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Lysosomes and the Connective Tissue Diseases LUCILLE BITENSKY from the Cellular Biology Division, Kennedy Institute of Rheumatology, Bute Gardens, London

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Lysosomes and the Connective Tissue Diseases LUCILLE BITENSKY from the Cellular Biology Division, Kennedy Institute of Rheumatology, Bute Gardens, London J Clin Pathol: first published as 10.1136/jcp.31.Suppl_12.105 on 1 January 1978. Downloaded from J. clin. Path., 31, Suppl. (Roy. Coll. Path.), 12, 105-116 Lysosomes and the connective tissue diseases LUCILLE BITENSKY From the Cellular Biology Division, Kennedy Institute of Rheumatology, Bute Gardens, London Lysosomes are small intracellular organelles present respondingly, in phagocytic cells primary lysosomes in most or all cells of animals of widely different become attached to endocytic vacuoles and release evolutionary development. In general their diameter their hydrolytic enzymes into these vacuoles, digest- may vary from 0(2 to 0(5 ,tm so that they overlap the ing the endocytosed matter (Cohn and Fedorko, dimensions of mitochondria. In the original differen- 1969). tial centrifugation studies they were isolated as the 'light mitochondrial' fraction. It is difficult to give Methods for studying lysosomes an exact size to these organelles because the term 'Iysosome' covers a wide range of structures from the BIOCHEMICAL small primary lysosomes budded off from the Golgi The term 'Iysosome' was coined by de Duve (de apparatus, to secondary lysosomes caused by the Duve et al., 1955; de Duve, 1969) for particles fusion of primary lysosomes with endocytotic isolated by homogenisation and differential centri- vacuoles, to complex fusion-structures such as het- fugation that contained acid hydrolases in a latent erolysosomes and autophagic vacuoles. Indeed, form. The latency of the enzymic activities was the de Duve (1969) suggested that lysosomes are only a most surprising feature of this organelle. Now these part of the intracellular vacuolar system. Con- structures are usually separated from mitochondria by copyright. sequently the term 'Iysosome' includes granules of by recentrifuging the crude 'mitochondrial' pellet to various sizes, specific densities, and enzymatic equilibrium in a medium that has a gradient of content. different specific gravities. The lysosomes equilibrate Possibly these structures have different comple- at a specific gravity of about 1-22 whereas the mito- ments of enzymes in the various tissues of the same chondria accumulate at a specific gravity of about animal. The lysosomes of polymorphonuclear cells, 1 19 (Beaufay, 1969). Occasionally the specific the azurophil, and possibly the specific granules gravity of the lysosomes is deliberately altered by (Goldstein, 1974) are peculiar in both their relatively using their ability to concentrate material of low great size and enzymic complement. Seemingly, density such as Triton WR-1339 or material of high however, the main cellular content of hydrolytic density such as Dextran 500, iron, or gold (Dean and http://jcp.bmj.com/ enzymes, particularly those acting optimally at an Barrett, 1976; Dean, 1977b). acidic pH, is within the lysosomes and is one of the If lysosomes were completely stable possibly the characterising features of these organelles. They may activities of none of their characteristic enzymes be defined as small intracellular organelles that con- could be demonstrated biochemically because all tain acid hydrolases bounded by a semi-permeable would be sequestered behind the semi-permeable membrane which is said to control the latency of the membrane of the organelle. Treatment with acetate intralysosomal enzymes and to allow the organelles at pH 5 0 at 37°C, or more crudely with a surfactant to behave as osmometers (de Duve, 1959). By such as Triton X-100, modifies or otherwise disrupts on September 29, 2021 by guest. Protected electron microscopy they are seen to be bounded by a the lysosomal membrane and releases the enzymes, single membrane (Daems et al., 1969; Schellens which then express their full biochemical activity. et al., 1977). When there is no 'free' activity, so that the activity Lysosomes can be seen when living cells are of the intralysosomal enzymes is totally latent, the examined by phase-contrast microscopy, when membrane is considered fully stable. This type of their greater refractility distinguishes them from the finding led to the idea that these organelles were smaller mitochondria or chondriosomes. Indeed, it 'suicide particles' by analogy with the suicide is now apparent that the lipochondria of the older pastilles that could safely be held in the mouth until microscopists probably correspond to lysosomes the casing was broken. However, it was then found (Lane, 1968; Munro et al., 1964). Such bodies were that in different physiological and pathological seen to congregate around the food vacuole in conditions more or less