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

The etiologic origins for chronic obstructive pulmonary disease

COPD, characterized by long-term poorly irreversible airway limitation and persistent respiratory symptoms, has resulted in enormous challenges to human health worldwide, with increasing rates of prevalence, death, and disability. Although its origin was thought to be in the interactions of genetic with environmental factors, the effects of environmental factors on the disease during different life stages remain little known. Without clear mechanisms and radical cure for it, early screening and prevention of COPD seem to be important. In this review, we will discuss the etiologic origins for poor lung function and COPD caused by specific adverse effects during corresponding life stages, as well as try to find new insights and potential prevention strategies for this disease.

Maternal serum PFOA concentration and DNA methylation in cord blood: A pilot study

Perfluorooctanoic acid (PFOA), a perfluoroalkyl substance, is commonly detected in the serum of pregnant women and may impact fetal development via epigenetic re-programming. In a pilot study, we explored associations between serum PFOA concentrations during pregnancy and offspring peripheral leukocyte DNA methylation at delivery in women with high (n = 22, range: 12-26ng/mL) and low (n = 22, range: 1.1-3.1ng/mL) PFOA concentrations. After adjusting for cell type, child sex, and income, we did not find differences in CpG methylation in the two exposure groups that reached epigenome-wide significance. Among the 20 CpGs with the lowest p-values we found that seven CpG sites in three genes differed by exposure status. In a confirmatory cluster analysis, these 20 CpGs clustered into two groups that perfectly identified exposure status. Future studies with larger sample sizes should confirm these findings and determine if PFOA-associated changes in DNA methylation underlie potential health effects of PFOA.

The Ras/Rap GTPase activating protein RASA3: from gene structure to in vivo functions

RASA3 (or GTPase Activating Protein III, R-Ras GTPase-activating protein, GAP1(IP4BP)) is a GTPase activating protein of the GAP1 subfamily which targets Ras and Rap1. RASA3 was originally purified from pig platelet membranes through its intrinsic ability to bind inositol 1,3,4,5-tetrakisphosphate (I(1,3,4,5)P4) with high affinity, hence its first name GAP1(IP4BP) (for GAP1 subfamily member which binds I(1,3,4,5)P4). RASA3 was thus the first I(1,3,4,5)P4 receptor identified and cloned. The in vitro and in vivo functions of RASA3 remained somewhat elusive for a long time. However, recently, using genetically-modified mice and cells derived from these mice, the function of RASA3 during megakaryopoiesis, megakaryocyte adhesion and migration as well as integrin signaling has been reported. The goal of this review is thus to summarize and comment recent and less recent data in the literature on RASA3, in particular on the in vivo function of this specific GAP1 subfamily member.

Regulation of autophagy in cardiomyocytes by Ins(1,4,5)P(3) and IP(3)-receptors

Autophagy is a process that removes damaged proteins and organelles and is of particular importance in terminally differentiated cells such as cardiomyocytes, where it has primarily a protective role. We investigated the involvement of inositol(1,4,5)trisphosphate (Ins(1,4,5)P(3)) and its receptors in autophagic responses in neonatal rat ventricular myocytes (NRVM). Treatment with the IP(3)-receptor (IP(3)-R) antagonist 2-aminoethoxydiphenyl borate (2-APB) at 5 or 20 μmol/L resulted in an increase in autophagosome content, defined as puncta labeled by antibody to microtubule associated light chain 3 (LC3). 2-APB also increased autophagic flux, indicated by heightened LC3II accumulation, which was further enhanced by bafilomycin (10nmol/L). Expression of Ins(1,4,5)P(3) 5-phosphatase (IP(3)-5-Pase) to deplete Ins(1,4,5)P(3) also increased LC3-labeled puncta and LC3II content, suggesting that Ins(1,4,5)P(3) inhibits autophagy. The IP(3)-R can act as an inhibitory scaffold sequestering the autophagic effector, beclin-1 to its ligand binding domain (LBD). Expression of GFP-IP(3)-R-LBD inhibited autophagic signaling and furthermore, beclin-1 co-immunoprecipitated with the IP(3)-R-LBD. A mutant GFP-IP(3)-R-LBD with reduced ability to bind Ins(1,4,5)P(3) bound beclin-1 and inhibited autophagy similarly to the wild type sequence. These data provide evidence that Ins(1,4,5)P(3) and IP(3)-R act as inhibitors of autophagic responses in cardiomyocytes. By suppressing autophagy, IP(3)-R may contribute to cardiac pathology.

Defining signal transduction by inositol phosphates

Ins(1,4,5)P(3) is a classical intracellular messenger: stimulus-dependent changes in its levels elicits biological effects through its release of intracellular Ca(2+) stores. The Ins(1,4,5)P(3) response is "switched off" by its metabolism to a range of additional inositol phosphates. These metabolites have themselves come to be collectively described as a signaling "family". The validity of that latter definition is critically examined in this review. That is, we assess the strength of the hypothesis that Ins(1,4,5)P(3) metabolites are themselves "classical" signals. Put another way, what is the evidence that the biological function of a particular inositol phosphate depends upon stimulus dependent changes in its levels? In this assessment, examples of an inositol phosphate acting as a cofactor (i.e. its function is not stimulus-dependent) do not satisfy our signaling criteria. We conclude that Ins(3,4,5,6)P(4) is, to date, the only Ins(1,4,5)P(3) metabolite that has been validated to act as a second messenger.

