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Cell Host & Microbe
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production of polyamines is normally
restrictive to S. aureus growth, given the
sensitivity of most strains to these mole-
cules (Joshi et al., 2011). However,
USA300 strains contain ACME-encoded
speG, which confers resistance to poly-
amine toxicity in vitro (Joshi et al., 2011).
Here, the authors extend this finding using
in vivo models to demonstrate that speG
mutants in USA300 display increased
sensitivity to polyamines during the
postinflammatory phase of skin and soft
tissue infection. Further, their data indi-
cate that bacterial burden in late-stage
abscesses is largely dependent on
SpeG, a novel finding that links ACME to
persistence during skin and soft tissue
infection.
Thurlow et al. continue their investiga-
tion into the role of ACME in skin infections
by addressing an apparent paradox
between their findings and that of earlier
studies, which were unable to uncover
a role for ACME in skin and soft tissue
infections (Montgomery et al., 2009). The
authors demonstrate that if speG is
deleted, bacterial burden is significantly
reduced and clearance occurs more
quickly in vivo. However, it was previously
observed that an ACME mutant (lacking
speG and arc) is equally as virulent as an
ACME-containing strain (Montgomery
et al., 2009). Thus, it is somewhat perplex-
ing that an ACMEmutant is not also atten-
uated in vivo. Through the use of murine
infection models, the authors rationalize
these findings by demonstrating that
S. aureus Arc diverts L-arginine away
from NO$ production by the host, thereby
fostering polyamine synthesis, a result of
the increased production of L-ornithine
by the bacterium (Figure 1). The presence
of Arc appears to enhance host polyamine
production and requires SpeG to detoxify
the system. Indeed, deletion of arc in
a speG mutant increases the virulence of
the strain to WT levels. Thus, the fitness
advantage conferred upon USA300 via
ACME requires both Arc and SpeG.
All together, the work presented in this
new study both describes a novel role
for ACME as an important virulence deter-
minant in USA300 skin and soft tissue
infection and provides a unique rationale
for how this strain may have rapidly out-
competed other S. aureus clones for
dominance over the human skin niche.
Furthermore, it highlights the remarkable
adaptability of S. aureus in the face of
host immunity, one that extends beyond
traditional mechanisms of pathogenesis
(toxins, immune-modulatory molecules,
etc.). It is worth mentioning that these
findings do not discount the role of major
virulence factors in S. aureus skin and soft
tissue infections; rather, they add to the
Cell Host & Microbe
ever-evolving armament of S. aureus
survival mechanisms.
REFERENCES
Diep, B.A., Stone, G.G., Basuino, L., Graber, C.J.,Miller, A., des Etages, S.A., Jones, A., Palazzolo-Ballance, A.M., Perdreau-Remington, F., Sensa-baugh, G.F., et al. (2008). J. Infect. Dis. 197,1523–1530.
Foster, T.J. (2005). Nat. Rev. Microbiol. 3,948–958.
Joshi, G.S., Spontak, J.S., Klapper, D.G., and Ri-chardson, A.R. (2011). Mol. Microbiol. 82, 9–20.
Li, M., Diep, B.A., Villaruz, A.E., Braughton, K.R.,Jiang, X., DeLeo, F.R., Chambers, H.F., Lu, Y.,and Otto, M. (2009). Proc. Natl. Acad. Sci. USA106, 5883–5888.
Miko, B.A., Uhlemann, A.C., Gelman, A., Lee, C.J.,Hafer, C.A., Sullivan, S.B., Shi, Q., Miller, M., Zenil-man, J., and Lowy, F.D. (2012). Microbes Infect.14, 1040–1043.
Miller, L.S., and Cho, J.S. (2011). Nat. Rev. Immu-nol. 11, 505–518.
Montgomery, C.P., Boyle-Vavra, S., Adem, P.V.,Lee, J.C., Husain, A.N., Clasen, J., and Daum,R.S. (2008). J. Infect. Dis. 198, 561–570.
Montgomery, C.P., Boyle-Vavra, S., and Daum,R.S. (2009). Infect. Immun. 77, 2650–2656.
Richardson, A.R., Libby, S.J., and Fang, F.C.(2008). Science 319, 1672–1676.
Thurlow, L.R., Joshi, G.S., Clark, J., Spontak, J.,Neely, C.J., Maile, R., and Richardson, A.R.(2013). Cell Host Microbe 13, this issue, 100–107.
How Trypanosoma cruzi Feastsupon Its Mammalian Host
Nicola S. Carter1 and Buddy Ullman1,*1Department of Biochemistry and Molecular Biology, Oregon Health & Science University, 3181 SW Sam Jackson Park Road, Portland,OR 97239-3098, USA*Correspondence: ullmanb@ohsu.eduhttp://dx.doi.org/10.1016/j.chom.2013.01.003
Trypanosoma cruzi has a complex relationship with its mammalian host in which parasite and host metabolicnetworks are intertwined. A genome-wide functional screen of T. cruzi infection in HeLa cells (Caradonnaet al., 2013) divulges host metabolic functions and signaling pathways that impact intracellular parasitereplication and reveals potential targets for therapeutic exploitation.
