Hematopoietic sphingosine 1-phosphate lyase deficiency decreases atherosclerotic lesion development in LDL-receptor deficient mice

AIMS

Altered sphingosine 1-phosphate (S1P) homeostasis and signaling is implicated in various inflammatory diseases including atherosclerosis. As S1P levels are tightly controlled by S1P lyase, we investigated the impact of hematopoietic S1P lyase (Sgpl1(-/-)) deficiency on leukocyte subsets relevant to atherosclerosis.

METHODS AND RESULTS

LDL receptor deficient mice that were transplanted with Sgpl1(-/-) bone marrow showed disrupted S1P gradients translating into lymphopenia and abrogated lymphocyte mitogenic and cytokine response as compared to controls. Remarkably however, Sgpl1(-/-) chimeras displayed mild monocytosis, due to impeded stromal retention and myelopoiesis, and plasma cytokine and macrophage expression patterns, that were largely compatible with classical macrophage activation. Collectively these two phenotypic features of Sgpl1 deficiency culminated in diminished atherogenic response.

CONCLUSIONS

Here we not only firmly establish the critical role of hematopoietic S1P lyase in controlling S1P levels and T cell trafficking in blood and lymphoid tissue, but also identify leukocyte Sgpl1 as critical factor in monocyte macrophage differentiation and function. Its, partly counterbalancing, pro- and anti-inflammatory activity spectrum imply that intervention in S1P lyase function in inflammatory disorders such as atherosclerosis should be considered with caution.

Farnesoid X receptor activation increases cholesteryl ester transfer protein expression in humans and transgenic mice

Cholesteryl ester transfer protein (CETP) activity results in a proatherogenic lipoprotein profile. In cholestatic conditions, farnesoid X receptor (FXR) signaling by bile acids (BA) is activated and plasma HDL cholesterol (HDL-C) levels are low. This study tested the hypothesis that FXR-mediated induction of CETP contributes to this phenotype. Patients with cholestasis and high plasma BA had lower HDL-C levels and higher plasma CETP activity and mass compared with matched controls with low plasma BA (each P < 0.01). BA feeding in APOE3*Leiden transgenic mice expressing the human CETP transgene controlled by its endogenous promoter increased cholesterol within apoB-containing lipoproteins and decreased HDL-C (each P < 0.01), while hepatic CETP mRNA expression and plasma CETP activity and mass increased (each P < 0.01). In vitro studies confirmed that FXR agonists substantially augmented CETP mRNA expression in hepatocytes and macrophages dependent on functional FXR expression (each P < 0.001). These transcriptional effects are likely mediated by an ER8 FXR response element (FXRE) in the first intron. In conclusion, using a translational approach, this study identifies CETP as novel FXR target gene. By increasing CETP expression, FXR activation leads to a proatherogenic lipoprotein profile. These results have clinical relevance, especially when considering FXR agonists as emerging treatment strategy for metabolic disease and atherosclerosis.

Type I diabetes mellitus decreases in vivo macrophage-to-feces reverse cholesterol transport despite increased biliary sterol secretion in mice

