An early outcome of our work was to postulate the lipid raft concept, which continues to be both controversial and exciting. Our present view is that lipid rafts in the resting state are dynamic assemblies of sphingolipids, cholesterol, and proteins that continuously associate and dissociate. A cell membrane is made up of a lipid bilayer, a two-dimensional liquid that acts as a solvent for the membrane proteins. Our work suggests that this bilayer has the capacity to dynamically subcompartmentalize. Dynamic rafts can cluster to produce fluid platforms into which proteins can partition. These raft clusters play an important role in membrane trafficking and signal transduction.

Basically the raft concept is an outcome of earlier work on the biogenesis of polarity in epithelial cells which led directly to the lipid raft hypothesis which we published in 1997 under the title "Functional rafts in cell membranes" (Simons K and Ikonen E, Nature, 387 [6633]: 569-72, 5 June 1997). This postulates the existence of lipid rafts, which are dynamic liquid-ordered assemblies of cholesterol and sphingolipids in the lipid layer of cell membranes.

In the 2000 review we surveyed the data on the involvement of lipid rafts in signalling and postulated the concept of raft clustering. Initially our concept was controversial because it was difficult to demonstrate that rafts exist in cells. The problem was that the methods to analyze raft structure and function in cell membranes were quite crude in the first phase. It also took some time to understand the dynamic nature of rafts. In fact they can only be observed after stabilization by clustering.

The most important outcome of the review was that it opened up a new field for research. Lipids had long been neglected and through the excitement stirred by the review, they became a crucial issue again in membrane research. Most importantly, many biophysicists joined in, using new methods to clarify what was going on in model systems and in cell membranes. Improved methodologies have by now dispelled most of the doubts.

We proposed that signal transduction can be initiated by complex protein-protein interactions, leading to oligomerization of signalling proteins and association into raft clusters. In addition to the well-known interactions between ligands, receptors, and kinases, we showed that lipid micro-environments on the cell surface (the rafts) also take part in this process. Importantly, lipid rafts associating with proteins can change their size in response to stimuli. This behavior is driven by specific protein scaffolding, resulting in signalling cascades.

One of the most important properties of lipid rafts is that they can include or exclude proteins to variable extents. The clustering of separate rafts exposes proteins to a new membrane environment, containing specific enzymes. Even a small change of partitioning into a lipid raft can initiate signalling cascades. We predicted that these dynamic features are crucial for switching on many signal transduction pathways.

We established the case for an important function of rafts at the cell surface in signal transduction. The raft clusters can be pictured as concentrating platforms for individual receptors, activated by ligand binding. We noted that if receptor activation takes place in a lipid raft cluster, the signalling complex is protected from non-raft enzymes that could affect the signalling process.

The paper introduced clarity in the messy field of rafts, their biochemistry and function. We also established a nomenclature for rafts and clusters of rafts. Our description of this principle of membrane organization is widely adaptable and covers a huge variety of functions in cells, including signalling. This is now a textbook concept.

Our motivation was to understand what this toxic molecule, cholesterol, is good for. How is it distributed in the cell and how does the cell cope with its presence? Cholesterol plays an indispensable role in regulating membranes in mammalian cells. Its concentration must be very tightly regulated; otherwise, problems such as atherosclerosis arise. Our paper reviewed the movements of cholesterol within the cell and the control of cellular cholesterol from the new perspective of lipid raft assembly and function. This review focused the attention to the essential role of cholesterol in lipid raft organization. Our thesis was that one main function of cholesterol in our cell membranes is to keep rafts functional.

The interesting point here is that we formed raft clusters by cross linking. We showed if you add antibodies to form big patches on the cell surface—raft associated—it is possible to make an underlying domain structure visible. At the time, lateral assemblies of glycolipids and cholesterol—the rafts—had been implicated to play a role in cellular processes such as signal transduction and cell adhesion. We studied the structure of raft domains in the plasma membrane of non-polarized cells. Crucially, in that paper we give the first microscopic demonstration that clusters of rafts can segregate away from non-raft proteins.

We are committed to a better understanding of the raft-clustering principle. We are studying this process in different systems. An early outcome of our work was the postulation of the lipid raft concept as protein-lipid platforms functioning in membrane trafficking, polarization and signaling. We are now trying to unravel the role of rafts in these processes.

Our working hypothesis is that sorting of proteins and lipids in the trans-Golgi network to the apical surface in epithelial cells is such a raft clustering process. We have identified several proteins involved in this process and we are now isolating the apical carriers that transport raft lipids and proteins. In parallel we are also using yeast as a model to identify and characterize the machinery for raft protein and lipid sorting from the Golgi complex to the yeast cell surface. By using these two cell systems and by comparing them to each other, we hope to understand how the raft principle in membrane traffic operates.

There most certainly are! Take Alzheimer’s disease for example. We are analyzing how amyloidogenic processing of the amyloid precursor protein functions to generate plaques in Alzheimer’s disease. Our hypothesis is that this process involves a raft clustering process. To that end we are dissecting this mechanism in cells and by studies in vitro. We are trying to reconstitute the main players of amyloidogenic processing in liposomes. We want to find out whether we can directly demonstrate that raft lipids are required for the production of beta-amyloid. If we can establish how lipid rafts and specifically which raft lipids are involved in amyloidogenic processing, then this process could become a therapeutic target for the disease.