apoptosis is a morphologically distinct cell death that spontaneously occurs in many different tissues under various conditions ( falcieri et al . ( 1994 ), scan . microsc . 8 , 653 - 666 ). it occurs in distinctly separated cells and progresses very rapidly , never causing exudative inflammation in tissues . typically , apoptosis is characterized morphologically by cell shrinkage and hyperchromatic nuclear fragments and biochemically by chromatin cleavage into nucleosomal oligomers . infectious pancreatic necrosis virus ( ipnv ) is the prototype virus of the family birnaviridae . the virus is capable of infecting a number of different hosts and has a worldwide presence . ipnv is especially susceptible to salmonids , particularly trout , salmon , carp , perch , pike , and eel . the most susceptible fish for ipnv are trout and salmon , such as rainbow trout ( oncorhynchus mykiss ), brook trout ( salvelinus fontinolis ), chinook salmon ( oncorhynchus tshawytscha ), coho salmon ( oncorhynchus kisutch ), sockeye salmon ( oncorhynchus nerca ) and atlantic salmon ( salmo salar ). ipnv contains a bisegmented double stranded rna genome . the larger genome segment a ( approximately 3100 bp ) of ipnv has two open reading frames : ( 1 ) one large ipnv contains a bisegmented double stranded rna genome . the larger genome segment a ( approximately 3100 bp ) of ipnv has two open reading frames : ( 1 ) one large 2916 bp orf encoding a 106 kd polypeptide which can be cleaved into at least three protein ; and ( 2 ) one overlapping orf of 444 bp encoding a 17kd arginine rich polypeptide . the three proteins encoded by the large orf include : ( 1 ) a 60k to 62k precursor ( pvp2 ) of the 52k to 54k major capsid protein vp2 ; ( 2 ) a 29k non - structural protein ( ns ); and ( 3 ) a 31k minor capsid protein vp3 . vp3 is believed to be located internally , associated with the rna , but may be partly exposed on the surface of the capsid . the localization of the 17 kd polypeptide is not known . the smaller b segment ( approximately 2900 bp ) encodes a singe gene product ( vp1 ) with a molecular weight of approximately 94k , presumed to be the viral rna polymerase . vp1 is present as free polypeptide in the virion and as genome - linked protein , vpg . ipnv is a contagious disease , which has particular importance of its effects on hatchery raised fish . the virus replicates in the cytoplasm and a single cycle of replication normally takes 16 - 20 h at 22 ° c ., which results in a characteristic cytopathic effect ( cpe ). recently , hong et al . ( 1998 ), supra , reported that in chinook salmon embryo ( chse214 ) cells after being infected by ipnv , apoptosis occurred prior to necrosis . hong et al . were able to demonstrate that during apoptosis , no cell hydration took place , but nuclear and cytoplasmic condensation appeared , which was followed by the formation of numerous membrane - bound cell fragments termed “ apoptotic bodies .” in addition , in contrast to necrosis , the nuclear organization during apoptosis was completely lost , and profound chromatin rearrangements took place , followed by the formation of a variable number of compact , electron - dense micronuclei . however , despite the extensive nuclear changes , both and undergoes a “ secondary ” necrosis . in the present invention , further investigation into the apoptotic processes has been carried out . the investigation can be subdivided in two embodiments : ( 1 ) monitoring morphological changes during apoptosis ; and ( 2 ) control of cell death via an mcl - 1 dependent pathway . green fluorescent protein ( gfp ) from the jellyfish aequorea victoria is a revolutionary report molecule for monitoring gene expression and fusion protein localization in vivo or in situ and in real time . one of the most useful aspects of gfp for biological studies is that it can be monitored in living cells . in the present study , a variant type of gfp ( egfp ) serves as a marker for visualizing the dynamic apoptotic cell morphological changes and for tracing membrane integrity changes during apoptosis by fluorescence microscopy , as well as for quantitation of the intra - and extracellular release of egfp during apoptosis by western blotting and fluorometry . the use of egfp to study the apoptotic process is illustrated in the following experimental designs , results , and discussion : chinook salmon embryo cells ( chse - 214 ) were obtained from american type culture collection ( atcc ). cells were grown at 18 ° c . as monolayers in plastic tissue culture flasks ( nunc ) using eagle &# 39 ; s minimum essential medium ( mem ) supplemented with 10 % ( v / v ) fetal calf serum and 25 μg / ml gentamicin . gfp - producing cells were obtained by transfection of chse - 214 cells with a pegfp - n1 vector and selection with g418 . in these vectors , transcription of insert sequences is driven by the immediate - early promoter of human cytomegalovirus . the coding region contains the egfp gene , which contains a chromophore mutation which produces fluorescence 35 times more intense than that of wild - type gfp . the virus isolated , el - s , a member of the ab strain of ipnv , was obtained from japanese eel in taiwan . el - s was propagated in chse - 214 cell monolayers at a multiplicity of infection ( moi ) of 0 . 