Early B Cell Factor 1 (EBF1) Regulates Glomerular Development by Controlling Mesangial Maturation and Consequently COX-2 Expression.

BACKGROUND: We recently showed the transcription factor Early B cell factor 1 (EBF1) is essential for the last stages of metanephric development, and that mice globally deficient in EBF1 display impaired maturation of peripheral glomeruli. EBF1 is present within multiple glomerular cell types, including the glomerular mesangium and podocytes. METHODS: To identify which cell type is driving the glomerular developmental defects in the global EBF1 knockout mice, we deleted EBF1 from the mesangium/pericytes (Foxd1-cre) or podocytes (Podocin-cre) in mice. RESULTS: Deletion of EBF1 from Foxd1 lineage cells resulted in hypoplastic kidneys, poorly differentiated peripheral glomeruli, and decreased proximal tubular mass in the outer cortex. Renal insufficiency was apparent at P21 when proteinuria presents, fibrosis of both the glomeruli and interstitium rapidly progresses, microthrombi appear, and hematuria develops. Approximately half of the Foxd1+, Ebf1fl/fl mice die before they are 3 months old. Mice with podocyte-targeted deletion of EBF1 exhibited no developmental abnormalities. Mice with Ebf1 deficiency in Foxd1 lineage cells shared characteristics with Ptgs2/COX-2-insufficient models, and mechanistic investigation revealed impaired calcineurin/NFATc1 activation and decreased COX-2 expression. Deletion of COX-2 from the interstitial/mesangial lineage displayed a less severe phenotype than EBF1 deficiency in mice. Overexpressing COX-2 in the EBF1-deficient mice, however, partially restored glomerular development. CONCLUSIONS: The results suggest that EBF1 regulates metanephric development at the last stages of glomerular maturation through its actions in the stromal progenitor (Foxd1+) lineage where it mediates proper regulation of calcineurin/NFAT signaling and COX-2 expression.

Renin cells are precursors for multiple cell types that switch to the renin phenotype when homeostasis is threatened.

Renin-synthesizing cells are crucial in the regulation of blood pressure and fluid-electrolyte homeostasis. Adult mammals subjected to manipulations that threaten homeostasis increase circulating renin by increasing the number of renin-expressing/-releasing cells. We hypothesize that the ability of adult cells to synthesize renin does not occur randomly in any cell type, depending instead on the cell's lineage. To determine the fate of renin-expressing cells, we generated knockin mice expressing cre recombinase in renin-expressing cells and crossed them with reporter mice. Results show that renin-expressing cells are precursors for a variety of cells that differentiate into non-renin-expressing cells such as smooth-muscle, epithelial, mesangial, and extrarenal cells. In the kidney, these cells retain the capability to synthesize renin when additional hormone is required to reestablish homeostasis: specific subpopulations of apparently differentiated cells are "held in reserve" to respond (repeatedly) by de-differentiating and expressing renin in response to stress, and re-differentiating when the crisis passes.

CCR7 Is Important for Mesangial Cell Physiology and Repair.

The homeostatic chemokine receptor CCR7 serves as key molecule in lymphocyte homing into secondary lymphoid tissues. Previous experiments from our group identified CCR7 also to be expressed by human mesangial cells. Exposing cultured human mesangial cells to the receptor ligand CCL21 revealed a positive effect on these cells regarding proliferation, migration, and survival. In the present study, we localized CCR7 and CCL21 during murine nephrogenesis. Analyzing wild-type and CCR7 deficient (CCR7-/-) mice, we observed a retarded glomerulogenesis during renal development and a significantly decreased mesangial cellularity in adult CCR7-/- mice, as a consequence of less mesangial cell proliferation between embryonic day E17.5 and week 5 postpartum. Cell proliferation assays and cell-wounding experiments confirmed reduced proliferative and migratory properties of mesangial cells cultured from CCR7-/- kidneys. To further emphasize the role of CCR7 as important factor for mesangial biology, we examined the chemokine receptor expression in rats after induction of a mesangioproliferative glomerulonephritis. Here, we demonstrated for the first time that extra- and intraglomerular mesangial cells that were CCR7-negative in control rats exhibited a strong CCR7 expression during the phase of mesangial repopulation and proliferation.

