Increased Collagen Turnover Impairs Tendon Microstructure and Stability in Integrin α2β1-Deficient Mice.

Daniel Kronenberg,Jürgen Brinckmann,Eva Hochstrat,Beate Eckes,Ma Wei,Marcus Müller,Andre Frank,Uwe Hansen,Philipp A Michel,Richard Stange

doi:10.3390/ijms21082835

Abstract

Integrins are a family of transmembrane proteins, involved in substrate recognition and cell adhesion in cross-talk with the extra cellular matrix. In this study, we investigated the influence of integrin α2β1 on tendons, another collagen type I-rich tissue of the musculoskeletal system. Morphological, as well as functional, parameters were analyzed in vivo and in vitro, comparing wild-type against integrin α2β1 deficiency. Tenocytes lacking integrin α2β1 produced more collagen in vitro, which is similar to the situation in osseous tissue. Fibril morphology and biomechanical strength proved to be altered, as integrin α2β1 deficiency led to significantly smaller fibrils as well as changes in dynamic E-modulus in vivo. This discrepancy can be explained by a higher collagen turnover: integrin α2β1-deficient cells produced more matrix, and tendons contained more residual C-terminal fragments of type I collagen, as well as an increased matrix metalloproteinase-2 activity. A greatly decreased percentage of non-collagenous proteins may be the cause of changes in fibril diameter regulation and increased the proteolytic degradation of collagen in the integrin-deficient tendons. The results reveal a significant impact of integrin α2β1 on collagen modifications in tendons. Its role in tendon pathologies, like chronic degradation, will be the subject of future investigations.

Highlights

Integrins are a family of heterodimeric type I transmembrane receptors
Using the constitutive integrin α2β1 knockout mouse, we demonstrated in previous studies that its absence in bone cells led to increased production of collagen type I in vitro, as well as in situ, which mitigated the impact of age-related bone degradation in aging mice
Our results pointed to a regulatory function of integrin α2β1 in collagen morphology and homeostasis; we focused in this study on tendons, another collagen-based musculoskeletal tissue that severely relies biomechanically on a strictly ordered arrangement of collagen fibrils

Summary

Introduction

Integrins are a family of heterodimeric type I transmembrane receptors. Eighteen α- and 8 β-subunits have been characterized so far, leading to 24 described combinations in humans [1]. A group of integrin α-subunits, including the collagen-binding receptors, contains another structural feature, an additional domain called the αA-domain [3]. Accessible collagen molecules still serve as a valid binding partner for integrin α2β1, α10β1 and α11β1, leading to the conclusion that integrin binding detects collagen molecules in the damaged extra-cellular matrix and serves as a sensor for tissue damage. This holds true for collagen type II and chondrocytes. For collagen type I, the situation may be completely different, as the putative binding site for collagen-binding integrins, which is deemed to be cryptic, was shown to be accessible due to a proline-mediated side-specific flexibility discovered by structural analyses. [12,13] integrin α2β1 might play a more complex role in the tissue homeostasis of collagen type I-rich tissue, like bones, skin and tendons

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