01 particles per cell . infected cultures were incubated at 18 ° c . until an extensive cytopathic effect was observed . the cells were scraped into a tube with the tissue culture medium and chilled on ice , and the cells were then sonicated . this virus stock ( 5 × 10 7 to 1 × 10 8 pfu / ml ) was dispensed into 1 - ml samples and stored at − 70 ° c . virus plaque assays were performed on confluent monolayers of chse - 214 cells that were infected with the virus solution for 1 h at room temperature , overlaid with 0 . 6 % agarose containing 2 . 5 μg of trypsin per ml , and incubated for 3 days at 18 ° c . cells were then stained with 1 % crystal violet in 20 % ethanol ( 8 ). about 10 5 cells per ml were seeded on a 60 - mm - diameter petri dish and allowed to grow for more than 20 h . the cell monolayers were rinsed twice with phosphate - buffered saline ( pbs ), after which they were infected at an moi of 1 and incubated for 0 , 2 , 4 , 6 , 8 , 10 , 12 , and 24 h post infection ( p . i .). uninfected control cells were also incubated for the same periods of time . at the end of each incubation time , the culture medium was aspirated . the cells were washed with pbs and then lysed in 0 . 3 ml of lysis buffer ( 10 mm tris base , 20 % glycerol , 10 mm sodium dodecyl sulfate [ sds ], 2 % β - mercaptoethanol , ph 6 . 8 ). proteins were separated by sds - polyacrylamide gel electrophoresis , electroblotted , and subjected to immunodetection as described by kain et al . ( 1994 ), biotechniques , 17 : 982 - 987 . blots were incubated with a 1 : 7 , 500 dilution of an immunoglobulin fraction ( clontech ) and a 1 : 1 , 500 dilution of a peroxidase - labeled goat anti - rabbit conjugate ( amersham ). chemilumine - scence detection was performed in accordance with the instructions provided with the western exposure chemiluminescent detection system ( amersham ). chemiluminescent signals were imaged by exposure of kodak xar - 5 film ( eastman kodak , rochester , n . y .). stripping and reprobing of the western blot and removal of the primary and secondary antibodies from blot were achieved by incubation in stripping buffer containing 62 . 5 mm tris - hcl ( ph 6 . 8 ), 3 . 0 % ( wt / vol ) sds , and 50 mm 1 , 4dithiothreitol for 30 min at 55 ° c . with gentle shaking . the blot was washed three times in pbs containing 0 . 1 % ( vol / vol ) tween 20 for 10 min each time and reprobed with antibodies beginning at the membrane blocking step . experiments examining the potency of drugs for preventing morphological change and blocking membrane integrity loss and those examining subsequent egfp retention during virus infection and incubation were carried out as described above , except that extra chx ( 10 μg / ml ), aprotinin ( 400 μg / ml ), leupeptin ( 400 μg / ml ), genistein ( 100 μg / ml ), tyrphostin ( 100 μg / ml ), and edta ( 2 mm ) were added to chse - 214 cells before virus infection and incubation for 16 h . at the end of the incubation period , cells were harvested and samples were analyzed by western blotting . a chse - 214 - egfp monolayer infected with ipnv ( moi = 1 ) was examined by light and fluorescence microscopy using an olympus ix70 microscope equipped with a bp450 - 480 pass excitation filter and a ba515 barrier emission filter for observation of egfp fluorescence . photographs were taken with a c - 35 ad - 4 camera using kodak ektachrome 200 film . about 10 5 cells per ml were seeded on a 60 - mm - diameter petri dish and allowed to grow for more than 20 h . the cell monolayers received virus at an moi of 1 . 0 and were incubated for 8 h . uninfected control cells were also incubated for 8 h . the two groups were used for dna fragmentation studies . at the end of incubation , the cells were lysed with lysis buffer ( 10 mm tris - hcl , 0 . 25 % triton x - 100 , 1 mm edta , ph 7 . 4 ). after treatment with phenol - chloroform - isoamyl alcohol ( 25 : 24 : 1 ), the dna was precipitated in the presence of 0 . 3 m sodium acetate and cold absolute ethanol at − 70 ° c . for 2 h and then resuspended in 10 mm tris - hcl ( ph 7 . 4 )— 1 mm edta . aliquots of 20 μl containing approximately 5 to 10 μg of dna were then electrophoresed in 1 . 2 % agarose gels for 2 h at 40 v . gels were stained with ethidium bromide and photographed under uv transillumination . scanning electron microscopy analysis was carried out by cell seeding on a twochamber slide . chse - 214 cells were infected with virus at an moi of 1 and incubated for 0 , 4 , 8 , and 12 h . at the end point , cells were washed twice with pbs and fixed with 2 . 