Christopher A. Fraker

Preventing hypoxia-induced cell death in beta cells and islets via hydrolytically activated, oxygen-generating biomaterials

A major hindrance in engineering tissues containing highly metabolically active cells is the insufficient oxygenation of these implants, which results in dying or dysfunctional cells in portions of the graft. The development of methods to increase oxygen availability within tissue-engineered implants, particularly during the early engraftment period, would serve to allay hypoxia-induced cell death. Herein, we designed and developed a hydrolytically activated oxygen-generating biomaterial in the form of polydimethylsiloxane (PDMS)-encapsulated solid calcium peroxide, PDMS-CaO2. Encapsulation of solid peroxide within hydrophobic PDMS resulted in sustained oxygen generation, whereby a single disk generated oxygen for more than 6 wk at an average rate of 0.026 mM per day. The ability of this oxygen-generating material to support cell survival was evaluated using a β cell line and pancreatic rat islets. The presence of a single PDMS-CaO2 disk eliminated hypoxia-induced cell dysfunction and death for both cell types, resulting in metabolic function and glucose-dependent insulin secretion comparable to that in normoxic controls. A single PDMS-CaO2 disk also sustained enhanced β cell proliferation for more than 3 wk under hypoxic culture conditions. Incorporation of these materials within 3D constructs illustrated the benefits of these materials to prevent the development of detrimental oxygen gradients within large implants. Mathematical simulations permitted accurate prediction of oxygen gradients within 3D constructs and highlighted conditions under which supplementation of oxygen tension would serve to benefit cellular viability. Given the generality of this platform, the translation of these materials to other cell-based implants, as well as ischemic tissues in general, is envisioned.

Shipment of Human Islets for Transplantation

The use of regional human islet cell processing centers (ICPC) supporting distant clinical islet transplantation programs (CITP) has proven successful in recent clinical trials. Standardization of islet shipping protocols is needed to preserve cell product identity, quantity, quality and sterility, and to meet criteria for transplantation. We evaluated the use of gas‐permeable bags for human islet preparation shipment from a single ICPC to two remote CITPs. Product release tests (counts, purity, viability, sterility and potency) were performed at both centers using identical protocols to determine adequacy for transplantation. Thirty‐five islet preparations were shipped either immediately after isolation (n = 20) or following culture (n = 15). Islet recovery rate after shipment was higher in cultured preparations, when compared to those not cultured (91.2 ± 4.9% vs. 72.9 ± 4.7%, respectively; p < 0.05), though the overall recovery rate based on isolation and pre‐transplant counts was comparable (72.9 ± 4.7% vs. 70.4 ± 3.5%, respectively; p = N.S.). All preparations met product release criteria for transplantation. Additional experiments showed that gas‐permeable bags led to improved recovery and potency, when compared to 50‐mL conical tubes or to non‐gas‐permeable bags for shipment. Collectively, our data demonstrate that the use of gas‐permeable bags is efficient for clinical‐grade and should be preferred also for the shipment of research‐grade islet preparations.

Rapamycin impairs in vivo proliferation of islet beta-cells

Background. Progressive graft dysfunction is commonly observed in recipients of islet allografts treated with high doses of rapamycin. This study aimed at evaluating the effect of rapamycin on pancreatic islet cell proliferation in vivo. Methods. The murine pregnancy model was utilized, since a high rate of &bgr;-cell proliferation occurs in a well-defined time frame. Rapamycin (0.2 mg/kg/day) was given to C57BL/6 mice for 5–7 days starting on day 7.5 of pregnancy. Cell proliferation was evaluated by detection of bromodeoxyuridine incorporation by immunohistochemistry. Results. Pregnancy led to increased &bgr;-cell proliferation and islet yield with skewing in islet size distribution as well as higher pancreatic insulin content, when compared to that of nonpregnant females. These effects of pregnancy on &bgr;-cell proliferation and mass were significantly blunted by rapamycin treatment. Minimal effect of rapamycin was observed on islet function both in vivo and in vitro. Rapamycin treatment of islets in vitro resulted in reduced p70s6k phosphorylation, which was paralleled by increased ERK1/2 phosphorylation. Conclusions. Rapamycin treatment reduces the rate of &bgr;-cell proliferation in vivo. This phenomenon may contribute to impair &bgr;-cell renewal in transplanted patients and to the progressive dysfunction observed in islet graft recipients.

