CELL SIGNALING AND COMMUNICATION

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CELL SIGNALING AND COMMUNICATION

• CHAPTER 7

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Lecture Objectives

• What Are Signals, and How Do Cells Respond to
  Them?
• How Do Signal Receptors Initiate a Cellular
  Response?
• How Is the Response to a Signal Transduced
  through the Cell?
• How Do Cells Change in Response to Signals?
• How Do Cells Communicate Directly?

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Signal

• Any information from the environment that the
  cell can respond to
• Proper signal receptors required to get a
  response
• Three stages of cell signaling
• Reception
• Transduction
• Response

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A SIGNAL TRANSDUCTION PATHWAY
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AUTOCRINE PARACRINE SIGNALING
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HOW DO CELLS RESPOND TO SIGNALS?

• E. coli in our intestines respond to solute
  concentration outside

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• Receptor EnvZ
• Conformation change protein kinase
• Phosphorylation occurs
• Phosphate attaches to OmpR
• Signal transduction OmpR causes DNA expression
  to synthesize OmpC
• OmpC closes pores in cell membrane

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Steps in a Transduction Pathway

1. Signal (ligand) causes receptor shape change
2. Becomes a kinase, adding a phosphate to a
   responder
3. Signal is amplified, protein causes DNA gene
   expression
4. Cell activity altered (excitation or inhibition)

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INHIBITORS OR ANTAGONISTS

• R L ? RL
• ADENOSINE slows down brain activity
• CAFFEINE increases brain activity

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RECEPTORS

• Classified based on location and function
• On cell membrane surface (insulin)
• Cytoplasmic (estrogen)

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TYPES OF MEMBRANE RECEPTORS

• Ion channels
• Protein kinase receptors
• G protein-linked receptors

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GATED ION CHANNEL
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G-PROTEIN LINKED RECEPTORS
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• G proteins can either activate or inhibit an
  effector.
• Epinephrine (adrenalin) binds to G protein-linked
  receptor in heart muscle this activates an
  enzyme to produce cyclic AMP (cAMP).

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• G protein-mediated inhibition can occur with the
  same hormoneepinephrinein smooth muscle cells
• In this case the enzyme that produces cAMP is
  inhibited
• Same signal molecule has different effects in
  different cells

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CYTOPLASMIC RECEPTOR
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TRANSDUCTION THROUGH CELL
DIRECT TRANSDUCTION
INDIRECT TRANSDUCTION
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PROTEIN KINASE CASCADE
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SECOND MESSENGERS

• The second messenger is released into the
  cytoplasm after signal binds to receptor.
• Second messengers affect many processes in the
  cell.

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cAMP as a SECOND MESSENGER
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OTHER SECOND MESSENGERS
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• Other second messengers are derived from
  phospholipids in the plasma membrane, hydrolyzed
  by phospholipases.
• Example second messengers from PIP2
  (phosphatidyl inositol-bisphosphate)

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• PIP2
• Hydrophobic portion is diacylglycerol (DAG)
  embedded in plasma membrane
• Hydrophilic portion is inositol triphosphate
  (IP3) projecting into cytoplasm

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CALCIUM AS A SECOND MESSENGER IN FERTILIZATION
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• Low Ca2 concentrations in the cytoplasm are
  maintained by active transport proteins at plasma
  and ER membranes.
• Many signals can cause Ca2 channels to open,
  such as IP3, and entry of a sperm into an egg.
• Ca2 concentrations in cytoplasm increase
  rapidly.

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• Nitric oxide (NO) can act as a second messenger.
• This was discovered in studies of acetylcholine
  effects on smooth muscle tissue.
• Acetylcholine stimulates the IP3/DAG signal
  transduction pathway to cause influx of Ca2,
  which leads to a second messenger, cyclic GMP
  (cGMP).

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• But there was another second messenger.
• Acetylcholine stimulates IP3/DAG pathway of
  endothelial cells that line blood vessels.
• The influx of Ca2 activates an enzyme, NO
  synthase, that catalyzes production of NO from
  arginine.

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• Nitric Oxide diffuses to nearby smooth muscle
  cells where it stimulates synthesis of cGMP.
• Explains use of nitroglycerin to treat
  anginachest pain due to restricted cardiac
  arteries. Nitroglycerin releases NO.

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NITRIC OXIDE AS A SECOND MESSNEGER
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REGULATION OF SIGNAL TRANDUCTION
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EFFECTS OF A SIGNAL IN A CELL

• Opening ion channels
• Changing enzyme activity
• Differential gene transcription

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SENSE OF SMELL AND OPENING OF CHANNELS
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ALTERED ENZYME ACTIVITY
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SIGNALS INITIATE GENE TRANSCRIPTION

• In the Ras signaling pathway, the final protein
  kinase, MAPk, enters the nucleus and stimulates
  transcription of genes for cell division.

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COMMUNICATION BETWEEN CELLS

• GAP JUNCTIONS
• PLASMODESMATA (PLANT CELLS)