Cell & Molecular Biology - Cell Interactions Lecture

Learning Objectives

  • Understand how cells attach to each other.
  • Identify differences between prokaryotic and eukaryotic extracellular structures.
  • Define extracellular matrix and its components.
  • Describe the different types of junctions holding cells together.
  • Explain how a signal gets into a cell and how the cell responds.
  • Understand the major cell signaling pathways.

Cell–Cell Interactions

  • Cells in multicellular organisms must communicate and cooperate with each other:
    • They form an interdependent community of cells.

The Cell Surface

  • The plasma membrane is chiefly composed of a phospholipid bilayer that is studded with proteins.
  • Membrane proteins play vital roles in regulating transport and signaling:
    • They attach to cytoskeletal elements on the interior surface.
    • They connect to a complex array of extracellular structures.
  • The extracellular matrix (ECM) and cell walls provide not only structural support but also signaling capabilities.

Extracellular Structures

  • Organism Types and Their Structures:
    • Bacteria:
    • Cell Wall: Yes
    • Scaffolding Material: Peptidoglycan
    • Cushioning Material: Sugar base
    • Plants:
    • Cell Wall: Yes
    • Scaffolding Material: Cellulose
    • Cushioning Material: Pectin
    • Animals:
    • Cell Wall: No
    • Scaffolding Material: Collagen
    • Cushioning Material: Polysaccharides
  • Cell wall and ECM components are secreted by the respective cells:
    • Carbohydrates provide strong, rigid support within the plant cell wall.
    • Proteins contribute to strong, rigid support within the animal cell ECM.
    • Carbohydrates in the ECM also absorb water, which provides additional cushioning.

Bacterial Support Structures

  • Bacterial cell walls consist primarily of polysaccharide peptidoglycan polymers that are interconnected by peptide bonds.
  • These structures are critical for bacteria's structural integrity and are a target for antibiotic treatments.

The Primary Cell Wall in Plants

  • The primary cell wall serves several functions:
    • It defines the shape of the plant cell.
    • It counters turgor pressure that the cell experiences due to water influx through osmosis.

The Composition of the Primary Cell Wall in Plants

  • Newly formed plant cells secrete the primary cell wall, which consists of:
    • Long strands of polysaccharide cellulose, which aggregate into cable-like microfibrils.
    • A crisscrossed network composed of microfibrils, interspersed with gelatinous polysaccharides like pectin that keep the wall moist.
  • Notably, cellulose is recognized as the most abundant organic compound on Earth.

The Extracellular Matrix in Animals

  • Most animal cells produce an organized fiber composite known as the extracellular matrix (ECM), which:
    • Provides essential structural support.
    • Connects cells within tissues.

Composition of the Extracellular Matrix in Animals

  • Fibrous Components:
    • The predominant fibrous component is collagen:
    • Collagen molecules form groups of triple helices that coalesce into collagen fibrils.
  • Ground Substance:
    • Composed of proteoglycans, which are proteins attached to many polysaccharides, responsible for the rubber-like consistency of cartilage.
  • Notably, collagen constitutes about 30% of the total protein in the human body.

Tissue Variation in ECM

  • The amount of ECM and its composition varies depending on the tissue type:
    • Example: In lung tissue, elastin protein allows for stretchability.
    • Bone is predominantly composed of ECM, while skin contains minimal ECM.

Integrins Connect Cells to the ECM

  • Integrins are membrane proteins that bind to cross-linking proteins in the ECM, such as laminins and fibronectin:
    • They facilitate the attachment of ECM to the plasma membrane.
    • Integrins also anchor the cytoskeleton to the ECM, signaling the cell that it is anchored.

Cell Communication and Attachment

  • Direct physical connections among cells are fundamental for multicellularity:
    • They sustain the structure and functioning of tissues.
  • Significance of cell–cell attachments includes the materials and structures that bind cells together.

Types of Cell Junctions

  • Tight Junctions:
    • Comprise membrane proteins in adjacent animal cells that align and bind to one another, forming a waterproof seal between the two cells.
  • Desmosomes:
    • Strong cell–cell attachments that resist tearing, consisting of linking proteins called cadherins and cytosolic anchoring proteins.
    • Cytoskeletal intermediate filaments reinforce desmosomes.
  • Selective Cadherins:
    • Cadherins, the linking proteins in desmosomes, bind specifically to cadherins of the same type, facilitating tissue formation through specific attachments.
  • Gap Junctions:
    • In animal tissues, these junctions consist of protein channels that directly connect adjacent cells.
    • They permit the flow of small molecules between cells, serving as communication portals to coordinate cellular activities and allow for the rapid passage of regulatory ions and small molecules.
  • Plasmodesmata:
    • As found in plant cells, these gaps in their cell walls allow for connections by linking plasma membranes, cytoplasm, and smooth endoplasmic reticulum (ER) of adjacent cells.

