Role of mesangial cells in macula densa to afferent arteriole information transfer

Ion channels and channelopathies in glomeruli

An essential step in renal function entails the formation of an ultrafiltrate that is delivered to the renal tubules for subsequent processing. This process, known as glomerular filtration, is controlled by intrinsic regulatory systems and by paracrine, neuronal, and endocrine signals that converge onto glomerular cells. In addition, the characteristics of glomerular fluid flow, such as the glomerular filtration rate and the glomerular filtration fraction, play an important role in determining blood flow to the rest of the kidney. Consequently, disease processes that initially affect glomeruli are the most likely to lead to end-stage kidney failure. The cells that comprise the glomerular filter, especially podocytes and mesangial cells, express many different types of ion channels that regulate intrinsic aspects of cell function and cellular responses to the local environment, such as changes in glomerular capillary pressure. Dysregulation of glomerular ion channels, such as changes in TRPC6, can lead to devastating glomerular diseases, and a number of channels, including TRPC6, TRPC5, and various ionotropic receptors, are promising targets for drug development. This review discusses glomerular structure and glomerular disease processes. It also describes the types of plasma membrane ion channels that have been identified in glomerular cells, the physiological and pathophysiological contexts in which they operate, and the pathways by which they are regulated and dysregulated. The contributions of these channels to glomerular disease processes, such as focal segmental glomerulosclerosis (FSGS) and diabetic nephropathy, as well as the development of drugs that target these channels are also discussed.

Defining the variety of cell types in developing and adult human kidneys by single-cell RNA sequencing

The kidney is among the most complex organs in terms of the variety of cell types. The cellular complexity of human kidneys is not fully unraveled and this challenge is further complicated by the existence of multiple progenitor pools and differentiation pathways. Researchers disagree on the variety of renal cell types due to a lack of research providing a comprehensive picture and the challenge to translate findings between species. To find an answer to the number of human renal cell types, we discuss research that used single-cell RNA sequencing on developing and adult human kidney tissue and compares these findings to the literature of the pre-single-cell RNA sequencing era. We find that these publications show major steps towards the discovery of novel cell types and intermediate cell stages as well as complex molecular signatures and lineage pathways throughout development. The variety of cell types remains variable in the single-cell literature, which is due to the limitations of the technique. Nevertheless, our analysis approaches an accumulated number of 41 identified cell populations of renal lineage and 32 of non-renal lineage in the adult kidney, and there is certainly much more to discover. There is still a need for a consensus on a variety of definitions and standards in single-cell RNA sequencing research, such as the definition of what is a cell type. Nevertheless, this early-stage research already proves to be of significant impact for both clinical and regenerative medicine, and shows potential to enhance the generation of sophisticated in vitro kidney tissue.

Lack of Connexins 40 and 45 Reduces Local and Conducted Vasoconstrictor Responses in the Murine Afferent Arterioles

The juxtaglomerular apparatus (JGA) is an essential structure in the regulation of renal function. The JGA embodies two major functions: tubuloglomerular feedback (TGF) and renin secretion. TGF is one of the mechanisms mediating renal autoregulation. It is initiated by an increase in tubular NaCl concentration at the macula densa cells. This induces a local afferent arteriolar vasoconstriction and a conducted response that can be measured several 100 μm upstream from the juxtaglomerular segment. This spread of the vasomotor response into the surrounding vasculature likely plays a key role in renal autoregulation, and it requires the presence of gap junctions, intercellular pores based on connexin (Cx) proteins. Several Cx isoforms are expressed in the JGA and in the arteriolar wall. Disruption of this communication pathway is associated with reduced TGF, dysregulation of renin secretion, and hypertension. We examine if the absence of Cx40 or Cx45, expressed in the endothelial and vascular smooth muscle cells respectively, attenuates afferent arteriolar local and conducted vasoconstriction. Afferent arterioles from wildtype and Cx-deficient mice (Cx40 and Cx45) were studied using the isolated perfused juxtamedullary nephron preparation. Vasoconstriction was induced via electrical pulse stimulation at the glomerular entrance. Inner afferent arteriolar diameter was measured locally and upstream to evaluate conducted vasoconstriction. Electrical stimulation induced local vasoconstriction in all groups. The local vasoconstriction was significantly smaller when Cx40 was absent. The vasoconstriction decreased in magnitude with increasing distance from the stimulation site. In both Cx40 and Cx45 deficient mice, the vasoconstriction conducted a shorter distance along the vessel compared to wild-type mice. In Cx40 deficient arterioles, this may be caused by a smaller local vasoconstriction. Collectively, these findings imply that Cx40 and Cx45 are central for normal vascular reactivity and, therefore, likely play a key role in TGF-induced regulation of afferent arteriolar resistance.

