Myocardin-related transcription factor and serum response factor regulate cilium turnover by both transcriptional and local mechanisms.

Turnover of the primary cilium (PC) is critical for proliferation and tissue homeostasis. Each key component of the PC resorption machinery, the HEF1/Aurora kinase A (AurA)/HDAC6 pathway harbors cis-elements potentially targeted by the transcriptional co-activator myocardin-related transcription factor (MRTF) and/or its partner serum response factor (SRF). Thus we investigated if MRTF and/or SRF regulate PC turnover. Here we show that (1) both MRTF and SRF are indispensable for serum-induced PC resorption, and (2) they act via both transcriptional and local mechanisms. Intriguingly, MRTF and SRF are present in the basal body and/or the PC, and serum facilitates ciliary MRTF recruitment. MRTF promotes the stability and ciliary accumulation of AurA and facilitates SRF phosphorylation. Ciliary SRF interacts with AurA and HDAC6. MRTF also inhibits ciliogenesis. It interacts with and is required for the correct localization of the ciliogenesis modulator CEP290. Thus, MRTF and SRF are critical regulators of PC assembly and/or disassembly, acting both as transcription factors and as PC constituents.

Claudin-2 suppresses GEF-H1, RHOA, and MRTF, thereby impacting proliferation and profibrotic phenotype of tubular cells.

The tight junctional pore-forming protein claudin-2 (CLDN-2) mediates paracellular Na+ and water transport in leaky epithelia and alters cancer cell proliferation. Previously, we reported that tumor necrosis factor-α time-dependently alters CLDN-2 expression in tubular epithelial cells. Here, we found a similar expression pattern in a mouse kidney injury model (unilateral ureteral obstruction), consisting of an initial increase followed by a drop in CLDN-2 protein expression. CLDN-2 silencing in LLC-PK1 tubular cells induced activation and phosphorylation of guanine nucleotide exchange factor H1 (GEF-H1), leading to Ras homolog family member A (RHOA) activation. Silencing of other claudins had no such effects, and re-expression of an siRNA-resistant CLDN-2 prevented RHOA activation, indicating specific effects of CLDN-2 on RHOA. Moreover, kidneys from CLDN-2 knockout mice had elevated levels of active RHOA. Of note, CLDN-2 silencing reduced LLC-PK1 cell proliferation and elevated expression of cyclin-dependent kinase inhibitor P27 (P27KIP1) in a GEF-H1/RHOA-dependent manner. P27KIP1 silencing abrogated the effects of CLDN-2 depletion on proliferation. CLDN-2 loss also activated myocardin-related transcription factor (MRTF), a fibrogenic RHOA effector, and elevated expression of connective tissue growth factor and smooth muscle actin. Finally, CLDN-2 down-regulation contributed to RHOA activation and smooth muscle actin expression induced by prolonged tumor necrosis factor-α treatment, because they were mitigated by re-expression of CLDN-2. Our results indicate that CLDN-2 suppresses GEF-H1/RHOA. CLDN-2 down-regulation, for example, by inflammation, can reduce proliferation and promote MRTF activation through RHOA. These findings suggest that the initial CLDN-2 elevation might aid epithelial regeneration, and CLDN-2 loss could contribute to fibrotic reprogramming.

Profibrotic epithelial phenotype: a central role for MRTF and TAZ.

