An unconventional partnership encourages neuron migration

From the Cooper Lab, Basic Sciences Division

Understanding how neurons migrate from their birthplace to different, final locations during the mammalian brain development is an active area of research. From a basic science standpoint, the process is justifiably fascinating. But a fundamental understanding of the mechanisms underlying neuron migration is also important from a translational perspective, since dysregulation of neuron migration has been implicated neurodevelopmental disorders such as epilepsy and schizophrenia.

Scientists in the Cooper Laboratory (Basic Sciences Division) study the signaling pathways that direct neuron migration. Previously, Yves Jossin, then a post-doc at the Cooper laboratory, discovered that neuron migration in the developing brain requires the adhesion protein neuronal (N) cadherin while N cadherin’s adhesion was dispensable. “We thought some other aspect of N cadherin must be involved,” said Dr. Jonathan Cooper.

Dr. Cooper continues to collaborate with Dr. Jossin who now has his own lab at the Université Catholique de Louvain. The two labs uncovered a mechanism by which N cadherin ubiquitinates the fibroblast growth factor receptor (FGFR) and targets it for degradation during neuron migration. Their findings appeared in a recent issue of Elife. More recently, the scientists investigated the mechanisms underlying neuron migration control by N cadherin beyond its function in adhesion. The study was led by Youn Na, a former technician in the Cooper lab and is now available as a preprint on bioRxiv. Dr. Cooper commented on the importance of their study: “Cadherins have well-studied roles in attaching cells to each other in tissues, where a cadherin on one cell can bridge across to an identical cadherin on another cell. Their roles in cell migration are less well understood. Our previous work showed that neuronal (N) cadherin is required for neurons to migrate properly during brain development. But N cadherin bridging between cells was not required.”

PDB structure of N-cadherin with the binding site of Fbxo45 (Asn84 loop) indicated. Trp2 mediates the dimer of N cadherin that allows cell-cell adhesion
A protein structure model from a published structure of N-cadherin (PDB: 3q2w) with the binding site of Fbxo45 (Asn84 loop) indicated. Trp2 mediates the dimer of N cadherin that allows cell-cell adhesion. Figure provided by Jonathan Cooper

In order to better understand how N cadherin functions in neuron migration, Na started screening for other proteins that bind N cadherin. Through two independent screening approaches, the researchers identified MycBP2 and SPRY-domain protein Fbxo45, two components of an intracellular E3 ubiquitin ligase, as N cadherin binding partners. Not only did Fbxo45 bind the extracellular domain of N cadherin, it also stimulated neurite branching in culture. As a result, Fbxo45 is secreted by a non-canonical mechanism that does not involve a signal peptide and does not require endoplasmic reticulum to Golgi transport. “This was a surprise to us,” said Dr. Cooper. “Fbxo45 was previously thought to remain inside the cell. “We were surprised to see that it gets secreted, and above all, by an unusual route that does not require endoplasmic reticulum to Golgi transport nor a signal peptide!” he added. 

A diagram shows a model for how Fbxo45 may work during migration of immature neurons during brain development.
A diagram shows a model for how Fbxo45 may work during migration of immature neurons during brain development. Image courtesy of Jonathan Cooper

Another surprising finding to the team was that the EC1 domain of N cadherin had two distinct functions that can be mapped to separate domains which allowed them to separate N cadherin functions in cell-cell adhesion from its functions in neuron migration. By mutating the SPRY motif of N cadherin, the authors showed that Fbxo45 binding was lost and that neuron migration was disrupted while cell-cell adhesion was maintained. The results suggest that secreted Fbxo45 may regulate neurite branching and bind N-cadherin to orient multipolar neuron migration. However, some questions remain unanswered. Does Fbxo45 regulate migration of other cell types?  “We know that N cadherin is increased during EMT of cancer cells, and is expressed by neural tumor cells (such as glioblastoma) which are highly invasive. Exactly how it works isn't clear. Perhaps Fbxo45, or the Fbxo45-binding residues on N cadherin are critical.” Said Dr. Cooper. Ongoing research in the lab aims to answer these questions.

In the short term, Dr. Cooper and colleagues are working to address some of the limitations of the current model. “A critical limitation of our current model is that, even though the region of N cadherin that binds Fbxo45 is needed for neuron migration, we were unable to show that migration required binding of Fbxo45 and not some other molecule,” said Dr. Cooper. “There were technical reasons why this experiment failed, and we are now trying to overcome them using new approaches,” he added. 

This work was supported by funding from the US National Institute of Health and the National Fund for Scientific Research in Belgium.

UW/Fred Hutch Cancer Consortium member Jonathan Cooper contributed to this work.

Na Y, Kon E, Cao H, Jossin Y, Cooper J. 2019. N-cadherin SPRY motifs bind unconventionally-secreted Fbxo45 and regulate multipolar neuron migration. bioRxiv