Science Signaling Podcast: 31 July 2012

Technical Details

Length: 15 min

File size: 8.5 MB

File Format: mp3

RSS Feed: http://stke.sciencemag.org/rss/podcast.xml

Listen to Podcast: http://podcasts.aaas.org/science_signaling/ScienceSignaling_120731.mp3

Educational Details

Learning Resource Type: Audio

Context: High school upper division 11-12, undergraduate lower division 13-14, undergraduate upper division 15-16, graduate, professional, general public and informal education

Intended Users: Teacher, learner

Intended Educational Use: Learn, teach

Discipline: Cell biology, immunology

Keywords: Science Signaling, antibody, B cell receptor, BCR, IgG, IgM, immunoglobulin, immunological memory, isotype, memory B cell, PDZ domain, SAP97, scaffold

Transcript

Host – Annalisa VanHookWelcome to the Science Signaling podcast for July 31st, 2012. I’m Annalisa VanHook, and today I’ll be speaking with Susan Pierce about signaling events that are important for antibody production when the body becomes infected by a pathogen that it’s been exposed to previously.

The adaptive immune system allows vertebrates to not only recognize specific pathogens during an infection, but also to remember those pathogens so that a subsequent exposure to the same pathogen elicits an even stronger response, so that an individual may not even have any symptoms when reinfected. One of the cell types that’s important for the immune system’s ability to remember specific pathogens are memory B cells. So, collectively, B cells produce a huge variety of antibodies. There are billions of B cells in the human body, and each one produces a unique antibody on its outer surface that acts as a receptor for pathogens. During the first infection with any given pathogen, B cells that just happen to produce an antibody that binds to that pathogen become activated, and they differentiate into cells whose sole purpose is to crank out antibodies into the blood to help combat the infection. During this initial response to a pathogen, B cell memory is also generated. In specialized areas of the lymph nodes and the spleen called “germinal centers,” B cells that recognize the pathogen proliferate. And while they’re proliferating, the portion of the gene that encodes the pathogen-binding part of the B cell’s antibody is hypermutated, the idea being to make slightly different versions of the original antibody that might bind to the pathogen even better than the original antibody did. While the variable region of the gene is undergoing hypermutation, the DNA that encodes the constant region of the antibody—the part that’s shared between different B cells—switches from an IgM type to an IgG type. When these B cells that produce the hypermutated, high-affinity IgG antibodies bind to the pathogen, they’re selected to become memory B cells. Most B cells are pretty short-lived, but memory B cells can last a lifetime, and they respond quickly if that same pathogen infects the individual again.

IgG B cell receptors are important for selecting memory B cells in the first place, and also for triggering memory B cells to produce the antibody-secreting cells when they bind to their specific pathogen in subsequent infections. Sue Pierce is a researcher at the National Institute of Allergy and Infectious Diseases who studies the B cell receptor and B cell memory, and her group has just published a study in Science Signaling in which they address mechanisms by which the B cell receptor activates memory B cells (1). Pierce’s lab is at the NIH in Bethesda, but she spoke to me by telephone from Stockholm, where she’s a guest researcher at the Karolinska Institute in the Department of Microbiology, Tumor, and Cell Biology.

Interviewer – Annalisa VanHookWelcome, Dr. Pierce.

Interviewee – Susan PierceWell, thank you. Nice to be here.

Interviewer – Annalisa VanHookThe immunoglobulin portion of the B cell receptor varies between different types of B cells—there’s IgM, and there’s IgGs. Naïve B cells are characterized by having IgM-type B cell receptors, and the memory B cells are characterized by having the IgG-type B cell receptors. How do those two types of receptor differ, in terms of the cell biology?