Glomerular expression of connexin 40 and connexin 43 in rat experimental glomerulonephritis

BACKGROUND

Gap junctional intercellular communication is thought to play an important role in the maintenance of cell differentiation and homeostasis. Gap junctions connect glomerular mesangial cells to each other. In this study, we examined the glomerular expression of connexins (Cxs) 40 and 43 at both the protein and transcript levels in anti-Thy1.1 glomerulonephritis (GN).

METHODS

Anti-Thy1.1 GN was induced by intravenous injection of anti-Thy1.1 monoclonal antibody 1-22-3. Cx protein expression was examined by immunofluorescence, immunoelectron microscopy, and Western blotting. Changes in mRNA levels were detected by real-time reverse transcriptase-polymerase chain reaction.

RESULTS

Cx40 was detected in mesangial cells in normal rat glomeruli; its expression was reduced on days 3 and 7 and recovered to normal on day 14 following GN induction. Cx43 was detected in mesangial cells and podocytes in normal rat glomeruli, and its expression did not change during the disease course of GN. Expression of Cx40 and Cx43 was also detected in extraglomerular mesangial cells; this expression did not change during the disease course. Opposing patterns of expression between Cx40 and smooth muscle actin (SMA) were observed with double-immunofluorescence labeling. SMA is a differentiation marker of mesangial cells; it is often expressed during proliferation but not under physiological conditions.

CONCLUSION

These results suggest that Cx40 expression in mesangial cells is related to mesangial cell regeneration. Thus, Cx expression regulation could be a therapeutic target for glomerular diseases.

ATP as a mediator of macula densa cell signalling

Within each nephro-vascular unit, the tubule returns to the vicinity of its own glomerulus. At this site, there are specialised tubular cells, the macula densa cells, which sense changes in tubular fluid composition and transmit information to the glomerular arterioles resulting in alterations in glomerular filtration rate and blood flow. Work over the last few years has characterised the mechanisms that lead to the detection of changes in luminal sodium chloride and osmolality by the macula densa cells. These cells are true "sensor cells" since intracellular ion concentrations and membrane potential reflect the level of luminal sodium chloride concentration. An unresolved question has been the nature of the signalling molecule(s) released by the macula densa cells. Currently, there is evidence that macula densa cells produce nitric oxide via neuronal nitric oxide synthase (nNOS) and prostaglandin E(2) (PGE(2)) through cyclooxygenase 2 (COX 2)-microsomal prostaglandin E synthase (mPGES). However, both of these signalling molecules play a role in modulating or regulating the macula-tubuloglomerular feedback system. Direct macula densa signalling appears to involve the release of ATP across the basolateral membrane through a maxi-anion channel in response to an increase in luminal sodium chloride concentration. ATP that is released by macula densa cells may directly activate P2 receptors on adjacent mesangial cells and afferent arteriolar smooth muscle cells, or the ATP may be converted to adenosine. However, the critical step in signalling would appear to be the regulated release of ATP across the basolateral membrane of macula densa cells.

Gap junctional intercellular communication in the juxtaglomerular apparatus

The juxtaglomerular apparatus (JGA) is a specialized contact region between the glomerulus and the cortical thick ascending limb that plays an active role in the maintenance of ion homeostasis and control of blood pressure. The JGA accommodates several different cell types, including vascular smooth muscle cells, endothelial cells, mesangial cells, macula densa cells, and renin-secreting juxtaglomerular granular cells. These cells, with the exception of the macular densa cells, are tightly coupled by gap junctions. Gap junction-mediated intercellular communication in the JGA provides a pathway for signal transduction and coordination of multicellular functions. Disruption of cell-to-cell communication in the JGA results in altered preglomerular vascular tone and renin secretion. This review summarizes recent data about the roles of gap junctions in the JGA and illustrates how gap junction-mediated intercellular Ca(2+) signals determine physiological responses in the JGA.

What is the significance of CD34 immunostaining in the extraglomerular and intraglomerular mesangium?

