Neutralizing TGF-beta1 antibody infusion in neonatal rat delays in vivo glomerular capillary formation 1

Insulin-like growth factor-1 effects on kidney development in preterm piglets

BACKGROUND

Preterm birth disrupts fetal kidney development, potentially leading to postnatal acute kidney injury. Preterm infants are deficient in insulin-like growth factor 1 (IGF-1), a growth factor that stimulates organ development. By utilizing a preterm pig model, this study investigated whether IGF-1 supplementation enhances preterm kidney maturation.

METHODS

Cesarean-delivered preterm pigs were treated systemically IGF-1 or vehicle control for 5, 9 or 19 days after birth. Blood, urine, and kidney tissue were collected for biochemical, histological and gene expression analyses. Age-matched term-born pigs were sacrificed at similar postnatal ages and served as the reference group.

RESULTS

Compared with term pigs, preterm pigs exhibited impaired kidney maturation, as indicated by analyses of renal morphology, histopathology, and inflammatory and injury markers. Supplementation with IGF-1 reduced signs of kidney immaturity, particularly in the first week of life, as indicated by improved morphology, upregulated expression of key developmental genes, reduced severity and incidence of microscopic lesions, and decreased levels of inflammatory and injury markers. No association was seen between the symptoms of necrotizing enterocolitis and kidney defects.

CONCLUSION

Preterm birth in pigs impairs kidney maturation and exogenous IGF-1 treatment partially reverses this impairment. Early IGF-1 supplementation could support the development of preterm kidneys.

IMPACT

Preterm birth may disrupt kidney development in newborns, potentially leading to morphological changes, injury, and inflammation. Preterm pigs have previously been used as models for preterm infants, but not for kidney development. IGF-1 supplementation promotes kidney maturation and alleviates renal impairments in the first week of life in preterm pigs. IGF-1 may hold potential as a supportive therapy for preterm infants sensitive to acute kidney injury.

Protective Effects of Hydrogen-rich Water Intake on Renal Injury in Neonatal Rats with High Oxygen Loading

Objectives

This study aimed to investigate the protective effects of hydrogen-rich water (HW) intake on renal injury in neonatal rats with high oxygen loading.

Materials

We used pregnant and newborn Sprague-Dawley rats.

Methods

Four groups were set up, with mother and newborn rats immediately after delivery as one group: RA-PW (room air and purified water), RA-HW (room air and HW), O2-PW (80% oxygen and purified water), and O2-HW (80% oxygen and HW). The newborn rats were maintained in either a normoxic (room air, 21% oxygen) or controlled hyperoxic (80% oxygen) environment from birth. Then, HW (O2-HW and RA-HW groups) or PW (O2-PW and RA-PW groups) was administered to parents of each group.

Results

The number of immature glomeruli significantly increased in the O2-PW group (exposed to hyperoxia). Conversely, the O2-HW group had significantly fewer immature glomeruli than O2-PW group. In the RT-PCR analysis of kidney tissue, α-SMA, TGF-β, and TNF-α levels were significantly higher in the O2-PW group than in the RA-PW group and significantly lower in the O2-HW group than in the O2-PW group.

Conclusions

HW intake can potentially reduce oxidative stress and prevent renal injury in neonates with high oxygen loading.

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.

A novel podocyte protein, R3h domain containing-like, inhibits TGF-β-induced p38 MAPK and regulates the structure of podocytes and glomerular basement membrane

Not only in kidney glomerular physiological function but also glomerular pathology especially in diabetic condition, glomerular podocytes play pivotal roles. Therefore, it is important to increase our knowledge about the genes and proteins expressed in podocytes. Recently, we have identified a novel podocyte-expressed gene, R3h domain containing-like (R3hdml) and analyzed its function in vivo as well as in vitro. Transforming growth factor-β (TGF-β) signaling regulated the expression of R3hdml. And R3hdml inhibited p38 mitogen-activated protein kinase phosphorylation, which was induced by TGF-β, leading to the amelioration of podocyte apoptosis. Furthermore, a lack of R3hdml in mice significantly worsened glomerular function in streptozotocin (STZ)-induced diabetes, while overexpression of R3hdml ameliorated albuminuria in STZ-induced diabetes. Our results surmise that the functional analyses of R3hdml may lead to the development of novel therapeutic strategies for diabetic nephropathy in the future. KEY MESSAGES: • A novel podocyte expressed protein R3h domain containing-like was identified. • R3HDML inhibits podocyte apoptosis by inhibiting TGF-β-mediated p38 MAPK signaling. • Overexpression of R3HDML ameliorates albuminuria in STZ-induced diabetes mice. • R3HDML may prove to be a novel therapeutic strategy for diabetic nephropathy.

Slit2/Robo1 signaling is involved in angiogenesis of glomerular endothelial cells exposed to a diabetic-like environment

