PKC-ε pseudosubstrate and catalytic activity are necessary for membrane delivery during IgG-mediated phagocytosis

PKC-ε regulates vesicle delivery and focal exocytosis for efficient IgG-mediated phagocytosis

Protein kinase C (PKC)-ε is required for membrane addition during IgG-mediated phagocytosis, but its role in this process is ill defined. Here, we performed high-resolution imaging, which reveals that PKC-ε exits the Golgi and enters phagosomes on vesicles that then fuse. TNF and PKC-ε colocalize at the Golgi and on vesicles that enter the phagosome. Loss of PKC-ε and TNF delivery upon nocodazole treatment confirmed vesicular transport on microtubules. That TNF+ vesicles were not delivered in macrophages from PKC-ε null mice, or upon dissociation of the Golgi-associated pool of PKC-ε, implies that Golgi-tethered PKC-ε is a driver of Golgi-to-phagosome trafficking. Finally, we established that the regulatory domain of PKC-ε is sufficient for delivery of TNF+ vesicles to the phagosome. These studies reveal a novel role for PKC-ε in focal exocytosis - its regulatory domain drives Golgi-derived vesicles to the phagosome, whereas catalytic activity is required for their fusion. This is one of the first examples of a PKC requirement for vesicular trafficking and describes a novel function for a PKC regulatory domain. This article has an associated First Person interview with the first author of the paper.

Bone Marrow-Derived and Elicited Peritoneal Macrophages Are Not Created Equal: The Questions Asked Dictate the Cell Type Used

Macrophages are a heterogeneous and plastic population of cells whose phenotype changes in response to their environment. Macrophage biologists utilize peritoneal (pMAC) and bone marrow-derived macrophages (BMDM) for in vitro studies. Given that pMACs mature in vivo while BMDM are ex vivo differentiated from stem cells, it is likely that their responses differ under experimental conditions. Surprisingly little is known about how BMDM and pMACs responses compare under the same experimental conditionals. While morphologically similar with respect to forward and side scatter by flow cytometry, reports in the literature suggest that pMACs are more mature than their BMDM counterparts. Given the dearth of information comparing BMDM and pMACs, this work was undertaken to test the hypothesis that elicited pMACs are more responsive to defined conditions, including phagocytosis, respiratory burst, polarization, and cytokine and chemokine release. In all cases, our hypothesis was disproved. At steady state, BMDM are more phagocytic (both rate and extent) than elicited pMACs. In response to polarization, they upregulate chemokine and cytokine gene expression and release more cytokines. The results demonstrate that BMDM are generally more responsive and poised to respond to their environment, while pMAC responses are, in comparison, less pronounced. BMDM responses are a function of intrinsic differences, while pMAC responses reflect their differentiation in the context of the whole animal. This distinction may be important in knockout animals, where the pMAC phenotype may be influenced by the absence of the gene of interest.

Phosphorylation of SNAP-23 at Ser95 causes a structural alteration and negatively regulates Fc receptor-mediated phagosome formation and maturation in macrophages

SNAP-23 is a plasma membrane-localized soluble N-ethylmaleimide–sensitive factor attachment protein receptors (SNARE) involved in Fc receptor (FcR)-mediated phagocytosis. However, the regulatory mechanism underlying its function remains elusive. Using phosphorylation-specific antibodies, SNAP-23 was found to be phosphorylated at Ser95 in macrophages. To understand the role of this phosphorylation, we established macrophage lines overexpressing the nonphosphorylatable S95A or the phosphomimicking S95D mutation. The efficiency of phagosome formation and maturation was severely reduced in SNAP-23-S95D–overexpressing cells. To examine whether phosphorylation at Ser95 affected SNAP-23 structure, we constructed intramolecular Förster resonance energy transfer (FRET) probes of SNAP-23 designed to evaluate the approximation of the N termini of the two SNARE motifs. Interestingly, a high FRET efficiency was detected on the membrane when the S95D probe was used, indicating that phosphorylation at Ser95 caused a dynamic structural shift to the closed form. Coexpression of IκB kinase (IKK) 2 enhanced the FRET efficiency of the wild-type probe on the phagosome membrane. Furthermore, the enhanced phagosomal FRET signal in interferon-γ–activated macrophages was largely dependent on IKK2, and this kinase mediated a delay in phagosome-lysosome fusion. These results suggested that SNAP-23 phosphorylation at Ser95 played an important role in the regulation of SNARE-dependent membrane fusion during FcR-mediated phagocytosis.

