Signal Transduction and Cell Signaling Review
Lecture Overview
Lecture #18: Signal Transduction and Cell Signaling, Part 2
Major Topics Covered:
Chapter 15 (pages 690-704):
15.5 Regulating Protein Secretion and Muscle Contraction: Ca2+ Ions as Second Messengers in Multiple Signal Transduction Pathways
15.6 Vision: How the Eye Senses Light
Chapter 16 (pages 705-726):
16.1 Growth Factors and Their Receptor Tyrosine Kinases
16.2 The Ras/MAP Kinase Signal Transduction Pathway
16.3 Phosphoinositide Signal Transduction Pathways
Learning Objectives
Explain the difference between monomeric and trimeric G proteins.
Describe the different G protein accessory proteins.
Describe the basic mechanisms of activation and termination of a signaling pathway.
Important Concepts about GPCRs and Calcium Signaling
GPCR-G Protein Activation:
Activation of phospholipase C (PLC) generates two second messengers:
IP3 (Inositol trisphosphate) – soluble
DAG (Diacylglycerol) – membrane bound
Mechanism of Action:
IP3 triggers opening of IP3-gated Ca2+ channels in the endoplasmic reticulum, elevating cytosolic free Ca2+ concentration.
This process activates Protein Kinase C (PKC) and calmodulin.
Coordination in Glycogen Breakdown:
Neural and hormonal signals together regulate glycogen breakdown through Ca2+ and cyclic AMP (cAMP).
Acetylcholine Activation:
Acetylcholine activates its GPCR on endothelial cells, leading to the generation of nitric oxide (NO), which stimulates smooth muscle relaxation and vasodilation.
Cellular Responses to Hormone-Induced Rise in Cytosolic Ca2+
Table 15-4: Cellular Responses in Various Tissues
Tissue
Hormone Inducing Cellular Response
Response
Pancreas (acinar cells)
Acetylcholine
Secretion of digestive enzymes (amylase, trypsinogen)
Parotid gland
Acetylcholine
Secretion of amylase
Vascular smooth muscle
Acetylcholine
Contraction
Liver
Vasopressin
Conversion of glycogen to glucose
Blood platelets
Thrombin
Aggregation, shape change, secretion of hormones
Mast cells
Antigen
Histamine secretion
Fibroblasts
Peptide growth factors
DNA synthesis, cell division (e.g., bombesin, PDGF)
Mechanism: Hormone stimulation leads to production of inositol 1,4,5-trisphosphate (IP3), promoting the release of Ca2+ from the endoplasmic reticulum.
Synthesis of Second Messengers: DAG and IP3 from Phosphatidylinositol (PI)
Phosphoinositides (PI):
Phosphorylated forms of phosphatidylinositol.
Produced by specific kinases activated by inter- or intracellular events.
Bind and activate various proteins, promoting cellular activities such as migration.
Phospholipase C (PLC):
Activated by G proteins.
Cleaves PIP2 to yield IP3 and DAG.
Signal Termination:
Phosphatase removes the 5-phosphate from IP3.
A second phosphatase removes the 1-phosphate, leading to the recycling of inositol 4-phosphate back to PI 4-phosphate.
IP3/DAG Pathway: Elevation of Cytosolic Ca2+
Pathway Steps:
Step 1: GPCR activation engages the Gα subunit, activating PLC.
Step 2: PLC cleaves PIP2, producing IP3 and DAG.
Step 3: IP3 moves through the cytosol, opening IP3-gated Ca2+ channels in the ER.
Step 4: Ca2+ moves down its concentration gradient into the cytosol.
Step 5: Ca2+ activation of PKC occurs as it is recruited to the plasma membrane.
Step 6: DAG activates the membrane-bound PKC.
Step 7: Activated PKC-Ca2+ phosphorylates various enzymes and transcription factors that are vital for cell growth and metabolism.
The Phospholipase C Response and Calmodulin Activation
Ca2+-Dependent Responses:
Most responses are indirect; Ca2+ binds to regulatory proteins, including calmodulin, influencing a variety of cellular functions based on cell type.
Calmodulin:
A widely used Ca2+-dependent regulatory protein with several targets.
One family of targets is the CaM Kinases.
These proteins have low Ca2+ binding affinity, allowing activation at increased cytosolic Ca2+ concentrations.
