Tendon and ligament engineering in the adult organism: mesenchymal stem cells and gene-therapeutic approaches

Abstract

Tendons and ligaments are elastic collagenous tissues with similar composition and hierarchical structure, contributing to motion. Their strength is related to the number and size of the collagen fibrils. Collagen fibrils increase in size during development and in response to increased physical demands or training. Tendon disorders are commonly seen in clinical practice and give rise to significant morbidity. Treatment is difficult and patients often suffer from the symptoms for quite a long time. Despite remodelling, the biochemical and mechanical properties of healed tendon tissue never match those of intact tendon. The prerequisite for focussed treatment strategies in the future will be an improved understanding of the molecular events both in the embryo and contributing to regeneration in the adult organism. Novel approaches include the local delivery of growth factors, stem- and tendon-cell-derived therapy, the application of mechanical load and gene-therapeutic approaches based on vehicles encoding selected factors, or combinations of these. Important factors are proteins of the extracellular matrix like the metalloproteinases, growth factors like the bone morphogenetic proteins but also intracellular signalling mediator proteins, such as the Smads and transcription factors from the helix-loop-helix and other families. In this review, we focus specifically on such molecular approaches based on mesenchymal stem cells.

Fig. 1

The organisational structure of tendons

Fig. 2

a BMP-mediated Smad-signalling. The constitutively active BMP-type II receptors (blue) phosphorylate the BMP-type I receptors (ALKs; activin-like kinase receptors; green) on specific serine and threonine residues in the GS domain (yellow). These activated BMP-type I receptors propagate the signal downstream by directly phosphorylating the latent and inactive BMP-specific R-Smads Smad1, 5 or 8. The R-Smads form heteromeric complexes with the common mediator Smad4 that translocate into the nucleus. In combination with transcription factors (TF), Smads regulate gene transcription. b R-Smad and L+MH2 structures. The SxS motif located at the distal end at the carboxyterminus of R-Smads is directly phosphorylated by BMP type I receptors to convert the latent Smads to active signalling mediators. Since the Smad8 L+MH2 fragment lacks intrinsic DNA binding, its tenogenic actions are mediated by regulatory protein–protein interactions