Activation of mesangial cells by the phosphatase inhibitor vanadate. Potential implications for diabetic nephropathy.

The metalion vanadate has insulin-like effects and has been advocated for use in humans as a therapeutic modality for diabetes mellitus. However, since vanadate is a tyrosine phosphatase inhibitor, it may result in undesirable activation of target cells. We studied the effect of vanadate on human mesangial cells, an important target in diabetic nephropathy. Vanadate stimulated DNA synthesis and PDGF B chain gene expression. Vanadate also inhibited total tyrosine phosphatase activity and stimulated tyrosine phosphorylation of a set of cellular proteins. Two chemically and mechanistically dissimilar tyrosine kinase inhibitors, genistein and herbimycin A, blocked DNA synthesis induced by vanadate. Vanadate also stimulated phospholipase C and protein kinase C. Downregulation of protein kinase C abolished vanadate-induced DNA synthesis. Thus, vanadate-induced mitogenesis is dependent on tyrosine kinases and protein kinase C activation. The most likely mechanism for the effect of vanadate on these diverse processes involves the inhibition of cellular phosphotyrosine phosphatases. These studies demonstrating that vanadate activates mesangial cells may have major implications for the therapeutic potential of vanadate administration in diabetes. Although vanadate exerts beneficial insulin-like effects and potentiates the effect of insulin in sensitive tissue, it may result in undesirable activation of other target cells, such as mesangial cells.

The glomerular mesangium: studies of its developmental origin and markers in vivo and in vitro.

Cultured mesangial cells are widely used to explore their role in kidney glomerular functions, but methods to reliably identify these cells in vivo and in vitro are lacking. Furthermore, the proposed relationship of mesangial cells to e.g. fibroblasts and smooth muscle cells has not been systematically studied. Here we wanted to search for markers of practical use also in identifying cultured mesangial cells, and to apply these markers in a study of the origin of glomerular mesangium. No epitopes specific for only mesangial cells could be identified, and no evidence of their true relationship with neural or lymphocytic lineages could be found. Findings with the variety of markers used suggest that mesangial cells may be indistinguishable from smooth muscle cells and fibroblasts. A panel of antibodies, including those against Thy1.1, smooth muscle actin, desmin, cellular fibronectin and beta 1 integrin alpha 1 and alpha 5 chains, and Wistaria floribunda (WFA) and Ricinus communis (RCA I) lectins, were found useful for mesangial cell detection in vivo and in vitro. The origin of glomerular mesangial cells could not be conclusively determined, although the results indirectly suggest that mesangial cells together with endothelial cells migrate to the glomerulus from the outside.

Expression and role of angiotensin II type 2 receptor in the kidney and mesangial cells of spontaneously hypertensive rats.

Angiotensin II type 2 (AT2) receptor is developmentally regulated and exerts antiproliferative and proapoptotic actions. Genetic ablation of this receptor in mice affects regulation of blood pressure, but the involvement of the AT2 receptor in the pathogenesis of hypertension remains unknown. In the present study, we examined developmental changes of angiotensin receptor subtypes in the kidney of stroke-prone spontaneously hypertensive rats (SHRSP), and compared them with those in normotensive Wistar-Kyoto rats (WKY). We also investigated the regulation and functional role of the AT2 receptor in cultured mesangial cells. Receptor binding and Northern blot analyses revealed that AT2 receptor expression is significantly lower in the SHRSP kidney than in the WKY kidney during the perinatal period, while AT1 receptor expression is not different between them. In WKY mesangial cells, AT2 receptor stimulation exerted a potent antiproliferative effect; this effect was not observed in SHRSP cells lacking the AT2 receptor expression. The expression of interferon regulatory factor (IRF)-1 paralleled the growth-dependent induction of AT2 receptor in WKY mesangial cells, and transfection of IRF-1 antisense oligonucleotide significantly suppressed AT2 receptor expression, indicating IRF-1-dependent regulation of AT2 receptor expression in mesangial cells. However, this induction was inefficient in SHRSP cells. Thus, we found impaired AT2 receptor expression in the SHRSP kidney in vivo and in mesangial cells in vitro. The unbalanced expression of renal angiotensin receptor subtypes with exaggerated AT1 receptor signaling during early life in SHRSP may play a role in the programming for hypertension and related renal injury.

