New developmental and molecular mechanism of brain wiring

Nervous system development involves a process called axon guidance, which is the process of getting nerves to the right place in the brain. This is achieved through the interaction between neuronal cell surface receptors and their attractive or repulsive ligands present in the environment.

Bassem Hassan's team focused on the developmental and molecular mechanism of brain wiring in Drosophila.
Wiring the brain is more complex and is organized differently from wiring the spinal cord.
In the Drosophila spinal cord (called the ventral nerve cord), a line of non-neuronal cells in the midline of the structure act as a point source of guidance cues. Axons have receptors for these guidance cues, so depending on the kind of receptors they have they will approach the midline or not, be attracted or repelled from it, and then will cross it only once. In the spinal cord, glial cells secrete a protein called Slit to guide axons expressing the Slit receptor Robo.

However, brain wiring is more complex as there is no midline source of guidance cue.
Scientists from Bassem Hassan’s team found that Slit protein is expressed in the memory neurons of the fly brain (called the mushroom bodies) which send their extensions (axons and dendrites) to many places throughout the brain. That suggests a distributive source from one neuronal population towards others. Theses axons and dendrites express Slit to guide other axons expressing Robo.

Regarding Robo, they found that, neurons that express Slit do not express Robo. With the hypothesis of neurons talking to other neurons, they showed that it is not a binary decision of crossing or not crossing the midline, like it is in the spinal cord. Instead it is one neuron telling another neuron how much to grow. Neurons that do not have Robo, do not react to Slit. In other words, brain neurons self-organize their circuits.
This opens up the idea that you can reorganize the brain by changing the size of different neuron populations, changing the level of Slit and Robo might trigger different wiring events, creating greater flexibility across a larger area.

Interestingly, in the fly, only one population expresses Slit, but it is entirely possible that in more complex brains different neurons express Slit leading to some kind of interplay between cells avoiding each other.

The second issue they addressed was the regulation of Robo on the surface of the neurons. They found that the protein that regulates Robo in the ventral nerve cord, does not do it in the brain.
They suspected a certain type of transmembrane proteins known as a receptor phosphatases. Receptor phosphatases sit on the membrane of the axon with one part towards the outside of the cell and another part inside the cell acting as an enzyme that removes chemical modifications from other proteins.

They found that one such receptor phosphatase called RPTP69d regulates the quantity of Robo on the membrane. The more RPTP69d the more Robo there is on the membrane. Without RPTP69d, the axons behave as if they don’t have Robo.
They also showed that the entire phosphatase domain, the enzymatic activity, is not required. Instead RPTP69d formed a complex with Robo and Slit and stabilized Robo levels at the surface.

This work reveals a previously unknown function of the fly memory neurons, the mushroom bodies. In adults they mediate memory formation, but earlier during development, they help wire the brain.

Robo receptors are also important in human brain wiring and one of the ROBO proteins, Robo3, is mutated in patients with a neurological disease called horizontal gaze palsy with progressive scoliosis (HGPPS). Patients with HGPPS are unable to move their eyes from side to side horizontally and also suffer from an abnormal curvature of the spine. Furthermore, Robo receptors play a role in blood vessel formation and both Slit and ROBO are involved in many types of human cancer. So understanding how Robo is regulated might provide a way to target it in different disease conditions.

As Slit, Robo and RPTP69d are conserved regulators, it will be interesting to study this new molecular mechanism more broadly, such as in mammalian systems.