L6 - Examples of Cellular Response to Signals

• Steps in Cell to Cell Signalling:

  • A critical part of cell to cell communication is the release of signalling molecules
  • Lipid soluble molecules are released by diffusion as soon as they are synthesised
  • Water soluble molecules are released from vesicles via regulated exocytosis
  • As can’t diffuse thru membrane
  • Regulated by Calcium
• Exocytosis:

  • Vesicles are generated by the Golgi & tagged to specific destinations
  • These include the cell surface where they release their contents by exocytosis
  • Constitutive - extracellular matrix materials
  • Not regulated in obvious way
  • Regulated - signals (paracrine, endocrine & neuronal)
• A Vesicle:

  • A phospholipid bilayer
  • Water soluble molecules trapped inside
• Vesicles to correct site by proteins for Exocytosis:

  • Getting a vesicle to the correct site & causing exocytosis involves multiple proteins including:
  • SNARES & Rabs
  • & synaptotagmin which makes exocytosis Ca2+ dependent
  • Ca2+ binding to synaptotagmin changes its shape bringing the vesicle towards the cell membrane
  • Has a part attached to vesicle + mouth that binds Ca2+; when Ca2+ binds changes shape
  • Thus for regulated exocytosis to occur a cell must increase its Calcium concentration at the site of release
• Proteins involved in getting vesicle to correct site:

  • SNARES & Rabs
  • SNARES: 2 proteins; interact w/ each other bring vesicle to right place & hold it there
  • Rabs: protein combined to GTP; like a switch turning it on, telling it to move, & when reaches right place changes to stay at right place
• Cell Types:

  • 3 cell types to compare how they increase their intracellular Calcium concentration to cause exocytotic release of their signalling molecules

  1. Adrenal chromaffin cell
  2. Pancreatic 𝛽-cell
  3. Mast cell
• The Adrenal Chromaffin Cell:

  • Found within the adrenal medulla
  • Releases catecholamines (adrenaline & noradrenaline) in response to stress
  • Secretion stimulated by acetylcholine release from nerve
  • Activates nicotine acetylcholine receptor
  • Receptor channel opens & sodium (Na+) influxes
  • Cell depolarises & opens voltage gated Calcium (Ca2+) channels
  • Calcium influx causes exocytosis
  • Calcium can also stimulate tyrosine hydroxylase (TH) to make replacement catecholamines
• The Adrenal Chromaffin Cell:

  • Glucocorticoids - lipid soluble hormones cortisol
  • Faster action Epinephrine & norepinephrine for flight & fight response
  • Both activated by nerves → Splanchnic nerve
  • Ach act in receptors that bind & open, when Na+ goes in so does a positive charge → depolarising that cell
  • Now 2nd set of channels
  • Opened by change in voltage; open, let Ca2+ in
  • When undergo exocytosis release adrenaline
  • Also release ATP when release adrenaline
  • ATP can do other things → signalling (signal molecule)
  • Thru G-protein receptor can turn this off (neg feedback)
• The 𝛽-cells of the Pancreas:

  • Found within the islets of Langerhans
  • Release insulin in response to increased glucose concentration in the blood
  • To be stored in cells
  • Glucose is transported into the 𝛽-cell
  • Metabolised to form ATP
  • ATP acts to close a potassium (K+) channel
  • More +ve charge within the cell causes it to depolarise
  • Opens voltage gated Calcium (Ca2+) channels Ca2+ enter the cell
  • Stimulates exocytotic release of insulin
  • Also increases insulin synthesis
  • Insulin acts on insulin receptors to remove glucose from the blood into cells
• The 𝛽-cells of the Pancreas:

  • Pancreas secrete into digestive system + hormones into the blood
  • GLUT pumps take glucose out of extracellular environment into cell
  • Transported into beta cell
  • Feed into metabolic pathways
  • Output of this pathway = ATP; ATP involved in energy
  • Also has channels closed by ATP (K+/ATP pump)
  • K+ conc higher inside cell so moves out (taking pos charge w/ it)
  • Now K+ can leave cell & pos charge (ATP closed channel)
  • Pos charge increase, Voltage gated Ca2+ channel opens, allows Ca2+ to come in
  • Allow release of vesicles, release of insulin
• Negative Feedback:

  • Increase insulin release
  • What stops it from being release (negative feedback) is physiological level
  • Makes other muscles take up glucose
  • Glucose levels fall so no longer stimulus for insulin secretion
• The Mast Cells:

  • Cells of the immune system found within connective tissues
  • Help to deal with infection & inflammation
  • Contain multiple granules (vesicles)
  • Undergo massive exocytosis called degranulation
  • Essentially explodes / releases a lot
  • Resting = not stimulated; dark parts = vesicles, inside vesicles help to deal with reaction etc
• The Mast Cells:

  • Release a cocktail of biologically active molecules
  • Provide first line defence to foreign antigens, tissue damage & pathogens (dangerous & non dangerous)
  • Release things into extracellular fluid; such as histamine (causes you go to red, sneeze) etc + others released
  • Some act on cytokine receptors (IL - hours)
• The Mast Cells:

  • Allergens cross-link 2 IgE (Fcer1) receptors
  • All triggered by receptors
  • Type of antibody = part of receptor
  • That dimerises to make things happen
• The Mast Cells:

  • Trigger a pathway that releases Calcium (Ca2+) from the endoplasmic reticulum
  • Increased Calcium causes degranulation
  • When they cross-link stuff happens inside cell
  • Intracellular signalling pathways activated
  • Want exocytosis to occur
  • So got to increase Calcium
  • Receptor activation also activates other intracellular pathways that increase production of more cytokines
  • Complicated as trigger a lot of molecules / signals (from category of cytokines for a lot of them)
• A Real Vesicle:

  • Multiple vesicle associated proteins responsible for:
  • Building the vesicle
  • Loading of signal molecules
  • Guiding the vesicle to the correct location
  • Regulating exocytosis
  • Fuse to membrane, release its contents at the right time
  • Vesicle recovery after exocytosis
  • Want to reuse it
  • Proteins span across membrane & within vesicle