Is mesangial hypercellularity with glomerular immaturity a variant of glomerulosclerosis?

Our aim was to correlate an immunohistochemical pattern of selected podocyte cytoskeleton-associated proteins in children diagnosed with focal segmental glomerulosclerosis (FSGS) and diffuse mesangial proliferation accompanied by glomerular immaturity (Im-DMP) with the clinical courses of both diseases. The material included 33 renal biopsies obtained from children diagnosed with DMP with or without signs of glomerular immaturity (ten and 15 participants, respectively) or FSGS (eight patients). Ezrin, podocalyxin, synaptopodin and nephrin expression was evaluated by immunohistochemical assay. A positive reaction for ezrin, podocalyxin, synaptopodin and nephrin was observed in the most superficial, continuous 'layer' of podocytes in Im-DMP patients. This distribution closely mimicked the immunohistochemical pattern observed in FSGS. The severe initial course of Im-DMP was transient. Resistance to steroids (six children) and renal insufficiency (two patients) in these subjects subsided, whilst, in the FSGS patients, the resistance to steroids recognized in all the children and the renal insufficiency diagnosed in three of them were still present. Mimicry between the immunohistochemical pattern of glomerular immaturity in DMP and focal segmental glomerulosclerosis might explain the severe initial course of this nephrotic syndrome in children. The transient clinical character of the former may also indicate that it is not a variant of FSGS.

Leprecan distribution in the developing and adult kidney.

The temporal and spatial deposition of extracellular matrix proteins is critical for nephrogenesis and glomerular maturation. We previously characterized leprecan as a novel chondroitin sulfate proteoglycan which has been recently shown to have prolyl hydroxylase activity. In this study, we examine the distribution of leprecan during nephrogenesis and after a hypertrophic stimulus to the adult kidney. During development, leprecan was localized to mesenchymal aggregates, early comma- and S-phase structures as determined by immunohistochemistry and in situ hybridization. Leprecan mRNA was increased in cells around the vascular cleft of the S- and comma-phase glomeruli. Expression was found in podocytes, mesangial cells, and parietal epithelial cells of loop-phase glomeruli. Leprecan mRNA was substantially decreased in the glomeruli of the adult kidney compared to the developing kidney with a uniform distribution between the glomeruli and the tubules. Within adult glomeruli, leprecan was found in the mesangium mesangial matrix, podocytes, and in Bowman's capsule. In response to glomerular hypertrophy, produced by unilateral nephrectomy, leprecan synthesis was increased in the adult kidney. We suggest that the regulated expression of leprecan during glomerular development or hypertrophy coupled with its reported prolyl hydroxylase activity plays a role during basement membrane assembly.

Growth factors and the mesangium.

Growth factors are prime candidates to mediate and modulate the functions of the mesangium. Mesangial cells are effector cells producing a number of growth factors that act in an autocrine manner to regulate their own function. Mesangial cells are also targets for growth factors released from neighboring glomerular cells or infiltrating cells and platelets. Growth factors may promote hypertrophy, proliferation, matrix metabolism, and immune-inflammatory and vasoactive properties of mesangial cells. These peptides represent important mediators of mesangial cell responses to injury. Platelet-derived growth factor mediates predominantly cell proliferation, whereas transforming growth factor beta mediates mesangial cell matrix expansion. Mesangial cells may also modulate some of the hemodynamic effects of growth factors, such as the increased renal vascular resistance in response to platelet-derived growth factor and epidermal growth factor or the increased RBF and GFR in response to insulin-like growth factor-1. Changes in the expression of growth factors of their receptors during the course of glomerular injury point to a potential role in mediating some of the pathologic changes in vivo. Several agents appear to antagonize the mitogenic and perhaps other effects of growth factors in mesangial cells. Such agents include adenylate cyclase as well as guanylate cyclase agonists. Recent studies also suggest that some traditional vasoactive agents may activate metabolic processes in mesangial cells similar to peptide growth factors. Collectively, these studies point to the interaction of both hemodynamic and metabolic factors in the response and contribution of glomerular and specifically mesangial cells to injury.

