lecture 4

Second Messengers: Function and Regulation

Overview

  • The concept of second messengers involves intracellular signaling molecules released by cell membrane receptors in response to extracellular signaling (first messengers).

  • Key second messengers include:

    • Cyclic AMP (cAMP)

    • Cyclic GMP (cGMP)

    • Inositol phosphates (IP3)

    • Calcium ions (Ca²⁺)

    • Diacylglycerol (DAG)

GTP-Binding Proteins

  • Types of GTP-binding proteins:

    • (A) Heterotrimeric G-proteins: Composed of three distinct subunits (α, β, and γ).

    • Activation leads to GDP-GTP exchange, causing dissociation into α and βγ subunits.

    • Biological actions are terminated by GTP hydrolysis, enhanced by GTPase-activating proteins (GAPs).

    • (B) Monomeric G-proteins: Similar mechanisms, relay signals from activated receptors to intracellular targets.

    • Activity also terminated by GTP hydrolysis regulated by GAP proteins.

Cytosolic and Membrane-Associated Second Messengers

  • Cytosolic: cAMP, cGMP, inositol phosphates (IP3), Ca²⁺

  • Membrane-associated: Diacylglycerol (DAG), Phosphatidyl inositides (PIPs)

Formation and Degradation of cAMP

  • Formation:

    • Catalyzed by Adenylyl cyclases from ATP; two types:

    • Transmembrane ACs regulated by Gα and Gβγ subunits.

    • Soluble ACs regulated independently of heterotrimeric G proteins, activated by bicarbonate and Ca²⁺.

  • Degradation:

    • By phosphodiesterases (PDEs), which shape intracellular cAMP gradients.

    • 11 families of PDEs exist, differing in regulatory properties and specificity.

    • Classes include cAMP-specific, cGMP-specific, and dual specificity.

    • Regulatory influences such as calcium/calmodulin and protein phosphorylation affect PDEs.

Signaling Functions of cAMP

  • cAMP primarily activates Protein Kinase A (PKA) and influences a variety of cellular functions:

    • Glycolysis

    • Gluconeogenesis

    • Lipogenesis

    • Muscle contraction

    • Membrane secretion

    • Learning processes

    • Ion transport

    • Differentiation

    • Growth control

    • Apoptosis

Role of cAMP in Target Signaling

  • cAMP-gated ion channels: Regulate ion passage, e.g., Ca²⁺ channels in sensory signaling (e.g., olfaction).

  • Protein Kinase A (PKA): Mediates effects of cAMP by relieving inhibition and phosphorylating substrate proteins.

    • Leads to various physiological responses.

  • Exchange Proteins directly activated by cAMP (EPAC): Guanine nucleotide exchange factors for small GTPases like Rap1.

Guanylyl Cyclases and cGMP

  • cGMP is synthesized from GTP by guanylyl cyclases; plays roles in cellular signaling:

    • Activates cGMP-dependent protein kinases and ion channels.

    • Two types of guanylyl cyclases: receptor and soluble types.

    • Receptor guanylyl cyclases activated by peptide ligands; soluble ones regulated by nitric oxide (NO).

Calcium Ions (Ca²⁺) as Second Messengers

  • Ca²⁺ is involved in numerous pathways influencing metabolic and physiological reactions.

    • Release mechanisms include:

    • IP3, cADP ribose, nicotinic acid adenine dinucleotide phosphate (NAADP).

  • Calcium-induced release mechanisms in skeletal and cardiac muscle also highlighted:

    • Dihydropyridine and Ryanodine receptors play crucial roles.

    • Various pathways activate intracellular Ca²⁺ release to influence muscle contraction and relaxation.

Integrated Signaling Pathways

  • G-protein-coupled receptors (GPCRs) activate various signaling pathways such as Gs, Gi, and Gq, leading to diverse biological responses based on the receptor type.

  • Examples provided illustrate different pathways (e.g., adrenergic receptors vs. insulin receptors).

Insulin Signaling and CREB Activation

  • Insulin binding activates signaling pathways for glucose homeostasis, leading to effects on cell growth, metabolism, and gene expression.

  • Cyclic AMP response element-binding protein (CREB) activated by cAMP or Ca²⁺ levels, influencing a range of biological functions and gene regulation.

Nitric Oxide (NO) Signaling

  • NO acts as a significant signaling molecule, involved in cardiovascular function and smooth muscle relaxation.

    • Synthesized by nitric oxide synthase; regulates various biological activities through the activation of guanylyl cyclase leading to cGMP production.

Pharmacological Implications

  • Understanding the signaling pathways can aid in drug development, particularly targeting GPCRs and addressing conditions like heart disease and mental health disorders through modulation of these signaling pathways.

End of Notes