Molecular definitions of cell death subroutines: Recommendations of the Nomenclature Committee on Cell Death 2012

Article (PDF Available)inCell death and differentiation 19(1):107-20 · July 2011with221 Reads
DOI: 10.1038/cdd.2011.96 · Source: PubMed
In 2009, the Nomenclature Committee on Cell Death (NCCD) proposed a set of recommendations for the definition of distinct cell death morphologies and for the appropriate use of cell death-related terminology, including 'apoptosis', 'necrosis' and 'mitotic catastrophe'. In view of the substantial progress in the biochemical and genetic exploration of cell death, time has come to switch from morphological to molecular definitions of cell death modalities. Here we propose a functional classification of cell death subroutines that applies to both in vitro and in vivo settings and includes extrinsic apoptosis, caspase-dependent or -independent intrinsic apoptosis, regulated necrosis, autophagic cell death and mitotic catastrophe. Moreover, we discuss the utility of expressions indicating additional cell death modalities. On the basis of the new, revised NCCD classification, cell death subroutines are defined by a series of precise, measurable biochemical features.
Molecular definitions of cell death subroutines:
recommendations of the Nomenclature Committee
on Cell Death 2012
L Galluzzi
, I Vitale
, JM Abrams
, ES Alnemri
, EH Baehrecke
, MV Blagosklonny
, TM Dawson
, VL Dawson
, WS El-Deiry
S Fulda
, E Gottlieb
, DR Green
, MO Hengartner
, O Kepp
, RA Knight
, S Kumar
, SA Lipton
, F Madeo
W Malorni
, P Mehlen
, G Nun
, ME Peter
, M Piacentini
, DC Rubinsztein
, Y Shi
, H-U Simon
P Vandenabeele
, E White
, J Yuan
, B Zhivotovsky
, G Melino
and G Kroemer*
In 2009, the Nomenclature Committee on Cell Death (NCCD) proposed a set of recommendations for the definition of distinct cell
death morphologies and for the appropriate use of cell death-related terminology, including ‘apoptosis’, ‘necrosis’ and ‘mitotic
catastrophe’. In view of the substantial progress in the biochemical and genetic exploration of cell death, time has come to switch
from morphological to molecular definitions of cell death modalities. Here we propose a functional classification of cell
death subroutines that applies to both in vitro and in vivo settings and includes extrinsic apoptosis, caspase-dependent
or -independent intrinsic apoptosis, regulated necrosis, autophagic cell death and mitotic catastrophe. Moreover, we discuss the
utility of expressions indicating additional cell death modalities. On the basis of the new, revised NCCD classification, cell death
subroutines are defined by a series of precise, measurable biochemical features.
Cell Death and Differentiation (2012) 19, 107–120; doi:10.1038/cdd.2011.96; published online 15 July 2011
Received 16.5.11; accepted 13.6.11; Edited by V De Laurenzi; published online 15.7.11
INSERM U848, ‘Apoptosis, Cancer and Immunity’, 94805 Villejuif, France;
Institut Gustave Roussy, 94805 Villejuif, France;
Paris Sud-XI, 94805 Villejuif,
Department of Cell Biology, UT Southwestern Medical Center, Dallas, TX 75390, USA;
Department of Biochemistry and Molecular Biology, Center for
Apoptosis Research, Kimmel Cancer Institute, Thomas Jefferson University, Philadelphia, PA 19107, USA;
Department of Cancer Biology, University of Massachusetts
Medical School, Worcester, MA 01605, USA;
Department of Cell Stress Biology, Roswell Park Cancer Institute, Buffalo, NY 14263, USA;
Neuroregeneration and Stem
Cell Programs, Institute for Cell Engineering, Johns Hopkins University School of Medicine, Baltimore, MD 21205, USA;
Cancer Institute Penn State, Hershey Medical
Center, Philadelphia, PA 17033, USA;
Institute for Experimental Cancer Research in Pediatrics, Goethe University, Frankfurt 60528, Germany;
The Beatson Institute
for Cancer Research, Glasgow G61 1BD, UK;
Department of Immunology, St. Jude Children’s Research Hospital, Memphis, TN 38105, USA;
Institute of Molecular
Life Sciences, University of Zurich, 8057 Zurich, Switzerland;
Institute of Child Health, University College London, London WC1N 3JH, UK;
Centre for Cancer
Biology, SA Pathology, Adelaide, South Australia 5000, Australia;
Department of Medicine, University of Adelaide, Adelaide, South Australia 5005, Australia;
Sanford-Burnham Medical Research Institute, San Diego, CA 92037, USA;
Salk Institute for Biological Studies, , La Jolla, CA 92037, USA;
The Scripps Research
Institute, La Jolla, CA 92037, USA;
Univerisity of California, San Diego, La Jolla, CA 92093, USA;
Ludwig Institute for Cancer Research, Oxford OX3 7DQ, UK;
Institute of Molecular Biosciences, University of Graz, 8010 Graz, Austria;
Department of Therapeutic Research and Medicines Evaluation, Section of Cell Aging and
Degeneration, Istituto Superiore di Sanita
, 00161 Rome, Italy;
Istituto San Raffaele Sulmona, 67039 Sulmona, Italy;
Apoptosis, Cancer and Development, CRCL,
69008 Lyon, France;
INSERM, U1052, 69008 Lyon, France;
CNRS, UMR5286, 69008 Lyon, France;
Centre Le
on Be
rard, 69008 Lyon, France;
University of
Michigan Medical School, Ann Arbor, MI 48109, USA;
Northwestern University Feinberg School of Medicine, Chicago, IL 60637, USA;
Laboratory of Cell Biology,
National Institute for Infectious Diseases IRCCS ‘L Spallanzani’, 00149 Rome, Italy;
Department of Biology, University of Rome ‘Tor Vergata’, 00133 Rome, Italy;
Cambridge Institute for Medical Research, Cambridge CB2 0XY, UK;
Shanghai Institutes for Biological Sciences, 200031 Shanghai, China;
Institute of
Pharmacology, University of Bern, 3010 Bern, Switzerland;
Department for Molecular Biology, Gent University, 9052 Gent, Belgium;
Department for Molecular
Biomedical Research, VIB, 9052 Gent, Belgium;
The Cancer Institute of New Jersey, New Brunswick, NJ 08903, USA;
Department of Cell Biology, Harvard Medical
School, Boston, MA 02115, USA;
Institute of Environmental Medicine, Division of Toxicology, Karolinska Institutet, 17177 Stockholm, Sweden;
Laboratory IDI-IRCCS, Department of Experimental Medicine, University of Rome ‘Tor Vergata’, 00133 Rome, Italy;
Medical Research Council, Toxicology Unit,
Leicester University, Leicester LE1 9HN, UK;
Metabolomics Platform, Institut Gustave Roussy, 94805 Villejuif, France;
Centre de Recherche des Cordeliers, 75005
Paris, France;
le de Biologie, Ho
pital Europe
en Georges Pompidou, AP-HP, 75908 Paris, France;
Paris Descartes, Paris 5, 75270 Paris, France
*Corresponding author: G Kroemer, INSERM U848, ‘Apoptosis, Cancer and Immunity’, Institut Gustave Roussy, Pavillon de Recherche 1, 39 rue Camille Desmoulins,
94805 Villejuif, France. Tel: þ 33 1 4211 6046; Fax þ 33 1 4211 6047; E-mail:
Keywords: autophagy; mitochondrial membrane permeabilization; necroptosis; parthanatos; TNFR1; TP53
Abbreviations: AIM2, absent in melanoma 2; AIF, apoptosis-inducing factor; BID, BH3-interacting domain death agonist; cIAP, cellular inhibitor of apoptosis protein;
CrmA, cytokine response modifier A; CYTC, cytochrome c; Dc
, mitochondrial transmembrane potential; DAPK1, death-associated protein kinase 1; DCC, deleted in
colorectal carcinoma; DD, death domain; DIABLO, direct IAP-binding protein with low pI; DISC, death-inducing signaling complex; EGFR, epidermal growth factor
receptor; ENDOG, endonuclease G; FADD, FAS-associated protein with a death domain; GMCSF, granulocyte-macrophage colony-stimulating factor; HTRA2, high
temperature requirement protein A2; IL, interleukin; MOMP, mitochondrial outer membrane permeabilization; mTOR, mammalian target of rapamycin; NCCD,
Nomenclature Committee on Cell Death; PAR, poly(ADP-ribose); NETs, neutrophil extracellular traps; PARP, PAR polymerase; PP2A, protein phosphatase 2A; PS,
phosphatidylserine; RIP, receptor-interacting protein kinase; RNAi, RNA interference; ROS, reactive oxygen species; SMAC, second mitochondria-derived activator of
caspases; ROCK1, RHO-associated, coiled-coil containing protein kinase 1; SQSTM1, sequestosome 1; TAB, TAK1-binding protein; TAK1, TGFb-activated kinase 1;
tBID, truncated BID; TG, transglutaminase; TGFb, transforming growth factor b; TNFa, tumor necrosis factor a; TNFR, TNFa receptor; TRADD, TNFR-associated DD;
TRAF, TNFR-associated factor; TRAIL, TNFa-related apoptosis-inducing ligand; TRAILR, TRAIL receptor; Z-VAD-fmk, N-benzyloxycarbonyl-Val-Ala-Asp-
fluoromethylketone; Z-YVAD-fmk, N-benzyloxycarbonyl-Tyr-Val-Ala-DL-Asp-fluoromethylketone
Cell Death and Differentiation (2012) 19, 107–120
2012 Macmillan Publishers Limited All rights reserved 1350-9047/12
Since the first descriptions of programmed cell death
mechanisms, which date back to the mid-1960s,
attempts have been made to classify cell death subroutines
based on morphological characteristics. Thus, in 1973
Schweichel and Merker
proposed a classification of several
cell death modalities, including ‘type I cell death’ associated
with heterophagy, ‘type II cell death’ associated with
autophagy and ‘type III cell death’, which was not associated
with any type of digestion, corresponding to apoptosis,
autophagic cell death and necrosis, respectively.
