Matthias Gerstner

Exit from multipotency and lineage commitment of mesenchymal stroma cells (MSC) depends on microenvironmental cues from the stem‐cell niche but steering cell fate into the desired lineage in vivo remains a challenge. Increasing evidence suggests that glycosaminoglycans (GAGs) can be used to activate or sequester growth factors with the specific action depending on sulfation levels. It is postulated that differentially sulfated biomaterials can aid developmental lineage instruction of MSC to guide tissue morphogenesis in vivo. By application of a new injectable TGFβ‐loaded heparin‐PEG‐hydrogel the authors here gain the ability to control skeletal stem‐cell fate in vivo down the chondral versus the endochondral pathway depending on the sulfation status. High sulfation allows for long‐term TGFβ‐retention and silencing of Hedgehog‐, BMP‐, and WNT‐pathways installing prochondrogenic and antihypertrophic cues which permitted in vivo growth of permanent, collagen‐type‐II‐rich neocartilage with long‐term resistance to calcification and bone formation. Selective 6‐O/N‐desulfation of heparin supports Hedgehog/BMP/WNT‐signaling switching lineage commitment into endochondral differentiation with strong hypertrophic/osteogenic marker expression and tissue calcification. This work identifies GAG sulfation as a crucial niche instruction signal to determine the chondral stem‐cell fate via silencing of prohypertrophic pathways, providing the first proof‐of‐principle on how GAG modification‐patterns can determine cell lineage‐choice during tissue morphogenesis in vivo.

Osteoarthritis (OA) represents one major cause of disability worldwide still evading efficient pharmacological or cellular therapies. Severe degeneration of extracellular cartilage matrix precedes the loss of mobility and disabling pain perception in affected joints. Recent studies showed that a reduced heparan sulfate (HS) content protects cartilage from degradation in OA-animal models of joint destabilization but the underlying mechanisms remained unclear. We aimed to clarify whether low HS-content alters the mechano-response of chondrocytes and to uncover pathways relevant for HS-related chondro-protection in response to loading. Tissue-engineered cartilage with HS-deficiency was generated from rib chondrocytes of mice carrying a hypomorphic allele of Exostosin 1 (Ext1), one of the main HS-synthesizing enzymes, and wildtype (WT) littermate controls. Engineered cartilage matured for 2 weeks was exposed to cyclic unconfined compression in a bioreactor. The molecular loading response was determined by transcriptome profiling, bioinformatic data processing, and qPCR. HS-deficient chondrocytes expressed 3–6% of WT Ext1-mRNA levels. Both groups similarly raised Sox9, Col2a1 and Acan levels during maturation. However, HS-deficient chondrocytes synthesized and deposited 50% more GAG/DNA. TGFβ and FGF2-sensitivity of Ext1^gt/gt chondrocytes was similar to WT cells but their response to BMP-stimulation was enhanced. Loading induced similar activation of mechano-sensitive ERK and P38-signaling in WT and HS-reduced chondrocytes. Transcriptome analysis reflected regulation of cell migration as major load-induced biological process with similar stimulation of common (Fosl1, Itgα5, Timp1, and Ngf) as well as novel mechano-regulated genes (Inhba and Dhrs9). Remarkably, only Ext1-hypomorphic cartilage responded to loading by an expression signature of negative regulation of apoptosis with pro-apoptotic Bnip3 being selectively down-regulated. HS-deficiency enhanced BMP-sensitivity, GAG-production and fostered an anti-apoptotic expression signature after loading, all of which may protect cartilage from load-induced erosion.