Silk gland recombinant expression vector of silkworm (<i>Bombyx mori</i>) expressing human epidermal growth factor, preparation method and use thereof

Abstract

Disclosed is a silk gland recombinant expression vector of silkworm expressing human epidermal growth factor, a preparation method and use thereof. The recombinant expression vector is formed by fusing truncated silk fibroin light chain N-terminal domain, C-terminal domain, optimized human epidermal growth factor and silkworm nuclear polyhedrosis virus enhancer, and removes the redundant part of the extra domain of endogenous silk protein contained in the fusion protein to the maximum extent.

Claims

1. A silk gland recombinant expression vector of silkworm expressing human epidermal growth factor, wherein the silk gland recombinant expression vector of silkworm comprises a vector skeleton and a target gene expression frame, wherein the target gene expression frame comprises an optimized human epidermal growth factor gene sequence (EGF) and a silkworm nuclear polyhedrosis virus enhancer (hr3) sequence, and wherein the nucleotide sequences of the optimized human epidermal growth factor gene sequence and the silkworm nuclear polyhedrosis virus enhancer sequence are set forth in SEQ ID NO:3 and SEQ ID NO:1, respectively.

2. The silk gland recombinant expression vector of silkworm according to claim 1, wherein the target gene expression frame further comprises a sequence-optimized truncated promoter (FHP3.sub.s) sequence encoding amino acids at positions 1-21 of the N-terminal domain of SF-h protein, and wherein the nucleotide sequence of the sequence-optimized truncated promoter sequence is set forth in SEQ ID NO:2.

3. The silk gland recombinant expression vector of silkworm according to claim 1, wherein the target gene expression frame further comprises a truncated SF-h gene light chain binding site (LBS.sub.s) sequence encoding amino acids at positions 1-20 of the C-terminal domain of the SF-h protein, and wherein the nucleotide sequence of the truncated SF-h gene light chain binding site sequence is set forth in SEQ ID NO:5.

4. The silk gland recombinant expression vector of silkworm according to claim 3, wherein the vector skeleton is pBac{3P3-DsRed}.

5. A method for preparing the silk gland recombinant expression vector of silkworm according to claim 4, comprising: sequentially connecting hr3, FHP3.sub.s, EGF and LBS.sub.s to a pUC57-T-simple vector to obtain a recombinant vector PUC-hr3-FHP3.sub.s-EGF-LBS.sub.s containing an hr3-FHP3.sub.s-EGF-LBS.sub.s, expression frame; double-enzyme-cleaving the recombinant vector with AscI/FseI, recovering an hr3-FHP3.sub.s-EGF-LBS.sub.s expression frame fragment, and connecting the fragment to a pBac{3P3-DsRed} vector skeleton double-enzyme-cleaved by AscI/FseI to obtain a silkworm silk gland recombinant expression vector pBac {3P3-DsRed; FHP3-EGF-LBS}.

6. An exogenous fibroin, wherein the exogenous fibroin is expressed by the silk gland recombinant expression vector of silkworm of claim 1.

7. A silk material based on silk fibroin comprising an exogenous fibroin according to claim 6.

8. The silk material according to claim 7, comprising an additional component selected from the group consisting of a transgenic silk fibroin-based injectable hydrogel, a foam scaffold, a silk fibroin film, nanowires, and nanospheres.

9. The exogenous fibroin according to claim 6, wherein the exogenous fibroin is capable of preparing a silk material based on silk fibroin.

10. The exogenous fibroin according to claim 9, wherein the silk material is a transgenic silk fibroin-based injectable hydrogel, a foam scaffold, a silk fibroin film, nanowires and nanospheres.

11. The silk gland recombinant expression vector of silkworm according to claim 2, wherein the target gene expression frame further comprises a truncated SF-h gene light chain binding site (LBS.sub.s) sequence encoding amino acids at positions 1-20 of the C-terminal domain of SF-h protein, and the nucleotide sequence of LBS.sub.s, is set forth in SEQ ID NO:5.

