Page 1041 - Williams Hematology ( PDFDrive )

1016 Part VII: Neutrophils, Eosinophils, Basophils, and Mast Cells Chapter 66: Disorders of Neutrophil Function 1017

CD11b and FcγRIIIB has been demonstrated by experiments in which signal-related kinase (ERK)-2 and then myosin light-chain kinase.
240
capping of one receptor results in co-capping a substantial fraction of Following phosphorylation of myosin, reorganization of the actin
the other receptor. CD11b can also interact with the transmembrane cytoskeletal occurs leading to phagocytosis. Concomitant with the
FcγRII, and both of these molecules can modify each other’s signals. 234 activation of PLD, ceramide is generated by a neutral sphingomyeli-
The tyrosine kinase Syk, plays a critical role in the phagocytic path- nase activity found in the plasma membrane of neutrophils and it is
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way mediated by FcγRIIA. A cytoplasmic amino acid motif, known most likely important in attenuating the activity of the cells through
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as ITAM, is present on FcγRIIA and FcγRI/γ (a receptor found on IFN- inhibition of PLD. Following engagement of the Fc receptors and Syk
γ–stimulated myeloid cells) and is essential for the phagocytic response activation in the neutrophil, PI3K is also activated. Inhibition of PI3K
during crosslinking of these two Fc receptors. Binding of Src family pro- activity impedes phagocytosis. 237
tein tyrosine kinases to the ITAM leads to activation of Src family pro- Arachidonate Metabolism In addition to their participation as
tein tyrosine kinases and ITAM tyrosine phosphorylation. This serves putative second-messenger products in the stimulus–response coupling
to recruit phosphatidylinositol 3′-kinase (PI3K) and Syk, which when pathway, many lipid metabolites may be released from stimulated
activated phosphorylates multiple substrates, including neighboring neutrophils, and, in turn, modulate cell function by interacting with
ITAMs. Syk is recruited from the cytosolic pool. The essential role for receptors on other neutrophils. Phospholipase A , present on both the
2
249
Syk-affecting signal transduction is reflected by ITAM-dependent acti- granules and plasma membranes of neutrophils, as well as the cyto-
250
vation of actin assembly. Other tyrosine kinases, including Src kinases, sol, is activated during neutrophil stimulation, yielding arachidonic
especially Lyn, facilitate the formation of the phagosome. Once active acid as one of the major end products. Arachidonic acid is not only
236
microfilaments are formed, they enhance the activity of phospholipase released from stimulated neutrophils, but also serves as a regulator of
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D (PLD) to generate phosphatidic acid (PA), a necessary phospholipid phospholipase A (PLA ) activity and as a stimulus for these cells.
2
2
for phagocytosis to ensue. 237,238 Sensitivity of the cells to other stimuli can be enhanced with arachi-
donic acid and other long-chain fatty acids. 252
Phospholipid Metabolism and Tyrosine Kinase Activation Arachidonic acid can also be metabolized by the lipoxygenase
The next step in signal transduction can be attributed to interactions pathway to produce hydroxyeicosatetraenoic acids (HETEs), includ-
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of receptor-activated G proteins or through FcγRIIA and tyrosine ing 5-HETE, 12-HETE, and 5,12-diHETE. These compounds
254
kinases with phospholipases. 239,240 For instance, a membrane-associated have also been shown to induce several neutrophil responses.
