The fundamental building block of life is the cell, but that doesn’t mean that cells themselves are static members of a bigger being. Rather, cells are like tiny little factories that perform specific tasks and manufacture different products: fat cells metabolize lipids, red blood cells carry oxygen, muscle cells expand and contract to move your body. To achieve all these remarkable functions, however, subcellular compartments within the cell, called organelles, must work synergistically. Just like workers in a factory, organelles need to be able to communicate and exchange goods with one another, yet exactly how these organelles communicate is still a matter of much debate. Researchers at Yale University in the De Camilli and Reinisch labs tackled part of this question by identifying the role that a certain family of proteins plays in lipid exchange between different cellular organelles.
The two labs focused on the VPS13 family of proteins, which coordinate lipid transfer between the endoplasmic reticulum (ER), the organelle where proteins and lipids are made, and other organelles that have lipid membranes such as the mitochondria. The researchers found that VPS13 proteins coordinate lipid transport in two ways: first, by bringing the organelles closer together like a cellular tether, and second, by transporting lipids themselves in a special lipid storage cavity. What makes this group of proteins so noteworthy is the devastating consequence that occurs when they break—deletion or mutation to any of the VPS13 proteins is implicated in several neurodegenerative diseases including Parkinson’s disease.
Lipids: building walls inside the cell
Lipids are an essential building blocks of the cell. While functionally diverse, all lipids share a similar structure composed of a fatty acid chain capped with a polar or charged head group. Like oil in water, lipids don’t dissolve in aqueous solutions because the fatty acid group is hydrophobic, or “water fearing.” However, the polar head group can interact with water because water is also a polar molecule. This causes lipids to spontaneously assemble into bilayers in the water-based environment of cells, with the hydrophobic fatty acid chains pointing towards the center of the bilayer and the polar head groups on the outside. As a result, the hydrophobic parts of the lipids are protected from the aqueous solvent by the polar head groups.
Lipid bilayers are the basic component of cell membranes and organelle membranes, and hence play a crucial role in cell biology. For example, the cell membrane helps prevent the insides of cells from leaking out or unwanted substances from the extracellular environment from penetrating in. Within cells, membranes are used to isolate different metabolic processes, particularly processes that generate or use molecules that are toxic to cells.
Cells constantly build new membrane and recycle old membrane to maintain metabolic demands. This creates a need for cells to be able to transport lipids to different compartments. One way of doing this consists of lipids moving from one location to another on their own by assembling into structures called vesicles. Vesicles are enclosed bits or “bubbles” of cell membrane. When the vesicle “bubble” reaches the target organelle, it fuses with the target membrane, creating an internal cavity through which the lipids can be transported to the target location without ever being exposed to the cytosol, or the liquid inside cells. This is achieved by special transport proteins that bring lipids from one specific organelle to another.
From letters to emails: Evolution of communication
Cells constantly need new lipid membranes to grow and divide; therefore, it is incredibly important to understand the mechanisms by which specific organelles exchange lipids. Unfortunately, much is still unknown about the proteins that mediate lipid transport in different types of cells from different organisms. Marianna Leonzino, a post-doctoral fellow in the De Camilli lab and the first author of the study, was curious about lipid transfer in human neurons. “One thing that we didn’t know in humans is how two specific organelles were talking to each other, and these organelles were the endoplasmic reticulum and the mitochondria,” Leonzino said.
In lower order organisms, like yeast, this pathway is well characterized: a protein complex called ERMES mediates lipid transfer between the endoplasmic reticulum and the mitochondria. However, ERMES doesn’t exist in higher level organisms like mammals—somewhere along the path of evolution, ERMES was lost. “For me, that was a super intriguing question: why would we lose something along evolution that seems to be so fundamental?” Leonzino said. This led her to hypothesize that maybe there was another group of proteins fulfilling the role that ERMES plays in yeast. “There must be something else that works better that we maintained” Leonzino added.
Eventually, Leonzino arrived at the VPS13 family of proteins. Two and a half years ago, when she began working on the project, little was known about VPS13 proteins, especially their role in lipid metabolism. Previous studies on yeast, which have both ERMES and VPS13 proteins, demonstrated the functional similarity of VPS13 and ERMES—while deletion of ERMES had no effect on the yeast, deletion of both ERMES and the VPS13 proteins killed the yeast, suggesting that VPS13 proteins play a similar role to ERMES. On the biomedical side, several mutations in the VPS13 family were known to occur in neurological diseases like chorea acanthocytosis, a Huntington’s-like disease, and Cohen syndrome. Combined, studying the role of VPS13 proteins seemed like a promising start to figuring out how organelles transfer lipids, and what can happen when that process goes awry.
VPS13: the missing link between mitochondria and the ER
In their study, Leonzino and her colleagues focused specifically on VPS13A, the protein implicated in chorea acanthocytosis, and VPS13C, the protein associated with Parkinson’s. They began by studying the role of these proteins in fibroblasts, a type of cell found in connective tissue in animals. “[Fibroblasts] can be a good proxy to understand what the basic role of these proteins are,” Leonzino said. They found that the two proteins are located in areas where organelles are close together, known as contact sites. The VPS13 proteins act as tethers between the organelles to keep them close together while mediating lipid transport. One side of each VPS13 protein contains a conserved binding region that tethers it to the endoplasmic reticulum, while the other side contains a specific binding region for another target organelle, which is the endosome for VPS13C and the mitochondria for VPS13A.
Structural biology studies of the VPS13 proteins in the Reinisch lab also showed that the VPS13 proteins contain a large hydrophobic cavity meant for transporting lipids. Unlike other lipid transport proteins while can fit only one lipid molecule at a time, the VPS13 proteins can transport lipids in bulk due to the sheer size of its hydrophobic cavity. Interestingly, all of the VPS13 proteins are incredibly similar structurally, with minor differences in the binding regions that target them to specific organelles, yet
A key implication of this study is that defects in lipid transfer in cells can cause serious neurodegenerative diseases, which was previously unknown. According to William Hancock-Cerutti, a graduate student in the De Camilli lab, so little is known about the cellular basis behind neurodegenerative disease that it’s hard to pinpoint individual proteins within individual metabolic processes as the source of disease. “It’s striking that there is this family of four proteins that are all structurally similar, so presumably have some overlap in function, and they all cause neurological diseases, but they all cause different neurological diseases,” Hancock-Cerutti said.
However, despite the strong correlation between between VPS13 mutations and neurodegenerative disease, there is still a long way to go before any of this can be developed into a useful therapeutic strategy. “I think we’re still a long way away from thinking about therapeutic implications given that the field has so little understanding of where to intervene therapeutically to slow or halt these diseases.” Hancock-Cerutti said Leonzino agrees, saying that there is still a lot to figure out behind the basic cell biology.
According to Leonzino, the next step is to replicate these results in neurons. This is more complicated, since mutations or deletions of the VPS13 proteins in neurons should lead to cell death as observed in neurodegenerative disease. But there’s a lot of interesting questions to be answered on the horizon. “We will try to better identify the steps of the pathogenesis, how you go from not having that lipid transfer to later having neurons die,” Leonzino said. “Even though these diseases are all pretty early onset, early onset for neurodegenerative disease is anywhere from your twenties to forties, so it takes time for neurons to die. We will try to figure out what happens in that time” Leonzine concluded.