Platelet-derived growth factor receptor beta regulates migration and DNA synthesis in metanephric mesenchymal cells

Analysis of CD74 Occurrence in Oncogenic Fusion Proteins

CD74 is a type II cell surface receptor found to be highly expressed in several hematological and solid cancers, due to its ability to activate pathways associated with tumor cell survival and proliferation. Over the past 16 years, CD74 has emerged as a commonly detected fusion partner in multiple oncogenic fusion proteins. Studies have found CD74 fusion proteins in a range of cancers, including lung adenocarcinoma, inflammatory breast cancer, and pediatric acute lymphoblastic leukemia. To date, there are five known CD74 fusion proteins, CD74-ROS1, CD74-NTRK1, CD74-NRG1, CD74-NRG2α, and CD74-PDGFRB, with a total of 16 different variants, each with unique genetic signatures. Importantly, the occurrence of CD74 in the formation of fusion proteins has not been well explored despite the fact that ROS1 and NRG1 families utilize CD74 as the primary partner for the formation of oncogenic fusions. Fusion proteins known to be oncogenic drivers, including those of CD74, are typically detected and targeted after standard chemotherapeutic plans fail and the disease relapses. The analysis reported herein provides insights into the early intervention of CD74 fusions and highlights the need for improved routine assessment methods so that targeted therapies can be applied while they are most effective.

Biomimetic models of the glomerulus

The use of biomimetic models of the glomerulus has the potential to improve our understanding of the pathogenesis of kidney diseases and to enable progress in therapeutics. Current in vitro models comprise organ-on-a-chip, scaffold-based and organoid approaches. Glomerulus-on-a-chip designs mimic components of glomerular microfluidic flow but lack the inherent complexity of the glomerular filtration barrier. Scaffold-based 3D culture systems and organoids provide greater microenvironmental complexity but do not replicate fluid flows and dynamic responses to fluidic stimuli. As the available models do not accurately model the structure or filtration function of the glomerulus, their applications are limited. An optimal approach to glomerular modelling is yet to be developed, but the field will probably benefit from advances in biofabrication techniques. In particular, 3D bioprinting technologies could enable the fabrication of constructs that recapitulate the complex structure of the glomerulus and the glomerular filtration barrier. The next generation of in vitro glomerular models must be suitable for high(er)-content or/and high(er)-throughput screening to enable continuous and systematic monitoring. Moreover, coupling of glomerular or kidney models with those of other organs is a promising approach to enable modelling of partial or full-body responses to drugs and prediction of therapeutic outcomes.

Dysregulated mesenchymal PDGFR-β drives kidney fibrosis

Kidney fibrosis is characterized by expansion and activation of platelet-derived growth factor receptor-β (PDGFR-β)-positive mesenchymal cells. To study the consequences of PDGFR-β activation, we developed a model of primary renal fibrosis using transgenic mice with PDGFR-β activation specifically in renal mesenchymal cells, driving their pathological proliferation and phenotypic switch toward myofibroblasts. This resulted in progressive mesangioproliferative glomerulonephritis, mesangial sclerosis, and interstitial fibrosis with progressive anemia due to loss of erythropoietin production by fibroblasts. Fibrosis induced secondary tubular epithelial injury at later stages, coinciding with microinflammation, and aggravated the progression of hypertensive and obstructive nephropathy. Inhibition of PDGFR activation reversed fibrosis more effectively in the tubulointerstitium compared to glomeruli. Gene expression signatures in mice with PDGFR-β activation resembled those found in patients. In conclusion, PDGFR-β activation alone is sufficient to induce progressive renal fibrosis and failure, mimicking key aspects of chronic kidney disease in humans. Our data provide direct proof that fibrosis per se can drive chronic organ damage and establish a model of primary fibrosis allowing specific studies targeting fibrosis progression and regression.

Development of new method to enrich human iPSC-derived renal progenitors using cell surface markers

Cell therapy using renal progenitors differentiated from human embryonic stem cells (hESCs) or induced pluripotent stem cells (hiPSCs) has the potential to significantly reduce the number of patients receiving dialysis therapy. However, the differentiation cultures may contain undifferentiated or undesired cell types that cause unwanted side effects, such as neoplastic formation, when transplanted into a body. Moreover, the hESCs/iPSCs are often genetically modified in order to isolate the derived renal progenitors, hampering clinical applications. To establish an isolation method for renal progenitors induced from hESCs/iPSCs without genetic modifications, we screened antibodies against cell surface markers. We identified the combination of four markers, CD9⁻CD140a⁺CD140b⁺CD271⁺, which could enrich OSR1⁺SIX2⁺ renal progenitors. Furthermore, these isolated cells ameliorated renal injury in an acute kidney injury (AKI) mouse model when used for cell therapy. These cells could contribute to the development of hiPSC-based cell therapy and disease modeling against kidney diseases.

Mouse Metanephric Mesenchymal Cell-Derived Angioblasts Undergo Vasculogenesis in Three-Dimensional Culture

In vitro models for the investigation of renal vascular development are limited. We previously showed that isolated metanephric mesenchymal (MM) and ureteric bud (UB) cells grown in three-dimensional (3D) matrices formed organoids that consisted of primitive vascular structures surrounding a polarized epithelium. Here, we examined the potential of two principal effectors of vasculogenesis, vascular endothelial growth factor A (VEGF-A), and platelet-derived growth factor B chain (PDGF-BB), to stimulate MM cell differentiation. The results showed that MM cells possess angioblast characteristics by expressing phenotypic markers for endothelial and mesenchymal cells. UB cells synthesize VEGF-A and PDGF-BB proteins and RNA, whereas the MM cells express the respective cognate receptors, supporting their role in directional induction of vasculogenesis. VEGF-A stimulated proliferation of MM cells in monolayer and in 3D sponges but did not affect MM cell migration, organization, or vasculogenesis. However, PDGF-BB stimulated MM cell proliferation, migration, and vasculogenesis in monolayer and organization of the cells into primitive capillary-like assemblies in 3D sea sponge scaffolds in vitro. A role for PDGF-BB in vasculogenesis in the 3D MM/UB co-culture system was validated by direct interference with PDGF-BB or PDGF receptor-β cell interactions to implicate PDGF-BB as a primary effector of MM cell vasculogenesis. Thus, MM cells resemble early renal angioblasts that may provide an ideal platform for the investigation of renal vasculogenesis in vitro.

