I’ll admit. I’m not a worm person.
Don’t get me wrong, I am well-versed in the value of C. elegans to science in general and neuroscience in particular. A worm is a compact circuit of identifiable neurons, a bundle of highly-documented genetic material, and shows surprisingly robust behavioral states. My bias against worms–I like my model organisms warm and fuzzy–is somewhat symbolic of the larger mismatch between basic science and public interest. Namely, C. elegans seem so far removed from my own physiology that I can’t help but wonder how significant such research will be for my narcissistic self.
In a 2011 TED talk, titled “Lost in Translation”, Dr. Daniel Colón-Ramos gives an eloquent defense for the importance of basic science research. He discusses a conundrum in science, one I encounter often when I try to describe my research to people who are not academics. As he says, “There is a huge disconnect between what people like about science,” namely outcomes that make our lives safer, easier, and healthier, “and what scientists actually do.” There is a perception, particularly for the life-sciences, that all research should be specifically to solve a problem, or cure a disease. A perception that if science is not actively trying to improve our lives then it is not worth pursuing.
Occasionally statements like this come from confused friends and family members, and these concerns I’m happy to allay. But such sentiments are worrying when they come from certain public figures or politicians. As an answer to those who wonder why all this federal money goes to research on things like fruit flies and “bacteria sex,” Colón-Ramos discusses a series of experiments on his model organism of choice: C. elegans. He describes the work of Brenner, Holvitz, and Sulston, whose work in mapping the lineage of C. elegans cells led to discovering the genetic regulation of cell death. Work that has significant implications for cancer research, and for which they shared the 2002 Nobel Prize in physiology. “Is this example exceptional?” Colón-Ramos asks. “It’s exceptional in its outcome, but it’s not exceptional in its trajectory. This is how basic research is done.”
In discussing the meandering nature of basic research, Colón-Ramos poses the question: why does basic science work this way, and why are research projects not more linear? “The reason is because we are explorers. We don’t have the answers.” I find my inner expeditionary warmed at the thought.
The Colón-Ramos lab’s sliver of neuroscience frontier is understanding how synapses and neural connections are formed. In their 2013 paper, “Synapse location during growth depends on glia location,” they investigate how specific synaptic connections are managed during substantial physical growth. Synapses are often formed early in development, but as animals continue to grow the neurons must maintain specific connections. For example, C. elegans undergoes a four-fold increase in growth between larva and adult, yet must maintain the synaptic contacts they established prior to this expansion.
Despite my comments above, I do see great value in C. elegans as a model organism. Genetic manipulations that would be prohibitive in higher order animals–in terms of time, cost, and techniques–are readily available in nematodes. This allows a more granular investigation of synapse formation and glia interaction than in another system.
In this paper, they investigate abnormal synaptic contacts found at a symmetric pair of nematode interneurons called AIY–because its C. elegans and you can name every neuron–that are thought to play a role in integrating different sensory inputs. The aberrant synaptic connections are observed in a mutant known as cima-1, a loss-of-function mutant named for “circuit maintenance.”
Figure 1: (A) Schematic diagram of AIYs (grey) in the C. elegans head. Green marks presynaptic sites. They identify three regions along the AIY neurite: one near the AIY cell body that is devoid of synapses (Zone 1, dashed box); and two regions with substantial synaptic connections (Zones 2, 3). (E,I) Presynaptic regions marked with the protein GFP:SYD-1. (J) Quantification of the AIY presynaptic pattern. The length of the ventral presynaptic regions (Zones 1, 2) increases with age. (K) The length of ventral presynaptic region divided by total presynaptic region is shows the general pattern of AIY synapses.
The output of the AIY neurons is determined very early. Synapses are formed during embryogenesis, as the Colón-Ramos lab observed by tagging the synapses with GFP (Figure 1E,I). While the length of this region of the neuron increases as the animal grows from larval-stage 1 (L1) to adult, the pattern of synapses across this length remains constant in the wild-type (WT) worm (Figure 1 J,K).
Normally “Zone 1” (figure 1A) contains no synapses, but in the cima-1 mutant novel synapses form in this zone as the worm grows larger. This indicates cima-1 is involved in preventing ectopic, unwanted synapses during this post-developmental growth phase. These AIY neurons normally synapse on to a postsynaptic partner, a pair of neurons known as RIA. However, the ectopic synapses in the cima-1 mutant do not contact the RIA neuron. So what is inducing these synapses? It turns out the answer is every neuroscientist’s favorite nonneuronal cell: glia!
