signaling pathway
Meaning of signaling pathway: Relay races rather than single messengers: every handover amplifies the message and gives the cell another point at which it can intervene.
Definition of signaling pathway
A signaling pathway is a chain of molecules inside a cell that relays an instruction from the cell surface to the place where the response is carried out. The first molecule is switched on by an arriving signal, it activates the next, and so on until a final step alters gene activity, movement, division or survival. Cells run many such chains at once.
Why relay the message instead of sending it directly?
A single molecule arriving at the surface is a faint signal. Each step of the relay activates many copies of the next molecule, so a handful of arriving signals becomes thousands of active molecules deep inside the cell. The chain also creates places where the message can be checked, slowed, redirected or shut off, and it lets several different incoming signals feed into one route or a single signal branch into several outcomes. A direct connection would offer none of that flexibility.
What are the usual parts?
| Part | Job |
|---|---|
| Signal | The arriving molecule, such as a hormone or growth factor |
| Receptor | Recognizes the signal and changes shape |
| Relay molecules | Pass the message on, usually by adding or removing a phosphate group |
| Effector | Carries out the response, often by switching genes on or off |
| Off switch | Enzymes that strip the message away so the pathway resets |
The off switch matters as much as the rest. A pathway that cannot be turned off keeps issuing its instruction whether or not the original signal is still present.
What goes wrong in disease?
Most faults leave the pathway stuck on rather than broken. A relay molecule altered by a mutation may no longer need the step above it, so the instruction to divide is issued continuously. That pattern underlies a large share of cancer, and it explains why many targeted drugs are designed to block one step in a single route rather than to attack dividing cells in general. Faults in the same routes also appear in inherited developmental conditions, in inflammatory disease and in diabetes, because the pathways are shared across the body.
How is it different from a molecular pathway?
The wider term molecular pathway covers any linked series of molecular events, including the chemical reactions that build or break down substances. A signaling pathway is the subset concerned with carrying information rather than with producing material. The two names are often used loosely as though they meant the same thing.
Also known as
Used in a sentence
The tumor carried a mutation activating a signaling pathway downstream of the receptor.
Good to know
A mutation that leaves one relay step permanently active makes the pathway independent of the step above it, so blocking the original receptor has no effect on such a cell.