Caspases and Cellular Demolition
In 1998, I remember watching the TV to see the demolition of the Hudson’s Department Store building in downtown Detroit. The once grand 29-story building was destroyed by implosion, a form of controlled demolition used in dense urban settings that relies on controlled explosions to cause the building to collapse into itself. By having the building collapse inward, you limit the damage to any surrounding structures. An explosion, in contrast, would expel the building outward, destroying its neighbors and causing additional damage to the surrounding area. Explosions and implosions rely on the same tools – wiring and explosives – but can lead to dramatically different results. And it all depends on how the tools are used.
This dichotomy of implosions and explosions can be found in biology, as well. A prominent example is the death of our cells. Cells in our body are dying every day. Normally, we never notice. In these cases, the cells implode, disappearing without any damage to their neighbors. They go quietly. But sometimes, cells die by explosion. They spew damaging debris at their neighbors. They set off alarm bells and activate our body’s emergency response teams. When cells explode, you get inflammation and tissue damage.
So, what controls the path between one cell’s quiet implosion and another’s messy explosion?
Similar to how the outcome of building demolition depends on how explosives are used, the contrasting scenarios of cell death are contingent on the use of a family of proteins called caspases.
There have been twelve caspases identified in humans, and while some caspases are poorly characterized (caspase-2, -6, -12, and -14), the others can be broadly categorized by their general function. Caspase-8, -9, and -10 are called initiator caspases and are responsible for propagating a death signal from an origin – like a receptor on the surface of the cell – to another group of caspases called the effector or executioner caspases. Once activated, these effector caspases (caspase-3, -6, and -7) cut up key cellular proteins and lead to cell death. Returning to the building implosion metaphor, if a death receptor, such as Fas, is represented as a plunger trigger, the initiator caspases are the wires that connect the plunger to the sticks of dynamite (the effector caspases). The resulting cell death is called apoptosis and is the quiet, cellular implosion that doesn’t trigger inflammation.
In contrast, caspase-1, -4, -5, -11, and -12 promote inflammation and a type of cell death called pyroptosis, the immunologically loud cell death that activates our immune response and can damage surrounding cells and tissue. Pyroptosis results in the rupture – or explosion – of the cell’s membrane, sending the cell’s contents outward into its environment to wreak further damage.
It should also be noted that caspase-8, which is often associated with apoptosis, is also implicated in another type inflammatory form of cell death called necroptosis. Similar to pyroptosis, necroptosis leads to the rupture of the cell membrane and inflammation in the surrounding tissue environment. The progression to apoptosis versus necroptosis depends on the activation status of caspase-8.
Caspase-8, like the other caspases, initially exists in its inactive form and is called procaspase-8. In apoptosis, an extrinsic signal can bind to a receptor, like Fas. Inside the cell, procaspase-8 is recruited to the internal stalk of the receptor and is activated to form caspase-8. Once active, it acts as a molecular scissors to cleave off the inhibitory piece of the effector caspases, converting them from their inactive procaspase form to their active caspase form. These effector caspases, caspase-3 and -7, proceed to cut up additional proteins in the cell, resulting in the death of the cell.
Once active, caspase-8 also cleaves a protein called Bid, converting it into a form called tBid, which is involved in the permeabilization of the cell’s mitochondria. While normally involved in the cell’s metabolism, in apoptosis, the mitochondria release proteins that result in the activation of caspase-9, which in turn triggers caspase-3 and -7 to terminate the cell.
Sometimes, a death signal leads to a situation where caspase-8 isn’t activated. Instead, caspase-8 forms a complex called the ripoptosome, so named because its formation encourages the association of proteins called Rip1 and Rip3. The Rip1-Rip3 complex eventually results in the creation of pores in the cell membrane, promoting necroptosis and rupture of the cell membrane. Like a water balloon being popped, upon the cell’s explosion, its cellular components spread out into the surrounding environment and act as inflammatory signals that trigger an immune response and can damage the surrounding cells.
Similar to necroptosis, pyroptosis also involves the generation of holes in the cell that eventually leads to the breakdown of the cell’s membrane. Certain stimuli trigger the formation of a protein complex called the inflammasome. This complex activates the inflammatory caspase, caspase-1. Once activated, caspase-1 promotes inflammation in two key ways. Caspase-1 cleaves a protein called gasdermin D. One of the resulting halves of gasdermin D goes on to form pores in the cell’s membrane. With holes in its membrane, the cell is no longer able to maintain its once carefully regulated balance of ions and water, triggering pyroptosis and the rupture of the cell membrane. As seen in necroptosis, the breakdown of the cell membrane allows inflammatory factors to flow from the cell. Prior to membrane rupture, the gasdermin D pores also act as conduits for inflammatory cytokines, two of which are generated by caspase-1. Pro-IL-1b and Pro-IL-18 are maintained within the cell in their inactive form. Caspase-1 liberates them from their inhibitory component, generating the active IL-1b and IL-18. These proteins are inflammatory cytokines and are released out of the cell through the gasdermin D pores to trigger inflammation in the surrounding area.
While it plays vital roles throughout human development, cell death, whether by implosion or explosion, is also commonly implicated in disease. Cell implosion, or apoptosis, can result in the loss of key cells in disease. Similarly, disease-associated cell explosion from necroptosis or pyroptosis claims the lives of important cells but also induces inflammation that can cause more widespread and lasting damage. For these reasons, researchers have worked hard to identify ways to block these processes.
To prevent apoptosis, there are a variety of chemical compounds that can bind and inhibit caspase activity, like using a clamp to keep a pair of scissors closed. The challenge with this approach is that these compounds often indiscriminately bind the caspases and may lack the specificity needed in a disease context. Additionally, as we saw with caspase-8 and its activity, blocking apoptosis can simply force the cell death machinery to change gears toward necroptosis. Other approaches include targeting the site of initiation for the death signal or promoting naturally occurring cellular inhibitors of caspases to support cell survival.
Preventing pyroptosis has also been pursued, as this form of cell death is important in inflammatory and infectious diseases. Some researchers have tried to target the caspases themselves, but this has proven challenging due to the specificity issues mentioned above. Other groups have had success blocking the downstream products of caspase activation, such as IL-1b and its receptor. Three compounds that target IL-1b signaling have been approved by the FDA, specifically: anakinra, a naturally occurring receptor inhibitor; rilonacept, an IL-1 receptor decoy; and canakinumab, an antibody that binds to IL1b and prevents it from binding to the receptor.
Due to its role in pyroptosis, gasdermin D has also been targeted by researchers. One example is a peptide inhibitor that mimics the cleavage site of gasdermin D, preventing its cleavage by caspases. Necrosulfonamide, an inhibitor of necroptosis, has also shown efficacy in preventing pyroptosis by binding to gasdermin D, preventing the cell membrane pore formation necessary for pyroptosis.
Caspases play vital roles in the life cycle of our cells. They can also cause severe damage within our bodies. Their two-sided nature is like dynamite: they can be destructive and damaging, or they can productively help clear out things that have outlasted their usefulness. A tool is a tool. What matters is how it’s used.
This post was inspired and informed by: Kesavardhana S et al., Caspases in Cell Death, Inflammation, and Pyroptosis. Annu. Rev. Immunol. 2020. 38:567-595.
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