Functional Nucleic Acids for DNA Nanotechnology - DNA Nanotechnology: From Structure to Function - page 13

increase in binding affinity. AFM was also used to visualize such multicomponent
biomolecular interactions at single-molecule level.
The enzymatic cascade reactions always take place within the nanoscale range in
vivo. To mimic such biological condition, Willner's group adopted aforementioned
strategy of aptamer-ligand interaction as glue and designed a nanowire structure to
study the enzyme assembly for cascade reaction. The nanowire was constructed
with circular DNA and split cocaine aptamer as “glues.” Two forms of circular
DNA components contained two parts of split cocaine aptamer, respectively. They
were designed to link with each other after the recognition of cocaine molecule to
assemble into long DNA-cocaine nanowire-like structure [ 65 ]. By fabricating two
enzymatic proteins, glucose oxidase (GOx) and horseradish peroxidase (HRP), on
the adjacent circulars in special binding regions, the cascade biocatalytic reaction
was activated upon the assembly of nanostructure by addition of cocaine (Fig. 2.3 b).
GOx catalyzes the oxidation reaction of glucose by O 2 and yields H 2 O 2 ,andthen
the resulting H 2 O 2 oxidizes 2,2 0 -azino-bis[3-ethylbenzthiazoline-6-sulfonic acid]
(ABTS 2 ) with the facilitation of HRP. The enhanced biocatalytic activity was
observed due to spatial organization of two enzymes on DNA nanostructure that
led to high local concentrations of the reactive components at adjacent biocatalysts.
DNA nanostructures with aptamers can also be engineered for cell assembly. Cell
communication is highly important in the development, function, and immunity
of multicellular organisms, which happens when the cells are close enough to
exchange information and transport substances. A self-assembly DNA structure
with cell aptamers was reported to specifically induce cell-cell interactions [ 73 ].
The structure consists of multiple aptamers targeting two kinds of cells as a proof of
concept for controlling over the cell distance. As a model, two cell aptamers were
chosen, TE02 aptamer for Ramos cells and the LD201t1 aptamer for Jurkat cells.
Experimental results demonstrated that tetravalent aptamer nanostructure had the
highest binding affinity and nuclease resistance, compared to monomer, dimer, and
trimer. As shown in Fig. 2.4 a, two five-point-star rigid structures with four aptamers
and a linkage were built into a hetero-octamer aptamer system to assemble Ramos
and Jurkat cells. To analyze the assembly, Ramos and Jurkat cells were stained
with red and green dyes, respectively. In Fig. 2.4 b, Ramos-Jurkat cell conjugates
have been identified as a double-stained cell population in the dot plot of the flow
cytometry analysis. Meanwhile, based on forward scatter (FSC) and side scatter
(SSC) analysis for cell size, the population of double-stained cells has a relatively
larger size than that of one kind of cells. In Fig. 2.4 c, the cell-cell conjugates
were visualized under fluorescence confocal microscope (left), while minimal cell
conjugates when no aptamer was added (right). The use of aptamer as recognition
molecules can eliminate the need of chemical modification of cell surfaces and
thus maintain cell integrality and viability. DNA nanoscaffolds provide an excellent
platform to construct multivalent aptamer complexes with controllable distance,
orientation, and conformation at nanometer level for manipulation of cell-cell
interactions.