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Simon A Walker

Publications and source records attributed to Simon A Walker.

7 recordsLinked to original sources
PubMed2005

The frequencies of calcium oscillations are optimized for efficient calcium-mediated activation of Ras and the ERK/MAPK cascade.

Ras proteins are binary switches that, by cycling through inactive GDP- and active GTP-bound conformations, regulate multiple cellular signaling pathways, including those that control growth and differentiation. For some time, it has been known that receptor-mediated increases in the concentration of intracellular free calcium ([Ca(2+)](i)) can modulate Ras activation. Increases in [Ca(2+)](i) often occur as repetitive Ca(2+) spikes or oscillations. Induced by electrical or receptor stimuli, these repetitive Ca(2+) oscillations increase in frequency with the amplitude of receptor stimuli, a phenomenon critical for the induction of selective cellular functions. Here, we show that Ca(2+) oscillations are optimized for Ca(2+)-mediated activation of Ras and signaling through the extracellular signal-regulated kinase (ERK)/mitogen-activated protein kinase (MAPK) cascade. We present additional evidence that Ca(2+) oscillations reduce the effective Ca(2+) threshold for the activation of Ras and that the oscillatory frequency is optimized for activation of Ras and the ERK/MAPK pathway. Our results describe a hitherto unrecognized link between complex Ca(2+) signals and the modulation of the Ras/ERK/MAPK signaling cascade.

Benzimidazoles↗
PubMed2004

P-Rex2, a new guanine-nucleotide exchange factor for Rac.

We have identified a new guanine-nucleotide exchange factor, P-Rex2, and cloned it from human skeletal muscle and brain libraries. It has widespread tissue distribution but is not expressed in neutrophils. P-Rex2 is a 183 kDa protein that activates the small GTPase Rac and is regulated by phosphatidylinositol (3,4,5)-trisphosphate and the beta gamma subunits of heterotrimeric G proteins in vitro and in vivo. P-Rex2 has structure, activity and regulatory properties similar to P-Rex1 but has divergent tissue distribution, as P-Rex1 is mainly expressed in neutrophils. Together, they form an enzyme family capable of mediating Rac signalling downstream of G protein-coupled receptors and phosphoinositide 3-kinase.

Blotting, Northern↗
PubMed2004

Visualizing Ras signalling in real-time.

Ras GTPases are universal molecular switches that act as kinetic timers of signal transduction events. They are post-translationally modified by the addition of lipid groups to their hypervariable carboxyl termini, which plug the proteins to membranes and influence their dynamic sorting and trafficking. For the past twenty years, the plasma membrane has been considered to be the predominant platform from which Ras operates. Recent work using live-cell imaging and novel probes to visualize where and when Ras is active has supported this long-held belief. However, an equally fascinating aspect of these imaging studies has been the discovery of dynamic Ras activity, as well as distinct signal output, from intracellular organelles. Activation of Ras on the Golgi exhibits kinetics different from Ras activation on the plasma membrane, and compartmentalized Ras signalling seems particularly prominent in lymphocytes. However, data on the spatial and temporal regulation of Ras activity has frequently differed depending on the nature of the probe, the cell type and the stimulus. Nevertheless, because Ras traffics through endomembranes en route to the plasma membrane, it seems likely that Ras can signal from such compartments. The burning question in this field concerns the significance of this observation for endogenous Ras signalling output.

Animals↗
PubMed2004

Identification of a Ras GTPase-activating protein regulated by receptor-mediated Ca2+ oscillations.

Receptor-mediated increases in the concentration of intracellular free calcium ([Ca2+]i) are responsible for controlling a plethora of physiological processes including gene expression, secretion, contraction, proliferation, neural signalling, and learning. Increases in [Ca2+]i often occur as repetitive Ca2+ spikes or oscillations. Induced by electrical or receptor stimuli, these repetitive Ca2+ spikes increase their frequency with the amplitude of the receptor stimuli, a phenomenon that appears critical for the induction of selective cellular functions. Here we report the characterisation of RASAL, a Ras GTPase-activating protein that senses the frequency of repetitive Ca2+ spikes by undergoing synchronous oscillatory associations with the plasma membrane. Importantly, we show that only during periods of plasma membrane association does RASAL inactivate Ras signalling. Thus, RASAL senses the frequency of complex Ca2+ signals, decoding them through a regulation of the activation state of Ras. Our data provide a hitherto unrecognised link between complex Ca2+ signals and the regulation of Ras.

Amino Acid Sequence↗
PubMed2003

Control of Ras cycling by Ca2+.

Ras GTPases are binary switches, cycling between an inactive GDP-bound form and an active GTP-bound form at the membrane. They transduce signals into the cytoplasm via effector pathways that regulate cell growth, differentiation and apoptosis. Ras activation is enhanced by guanine nucleotide exchange factors (GEFs); deactivation is accelerated by GTPase-activating proteins (GAPs). Recently, new roles for Ca(2+) and diacylglycerol (DAG) in the control of Ras cycling have emerged with the discovery of a series of novel GEFs and GAPs. These regulators of Ras cycling are likely to play a key role in the information processing of Ca(2+) and DAG signals.

Animals↗
PubMed2002

Analyzing the role of the putative inositol 1,3,4,5-tetrakisphosphate receptor GAP1IP4BP in intracellular Ca2+ homeostasis.

Inositol 1,3,4,5-tetrakisphosphate (IP(4)) has been linked to a potential role in the regulation of intracellular free Ca(2+) concentration ([Ca(2+)](i)) following cellular stimulation with agonists that activate phosphoinositide-specific phospholipase C. However, despite many studies, the function of IP(4) remains unclear and indeed there is still some debate over whether it has a function at all. Here we have used various molecular approaches to address whether manipulation of the potential IP(4) receptor, GAP1(IP4BP), affects [Ca(2+)](i) following cellular stimulation. Using single cell imaging, we show that the overexpression of a constitutively active and a potential dominant negative form of GAP1(IP4BP) appear to have no effect on Ca(2+) mobilization or Ca(2+) entry following stimulation of HeLa cells with histamine. In addition, through the use of small interfering RNA duplexes, we have examined the effect of suppressing endogenous GAP1(IP4BP) production on [Ca(2+)](i). In HeLa cells in which the endogenous level of GAP1(IP4BP) has been suppressed by approximately 95%, we failed to observe any effect on Ca(2+) mobilization or Ca(2+) entry following histamine stimulation. Thus, using various approaches to manipulate the function of endogenous GAP1(IP4BP) in intact HeLa cells, we have been unable to observe any detectable effect of GAP1(IP4BP) on [Ca(2+)](i).

Base Sequence↗