cell eating and drinking

• Cells are busy!
• Cells


  • Have busy highways that connect organelles with each other and with the plasma membrane to transport and deliver cargoes in, out and within the cell
  • This intracellular traffic is tightly regulated by a plethora of proteins and signaling lipids that define the identity of membranous compartments (tags) so they are transported to their correct destinations

  
• Endocytosis


  All the routes that lead inward from the cell surface

  
• Endocytic pathways


  • Pinocytosis ("cell drinking")
  • Phagocytosis ("cell eating")
• Eukaryotic cells constantly form endocytic vesicles
• Intracellular pathogens such as viruses, bacteria, parasite and fungus exploit endocytosis to get inside their host cells
• Roles of endocytosis


  • Cell nutrition
  • Cell migration
  • Cell signaling
  • Neurotransmission
• Through endocytosis the cell regulates the composition of its plasma membrane in response to a dynamic extracellular environment
• Endocytosis


  1. Material to be ingested is enclosed by a small portion of the plasma membrane (nascent endosome)

  2. Endocytic vesicle (single membrane) traffics intracellularly

  3. Fuses with lysosomes

  4. Recycled to the plasma membrane

  5. Sorted towards the endosome to Golgi pathway

  
• Dynamin


  GTPase that forms helical complexes at the neck of the invagination, and their structural changes coupled with GTP hydrolysis to promote membrane fission

  
• Different endocytic routes


  • Phagocytosis
  • Constitutive micropinocytosis

  
• Phagocytosis


  Internalization of almost everything, mainly occurs in specialized immune cells called phagocytes

  
• Macropinocytosis


  Uptake of large amounts of fluids and nutrients (can also involve the uptake of big particles)
• Micropinocytosis (pinocytosis)


  Comprises many internalization pathways for the uptake of fluid, small particles and membrane receptors
• The fate of a maturing endosome could be: 1-fusing with lysosomes, 2- being recycled to the plasma membrane or 3- being sorted towards the endosome to Golgi pathway

  
• Clathrin-dependent endocytosis


  • Eukaryotic cells continually ingest portions of their plasma membrane in the form of small pinocytic vesicles
  • A nascent endosome that is transiently coated with the protein clathrin pinches off to form clathrin-coated endosomes that quickly sheds off the coat and fuse with other early endosomes

  
• Cells use clathrin mediated endocytosis to internalize low-density lipoprotein (LDL) particles to transport and release cholesterol inside the cell

  
• LDL internalization


  1. Receptor recognition and internalization

  2. LDL particles dissociates from its receptors in the "acidic" environment of the early endosome

  3. Receptor is recycled back to the plasma membrane

  4. Endosome matures and LDL particles are degraded to release free cholesterol

  
• Genetic defects in the LDL receptor reduce or eliminate LDL uptake, producing hypercholesterolemia, leading to cholesterol deposition in the walls of arteries and atherosclerosis

  
• Familial hypercholesterolemia, defects in the hepatic uptake and degradation of LDL. Commonly caused by a loss-of-function mutation in the LDL-receptor gene or by a mutation in the gene encoding apolipoprotein B (APOB) (the ligand)

  
• Specialized cells use clathrin mediated endocytosis to internalize iron (bone marrow, liver, among others)
• Transferrin-Fe internalization


  1. Transferrin-Fe complex interacts with the transferrin receptor in the cell membrane and triggers clathrin mediated endocytosis

  2. Endosome containing the transferrin-Fe complex + receptor matures to a late endosome

  3. Decrease in luminal pH triggers the dissociation of Fe3+ from the complex, its reduction to Fe2+ and subsequent transport to the cytosol

  4. Transferrin-receptor complex is recycled back to the plasma membrane

  5. Changes in the pH (neutral) trigger the dissociation of transferrin from its receptors, so the protein (now called apotransferrin) returns to the bloodstream in its iron-free form

  
• Receptor down-regulation


  The endocytosis and readily degradation of membrane signaling receptors is a mechanism used by cells to control the action of extracellular stimuli such as growth factors
• EGFR down-regulation


  1. After endocytosis the receptor-EGFP complex are degraded in lysosomes, thus reducing the cell signaling

  2. Sequestration of the complexes in intraluminal vesicles as the endosome matures takes away the complex from its contact with the cytosol, thereby preventing further downstream signaling

