lysosome

**Structure and function** - Membrane-bound organelle with an acidic lumen (pH ~4.5–5), maintained by an H+-ATPase proton pump. - Contains acid hydrolases: proteases, nucleases, lipases, glycosidases, sulfatases, and phosphatases. The marker enzyme is acid phosphatase. - Digests material delivered by several routes: - Phagocytosis (extracellular material, e.g., bacteria in macrophages): phagosome + lysosome → phagolysosome. - Endocytosis (receptor-mediated): endosome → lysosome. - Autophagy (the cell's own damaged organelles): autophagosome + lysosome → autolysosome. - Lysosomal membrane proteins are heavily glycosylated, which protects them from the organelle's own hydrolases.

**Biogenesis and targeting** - Hydrolases are synthesized in the rough ER and modified in the cis-Golgi. - Phosphotransferase (GlcNAc-1-phosphotransferase) adds mannose-6-phosphate (M6P) to N-linked oligosaccharides on lysosomal enzymes. - M6P receptors in the trans-Golgi network bind these enzymes and deliver them to lysosomes via vesicles. - **I-cell disease (inclusion-cell disease, mucolipidosis II):** - Cause: defective phosphotransferase, so M6P is not added. - Result: enzymes are secreted extracellularly instead of reaching lysosomes, and plasma lysosomal enzyme levels are elevated. - Findings: coarse facial features, clouded corneas, skeletal abnormalities, restricted joint movement, and kyphoscoliosis. Intellectual disability and failure to thrive follow. Death usually occurs in childhood (often by age 5–7). - Cells contain characteristic inclusion bodies.

**Lysosomal storage diseases (mostly autosomal recessive; Fabry, Hunter, and Danon are X-linked)**

| Disease | Deficient enzyme | Accumulated substrate | Key features | |---|---|---|---| | Tay-Sachs | Hexosaminidase A | GM2 ganglioside | Cherry-red macular spot, **no** hepatosplenomegaly, neurodegeneration, onion-skin lysosomes; Ashkenazi Jews | | Sandhoff | Hexosaminidase A and B | GM2 ganglioside, globoside | Like Tay-Sachs, plus hepatosplenomegaly | | Gaucher (most common) | Glucocerebrosidase | Glucocerebroside | Hepatosplenomegaly, pancytopenia, bone crises, avascular necrosis of the femoral head; "crumpled tissue paper" macrophages; treat with enzyme replacement | | Niemann-Pick | Sphingomyelinase | Sphingomyelin | Cherry-red spot, hepatosplenomegaly, neurodegeneration, "foam cells" | | Fabry | α-Galactosidase A | Ceramide trihexoside | X-linked; peripheral neuropathy (burning pain), angiokeratomas, hypohidrosis, renal failure, cardiomyopathy/stroke | | Krabbe | Galactocerebrosidase | Galactocerebroside | Peripheral neuropathy, optic atrophy, globoid cells | | Metachromatic leukodystrophy | Arylsulfatase A | Cerebroside sulfate | Central and peripheral demyelination, ataxia, dementia | | Hurler | α-L-iduronidase | Heparan and dermatan sulfate | Corneal clouding, coarse facies, hepatosplenomegaly, developmental delay | | Hunter | Iduronate sulfatase | Heparan and dermatan sulfate | X-linked; mild Hurler-like picture, **no** corneal clouding, aggressive behavior | | Pompe | Lysosomal α-1,4-glucosidase (acid maltase) | Glycogen | Cardiomegaly, hypotonia, early death in infantile form |

Mnemonics: - Fabry: "Fabry = **F**ucked up, **a**ngiokeratoma, **b**urning pain, **r**enal, **y**oung stroke." - Hunter: "Hunters see clearly (no corneal clouding) and aim for the X (X-linked)." - Cherry-red spot: Tay-Sachs, Niemann-Pick, and also central retinal artery occlusion. Tay-Sachs lacks hepatosplenomegaly; Niemann-Pick has it.

**Other clinical associations** - **Chédiak-Higashi syndrome:** LYST gene defect causes impaired phagolysosome fusion due to a lysosomal trafficking regulator defect. Findings: recurrent pyogenic infections, partial albinism, peripheral neuropathy, giant granules in leukocytes, and progressive neurodegeneration. - **Drug-induced lysosomal accumulation:** Chloroquine and hydroxychloroquine are weak bases that raise lysosomal pH, impairing function. - **Lysosomal membrane stabilization:** Glucocorticoids stabilize lysosomal membranes, which contributes to their anti-inflammatory effect. - **Gout:** Phagocytosed urate crystals rupture lysosomes in neutrophils, releasing enzymes that perpetuate inflammation. Colchicine inhibits microtubule-dependent neutrophil function, including phagocytosis. - **Silicosis/asbestosis:** Macrophages phagocytose particles that damage lysosomal membranes, causing enzyme release and fibrosis. - **Neutrophil function:** Lysosomal granules contain myeloperoxidase (in azurophilic granules), which generates HOCl for microbial killing. - **Pathology:** Lysosomal enzyme leakage contributes to cell injury and necrosis, and lipofuscin ("wear-and-tear" pigment) represents undigested residue within lysosomes, accumulating in aging cells.

**High-yield takeaways** - Lysosomes are acidic (pH ~4.5–5), and acid hydrolases are inactive at cytosolic pH. This limits damage if lysosomal contents leak. - M6P tagging in the Golgi directs enzymes to the lysosome. Its loss causes I-cell disease. - Most lysosomal storage diseases are autosomal recessive. The exceptions are X-linked: Fabry, Hunter, and Danon. - Think "storage disease" when you see neurodegeneration, hepatosplenomegaly, and a cherry-red macula in a child.