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Mitochondria in neutrophil apoptosis
van Raam, B.J.
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Citation for published version (APA):
van Raam, B. J. (2009). Mitochondria in neutrophil apoptosis
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Download date: 18 Jun 2017
Preface
Outline and Scope of the Thesis
Preface
Introduction
Neutrophils are the most abundant phagocytes of the human immune system, with a primary function in
phagocytosing invading micro-organisms and killing them by means of a rapid, NADPH-dependent, respiratory burst.
Neutrophils have a very short life-span of up to 24 hours in the circulation and several more days in the tissues, after
which they are programmed to die by apoptosis. The energy that neutrophils require for their functions is mostly
derived from glycolysis 1, whereas most other cells derive a substantial part of their energy from oxidative
phosphorylation (OXPHOS) by the respiratory chain, a series of four respiratory complexes (I-IV) within the
mitochondria. In fact, it has long been thought that neutrophils do not possess any, or very few, functional
mitochondria, since these organelles were hard to identify on electron micrographs and the cells did not seem to
respond to OXPHOS inhibitors 1,2. Several years ago, the introduction of novel fluorescent probes and refined
microscopy techniques led to the identification of a complex mitochondrial network in neutrophils 3,4.
The function of mitochondria is not restricted to metabolism. These organelles also play an important role in
apoptosis. A number of proteins contained within the mitochondria, such as cytochrome c, Smac/DIABLO, AIF and
Omi/Htra2, rapidly accelerate the process of apoptosis upon release 5,6. Although neutrophils contain very little
cytochrome c, the other cell-death proteins are clearly present in their mitochondria 4,7. Since the mitochondria in
neutrophils have no obvious role in cellular metabolism, it has been suggested that their function is restricted to the
regulation of apoptosis 4. However, neutrophil mitochondria do maintain a membrane potential (∆ψm), normally
indicative of mitochondrial respiration and OXPHOS-activity 3,4.
Scope of the Thesis
The scope of this thesis is to elucidate the functions and properties of mitochondria in neutrophils, as well as to
address the role these organelles play in neutrophil metabolism and apoptosis, respectively. Chapter 1 provides an
overview of the earlier literature on neutrophil mitochondria in apoptosis. In chapter 2, the metabolic properties of
neutrophil mitochondria are described. Neutrophils appear to maintain ∆ψm by respiratory chain complex-III activity
only, while respiratory supercomplexes, which are required for efficient electron-transfer along the respiratory chain,
are mostly absent from neutrophils. Chapter 3 addresses the functional consequences of this mitochondrial defect for
neutrophils. It appears that, while neutrophil mitochondria produce a relatively large amount of reactive oxygen
species (mROS) due to their inherent metabolic defect, these mROS do not induce apoptosis but fulfill an important
role as signaling intermediates in neutrophil survival. Chapter 4 places the metabolic properties of neutrophil
mitochondria in the context of Barth syndrome (BTHS), a genetically determined mitochondrial disease associated
with neutropenia. BTHS neutrophils produce elevated amounts of mROS and have a lowered ∆ψm, but display normal
survival in the presence of enhanced phosphatidyl serine (PS) exposure, normally a hallmark for apoptosis. The results
described in this chapter indicated that neutrophil mitochondria might play an important role in the regulation of the
intracellular Ca2+ homeostasis. Chapter 5 discusses the role of intracellular free Ca2+ and calpains, Ca2+-activated
cysteine proteases, in neutrophil apoptosis. It appeared that the intracellular free Ca2+ levels rise steadily during
neutrophil apoptosis, a process that accelerates apoptosis via calpain-mediated degradation of XIAP. The growth
factor G-CSF prevented this rise in Ca2+ levels and thus prevented calpain activation and slowed down apoptosis.
Chapter 6 discusses a gene array study with neutrophils from healthy volunteers treated with G-CSF and
dexamethasone, a common method for mobilizing neutrophils for granulocyte transfusion. From this study, it
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Preface
appeared that G-CSF also inhibits calpains by enhancing the expression of calpastatin, the endogenous inhibitor of
calpains. Finally, chapter 7 discusses the role of the mitochondria as Ca2+ buffers and the functional association of the
mitochondria with the ER, the main intracellular Ca2+ storage. The anti-apoptotic Bcl-2 family member Bfl-1 appeared
to have a role in the dissociation of these organelles during apoptosis, but the exact nature of this role has not yet been
elucidated.
In sum, this thesis demonstrates the complex and essential role that neutrophil mitochondria have, both in the
process of apoptosis and in mediating survival as well as in the continued functioning of the cells. These findings
imply that mitochondria play a more complex role in cell death and survival than previously assumed.
References
(1) Borregaard N, Herlin T. Energy metabolism of human neutrophils during phagocytosis. J Clin Invest. 1982;70:550-557.
(2) Hirsch JG, Fedorko ME. Ultrastructure of human leukocytes after simultaneous fixation with glutaraldehyde and osmium
tetroxide and "postfixation" in uranyl acetate. J Cell Biol. 1968;38:615-627.
(3) Fossati G, Moulding DA, Spiller DG et al. The mitochondrial network of human neutrophils: role in chemotaxis,
phagocytosis, respiratory burst activation, and commitment to apoptosis. J Immunol. 2003;170:1964-1972.
(4) Maianski NA, Geissler J, Srinivasula SM et al. Functional characterization of mitochondria in neutrophils: a role
restricted to apoptosis. Cell Death Differ. 2004;11:143-153.
(5) Saelens X, Festjens N, Walle LV et al. Toxic proteins released from mitochondria in cell death. Oncogene.
2004;23:2861-2874.
(6) Munoz-Pinedo C, Guio-Carrion A, Goldstein JC et al. Different mitochondrial intermembrane space proteins are released
during apoptosis in a manner that is coordinately initiated but can vary in duration. Proc Natl Acad Sci U S A.
2006;103:11573-11578.
(7) Murphy BM, O'Neill AJ, Adrain C, Watson RW, Martin SJ. The apoptosome pathway to caspase activation in primary
human neutrophils exhibits dramatically reduced requirements for cytochrome C. J Exp Med. 2003;197:625-632.
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