From repair to regeneration: biomaterials to reprogram the meniscus wound microenvironment

Abstract

When the field of tissue engineering first arose, scaffolds were conceived of as inert three-dimensional structures whose primary function was to support cellularity and tissue growth. Since then, advances in scaffold and biomaterial design have evolved to not only guide tissue formation, but also to interact dynamically with and manipulate the wound environment. At present, these efforts are being directed towards strategies that directly address limitations in endogenous wound repair, with the goal of reprogramming the local wound environment (and the cells within that locality) from a state that culminates in an inferior tissue repair into a state in which functional regeneration is achieved. This review will address this approach with a focus on recent advances in scaffold design towards the resolution of tears of the knee meniscus as a case example. The inherent limitations to endogenous repair will be discussed, as will specific examples of how biomaterials are being designed to overcome these limitations. Examples will include design of fibrous scaffolds that promote colonization by modulating local extracellular matrix density and delivering recruitment factors. Furthermore, we will discuss scaffolds that are themselves modulated by the wound environment to alter porosity and modulate therapeutic release through precise coordination of scaffold degradation. Finally, we will close with emerging concepts in local control of cell mechanics to improve interstitial cell migration and so advance repair. Overall, these examples will illustrate how emergent features within a biomaterial can be tuned to manipulate and harness the local tissue microenvironment in order to promote robust regeneration.

Figure 1. Impediments to meniscus repair

Schematic illustration of a meniscus defect and the underlying impediments to adult meniscus repair, including a lack of vascularity, a loss of cellularity, alterations in ECM density and mechanics, and inflammatory factors in the wound environment. These factors converge to limit endogenous repair and have been incorporated into pro-repair strategies aimed at improving regeneration. Adapted from [96] with permission.

Figure 2. Intrinsic meniscus repair decreases with tissue maturation

A) Histological sections (H&E staining) showing near complete and seamless repair of a fetal meniscus segment 8 weeks post injury, compared to persistent defects, fissures, and clefts in an adult tissue repair construct cultured similarly. B) Schematic illustration of the dynamic processes of fetal tissue repair and the intrinsic changes to the tissue that limit repair in the adult. Adapted from [4] and [96] with permission.

Figure 3. Scaffold porosity modulates cell infiltration and integration

A) Dynamic composite fibrous scaffold with a stable poly(ε-caprolactone) (PCL) fiber fraction (red) and a water-soluble poly(ethylene oxide) (PEO) fiber fraction (green). Here, the composite is undergoing a transition in porosity as a hydration front advances from the bottom to the top of the image, with the sacrificial PEO fibers being removed. B) Schematic of fiber composites engineered to present differing levels of porosity based on the fraction of sacrificial PEO fibers included in the network at the time of fabrication. C) Integration strength as a function of time and scaffold porosity, where scaffolds with higher porosity integrate with native tissue with higher mechanical strength than those with lower porosity. Adapted form [59] and [61] with permission.

Figure 4. Bioactive scaffolds modulate local ECM density to improve repair

A) Schematic of meniscus repair construct and B) demonstration of matrix degradation (removal of PGs) at the wound interface (Alcian Blue staining, 2X magnification) with local delivery of collagenase from the PEO fiber fraction (PEO-C). C) Improved tissue integration as a result of biomaterial-mediated delivery of collagenase (PEO-C) to the wound interface in a subcutaneous model of meniscus repair. Picrosirius red staining of collagen (viewed under polarized light) and H&E staining of the repair interface at 4 weeks (10× magnification). Asterisk indicates scaffold. Adapted form [64] and [96] with permission.

Figure 5. Chemotactic cues and novel delivery methods to improve colonization of the wound site

A) Bone marrow cell (BMC) chemotaxis in response to SDF and HA release from degradable HA hydrogels. B) Meniscus fibrochondrocyte (MFC) chemotaxis in response to serum (FBS), SDF, FGF, and PDGF (normalized to 1% FBS controls). C) SEM and fluorescent images of HA nanofibers. D) Crosslinking chemistries providing stable (MeHA) and hydrolytically degradable (HeMA-HA) HA-based materials (gels and fiber networks). E) FITC-conjugated BSA release from HA-based nanofibers as a function of time and degree of modification and type of crosslink. Adapted from [71] and [11, 77] with permission.

Figure 6. Scaffold mediated instruction upon arrival

A) Vinculin localization (green), actin cytoskeleton (red), and nuclei (blue) staining of human mesenchymal stem cells (MSCs) cultured for 24 hours on fibrous HA scaffolds with varied RGD density and fiber modulus. Scale bar: 50 μm. B) MSC expression of chondrogenic markers after 14 days of culture in chondrogenic medium on fibrous HA scaffolds with varied RGD density and fiber modulus. *denotes significance (p<0.05) between groups, and denotes significance (p<0.05) compared to other RGD densities within the same fiber stiffness condition. Adapted with permission from [11].

Figure 7. Emerging concepts: manipulating endogenous cell mechanics to improve migration to the wound site

Schematic illustration of interstitial 3D cell migration in dense fibrous networks. Nuclear mechanics, mediated by the amount and distribution of nucleo-structural filamentous components such as Lamin A/C, mediate the ability of cells to squeeze through small pores in dense connective tissues.