Cytology 2 - Golgi, Vesicles Transport, Lysosomes PDF

Golgi apparatus It is named after Camillo Golgi, who discovered it in 1898 while working in Pavia. He then won the Nobel prize for Medicine and physiology for the invention of a technique to stain cells (with osmium and silver impregnation) and make them visible. The Golgi apparatus is made of memb...

Golgi apparatus It is named after Camillo Golgi, who discovered it in 1898 while working in Pavia. He then won the Nobel prize for Medicine and physiology for the invention of a technique to stain cells (with osmium and silver impregnation) and make them visible. The Golgi apparatus is made of membrane-covered, stacked and atten sacs called cisternae. The sacs are both convex and concave and the have a curved disk-like shape After protein synthesis happens in the RER, the Golgi sorts them out and sends them to their nal destination (membrane, outside of the cell, organelles…). The Golgi apparatus can also modify proteins. Golgi apparatus at TEM ⬇ The Golgi apparatus is directional —> the upper part is called cis face (entrance, from RER) while the bottom part is called trans face (exit). Sometimes there may be an intermediate part between RER and cis face called ERGIC (endoplasmic reticulum-Golgi intermediate compartment). The cis, medial and trans faces are morphologically and biochemically different. In the cis face proteins are mostly phosphorylated while in the trans face they are mostly proteolysed (cut) The expansion and number of the Golgi’s cisternae depend on the cell activity. More than one Golgi apparatus can be present in a cell (for example in cells which are very metabolically active) FUNCTIONS: Modi cation of proteins and lipids by glycosylation (adding of carbs) Activation of peptides by proteolysis or phosphorylation (it can start in the RER but it is only nished in the Golgi) Synthesis of glycosaminoglycans and mucin Selection of enzymes to be delivered to lysosomes Sorting system and shipping system (through vesicles) of proteins and lipids RER —> protein synthesis SER —> lipid synthesis GOLGI —> carbohydrates synthesis 4 mechanisms of sorting: I Golgi modi es the proteins and then sends them back to RER (structural RER proteins) —> These proteins express a sequence of 4 amino acids (Lys-Asp-Glu-Leu) called KDEL which is then recognised by the KDEL receptor in the RER. Proteins without KDEL remain in the Golgi Proteins remain in the Golgi2 Protein go to lysosomes3—> proteins have a speci c marker (6-phosphate mannose) Protein undergo exocytosis (go out of the cell)4—> the proteins are in a transport vesicles which brings them outside of the cell, where they are then freed. There are two pathways of secretion: constitutive (spontaneous, proteins are coated by coating proteins (COPs), there’s no need for a stimulus, the secretion products are secreted as soon as they are made, continuous process) and regulated (vesicles are coated by clathrin-like proteins and it depends on a stimulus (for example pancreatic enzymes, neurotransmitters and hormones). the protein is ready to be secreted and close to the plasma membrane but only gets out when a stimulus arrives). 1 2 3 4 Vesicle traf c divides into endocytosis (from plasma membrane to intracellular matrix) exocytosis (from intracellular to extracellular) and gemmation (from inner part of the membrane to out but the vesicle doesn’t fuse with the membrane and keeps going out with its content (mucose)) Functions of the coating in vesicles —> favours the bending of the membrane during formation, allows selection of components, sets direction and destination COPs coating —> vesicles coated in COP II move from RER to Golgi (anterograde transport) while vesicles coated in COP I move from Golgi to RER (retrograde transport). Vesicles coated by Clathrin move from Golgi to plasma membrane or to endosomes (early lysosomes). In order for the vesicle to know where to go proteins are used. For example the transmembrane protein v-SNARE can only be recognised by the sequence t-SNARE on the target membrane. V: vesicle. T: target Types of endocytosis: Receptor-mediated endocytosis* Pinocytosis —> very tiny soluble molecules. Non speci c. It divides into macropinocytosis and micropinocytosis and it is associated with the presence of caveolin and otillin protein (found in lipid rafts). It is a clathrin-independent type of endocytosis Phagocytosis I—> engul ng of solid matter (bacteria, cells, 1 cell fragments). It is brought on by specialised blood- deriving cells called phagocytes (macrophages, neutrophils, leukocytes and granulocytes). After phagocytosis the matter is recognised, absorbed and digested by lysosomes. It is non speci c and it is clathrin- independent but actin-dependent Autophagy —> internal endocytosis *Chlatrin-dependent endocytosis —> extracellular ligands bind to speci c receptors (called cargo receptors) and associate with adaptor proteins (adaptin) which then bind to Clathrin. Clathrin starts forming a curved pit which deepens as the coat grows and eventually pinches off as a closed vesicle (thanks to dynamin which proteolyses it). After the internalisation of the vesicle the clathrin coat disassembles while the vesicle fuses with previously internalised membranes to form new endosomes. In the acidic early endosomes ligands are released from the receptors and the latter fuse back to the membrane. The coated vesicles are now delivered to the lysosomes. Lysosomes They were discovered by Christian de Duve while he was studying acid phosphatase (more produced as the cell was disrupting). Lysosomes contain more than 50 lytic enzymes that break down waste materials and cellular debris. Lysosomes have a single wall membrane Important —> the protein enzymes of the lysosomes are tagged with mannose 6-phosphate Lysosomes are heterogeneous. Their role is that of degrading endogenous proteins/ molecules and digesting endocytosed proteins and lipids. To do that their pH needs to be acid, around 5.0 when active and 7.2 when inactive (in the cytosol ) (so that the 50 different enzymes can be activated at times). The acidic pH is maintained by a proton pump which pushes hydrogen ions against their concentration gradient Lysosomes at TEM ⬇ Lysosomes are interlinked with three important intracellular processes: phagocytosis, endocytosis and autophagy. Around the Golgi apparatus many lysosomes can be found (recognisable thanks to their electrodense interior). Lysosomes digest materials to obtain useful compounds or to destroy microorganisms and phagocytosed damaged cells. abundant in immune cells In the epidermis skin melanocytes produce a pigment (melanin) from the tyrosine aminoacid, which is then stored in the melanosomes (early non acidic lysosomes) Peroxisomes Crucial organelles for the detoxi cation process of the cell. They are an heterogeneous group of membrane bound organelles (internal membrane) that contain several enzymes and that act in different metabolic activities. The key feature is the presence of the enzyme catalase (40% of all the enzymes) Characteristics: Variable shape (spherical, oval, rod-like) Variable in number (lots in hepatocytes and renal cells) Contain enzymes involved in fatty acids and aminoacid oxidation (they produce hydrogen peroxide as a byproduct. Catalase then turns it into water avoiding the diffusion of the toxic molecule) Functions: Oxidation of fatty acids and aminoacid (degradation) Inactivation of H2O2 by catalase Detoxi cation of dangerous substances Removal of free radicals and reactive oxygen species (ROS) that may damage cell DNA and proteins Oligodendrocytes support the axonal integrity thanks to their peroxisomes. If these peroxisomes are damaged the result is an axons loss and demyelination Biogenesis of peroxisomes —> they originate in the RER, where microvesicles containing peroxisomes proteins are made and that then fuse together to form the peroxisomes. They don’t pass through the Golgi Apparatus CLINICAL DROP Genetic diseases called “lysosomes storage diseases” originate from an abnormal accumulation of substances in lysosomes (which can’t degrade them). This is caused by defects in lysosomal enzymes required for the metabolism of lipids, glycoproteins or complex sugars —> in children: developmental delay, defects in organs, movement disorders, early death. Pathologies linked to this —> Tay-Sachs disease, Gaucher disease, Nremann-Pick disease, Hunter syndrome