Patent Application: US-201314440968-A

Abstract:
the disclosure relates to a method of screening a modulator of xkr8 , comprising the steps of : contacting xkr8 - expressing cells with a candidate of the modulator , and selecting the candidate when the candidate alters distribution of a phospholipid in plasma membrane of the cells .

Description:
xkr8 is a membrane protein having 6 transmembrane regions . the nucleic acid sequences of human and mouse xkr8 are disclosed under genbank nm — 018053 and genbank nm — 201368 . the amino acid sequences of mouse , rat , human , fugu , medaka , and xenopus xkr8 ( seq id nos : 1 - 6 ) are shown in fig5 a . “ a candidate of a modulator of xkr8 ” may be a natural or synthetic product . examples of the candidate of a modulator of xkr8 include low - molecular compounds , proteins , nucleic acid molecules , peptides , antibodies , or cell extract or culture supernatant of microorganisms , plants or animals . the candidate may be provided in a form of a library , such as a library of low - molecular compounds , peptides , or antibodies . as used herein , “ xkr8 - expressing cells ” includes cells that express xkr8 in nature from the genome , and cells that express xkr8 from a gene encoding xkr8 introduced into the cells . the cells may be derived from , but not limited to , human , monkey , mouse , or rabbit . for example , human hela , human ebv ( epstein barr virus )- transformed b cell line , mouse mef ( embryonal fibroblasts ), and mouse pro b cell line ba / f3 may be used in the invention . the gene encoding xkr8 may be introduced into cells to prepare xkr8 - expressing cells by methods well known in the art ( molecular cloning : a laboratory manual ( fourth edition ), cold spring harbor laboratory press ). the modulator of xkr8 includes both “ a modulator enhancing a function of xkr8 ” and “ a modulator suppressing a function of xkr8 ”. as used herein , “ enhancing ( or suppressing ) a function of xkr8 ” means potentiating ( or inhibiting ) the function of xkr8 as a lipid scramblase in cells or animals . the modulator of xkr8 may be an agent directly or indirectly affecting the function of xkr8 , or an agent increasing or decreasing expression of xkr8 . the agent increasing or decreasing expression of xkr8 includes an agent increasing or decreasing mrna expression from a gene encoding xkr8 , and an agent increasing or decreasing xkr8 protein expression . therefore , the modulator of xkr8 includes an agent affecting a regulatory sequence of the gene encoding xkr8 , such as a promoter or enhancer , and also includes an antisense oligonucleotide ( dna or rna ), sirna , mirna , and lybozyme prepared according to the sequence of the gene encoding xkr8 . since xkr8 is activated by cleavage with caspase , the modulator of xkr8 includes an agent increasing or decreasing the cleavage of xkr8 with caspase . in the method of the invention , the enzymatic activity of xkr8 as a phospholipid scramblase is measured . the phospholipid is selected from the group consisting of phosphatidylserine ( ptdser ), phosphatidylethanolamine ( ptdetn ), phosphatidylcholine ( ptdcho ), and sphingomyelin ( sm ). under the normal condition , ptdser and ptdetn are distributed in the inner leaflet of plasma membrane and ptdcho and sm are distributed in the outer leaflet of plasma membrane . xkr8 moves ptdser and ptdetn to the outer leaflet of plasma membrane ( i . e ., exposes ptdser and ptdetn ) and moves ptdcho and sm to the inner leaflet of plasma membrane ( i . e ., internalizes ptdcho and sm ). the enzymatic activity of xkr8 may be measured by determination of the lipid distribution in plasma membrane . a candidate is selected as a modulator enhancing a function of xkr8 when the candidate increases distribution of ptdser and ptdetn in the outer leaflet of plasma membrane ( i . e ., increases exposure of ptdser and ptdetn ) compared to the control . in contrast , a candidate is selected as a modulator suppressing a function of xkr8 when the candidate decreases distribution of ptdser and ptdetn in the outer leaflet of plasma membrane ( i . e ., decreases exposure of ptdser and ptdetn ) to the control . also , a candidate is selected as a modulator enhancing a function of xkr8 