Synthetic mimics of the extracellular matrix: how simple is complex enough?

Abstract

Cells reside in a complex and dynamic extracellular matrix where they interact with a myriad of biophysical and biochemical cues that direct their function and regulate tissue homeostasis, wound repair, and even pathophysiological events. There is a desire in the biomaterials community to develop synthetic hydrogels to recapitulate facets of the ECM for in vitro culture platforms and tissue engineering applications. Advances in synthetic hydrogel design and chemistries, including user-tunable platforms, have broadened the field's understanding of the role of matrix cues in directing cellular processes and enabled the design of improved tissue engineering scaffolds. This review focuses on recent advances in the development and fabrication of hydrogels and discusses what aspects of ECM signals can be incorporated to direct cell function in different contexts.

FIGURE 1

(a) The extracellular matrix is composed of many different proteins, including collagen, fibonectin, and laminin, which were carefully preserved in decellularized heart tissue in a study by Ott et al. (b) The cellular microenvironment is a complex biophysical and biochemical environment where cells reside. This microenvironment includes degradable structural fibers, adhesive binding domains, and proteoglycans for biomolecule sequestration.

FIGURE 2

Examples of prominent reactions used for bioconjugation and/or hydrogel crosslinking: (a) base catalyzed thiol-vinyl sulfone Michael addition, (b) radical mediated thiolene, (c) strain-promoted azide-alkyne cycloaddition (SPAAC).

FIGURE 3

(a) The ECM provides important biophysical cues, such as matrix elasticity, illustrated here with a stiff, dense matrix (Left) and a soft, loose matrix (Right). Synthetic hydrogels can recapitulate these properties by tuning their mechanical properties. (b) Hydrogels with phototunable elasticity provide a versatile platform to study the role of biophysical cues in directing cell function. Yang et al. cultured MSC on stiff substrates for 1 or 10 days and then softened the hydrogels in situ using a pre-determined dose of light. RUNX2 and YAP nuclear localization was then observed for up to 10 days, and they both followed similar trends. Shown here, the percent of hMSC with nuclear RUNX2 localization returned to basal levels after being cultured for only 1 day on stiff substrates (DSt1) and different durations on soft substrates. The stiff and soft controls correspond to the average RUNX2 localization for cells only cultured on stiff substrates (i.e., not softened with light) or only on soft substrates (i.e., softened with light) and represent full and basal levels of activation, respectively. (c) When hMSC were cultured on stiff substrates for 10 days (DSt10), RUNX2 localization persisted at active levels even after culture on soft substrates, indicating that 10 days on stiff substrates induced irreversible activation of RUNX2. The data are plotted as the mean ± SEM. NS, not significant; *p <0.05; **p<0.01; ***p <0.001.

FIGURE 4

(a) Techniques based on covalent and physical immobilization are being developed for spatial biomolecule patterning to recapitulate aspects of biomolecule sequestration. Further, peptide mimics have been shown to be effective for recapitulating cell adhesion in otherwise non-adhesive hydrogels. (b) Using barstar-barnase and biotin-streptavidin binding pairs, Wylie et al. spatially tethered fluorescently labeled sonic hedgehog and ciliary neurotrophic factor into a hydrogel through physical immobilization. (c) Moseweicz et al. spatially patterned PDGF-BB to direct the invasion of encapsulated MSC. A confocal micrograph shows that MSC, labeled with DAPI (blue), more efficiently invaded the patterned region of a hydrogel with covalently immobilized PDGF-BB (purple).

FIGURE 5

(a) Cellular remodeling and degradation play an important role in regulating the dynamic nature of the ECM. The introduction of hydrogels that degrade in response to cell-secreted enzymes begins to recapitulate this natural process. (b) Patterson et al. synthesized PEG hydrogels that were crosslinked with two different peptide sequences that vary in degradation rates. Cells from the aortic chick ring invaded the MMP-degradable network at different rates based on the cleavage rate of the peptide sequence, GPQGIWG (slower) and VPMSMRGG (faster).