DATE2023.07.25 #Press Releases
What’s in a fish brain? Clues for the evolution of our brain
Each region in the cerebrum of medaka fish’s brain differs in the regulation of gene expression, which leads to specialized regions with high synaptic density.
July 25, 2023
A team of researchers from the University of Tokyo and Harvard University analyzed the structure of the Japanese medaka fish cerebrum and gene expression within its cells. The team found that gene expression in each region of the cerebrum differs, which allowed them to uncover a specialized brain region with more synapses and high information processing ability.
The cerebrum in the vertebrate brain consists of different regions, including the hippocampus and cerebral cortex (involved in memory and intelligence). The structure of the cerebrum varies among vertebrates, but the evolutionary origin is a mystery. To understand the evolution of the cerebrum, researchers study the cerebrum of different vertebrate ancestors such as fish. But a lack of definite brain regions in the cerebrum of fish model systems such as Zebrafish makes it challenging. So, instead, Yasuko Isoe, Ryohei Nakamura, and their team studied the structure of medaka rice fish’s cerebrum.
Figure 1. Diversity of the vertebrate cerebrum.
The team probed the structure of the cerebral cells during the development of the fish. They first genetically labeled neural stem cells at the early development stage. The stem cells can turn into any brain cells later in the development of the brain. Such neural cells derived from a single stem cell are called clones. When they visualized the development of clones over time, they found that each region in the cerebrum of adult fish had different combinations of clones.
Next, to understand the gene expression in these clones, the team studied their chromatin structure. Chromatin is a mix of DNA and protein folded to form chromosomes. An unfolded or open chromatin state leads to an active regulation in gene expression. And in a closed chromatin state, gene expression is silenced. The team found that each clone within the fish cerebrum differed in chromatin state. As a result, the clones differed in the synaptic regulation-related gene expression with implications for complex information processing. The dorsal cerebrum had higher synaptic density suggesting specialized information processing in the region. It may thus be involved in intelligence and memory, giving researchers a clue about the evolution of vertebrate cerebrum.
Figure 2. Analysis of chromatin structure in medaka cerebrum. The team extracted chromosomes from each clone and analyzed the chromatin structure. When the chromatin is open, gene expression is regulated actively, whereas when it is in close structure, gene expression is silenced. The team found that chromatin structure differs in each clone, which led them to find a specialized region with heavy regulation in synaptic genes and high density of synapses.
The study highlights the importance of Japan’s small fish in helping scientists understand the evolution of cerebral diversity. The team hopes to understand the role of the specialized brain region in complicated cognitive behavior, such as social behavior.
For more details, please read the article:
Yasuko Isoe, Ryohei Nakamura, Shigenori Nonaka, Yasuhiro Kamei, Teruhiro Okuyama, Naoyuki Yamamoto, Hideaki Takeuchi, Hiroyuki Takeda. 2023. Epigenetically distinct synaptic architecture in clonal compartments in the teleostean dorsal pallium. eLife. DOI: 10.7554/eLife.85093