How to differentiate apoptotic from necroptotic cell death? Part III.

How to determine cell death?

Various assays exist to determine cell death and all, unfortunately, have their limitations. In this section, I’ll discuss all of the assays that I’ve got experience with. I’ll also indicate whether I deem an assay suitable for high throughput screening (HTS) or not. In general, I’d advise against using an assay that you don’t understand the principles of. Companies such as Promega are never very eager to reveal the underlying principles of their assays, as they’re afraid other companies will copy them, but if you don’t know what, for example, the ‘live-cell protease’ activity is that an assay such as Promega’s  MultiToxFluor assay measures, you can’t possibly determine whether your treatment is indeed affecting the cells’ health or just the activity of this mystery protease. If a company won’t tell me what it is exactly that their assay does and what the buffer components are, I won’t trust that assay.

MTT assay

Figure 1. MTT assay to determine TNFa toxicity on L929 cells.

A rather old fashioned method for determining cell death is the good old MTT assay. This assay depends on the reduction of tetrazole to formazan by oxireductase enzymes in living cells. Formazan forms a purple precipitate that can easily be detected with an absorption spectrometer. However, just like ATP assays, this assay also detects loss of cells, lack of proliferation or reduced metabolic activity, rather than cell death itself. MTT assays are certainly useful because they’re extremely cheap and easy to perform, but shouldn't be used to accurately determine cell death, let alone differentiate between modes of cell death. In addition, the assay only really works well for adherent cells and the cells are lysed in the process and the assay can therefore not be used in a multiplex set up.

Detection method: Absorption

Pros: Cheap and relatively easy

Cons: Sensitivity, not a cell death assay, doesn't discriminate, samples destroyed in the process

Crystal Violet

What I like about using crystal violet is that the assay doesn't depend on metabolic activity of the cells, but exclusively on the number of adherent cells remaining in your well. In addition, the morphology of the stained cells can easily be judged by simple light microscopy. Since the dye can be re-dissolved with methanol after staining and washing, the assay can also be used to accurately quantify the number of cells remaining in your well (Figure 2). Drawbacks are, of course, that the assay can only be used for adherent cells, that the cells are fixed during staining and that it’s hard to integrate the assay in a multiplex procedure. Since the assay only determines loss of cells (which can also be lack of proliferation) no conclusions about the mode of cell death can be drawn from crystal violet staining alone. I find this assay particularly useful for illustrating colony outgrowth after treatment, though it can also be used for routine screening. Because the cell mono-layer easily gets damaged in the execution of the assay during washing, fixing or staining, the assay is not very suitable for small well formats. It works well enough in larger wells, down to a 24-well format, since the relative contribution of some minor damage to the mono-layer won’t influence the outcome of the assay as much in larger wells as in smaller wells.

Figure 2. Crystal violet assay. A: Titration of L929 cells stained with crystal violet to generate a standard curve on a double log scale. Insert shows a macroscopic view of the wells with increasing amounts of cells. B: The reactive oxygen species (ROS) scavenger butylated hydroxyanisole (BHA) protects L929 cells from TNF/zVAD-induced necroptosis. However, higher concentrations of BHA are toxic. C: Macroscopic (top) and microscopic images (bottom) of L929 cells untreated (left), treated with TNF/zVAD (middle) or TNF/zVAD+necrostatin-1 (right). Cells were treated for 4h after which the medium was refreshed and cells were allowed to grow for a week.

