1. Cell Signaling and Signal Transduction: Communication Between Cells
CHAPTER 15
Cell Signaling and Signal Transduction: Communication Between Cells

2. Introduction
Cells must respond adequately to external stimuli to survive.
Cells respond to stimuli via cell signaling.
Some signal molecules enter cells; others bind to cell-surface receptors.

3. 15.1 The Basic Elements of Cell Signaling Systems (1)
Extracellular messenger molecules transmit messages between cells.
In autocrine signaling, the cell has receptors on its surface that respond to the messenger.
During paracrine signaling, messenger molecules travel short distances through extracellular space.
During endocrine signaling, messenger molecules reach their target cells through the bloodstream.

4. Types of intercelular signaling

5. The Basic Elements of Cell Signaling Systems (2)
Receptors on or in target cells receive the message.
Some cell surface receptors generate an intracellular second messenger through an enzyme called an effector.
Other surface receptors recruit proteins to their intracellular domains.

6. Overview of signaling pathways

7. The Basic Elements of Cell Signaling Systems (3)
Signaling pathways consist of a series of proteins.
Each protein in a pathway alters the conformation of the next protein.
Protein conformation is usually altered by phosphorylation.
Target proteins ultimately receive a message to alter cell activity.
This overall process is called signal transduction.

8. A signal transduction pathway

9. 15.2 A Survey of Extracellular Messengers and Their Receptors (1)
Extracellular messengers include:
Small molecules such as amino acids and their derivatives.
Gases such as NO and CO
Steroids
Eicosanoids, which are lipids derived from fatty acids.
Various peptides and proteins

10. A Survey of Extracellular Messengers and Their Receptors (2)
Receptor types include:
G-protein coupled receptors (GPCRs)
Receptor protein-tyrosine kinases (RTKs)
Ligand gated channels
Steroid hormone receptors
Specific receptors such as B-and T-cell receptors

11. 15.3 G Protein-Coupled Receptors and Their Second Messengers (1)
G protein-coupled receptors (GPCRs) are numerous.
GPCRs have seven transmembrane domains and interact with G proteins.

12. A GPCR and a G protein

13. A GPCR and a G protein

14. Examples of GPCRs and their ligands

15. G Protein-Coupled Receptors and Their Second Messengers (2)
Signal Transduction by G Protein-Coupled Receptors
Ligand binding on the extracellular domain changes the intracellular domain.
Affinity for G proteins increases, and the receptor binds a G protein intracellularly.
GDP is exchanged for GTP on the G protein, activating the G protein.
One ligand-bound receptor can activate many G proteins.

16. Mechanism of receptor-mediated activation/inhibition by G proteins

17. G Protein-Coupled Receptors and Their Second Messengers (3)
Termination of the Response
Desensitization – by blocking active receptors from turning on additional G proteins.
G protein-coupled receptor kinase (GRK) activates a GPCR via phosphorylation.
Proteins called arrestins compete with G proteins to bind GPCRs.
Termination of the response is accelerated by regulators of G protein signaling (RGSs).

18. G Protein-Coupled Receptors and Their Second Messengers (4)
Bacterial Toxins, such as cholera toxin and pertussis virulence factors, target GPCRs and G proteins.
Second Messengers
The Discovery of Cyclic AMP
It is a second messenger, which is released into the cytoplasm after binding of a ligand.
Second messengers amplify the response to a single extracellular ligand.

19. The localized formation of cAMP

20. G Protein-Coupled Receptors and Their Second Messengers (5)
Phosphatidylinositol-Derived Second Messengers
Some phospholipids of cell membranes are converted into second messengers by activated phospholipases.
Phosphatidylinositol Phosphorylation
Phosphoinositides (PI) are derivatives of phosphatodylinositol.

21. Phospholipid-based second messengers

22. G Protein-Coupled Receptors and Their Second Messengers (6)
Phosphatidylinositol-specific phospholipase C- produces second messengers derived from phosphatidylinositol-inositol triphosphate (IP3) and diacylglycerol (DAG).
DAG activates protein kinase C, which phosphorylates serine and threonine residues on target proteins.
The phosphorylated phosphoinositides form lipid-binding domains called PH domains.