Abnormal angiogenesis plays a pathological role in diabetic nephropathy (DN), contributing to glomerular hypertrophy and microalbuminuria. Slit2/Robo1 signaling participates in angiogenesis in some pathological contexts, but whether it is involved in glomerular abnormal angiogenesis of early DN is unclear. The present study evaluated the effects of Slit2/Robo1 signaling pathway on angiogenesis of human renal glomerular endothelial cells (HRGECs) exposed to a diabetic-like environment or recombinant Slit2-N. To remove the effect of Slit2 derived from mesangial cells, human renal mesangial cells (HRMCs) grown in high glucose (HG) medium (33 mM) were transfected with Slit2 siRNA and then the HG-HRMCs-CM with Slit2 depletion was collected after 48 h. HRGECs were cultured in the HG-HRMCs-CM or recombinant Slit2-N for 0, 6, 12, 24, or 48 h. The mRNA and protein expressions of Slit2/Robo1, PI3K/Akt and HIF-1α/VEGF signaling pathways were detected by quantitative real-time PCR, western blotting, and ELISA, respectively. The CCK-8 cell proliferation assay, flow cytometry and the scratch wound-healing assay were used to assess cell proliferation, cycles, and migration, respectively. Matrigel was used to perform a tubule formation assay. Our results showed that the HG-HRMCs-CM with Slit2 depletion enhanced the activation of Slit2/Robo1, PI3K/Akt, and HIF-1α/VEGF signaling in HRGECs in time-dependent manner (0-24 h post-treatment). In addition, the HG-HRMCs-CM with Slit2 depletion significantly promoted HRGECs proliferation, migration, and tube formation. Pretreatment of HRGECs with Robo1 siRNA suppressed the activation of PI3K/Akt and HIF-1α/VEGF signaling and inhibited angiogenesis, whereas PI3K inhibitor suppressed HIF-1α/VEGF signaling, without influencing Robo1 expression. In the HRGECs treated with Slit2-N, Slit2-N time-dependently enhanced the activation of Robo1/PI3K/Akt/VEGF pathway but not HIF-1α activity, and promoted HRGECs proliferation, migration, and tube formation. The effects induced by Slit2 were also abolished by Robo1 siRNA and PI3K inhibitor. Taken together, our findings indicate that in a diabetic-like environment, in addition to mesangial cells, autocrine activation of Slit2/Robo1 signaling of HRGECs may contribute to angiogenesis of HRGECs through PI3K/Akt/VEGF pathway; therefore, Slit2/Robo1 signaling may be a potent therapeutic target for the treatment of abnormal angiogenesis in early DN and may have broad implications for the treatment of other diseases dependent on pathologic angiogenesis.

Effects of a single intravitreal injection of aflibercept and ranibizumab on glomeruli of monkeys

PURPOSE

It is known that endothelial cells in the kidney are also strongly VEGF-dependent. Whether intravitreal drugs can be detected within the glomeruli or affect VEGF in glomerular podocytes is not known. Therefore, the aim of this pilot study was to investigate the effects of a single intravitreal injection of aflibercept and ranibizumab on glomeruli of monkeys.

METHODS

The kidneys of eight cynomolgus monkeys, which were intravitreally injected either with 2 mg of aflibercept or with 0.5 mg of ranibizumab, were investigated one and seven days after injection. Two animals served as controls. The distribution of aflibercept, ranibizumab and VEGF was evaluated using anti-Fc- or anti-F(ab)-fragment and anti-VEGF antibodies respectively. The ratio of stained area/nuclei was calculated using a semi-quantitative computer assisted method. Glomerular endothelial cell fenestration was quantified in electron microscopy using a systematic uniform random sampling protocol and estimating the ratio of fenestrae per µm.

RESULTS

Compared to the controls, the anti-VEGF stained area/nuclei ratio of the ranibizumab-treated animals showed no significant changes whereas the stained areas of the aflibercept-treated monkeys showed a significant decrease post-treatment. Immune reactivity (IR) against aflibercept or ranibizumab was detected in aflibercept- or ranibizumab treated animals respectively. The number of fenestrations of the glomerular endothelial cells has shown no significant differences except one day after aflibercept injection in which the number was increased.

CONCLUSION

Surprisingly, both drugs could be detected within the capillaries of the glomeruli. After a single intravitreal injection of aflibercept, VEGF IR in the podocytes was significantly reduced compared to controls. Ranibizumab injection had no significant effect on the glomeruli's VEGF level. Whether this is caused by aflibercept's higher affinity to VEGF or because it is used in a higher stoichiometric concentration compared to ranibizumab remains to be investigated.

TIMAP promotes angiogenesis by suppressing PTEN-mediated Akt inhibition in human glomerular endothelial cells

The function of TIMAP, an endothelial cell (EC)-predominant protein phosphatase 1-regulatory subunit, is poorly understood. We explored the potential role of TIMAP in the Akt-dependent regulation of glomerular EC proliferation, survival, and in vitro angiogenesis. To deplete TIMAP, the EC were transfected with TIMAP-specific or nonspecific small interfering (si) RNA. The rate of electrical impedance development across subconfluent EC monolayers, a measure of the time-dependent increase in EC number, was 93 ± 2% lower in TIMAP-depleted than in control EC. This effect on cell proliferation was associated with reduced DNA synthesis and increased apoptosis: TIMAP silencing reduced 5-ethynyl-2'-deoxyuridine incorporation by 38 ± 2% during the exponential phase of EC proliferation, and cleaved caspase 3 as well as caspase 3 activity increased in TIMAP-depleted relative to control cells. Furthermore, TIMAP depletion inhibited the formation of angiogenic sprouts by glomerular EC in three-dimensional culture. TIMAP depletion strongly diminished growth factor-stimulated Akt phosphorylation without altering ERK1/2 phosphorylation, suggesting a specific effect on the PI3K/Akt/PTEN pathway. Endogenous TIMAP and PTEN colocalized in EC and coimmunoprecipitated from EC lysates. The inhibitory PTEN phosphorylation on S370 was significantly reduced in TIMAP-depleted compared with control EC, while phosphorylation of PTEN on the S380/T382/T383 cluster remained unchanged. Finally, the PTEN inhibitor bpV(phen) fully reversed the suppressive effect of TIMAP depletion on Akt phosphorylation. The data indicate that in growing EC, TIMAP is necessary for Akt-dependent EC proliferation, survival, and angiogenic sprout formation and that this effect of TIMAP is mediated by inhibition of the tumor suppressor PTEN.

