7.2 Intermediate filaments and cellular structure

Intermediate filaments are crucial components of the cytoskeleton, providing mechanical stability and resistance to stress. They come in six main types, each with unique functions in different tissues, from maintaining cell shape to regulating gene expression.

These rope-like structures form a dynamic network extending from the nucleus to the cell membrane. They're essential for cell adhesion, forming desmosomes and hemidesmosomes that anchor cells to each other and the extracellular matrix, ensuring tissue integrity.

Intermediate Filaments and Cellular Structure

Structure of intermediate filaments

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• Intermediate filaments (IFs) are a major component of the cytoskeleton with a diameter of 10-12 nm, falling between the sizes of actin filaments (6 nm) and microtubules (25 nm)
• Composed of fibrous proteins that form rope-like structures through a hierarchical assembly process
  • Monomers assemble into dimers, which then form tetramers
  • Tetramers associate in a staggered fashion to form protofilaments
  • Protofilaments twist around each other to form the final IF structure
• Provide mechanical stability and resistance to mechanical stress by maintaining cell shape and organizing internal structures, contributing to the overall integrity of the cytoskeleton

Types of intermediate filaments

• Six main types of IF proteins, each with unique tissue distribution and functions
  • Type I and II: Acidic and basic keratins, respectively, found in epithelial cells (skin, hair) and provide mechanical support and maintain cell junctions
  • Type III: Vimentin, desmin, and glial fibrillary acidic protein (GFAP)
    • Vimentin expressed in mesenchymal cells (fibroblasts, endothelial cells) and involved in cell migration and wound healing
    • Desmin found in muscle cells and maintains structural integrity and force transmission
    • GFAP expressed in glial cells (astrocytes, Schwann cells) in the nervous system and maintains cell shape and provides support for neurons
  • Type IV: Neurofilaments (NF-L, NF-M, NF-H) and α-internexin found in neurons and provide structural support and regulate axon diameter
  • Type V: Nuclear lamins (A, B, and C) form the nuclear lamina, provide structural support for the nuclear envelope, and regulate gene expression
  • Type VI: Nestin expressed in stem cells and progenitor cells and involved in cell division and differentiation

Cellular integrity from filaments

• IFs form a dynamic network that extends from the nuclear envelope to the plasma membrane, interacting with other cytoskeletal components (actin filaments, microtubules) to maintain cell shape and resist mechanical stress
• Provide tensile strength and elasticity to cells, allowing them to stretch and recover without breaking when subjected to mechanical deformation
• Participate in the formation and maintenance of desmosomes and hemidesmosomes, anchoring cells to neighboring cells and the extracellular matrix, respectively
• Mutations in IF proteins can lead to various diseases characterized by reduced cell integrity and mechanical stability (epidermolysis bullosa simplex from keratin mutations, dilated cardiomyopathy from desmin mutations)

Filaments in cell adhesions

• IFs are essential components of desmosomes and hemidesmosomes
  • Desmosomes are specialized cell-cell adhesion structures that link the IFs of adjacent cells, providing mechanical strength and maintaining tissue integrity, particularly in tissues subject to mechanical stress (skin, heart)
  • Hemidesmosomes are specialized cell-matrix adhesion structures that link IFs to the extracellular matrix, anchoring cells to the basement membrane and providing stability and resistance to mechanical forces
• IFs interact with desmosomal and hemidesmosomal proteins to form stable adhesion complexes
  • Desmosomal proteins (desmoplakin, plakoglobin) link IFs to the desmosomal plaque
  • Hemidesmosomal proteins (plectin, BP230) connect IFs to the hemidesmosomal plaque and integrin receptors
• Disruption of IF-mediated adhesions can lead to tissue fragility and blistering disorders (pemphigus from desmosomal protein autoantibodies, epidermolysis bullosa from mutations in hemidesmosomal proteins)