PICK1 regulates AMPA receptor endocytosis via direct interactions with AP2 α-appendage and dynamin

Clathrin-mediated endocytosis (CME) is used to internalize a diverse range of cargo proteins from the cell surface, often in response to specific signals. In neurons, the rapid endocytosis of GluA2-containing AMPA receptors (AMPARs) in response to NMDA receptor (NMDAR) stimulation causes a reduction in synaptic strength and is the central mechanism for long-term depression, which underlies certain forms of learning. The mechanisms that link NMDAR activation to CME of AMPARs remain elusive. PICK1 is a BAR domain protein required for NMDAR-dependent reductions in surface GluA2; however, the molecular mechanisms involved are unclear. In this study, we show that PICK1 makes direct, NMDAR-dependent interactions with the core endocytic proteins AP2 and dynamin. PICK1-AP2 interactions are required for clustering AMPARs at endocytic zones in dendrites in response to NMDAR stimulation and for consequent AMPAR internalization. We further show that PICK1 stimulates dynamin polymerization. We propose that PICK1 is a cargo-specific endocytic accessory protein required for efficient, activity-dependent AMPAR endocytosis.

The PICK1 Ca2+ sensor modulates N-methyl-d-aspartate (NMDA) receptor-dependent microRNA-mediated translational repression in neurons

MicroRNAs (miRNAs) are important regulators of localized mRNA translation in neuronal dendrites. The presence of RNA-induced silencing complex proteins in these compartments and the dynamic miRNA expression changes that occur in response to neuronal stimulation highlight their importance in synaptic plasticity. Previously, we demonstrated a novel interaction between the major RNA-induced silencing complex component Argounaute-2 (Ago2) and the BAR (bin/amphiphysin/rvs) domain protein PICK1. PICK1 recruits Ago2 to recycling endosomes in dendrites, where it inhibits miRNA-mediated translational repression. Chemical induction of long-term depression via NMDA receptor activation causes the dissociation of Ago2 from PICK1 and a consequent increase in dendritic miRNA-mediated gene silencing. The mechanism that underlies the regulation of PICK1-Ago2 binding is unknown. In this study, we demonstrate that the PICK1-Ago2 interaction is directly sensitive to Ca²⁺ ions so that high [Ca²⁺]_free reduces PICK1 binding to Ago2. Mutating a stretch of C-terminal Ca²⁺-binding residues in PICK1 results in a complete block of NMDA-induced PICK1-Ago2 disassociation in cortical neurons. Furthermore, the same mutant also blocks NMDA-stimulated miRNA-mediated gene silencing. This study defines a novel mechanism whereby elevated [Ca²⁺] induced by NMDA receptor activation modulates Ago2 and miRNA activity via PICK1. Our work suggests a Ca²⁺-dependent process to regulate miRNA activity in neurons in response to the induction of long-term depression.

Phosphorylation of a novel cytoskeletal protein (RsmP) regulates rod-shaped morphology in Corynebacterium glutamicum

Corynebacteria grow by wall extension at the cell poles, with DivIVA being an essential protein orchestrating cell elongation and morphogenesis. DivIVA is considered a scaffolding protein able to recruit other proteins and enzymes involved in polar peptidoglycan biosynthesis. Partial depletion of DivIVA induced overexpression of cg3264, a previously uncharacterized gene that encodes a novel coiled coil-rich protein specific for corynebacteria and a few other actinomycetes. By partial depletion and overexpression of Cg3264, we demonstrated that this protein is an essential cytoskeletal element needed for maintenance of the rod-shaped morphology of Corynebacterium glutamicum, and it was therefore renamed RsmP (rod-shaped morphology protein). RsmP forms long polymers in vitro in the absence of any cofactors, thus resembling eukaryotic intermediate filaments. We also investigated whether RsmP could be regulated post-translationally by phosphorylation, like eukaryotic intermediate filaments. RsmP was phosphorylated in vitro by the PknA protein kinase and to a lesser extent by PknL. A mass spectrometric analysis indicated that phosphorylation exclusively occurred on a serine (Ser-6) and two threonine (Thr-168 and Thr-211) residues. We confirmed that mutagenesis to alanine (phosphoablative protein) totally abolished PknA-dependent phosphorylation of RsmP. Interestingly, when the three residues were converted to aspartic acid, the phosphomimetic protein accumulated at the cell poles instead of making filaments along the cell, as observed for the native or phosphoablative RsmP proteins, indicating that phosphorylation of RsmP is necessary for directing cell growth at the cell poles.

