Biology Reference
In-Depth Information
body itself. For example, a high level of uric acid in the blood circulation is associated with
tumor lysis syndrome and gout. Kemmer et al.
114
constructed a synthetic gene circuit that
senses endogenous uric acid levels through a
Deinococcus radiodurans
-derived protein, and
which subsequently triggers elimination of excessive uric acid through a secretion-
engineered
Aspergillus flavus
urate oxidase. Another example of a synthetic gene circuit
responding to endogenous signals in situ is a designer gene network for coordinating bovine
artificial insemination by the ovulation-triggered release of implanted sperm.
115
This
involves transgenic expression of cellulase in response to a lutenizing hormone surge
associated with ovulation. The cellulase enzyme then breaks down the cellulose
microcapsules containing the spermatozoa, thus effecting their release for fertilization at the
time of ovulation. Because certain diseases are characterized by changes in redox state, it
may also be therapeutically relevant to engineer synthetic gene circuits to be responsive to
the redox state within cells. Weber et al.
116
constructed a genetic redox sensor for
mammalian cells based on the
Streptomyces coelicolor
redox control system.
In addition to their response to soluble molecules, synthetic gene circuits can be engineered
to be responsive to molecular signals in the gas phase. The pioneering study of Weber
et al.
117
achieved gas-inducible transgene expression in mammalian cells by reengineering
the acetaldehyde-inducible AlcR-P
alcA
transactivator
promoter interaction found within
Aspergillus nidulans
. By utilizing this synthetic gene circuit, together with the conversion of
ethanol to acetyldehyde, a further study
118
established a synthetic airborne cell
communication network.
Besides response to molecular signals, it may be useful in some therapeutic applications for
synthetic gene circuits to be able to respond to physical stimuli. For example, it may be
necessary to engineer synthetic gene circuits to respond to electrical signals in order to
interface between implanted electronic/electrical devices and biological systems. Weber
et al.
119
created an electro-genetic transcription unit by using electrical power to convert
ethanol into acertaldehyde, which in turn interacted with the acetyldehyde-inducible gene
circuit discussed earlier.
117
Gene transcription can be adjusted according to the intensity of
an applied direct current, as well as to the amplitude or frequency of an applied alternating
current. Utilizing such an electro-genetic transcription unit, modulation of the beating
frequency of primary heart cells was demonstrated.
119
A more recent study by the
Fussenegger group
120
used a light-inducible synthetic gene circuit to control the transgenic
expression of GLP-1 in a type II diabetic mouse model. Using this system, the attenuation of
glycemic excursions and the enhancement of blood-glucose homeostasis were achieved in
diabetic mice.
167
Alternatives to Recombinant DNA: Synthetic/Modified RNA, Proteins,
and Other Analogue Molecules
The use of recombinant DNA in synthetic biology applications carries a risk of permanent
genetic alteration to cells, which in turn raises serious safety concerns. To address these
safety concerns, an alternative may be to utilize synthetic/modified RNA, proteins and other
analogue molecules (for example, peptide nucleic acid, locked nucleic acid) in place of
recombinant DNA for some, but not all, synthetic biology applications. Nevertheless, it
must be noted that because these molecules are not integrated into the cellular genome, and
are eventually degraded, their effect is only transient. Hence, these molecules are only suited
for synthetic biology applications that require just a transient change in gene/protein
expression.
For transient gene silencing through RNA interference, it may be advantageous to utilize
nucleic acid analogues, such as peptide nucleic acid (PNA)
122
and locked nucleic acid
(LNA),
123
rather than siRNA or microRNA. This is due to the greater stability and superior
hybridization capacity of PNA and LNA compared with RNA, which in turn translates into a
