New Model: Calcium Release Not Vital for Muscle Movement

The mechanism of skeletal muscle contraction is a process that relies on calcium signaling. However, the physiological role of calcium-induced calcium release (CICR) through the ryanodine receptor type 1 (RyR1) has remained unresolved for decades. A new study led by Associate Professor Takashi Murayama from Juntendo University in Japan, along with his team members Drs. Takuya Kobayashi and Nagomi Kurebayashi from Juntendo University, and Dr. Toshiko Yamazawa from Jikei University School of Medicine, published in Volume 122, Issue 34 of the journal PNAS on August 21, 2025, provides compelling evidence that CICR plays a minimal role in normal muscle contraction but is a major driver of life-threatening muscle disorders.

The research team developed a genetically engineered mouse model carrying a mutation (E3896A) in RyR1, which disrupts its calcium-binding site, effectively disabling CICR while preserving depolarization-induced calcium release (DICR), the well-established mechanism responsible for initiating muscle contraction. "This approach allowed us to selectively inhibit CICR in vivo without altering DICR, a critical advantage over previous models," explains Dr. Murayama.

Experiments on muscle fibers confirmed that the mutation completely abolished CICR activity without altering calcium transients triggered by electrical stimulation. Ex vivo testing of both fast-twitch (extensor digitorum longus) and slow-twitch (soleus) muscles showed no impairment in twitch or tetanic force generation. In vivo assessments, including grip strength measurements, wheel-running behavior, and CT scans of body composition, revealed no significant differences between mutant and wild-type animals.

While CICR was dispensable for normal physiology, the study revealed its critical role in disease. The researchers crossed the E3896A mice with a malignant hyperthermia (MH) model carrying the RyR1 R2509C mutation, known to cause life-threatening reactions to anesthesia and environmental heat stress. Remarkably, introducing the E3896A allele reversed hallmark MH features, including heightened resting calcium levels, exaggerated halothane-induced calcium release and severe caffeine-triggered contractures. In live animals, this double mutation dramatically reduced fatal hyperthermia episodes and improved survival during both isoflurane anesthesia and heat stress tests. "These results clearly support the contribution of RyR1-mediated CICR to thermogenesis in disease states," says Dr. Murayama.

The study also sheds light on evolutionary and physiological questions. RyR1 channels are arranged in skeletal muscle in a pattern where only half are physically coupled to voltage sensors, prompting researchers to propose that CICR may amplify calcium signals during contraction. This genetic model suggests otherwise, indicating that coupled gating or other mechanical interactions, rather than CICR, drive activation of uncoupled RyR1 channels. Interestingly, the researchers observed that the soleus muscle fibers were slightly smaller in mutants, hinting at a possible role for CICR in long-term muscle adaptation, hypertrophy, or metabolic regulation under stress conditions.

Beyond its implications for malignant hyperthermia, this research provides a valuable model to study other RyR1-related disorders, such as muscular dystrophy, cancer-associated muscle weakness, and age-related sarcopenia. Because RyR1 is also expressed in neurons, immune cells, and vascular smooth muscle, the E3896A model offers a unique tool to examine CICR's broader physiological roles.

By combining gene engineering, Ca²⁺ imaging, and detailed functional tests, the team shows with high precision that skeletal muscle contraction is mainly DICR-driven. This long-accepted mechanism is now confirmed at the molecular level, while CICR, once thought to boost Ca²⁺ signals, proves largely unnecessary in baseline muscle function. Instead, CICR acts as a dormant but dangerous silent risk factor, staying quiet under normal conditions yet becoming a major driver of pathology when RyR1 signaling is disrupted.

This model resolves a decades-old debate in muscle physiology by separating CICR from DICR without altering the core excitation–contraction (E–C) coupling pathway. The E3896A RyR1 mutation offers a powerful genetic tool that outperforms previous pharmacological or knockdown models, enabling researchers dissect Ca²⁺ flux at a channel-specific level. These findings do more than clarify theory; they pave the way for next-generation therapies to modulate RyR1 selectively, with direct applications in MH, congenital myopathies, and Ca²⁺ leak-driven muscle damage. The same model also allows exploration of CICR in neurons, smooth muscle, and immune cells, expanding its clinical impact. With this insight, drug design can now focus on precision control of Ca²⁺ release, lowering risk of hyperthermic crises, slowing sarcopenia, and rethinking the fine balance between robust E–C signaling and disease-prone Ca²⁺ dysregulation.

Reference

About Associate Professor Takashi Murayama from Juntendo University, Japan

Associate Professor Takashi Murayama is a leading researcher in muscle physiology and calcium signaling, currently based in the Department of Pharmacology, Juntendo University School of Medicine, Japan. With over 90 publications, he has made major contributions to understanding the ryanodine receptor (RyR) function and its role in skeletal muscle health and disease. His current research focuses on developing therapeutic strategies for RyR-related disorders, including malignant hyperthermia and muscle weakness syndromes. Dr. Murayama's work integrates molecular biology, pharmacology, and physiology, positioning him as a key figure in advancing targeted treatments for calcium channel-associated diseases.