Melbourne Uni's 'nano-beacon' plays tricks with DNA

In medicine, good things are coming in increasingly tiny packages: microscopic delivery systems made from polymers or other bio-friendly materials, and stuffed with therapeutic genes, proteins or small synthetic molecules, that can be implanted strategically in the brain or body.

It can take multiple trials and errors before researchers achieve the optimal pore size distribution for the nanocapsules to release their contents at right site, and the right rate -- and that will vary with the size and chemistry of encapsulated molecules.

University of Melbourne chemists have devised an ingenious solution to make drug developers' lives a little easier: a DNA-based 'nano-beacon' that lights up when things are working optimally.

Federation Fellow Prof Frank Caruso, director of the university's Centre for Nanoscience and Nanotechnology, is designing molecular transport systems, and describes the pore-size problem as "a major roadblock' to the development of nano-encapsulated drugs.

Caruso's colleague, Dr Angus Johnston, said the challenge of visualising a very small number of small molecules in transit through a membrane is complicated by the fact that attaching a molecular 'label' would increase their size, and give a false reading.

Their solution was to develop a system that employs a single strand of DNA, with a fluorophore -- a light emitting molecule -- at one end, and a quencher at the other.

The linear molecule contains a palindromic DNA sequence that loops on itself and undergoes complementary base pairing, apposing the fluorophore and its quencher together at the narrow end of the flask-shaped molecule.

When the 'nano-beacons' are placed inside the capsules, they are in their base-paired conformation, and emit no fluorescence.

Single-stranded DNA 'probes' of various lengths, but complementary to the DNA sequence of the beacon, are introduced into the medium containing the capsules. If they are of the right size to enter the microcapsule through its membrane pores, they hybridise competitively with one or other of the base-paired sequences of the 'flask'.

As they do so, they forcing the strands apart, separating the fluorophore from its quencher -- and the fluorophore emits light, with an intensity proportional to the number of 'probe' molecules that penetrate the microcapsule.

The slow increase in light output allows the drug developer to calculate how rapidly the DNA is penetrating.

Caruso said the probe DNA strands can be linked to nanoparticles, proteins, other DNA species, and potentially, RNA.

"We have focused on DNA because of its stability and relevance to biotechnology research," he said. "It's a rapid and facile way to measure permeability of the carrier nanocapsules -- once we've done that, we can start to design for specific loading and release characteristics."