Download CD95 ligation and intracellular membrane flow

Biochem. J. (2008) 413, e11–e12 (Printed in Great Britain)
e11
doi:10.1042/BJ20081094
COMMENTARY
CD95 ligation and intracellular membrane flow
Roland REINEHR and Dieter HÄUSSINGER1
Clinic for Gastroenterology, Hepatology and Infectiology, Heinrich-Heine-University Düsseldorf, Düsseldorf, Germany
Whereas ligation of the CD95 death receptor in the plasma membrane of so-called type I cells leads to a direct caspase 8-dependent
activation of downstream effector caspases, mitochondrial amplification of caspase 8-derived signals is required in so-called type II
cells in order to execute apoptotic cell death. In type I cells CD95L
(CD95 ligand) binding to CD95 results in a ceramide-dependent
formation of the DISC (death-inducing signalling complex) and
caspase 8-dependent CD95 clustering in the plasma membrane,
followed by an internalization of these multimeric-receptor–DISC
complexes. In contrast, in the hepatocyte, a type II cell, the bulk
of CD95 is stored intracellularly under resting conditions and
only a few ‘sentinel’ CD95 receptors are present in the plasma
membrane. However, their activation by CD95L is sufficient
to trigger a caspase 8-dependent endosomal acidification and a
ceramide-dependent trafficking of intracellularly stored CD95 to
the plasma membrane, thereby amplifying CD95 activation. Thus,
in both type I and type II cells, ceramide and CD95 receptor
endo- and exo-cytosis are involved in CD95-mediated apoptosis,
but apparently in different ways. This, however, is not the only
effect of CD95 ligation on intracellular membrane flow in type II
cells, and evidence has been presented that soon after CD95
ligation Golgi elements intermix caspase-dependently with mitochondria. In this issue of the Biochemical Journal, Matarrese
et al. report another aspect on endocytosis in response to CD95
ligation in type II cells, namely a caspase-independent endocytosis
with vesicle translocation to the mitochondrial compartment, suggestive of an interplay between both organelles in the sense of an
‘organelle scrambling’. Thus early effects of CD95 activation
on intracellular membrane flow may be much more complex
than previously thought, but much has still to be learned about
signalling mechanisms and the role they play in apoptosis.
CD95-mediated apoptosis is an important process with enormous
physiological and pathophysiological impact and cell-typespecific features. Whereas in so-called type I cells, the CD95L
(CD95 ligand)–CD95 (CD95 receptor) interaction is followed
by formation of the DISC (death-inducing signalling complex)
and caspase 8 activation, which is sufficient for activation of
downstream effector caspases and execution of cell death, in socalled type II cells, the apical caspase signals require mitochondrial amplification for induction of apoptotic cell death [1]. This
amplification is achieved by the caspase 8-dependent cleavage
of Bcl-2-family members, e.g. Bid, which induce a loss of the
MMP (mitochondrial membrane potential) and the release of proapoptotic factors such as cytochrome c, which finally trigger the
activation of downstream ‘effector caspases’ [1].
It has been reported recently that, in type I cells, CD95L–CD95
ligation is followed by a ceramide-dependent DISC formation and
subsequent caspase 8-dependent CD95 clustering at the plasma
membrane which is then followed by an internalization of these
multi-receptor–DISC conglomerates [2]. Interestingly, inhibition
of CD95 internalization is followed by activation of alternative
signalling cascades, i.e. activation of the proliferative ERK
(extracellular-signal-regulated kinase) and NF-κB (nuclear factor
κB) pathway [2], indicating that, depending on the signalling
context, the CD95L–CD95 interaction can couple to both cell
death and cell proliferation.
