Stiffness Matters: Fine-Tuned Hydrogel Elasticity Alters Chondrogenic Redifferentiation.

Barbara Bachmann,Barbara Schädl,Heinz Redl,Peter Ertl,Andreas H Teuschl,Sarah Spitz,Sylvia Nürnberger

doi:10.3389/fbioe.2020.00373

Abstract

Biomechanical cues such as shear stress, stretching, compression, and matrix elasticity are vital in the establishment of next generation physiological in vitro tissue models. Matrix elasticity, for instance, is known to guide stem cell differentiation, influence healing processes and modulate extracellular matrix (ECM) deposition needed for tissue development and maintenance. To better understand the biomechanical effect of matrix elasticity on the formation of articular cartilage analogs in vitro, this study aims at assessing the redifferentiation capacity of primary human chondrocytes in three different hydrogel matrices of predefined matrix elasticities. The hydrogel elasticities were chosen to represent a broad spectrum of tissue stiffness ranging from very soft tissues with a Young’s modulus of 1 kPa up to elasticities of 30 kPa, representative of the perichondral-space. In addition, the interplay of matrix elasticity and transforming growth factor beta-3 (TGF-β3) on the redifferentiation of primary human articular chondrocytes was studied by analyzing both qualitative (viability, morphology, histology) and quantitative (RT-qPCR, sGAG, DNA) parameters, crucial to the chondrotypic phenotype. Results show that fibrin hydrogels of 30 kPa Young’s modulus best guide chondrocyte redifferentiation resulting in a native-like morphology as well as induces the synthesis of physiologic ECM constituents such as glycosaminoglycans (sGAG) and collagen type II. This comprehensive study sheds light onto the mechanobiological impact of matrix elasticity on formation and maintenance of articular cartilage and thus represents a major step toward meeting the need for advanced in vitro tissue models to study both re- and degeneration of articular cartilage.

Highlights

Articular cartilage is a specialized load-bearing tissue providing the ability of frictionless movement within synovial joints such as the knee, hip or shoulder
This study was aimed at elucidating the effect of matrix elasticity on the redifferentiation of primary human chondrocytes to obtain an optimized in vitro model for articular cartilage trauma and disease
Fibrin, silk/fibrin and PEG-dextran were selected based on stiffnesstunability and characterized using oscillatory rheology

Summary

Introduction

Articular cartilage is a specialized load-bearing tissue providing the ability of frictionless movement within synovial joints such as the knee, hip or shoulder. Adult articular cartilage is a relatively simple tissue with the extracellular matrix (ECM) forming up to 99% of the tissue while the chondrocytes cease their proliferative activity and acquire a metabolically quiescent state (Camarero-Espinosa et al, 2016). Hydrogels are hydrophilic polymeric networks capable of absorbing aqueous solutions multiple times their dry weight and present an ideal 3D cellular microenvironment emulating the native ECM characteristics of articular cartilage (Liu et al, 2017). These ECMmimicking properties can further be exploited by the addition of chondrogenic molecules as well as by fine-tuning matrix elasticity. A short overview of current 3D studies investigating the interplay between matrix elasticity and chondrocyte behavior is provided in Table 1, further highlighting the heterogenicity of the state of the art

Objectives

Methods

Results

Conclusion