5 % glutaraldehyde in 0 . 1 m phosphate buffer . samples were postfixed with oso 4 , dehydrated in ethanol , critical point dried , and gold sputtered . a philips 515 scanning electron microscope was used to examine the specimens . chse - 214 - egfp cells were infected at an moi of 1 . infected and uninfected control cells were harvested 8 h after infection . thin - section electron microscopy and immunogold labeling were carried out as described by mcnulty et al . ( 1990 ), avian pathol ., 19 : 67 - 73 . the grids were stained with a 1 : 1 , 000 dilution of gfp - specific polyclonal antiserum and a 1 : 50 dilution of a 15 - nm gold - labeled goat anti - rabbit immunoglobulin g conjugate . cellular egfp and culture medium egfp protein samples were prepared for assay in egfp release experiments . about 10 5 cells per ml were seeded on a 60 - mm petri dish for more than 20 h . cell monolayers were rinsed twice with pbs and then cultured in 3 ml of 10 % fcs - containing mem . uninfected cells used as a normal control and cells that received virus at an moi of 1 were incubated for 0 . 2 , 4 , 6 , 8 , 10 , 12 , and 24 h p . i . at the end of each incubation period , the culture medium was collected to determine the concentration of retained egfp . cells were washed with pbs and then lysed in 0 . 3 ml of lysis buffer ( 10 mm tris base , 20 % glycerol , 10 mm sds , 2 % β - mercaptoethanot , ph 6 . 8 ). the assay procedure was as follows . first , recombinant gfp purchased from clontech was used as the standard . the gfp standard was diluted from 1 μg / 0 . 1 ml to 0 . 001 μg / 0 . 1 ml with 10 % fcs - containing mem . second , 5 μg of lysed cells per sample was diluted with 10 % fcs - containing mem to a final volume of 100 μl . third , the supernatant was assayed , and 30 μg of supernatant per sample was diluted with 10 % fcs - containing mem to a final volume of 100 μl . protein concentration was determined by the dye - binding method of bradford using a commercially available kit ( bio - rad , richmond , calif .) with bovine serum as the standard . fourth , the fluorescence intensity of three group samples was counted by a fluorolite 1000 ( dynex ). the egfp concentrations of the lysed cells and supernatant were evaluated by comparing them with that of the gfp standard by using a fluorolite 1000 and dividing by 35 . fig1 shows the sequential morphological changes that occurred in chse - 214 - egfp cells during virus infection ( moi = 1 ). these events were divided into three stages . first ( early stage ), detachment of the chse - 214 - egfp cell matrix was initiated between 0 and 3 h p . i . second ( middle stage ), the whole cell was rounded up and appeared morphologically more compact . in this period ( 3 to 6 h p . i . ), the cell volume decreased to one - third of its original size and the fluorescence intensity was enhanced . in the third ( pre - late ) stage , the cells at 6 to 7 h p . i . quickly underwent severe morphological changes . membrane vesicles ( mv ) were formed from the plasma membrane , and these vesicles eventually blebbed and finally pinched off from the cell membrane . the effect of ipnv infection on host dna in chse - 214 - egfp cells was examined by agarose gel electrophoresis . virus ( moi = 1 ) infected cells were examined for evidence of intemucleosomal fragmentation . dna fragmentation is a well - defined biochemical marker of apoptosis . fig2 shows the results of agarose gel electrophoresis which demonstrates that intense internucleosomal fragmentation of dna , a pattern highly specific to apoptosis , occurred in chse - 214 - egfp cells infected with ipnv . the ipnv induced dna fragmentation at 8 h p . i . is shown in fig2 lane 4 . the negative control which showed no dna fragmentation at 0 and 8 h of incubation is shown in fig2 lanes 2 and 3 . lane 1 of fig1 shows molecular weight markers that ranged from 500 bp to 1 kb ( from mbi fermantas inc .). ( 3 ) ultrastructural morphology changes in ipnv - infected chse - 214 cells detected by scanning electron microscopy apoptosis induces characteristic morphological changes in cells , such as condensation and fragmentation of the nucleus , as well as loss of cytoplasm . to substantiate further that ipnv - infected cells had undergone nontypical apoptotic morphological changes such as membrane integrity changed , negative control and ipnv - infected chse - 214 cells were harvested and processed for scanning electron microscopy as shown in fig3 . negative control cells are shown in fig3 a . ipnv - infected chse - 214 cells at 8 h p . i . displayed detachment , cell rounding , and blebbing of membrane vesicles ( mv ) from the plasma membrane at the pre - late stage of apoptosis ( 20 %; p & lt ; 0 . 