Factors controlling growth and matrix production in vascular smooth muscle and glomerular mesangial cells.

The vasculature wall is an active, integrated organ composed of endothelial cells and vascular smooth muscle cells, as well as other cell types depending on the specific vascular segment (e.g. fibroblasts in many vascular regions). The vascular wall is not static; the vascular components (cells and extracellular matrix) dynamically increase, decrease or reorganize, or both, in response to physiological and pathological stimuli. The vascular smooth muscle cells are the final common pathway for many of these dynamic changes in vascular wall structure. In the renal glomerulus, however, the glomerular mesangial cells-a cell phenotypically related to the vascular smooth muscle cells-also participate. Although sometimes beneficial, changes in vascular or glomerular structure often lead to cardiovascular (e.g. atherosclerosis, restenosis, intimal hyperplasia) and renovascular (e.g. glomerulosclerosis) diseases. Consequently, much effort has been expended to elucidate the mechanisms that control growth and extracellular matrix production by vascular smooth muscle cells and glomerular mesangial cells. The purpose of this review is to discuss recent developments.

The importance of cellular adhesion for mesangial cell proliferation in serum sickness nephritis of the rat: a co-culture study of glomerular macrophages and mesangial cells.

The role of cell-cell contact on activating mesangial cell proliferation by nephritic macrophage was investigated. Nephritic glomerular macrophages were obtained from serum sickness nephritis (SSN) rat kidneys at 14 days after the cessation of sensitization, when proliferating cells were most increased in the glomeruli in the course of the SSN. The effect of the nephritic macrophages on mesangial cell proliferation was greater than that of control by co-culture allowing cellular contact. However, nephritic macrophages did not enhance mesangial cell proliferation by co-culture without direct contact even though the nephritic macrophages were activated with lipopolysaccharides. Conditioned medium from co-culture of the nephritic macrophages and mesangial cells did not enhance mesangial cell proliferation. Anti-intercellular adhesion molecules (ICAM)-1 antibody inhibited mesangial cell proliferation by direct co-culture dose-dependently. From these results, cellular contact was important for stimulation of mesangial cell proliferation by macrophages and ICAM-1 participated in these interactions.

Cyclooxygenase inhibition enhances rat interleukin 1 beta-induced growth of rat mesangial cells in culture.

The cytokine interleukin 1 (IL-1) has growth-promoting activities on mesangial cells (MC) and enhances MC prostanoid formation. A possible role of endogenous cyclooxygenase products on IL-1-mediated growth of MC is, however, unknown. Therefore we evaluated the effect of cyclooxygenase inhibition on growth of mesangial cells in culture, which were exposed to DNA recombinant rat interleukin 1 beta (rIL-1 beta). rIL-1 beta increased [3H]thymidine uptake in MC by approximately 70% after 48 h. This growth-promoting activity of the cytokine was observed at 1 ng/ml and was not further enhanced by the increase of the IL-1 beta concentration less than or equal to 100-fold. IL-1 beta, however, dose dependently stimulated prostaglandin E2 (PGE2) formation by MC. When prostaglandin synthesis was inhibited by indomethacin (Indo, 1 microgram/ml), rIL-1 beta (10 ng/ml)-induced cell proliferation was sevenfold greater compared with rIL-1 beta alone. In the presence of Indo (1 microgram/ml), rIL-1 beta (1, 10, 50, and 100 ng/ml) dose dependently stimulated MC proliferation. The addition of exogenous PGE2 (10(-7) and 10(-8) M) to Indo-treated MC blocked the mitogenic response of IL-1 beta. We conclude that endogenous PGE2 formation, which is stimulated by IL-1 beta, antagonizes the growth-promoting activity of the cytokine. PGE2 may thus exert antiproliferative effects in glomerular diseases, whereas IL-1 might mediate cell growth.

