The author acknowledges Jennifer Tipper, Bilan Li, Jesse vanWestrienen, Kevin Harrod, Da-Yuan Chen, Farjana Ahmed, Sonya Mros, Kenneth Yamada, Richard Webby, the BEI Resources (NIAID), the Health Research Council of New Zealand, the Maurice and Phyllis Paykel Trust (New Zealand), the H.S. and J.C. Anderson Trust (Dunedin), and the Department of Microbiology and Immunology and School of Biomedical Sciences (University of Otago).

Regulatory approvals were obtained from the University of Otago Institutional Biological Safety Committee to work with the IAV and mammalian cells. Madin-Darby Canine Kidney (MDCK) or human lung alveolar epithelial A549 cells and IAV H1N1 subtypes were used for the present study. IAV was grown in chicken eggs, as described elsewhere17. Sterile and aseptic conditions were used to work with mammalian cells, and a Biosafety Level 2 (or Physical Containment 2) facility and Class II biosafety cabinet were used to work with IAV subtypes.
1. Assessing the cleavage or degradation of proteins in IAV-infected cells by caspases
<ol>
	<li>Seed 3 x 105 MDCK or A549 cells per well in a 12-well cell culture plate. 
	NOTE: If using MDCK cells, ensure that the antibody against the protein of interest recognizes its canine species.</li>
	<li>Seed the wells in pairs, i.e., a control pair-one well for the uninfected-mock sample and one for the infected-mock sample-and a test pair-one well for uninfected-inhibitor A sample and one for the infected-inhibitor A sample. Increase the number of pairs with each additional inhibitor (see references7,8).</li>
	<li>Incubate the cells at 37 &#176;C under a 5% CO2 atmosphere overnight.</li>
	<li>The next day, infect the cells with IAV (an H1N1 subtype or another subtype).
	<ol>
		<li>For this, prepare the virus inoculum by diluting virus stock in 400 &#181;L of serum-free minimum essential medium (MEM) (see Table of Materials) at a multiplicity of infection (MOI) of 0.5-3.0 plaque-forming units (pfu)/cell7,11 (a factor of the doubling of cell number when calculating the MOI). 
		NOTE: For infecting MDCK cells, supplement the virus inoculum with tosyl phenylalanyl chloromethyl ketone (TPCK)-trypsin at a final concentration of 1 &#181;g/mL.</li>
	</ol>
	</li>
	<li>Remove the old culture medium from the cells (step 1.3) and wash the cells with 1 mL/well of serum-free MEM 2x.</li>
	<li>Add 400 &#181;L of virus inoculum (step 1.4.1) to the cells and incubate them for 1 h at 35 &#176;C under a 5% CO2 atmosphere.</li>
	<li>In the meantime, dilute a caspase inhibitor (e.g., Z-DEVD-FMK), a lysosome Inhibitor (e.g., NH4Cl), or a proteasome inhibitor (e.g., MG132) (see Table of Materials) at a final concentration of 40 &#181;M, 20 mM, or 10 &#181;M, respectively, in serum-free MEM7 (see Table of Materials). The latter two inhibitors serve as a control. Dilute an equal volume of the solvent (if other than water) used to reconstitute one of the inhibitors in serum-free medium as a mock.</li>
	<li>Remove the virus inoculum and wash the cells as in step 1.5 (1x).</li>
	<li>Add the serum-free MEM, 1 mL/well, mock or supplemented with an inhibitor, to the cells and incubate the cells as in step 1.6. This time point is considered as 0 h infection.</li>
	<li>After 24 h, harvest the cells by scraping them with a 1 mL syringe plunger (rubber side) and transfer them into a 1.5 mL polypropylene tube.</li>
	<li>Centrifuge the tube at 12,000 x g for 1-2 min at room temperature. Collect the supernatant using a pipette. 
	NOTE: This supernatant could be discarded or used for a plaque assay8 to measure the titer of the released virus progeny.</li>
	<li>Wash the cell pellet with 250 &#181;L of phosphate-buffered saline (PBS) by re-centrifugation as in step 1.11.</li>
	<li>Remove the supernatant and lyse the cells by adding 80-100 &#181;L of cell lysis buffer (50 mM Tris-HCl, pH 7.4, 150 mM NaCl, 0.5% sodium dodecyl sulfate (SDS), 0.5% sodium deoxycholate, 1% Triton X-100, and 1x protease inhibitor cocktail, see Table of Materials) and vortexing.</li>
	<li>Heat the tube at 98 &#176;C for 10 min to completely lyse and prepare the total cell lysate. Store the lysates at 4 &#176;C for performing steps 1.15-1.17 the next day, but, for best results, finish these steps on the same day.</li>
	<li>Estimate the protein amount in each sample using a BCA assay kit (see Table of Materials).</li>
	<li>Resolve equal amounts of protein from uninfected and infected samples by standard SDS-polyacrylamide gel electrophoresis (SDS-PAGE)11 along with molecular weight markers11.</li>
	<li>Transfer the protein to a nitrocellulose or polyvinylidene difluoride (PVDF) membrane (see Table of Materials). 
	NOTE: PVDF membrane may give a high background in some western blot imagers; check for compatibility.</li>
	<li>Perform western blotting to detect the protein of interest using the method described elsewhere11.</li>
	<li>Compare the protein levels in the mock-treated and inhibitor-treated infected sample lanes. 
	NOTE: If the protein level has recovered in the caspase inhibitor-treated infected sample lane, then the protein is cleaved or degraded by caspases. Otherwise, it is degraded by either lysosome or proteasome.</li>
	<li>Quantify the protein recovery by measuring its band intensity in each lane from at least three replicates of the same experiment and normalizing it with the corresponding loading control band.</li>
	<li>Use any contemporary imager and associated software (see Table of Materials) to image the western blots and quantify the protein recovery.</li>
</ol>
2. Confirmation of caspase-mediated cleavage or degradation of proteins in IAV-infected cells by RNA interference
<ol>
	<li>Design or obtain a pre-design small-interfering RNA (siRNA) targeting either canine or human caspase 3, caspase 6, and caspase 7 (executioner caspases1) and a non-targeting control siRNA (see Table of Materials).</li>
	<li>Deliver each siRNA to MDCK or A549 cells by reverse transfection8,11.</li>
	<li>For this, dilute in separate tubes 100 nM of control siRNA or caspase siRNA or a recommended volume of transfection reagent (see Table of Materials) in 100 &#181;L of appropriate medium (like OptiMEM, see Table of Materials), and incubate for 5 min at room temperature.</li>
	<li>Prepare each dilution in duplicate. 
	NOTE: This duplicate includes one replicate for analyzing the knockdown of caspase expression by RT-qPCR or western blotting (if antibodies to caspases are available) and another for assessing the recovery of the protein of interest by western blotting.</li>
	<li>Mix 100 &#181;L of each siRNA solution with 100 &#181;L of transfection reagent solution (step 2.3) and incubate for 20-45 min at room temperature to form the siRNA-transfection reagent complex.</li>
	<li>In the meantime, split and add 1 x 105 MDCK or A549 cells in 800 &#181;L of the complete growth medium, mix with 200 &#181;L of siRNA-transfection reagent complex (step 2.5), and add 1 mL of the suspension to a well of a 12-well culture plate. This dilution brings the final concentration of control siRNA or caspase siRNA to 10 nM.</li>
	<li>Incubate the cells at 37 &#176;C under a 5% CO2 atmosphere for 72 h.</li>
	<li>Harvest the cells from one replicate as in step 1.10, and process them to validate or confirm the knockdown of the expression of caspase 3, caspase 6, and caspase 7 either by RT-qPCR or western blotting, respectively, using standard methods and protocols8,11.</li>
	<li>Infect the cells in the other replicate with IAV as described in steps 1.4-1.9 (without inhibitors).</li>
	<li>After 24 h, harvest, process, and analyze the cells by western blotting and measure and quantify the protein recovery as described in steps 1.10-1.20.</li>
</ol>
3. Site-directed mutagenesis for locating the caspase cleavage site(s) in the polypeptide
<ol>
	<li>Clone the gene-encoding protein of interest in a mammalian expression plasmid11 or obtain it from a research lab or commercial source (see Table of Materials). 
	NOTE: It is preferred if the gene is cloned with an epitope tag or as GFP fusion to distinguish the ectopically expressed polypeptide from the endogenously expressed one during western blotting.</li>
	<li>Using caspase substrate databases4 or protein alignment tools, locate the consensus caspase cleavage motifs, XXXD and DXXD, on the polypeptide. Almost all caspase cleavage motifs possess the aspartic acid at the cleavage site (P1)3,4.</li>
	<li>Mutate the putative P1 aspartic acid to glutamic acid or a non-polar amino acid (alanine, valine) by site-directed mutagenesis followed by DNA sequencing11 using standard methods and protocols.</li>
	<li>Deliver the wild-type (WT) and mutant plasmid DNA to MDCK or A549 cells by reverse transfection11.
	<ol>
		<li>For this, dilute 2 &#181;g of plasmid DNA or a recommended volume of the transfection reagent in 100 &#181;L of an appropriate medium (see step 2.3 and Table of Materials) in separate tubes, and incubate for 5 min at room temperature.</li>
		<li>Mix 100 &#181;L of each plasmid DNA solution with 100 &#181;L of the transfection reagent solution and incubate for 20-45 min at room temperature to form the DNA-transfection reagent complex.</li>
	</ol>
	</li>
	<li>In the meantime, split and add 3 x 105 MDCK or A549 cells to 800 &#181;L of the complete growth medium, mix with 200 &#181;L of DNA-transfection reagent complex, and add the 1 mL suspension to a well of a 12-well culture plate.</li>
	<li>Incubate the cells at 37 &#176;C under a 5% CO2 atmosphere.</li>
	<li>After 48 h, infect the cells with IAV as described in steps 1.4-1.9 (without adding the inhibitors).</li>
	<li>After 24 h, harvest, process, and analyze the cells by western blotting (if applicable, using the antibody to an epitope tag or GFP). Then, measure and quantify the protein recovery as described in steps 1.10-1.20.</li>
</ol>

