(Image caption: Neurons with the Unc5-receptor send their axons in a cell culture in all directions. The processes avoid the parallel orientated stripes containing the FLRT3-protein (red). Credit: ©Seiradake et al, Neuron 2014)

Navigation for nerve cells

During brain development, the precursors of nerve cells sometimes have to migrate long distances from their place of origin to their destination. In this process, proteins, such as FLRTs (pronounced “flirts”), act as guide molecules. Researchers at the Max Planck Institute of Neurobiology in Martinsried, together with colleagues at the Universities of Oxford and Frankfurt have now discovered that FLRT proteins on the surface of progenitor cells can induce repellent and attractant signals depending on its binding partner. The scientists used X-ray crystallography to reveal the structural bases for both FLRT-mediated adhesion and repulsion. They applied this knowledge to elucidate how these opposed signals control cellular migration. Which signal predominates depends on the particular type of cell migration. The results further show that FLRTs also exert attractant and repellent effects in the walls of blood vessels and therefore control the development of other tissue types as well.

Pyramidal cells are the central nerve cells in the cerebral cortex. During embryonic development, the precursors of pyramidal cells follow the paths of glial cell axons to migrate from their original location to the surface of the cerebral cortex. As soon as they reach their intended layer, they develop into mature pyramidal cells and interlink to form a functional network. Pyramidal cells also spread to a limited extent within these layers, though the importance of such tangential migration is still poorly understood.

This migration of precursor pyramidal cells is controlled by FLRTs (fibronectin-leucine-rich transmembrane proteins) located on the cell surface. According to the researchers at the Max Planck Institute in Martinsried, FLRTs and the Unc5 receptor form a group of guidance proteins with opposing effects on cell migration. On one hand, they act as a repellent. This is the case when a FLRT molecule binds to an Unc5 receptor on the surface of a progenitor cell. “In this way, as the precursor cell migrates radially, it receives a signal to continue migrating at an adjusted speed to not move prematurely into outer layers,” explains Rüdiger Klein from the Max Planck Institute of Neurobiology.

However, if two identical FLRT molecules bind to each other, this triggers an adhesive signal. The scientists’ results show that pyramidal cells are guided in this manner as they spread tangentially, without affecting their ability to find their target layer. Thus, there are proteins with attractant and repellent effects located on the surface of precursor pyramidal cells. “By integrating these opposing signals, cells can navigate through brain tissue. During radial migration FLRTs induce repulsion; during tangential dispersion FLRT attraction dominates,” says Klein.

In their study the scientists also investigated whether the mechanisms of FLRT adhesion and repulsion are present in other cell types. Their findings show that cells in the walls of blood vessels in the retina and the umbilical cord are also controlled by a combination of attractant and repellent signals modulated by FLRT and Unc5 proteins.

Sociable receptors: in pairs, in groups or in a crowd

When cells migrate in the body, for instance, during development, or when neurons establish new connections, cells need to know where they are going. A ‘wrong turn’ will generally cause disease or developmental disorders. The cells take direction cues from other cells with which they interact, and which they then repel after a short period of contact. Among those direction cues are ephrin ligands, recognized by Eph receptors on the cell. Together with colleagues from the Max Planck Institute of Molecular Physiology in Dortmund, scientists at the Max Planck Institute of Neurobiology in Martinsried have discovered that Eph receptors must form groups of three or four in order to become active and transmit the signal. Furthermore, the ratio of such multimers to inactive dimers determines the strength of the cellular repulsion response. The new findings help scientists understand how cells communicate and offer a point of departure for studying diseases related to breakdowns in this guidance system.

