Molecules illuminate Alzheimer’s roots

RICE U. (US) — A breakthrough in sensing technology could make finding signs of Alzheimer’s disease nearly as simple as switching on a light.

Bioengineers are testing metallic molecules that naturally attach themselves to a collection of beta amyloid proteins called fibrils that form plaques in the brains of Alzheimer’s sufferers. When the molecules latch onto amyloid fibrils, their photoluminescence increases 50-fold.

The large increase in fluorescence may be an alternative to molecules currently used to study amyloid fibrils, which researchers believe form when misfolded proteins begin to aggregate. Changes in fluorescence characterize the protein transition from disordered monomers to aggregated structures.

Amyloid fibrils, magnified 12,000 times. (Credit: Nathan Cook)

Details are reported in the Journal of the American Chemical Society.

Researchers discovered that the molecules, ruthenium complexes, have a distinctive ability to luminesce when combined in a solution with amyloid fibrils.

Such fibrils are simple to make in the lab, says lead author Nathan Cook, a graduate student working in the lab of Angel Marti, assistant professor of chemistry and bioengineering at Rice University.

Molecules of beta amyloid naturally aggregate in a solution, as they appear to do in the brain. Ruthenium-based molecules added to the amyloid monomers don’t fluoresce, but once the amyloids begin to aggregate into fibrils that resemble “microscopic strands of spaghetti,” hydrophobic parts of the metal complex are naturally drawn to them.

“The microenvironment around the aggregated peptide changes and flips the switch” that allows the metallic complexes to light up when excited by a spectroscope, Cook says.

Thioflavin T (ThT) dyes are the standard sensors for detecting amyloid fibrils and work much the same way. But they have a disadvantage because it fluoresces when excited at 440 nanometers and emits light at 480 nanometers—a 40-nanometer window, a gap known as the Stokes shift.

“In the case of our metal complexes, the Stokes is 180 nanometers,” says Martí. “We excite at 440 and detect in almost the near-infrared range, at 620 nanometers.

“That’s an advantage when we want to screen drugs to retard the growth of amyloid fibrils,” he says. “Some of these drugs are also fluorescent and can obscure the fluorescence of ThT, making assays unreliable.”

The researchers also exploited the metallic’s long-lived fluorescence by “time gating” spectroscopic assays.

“We specifically took the values only from 300 to 700 nanoseconds after excitation,” Cook explains. “At that point, all of the fluorescent media have pretty much disappeared, except for ours.

“The exciting part of this experiment is that traditional probes primarily measure fluorescence in two dimensions: intensity and wavelength. We have demonstrated that we can add a third dimension—time—to enhance the resolution of a fluorescent assay.”

The researchers said their complexes could be fitting partners in a new technique called fluorescence lifetime imaging microscopy, which discriminates microenvironments based on the length of a particle’s fluorescence rather than its wavelength.

The goal is to treat Alzheimer’s, and possibly such other diseases as Parkinson’s, through the technique, possibly combining the ruthenium complex’s ability to target fibrils and other molecules’ potential to dissolve them in the brain, Marti says. “That’s something we are actively trying to target.”

The Welch Foundation supported the research.

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