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Molecular Signaling within Neurons

Overview of the mechanisms through which neurons communicate chemically, highlighting various types of signaling and their functions.

Types of Neuronal Chemical Signaling

  • (a) Endocrine Signaling

    • Hormone secretion into the blood by endocrine glands

    • Targets distant cells in the body

    • Example: Hormones influencing the metabolism through bloodstream circulation.

  • (b) Paracrine Signaling

    • Local signaling where a cell secretes signals to adjacent target cells

    • Example: Growth factors stimulating nearby cells.

  • (c) Autocrine Signaling

    • Signals act on the same cell that secreted them

    • Important for self-regulation and feedback mechanisms.

  • (d) Synaptic Signaling

    • Neurotransmitter release into the synaptic cleft, targeting adjacent neurons

    • Facilitates rapid communication between neurons.

Classes of Signaling Molecules

  • Neurotransmitters:

    • Examples: NPY (Neuropeptide Y), NE (Norepinephrine), ACh (Acetylcholine), 5HT (Serotonin)

  • Nitric Oxide (NO)

  • Hormones: E17b, T (Testosterone), thyroxin

  • NCAM (Neural Cell Adhesion Molecules)

  • Chemical Classifications:

    • Cell-permeant: Can cross the membrane (e.g., steroid hormones).

    • Cell-impermeable: Require receptors (e.g., neurotransmitters).

    • Cell-associated: Bound to the cell membrane.

Cellular Receptor Categories

  • (A) Channel-Linked Receptors

    • Signal binds to the receptor, causing the channel to open and ions to flow across the membrane.

  • (B) Enzyme-Linked Receptors

    • Signal binding activates an enzyme which catalyzes a reaction.

  • (C) G-Protein-Coupled Receptors

    • Signal binding activates a G-protein that regulates cellular activity.

  • (D) Intracellular Receptors

    • Signal binds to receptors inside the cell, affecting gene transcription.

G-Protein-Linked Receptors

  • Mechanism of Action:

    • Activation of a G protein starts cellular responses.

  • Types of G Proteins:

    1. Affecters of channel proteins

    2. Stimulatory G proteins: Activate amplifying enzymes (e.g., adenylyl cyclase).

    3. Inhibitory G proteins: Inhibit amplifying enzymes.

G-Protein Complex

  • The complex of beta and gamma subunits (Gb,g) inhibits the alpha subunit (Ga).

  • Structure of Neurotransmitter G-protein Receptor includes transmembrane helices and regulatory regions.

Amplifier Enzymes: Adenylate Cyclase

  • Function: Converts ATP into cAMP which activates protein kinases.

  • Process:

    1. Binding of neurotransmitters A and B activates adenylate cyclase.

    2. Signaled through G-proteins (Gα and Gβγ).

cAMP and Protein Kinase Activation

  • cAMP activates Protein Kinase A (PKA) which phosphorylates substrates.

  • Phosphorylation regulates enzyme activity within cells.

Kinases and Phosphatases

  • Enzyme Functions:

    • Protein kinases: Transfer terminal phosphate from ATP to proteins.

    • Protein phosphatases: Remove phosphate groups, reversing kinase action.

  • Critical for various cellular signaling pathways.

G-Protein Activation Cascade

  • One messenger activates multiple G-proteins, amplifying the signal significantly.

    • Example: Each G-protein activates adenylyl cyclase leading to thousands of cAMP molecules.

    • Each cAMP activates Protein Kinase A, subsequently leading to the phosphorylation of millions of proteins.

Phospholipase C (PLC)

  • Mechanism of Action:

    • PLC converts PIP2 into IP3 and DAG, leading to varied cellular responses.

  • Key Components:

    • IP3 (Inositol triphosphate)

    • DAG (Diacylglycerol)

Receptor Tyrosine Kinase

  • Structure:

    • Ligand-binding site, transmembrane helix, and cytoplasmic domain.

