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The zipper mechanism in phagocytosis: energetic requirements and variability in phagocytic cup shape.

Tollis S, Dart AE, Tzircotis G, Endres RG - BMC Syst Biol (2010)

Bottom Line: Highly curved shapes are not taken up, in line with recent experimental results.This suggests that biochemical pathways render the evolutionary ancient process of phagocytic highly robust, allowing cells to engulf even very large particles.The particle-shape dependence of phagocytosis makes a systematic investigation of host-pathogen interactions and an efficient design of a vehicle for drug delivery possible.

View Article: PubMed Central - HTML - PubMed

Affiliation: Division of Molecular Biosciences, South Kensington Campus, Imperial College London, SW72AZ London, UK.

ABSTRACT

Background: Phagocytosis is the fundamental cellular process by which eukaryotic cells bind and engulf particles by their cell membrane. Particle engulfment involves particle recognition by cell-surface receptors, signaling and remodeling of the actin cytoskeleton to guide the membrane around the particle in a zipper-like fashion. Despite the signaling complexity, phagocytosis also depends strongly on biophysical parameters, such as particle shape, and the need for actin-driven force generation remains poorly understood.

Results: Here, we propose a novel, three-dimensional and stochastic biophysical model of phagocytosis, and study the engulfment of particles of various sizes and shapes, including spiral and rod-shaped particles reminiscent of bacteria. Highly curved shapes are not taken up, in line with recent experimental results. Furthermore, we surprisingly find that even without actin-driven force generation, engulfment proceeds in a large regime of parameter values, albeit more slowly and with highly variable phagocytic cups. We experimentally confirm these predictions using fibroblasts, transfected with immunoreceptor FcγRIIa for engulfment of immunoglobulin G-opsonized particles. Specifically, we compare the wild-type receptor with a mutant receptor, unable to signal to the actin cytoskeleton. Based on the reconstruction of phagocytic cups from imaging data, we indeed show that cells are able to engulf small particles even without support from biological actin-driven processes.

Conclusions: This suggests that biochemical pathways render the evolutionary ancient process of phagocytic highly robust, allowing cells to engulf even very large particles. The particle-shape dependence of phagocytosis makes a systematic investigation of host-pathogen interactions and an efficient design of a vehicle for drug delivery possible.

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Progression of phagocytic engulfment with time. Experimental data for small particles with 1.5 μm radius (A) and large particles with 3 μm radius (C). The data points represent averages of phagocytic cups obtained at given time points for the three different experimental conditions. Error bars represent the corresponding standard errors. (A, Inset) Proportion of almost completely taken up particles (engulfed surface area larger than 70%) after at least 6 minutes for the three conditions. (B, D) Progression of phagocytic engulfment with time in the simulations for small (B) and large (D) particles. Simulation time was converted to actual time so that 50% average engulfment of small particles by the active zipper corresponds to 6 min. No further adjustment of time conversion was done for large-particle simulations. The data points and error bars represent respectively the statistical average and standard error of phagocytic cups size from several simulations.
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Figure 6: Progression of phagocytic engulfment with time. Experimental data for small particles with 1.5 μm radius (A) and large particles with 3 μm radius (C). The data points represent averages of phagocytic cups obtained at given time points for the three different experimental conditions. Error bars represent the corresponding standard errors. (A, Inset) Proportion of almost completely taken up particles (engulfed surface area larger than 70%) after at least 6 minutes for the three conditions. (B, D) Progression of phagocytic engulfment with time in the simulations for small (B) and large (D) particles. Simulation time was converted to actual time so that 50% average engulfment of small particles by the active zipper corresponds to 6 min. No further adjustment of time conversion was done for large-particle simulations. The data points and error bars represent respectively the statistical average and standard error of phagocytic cups size from several simulations.

Mentions: Our model also predicts that uptake by the active zipper is significantly faster than with the passive zipper (see Figures 3B and 3D). We experimentally tested this prediction by determining the percentage of engulfed surface area for each particle for different time points after initiation of phagocytosis, and comparing this result with our simulations, in which simulation time was matched to actual time. Figure 6A shows that cells transfected with the wild-type receptor (active zipper) engulf significantly faster (three to four times) than cells under the other two conditions (passive zippers). This result is in quantitative accordance with our model predictions (Figure 6B). Furthermore, we determined the time dependence of phagocytic uptake for large particles. The active zipper, although slower for large than for small particles, still engulfs regularly, both in experiments (Figure 6C) and simulations (Figure 6D). Note that predicted and measured time courses are in very good agreement without rescaling the time axis of the large-particle simulation. Furthermore, Figures 6C and 6D demonstrate the inability of the passive zipper to take up large particles, in both experiments and simulations. After more than 10 minutes, the average engulfed surface area remains below 20%.