when the candidate increases distribution of ptdcho and sm in the inner leaflet of plasma membrane ( i . e ., increases internalization of ptdcho and sm ) compared to the control . in contrast , a candidate is selected as a modulator suppressing a function of xkr8 when the candidate decreases distribution of ptdcho and sm in the inner leaflet of plasma membrane ( i . e ., decreases internalization of ptdcho and sm ) compared to the control . as used herein , “ control ” means distribution of the same lipid in the same leaflet ( outer or inner leaflet ) in xkr8 - expressing cells in the absence of the candidate of the modulator . in the step of contacting xkr8 - expressing cells with a candidate of a modulator of xkr8 ( step ( 1 )), typically , the candidate is added to the culture medium of the xkr8 - expressing cells in the presence of an apoptotic stimulus . examples of the apoptotic stimulus include an apoptotic molecule , such as fas and staurosporine , and uv irradiation . the apoptotic stimulus may be added to the culture medium before or after the addition of the candidate to the culture medium , or may be added simultaneously with the candidate . distribution of ptdser in plasma membrane may be determined by detection of the binding between ptdser exposed on the cell surface and an agent having the property to bind to ptdser , such as annexinv or mfg - e8 ( also called as lactadherin ). for example , after xkr8 - expressing cells are treated with fluorescently - labelled annexinv , the amount of annexinv bound to the cell surface is measured . distribution of ptdser in plasma membrane also may be determined based on blood - clotting reaction . for example , in the presence of an apoptotic stimulus , xkr8 - expressing cells are mixed with a candidate of the modulator and agents required for blood coagulation such as factor xa , factor va , and prothrombin , and then production of thrombin is measured . alternatively , fibrinogen may be further added to the cell culture to measure production of fibrin . distribution of ptdetn in plasma membrane may be determined by detection of the binding between ptdetn exposed on the cell surface and an agent having the property to bind to ptdetn , such as a ptdetn - binding peptide , r009 - 0198 . for example , after xkr8 - expressing cells are treated with biotin - labelled r009 - 0198 and stained with fluorescently - labeled streptavidin , the amount of r009 - 0198 bound to the cell surface is measured . distribution of ptdcho and sm in plasma membrane may be determined with a fluorescently - labeled lipid . as a fluorescent label , nbd and topfluor may be used . for example , a fluorescently - labeled lipid is added to the culture medium such that the fluorescently - labeled lipid is incorporated into the outer leaflet of plasma membrane of xkr8 - expressing cells . when xkr8 functions as a lipid scramblase , the fluorescently - labeled lipid is moved to the inner leaflet of plasma membrane ( i . e ., internalized ). therefore , xkr8 - expressing cells may be treated with a candidate of the modulator together with an apoptotic stimulus in the presence of a fluorescently - labeled lipid such as nbd - pc or nbd - sm . the cells are then treated with bsa such that the fluorescently - labeled lipid unincorporated into the cells is removed . finally , the fluorescently - labeled lipid incorporated into the cells is measured by a flow cytometry . xkr8 is involved in the ptdser exposure in apoptotic cells . therefore , the invention is useful to develop therapeutic agents for apoptotic diseases . examples of the apoptotic diseases include autoimmune diseases , cancers , aids , and brain diseases such as alzheimer &# 39 ; s disease . the invention is further illustrated by , but not limited to , the following examples . mouse interleukin ( il - 3 )- dependent ba / f3 cells 50 were maintained in rpmi containing 10 % foetal calf serum ( fcs , gibco ), 45 units / ml mouse il - 3 , and 50 μm β - mercaptoethanol . human plb - 985 51 , jurkat ( atcc tib152 ), namalwa ( atcc crl - 1432 ), and raji ( atcc ccl - 86 ) cells were grown in rpmi1640 containing 10 % fcs and 50 μm β - mercaptoethanol . plat - e packaging cells 52 were grown in dmem containing 10 % fcs . recombinant mouse il - 3 53 , and human fasl 54 were prepared as described . rabbit anti - activated caspase 3 mab was from cell signaling . mouse anti - human icad mab was from medical & amp ; biological laboratories ( mbl ), and alexa 488 - and alexa 568 - labeled goat anti - rabbit igg were from invitrogen . staurosporine was provided by kyowa hakko kirin . using poly ( a ) rna from ba / f3 - ps19 cells , double - stranded cdna was synthesized with random hexamers as primers , and a bstxi adaptor was attached as described 55 . dna fragments of 1 . 