BIOL310 Lecture Study Guide

Cell Signaling and Morphogens

  • MAPK Signal Transduction:

    • Cells separate multiple MAPK pathways to ensure specificity in signaling responses.

    • Necessary to avoid crosstalk that may lead to improper responses or cellular miscommunication.

  • Morphogens:

    • Signaling molecules that stimulate cellular responses at different concentrations.

    • Examples:

    • Sonic hedgehog (Shh)

    • Wingless (Wnt)

  • Hedgehog & Smoothened vs. Patched Mutations in Flies:

    • Loss-of-function mutations in hedgehog (Hh) and smoothened (Smo) yield the same phenotype (absence of specific structures) due to disrupted signaling.

    • Patched (Ptch) mutations result in the opposite phenotype because Ptch normally inhibits Hh signaling; its mutation leads to uncontrolled Hh activity.

  • Delta Signaling Mechanism:

    • Delta’s unique localization to neighboring cells ensures only adjacent cells receive the signal, preventing widespread signaling which can disrupt tissue patterning.

  • γ-secretase Reaction:

    • Catalyzes the intramembrane cleavage of specific transmembrane proteins (e.g., APP).

    • Proposed as a drug target for Alzheimer’s due to its role in generating amyloid-beta from APP.

    • Possible side effects: Disruption of normal cleavage processes may lead to unintended cellular signaling issues, thereby presenting risks like altered neuronal function or developmental defects.

Intermediate Filaments

  • Functions of Intermediate Filaments:

    • Provide structural support to cells, contribute to cellular integrity, help in cell signaling, and maintain cell shape.

  • Non-transport Role:

    • Unlike microtubules or actin, intermediate filaments do not serve as tracks for vesicular transport due to their structural rigidity and more stable nature.

  • Monoclonal Antibodies for Tumor Identification:

    • For muscle cell origin sarcoma: target vimentin.

    • For epithelial cell carcinoma: target cytokeratin.

    • For astrocytoma: target glial fibrillary acidic protein (GFAP).

  • Hard and Soft Keratin:

    • Hard Keratin: Found in hair, nails, and horns; provides toughness and protection.

    • Soft Keratin: Found in skin; retains flexibility but offers some structural support.

  • Hair Perm Treatments:

    • Involves the breaking and reforming of disulfide bonds in the keratin structure to change hair shape.

    • Ammonium thioglycolate deactivation prevents the reformation of bonds, leading to ineffective perm treatments.

  • Nuclear Intermediate Filaments:

    • Composed primarily of lamins located at the inner nuclear envelope; provide structural support to the nucleus and regulate DNA replication and cell division.

Actin Dynamics

  • Treadmilling in Actin Filaments:

    • Definition: A process where the amount of polymerized actin remains constant while the filament length changes due to the addition of G-actin at the plus end and loss at the minus end.

    • Facilitated by ATP concentrations and actin-binding proteins; inhibited by cytoskeletal blockers.

  • Stages of Actin Polymerization:

    1. Nucleation: Formation of a stable actin nucleus.

    2. Elongation: Addition of G-actin to polymerized F-actin.

    3. Steady State: Rate of addition equals rate of disassembly, no net growth.

  • Impact of Blocking Antibodies:

    • Profilin: Decreased addition of G-actin.

    • Thymosin-β4: Reduced G-actin pool for filament assembly.

    • CapZ: Loss of actin filament stabilization at the plus end.

    • Arp2/3: Impaired nucleation of branched filaments, affecting overall actin structures.

  • Filament Polarity:

    • Actin filaments have inherent polarity due to subunit orientation; the plus end grows faster than the minus end due to ATP hydrolysis kinetics.

    • Detectable through techniques like fluorescence microscopy.

  • Formin vs. WASp Activation:

    • Formin: Activated by Rho GTPases, promotes unbranched filament formation.

    • WASp: Activated by receptors, leads to Arp2/3 complex activation, resulting in branched filament creation.

Microtubules

  • Microtubule Polarity:

    • Microtubules exhibit polarity with distinct plus (beta-tubulin exposed) and minus (alpha-tubulin exposed) ends, affecting organization and intracellular transport.

  • Dynamic Instability:

    • Refers to the rapid switching between polymerization and depolymerization; key for cellular functions during mitosis. Difference from treadmilling (which maintains filament length) is that in dynamic instability, the total length of filaments can fluctuate widely.

  • Protofilaments:

    • Linear chains of alternating alpha and beta tubulin dimers; typically, 13 protofilaments form a microtubule in vivo.

  • Effects of Mutant Beta-Tubulin:

    • A mutant that hydrolyzes GTP slower can lead to instability in microtubules, increasing their likelihood of disassembly due to reduced GTP cap.

  • Cytokinesis:

    • Plane of cytokinesis determined by microtubule organization and actin contractile rings. Microtubules pull chromosomes and organize the mitotic spindle, while actin filaments help in cleavage furrow formation.

  • Microtubule-Associated Proteins:

    • - Stathmin: Sequesters tubulin; regulates microtubule dynamics.

    • EB1: Stabilizes growing microtubules.

    • CLASP: Coordinates microtubule dynamics at sites of cell division.

    • MAP2/Tau Proteins: Stabilize and promote assembly of microtubules, enhancing structural integrity

  • Centrosome Functions:

    • Organizes microtubules and is critical in mitotic spindle formation.

    • Different from basal bodies, which organize cilia and flagella.

  • Mutant Gamma Tubulsdin Impact:

    • Dysfunction in gamma tubulin will impede the formation of microtubules, resulting in decreased cellular structure organization and problems with intracellular transport.