1. Diffusion Barriers, Mechanical Forces, and the Biophysics of Phagocytosis 
Philip P. Ostrowski, Sergio Grinstein, Spencer A. Freeman 
Developmental Cell 
Volume 38, Issue 2, Pages (July 2016)
DOI: /j.devcel
Copyright © 2016 Elsevier Inc. Terms and Conditions

2. Figure 1 First Contact between Phagocytes and Their Prey
(A) Phagocytic prey can strike the phagocyte surface by Brownian-driven collisions or as a result of directed motion propelled by flagella or by fluid flow. In addition, phagocytes can extend membrane protrusions to engage their targets.
(B) Glycoprotein pickets extend further than phagocytic receptors, presenting a steric barrier to receptor engagement.
(C) Three possible mechanisms for the prey to gain access to phagocytic receptors to initiate the activation process.
Developmental Cell  , DOI: ( /j.devcel )
Copyright © 2016 Elsevier Inc. Terms and Conditions

3. Figure 2
The Diffusion of Phagocytic Receptors and Glycoprotein Pickets
(A) The diffusion of phagocytic receptors including FcγRIIA is limited by the cortical cytoskeleton that is tethered to the plasma membrane by (glyco)protein pickets.
(B) The cytoplasmic domain of CD45 binds to ankyrin and spectrin, which in turn bind to F-actin filaments.
(C) Muc-1 may be tethered to the actin cytoskeleton via catenins.
(D) The cytoplasmic domain of CD44 binds to actin filaments via ERM proteins like ezrin. PIP2, PtdIns(4,5)P2 in this and subsequent figures.
Developmental Cell  , DOI: ( /j.devcel )
Copyright © 2016 Elsevier Inc. Terms and Conditions

4. Figure 3 The Resting and Primed Cytoskeleton
(A–D) Two types of actin polymerization networks, one controlled by Rho/formins/myosin (A, resting) and the other by WASp family proteins/Arp2/3 (D, probing) can mutually exclude each other. Priming of macrophages (B) by LPS, chemokines, or nucleotides favors the conversion from the resting to the probing mode. Priming also increases the diffusion of receptors and pickets (C). PIP3, PtdIns(3,4,5)P3 in this and subsequent figures.
Developmental Cell  , DOI: ( /j.devcel )
Copyright © 2016 Elsevier Inc. Terms and Conditions

5. Figure 4 Particle Engagement by Phagocytic Receptors
(A) SFKs are regulated by phosphorylation on inhibitory (orange) and activating (white) sites. Phosphate moieties can be removed by the transmembrane phosphatases CD45 and CD148. Phosphorylation of the inactivating tyrosine residue is mediated by Csk. Phosphorylation of the activating tyrosine site results from trans-phosphorylation following receptor engagement.
(B) CD45 and CD148 are removed from contact sites between phagocytes and target particles and their exclusion is maintained and expanded by diffusional barriers cemented by engaged receptors. This leads to zones of SFK activation where contact is made with the target ligands, and zones where SFKs are primed at the extending pseudopods.
(C) IgG opsonized particles cluster FcγIIA (and other stimulatory Fcγ receptors) that contain ITAMs. Receptor ITAMs are phosphorylated by SFKs and recruit Syk. Phosphorylated Syk recruits and can phosphorylate adaptors, kinases, and lipases that remodel the cytoskeleton, thereby contributing to increased diffusion of non-engaged receptors. Diffusible second messengers originating from Fcγ receptor microclusters lead to the inside-out activation of integrins via Rap GTPases. Integrins can engage a plethora of ligands on the target particle and provide a linkage between the target and the actin cytoskeleton via talin, kindlin, vinculin, etc. Integrins further exclude CD45, expanding the signaling from Fcγ receptors.
Developmental Cell  , DOI: ( /j.devcel )
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6. Figure 5 Pseudopod Extension and Particle Engulfment
As actin polymerization drives pseudopod extension around the particle, PI3K converts PtdIns(4,5)P2 to PtdIns(3,4,5)P3, which recruits myosins. Myosins exert contractile activity that functions as a purse string to facilitate phagosome closure. PIP3 also recruits Rho GAPs that inactivate Rac1 and Cdc42, terminating actin polymerization starting from the base of the phagocytic cup. Concurrently, phospholipase C cleaves PIP2 to DAG and IP3, and the loss of PIP2 releases actin-binding proteins such as cofilin, WASp, and ezrin. The loss of actin at the base of the phagocytic cup allows for fusion of vesicles with the plasma membrane.
Developmental Cell  , DOI: ( /j.devcel )
Copyright © 2016 Elsevier Inc. Terms and Conditions