The human brain contains around 100 billion neurons. The connections between these tiny neurons are called synapses, which interconnect to form various neural circuits. Neural circuits carry out specific functions, such as transmitting visual information from the eyes, auditory information from the ears, and motor functions for moving the body. The communication between the neurons in these neural circuits is what enables us to think and move our bodies.

Advances in optical microscopy has made it possible to directly visualize synapses in the brains of living animals, which has revealed that synapses continue to change throughout life.

“From childhood to adulthood, synapses are created with astonishing speed. But it is not the case that once something is created, it remains intact. During the developmental stage, extra synapses are produced, but the brain starts to remove unused connections in a natural process known as synaptic pruning. In autism spectrum disorders, for example, this process is thought to go awry, due to an increased number of synapses, or synapses not being reduced where they need to be.”

Researchers have also learned how synapses are altered by learning and disease.

“As we trained animals to learn new things, we observed that new synaptic connections would be formed or broken. Synapses in more active areas are strengthened and unused synapses are removed, though not to the extent that they are during childhood. It has also been shown that in Alzheimer's disease, synapses disconnect before nerve cells die. You could say that the synapse is the at the very root of what makes us human.”

It is thought that the imbalance in the way synapses are connected can cause some neuropsychiatric disorders and that synapses that remain disconnected can lead to neurological disorders such as spinal cord injury. These diseases, including autism spectrum disorders, schizophrenia, and Alzheimer's disease, are referred to as “synaptopathy.”

When the synapses connecting a neural circuit break, it is like a bridge across a river splits in two, and the neural circuit breaks down.

What is the material that acts as a bridge connecting these neurons? Researchers call this group of substances “synaptic organizers,” and significant progress has been made on their research in the last decade or so. In 2005, Professor Yuzaki’s research team discovered a synaptic connector, cerebellin (Cbln1), and published their findings in the scientific journal Nature Neuroscience.

“Cbln1 is a naturally occurring molecule. If we think of Cbln1 as a ‘key,’ then the neural cells that bind to it have ‘keyholes’ made of protein molecules. The keyholes on both sides of the synapse, and the Cbln1 between them, connect together just like Lego bricks.”

While studying these “keyholes,” Prof. Yuzaki’s team noticed that mice that become "keyhole free" from disabling (or “knocking out”) the gene that controls the keyhole molecule, and mice that became “keyless” by knocking out the gene that controls the expression of Cbln1 “key,” waddled in the same way.

“We thought that perhaps Cbln1 was binding to the keyholes of both sides at the same time, holding the neurons together to form synapses.”

Discovering that Cbln1 was a synaptic organizer, Prof. Yuzaki and his team went about elucidating its structure.

Neural circuits, like electrical circuits, convey information in the form of electrical signals. However, in an electrical circuit, electrical signals flow uninterruptedly, whereas in a neural circuit, the flow of electricity ceases once it reaches the synapse. When an electrical signal is transmitted to a presynapse, chemical messengers known as neurotransmitters that are stored in the cell get released and are then received at the postsynapse and converted back into an electrical signal. There are two types of synapses: those that excite and those that inhibit. Glutamate is used as a neurotransmitter at excitatory synapses, and there are glutamate receptors in the postsynaptic region to receive it. Interestingly, at excitatory synapses in the cerebellum, Cbln1 is also released from the presynaptic area via a pathway separate from glutamate. Cbln1 is the key to the formation of cerebellar synapses, simultaneously binding to presynaptic neurexin (Nrx) and postsynaptic glutamate receptor D2 (GluD2). (See Fig. 1, left)

“Cbln1, when administered to the cerebellum of mice, can create synapses in one to two days. But GluD2, the “keyhole” required for Cbln1 to work, is only present in the cerebellum. Therefore, we focused on AMPA-type glutamate receptors (GluA), which is present at most excitatory postsynaptic sites in the brain and spinal cord.”

Another synaptic organizer called neuronal pentraxin (NP1) is known to induce clustering of postsynaptic GluA. However, NP1 does not have a “keyhole” that connects to postsynaptic sites.

“We thought that if we could spit Cbln1 and NP1 in half and artificially combine the Nrx-binding elements of Cbln1 with the postsynaptic GluA-binding elements of NP1, we could create a synaptic organizer that would work in all areas of the brain and spinal cord. (See Fig. 1, right) This is what led to the creation of the synthetic synaptic organizer CPTX (Cbln1 + neuronal pentraxin-1).

When Prof. Yuzaki and his team confirmed the effects of CPTX in mice, they could not believe how effective it was. (See Fig. 2) A single dose of CPTX administered to mouse spinal cord injury models significantly accelerated the restoration of motor functions in their hind limbs over a period of two months. These were breakthrough results compared to conventional treatments.

