Nanotoxicology: Exposing hidden dangers
by Roma Subramanian
From drug delivery systems to cosmetics, the science of
nanotechnology has a wide range of applications. This science is
based on the unusual properties that materials exhibit when their
dimensions are reduced to the nanoscale, that is, to less than 100
nanometers (which is 1000 times smaller than the width of a human
hair). For example, while inert at normal scales, gold
nanoparticles are catalytically active and are being used to
improve the performance of fuel cells. Further, unlike its bulk
form, zinc oxide nanoparticles have better UV blocking properties,
making them ideal for use in sunscreens.
(From left to right) Michael Berg, Aishwarya Sooresh, Dr.
Christie Sayes, Nivedita Banerjee, and Amelia Romoser make up the
research team studying nanoparticles.
While the unique properties of nanoparticles are being exploited
to generate a range of products, questions about the potential
toxicity of these particles have not been adequately addressed. For
example, how do nanoparticles incorporated into food matrices to
increase longevity and freshness affect the immune system? How does
inhaling an aerosolized mixture of nanoparticles affect the
Such questions are the focus of Dr. Christie Sayes' research at
the Texas A&M University College of Veterinary Medicine &
Biomedical Sciences. Sayes, an assistant professor in the
department of Veterinary Physiology & Pharmacology, and her
research group are at the forefront of a relatively new and rapidly
The toxic effects of nanoparticles are attributed to their
various physicochemical properties, for example, size, surface
reactivity, optical and electronic properties, agglomeration state
"Studies in my laboratory and in the labs I was trained in, as
well as the experiences of our collaborators all over the world
[have indicated] that nanoparticle exposure results in inflammatory
responses [for example, through the generation of reactive oxygen
species]," Sayes said. "We've seen membrane oxidation, protein
oxidation and even DNA oxidation." In a previous study, Sayes
showed that at high concentrations, nanoscale titanium dioxide
particles, which are used in solar cells, for example, disrupted
the normal activity of human dermal fibroblasts and human lung
epithelial cells in culture through oxidative damage. Further, she
has shown that depending on their surface reactivity, nanoquartz
particles intratracheally instilled in rats produced different
degrees of pulmonary inflammation and cytotoxicity. These and other
studies conducted by her lab on the biological effects of different
types of nanoparticles have helped provide evidence for
Currently, Sayes' lab is investigating the mechanisms underlying
nanoparticle-induced inflammatory cascades and adverse immune
"Some of the cells in the body look at nanoparticles as
antigens, and it's the [resulting] increase in antibody production
that's really interesting," Sayes said. "We know that antibodies
are recruited, but do antibodies know to bind to nanoparticles to
get rid of them? Can macrophages [cells involved in the body's
defense response] see nanoparticles and if not, will the particles
persist in our body, or will we able to excrete them out?"
A human skin cell exposed to green quantum dots, which
preferentially associate with the cell's mitochondria.
The lab is also investigating the effect of a nanoparticle's
agglomeration state on cytotoxicity. Although nanocrystals, which
can be in the form of a powder, tend to agglomerate in water, this
agglomeration is not irreversible.
"As nanoparticles move from one compartment of the body to
another, they could de- and then reagglomerate," Sayes said. "For
example, when an agglomeration of nanoparticles gets immersed in
lung surfactant fluid [secreted by lung alveolar cells], you
sometimes immediately see a deagglomeration of particles, that is,
the particles separate from each other." Since cells appear to
treat individual nanoparticles differently from the way they treat
nanoparticle agglomerates, Sayes is interested in investigating the
differential cellular uptake mechanisms of individual versus
A human skin cell exposed to green quantum dots encapsulated in
polymer microcapsules. The red stain shows the healthy morphology
of the cell by highlighting the cytoplasm.
Also, in collaboration with Swansea University, UK, Sayes' lab
is investigating the toxicological effects of nanoparticles in
composited materials. The project will examine whether in a car
crash involving car bumpers made of nanomaterial composites, the
nanoparticles embedded in the composited material are released and
if so, whether they retain their physical and chemical
characteristics and what would be the implications for the
environment or human health.
"The dose makes the poison," is an oft repeated maxim in
toxicology. However, the absence of a standardized unit to express
the toxic dose of a nanoparticle means that different metrics, for
example, nanoparticle number and nanoparticle mass, are used for
nanoparticle toxicity. This "dosimetric conundrum," as Sayes refers
to it, makes it difficult to compare the results of nanotoxicity
In a recent study, Sayes and her coauthors suggested that
because there are few analytical methods currently available to
measure nanoparticle mass, nanoparticle number per unit volume be
used as a metric for expressing nanoparticle toxicity.