Type I diabetes mellitus (T1DM) increases atherosclerotic cardiovascular disease; however, the underlying pathophysiology is still incompletely understood. We investigated whether experimental T1DM impacts HDL-mediated reverse cholesterol transport (RCT). C57BL/6J mice with alloxan-induced T1DM had higher plasma cholesterol levels (P < 0.05), particularly within HDL, and increased hepatic cholesterol content (P < 0.001). T1DM resulted in increased bile flow (2.1-fold; P < 0.05) and biliary secretion of bile acids (BA, 10.5-fold; P < 0.001), phospholipids (4.5-fold; P < 0.001), and cholesterol (5.5-fold; P < 0.05). Hepatic cholesterol synthesis was unaltered, whereas BA synthesis was increased in T1DM (P < 0.001). Mass fecal BA output was significantly higher in T1DM mice (1.5-fold; P < 0.05), fecal neutral sterol excretion did not change due to increased intestinal cholesterol absorption (2.1-fold; P < 0.05). Overall in vivo macrophage-to-feces RCT, using [(3)H]cholesterol-loaded primary mouse macrophage foam cells, was 20% lower in T1DM (P < 0.05), mainly due to reduced tracer excretion within BA (P < 0.05). In vitro experiments revealed unchanged cholesterol efflux toward T1DM HDL, whereas scavenger receptor class BI-mediated selective uptake from T1DM HDL was lower in vitro and in vivo (HDL kinetic experiments) (P < 0.05), conceivably due to increased glycation of HDL-associated proteins (+65%, P < 0.01). In summary, despite higher mass biliary sterol secretion T1DM impairs macrophage-to-feces RCT, mainly by decreasing hepatic selective uptake, a mechanism conceivably contributing to increased cardiovascular disease in T1DM.

Regulation of microsomal triglyceride transfer protein

Microsomal triglyceride transfer protein (MTP) facilitates the transport of dietary and endogenous fat by the intestine and liver by assisting in the assembly and secretion of triglyceride-rich apolipoprotein B-containing lipoproteins. Higher concentrations of apolipoprotein B lipoproteins predispose individuals to various cardiovascular and metabolic diseases such as atherosclerosis, diabetes, obesity and the metabolic syndrome. These can potentially be avoided by reducing MTP activity. In this article, we discuss regulation of MTP during development, cellular differentiation and diurnal variation. Furthermore, we focus on the regulation of MTP that occurs at transcriptional, post-transcriptional and post-translational levels. Transcriptional regulation of MTP depends on a few highly conserved cis-elements in the promoter. Several transcription factors that bind to these elements and either increase or decrease MTP expression have been identified. Additionally, MTP is regulated by macronutrients, hormones and other factors. This article will address the many ways in which MTP is regulated and advance the idea that reducing MTP levels, rather than its inhibition, might be an option to lower plasma lipids.

Biliary sterol secretion is required for functional in vivo reverse cholesterol transport in mice

BACKGROUND & AIMS

High-density lipoproteins (HDLs) protect against atherosclerotic cardiovascular disease, mainly by promoting reverse cholesterol transport (RCT). Biliary sterol secretion supposedly represents the final step in RCT, but the relevance of this pathway has not been explored. We tested the dependency of RCT on functional biliary sterol secretion.

METHODS

Macrophage-to-feces RCT was studied in mice with abolished (bile duct ligation) or decreased biliary sterol secretion (adenosine triphosphate binding cassette transporter B4 (Abcb4)-/- mice, with and without administration of a liver X receptor [LXR] agonist) after intraperitoneal injection of (3)H-cholesterol-loaded primary macrophage foam cells from mice. Fecal tracer excretion and also fecal mass sterol excretion were measured. Metabolism and tissue uptake of HDL cholesteryl ester was assessed with HDL kinetic studies.

RESULTS

Bile-duct ligation completely abolished RCT from (3)H-cholesterol-loaded macrophages to feces (P < .001). In Abcb4-/- mice lacking biliary cholesterol secretion, RCT was decreased markedly; fecal (3)H-tracer excretion was almost absent within neutral sterols (P < .001) and reduced within bile acids (P < .05). LXR activation stimulated RCT in wild-type (5.5-fold; P < .001) but not Abcb4-/- mice, whereas mass fecal sterol excretion increased similarly in both models (P < .05). Kinetic studies revealed minimal uptake of HDL cholesteryl ester by the intestine, which decreased on LXR activation (P < .05).

CONCLUSIONS

Functional RCT depends on biliary sterol secretion; there is no compensatory increase in RCT via bile acids. The stimulating effect of LXR agonists on RCT requires biliary cholesterol secretion. These results have implications for therapies against atherosclerotic cardiovascular disease targeting the RCT pathway.