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Y., Husain, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Caspase-mediated+cleavage+of+human+cortactin+during+influenza+A+virus+infection+occurs+in+its+actin-binding+domains+and+is+associated+with+released+virus+titres.">Caspase-mediated cleavage of human cortactin during influenza A virus infection occurs in its actin-binding domains and is associated with released virus titres.</a> Viruses. 12 (1), 87 (2020).</li><li>Zhirnov, O. P., Syrtzev, V. V. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Influenza+virus+pathogenicity+is+determined+by+caspase+cleavage+motifs+located+in+the+viral+proteins.">Influenza virus pathogenicity is determined by caspase cleavage motifs located in the viral proteins.</a> Journal of Molecular and Genetic Medicine. 3 (1), 124-132 (2009).</li><li>Zhirnov, O. P., Klenk, H. -. D. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Alterations+in+caspase+cleavage+motifs+of+NP+and+M2+proteins+attenuate+virulence+of+a+highly+pathogenic+avian+influenza+virus.">Alterations in caspase cleavage motifs of NP and M2 proteins attenuate virulence of a highly pathogenic avian influenza virus.</a> Virology. 394 (1), 57-63 (2009).</li><li>Zhirnov, O. P., Konakova, T. E., Garten, W., Klenk, H. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Caspase-dependent+N-terminal+cleavage+of+influenza+virus+nucleocapsid+protein+in+infected+cells.">Caspase-dependent N-terminal cleavage of influenza virus nucleocapsid protein in infected cells.</a> Journal of Virology. 73 (12), 10158-10163 (1999).</li><li>Robinson, B. A., Van Winkle, J. A., McCune, B. T., Peters, A. M., Nice, T. 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(97), e52421 (2015).</li><li>L&#252;thi, A. U., Martin, S. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+CASBAH:+A+searchable+database+of+caspase+substrates.">The CASBAH: A searchable database of caspase substrates.</a> Cell Death &amp; Differentiation. 14 (4), 641-650 (2007).</li><li>Kumar, S., van Raam, B. J., Salvesen, G. S., Cieplak, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Caspase+cleavage+sites+in+the+human+proteome:+CaspDB,+a+database+of+predicted+substrates.">Caspase cleavage sites in the human proteome: CaspDB, a database of predicted substrates.</a> PLoS One. 9 (10), 110539 (2014).</li><li>Igarashi, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=CutDB:+A+proteolytic+event+database.">CutDB: A proteolytic event database.</a> Nucleic Acids Research. 35 (Database issue). 35, 546-549 (2007).</li><li>Crawford, E. D., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=The+DegraBase:+A+database+of+proteolysis+in+healthy+and+apoptotic+human+cells.">The DegraBase: A database of proteolysis in healthy and apoptotic human cells.</a> Molecular &amp; Cellular Proteomics. 12 (3), 813-824 (2013).</li><li>Rawlings, N. D., Tolle, D. P., Barrett, A. J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=MEROPS:+The+peptidase+database.">MEROPS: The peptidase database.</a> Nucleic Acids Research. 32, 160-164 (2004).</li><li>Lange, P. F., Overall, C. 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The author has no conflicts of interest to disclose.

