Autophagy, Lysosomes, Peroxisomes & Cell Inclusions (PDF)

21 Autophagy, Lysosomes, Peroxisomes & cell inclusions ILOs By the end of this lecture, students will be able to 1. Explain the role of autophagy as a cellular sink 2. Describe the origin of lysosomal enzymes and their function in health and disease....

21 Autophagy, Lysosomes, Peroxisomes & cell inclusions ILOs By the end of this lecture, students will be able to 1. Explain the role of autophagy as a cellular sink 2. Describe the origin of lysosomal enzymes and their function in health and disease. 3. Predict the role especially of autolysosome and phagolysosome in physiological conditions 4. Connect the structure of proteasome to its degrative function. 5. Predict the role of peroxisomes in cell adaptation to patterns of stress. 6. Correlate the types of cytoplasmic inclusions to patterns of cell activity 7. Justify the impact of its derangement on cellular health. 1. Lysosomes Lysosomes are membrane bounded cell organelles that have an acidic pH and contain hydrolytic enzymes. It contains at least 40 different types of acid hydrolases, such as sulfatases, proteases, nucleases, lipases that are active in acidic pH. These enzymes are manufactured in the same steps of protein synthesis following the same steps in rER, packed in Golgi complex and released in vesicles from trans Golgi network. Lysosomes receive contents to be digested from late endosomes. Lysosomes aid in digesting phagocytosed microorganisms, cellular debris, and cells but also excess or senescent organelles, such as mitochondria and RER. The various enzymes digest the engulfed material into small, soluble end products that are transported by carrier proteins in the lysosomal membrane from the lysosomes into the cytosol andare either reused by the cell or exported from the cell into the extracellular space. Transport of Substances into Lysosomes Substances destined for degradation within lysosomes reach these organelles in one of three ways: through phagosomes, pinocytotic vesicles, or autophagosomes. 1- Phagosomes: Phagocytosed material, contained within phagosomes, moves toward the interior of the cell. The phagosome joins either a lysosome or a late endosome. The hydrolytic enzymes digest most of the contents of the phagosome, especially the protein and carbohydrate components. Lipids, however, are more resistant to complete digestion, and they remain enclosed within the spent lysosome, now referred to as a residual body. (Fig. 1) 1  Fg 1. Pathways of intracellular digestion by lysosomes 2- Autophagy: The term autophagy is derived from the Greek word meaning 'self- devouring'. Senescent organelles such as mitochondria or the RER, need to be degraded. The organelles in question become surrounded by elements of the endoplasmic reticulum and are enclosed in vesicles called autophagosomes. Fate of autophagosomes: These structures fuse either with late endosomes or with lysosomes and share the same subsequent fate as the phagosome. (Fig. 1) Autophagy is a self-digesting mechanism responsible for removal of long-lived proteins, damaged organelles, and malformed proteins during biosynthesis by lysosome. Significance of autophagy Regulation of diverse cellular functions including growth, differentiation, response to nutrient deficit and oxidative stress, cell death, and macromolecule and organelle turnover. Mechanism Autophagosome formation is regulated by dozens of “autophagy-related genes” called Atgs. Mutation leads to formation of a double-membrane vesicle, which encapsulates cytoplasm, malformed proteins, long-lived proteins, and organelles and then fuses with lysosomes for degradation. 2  Autophagy Regulation Autophagy is activated in response to diverse stress and physiological conditions. For example, food deprivation, hyperthermia, and hypoxia, which are known as major environmental modulators of ageing, are also conditions that induce autophagy. Figure 2 - Stages of autophagy Autophagy and Diseases Autophagy is important in normal development and responds to changing environmental stimuli. On starvation, autophagy is greatly increased, allowing the cell to degrade proteins and organelles and thus obtain a source of macromolecular precursors, such as amino acids, fatty acids, and nucleotides, which would not be available otherwise. Autophagy roles in cancer are a topic of intense debate. In one hand, autophagy has an anticancer role. On the other hand, when tumor cells are starved due to limited angiogenesis, autophagy stops them from dying. Autophagy is important in numerous diseases, including bacterial and viral infections, neurodegenerative disorders, several myopathies, and cardiovascular diseases. 