23. The generation of second messengers as a result of breakdown of PI

24. G Protein-Coupled Receptors and Their Second Messengers (7)
One IP3 receptor is a calcium channel located at the surface of the smooth endoplasmic reticulum.
Binding of IP3 opens the channel and allows Ca2+ ions to diffuse out.

25. Examples of responses mediated by Protein Kinase C

26. Cellular responses elicited by adding IP3

27. G Protein-Coupled Receptors and Their Second Messengers (8)
Regulation of Blood Glucose Levels
Different stimuli acting on the same target cell may induce the same response.
Glucagon and epinephrine bind to different receptors on the same cell.
Both hormones stimulate glucoses breakdown and inhibit its synthesis.
cAMP is activated by the G protein of both hormone receptors
Responses are amplified by signal cascades.

28. The reactions that lead to glucose storage or mobilization

29. G Protein-Coupled Receptors and Their Second Messengers (9)
Glucose Metabolism: An Example of a Reponse Induced by cAMP
cAMP is synthesized by adenylyl cyclase.
cAMP evokes a reaction cascade that leads to glucose mobilization.
Once formed, cAMP molecules diffuse into the cytoplasm where they bind a cAMP-dependent protein kinase (protein kinase A, PKA).

30. Formation of cAMP from ATP

31. The response by a liver cell to glucagon or epinephrine

32. G Protein-Coupled Receptors and Their Second Messengers (10)
Other Aspects of cAMP Signal Transduction Pathways
Some PKA molecules phosphorylate nuclear proteins.
Phosphorylated transcription factors regulate gene expression.
Phosphatases halt the reaction cascade.
cAMP is produced as long as the external stimulus is present.

33. The variety of processes that can be affected by changes in [cAMP]

34. Examples of hormone-induced responses mediated by cAMP

35. PKA-anchoring protein signaling

36. G Protein-Coupled Receptors and Their Second Messengers (11)
The Role of GPCRs in Sensory Perception
Rhodopsin is a photosensitive protein for black-and-white vision that is also a GPCR.
Several color receptors are GPCRs.
Odorant receptors in the nose are GPCRs.
Taste receptors for bitter and some sweet flavors are GPCRs.

37. Several disorders are caused by defects in receptors or G proteins.
The Human Perspective: Disorders Associated with G Protein-Coupled Receptors (1)
Several disorders are caused by defects in receptors or G proteins.
Loss of function mutations result in nonfunctional signal pathways.
Retinitis pigmentosa, a progressive degeneration of the retina, can be caused my mutations in rhodopsin’s ability to activate a G protein.

38. Transmembrane receptor responsible for causing human diseases

39. Human diseases linked to the G protein pathway

40. The Human Perspective: Disorders Associated with G Protein-Coupled Receptors (2)
Gain of function mutations may create a constitutively activated G protein.
Some benign thyroid tumors are caused by a mutation in a receptor.
Certain polymorphisms in G protein-related genes may result in an increased susceptibility to asthma or high blood pressure, as well as decreased susceptibility to HIV.

41. 15.4 Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (1)
Protein-tyrosine kinases phosphorylate tyrosine residues on target proteins.
Protein-tyrosine kinases regulate cell growth, division, differentiation, survival, and migration.
Receptor protein-tyrosine kinases (RTKs) are cell surface receptors of the protein-tyrosine kinase family.

42. Receptor Dimerization
Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (2)
Receptor Dimerization
Results from ligand binding.
Protein kinase activity is activated.
Tyrosine kinase phosphorylates another subunit of the receptor (autophosphorylation).
RTKs phosphorylate tyrosines within phosphotyrosine motifs.

