Normally, phosphorylated IRBIT and Bcl2l10 act at the IP3 receptor to reduce flow of calcium ions from the endoplasmic reticulum (ER) to mitochondria, and suppress apoptosis. Dephosphorylated IRBIT pulls Bcl2l10 away from the ER membrane, which leads to increases the flow of calcium ions and thereby promotes apoptosis. In IRBIT knockout (KO) cells, the flow of calcium ions increases, but does not make it into the mitochondria because of smaller connections with the ER. [RIKEN]
Normally, phosphorylated IRBIT and Bcl2l10 act at the IP3 receptor to reduce flow of calcium ions from the endoplasmic reticulum (ER) to mitochondria, and suppress apoptosis. Dephosphorylated IRBIT pulls Bcl2l10 away from the ER membrane, which leads to increases the flow of calcium ions and thereby promotes apoptosis. In IRBIT knockout (KO) cells, the flow of calcium ions increases, but does not make it into the mitochondria because of smaller connections with the ER. [RIKEN]

A molecule that had been thought to keep cell death at bay can actually play the Grim Reaper, report scientists based at the RIKEN Brain Science Institute. The death-dealing molecule, IRBIT, has been known to pair with another molecule, Bcl2l10, which binds to receptors on mitochondrial membranes, restricting the flow of Ca2+ from the endoplasmic reticulum (ER) into mitochondria, and thereby suppressing Ca2+-induced cell death, or apoptosis. IRBIT may cooperate with Bcl2l10 in preventing apoptosis, or it may peel Bcl2l10 away, depending on whether or not it is phosphorylated.

The findings appeared December 20 in the journal eLife, in an article entitled, “IRBIT Controls Apoptosis by Interacting with the Bcl-2 Homolog, Bcl2l10, and by Promoting ER-Mitochondria Contact.” By clarifying the molecular events that lead to apoptosis, the new findings improve our understanding of the ways dysregulated apoptosis may have medical consequences.

“Excessive apoptosis in the brain is associated with several neurodegenerative diseases,” explained Katsuhiko Mikoshiba, the leader of the RIKEN team, “while impaired apoptosis is related to some cancers and tumor formation.”

Events that happen inside our cells are often controlled by interactions between proteins and modifications of proteins that change how they can interact with each other. These events have long been of interest to Mikoshiba and his team. In previous studies, these scientists showed that apoptosis in neurons can be initiated when certain proteins binds to IP3, or inositol 1,4,5-trisphosphate, receptors. In the new study, the team first showed that IRBIT and Bcl2l10 naturally attach to different places on the IP3 receptor, and that they attach to each other. This is why they initially thought that they work together to prevent apoptosis.

Further testing revealed an important event that always preceded apoptosis—IRBIT loses a phosphate group. Protein function is often altered through small additions or subtractions. The team showed that in this case, the loss of the phosphate group prevents IRBIT from staying attached to the IP3 receptor. Instead, it moves away from the ER and drags Bcl2l10 with it. When this happens, the team saw that without Bcl2l10, calcium flow into the mitochondria increased, which led to cell death.

“We found that these proteins [Bcl2l10 and IRBIT] associate in a complex in mitochondria-associated membranes (MAMs) and that their interplay is involved in apoptosis regulation,” wrote the authors of the eLife paper. “MAMs are a hotspot for Ca2+ transfer between endoplasmic reticulum (ER) and mitochondria, and massive Ca2+ release through IP3R in mitochondria induces cell death.”

Whereas Bcl2l10 and IRBIT interact and exert an additive inhibition of the IP3 receptor in the physiological state, their function is altered upon apoptotic stress. Essentially, IRBIT becomes dephosphorylated and then inhibits of Bcl2l10. Also, IRBIT promotes ER mitochondria contact.

“Our results suggest that by inhibiting Bcl2l10 activity and promoting contact between ER and mitochondria, IRBIT facilitates massive Ca2+ transfer to mitochondria and promotes apoptosis,” the authors continued. “This work then describes IRBIT as a new regulator of cell death.”

Understanding IRBIT's role in facilitating apoptosis has implications for treating cancer that is associated with low levels of IRBIT expression. “Because reduced IRBIT expression may contribute to tumor formation,” noted Benjamin Bonneau, the first author of the eLife paper, “the next step is to determine to what extent modulating IRBIT expression contributes to cancer formation, and if this is the case, what organs are the most sensitive.”

“Neurodegenerative diseases such as Huntington's disease and Parkinson's disease are characterized by excessive apoptosis,” added Mikoshiba. “Knowing IRBIT's involvement gives us a new target for investigation. As IRBIT is highly expressed in the brain, the chances are good that we will be able to find a connection, which could lead to new treatment possibilities.”