Epithelial injury is a key initiator of fibrosis but - in contrast to the previous paradigm - the epithelium in situ does not undergo wide-spread epithelial-mesenchymal/myofibroblast transition (EMT/EMyT). Instead, it assumes a Profibrotic Epithelial Phenotype (PEP) characterized by fibrogenic cytokine production. The transcriptional mechanisms underlying PEP are undefined. As we have shown that two RhoA/cytoskeleton-regulated transcriptional coactivators, Myocardin-related transcription factor (MRTF) and TAZ, are indispensable for EMyT, we asked if they might mediate PEP as well. Here we show that mechanical stress (cyclic stretch) increased the expression of transforming growth factor-ß1 (TGFß1), connective tissue growth factor (CTGF), platelet-derived growth factor and Indian Hedgehog mRNA in LLC-PK1 tubular cells. These responses were mitigated by siRNA-mediated silencing or pharmacological inhibition of MRTF (CCG-1423) or TAZ (verteporfin). RhoA inhibition exerted similar effects. Unilateral ureteral obstruction, a murine model of mechanically-triggered kidney fibrosis, induced tubular RhoA activation along with overexpression/nuclear accumulation of MRTF and TAZ, and increased transcription of the above-mentioned cytokines. Laser capture microdissection revealed TAZ, TGFß1 and CTGF induction specifically in the tubular epithelium. CCG-1423 suppressed total renal and tubular expression of these proteins. Thus, MRTF regulates epithelial TAZ expression, and both MRTF and TAZ are critical mediators of PEP-related epithelial cytokine production.

Mediated nuclear import and export of TAZ and the underlying molecular requirements.

Nucleocytoplasmic distribution of Yap/TAZ is regulated by the Hippo pathway and the cytoskeleton. While interactions with cytosolic and nuclear "retention factors" (14-3-3 and TEAD) are known to control their localization, fundamental aspects of Yap/TAZ shuttling remain undefined. It is unclear if translocation occurs only by passive diffusion or via mediated transport, and neither the potential nuclear localization and efflux signals (NLS, NES) nor their putative regulation have been identified. Here we show that TAZ cycling is a mediated process and identify the underlying NLS and NES. The C-terminal NLS, representing a new class of import motifs, is necessary and sufficient for efficient nuclear uptake via a RAN-independent mechanism. RhoA activity directly stimulates this import. The NES lies within the TEAD-binding domain and can be masked by TEAD, thereby preventing efflux. Thus, we describe a RhoA-regulated NLS, a TEAD-regulated NES and propose an improved model of nucleocytoplasmic TAZ shuttling beyond "retention".

TGF-ß1 regulates the expression and transcriptional activity of TAZ protein via a Smad3-independent, myocardin-related transcription factor-mediated mechanism.

Hippo pathway transcriptional coactivators TAZ and YAP and the TGF-ß1 (TGFß) effector Smad3 regulate a common set of genes, can physically interact, and exhibit multilevel cross-talk regulating cell fate-determining and fibrogenic pathways. However, a key aspect of this cross-talk, TGFß-mediated regulation of TAZ or YAP expression, remains uncharacterized. Here, we show that TGFß induces robust TAZ but not YAP protein expression in both mesenchymal and epithelial cells. TAZ levels, and to a lesser extent YAP levels, also increased during experimental kidney fibrosis. Pharmacological or genetic inhibition of Smad3 did not prevent the TGFß-induced TAZ up-regulation, indicating that this canonical pathway is dispensable. In contrast, inhibition of p38 MAPK, its downstream effector MK2 (e.g. by the clinically approved antifibrotic pirferidone), or Akt suppressed the TGFß-induced TAZ expression. Moreover, TGFß elevated TAZ mRNA in a p38-dependent manner. Myocardin-related transcription factor (MRTF) was a central mediator of this effect, as MRTF silencing/inhibition abolished the TGFß-induced TAZ expression. MRTF overexpression drove the TAZ promoter in a CC(A/T-rich)6GG (CArG) box-dependent manner and induced TAZ protein expression. TGFß did not act by promoting nuclear MRTF translocation; instead, it triggered p38- and MK2-mediated, Nox4-promoted MRTF phosphorylation and activation. Functionally, higher TAZ levels increased TAZ/TEAD-dependent transcription and primed cells for enhanced TAZ activity upon a second stimulus (i.e. sphingosine 1-phosphate) that induced nuclear TAZ translocation. In conclusion, our results uncover an important aspect of the cross-talk between TGFß and Hippo signaling, showing that TGFß induces TAZ via a Smad3-independent, p38- and MRTF-mediated and yet MRTF translocation-independent mechanism.

MicroRNA-21 preserves the fibrotic mechanical memory of mesenchymal stem cells.

