Identification of protein networks associated with the PAK1-betaPIX-GIT1-paxillin signaling complex by mass spectrometry

The process of cell motility involves coordinate signaling events among proteins associated in interactive integrin-linked networks. Mass spectrometric analysis of immunoprecipitation-derived protein mixtures have provided efficient means of identifying proteomes. In this study, we investigate strategies to enhance the detection of interactome proteins for the known signaling module: PAK1, betaPIX, GIT1, and paxillin. Our results indicate that near-endogenous expression levels of bait protein enhances the identification of associated proteins, and that phosphatase inhibition augments the detection of specific protein interactions. Following the analysis of a large pool of spectral data, we have identified and mapped clusters of proteins that either share common interactions among the four bait proteins of interest or are exclusive to single bait proteins. Taken together, these data indicate that biochemical manipulations can enhance the ability for LC-MS/MS to identify interactome proteins, and that qualitative screening of multiple samples leads to the compilation of proteins associated with a known plexus.

Functions of G protein-coupled receptor kinase interacting protein 1 in human neuronal (NT2N) cells

OBJECT

Promotion of the repair and regeneration of damaged adult neurons is a major goal of neurological science. In this study, the effects of G protein-coupled receptor kinase interacting protein 1 (GIT1) overexpression in human neuron cells were tested in human neuronal cells by using an adenoviral vector.

METHODS

A recombinant GIT1 and enhanced green fluorescent protein (EGFP) adenoviral vector (AdGIT1) was created by using a standard viral construction procedure. Human neuronal (NT2N) cells, which had been derived from an NT2 human teratocarcinoma cell line, were used in this experiment. Immunocytochemical methods were applied to identify NT2N cells with neural features and to probe the relationship among signaling proteins. Several biological activities were assessed, including neural spine formation, cell migration, and the levels of expression of growth-associated protein-43 (GAP-43) and active Cdc42. The number of cells with spine formation and the number of migrated cells were significantly higher in the AdGIT1-treated group of NT2N cells than in untreated (control) NT2N cells or in AdEGFP-treated NT2N cells. The levels of GAP-43 and active Cdc42 expression were significantly higher in the AdGITl-treated group than that in the other two cell groups.

CONCLUSIONS

The results of this study demonstrate that GIT1 overexpression has the potential to promote neural spine formation and cell migration in human neuronal cells. At the same time, the increased level of GAP-43 in GIT1-overexpressed cells indicates that GIT1 may have the potential to improve growth and regeneration of damaged axons. The GIT1-beta-PIX-Cdc42-PAK pathway may play an important role in neuronal outgrowth.

Endothelin-1 contributes to maintenance of systemic but not portal haemodynamics in patients with early cirrhosis: a randomised controlled trial

BACKGROUND AND AIMS

Increased endothelin (ET)-1 activity may contribute to the complications of cirrhosis and portal hypertension. The aim of this study was to assess the systemic and portal haemodynamic effects of selective ET-A and ET-B receptor antagonism in patients with cirrhosis.

METHODS

Sixteen patients with cirrhosis and portal hypertension (aged 52 (1) years, Pugh score 6.2 (0.3)) underwent 24 studies with infusions of: (A) selective ET-A antagonist, BQ-123 (n = 8), at 1000 and 3000 nmol/min; (B) selective ET-B antagonist, BQ-788 (n = 8), at 100 and 300 nmol/min; or (C) matched saline placebo (n = 8) in a double blind randomised manner. Haemodynamic measurements were performed through pulmonary artery, hepatic venous, and femoral artery catheters.

RESULTS

Baseline patient characteristics were well matched. Compared with placebo, BQ-123 decreased mean arterial pressure (MAP -15 (11) mm Hg (-18%); p<0.02) and pulmonary vascular resistance index (PVRI -81 (54) dyn x s x m2/cm5 (-64%); p<0.05), with no effect on hepatic venous pressure gradient (HVPG), cardiac index (CI), or systemic vascular resistance index (SVRI). Compared with placebo, BQ-788 increased MAP (+11 (3) mm Hg (+12%); p<0.03) and SVRI (+1101 (709) dyn x s x m2/cm5 (+50%); p<0.05), reduced CI (-1.0 (0.4) l/min/m2 (-29%); p = 0.05) with no effect on HVPG or PVRI.

