Apoptosis, or genetically programmed cell death, is a crucial cellular process that maintains the balance between life and death in cells. The precise molecular mechanism of apoptosis signaling and the manner in which type 1 and type 2 pathways of the apoptosis signaling network are differentially activated under distinct apoptotic stimuli is poorly understood. Based on Monte Carlo stochastic simulations, we show that the type 1 pathway becomes activated under strong apoptotic stimuli, whereas the type 2 mitochondrial pathway dominates apoptotic signaling in response to a weak death signal. Our results also show signaling in the type 2 pathway is stochastic; the population average over many cells does not capture the cell-to-cell fluctuations in the time course (approximately 1-10 h) of downstream caspase-3 activation. On the contrary, the probability distribution of caspase-3 activation for the mitochondrial pathway shows a distinct bimodal behavior that can be used to characterize the stochastic signaling in type 2 apoptosis and other similar complex signaling processes. Interestingly, such stochastic fluctuations in apoptosis signaling occur even in the presence of large numbers of signaling molecules.

SummaryOver the past decade, following the discovery of the human heme protein neuroglobin, many studies have searched for evidence for this protein's mechanism of action. Much data has accrued showing that high levels of neuroglobin will protect cells from apoptotic cell death, following a wide range of challenges. Various explanations of its actions, based on measured reactivity with oxygen, nitric oxide, or free radicals, have been proposed, but none have, as yet, been substantiated in vivo. Following preliminary experiments, it was previously hypothesised that ''the central role of neuroglobin in highly metabolically active cells and retinal and brain neurons is to reset the trigger level of mitochondrial cytochrome c release necessary to commit the cells to apoptosis'' (I.U.M.B.M. Life (2008) 60, 398). In this article, we review the evidence, which has accumulated to support this hypothesised mechanism of action of neuroglobin and integrate this data, with other reported intracellular functions of neuroglobin, to suggest a plausible central role for neuroglobin in the control of apoptosis. IUBMBIUBMB Life, 62(12): 878-885, 2010

Neutrophil rolling and transition to arrest on inflamed endothelium are dynamically regulated by the affinity of the ␤ 2 integrin CD11a/CD18 (leukocyte function associated antigen 1 (LFA-1)) for binding intercellular adhesion molecule (ICAM)-1. Conformational shifts are thought to regulate molecular affinity and adhesion stability. Also critical to adhesion efficiency is membrane redistribution of active LFA-1 into dense submicron clusters where multimeric interactions occur. We examined the influences of affinity and dimerization of LFA-1 on LFA-1/ICAM-1 binding by engineering a cell-free model in which two recombinant LFA-1 heterodimers are bound to respective Fab domains of an antibody attached to latex microspheres. Binding of monomeric and dimeric ICAM-1 to dimeric LFA-1 was measured in real time by fluorescence flow cytometry. ICAM-1 dissociation kinetics were measured while LFA-1 affinity was dynamically shifted by the addition of allosteric small molecules. High affinity LFA-1 dissociated 10-fold faster when bound to monomeric compared with dimeric ICAM-1, corresponding to bond lifetimes of 25 and 330 s, respectively. Downshifting LFA-1 into an intermediate affinity state with the small molecule I domain allosteric inhibitor IC487475 decreased the difference in dissociation rates between monomeric and dimeric ICAM-1 to 4-fold. When LFA-1 was shifted into the low affinity state by lovastatin, both monomeric and dimeric ICAM-1 dissociated in less than 1 s, and the dissociation rates were within 50% of each other. These data reveal the respective importance of LFA-1 affinity and proximity in tuning bond lifetime with ICAM-1 and demonstrate a nonlinear increase in the bond lifetime of the dimer versus the monomer at higher affinity.Neutrophils circulate in the bloodstream to sites of inflammation where they adhere and transmigrate through the endothelium as the initial step in combating infection and to facilitate wound healing. Recruitment from the circulation involves a multistep process of cell rolling, activation, and arrest. The heterodimeric integrin receptor LFA-1 1 is composed of the ␣L (CD11a) and ␤ 2 (CD18) subunits and is constitutively expressed in a low affinity conformation on the plasma membrane of leukocytes (1-3). Neutrophils encountering chemokines on inflamed endothelium are activated to shift LFA-1 from the low to high affinity conformation, which supports tight binding to endothelial ICAM-1. Increases in integrin affinity correlate in time with adhesion function as recently demonstrated in aggregation of cells expressing ␣4␤1 and vascular cell adhesion molecule (4). ICAM-1 recognizes LFA-1 through an inserted (I) domain in the ␣ subunit. There is strong evidence correlating shifts in I domain conformation to affinity changes in binding ICAM-1. Mutations in I domain residues stabilized distinct structural conformations correlating to LFA-1 affinity. ICAM-1 equilibrium binding constants increase over 4 orders of magnitude ranging between low (i.e. 1600 M), intermediate (i.e. 9 M), and high...

The mean first passage time (MFPT) for photoexcitations diffusion in a funneling potential of artificial tree-like light-harvesting antennae (phenylacetylene dendrimers with generation-dependent segment lengths) is computed. Effects of the non-linearity of the realistic funneling potential and slow random solvent fluctuations considerably slow down the center-bound diffusion beyond a temperature-dependent optimal size. Diffusion on a disordered Cayley tree with a linear potential is investigated analytically. At low temperatures we predict a phase in which the MFPT is dominated by a few paths.Dendrimers constitute a new class of nanomaterials with unusual tree-like geometry and interesting chemical, transport, and optical properties [1][2][3][4][5]. Two families of Phenylacetylene dendrimers have received considerable recent attention. In the compact family, the length of the linear segments is fixed, whereas in the extended family it increases towards the center creating an energy funnel in that direction (Fig. 1). This latter family may, therefore, serve as artificial light-harvesting antennas, as has been demonstrated experimentally [6]. It has been conjectured by Kopelman et. al., based on optical absorption spectra [1], that electronic excitations in these dendrimers are localized on the linear segments. This has been confirmed in theoretical studies which showed that the relative motion of photogenerated electron-hole pairs is confined to the various segments and energy-transfer may then be described by the Frenkel exciton model. The time it takes for an excitation that starts at the periphery to reach the center, and its dependence on the molecular size (number of generations g) and the funneling force, were calculated. The latter results from the interplay of entropic (or geometric, i.e. the branching ratio c = 2) and energetic factors [7].These pioneering studies, however, assumed the funneling force to be constant since the energy ǫ(n) varied linearly with n. (n=1,2,. . . ,g is the generation number, at which the segment length is l = g − n + 1 monomers. See Fig. 1.) In addition, ε(n) were assumed to be fixed. In reality, interactions with other degrees of freedom (solvent and intramolecular vibrations), induce fluctuations in ε(n) which may span many different timescales. Here we consider slow (quenched) fluctuations compared with the exciton trapping times which are typically in the picosecond range [6,8]. Nonlinear [9] and single molecule [10] spectroscopy in liquids, glasses and proteins typically show nanosecond to millisecond bath motions responsible for spectral diffusion. Slow vibrational motions that can be treated as static disorder dominate the photoinduced energy transfer dynamics of photosynthetic antenna complexes [11]. Fast (annealed) fluctuations do not change the behavior qualitatively. In addition, we explore different degrees of correlations among the various energy fluctuations. In the absence of correlations, we obtain the standard diagonal disorder (random energy) model. If the e...