the present disclosure will further be elaborated with illustrative embodiments below . the methods are intended to illustrate and not to limit this disclosure . specific experimental conditions not stated in the following embodiments can be operated according to the conventional conditions such as sambrook etc . molecular cloning : a laboratory manual ( new york : cold spring harbor laboratory press press , 1989 ), or manufacturer &# 39 ; s recommendations . example 1 . preparation of the gene encoding the recombinant hfsh - fc fusion protein the design of gene sequence was optimized on the basis of preferred codons of cho cells . the gene encoding the leader peptide and mature protein of hfsh β chain , ctp and mature protein of hfsh α chain were synthesized de novo . the resulting dna fragment of 756 bp in length was inserted into a holding vector such as pucs7 at the ecorv restriction enzyme site to give the phfsh plasmid . the sequence of the hfsh gene was confirmed by dna sequencing . the gene encoding the linker peptide ( l ) and human igg2fc variant ( vigg2fc ) with restriction sites of bamhi ( 5 ′) and ecori ( 3 ′) was synthesized de novo . resulting dna fragment of l - vigg2fc was inserted into a holding vector such as puc19 between the bamhi and ecori sites to give the pl - vigg2fc plasmid . the sequence of the pl - vigg2fc gene was confirmed by dna sequencing . to prepare the hfsh - l - fc fusion gene , the hfsh fragment containing the leader peptide sequence , hfsh β , ctp and hfsh α was excised from the phfsh plasmid with spei and bamhi , and then purified by agarose gel electrophoresis . the purified fragment was then inserted to the 5 ′- end of the peptide linker in the pl - vigg2fc plasmid , linked by t4 ligase to give the phfsh - l - vigg2fc plasmid . the resulting fusion gene of phfsh - l - vigg2fc plasmid comprised hfsh 0 , ctp , hfsh α , peptide linker , and fc variant gene . the single stranded structure is shown in fig2 a and dimeric structure is shown in fig2 b . to construct the expression vector for hfsh - fc , the hfsh - l - vigg2fc fragment was excised from the phfsh - l - vigg2fc plasmid with restriction enzyme spei and ecori and was purified by agarose gel electrophoresis . the purified fragment was then inserted into the corresponding restriction sites of the mammalian expression plasmid , such as pcdna3 ( invitrogen ), to give the pcdna3 - hfsh - l - vigg2fc plasmid ( pcdna3 - hfsh - fc ), as shown in fig4 . the plasmid comprised a cytomegalovirus ( cmv ) early gene promoter - enhancer which was required for high level expression of exogenous protein , two kinds of selective marker gene , leading to ampicillin resistance in bacteria and zeocin resistance in mammalian cells . in addition , this expression vector comprised the dihydrofolate reductase ( dhfr ) gene which was in a position to enable the co - amplification of the hfsh - l - vigg2fc fusion gene together with the dhfr gene in the presence of methotrexate ( mtx ) when the host cells were deficient in the dhfr gene expression . the presence of ctp between the hfsh β and hfsh α moieties ( and chemically bound to both ) is favorable to the right assembly of the two chains . the presence of a peptide linker , preferably a flexible linker , between the hfsh and fc moieties ( and chemically bound to both ) increases the flexibility of the hfsh domain and enhances its biological activity . for the present disclosure , a peptide linker of about 20 or fewer amino acids in length is preferred . while a single amino acid is within the scope of the present disclosure , it is preferred to have a flexible peptide linker of about 20 to about 2 amino acids in length . peptide linker containing or comprising of two or more of amino acids selected from the group consisting of glycine , serine , alanine , and threonine can be used preferably . one embodiment of the present disclosure has the peptide linker containing a gly - ser peptide component , and its amino acid sequence is glyserglyglyglyserglyglyglyglyserglyglyglyglyser . example 2 . stable expression of the recombinant hfsh - fc fusion protein in mammalian cells the pcdna3 - hfsh - l - vigg2fc expression vector plasmid made from example 1 was transfected into a mammalian host cell line to achieve the expression of the fusion protein . for