Adhesion molecules in the glomerular mesangium

Adhesion molecules in the glomerular mesangium. Experimental evidence indicates that extensive “cross-talk” exists between glomerular cells, extracellular matrix molecules and soluble mediator substances affecting the proliferative and secretory phenotype of glomerular mesangial cells. Both matrix and cytokines regulate mesangial cell behavior in vitro and in vivo after binding to specific cell surface receptors. It appears as if the concerted action of insoluble and soluble ligands on mesangial cells involves a reciprocal regulation of matrix molecules and cytokines as well as expression and affinity of their respective receptors. Elucidation of the potential biologic and clinical relevance of cell-matrix interactions in the glomerular mesangium represents a challenging goal in current kidney research. This brief review summarizes recent investigations concerning regulation of expression and function of adhesion molecules and matrix receptors in the mesangium. In addition to results from cell culture studies, descriptive findings on expression and regulation of adhesion molecules and their potential role for altered mesangial cell behavior in glomerular disease is considered

α8 Integrin in glomerular mesangial cells and in experimental glomerulonephritis

α8 Integrin in glomerular mesangial cells and in experimental glomerulonephritis.BackgroundMesangial cell (MC) proliferation and extracellular matrix accumulation are typical responses of renal glomeruli to injury. Extracellular matrix components are known to affect MC behavior, which is mediated primarily via integrin receptors of the β1 family. In addition to α1, α3, α5, and α6 chains of β1 integrins, recent studies have shown the α8 chain to be expressed in glomeruli and renal vasculature. α8β1 can serve as a receptor for fibronectin, which is abundant in the mesangium. We investigated the glomerular expression pattern of the α8 chain in renal tissues of mouse, rat, and humans as well as in cultured MCs. In addition, the regulation of α8 expression in MCs was studied in culture and in nephritic rats.MethodsThe expression of α8 protein in kidney tissue and cultured MCs was investigated by immunohistochemistry, immunocytochemistry, and Western blotting. The effects of TGF-β1 on α8 mRNA levels in MCs were studied by Northern blot analysis. In addition, time course studies of glomerular abundance and localization of α8 were performed in rats with mesangioproliferative anti-Thy1.1 nephritis.ResultsIn tissue sections of normal human, rat, and mouse kidney, we found strong immunohistochemical staining for α8 in the mesangium and in the media of renal arterioles. Double staining for α8 and Thy1.1, a surface antigen of rat MCs, showed α8 to be specifically expressed in MCs but not in glomerular endothelial and epithelial cells. In anti-Thy1.1 nephritis of rats, the glomerular abundance of α8 protein was reduced in the early mesangiolytic phase but was increased greatly with subsequent MC proliferation, peaking at day 6 of disease. At later stages of this reversible form of nephritis, the number of MCs and the extent mesangial α8 staining declined to control levels. Cell culture experiments revealed that freshly plated MCs organize α8 into focal contacts within one hour after attachment to fibronectin and vitronectin substrata, showing colocalization with focal contact proteins vinculin and talin. Stimulation of MCs with transforming growth factor-β1 led to increases of α8 mRNA and protein levels.ConclusionsThese results show that in human, rat, and mouse glomeruli, α8 integrin is strongly and exclusively expressed in MCs. Gene expression of α8 is regulated in cultured MCs, and α8 protein abundance is regulated in vivo and in MC culture. It is currently unclear what functional properties this integrin receptor protein has with regard to MC anchorage to extracellular matrix and modulation of the MC phenotype in normal and diseased glomeruli. However, in view of its abundance in the mesangium, α8β1 integrin could be an important MC receptor of matrix ligands and may play a role in the embryology, physiology, and pathophysiology of the glomerular capillary tuft

Expression of the transcriptional regulator Egr-1 in experimental glomerulonephritis: Requirement for mesangial cell proliferation