Even though deep insights into the molecular pathways that
regulate and execute cell death have been gained and
biochemical assays for monitoring cell death-related phenom-
ena have become laboratory routine, the scientific community
has not yet adopted a systematic classification of cell death
modalities based on biochemical rather than morphological
criteria. Nonetheless, there has been a tendency to dichot-
omize cell death events into either of two mutually exclusive
groups. Thus, caspase-dependent, tolerogenic, programmed
and physiological cell death instances have been contrasted
to their caspase-independent, immunogenic, accidental and
pathological counterparts, respectively.
The Nomenclature Committee on Cell Death (NCCD) has
formulated two subsequent rounds of recommendations in
2005 and 2009, in Cell Death and Differentiation.
unified criteria for the definition of cell death morphotypes
were proposed and guidelines on the use of cell death-related
terminology were given. The mission of the NCCD, as
formulated previously, is ‘to provide a forum in which names
describing distinct modalities of cell death are critically
evaluated and recommendations on their definition and use
are formulated, hoping that a non-rigid, yet uniform nomen-
clature will facilitate the communication among scientists and
ultimately accelerate the pace of discovery’.
In line with this
mission statement and following recent breakthroughs in cell
death research that have invalidated the notion that necrosis
would represent a merely accidental cell death mode (see
the NCCD believes that the time has become
appropriate for a novel systematic classification of cell death
based on measurable biochemical features.
Pros and Cons of Morphological Versus Biochemical
Classifications of Cell Death
The very first catalogs of cell death
necessarily relied on
morphological traits, because the biochemical tests that are
available nowadays for assessing the cell demise
were only
developed decades later. Nevertheless, morphological clas-
sifications have dominated the cell death research scene even
after the introduction of biochemical assays into the laboratory
routine. Several economical, methodological, educational and
theoretical reasons can be invoked to explain why the
scientific community has clung to a conservative, morpholo-
gical classification of cell death modalities. First, while
conventional light microscopy is available in all cell biology
laboratories, this is not the case for more sophisticated
equipment (e.g., fluorescence readers for monitoring caspase
activity). Second, virtually all cell biologists are familiar with
the observation of cell cultures under the microscope before
any sort of experimental intervention, a routine practice that
has certainly contributed to the persistence of morphological
classifications. Third, it has been assumed for a long time that
some degree of morphological uniformity would represent
the activation of identical or at least similar lethal signaling
cascades. Only recently has it become clear that apparently
similar cell death morphotypes most often hide a great degree
of functional, biochemical and immunological heterogene-
Moreover, it should always be remembered that the
presence of specific morphological features is not sufficient to
establish a causal link between a given process and cellular
Biochemical methods for assessing cell death have many
advantages over morphological techniques in that they are
quantitative, and hence less prone to operator-dependent
misinterpretations. However, these methods also have major
drawbacks and must be interpreted with caution, especially
when single parameters are being investigated.
Thus, it
should always be kept in mind that single biochemical
readouts cannot be used as unequivocal indicators of a
precise death modality, for a variety of reasons. First, a cell
death pathway that is often associated with a particular
biochemical process may be normally executed in the
absence of this process. Thus, at least in vitro, caspase
activation is not a strict requirement for multiple cases of
apoptosis (see below).
Similarly, phosphatidylserine (PS)
exposure, which is widely considered as an early marker of
apoptotic cell death,
reportedly does not occur in autop-
hagy-deficient cells succumbing to apoptosis.
Second, a
‘specific’ cell death-related phenomenon may occur along
with the execution of another cell death mode. For instance,
excessive generation of reactive oxygen species (ROS) and
reactive nitrogen species has been associated with several
cases of apoptosis,
yet it also occurs during regulated
Along similar lines, PS exposure is not a
prerogative of apoptotic cell death, as it also constitutes an
early feature of parthanatos and netosis (see below).
Third, a cell death-associated biochemical process can
develop at a sublethal or transient level, which does not lead
to the cell demise. Thus, while full-blown mitochondrial outer
membrane permeabilization (MOMP) constitutes a point-of-
no-return of intrinsic apoptosis (see below),
limited extents
of MOMP (i.e., concerning a fraction of the mitochondrial pool)
and the consequent (localized) activation of caspase-3 have
been shown to participate in several cell death-unrelated
programs such as the differentiation of megakaryocytes and
Definition of ‘Extrinsic Apoptosis’
The term ‘extrinsic apoptosis’ has been extensively used to
indicate instances of apoptotic cell death that are induced by
extracellular stress signals that are sensed and propagated by
specific transmembrane receptors.
Extrinsic apoptosis
can be initiated by the binding of lethal ligands, such as FAS/
CD95 ligand (FASL/CD95L), tumor necrosis factor a (TNFa)
and TNF (ligand) superfamily, member 10 (TNFSF10, best
known as TNF-related apoptosis inducing ligand, TRAIL), to
various death receptors (i.e., FAS/CD95, TNFa receptor 1
Functional classification of cell death modalities
L Galluzzi et al
Cell Death and Differentiation
(TNFR1) and TRAIL receptor (TRAILR)1–2, respectively).
Alternatively, an extrinsic pro-apoptotic signal can be dis-
patched by the so-called ‘dependence receptors’, including
netrin receptors (e.g., UNC5A-D and deleted in colorectal
carcinoma, DCC), which only exert lethal functions when the
concentration of their specific ligands falls below a critical
threshold level.
One prototypic signaling pathway leading to extrinsic
apoptosis is elicited by FAS ligation. In the absence of FASL,
FAS subunits spontaneously assemble at the plasma
membrane to generate trimers, owing to the so-called pre-
ligand assembly domain (PLAD).
Ligand binding stabilizes
such trimers while inducing a conformational change that
allows for the assembly of a dynamic multiprotein complex at
the cytosolic tail of the receptor. This occurs owing to a
conserved sequence of 80 residues that is shared by all death
receptors, the so-called ‘death domain’ (DD).
recruited at the DD of FAS include receptor-interacting protein
kinase 1 (RIPK1, best known as RIP1); FAS-associated
protein with a DD (FADD); multiple isoforms of c-FLIP;
cellular inhibitor of apoptosis proteins (cIAPs), E3 ubiquitin
ligases that also inhibit apoptosis owing to their ability to
interfere with caspase activation;
and pro-caspase-8 (or
The resulting supramolecular complex, which has
been dubbed ‘death-inducing signaling complex’ (DISC),
constitutes a platform that regulates the activation of
caspase-8 (or -10).
Of note, TNFR1-like proteins also require TNFR-associated
DD (TRADD) for recruiting FADD and caspase-8, whereas
FAS and TRAILR1/2 do not,
pointing to the existence of
subgroups of death receptors with specific signaling properties.
