St. Jude researchers find key step in programmed cell death (news release)

Investigators at St. Jude Children’s Research Hospital have discovered a dance of proteins that protects certain cells from undergoing apoptosis, also known as programmed cell death. Understanding the fine points of apoptosis is important to researchers seeking ways to control this process.

In a series of experiments, St. Jude researchers found that if any one of three molecules is missing, certain cells lose the ability to protect themselves from apoptosis. A report on this work appears in the advance online publication of “Nature.”

“This is probably the first description of what is happening mechanistically that contributes to the ability of cells to delay apoptosis,” said James Ihle, Ph.D., the paper’s senior author and chair of the St. Jude Department of Biochemistry. “It provides incredible insights into how three proteins work and how they can control apoptosis.”

The molecular interactions that St. Jude researchers describe in “Nature” play out in nerve cells and blood cells that develop from hematopoietic (blood-forming) stem cells.

A research team elsewhere recently reported that Kostmann’s syndrome, a potentially fatal inherited deficiency of granulocytes in children, caused by excessive apoptosis of granulocytes, results from a deficiency in one of the three proteins, called Hax1.

“This suggests that the protein is playing basically the same role in humans as we described in mice,” Ihle said.

Apoptosis rids the body of faulty or unneeded cells. However, molecular malfunctions that trigger apoptosis may cause some diseases, including Parkinson’s disease. Understanding the biochemical interactions that control the extent of programmed cell death could lead to new treatments.

St. Jude biochemists have long studied how cytokines—small proteins used by neurons and blood-borne cells to communicate messages—contribute to keeping cells alive. For example, they demonstrated earlier that most cytokines controlling hematopoietic cells require an enzyme called Jak2, or Jak3 in lymphocytes, at the receptors where cytokines attached to the cell.

In screening for components that are regulated by the Jak enzymes, the St. Jude team found the Hax1 protein.

“That was intriguing because several studies suggested that Hax1 was controlled by cytokine signaling,” Ihle said. “Also, studies have suggested that if you overexpressed Hax1 in cells, the cells were protected from undergoing apoptosis.”

To pursue this lead, the researchers genetically engineered mice that lacked the gene for Hax1. The results showed that apoptosis in the animals’ brain caused extensive nerve cell degeneration that killed the mice within 10 to 12 weeks. Second, apoptosis in immune-system lymphocytes occurred in the altered mice eight hours sooner than in those with the Hax1 gene, when limited amounts of cytokines were available.

“That additional window of survival is extremely important because in the body, cytokines are limiting.” Ihle said. “The key observation was that Hax1 was important in helping cells to survive. Importantly, what happened to the mice we generated was remarkably similar to what happens if you remove the mitochondrial enzymes called HtrA2 or Parl.”

Exploring the similarities, the investigators found that Hax1 and Parl pair up in the inner membrane of the mitochondria—tiny chemical packets that serve as the main energy source for cells. HtrA2 is made in the cell’s cytoplasm and is transported into the mitochondria, where the enzyme must have a region removed for it to be active. This requires snipping away 133 amino acids, the building blocks of proteins. The St. Jude researchers demonstrated that it is the Hax1/Parl pair that positions HtrA2 to allow the precise snipping that is required. Without Hax1, the snipping does not occur and HtrA2 remains inert.

In lymphocytes, members of the Bcl-2 family of proteins both protect and initiate apoptosis. For this reason, Ihle and the researchers explored this family of proteins to understand why lymphocytes needed an active HtrA2 mitochondrial enzyme. This led them to discover that if active HtrA2 were present, the incorporation of a protein called Bax into the mitochondrial outer membrane did not occur. This was significant since accumulation of Bax in the outer mitochondrial membrane allows the release of proteins that set off a chain of biochemical reactions, including the activation of enzymes that are responsible for cell death.

Other authors of this study include Jyh-Rong Chao, Kelli Boyd, Evan Parganas, Cheol Yi Hong and Joseph T. Opferman (all St. Jude).

This work was supported in part by grants from the National Institutes of Health and ALSAC.

St. Jude Children's Research Hospital is internationally recognized for its pioneering work in finding cures and saving children with cancer and other catastrophic diseases. Founded by late entertainer Danny Thomas and based in Memphis, Tenn., St. Jude freely shares its discoveries with scientific and medical communities around the world. No family ever pays for treatments not covered by insurance, and families without insurance are never asked to pay. St. Jude is financially supported by ALSAC, its fundraising organization.

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Key Step In Programmed Cell Death Discovered

Link to Nature abstract:

Hax1-mediated processing of HtrA2 by Parl allows survival of lymphocytes and neurons

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Cytokines affect a variety of cellular functions, including regulation of cell numbers by suppression of programmed cell death¹. Suppression of apoptosis requires receptor signalling through the activation of Janus kinases and the subsequent regulation of members of the B-cell lymphoma 2 (Bcl-2) family. Here we demonstrate that a Bcl-2-family-related protein, Hax1, is required to suppress apoptosis in lymphocytes and neurons. Suppression requires the interaction of Hax1 with the mitochondrial proteases Parl (presenilin-associated, rhomboid-like) and HtrA2 (high-temperature-regulated A2, also known as Omi). These interactions allow Hax1 to present HtrA2 to Parl, and thereby facilitates the processing of HtrA2 to the active protease localized in the mitochondrial intermembrane space. In mouse lymphocytes, the presence of processed HtrA2 prevents the accumulation of mitochondrial-outer-membrane-associated activated Bax, an event that initiates apoptosis. Together, the results identify a previously unknown sequence of interactions involving a Bcl-2-family-related protein and mitochondrial proteases in the ability to resist the induction of apoptosis when cytokines are limiting.

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The discovery provides insight into how certain proteins, including Hax1, work and how they control the process of apoptosis.