Lysosomes are enriched in hydrolytic enzymes working at low pH, which include proteases, lipases, glycosidases, nucleases, phosphatases, and sulfatases, and are responsible for elimination of “unwanted” molecules derived from both the cell itself (autophagy, cf. Fig. 74) or from outside of the cells (heterophagy). However, contrary to the historic view of the lysosome as a terminal degradation compartment and as a unit used in the cells mainly as a disposal for garbage, it is increasingly becoming clear that lysosomes are dynamic organelles that receive continuous input from three sides, via the biosynthetic, endocytic, and autophagic pathways. Today, lysosomes cannot be seen as terminal or dead-end compartments. They are able to fuse with late endosomes involving “kiss-and-run”-mechanisms and are also capable of fusing with the plasma membrane.

In panels A, B, and C, lysosomes (Ly) in the cytoplasm of rat small intestinal absorptive cells (panels A and B) and in a rat hepatocyte (panel C) are on display. In panel C, the liver cell lysosomes show a particularly distinct and intense contrast after special uranyl acetate/methyl cellulose adsorption staining, to which the ultrathin sections of the tissue embedded in Lowicryl K4M were exposed (cf. also Fig. 103). The extreme heterogeneity in the lysosomal sizes, shapes, and luminal materials is evident. Densely packed membrane “whirls,” as they occur at high frequency, are visible in most of the lysosomes in panels A and B and are particularly prominent in the large lysosome shown in the inset of panel B. Lysosomes are distributed throughout the cytoplasm of the cells but show higher concentrations in the perinuclear area close to the Golgi apparatus (panels A and C). Lysosome morphologies and the favorite localizations in the Golgi area are considered to be regulated by Rab7 and Rab34 proteins interacting with a particular region of a Rab-interacting lysosomal protein (RILP). In the absorptive cell shown in panel A, numerous lipoprotein particles (LP) are accumulated in dilated Golgi cisternae and large vesicular carrier compartments. The cell segments shown in the micrographs also contain autophagosomes (AV) and multiple mitochondria (M).

The lysosomal enzymes, synthesized and initially glycosylated within the rough endoplasmic reticulum, receive characteristic glycan modifications in the Golgi apparatus. By a two-step process that starts at the cis-Golgi side and continues in medial cisternae of the Golgi stacks, mannose-6-phosphate residues are added to the asparagine-linked oligosaccharides of newly synthesized lysosomal enzymes involving two sequentially acting enzymes, N-acetyl-glucosaminyl (GlcNAc)-phosphotransferase and GlcNAc-l-phosphodiester alpha-N-acetylglucosaminidase (diagram). Presence of the mannose-6-phosphate recognition marker provides the basis for effective sorting through the mannose-6-phosphate receptor of newly synthesized lysosomal enzymes into the lysosomal pathway (cf. Fig. 66).

Lysosomal enzymes and membrane proteins can be located by cytochemical and immunogold methods. Panel A shows enzyme cytochemical staining for acid phosphatase in an acinar cell of the rat pancreas. Electron-dense reaction product for acid phosphatase is visible within cisternae at the trans Golgi side, coated vesicles budding from those sites (arrow), within condensing vacuoles (CV), and lysosomes (Ly), but not within the mature secretory granules. The enzyme localizations at the trans Golgi side reflect the well-studied routes of lysosomal enzymes from the Golgi apparatus to endosomes and lysosomes, although they also are seen in connection with activities of endo-proteases or are involved in the breakdown of cytidine monophosphate during terminal glycosylation (cf. Fig. 37).

Lysosomal enzymes are sorted from the Golgi apparatus and trans Golgi network to the lysosomal pathway by specific binding of the mannose-6-phosphate groups present on the lysosomal enzyme precursors (cf. Fig. 65) by receptors in a divalent cation-dependent or independent manner. The respective receptors, the cation-dependent and independent mannose-6-phosphate receptors (CD-MPR and CI-MPR), are localized in the membranes of the trans Golgi network (TGN). By interactions of acidic cluster-dileucine signals present in their cytosolic domains with Golgi-localized, gamma ear-containing ARF-binding proteins (GGAs) and cooperation of GGAs with adapter protein-1 (AP-1), the lysosomal enzyme-MPR complexes are packaged into clathrin-coated carriers that bud from the TGN and function in transporting the lysosomal enzymes to early and late endosomes. The enzymes dissociate from the receptors at the acidic pH of the endosomes and, together with the fluid phase, are transferred to lysosomes. The receptors are retrieved to the TGN by mechanisms involving AP-1 and a complex of proteins called “retromer.”