The MurC ligase essential for peptidoglycan biosynthesis is regulated by the serine/threonine protein kinase PknA in Corynebacterium glutamicum

The Mur ligases play an essential role in the biosynthesis of bacterial cell-wall peptidoglycan and thus represent attractive targets for the design of novel antibacterials. These enzymes catalyze the stepwise formation of the peptide moiety of the peptidoglycan disaccharide peptide monomer unit. MurC is responsible of the addition of the first residue (L-alanine) onto the nucleotide precursor UDP-MurNAc. Phosphorylation of proteins by Ser/Thr protein kinases has recently emerged as a major physiological mechanism of regulation in prokaryotes. Herein, the hypothesis of a phosphorylation-dependent mechanism of regulation of the MurC activity was investigated in Corynebacterium glutamicum. We showed that MurC was phosphorylated in vitro by the PknA protein kinase. An analysis of the phosphoamino acid content indicated that phosphorylation exclusively occurred on threonine residues. Six phosphoacceptor residues were identified by mass spectrometry analysis, and we confirmed that mutagenesis to alanine residues totally abolished PknA-dependent phosphorylation of MurC. In vitro and in vivo ligase activity assays showed that the catalytic activity of MurC was impaired following mutation of these threonine residues. Further in vitro assays revealed that the activity of the MurC-phosphorylated isoform was severely decreased compared with the non-phosphorylated protein. To our knowledge, this is the first demonstration of a MurC ligase phosphorylation in vitro. The finding that phosphorylation is correlated with a decrease in MurC enzymatic activity could have significant consequences in the regulation of peptidoglycan biosynthesis.

From the characterization of the four serine/threonine protein kinases (PknA/B/G/L) of Corynebacterium glutamicum toward the role of PknA and PknB in cell division

Corynebacterium glutamicum contains four serine/threonine protein kinases (STPKs) named PknA, PknB, PknG, and PknL. Here we present the first biochemical and comparative analysis of all four C. glutamicum STPKs and investigate their potential role in cell shape control and peptidoglycan synthesis during cell division. In vitro assays demonstrated that, except for PknG, all STPKs exhibited autokinase activity. We provide evidence that activation of PknG is part of a phosphorylation cascade mechanism that relies on PknA activity. Following phosphorylation by PknA, PknG could transphosphorylate its specific substrate OdhI in vitro. A mass spectrometry profiling approach was also used to identify the phosphoresidues in all four STPKs. The results indicate that the nature, number, and localization of the phosphoacceptors varies from one kinase to the other. Disruption of either pknL or pknG in C. glutamicum resulted in viable mutants presenting a typical cell morphology and growth rate. In contrast, we failed to obtain null mutants of pknA or pknB, supporting the notion that these genes are essential. Conditional mutants of pknA or pknB were therefore created, leading to partial depletion of PknA or PknB. This resulted in elongated cells, indicative of a cell division defect. Moreover, overexpression of PknA or PknB in C. glutamicum resulted in a lack of apical growth and therefore a coccoid-like morphology. These findings indicate that pknA and pknB are key players in signal transduction pathways for the regulation of the cell shape and both are essential for sustaining corynebacterial growth.

[Incessant atrial tachycardia and peripartum cardiomyopathy--a therapeutic challenge]

Peripartum cardiomyopathy is an uncommon cause of heart failure but with serious prognosis. We report the case of a patient with peripartum cardiomyopathy presenting acute heart failure (severe biventricular systolic failure) and incessant atrial tachycardia, a rare arrhythmia difficult to control, that was responsible for cardiogenic shock, fetus death and multiple organ failure: renal failure (hemodialysis during 17 days), respiratory and hepatic failure and ischemic acute cholecystitis (treated surgically). After emergency cesarean section, heart rate control was obtained only after administering verapamil. Progressive clinical improvement with total recovery of hepatic and renal functions followed under treatment with vigorous multiple organ support. Six months after referral, the patients is doing well with normal daily life controlled with conventional therapy for heart failure, in stable sinus rhythm. Echocardiography shows a dilated left ventricle with partial resolution of systolic dysfunction rhythm. Echocardiography shows a dilated left ventricle with partial resolution of systolic dysfunction. The use of verapamil in severe biventricular systolic failure is discussed.

[The digital image archive--its clinical applications and importance in cardiology consulting by telemedicine]

The revolution in digital technology is rapidly changing the world of telecommunications. Its applications to Medicine, and in particular to Cardiology, offer enormous benefits since communication is an essential part of medical practice. The need to improve the management of medical information is critical because of the explosion of medical knowledge, and the need to provide comprehensive documentation on patient care. The availability of a network offers many possibilities for clinical, research and teaching activities. Relevant, up-to-date scientific information is instantly available for analysis and interaction. The authors review the issue of digital communications as well as its potential application to Telemedicine, and present their preliminary experience with digital analysis and storage of echocardiographic images.

