Lessons from a worm

NEUROSCIENCE

Cornelia I Bargmann watches two colleagues manipulate a microscopic roundworm in her lab. They have trapped it in a tiny groove on a clear plastic chip, with just its nose sticking into a channel.

Pheromones – signaling chemicals produced by other worms – are being pumped through the channel, and the researchers have genetically engineered two neurons in the worm’s head to glow bright green if a neuron responds.

These techniques for exploring a tiny animal’s behaviour are the fruit of many years’ work by Bargmann’s and other labs. The study of a roundworm’s nervous system offers one of the most promising approaches for understanding the human brain.

Caenorhabditis elegans, as the roundworm is known, is a tiny, transparent animal, a millimeter long. It feeds on the bacteria that thrive in rotting plants and animals. It is a favourite laboratory organism for several reasons, including the comparative simplicity of its brain, which has just 302 neurons and 8,000 synapses, or neuron-to-neuron connections.

These connections are pretty much the same from one individual to another, meaning that in all worms, the brain is wired up in essentially the same way. Such a system should be considerably easier to understand than the human brain, a structure with close to 100 million neurons, 100,000 miles of biological wiring and 100 trillion synapses.

The task of reconstructing the worm’s wiring system fell on John G White, now at the University of Wisconsin. After over a decade’s labour, which required examining 20,000 electron microscope cross sections of the worm’s anatomy, White worked out how the 302 neurons were interconnected. Bargmann was one of the first biologists to take White’s wiring diagram and see if it could be understood in other ways.

Why the roundworm?

For her postdoctoral work, Bargmann decided to work on animal behaviour. The mouse is a standard for such studies, but she chose a nonfurry alternative, the fruit fly. She interviewed with a leading laboratory in California, but her husband at the time did not wish to move there. That left the roundworm. There are now several worm labs around the world, of which some like Bargmann’s, focus on the worm’s nervous system. She joined the lab of H Robert Horvitz who had established one of the first serious worm labs in the US, and read everything written on the worm.

She noticed that a particular behaviour of C. elegans had been described but not well explored: It can taste waterborne chemicals and move toward those it finds attractive. With White’s wiring diagram, published in 1986 in hand, she told Horvitz she planned to identify which of the worm’s 302 neurons controlled its chemical-tracking behaviour.

Bargmann discovered, by accident, the neurons that control the worm’s switch into hibernation. Finally, she found the neurons that control taste, showing that without them the worm could not track chemicals, and that it retained this ability even if she killed all the other neurons in the worm’s body.

Sense of smell, taste

She also discovered that the worms have a sense of smell as well as a sense of taste.
In 1991, Richard Axel and Linda Buck discovered the molecular basis for the sense of smell: There are about a thousand genes, at least in rats, that make odorant receptors, proteins that stud the olfactory nerves’ endings in the nose and respond to specific odours.

The C. elegans genome had just been decoded, and Bargmann was able to identify the worm’s odorant receptor genes. In fact, they have 2,000 of them, twice as many as the rat.

By working with mutant worms, she showed that a specific odour receptor recognises a specific odour. She found that worms with a mutation in a gene called odr-10 could not smell diacetyl, a chemical that gives butter its odour and is also produced by a bacterium that is a favourite worm food. The odr-10 gene, which makes the odour receptor protein that detects diacetyl, is active in neurons that guide the worm toward a scent.

Bargmann switched things around so that odr-10 was expressed only in a neuron that detected scents repulsive to the worm. These worms backed away from the buttery odour, showing that it is not the odour receptors but the wiring of the nervous system that determines whether the worm deems an odour delicious or detestable.

Having studied the worm by mutating its genes, Bargmann then looked at natural variation in the genetic basis of worm behaviour. Most worms like to congregate, but the laboratory version of C. elegans has developed an unusual liking for being on its own.

Role of RMG neurons

It turns out that social behaviour in the worm is controlled by a pair of neurons called RMG. The RMG neurons receive inputs from various sensory neurons that detect the several environmental cues that make worms aggregate. RMG integrates this information and sends signals to the worm’s muscles. After studying the worm for 24 years, she believes she is closer to understanding how its nervous system works.

Why is the wiring diagram produced by White so hard to interpret? The diagram shows the electrical connections that each of the 302 neurons makes to others in the system. These are the same connections made by human neurons. But worms have another kind of connection.

Besides the synapses that mediate electrical signals, there are also ‘gap’ junctions that allow direct chemical communication between neurons. The wiring diagram for the gap junctions is different from that of the synapses. Not only does the worm’s connectome, as Bargmann calls it, have two separate wiring diagrams superimposed on each other, but there is a third system that keeps rewiring the wiring diagrams. This is based on neuropeptides, hormonelike chemicals released by neurons to affect other neurons.

The neuropeptides probably help control the brain’s general status, or mood. A strong hint of how they work comes from the npr-1 gene, which makes a protein that responds to neuropeptides. When the npr-1 gene is active, its neuron becomes unavailable to its local circuit. That may be a reason why the worm’s behaviour cannot be computed from the wiring diagram: The pattern of connections is changing all the time under the influence of the worm’s 250 neuropeptides.

The human brain, too, has neuropeptides that set mood and modify behaviour. The human brain, though vastly more complex than the worm’s, uses many of the same components, from neuropeptides to transmitters.

Though the worm’s nervous system is described as simple, that is true only in comparison with the human brain. The work of Bargmann’s and other labs has deconstructed many of its operational mechanisms.