Panels B and C show constituents of the lysosomal membrane in human cells. The lysosomal membrane, which contains proton (H⁺) pumps for transport of H ions into the lumen and proteins that transport the final products of digestion into the cytoplasm, has to resist hydrolysis by its own lytic enzymes. The structural membrane proteins, the lysosome-associated membrane proteins (lamps), lysosomal integral membrane proteins (limps), and lysosomal membrane glycoproteins (lgps) are highly glycosylated at the luminal surfaces. Human lamp-1 and lamp-2 have been characterized as major sialoglycoproteins carrying polylactosamine chains. Panel B shows immunogold labeling for lamp at the membranes of lysosomes (Ly) in HeLa cells. In panel C, polylactosamine is located in lysosomes of HeLa cells by means of gold-labeled Datura stramonium lectin.

In some cells, the lysosomal compartment is modified and also has a role as a secretory compartment. Although there are some exceptions, such as the melanocytes, most cells containing secretory lysosomes are derived from the hematopoietic lineage and include granulocytes (cf. Figs. 190 and 191), mast cells (cf. Fig. 159), macrophages (cf. Figs. 157 and 159) and dendritic cells, B and T lymphocytes (cf. Fig. 194), platelets (cf. Figs. 195 and 196), and osteoclasts (cf. Fig. 168). There is a close relationship to compartments termed “lysosome-related organelles,” which comprise cell type-specific organelles that show some features of endosomes and lysosomes and have functions in different cellular processes, such as immunity, pigmentation, and hemostasis. Secretory granules in cytotoxic T-lymphocytes and platelets, melanosomes, and Weibel-Palade bodies belong to this group.

The I-cell disease (mucolipidosis II) is a lysosomal storage disease and is inherited in an autosomal recessive manner. The defect lies in the biosynthesis of the mannose 6-phosphate recognition marker for the targeting of lysosomal enzymes into lysosomes. The effect is therefore most far reaching because it affects all lysosomal enzymes. Specifically, the N-acetylglucosaminyl-1-phosphotransferase, which catalyzes the first step in the synthesis of mannose 6-phosphate (cf. Fig. 65), is defective. The lack of the recognition marker on de novo synthesized lysosomal enzymes results in their secretion into the extracellular space. The detection of elevated serum concentrations of multiple lysosomal enzymes is therefore diagnostic. Clinically, I-cell disease is characterized by early onset and progressive severe psychomotoric retardation accompanied by skeletal abnormalities and coarse facial shape. Surprisingly, not all body cells are devoid of lysosomal enzymes, although they are all lacking phosphotransferase. This indicates that certain cells such as hepatocytes, Kupffer cells, and leukocytes are able to endocytose lysosomal enzymes through an unknown alternate, mannose 6-phosphate-independent mechanism.

Phosphotransferase is a complex molecule and is composed of three dimeric subunits (α₂β₂γ₂). The gene encoding the α and β subunits has been mapped to chromosome 12p and that for the γ subunit to chromosome 16p. The causative genetic defects are not well known except for the lack of transcripts of the α and β subunits, probably due to point mutations or small deletions.

Morphological investigations have revealed the presence of numerous membrane-limited vacuoles, which have given the disease its name: Inclusion cell disease. In panel A, numerous such inclusions are prominently seen in the cytoplasm of dermal fibroblasts of a skin biopsy from a patient. The inset shows the characteristic appearance of the inclusion bodies, which are part empty and part filled with fine granular material. Occasionally, larger electron-dense deposits exist, as seen in the inset. Panel B is a field from the same skin biopsy showing affected Schwann cells and fibroblasts (arrows). Myelinated nerve fibers (NF) are not affected. The unique ultrastructural features exhibited by the inclusions is highly indicative of I-cell disease.