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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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Dependence of cup shape on cell-membrane parameters, particle size and shape. (A) Influence of surface tension and volume constraint. Simulated phagocytic cup for (left) a large volume constraint (κP = 10-2 pNμm-5) and a relatively low surface tension (σ = 6.2 × 10-7 mNm-1), and (right) a high surface tension (σ = 6.2 × 10-5 mNm-1) and a nearly unconstrained cell volume (κP = 3.8 × 10-7 pNμm-5). Both sets of parameters produce approximately the same speed for engulfment. (B) Ranges of parameter values for successful engulfment. Simulation time was restricted to twice the engulfment time with Standard Parameters (SP) given in Methods and represented by vertical dashed line. Shown are fold changes of bending stiffness (blue), surface tension (green), ligand-receptor energy density (orange) and volume constraint (purple) along x-axis. Pointed arrows indicate that parameter regimes may extend beyond tested limits, blunt arrows indicate sharp limits beyond which full engulfment is not reached in simulation time. (C) Cross section of particle engulfment with particle radii R = 1.2 μm (left) and R = 3.8 μm (right). (D) Engulfment of a spheroidal particle with tip (left) and long side (right) first. Principal axis of the spheroidal particle are R1 = R2 = 1.5 μm, and R3 = 4.2 μm. Dashed curve indicates particle outline. (E) Stalled engulfment of spiral-shaped particle, characterised by a volume similar to a spherical particle of radius 2.2 μm.
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Figure 2: Dependence of cup shape on cell-membrane parameters, particle size and shape. (A) Influence of surface tension and volume constraint. Simulated phagocytic cup for (left) a large volume constraint (κP = 10-2 pNμm-5) and a relatively low surface tension (σ = 6.2 × 10-7 mNm-1), and (right) a high surface tension (σ = 6.2 × 10-5 mNm-1) and a nearly unconstrained cell volume (κP = 3.8 × 10-7 pNμm-5). Both sets of parameters produce approximately the same speed for engulfment. (B) Ranges of parameter values for successful engulfment. Simulation time was restricted to twice the engulfment time with Standard Parameters (SP) given in Methods and represented by vertical dashed line. Shown are fold changes of bending stiffness (blue), surface tension (green), ligand-receptor energy density (orange) and volume constraint (purple) along x-axis. Pointed arrows indicate that parameter regimes may extend beyond tested limits, blunt arrows indicate sharp limits beyond which full engulfment is not reached in simulation time. (C) Cross section of particle engulfment with particle radii R = 1.2 μm (left) and R = 3.8 μm (right). (D) Engulfment of a spheroidal particle with tip (left) and long side (right) first. Principal axis of the spheroidal particle are R1 = R2 = 1.5 μm, and R3 = 4.2 μm. Dashed curve indicates particle outline. (E) Stalled engulfment of spiral-shaped particle, characterised by a volume similar to a spherical particle of radius 2.2 μm.

Mentions: Using our model for the zipper mechanism, we have successfully simulated phagocytic engulfment in a broad range of parameter values (see Figure 2, and movie Additional file 2). Figure 2A shows two different characteristic cup shapes we obtained. Low surface tension (i.e. low energy cost for stretching the membrane and underlying actin cortex), and tight cell-volume constraint (i.e. high energy cost for increasing the cell volume), lead to a thin phagocytic cup since a thin cup requires extra membrane but little extra volume. In contrast, weak volume constraint and/or high surface tension produce a broad cup. Based on the parameters explored, we chose intermediate values for both surface tension and cell-volume constraint as our Standard Parameters (SP) for the remainder of our simulations in order to produce realistic cup shapes (see Methods for details). Figure 2B shows that most parameter values can be changed independently by at least one order of magnitude, without negatively affecting engulfment completion. Note that changing simultaneously several parameters may affect engulfment more drastically. Our simulations also show that cup shape depends on the kinetics of engulfment, determined by membrane fluctuations and therefore temperature (see Additional file 1, Figure S2). Additionally, preventing thermal fluctuations (by setting the temperature to zero Kelvin) during a simulation stops cup progression. This indicates that in our model membrane fluctuations are indeed required to bring receptors in close contact with ligand molecules on the particle, emphasizing their important role in the ratchet mechanism.