05 ), as shown in fig3 b . middlelate - stage apoptotic cells ( 23 %; p & lt ; 0 . 05 ) are shown in fig3 c . the cell membrane appears shrunken , and holes are present in the plasma membrane . the hole sizes ranged from 0 . 39 to 0 . 78 μm with about 10 to 20 holes per cell . a late - stage apoptotic cell ( 2 %; p & lt ; 0 . 05 ) is shown in fig3 d with the small holes still on the surface of the late - apoptotic cell . ( 4 ) egfp release is prevented by protein synthesis inhibitor and tyrosine kinase inhibitor in egfp release experiments , egfp was used to monitor the integrity of the plasma membrane during apoptosis . as described in ( 3 ) above , small holes appeared in middle - late - stage apoptotic cells ( fig3 c ). it is possible that intracellular material might leak out of these small holes to the extracellular region before secondary necrosis . the use of egfp to monitor the integrity of the plasma membrane of ipnv - infected chse - 214 - egfp cells is shown in fig4 . the egfp release western blot assay result is shown in fig4 a , fig4 a , part a , shows that the amount ot gfp decreased , especially between 8 and 16 h p . i . the internal control , actin protein , is shown in fig4 a , part b . detection of the egfp released from the intracellular to the extracellular region during ipnv infection is shown in fig4 a , part c . the increase of gfp release began between 8 and 16 h p . i ., which is consistent with fig4 a , part a . these data indicate that the membrane integrity changed quickly at the middle - late apoptotic stage . the fluorometric egfp release assay results are shown in fig4 b . the open squares show that the intracellular amount of egfp sharply decreased from 6 to 24 h p . i . but that the largest release of egfp occurred between 12 and 24 h p . i . the open diamonds show that the extracellular amount of egfp increased between 6 and 24 h p . i ., which matches the intracellular data described above . egfp was also used as a protein indicator to directly probe membrane integrity by immunoelectron microscopy . normal chse - 214 - egfp cells used as controls are shown in fig5 a . fig5 b shows that the small vesicle escaped from the membrane hole at the pre - late apoptotic cell stage and that the vesicle contains the same egfp labeled by an anti - gfp polyclonal antibody - gold complex . drugs , including the protein synthesis inhibitor cycloheximide ( chx ), the serine proteinase inhibitors aprotinin and leupeptin , the tyrosine kinase inhibitors genistein and tyrphostin , and the cation chelator edta , were used before ipnv infection to test the viability of these drugs on preventing membrane integrity change . some of the drugs , such as chx at 10 μg / ml and 2 mm edta , completely prevented egfp release , and genistein at 100 μg / ml partially prevented egfp release ( as shown in fig6 a ) to the extra - cellular region ( as shown in fig6 c ). the serine proteinase inhibitors aprotinin ( 400 μg / ml ) and leupeptin ( 400 μg / ml ) ( fig6 c , lanes 5 and 6 , respectively ) and the tyrosine kinase inhibitor tyrphostin ( 100 μg / ml ) ( fig6 c , lane 8 ) did not prevent egfp release . the internal control , actin protein , is shown in fig6 b for quantitation of protein loading per sample . the present invention provides the first evidence that gfp can be used to sequentially monitor apoptotic morphological changes in living cells . gfp is stable and species independent and can be monitored noninvasively in living cells . however , working with gfp raises practical problems . one such problem , common in fluorescence microscopy of live cells , is that of phototoxicity , which is thought to be caused mainly by fluorophore - mediated generation of free radicals . fortunately , the introduction of mutant gfps with higher quantum efficiencies , lower - energy excitation spectra , and better temperature stability has been advantageous and has significantly widened the applicability of gfp to the study of proteins of low abundance . for the purposes of studying the changes in membrane integrity after ipnv infection , a clone with strong fluorescence intensity and normal morphology , chse - 214 - egfp , was selected and subcloned as a cell line for experiments as shown in fig1 . this clone uses a variant type of gfp , egfp , as a probe . the use of egfp to study the morphological changes during apoptosis clearly has advantages over gfp because egfp produces fluorescence 35 times more intense than gfp . the clones with lower fluorescence intensity did not produce a good image in sequential morphology studies . the cloned chse - 214 - egfp cells were expressed with egfp ( 32 . 