A positive feedback loop involving Erk5 and Akt turns on mesangial cell proliferation in response to PDGF

Platelet-derived growth factor BB and its receptor (PDGFRβ) play a pivotal role in the development of renal glomerular mesangial cells. Their roles in increased mesangial cell proliferation during mesangioproliferative glomerulonephritis have long been noted, but the operating logic of signaling mechanisms regulating these changes remains poorly understood. We examined the role of a recently identified MAPK, Erk5, in this process. PDGF increased the activating phosphorylation of Erk5 and tyrosine phosphorylation of proteins in a time-dependent manner. A pharmacologic inhibitor of Erk5, XMD8-92, abrogated PDGF-induced DNA synthesis and mesangial cell proliferation. Similarly, expression of dominant negative Erk5 or siRNAs against Erk5 blocked PDGF-stimulated DNA synthesis and proliferation. Inhibition of Erk5 attenuated expression of cyclin D1 mRNA and protein, resulting in suppression of CDK4-mediated phosphorylation of the tumor suppressor protein pRb. Expression of cyclin D1 or CDK4 prevented the dominant negative Erk5- or siErk5-mediated inhibition of DNA synthesis and mesangial cell proliferation induced by PDGF. We have previously shown that phosphatidylinositol 3-kinase (PI3-kinase) contributes to PDGF-induced proliferation of mesangial cells. Inhibition of PI3-kinase blocked PDGF-induced phosphorylation of Erk5. Since PI3-kinase acts through Akt, we determined the role of Erk5 on Akt phosphorylation. XMD8-92, dominant negative Erk5, and siErk5 inhibited phosphorylation of Akt by PDGF. Interestingly, we found inhibition of PDGF-induced Erk5 phosphorylation by a pharmacological inhibitor of Akt kinase and kinase dead Akt in mesangial cells. Thus our data unfold the presence of a positive feedback microcircuit between Erk5 and Akt downstream of PI3-kinase nodal point for PDGF-induced mesangial cell proliferation.

Src tyrosine kinase mediates platelet-derived growth factor BB-induced and redox-dependent migration in metanephric mesenchymal cells

The adult kidney is derived from the interaction between the metanephric blastema and the ureteric bud. Platelet-derived growth factor (PDGF) receptor β is essential for the development of the mature glomerular tuft, as mice deficient for this receptor lack mesangial cells. This study investigated the role of Src tyrosine kinase in PDGF-mediated reactive oxygen species (ROS) generation and migration of metanephric mesenchymal cells (MMCs). Cultured embryonic MMCs from wild-type and PDGF receptor-deficient embryos were established. Migration was determined via wound-healing assay. Unlike PDGF AA, PDGF BB-induced greater migration in MMCs with respect to control. This was abrogated by neutralizing an antibody to PDGF BB. Phosphatidylinositol 3-kinase (PI3K) inhibitors suppressed PDGF BB-induced migration. Conversely, mitogen-activated protein kinase/extracellular signal-regulated kinase (MEK) inhibitors had no effect. Src inhibitors inhibited PDGF-induced cell migration, PI3K activity, and Akt phosphorylation. Adenoviral dominant negative Src (AD DN Src) abrogated PDGF BB-induced Akt phosphorylation. Hydrogen peroxide stimulated cell migration. PDGF BB-induced wound closure was inhibited by the antioxidants N-acetyl-l-cysteine, tiron, and the flavoprotein inhibitor diphenyleneiodonium. These cells express the NADPH oxidase homolog Nox4. Inhibiting Nox4 with antisense oligonucleotides or small interfering RNA (siRNA) suppressed PDGF-induced wound closure. Inhibition of Src with siRNA reduced PDGF BB-induced ROS generation as assessed by 2',7'-dichlorodihydrofluorescein diacetate fluorescence. Furthermore, PDGF BB-stimulated ROS generation and migration were similarly suppressed by Ad DN Src. In MMCs, PDGF BB-induced migration is mediated by PI3K and Src in a redox-dependent manner involving Nox4. Src may be upstream to PI3K and Nox4.

Direct and efficient cellular transformation of primary rat mesenchymal precursor cells by KSHV

Infections by viruses are associated with approximately 12% of human cancer. Kaposi's sarcoma-associated herpesvirus (KSHV) is causally linked to several malignancies commonly found in AIDS patients. The mechanism of KSHV-induced oncogenesis remains elusive, due in part to the lack of an adequate experimental system for cellular transformation of primary cells. Here, we report efficient infection and cellular transformation of primary rat embryonic metanephric mesenchymal precursor cells (MM cells) by KSHV. Cellular transformation occurred at as early as day 4 after infection and in nearly all infected cells. Transformed cells expressed hallmark vascular endothelial, lymphatic endothelial, and mesenchymal markers and efficiently induced tumors in nude mice. KSHV established latent infection in MM cells, and lytic induction resulted in low levels of detectable infectious virions despite robust expression of lytic genes. Most KSHV-induced tumor cells were in a latent state, although a few showed heterogeneous expression of lytic genes. This efficient system for KSHV cellular transformation of primary cells might facilitate the study of growth deregulation mechanisms resulting from KSHV infections.