Expansion on stiff culture substrates activates pro-fibrotic cell programs that are retained by mechanical memory. Here, we show that priming on physiologically soft silicone substrates suppresses fibrogenesis and desensitizes mesenchymal stem cells (MSCs) against subsequent mechanical activation in vitro and in vivo, and identify the microRNA miR-21 as a long-term memory keeper of the fibrogenic program in MSCs. During stiff priming, miR-21 levels were gradually increased by continued regulation through the acutely mechanosensitive myocardin-related transcription factor-A (MRTF-A/MLK-1) and remained high over 2 weeks after removal of the mechanical stimulus. Knocking down miR-21 once by the end of the stiff-priming period was sufficient to erase the mechanical memory and sensitize MSCs to subsequent exposure to soft substrates. Soft priming and erasing mechanical memory following cell culture expansion protects MSCs from fibrogenesis in the host wound environment and increases the chances for success of MSC therapy in tissue-repair applications.

Context-dependent switch in chemo/mechanotransduction via multilevel crosstalk among cytoskeleton-regulated MRTF and TAZ and TGFß-regulated Smad3.

Myocardin-related transcription factor (MRTF) and TAZ are major mechanosensitive transcriptional co-activators that link cytoskeleton organization to gene expression. Despite many similarities in their regulation, their physical and/or functional interactions are unknown. Here we show that MRTF and TAZ associate partly through a WW domain-dependent mechanism, and exhibit multilevel crosstalk affecting each other's expression, transport and transcriptional activity. Specifically, MRTF is essential for TAZ expression; TAZ and MRTF inhibit each other's cytosolic mobility and stimulus-induced nuclear accumulation; they antagonize each other's stimulatory effect on the α-smooth muscle actin (SMA) promoter, which harbours nearby cis-elements for both, but synergize on isolated TEAD-elements. Importantly, TAZ confers Smad3 sensitivity to the SMA promoter. Thus, TAZ is a context-dependent switch during mechanical versus mechano/chemical signalling, which inhibits stretch-induced but is indispensable for stretch+TGFß-induced SMA expression. Crosstalk between these cytoskeleton-regulated factors seems critical for fine-tuning mechanical and mechanochemical transcriptional programmes underlying myofibroblast transition, wound healing and fibrogenesis.

Myocardin-related Transcription Factor Regulates Nox4 Protein Expression: LINKING CYTOSKELETAL ORGANIZATION TO REDOX STATE.

TGFß-induced expression of the NADPH oxidase Nox4 is essential for fibroblast-myofibroblast transition. Rho has been implicated in Nox4 regulation, but the underlying mechanisms are largely unknown. Myocardin-related transcription factor (MRTF), a Rho/actin polymerization-controlled coactivator of serum response factor, drives myofibroblast transition from various precursors. We have shown that TGFß is necessary but insufficient for epithelial-myofibroblast transition in intact epithelia; the other prerequisite is the uncoupling of intercellular contacts, which induces Rho-dependent nuclear translocation of MRTF. Because the Nox4 promoter harbors a serum response factor/MRTF cis-element (CC(A/T)6GG box), we asked if MRTF (and thus cytoskeleton organization) could regulate Nox4 expression. We show that Nox4 protein is robustly induced in kidney tubular cells exclusively by combined application of contact uncoupling and TGFß. Nox4 knockdown abrogates epithelial-myofibroblast transition-associated reactive oxygen species production. Laser capture microdissection reveals increased Nox4 expression in the tubular epithelium also during obstructive nephropathy. MRTF down-regulation/inhibition suppresses TGFß/contact disruption-provoked Nox4 protein and mRNA expression, Nox4 promoter activation, and reactive oxygen species production. Mutation of the CC(A/T)6GG box eliminates the synergistic activation of the Nox4 promoter. Jasplakinolide-induced actin polymerization synergizes with TGFß to facilitate MRTF-dependent Nox4 mRNA expression/promoter activation. Moreover, MRTF inhibition prevents Nox4 expression during TGFß-induced fibroblast-myofibroblast transition as well. Although necessary, MRTF is insufficient; Nox4 expression also requires TGFß-activated Smad3 and TAZ/YAP, two contact- and cytoskeleton-regulated Smad3-interacting coactivators. Down-regulation/inhibition of TAZ/YAP mitigates injury-induced epithelial Nox4 expression in vitro and in vivo. These findings uncover new MRTF- and TAZ/YAP-dependent mechanisms, which link cytoskeleton remodeling and redox state and impact epithelial plasticity and myofibroblast transition.

