When cleaving one of its near-hundred substrates—including the amyloid precursor protein (APP)—γ-secretase is always number two, making its cut only after another protease has had a go. Or so scientists believed until now. Researchers led by Stefan Lichtenthaler, German Center for Neurodegenerative Diseases, and Edgar Meinl, Ludwig Maximilian University, both in Munich, report in the 11 June Nature Communications that γ-secretase cuts a survival factor for plasma B cells all on its own. This means γ-secretase itself oversees how much of the protein remains at the cell surface and regulates B cell activity. The finding assigns a new role to γ-secretase and may eventually yield insight into the mechanism of action.
“This substrate bucks the trend,” said David Bolduc, Brigham and Women’s Hospital, Boston, who was not involved in the study. “Everyone to this point has assumed that a prior cleavage needs to occur before γ-secretase can cleave its substrates.”
Gamma-secretase is a protease complex embedded in the cell membrane. It cuts many type 1 transmembrane proteins, most notably APP, important in Alzheimer’s disease, and Notch, important for cell-fate determination. The enzyme cleaves in steps, with an initial snip followed by further pruning. Before γ-secretase can get to work, α- or β-secretase must first cut a protein to trim it down to a manageable length. This shedding is necessary because γ-secretase can only cut extracellular domains that measure fewer than 300 amino acids (Struhl and Adachi, 2000).
When Meinl’s group observed a soluble N-terminal fragment of the B-cell maturation antigen (BCMA) floating around in people’s body fluids, they wondered which protease might have shed it. BCMA is another type 1 transmembrane protein that, when attached to the surface of B cells, boosts their activation and survival upon binding to its natural agonists BAFF or APRIL. Meinl asked Lichtenthaler to help find the one responsible.
Mass spectrometry revealed that soluble, or sBCMA, included the protein’s entire extracellular region plus a bit of the transmembrane domain, hinting that a single transmembrane protease had cleaved it. Gamma-secretase was a candidate because it cuts type 1 transmembrane proteins. Sure enough, several γ-secretase inhibitors blocked shedding in mice and raised the level of BCMA bound to plasma cells. This led to more plasma B cells in the bone marrow and stronger immune activation. By contrast, an inhibitor of α-secretase left levels unchanged. Mouse embryonic fibroblasts from knockouts of the γ-secretase components PS1 or PS2 retained BCMA on the B cell membranes. Together, the data suggest that γ-secretase directly cleaves BCMA from the membrane, and that inhibitors block that shedding (see image below).
Gamma-secretase cuts BCMA from the membrane (left), while inhibitors block that cleavage and increase BCMA on the surface (right). Both forms of BCMA can bind the ligand APRIL. [Courtesy of Laurent et al., Nature Communications. This work is licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/.]
“We may have found the first member of a new family of γ-secretase substrates,” Lichtenthaler told Alzforum. Since γ-secretase cuts BCMA directly, scientists can use it to simplify studies on the enzyme, he said. They can now monitor enzyme activity in vivo without overexpressing its substrates or taking into account cleavage by other proteases. That will make it easier to study, for instance, how γ-secretase recognizes its substrates. Since BCMA has an extracellular domain of only 54 amino acids, this study supports the idea that a short ectodomain is one criterion, Lichtenthaler said. Bolduc agreed. “My guess is there may be other substrates out there with small ectodomains that γ-secretase could cleave directly,” he told Alzforum.
The finding may have implications for Alzheimer’s, as scientists are exploring whether γ-secretase modulators or inhibitors could treat disease. Scientists may have to consider BCMA, and -perhaps similar, yet-unknown substrates, when assessing drug safety, Lichtenthaler said. Further research will reveal whether these therapies affect BCMA shedding, and therefore B cell function. Lichtenthaler speculated that a BCMA effect might help explain a transient increase in IgM antibodies that occurred in the recent Phase 3 trial of the γ-secretase semagacestat (Doody et al., 2013).
People with familial AD who have mutations in PS1 or PS2 have a dysfunctional γ-secretase complex. Do they have immune problems? Lichtenthaler is unaware of any such examples. Because these carriers have normal Notch signaling, implying that the mutation does not affect the first cut made by γ-secretase, he expects these AD mutations leave BCMA function alone as well.
As for autoimmune diseases, Lichtenthaler said there is no evidence suggesting a contribution by γ-secretase. However, the resulting soluble BCMA could be a biomarker for neuroinflammatory diseases that involve increased B cell activation. The authors found that sBCMA rises when more BCMA is expressed on membranes. Soluble BCMA correlated with IgG levels in the cerebrospinal fluid of people with multiple sclerosis, and blood levels of sBCMA tracked with disease activity in people with lupus. Even after it detaches from the cell, sBCMA can bind to APRIL, drawing the ligand away from membrane-bound receptors. This could be one of the regulatory mechanisms for B cell activation, Lichtenthaler said. Whether γ-secretase could be a drug target for autoimmune diseases remains to be seen, he told Alzforum.—Gwyneth Dickey Zakaib
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