Detection methods: Light microscopy, absorption (after re-dissolving dye in methanol)

Pros: Very cheap and relatively easy, objectivity

Cons: Sensitivity, not a cell death assay, doesn't discriminate, only works for adherent cells, cells fixed in the process, prone to errors when the cell mono-layer is scratched, toxicity of the reagents

ATP assays

Luciferase-based assay that determine cellular ATP levels, such as Promega’s Cell Titer Glo, have become very popular, especially in high throughput screens. Ease-of-use is the main selling point of these assays, as they only require a single reagent addition and a short incubation time. In addition, the assay is very sensitive. However, this assay does not determine cell death. Rather, a reduction in ATP levels can reflect several scenarios. First, a reduction in ATP levels in a given well might indicate a reduction in cell numbers as a consequence of death, but could also reflect a reduction in cell growth. If the treated cells expanded more slowly than the control cells, this would be reflected in a relative reduction in ATP levels. Second, overall ATP levels may be transiently reduced in cells under stress without this resulting in cell death. Third, during apoptosis ATP levels actually increase before cell death occurs. Thus, cell death assays based on determining relative ATP levels can very easily lead to invalid results and wrong conclusions. For an initial screen, such an assay might be useful but one should be extremely cautious to draw conclusions based on results obtained from ATP assays alone.

Detection method: Luminescence

Pros: Easy, single addition, suitable for HTS, very sensitive

Cons: Not a cell death assay, doesn't discriminate, samples destroyed in the process

Dye exclusion

In many older papers ‘apoptosis’ is equated with cell permeability for DNA-binding dyes such as propidium iodide (PI) or ethidium bromide (EthBr). However, apoptotic cells only become permeable to such dyes at a very late stage, while necroptotic cells lose plasma membrane integrity much earlier. Thus, dye uptake is either a sign of necroptosis, primary necrosis or late-stage apoptosis (Figure 3). Bear this in mind when reading older papers and don’t make this mistake yourself. Of course, this is not only true for old-fashioned dyes such as PI, but also for newer dyes such as the Sytoxdyes from Life Technologies (formerly Molecular Probes/Invitrogen). Although dye exclusion by itself has limited usability, it’s a method that can easily be combined with other methods in a multiplex assay, as I’ll discuss below.

Detection method: Fluorescence

Pros: Cheap and very easy, single addition and no washing required, suitable for both adherent and suspension cells, many dyes available in different colours, suitable for HTS

Cons: Only indicates necrosis or necroptosis, not early-stage apoptosis

LDH Release

Figure 3. Sytox green staining vs. LDH release from U937 cells
treated with TNFa in the presence of increasing amounts
of zVADfmk for 24 hours.

When cells lose membrane integrity, their content is spilled in the environment. This includes the enzyme lactate dehydrogenase (LDH). Activity of this enzyme can easily be detected with a commercially available colorimetric assay. The great advantage of this assay is that only the culture medium of the cells is required and that the cells themselves can therefore be used for other assays, for example Western blot or FACS. I found this assay to be very similar in sensitivity to dye exclusion (Figure 3). Of course, just like dye exclusion, the assay only determines the rate of necrosis which can be either a consequence of necroptosis, primary necrosis or secondary necrosis in late-stage apoptosis. An advantage over dye uptake is that you only need an absorption reader for detection and these are usually cheaper than fluorescence readers, although the assay itself is somewhat more expensive. 

Detection method: Absorption

Pros: Cheap and very easy, single addition to culture supernatant, cells can still be used for continued culture or other assays, suitable for both adherent and suspension cells

Cons: Only indicates necrosis or necroptosis, not early-stage apoptosis

Resorufin/Resazurin

The principle of this assay, sold as AlamarBlue or Cell Titer Blue, is the conversion of blue, non-fluorescent, resazurin to red, fluorescent, resorufin by living cells in an oxidation reaction. The assay has several advantages: the cells are not destroyed in the process, the results can be measured with either a fluorometer (red) or an absorbance spectrometer (red/blue) and can easily be combined with other assays that utilize different fluorescence wave lengths or luminescence. In addition, the results are aesthetically pleasing (Figure 3).

Figure 4. Cell Titer Blue on U937 cells treated with various stimuli/inhibitor titrations in a 96-well plate (A). Wells with living cells turn pink, others remain blue. In B I plotted the rate of resorufin turnover by cells treated overnight with TNFa in the presence of increasing amounts of zVAD versus Sytox Green staining.

