Effects of rAAV-mediated FGF-2 gene transfer and overexpression upon the chondrogenic differentiation processes in human bone marrow aspirates
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Application of genetically modified bone marrow concentrates in articular cartilage lesions is a promising approach to enhance cartilage repair by stimulating the chondrogenic differentiation processes in sites of injury.
In the present study, we examined the potential benefits of transferring the proliferative and pro-chondrogenic basic fibroblast growth factor (FGF-2) to human bone marrow aspirates in vitro using the clinically adapted recombinant adeno-associated virus (rAAV) vectors to monitor the biological and chondrogenic responses over time to the treatment compared with control (lacZ) gene application.
Effective, significant FGF-2 gene transfer and expression via rAAV was established in the aspirates relative to the lacZ condition (from ~ 97 to 36 pg rhFGF-2/mg total proteins over an extended period of 21 days). Administration of the candidate FGF-2 vector led to prolonged increases in cell proliferation, matrix synthesis, and chondrogenesis but also to hypertrophic and terminal differentiation in the aspirates.
The present evaluation shows the advantages of rAAV-mediated FGF-2 gene transfer to conveniently modify bone marrow concentrates as a future approach to directly treat articular cartilage lesions, provided that expression of the growth factor is tightly regulated to prevent premature hypertrophy in vivo.
KeywordsCartilage repair Gene therapy Human bone marrow aspirates rAAV FGF-2
The adult articular cartilage has a limited ability for self-repair (Buckwalter 1998) due to the absence of vascularization in this particular joint tissue that may be a potential source of chondrogenerative cells such as mesenchymal stem cells (MSCs) (Orth et al. 2014). While progenitor cells can be made accessible within sites of injury by procedures like marrow stimulation (Dewan et al. 2014; Madry et al. 2011), the outcomes of such techniques do not match the expected native hyaline cartilage and instead, a fibrocartilaginous repair tissue is formed in the lesions with mostly type-I versus type-II collagen and proteoglycans (Dewan et al. 2014). One-step administration of bone marrow concentrates containing chondrogenically competent MSCs (Anam and Davis 2013; Lennon et al. 2000) within cartilage defects rather than of isolated and/or expanded MSCs raised important attention as a new, convenient option to improve cartilage repair (Orth et al. 2014). Still, even when using such a straightforward approach (Enea et al. 2013; Gigante et al. 2012; Kim et al. 2014; Orozco et al. 2013; Skowronski et al. 2013; Slynarski et al. 2006; Wakitani et al. 2004), an original (structural and functional) articular cartilage repair tissue could not be regenerated in patients, demonstrating the necessity to develop improved therapeutic strategies to manage cartilage injuries.
The modification of bone marrow aspirates by gene transfer technologies is a promising, new avenue of translational research as a means to enhance the chondrogenic responses to injury by overexpressing target candidates with potential reparative activities (Cucchiarini et al. 2014; Frisch et al. 2015b). In this regard, a number of studies revealed the feasibility of applying adenoviral vectors at very high doses (1010-1011) to promote the expression of reporter genes (E. coli β-galactosidase, firefly luciferase, green fluorescent protein) (Pascher et al. 2004) or therapeutic sequences including the transforming growth factor beta (TGF-β) (Ivkovic et al. 2010), bone morphogenetic protein 2 (BMP-2) (Sieker et al. 2015), and Indian hedgehog (IHH) (Sieker et al. 2015) in marrow aspirates from rabbits and sheep but only for relatively short periods of time (some days). Instead, we recently provided evidence that recombinant adeno-associated virus (rAAV) vectors are adapted gene vehicles to competently transduce human marrow concentrates over extended periods of time at very high efficiencies (~90 % for at least 125 days) using much lower vector doses (8 × 105) and without detrimental effect (Rey-Rico et al. 2015) compared with the more immunogenic adenoviral vectors (Frisch et al. 2015b). We showed for instance that effective rAAV-mediated overexpression of the transcription factor SOX9 (Rey-Rico et al. 2015) or of insulin-like growth factor I (IGF-I) (Frisch et al. 2015a) was capable of activating the chondrogenic processes in freshly prepared human marrow samples. Here, we focused on delivering the basic fibroblast growth factor (FGF-2) to human marrow aspirates in light of the proliferative and pro-chondrogenic activities of this agent in isolated human MSCs when applied as a recombinant molecule (Solchaga et al. 2010; Solchaga et al. 2005; Tsutsumi et al. 2001) or upon administration of the current rAAV FGF-2 vector (Cucchiarini et al. 2011). The present study reveals that delivery of rAAV allows for an effective and durable production of FGF-2 in human marrow concentrates, allowing for enhanced levels of cell proliferation, matrix synthesis, and chondrogenic differentiation in such samples. Processes of hypertrophic and terminal differentiation were also activated, suggesting that a critical regulation of transgene expression will be needed for FGF-2 using rAAV vectors prior to translation of the approach in vivo. Nevertheless, these findings reflect the value of rAAV gene transfer to modify marrow concentrates for future procedures of implantation in sites of cartilage damage.
