Keystone Neuroinflammation 2017

Last month, I attended my first scientific research conference as a PhD student at the beautiful town of Keystone in Colorado. The Keystone Symposia on “Neuroinflammation: Concepts, Characteristics, Consequences” was a week-long meeting with around 300 participants focussed on the interaction between two complex systems – the Central Nervous System (CNS) and the Immune System. Being a relatively new area of investigation, the conference brought together neuroscientists and immunologists who’re working on unraveling the mysteries of the brain and the human body. A traditional immunologist entering this field will have to grasp the intricacies of the brain with respect to microglia, astrocytes, neural circuits, neurodevelopment, the blood brain barrier (BBB), electrophysiology, behavior, degeneration, and more. Similarly, a traditional neuroscientist new to the field is expected to be familiar with all the details of the immune system from the heterogeneous immune cell types to different markers, the complement system, cytokines and chemokines, and be able to define aging in the context of immune regulation. As for me, being new to both the disciplines, this was an exciting opportunity to discover bold ideas as well as revisit my research objectives from a fresh perspective. Not to forget how wonderful Keystone itself was and made me realize how much I love the mountains!

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Without going into much of the details, some highlights of the conference are summarized below.

Microglia – the good, the bad, and the ugly

As expected, microglia were a hot topic through the meeting. Being the only resident immune cells in the CNS, microglia make up around 10% of the total cells in the brain. Activated microglia exhibit self-renewal and proliferative capacity along with phagocytic capability by engulfing toxic misfoled proteins (such as the aggregated amyloid beta peptides in Alzheimer’s disease and the alpha syneucleins in Parkinson’s disease). These “good” microglia are therefore essential for the maintenance of brain homeostasis. But what happens when these macrophages go haywire and start chewing up healthy neural synapses and start making more pro-inflammatory cytokines than necessary? How do these “bad” microglia contribute to neuroinflammation and accelerate the neurodegenerative phenomena? More importantly, the M1/M2 phenotypic characterization of microglia that I have previously written about is now considered to be an obsolete concept. Currently, we know that these cells are much more complex than to exhibit two polarized states. Microglia are heterogeneous in that their microenvironment dictates their characterization and therefore can exhibit multiple phenotypes.

Secretome – Looking beyond the transcriptome

A common method to predict the cellular phenotype or function is to study the transcriptome profile of a group of cells (or a single cell) under different activation states. In his talk, Christopher Glass highlighted the differences between the transcriptomes of human microglia and mouse microglia, and stressed on the importance of working with microenvironment-dependent microglial gene expression data. His work also showed the major differences in the up-regulated vs down-regulated genes in human vs mouse genomes highlighting the drawbacks of utilizing mouse models for characterizing human microglial cell behavior. Hugh Perry, in his talk laid out the “secretome” profile (the cytokines and chemokines) of microglial cells in the brain. Dr. Perry described the journey of his lab’s work towards targeted immunomodulation in chronic neurodegeneration by using this data.

The BBB – Pushing the boundaries of neuroinflammation

Among many topics of discussion, an interesting area was the role of the BBB in influencing immune cell proliferation into the CNS under stress and injury. (Side note: Other interesting areas of discussion were – cool new tools being developed for drug discovery, the use of various animal models in studying neuroinflammation such as transgenic and wild type zebra fish, drosophila, induced pluripotent stem cells, humanized mouse models, and rats). Studying the BBB in the context of inflammation can lead to some interesting questions such as – “What are the differences in the markers expressed and cytokines produced by the infiltrating macrophages versus the resident microglial cells?“, “How can the resident microglia be characterized in the context of microglial functions such as phagocytosis?“, “How do the pericytes and the oligodendrocytes contribute to inflammation in different disease models?

It’s all about the microglial cross-talk with glia (astrocytes) and neurons

Recently, Ben Barres’ group showed that activated microglial cells induce the formation of toxic A1 astrocytes by releasing TNF-a, IL-1a, and C1q that causing neurotoxicity and the eventual degradation of neurons. This study was significant because it showed that the neural death occurs through astrocytes and not directly from microglia. The importance of the role of astrocytes in mediating neuroinflammation has increased since then. Of course, the entire process in itself is a cross-talk between various cells within a particular microenvironment. Hence, when we discovery and design drugs to target microglia, we should also consider the effects of the compounds on glia and neurons in vitro, before moving to the in vivo studies.

Is anti-inflammation and not pro-inflammation the culprit in Alzheimer’s disease?

Perhaps one of the most provocative talks of the conference was by David Hansen from Genentech who illuminated the role of Triggering Receptor Expressed On Myeloid Cells 2 (Trem2) and debunked the myth of pro-inflammatory cytokines in Alzheimer’s pathology (i.e., lot of inflammation causes Alzheimer’s). Dr. Hansen highlighted key genes that are evident in anti-inflammation and immune suppression to be up regulated in his studies pointing towards an alternative activation pathway for neuroinflammation. This is an example where looking into the secretome along with the transcriptome becomes crucial in characterizing the cells and their subtypes in the progression of the disease.

My poster – Combination drug repurposing for the synergistic effect of enhancing microglial phagocytosis and reducing neurotoxicity in Alzheimer’s disease 

I presented a poster on one of my ongoing projects on combination drug repurposing for Alzheimer’s disease. I am grateful for everyone who stopped by and for all the valuable feedback and comments that I received on my work. Many questions were aimed towards the computation wing of the project (that I don’t directly work on). Briefly, these compounds are identified by utilizing the interactome-based drug discovery pipeline (called CANDO) that maps the signatures derived from the interaction between all the proteins in the proteome and all the human approved compounds. The overlapping mechanisms or pathways between Alzheimer disease with other diseases such as diabetes, heart failure, inflammation, among others can be utilized to determine drug behavior and therefore utilized for predicting novel targets.

