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

Synthetic matrices that are cytocompatible, cell adhesive,andcell responsive are needed for the engineering of implantable, secretorysalivary gland constructs to treat radiation induced xerostomia ordry mouth. Here, taking advantage of the bioorthogonality of the Michael-typeaddition reaction, hydrogels with comparable stiffness but varyingdegrees of degradability (100% degradable, 100DEG; 50% degradable,50DEG; and nondegradable, 0DEG) by cell-secreted matrix metalloproteases(MMPs) were synthesized using thiolated HA (HA-SH), maleimide (MI)-conjugatedintegrin-binding peptide (RGD-MI), and MI-functionalized peptide cross-linkersthat are protease degradable (GIW-bisMI) or nondegradable (GIQ-bisMI).Organized multicellular structures developed readily in all hydrogelsfrom dispersed primary human salivary gland stem cells (hS/PCs). Asthe matrix became progressively degradable, cells proliferated morereadily, and the multicellular structures became larger, less spherical,and more lobular. Immunocytochemical analysis showed positive stainingfor stem/progenitor cell markers CD44 and keratin 5 (K5) in all threetypes of cultures and positive staining for the acinar marker & alpha;-amylaseunder 50DEG and 100DEG conditions. Quantitatively at the mRNA level,the expression levels of key stem/progenitor markers KIT, KRT5, and ETV4/5 were significantlyincreased in the degradable gels as compared to the nondegradablecounterparts. Western blot analyses revealed that imparting matrixdegradation led to >3.8-fold increase in KIT expression by day15.The MMP-degradable hydrogels also promoted the development of a secretaryphenotype, as evidenced by the upregulation of acinar markers & alpha;-amylase(AMY), aquaporin-5 (AQP5), and sodium-potassiumchloride cotransporter 1 (SLC12A2). Collectively,we show that cell-mediated matrix remodeling is necessary for thedevelopment of regenerative pro-acinar progenitor cells from hS/PCs.