All reagents were from Sigma (Munich, Germany), unless otherwise identified. Recombinant TGF-β (rTGF-β) was purchased at Peprotech (Hamburg, Germany) and the dimethylmethylene blue dye at Serva (Heidelberg, Germany). The anti-FGF-2 (C-18) and anti-SOX9 (C-20) antibodies were from Santa Cruz Biotechnology (Heidelberg, Germany), the anti-type-II collagen (II-II6B3) antibody from the NIH Hybridoma Bank (University of Iowa, Ames, USA), the anti-type-I collagen (AF-5610) antibody from Acris (Hiddenhausen, Germany), and the anti-type-X collagen (COL-10) antibody from Sigma. Biotinylated secondary antibodies and the ABC reagent were obtained from Vector Laboratories (Alexis Deutschland GmbH, Grünberg, Germany). The FGF-2 Quantikine ELISA (DFB50) was from R&D Systems (Wiesbaden, Germany) and the type-II, −I, and -X collagen ELISAs from Antibodies-Online (Aachen, Germany).
Plasmids and rAAV vectors
All plasmids are based on the same parental AAV-2 genomic clone, pSSV9 (Samulski et al. 1987; Samulski et al. 1989). rAAV-lacZ carries the lacZ gene encoding the E. coli β-galactosidase (β-gal) and rAAV-hFGF-2 a human basic fibroblast growth factor (hFGF-2) cDNA fragment (480 bp), both under the control of the cytomegalovirus immediate-early (CMV-IE) promoter (Cucchiarini et al. 2011; Frisch et al. 2015a; Rey-Rico et al. 2015). Conventional (not self-complementary) rAAV vectors were packaged using the 293 adenovirus-transformed embryonic kidney cell line. Helper functions were provided by Adenovirus 5 in combination with rep and cap functions of a pAd8 helper plasmid as previously described (Cucchiarini et al. 2011; Frisch et al. 2015b). Purification, dialysis, and titration of the vector preparations via real-time PCR were performed, averaging 1010 transgene copies/ml with approximately 1/500 functional recombinant viral particles (Cucchiarini et al. 2011; Frisch et al. 2015b; Rey-Rico et al. 2015).
rAAV-mediated gene transfer
Bone marrow was aspirated from the distal femurs of patients undergoing total knee arthroplasty (~10 ml, n = 3). Aspirates were immediately aliquoted in a volume of 100 μl per well in 96-well plates and transduced with 40 μl vector (i.e. 8 × 105 functional recombinant viral particles, MOI = 10 ± 3) (Frisch et al. 2015b; Rey-Rico et al. 2015). Samples were incubated for up to 21 days with chondrogenic medium (4.5 g/l DMEM high glucose, 100 U/ml penicillin, 100 μl/ml streptomycin, 6.25 μg/ml insulin, 6.25 μg/ml transferrin, 6.25 μg/ml selenious acid, 5.35 μg/ml linoleic acid, 1.25 μg/ml BSA, 1 mM sodium pyruvate, 37.5 μg/ml ascorbate 2-phosphate, 10−7 M dexamethasone, and 10 ng/ml TGF-β3) with careful weekly medium change as previously described (Cucchiarini et al. 2011; Frisch et al. 2015b).