0 to 2 . 5 kb in length were size - fractionated by electrophoresis through a 1 % agarose gel , and ligated into a bst xi - digested pmxs vector 56 . approximately 1 . 3 × 10 6 clones were produced by transforming e . coli dh10b cells ( electromax dh10b ; invitrogen ) by electroporation . using plasmid dna from the cdna library , retrovirus was produced in plat - e cells , concentrated by centrifugation , and used to infect ba / f3 cells as described 56 . cells treated with a23187 were stained on ice for 15 min with cy5 - annexin v ( biovision ) and for 2 min with 5 μg / ml propidium iodide ( pi ), and sorted with facsaria ( bd biosciences ). the cdna integrated into the retroviral vector was identified by pcr with the dna from ba / f3 cell transformants as described 55 . the full - length coding sequences for mxkr8 ( genbank nm — 201368 ) and hxkr8 ( genbank nm — 018053 ) were prepared by rt - pcr from ba / f3 cells and namalwa cells , respectively . primers used were as follows ( in each primer , the bam hi or eco ri recognition sequence is underlined ): the pmxs puro c - gfp was constructed by inserting the gfp sequence between the eco ri and xho i sites of pmxs puro . the xkr8 cdnas were then inserted into the bam hi / eco ri site of pmxs puro c - flag 55 or of pmxs puro c - gfp to express proteins tagged with flag or gfp at the c - terminus . to generate the d351a / d354a ( 2da ) mutants of mxkr8 and the d352a / d355a ( 2da ) mutants of hxkr8 , the mouse and human xkr8 cdnas were mutated by recombinant pcr 57 using 30 - nucleotide primers carrying the mutated nucleotides : to construct the c - terminal deletion mutants of hxkr8 , pcr was performed using a mutant reverse primer : 5 ′- cgagatctgaattctcagtctacctggtcagggtcgg - 3 ′ ( seq id no ; 15 ) ( the eco ri recognition sequence is underlined ), and the product was inserted into a pmxs puro vector . four shrna expression plasmids for mxkr8 in a prs shrna vector carrying the puromycin - resistance gene were purchased from origene . among the four sequences , the best target sequence for shrna was 5 ′- gaatctgtgccatcgccttgttctcagct - 3 ′ ( seq id no ; 16 ). the scrambled shrna in prs was also from origene . ba / f3 cells were infected with retrovirus containing the shrna , while wr19l were transfected by electroporation . stable transformants were selected by culturing in medium with 1 . 0 μg / ml puromycin , and subjected to cloning by limited dilution . the xkr8 mrna was quantified by real - time rt - pcr . xkr8 conditionally targeted mice were generated as a custom order by unitech . in brief , a neo - loxp cassette carrying the phosphoglycerate kinase ( pgk ) promoter - driven neomycin - resistance ( neo ) gene flanked by frt sequences was inserted into intron 3 of the xkr8 gene . a 1 . 0 - kb dna fragment containing exon 3 was replaced with a fragment carrying the corresponding sequence and a locus of crossing over in the p1 ( loxp ) sequence . the diphtheria toxin a - fragment ( dt - a ) driven by the thymidine kinase ( tk ) promoter was inserted at the 5 ′ end of the vector . mouse bruce - 4h embryonic stem ( es ) cells 58 were transfected with the targeting vector , and the g418 - resistant clones were screened for homologous recombination by pcr . positive clones were injected into blastocysts to generate xkr8 +/ neofrt mice . the xkr8 +/ neofrt mice were crossed with transgenic mice carrying the cytomegalovirus enhancer - chicken β - actin hybrid promoter ( cag )- driven flippase variant ( flpe ) gene ( cag - flpe ) 59 , and the resulting mice were backcrossed to c57bl / 6 to generate xkr8 +/ flox mice . all the mice were housed in a specific pathogen - free facility ( spf , a facility which is free of specific microorganisms or parasites ) at kyoto university , and all animal experiments were carried out in accordance with protocols approved by kyoto university . an immortalized foetal thymocyte cell line ( ifet ) was established by immortalizing foetal thymocytes with h - ras v12 and c - myc as described 60 , 61 . in brief , the xkr8 +/ flox mice were intercrossed , and foetal thymocytes were obtained on embryonic day ( e ) 14 . 