Calcium is widely known for its role in maintaining strong bones and teeth, but it is also one of the body's most important cellular messengers. Calcium signals help regulate muscle contraction, neural function, immune cell activation and many other physiological processes. Because cells rely on calcium signals to decide when and how strongly to respond, the movement of calcium must be tightly controlled.
At the cellular level, one major calcium signaling pathway is known as store-operated calcium entry, or SOCE. In this pathway, the endoplasmic reticulum—a major intracellular calcium store—acts like a sensor-and-supply system. When calcium levels inside the endoplasmic reticulum fall, the protein stromal interaction molecule 1 (STIM1) detects the change and activates ORAI channels in the plasma membrane. ORAI1 forms the pore of the calcium release-activated calcium channel, or CRAC channel, allowing calcium from outside the cell to enter the cytosol and trigger downstream signaling.
Understanding how this pathway works—and how it can be controlled when it doesn't—is the focus of research led by Yubin Zhou , director of the Center for Translational Cancer Research at the Texas A&M Health Institute of Biosciences and Technology and professor in the Texas A&M Naresh K. Vashisht College of Medicine . Working with collaborators Guolin Ma, PhD, of MD Anderson and Qing Deng, PhD, of Purdue University , Zhou's team recently published a study in Nature Communications describing engineered CRAC channel inhibitory binders, or CRABs, that can selectively interfere with STIM-ORAI communication and reduce calcium entry through CRAC channels.
Tien-Hung Lan, Texas A&M Health research project scientist in Zhou's lab and co-author in this study, emphasized the importance of calcium signaling for basic cell functioning.
"Calcium signals are essential for cells to function," Lan said. "There are several routes by which calcium can enter the cell, and CRAC channels are one of the major pathways. They are especially important in immune cells, including T cells."
T cells rely on CRAC channels to sustain calcium signals that activate transcription factors such as NFAT, which help drive immune cell activation and cytokine production. When this pathway is defective, immune cells may fail to respond properly. When it is excessive or chronically active, it can contribute to disease.
Previous research has shown that CRAC channel activity depends largely on two core components. ORAI1 forms the calcium-selective channel in the plasma membrane. STIM1, located in the endoplasmic reticulum membrane, senses when internal calcium stores are depleted. Once activated, STIM1 moves to ER-plasma membrane contact sites and binds ORAI1, opening the channel and allowing calcium influx.
"There have been ongoing efforts in the community to understand how STIM and ORAI proteins interact," Lan said. "While studying this interface, we realized that an ORAI-derived peptide could be used as a decoy to compete for STIM1 binding and prevent the endogenous channel from opening."
Competitive inhibition is a process by which a molecule can bind to a protein in a position meant for another molecule. Unlike a traditional channel blocker, the research team engineered peptide binders that prevent STIM1 from engaging ORAI channels and thereby reducing calcium influx and downstream signaling. To test whether these peptide binders could counteract pathological CRAC-channel activation, Zhou's lab used a zebrafish model of Stormorken syndrome. Stormorken syndrome is a rare multi-system disorder linked to excessive CRAC-channel activity. Patients may experience low platelet count ( thrombocytopenia ), bleeding problems, muscle weakness or cramping, miosis (pinpoint pupils) and other symptoms.
"With the gain-of-function mutations there will be consequences," Zhou said. "Cells will die because they're flooded by excess calcium. Second, it can cause muscle weakness and cramps, and many patients will also have bleeding issues."
In their study, the research team shows that their engineered binders—named CRABs, for CRAC channel inhibitory binders—specifically target the CRAC channel and help restore production of essential cells called thrombocyte progenitors, which are necessary for preventing abnormal bleeding.
Having designed the binders, Zhou's lab is now looking toward how the platform could be adapted for broader applications, including future strategies to improve cellular immunotherapies. CAR-T cell therapy has transformed treatment for some blood cancers, but safety and durability remain challenges. Excessive or chronic calcium signaling can contribute to tonic signaling, T cell exhaustion and cytokine production.
"There are significant side effects and durability challenges in CAR-T cell therapy, and calcium signaling is one pathway that can contribute to overactivation or exhaustion," Lan said. "If we can tune this pathway rather than permanently shut it off, we may be able to expand the therapeutic window and improve the performance of engineered immune cells."
What does this mean for patients diagnosed with CRAC-channel-related disorders or cancers treated with cellular immunotherapy? It could mean another step toward precision medicine. It offers a proof of concept for a more precise approach: genetically encoded, tunable control over a key calcium entry pathway with light or chemicals.
Still, Zhou said the findings point toward a broader goal in precision medicine.
"The long-term vision is to create molecular tools that can adjust cell signaling with precision," Zhou said. "CRABs give us a way to place an adjustable brake on T cell activity, which could be useful for studying disease mechanisms and, eventually, for designing safer and more controllable immune cell-based therapies."
By Ann M. McKelvey, Texas A&M Health
