Healing of a Large Long-Bone Defect through Serum-Free In Vitro Priming of Human Periosteum-Derived Cells

Abstract

Clinical translation of cell-based strategies for regenerative medicine demands predictable in vivo performance where the use of sera during in vitro preparation inherently limits the efficacy and reproducibility. Here, we present a bioinspired approach by serum-free pre-conditioning of human periosteum-derived cells, followed by their assembly into microaggregates simultaneously primed with bone morphogenetic protein 2 (BMP-2). Pre-conditioning resulted in a more potent progenitor cell population, while aggregation induced osteochondrogenic differentiation, further enhanced by BMP-2 stimulation. Ectopic implantation displayed a cascade of events that closely resembled the natural endochondral process resulting in bone ossicle formation. Assessment in a critical size long-bone defect in immunodeficient mice demonstrated successful bridging of the defect within 4 weeks, with active contribution of the implanted cells. In short, the presented serum-free process represents a biomimetic strategy, resulting in a cartilage tissue intermediate that, upon implantation, robustly leads to the healing of a large long-bone defect.

Keywords: bone morphogenetic proteins; cell-based constructs; critical bone fractures; fracture healing; fracture nonunion regenerative medicine; progenitor cells; serum free; stem cells.

Graphical abstract

Figure 1

Serum-free Pre-conditioning for 6 Days Affected Cellular Identity (A) DNA quantification in cells pre-conditioned in CDM or GM normalized to day 0. (B–E) DNA per cell after 6 days of pre-conditioning (B). Pre-conditioning induced expression of cell cycle regulators CDK1 (C), CCNE1(D), and BIRC5 (E). (F) Flow cytometry analysis after pre-conditioning for MSC markers CD73, CD90, and CD105 together with CD34. (G and H) Kinetics studies on (G) the mRNA transcript level of CD34 and (H) flow cytometry data on the number of CD34 molecules per cell. (I and J) Kinetic studies on (I) the mRNA transcript level of CD105 and (J) flow cytometry data on the number of CD105 molecules per cell. (K–M) mRNA transcript analysis of BMP type 1 and type 2 receptors (K), FGF2, VEGF, and MMP9 (L), confirmed on the protein level from conditioned medium (M). n = 3, ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001.

Figure 2

Serum-free Pre-conditioning led to Enhanced BMP-2-Induced Osteochondrogenic Differentiation In Vitro After 6 days of pre-conditioning followed by 6 days of BMP-2 stimulation in CDM, a difference in response to BMP-2 was observed. (A) Bright-field images after 6 days of BMP-2 stimulation indicating differentiated cells (black arrows). (B–E) BMP-2 stimulation induced elevated ALP activity(B). mRNA transcripts displayed elevated expression of SOX9 (C), OSX (D), and ID1 (E) after 24 hr, and 3 and 6 days of BMP-2 stimulation. (F) IHC for SOX9 (red) and OSX (green) with DAPI (blue) as nuclear stain after 6 days of BMP-2 stimulation. (G–J) Quantification of merged confocal images expressed as relative units (RU) normalized to DAPI (G). mRNA transcript analysis after pre-conditioning of hPDCs followed by 6 days of stimulation of BMP-2, -6, -7, -9, and GDF5 depicted by SOX9 (H), OSX (I), and ID1 (J). Scale bar, 50 μm; n = 3, ^∗/#p < 0.05, ^∗∗/##p < 0.01, ^{∗∗∗/###}p < 0.001 where ^# represents statistical significance to BMP-2 treated condition.

Figure 3

Altered Pathway Activation upon Pre-conditioning with CD34⁺ Cells Displaying a More Potent Osteochondro-Progenitor Cell Population (A–J) Pathway activation following pre-conditioning and BMP-2 stimulation investigated by western blot and subsequent quantification for (A) and (F) p-SMAD1/5/8, (B) and (G) p-Erk1/2, (C) and (H) p-p38, (D) and (I) active β-catenin, and (E) and (J) p-SMAD2/3. (K and L) Following separation of the CD34⁺ cell population, elevated expression of CD34 (K) and BMP receptors (L) were seen. (M) Following 6 days of BMP-2 stimulation the CD34⁺ hPDCs showed elevated differentiation depicted by SOX9 and OSX expression. (N) Cluster analysis displayed a correlation between the expression of CD34, BMP receptors, and differentiation markers. (O) A constellation plot showed clear grouping of CD34⁺ cells and the total cell population. n = 3, ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001.

