The lysosome, a multi-copy organelle, was chosen as an example to study intraorganelle dynamics. Lysosomal contents and membrane proteins were shown to intermix rapidly in fused mammalian cells, with a t_½ of ~30 min. Lysosomal content intermixing, shown by a sensitive invertase-lysosome/[¹⁴C]-sucrose-lysosome pairing assay, was inhibited greatly by ATP inhibitors and partially by cytochalasin D. Lysosomal membrane protein intermixing was shown by the transfer of LAMP-2, a mouse specific lysosomal membrane antigen, from mouse lysosomes to hamster sucrosomes, sucrose-swollen lysosomes. Lysosomal membrane protein intermixing was also shown by the co-localization of LIMP I, a rat specific lysosomal membrane antigen, and LAMP-1, a mouse specific lysosomal membrane antigen. Co-localization was assessed by both double immunofluorescent staining and double immunogold labeling of thin cryosections. Both lysosomal content and membrane protein intermixing were inhibited by nocodazole, a microtubule disruptor. In fused cells, lysosomes remained small, punctate and scattered throughout the cytoplasm. In comparison to lysosomes, the prelysosomal compartment (PLC), a single copy organelle which is related to the lysosome, congregated together to form an extended PLC complex associated with clustered nuclei. The intermixing of both resident and transient Golgi membrane proteins was studied in fused cells. Resident Golgi membrane protein intermixing was slow, with a t_½ of ~ 1.75 h; it was concomitant with the congregation of the Golgi units. In comparison, the transient Golgi membrane protein was transported much faster from Golgi units to the other Golgi units, with the t_½ ≤ 15 min. Transient Golgi membrane protein transport occurred between separate Golgi units. These results are consistent with two different pathways for resident and transient Golgi membrane protein transport: a slow, lateral diffusion along the Golgi connections transport pathway for resident Golgi membrane proteins; and a rapid, transient protein selective, vesicle-mediated transport pathway for transient Golgi membrane proteins. / Ph. D.

Indiana University-Purdue University Indianapolis (IUPUI) / Autophagy is a conserved cellular process that involves sequestration and degradation of cytosolic contents. The cell can engulf autophagic cargo (lipids, long-lived proteins, protein aggregates, and pathogens) through a double bound membrane called an autophagosome that fuses with a lysosome where hydrolases then degrade these contents. This process is one of the main defenses against starvation and is imperative for newborns at birth. Research on this process has increased exponentially in the last decade since its discovery almost a half a century ago. It has been found that autophagy is an important process in many diseases, continues to be at the forefront of research, and is clearly not fully understood. Our preliminary cell culture data in endothelial and epithelial cells show that a blockade of the de novo ceramide synthesis pathway, during treatment with an autophagy stimulus (cigarette smoke extract exposure), does not result in any reduction in autophagy or autophagic flux. Conversely, when acid sphingomyelinase (ASM) is pharmacologically inhibited, which prevents the generation of ceramide from sphingomyelin in an acidic environment, a profound increase in autophagy is observed. In this work, we hypothesize that (ASM) is an endogenous inhibitor of autophagy. ASM has two forms, a secreted form and a lysosomal form. N-terminal processing in the Golgi determines its cellular fate. In the lysosomal form, the phosphodiesterase is bound in the lysosomal membrane. The pharmacological inhibition mechanism is to release ASM from the membrane and allow other hydrolases to actively degrade the enzyme which, in turn, decreases the activity of ASM. This suggests that either the activity of ASM is a regulator of autophagy or that the presence of ASM, activity aside, is required for the lysosomal nutrient sensing machinery (LYNUS) to function properly. Here, we show that ASM is, in fact, an endogenous inhibitor of autophagy in vitro. The phosphorylation status of P70 S6k, a downstream effector of mammalian target of rapamycin (mTOR), which is part of the LYNUS, shows that dissociation of ASM from the membrane regulates mTOR and disturbs the LYNUS in such a manner as to signal autophagy.