Transcript Cell signaling 3 - Washington State University

Cell signaling 3
Gaseous signals: NO
• Nitric Oxide (NO): major paracrine signaling agent in the
nervous system and circulatory system.
• NO has a short half-life (about 0.5 sec) and thus can
only act over short distances
• Major effect is to mediate relaxation of smooth muscle –
originally called endothelium-derived relaxing factor.
• Pharmacologically, nitroglycerine, a drug for angina, is
converted to NO, which dilates systemic arterioles and
thus lowers systemic arterial blood pressure and
increases coronary blood flow.
Inputs to source tissue
NO synthetase
Arginine + O2
Citrulline + NO
Generalized Effector
Pathway for gaseous
messengers NO and
CO
Heme
oxygenase
Diffusion
to target
tissue
CO
Soluble guanylate cyclase
GTP
cGMP
cGMP phosphodiesterase
GMP
inactive
Protein Kinase G
Effects on
target cells
“endothelium-derived
relaxing factor” is NO
Drug targets
• Both nitric oxide synthases and
phosphodiesterases exist in multiple forms
that are tissue-specific
A specific example – genital vasculature and sildenafil
Acetylcholine – from PS postganglionic cells
binds to muscarinic receptors on endothelial
cells
NO synthetase
Arginine + O2
Citrulline + NO
Diffusion to corpora
cavernosa sm muscle
Sildenafil, etc
Soluble guanylate cyclase
GTP
cGMP
Tissue specific
phosphodiesterase PDE5
Dietary arginine supplementation may lower
blood pressure and improve sexual
response
GMP
Zowie
!
Protein Kinase G
Vasodilation
= erection
Histamine: sources
• Basophils - class of leucocytes
• Mast cells – scattered throughout tissues
• Enterochromaffin-like cells - in stomach
Basophil or mast cell
-mediates responses
to allergens and
infectious agents
NO, adenosine
Histamine is a final common pathway that integrates the
effects of neural and hormonal inputs on gastric acid
secretion
Eicosanoids
• 20-C unsaturated fatty acid derivatives
• Phospholipase A acts on membrane
phospholipids to produce arachidonic acid
• Tissue-specific lipoxygenases and synthases
then generate various prostaglandins,
thromboxanes, leucotrienes, endocannabinoids
and isoecosanoids
• All receptors for this class of signals are Gprotein coupled.
Arachidonic acid
can be
generated
directly by
activation of
Phospholipase
A, or indirectly
through the
generation of
DAG and
subsequent
formation of
arachidonic acid
from the DAG.
Analgesics and antiinflammatory
drugs (NSAIDS) target eicosanoids
• Aspirin and many other NSAIDs inhibit COX-1, a
constitutively expressed enzyme that produces
prostacyclins and thromboxanes– responsible for its
effects on blood clotting, gastric secretion and pain
perception.
• To some extent, aspirin and other NSAIDs also inhibit
COX-2: expression of COX-2 is induced by inflammation.
• COX-2 (cyclooxygenase) inhibitors – designed to be
highly specific therapy for rheumatoid arthritis and other
inflammatory disease– but specificity was not good
enough to eliminate side-effects on clot formation and
other physiological processes.
Ca++ as a 2nd messenger
• Cytoplasmic [Ca++] is tightly controlled at 10-7M
or lower.
• Rise in cytoplasmic [Ca++] can be the result of
Ca++ channel activation, or release from internal
stores (ER, mitochondria), or both at the same
time.
• Effects:
– vesicle fusion with plasma membrane in regulated
secretion
– synaptic vesicle release
– muscle contraction
Calcium-Calmodulin Signaling
• Calcium-binding proteins like calmodulin
have a dual function of buffering
intracellular Ca++ and transducing its
effects.
The Ca++/calmodulin complex can activate
three major enzyme families
• CaM kinases (all cells) - for example, glycogen
phosphorylase kinase is a heterotetramertroponin, a control protein that switches on
striated muscle contraction, is a trimer. In both
molecules one of the subunits is calmodulin.
