Extracellular NAD and ATP Partners in immune, studia, genomika
[ Pobierz całość w formacie PDF ]
Purinergic Signalling (2007) 3:71
–
81
DOI 10.1007/s11302-006-9038-7
REVIEW
Extracellular NAD and ATP: Partners in immune
cell modulation
Friedrich Haag
&
Sahil Adriouch
&
Anette Braß
&
Caroline Jung
&
Sina Möller
&
Felix Scheuplein
&
Peter Bannas
&
Michel Seman
&
Friedrich Koch-Nolte
Received: 8 February 2006 /Accepted: 22 October 2006 / Published online: 9 January 2007
#
Springer Science + Business Media B.V. 2007
Abstract Extracellular NAD and ATP exert multiple, par-
tially overlapping effects on immune cells. Catabolism of both
nucleotides by extracellular enzymes keeps extracellular
concentrations low under steady-state conditions and gener-
ates metabolites that are themselves signal transducers. ATP
and its metabolites signal through purinergic P2 and P1
receptors, whereas extracellular NAD exerts its effects by
serving as a substrate for ADP-ribosyltransferases (ARTs) and
NAD glycohydrolases/ADPR cyclases like CD38 and
CD157. Both nucleotides activate the P2X7 purinoceptor,
although by different mechanisms and with different charac-
teristics. While ATP activates P2X7 directly as a soluble
ligand, activation via NAD occurs by ART-dependent ADP-
ribosylation of cell surface proteins, providing an immobilised
ligand. P2X7 activation by either route leads to phosphati-
dylserine exposure, shedding of CD62L, and ultimately to
cell death. Activation by ATP requires high micromolar con-
centrations of nucleotide and is readily reversible, whereas
NAD-dependent stimulation begins at low micromolar con-
centrations and is more stable. Under conditions of cell stress
or inflammation, ATP and NAD are released into the extra-
cellular space from intracellular stores by lytic and non-lytic
mechanisms, and may serve as
inflamed tissue, NICD may inhibit bystander activation of
unprimed T cells, reducing the risk of autoimmunity. In
draining lymph nodes, NICDmay eliminate regulatory T cells
or provide space for the preferential expansion of primed
cells, and thus help to augment an immune response.
Key words ADP-ribosylation
.
ADP-Ribosyltransferases
.
apoptosis
.
ATP
.
ectoenzymes
.
extracellular purines
.
NAD
.
posttranslational protein modification
Abbreviations
ADP adenosine diphosphate
ADPR Adenosine diphosphate ribose
AMP Adenosine monophosphate
ART ADP-ribosyltransferase
ATP Adenosine triphosphate
E-NPP Ecto-nucleotide pyrophosphatase/
phosphodiesterase
E-NTPD Ecto-nucleoside triphosphate
diphosphohydrolase
FoxP3 Forkhead box P3
NAADP Nicotinic acid adenine dinucleotide phosphate
NAD
to alert the
immune response to tissue damage. Since ART expression is
limited to naïve/resting Tcells, P2X7-mediated NAD-induced
cell death (NICD) specifically targets this cell population. In
“
danger signals
”
Nicotinamide adenine dinucleotide
NADP
Nicotinamide adenine dinucleotide phosphate
NICD
NAD-induced cell death
PARP
Poly(ADP-ribose) polymerase
PS
Phosphatidyl serine
:
S. Adriouch
:
A. Braß
:
C. Jung
:
S. Möller
:
F. Scheuplein
F. Haag (
)
*
:
P. Bannas
:
F. Koch-Nolte
Institute of Immunology, University Hospital,
Martinistr. 52, 20246 Hamburg, Germany
e-mail: haag@uke.uni-hamburg.de
ATP and NAD in the extracellular compartment:
From their release to the induction of specific signalling
:
S. Adriouch
:
M. Seman
INSERM U519- EA1556, Faculté de Médecine et de Pharmacie,
Université de Rouen,
F-76183 Rouen Cedex, France
F. Haag
ATP and NAD+ are classic intracellular metabolites with
center-stage roles in energy metabolism and electron
transfer. In recent years, it has become evident that these
72
Purinergic Signalling (2007) 3:71
–
81
purine nucleotides play important roles also in the
extracellular environment, i.e., as substrates for a flurry
of nucleotide-metabolising ectoenzymes, and, in the case
of ATP, also as a ligand for cell surface receptors (Figs.
1
and
2
).
