Marine organisms have long been important models in neurobiology, evolutionary biology and biomedical research, allowing many essential mechanisms of diverse biological phenomena to be revealed. Two very prominent examples are the Nobel Prize-winning work of Alan Hodgkin and Andrew Huxley uncovering the mechanism of membrane excitability using the giant squid Loligo pealei, and the Nobel Prize-winning work of Eric Kandel elucidating mechanisms of memory formation using the sea slug Aplysia californica. The ground-breaking work of Hodgkin and Huxley carried out at the Marine Biological Association (MBA) in Plymouth, was especially important as it revealed fundamental insights into how cells communicate with each other and how signals are sent along nerve cells. Their work provided the basis of our understanding of how we see, hear and feel things and how our brains work, and laid the foundations for other Nobel Prize-winning work including that of Erwin Neher and Bert Sakmann for their discoveries concerning the function of single ion channels in cells, and Roderick MacKinnon for structural and mechanistic studies of ion channels.
Nerve cells are the key building blocks of our nervous system and are central to information processing and transport in the brain. These specialized cells transmit information from one cell to the other via synapses, specialized contact sites that allow nerve cells to exchange information rapidly (Figure 1). Upon calcium influx, chemical signals called neurotransmitters (highlighted as green bubbles in Figure 1) are released from presynaptic synapses and diffuse across the synaptic cleft to react with receptors on postsynaptic synapses. This process involves a network of synaptic proteins that forms the molecular machinery underlying neurotransmitter release from presynapses and activation of neurotransmitter receptors on postsynapses (Figure 1). It is interesting to note, that while we now understand much of the underlying molecular mechanism of how nerve cells communicate, knowledge about how and when synapses and nerve cells originated is still in its infancy. 
Understanding how nerve cells evolved is central to reconstructing how animals evolved such vast biological and behavioural diversity. When wondering about the origins of nerve cells we should not look at humans, fish or even worms. Many of the key synaptic proteins appeared long before animals, brains and nerve cells existed and can be found already in single-celled organisms, for example, in choanoflagellates. Let us first take a step back and look at what choanoflagellates are. Choanoflagellates are the closest known relatives of all animals (Figure 2A), making them a fascinating family of organisms for studying the origin and evolution of synaptic proteins. Choanoflagellates are a group of microbial eukaryotes (organisms whose cells contain a nucleus and other organelles enclosed within membranes) that live in many different aquatic (both marine and freshwater) environments. They are characterized by a single flagellum surrounded by a collar of actin-filled microvilli (Figure 2B). Choanoflagellates prey on bacteria; the undulation of the apical flagellum creates fluid currents that draw bacteria into the cells. Although all choanoflagellates have a single-celled phase in their life history, many species also form colonies composed of multiple cells (Figure 2C). Most importantly, choanoflagellates do not have nerve cells, but the rich repertoire of neuronal protein homologues in choanoflagellates and the close relationship to animals make choanoflagellates ideal candidates to study the origin of synapses and nerve cells (Burkhardt et al., 2014).
The release of neurotransmitters is mediated by a conserved set of proteins called SNARE and Munc18 proteins (Figure 2D). These proteins are found in every nerve cell and are essential so that nerve cells can talk to each other. We decided to look for them in choanoflagellates and to our great surprise, we not only found these proteins in choanoflagellates, but we were also able to show that the interaction between the two was the same as in nerve cells (Figure 2D) (Burkhardt et al., 2011). Thus, an ancient protein machinery for the secretion of neurotransmitters was already present in the last common ancestor of choanoflagellates and animals. This machinery likely served as a starting point that helped to develop a more complex apparatus found in many animal cells, including nerve cells. This finding is intriguing on its own, but much more significant when combined with a growing body of evidence that many components of our nerve cells and brains evolved before the first animals appeared. For example, choanoflagellates express the same calcium channels as those used by nerve cells (Cai, 2008), have the same sodium channels that nerve cells use to send electrical signals along their length (Liebeskind et al., 2011) and also have several proteins that nerve cells use to process signals from their neighbours (Alié & Manuel, 2010). Put together, these findings suggest that choanoflagellates have components for the three main functions of nerve cells: carrying electrical signals along their bodies; signalling to their neighbours with neurotransmitters; and receiving those signals (Marshall, 2011). It looks as though our nervous system was built up from several ‘simple’ systems and that these systems likely first evolved separately for different reasons.
Important questions still remain: How did the first nerve cells evolve? And did nerve cells originate more than once? To answer these questions we have to look at two of the most ancient animal lineages: the sponges and the ctenophores (Figure 3). Sponges have neither tissues nor organs, they are filter feeders which prey on bacteria as choanoflagellates do, and until very recently sponges were thought to be the most ancient animal lineage (Figure 3). This scenario fitted well with observed morphological traits, as sponges lack synapses, nerve cells and a nervous system. Recent reports have now instead suggested ctenophores as the most ancient animal lineage (Moroz et al., 2014; Ryan et al., 2013,) (Figure 3) generating much excitement for evolutionary biologists and neuroscientists. Ctenophores, or comb jellies, are a group of carnivorous marine animals. If ctenophores are indeed the most ancient animal lineage, it has important implications for the evolution of nerve cells, which are present in ctenophores, but absent in sponges (Figure 3). One can think of two scenarios related to the evolution of nerve cells: either all nerve cells arose from a single ancestral cell; or nerve cells originated independently in ctenophores and the rest of all animals.
Through our experimental work at the MBA focusing on high quality basic research using choanoflagellates, sponges and ctenophores as new model organisms we will be able to elucidate the evolutionary history of synaptic proteins and understand the evolution of the first synapses and nerve cells. By reconstructing the origin of nerve cells, this research aims to provide new insights into animal development, health and disease.
Pawel Burkhardt is a Research Fellow at the Marine Biological Association.
FURTHER READING
Alié, A. and Manuel, M. (2010). The backbone of the post-synaptic density originated in a unicellular ancestor of choanoflagellates and metazoans. BMC Evolutionary Biology 2010, 10:34  
Burkhardt, P., Stegmann, C. M., Cooper, B., Kloepper, T. H., Imig, C., Varoqueaux, F., Wahl, M. C., and Fasshauer, D. (2011). Primordial neurosecretory apparatus identified in the choanoflagellate Monosiga brevicollis. Proceedings of the National Academy of Sciences of the United States of America 108, 15264–15269.
Burkhardt, P., Grønborg, M., Mcdonald, K., Sulur, T., Wang, Q., and King, N. (2014). Evolutionary insights into premetazoan functions of the neuronal protein homer. Molecular Biology and Evolution 31, 2342–2355.
Cai, X. (2008). Unicellular Ca2+ signaling “toolkit” at the origin of metazoa. Molecular Biology and Evolution 25, 1357–61.
Liebeskind, B. J., Hillis, D. M., and Zakon, H. H. (2011). Evolution of sodium channels predates the origin of nervous systems in animals. Proceedings of the National Academy of Sciences of the United States of America 108, 9154–9.
Marshall, M. (2011). Your brain chemistry existed before animals did. New Scientist 2828, 11.
Moroz, L. L., Kocot, K. M., Citarella, M. R., Dosung, S., Norekian, T. P., Povolotskaya, I. S., Grigorenko, A. P., Dailey, C., Berezikov, E., Buckley, K. M., et al. (2014). The ctenophore genome and the evolutionary origins of neural systems. Nature 510, 109–114.
Ryan, J. F., Pang, K., Schnitzler, C. E., Nguyen, A., Moreland, R. T., Simmons, D. K., Koch, B. J., Francis, W. R., Havlak, P., Comparative, N., et al. (2013). The genome of the ctenophore Mnemiopsis leidyi and its implications for cell type evolution. Science 342, 1336–1344.