It is established that viruses tailor the host factors and pathways to their benefit. In turn, the host cells resist that by employing various strategies. One of those strategies is PANoptosis, which host cells use as an antiviral strategy against virus infections. However, viruses like IAV have evolved their own strategies to counter PANoptosis and exploit it to their advantage1,3,6. This interplay involves the cleavage of various host and viral proteins by caspases. Identifying those caspases and their cleavage sites in substrate proteins are important to elucidate the mechanisms and significance of such interplay.
To this end, standard biochemical and molecular methods were used to identify the caspases and their cleavage sites in host cortactin, HDAC4, and HDAC67,8,9,11. The protocols described above can be used for such studies involving influenza viruses, other mammalian viruses, other microbes, and external stimuli like toxins and chemicals. Furthermore, these methods can be replicated in a standard molecular biology laboratory with pertinent skills and do not require specialized equipment and software training. The 12-well cell culture plate format was successfully adapted for these and similar studies to generate sufficient sample amounts for downstream analyses by western blotting and RT-qPCR. Nevertheless, as needed, this format can be downscaled or upscaled accordingly. When doing an experiment involving a caspase inhibitor for the first time, using a pan-caspase inhibitor (Z-VAD-FMK) alongside a lysosome and proteasome inhibitor is suggested to assess whether the protein of interest undergoes caspase-mediated degradation. Further, in addition to NH4Cl and MG132, other lysosome and proteasome inhibitors like Bafilomycin A and Epoxomicin, respectively, can be used. After positive results with a pan-caspase inhibitor, either specific inhibitors of individual caspases can be used, or RNA interference can be employed. For the former, it is important to optimize the effective inhibitory concentrations to avoid toxicity to the target cell type and non-specific results.
A genetic tool like RNA interference or CRISPR must be employed next to confirm the involvement of caspases because inhibitors do cross-react. Alternatively, an RNA interference or CRISPR screen individually targeting all known caspases can be carried out. The siRNA, gRNA, and RT-qPCR primer pairs to all known caspases could be procured as pre-designed or designed using online tools. In addition, two or more caspases can be depleted simultaneously to assess the sequential or cooperative cleavage of polypeptides (this strategy was recently used in a yet-to-be-published study). RNA interference is believed to be an ideal tool for virus-host cell interaction research. The latter requires a controlled manipulation of host gene expression so that the host cells remain relatively viable for virus infection and completion of the life cycle to display the phenotype of that manipulation. However, optimizing an effective siRNA concentration and siRNA-transfection reagent combination is needed for each cell type for an optimal knockdown and viability ratio for subsequent infection.
To locate the putative caspase cleavage motifs, many databases such as CASBAH18, CaspDB19, CutDB20, DegraBase21, MEROPS22, and TopFIND23,24 have been developed. A visual screening was performed to locate these motifs on HDAC6 polypetide9, but for cortactin11, the CaspDB was used (though the CaspDB website has been inaccessible lately). Those motifs were successfully confirmed as caspase cleavage sites by mutating the P1 aspartic acid to glutamic acid. However, mutating the aspartic acid to a non-polar amino acid-like alanine or valine first is suggested because some caspases can cleave after P1 glutamic acid4. This exercise may promptly result in the identification of caspase cleavage site(s) but could require generating tens of mutants if the target polypeptide is rich in aspartic acids and the putative target motifs are non-canonical. For the delivery of plasmids (and siRNA) to cells, forward, instead of reverse, transfection can be carried out. Further, the infection dose and time kinetics should be performed using a 4%-20% gradient SDS-PAGE for western blotting to assess the degradation kinetics of the polypeptides and identify any smaller-sized cleavage products. Finally, to confirm the protein of interest as a direct caspase substrate, in vitro biochemical assays containing purified caspases and target proteins (WT or mutant) and any cofactors could be developed.

Influenza A virus (IAV) is the prototypic member of the Orthomyxoviridae family and is known to cause global epidemics and unpredictable pandemics. IAV causes human respiratory disease, influenza, commonly known as &#34;flu&#34;. The flu is an acute disease that results in the induction of host pro- and anti-inflammatory innate immune responses and the death of epithelial cells in the human respiratory tract. Both processes are governed by a phenomenon called programmed cell death1. The signaling for programmed cell death is induced as soon as various pathogen recognition receptors sense the incoming virus particles in host cells. This leads to the programming of the death of infected cells and signaling to the neighboring healthy cells by three interconnected pathways called pyroptosis, apoptosis, and necroptosis-recently coined as one process, PANoptosis1.
PANoptosis involves the proteolytic processing of many host and viral proteins from induction to execution. Such processing of proteins is primarily spearheaded by a family of cysteine proteases called caspases1,2. Up to 18 caspases (from caspase 1 to caspase 18) are known3. Most caspases are expressed as pro-caspases and activated by undergoing their own proteolytic processing either by autocatalysis or other caspases4 in response to a stimulus like a virus infection. The PANoptosis of IAV-infected cells was thought to be a host defense mechanism, but IAV has evolved ways to evade and exploit it to facilitate its replication1,2,5,6. One of them is to antagonize the host factors via caspase-mediated cleavage or degradation that are either inherently antiviral or interfere with one of the steps of the IAV life cycle. To this end, host factors, cortactin, HDAC4, and HDAC6 have been discovered to undergo caspase-mediated cleavage or degradation in IAV-infected epithelial cells7,8,9. The HDAC4 and HDAC6 are anti-IAV factors8,10, and cortactin interferes with IAV replication at a later stage of infection, potentially during viral assembly and budding11.
In addition, various caspases are also activated, which, in turn, cleave multiple proteins to activate the host inflammatory response during IAV infection1,2. Furthermore, nucleoprotein (NP), ion-channel M2 protein of IAV12,13,14, and various proteins of other viruses3,15,16 also undergo caspase-mediated cleavage during infection, which influences viral pathogenesis. Therefore, there is a continuous need to study caspase-mediated cleavage or degradation of host and viral proteins during IAV and other virus infections to understand the molecular basis of viral pathogenesis. Herein, the methods are presented to (1) assess the cleavage or degradation of such proteins by caspases, (2) identify those caspases, and (3) locate the cleavage sites.

