Primitive beginnings of a nervous system in animals without one hint at the origins of the synapse
By Melissa Lee Phillips | June 6, 2007
Biologists have captured a glimpse into the origins of the nervous system through an unlikely lens: sea sponges. According to a report in this month's Public Library of Science ONE, these primitive animals possess protein components of synapses, even though they don't have nervous systems.
The authors also found evidence that these synaptic proteins interact in the same way in sponges as they do in animals with nervous systems.
The findings "could provide some clues to how synapses were constructed during evolution," said Lu Chen of the University of California, Berkeley, who was not involved in the research. "It's very exciting work."
Led by Onur Sakarya of the University of California, Santa Barbara, (UCSB) and Kathryn Armstrong of the Massachusetts Institute of Technology in Cambridge, the researchers found homologs of most human post-synaptic genes in the genome of the sponge Amphimedon queenslandica.
The authors also found that sequences that help human synaptic proteins to bind to each other were highly conserved in the sponge protein sequences. "It suggests that there's some conservation of function at the protein level of these genes between sponges and humans," co-author Todd Oakley of UCSB told The Scientist.
Further analyses confirmed that many of the structural motifs found in synaptic proteins of higher animals are also found in the sponge proteins, which the authors say suggests that, in the sponge, the proteins can assemble in a scaffold similar to that found in the post-synaptic neuron. They also found that a set of key synaptic genes is expressed in a single sponge cell type -- further evidence that these proteins could interact with each other in the sponge.
Among the few post-synaptic human genes that did not have homologs in the sponge, most were membrane proteins that interact with the post-synaptic scaffold, such as glutamate receptors. This suggests that most of the innovation that led to a functional synapse came from novel binding partners, which interacted with an existing scaffold, the authors say.
"It's quite striking that they could find these post-synaptic scaffold protein homologs in such an early stage of evolution," Chen said.
Finding molecular components of a synapse in an animal without neurons actually "makes a lot of sense," according to Robert Meech of the University of Bristol, UK, who was not involved in the work. "You would imagine that a thing as complicated as [a synapse] must have been put together from components that were doing another job," Meech said. "It would be nice to know what sort of things those components are doing in sponges."
The researchers don't yet know what function these "proto-synaptic" proteins serve in the sponge, although "we have a few clues," Oakley told The Scientist. The cells that express the proteins have some sensory capacity, "so it may be that these genes... mediate some sort of sensory function, but we don't know that for sure yet," he said.
They also can't be sure that the proteins they found in the sponge really represent the evolutionary beginnings of a synapse, and aren't simply an evolutionary relic, Oakley said. "What we know is that these genes were present at the origin of animals, since sponges and other animals have these genes," he said. "But it remains possible that the ancestor of all animals had something like a synapse, but then it was lost in the lineage leading to sponges."
Melissa Lee Phillips
Links within this article:
M.L. Phillips, "Ocean home to new proteins, families," The Scientist, March 14, 2007.
O. Sakarya et al., "A post-synaptic scaffold at the origin of the animal kingdom," PLoS ONE, June 2007.
"Building the synapse," The Scientist, August 1, 2006.
M. Maltenfort, "Synaptic transmission tenets challenged," The Scientist, March 14, 2005.
M. Sheng, "Molecular organization of the postsynaptic specialization," PNAS, June 19, 2001.