FGF-2 production was monitored by ELISA at the denoted time points by absorbance measurements on a GENios spectrophotometer/fluorometer (Tecan, Crailsheim, Germany) and by immunohistochemistry using a specific FGF-2 antibody, a biotinylated secondary antibody, and diaminobenzidine as a chromogen (ABC method) (Cucchiarini et al. 2011). A control condition with omission of the primary antibody was included to check for secondary immunoglobulins. All sections were examined under light microscopy (Olympus BX45, Olympus, Hamburg, Germany).
The aspirates were resuspended in a total volume of 100 μl of fresh DMEM and digested with papain (final concentration 75 μg/ml) at 60 °C (Frisch et al. 2015b). The DNA contents were measured by fluorimetry using Hoechst 22358 and the proteoglycan contents by binding to dimethylmethylene blue dye (Frisch et al. 2015b). The type-II, −I, and -X collagen contents were determined by ELISA (Frisch et al. 2015a). Values were normalized to total cellular proteins monitored via Pierce Thermo Scientific Protein Assay (Fisher Scientific, Schwerte, Germany). All measurements were performed on a GENios spectrophotometer/fluorometer (Tecan).
Histological and immunohistochemical analyses
Aspirates were collected and fixed in 4 % formalin with subsequent dehydration in graded alcohols, paraffin embedding and sectioning at 3 μm. Hematoxylin eosin (H&E) staining was performed to evaluate cellularity and toluidine blue and alizarin red staining for the detection of matrix proteoglycans and matrix mineralization, respectively (Frisch et al. 2015a; Rey-Rico et al. 2015). The expression of type-II, −I, and -X collagen and of SOX9 was evaluated by immunohistochemistry using specific primary antibodies, biotinylated secondary antibodies and the ABC method (Frisch et al. 2015b; Rey-Rico et al. 2015). Control conditions were included by omitting the primary antibodies. All sections were examined under light microscopy (Olympus BX45).
Cell proliferation was evaluated by counting the total cells per standardized area on H&E-stained sections (Frisch et al. 2015b). The intensities of H&E, toluidine blue, and alizarin red staining and those of FGF-2, type-II, −I, and -X collagen, and SOX9 immunostaining were monitored at magnification ×20 by inverting the pictures to grayscale mode, adapting background DAB signal for comparable range, and measuring the mean gray value per total area covered with cells (mm2) (Frisch et al. 2015b; Rey-Rico et al. 2015). The data were recorded at three random standardized sites or with 10 serial histological and immunohistochemical sections for each parameter, test and replicate condition using the SIS analySIS program (Olympus) and Adobe Photoshop (Adobe Systems, Unterschleissheim, Germany) and are given as mean intensity of staining or immunostaining in pixels per mm2 of total cell area (Frisch et al. 2015b; Rey-Rico et al. 2015).
Real-time RT-PCR analyses
Primers used for real-time RT-PCR
Primer sequences (5’-3’)
Each condition was performed in duplicate in two independent experiments for each patient. Data are expressed as mean ± standard deviation (SD) of separate experiments. The t-test and Mann–Whitney Rank Sum Test were used where appropriate. Any P value of less than 0.05 was considered statistically significant.
Effective genetic modification of chondrogenically-induced human bone marrow aspirates to overexpress FGF-2 via rAAV gene transfer
Analysis of the levels of FGF-2 production in rAAV-transduced, chondrogenically-induced human bone marrow aspirates
Effects of FGF-2 overexpression via rAAV upon the proliferation and chondrogenic differentiation of human bone marrow aspirates
Chondrogenically-induced aspirates were next transduced with rAAV-hFGF-2 relative to rAAV-lacZ in order to monitor the proliferative activities and differentiation events in the samples following FGF-2 overexpression. As we previously reported a lack of deleterious effects of rAAV gene transfer upon the potency of bone marrow aspirates (Rey-Rico et al. 2015), we did not test a condition lacking vector treatment in this study.
Effects of FGF-2 overexpression via rAAV upon the hypertrophic and terminal differentiation of human bone marrow aspirates
Chondrogenically-induced aspirates treated with rAAV-hFGF-2 versus rAAV-lacZ were finally processed to evidence possible effects of transgenic FGF-2 expression upon possible hypertrophic and terminal differentiation events in the samples.
