B1100 Cell Biology Past Paper PDF

95 CH 8. LYSOSOMES I. GENERAL DESCRIPTION Lysosomes are small organelles, dense in EM, spherical to oval and bounded by a single bilayer of lipids (Figure 8.1) and rich in hydrolytic enzymes. It is common to identify lysosomes through detection of a Enz...

95 CH 8. LYSOSOMES I. GENERAL DESCRIPTION Lysosomes are small organelles, dense in EM, spherical to oval and bounded by a single bilayer of lipids (Figure 8.1) and rich in hydrolytic enzymes. It is common to identify lysosomes through detection of a Enzyme type Example of Enzymes Substrate of the Enzyme marker, which is Phosphatases Acid phosphatase Phosphomonoesters Acid phosphodiesterase Phosphodiesters the acid Nucleases Acid ribonuclease RNA phosphatase Acid deoxvribonuclease DNA present in their Proteases Cathepsin Proteins membrane. Collagenase Collagen GAG-hydrolyzing 8-galactosidase Keratan sulfates Lysosomes are enzymes a-N-acetylglucosaminidase Heparan sulfate actively involved in Polysaccharidases, a-glucosidase Glycogen diverse cell Oligosaccharidases a-mannosidase Mannosyloligosaccharides metabolic Sphingolipid Ceramidase Ceramide hydrolyzing enzymes Glucocerebrosidase Glucosvlceramide reactions such as Lipid hydrolyzing Acid lipase Triacylglycerol intracellular enzymes Phospholipase Phospholipids digestion and Table 8.1: Examples of lysosomal enzymes. storing of certain substances. Accurate lysosomes occur in animal cells whereas plant cells have vacuoles which share certain common functions and properties with lysosomes. In fact, vacuoles Acid hyclrola e 0 0 pH 5 0 0 @ 0 Figure 8.1: Lysosomes of diverse sizes and digested content (left). Proton pumps in lysosomal membrane (right). regulate the osmotic pressure inside the cell, perform some reactions of intracellular digestion and store many compounds. Lysosomes and vacuoles are not found in bacteria, except the specialized gas vacuoles in cyanobacteria that are totally different from plant vacuoles. However, a variety of hydrolases do exist in bacteria despite the absence of lysosomes, they are especially located in the space between the plasma membrane and the cell wall, named pericytoplasmic space or periplasmic space (Figure 3.8). Lysosomes contain about 50 different types of enzymes (Table 8.1) in their lumen as well as in their membranes. Taken together, lysosomal enzymes can hydrolyze virtually every type of biological macromolecules (proteins, lipids, nucleic acids and carbohydrates). Lysosomes play a key role in cell defense, cell nutrition as well as renewal of membranes and organelles. Lebanese University-FS1. B1100 - Cell Biology by Pr Ziad ABDEL-RAZZAK (2022)  96 The lysosomal enzymes are generally named acid hydrolases since their hydrolyzing activity is optimal at acidic pH. An ATP-dependent proton pump (an integral protein in the lysosomal membrane) transports protons toward the lumen so that intralysosomal pH is kept about 4.6 to 5 (Figure 8.1 ). This property of lysosomal enzymes provides protection to the cell since leaking of lysosomal enzymes to the 0 cytosol makes them inactive at the Plasma membram cytosolic pH which is about 7. t. Regarding the lumenal face of the... Endosome lysosomal membrane, it is protected from being digested by the enclosed enzymes by its high glycosylation state. Lysosomal enzymes find their Heterophagic vacuole substrates in the endosomes and phagosomes (autophagic and heterophagic) and digest them into low­ molecular-weight molecules that can be transported actively or passively across the lysosomal membrane to the cytosol Q ):- : :·. Acid in order to be used for cell metabolism. hydrolases....... '·.:-:· ·->·.·· Permeases present in the lysosomal 0 membrane may also transport certain substrates into the lysosome lumen for digestion. For some substrates digestion may ?., · {.. Figure 8.2: Summary of lysosomes origin and roles. be incomplete thereby producing wastes which must be expelled by exocytosis outside the cell. 1. Structure and Forms Lysosomes form a structurally heterogeneous group of vesicles that have variable size and morphology. Their size is comparable to that of small mitochondria and ranges between 50 nm to 1 µm in diameter (Figure ·. :..,. Exocytosis 8.1 ). The number of lysosomes is variable among cell types and depending on cell's physiological activity. They are bounded by a single membrane which is structurally similar to the other cell membranes. Its composition however differs, for instance it is poorer in cholesterol than the plasma membrane. Lysosomal integral proteins are four main types: - ATPase proton pumps