Signalling Network Notation
(a) Positive Link = A activates node B
Negative Link = C inhibits D
(b) Double Negative = E inhibits F, preventing inhibition of F on G = Positive Amplification of G

Positive Feedback Loops: Amplification of a pathway deviating away from the norm.

Node I positively induces node II to achieve an output. Node II also increases further positive signalling of node III. Node III positive induces node I causing a positive feedback loop.
Node I positive induces node II to achieve an output. Node II also increases further signalling of node I causing a positive feedback loop.
Node I positively induces node II to achieve an output. Node II inhibits node III which is itself inhibiting node I. This causing a positive signalling of node I leading to a positive feedback loop.

Amplification in signalling cascades: We can achieve an amplification of a signal involving a small input via signalling cascades. For instance an input activates kinase I, that go on to phosphorylate other molecules and kinase II, that then activate an even larger number of kinase III amplifying this signal to achieve an output. However, this does need to be tightly regulated with feedback loops to coordinate inputs and outputs of cell behaviour. This is often allosterically regulated and occurs in a switch like manner.

Negative Feedback Loop: Regulation of a signalling pathway to return signal back to the norm.

Node I activates node II to achieve an intended output. Node II directly inhibits node I maintaining signalling pathway to narrow bounds.
Node I activates node II to achieve an intended output. Node II is an inhibitor of itself and will therefore inhibit the signalling pathway if the concentration of node I is high.

Adaption: (Bacteria Examples) A chemorepellant input binds to a receptor, CheR methylase methylates the receptor. This activates kinase CheA to phosophorylate CheY --> Activated CheY-Pi which rotates the flagellum motor for tumbling. This then needs to be turned off after a while which is achieved with CheB demethylase. In addition to rotating the flagellum a slower pathway with a bit of delay CheA also phosphorylates CheB which then demethylates the receptor turning off the rotating motor.

	Created by Joakim Silva over 8 years ago

Degree Integrative Cell Biology (Cell Signalling) Note on General Principles II, created by Joakim Silva on 24/03/2016.