Transcript How does the ligand-receptor interaction of a hormone mediate

What is cell signaling?
The Basics:
• Cell signaling refers to how information or a message moves inside
of the cytoplasm of a cell.
• Signaling occurs in response to a ligand that binds to a specific
receptor causing changes inside the cell.
• Cell signaling can lead to:
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Muscle contraction
Changes in transcription/translation
Changes in protein secretion and protein activity
Apoptosis or cell division
• Hormones, neurotransmitters, environmental change result in
changes within the cell itself that are mediated by signal
transduction (cell signaling).
• How does a hormone on the outside change the inside?
How is the need for a change in cell
function communicated intracellularly?
• Ligand: is a generic term for something (like a
hormone) that binds a receptor protein and changes
the function on the receptor on the intracellular side.
• Classes of Signals carried between cells:
– Electrical: requires gap junctions to pass between cells,
voltage gated channels to pass along length of cell (singular)
– Endocrine: requires release of hormone (specific ligands)
that are carried by blood to target
– Paracrine: requires release of hormone (specific ligands) that
are carried only to nearby cells (not blood)
– Autocrine: ligand is released and finds receptors on the same
cell that released it.
The beauty of the receptor-ligand interaction.?
• Many ligands are unable to cross the lipid bilayer of the plasma
membrane, and cannot enter the cytosol due to their size or
polarity.
• Protein receptors can have very precise specificity for a classes of
ligand or even sub-classes.
• Target cells can express no receptors, just one type of receptor, or
many different types of receptor for the same or different ligands.
• The number and activity of the receptors on the plasma membrane
can be modified to reflect the cells needs/status.
• Biggest Ligand Limit: how do you ensure that enough
ligand is released so that enough reaches the target cell at
the right time and how expensive will it be to produce or
release enough ligand to create change?
Hormones are often present at mico or nanogram/liter quantities in the
blood. Amplification creates massive effects in the target cell!
Steroids and thyroxine are lipid soluble hormones requiring carriers for transport in the
blood, but they can freely diffuse THROUGH the plasma membrane . Once inside they
find new receptors that carry then to specific “Hormone Response Elements” on the
DNA modifying DNA transcription, thus changing production of things like different
receptor classes/subclasses that can change future responsiveness.
Receptor “isoforms” are receptor proteins with a similar but
not identical amino acid sequence. Receptor isoforms are
suggestive of a common genetic heritage and can have
similar or dissimilar enzymatic activity outcomes in cells.
Consider the isoforms of the histamine receptor and its
importance for letting different pharmacological mimics of
histidine control GERD and another to control your allergies.
Diphenydramine-Benadryl
Loratidine-Claritin
Cimetidine-Tagamet
Possible sleep disorder use
Possible
Asthma/Inflammatory use
Katzung, 2007
The serotonin family of ligand isoforms is a classic with diversity that evolved
to permit us to create specific tailored responses to a series of receptor
isoforms that can create a myraid of potential tailored cell responses.
Katzung 2007
Signal amplification can be tied to a myraid of different
reactions just inside the cell, there are four classics however
that predominate. We call these second messenger
systems, although some may be a third or even fourth order
product resulting from the original ligand-receptor binding
on the plasma membrane.
Classic Second Messenger Methods:
• G-proteins: release of inhibitor or stimulatory alpha-subunit
• Adenyl cyclasecAMP
• Guanyl cyclase  cGMP
• Phospholipase C  DAG + IP3
• Entry of calcium into the cytosolCa++ dependent change in
enzyme activity may occur.
• Activation or deactivation of Phosphokinase(s)/Phosphatase(s).
• You also need to have a way to “reverse” the signal cascade!
The number of receptors, G-proteins, types of alpha subunit and
amount of phosphodiesterase can all be used to regulate cellular
responsiveness.
Cholera toxin blocks Ga-s degradation.
Pertussis toxin
blocks
G-a-I activation
Epinephrine and norepinephrine have the ability to modify the
ability of CD4+ B-cells so they produce more IgG antibodies by
increasing cAMP. (Kin and Sanders, 2006)
cAMP inhibits T-cell activation through a PKA type I–Csk–Lck inhibitory
pathway. In T-cells, cAMP inhibits TCR-induced T-cell activation and
thereby exerts important immunoregulatory functions through a
receptor–G-protein–AC–cAMP–PKA type I–Csk inhibitory pathway
assembled in T-cell lipid rafts and acting on the Src-family kinase Lck
(EP-R, prostaglandin E2 receptor). Taske and Stokka. The molecular machinery for
cAMP-dependent immunomodulation in T-cells. Biochemical Society Transactions (2006: 34)
The PL-C product DAG is an important activator of PK-C, and PK-C is
significant for the function of the immune system and dysfunctions
related to cancer/tumor suppression.