Reciprocal induction of simple organogenesis by mouse kidney progenitor cells in three-dimensional co-culture

Kidney development is regulated by a coordinated reciprocal induction of metanephric mesenchymal (MM) and ureteric bud (UB) cells. Here, established MM and UB progenitor cell lines were recombined in three-dimensional Matrigel implants in SCID mice. Differentiation potential was examined for changes in phenotype, organization, and the presence of specialized proteins using immunofluorescence and bright-field and electron microscopy. Both cell types, when grown alone, did not develop into specialized structures. When combined, the cells organized into simple organoid structures of polarized epithelia with lumens surrounded by capillary-like structures. Tracker experiments indicated the UB cells formed the tubuloid structures, and the MM cells were the source of the capillary-like cells. The epithelial cells stained positive for pancytokeratin, the junctional complex protein ZO-1, collagen type IV, as well as UB and collecting duct markers, rearranged during transfection (RET), Dolichos biflorus lectin, EndoA cytokeratin, and aquaporin 2. The surrounding cells expressed α-smooth muscle actin, vimentin, platelet endothelial cell adhesion molecule 1 (PECAM), and aquaporin 1, a marker of vasculogenesis. The epithelium exhibited apical vacuoles, microvilli, junctional complexes, and linear basement membranes. Capillary-like structures showed endothelial features with occasional pericytes. UB cell epithelialization was augmented in the presence of MM cell-derived conditioned medium, glial-derived neurotrophic factor (GDNF), hepatocyte growth factor (HGF), or fibronectin. MM cells grown in the presence of UB-derived conditioned medium failed to undergo differentiation. However, UB cell-derived conditioned medium induced MM cell migration. These studies indicate that tubulogenesis and vasculogenesis can be partially recapitulated by recombining individual MM and UB cell lineages, providing a new model system to study organogenesis ex vivo.

Alpha-tocopherol prevents intrauterine undernutrition-induced oligonephronia in rats

The role of α-tocopherol during nephrogenesis was investigated in rats subjected to maternal undernutrition, which reduces the number of nephrons. α-tocopherol (350 mg/kg, p.o.) was administered daily to well-nourished or malnourished Wistar dams during pregnancy, or to prenatal undernourished rats during lactation. The kidneys of 1- and 25-day-old offspring were removed to evaluate expression of angiotensin II (Ang II) and to correlate this with expression of proliferating cell nuclear antigen, α-smooth muscle actin, fibronectin and vimentin in the glomeruli and tubulointerstitial space. One-day-old prenatally undernourished rats had reduced expression of Ang II and of kidney development markers, and presented with an enlarged nephrogenic zone. Maternal administration of α-tocopherol restored the features of normal kidney development in undernourished rats. Twenty-five-day-old prenatally undernourished progeny had fewer glomeruli than the control group. Conversely, animals from mothers that received α-tocopherol during lactation presented with the same number of glomeruli and the same glomerular morphometrical profile as the control group. Analyzing the levels of thiobarbituric acid reactive substances in the liver in conjunction with kidney development markers, it is plausible that α-tocopherol had antioxidant and non-antioxidant actions. This study provides evidence that α-tocopherol treatment restored Ang II expression, and subsequently restored renal structural development.

PDGF receptor-{beta} modulates metanephric mesenchyme chemotaxis induced by PDGF AA

PDGF B chain or PDGF receptor (PDGFR)-beta-deficient (-/-) mice lack mesangial cells. To study responses of alpha- and beta-receptor activation to PDGF ligands, metanephric mesenchymal cells (MMCs) were established from embryonic day E11.5 wild-type (+/+) and -/- mouse embryos. PDGF BB stimulated cell migration in +/+ cells, whereas PDGF AA did not. Conversely, PDGF AA was chemotactic for -/- MMCs. The mechanism by which PDGFR-beta inhibited AA-induced migration was investigated. PDGF BB, but not PDGF AA, increased intracellular Ca(2+) and the production of reactive oxygen species (ROS) in +/+ cells. Transfection of -/- MMCs with the wild-type beta-receptor restored cell migration and ROS generation in response to PDGF BB and inhibited AA-induced migration. Inhibition of Ca(2+) signaling facilitated PDGF AA-induced chemotaxis in the wild-type cells. The antioxidant N-acetyl-l-cysteine (NAC) or the NADPH oxidase inhibitor diphenyleneiodonium (DPI) abolished the BB-induced increase in intracellular Ca(2+) concentration, suggesting that ROS act as upstream mediators of Ca(2+) in suppressing PDGF AA-induced migration. These data indicate that ROS and Ca(2+) generated by active PDGFR-beta play an essential role in suppressing PDGF AA-induced migration in +/+ MMCs. During kidney development, PDGFR beta-mediated ROS generation and Ca(2+) influx suppress PDGF AA-induced chemotaxis in metanephric mesenchyme.

