ATP can induce pyroptosis through P2X7 dependent K efflux in macrophages leading to NLRP3 activation and induction of inflammasomes. ATP also opens pannexin1 channel that is required for inflammasome activation. Pannexin can also be opened by high levels of extracellular K+ and are known to mediate the release of intracellular ATP. Do you have any information on how much extraellular K+ and ATP are exactly required to induce this circulus vitiosus in vivo. Also, having this in mind, is any form of cell death possible without inducing highly inflammatory milieu? Eg. excessive K+ efflux is a key early step in apoptosis. If K+ efflux is strong enough to induce inflammasome activation can we really call it apoptosis?
Hi Jan,
This is a bit of a convoluted question, because the effect of elevated extracellular K+ is distinct depending on what cell type you use. Specifically, macrophages will tolerate high levels (130 mM) KCl for many hours and not trigger pyroptosis, whereas astrocytes will trigger pyroptosis with 130 mM extracellular KCl in as soon as 30 minutes after addition. I've linked my recent paper in Cell Death & Disease that uses extracellular KCl with J774A.1 macrophages as an inhibitor of ATP-induced inflammasome activation/pyroptosis as well as a JBC paper by Silverman et al. that show extracellular KCl alone trigger pyroptosis in astrocytes.
Inflammasome-associated cell death is pyroptosis, not apoptosis. Could you please clarify the second part of your question?
Article The Pannexin 1 Channel Activates the Inflammasome in Neurons...
Article K+ regulates Ca2+ to drive inflammasome signaling: Dynamic v...
As noted by Jordan, most in vitro studies using macrophages and macrophage cell lines have indicated that ATP or increased KCl concentration induces a cell death by pyroptosis that is highly inflammatory. However, it remains unclear whether these concentrations of ATP or KCl are achieved in vivo and lead to cell death by only pyroptosis. Some clarification is needed with the second part of your question.
Dear Jordan and Divaker,
thank You very much for your responses! Pyroptosis is not my primary area of research so I don't have specific lab data on this subject but I am trying to understand it in theory. If pyroptosis is defined as a highly inflammatory form of programmed cell death and can be induced by K+ elevation I am trying to understand what is it that assures apoptosis won't activate the pyroptotic cascade in vivo. Is it that local levels of K+ and ATP are lower in apoptosis than in pyroptosis? As I stated in my question, in the beginning of apoptosis there is an excessive K+ efflux. Why is that this efflux is not enough to start pyroptosis of surrounding cells? Is it not great enough?
I hope the second part of my question is a little bit clearer now.
Thank You for Your answers once again.
Best regards,
Jan
Hi Jan,
Pyroptosis is specifically defined as caspase-1/11-dependent cell death, not simply an inflammatory cell death. It is considered inflammatory because caspase-1 processes IL-1b and IL-18, which are central inflammatory cytokines, but the cell death itself is defined by the requirement for caspase-1 (and recently identified caspase-11) activation.
Apoptosis, on the other hand, is mediated by activation of caspases 3, 7 and 9 primarily.
The role of K+ is a confusing one for pyroptosis. The canonical system for pyroptosis is macrophages. In macrophages, K+ efflux from the cell is a crucial step for pyroptosis and the experimental addition of KCl outside of the cell prevents pyroptosis by blocking this K+ efflux. For macrophages (and dendritic cells) outside of the brain, high K+ outside of the cell does not cause pyroptosis.
Astrocytes have also been found to activate caspase-1, leading to pyroptotic cell death. In these cells, interestingly, high K+ outside of the cell causes spontaneous caspase-1 activation and pyroptosis. This is fairly unique to astrocytes and other immune cells of the brain and I am not aware of other cell types that have this relationship between caspase-1 activation and K+.
K+ efflux in apoptosis and pyroptosis are difficult to reconcile without knowing more about the system. As mentioned earlier in this response, the two cell death pathways are mediated by completely different effector caspases. Interestingly, caspase-1 can cleave caspase-3 and 7, leading to late-stage involvement of those caspases, but the pathway would still be defined as pyroptosis because caspase-1 activation was the initial step.
It is essential to know more about the system/environment you are talking about. If you are talking about the brain, then elevated levels of extracellular K+ are possibly pro-pyroptotic. Outside of the brain, elevated extracellular K+ would be anti-pyroptotic.
As Divaker said, it's difficult to find a relevant physiological situation where elevated K+ to the degree used to experimentally inhibit/induce pyroptosis would actually occur (i.e., 50-150 mM). While the intracellular K+ of the cell is 150 mM (cytosol) to 250 mM (mitochondria), you would need a lot of acute cell rupture to elevate the extracellular K+ levels, because the diffusion coefficient of K+ is quite high and equilibrium (or dilution) would be quick. It is generally understood that elevating extracellular KCl with regards to studying pyroptosis is a more manufactured, experimental procedure in order to determine the role of potassium efflux; not to simulate a real biological situation.
Dear Jordan ,
Thank You! Can you please also tell me a little bit more about the time frame for caspase-1 induced late-stage involvement of caspases-3 and 7. Are you talking about minutes,hours, days, months? Regarding the system, I am interested in the brain, more specifically in the Alzheimer's disease.
Thanks,
Jan
Hi Jan,
In the brain, Caspase-1 is activated about 30 minutes before activation of Caspase-3. I would recommend reading the section titled "Brain injury/neuronal cell death" in the paper linked here:
Article Caspase-1: is IL-1 just the tip of the ICEberg?
- Badges
- Science topic
- Similar topics
- Biological Science
- Molecular Cell Biology