TGF-β-activated kinase 1 is crucial in podocyte differentiation and glomerular capillary formation

TGF-β-activated kinase 1 (TAK1) is a key intermediate in signal transduction induced by TGF-β or inflammatory cytokines, such as TNF-α and IL-1, which are potent inducers of podocyte injury responses that lead to proteinuria and glomerulosclerosis. Nevertheless, little is known about the physiologic and pathologic roles of TAK1 in podocytes. To examine the in vivo role of TAK1, we generated podocyte-specific Tak1 knockout mice (Nphs2-Cre(+):Tak1(fx/fx); Tak1(∆/∆)). Targeted deletion of Tak1 in podocytes resulted in perinatal lethality, with approximately 50% of animals dying soon after birth and 90% of animals dying within 1 week of birth. Tak1(∆/∆) mice developed proteinuria from P1 and exhibited delayed glomerulogenesis and reduced expression of Wilms' tumor suppressor 1 and nephrin in podocytes. Compared with Tak1(fx/fx) mice, Tak1(∆/∆) mice exhibited impaired formation of podocyte foot processes that caused disruption of the podocyte architecture with prominent foot process effacement. Intriguingly, Tak1(∆/∆) mice displayed increased expression of vascular endothelial growth factor within the glomerulus and abnormally enlarged glomerular capillaries. Furthermore, 4- and 7-week-old Tak1(∆/∆) mice with proteinuria had increased collagen deposition in the mesangium and the adjacent tubulointerstitial area. Thus, loss of Tak1 in podocytes is associated with the development of proteinuria and glomerulosclerosis. Taken together, our data show that TAK1 regulates the expression of Wilms' tumor suppressor 1, nephrin, and vascular endothelial growth factor and that TAK1 signaling has a crucial role in podocyte differentiation and attainment of normal glomerular microvasculature during kidney development and glomerular filtration barrier homeostasis.

Early chronic low-level lead exposure produces glomerular hypertrophy in young C57BL/6J mice

Early chronic lead exposure continues to pose serious health risks for children, particularly those living in lower socioeconomic environments. This study examined effects on developing glomeruli in young C57BL/6J mice exposed to low (30 ppm), higher (330 ppm) or no lead via dams' drinking water from birth to sacrifice on post-natal day 28. Low-level lead exposed mice [BLL mean (SD); 3.19 (0.70) μg/dL] had an increase in glomerular volume but no change in podocyte number compared to control mice [0.03 (0.01) μg/dL]. Higher-level lead exposed mice [14.68 (2.74) μg/dL] had no change in either glomerular volume or podocyte number. The increase in glomerular volume was explained by increases in glomerular capillary and mesangial volumes with no change in podocyte volume. Early chronic lead exposure yielding very low blood lead levels alters glomerular development in pre-adolescent animals.

Transforming growth factor-β1 receptor inhibition preserves glomerulotubular integrity during ureteral obstruction in adults but worsens injury in neonatal mice

Unilateral ureteral obstruction (UUO), a widely used model of chronic kidney disease and congenital obstructive uropathy, causes proximal tubular injury and formation of atubular glomeruli. Because transforming growth factor-β1 (TGF-β1) is a central regulator of renal injury, neonatal and adult mice were subjected to complete UUO while under general anesthesia and treated with vehicle or ALK5 TGF-β1 receptor inhibitor (IN-1130, 30 mg·kg(-1)·day(-1)). After 14 days, glomerulotubular integrity and proximal tubular mass were determined by morphometry of Lotus tetragonolobus lectin distribution, and the fraction of atubular glomeruli was determined by serial section analysis of randomly selected individual glomeruli. Glomerular area, macrophage infiltration, fibronectin distribution, and interstitial collagen were measured by morphometry. Compared with placebo, inhibition of TGF-β1 by IN-1130 decreased apoptosis and formation of atubular glomeruli, prevented parenchymal loss, increased glomerular area and glomerulotubular integrity, and increased proximal tubule fraction of the adult obstructed kidney parenchyma from 17 to 30% (P < 0.05, respectively). IN-1130 decreased macrophage infiltration and fibronectin and collagen deposition in the adult obstructed kidney by ∼50% (P < 0.05, respectively). In contrast to these salutary effects in the adult, IN-1130 caused widespread necrosis in obstructed neonatal kidneys. We conclude that whereas IN-1130 reduces obstructive injury in adult kidneys through preservation of glomerulotubular integrity and proximal tubular mass, TGF-β1 inhibition aggravates obstructive injury in neonates. These results indicate that while caution is necessary in treating congenital uropathies, ALK5 inhibitors may prevent nephron loss due to adult kidney disease.

Expression patterns of connective tissue growth factor and of TGF-beta isoforms during glomerular injury recapitulate glomerulogenesis

Transforming growth factor (TGF)-beta(1), -beta(2), and -beta(3) are involved in control of wound repair and development of fibrosis. Connective tissue growth factor (CTGF) expression is stimulated by all TGF-beta isoforms and is abundant in glomerulosclerosis and other fibrotic disorders. CTGF is hypothesized to mediate profibrotic effects of TGF-beta(1) or to facilitate interaction of TGF-beta(1) with its receptor, but its interactions with TGF-beta isoforms in nonpathological conditions are unexplored so far. Tissue repair and remodeling may recapitulate gene transcription at play in organogenesis. To further delineate the relationship between CTGF and TGF-beta, we compared expression patterns of CTGF and TGF-beta isoforms in rat and human glomerulogenesis and in various human glomerulopathies. CTGF mRNA was present in the immediate precursors of glomerular visceral and parietal epithelial cells in the comma- and S-shaped stages, but not in earlier stages of nephron development. During the capillary loop and maturing glomerular stages and simultaneous with the presence of TGF-beta(1), -beta(2), and -beta(3) protein, CTGF mRNA expression was maximal and present only in differentiating glomerular epithelial cells. CTGF protein was also present on precursors of mesangium and glomerular endothelium, suggesting possible paracrine interaction. Concomitant with the presence of TGF-beta(2) and -beta(3) protein, and in the absence of TGF-beta(1), CTGF mRNA and protein expression was restricted to podocytes in normal adult glomeruli. However, TGF-beta(1) and CTGF were again coexpressed, often with TGF-beta(2) and -beta(3), in particular in podocytes in proliferative glomerulonephritis and also in mesangial cells in diabetic nephropathy and IgA nephropathy (IgA NP). Coordinated expression of TGF-beta isoforms and of CTGF may be involved in normal glomerulogenesis and possibly in maintenance of glomerular structure and function at adult age. Prolonged overexpression of TGF-beta(1) and CTGF is associated with development of severe glomerulonephritis and glomerulosclerosis.

