To confirm this, Budnik and colleagues inserted a Syt4 gene into the neurons of a Drosophila mutant completely lacking the normal protein. This restored Syt4 in both neurons and muscle cells. Further experiments suggested that the only source of Syt4 is the neuron. These observations were consistent with the model that Syt4 is actually transferred from neurons to muscle cells. As a transmembrane protein, however, Syt4 was thought to be unable to move from one cell to another through traditional avenues. How the Syt4 protein was moving from neuron to muscle cell was unclear.
Knowing that exosomes had been observed to carry transmembrane proteins in other systems and from their own work on the Drosophila NMJ, Budnik and colleagues began testing to see if exosomes could be the vehicle responsible for carrying Syt4 form neurons to muscles. "We had previously observed that it was possible to transfer transmembrane proteins across the NMJ through exosomes, a process also observed in the immune system," said Budnik. "We suspect this was how Syt4 was making its way from the neuron to the muscle."
When exosomes were purified from cultured cells containing Syt4, they found that exosomes indeed contained Syt4. In addition, when these purified exosomes were applied to cultured muscle cells from fly embryos, these cells were able to take up the purified Syt4 exosomes. Taken together, these findings indicate that Syt4 plays a critical role in the signaling process between synapse and muscle cell that allows for coordinated development of the NMJ. While Syt4 is required to release a retrograde signal from muscle to neuron, a component of this retrograde signal must be supplied from the neuron to the muscle. This establishes a positive feedback loop that ensures coordinated growth of the NMJ. Equally
|Contact: Jim Fessenden|
University of Massachusetts Medical School