The relative contributions of apoptosis and necrosis in brain injury have been a matter of much debate. Caspase-3 has been identified as a key protease in the execution of apoptosis, whereas calpains have mainly been implicated in excitotoxic neuronal injury. In a model of unilateral hypoxia-ischemia in 7-day-old rats, caspase-3-like activity increased 16-fold 24 h postinsult, coinciding with cleavage of the caspase-3 proenzyme and endogenous caspase-3 substrates. This activation was significantly decreased by pharmacological calpain inhibition, using CX295, a calpain inhibitor that did not inhibit purified caspase-3 in vitro. Activation of caspase-3 by m-calpain, but not μ-calpain, was facilitated in a dose-dependent manner in vitroby incubating cytosolic fractions, containing caspase-3 proform, with calpains. This facilitation required the presence of some active caspase-3 and could be abolished by including the specific calpain inhibitor calpastatin. This indicates that initial cleavage of caspase-3 by m-calpain, producing a 29-kDa fragment, facilitates the subsequent cleavage into active forms. This is the first report to our knowledge suggesting a direct link between the early, excitotoxic, calcium-mediated activation of calpain after cerebral hypoxia-ischemia and the subsequent activation of caspase-3, thus representing a tentative pathway of “pathological apoptosis.” The relative contributions of apoptosis and necrosis in brain injury have been a matter of much debate. Caspase-3 has been identified as a key protease in the execution of apoptosis, whereas calpains have mainly been implicated in excitotoxic neuronal injury. In a model of unilateral hypoxia-ischemia in 7-day-old rats, caspase-3-like activity increased 16-fold 24 h postinsult, coinciding with cleavage of the caspase-3 proenzyme and endogenous caspase-3 substrates. This activation was significantly decreased by pharmacological calpain inhibition, using CX295, a calpain inhibitor that did not inhibit purified caspase-3 in vitro. Activation of caspase-3 by m-calpain, but not μ-calpain, was facilitated in a dose-dependent manner in vitroby incubating cytosolic fractions, containing caspase-3 proform, with calpains. This facilitation required the presence of some active caspase-3 and could be abolished by including the specific calpain inhibitor calpastatin. This indicates that initial cleavage of caspase-3 by m-calpain, producing a 29-kDa fragment, facilitates the subsequent cleavage into active forms. This is the first report to our knowledge suggesting a direct link between the early, excitotoxic, calcium-mediated activation of calpain after cerebral hypoxia-ischemia and the subsequent activation of caspase-3, thus representing a tentative pathway of “pathological apoptosis.” hypoxia-ischemia aminomethylcoumarin DNA fragmentation factor 45 α-fodrin breakdown product inhibitor of caspase-activated DNase poly(ADP-ribose) polymerase dithiothreitol 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone t-butoxycarbonyl-Asp-(OMe)-fluoromethyl ketone phosphate-buffered saline polymerase chain reaction glyceraldehyde-3-phosphate dehydrogenase Leu-Tyr-AMC The relative contributions of necrosis and apoptosis to the injury that develops after cerebral hypoxia-ischemia (HI)1 has been a matter of much debate (1Lee J.M. Zipfel G.J. Choi D.W. Nature. 1999; 399: 7-14Crossref PubMed Scopus (1004) Google Scholar). 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Kawashima S. Hagberg H. J. Biol. Chem. 1999; 274: 14046-14052Abstract Full Text Full Text PDF PubMed Scopus (101) Google Scholar), where the second phase closely followed the activation of caspase-3. Reports demonstrating degradation of calpastatin by caspase-3 (32Pörn-Ares M.I. Samali A. Orrenius S. Cell Death Differ. 1998; 5: 1028-1033Crossref PubMed Scopus (184) Google Scholar, 33Wang K.K. Posmantur R. Nadimpalli R. Nath R. Mohan P. Nixon R.A. Talanian R.V. Keegan M. Herzog L. Allen H. Arch. Biochem. Biophys. 