CONCLUSIONS

ET-1 contributes to maintenance of systemic and pulmonary haemodynamics without acutely affecting HVPG in patients with early cirrhosis. In this group of patients, the use of selective ET-A and ET-B antagonists for the management of variceal haemorrhage is likely to be limited.

Paxillin phosphorylation at Ser273 localizes a GIT1-PIX-PAK complex and regulates adhesion and protrusion dynamics

Continuous adhesion formation and disassembly (adhesion turnover) in the protrusions of migrating cells is regulated by unclear mechanisms. We show that p21-activated kinase (PAK)–induced phosphorylation of serine 273 in paxillin is a critical regulator of this turnover. Paxillin-S273 phosphorylation dramatically increases migration, protrusion, and adhesion turnover by increasing paxillin–GIT1 binding and promoting the localization of a GIT1–PIX–PAK signaling module near the leading edge. Mutants that interfere with the formation of this ternary module abrogate the effects of paxillin-S273 phosphorylation. PAK-dependent paxillin-S273 phosphorylation functions in a positive-feedback loop, as active PAK, active Rac, and myosin II activity are all downstream effectors of this turnover pathway. Finally, our studies led us to identify in highly motile cells a class of small adhesions that reside near the leading edge, turnover in 20–30 s, and resemble those seen with paxillin-S273 phosphorylation. These adhesions appear to be regulated by the GIT1–PIX–PAK module near the leading edge.

Phosphorylation of serine 709 in GIT1 regulates protrusive activity in cells

G protein-coupled receptor kinase-interacting protein (GIT)1 is a multidomain, adaptor protein that regulates cellular processes, such as migration and protrusive activity, by bringing together various signaling molecules, including PIX, PAK, and paxillin. Mutants of GIT1, which lack the C-terminal paxillin binding domain, fail to mediate its effects on migration and protrusions, suggesting that sites within this domain are critical to GIT1 function. In this study, we show that serine 709, which is located within the paxillin binding domain, regulates GIT1 function. Phosphorylation of serine 709 is necessary for GIT1-induced effects on protrusions. Phosphorylation of this site also regulates GIT1 interaction with paxillin, which could serve to target GIT1 to the leading edge of cells. As shown by an in vitro kinase assay, PAK phosphorylates GIT1 on serine 709. Taken together, our results indicate that GIT1 phosphorylation on serine 709 increases its binding to paxillin and regulates protrusive activity in cells.

Selective endothelin B receptor blockade does not influence BNP-induced natriuresis in man

Brain natriuretic peptide (BNP) and endothelin-1 (ET-1) both exhibit natriuretic activity within the human kidney. Furthermore, they both act partly through activation of the endothelial nitric oxide pathway. Since ET-1 may cause vasodilation and natriuresis via stimulation of the ET-B receptor, the aim of the present study was to investigate whether renal ET-B receptors participate in the renal actions of BNP. In this placebo-controlled, crossover study, we infused BNP (4 pmol/kg/min) or placebo (i.v.) for 1 h, with or without co-infusion of the ET-B receptor antagonist BQ-788 (50 nmol/min) for 15 min on 4 separate days, in 10 healthy subjects (mean age 54+/-6 years.). During infusion, we measured effective renal plasma flow (ERPF), and glomerular filtration rate (GFR) using PAH/inulin clearance. Cardiac output was measured before and after infusion, using echocardiography. Blood pressure and heart rate (HR) were monitored as well. Urine and plasma samples were taken every hour to measure diuresis, natriuresis, cyclic 3',5' guanosine monophosphate, and ET-1 levels. BNP with or without ET-B receptor blockade increased natriuresis and diuresis. In addition, BNP alone increased GFR and filtered load, without changing ERPF. BQ-788 infusion did not affect renal hemodynamics or natriuresis. Neither BNP nor BQ-788 altered cardiac output, blood pressure, and heart rate. In conclusion, the present study shows that selective ET-B receptor blockade has no effect on the BNP-induced natriuresis and glomerular filtration rate.

Use of antiepileptic drugs and risk of fractures: Case-control study among patients with epilepsy

OBJECTIVE

To study the association between use of antiepileptic drugs (AEDs) and risk of fractures.