5 kda ; as shown in fig4 a , lane 2 ), which is larger in molecular size than wild - type gfp ( 27 kda ; as shown in fig4 a , lane 1 ). but other characteristics of egfp are similar to gfp . for example , gfp is fluorescent either as a monomer or as a dimer . the ratio of monomeric to dimeric forms depends on the protein concentration and the environment . egfp was also found in both control cells and ipnv - infected cells either as a monomer or as a dimer ( as shown in fig5 ). in fact , a doublet egfp was found in the released egfp , as shown in fig4 a ( part c , lane 6 ) and 6c . the results of using egfp to monitor the dynamic morphological changes in chse214 - egfp cells infected with ipnv are shown in fig1 . the series of events can be briefly divided into four stages : ( i ) the early apoptotic stage ( 0 to 3 h p . i . ), ( ii ) the middle apoptotic stage ( 3 to 6 h p . i . ), ( iii ) the pre - late apoptotic stage ( 6 to 7 h p . i . ), and ( iv ) the postapoptotic necrosis stage ( after 7 h p . i .). the morphological changes in apoptotic cells observed include cell detachment . rounding up , formation of mv , pinched off mv floating away in the culture medium , and finally , postapoptotic necrosis . the sequential morphological change events were different from typical apoptotic morphological changes , which are characterized as detachment , rounding up , membrane blebbing , and finally the formation of apoptotic bodies , as described by wyllie et al . supra . the apoptotic process of chse - 214 after ipnv infection is clearly defined as a “ non - typical apoptotic morphological change ” as summarized in fig7 which include all of the typical characteristics of apoptosis , namely , dna fragmentation , cell detachment and rounding up ( fig7 a - b ), membrane blebbing and formation of membrane vesicles ( mv ) ( fig7 c ), and mv pinching off from the membrane which left small holes on the cell membrane ( fig7 d ). however , at the late apoptotic stage , in addition to the formation of apoptotic bodies ( fig7 e ) as in the typical apoptosis , the apoptotitc cells can undergo post - apoptotic necrosis ( fig7 f ) which is characterized by the condensed chromatin enclosed by the nuclear membrane . mcl - 1 belongs to the bcl - 2 family , which is known as the “ apoptosis - inhibiting protein ”. the founding member of this family is the bcl - 2 protooncogene which was initially isolated from a follicular lymphoma ( bakhshi et al . ( 1985 ), cell 41 : 889 - 906 ). mcl - 1 was originally identified from the differentiating human myeloid leukemia cell line ml - 1 . its expression was found to increase early in the induction or “ programming ” of differentiation of ml - 1 cells before the appearance of differentiation markers . the coding region of mcl - 1 was sequenced and found to have a pronounced region of sequence homology to bcl - 2 in the carboxyl - terminal region ( kozopas et al . ( 1993 ), supra ). unlike bcl - 2 , mcl - 1 contains a strong pest sequence ( enriched in proline , glutamic acid , serine and threonine ) which is present in a variety of proteins that undergo rapid turnover . overexpression of exogenously introduced mcl - 1 has been shown to cause a prolongation of viability under conditions that normally cause apoptotic cell death , such as exposure to cytotoxic agents ( e . g ., the chemotherapeutic agent etoposide , calcium ionophore , or uv irradiation ) or the withdrawal of required growth factors ( zhou et al ( 1997 ), blood 89 : 630 - 643 ). although mcl - 1 has been studied in mammalian cells , no such study has been conducted in aquatic cells . in addition , the present invention is the first to study the down regulation of mcl - 1 protein expression by viral infection . the experimental designs , results , and discussion of this embodiment are illustrated as follows : chinook salmon embryo cells ( chse - 214 ) and el - s of ipn virus ab strain were prepared according to ( 1 ) of the experimental design in embodiment 1 , supra . the scanning electron microscopy was prepared according to ( 5 ) of the experimental design of embodiment 1 , supra . the dna preparation and agarose gel electrophoresis were prepared according to ( 4 ) of the experimental design of the embodiment 1 , supra . about 10 5 cells / ml were seeded on a 60 - mm . petri dish for growth for more than 20 h . monolayers were rinsed twice with phosphate - buffered saline ( pbs ). control cells or cells that received virus at a moi of 1 were incubated for 0 , 2 , 4 , 6 , 8 , 10 and 24 h . at the end of each incubation time the culture medium was aspirated . the cells were washed with pbs and then lysed in 0 . 3 nil lysis buffer [ 10 mm tris base , 20 % glycerol , 10 mm sodium dodecyl sulfate ( sds ), 2 % β - mercaptoethanol ( p - me ), ph 6 . 