The fate of the primary cilium during myofibroblast transition.

Myofibroblasts, the culprit of organ fibrosis, can originate from mesenchymal and epithelial precursors through fibroblast-myofibroblast and epithelial-myofibroblast transition (EMyT). Because certain ciliopathies are associated with fibrogenesis, we sought to explore the fate and potential role of the primary cilium during myofibroblast formation. Here we show that myofibroblast transition from either precursor results in the loss of the primary cilium. During EMyT, initial cilium growth is followed by complete deciliation. Both EMyT and cilium loss require two-hit conditions: disassembly/absence of intercellular contacts and transforming growth factor-ß1 (TGFß) exposure. Loss of E-cadherin-dependent junctions induces cilium elongation, whereas both stimuli are needed for deciliation. Accordingly, in a scratch-wounded epithelium, TGFß provokes cilium loss exclusively along the wound edge. Increased contractility, a key myofibroblast feature, is necessary and sufficient for deciliation, since constitutively active RhoA, Rac1, or myosin triggers, and down-regulation of myosin or myocardin-related transcription factor prevents, this process. Sustained myosin phosphorylation and consequent deciliation are mediated by a Smad3-, Rac1-, and reactive oxygen species-dependent process. Transitioned myofibroblasts exhibit impaired responsiveness to platelet-derived growth factor-AA and sonic hedgehog, two cilium-associated stimuli. Although the cilium is lost during EMyT, its initial presence contributes to the transition. Thus myofibroblasts represent a unique cilium-less entity with profoundly reprogrammed cilium-related signaling.

Differential topical susceptibility to TGFß in intact and injured regions of the epithelium: key role in myofibroblast transition.

Induction of epithelial-myofibroblast transition (EMyT), a robust fibrogenic phenotype change hallmarked by α-smooth muscle actin (SMA) expression, requires transforming growth factor-ß1 (TGFß) and the absence/uncoupling of intracellular contacts. This suggests that an "injured" epithelium may be topically susceptible to TGFß. To explore this concept, we use an epithelial wound model in which intact and contact-deprived regions of the same monolayer can be analyzed simultaneously. We show that TGFß elicits dramatically different responses at these two loci. SMA expression and initially enhanced nuclear Smad3 accumulation followed by Smad3 mRNA and protein down-regulation occur exclusively at the wound. Mechanistically, three transcriptional coactivators whose localization is regulated by cell contact integrity are critical for these local effects. These are myocardin-related transcription factor (MRTF), the driver of the SMA promoter; ß-catenin, which counteracts the known inhibitory effect of Smad3 on MRTF and maintains MRTF protein stability and mRNA expression in the wound; and TAZ, a Hippo effector and Smad3 retention factor. Remarkably, active TAZ stimulates the SMA and suppresses the Smad3 promoter, whereas TAZ silencing prevents wound-restricted expression of SMA and loss of Smad3. Such locus-specific reprogramming might play key roles in wound healing and the susceptibility of the injured epithelium to fibrogenesis.

Central role of the exchange factor GEF-H1 in TNF-α-induced sequential activation of Rac, ADAM17/TACE, and RhoA in tubular epithelial cells.