43. Steps in the activation of RTK

44. Phosphotyrosine-Dependent Protein-Protein Interactions
Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (3)
Phosphotyrosine-Dependent Protein-Protein Interactions
Phosphorylated tyrosines bind effector proteins that have SH2 domains and PTB domains.
SH2 and PTB domain proteins include:
Adaptor proteins that bind other proteins.
Docking proteins that supply receptors with other tyrosine phosphorylation sites.
Signaling enzymes (kinases) that lead to changes in cell.
Transcription factors

45. The interaction between SH2 domain and a peptide contain a phosphotyrosine

46. A diversity of signaling proteins

47. The Ras-MAP Kinase Pathway
Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (4)
The Ras-MAP Kinase Pathway
Ras is a G protein embedded in the membrane by a lipid group.
Ras is active when bound to GTP and inactive when bound to GDP.

48. The structure of a G protein and the G protein cycle

49. Ras-MAP kinase pathway (continued)
Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (5)
Ras-MAP kinase pathway (continued)
Accessory proteins play a role:
GTPase-activating proteins (GAPs) shorten the active time of Ras.
Guanine nucleotide-exchange factors (GEFs) stimulate the exchange of GDP for GTP.
Guanine nucleotide-dissociation inhibitors (GDIs) inhibit release of GDP.

50. Ras-MAP kinase pathway (continued)
Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (6)
Ras-MAP kinase pathway (continued)
The Ras-MAP kinase cascade is a cascade of enzymes resulting in activation of transcription factors.
Adapting the MAP kinase to transmit different types of information:
End result differs in different cells/situations.
Specificity of the MAP kinase response due to differences in the types of kinases participating and differences in spatial organization of components.

51. The steps of a generalized MAP kinase cascade

52. Signaling by the Insulin Receptor
Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (7)
Signaling by the Insulin Receptor
Insulin regulates blood glucose levels by increasing cellular uptake of glucose.
The insulin receptor is a protein-tyrosine kinase
Autophosphorylated receptor associates with insulin receptor substrate proteins (IRSs).
IRSs bind proteins with SH2 domains, which activate downstream signal molecules.
SH2 domain-containing proteins are kinases that phosphorylate a lipid, PI 3-kinase (PI3K).

53. The response of the insulin receptor to ligand binding

54. The role of tyrosine-phosphorylated IRS in activating a variety of signaling pathways

55. Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (8)
Glucose Transport
PKB regulates glucose uptake by GLUT4 transporters.
GLUT4 transporters reside in intracellular membrane vesicles.
Vesicles fuse with the membrane in response to ligand binding to the IR.
Diabetes mellitus is caused by defects in insulin signaling and Type 2 diabetes is caused by gradual insensitivity to insulin.

56. Regulation of glucose uptake in muscle and fat cells by insulin

57. Insulin Signaling and Lifespan
Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (9)
Insulin Signaling and Lifespan
Several studies demonstrate that the lifespan can be increased by decreasing the level of insulin.
Human who live along life show high insulin sensitivity.
Laboratory studies show that calorie restriction leads to decreased insulin levels and increased insulin sensitivity.

58. Signaling Pathways in Plants
Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (10)
Signaling Pathways in Plants
Plants lack cyclic nucleotides and RTKs.
Plants have protein kinases that phosphorylate histidine residues.
The downstream cascade is similar to MAP kinase cascade.
The target of the cascade is usually transcription factors.

59. 15.5 The Role of Calcium as an Intracellular Messenger (1)
Cytoplasmic calcium levels are determined by events within a membrane.
Calcium levels are low in the cytosol because it is pumped out into the extracellular space and the membrane is highly impermeable to the ion.
Calcium channels can be transiently opened by action potential or calcium itself.
Calcium binds to calcium-binding proteins (such as calmodulin), which affects other proteins.

60. Experimental demonstration of localized release of intracellular Ca2+

61. Calcium-induced calcium release

62. Calcium wave in a starfish egg

63. Examples of mammalian proteins activated by Ca2+

64. The Role of Calcium as an Intracellular Messenger (2)
Recent research indicates a phenomenon called store-operated calcium entry (SOCE).
During SOCE the depleted calcium levels trigger a response that lead to opening of calcium channels.
The mechanism responsible for SOCE is a signaling system between the ER and plasma membrane.