<table><tbody><tr><td>A549 cells</td><td>ATCC</td><td>CRM-CCL-185</td><td>Human, epithelial, lung</td></tr><tr><td>Ammonium chloride</td><td>Sigma-Aldrich</td><td>A9434</td><td></td></tr><tr><td>Caspase 3 Inhibitor</td><td>Sigma-Aldrich</td><td>264156-M</td><td>Also known as &#039;InSolution Caspase-3 Inhibitor II - Calbiochem&#039;</td></tr><tr><td>cOmplete, Mini Protease Inhibitor Cocktail</td><td>Roche</td><td>11836153001</td><td></td></tr><tr><td>Goat anti-NP antibody</td><td></td><td></td><td>Gift from Richard Webby (St Jude Children&amp;rsquo;s Research Hospital, Memphis, USA) to MH</td></tr><tr><td>Lipofectamine 2000 Transfection Reagent</td><td>ThermoFisher Scientific</td><td>31985062</td><td></td></tr><tr><td>Lipofectamine RNAiMAX Transfection Reagent</td><td>ThermoFisher Scientific</td><td>13778150</td><td></td></tr><tr><td>MDCK cells</td><td>ATCC</td><td>CCL-34</td><td>Dog, epithelial, kidney</td></tr><tr><td>MG132</td><td>Sigma-Aldrich</td><td>M7449</td><td></td></tr><tr><td>Minimum Essential Medium (MEM)</td><td>ThermoFisher Scientific</td><td>11095080</td><td>Add L-glutamine, antibiotics or other supplements as required</td></tr><tr><td>MISSION siRNA Universal Negative Control #1</td><td>Sigma-Aldrich</td><td>SIC001</td><td></td></tr><tr><td>Odyssey Fc imager with Image Studio Lite software 5.2&amp;nbsp;</td><td>LI-COR</td><td></td><td>Odyssey Fc has been replaced with Odyssey XF and Image Studio Lite software has been replaced with Empiria Studio software.</td></tr><tr><td>Pierce BCA Protein Assay Kit</td><td>ThermoFisher Scientific</td><td>23225</td><td></td></tr><tr><td>Plasmid expressing human cortactin-GFP fusion&amp;nbsp;</td><td>Addgene</td><td>50728</td><td>Gift from Kenneth Yamada to Addgene</td></tr><tr><td>Pre-designed small interferring RNA (siRNA) to caspase 3</td><td>Sigma-Aldrich</td><td>NM_004346</td><td>siRNA ID: SASI_Hs01_00139105</td></tr><tr><td>Pre-designed small interferring RNA to caspase 6</td><td>Sigma-Aldrich</td><td>NM_001226</td><td>siRNA ID: SASI_Hs01_00019062</td></tr><tr><td>Pre-designed small interferring RNA to caspase 7</td><td>Sigma-Aldrich</td><td>NM_001227</td><td>siRNA ID: SASI_Hs01_00128361</td></tr><tr><td>Pre-designed SYBR Green RT-qPCR Primer pairs</td><td>Sigma-Aldrich</td><td>KSPQ12012</td><td>Primer Pair IDs: H_CASP3_1; H_CASP6_1; H_CASP7_1</td></tr><tr><td>Protran Premium nitrocellulose membrane</td><td>Cytiva (Fomerly GE Healthcare)</td><td>10600003</td><td></td></tr><tr><td>Rabbit anti-actin antibody</td><td>Abcam</td><td>ab8227</td><td></td></tr><tr><td>Rabbit anti-cortactin antibody</td><td>Cell Signaling</td><td>3502</td><td></td></tr><tr><td>Rabbit anti-GFP antibody</td><td>Takara</td><td>632592</td><td></td></tr><tr><td>SeeBlue Pre-stained Protein Standard</td><td>ThermoFisher Scientific</td><td>LC5625</td><td></td></tr><tr><td>Transfection medium, Opti-MEM</td><td>ThermoFisher Scientific</td><td>11668019</td><td></td></tr><tr><td>Tris-HCl, NaCl, SDS, Sodium Deoxycholate, Triton X-100</td><td>Merck</td><td></td><td></td></tr><tr><td>Trypsin, TPCK-Treated</td><td>Sigma-Aldrich</td><td>4370285</td><td></td></tr></tbody></table>

identifying caspases and their motifs that cleave proteins during influenza a virus infection

Caspases, a family of cysteine proteases, orchestrate programmed cell death in response to various stimuli, including microbial infections. Initially described to occur by apoptosis, programmed cell death is now known to encompass three interconnected pathways: pyroptosis, apoptosis, and necroptosis, together coined as one process, PANoptosis. Influence A virus (IAV) infection induces PANoptosis in mammalian cells by inducing the activation of different caspases, which, in turn, cleave various host as well as viral proteins, leading to processes like the activation of the host innate antiviral response or the degradation of antagonistic host proteins. In this regard, caspase 3-mediated cleavage of host cortactin, histone deacetylase 4 (HDAC4), and histone deacetylase 6 (HDAC6) has been discovered in both animal and human epithelial cells in response to the IAV infection. To demonstrate this, inhibitors, RNA interference, and site-directed mutagenesis were employed, and, subsequently, the cleavage or resistance to cleavage and the recovery of cortactin, HDAC4, and HDAC6 polypeptides were measured by western blotting. These methods, in conjunction with RT-qPCR, form a simple yet effective strategy to identify the host as well as viral proteins undergoing caspase-mediated cleavage during an infection of IAV or other human and animal viruses. The present protocol elaborates the representative results of this strategy, and the ways to make it more effective are also discussed.

