Evaluating the Effect of Serine Proteases on Collagenase Activity during Human Islet Isolation

A model to evaluate toxic factors influencing islets during collagenase digestion: the role of serine protease inhibitor in the protection of islets

The recovery of all of the islets contained in a pancreas is the goal of islet isolation for transplantation. This study reveals an environment that injures the isolated islets during digestion and proposes a new model for optimal islet isolation. Islets were isolated from Wistar rat pancreases by stationary collagenase digestion while the digestion time was varied at 15, 30, 60, and 120 min. The digested pancreas and islets were analyzed histologically and adenosine nucleotides were measured. Overnight cultured islets (40 islets) were cocultured for 30 min with the supernatants obtained from pancreatic collagenase digestion at different digestion periods in order to assess the toxic environment. The peak yields of islets were obtained at 30 min of digestion. The histological study of digested pancreas showed that the exocrine cells lost their cellular integrity at 120 min of digestion, but the islet cells were left intact. Accordingly, the ATP levels of the pancreatic tissue decreased during the digestion period. The coculture experiment demonstrated that the islets cultured with the supernatants from the collagenase digestion showed digestion time-dependent disruption of the cellular integrity of islets in accordance with a rapid decrease of ATP levels in the islets. The addition of serine protease inhibitors into this coculture clearly showed protection of islets, which maintained high ATP levels in association with intact membrane integrity as assessed by AO/PI staining. Morphological deterioration of islets as well as a marked ATP decrease was evident in the entire digested pancreas as well as in islets cocultured in the supernatants from the collagenase digestion. Various factors toxic to the islets can therefore be analyzed in future experiments using this coculture model for obtaining a good yield of viable islets.

Adenine nucleotide levels in a closed enzymatic digestion system for porcine islet isolation

Obtaining viable islets is a crucial step for successful islet transplantation. Adenosine triphosphate (ATP) is a marker of cell viability. However, little is known about any changes in the energy status of the tissues that are being digested during the digestion phase. We herein examined whether the ATP content in serially digested pancreatic tissue samples could be specific objective parameters that signal the optimal point to stop the digestion process. We obtained partial pancreata (body to tail) from 4- to 5-year-old pigs from a slaughterhouse. The tissue samples were preserved in M-Kyoto solution for less than 3 h. They were digested using an automated enzymatic and mechanical dissociation system at 37°C for 90 min following intraductal injection of Liberase HI. Samples were collected from the digestive circuit every 5 or 10 min to determine the ATP level, total adenine nucleotide (TAN) level, islet count (count/g), and yield of islet equivalent (IEQ) in the serial digestive fluids. The ATP and TAN levels, IEQ and islet count were increased and then decreased during digestion process. The profile of these parameters differed from case to case. However, when ATP changing ratio (respective value/precedent value) was compared with IEQ changing ratio, a greater than threefold increase in the ATP changing ratio followed by an increase in the islet count changing ratio within 5 min was consistently observed, indicating the optimal time to stop the digestion. The ATP levels of the handpicked islets in the digested samples were lower in the overdigested phase in comparison to those in the earlier digested phase. These results indicate that the ATP level in digested fluid could be an effective indicator to estimate the viability of cells as well as determine the optimal time to terminate the digestion process in order to obtain viable islets.

The use of multiparametric monitoring during islet cell isolation and culture: a potential tool for in-process corrections of critical physiological factors

Variables such as pH, pCO2, and PO2 have been established in the literature as critical factors that could affect the outcome of the islet cell processing and, therefore, the quality of the cells that could be transplanted. This report describes a highly accurate continuous multiparametric monitoring system and its evaluation for continuous monitoring of physiological variables during critical steps of the islet isolation procedure as well as during in vitro culture of the insulin-producing cells. Close monitoring of these variables could be of assistance to improve the outcome of islet cell processing, allowing to identify as soon as possible problems that could be corrected during the procedure, as well as during in vitro preservation, or shipment to remote sites.

Assessment of a novel two-component enzyme preparation for human islet isolation and transplantation

BACKGROUND

Biologic characteristics of enzyme products used for islet isolation are critical for the success of islet transplantation. In particular, lot-to-lot variability significantly affects the yields of the isolation procedure. In this study, we have evaluated a new enzyme preparation in which highly purified collagenase can be blended with separately supplied neutral protease in a predetermined ratio.

