1. Cell Communication
Chapter 9

2. Overview Communication between cells requires
Ligand – signaling molecule
Receptor protein – molecule to which the receptor binds
Interaction of these two components initiates the process of signal transduction, which converts the information in the signal into a cellular response

3. Ligand-Receptor

5. LIGANDS

6. Four basic mechanisms for cellular communication
Direct contact
Synaptic signaling
Paracrine signaling
Endocrine signaling
Some cells send signals to themselves (autocrine signaling)

7. Direct Contact
Molecules on the surface of one cell are recognized by receptors on the adjacent cell
Important in early development
Gap junctions

8. Synaptic signaling Animals
Nerve cells release the signal (neurotransmitter) which binds to receptors on nearby cells
Association of neuron and target cell is a chemical synapse

9. Paracrine signaling
Signal released from a cell has an effect on neighboring cells
Important in early development
Coordinates clusters of neighboring cells
Signaling between immune cells

10. Endocrine signaling
Hormones released from a cell travel through circulatory system to affect other cells throughout the body
Both animals and plants use this mechanism extensively

11. Signal Transduction
Events within the cell that occur in response to a signal
When a ligand binds to a receptor protein, the cell has a response
Different cell types can respond differently to the same signal
Epinephrine example (p. 170)
Glucagon example (p. 170)

12. Hormone: Epinephrine Flight or Fight
Differing effects of epinephrine depend of the different cell types with receptors for this hormone, but different sets of proteins respond to that signal
Liver – it mobilizes glucose
Heart muscle-contract more forcefully
Blood vessels-contract or dilate

13. Hormone: Glucagon
A variety of cell respond the same way to control blood glucose
Raise blood sugar
Works on liver cells and adipose tissue
Breaks down stored glycogen into glucose by turning on the genes that make the enzymes that synthesize glucose

14. Phosphorylation Addition of a phosphate group!
A cell’s response to a signal often involves activating or inactivating proteins
Phosphorylation is a common way to change the activity of a protein
Protein kinase – an enzyme that adds a phosphate to a protein
Phosphatase – an enzyme that removes a phosphate from a protein
Don’t get confused!

16. Receptors

17. Receptor Types Receptors can be defined by their location
Intracellular receptor – located within the cell
Cell surface receptor or membrane receptor – located on the plasma membrane to bind a ligand outside the cell
Transmembrane protein in contact with both the cytoplasm and the extracellular environment

18. Intracellular Receptors
Steroid hormones
Common nonpolar, lipid-soluble structure
Can cross the plasma membrane to a steroid receptor
Binding of the hormone to the receptor causes the complex to shift from the cytoplasm to the nucleus
Act as regulators of gene expression

20. A steroid receptor has 3 functional domains
Hormone-binding domain
DNA-binding domain
Domain that interacts with coactivators to affect level of gene transcription
In its inactive state, the receptor typically cannot bind to DNA because an inhibitor protein occupies the DNA binding site
Binding of ligand changes conformation

21. Coactivators
Target cell’s response to a lipid-soluble cell signal can vary enormously, depending on the nature of the cell
Even the same type of cell may have different responses
Depends on coactivators present
Estrogen has different effects in uterine tissue than mammary tissue
Not presence or absence of receptor but, the Presence or absence of coactivator

22. Cell Surface Receptors/ Membrane receptors
Chemically gated ion channels – channel-linked receptors that open to let a specific ion pass in response to a ligand
Enzymatic receptors – receptor is an enzyme that is activated by the ligand
Almost all are protein kinases
G protein-coupled receptor – a G-protein (bound to GTP) assists in transmitting the signal from receptor to enzyme (effector)

24. Chemically gated ion channels

25. Receptor Kinases
Protein kinases phosphorylate proteins to alter protein function
Receptor tyrosine kinases (RTK)
Influence cell cycle, cell migration, cell metabolism, and cell proliferation
Alteration to function can lead to cancer
Membrane receptor
Plants possess receptors with a similar overall structure and function

28. Insulin receptor (… how RTK works to lower blood sugar)
Insulin is a hormone that helps to maintain a constant blood glucose level
Glucose is bonded with other glucose molecules and converted to a glycogen molecule
Lowers blood glucose

29. Summary of RTKs A single transmembrane domain
Anchors them in membrane
Extracellular ligand-binding domain
Intracellular kinase domain
Catalytic site of receptor acts as protein kinase
Process:
When a ligand bind:
dimerization and autophosphorylation occurs
Cellular response follows – depends on cellular response proteins

30. G-Protein Coupled Receptors
G Proteins are transmembrane receptors
Single largest category of receptor type in animal cells is GPCRs

31. G-Protein Coupled Receptors
Sense molecules on OUTSIDE of cell (ligand)
Hormones, light sensitive compounds, neurotranmsitters, pheromones, odors
Activate a signal transduction INSIDE of the cell– ultimately leading to a response.
Open a gated channel
Enzymatic reaction

32.
G-Protein

34. Smell you later…

35. Often, the effector proteins activated by G proteins produce a second messenger
2 common effectors
Adenylyl cyclase
Produces cAMP (helpful for hormones that cannot get through cell membrane)
cAMP binds to and activates the enzyme protein kinase A (PKA)
PKA adds phosphates to specific proteins
Phospholipase C
PIP2 is acted on by effector protein phospholipase C
Produces IP3 plus DAG
Both act as second messengers

37. http://www. rci. rutgers. edu/~uzwiak/AnatPhys/ChemicalSomaticSenses

38. Different Signals, Same Effect
Different receptors can produce the same second messengers
Hormones glucagon and epinephrine can both stimulate liver cells to mobilize glucose
Different signals, same effect
Both act by same signal transduction pathway

40. Receptor for epinephrine has 9 isoforms
Single signaling molecule can have different effects in different cells
Existence of multiple forms of the same receptor (subtypes or isoforms)
Receptor for epinephrine has 9 isoforms
Encoded by different genes
Sequences are similar but differ in their cytoplasmic domains
Different isoforms activate different G proteins leading to different signal transduction pathways