However, just like ATP assays and such, the assay doesn't indicate cell death but rather cell metabolism. Since different cells display different rates of oxidative metabolism under different circumstances, it will take some experimenting before reliable results can be obtained from this assay. I found that it takes jurkat or U937 cells about 2-4 hours to metabolize resazurin, but I found that L929 cells act rather unpredictably in this assay. The assay can be used as either an end-point read out or kinetic read out. In the form of Amplex Red, resorufin can also be used to determine a rapid oxidative burst. Back when I worked on neutrophils we used Amplex Red routinely to determine neutrophil responses to certain stimuli.

Detection method: Absorption or fluorescence

Pros: Fairly cheap (I only once got a 10 mL sample of Cell Titer Blue and used it for years) and easy, single addition followed by incubation and measurement, suitable for multiplex assays, suitable for HTS, cells remain alive and intact

Cons: Indicates oxidative metabolism, not a cell death assay, doesn’t discriminate

Annexin V Binding

Probably my favourite apoptosis assay is binding of fluorescently labelled Annexin V binding followed by flow cytometry (FACS). Annexin V binds to phosphatidylserine (PS) which is exposed on the outer membrane of apoptotic cells. PS exposure is a passive process and happens when a cell’s active mechanism for retaining PS on the inside of the plasma membrane (the enzyme family collectively known as ‘flippases’) is compromised. Flippases are ATP-dependent enzymes that are sensitive to Ca²⁺. Thus, a drop in cellular ATP levels or a rise in intracellular Ca²⁺ levels will inhibit flippases and lead to PS exposure.

Figure 5. Membrane asymmetry in healthy cells versus PS exposure in apoptotic cells. An image I drew years ago.

Not too much is actually known about the regulation of these flippases during apoptosis, but PS exposure certainly is a very accurate hallmark of early apoptosis. However, PS exposure isreversible, some cells (such as macrophages) constitutively bind low levels of Annexin V and PS exposure can occur under certain rare  conditions (such as Barth syndrome) in the absence of apoptosis. Nevertheless, I’ve generally found Annexin V staining to correlate nicely with apoptosis. Of course, when the cell membrane integrity is compromised, Annexin V will also enter the cell and stain both sides of the membrane. Thus, high Annexin V staining alone can be an indication of either apoptosis or necrosis. Whatever the case, when a cell displays high Annexin V positivity something’s wrong. Annexin V binding can (and should) easily be combined with dye exclusion for accurate differentiation of (early) apoptotic cells and necrotic cells. Bear in mind that Annexin V binding is Ca²⁺-dependent and your binding buffer should therefore always contain ~2.5 mM CaCl₂. If you wash away the Ca²⁺, the Annexin V will also fall off. Finally, living cells will constantly expose low amounts of PS that are actively transported back in side and therefore constant exposure of living cells to Annexin V will very slowly lead to the uptake of the Annexin V and the staining of the cells. However, if you keep the cells on ice, you effectively fix the plasma membranes and the PS levels in the outer membrane won’t change anymore, even if you leave the cells unfixed.

Detection method: Fluorescence

Pros: Accurate assay for apoptosis, sensitive, easily combined with other assays

Cons: Indicates both early apoptosis and necrosis, not suitable for HTS

Figure 5. Annexin V staining of jurkat cells either deficient in FADD (DEF) or reconstituted with FADD (REC) treated with TNFa or TRAIL in the presence of the indicated inhibitors. Samples were taken every 2 hours, stained with Annexin-V-FITC and analyzed by flow cytometry.