METHODS

The authors obtained data from the General Practice Research Database (GPRD). A case-control study was nested within a cohort of patients with active epilepsy. Cases were patients with a first fracture after cohort entry. Up to four controls were matched to each case by practice, sex, year of birth, timing of first epilepsy diagnosis, index date, and duration of GPRD history. Cumulative exposure to AEDs was assessed by summing the duration of all AED prescriptions. A distinction was made between AEDs that induce the hepatic cytochrome P-450 enzyme system and AEDs that do not. Medical conditions and drugs known to be associated with bone metabolism or falls were evaluated as potential confounders. Conditional logistic regression analysis was used to calculate odds ratios (ORs) and 95% CIs.

RESULTS

The study population comprised 1,018 cases and 1,842 matched controls. The risk of fractures increased with cumulative duration of exposure (p for trend < 0.001), with the strongest association for greater than 12 years of use: adjusted OR 4.15 (95% CI 2.71 to 6.34). Risk estimates were higher in women than in men. There was no difference between users of AEDs that induce and AEDs that do not induce the hepatic cytochrome P-450 system.

CONCLUSIONS

Long-term use of AEDs was associated with an increased risk of fractures, especially in women. More research on mechanisms of AED-induced bone breakdown and female vulnerability to the effects of AEDs on bone health is warranted.

A GIT1/PIX/Rac/PAK signaling module regulates spine morphogenesis and synapse formation through MLC

Three of seven recently identified genes mutated in nonsyndromic mental retardation are involved in Rho family signaling. Two of the gene products, alpha-p-21-activated kinase (PAK) interacting exchange factor (alphaPIX) and PAK3, form a complex with the synaptic adaptor protein G-protein-coupled receptor kinase-interacting protein 1 (GIT1). Using an RNA interference approach, we show that GIT1 is critical for spine and synapse formation. We also show that Rac is locally activated in dendritic spines using fluorescence resonance energy transfer. This local activation of Rac is regulated by PIX, a Rac guanine nucleotide exchange factor. PAK1 and PAK3 serve as downstream effectors of Rac in regulating spine and synapse formation. Active PAK promotes the formation of spines and dendritic protrusions, which correlates with an increase in the number of excitatory synapses. These effects are dependent on the kinase activity of PAK, and PAK functions through phosphorylating myosin II regulatory light chain (MLC). Activated MLC causes an increase in dendritic spine and synapse formation, whereas inhibiting myosin ATPase activity results in decreased spine and synapse formation. Finally, both activated PAK and activated MLC can rescue the defects of GIT1 knockdown, suggesting that PAK and MLC are downstream of GIT1 in regulating spine and synapse formation. Our results point to a signaling complex, consisting of GIT1, PIX, Rac, and PAK, that plays an essential role in the regulation of dendritic spine and synapse formation and provides a potential mechanism by which alphaPIX and PAK3 mutations affect cognitive functions in mental retardation.

Methods for the detection of paxillin post-translational modifications and interacting proteins by mass spectrometry

Methods for the simultaneous identification of interacting proteins and post-translational modifications of the focal adhesion adapter protein, paxillin, are presented. The strategy includes (1) lower-level, transient transfection of FLAG-GFP-Paxillin into HEK293 cells, (2) incubation of cells with phosphatase inhibitors prior to lysis, (3) purification of paxillin by anti-FLAG immunoprecipitation, (4) analysis of peptides generated from on-beads digestion using LTQ-FT or LTQ-ETD mass spectrometry, and (5) enrichment of phosphopeptide methyl esters with IMAC. Using the above strategies, we identify 29 phosphorylation sites (19 novel and 10 previously reported) and a novel glycosylation site on Ser 74. Furthermore, with this method, we simultaneously detect 10 co-purifying proteins which are present in focal adhesion complexes. Extension of these methods to other substrates should facilitate generation of global phosphorylation maps and protein-protein interactions for any protein of interest.

Cortactin promotes cell motility by enhancing lamellipodial persistence

BACKGROUND

Lamellipodial protrusion, which is the first step in cell movement, is driven by actin assembly and requires activity of the Arp2/3 actin-nucleating complex. However, it is unclear how actin assembly is dynamically regulated to support effective cell migration.

