Cell Signaling and Signal Transduction

16. Signaling Molecules and Their Receptors

• Cell signaling is essential for cell survival, enabling them to monitor the environment and respond to external stimuli.
• Cell signaling, also known as signal transduction, involves:
  • Detection of a stimulus, typically a molecule secreted by another cell, on the plasma membrane surface.
  • Transfer of the signal to the cytoplasmic side of the membrane.
  • Transmission of the signal to effector molecules down a signaling pathway.
  • Proteins change conformation to pass the signal down the pathway.
  • This process triggers a cellular response, such as the activation of gene transcription.

Responding to Signals

• Bacteria and unicellular eukaryotes respond to:
  • Environmental signals like food.
  • Signaling molecules secreted by other cells for mating, aggregate formation, and communication.
• In multicellular organisms:
  • Cell-cell communication is more complex.

Cell Communication Examples

• Yeast Mating
  • Peptides secreted by one yeast cell signal mating to another.
  • Attraction occurs between \"a\" and \"α\" yeast mating types.
• Bacterial Quorum Sensing
  • Bacteria coordinate behavior based on population density.

Signaling Molecules

• Various molecules (e.g., hormones, neurotransmitters) transmit signals.
• They travel through circulatory systems to reach target cells.

Types of Signaling

• Nervous System Signaling
  • Nerve impulses transmit rapid signals for immediate responses.
  • Coordinates movement, sensory perception, and more.
• Hormonal Signaling
  • Hormones regulate processes over longer distances and time frames.
  • Controls growth, development, and homeostasis.
• Immune System Signaling
  • Immune cells release signals to communicate infection or damage.
  • Mobilizes responses for defense and healing.

Social Insect Communication

• Ants, bees, and termites exhibit advanced communication through chemical signals (pheromones) to coordinate tasks and defense.

Pheromone Types

• Aggregation Pheromones: Gather individuals into groups.
• Alarm Pheromones: Trigger defensive responses.
• Sex Pheromones: Indicate readiness for mating.
• Trail Pheromones: Guide others to a location, common in social insects; used in insect traps.

Signal Complexity and Transmission

• Signaling molecules range in complexity from simple gases to proteins.
• Signal transmission varies:
  • Some signals travel over long distances.
  • Others act locally.
• Modes of action:
  • Large, hydrophilic molecules bind to cell surface receptors.
  • Small, hydrophobic molecules cross the plasma membrane and bind to intracellular receptors.

Juxtacrine Signaling

• Signaling (juxtacrine signaling) involves communication between cells in direct contact.
• Regulates cell behavior in animal tissues.
• Integrins and cadherins serve as adhesion and signaling molecules.
• Influences cell proliferation and survival.
• Communication is mediated by gap junctions in animal cells and plasmodesmata in plant cells.
• Cell Surface Receptors
  • Cells express receptors interacting with neighboring cells.
  • Critical in regulating interactions during development and tissue maintenance.

Acetylcholine

• Acetylcholine binds to specific receptors on heart pacemaker cells, salivary glands, and skeletal muscles.
• Receptors are nicotinic or muscarinic, depending on location and function.

Types of Signaling Based on Distance

• Signaling molecule travel distance:
  • Endocrine signaling
  • Paracrine signaling
  • Autocrine signaling

Endocrine Signaling

• Hormones are produced by an endocrine gland and travel through the bloodstream to distant cells.
• Hormones can be small lipophilic molecules that diffuse through the cell membrane to reach cytosolic or nuclear receptors.
• Examples: Pituitary, parathyroid, pancreatic hormones, progesterone, testosterone, thyroid hormones.
• Example: Estrogen
  • Secreted by ovaries for development and maintenance of the female reproductive system.
  • Responsible for secondary sexual characteristics.

Neuronal Signaling

• Neuronal signals are transmitted electrically along a nerve cell axon.
• When the electrical signal reaches the nerve terminal, it causes neurotransmitter release.
• Examples: Conduction of an electric signal from one nerve cell to another or to a muscle cell.
• Follows both endocrine and paracrine signaling - travels long distances but personal.
• Synaptic signaling – neurotransmitters sent to adjacent cells in nervous systems of animals.
• E.g., acetylcholine stimulates skeletal muscle contraction

Paracrine Signaling

• Paracrine signaling affects only target cells near the signaling cell.
• Examples: Cells of the immune system regulating inflammation at an infection site or controlling cell proliferation in a healing wound.
• Neurotransmitters at nerve cell synapses.

Autocrine Signaling

• Autocrine signaling acts locally on the same types of cells, including themselves.
• Example: T lymphocytes respond to antigens by making a growth factor that drives their own proliferation, amplifying the immune response.

