Cell Communication Overview

Cell Communication Overview

• Intercellular communication (cell-to-cell) is crucial for multicellular organisms.
• Scientists have found universal cellular regulation mechanisms in diverse organisms (microbes and mammals).
• This suggests a shared evolutionary ancestry.
• Examples show multiple molecules controlling complex functions like blood vessel dilation.

Learning Objectives

• Understand the mechanisms of cell-to-cell communication, differentiating between local and long-distance signaling.
• Describe the stages of a cell signaling process.
• Identify and explain the function of various intracellular and plasma membrane receptors.
• Outline the role of second messengers in signal transduction cascades.
• Distinguish intercellular junctions in animal and plant cells and their functions.

Local Signaling

• Direct contact: Cells communicate through cell junctions (animal and plant cells) or recognizing each other via surface molecules (animal cells).
• Local regulators: In paracrine signaling, messenger molecules travel short distances to target cells (e.g., growth factors). Synaptic signaling uses neurotransmitters released at synapses between nerve cells.

Cell Junctions

• Gap junctions (animal cells), plasmodesmata (plant cells): Channels that connect adjacent cell cytoplasms, coordinating tissue function.

Cell-Cell Recognition

• Animal cells often communicate through direct contact using surface molecules (e.g., membrane carbohydrates).

Communication by Local Regulators

• Paracrine and synaptic signaling enable animals to communicate using local regulators (e.g., growth factors, neurotransmitters). These travel only short distances.

Types of Intercellular Junctions in Animals

• Tight junctions: Prevent intercellular communication and often block material exchange in cells that need to act as a barrier.
• Desmosomes: Anchor cells through the extracellular matrix.
• Gap junctions: Channels between cells permitting molecule exchange.

Gap Junctions

• Cytoplasmic channels (connexins) connect adjacent cells.
• Essential for cell-to-cell communication.
• Allow the exchange of small molecules and ions (e.g., cAMP, Ca+2).
• Found in various tissues (e.g., epithelial cells, heart muscle).
• Essential for the coordinated contraction of certain tissues like intestines and uterus during childbirth.

Tight Junctions

• Located beneath the apical surface of epithelial cells.
• Prevent cell-to-cell communication in cases where material exchange is unwanted or needs to be controlled.
• Create a watertight seal preventing the leakage of extracellular fluid.
• Formed by claudin and occludin transmembrane proteins.
• Occludin's cytoplasmic part connects to actin microfilaments.

Anchoring Junctions

• Connect neighboring cells (cell-to-cell) and cells to the ECM (cell-to-ECM connections).
• Desmosomes: Function like rivets fastening cells together into strong sheets, anchoring cells through intermediate filaments (e.g., keratin).
• Adherens junctions: Create an adhesion zone around the epithelial cells. Connect plasma membranes via transmembrane adhesion proteins (cadherins). These connect intracellularly with actin microfilaments with the help of catenins, vinculin, and a-actinin.

Focal Adhesions (Cell-ECM connection)

• Connect cells to the extracellular matrix (ECM) through integrins.
• Integrin cytoplasmic domain connects with actin microfilaments through proteins like talin, α-actinin, and vinculin.

Hemidesmosomes (Cell-ECM connection)

• Found at the basal surface of epithelial cells.
• Stabilize cells to the extracellular matrix (ECM) through integrins binding to ECM proteins like laminin.
• Intracellularly connect with keratin intermediate filaments through plectin.

Long-Distance Signaling

• Both plants and animals use hormones for long-distance communication.
• Hormonal signaling (endocrine signaling) is how cells specialize to release hormones (chemical messengers) into body fluids. The circulatory system conveys hormones to target cells (often in other parts of the body) which act in response.

Stages of Cell Signaling

• 1. Reception: A signaling molecule (ligand) binds to a receptor protein.
• 2. Transduction: The receptor protein's change in shape initiates a cascade of molecular changes.
• 3. Response: The transduced signal triggers a specific cellular response (e.g., change in enzyme activity, gene expression).

Receptor Types

• Intracellular receptors: Located inside the cell. Small or hydrophobic signaling molecules (like steroid hormones) can easily pass through the cell membrane.
• Plasma membrane receptors: Located on the cell surface. Large or hydrophilic signaling molecules cannot directly enter the cell, so they interact with the receptor at the cell's outer surface. They are further divided into distinct types, including:
  • G-protein-coupled receptors
  • Tyrosine kinases
  • Ion channels

Second Messengers

• Small, non-protein, water-soluble molecules or ions (e.g., cAMP, Ca+2, DAG, IP3) that relay signals in transduction pathways.
• cAMP is produced from ATP through adenylyl cyclase.
• Calcium ions (Ca2+) act as second messengers, regulating diverse cellular responses.

Transduction Pathways

• Signal transduction pathways utilize cascades of molecular interactions transmitting signals from receptors to cellular targets.
• At each step, signals are changed into another form (often a conformational change in a protein molecule).

Signal Amplification

• At each step of a multistep pathway, the number of activated products increases dramatically, amplifying the original signal.

Signal Specificity/Pathways

• Different cell types have specialized combinations of proteins.
• The same signaling molecule can trigger different responses in different cells, reflecting the diversity of proteins and pathways unique to a particular cell.
• Signaling efficiency can be enhanced by groups of signaling proteins that cluster relay proteins in the pathway (scaffolding proteins).

Signal Termination

• Signals are promptly turned off to prevent overreactions.
• Signal termination may involve the reversal of ligand binding to receptors.

Clinical Correlations (receptor examples)

• Tyrosine Kinases: Some cancer types are linked to overactivation of tyrosine kinase receptors (e.g., amplification or mutations), which is a target for anti-cancer drugs.
• G-Protein-Coupled Receptors: Bacterial toxins interfere with G protein function.

Explore the mechanisms of intercellular communication essential for multicellular organisms. This quiz covers local and long-distance signaling, stages of cell signaling, and the role of receptors and second messengers. Gain insights into the shared evolutionary traits in cellular regulation across different organisms.