• Myosin light chain kinases (MLCK) (cardiac
and smooth muscle cells – we will see a specific
example of this in the smooth muscle lectures)
• Protein phosphatase B (calcineurin) (most
excitable cell types)
Steroid hormones and thyroid
hormones
• act through cytoplasmic or nuclear
receptors
• Main mode of action is to influence gene
expression
Domains of the steroid receptor are
specialized for particular functions
•Attachment to cytoplasmic heat shock proteins
•Interaction with steroid
•Transition through nuclear pores
•Attachment to DNA
•Interaction with transcription control factors
This cartoon
shows a
generalized
picture of how
steroid
receptors work;
but there are
some variations
as shown in the
next slide
“Glucocorticoid” refers mainly to cortisol, the
main hormone of the adrenal cortex, which is
involved in modulating response to stress. The
unoccupied receptor is cytoplasmic; the
complex of hormone and receptor is
translocated to the nucleus, where it
modulates transcription of specific genes.
In contrast, the estrogen receptor, although
very similar to the glucocorticoid receptor, is
confined to the nucleus.
Thyroid hormones (T4
tetraiodothyronine or thyroxine; T3
triiodothyronine – these are tyrosine
derivatives and aren’t steroids).
Here, the receptor is already bound
to the regulatory region of responsive
genes and hormone binding causes
a reporesor protein to be replaced by
a coactivator.
All of the major 2nd messenger systems we’ve
seen so far, and some we haven’t seen yet,
have the potential to affect gene expression –
not just the ones for steroids and thyroid
hormones.
Clearly, we need to know something about
control of gene expression. Chapter 5 in
Goodman, starting about p 178, may be helpful.
What is a gene?
• Confers a particular trait?
• Piece of DNA that begins with a start
signal and sequence and ends with a stop
signal?
• Codes for a particular protein (or at least,
a functional RNA)?
• When I say a gene is expressed, I mean
that a particular protein is synthesized.
Gene
expression
can be
regulated at
multiple
steps. The
one we really
care about
now is 2. You
already know
about 1 (think
of Barr
bodies), and
5, 6 and 7.
General picture of
transcription control in
eucaryotes
At the top, the structure
of the chromatin is
condensed, and those
genes cannot be
expressed. Cells can
only express those
portions of the DNA that
have been somewhat
unwound. The tan discs
are histones, which
control the metastructure
of the chromatin.
Steroid and thyroid
receptors (and
mediators in the
other systems) are
activators of
transcription In order
to do its thing, an
activator must bind
to an upstream
region of the DNA
called a response
element, and then
bend around to
interact with the
RNA polymerase at
the start site to start
transcription.
Repressors can
interfere with this
effect.
It takes two to tango - steroid and thyroid
hormone receptors gotta dimerize
For some of these
receptors, the dimers
are homodimers, for
others, the receptor
must pair with a
retinoic acid receptor –
if you are a thyroid
receptor or vitamin D
receptor ya don’t have
to dance with the one
that brung ya.
Now we can look at how the
cAMP 2nd messenger system
affects gene expression. You
already saw how the
cytoplasmic elements of this
pathway worked in the last
lecture. When activated PKA
enters the nucleus, it
phosphorylates CREB proteins
that are associated with the
cAMP response element CRE.
A nuclear protein called CBP
can then bind to the CREBs
and activate RNA polymerase
to start transcription.
And, we can see how receptor tyrosine kinases affect gene expressionagain, you’ve seen the first steps of this already.
Don’t miss the forest for the trees…
• This is a complex pathway, but • Forget about SOS
• Ras is like a G protein alpha subunit– you know about
them already
• Ras and Raf are like Beavis and Butthead – they just
always seem to go together.
• MEK (“MAP kinase”) - is just a message-carrier for Ras
and Raf
• MAPK (stands for mitogen-activated protein kinase) is
the one that really does the heavy lifting at the end of
this cascade – it mediates both the cytoplasmic effects
and the nuclear effects. In the nucleus, it phosphorylates
at least 6 known transcription factors, some of which act
on multiple genes
In the next installment of this series, we will look at some
specific examples of hormones at work.