8 ]. proteins were separated by sds - polyacrylamide gel electrophoresis ( laemmli , 1970 ), electroblotted , and subjected to immunodetection as described by kain et al . ( 1994 ). blots were incubated with a 1 : 1500 dilution of anti - human mcl - 1 polyclonal antibodies ( pharmingen ) or a 1 : 7500 dilution of a peroxidase - labeled goat anti - rabbit conjugate ( amershan ). chemi - luminescent detection was performed according to the instructions provided with the western exposure chemiluminescent dectection system ( amershan ). chemiluminescent signals were imaged by exposure to kodak xar - 5 film ( eastman kodak , rochester , n . y ., usa ). primary ( mcl - 1 ) and secondary antibodies ( peroxidase - labeled goat anti - rabbit conjugate ) were stripped from blots by incubation in stripping buffer containing 62 . 5 mm tris - hci ( ph 6 . 8 ), 3 . 0 % ( w / v ) sds and 50 mm 1 , 4 - dithiothreitol for 30 min at 55 ° c . with gentle shaking . the blots were then washed four times for 10 min each time in pbs containing 0 . 1 % ( v / v ) tween 20 and reprobed with mouse actin monoclonal antibody ( 1 / 1500 , chemicon ) and a 1 : 7500 dilution of a peroxidase - labeled sheep anti - mouse conjugate ( amersham ). the potent drugs on effect of blockage on viral protein expression experiment , the cell preparation was as described above except that extra , cycloheximide ( 10 μg4 g / ml ), aprotinin ( 400 μg / ml ), leupeptin ( 400 μg / ml ), genistein ( 100 μg / ml ), tryphostin ( 100 μg / ml ) and edta ( 2 mm ) were added to 3 nil of mem medium on chse - 214 cells before virus infection and incubation for 16 h . at the end of the incubation period , cells were harvested and the samples were analyzed by western blot method . ( 1 ). ultrastructure morphological changes in chse - 214 cells with ipnv infection by scanning electron microscopy apoptosis induces characteristic morphological changes in cells , such as condensation and fragmentation of the nucleus as well as loss of cytoplasm ( wyllie et al . ( 1984 ), supra ). to substantiate that ipnv - infected cells had undergone apoptosis , negative control and ipnv - infected cells were harvested and processed for scanning electron microscopy . normal negative control cells are shown in fig8 a . ipnv - infected chse - 214 cells displaying detachment and blebbing of the plasma membrane are shown in fig8 b . dna fragmentation is a well - defined biochemical marker of apoptosis . el - s of ipn virus ab strain ( moi of 1 ) infected cells were examined for evidence of internucleosomal fragmentation . intense internucleosomal fragmentation of dna , a pattern highly specific to apoptosis , was observed in chse - 214 cells infected with ipnv ( fig9 ). the ipnv induced dna fragmentation at 8 h and 12 h postinfection was identified by gel electrophoresis ( fig9 lanes 4 and 5 ). the gel of the negative control at 0 h incubation and 4 h postinfection showed no dna fragmentation ( fig9 lanes 2 and 3 ). the characterization of the viral protein size and mcl - 1 expression was directly quantified by western blots from chse - 214 cells . fig1 shows the major protein expression pattern during infection of chse - 214 cells by a moi of 1 of el - s . the viral proteins had a large expression after 4 h post - infection . fig1 shows the mcl - 1 protein expression pattern during infection of chse - 214 cell with a moi of 1 of el - s . the down - regulation of mcl - 1 expression occurred between 6 h and 8 h post - infection ( as shown in fig1 a , lanes 4 and 5 ). the internal control actin is shown in fig1 b . ( 4 ) blocking of virus replication for prevention of down - regulation of the mcl - 1 protein by certain drugs to confirm whether viral replication is involved in down - regulation of mcl - 1 , viral replication in host cell was blocked by treatment with certain drugs . when the protein synthesis inhibitors , 10 μg / ml cyclohexamide , tyrosine kinase inhibitor , 100 μg / ml of genistein or the cation chelator , 2 mm of edta , were added to chse - 214 cells before ipnv infection , viral replication was prevented ( as shown in fig1 a ). at the same time , these same drugs partially prevented down - regulation of mcl - 1 protein expression ( as shown in fig1 b ). however , the serine proteinase inhibitor aprotinin 400 μg / ml and leupeptin 400 μg / ml ( as shown in fig1 b , lanes 5 - 6 ) and the tyrosine kinase inhibitor tryphostin 100 μg / ml ( as shown in fig1 b , lane 8 ) could not . the internal control actin protein is shown in fig1 c . the dna internucleosomes were assayed under the same conditions described above . the blocking of viral replication groups consistently and strongly prevented the induction of internucleosomal cleavage by ipnv ( as shown in fig1 ), with the exception of the 2 mm edta treatment group which displayed minor internucleosomal cleavage ( as shown in fig1 , lanes 14 and 15 ). in order to determine whether ipnv uses the vp3 protein in triggering host cell apoptosis , we designed different sized vp3 antisense rna molecules ( i . e ., a200 , a400 , and a700 ) and inserted them into a blank plasmid pcdna3 to form pa200 ( nt . 