Matrix Degradability Contributes to the Development of Salivary Gland Progenitor Cells with Secretory Functions

Design and Characterization of Thioester Networks with Adaptable and Enzymatically Degradable Cross-Links

Viscoelastic properties of the extracellular matrix (ECM) impact cell processes including proliferation, spreading, and migration. During these basic cellular processes, cells remodel the ECM by secreting enzymes and applying cytoskeletal tension to the network. To design cell delivery platforms that mimic physical ECM properties, new designs incorporate viscoelasticity and moieties that enable cell-mediated network remodeling. In this work, we design and characterize networks with two different types of cross-links, covalent adaptable and enzymatically degradable. Our networks consist of 8-arm poly(ethylene glycol) (PEG)-thiol, PEG-thioester norbornene, and a norbornene functionalized matrix metalloproteinase (MMP)-degradable peptide, KKGPQG↓IWGQKK. We characterize three network compositions with a ratio of 1:1, 3:1, and 4:1 adaptable to MMP-degradable cross-links. We characterize network mechanical properties using bulk rheology. Using multiple particle tracking microrheology (MPT), we measure the evolving microstructure of the network during degradation. MPT measures Brownian motion of fluorescently labeled probe particles, which can be used to calculate rheological properties. Our results show that the elastic modulus increases with an increasing ratio of adaptable to MMP-degradable cross-links, and all networks have the same extent of stress relaxation. We then measure degradation of these networks by incubating in l-cysteine, which degrades only the adaptable cross-links by the thioester exchange reaction. We measure complete degradation of all three compositions using bulk rheology. Networks with 4:1 adaptable to MMP-degradable cross-links are the slowest to degrade and networks with 3:1 adaptable to MMP-degradable cross-links are the fastest to degrade. MPT measurements during degradation show networks with 1:1 and 4:1 adaptable to MMP-degradable cross-links rearrange multiple times before complete degradation. In networks with 3:1 adaptable to MMP-degradable cross-links, we measure fewer network rearrangements prior to degradation. Using time-cure superposition (TCS), we measure the network structure at the phase transition. Networks with 1:1 and 4:1 adaptable to MMP-degradable cross-links are elastic and tightly cross-linked and networks with 3:1 adaptable to MMP-degradable cross-links can range from elastic to open networks. The most open network structure, networks with 3:1 adaptable to MMP-degradable cross-links, degrade on the shortest time scale. We also measure ≥70% hMSC viability in each network after 3D encapsulation. In this work, we characterize different compositions of hybrid networks that incorporate both adaptable and enzymatically degradable cross-links. This work can enable design that specifies the mechanical properties and degradation behavior of the material to better mimic aspects of the native ECM.

Promoting Polarization and Differentiation of Primary Human Salivary Gland Stem/Progenitor Cells in Protease-Degradable Hydrogels via ROCK Inhibition

Toward the goal of in vitro engineering of functional salivary gland tissues, we cultured primary human salivary stem/progenitor cells (hS/PCs) in hyaluronic acid-based matrices with varying percentages of proteolytically degradable crosslinks in the presence of Rho kinase (ROCK) inhibitor. Single cells encapsulated in the hydrogel grew into organized multicellular structures by day 15, and over 60% of the structures developed in the nondegradable and 50% degradable hydrogels contained a central lumen. Importantly, ROCK inhibition led to the establishment of multicellular structures that were correctly polarized, as evidenced by apical localization of a Golgi marker GM130, apical/lateral localization of tight junction protein zonula occludens-1 (ZO-1), and basal localization of integrin β1 and basement membrane proteins laminin α1 and collagen IV. Cultures maintained in 50% degradable gels with ROCK inhibition exhibited an increased expression of acinar markers aquaporin 5 (AQP5, AQP5) and sodium-potassium-chloride cotransporter 1 (SLC12A2, NKCC1) at the transcript and the protein levels, respectively, as compared to those without ROCK inhibition. Upon stimulation with isoproterenol, α-amylase secretion into the lumen was observed. Particle-tracking microrheology was employed to analyze the stiffness of cells using mitochondria as the passive tracer particles. Our results indicated that cells grown in 100% degradable gels were stiffer than those maintained in nondegradable gels, and cells cultured with the ROCK inhibitor were softer than those maintained without the inhibitor. We conclude that reducing cellular contractility via ROCK inhibition while retaining some degree of matrix confinement promotes the establishment of multicellular structures containing pro-acinar cells with correct apicobasal polarization.

