Cell Communication

Cell Communication

Overview: The Cellular Internet

  • Cell-to-cell communication is absolutely essential for multicellular organisms.

Signaling Types

External Signals and Cellular Responses

  • External signals are converted into responses within the cell.

  • Example: Epinephrine affects cellular processes.

Cell Junctions

  • Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells.

    • Plasma membranes facilitate communication.

    • Cell wall in plants.

    • Gap junctions between animal cells and Plasmodesmata between plant cells.

Local Signaling

  • In local signaling, animal cells may communicate via direct contact or use mechanisms such as:

    1. Paracrine signaling: A secreting cell acts on nearby target cells by discharging molecules of a local regulator (e.g., growth factors).

    2. Synaptic signaling: A nerve cell releases neurotransmitter molecules into a synapse, stimulating a target cell.

    • Local regulator diffuses through extracellular fluid.

    • Secretory vesicles release neurotransmitters triggered by electrical signals along nerve cells.

Long Distance Signaling

  • Long-distance signaling occurs in both plants and animals using hormones.

  • Includes the transmission of signals through the nervous system.

  • Hormones travel in the bloodstream to target cells:

    • Specialized endocrine cells secrete hormones into bodily fluids, often blood.

    • Hormones can reach virtually all body cells.

The Three Stages of Cell Signaling

  • Discovered by Earl W. Sutherland, the process includes:

    1. Reception: Target cells detect a signal molecule from outside the cell.

    2. Transduction: Binding of the signal molecule changes the receptor, initiating transduction.

    3. Response: The transduced signal triggers a cellular response.

Overview of Cell Signaling

  • Cytoplasm :

    • Contains plasma membrane and extracellular fluid with receptor signaling molecules.

    • Three main steps:

    1. Reception: Activation of receptor.

    2. Transduction: Relay of molecules.

    3. Response: Activation of cellular response.

Reception of Signals

  • A signal molecule binds to a receptor protein, causing a shape change:

    • Specificity: The binding between signal molecules (ligands) and receptors is highly specific.

    • The conformational change in a receptor initiates transduction by enabling interaction with other cellular molecules.

    • Most signal receptors are plasma membrane proteins; some, like steroid hormones, can be intracellular.

Intracellular Receptors

  • Intracellular receptors are located in the cytoplasm or nucleus.

  • Small or hydrophobic signal molecules can readily cross the plasma membrane to use these receptors.

  • Structure includes:

    • Extracellular domain.

    • Cytoplasmic/intracellular domain.

    • Transmembrane domain.

G-Proteins and Receptors

  • G-proteins are associated with the cytoplasmic domain of receptors:

    • Trimeric structure: consists of alpha, beta, and gamma components.

    • Acts as a molecular switch:

    • Bound to GDP: “off” state.

    • Bound to GTP: “on” state.

Action of G-Proteins

  • Involved in a slow ligand-gated ion channel response:

    1. Messenger binds to a receptor.

    2. Conformational change of G protein occurs.

    3. Changes in ion movement across the cell membrane affect the electrical properties of the cell.

Epinephrine and Its Pathway

  • Related to the β-adrenergic receptor pathway:

    • Signaling molecule epinephrine interacts with the receptor, causing the activation of adenylate cyclase (an enzyme).

    • Results in conversion of ATP to cyclic AMP (cAMP), triggering protein kinase pathways.

  • Process:

    1. Epinephrine binds to the β-adrenergic receptor.

    2. Activation of adenylate cyclase, stimulating cAMP production.

    3. cAMP activates protein kinase A, resulting in various cellular responses.

Example of Intracellular Receptors

  • Steroid hormone signaling (e.g., testosterone):

    1. Hormone passes through the plasma membrane.

    2. Binds to receptor protein in the cytoplasm activating it.

    3. The complex enters the nucleus and binds specific genes, stimulating transcription into mRNA.

    4. mRNA is translated into a specific protein.

Types of Plasma Membrane Receptors

  1. G-protein-linked receptors.

  2. Tyrosine kinases.

  3. Ion channel receptors.

G-Protein-Linked Receptor Process

  1. Ligand binds receptor.

  2. The binding results in a conformational change in the receptor's cytoplasmic domain.

  3. GDP is replaced by GTP on the G-protein; it's activated.

  4. The activated G-protein interacts with adenylyl cyclase, triggering the pathway.

Function of cAMP in Signaling

  • cAMP acts as a second messenger in cellular pathways activated by G-proteins:

  • It is produced from ATP by adenylyl cyclase and is key for transmitting signals from receptors to target molecules within the cell.

Multistep Pathways of Transduction

  • Transduction pathways involve cascades of molecular interactions which relay signals:

    • Amplification of signals occurs within these pathways.

    • Allows better coordination and regulation of cellular responses through phosphorylation cascades.

Protein Phosphorylation and Dephosphorylation

  • Signal pathways often use phosphorylation cascades:

    1. A series of kinases add phosphate groups to activate subsequent kinases.

    2. Phosphatases remove these phosphates, rendering proteins inactive and reusable.

Calcium Ions and Inositol Triphosphate (IP3)

  • Calcium ions serve as a key second messenger:

    • When released into the cytosol, they activate various signaling pathways.

    • Cells regulate calcium concentrations to maintain cellular function and signaling.