Joined: 03 Oct 2005
|Posted: Wed Dec 14, 2005 10:38 am Post subject: Scientists Identify Process for Cellular Uptake of Nanotubes
|Stanford Researchers Identify the Processes Involved for the Cellular Uptake of Carbon Nanotubes
Carbon nanotubes are like the tiniest of needles and have the potential to channel pharmaceutical agents into targeted living cells (see earlier story). Now, thanks to research published in the journal Angewandte Chemie, researchers have a clearer picture of how carbon nanotubes are able to breach the cell’s membrane and function as efficient drug delivery vehicles.
Hongjie Dai, Ph.D., and his colleagues at Stanford University discuss their efforts at identifying the cellular processes that are involved in cellular uptake of carbon nanotubes. In order to develop tailored nano-transporters that duly deliver their cargo, the investigators believe it is important to know which route they take through the cell membrane.
Molecules can get into the interior of a cell by various means. So-called passive transport mechanisms do not consume energy; molecules just pass the membrane, while active transport requires a cellular energy source, and can occur via several routes. Nanotubes might, for example, enter the cell by so-called endocytosis, in which the cell membrane forms a cavity-like structure that surrounds a molecule or nanostructure and pulls it into the cell. This process requires energy in the form of ATP and sufficiently high temperatures. Dr. Dai and his colleagues cooled some cell cultures and reacted others with an inhibitor that stops ATP production. In both cases the cells were no longer able to absorb nanotubes. “We conclude that this is an energy-dependent endocytosis mechanism,” says Dr. Dai.
For nanotubes, the researchers suspected one of two mechanisms was involved: “caveolae-mediated” and “clathrin-dependent” endocytosis. Caveolae are little indentations made of lipids (fatty molecules) in the cell membrane. Molecules from the medium enter the indentation, which then closes itself off into a bubble that migrates into the cell interior. By means of inhibitors, the researchers disrupted the lipid distribution in the cell membrane, thus disrupting the caveolae. Nonetheless, nanotubes were able to enter the cell unhampered by the lack of caveolae.
The clathrin-dependent mechanism involves nanoparticles docking outside of the cell at special docking stations on the exterior of the membrane. Tripod-shaped protein molecules, known as clathrin, are present within the docking site on the inside part of the cell membrane. There, clathrin molecules aggregate into a two-dimensional network that forms an arch, producing a cavity in the membrane. This prcoess again results in a bubble that closes itself off and moves into the interior of the cell.
Sugar-containing or potassium-free media destroy clathrin sheets. When the investigators conducted their uptake experiments using sugar-rich or potassium-deficient growth medium, cells no longer took in the nanotubes. “This clearly indicates clathrin-dependent endocytosis for carbon nanotubes used in our work,” says Dr. Dai. He did note that these results contradict those from another group who propose a non-endocytotic mechanism. The reasons for the discrepancy have yet to be determined.
This work is detailed in a paper titled, “Carbon nanotubes as intracellular transporters for proteins and DNA: an investigation of the uptake mechanism and pathway.” This paper was published online in advance of print publication. An abstract is not yet available, though a short item on the paper is available at the journal’s website.
Source: NCI Alliance for Nanotechnology in Cancer.
This story was posted on 13 December 2005.