stable high levels of expression , a preferred host cell line was cho cells deficient in the dhfr enzyme ( cho dhfr -). fig2 b shows the schematic diagram of the recombinant dimerized hfsh - fc fusion protein . a preferred method of transfection was electroporation . ten μg of plasmid dna linearized with pvui was added to 2 to 5 × 10 7 cells in a cuvette using gene pulser electroporator ( bio - rad laboratories , hercules , calif .) set at an electric field of 250 v and a capacitance of 960 μfd . two days following the transfection , the media was replaced with growth media containing resistant marker gene of 100 μg / ml zeocin . transifectants resistant to the selection chemical were analyzed for the expression level of hfsh - fc protein by western blotting using anti hfsh antibody . to achieve higher levels of the fusion protein expression , co - amplification was carried out by utilizing the gene of dhfr that could be inhibited by the mtx drug . in growth media containing increasing concentrations of mtx , the transfected fusion protein gene was co - amplified with the dhfr gene . transfectants capable of growing in media with up to 10 μm / ml of mtx were then subcloned by limiting dilutions . the subcloned cells were further analyzed by measuring the secretion rates . to obtain stable cell lines of high expression of recombinant hfsh - fc fusion protein , cell clones yielding secretion rate levels over 10 μg / million cells / 24 h ( preferably about 20 μg / million cells / 24 h ) were adapted to suspension culture using serum - free growth media . the high expression cell line from example 2 was first undergone a domestication process using serum - free medium in the culture dish , and then transferred to the shake flask for suspension culture . during the above serum - free medium culture process , medium optimization was also carried out for testing different ingredients to detect various parameters , such as the growth state , growth trend , bioactivity and sialic acid etc . the following conditions of cell culture were preferred : basic medium comprising 100 μm cu 2 + , feeding medium comprising 2 mm mannac ( n - acetyl amino mannose ). this culture condition can increase the glycosylation content of the recombinant hfsh - fc fusion protein , such as about 20 % increase of the content of sialic acid . for cell growth at a 7 l bioreactor , when the cell density reached 1 × 10 7 / ml at 37 ° c ., the culture temperature was adjusted to 33 ° c . to allow longer accumulation and more stable of secreted fusion protein than those at 37 ° c . the optimum culture period for one batch of cell production was approximately 20 days . small amount of the recombinant fusion protein was initially purified by chromatography of 1 ml protein a column to determine the expression level , as shown in fig5 , the cumulative yield of recombinant hfsh - l - vigg2fc cell line was 1 . 87 g / l . the purification of the recombinant hfsh - fc fusion protein included the following steps : 1 ) protein a affinity chromatography : the culture media containing the hfsh - fc fusion protein were centrifugated , and the supernatant was collected for subsequent loading onto a protein a column pre - equilibrated in phosphate - buffered saline ( pbs ). after binding of the fusion protein to protein a resin , the flow - through fractions were discarded . the column was then washed with pbs until od at 280 nm was below 0 . 01 . the bounded fusion protein was eluted with 20 mm sodium acetate buffer ( ph 4 . 0 ), and the elution was neutralized with 1m tris - hcl buffer ( ph10 . 0 ). the purity of the hfsh - fc protein after this step could reach 95 % or more . 2 ) hydrophobic chromatography : the elution from above protein a chromatography was buffer changed to 20 mm tris - hcl - 1 . 5 m nacl ( ph8 . 0 ) with ultrafiltration method , and loaded onto a phenyl - 6 fast flow column equilibrated in 20 mm tris - hcl - 1 . 5 m nacl ( ph8 . 0 ) buffer . the column was washed with the same equilibration buffer , and then washed with 20 mm tris - hcl - 1 . 35m nacl ( ph8 . 0 ) before its elution with 2 mm tris - hcl - 0 . 5m nacl ( ph8 . 