Researchers from CSIR-Institute of Genomics and Integrative Biology, New Delhi, India, and the Ecole Normale Supérieure de Paris, France, recently discovered ARL8B (ADP-ribosyltransferase (Arf)-like 8), a small GTPase acting as the initial mediator of lysosome-dependent lipid degradation in human macrophages. The findings suggest implications for conditions involving lipid accumulation, such as fatty liver disease and atherosclerosis, expanding research interests in this field.
Long assumed as mere fat reservoirs, lipid droplets (LDs) are recently gaining attention for their association with cellular metabolism to many diseases. Siddhesh S. Kamat, Associate Professor, Indian Institute of Science Education and Research Pune studies lipid signalling. He notes “Lipid metabolism is central to most physiological processes and LDs are emerging as important cellular organelles.”
LDs are also induced by pathogens. For instance, LDs are found to be abundant in macrophages after tuberculosis infection. Following inhalation, Mycobacterium tuberculosis is engulfed by macrophages, our professional killer cells. But these notorious mycobacteria have developed counterstrategies too. The survivors thus reside within those macrophages and reprogram the cellular lipid metabolism to ensure their food supply. The resultant fat-filled macrophages are the primary cause of tissue destruction and lung cavities observed in tuberculosis patients.
“There is, however, very little understanding of the role of LDs in macrophages” comments Sheetal Gandotra, Principal Scientist, CSIR-Institute of Genomics and Integrative Biology (CSIR-IGIB), New Delhi. In collaboration with researchers from the Ecole Normale Supérieure de Paris, France, and CSIR-IGIB, New Delhi, Gandotra Lab recently discovered ARL8B (ADP-ribosyltransferase (Arf)-like 8), a small GTPase as the first player mediating the lysosome-dependent lipid degradation in human macrophages.
Gandotra studies tuberculosis infection and her group previously discovered that lipids found in LDs regulate the immune response to tuberculosis infection. While LD turnover by lysosomes is known, none of the mediators involved in the process have been identified yet. Initially, they explored the known cellular processes of lipid degradation to identify the potential mediators. Gandotra says, “Unable to find any in our large-scale screens of proteins on LDs from infected macrophages, we wondered if the transport machinery could be another way of mobilising lipids from LDs.”
The researchers became interested in ARL8B, as known to have role in cellular cargo delivery to lysosomes, they found its abundance on the surface of LDs from infected macrophages. These GTPase proteins, acting as molecular switches in cellular signalling, depend on their shape when bound to either a GTP (Guanosine triphosphate) or GDP (Guanosine diphosphate) molecule. For instance, GTP-bound ARL8B binds lysosome, helping in its movement inside cells. Gandotra says,
When we pick a protein most of the time we rely on a priori knowledge, but sometimes you just have to go with the hunch.
Through extensive analyses, the researchers here vividly show that GDP-bound ARL8B can bind to LDs and the complex formed by both GTP and GDP-bound ARL8B is necessary for establishing contact between LDs and lysosomes, enabling the transfer of lipids from LDs to lysosomes. Using inhibitors for both cytosolic and lysosomal lipid degrading enzymes, coupled with silencing ARL8B expression, they further confirmed that the ARL8B-regulated lysosomal pathway is the primary route for the turnover of resident lipids of LDs, They thus demonstrate that ARL8B is crucial for the turnover of the long-chain unsaturated fatty acids associated with immune response.
Kamat comments, “Here, the authors report a new biological mechanism by which the protein ARL8B regulates the metabolism of LDs, via enhancing their transport to lysosome.” He also thinks that, though preliminary at this stage to say, the interesting findings in this paper might have important biomedical applications in fighting pathogenic microorganisms such as M. tuberculosis, which hijack diverse lipid pathways and LD metabolism to evade and survive in our immune system.
To understand the mechanism better, the group is now planning to identify the external stimuli and other proteins regulating the function of ARL8B. Gandotra adds,
We also hope that this study will piqué the interest of researchers working in the area of fatty liver disease, atherosclerosis, and other conditions where lipid accumulation is part of the pathology.