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

[Almost] one year milestone – my first advisory committee meeting

Advisory committee meetings are held once every year (or twice every year, if the student or the committee chooses to do so) to asses the progress of a grad student’s PhD thesis. The meeting involves a written report that is to be submitted to the committee a week prior to the meeting and an oral presentation on the D-Day. During the presentation, the validity of the research work is thoroughly discussed along with the future direction(s) of the project(s) being undertaken. The advisory committee meetings are extremely important for the successful advancement and completion of a thesis – it is where brutal yet honest feedback is conveyed. We as grad students are forced to think critically of our work and defend our hypotheses as well as our results.

My first advisory committee meeting was an intense two-hour long session on a rather dull Tuesday afternoon. As I explained the premise of my work and my goals for the next year, my committee members brought up important questions that I had not previously ever considered. All the members of my committee, including my advisor, were supportive and encouraging. I learned some valuable lessons from the entire experience and got some great feedback from everyone. Some interesting and important points highlighted in my feedback assessment were –

  • Think carefully about how to present data and set up an argument in my presentation.
  • Work on clearly identifying the premise that sets the stage for my hypotheses.
  • Be critical about my data.
  • Continue to read literature: more reading, and reading more critically.
  • Focus on developing more robust immunological assays to answer the questions in my aims.
  • Interact more with colleagues on campus and at other schools to learn and get insight into techniques and relevant assays (wrt understanding what works and what doesn’t).
  • Explaining the experiments in detail before delving into my results (every assay is unique and has a question to be answered).
  • Think about how I want to present the previous studies done in the field that are relevant to my questions.
  • My hypotheses should be provided with a context (what is the data in support or against my hypotheses?)

These were just some of the significant parts of the feedback that I received. Now it’s time to put these into action and definitely work on continuing to build on my project more confidently. More later.

Metabolic interplay

fimmu-08-00248-g001
Renner K et al. Front Immunol. (2017)

I recently came across this figure that shows the key metabolic processes that dictates an immune cell behavior and function. Biochemists and pharmacologists sometimes focus on one or two key pathways in a disease model and forget that proteins don’t function in isolation. Protein networks are complex pathways with many overlays. A drug designed to inhibit or activate a specific protein can also affect other proteins in the connected pathways. This figure is focussed on an immune cell (natural killer cell) and its interaction with a tumor cell. The interplay between the different metabolic pathways applies to all kinds of cells in the body.

This figure is also quite interesting to me because I have been studying the arginase-1 (Arg1) pathway in microglial cells and this gives me a brief overview of where my study lies in the spectrum of key cellular metabolic pathways. Arg1 is an enzyme that metabolizes L-arginine to L-ornithine and urea in the urea cycle. With the help of ornithine decarboxylase (ODC), L-ornithine further makes polyamines that are important (? – it depends) for cell growth and survival (? – it depends). I think it is quite interesting to see how Arg1 and ODC would dictate the phenotypes of the microglial cells in the brain. Microglia are the brain’s resident immune cells – they chew up all the toxic stuff and get rid of them (this is known as phagocytosis). We have always studied these cells based on their two active states (M1 or M2). There has been evidence in the recent years to show that these cells in fact may exhibit multiple activated states (not just M1 and M2). Just like many immune cells in the body that exhibit a heterogenous phenotype, microglia in the brain may be no different. I’m curious if Arg1 and ODC may be involved in regulating a similar mechanism in microglial cells during neurodegeneration..

Source: Renner K., Singer K., et al. Metabolic Hallmarks of Tumor and Immune Cells in the Tumor Microenvironment. Front Immunol. 2017; 8: 248.

 

It’s brain awareness week!

Hello all!  I wanted to take a few minutes to write something for the brain awareness week. This is important to me because my research focusses on understanding the role of the immune system in the brain. For a very long time, the brain was thought to be an “immune privileged” organ i.e., it was thought that the brain is protected from all the peripheral insults and that it is “divorced” from the rest of the body. In 2015, it was shown that there exists certain lymphatic vessels that connect the CNS to the rest of the body (1). The lymphatic system carries immune cells through a network of vessels and tissues; it connects the bloodstream and tissues in order to remove dead cells and other debris. The discovery of the new “glymphatic system” has opened new avenues to study the connection between the brain and the rest of the body. This is especially helpful in understanding the role of the peripheral immune system on the CNS during infections, injury, and other disease insults.

glymphatic system
Old lymphatic system (left) and newly discovered lymphatic system in the CNS (right). Source: University of Virginia Health System

