BMP-12 Treatment of Adult Mesenchymal Stem Cells In Vitro Augments Tendon-Like Tissue Formation and Defect Repair In Vivo

We characterized the differentiation of rat bone marrow-derived mesenchymal stem cells (BM-MSCs) into tenocyte-like cells in response to bone morphogenetic protein-12 (BMP-12). BM-MSCs were prepared from Sprague-Dawley rats and cultured as monolayers. Recombinant BMP-12 treatment (10 ng/ml) of BM-MSCs for 12 hours in vitro markedly increased expression of the tenocyte lineage markers scleraxis (Scx) and tenomodulin (Tnmd) over 14 days. Treatment with BMP-12 for a further 12-hour period had no additional effect. Colony formation assays revealed that ∼80% of treated cells and their progeny were Scx- and Tnmd-positive. BM-MSCs seeded in collagen scaffolds and similarly treated with a single dose of BMP-12 also expressed high levels of Scx and Tnmd, as well as type I collagen and tenascin-c. Furthermore, when the treated BM-MSC-seeded scaffolds were implanted into surgically created tendon defects in vivo, robust formation of tendon-like tissue was observed after 21 days as evidenced by increased cell number, elongation and alignment along the tensile axis, greater matrix deposition and the elevated expression of tendon markers. These results indicate that brief stimulation with BMP-12 in vitro is sufficient to induce BM-MSC differentiation into tenocytes, and that this phenotype is sustained in vivo. This strategy of pretreating BM-MSCs with BMP-12 prior to in vivo transplantation may be useful in MSC-based tendon reconstruction or tissue engineering

Effect of BMP-12 on Scx and Tnmd protein expression in colonies derived from rat BM-MSCs.

<p>Colony forming assays were performed on untreated cells, or on cells treated with BMP-12 for 12 h (“1-hit”) or for 12 h + 12 h. Total, Scx-positive and Tnmd-positive colonies were counted on Day 14 after plating. Data are presented as mean ± S.D. (n = 6); * represents P<0.05 for BMP-12 treated cells compared to untreated controls.</p

Enhanced cell alignment following BMP-12-treatment of scaffolds seeded with BM-MSCs.

<p>Rat BM-MSCs were cultured and implanted as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0017531#pone-0017531-g002" target="_blank">Fig. 2</a>. (<b>A</b>) Toluidine blue staining (20X magnification) revealed increased cell elongation and cellular alignment/organization, within the BMP-12-treated BM-MSCs implants. (<b>B</b>) Nuclear aspect ratio (width <i>vs</i> length of nucleus), and (<b>C</b>) Angular deviation (angle between individual nuclear axis and longitudinal axis based on general alignment). A smaller value of nuclear aspect ratio and nuclear orientation angle indicated greater cellular elongation and alignment in cells treated with BMP-12, as compared to untreated cells. * represents p<0.05.</p

Effects of BMP-12 on the rat BM-MSC differentiation toward tenocyte lineage <i>in vitro</i>.

<p>(<b>A</b>) BMP-12-induced expression of Scx and Tnmd in monolayer cultures. Rat BM-MSCs at passage 1 were plated, and 24 hours later were treated with 10 ng/ml of BMP12 for 12 hours (“1-hit”) or for 24 hours (12 hours plus another 12 hours). Following BMP-12 stimulation, cells were cultured in the absence of BMP-12 for the indicated times. mRNA levels were determined by qRT-PCR. Data are expressed as mean ± S.D. (<i>n</i> = 3). * represents p<0.05. (<b>B</b>, <b>C</b>) BMP-12-mediated tenocytic differentiation of rat BM-MSC in collagen scaffolds. BM-MSCs (2.5×10⁵ cells) were seeded onto sterilized 5 mm×2 mm collagen sponge scaffolds and incubated in growth media. After 24-hour culture, cells were left untreated or treated with 10 ng/mL of BMP-12 for 12 hours. The media was then replaced with fresh growth medium and the scaffolds were cultured in the absence of BMP-12 for 14 days. At the end of culture, cells were stained with methylene blue (B, left panels, 20X magnification) or subjected to immunohistochemical staining for analysis of Scx and Tnmd protein (Middle and right panels, 20X magnification). Cells were also lysed with Trizol and gene expression was determined by qRT-PCR (C). Data shown in (B) are representative of 3 independent experiments. Data in (C) are expressed as mean ± S.D. (<i>n</i> = 3). * represents p<0.05.</p

Fomation of tendon-like tissues by BMP-12-treated rat BM-MSCs in calcaneal tendon defects <i>in vivo</i>.

<p>(<b>A</b>) Exposed rat calcaneal tendon of the left hind-limb (Left panel). Scaffolds were implanted into half-width, 5 mm-long partial calcaneal tendon defects using 10-0 nylon (Middle panel). A schematic drawing highlights the spatial relationship between the tendon and the implant (Right panel). “T” denotes tendon; “Im” denotes implant; arrows denote the tendon-implant interface. Note that 5 interrupted 10-0 nylon sutures were used to secure the implant in the defect and that the implant completely filled in the 5 mm×2 mm calcaneal tendon defect. (<b>B</b>) Cells were cultured and treated as described for <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0017531#pone-0017531-g001" target="_blank">Fig. 1B, C</a>. Scaffolds with or without cell seeding were implanted as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0017531#pone-0017531-g002" target="_blank">Fig. 2A</a>. After 3 weeks, implants and Achilles tendons from naïve animals were dissected and subjected to histological analysis (Masson's Trichrome or H&E staining). Seeded BMP-12-treated implants but not unseeded and non-BMP-12-treated group exhibited higher cellularity, increased formation of collagen, and organized fibrous structures, indicating robust formation of tendon-like tissues. 20X magnification.</p

Induction of tendon cell-related genes by BMP-12 in rat BM-MSC implants in calcaneal tendon <i>in vivo</i>.

<p>Samples were collected as described in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0017531#s2" target="_blank">Methods and Materials</a>. mRNA levels were determined by qRT-PCR and normalized against samples of the non-BMP-12-treated group. The gene expression of all experimental groups was determined as fold changes relative to native tendon samples. While unseeded implants expressed minimal Scx, Tnmd, Col I, and Tn-C, BMP-12-treated implants had significantly increased expression of these genes compared to non-BMP-12-treated implants. Data are expressed as mean ± S.D. (n = 3). * represents p<0.05.</p

Increased expression of Scx and Tnmd proteins in seeded BMP-12-treated implants.

<p>Implants as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0017531#pone-0017531-g002" target="_blank">Fig. 2</a> and naïve Achilles tendons were dissected and tissue sections were immunohistochemically stained with specific anti-Scx or anti-Tnmd antibodies as described in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0017531#s2" target="_blank">Material and Methods</a>. Data shown are representative of three independent experiments (20X magnification).</p