Chapter 19: Cell Junctions and the Extracellular Matrix

Cell-Cell junctions

• Cell-cell junctions are specialized structures that mediate interactions and connections between adjacent cells.
• They play a crucial role in maintaining tissue integrity, providing mechanical strength, and facilitating communication between cells.

Tight Junctions:

• Tight junctions (also known as occluding junctions) form a seal between adjacent cells, preventing the passage of molecules and ions between the cells.
• They are composed of transmembrane proteins, including claudins and occludins, which interact with proteins in neighboring cells.
• Tight junctions are primarily found in epithelial and endothelial tissues and contribute to the establishment of tissue barriers.

Adherens Junctions:

• Adherens junctions are involved in cell-cell adhesion and provide mechanical strength to tissues.
• They are characterized by the presence of cadherin proteins, which mediate calcium-dependent homophilic interactions between adjacent cells.
• The intracellular region of cadherins interacts with cytoplasmic proteins, such as β-catenin and α-catenin, which link the junctions to the actin cytoskeleton.
• Adherens junctions are critical for maintaining tissue integrity, cell shape, and cell migration during development and wound healing.

Desmosomes:

• Desmosomes are strong intercellular junctions that provide mechanical stability and resistance to shearing forces.
• They are composed of desmogleins and desmocollins, which are transmembrane proteins that interact with cadherin-like proteins in adjacent cells.
• Inside the cell, desmosomal cadherins are linked to intermediate filaments, such as keratin filaments, which extend throughout the cytoplasm.
• Desmosomes are particularly abundant in tissues subjected to mechanical stress, such as the skin, heart, and uterus.

Gap Junctions:

• Gap junctions are specialized channels that allow direct communication and exchange of small molecules between adjacent cells.
• They are composed of connexin proteins, which form hemichannels called connexons in the plasma membrane.
• Connexons from two neighboring cells align and dock together, creating a pore that facilitates the passage of ions, small metabolites, and signaling molecules.
• Gap junctions are involved in coordinating cellular activities, electrical signaling, and metabolic coupling in tissues like cardiac and neural tissues.

Hemidesmosomes:

• Hemidesmosomes anchor epithelial cells to the underlying extracellular matrix, providing stability and mechanical support.

• They are composed of transmembrane proteins, such as integrins, which interact with extracellular matrix components like laminin and collagen.

• Inside the cell, hemidesmosomal proteins are linked to intermediate filaments, specifically keratin filaments.

• Hemidesmosomes are particularly important in tissues subjected to mechanical stress, such as the skin and epithelial linings of organs.

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The Extracellular Matrix of Animals

• The extracellular matrix (ECM) is a complex and dynamic network of macromolecules found in the extracellular space of animal tissues.
• The ECM provides structural support, mechanical strength, and biochemical cues to cells within tissues.
• Major components of the ECM include fibrous proteins, glycosaminoglycans (GAGs), proteoglycans, and glycoproteins.
• Collagen is the most abundant fibrous protein in the ECM, forming strong and flexible fibrils that contribute to tissue integrity and strength.
• Other fibrous proteins in the ECM include elastin, which provides elasticity to tissues, and fibronectin, which plays a role in cell adhesion and signaling.
• GAGs are long, unbranched polysaccharide chains that interact with proteins to form proteoglycans. They provide hydration, support, and resilience to the ECM.
• Proteoglycans consist of a core protein to which GAG chains are attached. They help maintain the ECM structure, regulate cell behavior, and bind growth factors.
• Glycoproteins, such as laminin and fibronectin, are involved in cell adhesion, migration, and signaling within the ECM.
• The ECM is organized into specialized structures, such as basement membranes and fibrillar networks, which vary depending on the tissue type and function.
• Basement membranes are thin, sheet-like structures that underlie epithelial and endothelial tissues, providing a scaffold for cell attachment and separating tissue compartments.
• Fibrillar networks, composed of collagen and other fibrous proteins, provide structural support and organization in connective tissues like tendons, cartilage, and bone.
• Cells interact with the ECM through cell surface receptors, including integrins, which mediate adhesion and transmit signals bidirectionally between the ECM and the cell.
• Integrins and other ECM receptors can activate signaling pathways that regulate cell proliferation, migration, differentiation, and survival.
• Remodeling of the ECM is tightly regulated by various enzymes, including matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs), to maintain tissue homeostasis and repair.

Cell-Matrix Junctions

• Cell-matrix junctions are specialized structures that mediate the interaction between cells and the extracellular matrix (ECM).
• These junctions play crucial roles in cell adhesion, signaling, migration, and tissue development.
• The main types of cell-matrix junctions include focal adhesions, hemidesmosomes, and podosomes.

