4.5 Second Messenger Systems
Second messengers are intracellular signaling molecules released or generated in response to first messengers (hormones, neurotransmitters). They amplify signals and coordinate diverse cellular responses through activation of protein kinases and other effectors.
cAMP (Cyclic Adenosine Monophosphate)
Synthesis and Degradation
Synthesis: Adenylyl cyclase (AC) converts ATP → cAMP
Activation: Gαs stimulates AC; Gαi inhibits AC
Degradation: Phosphodiesterases (PDEs) hydrolyze cAMP → AMP
Downstream Effectors
PKA (Protein Kinase A): cAMP binds regulatory subunits → release of catalytic subunits → phosphorylation of target proteins
Epac (Exchange protein activated by cAMP): Guanine nucleotide exchange factor for Rap GTPases
Cyclic nucleotide-gated channels: Direct ion channel activation
Physiological Effects
- • Cardiac: ↑ heart rate, contractility (β1-adrenergic → Gαs → cAMP → PKA)
- • Smooth muscle: Bronchodilation (β2), vasodilation
- • Metabolism: Glycogenolysis, gluconeogenesis, lipolysis
- • Renal: Aquaporin insertion (vasopressin V2 receptors)
- • Gene transcription: PKA phosphorylates CREB → CRE-mediated transcription
Pharmacological Modulation
PDE inhibitors: ↑ cAMP by preventing degradation
- • Theophylline (non-selective PDE inhibitor, bronchodilator)
- • Milrinone (PDE3 inhibitor, inotrope in heart failure)
- • Sildenafil, tadalafil (PDE5 inhibitors for cGMP, but illustrates concept)
- • Roflumilast (PDE4 inhibitor, COPD)
Calcium (Ca²⁺)
Calcium Homeostasis
Resting [Ca²⁺]: ~100 nM cytoplasmic; ~1 mM extracellular
Storage: Endoplasmic/sarcoplasmic reticulum (ER/SR), mitochondria
Signaling: Rapid ↑ to 1-10 μM creates signal
Sources of Ca²⁺ Signals
Extracellular entry:
- • Voltage-gated Ca²⁺ channels (L, N, P/Q, R, T-types)
- • Ligand-gated channels (NMDA receptors)
- • Store-operated Ca²⁺ entry (SOCE via Orai/STIM)
Intracellular release:
- • IP3 receptors (IP3-gated Ca²⁺ channels on ER)
- • Ryanodine receptors (RyR, Ca²⁺-induced Ca²⁺ release)
Ca²⁺-Binding Proteins
Calmodulin (CaM): Primary Ca²⁺ sensor; binds 4 Ca²⁺ → conformational change → activates target enzymes
CaM targets: CaM kinases (CaMKII, CaMKIV), calcineurin (phosphatase), adenylyl cyclases, phosphodiesterases, nitric oxide synthase
Troponin C: Muscle contraction (binds Ca²⁺ → removes tropomyosin inhibition)
Physiological Roles
- • Muscle contraction (skeletal, cardiac, smooth)
- • Neurotransmitter release (synaptic vesicle fusion)
- • Secretion (hormones, enzymes)
- • Cell proliferation and differentiation
- • Gene transcription (NFAT pathway via calcineurin)
- • Apoptosis (excessive or sustained elevation)
IP3/DAG System (Phospholipase C Pathway)
Activation Cascade
1. GPCR activation (Gαq) or RTK activation → Phospholipase C (PLCβ or PLCγ)
2. PLC cleaves PIP2 (phosphatidylinositol 4,5-bisphosphate) → IP3 + DAG
3. IP3 → binds IP3 receptors on ER → Ca²⁺ release
4. DAG + Ca²⁺ → activate protein kinase C (PKC)
IP3 (Inositol 1,4,5-trisphosphate)
Function: Rapidly diffuses to ER, opens IP3-gated Ca²⁺ channels
Termination: Dephosphorylated by phosphatases or further phosphorylated
Receptors activated by IP3: α1-adrenergic, M1/M3/M5 muscarinic, H1 histamine, angiotensin II, vasopressin V1
DAG (Diacylglycerol)
Function: Membrane-bound second messenger, activates PKC
PKC isoforms: Conventional (require Ca²⁺ + DAG), novel (DAG only), atypical (neither)
PKC substrates: MARCKS, transcription factors, ion channels, receptors
Pharmacological Modulators
PKC activators: Phorbol esters (tumor promoters, research tools)
PKC inhibitors: Staurosporine, bisindolylmaleimide (research), sotrastaurin (investigational immunosuppressant)
Nitric Oxide (NO)
Synthesis and Properties
Synthesis: L-arginine + O2 → NO + L-citrulline (via NOS enzymes)
NOS isoforms:
- • eNOS (endothelial): Constitutive, Ca²⁺/CaM-dependent, vascular tone
- • nNOS (neuronal): Constitutive, Ca²⁺/CaM-dependent, neurotransmission
- • iNOS (inducible): Ca²⁺-independent, induced by cytokines, immune response
Properties: Lipophilic gas, freely diffuses across membranes, short half-life (~seconds)
Signaling Mechanism
Primary pathway: NO → binds soluble guanylyl cyclase (sGC) → GTP → cGMP
cGMP effectors:
- • PKG (cGMP-dependent protein kinase): Smooth muscle relaxation
- • Phosphodiesterases: Some activated, some inhibited by cGMP
- • Cyclic nucleotide-gated channels
Physiological Roles
- • Vascular: Vasodilation (endothelium-derived relaxing factor, EDRF)
- • Platelet: Inhibits aggregation and adhesion
- • Neuronal: Neurotransmission, synaptic plasticity (LTP/LTD)
- • Immune: Antimicrobial, tumoricidal (high concentrations from iNOS)
- • Penile: Erection (smooth muscle relaxation)
Therapeutic Agents
NO donors: Nitroglycerin, isosorbide dinitrate (angina - vasodilation), sodium nitroprusside (hypertensive emergency)
PDE5 inhibitors: Sildenafil, tadalafil, vardenafil (↑ cGMP by preventing degradation - erectile dysfunction, pulmonary hypertension)
sGC stimulators: Riociguat (pulmonary hypertension - directly activates sGC)
Integration and Cross-Talk
Signal Amplification
Single receptor activation → hundreds of G-proteins → thousands of cAMP molecules → millions of phosphorylated substrates. Enzymatic cascades provide exponential amplification.
Temporal Dynamics
Ca²⁺ oscillations, cAMP pulses encode information. Frequency and amplitude modulation allow distinct cellular responses to same messenger.
Pathway Cross-Talk
Ca²⁺ activates some AC isoforms, inhibits others. PKA phosphorylates PDE (negative feedback). PKC desensitizes GPCRs. Multiple integration points.
Compartmentalization
Scaffold proteins (AKAPs) localize signaling complexes. Creates microdomains with distinct [cAMP] or [Ca²⁺]. Enables specificity despite ubiquitous messengers.