which trasport H+ into the lumen of the lysosomes to maintain an acid pH (5). - Permeases: some allow the direct entry, Mitchondria from the cytosol into the lumen, of the material to be digested. Others serve to export, from - > - -- lumen to the cytosol, the end products of J ---, - ,7 digestion. - Enzymatic glycoproteins, such as acid Figure 8.3: Fate of phagocytic vesicles and :r,r '- ii phosphatase. digestion by lysosomes. Lebanese University-FS1. B1100 - Cell Biology by Pr Ziad ABDEL-RAZZAK (2022)  97 - Non-enzymatic glycoproteins, such as lamp1 and lamp2 (Lysosome-associated membrane proteins) that are important in terms of lysosome genesis. Lysosomes are divided into the three following types: a. Primary lysosomes Primary lysosomes, also named protolysosomes, are newly produced organelles that have derived from the TGN. They are small vesicles delimited by a single membrane and contain digestive enzymes which have not yet taken part in hydrolysis reactions. The enzymes of primary lysosomes were synthesized by the attached ribosomes and matured in the RER and then in the Golgi apparatus where they are sorted and packed into clathrin-coated vesicles. In the last compartment of Golgi, these enzymes are recognized through their specific address code or signal (e.g. mannose-6-phosphate). Proteins with such signal are packed in clathrin coated vesicles which are destined to form the primary lysosomes after removal of their coats (Figure 7.4, Figure 8.2). b. Secondary lysosomes Secondary lysosomes result from fusion of one or more primary lysosomes with vesicles containing material to be hydrolyzed. The digested substances (particles) may derive from the cell itself or from the outside. In fact, particles which are captured by phagocytosis Exocytosis are enclosed in heterophagic vacuoles or heterophagosomes (Figure 8.3). Primary lysosomes fuse with them and form heterolysosomes or heterophagolysosomes Upofuscin where digestion takes place. Among the - gment granule endosomes and phagosomes there are those Residual body containing pathogens (virus, bacteria,... ) which will be digested. Lysosomes play therefore defense role. Lysosome Autophagic vacuoles or autophagosomes result from enveloping of an organelle to be.-®p \\i destroyed by an ER fragments (Figure 8.2, Figure Golgi complex Autophagolysosome t 8.4). This is known as macroautophagocytosis. Forming Afterward, primary lysosomes fuse with this autophagic ER _ - _:- , autophagic vacuole or autophagosomes forming autophagolysosomes where digestion is performed. (note: the term vacuole is not preferred in animal cells). Microautophagocytosis occurs for molecules such as the peptides released by the proteasome. Such peptides are transported into the lysosome through the permeases of the lysosomal membrane. This process of autodigestion plays a key role in organelles turnover (destruction and replacement). Actually, this is a common event during cell growth, tissue repair, cell differentiation, cell dedifferentiation, Figure 8.4: Autophagocytosis and metamorphosis of larva of insects and the lysosomal body digestion producing residual secreted by exocytosis or reduction of the uterus size after delivery. In accumulated as lipofuscin granules (top). addition, when the cells are deprived of nutrients TEM image of autaphagy of a autophagy will increase in order to supply the mitochondrion (bottom). Lebanese University-FS1. B1100 - Cell Biology by Pr Ziad ABDEL-RAZZAK (2022)  98 missing components. Clearly, according to the metabolic state and the performed functions, a cell has to adjust its content in terms of number and size of mitochondria, dictyosomes, density of ER channels and cisternae,... In the endocrine glands, autophagy plays a critical role in controlling intracellular hormone levels. In peptide-secreting cells of glands such as the pituitary gland, crinophagy, a specific form of autophagy, targets the secretory granules to control the levels of stored hormone inside the cell. The excess of hormone-containing secretory vesicles merge with primary lysosomes to form crinolysosomes. As digestion progresses in both vacuole types, autophagolysosome and heterophagolysosome, it becomes increasingly difficult to distinguish the nature of the original secondary lysosome and, therefore, they are referred to as digestive vacuoles, or secondary lysosomes. Substances obtained after digestion may be totally or partially recycled by the cell, this process requires active and passive transport of molecules across the lysosomal membrane toward the cytosol. c. Residual bodies and Lipofuscin pigment