PI4P and BLOC-1 remodel endosomal membranes into tubules

Morphodynamical adaptation of the endolysosomal system to stress

In eukaryotes, the spatiotemporal control of endolysosomal organelles is central to the maintenance of homeostasis. By providing an interface between the cytoplasm and external environment, the endolysosomal system is placed at the forefront of the response to a wide range of stresses faced by cells. Endosomes are equipped with a dedicated set of membrane-associated proteins that ensure endosomal functions as well as crosstalk with the secretory or the autophagy pathways. Morphodynamical processes operate through local spatialization of subdomains, enabling specific remodeling and membrane contact capabilities. Consequently, the plasticity of endolysosomal organelles can be considered a robust and flexible tool exploited by cells to cope with homeostatic deviations. In this review, we provide insights into how the cellular responses to various stresses (osmotic, UV, nutrient deprivation, or pathogen infections) rely on the adaptation of the endolysosomal system morphodynamics.

Arf1-PI4KIIIβ positive vesicles regulate PI(3)P signaling to facilitate lysosomal tubule fission

Formation and fission of tubules from autolysosomes, endolysosomes, or phagolysosomes are required for lysosome reformation. However, the mechanisms governing these processes in these different lysosomal organelles are poorly understood. Thus, the role of phosphatidylinositol-4-phosphate (PI(4)P) is unclear as it was shown to promote the formation of tubules from phagolysosomes but was proposed to inhibit tubule formation on autolysosomes because the loss of PI4KIIIβ causes extensive lysosomal tubulation. Using super-resolution live-cell imaging, we show that Arf1-PI4KIIIβ positive vesicles are recruited to tubule fission sites from autolysosomes, endolysosomes, and phagolysosomes. Moreover, we show that PI(4)P is required to form autolysosomal tubules and that increased lysosomal tubulation caused by loss of PI4KIIIβ represents impaired tubule fission. At the site of fission, we propose that Arf1-PI4KIIIβ positive vesicles mediate a PI(3)P signal on lysosomes in a process requiring the lipid transfer protein SEC14L2. Our findings indicate that Arf1-PI4KIIIβ positive vesicles and their regulation of PI(3)P are critical components of the lysosomal tubule fission machinery.

Phagocytosis: Phagolysosome vesiculation promotes cell corpse degradation

Cutting up food into small pieces is well known to improve digestion. New work now shows that this concept also applies in the cellular world, by demonstrating that phagolysosome vesiculation promotes cell corpse degradation in Caenorhabditis elegans blastomeres.

Type II phosphatidylinositol 4-kinases function sequentially in cargo delivery from early endosomes to melanosomes

Melanosomes are pigment cell-specific lysosome-related organelles in which melanin pigments are synthesized and stored. Melanosome maturation requires delivery of melanogenic cargoes via tubular transport carriers that emanate from early endosomes and that require BLOC-1 for their formation. Here we show that phosphatidylinositol-4-phosphate (PtdIns4P) and the type II PtdIns-4-kinases (PI4KIIα and PI4KIIβ) support BLOC-1-dependent tubule formation to regulate melanosome biogenesis. Depletion of either PI4KIIα or PI4KIIβ with shRNAs in melanocytes reduced melanin content and misrouted BLOC-1-dependent cargoes to late endosomes/lysosomes. Genetic epistasis, cell fractionation, and quantitative live-cell imaging analyses show that PI4KIIα and PI4KIIβ function sequentially and non-redundantly downstream of BLOC-1 during tubule elongation toward melanosomes by generating local pools of PtdIns4P. The data show that both type II PtdIns-4-kinases are necessary for efficient BLOC-1-dependent tubule elongation and subsequent melanosome contact and content delivery during melanosome biogenesis. The independent functions of PtdIns-4-kinases in tubule extension are downstream of likely redundant functions in BLOC-1-dependent tubule initiation.