Cell–Cell Signaling Mechanisms

  • Distant cells communicate through signaling molecules:
    • Neurotransmitters can open or close channels in nearby cells.
    • Hormones act as information-carrying molecules, secreted from one cell, circulating throughout the body, and affecting target cells far from the signaling cell.

Steps of Cell-Cell Signaling

  1. Signal Reception:

    • Signal molecules (ligands) bind to receptor molecules, triggering their activity after conformational changes.
    • Only cells with appropriate receptors will respond to specific signaling molecules.
    • Types of receptors include:
      • Steroid Hormone Receptors
      • G-Protein Coupled Receptors
      • Enzyme Linked Receptors

  2. Signal Transduction:

    • This process involves the conversion of an extracellular signal into an intracellular signal, often through a conformational change in the receptor.
    • Amplification occurs, leading to a larger cellular response, mediated by second messengers or phosphorylation cascades.

  3. Cellular Response:

    • Changes may include alterations in gene expression and modifications in the activity of existing proteins, such as increased glucose production or decreased glycogen levels.

Insulin Signaling Example

  • High blood glucose levels trigger insulin hormone secretion from the pancreas:
    • Insulin travels throughout the body, binding to receptors on target cells, stimulating the uptake of glucose into these cells.
    • The overall effect is a reduction in blood glucose levels, hence supplying energy to the cells.

Steroid Hormone Receptors

  • Lipid-soluble signaling molecules can diffuse across the plasma membrane:
    1. Receptors for steroids are located in the target cells’ cytoplasm.
    2. Upon ligand binding, a conformational change occurs in the receptor.
    3. The receptor then initiates a signaling response by altering gene expression.

Signal Transduction Mechanism for Membrane Bound Receptors

  • Lipid-insoluble signaling molecules do not cross the plasma membrane:
    1. They require recognition by cell surface receptors.
    2. Signal transduction and amplification occur through second messengers or phosphorylation cascades.
    3. The resulting transcription factors influence gene expression or alter protein activity.

G-Protein Coupled Receptors (GPCR)

  • G proteins are peripheral membrane proteins that are regulated by guanine nucleotides:
    • Binding of guanosine triphosphate (GTP) activates the G protein by altering its shape.
    • Removal of the phosphate group to form guanosine diphosphate (GDP) deactivates the G protein.
    • The cycle of activation and deactivation manages cellular signaling efficiency.

Mechanism of G-Protein Coupled Receptors

  1. The transmembrane receptor is activated by binding to the signaling ligand, causing a conformational change.
  2. An associated G protein exchanges GDP for GTP, activating the G protein subunit, which in turn activates an enzyme nearby, leading to the production of a second messenger.
  3. Ultimately, this leads to a cellular response.

Enzyme-Linked Receptors

  • Enzyme-linked receptors function through the following steps:
    1. Activation occurs via conformational change in the transmembrane receptor upon ligand binding.
    2. The receptor activates a kinase by facilitating the exchange of GDP for GTP.
    3. This kinase initiates a phosphorylation cascade, ultimately resulting in a cellular response.

Signal Deactivation

  • Cells possess mechanisms to turn off intracellular signals:
    • Phosphatases remove phosphate groups from proteins within phosphorylation cascades, allowing for signal termination and ensuring cellular sensitivity to slight signaling changes.

Summary of Steps in Cell-Cell Signaling

  • 1. Signal reception by ligand binding to the receptor.
  • 2. Signal transduction with conformational changes, amplification via second messengers, or phosphorylation cascades.
  • 3. Cellular response includes gene expression changes or alterations in protein activity.

Review of Cell-Cell Signaling Components

  • Overview of signaling pathways:
    • Nucleus: involved in gene regulation.
    • Plasma Membrane: site of receptor action.
    • Types of Ligands: lipid-soluble and lipid-insoluble ligands.
    • Examples include steroid hormone receptors, G-protein coupled receptors, and enzyme-linked receptors.
    • Change in gene expression might include regulation of processes such as cell division, insulin production, and metabolic rate adjustments.

Insulin Signaling Differences in Diabetic Individuals

  • Healthy individual vs. diabetic individuals (Type 1 and Type 2):
    • Healthy: pancreas secretes insulin to manage blood glucose levels, effectively removing glucose from the bloodstream into body cells for energy.
    • Type 1 Diabetes: pancreas produces little or no insulin due to cell destruction; glucose remains in the bloodstream, leading to high blood glucose levels.
    • Type 2 Diabetes: pancreas produces insulin, but the body's cells are resistant, preventing effective glucose uptake, causing glucose accumulation in blood and damaging nerves and blood vessels.

Regulating Signal Reception

  • Signal receptors are dynamic and can exhibit changes in:
    • The number of receptors present in a cell.
    • The capability of receptors to bind signaling molecules.
    • The overall sensitivity of a cell to specific signaling molecules.
  • Receptor blocking strategies include the use of beta-blocker drugs, which inhibit the interaction between hormones and their respective receptors.