The hepatocyte growth factor/scatter factor (HGF/SF) receptor, met, transduces a morphogenetic signal in renal glomerular fibromuscular mesangial cells.

Previous studies have demonstrated that hepatocyte growth factor/scatter factor (HGF/SF) is secreted by mesenchymal cells and that it elicits motility, morphogenesis and proliferation of epithelia expressing the met receptor. We now report that HGF/SF may act as an autocrine factor in fibromuscular renal mesangial cells. These cells mechanically support glomerular endothelia, control the rate of plasma ultrafiltration and are implicated in the pathogenesis of a variety of chronic renal diseases. We detected met protein in the vascular stalk of metanephric glomeruli and in the mature mesangium. Mesangial lines from a mouse transgenic for a temperature-sensitive simian virus 40 T antigen expressed met mRNA and protein, and recombinant HGF/SF phosphorylated the met receptor tyrosine kinase. Cells were immortal in the permissive condition and HGF/SF enhanced proliferation in a defined medium. In the absence of the immortalising protein, division ceased and recombinant HGF/SF caused multipolar cells to become bipolar. The factor diminished stress fibres, their focal contacts and immunostaining for extracellular fibronectin, hence suggesting reduced substratum adhesion and enhanced motility. Mesangial lines also expressed HGF/SF mRNA and secreted bioactive factor; immunocytochemistry showed both ligand and receptor in individual cells. HGF/SF blocking antibody aggregated the cells, suggesting that mesangial-derived factor affects basal cell conformation in an autocrine manner. We conclude that mesangial cells express both HGF/SF and met, and the factor induces morphogenesis of cultured mesangial cells. Therefore HGF/SF may have an autocrine role in mesangial biology but further studies are now required to investigate the potential importance of the factor in vivo.

Role of platelet-derived growth factor in mesangium development and vasculopathies: lessons from platelet-derived growth factor and platelet-derived growth factor receptor mutations in mice.

PURPOSE OF REVIEW: The phenotypic consequences of null mutations in the platelet-derived growth factor-B and the platelet-derived growth factor beta-receptor genes in mice have demonstrated that these proteins play pivotal roles in the development of the vascular smooth muscle cell lineage, including pericytes and mesangial cells. RECENT FINDINGS: The lethality of these mutants has precluded analysis of the physiological and pathophysiological consequences of platelet-derived growth factor-B and platelet-derived growth factor beta-receptor deficiency in adults. This review summarizes and discusses recent data from certain tissue-specific and subtle mutations in the platelet-derived growth factor-B and platelet-derived growth factor beta-receptor genes that are compatible with postnatal viability in spite of severe developmental deficits in pericyte and mesangial cell recruitment. In the postnatal period, the animals studied developed a characteristic set of pathological changes to small blood vessels of the retina and the kidney glomerulus, which sheds light on the importance of pericytes and mesangial cells for vascular integrity and function after birth. SUMMARY: These microvascular abnormalities and their consequences bear a resemblance to diabetic microangiopathy and nephropathy. The platelet-derived growth factor-B and platelet-derived growth factor beta-receptor mutant mouse models, therefore, might serve as valuable tools in the dissection of some of the pathogenic events in diabetic microangiopathy.

Protein kinase C beta I and beta II are differentially expressed in the developing glomerulus.