Mitogenic signaling via platelet-derived growth factor beta in metanephric mesenchymal cells

Mice deficient in either platelet-derived growth factor (PDGF) B chain or PDGF receptor (PDGFR) beta lack mesangial cells. PDGF stimulates proliferation and migration of metanephric mesenchymal cells, from which mesangial cells are derived. Binding of PDGF to PDGFR-beta induces autophosphorylation at specific tyrosine residues and activates various effector proteins, including phosphatidylinositol-3-kinase (PI3-K). This study explored the role of PI 3-K and reactive oxygen species (ROS) in PDGF-mediated signaling using cells established from wild-type and PDGFR-beta -/- metanephric blastemas at 11.5 days post-conception. PDGF-induced effects that were dependent on PI3-K activation were determined using PDGFR-beta -/- cells made to express "add-back" mutant PDGFR-beta capable of binding PI3-K. We found that PDGF is mitogenic for mesenchymal cells expressing PDGFR-beta, and PI3-K is an important regulator of PDGF-induced DNA synthesis. Activation of ERK1/2 is partially dependent on PI3-K, and both the PI3-K and MEK-ERK1/2 pathways contribute to PI3-K-dependent mitogenesis. In addition, PDGF-induced DNA synthesis in wild-type cells was found to be dependent on ROS that are generated downstream of PI3-K activation. Using antisense oligonucleotides and small interfering RNA, we determined that the NAD(P)H oxidase Nox4 produces these ROS that activate Akt and the MEK-ERK1/2 mitogenic cascade. In conclusion, the present study demonstrates Nox4 involvement in PDGF-induced DNA synthesis in metanephric mesenchymal cells and provides the first evidence that PDGF-induced PI3-K activity enhances production of ROS by Nox4.

Abnormal development of glomerular endothelial and mesangial cells in mice with targeted disruption of the lama3 gene

Mice with targeted disruption of the lama3 gene, which encodes the alpha3 chain of laminin-5 (alpha3beta3gamma2, 332), develop a blistering skin disease similar to junctional epidermolysis bullosa in humans. These animals also develop abnormalities in glomerulogenesis. In both wild-type and mutant animals (lama3(-/-)), podocytes secrete glomerular basement membrane and develop foot processes. Endothelial cells migrate into this scaffolding and secrete a layer of basement membrane that fuses with the one formed by the podocyte. In lama3(-/-) animals, glomerular maturation arrests at this stage. Endothelial cells do not attenuate, develop fenestrae, or form typical lumens, and mesangial cells (MCs) were not identified. LN alpha3 subunit (LAMA3) protein was identified in the basement membrane adjacent to glomerular endothelial cells (GEnCs) in normal rats and mice. In developing rat glomeruli, the LAMA3 subunit was first detectable in the early capillary loop stage, which corresponds to the stage at which maturation arrest was observed in the mutant mice. Lama3 mRNA and protein were identified in isolated rat and mouse glomeruli and cultured rat GEnCs, but not MC. These data document expression of LAMA3 in glomeruli and support a critical role for it in GEnC differentiation. Furthermore, LAMA3 chain expression and/or another product of endothelial cells are required for MC migration into the developing glomerulus.

Rat glomerular mesangial cells require laminin-9 to migrate in response to insulin-like growth factor binding protein-5

Temporal and spatial differences in extracellular matrix play critical roles in cell proliferation, differentiation and migration. Different migratory stimuli use different substrates and receptors to achieve cell migration. To understand the mechanism of insulin-like growth factor binding protein-5 (IGFBP-5)-induced migration in mesangial cells, the roles of integrins and substrates were examined. IGFBP-5 induced an increase in mRNA expression for laminin (LN) chains lama4, lamb2, and lamc1, suggesting that LN-9 might be required for migration. Antibodies to the LNalpha(4) and LNbeta(2) chains, but not LNbeta(1), blocked IGFBP-5-induced migration. Anti-sense morpholino oligonucleotide inhibition of expression of LNalpha(4) substantially reduced expression of LN-8/9 (alpha(4)beta(1)gamma(1)/alpha(4)beta(2)gamma(1), 411/421) and prevented IGFBP-5-induced migration. Anti-sense inhibition of lamb2 reduced expression of LN-9. Absence of LN-9 prevented IGFBP-5-induced migration, which was not preserved by continued expression of LN-8. The requirement for LN-9 was further supported by studies of T98G cells, which express predominantly LN-8. IGFBP-5 had little effect on migration in these cells, but increased migration when T98G cells were plated on LN-8/9. IGFBP-5-mediated mesangial cell migration was inhibited by antibodies that block attachment to alpha(6)beta(1)-integrins but was unaffected by antibodies and disintegrins that block binding to other integrins. Furthermore, in cells with anti-sense inhibited expression of LN-9, integrin alpha(6)beta(1) was no longer detected on the cell surface. These studies suggest the specificity of mechanisms of migration induced by specific stimuli and for the first time demonstrate a unique function for LN-9 in mediating IGFBP-5-induced migration.

Fibronectin induces ureteric bud cells branching and cellular cord and tubule formation

BACKGROUND

The extracellular matrix (ECM) protein fibronectin is involved in several stages of embryogenesis. Fibronectin exerts its effect through interaction with cellular integrin and nonintegrin receptors.

METHODS

We investigated the effect of fibronectin on branching and tubulogenesis of ureteric bud cells in a three-dimensional gel culture system. Primary ureteric bud cells from mouse embryos at gestation 11 days (E11) were isolated and established in culture. Fibronectin and integrin subunits were localized using immunoperoxidase staining.

RESULTS

In three-dimensional collagen type I gel culture of ureteric bud cell, fibronectin dose dependently induces cord and tubule formation. Both ureteric bud cells and ureteric bud branches in embryonic kidney express the same multiple integrin subunits that include beta(1), beta(3), alpha(3), alpha(4) and alpha(v). Embryonic kidneys examined at E12, E14, and E16 days of gestation express fibronectin in the undifferentiated mesenchyme especially next to ureteric bud branches and in the interstitium around glomerulotubular structures and blood vessels. Fibronectin expression was similar at the tips and stalks of branching ureteric bud. Fibronectin expression is maximum at E12 and decreases with advanced gestation. Cultured ureteric bud cells also express fibronectin. RGD peptides inhibit cord and tubular formation in the three-dimensional gel. Anti-alpha(3)beta(1) antibody partially inhibits fibronectin-induced cord and tubule formation. Hepatocyte growth factor (HGF), fibroblast growth factor (FGF), and glial cell line-derived neurotrophic factor (GDNF) induce ureteric bud cell cord formation in three-dimensional gel. The effects of growth factors are delayed and quantitatively less compared to the effect of fibronectin.

