_10__Apoptosis-Mito

Cell Cycle Regulation

• Cell Cycle Timing: Regulated by synthesis and degradation of Cyclins.

• Cyclin/Cdk Complexes: Phosphorylate targets to progress the cell cycle.

• Adjustment Signals:

  • Extrinsic: Growth factors.

  • Intrinsic: DNA damage.

• Dysregulation Consequences:

  • Too much regulation may lead to Cancer.

  • Too little regulation may lead to Degenerative Diseases.

Cell Cycle Progression and Apoptosis

• Slowing/Stopping Mechanisms: Apoptosis can be triggered by signals such as:

  • DNA damage.

  • Trophic factor insufficiency.

• Mechanism:

  • DNA Damage Activation: Damaged DNA activates pathways leading to apoptosis.

  • Key Players:

    • Mdm2: degrades p53.

    • p53: induces cell cycle arrest via p21 (Cdk inhibitor) and Puma (pro-apoptotic).

    • Bcl-2: inhibits apoptosis.

    • Rb protein: phosphorylated by Cdk-cyclin complex.

• Result of Signals:

  • Cell cycle arrest.

  • Triggered apoptosis (cell death).

Normal and Abnormal Apoptosis

• Normal Maintenance of Tissue:

  • Induced by DNA damage during development (e.g., pruning of tissue).

• Abnormal Disease States:

  • Excess Apoptosis: Neurodegenerative diseases (e.g., Alzheimer’s, Huntington’s, Parkinson’s).

  • Insufficient Apoptosis: Cancer.

• Apoptosis Definition: Programmed cell death leading to orderly loss of individual cells.

Apoptosis and Development

• Occurrence in Development: Common for organismal development (e.g., webbing between digits).

• Example: Syndactyly (fused fingers).

Apoptosis Trigger Mechanisms

• Apoptosis Initiation: Caused by DNA damage or insufficient trophic factors which lead to cytochrome C release from mitochondria.

• p53 Role: Can trigger apoptosis through various pathways.

Mitochondrial Role in Apoptosis

• Cytochrome C Release:

  • Regulated by BCL-2 protein family (anti-apoptotic and pro-apoptotic).

• Function: Pro-apoptotic proteins (e.g., Bax) form pores in mitochondria, leading to cytochrome C release.

BCL-2 Protein Family

• Members:

  • Anti-apoptotic: (e.g., Bcl2, Bcl-X1).

  • Pro-apoptotic:

    • BH123 proteins (e.g., Bax, Bak).

    • BH3-only proteins (e.g., Bad, Bim, Bid, Puma, Noxa).

Apoptosis Mechanisms

• Apoptosome Formation:

  • Released cytochrome C activates Apaf1 to form apoptosome (caspase recruitment domain).

  • Caspase Activation:

    • Activated procaspase-9 by apoptosome.

    • Leads to a caspase cascade culminating in apoptosis.

Caspases in Apoptosis

• Caspase Family: Proteases that cleave cellular proteins affecting cell integrity.

• Activation: Caspases need to be cleaved to become active (inactive zymogens).

• Caspase Functions: Cascades consist of initiator (2, 8, 9, 10) and effector (3, 6, 7) caspases.

Morphological Changes in Apoptosis

• Apoptotic Characteristics:

  • Chromatin condensation and DNA fragmentation (200bp).

  • Nuclear envelope disassembly and cytoskeleton collapse.

  • Cytoplasm shrinkage, forming apoptotic bodies.

• Phagocytosis: Apoptotic bodies are engulfed by macrophages, preventing inflammation.

Phagocytic Clearance of Apoptotic Cells

• "Eat Me" Signal:

  • Phosphatidylserine exposed on bloated membranes attracts phagocytes.

• Lysosomal Destruction: Apoptotic cells engulfed into lysosomes of phagocytes for degradation.

Mitochondria in Apoptosis

• Regulation: Mitochondria release cytochrome C which activates apoptosis.

• Energy Production: Also involved in ATP production for cellular functions.

Energy Metabolism Overview

• ATP Generation:

  • Main energy source from glucose metabolism, with contributions from other macromolecules.

  • Critical for cellular processes like membrane pumps and vesicle movement.

• Cellular Respiration:

  • Functions: Glycolysis in cytosol; Linking step and Kreb's cycle in mitochondria; Electron transport and ATP synthesis in inner membrane.

Glycolysis and Pyruvate Processing

• Glycolysis: Occurs in cytosol, producing some ATP and NADH.

• Linking Step: Converts pyruvate into Acetyl CoA in mitochondrial matrix.

Kreb’s Cycle (TCA)

• Function: Converts Acetyl CoA into NADH, FADH2, CO2, and a small ATP yield.

Electron Transport Chain

• Energy Transfer: Utilizes NADH and FADH2 to pump H+ ions, forming an electrochemical gradient.

ATP Synthase Functionality

• Gradient Use: H+ ions flow through ATP synthase to convert ADP to ATP (oxidative phosphorylation).

ATP Release in Cellular Function

• Utilization: Released ATP enters cytoplasm for energy-required cellular processes.