501 - 711 ), pa400 ( nt . 285 - 711 ) and pa700 ( nt . 1 - 711 ) plasmids . these and blank insert pcdna3 ( invitrogen , u . s . a .) and used selection with g418 to generate stable cell lines . the stable cells were then infected with ipnv ( moi 1 ). at the end of incubation time , the cell survival rate was assayed by use of the trypane blue method ( results shown in fig1 [ phase - contrast images ] and fig1 ). in particular , at 24 hours post - incubation ( p . i . ), it was found that pa200 could increase survival up to 40 %. vp3 antisense rna molecules pa200 , pa400 and pa700 were prepared using vp3 specific primers p1 , p2 , p3 , and p4 for amplification . specifically , vp3specific primers p1 and p2 were used to amplify and generate a200 , which contained base no . 501 to 711 of the vp3 antisense rna , by pcr using taq polymerase ; vp3 - specific primers p1 and p3 were used to amplify and generate a400 , which contained base no . 285 to 711 of the vp3 antisense rna ; vp3 - specific primers p1 and p4 were used to amplify and generate a700 , which contained base no . 1 to 711 of the vp3 antisense rna . the a200 , a400 , and a700 vp3 antisense rna molecules were constructed to pcdna3 . 11v5his - topo vector as pa200 , pa400 , and pa700 , respectively . the fused gene , c1 - egfp - vp3 , was inserted into a plasmid . pegfp - c1 is a commercially available vector which contains a strong promoter ( cmv promoter ) and the egfp gene . to produce the fused gene , vp3 gene was amplified using vp3 - specific primers p1 and p2 by pcr and ligated to a pcdna3 . 1 vector . the inserted vp3 in pcdna3 . 1 vector was then cut with restriction enzymes hind iii and ecori , and re - constructed to a pegfp - c 1 vector from the hindiii and ecori sites . individual gene function was tested in several different cell lines , including fish chse - 214 , mammalian cells nih3t3 ( rat ) and cho ( hamster ) cells and tumor cell as hepa - 3b and hepa - g2 ( human ). the egfp - vp3 was transfected to cell lines with lipofectamine . lipofectamine resembles a liposome which has the capability of forming a lipid bilayer to assist dna transfection . vp3 induced nih3t3 cell apoptosis ( as shown in fig1 ), as indicated by the percentage of positive apoptotic cells ( as shown in fig1 ) that were induced ( about 20 % at 36 hours p . i .). apoptosis could be similarly induced by vp3 in other cell lines . ( 7 ). vp3 induced zebrafish embryonic cell death and suppression by vp3 antisense rna using zebrafish embryos as an in vivo assay system , one or two cell stage embryos were transfected with about 20 pg of vp3 using microinjection . ( george streisinger , “ zebrafish ”, university of oregon press , edition 2 . 1 , ( 1994 )). dna fragmentation was analyzed in zebrafish embryonic cells injected with vp3 . at end of the incubation time , the genomic dna from the live and dead embryos was isolated and analyzed in 1 . 2 % agarose gel ( as shown in fig1 ). it was found that vp3 induced embryo cell death by increasing dna laddering ( as shown in lane 6 of fig1 ), as compared to live embryos ( as shown in lanes 3 and 6 of fig1 ) and with pegfp - vp3 microinjection for 4 hours ( as shown in lane 5 of fig1 ). overexpression of the ipnv - vp3 induced embryonic cell death . also shown is the micrographs of zebrafish embryos where apoptosis was triggered by microinjection with pegfp - vp3 ( as shown in fig1 ( c ) & amp ; d ), as contrasting to embryos microinjected with pegfp - c1 , the negative control ( fig1 ( a ) & amp ; ( b )). also , as shown in fig1 d , flattened dead cells and damaged somite appeared in the embryo microinjected with pegfp - vp3 for 24 hours , probably due to overexpression of vp3 . the damaged somite was more severe after 48 hours of microinjection with pegfp - vp3 into zebrafish embryos , as seen in fig2 d . interestingly , when antisense rna of vp3 was provided together with pegfp - vp3 , the % of alive embryos after microinjection with pegfp - vp3 increased proportionally , as shown in fig2 , demonstrating that antisense vp3 rna has the capability of overcoming cell death caused by overexpression of vp3 . the same phenomenon was further supported by fig2 , where the % of defected embryos after 60 hours of microinjection with pegfpvp3 and pegfp - vp3 plus antisense vp3 rna was measured . in the group where pegfpvp3 plus antisense vp3 rna was microinjected , the % of defected embryos was about 13 %, contrasting to about 29 % of defected embryos in the group where pegfp - vp3 was microinjected . further evidence showing that the leakage of defected zebrafish embryos was rescued by the addition of antisense vp3 rna is provided in fig2 c . zfmcl - 1a and zfbcl - xl were cloned by chen et al ( chen m . c ., gong h . y ., cheng c . y ., wang j . p , hong , j . r . and wu j . l . ( 2000 ) biochem . biophys res . com . 