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.
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).
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.
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
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.
The thing with first-year rotations in a Ph.D. program is that anxiety starts kicking in somewhere along the way when you consciously identify the lab that you want to join and want to get started right away. Having realized that this is going to be a long journey and rushing into things may not help, I am now gaining patience and perspective, and hope to make the most of the remaining time of my first year.
Rotations are a great way to learn about a lab and get involved in the nitty-gritty of research. I was warned at the beginning by a few seniors that I would either love a lab or reject it within the first few weeks of the rotation. Mind you – this has nothing to do with the science pursued in the lab (one wouldn’t decide to rotate in a lab if they didn’t find the research interesting in the first place). This is more about getting comfortable with the way a lab functions and deciding if the environment is a good fit for you. An eight-week lab rotation is really like an eight-week long interview with a potential PI and the lab! It is essential to identify the kind of relationship you foresee having with your advisor for the next couple of years (and beyond). This is perhaps one of the most important aspects of a rotation for me, next to the research work. A good mentor-mentee relationship can go a long way and can be extremely beneficial to one’s academic/professional career. I prefer having an open channel of communication with my mentor and learn as much as possible from him/her.
Not all graduate programs require laboratory rotations. Many departments or programs accept or reject students simply based on their application and/or an interview. In the UK for example, students are recruited to work on specific projects and grants as a part of their Ph.D. for the time period of around 3 years. This may not benefit the candidates who wish to propose their own ideas and develop their own thesis based on their individual research interests. In the US, for most graduate programs in the life sciences (mainly biology and chemistry), the average time for graduation is around 5-6 years. I believe that the freedom and independence of this system trump the short graduation time of the other systems. Although I am certain that both sides have their set of merits and demerits, at the end of the day, the journey is unique to each one of us and what we make of the experience matters the most.
Glioblastoma multiforme (GBM) is one of the most invasive forms of malignant brain tumors. By the time the tumor is removed from a region in the brain, the cancer cells rapidly metastasize and spread throughout the brain. Common treatment procedures such as chemotherapy and radiotherapy are not completely effective due to the aggressive nature of the tumor invasion. Therefore, many treatments also target the migratory properties of the tumor cells. Several proteins (focal adhesion kinase, paxillin, vinculin) are over-expressed in the extracellular matrix of the tumor microenvironment and help the GBM cells proliferate through the brain tissues. A treatment approach is to target these proteins and hopefully prevent -or at least reduce- the tumor cell invasion.
Studying this type of a cancer model is tricky. Most of the work that has been done in the traditional 2-dimensional cell culture environment cannot be translated into the 3-dimensional environment of the brain. We need a system that mimics the brain to realistically model the tumor cell growth and migration. As a part of my third rotation, I have been investigating the migration characteristics of GBM cells in tissue-engineered, 3-dimensional cell culture matrices that mimic the brain environment. The cells are grown in a collagen matrix containing components of the brain extracellular matrix (hyaluronan, astrocytes, etcetera) and the tumor cell migration is studied by tracking the focal adhesion proteins. However, this is not easy. Cancer cells have shown to modify their migratory patterns based on the physical conditions of the tumor microenvironment (Herrera-Perrez et al. 2015 Tissue Engineering Part A). This makes it more difficult to target the adhesion receptors to ultimately inhibit tumor invasion. It is also challenging to prevent the disruption of the cross-talk between the targeted receptor protein and other important signaling molecules during evaluation of the treatment procedures. Overall, new innovative strategies are required that focus on the diversity and adaptability of tumor cell invasion and migration.
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.
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.
The entire set of 518 protein kinases in the human genome makes up one of the largest of all human gene families. These enzymes catalyze the phosphorylation of proteins, specifically serine/threonine and tyrosine residues – an important reaction that regulates key cellular functions like cell division, metabolism, and apoptosis in normal and disease states. This makes kinases key therapeutic targets in several diseases such as cancer, neurodegeneration, behavioral disorders, diabetes, and cardiovascular diseases. Interestingly, both the labs that I’ve been in so far are focussed on kinases involved in pancreatic/prostrate cancer and GPCR signaling in the light of alcohol/drug addiction. Leaving this nice phylogenetic tree here as a reminder and reflection of kinome research!
I got a grant for my ion channel research project! Receiving the graduate research grant award has immensely boosted my morale, and has encouraged me towards the scientific career path that I wish to pursue.
“On behalf of the Graduate Studies Office, I am pleased to inform you that you are a recipient of a Graduate Research Grant for the 2014-2015 Academic Year. Those who evaluated your proposal were impressed with the project. We congratulate you and look forward to your successful completion of the proposed research. We will be interested in your progress as you move along in your research, and look forward to hearing your presentation at Student Research Day next spring.
The Graduate School seeks, through such awards, to encourage the highest standards of scholarship among its graduate students and we are extremely gratified to provide this opportunity for you to pursue your theory.”
Our lab finally got a new fluorescence/chemiluminescence imager and it has been great for our investigations so far. I have also been getting many positive western blot results. Evaluating the right antibody concentrations for the blots and tweaking the immunoprecipitation protocols to fit my experiments was a considerable amount of work. My next goal is to quantify the protein bands and figure out the statistical significance of GIRK channel subunit interactions. I am so glad to have had worked on it during the fall semester. I can now concentrate on putting my findings together by the time I graduate in spring(!).
Besides this, I am looking forward to working on the electrophysiology rig with my lab mates next semester. We have been concentrating on the biochemical experiments since the past couple of months. It would be interesting to perform preliminary patch clamping experiments on the cells expressing the mutated and wildtype channel subunits.
I finally got down to updating this space after a long break. I have been very busy with the research work, classes and teaching. It feels great to finally break the writing dry-spell.