Similarly, the DDs of some death receptors, for instance,
TNFR1, recruit several other proteins that are not found at
FADD-assembled DISCs, including TNFR-associated factor 2
(TRAF2) and TRAF5.
In this specific context, RIP1 is
polyubiquitinated by cIAPs,
allowing for the recruitment of
transforming growth factor b (TGFb)-activated kinase 1
(TAK1), TAK1-binding protein 2 (TAB2) and TAB3, which
together can stimulate the canonical activation pathway for the
multifunctional transcription factor NF-kB.
Thus, death
receptor activation not always entails the transduction of a
lethal signal. This is particularly true for TNFR1, which has been
shown to mediate cellular outcomes as different as proliferation
and (distinct modalities of) cell death (see below). Irrespective
of these variations, both FAS- and TNFR1-elicited signaling
pathways appear to be subjected to a consistent degree of
regulation upon receptor compartmentalization. A detailed
discussion of these aspects goes largely beyond the scope of
this paper and can be found in Schutze et al.
In some cell types including lymphocytes (which have been
dubbed ‘type I cells’),
active caspase-8 directly catalyzes
the proteolytic maturation of caspase-3, thereby triggering the
executioner phase of caspase-dependent apoptosis in a
mitochondrion-independent manner.
In other cells such as
hepatocytes and pancreatic b cells (‘type II cells’),
caspase-8 mediates the proteolytic cleavage of BH3-interact-
ing domain death agonist (BID), leading to the generation of a
mitochondrion-permeabilizing fragment (known as truncated
Thus, while type I cells undergo extrinsic
apoptosis irrespective of any contribution by mitochondria
(tBID and MOMP can occur in these cells, but they are
dispensable for the execution of extrinsic apoptosis), type II
cells succumb from the activation of death receptors while
showing signs of MOMP, including the dissipation of
mitochondrial transmembrane potential (Dc
) and the release
of toxic proteins that are normally retained within the
mitochondrial intermembrane space (IMS).
Among these,
cytochrome c (CYTC) drives together with the cytoplasmic
adaptor protein APAF1 and dATP the assembly of the
apoptosome, another caspase-activating multiprotein com-
plex (see below).
The actual contribution of caspase-10, a
close homolog of caspase-8, to extrinsic apoptosis remains
unclear. Thus, whereas several reports indicate that caspase-
10 is recruited at the DISC and gets activated in response
to death receptor signaling,
it seems that caspase-10
cannot functionally substitute for caspase-8.
Moreover, it
has recently been suggested that caspase-10 might be
required for the lethal signaling cascade ignited by death
receptors in the presence of caspase inhibitors (see below).
The molecular routes by which dependence receptors are
connected to the rapid activation of executioner caspases, in
particular caspase-3, have only recently begun to emerge.
Thus, in the absence of their ligands, some dependence
receptors like Patched and DCC appear to interact with the
cytoplasmic adaptor protein DRAL to assemble a caspase-9-
activating platform.
Another dependence receptor, UNC5B,
responds to the withdrawal of netrin-1 by recruiting a signaling
complex that includes protein phosphatase 2A (PP2A) and
death-associated protein kinase 1 (DAPK1).
This multi-
protein interaction would lead to the PP2A-mediated depho-
sphorylation of DAPK, in turn unleashing its multifaceted
pro-apoptotic potential.
As a note, there are several other transmembrane proteins
that at least under selected circumstances can transduce
lethal signals in response to ligand binding, including
(although presumably not limited to) CD2,
and class I/II MHC molecules.
Similar to TNFR1, most of
these proteins have dual functions: depending on the cellular
context and triggering stimulus they can engage either
pro-survival or pro-death signals. However, the molecular
cascades triggered by these receptors are complex and often
poorly elucidated, in particular with regard to their dependency
on caspases.
On the basis of these considerations, we propose the
following operational definition of extrinsic apoptosis. Extrinsic
apoptosis is a caspase-dependent cell death subroutine,
and hence can be suppressed (at least theoretically) by pan-
caspase chemical inhibitors such as N-benzyloxycarbonyl-
Val-Ala-Asp-fluoromethylketone (Z-VAD-fmk) or by the
overexpression of viral inhibitors of caspases like cytokine
response modifier A (CrmA).
Extrinsic apoptosis would
feature one among three major lethal signaling cascades:
(i) death receptor signaling and activation of the caspase-8
(or -10)-caspase-3 cascade; (ii) death receptor signaling and
activation of the caspase-8-tBID-MOMP-caspase-9-
caspase-3 pathway; or (iii) ligand deprivation-induced depen-
dence receptor signaling followed by (direct or MOMP-depen-
dent) activation of the caspase-9-caspase-3 cascade (Table 1
and Figure 1).
Functional classification of cell death modalities
L Galluzzi et al
Cell Death and Differentiation
Definition of caspase-dependent and caspase-
independent ‘intrinsic apoptosis’. The apoptotic demise
of cells can be triggered by a plethora of intracellular stress
conditions, including DNA damage, oxidative stress,
cytosolic Ca
2 þ
overload, mild excitotoxicity (related to
glutamate receptor overstimulation in the nervous system),
accumulation of unfolded proteins in the endoplasmic
reticulum (ER) and many others. Although the signaling
cascades that trigger intrinsic apoptosis are highly
heterogeneous as far as the initiating stimuli are
concerned, they are all wired to a mitochondrion-centered
control mechanism.
Frequently, along with the propagation
of the pro-apoptotic signaling cascade, anti-apoptotic
mechanisms are also engaged, in an attempt to allow cells
to cope with stress. In this scenario, both pro- and anti-
apoptotic signals converge at mitochondrial membranes,
which become permeabilized when the former predominate
over the latter.
MOMP can start at the outer mitochondrial
Table 1 Functional classification of regulated cell death modes
Main biochemical features
dependence Examples of inhibitory interventions
Anoikis Downregulation of EGFR
Inhibition of ERK1 signaling
Lack of b1-integrin engagement
Overexpression of BIM
Caspase-3 (-6,-7) activation
++ BCL-2 overexpression
Z-VAD-fmk administration
Autophagic cell death MAP1LC3 lipidation
SQSTM1 degradation
 VPS34 inhibitors
or BCN1 genetic inhibition
intrinsic apoptosis
intrinsic apoptosis
Irreversible Dc
Release of IMS proteins
Respiratory chain inhibition
BCL-2 overexpression
Z-VAD-fmk administration
BCL-2 overexpression
Cornification Activation of transglutaminases
Caspase-14 activation
+ Genetic inhibition of TG1, TG3 or TG5
Genetic inhibition of caspase-14
Entosis RHO activation
ROCK1 activation
 Genetic inhibition of metallothionein 2A
Lysosomal inhibitors
Extrinsic apoptosis by death
Death receptor signaling
Caspase-8 (-10) activation
BID cleavage and MOMP (in type II cells)
Caspase-3 (-6,-7) activation
++ CrmA expression
Genetic inhibition of caspases (8 and 3)
Z-VAD-fmk administration
Extrinsic apoptosis by
dependence receptors
Dependence receptor signaling
PP2A activation
DAPK1 activation
Caspase-9 activation
Caspase-3 (-6,-7) activation
++ Genetic inhibition of caspases (9 and 3)
Genetic inhibition of PP2A
Z-VAD-fmk administration
Mitotic catastrophe Caspase-2 activation (in some instances)
TP53 or TP73 activation (in some instances)
Mitotic arrest
 Genetic inhibition of TP53 (in some instances)
Pharmacological or genetic inhibition of
caspase-2 (in some instances)
Necroptosis Death receptor signaling
Caspase inhibition
RIP1 and/or RIP3 activation
 Administration of necrostatin(s)
Genetic inhibition of RIP1/RIP3
Netosis Caspase inhibition
NADPH oxidase activation
NET release (in some instances)
 Autophagy inhibition
NADPH oxidase inhibition
Genetic inhibition of PAD4
Parthanatos PARP1-mediated PAR accumulation
Irreversible Dc
ATP and NADH depletion
PAR binding to AIF and AIF nuclear
 Genetic inhibition of AIF
Pharmacological or genetic
inhibition of PARP1
Pyroptosis Caspase-1 activation
Caspase-7 activation
Secretion of IL-1b and IL-18
++ Administration of Z-YVAD-fmk
Genetic inhibition of caspase-1
Abbreviations: ATG, autophagy; BCN1, beclin 1; Dc
, mitochondrial transmembrane potential; CrmA, cytokine response modifier A; DAPK1, death-associated
protein kinase 1; EGFR, epidermal growth factor receptor; ERK1, extracellular-regulated kinase 1; IL, interleukin; MAP1LC3, microtubule-associated protein 1 light
chain 3; MOMP, mitochondrial outer membrane permeabilization; NET, neutrophil extracellular trap; PAD4, peptidylarginine deiminase 4; PAR, poly(ADP-ribose);
PARP1, poly(ADP-ribose) polymerase 1; PP2A, protein phosphatase 2A; ROCK1, RHO-associated, coiled-coil containing protein kinase 1; SQSTM1,
sequestosome 1; TG, transglutaminase; Z-VAD-fmk, N-benzyloxycarbonyl-Val-Ala-Asp-fluoromethylketone; Z-YVAD-fmk, N-benzyloxycarbonyl-Tyr-Val-Ala-DL-
For classification purposes, pharmacological and genetic interventions should be considered inhibitory when they truly reduce the
incidence of cell death, but not when they only provoke a shift between different cell death modalities or when they change the morphology of cell death. For further
details, please refer to the ‘Notes of Caution’ and ‘Concluding Remarks’ sec tions
Functional classification of cell death modalities
L Galluzzi et al
Cell Death and Differentiation
membrane owing to the pore-forming activity of pro-apoptotic
members of the BCL-2 protein family such as BAK and
BAX or can result from a phenomenon (the so-called
mitochondrial permeability transition, MPT) that originates
at the inner mitochondrial membrane due to the opening of a
multiprotein complex known as the permeability transition
pore complex (PTPC).
