Download Factor XIII: sticking it to platelets

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4. Ratner L, Lee J, Tang S, et al; AIDS Malignancy
Consortium. Chemotherapy for human immunodeficiency
virus-associated non-Hodgkin’s lymphoma in combination
with highly active antiretroviral therapy. J Clin Oncol.
2001;19(8):2171-2178.
5. Little RF, Pittaluga S, Grant N, et al. Highly effective
treatment of acquired immunodeficiency syndrome-related
lymphoma with dose-adjusted EPOCH: impact of
antiretroviral therapy suspension and tumor biology. Blood.
2003;101(12):4653-4659.
6. Levine AM, Noy A, Lee JY, et al. Pegylated liposomal
doxorubicin, rituximab, cyclophosphamide, vincristine, and
prednisone in AIDS-related lymphoma: AIDS Malignancy
Consortium Study 047. J Clin Oncol. 2013;31(1):58-64.
7. Coiffier B, Lepage E, Briere J, et al. CHOP
chemotherapy plus rituximab compared with CHOP alone
in elderly patients with diffuse large-B-cell lymphoma.
N Engl J Med. 2002;346(4):235-242.
8. Kaplan LD, Lee JY, Ambinder RF, et al. Rituximab
does not improve clinical outcome in a randomized
phase 3 trial of CHOP with or without rituximab in
patients with HIV-associated non-Hodgkin lymphoma:
AIDS-Malignancies Consortium Trial 010. Blood. 2005;
106(5):1538-1543.
9. Sparano JA, Lee JY, Kaplan LD, et al; AIDS
Malignancy Consortium. Rituximab plus concurrent
infusional EPOCH chemotherapy is highly effective in
HIV-associated B-cell non-Hodgkin lymphoma. Blood.
2010;115(15):3008-3016.
10. Mounier N, Spina M, Gabarre J, et al. AIDS-related
non-Hodgkin lymphoma: final analysis of 485 patients
treated with risk-adapted intensive chemotherapy. Blood.
2006;107(10):3832-3840.
© 2013 by The American Society of Hematology
l l l PLATELETS & THROMBOPOIESIS
Comment on Kasahara et al, page 3340
Factor
XIII: sticking it to platelets
----------------------------------------------------------------------------------------------------Adam D. Munday1 and José A. López1
1
PUGET SOUND BLOOD CENTER RESEARCH INSTITUTE
In this issue of Blood, Kasahara et al report that platelet-dependent clot retraction
requires factor XIII (FXIII), which covalently associates fibrin polymers with
protein located within the platelet plasma membrane at lipid rafts.1
T
he formation of hemostatic clots begins
with deposition of platelets in the area
of vascular injury. The activated platelets
adhere to each other, with fibrinogen and von
Willebrand factor serving as intermediaries,
and provide a surface for the assembly
Schematic representation of translocation of fibrin to lipid rafts. On platelet activation, fibrin(ogen) binds to integrin
aIIbb3. In the presence of FXIII, the aIIbb3-finbrin(ogen) complex translocates to SM-rich rafts. FXIIIa crosslinks fibrin
fibrils to each other and covalently links fibrin to aIIbb3 and possibly other, as yet unidentified, receptors. Concurrently,
translocation of cFXIII to the lipid raft–associated actin cytoskeleton would promote crosslinking of receptor cytoplasmic
domains to the cytoskeleton. Together, FXIII-dependent crosslinking would cement extracellular fibrin cables to the
intracellular platelet cytosolic motors to promote clot retraction.
3246
of coagulation complexes that produce
thrombin, which catalyzes the formation of
fibrin polymers from fibrinogen. After the clot
forms, it retracts to consolidate volume and
minimize leakage, a process during which
platelets pull on the fibrin polymers.
The thrombin generated on the platelet
surface also converts plasma FXIII to its
active form FXIIIa. FXIII is a member of the
transglutaminase family of enzymes, which
crosslink proteins by catalyzing the formation
of isopeptide bonds between glutamine and
lysine residues.2 FXIII has two forms:
a plasma form (pFXIII) that is a tetramer
of two carrier B-subunits and two catalytic
A-subunits3 and an intracellular form
(cFXIII) that consists of two catalytic
A-subunits. pFXIII circulates as a zymogen
that must be activated by thrombin; cFXIII is
constitutively active.2 FXIIIa crosslinks not
only fibrin, but also crosslinks a2 antiplasmin
to fibrin (which inhibits clot lysis) and
fibronectin to fibrin. FXIII also has other
substrates, which include factor V, Plasminogen
activator inhibitor-2, collagen, and the
intracellular proteins filamin, actin, and myosin.
