Imig C, López-Murcia FJ, Maus L, García-Plaza IH, Mortensen LS, Schwark M, Schwarze V, Angibaud J, Nägerl UV, Taschenberger H, Brose N, Cooper BH. (2020). Ultrastructural Imaging of Activity-Dependent Synaptic Membrane-Trafficking Events in Cultured Brain Slices. Neuron. 108(5):843-860.e8.
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Cells can cut up rough: They will sometimes even gnaw at a partner in order to get rid of it as quickly as possible. This mechanism, known as trogocytosis (Greek: trogo, to gnaw), is important for cell sorting and plays a role in processes as diverse as embryonic development and the interaction between tumor and immune cells. Researchers have now discovered important factors that coordinate this special type of cell interaction.
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“Till death us do part” – cells don’t usually go to this extreme, but separation by trogocytosis does, in fact, involve a small amount of cannibalism.

An important way for cells to get in touch is the ephrin/Eph signaling pathway. The ephrin ligand in one cell (red in the graphic) binds with the Eph receptor in a neighboring cell (green). The neighboring cell then devours the relatively large receptor-ligand complex (yellow), virtually biting off its neighbor’s outstretched hand to enable separation. At least both cells survive the process, in contrast to phagocytosis.

The researchers found that this ephrin/Eph mediated trans-endocytosis resembles features of trogocytosis. “Trogocytosis is not well understood and different from phagocytosis, a well-studied process where one entire cell is eaten up by another”, relays Jingyi Gong from the Max Planck Institute of Neurobiology in Martinsried. “We wanted to decipher how trogocytosis is regulated.”

Trogocytosis is known to play an important role in the development of the nervous system. A better understanding of trogocytosis and linking it to ephrin/Eph could open the way for new regenerative therapies for injuries or diseases of the nervous system.
What is involved in this cell nibbling process?

The international team of researchers from Rüdiger Klein’s department and from the University of Toronto (Canada) used fluorescence microscopy to view trogocytosis in action and analyzed the proteins involved.

The researchers discovered that clusters of Gulp1, a protein familiar to phagocytosis, accumulate temporarily on the ephrin/Eph complex of both cells. Subsequently, the ephrin/Eph complex is devoured by the cells. Tiam2, a protein that stimulates the reorganization of the cytoskeleton, supports Gulp1 in this process. In addition, Gulp1 also recruits the protein dynamin, initiating the internalization process on the membrane.

“We could thus demonstrate that mechanisms which play a role in the ephrin/Eph trogocytosis overlap with those involved in phagocytosis,” explains Thomas Gaitanos, joint lead author of the study. The team now plans to investigate under what conditions trogocytosis can occur in the brain and whether this ability can be exploited for the regeneration of injured brain areas.

Jingyi Gong, Thomas N. Gaitanos, Olivia Luu, Yunyun Huang, Louise Gaitanos, Jana Lindner, Rudolf Winklbauer, and Rüdiger Klein (2019).
Gulp1 controls Eph/ephrin trogocytosis and is important for cell rearrangements during development. Journal of Cell Biology  218: 3455.
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Abstract

GABAergic synapses in brain circuits generate inhibitory output signals with submillisecond latency and temporal precision. Whether the molecular identity of the release sensor contributes to these signaling properties remains unclear. Here, we examined the Ca2+ sensor of exocytosis at GABAergic basket cell (BC) to Purkinje cell (PC) synapses in cerebellum. Immunolabeling suggested that BC terminals selectively expressed synaptotagmin 2 (Syt2), whereas synaptotagmin 1 (Syt1) was enriched in excitatory terminals. Genetic elimination of Syt2 reduced action potential-evoked release to ∼10%, identifying Syt2 as the major Ca2+ sensor at BC-PC synapses. Differential adenovirus-mediated rescue revealed that Syt2 triggered release with shorter latency and higher temporal precision and mediated faster vesicle pool replenishment than Syt1. Furthermore, deletion of Syt2 severely reduced and delayed disynaptic inhibition following parallel fiber stimulation. Thus, the selective use of Syt2 as release sensor at BC-PC synapses ensures fast and efficient feedforward inhibition in cerebellar microcircuits.