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)

Dependence of cup shape on cell-membrane parameters, particle size and shape. (A) Influence of surface tension and volume constraint. Simulated phagocytic cup for (left) a large volume constraint (κP = 10-2 pNμm-5) and a relatively low surface tension (σ = 6.2 × 10-7 mNm-1), and (right) a high surface tension (σ = 6.2 × 10-5 mNm-1) and a nearly unconstrained cell volume (κP = 3.8 × 10-7 pNμm-5). Both sets of parameters produce approximately the same speed for engulfment. (B) Ranges of parameter values for successful engulfment. Simulation time was restricted to twice the engulfment time with Standard Parameters (SP) given in Methods and represented by vertical dashed line. Shown are fold changes of bending stiffness (blue), surface tension (green), ligand-receptor energy density (orange) and volume constraint (purple) along x-axis. Pointed arrows indicate that parameter regimes may extend beyond tested limits, blunt arrows indicate sharp limits beyond which full engulfment is not reached in simulation time. (C) Cross section of particle engulfment with particle radii R = 1.2 μm (left) and R = 3.8 μm (right). (D) Engulfment of a spheroidal particle with tip (left) and long side (right) first. Principal axis of the spheroidal particle are R1 = R2 = 1.5 μm, and R3 = 4.2 μm. Dashed curve indicates particle outline. (E) Stalled engulfment of spiral-shaped particle, characterised by a volume similar to a spherical particle of radius 2.2 μm.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 2: Dependence of cup shape on cell-membrane parameters, particle size and shape. (A) Influence of surface tension and volume constraint. Simulated phagocytic cup for (left) a large volume constraint (κP = 10-2 pNμm-5) and a relatively low surface tension (σ = 6.2 × 10-7 mNm-1), and (right) a high surface tension (σ = 6.2 × 10-5 mNm-1) and a nearly unconstrained cell volume (κP = 3.8 × 10-7 pNμm-5). Both sets of parameters produce approximately the same speed for engulfment. (B) Ranges of parameter values for successful engulfment. Simulation time was restricted to twice the engulfment time with Standard Parameters (SP) given in Methods and represented by vertical dashed line. Shown are fold changes of bending stiffness (blue), surface tension (green), ligand-receptor energy density (orange) and volume constraint (purple) along x-axis. Pointed arrows indicate that parameter regimes may extend beyond tested limits, blunt arrows indicate sharp limits beyond which full engulfment is not reached in simulation time. (C) Cross section of particle engulfment with particle radii R = 1.2 μm (left) and R = 3.8 μm (right). (D) Engulfment of a spheroidal particle with tip (left) and long side (right) first. Principal axis of the spheroidal particle are R1 = R2 = 1.5 μm, and R3 = 4.2 μm. Dashed curve indicates particle outline. (E) Stalled engulfment of spiral-shaped particle, characterised by a volume similar to a spherical particle of radius 2.2 μm.
Mentions: Using our model for the zipper mechanism, we have successfully simulated phagocytic engulfment in a broad range of parameter values (see Figure 2, and movie Additional file 2). Figure 2A shows two different characteristic cup shapes we obtained. Low surface tension (i.e. low energy cost for stretching the membrane and underlying actin cortex), and tight cell-volume constraint (i.e. high energy cost for increasing the cell volume), lead to a thin phagocytic cup since a thin cup requires extra membrane but little extra volume. In contrast, weak volume constraint and/or high surface tension produce a broad cup. Based on the parameters explored, we chose intermediate values for both surface tension and cell-volume constraint as our Standard Parameters (SP) for the remainder of our simulations in order to produce realistic cup shapes (see Methods for details). Figure 2B shows that most parameter values can be changed independently by at least one order of magnitude, without negatively affecting engulfment completion. Note that changing simultaneously several parameters may affect engulfment more drastically. Our simulations also show that cup shape depends on the kinetics of engulfment, determined by membrane fluctuations and therefore temperature (see Additional file 1, Figure S2). Additionally, preventing thermal fluctuations (by setting the temperature to zero Kelvin) during a simulation stops cup progression. This indicates that in our model membrane fluctuations are indeed required to bring receptors in close contact with ligand molecules on the particle, emphasizing their important role in the ratchet mechanism.

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