial positive signal in the budding process, but the negative regulation of this process remains unclear. Here, we examined the role of activin A, a member of TGF-beta family, in bud formation using an in vitro WD culture system. When cultured with the surrounding mesonephros, WDs formed many ectopic buds in response to GDNF. While the activin signaling pathway is normally active along the non-budding WD (as measured by expression of activin A and phospho-Smad2/3), activin A was absent and phospho-Smad2/3 was undetectable in the ectopic buds induced by GDNF. To examine the role of activin A in bud formation, we attempted to inactivate activin action. Interestingly, the addition of neutralizing anti-activin A antibody potentiated GDNF action. To further clarify the role of activin A, we also tested the effect of activin blockade on the WD cultured in the absence of mesonephros. WDs without mesonephros did not form ectopic buds even in the presence of GDNF. In contrast, blockade of activin action with a variety of agents acting through different mechanisms (natural antagonist, neutralizing antibodies, siRNA) enabled GDNF to induce ectopic buds. Inhibition of GDNF-induced bud formation by activin A was accompanied by inhibition of cell proliferation, reduced expression of Pax-2, and decreased phosphorylation of PI3-kinase and MAP kinase in the WD. Our data suggest that activin A is an endogenous inhibitor of bud formation and that cancellation of activin A autocrine action may be critical for the initiation of this process.

Attenuation of bleomycin-induced pulmonary fibrosis by follistatin

RATIONALE

Activins are members of the transforming growth factor-beta superfamily thought to be involved in repair processes after tissue injury.

OBJECTIVES

The aim of this study was to clarify whether activin and its antagonist, follistatin, played a significant role in lung injury and fibrosis.

METHODS AND RESULTS

In bleomycin (BLM)-treated rat lung, mRNA for the beta(A) subunit of activin was upregulated on Days 3 and 7 and decreased gradually thereafter. Immunoreactive activin A was abundantly expressed in macrophages infiltrated in the lung, and was detected in fibroblasts accumulated in the fibrotic area on Day 28. We then administered follistatin, an activin antagonist, to BLM-treated rats. Follistatin significantly reduced the number of macrophages and neutrophils in bronchoalveolar lavage and reduced the protein content. Histologically, follistatin markedly reduced the number of infiltrating cells, ameliorated the destruction of lung architecture on Day 7, and attenuated lung fibrosis on Day 28. The hydroxyproline content was significantly lower in follistatin-treated rats. In cultured lung fibroblasts, production of activin A was augmented by transforming growth factor-beta, and activin antagonist follistatin significantly inhibited transforming growth factor-beta-induced fibroblast activation. These results suggest that activin A was produced in the lung after BLM treatment and promoted acute inflammation and subsequent fibrosis.

CONCLUSIONS

Follistatin is effective in treating acute lung injury and BLM-induced fibrosis by blocking the actions of activin and transforming growth factor-beta.

Follistatin regulates enamel patterning in mouse incisors by asymmetrically inhibiting BMP signaling and ameloblast differentiation

Rodent incisors are covered by enamel only on their labial side. This asymmetric distribution of enamel is instrumental to making the cutting edge sharp. Enamel matrix is secreted by ameloblasts derived from dental epithelium. Here we show that overexpression of follistatin in the dental epithelium inhibits ameloblast differentiation in transgenic mouse incisors, whereas in follistatin knockout mice, ameloblasts differentiate ectopically on the lingual enamel-free surface. Consistent with this, in wild-type mice, follistatin was continuously expressed in the lingual dental epithelium but downregulated in the labial epithelium. Experiments on cultured tooth explants indicated that follistatin inhibits the ameloblast-inducing activity of BMP4 from the underlying mesenchymal odontoblasts and that follistatin expression is induced by activin from the surrounding dental follicle. Hence, ameloblast differentiation is regulated by antagonistic actions of BMP4 and activin A from two mesenchymal cell layers flanking the dental epithelium, and asymmetrically expressed follistatin regulates the labial-lingual patterning of enamel formation.

Assessment of the function of the betaC-subunit of activin in cultured hepatocytes

We assessed the function of the beta(C)-subunit of activin in hepatocytes. We studied the effect of conditioned medium of Chinese hamster ovary (CHO) cell line stably expressing the beta(C) gene (CHO-beta(C)) on growth of AML12 hepatocytes. We also examined the effect of recombinant activin C and transfection of the beta(C) gene by using adenovirus vector. CHO-beta(C) secreted activin C, a homodimer of the beta(C), as well as precursors of the beta(C). The conditioned medium of CHO-beta(C) increased both [(3)H]thymidine incorporation and the cell number in AML12 cells. It also supported survival of AML12 cells in a serum-free condition. Recombinant human activin C also increased both [(3)H]thymidine incorporation and the number of AML12 cells. Transfection of AML12 cells with the beta(C)-subunit led to the stimulation of [(3)H]thymidine incorporation. Analysis of the conditioned medium revealed that the beta(C)-subunit formed a heterodimer with the endogenous beta(A), the formation of which was dependent on the amount of beta(C) expressed. Recombinant activin C did not affect the binding of (125)I-activin A to its receptor or follistatin. These results indicate that activin C stimulates growth of AML12 cells. The beta(C)-subunit modifies the function of the beta(A)-subunit by multiple mechanisms.