CONCLUSION

Fibronectin induces ureteric bud cells branching and tubulogenesis through interaction with multiple integrin receptors. Cultured ureteric bud cells express fibronectin and the origin of fibronectin at mesenchyme-ureteric bud interface is likely both the metanephric mesenchyme and ureteric bud epithelium. Addition of individual neutralizing antibodies to beta(1), beta(3), alpha(3), alpha(4,)alpha(6) and alpha(v) integrin subunits does not block the effect of fibronectin. Only an antibody to alpha(3)beta(1) integrin substantially blocks the effect of fibronectin. Other mechanisms, including unidentified integrins, are likely involved in fibronectin-induced cord and tubule formation.

Platelet-derived growth factor plays a critical role to convert bone marrow cells into glomerular mesangial-like cells

BACKGROUND

Despite increasing interest in bone marrow-derived stem cells, little is known about critical factors that determine their fates both in vitro and in vivo. Recently, we have reported that bone marrow is a reservoir for glomerular mesangial cells in rats. To find a key factor responsible for the differentiation of bone marrow-derived cells into mesangial cells, we established a new culture system of rat bone marrow, which is based on serial replating and differential attachment to collagen types I and IV.

METHODS

Bone marrow cells that did not adhere to collagen type I within 24 hours were transferred to collagen type IV-coated dishes. Then, the cells attached to collagen type IV in the following 24 hours were maintained in the presence of 2% horse serum, 200 ng/mL of platelet-derived growth factor (PDGF)-BB, and 1 micromol/L of all-trans retinoic acid. In vivo effect of PDGF-B was also examined by introducing human PDGF-B gene into glomeruli.

RESULTS

After cultivation under the above condition for 7 days, approximately 14% of cells expressed Thy-1 and desmin, both of which are markers for rat mesangial cells. Thy-1++/desmin+ cells were stellate-shaped, and contracted in response to angiotensin II. When human PDGF-B gene was overexpressed in the glomeruli of chimeric rats whose bone marrow was transplanted from enhanced green florescent protein (EGFP) transgenic rats, the number of EGFP+ mesangial cells increased. This effect was canceled by prior introduction of a neutralizing molecule that is composed of PDGF receptor-beta ligand binding site and IgG-Fc.

CONCLUSION

These results indicate that PDGF-B plays a critical role to direct bone marrow-derived cells toward mesangial-like cells both in vitro and in vivo.

Roles of PDGF in animal development

Recent advances in genetic manipulation have greatly expanded our understanding of cellular responses to platelet-derived growth factors (PDGFs) during animal development. In addition to driving mesenchymal proliferation, PDGFs have been shown to direct the migration, differentiation and function of a variety of specialized mesenchymal and migratory cell types, both during development and in the adult animal. Furthermore, the availability of genomic sequence data has facilitated the identification of novel PDGF and PDGF receptor (PDGFR) family members in C. elegans, Drosophila, Xenopus, zebrafish and mouse. Early data from these different systems suggest that some functions of PDGFs have been evolutionarily conserved.

Laminin-8/9 is synthesized by rat glomerular mesangial cells and is required for PDGF-induced mesangial cell migration

BACKGROUND

Laminin (LM), the major glycoprotein component of basement membranes is expressed as multiple isoforms in a developmentally regulated and tissue-specific manner. LM alpha4 has a limited tissue distribution and is highly expressed in the developing glomerulus. In the present study, we investigate the in vivo and in vitro expression and function of LM alpha4 in the glomerulus.

METHODS

LM alpha4 expression was examined by Northern blot, reverse transcription polymerase chain reaction (RT-PCR), Western blot, and immunofluorescence microscopy. Mesangial cells (MC) were plated on purified LM-1, LM-2, and LM-8/9. Immunofluorescence microscopy was performed to examine the cellular phenotypes induced by LM-1 and LM-8/9. A modified Boyden chamber method was used to assess laminin participation in platelet-derived growth factor (PDGF)-stimulated migration.

RESULTS

mRNA for LMalpha4 is expressed in cultured rat MC, and isolated rat and mouse glomeruli, but not in cultured rat glomerular epithelial cells or glomerular endothelial cells. Using antibodies specific for LM alpha4, a 240 kD band was detected in MC extract and a slightly smaller band was identified in extracted rat glomeruli. Purified LM-8/9 had MC adhesive activity comparable to LM-1 and LM-2. MC attached to LM-8/9 exhibited a unique phenotype. In contrast to LM-1, attachment of MC to LM-8/9 produced a highly arborized cell morphology with significantly reduced formation of focal contacts or stress fibers. LM alpha4 is utilized by MC during PDGF-stimulated migration.

CONCLUSION

LM alpha4 is synthesized by MC and persists in the mature glomerulus. LM-8/9 stimulates a unique cellular morphology, and they are utilized in PDGF-induced migration. These factors suggest that LM alpha4 plays an important role in MC differentiation and in the maintenance of MC phenotype.

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.