RESULTS

Cells deficient in cortactin have impaired cell migration and invasion. Kymography analyses of live-cell imaging studies demonstrate that cortactin-knockdown cells have a selective defect in the persistence of lamellipodial protrusions. The motility and protrusion defects are fully rescued by cortactin molecules, provided both the Arp2/3 complex and F-actin binding sites are intact. Consistent with this requirement for simultaneous contacts with Arp2/3 and F-actin, cortactin is recruited by Arp2/3 complex to lamellipodia and binds with a higher affinity to ATP/ADP-Pi-F-actin than to ADP-F-actin. In situ labeling of lamellipodia revealed that the relative levels of free barbed ends of actin filaments are reduced by over 30% in the cortactin-knockdown cells; however, there is no change in Arp2/3-complex localization to lamellipodia. Cortactin-knockdown cells also have a selective defect in the assembly of new adhesions in protrusions, as assessed by analysis of GFP-paxillin dynamics in living cells.

CONCLUSIONS

Cortactin enhances lamellipodial persistence, at least in part through regulation of Arp2/3 complex. The presence of cortactin also enhances the rate of new adhesion formation in lamellipodia. In vivo, these functions may be important during directed cell motility.

Specific inhibition of the endothelin A receptor with ZD4054: clinical and pre-clinical evidence

Activation of the endothelin A receptor (ET(A)) by endothelin-1 (ET-1) mediates events that regulate mitogenesis, apoptosis, angiogenesis and metastasis in tumours. Specific blockade of ET(A) may have anticancer effects, while retaining beneficial endothelin B receptor (ET(B))-mediated effects such as apoptosis and clearance of ET-1. ZD4054 is an orally active, specific ET(A) antagonist in clinical development. In receptor-binding studies, ZD4054 specifically bound to ET(A) with high affinity; no binding was detected at ET(B). In a randomised placebo-controlled trial in eight healthy volunteers, a single oral dose of ZD4054 reduced forearm vasoconstriction in response to brachial artery infusion of ET-1, thus providing clinical evidence of ET(A) blockade. ET(B) blockade was assessed in an ascending, single-dose, placebo-controlled trial in 28 volunteers. For all doses of ZD4054, mean plasma ET-1 concentrations measured at 4 and 24 h were within the placebo reference range (a rise in ET-1 would indicate ET(B) blockade) and there was no evidence of dose-related changes. These data confirm the specificity of ZD4054 for ET(A), with no activity at ET(B) in a clinical or preclinical setting. As a result of this specificity, ZD4054 has the potential to block multiple ET(A)-induced pathological processes, while allowing beneficial ET(B)-mediated processes to continue, which may, in turn, lead to an effective cancer therapy.

Pulse transit time measured from the ECG: an unreliable marker of beat-to-beat blood pressure

The arterial pulse-wave transit time can be measured between the ECG R-wave and the finger pulse (rPTT), and has been shown previously to have a linear correlation with blood pressure (BP). We hypothesized that the relationship between rPTT, preejection period (PEP; the R-wave/mechanical cardiac delay), and BP would vary with different vasoactive drugs. Twelve healthy men (mean age 22 yr) were studied. Beat-to-beat measurements were made of rPTT (using ECG and photoplethysmograph finger probe), intra-arterial radial pressure, PEP (using cardiac bioimpedance), and transit time minus PEP (pPTT). Four drugs (glyceryl trinitrate, angiotensin II, norepinephrine, salbutamol) were administered intravenously over 15 min, with stepped dosage increase every 5 min and a 25-min saline washout between agents. All subjects in all conditions had a negative linear correlation (R2 = 0.39) between rPTT and systolic BP (SBP), generally constant between different drugs, apart from four subjects who had a positive rPTT/SBP correlation with salbutamol. The 95% limits of agreement between measured and rPTT-predicted SBP were +/-17.0 mmHg. Beat-to-beat variability of rPTT showed better coherence with SBP variability than it did with heart rate variability (P < 0.001). PEP accounted for a substantial and variable proportion of rPTT (12-35%). Diastolic (DBP) and mean arterial BP (MAP) correlated poorly with rPTT (R2 = 0.02 and 0.08, respectively) but better with pPTT (rPTT corrected for PEP, R2 = 0.41 and 0.45, respectively). The 95% limits of agreement between measured and pPTT-predicted DBP were +/- 17.3 mmHg. In conclusion, the negative correlation between rPTT and SBP is generally constant, even with marked hemodynamic perturbations. However, the relationship is not reliable enough for rPTT to be used as a surrogate marker of SBP, although it may be useful in assessing BP variability. DBP and MAP cannot be predicted from rPTT without correction for PEP. The significant contribution of PEP to rPTT means that rPTT should not be used as a marker of purely vascular function.