Diseases Related to Signaling

• Endocrine - diabetes, congenital dwarfism, thyroid problems.
• Paracrine - Neuronal (synaptic).
• Autocrine - T lymphocytes.

Signal Molecules, Site of Origin, Chemical Nature, and Actions

• Hormones
  • Epinephrine (adrenaline): Adrenal gland, tyrosine derivative; increases blood pressure, heart rate, and metabolism.
  • Cortisol: Adrenal gland, steroid (cholesterol derivative); affects metabolism of proteins, carbohydrates, and lipids.
  • Estradiol: Ovary, steroid (cholesterol derivative); induces and maintains secondary female sexual characteristics.
  • Insulin: \"ẞ\" cells of pancreas, protein; stimulates glucose uptake, protein and lipid synthesis.
  • Testosterone: Testis, steroid (cholesterol derivative); induces and maintains secondary male sexual characteristics.
  • Thyroid hormone (thyroxine): Thyroid gland, tyrosine derivative; stimulates metabolism.
  • Ethylene: Plant tissues, gaseous hydrocarbon; regulates developmental processes.
• Local Mediators
  • Epidermal growth factor (EGF): Various cells, protein; stimulates epidermal and other cell proliferation.
  • Platelet-derived growth factor (PDGF): Various cells, including blood platelets, protein; stimulates many cell types to proliferate.
  • Nerve growth factor (NGF): Various innervated tissues, protein; promotes survival and axonal growth of certain neurons.
  • Histamine: Mast cells, histidine derivative; causes blood vessels to dilate, causing inflammation.
  • Nitric oxide (NO): Nerve cells; endothelial cells, dissolved gas; relaxes smooth muscle; regulates nerve-cell activity.
• Neurotransmitters
  • Acetylcholine: Nerve terminals, choline derivative; excitatory neurotransmitter at nerve-muscle synapses and in central nervous system.
  • Y-Aminobutyric acid (GABA): Nerve terminals, glutamic acid derivative; inhibitory neurotransmitter in central nervous system.
• Contact-dependent Signal Molecules
  • Delta: Prospective neurons; transmembrane protein; inhibits neighboring cells from specializing similarly.

Receptor Locations

• Receptors are located on the cell surface or inside the cell (cytosol or nucleus).
• Hydrophilic Signals
  • e.g., Protein hormones, growth factors, neurotransmitters.
  • Use cell-surface receptors because they can't penetrate the cell membrane.
• Hydrophobic Signals
  • e.g., Steroid hormones, thyroid hormone, vitamin D3, retinoic acid.
  • Small, diffuse through the plasma membrane, bind to intracellular receptors.

Steroid Hormones

• Steroid hormones are synthesized from cholesterol:
  • Testosterone, estrogen, and progesterone are sex steroids, produced by the gonads.
  • Corticosteroids are produced from the adrenal gland:
    • Glucocorticoids stimulate glucose production.
    • Mineralocorticoids regulate salt and water balance in the kidney.
• Hydrophobic molecules diffuse through the plasma membrane to bind to intracellular receptors belonging to the nuclear receptor superfamily.
• Function as ligand-activated transcription factors, transcriptional regulators in animals.

Other Hormones and Molecules

• Ecdysone is an insect hormone that triggers metamorphosis of larvae to adults.
• Brassinosteroids are plant steroid hormones that control cell growth and differentiation.
• Thyroid hormone is synthesized from tyrosine in the thyroid gland and is critical in development and metabolism.
• Retinoic acid and retinoids are synthesized from vitamin A and are important in vertebrate development.
• Vitamin D3 regulates $Ca^{2+}$ metabolism and bone growth.

Glucocorticoids

• Produced by the adrenal gland during stress.
• Regulate glucose metabolism, immune response, and inflammation.
• Cellular Mechanism:
  • Bind specific receptors in the cytoplasm of target cells.
  • Receptors translocate to the nucleus, modulating gene expression.
• Nuclear Action:
  • Activated receptors bind DNA and recruit coactivators with HAT activity.
  • Stimulate transcription of target genes.

Thyroid Hormones

• Produced by the thyroid gland, vital for metabolism and growth regulation.
• Thyroid Hormone Receptor:
  • Binds DNA regardless of hormone presence.
  • Hormone binding shifts the receptor from repressor to activator of gene transcription.
• Receptor Actions:
  • Without hormone: Associates with corepressors (HDAC activity).
  • With hormone: Associates with coactivators (HAT activity).

