Major depressive disorder (MDD) is one of the most common psychiatric illnesses worldwide, but its molecular causes* have still not been clearly identified. A domestic research team has discovered that depression may not simply be caused by neuronal damage, but can also arise from the dysregulation of specific neural signaling pathways. In particular, they identified the molecular reason why elderly patients with depression do not respond to conventional antidepressants. This study suggests the possibility of therapeutic approaches using optogenetic technology to regulate neural signaling, and it provides clues for the development of new treatment strategies targeting the protein ‘Numb’ protein for elderly patients with depression.

*Molecular causes: explanations for the origin of a disease at the level of molecules, proteins, or genes in the brain.

KAIST (President Kwang Hyung Lee) announced on the 19th of August that a research team led by Distinguished Professor Won Do Heo of the Department of Biological Sciences at KAIST, in collaboration with forensic pathologist Minju Lee of the National Forensic Service (Director Bong Woo Lee) and Professor Seokhwi Kim of the Department of Pathology at Ajou University Medical Center (Director Sangwook Han), identified a new molecular mechanism for depression through RNA sequencing and the immunohistochemical analysis of brain tissue from patients who had committed suicide. Furthermore, they demonstrated in animal models that antidepressant effects can be restored by regulating the signaling pathway that induces neural recovery using optogenetic technology.

The research team focused on the hippocampus, the brain region responsible for memory and emotion, and in particular on the dentate gyrus (DG). The DG is the entry point of information into the hippocampus, playing a role in new memory formation, neurogenesis, and emotional regulation, and is closely linked with depression.

Using two representative mouse models for depression (the corticosterone stress model and the chronic unpredictable stress model), the team found that stress induced a striking increase in the signaling receptor FGFR1 (Fibroblast Growth Factor Receptor 1) in the DG. FGFR1 receives growth factor (FGF) signals and transmits growth and differentiation commands within cells.

Subsequently, using conditional knockout (cKO) mice in which the FGFR1 gene was deleted, the researchers revealed that the absence of FGFR1 made mice more vulnerable to stress and led them to exhibit depressive symptoms more quickly. This indicates that FGFR1 plays a critical role in proper neural regulation and stress resistance.

The team then developed an ‘optoFGFR1 system’ using optogenetics, enabling FGFR1 —essential for stress resistance—to be activated by light. They observed that activating FGFR1 in depression mouse models lacking FGFR1 restored antidepressant effects. In other words, they experimentally demonstrated that the activation of FGFR1 signaling alone could improve depressive behavior.

Surprisingly, however, in aged depression mouse models, the activation of FGFR1 signaling through the optoFGFR1 system did not yield antidepressant effects. Investigating further, the researchers found that in the aged brains, a protein called ‘Numb’ was excessively expressed and interfered with FGFR1 signaling.

Indeed, analysis of postmortem human brain tissue also showed the specific overexpression of Numb protein only in elderly patients with depression. When the researchers suppressed Numb using a gene regulatory tool (shRNA) while simultaneously activating FGFR1 signaling in mouse models, neurogenesis and behavior—previously unrecoverable—returned to normal even in aged depression models. This shows that the Numb protein acts as a “blocker” of FGFR1 signaling and is a key factor preventing the hippocampus from executing antidepressant mechanisms.

Distinguished Professor Won Do Heo of KAIST said, “This study is meaningful in that it revealed that depression may not only result from simple neuronal damage, but can also arise from the dysregulation of specific neural signaling pathways. In particular, we identified the molecular reason why antidepressants are less effective in elderly patients, and we expect this to provide a clue for the development of new therapeutic strategies targeting the Numb protein.”

He added, “Moreover, this interdisciplinary study, which combined KAIST’s expertise in neuroscience with the National Forensic Service’s forensic brain analysis technologies, is expected to serve as a bridge between basic research on psychiatric disorders and clinical applications.”

This study, led by first author Jongpil Shin, a PhD student in the Department of Biological Sciences at KAIST, was published on August 15, 2025, in the international journal Experimental & Molecular Medicine.
Paper title: “Dysregulation of FGFR1 signaling in the hippocampus facilitates depressive disorder”
DOI: https://doi.org/10.1038/s12276-025-01519-9

This research was supported by the Ministry of Science and ICT’s National Research Foundation of Korea through the ASTRA program and the Bio-Medical Technology Development project.