Heme oxygenase-1 fused to a TAT peptide transduces and protects pancreatic β-cells

Abstract Transplantation of islets is becoming an established method for treating type 1 diabetes. However, viability of islets is greatly affected by necrosis/apoptosis induced by oxidative stress and other insults during isolation and subsequent in vitro culture. Expression of cytoprotective proteins, such as heme oxygenase-1 (HO-1), reduces the deleterious effects of oxidative stress in transplantable islets. We have generated a fusion protein composed of HO-1 and TAT protein transduction domain (TAT/PTD), an 11-aa cell penetrating peptide from the human immunodeficiency virus TAT protein. Transduction of TAT/PTD–HO-1 to insulin-producing cells protects against TNF-α-mediated cytotoxicity. TAT/PTD–HO-1 transduction to islets does not impair islet physiology, as assessed by reversion of chemically induced diabetes in immunodeficient mice. Finally, we report that transduction of HO-1 fusion protein into islets improves islet viability in culture. This approach might have a positive impact on the availability of islets for transplantation.

Device design and materials optimization of conformal coating for islets of Langerhans

Significance Cell encapsulation with biocompatible and permeable hydrogels may allow transplantation without immunosuppression. As an alternative to standard microencapsulation approaches that create single-sized capsules around cell clusters of different sizes, we have designed and optimized a novel approach for conformal coating of islets of Langerhans, resulting in thin, complete, and uniform coatings of similar thickness on differently sized islets. Coated islets exhibited no delay in glucose-stimulated insulin release or loss of function during culture, which is often observed with naked islets. The conformal coating reduces transplant volume relative to traditional encapsulation approaches. When transplanted in syngeneic diabetic mice, conformally coated islets restored and maintained euglycemia for more than 100 d with no foreign body reaction and normal revascularization. Encapsulation of islets of Langerhans may represent a way to transplant islets in the absence of immunosuppression. Traditional methods for encapsulation lead to diffusional limitations imposed by the size of the capsules (600–1,000 μm in diameter), which results in core hypoxia and delayed insulin secretion in response to glucose. Moreover, the large volume of encapsulated cells does not allow implantation in sites that might be more favorable to islet cell engraftment. To address these issues, we have developed an encapsulation method that allows conformal coating of islets through microfluidics and minimizes capsule size and graft volume. In this method, capsule thickness, rather than capsule diameter, is constant and tightly defined by the microdevice geometry and the rheological properties of the immiscible fluids used for encapsulation within the microfluidic system. We have optimized the method both computationally and experimentally, and found that conformal coating allows for complete encapsulation of islets with a thin (a few tens of micrometers) continuous layer of hydrogel. Both in vitro and in vivo in syngeneic murine models of islet transplantation, the function of conformally coated islets was not compromised by encapsulation and was comparable to that of unencapsulated islets. We have further demonstrated that the structural support conferred by the coating materials protected islets from the loss of function experienced by uncoated islets during ex vivo culture.

Improved Human Islet Isolation Using Nicotinamide

We investigated the effects of nicotinamide (NA) supplementation of the processing medium during islet isolation. One hundred and two human pancreata were processed for clinical transplantation after preservation either in the University of Wisconsin (UW) or using the two‐layer method (TLM). Pancreata were then divided into four groups and retrospectively analyzed. Group I: UW preservation followed by processing without NA, Group II: UW preservation and processing with NA, Group III: TLM preservation without NA, Group IV: TLM preservation with NA. We observed a significant increase in islet yield in Group II (4343 ± 348 IEQ/g) [mean ± SEM], compared to Group I (2789 ± 348 IEQ/g) (p = 0.005). Similarly, a significant increase in islet yield was observed when NA was used in the processing of organs preserved with TLM (Group IV: 5538 ± 413 vs. Group III: 3500 ± 629; p = 0.02). Furthermore islet yield was higher in Group IV than in Group II (p < 0.05). The percentages of preparations that qualified for transplantation were 25, 47, 45, 69% in Groups I, II, III, IV, respectively. Addition of NA to the processing medium significantly improved islet yields in both the UW and TLM preservation protocols, allowing for a higher percentage of islet preparations to qualify for clinical transplantation.

