Lysosomal solute and water transport

Abstract

Lysosomes mediate hydrolase-catalyzed macromolecule degradation to produce building block catabolites for reuse. Lysosome function requires an osmo-sensing machinery that regulates osmolytes (ions and organic solutes) and water flux. During hypoosmotic stress or when undigested materials accumulate, lysosomes become swollen and hypo-functional. As a membranous organelle filled with cargo macromolecules, catabolites, ions, and hydrolases, the lysosome must have mechanisms that regulate its shape and size while coordinating content exchange. In this review, we discussed the mechanisms that regulate lysosomal fusion and fission as well as swelling and condensation, with a focus on solute and water transport mechanisms across lysosomal membranes. Lysosomal H+, Na+, K+, Ca2+, and Cl- channels and transporters sense trafficking and osmotic cues to regulate both solute flux and membrane trafficking. We also provide perspectives on how lysosomes may adjust the volume of themselves, the cytosol, and the cytoplasm through the control of lysosomal solute and water transport.

Figure 1.

Osmolarity changes in the lysosome lumen. Cargo macromolecules (white chains) are delivered to the lysosomes through endosome-lysosome and autophagosome-lysosome membrane fusion. Upon lysosomal degradation mediated by hydrolases (green), macromolecules are digested into many molecules of catabolites (white circles), and the process consumes many molecules of H₂O. Subsequently, there is a transient increase in luminal osmolality and a transient decrease in luminal water potential. Water influx is mainly mediated by putative lysosomal water channels (blue), and catabolite efflux is mediated by various carriers and transporters (brown). Prior to lysosome fission/reformation, solute and water efflux are required for osmotic condensation of endolysosomes or autolysosomes.

Figure 2.

Plasmalemmal ion and water transport in cell volume regulation. Upon hypotonic and hypertonic challenges, cells undergo swelling and shrinkage, respectively. In order to maintain and restore the volume constancy for cellular metabolism, cells have evolved regulatory volume decrease (RVD) and increase (RVI) responses. During RVD, various K⁺ and Cl⁻ channels and transporters are activated in response to extracellular hypotonicity, which include VRAC (Cl⁻ and organic anions), CaCC (Cl⁻), osmo-sensitive and calcium-activated K⁺ channels (K⁺), H(ypo)ICCs (Ca²⁺), KCCs (K⁺ and Cl⁻), and NKCC1 (Na⁺, K⁺, and Cl⁻). Subsequently, osmotically obligated water efflux is mediated by AQPs. During RVI, various channels and transporters are activated in response to extracellular hypertonicity to transport solutes into cytosol, which include anion exchangers (Cl⁻ and HCO₃⁻), NHEs (Na⁺ and H⁺), KCCs, and NKCC1. Osmotically obligated water influx is also mediated by AQPs.

Figure 3.

Lysosomal ion channels and catabolite exporters Relative to the cytosol, lysosome lumen is high in [H⁺], [Ca²⁺], and [Na⁺], but low in [K⁺]. Lysosomal patch-clamp studies have identified lysosomal Ca²⁺-permeable TRPMLs and P2X4 channels, H⁺/K⁺-permeable TMEM175 channels, Na⁺/Ca²⁺-permeable TPC channels, K⁺-permeable Lyso-BK and TWIK2 channels, and Cl⁻-permeable CLC7 transporters, CLN7 channels, and Lyso-VRAC channels. The proton pump V-ATPase and LysoH/TMEM175 channels regulate lysosomal acidity. Catabolite efflux is mediated by SLC family transporters, which include carbohydrate transporter SLC17A5 (H⁺-coupled), cationic amino acid (AA) uniporter SLC66A1/PQLC2, polar AA (Glu/Asn/Leu) transporter SLC38A9 (H⁺-coupled), cystine transporter SLC66A4/Cystonisin (H⁺-coupled), and neutral AA transporters SLC38A7/SNAT7 (Na⁺-coupled) and SLC36A1/PAT1 (H⁺-coupled). ATP13A2 may mediate polyamine export and possibly Ca²⁺ import. Water flux across the lysosomal membrane is mediated by unidentified water channels (Lyso-AQPs), Lyso-VRAC, and possibly certain catabolite transporters.

Figure 4.

Lysosome volume/size regulation: swelling and condensation, Homotypic fusion of two quasi-spherical lysosomes results in a larger quasi-spherical lysosome. In the absence of water and solute efflux, there is ∼20% excess folded membrane in the newly formed lysosome, i.e., wrinkled membrane with low membrane tension. Upon solute flux, which could be stimulated by fusion cues and/or changes in membrane curvature through yet-to-be-identified solute channels/transporters, followed by subsequent water flux, the newly formed lysosome is enlarged to contain additional ∼40% volume, i.e., turgid membrane with high membrane tension. Upon TPC-mediated Na⁺ release and subsequent water flux, the enlarged lysosome shrinks to reduce the volume and surface area (membrane).

Figure 5.

Lysosomes are water-storage organelles that mediate exocytosis-dependent water secretion. When mammalian cells are exposed to a hypotonic environment, cytosolic [H₂O] increases rapidly through plasma membrane AQPs. Lysosomes, but not early endosomes or mitochondria, selectively uptake water. Cytoplasmic volume = cytosolic volume + organellar volume. By acting as intracellular water storage compartments, lysosomes contribute to cytosolic volume decrease (cytosolic RVD). Upon exocytosis of these watery lysosomes, cytoplasmic volume is decreased (cytoplasmic RVD).