“We also tried administering CPTX to mouse models for Alzheimer's disease. These mice couldn't at all remember where their food was and would wander around looking for it whenever they were hungry, but with a single dose of CPTX injected into the hippocampus, synaptic formation was restored within days, dramatically improving their memory recall.”

Mouse models for cerebellar ataxia were again able to walk in a balanced and stable manner after being treated with CPTX.

“I never expected that CPTX would be so effective. There are still many issues left to be addressed, but we plan to conduct further research for clinical trials.”

The development of CPTX, the world's first synthetic synaptic organizer, is the result of international collaboration conducted between Prof. Yuzaki's research team at Keio, Aichi Medical University, and Hokkaido University as well as research teams in the UK and Germany. Although international research collaborations can often involve many hardships, Prof. Yuzaki says that he really enjoyed the experience.

Prof. Yuzaki worked as a Principal Investigator at St. Jude Children's Research Hospital in the U.S., where he tried to understand how GluD2 functions in the cerebellum where it was so highly expressed. Prof. Yuzaki began his research into Cbln1 on a whim after a neighboring researcher asked him to look into Cbln1, which was also highly expressed in the cerebellum with no known functions.

“One day, out of the blue, I received an e-mail from Dr. Radu Aricescu, a joint researcher who was involved in this study. At the time, he was based out of the University of Oxford in the UK. He was interested in the structure of glutamate receptors and wanted to use GluD2 as a model. I didn't know Dr. Aricescu at all, but after an initial video chat, we immediately hit it off.”

Cbln1 is made in the nerve cell and then released and accumulates in the synapse. This work led Prof. Yuzaki to an international conference where he had a chance to speak with German researcher Dr. Alexander Dityatev, who has spent years studying the extracellular matrix, proteins outside of neurons. Dr. Dityatev had been conducting research on Alzheimer's disease using animal models.

Prof. Yuzaki and his team later discovered that Cbln1 and GluD2, which were thought to be completely unrelated, work as synaptic organizers in the cerebellum like a key and a keyhole. They published their findings in Science in 2010.

“I thought that if the three of us, each with different areas of expertise, could combine our efforts, we could do something interesting. Although the three of us had not yet all met in person, we were excited by the idea of developing a new synthetic synaptic organizer based on characteristics of Cbln1. That is how our work at Keio became an international collaboration between Japan, Germany, and the UK.”

Prof. Yuzaki says that without any one of these research teams, they could not have made it this far.

“Every year at our lab’s welcome party, I always say the same thing: "We are in the same boat." This is because I believe that our ultimate goal as researchers is to give back to society through our research findings and contribute to culture and the welfare of humankind. You could say that we are all sailors of the same ship. In this study, the UK team demonstrated their ability in clarifying the crystal structure of proteins, and the German team clarified how CPTX worked in the Alzheimer’s disease model mice. I really enjoyed working together with this multinational team that was brought together around Cbln1.”

The driving force of science is curiosity and fun. To convey the fascinating aspects of neuroscience to the next generation, Prof. Yuzaki has gotten involved with the NPO Brain Century Promotion Conference, organizing tours of Keio's brain-related laboratories for high school students and assisting with the International Brain Bee for junior and senior high school students.

“Prof. Shinzo Koizumi, a former president of Keio University, always said this when talking about academics: ‘What immediately becomes useful soon becomes useless.’ If I think too much about conventional treatments, my ideas won’t expand much beyond them. I would like young people to engage in research with great curiosity in the hopes that it can one day be used to treat disease. Of course, I believe it is important to conduct research that can be immediately useful to patients, but it is also critical to start with basic research in order to create something that will be beneficial in the long term.”

CPTX was initially created by modifying synaptic organizers in the human brain, and it is thought that its administration in human patients may produce similar effects as in mice.

“I’d first like to make CPTX widely available as a medicine,” Prof. Yuzaki says.

He and his team are now working to overcome various hurdles and safety issues in order to conduct clinical trials in humans. This is one of many medical challenges, once thought to be impossible, that is one step closer to being solved.

Michisuke Yuzaki
In 1985, Prof. Yuzaki graduated from the School of Medicine at Jichi Medical University, Tochigi, Japan (M.D.) and had his credentials verified by the Educational Commission for Foreign Medical Graduates (ECFMG) in the USA. In 1993, he obtained a Ph.D. from the Graduate School of Medicine, Jichi Medical University, and worked in the USA at the Roche Institute of Molecular Biology as an HFSP Long-term fellow. In 1995, he was appointed as an Assistant Professor at St. Jude Children's Research Hospital in Memphis, Tennessee, USA, and became an Associate Professor in 2002. Since 2003, Prof. Yuzaki has served as a Professor at the Keio University Medical Department of Neurophysiology. In 2020, he was appointed as President of the Japan Neuroscience Society