Treatment with caspase 3 inhibitor 
It has been discovered that host cortactin, HDAC4, and HDAC6 polypeptides undergo degradation in response to IAV infection in both canine (MDCK) and human (A549, NHBE) cells7,8,9. By using the above approaches, it was uncovered that IAV-induced host caspases, particularly caspase 3, cause their degradation7,8,9. The cortactin was degraded in an infection dose- and time-dependent manner and a host cell- and IAV strain-independent manner7. Similar results were obtained for HDAC48 and HDAC69. Interestingly, the degradation of all three host proteins coincided with the cleavage of viral NP7,8,9, which is known to undergo caspase-mediated cleavage14. Thus, it was hypothesized that cortactin, HDAC4, and HDAC6 also undergo caspase-mediated cleavage and subsequent degradation. To test this, first, the IAV-infected cells were treated with a caspase 3 inhibitor, and the rescue of cortactin, HDAC4, and HDAC6 polypeptides was analyzed by western blotting. All three polypeptides (as well as viral NP) recovered from the IAV infection-induced degradation after caspase 3 inhibitor treatment7,8,9. The representative data7 for cortactin are shown in Figure 1A. This recovery was quantified by measuring the intensity of protein bands and normalizing the value of the cortactin band by the value of the corresponding actin (loading control) band. Subsequently, the normalized value of cortactin in the corresponding uninfected samples was considered 100% to determine its value in the infected samples. This quantitation method accounted the recovery of cortactin polypeptide in caspase 3 inhibitor-treated infected cells as 78.8% from 20.7% in untreated infected cells7 (Figure 1B).
RNA interference-mediated knockdown of caspase 3 expression 
Next, a genetic tool, RNA interference (RNAi), was employed, followed by western blotting to confirm the role of caspases in IAV infection-induced degradation of cortactin11 and HDAC48. In caspase 3-depleted cells (Figure 2C)11, both cortactin (Figure 2A)11 and HDAC48 polypeptides recovered from the IAV infection-induced degradation. However, in caspase 6- or caspase 7-depleted cells (Figure 2C)11, no significant recovery in cortactin polypeptide levels was observed (Figure 2A)11. Specifically, when quantified and compared to their levels in corresponding uninfected cells as mentioned above, the cortactin polypeptide level in infected cells with depleted caspase 3 expression recovered to 83% from 28.6% in infected cells with normal caspase 3 expression (Figure 2B)11.
Caspase cleavage motifs in cortactin and HDAC6 polypeptides 
Finally, the amino acid sequence in cortactin and HDAC6 polypeptides were screened and, in putative caspase cleavage motifs, the aspartic acid at the P1 position was mutated to glutamic acid9,11. This approach identified the aspartic acid 116 in cortactin polypeptide11 and the aspartic acid 1088 in HDAC6 polypeptide9 as a caspase cleavage site. The mutation of both aspartic acid residues to glutamic acid rendered the cortactin (Figure 3A)11 and HDAC69 polypeptides resistant to IAV infection-induced degradation. Specifically, when quantified and compared to their levels in corresponding uninfected cells as mentioned above, the level of plasmid-expressed mutant cortactin polypeptide in infected cells was 77.9%, whereas the level of plasmid-expressed wild-type cortactin polypeptide in infected cells was 23.6% (Figure 3B)11.
<img alt="Figure 1" class="xfigimg" src="/files/ftp_upload/64189/64189fig01.jpg" /> 
Figure 1: Cortactin polypeptide undergoes caspase 3-mediated degradation in IAV-infected cells. (A) MDCK cells were infected with influenza virus A/New Caledonia/20/1999/H1N1 strain at an MOI of 0.5 and subsequently treated as mock or with caspase 3 inhibitor (Cas3-I, 40 &#181;M). After 24 h, the cortactin, viral NP, and actin polypeptides were detected in total cell lysates by western blotting. UNI, uninfected; INF, infected. Arrows point to the full-length and cleaved NP. (B) The intensity of cortactin and actin bands was quantified. Then, the value of cortactin was normalized with the value of actin. The normalized amount of cortactin in UNI samples was considered 100% to determine its amount in the corresponding INF samples. Data presented are means &#177; SE of three biological replicates. ***P = 0.0006, calculated using two-way ANOVA. ns, not significant. This figure has been modified from Chen et al.7. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/64189/64189fig01large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" class="xfigimg" src="/files/ftp_upload/64189/64189fig02.jpg" /> 
Figure 2: Knockdown of caspase 3 expression rescued cortactin polypeptide degradation in IAV-infected cells. (A) A549 cells were transfected in duplicates with 10 nM of control (Ctrl), caspase 3 (Cas3), caspase 6 (Cas6), or caspase 7 (Cas7) siRNA. After 72 h, the cells in one replicate were harvested and processed to extract total RNA. The cells in the other replicate were infected with influenza virus A/WSN/1933/H1N1 subtype at an MOI of 3.0. After 24 h, the cortactin, viral NP, and actin polypeptides were detected by western blotting. UNI, uninfected; INF, infected. (B) The intensity of cortactin and actin bands was quantified, and the value of cortactin was normalized with the value of actin. Then, the normalized amount of cortactin in the UNI samples was considered 100% to determine its amount in the corresponding INF samples. Data presented are means &#177; SE of three biological replicates. ****P &#60; 0.0001, ***P = 0.0007, ***P = 0.0002, calculated using two-way ANOVA. ns, not significant. (C) Total RNA extracted above was converted to cDNA, which was then used as a template to detect caspase 3, caspase 6, caspase 7, and actin mRNA levels by RT-qPCR. The actin mRNA level was used as a reference, and the mRNA level of each caspase in control siRNA (Ctrl) and respective caspase siRNA (Cas) transfected cells was calculated using the standard 2&#8722;&#916;&#916;CT method. ****P &#60; 0.0001, calculated using two-way ANOVA. This figure has been modified from Chen et al.11. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/64189/64189fig02large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" class="xfigimg" src="/files/ftp_upload/64189/64189fig03.jpg" /> 
Figure 3: Mutation of aspartic acid 116 to glutamic acid rendered the cortactin polypeptide resistant to IAV infection-induced degradation. (A) MDCK cells were transfected with a plasmid expressing the wild-type (WT) or mutated (D116E) GFP-cortactin fusion. After 48 h, cells were infected with influenza virus A/WSN/1933/H1N1 subtype at an MOI of 3.0. After 24 h, the GFP-cortactin, viral NP, and actin polypeptides were detected by western blotting. UNI, uninfected; INF, infected. (B) The intensity of cortactin and actin bands was quantified, and the value of cortactin was normalized with the value of actin. Then, the normalized amount of cortactin in UNI samples was considered 100% to determine its amount in the corresponding INF samples. Data presented are means &#177; SE of three biological replicates. **P = 0.001, calculated using two-way ANOVA. ns, not significant. This figure has been modified from Chen et al.11. <a href="https://www-jove-com.bdigitaluss.remotexs.co/files/ftp_upload/64189/64189fig03large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Identifying Caspases and their Motifs that Cleave Proteins During Influenza A Virus Infection at JoVE.com

Identifying Caspases and their Motifs that Cleave Proteins During Influenza A Virus Infection

Influenza A virus (IAV) infection activates the caspases that cleave host and viral proteins, which, in turn, have pro- and antiviral functions. By employing inhibitors, RNA interference, site-directed mutagenesis, and western blotting and RT-qPCR techniques, caspases in infected mammalian cells that cleave host cortactin and histone deacetylases were identified.