5 . retrovirus carrying the genes for h - ras v12 and c - myc was produced in plat - e cells with the pcx4 vector 62 , and bound to retronectin - coated plates ( takara bio ) by centrifugation at 2 , 000 × g for 2 - 3 h at room temperature . the thymocytes were attached to the retrovirus - coated plate by centrifugation at 400 × g for 5 min , and cultured in dmem containing 10 % fcs , 1 × non - essential amino acids , 10 mm hepes - naoh buffer ( ph 7 . 4 ), 50 μm b - mercaptoethanol , 5 ng / ml mouse il - 7 63 ( peprotech ), and glutamax ™ ( gibco ). the resultant ifet cells were infected with adeno - cre ( adenovirus cre / loxp , takara bio ), and subjected to cloning by limited dilution . clones carrying the xkr8 −/− allele were selected by pcr with the following primers ( wild - type - specific sense primer : 5 ′- ctcattgctgatgtgggtgacaata - 3 ′ ( seq id no ; 17 ); mutant - specific sense primer : 5 ′- aggcttttctctacttttgatggag - 3 ′ ( seq id no ; 18 ); and common anti - sense primer , 5 ′- cattatcttcctcactggctgaatc - 3 ′ ( seq id no ; 19 )). retroviruses carrying mouse and human xkr8 cdna were produced by introducing the pmx - puro vector into plat - e cells , concentrated by centrifugation , and used to infect ba / f3 and xkr8 −/− ifet cells . stable transformants were selected in medium containing puromycin ( 1 . 0 μg / ml and 2 . 0 μg / ml for ba / f3 and ifet cells , respectively ), and the expression of the recombinant protein was confirmed by western blotting with an anti - flag ( clone m2 , sigma ) or anti - gfp ( clone jl8 , clontech ). mouse fas cdna 64 was introduced into ifet cells by retrovirus - mediated transformation , and its expression was confirmed by flow cytometry with the anti - mouse fas mab ( jo2 ) 65 . human plb - 985 and mouse wr19l cells were transformed by retrovirus infection with amphotropic retrovirus envelope or vsvγ envelope . in brief , retrovirus was generated by co - transfecting 293t cells with the pmxs retrovirus vector , pgp ( takara bio ) for gag and pol - fusion protein , and pe - ampho ( takara bio ) or pcmv - vsv - g - rsv - rev ( provided by dr . h . miyoshi , riken ). the virus particles in the culture supernatant were concentrated by centrifugation and used to transform cell lines . to express xkr8 - gfp in 293t cells , 293t cells were transfected with pmxs puroxkr8 - gfp by lipofection with fugene 6 ( promega ). stable transformants were selected in medium containing 1 . 0 μg / ml puromycin . ( 8 ) induction of apoptosis , treatment with ca 2 + - ionophore , and flow cytometry apoptosis was induced by treating cells with fasl or staurosporine , or exposing them to uv . in brief , 5 × 10 5 cells in 500 μl of culture medium were incubated at 37 ° c . with 10 - 400 units / ml hfasl for 1 . 2 - 2 . 0 h or with 10 μm staurosporine for 1 . 5 - 4 . 0 h . for uv exposure , 1 × 10 6 cells in 2 ml of pbs were exposed to 500 - 2000 j / m uv radiation ( 254 nm ) in a stratalinker uv oven ( stratagene ), and incubated at 37 ° c . for 1 . 5 - 2 . 0 h in 4 ml of rpmi1640 containing 10 % fcs . to monitor ca 2 ′- ionophore - induced ptdser exposure , cells ( 5 × 10 5 cells ) in 500 μl of annexin v staining buffer ( 10 mm hepes - naoh buffer [ ph7 . 4 ] containing 140 mm nacl and 2 . 5 mm cacl 2 ) were incubated at 20 ° c . for 3 min , treated with 3 . 0 - 10 μm a23187 , and analyzed with facsaria at 20 ° c . to detect active caspase 3 in cells , cells ( 1 × 10 6 cells ) were fixed by incubation at 37 ° c . for 10 min in pbs containing 1 % paraformaldehyde ( pfa ). after washing with chilled pbs containing 0 . 5 % bsa , the cells were permeabilized by overnight incubation at − 20 ° c . in 90 % methanol . the cells were then incubated with 200 - fold - diluted rabbit anti - active caspase 3 at room temperature for 30 min , followed by incubation for 30 min with 1 , 000 - fold - diluted alexa 488 - or alexa 568 - labeled goat anti - rabbit igg . after washing with pbs containing 0 . 