Morphological insights into the origin of glomerular endothelial and mesangial cells and their precursors

Glomerular endothelial and mesangial cells may originate from the metanephric mesenchyme. We used the MAb Thy1.1, a mesangial cell marker in the adult rat kidney, and rat endothelial cell markers MAb RECA-1, MAb PECAM-1 (CD31), and MAb Flk-1 as potential markers to characterize the spatial and temporal distribution of mesangial and endothelial cell precursors during nephrogenesis in the rat. At early stages of glomerulogenesis, RECA-1- and Thy1.1-positive cells were detected in the metanephric blastema at 14 days post conception (dpc) embryos and 15 dpc, respectively, with Thy1.1 expression in cells surrounding the ureteric bud. At 17 and 18 dpc, both RECA-1- and Thy1.1-positive cells were found in the cleft of the S-shaped bodies and in the capillary loops of maturing glomeruli. Double staining for BrdU, a marker of proliferation, and for RECA-1 or BrdU and Thy1.1 also localize in the cleft of S-shaped bodies and in glomerular capillary loops at later stages of development. PDGFRbeta co-localizes in cells expressing endothelial or mesangial markers. The data suggest that endothelial and mesangial cell precursors share common markers during the course of glomerulogenesis and that full differentiation of these cells occurs at late stages of glomerular maturation. Thy1.1- and RECA-1-positive cells may be derived from the metanephric blastemal cells at early stages of kidney development. A subpopulation of these Thy1.1- or RECA-1-positive cells may be precursors that can migrate into the cleft of comma and S-shaped bodies and proliferate in situ to form glomerular capillary tufts.

Induction of DNA synthesis by ligation of the CD53 tetraspanin antigen in primary cultures of mesangial cells

BACKGROUND

The interaction of mesangial cells with the extracellular matrix plays a major role in kidney biology. Tetraspanin proteins modulate cell interaction with the extracellular matrix. Tetraspanins form supramolecular structures on the cell membrane that send signals after engagement by unknown ligands, modulate different signaling processes, and regulate cell adhesion and motility.

METHODS

CD53 was determined by immunohistochemistry, and on the cell surface of cultured rat mesangial cells by flow cytometry. Mesangial cell cultures were stimulated with MRC OX-44 antibody. DNA synthesis was measured by thymidine incorporation. Extracellular signal-regulated kinase (ERK) activation was determined by Western blot.

RESULTS

CD53 was present in mesangial cells in vivo and in culture. Ligation of CD53 antigen with a monoclonal antibody triggered the induction of DNA synthesis, which was not sensitive to inhibitors of signaling pathways that use phosphatidylinositol 3-kinase (PI3K) and protein kinase C, or to calcium channel inhibitors, such as thapsigargin and verapamil. The DNA synthesis was inhibited by PD98059, a specific inhibitor of MEK that prevents ERK1/ERK2 activation. In addition, ERK1 and ERK2 activation by phosphorylation occurred following CD53 antigen ligation. The DNA synthesis was due to de novo synthesis and not to DNA repair as a consequence of the initiation of apoptosis, determined by flow cytometry, and lack of proteolytic activation of PARP by caspase 3. CD53 antigen ligation also induced an increase in mitochondrial activity.

CONCLUSIONS

To our knowledge this is the first identification of a tetraspanin protein in mesangial cells. CD53 antigen delivers a signal that initiates DNA synthesis. This signal is mediated by ERK1/ERK2 activation, but it is not sufficient to complete the cell cycle.

Differentiative potential of human metanephric mesenchymal cells

OBJECTIVE

To evaluate the ability of mesenchymal cells derived from nonhematopoietic organs to form blood and other tissues in vitro and in vivo.

MATERIALS AND METHODS

Because of its mesodermic derivation, human fetal kidney was used as a source of mesenchymal cells. Two populations of kidney cells were studied at a nonclonal level: a crude preparation, and an adherent fraction that was derived from the first by propagation in vitro (MNMC). Both populations were transplanted into sheep fetuses and analyzed at intervals for the presence of human cells in different organs by flow cytometry, PCR, immunohistochemistry, and in situ hybridization. Secondary transplantation studies were performed using human hematopoietic cells obtained from the bone marrow (BM) of primary recipients.

RESULTS

MNMC were Thy-1(+), CD51(+), CD44(+), CD45(-), and vimentin(+), a phenotype consistent with that of metanephric mesenchyme. The crude population displayed the same phenotype but was contaminated with 0.4% CD34(+)CD45(+) cells. Cells with hepatocyte-like morphology and phenotype were obtained from the MNMC after culture in specific inducing media. After transplantation, both populations of cells produced multilineage hematopoietic engraftment and gave rise to CD34(+) cells. Successful hematopoietic engraftment in secondary recipients demonstrated the generation of long-term engrafting hematopoietic stem cells from MNMC. PCR analysis confirmed human hematopoietic engraftment and revealed that human cells were also present within other organs. Liver sections of transplanted animals contained human albumin-producing hepatocyte-like cells.

CONCLUSION

A human metanephric mesenchymal cell population simultaneously gave rise to human blood and liver-like cells, suggesting that mesenchymal cells may represent a broad population of putative stem cells in multiple adult organs.

Identification of developmentally regulated mesodermal-specific transcript in mouse embryonic metanephros

Mesodermal-specific cDNA or transcript (MEST) was identified by suppression subtractive hybridization-PCR of cDNA isolated from embryonic day 13 vs. newborn mice kidneys. At day 13 of mouse gestation, a high expression of MEST, with a single approximately 2.7-kb transcript that was exclusively localized to the metanephric mesenchyme was observed. The MEST mRNA expression gradually decreased during the later stages and then abruptly decreased in the newborn kidneys and subsequent postnatal life, after which a very mild expression persisted in the glomerular mesangium. Regression in mRNA expression during embryonic renal development appears to be related to methylation of the MEST gene. Treatment of metanephroi, harvested at day 13 of gestation with MEST-specific antisense oligodeoxynucleotide resulted in a dose-dependent decrease in the size of the explants and the nephron population. This was associated with a selective decrease in MEST mRNA expression and accelerated apoptosis of the mesenchyme. These findings suggest that MEST, a gene with a putative mesenchymal cell-derived protein, conceivably plays a role in mammalian metanephric development.

