"This is a unique signaling pathway," says Randal Kaufman, Ph.D., a professor of biological chemistry in the U-M Medical School and an HHMI investigator. "In most pathways, there are multiple components and crosstalk. But there's only one IRE1 gene and one protein that carries out a unique biochemical reaction. This makes IRE1 a perfect target for pharmacological intervention for B cell-driven autoimmune diseases, like myasthenia gravis or systemic lupus erythematosus."
Results from the study will be published in the Feb. 1 issue of the Journal of Clinical Investigation.
Kaufman studies fundamental signaling pathways involved in the production of new proteins in the cell's endoplasmic reticulum (ER). The process begins with chains of amino acids, which are deposited in the ER membrane in response to coded instructions from genes. Chaperone proteins fold these amino acid chains into specific shapes and direct them to different areas of the ER for processing.
When too many amino acids pile up in the ER membrane, or when something goes wrong with the folding process, the entire system can get clogged with unfolded proteins. To prevent this, the cell activates the unfolded protein response (UPR).
"The UPR has three main aspects," Kaufman explains. "First, it signals the nucleus to stop synthesizing new proteins to give the ER time to catch up. Second, it induces expression and synthesis of more chaperone proteins to assist with protein folding and help with protein trafficking. And third, it activates genes involved in protein degradation to get rid of excess unfolded proteins. So you reduce the load, get rid of garbage and increase efficiency. If these don't work, the cell will die."
"Recently, scientists have discovered that the unfolded protein response is activated in many diseases from viral infections to stroke to Alzheimer's disease," says Kezhong Zhang, Ph.D., a senior research associate in biological chemistry and first author of the paper. "All these things disrupt protein folding and put stress on the cell. By doing so, they activate the UPR."
Several years ago, Kaufman and his research colleagues discovered that IRE1 triggers the unfolded protein response by splicing a segment of messenger RNA called XBP1. This leads to production of a new protein, which turns on genes in the UPR signaling pathway and directs cells to secrete large amounts of protein. The University of Michigan has filed a U.S. patent application covering the IRE1-mediated processing of XBP1 mRNA.
Other scientists have shown that XBP1 helps immune cells called B lymphocytes develop into antibody-generating plasma cells. Plasma cells secrete large amounts of protein antibodies into the bloodstream. Antibodies lock on to antigens ?substances that trigger the immune response ?until they can be removed or destroyed by other immune cells. Knowing that XBP1 was required to create plasma cells, Zhang designed a study to see if IRE1 was involved, also.
Zhang studied mouse embryos that were missing the gene for IRE1. He found that mice lacking both copies of IRE1 died before birth and had fewer hematopoietic stem cells ?precursor cells which develop into many different types of blood and immune cells ?than normal mice.
Zhang transplanted hematopoietic stem cells from IRE1 knock-out embryos into mice whose bone marrow had been destroyed by radiation. He found that stem cells deficient in IRE1 could reconstitute all types of blood and immune cells in the transplanted mice, but their mature B l ymphocytes were defective. Without IRE1, lymphocytes don't develop a B cell receptor and don't have the right rearrangements of immunoglobulin genes to function properly.
"Our findings show that IRE1 is required for both the early and late stages of B lymphocyte development, and that it works through a different mechanism at each stage," Zhang says.
"This is the first indication that a sensor for protein folding in the endoplasmic reticulum can also signal the nucleus to regulate gene rearrangements required for B cell differentiation," Kaufman says. "It's a completely unexpected observation. Learning more about the IRE1 signaling pathway may lead to new ways to suppress B cell development, which could be helpful in autoimmune diseases like diabetes and lupus."