Turning a cell’s scaffolding into a capsule that opens on cue
How a team in Japan rebuilt tubulin, the protein of microtubules, into hollow capsules that open in response to GTP, and what our simulations added.
Most anticancer drugs travel everywhere in the body, not only where they are needed. One way around this is to pack a drug into a tiny container that stays closed in healthy tissue and opens only where it finds a chemical signal typical of disease. The hard part is choosing a signal that healthy cells do not also send.
The right key
Cells run on small energy-carrying molecules. ATP is the best known, and earlier containers were designed to open in response to it. The trouble is that normal cells contain plenty of ATP too, more than 1 mM. Its close relative GTP is different: in normal cells it stays below about 0.3 mM, while fast-growing cancer cells and cells infected by RNA viruses hold 1.5 to 4.5 mM. A container that responds to GTP, and ignores ATP, would have a much more selective trigger. Until this study, no such container had been reported.
Borrowing from the cytoskeleton
Nature already has a protein that reacts to GTP. Tubulin is the building block of microtubules, the long tubes that cells assemble and take apart, for example when they divide. Tubulin uses GTP to assemble, and when that GTP is broken down the tubes fall apart.
The team, led by Takuzo Aida’s group in Japan, started from tubulin purified from pig brain. They mixed it with GTP and with a version of GTP that cannot be broken down, in a ratio of roughly one to five. They were trying to tune how stable microtubules are, and instead found something unexpected: the tubulin formed flat, leaf-like sheets about 5 nm thick rather than tubes. When they then added a “molecular glue” developed in their lab, the sheets rolled up into hollow spheres. A brief exposure to UV light stitched the shell together so that the capsules held up even in serum.
What the simulations showed
Each capsule is far larger than a single sheet, so at least a thousand sheets have to come together. That raised a question: what does the glue do to them? I was part of the group, with colleagues in Turin and Lugano, that ran and analysed atomistic molecular dynamics simulations of a small patch of tubulin with the glue molecules attached. In the simulations, the glue clung to negatively charged spots on the protein surface and linked up into a network. The share of water-repelling surface rose from about 48% to 57%, and the patch became slightly flatter while staying flexible. A stickier, flatter sheet is one plausible reason why many of them stack into a curved, layered shell.
Opening on demand
The capsules stayed intact at 0.2 mM GTP but fell apart at 0.5 mM. ATP and two other related molecules, CTP and UTP, left them unchanged. The team loaded the capsules with doxorubicin, a common chemotherapy drug, and gave them to human liver cancer cells grown in the lab. The cells took the capsules up, and the drug killed them more effectively than the same dose given on its own: cell viability fell to about 30%, against 48% for the free drug.
These are experiments in cell cultures, not in animals or patients. The authors see the capsules as candidate drug carriers for cancer and, possibly, for diseases caused by RNA viruses, and they note that testing them in living organisms is the next step worth taking. What I find most appealing is the idea itself: using the cell’s own machinery, and its own chemistry, as the lock and key.
Read the paper: Uchida, Kohata, Okuro, Cardellini, Lionello, Zizzi, Deriu, Pavan, Tomishige, Hikima, Aida, Nature Communications 13, 5424 (2022). doi:10.1038/s41467-022-33156-5