Protein Kinase C-epsilon in Membrane Delivery during Phagocytosis

During phagocytosis, internal membranes are recruited to the site of pathogen binding and fuse with the plasma membrane, providing the membrane needed for pseudopod extension and target uptake. The mechanism by which vesicles destined for the phagosome are generated, targeted, and fuse is unknown. We established that Golgi-associated protein kinase C-epsilon (PKC-ε) is necessary for the addition of membrane during FcyR-mediated phagocytosis. PKC-ε is tethered to the Golgi through interactions between its' regulatory domain and the Golgi lipids PI4P and diacylglycerol; disruption of these interactions prevents PKC-ε concentration at phagosomes and decreases phagocytosis. The accumulated evidence suggests that PKC-ε orchestrates vesicle formation at the Golgi by a mechanism requiring lipid binding but not enzymatic activity. This review discusses how PKC-ε might mediate vesicle formation at the level of budding and fission. Specifically, we discuss PKC-ε binding partners, the formation of lipid subdomains to generate membrane curvature, and PKC-ε mediated links to the actin and microtubule cytoskeleton to provide tension for vesicle fission. Assimilating information from several model systems, we propose a model for PKC-ε mediated vesicle formation for exocytosis during phagocytosis that may be applicable to other processes that require directed membrane delivery and fusion.

Golgi-Associated Protein Kinase C-ε Is Delivered to Phagocytic Cups: Role of Phosphatidylinositol 4-Phosphate

Protein kinase C-ε (PKC-ε) at phagocytic cups mediates the membrane fusion necessary for efficient IgG-mediated phagocytosis. The C1B and pseudosubstrate (εPS) domains are necessary and sufficient for this concentration. C1B binds diacylglycerol; the docking partner for εPS is unknown. Liposome assays revealed that the εPS binds phosphatidylinositol 4-phosphate (PI4P) and PI(3,5)P₂ Wortmannin, but not LY294002, inhibits PKC-ε concentration at cups and significantly reduces the rate of phagocytosis. As Wortmannin inhibits PI4 kinase, we hypothesized that PI4P mediates the PKC-ε concentration at cups and the rate of phagocytosis. PKC-ε colocalizes with the trans-Golgi network (TGN) PI4P reporter, P4M, suggesting it is tethered at the TGN. Real-time imaging of GFP-PKC-ε-expressing macrophages revealed a loss of Golgi-associated PKC-ε during phagocytosis, consistent with a Golgi-to-phagosome translocation. Treatment with PIK93, a PI4 kinase inhibitor, reduces PKC-ε at both the TGN and the cup, decreases phagocytosis, and prevents the increase in capacitance that accompanies membrane fusion. Finally, expression of the Golgi-directed PI4P phosphatase, hSac1-K2A, recapitulates the PIK93 phenotype, confirming that Golgi-associated PI4P is critical for efficient phagocytosis. Together these data are consistent with a model in which PKC-ε is tethered to the TGN via an εPS-PI4P interaction. The TGN-associated pool of PKC-ε concentrates at the phagocytic cup where it mediates the membrane fusion necessary for phagocytosis. The novelty of these data lies in the demonstration that εPS binds PI4P and PI(3,5)P₂ and that PI4P is necessary for PKC-ε localization at the TGN, its translocation to the phagocytic cup, and the membrane fusion required for efficient Fc [γ] receptor-mediated phagocytosis.

The effect of engineered nanotopography of electrospun microfibers on fiber rigidity and macrophage cytokine production