Caspase activity

Caspase activity can be determined in a variety of ways and is a fairly reliably indicator of apoptosis. Of course, as I mentioned in the first post of this series, caspases are not exclusively activated during apoptosis and it’s not trivial to tell the activity of one caspase accurately apart from the activity of another caspase. However, caspase-3 activity in particular is certainly a hallmark of apoptosis. Thus, you’ll always find caspase-3 to be very active in apoptotic cells, although limited caspase-3 activation can occur in non-apoptotic cells. To determine caspase-3 activity any assay containing the tetra-peptide substrate ‘DEVD’ will do. I prefer Ac-DEVD-AFC over AMC labelled substrates, since they seem to be more sensitive and AFC will also turn yellow when released from the DEVD moiety, which can even be detected by absorption. Those are available as fluorescent or luminescent assays but you can also buy the substrate and make your own lysis buffer. The substrate is also somewhat cell permeable and can therefore be added to cells before inducing apoptosis and then be used to kinetically determine the increase in apoptosis in a well. However, such an approach is more likely to detect late-stage apoptosis, when the plasma membrane of the cells becomes compromised. In addition, the DEVD is also consumed by the proteasome, so healthy cells will hydrolyse it very slowly.  Therefore, rather than stating that you’re measuring caspase-3 activity when using DEVD as a substrate, state that you’re measuring DEVDase activity as you can’t be absolutely certain that the activity you measure is derived from caspase-3. Still, a fluorescently labelled tetra peptide substrate can easily be combined with dye exclusion and viability (resazurin/resorufin) to determine whether your cells have become apoptotic or necrotic after treatment.

If you want to obtain more accurate information about the particular caspase involved, you could consider using an antibody that only detects the cleaved for of the caspase, tag it with a fluorescent label and perform flow cytometry. However, only the executioner caspases (3, 6 and 7) require cleavage for activation and the available antibodies detect other proteins with the same cleavage site as well. These are often cleaved as a consequence of caspase activity (caspases like to cleave their own linkers and will cleave every other protein with the same epitope as well) which is why Western blots with active caspase antibodies will often show a large amount of bands. In flow cytometry or microscopy assays you get no information about the size of the proteins labelled and therefore no accurate information about whether you’re really looking at the caspase or a product of caspase activity. A better method to determine which caspase has been activated is to label all active caspases with a biotinylated substrate, such as bVAD, bEVD or bVEID, perform a pull-down with streptavidin beads and detect your active caspases on Western blot.

Figure 6. Active caspase detection in cell extracts. Extracts were activated by addition of cytochomre c and caspase activity was detected by hydrolysis of the substrate Ac-DEVD-AFC (A) at the indicated time points or active caspases were labeled with bEVD-AOMK, pulled down with streptavidin beads and analyzed on Western blot (B). Active caspase-6 and -3 could be detected. Caspase-8 is cleaved (by caspase-6) but not activated under these conditions. See van Raam et al. for further details.

Detection method: Fluorescence, luminescence or absorbance

Pros: Accurate assay for apoptosis, sensitive, suitable for multiplex, suitable for HTS

Cons: Indicates mostly late apoptosis, risk of false positives

The ultimate multiplex assay?

In a good multiplex assay, you want to combine at least one parameter to detect necroptotic cells and one to detect apoptotic cells. I generally prefer to combine Annexin V binding with dye exclusion on a flow cytometer, as flow cytometry provides a very versatile platform and also provides you with valuable information about cell morphology, besides fluorescence. I've personally come to prefer FITC-labelled Annexin V (I get it from Bender Med, now eBiosciences) with Sytox Red. FITC fluorescence and Sytox Red are excited by different lasers and there’s therefore no need for compensation, while PI and FITC are excited by the same laser and their fluorescent peaks are close together. However, this assay is less suitable for HTS, although most steps can easily be automated. I know the Vandenabeele lab has developed an assay wherein they combine Sytox Green with Ac-DEVD-AMC to detect caspase activity. This seems to work well for them, although I’d combine it with a viability assay in the form of Cell Titer Blue. Assays that can be performed kinetically are always superior to end-point assays, but in the end the use of inhibitors can provide you with the most accurate information.

How to differentiate apoptotic from necroptotic cell death? Part II.

