Discovery & Architecture

1. De Duve’s 1955 Discovery

Christian de Duve, at the Université catholique de Louvain, was working through the 1950s on rat liver fractionation to localise enzymatic activities. His laboratory separated cellular components by differential centrifugation — nuclei, mitochondria, microsomes, and a dense “light mitochondrial” fraction. He observed that acid phosphatase activity in this fraction was latent: inactive on intact particles, but fully revealed after disruption with detergent or freeze-thaw.

The interpretation: a membrane-bounded organelle contains hydrolases that are kept sequestered from the cytosol. De Duve named it the lysosome (“dissolving body”, from Greek lysis) in 1955. The accompanying electron microscopy (Novikoff 1956) confirmed the morphology: a 250–1000 nm pleomorphic membrane-bound organelle in the cytoplasm.

De Duve shared the 1974 Nobel Prize in Physiology or Medicine with Albert Claude and George Palade for the biochemical and structural discoveries of the intracellular compartments. His insight — that biochemical latency revealed structural compartmentation — was a founding methodology of modern cell biology.

2. The Limiting Membrane

The lysosome’s single limiting membrane must contain ~60 hydrolases at millimolar concentrations without being degraded itself. Two adaptations:

• Heavy glycosylation: the luminal faces of the ~25 dominant lysosomal membrane proteins (LAMP1/2/3, LIMP1/2, CD63, LAPTM4A/B, etc.) are covered in dense N- and O-glycans forming a “glycocalyx” ~8 nm thick that physically prevents hydrolases from reaching the membrane surface.
• Rapid turnover: lysosomal membrane proteins are actively replaced from the Golgi, so damaged or cleaved pieces are replenished. LAMP1/2 alone account for ~40% of the limiting-membrane protein mass.

LAMP2 deficiency causes Danon disease: cardiomyopathy, myopathy, and cognitive impairment — a direct proof that the glycocalyx is essential for lysosomal function, and not a bystander. LAMP2A, a splice isoform, additionally serves as the receptor for chaperone-mediated autophagy (Module 3).

3. Morphology & Heterogeneity

Lysosomes are heterogeneous in size, shape, content, and cellular position. Sub-types:

• Primary lysosomes: newly formed from the TGN, empty of cargo.
• Secondary lysosomes: fused with endosomes / phagosomes / autophagosomes; contain cargo being digested.
• Residual bodies: contain undigested material, often lipofuscin (oxidised protein-lipid aggregate); accumulate with age.
• Specialised lysosomes: lysosome-related organelles (LROs) — melanosomes (melanocytes), lytic granules of cytotoxic T cells, platelet dense granules, Weibel-Palade bodies, Birbeck granules. Share biogenesis with lysosomes but have specialised function.

Per-cell lysosome number varies widely: ~50 in fibroblasts, ~100–500 in macrophages, thousands in cells with high autophagy burden.

4. Endolysosomal Maturation

A lysosome is the terminal member of a maturation ladder: early endosome (Rab5-positive, pH ~6.3) → late endosome/multivesicular body (Rab7, pH 5.5) → lysosome (LAMP1, pH 4.7). Transitions involve Rab5-to-Rab7 conversion, ESCRT- mediated intraluminal vesicle formation (late endosome becomes MVB), HOPS complex tethering for fusion, and additional V-ATPase loading. The pH gradient is progressive, and different classes of hydrolases are maximally active at different rungs of the ladder.

5. Intracellular Positioning & Dynamics

Lysosomes are not static. They move on microtubules via kinesin (plus-end, toward cell periphery) and dynein (minus-end, toward perinuclear region). Under nutrient abundance, kinesin dominates and lysosomes distribute toward the periphery; under starvation, dynein dominates and they cluster perinuclearly, promoting autolysosome formation. Positioning is regulated by Arl8b/SKIP, Rab7-RILP, BORC, and TRPML1-dependent Ca²⁺ release. The positioning is itself a variable in cell-state signalling — one of the less obvious elements of lysosome biology.