Transactivation of the epidermal growth factor receptor (EGFR) by tumor necrosis factor-α (TNF-α) is a key step in mediating RhoA activation and cytoskeleton and junction remodeling in the tubular epithelium. In this study we explore the mechanisms underlying TNF-α-induced EGFR activation. We show that TNF-α stimulates the TNF-α convertase enzyme (TACE/a disintegrin and metalloproteinase-17), leading to activation of the EGFR/ERK pathway. TACE activation requires the mitogen-activated protein kinase p38, which is activated through the small GTPase Rac. TNF-α stimulates both Rac and RhoA through the guanine nucleotide exchange factor (GEF)-H1 but by different mechanisms. EGFR- and ERK-dependent phosphorylation at the T678 site of GEF-H1 is a prerequisite for RhoA activation only, whereas both Rac and RhoA activation require GEF-H1 phosphorylation on S885. Of interest, GEF-H1-mediated Rac activation is upstream from the TACE/EGFR/ERK pathway and regulates T678 phosphorylation. We also show that TNF-α enhances epithelial wound healing through TACE, ERK, and GEF-H1. Taken together, our findings can explain the mechanisms leading to hierarchical activation of Rac and RhoA by TNF-α through a single GEF. This mechanism could coordinate GEF functions and fine-tune Rac and RhoA activation in epithelial cells, thereby promoting complex functions such as sheet migration.

How do your contacts (or their absence) shape your fate?

Tissue accumulation of contractile myofibroblasts is a key feature of a multitude of fibrotic diseases. Myofibroblast generation either from epithelial or mesenchymal precursors involves the activation of a myogenic program, hallmarked by the expression of α-smooth muscle actin (SMA). Recent research suggests that this robust phenotypic reprogramming requires two critical inputs: the fibrogenic cytokine transforming growth factor-ß1 (TGFß) and an injury (or absence) of intercellular junctions. This two-hit paradigm of epithelial-myofibroblast transition (EMyT) postulates that the injured (contact-deprived) epithelium is locally and selectively sensitive (topically susceptible) to the transforming effect of TGFß, while the intact areas are quite resistant to the phenotype-changing effect of this cytokine. Searching for molecular mechanisms underlying the synergy between contact injury and TGFß, we found that an interplay among three multifunctional transcriptional (co)activators, the junction component ß-catenin, the TGFß receptor target Smad3, and the actin cytoskeleton-regulated myocardin-related transcription factor (MRTF) controls the magnitude and timing of SMA expression.(1) Moreover, this regulation is realized not only at the transcriptional level. Notably, these factors form a pretranscriptional circuit, in which they impact each other's activity and stability. Based on this recent paper we ponder about the mechanisms of cellular plasticity in the context of EMyT. We propose that topical susceptibility to TGFß, triggered by cell contact-modulated pretranscriptional and transcriptional control is realized through the crosstalk of a few master regulators, whose coordinated action tailors SMA expression and contributes to the major decision of whether injury leads to healing or fibrosis.

Hyperosmotic stress regulates the distribution and stability of myocardin-related transcription factor, a key modulator of the cytoskeleton.

Hyperosmotic stress initiates several adaptive responses, including the remodeling of the cytoskeleton. Besides maintaining structural integrity, the cytoskeleton has emerged as an important regulator of gene transcription. Myocardin-related transcription factor (MRTF), an actin-regulated coactivator of serum response factor, is a major link between the actin skeleton and transcriptional control. We therefore investigated whether MRTF is regulated by hyperosmotic stress. Here we show that hypertonicity induces robust, rapid, and transient translocation of MRTF from the cytosol to the nucleus in kidney tubular cells. We found that the hyperosmolarity-triggered MRTF translocation is mediated by the RhoA/Rho kinase (ROK) pathway. Moreover, the Rho guanine nucleotide exchange factor GEF-H1 is activated by hyperosmotic stress, and it is a key contributor to the ensuing RhoA activation and MRTF translocation, since siRNA-mediated GEF-H1 downregulation suppresses these responses. While the osmotically induced RhoA activation promotes nuclear MRTF accumulation, the concomitant activation of p38 MAP kinase mitigates this effect. Moderate hyperosmotic stress (600 mosM) drives MRTF-dependent transcription through the cis-element CArG box. Silencing or pharmacological inhibition of MRTF prevents the osmotic stimulation of CArG-dependent transcription and renders the cells susceptible to osmotic shock-induced structural damage. Interestingly, strong hyperosmolarity promotes proteasomal degradation of MRTF, concomitant with apoptosis. Thus, MRTF is an osmosensitive and osmoprotective transcription factor, whose intracellular distribution is regulated by the GEF-H1/RhoA/ROK and p38 pathways. However, strong osmotic stress destabilizes MRTF, concomitant with apoptosis, implying that hyperosmotically induced cell death takes precedence over epithelial-myofibroblast transition, a potential consequence of MRTF-mediated phenotypic reprogramming.