Another challenge in conducting nanotoxicology studies is the
lack of correlation between the results of in vitro and in vivo
studies. This means that in vitro assays need to be further
standardized and validated before they can be used to effectively
screen nanomaterials for toxicity.
Developing such in vitro assays would require careful
characterization of the properties of the nanoparticles being
evaluated as these properties, for example, catalytic activity or
high adsorption capacity, might interfere with the results of the
"A detailed and comprehensive physicochemical characterization
of the test material being studied...is a critical factor for
correlating the nanoparticle surface characteristics with any
measured biological/toxicological responses as well as [for
providing] an adequate reference point for comparing toxicity
results with the hazard-based findings of other investigators,"
Sayes explained in a paper she recently coauthored, stressing the
importance of determining nanoparticle properties.
Other projects and future directions
In addition to studies on nanoparticle toxicity and
characterization, Sayes is investigating the use of nanoparticles
in drug delivery and diagnostics.
For example, a recent grant to the lab awarded from the Texas
AgriLife Research Vector-Borne Disease division will be directed
toward using nanoparticles to prevent the toxic effects of
pyrethroid pesticides after accidental exposure.
In another study, in collaboration with the university's Department
of Biomedical Engineering, Sayes is investigating how quantum dots,
nanocrystal semiconductors with fluorescent properties, can be used
as biosensors to monitor changes in blood chemistry.
Researchers confirm cause of proventricular dilatation
by Roma Subramanian
The cause of proventricular dilatation disease (PDD), a fatal
neurological disorder that affects mainly captive parrots, is avian
bornavirus (ABV). A group led by researchers at the Schubot Exotic
Bird Health Center of the Texas A&M University College of
Veterinary Medicine & Biomedical Sciences confirmed this
revelation in a recent study.
Members of the Schubot research team are (from left) Dr.
Patricia Gray, Dr. Itamar Villanueva, and Dr. Paulette
The study, published in the March 2010 issue of the journal
Emerging Infectious Diseases, is based on the fulfillment of Koch's
postulates for ABV. The postulates are a set of criteria that must
be met to prove that a particular pathogenic agent causes a
First identified in 2008 in birds affected with PDD, ABV has
been suggested as the possible cause of this disease. However, thus
far, conclusive evidence demonstrating the virus actually causes
PDD has not been presented.
Establishing such a causal relationship between ABV and PDD, the
researchers explain in the study, would require satisfying Koch's
postulates, that is, "isolation of the agent [in this case, ABV]
from infected birds; its propagation in culture; and, after
reintroduction of the isolate into a susceptible host,
manifestation of the disease." The researchers have demonstrated
precisely these steps.
The group isolated ABV from the brain tissues of eight parrots
with PDD. The virus was then propagated under laboratory
conditions; specifically, the virus was grown in a culture of duck
embryonic fibroblasts. Fibroblasts infected with the virus were
then injected into two PDD-free Patagonian conures. One Patagonian
conure was injected with fibroblasts that did not contain the
The two Patagonian conures infected with ABV developed clinical
signs of PDD. Further, brain tissues from these birds tested
positive for ABV. The conure that was not infected with the virus
did not develop PDD.
"It's the final act in proving that the virus actually causes
PDD," said Dr. Ian Tizard, director of the Schubot Center and head
of the research group, commenting on the successful experimental
reproduction of the disease in healthy birds.
A slide of the PDD virus.
The study, funded by Texas A&M University's Richard M. Schubot
Endowment, represents a major step forward in understanding PDD, a
disease that has befuddled scientists for more than 30 years.
First reported in the late 1970s, PDD affects more than 50
species of parrots as well as other birds. Many of these species
are endangered and are raised in captivity, making PDD a serious
threat to their conservation.
The disease is characterized by damage to the nerve supply in
the organs in the gastrointestinal tract. This affects the birds'
ability to digest food, resulting in the accumulation of undigested
food in the proventriculus, the first part of the stomach, which
consequently dilates (hence the name of the disease).
Common clinical signs include weight loss, regurgitation of
undigested seeds and loss of appetite. PDD can also damage nerves
in the brain and spinal cord, resulting in neurological symptoms
such as imbalance and lack of coordination. The disease eventually
results in death.
Further studies at the Schubot Center will focus on the origin,
prevention and treatment of the disease.
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