12. The silk gland recombinant expression vector of silkworm according to claim 11, wherein the vector skeleton is pBac{3P3-DsRed}.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIGS. 1A-B are schematic diagrams of expression frames of two different F-POI (EGF) genes and the corresponding transgenic silkworm recombinant vectors. FIG. 1A: Structural diagrams of silkworm fibroin heavy chain gene and expression frames of two different F-POI (EGF) genes; FIG. 1B: Structure diagram of the transgenic silkworm recombinant vector expressing two different F-POI (EGF) proteins.

(2) FIGS. 2A-D are diagrams showing the identification results of transgenic silkworm and transgenic cocoons expressing different F-POI (EGF) proteins. FIG. 2A: Compound eye photos of wild-type silkworm and two transgenic silkworm expressing different F-POI (EGF) proteins;

(3) FIG. 2B: Structural diagrams of the fibroin heavy-chain protein and two different F-POI (EGF) proteins; FIG. 2C: Immunofluorescence detection photos of wild-type monofilament sections before and after degumming and transgenic monofilament sections respectively containing two different F-POI (EGF) proteins; FIG. 2D: SDS-PAGE and Western blot electrophoretogram of the transgenic cocoon silk solution containing two different F-POI (EGF) proteins before and after degumming. Specific F-POI (EGF) proteins with different molecular weights are detected in NEC and N.sub.sEC.sub.s cocoon silk protein samples (indicated by asterisks and triangles, respectively).

(4) FIG. 3 shows digital photographs and scanning electron microscope photographs of different types of silk fibroin-based biological materials (injectable hydrogels, foam scaffolds, silk fibroin films, nanowires and nanospheres) prepared by using wild-type silk fibroin and two regenerated aqueous solutions of transgenic silk fibroin containing different F-POI (EGF) proteins.

(5) FIGS. 4A-D are diagrams showing the results of the identification of the cell growth-promoting activity of the transgenic silk fibroin-based biological material containing the F-POI protein. FIG. 4A: Immunofluorescence images of nucleus, the Ki-67 protein and F-actin after NIH/3T3 cells were treated with different F-POI (EGF) extracted from the transgenic silk fibroin film for 24 h, respectively. FIG. 4B: Cell proliferation state after 24 h of growth detected by Ki-67 staining. FIG. 4C: Cell metabolic activity after 72 h growth detected by using a CCK-8 kit; FIG. 4D: Cell proliferation rate within 72 h of growth detected by QuantiFluor dsDNA kit.

(6) FIGS. 5A-B are result diagrams showing the structural similarity between different recombinant fusion human epidermal growth factor proteins expressed in transgenic silk and the natural EGF protein by a de novo prediction 3D modeling method. FIG. 5A: Multi-sequence alignment of amino acids of two F-POI (EGF) proteins, the recombinant EGF protein (TSF-P.sub.(s)) expressed in transgenic silkworm in the published literature and the natural EGF protein. The amino acids of the N-terminal domain, the EGF sequence and the C-terminal domain are represented by blue, orange and purple backgrounds, respectively. The amino acids at positions 1-21 between the two inverted triangles of the N-terminal domain are the signal peptide sequences of SF-h (in modeling during protein secretion), and the light blue frame ( labeled) and purple frame ( labeled) contained in the N-terminal domain and the C-terminal domain, respectively, indicate the amino acids predicted to undergo phosphorylation modification. The dark blue box ( labeled) contained in the N-terminal domain indicates the amino acids predicted to undergo glycosylation modification, and the black frame ( labeled) contained in the C-terminal domain indicates the cysteine that will form intramolecular or intermolecular disulfide bonds. FIG. 5B: Structural models of two F-POI (EGF) proteins, TSF-P.sub.(s) and the natural EGF protein, wherein purple, orange and blue respectively represent an N-terminal domain of the SF-h protein, a structural fragment of the natural EGF protein and a C-terminal domain (or a truncated C-terminal domain) of the SF-h protein; the blue and purple spheres respectively represent predicted phosphorylation sites on the N-terminal and C-terminal domains; the dark blue sphere indicates predicted glycosylation sites on the N-terminal domain; the pink line indicates the cysteine residue of the C-terminal domain (or the truncated C-terminal domain), and the corresponding surface model is shown with a black dotted box.

