A world-first knock-in mouse allows researchers to observe both the decline and recovery of male fertility within the same individual. While the image of 'glowing sperm' may sound unusual, the scientific motivation behind this model is straightforward: to measure spermatogenesis repeatedly, quantitatively, and without sacrificing animals.
An international collaborative research group including Associate Professor Hisanori Fukunaga (Deputy Director of the Center for Environmental and Health Sciences) of the Faculty of Health Sciences and Professor Hiroki Shirato of the Graduate School of Medicine at Hokkaido University, Associate Professor Haruhiko Miyata of the Research Institute for Microbial Diseases at the University of Osaka, and Professor Kevin M. Prise of Queen's University Belfast in the UK has succeeded in developing a new genetically modified animal model that enables real-time in vivo visualization of spermatogenesis in male mice.
Reproductive toxicity testing, which examines the effects of pharmaceuticals, environmental chemicals, radiation, and other agents on male reproductive function, is essential for new drug development and environmental risk assessment. Conventional approaches, however, have relied primarily on costly and labor-intensive methods requiring large numbers of animals, such as mating trials to confirm conception and dissection for histological analysis of testicular tissue.
The "Acr-Luc knock-in mouse" developed in this study emits light through a luciferin reaction in germ cells, allowing the spermatogenesis process to be measured non-invasively and continuously from outside the body using a specialized camera. This innovative model is expected to find diverse applications not only in streamlining reproductive toxicity testing but also in drug discovery research, environmental exposure assessment, and studies of male infertility risk following cancer treatment. As it can reduce the number of animals required, the model is also anticipated to serve as a technical foundation for aligning with the "Reduction" principle of the 3Rs in animal experimentation. The findings were published online on January 4, 2026, in the journal MedComm.
Provided by Associate Professor Hisanori Fukunaga, Hokkaido University.
Due to dissection being unavoidable for assessing uncertainties inherent in mating, pregnancy, and birth, as well as for determining at which stage of spermatogenesis reproductive toxicity occurs, reproductive toxicity testing requires large numbers of mature male and female animals (rodents). As a result, progress toward the internationally promoted "principles of the 3Rs," particularly "reduction" (reducing the number of animals used), has lagged behind in this field.
The research group used a genetic engineering approach in which the reporter gene Luciferase (Luc) was knocked into the locus of Acrosin (Acr), a protein expressed specifically during meiosis. This produced a genetically modified animal (the Acr-Luc knock-in mouse) in which germ cells react with luciferin and emit light in step with the progress of spermatogenesis. The model was then validated as a new method for reproductive toxicity assessment.
The group successfully developed the world's first Acr-Luc knock-in mouse, in which only the germ cells involved in spermatogenesis emit light upon luciferin administration, and established a new visualization technology for long-term, non-invasive in vivo observation of male reproductive function.
First, the team confirmed that bioluminescence intensity strongly correlates with germ cell number, demonstrating that the luminescent signal following luciferin administration serves as an indicator of spermatogenesis. They also confirmed that the mice maintain stable luminescence over a period of more than one year. This realized an innovative evaluation approach that allows physiological changes in spermatogenic function, chronic toxicity, and the effects of environmental exposure to be tracked in the same individual over a long period.
(A) Time-dependent changes in in vivo bioluminescence intensity in Acr-Luc knock-in mice exposed to 0 Gy, 5 Gy, or 10 Gy of X-ray irradiation.
(B) Comparison of luminescence intensity among groups at 4, 8, and 12 weeks after irradiation.
Provided by Associate Professor Hisanori Fukunaga, Hokkaido University.
Verification experiments using radiation exposure also succeeded in directly and continuously visualizing "radiation responses typical of the germ cell lineage" in vivo:
- Luminescence completely disappeared at four weeks post-irradiation, indicating arrest of spermatogenesis
- In the 5 Gy (absorbed dose) group, luminescence recovered between 8 and 12 weeks, showing reversible improvement of temporary infertility
- In the 10 Gy group, luminescence did not recover during the observation period, indicating irreversible damage.
Furthermore, by combining these findings with sophisticated testicular dose estimation using the particle and heavy ion transport code system, a technical foundation was established for integrated assessment of radiation dose and infertility risk.
These results demonstrate that the newly developed Acr-Luc knock-in mouse is a promising innovative preclinical platform that enables the visualization, quantitatively and over time, of the onset and recovery of reproductive toxicity, something that was difficult to achieve with conventional methods relying on mating trials and dissection.
The Acr-Luc knock-in mouse, with its unique feature of "glowing sperm," is an innovative model expected to be used across diverse fields-not only for streamlining reproductive toxicity testing, but also for drug discovery research, environmental exposure assessment, and evaluation of male infertility risk following cancer treatment.
In reproductive toxicity testing, the model is expected to greatly reduce the processes involving uncertainties such as mating, pregnancy, and birth, thereby contributing to a reduction in the number of animals used (realizing the 3Rs) and a dramatic improvement in evaluation efficiency. It may also serve as a useful complementary method in safety evaluations conducted under OECD Test Guidelines and ICH S5(R3). In drug development and risk assessment of environmental chemicals, the ability to visualize not only the presence or absence of reproductive toxicity, but also at which stage germ cells are affected and how they recover, is expected to help elucidate mechanisms of action. Furthermore, in research on cancer treatment and male infertility, the model's capacity to track spermatogenic dysfunction and recovery after radiation or chemotherapy with high precision could make it a foundational technology supporting the development of strategies for protecting and restoring reproductive function.
This work was supported by the JST FOREST Program (JPMJFR211E), JSPS KAKENHI (JP24K03079), and the Takeda Science Foundation.
Journal Information
Publication: MedComm
Title: Longitudinal Analysis of Male Fertility Using an Acr-Luc Knock-In Mouse Model: A Preclinical Platform for Reproductive Toxicity Testing
DOI: 10.1002/mco2.70568