Roles of SLC/CCL21 and CCR7 in human kidney for mesangial proliferation, migration, apoptosis, and tissue homeostasis

The release of chemokines by intrinsic renal cells is an important mechanism for the regulation of leukocyte trafficking during renal inflammation. The expression of chemokine receptors by intrinsic renal cells such as mesangial cells (MC) suggests an expanded role for chemokine-chemokine receptor biology in local immunomodulation and potentially glomerular homeostasis. By immunohistochemistry we found the chemokine receptor CCR7 expressed in a mesangial pattern while the CCR7 ligand SLC/CCL21 showed a podocyte-specific expression. CCR7 expression was further characterized by RT-PCR, RNase protection assays, and FACS analysis of cultured human MC, and was found to be constitutively present. Real-time PCR of microdissected glomeruli confirmed the expression of SLC/CCL21. A functional role for CCR7 was demonstrated for human MC migration and proliferation. A protective effect of SLC/CCL21 was shown for MC survival in Fas Ab-induced apoptosis. Finally, "wound healing" was enhanced in the presence of SLC/CCL21 in an in vitro injury model. The constitutive glomerular expression of CCR7 and its ligand SLC/CCL21 in adjacent cell types of the human kidney suggests novel biological functions of this chemokine/chemokine receptor pair and a potential role in processes involved in glomerular homeostasis and regeneration.

Effect of platelet-derived growth factor isoforms in rat metanephric mesenchymal cells

Platelet-derived growth factor (PDGF) B-chain or PDGF beta-receptor-deficient mice lack mesangial cells. To explore potential mechanisms for failure of PDGF A-chain to rescue mesangial cell phenotype, we investigated the biological effects and signaling pathways of PDGF AA and PDGF BB in metanephric mesenchymal (MM) cells isolated from rat kidney. PDGF AA caused modest cell migration but had no effect on DNA synthesis, unlike PDGF BB, which potently stimulated migration and DNA synthesis. PDGF AA and PDGF BB significantly increased the activities of phosphatidylinositol 3-kinase (PI 3-K) and mitogen-activated protein kinase (MAPK). PDGF BB was more potent than PDGF AA in activating PI 3-K or MAPK in these cells. Pretreatment of MM cells with the MAPK kinase (MEK) inhibitor PD-098059 abrogated PDGF BB-induced DNA synthesis, whereas the PI 3-K inhibitor wortmannin had a very modest inhibitory effect on DNA synthesis (approximately Delta20%). On the other hand, wortmannin completely blocked PDGF AA- and PDGF BB-induced migration, whereas PD-098059 had a modest inhibitory effect on cell migration. These data demonstrate that activation of MAPK is necessary for the mitogenic effect of PDGF BB, whereas PI 3-K is required for the chemotactic effect of PDGF AA and PDGF BB. Although PDGF AA stimulates PI 3-K and MAPK activity, it is not mitogenic and only modestly chemotactic. Collectively, the data may have implications related to the failure of PDGF AA to rescue mesangial cell phenotype in PDGF B-chain or PDGF-beta-receptor deficiency.

Bone marrow is a reservoir of repopulating mesangial cells during glomerular remodeling

The renal glomerulus, whose cellular components are developmentally derived from the mesenchyme, plays a pivotal role in filtratating plasma. Irretrievable changes of glomerular components are responsible for the initiation and progression of impaired renal function. Recently, it has been shown that functional stem cells exist in the bone marrow of adult bodies and that they can reconstitute damaged tissues of the mesenchymal origin. To examine whether the bone marrow provides stem cells to damaged glomeruli, transgenic rats carrying enhanced green fluorescence protein (EGFP rat) were established in a systemic and constitutive manner. After transplanting the bone marrow of EGFP rats into wild-type rats, the progeny of the transplanted marrow cells were tracked with a tag of EGFP. Recruitment of bone marrow-derived cells into glomeruli was dramatically facilitated in response to mesangiolysis evoked in anti-Thy1 antibody-mediated glomerulonephritis. In the restored glomeruli, 11% to 12% of glomerular cells were derived from the transplanted bone marrow. The number of bone marrow-derived CD45(+) cells transiently increased during the disease process, and CD45-negative cells constantly accounted for more than half of the bone marrow-derived population in glomeruli. Bone marrow-derived Thy1(+) cells kept increasing in number until the remodeling ceased and finally made up 7% to 8% of glomerular cells. Laser scanning microscopy displayed that the bone marrow-derived Thy1(+) cells provide structural support for glomerular capillaries, which indicates that they are mesangial cells. Although CD45(-)Thy1(-) bone marrow-derived cells exist during the remodeling of glomeruli, none of them expressed endothelial markers such as Factor VIII and RECA1 as long as they were tested. The results indicate that the bone marrow can give rise to mesangial cells in vivo.

Akt serine threonine kinase regulates platelet-derived growth factor-induced DNA synthesis in glomerular mesangial cells: regulation of c-fos AND p27(kip1) gene expression

