Biology Reference
In-Depth Information
Treatment of Metabolic Disorders
The use of synthetic gene circuits to correct aberrant metabolic conditions is another major
area of interest in synthetic biology. Obesity has recently become a major health concern in
developed countries, due primarily to lifestyle and dietary factors. Dean et al.
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incorporated a synthetic gene circuit encoding the glyco-oxylate shunt pathway into mice
liver cells, resulting in increased fatty acid oxidation. Upon being placed on a high-fat diet,
the mice expressing the glyco-oxylate shunt pathway in their liver cells showed increased
resistance to diet-induced obesity, as well as reduced plasma triglyceride and cholesterol
levels.
A metabolic disorder closely associated with obesity is type II diabetes, the incidence of
which is increasing rapidly in developed countries.
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In a groundbreaking study by the
Fussenegger group,
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a light-responsive synthetic gene regulatory circuit expressing
glucagon-like peptide I (GLP-I) was shown to be capable of attenuating glycemic excursions
in a diabetic mouse model. Exogenous light stimulation was delivered through implanted
fiber optic cables. In an earlier study by the Fussenegger group, aberrant uric acid
metabolism associated with gout and tumor lysis syndrome was corrected by a synthetic
gene circuit expressing uricase mUox, which converted the toxic urate metabolite to
allantoin for easy excretion.
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Responsive Biomaterials and Controlled Release of Biopharmaceuticals
A newly emerging application of synthetic biology is in the fabrication of
responsive
biomaterials that may be capable of controlled release of biopharmaceuticals. This is best
exemplified by hydrogels composed of different high-molecular-weight polymers cross-
linked by either DNA or proteins.
15
'
smart
'
DNA-based responsive hydrogels exploit the ability of a defined sequence of single-stranded
DNA (ssDNA) to bind specifically to either its complementary ssDNA strand or to various
small molecules and proteins. Tierney & Stokke
141
achieved crosslinking within a hydrogel
with complementary ssDNA oligonucelotide sequences functionalized to polyacrylamide
chains. Upon exposure to free ssDNA oligonucleotides with complementary sequences to
the crosslinking DNA, competitive DNA hybridization took place, resulting in disruption of
the crosslink and subsequent dissolution of the hydrogel.
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In an alternative hydrogel configuration reported by Lin et al.,
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polyacrylamide chains
were functionalized to two different noncomplementary ssDNA oligonucleotide sequences.
Crosslinking was then achieved with a third ssDNA oligonucleotide that had terminal
complementary sequences to the two ssDNA oligonucleotides functionalized on the
polyacrylamide chains. It was then demonstrated that the stiffness of the hydrogel could be
modulated with a fourth ssDNA oligonucleotide that had a complementary sequence to the
nonhybridized mid-portion of the third oligonucleotide.
The existence of well-defined DNA sequences that bind specifically to various small
molecules (aptamers) and protein transcription factors (operator/promoter sequences) may
be exploited to further expand the functionality of DNA-based hydrogels to enable the
controlled release of biopharmaceuticals. In the study of He et al.,
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aptamer sequences
that bind specifically to small molecules, such as adenosine-5
0
-triphosphate and human
α
-thrombin, were localized within the mid-section of ssDNA that crosslinks polyacrylamide
chains functionalized with complementary DNA oligonucleotide sequences. Crosslinking
was achieved through hybridization at the terminal ends of the ssDNA, while the mid-
section aptamer domain remained free to bind to small molecules. Subsequent exposure to
free ssDNA of complementary sequences resulted in competitive DNA hybridization and
displacement of the small molecule, as well as disruption of crosslinking and dissolution of
the hydrogel. This effected controlled release of the small molecule. In an alternative