Laminin alpha4-null mutant mice develop chronic kidney disease with persistent overexpression of platelet-derived growth factor

Each extracellular matrix compartment in the kidney has a unique composition, with regional specificity in the expression of various laminin isoforms. Although null mutations in the majority of laminin chains lead to specific developmental abnormalities in the kidney, Lama4-/- mice have progressive glomerular and tubulointerstitial fibrosis. These mice have a significant increase in expression of platelet-derived growth factor (PDGF)-BB, PDGF-DD, and PDGF receptor beta in association with immature glomerular and peritubular capillaries. In addition, mesangial cell exposure to alpha4-containing laminins, but not other isoforms, results in down-regulation of PDGF receptor mRNA and protein, suggesting a direct effect of LN411/LN421 on vessel maturation. Given the known role of overexpression of PDGF-BB and PDGF-DD on glomerular and tubulointerstitial fibrosis, these data suggest that failure of laminin alpha4-mediated down-regulation of PDGF activity contributes to the progressive renal lesions in this animal model. Given the recent demonstration that individuals with laminin alpha4 mutations develop cardiomyopathy, these findings may be relevant to kidney disease in humans.

Glomerular endothelial cell fenestrations: an integral component of the glomerular filtration barrier

Glomerular endothelial cell (GEnC) fenestrations are analogous to podocyte filtration slits, but their important contribution to the glomerular filtration barrier has not received corresponding attention. GEnC fenestrations are transcytoplasmic holes, specialized for their unique role as a prerequisite for filtration across the glomerular capillary wall. Glomerular filtration rate is dependent on the fractional area of the fenestrations and, through the glycocalyx they contain, GEnC fenestrations are important in restriction of protein passage. Hence, dysregulation of GEnC fenestrations may be associated with both renal failure and proteinuria, and the pathophysiological importance of GEnC fenestrations is well characterized in conditions such as preeclampsia. Recent evidence suggests a wider significance in repair of glomerular injury and in common, yet serious, conditions, including diabetic nephropathy. Study of endothelial cell fenestrations is challenging because of limited availability of suitable in vitro models and by the requirement for electron microscopy to image these sub-100-nm structures. However, extensive evidence, from glomerular development in rodents to in vitro studies in human GEnC, points to vascular endothelial growth factor (VEGF) as a key inducer of fenestrations. In systemic endothelial fenestrations, the intracellular pathways through which VEGF acts to induce fenestrations include a key role for the fenestral diaphragm protein plasmalemmal vesicle-associated protein-1 (PV-1). The role of PV-1 in GEnC is less clear, not least because of controversy over existence of GEnC fenestral diaphragms. In this article, the structure-function relationships of GEnC fenestrations will be evaluated in depth, their role in health and disease explored, and the outlook for future study and therapeutic implications of these peculiar structures will be approached.

Circulating angiogenic factors in gestational proteinuria without hypertension

OBJECTIVE

Our goal was to determine whether obstetric outcomes and serum angiogenic factors are altered in women with gestational proteinuria without hypertension.

STUDY DESIGN

We performed a nested case-control study of 108 women with gestational proteinuria and compared them with 1564 randomly selected women with normotension without proteinuria during pregnancy (control subjects) and with 319 women who experienced preeclampsia.

RESULTS

Women with gestational proteinuria had greater body-mass index and higher blood pressure at study enrollment. Adverse obstetric outcomes were infrequent. Levels of free placental growth factor were lower than control levels beginning early in gestation. Compared with gestational-age matched control subjects, free placental growth factor was reduced beginning 6-8 weeks before proteinuria. Although soluble fms-like tyrosine kinase 1 and soluble endoglin concentrations were elevated 1-2 weeks before proteinuria, these elevations were modest and transient. After the onset of proteinuria, angiogenic factor levels generally did not differ significantly from control levels.

CONCLUSION

Gestational proteinuria in healthy nulliparous women appears to be a mild variant of preeclampsia.

Alteration of glomerulogenesis- and podocyte structure-related gene expression in early diabetic nephropathy

Diabetic nephropathy is a leading cause of end-stage renal disease. Several pathways, including the renin-angiotensin system, have been postulated as potential mechanisms of diabetic nephropathy. In addition, glomerulogenesis-related molecules are involved in the pathogenesis of diabetic nephropathy, especially at the early stage. They can be divided into three groups by function, that is, fibrosis-related, podocyte differentiation-related and angiogenesis-related molecules. Most of the molecules are expressed in the podocyte and upregulated, even during the normoalbuminuric stage. Expression of several podocyte structure-related molecules are also altered at the normoalbuminuric stage. They can contribute to the structural alteration of the podocyte in diabetic nephropathy. Thus, normalization of the expression of glomerulogenesis-related molecules could be a new target for preventing the initiation and progression of diabetic nephropathy.

Contribution of the endothelium to the glomerular permselectivity barrier in health and disease

BACKGROUND

The endothelium that lines glomerular capillaries shares many properties with endothelial cells in general, but unlike most endothelial cells, it is extremely flat and densely perforated by transendothelial cell pores, the fenestrae. Until recently, it was believed that the fenestrae allow free passage of large proteins, and that the glomerular endothelium contributes little to the permselectivity of the glomerular capillary wall.