Model systems for necroptosis research

Since necroptosis is what happens when caspase-8 fails to activate, the conditional caspase-8 knockout mouse reveals which tissues are susceptible to this form of cell death during development. Primarily endothelial cells, hematopoietic progenitor cells and leukocytes are susceptible to developmental necroptosis in the absence of caspase-8. CD8⁺ T-cells, monocytes and neutrophils require caspase-8 activity to properly develop and expand and are also quite susceptible to TNFa-induced necroptosis. Ischaemia/reperfusion injury also triggers necroptotic cell death in liver and kidney cells but this may represent a different form of necroptosis, distinct from TNFa-induced necroptosis. Necroptosis can be triggered by several other stimuli, such as immune receptor activation, TLR3 ligation, and RIG-I signalling. There may be other stimuli that induce necroptosis, but suffice to say that various tissues and cells under various conditions are susceptible to this form of cell death. Necroptosis can happen any time, you never know where and you never know when it will strike…

Cellular models of necroptosis

The most commonly used model cell line for necroptosis are the murine fibroblast cells L929. These cells are extremely sensitive to TNFa-induced necroptosis in the presence of the broad-spectrum caspase inhibitor z-VAD-fmk. In fact, they’ll also undergo necroptosis with z-VAD-fmk alone because the L929 cells produce low amounts of TNFa (and other cytokines!) constitutively. L929 cells are useful for the screening of anti- or pro-necroptotic compounds but shouldn’t be considered ‘real’ cells. They really respond very oddly in a number of ways and can’t be trusted entirely, in my experience. They express very high levels of RIPK3, the downstream effector of RIPK1 and this is most likely what makes them so susceptible to necroptotic death.

Mouse embryonic fibroblasts (MEFs) are sometimes susceptible to necroptosis as well, but not always. Bear in mind that authors tend to publish their successful experiments, rather than their failures, so when you see a paper in which necroptosis was induced in MEFs, don’t assume that your MEFs will respond the same way. It will work for some MEFs, but not for others and it remains hard to predict how MEFs will respond.

When dealing with human cells, either primary or cell lines, bear in mind that humans express caspase-10 besides caspase-8. Caspase-10 is activated in the same pathways as 8 and caspase-10 expression appears to be sufficient to prevent necroptosis (since patients deficient in either caspase-8 or -10 are quite viable but often develop Acute Lymhoid Proliferation Syndrome; ALPS) even though caspase-10 can’t substitute entirely for caspase-8. Caspase-10 didn’t evolve in humans but is in fact much older, the rodent lineage simply lost the gene (Figure 1). Presumably, rodent caspase-8 has taken over the functions of both caspase-8 and -10 (see also my review on the subject).

Figure 1: Evolution of caspase-8. Bony fish and their ancestors express two caspase-8 variants: the direct precursor to caspase-8 and -10 ('caspase-810') as well as caspase-18. Caspase-810 splits into two distinct genes (caspase-8 and caspase-10) just before tetrapods, while mammals lost caspase-18. Rodents, finally, lost caspase-10 as well. From my review.

Most primary human leukocytes appear to be susceptible to necroptosis, although no reports of a clear comparison has been published. In the older literature, TNFa or Fas-induced cell death in the presence of z-VAD-fmk is mentioned several times (here, here, here and here, for example) and it seems safe to assume that in most of these cases the cells succumbed to necroptotic cell death.

Certain clones of Jurkat cells are susceptible to necroptosis, but not all of them. In my experience, the FADD-deficient clone 5C3 is highly susceptible to necroptotic death induced by TNFa. The ‘wild type’ jurkat cell line A3 is not and the caspase-8 deficient line I9.2 is only mildly susceptible by itself, but, surprisingly, becomes more susceptible upon addition of z-VAD-fmk, suggesting that caspase-8 is not essential to prevent necroptosis in these cells. The RIPK1-deficient jurkat cell line is not susceptible to necroptosis unless reconstituted with RIPK1 harbouring a cleavage site mutation, The parental clone doesn’t undergo necroptosis upon TNFa stimulation in the presence of z-VAD-fmk. However, the RIPK1 deficient cells are extremely susceptible to all forms of apoptosis, suggesting either an important role of RIPK1 in preventing apoptosis or that these cells lack another anti-apoptotic factor in addition to RIPK1. These cells were initially generated by selecting randomly mutated jurkat clones against the ability to activate NF-kB, although later research has shown that RIPK1 is dispensable for NF-kB activation downstream of TNFa signalling.

