Biology Reference
In-Depth Information
configuration, binding of small molecules to the aptamer domain of crosslinking ssDNA
could be used to effect dissolution of the hydrogel. Yang et al.
144
reported on DNA-based
hydrogels that underwent dissolution in the presence of adenosine or thrombin.
The interaction of specific protein transcription factors with their cognate promoter/operator
sequences has been well-studied,
145
and can be potentially exploited to further expand the
functionality of DNA-based hydrogels. Of particular interest is the fact that many of these
protein transcription factors also bind to various small-molecule inducer/repressors that in
turn modulate their binding affinity to their cognate operator/promoter sequences. Christen
et al.
146
fabricated a tetracycline-responsive DNA-based hydrogel based on the TetR-tetO
interaction.
94
Polyacrylamide chains within the hydrogel were functionalized with either the
TetR protein or DNA oligonucleotide sequences corresponding to tetO. Hence, exposure to
tetracycline led to disruption of the TetR-tetO crosslink, resulting in dissolution of the
hydrogel.
Besides DNA-based hydrogels, the use of proteins to crosslink high-molecular-weight
polymers in hydrogels has also been reported. Kampf et al.
147
created a hydrogel by
crosslinking polyacrylamide chains with Fm dimer proteins. In the presence of the
immunosuppressive drug FK506 (tacrolimus), the Fm dimers dissociated, disrupting the
crosslink and causing dissolution of the hydrogel. Subsequently, Kampf et al.
147
demonstrated that this FK506-responsive hydrogel could be utilized for the controlled
release of vascular endothelial growth factor (VEGF) in a mouse model. The ability of some
proteins to undergo drastic conformational change in the presence/absence of their cognate
ligands can also be exploited in the fabrication of responsive hydrogels. In the study of King
et al.,
148
the protein calmodulin was utilized as a crosslinker for a polyethyleneglycol
(PEG)-based hydrogel. The addition or withdrawal of its cognate ligand trifluoperazine
(TFP) induced drastic conformational changes, which in turn were translated into swelling
or shrinking of the hydrogel.
171
Drug Screening and Discovery
Synthetic biology has also found increasing applications in the field of drug screening and
discovery. Synthetic gene circuits engineered into mammalian cells, and incorporating
suitable reporter genes, enable an easily measurable and quantifiable readout upon
pharmaceutical induction of the target cellular signaling pathway. Moreover, the use of
mammalian cells as screening tools also allows simultaneous evaluation of cytotoxicity and
cell penetration ability, in addition to assessment of its pharmacological effect on the target
signaling pathway. Mammalian cell-based screening technology can be combined with a
high-throughput analyzer for rapid screening and identification of pharmacologically active
compounds from numerous potential candidates extracted from natural products or
chemically synthesized in the laboratory.
Ligands to the G-protein-coupled receptors (GPCR) form an important class of potential
drug candidates.
149
Currently, there is a commercially available mammalian cell-based assay
that specifically identifies GPCR-binding compounds (marketed by Life Technologies Inc.,
Gaithersburg, MD, USA, under the trademark name of Tango
s
assay). A synthetic gene
circuit incorporating the
β
-lactam reporter gene is used to provide the readout for receptor
binding and activation.
150
Another important category of potential drug candidates is anticancer drugs, which must
have the ability to kill malignant neoplastic cells while at the same time have minimal
cytotoxic effect on normal cells. An initial first screen of potential anticancer drug candidates
would be to identify compounds that selectively kill rapidly dividing cells, while having no
effect on mitotically inactive cells. Gonzalez-Niccolini et al.
151
engineered a synthetic gene
network into Chinese hamster ovary (CHO) cells that allowed mitosis to take place only in