Therapeutic gene transfer via clinically adapted rAAV vectors is an attractive strategy to effectively and durably enhance the chondrogenic processes in bone marrow concentrates prior to re-implantation in sites of cartilage injury (Cucchiarini et al. 2014; Frisch et al. 2015a). The goal of this study was to demonstrate the possibility of applying a functional rAAV construct carrying the sequence for the proliferative and pro-chondrogenic FGF-2 (Solchaga et al. 2010; Solchaga et al. 2005; Tsutsumi et al. 2001) to freshly prepared human marrow aspirates in light of our previous findings in isolated human MSCs (Cucchiarini et al. 2011) and of evidence showing the potential of using such an approach to deliver reporter genes and other candidate sequences (SOX9, IGF-I) (Frisch et al. 2015b; Rey-Rico et al. 2015).
The results demonstrate that significant levels of FGF-2 expression were achieved in the aspirates upon rAAV-mediated gene transfer compared with control (lacZ) transduction over the whole period of evaluation (21 days), probably due to a good maintenance of this vector class in these targets (Rey-Rico et al. 2015), in agreement with our previous findings using the same vector to target isolated human MSCs (Cucchiarini et al. 2011). The decrease in FGF-2 expression noted over time may be due to a dilution of the transgene sequences upon cell proliferation and expansion as the rAAV vectors are mostly kept under (stable) episomal forms in their targets. Interestingly, the amounts of factor produced in the aspirates over time following rAAV-hFGF-2 application (~36 pg rhFGF-2/mg total proteins/24 h after 21 days in chondrogenic medium) were lower than those reported in MSCs (~137 pg/ml/24 h using 2 × 105 cells in chondrogenically-induced aggregate cultures, i.e. ~ 360 pg rhFGF-2/mg total proteins/24 h and a 10-fold difference) (Cucchiarini et al. 2011), reflecting the presence of other cell subpopulations in the aspirates (hematopoietic progenitor cells, hematopoietic cells, fibroblasts) that may not be a source of FGF-2 expression after 21 days of continuous chondrogenic induction as mostly the MSCs specifically and extensively commit toward the chondrocyte phenotype in this environment (Anam and Davis 2013; Frisch et al. 2015b; Lennon et al. 2000), being the major contributor of production over time.
Such sustained levels of FGF-2 generated from rAAV gene delivery were capable of stimulating the proliferative, biosynthetic, and chondrogenic activities in the aspirates over time versus lacZ treatment, consistent with the effects of FGF-2 when used in a recombinant form (Solchaga et al. 2010; Solchaga et al. 2005; Tsutsumi et al. 2001) or following application of the candidate rAAV-hFGF-2 construct to isolated human MSCs (Cucchiarini et al. 2011). The levels of proliferation reached over time in the aspirates via FGF-2 gene transfer were higher than those observed in isolated MSCs (~2.3 μg versus ~ 1.1 ng DNA/mg total proteins after 21 days, respectively, i.e. an ~ 2.1 × 103-fold difference) (Cucchiarini et al. 2011), probably due to the presence of other mitogenic factors in the samples secreted by all marrow populations. Also, the levels of matrix biosynthesis in the aspirates mediated over time by FGF-2 overexpression were higher than those noted in isolated MSCs (~1087 versus ~ 170 pixels of standardized toluidine blue staining, ~ 66 versus ~ 0.9 ng proteoglycans/mg total proteins, and ~ 3.4 versus ~ 0.025 ng type-II collagen/mg total proteins on day 21, respectively, i.e. an up to 136-fold difference) (Cucchiarini et al. 2011), again possibly due to a more favorable biochemical and cellular environment for MSC chondrogenesis in heterogeneous aspirates or to an up-regulation of the levels of highly chondrogenic transcription factor SOX9 (Bell et al. 1997; Lefebvre et al. 1997) by FGF-2 in these samples versus isolated MSCs (Cucchiarini et al. 2011), concordant with the reported stimulating effects of the growth factor on the activity of the sox9 gene (Murakami et al. 2000).