Overexpression of apolipoprotein O does not impact on plasma HDL levels or functionality in human apolipoprotein A-I transgenic mice

Apolipoprotein (apo) O is a newly discovered apolipoprotein preferentially contained within HDL; however, currently, no data are available on the (patho)physiological effects of apoO. Therefore, the present study assessed the impact of apoO overexpression on (i) plasma lipids and lipoproteins as well as on (ii) HDL functionality. Human apoO was overexpressed by means of recombinant adenovirus (AdhapoO) in human apoA-I transgenic mice, a humanized mouse model of HDL metabolism. AdhapoO substantially increased apoO in plasma and within HDL. However, plasma triglycerides, phospholipids, total cholesterol and HDL cholesterol did not change. HDL size distribution, lipid composition and the apoA-I and the apoO distribution over the different HDL fractions separated by FPLC remained unaltered. Furthermore, enrichment of HDL with apoO did not impact on HDL functionality assessed in four independent ways, namely (i) stimulation of cholesterol efflux from macrophage foam cells, (ii) protection against LDL oxidation, (iii) anti-inflammatory activity on endothelial cells, and (iv) induction of vasodilation in isolated aortic rings ex vivo as a measure of stimulating vascular NO production. These results demonstrate that although overexpression of apoO results in a substantial enrichment of HDL particles with this novel apolipoprotein, apoO does not impact the plasma lipoprotein profile or HDL functionality.

P2Y13 receptor is critical for reverse cholesterol transport

A major atheroprotective functionality of high-density lipoproteins (HDLs) is to promote "reverse cholesterol transport" (RCT). In this process, HDLs mediate the efflux and transport of cholesterol from peripheral cells and its subsequent transport to the liver for further metabolism and biliary excretion. We have previously demonstrated in cultured hepatocytes that P2Y(13) (purinergic receptor P2Y, G protein-coupled, 13) activation is essential for HDL uptake but the potential of P2Y(13) as a target to promote RCT has not been documented. Here, we show that P2Y(13)-deficient mice exhibited a decrease in hepatic HDL cholesterol uptake, hepatic cholesterol content, and biliary cholesterol output, although their plasma HDL and other lipid levels were normal. These changes translated into a substantial decrease in the rate of macrophage-to-feces RCT. Therefore, hallmark features of RCT are impaired in P2Y(13)-deficient mice. Furthermore, cangrelor, a partial agonist of P2Y(13), stimulated hepatic HDL uptake and biliary lipid secretions in normal mice and in mice with a targeted deletion of scavenger receptor class B type I (SR-BI) in liver (hypomSR-BI-knockout(liver)) but had no effect in P2Y(13) knockout mice, which indicate that P2Y(13)-mediated HDL uptake pathway is independent of SR-BI-mediated HDL selective cholesteryl ester uptake.

CONCLUSION

These results establish P2Y(13) as an attractive novel target for modulating RCT and support the emerging view that steady-state plasma HDL levels do not necessarily reflect the capacity of HDL to promote RCT.

Voluntary exercise increases cholesterol efflux but not macrophage reverse cholesterol transport in vivo in mice