Proliferation of mesangial cells requires platelet-derived growth factor receptor beta (PDGFR)-mediated signal transduction. We have previously shown that activation of phosphatidylinositol (PI) 3-kinase is necessary for PDGFR-induced DNA synthesis in these cells. The mechanism by which PI 3-kinase stimulates DNA synthesis is not known. One target of PI 3-kinase, Akt serine threonine kinase, regulates survival of many cells by inhibiting the actions of certain proapoptotic proteins. In this study, we investigated the role of Akt in PDGF-induced DNA synthesis in mesangial cells. PDGF increased Akt serine threonine kinase activity in a time- and PI 3-kinase-dependent manner. Expression of dominant negative Akt by adenovirus-mediated gene transfer blocked PDGF-induced activation of endogenous Akt in mesangial cells, resulting in complete inhibition of DNA synthesis. On the other hand, inhibition of MAPK attenuated PDGF-induced DNA synthesis only partially. Inhibition of Akt also attenuated PDGF-induced c-fos gene transcription, with concomitant inhibition of Elk-1-dependent transcription, indicating positive regulation of this early response gene by Akt. To further determine the role of Akt in PDGF-induced DNA synthesis, we investigated its effect on cyclin-dependent kinase 2 (CDK2). PDGF stimulated CDK2 activity in mesangial cells and decreased the level of p27(kip1) cyclin kinase inhibitor protein. Expression of dominant negative Akt increased p27(kip1) protein and resulted in inhibition of CDK2 activity. The increase in p27(kip1) expression in response to Akt kinase inhibition was due to increased transcription of the p27(kip1) gene. p27(kip1) transcription similarly was decreased by expression of constitutively active Akt kinase in mesangial cells. These data provide the first evidence that Akt kinase regulates PDGF-induced DNA synthesis by regulating CDK2 activity and define Akt-mediated inhibition of transcription of p27(kip1) as one of the mechanisms for PDGF-induced DNA synthesis in mesangial cells.

Ceramide blocks PDGF-induced DNA synthesis in mesangial cells via inhibition of Akt kinase in the absence of apoptosis

The mechanism of action of ceramide in glomerular mesangial cells has not been studied. We investigated the effect of C2 ceramide on the mitogenic signal transduction pathways induced by PDGF in mesangial cells. Increasing concentrations of C2 ceramide inhibited PDGF-induced DNA synthesis in a dose-dependent manner with maximum inhibition at 15 microM. This inhibition of DNA synthesis was associated with attenuation of PDGF-induced early response gene c-fos transcription. PDGF receptor beta immunecomplex kinase assay showed no inhibitory effect of C2 ceramide on PDGF receptor tyrosine kinase activity. We have recently shown that the mitogenic effect of PDGF is mediated by the enzyme phosphatidylinositol (PI) 3 kinase in mesangial cells. C2 ceramide had no effect on PDGF-induced PDGFR-associated PI 3 kinase activity. These data indicate that inhibitory effect of C2 on PDGF-induced DNA synthesis is likely due to post-receptor and post-PI 3 kinase events. To address the mechanism of C2-mediated inhibition of DNA synthesis, we investigated the downstream target of PI 3 kinase, Akt. PDGF time-dependently increased Akt kinase activity in a PI 3 kinase-dependent manner. Incubation of mesangial cells with C2 ceramide inhibited PDGF-induced Akt activity. Akt kinase inhibits apoptosis of cells via phosphorylation of multiple proapoptotic proteins. However, inhibition of Akt activity by C2 ceramide did not induce apoptosis in mesangial cells. These data provide the first evidence that in mesangial cells, ceramide cross-talks with PI 3 kinase-dependent Akt kinase to inhibit PDGF-induced DNA synthesis without inducing apoptosis.

Angiotensin II activates Akt/protein kinase B by an arachidonic acid/redox-dependent pathway and independent of phosphoinositide 3-kinase

Angiotensin II (Ang II) exerts contractile and trophic effects in glomerular mesangial cells (MCs). One potential downstream target of Ang II is the protein kinase Akt/protein kinase B (PKB). We investigated the effect of Ang II on Akt/PKB activity in MCs. Ang II causes rapid activation of Akt/PKB (5-10 min) but delayed activation of phosphoinositide 3-kinase (PI3-K) (30 min). Activation of Akt/PKB by Ang II was not abrogated by the PI3-K inhibitors or by the introduction of a dominant negative PI3-K, indicating that in MCs, PI3-K is not an upstream mediator of Akt/PKB activation by Ang II. Incubation of MCs with phospholipase A2 inhibitors also blocked Akt/PKB activation by Ang II. AA mimicked the effect of Ang II. Inhibitors of cyclooxygenase-, lipoxyogenase-, and cytochrome P450-dependent metabolism did not influence AA-induced Akt/PKB activation. However, the antioxidants N-acetylcysteine and diphenylene iodonium inhibited both AA- and Ang II-induced Akt/PKB activation. Dominant negative mutant of Akt/PKB or antioxidants, but not the dominant negative form of PI3-K, inhibited Ang II-induced protein synthesis and cell hypertrophy. These data provide the first evidence that Ang II induces protein synthesis and hypertrophy in MCs through AA/redox-dependent pathway and Akt/PKB activation independent of PI3-K.

PDGF-D is a specific, protease-activated ligand for the PDGF beta-receptor

The term 'platelet-derived growth factor' (PDGF) refers to a family of disulphide-bonded dimeric isoforms that are important for growth, survival and function in several types of connective tissue cell. So far, three different PDGF chains have been identified - the classical PDGF-A and PDGF-B and the recently identified PDGF-C. PDGF isoforms (PDGF-AA, AB, BB and CC) exert their cellular effects by differential binding to two receptor tyrosine kinases. The PDGF alpha-receptor (PDGFR-alpha) binds to all three PDGF chains, whereas the beta-receptor (PDGFR-beta) binds only to PDGF-B. Gene-targeting studies using mice have shown that the genes for PDGF-A and PDGF-B, as well as the two PDGFR genes, are essential for normal development. Furthermore, overexpression of PDGFs is linked to different pathological conditions, including malignancies, atherosclerosis and fibroproliferative diseases. Here we have identify and characterize a fourth member of the PDGF family, PDGF-D. PDGF-D has a two-domain structure similar to PDGF-C and is secreted as a disulphide-linked homodimer, PDGF-DD. Upon limited proteolysis, PDGF-DD is activated and becomes a specific agonistic ligand for PDGFR-beta. PDGF-DD is the first known PDGFR-beta-specific ligand, and its unique receptor specificity indicates that it may be important for development and pathophysiology in several organs.