In the pre- and postnatal period of kidney development, proliferation with subsequent functional maturation of intrinsic glomerular mesangial cells (MCs) continues within the existing framework. Recent work has suggested that PKC beta isoform is responsible for the proliferation observed during maturation. We sought to ascertain whether PKC beta isoform expression is altered during the development of the mesangium. MCs were subcultured from glomerular explants of Sprague-Dawley rat kidneys, days 1, 3, 5, 8, 15 postnatally, and adult. MCs from rat kidneys postnatally days 1-5 proliferated at a significantly greater rate than adult [ > 1.169-fold, P < 0.01] but term day 8 cells did not [ < 1.34-fold, not significant)] as assessed by [3H]thymidine incorporation. Western blot analysis using isoform specific antibodies was performed on confluent neonatal and adult MC. We observed that all neonatal and adult MC express beta I PKC (n = 8 kidneys from separate primaries for each date and adult). However, unlike adult MCs, neonatal MC express beta II in postnatal days 1-5 and none thereafter. Immunofluorescent staining of postnatal kidneys confirmed that PKC beta II is present in neonatal MC up to day 5. By day 8, staining of mesangium with PKC beta II begins to disappear and assumes a parietal epithelial pattern. In adult kidneys, there was only PKC beta II staining of the parietal epithelial cells. Our results demonstrate that differential expression of PKC beta II closely parallels the proliferative behavior of the MCs of the maturing glomerulus. Therefore, PKC beta II expression and activation may play a critical role in development.

Glomerular matrix: synthesis, turnover and role in mesangial expansion.

The extracellular matrix has an integral role in development, homeostasis and pathology of the glomerulus. Three spatially distinct matrices are present in the glomerulus: the mesangium, and basement membranes of the capillary loops and Bowman's capsule. Each is dominated by basement membrane components, but is distinct in organization and composition. Many matrix components influence cell behavior directly, through specific interactions with receptors, or indirectly through growth factor sequestration. Growth factors may be of great importance in development and disease progression in the glomerulus, and may be central to mesangial expansion. In addition, changes in matrix composition accompany and contribute to the pathological condition, such as the accumulation of matrix in diabetes mellitus.

Development of the glomerular mesangium.

The development of the glomerular mesangium was studied in fetal and newborn rat kidneys by using a widefield electron microscope which can cover a whole glomerulus within one low-power viewfield. A three-dimensional observation of the immature glomeruli was done by performing ultrathin serial sectionings of the specimen for electron microscopy. Scanning electron microscopic observation of the developing glomeruli was also performed. The developmental distribution of contractile protein (actin) in mesangial cells and the main intrinsic component of the extracellular matrix protein (type IV collagen) of the mesangium were examined by immunohistological techniques. The widefield electron micrograph revealed a precise relationship between the mesangium and other components of the glomerulus. The results confirmed that the capillary extends into the S-shaped body from the surrounding vascular system at the initiation of nephronogenesis. The mesangial cells are always continuous to the vascular pericyte-smooth muscle cell system during the whole course of glomerular development and they participate in the subdivision of the capillary network during glomerulogenesis. Morphological findings and the changing distribution of intra- and extracellular proteins of the mesangium during development suggest that the mesangial cell differentiates from the primitive pericyte of the immature capillary.

Autocrine growth regulation of human glomerular mesangial cells is primarily mediated by basic fibroblast growth factor.