1998; 356: 187-196Crossref PubMed Scopus (231) Google Scholar) prompted us to investigate further the spatial and temporal activation of these two proteases and possible interactions in this model. Unilateral HI was induced in 7-day-old Wistar F rats of both sexes (46Rice J. Vannucci R. Brierley J. Ann. Neurol. 1981; 9: 131-141Crossref PubMed Scopus (1935) Google Scholar, 47Hagberg H. Gilland E. Diemer N. Andine P. Biol. Neonate. 1994; 66: 205-213Crossref PubMed Scopus (148) Google Scholar). The pups were anesthetized with halothane (3.0β for induction and 1.0–1.5β for maintenance) in a mixture of nitrous oxide and oxygen (1:1), and the duration of anesthesia was <10 min. The left common carotid artery was cut between double ligatures of prolene sutures (6–0). After the surgical procedure, the wounds were infiltrated with a local anesthetic, and the pups were allowed to recover for 1–2 h. The litters were then placed in a chamber perfused with a humidified gas mixture (7.70 ± 0.01β oxygen in nitrogen) for 70 min. The temperature in the gas chamber was kept at 36 °C. Following hypoxic exposure, the pups were returned to their biological dams until sacrificed. Control animals were operated and ligated but not subjected to hypoxia. All animal experimentation was approved by the Ethical Committee of Göteborg (approval number 225-97). Three litters (n= 28) were treated with the calpain inhibitor CX295 (Z-Leu-aminobutyric acid-CONH(CH2)3-morpholine; Cortex Pharmaceuticals, Irvine, CA) or vehicle. The first dose, 200 μl of 5 mm CX295 in 100 mm NaCl (equivalent to ∼80 μmol/kg or 40 mg/kg body weight), was administered subcutaneously immediately after HI. Subsequently, animals were injected with 100 μl of the CX295 solution (equivalent to ∼40 μmol/kg or 20 mg/kg body weight) every 3 h for 24 h. Control animals were injected with 100 mm NaCl. The animals were sacrificed by decapitation, and the brains were rapidly dissected out on a bed of ice, weighed, quickly frozen in isopenthane and dry ice, and stored at −80 °C. Cortical tissue rostral to the hippocampus, ∼50 mg, was dissected out from each hemisphere at −10 °C. The tissue was homogenized by sonication in 10 volumes of ice-cold 50 mmTris-HCl (pH 7.3), containing 5 mm EDTA, aliquoted, and stored at −80 °C. Homogenate samples were mixed with an equal volume of concentrated (3×) SDS-polyacrylamide gel electrophoresis buffer and heated (96 °C) for 5 min. Recombinant, active human caspase-3 (MBL, Nagoya, Japan), 2.0 μl of reconstituted solution (the absolute amount of caspase-3 is not known), was preincubated with 50 μl of protease inhibitor solution (see below) for 10 min and then mixed with 100 μl of 50 μmDEVD-7-amino-4-methylcoumarin (DEVD-AMC) substrate (Bachem, Bubendorf, Switzerland) in 50 mm Tris-HCl (pH 7.3) containing 100 mm NaCl, 10 mm DTT, 5 mm EDTA, 1 mm EGTA, 0.2β CHAPS, and 3 mmNaN3. Cleavage of DEVD-AMC was measured at 37 °C using a Spectramax Gemini microplate fluorometer (Molecular Devices, Sunnyvale, CA) with an excitation wavelength of 380 nm and an emission wavelength of 440 nm. DEVD-AMC cleavage was calculated from theVmax and expressed as relative fluorescence units (RFU)/s/ml. Rabbit lung μ- or m-calpain (48Karlsson J.O. Gustavsson S. Hall C. Nilsson E. Biochem. J. 1985; 231: 201-204Crossref PubMed Scopus (45) Google Scholar) (480 pmol of AMC produced/min/ml from LY-AMC at 37 °C, the same for both isozymes) in 50 μl of buffer was preincubated with 50 μl of inhibitor solution (see below) for 10 min and then mixed with 100 μl of 1 mm LY-AMC in 20 mm Tris-HCl (pH 7.5) containing 4 mm CaCl2, 4 mmDTT, 3β Me2SO, and 3 mmNaN3. Substrate cleavage was evaluated as described for the caspase-3 assay. Inhibitors were as follows: CX295 from Cortex Pharmaceuticals (Irvine, CA) and benzyloxycarbonyl-Val-Ala-Asp-fluoromethyl ketone (ZVAD) andt-butoxycarbonyl-Asp-(OMe)-fluoromethyl ketone (BAF) from Enzyme Systems Products (Livermore, CA). Samples of homogenate (50 μl) were mixed with 50 μl of extraction buffer, containing 50 mm Tris-HCl (pH 7.3), 100 mm NaCl, 5 mm EDTA, 1 mm EGTA, 3 mmNaN3, 1 mm phenylmethylsulfonyl fluoride, 1 μg/ml pepstatin, 2.5 μg/ml leupeptin, 10 μg/ml aprotinin, and 0.2β CHAPS, on