5 % bsa , the cells were filtered into facs tube and analyzed by facsaria . to detect ptdser and ptdetn exposed on the cell surface , cells were stained on ice for 15 min with 2500 - 5000 - fold diluted cy5 - annexin v ( biovision ) or 800 - fold diluted - biotin - ro09 - 0198 ° followed by 1 . 0 μg / ml apc - streptavidin in annexin v staining buffer in the presence of 5 μg / ml propidium iodide ( pi ), and analyzed by facsaria or facscalibur ( bd biosciences ). to assay the internalization of ptdcho and sm , 1 × 10 6 cells in 0 . 5 ml of hbss containing 1 mm cacl 2 ( hbss - ca ) were incubated on ice for 7 min . an equal volume of 200 nm 1 - oleoyl - 2 -{ 6 -[( 7 - nitro - 2 - 1 , 3 - benzoxadiazol - 4 - yl ) amino ] hexanoyl }- sn - glycero - 3 - phosphocholine ( nbd - pc ) ( avanti polar lipids ), or n -[ 6 -[( 7 - nitro - 2 - 1 , 3 - benzoxadiazol - 4 - yl ) amino ] hexanoyl ]- sphingosine - 1 - phosphocholine ( nbd - sm ) ( avanti polar lipids ) in hbss - ca was added , and incubated at 20 ° c . aliquots ( 150 μl ) were mixed with 150 μl hbss containing 5 μg / ml fatty - acid free bsa ( sigma - aldrich ) and 500 nm sytoxblue ( molecular probes ), and analyzed by facsaria . to treat human plb - 985 cells with 5 - aza - 2 ′- deoxycytidine ( dac , sigma - aldrich ), 1 . 0 × 10 6 cells in 10 ml of rpmi containing 10 % fcs were incubated with 0 . 5 μm dac for up to 7 days . since dac is an unstable compound , the medium containing dac was changed every 24 hours . after dac treatment , the cells were divided into three portions : one portion for facs to analyze the ptdser exposure , one for real - time rt - pcr for xkr8 gene expression , and one for the methylation - specific pcr analysis 67 . for the bisulfite genomic sequencing , the dna was modified with bisulfite using a kit ( methyeasy xceed , human genetic signatures ). in brief , 3 μg dna was denatured by incubation at 37 ° c . for 15 min in 0 . 3 m naoh , and treated with sodium bisulfite according to the protocol provided by the supplier except that the incubation time was changed to 90 min . the modified dna was denatured at 95 ° c . for 20 min , and amplified by pcr using primers specific for the treated dna ( ttagggattagaatgtgttt ( seq id no ; 20 ) and cctatacaaataacccaact ( seq id no ; 21 )). pcr was carried out with epitaq hs polymerase ( takara bio ) for 7 - 40 cycles , and the product was cloned in a pgem - teasy vector for sequencing . total rna from human and mouse cell lines and various mouse tissues was reverse - transcribed using superscript iii reverse - transcriptase ( invitrogen ) or the high capacity rna - to - cdna ™ kit ( applied biosystems ). aliquots of the products were amplified in a reaction mixture containing lightcycler ™ 480 sybr green i master ( roche diagnostics ). the primers for real - time rt - pcr were : the specific mrna was quantified at the point where the lightcycler system detected the upstroke of the exponential phase of pcr accumulation with the respective linearized plasmid dna as reference . membrane fractions were prepared from plb - 985 cell transformants expressing hxkr8 - gfp or hxkr8 2da - gfp as described previously 53 . membranes were then solubilized by suspending in lysis buffer ( 20 mm tris - hcl [ ph 7 . 2 ], 140 mm nacl , 1 % triton x - 100 , 10 % glycerol , and 1 mm ( p - aminophenyl ) methanesulfonyl fluoride ( apmsf )). after insoluble materials were removed by centrifugation , the membrane proteins ( 20 μg ) were incubated at 37 ° c . for 1 h with 3 units of each recombinant human caspase ( biovision ) in 100 μl of 50 mm hepes - naoh ( ph 7 . 4 ), 50 mm nacl , 5 % ( v / v ) glycerol , 5 mm dtt , 10 mm edta , 0 . 1 mm apmsf , and 0 . 1 % chaps , and analyzed by western blotting . the cells were lysed in ripa buffer ( 50 mm hepes - naoh buffer [ ph 8 . 0 ] containing 1 % np - 40 , 0 . 1 % sds , 0 . 5 % sodium deoxycholate , 150 mm nacl , and 10 % protease inhibitor cocktail ). the lysates were mixed with 5 × sds sample buffer ( 200 mm tris - hcl [ ph 6 . 8 ], 10 % sds , 25 % glycerol , 5 % b - mercaptoethanol , and 0 . 