DETAILED DESCRIPTION OF THE EMBODIMENTS

(7) Preferred embodiments of the present disclosure will be described in detail with reference to the accompanying drawings. Experimental methods for which specific conditions are not specified in the examples are usually performed under conventional conditions such as those described in Molecular Cloning A Laboratory Manual (3rd edition, J. Sambrook et al.), or as recommended by the manufacturer. Contents of the sequence identification file designated Sequence-Listing-ST25.txt, created Oct. 4, 2023 and having a file size of 9.77 KB are incorporated by reference.

(8) The mouse embryonic fibroblast NIH/3T3 cell line (ATCC) used in Examples of the present disclosure was cultured with Dulbecco's Modified Eagle's Medium (DMEM, all from Gibco) containing 10% fetal bovine serum (FBS), 50 mg/mL streptomycin, and 100 mg/mL ampicillin.

Example 1

(9) Construction of the Silk Gland Recombinant Expression Vector of Silkworm (Bombyx mori) Expressing Human Epidermal Growth Factor

(10) The silkworm silk fibroin heavy chain promoter sequence (FHP3) and the light chain binding site sequence (LBS) in the silkworm SF-h gene (NCBI gene ID: 693030) and the silkworm traditional silk fibroin expression system in the Chinese invention patent (publication NO: CN101195834A), the silkworm nuclear polyhedrosis virus enhancer hr3 sequence (nucleotide sequence of SEQ ID NO:1), the sequence-optimized truncated promoter sequence (FHP3.sub.s, nucleotide sequence of SEQ ID NO:2) encoding amino acids at positions 1-21 of the N-terminal domain of SF-h protein, the optimized human epidermal growth factor (EGF) gene (nucleotide sequence of SEQ ID NO:3, amino acid sequence of SEQ ID NO:4, POI (EGF)) and the light chain binding site sequence (LBS.sub.s, nucleotide sequence of SEQ ID NO:5) of the truncated SF-h gene encoding amino acids at positions 1-20 of the C-terminal domain of SF-h protein were synthesized by GenScript (Nanjing) Co.Ltd., China.

(11) The above gene sequences were spliced into FHP3-EGF-LBS and HR3-FHP3.sub.s-EGF-LBS.sub.s expression frames respectively through seamless connection by the engineer of GenScript (Nanjing) Co.Ltd. Then the expression frames were ligated respectively to region between AscI and FseI restriction sites of the pUC57-T-simple vector to obtain the recombinant vectors pUC-FHP3-EGF-LBS and PUC-hr3-FHP3.sub.s-EGF-LBS.sub.s. Followed by double enzyme digestion of the recombinant vector by AscI/FseI, the corresponding FHP3-EGF-LBS and hr3-FHP3.sub.s-EGF-LBS.sub.s, expression frame fragments were respectively recovered, and then the recovered expression frame fragments were connected into the same enzyme-digested pBac{3P3-DsRed} vector to obtain the transgenic recombinant vector PBAC {3P3-DsRed; FHP3-EGF-LBS} (hereinafter referred to as NEC), and pBac{3P3-DsRed; hr3-FHP3.sub.s-EGF-LBS.sub.s} (hereinafter referred to as N.sub.sEC.sub.s), as shown in FIG. 1A-FIG. 1B. The two constructed transgenic recombinant vectors both contained a red fluorescent protein (DsRed) gene expression frame initiated by a promoter 3P3, and the red fluorescent protein specifically expressed in the eyes and nerves of silkworm would be used as a screening marker for positive transgenic silkworm.

