One Tough Molecule: Cholesterol

In praise of cholesterol.

Membranes are an underappreciated aspect of biology. The recent pandemic was caused by a virus that has a very sophisticated system to commandeer many aspects of our cellular apparatus, including our membrane systems, creating complicated vesicular bodies in which to develop and hide. Membranes may not have participated in the very origin of life, (which seems to have involved energy-rich mineral systems), but were essential at the origin of cells, as all cells are surrounded by a classic bilayer membrane, composed of two-faced molecules with water- soluble heads and fatty tails, the latter of which make up the middle of the bilayer.

Membranes everywhere. Eukaryotic cells are filled with membrane-bound compartments. Here, Covid-causing virus (black arrows) hides out in vesicles enclosed within additional membranes. These are post-mortem samples, examined by electron microscopy. In E, from lung cells, asterisks mark the presence of viral particles, while the number sign marks another lamellar structure of membranes involved in lung surfactant synthesis and secretion.

Membranes were also central to the next greatest innovation in life, the eukaryotic cell. Not only are eukaryotes full of membrane-bound compartments, like mitochondria, endosomes, lysosomes, endoplasmic reticulum, golgi apparatus, and others, but their membrane composition changed as well, with the advent of sterol-related molecules. Plants use phytosterols, while animals use cholesterol as an additive to their membranes. Cholesterol has gotten decades of bad press due to its association with atherosclerosis and the whole bad/good HDL story, about the particles that carry cholesterol around the body. But cholesterol is an essential and amazing molecule, painstakingly developed through evolution to strengthen our membranes and provide special nano-localization services.

Cholesterol (right) compared with a normal phospholipid that makes up the bulk of most membranes. Hydrophilic areas are in red/purple/blue, while hydrophobic areas are gray. The phospholipid is sphingomyelin, which appears to be fully saturated, meaning it has no double bonds or kinks in its hydrophobic tails. These on their own tend to be highly floppy, while cholesterol is far more structurally stable.

Cholesterol is a shockingly complex and expensive molecule to make. Its synthesis requires 37 steps, lots of molecular oxygen, and a hundred molecules of ATP. No wonder few bacteria make anything like it in such vast amounts. At the same time, there must be simpler chemicals that could afford similar functions- cholesterol is probably a relic from a lengthy exploration of membrane additives, to find one that is empirically ideal. Historically, cholesterol seems to have arisen after the general oxygenation event, enabling its peculiar synthesis, the symbiosis with mitochondria, and the evolution of eukaryotes generally. Our cells can still all make their own cholesterol, and our bodies have extensive means to regulate amounts, though evidently these mechanisms don't always work optimally for modern, aging humans. 

At any rate, it is now realized that dietary cholesterol has relatively little impact on internal levels or health outcomes. In our cells, cholesterol concentrations are rigorously controlled and highly diverse, being as high as ~40% of all lipids on the external face of the plasma membrane, while only 5% in the mitochondrial membranes. The reasons for this distribution are not entirely understood, but our genomes encode numerous proteins devoted to transferring cholesterol and phospholipids to various places and sides of membranes. A recent paper discussed the fact that cholesterol significantly strengthens membranes, allowing eukaryotes to attain the amoeboid lifestyle, rather than having to grow exoskeletons (i.e. cell walls) as bacteria generally do. 

Cholesterol makes membranes significantly stronger, less bendable, more viscous, and yet does not impair lateral fluidity.

The surface area per lipid goes down drastically (and strength and stiffness go up) as cholesterol is added to a regular phospholipid membrane. This is less meaningful than portrayed in the paper, however, since cholesterol counts as a lipid in this calculation, and with only one fat tail vs two slender tails, it is likely that the reduction in surface area arises as much from cholesterol's smaller cross-section (see cartoon above) as from its organizing / ordering effects on the neighboring phospholipids. 

Not only does it make membranes tougher, but it alters their thickness (by straightening up the phospholipid tails) and selectively prefers to bind certain partner phospholipids (sphingolipids), thereby creating nano-domains. These domains are called "lipid rafts" and at 50 nanometers across, they are exceedingly small, given that membranes are about 5 nanometers thick. These rafts are the prefered places for many hormone and immune system receptors to operate, which, when bound to their partners, lead to greater raft agglomerations that facilitate signaling and particularly the separation of some signals from others. This is just one example of the many roles that cholesterol has gained in cell and molecular biology.

Some reviewers note that while we often imagine nano-tech and nano-bots to be machines of metal, essentially miniaturized versions of our macro-tech, with tiny gears, etc., real nano tech may more properly lie in soft materials that are resilient at this scale, adapted to its challenges of constant thermal motion and mutable structure. Reeds that bend in the wind, not rocks that slowly break down in it. Membranes are being used in the form of liposomes as drug and vaccine delivery vehicles, and deserve a greater appreciation from both biological and technical perspectives.

This video, produced by detailed atomic computer simulation, illustrates how frenetic Brownian motion is. The membrane molecules (teal) are in constant motion, fending off the water molecules (red/white). The adoption of a second membrane component that intercalates, strengthens, and imposes some order here is a highly significant advancement.

• Maybe giving in to nuclear bluffing and blackmail is not a good idea.