Production of transgenic tilapia homozygous for a humanized insulin gene

Ancestral genomic duplication of the insulin gene in tilapia: An analysis of possible implications for clinical islet xenotransplantation using donor islets from transgenic tilapia expressing a humanized insulin gene

Tilapia, a teleost fish, have multiple large anatomically discrete islets which are easy to harvest, and when transplanted into diabetic murine recipients, provide normoglycemia and mammalian-like glucose tolerance profiles. Tilapia insulin differs structurally from human insulin which could preclude their use as islet donors for xenotransplantation. Therefore, we produced transgenic tilapia with islets expressing a humanized insulin gene. It is now known that fish genomes may possess an ancestral duplication and so tilapia may have a second insulin gene. Therefore, we cloned, sequenced, and characterized the tilapia insulin 2 transcript and found that its expression is negligible in islets, is not islet-specific, and would not likely need to be silenced in our transgenic fish.

Transgenic fish systems and their application in ecotoxicology

The use of transgenics in fish is a relatively recent development for advancing understanding of genetic mechanisms and developmental processes, improving aquaculture, and for pharmaceutical discovery. Transgenic fish have also been applied in ecotoxicology where they have the potential to provide more advanced and integrated systems for assessing health impacts of chemicals. The zebrafish (Daniorerio) is the most popular fish for transgenic models, for reasons including their high fecundity, transparency of their embryos, rapid organogenesis and availability of extensive genetic resources. The most commonly used technique for producing transgenic zebrafish is via microinjection of transgenes into fertilized eggs. Transposon and meganuclease have become the most reliable methods for insertion of the genetic construct in the production of stable transgenic fish lines. The GAL4-UAS system, where GAL4 is placed under the control of a desired promoter and UAS is fused with a fluorescent marker, has greatly enhanced model development for studies in ecotoxicology. Transgenic fish have been developed to study for the effects of heavy metal toxicity (via heat-shock protein genes), oxidative stress (via an electrophile-responsive element), for various organic chemicals acting through the aryl hydrocarbon receptor, thyroid and glucocorticoid response pathways, and estrogenicity. These models vary in their sensitivity with only very few able to detect responses for environmentally relevant exposures. Nevertheless, the potential of these systems for analyses of chemical effects in real time and across multiple targets in intact organisms is considerable. Here we illustrate the techniques used for generating transgenic zebrafish and assess progress in the development and application of transgenic fish (principally zebrafish) for studies in environmental toxicology. We further provide a viewpoint on future development opportunities.

A review of piscine islet xenotransplantation using wild-type tilapia donors and the production of transgenic tilapia expressing a "humanized" tilapia insulin

Most islet xenotransplantation laboratories have focused on porcine islets, which are both costly and difficult to isolate. Teleost (bony) fish, such as tilapia, possess macroscopically visible distinct islet organs called Brockmann bodies which can be inexpensively harvested. When transplanted into diabetic nude mice, tilapia islets maintain long-term normoglycemia and provide human-like glucose tolerance profiles. Like porcine islets, when transplanted into euthymic mice, they are rejected in a CD4 T-cell-dependent manner. However, unlike pigs, tilapia are so phylogenetically primitive that their cells do not express α(1,3)Gal and, because tilapia are highly evolved to live in warm stagnant waters nearly devoid of dissolved oxygen, their islet cells are exceedingly resistant to hypoxia, making them ideal for transplantation within encapsulation devices. Encapsulation, especially when combined with co-stimulatory blockade, markedly prolongs tilapia islet xenograft survival in small animal recipients, and a collaborator has shown function in diabetic cynomolgus monkeys. In anticipation of preclinical xenotransplantation studies, we have extensively characterized tilapia islets (morphology, embryologic development, cell biology, peptides, etc.) and their regulation of glucose homeostasis. Because tilapia insulin differs structurally from human insulin by 17 amino acids, we have produced transgenic tilapia whose islets stably express physiological levels of humanized insulin and have now bred these to homozygosity. These transgenic fish can serve as a platform for further development into a cell therapy product for diabetes.