The monocytic cell line U937 is also extremely susceptible to TNFa/z-VAD-fmk-induced necroptosis. These cells, as mentioned before ,will also produce high amount of TNFa in a RIPK1-dependent manner when stressed with a variety of stimuli in the presence of z-VAD-fmk. Other monocytic cell lines I tried, THP1 and NB4, are not susceptible to necroptotic cell death. Interestingly, in contrast to jurkat cells, U937 cells also undergo necroptosis when stimulated with TRAIL in the presence of z-VAD-fmk (Figure 2), even though jurkat cells are susceptible to TRAIL-induced apoptosis when FADD is expressed. This suggests that TRAIL may signal differently in U937 cells than it does in jurkat cells. I don’t know why this is, but I’ll share my observation and if anyone has an explanation, I’d be happy to collaborate.

Figure 2: TRAIL induces necroptosis in U937 cells.Treating U937 cells with TRAIL induced necrosis (Sytox Red uptake) which could not be prevented by zVAD, unlike TRAIL-induced apoptosis in Jurkat (A3) cells (A). Necrostatin only prevented necrosis in TRAIL+zVAD treated U937s (B). On Western blot, RIP1 cleavage was barely affected by zVAD in U937 cells, whereas it could be prevented in Jurkat cells. PARP cleavage was affected equally in both cell types (C and D).

These are all the necroptosis models that I’m experienced with. I’m sure there are others but you should run some tests to determine whether or not your favourite cell line is susceptible to necroptotic cell death. In the next chapter, I’ll outline several methods for determining cell death that are suitable for necroptosis research.

What determines whether a given cell is susceptible to necroptosis?

The most important factor that determines a cell’s susceptibility to necroptosis is the expression level of RIPK3 as well as expression of the downstream effector MLKL (mixed lineage kinase domain-like). Those cells that are most susceptible to necroptosis, appear to be those that express the highest levels of RIPK3 (such as L929 cells) while RIPK1 levels are normally quite stable among different cells. In fact, ordinary cells such as HeLa cells can be made susceptible to necroptosis by over-expression of RIPK3.

A recent paper in Science indicates that, indeed, RIPK3 kinase activity is required for necroptotic signalling, as mice expressing a kinase death mutant of RIPK3 did not succumb to necroptosis in the absence of caspase-8.  However, in the presence of caspase-8, these mice succumbed to massive caspase-8-dependent apoptosis. Thus, the kinase activity of RIPK3 both induces necroptosis while RIPK3 can act as a scaffold to promote apoptosis in the absence of kinase activity.

How to differentiate apoptotic from necroptotic cell death? Part I.

Figure 1. Example of a necroptotic cell (left) versus an
apoptotic cell (right). Image courtesy of the Vadenabeele lab.

Since the discovery that at least two forms of programmed cell death exist, apoptosis and necroptosis, a need has arisen to accurately discriminate between these two forms of cell death. Determining whether a cell is death or alive seems straightforward enough but all commonly used cell death assays have certain caveats and exceptions that one should be aware of before accurate conclusions can be drawn. In the following essays, I’ll go over the different cell death assays that I’m familiar with and discuss their applicability in discriminating between apoptotic and necroptotic cell death as well as common pitfalls and limitations using examples from my own experiments.

What discriminates apoptotic from necroptotic cell death?

