Membrane Function

1
Membrane Function

• Signal Transduction

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I. Introduction to Receptors Signal
Transduction
3
The Players

• Signaling molecules
• Receptors
• G-proteins
• Second messenger systems
• Effector proteins

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Signaling Molecules

• Neurotransmitters
• Hormones
• Growth factors
• Drugs
• Other nomenclature
• Ligand
• Agonist / Antagonist

5
Receptors

• Receptors are proteins associated with cell
  membranes
• Receptors recognize signaling molecules by
  binding to them.
• Binding of receptors by signaling molecules ---gt
  Cell behavior change

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Figure 1 Overview of Signaling
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Neurotansmitters Biogenic Amines.

• Catecholamines
• Epinephrine
• Norepinephrine
• Dopamine
• Esters Acetylcholine
• Indolamines
• Histamine
• 5-HT

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Neurotransmitters Peptides

• Substance P
• Neuropeptide Y (NPY)
• Enkephalins
• Somatostatin
• VIP

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Neurotransmitters Amino Acids

• Excitatory
• Glutamate
• Aspartate
• Inhibitory
• g-aminobutyric acid (GABA)
• Glycine

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Neurotranmitters Other

• Nitric Oxide
• Arachadonic acid
• Carbon Monoxide
• PAF
• Zinc

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The G-Proteins

• Involved in most signaling processes
• Link receptor proteins to effector proteins.
• Trimeric proteins composed of a, b, and
  g-subunits.

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Figure 2 G-Protein Cycling
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Functional G-Protein Units

• GTP-activated a-subunit
• produce second messenger
• and/or opens ion channels.
• bg-complexes
• Initially thought to be inert.
• Probably not inert
• Exact role currently ill-defined.

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Second messengers produced by G-protein
activation.

• Adenylate Cyclase
• cAMP
• Phospholipase C (PLC)
• Inositol triphosphate (IP3)
• Diacylglycerol (DAG)
• Ion Channel Activity

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Families of G-proteins

• Unique structure of their a-subunits.
• bg subunits appear to be similar across families.
• Main families
• Gas
• Gai
• Gaq

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II. cAMP Second Messenger System
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Figure 3 Adenylate Cyclase
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Summary of Adenylate Cyclase Activation

• Receptors which associate with Gs -type G-protein
• Stimulates adenylate cyclase.
• Increases cAMP
• Receptors which associate with Gi -type G-protein
• Inhibit adenylate cyclase.
• Decreases cAMP

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Summary of cAMP action

• cAMP exerts its effect by activating protein
  kinase A (PKA)
• PKA phosphorylates proteins
• Enzymes, pumps, and channels
• Phosphorylation can either increase or decrease
  activity depending on the protein.

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Adenylate Cyclase

• Family of membrane spanning enzymes.
• Types I through IV have been well characterized.
• Additional types probably exist.
• Types differ with respect to activity modulation
  by other second messenger systems

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Adenylate Cyclase Activity and Other Messenger
Systems

• Kinases (PKA, PKC, other) can phosphorylate
  adenylate cyclase in some cells.
• Binding of adenylate cyclase by
• bg-subunits of other G-proteins
• Ca/calmodulin complexes
• Allows other second messenger systems to interact
  with cAMP system

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III. The Phospholipase C Second Messenger System

• IP3 and DAG

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Figure 4 Phospholipase C System
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Summary of the Phospholipase C Messengers

• Agonist binds receptor
• Occupied Receptor ---gt activation of PLC (Gq
  -mediated)
• PLC Produces second messengers IP3 and DAG
• PLC activation associated with Ca-channel
  activation

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Action of IP3

• IP3 binds to IP3-receptors on the endoplasmic
  reticulum
• Releases intracellular Ca stores.

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Action of DAG

• Remains membrane associated.
• Activates Protein kinase C (PKC) which
  translocates from the cytosol to the membrane
• Activated PKC phosphorylates other proteins and
  alters their function state.

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PLC System and Calcium

• PLC causes the IP3-mediated Calcium
• PLC also causes the influx of Ca.
• Ca binds one of a family of Ca binding
  proteins (calmodulin).
• Ca/calmodulin complex
• binds to yet other proteins and changes their
  functional activity.

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IV. Guanylate Cyclase cGMP and Nitric Oxide As
Second Messengers
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Figure 5 Nitric Oxide and cGMP
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NO is Membrane Soluble.

• Diffusion to nearby cells
• Increase cGMP levels in nearby cells
• Vascular endothelial cells and nearby smooth
  muscle cells.

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V. SIGNALING BY ACETYLCHOLINE
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Acetylcholine As a Neurotransmitter

• Both the central and peripheral nervous systems.
• Binds two broad classes of receptors
• Nicotinic receptors
• Muscarinic receptors.

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Nicotinic Receptor Features

• Composed of 5 subunits
• 2 a, b, g and d.
• Subunits are arranged to form a central cavity
  that extends across the membrane.
• Nicotinic receptors are also channels
• ACh-binding opens gates and allows ion fluxes
  across the channel

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Figure 6 Nicotinic Receptor
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Subclasses of Nicotinic Receptors

• Skeletal muscle (N1 or Nm)
• Unique a and b subunits
• Autonomic ganglia (N2 or Ng).
• Both N1 and N2 are gene-product families not
  single receptor types.

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Other Ligand-Gated Channels

• Structural and sequence similarity to nicotinic
  receptors.
• Example agonists for these channels include
• Serotonin (5-HT)
• Glutamate
• GABA
• Glycine

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Muscarinic receptors

• Muscarinic receptors are not channels.
• Operate through G-proteins to alter second
  messenger systems.
• 5 muscarinic subtypes have been cloned and
  sequenced (M1, M2, M3, M4, M5).

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Grouping Muscarinic Receptors

• M1, M3, and M5 receptors Activate Phospholipase
  C through Gq.
• PLC activation ---gt increased IP3 --gt increased
  intracellular Ca
• Increased intracellular Ca ---gtActivation of
  Ca-sensitive K Cl- channels.

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Grouping Muscarinic Receptors

• M2 and M4 receptors
• Gi -coupled inhibition of adenylate cyclase
• Go or Gi -coupled regulation of certain Ca
  K channels.

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VI. Signaling by Epinephrine and Norepinephrine
and Coupling Through Adrenergic Receptors
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Three Families of Adrenergic Receptors

• b -receptors Three subtypes b1, b2, and b3.
• a1 -receptors Three subtypes a1A , a1B , and
  a1C
• a2 -receptors Three subtypes a2A , a2B , and a2C

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.

• All adrenergic receptors appear to be coupled to
  cellular processes through G-proteins

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Occupation of b - Adrenergic Receptors

• Gs-mediated stimulation of adenylate cyclase
• Increased cAMP
• Increased PKA activity.

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Occupation of a1 -Adrenergic Receptors

• Mechanistic details sketchy
• Possibly Gq-mediated PLC activation
• Increases IP3 and DAG for some subtypes (1B)?
• Activates Ca-channels for other subtypes (1A)?

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Occupation of a2 -Adrenergic Receptors

• Gi -mediated inhibition of adenylate cyclase.
• Decreased cAMP
• Decreased PKA activity.