METHODS

Key studies addressing the nature of the glomerular capillary endothelium and its contribution to glomerular permselectivity were reviewed.

RESULTS

Glomerular endothelial cell flattening and fenestrae formation requires signals from differentiated podocytes, and from the glomerular basement membrane. Deletion of VEGF-A from podocytes prevents flattening and fenestration of glomerular endothelium. Application of VEGF-A to endothelial cells in vivo stimulates fenestrae formation, and neutralization of VEGF-A by soluble VEGF receptor 1 (sFlt-1) or anti-VEGF antibodies results in loss of glomerular fenestrae, and proteinuria. Neutralizing TGF-beta1 antibodies, deletion of laminin alpha3 in mice or laminin beta3 in humans cause similar defects. The glomerular endotheliosis lesion of pre-eclampsia is due to the placenta-derived inhibitors sFlt-1 and sEndoglin, which block the VEGF-A/VEGF receptor and TGF-beta/endoglin signaling, respectively, causing the loss of glomerular endothelial cell fenestrae, cell swelling and proteinuria. The glomerular endothelium is covered by a glycocalyx that extends into the fenestrae and by a more loosely associated endothelial cell surface layer of glycoproteins. Mathematical analyses of functional permselectivity studies have concluded that the glomerular endothelial cell glycocalyx and its associated surface layer account for the retention of up to 95% of proteins within the circulation. Furthermore, the fenestrae are critical for the maintenance of the high hydraulic conductivity of the glomerular capillary wall, and their loss results in a reduction in the glomerular filtration rate.

CONCLUSIONS

Loss of GFR and proteinuria can result from glomerular endothelial cell injury.

Glomerular endothelial cell differentiation

BACKGROUND

Glomerular endothelial cells differ from most other endothelial cells in that they are extraordinarily flattened and highly fenestrated. In this differentiated form, they allow formation of glomerular ultrafiltrate at a prodigious rate.

METHODS

Molecular processes that dictate the development and differentiation of glomerular endothelium are reviewed.

RESULTS

During glomerular development, angioblasts already present in the metanephric blastema well before any organized angiogenic sprouts invade the capillary cleft of developing nephrons at the comma and S-shape stages in response to chemotactic and guiding cues from primitive podocytes. The angioblasts then undergo homotypic aggregation into precapillary cords as yet devoid of a lumen. Lumen development then proceeds through the loss of superfluous endothelial cells by apoptosis as well as flattening of the remaining viable endothelial cells. The final step, fenestration, is critically dependent on appropriate stimuli, most notably vascular endothelial growth factor A (VEGF-A), from differentiated podocytes. Current evidence suggests that the fenestrae of fully differentiated glomerular endothelium can be lost within hours if the VEGF-A stimulus is removed, and that the glomerular endotheliosis, loss of glomerular filtration rate (GFR) and proteinuria observed in preeclampsia are due to the circulating inhibitor of VEGF-A, soluble VEGF receptor 1 (VEGFR-1).

CONCLUSION

Differentiation of the glomerular endothelium is highly dependent on podocyte-derived stimuli and their loss leads to the derangements of glomerular function in preeclampsia.

Transforming growth factor-beta and Smad signalling in kidney diseases

Extensive studies have demonstrated that transforming growth factor-beta (TGF-beta) plays an important role in the progression of renal diseases. TGF-beta exerts its biological functions mainly through its downstream signalling molecules, Smad2 and Smad3. It is now clear that Smad3 is critical for TGF-beta's pro-fibrotic effect, whereas the functions of Smad2 in fibrosis in response to TGF-beta still need to be determined. Our recent studies have demonstrated that Smad signalling is also a critical pathway for renal fibrosis induced by other pro-fibrotic factors, such as angiotensin II and advanced glycation end products (AGE). These pro-fibrotic factors can activate Smads directly and independently of TGF-beta. They can also cause renal fibrosis via the ERK/p38 MAP kinase-Smad signalling cross-talk pathway. In contrast, blockade of Smad2/3 activation by overexpression of an inhibitory Smad7 prevents collagen matrix production induced by TGF-beta, angiotensin II, high glucose and AGE and attenuates renal fibrosis in various animal models including rat obstructive kidney, remnant kidney and diabetic kidney diseases. Results from these studies indicate that Smad signalling is a key and final common pathway of renal fibrosis. In addition, TGF-beta has anti-inflammatory and immune-regulatory properties. Our most recent studies demonstrated that TGF-beta transgenic mice are protected against renal inflammation in mouse obstructive and diabetic models. Upregulation of renal Smad7, thereby blocking NF.kappaB activation via induction of IkappaBalpha, is a central mechanism by which TGF-beta inhibits renal inflammation. In conclusion, TGF-beta signals through Smad2/3 to mediate renal fibrosis, whereas induction of Smad7 inhibits renal fibrosis and inflammation. Thus, targeting Smad signalling by overexpression of Smad7 may have great therapeutic potential for kidney diseases.

