Proc. Nati. Acad. Sci. USA Vol. 73, No. 8, pp.,2781-2787, August 1976 Cell Biology The endoplasmic reticulum: A cytochemist's view (A Review) * (electron microscopy/enzyme localizations/GERL/Golgi apparatus) ALEX B. NOVIKOFF Department of Pathology, Albert Einstein College of Medicine of Yeshiva University, Bronx, New York 10461 Contributed by Alex B. Novikoff, March 25,1976 ABSTRACT Enzyme cytochemistry has been used, at the The contributions of AcPase cytochemistry to elucidating light and electron microscope levels, to "mark" cytoplasmic the forms and functions of lysosomes have been described by organelles of mammalian cells. Catalase cytochemistry per- de Duve (19). Light microscopy of lysosomes with the AcPase mitted identification of microperoxisomes, apparently ubiq- uitous organelles that are attached by numerous slender cofl- procedure and of the Golgi apparatus with the TPPase proce- nections to the endoplasmic reticulum. Thiamine pyrophos- dure, in the small neurons of rat dorsal root ganglia, initiated phatase and acid phosphatase cytochemistry can be used to a series of publications which led to appreciation of specialized distinguish between the Golgi apparatus and a specialized regions of ER. acid-phosphatase-rich region of smooth endoplasmic reticulum (ER) that appears to be involved in: (a) the formation of lyso- GERL and lysosomes somes and melanin granules: (b) the processing and ackaging of secretory materials in endocrine and exocrine cells; and (c) Light microscopic studies of a wide variety of cell types led us the metabolism of lipid. The acronym GERL has been given to to identify a close relationship between the location of lysosomes this region of smooth ER because it is located at the inner or and the form and distribution of the apparatus (20, 2, 21). "trans" aspect of the Golgi apparatus and because it appears Golgi to produce various types of Lysosomes. In AcPase preparations of rat dorsal root ganglia not only the stained spherical lysosomes but also some larger stained areas Cytochemistry, "marker enzymes," and specialized were encountered. From their distribution, size, and shape I endoplasmic reticulum regions considered that these areas could be fitted within the cres- By making possible the visualization of organelles in situ, cy- cent-shaped regions formed by portions of the Golgi apparatus tochemistry has aided interpretation of biochemical analyses of subcellular fractions isolated from homogenates. It has added Table 1. Cytochemical marker enzymes a new dimension to understanding electron microscope ob- servations. Cytoplasmic organelles not usually identifiable by Refer- light microscopy can be observed in normal and altered cell Organelle Enzyme ence states. In our laboratory we have chosen enzyme activities that Endoplasmic reticulum survive the sufficiently long aldehyde fixation required for in some cell types Nucleoside diphospha- 1, 2 adequate structural preservation of the organelles and that stain tase (NDPase) (EC ("mark") one or more cytoplasmic organelle (Table 1). The 3.6.1.6) manner of preparing the tissue sections and incubation media in a few cell types Peroxidase 3, 4 is described in the references listed in the table. (EC 1.11.1.7) During 1966-1970 a series of publications drew attention to GERL and Acid phosphatase 5 an effect of lead ions employed to trap the phosphate released lysosomes (AcPase) [orthophos- in the course of the phosphatase procedures: that the lead ions phoric-monoester could themselves induce hydrolysis of ATP or other phosphate phosphohydrolase esters. These papers (12-15) cast doubt upon the validity of the (acid optimum), cytochemical procedures. In reply I pointed to: (a) specificities EC 3.1.3.2] of the localizations obtained with specific substrates; (b) the Tyrosinase (mono- 6 similar localizations obtained under conditions where lead- phenol monoxy- induced hydrolysis does not occur; and (c) other observations genase) (EC 1.14.18.1) which made me confident that the observed localizations did (in melanocytes) indeed reveal the intracellular sites of the different phosphatases Peroxisomes, Catalase 7, 8 (16-18). Thus, in 1963 we described the NDPase localization including (EC 1.11.1.6) in the endoplasmic reticulum (ER) of rat liver (Fig. 1) and we microperoxisomes confirmed it by assay of subcellular fractions (1). In addition, Golgi apparatus Thiamine pyrophospha- 9, 2 when purified from isolated microsome fractions the enzyme tase (TPPase) the demonstrates same substrate specificity and optimal con- Mitochondria Enzymes, coenzymes 7, 10 ditions for activity as when studied by the cytochemical pro- that oxidize 3,3'-di- cedure (2). aminobenzidine Abbreviations: AcPase, acid phosphatase; ER, endoplasmic reticulum; NADH-tetrazolium re- 11 NDPase, nucleoside diphosphatase; TPPase, thiamine pyrophospha- ductase (for light tase. microscopy only) * By invitation. From time to time, reviews on scientific and techno- Plasma membrane Nucleoside phospha- 2 logical matters of broad interest are published in the PROCEED- tases INGS. 2781 Downloaded by guest on September 29, 2021 2782 Cell Biology: Novikoff Proc. Natl. Acad. Sci. USA 73 (1976) 4th)*>t<e w ;ot . t-t t. W,4)-''",''k., 0 At .. 