Chapter 17D: The Cell Cycle

Contractile Ring Formation

  • Rho Signaling: Essential for contractile ring formation at the center of dividing cells.

  • Central Spindlein: Protein that aids Rho signaling, ensuring proper contractile ring positioning.

  • RhoA Activation:

    • RhoA is a GTPase; when active, it activates formins.

    • Formins (from Chapter 16) promote actin filament formation to assemble the actin-myosin ring.

    • RhoA also activates Rho-activated kinases (ROCK), such as ROCK, which inhibits phosphatase activity, leading to myosin light chain phosphorylation and myosin ring formation.

    • This process requires RhoA kinase activity.

  • Regulation of RhoA:

    • RhoGAP inactivates RhoA.

    • RhoGEF (specifically ECT2) activates RhoA.

    • Aurora B kinase phosphorylates and activates central spindlein, which activates ECT2.

    • This ensures the contractile ring forms at the equatorial cortex.

Aurora B Kinase

  • Critical for mitosis progression.

  • Regulates chromosome condensation and segregation.

  • Dysfunction leads to defects in chromosome condensation and segregation, disrupting mitosis.

Formins

  • Activated by RhoA signaling.

  • Form a dimeric complex that nucleates actin filaments at the plus end.

  • Connected to the plasma membrane to deposit actin filaments.

  • RhoA is deposited at the equatorial cortex, under the plasma membrane.

Central Spindlein

  • Essential for RhoA activation.

  • Associates with overlapping ends of interpolar microtubules.

  • Cross-section of the center shows a hollow region with interspersed central spindlein and the actin-myosin ring.

Cytokinesis

  • Central spindlein and ECT2 activate RhoA specifically at the equator.

  • Astral microtubules inhibit ECT2 activation at the corners, preventing contractile ring formation there.

  • Mechanism of central deposition is still under study.

Abscission

  • The midbody is pinched during abscission, not the contractile ring.

  • Midbody consists of spindle remnants, cytoplasm, and membrane.

  • Pinches the membrane to separate the two cells, completing cytokinesis.

RhoA Inhibition

  • If RhoA is not activated at the cortex between spindle poles, the contraction of the actin-myosin ring is inhibited.

  • Central spindlein and Aurora kinase activity are independent and upstream of RhoA signaling.

Cytokinesis in Plant Cells

  • A new cell wall is deposited between cells, without pinching.

  • Microtubules assemble in a pre-prophase band, centered where the cells will separate.

  • Vesicles from the Golgi, containing polysaccharides and glycoproteins, deposit cell wall material.

  • The early cell plate expands with new materials, forming a cell wall.

  • Phragmoplast microtubules help form the cell wall.

  • Plates expand until they reach the plasma membrane, forming a new cell wall during telophase.

Asymmetric Cell Division

  • Spindle repositioning can result in unequal cell sizes.

  • Example: Germ cell development in C. Elegans, where one cell becomes a germ cell.

  • P granules are segregated into the smaller cell, defining it as a germ cell.

Syncytium

  • Multiple nuclear divisions without cytokinesis.

  • Results in multiple nuclei within a single cytoplasm.

  • Example: Drosophila embryo development.

  • Nuclei migrate to the cell cortex and cellularization occurs, forming cell boundaries.

Mitogens, Growth Factors, and Survival Factors

  • Mitogens: Extracellular factors that stimulate cell division by helping cells pass the restriction point.

  • Growth Factors: Trigger cell growth, increasing cell mass and volume.

  • Survival Factors: Promote cell survival.

  • These categories are not mutually exclusive.

Mitogen Activation Pathway

  • Mitogen binds to a receptor, activating the RAS signaling pathway.

  • RAS activates the MAP kinase cascade, leading to immediate early gene expression in the nucleus, including MYC.

  • MYC promotes cyclin synthesis (G1 cyclin), which binds to G1 Cdk, activating it.

  • G1 Cdk phosphorylates retinoblastoma (Rb), inactivating it and releasing E2F.

  • E2F increases S phase gene transcription, activating G1/S and S Cdks.

  • Positive feedback loop: G1/S Cdks further phosphorylate and inactivate Rb, maintaining E2F activation.

  • E2F also promotes its own synthesis.

Retinoblastoma (Rb)

  • A tumor suppressor named after a childhood cancer.

  • Inactivation leads to continuous DNA synthesis and cell cycle progression.

Checkpoint Signaling Pathways

  • Regulate the cell cycle in response to DNA damage.

  • Two main pathways: p53 and Checkpoint Kinase 1 (Chk1).

  • Both pathways inhibit cyclin-Cdks, stopping cell division.

  • Majority of human cancers have mutations in p53.

p53 Pathway

  • High levels of X-rays trigger DNA damage, activating ATM/ATR sensor kinases.

  • ATM/ATR activate checkpoint kinase 2 (Chk2), which phosphorylates p53.

  • Phosphorylation of p53 disrupts its binding to MDM2, stabilizing and activating p53.

  • p53 acts as a transcription factor, promoting the expression of p21.

  • p21 is a Cdk inhibitor (CKI) that binds to cyclin-Cdk complexes, inhibiting their activity.

  • This stops cell cycle progression, allowing time for DNA repair.

  • p53 can also trigger cell death if the DNA damage is too extensive.

Checkpoint Kinase 1 (Chk1) Pathway

  • Activated by DNA damage (single or double-strand breaks).

  • Inhibits Cdk-activating phosphatase (Cdc25).

  • This is more prominent in S and G2 phases of the cell cycle.

Growth Factors and Cell Growth

  • Triggered by growth factors and nutrients (amino acids).

  • mTORC1 pathway is activated, leading to increased protein and lipid synthesis.

  • mTORC1 activates initiation factors, promoting protein synthesis.

  • Combined effects increase cell volume and growth.

Coordination of Growth and Division

  • Extracellular factors trigger both growth and cell division.

  • Growth factors and mitogens act simultaneously to promote cell growth and division.