When the nervous system is developing neurons send their axons, the parts of neurons that make up the body’s nerves, to make connections with other neurons. For instance, a neuron in the dorsal spinal cord grows towards attractive signals coming from the ventral spinal cord and cross the midline. The front ends of growing axons have ‘growth cones’ that sniff out attractive and repulsive signals. If a signal of attraction is on the left, new pieces of axon membrane are inserted into the left side of the growth cone, which makes the axon grow towards the attractive signal.

While it is known that attractive signals trigger calcium ions to be released from storage inside the growth cone, it is still unclear how this causes new pieces of the axon to be directed into the correct path of the growth cone.  Now, a study from researchers at the RIKEN Brain Science Institute identifies a protein complex, myosin-Va, that helps direct the growth of axons.  The team state that their findings show that myosin-Va is the missing link between Ca²⁺ and polarized membrane delivery to the growth cone, telling new pieces of axon where they should go.  The opensource study is published in the journal Cell Reports.

Previous studies show that there are two receptors for the calcium which is released from the endoplasmic reticulum of growth cones. The two receptors, RyR3 and IP₃R, are associated with a different kind of calcium release from the endoplasmic reticulum, and with axon growth toward attractive signals. Therefore, the lab hypothesized that in both cases, calcium was being detected by the same sensor, triggering export of membrane vesicles to one side of the growth cone.  The current study investigates what the common calcium sensor is by examining the structures of the two receptors to look for matching docking or binding sites.

The current study shows that the protein myosin-Va interacts with a matching site on both receptors; to test if myosin-Va is needed for axon growth, in vitro neurons were injected with peptides made from the myosin-Va binding region. Results show that Myosin-Va bound to these peptides instead of the Ry3 and IP₃ receptors, with the axons repulsed from the signals of attraction.  Data findings show that focal laser-induced photolysis can be used to direct axons to grow in a particular direction without using calcium.

The group also looked at how myosin-Va is involved in transporting membrane vesicles to the growth cone. To do so, they fluorescently labeled VAMP2, a molecule found in the vesicles, and observed what happened when myosin-Va was prevented from binding to the receptors. Data findings show that vesicles export increased, preventing growth in the correct direction.

The team surmise that they show their peptides prevent axons from crossing the spinal cord midline in vivo. For the future, the researchers state that they now plan to use light-mediated control of membrane dynamics to guide axons in vivo, and to manipulate other cell functions such as synaptic growth.

Source: RIKEN Brain Science Institute