The zipper mechanism in phagocytosis: energetic requirements and variability in phagocytic cup shape.

Tollis S, Dart AE, Tzircotis G, Endres RG - BMC Syst Biol (2010)

Progression of phagocytic engulfment with time. Experimental data for small particles with 1.5 μm radius (A) and large particles with 3 μm radius (C). The data points represent averages of phagocytic cups obtained at given time points for the three different experimental conditions. Error bars represent the corresponding standard errors. (A, Inset) Proportion of almost completely taken up particles (engulfed surface area larger than 70%) after at least 6 minutes for the three conditions. (B, D) Progression of phagocytic engulfment with time in the simulations for small (B) and large (D) particles. Simulation time was converted to actual time so that 50% average engulfment of small particles by the active zipper corresponds to 6 min. No further adjustment of time conversion was done for large-particle simulations. The data points and error bars represent respectively the statistical average and standard error of phagocytic cups size from several simulations.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2991294&req=5

Figure 6: Progression of phagocytic engulfment with time. Experimental data for small particles with 1.5 μm radius (A) and large particles with 3 μm radius (C). The data points represent averages of phagocytic cups obtained at given time points for the three different experimental conditions. Error bars represent the corresponding standard errors. (A, Inset) Proportion of almost completely taken up particles (engulfed surface area larger than 70%) after at least 6 minutes for the three conditions. (B, D) Progression of phagocytic engulfment with time in the simulations for small (B) and large (D) particles. Simulation time was converted to actual time so that 50% average engulfment of small particles by the active zipper corresponds to 6 min. No further adjustment of time conversion was done for large-particle simulations. The data points and error bars represent respectively the statistical average and standard error of phagocytic cups size from several simulations.
Mentions: Our model also predicts that uptake by the active zipper is significantly faster than with the passive zipper (see Figures 3B and 3D). We experimentally tested this prediction by determining the percentage of engulfed surface area for each particle for different time points after initiation of phagocytosis, and comparing this result with our simulations, in which simulation time was matched to actual time. Figure 6A shows that cells transfected with the wild-type receptor (active zipper) engulf significantly faster (three to four times) than cells under the other two conditions (passive zippers). This result is in quantitative accordance with our model predictions (Figure 6B). Furthermore, we determined the time dependence of phagocytic uptake for large particles. The active zipper, although slower for large than for small particles, still engulfs regularly, both in experiments (Figure 6C) and simulations (Figure 6D). Note that predicted and measured time courses are in very good agreement without rescaling the time axis of the large-particle simulation. Furthermore, Figures 6C and 6D demonstrate the inability of the passive zipper to take up large particles, in both experiments and simulations. After more than 10 minutes, the average engulfed surface area remains below 20%.

Bottom Line: Highly curved shapes are not taken up, in line with recent experimental results.This suggests that biochemical pathways render the evolutionary ancient process of phagocytic highly robust, allowing cells to engulf even very large particles.The particle-shape dependence of phagocytosis makes a systematic investigation of host-pathogen interactions and an efficient design of a vehicle for drug delivery possible.

View Article: PubMed Central - HTML - PubMed

Affiliation: Division of Molecular Biosciences, South Kensington Campus, Imperial College London, SW72AZ London, UK.

ABSTRACT

Background: Phagocytosis is the fundamental cellular process by which eukaryotic cells bind and engulf particles by their cell membrane. Particle engulfment involves particle recognition by cell-surface receptors, signaling and remodeling of the actin cytoskeleton to guide the membrane around the particle in a zipper-like fashion. Despite the signaling complexity, phagocytosis also depends strongly on biophysical parameters, such as particle shape, and the need for actin-driven force generation remains poorly understood.

Results: Here, we propose a novel, three-dimensional and stochastic biophysical model of phagocytosis, and study the engulfment of particles of various sizes and shapes, including spiral and rod-shaped particles reminiscent of bacteria. Highly curved shapes are not taken up, in line with recent experimental results. Furthermore, we surprisingly find that even without actin-driven force generation, engulfment proceeds in a large regime of parameter values, albeit more slowly and with highly variable phagocytic cups. We experimentally confirm these predictions using fibroblasts, transfected with immunoreceptor FcγRIIa for engulfment of immunoglobulin G-opsonized particles. Specifically, we compare the wild-type receptor with a mutant receptor, unable to signal to the actin cytoskeleton. Based on the reconstruction of phagocytic cups from imaging data, we indeed show that cells are able to engulf small particles even without support from biological actin-driven processes.

Conclusions: This suggests that biochemical pathways render the evolutionary ancient process of phagocytic highly robust, allowing cells to engulf even very large particles. The particle-shape dependence of phagocytosis makes a systematic investigation of host-pathogen interactions and an efficient design of a vehicle for drug delivery possible.

Show MeSH
Related in: MedlinePlus