Heart disease and stroke are two of the leading causes of death globally — and high levels of blood cholesterol increases the chances of both of them. While significant progress has been made in developing a slate of medicines that can maintain normal levels and reduce the associated risks, we’re still a long way from understanding all the ways cholesterol acts and affects humans. Now, a new study published in the journal Proceedings of the National Academy of Sciences suggests that the ultimate solution to managing this disease might come from songbirds.
Cholesterol is a fatty substance that our body produces and acquires through food. Despite its negative image, this molecule is an integral part of most animal bodies. It is an essential component of the membranous packaging of our cells. Cholesterol is also needed to synthesize various vitamins and hormones crucial for the smooth functioning of our body and for the bile necessary for digesting fats. The problem arises when too much of it starts floating around in the blood, clumping together to form plaque on vessel walls and blocking the flow of blood to organs.
Just like oil and water don’t mix, cholesterol also remains insoluble in blood due to its waxy, lipid-like properties. Most vertebrate animals have a system wherein cholesterol is packaged with lipids and proteins called “apolipoproteins” to form small ‘lipoprotein’ droplets that can be easily moved around in blood. Depending on the amount of lipid and the type of protein, these droplets can be ‘low density lipoproteins’ (LDL) or ‘high density lipoproteins’ (HDL). LDL carries cholesterol to the cells which need it, where it attaches with a protein on the cell surface called LDL-receptor (LDLR) and gets engulfed by the cell. Inside, the cholesterol is taken out of the packaging and LDLR is returned to the cell surface. On the other hand, HDL picks up excess cholesterol from all around and takes it to the liver to be excreted out of the body.
Sometimes, cells do not gulp LDL very efficiently. This could either be because they already have enough of it for cellular function, so they stop decorating their surface with LDLRs. This can lead to high levels in the blood, causing a build-up of plaque on artery walls. If not checked in time, the plaque can grow and plug up the artery. It’s like a really slow pile up of cars on the highway.
Genetic causes like a mutation in the LDLR gene can also cause the transport and uptake of these lipoproteins to malfunction, resulting in a genetic disorder called “familial hypercholesterolemia.” Approximately 1 in 250 people suffer from this condition, in which levels of LDL cholesterol in the blood can shoot up to very high values and lead to heart attacks at young ages. Until recently, scientists believed this was true for all vertebrate animals. In a rather serendipitous turn of events, a group of researchers from Brazil and the U.S. discovered an exception that left them astonished. They found that zebra finches, as well as other songbirds, lacked three major regions of the LDLR gene, which possibly made the receptor non-functional, yet did not develop high levels of LDL cholesterol or have any heart related issues.
This is not something the team could have hypothesized out of the blue. The story begins somewhere else entirely. Many animal experiments involve introducing foreign genes into embryos to see the effect on things like animal growth, development, or behavior. This is regularly done using viruses engineered with the gene one wants to insert. Engineered viruses decorated with a protein called Vesicular Stomatitis Virus G-protein (VSV-G) are a common tool used to manipulate avian embryos. Zebra finch eggs are notoriously resistant to such virus-aided gene expression. The authors were interested in finding out why. “I’ve had to inject probably 6000 eggs, if not more, [in the course of my research],” said Tarciso Velho, a neurobiologist at Federal University of Rio Grande do Norte in Brazil and the lead author of the study. “I’ve been trying for several years to make transgenic zebra finches, but it has been difficult. We never quite understood why.”
In 2014, a study described how the VSV-G protein attaches to LDLRs and enters the cells. All cells which did not have any low density lipoprotein receptors showed resistance to the entry of the virus. This was the perfect starting point for understanding the poor infection rates in zebra finch embryos. The authors combed the finch genome to see if the LDLR gene was present. They found it, but saw that it was missing big chunks important for placing the receptor on the surface of the cell an interaction with G-protein. Essentially, they’d hit the jackpot in their quest. To make sure that this divergent receptor was the reason for the poor infection rates, they introduced the full human low-density lipoprotein receptor, or LDLR, into the finch eggs and then tried introducing the virus with the G protein. These engineered eggs showed a remarkable improvement in expression of the virus-aided genes, proving that the variant LDLR was responsible for the resistance.
But what perplexed the authors was how these birds remained alive and healthy without receptors to clear low-density cholesterol from their blood. They examined the lipoprotein levels of the birds and found that they had no low-density lipoproteins at all. All of their cholesterol was bound to high-density lipoproteins.
No LDLR, no LDL, no plaque build-up, no problem, right? Not quite. Zebra finch cells have the same need for cholesterol as other animals, or at least that’s the assumption for now. What transports it for them? “We still have no idea,” Tarciso told me. “This is something we’re super curious about. The cholesterol must be (transported) to the peripheral tissue somehow via HDL.” So, then what is this variant LDL receptor even doing? “We don’t quite know if this receptor is capable of actually taking up cholesterol or not. This is something that we’re pursuing, by putting the receptor into a cell and seeing how it behaves and what it’s capable of.”
These surprising findings tell us that even the most conserved and canonical biological processes can be altered, given the right mix of evolutionary pressures. It’s quite possible that songbirds or their common ancestors lost these receptors as a way to develop resistance against VSV infections, and in doing so, developed a unique system of cholesterol transport. This opens up a host of new avenues to pursue in the attempts to treat the genetic causes of high cholesterol in humans. “Now we know there are different ways to transport cholesterol,” Tarciso said. “If we could precisely identify the mechanism and a common receptor that is performing this role, [we] could direct cholesterol transport to that, instead of LDLR.” Though controversial, using genetic approaches such as CRISPR to re-route the movement of cholesterol in patients lacking the necessary receptor could make medication for this condition obsolete.
There’s a ton of work to be done before we fully understand the mysteries of songbird cholesterol. But there’s also much to look forward to during the journey. Unexpected changes in direction, like this one, are the basis on which many serendipitous discoveries stand. As Tarciso puts it, “the unexpected things, how science takes you on turns that you’re completely unprepared for,” those are what push you to ask questions like, ‘What can I learn about my body from studying this tiny bird?’