Recent findings by medical researchers indicate that naturally occurring nanotubes may serve as tunnels that protect retroviruses and bacteria in transit from diseased to healthy cells — a fact that may explain why vaccines fare poorly against some invaders.

To better study the missions of these intercellular nanotubes, scientists have sought the means to form them quickly and easily in test tubes.

Sandia National Laboratories researchers have now learned serendipitously to form nanotubes with surprising ease.

“Our work is the first to show that the formation of nanotubes is not complicated, but can be a general effect of protein-membrane interactions alone,” says Darryl Sasaki of Sandia’s Bioscience and Energy Center.

Sandia is a National Nuclear Security Administration laboratory.

The tunnel-like structures have been recognized only recently as tiny but important bodily channels for the good, the bad, and the informational.

In addition to providing protected transport to certain diseases, the nanotubes also seem to help trundle bacteria to their doom in the tentacles of microphages. Lastly, the nanotubes may provide avenues to send and receive information (in the form of chemical molecules) from cell to cell far faster than their random dispersal into the bloodstream would permit.

Given the discovery of this radically different transportation system operating within human tissues, it was natural for researchers to attempt to duplicate the formation of the nanotubes. In their labs, they experimented with giant lipid vesicles that appeared to mimic key aspects of the cellular membrane.

Giant lipid vesicles resemble micron-sized spherical soap bubbles that exist in water. They are composed of a lipid bilayer membrane only five nanometers thick.

The object for experimenters was to create conditions in which the spheres would morph into cylinders of nanometer radii.

But researchers had difficulties, says Sasaki, perhaps because they used a composite lipid called egg PC that requires unnecessarily high energies to bend into a tubular shape.

Egg PC is inexpensive, readily available, and offers good, stable membrane properties. It is the usual lipid of choice in forming nanocylinders via mechanical stretching techniques.

But Sandia postdoctoral researcher Haiqing Lui instead used POPC — a single pure lipid requiring half the bending energy of egg PC.

She was trying to generate nanotubes by a completely different approach that involved the use of motor proteins to stretch naturally occurring membranes into tubes.

Working with Sandia researcher George Bachand, she serendipitously found that interaction of the POPC membrane with a high affinity protein called streptavidin alone was enough to form the nanotubes.

“Perhaps this information — linking membrane bending energy with nanotube formation — may provide some clue about the membrane structure and the cell’s ability to form such intercellular connections,” Sasaki says.

The formation was confirmed by Sandia researcher Carl Hayden, who characterized the nanotube formation through a confocal imaging microscope. The custom instrument allows pixel-by-pixel examination of the protein interaction with the membranes comprising the nanotubes by detecting the spectrum and lifetimes of fluorescent labels on the proteins.

Nanotube formation had been noticed previously by cell biologists, but they had dismissed the tiny outgrowths as “junk — an aberration of cells growing in culture,” says Sasaki. “The reason they were only noticed recently as trafficking routes is because of labeling studies that marked organelles and proteins. This allowed a focused look at what these nanostructures might be used for.”

It became clear, says Sasaki, that the organelles were being transported with “specific directionality” on the backs of motor proteins within the tubes, rather than randomly.

Three-dimensional networks of nanotubes also are found to be created by macrophages — part of the police force of the body — grown in culture, says George. The tubes in appearance and function resemble a kind of spider web, capturing bacterium and transporting them to the macrophages, which eat them.

Source: DOE/Sandia National Laboratories

Molecular transport across cellular membranes is essential to many of life’s processes, for example electrical signaling in nerves, muscles and synapses.

In biological systems, the membranes often contain a slippery inner surface with selective filter regions made up of specialized protein channels of sub-nanometer size. These pores regulate cellular traffic, allowing some of the smallest molecules in the world to traverse the membrane extremely quickly, while at the same time rejecting other small molecules and ions.

Researchers at Lawrence Livermore National Laboratory are mimicking that process with manmade carbon nanotube membranes, which have pores that are 100,000 times smaller than a human hair, and were able to determine the rejection mechanism within the pores.

“Hydrophobic, narrow diameter carbon nanotubes can provide a simplified model of membrane channels by reproducing these critical features in a simpler and more robust platform,” said Olgica Bakajin, who led the LLNL team whose study appeared in the June 6 online edition of the journal Proceedings of the National Academy of Sciences.

In the initial discovery, reported in the May 19, 2006 issue of the journal Science, the LLNL team found that water molecules in a carbon nanotube move fast and do not stick to the nanotube’s super smooth surface, much like water moves through biological channels. The water molecules travel in chains - because they interact with each other strongly via hydrogen bonds.

“You can visualize it as mini-freight trains of chain-bonded water molecules flying at high speed through a narrow nanotube tunnel,” said Hyung Gyu Park, an LLNL postdoctoral researcher and a team member.

One of the most promising applications for carbon nanotube membranes is sea water desalination. These membranes will some day be able to replace conventional membranes and greatly reduce energy use for desalination.

In the recent study, the researchers wanted to find out if the membranes with 1.6 nanometer (nm) pores reject ions that make up common salts. In fact, the pores did reject the ions and the team was able to understand the rejection mechanism.

“Our study showed that pores with a diameter of 1.6nm on the average, the salts get rejected due to the charge at the ends of the carbon nanotubes,” said Francesco Fornasiero, an LLNL postdoctoral researcher, team member and the study’s first author

Fast flow through carbon nanotube pores makes nanotube membranes more permeable than other membranes with the same pore sizes. Yet, just like conventional membranes, nanotube membranes exclude ions and other particles due to a combination of small pore size and pore charge effects.

“While carbon nanotube membranes can achieve similar rejection as membranes with similarly sized pores, they will provide considerably higher permeability, which makes them potentially much more efficient than the current generation of membranes,” said Aleksandr Noy, a senior member of the LLNL team.

Researchers will be able to build better membranes when they can independently change pore diameter, charge and material that fills gaps between carbon nanotubes.

Source: DOE/Lawrence Livermore National Laboratory

Francesco Fornasiero, Hyung Gyu Park, Jason K. Holt, Michael Stadermann, Costas P. Grigoropoulos, Aleksandr Noy, and Olgica Bakajin. Colloquium Paper: Ion exclusion by sub-2-nm carbon nanotube pores. PNAS published June 6, 2008, 10.1073/pnas.0710437105

Josh says:

This does have a lot of implications. They must work very similar to ion channels in membranes, where only certain ions can travel through. I wonder if it would be possible to design custom transmembrane membrane channels to help with drug delivery?