Physical exercise beneficially impacts on the plasma lipoprotein profile as well as on the incidence of cardiovascular events and is therefore recommended in primary and secondary prevention strategies against atherosclerotic cardiovascular disease. However, the underlying mechanisms of the protective effect of exercise remain largely unknown. Therefore, the present study tested the hypothesis that voluntary exercise in mice impacts on cholesterol efflux and in vivo reverse cholesterol transport (RCT). After two weeks of voluntary wheel running (average 10.1 ± 1.4 km/day) plasma triglycerides were lower (p < 0.05), while otherwise lipid and lipoprotein levels did not change. Macrophage cholesterol efflux towards plasma was significantly increased in running (n = 8) compared to sedentary (n = 6) mice (14.93 ± 1.40 vs. 12.33 ± 2.60%, p < 0.05). In addition, fecal excretion of bile acids (3.86 ± 0.50 vs. 2.90 ± 0.51 nmol/d, p = 0.001) and neutral sterols (2.75 ± 0.43 vs. 1.94 ± 0.22 nmol/d, p < 0.01) was significantly higher in running mice. However, RCT from macrophages to feces remained essentially unchanged in running mice compared with sedentary controls (bile acids: 3.2 ± 1.0 vs. 2.9 ± 1.1 % of injected dose, n.s.; neutral sterols: 1.4 ± 0.7 vs. 1.1 ± 0.5 % injected dose, n.s.). Judged by the plasma lathosterol to cholesterol ratio, endogenous cholesterol synthesis was increased in exercising mice (0.15 ± 0.03 vs. 0.11 ± 0.02, p < 0.05), while the hepatic mRNA expression of key transporters for biliary cholesterol (Abcg5/g8, Sr-bI) as well as bile acid (Abcb11) and phospholipd (Abcb4) excretion did not change. These data indicate that the beneficial effects of exercise on cardiovascular health include increased cholesterol efflux, but do not extend to other components of RCT. The increased fecal cholesterol excretion observed in running mice is likely explained by higher endogenous cholesterol synthesis, however, it does not reflect increased RCT in the face of unchanged expression of key transporters for biliary sterol secretion.

Myeloperoxidase and serum amyloid A contribute to impaired in vivo reverse cholesterol transport during the acute phase response but not group IIA secretory phospholipase A(2)

Atherosclerosis is linked to inflammation. HDL protects against atherosclerotic cardiovascular disease, mainly by mediating cholesterol efflux and reverse cholesterol transport (RCT). The present study aimed to test the impact of acute inflammation as well as selected acute phase proteins on RCT with a macrophage-to-feces in vivo RCT assay using intraperitoneal administration of [(3)H]cholesterol-labeled macrophage foam cells. In patients with acute sepsis, cholesterol efflux toward plasma and HDL were significantly decreased (P < 0.001). In mice, acute inflammation (75 microg/mouse lipopolysaccharide) decreased [(3)H]cholesterol appearance in plasma (P < 0.05) and tracer excretion into feces both within bile acids (-84%) and neutral sterols (-79%, each P < 0.001). In the absence of systemic inflammation, overexpression of serum amyloid A (SAA, adenovirus) reduced overall RCT (P < 0.05), whereas secretory phospholipase A(2) (sPLA(2), transgenic mice) had no effect. Myeloperoxidase injection reduced tracer appearance in plasma (P < 0.05) as well as RCT (-36%, P < 0.05). Hepatic expression of bile acid synthesis genes (P < 0.01) and transporters mediating biliary sterol excretion (P < 0.01) was decreased by inflammation. In conclusion, our data demonstrate that acute inflammation impairs cholesterol efflux in patients and macrophage-to-feces RCT in vivo in mice. Myeloperoxidase and SAA contribute to a certain extent to reduced RCT during inflammation but not sPLA(2). However, reduced bile acid formation and decreased biliary sterol excretion might represent major contributing factors to decreased RCT in inflammation.

Scavenger receptor class B type I mediates biliary cholesterol secretion independent of ATP-binding cassette transporter g5/g8 in mice