CD34, traditionally a marker of hematopoietic stem cells (HSCs), was found on endothelial cells and fibroblasts as well. At the level of the extraglomerular or intraglomerular mesangium, CD34 may signal either the presence of HSCs or, conversely, may be a marker of transdifferentiation. CD34-positive cells of the extraglomerular mesangium could migrate into the intraglomerular mesangium and participate in reparative processes at this level. The aim of our study was to analyze the presence of CD34 at the level of the extraglomerular and intraglomerular mesangium and its relationship with histological markers of activity and chronicity, as well as with other immunohistochemical markers in glomerulonephritis (GN). A cross-sectional study of 36 patients with GN was conducted. Conventional stains: hematoxylin-eosin, periodic acid Schiff, and Trichrome Gömöri, as well as immunohistochemistry: CD34, alpha smooth muscle actin (alpha SMA), vimentin, and proliferating cell nuclear antigen (PCNA) were employed. Activity and chronicity of GN were evaluated according to a scoring system initially used for lupus nephritis and antineutrophil-cytoplasmic-antibody-associated vasculitis. Immunohistochemistry was assessed using a semiquantitative score. The mean age was 46.44 +/- 12.97 years; 22 were male and 14 were female. The extraglomerular mesangium was visible on specimens in 30 patients. CD34 was present in the extraglomerular mesangium in 15 patients: 11 of these patients showed concomitant intraglomerular and extraglomerular mesangial CD34 immunostaining, while four showed only extraglomerular mesangial immunostaining. In three patients, CD34 immunostaining was present only in the intraglomerular mesangium. Twelve patients showed negative immunostaining in both the extraglomerular and the intraglomerular mesangium. Overall, there was a fair degree of relationship, which did not reach statistical significance between CD34 in the extraglomerular mesangium and CD34 in the intraglomerular mesangium across the 36 patients. In the intraglomerular mesangium, CD34 did not significantly correlate with mesangial alpha SMA, vimentin, PCNA, and activity or chronicity index. In the extraglomerular mesangium, CD34 did not show a significant correlation with alpha SMA, vimentin, or PCNA. The activity index and the chronicity index showed a good correlation with serum creatinine. Mesangial cell proliferation correlated well with the mesangial matrix increase, while interstitial vimentin showed a good correlation with interstitial alpha SMA. We demonstrated the presence of CD34 in the extraglomerular mesangium, which could be related to transdifferentiated mesangial cells or to HSCs in the absence of blood vessels at this level. Our study shows the value of histological indices for evaluating GN but cannot assign significance to CD34 immunolabeling for the assessment of GN.

Function of connexins in the renal circulation

Connexins form intercellular channels that span two plasma membranes and directly couple the cytoplasm of adjacent cells. This morphological contact enables the exchange of ions, second messengers, and metabolites, which act to regulate several biological functions. This review focuses on the significance of connexins in the renal circulation. Cells of the renal vasculature are coupled and express connexins in a vessel and cell-specific pattern. This finding indicates that renal connexins likely play an important role in renal autoregulatory mechanisms (Bayliss effect, tubuloglomerular feedback) and in the control of vasomotor responses. The described coupling of endothelial and vascular smooth muscle cells in the afferent arterioles may also contribute to the communication of neighboring nephrons, called 'nephron coupling.' Furthermore, deletion of the Cx40 and Cx43 genes results in an altered functional behavior of the renin-producing cells, suggesting involvement of these connexin isoforms in the regulation of renin secretion and synthesis. In addition, this review discusses the role of renal connexin expression in the pathogenesis of hypertension or diabetes.

Pathophysiological roles of gap junction in glomerular mesangial cells

Glomerular mesangial cells (MCs) are specialized vascular smooth muscle cells that play a critical role in the control of glomerular hemodynamics. One of the intriguing features of MCs is their extraordinary abundance in gap junctions (GJs). It has long been speculated that GJs may bridge MCs together and provide the mesangium with the characteristics of a functional syncytium. Accumulating scientific evidence supports this idea. GJs are reported to be critically involved in important physiological processes like tubuloglomerular feedback and glomerular filtration. In addition, GJs are implicated in the control of many cellular processes of MCs, including growth, differentiation and survival. This article summarizes the current knowledge on the roles of GJs in glomerular pathophysiology.

