Receptors and Signal Transduction Mechanisms

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Receptors and Signal Transduction Mechanisms

• Dr. Melanie Kelly, 2007
• Introduction to Pharmacology I
• Biology 4404A, Biochemistry 4804A, Neuroscience
  4374A, Pharmacology 5406A
• melanie.kelly_at_dal.ca

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Cell Signaling
Communication by extracellular signals usually
involves 6 steps (1) Synthesis (2) Release of
transmitter (3) transport to the target cells (4)
detection of the signal by a specific receptor
protein (5) A change in cellular metabolism
triggered by the receptor signaling molecule
complex (6) removal of the signal often
terminating the cellular response
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Types of signaling
Paracrine, autocrine, synaptic, endocrine.
Paracrine is a form of signaling in which the
signaling cell releases mediator onto a different
target cell that is close (para next too) to
target cell. Autocrine is when the releasing
cell and target are the same cell
type. Endocrine is the secretion of mediator by
cells (e.g adrenal) into bloodstream to diffuse
throughout body and act on multiple target
cells. Synaptic signaling occurs when
presynaptic neuron releases neuroactive substance
into synapse to act on receptors on postsynaptic
cell.
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Signal transduction and second messengers
Released molecules - hormones and
neurotransmitters with limited access across the
lipid membrane (first messengers). Stimulation
of cells with first messengers results in
receptor activation and transduction of the
signal via increases in cytosolic second
messenger molecules e.g cyclic nucleotides,
phospholipids, ions.
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Signaling mechanisms
Each cell expresses a unique combination of
receptors that allow it to respond to signaling
molecules.
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1. Intracellular receptor - Lipid soluble drug
crosses the PM and acts on intracellular
receptors (enzyme or gene regulator). e.g 2.
Transmembrane receptor - Drug binds to the
extracellular domain of transmembrane protein
activating cytoplasmic enzyme activity e.g 3.
Ligand-gated Ion channel- Drug binds to and
directly regulates the opening of an ion channel.
e.g 4. G-protein -gated receptor - Drug binds
to cell surface receptor linked to an effector
enzyme by a G protein. e.g
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1. Intracellular Receptors

• Steroid receptors - Ligands that are sufficiently
  soluble to cross the plasma
• membrane exert their actions by binding to a
  group of intracellular proteins called
• the steroid receptor superfamily. Activation of
  these receptors results in increased
• transcription of specific genes within the target
  cells.

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B. Enzymes as Receptors Nitric Oxide and
guanylate synthase
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Summary of Intracellular Receptors

• Receptors that Control Gene transcription.
• Ligands including steroid hormones, vit D and
  retinoic acid.
• Receptors are intracellular proteins.
• Receptors consist of conserved DNA-binding domain
  attached to variable ligand-binding and
  transcriptional control domains.
• DNA binding domain recognises specific DNA bases
  sequences promoting or repressing gene
  transcription.
• Effects are produced as result of altered protein
  synthesis so are slow in onset.

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2. Enzyme-linked Receptors
A. Tyrosine Kinases-containing Receptors -
Receptors that have intrinisic catalytic
activity e.g growth factors such as insulin,
epidermal growth factor. Receptors are single
polypeptide chains that traverse the
membrane creating 3 domains extracellular ligand
binding domain, a transmembrane domain and an
intracellular kinase domain (contains the portion
responsible for the enzymatic activity of the
receptor). Cross-linking of the receptors is
essential for activation. Activation of
downstream proteins occurs via binding to the
activated receptor and phosphorylation of
substrate proteins, leading to alterations in
gene transcription and cellular proliferation or
differentiation.
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B. Receptor Guanylyl cyclases - Single pass
transmembrane protein that has extracellular
binding site for Atrial natriuretic peptides
(ANPs) and an intracellular guanylyl cyclase
catalytic unit. Binding of ANP activates cylase
to produce cGMP which in turn activates
cGMP-dependent protein kinase (G-Kinase)
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C. Tyrosine Kinase Associated Receptors a third
type of enzyme-linked receptors are receptors
that have no intrinsic catalytic activity but
associate with a tyrosine protein kinase e.g
Interferon receptors, immunoglobulins,
interleukins. These receptors are single
membrane-spanning receptors that do not have
intrinsic catalytic activity but interact with
tyrosine kinases from the src tyrosine kinase
family.
Src tyrosine kinase family
P56LCK p55BLK p59YES p59HCK p59FYN p55FGR p62YES
Abbreviations IL-2R, interferon receptor 2, CD,
cluster of differentiation
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Kinase-linked Receptors