The importance of FXIII is underscored
by the clinical signs and symptoms of its
deficiency. Clots formed in the absence of
FXIII are unstable and easier to lyse by the
fibrinolytic system, the consequence being
delayed bleeding after injury or surgery.
Affected individuals also experience delayed
wound healing, possibly due to the inability of
clots to retract properly. In affected women,
habitual abortion is common, most likely due
to combined abrogation of fibrin/fibronectin
crosslinking by FXIII and impairment of firm
adhesion of the placenta to the endometrium.
Clot retraction requires both platelets
and fibrin. Platelets anchor fibrin to their
surface and intracellular motors act as a
winch to contract the polymerized fibrin
fibers. Integrin aIIbb3 on platelets is required
to retract clots, because patients with
Glanzmann thrombasthenia, who lack the
receptor, exhibit defective clot retraction.
In mice, FXIII has also been shown to be
necessary.4
Using elegant microscopy and biochemical
approaches, Kasahara et al show for the
first time that fibrin associates with lipid rafts
on the platelet surface and that raft integrity
is required for clot retraction; raft disruption
with methyl-b-cyclodextrin (which removes
membrane cholesterol) prevented fibrin
BLOOD, 7 NOVEMBER 2013 x VOLUME 122, NUMBER 19
From www.bloodjournal.org by guest on June 15, 2017. For personal use only.
association and clot retraction. Using raftspecific affinity labels, they demonstrated that
fibrin associates with a subset of lipid rafts
that are rich in sphingomyelin (SM) (as
opposed to the cholesterol-rich raft population).
Platelets from mice lacking either of the
enzymes that synthesize sphingomyelin
retracted clots poorly.
Previous studies have implicated both
pFXIII and cFXIII in fibrin association with
the platelet surface on thrombin receptor
activation.5 Here, Kasahara et al demonstrate
that FXIII is required for translocation of
fibrin to SM-rich rafts and that this fibrin
is a substrate for FXIII. The translocation
required the g-chain of fibrin: a recombinant
g-chain fusion protein bound to rafts in
a FXIII-dependent manner in thrombinactivated platelets. Covalent crosslinking was
required for this association because mutating
the FXIII crosslinking sites prevented
association of the g-chain fusion protein with
rafts, as did the transglutaminase inhibitor
cystamine.
These results point to a novel 2-step
mechanism whereby fibrin(ogen) binds to
integrin aIIbb3, translocates to SM-rich
lipid rafts in a FXIII-dependent manner,
and covalently associates with an as yet
unidentified receptor. Considered in light of
the data indicating that cFXIII translocates to
the cytoskeleton of activated platelets6 (which
in activated platelets attaches to lipid rafts7),
a mechanism emerges whereby, within
SM-rich rafts, FXIII crosslinks cytoskeletal
and motor elements to promote force
transduction from the platelet interior
(see figure). The raft-associated fibrin is
crosslinked to an unknown receptor (one
candidate being avb3), which itself might
be crosslinked to the cytoskeleton on the
cytoplasmic side to allow the strongest possible
linkage between the cytosolic motors and
the extracellular cables they tug on to retract
the clot. Lack of FXIII would not only be
expected to increase fibrin clot lysis, it could
also impair that ability of the fibrin to attach to
the underlying platelets. These findings also
suggest that changes in lipid raft structure
or organization could alter clot retraction
and impair wound healing.
As with many thought-provoking papers,
the current paper raises many new questions
and opens new areas of research. These
include the following. What is the receptor
for fibrin within the SM-rich rafts? Is this
mechanism specific for thrombin-activated
platelets or is it applicable to platelet
activation by other agonists? Is there a role for
cFXIII-mediated crosslinking of intracellular
proteins within lipid rafts? What effect does
this crosslinking have on the platelets’ ability
to generate contractile forces? Answers to
these and other questions will provide further
insight into the clinical signs and symptoms of
FXIII deficiency.