Cisplatin-induced renal interstitial fibrosis in neonatal rats, developing as solitary nephron unit lesions

Cisplatin (CDDP)-induced renal lesions in rats prove a useful model for analysis of the pathogenesis of post-tubular injury-renal interstitial fibrosis. This study investigated the histopathological changes in 10-day-old neonatal rats induced by a single injection of CDDP (4.5 mg/kg). Compared with age-matched controls, on postinjection (PI) days 1 to 6, the number of apoptotic cells, demonstrable with TUNEL method, was significantly increased in CDDP-treated neonates, and there was no marked epithelial necrosis nor fibrotic lesions. Fibrotic lesions began to be developed solitarily around some nephrons with dilated ducts in the corticomedullary junction on PI day 10 and the lesions became more prominent until PI day 20. The alpha-SMA-positive myofibroblastic cells were seen exclusively in the fibrotic lesions. Additionally, the numbers of macrophages reacting with EDI (specific for exudate macrophages), ED2 (for resident macrophages), and OX6 (recognizing MHC class II antigens expressed in antigen-presenting macrophages/dendritic cells) were significantly increased around the affected renal tubules. A greater immunoreaction for TGF-beta1 was seen mostly in the renal epithelial cells of CDDP-treated neonates. These findings indicated that macrophage populations and myofibrolastic cells as well as TGF-beta1 may be responsible for the production of neonatal renal interstitial fibrosis. Compared with CDDP-injected adult rats that develop extensive interstitial fibrosis (Yamate et al., J Comp Pathol, 1995), the formation of fibrotic lesions was delayed, and the lesions were limited to the area around the affected nephrons; this could be attributable to differences in renal morphology between neonates and mature kidney of adult rats.

Endothelial cell apoptosis during glomerular capillary lumen formation in vivo

Transforming growth factor-beta (TGF-beta) stimulates endothelial cell apoptosis in vitro, and inhibition of TGF-beta1 leads to retention of undifferentiated endothelial cells in developing glomerular capillaries and reduced lumen formation in vivo. This study explored the question whether glomerular capillary lumen formation in vivo may involve TGF-beta1-dependent endothelial cell apoptosis. Neutralizing anti-TGF-beta1 or non-immune IgY were infused into the renal arteries of 3-d-old rats, and the kidneys were examined 2 d later. By transmission electron microscopy, endocapillary apoptotic cells were observed at a frequency of 0.10/loop in immature glomeruli of 3-d-old rat pups. In 5-d-old rat pups given neutralizing TGF-beta1 antibody or control IgY, the frequency of endocapillary apoptotic cells was 0.03 and 0.09/loop, respectively (P < 0.001, chi(2)). Dual labeling with TUNEL and anti-von Willebrand factor (vWF) antibody showed that apoptotic cells in immature glomeruli of 5-d-old rat pups are endothelial cells. Quantitative analysis showed significantly fewer TUNEL/vWF-labeled cells in glomeruli after anti-TGF-beta1 antibody infusion than in controls. No endocapillary apoptotic cells were observed in any group in C-shaped or S-shaped bodies, and the TUNEL assay revealed no glomerular apoptotic cells in kidneys from mature rats. These findings suggest that superfluous endothelial cells are cleared from immature glomerular capillaries by apoptosis, a process regulated by TGF-beta1. Taken together with the previous finding, that TGF-beta1 blockade blunts glomerular capillary lumen formation in vivo, it is proposed that TGF-beta1-dependent apoptosis serves to open capillary lumens in this vascular bed during glomerular development.

Tubulointerstitial disease: role of ischemia and microvascular disease

PURPOSE OF REVIEW

Tubulointerstitial injury is characteristic of aging-associated renal injury and progressive renal disease. Salt-sensitive hypertension is also associated with tubulointerstitial inflammation, especially when accompanied by microvascular disease. Here we summarize recent studies on the pathogenesis and consequences of tubulointerstitial disease, emphasizing the role of ischemia and the microvasculature.

RECENT FINDINGS

Tubulointerstitial injury occurs via several mechanisms of which one of the most important is chronic ischemia. Recent studies suggest that chronic vasoconstriction may contribute to the renal injury associated with angiotensin II, catecholamines, nitric oxide inhibition, hypokalemia, hyperuricemia, and cyclosporine nephropathy. Salt-sensitivity may result as a consequence of the tubulointerstitial inflammatory response to these conditions, and this appears to be perpetuated by the development of preglomerular vascular disease. With progression of tubulointerstitial disease there is also a loss of peritubular capillaries, and stimulating microvascular growth with angiogenic factors can stabilize renal function in these models.

SUMMARY

Ischemia secondary to vasoconstriction or to structural changes of the renal vasculature may have important consequences both in terms of mediating salt-sensitive hypertension and renal progression. Angiogenic factors may have potential benefit in preventing or treating these conditions.

Cell biology of the glomerular podocyte

Glomerular podocytes are highly specialized cells with a complex cytoarchitecture. Their most prominent features are interdigitated foot processes with filtration slits in between. These are bridged by the slit diaphragm, which plays a major role in establishing the selective permeability of the glomerular filtration barrier. Injury to podocytes leads to proteinuria, a hallmark of most glomerular diseases. New technical approaches have led to a considerable increase in our understanding of podocyte biology including protein inventory, composition and arrangement of the cytoskeleton, receptor equipment, and signaling pathways involved in the control of ultrafiltration. Moreover, disturbances of podocyte architecture resulting in the retraction of foot processes and proteinuria appear to be a common theme in the progression of acquired glomerular disease. In hereditary nephrotic syndromes identified over the last 2 years, all mutated gene products were localized in podocytes. This review integrates our recent physiological and molecular understanding of the role of podocytes during the maintenance and failure of the glomerular filtration barrier.

A new method for large scale isolation of kidney glomeruli from mice

Here we report a new isolation method for mouse glomeruli. The method is fast and simple and allows for the isolation of virtually all glomeruli present in the adult mouse kidney with minimal contamination of nonglomerular cells. Mice were perfused through the heart with magnetic 4.5- micro m diameter Dynabeads. Kidneys were minced into small pieces, digested by collagenase, filtered, and collected using a magnet. The number of glomeruli retrieved from one adult mouse was 20,131 +/- 4699 (mean +/- SD, n = 14) with a purity of 97.5 +/- 1.7%. The isolated glomeruli retained intact morphology, as confirmed by light and electron microscopy, as well as intact mRNA integrity, as confirmed by Northern blot analysis. The method was applicable also to newborn mice, which allows for the isolation of immature developmental stage glomeruli. This method makes feasible transcript profiling and proteomic analysis of the developing, healthy and diseased mouse glomerulus.

