Epac activation converts cAMP from a proliferative into a differentiation signal in PC12 cells

Elevation of the intracellular cAMP concentration ([cAMP]i) regulates metabolism, cell proliferation, and differentiation and plays roles in memory formation and neoplastic growth. cAMP mediates its effects mainly through activation of protein kinase A (PKA) as well as Epac1 and Epac2, exchange factors activating the small GTPases Rap1 and Rap2. However, how cAMP utilizes these effectors to induce distinct biological responses is unknown. We here studied the specific roles of PKA and Epac in neuroendocrine PC12 cells. In these cells, elevation of [cAMP]i activates extracellular signal-regulated kinase (ERK) 1/2 and induces low-degree neurite outgrowth. The present study showed that specific stimulation of PKA triggered ERK1/2 activation that was considerably more transient than that observed upon simultaneous activation of both PKA and Epac. Unexpectedly, the PKA-specific cAMP analog induced cell proliferation rather than neurite outgrowth. The proliferative signaling pathway activated by the PKA-specific cAMP analog involved activation of the epidermal growth factor receptor and ERK1/2. Activation of Epac appeared to extend the duration of PKA-dependent ERK1/2 activation and converted cAMP from a proliferative into an anti-proliferative, neurite outgrowth-promoting signal. Thus, the present study showed that the outcome of cAMP signaling can depend heavily on the set of cAMP effectors activated.     

Monitoring proton dissociation and solution conformation of chiral ytterbium complexes with near-IR CD

The ytterbium complex [Yb((S)-THP)](3+) ((S)-THP = (1S,4S,7S,10S-tetrakis(2-hydroxypropyl)-1,4,7,10-tetraazacyclododecane) is investigated in solution through NMR, near-IR absorption, and CD spectroscopy. Quantitative analysis of the paramagnetic pseudocontact NMR shift shows Lambda helicity of the ligand cage around the metal. The NIR CD spectrum recorded at acidic pH is found to be very similar to that of [Yb((R)-DOTMA)](-) ((R)-DOTMA = (1R,4R,7R,10R)-alpha,alpha',alpha',alpha'-tetramethyl-1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid), which in solution assumes a twisted square antiprism (TSA) conformation. The similarity of the NIR CD spectra is discussed, and it is the first proof of the Lambda(lambda,lambda,lambda,lambda) conformation of [Yb((S)-THP)](3+). NIR CD spectra recorded in the pH range of 2-9 allow one to easily follow proton dissociation and to calculate the pK of this equilibrium in water (pK(A) = 6.4 +/- 0.1). This value agrees well with that determined for [Lu((S)-THP)](3+) using potentiometric methods. This demonstrates once again that NIR CD spectroscopy is a powerful technique for investigating the solution structure and dynamics of these complexes.     

Design of HIV-1-PR inhibitors that do not create resistance: blocking the folding of single monomers

The main problems found in designing drugs are those of optimizing the drug-target interaction and of avoiding the insurgence of resistance. We suggest a scheme for the design of inhibitors that can be used as leads for the development of a drug and that do not face either of these problems, and then apply it to the case of HIV-1-PR. It is based on the knowledge that the folding of single-domain proteins, such as each of the monomers forming the HIV-1-PR homodimer, is controlled by local elementary structures (LES), stabilized by local contacts among hydrophobic, strongly interacting, and highly conserved amino acids that play a central role in the folding process. Because LES have evolved over many generations to recognize and strongly interact with each other so as to make the protein fold fast and avoid aggregation with other proteins, highly specific (and thus little toxic) as well as effective folding-inhibitor molecules suggest themselves: short peptides (or eventually their mimetic molecules) displaying the same amino acid sequence of that of LES (p-LES). Aside from being specific and efficient, these inhibitors are expected not to induce resistance; in fact, mutations in HIV-1-PR that successfully avoid the action of p-LES imply the destabilization of one or more LES and thus should lead to protein denaturation. Making use of Monte Carlo simulations, we first identify the LES of the HIV-1-PR and then show that the corresponding p-LES peptides act as effective inhibitors of the folding of the protease.     

