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 inter-cellular signaling
FGF
Insulin
Neurotransmitters

5. The Basic Elements of Cell Signaling Systems (2)
Receptors 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 pathwaysPM
Signaling
Events
(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. The Overall Flow of Information During Cell Signaling
Small Kd
– high affinity receptors
Recruitment of proteins 2’
2’’

10. Extracellular messengers include:
15.2 A Survey of Extracellular Messengers (primary) and Their Receptors (1)
Extracellular messengers include:
Small molecules such as amino acids and their derivatives.
Gases such as NO and CO (no O2 or CO2)
Steroids
Eicosanoids, which are lipids derived from fatty acids.
Various peptides and proteins

11. 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

12. Hormone systems
Hormones – primary messengers secreted by one tissue that regulate the function of other cells or tissues in the same organism
Act over large distances
Intracellular receptors,
Glucocorticoid
Testosterone
Estrogen

13. Activation of Gene Transcription by Glucocorticoid Receptors
Specific mechanisms
for nuclear internalization

14. Comparison of the DNA Sequence of Several Hormone Response Elements

15. 15.3 G Protein-Coupled Receptors and Their Second Messengers (1)
G protein-coupled receptors (GPCRs) are numerous.
GPCRs have seven trans-membrane domains and interact with G proteins.

16. Examples of GPCRs and their ligands

17. A GPCR and a G protein

18. A GPCR and a G protein

19. The G Protein Activation/Inactivation Cycle
Effector = generation of second messenger

20. 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.

21. Mechanism of receptor-mediated activation / inhibition by G proteins
Adenylyl Cyclase
cAMP
ATP

22. 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 (RGP(S)s).

24. G Protein-Coupled Receptors and Their Second Messengers (4)
Cholera toxin – locks Gs in GTP bound state
cAMP high, intestine secretes salt & water.
Pertussis toxin – prevents GTP exchange on Gi
shuts off adenylyl cyclase,whooping cough.
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.

25. The localized formation of cAMP

26. 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.
Phosphoinositides (PI) are derivatives of phosphatodylinositol.

27. Phospholipid-based second messengers

28. 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.

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

30. 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.

31. Cellular responses elicited by adding IP3

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

33. Examples of responses mediated by Protein Kinase C

34. 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 glycogen breakdown and also inhibit its synthesis.
cAMP is activated by the G protein of both hormone receptors
Responses are amplified by signal cascades.

35. The reactions that lead to glucose storage or mobilization
Epinephrine/ Glucagon ==>glucose

36. 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).

37. Formation of cAMP from ATP

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

39. 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.

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

41. Examples of hormone-induced responses mediated by cAMP

42. 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.

43. 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.

44. Trans-membrane receptor responsible for causing human diseases

45. Human diseases linked to the G protein pathway

46. 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.

47. 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 (trans-autophosphorylation).
RTKs phosphorylate tyrosines within phosphotyrosine motifs.

48. Steps in the activation of RTK

49. 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

50. A diversity of signaling proteins

51. 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.
Mitogenic Activating Proteins=MAP

52. 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. (Inactivate)
Guanine nucleotide-exchange factors (GEFs) stimulate the exchange of GDP for GTP. (activate)
Guanine nucleotide-dissociation inhibitors (GDIs) inhibit release of GDP. (keep them inactive)

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

54. 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/time organization of components.

55. The steps of a generalized MAP kinase cascade

56. The Structure and Activation of Epidermal Growth Factor (EGF) Receptor
Autophosphorylation

57. Signal Transduction Through Epidermal Growth Factor Receptor
GEF

58. Fibroblast growth factor
Fibroblast growth factor (FGF)
Fibroblast growth factor receptor (FGFR)
Important for embryonic development
mesoderm layer  muscle, cartilage, bone, blood cells
forerunner of vertebral column

59. Disruption of FGF -Receptor Function

60. Fibroblast Growth Factor Signaling is Essential for Mesoderm Production in Frog Embryos

61. Dominant mutations in human FGFR-3 (TM domain)
Achondroplasia

62. 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).

63. The response of the insulin receptor to ligand binding

64. The role of tyrosine-phosphorylated IRS in activating a variety of signaling pathways
/AKT

65. Protein-Tyrosine Phosphorylation as a Mechanism for Signal Transduction (8)
Glucose Transport
PKB (AKT) regulates glucose uptake by GLUT4 transporters.
GLUT4 transporters reside in intracellular membrane vesicles.
Vesicles fuse with the plasma 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.

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

67. 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 a long life show high insulin sensitivity (not too much)
Laboratory studies show that calorie restriction leads to decreased insulin levels and increased insulin sensitivity.

68. 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.

69. 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.

70. Experimental demonstration of localized release of intracellular Ca2+

71. Calcium-induced calcium release

72. Calcium wave in a starfish egg

73. 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.

74. A model for store-operated calcium entry

75. Calmodulin

76. The Structure and Function of the Calcium-Calmodulin Complex
Kinases
Phosphatases

77. An Overview of Calcium Regulation in Cells
Cytosolic
Function of
Ca2+?

78. Examples of mammalian proteins activated by Ca2+

79. 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.

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

81. 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.

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

83. 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.

84. Grb2/SOS

85. 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.

86. An example of crosstalk between two major signaling pathways

87. 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.

88. Signal transduction by means of NO and cGMP
Nitroglycerin

89. 15.8 Apoptosis (Programmed Cell Death) (1)
Apoptosis is an ordered process involving cell self-destruction (i.e., shrinkage, loss of adhesion to other cells, dissection of chromatin) and engulfment by phagocytosis.
Necrosis= injury
Apoptosis= program for cell death

90. A comparison of normal and apoptotic cells

91. ‘bubble”

92. 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)

93. 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.

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

95. 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.

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

97. Release of cytochrome c and nuclear fragmentation during apoptosis

98. IGFR=insulin-like growth factor receptor INSR= insulin receptor
Induction of Apoptosis by Cell Death Signals or by Withdrawal of Survival Factors
Killer lymphocytes
Autoproteolysis
ATP
proteolysis
IGFR=insulin-like growth factor receptor
INSR= insulin receptor

99. 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.

100. Clearance of apoptotic cells is accompanied by phagocytosis