Conflict-of-interest disclosure: The authors declare
no competing financial interests. n
REFERENCES
1. Kasahara K, Kaneda M, Miki T, et al. Clot retraction is
mediated by factor XIII-dependent fibrin-aIIbb3-myosin
axis in platelet sphingomyelin-rich membrane rafts. Blood.
2013;122(19):3340-3348.
2. Muszbek L, Yee VC, Hevessy Z. Blood coagulation factor
XIII: structure and function. Thromb Res. 1999;94(5):271-305.
3. Lorand L. Factor XIII and the clotting of fibrinogen:
from basic research to medicine. J Thromb Haemost. 2005;
3(7):1337-1348.
4. Kasahara K, Souri M, Kaneda M, Miki T, Yamamoto
N, Ichinose A. Impaired clot retraction in factor XIII A
subunit-deficient mice. Blood. 2010;115(6):1277-1279.
5. Jayo A, Conde I, Lastres P, Jiménez-Yuste V,
González-Manchón C. New insights into the expression
and role of platelet factor XIII-A. J Thromb Haemost. 2009;
7(7):1184-1191.
6. Serrano K, Devine DV. Intracellular factor XIII
crosslinks platelet cytoskeletal elements upon platelet
activation. Thromb Haemost. 2002;88(2):315-320.
7. Munday AD, Gaus K, López JA. The platelet
glycoprotein Ib-IX-V complex anchors lipid rafts to the
membrane skeleton: implications for activation-dependent
cytoskeletal translocation of signaling molecules. J Thromb
Haemost. 2010;8(1):163-172.
© 2013 by The American Society of Hematology
l l l PLATELETS & THROMBOPOIESIS
Comment on Kahr et al, page 3349
a-Granules
at the BEACH
----------------------------------------------------------------------------------------------------Sidney W. Whiteheart1
1
UNIVERSITY OF KENTUCKY COLLEGE OF MEDICINE
In this issue of Blood, Kahr et al report the description of a mouse model lacking
the Nbeal2 protein and demonstrate its utility in the study of a-granule biogenesis
and megakaryocyte development in gray platelet syndrome (GPS).1
G
PS (Mendelian Inheritance in Man
[MIM] 139090) was first identified
by Raccuglia in 1971.2 The first patient, an
11-year-old boy, presented with
thrombocytopenia and platelets that had
a “peculiar gray color” on Wright-stained
blood smears. Analysis showed that these
platelets lacked a class of granules that are
now known as a-granules. GPS is a rare
autosomal recessive disorder associated with
macrothrombocytopenia, splenomegaly,
myelofibrosis, increased serum B12, and mildto-moderate bleeding tendencies.3 Although
fatal in some cases, in other cases, patients
have lived into their 7th decade. However,
where data are available, it seems that GPS
is progressive, because the thrombocytopenia
and myelofibrosis worsen with age.
Additionally, there is evidence that genetic
modifiers affect disease outcomes. Individuals
with identical mutations, but from different
families, have discordant disease severities.4
The mouse strain reported by Kahr et al1
BLOOD, 7 NOVEMBER 2013 x VOLUME 122, NUMBER 19
and also by Deppermann et al5 represents
the first animal model for GPS.
The causative gene for GPS was identified
on chromosome 3p by 3 groups in 2011.4,6,7
These studies showed that mutations in the
coding region of the neurobeachin-like 2
(NBEAL2) gene correlated with GPS in
humans. Nbeal2 is a member of a family of
proteins known as BEACH–domain-containing
proteins (BDCP).8 The BEACH domain
gained its name because the charter family
member—LYST/CHS1 (lysosomal
trafficking regulatory/Chediak-Higashi
syndrome 1 protein), defective in the beige
mouse and in human Chediak-Higashi
syndrome (MIM 214500)—contained this
unusual ;300 amino acid domain.9 This
domain has a unique peptide-backbone fold,
in that its core does not assume regular
secondary structures.10 The highly conserved
BEACH domain has now been identified in
9 human proteins and in numerous species.8
Most members of this family of generally
high molecular weight proteins contain
3247
From www.bloodjournal.org by guest on June 15, 2017. For personal use only.
2013 122: 3246-3247
doi:10.1182/blood-2013-09-526426
Factor XIII: sticking it to platelets
Adam D. Munday and José A. López
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