Science Up Front

Science Up Front: Raju Tomer and Detlev Arendt on Worm Brains and the Evolution of the Cerebral Cortex

 

Kara Rogers - October 15th, 2010

The last place one might think to look for insights into the complexities of the human brain would be the brains of worms. But a tiny annelid is precisely where researchers Raju Tomer and Detlev Arendt, at the European Molecular Biology Laboratory in Heidelberg, Germany, stumbled upon the “mushroom body”—an invertebrate brain structure that is the equivalent of the vertebrate cerebral cortex, or pallium, which in humans handles complex cognitive functions, such as thought and memory.

 

The subject of Tomer and Arendt’s study, the marine worm Platynereis dumerilii. (Photo courtesy of EMBL/U. Ringeisen) 

Tomer and Arendt’s study, which was published in September in the journal Cell, offers an extraordinary glimpse into the history of the human cortex and suggests that this part of our brain the most evolutionarily ancient is actually far older than was assumed, possibly originating as many as 600 million years ago. At that time, the chordates, who would become the world’s most highly evolved animals, split from the invertebrate lineage.

To better understand the evolutionary origins of the cortex, Tomer and Arendt needed a living fossil, an organism little changed from its most ancient ancestors. That organism was the marine worm Platynereis dumerilii.

“Platynereis lives in the ocean, showing a complex life cycle with planktonic swimming larvae and worm-shaped adults that live in self-made tubes on algae,” Tomer said. “They actively explore their environment for food and show signs of learning.” He explained too that studies have shown that the worm has many ancient morphological and genetic characteristics.

 

A close-up of Platynereis dumerilii’s head. (Photo courtesy of EMBL/U. Ringeisen) 

When Tomer and Arendt probed deep into the brain of P. dumerilii, they discovered that the small mushroom bodies are connected to chemical sense organs, indicating that they process olfactory, or smell, information, a function that in humans is performed by the cerebral cortex. The ability of the mushroom bodies to process information about smell presumably allowed Platynereis to learn to distinguish between food and nonfood, which Tomer suspects gave it a competitive edge in the Precambrian sea.

According to Arendt, the worm’s mushroom bodies are made up of bundles of interconnected axon fibers that receive and transmit information. “From this we can conclude that the mushroom bodies function like huge sensory integration centers that also allow some sort of learning,” he said. “Thus, the basic principles of mushroom body function may well resemble those of a primitive cortex in lower vertebrates, which also functions as a center for integrating information from different senses, foremost smell.”

Tomer and Arendt made their discoveries using a technique they developed known as profiling by image registration (PrImR). “[PrImR] builds upon advanced algorithms for image registration and analysis,” explained Tomer. “The basic idea is to acquire microscopy images in two colors, with one color visualizing the scaffold of neurons in the brain and the other color the expression pattern of a given gene.”

By registering the activity of a large number of genes at cellular resolution in the whole organism, the team was able to distinguish different cell types in the mushroom bodies according to their so-called molecular fingerprints. Arendt said, “At the current state of analysis, we know that vertebrates and annelid worms show a very similar ‘molecular map’ of differentially expressed genes during brain development and that mushroom bodies and the cerebral cortex form at very similar positions in this conserved map.”

 

 A virtual Platynereis brain (left), created by averaging microscopy images of the brains of 36 different individuals, onto which gene activity was then mapped (right). Perspective shows the brain as viewed from inside a 48-hour-old Platynereis larvae. (EMBL/R. Tomer)

In other words, mushroom bodies and the human cerebral cortex both express specific combinations of genes that encode proteins needed for brain development. “For example, the annelid mushroom bodies include cells that specifically express a transcription factor called emx,” Arendt noted. Transcription factors are proteins that help cells decipher the information contained in DNA. Emx in particular is known to be active in the developing vertebrate cortex and in the insect brain.

Emx is an ancient molecule, which Arendt said occurred in the common ancestor of jellyfish and animals with bilateral symmetry—animals like ourselves. But in our most remote ancestors, the types of cells that expressed emx, the functions these ancestral cells performed, and how they ended up in the mushroom bodies of Platynereis and in the modern human cortex are questions yet unanswered.

Mushroom bodies are found in only a handful of animals, namely insects, spiders, and annelid worms. “Other groups, such as crustaceans, nematodes, and mollusks, don’t have them,” Arendt explained. “If the vertebrate cerebral cortex and the annelid mushroom bodies are indeed evolutionarily ancient and trace back to a common precursor structure, then we expect mushroom body-like brain centers also in these other groups, including, for example, cuttlefish or crabs.”

So, current understanding of the evolutionary history of the cerebral cortex contains an extraordinary gap. This gap is likely associated with constraints placed on the brain as a result of an animal’s evolutionary environment. But Tomer and Arendt are hopeful that they will be able to fill in at least some of the missing links.

“With our study we have defined a set of genes that should, by specific expression, unravel [brain] centers [in other animals] that do not look like mushroom bodies but should perform similar functions,” Arendt added. “This way new insight would be gained about the morphological plasticity of conserved brain centers due to environmental constraints.”