1. Cell Communication
Part 3

2. Three Stages of Cell Signaling
Reception
Signal transduction
Response

3. 3. Cellular Response
What happens within a cell after the signaling pathway has been completed?
Several different types of cellular responses:
Secretion of a hormone, synthesis of an enzyme, etc.

4. Cytoplasmic & Nuclear Responses
Response can occur at either level.
May cause the opening or closing of an ion channel.
Regulates synthesis of enzymes by activation of genes within the nucleus.
Initiates or inhibits production of transcription factors

5. Fine Tuning The Response
Multiplicity of pathway signals serves 2 main purposes:
Signal amplification
Response specificity

6. Signal Amplification
Several steps within the cascade allows for the amplification of product to be made during each step.
For example: the pathway of glycogen breakdown produces increasing amounts of product at each step of the pathway.

8. Specificity of Cell Signaling
Different cells in the body are exposed to the same signals, but do not respond to all of them.
How are some signals responded to, and others ignored?
Some signals act on 2 different kinds of cells but elicit different responses.
How can the same molecule elicit different responses?

9. Specificity For example:
Liver cells and cardiac muscle cells are both exposed to epinephrine in the bloodstream.
However, epinephrine causes glycogen breakdown in the liver, and cardiac muscle contraction in the heart.

10. Different Cells = Different Proteins
Specificity of response depends on several factors within each cell:
Signal receptor proteins
Relay proteins
Proteins that carry out the cellular response
Therefore, different cells will have different responses to the same signal.

11. Signaling Efficiency Scaffolding proteins
Carry several protein kinases, therefore can phosphorylate several proteins at one time.
Allows for amplification of cascade.
Pathways are not always linear:
Several pathways can overlap and intertwine

12. When Pathways Run Amok
When one or more proteins involved in a pathway are damaged or missing, the results can be severe:
For example, Wiskott-Aldrich Syndrome (WAS) is characterized by the absence of 1 signal relay protein.
Widespread effects:
Abnormal bleeding
Eczema
Susceptibility to infections
Leukemia

13. Signal Termination Signaling pathways should not always be activated.
Every step of a cascade is reversible.
Works by feedback mechanisms.
Positive feedback
Negative feedback