Currently, it is unknown how the mechanical properties of electrospun fibers, and the presentation of surface nanotopography influence macrophage gene expression and protein production. By further elucidating how specific fiber properties (mechanical properties or surface properties) alter macrophage behavior, it may be possible to create electrospun fiber scaffolds capable of initiating unique cellular and tissue responses. In this study, we determined the elastic modulus and rigidity of fibers with varying topographies created by finely controlling humidity and including a non-solvent during electrospinning. In total,five fiber scaffold types were produced. Analysis of fiber physical properties demonstrated no change in fiber diameter amongst the five different fiber groups. However, the four different fibrous scaffolds with nanopits or divots each possessed different numbers of pits with different nanoscale dimensions. Unpolarized bone marrow derived murine macrophages (M0), macrophages polarized towards a pro-inflammatory phenotype (M1), or macrophages polarized towards anti-inflammatory phenotype (M2b) were placed onto each of the scaffolds and cytokine RNA expression and protein production were analyzed. Specific nanotopographies did not appreciably alter cytokine production from undifferentiated macrophages (M0) or anti-inflammatory macrophages (M2b), but a specific fiber (with many small pits) did increase IL-12 transcript and IL-12 protein production compared to fibers with small divots. When analyzing the mechanical properties between fibers with divots or with many small pits,divoted fibers possessed similar elastic moduli but different stiffness values. In total,we present techniques capable of creating unique electrospun fibers. These unique fibers have varying fiber mechanical characteristics and modestly modulate macrophage cytokine expression.

A novel phosphorylation site at Ser130 adjacent to the pseudosubstrate domain contributes to the activation of protein kinase C-δ

Protein kinase C-δ (PKCδ) is a signalling kinase that regulates many cellular responses. Although most studies focus on allosteric mechanisms that activate PKCδ at membranes, PKCδ also is controlled via multi-site phosphorylation [Gong et al. (2015) Mol. Cell. Biol. 35: , 1727-1740]. The present study uses MS-based methods to identify PKCδ phosphorylation at Thr(50) and Ser(645) (in resting and PMA-treated cardiomyocytes) as well as Thr(37), Thr(38), Ser(130), Thr(164), Thr(211), Thr(215), Ser(218), Thr(295), Ser(299) and Thr(656) (as sites that increase with PMA). We focused on the consequences of phosphorylation at Ser(130) and Thr(141) (sites just N-terminal to the pseudosubstrate domain). We show that S130D and T141E substitutions co-operate to increase PKCδ's basal lipid-independent activity and that Ser(130)/Thr(141) di-phosphorylation influences PKCδ's substrate specificity. We recently reported that PKCδ preferentially phosphorylates substrates with a phosphoacceptor serine residue and that this is due to constitutive phosphorylation at Ser(357), an ATP-positioning G-loop site that limits PKCδ's threonine kinase activity [Gong et al. (2015) Mol. Cell. Biol. 35: , 1727-1740]. The present study shows that S130D and T141E substitutions increase PKCδ's threonine kinase activity indirectly by decreasing G loop phosphorylation at Ser(357). A S130F substitution [that mimics a S130F single-nt polymorphism (SNP) identified in some human populations] also increases PKCδ's maximal lipid-dependent catalytic activity and confers threonine kinase activity. Finally, we show that Ser(130)/Thr(141) phosphorylations relieve auto-inhibitory constraints that limit PKCδ's activity and substrate specificity in a cell-based context. Since phosphorylation sites map to similar positions relative to the pseudosubstrate domains of other PKCs, our results suggest that phosphorylation in this region of the enzyme may constitute a general mechanism to control PKC isoform activity.

Specifically targeted delivery of protein to phagocytic macrophages

Macrophages play important roles in the pathogenesis of various diseases, and are important potential therapeutic targets. Furthermore, macrophages are key antigen-presenting cells and important in vaccine design. In this study, we report on the novel formulation (bovine serum albumin [BSA]-loaded glucan particles [GMP-BSA]) based on β-glucan particles from cell walls of baker’s yeast for the targeted delivery of protein to macrophages. Using this formulation, chitosan, tripolyphosphate, and alginate were used to fabricate colloidal particles with the model protein BSA via electrostatic interactions, which were caged and incorporated BSA very tightly within the β-glucan particle shells. The prepared GMP-BSA exhibited good protein-release behavior and avoided protein leakage. The particles were also highly specific to phagocytic macrophages, such as Raw 264.7 cells, primary bone marrow-derived macrophages, and peritoneal exudate macrophages, whereas the particles were not taken up by nonphagocytic cells, including NIH3T3, AD293, HeLa, and Caco-2. We hypothesize that these tightly encapsulated protein-loaded glucan particles deliver various types of proteins to macrophages with notably high selectivity, and may have broad applications in targeted drug delivery or vaccine design against macrophages.