Patterns of spatiotemporal organization in an "ambiquitous" enzyme model

Glycolysis: A multifaceted metabolic pathway and signaling hub

Glycolysis is a highly conserved metabolic pathway responsible for the anaerobic production of adenosine triphosphate (ATP) from the breakdown of glucose molecules. While serving as a primary metabolic pathway in prokaryotes, glycolysis is also utilized by respiring eukaryotic cells, providing pyruvate to fuel oxidative metabolism. Furthermore, glycolysis is the primary source of ATP production in multiple cellular states (e.g., hypoxia) and is particularly important in maintaining bioenergetic homeostasis in the most abundant cell type in the human body, the erythrocyte. Beyond its role in ATP production, glycolysis also functions as a signaling hub, producing several metabolic intermediates which serve roles in both signaling and metabolic processes. These signals emanating from the glycolytic pathway can profoundly impact cell function, phenotype, and fate and have previously been overlooked. In this review, we will discuss the role of the glycolytic pathway as a source of signaling molecules in eukaryotic cells, emphasizing the newfound recognition of glycolysis' multifaceted nature and its importance in maintaining cellular homeostasis, beyond its traditional role in ATP synthesis.

Simple and complex spatiotemporal structures in a glycolytic allosteric enzyme model

Pattern formation in glycolysis is studied with a classical reaction-diffusion allosteric enzyme model. It is found that, similar to recent experimental reports in the yeast extracts, a small magnitude local perturbation can induce transient target waves in a two dimensional oscillatory medium. An above threshold stimulation generates target waves which eventually evolve into spatiotemporal chaos upon collisions with the boundary or other wave activities. Detailed simulation studies show that the studied simple glycolytic reaction-diffusion model can support three types of spatiotemporal behaviors which are independent of the boundary conditions: (1) a spatially uniform stable steady state, (2) periodic global oscillations and (3) spatiotemporal chaos.

Spatiotemporal chemical dynamics in living cells: from information trafficking to cell physiology

Biological thought in the 20th century was dominated by the study of structures at increasingly minute levels. For biology to advance beyond structural reductionism and contribute its full measure to clinical care, living biological structures must be understood in the context of their collective chemical processes at the relevant chemical time-scales. Using high-speed fluorescence microscopy, we have studied intra- and inter-cellular signaling using shutter speeds ( approximately 100 ns) that remove the effects of wave motion and diffusion from optical images. By collecting a series of such images, stop-action movies of signal trafficking in living cells are created; these have revealed a new level of spatiotemporal chemical organization within cells. Numerous types of chemical waves have been found in living cells expressing a great variety of physical properties. In this article I will review some of these basic findings, discuss these events in the context of information trafficking, and illustrate the potential implications of this work in medicine.

Apparent role of traveling metabolic waves in oxidant release by living neutrophils

Cell metabolism self-organizes into two types of dissipative structures: chemical oscillations and traveling metabolic waves. In the present study we test the hypothesis that traveling NAD(P)H waves within neutrophils are associated spatially and temporally with the release of reactive oxygen metabolites (ROMs). Using high-speed optical microscopy and taking advantage of the autofluorescence of NAD(P)H, we have observed the propagation of NAD(P)H waves within cells. When NAD(P)H waves reach the lamellipodium of morphologically polarized neutrophils, a diffusing plume of superoxide is released as evidenced by the conversion of hydroethidine in the extracellular environment to ethidium bromide. Parallel results were obtained by using high-speed emission microspectrophotometry. These experiments indicate that the spatial and temporal properties of NAD(P)H waves are transformed into ROM pulses in the extracellular environment. Propagating NAD(P)H waves allow neutrophils to specifically deliver substrate to the lamellipodium at high concentrations, thus facilitating the local and periodic release of ROMs in the direction of cell movement and/or a target.

Hexokinase translocation during neutrophil activation, chemotaxis, and phagocytosis: disruption by cytochalasin D, dexamethasone, and indomethacin

Neutrophils expend large amounts of energy to perform demanding cell functions. To better understand energy production and flow during cell activation, immunofluorescence microscopy was employed to determine the location of the key metabolic enzyme hexokinase during various conditions. Hexokinase is translocated from the neutrophil's cytosol to its periphery in response to N-formyl-methionyl-leucyl-phenylalanine (fMLP) and other activating stimuli, but not during exposure to the formyl peptide receptor antagonist N-tert-BOC-phe-leu-phe-leu-phe (Boc-PLPLP). Translocation was observed from 10(-6) to 10(-9)M fMLP. However, fMLP did not affect the intracellular distribution of lactate dehydrogenase. Hexokinase accumulated at the lamellipodium of cells exposured to an fMLP gradient whereas it localized to the phagosome after latex bead uptake. Thus, hexokinase is differentially translocated within cells depending upon the prevailing physiological conditions. Further studies noted that cytochalasin D, dexamethasone, and indomethacin blocked hexokinase translocation. Parallel regulation of reactive oxygen metabolite (ROM) production was shown. We speculate that hexokinase translocation participates in neutrophil activation.