05 % bromophenolblue ), and incubated at room temperature for 1 h to detect xkr8 - gfp , or boiled for 5 min to detect other proteins . proteins were separated by electrophoresis on a 10 - 20 % gradient sds - page ( bio craft ), and transferred to a pvdf membrane ( millipore ). the membranes were probed with 3000 - fold - diluted mouse anti - gfp mab , 3000 - fold - diluted mouse anti - human icad mab , or 3000 - fold - diluted rabbit anti - active caspase 3 mab followed by incubation with 1 , 000 - fold - diluted hrp - conjugated goat anti - mouse or rabbit immunoglobulins ( dako ). the peroxidase activity was detected by the western lightning ™- ecl system ( perkinelmer ). by repeated facs sorting and expansion of cells that efficiently exposed ptdser , we previously established a subline of mouse ba / f3 cells ( ba / f3 - ps19 ) that exposes ptdser at a high level 15 . tmem16f , a ca 2 + - dependent phospholipid scramblase , was cloned from a ba / f3 - ps19 cdna library constructed with cdnas larger than 2 . 5 kb . to search for the scramblase responsible for apoptotic ptdser exposure , a cdna library was prepared with ba / f3 - ps19 cdnas of 1 . 0 to 2 . 5 kb long , and introduced into ba / f3 cells . facs sorting and expansion of the cells that efficiently exposed ptdser was repeated 5 times , and the sorted cells were subjected to limiting dilution . using this process , we established a cell line ( ld - ps5 - 2 - 2 ) that constitutively exposed ptdser ( fig1 a ). the ld - ps5 - 2 - 2 cells carried a single integrated cdna that codes for mouse xkr8 , a protein of 401 amino acids with several transmembrane segments . to examine whether xkr8 is involved in apoptotic ptdser exposure , ba / f3 line and mouse t - cell line ( wr19l ) that expressed mouse fas were established ( baf - fas and wr - fas ). fas ligand ( fasl ) efficiently induced the apoptosis of wr - fas cells , accompanied by the caspase - 3 activation and ptdser exposure . this fasl - induced ptdser exposure was strongly enhanced by transforming the cells with mouse ( m ) xkr8 - gfp but not with mtmem16f - gfp ( fig1 b ). the endogenous mxkr8 gene expression in baf - fas and wr - fas was then knocked down by expressing the mxkr8 short hairpin rna ( shrna ). the mxkr8 mrna expression levels in transformants expressing the shrna decreased to 18 - 24 % of their levels in wr - fas and baf - fas cells expressing a control shrna ( fig1 c and fig7 a ). accordingly , the fasl - induced ptdser exposure was slowed in these transformants , although caspase 3 was activated similarly as in the parental cells ( fig1 d ). human ( h ) xkr8 cdna , of which the sequence corresponding to the shrna differs from mxkr8 , fully rescued the inhibitory effect of mxkr8 shrna ( fig1 d , fig7 b ). observation of human 293t cell transformants expressing mxkr8 - gfp ( fig1 e ) suggested that hxkr8 was located at the plasma membrane . these results indicated that xkr8 at the plasma membrane could be a phospholipid scramblase activated by apoptotic stimuli . in agreement with previous reports 17 , 18 , human plb - 985 leukemia and raji lymphoma lines did not expose ptdser upon apoptotic stimuli such as staurosporine , uv , or fasl , which is in sharp contrast to human namalwa and jurkat cells that responded to staurosporine by exposing ptdser ( fig2 a ). real - time rt - pcr analysis indicated that the xkr8 mrna levels in the plb - 985 and raji cells were 8 and 9 % of that in namalwa cells ( fig2 b ), respectively . when the plb - 985 or raji cells were transfected with an hxkr8 expression plasmid , the transformants efficiently responded to staurosporine , uv - irradiation or fasl by exposing ptdser , without enhancement of caspase activation ( fig2 c ). these results indicated that the inability of plb - 985 and raji cells to expose ptdser was due to the lack of the xkr8 gene expression . a pcr analysis showed no gross abnormality in the hxkr8 genomic structure in the plb - 985 cells . however , an analysis of the hxkr8 gene for the presence of cpg islands using the program “ cpg island searcher ” from the university of southern california ( http :// cpgislands . usc . edu /) indicated two cpg islands in the 1 . 