Fellow's research: Identification of an enzyme that neutralizes neurodegeneration-causing oxidized phospholipids
30 Jan 2019
Siddhesh S. Kamat, Intermediate Fellow
In our recently published article, we identified an enzyme that detoxifies lipids oxidized by reactive oxygen species (ROS) produced as by-products of oxygen metabolism in cells. These oxidized lipids trigger apoptosis, a form of programmed cell death occurring normally, and thus, the enzymes metabolizing these oxidized lipids save cells from undergoing premature apoptosis.
Oxidative stress is an imbalance between the production of oxidants and their neutralization by antioxidants in the body. Prominent amongst these oxidants are the ROS, comprising of superoxide, hydroxyl radical, and hydrogen peroxide. During oxidative stress, the excess ROS damage important biomolecules like DNA, proteins and lipids, resulting in perturbation of regular cellular functions. In the cell, lipid membranes serve as the first line of defence against ROS by providing a physical barrier to their diffusion and are therefore the primary targets for oxidative damage by ROS. In particular, the phosphatidylserine (PS) class of lipids is important, as these lipids when oxidized trigger cellular apoptosis. While several studies describe the production of oxidized PS in mammalian systems, virtually nothing is known about the enzymatic pathways that metabolize these oxidized lipid products in vivo. In mammals, several cell types (e.g. neurons, macrophages, cancer cells) have inherently very high oxygen consumption and in turn, elevated ROS. However, these cells survive well under such conditions without undergoing apoptosis, presumably because of the presence of enzymes that can detoxify the oxidized PS lipids.
We had two main objectives in this study: First, to uncover the chemical structures of the oxidized PS lipids produced in presence of surplus ROS; and second, to identify an enzyme that could metabolize these oxidized PS lipids in vivo.
To achieve our objectives, we first needed to simulate conditions of chronic oxidative stress in mammalian cells. In collaboration with Harinath Chakrapani’s lab at IISER Pune, we synthesized and characterized a novel small molecule that robustly generates ROS in different mammalian cells. Using this chemical probe, we successfully produced oxidized PS in mammalian cells, and in conjunction with advanced mass spectrometry and nuclear magnetic resonance techniques, we, for the first time, were able to elucidate the structure of oxidized PS lipids in vivo.
Next, to identify the enzyme that metabolizes these oxidized PS lipids, we developed a chemical genetic screen. This screen identified that PHARC (polyneuropathy, hearing loss, ataxia, retinitis pigmentosa, and cataract) associated serine hydrolase ABHD12 is a major oxidized PS lipase. Further, we showed that ABHD12 regulates levels of oxidized PS lipids under severe inflammatory stimuli in the mammalian brain and innate immune system. We thus build on the existing model of PHARC, and posit a role for oxidized PS lipids in neurodegeneration.
A chemical-genetic screen identifies ABHD12 as an oxidized-phosphatidylserine lipase Dhanashree S. Kelkar1, Govindan Ravikumar1, Neelay Mehendale1, Shubham Singh1, Alaumy Joshi, Ajay Kumar Sharma, Amol Mhetre, Abinaya Rajendran, Harinath Chakrapani, Siddhesh S. Kamat. Nature Chemical Biology. January 2019.
1These authors contributed equally to this work.
Banner image credits: Movie of apoptosis, confocal. Dr Jeremy Skepper.