Irrespective of the precise
biochemical and physical mechanisms through which it
develops, irreversible MOMP affecting most mitochondria
within a single cell has multiple lethal consequences: (i) the
dissipation of the Dc
, with cessation of mitochondrial ATP
synthesis and Dc
-dependent transport activities; (ii) the
release of toxic proteins from the IMS into the cytosol, as
this applies to CYTC, apoptosis-inducing factor (AIF),
endonuclease G (ENDOG), direct IAP-binding protein with
low pI (DIABLO, also known as second mitochondria-derived
activator of caspases, SMAC) and high temperature
requirement protein A2 (HTRA2); and (iii) the inhibition of
the respiratory chain (favored by the loss of CYTC), eliciting
or aggravating ROS overproduction and hence activating a
feed-forward circuit for the amplification of the apoptotic
Thus, intrinsic apoptosis results from a bioenergetic and
metabolic catastrophe coupled to multiple active executioner
mechanisms. Upon MOMP, cytosolic CYTC participates with
APAF1 and dATP in the formation of the apoptosome, which
triggers the caspase-9-caspase-3 proteolytic cascade.
AIF and ENDOG relocate to the nucleus, where they mediate
large-scale DNA fragmentation independently of cas-
SMAC/DIABLO and HTRA2 inhibit the anti-
apoptotic function of several members of the IAP family,
thereby derepressing caspase activation.
In addition,
HTRA2 exerts caspase-independent pro-apoptotic effects by
virtue of its serine protease activity.
These mechanisms
present a considerable degree of redundancy, as demon-
strated by the fact that the knockout or genetic inhibition of
single IMS proteins not always affects the execution of
intrinsic apoptosis.
Moreover, the relative contribution of
these processes to intrinsic apoptosis varies in distinct
physiological, pathological and experimental scenarios. Thus,
while ENDOG appears to be dispensable for intrinsic
apoptosis in mammalian models,
Nuc1p, the yeast ortholog
of ENDOG, has an important role during the apoptotic
response of Saccharomyces cerevisiae to chronological
aging in non-fermentable carbon sources (which potentiate
mitochondrial respiration).
DRONC, the ortholog of cas-
pase-9 in Drosophila melanogaster, is required for many
forms of developmental cell deaths and apoptosis induced by
DNA damage in vivo.
Conversely, caspase activation
seems to have a prominent role in a limited number of
instances of stress-induced intrinsic apoptosis in vitro,as
demonstrated by the fact that in contrast to extrinsic
apoptosis chemical and/or genetic inhibition of caspases
rarely, if ever, confers long-term cytoprotective effects and
truly prevents cell death. In this context, caspase inhibition
only delays the execution of cell death, which eventually can
even exhibit morphological features of necrosis.
In view of these observations, we suggest to define ‘intrinsic
apoptosis’ as a cell death process that is mediated by MOMP
and hence is always associated with (i) generalized and
irreversible Dc
dissipation, (ii) release of IMS proteins into
the cytosol (and their possible relocalization to other
subcellular compartments) and (iii) respiratory chain inhibi-
tion. We propose to differentiate between caspase-dependent
and caspase-independent intrinsic apoptosis based on the
extent of cytoprotection conferred by (pharmacological or
genetic) inhibition of caspases (Table 1 and Figure 2). This
distinction is particularly relevant in vivo, as in some
not all)
instances of developmental cell death, caspase
inhibition has been shown to provide stable cytoprotection.
In vitro, in the long run, caspase-independent mechanisms,
be they active (e.g., AIF, ENDOG) or passive (e.g., ATP
depletion), tend to prevail over caspase inhibition and to kill
cells even in instances of intrinsic apoptosis that would have
normally been rapidly executed by the caspase cascade.
Figure 1 Extrinsic apoptosis. Upon FAS ligand (FASL) binding, the cytoplasmic
tails of FAS (also known as CD95, a prototypic death receptor) trimers recruit
(among other proteins) FAS-associated protein with a death domain (FADD),
cellular inhibitor of apoptosis proteins (cIAPs), c-FLIPs and pro-caspase-8 (or -10).
This supramolecular platform, which has been dubbed ‘death-inducing signaling
complex’ (DISC), controls the activation of caspase-8 (-10). Within the DISC,
c-FLIPs and cIAPs exert pro-survival functions. However, when lethal signals
prevail, caspase-8 gets activated and can directly trigger the caspase cascade by
mediating the proteolytic maturation of caspase-3 (in type I cells) or stimulate
mitochondrial outer membrane permeabilization (MOMP) by cleaving the BH3-only
protein BID (in type II cells). Extrinsic apoptosis can also be ignited by dependence
receptors like DCC or UNC5B, which relay lethal signals in the absence of their
ligand (netrin-1). In the case of DCC and UNC5B, the pro-apoptotic signaling
proceeds through the assembly of a DRAL- and TUCAN- (or NLRP1-) containing
caspase-9-activating platform or by the dephosphorylation-mediated activation of
death-associated protein kinase 1 (DAPK1) by UNC5B-bound protein phosphatase
2A (PP2A), respectively. DAPK1 can mediate the direct activation of executioner
caspases or favor MOMP. tBID, truncated BID
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L Galluzzi et al
Cell Death and Differentiation
Thus, in vitro, intrinsic apoptosis appears to entail a caspase-
dependent and a caspase-independent phase, whose relative
contribution to the execution of cell death might be estimated
by the extent of short-term (24–48 h) cytoprotection conferred
by caspase inhibitors.
Definition of ‘Regulated Necrosis’
For a long time, necrosis has been considered as a merely
accidental cell death mechanism and was defined by the
absence of morphological traits of apoptosis or autophagy.
Owing to the work of several laboratories,
it is now clear
that necrosis can occur in a regulated manner, and that
necrotic cell death has a prominent role in multiple physiolo-
gical and pathological settings.
Several triggers can induce
regulated necrosis, including alkylating DNA damage, ex-
citotoxins and the ligation of death receptors, at least under
selected circumstances.
Indeed, when caspases
(and in particular caspase-8) are inhibited by genetic
manipulations (e.g., by gene knockout or RNA interference,
RNAi) or blocked by pharmacological agents (e.g., chemical
caspase inhibitors), RIP1 and its homolog RIP3 are not
degraded and rather engage in physical and functional
interactions that ultimately activate the execution of necrotic
cell death.
Regulated necrosis can be further characterized with
regard to its dependence on specific signaling modules, and
should be named consequently. For instance, cases of
regulated necrosis that exhibit RIP1 activation (which can be
measured by enzymatic assays or by monitoring RIP1
phosphorylation on S161)
and that can be suppressed
by RIP1 inhibitors including necrostatin-1
should be
labeled ‘RIP1-dependent regulated necrosis’. Of note, RIP3-
dependent, but RIP1-independent instances of regulated
necrosis have recently been identified,
and these are
insensitive to necrostatins.