Simply put: apoptosis is caspase-dependent cell death, whereas necroptosis is RIP-kinase-dependent cell death (for a brief review see Walsh, 2014) . Apoptosis occurs in an orderly manner: the cell’s DNA is digested into chunks of roughly 300 base pairs and the cell contents are packaged in small vesicles, the apoptotic bodies, that are phagocytosed by neighbouring cells and tissue macrophages for recycling. During necroptosis, on the other hand, the plasma membrane ruptures and the cell contents are spilled into the environment, stimulating a local inflammatory response. Apoptosis is the dominant form of cell death, since caspases have the ability to inactivate the RIPK signalling pathway. Thus, if your cells are dying while caspases are active, they’re most likely dying by apoptosis. However, caspase activity also occurs outside apoptosis and although vendors might claim that their product accurately detects the activity of one caspase or another, no simple method exists to make this distinction.

In general, caspase activity can be measured by the rate of cleavage of a tetra peptide linked to a fluorophore. For example, products to determine caspase-3/7 activity are usually based on the tetra peptide ‘DEVD’ linked to, for example, an AMC or AFC fluorescent moiety. Active caspase-3 will cleave the peptide after the last ‘D’, releasing the fluorescent moiety. An increase in fluorescence can then be said to correlate with an increase of caspase-3 activity. However, no single tetra peptide is exclusively cleaved by one caspase or another. DEVD is indeed a preferred substrate of both caspase-3 and -7 but can also be processed readily by caspase-8 and even the proteasome. Thus, an increase in DEVDase activity in your sample doesn’t necessarily indicate an increase in caspase-3 activity. The same holds true for every other tetra-peptide-based assay.

In addition, as I mentioned earlier, there are many scenarios wherein moderate caspase activity is not followed by cell death. Caspase-3 activity, for example, normally associated with end-stage apoptosis, also plays a role in memory formation in the brain in the absence of cell death. Caspase-7 may be involved in inflammation and initial activation of caspase-8 signals cell survival, rather than death. Thus, although apoptosis is invariably associated with caspase activity, caspase activity does not necessarily lead to apoptosis.

Unfortunately, no simple methods exists to determine the activity of the RIP kinases or their downstream effectors. However, there are several potent inhibitors of RIPK1 on the market: the necrostatins. The first of these, necrostatin-1, has now been shown to inhibit at least one additional enzyme, indoleamine-2,3-dioxygenase (IDO; Vandenabeele et al), and should therefore be used with caution. An alternative is now available in the form of necrostatin-1s (Nec-1s) which still prevents necroptotic cell death, in the absence of IDO inhibition. However, we can’t know what we haven’t looked for and even this inhibitor may have off-target effects.

Notwithstanding the fact that all chemical inhibitors may, to a greater or lesser extent, have off-target effects, utilization of these inhibitors still provides us with relatively simple means of discriminating the two forms of cell death. Thus, if you want to investigate whether a death-inducing compound kills your cells by apoptosis or necroptosis you could do a control experiment in the presence of a broad spectrum caspase inhibitor, such as z-VAD-fmk or boc-D-fmk, and/or necrostatin. If the caspase inhibitor rescues the death phenotype, the cells were most likely killed by apoptosis, if the necrostatin rescues the phenotype, the cells were most likely dying by necroptosis. However, there is a caveat. Certain substances have the ability to induce necroptotic cell death, but only in the absence of caspase activity as a consequence of autocrine TNFa signalling. To determine whether your compound induces such ‘secondary necroptosis’ I advice using a combination of z-VAD-fmk and necrostatin as a control besides z-VAD-fmk and necrostatin alone. If your compound appears to induce secondary necroptosis, perform a control in the presence of an anti-TNFa antibody to make sure that the observed cell death isn't a consequence of autocrine TNFa signalling. 

Figure 2. Secondary necroptosis in U937 cells. For this experiment, U937 cells where stimulated with the DNA-damaging agent etoposide in the presence of various inhibitors, as indicated. As you can see in panel A, the cells produce large amounts of TNFa upon etoposide stimulation in the presence of zVAD. Addition of necrostatin-1 (Nec1) inhibits this TNFa production. In panel B, you can see that the TNFa production is so high that the cells undergo secondary necroptosis. Vability is restored by addition of necrostatin-1 or an anti-TNFa antibody (aTNF). See van Raam et al., 2012 for a detailed description.