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Studies on the Mechanisms of Autophagy: Formation of the Autophagic Vacuole W

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Studies on the Mechanisms of Autophagy: Formation of the Autophagic Vacuole W Studies on the Mechanisms of Autophagy: Formation of the Autophagic Vacuole W. A. Dunn, Jr. Department of Anatomy and Cell Biology, University of Florida College of Medicine, Gainesville, Florida 32610 Abstract. Autophagic vacuoles form within 15 min of tophagic vacuoles. All these results suggested that au- perfusing a liver with amino acid-depleted medium. tophagic vacuoles were not formed from plasma mem- These vacuoles are bound by a "smooth" double mem- brane, Golgi apparatus, or endosome constituents. An- brane and do not contain acid phosphatase activity. In tisera prepared against integral membrane proteins (14, Downloaded from http://rupress.org/jcb/article-pdf/110/6/1923/1059547/1923.pdf by guest on 26 September 2021 an attempt to identify the membrane source of these 25, and 40 kD) of the RER was found to label the in- vacuoles, I have used morphological techniques com- ner and outer limiting membranes of almost all na- bined with immunological probes to localize specific scent autophagic vacuoles. In addition, ribophorin II membrane antigens to the limiting membranes of was identified at the limiting membranes of many na- newly formed or nascent autophagic vacuoles. Anti- scent autophagic vacuoles. Finally, secretory proteins, bodies to three integral membrane proteins of the rat serum albumin and alpha2o-globulin, were localized plasma membrane (CE9, HA4, and epidermal growth to the lumen of the RER and to the intramembrane factor receptor) and one of the Golgi apparatus space between the inner and outer membranes of some (sialyltransferase) did not label these vacuoles. Inter- of these vacuoles. The results were consistent with the nalized epidermal growth factor and its membrane formation of autophagic vacuoles from ribosome-free receptor were not found in nascent autophagic vacu- regions of the RER. oles but were present in lysosome-like degradative au- ELL growth is governed by a delicate balance between or autophagosome (AVi)~ (1, 13, 37, 38). Acid hydrolases the synthesis and degradation of proteins. Protein are then introduced by fusion of the initial vacuole with C degradation may occur by a lysosomal or nonlyso- preexisting lysosomes or Golgi elements to form a single- somal mechanism (8, 16, 26, 31). Entry of proteins into lyso- membrane bound degradative vacuole or autolysosome somes can occur by fusion of endocytic or autophagic vesi- (AVd). The loss of the inner membrane is presumably due cles, by invagination of the lysosomal membrane, or by to hydrolysis or fusion with the outer membrane. The evi- prp73-mediated transport (5, 8, 10, 12, 26, 29). Although en- dence suggests that vacuole formation can be regulated by a docytosis may be important in the turnover of plasma mem- variety of physiological (e.g., amino acids and hormonesJ as brane proteins (12), lysosomal degradation of endogenous well as artificial (e.g., stress) effectors (26, 33). In addition proteins occurs primarily through autophagic mechanisms to being the site of regulation, this sequestration event may (26, 32, 38). Autophagy has been implicated in the degrada- also dictate those cellular components destined for degrada- tion of normal proteins in response to nutrient deprivation; tion (8, 20, 23). Therefore, it becomes important to under- in the cellular remodeling that occurs during differentiation, stand the mechanisms and regulation of the formation of the metamorphosis, aging, and transformation; in growth of the autophagic vacuole. uterus during gestation and its atrophy after birth; and in the Despite earlier attempts, the source of the limiting double removal of abnormal cellular components that accumulate membrane remains unclear (I, 13, 17, 29, 36). In this report, during cell toxicity or death (6, 26, 27, 45, 46). In addition, an immunological approach was used to characterize those the inhibition of autophagy is coincident with a decrease in protein constituents of the limiting membranes of the newly protein degradation and net cell growth (26, 28, 30, 39). formed autophagic vacuole. Membrane antigens of plasma Autophagy is a general mechanism occurring in many cell membrane, Golgi apparatus, and endosomes were not found types whereby intracellular organelles and cytosol are first in these vacuoles. However, specific membrane proteins of sequestered away from the remaining cytoplasm and then the RER were localized to the limiting membranes of AVis. degraded within lysosomes. The first recognizable step in this process is the sequestration of a region of cytoplasm by 1. Abbreviations used in this paper: AVd, degradativeautophagic vacuole a double-membrane bound vacuole lacking acid phosphatase or