Example 2

(12) Preparation of Transgenic Silkworm

(13) With commercial diapause silkworm strain 932 as the original material, the parental silkworm eggs were treated with low temperature induction at 16 C. to terminate diapause of the offspring silkworm eggs. Then the recombinant vector NEC or N.sub.sEC.sub.s was mixed with the auxiliary plasmid pHA3PIG in a molar ratio of 1:1, the obtained mixture was injected into G0 generation silkworm eggs that had been terminated from diapause, and the result silkworm eggs were placed in an environment with the relative humidity of 85% at 25 C. for incubation after sealed by non-toxic glue. The hatched G0 generation newly-hatched silkworm was fed with mulberry leaves to moths, the obtained G0 generation silkworm moths were seeded by backcrossing or self-inbring, and G1 generation silkworm eggs were collected. Positive transgenic silkworms with red fluorescence in eyes or the nervous system were screened with Olympus motorized macroscopic fluorescence microscopy from G1 generation silkworm eggs which were diapause-terminated. And the obtained positive transgenic silkworms were fed to cocoon collection and further self-inbred and selected to obtain the transgenic silkworm strain capable of stably inherit. Table 1 shows the statistics of transgenic vector injection and screening of positive transgenic silkworms.

(14) TABLE-US-00001 TABLE 1 Statistics of Transgenic Vector Injection and Screening of Positive Transgenic silkworm Hatched Positive G0 Screened moth Injected generation G1 compartment G0 silkworm eggs generation number in G1 generation number moth generation Injecting silkworm eggs (hatching compartment (positive rate, plasmids number rate, %) number %) Strain pBac{3 P3- 353 153 76 4 NEC DsRed; FHP3-EGF- (43.3%) (5.3%) LBS} + pHA3PIG pBac{3 P3- 373 167 84 5 N.sub.sEC.sub.s DsRed; hr3-FHP3.sub.s-EGF- (44.8%) (6%) LBS.sub.s} + pHA3PIG

(15) The results were shown in FIG. 2A, FIG. 2B and Table 1, the red fluorescence caused by DsRed-Express was observed in the eyes of the obtained NEC and N.sub.sEC.sub.s silkworm moths (adults), while no fluorescence in the eyes of wild-type (WT) silkworm moths was observed (FIG. 2A), indicating that all the designed transgenic expression frames were successfully inserted into the NEC and N.sub.sEC.sub.s silkworm genomes; FIG. 2 B showed the structural schematic diagrams of two F-POI (EGF) proteins expressed in vivo and in the silkworm cocoons of NEC and N.sub.sEC.sub.s. The F-POI (EGF) protein expressed by NEC contained the complete N-terminal domain and C-terminal domain of SF-h protein, and the F-POI (EGF) protein expressed by N.sub.sEC.sub.s contained the optimized N-terminal domain encoding SF-h protein and the optimized C-terminal domain encoding SF-h protein. The results in Table 1 showed that, by fluorescence detection and molecular identification, the obtained genomes contained the transgenic silkworm strains NEC and N.sub.sEC.sub.s with the expression frames of FHP3-EGF-LBS and hr3-FHP3.sub.s-EGF-LBS.sub.s inserted, respectively.

Example 3

(16) Detection of F-POI (EGF) Protein Expression in Transgenic Cocoon Silk

(17) In order to extract the mixed SF and F-POI (EGF) from the cocoon for further material preparation, the cocoon was cut into pieces and degummed to remove the sericin layer. The specific operation steps of degumming treatment were as follows: NEC and N.sub.sEC.sub.s silkworm cocoons expressing two different F-POI (EGF) proteins were randomly selected and cut into 0.5-1 cm pieces, immersed in a mixture of 0.08% (w v.sup.1) sodium carbonate and 0.12% (w v.sup.1) Marseille soap, degummed for 45 min at 70 C., and rinsed for 3-4 times with ultra-pure water of Milli-Q system (Millipore, Billerica, MA) to remove sericin.