Bio-orthogonal tuning of matrix properties during 3D cell culture to induce morphological and phenotypic changes

Described herein is a protocol for producing a synthetic extracellular matrix that can be modified in situ during three-dimensional cell culture. The hydrogel platform is established using modular building blocks employing bio-orthogonal tetrazine (Tz) ligation with slow (norbornene, Nb) and fast (trans-cyclooctene, TCO) dienophiles. A cell-laden gel construct is created via the slow, off-stoichiometric Tz/Nb reaction. After a few days of culture, matrix properties can be altered by supplementing the cell culture media with TCO-tagged molecules through the rapid reaction with the remaining Tz groups in the network at the gel-liquid interface. As the Tz/TCO reaction is faster than molecular diffusion, matrix properties can be modified in a spatiotemporal fashion simply by altering the identity of the diffusive species and the diffusion time/path. Our strategy does not interfere with native biochemical processes nor does it require external triggers or a second, independent chemistry. The biomimetic three-dimensional cultures can be analyzed by standard molecular and cellular techniques and visualized by confocal microscopy. We have previously used this method to demonstrate how in situ modulation of matrix properties induces epithelial-to-mesenchymal transition, elicits fibroblast transition from mesenchymal stem cells and regulates myofibroblast differentiation. Following the detailed procedures, individuals with a bachelor's in science and engineering fields can successfully complete the protocol in 4-5 weeks. This protocol can be applied to model tissue morphogenesis and disease progression and it can also be used to establish engineered constructs with tissue-like anisotropy and tissue-specific functions.

Promoting Polarization and Differentiation of Primary Human Salivary Gland Stem/Progenitor Cells in Protease-Degradable Hydrogels via ROCK Inhibition

Towards the goal of in vitro engineering of functional salivary gland tissues, we cultured primary human salivary stem/progenitor cells (hS/PCs) in hyaluronic acid-based matrices with varying percentages of proteolytically degradable crosslinks in the presence of Rho kinase (ROCK) inhibitor. Single cells encapsulated in the hydrogel grew into organized multicellular structures by day 15, and over 60% of the structures developed in the non-degradable and 50% degradable hydrogels contained a central lumen. Importantly, ROCK inhibition led to the establishment of multicellular structures that were correctly polarized, as evidenced by apical localization of a Golgi marker GM130, apical/lateral localization of tight junction protein zonula occludens-1 (ZO-1), and basal localization of integrin β1 and basement membrane proteins laminin α1 and collagen IV. Cultures maintained in 50% degradable gels with ROCK inhibition exhibited an increased expression of acinar markers AQP5 and SLC12A2 (at the transcript level) and AQP5 and NKCC1 (at the protein level) as compared to those without ROCK inhibition. Upon stimulation with isoproterenol, α-amylase secretion into the lumen was observed. Particle-tracking microrheology was employed to analyze the stiffness of cells using mitochondria as the passive tracer particles. Our results indicated that cells grown in 100% degradable gels were stiffer than those maintained in non-degradable gels, and cells cultured with the ROCK inhibitor were softer than those maintained without the inhibitor. We conclude that reducing cellular contractility via ROCK inhibition while retaining some degree of matrix confinement promotes the establishment of multicellular structures containing pro-acinar cells with correct apicobasal polarization.

Basement Membrane Mimetic Hydrogel Cooperates with Rho-Associated Protein Kinase Inhibitor to Promote the Development of Acini-Like Salivary Gland Spheroids

Successful engineering of functional salivary glands necessitates the creation of cell-instructive environments for ex vivo expansion and lineage specification of primary human salivary gland stem cells (hS/PCs). Herein, basement membrane mimetic hydrogels were prepared using hyaluronic acid, cell adhesive peptides, and hyperbranched polyglycerol (HPG), with or without sulfate groups, to produce "hyperGel+" or "hyperGel", respectively. Differential scanning fluorescence experiments confirmed the ability of the sulphated HPG precursor to stabilize fibroblast growth factor 10. The hydrogels were nanoporous, cytocompatibile and cell-permissive, enabling the development of multicellular hS/PC spheroids in 14 days. Incorporation of sulfated HPG species in the hydrogel enhanced cell proliferation. Culture of hS/PCs in hyperGel+ in the presence of a Rho kinase inhibitor, Y-27632 (Y-27), led to the development of spheroids with a central lumen, increased the expression of acinar marker aquaporin-3 at the transcript level (AQP3), and decreased the expression of ductal marker keratin 7 at both the transcript (KRT7) and the protein levels (K7). Reduced expression of transforming growth factor beta (TGF-β) targets SMAD2/3 was also observed in Y27-treated cultures, suggesting attenuation of TGF-β signaling. Thus, hyperGel+ cooperates with the ROCK inhibitor to promote the development of lumened spheroids with enhanced expression of acinar markers.