The term ‘necroptosis’ has recently been used as a
synonym of regulated necrosis, but it was originally introduced
to indicate a specific case or regulated necrosis, which is
ignited by TNFR1 ligation and can be inhibited by the RIP1-
targeting chemical necrostatin-1 (Table 1 and Figure 3).
NCCD encourages scientists and authors of scientific pub-
lications to prefer the use of general versus specific
nomenclature (see below). However, ‘necroptosis’ can be
used to indicate RIP1- and/or RIP3-dependent regulated
necrosis, provided that this expression is explicitly defined at
its first appearance and used consistently thereafter.
Definition of ‘Autophagic Cell Death’
On the basis of morphological features, the term ‘autophagic
cell death’ has widely been used to indicate instances of cell
death that are accompanied by a massive cytoplasmic
vacuolization, which often (although not always) indicates
increased autophagic flux.
Although originally this expres-
sion did not imply any functional consideration, scientists have
quickly adopted the term ‘autophagic cell death’ and used it to
imply that autophagy would actually execute the cell
This applies to at least two very distinct settings.
First, autophagy has been shown to mediate physiological
cell death in vivo, during the developmental program of
D. melanogaster.
Second, autophagy appears to be
responsible for the death of some cancer cells (especially
when they lack essential apoptotic modulators like BAX and
BAK or caspases)
that respond to a selected panel of
chemotherapeutic agents in vitro.
Nonetheless, in most
known cases, autophagy constitutes a cytoprotective
response activated by dying cells in the attempt to cope with
stress, and its inhibition accelerates, rather than prevents,
cell death.
Several methods may be used to determine whether the
autophagic pathway is activated above baseline levels in the
context of the cellular demise. Discussing the advantages and
Figure 2 Intrinsic apoptosis. In response to multiple intracellular stress
conditions (e.g., DNA damage, cytosolic Ca
2 þ
overload), pro-survival and pro-
death signals are generated and converge to a mitochondrion-centered control
mechanism. When lethal signals prevail, mitochondrial outer membrane
permeabilization (MOMP) occurs and leads to mitochondrial transmembrane
potential (Dc
) dissipation, arrest of mitochondrial ATP synthesis and Dc
dependent transport activities. Moreover, the respiratory chains gets uncoupled,
leading to reactive oxygen species (ROS) overgeneration, and proteins that are
normally confined within the mitochondrial intermembrane space (IMS) are released
into the cytosol. Among these, cytochrome c (CYTC) drives together with the
cytoplasmic adaptor protein APAF1 and dATP the assembly of the so-called
apoptosome, a multiprotein complex that triggers the caspase-9-caspase-3
proteolytic cascade. Direct IAP-binding protein with low pI (DIABLO, also known as
second mitochondria-derived activator of caspases, SMAC) and high temperature
requirement protein A2 (HTRA2) facilitate caspase activation by sequestering and/
or degrading several members of the inhibitor of apoptosis protein (IAP) family.
On the contrary, apoptosis-inducing factor (AIF) and endonuclease G (ENDOG)
function in a caspase-independent manner by relocating to the nucleus and
mediating large-scale DNA fragmentation. Of note, the serine protease HTRA2 also
contributes to caspase-independent apoptosis by cleaving a wide array of cellular
substrates (including cytoskeletal proteins). IM, mitochondrial inner membrane; OM,
mitochondrial outer membrane; PTPC, permeability transition pore complex
Functional classification of cell death modalities
L Galluzzi et al
Cell Death and Differentiation
pitfalls of these techniques is beyond the scope of this article,
and several excellent reviews on methods for monitoring the
autophagic flux have recently been published.
theless, it should be remembered that steady-state methods
do not provide any reliable estimation of autophagic activity,
as they are intrinsically unable to discriminate between
enhanced rates of autophagy (increased on-rate) and
situations in which the late steps of the pathways are blocked
(decreased off-rate).
From a purely morphological perspective, the term ‘autop-
hagic cell death’ is highly prone to misinterpretation and hence
must be used with caution.
On the contrary, we suggest to
reintroduce the term ‘autophagic cell death’ based on
biochemical and functional considerations, to indicate a cell
death instance that is mediated by autophagy, that is, that can
be suppressed by the inhibition of the autophagic pathway by
chemicals (e.g., agents that target VPS34) and/or genetic
means (e.g., gene knockout/mutation or RNAi targeting
essential autophagic modulators like AMBRA1, ATG5,
ATG12 or beclin 1
(Table 1 and Figure 4). As some
ATG proteins may have autophagy-independent functions
and may even be converted from pro-autophagy to pro-death
proteins by proteolytic cleavage (e.g., ATG5 and
it may be advisable to interrogate possible
cases of autophagic cell death by knocking down at least two
distinct essential autophagic proteins. On the basis of our
definition, all cases of cell death that exhibit markers of
autophagy such as the lipidation of microtubule-associated
protein 1 light chain 3 (better known as LC3/Atg8) or an
increased degradation of autophagic substrates like seques-
tosome 1 (SQSTM1), but cannot be blocked by autophagy
inhibition should not be classified as autophagic cell death.
Definition of ‘Mitotic Catastrophe’
During the past decade, several attempts have been made
to delineate the molecular pathways leading to mitotic
Occasionally, researchers restrictively
employ the term ‘mitotic catastrophe’ for cell death occurring
in mitosis.
More frequently, mitotic catastrophe refers to
cases of cell death that are triggered by aberrant mitosis and
executed either during mitosis or in the subsequent inter-
Recently, it has been proposed that mitotic
catastrophe might not even constitute a bona fide cell death
executioner mechanism, but an oncosuppressive pathway
that precedes and is distinct from, yet operates through, cell
death or senescence.
After aberrant mitosis, cells frequently exhibit gross nuclear
alterations (e.g., micro- and multinucleation), which have
been used as morphological markers for the detection of
mitotic catastrophe.
However, apoptotic and necrotic traits
have also been detected in such cells, either concomitant with
or following multinucleation.
Thus, end-point techni-
ques are intrinsically unsuitable for assessing mitotic cata-
strophe, as they cannot reconstruct the sequence of events
that have lead to cell death. To circumvent this issue, novel
methods relying on high-throughput video microscopy or
time-lapse fluorescence microscopy are under develop-
Several processes were originally associated
with and were then shown to be dispensable for (at least some
instances of) mitotic catastrophe. These include, but are not
limited to, the activation of the DNA damage-responsive
protease caspase-2,
of the tumor suppressor TP53
and of other members of the TP53 family, including the TP73
variant TAp73.
In view of recent results from several laboratories indicating
that mitotic aberrations can induce cell senescence,
and that cell death can be either apoptotic or necrotic,
have recently proposed a novel definition and categorization
of mitotic catastrophe based on purely functional considera-
Thus, mitotic catastrophe would not constitute a
‘pure’ cell death executioner pathway, but an oncosuppres-
sive mechanism that: (i) is initiated by perturbations of the
mitotic apparatus (i.e., chromosomes and the complex
Figure 3 Regulated necrosis. Upon tumor necrosis factor a (TNFa) binding, the
cytoplasmic tails of TNF receptor 1 (TNFR1, a prototypic death receptor) trimers
recruit TNFR-associated death domain (TRADD), receptor-interacting protein
kinase 1 (RIP1), cellular inhibitor of apoptosis 1 (cIAP1), cIAP2, TNFR-associated
factor 2 (TRAF2) and TRAF5. Within the so-called complex I, RIP1 is
polyubiquitinated by cIAPs, thereby providing a docking site for the recruitment of
transforming growth factor b (TGFb)-activated kinase 1 (TAK1), TAK1-binding
protein 2 (TAB2) and TAB3 (which together deliver a pro-survival signal by
activating the transcription factor NF-kB). In some pathophysiological and
experimental settings, and in particular when caspase-8 is absent or when
caspases are inhibited by pharmacological agents, cylindromatosis (CYLD)-
deubiquitinated RIP1 engage in physical and functional interactions with its homolog
RIP3, ultimately activating the execution of necrotic cell death. Regulated necrosis
can also be induced by alkylating DNA damage (possibly by the overactivation of
poly(ADP-ribose) polymerase 1, PARP1). In some (but not all) instances, regulated
necrosis requires the kinase activity of RIP1, that is, it can be blocked by the RIP1-
targeting compounds necrostatins. FADD, FAS-associated protein with a death
Functional classification of cell death modalities
L Galluzzi et al
Cell Death and Differentiation
machinery that ensure their faithful segregation); (ii) is
initiated during the M phase of the cell cycle; (iii) is paralleled
by some degree of mitotic arrest; and (iv) ultimately triggers
cell death or senescence (Table 1 and Figure 5). It remains
an open conundrum whether the duration of the mitotic
arrest truly influences
or not
the cell fate after mitotic
catastrophe. However, it appears that the crosstalk between
TP53- and mammalian target of rapamycin (mTOR)-relayed
signals might (at least partially) determine cell senes-
In line with our definition, the DNA damage-
induced signaling that is initiated at the G
/M transition by the
checkpoint (which often, although not always, is mediated by
members of the TP53 family)
does not constitute a case of
mitotic catastrophe. Moreover, instances of mitotic arrest that are
followed by the re-establishment of homeostasis and resumed
proliferation cannot be considered as events of mitotic
catastrophe, even when they lead to the gain or loss of
chromosomes, and hence to the generation of aneuploid
Tentative Definition of Other Cell Death Modalities
Anoikis. Literally meaning ‘the state of being homeless’, this
term of ancient Greek derivation was introduced by Frisch
and Francis in 1994 to describe the apoptotic response
of adherent cells due to the absence of cell-to-matrix
The survival of non-transformed adherent
cells does indeed depend on signals transduced by integrins
and by some growth factor receptors (such as the epidermal
growth factor receptor (EGFR)) upon interaction with the
extracellular matrix (ECM).
