Mouse islet of Langerhans isolation using a combination of purified collagenase and neutral protease

The interrogation of beta cell gene expression and function in vitro has squarely shifted over the years from the study of rodent tumorigenic cell lines to the study of isolated rodent islets. Primary islets offer the distinct advantage that they more faithfully reflect the biology of intracellular signaling pathways and secretory responses. Whereas the method of islet isolation using tissue dissociating enzyme (TDE) preparations has been well established in many laboratories^1-4, variations in the consistency of islet yield and quality from any given rodent strain limit the extent and feasibility of primary islet studies. These variations often occur as a result of the crude partially purified TDEs used in the islet isolation procedure; TDEs frequently exhibit lot-to-lot variations in activity and often require adjustments to the dose of enzyme used. A small number of reports have used purified TDEs for rodent cell isolations^{5, 6}, but the practice is not widespread despite the routine use and advantages of purified TDEs for human islet isolations. In collaboration with VitaCyte, LLC (Indianapolis, IN), we developed a modified mouse islet isolation protocol based on that described by Gotoh^{7, 8}, in which the TDEs are perfused directly into the pancreatic duct of mice, followed by crude tissue fractionation through a Histopaque gradient⁹, and isolation of purified islets. A significant difference in our protocol is the use of purified collagenase (CIzyme MA) and neutral protease (CIzyme BP) combination. The collagenase was characterized by the use of a⁶ fluorescence collagen degrading activity (CDA) assay that utilized fluorescently labeled soluble calf skin fibrils as substrate⁶. This substrate is more predictive of the kinetics of collagen degradation in the tissue matrix because it relies on native collagen as the substrate. The protease was characterized with a sensitive fluorescent kinetic assay¹⁰. Utilizing these improved assays along with more traditional biochemical analysis enable the TDE to be manufactured more consistently, leading to improved performance consistency between lots. The protocol described in here was optimized for maximal islet yield and optimal islet morphology using C57BL/6 mice. During the development of this protocol, several combinations of collagenase and neutral proteases were evaluated at different concentrations, and the final ratio of collagenase:neutral protease of 35:10 represents enzyme performance comparable to Sigma Type XI. Because significant variability in average islet yields from different strains of rats and mice have been reported, additional modifications of the TDE composition should be made to improve the yield and quality of islets recovered from different species and strains.

Cell-free expression and functionality analysis of the tobacco lectin

The Nicotiana tabacum lectin, called Nictaba, is a nucleocytoplasmic plant lectin expressed in tobacco leaves after exogenous application of specific jasmonates and upon insect herbivory. Since the lectin concentrations are rather low, huge amounts of plant material are needed to purify milligram quantities of the protein. In addition, the purified lectin fractions are always contaminated with low molecular weight compounds such as phenols. In an attempt to improve and facilitate the purification of the tobacco lectin in reasonable amounts, an in vitro-coupled transcription/translation system based on an Escherichia coli lysate was used to express the lectin gene. Recombinant expression levels could be enhanced by an adapted codon usage. Recombinant lectin was purified, biochemically characterized and found to be biologically active. The biological activity of the recombinant lectin towards insect epithelial midgut cells was clearly demonstrated in a functional bio-assay and the internal cellular localization was analyzed using immunocytochemical techniques.