For various forms of human glomerulonephritis a close relationship between inflammatory injury and a local mesangial proliferative response has been described. Herein, we used primary cultures of human glomerular mesangial cells (HMCs) from five different donors to determine the autocrine growth-inducing capacity of their supernatants after stimulation with different cytokines and lipopolysaccharide (LPS) to determine whether this effect is due to basic fibroblast growth factor (bFGF). The basal growth-inducing capacity of supernatants collected from serum-free cultured HMC and concentrated 100-fold above a cut-off size of 10 kd was significantly increased by interleukin (IL)-1 beta, platelet-derived growth factor (PDGF), and LPS up to 15-fold, but not by IL-1 alpha, IL-6, or bFGF. An anti-human bFGF antibody blocked the majority of IL-1 or LPS-induced proliferative effect of supernatants; complete inhibition was achieved by a combination of anti-human bFGF- and anti-human platelet-derived growth factor antibodies. HMCs express different isoforms of bFGF (18, 21.5, and 24 kd) in membrane, cytosolic, and nuclear fractions. All isoforms of bFGF were found in the nuclear fraction of HMC, whether stimulated or not. Immunoblots for bFGF protein of HMC demonstrated that only a approximate to 16 kd bFGF protein was released into HMC supernatants after stimulation with IL-1 beta, platelet-derived growth factor-BB, and LPS. The 18 kd isoform of bFGF accumulated in the membranes but was not released after stimulation with IL-1 alpha, IL-6, and bFGF, suggesting that its release was a prerequisite for autocrine growth stimulation. By means of reverse transcription polymerase chain reaction controlled by Southern blots, bFGF-mRNA expression of HMC was enhanced by IL-1 alpha, IL-1 beta, and LPS. Finally, we were able to show that HMCs are expressing bFGF receptors. In summary, our data demonstrate for the first time that the autocrine proliferative response of HMC to major inflammatory factors may primarily be mediated by bFGF.

The three-dimensional structure of the neonatal mouse kidney as revealed by scanning electron microscopy after KOH treatment.

The shape and arrangement of the developing nephrons were studied three-dimensionally by scanning electron microscopy (SEM) of the neonatal mouse kidney. The specimens were treated with the KOH digestion method in order to remove extracellular connective tissue components, thus enabling the direct observation of the developing nephrons at various stages. At the subcapsular region of the renal cortex, the ureteric ducts were observed as branched tubules with terminal swellings or ampullae. Newly formed blood vessels were often associated with terminals of these ureteric ducts. The cup-shaped renal corpuscles had aggregations of mesangial cells with blood vessels in the groove. At the vascular pole of mature nephrons, extraglomerular mesangial cells were observed as a cellular sheet, which was continuous with the smooth muscle layer of afferent and efferent blood vessels. The present study also demonstrated the shape of the immature podocytes in relation to the endothelial morphology of glomerular capillaries.

Development of pH regulatory transport in glomerular mesangial cells.

Developmental changes in activity or expression of transporters may account for alterations in cell behavior as the nonpolarized cell matures. We sought to ascertain whether there is a maturational change in each of the major acid-base transporters in the developing mesangial cell (MC), the Na/H exchanger, Na-dependent Cl/HCO3 exchanger, and the Cl/HCO3 exchanger. Intracellular pH (pHi) was determined by the use of the fluorescent pH-sensitive dye, 2',7'-bis(2- carboxyethyl)-5(6)-carboxyfluorescein (BCECF). We assessed transporter activity by studying recovery from an acid load (NH4/NH3) in CO2/HCO3. In adult MCs, Na/H exchanger was responsible for 35.2 +/- 4.3% of steady-state pHi, whereas the Na-dependent Cl/HCO3 exchanger contributed 58.7 +/- 6.1 (n = 14). In term MCs (tMCs), from days 1-3 after birth, the Na/H exchanger contributes 62.9 +/- 7.8% (n = 11, P < 0.001 vs. adult), whereas the Na-dependent Cl/HCO3 exchanger contributes 34.0 +/- 5.7% (n = 12, P < 0.001 vs. adult), to the rate of recovery from an acid load in these cells. However, in tMCs (days 4-6), the Na/H contributes 47.2 +/- 5.9% (n = 8, P < 0.05 vs. adult), whereas the Na-dependent Cl/HCO3 exchanger contributes 48.7 +/- 7.3% (n = 13, P < 0.05 vs. adult), to the rate of recovery. tMCs (days 6-12) yielded transporter activity that was not statistically different than adult MCs (37.8 +/- 4.9 and 54.3 +/- 10.2% for Na/H and Na-dependent Cl/HCO3, respectively). The magnitude of the stimulated response to angiotensin II by Na/H and Na-dependent Cl/HCO3 exchanger in adult and tMCs is unchanged throughout development. The Na/H exchanger appears to play a greater role in pHi homeostasis earlier on in development, and this may reflect developmental needs of the maturing cell.