Cytoskeleton and Muscle Contraction

Cytoskeletal Systems

• The cytoskeleton is a network of interconnected filaments and tubules in the cytosol.

• It provides structure to the cell, plays pivotal roles in cell movement and division, and is dynamic, allowing for rapid changes through polymerization and depolymerization.

• Major structural elements:

  • Microtubules

  • Microfilaments

  • Intermediate filaments

Goal of This Chapter

• Understand major structural elements of the cytoskeleton.

• Focus areas:

  • Microfilaments and Microtubules:

  • Size, subunit, polarity

  • Motor proteins involved

  • Structures they support

  • Activities and functions, especially muscle contraction and the cross-bridge cycle

  • Similarities and differences between microfilaments and microtubules

  • Intermediate filaments: size, subunit, polarity, and presence of motor proteins.

  • Techniques applied to study cytoskeletal dynamics.

  • HIV trafficking’s reliance on the cytoskeleton.

Major Structural Elements of the Cytoskeleton

Microtubules

• Microtubules are composed of tubulin subunits (alpha and beta dimers).

• Characteristics:

  • Diameter: 25 nm

  • Polarity: (+) end has beta-tubulin and is favored for polymerization, while (-) end has alpha-tubulin.

• Functions include:

  • Maintaining axon structure

  • Forming mitotic and meiotic spindles

  • Supporting cilia and flagella movement

  • Modulating cell shape

  • Facilitating vesicle movement

Microfilaments

• Microfilaments (comprised of actin):

• Polymer structure:

  • Composed of two intertwined chains of F-actin.

  • Diameter: 7-9 nm

  • Monomers: G-actin

  • Polarity: (+) end and (-) end allowing directional growth.

• Functions:

  • Muscle contraction

  • Cell motility (locomotion)

  • Cytoplasmic streaming

  • Cytokinesis

  • Shaping and support of cells

  • Transport within cells.

Actin Polymerization Dynamics

• Actin polymerizes through ATP-actin assembly.

• The cycle:

  1. ATP-actin binds and is added to a filament.

  2. ATP hydrolysis converts it to ADP-actin.

  3. Phosphate (Pi) is released, leading to dissociation of ADP-actin.

• Treadmilling phenomenon where:

  • (+) end grows faster, while (-) end shrinks.

Myosin: Motor Protein of Actin

• Myosin is a type of motor protein that interacts with actin filaments.

• Mechanism:

  • Powered by ATP hydrolysis, turning chemical energy into mechanical work.

• Types of Myosin and Functions:

  • Myosin II: Responsible for muscle contraction.

  • Myosin V: Transports cargo along actin fibers.

• Myosin moves towards the (+) end of actin.

Muscle Organization

• Muscles consist of elongated cells called muscle fibers surrounded by connective tissue.

• Structure:

  • Muscle fibers contain myofibrils organized into sarcomeres (contractile units).

• Sarcomere composition:

  • Thin filaments (actin)

  • Thick filaments (myosin)

  • Z discs help anchor these filaments

• Muscle contraction involves increasing overlap between thin and thick filaments.

Sliding Filament Model

• In muscle fibers, the contraction results in overlapping thin (actin) and thick (myosin) filaments.

• Contraction Mechanics:

  • Myosin head undergoes conformational changes powered by ATP to pull actin filaments toward the center of the sarcomere.

Cross Bridge Cycle

• The cross-bridge cycle details how myosin heads pull on actin during muscle contraction:

  1. Myosin binds to actin (tight binding in the absence of ATP).

  2. ATP hydrolysis leads to conformational changes in the myosin head.

  3. Myosin head attaches to actin and performs a power stroke (pulling the actin).

  4. ADP is released; ATP binds leading to myosin detaching from actin.

Muscle Contraction Trigger Mechanism

• Skeletal muscle contraction requires:

  • Nerve stimulation

  • Release and binding of neurotransmitter acetylcholine (ACh) to muscle receptors.

  • Propagation of electrical signals leading to an increase in intracellular Ca2+ levels, triggering contraction.

• Excitation-contraction coupling connects these electrical signals to muscle contraction.

Sarcoplasmic Reticulum and T Tubules

• The sarcoplasmic reticulum (SR) retains Ca2+ necessary for muscle contraction.

• T tubules allow rapid transmission of the action potential, ensuring synchronous contraction by linking signals between the SR and the muscle fiber.

Control of Muscle Contraction

• Control involves calcium-mediated structural changes:

  • Calcium binds to troponin, altering the conformation of tropomyosin and permitting myosin binding to actin.

• The sliding-filament model illustrates contraction mechanisms impacting muscle structure and function.

Intermediate Filaments

• Intermediate filaments (IF) are structurally diverse and vary in protein composition.

• Structure:

  • Diameter: 8-12 nm, providing tensile strength.

• Types include keratins (found in epithelial tissues) and nuclear lamins (giving structure to the nucleus).

Techniques to Study Cytoskeleton

• Immunofluorescence Microscopy: Utilizing antibodies specific to proteins to visualize their distribution in cells.

• Video Microscopy: Allows dynamic observation of cytoskeletal processes in living cells.

• Genetically Engineered Cells: Creates altered gene products to study their function in cellular contexts.

HIV Trafficking and Cytoskeleton

• Understanding the reliance of HIV on cytoskeletal elements for its replication and dissemination.

• Different microtubule-associated proteins (MAPs) play key roles in cellular transport processes including the movement of HIV components.

• Interplay of dynein and kinesin motor proteins for cargo transport within the cell.