Nematodes Are Smarter than You Think

Weighing in at about 5 μg, with 302 neurons and 5000 synapses, C. elegans is unlikely to prove theorems, write poetry, or challenge Mike Tyson. Still, remarkable behavioral complexity is packed into this tiny worm. Worms live in a sensory world very different from our own, one dominated by touch, taste, and smell, one in which bacteria are the size of cherries, and Brownian motion and surface tension take the place of wind and gravity. It is difficult for us to intuit what their world is like, but one measure of what the worm cares about is the behavioral and genetic complexity devoted to various sensory modes. By this measure, thermotaxis must matter a lot to the worm. This behavior was first described in 1975 by Hedgecock and Russell, who noted, among other things, two remarkable features. First, worms are capable of moving along curving isothermal lines in their medium with precision, deviating as little as 0.1°C from their preferred temperature. Second, thermal preference is plastic, being conditioned by the presence or absence of food.The plasticity in C. elegans thermotaxis is apt. When a worm experiences prosperity (abundant food) at a given temperature, it acquires a preference for this temperature over others. When the temperature is held steady and food is removed, this preference degrades with time and is replaced by a temperature-independent searching mode of locomotion. When food remains abundant and temperature is changed, the result is a gradual shift in thermal preference toward the new growth temperature. The assay for these changes is simple (Hedgecock and Russell, 1975xHedgecock, E.M. and Russell, R.L. PNAS. 1975; 72: 4061–4065Crossref | PubMedSee all References(Hedgecock and Russell, 1975). A conditioned worm is placed on an agar surface (without food) in which a radial thermal gradient has been formed. As the worm moves, it leaves behind a detectable indentation in the agar, providing a record of its locomotory behavior over time. A happily conditioned worm will spend most of its time making precise circles along its preferred isotherm. In the absence of other cues, the worm may anticipate that food is likely to be found at this temperature or it may simply be physiologically optimized at this temperature. In contrast, a worm conditioned by the absence of food will move in the same assay without regard for temperature as long as it remains within a tolerable range (Wittenburg and Baumeister, 1999xWittenburg, N. and Baumeister, R. PNAS. 1999; 96: 10477–10482Crossref | PubMed | Scopus (79)See all References(Wittenburg and Baumeister, 1999). The main features of the neural circuit controlling thermotaxis are known. A single class of thermosensory neuron (AFD) makes synaptic output to a single class of interneuron (AIY). AIY in turn makes synapses mostly to three other classes of interneurons: AIZ, RIA, and RIB. AIZ makes synapses back to AIY and to various other interneurons. Based on synaptic connections, laser ablation studies, and genetic perturbations, AIY and AIZ are particularly important for thermotaxis, whereas RIA and RIB seem to be integrative interneurons that respond to many sensory cues (Mori and Ohshima 1995xMori, I. and Ohshima, Y. Nature. 1995; 376: 344–348Crossref | PubMedSee all References, White et al. 1986xWhite, J., Southgate, E., Thomson, J.N., and Brenner, S. Phil. Trans. R. Soc. Lond. B. 1986; 314: 1–340Crossref | PubMedSee all References, Hobert et al. 1997xHobert, O., Mori, I., Yamashita, Y., Honda, H., Ohshima, Y., Liu, Y., and Ruvkun, G. Neuron. 1997; 19: 345–357Abstract | Full Text | Full Text PDF | PubMed | Scopus (171)See all References). AIY and AIZ play antagonistic roles in thermotaxis: AIY activity favors migration to high temperatures and AIZ activity favors low temperatures (Mori and Ohshima, 1995xMori, I. and Ohshima, Y. Nature. 1995; 376: 344–348Crossref | PubMedSee all References(Mori and Ohshima, 1995).The paper by Gomez et al. (2001)xGomez, M., De Castro, E., Guarin, E., Sasakura, H., Kuhara, A., Mori, I., Bartfai, T., Bargmann, C.I., and Nef, P. Neuron. 