• Includes receptors for many hormones (insulin)
  and growth factors.
• Receptors share common architecture with
  extracellular ligand binding domain connected via
  single ?-helix to catalytic domain.
• Signal transduction involved receptor
  dimerization and autophosphorylation via
  intrinsic catalytic acitivyt of enzyme
  recruitment.
• They are involved in events control cell growth
  and differentiation.
• Receptors linked to Ras/raf or Jak/Stat signaling
  pathways important in cell division, growth,
  differentiation.
• Few receptors couple to guanylyl cycalse e.g ANP

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3. Extracellular Ligand-Gated Ion Channels
Also known as transmitter-gated ion
channels Specialized for rapid synaptic
signaling They conduct ions through the
impermeable cell membrane. They select among
different ions (specificity). Belong to a
family of homologous multi-pass transmembrane
proteins
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Ligand-gated ion channels

• Sometimes called ionotropic receptors
• Involved mainly in fast synaptic transmission
• Several structurally similar families commonly
  having 4-5 subunits with transmembrane helices
  arranged around a central aqueous channel.
• Ligand binding occur on a millisecond timescale.
• Examples include, nicotinic Ach, GABAA, -HT3.

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4. G protein coupled receptors
Family of receptors with over 100 members Have a
common 7 transmemrbane serpentine
structure endogenous ligand binds to
extra-cellular side of receptor receptor/ligand
complex associates with a G protein which binds
to an intracellular portion of the
receptor. Interactions with the G protein
stabilize the active confirmation of the receptor
and allow regulation of effector and generation
of second messenger molecules.
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Guanine nucleotide binding proteins
1. GTP binding proteins act as transducers
between receptors and effectors 2.
Heterotrimeric G proteins ? subunits of
trimeric G proteins (39-52 kDa) characterizes the
individual G protein and has GTPase activity.
?? subunits are 35-36 kDa and the ?-subunit is
7-16 kDa. 3. GTP-bound state of G protein is
active and associates with downstream effector
molecules. 4. Low-molecular weight monomeric G
proteins are 18-32 kDa and include ras-related
proteins and many oncogene products.
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Mechanism of G protein Activation
(a) Agonist binds to receptor (b) G protein
interacts with activated receptor and GDP
exchanged for GTP on ? subunit and subunits
dissociate (c) Activated ?? subunit and ??
subunits interact with other signaling
molecules (d) ? -subunit possess slowly
hydrolysing intrinsic GTPase activity Upon
hydrolysis of GTP to GDP, ?-subunit inactivated
and recycled back to associate with ?? subunits.
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Heterotrimeric G protein diversity
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Effector enzymes regulated by G proteins
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G-protein-coupled Receptors

• Sometimes called metabotropic receptors
• All monomers consiting of 7-TM spanning segments-
  some evidence for dimers
• One of the intracellular loops of GPCR is larger
  and interacts with G-protein.
• G-protein is membrane protein consisting of 3
  subunits ( ???). The ?-subunit has GTPase
  activity.
• Dissociation of G-protein subunits after
  interacting with activated receptor and
  exchanging GDP for GTP on ?-subunit allows
  activation of effector proteins (ion channel
  enzymes).
• Activation of effector is terminated when bound
  GTP molecule is hyrolyzed allowing G-protein
  subunits to reassociate.
• Several types G-protein which interact with
  different receptors e.g muscarinic,
  adrenoreceptors, neuropeptide receptors.