Enhanced oxygenation promotes β-cell differentiation in vitro

Despite progress in our knowledge about pancreatic islet specification, most attempts at differentiating stem/progenitor cells into functional, transplantable β cells have met only with moderate success thus far. A major challenge is the intrinsic simplicity of in vitro culture systems, which cannot approximate the physiological complexity of in vivo microenvironments. Oxygenation is a critical limitation of standard culture methods, and one of special relevance for the development of β cells, known for their high O2 requirements. Based on our understanding of islet physiology, we have tested the hypothesis that enhanced O2 delivery (as provided by novel perfluorocarbon‐based culture devices) may result in higher levels of β‐cell differentiation from progenitor cells in vitro. Using a mouse model of pancreatic development, we demonstrate that a physiological‐like mode of O2 delivery results in a very significant upregulation of endocrine differentiation markers (up to 30‐fold for insulin one and 2), comparable to relevant in vivo controls. This effect was not observed by merely increasing environmental O2 concentrations in conventional settings. Our findings indicate that O2 plays an important role in the differentiation of β cells from their progenitors and may open the door to more efficient islet differentiation protocols from embryonic and/or adult stem cells.

Macroporous three-dimensional PDMS scaffolds for extrahepatic islet transplantation.

Clinical islet transplantation has demonstrated success in treating type 1 diabetes. A current limitation is the intrahepatic portal vein transplant site, which is prone to mechanical stress and inflammation. Transplantation of pancreatic islets into alternative sites is preferable, but challenging, as it may require a three-dimensional vehicle to confer mechanical protection and to confine islets to a well-defined, retrievable space where islet neovascularization can occur. We have fabricated biostable, macroporous scaffolds from poly(dimethylsiloxane) (PDMS) and investigated islet retention and distribution, metabolic function, and glucose-dependent insulin secretion within these scaffolds. Islets from multiple sources, including rodents, nonhuman primates, and humans, were tested in vitro. We observed high islet retention and distribution within PDMS scaffolds, with retention of small islets (<100 μm) improved through the postloading addition of fibrin gel. Islets loaded within PDMS scaffolds exhibited viability and function comparable to standard culture conditions when incubated under normal oxygen tensions, but displayed improved viability compared to standard two-dimensional culture controls under low oxygen tensions. In vivo efficacy of scaffolds to support islet grafts was evaluated after transplantation in the omental pouch of chemically induced diabetic syngeneic rats, which promptly achieved normoglycemia. Collectively, these results are promising in that they indicate the potential for transplanting islets into a clinically relevant, extrahepatic site that provides spatial distribution of islets as well as intradevice vascularization.

Oxygen: a master regulator of pancreatic development?

Beyond its role as an electron acceptor in aerobic respiration, oxygen is also a key effector of many developmental events. The oxygen‐sensing machinery and the very fabric of cell identity and function have been shown to be deeply intertwined. Here we take a first look at how oxygen might lie at the crossroads of at least two of the major molecular pathways that shape pancreatic development. Based on recent evidence and a thorough review of the literature, we present a theoretical model whereby evolving oxygen tensions might choreograph to a large extent the sequence of molecular events resulting in the development of the organ. In particular, we propose that lower oxygenation prior to the expansion of the vasculature may favour HIF (hypoxia inducible factor)‐mediated activation of Notch and repression of Wnt/β‐catenin signalling, limiting endocrine cell differentiation. With the development of vasculature and improved oxygen delivery to the developing organ, HIF‐mediated support for Notch signalling may decline while the β‐catenin‐directed Wnt signalling is favoured, which would support endocrine cell differentiation and perhaps exocrine cell proliferation/differentiation.

Complementary methods for the determination of dissolved oxygen content in perfluorocarbon emulsions and other solutions.

Perfluorocarbons (PFCs) are compounds with increased oxygen solubility and effective diffusivity, making them ideal candidates for improving oxygen mass transfer in numerous biological applications. Historically, quantification of the mass transfer characteristics of these liquids has relied on the use of elaborate laboratory equipment and complicated methodologies, such as in-line gas chromatography coupled with temperature-controlled glass fritted diffusion cells. In this work, we present an alternative method for the determination of dissolved oxygen content in PFC emulsions and, by extrapolation, pure PFCs. We implemented a simple stirred oxygen consumption microchamber coupled with an enzymatic reaction for the quantitative determination of oxygen by optical density measurements. Chambers were also custom fitted with lifetime oxygen sensors to permit simultaneous measurement of internal chamber oxygen levels. Analyzing the consumption of oxygen during the enzymatic reaction via recorded oxygen depletion traces, we found a strong degree of correlation between the zero-order reaction rate and the total measured oxygen concentrations, relative to control solutions. The values obtained were in close agreement with published values in the literature, establishing the accuracy of this method. Overall, this method allows for easy, reliable, and reproducible measurements of oxygen content in aqueous solutions, including, but not limited to PFC emulsions.