  • Cascade:

    • Dimerization leads to autophosphorylation and recruitment of relay proteins for cellular responses.

Steroid Hormone Action

  • Steroid hormones pass through the cell membrane and bind to intracellular receptors.

  • The hormone-receptor complex then influences gene expression by interacting with DNA.

Signaling Pathways Associated with G-Protein-Coupled Receptors

  • Examples:

    • Dopamine and norepinephrine influence various pathways via adenylyl cyclase or phospholipase C.

  • Target actions induce protein phosphorylation and calcium release.

GTP-binding Proteins

  • Types:

    • Heterotrimeric G-proteins: Composed of three subunits (alpha, beta, gamma).

    • Monomeric G-proteins: Small G-proteins (e.g., Ras) that are activated in cancer.

  • GAPs (GTPase-Activating Proteins): Regulate GTP hydrolysis.

Cancer and Cell Regulation

  • Ras proteins implicated in cancer progression.

  • Drugs targeting Ras present challenges due to its binding properties.

Second Messengers in Cellular Signaling

  • Cyclic Nucleotides

    • cAMP: Produced from ATP, activating PKA.

    • cGMP: Produced from GTP, activating PKG.

Calcium as a Second Messenger

  • Sources include voltage-gated and ligand-gated calcium channels.

  • Calcium mediates downstream signaling by binding to various proteins like calmodulin.

Protein Kinase Families

  • PKA: Phosphorylates serine and threonine residues.

  • PKC: Requires Ca2+ binding and phosphorylates serine and threonine.

  • CaMK: Calcium/calmodulin-dependent, also phosphorylates serine and threonine.

Receptor Regulation

  • Up-regulation: Increased receptor synthesis in response to decreased signaling.

  • Down-regulation: Decreased receptor availability via internalization following prolonged stimulation.

G Protein-Coupled Receptor Internalization

  • Steps leading to receptor desensitization and internalization via phosphorylation and arrestins.

CREB & c-Fos in Neuronal Signaling

  • CREB activation leads to c-Fos expression, initiating a cascade that influences gene transcription.

Signal Transduction Pathways

  • Pathways are intricate and interlinked, with many components required for activation and deactivation.

Epigenetic Regulation

  • DNA Methylation and Histone Modifications: Affect gene expression by altering chromatin structure and accessibility.

Histone Modifiers

  • Histone Acetylation and Deacetylation: Regulate gene expression by modifying histones through enzymes such as HATs and HDACs.

MicroRNA & Gene Regulation

  • miRNA: Inhibit translation by binding to target mRNA sequences.

  • siRNA: Structure allows for degradation of complementary mRNA sequences.

CRISPR Technology

  • Gene Editing: Utilize guide RNA and Cas9 to target and modify specific genes through either non-homologous end joining or homology-directed repair.

Optogenetics in Neuroscience

  • Mechanism: Using light-sensitive proteins to control neuronal activity with light exposure, enabling control of brain circuits and behavior modification.

Neuroendocrine Response to Stressors

  • CRH (Corticotropin-Releasing Hormone): Released in response to stress, influencing physiological responses through the hypothalamic-pituitary-adrenal axis.

Autonomic Nervous System Overview

  • Parasympathetic Division: Responsible for conserving energy; promotes rest-digest state.

  • Sympathetic Division: Activates the body’s fight-or-flight response during threats.

Brain Centers in Stress Response

  • Hypothalamus coordinates physiological response to stress via neurosecretory pathways that influence hormone release.

Mechanism of Action of Cortisol

  • Cortisol's effects include energy metabolism regulation and anti-inflammatory responses.

Summary of Stress Adaptation

  • Provides a balance between immediate and prolonged stress responses by reallocating bodily resources.

Glucocorticoid Receptor Function

  • Activation of genes involved in glucose metabolism and stress responses through glucocorticoid receptors triggers metabolic needs and maintains homeostasis nationwide.