[Thrombolytic therapy in acute myocardial infarct--is it being used to the fullest?]

OBJECTIVE

To evaluate the experience with thrombolytic therapy (TT) in the treatment of patients with the discharge diagnosis of acute myocardial infarction (AMI).

DESIGN

Retrospective analysis of the clinical records of patients with the discharge diagnosis of AMI, between May 1988 and December 1995.

SETTING

Intensive Cardiac Care Unit (UCIM-Medicina IV) of Santa Maria University Hospital, Lisbon, Portugal.

PATIENTS

1319 patients, 958 men (73%) and 361 women, mean age 64 +/- 12 years.

MAIN RESULTS

Thrombolytic therapy was administered in 391 patients (30%). Female patients received less thrombolytic therapy compared with male (17% vs. 34%; p < 0.001). Anterior wall infarction was more frequent (38%). Patients in Killip class I-II (77%) received more thrombolytic therapy than class III-IV (33% vs. 17%). The utilization rate of thrombolytic therapy increased from 25% in 1988 to 34% in 1995. Two major categories of patients were not treated with thrombolysis: 1) patients presented more than 12 hours after pain onset (38%); 2) patients without ST segment elevation or LBBB on the ECG presented (37%). Overall hospital mortality was 15.6%. The mortality in patients receiving thrombolytic therapy was significantly lower than in those excluded (9.4% vs. 18.2%; p < 0.001).

CONCLUSION

Only 30% of patients with AMI receive TT. Of those excluded from this important therapeutic intervention, 75% had no ECG criteria or were outside the standard therapeutic "window". This significant group of patients need new diagnostic and intervention strategies.

[Dobutamine stress echocardiography in patients after an acute myocardial infarct: what is the significance of arterial patency related to the infarct?]

BACKGROUND

Dobutamine stress echocardiography is a very promising technique for assessing myocardial viability, ischemia and prognosis in patients with a recent acute myocardial infarction. Recent studies have shown that the type of perfusion in the infarct-related artery also plays a role in the prognosis of these patients.

PURPOSE

To evaluated in patients with non complicated infarction the ability of low-dose and high-dose dobutamine stress echocardiography to assess the presence of both viability and inducible ischemia and correlate the results with the patency of the infarct related artery and the accuracy to predict cardiac events during one year follow-up.

PATIENTS AND METHODS

Fifty one consecutive in-hospital patients (47 male and four female, mean age 52 +/- 11 years, range 31-75 years) with acute myocardial infarction were evaluated both by dobutamine stress echocardiography and by coronary angiography and followed-up for a mean of 12 +/- 8 months.

RESULTS

With low-doses of dobutamine, the incidence of myocardial viability was 33% and with high-doses, the incidence of residual inducible ischemia was 41%. The patency of infarct-related artery showed significant correlation with score wall motion index for viability and for myocardial ischemia. Twenty four (47%) patients suffered cardiac events. Dobutamine stress echocardiography showed a positive predictive value of 67% and a negative of 70% and identified high and low risk patients.

CONCLUSIONS

Dobutamine stress echocardiography in patients with acute myocardial infarction is a safe and well tolerated test, and provides in a single test useful information regarding viability, ischemia and prognosis.

[Stress echocardiography with dobutamine: the importance of adding atropine in inconclusive tests]

OBJECTIVES

To study the feasibility of adding atropine after dobutamine infusion in patients with known or suspected coronary artery disease not achieving 85% of predicted maximal heart rate.

PATIENTS

We studied with dobutamine stress echocardiography 219 patients (174 men e 45 women; mean age +/- SD: 58 +/- 11 years), 126 patients with a previous myocardial infarction and 93 with angor. One hundred and thirty three (61%) also performed coronariography.

METHODS

Dobutamine stress echocardiography protocol consisted of a step-wise infusion of dobutamine from 5 micrograms/kg/min to a maximal dose of 40 micrograms/kg/min or until a new or a worsening wall motion abnormality, 85% of predicted maximal heart rate or any indication for interruption. In patients not achieving any of these end points, intravenous atropine was administered up to 1 mg. Patients were divided in two groups: Group A: infusion of dobutamine alone and Group B: dobutamine plus atropine.

RESULTS

With this protocol the test was interrupted in 41 patients (19%) because of adverse effects that were mild and short lived and resolved with discontinuation of the test. After dobutamine infusion 46% of the tests were nonconclusive, the majority were on beta blocker therapy. After atropine administration there was a significant reduction of the nonconclusive studies to 20% (p < 0.001). In patients with a normal rest echocardiogram the sensitivity and specificity of the test to diagnose significant coronary artery disease was respectively 89% e 100% after atropine. The capacity of the test to detect multivessel disease was also increased after atropine, allowing to reach a sensitivity of 85% and a specificity of 90%.