autolysosome; AVi, nascent autophagic vacuole or autophagosome; and aryl sulfata~e activities: the nascent autophagic vacuole TBST, 10 mM Tris, pH 7.2/0.5 M NaCI/0.3% Tween 20. © The RockefellerUniversity Press, 0021-9525/90/06/1923/11 $2.00 The Journal of Cell Biology,Volume 110, June 1990 1923-1933 1923 Downloaded from http://rupress.org/jcb/article-pdf/110/6/1923/1059547/1923.pdf by guest on 26 September 2021 Figure 1. Morphological characterization of autophagic vacuoles. Livers were perfused for 90 min with media enriched with amino acids and containing insulin (a) or depleted of amino acids (b-e), fixed with 2% glutaraldehyde/2% paraformaldehyde/0.1 M cacodylate, pH 7.4, and processed for acid phosphatase cytochemistry. The omission of amino acids from the perfusion medium resulted in the accumula- tion of numerous autophagic vacuoles (b). These vacuoles were routinely found to contain mitochondria, peroxisomes, and cytoplasmic components, e.g., ribosomes. However, vacuoles containing recognizable RER were not visualized. Despite their diverse morphological appearance, these vacuoles have been classified morphologically and cytochemically into two groups: (1) nascent autophagic vacuoles or autophagosomes (AVi) and (2) degradative autophagic vacuoles or autolysosomes (AVd) (see text). Bar, 1.0/~m. Materials and Methods Polysciences, Inc. (Warrington, PA); and horseradish peroxidase-con- jugated sheep anti-mouse Fab and horseradish peroxidase-conjugated sheep anti-rabbit Fab from Pasteur Productions (Marries La Coquette, Materials France). Epidermal growth factor was conjugated to horseradish peroxidase Most reagents were purchased from Sigma Chemical Co. (St. Louis, MO), as previously described (10). Protein A (Pharmacia Inc., Piscataway, NJ) from Bio-Rad Laboratories (New York, NY), Or from Fisher Scientific was iodinated using the chloramine T method (12). (Springfield, NJ). Sprague-Dawley rats were purchased from Charles River Breeding Laboratories (Wilmington, MA) or University of Florida Breed- Preparation and Characterization of Antibodies ing Facility (Gainesville, FL). New Zealand white female rabbits were ob- tained from Bunnyville Farms, Littleton, PA. Rabbit anti-rat serum albu- Antibodies recognizing plasma membrane proteins, CE9, HA4, and EGF min was purchased from Cooper Biomedical (Malvern, PA); Freund's receptor, have been previously characterized (12, 19). Antiserum to c~2,6 adjuvant from Difco Laboratories Inc. (Detroit, MI); LRGold resin from Gal/31, 4 GIcNAc sialyltransferase was prepared by Dr. G. Hart (Johns Hop- The Journal of Cell Biology, Volume 110, 1990 1924 0,75 obtained from rat liver homogenates as previously described (22). This fraction contained •33% of the NADH cytochrome C reductase and AVd 60-70 % of the RNA and ribophorin II present in the postmitochondrial su- ~uJ 0,50 pernatant (see Fig. 5). Morphologically, >90% of the vesicle profiles pres- ent in this fraction had ribosomes bound to their outer surfaces. Minor con- tamination by lysosomes, mitochondria, and a few smooth vesicles was also observed. The vesicles were stripped of ribosomes and content by carbonate 0.25 lysis (14). The stripped rough microsomes (50 #g) were emulsified in Freund's complete adjuvant and injected into the lymph nodes of a rabbit (24). A regimen of boost injections was performed that included 10-50 #g of antigen in either PBS or Freund's incomplete adjuvant injected intrader- 0 ,,.A15 20 loll25 30 illl45 60 mally (at 6 wk after initial immunization), intramuscularly (at 6.5 wk), or TIME (min) intravenously (at 7-8 wk). On Western blots, the antiserum recognized Figure 2. Morphometric quantitation of the autophagic response. seven to eight proteins of the rough microsomes and three lysosomal pro- Livers from previously fasted rats were excised and perfused with teins. Antibodies specific for the RER were purified on a column containing media enriched with amino acids and containing insulin (14 nm). stripped rough microsomes bound to Sepharose 4B (12). At 90 min, the media was switched to one depleted of amino acids Antisera against ribophorin II purified from canine liver rough micro- and containing glucagon (86 nm). The perfusion was continued for somes was prepared by Dr. D. Meyer (18), In addition, ribophorin 11 was selected periods of time, the liver fixed by perfusion and processed isolated from rat liver rough microsomes by affinity chromatography on a for acid phosphatase cytochemistry. The relative volumes of na- column of anti-ribophorin II mouse lg conjugated to Sepharose 4B and fur- ther purified by preparative SDS-PAGE (3). The protein band was cut from scent and degradative autophagic vacuoles were measured and ex- the gel and used to immunize a rabbit. Antisera to both canine and rat pressed as a percentage of cell volume as described in Materials ribophorins recognized only the 65-kD
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