Amino acid signaling through the mammalian target of rapamycin (mTOR) pathway: Role of glutamine and of cell shrinkage

Mammalian target of rapamycin (mTOR) mediates a signaling pathway that couples amino acid availability to S6 kinase (S6K) activation, translational initiation and cell growth rate, participating to a versatile checkpoint that inspects the energy status of the cell. The pathway is activated by branched-chain amino acids (BCAA), leucine being the most effective, whereas amino acid dearth and ATP shortage lead to its deactivation. Glutamine- or amino acid-deprivation and hyperosmotic stress induce a fast cell shrinkage (with marked decrease of the intracellular water volume) associated to mTOR-dependent S6K1 dephosphorylation. Using cultured Jurkat cells, we have measured the changes of cell content and intracellular concentration of ATP, of relevant amino acids (BCAA) and of ninhydrin-positive substances (NPS, as measure of NH(2)-bearing organic osmolytes) under conditions that deactivate (leucine-deprivation, glutamine-deprivation, amino acid withdrawal, sorbitol-induced hyperosmotic stress) or reactivate a previously deactivated, mTOR-S6K1 pathway. We have also assessed the mitochondrial function by measurements of mitochondrial transmembrane potential in cells subjected to hypertonic stress. Our results indicate that diverse control signals converge on the mTOR-S6K1 signaling pathway. In the presence of adequate energy resources, the pathway senses the amino acid availability as inward transport of effective amino acids (as BCAA and especially leucine), but its activation occurs only in the presence of an extracellular amino acid complement, with glutamine as obligatory component, and does not tolerate decrements of cell water volume incapable of maintaining adequate intracellular physicochemical conditions.     

{beta}1 Integrin and IL-3R coordinately regulate STAT5 activation and anchorage-dependent proliferation

We previously demonstrated that integrin-dependent adhesion activates STAT5A, a well known target of IL-3-mediated signaling. Here, we show that in endothelial cells the active beta1 integrin constitutively associates with the unphosphorylated IL-3 receptor (IL-3R) beta common subunit. This association is not sufficient for activating downstream signals. Indeed, only upon fibronectin adhesion is Janus Kinase 2 (JAK2) recruited to the beta1 integrin-IL-3R complex and triggers IL-3R beta common phosphorylation, leading to the formation of docking sites for activated STAT5A. These events are IL-3 independent but require the integrity of the IL-3R beta common. IL-3 treatment increases JAK2 activation and STAT5A and STAT5B tyrosine and serine phosphorylation and leads to cell cycle progression in adherent cells. Expression of an inactive STAT5A inhibits cell cycle progression upon IL-3 treatment, identifying integrin-dependent STAT5A activation as a priming event for IL-3-mediated S phase entry. Consistently, overexpression of a constitutive active STAT5A leads to anchorage-independent cell cycle progression. Therefore, these data provide strong evidence that integrin-dependent STAT5A activation controls IL-3-mediated proliferation.     

Complement inhibitor of C5 activation from the soft tick Ornithodoros moubata

Blood-feeding ticks must control C activation or be damaged by the host inflammatory response. We report the characterization and expression of a novel, relatively small, broad-acting C inhibitory protein (termed OmCI) from the soft tick Ornithodoros moubata. The native 17-kDa nonglycosylated protein inhibits both human and guinea pig classical and alternative C activation pathways. The IC50 values for each pathway were 12 and 27 nM, respectively, in hemolytic assays using human serum diluted 40-fold. The cDNA encodes a protein of 168 aa, including an 18-aa secretion signal sequence that is absent in the mature form. The inhibitor has 46% amino acid identity with moubatin, a platelet aggregation inhibitor also from O. moubata that is an outlying member of the lipocalin family. Native OmCI had no inhibitory effect on the addition of C8 and C9 to preformed C5b-C7 and C5b-C8 to form the membrane attack complex and no effect on the rate of C3a production by the C3 convertase enzymes C4bC2a, C3(H2O)Bb, or C3bBb. Both recombinant and native OmCI abolish production of C5a by human classical (C4bC3bC2a) and alternative (C3bC3bBb) C5 convertases. Addition of excess C5 but not C3 competes away the inhibitory activity of OmCI, indicating that OmCI targets C5 itself rather than inhibiting the C5 convertase C4bC3bC2a itself. Direct binding of OmCI to C5 was demonstrated by Western blotting and gel filtration chromatography using 125I-labeled proteins. OmCI is the first lipocalin family member shown to inhibit C and also the first natural inhibitor that specifically targets the C5 activation step.     