Focal Adhesions:

• Focal adhesions are large, dynamic structures that connect the ECM to the actin cytoskeleton within cells.
• They are formed by the clustering of integrin transmembrane receptors at specific sites on the plasma membrane.
• Integrins, such as α and β subunits, bind to ECM components, such as fibronectin or collagen, through their extracellular domains.
• The intracellular domains of integrins interact with a variety of adaptor proteins, including talin, vinculin, and focal adhesion kinase (FAK).
• Adaptor proteins link integrins to the actin cytoskeleton through interactions with actin-binding proteins like actinin, paxillin, and α-actinin.
• Focal adhesions transmit mechanical forces, regulate cell adhesion strength, and activate intracellular signaling pathways, including those involved in cell proliferation, migration, and survival.
• Signaling molecules, such as kinases, GTPases, and focal adhesion-associated proteins, are recruited to focal adhesions, modulating cellular processes and responses to the ECM.

Hemidesmosomes:

• Hemidesmosomes are specialized junctions that anchor epithelial cells to the underlying basement membrane.
• They are primarily found in tissues subjected to mechanical stress, such as the skin and mucosal linings.
• Hemidesmosomes consist of transmembrane integrin α6β4 receptors, which interact with laminins and collagen within the basement membrane.
• Intracellularly, integrin α6β4 associates with plectin and BPAG1, which connect to intermediate filaments, such as keratins.
• The anchorage provided by hemidesmosomes helps maintain tissue integrity, resist mechanical forces, and regulate epithelial cell polarity.

Podosomes:

• Podosomes are dynamic, actin-rich structures found in specialized cells like osteoclasts and macrophages.
• They function as sites of cell adhesion, extracellular matrix degradation, and cell migration.
• Podosomes are characterized by a core of actin filaments surrounded by a ring of adhesion molecules, such as integrins and vinculin.
• Proteolytic enzymes, like matrix metalloproteinases, localize to podosomes and facilitate ECM remodeling.
• Podosomes play roles in tissue remodeling, immune response, and cell invasion processes.

Regulation and Signaling:

• Cell-matrix junctions are regulated by numerous signaling pathways, including those involving integrin activation, cytoskeletal rearrangement, and kinase signaling.
• Various intracellular signaling molecules, such as focal adhesion kinase (FAK), Src family kinases, and small GTPases (e.g., Rho, Rac, and Cdc42), are involved in the regulation of cell-matrix junctions.
• Signaling through cell-matrix junctions can influence cell behavior, including adhesion strength, migration speed, proliferation, differentiation, and survival.
• Dysregulation of cell-matrix junctions and associated signaling pathways can contribute to disease conditions, including cancer metastasis, tissue fibrosis, and autoimmune disorders.

The Plant Cell Wall

• The plant cell wall is a complex and rigid structure that surrounds the plasma membrane of plant cells.

• It provides structural support, mechanical strength, and protection to plant cells and tissues.

• The plant cell wall is primarily composed of cellulose, hemicelluloses, pectins, and lignin.

  • Cellulose, a polysaccharide made up of glucose units, forms long, linear chains that are bundled together to create strong microfibrils.
  • Hemicelluloses are a diverse group of polysaccharides that interact with cellulose, providing cross-linkages and contributing to the flexibility and stability of the cell wall.
  • Pectins are complex polysaccharides that form a gel-like matrix between cellulose microfibrils, imparting adhesive properties and regulating cell-cell communication.
  • Lignin is a complex phenolic polymer that fills the spaces between cellulose microfibrils, providing rigidity and hydrophobicity to the cell wall.

  

• In addition to these major components, the plant cell wall may also contain proteins, enzymes, lipids, and other secondary metabolites.

• The cell wall is organized into primary and secondary cell walls, each with distinct properties and functions.

• The primary cell wall is flexible and extensible, allowing cell growth and expansion during development.

• As cells mature, some plants develop a secondary cell wall inside the primary cell wall, which provides additional strength and protection.

• The secondary cell wall is often thicker and contains a higher proportion of lignin, making it more rigid and impermeable.

• The plant cell wall interacts with the cytoskeleton and plasma membrane, providing structural support and facilitating cell-cell adhesion.

• The cell wall also acts as a barrier against pathogens, pests, and environmental stresses.

• Plant cell walls have specialized regions called plasmodesmata, which are channels that connect adjacent cells, allowing the exchange of molecules and signaling between cells.

• The composition and structure of the cell wall can vary across different plant tissues and cell types, allowing for functional diversity.

• The synthesis, modification, and degradation of the cell wall are tightly regulated by various enzymes, transporters, and signaling pathways.

• Plant cell walls are dynamic structures that can remodel in response to developmental and environmental cues.

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