Caspases, a family of cysteine proteases, orchestrate programmed cell death in response to various stimuli, including microbial infections. Initially ...

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Research

JoVE Journal

Immunology and Infection

1.4K Views.  University of Otago. Influenza A virus (IAV) infection activates the caspases that cleave host and viral proteins, which, in turn, have pro- and antiviral functions. By employing inhibitors, RNA interference, site-directed mutagenesis, and western blotting and RT-qPCR techniques, caspases in infected mammalian cells that cleave host cortactin and histone deacetylases were identified.

Video: Identifying Caspases and their Motifs that Cleave Proteins During Influenza A Virus Infection

Lighting Up the Pathways to Caspase Activation Using Bimolecular Fluorescence Complementation

The caspase family of proteases play essential roles in apoptosis and innate immunity. Among these, a subgroup known as initiator caspases are the first to be activated in these pathways. This group includes caspase-2, -8, and -9, as well as the inflammatory caspases, caspase-1, -4, and -5. The initiator caspases are all activated by dimerization following recruitment to specific multiprotein complexes called activation platforms. Caspase Bimolecular Fluorescence Complementation (BiFC) is an imaging-based approach where split fluorescent proteins fused to initiator caspases are used to visualize the recruitment of initiator caspases to their activation platforms and the resulting induced proximity. This fluorescence provides a readout of one of the earliest steps required for initiator caspase activation. Using a number of different microscopy-based approaches, this technique can provide quantitative data on the efficiency of caspase activation on a population level as well as the kinetics of caspase activation and the size and number of caspase activating complexes on a per cell basis.

This protocol describes caspase Bimolecular Fluorescence Complementation (BiFC); an imaging-based method that can be used to visualize induced proximity of initiator caspases, which is the first step in their activation.

The caspase family of proteases play essential roles in apoptosis and innate immunity. Among these, a subgroup known as initiator caspases are the first ...

Biochemistry

Affinity Purification of Influenza Virus Ribonucleoprotein Complexes from the Chromatin of Infected Cells

Like all negative-strand RNA viruses, the genome of influenza viruses is packaged in the form of viral ribonucleoprotein complexes (vRNP), in which the single-stranded genome is encapsidated by the nucleoprotein (NP), and associated with the trimeric polymerase complex consisting of the PA, PB1, and PB2 subunits. However, in contrast to most RNA viruses, influenza viruses perform viral RNA synthesis in the nuclei of infected cells. Interestingly, viral mRNA synthesis uses cellular pre-mRNAs as primers, and it has been proposed that this process takes place on chromatin1. Interactions between the viral polymerase and the host RNA polymerase II, as well as between NP and host nucleosomes have also been characterized1,2.
Recently, the generation of recombinant influenza viruses encoding a One-Strep-Tag genetically fused to the C-terminus of the PB2 subunit of the viral polymerase (rWSN-PB2-Strep3) has been described. These recombinant viruses allow the purification of PB2-containing complexes, including vRNPs, from infected cells. To obtain purified vRNPs, cell cultures are infected, and vRNPs are affinity purified from lysates derived from these cells. However, the lysis procedures used to date have been based on one-step detergent lysis, which, despite the presence of a general nuclease, often extract chromatin-bound material only inefficiently. 
Our preliminary work suggested that a large portion of nuclear vRNPs were not extracted during traditional cell lysis, and therefore could not be affinity purified. To increase this extraction efficiency, and to separate chromatin-bound from non-chromatin-bound nuclear vRNPs, we adapted a step-wise subcellular extraction protocol to influenza virus-infected cells. Briefly, this procedure first separates the nuclei from the cell and then extracts soluble nuclear proteins (here termed the "nucleoplasmic" fraction). The remaining insoluble nuclear material is then digested with Benzonase, an unspecific DNA/RNA nuclease, followed by two salt extraction steps: first using 150 mM NaCl (termed "ch150"), then 500 mM NaCl ("ch500") (Fig. 1). These salt extraction steps were chosen based on our observation that 500 mM NaCl was sufficient to solubilize over 85% of nuclear vRNPs yet still allow binding of tagged vRNPs to the affinity matrix.
After subcellular fractionation of infected cells, it is possible to affinity purify PB2-tagged vRNPs from each individual fraction and analyze their protein and RNA components using Western Blot and primer extension, respectively. Recently, we utilized this method to discover that vRNP export complexes form during late points after infection on the chromatin fraction extracted with 500 mM NaCl (ch500)3.

Influenza viruses replicate their RNA genome in association with host-cell chromatin. Here, we present a method to purify intact viral ribonucleoprotein complexes from the chromatin of infected cells. Purified viral complexes can be analyzed by both Western blot and primer extension of protein and RNA content, respectively.

Like all negative-strand RNA viruses, the genome of influenza viruses is packaged in the form of viral ribonucleoprotein complexes (vRNP), in which the ...

A Fluorogenic Peptide Cleavage Assay to Screen for Proteolytic Activity: Applications for coronavirus spike protein activation

Enveloped viruses such as coronaviruses or influenza virus require proteolytic cleavage of their fusion protein to be able to infect the host cell. Often viruses exhibit cell and tissue tropism and are adapted to specific cell or tissue proteases. Moreover, these viruses can introduce mutations or insertions into their genome during replication that may affect the cleavage, and thus can contribute to adaptations to a new host. Here, we present a fluorogenic peptide cleavage assay that allows a rapid screening of peptides mimicking the cleavage site of viral fusion proteins. The technique is very flexible and can be used to investigate the proteolytic activity of a single protease on many different substrates, and in addition, it also allows exploration of the activity of multiple proteases on one or more peptide substrates. In this study, we used peptides mimicking the cleavage site motifs of the coronavirus spike protein. We tested human and camel derived Middle East Respiratory Syndrome coronaviruses (MERS-CoV) to demonstrate that single and double substitutions in the cleavage site can alter the activity of furin and dramatically change cleavage efficiency. We also used this method in combination with bioinformatics to test furin cleavage activity of feline coronavirus spike proteins from different serotypes and strains. This peptide-based method is less labor- and time intensive than conventional methods used for the analysis of proteolytic activity for viruses, and results can be obtained within a single day.

