Like an invisible conductor
Scientists from Berlin have succeeded in explaining the molecular machinery of a central cellular transport process. Using chemical probes and high-resolution fluorescence microscopy, the molecular biologists were able to track the components involved in the process called endocytosis in detail and even produce short film shots of the cells. The scientists involved came from the Leibniz-Institut für Molekulare Pharmakologie (FMP), Freie Universität Berlin and the NeuroCure Cluster of Excellence of the Charité – Universitätsmedizin Berlin, the joint medical faculty of Humboldt-Universität zu Berlin and Freie Universität Berlin. The work was published in the current edition of the renowned journal Nature.The work was published in the current edition of the renowned journal Nature. The cellular transport investigated is important for numerous bodily functions, such as the uptake of nutrients from the blood or stimulus conduction in the brain. It also plays a role in the development of cancer and neurodegenerative diseases such as Alzheimer's. Of decisive importance in this process are lipid molecules that serve as identification markers in the cell membrane. These lipid molecules can be modified by enzymes in the blink of an eye and thus dictate the direction of transport.
At first sight, the processes taking place in living cells appear to be totally chaotic: substances are permanently being synthesised and broken down again, three-dimensional structures are formed and decay again. In order to take up substances from the surroundings and transport them, the cell invaginates its external membrane and constricts tiny vesicles in a process called endocytosis. As if directed by an invisible conductor, the vessels then migrate into the interior of the cell. But where does the order come from in this apparent chaos? In their paper, the research group led by Prof. Dr. Volker Haucke has shown how such a complicated process organises itself, the individual components, optimised over millions of years, engaging like cogwheels.
It was already known that certain components of the cell membrane collect at the point at which the cell is going to invaginate. These substances are phosphoinositides, called PIPs in the laboratory jargon: they consist of a fat-soluble tail, anchored in the lipid membrane, and a water-soluble head, which projects very slightly into the interior of the cell. These heads are particularly characteristic in their chemical properties, so that other cell components such as protein molecules recognise them and can bind to them. This is how the formation or the transport of vesicles is driven.
At the same time, the PIP heads can be easily modified, since perfectly fitting enzymes can detach the phosphate groups and reattach them in different orientations, thus giving the head a different face. In a complicated search for evidence, the group's leader Volker Haucke, his doctoral candidate York Posor and other researchers involved show how a certain enzyme accumulates upon invagination and transforms the initial PIP within seconds into another, previously less characterised PIP. When York Posor blocked this enzyme using genetic-engineering methods, the system froze so to speak. In comparative film sequences, he demonstrated how the invaginations remained suspended on the membrane. In the normal course of endocytotic vesicle transport, in contrast, the transformed PIP then attracts a special protein, which advances the further invagination and detachment of the vesicles. This in turn calls new enzymes into action, which further transform the PIPs. A chain of chemical reactions thus leads to a spatial-temporal dynamic with a specified direction.
"We can now quite precisely determine which molecules and how many of them are to be found at a particular time and place," explains Volker Haucke. "This can even be expressed in mathematical models, and we are currently preparing a further publication on this subject." The whole system runs under its own organisation, but also reacts to external influences. "We suspect that the enzymes that produce or break down the PIPs also serve as a sensor, in order to ensure the supply of nutrients to the cell and react appropriately. Among other things, this sensor function determines whether a cell grows and divides, which is of importance in the development of cancer. At the same time, the PIPs influence the communication between cells, for example in the brain, or the breakdown of clumped protein molecules, a central cause of neurodegenerative diseases such as Alzheimer's."
Spatiotemporal control of endocytosis by phophatidylinositol-3,4,bisphosphate: 11 July 2013 issue of Nature; Advance Online Publication (AOP) on http://www.nature.com/nature on 03 July 2013 at 1800 London time / 1300 US Eastern Time. DOI: 10.1038/nature12360
Prof. Dr. Volker Haucke
Leibniz-Institut für Molekulare Pharmakologie
13125 Berlin, Germany