0 ). as shown in fig6 , western blotting analysis indicated that the recombinant hfsh - fc fusion protein in cho cells was successfully expressed ; non - reduced sds - page showed hybridization bands of target protein respectively : 1 ) 43 kda band , human urine hfsh ( commercial product ); 2 ) 140 kda band , recombinant hfsh - fc fusion protein ( the present disclosure ), proving that the recombinant hfsh - fc fusion protein comprised hfsh component . fig7 indicated sds - page of the hfsh - fc fusion protein under reduced and non - reduced conditions . the result demonstrated that the purity of the hfsh - fc protein could reach 98 % or more , and the molecular weight of the hfsh - fc protein under reduced condition was half of that under non - reduced condition . in vitro bioactivity of the recombinant hfsh - fc fusion protein ( immunological activity ) was assayed by elisa kit from biocheck ( usa ) company . experimental procedure was conducted according to specifications of the kit . in vivo activity was assayed by measuring ovarian weight gain based on the 2010 edition of the british pharmacopoeia . protein quantitation was determined using the traditional lowry method . according to british pharmacopoeia , small amount of background hcg ( 701 u / ml ) was used to increase assay sensitivity for ovarian weight gain in rats . sample diluent comprising 701 u / ml hcg was prepared by adding 0 . 1 % albumin phosphate buffer ( ph7 . 2 ± 0 . 2 ). for in vivo assay , each of sample diluent ( ph7 . 2 ± 0 . 2 ) was prepared to equally contain 1 . 67 iu / ml fsh based on elisa activity of test samples , that includes the fsh standard sample ( positive control ), recombinant hfsh , human urine fsh and recombinant hfsh - fc fusion protein . female wistar rats of 19 to 28 days old could be used , however , one experiment was required for those rats of age differences no more than 3 days and weight differences no more than 10 grams to minimize experimental variation . the 24 rats were equally divided into four groups for the following samples : fsh standard sample ( positive control ), recombinant hfsh ( commercial product ), human urine fsh and recombinant hfsh - fc fusion protein . each group were injected with corresponding above sample subcutaneously at the same time every day , two times a day , 0 . 2 ml each time for 3 consecutive days . twenty - four hours after the last injection , animals were killed in accordance with the sequence of administration by cervical vertebra dislocation , and ovaries were drawed and weighed after drying the surfaces . the in vivo activities of recombinant hfsh , human urine hfsh and recombinant hfsh - fc were assayed by the quantity response parallel line method based on the ovarian weight gain of the standard group ( positive control ). the data showed in vitro bioactivity of recombinant hfsh - fc , recombinant hfsh and human urine fsh were 10105 , 9928 and 9321 iu / ml , respectively ; and in vivo bioactivity were 10230 , 8190 and 9051 iu / ml , respectively . these results indicated that , the recombinant hfsh - fc fusion protein of present disclosure has both in vitro and in vivo biological activity . fifteen male wistar rats with weight between 200 - 250 g were randomly and equally divided into 3 groups for recombinant hfsh - fc , human urine fsh and recombinant hfsh . all groups were subcutaneously injected with single dose corresponding sample protein of 15 iu / kg . the blood samples were collected at different time intervals , i . e ., at 1 , 2 , 3 , 4 , 6 , 8 , 12 , 36 , 56 h for human urine fsh and recombinant hfsh groups , at 1 , 2 , 4 , 8 , 12 , 24 , 56 , 120 , 176 , 200 , 264 , 340 h for recombinant hfsh - fc group . the above samples were centrifuged at 3000 rpm for 5 min and plasma supernatants were stored at − 20 ° c . until assays were performed . the immunological activities of fsh in plasma at each time point were tested by elisa kit ( biocheck , usa ). the pharmacokinetic parameters were calculated by statistical method using pksolver 2 software . the pharmacokinetic curves of each group were shown in fig8 , and the data of half - life were shown in table 1 . the results indicated that the half - life of recombinant hfsh and human urine fsh in rats were 11 . 35 ± 1 . 0 h and 12 . 7 ± 2 . 8 h , respectively . the half - life of recombinant hfsh - fc fusion protein of present disclosure was 47 . 24 ± 13 . 