4.* B B 'Ar 0'0 G ' t < E 'S5.X~. tR - NA 't-49i.,. t *?.!;,r N!4 N~~~~~~~~~~~~~4 4& 1 :r~~:~~ ~ ~ ' ->i< ~ 0e;It * it * .. 6 * .f, W 4.KE#' :.,<+ > c -t, .4 . r N ~i _3 <-{ s atK ¼-i-<5 ( .I.:b .~~~~A ..C.... I-.0,r..w ....1. /- t + . * .3<s.;^. ' C 7 @8ini K?) a -A S FIGS. 1-8. (Legend appears at bottom of the following page.) Downloaded by guest on September 29, 2021 Proc. Natl. Acad. Sci. USA 73 (1976) Endoplasmic reticulum (Review) 2783 (22). This was confirmed by incubating sections for AcPase contents of small ferritin-like grains, myelin-like figures, and activity and then for TPPase activity. With electron microscopy other structures (25). it was determined that the AcPase-rich region was smooth ER. I named it GERL (23) because this region of smooth ER is lo- GERL and secretion processing and packaging cated at the inner or "trans" (24) aspect of the Golgi apparatus The cytochemical visualization of TPPase and AcPase activities and because it appears to produce various types of Lysosomes has shed new light on the packaging, and perhaps the final (25). processing, steps of secretory materials. With E. Essner, in the The structural relation of GERL and Golgi apparatus has early sixties, we had demonstrated the presence of AcPase ac- been studied best in the neurons of dorsal root ganglia of the rat tivity in nascent secretory granules of both endocrine and ex- (5) and mouse (26). In these cells the relationship appears to be ocrine glands (20). Recently we have returned to studies en- more complex than in other cell types studied in our laboratory gendered by these observations. In cells where TPPase activity (6, 4, 27). Figs. 5-7 illustrate some aspects of this relationship; is demonstrable in only one element of the Golgi apparatus it also see Fig. 40 in ref. 5. Smooth-surfaced tubules, probably part is the trans element, adjacent to GERL, that shows the activity. of GERL, extend into each polygonal compartment of the trans Thus, by incubating sections for TPPase activity and parallel element of the Golgi apparatus. The polygonal arrangement ones for AcPase activity it can readily be determined whether provides a large surface area where molecular interchanges secretory granules arise from GERL or from the trans element between GERL and Golgi apparatus could occur. However, the of the Golgi apparatus. Thus far the findings are different from functional significance of the spatial relations of GERL and what is generally assumed (30, 31). Golgi apparatus remains to be elucidated. This is particularly In an insulin-producing transplantable hamster tumor we important for understanding events in the secretory cells con- conclude (32) that most secretory granules arise from GERL sidered in the next section. and none from the Golgi apparatus, because the granules show Four types of lysosomes seem to arise from GERL: (a) re- AcPase activity and not TPPase activity (Fig. 9). sidual bodies (Fig. 8) in which presumably indigestible residues In the fi cells of the rat pancreas the granules also arise from of intracellular digestion are visible; (b) coated vesicles which GERL and not the Golgi apparatus as widely thought. Again may carry lysosomal hydrolases to other cell structures (if so the nascent granules show AcPase activity (Fig. 11) and no they would be primary lysosomes); (c) autophagic vacuoles, TPPase activity. Unlike the insulinoma, however, the more type 1, in which portions of GERL envelop regions of cytoplasm mature secretory granules do not have demonstrable AcPase containing organelles like mitochondria, ER, or peroxisomes. activity. (When sealed off, lysosomal hydrolases are apparently released We (32) have indicated possible advantages of the trans- into the vacuole and the sequestered structures undergo deg- plantable insulinoma for biochemical studies required to test radation to become a residual body.); and (d) autophagic vac- the assumption that when acid phosphatase activity is present uoles, type 2, in which a region of GERL enlarges and portions in granules it is likely that other lysosomal enzymes are there of its membrane are internalized, bringing in bits of cytoplasm as well, and to resolve the issues considered by Steiner et al. (31) like cytosol or glycogen. Degradation by lysosomal enzymes in their discussion of the possible involvement of lysosomes in leads to residual body formation, as in autophagic vacuoles, type converting proinsulin to insulin. Unfortunately there is cur- 1. For a fuller description see another review (25). rently no valid cytochemical procedure generally accepted for An assumption made by cytochemists, apparently valid thus visualizing, at the electron microscope level, the sites of the far, is that when AcPase activity is found in a cell organelle other proteolytic enzymes involved in converting proinsulin to in- lysosomal hydrolases are probably there as well.