Chagas’ disease is a devastating, perni-
cious, and often fatal disease of the
cardiovascular system for which the
hemoflagellate protozoan parasite, Try-
panosoma cruzi, is the etiologic agent.
The neurological system and digestive
tract can also be impacted by T. cruzi
infection. Chagas’ disease is endemic to
all 22 countries of South and Central
13, January 16, 2013 ª2013 Elsevier Inc. 5
Cell Host & Microbe
Previews
America and causes the greatest disease
burden of any parasitic disease in
the western hemisphere. Approximately
8–11 million people are affected, with
�10,000 deaths per year and an addi-
tional >25,000,000 people at risk for
infection (http://www.cdc.gov/parasites/
chagas/epi.html). Autochthonous infec-
tion in the United States has also
been documented, and the Centers
for Disease Control now estimate that
�300,000 individuals in the United
States are currently seropositive for the
parasite (Bern and Montgomery, 2009).
Transmitted to humans by insects known
as ‘‘kissing bugs,’’ insects that are
endemic to the southern half of the
United States, the parasite—which can
also be acquired by blood transfusion,
food contamination, organ transplanta-
tion, transplacental passage, birthing, or
nursing—exists in two forms in the human
host: the nondividing flagellated trypo-
mastigote stage that is primarily found in
the bloodstream, and the aflagellated
amastigote stage that replicates within a
variety of host cells (http://www.cdc.gov/
parasites/chagas/biology.html). A major
goal for treating Chagas’ disease is to
stem disease progression; however, con-
ventional Chagas’ remedies are based
upon antitrypanosomal therapeutics, and
the few drugs that are available for treat-
ment are compromised by toxicity and
demonstrate limited efficacy once chronic
cardiomyopathy develops. An alternative
approach for therapeutic development—
one adopted by several laboratories,
including that of Caradonna et al. (2013),
whose research is featured here—has
focused instead on delineating hostmeta-
bolic pathways that are vital for disease
progression and parasite persistence.
Previous investigations evaluating host
factors perturbed by T. cruzi invasion
have primarily focused on profiling the
host transcriptional response after
T. cruzi infection of cardiomyocytes and
fibroblasts, as well as a variety of other
cells, using classical DNA microarray
technology. These studies, which have
contributed tremendously to our knowl-
edge about the invasion processes
utilized by T. cruzi trypomastigotes, as
well as provided links between host dys-
regulation and disease progression,
have shed little light on the host metabolic
processes important for sustaining intra-
cellular T. cruzi infections that are requi-
6 Cell Host & Microbe 13, January 16, 2013 ª
site for the long-term persistence of
this parasite. In this issue of Cell Host &
Microbe, Caradonna et al. describe
a particularly sophisticated set of studies
founded upon an unbiased, genome-
scale, forward genetic screen to identify
host genes and proteins, as well as
their participatory metabolic partners,
that impact the intracellular growth of
T. cruzi. The primary screen conducted
in HeLa cells consisted of a genome-
wide knockdown of 18,263 host mRNAs
and their corresponding proteins using
siRNA technology and offers a powerful,
all-encompassing approach for the iden-
tification of both positive and negative
regulators of T. cruzi growth. The sig-
nificance of the data from the primary
screen, which yielded hundreds of
hits (see Table S1 in Caradonna et al.,
2013), was realized by the implemen-
tation of an ingenious secondary screen
to distinguish those host proteins
important for T. cruzi early infection—
i.e., trypomastigote invasion and early
establishment in the host cell (<18 hr
postinfection)—from those important for
later amastigote growth and viability
(18–72 hr postinfection).
While the results from the early infection
data were largely supportive of the known
models for T. cruzi host cell invasion and
parasitism (reviewed in Caradonna and
Burleigh, 2011), they did identify several
additional new factors that are likely
complicit in parasite attachment and
invasion. However, the major thrust of
this elegant composition was the elucida-
tion of host constituents and processes
that impact amastigote growth and
viability. In one of the most wide-ranging
examinations of intracellular host meta-
bolism and its relationship to intracellular
parasite proliferation and viability,
Caradonna et al. have succeeded in
uncovering several unique facets of host
metabolism that offer opportunities for
future drug exploration. One intriguing
discovery was the apparent shift in
host cell energy production away from
the oxidization of glucose—and more
specifically the conversion of pyruvate
to acetyl-CoA—to the b-oxidation of
long chain and very long chain fatty
acids in the infected host cell. Indeed,
the siRNA data, accompanied by addi-
tional experiments with fibroblasts con-
taining a genetic deficiency in the first
mitochondrial step of the b-oxidation
2013 Elsevier Inc.
of long chain and very long chain fatty
acids, suggest that the production of
very long chain fatty acids by the host
cell and their subsequent oxidation is
likely a requisite for parasite viability and
ultimately, perhaps, for parasite persis-
tence. A key question for future studies
will be how the parasite manipulates and
ultimately benefits from these changes in
the host metabolic landscape. Whether
this is a ploy to mobilize and obtain fatty
acid precursors necessary for the syn-
thesis of membrane in the dividing
amastigote population or, alternatively,
to bolster parasite energy production
by providing reducing intermediates (i.e.,
NADH and FADH2) generated during
the oxidation process remains to be
elucidated. Another intrigue concerns
the fate of pyruvate. The siRNA results,
combined with additional experimental
data, indicate that downregulation of
pyruvate metabolism to either acetyl-
CoA or lactate is beneficial for intracellular
amastigote development. While this
might intimate that the accumulation of
pyruvate in the host cell is of benefit to
the parasite, perhaps as an energy
precursor, pyruvate transport into the
amastigote has yet to be confirmed.
