Dissipation of lysosome pH impairs formation and collapses existing LPS-induced lysosome tubules in macrophages

While lysosomes are typically globular in morphology, lipopolysaccharide-activated macrophages reorganize lysosomes into an expanded tubular network. Here, we sought to determine if the V-ATPase and the lysosomal pH contribute to LPS-mediated lysosome tubulation in macrophages. We found that inhibition of the V-ATPase prevented lysosome tubulation and collapsed preformed tubules. However, the V-ATPase also controls mTORC1 activity, which itself is needed for tubulation. To distinguish between lysosomal pH and mTORC1, we turned to NH4Cl, which alkalinized lysosomal pH but did not interfere with mTORC1; yet NH4Cl blocked tubulation showing that an acidic lysosomal pH is needed for lysosome remodelling. Moreover, clamping the pH to either acidic or alkaline pH caused tubules to collapse, indicating that the pH gradient promotes tubulation, rather than a specific pH. These effects were not due to altered microtubule organization or impaired lysosome motility, suggesting that motors remained associated with lysosomes. On the other hand, while LPS did not alter the average pH of spherical or tubular lysosomes, growing tubules displayed a more acidic peripheral end relative to the pericentral end. Based on this observation, we propose that a localized pH gradient along the tubule may enable tubulation by modulating factors that catalyse tubule growth.