Vi Pham

SystemC has gained popularity as a modeling language in the design of highly complex, heterogeneous, and large concurrent systems. Efficient and accurate simulation of the SystemC designs has become increasingly important. In this paper, we analyze the synchronization dependencies of concurrent systems modeled in the SystemC environment, where SystemC models are simulated through a discrete event simulation kernel that schedules events at runtime. We discuss different possibilities of SystemC communication constructs that may lead to deadlocks. We create a framework for system level designers to detect the deadlocks as soon as they occur, thus avoiding more complicated scenarios later in the simulation. This is accomplished by extension of the SystemC simulation kernel to build a dynamic dependency graph incrementally at runtime, and then applying an incremental deadlock detection algorithm to the graph. We demonstrate our approach through the well established dining philosopher problem and two real world designs, CS6100 JPEG encoder and MPEG-2 decoder. The overhead on the overall simulation is shown to be insignificant

Insulin-regulated aminopeptidase (IRAP), a marker of glucose transporter 4 (GLUT4) storage vesicles (GSVs), is the only protein known to traffic with GLUT4. In the basal state, GSVs are sequestered from the constitutively recycling endosomal system to an insulin-responsive, intracellular pool. Insulin induces a rapid translocation of GSVs to the cell surface from this pool, resulting in the incorporation of IRAP and GLUT4 into the plasma membrane. We sought to identify proteins that interact with IRAP to further understand this GSV trafficking process. This study describes our identification of a novel interaction between the amino terminus of IRAP and the Akt substrate, AS160 (Akt substrate of 160 kDa). The validity of this interaction was confirmed by coimmunoprecipitation of both overexpressed and endogenous proteins. Moreover, confocal microscopy demonstrated colocalization of these proteins. In addition, we demonstrate that the IRAP-binding domain of AS160 falls within its second phosphotyrosine-binding domain and the interaction is not regulated by AS160 phosphorylation. We hypothesize that AS160 is localized to GLUT4-containing vesicles via its interaction with IRAP where it inhibits the activity of Rab substrates in its vicinity, effectively tethering the vesicles intracellularly.

The physiological importance of the insulin responsive glucose transporter GLUT4 in adipocytes and muscle in maintaining glucose homeostasis is well established. A key protein associated with this process is the aminopeptidase IRAP which co-localizes with GLUT4 in specialized vesicles, where it plays a tethering role. In this study, we investigated the distribution of both GLUT4 and IRAP in the kidney to gain insights into the potential roles of these proteins in this organ. Both IRAP and GLUT4 immunostaining was observed in the epithelial cells of the proximal and distal tubules and thick ascending limbs in the cortex, but very little overlap between GLUT4 and IRAP immunoreactivity was observed. GLUT4 staining was consistent with a vesicular localization, whereas IRAP staining was predominantly on the luminal surface. In the principal cells of the inner medulla collecting duct (IMCD), IRAP immunoreactivity was detected throughout the cell, with limited overlap with the vasopressin responsive water channel aquaporin-2 (AQP-2). AQP-2 levels were observed to be two-fold higher in IRAP knockout mice. Based on our results, we propose that GLUT4 plays a role in shunting glucose across epithelial cells. In the kidney cortex, IRAP, in concert with other peptidases, may be important in the generation of free amino acids for uptake, whereas in the principal cells of the inner medulla IRAP may play a localized role in the regulation of vasopressin bioactivity.

The AT(4) ligands, angiotensin IV and LVV-hemorphin 7, elicit robust effects on facilitating memory by binding to a specific site in the brain historically termed the angiotensin AT(4) receptor. The identification of the AT(4) receptor as insulin-regulated aminopeptidase (IRAP) is controversial, with other proteins speculated to be the target(s) of these peptides. In this study we have utilized IRAP knockout mice to investigate IRAP in the brain. We demonstrate that the high-affinity binding site for angiotensin IV is absent in IRAP knockout mice brain sections in parallel with the loss of IRAP immunostaining, providing irrefutable proof that IRAP is the specific high-affinity binding site for AT(4) ligands. However, our characterization of the behavioural phenotype of the IRAP knockout mice revealed a totally unexpected finding. In contrast to the acute effects of IRAP inhibitors in enhancing memory, deletion of the IRAP gene resulted in mice with an accelerated, age-related decline in spatial memory that was only detected in the Y maze paradigm. Moreover, no alterations in behaviour of the IRAP knockout mice were observed that could assist in elucidating the endogenous substrate(s). Our results highlight the importance of analysing the behavioural phenotype of knockout mice across different ages and in distinct memory paradigms.

A number of studies have suggested that angiotensin IV is able to mediate a range of signalling events through a receptor distinct to the well-characterised angiotensin AT1 and AT2 receptors. This receptor was termed the AT4 receptor, but was subsequently identified to be the transmembrane enzyme, insulin regulated aminopeptidase, IRAP. Using HEK293T cells transfected with IRAP we investigated whether angiotensin IV was able to mediate signalling events via this aminopeptidase. No effect of the angiotensin IV analogue, Nle1-Ang IV, on intracellular calcium or ERK phosphorylation was observed. In addition, the effect of Nle1-Ang IV on IRAP internalization was investigated and, in contrast to classical ligand-mediated receptor endocytosis, Nle1-Ang IV (10−6 M) extends the half-life of IRAP at the plasma membrane. Our results do not support a direct role for Ang IV signalling via IRAP in this system.

The family of G protein-coupled receptors constitutes about 50% of the therapeutic drug targets used in clinical medicine today, although the mechanisms of ligand binding, activation and signal transduction for G protein-coupled receptors are not yet well defined. This review discusses ongoing research using the photoaffinity scanning method to map the bimolecular interface between class II G protein-coupled receptors and their ligands. Furthermore the available computer model of class II peptide ligand docking into the receptor, based on the positional constraints imposed by the photoaffinity scanning analyses, will be discussed briefly. The ultimate goal of these efforts is to understand the molecular basis of receptor binding and therefore to generate a template for rational drug design. Copyright © 2003 European Peptide Society and John Wiley & Sons, Ltd.

Understanding the molecular mechanism underlying how the peptide ligands bind to their receptors with subsequent receptor activation and cellular response is of great long-term value in designing receptor-targeted drugs. This is more difficult for class-II G protein-coupled receptors as only minimal structural data is available and their natural peptide ligands contain a large and diffuse pharmacophore. To address this problem, photoaffinity labeling studies have been developed to identify the spatial proximity between the photophore-modified ligand and its receptor. This minireview looks at the application of this approach in determining the proximal sites between class-II G protein-coupled receptor peptide ligands and their corresponding receptors, including parathyroid hormone, secretin and vasoactive intestinal polypeptide. More specifically, we will highlight interaction sites between positions 19, 16 and 26 of calcitonin with C134–K141, and F137 and T30 of the receptor, respectively.

Understanding the molecular mechanism underlying how the peptide ligands bind to their receptors with subsequent receptor activation and cellular response is of great long-term value in designing receptor-targeted drugs. This is more difficult for class-II G protein-coupled receptors as only minimal structural data is available and their natural peptide ligands contain a large and diffuse pharmacophore. To address this problem, photoaffinity labeling studies have been developed to identify the spatial proximity between the photophore-modified ligand and its receptor. This minireview looks at the application of this approach in determining the proximal sites between class-II G protein-coupled receptor peptide ligands and their corresponding receptors, including parathyroid hormone, secretin and vasoactive intestinal polypeptide. More specifically, we will highlight interaction sites between positions 19, 16 and 26 of calcitonin with C134−K141, and F137 and T30 of the receptor, respectively.