When people get together, there is usually a lot of interaction. Our cells behave similarly. When cells grow close to each other during development, they need to communicate with the surrounding cells to establish whether they are in the right place in the organism and which cells they should connect with. This communication is especially critical in the brain, where adhesion and repulsion processes between neurons occur continuously. It is only when the right cells connect that something new can be learned, for example. Emerging tumours also must exchange information with the cells around them to be able to grow. “It is of fundamental importance to understand how cells communicate with one another”, says Rüdiger Klein, Director at the Max Planck Institute of Neurobiology. He has been studying the language of the cells for years together with colleagues in his department. Their research focuses on the so-called Eph receptors and their ephrin ligands.

Cell communication via ephrin/Eph receptors comes into play in most encounters between cells. As a result of this communication, one cell usually repels the other, which continues to grow in another direction. Many such instances of interaction guide the cell to the right place. The guidance system itself – the ephrins and Eph receptors – are found on the cell surface. When the ephrin and the Eph receptor of two opposing cells meet, they form an ephrin/Eph complex. This triggers cellular processes in one or both of the cells, which eventually cause the detachment of the ephrin/Eph complex and the repulsion of the two cells from one another.

“Many receptor systems have developed a security mechanism to prevent false alarms from triggering the cellular processes”, explains Rüdiger Klein. “A signal is only transmitted to the cell if two receptor/ligand pairs form a dimer.” However, in the case of ephrins and Eph receptors, things are different. Ephrin/Eph complexes form dimers, but often also larger groups on the cell membranes. Scientists were previously not sure how this affects repulsion and repulsive signalling strength.

The neurobiologists in Martinsried and their colleagues from the Max Planck Institute of Molecular Physiology in Dortmund have now been able to artificially trigger and study the formation of groups of Eph receptors in cell culture. The results show that the otherwise usual dimers are inactive when made up of Eph receptors. Only trimers and tetramers triggered the signals that caused cell repulsion. However, the scientists’ working hypothesis that a larger group would trigger a stronger signal turned out to be too simple. “It took us quite some time to figure out the system”, says Andreas Schaupp, first author of the study. “In fact, it is not the size of each individual group that matters, but the composition of the entire population of groups.”

The more trimers and tetramers and the fewer dimers present in the cell membrane, the stronger the repulsion signal. In contrast, a higher abundance of dimers and a smaller number of multimers produce a weaker reaction or none at all. “Thanks to this mechanism, a cell can grade its response from forcing another cell to make a U-turn to simply guiding it past at close range”, Rüdiger Klein says. This is an important step in understanding how migrating and growing cells navigate, and why this guidance system breaks down in some diseases.

At any given moment, millions of cells are on the move in the human body, typically on their way to aid in immune response, make repairs, or provide some other benefit to the structures around them. When the migration process goes wrong, however, the results can include tumor formation and metastatic cancer. Little has been known about how cell migration actually works, but now, with the help of some tiny worms, researchers at the California Institute of Technology (Caltech) have gained new insight into this highly complex task.

The team’s findings are outlined this week online in the early edition of the Proceedings of the National Academy of Sciences (PNAS).

Proteins adorning the surfaces of human cells perform an array of essential functions, including cell signaling, communication and the transport of vital substances into and out of cells. They are critical targets for drug delivery and many proteins are now being identified as disease biomarkers—early warning beacons announcing the pre-symptomatic presence of cancers and other diseases.

While study of the binding properties of membrane proteins is essential, detailed analysis of these complex entities is tricky. Now, Nongjian (NJ) Tao, Professor of Electrical Engineering, and director of the Center for Bioelectronics and Biosensors at Arizona State University’s Biodesign Institute has devised a new technique for examining the binding kinetics of membrane proteins.

“This is a very important but very difficult problem to solve,” Tao notes. “We demonstrate a new method of approaching the issue, which provides a quantitative analysis of protein interactions on the surface of a cell.”

The technique—known as SPR microscopy—holds the potential to simplify the study of membrane proteins, thereby streamlining the development of new drugs, aiding the identification of diagnostic biomarkers and improving the understanding of cell-pathogen interactions.

The group’s results appear in this week’s advanced online issue of the journal Nature Chemistry.