Whereas in most type I cells the bulk of CD95 is already
located at the plasma membrane under control conditions, it has
been reported, for at least some type II cells, that both CD95Ldependent [3] and -independent activation of the CD95 system
[4] involves substantial trafficking of the CD95 receptor from the
cellular interior to the plasma membrane. In prototypical type II
cells, such as rat hepatocytes, the bulk of CD95 is stored intracellularly, with only very few ‘sentinel’ receptors at the plasma
membrane. Their ligation triggers a caspase 8-dependent rise of
the intracellular chloride concentration, leading to an endosomal
acidification, subsequent activation of acidic sphingomyelinase
and ceramide formation [5]. These events occur within seconds
after CD95 ligation and trigger a PKCζ (protein kinase
Cζ )-mediated activation of NADPH oxidase with subsequent
formation of ROS (reactive oxygen species) [5]. This ROS
response then leads to a Yes-mediated EGFR (epidermal growth
factor receptor) transactivation and a JNK (c-Jun N-terminal
kinase)-dependent association of the CD95 with the EGFR, which
allows for EGFR-mediated CD95 tyrosine phosphorylation [3].
CD95 tyrosine phosphorylation is required for further recruitment
of CD95 to the CD95–EGFR complex (oligomerization) and
subsequent trafficking of the CD95–EGFR complex to the
plasma membrane, where DISC formation and substantial caspase
8 activation occurs [3]. Inhibition of the vacuolar-type H+ ATPase by bafilomycin abolishes CD95L-induced endosomal
acidification and apoptosis induction [5]. Exocytic recruitment
of CD95–EGFR complexes to the plasma membrane in type II
cells, however, is only one aspect of the early alterations of
intracellular membrane flow induced by CD95 ligation. Evidence
has been presented for a complex mixing of Golgi elements
with mitochondria and a caspase-dependent outward movement
of secretory endomembranes [6].
In this issue of the Biochemical Journal, Matarrese et al. [7] now
report on another role of endosomes in CD95-dependent type II
cell death by demonstrating an induction of endocytosis in the
human lymphoblastoid T cell line CEM in response to CD95
ligation. Transmission electron microscopy and immunogold
1
Key words: apoptosis, cell volume, endosomal compartment,
endocytosis, CD95/Fas/APO-1 receptor, CD95 trafficking, mitochondrial amplification, mitochondrial membrane potential
(MMP), type II cell.
To whom correspondence should be addressed (email [email protected]).
c The Authors Journal compilation c 2008 Biochemical Society
e12
R. Reinehr and D. Häussinger
labelling studies on endocytosed BSA revealed not only an
increase of endocytotic vacuoles, but also provided evidence for a
scrambling of such vesicles with mitochondrial membranes. This
cross-talk between endosomal and mitochondrial compartments
was accompanied by a loss of the MMP and apoptosis execution.
This cell remodelling was sensitive to monensin and was absent
not only after induction of receptor-independent cell death with
staurosporine, but also in cells selected for multidrug resistance.
As shown in sucrose gradient experiments, these endocytotic
vesicles probably did not contain CD95 receptors. In contrast
with the reported CD95–DISC internalization upon the CD95L–
CD95 interaction in type I cells [2] and the scrambling between
Golgi-derived and mitochondrial membranes in type II cells [6],
the reported increase in type II cell endocytosis was caspaseindependent. From these results the authors concluded that the
endosomal/mitochondrial ‘organelle cross-talk’ may play a key
role in both propagation and amplification of CD95-mediated
apoptosis. Thus CD95 ligation in type II cells induces early and
complex alterations in intracellular membrane flow, involving
endosomes, mitochondria and Golgi, which may contribute to the
induction of cell death. The mechanisms and signalling pathways
towards CD95-triggered endocytosis, however, remain unclear.
Ceramide is formed very rapidly in response to CD95 ligation
and may be a key compound in this respect, because there is
evidence for its involvement in CD95 trafficking to the plasma
membrane [5], in Golgi fragmentation prior to, or parallel with
caspase activation [8] and in CD95 clustering in the plasma
membrane (for a review, see [9]). Furthermore, ceramide can
artificially alter membrane traffic and the lipid composition of
diverse organelles [10]. On the other hand, CD95 engagement was
reported to induce a rapid apoptotic cell volume decrease, which
in part involves opening of ion channels in the plasma membrane
and the release of organic osmolytes, such as taurine (for a review,
see [9]). The role of apoptotic volume decrease for execution of
apoptosis is under discussion. However, hyperosmotic hepatocyte
shrinkage not only triggers volume regulatory responses, but also
induces ligand-independent activation of the CD95 death receptor
[3]. Thus the possibility must be considered that CD95 ligationinduced endocytosis is part of a volume-regulatory response,
whose role in apoptosis induction is far from being clear.
