A new role for a class of waxy lipids, ceramides, in helping cells engulf solid matter, including pathogens
18 Jul 2018
By Dr. Siddhesh S. Kamat, Intermediate Fellow
Phagocytosis is the process of internalization of pathogens or cellular debris (generally ≥ 500 nm in size) by immune cells, predominantly macrophages. Once internalized, these particles acquire cellular membranes, form phagosomes, and undergo a sequence of events collectively termed as ‘phagosomal maturation’ culminating in fusion of the phagosomes with lysosomes. During this transformation, the maturing phagosome is transported by motor proteins along microtubules inside cells, and changes profoundly in its protein and lipid composition through interactions with different cellular components and pathways. While several studies describe the changes in the protein composition, little is known about the lipid profile of phagosomes as a function of their maturation.
Recent studies suggest that during maturation, phagosomes exist in two distinct forms: the early phagosome (EP) and the late phagosome (LP). Roop Mallik’s lab at TIFR Mumbai, team showed that EPs move bi-directionally on microtubules, but LPs move uni-directionally; this movement drives the maturing phagosomes to the lysosome for eventual clearance. This unidirectional motion of LPs was attributed to clustering of the motor protein dynein to cholesterol-rich lipid microdomains formed on LPs during maturation. Given the importance of lipids in this process and as a corollary to the aforementioned work, our lab in collaboration with Roop Mallik’s lab, set out to map the lipid profile of maturing phagosomes and identify new lipid pathways in this process.
We use “lipidomics”, a cutting edge mass spectrometry-based technique, to profile lipids in complex biological samples (e.g. tissues, cells, blood, serum, and phagosomes) quantitatively. Lipidomics platform is used for globally profiling the lipid content of maturing phagosomes. We quantitatively profiled 22 lipid classes and over 400 unique lipids and found that the ceramides (a class of waxy lipid), especially very long-chain fatty-acid-containing ceramides, were enriched on mature phagosomes (LPs). Further, we found that ceramide synthase, the major ceramide biosynthetic enzyme, controls the flux of ceramides on maturing phagosomes; pharmacological disruption of this enzyme hampers phagosomal maturation.
Results of our lipidomics studies suggest that at least two more as-of-yet unknown enzymes exist, which regulate intricate lipid networks in phagocytosis, and open new research avenues for studying this exciting evolutionary conserved immunological process.
Lipidomics suggests a new role for ceramide synthase in phagocytosis. Divya Pathak1, Neelay Mehendale1, Shubham Singh, Roop Mallik*, Siddhesh S. Kamat. ACS Chemical Biology, July 2018.
1These authors contributed equally to this work.
*Roop Mallik is an India Alliance Senior Fellow.
Banner image: Wellcome Collection. TEM of cell performing phagocytosis.