2001; 30: 241–248Abstract | Full Text | Full Text PDF | PubMed | Scopus (161)See all ReferencesGomez et al. (2001) in this issue of Neuron reports a role for a C. elegans neuronal Ca2+ sensor protein (NCS-1) in thermotaxis accuracy and plasticity. NCS-1 belongs to a large family of EF hand containing calcium binding proteins and is highly conserved across species, including yeast, Drosophila, C. elegans, rodents, and humans. They report that NCS-1 is expressed in 13 classes of neurons and one muscle cell. The neurons are mostly sensory, including the thermosensory neuron AFD, but they also include the interneuron AIY. The authors generate a null deletion allele of NCS-1 using reverse genetic methods (Plasterk, 1995xPlasterk, R. Methods Cell Biol. 1995; 48: 59–80Crossref | PubMed | Scopus (43)See all References(Plasterk, 1995). ncs-1(null) mutants develop normally and perform normally in chemotactic odorant responses, suggesting that their fine locomotory and taxis systems are unperturbed. However, their performance in isothermal tracking is substantially degraded, resembling that seen when the thermosensory neuron AFD is removed. Transgenic experiments with altered ncs-1 genes show that its function in thermotaxis depends on its Ca2+ binding sites and on expression specifically in the interneuron AIY.Up to this point, these results are interesting but, one might argue, not particularly exceptional: NCS-1 functions in AIY in its familiar Ca2+ binding role to mediate thermosensory response. However, perhaps inspired by similar approaches to the study of Drosophila learning and memory (Yin et al., 1995xYin, J.C., Del Vecchio, M., Zhou, H., and Tully, T. Cell. 1995; 81: 107–115Abstract | Full Text PDF | PubMed | Scopus (484)See all References(Yin et al., 1995), the authors go on to make a very striking set of observations. Transgenic overexpression of normal NCS-1 protein from its own promoter enhances peak isothermal tracking accuracy, speeds the acquisition of a new thermal preference after temperature shift paired with food, and delays the extinction of thermal preference when food is removed. This is the kind of result every scientist dreams of: simple and compelling. It is hard to escape the conclusion that NCS-1 is a critical component of a process that mediates thermotaxis plasticity and memory. Are worms with more NCS-1 smarter? I doubt this is the right way to think about it. We can presume that the quality of isothermal tracking and the rate of change in thermal preference are adaptive traits. As with sensory attentiveness and long-term memory in humans, it is presumably important for worms to place a selectable weight on particular sensory information and to modify existing associations with appropriate deliberation.Naturally, it is tempting to speculate about the mechanistic role of NCS-1 in plasticity. However, the NCS family of proteins is sizeable, diverse, and largely unexplored. C. elegans appears to have five NCS-related genes and humans have perhaps a dozen (Burgoyne and Weiss, 2001xBurgoyne, R.D. and Weiss, J.L. Biochem. J. 2001; 353: 1–12Crossref | PubMed | Scopus (367)See all References(Burgoyne and Weiss, 2001), with different members implicated in processes as diverse as guanylyl-cyclase regulation (Palczewski et al., 1994xPalczewski, K., Subbaraya, I., Gorczyca, W.A., Helekar, B.S., Ruiz, C.C., Ohguro, H., Huang, J., Zhao, X., Crabb, J.W., Johnson, R.S. et al. Neuron. 1994; 13: 395–404Abstract | Full Text PDF | PubMed | Scopus (270)See all References(Palczewski et al., 1994), K+ channel modulation (An et al., 2000xAn, W.F., Bowlby, M.R., Betty, M., Cao, J., Ling, H.P., Mendoza, G., Hinson, J.W., Mattsson, K.I., Strassle, B.W., Trimmer, J.S., and Rhodes, K.J. Nature. 2000; 403: 553–556Crossref | PubMed | Scopus (698)See all References(An et al., 2000), and protein kinase inhibition (Chen et al., 1995xChen, C.K., Inglese, J., Lefkowitz, R.J., and Hurley, J.B. J. Biol. Chem. 1995; 270: 18060–18066Crossref | PubMedSee all References(Chen et al., 1995). A role in control of synaptic strength (e.g., Pongs et al., 1993xPongs, O., Lindemeier, J., Zhu, X.R., Theil, T., Engelkamp, D., Krah-Jentgens, I., Lambrecht, H.G., Koch, K.W., Schwemer, J. et al. Neuron. 1993; 11: 15–28Abstract | Full Text PDF | PubMed | Scopus (236)See all ReferencesPongs et al., 1993), though mechanistically not yet understood, is the most promising in explaining the current results. This NCS-1 function is likely to be in the interneuron AIY, but whether it functions pre- or postsynaptically, at what synapse, and by what biochemical mechanism are all unknown. The pump has been primed, but much remains to be investigated.