2 - kb region near the transcription start site of the hxkr8 gene ( fig3 a ). the second island was in the promoter region , where recognition sites for the sp1 and nf - κb transcription factors were clustered . to determine the methylation status of each cpg site between − 239 and + 7 of the hxkr8 gene , dna was prepared from plb - 985 , raji , and namalwa cells , and from peripheral blood leukocytes from a healthy person , and analyzed by bisulfite dna sequencing 19 . none of the 23 cpg sites was methylated in the dna from the peripheral blood leukocytes , jurkat or namalwa cells ( fig3 b ). in contrast , all the cpg sites were heavily , or with more than 90 % probability , methylated in the plb - 985 and raji cells . treatment of the plb - 985 cells with a demethylating agent , 5 - aza - 2 ′- deoxycytidine ( dac ), gradually increased their xkr8 mrna level ( fig3 c ). on day 7 , all 23 of the cpg sites between − 239 and + 7 in the plb - 985 cells were highly demethylated ( fig3 b ), and the xkr8 mrna level reached about 91 % of that in namalwa cells ( fig3 c ). accordingly , the dac - treated plb - 985 cells gained the ability to expose ptdser in response to uv irradiation ( fig3 d ). these results indicate that the cpg islands in the promoter region of the hxkr8 gene were heavily methylated in the plb - 985 and raji cells , which blocked the xkr8 gene expression , leading to the defective ptdser exposure during apoptosis . phospholipid scramblase is defined as an enzyme that can non - specifically scramble phospholipids in the plasma membrane 20 . to characterize the scramblase activity mediated by xkr8 , human plb - 985 and its transformants expressing hxkr8 were treated with staurosporine . four hours later , icad ( icad - l and icad - s ) was cleaved equally well in plb - 985 and its hxkr8 - expressing transformant ( fig4 a ). as found for the ptdser exposure , the parental plb - 985 cells and hxkr8 transformants did not expose ptdetn when they were growing as recognized by the ptdetn - binding peptide ro09 - 0198 21 ( ro peptide ) ( fig4 b ). staurosporine - treated hxkr8 - expressing plb - 985 cells , but not the parental cells , were stained with the ro peptide , indicating that ptdetn was exposed . the scrambling activity for ptdcho and sm was examined by assaying the internalization of l - oleoyl - 2 -{ 6 -[( 7 - nitro - 2 - 1 , 3 - benzoxadiazol - 4 - yl ) amino ] hexanoyl }- sn - glycero - 3 - phosphocholine ( nbd - pc ), or n -[ 6 -[( 7 - nitro - 2 - 1 , 3 - benzoxadiazol - 4 - yl ) amino ] hexanoyl ]- sphingosine - 1 - phosphocholine ( nbd - sm ). as shown in fig4 c and 4 d , nbd - pc and nbd - sm were not internalized in the growing plb - 985 cells or the hxkr8 transformants . however , nbd - pc and nbd - sm were internalized in the staurosporine - treated hxkr8 transformants , but not in the parental cells . these results indicated that xkr8 was activated during apoptotic cell death and served as a non - specific scramblase . this non - specific scrambling activity of xkr8 is similar to that of tmem16f , a ca 2 - dependent phospholipid scramblase 15 . however , unlike the apoptosis - induced ptdser exposure , the ca 2 + - induced ptdser exposure occurred in the plb - 985 cells and the hxkr8 transformants with the same kinetics ( fig8 ), suggesting that xkr8 had no effect on the ca 2 + - dependent ptdser exposure . xkr8 is a member of the xk family 22 , and its homologues are present in mammals , fishes , and amphibians ( fig5 a ). analyses of the amino acid sequences of xkr8 from various species with topology prediction programs including tmpred ( http :// www . ch . embnet . org / software / tmpred_form . html ) and tmhmm ( http :// www . cbs . dtu . dk / services / tmhmm - 2 . 