Nitric Oxide (NO)

• Major paracrine signaling molecule in nervous, immune, and circulatory systems.
• Diffuses directly across the plasma membrane.
• NO is synthesized from arginine; its action is restricted to local effects due to its instability (half-life of a few seconds).
• Alters guanylyl cyclase activity, leading to cyclic GMP synthesis.
• Nitric oxide produced by endothelial cells causes relaxation in vascular smooth muscle, leading to blood vessel dilation.

Blood Vessel Dilation

• Neurotransmitters (e.g., acetylcholine) stimulate NO synthesis.
• NO diffuses to smooth muscle cells, activating guanylyl cyclase.
• Results in cyclic GMP production, leading to muscle relaxation and vessel dilation.
• Clinical Application of NO
  • Nitroglycerin in heart disease treatment converts to NO, increasing blood flow.

Neurotransmitters

• Small hydrophilic molecules (e.g., acetylcholine, GABA) carry signals between neurons and target cells.
• Signal release is triggered by action potential arrival.
• Neurotransmitter Action:
  • Neurotransmitters are hydrophilic and cannot cross plasma membranes; they must bind to cell surface receptors.
  • Diffuse across the synaptic cleft and bind to cell surface receptors.
  • Activate ligand-gated ion channels or G protein-coupled receptors.

Plant Hormones

• Small molecules that regulate plant growth and development.
  • Auxins - induce plant cell elongation by weakening the cell wall. Also regulate cell division and differentiation.
  • Gibberellins – stem elongation.
  • Ethylene - fruit ripening.
  • Cytokinin - cell division.
  • Abscisic acid - promotes dormancy in seeds and buds.

Peptide Signaling Molecules

• Include peptide hormones, neuropeptides, and polypeptide growth factors.
• Peptide hormones include insulin, glucagon, and pituitary gland hormones (e.g., growth hormone, follicle stimulating hormone, prolactin).
• Neuropeptides are secreted by some neurons instead of small-molecule neurotransmitters.
• Peptide growth factors include a wide variety of signaling molecules that control animal growth and differentiation.

Examples of Peptide Hormones and Growth Factors

• Includes Insulin, Glucagon, Growth hormone, Follicle-stimulating hormone, Prolactin, Substance P, Oxytocin, Vasopressin, Enkephalin, B-Endorphin, Nerve growth factor (NGF), Epidermal growth factor (EGF), Platelet-derived growth factor (PDGF), Interleukin-2, Erythropoietin.

Growth Factors

• Nerve growth factor (NGF) is a member of the neurotrophin family that regulates the development and survival of neurons.
• Epidermal growth factor (EGF) stimulates cell proliferation and is the prototype for the study of growth factors.
• Platelet-derived growth factor (PDGF) is stored in blood platelets and released during blood clotting at a wound site; it stimulates fibroblast proliferation, contributing to regrowth of the damaged tissue.

Lipid Signaling Molecules

• Include prostaglandins, prostacyclin, thromboxanes, and leukotrienes.
• They break down rapidly and act in autocrine or paracrine pathways.
• Arachidonic acid is converted to prostaglandin $H_2$ by cyclooxygenase.
• This enzyme is the target of aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs).
• Inhibiting prostaglandin synthesis reduces inflammation and pain.
• Aspirin reduces platelet aggregation and blood clotting by inhibiting thromboxane synthesis; thus, low daily doses of aspirin are often prescribed for stroke prevention.
• Synthesized from arachidonic acid, which is formed by the hydrolysis of phospholipids catalyzed by the enzyme phospholipases $A_2$.
• Arachidonic acid can then be metabolized via 2 alternative pathways:
  • COX - cyclooxygenase
  • LOX -lipoxygenase

Intracellular Signaling Pathways

• Extracellular signals activate intracellular signaling pathways to change the behavior of the target cell.
• A cell-surface receptor protein activates one or more intracellular signaling pathways, each mediated by a series of intracellular signaling molecules (proteins or small messenger molecules).
• Signaling molecules eventually interact with specific effector proteins, altering them to change cell behavior.
  • They can alter metabolism with metabolic enzymes
  • They can alter cell shape or movement using cytoskeletal protein.
  • They can alter gene expression with transcription regulators.

Intracellular Signaling Proteins

• Intracellular signaling proteins can relay, amplify, integrate, distribute, and modulate via feedback signals.

Molecular Switches

• Proteins are activated or inactivated by phosphorylation, involving protein kinases (serine, threonine, or tyrosine) and protein phosphatases.
• GTP-binding proteins toggle between GTP-bound (active) and GDP-bound (inactive) states.
• Many intracellular signaling proteins behave as molecular switches.
• Molecular switches are of 2 types.