ß-catenin and Smad3 regulate the activity and stability of myocardin-related transcription factor during epithelial-myofibroblast transition.

Injury to the adherens junctions (AJs) synergizes with transforming growth factor-ß1 (TGFß) to activate a myogenic program (α-smooth muscle actin [SMA] expression) in the epithelium during epithelial-myofibroblast transition (EMyT). Although this synergy plays a key role in organ fibrosis, the underlying mechanisms have not been fully defined. Because we recently showed that Smad3 inhibits myocardin-related transcription factor (MRTF), the driver of the SMA promoter and many other CC(A/T)-rich GG element (CArG) box-dependent cytoskeletal genes, we asked whether AJ components might affect SMA expression through interfering with Smad3. We demonstrate that E-cadherin down-regulation potentiates, whereas ß-catenin knockdown inhibits, SMA expression. Contact injury and TGFß enhance the binding of ß-catenin to Smad3, and this interaction facilitates MRTF signaling by two novel mechanisms. First, it inhibits the Smad3/MRTF association and thereby allows the binding of MRTF to its myogenic partner, serum response factor (SRF). Accordingly, ß-catenin down-regulation disrupts the SRF/MRTF complex. Second, ß-catenin maintains the stability of MRTF by suppressing the Smad3-mediated recruitment of glycogen synthase kinase-3ß to MRTF, an event that otherwise leads to MRTF ubiquitination and degradation and the consequent loss of SRF/MRTF-dependent proteins. Thus ß-catenin controls MRTF-dependent transcription and emerges as a critical regulator of an array of cytoskeletal genes, the "CArGome."

Extracellular signal-regulated kinase and GEF-H1 mediate depolarization-induced Rho activation and paracellular permeability increase.

Plasma membrane depolarization activates the Rho/Rho kinase (ROK) pathway and thereby enhances myosin light chain (MLC) phosphorylation, which in turn is thought to be a key regulator of paracellular permeability. However, the upstream mechanisms that couple depolarization to Rho activation and permeability changes are unknown. Here we show that three different depolarizing stimuli (high extracellular K(+) concentration, the lipophilic cation tetraphenylphosphonium, or l-alanine, which is taken up by electrogenic Na(+) cotransport) all provoke robust phosphorylation of ERK in LLC-PK1 and Madin-Darby canine kidney (MDCK) cells. Importantly, inhibition of ERK prevented the depolarization-induced activation of Rho. Searching for the underlying mechanism, we have identified the GTP/GDP exchange factor GEF-H1 as the ERK-regulated critical exchange factor responsible for the depolarization-induced Rho activation. This conclusion is based on our findings that 1) depolarization activated GEF-H1 but not p115RhoGEF, 2) short interfering RNA-mediated GEF-H1 silencing eliminated the activation of the Rho pathway, and 3) ERK inhibition prevented the activation of GEF-H1. Moreover, we found that the Na(+)-K(+) pump inhibitor ouabain also caused ERK, GEF-H1, and Rho activation, partially due to its depolarizing effect. Regarding the functional consequences of this newly identified pathway, we found that depolarization increased paracellular permeability in LLC-PK1 and MDCK cells and that this effect was mitigated by inhibiting myosin using blebbistatin or a dominant negative (phosphorylation incompetent) MLC. Taken together, we propose that the ERK/GEF-H1/Rho/ROK/pMLC pathway could be a central mechanism whereby electrogenic transmembrane transport processes control myosin phosphorylation and regulate paracellular transport in the tubular epithelium.