0 /) yielded inconsistent predictions of six to eight transmembrane regions . assuming that the overall structure of the xkr8 protein is conserved among species , it is tentatively ascertained that xkr8 carries six transmembrane regions with cytosolic n - and c - termini ( fig5 b ). using the online search tool ( casvm , http :// www . casbase . org / casvm / index . html ), a well - conserved caspase 3 - recognition sequence 23 was identified in the c - terminal cytoplasmic region of human , mouse , rat , fugu , medaka , and xenopus xkr8 ( fig5 a ). apoptotic ptdser exposure is caspase - dependent in many cases 24 . the presence of a caspase recognition sequence in xkr8 suggested that xkr8 could be a direct target of caspase 3 . to examine whether the cleavage of xkr8 at this position was required for its scrambling activity , the p d qv d g sequence at amino - acid position 355 of hxkr8 was mutated to p a qv a g ( 2da ) ( fig5 c ), fused to gfp at its c - terminus , and introduced into plb - 985 cells . in contrast to plb - 985 transformant expressing wild - type hxkr8 - gfp , the cells transformed by the 2da mutant hxkr8 - gfp showed almost no ptdser exposure in response to staurosporine ( fig5 d ). to confirm that hxkr8 was cleaved during apoptosis , plb - 985 cells expressing hxkr8 - gfp were treated with staurosporine , then analyzed by western blotting with anti - gfp antibody ( fig5 e ). in growing cells , hxkr8 - gfp showed a band of 52 kda , the size of the hxkr8 - gfp fusion protein . when the cells were treated with staurosporine , the 52 kda band shifted to 29 kda , which was slightly larger than authentic gfp . on the other hand , the 2da mutant of hxkr8 was barely processed by staurosporine treatment . endogenous icad was cleaved similarly in plb - 985 transformants expressing the wild - type or caspase - resistant hxkr8 , indicating that caspase 3 was activated equally in them . similar processing of mxkr8 at the caspase - recognition site ( pdlvdg at position 354 ) during apoptotic cell death was observed when wr - fas cells expressing the wild - type mxkr8 - gfp fusion protein were treated with fasl ( fig5 f ). the triton x - 100 - solubilized membrane fraction was then prepared from cells expressing the gfp - fusion protein , and incubated with various recombinant human caspases . western blot analysis with anti - gfp antibody showed that caspase 3 and caspase 7 cleaved the wild - type but not 2da mutant hxkr8 ( fig5 g ). these results indicated that cleavage of hxkr8 at asp - 355 or of mxkr8 at asp - 354 in the c - terminal tail enabled the molecule to function as a phospholipid scramblase . next , hxkr8 was truncated at asp - 355 ( fig5 c ) and introduced into plb - 985 cells with or without fused gfp to examine whether it acted as a constitutively active form . however , the truncated hxkr8s did not mediate ptdser exposure in either growing or apoptotic cells ( fig5 d ). fluorescence microscopy of human 293t cells expressing the truncated hxkr8 - gfp fusion protein showed that it to be present in the cytoplasm , probably in the endoplasmic reticulum ( fig5 h ). a motif of dibasic [ r / k ( x ) r / k ] or diaromatic amino acids ( ff , fy or yy ) present in various transmembrane proteins serves as an endoplasmic reticulum export signal 25 , 26 . there were two or three of these motifs in the xkr8 cytoplasmic tail of all the species listed in fig5 a . since these motifs were located downstream of the caspase - recognition site in hxkr8 , it is likely that the truncated hxkr8 could not be transported to the plasma membrane . ptdser is exposed on the cell surface of most cells undergoing apoptosis . accordingly , mxkr8 mrna was ubiquitously and similarly expressed in various mouse tissues ( fig6 a ), except that it was extremely high in the testis , and low in the heart and muscle . to confirm xkr8 &# 39 ; s role in phospholipid scrambling , and to compare its phospholipid scrambling activity with that of tmem16f , an xkr8 - deficient foetal thymocyte cell line ( ifet ) was established ( fig6 b ). in response to fasl , the xkr8 flox / flox and tmem16f −/− ifet cells quickly exposed ptdser ( fig6 c ). in contrast , xkr8 −/− ifet cells did not expose ptdser in response to this treatment , although caspase 3 was similarly activated . when the xkr8 −/− ifet cells were infected by a retrovirus carrying mxkr8 cdna , the transformants exposed ptdser in response to fasl . in contrast , ca 2 + ionophore induced ptdser exposure in the xkr8 flox / flox and xkr8 −/− , but not in the tmem16f −/− ifet cells ( fig6 d ). these results indicate that xkr8 is responsible for the apoptotic ptdser exposure , while tmem16f is responsible for ca 2 + - 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