mTOR induces lysosome expansion by selective translation of lysosomal transcripts during phagocyte activation

Abstract

The molecular mechanisms that govern and adapt organelle number, size, morphology and activities to suit the needs of many cell types and the conditions that a cell may encounter remain poorly defined. Lysosomes are organelles that degrade cargo from a variety of routes including endocytosis, phagocytosis and autophagy. Lysosomes have emerged as a signalling platform that senses and couples stress signals such as nutrient deprivation to regulatory kinase hubs like mTOR and AMPK to modulate metabolic activity. For phagocytes and antigen-presenting cells like macrophages and dendritic cells, lysosomes are a kingpin organelle since they are essential to kill and process pathogens, and present antigens. During phagocyte activation, lysosomes undergo a striking reorganization, changing from dozens of globular structures to a tubular network, in a process that requires the phosphatidylinositol-3-kinase-Akt-mTOR signalling pathway. Ultimately, lysosome tubulation is thought to promote pinocytic retention and antigen presentation. We show that lysosome tubulation is accompanied by a rapid boost in lysosome volume and holding capacity during phagocyte activation with lipopolysaccharides. Unexpectedly, lysosome expansion was paralleled with the induction of lysosomal proteins, which was independent of TFEB and TFE3, transcription factors known to scale up lysosome biogenesis. Instead, we demonstrate a hitherto unappreciated mechanism of lysosome expansion via mTOR-dependent increase in translation of mRNAs encoding key lysosomal proteins including LAMP1 and V-ATPase subunits. Collectively, we identified a mechanism of rapid organelle expansion and remodelling driven by selective enhancement of protein synthesis.