279 , 725 - 731 ). in order to test whether this protein could block vp3 function in zebrafish embryo , vp3 was microinjected along with zfmcl - 1a or zfbcl - xl ( 1 : 1 = 20 pg : 20 pg ). the protocol for microinjection of vp3 and zfmcl - 1a plasmid dna was according to chapter 5 , george streisinger , “ zebrafish ”, university of orgon press , edition 2 . 1 , ( 1994 ), which is herein incorporated by reference . the results ( fig2 and 25 ), which were determined after 36 hours of microinjection , demonstrate that zfmcl - 1a enhanced embryonic survival by about 38 % ( fig2 , columns 3 and 4 ) and decreased embryonic defects about 20 % ( fig2 , columns 3 and 4 ). ipnv is a highly contagious disease of susceptible hatchery - reared trout and japanese eel in taiwan . ipnv replicates in a variety of continuous cell lines from teleost fish at temperatures below 24 ° c . the virus replicates in the cytoplasm and a single cycle of replication takes 16 - 20 h at 22 ° c . resulting in a characteristic cytopathic effect ( cpe ). rainbow trout gonad ( rtg - 2 ) cells infected with ipnv yielded infectious titers of only 10 6 - 10 7 pfu / ml , because the rtg - 2 cells can produce interferon when ipnv infected and are themselves sensitive to interferon treatment , as reported by macdonald and kennedy ( virology ( 1979 ), 95 : 260 - 264 ). on the other hand , chinook salmon embryo ( chse - 214 ) cells do not produce interferon , and are insensitive to interferon treatment , resulting in virus yields of 2 - 5 × 10 8 pfu / ml or higher . in the present invention , by assaying for cell ultra - morphological features ( as shown in fig8 ) and dna fragmentation ( as shown in fig9 ), demonstrates that apoptosis contributed to the death of ipnv infected cells . also , viral replication ( as shown in fig1 ) correlates with down - regulation of mcl - 1 protein expression ( as shown in fig1 a ). this is the first study that confirms down - regulation of mcl - 1 ( a member of the bcl - 2 family ) protein expression by viral infection . in view of the capacity of mcl - 1 to block or delay apoptosis and its sequence feature as a pest ( proline , glutamic acid , serine , threonine ) protein that can be degraded rapidly , one possible role of this protein is as a rapid turnover effector that controls the rate of apoptosis . however , when treatment with some drugs before ipnv - infected chse - 214 cells was performed , the protein synthesis inhibitor , cycloheximide , or the tyrosine kinase inhibitor , genistein , and the cation chelator , edta , all blocked viral protein expression ( as shown in fig1 a ). and at the same time , the same drugs helped to maintain mcl - 1 expression level ( as shown in fig1 ) and blocked the induction of dna intemucleosomal cleavage ( as shown in fig1 ) for rescue or delay of apoptotic cell death . vp3 , a 32 - kda protein derived from the ipnv segment a , induces apoptosis in fish chse - 214 and mammalian cells , including the nih3t3 ( mouse ) and cho ( hamster ) cells and tumor cell as hepa - 3b and hepa - g2 ( human ). the results presented herein above demonstrate that vp3 can be suppressed by vp3 antisense rna thereby enhancing the survival rate during ipnv infection . on the other hand , the vp3 expression alone can induce zebrafish embryonic cell death and dna fragmentation . death caused by vp3 can be prevented by anti - apoptosis protein zfmcl - 1a , a bcl - 2 family member from zebrafish . it appears that vp3 is a functional death gene in late ipnv replication cycle that the effect of vp3 can be blocked by vp3 antisense rna or functionally blocked by anti - apoptotic gene zfmcl - 1a . having described the invention in detail and by reference to the preferred embodiments it will be apparent to those skilled in the art that modifications and variations are possible without departing from the scope of the invention as defined in the following appended claims .
the present invention provides a mechanism for studies of apoptosis in aquatic organisms by infecting the aquatic organisms with aquabirnavirus , especially infectious pancreatic necrosis virus . the infection of ipnv in an aquatic cell such as a chinook salmon embryo cell converts the cell into an apoptotic cell . the present invention also provides a method for monitoring the morphological changes during apoptosis by cloning egfp to an aquatic cell and monitoring the fluorescence using microscopic techniques . the intensity of the fluorescence reflects the expression of egep in cells . finally , the present invention provides means for inducing or preventing / rescuing apoptosis in a host , which include aquatic and vertebrate . the apoptosis can be induced by ipnv infection or vp3 transfection . that vp3 is a 32 kda protein derived from ipnv segment a . the apoptosis can be prevented or rescued by an antisense vp3 rna or a zfmcl - 1a protein .