Surprisingly, collapse of the MMP and apoptosis induction in
response to CD95L were caspase-inhibitor-sensitive, whereas
CD95L-induced endocytosis was not. This is difficult to reconcile
with the proposed key role of endocytosis in the propagation
and amplification of CD95-activated signalling. In the study by
Received 29 May 2008/3 June 2008; accepted 4 June 2008
Published on the Internet 15 July 2008, doi:10.1042/BJ20081094
c The Authors Journal compilation c 2008 Biochemical Society
Matarrese et al. [7], monensin inhibited both CD95L-induced
endocytosis and apoptosis, suggesting that endocytosis is important for apoptosis induction. However, monensin may also inhibit
acidification of intracellular vacuoles and prevent their exocytosis,
which is required for the recruitment of intracellularly stored
CD95 to the plasma membrane in order to amplify early CD95
signalling [5]. Despite these uncertainties in defining the role
of endocytosis in apoptosis induction in type II cells, these and
previous observations by Matarrese et al. [6,7] open an interesting
field of research on the interplay between death receptor engagement and intracellular membrane flow. Clearly, the consequences
of CD95 receptor ligation are much more complex than previously
thought, and it appears that multiple amplification loops become
activated very early in response to CD95 ligation, which can
transform this ‘snowball’ into the ‘avalanche’ of cell death.
REFERENCES
1 Scaffidi, C., Fulda, S., Srinivasan, A., Friesen, C., Li, F., Tomaselli, K. J., Debatin, K. M.,
Krammer, P. H. and Peter, M. E. (1998) Two CD95 (APO-1/Fas) signalling pathways.
EMBO J. 17, 1675–1687
2 Lee, K. H., Feig, C., Tchikov, V., Schickel, R., Hallas, C., Schütze, S., Peter, M. E. and
Chan, A. C. (2006) The role of receptor internalization in CD95 signaling. EMBO J. 25,
1009–1023
3 Reinehr, R., Schliess, F. and Häussinger, D. (2003) Hyperosmolarity and CD95L trigger
CD95/EGFR association and tyrosine phosphorylation of CD95 as prerequisites for CD95
membrane trafficking and DISC formation. FASEB J. 17, 731–733
4 Sodeman, T., Bronk, S. F., Roberts, P. J., Miyoshi, H. and Gores, G. J. (2000) Bile salts
mediate hepatocyte apoptosis by increasing cell surface trafficking of Fas. Am. J. Physiol.
Gastrointest. Liver Physiol. 278, G992–G999
5 Reinehr, R., Sommerfeld, A., Keitel, V., Grether-Beck, S. and Häussinger, D. (2008)
Amplification of CD95 activation by caspase 8-induced endosomal acidification in rat
hepatocytes. J. Biol. Chem. 283, 2211–2222
6 Ouasti, S., Matarrese, P., Paddon, R., Khosravi-Far, R., Sorice, M., Tinari, A., Malorni, W.
and Degli Esposti, M. (2007) Death receptor ligation triggers membrane scrambling
between Golgi and mitochondria. Cell Death Differ. 14, 453–461
7 Matarrese, P., Manganelli, V., Garofalo, T., Tinari, A., Gambardella, L., Ndebele, K.,
Khosravi-Far, R., Sorice, M., Degli Esposti, M. and Malorni, W. (2008) Endosomal
compartment contributes to the propagation of CD95/Fas-mediated signals in type II
cells. Biochem. J. 413, 467–478
8 Hu, W., Xu, R., Zhang, G., Jin, J., Szulc, Z. M., Bielawski, J., Hannun, Y. A., Obeid, L. M.
and Mao, C. (2005) Golgi fragmentation is associated with ceramide-induced cellular
effects. Mol. Biol. Cell 16, 1555–1567
9 Gulbins, E., Jekle, A., Ferlinz, K., Grassmé, H. and Lang, F. (2000) Physiology of
apoptosis. Am. J. Physiol. Renal Physiol. 279, 605–615
10 Cristea, I. M. and Degli Esposti, M. (2004) Membrane lipids and cell death: an overview.
Chem. Phys. Lipids 129, 133–160