Apoptosis | Difference between apoptosis and necrosis | Caspases

Apoptosis: – Apoptosis is the process of programmed cell death (PCD). The word apoptosis, derived from the Greek word for a natural process of leaves falling from trees or petals from flowers. Apoptosis is a very essential part of life cycle of organisms, in which that it regulates and adjusts cell population. Apoptosis also exerts a role opposite to mitosis in the maintenance of cell populations. As many as 10 ¹¹cells die in an adult human per day to ensure tissue homeostasis.

Apoptosis leads to several distinct cellular changes, including:

1. Cell Shrinkage –   The cell becomes smaller in size due to loss of cytoplasmic volume.
2. Membrane Blebbing –  The plasma membrane shows bubble-like protrusions without losing its integrity.
3. Chromatin Condensation (Pyknosis) –  The nucleus becomes dense due to condensation of chromatin.
4. Nuclear Fragmentation (Karyorrhexis) –  The condensed nucleus breaks into several fragments.
5. Cytoplasmic Fragmentation –  The cytoplasm is divided into smaller, membrane-bound fragments.
6. Formation of Apoptotic Bodies –  The cell is broken into small vesicles containing nuclear and cytoplasmic material, including intact organelles.
7. Phagocytosis by Neighbouring Cells or Macrophages –  Apoptotic bodies are rapidly cleared without inducing inflammation.

Cell death

Cell death is a normal process that has two functions: tissue remodelling and removal of damaged cells that harm the rest of the body. Integration of proliferation( cell division) and death is essential for normal homeostasis. There are two possible reasons for cell death:

* The cells may no longer be needed. When in a developing embryo, fingers are connected to each other by connective tissue, to initiate apoptosis connective tissue disappears and separate the fingers.

* Apoptosis may be a mechanism for preventing cancer when the cells have a genetic problem or damage that lead to cancer.

There are two types of cell death: apoptosis and necrosis. Apoptosis is a regulated process for numbers of cells but necrosis is dying cells by swelling and bursting. Unlikely necrosis, apoptosis is a regulated and controlled process. These cell death have different morphology and molecular features are the following:

Difference between apoptosis and necrosis

Apoptosis
Apoptosis is a highly regulated, energy-dependent form of programmed cell death that plays a crucial role in development, immune regulation, and removal of damaged or unwanted cells. It occurs in a controlled manner without causing harm to neighbouring cells.

Necrosis
Necrosis is an uncontrolled and accidental form of cell death that typically results from external injury, such as infection, toxins, trauma, or lack of blood supply (ischemia). It often leads to inflammation and damage to surrounding tissues.

Removal of Dead Cells

Now LET’S TALK about how the apoptotic bodies are recognized by Macrophage. The removal (engulfment) of dead cells by phagocytic cells known as Phagocytosis. In a living cell, the phosphatidylserine is found on the cytosolic surface of the plasma membrane. The protein Flippase maintain the phosphatidylserine cytosolic surface of the plasma membrane. However, in the dead cell, its come out through Flip-flop movement and mark the cell for phagocytosis. The macrophages bind phosphatidylserine exposed only in apoptotic bodies through phosphatidylserine receptors. Tissue macrophages rapidly recognize and engulf apoptotic cells. These events require the display of so-called eat-me signals on the apoptotic cell surface, the most fundamental of which is phosphatidylserine (PtdSer). Eat-me signals also trigger macrophages to engulf virus-infected or metabolically traumatized, but still living, cells, and this ‘murder by phagocytosis’ may be a common phenomenon.

Molecular mechanisms of an apoptosis signalling pathway

Apoptosis can be initiated by various stimuli from outside or inside the cell, e.g. by a specific ligand of cell surface receptors, by DNA damage, failure of DNA repair mechanisms, certain cytotoxic drugs or irradiation of cell, by a lack of survival signals.

Caspases in Apoptosis

Now LET’S TALK about why they called as caspases and what is the role of caspases. Caspases are cysteine proteases. Caspases are endoproteases have an active site Cysteine (C) and which cleave at the C- terminal side of Aspartate  residues ( asp ) and hence are known as caspases. Their catalytical activity depends on a critical cysteine-residue within a highly conserved active-site consist of pentapeptide Glutamine, Alanine, Cysteine, Arginine Glycine (QACRG). Caspases are essential enzymes that drive programmed cell death, or apoptosis, by cleaving cellular proteins and orchestrating the sequential events leading to cell demise. So far, 7 different caspases have been identified in Drosophila, and 14 in mammals, with caspase-11 and caspase–12 only identified in the mouse. These caspases form a cascade of proteases which are activated in this process. Caspases are expressed as zymogens (procaspases), which are activated by different apoptosis inducers.

Caspases is classified into three groups on the basis of their functions:

Caspases (Cysteine-aspartic proteases) are a family of protease enzymes that play essential roles in apoptosis (programmed cell death), necrosis, and inflammation. Based on their functions, caspases are classified into three main groups:

1. Initiator Caspases
   • Function: These caspases initiate the apoptosis signaling pathways.
   • Examples: Caspase-2, Caspase-8, Caspase-9, and Caspase-10
   • Characteristic: They have long pro-domains (such as CARD or DED) that allow them to interact with adaptor proteins in apoptotic complexes like the apoptosome (for caspase-9) or death-inducing signaling complex (DISC, for caspase-8/10).

1. Executioner (Effector) Caspases
   • Function: These caspases execute apoptosis by cleaving various cellular substrates.
   • Examples: Caspase-3, Caspase-6, and Caspase-7
   • Characteristic: They are activated by initiator caspases and carry out the degradation of cellular components leading to apoptosis.

1. Inflammatory Caspases
   • Function: These caspases are involved in cytokine maturation and inflammation rather than apoptosis.
   • Examples: Caspase-1, Caspase-4, Caspase-5 (humans), and Caspase-11 (mice)
   • Characteristic: Caspase-1 activates pro-inflammatory cytokines like IL-1β and IL-18 and is involved in pyroptosis, a type of inflammatory cell death.

Caspase Activation Process

Caspases are synthesized as inactive zymogens (procaspases) and require proteolytic cleavage for activation. The activation involves multiple, highly specific hydrolysis steps, as follows:

1. Cleavage of N-terminal Propeptide
   • The first step involves hydrolysis at a specific site between the N-terminal propeptide and the large subunit of the procaspase.
   • This removes the regulatory pro-domain.
2. Cleavage Between Subunits
   • A second hydrolysis occurs between the large (p20) and small (p10) catalytic subunits.
   • This cleavage releases both subunits from the precursor form.
3. Dimerization
   • The large and small subunits form a heterodimer (p20/p10).
   • Two heterodimers then associate to form the active caspase tetramer (p20/p10–p20/p10).
4. Active Enzyme Formation
   • The final active enzyme is a tetramer composed of two large and two small subunits, functioning as a protease to cleave specific substrates during apoptosis or inflammation.