phosphoinositide-specific phospholipase is activated upon stimulation Stimulated neutrophils also produce the diHETE LTB through the
4
with chemotactic stimuli. In particular, phospholipase C (PLC) hydrolyzes lipoxygenase pathway. LTB and other leukotrienes can be released
4
255
phosphatidylinositol-4,5-bisphosphate (PIP ) and phosphatidyl in response to a variety of stimuli. Receptors for LTB have been
4
2
inositol-4-monophosphate (PIP ) to the putative second-messenger partially purified, and their activation serves as a potent stimulus for
1
products inositol 1,4,5-trisphosphate (IP ) and 1,2-diacylglycerol (DAG) chemotaxis and adherence. 256
3
241
(see Fig. 66–4). In neutrophils, IP interacts with a specific intracel- Another potent mediator of inflammation produced by stimulated
3
2+
lular receptor and stimulates the release of Ca , as well as opens Ca neutrophils is 1-O-alkyl-2-acetyl-sn-glyceryl-3-phosphoryl choline,
2+
channels on the plasma membrane, resulting in rises in intracellular also known as PAF. Not only is PAF synthesized by neutrophils and
21
Ca . Activation of the small GTP-binding proteins of the Rac, Rho, activated endothelial cells, but it induces degranulation, aggregation,
2+ 242
29
and Cdc42 families regulates actin-dependent processes such as mem- and superoxide generation. Inflamed endothelium generates PAF,
brane ruffling, formation of pseudopodia, and stress fibers leading to which serves to immobilize neutrophils on the luminal surface of the
cell adhesion and motility, and appears critical in neutrophil func- endothelial cells, thereby facilitating the interaction of the neutrophil
tion, 243,244 while working in concert with the phospholipases. integrin receptors with the ICAM ligands on the endothelial cells.
Even in the absence of PLC metabolism, there is a significant increase
2+
in DAG and Ca intracellularly that accompanies phagocytosis. Ca Degranulation and Membrane Fusion
2+
245
is necessary for granule phagosome fusion and DAG has been linked to In stimulated cells the signal transduction cascade activates G proteins,
246
2+
both particle ingestion and degranulation. Both can be formed by the followed by enhanced intracellular Ca , lipid remodeling, and pro-
activation of PLD, which hydrolyzes phosphatidylcholine to produce PA tein kinase activation. These events culminate in secretion. This ulti-
and choline. Activation of PLD is mediated by Rho and/or ADP-ribosyla- mate event—the fusion of granule membranes with phagosomes or the
tion factor (ARF). Diacylglycerol is then generated by PA phosphohy- plasma membrane—occurs rapidly and is highly efficient.
247
drolase, which catalyzes the dephosphorylation of PA. The hallmark of Fusion Proteins Over the past 20 years the SNARE (soluble N-
the phosphatidylcholine-derived DAG is the presence of 1-O-alkyl link- ethylmaleimide-sensitive factor attachment protein receptor) hypothesis
ages. During PA formation by the action of PLD on phosphatidylcholine, has become the reigning paradigm for fusion of biomembranes.
104
PA can act as a Ca ionophore, thereby initiating fusogenic activity. The hypothesis is centered around the protein that is sensitive to N-
2+
248
Thus, the phosphatidylcholine acid generated during phagocytosis may ethylmaleimide (designated NEM-sensitive fusion protein or NSF) and
promote fusion of neutrophil granules with newly formed phagosomes. several SNAREs on the participating membranes. The SNAREs are
Another downstream target of DAG in phagocytosis is the acti- divided into the v-SNAREs, being found on vesicles or granules, and
vation of protein kinase C (PKC), particularly PKCδ, a Ca -independent t-SNAREs, being found on the target plasma membranes. The SNARE
2+
isozyme of PKC found in neutrophils. PKCδ is one of four PKC hypothesis has proven to have great predictive value as the constellation
237
isozymes that translocate to the plasma membrane during phagocy- of fusion proteins and their interactions appears in almost all species
tosis. During phagocytosis, PKCδ is translocated from the cytosol to and tissues. Initial docking of granules with the membrane to which
the plasma membrane. Accompanying the translocation of PKCδ to they fuse is likely mediated by Rab-GTPases. Once docking is obtained,
the membrane, RAF-1 translocation is promoted. Following translo- SNAREs are recruited to both membranes and interact and mediate
cation of these two key components, mitogen-activated extracellular actual fusion assisted by SNARE-interacting proteins such as sSec1/
2+
signal-regulated kinase (MEK) activation occurs, which is followed Munc18 proteins and a local rise in Ca . Disassembly of the fusion com-
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by activation of mitogen-activated protein (MAP) kinase/extracellular plex is mediated by NSF in an ATP-dependent process. The t-SNARE
Kaushansky_chapter 66_p1005-1042.indd 1016 9/21/15 10:48 AM