Integrating concepts of material mechanics, ligand chemistry, dimensionality and degradation to control differentiation of mesenchymal stem cells

https://doi.org/10.1016/j.cossms.2016.04.001Get rights and content

Highlights

  • Ligand chemistry and substrate mechanics are integrated to define the mechanical resistance presented to MSCs.

  • Distinctive changes in mechanics moving from 2D to 3D macro-porous or non-macro-porous substrates.

  • Macro-porous substrates present a heterogeneous mechanical environment.

  • The specificity of integrin–ligand interactions is altered within non-macro-porous substrates.

  • Mechanotransduction within covalently crosslinked, non-macro-porous substrates requires degradation.

Abstract

The role of substrate mechanics in guiding mesenchymal stem cell (MSC) fate has been the focus of much research over the last decade. More recently, the complex interplay between substrate mechanics and other material properties such as ligand chemistry and substrate degradability to regulate MSC differentiation has begun to be elucidated. Additionally, there are several changes in the presentation of these material properties as the dimensionality is altered from two- to three-dimensional substrates, which may fundamentally alter our understanding of substrate-induced mechanotransduction processes. In this review, an overview of recent findings that highlight the material properties that are important in guiding MSC fate decisions is presented, with a focus on underlining gaps in our existing knowledge and proposing potential directions for future research.

Introduction

Mesenchymal stem cells (MSCs) isolated from bone marrow have great potential as a cell source for regenerative medicine due to both their relative ease of isolation and their ability to undergo differentiation towards multiple lineages [1], [2], [3]. Initially, the use of biochemical factors to induce controlled differentiation was seen as a key aspect of their effective clinical translation [4]. However, over the last decade there has been much research carried out on the role of the material properties of the substrates that MSCs are seeded onto, or embedded within, in guiding differentiation. While substrate mechanics has long been known to have an impact on cellular activity [5], a seminal manuscript by Engler et al. first provided evidence that MSC differentiation could be directed by substrate mechanics [6]. This discovery brought about renewed interest in the field of cellular mechanotransduction, with much of the research focused on characterizing the cellular signaling mechanisms involved in sensing and responding to two-dimensional substrate stiffness. In this review, we do not cover the various different cellular processes thought to be involved in mechanotransduction, but point the interested reader to several excellent reviews on this topic [5], [7], [8], [9]. Instead, here we focus on the material parameters that are known to impact cellular mechanotransduction and present these as design choices that must be carefully considered when developing a biomaterials-based study. To illustrate the importance of these material parameters, in Section 2 we introduce the molecular clutch hypothesis, as it provides an excellent framework within which to explore the role of biomaterials design choices.

The majority of existing research has focused on the role of substrate stiffness in guiding cell fate. Conversely, the relative importance of other material properties and how they may interact with stiffness cues has seen less focus, leaving some gaps in our knowledge and presenting opportunities for further discoveries. It has become increasingly apparent that elastic modulus alone is not the sole material property governing the mechanotransduction response of MSCs and that there is significant interplay between mechanics and properties such as ligand chemistry and substrate degradation [10], [11], [12], [13], [14]. Additionally, the majority of existing research has been carried out using two dimensional (2D) substrates. Mechanotransduction is likely to be inherently different in the three dimensional (3D) environments employed in most therapeutic strategies using MSCs [15]. In the few studies that have investigated MSC response to 3D materials, there have been notable differences in comparison to behavior on 2D surfaces [11], [12], [16]. As a result, there is a need for investigation of MSC response to material properties in 3D substrates, particularly in macro-porous environments, which are considerably more complex from a topographical and mechanical viewpoint [17]. Furthermore, due to the complexity of separating material variables in such experiments, novel materials science approaches are required to enable single variable studies to reveal their relative importance and potential non-additive outcomes.

In this review, we summarize recent findings that highlight the importance of materials in guiding MSC fate decisions, underline gaps in our existing knowledge, and propose potential directions for future research. First, we describe the development of myosin-mediated traction, which is the basic mechanism underlying cellular mechanosensation of substrate mechanics. Following this, we highlight several specific material properties that recent studies have revealed to be important in directing MSC differentiation. To conclude the review, we discuss new materials strategies and experimental techniques that have the potential to lead to an increased understanding of these phenomena towards the ultimate goal of engineering effective regenerative medicine therapies.

Section snippets

Cellular mechanotransduction of material stiffness

In order to appreciate the importance of material properties in directing MSC fate, it is first necessary to understand the basic principals by which cells sense and respond to their local mechanical environment, a process which is termed mechanotransduction (Fig. 1). In this context, the dominant mechanism proposed in the field is that cells sense the stiffness or rigidity of the surrounding substrate through integrin–ligand attachments [8]. At the interface between cells and the substrate,

Material properties and MSC differentiation

In this section, we highlight recent studies that have advanced our understanding of the relationship between material properties, cellular mechanotransduction and MSC differentiation. Initially, we discuss intrinsic material properties, before moving on to discussing the changes brought about by transitioning dimensionality from 2D to 3D.

Future opportunities and conclusions

Despite the growing body of data demonstrating that material properties have a strong role in guiding MSC fate, there are still no guidelines for the design of therapeutically relevant, 3D materials to direct differentiation towards specific lineages. This is partly explained by the complex interaction between variables such as ligand chemistry, substrate mechanics and dimensionality in defining the mechanical environment presented to MSCs and the difficulty in separating these variables. In

Acknowledgements

The authors would like to thank Dr. Ted Vaughan and Dr. Laoise McNamara (NUI Galway, Ireland) for insightful discussion. The authors acknowledge funding provided by NIH R21 EB018407-01, NIH U19 AI116484-01, NIH R21 EB020235-01, NSF DMR 1508006, California Institute for Regenerative Medicine RT3-07948, and Stanford University SICB-112878, Bio-X IIP-7-75 (S.C.H.), and an Irish Research Council ELEVATE fellowship ELEVATEPD/2013/30 (M.G.H.).

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