The Study of Non-Viral Nanoscale Delivery Systems for Islet Transplantation

Our laboratory described that human islet cells can be driven to proliferate by expressing specific cell cycle proteins via adenoviral vectors [1]. Adenoviral vectors have several limitations including poor penetration into intact islets; only the most superficial cell layers get infected. Due to safety concerns associated with using viral systems clinically to expand islet cells and make them available to many more patients, significant emphasis has been placed on producing a safe and effective non-viral delivery system for biological research and gene therapy. To obtain this goal, we propose the use of an innovative technology that utilizes gold nanoparticles (AuNPs) as a non-viral method of delivery. Our laboratory was one of the first to describe the use of AuNPs in human islets and observe AuNPs can penetrate into the core of islets to deliver a gene to the vast majority of the cells, without damaging the cell [2]. Gold nanoparticles proved to be a biocompatible delivery system both in vitro and in vivo [2]. Thus far, gene therapy and molecular biology have focused primarily on delivering DNA of a specific gene into cells. The risk of this approach is that the DNA can be permanently incorporated into the genome and lead to damages in the cell that could result in overexpression of cancerous tumor cells. This risk does not exist with the use of mRNA. Many researchers believe mRNA is too unstable to be used as a molecular tool to overexpress specific proteins. With advances in nanotechnology, and better understanding of the translation process, methods have been developed that allow for expression of specific proteins by intracellular delivery of protein-encoding mRNA [3]. We used AuNPs conjugated to mCherry mRNA to establish a proof of concept of the feasibility of using AuNP-mRNA to achieve increased expression of a specific protein within cells. To do this, we conjugated mCherry mRNA to AuNPs and tested the feasibility for increasing delivery efficacy and preserve functionality of human pancreatic islets. We believe that with this novel xii technology we can create AuNPs that allow specific mRNA to enter islets and lead to the production of a specific protein within the cell, with the aim to induce beta cell proliferation. In a previous experiment with single cells, the highest amount of protein expression was observed after 24 hours incubation with mCherry conjugated AuNPs. Based on this, human islets were treated with 12 nm, 7 nm and 2 nm mCherry AuNPs for 24 hours. The expression of of mCherry protein in human islets was analyzed by 3D image reconstruction of z-stack images acquired by confocal microscopy. A minimal amount of mCherry protein was expressed in human islets when treated with mCherry mRNA coupled to the 12 nm size AuNP. Decreasing the size of the AuNPs to 7 nm or 2 nm resulted in substantial increase in mCherry protein expression throughout human pancreatic islets when treated at concentrations of 20 nM and 50 nM with mCherry mRNA AuNPs for 24 hours. We used measurements of calcium influx, KCL and mitochondrial potential to determine the effect of AuNP-mCherry mRNA treatment on islet cell function. The area under the curve was computed for intracellular calcium influx of three different islet preparations. There was no statistically significance difference between (2 nm) 20 nM versus (7 nm) 20 nM, (2 nm) 20 nM versus (7 nm) 50 nM, (2 nm) 50 nM versus (7 nm) 20 nM, (2 nm) 50 nM versus (7 nm) 50 nM. For the area under the curve for the KCL there was no significant statistical difference between the groups. In addition, mitochondrial potential indices demonstrated similarity between the control group and mCherry mRNA AuNPs treated human pancreatic islets, there was no statistical difference between the three different sizes and concentrations when compared to the non-treated group. Taken together, AuNP did not impair islet function when concentration was increased. Although, the optimal size of AuNP that was easily seen to express mCherry protein was 7 nm, when human islet cells were treated with AuNP coupled to mRNA for E2F3 (the β-cell xiii proliferation inducing protein), to observe whether there was any sign of enhanced β-cell proliferation, the 12 nm sized AuNP seemed to give a slight increase in β-cell proliferation. Transmission electron microscopy (TEM) was used to determine where within the islets the AuNPs were localized. This validated that both the 12 nm and 7 nm size AuNPs crossed the cell membrane and were found within vesicles, mitochondria and in one case the insulin granules of the islets. A notable difference that was detected under TEM for the two size of AuNPs was that the 12nm appeared predominantly in clusters where as the 7nm AuNP was more evenly distributed within the cell. Further analysis with TEM may provide provide insight on how the size, concentration and kinetics of the AuNPs will influence protein expression and β-cell expansion within human pancreatic islets.