We present a fluorogenic peptide cleavage assay that allows a rapid screening of the proteolytic activity of proteases on peptides representing the cleavage site of viral fusion peptides. This method can also be used on any other amino acid motif within a protein sequence to test for the protease activity.

Enveloped viruses such as coronaviruses or influenza virus require proteolytic cleavage of their fusion protein to be able to infect the host cell. Often ...

Activation and Measurement of NLRP3 Inflammasome Activity Using IL-1&#946; in Human Monocyte-derived Dendritic Cells

Inflammatory processes resulting from the secretion of Interleukin (IL)-1 family cytokines by immune cells lead to local or systemic inflammation, tissue remodeling and repair, and virologic control1,2 . Interleukin-1&#946; is an essential element of the innate immune response and contributes to eliminate invading pathogens while preventing the establishment of persistent infection1-5.
Inflammasomes are the key signaling platform for the activation of interleukin 1 converting enzyme (ICE or Caspase-1). The NLRP3 inflammasome requires at least two signals in DCs to cause IL-1&#946; secretion6. Pro-IL-1&#946; protein expression is limited in resting cells; therefore a priming signal is required for IL-1&#946; transcription and protein expression. A second signal sensed by NLRP3 results in the formation of the multi-protein NLRP3 inflammasome. The ability of dendritic cells to respond to the signals required for IL-1&#946; secretion can be tested using a synthetic purine, R848, which is sensed by TLR8 in human monocyte derived dendritic cells&#160;(moDCs) to prime cells, followed by activation of the NLRP3 inflammasome with the bacterial toxin and potassium ionophore, nigericin.
Monocyte derived DCs&#160;are easily produced in culture and provide significantly more cells than purified human myeloid DCs. The method presented here differs from other inflammasome assays in that it uses in vitro human, instead of mouse derived, DCs thus allowing for the study of the inflammasome in human disease and infection.

Dendritic cells (DCs)&#160;secrete IL-1&#946; in response to TLR8 recognition of synthetic purine, R848, followed by NLRP3 inflammasome activation with nigericin, therefore, IL-1&#946; can be used to measure NLRP3 inflammasome activity. Intracellular cytokine staining, immunoblotting, and ELISA are used to accurately measure NLRP3 inflammasome priming and activation via IL-1&#946; expression.

Inflammatory processes resulting from the secretion of Interleukin (IL)-1 family cytokines by immune cells lead to local or systemic inflammation, tissue ...

In Vitro Disassembly of Influenza A Virus Capsids by Gradient Centrifugation

Acid-triggered molecular processes closely control cell entry of many viruses that enter through the endocytic system. In the case of influenza A virus (IAV), virus fusion with the endosomal membrane as well as the subsequent disassembly of the viral capsid, called uncoating, is governed by the ionic conditions inside endocytic vesicles. The early steps in the virus life cycle are hard to study because endosomes cannot be directly accessed experimentally, creating the need for an in vitro approach. Here, we describe a method based on velocity gradient centrifugation of purified virions through a two-layer glycerol gradient, which enables analysis of the IAV core and its stability. The gradient contains a non-ionic detergent (NP-40) in its lower layer to remove the viral membrane by solubilization as the virus sediments toward the bottom. At neutral pH, viral cores are pelleted as stable structures. The major core components, matrix protein (M1) and the viral ribonucleoproteins (vRNPs), can be clearly identified in the pellet fraction by SDS-PAGE. Decreasing the pH to 6.0 or lower in the bottom layer selectively removes M1 from the pellet followed by release of vRNPs at more acidic conditions. Viral protein bands on Coomassie-stained gels can be subjected to densitometric quantification to monitor intermediate states of IAV disassembly. Besides pH, other factors that influence viral core stability can be assessed, such as salt concentration and putative viral uncoating factors, simply by modifying the detergent-containing glycerol layer accordingly. Taken together, the presented technique allows highly reproducible and quantitative analysis of viral uncoating in vitro. It can be applied to other enveloped viruses that undergo complex uncoating processes.

In Vitro Disassembly of Influenza A Virus Capsids by Gradient Centrifugation

Disassembly of influenza A virus cores during virus entry into host cells is a multistep process. We describe an in vitro method to analyze the early stages of viral uncoating. In this approach, velocity gradient centrifugation is used to biochemically dissect the steps that initiate uncoating under defined conditions.

Acid-triggered molecular processes closely control cell entry of many viruses that enter through the endocytic system. In the case of influenza A virus ...

Measuring Caspase Activity Using a Fluorometric Assay or Flow Cytometry

The activation of cysteine proteases, known as caspases, remains an important process in multiple forms of cell death. Caspases are critical initiators and executioners of apoptosis, the most studied form of programmed cell death. Apoptosis occurs during developmental processes and is a necessary event in tissue homeostasis. Pyroptosis is another form of cell death that utilizes caspases and is a critical process in activating the immune system through the activation of the inflammasome, which results in the release of members of the interleukin-1 (IL-1) family. To assess caspase activity, target substrates can be assessed. However, sensitivity can be an issue when examining single cells or low-level activity. We demonstrate how a fluorogenic substrate can be used with a population-based assay or single-cell assay by flow cytometry. With proper controls, different amino acid sequences can be used to identify which caspases are active. Using these assays, the simultaneous loss of the inhibitors of apoptosis proteins upon tumor necrosis factor (TNF) stimulation has been identified, which primarily induces apoptosis in macrophages rather than other forms of cell death.

The present protocol describes two methods to measure caspase activity through a fluorogenic substrate using flow cytometry or a spectrofluorometer.

The activation of cysteine proteases, known as caspases, remains an important process in multiple forms of cell death. Caspases are critical initiators ...