granules Residual bodies are larger than primary lysosomes and irregular in shape. They derive from the secondary lysosomes after the end of digestion. In fact, in both types of secondary lysosomes digestion and hydrolysis is not necessarily complete and, therefore, it produces wastes that are enclosed in a vesicle named residual bodies (Figure 8.2, Figure 8.4). Residual bodies are expelled outside the cell by exocytosis. Nevertheless, certain wastes are accumulated in the cytosol especially in long-lived cells such as neurons. Residual bodies that are indefinitely kept in the cell are named lipofuscin granules which are finely granular yellow-brown pigment granules composed of lipid undigested residues. Accumulation of such granules disturbs cell metabolism and causes aging of cells and organs. Note: rare cells may secrete active lysosomal content (enzymes) such as macrophages and certain eiosinophil cells. Moreover, the acrosome of a spermatozoon is a giant lysosome responsible for digestion of the zona pellucida of the oocyte during fertilization. 2. Origin of lysosomes# As previously mentioned, the lysosomal enzymes and proteins are synthesized by membrane-bound ribosomes and, therefore, transit in the RER and the Golgi complex (Figure 7.4, Figure 8.2). In the trans cisternae, lysosomal enzymes are distinguished from other protein by their phosphorylated mannose residues which is a signal recognized by specific receptors (integral proteins) of the TGN. Lysosomal enzymes recognized through that signal are concentrated in clathrin-coated buds which form in the trans cisternae. In fact, the mannose 6-phosphate receptors have clathrin binding site on the cytosolic face so that lysosomal enzymes are packed in clathrin coated vesicles only. Afterward, the buds Figure 8.5: Numerous small vacuoles in a separate completely from the TGN membrane. dividing plant cell (top) and a vacuole As coated vesicles migrate in the cytosol occupying most of cytoplasm volume in a non dividing plant cell (bottom) (TEM mannose 6-phosphate dissociates from its receptor image). Lebanese University-FS1. B1100 - Cell Biology by Pr Ziad ABDEL-RAZZAK (2022)  99 and clathrin coat is therefore released. Then, mannose 6-phosphate receptors are returned back to the TGN in order to contribute to another budding and sorting process. II. PLANT VACUOLES Lysosomes are absent in plant cells since endocytosis and phagocytosis are impossible due to the rigid cell wall. However, plant vacuoles are similar to lysosomes in many respects. Vacuoles may be named phytolysosomes. There are many small vacuoles in dividing plant cells (meristems). When the cell matures and stops dividing, these numerous small vacuoles merge together forming a single large vacuole which occupies as much as 90% of the plant cell volume (Figure 8.5). A vacuole is bounded by one membrane, named tonoplast which is provided with many active transport systems that pump ions (including proton pumps) toward the vacuole lumen. Consequently, ions concentration is higher in the vacuole than in the cytosol. Vacuole lumen is therefore hypertonic. As a result, water enters the vacuole by osmosis and turgor pressure is generated because the vacule is full of water. This fact provides mechanical support to the soft unwoody tissues of a plant, it also helps stretching the cell wall during cell growth. Similarly to lysosomes, pH of the intravacuolar medium is low due to the activity of proton pumps in the tonoplast. Another transport mode of substance into the vacuole is intravacuolar pinocytosis. Moreover, vacuoles contributes to organelles turnover upon autophagy. Although structurally simple, vacuoles carry out a variety of essential functions. Most molecules in a cell ("aa", carbohydrates, proteins, ions, carboxylic acids, phenolic compounds, pigments, alkaloids) may be temporarily stored in the vacuole lumen. Sometimes, vacuole is a storage site for certain toxic compounds that are necessary for plant defense against pest. On the other hand, vacuoles may be a site for storage of undesired molecules and pollutants since plant cells and tissues lack efficient excretion systems. Leaves' falling eliminates the undesired molecules. Moreover, intracellular digestion may occur in vacuoles due to the presence of hydrolytic enzymes (certain acidic hydrolases are shared with lysosomes). As previously mentioned, proteins of vacuole and tonoplast are produced at the level of RER by attached ribosomes and then exported toward Golgi complex for maturation. Lebanese University-FS1. B1100 - Cell Biology by Pr Ziad ABDEL-RAZZAK (2022)