Figure 4

The Number of Cells per Aggregate Affected Osteogenic and Chondrogenic Cell Specification To determine optimal size, aggregates of 50, 100, and 250 cells/aggregate were investigated after 6 days of aggregation. (A–D) Aggregation and BMP-2 stimulation induced a shift in MSC marker expression and a reduction when both factors were combined (A). mRNA transcript analysis of (B) chondrogenic, (C) osteogenic, and (D) BMP signaling and angiogenic markers indicated that both aggregation, aggregation size and BMP-2 stimulation affected cell differentiation. n = 3, statistical significance to non-BMP-2-stimulated conditions: ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001 and to 2D BMP-2 conditions: ^#p < 0.05, ^##p < 0.01, ^###p < 0.001.

Figure 5

In Vitro Priming Leads to an In Vivo Cartilage Intermediate 1 Week Post Transplantation Histology of tissue constructs 1 week after in vivo subcutaneous implantation displayed that both BMP-2 stimulation and aggregation affected in vivo tissue formation. (A) H&E staining showed microvessel formation (black arrows). (B and C) Denser AB staining revealed more cartilage matrix accumulation (black arrows) (B), confirmed by denser collagen staining by MT staining (black arrows) (C). (D) Upon IHC for p-SMAD1/5/8 as a marker for active BMP signaling, positive nuclei (brown stain, black arrows) were seen in samples stimulated by BMP-2 and/or aggregation. (E and F) Quantification showed that the combined stimulation of BMP-2 and aggregation elevated the number of microvessels (E) and positive nuclei for p-SMAD1/5/8 (F). (G) BMP-2 production by stimulated cells were confirmed in conditioned medium by ELISA. (H) Enhanced transcripts for BMP-2 on implants at the time of implantation. Scale bars, 20 μm. n = 4, ^∗p < 0.05, ^∗∗p < 0.01, ^∗∗∗p < 0.001.

Figure 6

The Combined In Vitro Priming by BMP-2 Treatment and Aggregation Led to Endochondral Bone Formation In Vivo Histology on tissue constructs 3 weeks after in vivo subcutaneous implantation demonstrated that the simultaneous stimulation by aggregation and BMP-2 affected in vivo tissue formation. (A) H&E staining displayed the presence hypertrophic chondrocytes (black arrows) next to denser ECM areas (white arrows). (B) Denser AB stain lines the zones of hypertrophic chondrocytes (white arrows) surrounded by GAG-rich areas (white arrows). (C) The MT stain confirmed the formation of denser areas (black arrows). (D and E) TRAP-positive areas (black arrows) surrounding the hypertrophic chondrocytes suggest remodeling (D) and quantification (E). (F) IHC for DIPEN showed breakdown of GAG-rich areas. (G and H) IHC for S100 displayed more mature cartilage tissue in BMP-2 stimulated aggregates (G), confirmed by quantification (H). (I and J) IHC for Ihh showed the presence of hypertrophic chondrocytes (I), which upon quantification was a 4-fold higher compared with 2D-stimulated cells (J). (K) At 6 weeks, the endochondral tissue intermediate had developed into a mineralized ossicle. (L) H&E staining confirmed de novo formed bone (black arrows) and bone marrow (red arrows), while MT staining displayed zones of mature bone (green arrows). TRAP staining suggested ongoing remodeling of immature bone (blue arrows) and IHC for hOCN confirmed the contribution of the implanted cells. Unlabeled scale bars, 20 μm. n = 4, ^∗p < 0.05, ^∗∗p < 0.01.

Figure 7

The In Vitro Primed Cell-based Tissue Construct Led to the Healing of a Critical Size Long-Bone Defect (A) In vivo X-ray monitoring of fracture healing upon transplantation of in vitro BMP-2-stimulated microaggregates. (B) Reconstructed images from nano-computed tomography scanned explants displayed mineralized healing of the critical-sized defect 4 weeks post transplantation. (C–G) Qualitative analysis of sectioned explants 2, 4, and 8 weeks post transplantation, including control confirming non-union. Cartilage and bone formation was confirmed by H&E (C), Safranin O and fast green (D), and MT (E) staining. Tissue remodeling was investigated by a TRAP stain (F), whereas contribution of implanted cells was confirmed by IHC for hOCN (G). Arrows indicate specific tissues as follows: cartilage (gray), bone (black), mature bone (yellow), TRAP-positive areas (blue), hOCN-positive cells, and matrix (white). Unlabeled scale bar, 50 μm.