TIMAP, a novel CAAX box protein regulated by TGF-beta1 and expressed in endothelial cells

Representational difference analysis of the glomerular endothelial cell response to transforming growth factor-beta1 (TGF-beta1) revealed a novel gene, TIMAP (TGF-beta-inhibited membrane-associated protein), which contains 10 exons and maps to human chromosome 20.q11.22. By Northern blot, TIMAP mRNA is highly expressed in all cultured endothelial and hematopoietic cells. The frequency of the TIMAP SAGE tag is much greater in endothelial cell SAGE databases than in nonendothelial cells. Immunofluorescence studies of rat tissues show that anti-TIMAP antibodies localize to vascular endothelium. TGF-beta1 represses TIMAP through a protein synthesis- and histone deacetylase-dependent process. The TIMAP protein contains five ankyrin repeats, a protein phosphatase-1 (PP1)-interacting domain, a COOH-terminal CAAX box, a domain arrangement similar to that of MYPT3, and a PP1 inhibitor. A green fluorescent protein-TIMAP fusion protein localized to the plasma membrane in a CAAX box-dependent fashion. Hence, TIMAP is a novel gene highly expressed in endothelial and hematopoietic cells and regulated by TGF-beta1. On the basis of its domain structure, TIMAP may serve a signaling function, potentially through interaction with PP1.

Temporally compartmentalized expression of ephrin-B2 during renal glomerular development

Glomerular development proceeds through the spatially ordered and sequential recruitment, proliferation, assembly, and differentiation of endothelial, mesangial, and epithelial progenitors. The molecular determinants of cell-cell recognition and targeting in this process have yet to be defined. The Eph/ephrin family of membrane receptors and counter-receptors are critical participants of developmental vascular assembly in extrarenal sites. Renal expression patterns of ephrin-B2 and EphB4 were investigated using mice expressing beta-galactosidase under control of ephrin-B2 or EphB4 promoters. The earliest glomerular expression of ephrin-B2 was identified in a subset of differentiating comma-stage glomerular epithelial cells (podocyte progenitors) adjacent to the vascular cleft where endothelial progenitors are subsequently recruited. Epithelial ephrin-B2 expression was accompanied by expression in endothelial and mesangial cells as capillary assembly progressed. At or near completion of glomerular maturation, epithelial ephrin-B2 expression was extinguished, with persistence in glomerular endothelial cells. Throughout development, one of several ephrin-B2 receptors, EphB4, was persistently and exclusively expressed in endothelial cells of venous structures. The findings show sequential ephrin-B2 expression across glomerular lineages, first in a distinct subset of podocyte progenitors and subsequently in endothelial cells of the developing glomerulus. Given targeting functions for Eph/ephrin family proteins, the findings suggest that ephrin-B2 expression marks podocyte progenitors at the site of vascular cleft formation, where expression may establish an "address" to which endothelial and mesangial progenitors are recruited. Thus, the present results suggest that ephrin-B2 and EphB interactions play an important role in glomerular microvascular assembly.

Multiple growth factors regulate coronary embryonic vasculogenesis

Mechanisms regulating coronary vascularization are not well understood. To test hypotheses regarding the influence of key growth factors and their interactions, we studied vascular tube formation (vasculogenesis) in collagen gels onto which quail embryonic ventricles were placed and incubated in the presence of growth factors or inhibitors. Vasculogenesis in this model is dependent on tyrosine kinase receptors, since tube formation was totally blocked by genestein. Tube formation was attenuated when anti-bFGF or anti-VEGF neutralizing antibodies were added to the medium and nearly completely inhibited when the both were added. The attenuation associated with anti-VEGF was due primarily to a decrease in assembly of endothelial cells, while that associated with bFGF was primarily due to a reduction in endothelial cells. Soluble tie-2, the receptor for angiopoietins, also had an inhibitory effect and, when added with either anti-bFGF or anti-VEGF, markedly attenuated tube formation. At optimal doses, tube formation was enhanced 6.5-fold by bFGF and 2.5-fold by VEGF over the controls. Each of these growth factors was dependent upon the other for optimal induction of tube formation, since neutralizing antibodies to one markedly reduced the potency of the other. VEGF potency was also markedly reduced when soluble tie-2 was added to the medium. Tube formation was virtually totally blocked by exogenous TGF-beta at doses > 1 ng/ml, while neutralizing TGF-beta antibodies enhanced tube formation 2-fold in the 30 ng-30 microg range. These data provide the first documentation of multiple growth factor regulation of coronary tube formation.

Impaired angiogenesis in the remnant kidney model: I. Potential role of vascular endothelial growth factor and thrombospondin-1

Few studies have examined the role of the microvasculature in progressive renal disease. It was hypothesized that impaired angiogenesis might occur in the diseased kidney and could contribute to renal scarring. Progressive renal disease was induced in rats by 5/6 renal ablation and those rats were compared with sham-operated control animals at multiple time points, for examination of changes in the microvasculature and the expression of angiogenic factors. An early angiogenic response was documented in remnant kidneys, with increases in the proliferation of peritubular (1 wk) and glomerular (2 wk) endothelial cells. Subsequently, however, there was a decrease in endothelial cell proliferation, which was reduced to levels below those of sham-treated animals, in conjunction with interstitial expression of the antiangiogenic factor thrombospondin-1 (TSP-1) and decreased tubular expression of the proangiogenic factor vascular endothelial growth factor (VEGF). Both the increase in TSP-1 expression and the loss of VEGF expression were correlated with capillary loss and the development of glomerulosclerosis and interstitial fibrosis. Progressive macrophage infiltration was correlated both spatially and quantitatively with the sites of absent or diminished VEGF expression. In addition, macrophage-associated cytokines (interleukin-1beta, interleukin-6, and tumor necrosis factor-alpha) inhibited VEGF mRNA expression and protein secretion by cultured tubular epithelial cells of the medullary thick ascending limb, under both normoxic and hypoxic conditions. Impaired angiogenesis characterizes the remnant kidney model and is correlated with progression. The impaired angiogenesis may be mediated by alterations in the renal expression of TSP-1 and VEGF, with the latter being regulated by macrophage-associated cytokines.