Shiga toxin-1 affects nitric oxide production by human glomerular endothelial and mesangial cells

Acute renal failure hallmarks the pathogenesis of the epidemic form of hemolytic uremic syndrome (D+HUS), which is caused by E. coli strains that produce Shiga-like toxin (Stx). In this study, we investigated the influence of Stx-1 on nitric oxide (NO) production by human glomerular microvascular endothelial cells (GMVEC) and human mesangial cells. NO synthesis by human mesangial cells is in the micromolar range and that of GMVEC in the picomolar range. Stx-1 reduced NO production in non-stimulated GMVEC (5 nmol/l Stx-1 required) without inhibition of protein synthesis. In non-stimulated and TNFalpha-pretreated mesangial cells, NO production was reduced with a maximal reduction at 10 fmol/l shiga toxin. The cellular iNOS antigen content in mesangial cells was reduced in a concentration-dependent way (10 fmol/l-100 pmol/l), while partial inhibition of protein synthesis required 10 nmol/l Stx-1 in these cells. Our in vitro data suggest that Stx may reduce NO synthesis during the course of HUS development, contributing to the aggravation of the thrombotic microangiopathy and renal failure as observed in HUS.

Synergistic effects of PDGF-BB and cAMP-elevating agents on expression of connexin43 in mesangial cells

The gap junction plays an important role in the regulation of cell growth, migration, and differentiation. Platelet-derived growth factor (PDGF) is reported to be a potent inhibitor of gap junctional intercellular communication (GJIC). Short-term exposure of cells to PDGF causes rapid and transient disruption of GJIC without altering connexin43 (Cx43) protein level. In this study, we investigated long-term effects of PDGF-BB on Cx43 expression in mesangial cells (MCs). Exposure of MCs to PDGF-BB affected neither the Cx43 protein level nor GJIC. However, in the presence of cAMP-elevating agents, PDGF-BB dramatically increased the expression of Cx43, which was accompanied by obviously augmented membrane distribution of Cx43 and functional GJIC. The increased expression of Cx43 was closely correlated with reduction in alpha-actin, a dedifferentiation marker of MCs. The effect of PDGF on Cx43 was largely prevented by inhibitors of phosphatidylinositol 3'-kinase or mitogen-activated protein kinase, but not by inhibition of protein kinase C. Exposure of MCs to PDGF-BB caused elevation in intracellular cAMP, and it was abolished by indomethacin, a cyclooxygenase inhibitor. However, indomethacin did not affect the synergistic effect. In addition, PDGF-BB also did not affect the degradation of Cx43. With the use of MCs transfected with a Cx43 promoter-luciferase vector, cooperative activation of Cx43 promoter by PDGF and cAMP was found. Together, our data reveal, for the first time, unexpected synergy between PDGF-BB and cAMP-elevating agents in the induction of Cx43 and MC differentiation. Regulation of GJIC could be an important mechanism via which PDGF modulates MC phenotypes.

Differential connexin expression in preglomerular and postglomerular vasculature: accentuation during diabetes

BACKGROUND

Gap junctions may play an important role in regulating renal blood flow and glomerular responses. We have therefore made a comprehensive analysis of connexin expression in the renal vasculature of control and diabetic mice since elevated glucose has been reported to down-regulate connexin 43 in vascular cells in vitro.

METHODS

Connexin distribution was determined with immunohistochemistry using subtype-specific and cell type-specific antibodies. Diabetes was induced with streptozotocin (120/80 mg/kg, intraperitoneally) in C57BL/6 mice.

RESULTS

Connexins 37, 40, and 43 were expressed in endothelial cells of the renal, lobar, arcuate, and interlobular arteries and afferent arterioles, although connexin 43 was weak in the renal and arcuate arteries. Connexin 37 was detected in the media of arcuate, interlobular arteries and afferent arterioles and connexins 37 and 40 were found in renin-secreting cells. Both connexins 37 and 40 were expressed in extraglomerular mesangial cells, connexin 40 was abundantly expressed in intraglomerular mesangial cells, but connexin 37 was limited to mesangial cells at the vascular pole. In contrast, only connexin 43 was detected in endothelial cells of efferent arterioles and there was no connexin staining in the media. In diabetes, connexin 40 was expressed in smooth muscle cells along afferent arterioles, glomerular connexin staining was more extensive and connexin 43 was detected in renin-secreting cells. In contrast connexin 43 expression in endothelial cells of efferent arterioles was markedly reduced.