(18) To identify the distribution of two F-POI (EGF) proteins in NEC and N.sub.sEC.sub.s silk fibers, a transgenic silk fiber sample for tissue immunofluorescence examination was prepared. The specific operation steps were as follows: The non-degummed or degummed NEC and N.sub.sEC.sub.s silk fibers were fixed overnight with 10% (v v.sup.1) formalin, the silk fibers were wound into bundles (about 50-100 silk fibers), embedded in Tissue-Tek OCT Tissue Freezing Medium (Sakura Finetek, Torrance, CA, US), and then frozen overnight at 40 C. Subsequently, the frozen embedded samples were cut into 10 m thick sections, and the sections were immunohistochemical stained with anti-EGF antibodies (ab155576, Abcam, US) as primary antibodies and goat anti-mouse IgG H&L (Alexa-Fluor 594) (ab 150116, Abcam, US) carrying fluorescent groups as secondary antibodies, and observed under fluorescence microscopy (BX51RF Olympus, Tokyo, Japan). The results were shown in FIG. 2 C. The red fluorescence signals were observed in the silk fibroin layer of both NEC and N.sub.sEC.sub.s silk fibers before degumming, but no fluorescence signal in the sericin layer was observed. However, relatively strong red fluorescence signals were still retained in the degummed NEC and N.sub.sEC.sub.s silk fibroin. The results showed that the truncated N-terminal and C-terminal domains could also ensure the specific secretion of the F-POI (EGF) protein into the silk fibroin layer of cocoon silk, and maintain the stability of the F-POI (EGF) protein in the silk fibroin layer during the degumming process.

(19) The loss rate of the F-POI (EGF) protein caused by the degumming process was evaluated by SDS-PAGE and Western blot detection and calculation of the expression levels of two different F-POI (EGF) proteins in the cocoons of NEC and N.sub.sEC.sub.s, respectively. The specific operations were as follows: 30 mg of degummed or non-degummed NEC and N.sub.sEC.sub.s cocoon pieces were respectively weighed and immersed in 500 L of 9.3M lithium bromide (LiBr) solution and dissolved at 60 C. for 2.5 h. The regenerated solution was dialyzed by using dialysis membrane (MWCO 3500, Pierce) to remove salts. The concentration of cocoon silk protein solution after dialysis was detected by dicyandiamide (BCA) protein detection kit (Beyotine Biotech). 2% (v v.sup.1) of -galactosidase (2-ME) was added into the protein sample and boiled for 5 min, followed by SDS-PAGE (15%, Bio-Rad) gel for electrophoretic separation. The commercially available EGF standard (AF-100-15, PeproTech, US) was used as the positive control and wild-type (WT, common cocoon) degummed cocoon silk was used as the negative control. The SDS-PAGE gel after electrophoresis was stained with 0.1% (v v.sup.1) Coomassie brilliant blue R-250, 10% (v v.sup.1) acetic acid, and 50% (v v.sup.1) methanol. Western blotting was performed using commercial anti-EGF antibodies (ab155576, Abcam, US) as the primary antibody and the corresponding goat anti-mouse H&L IgG (HRP) (ab205719, Abcam, US) as the secondary antibody according to the operation procedures described in the kit instructions. Immunoreactive protein hybridization signals were detected by the Clinx chemiluminescent imaging system (Chemicope 5300, Shanghai, China) using ECL+Western Blot assay (Beyotime, Jiangsu, China) according to the manufacturer's instructions. The intensity of the F-POI (EGF) protein hybridization signal was calculated using Image-J software, and the results of three independent experiments were taken and conducted for error analysis (the results were expressed as meanstandard deviation). The results were as shown in FIG. 2 D, Table 2 and Table 3. Compared with the wild-type (WT) cocoon silk sample, specific F-POI (EGF) proteins with different molecular weights were detected in NEC and N.sub.sEC.sub.s cocoon silk protein samples (FIG. 2D). Through comparison with protein Marker, it was found that the actual molecular weight of the F-POI (EGF) protein in NEC cocoon silk protein sample (about 36 kDa) was greater than its theoretical molecular weight (25.9 kDa), while the actual molecular weight of the F-POI (EGF) protein in N.sub.sEC.sub.s cocoon silk protein sample (about 9 kDa) was similar to its theoretical molecular weight (8.5 kDa) (Table 2). By analyzing the Western blotting hybrid bands by Image-J software, the results showed that the proportions of the two F-POI (EGF) proteins in the total unregummed NEC and NECs cocoon shell proteins were 14.281.09% and 2.750.51%, respectively; and after the degumming treatment, the proportions of the two F-POI (EGF) proteins in NEC and N.sub.sEC.sub.s silk fibroin protein solutions were 7.131.79% and 1.160.23%, respectively (Table 3). Summing up the above research results, it shows that the actual molecular weight of the F-POI (EGF) protein in the N.sub.sEC.sub.s cocoon silk could can be consistent with and the theoretical molecular weight by using the truncated N-terminal domain and the C-terminal domain expressed by the novel silk gland expression system disclosed by the present disclosure, that is, additional post-translational modification of the F-POI (EGF) protein could also be avoided. In addition, the loss rates of the two F-POI (EGF) proteins in the degumming process were both 50%, indicating that the truncated N-terminal domain and C-terminal domain were consistent with the traditional full-length N-terminal domain and C-terminal domain in affecting the degumming loss of the F-POI (EGF) protein. The truncated N-terminal domain and C-terminal domain did not lead to an increase in the loss rate of the F-POI (EGF) protein degumming. (Table 2: Theoretical and actual molecular weight statistics of the two different F-POI (EGF) proteins contained in the transgenic cocoon silk and the EGF standard; Table 3: The percentage of two different F-POI (EGF) proteins contained in transgenic cocoon silk in the total protein mass of cocoon silk before and after degumming treatment)

