Wilfling F, Wang H, Haas JT, Krahmer N, Gould TJ, Uchida A, Cheng JX, Graham M, Christiano R, Fröhlich F, Liu X, Buhman KK, Coleman RA, Bewersdorf J, Farese RV Jr, Walther TC. (2013). Triacylglycerol Synthesis Enzymes Mediate Lipid Droplet Growth by Relocalizing from the ER to Lipid Droplets. Dev Cell Feb 13.
Lipid droplets have suffered from an image problem, and Tobias Walther who studies them at Yale has his theories. "At least part of the answer is that they got a boring name. If you're a mitochondrion or an endoplasmic reticulum, it sounds good. If you're called a ‘droplet,' a lot of people naturally assume that you're an inert blob that doesn't do anything," he says.
But lipid droplets (LDs) are anything but inert blobs. As we have learned in growing detail in recent years, LDs are dynamic organelles that function in the storage and mobilization of neutral lipids (i.e., fat). Life is metabolic energy moving through biological systems, says Walther, and when there is an energy surplus, cells, from yeast to human, usually convert it into triacylglycerols (TGs) stashed in LDs. This leading role in fat storage has put LDs square in the research spotlight and LDs are now implicated in a host of human pathological conditions, the obvious ones being obesity, Type II diabetes, and atherosclerosis. Less obvious but now well-documented are the ways LDs are exploited as targets in Hepatitis C, dengue virus, and Chlamydia trachomatis infections. Yet despite their evident importance, great mysteries still surround LDs, including the most obvious one: Where do they come from?
From microscopy, it's obvious that LDs are closely associated with the endoplasmic reticulum (ER) but do they "bud" off from the ER or do LDs grow by synthesizing TGs themselves? In a new paper in Developmental Cell, Walther, Robert Farese, Jr., at the Gladstone Institutes, San Francisco, and colleagues at Purdue University and the University of North Carolina, Chapel Hill, demonstrate that a subset of lipid droplets grow in size due to the delivery of lipid synthesis machinery via "membrane bridges" from the ER to LDs. This delivery enables local lipid synthesis directly on the outside of the phospholipid membrane that surrounds lipid droplets.
That there are two subpopulations of LDs—small ones that appear static vs. large ones that grow in size—was one surprise, Walther reports. That the TG synthesis machinery assembled only on the outside of the large and growing LDs was another. It's this local assembly of synthesis enzymes on the outside of LD membranes that differentiates the two subpopulations since the small LDs and the large ones contain more or less the same thing—sterol esters and TGs, says Walter.
The transfer of the enzymatic machinery for TG synthesis from the ER to the membrane of the large LDs was another surprise. Led by gylcerol-3-phosphate acyltransferase (GPAT4), which catalyzes the first step and is the rate limiter of TG synthesis, the isoenzymes move out of the ER across flexible tubules or membrane bridges onto the LD outer membrane where they spread in a film across the surface. Walther admits that he had a hard time accepting the membrane bridges and that his laboratory repeated the FRAP (fluorescence recovery after photobleaching) experiments over and over. "I kept saying I don't believe this. Show me more," he recalls, until the consistent FRAP results forced him to reconsider. The existence of connections between the tubules and the LD monolayer was further confirmed by thin section electron microscopy.
Why TG synthesis would take place outside the ER and even outside the LD is intriguing, says Walther. It's speculative, he admits, but the segregation of (TG) synthesis from the ER by putting all the machinery and the potentially harmful things on the outside of the droplets makes evolutionary sense. Many of the intermediates in TG synthesis are powerful signaling molecules in their own right. Walther explains. "You don't want to have any potentially toxic molecules accumulating and floating around the ER."
—James Olzmann & John Fleischman
Created on Tuesday, March 19, 2013