Cytoskeleton: Filaments and Functions

The Cytoskeleton: A Cell's Internal Framework

The cytoskeleton is a dynamic network of filamentous proteins extending throughout the cytoplasm, providing essential mechanical support, maintaining cell shape, and enabling various cellular movements.

I. Cytoplasm Overview

• Definition: The region between the cell membrane and the nucleus membrane.

• Components: Jelly-like fluid (cytosol/hyaloplasm), cell organelles, cytoskeleton, and nutrients.

• Composition of Cytosol:

  • Water: ~70%

  • Small molecules (low MW): Ions, gases, glucose, amino acids, fatty acids, urea, glycerol.

  • Large molecules/Macromolecules (high MW):

    • Proteins: Granular & filamentous (Cytoskeleton)

    • Lipids

    • Carbohydrates

    • Nucleic acids (RNA)

  • Inert inclusions (cell reservation forms):

    • Proteins: Cylinder-shaped (e.g., proteasome)

    • Lipids: Droplets (lipid droplets)

    • Carbohydrates: Little roses or clouds

II. Cytoskeletal Components

The cytoskeleton consists of three main types of protein filaments, differing in diameter and function:

III. General Functions of the Cytoskeleton

• Mechanical support and maintenance of cell shape (3D).

• Ensuring cell motility.

• Chromosome migration during mitosis.

• Organelle movement and positioning.

• Facilitating biochemical reactions.

IV. Actin Microfilaments (AF)

• Structure: Double-helical polymers of actin.

  • Monomer: G-Actin (globular protein with 1 ATP groove).

  • Polymer: F-Actin (filamentous actin) formed by G-actin binding, creating 2 twisted chains. This polymerization is called Actin nucleation or assembly and involves the ARP2/3 complex.

• Polarity: F-Actin has a minus end ("pointed end") and a plus end ("barbed end").

• Dynamics:

  • Polymerization: Favored when G-Actin/ATP concentration increases.

  • Depolymerization: Favored when G-Actin/ATP concentration decreases.

• Key Functions:

  • Maintain 3D shape: Via the cellular cortex (layer under plasma membrane).

  • Cytoplasmic currents: Guide movement of molecules & organelles.

  • Supporting cell structures: E.g., microvilli in intestinal and kidney cells.

  • Vesicular exchanges: Endocytosis/exocytosis traffic.

  • Cell division: Form the contractile ring during cytokinesis.

  • Diapedesis: Extravasation of white blood cells.

  • Cell migration: Achieved by polymerization and depolymerization (e.g., macrophages).

• Associated Proteins:

  • ARP2/3: Essential for actin assembly (nucleation).

  • CapZ: Stops AMF polymerization.

  • Tropomyosin: Protects AMF from depolymerization.

  • Troponin: Protects myosin binding sites.

  • Severin: Cuts AMF to create new extremities.

  • Angulation proteins: Fimbrin, filamin, -Actinin (involved in actin bundling/cross-linking).

  • Myosin I: Links cellular cortex to membrane (e.g., to spectrin in red cells, dystrophin in muscle cells).

  • Myosin II: Motor protein, responsible for muscle contraction.

  • Myosin IV: Motor protein for molecule/organelle transport.

  Myosins convert chemical energy (ATP hydrolysis) into mechanical energy to "walk" on actin filaments.

• Small GTPases (Rho family): Regulatory proteins that bind GTP (active) or GDP (inactive), bringing energy to cytosolic mechanisms.

  • Ras: Signaling pathways.

  • Rac: Cell migration.

  • Rho: Cell tension.

  • Cdc42: Migration, polarity, cell cycle.

  • Rab: Vesicular traffic.

  • Ran: Nucleus molecule transport.

V. Microtubules (MT)

• Structure: Hollow cylinders made of tubulin protein.

  • Monomer: α-tubulin and β-tubulin.

  • Tubulin dimers: α and β tubulins combine (using GTP).

  • Protofilament: Formed by combination of 3-4 dimers (using GTP).

  • MT Formation: 13 protofilaments bind to γ-Tubulin Ring Complex (γ-TuRC) for nucleation.

• Key Functions:

  • Organize positions of organelles.

  • Direct intracellular transport.

  • Maintain 3D shape.

  • Cytoplasmic currents: Guidance and movement of molecules & organelles.

  • Supporting cell structures: Cilia, Flagella, Axons, Centrioles.

  • Vesicular exchanges: Endocytosis/exocytosis traffic.

  • Cell division: Form the mitotic spindle.

  • Diapedesis: Extravasation of white blood cells.

  • Cell migration: (also supported by MTs).

• Dynamics: Dependent on the equilibrium between free and linked tubulin.

  • Polymerization: Increases if free tubulins in GTP form increase.

  • Depolymerization: Increases if free tubulins in GTP form decrease.

  • Disaster phenomenon: MT becomes unstable and collapses if the balance between poly/depolymerization is lost (e.g., decrease in free GTP tubulins, disappearance of MAP2, tension).

• Associated Proteins & Modulators:

  • γ-TuRC: MT nucleation.

  • MAP2: MT stability.

  • Kinetochore: Attaches chromosomes to mitotic spindle (a chromosome's motor protein).

  • Motor Proteins: Transform chemical energy (ATP or GTP) into mechanical energy.

    • Kinesin

    • Dynein

    Both have globular heads on MTs and a cytosolic part with a cargo seat.

  • Antimitotics:

    • Colchicine: Binds to free tubulin, prevents polymerization.

    • Other alkaloids (vinblastin, vincristin, podophyllin): Similar antimitotic effects.

    • Taxol: Prevents MT depolymerization of the mitotic spindle, stopping cell division (used in chemotherapy for breast cancer).

• Case Study: Cilia & Flagella:

  • MTs control their beating.

  • Defects in cilia MTs (e.g., in trachea) can cause mucoviscidosis.

  • Defects in flagella MTs (e.g., in spermatozoa) can induce masculine sterility.

VI. Intermediate Filaments (IF)

• Structure: Made of filamentous monomers with two heads.

  • Monomers dimerize, then tetramerize.

  • Tetramers alternate in space to form a rope-like structure.

• Key Characteristics:

  • Stable: Do not readily polymerize/depolymerize.

  • Act as the "true cytoskeleton" to maintain cell shape.

• Key Functions:

  • Cell Morphogenesis: Provide cell scaffolding (e.g., orient actin & myosin in muscle cells).

  • Cell Resistance: Attached to Cell Adhesion Molecules (CAM) and Substrate Adhesion Molecules (SAM), giving cells resistance to tension, attraction, and strengthening cell adhesion stability.

  • Form nuclear lamina: In the nucleus, 3 lamins (A, B, C) form a layer against the inner nucleus, reinforcing the nuclear envelope.

• Diversity & Clinical Relevance: There are many types, specific to different cell types.

  • Cytokeratin: In epithelial cells (20 types); specific for carcinoma tumors.

  • Desmin: In muscle cells; associated with myoblastoma.

  • Vimentin: In connective tissue; associated with sarcoma.

  • Neurofilaments: In neurons; associated with neuroblastoma.

  • GFAP (Glial Fibrillary Acidic Protein): In glial nerve cells; associated with glioma.

  IFs are used to determine cancer type by Immunohistochemistry (IHC).