1. Cell Communication

2. Cell signaling
The mechanisms by which cells communicate with each other
Absolutely essential in multicellular organisms
Can communicate in multiple ways

3. Sending Signals 4 basic “styles” of signaling Endocrine
Contact- dependent
Paracrine
Neuronal

4. Local Signaling Ex. Paracrine & Neurons Paracrine Signaling
Signaling molecule that diffuses through the interstitial fluid
Ex. Histamine
Stored in certain cells of immune system
Released in response to allergic reactions, injury, or infection
Causes blood vessels to dilate and capillaries to become more permeable

5. Local Signaling Neuron Signaling
Most signal by releasing chemical compounds called neurotransmitters
Diffuse across synapses = tiny gaps between neurons
Ex. Acetylcholine, dopamine, serotonin

7. Long Distance Signals Ex. Endocrine/Hormone Signaling
Hormones are secreted by endocrine glands
Secreted into extracellular fluid
Diffuses into capillaries and transported by blood to target cell

9. Cell Signaling Overview

10. Step 1: Reception A signal molecule binds to a receptor protein
Signal molecule = ligand
Causes protein to change shape
Most receptors are plasma membrane proteins

14. Intracellular Receptors
Found in cytosol or nucleus
Activated by small or hydrophobic molecules that can cross membrane
Ex. Steroid and thyroid hormones
Can act as a transcription factor, turning on specific genes

16. Step 2: Transduction
Cascades of molecular interactions relay signals from receptors to target molecules in the cell
Usually involves multiple steps
Can amplify a signal

17. Transduction Pathways
The molecules that relay signals are mostly proteins
Activated from receptor, which activates another protein, which activates another, etc. until response is activated

18. Protein Phosphorylation
The signal is transmitted by phosphorylations
Protein kinases transfer phosphates from ATP to protein
Protein phosphatases remove the phosphates, called dephosphorylation
Acts as on/off switch

20. Second Messengers The ligand is considered the “1st messenger”
Second messengers are small, nonprotein molecules that spread by diffusion
Participate in pathways initiated by G protein coupled receptors & receptor tyrosine kinases
Ex. Cyclic AMP

21. Cyclic AMP (cAMP) One of the most widely used second messengers
Adenylyl cyclase, an enzyme in plasma membrane, coverts ATP to cAMP

23. Many ligands trigger formation of cAMP
Usually activates protein kinase A
Phosphorylates various other proteins

25. Step 3: Response Regulation of transcription or cytoplasmic activities
Many pathways regulate the synthesis of enzymes or other proteins
By turning on/off genes
Other pathways regulate the activity of enzymes
Can also affect physical characteristics of a cell

28. Fine-tuning the Response
Multistep pathways have 2 important benefits:
Amplifying the signal
Contributing to the specificity of the response

29. Signal Amplification Enzyme cascades amplify the cell’s response
At each step, the number of activated products is much greater than in the preceding step

30. Specificity of Signaling & Response
Different kinds of cells have different collections of proteins
Allow cells to detect and respond to different signals
The same signal can have different effects in cells with different proteins & pathways

33. Termination of Signal
Receptor and each of the components of the transduction pathway return to inactive states
Failure to terminate can lead to undesirable consequences
Ex. Cholera