Dissipative metabolic patterns respond during neutrophil transmembrane signaling

Self-organization is a common theme in biology. One mechanism of self-organization is the creation of chemical patterns by the diffusion of chemical reactants and their nonlinear interactions. We have recently observed sustained unidirectional traveling chemical redox [NAD(P)H - NAD(P)(+)] waves within living polarized neutrophils. The present study shows that an intracellular metabolic wave responds to formyl peptide receptor agonists, but not antagonists, by splitting into two waves traveling in opposite directions along a cell's long axis. Similar effects were noted with other neutrophil-activating substances. Moreover, when cells were exposed to an N-formyl-methionyl-leucyl-phenylalanine (FMLP) gradient whose source was perpendicular to the cell's long axis, cell metabolism was locally perturbed with reorientation of the pattern in a direction perpendicular to the initial cellular axis. Thus, extracellular activating signals and the signals' spatial cues are translated into distinct intracellular dissipative structures.

Simulation of enzymatic cellular reactions complicated by phase separation

We present two-dimensional Monte Carlo simulations of enzymatic cellular reaction occurring via the Michaelis-Menten scheme in the case of attractive interactions between the reaction products. The model employed predicts phase separation in the cell provided that the reaction is relatively fast. The shape of the corresponding patterns varies from a few separate islands to a large patch located in the center of the cell. The fluctuations of the reaction rate during such regimes are found to be much higher than those predicted by the Poissonian distribution.

Imaging sustained dissipative patterns in the metabolism of individual living cells

Theoretical studies have predicted spatiotemporal organization of cell metabolism. Using a rapidly gated CCD camera, we demonstrate for the first time sustained traveling waves of NAD(P)H autofluorescence and protons in individual morphologically polarized living cells. Chemical concentration fronts moved in the direction of cell orientation, thus correlating dissipative structures with cell shape.

Traveling waves in yeast extract and in cultures of Dictyostelium discoideum

Biological self-organization was investigated in a biochemical and a cellular system: yeast extract and cultures of the slime mold Dictyostelium discoideum. In both systems traveling reaction-diffusion waves occur in response to oscillatory reactions. Glycolytic degradation of sugar in a yeast extract leads to the spontaneous formation of NADH and proton waves. Manipulation of the adenine nucleotide pool by addition of purified plasma membrane ATPase favors the formation of both reaction-diffusion waves and phase waves. The results indicate that the energy charge has an important impact for the dynamics of glycolytic patterns. When affecting the lower part of glycolysis by pyruvate addition the frequency of wave generation was increased with concomitant formation of rotating NADH and proton spirals. During morphogenesis of the cellular system Dictyostelium discoideum, circular and spiral shaped aggregation patterns of motile amoeboid cells form in response to traveling cAMP waves. Velocity analysis of the cell movements reveals that the cAMP waves guide the cells towards the site of wave initiation along optimized trajectories. The minimization of aggregation paths is based on a mechanism exploiting general properties of excitation waves. The resulting aggregation territories are reminiscent of Voronoi diagrams.

Effect of enzyme organization on the stability of Yates-Pardee pathways

Yates-Pardee-type metabolic pathways in a heterogeneous cell milieu are modeled as a system of coupled non-linear partial differential equations. A numerical solution to this system is described and some properties of such a physiological system are studied. Confinement with and without a membrane is considered and it is shown how confinement results in an increase in the stability of the metabolite concentrations. These results suggest that the enzyme organization may contribute to the stability of the cellular metabolism.

Traveling NADH and proton waves during oscillatory glycolysis in vitro

Propagation and mutual annihilation of circular and spiral NADH and proton waves were detected by spatially resolved spectrophotometry and fluorescent proton indicators in a biological in vitro system: an organelle-free yeast extract. Spontaneous wave generation during glycolytic sugar degradation is established after an induction period of about 1 h. Controlled wave initiation could be performed by local injection of the strong activator of phosphofructokinase, fructose 2,6-bisphosphate. A crucial role for wave initiation and control of pattern dynamics is attributed to the key enzyme of glycolysis, the allosterically regulated phosphofructokinase. An overall increase in the concentration of its positive effector AMP leads to the formation of rotating spirals. The dynamics of the observed wave patterns resemble that of self-organized calcium waves as recently found in frog eggs and heart cells.