a microtiter plate (Microfluor; Dynex Technologies, Chantilly, VA). After incubation for 15 min at room temperature, 100 μl of peptide substrate, 50 μm Ac-DEVD-AMC (Enzyme Systems Products, Livermore, CA) in extraction buffer without inhibitors or CHAPS, but with 4 mm DTT, was added. Cleavage of DEVD-AMC was measured as described above and expressed as pmol of AMC formed per mg of protein and minute. The caspase-3 antibodies were polyclonal and raised either against the full-length precursor of human caspase-3 (H-277; Santa Cruz Biotechnology, Inc., Santa Cruz, CA) or against the p17 fragment (residues 176–277) (number 280, a kind gift from Dr. Donald W. Nicholson, Merck Frosst Center for Therapeutic Research, Quebec, Canada). The latter antibody specifically recognized the active form of caspase-3 in tissue sections (6Zhu C. Wang X. Hagberg H. Blomgren K. J. Neurochem. 2000; 75: 819-829Crossref PubMed Scopus (111) Google Scholar). A polyclonal antibody that specifically recognizes the amino-terminal proteolytic 145-kDa breakdown product of rat α-fodrin (FBDPN) was used to detect calpain-induced fodrin cleavage (49Bahr B. Tiriveedhi S. Park G. Lynch G. J. Pharmacol. Exp. Ther. 1995; 273: 902-908PubMed Google Scholar). The antibody against DFF-45/ICAD was polyclonal and was generated by immunizing rabbits with a recombinant DFF-45 fusion protein (50Liu X. Zou H. Slaughter C. Wang X. Cell. 1997; 89: 175-184Abstract Full Text Full Text PDF PubMed Scopus (1649) Google Scholar) (a kind gift from Dr. Xiaodong Wang, University of Texas Southwestern Medical Center). The antibodies against tubulin (clone TU-01; Sanbio, Uden, The Netherlands), PARP (clone C-2–101, Zymed Laboratories Inc., San Francisco, CA), fodrin (FG 6090, Affiniti Research Products, Mamhead, UK), and MAP 2 (clone HM-2, SIGMA) were monoclonal. All secondary antibodies and avidins (horseradish peroxidase-, biotin-, Texas Red-, or fluorescein isothiocyanate-conjugated) were from Vector Laboratories (Burlingame, CA). The specificity of the FBDP antibody for calpain-cleaved fodrin was verified by incubating a fodrin-enriched (P2) fraction with either caspase-3 or m-calpain at 37 °C. The incubations were interrupted at various time points, and the samples were subjected to SDS-polyacrylamide gel electrophoresis and immunoblotting. In the fraction incubated with m-calpain, a single, prominent 145-kDa band appeared, increasing with time. In the fraction incubated with caspase-3, no bands could be detected using the FBDP antibody, although extensive degradation of fodrin was taking place, as verified using the FG 6090 antibody that recognizes the intact fodrin as well as several cleavage products (not shown). The protein concentration of homogenates in SDS-polyacrylamide gel electrophoresis buffer was determined according to Karlsson et al. (51Karlsson J. Ostwald K. Kåbjorn C. Andersson M. Anal. Biochem. 1994; 219: 144-146Crossref PubMed Scopus (59) Google Scholar). Samples corresponding to 20 μg of bovine serum albumin were electrophoresed on NOVEX 8–16β Tris-glycine gels (NOVEX, San Diego, CA) and transferred to polyvinylidene difluoride (Hybond-P; Amersham Pharmacia Biotech) or nitrocellulose (0.45 μm, Schleicher & Schuell) membranes. Immunoreactive species were visualized using peroxidase-conjugated secondary antibodies; Super Signal Western, PICO, DURA, or FEMTO chemiluminescent substrates (Pierce); and Fuji RX film (Fuji Photo Film Co., Tokyo, Japan). Films were scanned, and immunoreactive bands were quantified using the software IPLab Gel 1.5f (Scanalytics Corp., Fairfax, VA). Alternatively, membranes were exposed in a LAS 1000 cooled CCD camera, and immunoreactive bands were quantified using the software Image Gauge (Fujifilm, Tokyo, Japan). Every sample was analyzed 1–4 times, and when multiple determinations were performed the average value was used as n = 1. Stripping of membranes for reprobing purposes was performed by incubation in 62.5 mm Tris-HCl (pH 6.7), 100 mmβ-mercaptoethanol, 2β SDS, at 50 °C for 30 min. All membranes were reprobed with the antibody against α-tubulin. Tubulin was used to normalize between samples (10Namura S. Zhu J. Fink K. Endres M. Srinivasan A. Tomaselli K.J. Yuan J. Moskowitz M.A. J. Neurosci. 