CONCLUSION

The renal vasculature and mesangial cells are well coupled on the preglomerular side but there is little evidence that the coupling extends into the efferent arteriole. This pattern of cell coupling is accentuated during diabetes.

SLC26A6 and SLC26A7 Anion Exchangers Have a Distinct Distribution in Human Kidney

BACKGROUND

The anion transporters SLC26A6 (PAT1) and SLC26A7, transporting at least chloride, oxalate, sulfate and bicarbonate, show a distinct expression and function in different mammalian species. They are expressed in kidney, but their exact localization in human kidney has not been studied. We therefore examined SLC26A6 and A7 expression in human kidneys.

METHODS

The localization of SLC26A6 and A7 in different segments of human nephrons was studied by RT-PCR and immunohistochemistry by comparing to the tubular markers PNRA, CD10, Tamm-Horsfall antigen, high molecular weight cytokeratin, CK7, AQP2 and H(+)V-ATPase.

RESULTS

In human kidney, SLC26A6 is expressed in distal segments of proximal tubules, parts of the thin and thick ascending limbs of Henle's loops, macula densa, distal convoluted tubules and a subpopulation of intercalated cells of collecting ducts. SLC26A7 is expressed in extraglomerular mesangial cells and a subpopulation of intercalated cells of collecting ducts.

CONCLUSION

Our results show that in human kidney SLC26A6 and A7 have a distinct, partially overlapping expression in distal segments of nephrons. The distribution partly differs from that found previously in rodent kidneys.

Nitric oxide-mediated regulation of connexin43 expression and gap junctional intercellular communication in mesangial cells

This study investigated a potential role of nitric oxide (NO) in the regulation of gap junctional intercellular communication (GJIC). Incubation of mesangial cells (MC) with NO donor S-nitroso-N-acetylpenicillamine (SNAP) enhanced both basal and 8-bromo-cAMP-stimulated GJIC as well as expression of gap junction protein connexin43 (Cx43). This potentiating action of SNAP on Cx43 expression was mimicked by two other NO donors and significantly blocked by soluble guanylate cyclase inhibitor 1H-[1,2,4]oxadiazolo[4,3,-alpha]quinoxalin-1-1. Guanosine 3',5'-cyclic monophosphate (cGMP) analogue 8-bromo-cGMP exerted an effect similar to NO, whereas another cGMP analogue, 8-pCPT-cGMP, which selectively activates cGMP-dependent kinase without affecting cGMP-inhibited phosphodiesterase (PDE3), had no effect. Moreover, the synergistic action of NO on Cx43 expression was completely prevented by protein kinase A inhibitor H89 but not by cGMP-dependent kinase inhibitor Rp-8-Br-PET-cGMP. These results suggested a possible involvement of NO-cAMP interaction via cGMP-mediated inhibition of PDE3. Indeed, PDE3 inhibitor cilostamide caused potentiation of 8-bromo-cAMP-elicited elevations of Cx43 expression that is similar to the effect of SNAP, and an elevation of intracellular cAMP was detected in SNAP-treated cells. With the use of genetically engineered reporter MC that express secreted alkaline phosphatase under the control of the cAMP response element, significant potentiation of cAMP-elicited activation of cAMP response element by SNAP was found. This effect was abrogated in the presence of PDE3 inhibitor cilostamide. Taken together, the results suggest that NO is involved in the control of GJIC and Cx43 expression. This effect of NO is due to activation of protein kinase A via cGMP-dependent inhibition of PDE3 activity.

Phospholemman expression in extraglomerular mesangium and afferent arteriole of the juxtaglomerular apparatus

The molecular mechanisms with which the juxtaglomerular apparatus accomplishes its twin functions, acute regulation of glomerular blood flow and secretion of renin, are still not clearly understood. Least understood is the role of the extraglomerular mesangial (EM) cells, also known as lacis or Goormaghtigh cells, which lie sandwiched between the macula densa and the afferent and efferent arterioles. Here, we report that immunoreactivity for phospholemman (FXYD1), a single-span membrane protein homologous to the gamma (gamma) sub-unit of the Na,K-ATPase, is found in the kidney in EM cells with the Na,K-ATPase beta2-subunit and in cortical blood vessels and the afferent arteriole with Na,K-ATPase alpha2 and beta2. Phospholemman's distribution in EM cells is distinct from that of the Na,K-ATPase gamma-subunit, which is found on the basolateral surface of macula densa cells with Na,K-ATPase alpha1 and beta1. Phospholemman is a major kinase target, and its location in the juxtaglomerular apparatus suggests that it is involved in tubuloglomerular feedback.

