Post on 16-Dec-2015
Anthrax
Another Reason to Fear Your MailmanBy Stefko Waschuk
R.C
. Lid
ding
ton,
Nat
ure,
415
: 373
-374
(20
02)
Outline
General Information
Pathogenic components
Treatment / Management
Therapeutic uses
General Information
• From Bacillus anthracis
• Two primary forms– cutaneous anthrax (usually curable)– systemic anthrax (usually lethal)
• Encoded by 2 additional plasmids in genome– pXO1 (184.5 kbp)
• anthrax toxin oedema factor (EF), lethal factor (LF), and protective antigen (PA)
– pXO2 (95.3 kbp)• poly-D-glutamic acid capsule
B. Anthracis cycle
M. M
ock,
A. F
ouet
, Ann
u. R
ev. M
icro
biol
. 55:
647
-671
(20
01)
Cutaneous Anthrax
• 95% of all cases
• Characterized by– tissue swelling (oedema)– skin lesion– impaired neutrophil function
• Usually self-limiting– 80-90% of cases resolve without complication
Cutaneous Anthrax
T.C. Dixon. et al. New England Journal of Medicine, 341: 815-826 (1999)
Systemic Anthrax
• Mortality rate ~100%
• Spores germinate within macrophage
• Toxin released into bloodstream– Toxemia and septicemia
• Shock and death
So, How Does It Kill Me?
M. M
oure
z et
al.
Tre
nds
Mic
robi
ol. 1
0:28
7-29
3 (2
002)
Requirements for Pathogenesis
• Anthrax Toxin Receptor
• Protective Antigen
• Lethal Factor
and/or
• Oedema Factor
Anthrax Toxin Receptor (ATR)
• Type I membrane protein
• Extracellular von Willebrand factor A domain– Directly binds to PA
• large extracellular domain with 3 N-linked
glycosylation sites
• Highly conserved between different species
Protective Antigen (PA)
• 83 kDa protein
• 4 domains
• Binds ATR
• Activation requires cleavage
• Mediates delivery of EF & LF into host cells
Protective Antigen
M. M
oure
z et
al.
Tre
nds
Mic
robi
ol. 1
0:28
7-29
3 (2
002)
Mode of Anthrax Toxin Entry
M. M
ou
rez
et a
l. N
atu
re B
iote
ch.,
19:
958-
961
(200
1)
PA Heptamer
M. M
oure
z et
al.
Tre
nds
Mic
robi
ol. 1
0:28
7-29
3 (2
002)
Lethal Factor (LF)
• 90 kDa zinc-dependent protease
• 7 N-terminal residues critical for PA
binding
• Large homology with EF
PA Recognition Site on LF/EF
Lethal Factor Structure
A.D
. Pan
nife
r et
al.
Nat
ure,
414
: 22
9-23
3 (2
001)
Lethal Factor
• Surgical protease– Cleaves 1 specific bond near N-terminus of six known
MAPKKs• Removes the docking sequence for MAPK
– Lethal effects by unknown mechanism
• Cleavage of MAPKK inhibits release of pro-
inflammatory cytokines
Oedema Factor (EF)
• 89 kDa adenylate cyclase
• Contributes to both cutaneous and systemic
anthrax
• Impairs phagocytosis in macrophages
• Identical 7 PA binding residues as LF
• Requires activation by calmodulin (CaM)
Oedema Factor Structure (inactive)
• Active site in interface of CA and CB
• Catalytic machinery is present, but disordered
C.L
. Dru
m e
t al.
Nat
ure,
415
: 39
6-40
2 (2
002)
Oedema Factor Structure (active)
• CaM displaces helical domain• Switch B becomes ordered
– binds ATP– stabilizes EF catalytic residues
C.L
. Dru
m e
t al.
Nat
ure,
415
: 39
6-40
2 (2
002)
More Fun with CaM binding
• Large binding surface stabilizes structural
changes
• ATP locked into catalytic site by salt bridge
• Conformational changes to active site do not
directly involve catalytic residues– become exposed to solvent in active state
Effects of EF Activation
• EF-CaM forms an irreversible complex– CaM forced into extended conformation
• Adenylate cyclase becomes active• Conversion of ATP cAMP• Increased [cAMP] perturbs immune effector cell
functions– Phagocytosis– Chemotactic response– Cytokine expression
Summary: Anthrax Toxin Action
M. M
oure
z et
al.
Tre
nds
Mic
robi
ol. 1
0:28
7-29
3 (2
002)
Anthrax Toxin Management
• Vaccinations
• Antibiotics
• Other strategies– Polyclonal antibodies– Synthetic inhibitors
Vaccinations
• Anthrax vaccine adsorbed (AVA)
• Made from protective antigen
Antibiotics
• Ciprofloxacin Hydrochloride– C17H18FN3O3.HCl.H2O
• Cutaneous Anthrax ~100% effective
• Systemic Anthrax before symptomatic
Synthetic Inhibitors
• EF/LF binding analogues
• Mutated PA
• Soluble ATR
EF/LF Binding Analogues
M. Mourez et al. Trends Microbiol. 10:287-293 (2002)
Domain II Mutant PA
M. M
oure
z et
al.
Tre
nds
Mic
robi
ol. 1
0:28
7-29
3 (2
002)
Domain III Mutant PA
J. M
og
rid
ge
et a
l. P
NA
S. 9
9: 7
045-
7048
(2
002)
Soluble ATR
M. Mourez et al. Trends Microbiol. 10:287-293 (2002)
Therapeutic Uses of Anthrax
• LFN and EFN can be bound to drugs, imported
through ATR & PA
• Cancer Treatments– Oncogenic proteins (Ras) activate MAPKs– Expression of matrix metalloproteases