As the resistance to anoikis of
epithelial cancer cells sustains invasiveness and metastatic
Figure 5 Mitotic catastrophe. (a) In the absence of chemical and genetic perturbations of the mitotic apparatus (including chromosomes and the molecular machinery that
ensures their faithful segregation), cells progress through the different phases of the cell cycle to generate a diploid offspring. On the contrary, if chromosomal defects or
problems affecting the mitotic machinery are sensed during the M phase, cells become arrested in mitosis due to the activation of mitotic catastrophe (b–d). These cells can
undergo different fates: they can die without exiting mitosis (b), reach the G
phase of the subsequent cell cycle (through a phenomenon that is known as mitotic slippage) and
then die (c), or exit mitosis and undergo senescence (d). Irrespective of this diversity of outcomes, mitotic catastrophe can be defined as an oncosuppressive mechanism that
precedes and is distinct from, but operates through, cell death and senescence
Figure 4 Autophagic cell death. In response to stress and during development, eukaryotic cells often activate autophagy, a mechanism whereby organelles and portion of
the cytoplasm are sequestered in double-membraned vesicles (autophagosomes) that are delivered to lysosomes for degradation. Stress-induced autophagy most often
exerts cytoprotective functions and favors the re-establishment of homeostasis and survival (a). In this setting, pharmacological or genetic inhibition of autophagy accelerates
cell death. On the contrary, these interventions frequently inhibit developmental cell death, indicating that autophagy also constitutes a lethal mechanism that mediates
‘autophagic cell death’ (b)
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L Galluzzi et al
Cell Death and Differentiation
potential, great efforts have been undertaken to precisely
characterize the underlying molecular cascades.
On the
basis of biochemical and functional considerations, anoikis
may currently be defined as an adherent cell-restricted lethal
cascade that is ignited by detachment from the matrix and
that is characterized by (i) lack of b1-integrin engagement,
(ii) downregulation of EGFR expression, (iii) inhibition of
extracellular-regulated kinase 1 (ERK1) signaling, and (iv)
overexpression of the BCL-2 family member BIM
(Table 1).
It should be noted that in most, if not all,
instances, the cell death program ignited by anoikis is
executed by the molecular machinery for intrinsic apoptosis
(see above).
Entosis. In 2007, Overholtzer et al.
introduced the term
‘entosis’ to describe a cell death mechanism linked to the
‘cell-in-cell’ phenotype that is frequently exhibited by non-
phagocytic cells in clinical tumor samples. Of note, Mormone
et al.
had previously reported a similar phenotype in
lymphoblasts from patients with Huntington’s disease, and
had dubbed it ‘cell cannibalism’.
Entosis would be
provoked by the loss of ECM interaction, but would not entail
the activation of apoptotic executioners, thereby constituting
a cell death mode distinct from anoikis.
Recently, the
RNAi-mediated downregulation of metallothionein 2A has
been shown to promote the ‘cell-in-cell’ phenotype, pointing
to a prominent role for adherens junctions in the entotic
Entosis reportedly proceeds in the absence of
caspase activation, is insensitive to inhibition by BCL-2 and is
paralleled by the activation of the small GTPase RHO and
the RHO-associated, coiled-coil containing protein kinase 1
(ROCK1) in the engulfing cell.
Internalized cells appear
virtually normal and later disappear, leading to the
hypothesis that they are degraded by lysosomal
hydrolases. In some instances, however, such cells have
been shown to divide and eventually to be released, raising
doubts on the inexorable fate of engulfed cells.
cell fate experiments are urgently awaited to resolve this
debate. Meanwhile, we propose to define entosis as an
instance of cell death only when all the three following
conditions are met. First, the engulfed cells should never exit
the phagosome (as detectable by time-lapse microscopy or
videomicroscopy) and should be degraded within the
lysosome (implying that entosis can be blocked by
lysosomal inhibitors). Second, the ‘cell-in-cell’ phenotype
should arise from homotypic interactions (i.e., it should
involve cells of the same type) and should not engage
professional phagocytes. Third, the process should be
insensitive to chemical and genetic interventions that
normally block caspase-dependent and -independent
intrinsic apoptosis (e.g., caspase inhibitors, BCL-2
overexpression) (Table 1).
Parthanatos. Coined after Thanatos, the personification of
death in Greek mythology, the term ‘parthanatos’ has been
introduced to indicate a particular cell death mode involving
the DNA damage-responsive enzymes poly(ADP-ribose)
polymerases (PARPs), and in particular PARP1, which
alone accounts for more than 90% cellular PARP
In physiological conditions, PARP1 cooperates
with the DNA repair machinery to ensure genomic
homeostasis upon mild DNA damage.
PARP1 overactivation has several toxic consequences,
including NAD
and ATP depletion, as well as the
accumulation of mitochondriotoxic PAR, which favors Dc
dissipation and AIF release.
Of note, AIF has recently
been shown to possess a high-affinity PAR-binding site, and
the physical interaction between PAR and AIF appears to be
required for parthanatos, both in vitro and in vivo.
Parthanatos have a role in multiple experimental and
physiopathological scenarios, including stroke, diabetes,
inflammation and neurodegeneration.
In line with the
original definition of parthanatos, cell death instances should
be considered as parthanatic when they depend on early
PARP1 activation (i.e., they can be blocked by its chemical
and/or genetic inhibition),
and exhibit NAD
plus ATP
depletion paralleled by AIF-mediated chromatinolysis.
Parthanatos constitutes a caspase-independent cell death
pathway (Table 1),
and possibly represents together
with necroptosis a particular case of regulated necrosis
(see above).
Pyroptosis. The term ‘pyroptosis’ has been introduced in
2000 by Brennan and Cookson
to functionally describe the
peculiar death of macrophages infected by Salmonella
typhimurium. Several other bacterial triggers of this atypical
cell death modality have been identified, including Shigella
flexneri, Listeria monocytogenes, Pseudomonas aeruginosa,
Francisella tularensis and the Bacillus anthracis toxin.
However, it has become clear that pyroptosis neither
constitutes a macrophage-specific process nor a cell death
subroutine that only results from bacterial infection.
note, pyroptotic cells can exhibit apoptotic and/or necrotic
morphological features.
The most distinctive biochemical feature of pyroptosis is the
early, induced proximity-mediated activation of caspase-1.
The pyroptotic activation of caspase-1 can occur in the
context of a multiprotein platform known as the inflamma-
some, which involves the adaptor protein ASC and NOD-like
receptors (NLRs) or the cytosolic DNA sensor absent in
melanoma 2 (AIM2).
Alternatively, caspase-1 can
be activated by the so-called pyroptosome, a supramolecular
assembly of ASC dimers.
In both cases, active caspase-1
catalyzes the proteolytic maturation and release of pyrogenic
interleukin-1b (IL-1b) and IL-18.
