#2: Paracrine & Autocrine Signaling
In paracrine signaling, one cell (or group of cells) secretes signaling molecules that are
received by receptors in nearby cells, changing those cells’ activities. This occurs between cells
that are close together, but not directly touching one another.
Paracrine signals move by diffusion through the extracellular matrix. These types of
signals usually elicit quick responses that last only a short amount of time. In order to keep the
response localized, paracrine ligand molecules are normally quickly degraded by enzymes or
removed by neighboring cells. Removing the signals will reestablish the concentration gradient for the signal, allowing them to
quickly diffuse through the intracellular space if released again.
Paracrine signaling allows cells to locally coordinate activities with their neighbors.
Although they're used in many different tissues and contexts, one example of
paracrine signaling is the transfer of signals across synapses between nerve cells. The
small space between nerve cells where this signaling occurs is called a synapse. In order
to send a signal from one nerve cell to another, one cell must release
neurotransmitters (the ligands) into the synapse. The neurotransmitters travel the
short distance between the two nerve cells, where they then bind to the receptors on
the next cell. The small distance allows the signal to travel quickly; this enables an
immediate response, such as, “Ow, take your hand off the stove, it’s hot!!!!” The
neurotransmitters that are released into the synapse are quickly degraded or taken
back up by the sending cell. This "resets" the system so the synapse is prepared to
respond quickly to the next signal.
In autocrine signaling, a cell signals to itself. It does so by releasing a ligand that then
binds to receptors on its own surface (or, depending on the type of signal, to receptors inside of
the cell). The prefix “auto-” means “self”, a reminder that the signaling cell sends a signal to itself.
It may seem odd for a cell to signal to itself, but autocrine signaling plays an important
role in many processes. One example of autocrine signaling is in cells infected with a virus.
Infected cells may release signals that result in the cell undergoing apoptosis, or
programmed cell death. While this results in the cell lysing and dying, it also kills the
infecting virus in the process, helping protect other cells that may not have been infected
yet. Another example of autocrine signaling is through the use of a negative feedback
loop. In this scenario, a ligand released by a cell signals to the cell to slow down or stop
production of more of the ligand. This type of feedback loop could ensure the cell
produces the correct amount of a certain molecule, without producing too much.
Both paracrine and autocrine signaling involve a signal being sent out in a small local area. However, in paracrine
signaling, the cell sending out the signal might not also be a target cell for that particular ligand. Instead, nearby cells are target
cells, receiving the signal and responding, while the cell that sent out the signal may not have the proper receptors to receive
and respond to its own signal, like in autocrine signaling. It’s also possible that a cell could take part in both paracrine and
autocrine signaling, if the ligand(s) it releases binds to other nearby cells and itself!