And finally – Colorado!

I cannot leave out Colorado in this conversation about Keystone Symposia. I have been living on the plain lands among endless corn fields for almost four years now. Being around snow-capped mountains was an absolute treat to the eye (and to the soul). During the afternoon break sessions, we drove to various locations such as the Loveland pass which is around 11,990 feet above sea level in the Rocky mountains and the riven run area.  Intense winds and long stretches of snow welcomed us after a beautiful drive up the Rocky’s. I’ll let the photos do rest of the talking. Overall, attending the Keystone conference at this time in my research career was a great decision. I now have a better understanding of the field and my own project. I should really thank my advisor since it was he who pushed this idea and gave me valuable insight and advice throughout.

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Does arginase mediate immune suppression in the brain?

This week I want to talk about an interesting enzyme: Arginase-I (Arg1) and its metabolic pathway in an immune microenvironment. Arg1 is a cytosolic protein that is involved in the urea cycle. Specifically, it catalyzes the hydrolysis of L-arginine to L-ornithine and urea. It has been shown that the expression of Arg1 by macrophages has an important role in tumor growth. Macrophages that are recruited into the tumor microenvironment (tumor associated macrophages) express high levels of Arg1 resulting in the depletion of arginine – an essential nutrient required for T cell metabolism. Cytotoxic T cells, therefore, can no longer function and inhibit the tumor cells from proliferating. Arg1 is implicated in several inflammatory diseases as well as in autoimmunity. Arg1 plays a significant role in mediating immune suppression and blocking its metabolism is a novel strategy in preventing tumor growth and other inflammation-related conditions.

T cell suppression by MDSC
Myeloid-Derived Suppressor Cell suppresses cytotoxic T cell function through Arg1 metabolism. (IFNγ, IL-4, and IL-13 are cytokines that induced MDSC activation)

One hypothesis is that the immune suppressive cells in other immune microenvironments in the body must be similar to the Myeloid-Derived Suppressor Cells (MDSCs) that induce T cell suppression in the cancer microenvironment. The question is, do such immunosuppressive cells express increased levels of Arg1 and act through the Arg1 metabolic pathway? Since I am interested in the brain and neurodegeneration, this hypothesis can be extended to the brain immune microenvironment. Microglial cells in the brain also upregulate Arg1 and are neuroprotective in nature (in a healthy brain). These cells are the resident macrophages of the central nervous system and function by phagocytosing cell debris and toxic misfolded proteins (that eventually form aggregates and lead to neuronal death as seen in Alzheimer’s disease) out of the brain environment. The question now is – do the microglial cells exhibit immunosuppressive behavior by altering their Arg1 metabolism?

Kan et al., 2015, recently showed that CD11c positive microglial cells are immunosuppressive in the CVN-AD mouse model and that immune suppression is caused due to the deprivation of arginine (increased levels of extracellular Arg1 causing decreased levels of total brain arginine). What isn’t explicitly mentioned in this study is that arginine is also the substrate for nitric oxide synthase (NOS) that makes nitric oxide (NO) in an alternate L-arginine metabolic pathway. L-arginine is a substrate for both Arg1 and NOS. The Arg1 pathway polarizes the macrophages to M2 phenotype and the NOS pathway polarizes the macrophages to the M1 phenotype (Rath et al., 2014). The current model of microglial activation in the CNS is limited to these two polarized states, where, the M1 microglia are neurotoxic and the M2 microglia are neuroprotective. Arg1 is upregulated in microglia in the healthy brain and aids in phagocytosis of misfolded proteins and other cell debris. The classical microglial activation is through the M2 phenotype wherein the induced nitric oxide synthase (iNOS) is upregulated thereby accelerating inflammation in the brain (neuroinflammation is one of the hallmark characteristics of several neurological diseases such as Alzheimer’s Parkinson’s, Multiple Sclerosis, Traumatic Brain Injury, etc).

M1 and M2 microglia
The current model of microglial activation is limited to the Arg1-mediated M1 and iNOS-mediated M2 polarized states.

So, if an immunosuppressive cell exists in the brain, is it possible that the immune suppression is regulated through the M2 activation and that M1 activation is absent? In other words, L-arginine is metabolized through the Arg1 pathway and not through the NOS pathway. Other questions to consider: MDSCs upregulate both Arg1 and iNOS – so how does that fit into the two-state polarization model? How does iNOS modulate MDSC activity? We know that increased NO expression by MDSCs increases T cell suppression in the tumor microenvironment. Currently, both Arg1 and iNOS inhibitors are being developed to block the immune suppressive activity of MDSCs in the cancer microenvironment. However, understanding immunosuppression in the brain is still a long way to go and the idea is not widely accepted within the neurobiology community (my understanding from the currently available literature or published studies). Investigating this mechanism in the brain will be useful in developing potential therapeutic strategies for treating neuroinflammation and neurodegeneration.

References:

  • Kan MJ, Lee JE, Wilson JG, et al. Arginine Deprivation and Immune Suppression in a Mouse Model of Alzheimer’s Disease. The Journal of Neuroscience. 2015;35(15):5969-5982. DOI:10.1523/JNEUROSCI.4668-14.2015.
  • Rath M, Müller I, et al. Metabolism via arginase or nitric oxide synthase: two competing arginine pathways in macrophages. Front. Immunol., 27 October 2014. DOI: https://doi.org/10.3389/fimmu.2014.00532