Moreover, in some (but not
all) instances, this is followed by caspase activation and cell
Active caspase-1 reportedly mediates the proteoly-
tic activation of caspase-7 (rather than that of caspase-3),
suggesting that pyroptotic cell death might proceed via
an unusual caspase-1-caspase-7 cascade with limited
(and perhaps caspase-1-independent) involvement of
Still, the molecular mechanisms determining
why caspase-1 activation sometimes results in cytokine
secretion without death and sometimes culminates in the
cellular demise remain to be elucidated. We therefore propose
to define pyroptosis as a caspase-1-dependent cell death
subroutine (i.e., that can be suppressed by the genetic
knockout/knockdown of caspase-1 or by caspase-1-specific
blockers like N-benzyloxycarbonyl-Tyr-Val-Ala-
omethylketone (Z-YVAD-fmk)) that is associated with the
Functional classification of cell death modalities
L Galluzzi et al
Cell Death and Differentiation
generation of pyrogenic mediators such as IL-1b and IL-18
(Table 1). It remains to be clarified whether pyroptosis truly
constitutes a cell death subroutine on its own or whether it
represents a particular case of caspase-dependent intrinsic
apoptosis (see above).
Netosis. In response to several stimuli, neutrophils and
eosinophils can release the so-called neutrophil extracellular
traps (NETs), that is, microbicidal structures composed of
nuclear chromatin, histones and granular antimicrobial
Upon the administration of granulocyte–
macrophage colony-stimulating factor (GMCSF) or short-term
stimulations with the complement fragment C5a, two rather
physiological conditions, NETs are generated by viable cells, as
demonstrated by several observations: (i) no cytosolic proteins
are detected in NETs; (ii) NET-releasing cells fail to take up
exclusion dyes; and (iii) NETs can be detected within 30–60
min after stimulation with IL-8 and lipopolysaccharide.
However, following non-physiological stimulation with phorbol-
12-myristate-13-acetate, NETs are released by a subset of
neutrophils undergoing a peculiar form of cell death,
has been dubbed ‘netosis’.
Netotic cells exhibit massive vacuolization of the cytoplasm,
rapid chromatin decondensation and breakdown of both the
nuclear and granular membranes, which is required for proper
NET formation.
Netosis is insensitive to caspase inhibitors
and necrostatin-1,
further demonstrating that it constitutes
a cell death subroutine distinct from apoptosis and regulated
necrosis. However, the netotic process can be suppressed by
pharmacological inhibition of NADPH oxidase (which is
responsible for the oxidative burst occurring during neutrophil
activation) or autophagy.
Of note, ROS appear to be
required but insufficient for netosis to occur, suggesting that
the autophagic component of netosis cannot be entirely
attributed to the autophagy-promoting activity of ROS.
Histone citrullination (i.e., the conversion of positively charged
arginine side chains into polar, but uncharged citrulline side
chains) also appears to participate in netosis by mediating
chromatin decondensation and NET formation.
genetic inhibition of the major histone-citrullinating enzyme,
that is, peptidylarginine deiminase 4 (PAD4), interfered with
NET release by HL-60 cells stimulated with Ca
2 þ
or Shigella flexneri.
Importantly, cell death with netotic
features has been observed in a subset of cytokine-primed
neutrophils following ligation of Siglec-9 and CD44.
response to these rather physiological stimuli, dying neutro-
phils reportedly do not release NETs.
Thus, whereas NET
formation may be paralleled by netosis, this is not always the
case: netotic cell death can occur in the absence of NET
release and, vice versa, NET can be generated in the absence
of cell death.
In view of these observations, netosis might be defined as a
cell death subroutine that is: (i) restricted to granulocytic cells;
(ii) insensitive to (and perhaps dependent on)
inhibition; (iii) insensitive to necrostatin; (iv) dependent on
NAPDH oxidase-mediated superoxide generation; and (v)
dependent on (components of) the autophagic machinery
(Table 1). As it stands, netosis shares biochemical features
with both autophagic cell death and regulated necrosis.
Further investigation is required to elucidate whether netosis
is a specific case of either these cell death subroutines or
whether it constitutes a cell death mechanism per se.
Cornification. Although cells belonging to the basal layer of
the epidermis respond to insults, for instance UV irradiation,
by undergoing apoptosis or necrosis, cells of the external
layer continuously undergo a physiological cell death
subroutine that has been dubbed ‘cornification’.
underlies the generation of the stratum corneum, a layer of
dead keratinocytes (so-called ‘corneocytes’) containing an
mixture of specific proteins (e.g., keratin, loricrin, SPR
and involucrin) and lipids (e.g., fatty acids and ceramides)
that confers to the skin structural stability, mechanical
resistance, elasticity and water repellence.
Also known
as ‘keratinization’ or ‘cornified envelope formation’, cornifica-
tion is often viewed as a program of terminal differentiation
similar to those underlying the maturation of other anucleated
tissues (e.g., the lens epithelium, red blood cells).
these processes are indeed associated with the (partial)
activation of cell death executioners, notably caspases.
However, lens and red blood cells preserve the ability to
succumb from stress-induced cell death,
corneocytes do not, suggesting that only cornification
constitutes a bona fide cell death program.
From a biochemical perspective, cornification is associated
with the synthesis of the enzymes and substrates that are
required for the generation of the stratum corneum. Enzymes
include, although presumably are not limited to, caspase-
and transglutaminase (TG)-1, -3 and -5, which catalyze
crosslinking reactions.
Substrates include proteins (e.g.,
filaggrin, loricrin, SPR, involucrin and SP100), but also lipids,
which are extruded into the extracellular space and covalently
attached to cornified envelope proteins to ensure skin
The skin of Casp14
mice presents an
altered composition of the stratum corneum and is character-
ized by reduced hydration levels, increased water loss and
high sensitivity to UV-induced DNA damage,
pointing to a
critical role for caspase-14 in cornification.
On the basis of these considerations, cornification can be
defined as cell death subroutine that: (i) is restricted to
keratinocytes; (ii) is functionally linked to the generation of the
stratum corneum of the epidermis; and (iii) can be altered,
although not blocked, by the inhibition of transglutaminases or
caspase-14 (Table 1).
Notes of Caution
There are several issues that should be taken into considera-
tion for appropriately classifying cell death subroutines based
on biochemical parameters.
(1) Physiopathological relevance: Here, we provide a
functional classification that can be applied to both in vitro
and in vivo observations. This said, cell death subroutines
should be considered relevant from a physiopathological
perspective only when they are shown to occur in vivo and to
be critical either for embryonic/post-embryonic development
or as an etiological determinant of disease. There probably is
a plethora of genes that can influence cell death in
physiopathologically relevant settings, but they are never
tested under such conditions and most of them remain
Functional classification of cell death modalities
L Galluzzi et al
Cell Death and Differentiation
therefore unidentified. Thus, in vivo studies constitute the
ultimate tool to recognize the true importance of cell death
signaling pathways and to understand their regulation.
(2) Pharmacological modulators: Chemical agents includ-
ing the RIP1 inhibitor necrostatin-1, the pan-caspase inhibitor
Z-VAD-fmk and the VPS34 inhibitors 3-methyladenine and
wortmannin have been widely employed in cell death
research, and surely contributed to important discoveries,
for example, that of regulated necrosis.
However, most of
these compounds lack adequate specificity to precisely define
a cell death program. For instance, Z-VAD-fmk has repeatedly
been shown to inhibit non-caspase proteases including
Similarly, 3-methyladenine affects multiple
facets of cellular metabolism.
Thus, while tests with
pharmacological inhibitors constitute useful starting points
for experimentation, they cannot be employed as surrogates
of genetic studies based on gene knockout or RNAi.
(3) Specificity of signaling: Most if not all proteins that ignite
or mediate cell death subroutines have multiple and
sometimes cell-death-unrelated functions.
Thus, whereas
cytosolic CYTC activates the apoptosome,
in the mitochon-
dria CYTC functions as an electron shuttle within the
respiratory chain,
and its complete absence is incompatible
with life.
RIP1 has a plethora of downstream targets that
mediate different biological outputs, and not only is it involved
in apoptotic and necrotic cell death,
but is also critical for
pro-survival NF-kB signaling.
Along similar lines, the VPS34
complex regulates autophagy as well as other vesicle
trafficking pathways including endocytosis.
Hence, results
coming from either the ablation or genetic inhibition of
pleiotropic modulators including (but not limited to) RIP1 and
VPS34 should be interpreted with caution, as it is easy to
underestimate the number of signaling cascades that have
been affected by apparently highly specific perturbations.
(4) Crosstalk between different cell death subroutines:It
should always be kept in mind that, in the vast majority of
settings (in particular in vivo), cell death subroutines are
neither isolated nor mutually exclusive signaling cascades.
