Signal Transduction and Cell Signaling, Part 2

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

Explain the difference between monomeric and trimeric G proteins.
- Monomeric G proteins consist of a single subunit (such as Ras), whereas trimeric G proteins (e.g., Gs, Gi, Gq) consist of three subunits: alpha (α), beta (β), and gamma (γ).
Describe the different G protein accessory proteins
Basic mechanisms of activation and termination of a signaling pathway

GPCRs initiate intracellular signaling via the activation of G proteins, leading to elevated intracellular calcium levels.
Activation of phospholipase C generates two important second messengers: inositol trisphosphate (IP3) and diacylglycerol (DAG) from phosphatidylinositol 4,5-bisphosphate (PIP2).
- IP3 is a soluble second messenger that diffuse through the cytosol.
- DAG is a membrane-bound second messenger.
IP3 Triggers Calcium Release: IP3 activates IP3-gated calcium channels in the endoplasmic reticulum, raising cytosolic Ca²⁺ levels, which subsequently activates proteins such as Protein Kinase C (PKC) and calmodulin.
Modulation of Glycogen Breakdown: GPCRs coordinate glycogen breakdown through the combined actions of Ca²⁺ and cyclic AMP (cAMP).
Acetylcholine and Nitric Oxide (NO): Acetylcholine’s engagement with GPCRs on endothelial cells leads to the production of NO, promoting smooth muscle relaxation and vasodilation.

Tissue-Specific Responses:
- Pancreas (Acinar Cells): Acetylcholine stimulates digestive enzyme secretion.
- Parotid Gland: Acetylcholine stimulates amylase secretion.
- Smooth Muscle (Vascular/Stomach): Contraction induced by acetylcholine.
- Liver: Vasopressin stimulates glycogen conversion to glucose.
- Blood Platelets: Thrombin induces aggregation and hormone secretion.
- Mast Cells: Antigen stimulation leads to histamine secretion.
- Fibroblasts: Growth factors induce DNA synthesis and division.
Mechanism: Hormonal stimulation utilizes IP3 to release Ca²⁺ from the endoplasmic reticulum.

Phosphoinositides: These are phosphorylated derivatives of phosphatidylinositol, produced by kinases activated by inter- or intracellular events, acting as precursors for secondary messengers (IP3 and DAG).
Phospholipase C (PLC): Activated by G proteins, PLC cleaves PIP2 to generate IP3 and DAG.
Signal Termination: The activity of IP3 is terminated by phosphatases, which remove phosphate groups, recycling inositol 4-phosphate for PI synthesis.

IP/DAG Pathway:
- Elevation of intracellular Ca²⁺ to concentrations between 10^{-7} to 10^{-4} M involves various cellular mechanisms:
1. GPCR Activation: Gα subunit activates PLC.
2. PLC Activity: Cleaves PIP2 into IP3 and DAG.
3. IP3 Diffusion: Triggers opening of calcium channels in ER.
4. Calcium Influx: Ca²⁺ moves through channels down the concentration gradient, activating PKC.
5. PKC Activation: DAG recruits PKC to the membrane.
6. Cellular Responses: Active PKC phosphorylates various enzymes and transcription factors.
Calcium as a Secondary Messenger:
- Ca²⁺ binding activates calmodulin and various regulatory proteins, leading to cellular responses.

Ca²⁺ Reservoir: The endoplasmic reticulum (ER) serves as the main intracellular store for Ca²⁺, controlled by Ca²⁺ ATPases that maintain a low cytosolic concentration (around 100 nM) under resting conditions.
Release Mechanisms: Signals can increase intracellular Ca²⁺ concentration up to 500–1,000 nM.
Calmodulin Sensitivity: Calmodulin responds to higher concentrations due to lower binding affinity.

Ca²+ Dynamics:
1. IP3Gated Channels: IP3 opens Ca²⁺ channels in the ER leading to cytosolic influx.
2. Mitochondrial Interaction: VDACs connect ER and mitochondria, facilitating Ca²⁺ passage through the mitochondrial calcium uniporter (MCU).
3. Ca²+ in Mitochondrial Matrix: Ca²⁺ influx stimulates ATP synthesis.
4. Preventing Toxicity: Ca²⁺ is released slowly from the mitochondrial matrix; transport proteins prevent overload.
5. Restorative Mechanisms: Ca²⁺ ATPases on the ER restore Ca²⁺ levels post-signal.

Signal Initiation:
1. First messenger binds and induces a conformational change in the receptor.
2. Activated receptor transmits signal to a trimeric G protein, activating downstream effector.
3. Effectors generate second messengers (cAMP, IP3, DAG)
Target Protein Activation:
- cAMP activates Protein Kinase A (PKA).
- IP3 opens calcium channels in the ER.
- DAG activates PKC.

NO as a Second Messenger:
- NO is produced by nitric oxide synthase and acts both intracellularly and extracellularly, modulating various physiological functions.
Signaling Mechanism: Acetylcholine activation of GPCRs generates cGMP via NO, which decreases cytosolic Ca²⁺ and induces smooth muscle relaxation.

TGF-β Family Signaling: TGF-β regulates cellular processes, inhibiting proliferation and guiding development.
- Ligands: TGF-β ligands activate receptors with serine/threonine kinase activity, leading to Smads phosphorylation which regulates gene transcription.

Both receptor types activate similar downstream pathways involving Jak/STAT signaling,
RTK and Cytokine Receptor Functionality:
- Dimerization activates autophosphorylation and recognition of phosphorylated sites by SH2-domain proteins which leads to further signaling.

Short-term Termination: Phosphotyrosine phosphatases and receptor internalization lead to downregulation.
Long-term Mechanisms: SOCS proteins mediate negative feedback by targeting receptors for degradation.

Activation and Mechanism: RTKs activate Ras, which triggers Raf-MEK-MAPK cascade leading to distinct cellular responses, including gene transcription regulation and cell growth.
Specificity in Signaling: Scaffold proteins segregate pathways to ensure precise regulation and response within different cellular contexts.