Cell Communication
I.
General Info
A. What is cell communication?
other using chemicals.
Cells talk to each
Some of the things cells
might say to each other include:
i. Are we sexually compatible? Ex- type alpha
and type a yeast
ii. How many of us are there?
Ex- Vibrio
fisheri, bioluminescent bacteria that only
luminesce when they are crowded together
iii. If the cells are part of a multicellular
organism, cells talk to each other to alter
and coordinate each other’s activity and
keep the organism alive.
This is what we
will focus on!
B. In a multicellular organism, cells communicate
with chemicals in 3 general ways:
i. Synapses- a neuron spits chemicals
(neurotransmitters) onto a cell, either
another neuron or another type of cell.
For
example, your muscle cells contract because
a neuron told them to.
ii. Paracrine communication- cells in a local
area can talk to each other.
iii. Endocrine communication- cells in one area
of the body can talk to cells far away.
Chemicals that are used for this type of
long-distance communication are called
hormones.
In animals, hormones travel
through the circulatory system.
These are
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the chemicals we will draw our examples
from.
-from here on out I will refer to chemical signals, whether
they are neurotransmitters, paracrine factors, or hormones,
as “signals” unless I am using a specific example.C. A cell can only “hear” a chemical signal from
another cell if it has a receptor for that
signal.
A receptor is a protein that can
specifically bind to the chemical signal.
Receptors may be embedded into the membrane and
stick into the exterior space or may be in the
cytosol.
Generally, water-soluble signals bind
to receptors on the EXTERIOR surface of the cell.
These signals do NOT enter the cell.
Lipid-
soluble signals generally diffuse into the cell
and bind a receptor in the cytosol.
II.
The action of chemical signals: how they send a
message and how receiving cells respond to that
message
A. What signals cause cells to do:
i. Turn genes on: remember that genes generally
code for proteins, so "turning on" a gene
will cause a protein (such as an enzyme or
channel) to be made.
ii. Activate
or inactivate an existing enzyme
or other protein in the cell
- Overall, the effects of these actions can:
1. Alter membrane permeability or solute
absorption by opening or closing
channels (#ii), or causing them to be
made (#i) (for example, calcitriol
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causes the production of Ca2+ channels
in cells lining the small intestine, so
that dietary Ca2+ may be absorbed;
insulin causes the activation of
glucose channels so that cells may take
in glucose from the blood)
2. Induce secretory activity; for example,
by the production or activation of
enzymes involved with making products
for secretion
3. Stimulate cell division; for example,
by the production or activation of
enzymes involved with replication of
organelles or DNA
4. Alter metabolic activity; for example,
by the production or activation of
enzymes involved with ATP production
5. Alter the rate of transcription or
translation, ie, make particular
proteins/enzymes faster or slower.
B. How Water-Soluble Signals work
i. There are 3 general types of receptors that
can bind to water-soluble signals:
1. Ion channel receptors: these are
actually gated channels.
binds to them, they open.
When a signal
For example,
when a neuron tells a muscle cell to
contract, it spits the neurotransmitter
acetylcholine onto the muscle cell.
Acetylcholine binds to a receptor on
the muscle cell.
This receptor is
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actually a Na+ channel that has been
closed.
It opens when Acetylcholine
binds, and Na+ rushes in.
That event
will eventually lead to a muscle
contraction.
2. Receptor tyrosine kinases- more about
these later, but an example of a signal
that binds this type of receptor is the
hormone insulin.
3. G-protein linked receptors- more about
these later, but an example of a signal
that binds this type of receptor is the
hormone epinephrine (adrenaline).
ii. How a cell responds when water-soluble
signals bind to tyrosine kinases and Gprotein linked receptors: signal
transduction
1. When a signal binds one of these
receptors, the receptor will change
shape or move on the INTERIOR part of
the cell.
This event will set forth a
series of chemical reactions that will
eventually lead to the activation or
inactivation of specific proteins.
2. As stated above, binding of signal to
the receptors causes a cascade of
chemical events within the cell. In
many pathways, the ultimate goal is to
activate protein kinases, enzymes that
phosphorylate other chemicals (for
example, kinases could phosphorylate
enzymes that drive the production of
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thyroid hormones in thyroid cells). Why
does this matter? Many proteins are
activated or inactivated by the
addition or removal of phosphate; so by
adding phosphate to proteins, kinases
can turn them "on" or "off."