Fig. 1. Cartoon of two nerve cells with their most prominent feature: the synapse. Chemical signals called neurotransmitters (shown as green bubbles) are released from presynaptic synapses and diffuse across the synaptic cleft to react with receptors on postsynaptic synapses. Enlarged view: the network of synaptic proteins that forms the molecular machinery underlying neurotransmitter release. Cartoon modified from ClipArtHut.com.

Fig. 2. (A) Choanoflagellates are the closest living relatives of animals. (B and C) Choanoflagellates have a single flagellum (fl) surrounded by a collar (bracket). They have a single-celled phase in their life history, but many species also form colonies. (D) The protein machinery for the secretion of neurotransmitters is conserved in choanoflagellates and rats. Choanoflagellate and rat illustrations were reused with modifications from phylopic.org. Scale bars: 2 μm.

Fig. 3. Uncertainty at the base of the animal tree; sponges or ctenophores are currently considered to be the most ancient animal lineage. This has important implications for our understanding of the evolution of nerve cells, which are present in ctenophores, but absent in sponges. Either all nerve cells arose from a single ancestral cell or nerve cells originated independently in ctenophores and the rest of all animals. Shown are the choanoflagellate Salpingoeca rosetta, the sponge Sycon ciliatum and the ctenophore Pleurobrachia pileus, new model organisms which are cultured at the laboratory of the Marine Biological Association in Plymouth. Scale bars: choanoflagellates: 2 μm; sponges and ctenophores: 0.5 cm.

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Pawel Burkhardt

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