Welcome to FOCUS In Sound, the podcast series from the FOCUS newsletter published by the Burroughs Wellcome Fund. I’m your host, science writer Ernie Hood.
On this edition of Focus In Sound, we meet a young investigator who is making significant contributions to our basic knowledge of cell signaling pathways, cell death mechanisms, innate immunity, infectious diseases and autoimmune disorders.
Dr. Maya Saleh of McGill University was named an Investigator in the Pathogenesis of Infectious Disease by the Burroughs Wellcome Fund in 2009. Maya received her Ph.D. at McGill in 2001, served post-docs at Merck and the La Jolla Institute, and in 2005 returned to McGill to join the faculty. She is an Associate Professor in the Departments of Medicine and Biochemistry and Director of the Inflammation and Cancer Program. She is also an Associate Member in the Department of Microbiology and Immunology, an Associate Member of the Goodman Cancer Centre, and a member of the Center for the Study of Host Resistance and the Division of Critical Care of the McGill University Health Centre Research Institute.
Maya Saleh, welcome to FOCUS In Sound…
Thank you for having me.
I’m sure our listeners could tell just from that introduction that your laboratory’s research really touches a variety of different areas. Is there one central focus from which everything else emanates?
Yes, absolutely. It seems that we are researching a variety of different areas, but in fact they are all inter-related, and that’s what is fascinating for us. We are investigating specifically immune-mediated inflammatory diseases, including infectious diseases, inflammatory bowel disease, starting to look at metabolic diseases and cancer. And the common link between all of these is inflammation. So our interest at the cellular and molecular levels is to understand how the inflammatory response is triggered and executed. This has directed our focus to studying how the host initially senses the presence of microbes, both commensal and pathogenic, or that of stress, cellular transformation as in cancer, or metabolic perturbation. As you know, this early detection is a function of specific receptors of our innate immune system, which is our first line of defense. Now these receptors are termed pattern recognition receptors. They’re highly conserved from plants to humans, and have evolved to confer host resistance in case of infection, injury or stress. So the way they function is that, when they sense the presence of microbial motifs or endogenous danger signals, such as those produced in the cancer microenvironment or during metabolic stress, these receptors activate a plethora of effector mechanisms that initiate the inflammatory response. And there are a number of these receptors in the innate immune system, different families, and a number of members per family. And the excitement in the field right now is to understand the specific role of each of these in health and disease. So in the last few years, my lab has studied a subfamily of these receptors known as NLRs or Nod-like receptors, along with their associated inflammasomes. These NLRs are mutated or deregulated in multiple diseases like IBD (inflammatory bowel disease), cancers, hereditary periodic fever syndromes, diabetes, Alzheimer’s, and so on. And our goal is to understand how they operate in physiological and pathological conditions.
Your group recently published an important paper in Nature illuminating the co-evolution of pathways involving apoptosis, or programmed cell death, and innate immunity. Those would seem to be very different, even opposing, phenomena…can you tell us more about what you’ve discovered?
In this context we’ve been particularly interested in understanding how the host innate immune system, in particular these microbial sensors, dialogue with the microbiota in the intestine. It’s now clear from our work and that of many other labs that NLRs are necessary to maintain intestinal homeostasis and immune tolerance towards these commensal microorganisms, and consequently their deregulation is at the basis of these inflammatory bowel diseases like Crohn’s disease and ulcerative colitis. In particular we focused on two receptors that sense bacterial peptidoglycans, specifically, namely NOD1 and NOD2, as these have been genetically linked to these inflammatory bowel disorders. Yet although we know these two receptors are necessary in the intestine, we still lack information on how they function in the gut and how they translate the bacterial sensing signal to induction of homeostatic responses.
