Stiffness-tunable biomaterials provide a good extracellular matrix environment for axon growth and regeneration

Abstract

Neuronal growth, extension, branching, and formation of neural networks are markedly influenced by the extracellular matrix-a complex network composed of proteins and carbohydrates secreted by cells. In addition to providing physical support for cells, the extracellular matrix also conveys critical mechanical stiffness cues. During the development of the nervous system, extracellular matrix stiffness plays a central role in guiding neuronal growth, particularly in the context of axonal extension, which is crucial for the formation of neural networks. In neural tissue engineering, manipulation of biomaterial stiffness is a promising strategy to provide a permissive environment for the repair and regeneration of injured nervous tissue. Recent research has fine-tuned synthetic biomaterials to fabricate scaffolds that closely replicate the stiffness profiles observed in the nervous system. In this review, we highlight the molecular mechanisms by which extracellular matrix stiffness regulates axonal growth and regeneration. We highlight the progress made in the development of stiffness-tunable biomaterials to emulate in vivo extracellular matrix environments, with an emphasis on their application in neural repair and regeneration, along with a discussion of the current limitations and future prospects. The exploration and optimization of the stiffness-tunable biomaterials has the potential to markedly advance the development of neural tissue engineering.

Figure 1

Schematic illustration of neuronal growth cones directing protrusion growth. The formation of focal adhesions mediated by integrins diminishes the actin flow while prompting the rearrangement of actin in the direction of growth. This alignment serves to guide the pathway for the advancement of organelles and microtubules. Reorganization of the cytoskeleton provides traction essential for the extension of neuronal protrusions, thereby driving protrusion growth. Created using Microsoft PowerPoint 2016 and Scienceslides from Visiscience. F-actin: Filamentous actin; G-actin: globular actin.

Figure 2

Neural response to ECM stiffness via integrin-mediated cytoskeleton remodeling. The bonding of ECM to integrin initiates the integrin activation through a conformational change from low- to high-affinity states. Stiffness signals can be transmitted via integrin to intracellular force-sensitive proteins, leading to phosphorylation of several intracellular tyrosine kinases. The phosphorylated FAK interacts with Src kinase, further promoting phosphorylation at additional sites on the FAK. The recruitment of proteins by phosphorylated FAK enables the assembly and formation of focal adhesions, actin polymerization, and cytoskeleton remodeling, ultimately supporting axon adhesion and extension. Created using Microsoft PowerPoint 2016 and Scienceslides from Visiscience. Crk: Cysteine-rich receptor protein kinases; DOCK: dedicator of cytokinesis; Erk: extracellular signal-regulated kinase; FAK: focal adhesion kinase; Grb2: growth factor receptor-bound protein-2; MLCK: myosin light-chain kinase; pSrc: phosphorylated sarcoma receptor coactivator; p130Cas: p130Crk-associated substrate; pY397: phosphorylated tyrosine residue 397 of FAK; RhoA: Ras homolog family member A; ROCK: Rho-associated protein kinase; Rac: Ras-related C3 botulinum toxin substrate; Ras-MAPK: rat sarcoma-mitogen–activated protein kinase.

Figure 3

History of application of stiffness-tunable biomaterials in nerve injury repair and regeneration. Created using Microsoft PowerPoint 2016. 3D: Three-dimensional.

Figure 4

Stiffness-tunable biomaterial strategies for neural repair and regeneration. The versatile biomaterials with various stiffness range including polyacrylamide (PA), alginate, and polydimethylsiloxane (PDMS) are commonly employed to modulate neural behavior (left panel). These materials can be assembled into multi-functional scaffolds with finely tuned properties. By further incorporating different neural cells, supporting cells, and bioactive molecules (i.e., adhesion proteins, neural growth factors) into these multi-functional scaffolds, a permissive environment is created to facilitate neural repair and regeneration (right panel). Created using Microsoft PowerPoint 2016 and Scienceslides from Visiscience. 3D: Three-dimensional.