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A question for you- have you ever played the game “telephone?”
You know, the game where someone whispers something to someone and then that person
whispers it to someone else and then that person whispers it to someone else…and by
the time you get through everyone playing, the original message is all messed up?
I used to kind of dread that game---most people seemed to like it---but somehow whenever it
reached me, the message was always really messed up so I felt like I was spreading nonsense.
Well, either that, or it was me all along that misheard it.
Anyway, the game is all about communication and how things spread.
Communication is incredibly important not just for us but for the things that we are
made up of.
Like our cells!
Cells make up all living things.
And while they don’t talk in the way that you and I do, it’s important for their messages---their
signaling---to be transmitted and received appropriately.
Multicellular organisms need their many cells to be able to work together to carry out functions.
Just consider all of the cells working together in one of your organs—like you heart—for
example!
First some vocabulary---we’re going to be talking a lot about receptors.
A receptor is a molecule---such as a protein---where a signal molecule can bind.
One place you can find receptors is on the surface of a cell membrane.
When a signal molecule binds the receptor, amazing things can happen.
The receptor might start activating another molecule for an action to happen---the receptor
often changes its shape slightly in the process---more about that later.
So receptor.
Signal molecule.
The signal molecule can be considered a ligand, a fancy term which basically means it’s
the smaller molecule that binds to a typically larger molecule.
Signal molecules can be a variety of things such as gas molecules or hydrophobic biomolecules
like lipids or hydrophilic biomolecules like some kinds of proteins.
But ligands are generally smaller than the receptors they bind.
Ligands and receptors can have a very specific fit as well.
Let’s talk about the general sequence of cell signaling.
First, Reception.
Typically, a signal molecule binds a receptor.
Second, Transduction.
The receptor gets activated by this binding.
This often means the receptor will change its shape.
It could even involve a whole series of molecules changing their conformation in something called
a signal transduction pathway.
This can amplify the original signal.
Third, Response.
There is some kind of response that is going to happen.
A portion of DNA that is found in the nucleus getting transcribed for example, that’s
a type of response.
Now cell signaling can involve intracellular signaling---which occurs within the cell itself---
and intercellular signaling where a cell communicates with another cell.
In many cases, signaling involves both: signaling between cells and then also the signaling
within cells.
Now, when we’re talking about signals traveling from one cell to another--- distance matters.
Some cells are close and have direct contact.
In the case of these two animal cells, they are gap junction close.
Gap junctions in these animal cells---or plasmodesmata in plant cells---are connections between two
close together cells that can allow ions or other small molecules to pass and by doing
so, they don’t have to pass across the plasma membrane.
Paracrine signaling allows a cell to target another cell by a signal molecule that may
diffuse between them---these cells are still close---but need not be connected.
The ligands in paracrine signaling tend to be rapidly reabsorbed and rapidly degraded;
the ligands are typically not traveling far as this is local signaling.
Synaptic signaling which specifically involves neurotransmitters in a synapse is another
example of this local signaling.
And what about long distance?
Endocrine signaling can allow a cell to communicate with a target cell from far away.
Signals may be carried in the bloodstream.
Hormones released by certain types of endocrine cells are a great example.
We also want to point out that a cell could just signal itself.
For example, in autocrine signaling, a cell could secrete a certain type of molecule which
then binds to its own receptor and causes a response.
A cell releasing its own growth factor could be an example of this.
Let’s just give a few signaling examples so we can get a basic understanding of the
vocab and what this can look like.
Let’s say we have a steroid hormone that travels through the cell’s semi-permeable
membrane.
Remember that would mean the steroid hormone is our signal molecule, our ligand.
Once inside the cell, it binds a protein receptor within the cell.
Now the protein receptor is active.
The protein receptor travels into the nucleus where the cell’s DNA is found.
This protein receptor binds to DNA and is involved in getting transcription of a certain
gene going, which eventually can be used to produce a specific protein.
This is an example of signaling inside the cell, simplified a bit.
Now in that example, the receptor was inside the cell, a cytoplasmic receptor.
But it doesn’t have to be.
Receptors are frequently part of the cell’s membrane surface.
If a receptor is sitting outside on the cell membrane surface, then the ligand doesn’t
have to come in.
In fact, there could be properties of the ligand that may not allow it to pass the membrane:
the ligand could be hydrophilic which would make it hard to pass through.
Let’s mention an example of a cell surface receptor type: a ligand-gated ion channel.
Ions normally don’t go unassisted through the cell membrane --- they are charged after
all --- see our cell transport video.
But a ligand-gated ion channel gives them a way through---through a channel!
But it is controlled.
In this example, the channel is closed.
But here comes a signal molecule, a ligand, it binds the receptor---which is the channel
protein in this case.
Now the channel protein responds by opening.
The ions - not the ligand- go through.
Once ions get through, the concentration of ions increases inside the cell.
And you may wonder, “Ok, so now there are ions in the cell, why does that matter?”
Well the increasing ion concentration can trigger a cellular response.
After reaching a certain concentration and stimulating a cell response, the ligand can
leave its binding site from the receptor and the channel can close.
Ligand gated ion channels can be used by neurons--- a neurotransmitter may be the ligand for the
channel to open.
This could happen at a synapse.
But just be aware that not all ion channels are ligand-gated ion channels.
Ion channels can be gated by other things.
A voltage gated ion channel, for example, depends on electrical membrane potential---not
a ligand.
Voltage gated ion channels are used by neurons too.
Check out action potential in neurons to learn more.
There are other types of cell membrane surface receptors we don’t have time to go into
in this video.
G-protein linked receptors and enzyme linked receptors are two other types that we encourage
you to explore!
So, in summary, why do we care about this cell signaling thing?
Well, realize that your body processes that keep you alive rely on your cells’ ability
of cell signaling.
From the regulation of your heartbeat to the hormone signals traveling long distances in
your body to the ways the neurons in your brain communicate---your multicellular self
needs cell signaling.
But there are many disorders where cell signaling does not work as it should and so understanding
all of the complexities of cell signaling is critical in order to find ways to treat
them.
Cancer is an example of a disease that can involve body cells with problems in cell signaling.
When we mentioned autocrine signaling – a cancer cell could have a problem where it
produces too much of its own growth factor causing excessive division.
Cancer cells can also have many other cell signaling difficulties where they do not function
like normal, healthy cells.
Or another example, there are also pathogens – such as viruses or bacteria – that can
take advantage of cell signaling.
Consider the virus HIV which targets Helper T cells.
Helper T cells are important immune cells in your body, and they have something called
a CD4 receptor on their surface.
That receptor is important so that Helper T cells can communicate with other immune
cells.
But as mentioned in our viruses video, HIV targets that CD4 receptor.
It is because of the virus binding to that receptor on Helper T cells that the virus
can attach and infect the cell in the first place.
A fact that continues to be researched for treatment options.
The understanding of the details of cell signaling continues to expand.
Well that’s it for the Amoeba Sisters, and we remind you to stay curious.