My work focusses on a specific cell type in the brain known as microglia which are are the resident macrophages of the CNS (they eat up and clear out the bad stuff in the brain like dead cells and mis-folded proteins). Microglia are the only known immune cells of the brain. Compared to all that’s known about the cells of our body’s immune system (B cells, T cells, NK cells, neutrophils, basophils, Treg cells, MDSCs, TH1, TH2, and many many more with several subtypes of each cell), it is safe to say that cells of the CNS are poorly understood. My efforts are focussed towards understanding the role of microglial cells in neurodegenerative diseases such as Alzheimer’s Diseases (AD) , Parkinson’s Disease (PD), Multiple Sclerosis (MS), etcetera. These diseases are characterized by mis-folded proteins that aggregate in the different regions of the brain tissues causing the neurons to degenerate and eventually die. The microglial cells in these disorders play a major role in disease progression by regulating many pathways involved in cell-cell communication, cell survival, and cell death. This is a relatively new and an exciting area of study with many missing links and questions to be answered. I will try my best to keep this space alive with updates and stories! In the meantime, here’s a fun read on Leonardo da Vinci’s contributions to neuroscience: http://www.sciencedirect.com/science/article/pii/S0166223600021214

And here’s a 1504-1506 drawing of the human brain by da Vinci:

Leonardo da Vinci's contributions to neuroscience
In the upper figure, the three ventricles are labeled imprensiva (anterior ventricle, corresponding to the paired lateral ventricles), senso comune (third ventricle), and memoria (posterior or fourth ventricle). Below the ventricles, seven pairs of cranial nerves are shown. The lower figure shows a human head in an exploded view, with the skull raised over the brain and from the head. Source: 

Sources:

  1. Louveau A, et al. Structural and functional features of central nervous system lymphatic vessels. Nature. 2015;523(7560):337–341. doi: 10.1038/nature14432.

Blots, cultures and assays concludes rotation two

This week officially concludes my second laboratory rotation in the neuropharmacology lab with research focussed on  G protein-coupled receptors and their application in several neurological disorders such as depression and anxiety. In the eight week duration of my rotation, a few things were achieved with respect to validating the activity of the newly developed M4R-DREADD (a designer M4 muscarinic receptor exclusively activated by a designer drug). Designer receptors are engineered such that they are solely activated by a synthetic ligand. This opens new avenues in the activation and control of G protein-coupled receptors’ function in vivo.

After a long break from my Master’s research, I got back to maintaining two cell lines – CHO (Chinese Hamster Ovary) and HEK293 (Human Embryonic Kidney) cells, in which the opioid receptors were expressed for all my experiments. These cells were used to characterize the receptor signaling by western blot analysis of the downstream MAPK/ERK signaling  upon stimulation by a few agonists/drugs of interest. Luckily, the lab acquired a new fluorescence microscope during this period which helped us observe the recruitment of the β-arretin2 protein by δ-opioid receptors in HEK293 cells stimulated with clozapine-n-oxide, a synthetic ligand.

mrrd-gfp barrest-cherr cno 0 min 20x_Overlay copy
HEK293 with M4R-dreadd 20x
mrrd-gfp barrest-cherr cno 10 min 20x_Overlay copy
HEK293 with M4R-dreadd 20x

This week, I had a lot of difficulty in handling the mice. Being my first experience with animal work, watching the mice anxious and struggle while we held them down was hard. I am still pretty unsure about how I feel about animal work (if I HAVE to do it to save my research in the future, I will) but I definitely need more exposure and practice with them.

Overall, this lab taught me a lot, even if some days were stressful and  tiring. I feel like I learned and enhanced many skills in the process (primer design, restriction analysis, cell culture, cloning, western blot, cAMP assay), and got a feel for the lab at the same time. Through the course of these past two rotations, I have met some really smart and dedicated people. In the end, I am grateful to have had this opportunity.

Two months in: Last day in cancer lab

Today marks the last day of my first laboratory rotation. I want to pen down a few things that I learned and experienced during my time in the cancer lab:

  • Starting fresh in a new field of research was challenging at first, but got interesting once the different pieces of the puzzle were pieced together along the way.
  • Understanding the nitty-gritty of the investigation entails failures, failures, failures, followed by lots of optimizations and practice. Patience and perseverance is the key.
  • Staying positive and motivated throughout the journey can go a long way. My mentor/ senior grad student in the lab is one of the most optimistic people I’ve met in recent times.
  • Almost always, grad students manage multiple projects at the same time. It is essential to have a main project (or two) and a few side projects to keep the lab active, and research moving forward.
  • Taking a computing course along with the Biochemistry course has kept my study diverse and helped me broaden my thought bubble. At the same time, it has contributed to an additional pressure of having to take exams and submit assignments frequently (which I don’t want to be doing a lot of at grad school).
  • I nearly broke my arm while working with the french press for cell lysis (it really is a workout in itself!)
  • Having flexible working hours in the lab was good, but there were days when I took this for granted and ended up being awfully lazy. I am still learning to implement a fixed schedule for the weekdays to increase my productivity through the week.
  • Caffeine addiction is a real thing. I now even have a brew preference to satisfy my taste buds and more importantly, my brain.