Background

In the 1970s, the field of molecular biology could not explain how the human immune system distinguishes countless pathogens to form antibodies. The principle of constructing billions of antigen-binding sites with limited genomic information was a major challenge in life sciences at the time. Under the classical genetic principle that inherited genes remain invariant, the germline theory and somatic mutation theory were in fierce opposition. How could the human body confront near-infinite antigens with a limited set of genes? With the technology of that era, it was difficult to track genetic changes within mature immune cells.

Even after elucidating the principles of the immune system, his focus did not rest. In the late 1980s, he moved to the Massachusetts Institute of Technology (MIT) in the United States and stepped into the unfamiliar territory of neurobiology. Where are memories physically etched as traces in the brain? The memory engram hypothesis, proposed in the early 20th century, remained an unprovable theory for nearly a hundred years. There was a lack of methodology to selectively identify and manipulate only the specific neural cell populations responsible for particular memories.

Key Discoveries

During his time at the Institute for Immunology in Basel, Switzerland, he closely compared the immunoglobulin gene structures of mouse embryonic cells and adult B cells from myeloma cell lines. The method involved separating deoxyribonucleic acid (DNA) fragments cut with restriction enzymes via electrophoresis and applying messenger ribonucleic acid (mRNA) hybridization analysis to track sequence differences between the two cell populations. The results shook the academic establishment. He confirmed that the variable (V) and constant (C) region genes, which are located far apart in the embryonic state, undergo physical recombination and join during B cell differentiation. This was the moment he first elucidated the mechanism of somatic recombination, in which gene segments are cleaved and rejoined during differentiation. In 1976, he demonstrated that the combination of V, D (diversity), and J (joining) segments generates hundreds of millions of antibody types, earning him the sole Nobel Prize in Physiology or Medicine in 1987.

In the field of neuroscience, genetic precision has been integrated into brain circuit analysis. In the 1990s, by creating mice with selectively deficient NMDA receptors in the hippocampal CA1 region, he clearly demonstrated the correlation between long-term potentiation (LTP) at synapses and spatial memory. In 2012, he perfectly proved the engram hypothesis in the laboratory by integrating optogenetics. He used an adeno-associated virus (AAV) to deliver an expression cassette containing the c-Fos promoter (a marker of neuronal activity) and channelrhodopsin-2 (ChR2) into the dentate gyrus of mice. The strategy was designed to ensure that blue light-responsive proteins were labeled only in cells activated by electrical stimulation. When mice with no prior learning experience were placed in a novel box and exposed to a 473-nanometer wavelength blue laser, they exhibited freezing behavior as they recalled fear memories. In 2013, this led to the achievement of creating false memories by combining light stimulation with new environmental stimuli, inducing the recall of non-existent memories.

Significance and Outlook

The elucidated principle of gene rearrangement has transformed the landscape of modern biotechnology and medicine. The fact that antibody-binding sites form through recombination facilitated the development of monoclonal antibodies that target specific antigens, serving as the foundation for today's anti-cancer antibody therapies and immunodiagnostic technologies. It was a decisive milestone in proving the functioning of the immune system against pathogens at the molecular level.

The legacy left in the field of neuroscience is also producing a strong impact. By defining memory not as an abstract concept but as a physically manipulatable collection of cells, he opened new paths for research into neurodegenerative diseases and post-traumatic stress disorder (PTSD). A representative example is follow-up research revealing that early memory loss in Alzheimer's disease stems from retrieval circuit abnormalities rather than storage defects.

High technical barriers remain for directly applying achievements obtained in animal models to the human brain. The challenge remains to determine whether the manipulation of neural networks in the hippocampal dentate gyrus fully represents the complex, long-term memory networks scattered across the entire cerebral cortex. Clinical limitations, such as inserting invasive optical fibers or injecting viral vectors into brain tissue, also need to be overcome. The academic footsteps of Professor Tonegawa, who has conquered uncharted territories with the rigor of molecular genetics, remain an eternal compass for future life science research.