Ion Channel Coupled Receptors

• Open upon binding to an extracellular signal.
• Change the permeability of the plasma membrane to selected ions, altering the membrane potential and, if conditions are right, producing an electrical current.

G Protein and Enzyme Receptors

• G Protein-Coupled Receptors: The conformational change in the receptor upon ligand binding activates a G protein, which in turn activates an effector protein that generates a second messenger.
• Enzyme (Tyrosine Kinase) Receptors: Binding of an often-dimeric ligand induces dimerization of the receptors that leads to cross-phosphorylation of the cytosolic domains and phosphorylation of other proteins.
• There are several receptor classes that are used in different signaling pathways. The two more predominant are:

G Protein-Coupled Receptors

• The largest family of cell surface receptors.
• Signals are transmitted via guanine nucleotide-binding proteins (G proteins).
• The receptors have seven membrane-spanning α helices.
• G proteins have three subunits designated α, β, and γ (heterotrimeric G proteins).
• The largest family of G protein-coupled receptors are responsible for odor detection and recognition.

G Protein Activation

• In the inactive state, α is bound to GDP in a complex with β and γ.
• Hormone binding to the receptor causes exchange of GTP for GDP.
• The α and βγ complex then dissociate from the receptor and interact with their targets.

Regulation of G Protein Activity

• Activity of the α subunit is terminated by hydrolysis of the bound GTP, stimulated by RGS proteins.
• The inactive GDP-bound α subunit then reassociates with the βγ complex.

Regulation of Glycogen Metabolism

• Epinephrine stimulates glycogen breakdown via a G protein-coupled receptor.

Cyclic AMP (cAMP)

• Cyclic AMP (cAMP) is synthesized from ATP by adenylyl cyclase and degraded to AMP by cAMP phosphodiesterase.

G Proteins and Ion Channels

• A large array of G proteins connect receptors to distinct targets, including ion channels.
• Example: Action of the neurotransmitter acetylcholine on heart muscle.
• Heart muscle cells have a different acetylcholine receptor than nerve and skeletal muscle cells; this receptor is G protein-coupled.
• The α subunit of this G protein (Gi ) inhibits adenylyl cyclase.
• The Gi βγ subunits open $K^+$ channels in the plasma membrane, slowing heart muscle contraction.

Active G-Protein and Calcium Release

• Active G-protein leads to release of $Ca^{2+}$ from ER to cytoplasm, an example of activation of αGq

Tyrosine Kinases

• Largest class of enzyme-linked receptors, which phosphorylate their substrate proteins on tyrosine residues (receptor tyrosine kinases).
• Tyrosine kinases are key elements of signaling pathways involved in the control of animal cell growth and differentiation.
• The human genome encodes 58 receptor tyrosine kinases, including the receptors for EGF, NGF, PDGF, insulin, and many other growth factors.

Growth Factor Receptors

• Each receptor consists of an N-terminal extracellular ligand-binding domain, a single transmembrane α helix, and a cytosolic C-terminal domain with tyrosine kinase activity.
• Growth factor binding induces receptor dimerization, which results in receptor autophosphorylation as the two polypeptide chains cross-phosphorylate one another.

Steps of Receptor Activation

• The first step is ligand-induced receptor dimerization.
• This results in receptor autophosphorylation, as the two polypeptide chains cross-phosphorylate each other.
• Autophosphorylation has two roles:
  • Phosphorylation of tyrosine in the catalytic domain increases protein kinase activity.
  • Phosphorylation of tyrosine outside the catalytic domain creates binding sites for other proteins that transmit signals downstream from the activated receptors.

Receptor Phosphorylation and Inactivation

• Receptor phosphorylation creates binding sites for downstream signaling molecules.
• SH2 domains bind to specific phosphotyrosine-containing peptides of the activated receptor.
• Inactivation is achieved by protein tyrosine phosphatases
• Activated receptors are often destroyed by endocytosis and then digestion in lysosomes.

GPCR-Activated Intracellular Signaling Pathways

• All pathways eventually activate protein kinases, activating multiple intracellular signaling pathways. These include the GPCR, G protein, phospholipase C, adenylyl cyclase, Protein Kinase A (PKA), CaM-kinase, Protein Kinase C (PKC), Ras-GEF, Ras, PI 3-kinase, MAP kinase kinase kinase, MAP kinase kinase, protein kinase 1, Akt kinase, and transcription regulators

Signaling Pathway Integration

• Signaling pathways don’t operate in isolation; intracellular signal transduction is an integrated network of connected pathways.
• Computational modeling of signaling networks is a major challenge in cell biology.