(20) TABLE-US-00002 TABLE 2 F-POI or Theoretical Actual standard (kDa) (kDa) NEC 25.9 36 N.sub.sEC.sub.s 8.5 9 EGF 6.2 6 standard

(21) TABLE-US-00003 TABLE 3 F-POI Cocoon (%) Solution (%) NEC 14.28 1.09 7.13 1.79 N.sub.sEC.sub.s 2.75 0.51 1.16 0.23

Example 4

(22) Preparation of Different Types of Functional Silk Fibroin-Based Biological Materials Using F-POI (EGF) Protein Cocoon Silk Injectable Hydrogels, Foam Scaffolds, Silk Fibroin Films, Nanowires and Nanospheres

(23) Using the NEC and NECs silk fibroin protein aqueous solutions respectively containing two different F-POI (EGF) proteins obtained in Example 3 as raw materials, a transgenic silk fibroin-based injectable hydrogel, foam scaffold, silk fibroin film, nanowire and nanosphere (FIG. 3) were prepared according to the processing method of the conventional silk fibroin-based biological material, thereby confirming that the two SF/F-POI (EGF) mixed aqueous solutions obtained by the present disclosure could be used for preparing various biological materials similar to the conventional natural silk fibroin-based tissue engineering applications. The specific preparation steps were as follows:

(24) Injectable hydrogels were prepared by an ultrasonic method: i) a NEC (or N.sub.sEC.sub.s) silk fibroin solution was diluted with deionized water to a concentration of 20 mg/mL; ii) the silk fibroin solution was ultrasonically treated by an ultrasonic instrument at an amplitude of 20% for 60 s, and paused for 1 s every 1 s of ultrasound; iii) The ultrasonic silk fibroin solution was injected into a syringe and placed at 4 C. overnight to obtain NEC (or N.sub.sEC.sub.s) silk fibroin-based injectable hydrogel.

(25) Foam scaffolds were prepared by casting: i) a NEC (or N.sub.sEC.sub.s) silk fibroin solution was diluted with deionized water to a concentration of 20 mg/mL and transferred to a plastic culture dish; ii) the culture dish was placed in a refrigerating chamber to cool the silk fibroin solution to 4 C., and then the culture dish was transferred to a 20 C. refrigerator and left overnight; iii) the culture dish was subsequently transferred to a refrigerator at the temperature of 80 C. for further storage for 24 h; iv) then the silk fibroin was freeze-dried, the freeze dried silk fibroin was annealed overnight at room temperature with water vapor; and v) finally, the obtained silk fibroin foam scaffolds were cut into cubes.