Scavenger receptor class B type I (SR-BI) mediates selective uptake of cholesterol from high-density lipoprotein (HDL) particles by the liver and influences biliary cholesterol secretion. However, it is not clear, if this effect is direct or indirect. The aim of this study was to determine the impact of SR-BI on biliary cholesterol secretion, especially in a functional context with ATP-binding cassette transporter g5 (Abcg5)/Abcg8 and Abcb4. SR-BI was overexpressed by means of adenovirus (AdSR-BI) in livers of wild-type, liver X receptor-null (Lxr(-/-)), Abcg5(-/-), and Abcb4(-/-) mice. Consistent with previous reports, AdSR-BI decreased plasma HDL cholesterol levels in all models (P < 0.001). Hepatic cholesterol content increased (at least P < 0.05), whereas expression of sterol regulatory element binding protein 2 target genes was decreased (at least P < 0.05,) and established Lxr target genes were unaltered. Biliary cholesterol secretion was increased by AdSR-BI in wild-type as well as in Lxr(-/-) and Abcg5(-/-) mice, and considerably less in Abcb4(-/-) mice (each P < 0.001), independent of bile acid and phospholipid secretion. Immunogold electron microscopy and western blot showed a substantial increase of SR-BI protein localized to basolateral and canalicular membranes in response to SR-BI overexpression. Subcellular fractionation revealed a significantly higher cholesterol content of canalicular membranes (P < 0.001) upon SR-BI overexpression. Inhibition of microtubule function did not affect SR-BI-mediated biliary cholesterol secretion, indicating that transcytosis pathways are not involved.

CONCLUSION

Our data indicate that SR-BI mediates biliary cholesterol secretion independent of Abcg5, yet largely depends on Abcb4-mediated phospholipid secretion and mixed micelles as acceptors in bile. SR-BI-mediated biliary cholesterol secretion has a high capacity, can compensate for the absence of Abcg5, and does not require transcytosis pathways.

Scavenger receptor BI facilitates hepatic very low density lipoprotein production in mice

Scavenger receptor BI (SR-BI) is a selective uptake receptor for HDL cholesterol but is also involved in the catabolism of apolipoprotein (apo)B-containing lipoproteins. However, plasma levels of apoB-containing lipoproteins increase following hepatic SR-BI overexpression, suggesting that SR-BI not solely mediates their catabolism. We therefore tested the hypothesis that hepatic SR-BI impacts on VLDL production. On day 7 following adenovirus (Ad)-mediated overexpression of SR-BI, VLDL-triglyceride and VLDL-apoB production rates were significantly increased (P < 0.001), whereas VLDL production was significantly lower in SR-BI knockout mice compared with controls (P < 0.05). In mice injected with AdSR-BI, hepatic cholesterol content increased (P < 0.001), microsomal triglyceride transfer protein activity was higher (P < 0.01) and expression of sterol-regulatory element binding protein (SREBP)2 and its target genes was decreased (P < 0.01). Conversely, in SR-BI knockout mice, microsomal triglyceride transfer protein activity was lower and expression of SREBP2 target genes was increased (P < 0.01). Finally, we demonstrate in vitro in isolated primary hepatocytes as well as in vivo that cholesterol derived from HDL and taken up via SR-BI into the liver can be resecreted within VLDL. These data indicate that hepatic SR-BI expression is linked to VLDL production, and within liver, a metabolic shunt might exist that delivers HDL cholesterol, at least in part, to a pool from which cholesterol is mobilized for VLDL production. These results might have implications for HDL-based therapies against atherosclerotic cardiovascular disease, especially with SR-BI as target.

Hepatic SR-BI, not endothelial lipase, expression determines biliary cholesterol secretion in mice