So in this particular paper, we’ve decided to take a functional genomic approach to define the key molecules and pathways required for NOD function in intestinal epithelial cells following bacterial peptidoglycan sensing by these receptors. Our rationale was to refine the molecular portrait of NOD signaling, first, to understand the function of these receptors at the fundamental level, and second, to identify therapeutic targets for IBD. So the approach we’ve used was to systematically silence every druggable gene in the genome, and question which of these is needed to either stimulate or inhibit the inflammatory response following NOD activation by bacterial peptidoglycan. One interesting facet that emerged from this work was the finding of an enrichment of apoptosis effector proteins in this innate immunity pathway. In other words, as you’ve mentioned, it appears that proteins that were thought to primarily function in the process of programmed cell death might have evolved additional vital functions in innate immunity. So on the surface these functions appear contradictory, but in the big scheme of host resistance, they are actually not. If you think about it, in primitive organisms, cell death is the main host defense strategy against pathogenic infection, whereby an infected cell commits suicide to destroy the microorganism and prevent its spread to neighboring cells. In more complex organisms, like ours, other innate immunity mechanisms, including the inflammatory process, were essentially grafted to this primitive cell death response. But of course when these fail, the last resort remains cell death. In fact there are a multitude of parallels between the cell death machinery and the NLR pathways. First, both are activated by molecules released from the mitochondria, which as you can imagine is reminiscent of an intracellular bacteria. Second, these NLR receptors are very similar structurally to the molecule that triggers apoptosis, or the apoptosis activating factor. Thirdly, the inflammasome complex that forms when these NLRs are activated, again, resembles structurally the apoptosome complex. And finally, the proteases, or the enzymes that are engaged in both complexes, known as caspases, that are necessary to execute cell death or regulate inflammation, belong to the same family, thus in theory have evolved from the same ancestral gene.
So these are actually complementary functions, then.
Yes, we think so.
I understand from what you’ve said already, Maya, that this particular work may well have clinical implications as well, perhaps helping to identify new targets for therapy in disorders that you’ve mentioned, such as inflammatory bowel disease. How would that work?
Yes, absolutely. By knowing the key players that direct the NLR responses, one could design new therapeutics to target these proteins in pathological conditions. So in this Nature paper we’ve characterized specifically one such molecule, known as BID, and we know exactly which domain is involved in interaction with the NLRs, and we know that without this molecule the NLRs are unable to transduce inflammatory signaling. So one can now design therapeutic approaches around targeting the cross-talk between this BID protein and the NLR receptors.
So is that a direction that you will continue to pursue, you and your group, or would that development of new therapies fall to others?
No, we are of course interested in that, and we are now refining the region in the BID molecule that can interfere with the NOD function. We have at McGill actual programs that would allow us to translate our basic science finding further into the clinic.
Terrific, well we will certainly look forward to your results from that work. Maya, you’ve already touched on this, but I’d like to get you to elaborate just a little bit…you and your colleagues have also made great strides in characterizing what’s going on in the gut in terms of innate immunity. As we’ve learned so much recently about the role of the gut microbiota, obviously the gut epithelial cells have a way to recognize the benign, or so-called “commensal” microorganisms that colonize the gut, while also being able to sense and attack pathological microbes that might invade. I’m sure that’s a very simplistic explanation, but tell us a little bit more about the insights you’ve provided into both gut homeostasis and disease-causing dysfunctions in this very complex but vital biological system…
Last year we published a report in the journal Immunity examining the role of the inflammasome in intestinal homeostasis, colitis and colitis-associated colorectal cancer. So as I mentioned, when these NLR receptors are activated, they assemble a large complex referred to as the inflammasome that recruits and activates this protease enzyme named caspase-1. Now this enzyme is inflammatory, it processes some dormant cytokines such as interleukin-1β and interleukin-18 into their active forms and enables their release from the cell. So we’ve asked the simple question of whether the inflammasome, this inflammatory entity, contributes to the inflammatory pathology that occurs in IBD and colorectal cancer, or whether it’s, on the contrary, protective, and what we found was that the latter is correct. So it seems that the cross-talk between the microbiota and the inflammasome in intestinal epithelial cells is necessary to induce a physiological level of inflammation, which we’ve characterized to be specifically mediated by the inflammasome-dependent cytokine interleukin-18. And that is needed, first, to maintain the epithelial barrier intact or to induce its repair following either a physical, chemical or microbial insult, second, to maintain a symbiotic microbial ecology. Indeed, what we’ve shown is that a defect in inflammasome signaling and lack of the steady state production of interleukin-18 leads to colitis and colitis-associated colorectal cancer. And we were able to reverse these pathologies through the administration of interleukin-18 to colitic animals, and others have shown that defects in inflammasome signaling also lead to dysbiotic expansion of colitogenic bacteria in the gut that further fuels the vicious cycle of chronic inflammation such as what occurs in IBD. So this work argues against full inhibition of caspase-1 or total neutralization of interleukin-18 as therapeutic approaches in IBD.
