Media Release: 28 September 2011
For the first time, Australian scientists have detailed the proteins, or functional molecules, inside and around the ‘plasma membrane’ of a fat cell, the permeable barrier between the cell’s inner workings and the rest of the body.
Mapping a healthy fat cell at a basal level, or in a ‘pure’ state unaffected by its environment, allows us to understand exactly how it responds when exposed to hormones and other substances that blood carries around the body.
Why a fat cell? Because it plays a central role in metabolism, and its dysfunction is one of the factors that leads to the complex lifestyle-related illness we call Type 2 diabetes.
Professor David James, Leader of the Diabetes Program at Sydney’s Garvan Institute of Medical Research, along with Drs Matthew Prior and Mark Larance from his lab, focused particularly on how fat cells respond to insulin, the hormone that facilitates movement of fats and sugars from the blood to the cell interior, where they can be burned for energy.
When a fat cell is exposed to insulin, armies of proteins spring into action. Receptor proteins on the cell surface send signals into the cell, forcing a chain of events. Glucose transport proteins speed along intracellular tramways towards the cell surface where they can pump glucose into the cell; motor proteins help push them, so that they glide easily to the surface. Pore-opening proteins allow entry of nutrients, as well as ‘ions’, charged molecules that bring about important biochemical changes in the cell, such as a rise in pH levels.
All this activity takes place within seconds – thousands of identical reactions within millions of fat cells. The complexity is almost unimaginable.
For that reason, James, Prior and Larance used a sophisticated mass spectrometer, along with mathematical analysis, to allow them to glimpse what the eye cannot see – even with the most powerful electron microscopes.
Their findings, which took around two years to compile and analyse, are now published online in the Journal of Proteome Research. In addition to the descriptive analysis, the research team has uploaded its data to an online repository, available to all scientists in the field.
“This is very novel - a detailed study of the plasma membrane has not been done on any cell, let alone a fat cell,” said Dr Matthew Prior.
“Proteins carry out the work of the cell, and their precise location tells us a lot about what they do. This study highlights the proteins that are either embedded within, or have a strong association with, the plasma membrane. It reveals aspects of function, which in some cases was a mystery until now.”
“We isolated cells and brought them to basal level – so we could work out the proteome of the cell surface in the absence of stimulation.”
“By then looking at the cell surface in the presence of insulin, we could see which proteins changed. Some proteins moved away from the cell surface, while other proteins moved towards it. A good example is the glucose transporter GLUT 4 – which in response to insulin moves to the cell surface to facilitate the entry of glucose into the cell.”
“GLUT 4 was our positive control – as we already know that it’s regulated by insulin. As well as seeing a rise in GLUT 4 levels, we also saw a number of other proteins, not previously known to be insulin-responsive, move to the cell surface.”
“It’s already known that when you put insulin onto a cell, the pH goes up. One of the abundant proteins we identified is involved with intracellular pH.”
“Metabolism of food – facilitated by insulin – generates lactic acid and other acidic metabolites. Because of this, we surmise that insulin increases the cell’s pH as a way of buffering it against acidity.”
“In all, there were around 10 proteins that robustly changed with insulin exposure.”
Most importantly, the study gives the science community the fat cell fingerprint, the receptors on the surface of a fat cell being very different from those found on a muscle cell or a bone cell.
This kind of cellular specificity is important in the development of new drug targets, the best of which are cell surface proteins.
Notes to Editors
Sequencing of the human genome has provided a great resource to the bio community. The challenge in front of scientists is to understand the function of every gene and how, or if, it is involved in disease. A signature feature of function is intracellular location. Transcription factors live in the nucleus, fat burning enzymes in the mitochondria and protein synthesising proteins on the ribosome. In this study, Prior and colleagues have used omics of a different kind to identify indiscriminately as many proteins as possible on the plasma membrane of a fat cell. This provides another step forward in dissecting function, particularly of unknown genes. Notably, in their analysis of the plasma membrane, they identified several genes that were annotated as unknown function. Now that we know they are on the plasma membrane, this will help us unravel their function.