The ongoing investigation
of treatment options for human type I diabetes mellitus requires the continual development of innovative strategies that more effectively restore long-term physiological function, while still maintaining a simple approach with minimal invasiveness. In the past decade, islet engraftment has been heavily investigated as one such promising treatment for type I diabetic patients, offering a less invasive alternative to full pancreas replacement.1
In spite of the numerous efforts to improve islet engraftment, clinical efficacy is still lacking because the primary focus has been on avoiding host immune response via the development of semipermeable and biocompatible membranes, while relatively ignoring the potential benefits of a more biomimetic engraftment material.2
The need for improving the biomimetic character of islet scaffolds is supported by recent literature demonstrating that substantial β-cell loss during the peritransplant period is detrimental to the efficacy of islet transplantation.3
Specifically, the loss of β-cell function during islet isolation is believed to occur from the destruction of the native islet microenvironment, thereby triggering islet death.4
In addition, the disruption of islet–extracellular matrix (ECM) interactions exposes the islets to a variety of cellular stresses that further contribute to loss of biological functions. Therefore, biomimetic materials that create more ECM-mimicking environments are needed to better maintain islet function and survival during the intermediate stage between implantation and fully restored host integration.
The importance of islet-ECM interactions is well documented by a number of studies that demonstrate improved islet survival and function facilitated through the use of scaffolds containing isolated ECM proteins. For example, Nagata et al.
demonstrated that a mixture of different types of collagen increased both rat islet β-cell viability and glucose-stimulated insulin secretion.5
Human pancreatic β-cells grown on a bovine corneal endothelial cell matrix have also been found to maintain glucose-stimulated insulin secretion.6
Additionally, laminin-5-rich ECM substrates derived from rat bladder carcinoma cells have been shown to enhance the spreading of rat islet β-cells and improve glucose-stimulated insulin secretion.7
Thus, many research groups have begun incorporating ECM proteins into synthetic biomaterials for the encapsulation of islets or β-cells. To this effect, collagen IV absorbed in a poly(lactide-co-glycolide) scaffold showed enhanced function of transplanted islets.8
Similarly, MIN6 β-cells showed reduced apoptosis and maintained insulin secretion when cultured in poly(ethylene glycol) (PEG) hydrogels containing ECM proteins compared to control hydrogels alone.9
However, for clinical applications, the use of ECM proteins has some potential problems, such as undesirable immune responses, higher infection risks, variability in biological sources, and increased costs.10
To overcome these limitations, small peptide sequences derived from ECM proteins have been employed to modify the different types of polymers used for islet engraftment. For example, the growth and function of MIN6 cells were enhanced when encapsulated in either a PEG-β-based thermo-reversible gel conjugated with Gly-Arg-Gly-Asp-Ser (GRGDS) or photopolymerized PEG hydrogels modified with various laminin-derived peptides or type I collagen-derived peptides.11,12
Despite the benefits of the isolated ECM peptides, limitations of encapsulating biomaterials still need to be given consideration. In particular, the entrapment of cells in photopolymerized biomaterials can lead to many potential problems after implantation, such as the formation of fibrotic processes, poor degradation of the scaffold, and local and/or systemic toxicity.13
Differing compositions and concentrations of alginate have also been found to affect the cellular overgrowth of implanted capsules, as metabolic barriers to nutrient diffusion can form around the implant if nonoptimal levels of the material are used, despite the established biocompatibility of alginate.14
As a promising solution to overcome these drawbacks, peptide amphiphile (PA) nanomatrix gels would offer improved efficacy in pancreatic islet engraftment. The use of the PA nanomatrix gel as a transplant intermediary for islets is potentially advantageous because they meet the essential design criteria for synthetically recapitulating the ECM: rapid gel-like 3D network formation by self-assembly, versatility to incorporate various cell adhesive moieties, and cell-mediated degradable sites (matrix metalloproteinase-2 [MMP-2]) for progressive scaffold degradation and eventual replacement by host-ECM.15
Structurally, the PA consists of a hydrophilic functional peptide sequence attached to a hydrophobic alkyl tail, and the internal peptide structure can be adapted to mimic the characteristic properties of the natural ECM.16–20
Further, PAs self-assemble into long cylindrical structures that are 8–10
nm in diameter with a length up to several microns in length, and the self-assembly process is initiated by lowering the pH or adding multivalent ions, providing biocompatible means to encapsulate islets for engraftment.16,17
In this study, isolated rodent islets were incorporated into the PA nanomatrix gel containing a cell-adhesive ligand isolated from ECM proteins, arginine-glycine-aspartic acid (RGD), as well as an MMP-2 sensitive sequence. The interactions between islets and ECM are known to be important for β-cell viability and function, especially integrin signaling via RGD, which has been shown to decrease apoptosis of islets.21
Moreover, the MMP-2 enzyme is activated during rat pancreatic development, enabling cell migration of pancreatic endocrine cells throughout the ECM during islet morphogenesis.22
Thus, it is believed that the PA nanomatrix gel will facilitate progressive degradation and replacement by host ECM after transplantation in vivo
. Importantly, the PA forms a rapid, viscoelastic 3D microenvironment without any organic solvents or chemicals, providing the islets with a protective and nurturing environment to promote islet survival and function. Hence, we hypothesize that the PA nanomatrix gel will provide an ECM-mimicking microenvironment that imitates the native ECM microenvironment between islets and ECM, thereby improving islet survival and function. To evaluate this hypothesis, rat islets encapsulated within the PA nanomatrix gel were studied over 14 days of cultivation.
Since traditional islet culturing methods cause variability in the results due to physical loss during cultivation, we have also developed a new in vitro
culture method consisting of a modified insert chamber with a 5
μm nylon mesh sheet. This system allows long-term cultivation with a minimal loss of islets. Thus, we were able to assess the islet survival and function more consistently and precisely. Overall, our results demonstrated that islets encapsulated in the PA nanomatrix gel maintained their function with increased glucose-stimulated insulin secretion throughout the 14 days compared to the other groups. This demonstrates the potential of the PA nanomatrix gel to restore islet-ECM interactions and provide more precise evaluations of islet responses using the devised culture method. Therefore, this study proposes an innovative strategy to tackle some of limitations facing the clinical practice of islet transplantation.