Cell Biology Study Guide: Cytoskeleton, Cellular Motility, Neuronal and Cell Signaling

Cytoskeleton

Concepts and Components of Cytoskeletal Elements

The cytoskeleton is a dynamic network of protein filaments that provides structural support, facilitates intracellular transport, and enables cellular movement.

• Microfilaments (Actin Filaments):

  • Regulation of Actin Polymerization and Bundling: Actin filaments are regulated by proteins that control their assembly, disassembly, and organization into bundles or networks.

  • Key Proteins:

    • Profilin: Promotes actin polymerization by facilitating the addition of actin monomers.

    • Formin: Nucleates actin filaments and promotes elongation.

    • Arp2/3 Complex: Initiates branching of actin filaments, creating a network structure.

    • Gelsolin: Severs actin filaments and caps the newly exposed ends.

    • CapZ: Caps the barbed ends of actin filaments, regulating their length.

  • Microvilli: Finger-like projections on epithelial cells, supported by actin bundles, increasing surface area for absorption.

  • Villi: Larger structures in the intestine, containing microvilli on their surface.

  • Filopodia and Lamellipodia: Cellular protrusions involved in cell movement and sensing the environment.

  • Actin Filament Association with Membranes: Actin filaments are linked to the plasma membrane via proteins such as spectrin and ankyrin, providing structural integrity.

• Intermediate Filaments:

  • Provide mechanical strength to cells and tissues.

  • Examples include keratins (epithelial cells), vimentin (mesenchymal cells), and neurofilaments (neurons).

Cellular Motility and Contractility

General Characteristics of Motor Proteins and Diversity of Microtubule-Based Movement

Motor proteins convert chemical energy from ATP hydrolysis into mechanical work, enabling movement along cytoskeletal filaments.

• Microtubules:

  • Motor Proteins:

    • Kinesins: Move toward the plus end of microtubules, involved in anterograde transport.

    • Dyneins: Move toward the minus end, involved in retrograde transport and ciliary/flagellar movement.

  • Types of Movement: Includes vesicle transport, organelle positioning, and chromosome segregation during mitosis.

  • Mechanism: Motor proteins use ATP hydrolysis to "walk" along microtubules.

• Cilia and Flagella:

  • Structures composed of microtubules and dynein arms, responsible for cell movement and fluid flow across cell surfaces.

• Muscle Contraction:

  • Actin and Myosin: Myosin heads bind to actin filaments, hydrolyze ATP, and generate force for contraction.

  • Thick and Thin Filaments: Thick filaments are composed of myosin; thin filaments are composed of actin.

  • Mechanism: Muscle contraction occurs via the sliding filament model, where myosin pulls actin filaments past itself.

  • ATP Role: ATP binding and hydrolysis are essential for myosin head movement and detachment from actin.

Neuronal Signaling

Types of Cells in the Nervous System

The nervous system consists of neurons and supporting glial cells, each with specialized functions.

• Neurons: Primary signaling cells, transmit electrical and chemical signals.

• Glial Cells:

  • Microglia: Immune cells of the CNS.

  • Oligodendrocytes and Schwann Cells: Form myelin sheaths around axons in the CNS and PNS, respectively.

  • Astrocytes: Support neurons, regulate the extracellular environment.

Anatomy of a Neuron

• Cell Body (Soma): Contains the nucleus and organelles.

• Dendrites: Receive incoming signals.

• Axon: Transmits electrical impulses away from the cell body.

• Myelin Sheath: Insulates axons, increasing signal speed.

• Synapse: Junction between neurons for signal transmission.

Membrane Potential

Membrane potential is the voltage difference across the cell membrane, essential for neuronal signaling.

• Resting Membrane Potential: Maintained by ion gradients and selective permeability.

• Key Ions: Na+, K+, Cl-, Ca2+

• Goldman Equation: Used to calculate membrane potential based on ion concentrations and permeability. $V_m = \frac{RT}{F} \ln \left( \frac{P_{K^+}[K^+]_{out} + P_{Na^+}[Na^+]_{out} + P_{Cl^-}[Cl^-]_{in}}{P_{K^+}[K^+]_{in} + P_{Na^+}[Na^+]_{in} + P_{Cl^-}[Cl^-]_{out}} \right)$

Action Potential

An action potential is a rapid, transient change in membrane potential that propagates along the axon.

• Phases: Depolarization, repolarization, and hyperpolarization.

• Key Channels: Voltage-gated Na+ and K+ channels.

• Propagation: Action potentials travel unidirectionally due to refractory periods.

Synaptic Transmission

Neurons communicate at synapses via chemical neurotransmitters or electrical signals.

• Chemical Synapses: Neurotransmitter release, receptor binding, and postsynaptic response.

• Electrical Synapses: Direct ion flow through gap junctions.

Cell Signaling

Overview of Cell Signaling

Cell signaling involves the transmission of information from the environment or other cells to elicit a cellular response.

• Key Terms: Receptors, ligands, signaling pathways, effectors.

• Types of Signaling:

  • Contact-dependent: Direct cell-to-cell contact.

  • Paracrine: Local signaling to nearby cells.

  • Synaptic: Neuronal signaling via synapses.

  • Endocrine: Hormonal signaling via the bloodstream.

• Signal Transduction: Conversion of extracellular signals into intracellular responses.

Dissociation Constants and Ligand Binding

• Dissociation Constant (Kd): Measures the affinity between a ligand and its receptor. $K_d = \frac{[R][L]}{[RL]}$

• Types of Ligands: Small molecules, peptides, proteins.

Signal Pathways and Feedback Loops

• Second Messengers: cAMP, IP3, DAG, Ca2+

• Feedback Loops: Mechanisms that regulate pathway activity, ensuring proper cellular responses.

Enzyme-Coupled Receptors and Kinases

• Tyrosine Kinases: Enzymes that phosphorylate tyrosine residues on target proteins, initiating signaling cascades.

• Serine/Threonine Kinases: Phosphorylate serine or threonine residues.

• Calcium/Calmodulin and Calcium-Calmodulin-Dependent Kinase: Calcium ions bind calmodulin, activating kinases involved in various cellular processes.

Examples of Signaling Pathways

• EGF and EGFR: Epidermal growth factor binds its receptor, triggering cell proliferation.

• JAK-STAT Pathway: Cytokine signaling pathway involved in immune responses.

Additional info:

• Some details inferred from standard cell biology curriculum, such as the sliding filament model and the role of ATP in muscle contraction.

• Specific equations and protein functions expanded for clarity and completeness.