Regulation of immune cell development through soluble inositol-1,3,4,5-tetrakisphosphate

The membrane lipid phosphatidylinositol-3,4,5-trisphosphate (PtdInsP(3)) regulates membrane receptor signalling in many cells, including immunoreceptor signalling. Here, we review recent data that have indicated essential roles for the soluble PtdInsP(3) analogue inositol-1,3,4,5-tetrakisphosphate (InsP(4)) in T cell, B cell and neutrophil development and function. Decreased InsP(4) production in leukocytes causes immunodeficiency in mice and might contribute to inflammatory vasculitis in Kawasaki disease in humans. InsP(4)-producing kinases could therefore provide attractive drug targets for inflammatory and infectious diseases.

TRPC channels and store-operated Ca(2+) entry

Store-operated calcium entry (SOCE) is a major mechanism for Ca(2+) influx. Since SOCE was first proposed two decades ago many techniques have been used in attempting to identify the nature of store-operated Ca(2+) (SOC) channels. The first identified and best-characterised store-operated current is I(CRAC), but a number of other currents activated by Ca(2+) store depletion have also been described. TRPC proteins have long been proposed as SOC channel candidates; however, whether any of the TRPCs function as SOC channels remains controversial. This review attempts to provide an overview of the arguments in favour and against the role of TRPC proteins in the store-operated mechanisms of agonist-activated Ca(2+) entry.

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.

Role of nitric oxide in NAG-ST induced store-operated calcium entry in rat intestinal epithelial cells

This study was undertaken to find out the mechanism of non-agglutinable Vibrio cholerae heat-stable enterotoxin (NAG-ST)-induced calcium influx across the plasma membrane. Adriamycin, an inhibitor of IP3-specific 3-kinase, could not inhibit NAG-ST-induced calcium influx in rat intestinal epithelial cells, which suggested that inositol 1,3,4,5-tetrakisphosphate (IP4) had no role in NAG-ST-induced calcium influx. NAG-ST increased intracellular nitric oxide level of rat enterocytes as measured by a fluorimetric method using a fluoroprobe 4,5-diaminofluorescein-2-diacetate (DAF-2DA). N-Nitro-L-arginine, an inhibitor of nitric oxide synthase, inhibited NAG-ST-induced rise in nitric oxide level and also calcium influx. Inhibition of inositol trisphosphate (IP3)-mediated intracellular calcium mobilization by Dantrolene could also inhibit NAG-ST-induced rise in intracellular nitric oxide level. Moreover, inhibition of soluble guanylate cyclase by inhibitors (ODQ, LY83583) could inhibit the NAG-ST-induced rise in cyclic guanosine-3',5'-monophosphate (cGMP) level and calcium influx. From this study, it is evident that NAG-ST causes IP3-mediated calcium release from intracellular calcium store, which then stimulates nitric oxide production by activating nitric oxide synthase and the nitric oxide through cGMP activates calcium influx.

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.

Engineering the phosphoinositide-binding profile of a class I pleckstrin homology domain

Pleckstrin homology (PH) domains are protein modules that bind with varying degrees of affinity and specificity membrane phosphoinositides. Previously we have shown that although the PH domains of the Ras GTPase-activating proteins GAP1m and GAP1IP4BP are 63% identical at the amino acid level they possess distinct phosphoinositide-binding profiles. The GAP1m PH domain binds phosphatidylinositol 3,4,5-trisphosphate (PtdIns(3,4,5)P3), whereas the domain from GAP1IP4BP binds PtdIns(3,4,5)P3 and phosphatidylinositol 4,5-bisphosphate (PtdIns(4,5)P2) equally well. These phosphoinositide specificities are translated into distinct subcellular localizations. GAP1m is cytosolic and undergoes a rapid PtdIns(3,4,5)P3-dependent association with the plasma membrane following growth factor stimulation. In contrast, GAP1IP4BP is constitutively associated, in a PtdIns(4,5)P2-dependent manner, with the plasma membrane (Cozier, G. E., Lockyer, P. J., Reynolds, J. S., Kupzig, S., Bottomley, J. R., Millard, T., Banting, G., and Cullen, P. J. (2000) J. Biol. Chem. 275, 28261-28268). In the present study, we have used molecular modeling to identify residues in the GAP1IP4BP PH domain predicted to be required for high affinity binding to PtdIns(4,5)P2. This has allowed the isolation of a mutant, GAP1IP4BP-(K591T), which while retaining high affinity for PtdIns(3,4,5)P3 has a 6-fold reduction in its affinity for PtdIns(4,5)P2. Importantly, GAP1IP4BP-(K591T) is predominantly localized to the cytosol and undergoes a PtdIns(3,4,5)P3-dependent association with the plasma membrane following growth factor stimulation. We have therefore engineered the phosphoinositide-binding profile of the GAP1IP4BP PH domain, thereby emphasizing that subtle changes in PH domain structure can have a pronounced effect on phosphoinositide binding and the subcellular localization of GAP1IP4BP.

Inositol 1,3,4,5-tetrakisphosphate is essential for T lymphocyte development

Inositol 1,4,5-trisphosphate (Ins(1,4,5)P(3)) is phosphorylated by Ins(1,4,5)P(3) 3-kinase, generating inositol 1,3,4,5-tetrakisphosphate (Ins(1,3,4,5)P(4)). The physiological function of Ins(1,3,4,5)P(4) is still unclear, but it has been reported to be a potential modulator of calcium mobilization. Disruption of the gene encoding the ubiquitously expressed Ins(1,4,5)P(3) 3-kinase isoform B (Itpkb) in mice caused a severe T cell deficiency due to major alterations in thymocyte responsiveness and selection. However, we were unable to detect substantial defects in Ins(1,4,5)P(3) amounts or calcium mobilization in Itpkb(-/-) thymocytes. These data indicate that Itpkb and Ins(1,3,4,5)P(4) define an essential signaling pathway for T cell precursor responsiveness and development.