(26) Silk fibroin films were prepared by film casting: i) a NEC (or N.sub.sEC.sub.s) silk fibroin solution was diluted with deionized water to a concentration of 20 mg/mL and cast on a plastic culture dish; ii) the silk fibroin solution was annealed overnight at room temperature with water vapor after naturally air-drying; and iii) finally, the obtained silk fibroin films were punched into disc shape by using a hole punch.

(27) The nanofiber mesh was prepared by an electrospinning method: i) polyethylene oxide and NEC (or N.sub.sEC.sub.s) silk fibroin solution (concentration: 75 mg/mL) were mixed and dissolved in a mass ratio of 0.3:1; ii) the mixed solutions was electrospinned using an electrostatic spinning machine (flow rate of 20 L/min, voltage of 20 to 25 kV, distance of 20 cm); iii) the nanofiber meshes were annealed overnight at room temperature with water vapor after electrospinning.

(28) The nanospheres were prepared by acetone precipitation method: i) a NEC (or N.sub.sEC.sub.s) silk fibroin solution was diluted with deionized water to a concentration of 40 mg/mL, and then dropwise added into an acetone solution to make the silk fibroin form nanospheres and separate out. ii) the nanospheres were washed with deionized water, and centrifuged at 5000 rpm for 1 h. The nanospheres were collected three times in total.

(29) The cell growth promoting activities of transgenic silk fibroin-based biological materials containing two different F-POI (EGF) proteins were further compared. The biological activities of two different F-POI (EGF) proteins in silk fibroin films were detected by using mouse fibroblast NIH/3T3 cells. Two kinds of F-POI (EGF) proteins solutions extracted from the silk fibroin films of the NEC and N.sub.sEC.sub.s groups and a commercial EGF standard (AF-100-15, PeproTech, US) solution (as the positive control group) were diluted to a concentration of 0.25 nM (0.25 nM was the optimal stimulation concentration of the experimentally identified EGF protein on NIH/3T3 cells) and the NIH/3T3 cells were treated with the diluted solution. Since the diluted wild-type (WT) silk fibroin film extract had no significant effect on cell proliferation, a cell culture medium free of EGF was used as the negative control group.

(30) The metabolic activity of cells was determined by detecting the protein content of Ki-67, a proliferation marker related to ribosomal RNA transcription, in different treatment groups. The results were as shown in FIG. 4A, FIG. 4B and FIG. 4C. After treatment for 24 h, the percentages of positive proliferating cells containing Ki-67 in N.sub.sEC.sub.s and positive control group were significantly higher than those in NEC group (P<0.05) and negative control group (P<0.01) (FIG. 4A and FIG. 4B). From the experimental results, it could be shown that compared with the traditional F-POI (EGF) protein, the F-POI (EGF) protein containing the truncated N-terminal domain and the C-terminal domain had biological activity for promoting cell proliferation comparable to that of the commercially available EGF standard. After 72 h of treatment, the detection result of Ki-67 protein was confirmed by CCK-8 detection. Compared with the traditional F-POI (EGF) protein, the F-POI (EGF) protein containing the truncated N-terminal domain and C-terminal domain and the commercially available EGF standard treatment group significantly improved the metabolic activity of the cells (P<0.05) (FIG. 4C).

(31) In addition, cell proliferative activity was quantified by measuring the DNA content of cells from different treatment groups. The results were as shown in FIG. 4 D. Within the first 12 h, the cell proliferation situations of the four experimental groups were similar. After 24 h, the numbers of cells in the positive control group and the N.sub.sEC.sub.s group were significantly higher than that in the negative control group (P<0.05), and the difference became larger with the culture time prolonging. After 72 h, the cell proliferation rates in the positive control group and the N.sub.sEC.sub.s group were significantly higher than that in the NEC group (P<0.01), while the number of cells in the NEC group was significantly higher than that in the negative control group (P<0.05). In summary, the biological activity of the F-POI (EGF) protein containing the full-length N-terminal domain and the C-terminal domain expressed by the traditional silk gland expression system is obviously lower than that of the F-POI (EGF) protein containing the truncated N-terminal domain and the C-terminal domain expressed by the novel silk gland expression system disclosed by the present disclosure. In addition, the biological activity of F-POI (EGF) containing the truncated N-terminal domain and the C-terminal domain was even comparable to that of commercially available EGF standard.