  
• Clathrin-independent endocytosis


  Cells can form other types of micropinocytic vesicles besides clathrin-coated pits and vesicles
• Caveolae


  • Best-understood clathrin-independent invaginations, associated with lipid rafts
  • Major structural proteins are caveolins, which are integral membrane proteins that insert but do not extend across the lipid bilayer
  • Usually static structures that can serve as a reservoir of additional plasma membrane, rapid disassembly of the protein scaffold in response to mechanical force allow the underlying membrane to temporarily increase the surface area of the cell
• Endosome maturation


  1. Depends on the specific recruitment of membrane trafficking proteins (Rab GTPases) and the synthesis of specific signaling lipids (PtdIns) on the cytosolic leaflet of the membrane

  2. If the fate is lysosomal degradation, the endocytic compartment will acquire degradative characteristics: low pH (proton pump, V-ATPase) and hydrolytic activity (acquisition of hydrolytic enzymes)

  3. Endosomal transport requires dynein and kinesin motors

  
• Both, the switch of Rab 7 GTPase for Rab5 GTPase and the conversion of PtdIns(3)P to PtdIns(3,5)P2 on the membrane of endosomes are critical for endosomal maturation

  
• Early endosome


  • Sorting platform in eukaryotic cells
  • Short-lived organelle (approx. 10 min) characterized for the presence of specific markers on its membrane (Rab5, PtdIns3P)
  • Fuses with incoming vesicular carriers from the plasma membrane, while also undergo multiple fusion and fission events to form carrier vesicles that bud from them and go to other destinations
  • If not recycled or retrieved, it will undergo further maturation to become a late endosome

  
• Late endosome


  • Takes approximately 20 min to mature, acquires specific markers on its membrane (Rab7, PtdIns3,5P2)
  • Shares some membrane markers with lysosomes, but is not yet a fully degradative organelle
  • Has a lower pH than the early endosome because of the acquisition of H+ V-ATPase
  • Characterized for the formation of intraluminal vesicles with important roles in recycling and cell signaling regulation

  
• ESCRT protein complexes


  Sequentially sort ubiquitinated membrane proteins into endosomes to form intraluminal vesicles in multivesicular bodies

  
• Lysosomes


  • Heterogeneous single membrane-enclosed organelles filled with soluble hydrolytic enzymes that digest macromolecules
  • Meeting places where both the endocytic and autophagy pathways converge
  • Most of the lysosome membrane proteins, for example LAMP proteins, are highly glycosylated, which helps to protect them from the lysosome proteases in the lumen
  • Transport proteins in the lysosome membrane carry the final products of the digestion of macromolecules to the cytosol, where the cell can either reuse or excrete them

  
• Most plant and fungal cells contain one or several very large, fluid-filled vesicles called vacuoles that can function as lysosomes
• If a lysosome breaks, it will recruit galectins to the damaged membrane

  
• Macropinocytosis


  • Nonselective process for bringing fluid into the cell under certain conditions
  • Involves the generation of membrane protrusions (ruffles) lead by actin polymerization and dynamics, forming very large endocytic vesicles called macropinosomes
  • Macropinosomes mature and fuse with late endosomes or lysosomes, without recycling their cargo back to the plasma membrane

  
• Phagocytosis


  • Fundamental process of immunity, carries out the engulfment and destruction of pathogens, dead cells, and other foreign particles
• Macropinocytosis


  • It can be constitutive or induced depending on the cell type
  • It involves the generation of membrane ruffles, driven by actin polymerization and dynamics
  • Macropinosomes can undergo any of the 3 fates of a maturing endosome (degradation, recycling or retrieval to the Golgi complex)

  
• Phagocytosis


  • A fundamental process of immunity, carried out mainly by specialized cells called phagocytes
  • Consists in recognition, engulfment and digestion of particles larger than 0.5 μm
  • Phagocytosis is receptor mediated

  
• Phagocytes can easily recognize and phagocyte non-opsonized and opsonized microbes or dead cells
• Phagocytes are equipped with a plethora of pattern recognition receptors that binds to pathogen associated molecular patterns