Apoptosis is a form of programmed cell death (PCD). It is regulated, caspase-dependent, and non-inflammatory. The cell dismantles itself in a controlled, orderly manner. Cellular contents are packaged into small vesicles called apoptotic bodies. These are quickly cleared by phagocytes without causing inflammation. Apoptosis is different from necrosis. In necrosis, the cell bursts and releases its contents, triggering inflammation and damage to surrounding tissue. In apoptosis, the cell dies cleanly. There are two main pathways of apoptosis: 1) Extrinsic pathway — triggered by external death signals (such as FasL or TNF) through death receptors on the cell surface. Initiator caspase here is caspase-8. 2) Intrinsic pathway of apoptosis— triggered by internal cellular stress signals. This pathway is centred on the mitochondrion. Initiator caspase here is caspase-9.
This post covers the intrinsic (mitochondrial) pathway of Apoptosis in full detail.
Intrinsic pathway
DEFINITION: The intrinsic pathway of apoptosis is a mitochondria-centred cell death pathway activated by internal cellular stress. It involves the BCL-2 protein family, mitochondrial outer membrane permeabilisation (MOMP), cytochrome c release, apoptosome formation, and a caspase cascade.
Step 1 — Initiation Signals
The intrinsic pathway is activated when the cell detects severe internal stress. The following stimuli can initiate it:
- DNA damage (caused by radiation or chemotherapy agents)
- Oxidative stress (excess reactive oxygen species)
- Hypoxia (oxygen deprivation)
- Loss of survival signals or growth factors
- Endoplasmic reticulum (ER) stress
- Cytotoxic drugs
No matter what the initial insult is, all these signals converge on the mitochondrion. The mitochondrion is the central decision-making organelle for the intrinsic pathway.
EXAM TIP: List at least 4–5 initiation signals with examples. Always state that all signals converge on the mitochondrion. Examiners look for this convergence statement.
Step 2 — The BCL-2 Family: Guardians and Executioners
The BCL-2 (B-cell lymphoma 2) protein family is the master regulator of the intrinsic pathway. These proteins control whether the cell lives or dies by regulating the integrity of the outer mitochondrial membrane.
There are three subfamilies:
Subfamily 1 — Anti-Apoptotic Proteins (Pro-Survival)
These proteins protect the cell from apoptosis. They bind to and hold the effector proteins BAX and BAK in an inactive state.
- Members: BCL-2, BCL-XL, MCL-1
Subfamily 2 — BH3-Only Proteins (Stress Sensors)
These proteins detect and respond to cellular stress. When activated, they bind to and neutralise the anti-apoptotic proteins (BCL-2, BCL-XL, MCL-1). This releases BAX and BAK from inhibition.
- Members and their activators:
- BID — activated by caspase-8 (links extrinsic pathway to intrinsic pathway)
- BIM — activated during cytokine withdrawal and ER stress
- PUMA — transcriptionally induced by p53 after DNA damage
- NOXA — also induced by p53
- BAD — activated when growth factor signalling is withdrawn
Subfamily 3 — Effector Proteins (Executioners)
These proteins execute mitochondrial membrane permeabilisation. Once freed from BCL-2/BCL-XL control, BAX and BAK oligomerise and form pores in the outer mitochondrial membrane.
- Members: BAX, BAK
KEY CONCEPT — THE BCL-2/BAX RATIO:
The ratio of anti-apoptotic to pro-apoptotic proteins determines the cell’s fate.
- High BCL-2/BAX ratio → Cell survives
- Low BCL-2/BAX ratio → Cell undergoes apoptosis
This ratio is one of the most frequently tested points in life science exams. Write it explicitly in every 10-mark answer on this topic.
EXAM TIP: Name all three subfamilies with a minimum of two examples each. A vague mention of “BCL-2 family proteins” without classification will not earn full marks.
Step 3 — MOMP: The Point of No Return
MOMP stands for Mitochondrial Outer Membrane Permeabilisation.
When BH3-only proteins neutralise BCL-2 and BCL-XL, BAX and BAK are freed. They then:
- Undergo conformational changes (change shape).
- Oligomerise — cluster together on the outer mitochondrial membrane.
- Form large lipidic pores in the outer mitochondrial membrane.
