Cell Communication

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Cell Communication

• Part 3

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Three Stages of Cell Signaling

1. Reception
2. Signal transduction
3. Response

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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.

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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

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Fine Tuning The Response

• Multiplicity of pathway signals serves 2 main
  purposes
• Signal amplification
• Response specificity

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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.

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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?

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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.

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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.

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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

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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

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Signal Termination

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