I see. I’m sure you’ll be continuing to develop that work as well. The work you do, Maya, seems to be really kind of unique in that it is on a very basic level in terms of gaining new biological knowledge, but at the same time it also seems to have great translational potential. Is that part of your approach, to always keep that in mind?
Indeed, I believe that my training path has greatly contributed to the translational approach of my research. I am a basic scientist who has trained in both academia and industry, and thus part of, I would say, a rare group of individuals that strive to constantly translate basic scientific discoveries into potential therapeutic treatments. Academically, I have a diversified background in genetics, biochemistry and immunology, which I think is an asset to perceiving complex disease mechanisms and tackling relevant research questions.
It must keep you very engaged on a daily basis, I imagine…
Maya, how did you happen to pursue this particular type of research?
We were talking about this translational aspect…I always had an attraction towards interacting with patients and clinical research. So during my Master’s studies at the American University of Beirut, my work focused on the genetic basis of b-thalassemia, which is a severe anemia-causing disorder endemic to the Mediterranean region. So at the time, I identified the spectrum of the b-thalassemia mutations in Lebanon, and this had a direct impact on the development of the first pre-natal screening and counseling facility in the country. And then from there I moved to my doctoral studies at McGill, where I investigated embryonic development, specifically biochemical events that switch on or off decisive transcriptional programs required for embryogenesis. So all of this is very far from what I’m doing right now. However, during my post-doc, I joined the laboratory of Dr. Donald Nicholson at Merck, first in Montreal then in San Diego. I think this unique training experience in the industry contributed to this translational approach of my research. My work at Merck focused on understanding the biology of these enzymes that we’ve talked about, the caspases. However, first the focus was on understanding their role in cell death and in neurodegenerative diseases. Then in 2004, I cloned a new caspase gene termed caspase-12, and identified a mutation in this gene that confines its expression to the African population and African descendants, but not to other ethnicities in the human population. In addition, we’ve demonstrated that African-American individuals that express caspase-12 were predisposed to infection and that this caspase-12 allele was associated with severe sepsis in the clinic. This discovery was published in Nature in 2004, and led me to embark on a second post-doctoral fellowship now in the laboratory of Dr. Douglas Green, then at the La Jolla Institute of Allergy and Immunology, to dissect the mechanism by which this protein impacted inflammation and the innate immune response. My work in Doug’s lab, which I completed as an independent PI at McGill, resulted in a second publication, also in Nature, in 2006, where we’ve characterized the mechanism by which caspase-12 inhibited the generation of a necessary inflammatory response during infection, and then identified that at the molecular level. The way this protein functions is by antagonizing, or acting as an inhibitor to the inflammasome complex that we’ve discussed. Our work was among the first to emphasize the key role of the inflammasome in host defense during bacterial infection and sepsis and this has then directed our efforts to further contribute to this emerging field of NLR-mediated innate immunity.
You’ve already made some really significant contributions and had some very important publications, as you’ve shared with us, and congratulations for all of that. So where is your research headed from here? What are the burning questions that remain to be answered?
The burning questions now are, first, to understand whether these innate immunity receptors have distinct or overlapping functions in different physiological or pathological contexts, and to understand what agonists they recognize, both during infection but also in sterile inflammatory diseases, such as in metabolic diseases or cancer. And then, to understand their predominant effector mechanisms. How do they function following sensing of either the danger signals produced in sterile conditions or during infection, and this would provide therapeutic opportunities, as we’ve discussed. But we are also very interested in understanding the role of the microbiota and sensing of the microbiota by these receptors not only in the intestine but also in relatively sterile tissues such as the lungs or in the mammary gland, and understand their contribution to diseases at these sites.
Terrific. It sounds like you have plenty to keep you busy for years to come. Maya, it’s been a great pleasure for us to get to know you and your work, which is shedding so much light on basic cellular mechanisms while illuminating a path toward improved treatments and outcomes. We wish you the best of luck for continued success, and thanks so much for joining us today on FOCUS In Sound…
Thank you for having me.
We hope you’ve enjoyed listening to this edition of the FOCUS In Sound podcast. Until next time, this is Ernie Hood. Thanks for listening!