1998; 18: 3659-3668Crossref PubMed Google Scholar). Pups were deeply anesthetized and perfusion-fixed with 5β formaldehyde in 0.1m phosphate buffer. The brains were rapidly removed and immersion-fixed at 4 °C for 24 h. After dehydration with graded ethanol concentrations and xylene, the brains were paraffin-embedded and cut into 4-μm coronal sections. Sections were deparaffinized in xylene and rehydrated in graded ethanol concentrations before staining. Immunopositive cells were counted in a MAP 2-negative area of parietal cortex 300 × 660 μm in size and expressed as positive cells/mm2. Sections were pretreated with proteinase K (Roche Molecular Biochemicals), 10 μg/ml in PBS for 10 min, at room temperature. Antigen recovery was performed by boiling the sections in 10 mm sodium citrate buffer (pH 6.0) for 10 min. Nonspecific binding was blocked for 30 min with 4β goat serum in PBS. Anti-caspase-3 p17 was applied diluted 1:500 in PBS and incubated for 60 min at room temperature, followed by biotinylated goat anti-rabbit IgG (6 μg/ml in PBS) or fluorescein isothiocyanate-labeled goat anti-rabbit IgG (6 μg/ml) for 60 min. Visualization was performed using Vectastain ABC Elite or fluorescence microscopy. Antigen recovery and blocking were performed as above. The anti-FBDP was applied diluted 1:50 in PBS containing 0.2β Triton X-100 and incubated for 60 min at room temperature, followed by biotinylated goat anti-rabbit IgG (11 μg/ml in PBS) or fluorescein isothiocyanate-labeled goat anti-rabbit IgG (6 μg/ml) for 60 min. Visualization was performed using Vectastain ABC Elite or fluorescence microscopy. Six pups for each time point were decapitated at 0 h, 1 h, 3 h, 6 h, 12 h, 24 h, 72 h, and 14 days of recovery after hypoxia-ischemia. Control pups (n = 6) were decapitated on postnatal days 7, 8, 10, and 21. The brains were rapidly removed and frozen in liquid nitrogen. Total RNA was extracted from each hemisphere (52Chomczynski P. Sacchi N. Anal. Biochem. 1987; 162: 156-159Crossref PubMed Scopus (63187) Google Scholar), quantified spectrophotometrically at 260 nm, and stored at −80 °C. First strand cDNA synthesis was performed with the Superscript RNase H− Reverse Transcriptase kit (Life Technologies, Inc.) and random hexamer primers (Roche Molecular Biochemicals). Total RNA (2 μg), random primers (500 ng), and RNase-free water to 24 μl were incubated at 70 °C for 10 min. The mixture was chilled on ice, and 8 μl of 5× first strand buffer (250 mm Tris-HCl (pH 8.3), 375 mm KCl, 15 mm MgCl2), 4 μl of 0.1 m DTT, and 2 μl of 10 mm each of dATP, dGTP, dCTP, and dTTP (Roche Molecular Biochemicals) were added and incubated at 25 °C for 10 min followed by 2 min at 42 °C. RT enzyme (2 μl (400 units)) was added, and the reaction was allowed to proceed for 60 min at 42 °C, followed by 15 min of inactivation at 70 °C. The template cDNA thus obtained was diluted to 100 μl with water and stored at −20 °C. Each subsequent PCR (25 μl) contained 4 μl of template cDNA, a 0.2 mmconcentration each of dATP, dGTP, dCTP, and dTTP (Roche Molecular Biochemicals), a 1 μm concentration of each primer, 1 unit of Taq DNA polymerase (Sigma), and 2.5 μl of 10× PCR buffer (Sigma). Primers were as follows: caspase-3 (GenBankTM accession number U49930) 5′-408TTTGGAACGAACGGACCTGT-3′ (upstream) and 5′-798CACGGGATCTGTTTCTTTGC-3′ (downstream); GAPDH (GenBankTM accession number M17701), 5′-331ACCACCATGGAGAAGGCTGG-3′ (upstream) and 5′-839CTCAGTGTAGCCCAGGATGC-3′ (downstream). All primers were from Kebo Lab (Stockholm, Sweden). PCR cycling for caspase-3 was as follows: step 1, 94 °C for 5 min; step 2, 24 cycles of 94 °C for 20 s, 62 °C for 20 s, 72 °C for 20 s; step 3, 72 °C for 5 min. PCR cycling for GAPDH was as follows: step 1, 94 °C for 5 min; step 2, 20 cycles of 94 °C for 20 s, 60 °C for 20 s, 72 °C for 20 s; step 3, 72 °C for 5 min. The annealing temperatures and cycle numbers were chosen such that both the caspase-3 and the