Intercellular communication between renin expressing As4.1 cells, endothelial cells and smooth muscle cells

Angiotensin II (AII) regulation of renin production by the juxtaglomerular (JG) cells of the kidney is commonly thought to occur through a direct feedback mechanism. However, recent evidence suggests that other cells in the vicinity may indirectly mediate AII's effect on renin production. Therefore we investigated whether an in vitro model of JG cells (As4.1) could have intercellular communication with endothelial or smooth muscle cells, which are in proximity to JG cells in vivo. 6-carboxyfluorescein was introduced to individual bovine aortic endothelial cells in co-culture with As4.1 cells. Coupling was observed 84% of the time at resting membrane potential and was attenuated by membrane depolarization or octanol (1 mM). Calcein green transfer between human aortic smooth muscle and As4.1 cells occurred 82% of the time and was inhibited by octanol. Expression of connexin 37, 40, 43, and 45 were detected in As4.1 cells using RT-PCR. Stimulation of As4.1 cells by AII failed to alter [Ca(2+)](i) or renin mRNA levels. These findings support the existence of gap junctions between renin producing cells and other cell types of the JG region. Moreover the lack of effect by AII suggest that feedback regulation of renin by AII may be due in part to intercellular communication with cells in proximity to JG cells.

Role of mesangial cells and gap junctions in tubuloglomerular feedback

BACKGROUND

Tubuloglomerular feedback (TGF) is a process whereby the resistance of the afferent arterioles delivering blood to the glomeruli is regulated by the NaCl concentration of the forming urine in the lumen of the macula densa. Intraglomerular mesangial cells are located between capillaries within the glomerulus, while extraglomerular mesangial cells are located between the macula densa and the afferent arteriole. They are electrically and chemically coupled via gap junctions. The purpose of this study was to investigate the role of mesangial cells and gap junctions in TGF using the isolated, perfused juxtaglomerular apparatus.

METHOD

Juxtaglomerular apparatuses were dissected from male New Zealand white rabbits and perfused in vitro. The NaCl concentration at the macula densa was changed from 17/2 to 65/50 Na/Cl to initiate a TGF response. Afferent arterioles were perfused at 60 mm Hg throughout the experiment. Changes in luminal diameter caused by increasing the NaCl concentration at the macula densa were taken as the TGF response. TGF was measured before and after disrupting the gap junctions or damaging the mesangial cells in paired experiments.

RESULTS

During the control period, TGF decreased afferent arteriole diameter by 2.9 +/- 0.2 microm. After mesangial cells were damaged by perfusing Thy 1-1 antibody and complement into the afferent arteriole, the TGF response was completely eliminated. Separate experiments showed no statistically significant change in TGF response with time, or when antibody and complement were perfused into the macula densa lumen. The presence of Thy 1-1 antibody and complement in the afferent arteriole perfusate did not alter the ability of norepinephrine to constrict or acetylcholine to dilate the afferent arteriole. To investigate the role of gap junctions in TGF, we used heptanol to disrupt them. During the control period, TGF decreased afferent arteriole diameter by 2.9 +/- 0.4 microm. After perfusing heptanol into the lumen of the afferent arteriole, the TGF response was completely eliminated. When heptanol was added to the bath, it had no significant effect on TGF response.

DISCUSSION

The data show that after mesangial cells were selectively damaged, the constriction of the afferent arteriole induced by increasing the NaCl concentration at the macula densa was eliminated. However, such treatment had no effect when Thy 1-1 was perfused into the macula densa lumen, and did not alter the response of the afferent arteriole to norepinephrine or acetylcholine. Disruption of the gap junctions also eliminated the TGF response. These data indicate that the mesangial cells play a key role in mediating the TGF response, and that gap junctions among mesangial cells and between mesangial cells and vascular smooth muscle cells communicate the TGF signal to the afferent arteriole.