Fate-determining mechanisms in epithelial-myofibroblast transition: major inhibitory role for Smad3.

Epithelial-myofibroblast (MF) transition (EMyT) is a critical process in organ fibrosis, leading to alpha-smooth muscle actin (SMA) expression in the epithelium. The mechanism underlying the activation of this myogenic program is unknown. We have shown previously that both injury to intercellular contacts and transforming growth factor beta (TGF-beta) are indispensable for SMA expression (two-hit model) and that contact disruption induces nuclear translocation of myocardin-related transcription factor (MRTF). Because the SMA promoter harbors both MRTF-responsive CC(A/T)-rich GG element (CArG) boxes and TGF-beta-responsive Smad-binding elements, we hypothesized that the myogenic program is mobilized by a synergy between MRTF and Smad3. In this study, we show that the synergy between injury and TGF-beta exclusively requires CArG elements. Surprisingly, Smad3 inhibits MRTF-driven activation of the SMA promoter, and Smad3 silencing renders injury sufficient to induce SMA expression. Furthermore, Smad3 is degraded under two-hit conditions, thereby liberating the myogenic program. Thus, Smad3 is a critical timer/delayer of MF commitment in the epithelium, and EMyT can be dissected into Smad3-promoted (mesenchymal) and Smad3-inhibited (myogenic) phases.

GEF-H1 mediates tumor necrosis factor-alpha-induced Rho activation and myosin phosphorylation: role in the regulation of tubular paracellular permeability.

Tumor necrosis factor-alpha (TNF-alpha), an inflammatory cytokine, has been shown to activate the small GTPase Rho, but the underlying signaling mechanisms remained undefined. This general problem is particularly important in the kidney, because TNF-alpha, a major mediator of kidney injury, is known to increase paracellular permeability in tubular epithelia. Here we aimed to determine the effect of TNF-alpha on the Rho pathway in tubular cells (LLC-PK(1) and Madin-Darby canine kidney), define the upstream signaling, and investigate the role of the Rho pathway in the TNF-alpha-induced alterations of paracellular permeability. We show that TNF-alpha induced a rapid and sustained RhoA activation that led to stress fiber formation and Rho kinase-dependent myosin light chain (MLC) phosphorylation. To identify new regulators connecting the TNF receptor to Rho signaling, we applied an affinity precipitation assay with a Rho mutant (RhoG17A), which captures activated GDP-GTP exchange factors (GEFs). Mass spectrometry analysis of the RhoG17A-precipitated proteins identified GEF-H1 as a TNF-alpha-activated Rho GEF. Consistent with a central role of GEF-H1, its down-regulation by small interfering RNA prevented the activation of the Rho pathway. Moreover GEF-H1 and Rho activation are downstream of ERK signaling as the MEK1/2 inhibitor PD98059 mitigated TNF-alpha-induced activation of these proteins. Importantly TNF-alpha enhanced the ERK pathway-dependent phosphorylation of Thr-678 of GEF-H1 that was key for activation. Finally the TNF-alpha-induced paracellular permeability increase was absent in LLC-PK(1) cells stably expressing a non-phosphorylatable, dominant negative MLC. In summary, we have identified the ERK/GEF-H1/Rho/Rho kinase/phospho-MLC pathway as the mechanism mediating TNF-alpha-induced elevation of tubular epithelial permeability, which in turn might contribute to kidney injury.

Hyperosmotic stress induces Rho/Rho kinase/LIM kinase-mediated cofilin phosphorylation in tubular cells: key role in the osmotically triggered F-actin response.

