Key role of a protein complex in the migration of neurons and neuroblastoma tumor cells

During brain development, neurons must migrate long distances through complex environments to reach their final location. To guide themselves, they must establish various interactions —today still difficult to study— between their receptors and the surrounding molecules.

Now, a study has identified how two different proteins, the neuronal receptor Unc5 and the molecule Glipican 3 (GPC3), decisively collaborate in guiding neurons in two very different processes: the formation of the brain and the spread of tumor cells of cerebral origin (from neuroblastoma).

The work identifies the points where both proteins connect, reveals the structure of the new Unc5-GPC3 complex and determines its key role in the migration of neurons and in certain tumors. It also delves into the mechanisms that regulate cell migration very precisely, and notes how some molecular mechanisms that regulate cell migration are highly conserved between neurons and some brain tumors.

The team that has directed the work is made up of the experts Daniel del Toro, from the Faculty of Medicine and Health Sciences and the Institute of Neurosciences (UBNeuro) of the University of Barcelona (UB), the August Pi Biomedical Research Institute and Sunyer (IDIBAPS) and the Center for Network Biomedical Research on Neurodegenerative Diseases (CIBERNED), in Spain; Valerie Castellani, from the University of Lyon (France); and Elena Seiradake, from the University of Oxford (United Kingdom).

Mouse brain showing radial glia in green (GFP fluorescent protein) and migrating neurons on fibers in red (mCherry fluorescent protein). (Images: UB)

During the development of the cerebral cortex, neurons use fibers that originate from their stem cells, called radial glia, as if they were highways to reach their final position. During migration, neurons must interact with these fibers, but little has been known about the proteins involved and how they interact.

Gaining a detailed insight into the molecular guidance mechanisms of brain cells during brain development is quite a challenging goal. In this work, the collaboration and experience of the three scientific teams have made it possible to identify the interaction between the two proteins Unc5 and GPC3 and a detailed view of how they bind through the carbohydrates they bring to the surface.

“The study reveals for the first time a new complex formed by the protein Unc5, present in neurons, and the compound GPC3, which is found in the fibers through which these cells migrate. When they interact with each other, the protein complex Unc5-GPC3 is formed, which facilitates the migration of the neurons on the fibres”, explains Professor Daniel del Toro, member of the UB Department of Biomedicine.

The discovery of the molecular guidance mechanisms of brain cells was made possible by structural analyzes of the protein complex. By identifying the binding sites between the proteins, the team has been able to generate tools to monitor their interaction and identify the specific function of this protein complex. “A very surprising result was to find that the complex itself regulates the migration of cells as diverse as neurons and certain brain tumors such as neuroblastoma,” says the researcher.

The work has focused on the study of the migration of the main neurons of the cerebral cortex, a decisive process for the correct formation of the neuronal circuits that regulate the most sophisticated cognitive functions (language, cognition, abstract thought, etc.). However, the team has found that the Unc5-GPC3 complex is present in other regions of the brain and, consequently, other neurons could also use it to migrate. “For example, other neurons —known as interneurons— that also express proteins from this complex also reach the cerebral cortex. Therefore, it will be very interesting to check the functions of this complex in other types of neurons in future studies”, points out Daniel del Toro.

Until now, it was thought that cells used different mechanisms to migrate in completely independent biological environments, but the new results suggest that the guiding mechanisms during cell migration can be shared and reused by different cell types. This implies that the knowledge tools generated in this work can be applied in very different contexts, such as the migration of other cell types, or the application of new strategies to control this process in certain pathologies, such as cancer.

It should be remembered that in the case of certain tumors, such as neuroblastoma, the expression of the Unc5 protein is very high. “For this reason, we think that this factor could also regulate the migration and dissemination of this tumor cell, which is the field of study of the Lyon team. Given that the other protein in the complex, GPC3, is highly expressed in other types of tumours, it is very likely that the same complex can form in other cancers. Thus, the tools developed in this work will make it possible to study it in future works”, underlines the expert.

The UB team has focused on studying the role played by the Unc5-GPC3 complex in neuronal migration. In the laboratories, the team identified the presence of this new protein complex in the brain during the development of this organ. Thanks to this finding, they focused their research on the study of this complex during the migration of neurons in the cerebral cortex. Using different techniques, they modified the binding sites of these proteins in mouse brains and this has made it possible to demonstrate their function during the process.

Methodologically, obtaining the structure of the protein complex through X-ray crystallography has also been decisive in order to discover the binding sites between the proteins. Thanks to this contribution from the Oxford group, it has been possible to develop very small antibodies (nanoantibodies) that can facilitate or block the formation of this complex. ‘We have managed to introduce these nanoantibodies into the brain while it was developing to see how this complex regulates the migration of neurons. In addition, we have also applied microfluidic methods to study the behavior of cortical neurons through structure-based protein engineering”, Del Toro points out.

Alterations in the migration process of neurons in the cerebral cortex can cause cognitive alterations and learning problems, among others. From a more clinical perspective, in the case of tumors such as neuroblastoma, all the processes that regulate their dissemination have a great impact on the prognosis of the pathology.

«The results and tools generated by the study of the Unc5-GPC3 complex in the migration of neurons are ideal for future studies on its function in the brain. Specifically, this work gives us the ideal instruments to study the functions of the complex in other systems in which it operates, such as in different regions of the brain during development or in the migration of other tumors. Given that the proteins that make up the complex can interact with other components, it will be interesting to discover if this complex can be modified by incorporating new proteins in order to adapt the response of cells when migrating through different environments”, concludes Professor Daniel del Toro.

The study is titled “GPC3-Unc5D complex structure and role in cell migration.” And it has been published in the academic journal Cell. (Source: UB)