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.