Interviewee – Susan PierceThat’s what we really don’t know. So, the receptor has to perform different functions at different developmental points in an immune response, which is mirrored by stages in B cell development. And the way I view it is the threshold for that IgM B cell to be activated is reasonably high—there are a lot of checks and balances. T cells, for example, have to be alerted and okay that response. The innate immune system sensing common features of pathogens has to be alerted and okay that response. So, the M molecule is expressed by a cell that doesn’t have a complete green light yet. And it’s a cell that, when a pathogen comes in, it’s going to—under the right conditions—enter a very specialized microenvironment called the “germinal center.” And there it’s going to begin to respond to antigen. And one of the key enzymes that’s expressed is one that switches—literally recombines the variable region—from its site in front of IgM to in front of the IgG gene, and to make a whole new antibody.

Interviewer – Annalisa VanHookWhat sort of response is elicited in the B cell when an antigen engages an antibody that’s complexed with the IgG type of immunoglobulin?

Interviewee – Susan PierceYes, so, that’s really where the field is right now, asking that question. What is the IgM receptor capable of doing? What type of cascade of signaling does it trigger? And is that similar or not to what the IgG receptor does? Now, the biology is, we know, is after you’ve recovered from a pathogen response when you first encounter a pathogen, your immune system slowly—after a lag period—begins to produce secreted IgM, and then maybe a little bit of IgG towards the end. And what you’re left with after that response, if you succeeded, is now an expanded population of IgG-expressing cells, and what you know is those cells, well, when you see the pathogen again, you’re going to get a rapid rise in antibody titers—antibody concentration—and that is also going to be higher-affinity antibody. So it suggests that there’s something encoded in the G receptor that makes it able to do that, where an M receptor can’t, so it’s assumed that there’s some qualitative difference between the two receptors.

Interviewer – Annalisa VanHookAnd so that’s the question—the specificity of the signaling, or quantity of the signaling, in the IgM versus IgG receptors—that you looked at in this study.

Interviewee – Susan PierceYes, that’s right. A lot of work preceded ours, some important work. So, the IgG receptor has a longer 28–amino acid cytoplasmic domain, and the M receptor has none, it has just enough—a couple of amino acids—to allow it to be anchored into the membrane. And work that preceded us had shown that that tail was important to have these memory-like responses. So, what we brought to the study was live cell imaging of the B cells and the B cell receptors. What our imaging technology allowed us to do was to literally look within the several seconds to two minutes after the receptor had engaged antigen to ask,”How is the M receptor behaving versus the G, in terms of ultimately being able to trigger the signaling cascade?” So, we looked at, if you will, B cell receptor–intrinsic properties between the Ms and Gs that led to—would lead to—the ultimate clustered B cell receptor and active signaling.

Interviewer – Annalisa VanHookAnd how did the two types of receptors behave, in terms of clustering and synapse formation?

Interviewee – Susan PierceYes, so it turned out that the Gs at every step were just better. So, we engineered the cells to have exactly the same variable region—the antigen binding end—and then just differed in their constant region, M or G. So, after antigen binding, which they did identically—and if you can imagine this clustering is a reversible event. To cluster, a B cell receptor has to be holding onto the antigen. If it falls off, then the cluster will fall apart, and then rebinds, and the clustering will start again. So Gs just very rapidly go through oligomerization, clustering, and then the cluster grows much faster than the Ms, and there’s also phenomena that some of the G receptors on a B cell that aren’t even engaged in the antigen are pulled into these clusters in a ligand-independent event. So it’s as though the G receptor is designed to hop through these steps in a very rapid, efficient way.

Interviewer – Annalisa VanHookAnd so it’s the clustering of the receptor that really promotes maximal signaling through that receptor.

Interviewee – Susan PierceYes, that’s the required step, right.

Interviewer – Annalisa VanHookYou mentioned that the IgM and the IgG differ in their cytoplasmic tail regions.