CONCLUSIONS

Dobutamine stress echocardiogram supplemented with atropine is a safe and accurate method to diagnose significant coronary artery disease and to detect multivessel disease in patients with a previous myocardial infarction with rest wall motion abnormalities.

[The classic exercise test or by imaging? New applications of Doppler echocardiography]

Classic electrocardiographic exercise test was for many years the only stress modality to exercise the heart, and remains the most widely performed method for provoking ischemia. ST segment deviation on the ECG is the most often utilized marker of abnormal regional myocardial perfusion. But it has traditionally been considered to possess unsatisfactory accuracy in certain categories of patients, namely those with baseline ECG changes that preclude the diagnosis of coronary artery disease. To overcome these limitations, other stress modalities have been introduced recently, namely the radionuclide techniques like stress thallium or MIBI scintigraphy, utilizing inhomogeneous uptake of the radionuclide as a marker of abnormal regional myocardial perfusion, and represents a major improvement in diagnostic accuracy compared with exercise ECG. But these stress modalities are very expensive, and need radiation exposure. More recently stress echocardiography has been introduced and recommended by the American College of Cardiology as a valid and clinically useful technique, presenting several advantages over the radionuclide techniques: a relatively inexpensive test, provides immediate results, no radiation exposure. Because many patients cannot perform dynamic exercise, pharmacological stress testing (dipyridamole, adenosine, dobutamine) coupled with imaging techniques, in particular echocardiography has become very popular with high sensitivity and specificity for the non invasive diagnosis of coronary artery disease. The analysis of the results obtained from several studies, showed that these new stress modalities, are ready for routine clinical use.

[Doppler echocardiography in the diagnosis of mechanic complications of acute myocardial infarction]

OBJECTIVES

To evaluate the ability of bedside emergency Doppler/Echocardiographic (ECOCG/DP) studies in the diagnosis of mechanic complications during acute myocardial infarction (AMI).

DESIGN

Retrospective analysis of 44 fatal AMI cases, studied by ECOCG/DP and with diagnostic confirmation by surgery and/or necropsy.

SETTING

Patients (pts) with AMI admitted to an Intensive Care Unit of a tertiary Hospital (UCIM), Hospital de Santa Maria.

PATIENTS

44 fatal AMI cases were analysed (24 men and 20 women; mean age +/- SD: 72 +/- 9 years) and were divided in two groups according to Killip classification in Group 1 (III/IV): 35 pts and Group 2 (I/II): 9 pts.

METHODS

ECOCG/DP was performed in a routine basis at admission, using all standard views and by subcostal view when in an emergency scenario.

RESULTS

In 20 pts with bad left ventricular function (LVF) (Group 1) at admission, ECOCG/DP monitoring showed that death was due to worsening of LVF, which was confirmed by necropsy. In the other 15 pts of this group, ECOCG/DP documented the clinical diagnosis of cardiac rupture (free wall: 4 pts; papillary muscle: 4 pts; interventricular septum: 7 pts) which was confirmed by surgery and/or necropsy. In the 9 pts of Group 2, ECOCG/DP disclosed, at admission, good LVF in all. In 5 pts there was a sudden worsening clinical status, and ECOCG/DP showed a severe pericardial effusion with right chambers collapse, highly suggestive of free wall rupture also confirmed at necropsy. In the other 4 pts, ECOCG/DP showed aggravation of wall motion abnormalities and of LVF without rupture, once again in agreement with necropsy. Five clinical cases are presented for illustration of this issue.

CONCLUSION

In the 44 fatal AMI cases of our study there was complete agreement between the ECOCG/DP and necropsy studies. In AMI patients, ECOCG/DP monitoring can in a routine basis, evaluate wall motion abnormalities and LVF. In an emergency setting ECOCG/DP can diagnose all the mechanic complications with a great certainty.

[Complete interruption of the aortic arch in adults]

We describe a case of total interruption of the aortic arch (Steidele Complex) that survived without surgical operation and reached adulthood. The patient first presented with stroke. The diagnosis was made by cardiac catheterization. The presence of fixed pulmonary arterial hypertension excluded the possibility of surgical correction.

[Silent myocardial ischemia: 3 years' experience with Holter]

The authors describe their experience concerning silent myocardial ischemia diagnosed by Holter monitoring technique, and present an update review about this issue. The new concepts of stunned and hibernated myocardium are analyzed, as well as the concepts of ischemic cascade, total ischemic burden and variable residual coronary reserve. Diagnostic methods and therapeutic decisions are also reviewed.