92 h , which was at least 4 times of that of recombinant hfsh and human urine fsh . ninety female sd rats with weight between 200 - 250 g were fed and observed for more than 7 days in the experimental environment to ensure their healthy conditions . sixty of the healthy female sd rats were selected , randomly and equally divided into 4 groups : negative control , recombinant hfsh - fc fusion protein and recombinant hfsh , human urine fsh . in addition , the weight distribution of each group rats was similar . firstly , one estrous cycle of these rats was observed and monitored by the method of vagina smear . the group of recombinant hfsh - fc fusion protein was subcutaneously injected with corresponding drug ( fusion protein ) only one time on the first day among diestrous with a dose of 45 iu / kg per rat ; the recombinant hfsh group and human urine fsh group were subcutaneously injected with corresponding drug on the first day and consecutive two days among diestrous ( 15 iu / kg / day ) to give a total dose of 45 iu / kg per rat ; negative control group was subcutaneously injected with same volume of normal saline . all groups were injected with hcg ( 10 iu / each rat ) subcutaneously on the fourth day ( proestrus ). on the fifth day ( estrus ), all rats were anesthetized , ovaries were drawn and weighed . the separated ovaries were fixed by bouin &# 39 ; s fluid and embedded in paraffin . consecutive sections of fixed ovary at 6 m intervals and microscopic organizational observation were performed , and the number of follicles was also recorded . the number of follicles each group was shown in table 2 . the result indicated that , compared with the negative control group , recombinant hfsh - fc fusion protein , recombinant hfsh and human urine fsh can all promote superovulation significantly ( p & lt ; 0 . 01 ) on female rats . importantly , for achieving equivalent efficacy , recombinant hfsh - fc fusion protein of present disclosure only needs one time administration whereas both the recombinant hfsh and human urine fsh need three times administration . seventy - five juvenile sd female rats of 9 days old were randomly divided into two groups : normal control group ( 15 rats ) and animal model group ( 60 rats ). the normal control group was subcutaneously injected once with 0 . 05 ml neutral tea - seed oil on the nape of neck , the animal model group was subcutaneously injected once with 1 . 25 mg testosterone propionate on the nape of neck . vagina was opened at 70th day , and two cycles of consecutive vaginal smear ( total 10 days as one cycle lasting for 5 days ) were performed . the normal control group should have typical estrous cycle ( proestrus , estrus , metestrus , diestrus ) with consecutive vaginal smear , and any rats with abnormal estrous cycle was eliminated from the normal control group . the animal model group &# 39 ; s vaginal epithelial cells should be free of estrous cycle with sustained keratinization , indicating that the anovulatory rat model was successful , and any rats with abnormal performance were eliminated from the model group . at 81th day and later on , 13 rats from the normal control group were injected subcutaneously with distilled water of 10 ml / kg / day . fifty - two rats from the anovulatory model mentioned above were randomly and equally divided into 4 groups : negative control ( animal model ), recombinant hfsh - fc , recombinant hfsh and human urine fsh . rats of the negative control group were injected subcutaneously with distilled water of 10 ml / kg / day ; rats of the recombinant hfsh - fc group were injected subcutaneously with hfsh - fc of 45 iu / kg / time , once every three days lasting for 15 days to give a total of 5 injections ; rats from the recombinant hfsh group and human urine fsh group were injected subcutaneously with corresponding drugs of 7 . 