Chen, C., Arai, I., Satterfield, R., Jr, S.M.Y., and Jonas, P. (2017). Synaptotagmin 2 Is the Fast Ca2+ Sensor at a Central Inhibitory Synapse. Cell Reports 18, 723–736.
DO: http://dx.doi.org/10.1016/j.celrep.2016.12.067

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When cells grow and divide, they come into contact with other cells. This happens not only during development and regeneration and after injury, but also during cancer growth and the formation of metastases. When cells come into contact with each other in this way, information is exchanged by proteins, which are embedded in the cell membranes and form tight lock-and-key complexes with each other. These connections must be severed if the cells want to transmit a repulsion signal. It appears that the fastest way to do this is for the cells to engulf the protein complex from the membrane of the neighboring cell. Scientists from the Max Planck Institute of Neurobiology in Martinsried have now identified the molecules that control this process.

Development is an extremely rapid process. Increasing numbers of cells are formed which must find their correct position in the body, clearly demarcate themselves from each other to form tissue, or – as is the case in the nervous system – establish contact with partner cells in remote locations. “The crowding is accompanied by orderly pushing and shoving,” says Rüdiger Klein, whose Department at the Max Planck Institute of Neurobiology studies how cells get their bearings. “A popular way for one cell to show another which direction to take is for it to repel the other cell following brief contact.” According to the scientists’ observations, the cells do not exactly treat each other with kid gloves and even go so far as to engulf entire pieces from the membranes of other cells.

When cells come into contact with each other, ephrin and Eph receptors are often involved. These proteins are located on the surface of almost all cells. When two cells meet, their ephrin and Eph receptors connect to form tight ephrin/Eph complexes. These complexes then trigger the repulsion process through intracellular signaling pathways. “This is where the problem arises, as it appears that the cells then want to separate as quickly as possible – however, the two cells are attached to each other through the tight ephrin/Eph complex,” explains Klein. So the cells do something else: they extend their own cell membranes so far over the individual complexes that the complex and the surrounding membrane detaches from the neighboring cell and is fully incorporated into the cell.

Left: Ephrin and Eph receptors are found on the surface of almost all cells. Center: When cells come into contact with each other, the two proteins form a tight complex. This triggers a signaling chain which causes the cell membrane to protrude. This process is controlled by the Tiam and Rac molecules and results in the reformation of the actin cytoskeleton. Right: The cells separate when one cell fully engulfs the ephrin/Eph complex through endocytosis.
© MPI of Neurobiology/Gaitanos

The Max Planck researchers discovered as early as 2003 that cells can use this process, known as endocytosis, to separate from each other. Thanks to progress made in molecular biology since then, they have now managed to show how the process is controlled in detail.

With the help of a series of genetic modifications and the targeted deactivation of individual cell components, the scientists succeeded in demonstrating that Tiam signaling proteins are activated through the formation of the ephrin/Eph complex. As a result, Rac enzymes become active which, in turn, cause the engulfment of the ephrin/Eph complexes by the cell membrane through the local restructuring of the actin cytoskeleton. If one of these components is missing, this engulfing process through endocytosis is blocked and the cells do not repel each other but remain attached.

The clarification of this signaling pathway is important, as it provides a better understanding of the development of neuronal networks and other organ systems. The findings are also of considerable interest for cancer research: thanks to their ability to control cell repulsion, ephrin and Eph receptors play a major role in the penetration of tissue by cancer cells and in the formation of metastases. For this reason, receptors and their connection partners are the focus of current medical research. Better understanding of this signaling pathway, through which cell repulsion is controlled, could enable the development of new drugs to combat cancer.

Original Publication:
Thomas N. Gaitanos, Jorg Koerner, Rüdiger Klein
Tiam/Rac signaling mediates trans-endocytosis of ephrin receptor EphB2 and is important for cell repulsion.
Journal of Cell Biology; 5 September, 2016
https://dx.doi.org/10.1083/jcb.201512010

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