The activity of cGMP or cAMP is often regulated indirectly by
stimulating or inhibiting phosphodiesterase that destroy the
original signal.
There are two specific types of phosphodiesterase:
1) cAMP + H2O  AMP + H+
Inhibited by theophylline or caffeine
2) cGMP + H2O  GMP + H+
Many important inhibitors
Viagra is a classic example
Guanylyl cyclases can be a membrane-ligand/receptor form
or a soluble cytosolic form.
• cGMP has a less global role than cAMP, but is no less critical for our
survival!
• Guanyly cyclase has homology with adenylyl cyclase
THERE ARE TWO FORMS OF GUANYLYL CYCLASE:
• 1) The classic soluble guanylyl cyclase is activated by nitric oxide (NO).
NO binds the haem at the center of the enzyme and stimulates cGMP
production.
• 2) Membrane-Receptor-Ligand may be important for the function of
atria natiuretic peptide and the promotion of Na+ diuresis in the
kidney when your blood pressure is too high.
• cGMP Roles:
– Regulation of cGMP gated ion channels in retina
– Regulation of protein kinase G (PKG)
– Regulation of cGMP dependent phosphodiesterases
How does cGMP mediate vasodilation? Viagra inhibits cGMP phosphodiestrase
resulting in excessive activation of protein kinase G (PKG).
We will see this when we travel to Gunderson-Lutheran in the form of a nitro pill
given to persons who experience angina prior to or during an angioplasty.
Receptor Tyrosine Kinases (RTK) represent a classic and
simple way to mediate signal transduction.
Proteins encoding RTKs contain four major domains (regions):
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An extracellular ligand binding domain what binds here?
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An intracellular tyrosine kinase domain what happens here?
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An intracellular regulatory domain what binds here?
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A transmembrane domain what type of amino acids would you expect here?
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In short: RTKs permit the receptor to remove a phosphate from ATP and attach it to a
specific tyrosine on the target molecule.
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In short: Some RTKs autophosphorylate themselves.
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In short: the highly electronegative phosphate alters the conformation of the target protein
and changes its function.
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Evolutionarily: the amino acid sequences of the tyrosine kinase domains of RTKs are highly
conserved with those of cAMP-dependent protein kinase (PKA) within the ATP binding and
substrate binding regions.
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Based upon the presence of these various extracellular domains the RTKs have been subdivided into at least 14 different families, seven classic (common) classes are as follows:
RTKs are monomers that “may” form dimers in the plasma membrane.
Amino Acid Consensus Sequence on RTK Target peptide:
-(Lys/Arg)-X-X-(Asp/Glu)-X-X-X-Tyr- or -(Lys/Arg)-X-X-(Asp/Glu)-X-X-TyrMany types exist that differ mostly with respect to what is located on the
extracellular side, whether the “form” a dimer and the number of cytosolic
kinase domains.
These RTK actions determine normal cell growth, apoptosis and
movement of glucose transporters to the plasma membrane.
How does interferon moderate cell function by changing
transcription with a tyrosine kinase? (DeFranco, Locksley
and Roberston 2007)
Protein Kinase C (PKC): composed of a family of at
least 10 isomers, generally bound to plasma
membrane
PKC Activation typically linked to DAG (diacylglycerol)
Some isoform activity regulated by Ca++ and Zn++
Target sequence for phosphorylation: -Ser/Thr-X-Arg/LysActivation of PKC may also modify Ca++ permeability
Ca++/Calmodulin Dependent Protein Kinase
This is a big-league player! Increased Ca++ is the signal
Calmodulin looks like a dumb bell and when 4 Ca++ are attached
it moves ONTO its target proteins
ATTACHMENT of Calmodulin means Ca++ is present and kinase
activity is initiated.
Capability of autophosphorylation and targets with Ser/Thr rich
amino ends of pertide chains
Kinase itself has three domains: catalytic-regulatory-association
Classic targets: myosin light chain kinase and transcription
factors
What goes up must come down:
The role of phosphatases
• Phosphatases remove phosphates from specific
targets and therefore reverse the signals generated
by kinases.
• Specific for Targets:
Tyrosine-PO4 or Serine/Threonine-PO4
• Activity tends to be constitual (not regulated),
however there are a few VERY important exceptions
to this rule….i.e. glycogen phosphorylase
How do all these signal systems work together to let an antigen
presenting dendritic cell cause a T-cell to change its function?
(Part #1) (DeFranco, Locksley and Robertson 2007)
Green=MHC
Purple: receptor for
Antigen presentign
cell detects problem
How do phospholipase-C, IP3 , calcium, phosphatases, and
protein kinase D mediate this effect fo change transcription in the
T-cell? (Part #2) (DeFranco, Locksley and Robertson 2007)