5 iu / kg / time , twice a day lasting for 15 days to give a total of 30 injections . after administration , rats in each group were given a continuous vaginal smear for two estrous cycles to examine their ovulation condition . rats in each group were weighed and anesthetized by intraperitoneal injection with urethan before the estrous cycle . the left ovary and uterus tissue specimens were fixed in 10 % neutral formaldehyde for pathorphological observation of ovary and uterus . quantitative indexes of ovarian observation include the number of follicles at different stages , the number of corpus luteums ; and index of uterus observation was measured by endometrial thickness . table 3 showed results of each group &# 39 ; s estrous cycles , table 4 showed results of each group &# 39 ; s pathological changes of uterus and ovaries . compared with the negative control group of animal model , the recombinant hfsh - fc fusion protein of present disclosure , recombinant hfsh and human urine fsh had a significant promoting effect on inducing estrus of anovulatory female rats ; the number of large follicles and corpus luteums of these later three groups were significantly higher ( p & lt ; 0 . 01 ) than those of the negative control group within anovulatory model . however , the recombinant hfsh and human urine fsh both were required for total 30 times administration and the recombinant hfsh - fc fusion protein was only required for 5 times to achieve the same potency . results showed that the recombinant hfsh - fc fusion protein of present disclosure , recombinant hfsh and human urine fsh all had a significant promoting effect on inducing estrus of anovulatory female rats , but the dosing frequency of the fusion protein from present disclosure was significantly less than that of the recombinant hfsh and human urine fsh . forty female wistar rats of 22 - day old were randomly and equally divided into 4 groups : saline buffer ( negative control ), recombinant hfsh - fc fusion protein , recombinant hfsh ( positive control ), human urine fsh . the last three groups of rats were injected subcutaneously for consecutive four days daily with corresponding protein of 10 iu / day / rat of the recombinant hfsh - fc fusion protein , recombinant hfsh and human urine fsh . at 26 - day , these three groups were injected subcutaneously with 30 iu hcg ; for the negative control group , the 22 - day to 26 - day age of rats was daily injected subcutaneously with the same volume of saline buffer . at 28 - day age , all four groups of rats were injected with 0 . 1 ml of 1 % blue evans ( eb , sigma ) for 30 minutes staining . all rats were then killed by cervical vertebra luxation and abdominal cavities were cut open to observe whether there was ascites . the rats with little or no ascites were injected intraperitoneally with 5 ml of saline , and the perfusate was collected and diluted to 10 ml in a test tube . then , 0 . 05 ml of 0 . 1 m naoh solution was added to the test tube , centrifuging for 10 minutes at 3000 rpm at room temperature . spectrophotometer was used to determine the od at 600 nm . finally , the ovaries of both sides were detached and weighed with electronic balance immediately . observation indexes include : ( 1 ). permeability of abdominal capillary : the eb content of peritoneal lavage fluid was calculated according to the standard curve ploted thereby . ( 2 ). ascites grading : grade 1 : no ascites ; grade 2 : a small amount of ascites ; grade 3 : medium amount of ascites ; grade 4 : large amount of ascites ; grade 5 : massive ascites or ascites overflows from abdominal incision . ( 3 ). ovarian weight : according to golan &# 39 ; s criteria for the diagnosis of ovarian hyperstimulation syndrome , only all three criteria of ovarian volume increasing , abdominal capillary permeability increasing and ascites emerging were met together that ovarian hyperstimulation syndrome can be diagnosed . after 4 days administration of same dose of protein sample , as shown in table 5 , the eb content and ascites score of the recombinant hfsh - fc group had no significant difference from that of the saline control group . however , in comparison with the saline control group , the ovarian weight of the recombinant hfsh - fc group was significantly increased but no ovarian hyperstimulation syndrome was observed . in contrast , the eb content , ovarian weight and ascites score of recombinant hfsh group and human urine fsh group were significantly higher than thseat of the saline control group and recombinant hfsh - fc group , indicating that the recombinant hfsh and human urine fsh could cause ovarian hyperstimulation syndrome . these results demonstrated that the side effect of recombinant hfsh - fc was relatively smaller than that of recombinant hfsh and human urine fsh , indicating recombinant hfsh - fc protein was more safe for clinical application .