The basics of signal transduction

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The basics of signal transduction

• Inside the cell. NIH publication No. 05-1051
  http//www.nigms.nih.gov
• Cooper G. M, Hausman R. E. The cell. A molecular
  approach, 4th addition, ASM Press, 2007

htt//plab.ku.dk
Professors
Vladimir Berezin
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Signaling molecules and their receptors

• All cells receive and respond to signals from
  their environment.

• A variety of signaling molecules are secreted by
  cells or are pressed on the surface of the cells.

• Signaling molecules interact with their specific
  receptor molecules expressed by the cells.

• The binding of most signaling molecules to their
  receptors initiates a series of intracellular
  interactions that regulate virtually all aspects
  of cell behavior, including metabolism, movement,
  proliferation, survival and differentiation.

• Many of the current insights into cell signaling
  mechanisms have come from the study of cancer
  cells a striking example of the fruitful
  interplay between medicine and basic research in
  cell and molecular biology.

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What is signal transduction?
The conversion of a signal from one type to
another (e.g. chemical to electrical, electrical
to second messenger pathway, extracellular to
intracellular)
Signal-response relationship
The conversion of signals into cellular responses
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How do signaling molecules reach the cell?
(C) AUTOCRINE
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Ligands
Ligand, a molecule, or a molecular group that
binds to another chemical entity to form a larger
complex
Synthetic analogues, agonists, mimetics.
antagonists
Agonist, a molecule that can bind to a receptor
on a cell to produce a physiologic reaction
typical of a naturally occurring substance
Antagonist, a molecule that can bind to a
receptor on a cell to block its physiological
activity
Agonist promotes relaxation of bronchial smooth
muscle, 10 x stronger than EP
Antagonist (beta-blocker) binds with high
affinity to cardiac muscle EP-R, slow heart
contraction (arrhytmias, angina)
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Receptors
-Ligand-binding specificity
-Effector specificity (signaling pathways)
Note
-Different receptors with different effector
specificity can bind the same ligand
-In striated muscle (contraction) ion channel
AChR
-In heart muscle (slow the rate of contraction
G-proteins
-In pancreatic acinar cells (secretion)
exocytosis of secretory granules with digestive
enzymes
-Different receptors with different binding
specificity may have the same effector
specificity
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Sensitivity to external signals
-Signal molecule concentration
-Binding affinity
RL
1
____
________

RT
1 Kd/L
-Number of receptor molecules
-Number of receptor molecules occupied by a
ligand to trigger cellular response (threshold)
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Extracellular Signals (e.g. hormones, cytokines,
ECM-molecules, CAM, neurotransmitters, UV)
Membrane Cell Receptors
Cytosolic Receptors
Membrane-to-Nucleus Signaling Modules (signal
transduction cascades)
Nuclear Receptors
Transcription Factors (e.g. c-Fos, c-Jun, CREB,
elk1, Hes-1)
Remodelling of the Cytoskeleton
Regulation of Gene Expression
Cellular Response (e.g. adhesion, spreading,
motility, cell cycle progression, cell
differentiation, apoptosis, other changes in
cellular phenotype)
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Receptor-mediated signal transduction
Basic principles
- Many receptors, few second messengers
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Major receptor classes
1. Trimeric G protein-linked/coupled receptors
(GPCRs) (e.g. glucagon-, serotonin-,
epinephrine-receptors)
2. Ion-channel receptors (ligand-gated
ion-channels, e.g. the acetylcholine receptor)
3. Receptors lacking intrinsic catalytic activity
but directly associated with cytosolic protein
tyrosine kinases
4. Receptors with intrinsic enzymatic
activity (e.g. guanylate cyclase activity,
protein phosphatase, serine/threonine kinase or
tyrosine kinase activiy)
5. Cell adhesion molecules
6. Intracellular and nuclear receptors
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GPCRs

• Among membrane-bound receptors,
• the G protein-coupled receptors
• (GPCRs) are the most diverse.
• In vertebrates, this family contains
• 1000 2000 members (gt1 of the genome).
• - GPCRs have been very successful during
• evolution, being capable of transducing
• messages as different as photons, organic
• odorants, necleotides, peptides, lipids
• and proteins.
• GPCRs have a common central core,
• composed of 7 transmembrane helical
• domains.
• - The fine-tuning of coupling of the receptor
• to G proteins is regulated by splicing,

Illustration of the central core of rhodopsin.
The core is viewed from the cytoplasm.
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Major receptor classes
1. Trimeric G protein-linked/coupled receptors
(GPCRs) (e.g. glucagon-, serotonin-,
epinephrine-receptors)
2. Ion-channel receptors (ligand-gated
ion-channels, e.g. the acetylcholine receptor)
3. Receptors lacking intrinsic catalytic activity
but directly associated with cytosolic protein
tyrosine kinases
4. Receptors with intrinsic enzymatic
activity (e.g. guanylate cyclase activity,
protein phosphatase, serine/threonine kinase or
tyrosine kinase activiy)
5. Cell adhesion molecules
6. Intracellular and nuclear receptors
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Cytokine receptors
Cytokines are relatively small, secreted
proteins that control many aspects of growth
and differentiation of specific types of cells
(e.g. prolactin, interleukins, interferons,
granulocyte colony stimulating factor,
erythropoietin or Epo)
Low O2
Kidney cells
HIF-1a
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Cytokine receptors
JAK-STAT signaling pathway
JAK is an associated tyrosine kinase. Upon
receptor activation, it phospho- rylates several
tyrosine residues on the receptor. An inactive
STAT (transcrip- tion factor) binds the
phosphorylated receptor. It is then
phosphorylated by JAK and dissociates.
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Receptor tyrosine kinases (RTK)
Ligand-binding results in receptor dimerization
or activation of a preexisting dimer
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Intracellular signal propagation
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Intracellular signal propagation
Human carcinomas frequently express high levels
of receptors in the EGF receptor family. In the
last two decades monoclonal antibodies (MAbs)
which block activation of the EGFR and ErbB2 have
been developed. A humanized anti-ErbB2 MAb, is
active and was recently approved in combination
with paclitaxel for the therapy of patients with
metastatic ErbB2 -overexpressing breast cancer.
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Intracellular signal propagation
There are three MAPK pathways, and their
activation results in phosphorylation of a
variety of transcription factors. There is a
crosstalk at the kinase levels between different
MAPK pathways.
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Conclusions
Binding of extracellular signaling molecules to
cell surface receptors triggers
intracellular signal transduction pathways that
ultimately modulate cellular metabolism,
function, or gene expression.
Signals from one cell can act on distant cells
(endocrine), nearby cells or on the same cell.
Trimeric G proteins transduce signals from
coupled cell surface receptors to associated
effector proteins, which are either enzymes that
form effector proteins or cation channels
proteins.
An external signal is amplified downstream from a
cell surface receptor.
Two receptor classes, cytokine receptors and
receptor tyrosine kinases, transduce signals
via their associated or intrinsic protein
kinases. Ligand binding triggers the formation of
functional dimeric receptors and phosphotylation
of the activation lip in the kinases, enhancing
their catalytic activity.
All cytokine receptors are closely associated
with a JAK protein tyrosine kinase, which
can activate several downstream signaling
pathways leading to changes in transcription of
target genes.
Ligand binding leads to activation of intrinsic
protein kinase activity of RTKs
and phosphorylation of tyrosine residues in its
cytosolic domain. RTKs are linked indirectly to
Ras, an intracellular GTPase switch protein which
can activate the MAP kinase enzymatic
cascade, leading to alteration in gene
transcription.