Interviewee – Susan PierceYes. So that was actually the subject of the Science Signaling paper. So, once we understood that the Gs were very efficient, then the question is, “Why? What part of the G?” We already had a clue from the field—a very big clue—that the tail was going to be important. And, in fact, we narrowed the G behavior down to a 15-residue part of the 28-residue tail, and this so happened to be the closest to the membrane part of the tail. And if you had that 15 residues, then you look like a G, and if you didn’t, you look like an M, behave like an M.

Interviewer – Annalisa VanHookWhat does that 15-residue region of the cytoplasmic tail do? How does it confer those behavioral differences?

Interviewee – Susan PierceSo, we imagined that it might be associating with something else—that’s what receptors do. And when we looked at the sequence of all the known G tails—and these range from human to lizard and platypus—that region is absolutely conserved. And it’s conserved among what we call “G subtypes”—so there’s G1, G2, which all can be receptors. They all had precisely that sequence. And within that sequence, what we noticed was there was a motif, which proteins called a PDZ domain could bind. So, what are PDZ domain–containing proteins? They tend to be large scaffolding proteins. So what we thought was is, well maybe that’s why the G is so good, is that it oligomerizes, and then its tail associates with a big scaffolding protein that can stabilize the growing oligomer, bring new signaling molecules into the mix. So we went fishing with the 15–amino acid tail, looking for a PDZ-binding domain protein. And at the end of the day, what we fished out was a member of what’s called a MAGUK family of PDZ-binding domain proteins—a big family. And if you ask the average immunologist what they do, they tend to not know because they were first characterized by neurobiologists, because these proteins play very important roles at the neurological synapses in neurons. So, what they do in neurons is bind to tails of receptors—the glutamate receptor, for example—and they stabilize the expression. They’re also involved in transporting the receptors to the neuron surface. So, we thought that made a lot of sense. And then the job was to demonstrate that this protein that we knew bound to the tail was actually necessary for the G response. And basically we did that by knocking it out and seeing that a G receptor then looked like an M receptor. The particular one that a B cell expresses is called SAP97. If you didn’t have SAP97, the G looked like an M. And, in fact, if you just didn’t have that PDZ-binding domain in the tail, then the G receptor acted like an M receptor.

Interviewer – Annalisa VanHookSAP97—or molecules related to it—function in neurological synapses, and now you’ve shown that they’re also functioning in the immunological synapse.

Interviewee – Susan PierceYeah.

Interviewer – Annalisa VanHookHow similar are the two structures? I mean, they’re both involved in receptor clustering and getting proteins together through scaffolding proteins.

Interviewee – Susan PierceYeah. I think that conceptually, they’re very similar, but—conceptually in the sense of needing to activate a cell to [respond to] stimuli that the system can’t predict whether it’s going to get it or not. You never know what neurological stimulus you’re going to see in your world; you don’t know what antigen you’re going to see. So, the purpose of the synapses seem similar, and then what proteins they have in common… I think this—I mean, as you ask it I have to think a little bit about it—this is one of the first proteins that has been identified in both synapses.

Interviewer – Annalisa VanHookIf any of the proteins would be expected to be shared, you would think about the scaffolds. That seems to make the most sense.

Interviewee – Susan PierceYeah.

Interviewer – Annalisa VanHookThank you, Dr. Pierce, for speaking with me.

Interviewee – Susan PierceWell, thank you for taking the time. Bye-bye.

Host – Annalisa VanHookThat was Sue Pierce, senior author of a Research Article published in the July 31st issue of Science Signaling. That paper is by Liu and colleagues and it’s titled “The Scaffolding Protein Synapse-Associated Protein 97 Is Required for Enhanced Signaling Through Isotype-Switched IgG Memory B Cell Receptors” (1).

music

And that wraps up this Science Signaling Podcast. If you have any questions or suggestions, you can write to us at sciencesignalingeditors{at}aaas.org. This show is a production of Science Signaling and of AAAS—Advancing Science, Serving Society. I'm Annalisa VanHook, and on behalf of Science Signaling and its publisher, the American Association for the Advancement of Science, thanks for listening.