Storing,linking, and mining microarray databases using SRS

Background  

SRS (Sequence Retrieval System) has proven to be a valuable platform for storing, linking, and querying biological databases. Due to the availability of a broad range of different scientific databases in SRS, it has become a useful platform to incorporate and mine microarray data to facilitate the analyses of biological questions and non-hypothesis driven quests. Here we report various solutions and tools for integrating and mining annotated expression data in SRS.     

Circadian regulation of phospholipid metabolism in retinal photoreceptors and ganglion cells

The neural retina is a key component of the vertebrate circadian system that is responsible for synchronizing the central circadian pacemaker to external light-dark (LD) cycles. The retina is itself rhythmic, showing circadian cycles in melatonin levels and gene expression. We assessed the in vivo incorporation of 32P-phosphate and 3H-glycerol into phospholipids of photoreceptor cells (PRCs) and retina ganglion cells (GCs) from chicks in constant illumination conditions (dark: DD or light: LL) over a 24-h period. Our findings showed that in DD there was a daily oscillation in 32P-labeling of total phospholipids synthesized in GCs and axonally transported to the brain. This metabolic fluctuation peaked during the subjective night (zeitgeber time [ZT] 20), persisted for several hours well into the subjective day and declined at subjective dusk (ZT 10-12). PRCs also exhibited an in vivo rhythm of 32P-phospholipid synthesis in DD. This rhythm peaked around ZT 22, continued a few hours into the day and declined by the end of subjective dusk. The major individual species labeled 1 h after 32P administration was phosphatidylinositol (PI) in both PRCs and GCs. Rhythmic phospholipid biosynthesis was also observed in DD after 3H-glycerol administration, with levels in GCs elevated from midday to early night. PRCs exhibited a similar rhythmic profile with the lowest levels of labeling during midnight. Phosphatidylcholine (PC) accounted for the individual species with the highest ratio of 3H-glycerol incorporation in both cell populations at all phases examined. By contrast, in LL the rhythm of 3H-glycerol labeling of phospholipids damped out in both cell layers. Our findings support the idea that, in constant darkness, the metabolism of retinal phospholipids, including their de novo biosynthesis, is regulated by an endogenous circadian clock.     

Mitotic catastrophe: a special case of apoptosis

Mitotic catastrophe is a poorly defined type of cell death linked to the abnormal activation of cyclin B/Cdk1. Here we propose that a conflict in cell cycle progression or DNA damage can lead to mitotic catastrophe, provided that cell cycle checkpoints are inhibited, in particular the DNA structure checkpoints and the spindle assembly checkpoint. Two subtypes of mitotic catastrophe can be distinguished. First, mitotic catastrophe can kill the cell during or close to the metaphase, in a p53-independent fashion, as this occurs in Chk2-inhibited heterokarya generated by fusion. Second, mitotic catastrophe can occur after failed mitosis, during the activation of the polyploidy checkpoint, in a partially p53-dependent fashion. In these conditions, cells die as a result of caspase activation and mitochondrial membrane permeabilization that constitute hallmarks of apoptosis. Prevention of caspase activation and/or mitochondrial damage avoids mitotic catastrophe, indicating that this form of cell death indeed constitutes a special case of apoptosis. Importantly, the suppression of mitotic catastrophe can favor asymmetric division and the generation of aneuploid cells. This delineates a molecular pathway through which failure to arrest the cell cycle and inhibition of apoptosis can favor the occurrence of cytogenetic abnormalities which are likely to participate in oncogenesis.