Coordination of mesangial cell contraction by gap junction--mediated intercellular Ca(2+) wave

Gap junction intercellular communication (GJIC) plays a fundamental role in mediating intercellular signals and coordinating multicellular behavior in various tissues and organs. Glomerular mesangial cells (MC) are rich in GJ, but the functional associations of these intercellular channels are still unclear. This study examines the potential role of GJ in the transmission of intercellular Ca(2+) signals and in the coordination of MC contraction. First, the presence of GJ protein Cx43 and functional GJIC was confirmed in MC by using immunochemical staining or transfer of Lucifer yellow (LY) after a single cell injection, respectively. Second, mechanical stimulation of a single MC initiated propagation of an intercellular Ca(2+) wave, which was preventable by the GJ inhibitor heptanol but was not altered by pretreatment of MC with ATP or addition of apyrase into the assay system. Third, the phospholipase C (PLC) inhibitor U73122 could largely eliminate the mechanically elicited propagation of intercellular Ca(2+) waves, suggesting a possibly mediating role of inositol trisphosphate (IP(3)) in the initiation and transmission of intercellular Ca(2+) signaling. Fourth, injection of IP(3) into a single cell caused contraction, not only in the targeted cell, but also in the adjacent cells, as indicated by the reduction of cellular planar area. Fifth, addition of two structurally unrelated GJ inhibitors, heptanol and alpha-glycyrrhetinic acid (GA), into MC embedded in collagen gels significantly attenuated the reduction of gel areas after exposure to serum. This study provides the first functional evidence supporting the critical role of GJIC in the synchronization of MC behaviors.

PDGF regulates gap junction communication and connexin43 phosphorylation by PI 3-kinase in mesangial cells

BACKGROUND

Gap junctional intercellular communication (GJIC) plays an important role in the regulation of cell growth, migration, and differentiation. Ultrastructural and histochemical studies indicate the existence of a high density of gap junctions among mesangial cells (MCs), but little is known about their regulation. Because of the close link between growth and GJIC, we examined how platelet-derived growth factor (PDGF) may affect GJIC in cultured MCs.

METHODS

MCs were exposed to PDGF in the presence or absence of phosphatidylinositol 3' kinase (PI3K) inhibitors, and GJIC was evaluated by the transfer of Lucifer yellow. The gap junction protein connexin43 (Cx43) was examined by immunohistochemistry, immunoprecipitation, and Western blot.

RESULTS

The addition of PDGF into MC culture caused a rapid and transient inhibition of GJIC, with maximal inhibition (80%) occurring 15 minutes after PDGF exposure and returning to control levels after 90 minutes. This action of PDGF could be largely prevented by pretreatment of MCs with the PI3K inhibitor LY294002. Immunochemical staining showed that PDGF did not alter the localization and distribution of Cx43. Immunoprecipitation studies demonstrated that PDGF induced a rapid and transient increase of tyrosine phosphorylation of Cx43 protein, which was dose dependent and in accordance with the time course of the disruption of GJIC. PDGF also elicited activation of extracellular signal-regulated kinase (ERK). Using two structurally unrelated PI3K inhibitors, wortmanin and LY294002, both tyrosine phosphorylation of Cx43 and activation of ERK stimulated by PDGF were largely blocked.

CONCLUSION

These results suggest that PDGF abrogates GJIC function in MCs via the PI3K-dependent signaling pathway. Disruption of GJIC by PDGF could be one mechanism by which PDGF modulates MC behavior. Participation of PI3K in the regulation of GJIC demonstrates the complex coordination of molecular events that accompany MC mitogenesis.

Effects of cytosolic Ca2+ on membrane voltage and conductance of cultured mesangial cells from stroke-prone spontaneously hypertensive rats and WKY rats

Mesangial cells (MC) are considered to play an important role in the development of hypertension. The purpose of this study was to characterize the effects of cytosolic Ca2+ on membrane voltage and conductance of MC using stroke-prone spontaneously hypertensive rats (SHRSP) and Wistar Kyoto rats (WKY). We applied the patch-clamp technique in the whole-cell configuration to measure membrane potential (Vm) and ion currents. There was no significant difference in resting Vm values between MC from WKY and SHRSP. The cytosolic Ca2+ increase induced membrane depolarization and the increase of Cl- currents in MC from WKY but not in MC from SHRSP. On the other hand, the Ca2+ increase induced membrane hyperpolarization and the increase of K+ currents in MC from SHRSP but not in MC from WKY. Such differences between MC from two rat strains may play an important role in the alterations in renal hemodynamics observed in hypertension.