Biosynthesis of NAD presumably takes place in several
locations in the cell [
1
]. Under physiological conditions
most (more than 70%) of the cellular NAD content is
stored and is utilised in the mitochondria primarily for
metabolic purposes. In the cytoplasm and in the nucleus
NAD serves cell signalling functions, as a precursor for
calcium mobilising metabolites and as a substrate for two
families of nuclear enzymes, i.e., poly-ADP-ribosyl
polymerases (PARPs) and the sirtuin (homologues of
the yeast
recently been suggested to serve as a
“
danger signal
”
that
may alert the immune system to tissue damage [
8
–
10
].
Immune modulation by extracellular ATP
Once released, extracellular ATP and NAD can be degraded
into further metabolites such as ADP, AMP or adenosine by
extracellular enzymes, i.e., ecto-nucleoside triphosphate
diphosphohydrolase (E-NTPDs), ecto-nucleotide pyrophos-
phatase/phosphodiesterase (E-NPPs), and the ecto-5
′
-nucle-
otidase CD73 (Figs.
1
,
2
). ATP or its by-products activate
different members of the purinoceptor family of receptors.
Purinoceptors comprise adenosine-sensitive P1 receptors
(A1, A2a, A2b, and A3) and P2 receptors, which are
activated by ATP, ADP, UTP, UDP or UDP-glucose [
11
,
12
] (and NAD, see note added in proof). P2 receptors are
further divided into two groups: the G protein-coupled
seven-transmembrane P2Y receptors (P2Y1, -2, -4, -6, -11,
-12, -13, -14), and the P2X ligand-gated ion channels
(P2X1-7) [
13
,
14
]. Triggering of purinoceptors by their
ligands regulates important physiological functions such as
platelet aggregation, local regulation of blood pressure,
modulation of cardiac functions in ischemic conditions or
regulation of the development of inflammation [
11
,
15
,
16
].
Regulation of immune functions by ATP and its
metabolites has been reviewed elsewhere [
8
,
10
]. ATP can
in principle transmit signals through several different
receptors, including the complete P2X family and a
subgroup of P2Y receptors (P2Y1, 2, 11, 12, 13) [
12
].
These receptors differ greatly in their relative sensitivities to
ATP, with EC50s in the nanomolar (P2Y), low micromolar
”
(
Sir2
) gene) family of NAD-dependant lysine deacetylases,
both of which play important roles in coordinating DNA
repair, regulating transcription levels and controlling pro-
gression towards apoptosis [
2
“
silent mating type information regulation 2
4
]. Under pathophysiologi-
cal conditions, such as ischemia, oxidative stress or
DNA-damaging agents, cells release their mitochondrial
NAD content to the cytoplasm and the nucleus by still
unknown mechanisms [
4
]. It is not surprising, then, that
NAD plays an essential role in the cellular response to
stress.
Similarly, following the induction of cellular stress part
of this cellular content of NAD and ATP may be released
into the extracellular space. This may occur by several
mechanisms involving active exocytosis or diffusion
through transmembrane transporters in living cells or
passive leakage across the membrane in dying cells [
5
–
7
].
Of note, release of purines by injured or dying cells has
–
Fig. 1 Chemical structure of
ATP and NAD, and sites of
cleavage by different
ecto-enzymes
Purinergic Signalling (2007) 3:71
–
81
73
receptors. Extracellular NAD serves as a substrate for cell-surface
ADP-ribosyltransferases (ART2), or is hydrolyzed to ADPribose by
CD38. CD38 can also synthesise cyclic ADP-ribose, a known
intracellular calcium mobilising agent. It is not known how cADPR
gains access to the intracellular compartment. NAD (and ATP) may
also be hydrolysed by ecto-nucleotide pyrophosphatase/phosphodies-
terases (E-NPPs) to AMP, which in turn is hydrolysed by CD73 to
adenosine. See text for details
Fig. 2 Action of extracellular ATP and NAD and their metabolites on
different cell surface receptors. Extracellular ATP present in high,
intermediate, or low concentrations can activate P2X7, other P2X, or
P2Y receptors, respectively, or is hydrolysed by the sequential action
of ecto-nucleoside triphosphate diphosphohydrolases (E-NTPDs) such
as CD39 and ecto-5
-nucleotidase (CD73) to ADP and adenosine
(ADO). For clarity, P2X receptors other than P2X7 are not shown,
since their presence on immune cells is not well documented. ADP can
act on P2Y receptors, and adenosine can activate G protein-coupled P1
′
(most P2X) or high micromolar (P2X7) ranges [
8
]. The
situation is further complicated by the fact that extracellular
