No, actually they don’t — but they do have some proteins that are essential components of synapses, and it tells us something important about the evolution of the nervous system. A new paper by Sakarya et al. really isn’t particularly revolutionary, but it is very interesting, and it does confirm something many of us suspected.
First, in case you don’t know what I’m talking about, here’s a synapse:
A synapse is a kind of gateway for the transmission of electrical impulses in the nervous system. What’s portrayed above is the terminal of a generic neuron; electrical signals travel down from above to the end, where packets of a chemical transmitter are stored (the round brown blobs in the diagram). The signal triggers various ions and other agents to cause some of the vesicles full of transmitter to fuse with the presynaptic membrane, dumping their contents (the scattered small dots) into the synaptic cleft, where they diffuse across to receptors on the postsynaptic membrane. When the receptors bind the transmitter, they cause a change in the electrical properties of their cell.
Synapses have not been found in sponges. They don’t have a nervous system.
However, what has been found is very interesting. The above diagram is greatly simplified. In reality, there is a whole complex network of proteins on both the pre- and postsynaptic side that organize, structure, and transduce signals. For instance, on the postsynaptic side, there is an array of receptors to receive the transmitter, and they are not just floating free in the membrane—they are clustered just where the synapse is made, and are linked together by a meshwork of proteins. That meshwork isn’t shown up there, but here’s another diagram of just the post-synaptic membrane.
The protein complex underlying the receptors is shown in blue. They link the receptors (in green) together, and are also connected to the cytoskeleton of the cell. Activity in the receptors can be mechanically and chemically transduced into changing the activity of the cell through these linkages.
These post-synaptic proteins contain a domain called PDZ that mediates these protein-protein interactions — think of it as a kind of general purpose glue found in many of these proteins proteins that makes them stick to one another. PDZ is a cryptic acronym: it refers to the first three proteins in which they were found, which were: PSD-95 (post-synaptic density protein of 95kD MW), a protein important in clustering glutamate receptors; the dlg tumor suppressor protein from Drosophila; and another protein called zo-1. The PDZ domain is found all over the place, in bacteria, plants, fungi, and animals. Sponges have PDZ domains, too—that isn’t a big deal at all.
Here’s what’s interesting. We can also characterize proteins by the arrangement of these domains within the protein. For example, dlg has, in order from the amino terminus, an L27 domain, 3 PDZ domains, a Src homology 3 (SH3) domain, and a guanylate kinase-like domain (GK). If you isolate a PDZ protein from a bacterium, it won’t have that same arrangement of different motifs. We know that certain arrangements of motifs are characteristic of the PDZ proteins found in postsynaptic protein complexes; the PSD-95 protein has these domains, and links to one another and to other proteins to make that post-synaptic meshwork. And sponges do have proteins just like PSD-95 and dlg that have previously been associated with post-synaptic protein complexes.
That’s the important distinction. Sponges do not have a nervous system, and they do not make synapses, but they do make proteins similar to those that underlie our synapses.
In addition, they’ve found that many of these same postsynaptic proteins are present in choanoflagellates—organisms that aren’t multicellular at all. Others are found in Nematostella, a sea anemone, which has a very simple nerve net with some signs of condensation of ganglia. By taking a phylogenetic approach, researchers have figured out when these various components evolved, and the message is that the pieces of this complex structure were present early in evolution. Very early.
The diagram below illustrates more details of the post-synaptic proteins. The small ovals are the PDZ domains, and this also illustrates the ligands and various other proteins present in the post-synaptic density. Everything is also color coded. The blue proteins are present in sponges, and evolved before the kingdom Animalia branched off—notice that the diagram is a sea of blue. The yellow proteins are the next most recent addition, and include important elements like the glutamate receptors mentioned above. These are present in sea anemones. Next to evolve are the green proteins, in the evolution of the bilateria. There aren’t so many that are unique to us. And finally, there a very few orange proteins that evolved after the protostome-deuterostome split.
Almost everything in this diagram evolved before bilaterally symmetric animals did. Most of it was present in sponges.
What this is saying is that all of that complex functionality in a structure we usually consider a key piece of something relatively sophisticated, the physiology of signaling in our brain, evolved before the Cambrian, and that much of it was already present in single-celled organisms.
This is a wonderful example of exaptation. These components of the post-synaptic membrane actually evolved to faciliate interactions between single-celled organisms, with no neural activity involved. After all, bacteria stick to objects in their environment all the time, and the binding triggers changes in internal activity. Our evolving nervous systems simply reused that same machinery to mediate connections in the brain—a bacterium sticking in a biofilm and making an appropriate physiological response is in some ways very similar to a neuron sticking to a target input and making an appropriate physiological response to its activity.
Another important message is that it should make the idea of a Cambrian “explosion” a little less dramatic. Key molecular elements that underlie the apparent morphological revolution all arose in the Neoproterozoic, in bacteria and protists, and the Cambrian was a period when that rich repertoire of molecular signaling tools were simply redeployed, refined, and specialized to allow for multicellular diversity to emerge.
Sakarya O, Armstrong KA, Adamska M, Adamski M, Wang I, et al. (2007) A Post-Synaptic Scaffold at the Origin of the Animal Kingdom. PLoS ONE 2(6): e506. doi:10.1371/journal.pone.0000506