of the enzymic activities amoebae shortly after the uptake of food. Cor- were present in the fluid in which the purified lyso- 105 J Clin Pathol: first published as 10.1136/jcp.31.Suppl_12.105 on 1 January 1978. Downloaded from 106 Lucille Bitensky somal fraction was isolated. This gave rise to many lysosomal activity in histologically identified cells studies (for example, de Duve, 1959; Peters et al., can be measured in individual cells by micro- 1975) in which the 'free' activity was measured densitometry. separately from that activity which still remained latent, or bound, within the organelles and which Contents of lysosomes and role in infmmation could be measured only after the particles had been physically or chemically disrupted. This 'free-to- Lysosomes in different cell-types contain different bound' ratio seemed to relate to physiological or enzymes and bioactive molecules, and possibly the pathological activity in the cells. lysosomes of any one cell type are not homogeneous. Nevertheless, the lysosomes of most if not all cells ELECTRON MICROSCOPY contain an array of hydrolytic enzymes including Lysosomes and related structures are recognised by those that will hydrolyse proteins, nucleic acids, the characteristic single outer membrane bounding a polysaccharides, and phospholipids. The pH opti- structure that may be electron-dense and contains ma of all these enzymes, with a few exceptions, are acid hydrolase activity. The application of electron in the acidic range. Barrett and Dean (1976; Barrett microscopy to the study of lysosomes has been fully and Heath, 1977) have listed all the lysosomal reviewed by Daems (Daems et al., 1969; Schellens enzymes that have been encountered. The possible et al., 1977) role of the proteinases in the degradation of the protein matrix of cartilage in rheumatoid arthritis CYTOCHEMISTRY has been much studied (Dingle, 1962; Weissmann, Lysosomes can be recognised by their concentration 1966). The phospholipases are thought to have a of acridine orange so that they appear as vivid red potential function in the liberation of unsaturated granules when viewed by fluorescence microscopy fatty acids that initiates the formation of prosta- (Robbins and Marcus, 1963; Allison and Young, glandins (Anderson et al., 1971; Zurier, 1974). 1969). They can usually be stained histochemically by Prostaglandins alone can cause resorption of bone a suitable method for acid phosphatase activity (Robinson et al., 1975) and their production can be by copyright. (Holt, 1959; Bitensky, 1963a, b) because the lyso- blocked at a very early stage by glucocorticoids somes of most but not ofall cells contain active acid (Kantrowitz et al., 1975) which influence lysosomal phosphatases. The presence of lysosomal enzymes, function (see later). in characteristic particles, can also be visualised by Lysosomes of many types of cell also contain immunohistochemical methods (Weston and Poole, amino-acid arylamidases capable of generating 1973). bradykinin or other inflammatory amines from their Quantitative cytochemical methods applicable to inactive precursors (Hopsu-Havu et al., 1966). Apart the activity of many characteristically lysosomal from such enzymes lysosomes of polymorphonuclear enzymes are now well established (Bitensky and leucocytes contain a number of inflammatory media- Chayen, 1977). Two of these techniques allow the tors 1974) cationic (Goldstein, including proteins http://jcp.bmj.com/ assessment of 'freely manifest' and of 'bound', or (Janoff and Zweifach, 1964), an activator of plas- latent, activity, approximating to the free-to-bound minogen (Lack and Ali, 1964), a protease that causes ratios used by biochemists. These cytochemical changes in the permeability of capillaries (Movat techniques are considerably more sensitive than the et al., 1964; Uriuhara et al., 1965), as well as haemo- biochemical tests for assessing the function of the lysins (Desai and Tappel, 1965). Thus, not sur- lysosomal membranes. For example, they have prisingly, when the contents of lysosomes isolated shown that thyrotrophin, acting on thyroid follicle biochemically were injected under the skin or into cells, increases the instability of the lysosomal mem- joints (Weissmann et al., 1969) they proved highly on September 29, 2021 by guest. Protected branes as a prelude to lysosomal involvement inflammatory. Repeated intra-articular injection of in the production of thyroid hormones. lysates of lysosomes isolated from rabbit leucocytes The cytochemical bioassay of thyrotrophin produced hypertrophy and hyperplasia of the cells (Bitensky et al., 1974a), 10 000 times more sensitive lining the synovium, round cell infiltration of the than the radioimmunoassay, is now the micro- synovium close to blood vessels, the formation of a bioassay recommended by the World Health pannus, and erosion of cartilage (Weissmann et al., Organization. The cytochemical ratio of freely 1969; Page-Thomas, 1969).
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