Research Report 2010 2011 - Helmholtz-Zentrum für ...

96 SCIENTIFIC REPORTS | Infection and Immunity | Infl ammation and Immunity
03.5 Cell-death Mechanisms in Immunity
PROJECT LEADER | Prof. Dr. Ingo Schmitz | Research Group Systems-oriented Immunology and Inflammation
Research | isc09@helmholtz-hzi.de
PROJECT MEMBERS | Michaela Annemann | Frida Ewald | Ralf Höcker | Dr. Yvonne Rauter | Marc Schuster | Tanja Telieps
Types of cell death Multicellular organisms have evolved
multiple cell-death mechanisms which are activated upon
different stimuli and cell type-dependent to allow a finetuning
of cellular responses to environmental conditions.
Necrosis is characterized by swelling and rupture of the cells
so that intracellular contents leak out of the cell and initiate
an inflammatory immune response. During apoptosis, on
the other hand, cells shrink and the integrity of the plasma
membrane stays intact. Therefore, leakage of intracellular
components does not take place and activation of the immune
system is inhibited. Apoptosis plays an important role
in the interplay between pathogens and the host cell. In
this regard, immune cells may kill infected tissue cells by
the induction of apoptosis. On the other hand, viruses and
bacteria are able to inhibit host cell apoptosis to ensure their
own survival and further spreading. Deregulation of apoptosis
may lead to various diseases such as viral hepatitis (too
much apoptosis) or cancer (too little apoptosis).
Apoptotic signal transduction Two major apoptosis pathways
exist referred to as the extrinsic and intrinsic pathway,
respectively. The intrinsic pathway acts on the mitochondrium
and is regulated by proteins of the Bcl-2 family. The
extrinsic signalling cascade is initiated at the cell surface
by so-called death receptors with CD95 being a prototypical
family member. CD95 plays a crucial role in a number of
immunological processes such as T cell-dependent immune
responses but also in tumour development. Not much is
known about the apoptosis signalling mechanisms which
operate during acute and chronic infections in vivo. The aim
of our project is to analyse these signalling pathways using
different transgenic mice strains and apoptosis-modulating
substances (e.g. caspase inhibitors). One family of proteins
we may focus on are the FLICE-inhibitory proteins (FLIP),
important regulators of CD95 signaling. FLIP proteins have
been identified in mammalian cells (cellular FLIP = c-FLIP)
as well as in γ-herpesviruses (v-FLIP). We could recently
show that, though structurally very similar, viral and cellular
FLIP proteins act by different biochemical mechanisms. The
functional consequences and potential therapeutic windows
opened by these differences will be analysed in the future.
The intrinsic apoptosis pathway is activated by different stimuli
such as UV irradiation or growth factor deprivation. Subsequently,
pro-apoptotic BH3-only proteins are activated to inhibit the
anti-apoptotic Bcl-2 family members. This event allows Bax and
Bak to form pores in the outer mitochondrial membrane so that
apoptogenic factors such as cytochrome c are released. In the
cytosol, cytochrome c binds to Apaf-1 to form a multiprotein complex,
called the apoptosome, at which pro-caspase-9 is activated.
Caspase-9 then cleaves caspase-3 to start a proteolytical cascade
ultimately leading to cell death. The extrinsic apoptosis pathway
is activated by death receptors. Upon binding of trimeric ligands,
the receptor oligomerizes and recruits the adapter molecule
FADD and pro-caspase-8 to form a death inducing signaling complex
(DISC). Caspase-8 is activated at the DISC and can directly
activate caspase-3 or it cleaves the BH3-only protein Bid to tBid
leading to activation of the intrinsic pathway. c-FLIP proteins
can inhibit the activation of caspase-8 at the DISC.
Autophagy Autophagy is a lysosomal degradation pathway
that is important for cellular homeostasis, mammalian development
and immunity. On the one hand, autophagy is able to
eliminate intracellular pathogens. However, some pathogens
use the autophagic pathway to escape immune recognition.
Though the process of autophagy has been elucidated on
the molecular level in recent years, little is known about
signaling mechanisms regulating the effector functions of
these molecules. We have recently uncovered a novel regulatory
mechanism involving the stress kinase p38 and Atg5,
the latter being an essential component of the autophagic
pathway. We will address the role of autophagy regulation for
the intracellular survival of Staphylococcus aureus in mammalian
cells.
Ueffing N, Keil E, Freund C, Kühne R, Schulze-Osthoff K, Schmitz I (2008) “Structural
and mutational analyses of c-FLIPR, the only murine short FLIP isoform, reveal requirements
for DISC recruitment”. Cell Death and Differ 15(4):773-82.
Ueffing N, Schuster M, Keil E, Schulze-Osthoff K, Schmitz I (2008) “Upregulation of
c-FLIPshort by NFAT contributes to apoptosis resistance of short-term activated T cells”.
Blood 112(3):690-8.
Ueffing N, Singh KK, Christians A, Thorns C, Feller AC, Nagl F, Fend F, Heikaus S, Marx
A, Zotz RB, Brade J, Schulz WA, Schulze-Osthoff K, Schmitz I*, Schwerk C (2009) “A
SNP in the cellular FLICE-inhibitory protein (c-FLIP) gene determines protein isoform
production and is associated with risk of follicular lymphoma in humans”. Blood
114(3):572-9.
* corresponding author