Interestingly, the results also indicate that administration of the rAAV FGF-2 construct promoted the development of hypertrophic and terminal differentiation events in the aspirates in the particular conditions selected, with enhanced levels of MMP13, ALP, and RUNX2 (markers of terminal and osteogenic differentiation, transcription factor controlling the osteoblastic expression of COL1, COL10, and MMP13) as previously described with recombinant FGF-2 (Solchaga et al. 2010; Solchaga et al. 2005; Tsutsumi et al. 2001) but in contrast with our findings when applying the current vector to isolated MSCs (Cucchiarini et al. 2011). Such discrepancy may be due to pro-hypertrophic activities induced by other marrow subpopulations.
This study demonstrate the possibility of delivering a candidate FGF-2 sequence to human bone marrow concentrates via rAAV vectors as a means to expand the processes of chondrogenic differentiation in such samples in vitro. It remains to be seen whether prolonged FGF-2 secretion via rAAV may not lead to side (paracrine and autocrine) effects (activation of proliferation/differentiation of undesirable cell populations), even though we did not observe such problems when applying directly the current vector to focal experimental articular cartilage defects in vivo (Cucchiarini et al. 2005). Work is now ongoing to define the subpopulations participating in the chondrogenic events in order to investigate cell heterogeneity within the aspirates (Anam and Davis 2013; Jones et al. 2002; Lennon et al. 2000). We are also currently testing the feasibility of translating the approach in vivo by implanting genetically modified marrow aspirates within experimental models of articular cartilage defects to corroborate the observations in vitro (Cucchiarini et al. 2013; Ivkovic et al. 2010; Pascher et al. 2004; Sieker et al. 2015). As application of rAAV FGF-2 led to premature hypertrophy and terminal differentiation in the aspirates, controlled expression of this factor as afforded by using tissue-specific (type-II collagen, SOX9) or regulatable promoters (tetracycline-sensitive) may allow to restrain such possible, undesirable events in vivo. Alternatively, combined gene transfer using extra, anti-hypertrophic agents might be capable of circumventing these effects in vivo like by co-applying the SOX9 transcription factor (Cucchiarini et al. 2013; Rey-Rico et al. 2015). Overall, the present findings provide evidence on the benefits of conveniently modifiying marrow aspirates by rAAV gene transfer for future applications to treat articular cartilage defects.
Genetic modification of bone marrow aspirates is a promising avenue of translational research to treat cartilage defects in patients especially using clinically adapted therapeutic rAAV vectors. The current study provides evidence of the possibility to deliver the proliferative and pro-chondrogenic FGF-2 to primary human marrow aspirates via rAAV for the durable commitment of the samples toward chondrogenic differentiation. The occurrence of hypertrophic events in vitro suggests that controlled expression of the growth factor will be a prerequisite for future applications in vivo to effectively treat cartilage lesions in the affected human population.
This study was funded by the German Osteoarthritis Foundation (Deutsche Arthrose-Hilfe e.V.). We thank R. J. Samulski (The Gene Therapy Center, University of North Carolina, Chapel Hill, NC), X. Xiao (The Gene Therapy Center, University of Pittsburgh, Pittsburgh, PA), and E. F. Terwilliger (Division of Experimental Medicine, Harvard Institutes of Medicine and Beth Israel Deaconess Medical Center, Boston, MA) for providing genomic AAV-2 plasmid clones and the 293 cell line, and M. Seno (Department of Bioscience and Biotechnology, Faculty of Engineering, Okayama University, Japan) for the hFGF-2 cDNA.
Conceived and designed the experiments: MC. Performed the experiments: JF JKV ARR AMZ GS. Analyzed the data: MC HM JF JKV ARR AMZ GS. Contributed reagents/materials/analysis tools: MC JF JKV ARR AMZ GS. Wrote the paper: MC. Pre-submission manuscript review and correction: MC HM JF JKV ARR AMZ GS. All authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Ethics approval and consent to participate
All patients included in the study provided informed consent and the procedures were in accordance with the Helsinki Declaration. The study was approved by the Ethics Committee of the Saarland Physicians Council (Nr. 06/08). The project was funded by German Osteoarthritis Foundation (Deutsche Arthrose-Hilfe e.V.).
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