Hyperosmotic stress induces cytoskeleton reorganization and a net increase in cellular F-actin, but the underlying mechanisms are incompletely understood. Whereas de novo F-actin polymerization likely contributes to the actin response, the role of F-actin severing is unknown. To address this problem, we investigated whether hyperosmolarity regulates cofilin, a key actin-severing protein, the activity of which is inhibited by phosphorylation. Since the small GTPases Rho and Rac are sensitive to cell volume changes and can regulate cofilin phosphorylation, we also asked whether they might link osmostress to cofilin. Here we show that hyperosmolarity induced rapid, sustained, and reversible phosphorylation of cofilin in kidney tubular (LLC-PK1 and Madin-Darby canine kidney) cells. Hyperosmolarity-provoked cofilin phosphorylation was mediated by the Rho/Rho kinase (ROCK)/LIM kinase (LIMK) but not the Rac/PAK/LIMK pathway, because 1) dominant negative (DN) Rho and DN-ROCK but not DN-Rac and DN-PAK inhibited cofilin phosphorylation; 2) constitutively active (CA) Rho and CA-ROCK but not CA-Rac and CA-PAK induced cofilin phosphorylation; 3) hyperosmolarity induced LIMK-2 phosphorylation, and 4) inhibition of ROCK by Y-27632 suppressed the hypertonicity-triggered LIMK-2 and cofilin phosphorylation.We thenexamined whether cofilin and its phosphorylation play a role in the hypertonicity-triggered F-actin changes. Downregulation of cofilin by small interfering RNA increased the resting F-actin level and eliminated any further rise upon hypertonic treatment. Inhibition of cofilin phosphorylation by Y-27632 prevented the hyperosmolarity-provoked F-actin increase. Taken together, cofilin is necessary for maintaining the osmotic responsiveness of the cytoskeleton in tubular cells, and the Rho/ROCK/LIMK-mediated cofilin phosphorylation is a key mechanism in the hyperosmotic stress-induced F-actin increase.

Reanalysis of structure/function correlations in the region of transmembrane segments 4 and 5 of the rabbit sodium/glucose cotransporter.

The predicted topology of the mammalian high-affinity sodium/glucose cotransporter (SGLT1), in the region surrounding transmembrane segments 4 and 5, disagrees with the recent published crystal structure of bacterial SGLT from Vibrio parahaemolyticus (vSGLT). To investigate this issue further, 38 residues from I143 to A180 in the N-terminal half of rabbit SGLT1 were each replaced with cysteine and then expressed in COS-7 cells or Xenopus laevis oocytes. The membrane orientations of the substituted cysteines were determined by treatment with the thiol-specific reagent N-Biotinoylaminoethyl methanethiosulfonate (biotin-MTSEA), combined with the membrane impermeant thiol-specific reagent sodium (2-sulfonatoethyl) methanethiosulfonate (MTSES). The present results combined with previous structure/function studies of SGLT1, suggest that transmembrane domain (TM) 4 of mammalian SGLT1 extends from residue 143-171 and support the topology observed in the crystal structure of vSGLT.

Transmembrane IV of the high-affinity sodium-glucose cotransporter participates in sugar binding.

Investigation of the structure/function relationships of the sodium-glucose transporter (SGLT1) is crucial to understanding the cotransporter mechanism. In the present study, we used cysteine-scanning mutagenesis and chemical modification by methanethiosulfonate (MTS) derivatives to test whether predicted transmembrane IV participates in sugar binding. Five charged and polar residues (K139, Q142, T156, K157, and D161) and two glucose/galactose malabsorption missense mutations (I147 and S159) were replaced with cysteine. Mutants I147C, T156C, and K157C exhibited sufficient expression to be studied in detail using the two-electrode voltage-clamp method in Xenopus laevis oocytes and COS-7 cells. I147C was similar in function to wild-type and was not studied further. Mutation of lysine-157 to cysteine (K157C) causes loss of phloridzin and alpha-methyl-D-glucopyranoside (alphaMG) binding. These functions are restored by chemical modification with positively charged (2-aminoethyl) methanethiosulfonate hydrobromide (MTSEA). Mutation of threonine-156 to cysteine (T156C) reduces the affinity of alphaMG and phloridzin for T156C by approximately 5-fold and approximately 20-fold, respectively. In addition, phloridzin protects cysteine-156 in T156C from alkylation by MTSEA. Therefore, the presence of a positive charge or a polar residue at 157 and 156, respectively, affects sugar binding and sugar-induced Na(+) currents.