Calcium Movement Between Cytosol, Mitochondria, and Endoplasmic Reticulum
Stepwise Movement of Ca2+:
Step 1a: IP3 binding opens Ca2+ channels in the ER membrane, releasing Ca2+ into the cytosol.
Step 1b: IP3 also opens channels in the mitochondria-associated membranes (MAMs).
Step 2: Outer mitochondrial membrane VDACs (voltage-dependent anion channels) facilitate efficient Ca2+ transfer.
Step 3: High intermembrane space Ca2+ concentration opens mitochondrial Ca2+ uniporters (MCU).
Step 4: Ca2+ released over time into the intermembrane space to maintain homeostasis and prevent toxicity.
Step 5: ER Ca2+-ATPases pump Ca2+ back into the ER to restore equilibrium.
Summary of G-Protein Coupled Pathways
Mechanisms:
First messenger binds to receptor, inducing receptor change.
Activated receptor stimulates trimeric G protein.
G protein activates effector proteins, generating second messengers:
Adenylyl cyclase generates cAMP.
Phospholipase C yields IP3 and DAG.
Second Messengers' Actions:
cAMP activates PKA.
IP3 opens calcium channels in the ER membrane.
DAG recruits and activates PKC.
Role of Nitric Oxide (NO) in Cell Signaling
NO as a Messenger:
Produced by nitric oxide synthase in response to various stimuli such as acetylcholine activating GPCR.
Mechanism:
Activation of PLC leads to the production of IP3 (+ DAG), raising cytosolic Ca2+ levels that activate calmodulin, which in turn activates NO synthase.
NO stimulates guanylyl cyclase leading to cGMP production, resulting in decreased cytosolic calcium and smooth muscle relaxation.
Learning Objectives for Next Sessions
Describe receptor serine kinase pathways.
Describe cytokine receptor and JAK/STAT pathways.
Describe receptor tyrosine kinase signaling pathways.
Explain Ras protein and how the MAP kinase cascade controls cell function.
Common Types of Cell-Surface Receptors and Signal Transduction Pathways
Categories of Receptors:
Receptor-associated kinases
Cytosolic kinases
Protein subunit dissociation pathways
Protein cleavage pathways (irreversible signaling)
Receptor-Associated Kinases
Mechanism:
Receptor dimerization upon ligand binding activates kinase activity.
Direct phosphorylation of transcription factors or signaling proteins.
Connection to GTP-Binding Proteins:
Small GTP-binding proteins (e.g., Ras) are often activated in response to receptor signaling, leading to kinase cascade activation.
Cytosolic Kinases and Their Function
Mechanisms of Activation:
Cytoplasmic domain binds to receptor-associated kinases or can directly trigger downstream signaling cascades.
Termination of Signaling Pathways
Short-Term Regulation:
Inactivation via distinct phosphatases.
Long-Term Regulation:
Endocytosis and lysosomal degradation of activated receptors, modulating responsiveness to extracellular signals.
Important Concepts on Receptor Tyrosine Kinases (RTKs) and Cytokine Receptors
RTKs:
Kinase is intrinsic to receptor; activates downstream pathways through dimerization and autophosphorylation.
Cytokine Receptors:
Activate separate kinases like JAK upon ligand binding, which phosphorylate tyrosine residues to modulate cellular responses.
Protein-Tyrosine Phosphorylation Mechanism
Protein-Tyrosine Kinases (PTKs):
Phosphorylate tyrosine residues, impacting cell functions like growth and differentiation.
Activation through dimerization of receptors allows for cross-activation of downstream signaling molecules.
SH2 and PTB Domains:
Protein domains that allow specific binding to phosphorylated tyrosine residues, playing crucial roles in downstream signaling pathways.
Activation and Structure of STAT Proteins
STAT Activation:
JAK kinases activate Signal Transducer and Activator of Transcription (STAT) transcription factors through phosphorylation and dimerization, regulating gene expression in response to growth factors.
Mechanisms for Terminating Cytokine Signal Transduction
Short-Term Regulation:
Phosphotyrosine phosphatases deactivate JAK kinases.
Long-Term Regulation:
SOCS proteins form negative feedback loops, target receptors for degradation, and prevent overstimulation of signaling pathways.