In the adult worm, cima-1 is largely expressed in epidermal cells (4A), but not neurons. In cima-1 mutants, expression of functional cima-1 cDNA rescues the normal synaptic pattern, abolishing the ectopic connections, when the cDNA is added to epidermal cell lineages (4E), but not when added specifically to AIY neurons, or to all neurons.
Figure 4: (A) A larval animal displaying the endogenous cima-1 expression. (E) Quantification of tissue-specific rescue. Expression of cima-1 cDNA in AIY, or pan-neuronally does not prevent abnormal synapses. However, expression of cima-1 cDNA in epidermal cells does
Thus cima-1 expression is necessary to prevent weird, unwanted synapses as the worm grows larger. However, this cima-1 expression is in epidermal cells, not neurons. These epidermal cells don’t directly contact the AIY neurons. Instead, the ventral cephalic sheath cells (VCSC), which are nematode glia similar to vertebrate astrocytes, are located between the cima-1 expressing epidermal cell and the AIY interneurons (figure 5A).
The cima-1 mutants VCSCs have abnormally long endfeet that extend into Zone 1, where the ectopic AIY synapses form (figure 5M). The wacky endfeet show a strong correlation to the emergence of abnormal synapses. When cima-1 is expressed only in epidermal cells, the normal glia shape is rescued. Thus cima-1 in epidermal cells maintains proper glia morphology during growth.
Figure 5: (A) A cross section EM image of a wild-type animal in the Zone 2 region of AIY. VCSC glia (red) lie between the epidermal cells (purple) and AIY Zone 2 synapses (green). (E, I, M, and
Q) Are cartoons of data showing simultaneous visualization of AIY presynaptic sites (green) and VCSC glia (red) in each experiment type and stage. In Q, rescue occurred by expressing cima-1 cDNA in epidermal cells
So, what does the cima-1 in epidermal cells actually do? It appears to negatively regulate the fibroblast growth factor known as EGL-15(5A). Losing EGL-15 in the cima-1 mutants restores glia morphology and prevents the ectopic synapse formation. Going the opposite direction, expressing excess EGL-15 in wild-type animals, induces the wacky glia and abnormal synaptic sites in the AIY neuron (figure 7L).
Figure 7: Schematic for the interaction between cima-1 and egl-15(5A) in epidermal cells (purple) regulating VCSC glia (red) morphology and AIY synapses (green).
The Colón-Ramos lab establishes that the cima-1(wy84) allele corresponds to a missense mutation in the unnamed gene F45E4.11–you can’t name everything, even in nematodes, apparently. It codes for a transmembrane protein, belonging to a family of solute-carrier transporters. Although the specific cargo for the CIMA-1 protein is not yet known, the lab hypothesizes that the membrane transporter moves acidic monosaccharides that modify or regulate EGL-15 levels.
Tl;dr this paper identifies the cellular and molecular mechanism for maintaining the distribution of presynaptic sites during C. elegans growth, after synaptic contacts have been made.
If you are interested in hearing more about how C. elegans manage their synapses, come see Dr. Colón-Ramos’ DART seminar, “Molecular mechanisms of synaptic development and function: lessons from C.elegans,” November 17 @ 4:00pm, CNCB.
And don’t miss Journal Club, November 16 @ 5:00pm, when Alie Caldwell will present this paper and discuss Daniel Colón-Ramos’ outreach/science communication work.
- Shao et al. “Synapse location during growth depends on glia location” (2013) Cell 154, 337-350.
- “Lost in Translation: the Value of Basic Research in Medicine” (2011) TEDxSanJuan *Bonus: it includes a picture of his adorable triplet daughters scientifically investigating the edibility of sand
Bethanny Danskin is a first-year student in the Neurosciences PhD program at UCSD. She is currently rotating with Dr. Byungkook Lim, and is interested in tracing and manipulating in vivo circuits. She appreciates that frontiers are now knowledge-based, which she can explore from the comfort of her blankets. She can occasionally be found sidewalk-ranting at passersby about the importance of basic science.