The manipulation of host cell energy
production may not be without con-
sequence, and perturbation of mito-
chondrial function in particular is clearly
associated with programmed cell death
(Kroemer et al., 1995). From the siRNA
screen, a strong relationship between
host cell ATP currency and parasite
growth and viability emerged. In partic-
ular, the downregulation of ATP5B,
a subunit of ATP synthase and presum-
ably a determinant of its activity, nega-
tively impacted parasite growth. In
addition, the opposing effects of siRNA
against AMP-activated kinase (AMPK)
and AKT1, with silencing of the former
boosting parasite intracellular growth
and survival and of the latter being associ-
ated with a reduction in parasitemia,
seems to signify an important connection
between host cell energy balance and
parasite viability. AKT1 is a protein kinase
strongly associated with cell survival, in
part through its repression of proapopto-
tic proteins and activation of TOR kinase
complex 1 (mTORC1), which promotes
protein synthesis and inhibits autophagy.
On the other hand, AMPK, which is
activated in response to rising cellular
Cell Host & Microbe
Previews
levels of AMP, a signal that cellular ATP
levels are decreased, opposes many of
the functions of AKT1 and is itself nega-
tively regulated by AKT1 (Mankouri et al.,
2010). Whether and how the parasite
intervenes in the complex interplay
between these two kinases and their
signaling cascades that mediate cell fate
is unclear but will be of obvious conse-
quence for future studies.
Perhaps the most surprising finding
from this screen, which demonstrates
the power of an unbiased screen such
as this one, was the discovery of the
apparent dependence of the T. cruzi
amastigote on the host for the provision
of preformed pyrimidines, which was
functionally authenticated by the authors
by media supplementation, as well as
the preference for salvageable purine
nucleosides over nucleobases, which
remains to be functionally tested. These
parasites have a full complement of
pyrimidine biosynthesis enzymes located
in a cluster on chromosome 21, as well
as several putative orthologs within their
genome to the leishmanial nucleoside
and nucleobase transporters NT1–NT4
(http://tritrypdb.org/tritrypdb/ and Carter
et al., 2008). Thus, a nutritional depen-
dency on salvageable pyrimidines and
purine nucleosides is quite unexpected.
It is noteworthy that recent studies with
T. cruzi engineered to lack carbamoyl-
phosphate synthetase II (CPSII), the
first enzyme in pyrimidine biosynthesis,
also suggested a functional significance
between this first pyrimidine biosynthetic
step and intracellular parasite develop-
ment (Hashimoto et al., 2012), under-
scoring the significance of pyrimidine
metabolism and the contributions of
both parasite salvage and biosynthesis
pathways for host cell parasitism. Other
nutritional interactions revealed by this
screen between parasite and host, for
example, pteridine metabolism, were
less surprising but confirmed the power
of the general forward genetic approach.
Overall, this investigation has provided
numerous incisive insights into the host
response to T. cruzi infection and persis-
tence and lends itself to the exploita-
tion of previously unpredictable host
Cell Host & Microbe
determinants that could be pharmaco-
logically targeted in a fashion that mini-
mizes the emergence of long-term drug
resistance.
REFERENCES
Bern, C., andMontgomery, S.P. (2009). Clin. Infect.Dis. 49, e52–e54.
Caradonna, K.L., and Burleigh, B.A. (2011). Adv.Parasitol. 76, 33–61.
Caradonna, K.L., Engel, J.C., Jacobi, D., Lee,C.-H., and Burleigh, B.A. (2013). Cell Host Microbe13, this issue, 108–117.
Carter, N.S., Yates, P., Arendt, C.S., Boitz, J.M.,and Ullman, B. (2008). Adv. Exp. Med. Biol. 625,141–154.
Hashimoto, M., Morales, J., Fukai, Y., Suzuki, S.,Takamiya, S., Tsubouchi, A., Inoue, S., Inoue, M.,Kita, K., Harada, S., et al. (2012). Biochem.Biophys. Res. Commun. 417, 1002–1006.
Kroemer, G., Petit, P., Zamzami, N., Vayssiere,J.L., and Mignotte, B. (1995). FASEB J. 9, 1277–1287.
Mankouri, J., Tedbury, P.R., Gretton, S., Hughes,M.E., Griffin, S.D., Dallas, M.L., Green, K.A.,Hardie, D.G., Peers, C., and Harris, M. (2010).Proc. Natl. Acad. Sci. USA 107, 11549–11554.
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