Following the motions of a specific molecule inside a cell is no easy task
In "Too Hard for Science?" I interview scientists about ideas they would love to explore that they don't think could be investigated. For instance, they might involve machines beyond the realm of possibility, such as particle accelerators as big as the sun, or they might be completely unethical, such as lethal experiments involving people. This feature aims to look at the impossible dreams, the seemingly intractable problems in science. However, the question mark at the end of "Too Hard for Science?" suggests that nothing might be impossible.
Jeanne is the first scientist to respond to my open request for ideas that might be too hard for science. If you are a scientist with what you think might be an impossible dream, contact me at email@example.com!
The scientist: Jeanne Garbarino, a postdoctoral researcher in the Laboratory of Biochemical Genetics and Metabolism at The Rockefeller University. Be sure to read her blog and follow her on Twitter @jeannegarb.
The idea: The operations of a cell depend on the interplay between a countless array of complex biomolecules. To comprehend life at its most basic level, scientists must track where these biomolecules go in cells to pinpoint what they interact with — what changes they might enable or undergo. The capability to understand the inner workings of cells might help grant the ability to control them as needed.
Garbarino researches how cholesterol is transported within cells. At the cellular level, cholesterol is a major determinant of the chemical properties of the membrane enclosing a cell's innards — "too much cholesterol and the membrane becomes too rigid; too little cholesterol and the membrane becomes too fluid, drastically increasing permeability," she explains. As such, cholesterol is a key factor behind what gets into cells and what does not, such as nutrients or signals from other cells. At the level of an organism, excess cholesterol is tightly linked with cardiovascular disease, Alzheimer's disease and even some cancers.
"My wish is that I could determine the exact contribution for each transport mechanism of cholesterol when it comes to delivering cholesterol to areas within the cell," Garbarino says. "One must consider cholesterol as a molecule of utmost importance. Understanding its metabolism would greatly aid in the development of treatments or preventative measures or both that would combat illnesses where abnormal cholesterol levels was a major risk factor."
The problem: Following the motions of a specific molecule within a cell is no easy task. To start with, scientists have to distinguish it from every other molecule within the crowded interior of a cell. "One method that cell biologists would use to follow how something moves within a cell is to put some sort of tag on the molecule — for example, a fluorescent tag," Garbarino says. "However, because the structure of cholesterol is so important to its function, adding a bulky tag would certainly alter how this molecule would normally behave."
"There is a naturally fluorescent molecule produced by microorganisms called dehydroergosterol or DHE that closely resembles cholesterol, and scientists have been using this as a tool to study cholesterol movement," Garbarino adds. "But, because there are differences in structure, the behavior of DHE is not exactly the same as cholesterol, and it is not entirely possible to assume that the information obtained from DHE studies is exactly how we would expect cholesterol to behave."
Also, cholesterol moves with impressive speed in and out of cell membranes. "In yeast, it has been estimated that the yeast version of cholesterol, called ergosterol, moves in and out of the plasma membrane at a rate of 100,000 molecules per second — when it comes to human cells, the rate of cholesterol movement in and out of the plasma membrane is likely very similar," Garbarino says.
The speed at which cholesterol is transferred to and from its carriers, the variety of carriers for cholesterol (including many different proteins and little capsules known as vesicles), the fact all these carriers act simultaneously and the sheer number of cholesterol transfers that occur at any given moment "makes it near impossible to visualize the individual steps of the cholesterol transport process," Garbarino says. "Our current microscopes and cameras just cannot accommodate this speed and level of resolution."
If researchers wanted to complicate the problem even further, instead of just following the paths that cholesterol and its carriers take in the cell, they might actually want to see what other molecules that might have interacted with along the way from one place to another. This could involve tagging an unknown number of molecules, and one could always miss tagging one or more key compounds.
The problems that scientists face with following the movements of cholesterol within cells are much the same ones researchers confront with tracking other complex biomolecules, Garbarino adds.
The solution? Instead of trying to follow cholesterol throughout its entire journey inside the cell, scientists often analyze its transport piece by piece, such as by examining transfer rates between vesicles. Garbarino indirectly measures how cholesterol is transported to specific areas, by analyzing regions such as the endoplasmic reticulum for concentrations of compounds that use cholesterol as a building block. "I also look at how the major cholesterol sensor in the cell is behaving," she adds. "This is the transcription factor called SREBP — when cholesterol is low, it gets cleaved and transported into the nucleus."
"I think that the improvements we will see in the near future will involve more sensitive cameras and microscopes," Garbarino says. "Additionally, the development of quantum dot technology for labeling single molecules seems like a promising avenue. However, I doubt we will ever be able to fully tease out every aspect of intracellular cholesterol transport, at least in our lifetimes."
Next week, Too Hard for Science? will host its first entry in physics, having to do with singularities. Stay tuned!
If you have a scientist you would like to recommend I question, or you are a scientist with an idea you think might be too hard for science, email me at firstname.lastname@example.org.
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About the Author: Charles Q. Choi is a frequent contributor to Scientific American. His work has also appeared in The New York Times, Science, Nature, Wired, and LiveScience, among others. In his spare time, he has traveled to all seven continents. Follow him on Twitter @cqchoi.
The views expressed are those of the author and are not necessarily those of Scientific American.