METHODS

We compared the results of human-islet isolations performed either with Collagenase NB1 supplemented with Neutral Protease NB (group I, n=9) or with Liberase (group II, n=9).

RESULTS

Endotoxin contents of the enzyme preparations were lower in group I. Total islet yields were similar in both groups, but islet equivalents per gram of pancreas was higher in group I (4,020+/-1,240 vs. 2,360+/-1,350; P<0.05). Islet morphology was improved in group I with significantly higher proportion of free and intact islets (71+/-9% vs. 52+/-14%; P<0.01). In vitro function was improved and apoptosis rate was lower in group I.

CONCLUSIONS

This new enzyme blend was as efficient as Liberase in terms of islet yields and showed improvements in islet morphology, viability, and in vitro function. The possibility to control lot-to-lot variability and the low endotoxin contents make Collagenase NB1 a promising product for human-islet isolation.

Gene expression profiling of cultured human islet preparations

The expression of functional and regulatory genes by islet cells is a key determinant for the success of islet transplantation. The aim of this study is twofold: first, to characterize the cluster of genes expressed in human islet isolations; and second, to validate the capability of gene array technology to assess with accuracy the expression of various transcripts. RNA from isolated islet preparations obtained from three independent donors was converted to cDNA and then transcribed to cRNA. Individual cRNA preparations were then hybridized to U133A microarrays carrying approximately 23,000 genes, and analyzed using GeneSpring (SiliconGenetics, Redwood City, CA) software. Real-time reverse transcription-polymerase chain reaction was performed to validate results obtained by microarray analysis. Microarray analysis identified the expression of about 7,000 genes transcribed in cultured human islet preparations. Enzymes represented the most abundant class of genes identified, followed by nuclear binding proteins, signal transduction molecules, transport proteins, and growth factor receptors and their ligands. Real-time polymerase chain reaction confirmed the identification of various islet-specific genes detected by microarray analysis, but also showed that such genes as pancreatic duodenal homeobox 1 protein and glucagon-like peptide 1 receptor, which were not detected by gene array, can be readily identified and quantified. In addition, gene array produced a suboptimal quantification of genes expressed in large amounts by islet cells. Indeed, the abundance of mRNA for insulin when compared with the level of somatostatin mRNA was not as different as one would have predicated based on the classic knowledge of islet physiology. Gene array analysis appears to be a valuable tool to obtain preliminary information of genes expressed by a given tissue. The expression levels of transcripts expressed in very low or very high quantities need to be confirmed by an independent technique.

An evaluation of endogenous pancreatic enzyme levels following enzymatic digestion

Human islet isolation consists of a digestion phase, dilution phase, and purification phase. Recent evidence suggests that inconsistencies in islet yields are attributed to the activation of endogenous enzymes of the donor pancreas during the digestion phase. Therefore, following the digestion phase, it is important to inhibit these enzymes by the addition of an inhibitor to the dilution phase. In this study, we report the endogenous pancreatic enzyme levels after the purification phase and the effects of potential inhibitors on the proteases of interest. Results at the end of the purification phase indicated that chymotrypsin retained approximately 20% of the activity observed after the digestion phase, whereas trypsin, elastase, and collagenase retained approximately 2.5%, 2%, and 3.5% of the activity, respectively, demonstrating that added inhibitors are not fully effective. Potential enzyme inhibitors, human albumin, fetal calf serum, and aprotinin, were incubated with trypsin, chymotrypsin, elastase, and collagenase and assayed for activity. Fetal calf serum and aprotinin showed strong inhibitory actions toward trypsin and chymotrypsin. Aprotinin completely inhibited the tryptic activity; however, it did not inhibit human chymotrypsin or elastase activity. Human albumin showed minimal inhibition and was shown to act as a competitive inhibitor. This study clearly demonstrates that low amounts of endogenous pancreatic enzymes remain active at the end of the human islet isolation procedure and that the added inhibitors at the dilution are not fully effective at inhibiting the enzymes.