Example 5

(32) Comparison of Structural Similarities Between Different Recombinant Fused Human Epidermal Growth Factor Proteins and the Natural EGF Protein Using the 3D Modeling Method

(33) In a paper entitled Creation and research of novel silk material with hEGF bioactivity, a recombinant epidermal growth factor active protein expressed in silk named TSF-P.sub.(s) was disclosed (Lu Weijian. Creation and research of novel silk material with hEGF bioactivity. Dissertation for Master's Degree of Southwest University, 2015). Compared with the POI (EGF) protein sequence of N.sub.sEC.sub.s group in Example 1, the amino acid sequence of the disclosed EGF protein has 4 more amino acids upstream and 4 more amino acids downstream (FIG. 5A). From the perspective of biological research, it is recognized that the activity or function of a protein is determined by its three-dimensional structure. In order to evaluate the structural similarity between the two F-POI (EGF) proteins, the TSF-P.sub.(s) protein and the natural EGF protein, the 3D structural models of the two F-POI (EGF) proteins, the TSF-P.sub.(s) protein and the natural EGF protein were constructed using DMPfold (http://bioinf.cs.ucl.ac.uk/psipred/) for de novo modeling of proteins. As shown in the structural model of FIG. 5B, the F-POI (EGF) protein from the NECs group, the TSF-P.sub.(s) protein and the natural EGF protein had similar center-aligned -sheet structures, while F-POI (EGF) from the NEC group did not have this center-aligned structure. As shown in Table 4, the C-alpha RMSD values (3.173 ) between the F-POI (EGF) protein in the N.sub.sEC.sub.s group and the native EGF protein were much smaller than the C-alpha RMSD values (11.200 and 9.413 , respectively) between the F-POI (EGF) or TSF-P.sub.(s) protein and the native EGF protein in the NEC group. In addition, the TM-score of the predicted structure of the F-POI (EGF) protein in the N.sub.sEC.sub.s group relative to the natural EGF protein was 0.413, much higher than the TM-score (0.066) of the predicted structure of the F-POI (EGF) protein in the NEC group relative to the natural EGF protein, and much higher than the TM-score (0.125) of the predicted structure of the TSF-P.sub.(s) protein relative to the natural EGF protein. Among them, the TM-score range of 0-0.1747 indicated the randomness or dissimilarity between the two protein structures. In conclusion, the data from the 3D structural model of the protein indicated that, compared with the other two recombinant EGF proteins, the F-POI (EGF) protein with the redundant post-translational modification sites removed from the N.sub.sEC.sub.s group had the highest structural similarity with the natural EGF protein, thereby effectively enhancing the biological activity of expressing the F-POI (EGF) protein in the cocoon silk material from the N.sub.sEC.sub.s group. (Table 4: Root mean square deviation (RMSD) and TM-score of C-alpha atoms between different F-POI (EGF) and TSF-P.sub.(s) protein models and natural EGF protein models)

(34) TABLE-US-00004 TABLE 4 Protein RMSD TM- models () score NEC 11.200 0.066 N.sub.sEC.sub.s 3.173 0.413 TSF-P.sub.(s) 9.413 0.125

(35) Finally, the above examples are only for illustrating the technical scheme of the present disclosure and are not limiting. Although the technical scheme of the present disclosure has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that, without departing from the spirit and scope of the present disclosure, several modifications or equivalent substitutions can be made, and these modifications and equivalent substitutions should also fall within the scope of the claims of the present disclosure.