ATP is rapidly metabolised, and its break-down products,
notably ADP and adenosine, have signalling functions of
their own through different receptors. Both pro- and anti-
inflammatory effects of ATP on immune cells have been
reported, depending on the cell type and the available
concentration of ATP. In general, P2X7, which requires
high ATP concentrations acting for a short time, mediates
mainly pro-inflammatory effects, such as the processing
and release of interleukin- (IL-) 1 and IL-18 [
17
,
18
], in
dendritic cells and macrophages, and induces cell death in
T cells. Activation of P2X7 also stimulates the production
of tumor necrosis factor alpha (TNFa) in microglial cells
[
19
]. Low concentrations of ATP present during the
maturation of DCs reduce their capacity to induce Th1-
typical responses in primed T cells [
20
,
21
]. These anti-
inflammatory effects may be due either to direct action on
74
Purinergic Signalling (2007) 3:71
–
81
ADP-ribose. It is conceivable that NAD may exert distant effects by
reaching lymph nodes draining inflammatory sites in physiologically
relevant concentrations. Both ATP and NAD are degraded by
metabolising enyzmes to yield other signalling molecules, notably
adenosine (ADO), which exerts predominantly anti-inflammatory
effects through P1 receptors of the A2-subfamily. See text for details
Fig. 3 Hypothetical scheme of the interplay of purine sensors during
an immune response. ATP and NAD are released locally at sites of
tissue injury or inflammation. At high concentrations, ATP acts on the
P2X7R receptor to exert pro-inflammatory effects on antigen present-
ing cells or to kill T cells; at low concentrations it acts on other P2
receptors to downregulate the initiation of Th1 responses. NAD is used
by ARTs on T cells to activate P2X7, or by CD38 to generate cyclic
some P2Y receptors like P2Y11, or by adenosine signalling
through P1 receptors (Figs.
2
and
3
).
CD157 enzyme, possesses NAD-glycohydrolase and ADP-
ribose cyclase activities. They catalyse cleavage of NAD
into ADP-ribose or cyclic ADP-ribose and nicotinamide
[
22
], as well as the transglycosidation of NADP and
nicotinic acid to yield NAADP [
23
]. Cyclic ADP-ribose
and NAADP are newly recognised second messenger
molecules, which trigger calcium release from IP3-inde-
pendent intracellular stores, and which may thus play
important regulatory roles [
24
,
25
]. However, it is contro-
versial whether these second messengers are generated by
extracellular CD38 and are then translocated to the cytosol
by hitherto unknown mechanisms, or whether they are
generated from intracellular NAD by an intracellular
isoform of CD38. CD38 may also be involved in the
Immune modulation by extracellular NAD
Similar to ATP, NAD is also degraded in the extracellular
compartment, giving rise to the generation of metabolites
like cyclic ADP-ribose or adenosine that are active signal
transducers (Fig.
2
). In contrast to ATP, signalling through
intact NAD does not involve specific membrane receptors
(see note added in proof). Nevertheless, NAD may regulate
cellular functions through two known enzyme families. The
first family, comprising CD38 and the functionally related
Purinergic Signalling (2007) 3:71
–
81
75
regulation of immune functions by limiting the substrate
availability for ADP-ribosyltransferases (see below) [
26
].
Mice lacking the CD38 glycohydrolase/ADP-ribosyl cy-
clase show an impaired antibody response to T-cell
dependent antigens [
27
], which may be due to a defect
in the migratory capacity of dendritic cells [
28
].
The second family of enzymes mediating signalling by
NAD comprises the mono(ADP-ribosyl)transferases
(ARTs), which are structurally related to ADP-ribosylating
bacterial toxins. These enzymes catalyse a posttranslational
modification of proteins by transferring the ADP-ribose
moiety from NAD to specific amino acids, e.g., arginine
residues, on target proteins [
29
]. This family contains five
known mammalian members, ART1-ART5, which are GPI-
anchored membrane proteins (ART1-ART4) or secreted
enzymes (ART5) [
30
]. Human ART1 was recently assigned
the CD number CD296 [
31
]; it is expressed by activated
granulocytes as well as by skeletal muscle, heart, and
epithelial cells [
32
ultimately cell death [
38
,
39
]. Of the P2Y receptors, P2Y6
and P2Y14 have been described on T cells [
40
,
41
], but
these receptors are sensitive to UDP and UDP-glucose,
respectively.
