Authors (left to right) Eugenie Shieh, Adam Castoreno,
Larissa Jarzylo and Axel Nohturfft
The ingredients of a human cell include about 100,000 different proteins, 500 kinds of fat, 25 different molecules of DNA, something called RNA, a lot of water, minerals and some “proprietary flavors.” An overarching problem in molecular and cellular biology is the question of how all these molecules are (a) made and then (b) assembled to build a cell.
The part of this problem that we focus on in our lab is to understand how cell membranes are built. Membranes are thin sheets that surround each cell and that define the size and shape of cellular sub-compartments such as mitochondria, the nucleus, Golgi apparatus, etc.
The principal building blocks of membranes are fat-like molecules called lipids. The number of different lipids found in a given membrane is about 100, and to ensure proper structure and function each kind must be represented in specific and characteristic amounts.
The biochemical pathways that lead to the synthesis of membrane lipids are well studied. But how these pathways are orchestrated to produce each lipid at just the right amount is poorly understood.
Our operating hypothesis is that membrane synthesis works basically like a symphony orchestra: imagine you are playing the English horn in Richard Strauss’ “Also sprach Zarathustra.” To hit the fifth bar at just the right time and volume, you have to adjust your breathing to three sources of input: the gesticulations of the conductor, the sound of your own instrument and the sound of all the other instruments.
The regulation of lipid-synthesizing enzymes is likely to be similar. Take for example cholesterol. The ratio of cholesterol to other lipids (“phospholipids”), we know, is a constant. Cholesterol synthesis is activated in response to two signals, (a) whenever there is a general demand for more membranes and (b) when the cholesterol-to-phospholipid ratio changes. The first situation should require the coordination by a central “conductor” because all lipids would have to be synthesized at the same time. A second process exists that throttles cholesterol synthesis when its level with respect to phospholipids teeters off balance. This balance is of course determined by two variables, cholesterol (the English horn) and phospholipids (the other instruments).
A problem we were initially faced with was to find a suitable experimental system with which to study membrane synthesis. Large amounts of membrane are made every time a cell divides; however, trying to look at lipid synthesis in dividing cells is like trying to hear a bird sing in Harvard Square in the summer when a cacophony of sounds is coming from every corner.
In a paper appearing in the September 13, 2005 issue of the Proceedings of the National Academy of Sciences USA, we are describing a way to study membrane synthesis in relative isolation. One class of cells that met our criteria is phagocytes. Phagocytes have the amazing ability to “eat” other cells or particles of up twice their own size. During phagocytosis, the particle is wrapped up by a patch of membrane to form what’s called a phagosome. Since every phagosome is made at the expense of a patch of membrane, the cells re-synthesize the “lost” lipids within just a few hours.
By focusing on phagocytes, we discovered that membrane lipid synthesis is controlled primarily on the level of transcription. Moreover, we identified the “conductor”: a transcription factor that controls the expression of genes required for making the entire “orchestra” of membrane lipids, from triangles to trombones.
Surprisingly, this transcription factor, known as SREBP (for “sterol regulatory element binding protein”), was previously known to also control cholesterol synthesis in response to changes in the cholesterol-to-phospholipid ratio. Finding out how this protein is regulated by changes in membrane demand as well as changes in membrane composition will be one of our goals for the future.
Another interesting conundrum raised by our studies is in the fact that phagocytosis triggered membrane synthesis in the first place. After all, the lipids stayed inside the cell; why make more? Moreover, the amount of synthesized lipids exactly matches the surface of the internalized particles. How did the cells know when to stop? Most likely, membrane synthesis is coupled to a functional aspect of the organelle(s) that provide the membranes for making phagosomes. What that function might be and how it relates to lipid synthesis remain to be explored.
Abstract of PNAS paper, on PubMed: html