In vitro studies on the roles of transforming growth factor-beta 1 in rat metanephric development

BACKGROUND

The development of the permanent kidney (metanephros) involves the interplay between both positive and negative regulatory molecules. Transforming growth factor-beta1 (TGF-beta 1) has previously been shown to negatively regulate ureteric duct growth. However, its potential role in nephron development and glomerulogenesis has been largely ignored.

METHODS

In situ hybridization and reverse transcription-polymerase chain reaction were employed to examine the temporal and spatial localization of TGF-beta 1 mRNA and a TGF-beta type I receptor (activin-like receptor kinase-5; ALK-5) mRNA in developing rat metanephroi. The addition of exogenous TGF-beta 1 to rat metanephric organ culture at different time points was used to examine the role of TGF-beta 1 in ureteric duct growth and nephron development.

RESULTS

TGF-beta 1 mRNA did not colocalize with ALK-5 mRNA. Instead, TGF-beta1 mRNA colocalized with the TGF-beta type II receptor mRNA. The addition of recombinant human TGF-beta 1 to rat metanephric organ culture at the beginning of the culture period inhibited total metanephric growth and the growth of the ureteric tree, resulting in a decrease in nephron number. Similarly, the addition of TGF-beta 1 to metanephroi after 48 hours of culture inhibited ureteric duct growth, decreasing nephron number. The addition of TGF-beta 1 at days 0 or 2 of culture promoted hypertrophy of the renal capsule.

CONCLUSIONS

These findings confirm that TGF-beta 1 inhibits ureteric duct growth and thereby nephron endowment in developing rat metanephroi in vitro. However, TGF-beta 1 does not appear to play a significant role in nephron development per se once the epithelial vesicle has formed.

The latent transforming growth factor-beta-binding protein-1 promotes in vitro differentiation of embryonic stem cells into endothelium

The latent transforming growth factor-beta-binding protein-1 (LTBP-1) belongs to a family of extracellular glycoproteins that includes three additional isoforms (LTBP-2, -3, and -4) and the matrix proteins fibrillin-1 and -2. Originally described as a TGF-beta-masking protein, LTBP-1 is involved both in the sequestration of latent TGF-beta in the extracellular matrix and the regulation of its activation in the extracellular environment. Whereas the expression of LTBP-1 has been analyzed in normal and malignant cells and rodent and human tissues, little is known about LTBP-1 in embryonic development. To address this question, we used murine embryonic stem (ES) cells to analyze the appearance and role of LTBP-1 during ES cell differentiation. In vitro, ES cells aggregate to form embryoid bodies (EBs), which differentiate into multiple cell lineages. We analyzed LTBP-1 gene expression and LTBP-1 fiber appearance with respect to the emergence and distribution of cell types in differentiating EBs. LTBP-1 expression increased during the first 12 d in culture, appeared to remain constant between d 12 and 24, and declined thereafter. By immunostaining, fibrillar LTBP-1 was observed in those regions of the culture containing endothelial, smooth muscle, and epithelial cells. We found that inclusion of a polyclonal antibody to LTBP-1 during EB differentiation suppressed the expression of the endothelial specific genes ICAM-2 and von Willebrand factor and delayed the organization of differentiated endothelial cells into cord-like structures within the growing EBs. The same effect was observed when cultures were treated with either antibodies to TGF-beta or the latency associated peptide, which neutralize TGF-beta. Conversely, the organization of endothelial cells was enhanced by incubation with TGF-beta 1. These results suggest that during differentiation of ES cells LTBP-1 facilitates endothelial cell organization via a TGF-beta-dependent mechanism.

Endothelial signal integration in vascular assembly

Regulated assembly of a highly specialized interconnecting network of vascular endothelial and supportive cells is fundamental to embryonic development and organogenesis, as well as to postnatal tissue repair in metazoans. This review advances an "endotheliocentric" model that defines tasks required of endothelial cells and describes molecular controls that regulate steps in activation, assembly, and maturation of new vessels. In addition to the classical assembly mechanisms--angiogenesis and vasculogenesis--endothelial cells are also recruited into vascular structures from the circulatory system in adult animals and from resident mesenchymally derived progenitors during organogenesis of kidney and other organs. Paracrine signaling cascades regulated by hypoxia initiate a sequentially coordinated series of endothelial responses, including matrix degradation, migration, proliferation, and morphogenetic remodeling. Surface receptors on committed endothelial lineage progenitors transduce cues from extracellular-matrix-associated proteins and cell-cell contact to direct migration, matrix attachment, proliferation, targeting and cell-cell assembly, and vessel maturation. Through their capacity to spatially segregate and temporally integrate a diverse range of extracellular signals, endothelial cells determine their migratory paths, cellular partners, and life-or-death responses to local cues.

Advances in renal development

Branching morphogenesis, mesenchymal cell condensation and mesenchymal-to-epithelial conversion are key steps in kidney development and morphogenesis of several other organs. Review articles describing our current knowledge of the genetic and molecular regulation of these processes have been published. Therefore, this review focuses on the important question of cell lineage specification during kidney development. We describe how new insights are being made into the specification of kidney cells in the intermediate mesoderm, as well as specification of cells that will form the renal mesenchyme, ureteric duct, nephron tubule epithelium and renal vasculature.