Self-Assembling Peptide Hydrogels as Functional Tools to Tackle Intervertebral Disc Degeneration

Abstract

Low back pain (LBP), caused by intervertebral disc (IVD) degeneration, is a major contributor to global disability. In its healthy state, the IVD is a tough and well-hydrated tissue, able to act as a shock absorber along the spine. During degeneration, the IVD is hit by a cell-driven cascade of events, which progressively lead to extracellular matrix (ECM) degradation, chronic inflammation, and pain. Current treatments are divided into palliative care (early stage degeneration) and surgical interventions (late-stage degeneration), which are invasive and poorly efficient in the long term. To overcome these limitations, alternative tissue engineering and regenerative medicine strategies, in which soft biomaterials are used as injectable carriers of cells and/or biomolecules to be delivered to the injury site and restore tissue function, are currently being explored. Self-assembling peptide hydrogels (SAPHs) represent a promising class of de novo synthetic biomaterials able to merge the strengths of both natural and synthetic hydrogels for biomedical applications. Inherent features, such as shear-thinning behaviour, high biocompatibility, ECM biomimicry, and tuneable physiochemical properties make these hydrogels appropriate and functional tools to tackle IVD degeneration. This review will describe the pathogenesis of IVD degeneration, list biomaterials requirements to attempt IVD repair, and focus on current peptide hydrogel materials exploited for this purpose.

Keywords: intervertebral disc; self-assembling peptide hydrogels; tissue engineering.

Figure 1

(A) Graphical representation of adjacent vertebral units in sagittal view. Each unit consists of vertebral bodies surrounding an IVD. The nerve supply of the IVD and vertebral bodies consists of the spinal cord disposed longitudinally along the vertebrae and passing through the intervertebral foramen. Vertebrae are kept in place by anterior and posterior ligaments. (B) Schematic representation of the IVD, showing the NP, AF, and CEP regions.

Figure 2

Schematic representation of the human IVD, showing its water and ECM content. The IVD is surrounded by blood and nerve vessels. Capillaries penetrate a few millimetres into the outer AF to provide nutrients and waste exchange. Cells from the avascular NP and inner AF receive nutrients and are able to exchange waste products through a bidirectional flow occurring via blood capillaries that penetrate the subchondral plate and reach the CEP.

Figure 3

Macroscopic and microscopic changes of the IVD during degeneration. Characteristic changes of the NP’s ECM after degeneration are illustrated in the dashed circles. ECM of degenerated discs (dashed red circle) shows shorter aggrecan macromolecules and more collagen type I fibres (thicker fibre bundles) than collagen type II (thinner collagen bundles), which are largely abundant in the ECM of healthy discs (dashed green circle).

Figure 4

Comparison between different stages of intervertebral disc degeneration (I to V, left to right) according to the Pfirrmann grade scale based on MRI images. Reprinted and adapted with permission from Ref. [103]. Copyright 2022, Elsevier.

Figure 5

List of 20 natural amino acids. For each amino acid, its 3-letter abbreviation, 1-letter code, and property (‘hydrophobic’, ‘negatively charged’, ‘positively charged’, ‘hydrophilic, uncharged’, and ‘other’) are provided. Cys, Gly, and Pro are listed as ‘other’ since they have specific roles in peptide self-assembly that cannot be associated with the other properties listed.

Figure 6

Beta-sheet and beta-hairpin peptide system designs. (A) Schematic representation of β-sheet-forming peptide hydrogel formation. Peptide sequences under external stimuli (e.g., pH, enzyme, temperature, light, time) self-assemble into β-sheet fibrils and fibres, which above a critical gelation concentration entrap water in water-swollen networks, i.e., hydrogels. (B) Detail of a β-sheet-forming polypeptide (i) reacting in water with another polypeptide via stacking of hydrophobic regions (ii). (C) Design of a β-hairpin sequence (i.e., MAX1), in which two valine-based peptides are linked together by a -V^DPPT- turn. This tetrapeptide based on D-isomer valine induces a trans-prolyl amide bond re-arrangement favouring the β-hairpin formation. (D) Folding/self-assembly pathways of β-hairpins. Adapted with permission from Ref. [200]. Copyright 2022, American Chemical Society.

Figure 7

Coiled-coil and α-helix barrel peptide system designs. (A) Heptad wheel representation of parallel and anti-parallel coiled coils. Supramolecular structures form due to the interactions occurring between ‘a’ and ‘d’ (usually hydrophobic), while ‘g’ and ‘e’ (usually charged) stabilise the assembly. (B) Larger coiled-coil structures form α-helical barrels (αHBs) with accessible central channels. From right (red) to left (blue), X-ray crystal structures of multiple pentameric to nonameric αHBs. Images were adapted with permission from Ref. [206]. Copyright 2022, Royal Society of Chemistry.

Figure 8

Examples of short aromatic and peptide amphiphile designs. (A) Chemical structures of Fmoc-FF and Fmoc-RGD assembling into nanofibrils with RGD sequences on the fibre surface. (B) Example of a PA sequence consisting of a hydrophobic tail, a β-forming segment, an ionisable motif, and a hydrophilic bioactive epitope. Self-assembled PAs show bioactive epitope on the surface (green appendages) and hydrophobic tails in the core (dark grey). Reprinted and adapted with permission from Ref. [223]. Copyright 2022, Elsevier.

Figure 9

Representative studies employing peptide-based hydrogels for IVD-repair applications. (A) PA structures mimicking collagen fibres for IVD applications. MSCs cultured on PA nanofibres showed successful differentiation after 14 days and abundant GAG deposition (Safranin O and Alcian Blue stainings shown). (B) Self-assembly and formulation of GO-containing F8 hydrogels. TGF-β3-coated GO-F8 hydrogels showed increased gene and protein expression of NP cells after 3 weeks of 3D culture; *** p-value < 0.001. Reprinted and adapted with permission from Refs. [233,234]. Copyright 2022, Elsevier. Reprinted and adapted with permission from [236]. Copyright 2022, American Chemical Society.