Processes Bio230

1. Cell Polarity and Organization

Occurs during cell differentiation and tissue formation.

  1. Establish Polarity:

    • Proteins and lipids are sorted into apical and basolateral domains.

    • Vesicle trafficking delivers proteins to their respective regions.

  2. Maintain Polarity:

    • Recycling pathways via endosomes ensure proteins stay in the correct domain.

    • Phosphoinositides (PIPs) mark distinct membrane domains.

2. Cytoskeletal Dynamics

Supports cell shape, intracellular transport, and motility across various cycles (e.g., migration, mitosis).

  1. Microtubule Assembly:

    • GTP-tubulin adds to the plus end, growing the microtubule network.

    • γ-Tubulin anchors minus ends at organizing centers.

  2. Intracellular Transport:

    • Kinesins and dyneins move vesicles or organelles along microtubules.

  3. Actin Treadmilling:

    • ATP-actin polymerizes at the plus end while ADP-actin depolymerizes at the minus end.

  4. Cytoskeletal Reorganization:

    • During cell crawling, actin polymerizes at the leading edge, and myosin contracts at the rear.

3. Cell Adhesion

Occurs in the initial stages of tissue formation and morphogenesis.

  1. Adherens Junction Formation:

    • Cadherins link adjacent cells and connect to the actin cytoskeleton.

  2. Tight Junction Establishment:

    • Occludins and claudins form seals to maintain polarity.

  3. Integrin Activation:

    • Integrins anchor cells to the ECM and transmit survival signals.

4. Morphogenesis

Defines tissue architecture during early development.

  1. Cell Internalization:

    • Ingression: Individual cells migrate inward.

    • Invagination: Epithelial sheets fold into deeper layers.

  2. Elongation:

    • Convergent extension aligns and intercalates cells.

    • Axial elongation occurs through microtubule guidance.

  3. Cell Repositioning:

    • Axons extend, guided by extracellular signals.

5. Tissue Patterning

Occurs after initial morphogenesis to define regions within tissues.

  1. Lateral Inhibition:

    • Notch-Delta signaling ensures neighboring cells adopt distinct fates.

  2. Morphogen Gradients:

    • Cells respond to concentration-dependent cues, forming spatial patterns.

  3. Regulatory Hierarchies:

    • Sequential activation of transcription factors creates layers of patterning.

6. Stem Cells and Tissue Renewal

Happens continuously in adult tissues and during embryogenesis.

  1. Stem Cell Division:

    • Asymmetric division produces one stem cell and one differentiated cell.

  2. Progenitor Expansion:

    • Transit-amplifying cells rapidly proliferate before differentiation.

  3. Differentiation:

    • Signals from stem cell niches direct differentiation into specialized cells.

7. Cell Communication

Happens continuously for environmental response and multicellular coordination.

  1. Signal Perception:

    • Extracellular signals (e.g., hormones, neurotransmitters) bind receptors.

  2. Signal Transmission:

    • Intracellular pathways activate via phosphorylation or secondary messengers (e.g., cAMP).

  3. Cellular Response:

    • Changes in gene expression, protein activity, or membrane potential occur.

8. Cell Cycle Control

Ensures orderly progression of growth, DNA replication, and division.

  1. G1 Phase (Preparation):

    • Growth factors increase Myc levels, activating G1-Cdk.

    • G1-Cdk phosphorylates Rb, releasing E2F to promote S-phase genes.

  2. S Phase (DNA Replication):

    • S-Cdk activates helicases to initiate replication.

  3. G2 Phase (Preparation for Mitosis):

    • Cyclin synthesis prepares M-Cdk for mitotic events.

  4. M Phase (Mitosis):

    • M-Cdk triggers spindle assembly and nuclear envelope breakdown.

    • Cytokinesis divides the cytoplasm into two daughter cells.

9. Programmed Cell Death (Apoptosis)

Occurs during development or in response to stress.

  1. Intrinsic Pathway:

    • Mitochondrial outer membrane permeabilization (MOMP) releases cytochrome c.

    • Cytochrome c and Apaf1 form the apoptosome, activating caspase-9.

  2. Extrinsic Pathway:

    • Fas receptors bind Fas ligands, forming the death-inducing signaling complex (DISC).

    • DISC activates caspase-8.

  3. Execution:

    • Caspase cascades degrade cytoskeletal and nuclear components.

    • Lipid changes signal phagocytosis of apoptotic bodies.

10. Cancer Development

Occurs when normal processes of cell cycle control and apoptosis fail.

  1. Mutation Accumulation:

    • Oncogenes (e.g., Ras, Myc) are activated.

    • Tumor suppressor genes (e.g., Rb, p53) are inactivated.

  2. Tumor Formation:

    • Cells gain unchecked proliferation and evade apoptosis.

  3. Metastasis:

    • Cells invade surrounding tissues, enter the bloodstream, and colonize distant sites.

  4. Treatment:

    • Targeted therapies (e.g., Gleevec) inhibit specific oncogenic pathways.