Direct comparison of selective endothelin A and non-selective endothelin A/B receptor blockade in chronic heart failure

OBJECTIVE

To investigate the potential differential effects of selective endothelin (ET) A and dual ET-A/B receptor blockade in patients with chronic heart failure.

METHODS

Nine patients with chronic heart failure (New York Heart Association class II-III) each received intravenous infusions of BQ-123 alone (selective ET-A blockade) and combined BQ-123 and BQ-788 (dual ET-A/B blockade) in a randomised, placebo controlled, three way crossover study.

RESULTS

Selective ET-A blockade increased cardiac output (maximum mean (SEM) 33 (12)%, p < 0.001) and reduced mean arterial pressure (maximum -13 (4)%, p < 0.001) and systemic vascular resistance (maximum -26 (8)%, p < 0.001), without changing heart rate (p = 0.38). Dual ET-A/B blockade significantly reduced the changes in all these haemodynamic variables compared with selective ET-A blockade (p < 0.05). Selective ET-A blockade reduced pulmonary artery pressure (maximum 25 (7)%, p = 0.01) and pulmonary vascular resistance (maximum 72 (39)%, p < 0.001). However, there was no difference between these effects and those seen with dual ET-A/B blockade. Unlike selective ET-A blockade, dual ET-A/B blockade increased plasma ET-1 concentrations (by 47 (4)% with low dose and 61 (8)% with high dose, both p < 0.05).

CONCLUSIONS

While there appeared to be similar reductions in pulmonary pressures with selective ET-A and dual ET-A/B blockade, selective ET-A blockade caused greater systemic vasodilatation and did not affect ET-1 clearance. In conclusion, there are significant haemodynamic differences between selective ET-A and dual ET-A/B blockade, which may determine responses in individual patients.

Cell migration: an overview

Cell migration is an essential process for normal development and homeostasis that can also contribute to important pathologies. Not surprisingly, there is considerable interest in understanding migration on a molecular level, but this is a difficult task. However, technologies are rapidly emerging to address the major intellectual challenges associated with migration. In this chapter, we outline the basics of cell migration with an emphasis on the diverse systems, methodologies, and techniques described in this book. From the contributions presented, it is apparent that the next few years should produce major advances in our understanding of cell migration.

Spatial mapping of integrin interactions and dynamics during cell migration by image correlation microscopy

Image correlation microscopy methodology was extended and used to determine retrospectively the density, dynamics and interactions of alpha5-integrin in migrating cells. Alpha5-integrin is present in submicroscopic clusters containing 3-4 integrins before it is discernibly organized. The integrin in nascent adhesions, as identified by the presence of paxillin, is approximately 1.4 times more concentrated, approximately 4.5 times more clustered and much less mobile than in surrounding regions. Thus, while integrins are clustered throughout the cell, they differ in nascent adhesions and appear to initiate adhesion formation, despite their lack of visible organization. In more mature adhesions where the integrin is visibly organized there are approximately 900 integrins microm(-2) (about fivefold higher than surrounding regions). Interestingly, alpha5-integrin and alpha-actinin, but not paxillin, reside in a complex throughout the cell, where they diffuse and flow together, even in regions where they are not organized. During adhesion disassembly some integrins diffuse away slowly, alpha-actinin undergoes a directed movement at speeds similar to actin retrograde flow (0.29 microm min(-1)), while all of the paxillin diffuses away rapidly.

Illuminating adhesion complexes in migrating cells: moving toward a bright future

Cell migration is a complex, tightly regulated process that involves the continuous formation and disassembly of adhesions. Despite the importance of these processes, very little is known about the factors that regulate adhesion dynamics during migration. Recent advances in imaging technologies are allowing monitoring of these processes during migration and are providing insight into the mechanisms that regulate them.