P2X7 is also expressed on antigen-presenting cells,
including dendritic cells and macrophages, where it
mediates release of the non-classically secreted cytokines
IL-1
and IL-18 [
18
,
42
], and promotes phagosome/
lysosome fusion [
43
β
45
]. P2X7 is not expressed on resting
B cells in the mouse, but in the human has been identified
on a subset of chronic B-cell lymphomas (B-CLL) [
46
,
47
].
Immature dendritic cells also express the P2Y11 receptor
[
48
]. This receptor, which is sensitive to nanomolar
concentrations of ATP (see note added in proof), has been
implicated in several responses of DCs to ATP. Low doses
of ATP synergise with other stimuli like TNFa or LPS to
enhance DC maturation, but the net effect is to reduce the
production of IL-12p70 and increase the production of IL-
10 [
49
]. As is the case for the anti-inflammatory A2
subgroup of P1 receptors (see below), stimulation of P2Y11
causes an elevation of intracellular cAMP in DCs, which
mediates its effects on DC maturation [
48
]. Using a
different biochemical pathway, P2Y11 also inhibits the
migratory response of immature DCs to chemotactic
gradients, causing these cells to remain longer at sites of
tissue damage [
50
].
The G protein-coupled P1 receptors fall into two
functional groups, which serve to lower (A1 and A3
receptors) or to increase (A2a and A2b receptors) intracel-
lular levels of cyclic AMP (cAMP). A1/A3 receptors are
expressed on immature DCs, where they induce calcium
flux and promote chemotaxis [
51
,
52
]. A2a/b receptors
down-regulate the production of IL-12 in LPS-matured
DCs and thus inhibit the differentiation of naive CD4+ T
cells towards a Th1 phenotype. T cells also express A2a
receptors. Stimulation of these receptors by adenosine
inhibits TCR-mediated T cell proliferation and upregulation
of the IL-2 receptor, as well as most of the effector
functions of cytotoxic T cells [
53
,
54
]. The A2a/b receptors
are the most prominent P1 receptors on immune cells, and
are responsible for the dominant anti-inflammatory effects
of adenosine on the immune system (recently reviewed in
[
55
]). Nucleotide-metabolising enzymes are widely distrib-
uted among cells of the immune system. Ecto-ATPase and
5
–
34
]. ART4 has been identified as the
carrier of the Dombrock blood group alloantigens and was
recently assigned the CD number CD297 [
35
]. ART4 is
expressed prominently by erythrocytes and at lower levels
also on monocytes and splenic macrophages. Only ART1,
ART2 and ART5 have been shown so far to possess
arginine-specific activity, while ART3 and ART4 may have
acquired a new target specificity. Akin to the well-known
phosphorylation reaction, posttranslational protein modifi-
cation by ADP-ribosylation regulates (inhibits or activates)
the functions of target proteins [
30
,
36
]. The ART enzyme
family members thus represent new players in the epige-
netic regulation of protein functions.
It has been shown that ART2, like many other GPI-
anchored proteins, is segregated into specialised cholester-
ol- and ganglioside-enriched microdomains on the cell
surface termed lipid rafts [
37
]. Localisation into lipid rafts
is important for the activity of ART2, presumably by
focussing it on its target molecules. Indeed, substantial
fractions of two known non-GPI-anchored target proteins of
ART2, i.e., LFA-1 and P2X7, may also be recruited into
lipid rafts [
37
].
–
Purine sensors on cells of the immune system
Cells of the immune system express a variety of purine
sensors on their surfaces, either as purinoreceptors or as
ecto-enzymes that metabolise purine nucleotides (Fig.
3
).
For many of the molecules, a detailed expression analysis is
still hampered by a lack of suitable antibodies.
The only ATP-sensitive purinoreceptor that has been
positively identified on peripheral T cells to date is P2X7.
In these cells, P2X7 mediates ATP- and NAD-dependent
phosphatidyl serine (PS) exposure, CD62L shedding, and
-nucleotidase activities, which are sufficient to convert
ATP into adenosine, are found on many lymphocytes and
antigen presenting cells. It is worth noting that ecto-
adenylate kinase, the enzyme catalysing the reverse
pathway, i.e., the generation of ATP from extracellular
adenosine, is also present on the surface of lymphocytes
[
56
]. Adenosine can also be generated from extracellular
NAD by the sequential action of E-NPPs and 5
′
-nucleotid-
ase. Although the expression of E-NPPs on immune cells
′
[ Pobierz całość w formacie PDF ]