Expression of the transcriptional regulator Egr-1 in experimental glomerulonephritis: Requirement for mesangial cell proliferation. The early growth response gene-1 (Egr-1), a zinc finger transcriptional regulator, was induced in a rat model of mesangioproliferative glomerulonephritis (GN). Northern blot analysis revealed a maximal 14.9-fold increase in glomerular Egr-1 mRNA at day 6 of GN. By immunohistochemistry Egr-1 protein expression was demonstrated to be mainly confined to glomerular mesangial cells (MC). To test whether Egr-1 directly regulates MC proliferation, cultured MCs were stimulated with platelet-derived growth factor (PDGF) after preincubation with different Egr-1 antisense oligonucleotides (ASOs). PDGF-induced Egr-1 mRNA levels were inhibited by up to 75% and protein levels by up to 91%. In addition Egr-1-specific ASOs blocked PDGF-induced rise in 3H-thymidine uptake by 83% and almost completely abrogated increase in MC number. We conclude that Egr-1 induction is of critical importance for PDGF-induced mitogenic signaling in MCs, and inhibition of Egr-1 in vivo may offer an approach to oppose glomerular MC proliferation in glomerular inflammatory disease

Glomerular epithelial cell products stimulate mesangial cell proliferation in culture

Glomerular epithelial cell products stimulate mesangial cell proliferation in culture. Glomerular epithelial cells (GEC) and mesangial cells (MC) are both involved in glomerular diseases. To elucidate potential interactions between these glomerular cell types, we examined whether products of GEC affect the proliferative activity of MC. We found that cultured rat GEC secrete soluble factors into the supernate (GEC-CM) that induce proliferation of quiescent rat MC. The mitogenic activity was trypsin sensitive and partially heat-labile. Biochemical analysis of GEC-CM by gel filtration HPLC, reverse phase HPLC, and isoelectric focusing revealed at least three mitogenic fractions as well as inhibitory activity present in GEC-CM. Competitive binding assays with 125I-labeled PDGF did not show significant amounts of PDGF in GEC-CM. The biochemical features of the GEC-derived MC growth factors are distinct from IL-6, PDGF, bFGF, and endothelin, previously described GEC-derived MC growth factors. Additionally, significant contributions of known growth factors such as IL-1, IL-2, IL-3, IL-4, IL-5, TNFα, TGFβ, and GM-CSF are unlikely. The results indicate that GEC produce several biochemically-distinct MC growth regulators. While these epithelial cell-derived mitogens for MC require further characterization, they may play an important role in the regulation of MC replication, such as during embryogenesis and glomerular disease

Polymerase chain reaction and focal contact formation indicate integrin expression in mesangial cells

Polymerase chain reaction and focal contact formation indicate integrin expression in mesangial cells. Cultured kidney glomerular mesangial cells (MCs) allow the role of extracellular matrix (ECM) and growth factors in glomerular inflammatory disease to be studied. To investigate the potential of MCs to interact with matrix components, the expression of integrin mRNA in cultured MCs was examined by polymerase chain reaction (PCR), by Northern blotting and by immunofluorescence. In addition, the effect of matrix substrates on mRNA expression was assessed by PCR. Northern blots with cDNA probes to integrin α-chains revealed that MCs expressed α1, α3 and α5 integrin mRNA. α1 and α3 were the major messages. No α2, α4 or α6 were detectable. RT-PCR revealed that α2 and α6 were also expressed at low levels. The control cells, HT1080, expressed α2, α3, α4, α5 and α6 mRNA, and Rugli expressed α1, α3, and α5, supporting previous studies. Immunocytochemistry confirmed that α1β1, α2β1, α3β1 and α5β1 integrins were expressed and that they were concentrated into focal adhesions (α1β1 on type I collagen and laminin; α2β1 on type I collagen; α3β1 on type I collagen, laminin and fibronectin; α5β1 on fibronectin). α6β1 was not detected in focal contacts. Attachment, spreading, and formation of talin and integrin containing focal contacts still occurred when endogenous protein synthesis was blocked with 30 µg · ml-1 cycloheximide. Variation of substrate did not lead to a rapid degradation of integrin α-chain mRNA as assessed by RT-PCR. These results provide a basis for studying the regulation of interactions between MCs and extracellular matrix mediated by integrins