65. A model for store-operated calcium entry

66. Calmodulin

67. The Role of Calcium as an Intracellular Messenger (3)
Regulating Calcium Concentrations in Plant Cells
Cytosolic calcium changes in response to several stimuli, including light, pressure, gravity, and hormones.
Calcium signaling aids in decreasing turgor pressure in guard cells.

68. A model of the role of Ca2+ in guard cell closure

69. Signaling pathways can converge , diverge, and crosstalk as follows:
15.6 Convergence, Divergence and Crosstalk Among Different Signaling Pathways (1)
Signaling pathways can converge , diverge, and crosstalk as follows:
Signals form unrelated receptors can converge to activate a common effector.
Identical signals can diverge to activate a variety of effectors.
Signals can be passed back and forth between pathways as a result of crosstalk.

70. Examples of convergence, divergence, and crosstalk among signal transduction pathways

71. Convergence, Divergence and Crosstalk Among Different Signaling Pathways (2)
Convergence – GPCRs, receptor tyrosine kinases, and integrins bind to different ligands but they all can lead to a docking site for Gbr2.

72. Convergence of signals transmitted from a GPCR, an integrin, and receptor tyrosine kinase

73. Convergence, Divergence and Crosstalk Among Different Signaling Pathways (3)
Divergence – all of the examples of signal transduction so far are evidence of divergence of how a single stimulus sends signals along a variety of different pathways.

74. Convergence, Divergence and Crosstalk Among Different Signaling Pathways (4)
Crosstalk – more and more crosstalk is found between signaling pathways:
cAMP can block signals transmitted through the MAP kinase cascade.
Ca2+ and cAMP can influence each other’s pathways.

75. An example of crosstalk between two major signaling pathways

76. 15.7 The Role of NO as an Intracellular Pathway
Nitric oxide (NO) is both an extracellular and intercellular messenger with a variety of functions.
NO is produced by nitric oxide synthase.
NO stimulates guanylyl cyclase, making cGMP.
cGMP decreases cytosolic calcium and relaxes smooth muscle.
NO also plays a role in male arousal.

77. Signal transduction by means of NO and cGMP

78. 15.8 Apoptosis (Programmed Cell Death) (1)
Apoptosis is an ordered process involving cell shrinkage, loss of adhesion to other cells, dissection of chromatin, and engulfment by pahgocytosis.

79. A comparison of normal and apoptotic cells

80. Apoptosis (Programmed Cell Death) (2)
Apoptotic changes are activated by proteolytic enzymes called caspases, which target:
Protein kinases, some of which cause detachment of cells.
Lamins, which line the nuclear envelope.
Proteins of the cytoskeleton
Caspase activated DNase (CAD)

81. Apoptosis (Programmed Cell Death) (3)
The Extrinsic Pathway of Apoptosis
It is initiated by external stimuli:
Tumor necrosis factor (TNF) is detected by a TNF cell surface receptor.
Bound TNF receptors recruit “procaspases” to the intracellular domain of the receptor.
Procaspases convert other procaspases to caspases.
Caspases activate executioner caspases, leading to apoptosis.

82. The extrinsic (receptor-mediated) pathway of apoptosis

83. Apoptosis (Programmed Cell Death) (4)
The Intrinsic Pathway of Apoptosis
It is initiated by intracellular stimuli.
Proapoptotic proteins stimulate mitochondria to leak proteins, mostly cytochrome c.
Release of apoptotic mitpchondrial proteins irreversibly commits the cell to apoptosis.

84. The intrinsic (mitochondria-mediated) pathway of apoptosis

85. Release of cytochrome c and nuclear fragmentation during apoptosis

86. Apoptosis (Programmed Cell Death) (5)
Antiapoptotic proteins promote survival.
Cell fate depends on the balance between pro- and anti-apoptotic signals.
Apoptotic cell death occurs without spilling cellular contents to prevent inflammation.
Apoptotic cells are cleared by phagocytosis.

87. Clearance of apoptotic cells is accompanied by phagocytosis