Extracellular nitric oxide release mediates soluble guanylate cyclase-independent vasodilator action of spermine NONOate: comparison with other nitric oxide donors in isolated rat femoral arteries

Nitric oxide (NO) and NO donors exhibit actions that are not entirely mediated by soluble guanylate cyclase (sGC). The site of NO release may influence the involvement of sGC-independent effects. Here we use spermine NONOate (SPER/NO) to release NO extracellularly, compared with other NO donors. Isolated rat femoral arteries were perfused luminally and perfusion pressure monitored. Vessels were contracted with phenylephrine (2-14 microM) in the presence of an NO synthase inhibitor (N(omega)-nitro-L-arginine methyl ester; 20 microM). Vasodilator responses to NO donors were assessed before and after perfusion of an sGC inhibitor (1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one; ODQ; 20 microM), NO scavengers (hemoglobin; Hb & hydroquinone; HQ), and a superoxide generator (duroquinone; DQ). ODQ (20 microM) abolished the vasodilator responses to glyceryl trinitrate (10(-8) - 10(-3) M), and sodium nitroprusside (10(-8) - 10(-4) M), which release NO intracellularly. ODQ (20 microM) attenuated, but failed to abolish, the vasodilator responses to SPER/NO (10(-6) - 10(-3) M). ODQ abolished responses to S-nitrosoglutathione and S-nitroso-N-valeryl-D-penicillamine (10(-8) - 10(-4) M), but a small residual vasodilatation remained in response to 10(-3) M. In the presence of ODQ, the remaining vasodilatation to SPER/NO was all but abolished by scavengers of extracellular NO (Hb; 10 microM, HQ; 100 microM). Superoxide generation (DQ; 100 microM) also attenuated ODQ-resistant vasodilatation. The data suggest that, in rat femoral arteries, NO donors that are capable of releasing extracellular NO cause vasodilatation that is only partially mediated by sGC. Lack of augmentation of sGC-independent effects by superoxide suggests that they are not mediated by peroxynitrite.

Hyperuricaemia does not impair cardiovascular function in healthy adults

OBJECTIVE

To investigate the possibility that uric acid (UA) can impair endothelial function, an important surrogate for atherosclerosis.

DESIGN

UA was administered locally or systemically to healthy adult men and women in a series of randomised placebo controlled studies. This temporarily raised serum UA concentrations, so that the potential effects of hyperuricaemia on mechanisms of cardiovascular disease could be studied.

MAIN OUTCOME MEASURES

The effects of UA administration on basal blood flow and responses to locally administered acetylcholine, sodium nitroprusside, and L-N(G)-monomethylarginine were studied in the forearm vascular bed with venous occlusion plethysmography. The effects of hyperuricaemia on systemic vascular resistance, large artery compliance, and baroreflex sensitivity were examined by validated non-invasive techniques.

RESULTS

UA administration caused a twofold increase in serum concentrations. However, there were no acute effects on haemodynamic variables, basal forearm blood flow, or nitric oxide dependent endothelial function.

CONCLUSION

Unlike other risk factors associated with endothelial dysfunction, acute exposure to high concentrations of UA does not impair cardiovascular function in healthy men. These findings do not support a causal link between hyperuricaemia and atherosclerosis.

Talin: an emerging focal point of adhesion dynamics

The adhesion protein talin and the phosphoinositide PIP2 are emerging as key modulators of adhesion dynamics. Recent genetic studies on talin demonstrate its physiological role in organizing adhesions, stabilizing integrin-actin linkages and mediating integrin signaling in vivo. Biophysical force measurements provide further evidence that it is required for the reinforcement of the extracellular matrix-integrin-actin connection. Knockdown data along with structural analyses establish a major role for talin in 'inside-out' integrin activation through its direct interaction with integrin cytoplasmic domains. A recently uncovered role for talin is the recruitment of a PIPKI gamma isoform to adhesions. This introduces a novel connection between talin and PIP2 generation. Finally, PIP2 also stimulates the transient, direct binding interaction of the Arp2/3 complex with vinculin and thus may couple adhesion to actin assembly.