Once these pores form, proteins trapped in the intermembrane space are released into the cytosol.
MOMP is called the “point of no return” because the cell is irreversibly committed to death at this step. The pores formed by BAX/BAK oligomers in the outer membrane cannot be repaired. The cell cannot recover.
EXAM WARNING: Always state that MOMP is the point of no return AND explain why it is irreversible. Simply using the term without explanation will not earn full marks. State clearly: BAX/BAK oligomers form irreversible pores — the cell cannot recover from this step.
Step 4 — Cytochrome c Release
Once MOMP pores form, the contents of the intermembrane space (the space between the inner and outer mitochondrial membranes) escape into the cytosol.
The most important protein released is cytochrome c.
- Cytochrome c is normally a component of the electron transport chain.
- It is located in the intermembrane space.
- After MOMP, it exits through BAX/BAK pores into the cytosol.
COMMON MISTAKE — READ CAREFULLY: Cytochrome c EXITS the mitochondria. It does NOT enter. Many students write “cytochrome c enters the mitochondria.” This is factually incorrect. It is released FROM the intermembrane space INTO the cytosol. Always write it this way.
Step 5 — Apoptosome Formation
Once cytochrome c reaches the cytosol, it triggers the assembly of the apoptosome — a large multi-protein complex that activates the initiator caspase of the intrinsic pathway.
Apoptosome assembly occurs in the following steps:
- Cytochrome c (released into cytosol) binds to Apaf-1 (Apoptotic Protease Activating Factor-1).
- This binding, in the presence of dATP, causes a conformational change in Apaf-1.
- Seven Apaf-1 molecules assemble into a wheel-like, heptameric (7-subunit) ring complex — this is the apoptosome.
- The apoptosome recruits procaspase-9 and activates it into active caspase-9.
APOPTOSOME — QUICK REFERENCE:
- Components: Cytochrome c + Apaf-1 + dATP
- Structure: Wheel-like, heptameric (7-subunit) complex
- Function: Recruits and activates procaspase-9 → active caspase-9 (initiator caspase of intrinsic pathway)
EXAM TIP: A mere mention of the apoptosome is insufficient for full marks. You must state: (a) it is a 7-subunit complex, (b) components are cytochrome c + Apaf-1 + dATP, (c) it activates procaspase-9. All three details are expected.
COMMON MISTAKE: Caspase-8 is the initiator caspase of the EXTRINSIC pathway. Caspase-9 is the initiator caspase of the INTRINSIC pathway. Do not confuse the two. The apoptosome activates caspase-9, not caspase-8.
Step 6 — The Caspase Cascade
Caspases are cysteine-aspartate proteases. They act as molecular scissors and cleave specific target proteins to dismantle the cell. They exist as inactive precursors called procaspases.
Two types are involved:
- Initiator caspases — activated first; activate the executioner caspases. In the intrinsic pathway: caspase-9.
- Executioner caspases — directly cleave cellular substrates to carry out cell death. In both pathways: caspase-3 and caspase-7.
How the Cascade Proceeds
Active caspase-9 (initiator) cleaves procaspase-3 and procaspase-7, converting them into active caspase-3 and caspase-7 (executioners). Active caspase-3 and caspase-7 then cleave the following key substrates:
- PARP (Poly ADP-ribose polymerase) — DNA repair is shut off. The cell can no longer fix its damaged DNA.
- Lamin A — The nuclear lamina (protein scaffold of the nucleus) collapses. This causes nuclear condensation and fragmentation.
- ICAD (Inhibitor of CAD) — Cleaving ICAD releases CAD (Caspase-Activated DNase). CAD enters the nucleus and cleaves DNA into ~180 base pair fragments. This produces the characteristic DNA ladder pattern seen on gel electrophoresis — a hallmark of apoptosis.
- Cytoskeletal proteins — The cell shrinks. Blebs (membrane protrusions) form on the plasma membrane surface.
The result is a cell that systematically packages itself into apoptotic bodies — ready for phagocytic clearance.
Step 7 — Other Mitochondrial Factors Released After MOMP
Cytochrome c is not the only protein released from the intermembrane space after MOMP. Two other important factors are released:
Smac/DIABLO
(Second Mitochondria-derived Activator of Caspases / Direct IAP Binding protein with Low pI)
- Released from the intermembrane space after MOMP.