3 Autophagy and weight loss A type of intermittent fasting is used to stimulate autophagy and to 'trick' one's metabolism into working longer hours and burning more fat. Notably, pharmacological stimulation of autophagy can reduce both weight gain and obesity-associated alterations upon hypercaloric regimens usage. Proteasomes Proteasomes are small organelles composed of protein complexes (proteases) that are responsible for proteolysis (protein breakdown) of malformed and ubiquitin-tagged proteins. Proteasomes monitor the protein content of the cell to ensure degradation of unwanted proteins, such as excess enzymes and other proteins that become unnecessary to the cell after they perform their normal functions, and malformed proteins. Protein encoded by virus should also be destroyed. The process of cytosolic proteolysis is carefully controlled by the cell, and it requires that the protein be recognized as a potential candidate for degradation. This recognition involves ubiquination, a process whereby several ubiquitin molecules (a 76-amino acid long polypeptide chain) are attached to the candidate protein using ATP. Once a protein has been marked, it is degraded by proteasomes. (Fig 2) During proteolysis, the ubiquitin molecules are released and re-enter the cytosolic pool to be re used. Fig. 3. The structure and function of the proteasome 4 Protein degradation by proteasomes in health and disease Proteins destined for degradation are labeled with ubiquitin through covalent attachment to a lysine side chain. The amino acid composition at the amino terminus determines how quickly the protein will be ubiquinated and thus the half-life of the protein. Some proteins have very long half-lives, such as the crystallins in the lens of the eye; these proteins do not turn over significantly during the human life span. Because they were synthesized largely in utero, about half the crystallins in the adult lens are older than the person. Other proteins have half-lives of 4 months (proteins such as hemoglobin that last as long as the red blood cell), or the half-life can be very short, such as for ornithine decarboxylase, which has a half-life of 11 minutes. The half-lives of proteins is influenced by the amino (N)-terminal residue, the so- called N-end rule. Destabilizing N-terminal amino acids (causing short half-life) include arginine and acetylated alanine. In contrast, serine is a stabilizing amino acid. Additionally, proteins rich in sequences containing proline, glutamate, serine, and threonine (called PEST sequences) are rapidly ubiquinated and degraded and, therefore, have short half-lives Poly-ubiquination, which increases the rate of turnover/degradation of a protein, occurs by successive addition of free ubiquitin to that which is already bound to the protein. Failure of degradation of misfolded proteins by proteasomes, can lead to accumulation of abnormal proteins and development of certain diseases such as Alzheimer’s disease and Creutzfeldt–Jakob disease (Mad-cow disease). Peroxisomes Peroxisomes are small membrane bounded, self-replicating organelles. They contain more than 40 oxidative enzymes, especially urate oxidase, and D- amino acid oxidase that contain oxidative enzymes. Peroxisomes function in the catabolism of long-chained fatty acids (beta oxidation), forming acetyl coenzyme A (CoA) as well as hydrogen peroxide (H2O2) by combining hydrogen from the fatty acid with molecular oxygen. Similar to mitochondria, peroxisomes increase in size and undergo fission to form new peroxisomes; however, they possess no genetic material of their own. Inclusions Inclusions are non-living components of the cell that do not possess metabolic activity and are not bounded by membranes. The most common inclusions are glycogen, lipid droplets, pigments, and crystals. 5  1. Glycogen Glycogen is the most common storage form of glucose in human and is especially abundant in cells of muscle and liver. It appears in electron micrographs as clusters, or rosettes, of β particles (and larger α particles in the liver) that resemble ribosomes, located in the vicinity of the SER. On demand, enzymes responsible for glycogenolysis degrade glycogen into individual molecules of glucose. 2. Lipids Lipids, triglycerides in storage form, not only are stored in specialized cells (adipocytes) but also are located as individual droplets in various cell types, especially hepatocytes. Lipids are considered as potential source of energy within the cells. 3. Pigments It could be natural pigments as haemoglobin of red blood cells, melanin in the skin and hair and a yellow-to-brown pigment, lipofuscin in the long-lived cells, such as neurons and cardiac muscle. Tattoos is the injection of ink intracellular that could be phagocytosed by macrophages leading to its permanent effect. Fig. 4 Types of inclusions A. TEM of glycogen ganules in rosettes B. Lipid droplets in fat cell Clinical hint: abnormal accumulations I. Lipids 1-Steatosis (Fatty Change) Means excessive, abnormal accumulations of triglycerides within parenchymal cells due to alcohol abuse, diabetes mellitus, obesity, toxins, protein malnutrition, and anoxia 2- Cholesterol and Cholesterol Esters as in atherosclerosis. 6  I. Proteins as inAlzheimer disease. II. Glycogenas in Diabetes mellitus and Glycogen storage diseases. III. Pigments; Exogenous Carbon (coal dust),The most common air pollutant in urban areas. Its accumulation could lead to Anthracosis occurs in heavy smokers, and coal mines workers with accumulation of carbon pigment within lungs and regional lymph nodes. Endogenous Pigments Lipofuscin Patients with severe malnutrition& Cancer cachexia. Melanin: Hyperpigmentation generalized due to excessive sun exposure or localized as in benign (nevus) and malignant cutaneous tumors. Hypopigmentation Generalized as in albinism or localized as in vitiligo (autoimmune disorder). 7