Encapsulated piscine (tilapia) islets for diabetes therapy: studies in diabetic NOD and NOD-SCID mice

BACKGROUND

Our goal was to improve islet transplantation as a therapy for patients with type I diabetes mellitus. Because human donor islets are scarce, we are studying islet xenografts in the diabetic NOD mouse model. We hypothesize that optimal xenoislet survival will be achieved by the combination of donor islet immunoisolation with recipient immunosuppression. We and others have studied adult and neonatal porcine islets as sources of tissue for microencapsulated islet xenografts, but we believe it is also advantageous to consider using islets from fish, which can be raised in large numbers relatively quickly and economically. Therefore, in this study, we have evaluated the function of microencapsulated xenogeneic piscine (tilapia) islets transplanted intraperitoneally (IP) in NOD mice in the presence of CD4(+) T-cell depletion and/or costimulatory blockade.

METHODS

Spontaneously diabetic NOD mice or streptozotocin (STZ)-diabetic NOD-SCID mice were transplanted IP with microencapsulated tilapia islets. Recipient immunosuppression included anti-CD4 mAb, CTLA4-Ig, anti-CD80 mAb, anti-CD86 mAb, or anti-CD154 mAb, alone or in combination. Graft function was evaluated by blood glucose (BG) levels, intravenous (IV) and oral glucose tolerance tests (GTTs), histologic and immunohistochemical analyses of grafts, and flow cytometric analysis of peritoneal cells.

RESULTS

Encapsulated tilapia islets normalized random BG levels for up to 210 days in NOD-SCID mice. In diabetic NOD mice, encapsulated tilapia islets were rejected on day 11 ± 4 with a peritoneal infiltrate of macrophages, eosinophils, B cells, occasional neutrophils, but few T cells. Immunohistochemical staining demonstrated the presence of murine IgG on tilapia islets within capsules of rejecting, non-immunosuppressed mice, as well as murine IgG-positive lymphocytes in the layer of host cells surrounding those capsules. These findings suggested that our barium (Ba)-gelled alginate capsules are permeable to IgG and that anti-piscine antibodies may be involved in the rejection of encapsulated tilapia islets in untreated mice. No single immunosuppressive agent prolonged encapsulated tilapia islet survival in NOD mice, but the combination of CTLA4-Ig plus anti-CD154 mAb extended tilapia islet graft survival until rejection at 119 ± 20 days and inhibited host cell recruitment to the peritoneal cavity. Triple treatment with CTLA4-Ig, anti-CD154 mAb, and anti-CD4 mAb allowed graft survival for 157 ± 35 days with little evidence of a host cellular reaction. IV and oral glucose tolerance tests (GTTs) of recipients with functioning xenografts demonstrated remarkably normal metabolic function.

CONCLUSIONS

We conclude that microencapsulated tilapia islets can survive long term with excellent metabolic control in diabetic mice given targeted immunosuppression, suggesting that cross-species physiological incompatibility may not compromise the applicability of this novel approach for future clinical applications. We predict that an improved microcapsule that prevents the entrance of IgG will enhance tilapia islet survival in this model, possibly allowing the application of this technique with limited or no immunosuppression.

Animal transgenesis: an overview

Transgenic animals are extensively used to study in vivo gene function as well as to model human diseases. The technology for producing transgenic animals exists for a variety of vertebrate and invertebrate species. The mouse is the most utilized organism for research in neurodegenerative diseases. The most commonly used techniques for producing transgenic mice involves either the pronuclear injection of transgenes into fertilized oocytes or embryonic stem cell-mediated gene targeting. Embryonic stem cell technology has been most often used to produce null mutants (gene knockouts) but may also be used to introduce subtle genetic modifications down to the level of making single nucleotide changes in endogenous mouse genes. Methods are also available for inducing conditional gene knockouts as well as inducible control of transgene expression. Here, we review the main strategies for introducing genetic modifications into the mouse, as well as in other vertebrate and invertebrate species. We also review a number of recent methodologies for the production of transgenic animals including retrovirus-mediated gene transfer, RNAi-mediated gene knockdown and somatic cell mutagenesis combined with nuclear transfer, methods that may be more broadly applicable to species where both pronuclear injection and ES cell technology have proven less practical.