- Binds to and neutralises IAPs (Inhibitor of Apoptosis Proteins).
- This removes the inhibition on caspase-3, caspase-7, and caspase-9.
- Result: caspase activity is amplified.
AIF (Apoptosis-Inducing Factor)
- Released from the intermembrane space after MOMP.
- Translocates directly to the nucleus.
- Triggers chromatin condensation and large-scale DNA fragmentation.
- This process is caspase-independent — it occurs even when caspases are inhibited.
- Result: AIF provides a backup demolition mechanism.
Step 8 — IAP Regulation and the Smac/DIABLO Counter-Check
IAPs (Inhibitor of Apoptosis Proteins) are a family of endogenous caspase inhibitors. The most important IAP is XIAP (X-linked IAP). XIAP directly binds to and inhibits: Caspase-3, Caspase-7, Caspase-9
This means that even after caspase activation, the cell has a built-in brake mechanism. IAPs prevent accidental or incomplete apoptosis.
However, after MOMP, Smac/DIABLO is released from the intermembrane space. Smac/DIABLO binds to XIAP and competitively displaces the caspases. This lifts the inhibition and allows the caspase cascade to proceed to completion.
This represents a regulatory tug-of-war: IAPs try to suppress apoptosis, while Smac/DIABLO ensures apoptosis is completed once MOMP has occurred.
EXAM TIP: Mention IAPs and Smac/DIABLO for full marks in a 10-mark answer. Many students omit this regulatory layer entirely.
Intrinsic Pathway of Apoptosis Flowchart :
▸ Lamin A — nuclear collapse
▸ ICAD → CAD → DNA ladder
▸ Cytoskeletal proteins cleaved
Common Mistakes to Avoid (Exam Cheat Sheet)
- ❌ Writing that caspase-8 is the initiator caspase of the intrinsic pathway.
- ✅ Correct: Caspase-8 is the initiator of the EXTRINSIC pathway. Caspase-9 is the initiator of the INTRINSIC pathway.
- ❌ Writing that cytochrome c enters the mitochondria.
- ✅ Correct: Cytochrome c is released FROM the intermembrane space INTO the cytosol through MOMP pores.
- ❌ Writing that BCL-2 promotes apoptosis.
- ✅ Correct: BCL-2 is ANTI-apoptotic. It suppresses apoptosis by keeping BAX/BAK inactive. BCL-2 overexpression is seen in many cancers because it prevents cancer cells from undergoing apoptosis.
- ❌ Saying “MOMP is the key step” without explaining why it is the point of no return.
- ✅ Correct: Explain that BAX/BAK oligomers form irreversible pores. The cell cannot repair these pores and cannot recover once MOMP has occurred.
- ❌ Describing the apoptosome as “a complex formed by cytochrome c and Apaf-1” without structural detail.
- ✅ Correct: State it is a heptameric (7-subunit), wheel-like complex formed by cytochrome c + Apaf-1 + dATP.
- ❌ Forgetting to mention IAPs and Smac/DIABLO.
- ✅ Correct: Mention XIAP as the caspase inhibitor and Smac/DIABLO as its antagonist. This regulatory layer is expected in full-mark answers.
Summary
The intrinsic pathway of apoptosis is a highly regulated, multi-step cell death mechanism centred on the mitochondrion. Here is the core logic:
- Internal stress activates BH3-only proteins.
- BH3-only proteins neutralise BCL-2/BCL-XL, releasing BAX/BAK.
- BAX/BAK oligomerise and form MOMP pores — the irreversible commitment step.
- Cytochrome c escapes from the intermembrane space into the cytosol.
- Cytochrome c + Apaf-1 + dATP → apoptosome (7-subunit complex).
- Apoptosome activates caspase-9 (initiator).
- Caspase-9 activates caspase-3/7 (executioners).
- Executioner caspases dismantle the cell: DNA repair off, nuclear collapse, DNA ladder, membrane blebbing.
- Smac/DIABLO counters IAPs to ensure the cascade proceeds.
- AIF provides a caspase-independent backup pathway.
The BCL-2/BAX ratio and MOMP are the two most critical regulatory checkpoints. Master these two concepts and the rest of the pathway falls into place.