High density lipoprotein cholesterol is thought to represent a preferred source of sterols secreted into bile following hepatic uptake by scavenger receptor class B type I (SR-BI). The present study aimed to determine the metabolic effects of an endothelial lipase (EL)-mediated stimulation of HDL cholesterol uptake on liver lipid metabolism and biliary cholesterol secretion in wild-type, SR-BI knockout, and SR-BI overexpressing mice. In each model, injection of an EL expressing adenovirus decreased plasma HDL cholesterol (P < 0.001) whereas hepatic cholesterol content increased (P < 0.05), translating into decreased expression of sterol-regulatory element binding protein 2 (SREBP2) and its target genes HMG-CoA reductase and LDL receptor (each P < 0.01). Biliary cholesterol secretion was dependent on hepatic SR-BI expression, being decreased in SR-BI knockouts (P < 0.001) and increased following hepatic SR-BI overexpression (P < 0.001). However, in each model, biliary secretion of cholesterol, bile acids, and phospholipids as well as fecal bile acid and neutral sterol content, remained unchanged in response to EL overexpression. Importantly, hepatic ABCG5/G8 expression did not correlate with biliary cholesterol secretion rates under these conditions. These results demonstrate that an acute decrease of plasma HDL cholesterol levels by overexpressing EL increases hepatic cholesterol content but leaves biliary sterol secretion unaltered. Instead, biliary cholesterol secretion rates are related to the hepatic expression level of SR-BI. These data stress the importance of SR-BI for biliary cholesterol secretion and might have relevance for concepts of reverse cholesterol transport.

Scavenger receptor BI-mediated selective uptake is required for the remodeling of high density lipoprotein by endothelial lipase

Endothelial lipase (EL) is a negative regulator of high density lipoprotein (HDL) cholesterol plasma levels, and scavenger receptor BI (SR-BI) is involved in remodeling of HDL. The present study investigates the requirement of SR-BI for the effects of EL-mediated phospholipid hydrolysis on HDL metabolism in vivo. In vitro, selective uptake from EL-modified HDL was 129% higher than selective uptake from control HDL in SR-BI-overexpressing cells (p=0.01). In vivo overexpression of human EL by means of recombinant adenovirus decreased HDL plasma levels significantly (p<0.01). Fast protein liquid chromatography analysis and agarose gel electrophoresis revealed that EL expression resulted in the generation of small pre-beta HDL particles in wild-type mice, whereas in SR-BI-/- mice small HDL were preferentially removed. In kinetic experiments the fractional catabolic rate (FCR) of HDL cholesteryl ester increased by 110% (p<0.001), and the FCR of HDL apolipoproteins increased by 64% (p<0.001) in response to EL overexpression in wild-type mice. In SR-BI-/- mice a similar increase in the HDL apolipoprotein FCR occurred (p<0.001); however, there was no further increase in HDL cholesteryl ester catabolism. The apparent whole body selective uptake was increased 3-fold by EL in wild-type mice (p<0.001), whereas there was no selective uptake in SR-BI knock-out mice. EL overexpression increased hepatic selective uptake as well as holoparticle uptake (each p<0.01) in wild-type mice, whereas in SR-BI knock-out mice only holoparticle uptake increased (p<0.01). Our results indicate that SR-BI-mediated selective uptake of HDL cholesteryl ester is essential for the remodeling of large alpha-migrating HDL particles by EL.

Secretory phospholipase A2 increases SR-BI-mediated selective uptake from HDL but not biliary cholesterol secretion

High density lipoprotein cholesterol represents a major source of biliary cholesterol. Secretory phospholipase A2 (sPLA2) is an acute phase enzyme mediating decreased plasma HDL cholesterol levels. Clinical studies reported a link between increased sPLA2 expression and the presence of cholesterol gallstones. The aim of our study was to investigate whether the overexpression of human sPLA2 in transgenic mice affects biliary cholesterol secretion and gallstone formation. Liver weight (P < 0.01) and hepatic cholesterol content (P < 0.01) were significantly increased in sPLA2 transgenic mice compared with controls as a result of increased scavenger receptor class B type I (SR-BI)-mediated hepatic selective uptake of HDL cholesterol (P < 0.01), whereas hepatic SR-BI expression remained unchanged. However, biliary cholesterol secretion as well as fecal neutral sterol and fecal bile salt excretion remained unchanged in sPLA2 transgenic mice. Furthermore, gallstone prevalence in response to a lithogenic diet was identical in both groups. These data demonstrate that i) increased flux of cholesterol from HDL into the liver via SR-BI as a result of phospholipase modification of the HDL particle translates neither into increased biliary and fecal sterol output nor into increased gallstone formation, and ii) increased sPLA2 expression in patients with cholesterol gallstones might be a consequence rather than the underlying cause of the disease.