Most often, pro-survival pathways are engaged along with the
propagation of lethal signals. Moreover, stress conditions can
result in the activation of multiple lethal mechanisms, which
can exhibit variable degrees of overlap. It is the crosstalk
between pro-survival and pro-death pathways that deter-
mines if and by which subroutine the cell will eventually die.
This level of complexity must be taken into account for the
classification of cell death modalities, as the inhibition of one
specific pathway often unveils the existence of backup
mechanisms instead of truly blocking the cell demise.
(5) ‘Programmed’, ‘regulated’ and ‘accidental’ cell death:
We suggest to preserve the adjective ‘programmed’ for those
physiological instances of cell death irrespective of the
modality by which they are executed that occur in the
context of embryonic/post-embryonic development and tissue
homeostasis. ‘Regulated’ should be used to indicate cases of
cell death be they programmed or not whose initiation and/
or execution is mediated by a dedicated molecular machinery,
implying that they can be inhibited by targeted pharmacolo-
gical and/or genetic interventions. Finally, the expression
‘accidental’ should be employed to indicate cell death
triggered by extremely harsh physical conditions (e.g.,
freeze–thawing cycles, high concentrations of pro-oxidants),
which cannot be inhibited by pharmacological and/or genetic
manipulations and usually exhibits morphological features of
(6) General versus specific nomenclature: During the past
decade, several neologisms have been introduced to indicate
very specific signaling pathways that lead to cell death,
including parthanatos, necroptosis, paraptosis, pyronecrosis
and several others.
Although some of these expressions
(e.g., necroptosis, parthanatos) have been used in a relatively
homogeneous manner, others (e.g., paraptosis, pyronecro-
sis) have acquired a variety of connotations, facilitating
confusion. For this reason, and in line with our mandate
(i.e., to provide truly functional definitions of cell death
subroutines), we encourage scientists and authors of scien-
tific publications to (i) prefer the use of general terms that bear
functional connotations to that of specific names, and (ii) to
avoid the introduction of neologisms.
Concluding Remarks
Until now, the field of cell death research has been dominated
by morphological definitions that ignore our relentlessly
increasing knowledge of the biochemical features of distinct
cell death subroutines. Here, the NCCD proposes a new
classification of lethal signaling pathways based on biochem-
ical and functional considerations. In this context, ‘loss-of-
function’ and ‘gain-of function’ genetic interventions (e.g.,
RNAi, knockout models and plasmid-driven overexpression
systems), as well as chemical inhibitors or activators of
important signaling nodes, constitute irreplaceable tools to
characterize cell death. During the process of functional
characterization, great attention should be paid to ensure that
genetic and chemical interventions truly modify the incidence
of cell death (as assessed by clonogenic cell survival),
rather than activate alternative lethal pathways. With these
recommendations in mind and the appropriate tools at hand,
researchers have the possibility to label cell death instances
with functional and biochemical tags. Our belief is that this
classification, if properly applied, will facilitate the under-
standing of scientific reports, stimulate the communication
among scientists and ultimately accelerate the pace of cell
death discovery.
Conflict of Interest
The authors declare no conflict of interest.
. The NCCD acknowledges the valuable input of all
Editors of Cell Death and Differentiation as well as that of numerous
colleagues that helped shaping the present recommendations in scientific meetings.
LG is financed by APO-SYS. DCR is a Wellcome Trust Senior Fellow.
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Functional classification of cell death modalities
L Galluzzi et al
Cell Death and Differentiation
    • "The induction of apoptosis and/or necrosis by shark liver oil has been attributed to the presence of AKG on its composition (Krotkiewski et al., 2003), but UDG has not been reported as a usual component of that oil. For a long time, necrosis has been considered as a merely accidental cell death mechanism and was defined by the absence of morphological traits of apoptosis or autophagy, but now is clear that necrotic cell death has a prominent role in multiple physiological and pathological settings (Galluzzi et al., 2012). Multidrug-resistant tumors remain susceptible to necrotic death (Sasi et al., 2009 ). "
    [Show abstract] [Hide abstract] ABSTRACT: Context: 1-O-alkylglycerols are ether-linked glycerols derived from shark liver oil and found in small amounts in human milk. Previous studies showed antineoplastic activity for this family of compounds, structurally related to alkylphospholipids, but the activity of linear chain synthetic alkylglycerols in cancer cell lines is less documented. Melanoma is a high incidence cancer, highly resistant to potential treatments. Finding new anti-cancer compounds to improve melanoma prognosis is a relevant research issue. Aims: To study the cytotoxic effect of 1-O-undecylglycerol in primary cultured normal fibroblasts and A375 human melanoma cell line. Methods: Cells were treated with different concentrations of 1-O-undecylglycerol and viability assessed by MTT assay. Morphological changes were visualized by DAPI and acridine orange-ethidium bromide staining. Mitochondrial membrane potential was evaluated, and gene expression of P53 and BcL-2 was semi-quantified. Results: 1-O-undecylglycerol decreased viability of A375 cells and exerted very low cytotoxicity on primary cultured normal fibroblasts. Necrosis appeared in A375 cells but not in fibroblasts, and no apoptotic changes were visualized in DAPI staining experiments. After 24 h fibroblasts and melanoma cells developed mitochondrial potential changes similar to valinomycin. The gene expression of P53 and BcL-2 decreased in treated cells. Conclusions: 1-O-undecylglycerol exhibited selective cytotoxic activity in A375 melanoma cells when compared with primary cultured fibroblast. Its toxicity is mediated by necrosis that may be related with mitochondrial events and decrease in P53 and BcL-2 expression. The results suggest that UDG could be a useful strategy to combine with other chemotherapeutic agents in melanoma treatment.
    Full-text · Article · Apr 2016
    • "The signaling pathways leading to RN exhibit a consistent degree of crosstalk with the molecular cascades that control and execute apoptosis [2] [23]. This functional interplay – which often occurs in the context of cell-wide responses to stress [152] – mainly reflects: (1) the existence of shared signal transducers, which can activate either apoptotic or necrotic cell death (e.g., TNFR1, AIF, p53), depending on the specific circumstances [19] [85] [94]; and (2) the existence of negative feedback circuitries whereby one cell death subroutine (most often apoptosis) actively inhibits the other (most frequently necrosis) (Figure 2) [153]. "
    [Show abstract] [Hide abstract] ABSTRACT: It is now clear that apoptosis does not constitute the sole genetically-encoded form of cell death. Rather, cells can spontaneously undertake or exogenously be driven into a cell death subroutine that manifests with necrotic features, yet can be inhibited by pharmacological and genetic interventions. As regulated necrosis (RN) plays a major role in both physiological scenarios (e.g., embryonic development) and pathological settings (e.g., ischemic disorders), consistent efforts have been made throughout the last decade toward the characterization of the molecular mechanisms that underlie this cell death modality. Contrarily to initial beliefs, RN does not invariably result from the activation of a receptor interacting protein kinase 3 (RIPK3)-dependent signaling pathway, but may be ignited by distinct molecular networks. Nowadays, various types of RN have been characterized, including (but not limited to) necroptosis, mitochondrial permeability transition (MPT)-dependent RN and parthanatos. Of note, the inhibition of only one of these modules generally exerts limited cytoprotective effects in vivo, underscoring the degree of interconnectivity that characterizes RN. Here, we review the signaling pathways, pathophysiological relevance and therapeutic implications of the major molecular cascades that underlie RN.
    Full-text · Article · Nov 2014
    • "This aberrant mitosis may lead to apoptosis or necrosis (41). Of note, mitotic catastrophe is not considered a form of cell death, but rather an irreversible trigger for cell death (22). "
    [Show abstract] [Hide abstract] ABSTRACT: Histological tumor necrosis (TN) has been reported to indicate a poor prognosis for different human cancers. It is generally accepted that TN results from chronic ischemic injury due to rapid tumor growth. However, whether insufficient tumor vascularization and inadequate tumor cell oxygenation are the only factors causing TN remains controversial. Mitotic catastrophe is considered to occur as a result of dysregulated/failed mitosis, leading to cell death. We hypothesize that mitotic catastrophe, induced by hypoxic stress, may lead to the TN which is observed in high grade carcinomas. The current review describes the morphological features of TN in malignant epithelial tumors. In addition, evidence regarding the involvement of mitotic catastrophe in the induction of TN in human carcinomas is discussed.
    Full-text · Article · Oct 2014
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