Lecture 15: Cell Death

Apoptosis

• Programmed cell death

• A process by which a cell is removed without damaging its neighbors or the rest of the tissue

• Controlled and regulated

• Cells receive cues, which let them proceed through apoptosis in an orderly manner

Reasons Why The Process of Cell Death is Actively Initiated

• Overall, cell death is a routine and essential part of life

• Death of diseased or damaged cells

  • Ensures that these cells are removed before they threaten the health of the organism

  • Ex: Virus-infected cells & cells with severely damaged DNA

• Death during development

  • Carefully orchestrated patterns of cell death help determine the size and shape of limbs and other tissues

• Death to maintain tissue/organ size

  • Adult tissues that are neither growing nor shrinking maintain a constant size through a balanced rate of cell division and cell death

Examples of Death During Development

• Cells die to remove structures that are no longer needed

  • Apoptosis is induced during frog morphogensis in cells of the tadpole’s tail b/c it is not needed in the adult frog

• Cells between digits die during embryonic development

  • Apoptosis in the developing human hand sculpts the digits (likewise in mice paws)

• Excess neurons die during development

  • During nervous system development, there are often more nerve cells initially than target cells (which secrete survival factors)

  • As a result, there are too many nerve cells and not enough survival factors for all the nerve cells to survive

  • The excess nerve cells undergo apoptosis, adjusting the number of nerve cells to match the number of target cells

How Cancer Can Arise

• Cancer can arise from an imbalance between cell division and apoptosis

• Increased cell division but normal apoptosis can lead to overgrowth

• Normal cell division but decreased apoptosis can lead to overgrowth

• As a result, it is important to maintain a proper balance between cell division and cell death

Cell Death: Necrosis Vs. Apoptosis

• Necrosis

  • Cell death in response to an acute insult (e.g., trauma)

  • Cells swell, burst, and spill their contents

  • Leakage of enzymes can damage nearby cells

  • Often triggers an inflammatory response

  • As a result, it is very messy!!

• Apoptosis

  • Programmed cell death

  • Cells shrink, the cytoskeleton collapses, the nuclear envelope disassembles

  • DNA fragments are created

  • The cell surface becomes chemically altered

  • The dying cell breaks into membrane-enclosed fragments

  • Dead cells are quickly disposed of by phagocytosis

  • As a result, it is neat and tidy!!

Phagocytic Cells

• Play an important role in scavenging dead and damaged cells, and cell debris

• They also remove the membrane-enclosed fragments of cell undergoing apoptosis

Caspases

• Family of proteases that are required for apoptosis

  • Proteases cleave proteins

• Initiator caspases: Initiate apoptosis

• Executioner caspases: Cleave substrates to dismantle the cell

• Important: Initiator caspases activate the executioner caspases

Caspase activation during apoptosis - Initiator caspase

• Caspases are always present in cells, but are made as inactive precursors (“procaspases”) that are only activated during apoptosis

• Initiator procaspases exist as inactive monomers

• Adaptor proteins facilitate procaspase dimerization, which activates the initiator caspases in the dimer

• These protease domains of the associated initiator caspases perform cross-cleavage

  • One domain cleaves the other

• Rearrangement of large and small subunits that arise locks the active complex in a highly stable conformation

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Caspase activation during apoptosis - Executioner caspase

• Executioner procaspases exist as inactive dimers

  • Don’t need adaptor proteins

• The activated initiator caspases cleave the executioner procaspases, which are activated

• The protease domains within executioner caspases rearrange after cleavage, keeping the active complex on

• Once activated, executioner caspases can cleave substrates and dismantle cellular components

Caspases Cleave Thousands of Different Proteins

• A few of the proteins cleaved by caspases:

  • Nuclear lamins → Breakdown of the nuclear envelope

  • Components of the cytoskeleton → Collapse of cellular structure

  • Cell-cell adhesion proteins → Detachment from neighboring cells

  • An inhibitor of the CAD nuclease (cleaves DNA/RNA) → Triggers DNA fragmentation by CAD

CAD Nuclease

• CAD is a nuclease that can cut up DNA

• CAD is bound by an inhibitor called iCAD

• An executioner caspase cleaves iCAD

• Activation of CAD triggers DNA fragmentation during apoptosis

Extrinsic Vs. Intrinsic Apoptosis

• Major difference: What type of signal initiates the apoptotic program

• Extrinsic

  • External signals tell the cell to die

  • The cell obliges by inducing a death program

• Intrinsic

  • The cell responds to damage or absence of critical survival factors by inducing the death program

The Extrinsic Pathway of Apoptosis

• Damaged cells induce the expression of death receptors called Fas receptors

• Fas receptors are activated by Fas ligands, which are on external killer lymphocytes

• This induces the formation of the death induced signaling complex (DISC) in the damaged cell

  • DISC recruits adaptor proteins and initiator procaspases

  • DISC specifically enables dimerization of initiator procaspase, caspase-8

The Intrinsic Pathway of Apoptosis

• A cell triggers its own destruction in intrinsic apoptosis

• Mitochondria play a key role by releasing cytochrome C

  • Release is mediated by Bax/Bak

  • Cytochrome C is a component of the respiratory energy regeneration system

Cytosolic Cytochrome C

• Activates an adaptor protein that leads to apoptosome assembly

• Apoptosome: Wheel-like heptamer of adaptor proteins that can recruit and activate seven initiator procaspases

  • Specifically, procaspase-9

Survival Factors can Inhibit Apoptosis

• Apoptosis is highly controlled

• There are signals that can suppress apoptosis, called survival factors, to ensure that cells only die when they need to

• Process

  • Survivial factors bind cell surface receptors

  • This triggers signaling cascades that lead to activation of a transcription factor

    • Which binds to DNA and stimulates the production of particular RNAs

  • The transcription factor goes into the nucleus and stimulates the production of Bcl2, which is an anti-apoptotic protein

Balance Between Pro- and Anti-apoptotic Factors

• Pro- and Anti-apoptotic proteins belong to the same family

  • The Bcl2 family

• Anti-apoptotic Bcl2 family proteins

  • Contain Bcl2 itself and others

• Pro-apoptotic Bcl2 family proteins

  • Include Bax and Bak

Inhibition of Bax/Bak by Bcl2

• Bcl2 can inhibit Bax/Bak, preventing cytochrome C release

• Process

  • Bax/Bak must oligomerize to form channels that can release cytochrome C

  • Bcl2 antagonizes the formation of these channels

Inhibition of Bcl2

• Following an apoptotic stimulus, the anti-apoptotic protein Bcl2 is inhibited

• Bax/Bak aggregate on the mitochondrial membrane and release cytochrome C

Too Much or Too Little Apoptosis Can Be a Bad Thing

• Too much

  • Excessive numbers of cells undergoing apoptosis can cause tissue damage

  • Examples

    • Heart attacks (lack of oxygen triggers apoptosis in heart tissue)

  • Strokes *lack of oxygen triggers apoptosis in brain tissue)

• Too little

  • Prevention of normal apoptotic death

  • Examples

    • Prevention of apoptosis contributes to many cancers

      • Some cancer cells overproduce anti-apoptotic protein Bcl2

      • This suppresses apoptosis and promotes over-proliferation of cells, leading to tumor formation