Tyrosine phosphorylation controls cortactin binding to two enterohaemorrhagic Escherichia coli effectors: Tir and EspFu/TccP

Enterohaemorrhagic Escherichia coli (EHEC) is an important food-borne pathogen that, upon infection, causes destruction of the microvilli brush border of intestinal cells. EHEC is able to recruit several host cell proteins and induce actin accumulation beneath its adherence site, forming a pedestal-like structure upon which the bacterium is firmly attached. Injection of bacterial effectors into the host cells is required to trigger the recruitment and activation of proteins, such as cortactin, neural Wiskott-Aldrich syndrome protein (N-WASP) and Arp2/3 complex, directly involved in the actin polymerization process. We found that cortactin, an actin-binding protein, has a pivotal role during pedestal formation by EHEC. Cortactin was found to bind directly to two important virulence factors of EHEC, Tir and EspF(u), which are translocated into the host cells during infection. Binding of cortactin to these effectors is dependent upon tyrosine phosphorylation and a balance between tyrosine phosphorylation and dephosphorylation of cortactin is required to regulate pedestal formation by EHEC.

Cortactin is essential for F-actin assembly in enteropathogenic Escherichia coli (EPEC)- and enterohaemorrhagic E. coli (EHEC)-induced pedestals and the alpha-helical region is involved in the localization of cortactin to bacterial attachment sites

Enteropathogenic Escherichia coli (EPEC) and enterohaemorrhagic E. coli (EHEC) are important human pathogens. Upon attachment to host cells, EPEC and EHEC are able to induce actin polymerization, which accumulates, forming a pedestal-like structure beneath the attached bacteria. Using siRNA, we show here that EPEC- and EHEC-induced pedestals are dependent on cortactin, an F-actin-binding protein found in the mammalian cell cortex. Knock-down of cortactin by siRNA resulted in a dramatic reduction of the pedestal formation induced by both pathogens. We also show that disruption of the Src homology 3 (SH3) domain of cortactin, or its downregulation by specific point mutations, negatively affects pedestal formation, suggesting that this domain is important for regulation of F-actin assembly by EPEC and EHEC. Green fluorescent protein (GFP) fused with the SH3 domain (GFP-SH3), proline-rich region (GFP-PRR) or alpha-helical region of cortactin markedly reduced the amount of F-actin at the bacterial attachment sites. Interestingly, neither GFP-SH3 nor GFP-PRR was recruited to the vicinity of the bacterial adherence sites; however, GFP fused to the alpha-helical region was efficiently recruited and colocalized with the attached bacteria. These results demonstrate that cortactin is a requirement for pedestal formation and suggest a novel function for the predicted alpha-helical region of cortactin in actin assembly induced by EPEC and EHEC.

Covalently dimerized SecA is functional in protein translocation

The ATPase SecA provides the driving force for the transport of secretory proteins across the cytoplasmic membrane of Escherichia coli. SecA exists as a dimer in solution, but the exact oligomeric state of SecA during membrane binding and preprotein translocation is a topic of debate. To study the requirements of oligomeric changes in SecA during protein translocation, a non-dissociable SecA dimer was formed by oxidation of the carboxyl-terminal cysteines. The cross-linked SecA dimer interacts with the SecYEG complex with a similar stoichiometry as non-cross-linked SecA. Cross-linking reversibly disrupts the SecB binding site on SecA. However, in the absence of SecB, the activity of the disulfide-bonded SecA dimer is indistinguishable from wild-type SecA. Moreover, SecYEG binding stabilizes a cold sodium dodecylsulfate-resistant dimeric state of SecA. The results demonstrate that dissociation of the SecA dimer is not an essential feature of the protein translocation reaction.