Tubular lysosome biogenesis in innate immune cells

Lysosomes are essential organelles required for breakdown of endocytic and biosynthetic cargo, pathogen killing and autophagy. In most cells, lysosomes are typically small punctate structures. By contrast, innate immune cells like macrophages and dendeitic cells that have been exposed to bacterial lipopolysaccharids (LPS) exhibit strikingly tubular lysosomes (TLs) and lysosome-related major histocompatility class II (MHCII) compartments (MIIC), respectively. TLs are suggested to play a role in phagosome maturation and retention of fluid-phase endocytic uptake in activated macrophages. In addition, the dendritic cell tubular MIIC (tMIIC) may be involved in antigen presentation. Since remarkably little was known about how tubular lysosomes form, I took to investigate the molecular requirements for this process in macrophages and dendritic cells and present my findings in this thesis. Here I confirm that microtubules are necessary as a template for lysosome tabulation, along with dynein and kinesin microtubule-dependendent motors. We were first to identify molecular components necessary for lysosome tabulation; TL biogenesis required the concerted action of the Ar18b GTPasem along with its effector SKIP, a kinesin adaptor proteins for dynein and kinesin, respectively. Importantly, we observed that TLs are highly dynamic structures whereas punctate lysosomes are conspicuously more static. I also present evidence that mTOR, a lysosomal protein kinase, is required for LPS-induced TL biogenesis and cell surface delivery of MHCII in macrophages and dendritic cells. First, I show that the MyD88-P13K-Akt-mTOR signaling pathway regulates LPS-induced lysosome tabulation. Second, I demonstrate that mTOR is required for anterograde lysosomal transport suggesting that this kinase may regulate tabulation and antigen presentation by modulating the microtubule-based motor activity of lysosomes. Finally I present preliminary data on the properties of tubular lysosomes compare to punctate lysosomes in an effort to characterize these organelles. Among the data presented is evidence that total lysosomal volume increases significantly upon tabulation, which may have important underlying implications in antigen sampling and processing. Overall, my work has expanded on our knowledge on our knowledge of how morphology and trafficking of lysosomes is modulated in immune cells, which may alter cell function.