GAPDH PCR products would be in the linear phase of amplification and of similar intensity (data not shown). The PCR products (412 bp for caspase-3 and 528 bp for GAPDH) were separated on 1.5β agarose gels, stained with ethidium bromide, and photographed under UV light. The pictures were scanned, and the bands were quantified using the software IPLab Gel 1.5f (Scanalytics Corp., Fairfax, VA). The relative amount of caspase-3 mRNA was calculated after normalization to GAPDH, to compensate for errors introduced during the preparation of RNA, the production of cDNA, or the PCR. Forebrain hemispheres of P7 control animals (n = 7) were homogenized in 10 volumes of 50 mm Tris-HCl (pH 7.3), 5 mm EDTA and centrifuged at 200,000 × g for 45 min to obtain cytosolic (S3) fractions. Aliquots of 100 μl of S3, 3.0 μl of 100 mmdithiothreitol, 9.0 μl of 100 mm CaCl2, 1.0 μl of 0.5 m NaOH (to compensate for the drop in pH occurring when Ca2+ ions replace protons in the EDTA molecules), and 58.0 μl of homogenizing buffer were incubated for 30 min at 37 °C. Purified μ- or m-calpain, recombinant caspase-3, purified calpastatin (14 units/ml), calpastatin peptide (3 μg/ml; Sigma), CX295 (1 mm), ZVAD (0.7 mm), or BAF (0.88 mm) was included in some incubations, replacing partly the homogenizing buffer. When inhibitors were added, they were preincubated for 10 min with the enzymes at room temperature before being added to the S3 mixture. The reactions were stopped by adding 8.0 μl of 100 mm EDTA. Aliquots of 50 μl were assayed for DEVD cleavage, and portions equivalent to 90 μg of total protein were subjected to SDS-polyacrylamide gel electrophoresis and immunoblotting. The protein concentrations were determined according to Whitaker and Granum (53Whitaker J.R. Granum P.E. Anal. Biochem. 1980; 109: 156-159Crossref PubMed Scopus (482) Google Scholar), adapted for microplates, using a Spectramax Plus plate reader (Molecular Devices). Crude calpastatin (100 units/ml) was purified from rabbit lung via hydrophobic interaction chromatography, as previously described (48Karlsson J.O. Gustavsson S. Hall C. Nilsson E. Biochem. J. 1985; 231: 201-204Crossref PubMed Scopus (45) Google Scholar) and further purified via trichloroacetic acid precipitation and gel filtration (54Belles B. Hescheler J. Trautwein W. Blomgren K. Karlsson J. Pflugers Arch. 1988; 412: 554-556Crossref PubMed Scopus (64) Google Scholar). Aliquots (50 μl) of m-calpain were incubated for 15 min at room temperature under conditions where the enzyme was half-maximally activated (0.36 mm Ca2+). Increasing amounts of active caspase-3 (1–60 units per incubation, where 1 unit is defined as the amount of caspase-3 that will release 1.0 pmol of AMC/min/ml in the DEVD-cleaving assay described above) were included to see if caspase-3 could increase the m-calpain activity directly. Calpain activity was measured as described above. Student's unpaired t test or analysis of variance with Scheffe's post hoc test were used. DEVD-cleaving activity was detectable in neonatal brain samples and increased severalfold in the ipsilateral compared with the contralateral hemispheres, in accordance with earlier findings (9Cheng Y. Deshmukh M. DaCosta A. Demaro J.A. Gidday J.M. Shah A. Sun Y. Jacquin M.F. Johnson E.M. Holtzman D.M. J. Clin. Invest. 1998; 101: 1992-1999Crossref PubMed Scopus (481) Google Scholar), and this increase displayed a maximum 24 h post-HI (not shown). The average ratio between the ipsi- and contralateral hemispheres was significantly decreased after calpain inhibition, from 1618β in the vehicle-treated animals to 606.1β in the CX295-treated animals (p = 0.0004) (TableI).Table IEffects of in vivo calpain inhibition on the activation of caspase-3RatioS.D.npββCaspase-3DEVD cleavageVehicle1618.0564.490.0004CX295606.1289.4832 kDaVehicle72.818.3110.005CX29596.017.41229/(29 + 32) kDaVehicle15.211.1110.031CX2956.885.4612ICAD45 + 32 kDaVehicle77.518.2110.006CX29599.914.510The caspase-3-like activity (DEVD cleavage), cleavage of the caspase-3 proform and degradation of endogenous caspase-3 substrate ICAD were assessed