Biomedical Engineering Reference
In-Depth Information
functionalization.of.nanomaterials.with.nucleic.acid.aptamers.is.promising.for.a.variety.of.
cell.recognition-based.biological.and.biomedical.applications.
In.this.chapter,.several.important.applications.of.aptamer-functionalized.nanomaterials.
are. discussed.. The. irst. part. of. this. chapter. briely. introduces. the. procedure. of. aptamer.
discovery,. including. both. aptamer. selection. and. aptamer. truncation.. The. second. part.
discusses.different.aptamer-functionalized.nanomaterials,.including.aptamer-based.syn-
thetic. antibodies,. aptamer-functionalized. nanoparticles,. and. aptamer-based. multifunc-
tional.nucleic.acid.molecules.
2.2 Nucleic Acid Aptamer
Nucleic.acid.aptamers.are.single-stranded.nucleic.acid.molecules.that.can.recognize.target.
molecules.with.high.afinity.and.speciicity..The.term.“aptamer”.comes.from.two.words:.
the.Latin.
aptus
,.which.means.“to.it,”.and.the.Greek.
meros
,.which.means.“part”.(Ellington.
and.Szostak.1990,.Ng.et.al..2006).
Nucleic. acid. aptamers. have. been. studied. for. many. applications. because. they. have.
numerous. merits. as. afinity. molecules.. First,. in. principle,. nucleic. acid. aptamers. can. be.
selected. for. any. interesting. target. under. physiological. or. nonphysiological. conditions.
(Jenison.et.al..1994,.Liu.et.al..2003,.Proske.et.al..2005,.Jaeger.and.Chworos.2006,.Lee.et.al..
2006,.Bruno.et.al..2009)..The.targets.of.nucleic.acid.aptamers.range.from.small.inorganic.
and.organic.molecules.(Hg
2+
,.K
+
,.cocaine,.ATP).to.large.biomolecules.(peptides,.proteins).
or.even.whole.organisms.(e.g.,.cells.or.tissues).(Ueyama.et.al..2002,.Ono.and.Togashi.2004,.
He.et.al..2005,.Shangguan.et.al..2006,.Chen.et.al..2007)..Second,.unlike.natural.antibodies,.
aptamers.can.be.easily.synthesized.via.a.chemical.process..Therefore,.aptamers.are.also.
recognized. as. “chemical. antibodies.”. Third,. nucleic. acid. aptamers. can. discriminate.
different.targets.based.on.subtle.structural.differences.such.as.the.presence.or.absence.of.
a.hydroxyl.or.methyl.group.in.proteins..Fourth,.aptamers.have.tunable.stability,.which.can.
be.controlled.by.the.modiication.of.nucleotides.(Lee.et.al..2006)..For.example,.a.methylated.
aptamer.was.kept.stable.in.plasma.for.96.h.(Burmeister.et.al..2005)..Fifth,.the.binding.afinity.
of. aptamers. to. their. targets. can. be. changed. under. certain. conditions. (Hicke. et. al.. 1996,.
2001,.O'Connell.et.al..1996,.Rusconi.et.al..2002)..For.instance,.some.aptamers.that.exhibit.
high.binding.afinity.at.4°C.have.very.low.binding.afinity.at.37°C.(Hicke.et.al..1996,.2001,.
O'Connell.et.al..1996)..Sixth,.aptamers.are.small.in.size,.usually.having.20-80.nucleotides.
(∼6-24.kDa)..In.contrast,.natural.antibodies.such.as.IgG.have.a.higher.molecular.weight.of.
∼150.kDa.(Cohen.and.Milstein.1967).
The. procedure. of. discovering. nucleic. acid. aptamers. includes. two. steps:. upstream.
selection. and. downstream. truncation.. Upstream. selection. is. used. to. ind. full-length.
aptamers.with.a.dissociation.constant.in.micro-.to.picomolar.range.(Jenison.et.al..1994)..
Downstream.truncation.is.used.to.identify.essential.nucleotides.and.remove.nonessential.
nucleotides.of.full-length.aptamers.(Zhou.et.al..2011).
2.2.1 Aptamer Selection
Nucleic. acid. aptamers. are. selected. from. single-stranded. DNA. or. RNA. libraries. through.
SELEX.(i.e.,.systematic.evolution.of.ligands.by.exponential.enrichment)..Figure.2.1.shows.
a. typical. process. of. SELEX,. which. includes. iterative. cycles. of. incubating. the. DNA/RNA.
