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

Enzyme Development for Human Islet Isolation: Five Decades of Progress or Stagnation?

In comparison to procedures used for the separation of individual cell types from other organs, the process of human pancreatic islet isolation aims to digest the pancreatic exocrine matrix completely without dispersing the individual cells within the endocrine cell cluster. This objective is unique within the field of tissue separation, and outlines the challenge of islet isolation to balance two opposing priorities. Although significant progress has been made in the characterization and production of enzyme blends for islet isolation, there are still numerous areas which require improvement. The ultimate goal of enzyme production, namely the routine production of a consistent and standardized enzyme blend, has still not been realized. This seems to be mainly the result of a lack of detailed knowledge regarding the structure of the pancreatic extracellular matrix and the synergistic interplay between collagenase and different supplementary proteases during the degradation of the extracellular matrix. Furthermore, the activation of intrinsic proteolytic enzymes produced by the pancreatic acinar cells, also impacts on the chance of a successful outcome of human islet isolation. This overview discusses the challenges of pancreatic enzymatic digestion during human islet isolation, and outlines the developments in this field over the past 5 decades.

Fluorogenic Peptide Substrate for Quantification of Bacterial Enzyme Activities

A novel peptide substrate (A G G P L G P P G P G G) was developed for quantifying the activities of bacterial enzymes using a highly sensitive Fluorescence Resonance Energy Transfer (FRET) based assay. The peptide substrate was cleaved by collagenase class I, II, Liberase MTF C/T, collagenase NB1, and thermolysin/neutral protease, which was significantly enhanced in the presence of CaCl2. However, the activities of these enzymes were significantly decreased in the presence of ZnSO4 or ZnCl2. Collagenase I, II, Liberase MTF C/T, thermolysin/neutral protease share similar cleavage sites, L↓G and P↓G. However, collagenase NB1 cleaves the peptide substrate at G↓P and P↓L, in addition to P↓G. The enzyme activity is pH dependent, within a range of 6.8 to 7.5, but was significantly diminished at pH 8.0. Interestingly, the peptide substrate was not cleaved by endogenous pancreatic protease such as trypsin, chymotrypsin, and elastase. In conclusion, the novel peptide substrate is collagenase, thermolysin/neutral protease specific and can be applied to quantify enzyme activities from different microbes. Furthermore, the assay can be used for fine-tuning reaction mixtures of various agents to enhance the overall activity of a cocktail of multiple enzymes and achieve optimal organ/tissue digestion, while protecting the integrity of the target cells.

Sericin in the isolating solution improves the yield of islets isolated from the pancreas

Approximately half of the transplantable pancreatic islet tissue is lost during isolation, including the digestion and purification steps. Modifying the isolation method could increase the yield. This would enable the one donor-one recipient concept and improve the therapeutic effects of islet transplantation. This study aims to improve islet transplantation by increasing the yield of islets from the pancreas, both the number of islets and their size. Therefore, we used a sericin-containing isolating solution. Rat pancreatic islets were isolated by collagenase digestion and hand picking. We refer to islets isolated with or without sericin in the isolation solution as the sericin and control group, respectively. Volume yield, endocrine function, and islet morphology were compared between the groups. Histological distribution of sericin was evaluated by immunofluorescence staining to examine its mechanism of action in pancreatic islets. The pancreatic islet yield in the sericin group was significantly higher than that in the control group. The endocrine function of islets in the sericin group was comparable to that of islets isolated by conventional methods. Sericin adhered to the surface of isolated pancreatic islets and colocalized with E-cadherin, a cell membrane protein, which might explain the cytoprotective effects of sericin. The islet morphology tended to be better preserved in the sericin group. Sericin could prevent cytoarchitectural damage during the isolation and purification process, resulting in increased pancreatic islet yield. This suggests that sericin could contribute to islet therapy by enhancing the stability of islets.

Synergistic Effect of Neutral Protease and Clostripain on Rat Pancreatic Islet Isolation

BACKGROUND

Islet isolation currently requires collagenase, neutral protease and other components. Thermolysin (TL) from Bacillus thermoproteolyticus is the gold standard neutral protease. However, we speculated that neutral protease derived from Clostridium histolyticum (Ch; ChNP) would be biologically superior for islet isolation. Tryptic-like activity has also been reported to be important. Therefore, we focused on clostripain (CP), since it is one of the main proteases in Clostridium histolyticum which possesses tryptic-like activity. We then examined the synergistic effects of highly purified ChNP and CP on rat islet isolation.

METHODS

The same amount of collagenase was used in all four groups (TL, ChNP, TL+CP and ChNP+CP; n = 12/group). The efficiency was evaluated by the islet yield and function. An immunohistochemical analysis, in vitro digestion assay for each enzyme component and evaluation of the activation of endogenous exocrine proteases during islet isolation were also performed.

RESULTS

The islet yield of the TL group was significantly higher than that of the ChNP group (P < 0.01). The islet yield was dose dependently increased in the ChNP+CP group, but was decreased in the TL + CP group. The islet yield in the ChNP + CP group was significantly higher than that in the TL group, but their islet function was similar. Different specificities for laminin, especially laminin-511, were observed in the TL, ChNP, and CP groups.

CONCLUSIONS

Clostripain had a strong synergistic effect with ChNP, but not with TL. Therefore, ChNP and CP, in combination with collagenase derived from the same bacteria, may effectively increase the isolation efficiency without affecting the quality of islets.

Optimization and Scale-up Isolation and Culture of Neonatal Porcine Islets: Potential for Clinical Application

One challenge that must be overcome to allow transplantation of neonatal porcine islets (NPIs) to become a clinical reality is defining a reproducible and scalable protocol for the efficient preparation of therapeutic quantities of clinical grade NPIs. In our standard protocol, we routinely isolate NPIs from a maximum of four pancreases, requiring tissue culture in 16 Petri dishes (four per pancreas) in Ham's F10 and bovine serum albumin (BSA). We have now developed a scalable and technically simpler protocol that allows us to isolate NPIs from a minimum of 12 pancreases at a time by employing automated tissue chopping, collagenase digestion in a single vessel, and tissue culture/media changes in 75% fewer Petri dishes. For culture, BSA is replaced with human serum albumin and supplemented with Z-VAD-FMK general caspase inhibitor and a protease inhibitor cocktail. The caspase inhibitor was added to the media for only the first 90 min of culture. NPIs isolated using the scalable protocol had significantly more cellular insulin recovered (56.9 ± 1.4 µg) when compared to the standard protocol (15.0 ± 0.5 µg; p < 0.05). Compared to our standard protocol, recovery of β-cells (6.0 × 10(6) ± 0.2 vs. 10.0 × 10(6) ± 0.4; p < 0.05) and islet equivalents (35,135 ± 186 vs. 41,810 ± 226; p < 0.05) was significantly higher using the scalable protocol. During a static glucose stimulation assay, the SI of islets isolated by the standard protocol were significantly lower than the scale-up protocol (4.3 ± 0.2 vs. 5.5 ± 0.1; p < 0.05). Mice transplanted with NPIs using the scalable protocol had significantly lower blood glucose levels than the mice that receiving NPIs from the standard protocol (p < 0.01) and responded significantly better to a glucose tolerance test. Based on the above findings, this improved simpler scalable protocol is a significantly more efficient means for preparing therapeutic quantities of clinical grade NPIs.