Paper Journal

Oscillatory NAD(P)H Waves and Calcium Oscillations in Neutrophils? A Modeling Study of Feasibility

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Abstract
The group of Howard Petty has claimed exotic metabolic wave phenomena together with mutually phase-coupled NAD(P)H- and calcium-oscillations in human neutrophils. At least parts of these phenomena are highly doubtful due to extensive failure of reproducibility by several othergroups and hints that unreliable data from the Petty lab are involved in publications concerning circular calcium waves. The aim of our theoretical spatiotemporal modeling approach is to propose a possible and plausible biochemical mechanism which would, in principle, be able to explain metabolic oscillations and wave phenomena in neutrophils. Our modeling suggests the possibility of acalcium-controlled glucose influx as a driving force of metabolic oscillations and a potential role of polarized cell geometry and differential enzyme distribution for various NAD(P)H wave phenomena. The modeling results are supposed to stimulate further controversial discussions of such phenomena and potential mechanisms and experimental efforts to finally clarify the existence and biochemical basis of anykind of temporal and spatiotemporal patterns of calcium signals and metabolic dynamics in human neutrophils. Independent of Petty's observations, they present a general feasibility study of such phenomena in cells.
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Introduction

Human immune cells are classified into different types according to their functions in adaptive and innate immunity (1). Whereas cells of adaptive immunityrespond in an antigen-specific way to invading pathogens, cells of innate immunity recognize molecular components that are conserved among a large group of microorganisms.
An important cell type of innate immunity is the neutrophil, a terminally differentiated peripheral blood cell (2). Neutrophils are part of the first line of defense against microorganisms and usually circulate in the bloodstream. They leave the vascular system at a location of tissue inflammation after becoming adherent to the vessel wall acquiring a flat elongated and polarized shape and chemotactically migrate toward the focus of inflammation within the extracellular matrix. After recognition by cell surface receptors, the pathogens are engulfed via phagocytosis and killed by phagolysosome formation and production ofreactive oxygen metabolites.
Recently, Petty et al. claimed the observation of self-organized patterns in terms of spatiotemporal NAD(P)H, which denotes the sum of nicotinamide adenine dinucleotide (NADH) and nicotinamide adenine dinucleotide phosphate-oxidase (NADPH), concentration waves in neutrophils using fluorescence microscopy (3,4). In addition to these waves, sinusoidal temporaloscillations of NAD(P)H with periods of 20 and 10 s have been detected by fluorimetric whole-cell measurements of NAD(P)H autofluorescence (5).
Interestingly, Petty et al. have observed similar oscillatory properties for Ca2+ signals. They observe that calcium spikes occur with the same frequency and a constant phase relation to NAD(P)H (5). Both oscillators consistently show an increase in frequencyfrom ~20s to a 10 s period after activation of neutrophils with proinflammatory stimuli like fMLP or cytokines (6,7) while retaining their phase locking.
In addition to the oscillations, “high-speed-fluorescence-microscopy” (8) results from the Petty group show NAD(P)H-waves in the nonactivated state being ignited exclusively at the rear of the cell (uropod) propagating unidirectionally toward thefront (lamellipodium) with a velocity of ~15–50 μm/s (see Fig. 1 and (3,4,9)). Each wave is annihilated after reaching the cell front and a new wave starts at the rear. After proinflammatory stimulation, two NAD(P)H waves are observed propagating in opposite directions. These are reflected at the front and the rear end of the cell, respectively, and do not affect each other when colliding in the...