Bone Marrow Mesenchymal Stem Cells Spontaneously Make Cartilage After Blockage of VEGF Signaling


Bone marrow-derived mesenchymal stem cells (MSCs) can be induced to make cartilage by incubating the cells with particular growth factors.  Unfortunately, batches of MSCs show respectable variability from patient-to-patient.  Therefore the growth factor-dependent method suffers from poor efficacy, limited reproducibility from batch-to-batch, and the cell types that are induced are not always terribly stable.  Finding a better way to make cartilage would certainly be a welcome addition to regenerative treatments,

Cartilage that coats the ends of bones is known as articulate cartilage, and articular cartilage lacks blood vessels.  Therefore, is it possible that inhibiting blood vessel formation could conveniently push MSCs to differentiate into cartilage-making chondrocytes?

A new paper by Ivan Martin and Andrea Basil from the University Hospital Basel and their colleagues have used this very strategy to induce cartilage formation in MSCs from bone marrow.

Martin and others isolated MSCs from bone marrow aspirates from human donors.  These cultured human MSCs were then genetically engineered with modified viruses to express a receptor for soluble vascular endothelial growth factor (VEGF) that binds this growth factor, but fails to induce any intracellular signals.  Such a receptor that binds the growth factor but does not induce any biological effects is called a “decoy receptor,” and decoy receptors efficiently sequester or vacuum up all the endogenous VEGF.  VEGF is the major blood vessel-inducing growth factor and it is heavily expressed during development, by cancer cells, and during healing.

After expressing the decoy VEGF receptor in these human MSCs, these genetically engineered cells were grown on collagen sponges and then implanted in immunodeficient mice.  If the implanted MSCs were not genetically engineered to express decoy VEGF receptors, they induced for formation of vascularized fibrous tissue.  However, the implantation of genetically engineered MSCs that expressed the decoy VEGF receptor efficiently and reproducibly differentiated into chondrocytes and formed hyaline cartilage. This is significant because headline cartilage is the very type of cartilage found at articular surfaces where the ends of bones come together to form joints.

In vivo chondrogenesis. Histological staining with Safranin-O for glycosaminoglycans and immunohistochemistry for type II collagen of engineered tissue generated by naïve (control) or sFlk-1 MSCs after 4 (A) or 12 (B) weeks in vivo. Fluorescence staining with DAPI (in blue) and a specific anti-human nuclei antibody (in red) of constructs generated by control or sFlk-1 MSCs after 4 (A) or 12 (B) weeks in vivo. Scale bar = 100 µm. Abbreviations: DAPI, 4′,6-diamidino-2-phenylindole; MSC, bone marrow-derived mesenchymal stromal/stem cell.
In vivo chondrogenesis. Histological staining with Safranin-O for glycosaminoglycans and immunohistochemistry for type II collagen of engineered tissue generated by naïve (control) or sFlk-1 MSCs after 4 (A) or 12 (B) weeks in vivo. Fluorescence staining with DAPI (in blue) and a specific anti-human nuclei antibody (in red) of constructs generated by control or sFlk-1 MSCs after 4 (A) or 12 (B) weeks in vivo. Scale bar = 100 µm. Abbreviations: DAPI, 4′,6-diamidino-2-phenylindole; MSC, bone marrow-derived mesenchymal stromal/stem cell.

This articular cartilage was quite stable and showed no signs of undergoing the chondrocytes enlargement found in terminally differentiated cartilage that is ready to form bone.  This stability was maintained for up to 12 weeks.

In vivo cartilage stability. Immunohistochemistry for type X collagen, BSP, and MMP-13 on sections of hypertrophic cartilage generated in vitro by MSCs (as a positive control) and on sections of the cartilaginous constructs generated in vivo by sFlk1 MSCs 12 weeks after implantation. Scale bar = 50 µm. Abbreviations: BSP, bone sialoprotein; MMP-13, metalloproteinase-13; MSC, bone marrow-derived mesenchymal stromal/stem cell.
In vivo cartilage stability. Immunohistochemistry for type X collagen, BSP, and MMP-13 on sections of hypertrophic cartilage generated in vitro by MSCs (as a positive control) and on sections of the cartilaginous constructs generated in vivo by sFlk1 MSCs 12 weeks after implantation. Scale bar = 50 µm. Abbreviations: BSP, bone sialoprotein; MMP-13, metalloproteinase-13; MSC, bone marrow-derived mesenchymal stromal/stem cell.

Why did inhibition of VEGF signaling induce cartilage?  Inhibition of angiogenesis induced low oxygen tensions, which activated a growth factor called transforming growth factor-β.  Activation of the TGF-beta pathway robustly enhanced the formation of articular cartilage.

In vitro chondrogenesis at different oxygen tensions. Histological staining with Safranin-O and immunohistochemistry for type II collagen on constructs generated in vitro by naïve MSC cultured with (A) or without (B) TGFβ3 supplementation at 2% or 20% of oxygen tension. Scale bar = 50 µm. Expression levels of the mRNA for type II and X collagen, Gremlin-1, IHH TGFβ1 were quantified in pellets generated by naïve bone marrow-derived mesenchymal stromal/stem cells (C, D) cultured in the two different oxygen tensions. ∆Ct values were normalized to expression of the GAPDH housekeeping gene, and results are shown as mean ± SD (n = 6 samples/group from 3 independent experiments). ∗, p < .05, ∗∗∗, p < .001. Abbreviations: GAPDH, glyceraldehyde-3-phosphate dehydrogenase; IHH, Indian hedgehog; TGFβ, transforming grown factor-β.
In vitro chondrogenesis at different oxygen tensions. Histological staining with Safranin-O and immunohistochemistry for type II collagen on constructs generated in vitro by naïve MSC cultured with (A) or without (B) TGFβ3 supplementation at 2% or 20% of oxygen tension. Scale bar = 50 µm. Expression levels of the mRNA for type II and X collagen, Gremlin-1, IHH TGFβ1 were quantified in pellets generated by naïve bone marrow-derived mesenchymal stromal/stem cells (C, D) cultured in the two different oxygen tensions. ∆Ct values were normalized to expression of the GAPDH housekeeping gene, and results are shown as mean ± SD (n = 6 samples/group from 3 independent experiments). ∗, p < .05, ∗∗∗, p < .001. Abbreviations: GAPDH, glyceraldehyde-3-phosphate dehydrogenase; IHH, Indian hedgehog; TGFβ, transforming grown factor-β.

Cartilage formation from MSCs was induced by blocking VEGF-mediated angiogenesis.  These results represent a remarkable advance in cartilage formation that can be used for regenerative treatments.  This cartilage formation was spontaneous and efficient and if it can be carried out with VEGF-inhibiting drugs rather than genetic engineering techniques, then we might have a transferable technique for making cartilage in the laboratory to treat osteoarthritis and other joint-based maladies.  Clinical trials will be required, but this is certainly an auspicious start.

Advertisements

Published by

mburatov

Professor of Biochemistry at Spring Arbor University (SAU) in Spring Arbor, MI. Have been at SAU since 1999. Author of The Stem Cell Epistles. Before that I was a postdoctoral research fellow at the University of Pennsylvania in Philadelphia, PA (1997-1999), and Sussex University, Falmer, UK (1994-1997). I studied Cell and Developmental Biology at UC Irvine (PhD 1994), and Microbiology at UC Davis (MA 1986, BS 1984).