3. The substance that activates the
kinases is called a second messenger.
Two common second messengers that are
used in signal transduction pathways
are cyclicAMP and Ca2+.
4. I will ask you to know the events that
occur when a signal binds to a Gprotein linked receptor using cAMP as a
second messenger.
I will not ask you
to know the events of the tyrosine
kinases, or the events that occur when
Ca2+ is used as a second messenger.
Here’s how the G-protein linked receptor/cAMP system works:
First, be aware that there are 3 types of membrane
proteins you need to know about: 1) the hormone
receptor, which spans the membrane from the outside
surface to the interior 2) the G-protein, which is
bound to the receptor on the interior portion of the
membrane. The G-protein has a molecule of GDP
attached to it. 3) Adenylate Cyclase, an enzyme that
spans the membrane. Take a minute to draw a portion of
the membrane with these 3 proteins. As you read each
of the following steps, redraw the pictures, showing
what's going on at each step.
Let's use a liver cell responding to epinephrine. One
of the things that liver cells do in response to E is
break down glycogen to release glucose to the blood
(what other hormone has this effect?). When
epinephrine binds to its receptor, the receptor
changes shape and causes the G-protein to eject its
GDP. When GDP is ejected, the molecule GTP takes its
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place. The G-protein is now activated and released
from the receptor.
The G-protein moves along the interior part of the
cell membrane once it's released. It will bump into
and bind an Adenylate Cyclase.
Adenylate Cyclase, upon being bumped and bound, will
drive the reaction:
ATP --> cAMP. That is, it will convert ATP to cAMP.
Again, cAMP is the 2nd messenger, whose job it is to
activate kinases. So, cAMP cruises around the cytosol,
activating kinases. Incidentally, there are enzymes
ready to degrade cAMP almost as soon as it's made.
Now, we have a bunch of kinases running around really
getting the job done: activating or inactivating
proteins that will have the effect desired by the
hormone. For example, in liver cells, one of the
enzymes that gets activated by these kinases is
responsible for chopping glucose units off of glycogen
chains.
C. How lipid-soluble signals work: these are a
little more straightforward. Lipid-soluble
signals, such as steroid hormones, diffuse into
the cell freely. They will bind with a receptor
in the cytosol (or sometimes in the nucleus).
Together, the signal chemical and the receptor
will go into the nucleus and bind DNA adjacent to
specific genes. That binding will cause genes to
be activated, and the cell will build more of the
target protein. For example, in muscle cells,
testosterone activates genes that code for
contractile proteins (actin and myosin).
III. Three specific examples of hormone effects on a
liver cell’s use of glucose. Some background- when
you eat, liver cells store glucose as glycogen.
After you fast for a few hours, glucose levels in
the blood fall so the liver cells break down the
glycogen and put that glucose into the blood. If
you fast for several hours to days, the liver cells
will build new glucose using other sources, like
amino acids. The liver is largely responsible for
making sure that there is enough (but not too much)
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glucose in the blood. What you need to know from
this is ONLY the name of the hormone and which type
of receptor it uses. The names of the enzymes and
their actions are just for your interest!
A. Insulin- when you have just eaten, insulin is
released. Binds a tyrosine kinase. A very
complex series of activations leads to the
activation of protein phosphatase-1, which will:
i. Activate glycogen synthase, an enzyme that
builds glycogen
ii. Inactivate glycogen phosphorylase- an enzyme
that breaks down glycogen
B. Epinephrine (adrenaline)- after a fast,
epinephrine is released. Binds a G-linked
receptor. Uses cAMP as a second messenger. cAMP
activates protein kinase a, which then activates
phosphorylase kinase. Phosphorylase kinase will:
i. Activate glycogen phosphorylase
ii. Inactivate glycogen synthase
C. Cortisol- after a fast, cortisol is released.
This is a lipid-soluble hormone. It will enter
the nucleus and activate genes that code for
enzymes that build glucose, for example, from
amino acids.
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