Lysosomal physiology

Abstract

Lysosomes are acidic compartments filled with more than 60 different types of hydrolases. They mediate the degradation of extracellular particles from endocytosis and of intracellular components from autophagy. The digested products are transported out of the lysosome via specific catabolite exporters or via vesicular membrane trafficking. Lysosomes also contain more than 50 membrane proteins and are equipped with the machinery to sense nutrient availability, which determines the distribution, number, size, and activity of lysosomes to control the specificity of cargo flux and timing (the initiation and termination) of degradation. Defects in degradation, export, or trafficking result in lysosomal dysfunction and lysosomal storage diseases (LSDs). Lysosomal channels and transporters mediate ion flux across perimeter membranes to regulate lysosomal ion homeostasis, membrane potential, catabolite export, membrane trafficking, and nutrient sensing. Dysregulation of lysosomal channels underlies the pathogenesis of many LSDs and possibly that of metabolic and common neurodegenerative diseases.

Keywords: TFEB; TPC1; TPC2; TRPML1; lysosomal exocytosis; lysosomal storage disease; mTOR.

Figure 1

(Upper panel) Lysosomal export and trafficking pathways. Lysosomes receive inputs from both endocytic and autophagic pathways. Endocytic substrates are delivered first to early endosomes and then to late endosomes (LEs). LEs then fuse with lysosomes to form endolysosome (EL) hybrids. Damaged intracellular organelles enter the autophagic pathway in autophagosomes (APs), which then fuse with lysosomes to form autolysosomes (ALs). Endocytic and autophagic substrates are degraded in ELs and ALs by lysosomal hydrolases. ALs and ELs may exchange materials or fuse directly to form EL-AL hybrids (139). Upon degradation, insoluble catabolites, such as lipids, can be transported to the trans-Golgi network (TGN) for reutilization via transport vesicles (TVs) in the retrograde trafficking pathway or can be released into the extracellular medium via lysosomal exocytosis. Upon completion of lysosomal degradation, ALs undergo extensive tubulation to become tubular ALs (Tu-ALs), from which protolysosomes are regenerated via a fission-based budding-off mechanism. Several trafficking steps are Ca²⁺ sensitive as indicated (2, 18, 25, 83). The plasma membrane, early endosomes (EEs), LEs, and lysosomes contain compartment-specific phosphoinositides [PI(3)P, PI,(4,5)P₂, and PI(3,5)P₂] to regulate membrane trafficking and activity of ion channels in a compartment-specific manner. Increased acidification (from pH 7.4 to pH 4.6) is associated with endosomal maturation. (Lower panel) Lamp1 (Lysosome-associated membrane protein 1) surface staining allows one to monitor the insertion of lysosomal membrane proteins into the plasma membrane by using a monoclonal antibody against a luminal epitope of Lamp1 in non-permeabilized cells. Mucolipin synthetic agonist 1 (ML-SA1) treatment resulted in the localization of Lamp1 (red) on the plasma membrane in non-permeabilized wild-type, but not trpml1 knockout, macrophages. The total amount of Lamp1 (green) proteins was detected by using the same antibody in permeabilized cells. Abbreviations: LY, lysosome MVB, multivesicular body. Lower left panel was modified from Reference 35 with permission.

Figure 2

Ion channels and transporters in the lysosome. (a) An illustration of a whole-lysosome patch-clamp recording (23, 42). Right panel was modified from Reference 42 with permission. (b) (Right) [←**The lysosome is an acidic compartment and with relatively high [Ca²⁺]. These gradients are established by V-ATPases and putative Ca²⁺ transporters. The lysosomal membrane potential (Δψ) is cytosolic-side negative, with Δψ ranging from −20 to −40 mV. The products of lysosomal degradation, such as amino acids (AAs), sugars, and lipids, are transported out of the lysosome via specific exporters. Among the conductances that have been identified, two-pore channels (TPCs) encode I_Na, and Mucolipin TRP channels (TRPMLs) encode I_Ca and I_Fe; ClC-7 is presumed to encode I_Cl. Several other conductances, such as I_H and I_K, have been electrophysiologically but not molecularly characterized. (Left) I-Ψ curves of four lysosomal channels or currents (TPCs, TRPMLs, I_Cl, and I_K) are shown on the left.

Figure 3

A model for nutrient sensing by two-pore channels (TPCs). (Stage 1) In nutrient-replete cells, TPCs are minimally open at resting lysosomal membrane potential (Δψ). (Stage 2) Upon a drop in [ATP], TPC2 opens to promote Na⁺ efflux (into the cytosol) and to depolarize the membrane (lumen becomes less positive). In TPC1-expressing organelles, a ΔΨ above TPC1’s activation threshold also triggers TPC1 opening. (Stage 3) Following lysosomal alkalinization, TPC1 lowers its activation threshold. Membrane depolarization and Na⁺ efflux potentiate H⁺ pumping by the V-ATPase and help stabilize pH. Strong positive feedback between Δψ and TPC1 can also lead to plateau potentials and membrane bistability. Ψ_Na denotes Na⁺ Nernst potential. See References 53 and for details.