Thymic generation and regeneration

The thymus is a complex epithelial organ in which thymocyte development is dependent upon the sequential contribution of morphologically and phenotypically distinct stromal cell compartments. It is these microenvironments that provide the unique combination of cellular interactions, cytokines, and chemokines to induce thymocyte precursors to undergo a differentiation program that leads to the generation of functional T cells. Despite the indispensable role of thymic epithelium in the generation of T cells, the mediators of this process and the differentiation pathway undertaken by the primordial thymic epithelial cells are not well defined. There is a lack of lineage-specific cell-surface-associated markers, which are needed to characterize putative thymic epithelial stem cell populations. This review explores the role of thymic stromal cells in T-cell development and thymic organogenesis, as well as the molecular signals that contribute to the growth and expansion of primordial thymic epithelial cells. It highlights recent advances in these areas, which have allowed for a lineage relationship amongst thymic epithelial cell subsets to be proposed. While many fundamental questions remain to be addressed, collectively these works have broadened our understanding of how the thymic epithelium becomes specialized in the ability to support thymocyte differentiation. They should also facilitate the development of novel, rationally based therapeutic strategies for the regeneration and manipulation of thymic function in the treatment of many clinical conditions in which defective T cells have an important etiological role.

The activin-follistatin system in the neonatal ovine uterus

Uterine gland development or adenogenesis in the neonatal ovine uterus involves budding and tubulogenesis followed by coiling and branching morphogenesis of the glandular epithelium (GE) from the luminal epithelium (LE) between birth (Postnatal Day [PND] 0) and PND 56. Activins, which are members of the transforming growth factor beta superfamily, and follistatin, an inhibitor of activins, regulate epithelial branching morphogenesis in other organs. The objective of the present study was to determine effects of postnatal age on expression of follistatin, inhibin alpha subunit, betaA subunit, betaB subunit, activin receptor (ActR) type IA, ActRIB, and ActRII in the developing ovine uterus. Ewes were ovariohysterectomized on PND 0, 7, 14, 21, 28, 35, 42, 49, or 56. The uterus was analyzed by in situ hybridization and immunohistochemistry. Neither inhibin alpha subunit mRNA or protein was detected in the neonatal uterus. Expression of betaA and betaB subunits was detected predominantly in the endometrial LE and GE and myometrium between PND 0 and PND 56. In all uterine cell types, ActRIA, ActRIB, and ActRII were expressed, with the highest levels observed in the endometrial LE and GE and myometrium. Between PND 0 and PND 14, follistatin was detected in all uterine cell types. However, between PND 21 and PND 56, follistatin was only detected in the stroma and myometrium and not in the developing GE. Collectively, the present results indicate that components of the activin-follistatin system are expressed in the developing neonatal ovine uterus and are potential regulators of endometrial gland morphogenesis.

Activin a produced by ureteric bud is a differentiation factor for metanephric mesenchyme

The present study was conducted to investigate the role of the activin-follistatin system in the development of metanephros. Organ culture system and cultured metanephric mesenchymal cells were used to address this issue. Activin A was localized in ureteric bud. Activin type II receptor was localized in ureteric bud as well as metanephric mesenchyme. In an organ culture system, exogenous activin A reduced the size of cultured metanephroi, delayed ureteric bud branching, and enlarged the tips of ureteric bud. Follistatin, an antagonist of activin A was used to clarify the role of endogenous activin A. Exogenous follistatin enlarged the size of cultured metanephroi, increased ureteric bud branching, and promoted cell growth in ureteric bud. Blockade of activin signaling by adenoviral transfection of dominantly negative activin mutant receptor mimics the effect of follistatin. In cultured metanephric mesenchymal cells, activin A promoted cell growth; conversely, follistatin induced apoptosis. Furthermore, activin A induced the expressions of epithelial differentiation markers in these cells. These results suggest that activin A produced by ureteric bud is not only an important regulator of ureteric bud branching, but also a differentiation factor for metanephric mesenchyme during kidney development.

Gene expression during the priming phase of liver regeneration after partial hepatectomy in mice

Understanding the gene-expression patterns during liver regeneration may help to reveal how regenerative processes are initiated and controlled as well as shed new light onto processes that lead to liver disease. Using high-density oligonucleotide arrays, we have examined the gene-expression program in the livers of mice after partial hepatectomy. A time course was constructed for gene expression between 0 and 4 h after partial hepatectomy, corresponding to the priming phase of liver regeneration. The genomic program for liver regeneration involves transcription-factor generation, stress and inflammatory responses, cytoskeletal and extracellular matrix modification, and regulation of cell-cycle entry. The genome-wide changes that are observed provide a detailed and comprehensive map of the initial priming stage of liver regeneration.