The transport of lysosomal enzymes

Abstract

This paper reviews the experimental evidence for the proposal that hydrolytic enzymes are introduced into lysosomes of cultured fibroblasts only after secretion and receptormediated recapture.Key words: lysosornes, lysosomal enzymes, pinocytosis, secretion, of-biduronidase How are hydrolytic enzymes transferred from the site of their synthesis to lysosomes? In the usual concept of lysosome formation, the hydrolases proceed from rough t o smooth endoplasmic reticulum, then either t o the Golgi apparatus or t o a specialized region of the smooth endoplasmic reticulum known as GERL. There they are concentrated into small vesicles (primary lysosomes) which detach from the Golgi or from GERL. Eventually these vesicles fuse with phagocytic, pinocytic, or autophagic vacuoles which contain macromolecules t o be hydrolyzed but not the necessary enzymes. The organelles that result from the fusion -i.e., the secondary lysosomes -contain enzymes, substrates, and the appropriately acid environment for hydrolysis to take place (for reviews, see Refs. 1-4). The best evidence for the transport of the hydrolytic enzymes by way of primary lysosomes has been obtained in polymorphonuclear leukocytes. Particles which are rich in acid hydrolases and which correspond t o primary lysosomes -the azurophilic granuleshave been isolated by centrifugation and have been seen t o fuse with phagocytic vacuoles after ingestion of bacteria (2). However, the evidence is weaker for nonphagocytic mammalian cells, since primary lysosomes have not been isolated and their role in enzyme transport has been deduced from static morphological studies by electron microscopy. THE SECRETION-RECAPTURE HYPOTHESISAn alternative hypothesis would have hydrolytic enzymes secreted t o the cell exterior (perhaps through secretory vesicles), recaptured by a receptor-mediated endocytosis, and only then packaged into lysosomes (Fig. 1). Prior t o secretion, the enzymes would be equipped with a structural feature (a "recognition marker") to insure binding to the *Postdoctoral Fellow of the Arthritis Foundation

An alternative hypothesis of cellular transport of lysosomal enzymes in fibroblasts. Effect of inhibitors of lysosomal enzyme endocytosis on intra- and extra-cellular lysosomal enzyme activities

“…According to the Hickman-Neufeld hypothesis for transport of lysosomal enzymes in fibroblasts, these enzymes have to be secreted before they reach the lysosomes via a receptor-mediated recapture (Hickman & Neufeld, 1972;Neufeld et al, 1977). Selective prevention of lysosomal-enzyme recapture should result in an intracellular decrease and extracellular increase in lysosomal-enzyme activities.…”

“…The rate of intracellular lysosomal-enzyme decrease would be determined by the intracellular half-life of these enzymes. Previous experiments have established a half-life of 9 days for fi-N-acetylglucosaminidase prepared from fibroblast secretions (Hickman & Neufeld, 1972;Neufeld et al, 1977) and even shorter half-lives for other lysosomal enzymes from non-fibroblast sources (Hickman & Neufeld, 1972;von Figura & Kresse, 1974). None of the conditions by which recapture of released lysosomal enzymes was prevented over a period of 12-21 days led to a decrease in intracellular lysosomal enzyme activities.…”

Angiotensin-converting enzyme in cultured endothelial cells. Synthesis, degradation, and transfer to culture medium.

“…Lysosomal enzymes can be secreted from fibroblasts into the extracellular space and then taken back to the cells. 15 " 17 Miller et al 1819 clearly showed that the soluble glycoproteins (named collectively epiglycanin) released by TA3 Ha tumor cells are shed from a surface-bound pool. Finally, Rogers et al 20 and Alt et al 21 have shown that the membrane-bound and secreted form of the n chain of IgM are coded for by different mRNA's and hence must be independently synthesized.…”

Selective noncompetitive assimilation of bovine testicular beta-galactosidase and bovine liver beta-glucuronidase by generalized gangliosidosis fibroblasts.

“…Neufeld et al (12) have postulated that extracellular enzymes are selectively assimilated by a series of steps, including binding of the enzyme to the cell surface through carbohydrate residues, endocytosis, and, finally, fusion of the endocytotic vacuoles with the lysosomes. A human platelet ,B-glucuronidase (17), a human aiduronidase (18), bovine testicular ,8-galactosidase (19), and a variety of other human lysosomal enzymes (20) have been suggested as recognition imarkers for the selective uptake of the respective enzymes by fibroblasts.…”