Analysis of Group IV Viral SSHHPS Using In Vitro and In Silico Methods

Alphaviral enzymes are synthesized in a single polypeptide. The nonstructural polyprotein (nsP) is processed by its nsP2 cysteine protease to produce active enzymes essential for viral replication. Viral proteases are highly specific and recognize conserved cleavage site motif sequences (~6-8 amino acids). In several Group IV viruses, the nsP protease(s) cleavage site motif sequences can be found in specific host proteins involved in generating the innate immune responses and, in some cases, the targeted proteins appear to be linked to the virus-induced phenotype. These viruses utilize short stretches of homologous host-pathogen protein sequences (SSHHPS) for targeted destruction of host proteins. To identify SSHHPS the viral protease cleavage site motif sequences can be inputted into BLAST and the host genome(s) can be searched. Cleavage initially can be tested using the purified nsP viral protease and fluorescence resonance energy transfer (FRET) substrates made in E. coli. The FRET substrates contain cyan and yellow fluorescent protein and the cleavage site sequence (CFP-sequence-YFP). This protease assay can be used continuously in a plate reader or discontinuously in SDS-PAGE gels. Models of the bound peptide substrates can be generated in silico to guide substrate selection and mutagenesis studies. CFP/YFP substrates have also been utilized to identify protease inhibitors. These in vitro and in silico methods can be used in combination with cell-based assays to determine if the targeted host protein affects viral replication.

We present a general protocol for identifying short stretches of homologous host-pathogen protein sequences (SSHHPS) embedded in the viral polyprotein. SSHHPS are recognized by viral proteases and direct the targeted destruction of specific host proteins by several Group IV viruses.

Alphaviral enzymes are synthesized in a single polypeptide. The nonstructural polyprotein (nsP) is processed by its nsP2 cysteine protease to produce ...

Visualization of Inflammatory Caspases Induced Proximity in Human Monocyte-Derived Macrophages

Inflammatory caspases include caspase-1, -4, -5, -11, and -12 and belong to the subgroup of initiator caspases. Caspase-1 is required to ensure correct regulation of inflammatory signaling and is activated by proximity-induced dimerization following recruitment to inflammasomes. Caspase-1 is abundant in the monocytic cell lineage and induces maturation of the pro-inflammatory cytokines interleukin (IL)-1&#946; and IL-18 to active secreted molecules. The other inflammatory caspases, caspase-4 and -5 (and their murine homolog caspase-11) promote IL-1&#946; release by inducing pyroptosis. Caspase Bimolecular Fluorescence Complementation (BiFC) is a tool used to measure inflammatory caspase induced proximity as a readout of caspase activation. The caspase-1, -4, or -5 prodomain, which contains the region that binds to the inflammasome, is fused to non-fluorescent fragments of the yellow fluorescent protein Venus (Venus-N [VN] or Venus-C [VC]) that associate to reform the fluorescent Venus complex when the caspases undergo induced proximity. This protocol describes how to introduce these reporters into primary human monocyte-derived macrophages (MDM) using nucleofection, treat the cells to induce inflammatory caspase activation, and measure caspase activation using fluorescence and confocal microscopy. The advantage of this approach is that it can be used to identify the components, requirements, and localization of the inflammatory caspase activation complex in living cells. However, careful controls need to be considered to avoid compromising cell viability and behavior. This technique is a powerful tool for the analysis of dynamic caspase interactions at the inflammasome level as well as for the interrogation of the inflammatory signaling cascades in living MDM and monocytes derived from human blood samples.

This protocol describes the workflow to obtain monocytes-derived macrophages (MDM) from human blood samples, a simple method to efficiently introduce inflammatory caspase Bimolecular Fluorescence Complementation (BiFC) reporters into human MDM without compromising cell viability and behavior, and an imaging-based approach to measure inflammatory caspase activation in living cells.

Inflammatory caspases include caspase-1, -4, -5, -11, and -12 and belong to the subgroup of initiator caspases. Caspase-1 is required to ensure correct ...

Evaluation of Caspase Activation to Assess Innate Immune Cell Death

Innate immunity provides the critical first line of defense in response to pathogens and sterile insults. A key mechanistic component of this response is the initiation of innate immune programmed cell death (PCD) to eliminate infected or damaged cells and propagate immune responses. However, excess PCD is associated with inflammation and pathology. Therefore, understanding the activation and regulation of PCD is a central aspect of characterizing innate immune responses and identifying new therapeutic targets across the disease spectrum.
This protocol provides methods for characterizing innate immune PCD activation by monitoring caspases, a family of cysteine-dependent proteases that are often associated with diverse PCD pathways, including&#160;apoptosis,&#160;pyroptosis, necroptosis, and PANoptosis. Initial reports characterized caspase-2, caspase-8, caspase-9, and caspase-10 as initiator caspases and caspase-3, caspase-6, and caspase-7 as effector caspases in apoptosis, while later studies found the inflammatory caspases, caspase-1, caspase-4, caspase-5, and caspase-11, drive pyroptosis. It is now known that there is extensive crosstalk between the caspases and other innate immune and&#160;cell death molecules across the previously defined PCD pathways, identifying a key knowledge gap in the mechanistic understanding of innate immunity and PCD and&#160;leading to the characterization of PANoptosis. PANoptosis is a unique innate immune inflammatory PCD pathway regulated by PANoptosome complexes, which integrate components, including caspases,&#160;from other cell death pathways.
Here, methods for assessing the activation of caspases in response to various stimuli are provided. These methods allow for the characterization of PCD pathways both in vitro and in vivo, as activated caspases undergo proteolytic cleavage that can be visualized by western blotting using optimal antibodies and blotting conditions. A protocol and western blotting workflow have been established that allow for the assessment of the activation of multiple caspases from the same cellular population, providing a comprehensive characterization of the PCD processes. This method can be applied across research areas in development, homeostasis, infection, inflammation, and cancer to evaluate PCD pathways throughout cellular processes in health and disease.

This protocol describes a comprehensive method for assessing caspase activation (caspase-1, caspase-3, caspase-7, caspase-8, caspase-9, and caspase-11) in response to both in vitro and in vivo (in mice) models of infection, sterile insults, and cancer to determine the initiation of cell death pathways, such as pyroptosis, apoptosis, necroptosis, and PANoptosis.

Innate immunity provides the critical first line of defense in response to pathogens and sterile insults. A key mechanistic component of this response is ...

Assessment of the Activation of Multiple Caspases in Macrophages

Source: Han, J. et al., <a href="https://app-jove-com.bdigitaluss.remotexs.co/v/64308">Evaluation of Caspase Activation to Assess Innate Immune Cell Death.</a>&#160;J. Vis. Exp.&#160;(2023)
This video demonstrates the assessment of multiple caspase activations in bone marrow-derived macrophages through stimulation with the influenza virus. Following macrophage lysis, heat-inactivation of lysate, and centrifugation, intracellular caspases are collected for subsequent immunological analysis.

Identifying Caspases and their Motifs that Cleave Proteins During Influenza A Virus Infection

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